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

A survey for near-infrared H 2 emission in Herbig Ae/Be stars: emission from the outer disks of HD 97048 and HD 100546

Astronomy & Astrophysics, 2011

We report on a sensitive search for H 2 1-0 S(1), 1-0 S(0) and 2-1 S(1) ro-vibrational emission at 2.12, 2.22 and 2.25 µm in a sample of 15 Herbig Ae/Be stars employing CRIRES, the ESO-VLT near-infrared high-resolution spectrograph, at R∼90,000. We report the detection of the H 2 1-0 S(1) line toward HD 100546 and HD 97048. In the other 13 targets, the line is not detected. The H 2 1-0 S(0) and 2-1 S(1) lines are undetected in all sources. These observations are the first detection of near-IR H 2 emission in HD 100546. The H 2 1-0 S(1) lines observed in HD 100546 and HD 97048 are observed at a velocity consistent with the rest velocity of both stars, suggesting that they are produced in the circumstellar disk. In HD 97048, the emission is spatially resolved and it is observed to extend at least up to 200 AU from the star. We report an increase of one order of magnitude in the H 2 1-0 S(1) line flux with respect to previous measurements taken in 2003 for this star, which suggests line variability. In HD 100546 the emission is tentatively spatially resolved and may extend at least up to 50 AU from the star. Modeling of the H 2 1-0 S(1) line profiles and their spatial extent with flat keplerian disks shows that most of the emission is produced at a radius larger than 5 AU. Upper limits to the H 2 1-0 S(0)/ 1-0 S(1) and H 2 2-1 S(1)/1-0 S(1) line ratios in HD 97048 are consistent with H 2 gas at T>2000 K and suggest that the emission observed may be produced by X-ray excitation. The upper limits for the line ratios for HD 100546 are inconclusive. Because the H 2 emission is located at large radii, for both sources a thermal emission scenario (i.e., gas heated by collisions with dust) is implausible. We argue that the observation of H 2 emission at large radii may be indicative of an extended disk atmosphere at radii > 5 AU. This may be explained by a hydrostatic disk in which gas and dust are thermally decoupled or by a disk wind caused by photoevaporation.

The TEXES Survey for H 2 Emission from Protoplanetary Disks

The Astrophysical Journal, 2008

We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H 2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H 2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H 2 emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (∼10 km s −1), centered at the stellar velocity, and spatially unresolved at scales of 0.4 ′′ , which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we derive temperatures 500 K from ∼1 M ⊕ of gas. Our upper limits for the non-detections place upper limits on the amount of H 2 gas with T >500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions we are studying and that processes such as UV, X-ray, and accretion heating may be important.

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

2007

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

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

Astronomy and Astrophysics, 2010

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

Looking for Pure Rotational H[TINF]2[/TINF] Emission from Protoplanetary Disks

The Astrophysical Journal, 2002

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 (2σ) 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.

EMISSION FROM DISKS AROUND HERBIG Ae AND T TAURI STARS W.F. Thi

We present the initial results of a deep ISO-SWS survey for the low J pure rotational emission lines of H 2 toward a number of Herbig Ae and T Tauri stars. The objects are selected to be as isolated as possible from molecular clouds, with a spectral energy distribution characteristic of a circumstellar disk. For most of them the presence of a disk has been established directly by millimeter interferometry. The S(1) line is detected in most sources with a peak ux of 0.3{1 Jy. The S(0) line is de nitely seen in 2 objects: GG Tau and HD 163296. The observations suggest the presence of \warm" gas at T kin 100 K with a mass of a few % of the total gas + dust mass, derived assuming a gas-to-dust ratio of 100:1. The S(1) peak ux does not show a strong correlation with spectral type of the central star or continuum ux at 1.3 millimeter. Possible origins for the warm gas seen in H 2 are discussed, and comparisons with model calculations are made.

