Cool Stars in Hot Places
Cool Stars in Hot Places
ASP Conference Series, Vol. To appear in proceedings of Cool Stars 14,
Ed. Gerard Van Belle
Cool Stars in Hot Places
S. T. Megeath
Ritter Observatory, Department of Physics and Astronomy, University
of Toledo, Toledo, OH 43606
E. Gaidos
Department of Geology & Geophysics, University of Hawaii, Honolulu,
HI 96822
J. J. Hester
Arizona State University, Department of Physics & Astronomy, Tempe,
AZ 85287
F. C. Adams
Physics Department, University of Michigan, Ann Arbor, MI 48109
J. Bally
Center for Astrophysics and Space Astronomy, University of Colorado,
Boulder, CO 80309
J.-E. Lee
Physics and Astronomy Department, The University of California at
Los Angeles, Los Angeles, CA 90095
S. Wolk
Harvard Smithsonian Center for Astrophysics, Cambridge, MA 02138
Abstract.
During the last three decades, evidence has mounted that star and planet
formation is not an isolated process, but is influenced by current and previous
generations of stars. Although cool stars form in a range of environments, from
isolated globules to rich embedded clusters, the influences of other stars on cool
star and planet formation may be most significant in embedded clusters, where
hundreds to thousands of cool stars form in close proximity to OB stars. At
the cool stars 14 meeting, a splinter session was convened to discuss the role
of environment in the formation of cool stars and planetary systems; with an
emphasis on the “hot” environment found in rich clusters. We review here the
basic results, ideas and questions presented at the session. We have organized
this contribution into five basic questions: what is the typical environment of
cool star formation, what role do hot star play in cool star formation, what role
does environment play in planet formation, what is the role of hot star winds and
supernovae, and what was the formation environment of the Sun? The intention
is to review progress made in addressing each question, and to underscore areas
of agreement and contention.
http://arxiv.org/abs/0704.1045v1
2 Megeath et al.
1. What is the Typical Environment of Cool Star Formation?
Cool stars form in a range of environments, from isolated Bok globules, to modest
sized clusters containing 100-200 stars, and finally to large, dense clusters with
thousands of cool stars and several to tens of OB stars. This is in sharp contrast
to OB stars, which form almost entirely in large clusters. This motivates the
question: in what environment do most cool stars form?
Surveys of the molecular gas in our Galaxy indicate that most of the cold
molecular gas is in giant molecular clouds (GMCs) with masses of 105 to 106 M⊙
(Heyer & Terebey 1998). These massive molecular clouds are thought to form
entire associations of hot OB stars as well thousands of low mass stars. Coupled
with analyses indicating that 80-90% of cool stars form in large clusters (Porras
2003; Lada & Lada 2003; Carpenter 2000); these results seemed to point to a
galaxy in which the vast majority of cool star formation takes place in rich
crowded clusters in close proximity to hot stars. However, since there was little
information on the numbers of isolated stars, the analyses of Porras (2003) and
Lada & Lada (2003) considered only stars in groups and clusters. In an analysis
of the 2MASS point source catalog toward several molecular clouds, Carpenter
(2000) found evidence for substantial numbers of isolated stars, but the estimates
contained significant uncertainties.
More recently, surveys of giant molecular clouds with the Spitzer space
telescope provided the means to identify isolated young stars and protostars
through the infrared excesses from their disks and envelopes (Allen et al. 2007).
Spitzer surveys of four giant molecular clouds containing young massive hot stars,
the Orion A cloud, Orion B cloud, Cep OB3 cloud and Mon R2 cloud, show that
in addition to clusters associated with regions of massive star formation, there
are large number of stars in small groups or isolation. In these clouds, 46% of
the young stars with excesses are found in clusters with over 90 sources, 11%
are found in small clusters of 90-30 stars, 8% in groups of 30-10 stars, and the
remaining 35% in groups with less than 10 members or isolation (Megeath et al.
2007; Gutermuth et al. 2007). About 33% of the stars are found in the two
largest clusters with over 700 members each. Thus, although most cool stars
may form in OB associations, young cool stars in OB associations are not found
primarily in large clusters. Instead, they are found in a range of environments,
with a significant fraction of stars forming in relative isolation several to tens of
parsecs away from the nearest OB stars (Fig. 1).
