arXiv subjects

G. A. Blake

Publications and source records attributed to G. A. Blake.

At least 91 records · Page 5Linked to original sources

arXiv2005

Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry

Spitzer Space Telescope spectra of the low mass young stellar object (YSO) IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of a sample of ~100 YSO's that shows these features and the first time they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra gives excitation temperatures of > 350 K and abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line is barely detected with the James Clerk Maxwell Telescope, indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limits the mass and temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K, respectively. This excludes a hot-core type region as found in high-mass YSO's. The most plausible origin of this hot gas rich in organic molecules is in the inner (<6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe organic chemistry, gas temperatures and kinematics in the planet-forming zones close to a young star.

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arXiv2005

Spitzer Mid-infrared Spectroscopy of Ices Toward Extincted Background Stars

A powerful way to observe directly the solid state inventory of dense molecular clouds is by infrared spectroscopy of background stars. We present Spitzer/IRS 5-20 micron spectra of ices toward stars behind the Serpens and Taurus molecular clouds, probing visual extinctions of 10-34 mag. These data provide the first complete inventory of solid-state material in dense clouds before star formation begins. The spectra show prominent 6.0 and 6.85 micron bands. In contrast to some young stellar objects (YSOs), most (~75%) of the 6.0 micron band is explained by the bending mode of pure water ice. In realistic mixtures this number increases to 85%, because the peak strength of the water bending mode is very sensitive to the molecular environment. The strength of the 6.85 micron band is comparable to what is observed toward YSOs. Thus, the production of the carrier of this band does not depend on the energetic input of a nearby source. The spectra show large abundances of carbon monoxide and carbon dioxide (20-40% with respect to water ice). Compared to YSOs, the band profile of the 15 micron carbon dioxide bending mode lacks the signatures of crystallization, confirming the cold, pristine nature of these lines of sight. After the dominant species are removed, there are residuals that suggest the presence of minor species such as formic acid and possibly ammonia. Clearly, models of star formation should begin with dust models already coated with a fairly complex mixture of ices.

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arXiv2005

C2D Spitzer-IRS spectra of disks around T Tauri stars: I. Silicate emission and grain growth

Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar-mass stars limited primarily to the 10 um region. The observed 10 and 20 um silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 um indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 um ratios and require even larger grains emitting at 20 um than at 10 um. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 um feature strength vs. shape trend is not correlated with age or Halpha equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength vs. shape trend is related to spectral type, however, with M stars showing significantly flatter 10 um features (larger grain sizes) than A/B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation, stellar/disk winds, etc.).

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arXiv2005

Protostellar holes: Spitzer Space Telescope observations of the protostellar binary IRAS16293-2422

Mid-infrared (23-35 micron) emission from the deeply embedded "Class 0" protostar IRAS16293-2422 is detected with the Spitzer Space Telescope infrared spectrograph. A detailed radiative transfer model reproducing the full spectral energy distribution (SED) from 23 micron to 1.3 mm requires a large inner cavity of radius 600 AU in the envelope to avoid quenching the emission from the central sources. This is consistent with a previous suggestion based on high angular resolution millimeter interferometric data. An alternative interpretation using a 2D model of the envelope with an outflow cavity can reproduce the SED but not the interferometer visibilities. The cavity size is comparable to the centrifugal radius of the envelope and therefore appears to be a natural consequence of the rotation of the protostellar core, which has also caused the fragmentation leading to the central protostellar binary. With a large cavity such as required by the data, the average temperature at a given radius does not increase above 60-80 K and although hot spots with higher temperatures may be present close to each protostar, these constitute a small fraction of the material in the inner envelope. The proposed cavity will also have consequences for the interpretation of molecular line data, especially of complex species probing high temperatures in the inner regions of the envelope.

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arXiv2005

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. I. Chamaeleon II Observed with MIPS

We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70, and 160 micron observed with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) and a 1.2 square degree millimeter map from SIMBA on the Swedish-ESO Submillimetre Telescope (SEST). The c2d Spitzer Legacy Team's data reduction pipeline is described in detail. Over 1500 24 micron sources and 41 70 micron sources were detected by MIPS with fluxes greater than 10-sigma. More than 40 potential YSOs are identified with a MIPS and 2MASS color-color diagram and by their spectral indices, including two previously unknown sources with 24 micron excesses. Our new SIMBA millimeter map of Cha II shows that only a small fraction of the gas is in compact structures with high column densities. The extended emission seen by MIPS is compared with previous CO observations. Some selected interesting sources, including two detected at 1 mm, associated with Cha II are discussed in detail and their SEDs presented. The classification of these sources using MIPS data is found to be consistent with previous studies.

