485 research outputs found
Accretion in Evolved and Transitional Disks in Cep OB2: Looking for the Origin of the Inner Holes
We present accretion rates for a large number of solar-type stars in the Cep
OB2 region, based on U band observations. Our study comprises 95 members of the
~4 Myr-old cluster Tr 37 (including 20 "transition" objects; TO), as well as
the only CTTS in the ~12 Myr-old cluster NGC 7160. The stars show different
disk morphologies, with the majority of them having evolved and flattened
disks. The typical accretion rates are about one order of magnitude lower than
in regions aged 1-2 Myr, and we find no strong correlation between disk
morphology and accretion rates. Although half of the TO are not accreting, the
median accretion rates of normal CTTS and accreting "transition" disks are
similar (~3 10^{-9} and 2 10^{-9} Msun/yr, respectively). Comparison with other
regions suggests that the TO observed at different ages do not necessarily
represent the same type of objects, which is consistent with the fact that the
different processes that can lead to reduced IR excess/inner disk clearing
(e.g., binarity, dust coagulation/settling, photoevaporation, giant planet
formation) do not operate on the same timescales. Accreting TO in Tr 37 are
probably suffering strong dust coagulation/settling. Regarding the equally
large number of non-accreting TO in the region, other processes, like
photoevaporation, the presence of stellar/substellar companions, and/or giant
planet formation may account for their "transitional" SEDs and negligible
accretion rates.Comment: 37 pages, 5 figures, 6 tables Accepted by Ap
The Transitional Protoplanetary Disk Frequency as a Function of Age: Disk Evolution in the Coronet Cluster, Taurus, and Other 1--8 Myr-old Regions
We present Spitzer 3.6--24 micron photometry and spectroscopy for stars in
the 1--3 Myr-old Coronet Cluster, expanding upon the survey of Sicilia-Aguilar
et al. (2008). Using sophisticated radiative transfer models, we analyze these
new data and those from Sicilia-Aguilar et al. (2008) to identify disks with
evidence for substantial dust evolution consistent with disk clearing:
transitional disks. We then analyze data in Taurus and others young clusters --
IC 348, NGC 2362, and eta Cha -- to constrain the transitional disk frequency
as a function of time. Our analysis confirms previous results finding evidence
for two types of transitional disks -- those with inner holes and those that
are homologously depleted. The percentage of disks in the transitional phase
increases from ~ 15--20% at 1--2 Myr to > 50% at 5--8 Myr; the mean
transitional disk lifetime is closer to ~ 1 Myr than 0.1--0.5 Myr, consistent
with previous studies by Currie et al. (2009) and Sicilia-Aguilar et al.
(2009). In the Coronet Cluster and IC 348, transitional disks are more numerous
for very low-mass M3--M6 stars than for more massive K5--M2 stars, while Taurus
lacks a strong spectral type-dependent frequency. Assuming standard values for
the gas-to-dust ratio and other disk properties, the lower limit for the masses
of optically-thick primordial disks is Mdisk ~ 0.001--0.003 M*. We find that
single color-color diagrams do not by themselves uniquely identify transitional
disks or primordial disks. Full SED modeling is required to accurately assess
disk evolution for individual sources and inform statistical estimates of the
transitional disk population in large samples using mid-IR colors.Comment: 43 pages in emulateapj format, Accepted for publication in Ap
Protostars and stars in the Coronet cluster: Age, evolution, and cluster structure
We present new optical spectroscopy with FLAMES/VLT, near-IR imaging with
HAWK-I/VLT, and 870 micron mapping with APEX/LABOCA of the Coronet cluster. The
optical data allow to estimate spectral types, extinction and the presence of
accretion in 6 more M-type members, in addition to the 12 that we had
previously studied. The submillimeter maps and near-IR data reveal the presence
of nebular structures and high extinction regions, which are in some cases
associated to known IR, optical, and X-ray sources. Most star formation is
associated to two elongated structures crossing in the central part of the
cluster. Placing all the 18 objects with known spectral types and extinction in
the HR diagram suggests that the cluster is younger than previously thought (<2
Myr, and probably ~0.5-1 Myr). The new age estimate is in agreement with the
evolutionary status of the various protostars in the region and with its
compactness (<1.3 pc across), but results in a conflict with the low disk and
accretion fraction (only 50-65% of low-mass stars appear to have protoplanetary
disks, and most transitional and homologously depleted disks are consistent
with no accretion) and with the evolutionary features observed in the mid-IR
spectra and spectral energy distributions of the disks.Comment: 30 pages, 9 figures, 6 tables, ApJ in pres
Accretion properties of T Tauri stars in sigma Ori
Accretion disks around young stars evolve in time with time scales of few
million years. We present here a study of the accretion properties of a sample
of 35 stars in the ~3 million year old star-forming region sigma Ori. Of these,
31 are objects with evidence of disks, based on their IR excess emission. We
use near-IR hydrogen recombination lines (Pa_gamma) to measure their mass
accretion rate. We find that the accretion rates are significantly lower in
sigma Ori than in younger regions, such as rho-Oph, consistently with viscous
disk evolution. The He I 1.083 micron line is detected (either in absorption or
in emission) in 72% of the stars with disks, providing evidence of
accretion-powered activity also in very low accretors, where other accretion
indicators dissapear.Comment: Astronomy and Astrophysics, accepte
Wide-Field Survey of Emission-line Stars in IC 1396
We have made an extensive survey of emission-line stars in the IC 1396 HII
region to investigate the low-mass population of pre-main sequence (PMS) stars.