2. What Role do Hot Stars play in Cool Star Formation?
Although cool stars dominate star-forming regions in both number and total
stellar mass, hot stars are thought to be the primary agents of molecular cloud
evolution. The extreme-UV radiation from young O and early B-type stars pho-
toionizes the surfaces of molecular clouds, resulting in flows of ionized gas which
erode the clouds. Far-UV radiation may play a similar role in regions where only
B-stars are present by heating and photodissociating the molecular gas. Clus-
ters with O and/or B stars and ages of only a few million years appear to have
partially or fully dispersed their molecular clouds. An example is the 2.5 Myr
old σ Ori cluster (Sherry et al. 2004); this cluster sits outside the Orion B cloud
Cool Stars in Hot Places 3
Figure 1. The Orion OB association. The contours show an AV map of
the Orion region made from the 2MASS database (Gutermuth, p. com.), the
gray circles are the O stars (large circles) and B stars (small circles) from
Brown et al. (1994) and the dots are Spitzer identified young cool stars and
protostars (Megeath et al. 2007; Hernandez et al. 2007). Only regions of
high molecular column density and the σ Ori cluster have been surveyed by
Spitzer, and many more young cool stars certainly exist in the OB association
(Fig. 1). Other examples are discussed in Allen et al. (2007). It is estimated
that only 10% of embedded clusters survive gas dispersal and presist as clusters
for more than 10 Myr (Lada & Lada 2003).
The detection of Evaporating Gaseous Globules (EGGs), 1000 AU diameter
photoevaporating dark globules, demonstrated that hot stars may directly im-
pact protostellar evolution (Hester et al. 1996). EGGs appear to be protostellar
or prestellar cores which emerge from their parental clouds as the surrounding
lower density gas is ionized (Hester & Desch 2005). In M16, 15% of the EGGs
contain embedded stars, indicating that they are the sites of recent or ongoing
star formation (McCaughrean & Anderson 2002). This suggests that hot stars
can directly affect protostellar evolution by photoevaporating the infalling gas
4 Megeath et al.
and limiting the ultimate mass of the nascent star. However, it is not known
what fraction of stars emerge from their clouds in EGGs.
A more controversial issue is whether OB stars trigger cool star forma-
tion. This possibility has been discussed in the literature for decades (e.g.
Elmegreen & Lada 1977). Hester & Desch (2005) proposed that in regions with
hot stars, cool star formation is driven primarily by shock fronts preceeding ad-
vancing ionization fronts. The shock fronts overtake and compress pre-existing
density enhancements, inducing collapse and the formation of clusters of low
mass stars. Evidence for this is found in the detection of clusters of young stars
at the surfaces of molecular faces being eroded by hot stars (Sugitani et al. 1995;
Megeath et al. 2004; Allen et al. 2006). However, additional evidence, such as
the detection of the shock fronts, is needed to determine whether the clusters
have been triggered, or whether they are regions of ongoing star formation which
have been overtaken by ionization fronts (Megeath & Wilson 1997).
Although there is growing evidence that triggering does happen, it is not
clear what fraction of cool star formation is triggered. Assessing the overall
importance of triggered star formation can be difficult due to the rapid evolution
and even rapid motions of OB stars. For example, Hoogerwerf et al. (2001)
argued that the interaction of the ι Ori binary system with a second system led
to the ejection of the runaway stars AE Aur and µ Col 2.5 Myr ago (both are 09.5
stars). Although they suggested that these stars originated in the Orion Nebula
Cluster, the lack of a visible HII region surrounding ι Or, an O9 III star which in
projection appears conicident with the Orion A cloud, suggests that it is several
to tens of parsecs away from the Orion A molecular cloud and is part of the 5 Myr
OB1c association (Brown et al. 1994). At the time of their ejection, these three
O–stars may have had a significant impact on the Orion A cloud, and could
have been responsible for triggering star formation in the Orion Nebula Cluster.
Another possible example is the LDN 1551 dark cloud in the Taurus dark cloud
complex. This cloud has a cometary morphology with the “head” of the comet
pointing toward the Orion constellation. Moriarty-Schieven et al. (2005) argued
that the cometary shape may be due to the interaction of LDN 1551 (149 pc from
the Sun) with the B8I star Rigel (Hipparcos distance is 240 pc) and the M2I star
Betelgeuse (Hipparcos distance 130 pc). The high proper motion of Betelgeuse
would place it southeast of LDN 1551 several million years ago; hence, both
Betelgeuse and Rigel could have plausibly interacted with LDN 1551, creating
the cometary morphology.
