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

Context. Mass-loss from evolved stars chemically enriches the interstellar medium (ISM). Stellar winds from massive stars and their explosions as supernovae shape the ISM and trigger star formation. Studying evolved stars is fundamental for understanding galaxy formation and evolution at any redshift. Aims. 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 these samples as tracers of Galactic structure. Methods. We searched for counterparts of known Galactic WR, LBV, RSG, and O-rich AGB stars in the 2MASS, GLIMPSE, and MSX catalogs, and we analyzed their properties with near-and mid-infrared color-color diagrams. Results. We used the Q1 parameter, which is a measure of the deviation from the interstellar reddening vector in the J - H versus H-K-s diagram, and we defined a new parameter, Q2, which is a measure of the deviation from the interstellar reddening vector in the J-K-s versus K-s-[8.0] diagram. The latter plane enables to distinguish between interstellar and circumstellar reddening, and to identify stars with circumstellar envelopes. WR stars and late-type mass-losing stars (AGBs and RSGs) are distributed in two different regions of the Q1 versus K-s-[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/IR stars) is found. Spectra of Miras and OH/IR stars have stronger water absorption at 3.0 mu m than SR stars or most of the RSGs. Masing Miras stars have water, but stronger SiO (similar to 4 mu m) and CO2 absorption (similar to 4.25 mu m), as suggested by their [3.6]-[4.5] colors, bluer than those of non masing stars. 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. Conclusions. The GLIMPSE catalog is a powerful tool for photometric classification of Galactic mass-losing evolved stars. Our new criteria will yield many new RSGs and WRs.