45 research outputs found
The Physical Structure of Protoplanetary Disks: the Serpens Cluster Compared with Other Regions
Spectral energy distributions are presented for 94 young stars surrounded by
disks in the Serpens Molecular Cloud, based on photometry and Spitzer IRS
spectra. Taking a distance to the cloud of 415 pc rather than 259 pc, the
distribution of ages is shifted to lower values, in the 1-3 Myr range, with a
tail up to 10 Myr. The mass distribution spans 0.2-1.2 Msun, with median mass
of 0.7 Msun. The distribution of fractional disk luminosities in Serpens
resembles that of the young Taurus Molecular Cloud, with most disks consistent
with optically thick, passively irradiated disks in a variety of disk
geometries (Ldisk/Lstar ~ 0.1). In contrast, the distributions for the older
Upper Scorpius and Eta Chamaeleontis clusters are dominated by optically thin
lower luminosity disks (Ldisk/Lstar ~ 0.02). This evolution in fractional disk
luminosities is concurrent with that of disk fractions. The actively accreting
and non-accreting stars (based on Ha data) in Serpens show very similar
distributions in fractional disk luminosities, differing only in the brighter
tail dominated by strongly accreting stars. In contrast with a sample of Herbig
Ae/Be stars, the T Tauri stars in Serpens do not have a clear separation in
fractional disk luminosities for different disk geometries: both flared and
flat disks present wider, overlapping distributions. This result is consistent
with previous suggestions of a faster evolution for disks around Herbig Ae/Be
stars. Furthermore, the results for the mineralogy of the dust in the disk
surface do not show any correlation to either stellar and disk characteristics
or mean cluster age in the 1-10 Myr range probed here. A possible explanation
for the lack of correlation is that the processes affecting the dust within
disks have short timescales, happening repeatedly, making it difficult to
distinguish long lasting evolutionary effects. [abridged]Comment: ApJ in pres
A Herschel view of IC 1396 A: Unveiling the different sequences of star formation
The IC1396A globule in the young cluster Tr37, hosting many young stars and
protostars, is assumed to be a site of triggered star formation. We mapped
IC1396A with Herschel/PACS at 70 and 160 micron. The Herschel maps trace in
great detail the very embedded protostellar objects and the structure of the
cloud. PACS data reveal a previously unknown Class 0 object (IC1396A-PACS-1)
located behind the ionization front. IC1396A-PACS-1 is not detectable with
Spitzer, but shows marginal X-ray emission. The data also allowed to study
three of the Class I intermediate-mass objects within the cloud. We derived
approximate cloud temperatures to study the effect and potential interactions
between the protostars and the cloud. The Class 0 object is associated with the
densest and colder part of IC1396A. Heating in the cloud is dominated by the
winds and radiation of the O6.5 star HD 206267 and, to a lesser extent, by the
effects of the Herbig Ae star V 390 Cep. The surroundings of the Class I and
Class II objects embedded in the cloud also appear warmer than the sourceless
areas, although most of the low-mass objects cannot be individually extracted
due to distance and beam dilution. The observations suggest that at least two
episodes of star formation have occurred in IC1396A. One would have originated
the known, ~1 Myr-old Class I and II objects in the cloud, and a new wave of
star formation would have produced the Class 0 source at the tip of the
brigth-rimmed cloud. From its location and properties, IC1396A-PACS-1 is
consistent with triggering via radiative driven implosion (RDI) induced by HD
206267. The mechanisms behind the formation of the more evolved population of
Class I/II/III objects in the cloud are uncertain. Heating of most of the
remaining cloud by Class I/Class II objects and by HD 206267 itself may
preclude further star formation in the region.Comment: Accepted by A&A, 9 pages, 5 figure
Herschel Observations of the T Cha Transition Disk: Constraining the Outer Disk Properties
T Cha is a nearby (d similar to 100 pc) transition disk known to have an optically thin gap separating optically thick inner and outer disk components. Huelamo et al. recently reported the presence of a low-mass object candidate within the gap of the T Cha disk, giving credence to the suspected planetary origin of this gap. Here we present the Herschel photometry (70, 160, 250, 350, and 500 mu m) of T Cha from the "Dust, Ice, and Gas in Time" Key Program, which bridges the wavelength range between existing Spitzer and millimeter data and provide important constraints on the outer disk properties of this extraordinary system. We model the entire optical to millimeter wavelength spectral energy distribution (SED) of T Cha (19 data points between 0.36 and 3300 mu m without any major gaps in wavelength coverage). T Cha shows a steep spectral slope in the far-IR, which we find clearly favors models with outer disks containing little or no dust beyond similar to 40 AU. The full SED can be modeled equally well with either an outer disk that is very compact (only a few AU wide) or a much larger one that has a very steep surface density profile. That is, T Cha's outer disk seems to be either very small or very tenuous. Both scenarios suggest a highly unusual outer disk and have important but different implications for the nature of T Cha. Spatially resolved images are needed to distinguish between the two scenarios.DIGIT Herschel Open Time Key ProgramNASAAlexander von Humboldt FoundationEuropean CommissionAgence Nationale pour la Recherche of France PERG06-GA-2009-256513, ANR-07-BLAN-0221, ANR-2010-JCJC-0504-01CNRS/INSU, FranceAstronom
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed With IRAC and MIPS
We present c2d Spitzer/IRAC observations of the Lupus I, III and IV dark
clouds and discuss them in combination with optical and near-infrared and c2d
MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times
larger than the previous one. It is dominated by low- and very-low mass stars
and it is complete down to M 0.1M. We find 30-40 % binaries
with separations between 100 to 2000 AU with no apparent effect in the disk
properties of the members. A large majority of the objects are Class II or
Class III objects, with only 20 (12%) of Class I or Flat spectrum sources. The
disk sample is complete down to ``debris''-like systems in stars as small as M
0.2 M and includes sub-stellar objects with larger IR
excesses. The disk fraction in Lupus is 70 -- 80%, consistent with an age of 1
-- 2 Myr. However, the young population contains 20% optically thick accretion
disks and 40% relatively less flared disks. A growing variety of inner disk
structures is found for larger inner disk clearings for equal disk masses.
