arXiv subjects

Dan M. Watson

Publications and source records attributed to Dan M. Watson.

53 records · Page 3Linked to original sources

arXiv2010

A Spitzer IRS Study of Infrared Variability in Transitional and Pre-Transitional Disks around T Tauri Stars

We present a Spitzer IRS study of variability in 14 T Tauri stars in the Taurus and Chamaeleon star-forming regions. The sample is composed of transitional and pre-transitional objects which contain holes and gaps in their disks. We detect variability between 5-38 microns in all but two of our objects on timescales of 2-3 years. Most of the variability observed can be classified as seesaw behavior, whereby the emission at shorter wavelengths varies inversely with the emission at longer wavelengths. For many of the objects we can reasonably reproduce the observed variability using irradiated disk models, particularly by changing the height of the inner disk wall by ~20%. When the inner wall is taller, the emission at the shorter wavelengths is higher since the inner wall dominates the emission at 2-8 microns. The taller inner wall casts a larger shadow on the outer disk wall, leading to less emission at wavelengths beyond 20 microns where the outer wall dominates. We discuss how the possible presence of planets in these disks could lead to warps which cause changes in the height of the inner wall. We also find that crystalline silicates are common in the outer disks of our objects and that in the four disks in the sample with the most crystalline silicates, variability on timescales of 1 week is present. In addition to explaining the infrared variability described above, planets can create shocks and collisions which can crystallize the dust and lead to short timescale variability.

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arXiv2010

Spitzer Spectroscopy of the Circumprimary Disk in the Binary Brown Dwarf 2MASS J04414489+2301513

Using the Spitzer Infrared Spectrograph, we have performed mid-infrared spectroscopy on the young binary brown dwarf 2MASS J04414489+2301513 (15 AU) in the Taurus star-forming region. The spectrum exhibits excess continuum emission that likely arises from a circumstellar disk around the primary. Silicate emission is not detected in these data, indicating the presence of significant grain growth. This is one of the few brown dwarf disks at such a young age (~1 Myr) that has been found to lack silicate emission. To quantitatively constrain the properties of the disk, we have compared the spectral energy distribution of 2MASS J04414489+2301513 to the predictions of our vertical structure codes for irradiated accretion disks. Our models suggest that the remaining atmospheric grains of moderately depleted layers may have grown to a size of $\gtrsim5$ micron. In addition, our model fits indicate an outer radius of 0.2-0.3 AU for the disk. The small size of this circumprimary disk could be due to truncation by the secondary. The absence of an outer disk containing a reservoir of small, primordial grains, combined with a weak turbulent mechanism, may be responsible for the advanced grain growth in this disk.

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arXiv2010

Spitzer Spectroscopy of the Transition Object TW Hya

We report sensitive Spitzer IRS spectroscopy in the 10-20 micron region of TW Hya, a nearby T Tauri star. The unusual spectral energy distribution of the source, that of a transition object, indicates that the circumstellar disk in the system has experienced significant evolution, possibly as a result of planet formation. The spectrum we measure is strikingly different from that of other classical T Tauri stars reported in the literature, displaying no strong emission features of water, acetylene, or HCN. The difference indicates that the inner planet formation region (within 5 AU) of the gaseous disk has evolved physically and/or chemically away from the classical T Tauri norm. Nevertheless, TW Hya does show a rich spectrum of emission features of atoms (HI, [NeII], and [NeIII]) and molecules (H2, OH, CO2, HCO+, and possibly CH3), some of which are also detected in classical T Tauri spectra. The properties of the neon emission are consistent with an origin for the emission in a disk irradiated by X-rays (with a possible role for additional irradiation by stellar EUV). The OH emission we detect, which also likely originates in the disk, is hot, arising from energy levels up to 23,000 K above ground, and may be produced by the UV photodissociation of water. The HI emission is surprisingly strong, with relative strengths that are consistent with case B recombination. While the absence of strong molecular emission in the 10-20 micron region may indicate that the inner region of the gaseous disk has been partly cleared by an orbiting giant planet, chemical and/or excitation effects may be responsible instead. We discuss these issues and how our results bear on our understanding of the evolutionary state of the TW Hya disk.

