Mass and Temperature of the TWA 7 Debris Disk
Draft version October 23, 2018
Preprint typeset using LATEX style emulateapj v. 08/13/06
MASS AND TEMPERATURE OF THE TWA 7 DEBRIS DISK
Brenda C. Matthews
Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
Paul G. Kalas
Department of Astronomy, University of California, Berkeley, CA, 94720-3411, U.S.A.
Mark C. Wyatt
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, U.K.
Draft version October 23, 2018
ABSTRACT
We present photometric detections of dust emission at 850 and 450 µm around the pre-main sequence
M1 dwarf TWA 7 using the SCUBA camera on the James Clerk Maxwell Telescope. These data
confirm the presence of a cold dust disk around TWA 7, a member of the TW Hydrae Association.
Based on the 850 µm flux, we estimate the mass of the disk to be 18 Mlunar (0.2 M⊕) assuming a mass
opacity of 1.7 cm2 g−1 with a temperature of 45 K. This makes the TWA 7 disk (d = 55 pc) an order
of magnitude more massive than the disk reported around AU Microscopii (GL 803), the closest (9.9
pc) debris disk detected around an M dwarf. This is consistent with TWA 7 being slightly younger
than AU Mic. We find that the mid-IR and submillimeter data require the disk to be comprised of
dust at a range of temperatures. A model in which the dust is at a single radius from the star, with
a range of temperatures according to grain size, is as effective at fitting the emission spectrum as a
model in which the dust is of uniform size, but has a range of temperatures according to distance. We
discuss this disk in the context of known disks in the TW Hydrae Association and around low-mass
stars; a comparison of masses of disks in the TWA reveals no trend in mass or evolutionary state
(gas-rich vs. debris) as a function of spectral type.
Subject headings: stars: circumstellar matter, pre-main sequence — stars: individual (TWA 7) —
submillimeter
1. INTRODUCTION
TWA 7 (2MASS J10423011-3340162, TWA 7A) is
a weak-line T Tauri star identified as part of the
TW Hydrae Association (TWA, Kastner et al. 1997) by
Webb et al. (1999) based on proper motion studies in
conjunction with youth indicators such as high lithium
abundance, X-ray activity and evidence of strong chro-
mospheric activity. Disk systems were inferred around
four of the 18 association members (Zuckerman & Song
2004) from measurements of IR excess with the In-
frared Astronomical Satellite (IRAS). TW Hydra itself
(a K7 pre-main sequence star) hosts the nearest proto-
stellar disk to the Sun. Another accreting disk is ob-
served around one member of the triple system Hen 3-
600 (Muzerolle et al. 2000), and two debris disk systems
have been detected, around the A0 star HR 4796A (Jura
1991; Schneider et al. 1999) and one of two spectroscopic
binary components of the quadruple system HD 98800
(Jayawardhana et al. 1999; Gehrz et al. 1999), which is
a K5 dwarf. TWA 7 was not detected by IRAS.
Based on the width of the Li 6707 Ȧ line,
Neuhäuser et al. (2000) deduced that TWA 7 is a
pre-main sequence star. TWA 7 was not detected
by Hipparcos, but its membership in the TWA sets
its distance to be 55 ± 16 pc (Neuhäuser et al. 2000;
Weinberger et al. 2004; Low et al. 2005). Its spectral
Electronic address: brenda.matthews@nrc-cnrc.gc.ca
Electronic address: kalas@astron.berkeley.edu
Electronic address: wyatt@ast.cam.ac.uk
type is M1 based on LRIS spectra (Webb et al. 1999).
Based on existing photometry, evolutionary tracks and
isochrone fitting, Neuhäuser et al. (2000) derived an
age of 1-6 Myr (i.e., roughly coeval with other TWA
stars) and a mass of 0.55 ± 0.15 M⊙. The age of
the association is generally taken to be ∼ 8 − 10
Myr (Stauffer, Hartmann & Barrado y Navascues 1995;
Zuckerman & Song 2004). This is the age when planet
formation is thought to be ongoing and when disk dissi-
pation is occurring. Thus, the TWA is the ideal cluster in
which to observe the transitions from pre-main sequence
stars with proto-planetary (gas-rich) disks to main se-
quence stars with debris (gas-poor) disks.
TWA 7 has been observed for infrared excess emission
several times. The presence of a disk around TWA 7 was
first noted in submillimeter observations byWebb (2000).
They measured a flux of 15.5± 2.4 mJy at 850 µm. Nei-
ther Jayawardhana et al. (1999) nor Weinberger et al.
