arXiv subjects

Geoffrey A. Blake

Publications and source records attributed to Geoffrey A. Blake.

At least 145 records · Page 8Linked to original sources

arXiv2010

Spectrally resolved pure rotational lines of water in protoplanetary disks

We present ground-based high resolution N-band spectra (Δv = 15 km/s) of pure rotational lines of water vapor in two protoplanetary disks surrounding the pre-main sequence stars AS 205N and RNO 90, selected based on detections of rotational water lines by the Spitzer IRS. Using VISIR on the Very Large Telescope, we spectrally resolve individual lines and show that they have widths of 30-60 km/s, consistent with an origin in Keplerian disks at radii of ~1 AU. The water lines have similar widths to those of the CO at 4.67 micron, indicating that the mid-infrared water lines trace similar radii. The rotational temperatures of the water are 540 and 600K in the two disks, respectively. However, the lines ratios show evidence of non-LTE excitation, with low-excitation line fluxes being over-predicted by 2-dimensional disk LTE models. Due to the limited number of observed lines and the non-LTE line ratios, an accurate measure of the water ortho/para ratio is not available, but a best estimate for AS 205N is ortho/para = 4.5 +/- 1.0, apparently ruling out a low-temperature origin of the water. The spectra demonstrate that high resolution spectroscopy of rotational water lines is feasible from the ground, and further that ground-based high resolution spectroscopy is likely to significantly improve our understanding of the inner disk chemistry recently revealed by recent Spitzer observations.

astro-ph.EP↗

arXiv2010

A Spitzer c2d Legacy Survey to Identify and Characterize Disks with Inner Dust Holes

Understanding how disks dissipate is essential to studies of planet formation. However, identifying exactly how dust and gas dissipates is complicated due to difficulty in finding objects clearly in the transition of losing their surrounding material. We use Spitzer IRS spectra to examine 35 photometrically-selected candidate cold disks (disks with large inner dust holes). The infrared spectra are supplemented with optical spectra to determine stellar and accretion properties and 1.3mm photometry to measure disk masses. Based on detailed SED modeling, we identify 15 new cold disks. The remaining 20 objects have IRS spectra that are consistent with disks without holes, disks that are observed close to edge-on, or stars with background emission. Based on these results, we determine reliable criteria for identifying disks with inner holes from Spitzer photometry and examine criteria already in the literature. Applying these criteria to the c2d surveyed star-forming regions gives a frequency of such objects of at least 4% and most likely of order 12% of the YSO population identified by Spitzer. We also examine the properties of these new cold disks in combination with cold disks from the literature. Hole sizes in this sample are generally smaller than for previously discovered disks and reflect a distribution in better agreement with exoplanet orbit radii. We find correlations between hole size and both disk and stellar masses. Silicate features, including crystalline features, are present in the overwhelming majority of the sample although 10 micron feature strength above the continuum declines for holes with radii larger than ~7 AU. In contrast, PAHs are only detected in 2 out of 15 sources. Only a quarter of the cold disk sample shows no signs of accretion, making it unlikely that photoevaporation is the dominant hole forming process in most cases.

astro-ph.SR↗

arXiv2010

A Spitzer survey of mid-infrared molecular emission from protoplanetary disks I: Detection rates

We present a Spitzer InfraRed Spectrometer search for 10-36 micron molecular emission from a large sample of protoplanetary disks, including lines from H2O, OH, C2H2, HCN and CO2. This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star forming regions, including Ophiuchus, Lupus and Chamaeleon. In total, we identify 22 T Tauri stars with strong mid-infrared H2O emission. Integrated water line luminosities, where water vapor is detected, range from 5x10^-4 to 9x10^-3 Lsun, likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the 5 transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H2). We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1-2% line-to-continuum ratios, in the mid-infrared, while almost 2/3 of the disks around K stars show sufficiently intense water emission to be detected by Spitzer. Some Herbig Ae/Be stars show tentative H2O/OH emission features beyond 20 micron at the 1-2 level, however, and one of them shows CO2 in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks is due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks.

