Near-infrared observations and optical identifications of a few unassociated IRAS sources with dust shells

Abstract

We have carried out near infrared observations of twenty six unassociated IRAS sources with dust shells. Twenty of these belong to the group that have IRAS low resolution spectral characteristic (LRSCHAR =4 n), classified as stars with carbon rich envelopes. Optical counterparts of 18 of these unassociated sources were identified using Palomar Observatory Sky Survey (POSS) prints. The programme sources with LRSCHAR=4n as a class appear to have thicker dust shells than Two Micron Sky Survey (TM SS) sources of similar LRSCHAR, as evidenced by their larger K-[12] colours (>4.3). They thus represent more evolved sources, with high mass loss rates and are perhaps variables with periods >400 days. Their [12]-[25] and [25]-[60] colours are typical of black bodies with temperatures ranging from 400 to 1200K. The average K-[12] and [12]-[25] colours decrease with increasing SiC index, suggesting that the fractional abundance of SiC grains decreases with increasing mass loss rate.