arXiv subjects

Geoffrey A. Blake

Publications and source records attributed to Geoffrey A. Blake.

At least 163 records · Page 9Linked to original sources

arXiv2007

Deep Spitzer spectroscopy of the `Flying Saucer' edge-on disk: Large grains beyond 50 AU

We present deep Spitzer-IRS low-resolution (lambda/Delta lambda ~ 100) 5-35 micron spectroscopy of the edge-on disk ``the Flying Saucer'' (2MASS J16281370-2431391) in the Ophiuchus molecular cloud. The spectral energy distribution exhibits the characteristic two-peak shape predicted for a circumstellar disk viewed very close to edge-on. The short-wavelength peak is entirely due to photons scattered off the surface of the disk, while the long-wavelength peak is due to thermal emission from the disk itself. The Spitzer spectrum represents the first spectroscopic detection of scattered light out to 15 micron from a bona-fide, isolated edge-on disk around a T Tauri star. The depth and the wavelength of the mid-infrared "valley" of the SED give direct constraints on the size distribution of large grains in the disk. Using a 2D continuum radiative transfer model, we find that a significant amount of 5-10 micron-sized grains is required in the surface layers of the disk at radii of 50-300 AU. The detection of relatively large grains in the upper layers implies that vertical mixing is effective, since grain growth models predict the grains would otherwise settle deep in the disk on short time scales. Additionally, we tentatively detect the 9.66 micron S(3) line of H2 and the 11.2 micron emission feature due to PAHs.

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arXiv2006

The Spitzer c2d survey of Large, Nearby, Interstellar Clouds. V. Chamaeleon II Observed with IRAC

We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud. The observed area covers about 1 square degree defined by $A_V >2$. Analysis of the data in the 2005 c2d catalogs reveals a small number of sources (40) with properties similar to those of young stellaror substellar objects (YSOs). The surface density of these YSO candidates is low, and contamination by background galaxies appears to be substantial, especially for sources classified as Class I or flat SED. We discuss this problem in some detail and conclude that very few of the candidate YSOs in early evolutionary stages are actually in the Cha II cloud. Using a refined set of criteria, we define a smaller, but more reliable, set of 24 YSO candidates.

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arXiv2006

The Spitzer c2d Survey of Nearby Dense Cores: IV. Revealing the Embedded Cluster in B59

Infrared images of the dark cloud core B59 were obtained with the Spitzer Space Telescope as part of the "Cores to Disks" Legacy Science project. Photometry from 3.6-70 microns indicates at least 20 candidate low-mass young stars near the core, more than doubling the previously known population. Out of this group, 13 are located within about 0.1 pc in projection of the molecular gas peak, where a new embedded source is detected. Spectral energy distributions span the range from small excesses above photospheric levels to rising in the mid-infrared. One other embedded object, probably associated with the millimeter source B59-MMS1, with a bolometric luminosity L(bol) roughly 2 L(sun), has extended structure at 3.6 and 4.5 microns, possibly tracing the edges of an outflow cavity. The measured extinction through the central part of the core is A(V) greater than of order 45 mag. The B59 core is producing young stars with a high efficiency.

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arXiv2006

Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star

We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star VV Ser. We combine the Spitzer data with interferometric and spectroscopic data from the literature covering UV to submillimeter wavelengths. The full set of data are modeled by a two-dimensional axisymmetric Monte Carlo radiative transfer code. The model is used to test the prediction of (Dullemond et al. 2003) that disks around UX Orionis stars must have a self-shadowed shape, and that these disks are seen nearly edge-on, looking just over the edge of a puffed-up inner rim, formed roughly at the dust sublimation radius. We find that a single, relatively simple model is consistent with all the available observational constraints spanning 4 orders of magnitude in wavelength and spatial scales, providing strong support for this interpretation of UX Orionis stars. The grains in the upper layers of the puffed-up inner rim must be small (0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events, while the shape and strength of the mid-infrared silicate emission features indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0 micron). From the model fit, the location of the puffed-up inner rim is estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small grains. This is almost twice the rim radius estimated from near-infrared interferometry. A best fitting model for the inner rim in which large grains in the disk mid-plane reach to within 0.25 AU of the star, while small grains in the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to reproduce all the data, including the near-infrared visibilities. [Abstract abridged]

