282 research outputs found
Herschel evidence for disk flattening or gas depletion in transitional disks
Transitional disks are protoplanetary disks characterized by reduced near-
and mid-infrared emission with respect to full disks. This characteristic
spectral energy distribution indicates the presence of an optically thin inner
cavity within the dust disk believed to mark the disappearance of the
primordial massive disk. We present new Herschel Space Observatory PACS spectra
of [OI] 63 micron for 21 transitional disks. Our survey complements the larger
Herschel GASPS program "Gas in Protoplanetary Systems" (Dent et al. 2013) by
quadrupling the number of transitional disks observed with PACS at this
wavelength. [OI] 63 micron traces material in the outer regions of the disk,
beyond the inner cavity of most transitional disks. We find that transitional
disks have [OI] 63 micron line luminosities two times fainter than their full
disk counterparts. We self consistently determine various stellar properties
(e.g. bolometric luminosity, FUV excess, etc.) and disk properties (e.g. disk
dust mass, etc.) that could influence the [OI] 63 micron line luminosity and we
find no correlations that can explain the lower [OI] 63 micron line
luminosities in transitional disks. Using a grid of thermo-chemical
protoplanetary disk models, we conclude that either transitional disks are less
flared than full disks or they possess lower gas-to-dust ratios due to a
depletion of gas mass. This result suggests that transitional disks are more
evolved than their full disk counterparts, possibly even at large radii.Comment: Accepted for publication in ApJ; 52 pages, 16 figures, 8 table
Structure and Composition of Two Transitional Circumstellar Disks in Corona Australis
The late stages of evolution of the primordial circumstellar disks
surrounding young stars are poorly understood, yet vital to constrain theories
of planet formation. We consider basic structural models for the disks around
two ~10 Myr-old members of the nearby RCrA association, RX J1842.9-3532 and RX
J1852.3-3700. We present new arcsecond-resolution maps of their 230 GHz
continuum emission from the Submillimeter Array and unresolved CO(3-2) spectra
from the Atacama Submillimeter Telescope Experiment. By combining these data
with broadband fluxes from the literature and infrared fluxes and spectra from
the catalog of the Formation and Evolution of Planetary Systems (FEPS) Legacy
program on the Spitzer Space Telescope, we assemble a multiwavelength data set
probing the gas and dust disks. Using the Monte Carlo radiative transfer code
RADMC to model simultaneously the SED and millimeter continuum visibilities, we
derive basic dust disk properties and identify an inner cavity of radius 16 AU
in the disk around RX J1852.3-3700. We also identify an optically thin 5 AU
cavity in the disk around RX J1842.9-3532, with a small amount of optically
thick material close to the star. The molecular line observations suggest an
intermediate disk inclination in RX J1842.9-3532, consistent with the continuum
emission. In combination with the dust models, the molecular data allow us to
derive a lower CO content than expected, suggesting that the process of gas
clearing is likely underway in both systems, perhaps simultaneously with planet
formation.Comment: 11 pages, 5 figures, accepted for publication in A
Infrared Variability of Evolved Protoplanetary Disks: Evidence for Scale Height Variations in the Inner Disk
We present the results of a multi-wavelength multi-epoch survey of five
evolved protoplanetary disks in the IC 348 cluster that show significant
infrared variability. Using 3-8micron and 24micron photometry along with
5-40micron spectroscopy from the Spitzer Space Telescope, as well as
ground-based 0.8-5micron spectroscopy, optical spectroscopy and near-infrared
photometry, covering timescales of days to years, we examine the variability in
the disk, stellar and accretion flux. We find substantial variations (10-60%)
at all infrared wavelengths on timescales of weeks to months for all of these
young stellar objects. This behavior is not unique when compared to other
cluster members and is consistent with changes in the structure of the inner
disk, most likely scale height fluctuations on a dynamical timescale. Previous
observations, along with our near-infrared photometry, indicate that the
stellar fluxes are relatively constant; stellar variability does not appear to
