Interaction of Supernova Ejecta with Nearby Protoplanetary Disks
Interaction of Supernova Ejecta with Nearby Protoplanetary
Disks
N. Ouellette
Department of Physics, Arizona State University, PO Box 871504, Tempe, AZ 85287-1504
S. J. Desch and J. J. Hester
School of Earth and Space exploration, Arizona State University, PO Box 871404, Tempe,
AZ 85287-1404
Received ; accepted
http://arxiv.org/abs/0704.1652v1
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ABSTRACT
The early Solar System contained short-lived radionuclides such as 60Fe
(t1/2 = 1.5 Myr) whose most likely source was a nearby supernova. Previous
models of Solar System formation considered a supernova shock that triggered
the collapse of the Sun’s nascent molecular cloud. We advocate an alternative hy-
pothesis, that the Solar System’s protoplanetary disk had already formed when a
very close (< 1 pc) supernova injected radioactive material directly into the disk.
We conduct the first numerical simulations designed to answer two questions re-
lated to this hypothesis: will the disk be destroyed by such a close supernova;
and will any of the ejecta be mixed into the disk? Our simulations demonstrate
that the disk does not absorb enough momentum from the shock to escape the
protostar to which it is bound. Only low amounts (< 1%) of mass loss occur, due
to stripping by Kelvin-Helmholtz instabilities across the top of the disk, which
also mix into the disk about 1% of the intercepted ejecta. These low efficiencies
of destruction and injectation are due to the fact that the high disk pressures
prevent the ejecta from penetrating far into the disk before stalling. Injection
of gas-phase ejecta is too inefficient to be consistent with the abundances of ra-
dionuclides inferred from meteorites. On the other hand, the radionuclides found
in meteorites would have condensed into dust grains in the supernova ejecta, and
we argue that such grains will be injected directly into the disk with nearly 100%
efficiency. The meteoritic abundances of the short-lived radionuclides such as
60Fe therefore are consistent with injection of grains condensed from the ejecta
of a nearby (< 1 pc) supernova, into an already-formed protoplanetary disk.
Subject headings: methods: numerical—shock waves—solar system: formation—stars:
formation—supernovae: general
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1. Introduction
Many aspects of the formation of the Solar System are fundamentally affected by the
Sun’s stellar birth environment, but to this day the type of environment has not been well
constrained. Did the Sun form in a quiescent molecular cloud like the Taurus molecular
cloud in which many T Tauri stars are observed today? Or did the Sun form in the vicinity
of massive O stars that ionized surrounding gas, creating an H ii region before exploding as
core-collapse supernovae? Recent isotopic analyses of meteorites reveal that the early Solar
System held live 60Fe at moderately high abundances, 60Fe/56Fe ∼ 3− 7× 10−7 (Tachibana
& Huss 2003; Huss & Tachibana 2004; Mostefaoui et al. 2004, 2005; Quitte et al. 2005;
Tachibana et al. 2006). Given these high initial abundances, the origin of this short-lived
radionuclide (SLR), with a half-life of 1.5 Myr, is almost certainly a nearby supernova, and
these meteoritic isotopic measurements severely constrain the Sun’s birth environment.
Since its discovery, the high initial abundance of 60Fe in the early Solar System has
been recognized as demanding an origin in a nearby stellar nucleosynthetic source, almost
certainly a supernova (Jacobsen 2005; Goswami et al. 2005; Ouellette et al. 2005; Tachibana
et al. 2006, Looney et al. 2006). Inheritance from the interstellar medium (ISM) can be
ruled out: the average abundance of 60Fe maintained by ongoing Galactic nucleosynthesis in
supernovae and asymptotic-giant-branch (AGB) stars is estimated at 60Fe/56Fe = 3× 10−8
(Wasserburg et al. 1998) to 3 × 10−7 (Harper 1996), lower than the meteoritic ratio.
Moreover, this 60Fe is injected into the hot phase of the ISM (Meyer & Clayton 2000), and
incorporation into molecular clouds and solar systems takes ∼ 107 years or more (Meyer &
Clayton 2000; Jacobsen 2005), by which time the 60Fe has decayed. A late source is argued
for (Jacobsen 2005; see also Harper 1996, Meyer & Clayton 2000). Production within the
Solar System itself by irradiation of rocky material by solar energetic particles has been
proposed for the origin of other SLRs (e.g., Lee et al. 1998; Gounelle et al. 2001), but
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neutron-rich 60Fe is produced in very low yields by this process. Predicted abundances are
60Fe/56Fe ∼ 10−11, too low by orders of magnitude to explain the meteoritic abundance
(Lee et al. 1998; Leya et al. 2003; Gounelle et al. 2006). The late source is therefore a stellar
nucleosynthetic source, either a supernova or an AGB star. AGB stars are not associated
with star-forming regions: Kastner & Myers (1994) used astronomical observations to
estimate a firm upper limit of ≈ 3× 10−6 per Myr to the probability that our Solar System
was contaminated by material from an AGB star. The yields of 60Fe from an AGB star also
may not be sufficient to explain the meteoritic ratio (Tachibana et al. 2006). Supernovae,
on the other hand, are commonly associated with star-forming regions, and a core-collapse
supernova is by far the most plausible source of the Solar System’s 60Fe.
Supernovae are naturally associated with star-forming regions because the typical
lifetimes of the stars massive enough to explode as supernovae (>∼ 8M⊙) are
7 yr, too
short a time for them to disperse away from the star-forming region they were born in.
Low-mass (∼ 1M⊙) stars are also born in such regions. In fact, astronomical observations
indicate that the majority of low-mass stars form in association with massive stars. Lada &
Lada (2003) conducted a census of protostars in deeply embedded clusters complete to 2 kpc
and found that 70-90% of stars form in clusters with > 100 stars. Integration of the cluster
initial mass function indicates that of all stars born in clusters of at least 100 members,
about 70% will form in clusters with at least one star massive enough to supernova (Adams
& Laughlin 2001; Hester & Desch 2005). Thus at least 50% of all low-mass stars form in
association with a supernova, and it is reasonable to assume the Sun was one such star.
Astronomical observations are consistent with, and the presence of 60Fe demands, formation
of the Sun in association with at least one massive star that went supernova.
While the case for a supernova is strong, constraining the proximity and the timing
of the supernova is more difficult. The SLRs in meteorites provide some constraints on
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the timing. The SLR 60Fe must have made its way from the supernova to the Solar
System in only a few half-lives; models in which multiple SLRs are injected by a single
supernova provide a good match to meteoritic data only if the meteoritic components
containing the SLRs formed <∼ 1 Myr after the supernova (e.g., Meyer 2005, Looney et al.
2007). The significance of this tight timing constraint is that the formation of the Solar
System was somehow associated with the supernova. Cameron & Truran (1977) suggested
that the formation of the Solar System was triggered by the shock wave from the same
supernova that injected the SLRs, and subseqeuent numerical simulations show this is a
viable mechanism, provided several parsecs of molecular gas lies between the supernova
and the Solar System’s cloud core, or else the supernova shock will shred the molecular
cloud (Vanhala & Boss 2000, 2002). The likelihood of this initial condition has not yet been
established by astronomical observations. Also in 1977, T. Gold proposed that the Solar
System acquired its radionuclides from a nearby supernova, after its protoplanetary disk had
already formed (Clayton 1977). Astronomical observations strongly support this scenario,
especially since protoplanetary disks were directly imaged ∼ 0.2 pc from the massive star
θ1 Ori C in the Orion Nebula (McCaughrean & O’Dell 1996). Further imaging has revealed
protostars with disks near (≤ 1 pc) massive stars in the Carina Nebula (Smith et al. 2003),
NGC 6611 (Oliveira et al. 2005), and M17 and Pismis 24 (de Marco et al. 2006). This
hypothesis, that the Solar System acquired SLRs from a supernova that occurred < 1 pc
away, after the Sun’s protoplanetary disk had formed, is the focus of this paper.
In this paper we address two main questions pertinent to this model. First, are
protoplanetary disks destroyed by the explosion of a supernova a fraction of a parsec away?