A total of 639 H-alpha emission-line stars were detected in an area of 4.2
deg^2 and their i'-photometry was measured. Their spatial distribution exhibits
several aggregates near the elephant trunk globule (Rim A) and bright-rimmed
clouds at the edge of the HII region (Rim B and SFO 37, 38, 39, 41), and near
HD 206267, which is the main exciting star of the HII region. Based on the
extinction estimated from the near-infrared (NIR) color-color diagram, we have
selected pre-main sequence star candidates associated with IC 1396. The age and
mass were derived from the extinction corrected color-magnitude diagram and
theoretical pre-main sequence tracks. Most of our PMS candidates have ages of <
3 Myr and masses of 0.2-0.6 Mo. Although it appears that only a few stars were
formed in the last 1 Myr in the east region of the exciting star, the age
difference among subregions in our surveyed area is not clear from the
statistical test. Our results may suggest that massive stars were born after
the continuous formation of low-mass stars for 10 Myr. The birth of the
exciting star could be the late stage of slow but contiguous star formation in
the natal molecular cloud. It may have triggered to form many low-mass stars at
the dense inhomogeneity in and around the HII region by a radiation-driven
implosion.Comment: 48 pages, 12 figures, 5 tables, accepted for publication in A
Disk Evolution in OB Associations - Deep Spitzer/IRAC Observations of IC 1795
We present a deep Spitzer/IRAC survey of the OB association IC 1795 carried
out to investigate the evolution of protoplanetary disks in regions of massive
star formation. Combining Spitzer/IRAC data with Chandra/ACIS observations, we
find 289 cluster members. An additional 340 sources with an infrared excess,
but without X-ray counterpart, are classified as cluster member candidates.
Both surveys are complete down to stellar masses of about 1 Msun. We present
pre-main sequence isochrones computed for the first time in the Spitzer/IRAC
colors. The age of the cluster, determined via the location of the Class III
sources in the [3.6]-[4.5]/[3.6] color-magnitude diagram, is in the range of 3
- 5 Myr. As theoretically expected, we do not find any systematic variation in
the spatial distribution of disks within 0.6 pc of either O-type star in the
association. However, the disk fraction in IC 1795 does depend on the stellar
mass: sources with masses >2 Msun have a disk fraction of ~20%, while lower
mass objects (2-0.8 Msun) have a disk fraction of ~50%. This implies that disks
around massive stars have a shorter dissipation timescale.Comment: Accepted for publication in Ap
Short Lifetime of Protoplanetary Disks in Low-metallicity Environments
We studied near-infrared disk fractions of six young clusters in the
low-metallicity environments with [O/H using deep images
with Subaru 8.2\,m telescope. We found that disk fraction of the
low-metallicity clusters declines rapidly in 1\,Myr, which is much faster
than the 5--7\,Myr observed for the solar-metallicity clusters,
suggesting that disk lifetime shortens with decreasing metallicity possibly
with an dependence. Since the shorter disk lifetime reduces the
time available for planet formation, this could be one of the major reasons for
the strong planet--metallicity correlation. Although more quantitative
observational and theoretical assessments are
necessary, our results present the first direct observational evidence
that can contribute to explaining the planet--metallicity correlation.Comment: 5 pages, 1 figure, and 2 tables. Accepted for publication in The
Astrophysical Journal Letter
Spitzer IRS Spectroscopy of the 10 Myr-old EF Cha Debris Disk: Evidence for Phyllosilicate-Rich Dust in the Terrestrial Zone
We describe Spitzer IRS spectroscopic observations of the 10 Myr-old star, EF
Cha. Compositional modeling of the spectra from 5 {\mu}m to 35 {\mu}m confirms
that it is surrounded by a luminous debris disk with LD/L\star ~ 10-3,
containing dust with temperatures between 225 K and 430 K characteristic of the
terrestrial zone. The EF Cha spectrum shows evidence for many solid-state
features, unlike most cold, low-luminosity debris disks but like some other
10-20 Myr-old luminous, warm debris disks (e.g. HD 113766A). The EF Cha debris
disk is unusually rich in a species or combination of species whose
emissivities resemble that of finely powdered, laboratory-measured
phyllosilicate species (talc, saponite, and smectite), which are likely
produced by aqueous alteration of primordial anhydrous rocky materials. The
dust and, by inference, the parent bodies of the debris also contain abundant
