Browsing by Subject "forming region"

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c2d Spitzer-IRS Spectra Of Disks Around T Tauri Stars IV. Crystalline Silicates
(2009-11) Olofsson, J.; Augereau, J. C.; van Dishoeck, E. F.; Merin, B.; Lahuis, F.; Kessler-Silacci, J.; Dullemond, C. P.; Oliveira, I.; Blake, G. A.; Boogert, A. C. A.; Brown, J. M.; Evans, Neal J.; Geers, V.; Knez, C.; Monin, J. L.; Pontoppidan, K.; Kessler-Silacci, J.; Evans, N. J.
Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 mu m where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at lambda > 20 mu m correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 mu m feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (<= 1 AU) emitting at mu similar to 10 mu m and a much colder region emitting at lambda > 20 mu m (<= 10 AU). We identify a crystallinity paradox, as the long-wavelength (lambda > 20 mu m) crystalline silicate features are detected 3.5 times more frequently (similar to 55% vs. similar to 15%) than the crystalline features arising from much warmer disk regions (lambda similar to 10 mu m). This suggests that the disk has an inhomogeneous dust composition within similar to 10 AU. The analysis of the shape and strength of both the amorphous 10 mu m feature and the crystalline feature around 23 mu m provides evidence for the prevalence of mu m-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of mu m-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.
c2d Spitzer-IRS Spectra Of Disks Around T Tauri Stars IV. Crystalline Silicates
(2009-11) Olofsson, J.; Augereau, J. C.; van Dishoeck, E. F.; Merin, B.; Lahuis, F.; Kessler-Silacci, J.; Dullemond, C. P.; Oliveira, I.; Blake, G. A.; Boogert, A. C. A.; Brown, J. M.; Evans, Neal J.; Geers, V.; Knez, C.; Monin, J. L.; Pontoppidan, K.; Kessler-Silacci, J.; Evans, N. J.
Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 mu m where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at lambda > 20 mu m correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 mu m feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (<= 1 AU) emitting at mu similar to 10 mu m and a much colder region emitting at lambda > 20 mu m (<= 10 AU). We identify a crystallinity paradox, as the long-wavelength (lambda > 20 mu m) crystalline silicate features are detected 3.5 times more frequently (similar to 55% vs. similar to 15%) than the crystalline features arising from much warmer disk regions (lambda similar to 10 mu m). This suggests that the disk has an inhomogeneous dust composition within similar to 10 AU. The analysis of the shape and strength of both the amorphous 10 mu m feature and the crystalline feature around 23 mu m provides evidence for the prevalence of mu m-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of mu m-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.
Emission From Water Vapor And Absorption From Other Gases At 5-7.5 Mu M In Spitzer-Irs Spectra Of Protoplanetary Disks
(2014-09) Sargent, B. A.; Forrest, W.; Watson, D. M.; D'Alessio, P.; Calvet, N.; Furlan, E.; Kim, K. H.; Green, J.; Pontoppidan, K.; Richter, I.; Tayrien, C.; Green, J.
We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming region showing emission in Spitzer Space Telescope Infrared Spectrograph 5-7.5 mu m spectra from water vapor and absorption from other gases in these stars' protoplanetary disks. Seven stars' spectra show an emission feature at 6.6 mu m due to the nu(2) = 1-0 bending mode of water vapor, with the shape of the spectrum suggesting water vapor temperatures >500 K, though some of these spectra also show indications of an absorption band, likely from another molecule. This water vapor emission contrasts with the absorption from warm water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other 6 of the 13 stars have spectra showing a strong absorption band, peaking in strength at 5.6-5.7 mu m, which for some is consistent with gaseous formaldehyde (H2CO) and for others is consistent with gaseous formic acid (HCOOH). There are indications that some of these six stars may also have weak water vapor emission. Modeling of these stars' spectra suggests these gases are present in the inner few AU of their host disks, consistent with recent studies of infrared spectra showing gas in protoplanetary disks.
A Herschel Survey Of Cold Dust In Disks Around Brown Dwarfs And Low-Mass Stars
(2012-08) Harvey, Paul M.; Henning, Thomas; Liu, Yao; Menard, Francois; Pinte, Christopher; Wolf, Sebastian; Cieza, Lucas A.; Evans, Neal J.; Pascucci, Ilaria; Harvey, Paul M.
We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs (BDs). We surveyed 50 fields containing 51 known or suspected BDs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70 mu m and 14 at 160 mu m with signal-to-noise ratio (S/N) greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]-[70] mu m colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 10(-6) M-circle dot up to 10(-3) M-circle dot with a median disk mass of the order of 3 x 10(-5) M-circle dot, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young BDs and low-mass stars are located span a range in estimated age from similar to 1-3 Myr to similar to 10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.
The Spitzer c2d Survey Of Large, Nearby, Interstellar Clouds. X. The Chamaeleon II Pre-Main-Sequence Population As Observed With IRAC And MIPS
(2008-03) Alcala, Juan M.; Spezzi, Loredana; Chapman, Nicholas; Evans, Neal J.; Huard, Tracy L.; Jorgensen, Jes K.; Merin, Bruno; Stapelfeldt, Karl R.; Covino, E.; Frasca, Antonio; Gandolfi, Davide; Oliveira, Isa; Evans, Neal J.
We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M-circle dot). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M-circle dot pc(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8 '' < theta < 6.0 '' (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Delta t similar to 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M-circle dot into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered.
The Young Population Of The Chamaeleon II Dark Cloud
(2008-06) Spezzi, Loredana; Alcala, Juan M.; Covino, Elvira; Frasca, Antonio; Gandolfi, Davide; Oliveira, Isa; Chapman, Nicholas; Evans, Neal J.; Huard, Tracy L.; Jorgensen, Jes K.; Merin, Bruno; Stapelfeldt, Karl R.; Evans, Neal J.
We discuss the results of the optical spectroscopic follow-up of pre-main-sequence (PMS) objects and candidates selected in the Chamaeleon II dark cloud based on data from the Spitzer Legacy survey "From Molecular Cores to Planet Forming Disks" (c2d) and from previous surveys. Our sample includes both objects with infrared excess selected according to c2d criteria and referred to as young stellar objects and other cloud members and candidates selected from complementary optical and near-infrared data. We characterize the sample of objects by deriving their physical parameters. The vast majority of objects have masses M <= 1 M(circle dot) and ages < 6 Myr. Several of the PMS objects and candidates lie very close to or below the hydrogen-burning limit. A first estimate of the slope of the initial mass function in Cha II is consistent with that of other T associations. The star formation efficiency in the cloud (1%-4%) is consistent with our own estimates for Taurus and Lupus, but significantly lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds may reflect a different history of star formation. We also find that the Cha II cloud is turning some 8 M(circle dot) into stars every megayear, which is less than the star formation rate in the other c2d clouds. However, the star formation rate is not steady and evidence is found that the star formation in Cha II might have occurred very rapidly. The H alpha emission of the Cha II PMS objects, as well as possible correlations between their stellar and disk properties, is also investigated.