Abundant cyanopolyynes as a probe of infall in the Serpens South cluster-forming region

TL;DR

This study detects HC7N emission in the Serpens South region, revealing infall motions onto filaments and demonstrating HC7N as a useful probe of early-stage star formation in active environments.

Contribution

It presents the first detailed mapping of HC7N in a dynamic, cluster-forming region, linking chemical signatures to infall motions and star formation processes.

Findings

HC7N is primarily found in cold, filamentary structures.

Infall motions are indicated by systematic line centroid shifts and increased non-thermal line widths.

HC7N is present in active star-forming regions, not just quiescent clouds.

Abstract

We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Where detected, the HC7N abundances are similar to those found in other nearby star forming regions. Toward some HC7N `clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n ~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are rarely co-located with those of either NH3 or continuum, it is likely that Serpens South is not particularly remarkable in its abundance of HC7N, but instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed us to detect HC7N at low abundances in regions where it otherwise may not have been looked for. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments.