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ABSTRACT: We present a discussion of the science drivers and design approach for a high-resolution, mid-infrared spectrograph for the Thirty-Meter Telescope. The instrument will be integrated with an adaptive optics system optimized for the mid-infrared; as a consequence it is not significantly larger or more complex than similar instruments designed for use on smaller telescopes. The high spatial and spectral resolution possible with such a design provides a unique scientific capability. The design provides spectral resolution of up to 120,000 for the 4.5-25 m region in a cross-dispersed format that provides continuous spectral coverage of up to 2% to 14 micrometers. The basic concept is derived from the successful TEXES mid-infrared spectrograph. To facilitate operation, there are separate imaging channels for the near-infrared and the mid-infrared; both can be used for acquisition and the mid-infrared imaging mode can be used for science imaging and for guiding. Because the spectrograph is matched to the diffraction limit of a 30-m telescope, gains in sensitivity are roughly proportional to the square of the telescope diameter, opening up a volume within the Galaxy a thousand times greater than existing instruments.
11/2012;
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ABSTRACT: We compare line emission calculated from theoretical disk models with optical to submillimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from ~10–4 to 10–5 M ☉ for 0.06 AU < r < 3.5 AU and ~0.06 M ☉ for 3.5 AU < r < 200 AU. We find that the inner dust hole (r < 3.5 AU) in the disk must be depleted of gas by ~1-2 orders of magnitude compared with the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within r ~ 0.5 AU from thermal excitation of gas. [O I] 6300 Å and 5577 Å forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at r 0.1 AU and at r ~ 4 AU. [Ne II] emission is consistent with an origin in X-ray heated neutral gas at r 10 AU, and may not require the presence of a significant extreme-ultraviolet (hν > 13.6 eV) flux from TW Hya. H2 pure rotational line emission comes primarily from r ~ 1 to 30 AU. [O I] 63 μm, HCO+, and CO pure rotational lines all arise from the outer disk at r ~ 30-120 AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass ~4-7 MJ situated at ~3 AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of 4 × 10–9 M ☉ yr–1 and a remaining disk lifetime of ~5 million years.
The Astrophysical Journal 06/2011; 735(2):90. · 6.02 Impact Factor
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B. Sturm,
J. Bouwman,
Th. Henning,
N. J. Evans II,
B. Acke,
G. D. Mulders,
L. B. F. M. Waters,
E. F. van Dishoeck,
G. Meeus,
J. D. Green, [......],
C. Knez,
J. H. Lacy,
J E Lee,
S. Maret,
R. Meijerink,
B. Merín,
L. Mundy,
K. M. Pontoppidan,
R. Visser,
U. A. Yíldíz
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ABSTRACT: We present far-infrared spectroscopic observations, taken with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory, of the protoplanetary disk around the pre-main-sequence star HD 100546. These observations are the first within the DIGIT Herschel key program, which aims to follow the evolution of dust, ice, and gas from young stellar objects still embedded in their parental molecular cloud core, through the final pre-main-sequence phases when the circumstellar disks are dissipated. Our aim is to improve the constraints on temperature and chemical composition of the crystalline olivines in the disk of HD 100546 and to give an inventory of the gas lines present in its far-infrared spectrum. The 69 \mu\m feature is analyzed in terms of position and shape to derive the dust temperature and composition. Furthermore, we detected 32 emission lines from five gaseous species and measured their line fluxes. The 69 \mu\m emission comes either from dust grains with ~70 K at radii larger than 50 AU, as suggested by blackbody fitting, or it arises from ~200 K dust at ~13 AU, close to the midplane, as supported by radiative transfer models. We also conclude that the forsterite crystals have few defects and contain at most a few percent iron by mass. Forbidden line emission from [CII] at 157 \mu\m and [OI] at 63 and 145 \mu\m, most likely due to photodissociation by stellar photons, is detected. Furthermore, five H2O and several OH lines are detected. We also found high-J rotational transition lines of CO, with rotational temperatures of ~300 K for the transitions up to J=22-21 and T~800 K for higher transitions.