(2004) detected any excess associated with TWA 7 in
the mid-infrared. However, Low et al. (2005) report de-
tections of IR excess at 24 and 70 µm toward TWA 7 with
the Spitzer Space Telescope. Based on these data and ex-
isting shorter wavelength data on the stellar photosphere,
Low et al. (2005) derive a disk temperature of 80 K and
a lower limit to the mass of 2.4 × 1023 g (3.3 × 10−3
Mlunar) for the disk. This is a lower limit because the 70
µm data are not sensitive to colder material in the outer
disk and dust grains exceeding a few hundred micron in
size. A search for substellar companions using the NIC-
http://arxiv.org/abs/0704.0463v1
mailto:brenda.matthews@nrc-cnrc.gc.ca
mailto:kalas@astron.berkeley.edu
mailto:wyatt@ast.cam.ac.uk
2 Matthews et al.
MOS chronograph on HST did not reveal any evidence
of the disk (Lowrance et al. 2005); a nearby point source
is identified as a background object.
The study of debris disks around members of an as-
sociation permits the study of the evolution of disks as
a function of spectral type alone, since the disks likely
formed coevally and with similar compositions. It is also
possible to judge whether the presence and evolution of
disks around multiple stellar systems is comparable to
those around single stars. The discovery of disks around
low-mass stars is relatively recent (Greaves et al. 1998;
Liu et al. 2004; Kalas et al. 2004). The low radiation
field of late-K and M dwarfs means that the disks are
faint in scattered light compared to disks around more
massive stars, and hence they were less frequently tar-
geted by scattered light searches for disks. However,
scattered light imaging is often a follow-up technique
used after an infrared excess has been discovered (e.g.,
AU Mic, Kalas et al. 2004). In fact, the low radiation
fields may favor long-lived and slowly evolving disks,
since some disk material may be unchanged from the
original proto-planetary disks (e.g., Graham et al. 2007).
Spitzer has detected evidence of disks around a few K
stars (Chen et al. 2005; Bryden et al. 2006; Uzpen et al.
2005; Gorlova et al. 2004; Beichman et al. 2005), but
Beichman et al. (2006) note that in a sample of 61 K1 to
M6 stars, no excess emission is detected at 70 µm. This is
well below the expected detection rate if the disk fraction
is at all comparable to the ∼ 15% observed around solar-
type stars. The discovery of disks around K- and M-type
dwarfs may be difficult at far-infrared wavelengths be-
cause material at similar radii to disks around early-type
stars will be cool and will not radiate sufficiently. How-
ever, submillimeter observing sensitivities and the dust
temperature conspire to allow these objects to be dis-
covered at submillimeter wavelengths (Zuckerman 2001).
Submillimeter observations are sensitive to colder, larger
grains, which are more likely to be optically thin than the
warmer far-infrared emitting dust (Hildebrand 1988).
We report here detections of submillimeter excess emis-
sion at 450 and 850 µm around TWA 7 using the James
Clerk Maxwell Telescope (JCMT). In § 2, we summarize
our observations. In § 3, we present our results. We
discuss the relevance of these data to the TWA and the
population of disks around low-mass stars in § 4; our
results are summarized in § 5.
2. OBSERVATIONS AND DATA REDUCTION
Observations were made in 2004 October 19 using the
photometry mode on the Submillimeter Common User
Bolometer Array (SCUBA) on the JCMT (Holland et al.
1999). The pointing center of the observation was α =
10h42m30.s3, δ = −33◦40′16.′′9 (J2000). The on-source in-
tegration time was 1.6 hours. Flux calibration was done
using Mars, yielding flux conversion factors (FCFs) of
289.2 ± 1.4 Jy beam−1 volt−1 at 850 µm and 367.5 +/-
15 Jy beam−1 volt−1 at 450 µm. The absolute flux cali-
bration is accurate to ∼ 20− 30%. Pointing was checked
on the source 1034 − 293. The weather was excellent
during the observations, with a CSO tau measurment at
225 GHz of ∼ 0.04. The extinction was corrected using
skydips to measure the tau at 850 and 450 µm. The
mean tau value was 0.15 at 850 µm in four skydips and
0.59 at 450 µm in three skydips. The extinction values
derived from the skydips were consistent with the values
extrapolated from the CSO tau values according to the
relations of Archibald et al. (2002).
The data were reduced using the Starlink SURF pack-
age (Jenness & Lightfoot 1998). After flatfielding and
extinction correction, we flagged noisy bolometers rig-
orously; eleven (of 37) bolometers were removed in the
long wavelength array and 26 (of 91) were removed from
the short wavelength array. The photometry data were
then clipped at the 5 σ level to remove extreme values.
Short timescale variations in the sky background were
then removed using the mean of all bolometers except
the central one in each array. The average and variance
were then taken of each individual integration for the
central bolometers of the long and short wavelength ar-
rays after a clip of 3 σ was applied. In the case of the
450 µm data, the signal-to-noise ratio was improved by
applying a subsequent 2σ clip to the remaining data.
3. RESULTS
We have detected emission toward TWA 7 at both 850
µm and 450 µm. The fluxes measured are 9.7 ± 1.6 mJy
(6.1σ) and 23.0 ± 7.2 mJy (3.2σ), respectively. Errors
are statistical, and do not include the typical flux uncer-
tainty of∼ 20−30% for submillimeter single-dish calibra-
tion. Utilizing the measured fluxes of the star (Table 1)
and recently published Spitzer data (Low et al. 2005), we
construct the spectral energy distribution (SED) for this
source (Figure 1). The submillimeter data clearly repre-
sent an excess of emission over the photospheric emission
from TWA 7, as detected at 70 µm by Low et al. (2005).