Second, can supernova ejecta containing SLRs be mixed into the disk? These questions
were analytically examined in a limited manner by Chevalier (2000). Here we present the
first multidimensional numerical simulations of the interaction of supernova ejecta with
protoplanetary disks. In §2 we describe the numerical code, Perseus, we have written to
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study this problem. In §3 we discuss the results of one canonical case in particular, run
at moderate spatial resolution. We examine closely the effects of our limited numerical
resolution in §4, and show that we have achieved sufficient convergence to draw conclusions
about the survivability of protoplanetary disks hit by supernova shocks. We conduct a
parameter study, investigating the effects of supernova energy and distance and disk mass,
as described in §5. Finally, we summarize our results in §6, in which we conclude that
disks are not destroyed by a nearby supernova, that gaseous ejecta is not effectively mixed
into the disks, but that solid grains from the supernova likely are, thereby explaining the
presence of SLRs like 60Fe in the early Solar System.
2. Perseus
We have written a 2-D (cylindrical) hydrodynamics code we call Perseus. Perseus (son
of Zeus) is based heavily on the Zeus algorithms (Stone & Norman 1992). The code evolves
the system while obeying the equations of conservation of mass, momentum and energy:
+ ρ∇ · ~v = 0 (1)
= −∇p− ρ∇Φ (2)
= −p∇ · ~v, (3)
where ρ is the mass density, ~v is the velocity, p is the pressure, e is the internal energy
density and Φ is the gravitational potential (externally imposed). The Lagrangean, or
comoving derivative D/Dt is defined as
+ ~v · ∇. (4)
The pressure and energy are related by the simple equation of state appropriate for the
ideal gas law, p = e(γ − 1), where γ is the adiabatic index. The term p∇ · ~v represents
mechanical work.
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Currently, the only gravitational potential Φ used is a simple point source, representing
a star at the center of a disk. This point mass is constrained to remain at the origin.
Technically this violates conservation of momentum by a minute amount by excluding
the gravitational force of the disk on the central star. As discussed in §4, the star should
acquire a velocity ∼ 102 cm s−1 at the end of our simulations. In future simulations we will
include this effect, but for the problem explored here this is completely negligible.
The variables evolved by Perseus are set on a cylindrical grid. The program is separated
in two steps: the source and the transport step. The source step calculates the changes
in velocity and energy due to sources and sinks. Using finite difference approximations, it
evolves ~v and e according to
= −∇p− ρ∇Φ−∇ ·Q (5)
= −p∇ · ~v −Q : ∇~v, (6)
where Q is the tensor artificial viscosity. Detailed expressions for the artificial viscosity can
be found in Stone & Norman (1992).
The transport step evolves the variables according to the velocities present on the
grid. For a given variable A, the conservation equation is solved, using finite difference
approximations:
AdV = −
A~v · d~S. (7)
The variables A advected in this way are density ρ, linear and angular momentum ρ~v and
Rρvφ, and energy density e. As in the Zeus code, A on each surface element is found with
an upwind interpolation scheme; we use second-order van Leer interpolation.
Perseus is an explicit code and must satisfy the Courant-Friedrichs-Lewis (CFL)
stability criterion. The amount of time advanced per timestep, essentially, must not exceed
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the time it could take for information to cross a grid zone in the physical system. In every
grid zone, the thermal time step δtcs = ∆x/(cs) is computed, where ∆x is the size of the
zone (smallest of the r and z dimension) and cs is the sound speed. Also computed are
δtr = δr/(|vr|) and δtz = δz/(|vz|), where ∆r and ∆z are the sizes of the zone in the r and
z directions respectively. Because of artificial viscosity, a viscous time step must also be
added for stability. For a given grid zone, the viscous time step δtvisc = max(|(l ∇ · ~v/δr
|(l ∇ · ~v/δz2)|) is computed, where l is a length chosen to be a 3 zone widths. The final ∆t
is taken to be
∆t = C0 (δt
+ δt−2r + δt
z + δt
visc)
−1/2, (8)
where C0 is the Courant number, a safety factor, taken to be C0=0.5. To insure stability,
∆t is computed over all zones, and the smallest value is kept for the next step of the
simulation.
Boundary conditions were implemented using ghost zones as in the Zeus code. To
allow for supernova ejecta to flow past the disk, inflow boundary conditions were used at
the upper boundary (z = zmax), and outflow boundary conditions were used at the lower
boundary (z = zmin) and outer boundary (r = rmax). Reflecting boundary conditions,
were used on the inner boundary (r = rmin 6= 0) to best model the symmetry about the
protoplanetary disk’s axis. The density and velocity of gas flowing into the upper boundary
were varied with time to match the ejecta properties (see §3).
A more detailed description of the algorithms used in Perseus can be found in Stone &
Norman (1992).
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2.1. Additions to Zeus
To consider the particular problem of high-velocity ejecta hitting a protoplanetary
disk, we wrote Perseus with the following additions to the Zeus code. One minor change
is the use of a non-uniform grid. In all of our simulations we used an orthogonal grid
with uniform spacing in r but non-uniform spacing in the z direction. For example, in the
canonical simulation (§3), the computational domain extends from r = 4 to 80 AU, with
spacing ∆r = 1AU, for a total of 76 zones in r. The computational domain extends from
z = −50AU to +90AU, but zone spacings vary with z, from ∆z = 0.2AU at z = 0, to
∆z ≈ 3AU at the upper boundary. Grid spacings increased geometrically by 5% per zone,
for a total of 120 zones in z.
Another addition was the use of a radiative cooling term. The simulations bear out
the expectation that almost all of the shocked supernova ejecta flow past the disk before
they have time to cool significantly. Cooling is significant only where the ejecta collide with
the dense gas of the disk itself, but there the cooling is sensitive to many unconstrained
physical properties to do with the the chemical state of the gas, properties of dust, etc.
To capture the gross effects of cooling (especially compression of gas near the dense disk
gas) in a computationally simple way, we have adopted the following additional term in the
energy equation, implemented in the source step:
= −nenpΛ, (9)
where ne and np are the number of protons and electrons in the gas, and Λ is the cooling
function. The densities ne and np are obtained simply by assuming the hydrogen gas is
fully ionized, so ne = np = ρ/1.4mH. For gas temperatures above 10
4K, we take Λ of
a solar-metallicity gas from Sutherland and Dopita (1993); Λ typically ranges between
10−24 erg cm3 s−1 (at T = 104K) and Λ = 10−21 erg cm3 s−1 (at T = 105K). Below 104K
we adopted a flat cooling function of Λ = 10−24 erg cm3 s−1. At very low temperatures it
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is necessary to include heating processes as well as cooling, or else the gas rapidly cools
to unreasonable temperatures. Rather than handle transfer of radiation from the central
star, we defined a minimum temperature below which the gas is not allowed to cool:
Tmin = 300 (r/1AU)
−3/4 K. Perseus uses a simple first-order, finite-difference equation to
handle cooling. Although this method is not as precise as a predictor-corrector method, in
§2.4 we show that it is sufficiently accurate for our purposes.
Because Perseus is an explicit code, the implementation of a cooling term demands the
introduction of a cooling time step to insure that the gas doesn’t cool too rapidly during
one time step, resulting in negative temperatures or other instabilities. For a radiating gas,
the cooling timescale can be approximated by tcool ≈ kBT/nΛ, where kB is the Boltzmann
constant, T is the temperature of the gas, n is the number density and Λ is the appropriate
cooling function. This cooling timescale is calculated on all the grid zones where the
temperature exceeds 103K, and the cooling time step δtcool is defined to be 0.025 times
the shortest cooling timescale on the grid. If the smallest cooling time step is shorter that
the previously calculated ∆t as defined by eq. [8], then it becomes the new time step. We
ignore zones where the temperature is below 103K because heating and cooling are not fully
calculated anyway, and because these zones are always associated with very high densities
and cool extremely rapidly, on timescales as short as hours, too rapidly to reasonably follow
anyway.
Finally, to follow the evolution of the ejecta gas with respect to the disk gas, a tracer
“color density” was added. By defining a different density, the color density ρc, it is possible
to follow the mixing of a two specific parts of a system, in this case the ejecta and the disk.
By comparing ρc to ρ, it is possible to know how much of the ejecta is present in a given
zone relative to the original material. It is important to note that ρc is a tracer and does
not affect the simulation in any way.
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2.2. Sod Shock-Tube
We have benchmarked the Perseus code against a well-known analytic solution, the
Sod shock tube (Sod 1978). Tests were performed to verify the validity of Perseus’s results.
