Abundance of palladium group elements in the atmospheres of evolved stars. III. Rhodium

Abstract

We have previously determined the abundance of molybdenum (27 = 42) and ruthenium (Z = 44) in the atmospheres of selected evolved stars [1, 2]. It is very difficult to determine the abundance of rhodium (Z = 45) in stellar atmospheres because of significant blending of its usually weak lines and the lack of reliable oscillator strengths. We know of only one attempt to evaluate the abundance of this element. A rhodium abundance in the atmosphere of a Tau (K5 III) of [Rh]~ Tau = -t-0.9 was found in [2] from a single Rhl line by the model atmosphere method (judging from the wavelength range, this line was 25686.360 A; the oscillator strength was taken from the list of Kurucz and Peytremann [4]). We have undertaken a search of rhodium lines suitable for analysis from the well-known atlases of stellar spectra with sufficiently high spectral resolution; synthetic spectra in the vicinity of the rhodium lines were constructed. It was found that there are no suitable lines in the spectral intervals encompassed by these atlases. Thus, we decided to make use of the equivalent widths obtained by Vetesnik [5] for nine RhI lines in the spectrum of fl Peg (M2.5 II-II)). He used the blend separation method to measure them from spectrograms with dispersion 4.1/~/mm. We assumed the following parameters of the model atmosphere of fl Peg [6]: effective temperature Tel f = 3600 K, acceleration of gravity at the surface logg = 1.2, and microturbulent velocity V t = 2.0 km/sec, which are based on a detailed quantitative analysis. A model atmosphere of the star was calculated from these parameters with the Tartu version of the SAM1 program [7]~ The oscillator strengths of the RhI lines were taken from the list of Kurucz and Peytremann [4]. The equivalent widths of the lines under investigation were calculated for this model. The rhodium abundance could be determined by comparison with observations of the equivalent widths. The results are given in Table 1. Although the equivalent widths of RhI lines measured in [5] differ in accuracy (mainly 20-50%), we have derived an average value (from nine lines) for the rhodium abundance in the atmosphere offl Peg of N(Rh) = 0.7 dex on the scale N(H) = 12.0 alex. The value N(Rh)| = 1.12 +_ 0.12 dex has been found for the solar atmosphere [8]. Consequently, the relative abundance of rhodium in the atmosphere of fl Peg [Rh]~ Peg = --0.4, There are grounds to assert that this value is understated. Let us examine the main sources of error. Besides the oscillator strengths of RhI lines given in [4] (they match the data of Corliss and Boseman [91), the recent determinations of Gurtovenko and Kostyk [10] are known. We have found from five lines in common