The Abundances of the Noble Gases Neon and Xenon in the Atmosphere of 53 Tau (HD 27295)

53 Tau (HD 27295, V = 5.5) is a bright late B-type chemically peculiar stars. Abt & Morrell (1995) classified 53 Tau as B9 Mn confirming that this star harbors an overabundance of manganese compared to normal late B-type stars. The spectra of 53 Tau do not show the Hg ii line at 3983.93 Å usually observed in the spectra of HgMn stars, accordingly this star is known as a Mn star rather than a HgMn star. 53 Tau is also characterized by a large excess in xenon (about 2240 times the solar abundance) as shown by Dworetsky et al. (2008). The FUV flux of 53 Tau as observed by IUE did not vary whereas its TESS lightcurve ¹ (Ricker et al. 2015) shows periodic low amplitude variations with a period close to 4.43 days (Monier 2022). Smith & Dworetsky (1993) have derived the abundances of several chemical elements in 53 Tau using the available International Ultraviolet Explorer spectra at that time.

The purpose of this note is to measure the neon and xenon abundances in 53 Tau using a high resolution and high signal-to-noise ratio optical spectrum and updated atomic data. Few HgMn stars actually have measurements of their abundances of neon and xenon. Dworetsky & Budaj (2000) have measured non-local thermodynamical equilibrium abundances of neon in seven normal late B-type stars and 20 HgMn stars using several Ne i lines. They found that the mean neon abundance in normal late B-type stars agrees well with the standard (solar) abundance of neon. However, neon is underabundant in all HgMn stars with uderabundances ranging from 0.1 dex to an order of magnitude or more. Dworetsky et al. (2008) have derived the xenon abundances in a sample of 21 HgMn stars. They found that xenon usually is largely overabundant in these stars by factors ranging from 3.0 to 5.0 dex.

A spectrum acquired with Ultraviolet and Visual Echelle Spectrograph (UVES) was retrieved from the ESO spectral archive site. ² This spectrum stretches from 3300 up to 11000 Å and has a signal-to-noise ratio is about 400. The resolving power is 42000. The instrument paper for UVES is Dekker et al. (2000). The program ID is 076.B-0055(A) and the PI is Dr. Silva.

Using Napiwotzky's UVBYBETA code (Napiwotzki et al. 1993) and the Strömgren photometry of 53 Tau taken from Hauck & Mermilliod (1998), I have derived the effective temperature, T_eff = 12,000 ± 200 K and the logarithm of the surface gravity, $\mathrm{log}\ g$ = 4.29 ± 0.20 dex. A model atmosphere was then computed using ATLAS9 (Kurucz 1992) assuming radiative, local thermodynamical and hydrostatic equilibrium for the effective temperature and surface gravity of 53 Tau. The atomic data for the various lines studied are the same as those adopted by Dworetsky & Budaj (2000) for Ne i and Dworetsky et al. (2008) for Xe ii.

The adjustment of a series of synthetic spectra conputed with SYNSPEC49 (Hubeny & Lanz 1992) to the observed spectrum yields new abundances for neon and xenon. The line of Ne i at 6402.05 Å is not present in the UVES spectrum, whereas spectrum synthesis shows that it should absorb about 5% of the continuum for a solar abundance (see the left panel of Figure 1). Decreasing the neon abundance to about 10⁻³ the solar abundance, produces no absorption in the synthetic spectrum which allows to set an upper limit to the neon abundance. Three lines of Xe ii, located at 4603.03, 4844.33 and 5292.22 Å have been modeled. The fit to the strongest line of Xe ii at 5292.22 Å is achieved for an overabundance of about 3000 times the solar value (see right panel of Figure 1).

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The conclusions of this short study is that the neon abundance in 53 Tau must be very low, with an upper limit of about 10⁻³ the solar abundance. In contrast, xenon is overabundant by a factor of about 3000, which is in good agreement with the abundance found by Dworetsky et al. (2008). The underabundance of neon is expected from radiative acceleration calculations which show that neon has a very small radiative acceleration in the photosphere. I am not aware of radiative acceleration computations for Xe ii for late B-type stars.