It is a 1-D test, and hence was only done in the z direction, as curvature effects would
render this test invalid in the r direction. Therefore, the gas was initially set spatially
uniform in r. 120 zones were used in the z direction. The other initial conditions of the Sod
shock-tube are as follows: the simulation domain is split in half and filled with a γ=1.4 gas;
in one half (z < 0.5 cm), the gas has a pressure of 1.0 dyne cm−2 and a density of 1.0 g cm−3,
while in the other half (z > 0.5 cm) the gas has a pressure of 0.1 dyne cm−2 and a density
of 0.125 g cm−3. The results of the simulation and the analytical solution at t = 0.245 s
are shown in Figure 1. The slight discrepancies between the analytic and numerical results
are attributable to numerical diffusion associated with the upwind interpolation (see Stone
& Norman 1992), match the results of Stone & Norman (1992) almost exactly, and are
entirely acceptable.
2.3. Gravitational Collapse
As a test problem involving curvature terms, we also simulated the pressure-free
gravitational collapse of a spherical clump of gas. A uniform density gas (ρ = 10−14 g cm−3)
was imposed everywhere within 30 AU of the star. As stated above, the only source of
gravitational acceleration in our simulations is the central protostar, with mass M = 1M⊙.
The grid on which this simulation takes place has 120 zones in the z direction and 80 in the
r direction The free-fall timescale under the gravitational potential of a 1 M⊙ star is 29.0
yrs. The results of the simulation can be seen in Figure 2. After 28 years, the 30AU clump
has contracted to the edge of the computational volume. Spherical symmetry is maintained
throughout as the gas is advected despite the presence of the inner boundary condition.
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2.4. Cooling
To test the accuracy of the cooling algorithm, a simple 2D grid of 64 zones by 64 zones
was set up. The simulation starts with gas at T = 1010K. The temperature of the gas is
followed until it reaches T = 104K. Simulations were run varying the cooling time step
δtcool. As the cooling subroutine does not use a predictor-corrector method, decreasing the
time step increases the precision. A range of cooling time steps, varying from 10 times what
is used in the code to 0.1 times what is used in the code, were tested. Since in the range of
T = 104K − 1010K, the cooling rate varies with temperature (according to Sutherland &
Dopita 1993), the size of the time step should affect the time evolution of the temperature.
This evolution is depicted in Figure 3, from which one can see that δtcool used in the code
is sufficient, as using smaller time steps gives the same result. In addition, we can see that
even the lesser precision runs give comparably good results, as the thermal time step of the
CFL condition prevents a catastrophically rapid cooling. The precision of the cooling is
limited by the accuracy of the cooling factors used, not the algorithm.
2.5. “Relaxed Disk”
Finally, we have modeled the long-term evolution of an isolated protoplanetary disk.
To begin, a minimum-mass solar nebula disk (Hayashi et al. 1985) in Keplerian rotation
is truncated at 30AU. The code then runs for 2000 years, allowing the disk to find its
equilibrium configuration under gravity from the central star (1M⊙), pressure and angular
momentum. We call this the “relaxed disk”, and use it as the initial state for the runs that
follow. To check the long term stability of the system, we allow the relaxed disk to evolve
an extra 2000 years. This test verifies the stability of the simulated disk against numerical
effects. In addition, using a color density, we can assess how much numerical diffusion
occurs in the code.
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After the extra 2000 years, the disk maintains its shape, and is deformed only at
its lowest isodensity contour, because of the gravitational infall of the surrounding gas
(Figure 4). Comparing this deformation to the results from the canonical run (§3), this
is a negligible effect. Some of the surrounding gas has accreted on the disk due to the
gravitational potential of the central star. The color density allows us to follow the location
of the accreted gas. After 2000 years, roughly 20% of the accreted mass has found its way
to the midplane of the disk due to the effects of numerical diffusion. Hence some numerical
diffusion exists and must be considered in what follows.
3. Canonical Case
In this section, we adopt a particular set of parameters pertinent to the disk and the
supernova, and follow the evolution of the disk and ejecta in some detail. The simulation
begins with our relaxed disk (§2.5), seen in Figure 5. Its mass is about 0.00838 M⊙, and
it extends from 4AU to 40AU, the inner parts of the disk being removed to improve
code performance. The gas density around the disk is taken to be a uniform 10 cm−3,
which is a typical density for an H ii region. This disk has similar characteristics to those
found in the Orion nebula, which have been photoevaporated down to tens of AU by the
radiation of nearby massive O stars (Johnstone, Hollenbach & Bally 1998). In setting up
our disk, we have ignored the effects of the UV flash that accompanies the supernova,
in which approximately 3 × 1047 erg of high-energy ultraviolet photons are emitted over
several days (Hamuy et al. 1988). The typical UV opacities of protoplanetary disk dust
are κ ∼ 102 cm2 g−1 (D’Allesio et al. 2006), so this UV energy does not penetrate below
a column density ∼ κ−1 ∼ 10−2 g cm−2. The gas density at the base of this layer is
typically ρ ∼ 10−15 g cm−3; if the gas reaches temperatures < 105K, tcool will not exceed
a few hours (§2.1). The upper layer of the disk absorbing the UV is not heated above a
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temperature T ∼ (EUV/4πd
2)mHκ/kB ∼ 10
5K. Because the gas in the disk absorbs and
then reradiates the energy it absorbs from the UV flash, we have ignored it. We have also
neglected low-density gas structures that are likely to have surrounded the disk, including
photoevaporative flows and bow shocks from stellar winds, as these are beyond the scope
of this paper. It is likely that the UV flash would greatly heat this low-density gas and
cause it to rapidly escape the disk anyway. Our “relaxed disk” initial state is a reasonable,
simplified model of the disks seen in H ii regions before they are struck by supernova shocks.
After a stable disk is obtained, supernova ejecta are added to the system. The canonical
simulation assumes Mej = 20M⊙ of material was ejected isotropically by a supernova
d = 0.3 pc away, with an explosion kinetic energy Eej = 10
51 erg, (1 f.o.e.). This is typical of
the mass ejected by a 25M⊙ progenitor star, as considered by Woosley & Weaver (1995),
and although more recent models show that progenitor winds are likely to reduce the ejecta
mass to < 10M⊙ (Woosley, Heger & Weaver 2002), we retain the larger ejecta mass as a
worst-case scenario for disk survivability. The ejecta are assumed to explode isotropically,
but with density and velocity decreasing with time. The time dependence is taken from
the scaling solutions of Matzner & McKee (1999); in analogy to their eq. [1], we define the
following quantities:
where R∗ is the radius of the exploding star, taken to be 50R⊙. The travel time from the
supernova to the disk is computed as ttrav = d/v∗, and is typically ∼ 100 years. Finally,
expressions for the time dependence of velocity, density and pressure of the ejecta, are
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obtained for any given time t after the shock strikes the disk:
vej(t) = v∗
ttrav
t+ ttrav
ρej(t) = ρ∗
ttrav
ttrav
t + ttrav
pej(t) = p∗
ttrav
ttrav
t+ ttrav
We acknowledge that supernova ejecta are not distributed homogeneously within the
progenitor (Matzner & McKee 1999), nor are they ejected isotropically (Woosley, Heger
& Weaver 2002), but more detailed modeling lies beyond the scope of this paper.
Our assumption of homologous expansion is in any case a worst-case scenario for disk
survivability in that the ejecta are front-loaded in a way that overestimates the ram pressure
(C. Matzner, private communication). As our parameter study (§5) shows, density and
velocity variations have little influence on the results.
The incoming ejecta and the shock they create while propagating through the
low-density gas of the H ii region can be seen in Figure 6. When the shock reaches the disk,
the lower-density outer edges are swept away, as the ram pressure of the ejecta is much
higher than the gas pressure in those areas. However, the shock stalls at the higher density
areas of the disk, as the gas pressure is higher there. A snapshot of the stalling shock can
be seen in Figure 7. As the ejecta hit the disk, they shock and thermalize, heating the
gas on the upper layers of the disk. This increases the pressure in that area, causing a
reverse shock to propagate into the incoming ejecta. The reverse shock will eventually stall,
forming a bow shock around the disk (Figures 8 and 9). Roughly 4 months have passed
between the initial contact and the formation of the bow shock.
