An Unbiased 1.3 mm Emission Line Survey of the Protoplanetary Disk Orbiting LkCa 15

TL;DR

This paper presents an unbiased 1.3 mm molecular line survey of the LkCa 15 protoplanetary disk, revealing key molecular emissions and their excitation conditions, providing a comprehensive chemical inventory to inform future high-resolution studies.

Contribution

It provides the first complete spectral coverage of certain hyperfine complexes in the LkCa 15 disk, enhancing understanding of disk chemistry and excitation conditions.

Findings

Detection of CO, 13CO, C18O, HCO+, HCN, CN, C2H, CS, and H2CO lines.

First complete coverage of CN (2-1) and C2H (3-2) hyperfine complexes.

Indications of cold or subthermally excited regions with enhanced molecular abundances.

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 Millimetrique (IRAM) 30 meter telescope. The survey demonstrates that, in this spectral region, the most readily detectable lines are those of CO and its isotopologues 13CO and C18O, as well as HCO+, HCN, CN, C2H, CS, and H2CO. 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 C2H (3-2) hyperfine complexes. Modeling of these emission complexes indicates that the CN and C2H 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.