astro-ph.EP↗

arXiv2010

c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines

A survey of mid-IR gas-phase emission lines of H2, H2O and various atoms toward a sample of 43 embedded low-mass young stars in nearby star-forming regions is presented. The sources are selected from the Spitzer "Cores to Disks" (c2d) legacy program. The environment of embedded protostars is complex both in its physical structure (envelopes, outflows, jets, protostellar disks) and the physical processes (accretion, irradiation by UV and/or X-rays, excitation through slow and fast shocks) which take place. A key point is to spatially resolve the emission in the Spitzer-IRS spectra. An optimal extraction method is used to separate both spatially unresolved (compact, up to a few 100 AU) and spatially resolved (extended, 1000 AU or more) emission from the IRS spectra. The results are compared with the c2d disk sample and literature PDR and shock models to address the physical nature of the sources. Both compact and extended emission features are observed. Warm (Tex few 100 K) H2, observed through the pure rotational H2 S(0), S(1) and S(2) lines, and [S I] 25 mu emission is observed primarily in the extended component. [S I] is observed uniquely toward truly embedded sources and not toward disks. On the other hand hot (Tex>700 K) H2, observed primarily through the S(4) line, and [Ne II] emission is seen mostly in the compact component. [Fe II] and [Si II] lines are observed in both spatial components. Hot H2O emission is found in the compact component of some sources. The observed emission on >=1000 AU scales is characteristic of PDR emission and likely originates in the outflow cavities in the remnant envelope created by the stellar wind and jets from the embedded young stars. Weak shocks along the outflow wall can also contribute. The compact emission is likely of mixed origin, comprised of optically thick circumstellar disk and/or jet/outflow emission from the protostellar object.

astro-ph.GA↗

arXiv2010

The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects. IV. NH3 and CH3OH

NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and HCOOCH3. Despite a number of recent studies, little is known about their abundances in the solid state. (...) In this work, we investigate the ~ 8-10 micron region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 micron silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ~9 micron in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 nu_2 umbrella mode, and derive abundances with respect to water between ~2 and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the nu_4 C-O stretch mode of this molecule at ~9.7 micron in 16 objects, yielding abundances consistent with those derived by Boogert et al. 2008 (hereafter paper I) based on a simultaneous 9.75 and 3.53 micron data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10-20 % of nitrogen is locked up in known ices.

astro-ph.GA↗

arXiv2010

Grain growth across protoplanetary discs: 10-micron silicate feature versus millimetre slope

Young stars are formed within dusty discs. The grains in the disc are originally of the same size as interstellar dust. Models predict that these grains will grow in size through coagulation. Observations of the silicate features at micron wavelengths are consistent with growth to micron sizes whereas the slope of the SED at longer wavelengths traces growth up to mm sizes. We here look for a correlation between these two grain growth indicators. A large sample of T-Tauri and Herbig-Ae/Be stars was observed with the Spitzer Space Telescope at 5-13 micron; a subsample was observed at mm wavelengths. We complement this subsample with data from the literature to maximise the overlap between micron and mm observations and search for correlations. Synthetic spectra are produced to determine which processes may produce the dust evolution. Dust disc masses in the range <1 to 7 x 10^-4 MSun are obtained. Most sources have a mm spectral slope consistent with grain growth. There is a tentative correlation between the 10-micron silicate feature and the mm slope of the SED. The observed sources seem to be grouped per star-forming region in the micron-vs-mm diagram. The modelling results show that the 10-micron feature becomes flatter and subsequently the mm slope becomes shallower. Grain size distributions shallower than that of the ISM and/or bright central stars are required to explain specific features. Settling of larger grains towards the disc midplane affects the 10-micron feature, but hardly the mm slope. The tentative correlation between the strength of the 10-micron feature and the mm slope suggests that the inner and outer disc evolve simultaneously. Dust with a mass dominated by mm-sized grains is required to explain the shallowest mm slopes. Other processes besides grain growth may also be responsible for the removal of small grains.

astro-ph.SR↗

arXiv2009

A new raytracer for modeling AU-scale imaging of lines from protoplanetary disks

The material that formed the present-day Solar System originated in feeding zones in the inner Solar Nebula located at distances within ~20 AU from the Sun, known as the planet-forming zone. Meteoritic and cometary material contain abundant evidence for the presence of a rich and active chemistry in the planet-forming zone during the gas-rich phase of Solar System formation. It is a natural conjecture that analogs can be found amoung the zoo of protoplanetary disks around nearby young stars. The study of the chemistry and dynamics of planet formation requires: 1) tracers of dense gas at 100-1000 K and 2) imaging capabilities of such tracers with 5-100 (0.5-20 AU) milli-arcsec resolution, corresponding to the planet-forming zone at the distance of the closest star-forming regions. Recognizing that the rich infrared (2-200 micron) molecular spectrum recently discovered to be common in protoplanetary disks represents such a tracer, we present a new general raytracing code, RADLite, that is optimized for producing infrared line spectra and images from axisymmetric structures. RADLite can consistently deal with a wide range of velocity gradients, such as those typical for the inner regions of protoplanetary disks. The code is intended as a backend for chemical and excitation codes, and can rapidly produce spectra of thousands of lines for grids of models for comparison with observations. Such radiative transfer tools will be crucial for constraining both the structure and chemistry of planet-forming regions, including data from current infrared imaging spectrometers and extending to the Atacama Large Millimeter Array and the next generation of Extremely Large Telescopes, the James Webb Space Telescope and beyond.