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arXiv2006

Modeling Spitzer observations of VV Ser. II. an extended quantum heated nebula and a disk shadow

We present mid-infrared Spitzer IRAC and MIPS images of the UX Orionis star VV Ser and the surrounding cloud. The 5.6--70 micron images show bright, localized and nebulous emission extended over 4 arcmin centered on VV Ser. We interpret the nebulosity as being due to transiently heated grains excited by UV photons emitted by VV Ser. A companion paper describes how the physical structure of the VV Ser disk has been constrained using a wide range of observational data modeled by an axisymmetric Monte Carlo radiative transfer code. In this paper we employ the model to study the nebulosity surrounding VV Ser using quantum-heated PAH molecules and Very Small Grains (VSGs) consisting of amorphous carbon in the thermal cooling approximation. Imprinted on the nebulosity is a wedge-shaped dark band, centered on the star. We interpret this dark wedge as the shadow cast by the inner regions of a near-edge-on disk in UV light, allowing the PAHs to be excited only outside of this shadow. The presence of a disk shadow strongly constrains the inclination as well as the position angle of the disk. Although depending on the adopted PAH opacity, the abundance of PAHs in the surrounding cloud is constrained to 5+/-2% of the total dust mass, given the opacity. The extent of the nebulosity constrains the density of the gas surrounding the VV Ser disk to 500+/-200 cm-3 for a gas-to-dust ratio of 100. This low density suggests that the quantum heated material is not part of the original envelope of VV Ser and that it is rather a quiescent part of the Serpens molecular cloud that the star has passed into after being formed. [Abstract abridged]

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arXiv2006

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. III. Perseus Observed with IRAC

We present observations of 3.86 sq. deg. of the Perseus molecular cloud complex with the Spitzer Space Telescope Infrared Array Camera (IRAC). The maps show strong extended emission arising from shocked H2 in outflows in the region and from polycyclic aromatic hydrocarbon features. More than 120,000 sources are extracted toward the cloud. Based on their IRAC colors and comparison to off-cloud and extragalactic fields, we identify 400 candidate young stellar objects. About two thirds of these are associated with the young clusters IC348 and NGC1333, while the last third is distributed over the remaining cloud. We classify the young stellar objects according to the traditional scheme based on the slope of their spectral energy distributions. Significant differences are found for the numbers of embedded Class I objects relative to more evolved Class II objects in IC348, NGC1333 and the remaining cloud with the embedded Class I and "flat spectrum" YSOs constituting 14%, 36% and 47% of the total number of YSOs identified in each of these regions. The high number of Class I objects in the extended cloud (61% of the Class I objects in the entire cloud) suggests that a significant fraction of the current star formation is occuring outside the two main clusters. Finally we discuss a number of outflows and identify their driving sources, including the known deeply embedded Class 0 sources outside the two major clusters. The Class 0 objects are found to be detected by Spitzer and have very red [3.6]-[4.5] colors but do not show similarly red [5.8]-[8.0] colors. The Class 0 objects are easily identifiable in color-color diagrams plotting these two colors but are problematic to extract automatically due to the extended emission from shocked gas or scattered light in cavities related to the associated outflows.

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arXiv2006

A Bima Array Survey of Molecules in Comets Linear (C/2002 T7) and Neat (C/2001 Q4)

We present an interferometric search for large molecules, including methanol, methyl cyanide, ethyl cyanide, ethanol, and methyl formate in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) array. In addition, we also searched for transitions of the simpler molecules CS, SiO, HNC, HN13C and 13CO . We detected transitions of methanol and CS around Comet LINEAR and one transition of methanol around Comet NEAT within a synthesized beam of ~20''. We calculated the total column density and production rate of each molecular species using the variable temperature and outflow velocity (VTOV) model described by Friedel et al.(2005).Considering the molecular production rate ratios with respect to water, Comet T7 LINEAR is more similar to Comet Hale-Bopp while Comet Q4 NEAT is more similar to Comet Hyakutake. It is unclear, however, due to such a small sample size, whether there is a clear distinction between a Hale-Bopp and Hyakutake class of comet or whether comets have a continuous range of molecular production rate ratios.