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arXiv2004

8--13 um spectroscopy of YSOs: Evolution of the silicate feature

In order to investigate possible connections between dust processing and disk properties, 8--13 um spectra of 34 young stars, with a range of circumstellar environments and spectral types A to M, were obtained using the Long Wavelength Spectrometer at the W. M. Keck Observatory. The broad 9.7 um amorphous silicate feature which dominates this wavelength regime evolves from absorption in young, embedded sources, to emission in optically revealed stars, and to complete absence in older debris disk systems for both low- and intermediate-mass stars. The peak wavelength and FWHM are centered about 9.7 and ~2.3 um, corresponding to amorphous olivine, with a larger spread in FWHM for embedded sources and in peak wavelength for disks. In a few of our objects that have been previously identified as class I low-mass YSOs, the observed silicate feature is complex, with absorption near 9.5 um and emission peaking around 10 um. Although most of the emission spectra show broad classical features attributed to amorphous silicates, variations in the shape/strength may be linked to dust processing, including grain growth and/or silicate crystallization. We study quantitatively the evidence for evolutionary trends in the 8--13 um spectra through a variety of spectral shape diagnostics. Based on the lack of correlation between these diagnostics and broad-band infrared luminosity characteristics for silicate emission sources, we conclude that although spectral signatures of dust processing are present, they can not be connected clearly to disk evolutionary stage (for optically thick disks) or optical depth (for optically thin disks). The diagnostics of silicate absorption features (other than the central wavelength of the feature), however, are tightly correlated with optical depth.

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arXiv2004

Methane Abundance Variations toward the Massive Protostar NGC 7538 : IRS9

Absorption and emission lines originating from the nu3 C-H stretching manifold of gas phase CH4 were discovered in the high resolution (R=25,000) infrared L band spectrum along the line of sight toward NGC 7538 : IRS9. These observations provide a diagnostic of the complex dynamics and chemistry in a massive star forming region. The line shapes resemble P Cygni profiles with the absorption and emission components shifted by ~7 km/s with respect to the systemic velocity. Similar velocity components were observed in CO at 4.7 um, but in contrast to CH4, the CO shows deep absorption due to a high velocity outflow as well as absorption at the systemic velocity due to the cold outer envelope. It is concluded that the gas phase CH4 abundance varies by an order of magnitude in this line of sight: it is low in the envelope and the outflow (X[CH4]<0.4e-6), and at least a factor of 10 larger in the central core. The discovery of solid CH4 in independent ground and space based data sets shows that methane is nearly entirely frozen onto grains in the envelope. It thus appears that CH4 is formed by grain surface reactions, evaporates into the gas phase in the warm inner regions of protostellar cores and is efficiently destroyed in shocks related to outflows.

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arXiv2004

A "Starless" Core that Isn't: Detection of a Source in the L1014 Dense Core with the Spitzer Space Telescope

We present observations of L1014, a dense core in the Cygnus region previously thought to be starless, but data from the Spitzer Space Telescope shows the presence of an embedded source. We propose a model for this source that includes a cold core, heated by the interstellar radiation field, and a low-luminosity internal source. The low luminosity of the internal source suggests a substellar object. If L1014 is representative, other "starless" cores may turn out to harbor central sources.

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arXiv2004

Imaging the Disk around TW Hya with the Submillimeter Array

We present ~2"-4" aperture synthesis observations of the circumstellar disk surrounding the nearby young star TW Hya in the CO J=2--1 and J=3--2 lines and associated dust continuum obtained with the partially completed Submillimeter Array. The extent and peak flux of the 230 and 345 GHz dust emission follow closely the predictions of the irradiated accretion disk model of Calvet et al. (2002). The resolved molecular line emission extends to a radius of at least 200 AU, the full extent of the disk visible in scattered light, and shows a clear pattern of Keplerian rotation. Comparison of the images with 2D Monte Carlo models constrains the disk inclination angle to 7+/-1 degrees. The CO emission is optically thick in both lines, and the kinetic temperature in the line formation region is ~20K. Substantial CO depletion, by an order of magnitude or more from canonical dark cloud values, is required to explain the characteristics of the line emission.