05/2010;
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T. A. van Kempen,
J. D. Green,
N. J. Evans,
E. F. van Dishoeck,
L. E. Kristensen,
G. J. Herczeg,
B. Merin,
J E Lee,
J. K. J. Joergensen,
J. Bouwman, [......],
L. Mundy, J. Najita,
J. Olofsson,
K. M. Pontoppidan,
C. Salyk,
B. Sturm,
R. Visser,
L. B. F. M. Waters,
C. Waelkens,
U. A. Yıldız
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ABSTRACT: DK Cha is an intermediate-mass star in transition from an embedded configuration to a star plus disk stage. We aim to study the composition and energetics of the circumstellar material during this pivotal stage. Using the Range Scan mode of PACS on the Herschel Space Observatory, we obtained a spectrum of DK Cha from 55 to 210 micron as part of the DIGIT Key Program. Almost 50 molecular and atomic lines were detected, many more than the 7 lines detected in ISO-LWS. Nearly the entire ladder of CO from J=14-13 to 38-37 (E_u/k = 4080 K), water from levels as excited as E_u/k = 843 K, and OH lines up to E_u/k = 290 K were detected. The continuum emission in our PACS SED scan matches the flux expected from a model consisting of a star, a surrounding disk of 0.03 Solar mass, and an envelope of a similar mass, supporting the suggestion that the object is emerging from its main accretion stage. Molecular, atomic, and ionic emission lines in the far-infrared reveal the outflow's influence on the envelope. The inferred hot gas can be photon-heated, but some emission could be due to C-shocks in the walls of the outflow cavity. Comment: 4 Page letter, To appear in A&A special issue on Herschel
05/2010;
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Dan M. Watson,
Jarron M. Leisenring,
Elise Furlan,
C. J. Bohac,
B. Sargent,
W. J. Forrest,
Nuria Calvet,
Lee Hartmann,
Jason T Nordhaus,
Joel D. Green,
K. H. Kim,
G. C. Sloan,
C. H. Chen,
L. D. Keller,
Paola d'Alessio, J. Najita,
Keven I. Uchida,
and J. R. Houck
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ABSTRACT: 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 the Spitzer Space Telescope mid-IR 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 photoevaporation 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 large-scale 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 seems 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.
The Astrophysical Journal Supplement Series 12/2008; 180(1):84. · 13.46 Impact Factor
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Lee Hartmann,
Nuria Calvet,
Dan M. Watson,
P. D'Alessio,
E. Furlan,
B. Sargent,
W. J. Forrest,
K. I. Uchida,
J. D. Green,
G. C. Sloan,
C. H. Chen, J. Najita,
F. Kemper,
T. L. Herter,
P. Morris,
D. J. Barry,
and P. Hall
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ABSTRACT: We present the infrared spectrum of the young binary system St 34 obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The IRS spectrum clearly shows excess dust emission, consistent with the suggestion of White & Hillenbrand that St 34 is accreting from a circumbinary disk. The disk emission of St 34 is low in comparison with the levels observed in typical T Tauri stars; silicate features at ~10 and 20 μm are much weaker than typically seen in T Tauri stars; and excess emission is nearly absent at the shortest wavelengths observed (~5 μm). These features of the infrared spectrum suggest substantial grain growth (to eliminate silicate features) and possible settling of dust to the disk midplane (to reduce the continuum excess emission levels), along with a relatively evacuated inner disk, as expected due to gravitational perturbations by the binary system. Although the position of St 34 in the H-R diagram suggests an age of 8 ± 3 Myr, assuming that it lies at the distance of the Taurus-Auriga molecular clouds, White & Hillenbrand could not detect any Li I absorption, which would indicate a Li depletion age of roughly 25 Myr or more. We suggest that St 34 is closer than the Taurus clouds by ~ 30-40 pc and has an age roughly consistent with Li depletion models. Such an advanced age would make St 34 the oldest known low-mass pre-main-sequence object with a dusty accretion disk. The persistence of optically thick dust emission well outside the binary orbit may indicate a failure to make giant planets that could effectively remove dust particles.