Fig. 1.— The spectral energy distribution of TWA 7. The optical
and near-infrared data (diamonds) are modeled with a NextGen
model (Hauschildt et al. 1999) scaled to the mass and luminosity
of TWA 7 (Low et al. 2005). The star has a temperature of ∼
3500 K (best fit to stellar photometry, Low et al. 2005; Webb et al.
1999). Triangles mark detections from the Spitzer Space Telescope
and submillimeter detections from the JCMT. These fluxes show
clear excess when compared to the stellar photosphere. The flux
values are reported in Table 1. Fits for two single temperature
blackbodies (parameters are described in Table 2) to the TWA 7
data show that no single temperature fits all four measurements
of excess emission. An 80 K blackbody, Model 1, (dotted line) fits
the 24 and 70 µm data (Low et al. 2005), but underestimates the
submillimeter fluxes. The 70, 450 and 850 µm data are well fit
by a 45 K blackbody, Model 2, (dashed line), but a disk this cold
cannot account for the observed 24 µm excess.
The flux we measure at 850 µm is only about 70%
that measured by Webb (2000). Taking into account
the typical 20 − 30% uncertainty in absolute flux cali-
bration between epochs, the fluxes become 15± 3.8 mJy
(2000) and 9.7 ± 2.5 mJy (2004), which are consistent
Properties of the TWA 7 Disk 3
TABLE 1
Fluxes of TWA 7
Wavelength Magnitude Flux Reference
[µm] [mJy]
0.44 (B) 12.55 39.4 HST Guide Star Catalog (Lasker et al. 1996)
0.44 (B) 12.3 49.7 USNO-A2.0 (Monet et al. 1998)
0.54 (V) 11.06 142.4 reported in Low et al. (2005)
0.64 (R) 11.2 97.4 USNO-A2.0 (Monet et al. 1998)
1.25 (J) 7.78 1259.5 Webb et al. (1999)
1.65 (H) 7.13 1476.3 Webb et al. (1999)
2.16 (Ks) 6.90 1159.1 2MASS PSC
2.18 (K) 6.89 1148.8 Webb et al. (1999)
12 – 70.4 ± 8.6 Weinberger et al. (2004)
24 – 30.2 ± 3.0 Low et al. (2005)
70 – 85 ± 17 Low et al. (2005)
450 – 23 ± 7.2 this work
850 – 9.7 ± 1.6 this work
Note. — Conversion from magnitudes to Janskys has been done using zero points from
Allen’s Astrophysical Quantities (Cox 1999). The 12 µm estimate by Weinberger et al.
(2004) agrees with that of Jayawardhana et al. (1999) to within 1 σ and is consistent with
the photospheric flux expected from TWA 7.
within the errors. The instability of flux calibration in
the submillimeter is well known, and may be assuaged
somewhat by the ability to flux calibrate more often with
SCUBA-2, the next generation submillimeter camera on
the JCMT. In the interests of consistency with the only
450 µm detection in this work, we adopt the 850 µm flux
from the later epoch and do not attempt to combine the
two datasets.
3.1. Submillimeter Excess and Temperature
Low et al. (2005) derive a disk temperature of 80 K for
TWA 7 based on its infrared excess values at 24 and 70
µm. The temperature of the star is derived to be 3500
K, which is consistent with the log(Teff) of 3.56 reported
by Neuhäuser et al. (2000) based on the stellar SED.
Figure 1 shows the flux density distribution toward
TWA 7. The stellar fluxes and fit to the stellar pho-
tosphere are taken from Low et al. (2005) based on
NextGen models by Hauschildt et al. (1999) using a grid
of Kurucz (1979). The submillimeter excesses are evi-
dent when compared with the stellar spectra. Based on
their detection of TWA 7 in the mid-infrared, Low et al.
(2005) determine that the dust orbiting TWA 7 exists at
radii ≥ 7 AU from the star and has a temperature of 80
K. We have constructed several models for dust emission
around a 0.55M⊙ star (see Table 2 for their parameters).
We present two single temperature models in Figure 1,
neither of which is capable of fitting the measured ex-
cess fluxes at all four wavelengths. Model 1 (dotted line
in Fig. 1) is that of Low et al. (2005): a blackbody of
80 K, which fits the 24 and 70 µm data, but cannot re-
produce the fluxes at longer wavelengths. Model 2 is a
colder black body at a temperature of 45 K (dashed line
of Fig. 1), which fits the three longest wavelength fluxes,
but cannot reproduce the 24 µm flux. Thus we conclude
that the dust in this disk cannot be at a single tempera-
ture; rather there are a range of temperatures responsible
for the observed emission.
Two models that are able to fit the observed emission
spectrum are shown in Figure 2. In both models the
dust has a range of temperatures, however there are two
different physical motivations behind the origin of this
range.
