John S. Carr

Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars

We report the results of a high-resolution spectroscopic survey for CO fundamental emission from T Tauri stars. CO fundamental emission is frequently detected, with the likely origin of the emission in the circumstellar disk. An initial assessment of the line profiles indicates that the emission region includes the equivalent of the terrestrial planet region of our solar system, a result that suggests the utility of CO fundamental emission as a probe of disks at planet formation distances. Since fundamental emission is detected frequently from both close binary and apparently single stars, it appears that both low column density regions, such as disk gaps, and temperature inversion regions in disk atmospheres can produce significant emission. The estimated excitation temperature of the emitting gas is unexpectedly warm for the disk radii that they appear to probe. Thus, the surface gaseous component of inner disks may be significantly warmer than the surface dust component. We also detect CO emission from a transitional T Tauri star. Because fundamental emission from CO and its isotopes is sensitive to a wide range of gas masses, including masses M⊕, CO fundamental emission may prove useful in measuring the residual gas content of dissipating disks. This may be an effective way to explore the gas dissipation timescale in inner disks and to thereby place constraints on the timescale for giant planet formation.

A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. I. Detection Rates

We present a Spitzer InfraRed Spectrometer search for 10-36 μm molecular emission from a large sample of protoplanetary disks, including lines from H_(2)O, OH, C_(2)H_2, HCN, and CO2. This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star-forming regions, including Ophiuchus, Lupus, and Chamaeleon. Spectra showing strong emission lines are used to identify which lines are the best tracers of various physical and chemical conditions within the disks. In total, we identify 22 T Tauri stars with strong mid-infrared H2O emission. Integrated water line luminosities, where water vapor is detected, range from 5 × 10^(–4) to 9 × 10^(–3) L_☉ sun, likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the five transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H_2), but the sample is too small to conclude whether this is a general property of transitional disks. We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1%-2% line-to-continuum ratios, in the mid-infrared, while more than half of disks around late-type stars (M-G) show sufficiently intense water emission to be detected by Spitzer, with a detection rate approaching 2/3 for disks around K stars. Some Herbig Ae/Be stars show tentative H_(2)O/OH emission features beyond 20 μm at the 1%-2% level, however, and one of them shows CO_2 in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks are due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks.

From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348

We investigate the low-mass population of the young cluster IC 348 down to the deuterium-burning limit, a —ducial boundary between brown dwarf and planetary mass objects, using a new and innovative method for the spectral classi—cation of late-type objects. Using photometric indices, constructed from HST /NICMOS narrowband imaging, that measure the strength of the 1.9 km water band, we determine the spectral type and reddening for every M-type star in the —eld, thereby separating cluster members from the interloper population. Due to the efficiency of our spectral classi—cation technique, our study is complete from D0.7 to The mass function derived for the cluster in this interval, dN/ 0.015 M _ . d log M P M0.5, is similar to that obtained for the Pleiades, but appears signi—cantly more abundant in brown dwarfs than the mass function for companions to nearby Sunlike stars. This provides compelling observational evidence for diUerent formation and evolutionary histories for substellar objects formed in isolation versus as companions. Because our determination of the IMF is complete to very low masses, we can place interesting constraints on the role of physical processes such as fragmentation in the star and planet formation process and the fraction of dark matter in the Galactic halo that resides in sub- stellar objects. Subject headings: stars: late-typestars: low-mass, brown dwarfs ¨ stars: luminosity function, mass functionstars: premain-sequence

A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. II. Correlations and Local Thermal Equilibrium Models

We present an analysis of Spitzer Infrared Spectrograph observations of H_(2)O, OH, HCN, C_(2)H_2, and CO_2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections and strengths of most mid-IR molecular emission features are correlated with each other, suggesting a common origin and similar excitation conditions for this mid-infrared line forest. Aside from the remarkable differences in molecular line strengths between T Tauri, Herbig Ae/Be, and transitional disks discussed in Pontoppidan et al., we note that the line detection efficiency is anti-correlated with the 13/30 μm spectral slope, which is a measure of the degree of grain settling in the disk atmosphere. We also note a correlation between detection efficiency and Hα equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H_(2)O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L_*. However, significant sample variation, especially in molecular line ratios, remains, and its origin has yet to be explained. Local thermal equilibrium (LTE) models of the H_(2)O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H_(2)O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near ~10^(18) cm^(–2) and 450 K for all sources, suggesting a high abundance of H_(2)O in many planet-forming regions. Other molecules have a range of excitation temperatures from ~500 to 1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of ~10^(–3) for all molecules, with the exception of CO, for which n(CO)/n(H_(2)O) ~ 1. However, LTE fitting caveats and differences in the way thermo-chemical modeling results are reported make comparisons with such models difficult, and highlight the need for additional observations coupled with the use of line-generating radiative transfer codes.

TRACING THE INNER EDGE OF THE DISK AROUND HD 100546 WITH ROVIBRATIONAL CO EMISSION LINES

In this paper we present high-resolution 4.7 μm spectra of the isolated Herbig Be star HD 100546. HD 100546 has been the subject of intense scrutiny because it is a young nearby star with a transitional disk. We observe the Δv = 1 rovibrational CO transitions in order to clarify the distribution of warm gas in the inner disk. Modeling of the CO spectrum indicates that the gas is vibrationally excited by collisions and UV fluorescence. The observed emission extends from 13 to 100 AU. The inner edge of the molecular gas emission is consistent with the inner edge of the optically thick dust disk indicating that the inner hole is not simply a hole in the dust opacity but is likely cleared of gas as well. The rotational temperature of the CO is ~1000 K—much hotter than the ~200 K CO in the otherwise similar transitional disk surrounding HD 141569. The origin of this discrepancy is likely linked to the brighter polycyclic aromatic hydrocarbon emission observed toward HD 100546. We use the excitation of the CO to constrain the geometry of the inner disk and comment on the evolutionary state of the system.

