The Ages of Optically Bright Sub-Clusters in the Serpens Star-Forming Region

TL;DR

This study develops a spectral energy distribution fitting method to estimate ages of young stars in the Serpens Molecular Cloud, revealing a prolonged star formation history extending from 4 to 22 million years.

Contribution

The paper introduces a new SED fitting technique for age estimation of evolved young stars, calibrated with APOGEE spectra, applied to Serpens YSOs using Gaia data.

Findings

YSOs in Serpens range from ~4 to ~22 Myr in age.

Serpens has been forming stars over a longer period than infrared surveys suggested.

Optical members are concentrated in distinct age subgroups.

Abstract

The Serpens Molecular Cloud is one of the most active star-forming regions within 500 pc, with over one thousand of YSOs at different evolutionary stages. The ages of the member stars inform us about the star formation history of the cloud. In this paper, we develop a spectral energy distribution (SED) fitting method for nearby evolved (diskless) young stars from members of the Pleiades to estimate their ages, with a temperature scale adopted from APOGEE spectra. When compared with literature temperatures of selected YSOs in Orion, the SED fits to cool (<5000 K) stars have temperatures that differ by an average of <~ 50 K and have a scatter of ~ 210 K for both disk-hosting and diskless stars. We then apply this method to YSOs in the Serpens Molecular Cloud to estimate ages of optical members previously identified from Gaia DR2 astrometry data. The optical members in Serpens are concentrated in different subgroups with ages from ~4 Myr to ~22 Myr; the youngest clusters, W40 and Serpens South, are dusty regions that lack enough optical members to be included in this analysis. These ages establish that the Serpens Molecular Cloud has been forming stars for much longer than has been inferred from infrared surveys.