Fig. 2.— The measured SED for TWA 7. Symbols and fit to the
stellar spectrum is as for Fig. 1. Two multi-temperature models are
fit to the excess emission arising from the disk. These are described
in detail in the text, and the parameters are summarized in Table
2. In Model 3 (dotted line), different temperatures arise due to
grains of different sizes located at a common radius from the star;
while Model 4 (dashed line) contains grains distributed at a range
of distances from the star.
In Model 3 (dotted line of Fig. 2), the dust is assumed
to all lie at the same distance from the star, r0. It
is assumed to have a range of sizes with a power law
distribution defined by the relation n(D) ∝ D(2−3q) for
grains of size D. This power law is assumed to be trun-
cated below dust of size Dmin = 0.9 µm, the size for
which β = Frad/Fgra = 0.5 for compact grains. The
dust is assumed to be composed of compact spherical
grains of a mixture of organic refractories and silicates
(Li & Greenberg 1997), and interaction with stellar radi-
ation determines the temperature that dust of different
sizes attains. The same model was used in Wyatt & Dent
(2002) to model the emission from the Fomalhaut disk,
where more detail can be found on the modelling method.
The emission spectrum could be fitted by dust at a ra-
dius r0 = 100 AU with a size distribution described by
q = 1.78. The temperatures of dust in this model range
from 21 K for the largest grains (> 100 µm) to 65 K for
the smallest grains (0.9 µm).
The size distribution used in Model 3 is close to that
expected in a collisional cascade wherein dust is replen-
ished by collisions between larger grains, since this re-
4 Matthews et al.
TABLE 2
Dust Model Parameters
Model Plotted as Temperature Dmin Dmax q Radius Mass (M⊕)
1 dotted Fig. 1 80 K – – – – 0.025
2 dashed Fig. 1 45 K – – – – 0.2
3 dotted Fig. 2 21 - 65 K 0.9 µm 1 m 1.78 100 AU 6.0
4 dashed Fig. 2 > 38 K – – – ≤ 35 AU 0.2
sults in a size distribution with q ∼ 1.83 and would be
truncated below the size of dust for which radiation pres-
sure would place the dust on hyperbolic orbits as soon
as they are created. The effect of radiation forces on
small grains is quantified by the parameter β = Frad/Fgra
(which is not to be confused with the index of dust emis-
sivity of grains that moderates the Rayleigh-Jeans tail
of the SED of cold dust), and it is dust with β > 0.5
which is unbound. However, due to the low luminosity
of M stars, it is not clear whether radiation pressure is
sufficient to remove dust grains from the disk system.
Figure 3 shows β as a function of dust grain size for
dust around TWA 7 (M∗ = 0.55 M⊙, L∗ = 0.31 L⊙).
While β is larger than 0.5 for compact (< 0.9 µm) grains,
this condition is only met for a narrow region of the
size distribution. Furthermore, if the dust grains are
porous, as around AU Mic (Graham et al. 2007), then
no grains will have β > 0.5. An alternative origin for
the small grain cut-off could be stellar wind forces, since
these provide a pressure force similar to radiation pres-
sure (Augereau & Beust 2006), and it is known that stel-
lar wind forces can be significantly stronger than radia-
tion forces for M stars (Plavchan, Jura & Lipscy 2005).
While the smallest grain size in the distribution may dif-
fer from our value of 0.9 µm, this does not affect the
ability of the model to fit the observed emission spec-
trum with suitable modifications to the slope in the size
distribution and radius of the dust belt.
In Model 4 (dashed line of Fig. 2), the dust grains
are assumed to lie at a range of distances from the star,
but they are all assumed to emit like black bodies, and
so have temperatures T = 278.3 L0.25
r. The spatial
distribution of the grains was taken from the model of
Wyatt (2005b) in which dust is created in a planetesimal
belt at r0, and then migrates inward due to Poynting-
Robertson (P-R) drag, but with some fraction of the
grains removed by mutual collisions on the way. In the
model the dust ends up with a spatial distribution which
can be described by the parameter η0, such that the sur-
face density falls off ∝ 1/(1 + 4 η0 (1 −
r/r0)). The
emission spectrum could be fitted using the parameters
r0 = 30 AU and η0 = 10, resulting in dust with tem-
peratures upwards of 38 K. However, the density of the
planetesimal belt required to scale the resulting emission
spectrum showed that removal by collisions dominates
over the P-R drag force in such a way that η0 should
be closer to 1400 in this disk. Thus, if this model is to
have a true physical motivation, then we need to invoke a
drag force which is ∼ 140 times stronger than P-R drag.
Such a force could come from the stellar wind, which
causes a drag force similar to P-R drag and which can
be incorporated into the model by reducing η0 by a fac-
tor 1 + (dMwind/dt) c
2/L∗ (Jura 2004). Thus to achieve
η0 = 10 in this way we would require the stellar wind to
be ∼ 140 times stronger than that of the Sun. The high
X-ray luminosity of TWA 7 (LX = 9.2±1.0×1029 erg/s,
Stelzer & Neuhauser 2000) may be indicative of a strong
stellar wind, since measured mass loss rates have been
found to increase with X-ray flux (Wood et al. 2005).
