3. Identification of IRAS infrared counterpart
Usually Be stars have considerable infrared color excess (Johnson, 1967). The special behaviors in spectra of 3G71 encourage us to search for its IRAS point source counterpart. The equatorial coordinates of 3G71 at equinox 1950.0 given by Wackerling's Catalog of Early-Type Stars with Emission Lines (Wackerling, 1970) are 22h 24m 47.8s and + 60o 58´ 59". But at the exact coordinate position, we could not find any candidate. Within 3 arc-minutes region nearby, there are no stars brighter than V = 16 except a V = 12.2 star and a fainter one (V = 14.1). Further observations by one of the authors (QZL) show that the latter is an F-type star so that it could not be 3G71. Considering the coordinates, the magnitude (V 11) and the behavior of hydrogen emission lines of 3G71, we suggest that the V = 12.2 star should be 3G71. The equatorial coordinates of 3G71 at equinox 1950.0 given by the GSC catalogue are 22h 24m 53.37s and + 60o 58´ 13.1". Examining IRAS Point Source Catalogue (IRAS Explanatory Supplement, 1985), we found an IRAS point source 22248 + 6058 with coordinates 22h 24m 53.8s and + 60o 58´ 15" at equinox 1950.0. The differences of position between 3G71 and the IRAS point source counterpart are only 0.4s in right ascension and 2" in declination, lying in the uncertainty ellipse of major axis 11" and minor axis 6" given in IRAS Point Source Catalogue. Within the uncertainty ellipse there are no other stars brighter than V = 16. Thus we associate the IRAS source with 3G71. The flux densities of 22248 + 6058 in various IR bands quoted from IRAS PSC and its converted magnitudes in the corresponding bands (no correction for interstellar reddening) using the IRAS definition for zero-magnitude are listed in Table 1.
Table 1. IRAS flux densities and magnitudes of 3G71.
The equivalent widths of the Na I D1 and D2 doublet absorption lines are 0.365 and 0.496, respectively. From the empirical formula between the distances and the average equivalent widths of Na I doublet lines (Allen, 1973), a distance of 0.86 kpc can be deduced. We estimate that the reduction of V magnitude due to interstellar extinction is less than 2 magnitude because the average interstellar extinction is about Av = 1.9 mag kpc-1. By comparing the far-infrared magnitudes listed in Table 1 with the visual magnitude (V = 12.2) of 3G71, it is obvious that this star has a very large infrared color excess.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999