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Register a new userRotational Line Emission from Water in Protoplanetary Disks
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Circumstellar disks provide the material reservoir for the growth of young stars and for planet formation. We combine a high-level radiative transfer program with a thermal-chemical model of a typical T Tauri star disk to investigate the diagnostic potential of the far-infrared lines of water for probing disk structure. We discuss the observability of pure rotational H2O lines with the Herschel Space Observatory, specifically the residual gas where water is mainly frozen out. We find that measuring both the line profile of the ground 110-101 ortho-H2O transition and the ratio of this line to the 312-303 and 221-212 line can provide information on the gas phase water between 5-100 AU, but not on the snow line which is expected to occur at smaller radii.
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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 NASAs 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 (2sigma) 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.
We present ground-based high resolution N-band spectra (Delta v = 15 km/s) of pure rotational lines of water vapor in two protoplanetary disks surrounding the pre-main sequence stars AS 205N and RNO 90, selected based on detections of rotational water lines by the Spitzer IRS. Using VISIR on the Very Large Telescope, we spectrally resolve individual lines and show that they have widths of 30-60 km/s, consistent with an origin in Keplerian disks at radii of ~1 AU. The water lines have similar widths to those of the CO at 4.67 micron, indicating that the mid-infrared water lines trace similar radii. The rotational temperatures of the water are 540 and 600K in the two disks, respectively. However, the lines ratios show evidence of non-LTE excitation, with low-excitation line fluxes being over-predicted by 2-dimensional disk LTE models. Due to the limited number of observed lines and the non-LTE line ratios, an accurate measure of the water ortho/para ratio is not available, but a best estimate for AS 205N is ortho/para = 4.5 +/- 1.0, apparently ruling out a low-temperature origin of the water. The spectra demonstrate that high resolution spectroscopy of rotational water lines is feasible from the ground, and further that ground-based high resolution spectroscopy is likely to significantly improve our understanding of the inner disk chemistry recently revealed by recent Spitzer observations.
We present the largest survey of spectrally resolved mid-infrared water emission to date, with spectra for 11 disks obtained with the Michelle and TEXES spectrographs on Gemini North. Water emission is detected in 6 of 8 disks around classical T Tauri stars. Water emission is not detected in the transitional disks SR 24 N and SR 24 S, in spite of SR 24 S having pre-transitional disk properties like DoAr 44, which does show water emission (Salyk et al. 2015). With R~100,000, the TEXES water spectra have the highest spectral resolution possible at this time, and allow for detailed lineshape analysis. We find that the mid-IR water emission lines are similar to the narrow component in CO rovibrational emission (Banzatti & Pontoppidan 2015), consistent with disk radii of a few AU. The emission lines are either single peaked, or consistent with a double peak. Single-peaked emission lines cannot be produced with a Keplerian disk model, and may suggest that water participates in the disk winds proposed to explain single-peaked CO emission lines (Bast et al. 2011, Pontoppidan et al. 2011). Double-peaked emission lines can be used to determine the radius at which the line emission luminosity drops off. For HL Tau, the lower limit on this measured dropoff radius is consistent with the 13 AU dark ring (ALMA partnership et al. 2015). We also report variable line/continuum ratios from the disks around DR Tau and RW Aur, which we attribute to continuum changes and line flux changes, respectively. The reduction in RW Aur line flux corresponds with an observed dimming at visible wavelengths (Rodriguez et al. 2013).
The low water content of the terrestrial planets in the solar system suggests that the protoplanets formed within the water snow line. Accurate prediction of the snow line location moving with time provides a clue to constrain the formation process of the planets. In this paper, we investigate the migration of the snow line in protoplanetary disks whose accretion is controlled by laminar magnetic fields, which have been proposed by various nonideal magnetohydrodynamic (MHD) simulations. We propose an empirical model of the disk temperature based on our nonideal MHD simulations, which show that the accretion heating is significantly less efficient than in turbulent disks, and calculate the snow line location over time. We find that the snow line in the magnetically accreting laminar disks moves inside the current Earths orbit within 1 Myr after star formation, whereas the time for the conventional turbulent disk is much longer than 1 Myr. This result suggests that either the rocky protoplanets formed in such an early phase of the disk evolution, or the protoplanets moved outward to the current orbits after they formed close to the protosun.
The study of warm molecular gas in the inner region (<10 AU) of circumstellar disks around young stars is of significant importance to understand how planets are forming. This inner zone of disks can now be explored in unprecedented detail with the high spectral (R=100000) and spatial resolution spectrometer CRIRES at the VLT. This paper investigates a set of disks that show CO ro-vibrational v=1-0 4.7 micron emission line profiles characterized by a single, narrow peak and a broad base extending to > 50 km/s, not readily explained by just Keplerian motions of gas in the inner disk. The line profiles are very symmetric, have high line/continuum ratios and have central velocity shifts of <5 km/s relative to the stellar radial velocity. The disks in this subsample are accreting onto their central stars at high rates relative to the parent sample. All disks show CO lines from v=2, suggesting that the lines are excited, at least in part, by UV fluorescence. Analysis of their spatial distribution shows that the lines are formed within a few AU of the central star. It is concluded that these broad centrally peaked line profiles are inconsistent with the double peaked profiles expected from just an inclined disk in Keplerian rotation. Alternative non-Keplerian line formation mechanisms are discussed, including thermally and magnetically launched winds and funnel flows. The most likely interpretation is that these profiles originate from a combination of emission from the inner part (< a few AU) of a circumstellar disk, perhaps with enhanced turbulence, and a slow moving disk wind, launched by either EUV emission or soft X-rays.
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