However the correlation found by (Wood et al. 2005, see
their Fig. 3) breaks down at X-ray fluxes an order of mag-
nitude lower than that of TWA 7 (for which FX ∼ 8×106
erg cm−2 s−1, and so it is not possible to use this flux to
estimate the mass loss rate with any certainty, and we
simply note that the mass loss rate required to achieve
η0 = 10 is not incompatible with observations of mass
loss rates of other stars. A stellar wind drag force has
also been invoked to explain structure in the AU Mic disk
(Strubbe & Chiang 2006; Augereau & Beust 2006).
We note that we are not claiming that the radius, r,
size distribution, q, or parameter η0 have been well con-
strained by these fits. The SED does indicate that the
disk around TWA 7 contains grains at a range of temper-
atures. The fits of Figure 2 illustrate two ways in which
multiple temperatures in the disk may arise from phys-
ically motivated models: the dust could have a range of
sizes, or it could be distributed over a range of distances.
Other models may also fit the data, including those in
which dust has a range of distances and sizes, and those
in which the dust originates in not one but multiple dust
zones, as has been inferred for AU Mic (Fitzgerald et al.,
in prep.).
Fig. 3.— Ratio of the radiation force acting on dust grains of
different size in the TWA 7 disk to the force of stellar gravity, β =
Frad/Fgra. Two different models are shown: compact grains (p =
0) and porous grains (p = 0.9). Dust with β > 0.5 is unbound from
the star as soon as it is released from a planetesimal. This figure
demonstrates that radiation pressure is not a significant mechanism
to remove dust grains from the TWA 7 system and is particularly
ineffective if the grains are highly porous.
The derived fits of Figure 2 do show differences in the
mid-IR spectrum. While this suggests that knowledge
of the mid-IR spectrum would enable us to distinguish
between the two models, it is worth pointing out that
Properties of the TWA 7 Disk 5
the exact shape of this spectrum is very sensitive to the
size distributions of small grains (in Model 3) and to the
radial distribution of grains interior to the planetesimal
belt (in Model 4). Thus it is possible that different as-
sumptions about these distributions could be made to
provide a fit to the same observed spectrum with both
models. The only way to definitively break the degen-
eracy is through imaging of the thermal emission from
the mid-IR to the submillimeter. The single radius disk
must look the same at all wavelengths, while a radially
distributed disk would look larger at longer wavelengths,
as more of the cold dust further from the star is detected.
3.2. Mass of the Disk
Low et al. (2005) estimate the minimum mass in dust
of the TWA 7 disk to be 0.0033 Mlunar, under the as-
sumption that the dust grain size is 2.8 µm. In deter-
mining the mass of the disk from submillimeter measure-
ments, a key parameter is the temperature of the dust.
Where possible, we discuss the derived mass for each of
the models discussed above. For submillimeter fluxes, we
can estimate the mass of the disk directly for an assumed
temperature and opacity from the relation:
Mdisk =
κν Bν(Td)
where Bν(Td) is the Planck function at the dust temper-
ature, Td, and κν is the absorption coefficient of the dust.
The derived mass is a strongly dependent function of the
value of κν . For debris disk studies, a value of 1.7 cm
g−1 is appropriate at 850 µm (Dent et al. 2000). This is
at the upper end of the values derived by Pollack et al.
(1994).
We can estimate the mass in Model 1 by using the
submillimeter flux predicted from the Rayleigh-Jeans tail
of the model (2.4 mJy) as well as the 80 K temperature
and standard opacity. This gives a mass of 0.025 M⊕
(∼ 2 Mlunar). This can be interpreted as the mass of hot
dust, with the proviso that the submillimeter observation
shows that there is more mass in colder dust as well.
Model 2 fits the submillimeter and 70 µm emission well.
Under the estimate of 45 K for the dust temperature of
the disk, the standard dust opacity and the measured 850
µm flux, the TWA 7 disk contains 0.2M⊕ of material (18
Mlunar). Based on the uncertainty in the flux (random
and systematic), the uncertainty on the mass estimates
are ∼ 30%. The mass of the TWA 7 disk is an order
of magnitude greater than the mass of 0.011 M⊕ for the
disk detected around AU Mic (Liu et al. 2004) that is
also derived based on a single temperature fit to far-IR
and submillimeter data with κν = 1.7 cm
2 g−1.
Model 3 contains dust grains at different temperatures
based on their size. In this case, the size distribution of
dust is well defined (i.e., with known scaling), the total
mass of dust is given by Mtot = 11 D
max, where Mtot is
the mass in Earth masses and Dmax is in meters. While
it is impossible to observationally knowDmax (since large
planetesimals are effectively invisible), in our model 95%
of the 850 µm flux comes from grains < 0.4 m. This
implies a mass of 6 M⊕, significantly higher than that
of Model 2. In the TWA 7 system, Dmax could be even
larger (or smaller) than this, so this discrepency carries
little definitive weight. The size distribution is relatively
shallow (i.e., there are lots of large grains) which explains
why the mass is much larger than in Model 2.