The Astrophysical Journal 12/2008; 628(2):L147. · 6.02 Impact Factor
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ABSTRACT: We present the mid-infrared spectrum, obtained with the Spitzer Infrared Spectrograph (IRS), of HD 98800, a quadruple star system located in the 10 Myr old TW Hydrae association. It has a known mid-infrared excess that arises from a circumbinary disk around the B components of the system. The IRS spectrum confirms that the disk around HD 98800B displays no excess emission below about 5.5 μm, implying an optically thick disk wall at 5.9 AU and an inner, cleared-out region; however, some optically thin dust, consisting mainly of 3 μm-sized silicate dust grains, orbits the binary in a ring between 1.5 and 2 AU. The peculiar structure and apparent lack of gas in the HD 98800B disk suggests that this system is likely already at the debris disks stage, with a tidally truncated circumbinary disk of larger dust particles and an inner, second-generation dust ring, possibly held up by the resonances of a planet. The unusually large infrared excess can be explained by gravitational perturbations of the Aa+Ab pair puffing up the outer dust ring and causing frequent collisions among the larger particles.
The Astrophysical Journal 12/2008; 664(2):1176. · 6.02 Impact Factor
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I. Pascucci,
D. Hollenbach, J. Najita,
J. Muzerolle,
U. Gorti,
G. J. Herczeg,
L. A. Hillenbrand,
J. S. Kim,
J. M. Carpenter,
M. R. Meyer,
E. E. Mamajek,
and J. Bouwman
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ABSTRACT: We report the detection of [Ne II] emission at 12.81 μm in four out of the six optically thick dust disks observed as part of the FEPS Spitzer Legacy program. In addition, we detect a H I (7-6) emission line at 12.37 μm from the source RX J1852.3-3700. Detections of [Ne II] lines are favored by low mid-infrared excess emission. Both stellar X-rays and extreme ultraviolet (EUV) photons can sufficiently ionize the disk surface to reproduce the observed line fluxes, suggesting that emission from Ne+ originates in the hot disk atmosphere. On the other hand, the H I (7-6) line is not associated with the gas in the disk surface, and magnetospheric accretion flows can account for at most ~30% of the observed flux. We conclude that accretion shock regions and/or the stellar corona could contribute to most of the H I (7-6) emission. Finally, we discuss the observations necessary to identify whether stellar X-rays or EUV photons are the dominant ionization mechanism for Ne atoms. Because the observed [Ne II] emission probes very small amounts of gas in the disk surface (~10-6 MJ) we suggest using this gas line to determine the presence or absence of gas in more evolved circumstellar disks.
The Astrophysical Journal 12/2008; 663(1):383. · 6.02 Impact Factor
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K. I. Uchida,
N. Calvet,
L. Hartmann,
F. Kemper,
W. J. Forrest,
D. M. Watson,
P. D'Alessio,
C. H. Chen,
E. Furlan,
B. Sargent,
B. R. Brandl,
T. L. Herter,
P. Morris,
P. C. Myers, J. Najita,
G. C. Sloan,
D. J. Barry,
J. Green,
L. D. Keller,
and P. Hall
[show abstract]
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ABSTRACT: We have used the Spitzer Space Telescope Infrared Spectrograph to observe seven members of the TW Hya association, the nearest stellar association whose age (~10 Myr) is similar to the timescales thought to apply to planet formation and disk dissipation. Only two of the seven targets display infrared excess emission, indicating that substantial amounts of dust still exist closer to the stars than is characteristic of debris disks; however, in both objects we confirm an abrupt short-wavelength edge to the excess, as is seen in disks with cleared-out central regions. The mid-infrared excesses in the spectra of Hen 3-600 and TW Hya include crystalline silicate emission features, indicating that the grains have undergone significant thermal processing. We offer a detailed comparison between the spectra of TW Hya and Hen 3-600, and a model that corroborates the spectral shape and our previous understanding of the radial structure of these protoplanetary disks.
