Intrinsically Red Sources observed by Spitzer in the Galactic Mid-Plane

TL;DR

This paper provides a comprehensive, reliable catalog of nearly 19,000 intrinsically red mid-infrared sources in the Galactic mid-plane, mainly young stellar objects and AGB stars, based on Spitzer data, with analysis of their distribution, classification, and variability.

Contribution

It offers the largest uniform census of AGB stars and YSOs in the Milky Way, with new selection criteria and detailed analysis of their properties and variability.

Findings

YSOs and AGB stars can be separated by simple color-magnitude criteria.

Planetary nebulae and background galaxies are a small fraction of the sources.

Over 1,000 sources show significant mid-infrared variability.

Abstract

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic mid-plane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope GLIMPSE I and II surveys of 274 deg^2 of the Galactic mid-plane, and consist mostly of high- and intermediate- mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of position- dependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in IRAC and MIPS 24 microns color-magnitude and color-color space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50-70% of YSOs and 30-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2-3% of all the red sources. 1,004 red sources in the GLIMPSE II region, mostly AGB stars with high mass- loss rates, show significant (>0.3 mag) variability at 4.5 and/or 8.0 microns. With over 11,000 likely YSOs and over 7,000 likely AGB stars, this is to date the largest uniform census of AGB stars and high- and intermediate mass YSOs in the Milky-Way Galaxy.