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arXiv2009

V819 Tau: A Rare Weak-Lined T Tauri Star with a Weak Infrared Excess

We use Spitzer data to infer that the small infrared excess of V819 Tau, a weak-lined T Tauri star in Taurus, is real and not attributable to a "companion" 10 arcsec to the south. We do not confirm the mid-infrared excess in HBC 427 and V410 X-ray 3, which are also non-accreting T Tauri stars in the same region; instead, for the former object, the excess arises from a red companion 9 arcsec to the east. A single-temperature blackbody fit to the continuum excess of V819 Tau implies a dust temperature of 143 K; however, a better fit is achieved when the weak 10 and 20 micron silicate emission features are also included. We infer a disk of sub-micron silicate grains between about 1 AU and several 100 AU with a constant surface density distribution. The mid-infrared excess of V819 Tau can be successfully modeled with dust composed mostly of small amorphous olivine grains at a temperature of 85 K, and most of the excess emission is optically thin. The disk could still be primordial, but gas-poor and therefore short-lived, or already at the debris disk stage, which would make it one of the youngest debris disk systems known.

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arXiv2009

Dust amorphization in protoplanetary disks

High-energy irradiation of the circumstellar material might impact the structure and the composition of a protoplanetary disk and hence the process of planet formation. In this paper, we present a study on the possible influence of the stellar irradiation, indicated by X-ray emission, on the crystalline structure of the circumstellar dust. The dust crystallinity is measured for 42 class II T Tauri stars in the Taurus star-forming region using a decomposition fit of the 10 micron silicate feature, measured with the Spitzer IRS instrument. Since the sample includes objects with disks of various evolutionary stages, we further confine the target selection, using the age of the objects as a selection parameter. We correlate the X-ray luminosity and the X-ray hardness of the central object with the crystalline mass fraction of the circumstellar dust and find a significant anti-correlation for 20 objects within an age range of approx. 1 to 4.5 Myr. We postulate that X-rays represent the stellar activity and consequently the energetic ions of the stellar winds which interact with the circumstellar disk. We show that the fluxes around 1 AU and ion energies of the present solar wind are sufficient to amorphize the upper layer of dust grains very efficiently, leading to an observable reduction of the crystalline mass fraction of the circumstellar, sub-micron sized dust. This effect could also erase other relations between crystallinity and disk/star parameters such as age or spectral type.

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arXiv2009

Solar System Analogs Around IRAS-Discovered Debris Disks

We have rereduced Spitzer IRS spectra and reanalyzed the SED's of three nearby debris disks: lambda Boo, HD 139664, and HR 8799. We find that that the thermal emission from these objects is well modeled using two single temperature black body components. For HR 8799 -- with no silicate emission features despite a relatively hot inner dust component (Tgr = 150 K) -- we infer the presence of an asteroid belt interior to and a Kuiper Belt exterior to the recently discovered orbiting planets. For HD 139664, which has been imaged in scattered light, we infer the presence of strongly forward scattering grains, consistent with porous grains, if the cold, outer disk component generates both the observed scattered light and thermal emission. Finally, careful analysis of the lambda Boo SED suggests that this system possesses a central clearing, indicating that selective accretion of solids onto the central star does not occur from a dusty disk.

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arXiv2009

Spitzer mapping of molecular hydrogen pure rotational lines in NGC 1333: A detailed study of feedback in star formation

We present mid-infrared spectral maps of the NGC 1333 star forming region, obtained with the the Infrared Spectrometer on board the Spitzer Space Telescope. Eight pure H2 rotational lines, from S (0) to S (7), are detected and mapped. The H2 emission appears to be associated with the warm gas shocked by the multiple outflows present in the region. A comparison between the observed intensities and the predictions of detailed shock models indicates that the emission arises in both slow (12 - 24 km/s) and fast (36 - 53 km/s) C-type shocks with an initial ortho-to-para ratio of ~ 1. The present H2 ortho-to-para ratio exhibits a large degree of spatial variations. In the post-shocked gas, it is usually about 2, i.e. close to the equilibrium value (~ 3). However, around at least two outflows, we observe a region with a much lower (~ 0.5) ortho-to-para ratio. This region probably corresponds to gas which has been heated-up recently by the passage of a shock front, but whose ortho-to-para has not reached equilibrium yet. This, together with the low initial ortho-to-para ratio needed to reproduce the observed emission, provide strong evidence that H2 is mostly in para form in cold molecular clouds. The H2 lines are found to contribute to 25 - 50% of the total outflow luminosity, and thus can be used to ascertain the importance of star formation feedback on the natal cloud. From these lines, we determine the outflow mass loss rate and, indirectly, the stellar infall rate, the outflow momentum and the kinetic energy injected into the cloud over the embedded phase. The latter is found to exceed the binding energy of individual cores, suggesting that outflows could be the main mechanism for core disruption.