Some stripping of the low density gas at the disk’s edge (> 30 AU) may occur as the
supernova ejecta is deflected around it, due primarily to the ram pressure of the ejecta. As
the stripped gas is removed from the top and the sides of the disk, it either is snowplowed
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away from the disk if enough momentum has been imparted to it, or it is pushed behind
the disk, where it can fall back onto it (Figure 10). In addition to stripping the outer
layers of the disk, the pressure of the thermalized shocked gas will compress the disk to a
smaller size; although they do not destroy the disk, the ejecta do temporarily deform the
disk considerably. Figure 11 shows the effect of the pressure on the disk, which has been
reduced in thickness and has shrunk to a radius of 30 AU. The extra external pressure
effectively aids gravity and allows the gas to orbit at a smaller radius with the same angular
momentum. As the ejecta is deflected across the top edge of the disk, some mixing between
the disk gas and the ejecta may occur through Kelvin-Helmholtz instabilities. Figure 12
shows a close up of the disk where a Kelvin-Helmholtz roll is occurring at the boundary
between the disk and the flowing ejecta. In addition, some ejecta mixed in with the stripped
material under the disk might also accrete onto the disk. As time goes by and slower ejecta
hit the disk, the ram pressure affecting the disk diminishes, and the disk slowly returns to
its original state, recovering almost completely after 2000 years (Figure 13).
The exchange of material between the disk and the ejecta is mediated through the
ejecta-disk interface, which in our simulations is only moderately well resolved. As discussed
in §4, the numerical resolution will affect how well we quantify both the destruction of the
disk and the mixing of ejecta into the disk. In the canonical run, at least, disk destruction
and gas mixing are minimal. Although some stripping has occurred while the disk was being
hit by the ejecta, it has lost less than 0.1% of its mass. The final disk mass, computed from
the zones where the density is greater than 100 cm−3, remains roughly at 0.00838 M⊙. Some
of the ejecta have also been mixed into the disk, but only with very low efficiency. A 30AU
disk sitting 0.3 pc from the supernova intercepts roughly one part in 1.7 × 107 of the total
ejecta from the supernova, assuming isotropic ejecta distribution. For 20 M⊙ of ejecta, this
corresponds to roughly 1.18 × 10−6M⊙ intercepted. At the end of the simulation, we find
only 1.48× 10−8M⊙ of supernova ejecta was injected in the disk, for an injection efficiency
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of about 1.3%. Some of the injected material could be attributed to numerical diffusion
between the outer parts of the disk and the inner layers: as seen in §2.5, Perseus is diffusive
over long periods of time. However, the distribution of the colored mass is qualitatively
different from that obtained from a simple numerical diffusion process. Figure 14 compares
the percentage of colored mass within a given isodensity contour for the canonical case and
the relaxed disk simulation of §2.5, at a time 500 years after the beginning of each of these
simulations. From this graph, it is clear that the process that injects the supernova ejecta is
not simply numerical diffusion, as it is much more efficient at injecting material deep within
the disk. The post-shock pressure of the ejecta gas, 100 years after initial contact, when
its forward progession in the disk has stalled is ∼ 2ρejv
ej/(γ + 1) = 2.8 × 10
−5 dyne cm−2.
(After 100 years, ρej = 2.2 × 10
−21 g cm−3 and vej = 1300 km s
−1.) The shock stalls where
the post-shock pressure is comparable to the disk pressure ∼ ρkBT/m̄. Hence at 20AU,
where the temperature of the disk is T ≈ 30K, the shock stalls at the isodensity contour
∼ 1.5 × 10−14 g cm−3. As about half of the color mass is mixed to just this depth, this is
further evidence that the color field in the disk represents a real physical mixing.
4. Numerical Resolution
The results of canonical run show many similarities to related problems that have
been studied extensively in the literature. The interaction of a supernova shock with a
protoplanetary disk resembles the interaction of a shock with a molecular cloud, as modeled
by Nittmann et al. (1982), Bedogni & Woodward (1990), Klein, McKee & Colella (1994;
hereafter KMC), Mac Low et al. (1994), Xu & Stone (1995), Orlando et al. (2005) and
Nakamura et al. (2006). Especially in Nakamura et al. (2006), the numerical resolutions
achieved in these simulations are state-of-the-art, reaching several ×103 zones per axis.
In those simulations, as in our canonical run, the evolution is dominated by two physical
– 18 –
effects: the transfer of momentum to the cloud or disk; and the onset of Kelvin-Helmholtz
(KH) instabilities that fragment and strip gas from the cloud or disk. KH instabilities
are the most difficult aspect of either simulation to model, because there is no practical
lower limit to the lengthscales on which KH instabilities operate (they are only suppressed
at scales smaller than the sheared surface). Increasing the numerical resolution generally
reveals increasingly small-scale structure at the interface between the shock and the cloud
or disk (see Figure 1 of Mac Low et al. 1994). The numerical resolution in our canonical
run is about 100 zones per axis; more specifically, there are about 26 zones in one disk
radius (of 30 AU), and about 20 zones across two scale heights of the disk (one scale-height
being about 2 AU at 20 AU). Our highest-resolution run used about 50 zones along the
radius of the disk, and placed about 30 zones across the disk vertically. In the notation of
KMC, then, our simulations employ about 20-30 zones per cloud radius, a factor of 3 lower
than the resolutions of 100 zones per cloud radius argued by Nakamura et al. (2006) to be
necessary to resolve the hydrodynamics of a shock hitting a molecular cloud.
Higher numerical resolutions are difficult to achieve; unlike the case of a supernova
shock with speed ∼ 2000 km s−1 striking a molecular cloud with radius of 1 pc, our
simulations deal with a shock with the same speed striking an object whose intrinsic
lengthscale is ∼ 0.1AU. Satisfying our CFL condition requires us to use timesteps that are
only ∼ 103 s, four orders of magnitude smaller than the timesteps needed for the case of a
molecular cloud. This and other factors conspire to make simulations of a shock striking
a protoplanetary disk about 100 times more computationally intensive than the case of a
shock striking a molecular cloud. Due to the numerous lengthscales in the problem imposed
by the star’s gravity and the rotation of the disk, it is not possible to run the simulations
at low Mach numbers and then scale the results to higher Mach numbers. We intend to
create a parallelized version of Perseus to run on a computer cluster in the near future, but
until then, our numerical resolution cannot match that of simulations of shocks interacting
– 19 –
with molecular clouds. This begs the question, if our resolution is not as good as has been
achieved by others, is it good enough?
To quantify what numerical resolutions are sufficient, we examine the physics of a
shock interacting with a molecular cloud, and review the convergence studies of the same
undertaken by previous authors. In the most well-known simulations (Nittmann et al.
1982; KMC; Mac Low et al. 1994; Nakamura et al. 2006), it is assumed that a low-density
molecular cloud with no gravity or magnetic fields is exposed to a steady shock. The
shock collides with the cloud, producing a reverse shock that develops into a bow shock;
a shock propagates through the cloud, passing through it in a “cloud-crushing” time tcc.
The cloud is accelerated, but as long as a velocity difference between the high-velocity gas
and the cloud exists, KH instabilities grow that create fragments with significant velocity
dispersions, ∼ 10% of the shock speed (Nakamura et al. 2006). Cloud destruction takes
place before the cloud is fully accelerated, and the cloud is effectively fragmented in a few
× tcc before the velocity difference diminishes. These fragments are not gravitationally
bound to the cloud and easily escape. As long as the shock remains steady for a few × tcc,
it is inevitable that the cloud is destroyed.
As KH instabilities are what fragment the cloud and accelerate the fragments, it is
important to model them carefully, with numerical resolution as high as can be achieved.
KMC stated in their abstract and throughout their paper that 100 zones per cloud
radius were required for “accurate results”; however, all definitions of what was meant
by “accurate”, or what were the physically relevant “results” were deferred to a future
“Paper II”. A companion paper by Mac Low et al. (1994) referred to the same Paper II
and repeated the claim that 100 zones per axis were required. Nakamura et al. (2006),
published this year, appears to be the Paper II that reports the relevant convergence
study and quantifies what is meant by accurate results. Global quantities, including the
– 20 –
morphology of the cloud, its forward mass and momentum, and the velocity dispersions
of cloud fragments, were defined and calculated at various levels of numerical resolution.
