Weak metallic emission lines in early B-type stars

Abstract

Previously unrecognized weak emission lines originating from high excitation states of Si ii (12.84 eV) and Al ii (13.08 eV) are detected in the red-region spectra of slowly rotating early B-type stars. We surveyed high-resolution spectra of 35 B-type stars covering spectral sub-types between B1 and B7 near the main sequence and found the emission line of Si ii at 6239.6 Å in all of the 13 stars that have spectral sub-types B2 and B2.5. There are 17 stars belonging to sub-type B3 among which seven stars are found to show the emission line of Si ii. The emission line of Al ii at 6243.4 Å is detected in a narrower temperature range (T eff between 19000 K and 23000 K) in nine of the stars. Both emission lines are not detected in cooler (T eff < 16000 K) stars in our sample. The emission line of Si ii at 6239.6 Å shows a single-peaked and symmetrical profile, and the line center has no shift in wavelength with respect to line centers of low excitation absorption lines of Si ii. The measured half-width of the emission line is the same as those of rotationally broadened, low-excitation absorption lines of Si ii. These observations imply that the emitting gas is not of circumstellar origin, but it is located at the outermost layer of the atmosphere covering the whole stellar surface, and corotates with the star.