Isotopic Pt II Abundances in the HgMn Star HR 7775

D. A. Bohlender

National Research Council of Canada
Herzberg Institute of Astrophysics
5071 West Saanich Road, Victoria, BC, Canada V8X 4M6
Electronic-mail: David.Bohlender@hia.nrc.ca

and

M. M. Dworetsky and, C. M. Jomaron

University College London
Gower Street, London, United Kingdom WC1E 6BT
Electronic-mail: mmd@star.ucl.ac.uk and cmj@star.ucl.ac.uk

Abstract:

High-resolution spectra have been obtained for the regions of the five strongest optical lines of Pt II in the spectrum of the cool HgMn star HR 7775, which is one of the sharpest-lined HgMn stars known. Model lines have been constructed from isotopic and hyperfine structure laboratory analyses and abundances of the individual isotopes have been determined from spectrum synthesis. The total abundance of Pt is 4.46 dex greater than the adopted solar abundance. The isotopic composition is clearly non-terrestrial, with a pronounced relative enhancement of the heaviest isotope, ¹⁹⁸Pt, and deficiencies of the isotopes lighter than ¹⁹⁶Pt. The pattern of isotopic composition does not follow the widely-assumed fractionation formalism; the lighter isotopes are far more deficient than a single-parameter fractionation pattern would predict.

Introduction

  The spectral peculiarities of the HgMn star HR 7775 (HD 193452 = $\beta^1$ Cap, V=6.10) were first noted by Preston, as reported by Dworetsky (1976), who described it as having v sin i < 2 km/s with strong lines of Hg II, Pt II, Ga II and Ga II, Y II and Sr II. Cowley & Aikman (1979) reported the presence of Au I and Au II; Jacobs & Dworetsky (1982) found Bi II in IUE spectra, which was confirmed by the optical analysis of Guthrie (1984), who also found evidence for the rare earth spectra of Pr II and Nd II. Recently, Adelman (1994) presented an abundance analysis based on DAO 2.4 Åmm^-1 spectrograms, in which most of these anomalies are confirmed. The general impression is that HR 7775 is the most extreme cool HgMn star yet discovered, with very strong Pt and Au lines, the strongest known Bi abundance enhancement, and a strong Ga enhancement usually found only in much hotter HgMn stars.

Observations and Models

  Spectra of HR 7775 were obtained 1995 September 8-10 UT at the CFHT with the high-resolution coudé spectrograph (``Gecko''). Four wavelength windows centered at 3984, 4034, 4055, and 4514Å were observed with the thin, backside illuminated Orbit1 2048 x 2048 CCD. The combination of 15m pixels and the 316 l/mm echelle grating produced spectra with resolutions varying from approximately 100,000 to 115,000. After complete processing and final continuum rectification the S/N of a typical 20 min exposure is approximately 400.

The models for the lines are based on isotopic and hyperfine structure measurements from the literature (Englemman 1989) and on gf-values from Dworetsky et al (1984) and Dworetsky (1980). Line profiles were calculated with the spectrum-synthesis code UCLSYN (Smith 1992).

Analysis and Results

  A representative example of the fit of our model to the observations is shown in Figure 1, while Table 1 gives a summary of the inferred abundances of the individual isotopes, and the total measured equivalent width and abundance of Pt summed over isotopes for each line. The abundances of the individual isotopes in the solar system are also shown for comparison.

In Table 2 we derive mean isotope ratios in HR 7775. We define the quantities $\alpha$ and q in a manner analogous to the definitions for Hg given by White et al. (1976):

These dimensionless fractionation parameters have been used by White et al. (1976) and others to investigate the relationship between the Hg II $\lambda$3984 centroid wavelength and parameters like .

 

Figure 1: Observed (histograms) and synthesized (continuous lines) profiles of the Pt II $\lambda$4046 line in HR 7775. The wavelengths of the even isotopes of Pt II are shown with long tick marks; the heaviest isotope (¹⁹⁸Pt) has the longest wavelength in each case, while isotopes 196, 194 and 192 are positioned successively at shorter wavelengths. The short tick marks A and B are the strongest components of the hyperfine structure of the odd isotope ¹⁹⁵Pt. The most important identified blends are also indicated.  

The usual practice in past work has been to adopt this formalism of fractionation, i.e., that for every isotopic mixture a single parameter q can be used to characterize the way in which a star's isotopic composition of Hg (or Pt) differs from solar system material. Clearly, Table 2 shows that this is not the case for Pt II in HR 7775. Smith (1997), using Lick Hamilton Échelle spectra obtained by MMD, has also pointed out that the Hg isotopic composition in HR 7775 does not follow the q formalism, in that the lighter isotopes are deficient relative to the model with q deduced from fits to the two heaviest isotopes. We see much the same pattern in the Pt isotopic abundances in HR 7775; if the value q = 0.67 found for isotopes 198 and 196 were used to predict the abundances of isotopes 195 and 194, ¹⁹⁵Pt would be 0.37 dex more abundant than observed, and the normally abundant isotope ¹⁹⁴Pt would be 1.31 dex more abundant than observed here.

Conclusion

  High-resolution spectra of five optical lines of Pt II have been obtained for the cool HgMn star HR 7775. The abundance of Pt is found to be enhanced by a huge factor, 4.46 dex, over its terrestrial value. The isotopic composition of Pt is markedly non-terrestrial, with enhancement of the heaviest isotope, ¹⁹⁸Pt, and deficiencies of the lighter isotopes. The relative abundances do not satisfy the fractionation formalism of White et al. (1976) in that the light isotopes are far more deficient than that model would predict.