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arXiv2009

Mineralization, Grain Growth and Disk Structure: Observations of the Evolution of Dust in Protoplanetary Disk

During the past five years, the Spitzer Space Telescope and improved ground-based facilities have enabled a huge increase in the number of circumstellar disks, around young stars of Solar mass or smaller, in which the composition of the solid component has been studied with complete mid-infrared spectra. With these samples we can assess observationally the evolution of dust through the planet-forming era, in parallel with the evolution of the composition and structure of protoplanetary disks. Here we will review the progress in this endeavour, with emphasis on objects in nearby associations and star-formation regions, and on the methods by which dust composition is determined from the infrared spectra of young stellar objects.

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arXiv2008

Spitzer IRS Observations of Class I/II Objects in Taurus: Composition and Thermal History of the Circumstellar Ices

We present observations of Taurus-Auriga Class I/II protostars obtained with the Spitzer InfraRed Spectrograph. Detailed spectral fits to the 6 and 15 micron features are made, using publicly-available laboratory data, to constrain the molecular composition, abundances, and levels of thermal processing along the lines of sight. We provide an inventory of the molecular environments observed, which have an average composition dominated by water ice with ~12% CO_2 (abundance relative to H_2O), >~2-9% CH_3OH, <~14% NH_3, ~4% CH_4, ~2% H_2CO, ~0.6% HCOOH, and ~0.5% SO_2. We find CO_2/H_2O ratios nearly equivalent to those observed in cold clouds and lines of sight toward the galactic center. The unidentified 6.8 micron profiles vary from source to source, and it is shown to be likely that even combinations of the most common candidates (NH_4+ and CH_3OH) are inadequate to explain the feature fully. We discuss correlations among SED spectral indices, abundance ratios, and thermally-processed ice fractions and their implications for CO_2 formation and evolution. Comparison of our spectral fits with cold molecular cloud sight-lines indicates abundant prestellar ice environments made even richer by the radiative effects of protostars. Our results add additional constraints and a finer level of detail to current full-scale models of protostellar and protoplanetary systems.

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arXiv2008

Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region

Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline silicates at temperatures lower than those required for spectra showing high abundance of other crystalline silicates. A few spectra show 10 micron complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 micron complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. Crystalline silicate abundances correlate positively with other such abundances, suggesting that crystalline silicates are processed directly from amorphous silicates and that neither forsterite, enstatite, nor silica are intermediate steps when producing either of the other two. Disks with more dust settling typically have greater crystalline abundances. Large-grain abundance is somewhat correlated with greater settling of disks. The lack of strong correlation is interpreted to mean that settling of large grains is sensitive to individual disk properties. Lower-mass stars have higher abundances of large grains in their inner regions.

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arXiv2008

A sub-AU outwardly truncated accretion disk around a classical T Tauri star

We present the Spitzer Infrared Spectrograph (IRS) spectrum of SR20, a 5--10 AU binary T Tauri system in the $ρ$ Ophiuchi star forming region. The spectrum has features consistent with the presence of a disk; however, the continuum slope is steeper than the $λ^{-4/3}$ slope of an infinite geometrically thin, optically thick disk, indicating that the disk is outwardly truncated. Comparison with photometry from the literature shows a large increase in the mid-infrared flux from 1993 to 1996. We model the spectral energy distribution and IRS spectrum with a wall + optically thick irradiated disk, yielding an outer radius of 0.39$_{+0.03}^{-0.01}$ AU, much smaller than predicted by models of binary orbits. Using a two temperature $χ^2$ minimization model to fit the dust composition of the IRS spectrum, we find the disk has experienced significant grain growth: its spectrum is well-fit using opacities of grains larger than 1 $μ$m. We conclude that the system experienced a significant gravitational perturbation in the 1990s.