To derive a mass for Model 4 we assume an opacity
of 1.7 cm2 g−1 and determine the mass of dust in the
model required to reproduce the observed 850 µm flux,
given that the dust has a range of temperatures. The
derived mass is almost exactly the same as that at of
Model 2 at 0.2M⊕. This is to be expected because the
submillimeter flux in both models is dominated by the
coldest dust at 38-45 K, and very little additional mass
is required in Model 4 to explain the mid-IR emission (as
illustrated by the low mass in Model 1).
Given the inherent assumptions and unknowns in each
of these models, we adopt the results of the highly sim-
ple Model 2 as our most robust estimate of the mass
of the dust disk in TWA 7. It depends on the tem-
perature of the grains producing the observed 850 µm
flux, which are well fit by the cold dust model of Fig-
ure 1. The mass is also highly dependent on the value
of κν , for which we have adopted a value in line with
other disk modeling work (Dent et al. 2000), and so can
be compared with the disk masses derived by other au-
thors (e.g., Wyatt, Dent & Greaves 2003; Liu et al. 2004;
Najita & Williams 2005).
4. DISCUSSION
4.1. Disks in the TW Hydrae Association
Recent MIPS data from the Spitzer Space Telescope
show that most stars in the TWA show no evidence of
circumstellar dust out to 70 µm (Low et al. 2005). These
results confirm the assertion of Weinberger et al. (2004)
and Greaves & Wyatt (2003) that there is a bimodal dis-
tribution in the TWA: either stars have strong excesses
associated with warm dust emission, or they have very
weak emission consistent with very cold disks. The ex-
ceptions are the proto-planetary disks around TW Hya
and Hen 3-600, which both show evidence of warm and
cold disk components (Wilner et al. 2000; Zuckerman
2001).
Lower limits on warm dust emission were set for all
TWA members (except TW Hya) by Weinberger et al.
(2004). The absence of dust near the star is often taken
as a signature of a centrally depleted debris, rather than
an accreting, proto-planetary disk. There are a few
main sequence stars for which warm dust is present (i.e.,
HD 69830 and η Corvi), but this may be a transient
phenomenon (Wyatt et al. 2007). Based on their data,
Weinberger et al. (2004) deduced that the non-detections
implied an absence of material in terrestrial planet region
and that, except for TW Hya, there were no long-lived
disks in the association which could still form planets.
An outstanding question is whether the stars without
warm disks have potential disk material locked up in
undetected planets, which would imply that dusty sys-
tems represent failed planetary systems, or whether non-
detections represent disk systems in which the dust is
too cold to be detected in the mid-IR. Only large-scale
searches for cold dust around a statistically significant
number of stars can clarify whether a sizable population
of disks exist which are too cold to detect at mid-IR
wavelengths. Such a survey is planned using the new
SCUBA-2 camera at the JCMT (Matthews et al. 2007).
The disks now known in the TWA each have very dif-
ferent properties. The disk around TW Hya (K7) still
6 Matthews et al.
TABLE 3
Fluxes and Masses of Disks in the TWA
Star Sp. Type Gas? Optical Depth Flux λ Mass Temp Reference
[mJy] [Mlunar] [K]
HR 4796A A0 no thin 19.1 ± 3.4 850 µm 19 99 Sheret et al. (2004)
HD 98800 K5 no thick 111.1 ± 0.01 800 µma 28 150 Prato et al. (2001)
TW Hya K7 yes thick 8± 1 7 mm 8× 105 – Wilner et al. (2000), SED fit
TWA 7 M1 no thin 9.7± 1.6 850 µm 18 45 this work
TWA 13 M1 no thin 27.6 ± 5.9 70 µm > 0.0019 65 Low et al. (2005)
Hen 3-600 M3 yes thick ∼ 65 850 µm 304b 20c Zuckerman (2001)
Note. — a κ850µm adjusted by 1/λ.
b mass estimate based on conservative value of κ850µm = 1.7 cm
2 g−1; c temperature
constraint on cold dust component only.
contains molecular gas (Kastner et al. 1997), meaning
it is proto-planetary or in a transition from a proto-
planetary to a debris disk. It is a broad, face-on disk
which extends to 135 AU with evidence of a dip in
flux at 85 AU (Krist et al. 2000; Wilner et al. 2000;
Trilling et al. 2001; Weinberger et al. 2002; Qi et al.
2004). Hen 3-600 also shows evidence of hosting an
accreting, gas-rich disk (Muzerolle et al. 2000). The
HR 4796A (TWA 11) disk contains a narrow dust ring
at 70 AU from the star (Jayawardhana et al. 1998;
Koerner et al. 1998; Schneider et al. 1999; Telesco et al.
