Actinides in HD 101065 (Przybylski’s Star)

Author:  Cowley, C. R.; Hubrig, S.; Bord, D. J.

American Astronomical Society Meeting 202, #32.09; Bulletin of the American Astronomical Society, Vol. 35, p.744

There are many strange things about the abundances in Przybylski’s star (HD 101065). The most recent study (MN, 217, 299, 2000) finds that among the elements through copper, the abundances scatter with no apparent pattern. The largest deviations from solar are found for magnesium, which may be deficient by somewhat more than 1 dex, and cobalt, which appears to be in excess by about the same amount.The heavier elements especially those beyond barium, and continuing through the actinides uranium and thorium, show a clear pattern and a remarkable przybylskis-starcoherence. We plot solar and stellar abundances on a logarithmic plot vs. atomic number Z, and displace the solar points upward by some 3 dex. While individual points do not overlap, there is a remarkable similarity in the overall trends from Z = 58-80, including the maxima caused in the solar case by the third r-process peak. Points for the displaced solar actinides fall near their stellar counterparts.

The processes that produced the large enhancements of these heavy elements have not caused large fractionations of adjacent elements.

We have new spectra from the ESO UVES spectrograph on UT2. Resolution is 80,000 (blue) and 110,000 (red), with S/N > 300. This material, and new oscillator strengths (AA 381, 1090,2002; AA 382, 368, 2002), allow a more complete analysis of U and Th. We find abundances near +2.5 for both elements (log(H) = 12), but uncertainties at the moment are surely +/- 0.3 dex. We are currently working to improve the accuracy. Current errors preclude the use of these observations for accurate cosmochronometry, even if the chemical differentiation were not relevant.

If a U/Th ratio of unity could be taken at face value, it would imply an r-process event some 103 years ago. Thanks to B. Pfeiffer help and advice.

Interpretation

It is generally accepted that chemical anomalies in main sequence stars are fractionations due to radiative and gravitational forces. This is a plausible explanation for HD 101065. We see for the heavier elements a displaced solar abundance pattern. This holds more or less from barium through uranium (but not for the lighter elements). We must assume that the excesses, which are of the order of 1000 to 10 000 times the solar values occurred more or less uniformly. There appear to be no heavy elements that are underabundant, which can occur for some HgMn stars. Thus, if the current U/Th ratio was originally solar, we assume each element was enhanced by roughly a factor of 600.

On the other hand, if the abundances of the heavy elements are due to nuclear processing, then the r-process must be invoked, since uranium and thorium are not produced in the s-process.

Neither Tables 3 nor 4 have entries of U/Th with the value 0.44, which we for HD 101065. But numerous subjective choices led to this result. Had we chosen to weight the “best” Th II line somewhat less–say 1/3 rather than 1/2 the average of the 12 lines in Table 5, we would have a ratio of 0.36, in reasonable agreement with the last entry of Table 3 (Model No. 1).

Model 2 would accommodate the ratio 0.44 for a supernova event 2 to 4 billion years ago (see Table 4). This would be reasonable for a star of mass 1.4 solar masses (F5). Przybylski would be happy with such a mass and the corresponding temperature (ca. 6300K). These stellar parameters conflict with the (subjective?) judgements of other workers, who believe the star is hotter than the 6600K value of Teff used by CRKBMB. They also conflict with the photometric estimate by North for the age of the star.

A resolution of this might be obtained if reliable information could be obtained for the 235U/238U ratio. Thus a further investigation is indicated.

 So anyways, I thought it was cool that our family has a star with the same name… Przybylski’s star. Google Your name and see what you find. Here is what I Get When I Google: Ed Przybylski

Thanks for reading.

Ed

 

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