The Abundance of Phosphorus in the HgMn Star 14 Sge

The odd-Z element phosphorus is the seventeenth most abundant element in the universe and an important element for life. It has one stable isotope which is thought to be primarily produced by neutron capture on isotopes of silicon in hydrostatic carbon and neon burning in massive stars (Woosley & Weaver 1995). Type Ia supernovae may also produce phosphorus (Leung & Nomoto 2018) but to a lesser extent than burning in massive stars. A little phosphorus may also be produced in AGB stars (Karakas & Lugaro 2016). Most phosphorus abundances have been derived for late-type stars (see Asplund et al. 2009 for a review). In the Sun, the photospheric abundance of phosphorus has been derived using high-excitation P i lines (Asplund et al. 2009). In late B stars, the phosphorus lines located in the red part of the optical spectrum are fairly weak high-excitation lines. They are detected only if the star has a sizable overabundance of phosphorus.

HR 7664 (14 Sge, HD 190229, B9pHgMn, V = 5.66) is a bright northern HgMn star which is one of the least studied of its class. Only 129 references can be found for this star in SIMBAD. ⁴ It is the most massive member of a spectroscopic binary (Pourbaix et al. 2009) with an orbital period P = 61.541 days, derived from the variations of its radial velocity (Stickland & Weatherby 1984).

We have recently started a systematic survey of all northern HgMn stars in order to derive the elemental abundances of these objects in a consistent manner, i.e., on a common effective temperature scale, using model atmospheres and synthetic spectra and the latest atomic data available including hyperfine structures of the various isotopes for a given element when available. The abundances of most chemical elements in chemically peculiar B-type stars are important diagnostics of the physical processes deemed to be at work in the atmospheres of these stars. Detection of overabundances or underabundances indicates that nonstandard processes (e.g., atomic diffusion, see Michaud et al. 2015) must be occurring in the atmospheres of these stars in order to support (or not) this element in the line forming region.

The purpose of this note is to report on the presence of strong P ii lines in newly acquired spectra of HR 7664 and derive the phosphorus abundance in this star using the latest atomic data.

The high resolution spectra of HR 7664 have been obtained in service observing mode with the new spectropolarimeter NeoNarval at Observatoire du Pic du Midi. ⁵ NeoNarval is a unique instrument that is able to acquire spectra with an accuracy of 3 m s⁻¹. The resolving power of the Stokes IV profiles is R = 65,000 from 3700 up to 10000 Å. The typical signal-to-noise ratios are about 300 for exposures of a few minutes.

The abundance of phosphorus has been derived using spectrum synthesis. Employing Napiwotzky's UVBYBETA code (Napiwotzki et al. 1993) and the Strömgren photometry of HR 7664 taken from Hauck & Mermilliod (1998), we derived T_eff = 13,240 ± 250 K and $\mathrm{log}\ g$ = 3.39 ± 0.25 dex for HR 7664. A model atmosphere was then computed for these parameters using ATLAS9 (Kurucz 1992) assuming local thermodynamical equilibrium, hydrostatic equilibrium and radiative equilibrium. Synthetic spectra were then computed using SYNSPEC49 (Hubeny & Lanz 1992) for different abundances and convolved with the appropriate ${v}_{{\rm{e}}}\,\sin \,i$ = 20 km s⁻¹  of HR 7664 taken from Abt et al. (2002) and with the FWHM of NeoNarval. The most recent linelist gfall08Oct17.dat retrieved fom B. Kurucz's site ⁶ is used in this work (Kurucz 2018).

The lines synthesized here are the high excitation lines λ 6024.18 Å, 6034.04 Å and 6043.13 Å of P ii which are unblended in the NeoNarval Stokes I spectra of HR 7664. The synthesis of the P ii lines of HR 7664 is shown in Figure 1 for an overabundance of phosphorus of about 60 times the solar value which provides the best fit. This overabundance was obtained by iteratively changing the abundance of phosphorus in the computation of the synthetic spectrum until the observed line profiles were properly reproduced. The overabundance of phosphorus is quite large in the atmosphere of HR 7664 but is less than the overabundances found in the slightly cooler HgMn stars HD 53929 and HD 63975 (Ndiaye et al. 2018). In the course of this study, we also derived the abundances of other chemical elements (quoted as a multiplying factor of the solar abundance): magnesium (0.10 times solar), silicon (1.50 solar), chromium (2.7 solar), manganese (20 solar), iron (2.5 solar), gallium (250 solar) and praseodymium (50 solar). The comparison of spectra taken at different epochs also shows large radial velocity variations. More observations of HR 7664 are planed over the next months in order to measure other elemental abundances, monitor the radial velocity variations and search for putative inhomogeneous distribution of elements (chemical spots) over the surface of HR 7664.

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