2000) and no detectable gas in emission line stud-
ies (Greaves, Mannings & Holland 2000) or more recent
searches for absorption due to circumstellar gas along the
line of sight (Chen & Kamp 2004), a technique which is
highly dependent on the temperature profile of the gas
because the disk is not edge-on. In both the Hen 3-600
and HD 98800 systems, the disk orbits only one mem-
ber of the binary (Jayawardhana et al. 1999; Gehrz et al.
1999) with evidence for cooler dust in circumbinary or-
bits (Zuckerman 2001). In fact, HD 98800 is a quadruple
system, so the dust is in a circumbinary orbit around a
spectroscopic binary.
The discussion of the TWA disk population can be il-
luminated by the results of recent Spitzer study of the
Upper Sco Association (with an estimated age of 3 − 5
Myr) by Carpenter et al. (2006). Their MIPS obser-
vation found optically thin disks around A-type stars,
no disks around solar-like stars, and optically thick, ac-
creting disks around K- and M-type dwarfs, suggest-
ing that disk evolution proceeds more quickly around
higher mass stars. A similar result was found for the
H and χ Persei double cluster at 13 Myr (Currie et al.
2007). The sample of stars in Upper Sco was 204
stars with 31 detections at 8 µm, well distributed across
spectral types. In the TWA, we have disk detections
around only a handful of members, and the associa-
tion is much smaller, with only 18 members. How-
ever, both the optically-thick, gas-rich disks in the TWA
are hosted by K and M stars, whereas the only A star
with a disk hosts an optically-thin debris disk. We
note that HD 98800 is noted to be gas-poor, but with
an optically-thick dust disk (Weintraub, Kastner & Bary
2000; Zuckerman & Becklin 1993).
The masses and temperatures of the disks are com-
pared for the TWA members with disks in Table 3. All
six disks are detected and their SEDs modeled in the
recent paper by Low et al. (2005). Where possible, the
masses in Table 3 are derived from submillimeter fluxes
or fits to SEDs, rather than infrared values which pro-
vide lower limits only. This is only an issue for TWA
13 which has not yet been detected at submillimeter or
millimeter wavelengths. Scaling κ850µm = 1.7 cm
2 g−1
to 70 µm implies a lower mass limit of 0.1 Mlunar based
on cold grains.
Of the debris disks, the most massive is the disk around
the HD 98800 system, but its mass is comparable to that
of the disk around the earlier star HR 4796A. Based on
submillimeter masses, the TWA 7 disk is roughly compa-
rable to that of HR 4796A. It is clear that we cannot yet
identify a systematic trend in mass or evolutionary phase
with spectral type in the TWA. We note that, given the
presence of massive disks around components of the mul-
tiple systems HD 98800 and HR 4796, their dust disk life-
times do not appear to be any shorter than that around
a single star.
4.2. Disks Around Late-type Stars
Table 4 shows the compilation of the few known debris
disks around late-type (K and M) stars. Although a de-
bris disk has been historically claimed around HD 233517
(Sylvester et al. 1989), Jura et al. (2006) conclude that
it is a giant, not a main sequence, star, and so we do
not include it. Similarly, we exclude HD 23362, although
it has a measured excess, because Kalas et al. (2002) at-
tribute the emission to a uniform surrounding dust cloud,
not a debris disk, around a K2III star at 187 pc dis-
tance. All fluxes are measured at 850 µm with SCUBA,
except where noted. For one of these stars, only an up-
per limit is observed in the 850 µm flux. Of the eight
solid detections, two (HD 92945, TWA 13) are at a sin-
gle wavelength in the mid-infrared, and two (GJ 182 and
GJ 842.2) are detected only in the submillimeter. For the
four remaining disks (HD 53143, ǫ Eri, AU Mic and TWA
7), there is a trend of increasing mass with later spectral
type, but it must be noted that the mass dependence
could also be attributed to the known trend of declining
dust masses around older stars (Rhee et al. 2006) in the
cases of HD 53143 and ǫ Eri since they are significantly
older than AUMic and TWA 7. The youngest star, TWA
7, has the most massive disk. We are obviously also bi-
ased toward more massive disks at larger distances. AU
Mic’s disk (9.9 pc) could not have been detected in the
observation which detected TWA 7’s disk at 55 pc.
