The nature of unidentified 12 µm IRAS sources at high Galactic latitudes

Abstract

A sample of 47 previously uncataloged objects detected at 12 µm by the Infrared Astromonical Satellite {IRAS), located above a Galactic latitude of 50° and selected to have relatively faint counterparts on the Palomar Observatory Sky Survey prints, has been studied using near-infrared photometry. The vast majority of the sample are distant (~3 kpc) red giant stars with spectral types later than about M6. These stars show 1-2 mag of excess at 12 µm, indicative of mild mass loss of about 10^(-7)-10^(-8) M_⊙ yr^(-1). The sample also contains one quasar, one oxygen-rich giant star undergoing rapid mass loss, and a remarkable cool (1230 K) carbon star. The absence of nearby, low-luminosity infrared sources in this sample limits the space density of field brown dwarf stars. The value of this limit depends on the unknown properties and distribution of ages and masses of brown dwarfs, and ranges from 0.12 M_⊙ pc^(-3), or comparable to the Oort limit for the local missing mass, up to 2.4 M_⊙ pc^(-3).