The Astrophysical Journal Supplement Series 12/2008; 154(1):439. · 13.46 Impact Factor
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G. C. Sloan,
L. D. Keller,
W. J. Forrest,
E. Leibensperger,
B. Sargent,
A. Li, J. Najita,
D. M. Watson,
B. R. Brandl,
C. H. Chen,
J. D. Green,
F. Markwick-Kemper,
T. L. Herter,
P. D'Alessio,
P. W. Morris,
D. J. Barry,
P. Hall,
P. C. Myers,
and J. R. Houck
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ABSTRACT: We present spectra of four Herbig Ae/Be stars obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. All four of the sources show strong emission from polycyclic aromatic hydrocarbons (PAHs), with the 6.2 μm emission feature shifted to 6.3 μm and the strongest C–C skeletal-mode feature occurring at 7.9 μm instead of at 7.7 μm, as is often seen. Remarkably, none of the four stars has silicate emission. The strength of the 7.9 μm feature varies with respect to the 11.3 μm feature among the sources, indicating that we have observed PAHs with a range of ionization fractions. The ionization fraction is higher for systems with hotter and brighter central stars. Two sources, HD 34282 and HD 169142, show emission features from aliphatic hydrocarbons at 6.85 and 7.25 μm. The spectrum of HD 141569 shows a previously undetected emission feature at 12.4 μm that may be related to the 12.7 μm PAH feature. The spectrum of HD 135344, the coolest star in our sample, shows an unusual profile in the 7-9 μm region, with the peak emission to the red of 8.0 μm and no 8.6 μm PAH feature.
The Astrophysical Journal 12/2008; 632(2):956. · 6.02 Impact Factor
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M. Jura,
C. H. Chen,
E. Furlan,
J. Green,
B. Sargent,
W. J. Forrest,
D. M. Watson,
D. J. Barry,
P. Hall,
T. L. Herter, [......],
K. Uchida,
P. D'Alessio,
B. R. Brandl,
L. D. Keller,
F. Kemper,
P. Morris, J. Najita,
N. Calvet,
L. Hartmann,
and P. C. Myers
[show abstract]
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ABSTRACT: We report spectra obtained with the Spitzer Space Telescope in the λ = 14-35 μm range of 19 nearby main-sequence stars with infrared excesses. The six stars with strong dust emission show no recognizable spectral features, suggesting that the bulk of the emitting particles have diameters larger than 10 μm. If the observed dust results from collisional grinding of larger solids, we infer minimum masses of the parent body population between 0.004 and 0.06 M⊕. We estimate grain production rates of ~1010 g s-1 around λ Boo and HR 1570; selective accretion of this matter may help explain their peculiar surface abundances. There appear to be inner truncations in the dust clouds at 48, 11, 52, and 54 AU around HR 333, HR 506, HR 1082, and HR 3927, respectively.
The Astrophysical Journal Supplement Series 12/2008; 154(1):453. · 13.46 Impact Factor
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Dan M. Watson,
F. Kemper,
Nuria Calvet,
Luke D. Keller,
Elise Furlan,
Lee Hartmann,
W. J. Forrest,
C. H. Chen,
Keven I. Uchida,
Joel D. Green, [......],
G. C. Sloan,
Terry L. Herter,
Bernhard R. Brandl,
J. R. Houck, J. Najita,
Paola D'Alessio,
P. C. Myers,
D. J. Barry,
P. Hall,
and P. W. Morris
[show abstract]
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ABSTRACT: We present Spitzer Space Telescope Infrared Spectrograph observations in the 5.3-20 μm range of five young stellar objects in Taurus that have Class I continuum spectral energy distributions (λFλαλn, n ≥ 0), often taken to represent the youngest stellar objects in this star formation region. The spectra include a rich collection of broad absorption features that we identify with amorphous silicates and various ices, notably those of carbon dioxide and water. We show that the absorption features are produced mainly in the envelopes of these systems. The apparent depths of silicate and 15.2 μm CO2 ice features vary among the objects in a manner that is consistent with a variation of inclination with respect to the line of sight, contribution to the silicate features from material throughout the envelopes, and an origin for the CO2 ice feature in the outer parts of the envelope. Thus, these features provide new and useful constraints on models of the physical structure of Class I protostars.