These observations demonstrate the difficulties in determining causal rela-
tionships between subsequent generations of star formation and establishing the
importance of triggering. Although ongoing triggering can be identified by the
detection of clusters near ionization fronts, in many cases, evidence of triggering
may be erased by the evolution and motion of massive stars.
3. What Role does Environment Play in Planet Formation?
Environment may also play a role in planet formation by altering the properties
of protoplanetary disks. We discuss here two mechanisms: tidal interactions
between stars in clusters and the photo-ablation of disks by UV photons from
nearby OB stars.
Cool Stars in Hot Places 5
Tidal interactions occur when a disk around a star in a cluster is distorted
or stripped during a close encounter with another cluster member. Such inter-
actions appear to be unimportant. Adopting a stellar density of 104 pc−3 (the
peak density for many embedded clusters) and assuming virialized velocities,
Gutermuth et al. (2005) used a simple mean free path argument to estimate the
frequency of close approaches. They estimated that even in the dense, central
cores of clusters, close approaches at distances of 100 AU would occur once
in a 10 Myr interval. However, the high stellar densities assumed by Guter-
muth et al. may only persist for a few million years before the clusters begin
to expand. This result is supported by N-body simulations of bound clusters
which show that such interactions are rare over the lifetime of an embedded
cluster (Adams et al. 2006; Throop & Bally in prep). Adams et al. (2006) find
that each star in a 1000-member (initially) embedded cluster will experience
one close-approach within 700-4000 AU over a 10 Myr interval. This distance
is more than three times the typical radius of observed circumstellar disks in
nearby dark clouds (Andrews et al. 2007) and much larger than the size of the
Solar System. Since the adopted timescale for gas removal in these simulations
was 5 Myr, longer than the observed timescale (Sec. 2), the close-approach dis-
tances should be considered lower limits. In summary, the results from three
independent investigations are in agreement; unless embedded clusters exist in
our galaxy with much higher stellar densities than observed in nearby regions
such as the Orion Nebula Cluster, tidal interactions in clusters rarely influence
disk evolution and planet formation.
In contrast, photoevaporation of disks by nearby OB stars appears to be a
much more influential process. The UV radiation from the OB stars heats the
gas in disks through photoionization and photodissociation, resulting in flows
of gas off the disks. This process was discovered in VLA and HST observations
of young stars in the Orion Nebula (Churchwell et al. 1987; O’Dell & Zheng
1994). The inferred mass loss rates were 10−7 M⊙, suggesting disk lifetimes of
only a few hundred thousand years (Bally et al. 1998). However, the mass loss
occurs in the outer disk where the thermal velocity of ionized gas exceeds the
escape velocity from the star, and the gas in the inner disk may not be strongly
affected. More recent calculations include the effect of the far-UV radiation
and the time dependent nature of the UV-field as the stars orbit within the
cluster potential. Adams et al. (2004) calculate the mass loss from a disk as a
function of the intensity of the far-UV radiation field. They find the radiation
field can truncate a disk to the size of our solar system in several million years;
the exact radius depends on the duration of the exposure to UV radiation, the
intensity of the UV radiation, and the mass of the central star (Adams et al.
2004). Throop & Bally (in prep) use N-body simulations to calculate the time
dependent flux of UV radiation incident on a young star with disk as it orbits
in a cluster which contains OB stars in its center. They find that typical stars
experience only a brief exposure to intense UV as they pass within 10,000 AU
of the central OB stars. Consequently, the UV flux incident on a disk varies in
an stochastic manner over the lifetime of the cluster.
Recently, Throop & Bally (2005) proposed that the photoevaporation of
disks may in fact trigger the formation of planets. In their model, grain growth
and dust settling concentrates dust grains in the midplane of the disk. Conse-
quently, the ablation of the gas from the disk surface (as well as the remaining
6 Megeath et al.
dust grains entrained in the gas) reduces the ratio of the gas surface density
to dust surface density. If the surface density of gas is reduced to less then
10 times the dust density, the disk becomes unstable to gravitational collapse
(Sekiya 1998; Youdin & Shu 2002).
