An ALMA Survey of Chemistry in Disks around M4-M5 Stars

Abstract

M-stars are the most common hosts of planetary systems in the Galaxy. Protoplanetary disks around M-stars thus offer a prime opportunity to study the chemistry of planet-forming environments. We present an ALMA survey of molecular line emission toward a sample of five protoplanetary disks around M4-M5 stars (FP Tau, J0432+1827, J1100-7619, J1545-3417, and Sz 69). These observations can resolve chemical structures down to tens of AU. Molecular lines of ${}^{12}$CO, ${}^{13}$CO, C${}^{18}$O, C${}_2$H, and HCN are detected toward all five disks. Lines of H${}_2$CO and DCN are detected toward 2/5 and 1/5 disks, respectively. For disks with resolved C${}^{18}$O, C${}_2$H, HCN, and H${}_2$CO emission, we observe substructures similar to those previously found in disks around solar-type stars (e.g., rings, holes, and plateaus). C${}_2$H and HCN excitation conditions estimated interior to the pebble disk edge for the bright disk J1100-7619 are consistent with previous measurements around solar-type stars. The correlation previously found between C${}_2$H and HCN fluxes for solar-type disks extends to our M4-M5 disk sample, but the typical C${}_2$H/HCN ratio is higher for the M4-M5 disk sample. This latter finding is reminiscent of the hydrocarbon enhancements found by previous observational infrared surveys in the innermost ($<$10AU) regions of M-star disks, which is intriguing since our disk-averaged fluxes are heavily influenced by flux levels in the outermost disk, exterior to the pebble disk edge. Overall, most of the observable chemistry at 10-100AU appears similar for solar-type and M4-M5 disks, but hydrocarbons may be more abundant around the cooler stars.

arXiv comment46 pages (20 pages in the main document, 26 pages in the appendix), 48 figures, 7 tables. Accepted in ApJ (February 2021)

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