The Astrophysical Journal Supplement Series 12/2008; 154(1):391. · 13.46 Impact Factor
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Luke D. Keller,
G. C. Sloan,
W. J. Forrest,
S. Ayala,
P. D'Alessio,
S. Shah,
N. Calvet, J. Najita,
A. Li,
L. Hartmann,
B. Sargent,
D. M. Watson,
and C. H. Chen
[show abstract]
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ABSTRACT: We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained with the Infrared Spectrograph on Spitzer. All but one of the Herbig stars show emission from PAHs, and seven of the spectra show PAH emission, but no silicate emission at 10 μm. The central wavelengths of the 6.2, 7.7-8.2, and 11.3 μm emission features decrease with stellar temperature, indicating that the PAHs are less photoprocessed in cooler radiation fields. The apparent low level of photoprocessing in HAeBe stars, relative to other PAH emission sources, implies that the PAHs are newly exposed to the UV-optical radiation fields from their host stars. HAeBe stars show a variety of PAH emission intensities and ionization fractions but a narrow range of PAH spectral classifications based on positions of major PAH feature centers. This may indicate that, regardless of their locations relative to the stars, the PAH molecules are altered by the same physical processes in the protoplanetary disks of intermediate-mass stars. Analysis of the mid-IR SEDs indicates that our sample likely includes both radially flared and more flattened/settled disk systems, but we do not see the expected correlation of overall PAH emission with disk geometry. We suggest that the strength of PAH emission from HAeBe stars may depend not only on the degree of radial flaring but also on the abundance of PAHs in illuminated regions of the disks and possibly on the vertical structure of the inner disk as well.
The Astrophysical Journal 12/2008; 684(1):411. · 6.02 Impact Factor
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C. H. Chen,
B. A. Sargent,
C. Bohac,
K. H. Kim,
E. Leibensperger,
M. Jura, J. Najita,
W. J. Forrest,
D. M. Watson,
G. C. Sloan,
and L. D. Keller
[show abstract]
[hide abstract]
ABSTRACT: We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS) 5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60 μm excess. The spectra of five objects possess spectral features that are well-modeled using micron-sized grains and silicates with crystalline mass fractions 0%-80%, consistent with T Tauri and Herbig AeBe stars. With the exception of η Crv, these objects are young with ages ≤50 Myr. Our fits require the presence of a cool blackbody continuum, Tgr = 80-200 K, in addition to hot, amorphous, and crystalline silicates, Tgr = 290-600 K, suggesting that multiple parent body belts are present in some debris disks, analogous to the asteroid and Kuiper belts in our solar system. The spectra for the majority of objects are featureless, suggesting that the emitting grains probably have radii a > 10 μm. We have modeled the excess continua using a continuous disk with a uniform surface density distribution, expected if Poynting-Robertson and stellar wind drag are the dominant grain removal processes, and using a single-temperature blackbody, expected if the dust is located in a narrow ring around the star. The IRS spectra of many objects are better modeled with a single-temperature blackbody, suggesting that the disks possess inner holes. The distribution of grain temperatures, based on our blackbody fits, peaks at Tgr = 110-120 K. Since the timescale for ice sublimation of micron-sized grains with Tgr > 110 K is a fraction of a Myr, the lack of warmer material may be explained if the grains are icy. If planets dynamically clear the central portions of debris disks, then the frequency of planets around other stars is probably high. We estimate that the majority of debris disk systems possess parent body masses, MPB < 1 M⊕. The low inferred parent body masses suggest that planet formation is an efficient process.
The Astrophysical Journal Supplement Series 12/2008; 166(1):351. · 13.46 Impact Factor
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W. J. Forrest,
B. Sargent,
E. Furlan,
P. D'Alessio,
N. Calvet,
L. Hartmann,
K. I. Uchida,
J. D. Green,
D. M. Watson,
C. H. Chen, [......],
L. D. Keller,
G. C. Sloan,
T. L. Herter,
B. R. Brandl,
J. R. Houck,
D. J. Barry,
P. Hall,
P. W. Morris, J. Najita,
and P. C. Myers
[show abstract]
[hide abstract]
ABSTRACT: We present the first Spitzer Space Telescope Infrared Spectrograph observations of the disks around classical T Tauri stars: spectra in the 5.2-30 μm range of six stars. The spectra are dominated by emission features from amorphous silicate dust, and a continuous component from 5 to 8 μm that in most cases comprises an excess above the photosphere throughout our spectral range. There is considerable variation in the silicate feature/continuum ratio, which implies variations of inclination, disk flaring, and stellar mass accretion rate. In most of our stars, structure in the silicate feature suggests the presence of a crystalline component. In one, CoKu Tau/4, no excess above the photosphere appears at wavelengths shortward of the silicate features, similar to 10 Myr old TW Hya, Hen 3-600, and HR 4796A. This indicates the optically thick inner disk is largely absent. The silicate emission features with peaks at 9.7 and 18 μm indicate small dust grains are present. The extremely low 10-20 μm color temperature of the dust excess, 135 K, indicates these grains are located more than 10 AU from the star. These features are suggestive of gravitational influence by planets or close stellar companions and grain growth in the region within 10 AU of the star, somewhat surprising for a star this young (1 Myr).