Spitzer has made great progress in the last year de-
tecting infrared excess from main sequence stars (see the
review by Werner et al. 2006). However, not many of
these have been around late-type stars, and for those
Properties of the TWA 7 Disk 7
TABLE 4
Masses and Mass Limits of Debris Disks around Late-type Main Sequence Stars
Star Spectral Distance Age Flux λ Mass Temp Mass Reference
Type [pc] [Myr] [mJy] [Mlunar] [K]
HD 69830a K0 12.6 600-2000 < 7 850 µm < 0.24 100 this work
HD 92945 K1 22 20-150 271 70 µm > 0.002 40 Chen et al. (2005)
HD 53143 K1 18 1000 82.0± 1.1 30-34 µm > 6.5× 10−6 120± 60 Chen et al. (2006)
– optical > 0.0096 60b Kalas et al. (2006)
ǫ Eri K2 3 730± 200 40 ± 1.5 850 µm 0.1 85 Sheret et al. (2004)
GJ 842.2 M0.5 20.9 200 25 ± 4.6 850 µm 28± 5 13 Lestrade et al. (2006)
GJ 182c M0.5 26.7 100
4.8± 1.2 850 µm > 2.1 40 + 150d Liu et al. (2004)
AU Micc M1 9.9 10 14.4± 1.8 850 µm 0.89 40 Liu et al. (2004)
TWA 7c M1 55 8 9.7± 1.6 850 µm 18 45 this work
TWA 13c M1 55 8 27.6± 5.9 70 µm > 0.0019 65 Low et al. (2005)
Note. — a We have used the adopted temperature of 100 K but revised downward the estimated flux and mass based on
re-analysis of data originally presented in Sheret et al. (2004). Wyatt et al. (2007) suggest that the warm dust around this
source must be transient. b Temperature from Zuckerman & Song (2004). c Ages derived from association membership. d
two-component fit.
detections which have been made (Gorlova et al. 2004;
Beichman et al. 2005; Uzpen et al. 2005; Bryden et al.
2006; Smith et al. 2006), no estimates of mass in the disk
exist. The detections are typically toward field stars or
very distant targets in the galactic plane, although one
(P922) is a member of the cluster M47 (Gorlova et al.
2004). We do not list these candidates in Table 4. One
exception is the excess around HD 92945 (K1V), for
which Chen et al. (2005) measure a minimum disk mass
of 2× 10−3 Mlunar.
As discussed in § 3.1, the degeneracies in the models
are best broken with thermal imaging. Of the disks com-
piled in Table 4, only ǫ Eri (3.3 pc) has been well resolved
at submillimeter wavelengths. The distance of the TWA
makes imaging of 100 AU (∼ 2′′) scale disks impossi-
ble with single dish telescopes in the submillimeter. ǫ
Eri would be exceedingly difficult to map if it were even
three times more distant. We will have to rely on arrays
with higher sensitivity to obtain maps like that of ǫ Eri
around most low mass stars unless many more are dis-
covered within 10 pc. In the long-term, mid-IR images
will be possible with the planned MIRI instrument on the
James Webb Space Telescope. In the short-term, submil-
limeter imaging will be possible with the Atacama Large
Millimeter-submillimeter Array (ALMA). Far-IR obser-
vations will be possible with the Herschel Space Obser-
vatory, although imaging of 100 AU disks will only be
possible for stars within 10 pc.
5. SUMMARY
We have detected submillimeter excess emission aris-
ing from the dust disk around TWA 7 at 450 and 850
µm using SCUBA on the JCMT. Based on our photom-
etry and recent data from Spitzer, we derive a disk mass
of 0.2 M⊕ (18 Mlunar) for a temperature of 45 K. This
model effectively fits the 70, 450 and 850 µm data with
a blackbody. To fit these data and the 24 µm flux re-
quires dust at a range of temperatures, and we show that
this could arise from dust at one radius with a range of
sizes, or from dust of one size at a range of distances from
the star. Based on the SED alone, it is not possible to
determine which physical model is dominant.
While the multiple system HD 98800 appears to har-
bour the most massive debris disk in the TWA, disks of
relatively comparable masses are observed around the A0
star HR 4796A and the M1 star TWA 7. Therefore, the
formation of debris disks does not appear to be solely a
function of the mass of the parent star. A comparison of
masses of disks in the TWA reveals no trend in mass or
evolutionary state (gas-rich, proto-planetary vs. debris)
as a function of spectral type, although the detection of
proto-planetary disks around the latest stars is consis-
tent with the results of Carpenter et al. (2006) toward
the Upper Sco Association and Currie et al. (2007) in
the double cluster H and χ Persei.
Kalas et al. (2006) came to the same conclusion with
regard to other debris disks. They surmise that nur-
ture could explain the presence or absence of disks at
later epochs. If the environment dynamically heats the
disk such that the large planets fail to form, then dust
remains for a longer timescale. The dynamically quiet
systems then may quickly form planets, leaving no disk
to be observed at later epochs, although there is as yet
no evidence for any correlation between stars with debris
and/or planets (Moro-Martin et al. 2006).
The authors acknowledge our anonymous referee for
an insightful and constructive report. As well, the au-
thors thank B. Zuckerman for providing us with the pre-
vious 850 µm flux measurement from the thesis of R.
Webb, and P. Smith for providing the stellar fit to TWA
7 of Low et al. (2005) to maximize consistency with their
analysis. We also acknowledge useful conversations with
P. Hauschildt and R. Gray regarding the spectra of M
dwarfs. We thank our telescope operator E. Lundin and
the staff at the JCMT for their support. B.C.M ac-
knowledges support of the National Research Council of
Canada through a Plaskett Fellowship. P.K. acknowl-
edges support from GO-10228 provided by STScI under
NASA contract NAS5-26555. M.C.W. acknowledges sup-
port of the Royal Society.
8 Matthews et al.
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http://arxiv.org/abs/astro-ph/0701441
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