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arXiv2005

BIMA Array Detections of HCN in Comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4)

We present interferometric detections of HCN in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) Array. With a 25.4"x20.3" synthesized beam around Comet LINEAR and using a variable temperature and outflow velocity (VTOV) model, we found an HCN column density of N_T=6.4+-2.1x10^12 cm^-2, and a production rate of Q(HCN)=6.5+-2.2x10^26 s^-1, giving a production rate ratio of HCN relative to H_2O of ~3.3+-1.1x10^-3 and relative to CN of ~4.6+-1.5. With a 21.3"x17.5" synthesized beam around Comet NEAT and using a VTOV model, we found an HCN column density of N_T=8.5+-4.5x10^11 cm^-2, and a production rate of Q(HCN)=8.9+-4.7x10^25 s^-1, giving a production rate ratio of HCN relative to H_2O of ~7.4+-3.9x10^-4 and relative to CN of ~0.3+-0.2. For both comets, the production rates relative to H_2O are similar to those found in previous comet observations. For Comet LINEAR the production rate relative to CN is consistent with HCN being the primary parent species of CN, while for Comet NEAT it is too low for this to be the case.

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arXiv2004

Ices in the edge-on disk CRBR 2422.8-3423: Spitzer spectroscopy and Monte Carlo radiative transfer modeling

We present 5.2-37.2 micron spectroscopy of the edge-on circumstellar disk CRBR 2422.8-3423 obtained using the InfraRed Spectrograph (IRS) of the Spitzer Space Telescope. The IRS spectrum is combined with ground-based 3-5 micron spectroscopy to obtain a complete inventory of solid state material present along the line of sight toward the source. We model the object with a 2D axisymmetric (effectively 3D) Monte Carlo radiative transfer code. It is found that the model disk, assuming a standard flaring structure, is too warm to contain the very large observed column density of pure CO ice, but is possibly responsible for up to 50% of the water, CO2 and minor ice species. In particular the 6.85 micron band, tentatively due to NH4+, exhibits a prominent red wing, indicating a significant contribution from warm ice in the disk. It is argued that the pure CO ice is located in the dense core Oph-F in front of the source seen in the submillimeter imaging, with the CO gas in the core highly depleted. The model is used to predict which circumstances are most favourable for direct observations of ices in edge-on circumstellar disks. Ice bands will in general be deepest for inclinations similar to the disk opening angle, i.e. ~70 degrees. Due to the high optical depths of typical disk mid-planes, ice absorption bands will often probe warmer ice located in the upper layers of nearly edge-on disks. The ratios between different ice bands are found to vary by up to an order of magnitude depending on disk inclination due to radiative transfer effects caused by the 2D structure of the disk. Ratios between ice bands of the same species can therefore be used to constrain the location of the ices in a circumstellar disk. [Abstract abridged]

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arXiv2004

H2 Mid-IR Pure Rotational Emission from Young Stars: The TEXES/IRTF Survey

We describe the TEXES survey for mid-IR H2 pure rotational emission from young stars and report early successes. H2 emission is a potential tracer of warm gas in circumstellar disks. Three pure rotational lines are available from the ground: the J=3=>1, J=4=>2, and J=6=>4, transitions at 17.035 microns, 12.279 microns, and 8.025 microns, respectively. Using TEXES at the NASA IRTF 3m, we are midway through a survey of roughly 30 pre-main-sequence stars. To date, detected lines are all resolved, generally with FWHM<10 km/s. Preliminary analysis suggests the gas temperatures are between 400 and 800 K. From the work so far, we conclude that high spectral and spatial resolution are critical to the investigation of H2 in disks.