Lupus III is the most centrally populated and rich, followed by Lupus I with a
filamentary structure and by Lupus IV, where a very high density core with
little star-formation activity has been found. We estimate star formation rates
in Lupus of 2 -- 10 M Myr and star formation efficiencies of a
few percent, apparently correlated with the associated cloud masses.Comment: Accepted for publication in the ApJS. Contains 101 pages, 23 figures,
and 13 tables. A version with full resolution figures can be found at
http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap102.pub.pd
The Herschel Digit Survey Of Weak-Line T Tauri Stars: Implications For Disk Evolution And Dissipation
As part of the "Dust, Ice, and Gas In Time (DIGIT)" Herschel Open Time Key Program, we present Herschel photometry (at 70, 160, 250, 350, and 500 mu m) of 31 weak-line T Tauri star (WTTS) candidates in order to investigate the evolutionary status of their circumstellar disks. Of the stars in our sample, 13 had circumstellar disks previously known from infrared observations at shorter wavelengths, while 18 of them had no previous evidence for a disk. We detect a total of 15 disks as all previously known disks are detected at one or more Herschel wavelengths and two additional disks are identified for the first time. The spectral energy distributions (SEDs) of our targets seem to trace the dissipation of the primordial disk and the transition to the debris disk regime. Of the 15 disks, 7 appear to be optically thick primordial disks, including 2 objects with SEDs indistinguishable from those of typical Classical T Tauri stars, 4 objects that have significant deficit of excess emission at all IR wavelengths, and 1 "pre-transitional" object with a known gap in the disk. Despite their previous WTTS classification, we find that the seven targets in our sample with optically thick disks show evidence for accretion. The remaining eight disks have weaker IR excesses similar to those of optically thin debris disks. Six of them are warm and show significant 24 mu m Spitzer excesses, while the last two are newly identified cold debris-like disks with photospheric 24 mu m fluxes, but significant excess emission at longer wavelengths. The Herschel photometry also places strong constraints on the non-detections, where systems with F-70/F-70,(*) greater than or similar to 5-15 and L-disk/L-* greater than or similar to 10(-3) to 10(-4) can be ruled out. We present preliminary models for both the optically thick and optically thin disks and discuss our results in the context of the evolution and dissipation of circumstellar disks.NASA through JPL/CaltechNASA through the Sagan Fellowship ProgramEuropean Commission PERG06-GA-2009-256513Agence Nationale pour la Recherche (ANR) of France ANR-2010-JCJC-0504-01CFHT 11AH96Astronom
Nature Of Transition Circumstellar Disks. I. The Ophiuchus Molecular Cloud
We have obtained millimeter-wavelength photometry, high-resolution optical spectroscopy, and adaptive optics near-infrared imaging for a sample of 26 Spitzer-selected transition circumstellar disks. All of our targets are located in the Ophiuchus molecular cloud (d similar to 125 pc) and have spectral energy distributions (SEDs) suggesting the presence of inner opacity holes. We use these ground-based data to estimate the disk mass, multiplicity, and accretion rate for each object in our sample in order to investigate the mechanisms potentially responsible for their inner holes. We find that transition disks are a heterogeneous group of objects, with disk masses ranging from <0.6 to 40 M(JUP) and accretion rates ranging from <10(-11) to 10(-7) M(circle dot) yr(-1), but most tend to have much lower masses and accretion rates than "full disks" (i.e., disks without opacity holes). Eight of our targets have stellar companions: six of them are binaries and the other two are triple systems. In four cases, the stellar companions are close enough to suspect they are responsible for the inferred inner holes. We find that nine of our 26 targets have low disk mass (<2.5 M(JUP)) and negligible accretion (<10(-11) M(circle dot) yr(-1)), and are thus consistent with photoevaporating (or photoevaporated) disks. Four of these nine non-accreting objects have fractional disk luminosities <10(-3) and could already be in a debris disk stage. Seventeen of our transition disks are accreting. Thirteen of these accreting objects are consistent with grain growth. The remaining four accreting objects have SEDs suggesting the presence of sharp inner holes, and thus are excellent candidates for harboring giant planets.NASA 1224608, 1230782, 1230779, 1407FONDECYT 1061199Basal CATA PFB 06/09ALMA FUND 31070021ALMA-Conicyt FUND 31060010National Science Foundation AST0-808144Spitzer Space Telescope Legacy Science ProgramAstronom