These were then compared to the same quantities calculated using the highest achievable
resolutions, about 500 zones per cloud radius (over 1000 zones per axis). The quantities
slowest to converge with higher numerical resolution were the velocity dispersions, probably,
they claim, because these quantities are so sensitive to the hydrodynamics at shocks and
contact discontinuities where the code becomes first-order accurate only. The velocity
dispersions converged to within 10% of the highest-resolution values only when at least
100 zones per cloud radius were used. For this single arbitrary reason, Nakamura et al.
(2006) claimed numerical resolutions of 100 zones per cloud radius were necessary. We note,
however, that the other quantities to do with cloud morphology and momentum were found
to converge much more readily; according to Figure 1 of Nakamura et al. (2006), numerical
resolutions of only 30 zones per cloud radius are sufficient to yield values within 10% of the
values found in the highest-resolution simulations. And although the velocity dispersions
are not so well converged at 30 zones per cloud radius, even then the errors do not exceed
a factor of 2. Assuming that the problem we have investigated is similar enough to that
investigated by Nakamura et al. (2006) so that their convergence study could be applied
to our problem, we would conclude that even our canonical run is sufficiently resolving
relevant physical quantities, the one possible exception being the velocities of fragments
generated by KH instabilities, where the errors could be a factor of 2.
Of course, the problem we have investigated, a supernova shock striking a
protoplanetary disk, is different in four very important ways from the cases considered by
KMC, Mac Low et al. (1994) and Nakamura et al. (2006). The most important fundamental
difference is that the disk is gravitationally bound to the central protostar. Thus, even
if gas is accelerated to supersonic speeds ∼ 10 km s−1, it is not guaranteed to escape the
star. Second, the densities of gas in the disk, ρdisk, are significantly higher than the density
– 21 –
in the gas colliding with the disk, ρej. In the notation of KMC, χ = ρdisk/ρej. Because
the disk density is not uniform, no single value of χ applies, but if χ is understood to
refer to different parcels of disk gas, χ would vary from 104 to over 108. This affects the
magnitudes of certain variables (see, e.g., Figure 17 of KMC regarding mix fractions), but
also qualitatively alters the problem: the densities and pressures in the disk are so high that
the supernova shock cannot cross through the disk, instead stalling at several scale heights
above the disk. Unlike the case of a shock shredding a molecular cloud, the cloud-crushing
timescale tcc is not even a relevant quantity for our calculations. The third difference is
that shocks cannot remain non-radiative when gas is as dense as it is near the disk. Using
ρ = 10−14 g cm−3 and Λ = 10−24 erg cm3 s−1, tcool is only a few hours, and shocks in the disk
are effectively isothermal. Shocks propagating into the disk therefore stall at somewhat
higher locations above the disk than they would have if they were adiabatic. Finally, the
fourth fundamental difference between our simulations and those investigated in KMC,
Mac Low et al. (1994) and Nakamura et al. (2006) is that we do not assume steady shocks.
For supernova shocks striking protoplanetary disks about 0.3 pc away, the most intense
effects are felt only for a time ∼ 102 years, and after only 2000 years the shock has for all
purposes passed. There are limits, therefore, to the energy and momentum that can be
delivered to the disk. Very much unlike the case of a steady, non-radiative shock striking
a low-density, gravitationally unbound molecular cloud, where ultimately destruction of
the cloud is inevitable, many factors contribute to the survivability of protoplanetary disks
struck by supernova shocks.
This conclusion is borne out by a resolution study we have conducted that shows
that the vertical momentum delivered to the disk is certainly too small to destroy it, and
that we are not significantly underresolving the KH instabilities at the top of the disk.
Using the parameters of our canonical case, we have conducted 6 simulations with different
numerical resolutions. The resolutions range from truly awful, with only 8 zones in the
– 22 –
radial direction (∆r = 10AU) and 18 zones in the vertical direction (with ∆z = 1AU at
the midplane, barely sufficient to resolve a scale height), to our canonical run (76 x 120),
to one high-resolution run with 152 radial zones (∆r = 0.5AU) and 240 vertical zones
(∆z = 0.13AU at the midplane). On an Apple G5 desktop with two 2.0-GHz processors,
these simulations took from less than a day to 80 days to run. To test for convergence,
we calculated several global quanities Q, including: the density-weighted cloud radius, a;
the density-weighted cloud thickness, c; the density-weighted vertical velocity, 〈vz〉; the
density-weighted velocity dispersion in r, δvr; the density-weighted velocity dispersion in z,
〈vz〉; as well as the mass of ejecta injected into the disk, Minj. Except for the last quantity,
these are defined exactly as in Nakamura et al. (2006), but using a density threshold
corresponding to 100 cm−3. Each global quantity was measured at a time 500 years into
each simulation. We define each global quantity Q as a function of numerical resolution n,
where n is the geometric mean of the number of zones along each axis, which ranges from
12 to 191. To compare to the resolutions of KMC, one must divide this number by about 3
to get the number of zones per “cloud radius” (two scale heights at 20 AU) in the vertical
direction, and divide by about 2 to get the number of zones per cloud radius in the radial
direction. The convergence is measured by computing |Q(n)−Q(nmax)| /Q(nmax), where
nmax = 191 corresponds to our highest resolution case. In Figure 15 we plot each quantity
Q(n) as a function of resolution n (except 〈vz〉). All of the quantities have converged
to within 10%, the criterion imposed by Nakamura et al. (2006) as signifying adequate
convergence. It is significant that δvr has converged to within 10%, because this is the
quantity relevant to disk destruction by KH instabilities. Material is stripped from the
disk only if supersonic gas streaming radially above the top of the disk can generate KH
instabilities and fragments of gas that can then be accelerated radially to escape velocities.
If we were underresolving this layer significantly, one would expect large differences in δvr
as the resolution was increased, but instead this quantity has converged. Higher-resolution
– 23 –
simulations are likely to reveal smaller-scale KH instabilities and perhaps more stripping of
the top of the disk, but not an order of mangitude more.
The convergence of 〈vz〉 with resolution is handled differently because unlike the other
quantities, 〈vz〉 can vanish at certain times. The disk absorbs the momentum of the ejecta
and is pushed downward, but unlike the case of an isolated molecular cloud, the disk feels
a restoring force from the gravity of the central star. The disk then undergoes damped
vertical oscillations about the origin as it collides with incoming ejecta at lower and lower
speeds. This behavior is illustrated by the time-dependence of 〈vz〉, shown in Figure 16
for two numerical resolutions, our canonical run (n = 95) and our highest-resolution run
(n = 191). Figure 16 shows that the vertical velocity of the disk oscillates about zero, but
with an amplitude ∼ 0.1 km s−1. The time-average of this amplitude can be quantified by
−< vz >2
, where the bar represents an average over time; the result is 825 cm s−1
for the highest-resolution run and is only 2% smaller for the canonical resolution. The
difference between the two runs is generally much smaller than this; except for a few times
around t = 150 yr, and t = 300 yr, when the discrepancies approach 30%, the agreement
between the two resolutions is within 10%. The time-averaged dispersion of the amplitude
of the difference (defined as above for 〈vz〉 itself) is only 12.0 cm s
−1, which is only 1.5% of
the value for 〈vz〉 itself. Taking a time average of |〈vz〉95 − 〈vz〉191| / |〈vz〉191| yields 8.7%.
We therefore claim convergence at about the 10% level for 〈vz〉 as well.
Using these velocities, we also note here that the neglect of the star’s motion is entirely
justified. The amplitude of 〈vz〉 is entirely understandable as reflecting the momentum
delivered to the disk by the supernova ejecta, which is ∼ 20M⊙ (πR
disk/4πd
2) Vej ∼
10−3M⊙ km s
−1, and which should yield a disk velocity ∼ 0.1 km s−1. The period of
oscillation is about 150 years, which is consistent with most of this momentum being
delivered to the outer reaches of the disk from 25 to 30 AU where the orbital periods are
– 24 –
125 to 165 years. These velocities are completely unaffected by the neglected velocity of the
central star, whose mass is 120 times greater than the disk’s mass. If the central star, with
mass ∼ 1M⊙, had been allowed to absorb the ejecta’s momentum, it would only move at
∼ 100 cm s−1 and be displaced at most 0.4 AU after 2000 years. This neglected velocity, is
much smaller than all other relevant velocities in the problem, including |〈vz〉| ∼ 800 cm s
as well as the escape velocities (∼ 10 km s−1), the velocities of gas flowing over the disk
(∼ 102 km s−1), and of course the shock speeds (∼ 103 km s−1).