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arXiv2004

On the origin of H2CO abundance enhancements in low-mass protostars

High angular resolution H2CO 218 GHz line observations have been carried out toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data reveal extended emission which is compared with that predicted by model envelopes constrained from single-dish data. For L1448-C the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For IRAS 16293-2422, a known proto-binary object, the interferometer observations indicate that the binary has cleared much of the material in the inner part of the envelope, out to the binary separation of ~800 AU. The morphology and velocity structure of the H2CO array data have been used to investigate whether the abundance enhancements inferred from single-dish modelling are due to thermal evaporation of ices or due to liberation of the ice mantles by shocks in the inner envelope. A scenario in which the H2CO abundance drops in the cold dense part of the envelope where CO is frozen out but is undepleted in the outermost region provides good fits to the single-dish and interferometer data on short baselines for both sources. Emission on the longer baselines is best reproduced if the H2CO abundance is increased by about an order of magnitude from ~1E-10 to ~1E-9 in the inner parts of the envelope due to thermal evaporation when the temperature exceeds ~50K. Other scenarios, including weak outflow-envelope interactions and photon heating of the envelope, are discussed and predictions for future generation interferometers are presented, illustrating their potential in distinguishing these competing scenarios.

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arXiv2004

Combined BIMA and OVRO observations of comet C/1999 S4 (LINEAR)

We present results from an observing campaign of the molecular content of the coma of comet C/1999 S4 (LINEAR) carried out jointly with the millimeter-arrays of the Berkeley-Illinois-Maryland Association (BIMA) and the Owens Valley Radio Observatory (OVRO). Using the BIMA array in autocorrelation (`single-dish') mode, we detected weak HCN J=1-0 emission from comet C/1999 S4 (LINEAR) at 14 +- 4 mK km/s averaged over the 143" beam. The three days over which emission was detected, 2000 July 21.9-24.2, immediately precede the reported full breakup of the nucleus of this comet. During this same period, we find an upper limit for HCN 1-0 of 144 mJy/beam km/s (203 mK km/s) in the 9"x12" synthesized beam of combined observations of BIMA and OVRO in cross-correlation (`imaging') mode. Together with reported values of HCN 1-0 emission in the 28" IRAM 30-meter beam, our data probe the spatial distribution of the HCN emission from radii of 1300 to 19,000 km. Using literature results of HCN excitation in cometary comae, we find that the relative line fluxes in the 12"x9", 28" and 143" beams are consistent with expectations for a nuclear source of HCN and expansion of the volatile gases and evaporating icy grains following a Haser model.

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arXiv2003

The impact of shocks on the chemistry of molecular clouds: high resolution images of chemical differentiation along the NGC1333-IRAS2A outflow

This paper presents a detailed study of the chemistry in the outflow associated with the low-mass protostar NGC1333-IRAS2A down to 3" (650 AU) scales. Millimeter-wavelength aperture-synthesis observations from the OVRO and BIMA interferometers and (sub)millimeter single-dish observations from the Onsala 20m telescope and CSO are presented. The interaction of the highly collimated protostellar outflow with a molecular condensation ~15000 AU from the central protostar is clearly traced by molecular species such as HCN, SiO, SO, CS, and CH3OH. Especially SiO traces a narrow high velocity component at the interface between the outflow and the molecular condensation. Multi-transition single-dish observations are used to distinguish the chemistry of the shock from that of the molecular condensation and to address the physical conditions therein. Statistical equilibrium calculations reveal temperatures of 20 and 70 K for the quiescent and shocked components, respectively, and densities near 10^6 cm^{-3}. Significant abundance enhancements of two to four orders of magnitude are found in the shocked region for molecules such as CH3OH, SiO and the sulfur-bearing molecules. HCO+ is seen only in the aftermath of the shock consistent with models where it is destroyed through release of H2O from grain mantles in the shock. N2H+ shows narrow lines, not affected by the outflow but rather probing the ambient cloud. Differences in abundances of HCN, H2CO and CS are seen between different outflow regions and are suggested to be related to differences in the atomic carbon abundance. Compared to the warm inner parts of protostellar envelopes, higher abundances of in particular CH3OH and SiO are found in the outflows, which may be related to density differences between the regions.

