Identification of IRAS Sources in the Outer Disk of the Galaxy
Abstract
Near infrared imaging and photometric observations at J, H and K (or K') bands were performed toward 95 IRAS sources in the outer disk of the Galaxy most of which had been searched for SiO maser emission and are candidates for variable late-type stars with cold circumstellar envelopes. Low resolution optical spectroscopic observations were made to 19 stars of them for classifying the spectral types. Thirty-eight of them, for which counterparts were not found at I band brighter than 19mag, were taken near infrared images for identification. All are identified in the near-infrared images brighter than 16 mag at K' band except two that could be in the faint phase of variation when observed. So most of the IRAS late-type stars perhaps have a near-infrared counterpart although many have no optical counterpart. In the near-infrared color-color diagram, the stars with O-rich and C-rich circumstellar envelopes(CSEs) are separated from each other. The stars with C-rich CSE are redder than those with O-rich CSE in general but there are three very red stars in this sample being SiO maser sources, i.e. with O-rich CSE. By combining the near-infrared colors and IRAS color, the two types of circumstellar envelopes are discriminated. 48 percent are stars with C-rich CSE in the entire sample and the number ratios of C-rich to O-rich stars are 14:43 and 32:6 for the optically identified and unidentified groups respectively. This indicates an increasing proportion of C-rich stars with galacto-centric distance and stellar evolutionary stage. The result is applied to explain the low detection rate of SiO maser emission in a similar and bigger sample in the outer disk of the Galaxy.
- Publication:
-
The Astronomical Journal
- Pub Date:
- April 1997
- DOI:
- 10.1086/118345
- Bibcode:
- 1997AJ....113.1315J