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arXiv2003

Continuum and CO/HCO+ Emission from the Disk Around the T Tauri Star LkCa 15

We present OVRO Millimeter Array lambda = 3.4 - 1.2 mm dust continuum and spectral line observations of the accretion disk encircling the T Tauri star LkCa 15. The 1.2 mm dust continuum emission is resolved, and gives a minimum diameter of 190 AU and an inclination angle of 57+/-5 degrees. There is a noticeable, but at present poorly constrained, decrease in the continuum spectral slope with frequency that may result from the coupled processes of grain growth and dust settling. Imaging of the fairly intense emission from the lowest rotational transitions of CO, 13CO and HCO+ reveals a rotating disk and emission extends to 750 AU and the characteristic radius of the disk is determined to be around 425 AU (HWHM) based on model fits to the CO velocity field. The disk mass derived from the CO isotopologues with ``typical'' dense cloud abundances is still nearly two orders of magnitude less than that inferred from the dust emission, which is probably due to extensive molecular depletion in the cold, dense disk midplane. N2H+ 1-0 emission has also been detected which, along with HCO+, sets a lower limit to the fractional ionization of 10^{-8} in the near-surface regions of protoplanetary disks. This first detection of N2H+ in circumstellar disks has also made possible a determination of the N2/CO ratio (~2) that is at least an order of magnitude larger than those in the envelopes of young stellar objects and dense clouds. The large N2/CO ratio indicates that our observations probe disk layers in which CO is depleted but some N2 remains in the gas phase. Such differential depletion can lead to large variations in the fractional ionization with height in the outer reaches of circumstellar disks, and may help to explain the relative nitrogen deficiency observed in comets.

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arXiv2003

Observations of rotationally resolved C3 in translucent sight lines

The rotationally resolved spectrum of the A ^1Pi_u <- X ^1Sigma^+_g 000-000 transition of C3, centered at 4051.6A, has been observed along 10 translucent lines of sight. To interpret these spectra, a new method for the determination of column densities and analysis of excitation profiles involving the simulation and fitting of observed spectra has been developed. The populations of lower rotational levels (J<14) in C3 are best fit by thermal distributions that are consistent with the kinetic temperatures determined from the excitation profile of C2. Just as in the case of C2, higher rotational levels (J>14) of C3 show increased nonthermal population distributions in clouds which have been determined to have total gas densities below ~500 cm-3.

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arXiv2003

Interferometric Observations of Formaldehyde in the Protoplanetary Disk around LkCa15

Emission from the $2_{12}-1_{11}$ line of H$_2$CO has been detected and marginally resolved toward LkCa15 by the Nobeyama Millimeter Array. The column density of H$_2$CO is higher than that observed in DM Tau and than predicted by theoretical models of disk chemistry; also the line-intensity profile is less centrally peaked than that for CO. A similar behavior is observed in other organic gaseous molecules in the LkCa 15 disk.

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arXiv2002

Millimeter-wave Searches for Cold Dust and Molecular Gas around T Tauri Stars in MBM 12

We report results of a sensitive search for cold dust and molecular gas in the disks around 8 T Tauri stars in the high-latitude cloud MBM 12. Interferometric observations of 3 mm continuum emission in 5 fields containing 6 of the objects, and literature values for the remaining two, limit the disk masses to M_disk < 0.04 - 0.09 M_sun (gas+dust), for a gas:dust mass ratio of 100 and a distance of 275 pc. By coadding the 3 mm data of our five fields, we set an upper limit to the average disk mass of (N=5) < 0.03 M_sun. Simultaneous observation of the CS J=2-1 and the N2H+ 1-0 lines show no emission. Single-dish observations of the 13CO 2-1 line limit the disk mass to (5 - 10) * 10^-4 M_sun for a standard CO abundance of 2 * 10^-4. Depletion of CO by up to two orders of magnitude, through freezing out or photodissociation, can reconcile these limits. These mass limits lie within the range found in the Taurus-Auriga and rho Oph star-forming regions (0.001 - 0.3 M_sun), and preclude conclusions about possible decrease in disk mass over the 1 - 2 Myr age range spanned by the latter two regions and MBM 12. Our observations can exclude the presence in MBM 12 of T Tauri stars with relatively bright and massive disks such as T Tau, DG Tau, and GG Tau.

