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 Btype 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 6239.6Å in all 13 stars having spectral sub-types B2 and B2.5. There are 17 stars belonging to sub-type B3 and seven stars among them are found to show the emission line of Si II. The emission line of Al II at 6243.4Å is detected in a narrower temperature range (T eff between 19000K and 23000 K) in nine stars. Both of these emission lines are not detected in cooler (T eff < 16000 K) stars in our sample. The emission line of Si II at 6239.6Å shows a single-peaked and symmetric profile and the line center has no shift in wavelength with respect to those of low excitation absorption lines of Si II. 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 circumstellar origin, but is located at the outermost layer of the atmosphere, covering the whole stellar surface and co-rotates with the star.

Non-local thermodynamic equilibrium line formation for Si i–ii–iii in A–B stars and the origin of Si ii emission lines in ι Her

“…Therefore, a correct evaluation of atmospheric parameters from the Si i/Si ii or Si ii/Si iii ionisation equilibrium method can only be possible, when neglecting the LTE assumption. Sadakane & Nishimura (2017, 2019 registered weak emission lines of Si ii in the red-region spectra of slowly rotating early B-type stars, and HD 160762 (ι Her, B3 IV) among them. The LTE calculations with classical hydrostatic model atmosphere can only yield an absorption line profile, but never the emission one.…”

A spectroscopic study of weak metallic emission lines in a B3 V star ι Herculis

“…We find several clear emission lines of Si II in ι Her. Two emission lines of Si II at 5688.82 Å and at 6239.61 Å reported inSadakane and Nishimura (2017) are weak. The newly recorded pair of Si II emission lines at 7848.82 Å and at 7849.72 Å (figure…”

“…Weak emission components of C I, Ti II, Cr II, and Mn II in a HgMn star HD 71066 and those of multiplet 13 lines of Mn II in a PGa star HD 19400 were suggested by Yüce et al (2011) and by Hubrig et al (2014), respectively. Sadakane and Nishimura (2017) presented observations of WELs of Si II and Al II near 6240 Å in many early B-type stars. They demonstrated that the WELs phenomenon is not an exceptional case observed only among chemically peculiar stars but generally observed in many normal stars.…”

Direct observation of the radial magnetic field gradient in HD58260 from spectropolarimetry of NLTE lines in emission

“…A conclusion theoretically supported by Alexeeva et al (2016Alexeeva et al ( , 2018Alexeeva et al ( , 2019; Mashonkina (2020); Mashonkina et al (2020). From an analysis of line profiles of a sample of early B-type stars, Sadakane & Nishimura (2017) concluded that emission and absorption lines are equally broadened by the stellar rotation, supplying an observational proof that emission lines are from the outer stellar atmosphere and not from a circumstellar environment. The present paper reports the results of a search for a radial gradient in the magnetic field of the early B-type star HD 58260, based on high-resolution Stokes I and V profiles of spectral lines in absorption and in emission, sampling at least 6 dex in optical depth.…”