First results of the Herschel key program "Dust, Ice and Gas In Time" (DIGIT): Dust and gas spectroscopy of HD 100546

Context. 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 HD100546. 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. Aims. Our aim is to improve the constraints on temperature and chemical composition of the crystalline olivines in the disk of HD100546 and to give an inventory of the gas lines present in its far-infrared spectrum. Methods. The 69 μm 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. Results. The 69 μm emission comes either from dust grains with ~70 K at radii larger than 50 AU, as suggested by blackbody fitting, or it arises from ~200K 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 [C_(II)] at 157 μm and [O_I] at 63 and 145 μm, most likely due to photodissociation by stellar photons, is detected. Furthermore, five H_2O and several OH lines are detected. We also found high-J rotational transition lines of CO, with rotational temperatures of ~300K for the transitions up to J = 22−21 and T ~ 800 K for higher transitions.

Spitzer Spectroscopy of the Transition Object TW?Hya

We report sensitive Spitzer IRS spectroscopy in the 10-20 ?m 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 H2O, C2H2, or HCN. The difference suggests that the inner planet formation region (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 (H I, [Ne II], and [Ne III]) 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 H I 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 ?m 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.

On the origin of [NeII] 12.81 μm emission from pre-main sequence stars: Disks, jets, and accretion

Context. Extreme-ultraviolet (EUV) and X-ray photons from classical T Tauri stars are powerful ionization and heating agents that drive disk chemistry, disk instabilities, and photoevaporative flows. The mid-infrared fine-structure line of [Ne II] at 12.81 mu m has been proposed to trace gas in disk surface layers heated and ionized by stellar X-ray and EUV radiation. Aims. We aim at locating the origin of [Ne II] line emission in circumstellar environments by studying distributions of [Ne II] emission and correlating the inferred [Ne II] luminosities, L([Ne II]), with stellar and circumstellar disk parameters. Methods. We have conducted a study of [Ne II] line emission based on a sample of 92 pre-main sequence stars mostly belonging to the infrared Class II, but including 13 accreting transition disk objects, and also 14 objects that drive known jets and outflows. Results. We find several significant correlations between L([Ne II]) and stellar parameters, in particular L(X) and the wind mass loss rate,. (M) over dot(loss). Most correlations are, however, strongly dominated by systematic scatter of unknown origin. While there is a positive correlation between L([Ne II]) and L(X), the stellar mass accretion rate,. (M) over dot(acc), induces a correlation only if we combine the largely different subsets of jet sources and stars without jets. Our results indeed suggest that L([Ne II]) is bi-modally distributed, with separate distributions for the two subsamples. The jet sources show systematically higher L([Ne II]), by 1-2 orders of magnitude with respect to objects without jets. Jet-driving stars also tend to show higher mass accretion rates. We therefore hypothesize that the trend with. (M) over dot(acc) only reflects a trend with (M) over dot(loss) that is more physically relevant for [Ne II] emission. Conclusions. The [Ne II] luminosities measured for objects without known outflows and jets are found to agree with simplified calculations of [Ne II] emission from disk surface layers if the measured stellar X-rays are responsible for heating and ionizing the gas. The large scatter in L([Ne II]) may be introduced by variations of disk properties and the irradiation spectrum, as previously suggested. If these additional factors can be sufficiently well constrained, then the [Ne II] 12.81 mu m line should be an important diagnostic for disk surface ionization and heating, at least in the inner disk region. This applies in particular to transition disks also included in our sample. The systematically enhanced [Ne II] flux from jet sources clearly suggests a role for the jets themselves, as previously demonstrated by a spatially resolved observation of the outflow system in the T Tau triple.

DIGIT survey of far-infrared lines from protoplanetary disks. I. [O i], [C ii], OH, H2O, and CH

Astronomy and Astrophysics 559 (2013): A77 reproduced with permission from Astronomy & Astrophysics

NEAR-IR DIRECT DETECTION OF WATER VAPOR IN TAU BOÖTIS b

We use high dynamic range, high-resolution L-band spectroscopy to measure the radial velocity (RV) variations of the hot Jupiter in the τ Bootis planetary system. The detection of an exoplanet by the shift in the stellar spectrum alone provides a measure of the planets minimum mass, with the true mass degenerate with the unknown orbital inclination. Treating the τ Boo system as a high flux ratio double-lined spectroscopic binary permits the direct measurement of the planets true mass as well as its atmospheric properties. After removing telluric absorption and cross-correlating with a model planetary spectrum dominated by water opacity, we measure a 6σ detection of the planet at K_p = 111 ± 5 km s^(−1), with a 1σ upper limit on the spectroscopic flux ratio of 10^(−4). This RV leads to a planetary orbital inclination of i=45^(+3)_(-4)° and a mass of M_p = 5.90^(+0.35)_(-0.20)M_Jup. We report the first detection of water vapor in the atmosphere of a non-transiting hot Jupiter, τ Boo b.