The meaning of WISE colours - I. The Galaxy and its satellites

TL;DR

This paper uses WISE infrared data to classify and map various stellar populations in our Galaxy and nearby satellites, revealing spatial distributions and dust properties of evolved stars and other objects.

Contribution

It develops WISE-based criteria to identify and distinguish AGB star types and creates all-sky maps showing their distribution and dust shell characteristics.

Findings

WISE colours effectively separate Galactic and extragalactic sources.

The C:O ratio of dusty AGB stars increases with distance from the LMC center.

Dust shell properties explain the segregation of object classes in WISE colour space.

Abstract

Through matches with the Sloan Digital Sky Survey (SDSS) catalogue we identify the location of various families of astronomical objects in WISE colour space. We identify reliable indicators that separate Galactic/local from extragalactic sources and concentrate here on the objects in our Galaxy and its closest satellites. We develop colour and magnitude criteria that are based only on WISE data to select asymptotic giant branch (AGB) stars with circumstellar dust shells, and separate them into O-rich and C-rich classes. With these criteria we produce an all-sky map for the count ratio of the two populations. The map reveals differences between the Galactic disc, the Magellanic Clouds and the Sgr Dwarf Spheroidal galaxy, as well as a radial gradient in the Large Magellanic Cloud (LMC) disc. We find that the C:O number ratio for dusty AGB stars increases with distance from the LMC centre about twice as fast as measured for near-IR selected samples of early AGB stars. Detailed radiative transfer models show that WISE colours are well explained by the emission of centrally heated dusty shells where the dust has standard properties of interstellar medium (ISM) grains. The segregation of different classes of objects in WISE colour space arises from differences in properties of the dust shells: those around young stellar objects have uniform density distributions while in evolved stars they have steep radial profiles.