astro-ph.EP↗

arXiv2009

The Evolution of Massive YSOs in the LMC: Part I. Identification and Spectral Classification

We present and categorize Spitzer IRS spectra of 294 objects in the Large Magellanic Cloud (LMC) to create the largest and most complete catalog of massive young stellar object (YSO) spectra in the LMC. Target sources were identified from infrared photometry and multi-wavelength images indicative of young, massive stars highly enshrouded in their natal gas and dust clouds. Our sample primarily consists of 277 objects we identify as having spectral features indicative of embedded YSOs. The remaining sources are comprised of 7 C-rich evolved sources, 8 sources dominated by broad silicate emission, and 1 source with multiple broad emission features. Those with YSO-like spectra show a range of spectral features including polycyclic aromatic hydrocarbon emission, deep silicate absorption, fine-structure lines, and ice absorption features. Based upon the relative strengths of these features, we have classified the YSO candidates into several distinct categories using the widely-used statistical procedure known as principal component analysis. We propose that these categories represent a spectrum of evolutionary stages during massive YSO formation. We conclude that massive pre-main sequence stars spend a majority of their massive, embedded lives emitting in the UV. Half of the sources in our study have features typical of compact HII regions, suggesting that massive YSOs can create a detectable compact HII region half-way through the formation time present in our sample. This study also provides a check on commonly used source-selection procedures including the use of photometry to identify YSOs. We determine a high success rate (>95%) of identifying objects with YSO-like spectra can be achieved through careful use of infrared CMDs, SEDs, and image inspections.

astro-ph.SR↗

arXiv2009

High Resolution Spectroscopy of [NeII] Emission from AA Tau and GM Aur

We present high resolution (R=80,000) spectroscopy of [NeII] emission from two young stars, GM Aur and AA Tau, which have moderate to high inclinations. The emission from both sources appears centered near the stellar velocity and is broader than the [NeII] emission measured previously for the face-on disk system TW Hya. These properties are consistent with a disk origin for the [NeII] emission we detect, with disk rotation (rather than photoevaporation or turbulence in a hot disk atmosphere) playing the dominant role in the origin of the line width. In the non-face-on systems, the [NeII] emission is narrower than the CO fundamental emission from the same sources. If the widths of both diagnostics are dominated by Keplerian rotation, this suggests that the [NeII] emission arises from larger disk radii on average than does the CO emission. The equivalent width of the [NeII] emission we detect is less than that of the spectrally unresolved [NeII] feature in the Spitzer spectra of the same sources. Variability in the [NeII] emission or the mid-infrared continuum, a spatially extended [NeII] component, or a very (spectrally) broad [NeII] component might account for the difference in the equivalent widths.

astro-ph.GA↗

arXiv2008

The Spitzer c2d Legacy Results: Star Formation Rates and Efficiencies; Evolution and Lifetimes

(Abridged) The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star formation rates and efficiencies, numbers and lifetimes for SED classes, and clustering properties. Current star formation efficiencies range from 3% to 6%. Taken together, the five clouds are producing about 260 solar masses of stars per Myr. The star formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, and the current stock of dense cores would be exhausted in 1.8 Myr on average. The derived lifetime for the Class I phase is 0.44 to 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.10 to 0.16 Myr, is longer than early estimates. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 solar mass per cubic pc. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 solar masses, given an efficiency of 0.3. Our data confirm and aggravate the "luminosity problem" for protostars. Our results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.