amorphous silicates and metal sulfides, and possibly water ice. The dust's
total olivine to pyroxene ratio of ~ 2 also provides evidence of aqueous
alteration. The large mass volume of grains with sizes comparable to or below
the radiation blow-out limit implies that planetesimals may be colliding at a
rate high enough to yield the emitting dust but not so high as to devolatize
the planetesimals via impact processing. Because phyllosilicates are produced
by the interactions between anhydrous rock and warm, reactive water, EF Cha's
disk is a likely signpost for water delivery to the terrestrial zone of a young
planetary system.Comment: 21 pages, 10 figures, accepted for publication in The Astrophysical
Journa
The low-mass stellar population in the young cluster Tr37: Disk evolution, accretion, and environment
We present a study of accretion and protoplanetary disks around M-type stars
in the 4 Myr-old cluster Tr37. With a well-studied solar-type population, Tr37
is a benchmark for disk evolution. We used low-resolution spectroscopy to
identify 141 members (78 new) and 64 probable members, mostly M-type stars.
H\alpha\ emission provides information about accretion. Optical, 2MASS,
Spitzer, and WISE data are used to trace the SEDs. We construct radiative
transfer models to explore the structures of full-disks, pre-transition,
transition, and dust-depleted disks. Including the new and previously known
members, we confirm that a substantial fraction (~2/5) of disks show signs of
evolution, either as radial dust evolution (transition/pre-transition disks) or
as a more global evolution (low small-dust masses, dust settling, and
weak/absent accretion signatures). Accretion is strongly dependent on the SED
type. About half of the transition objects are consistent with no accretion,
and dust-depleted disks have weak (or undetectable) accretion signatures,
especially among M-type stars. The analysis of accretion and disk structure
suggests a parallel evolution of dust and gas. We find several distinct classes
of evolved disks, based on SED type and accretion, pointing to different disk
dispersal mechanisms and probably different evolutionary paths. Dust depletion
and opening of inner holes appear to be independent processes: most transition
disks are not dust-depleted, and most dust-depleted disks do not require inner
holes. The differences in disk structure between M-type and solar-type stars in
Tr37 (4 Myr) are not as remarkable as in the young, sparse, Coronet cluster
(1-2 Myr), suggesting that other factors, like the environment/interactions,
are likely to play a role in the disk evolution and dispersal. Finally, we also
find some evidence of clumpy star formation or mini-clusters within Tr37.Comment: 21 pages, 16 figures, plus appendix with tables and figures. Accepted
by A&
Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B
The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB
association, one of the most active regions of star formation within 1 kpc,
have been observed with the IRAC detector on board the Spitzer Space Telescope.
The goals are to study protoplanetary disk evolution and processes of
sequential triggered star formation in the region. Out of ~400 pre-main
sequence (PMS) stars selected with an earlier Chandra X-ray Observatory
observation, 95% are identified with mid-infrared sources and most of these are
classified as diskless or disk-bearing stars. The discovery of the additional
>200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample
complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the
cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected
stellar sample give several results. Our major finding is a spatio-temporal
gradient of young stars from the hot molecular core towards the primary
ionizing O star HD 217086. This strongly supports the radiation driven
implosion (RDI) model of triggered star formation in the region. The empirical
estimate for the shock velocity of 1 km/s is very similar to theoretical models
of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1.
agreement of the disk fractions, their mass dependency, and fractions of
transition disks with other clusters; 2. confirmation of the youthfulness of
the embedded CepB cluster; 3. confirmation of the effect of suppression of
time-integrated X-ray emission in disk-bearing versus diskless systems.Comment: Accepted for publication in The Astrophysical Journal. 48 pages, 14
figures. For a version with high-quality figures, see
http://www.astro.psu.edu/users/gkosta/RESEARCH/cepb_spitzer_chandra.pd
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