A search for mid-infrared molecular hydrogen emission from protoplanetary disks

Astronomy & Astrophysics, 2007

We report on a sensitive search for mid-infrared molecular hydrogen emission from protoplanetary disks. We observed the Herbig Ae/Be stars UX Ori, HD 34282, HD 100453, HD 101412, HD 104237 and HD 142666, and the T Tauri star HD 319139, and searched for H 2 0−0 S (2) (J = 4−2) emission at 12.278 micron and H 2 0−0 S (1) (J = 3−1) emission at 17.035 micron with VISIR, ESO-VLT's high-resolution mid-infrared spectrograph. None of the sources present evidence for molecular hydrogen emission at the wavelengths observed. Stringent 3σ upper limits to the integrated line fluxes and the mass of optically thin warm gas (T = 150, 300 and 1000 K) in the disks are derived. The disks contain less than a few tenths of Jupiter mass of optically thin H 2 gas at 150 K, and less than a few Earth masses of optically thin H 2 gas at 300 K and higher temperatures. We compare our results to a Chiang & Goldreich (1997, ApJ, 490, 368, CG97) two-layer disk model of masses 0.02 M and 0.11 M. The upper limits to the disk's optically thin warm gas mass are smaller than the amount of warm gas in the interior layer of the disk, but they are much larger than the amount of molecular gas expected to be in the surface layer. If the two-layer approximation to the structure of the disk is correct, our non-detections are consistent with the low flux levels expected from the small amount of H 2 gas in the surface layer. We present a calculation of the expected thermal H 2 emission from optically thick disks, assuming a CG97 disk structure, a gas-to-dust ratio of 100 and T gas = T dust. We show that the expected H 2 thermal emission fluxes from typical disks around Herbig Ae/Be stars are of the order of 10 −16 to 10 −17 erg s −1 cm −2 for a distance of 140 pc. This is much lower than the detection limits of our observations (5 × 10 −15 erg s −1 cm −2). H 2 emission levels are very sensitive to departures from the thermal coupling between the molecular gas and dust in the surface layer. Additional sources of heating of gas in the disk's surface layer could have a major impact on the expected H 2 disk emission. Our results suggest that in the observed sources the molecular gas and dust in the surface layer have not significantly departed from thermal coupling (T gas /T dust < 2) and that the gas-to-dust ratio in the surface layer is very likely lower than 1000.

Far-Ultraviolet H2 Emission from Circumstellar Disks

Astrophysical Journal, 2009

We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ∼1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex and one, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of H 2 to study the evolution of the gas in the inner disk. We find that the H 2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting H 2 in the disk, the fact that the non-accreting sources show no H 2 emission implies that the H 2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600Å, we estimate that the column density of H 2 left in the inner regions of the debris disks in our sample is less than ∼ 3 × 10 −6 g cm −2 , nine orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU.

Searching for gas emission lines in Spitzer Infrared Spectrograph (IRS) spectra of young stars in Taurus

Astronomy & Astrophysics, 2011

Context. Our knowledge of circumstellar disks has traditionally been based on studies of dust. However, gas dominates the disk mass and its study is key to our understanding of accretion, outflows, and ultimately planet formation. The Spitzer Space Telescope provides access to gas emission lines in the mid-infrared, providing crucial new diagnostics of the physical conditions in accretion disks and outflows. Aims. We seek to identify gas emission lines in mid-infrared spectra of 64 pre-main-sequence stars in Taurus. Using line luminosities and other known star-disk-outflow parameters, we aim to identify correlations that will help to constrain gas heating, excitation mechanisms, and the line formation. Methods. We have based our study on Spitzer observations using the Infrared Spectrograph (IRS), mainly with the high-resolution modules. Line luminosities (or 3σ upper limits) have been obtained by fitting Gaussian profiles to the lines. We have further searched for correlations between the line luminosities and different parameters related to the star-disk system. Results. We have detected H 2 (17.03, 28.22 μm) emission in 6 objects, [Ne II] (12.81 μm) emission in 18 objects, and [Fe II] (17.93, 25.99 μm) emission in 7 objects. [Ne II] detections are found primarily in Class II objects. The luminosity of the [Ne II] line (L NeII) is in general higher for objects known to drive jets than for those without known jets, but the two groups are not statistically distinguishable. L NeII is correlated with X-ray luminosity, but for Class II objects only. L NeII is also correlated with disk mass and accretion rate when the sample is divided into high and low accretors. Furthermore, we find correlations of L NeII with mid-IR continuum luminosity and with luminosity of the [O I] (6300 Å) line, the latter being an outflow tracer. L [FeII] correlates withṀ acc. No correlations were found between L H 2 and several tested parameters. Conclusions. Our study reveals a general trend toward accretion-related phenomena as the origin of the gas emission lines. Shocks in jets and outflowing material are more likely to play a significant role than shocks in infalling material. The role of X-ray irradiation is less prominent but still present for [Ne II], in particular for Class II sources, while the lack of correlation between [Fe II] and [Ne II] points toward different emitting mechanisms.