Herschel dust emission as a probe of starless cores mass: MCLD 123.5+24.9 of the Polaris Flare

TL;DR

This study uses Herschel data and spectral modeling to analyze uncertainties in measuring the masses of starless cores in a molecular cloud, revealing significant potential underestimations and their implications for core stability.

Contribution

It systematically assesses uncertainties in Herschel-based mass estimates of starless cores, highlighting the impact of grain models and fitting methods on derived core properties.

Findings

Opacity models vary by over a factor of two, affecting mass estimates.

Blackbody fitting can underestimate high-density core masses by up to three times.

All cores are sub-virial and not self-gravitating, explaining their starless nature.

Abstract

We present newly processed archival Herschel images of molecular cloud MCLD 123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using the spectral synthesis code Cloudy, we explore uncertainties in the derivation of column densities, hence, masses of molecular cores from Herschel data. We first consider several detailed grain models that predict far-IR grain opacities. Opacities predicted by the models differ by more than a factor of two, leading to uncertainties in derived column densities by the same factor. Then we consider uncertainties associated with the modified blackbody fitting process used by observers to estimate column densities. For high column density clouds (N(H) $\gg$ 10$^{22}$ cm$^{-2}$), this fitting technique can underestimate column densities by about a factor of three. Finally, we consider the virial stability of the five starless cores in MCLD 123.5+24.9. All of these cores appear to have strongly sub-virial masses, assuming, as we argue, that $^{13}$CO line data provide reliable estimates of velocity dispersions. Evidently, they are not self-gravitating, so it is no surprise that they are starless.