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arXiv2003

The structure of the NGC1333-IRAS2 protostellar system on 500 AU scales: an infalling envelope, a circumstellar disk, multiple outflows, and chemistry

This paper investigates small-scale structures of dense gas and dust around the low-mass protostellar binary NGC1333-IRAS2 using millimeter-wavelength aperture-synthesis observations from the OVRO and BIMA interferometers. The detected 3 mm continuum emission is consistent with models of the envelope around IRAS2A, based on previously reported submillimeter continuum images, down to the 3", or 500 AU, resolution of the interferometer data. Our data constrain the contribution of an unresolved point source to 22 mJy. Within the accuracy of the parameters describing the envelope model, the point source flux has an uncertainty by up to 25%. We interpret this point source as a cold disk of mass \gtrsim 0.3 M_\odot. The same envelope model also reproduces aperture-synthesis line observations of the optically thin isotopic species C34S and H13CO+. The more optically thick main isotope lines show a variety of components in the protostellar environment: N2H+ is closely correlated with dust concentrations as seen at submillimeter wavelengths and is particularly strong toward the starless core IRAS2C. We hypothesize that N2H+ is destroyed through reactions with CO that is released from icy grains near the protostellar sources IRAS2A and B. CS, HCO+, and HCN have complex line shapes apparently affected by both outflow and infall. In addition to the east-west jet from IRAS2A, a north-south velocity gradient near this source indicates a second, perpendicular outflow. This suggests the presence of a binary companion within 0.3" (65 AU) from IRAS2A as driving source of this outflow. Alternative explanations of the velocity gradient such as rotation in a circumstellar envelope or a single, wide-angle outflow are less likely.

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arXiv2002

Does IRAS 16293-2422 contain a hot core? New interferometric results

High angular resolution H2CO line observations have been carried out for the low mass proto-binary star IRAS 16293-2422 using the OVRO millimeter array. Simultaneous continuum observations reveal that the source most probably contains two accretion disks, less than 250 AU in size, through which matter is fed onto the stars. The observations suggest that the binary has cleared most of the material in the inner part of the envelope, out to the binary separation. The H2CO interferometer observations indicate the presence of large scale rotation roughly perpendicular to the large scale CO outflow associated with this source. The H2CO emission is dominated by the compact dense and hot (T>100 K) gas close to the positions of the continuum peaks.

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arXiv2002

High Resolution 4.7 um Keck/NIRSPEC Spectra of Protostars. II: Detection of the 13CO Isotope in Icy Grain Mantles

The high resolution (R=25,000) infrared M band spectrum of the massive protostar NGC 7538 : IRS9 shows a narrow absorption feature at 4.779 um (2092.3 cm-1) which we attribute to the vibrational stretching mode of the 13CO isotope in pure CO icy grain mantles. This is the first detection of solid 13CO in the interstellar medium. The 13CO band is a factor of 2.3 narrower than the apolar component of the 12CO band. With this in mind, we discuss the mechanisms that broaden solid state absorption bands. It is shown that ellipsoidally shaped pure CO grains fit the bands of both isotopes at the same time. Reasonable fits are also obtained by CO embedded in N2-rich ices and thermally processed O2-rich ices. In addition, we report new insights into the the nature and evolution of interstellar CO ices by comparing the high quality multi-component solid 12CO spectrum of NGC 7538 : IRS9 with that of the previously studied low mass source L1489 IRS. The narrow absorption by apolar CO ices is present in both spectra, but much stronger in NGC 7538. It is superposed on a smooth broad absorption feature well fitted by a combination of CO2 and H2O-rich laboratory CO ices. The abundances of the latter two ices relative to the total H2O ice column are the same in both sources. We thus suggest that thermal processing manifests itself as evaporation of apolar ices only, and not the formation of CO2 or polar ices. Finally, the decomposition of the 12CO band is used to derive the 12CO/13CO abundance ratio in apolar ices. A ratio of 71+/-15 is deduced, in good agreement with gas phase CO studies (~77) and the solid 12CO2/13CO2 ratio of 80+/-11 found in the same line of sight. The implications for the chemical path along which CO2 is formed are discussed.

