The Large Xenon Overabundances of the Bright HgMn stars μ Lep and β Scl

We provide evidence for strong lines of Xe ii in the spectra of the two classical HgMn stars μ Lep and β Scl. Using updated atomic data, we derive large overabundances of xenon in the atmospheres of these two stars.

β Scl (HD 221507, V = 4.37) and μ Lep (HD 33904, V = 3.29) are two bright southern late B-type chemically peculiar stars. Abt & Morrell (2005) classified β Scl as B9.5 IIIp HgMnSi and Houk & Smith-Moore (1988) classified μ Lep as B9 IV: HgMn, confirming that both stars are HgMn-type stars, i.e., their atmospheres harbor overabundances of manganese and mercury compared to normal late B-type stars. Recently, β Scl has been observed with TESS³ (Ricker et al. 2015) and was unexpectedly found to have low-amplitude variability (141 ppm) with a period of 1.92 day (Balona et al. 2019). Inspection of the unpublished TESS lightcurve of μ Lep also reveals variability with a period which we estimate is close to 3 days. One interpretation is that this photometric variability is caused by rotational modulation by thermal or chemical spots on the surface of these stars.

The purpose of this note is to report on the detection of three lines of Xe ii in archival high resolution spectra of β Scl and μ Leporis acquired with the échelle spectrograph FEROS⁴ and retrieved from the ESO archive.⁵ FEROS (Fiber-fed Extended Range Optical Spectrograph) has a resolving power R = 48,000 and a large spectral coverage stretching from 3500 up to 9200 Å. The FEROS spectrum of μ Lep was acquired on 2011 December 8, that of β Scl on 2016 September 21, their signal-to-noise ratios run from 300 to 400. We also provide a determination of the abundances of xenon in the atmospheres of these stars using up-to-date atomic data for the Xe ii lines. The only systematic study of xenon in several HgMn stars is that by Dworetsky et al. (2008) who used equivalent widths and atomic data available at that time.

We have derived the effective temperatures and surface gravities (T_eff = 12,470 ± 250 K and $\mathrm{log}\,g=4.13\pm 0.25$ for β Scl and T_eff = 12,770 ± 250 K and $\mathrm{log}\,g=3.77\pm 0.25$ for μ Lep) by using the UVBYBETA code (Napiwotzki et al. 1993) and Strömgren photometry from Hauck & Mermilliod (1998). Hence β Scl and μ Lep have similar effective temperatures, however μ Lep is probably more evolved on the main sequence. For these parameters we have computed a model atmosphere using ATLAS9 (Kurucz 1993) for each star assuming local thermodynamical equilibrium, hydrostatic and radiative equilibrium.

A cross-correlation of several Fe ii lines of β Scl and μ Lep which have accurate wavelengths with a synthetic spectrum computed using SYNSPEC49 (Hubeny & Lanz 1995) for a solar iron abundance yields stellar radial velocities of 2.83 ± 0.20 km s⁻¹ for β Scl and 23.30 ± 0.20 km s⁻¹ for μ Lep. After correction for these radial velocities, the three lines of Xe ii at 4603.03, 4844.32 and 5419.15 Å are clearly present in the spectra of both stars. They absorb from 3% up to 7% of the local continuum.

We derived the abundances of xenon by adjusting a grid of synthetic spectra computed for various abundances with SYNSPEC49 using the most recent oscillator strengths provided in VALD.⁶ The best fit to the FEROS spectrum of μ Lep in the region from 5410 up to 5430 Å is displayed in Figure 1. The xenon abundances derived from the three lines run from 5 × 10⁴ to 6 × 10⁴ times the solar value respectively for μ Lep (about 2.4 times the value of Dworetsky et al. 2008) and 2.5 × 10⁴ times for β Scl respectively. The strong line observed at 5416.39 Å is a line of Ga ii at 5416.32 Å indicating a large overabundance of gallium (about 2.2 × 10⁴ times the solar abundance). The large excesses we find agree quite well with the overabundances of xenon in several other confirmed HgMn stars collected by Ghazaryan & Alecian (2016). The full analysis of the FEROS spectra of β Scl and μ Lep including the determination of the abundances of other chemical elements is in progress.

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