The Astrophysical Journal Supplement Series 12/2008; 154(1):443. · 13.46 Impact Factor
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E. Furlan,
N. Calvet,
P. D'Alessio,
L. Hartmann,
W. J. Forrest,
D. M. Watson,
K. L. Luhman,
K. I. Uchida,
J. D. Green,
B. Sargent, J. Najita,
G. C. Sloan,
L. D. Keller,
and T. L. Herter
[show abstract]
[hide abstract]
ABSTRACT: We present Spitzer Infrared Spectrograph measurements for two young stars near the hydrogen-burning mass limit in the Taurus star-forming region. While one of the objects, V410 X-ray 3, displays no mid-infrared excess, the other one, V410 Anon 13, shows a clear excess at wavelengths longward of 10 μm, indicative of a circumstellar disk. Moreover, the disk surrounding V410 Anon 13 is reminiscent of flared accretion disks around classical T Tauri stars; small dust grains in the disk photosphere generate the broad 10 μm silicate emission feature, whose structure suggests the presence of crystalline components. This demonstrates that very low mass objects, like their more massive counterparts, experience dust processing in their disks.
The Astrophysical Journal 12/2008; 621(2):L129. · 6.02 Impact Factor
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N. Calvet,
P. D'Alessio,
D. M. Watson,
R. Franco-Hernández,
E. Furlan,
J. Green,
P. M. Sutter,
W. J. Forrest,
L. Hartmann,
K. I. Uchida,
L. D. Keller,
B. Sargent, J. Najita,
T. L. Herter,
D. J. Barry,
and P. Hall
[show abstract]
[hide abstract]
ABSTRACT: We present Spitzer Infrared Spectrograph (IRS) observations of two objects of the Taurus population that show unambiguous signs of clearing in their inner disks. In one of the objects, DM Tau, the outer disk is truncated at 3 AU; this object is akin to another recently reported in Taurus, CoKu Tau/4, in that the inner disk region is free of small dust. Unlike CoKu Tau/4, however, this star is still accreting, so optically thin gas should still remain in the inner disk region. The other object, GM Aur, also accreting, has ~0.02 lunar masses of small dust in the inner disk region within ~5 AU, consistent with previous reports. However, the IRS spectrum clearly shows that the optically thick outer disk has an inner truncation at a much larger radius than previously suggested, ~24 AU. These observations provide strong evidence for the presence of gaps in protoplanetary disks.
The Astrophysical Journal 12/2008; 630(2):L185. · 6.02 Impact Factor
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I. Pascucci,
U. Gorti,
D. Hollenbach, J. Najita,
M. R. Meyer,
J. M. Carpenter,
L. A. Hillenbrand,
G. J. Herczeg,
D. L. Padgett,
E. E. Mamajek,
M. D. Silverstone,
W. M. Schlingman,
J. S. Kim,
E. B. Stobie,
J. Bouwman,
S. Wolf,
J. Rodmann,
D. C. Hines,
J. Lunine,
and R. Malhotra
[show abstract]
[hide abstract]
ABSTRACT: We have carried out a sensitive search for gas emission lines at IR and millimeter wavelengths for a sample of 15 young Sun-like stars selected from our dust disk survey with Spitzer. We have used mid-IR lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of 12CO to probe the cold (~20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti & Hollenbach that confirm and extend our simple analysis. These models show that the [S I] 25.23 μm line can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04MJ of gas within a few AU from the disk inner radius for disk radii from 1 to ~40 AU. These gas mass upper limits even in the eight systems younger than ~30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, which is mainly traced by our CO data, are <2 M⊕. If these systems are analogs of the solar system, they either have already formed Uranus- and Neptune-like planets or will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.