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arXiv2002

Does IRAS 16293-2422 have a hot core? Chemical inventory and abundance changes in its protostellar environment

A detailed radiative transfer analysis of the observed continuum and molecular line emission toward the deeply embedded young stellar object IRAS 16293-2422 is performed. The continuum modelling is used to constrain the temperature and density distributions in the envelope, enabling quantitative estimates of various molecular abundances. The molecular excitation analysis reveals that the emission from some molecular species is well reproduced assuming a constant fractional abundance throughout the envelope. The abundances and isotope ratios are generally close to typical values found in cold molecular clouds in these cases, and there is a high degree of deuterium fractionation. There are, however, a number of notable exceptions. Lines covering a wide range of excitation conditions indicate for some molecules, e.g., H2CO, CH3OH, SO, SO2 and OCS, a drastic increase in their abundances in the warm and dense inner region of the circumstellar envelope. The location at which this increase occurs is consistent with the radius at which ices are expected to thermally evaporate off the grains. In all, there is strong evidence for the presence of a `hot core' close to the protostar, whose physical properties are similar to those detected towards most high mass protostars except for a scaling factor. However, the small scale of the hot gas and the infalling nature of the envelope lead to very different chemical time scales between low mass and high mass hot cores, such that only very rapidly produced second-generation complex molecules can be formed in IRAS 16293-2422. Alternatively, the ices may be liberated due to grain-grain collisions in turbulent shear zones where the outflow interacts with the envelope.

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arXiv2002

The Environment and Nature of the Class I Protostar Elias 29: Molecular Gas Observations and the Location of Ices

A (sub-)millimeter line and continuum study of the class I protostar Elias 29 in the Rho Ophiuchi molecular cloud is presented, whose goals are to understand the nature of this source, and to locate the ices that are abundantly present along this line of sight. Within 15-60" beams, several different components contribute to the line emission. Two different foreground clouds are detected, an envelope/disk system and a dense ridge of HCO+ rich material. The latter two components are spatially separated in millimeter interferometer maps. We analyze the envelope/disk system by using inside-out collapse and flared disk models. The disk is in a relatively face-on orientation (<60 degrees), which explains many of the remarkable observational features of Elias 29, such as its flat SED, its brightness in the near infrared, the extended components found in speckle interferometry observations, and its high velocity molecular outflow. It cannot account for the ices seen along the line of sight, however. A small fraction of the ices is present in a (remnant) envelope of mass 0.12-0.33 Msun, but most of the ices (~70%) are present in cool (T<40 K) quiescent foreground clouds. This explains the observed absence of thermally processed ices (crystallized H2O) toward Elias 29. Nevertheless, the temperatures could be sufficiently high to account for the low abundance of apolar (CO, N2, O2) ices. This work shows that it is crucial to obtain spectrally and spatially resolved information from single-dish and interferometric molecular gas observations in order to determine the nature of protostars and to interpret infrared ISO satellite observations of ices and silicates along a pencil beam.

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arXiv2001

High Resolution 4.7 um Keck/NIRSPEC Spectra of Protostars. I: Ices and Infalling Gas in the Disk of L1489 IRS

We explore the infrared M band (4.7 um) spectrum of the class I protostar L1489 IRS in the Taurus Molecular Cloud. This is the highest resolution wide coverage spectrum at this wavelength of a low mass protostar observed to date (R=25,000; Dv=12 km/s). Many narrow absorption lines of gas phase 12CO, 13CO, and C18O are detected, as well as a prominent band of solid 12CO. The gas phase 12CO lines have red shifted absorption wings (up to 100 km/s), likely originating from warm disk material falling toward the central object. The isotopes and the 12CO line wings are successfully fitted with a contracting disk model of this evolutionary transitional object (Hogerheijde 2001). This shows that the inward motions seen in millimeter wave emission lines continue to within ~0.1 AU from the star. The colder parts of the disk are traced by the prominent CO ice band. The band profile results from CO in 'polar' ices (CO mixed with H2O), and CO in 'apolar' ices. At the high spectral resolution, the 'apolar' component is, for the first time, resolved into two distinct components, likely due to pure CO and CO mixed with CO2, O2 and/or N2. The ices have probably experienced thermal processing in the upper disk layer traced by our pencil absorption beam: much of the volatile 'apolar' ices has evaporated and the depletion factor of CO onto grains is remarkably low (~7%). This study shows that high spectral resolution 4.7 um observations provide important and unique information on the dynamics and structure of protostellar disks and the evolution of ices in these disks.

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