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