drive the large changes in the infrared fluxes. Based on our near-infrared
spectroscopy of the Pa-beta and Br-gamma lines we find that the accretion rates
are variable in most of the evolved disks but the overall rates are probably
too small to cause the infrared variability. We discuss other possible physical
causes for the variability, including the influence of a companion, magnetic
fields threading the disk, and X-ray flares.Comment: Accepted to ApJ. 33 pages, emulate apj forma
GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor
The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly
continuous monitoring of its early optical afterglow decay, as well as
high-quality optical spectra that determined a redshift of z3=2.328 for its
host galaxy, an active starburst galaxy with strong Lyman-alpha emission and
several absorption lines. Spectral observations show multiple absorbers at
z3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/s
respectively relative to the host galaxy Lyman-alpha emission. We argue that
these correspond to a fragmented shell nebula that has been radiatively
accelerated by the gamma-ray burst (GRB) afterglow at a distance greater than
0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratios
indicate that the nebula is overabundant in carbon and silicon. The high level
of carbon and silicon is consistent with a swept-up shell nebula gradually
enriched by a WCL progenitor wind over the lifetime of the nebula prior to the
GRB onset. The detection of statistically significant fluctuations and color
changes about the jet-like optical decay further supports this interpretation
since fluctuations must be present at some level due to inhomogeneities in a
clumpy stellar wind medium or if the progenitor has undergone massive ejection
prior to the GRB onset. This evidence suggests that the mass-loss process in a
Wolf-Rayet star might lead naturally to an iron-core collapse with sufficient
angular momentum that could serve as a suitable GRB progenitor.Comment: Replaced with version accepted by ApJ; 40 pages, 9 figure
The Disk Population of the Taurus Star-Forming Region
We have analyzed nearly all images of the Taurus star-forming region at
3.6-24um that were obtained during the cryogenic mission of the Spitzer Space
Telescope (46 deg^2) and have measured photometry for all known members of the
region that are within these data, corresponding to 348 sources. We have
classified the members of Taurus according to whether they show evidence of
disks and envelopes (classes I, II, and III). The disk fraction in Taurus is
75% for solar-mass stars and declines to 45% for low-mass stars and brown
dwarfs (0.01-0.3 M_sun). This dependence on stellar mass is similar to that
measured for Cha I, although the disk fraction in Taurus is slightly higher
overall, probably because of its younger age (1 vs. 2-3 Myr). In comparison,
the disk fraction for solar-mass stars is much lower (20%) in IC 348 and Sigma
Ori, which are denser than Taurus and Cha I and are roughly coeval with the
latter. These data indicate that disk lifetimes for solar-mass stars are longer
in regions that have lower stellar densities. Through an analysis of multiple
epochs of photometry that are available for ~200 Taurus members, we find that
stars with disks exhibit significantly greater mid-IR variability than diskless
stars. Finally, we have used our data in Taurus to refine the criteria for
primordial, evolved, and transitional disks. The number ratio of evolved and
transitional disks to primordial disks in Taurus is 15/98 for K5-M5, indicating
a timescale of 0.15 x tau(primordial)=0.45 Myr for the clearing of the inner
regions of optically thick disks. After applying the same criteria to older
clusters (2-10 Myr), we find that the proportions of evolved and transitional
disks in those populations are consistent with the measurements in Taurus when
their star formation histories are properly taken into account. ERRATUM: In
Table 7, we inadvertently omitted the spectral type bins in which class II
sources were placed in Table 8 based on their bolometric luminosities (applies
only to stars that lack spectroscopic classifications). The bins were K6-M3.5
for FT Tau, DK Tau B, and IRAS 04370+2559, M3.5-M6 for IRAS 04200+2759, IT Tau
B, and ITG 1, and M6-M8 for IRAS 04325+2402 C. In addition, the values of
K_s-[3.6] in Table 13 and Figure 26 for spectral types of M4-M9 are incorrect.