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arXiv2008

Crystalline silicates and dust processing in the protoplanetary disks of the Taurus young cluster

We characterize the crystalline silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster, using Spitzer Space Telescope mid-infrared spectra. In turn we use the results to analyze the evolution of structure and composition of these 1-2 Myr-old disks around Solar- and later-type young stars, and test the standard models of dust processing which result in the conversion of originally amorphous dust into minerals. We find strong evidence of evolution of the dust crystalline mass fraction in parallel with that of the structure of the disks, in the sense that increasing crystalline mass fraction is strongly linked to dust settling to the disk midplane. We also confirm that the crystalline silicates are confined to small radii, r < 10 AU. However, we see no significant correlation of crystalline mass fraction with stellar mass or luminosity, stellar accretion rate, disk mass, or disk/star mass ratio, as would be expected in the standard models of dust processing based upon photo-evaporation and condensation close to the central star, accretion-heating-driven annealing at r < 1 AU, or spiral-shock heating at r < 10 AU, with or without effective radial mixing mechanisms. Either another grain-crystallizing mechanism dominates over these, or another process must be at work within the disks to erase the correlations they produce. We propose one of each sort that seem to be worth further investigation, namely X-ray heating and annealing of dust grains, and modulation of disk structure by giant-planetary formation and migration.

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arXiv2008

Detection of Extended Hot Water in the Outflow from NGC 2071

We report the results of spectroscopic mapping observations carried out toward a ~1 min x 1 min region within the northern lobe of the outflow from NGC 2071 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations covered the 5.2-37 um spectral region and have led to the detection of a number of ionic, atomic, and molecular lines, including fine-structure emission of Si+, Fe+, S++, S, the S(0)-S(7) pure rotational lines of H2, the R(3) and R(4) transitions of HD, and at least 11 transitions of H2O. In addition, the 6.2, 7.4, 7.6, 7.9, 8.6 and 11.3 um PAH emission bands were also observed and several transitions of OH were tentatively detected. Most of the detected line transitions were strong enough to map including, for the first time, three transitions of hot H2O. We find that: (1) the water emission is extended; (2) the extended emission is aligned with the outflow; and, (3) the spatial distribution of the water emission generally follows that observed for H2. Based on the measured line intensities, we derive an HD abundance relative to H2 of 1.1-1.8 10^-5 and an H2O number density of 12-2 cm^3. The H2 density in the water-emitting region is not well constrained by our observations, but is likely between 3 10^4 and 10^6 cm^3, yielding an H2O abundance relative to H2 of between 2 10^-5 and 6 10^-4. Future observations planned for the Herschel Space Observatory should greatly improve the density estimate, and thus our knowledge of the H2O abundance, for the water-emitting regions reported here. Finally, we note a possible departure from the H2O ortho-to-para ratio of 3:1 expected for water formed in hot post-shocked gas, suggesting that a significant fraction of the water vapor we detect may arise from H2O sputtered from cold dust grains.

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arXiv2007

Hubble and Spitzer Observations of an Edge-on Circumstellar Disk around a Brown Dwarf

We present observations of a circumstellar disk that is inclined close to edge-on around a young brown dwarf in the Taurus star-forming region. Using data obtained with SpeX at the NASA Infrared Telescope Facility, we find that the slope of the 0.8-2.5 um spectrum of the brown dwarf 2MASS J04381486+2611399 cannot be reproduced with a photosphere reddened by normal extinction. Instead, the slope is consistent with scattered light, indicating that circumstellar material is occulting the brown dwarf. By combining the SpeX data with mid-IR photometry and spectroscopy from the Spitzer Space Telescope and previously published millimeter data from Scholz and coworkers, we construct the spectral energy distribution for 2MASS J04381486+2611399 and model it in terms of a young brown dwarf surrounded by an irradiated accretion disk. The presence of both silicate absorption at 10 um and silicate emission at 11 um constrains the inclination of the disk to be ~70 deg, i.e. ~20 deg from edge-on. Additional evidence of the high inclination of this disk is provided by our detection of asymmetric bipolar extended emission surrounding 2MASS J04381486+2611399 in high-resolution optical images obtained with the Hubble Space Telescope. According to our modeling for the SED and images of this system, the disk contains a large inner hole that is indicative of a transition disk (R_in~58 R_star~0.275 AU) and is somewhat larger than expected from embryo ejection models (R_out=20-40 AU vs. R_out<10-20 AU).