Our analysis shows that we have reached adequate convergence with our canonical
numerical resolution (n = 95). We observe KH instabilities in all of our simulations (except
n = 12), and we see the role they play in stripping the disk and mixing ejecta gas into it.
We are therefore confident that we are adequately resolving these hydrodynamic features;
nevertheless, we now consider a worst-case scenario in which we KH instabilities can strip
the disk with 100% efficiency where they act, and ask how much mass the disk could
possibly lose under such conditions.
Supernova ejecta that has passed through the bow shock and strikes the disk necessarily
stalls where the gas pressure in the disk exceeds the ram pressure of the ejecta. Below this
level, the momentum of the ejecta is transferred not as a shock but as a pressure (sound)
wave. Gas motions below this level are subsonic. Note that this is drastically different from
the case of an isolated molecular cloud as studied by KMC and others; the high pressure in
the disk is maintained only because of the gravitational pull of the central star.
The location where the incoming ejecta stall is easily found. Assuming the vertical
isothermal minimum-mass solar nebula disk of Hayashi et al. (1985), the gas density varies
as ρ(r, z) = 1.4×10−9 (r/1AU)−21/8 exp(−z2/2H2) g cm−3, where H = cs/Ω, cs is the sound
speed and Ω is the Keplerian orbital frequency. Using the maximum density and velocity
of the incoming ejecta (ρej = 1.2 × 10
−20 g cm−3 and Vej = 2200 km s
−1), the ram pressure
– 25 –
of the shock striking the disk does not exceed pram = ρejV
ej/4 = 1.5 × 10
−4 dyne cm−2 (the
factor of 1/4 arises because the gas must pass through the bow shock before it strikes the
disk). At 10 AU the pressure in the disk, ρc2s , exceeds the ram pressure at z = 2.7H , and at
20 AU the ejecta stall at z = 1.7H ; the gas densities at these locations are ≈ 10−13 g cm−3.
At later times, the ejecta stall even higher above the disk, because pram ∝ t
−5 (cf. eq. [11]).
For example, at t = 100 yr, the ram pressure drops below 1×10−5 dyne cm−2, and the ejecta
stall above z = 3.6H (10 AU) and z = 2.9H (20 AU).
The column density above a height z in a vertically isothermal disk is easily found to
be Σ(> z) ≈ ρ(z)H2/z = p(z)/(Ω2z). Integrating over radius, the total amount of disk gas
that ever comes into contact with ejecta is (approximating z = 2H):
Mss =
pramr
πpramR
. (12)
Using a disk radius Rd = 30AU, the maximum amount of disk gas that is actually
exposed to a shock at any time is only 1.5 × 10−5M⊙, or 0.2% of the disk mass. This
fraction decreases with time as pram ∝ t
−5 (eq. [11]); the integral over time of pram is
pram(t = 0) × ttrav/4. The ram pressure drops so quickly, that effectively ejecta interact
with this uppermost 0.2% of the disk mass only for about 30 years. This is equivalent to
one orbital timescale at 10 AU, so the amount of disk gas that is able to mix or otherwise
interact with the ejecta hitting the upper layers of the disk is very small, probably a few
percent at most. As for KH instabilities, they are initiated when the Richardson number
drops below a critical value, when
(∂U/∂z)2
, (13)
where g = −Ω2z is the vertical gravitational acceleration, Ω is the Keplerian orbital
frequency, and (∂U/∂z) is the velocity gradient at the top of the disk. Below the stall
point, all gas motions are subsonic and the velocity gradient would have to be execptionally
– 26 –
steep, with an unreasonably thin shear layer thickness, <∼H/10, to initiate KH instabilities.
Mixing of ejecta into the disk is quite effective above where the shock stalls, as illustrated
by Figure 14; it is in these same layers (experiencing supersonic velocities) that we expect
that KH instabilities to occur, but again <∼ 1% of the disk mass can be expected to interact
with these layers.
To summarize, our numerical simulations are run at a lower simulation (by a factor
of about 3) than has been claimed necessary to study the interaction of steady shocks
with gravitationally unbound molecular clouds, but the drastically different physics of the
problem studied here as allowed us to achieve numerical convergence and allowed us to reach
meaningful conclusions. Our global quantities have converged to within 10%, the same
criterion used by Nakamura et al. (2006) to claim convergence. The problem is so different
because the disk is tightly gravitationally bound to the star and the supernova shock is
of finite duration. The high pressure in the disk makes the concept of a cloud-crushing
time meaningless, because the ejecta stall before they drive through even 1% of the disk
gas. Rather than a sharp interface between the ejecta and the disk, the two interact via
sound waves within the disk, which entails smoother gradients. While we do resolve KH
instabilities in this interface, we allow that we may be underresolving this layer; but even
if we are, this will not affect our conclusions regarding the disk survival or the amount of
gas mixed into the disk. This is because we already find that mass is stripped from the
disk and ejecta are mixed into the disk very effectively (see Figure 14) above the layer
where the ejecta stall, and below this layer mixing is much less efficient and all the gas is
subsonic and bound to the star. It is inevitable that mass loss and mixing of ejecta should
be only at the ∼ 1% level. Similar studies using higher numerical resolutions are likely to
reveal more detailed structures at the disk-ejecta interface, but it is doubtful that more
than a few percent of the disk mass can be mixed-in ejecta, and it is even more doubtful
that even 1% of the disk mass can be lost. We therefore have sufficient confidence in our
– 27 –
canonical resolution to use it to test the effects of varying parameters on gas mixing and
disk destruction.
5. Parameter Study
5.1. Distance
Various parameters were changed from the canonical case to study their effect on the
survival of the disk and the injection efficiency of ejecta, including: the distance between the
supernova and the disk, d; the explosion energy of the supernova, Eej; and the mass of gas
in the disk, Mdisk. In all these scenarios, the resolution stayed the same as in the canonical
case. The first parameter studied was the distance between the supernova and the disk.
From the canonical distance of 0.3 pc, the disk was moved to 0.5 pc and 0.1 pc. The main
effect of this change is to vary the density of the ejecta hitting the disk (see eq. [11]). If the
disk is closer, the gaseous ejecta is less diluted as it hits the disk. Hence these simulations
are essentially equivalent to simulating a denser or a more tenuous clump of gas hitting the
disk in an non-homogeneous supernova explosion. The results of these simulations can be
seen in Table 2. The “% injected” column gives the percentage of the ejecta intercepted by
the disk [with an assumed cross-section of π(30 AU)2] that was actually mixed into the disk.
The third column gives the estimated 26Al/27Al ratio that one would expect in the disk
if the SLRs were delivered in the gas phase. This quantity was calculated using a disk
chemical composition taken from Lodders (2003), and the ejecta isotopic composition from
a 25 M⊙ supernova taken from Woosley & Weaver (1995), which ejects M = 1.27× 10
of 26Al. Although the injection efficiency increases for denser ejecta, and the geometric
dilution decreases for a closer supernova, gas-phase injection of ejecta into a disk at 0.1 pc
cannot explain the SLR ratios in meteorites. The 26Al/27Al ratio is off by roughly an order
of magnitude from the measured value of 5 × 10−5 (e.g., MacPherson et al. 1995). Stripping
– 28 –
was more important with denser ejecta (d = 0.1 pc), although still negligible compared to
the mass of the disk; only 0.7% of the disk mass was lost.
5.2. Explosion Energy
We next varied the explosion energy, which defines the velocity at which the ejecta
travel. The explosion energy was changed from 1 f.o.e. to 0.25 and 4 f.o.e., effectively
modifying the ejecta velocity from 2200 km/s to 1100 km/s and 4400 km/s, respectively.
The results of the simulations can be seen in Table 3. Slower ejecta thermalizes to a lower
temperature, and does not form such a strong reverse shock. Therefore, slower ejecta
is injected at a slightly higher efficiency into a disk. Primarily, though, the results are
insensitive to the velocity of the incoming supernova ejecta.
