UNBIASED MILLIMETER-WAVE LINE SURVEYS OF TW Hya AND V4046 Sgr: THE ENHANCED C₂H AND CN ABUNDANCES OF EVOLVED PROTOPLANETARY DISKS

Abstract

We have conducted the first comprehensive millimeter-wave molecular emission line surveys of the evolved circumstellar disks orbiting the nearby, roughly solar-mass, pre-main-sequence (T Tauri) stars, TW Hya (D = 54 pc) and V4046 Sgr AB (D = 73 pc). Both disks are known to retain significant residual gaseous components despite the advanced ages of their host stars (∼8 Myr and ∼21 Myr, respectively). Our unbiased broadband radio spectral surveys of the TW Hya and V4046 Sgr disks were performed with the Atacama Pathfinder Experiment 12 m telescope, and are intended to yield a complete census of the bright molecular emission lines in the range 275-357 GHz (1.1-0.85 mm). We find that lines of 12 CO, 13 CO, HCN, CN, and C 2 H, all of which lie in the higher frequency (>330 GHz) range, constitute the strongest molecular emission from both disks in the spectral region surveyed. The molecule C 2 H is detected here for the first time in both disks, as is CS in the TW Hya disk. The survey results also include the first measurements of the full suite of the hyperfine transitions of CN N = 3 → 2 and C 2 H N = 4 → 3 in both disks. Modeling of these CN and C 2 H hyperfine complexes in the spectrum of TW Hya indicates that the emission from both species is optically thick and may originate from very cold ( 10 K) disk regions. The latter result, if confirmed, would suggest the efficient production of CN and C 2 H in the outer disk and/or near the disk midplane. It furthermore appears that the fractional abundances of CN and C 2 H are significantly enhanced in these evolved protoplanetary disks, relative to the fractional abundances of the same molecules in the environments of deeply embedded protostars. These results, combined with previous determinations of the enhanced abundances of other species (such as HCO + ) in T Tauri star disks, underscore the importance of properly accounting for high-energy (FUV and X-ray) radiation from the central T Tauri star when modeling protoplanetary disk gas chemistry and physical conditions.

Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue

“…The derived total opacity is ≈3.4−5.4 for groups a and b lines, confirming previous studies (Kastner et al 2014). The weakest hf CN lines, in group a, which carry 2.7% of the total intensity (see Table 1), is thus less than 0.15, well into the optically thin regime.…”

“…The chosen CN hf transition at 340.020 GHz is readily shown to be optically thin by considering the total opacity of the CN line, which is of order 4−5 (Kastner et al 2014), and recognizing that this hf line carries 5.4% of the total intensity. The low optical depth is confirmed by hyperfine analysis of the ALMA spectra and by the identical lineshapes of the diskaveraged spectra of the 4 weakest hf transitions with relative intensities 2.7 and 5.4% (see Fig.…”

A Surface Density Perturbation in the TW Hydrae Disk at 95 au Traced by Molecular Emission

“…It is primarily depleted from the gas phase by freezing out onto grain surfaces. The predicted column densities for our unperturbed Model A at r = 100 au is N (CS) ∼ 10 13 cm −2 and are in good overall agreement with typically observed values in T Tauri disks (Guilloteau et al 2012) and estimated for TW Hya from single-dish observations (Kastner et al 2014).…”

Chemistry in disks

“…The detection of several molecules in small disks (radii ∼200 au) indicates the chemistry in these regions is not radically different from that of bigger disks, as already hinted at by the only known detailed case so far, TW Hya (Thi et al 2004;Kastner et al 2014). -As suggested by the previous study of AB Aur (Schreyer et al 2008), disks around HAe stars appear less rich in molecules.…”