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arXiv2006

Spitzer observations of hydrogen deuteride

We report the detection of interstellar hydrogen deuteride (HD) toward the supernova remnant IC443, and the tentative detection of HD toward the Herbig Haro objects HH54 and HH7 and the star forming region GGD37 (Cepheus A West). Our detections are based upon spectral line mapping observations of the R(3) and R(4) rotational lines of HD, at rest wavelengths of 28.502 and 23.034 micron respectively, obtained using the Infrared Spectrograph onboard the Spitzer Space Telescope. The HD R(4)/R(3) line intensity ratio promises to be a valuable probe of the gas pressure in regions where it can be observed. The derived HD/H2 abundance ratios are 1.19(+0.35/-0.24)E-5, 1.80(+0.54/-0.32)E-5, and 1.41(+0.46/-0.33)E-5 respectively (68.3% confidence limits, based upon statistical errors alone) for IC443 (clump C), HH54, and HH7. If HD is the only significant reservoir of gas-phase deuterium in these sources, the inferred HD/H2 ratios are all consistent with a gas-phase elemental abundance [n(D)/n(H)](gas) ~ 7.5E-6, a factor 2 - 3 below the values obtained previously from observations of atomic deuterium in the local bubble and the Galactic halo. However, similarly low gas-phase deuterium abundances have been inferred previously for molecular gas clouds in the Orion region, and in atomic clouds along sight-lines within the Galactic disk to stars more distant than 500 pc from the Sun.

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arXiv2006

Spitzer observations of HH54 and HH7-11: mapping the H2 ortho-to-para ratio in shocked molecular gas

We report the results of spectroscopic mapping observations carried out toward the Herbig-Haro objects HH7-11 and HH54 over the 5.2 - 37 micron region using the Infrared Spectrograph of the Spitzer Space Telescope. These observations have led to the detection and mapping of the S(0) - S(7) pure rotational lines of molecular hydrogen, together with emissions in fine structure transitions of Ne+, Si+, S, and Fe+. The H2 rotational emissions indicate the presence of warm gas with a mixture of temperatures in the range 400 - 1200 K, consistent with the expected temperature behind nondissociative shocks of velocity ~ 10 - 20 km/s, while the fine structure emissions originate in faster shocks of velocity 35 - 90 km/s that are dissociative and ionizing. Maps of the H2 line ratios reveal little spatial variation in the typical admixture of gas temperatures in the mapped regions, but show that the H2 ortho-to-para ratio is quite variable, typically falling substantially below the equilibrium value of 3 attained at the measured gas temperatures. The non-equilibrium ortho-to-para ratios are characteristic of temperatures as low as ~ 50 K, and are a remnant of an earlier epoch, before the gas temperature was elevated by the passage of a shock. Correlations between the gas temperature and H2 ortho-to-para ratio show that ortho-to-para ratios < 0.8 are attained only at gas temperatures below ~ 900 K; this behavior is consistent with theoretical models in which the conversion of para- to ortho-H2 behind the shock is driven by reactive collisions with atomic hydrogen, a process which possesses a substantial activation energy barrier (E_A/k ~ 4000 K) and is therefore very inefficient at low temperature.

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arXiv2004

The Truncated Disk of CoKu Tau/4

We present a model of a dusty disk with an inner hole which accounts for the Spitzer Space Telescope Infrared Spectrograph observations of the low-mass pre-main sequence star CoKu Tau/4. We have modeled the mid-IR spectrum (between 8 and 25 mic) as arising from the inner wall of a disk. Our model disk has an evacuated inner zone of radius ~ 10 AU, with a dusty inner ``wall'', of half-height ~ 2 AU, that is illuminated at normal incidence by the central star. The radiative equilibrium temperature decreases from the inner disk edge outward through the optically-thick disk; this temperature gradient is responsible for the emission of the silicate bands at 10 and 20 mic. The observed spectrum is consistent with being produced by Fe-Mg amorphous glassy olivine and/or pyroxene, with no evidence of a crystalline component. The mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older star TW Hya, where it has been suggested that the significant clearing of its inner disk is due to planet formation. However, no inner disk remains in CoKu Tau/4, consistent with the star being a weak-emission (non-accreting) T Tauri star. The relative youth of CoKu Tau/4 (~ 1 Myr) may indicate much more rapid planet formation than typically assumed.

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