Main-sequence stars masquerading as Young Stellar Objects in the central molecular zone

TL;DR

This study reveals that many objects previously classified as Young Stellar Objects in the Milky Way's central region are actually main-sequence stars, leading to overestimated star formation rates.

Contribution

The paper demonstrates that main-sequence stars can mimic YSOs at 24 microns, causing significant misclassification and overestimation of star formation rates in the Central Molecular Zone.

Findings

At least 63% of YSO candidates are misclassified main-sequence stars.

Main-sequence stars can be marginally resolved at 24 microns, unlike YSOs.

Star formation rate estimates from YSOs may be overestimated by a factor of three.

Abstract

In contrast to most other galaxies, star-formation rates in the Milky Way can be estimated directly from Young Stellar Objects (YSOs). In the Central Molecular Zone (CMZ) the star-formation rate calculated from the number of YSOs with 24 microns emission is up to order of magnitude higher than the value estimated from methods based on diffuse emission (such as free-free emission). Whether this effect is real or whether it indicates problems with either or both star formation rate measures is not currently known. In this paper, we investigate whether estimates based on YSOs could be heavily contaminated by more evolved objects such as main-sequence stars. We present radiative transfer models of YSOs and of main-sequence stars in a constant ambient medium which show that the main-sequence objects can indeed mimic YSOs at 24 microns. However, we show that in some cases the main-sequence models can be marginally resolved at 24 microns, whereas the YSO models are always unresolved. Based on the fraction of resolved MIPS 24 microns sources in the sample of YSOs previously used to compute the star formation rate, we estimate the fraction of misclassified "YSOs" to be at least 63%, which suggests that the star-formation rate previously determined from YSOs is likely to be at least a factor of three too high.