5.3. Disk Mass
The final parameter varied was the mass of the disk. From these simulations, the mass
of the the minimum mass disk used in the canonical simulation was increased by a factor
of 10, and decreased by a factor of 10. The results of the simulations can be seen in Table
4. Increasing the mass by a factor of 10 slightly increases, but this could be due to the fact
that the disk does not get compressed as much as the canonical disk (it has a higher density
and pressure at each radius). Hence the disk has a larger surface to intercept the ejecta
(the calculation for injection efficiency assumes a radius of 30 AU). Reducing the mass by
a factor of 10 increases the efficiency. As the gas density in the disk is less, the pressure
is less, and hence the ejecta is able to get closer to the midplane, increasing the amount
injected.
– 29 –
6. Conclusions
In this paper, we have described a 2-D cylindrical hydrodynamics code we wrote,
Perseus, and the results from the application of this code to the problem of the interaction
of supernova shocks with protoplanetary disks. A main conclusion of this paper is that disks
are not destroyed by a nearby supernova, even one as close as 0.1 pc. The robustness of
the disks is a fundamentally new result that differs from previous 1-D analytical estimates
(Chevalier 2000) and numerical simulations (Ouellette et al. 2005). In those simulations, in
which gas could not be deflected around the disk, the full momentum of the supernova ejecta
was transferred directly to each annulus of gas in the disk. Chevalier (2000) had estimated
that disk annuli would be stripped away from the disk wherever MejVej/4πd
2 > ΣdVesc,
where Σd is the surface density of the disk [Σd = 1700 (r/1AU)
−3/2 g cm−2 for a minimum
mass disk; Hayashi et al. 1985), and Vesc is the escape velocity at the radius of the annulus.
In the geometry considered here, the momentum is applied at right angles to the disk
rotation, so vesc can be replaced with the Keplerian orbital velocity, as the total kinetic
energy would then be sufficient for escape. Also, integrating the momentum transfer over
time (eq. [11]), we find Vej = 3v⋆/4. Therefore, using the criterion of Chevalier (2000), and
considering the parameters of the canonical case but with d = 0.1 pc, the disk should have
been destroyed everywhere outside of 30.2AU, representing a loss of 13% of the mass of a
40 AU radius disk. Comparable conclusions were reached by Ouellette et al. (2005).
In contrast, as these 2-D simulations show, the disk becomes surrounded by high-
pressure shocked gas that cushions the disk and deflects ejecta around the disk. This
high-pressure gas has many effects. First, the bow shock deviates the gas, making part
of the ejecta that would have normally hit the disk flow around it. From Figure 11, by
following the velocity vectors, it is possible to estimate that the gas initially on trajectories
withr > 20AU will be deflected by > 14◦ after passing through the bow showk, and will
– 30 –
miss the disk. For a disk 30 AU in size, this represents a reduction in the mass flux hitting
by ≈ 45%; more thorough calculations give a reduction of ≈ 50%. Second, the bow shock
reduces the forward velocity of the gas that does hit the disk. Gas deviated sideways about
14◦, will have lost more than 10% of its forward velocity upon reaching the disk. These
two effects combined conspire to reduce the amount of momentum hitting the disk by 55%
overall. By virtue of the smaller escape velocity and the lower disk surface density, gas at
the disk edges is most vulnerable to loss by the momentum of the shock, but it at the disk
edges that the momentum of the supernova shock is most sharply reduced. Because of
the loss of momentum, the disk in the previous paragraph could survive out to a radius of
about 45AU.
A third, significant effect of the surrounding high-pressure shocked gas, though, is
its ability to shrink the disk to a smaller radius. The pressure in the post-shock gas is
∼ 2ρejv
ej/(γ + 1) = 4.4 × 10
−4 dyne cm−2, so the average pressure gradient in the disk
between about 30 and 35 AU is ≈ 1.9 × 10−18 dyne cm−3. This is to be compared to
the gravitational force per volume at 35AU, ρg = 4.8 × 10−19 dyne cm−3 (at 35 AU,
ρ ∼ 1.0× 10−15 in the canonical disk.) The pressure of the shocked gas enhances the inward
gravitational force by a significant amount, causing gas of a given angular momentum
to orbit at a smaller radius than it would if in pure Keplerian rotation. When this high
pressure is relieved after the supernova shock has passed, the disk is restored to Keplerian
rotation and expands to its original size. While the shock is strongest, the high-pressure gas
forces a protoplanetary disk to orbit at a reduced size, ≈ 30AU, where it is invulnerable to
being stripped by direct transfers of momentum. Because of these combined effects of the
cushion of high-pressure shocked gas surrounding the disk—reduction in ejecta momentum
and squeezing of the disk—protoplanetary disks even 0.1 pc from the supernova lose < 1%
of their mass.
– 31 –
Destruction of the disk, if it occurs at all, is due to stripping of the low-density upper
layers of the disk by Kelvin-Helmholtz (KH) instabilities. We observed KH instabilities in
all of our simulations (except n = 12), and we observe their role in stripping gas from the
disk and mixing supernova ejecta into the disk (e.g., Figure 12). Our canonical numerical
resolution (n = 95) and our highest-resolution simulation (n = 191), corresponding to
effectively 20-30 zones per cloud radius in the terminology of KMC, are just adequate to
provide convergence at the 10% level, as described in §4. We are confident we are capturing
the relevant physics in our simulations, but we have shown that even if KH instabilities
are considerably more effective than we are modeling, that no more than about 1% of
the disk mass could ever be affected by KH instabilities. This is because the supernova
shock stalls where the ram pressure is balanced by the pressure in the disk, and for typical
protoplanetary disk conditions, this occurs several scale heights above the midplane. It is
unlikely that higher-resolution simulations would observe loss of more than ∼ 1% of the
disk mass. We observed that the ratio of injected mass to disk mass was typically ∼ 1% as
well, for similar reasons. We stipulate that mixing of ejecta into the disk is more subtle
than stripping of disk mass, but given the limited ability of the supernova ejecta to enter
the disk, we find it doubtful that higher-resolution simulations would increase by more than
a few the amount of gas-phase radionuclides injected into the disk. Therefore, while disks
like those observed in the Orion Nebula (McCaughrean & O’Dell 1995) should survive the
explosions of the massive stars in their vicinity, and while these disks would then contain
some fraction of the supernova’s gas-phase ejecta, they would not retain more than a small
fraction (∼ 1%) of the gaseous ejecta actually intercepted by the disk. If SLRs like 26Al are
in the gas phase of the supernova (as modeled here), they will not be injected into the disk
in quantities large enough to explain the observed meteoritic ratios, failing by 1-2 orders of
magnitude.
Of course, the SLRs inferred from meteorites, e.g., 60Fe and 26Al, would not be detected
– 32 –
if they were not refractory elements. These elements should condense out of the supernova
ejecta as dust grains before colliding with a disk (Ebel & Grossman 2001). Colgan et al.
(1994) observed the production of dust at the temperature at which FeS condenses, 640
days after SN 1987A, suggesting that the Fe and other refractory elements should condense
out of the cooling supernova ejecta in less than a few years. (The supernova ejecta is
actually quite cool because of the adiabatic expansion of the gas.) As the travel times from
the supernova to the disks in our simulations are typically 20 − 500 years, SLRs can be
expected to be sequestered into dust grains condensed from the supernova before striking a
disk.
Dust grains will be injected into the disk much more effectively than gas-phase ejecta.
When the ejecta gas and dust, moving together at the same speed, encounter the bow
shock, the gas is almost instantaneously deflected around the disk, but the dust grains will
continue forward by virtue of their momentum. The dust grains will be slowed only as fast
as drag forces can act. The drag force F on a highly supersonic particle is F ≈ πa2 ρg ∆v
where a is the dust radius, ρg is the gas density, and ∆v is the velocity difference between
the gas and the dust. Assuming the dust grains are spherical with internal density ρs, the
resultant acceleration is dv/dt = −(3ρg∆v
2)/(4ρsa). Immediately after passage through the
bow shock, the gas velocity has dropped to 1/4 of the ejecta velocity, so ∆v ≈ (3/4)vej.
