Near- and Mid-Infrared colors of evolved stars in the Galactic plane. The Q1 and Q2 parameters

TL;DR

This study develops a new infrared color-based classification scheme for evolved stars in the Galactic plane, enabling identification of different stellar types and improving understanding of their role in galactic evolution.

Contribution

The paper introduces the Q2 parameter and new photometric criteria for classifying mass-losing evolved stars using infrared data, enhancing detection and analysis methods.

Findings

Q2 distinguishes interstellar and circumstellar reddening.

Different evolved star types occupy distinct regions in color diagrams.

New criteria will increase the number of identified RSGs and WRs.

Abstract

Mass-loss from evolved stars chemically enriches the ISM. Stellar winds from massive stars and their explosions as SNs shape the ISM and trigger star formation. Studying evolved stars is fundamental for understanding galaxy formation and evolution, at any redshift. We aim to establish a photometric classification scheme for Galactic mass-losing evolved stars (e.g., WR, RSG, and AGB stars) with the goal of identifying new ones, and subsequently to use the sample as tracers of Galactic structure. We searched for counterparts of known Galactic WR, LBV, RSG, and O-rich AGBs in the 2MASS, GLIMPSE, and MSX catalogs, and we analyzed their properties with near- and mid-infrared color-color diagrams. We used the Q1 parameter, which measures the deviation from the interstellar reddening vector in the J-H versus H-Ks diagram, and we defined a new parameter, Q2, that measures the deviation from the interstellar reddening vector in the J-Ks versus Ks-[8.0] diagram. The latter plane enables to distinguish between interstellar and circumstellar reddening, and to identify stars with envelopes. WR stars and late-type mass-losing stars are distributed in two different regions of the Q1 versus Ks-[8.0] diagram. A sequence of increasing [3.6]-[4.5] and [3.6]-[8.0] colors with increasing pulsation amplitudes (SRs, Miras, and OH/IRs) is found. Spectra of Miras and OH/IRs have stronger H2O absorption at 3.0um than SRs or most of the RSGs. Masing Miras have H2O, but stronger SiO (~ 4 um) and CO2 absorption (~4.25 um), as suggested by their bluer [3.6]-[4.5] colors. A fraction of RSGs (22%) have the bluest [3.6]-[4.5] colors, but small Q2 values. We propose a new set of photometric criteria to distinguish among IR bright Galactic stars. The GLIMPSE catalog is a powerful tool for photometric classification of mass-losing evolved stars. Our new criteria will yield many new RSGs and WRs.