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HD 50138 is one of the brightest B[e] stars and is located at a distance of ∼380 pc with strong infrared excess. The star was observed in [O i] 63 μm and [C ii] 158 μm with high velocity resolution with upGREAT on SOFIA. The velocity-resolved [O i] emission provides evidence for a large gas disk, ∼760 au in size, around HD 50138. Whereas previous interferometric observations gave strong evidence for a hot gas and dust disk in Keplerian rotation, our observations are the first to provide unambiguous evidence for a large warm disk around the star. Herschel/PACS observations showed that the [C ii] emission is extended, therefore the [C ii] emission most likely originates from an ionized gas shell created by a past outflow event. We confirm the isolated nature of HD 50138. It is far from any star-forming region and has low proper motion. Neither is there any sign of a remnant cloud from which it could have formed. The extended disk around the star appears to be carbon-poor. It shows OH and [O i] emission, but no CO. The CO abundance appears to be at least an order of magnitude lower than that of OH. Furthermore, 13CO is enriched by more than a factor of five, confirming that the star is not a Herbig Be star. Finally, we note that our high-spectral-resolution [O i] and [C ii] observations provide a very accurate heliocentric velocity of the star, 40.8 ± 0.2 km s−1. [ABSTRACT FROM AUTHOR]

We present a new velocity-resolved survey of 2.9 μm spectra of hot H2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R ∼ 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9–33 μm that probe from the dust sublimation radius at ∼0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H2O line fluxes and the radius of CO gas emission, as measured in velocity-resolved 4.7 μm spectra (R ), which probes molecular gaps in inner disks. We find that H2O emission disappears from 2.9 μm (hotter water) to 33 μm (colder water) as increases and expands out to the snow line radius. These results suggest that the infrared water spectrum is a tracer of inside-out water depletion within the snow line. It also helps clarify an unsolved discrepancy between water observations and models by finding that disks around stars of generally have inner gaps with depleted molecular gas content. We measure radial trends in H2O, OH, and CO line fluxes that can be used as benchmarks for models to study the chemical composition and evolution of planet-forming disk regions at 0.05–20 au. We propose that JWST spectroscopy of molecular gas may be used as a probe of inner disk gas depletion, complementary to the larger gaps and holes detected by direct imaging and by ALMA. [ABSTRACT FROM AUTHOR]

The total gas mass of a protoplanetary disk is a fundamental, but poorly determined, quantity. A new technique has been demonstrated to assess directly the bulk molecular gas reservoir of molecular hydrogen using the HD J = 1–0 line at 112 μm. In this work we present a Herschel Space Observatory10 survey of six additional T Tauri disks in the HD line. Line emission is detected at >3σ significance in two cases: DM Tau and GM Aur. For the other four disks, we establish upper limits to the line flux. Using detailed disk structure and ray-tracing models, we calculate the temperature structure and dust mass from modeling the observed spectral energy distributions, and we include the effect of UV gas heating to determine the amount of gas required to fit the HD line. The ranges of gas masses are 1.0–4.7 × 10−2 for DM Tau and 2.5–20.4 × 10−2 for GM Aur. These values are larger than those found using CO for GM Aur, while the CO-derived gas mass for DM Tau is consistent with the lower end of our mass range. This suggests a CO chemical depletion from the gas phase of up to a factor of five for DM Tau and up to two orders of magnitude for GM Aur. We discuss how future analysis can narrow the mass ranges further. [ABSTRACT FROM AUTHOR]

From the masses of the planets orbiting the Sun, and the abundance of elements relative to hydrogen, it is estimated that when the Solar System formed, the circumstellar disk must have had a minimum mass of around 0.01 solar masses within about 100 astronomical units of the star. (One astronomical unit is the Earth-Sun distance.) The main constituent of the disk, gaseous molecular hydrogen, does not efficiently emit radiation from the disk mass reservoir, and so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally indicates properties of the disk surface, and the conversion from dust emission to gas mass requires knowledge of the grain properties and the gas-to-dust mass ratio, which probably differ from their interstellar values. As a result, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3-10 million years) star TW Hydrae, for which the range is 0.0005-0.06 solar masses. Here we report the detection of the fundamental rotational transition of hydrogen deuteride from the direction of TW Hydrae. Hydrogen deuteride is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The detection of hydrogen deuteride, combined with existing observations and detailed models, implies a disk mass of more than 0.05 solar masses, which is enough to form a planetary system like our own.