Although photoevaporation may be important in rich embedded clusters
with OB stars, many young cool stars in OB associations are not found in such
clusters. Young cool stars with disks identified in the Spitzer survey of the
Orion A cloud have a median projected distance of 4.1 pc to the nearest O to
B0 star, and a median projected distance of 2.1 pc to the nearest B1-B3 stars
(Megeath et al. 2007). Hence, in OB associations, most cool stars may form
at large distances from the central OB stars and are unaffected by their UV
radiation.
4. What is Role of Hot Star Winds and Supernovae?
Chandra X-ray observations of young stellar clusters have detected diffuse X-
ray emission in nine regions. The total luminosities of this gas range from
1− 200× 1033 erg s−1 (Wolk et al. 2002; Townsley 2006). Although supernovae
could generate this gas, in most cases the diffuse gas appears to be generated by
stellar winds from massive stars colliding with other winds or the surrounding
HII region. However, in the Carina region, a component of hot gas enriched in Fe
was likely created by a supernova (Townsley 2006). The impact of the extremely
hot gas on star and planet formation is not well understood. In addition to
destroying the surrounding the cloud, the blast waves from a supernova could
compress surrounding cores of gas causing them to collapse into stars (Boss 1995;
Melioli et al. 2006). Disks can survive at distances of ≤ 1 pc from a supernova
(Chevalier 2000); however, these disks will be heated by the radiation and blast
wave, and may also be stripped by the blast wave when the disks are only 0.25 pc
from the supernova (Chevalier 2000). The hot X-ray gas created by winds may
fill bubbles within the larger HII region. This hot, low density gas would be
transparent to UV photons, and hence any young stars within the bubble may
be exposed to a more intense UV field than those in the surrounding HII region.
5. What Was the Formation Environment of the Sun?
Did our Sun also form in the “hot” environment of a large embedded cluster?
Tremaine (1991) and Gaidos (1995) proposed that our Solar System might pre-
serve dynamical evidence of its birth environment. Gaidos (1995) and Adams & Laughlin
(2001) used the low inclination and eccentricity of Neptune to place constraints
on the time-integrated tidal field of a cluster and the closest stellar passage.
However, such reasoning must now be re-examined in light of the expectation
that most embedded clusters expand and disperse in a few Myr (although some
clusters would form bound open clusters, Sec. 2) and the realization that Nep-
tune (and Uranus) migrated outward to its present orbit by scattering in a
residual planetesimal disk, a process that was probably not completed until
after a parental cluster dispersed (Hahn & Malhotra 2005). Scattering inside
the disk itself, which dampens any non-circular motion, could have produced
the low eccentricity and inclination observed today. Similar arguments can
Cool Stars in Hot Places 7
be made that other parts of the outer Solar System (the Edgewood-Kuiper
belt, Oort Cloud) formed after the cluster evaporated (Levison & Morbidelli
2003). Kenyon & Bromley (2004) and Morbidelli & Levison (2004) proposed
that Sedna, a member of the scattered Kuiper Belt, was produced by the
close passage of a star, but there are other explanations (Barucci et al. 2005;
Gladman & Chan 2006). Thus, it is likely that the structure of the outer Solar
System post-dates an embedded cluster phase.
The strongest evidence for an early cluster environment is the inferred pres-
ence of short-lived radionuclides (SLRs) during the formation of solids now found
in meteorites. There are at least three possible sources of SLRs: particle irradi-
ation within the primordial solar nebula, the wind from a nearby AGB star, and
the wind and/or supernova ejecta from a nearby massive star. The discovery
of 60Fe in the early Solar System (Tachibana & Huss 2003) firmly establishes
that the Sun formed in a rich cluster containing massive stars (Hester 2004;
Hester & Desch 2005). Neutron-rich isotopes such as 60Fe cannot be produced
by particle irradiation. The uniform distribution of the SLR 26Al makes it
unlikely it was produced by irradiation (Thrane et al. 2006). Finally, it is sta-
tistically unlikely that the SLRs originated in an AGB star (Kastner & Myers
1994).