astro-ph↗

arXiv2008

Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps

We present velocity-resolved spectro-astrometric imaging of the 4.7 $μ$m rovibrational lines of CO gas in protoplanetary disks using the CRIRES high resolution infrared spectrometer on the Very Large Telescope (VLT). The method as applied to three disks with known dust gaps or inner holes out to 4-45 AU (SR 21, HD 135344B and TW Hya) achieves an unprecedented spatial resolution of $0.1-0.5$ AU. While one possible gap formation mechanism is dynamical clearing by giant planets, other equally good explanations (stellar companions, grain growth, photo-evaporation) exist. One way of distinguishing between different scenarios is the presence and distribution of gas inside the dust gaps. Keplerian disk models are fit to the spectro-astrometric position-velocity curves to derive geometrical parameters of the molecular gas. We determine the position angles and inclinations of the inner disks with accuracies as good as 1-2\degr, as well as the radial extent of the gas emission. Molecular gas is detected well inside the dust gaps in all three disks. The gas emission extends to within a radius of 0.5 AU for HD 135344B and to 0.1 AU for TW Hya, supporting partial clearing by a $< 1-10 M_{\rm Jup}$ planetary body as the cause of the observed dust gaps, or removal of the dust by extensive grain coagulation and planetesimal formation. The molecular gas emission in SR 21 appears to be truncated within $\sim 7 $AU, which may be caused by complete dynamical clearing by a more massive companion. We find a smaller inclination angle of the inner disk of TW Hya than that determined for the outer disk, suggestive of a disk warp. We also detect significant azimuthal asymmetries in the SR 21 and HD 135344B inner disks.

astro-ph↗

arXiv2008

The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4

CH4 is proposed to be the starting point of a rich organic chemistry. Solid CH4 abundances have previously been determined mostly toward high mass star forming regions. Spitzer/IRS now provides a unique opportunity to probe solid CH4 toward low mass star forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH4 abundance toward a large sample of low mass young stellar objects. 25 out of 52 ice sources in the $c2d$ (cores to disks) legacy have an absorption feature at 7.7 um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances are 2-8%, except for three sources with abundances as high as 11-13%. These latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above 2E18 cm-2, the CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation plots with solid H2O, CH3OH, CO2 and CO column densities and abundances relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with models where CH4 is formed through sequential hydrogenation of C on grain surfaces. Finally the equal or higher abundances toward low mass young stellar objects compared with high mass objects and the correlation studies support this formation pathway as well, but not the two competing theories: formation from CH3OH and formation in gas phase with subsequent freeze-out.

astro-ph↗

arXiv2007

The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects II: CO2

This paper presents Spitzer-IRS spectroscopy of the CO2 15.2 micron bending mode toward a sample of 50 embedded low-mass stars in nearby star-forming clouds, taken mostly from the ``Cores to Disks (c2d)'' Legacy program. The average abundance of solid CO2 relative to water in low-mass protostellar envelopes is 0.32 +/- 0.02, significantly higher than that found in quiescent molecular clouds and in massive star forming regions. It is found that a decomposition of all the observed CO2 bending mode profiles requires a minimum of five unique components. Roughly 2/3 of the CO2 ice is found in a water-rich environment, while most of the remaining 1/3 is found in a CO environment. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO2:CO component and suggest a model in which low-density clouds form the CO2:H2O component and higher density clouds form the CO2:CO ice during and after the freeze-out of gas-phase CO. It is suggested that the subsequent evolution of the CO2 and CO profiles toward low-mass protostars, in particular the appearance of the splitting of the CO2 bending mode due to pure, crystalline CO2, is first caused by distillation of the CO2:CO component through evaporation of CO due to thermal processing to ~20-30 K in the inner regions of infalling envelopes. The formation of pure CO2 via segregation from the H2O rich mantle may contribute to the band splitting at higher levels of thermal processing (>50 K), but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.

astro-ph↗

arXiv2007

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds: VII. Ophiuchus Observed with MIPS

We present maps of 14.4 deg^2 of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high quality maps depict both numerous point sources as well as extended dust emission within the star-forming and non-star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 um and 81 sources at 70 um at the 10 sigma level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus K$-$[24] color-magnitude diagrams as compared to 24 um detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all the ones with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates.

astro-ph↗

arXiv2007

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. IV. Lupus Observed with MIPS

We present maps of 7.78 square degrees of the Lupus molecular cloud complex at 24, 70, and $160\:μ$m. They were made with the Spitzer Space Telescope's Multiband Imaging Photometer for Spitzer (MIPS) instrument as part of the Spitzer Legacy Program, ``From Molecular Cores to Planet-Forming Disks'' (c2d). The maps cover three separate regions in Lupus, denoted I, III, and IV. We discuss the c2d pipeline and how our data processing differs from it. We compare source counts in the three regions with two other data sets and predicted star counts from the Wainscoat model. This comparison shows the contribution from background galaxies in Lupus I. We also create two color magnitude diagrams using the 2MASS and MIPS data. From these results, we can identify background galaxies and distinguish them from probable young stellar objects. The sources in our catalogs are classified based on their spectral energy distribution (SED) from 2MASS and Spitzer wavelengths to create a sample of young stellar object candidates. From 2MASS data, we create extinction maps for each region and note a strong corresponence between the extinction and the $160\:μ$m emission. The masses we derived in each Lupus cloud from our extinction maps are compared to masses estimated from $^{13}$CO and C$^{18}$O and found to be similar to our extinction masses in some regions, but significantly different in others. Finally, based on our color-magnitude diagrams, we selected 12 of our reddest candidate young stellar objects for individual discussion. Five of the 12 appear to be newly-discovered YSOs.

