Abstract
Little is known about the abundances of lanthanides in the Chemically Peculiar HgMn stars. Evidence is presented in this Note that they are not important absorbers in the classical HgMn stars μ Lep and β Scl. A fully consistent abundance analysis of lanthanides on a single temperature scale, using the same atomic data and including hyperfine structure for a larger number of HgMn stars is necessary.
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In stellar atmospheres, the lanthanides are usually present in the form of two or more stable isotopes (e.g., 151Eu and 153Eu for europium). These isotopes are thought to be produced by the r-process and a small contribution of the s-process (see Burris et al. 2000 for europium for instance). Among the tepid stars (from spectral types B8V to F5V), lanthanides are present and overabundant only in the Chemically Peculiar Am stars and the Ap stars of the SrCrEu group (having overabundances in strontium, chromium and europium). The SrCrEu Ap stars posses dipolar large-scale magnetic fields (up to 104 Gauss), effective temperatures running from 8000 to 10,000 K and surface gravities typical of main-sequence stars (Garrison & Gray 1994). In contrast Am stars are non magnetic stars and are frequently found in binary systems. The presence of lanthanides in the spectra of the hotter HgMn stars, which harbor large excesses of manganese and mercury, has been little studied. Only a handful of HgMn stars among the 107 HgMn stars having abundances in the recent compilation of Ghazaryan & Alecian (2016) have abundances for a few lanthanides.
The bright stars, μ Lep, β Scl and χ Lupi A are three classical HgMn stars. The purpose of this note is to report on the determination of the abundances of the lanthanides from the optical spectra of these three stars on the same temperature scale using spectrum synthesis and the latest available atomic data. The abundances of these elements, which are intrinsically low in the Standard Abundance Distribution, are important diagnostics of the physical processes deemed to be at work in the atmospheres of these stars. Detection of overabundances of the Rare Earths indicates that nonstandard transport processes (e.g., radiative diffusion) must be occurring in the atmospheres of these stars in order to support these heavy elements in the line forming region. In the atmospheres of HgMn stars, the dominant ionization stage for most lanthanides is twice ionized.
All the observed spectra used here are FEROS spectra (Kaufer et al. 1999), which were retrieved from the ESO Archive product page.1 FEROS is a high efficiency (20%) high resolution (R = 48,000) échelle spectrograph covering a large wavelength range from 3527 up to 9217 Å in one exposure. The exposure times run from 45 s to 150 s and the signal-to-noise ratio at 5000 Å range from 325 up to 600 at 5000 Å. The PIs of the programs are N. Przybilla (χ Lupi A) and S. Hubrig (β Scl). I have coadded the 4 spectra of μ Leporis in order to increase the signal-to-noise ratio.
The effective temperature and surface gravities were derived for each star using Napiwotzky's UVBYBETA code (Napiwotzki et al. 1993) and the Strömgren photometry taken from Hauck & Mermilliod (1998). A model atmosphere was then computed for each star using ATLAS9 (Kurucz 1992) assuming local thermodynamical equilibrium, hydrostatic equilibrium and radiative equilibrium. A grid of synthetic spectra was then computed using SYNSPEC49 (Hubeny & Lanz 1992) to derive the abundances of lanthanides from cerium up to ytterbium. The linelist includes the hyperfine structure transitions of the various isotopes, when available.
While several lanthanides are detected and overabundant in the sharp-lined χ Lupi A, their presence is more difficult to assess in μ Lep and β Scl. In several instances, the lines are blended because of the larger equatorial rotational velocities. In other instances, lines are not observed suggesting an upper limit equal to the solar abundance.
In Figure 1, I show the synthesis of the Gd ii line at 4037.32 Å of μ Leporis for an overabundance of gadolinium of 3000 times the solar value which provides the best fit. Gadolinium is by far the most overabundant lanthanide in μ Lep. The vertical line depicts the laboratory wavelength of the Gd ii line, which is well separated from the closeby Cr ii line at 4037.97 Å. The estimated signal-to-noise ratio of the coadded spectrum is 727 at this wavelength. No attempt has been made yet to reproduce all observed lines in this spectral range.
Figure 1. Likely detection of the Gd ii line at 4037.32 Å in the FEROS mean spectrum of μ Leporis (model in blue, observed spectrum in black).
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The lines of the lanthanides do not appear to be important absorbers in the optical spectra of μ Lep, β Scl analyzed here. One explanation is that the lanthanides may occur in spots which were not present in the spectra of μ Lep and β Scl analyzed here. Only a fully consistent abundance analysis of lanthanides on a single temperature scale, using the same atomic data and including hyperfine structure of a much larger number of HgMn stars will address the importance of lanthanides in the atmospheres of the HgMn stars. As these lines are expected to be faint, high resolution high signal-to-noise spectra are mandatory. Often the abundances were derived using only one line for a given element. Additional atomic data for more lines of twice ionized lanthanides, in particular hyperfine structure, is highly desirable.