The Astrophysical Journal 12/2008; 651(2):1177. · 6.02 Impact Factor
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B. Sargent,
W. J. Forrest,
P. D'Alessio,
A. Li, J. Najita,
D. M. Watson,
N. Calvet,
E. Furlan,
J. D. Green,
K. H. Kim,
G. C. Sloan,
C. H. Chen,
L. Hartmann,
and J. R. Houck
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ABSTRACT: The 8-14 μm emission spectra of 12 T Tauri stars in the Taurus/Auriga dark clouds and in the TW Hydrae association obtained with the Infrared Spectrograph (IRS) on board Spitzer are analyzed. Assuming that the 10 μm features originate from silicate grains in the optically thin surface layers of T Tauri disks, the 8-14 μm dust emissivity for each object is derived from its Spitzer spectrum. The emissivities are fit with the opacities of laboratory analogs of cosmic dust. The fits include small nonspherical grains of amorphous silicates (pyroxene and olivine), crystalline silicates (forsterite and pyroxene), and quartz, together with large fluffy amorphous silicate grains. A wide range in the fraction of crystalline silicate grains, as well as large silicate grains among these stars, are found. The dust in the transitional-disk objects CoKu Tau/4, GM Aur, and DM Tau has the simplest form of silicates, with almost no hint of crystalline components and modest amounts of large grains. This indicates that the dust grains in these objects have been modified little from their origin in the interstellar medium. Other stars show various amounts of crystalline silicates, similar to the wide dispersion of the degree of crystallinity reported for Herbig Ae/Be stars of mass <2.5 M☉. Late spectral type, low-mass stars can have significant fractions of crystalline silicate grains. Higher quartz mass fractions often accompany low amorphous olivine to amorphous pyroxene ratios. Lower contrast of the 10 μm feature accompanies greater crystallinity.
The Astrophysical Journal 12/2008; 645(1):395. · 6.02 Impact Factor
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D. Hollenbach,
U. Gorti,
M. Meyer,
J. S. Kim,
P. Morris, J. Najita,
I. Pascucci,
J. Carpenter,
J. Rodmann,
T. Brooke,
L. Hillenbrand,
E. Mamajek,
D. Padgett,
D. Soderblom,
S. Wolf,
and J. Lunine
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ABSTRACT: We report infrared spectroscopic observations of HD 105, a nearby (~40 pc) and relatively young (~30 Myr) G0 star with excess infrared continuum emission, which has been modeled as arising from an optically thin circumstellar dust disk with an inner hole of size 13 AU. We have used the high spectral resolution mode of the Infrared Spectrometer (IRS) on the Spitzer Space Telescope to search for gas emission lines from the disk. The observations reported here provide upper limits to the fluxes of H2 S(0) 28 μm, H2 S(1) 17 μm, H2 S(2) 12 μm, [Fe II] 26 μm, [Si II] 35 μm, and [S I] 25 μm infrared emission lines. The H2 line upper limits place direct constraints on the mass of warm molecular gas in the disk: M(H2) < 4.6, 3.8 × 10-2, and 3.0 × 10-3 MJ at T = 50, 100, and 200 K, respectively. We also compare the line flux upper limits to predictions from detailed thermal/chemical models of various gas distributions in the disk. These comparisons indicate that if the gas distribution has an inner hole with radius ri,gas, the surface density at that inner radius is limited to values ranging from 3 g cm-2 at ri,gas = 0.5 AU to 0.1 g cm-2 at ri,gas = 5-20 AU. These values are considerably below the value for a minimum mass solar nebula, and suggest that less than 1 Jupiter mass (MJ) of gas (at any temperature) exists in the 1-40 AU planet-forming region. Therefore, it is unlikely that there is sufficient gas for gas giant planet formation to occur in HD 105 at this time.
The Astrophysical Journal 12/2008; 631(2):1180. · 6.02 Impact Factor