Integrating the acceleration, we find the time t1/2 for the dust to lose half its initial velocity:
t1/2 =
16ρsa
9ρgvej
. (14)
Measurements of SiC grains with isotopic ratios indicative of formation in supernova ejecta
reveal typical radii of a ∼ 0.5µm (Amari et al. 1994; Hoppe et al. 2000). Assuming
similar values for all supernova grains, and an internal density ρs = 2.5 g cm
−3, and using
the maximum typical gas density in the region between the bow shock and the disk,
ρg ≈ 5 × 10
−20 g cm−3 , we find a minimum dust stopping time t1/2 ≈ 2 × 10
7 s. In that
– 33 –
time, the dust will have travelled about 300AU. As the bow shock lies about 20 AU from
the disk, the dust will encounter the protoplanetary disk well before travelling this distance,
and we conclude that the dust the size of typical supernova SiC grains is not deflected
around the disk. We estimate that nearly all the dust in the ejecta intercepted by the
disk will be injected into the disk. With nearly 100% injection efficiency, the abundances
of 26Al and 60Fe in a disk 0.15 pc from a supernova would be 26Al/27Al = 6.8 × 10−5
and 60Fe/56Fe = 4.8 × 10−7 (using the yields from Woosley & Weaver 1995). These
values compare quite favorably to the meteoritic ratios (26Al/27Al = 5.0 × 10−5 and
60Fe/56Fe = 3−7×10−7; MacPherson et al. 1995, Tachibana & Huss 2003), and we conclude
that injection of SLRs into an already formed protoplanetary disk by a nearby supernova
is a viable mechanism for delivering radionuclides to the early Solar System, provided the
SLRs have condensed into dust. In future work we will present numerical simulations of
this process (Ouellette et al. 2007, in preparation).
We thank an anonymous referee for two very thorough reviews that significantly
improved the manuscript. We also thank Chris Matzner for helpful discussions.
– 34 –
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This manuscript was prepared with the AAS LATEX macros v5.2.
– 38 –
Table 1 Mass injected
# of zones (r × z) % injected
8×18 0.83
16×30 0.77
30×54 0.96
39×74 1.28
60×88 1.31
76 ×120 1.26
152×240 1.25
– 39 –
Table 2 Effect of Distance
d % injected 26Al/27Al
0.1 pc 4.3 6.4 × 10−6
0.3 pc 1.3 2.2 × 10−7
0.5 pc 1.0 5.6 × 10−8
– 40 –
Table 3 Effect of Explosion Energy
Eej % injected
26Al/27Al
4.0 f.o.e. 1.0 1.7 × 10−7
1.0 f.o.e. 1.3 2.2 × 10−7
0.25 f.o.e. 1.7 2.8 × 10−7
– 41 –
Table 4 Effect of Disk Mass
Mdisk % injected
26Al/27Al
0.084 M⊙ 1.4 2.3 × 10
0.0084 M⊙ 1.3 2.2 × 10
0.00084 M⊙ 2.2 3.6 × 10
– 42 –
Fig. 1.— Sod shock-tube problem benchmark. The squares are the results of the simulation
using Perseus, and the solid line is the analytical solution from Hawley et al. (1984).
– 43 –
Fig. 2.— Pressure-free collapse of a 30 AU, uniform-density clump of gas, as simulated by
Perseus. Spherical symmetry is maintained despite the cylindrical geometry and the inner
boundary condition at r = 2 AU.
– 44 –
Fig. 3.— (a) Gas temperature evolution using various cooling time steps (tc is the time step
normally used in the code) (b) Close-up of the time interval 31-33 years. Convergence is
achieved using tc and higher resolution timesteps.
– 45 –
Fig. 4.— (a) Isodensity contours of an equilibrium (“relaxed”) protoplanetary disk. Con-
tours are spaced a factor of 10 apart, with the outermost contour representing a density
10−20 g cm−3. The rotating disk has already evolved for 2000 years and is stable; this is the
configuration used as the initial state for our subsequent runs. (b) The “relaxed” disk, after
an additional 2000 years of evolution. While some slight deformation of the lowest-density
contours is seen, attributable to gravitational infall of surrounding gas, the disk is stable and
non-evolving over the spans of time relevant to supernova shock passage, ≈ 2000 years.
– 46 –
Fig. 5.— Isodensity contours of the relaxed disk, just before impact of the supernova ejecta.
Contours are spaced a factor of 10 apart, with the outermost contour representing a density
10−21 g cm−3.
– 47 –
Fig. 6.— Protoplanetary disk immediately prior to impact by the supernova shock. Isoden-
sity contours are as in Figure 4. Arrows represent gas velocities. The supernova ejecta are
travelling through the H ii region toward the disk at about 2200 km s−1.
– 48 –
Fig. 7.— Protoplanetary disk 0.05 years after being first hit by supernova ejecta. As the
supernova shock sweeps around the disk edge, it snowplows the low-density disk material
with it, but the shock stalls in the high-density gas in the disk proper. Isodensity contours
and velocity vectors as in Figure 5.
– 49 –
Fig. 8.— Protoplanetary disk 0.1 years after first being hit by supernova ejecta. As the
pressure increases on the side of the disk facing the ejecta, a reverse shock forms, visible as
the outermost (dashed) isodensity contour. Isodensity contours and velocity vectors as in
Figure 5.
– 50 –
Fig. 9.— Protoplanetary disk 0.3 years after first being hit by supernova ejecta. The reverse
shock, visible as the outermost (dashed) contour, has stalled and formed a bow shock. The
bow shock deflects incoming gas around the disk, which is effectively protected in a high-
pressure “bubble” of gas. Isodensity contours and velocity vectors as in Figure 5.
– 51 –
Fig. 10.— Protoplanetary disk 4 years after first being hit by supernova ejecta. Gas is being
stripped from the disk (e.g., the clump between R = 35 and 45 AU). Gas stripped from the
top of the disk either is entrained in the flow of ejecta and escapes the simulation domain,
or flows under the disk and falls back onto it. Isodensity contours and velocity vectors as in
Figure 5.
– 52 –
Fig. 11.— Protoplanetary disk 50 years after first being hit by supernova ejecta. The disk is
substantially deformed by the high pressures in the surrounding shocked gas. The pressures
compress the disk in the z direction, and also effectively aid gravity in the r direction,
allowing the gas to orbit at smaller radii with the same angular momentum. Isodensity
contours and velocity vectors as in Figure 5.
– 53 –
Fig. 12.— (a) Protoplanetary disk 400 years after first being hit by supernova ejecta. At
this instant mass is being stripped off the top of the disk by a Kelvin-Helmholtz instability,
seen in detail in (b). Isodensity contours and velocity vectors as in Figure 5.
– 54 –
Fig. 13.— (a) Protoplanetary disk prior to impact by supernova ejecta (same as the relaxed
disk of Figure 4), and (b) 2000 years after first being struck by supernova ejecta. This “before
and after” picture of the disk illustrates how the disk recovers almost completely from the
shock of a nearby supernova. Isodensity contours and velocity vectors as in Figure 5.
– 55 –
Fig. 14.— Ratio of color mass to total mass within a given isodensity contour (abscissa).
The dashed line represents the mass ratio after allowing the relaxed disk to evolve for 2000
years in the absence of a supernova; the solid line represents the mass ratio after 2000 years
of interaction with the supernova shock (our canonical simulation). Supernova ejecta is
injected very effectively up to densities where the shock would stall (∼ 10−14 g cm−3), much
more effectively than can be accounted for by numerical diffusion alone.
– 56 –
Fig. 15.— Convergence properties of selected global variables. The global variables are: the
mass-weighted radius of the cloud, a; the mass-weighted cloud thickness, c; the dispersions
in the mass-weighted radial (δvr) and vertical (〈vz〉) velocities; and the mass of ejecta gas
injected into the disk, Minj. The quantities are calculated at a time t = 500 years, but using
6 different numerical resolutions, n = 12, 22, 40, 54, 98 and 191. The deviation of each global
quantity Q from the highest-resolution value Q191 is plotted against numerical resolution n.
For our canonical simulation (n = 98), all quantities have converged at about the 10% level.
– 57 –
Fig. 16.— Density-weighted velocity along the z axis using the highest numerical simulation
n = 191 (solid line) and the canonical resolution n = 98 (dotted line). The difference
between them is plotted as the dashed line. After absorbing the initial impulse of downward
momentum from the supernova ejecta, the disk oscillates vertically about the position of the
central protostar with a period ∼ 150 years, characteristic of the most affected gas at about
30 AU.
	Introduction
	Perseus
	Additions to Zeus
	Sod Shock-Tube
	Gravitational Collapse
	Cooling
	``Relaxed Disk''
	Canonical Case
	Numerical Resolution
	Parameter Study
	Distance
	Explosion Energy
	Disk Mass
	Conclusions