Further evidence is found in the mass-independent fractionation of the oxy-
gen isotopes (17O and 18O) in meteorites. Following a proposal by Clayton, Grossman & Mayeda
(1973), Lee et al. (2007) have made a theoretical analysis of the time-dependent
chemistry in a collapsing envelope subjected to an external UV field. Due to
“self-shielding” of the much more abundant C16O, the UV field preferentially
dissociates C18O and C17O, producing an enhancement of 18O and 17O in the
gaseous envelope. These heavier isotopes are then incorporated (as water) into
ice grains and transported into the inner region of the solar nebula. This pro-
cess depends on the intensity of the external UV radiation field (from OB stars)
so that the measured fractionation can constrain the formation environment of
the Sun. Lee et al. (2007) conclude that the observed isotopic ratios are best
explained by a radiation field 105 greater than the interstellar field, again sup-
porting the presence of nearby massive stars.
The current evidence firmly indicates that the Sun formed in a hot environ-
ment enriched by the ejecta of one or more nearby supernova; however, there
is a continuing debate over how the solar nebula was enriched. Cameron et al.
(1995) argued that the enrichment occurred when the collapse of the proto-
solar molecular cloud was triggered by the blast wave of a supernova (also see
Vanhala & Boss 2002). Hester & Desch (2005) question whether this process
could enrich the collapsing molecular gas. Alternatively, the protostellar en-
velope of the Sun may have been directly enriched while collapsing onto the
proto-Sun (Looney et al. 2006). For example, if the solar system formed in an
EGG, then it may have been subjected to a blast wave from a supernova. Finally,
the SLRs may have been injected directly into the disk of the solar nebula when
the Sun was in its T-Tauri phase; a possible mechanism for this is the “aerogel”
model, in which grains in SN ejecta are deaccelerated and vaporized within the
gaseous primordial disk (Ouellette et al. 2005). This scenario is supported by
observations showing that 40% of disks may persist for 4 Myr (Hernandez 2007),
the lifetime of a 60 M⊙ star.
8 Megeath et al.
Recent quantitative analyses have constrained the distance between the
Sun and the supernova from which the SLRs presumably originated. If the en-
richment occurred while the Sun was in a T Tauri phase with a 200 AU disk
(Andrews et al. 2007), the estimated distance is between 0.04-0.4 pc (Looney et al.
2006; Ellinger et al. in prep). If the enrichment occurred in the protostellar
phase (5000 AU diameter), the estimated distance is between 0.12- 1.6 pc (Looney et al.
2006). The question has been raised whether these distances are consistent with
observations showing that embedded clusters largely disrupt their parental cloud
and disperse in a few million years (see Sec. 2). The dispersal of the molecular
gas makes the presence of nearby protostars unlikely, and the subsequent expan-
sion of the cluster make the presence of young stars with disks less likely. There
are possible solutions to this problem. The Sun may have remained close to a
hot star as the cluster dispersed. Only one low mass star with a disk is found
within a projected distance of ∼ 0.3 pc of the O6 star HD206267 in the 4 Myr
old IC 1396 association (Sicilia-Aguilar et al. 2006), suggesting that this may be
rare occurrence. The Sun may have been a bound companion to a massive star,
such as the companions with disks found around the OB stars comprising the
Orion Trapezium (Schertl et al. 2003); however, it unclear how long such a disk
may survive. The Sun could have formed in a massive embedded cluster which
evolved into a bound open cluster. In this case, the solar system would have to
survive photoevaporation and perturbations from tidal interactions as it orbited
within the cluster (Adams & Laughlin 2001). Finally, the solar system may
have been enriched by the combined ejecta of many supernova (Hester & Desch
2005; Williams & Gaidos in prep). Additional data on SLRs in meteorites, de-
tailed modeling of the evolution and dispersal of embedded clusters, and the
study of other planetary systems in hot environments should bring a more de-
tailed understanding of our Sun’s formation environment.
The presence of SLRs may have had a significant impact on planet formation
in the solar nebula. Radioactive decay of 26Al and 60Fe provides by far the
largest source of energy for melting and differentiating planetesimals in the early
Solar System (Bizzarro et al. 2005; Hevey & Sanders 2006). In summary, it has
been amply demonstrated by observation and theory that environment plays
a significant role in the formation of cool stars and planets. A comprehensive
understanding of star and planet formation must not treat young stars and
protoplanetary solely as isolated objects, but as parts of larger associations and
clusters in which the formation of cool and hot stars are inextricably linked.
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