AN UNBIASED 1.3 mm EMISSION LINE SURVEY OF THE PROTOPLANETARY DISK ORBITING LkCa 15

Abstract

The outer (>30 AU) regions of the dusty circumstellar disk orbiting the ∼2-5 Myr-old, actively accreting solar analog LkCa 15 are known to be chemically rich, and the inner disk may host a young protoplanet within its central cavity. To obtain a complete census of the brightest molecular line emission emanating from the LkCa 15 disk over the 210-270 GHz (1.4 -1.1 mm) range, we have conducted an unbiased radio spectroscopic survey with the Institute de Radioastronomie Millimétrique (IRAM) 30 meter telescope. The survey demonstrates that, in this spectral region, the most readily detectable lines are those of CO and its isotopologues 13 CO and C 18 O, as well as HCO + , HCN, CN, C 2 H, CS, and H 2 CO. All of these species had been previously detected in the LkCa 15 disk; however, the present survey includes the first complete coverage of the CN (2-1) and C 2 H (3-2) hyperfine complexes. Modeling of these emission complexes indicates that the CN and C 2 H either reside in the coldest regions of the disk or are subthermally excited, and that their abundances are enhanced relative to molecular clouds and young stellar object environments. These results highlight the value of unbiased single-dish line surveys in guiding future high resolution interferometric imaging of disks.

Chemistry in disks

“…We note that excitation temperatures and column densities for C 2 H in LkCa15 found in this detailed disk modeling are quite different from those derived by Punzi et al (2015) from the N = 3−2 IRAM 30 m telescope spectra alone. This is presumably because Punzi et al (2015) assumed a uniform beam filling factor in the 30 m telescope beam, of order 1 given the outer radius of the disk, ∼500 au, and thus derive very low excitation temperatures (as the opacity of each of the detected hyperfine component is ∼0.5 from the hyperfine ratios).…”

“…A comparison with the unbiased frequency survey of Punzi et al (2015) shows that, although covering only half of the 64 GHz between 206 and 270 GHz, our study only misses three detectable transitions:…”

“…We used the HFS method of the CLASS 1 program to make a fit of all components together, assuming optically thin emission (total opacity <0.5). From the hyperfine line ratios, Punzi et al (2015) find non-negligible optical depths for CN and especially C 2 H in LkCa 15. Their quoted values for opacities are within those allowed by our (somewhat less sensitive) data: our best-fit total opacity for C 2 H is 4, but with large errors (∼2−3).…”

“…Also, more extended surveys (either in number of targeted disks or molecules) with single-dish radio antennas became feasible; however, the complete, unbiased frequency coverage between 208 and 270 GHz, performed at the IRAM 30 m telescope for the LkCa 15 transition disk by Punzi et al (2015), did not reveal any new molecule in this source. Guilloteau et al (2013) observed 13 CO, CN, H 2 CO, and SO in 42 disks around T Tauri and Herbig Ae stars of the Taurus-Auriga association with the IRAM 30 m telescope.…”

The Gas Disk: Evolution and Chemistry

“…Recent adaptive optics observations by Sallum et al (2015) reveal up to three accreting giant proto-planets in the large inner cavity (r cav ≈ 50 au, ) of this disk. At the same time the outer disk of this system still remains intact and shows similar observational signatures as classical disks (Öberg et al 2011;Punzi et al 2015).…”