We present corrected versions of Table 13 and Figure 26.Comment: revised version with Erratum (in press
Probing the Dust and Gas in the Transitional Disk of CS Cha with Spitzer
Here we present the Spitzer IRS spectrum of CS Cha, a member of the ~2 Myr
old Chamaeleon star-forming region, which reveals an optically thick
circumstellar disk truncated at ~43 AU, the largest hole modeled in a
transitional disk to date. Within this inner hole, ~5x10^-5 lunar masses of
dust are located in a small optically thin inner region which extends from 0.1
to 1 AU. In addition, the disk of CS Cha has bigger grain sizes and more
settling than the previously modeled transitional disks DM Tau, GM Aur, and
CoKu Tau/4, suggesting that CS Cha is in a more advanced state of dust
evolution. The Spitzer IRS spectrum also shows [Ne II] 12.81 micron
fine-structure emission with a luminosity of 1.3x10^29 ergs s^-1, indicating
that optically thin gas is present in this ~43 AU hole, in agreement with
H_alpha measurements and a UV excess which indicate that CS Cha is still
accreting 1.2x10^-8 M_sun yr^-1. We do not find a correlation of the [Ne II]
flux with L_X, however, there is a possible correlation with mass accretion
rate, which if confirmed would suggest that EUV fluxes due to accretion are the
main agent for formation of the [Ne II] line.Comment: accepted to ApJ Letter
A Spatially Resolved Inner Hole in the Disk around GM Aurigae
We present 0.3 arcsec resolution observations of the disk around GM Aurigae
with the Submillimeter Array (SMA) at a wavelength of 860 um and with the
Plateau de Bure Interferometer at a wavelength of 1.3 mm. These observations
probe the distribution of disk material on spatial scales commensurate with the
size of the inner hole predicted by models of the spectral energy distribution.
The data clearly indicate a sharp decrease in millimeter optical depth at the
disk center, consistent with a deficit of material at distances less than ~20
AU from the star. We refine the accretion disk model of Calvet et al. (2005)
based on the unresolved spectral energy distribution (SED) and demonstrate that
it reproduces well the spatially resolved millimeter continuum data at both
available wavelengths. We also present complementary SMA observations of CO
J=3-2 and J=2-1 emission from the disk at 2" resolution. The observed CO
morphology is consistent with the continuum model prediction, with two
significant deviations: (1) the emission displays a larger CO J=3-2/J=2-1 line
ratio than predicted, which may indicate additional heating of gas in the upper
disk layers; and (2) the position angle of the kinematic rotation pattern
differs by 11 +/- 2 degrees from that measured at smaller scales from the dust
continuum, which may indicate the presence of a warp. We note that
photoevaporation, grain growth, and binarity are unlikely mechanisms for
inducing the observed sharp decrease in opacity or surface density at the disk
center. The inner hole plausibly results from the dynamical influence of a
planet on the disk material. Warping induced by a planet could also potentially
explain the difference in position angle between the continuum and CO data
sets.Comment: 12 pages, 6 figures, accepted for publication in Ap
Resolving the gap and AU-scale asymmetries in the pre-transitional disk of V1247 Orionis
Pre-transitional disks are protoplanetary disks with a gapped disk structure,
potentially indicating the presence of young planets in these systems. In order
to explore the structure of these objects and their gap-opening mechanism, we
observed the pre-transitional disk V1247 Orionis using the Very Large Telescope
Interferometer, the Keck Interferometer, Keck-II, Gemini South, and IRTF. This
allows us spatially resolve the AU-scale disk structure from near- to
mid-infrared wavelengths (1.5 to 13 {\mu}m), tracing material at different
temperatures and over a wide range of stellocentric radii. Our observations
reveal a narrow, optically-thick inner-disk component (located at 0.18 AU from
the star) that is separated from the optically thick outer disk (radii >46 AU),
providing unambiguous evidence for the existence of a gap in this
pre-transitional disk. Surprisingly, we find that the gap region is filled with
significant amounts of optically thin material with a carbon-dominated dust
mineralogy. The presence of this optically thin gap material cannot be deduced
solely from the spectral energy distribution, yet it is the dominant
contributor at mid-infrared wavelengths. Furthermore, using Keck/NIRC2 aperture
masking observations in the H, K', and L' band, we detect asymmetries in the
brightness distribution on scales of about 15-40 AU, i.e. within the gap
region. The detected asymmetries are highly significant, yet their amplitude
and direction changes with wavelength, which is not consistent with a companion
interpretation but indicates an inhomogeneous distribution of the gap material.
We interpret this as strong evidence for the presence of complex density
structures, possibly reflecting the dynamical interaction of the disk material
with sub-stellar mass bodies that are responsible for the gap clearing.Comment: 16 pages, 17 Figures, accepted by Astrophysical Journa
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