astro-ph↗

arXiv2007

The Detection and Characterization of cm Radio Continuum Emission from the Low-mass Protostar L1014-IRS

Observations by the Cores to Disk Legacy Team with the Spitzer Space Telescope have identified a low luminosity, mid-infrared source within the dense core, Lynds 1014, which was previously thought to harbor no internal source. Followup near-infrared and submillimeter interferometric observations have confirmed the protostellar nature of this source by detecting scattered light from an outflow cavity and a weak molecular outflow. In this paper, we report the detection of cm continuum emission with the VLA. The emission is characterized by a quiescent, unresolved 90 uJy 6 cm source within 0.2" of the Spitzer source. The spectral index of the quiescent component is $α= 0.37\pm 0.34$ between 6 cm and 3.6 cm. A factor of two increase in 6 cm emission was detected during one epoch and circular polarization was marginally detected at the $5σ$ level with Stokes {V/I} $= 48 \pm 16$% . We have searched for 22 GHz H2O maser emission toward L1014-IRS, but no masers were detected during 7 epochs of observations between June 2004 and December 2006. L1014-IRS appears to be a low-mass, accreting protostar which exhibits cm emission from a thermal jet or a wind, with a variable non-thermal emission component. The quiescent cm radio emission is noticeably above the correlation of 3.6 cm and 6 cm luminosity versus bolometric luminosity, indicating more radio emission than expected. We characterize the cm continuum emission in terms of observations of other low-mass protostars, including updated correlations of centimeter continuum emission with bolometric luminosity and outflow force, and discuss the implications of recent larger distance estimates on the physical attributes of the protostar and dense molecular core.

astro-ph↗

arXiv2007

c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H_2 gas-phase lines

We present a survey of mid-infrared gas-phase lines toward a sample of 76 circumstellar disks around low mass pre-main sequence stars from the Spitzer "Cores to Disks" legacy program. We report the first detections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20% respectively ~9% of our sources. The observed [Ne II] line fluxes and upper limits are consistent with [Ne II] excitation in an X-ray irradiated disk around stars with X-ray luminosities L_X=10^{29}-10^{31} erg s^{-1}. [Fe I] is detected at ~10^-5-10^-4 L_Sun, but no [S I] or [Fe II] is detected down to ~10^{-6} L_Sun. The [Fe I] detections indicate the presence of gas-rich disks with masses of >~0.1 M_J. No H_2 0-0 S(0) and S(1) disk emission is detected, except for S(1) toward one source. These data give upper limits on the warm (T~100-200K) gas mass of a few Jovian masses, consistent with recent T Tauri disk models which include gas heating by stellar radiation. Compact disk emission of hot (T>~500K) gas is observed through the H_2 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The line fluxes are, however, higher by more than an order of magnitude than those predicted by recent disk models, even when X-ray and excess UV radiation are included. Similarly the [Ne II]/H_2 0-0 S(2) ratios for these sources are lower than predicted, consistent with the presence of an additional hot molecular gas component not included in current disk models. Oblique shocks of stellar winds interacting with the disk can explain many aspects of the hot gas emission, but are inconsistent with the non-detection of [S I] and [Fe II] lines.

astro-ph↗

arXiv2007

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds VIII. Serpens Observed with MIPS

We present maps of 1.5 square degrees of the Serpens dark cloud at 24, 70, and 160\micron observed with the Spitzer Space Telescope MIPS Camera. More than 2400 compact sources have been extracted at 24um, nearly 100 at 70um, and 4 at 160um. We estimate completeness limits for our 24um survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets, a 0.50 deg^2 set on a low-extinction region near the dark cloud, and a 5.3 deg^2 subset of the SWIRE ELAIS N1 data that was processed through our pipeline. These results show that there is an easily identifiable population of young stellar object candidates in the Serpens Cloud that is not present in either of the reference data sets. We also show a comparison of visual extinction and cool dust emission illustrating a close correlation between the two, and find that the most embedded YSO candidates are located in the areas of highest visual extinction.

astro-ph↗