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arXiv2002

Looking for Pure Rotational H_2 Emission from Protoplanetary Disks

We report on a limited search for pure-rotational molecular hydrogen emission associated with young, pre-main-sequence stars. We looked for H_2 v=0 J = 3->1 and J = 4->2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle Spectrograph (TEXES) at NASA's 3m Infrared Telescope Facility. The high spectral and spatial resolution of our observations lead to more stringent limits on narrow line emission close to the source than previously achieved. One star, AB Aur, shows a possible (2sigma) H_2 detection, but further observations are required to make a confident statement. Our non-detections suggest that a significant fraction, perhaps all, of previously reported H_2 emission towards these objects could be extended on scales of 5" or more.

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arXiv2001

Submillimeter lines from circumstellar disks around pre-main sequence stars

Observations of submillimeter lines of CO (up to J=6-5), HCO+, HCN (up to J=4-3) and their isotopes from circumstellar disks around low mass pre-main sequence stars are presented. The different line ratios and optical depths indicate that most of the observed line emission arises from an intermediate disk layer with high densities of 1E6-1E8 cm-3 and moderately warm temperatures in the outer regions. The data are compared with three disk models from the literature using a full 2D Monte Carlo radiative transfer code. The abundances of the molecules are constrained from the more optically thin 13C species and indicate depletions of 1-30 for LkCa 15 and very high depletions of > 100 for TW Hya with respect to dark cloud abundances. Evidence for significant freeze-out (factors 10 or larger) of CO and HCO+ onto grain surfaces at temperatures below 22 K is found, but the abundances of these molecules must also be low in the warmer upper layer, most likely as a result of photodissociation. A warm upper layer near the surface of a flaring disk heated by stellar and interstellar radiation is an appropriate description of the observations of TW Hya. LkCa 15 seems to be cooler at the surface, perhaps due to dust settling.

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arXiv2000

Structure and Evolution of the Envelopes of Deeply Embedded Massive Young Stars

The physical structure of the envelopes around a sample of fourteen massive (1000-100,000 solar L) young stars is investigated on 100- 100,000 AU scales using maps and spectra in submillimeter continuum and lines of C17O, CS and H2CO. The total column densities and the temperature profiles are obtained by fitting self-consistent dust models to submillimeter photometry. Both the molecular line and dust emission data indicate density gradients ~r^{-alpha}, with alpha=1.0-1.5, significantly flatter than the alpha=2.0 generally found for low-mass objects. This flattening may indicate that in massive young stellar objects, nonthermal pressure is more important for the support against gravitational collapse, while thermal pressure dominates for low-mass sources. We find alpha=2 for two hot core-type sources, but regard this as an upper limit since in these objects, the CS abundance may be enhanced in the warm gas close to the star.

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arXiv1999

Detection of H2 pure rotational line emission from the GG~Tau binary system

We present the first detection of the low-lying pure rotational emission lines of H2 from circumstellar disks around T~Tauri stars, using the Short Wavelength Spectrometer on the Infrared Space Observatory. These lines provide a direct measure of the total amount of warm molecular gas in disks. The J=2->0 S(0) line at 28.218 mum and the J=3->1 S(1) line at 17.035 mum have been observed toward the double binary system GG Tau. Together with limits on the J=5->3 S(3) and J=7->5 S(5) lines, the data suggest the presence of gas at T_kin=110+-10 K with a mass of (3.6+-2.0)x10^-3 M_sol (3sigma). This amounts to ~3% of the total gas + dust mass of the circumbinary disk as imaged by millimeter interferometry, but is larger than the estimated mass of the circumstellar disk(s). Possible origins for the warm gas seen in H2 are discussed in terms of photon and wind-shock heating mechanisms of the circumbinary material, and comparisons with model calculations are made.

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