Oct 152014

by Van Dixon, Scientist at STScI

Optical color-magnitude diagrams of Galactic globular clusters generally include one or two UV-bright stars, objects that are brighter than the horizontal branch and bluer than the red-giant branch.  When astronomers first observed these luminous, blue stars, they were a bit confused: What are young, massive stars doing in globular clusters?  The puzzle was solved with the realization that UV-bright stars are not main-sequence stars, but pre-white dwarfs.  Objects in transition, they are evolving from the asymptotic giant branch (or directly from the blue horizontal branch) to the hot tip of the white-dwarf cooling curve.  Because this phase of stellar evolution is short, lasting 103 to 105 years, these objects are rare; only a few dozen are known.

UV-bright stars in globular clusters can be used to address a number of questions. Their high temperatures (generally 10,000 to 100,000 K) make them ideal targets for far-ultraviolet spectroscopy, a powerful tool for measuring chemical abundances, particularly of species that are unobservable in cool, red-giant-branch (RGB) stars.  To first order, globular cluster stars have identical ages and initial chemical compositions, so any differences between a UV-bright star’s chemistry and that of its RGB siblings provide important constraints on theories of post-RGB evolution.  Astronomers studying the interstellar medium use UV-bright stars as background sources (with known distances!) for absorption-line studies of the ISM.

One of the most famous (and best-named) UV-bright stars is von Zeipel 1128 in the globular cluster M3 (hereafter vZ 1128; von Zeipel 1908, Strom & Strom 1970). Pierre Chayer (STScI) and I have analyzed archival FUSE, HST/STIS, and Keck HIRES spectra of this remarkable object.  By fitting the H I, He I, and He II lines in its optical spectrum with non-LTE models, we derive an effective temperature Teff = 36,000 K, a gravity log g = 3.95, and a helium abundance N(He)/N(H) = 0.15.  By comparing absorption features in the star’s FUSE and STIS spectra with a set of synthetic spectra, we can determine its photospheric abundances of C, N, O, Al, Si, P, S, Fe, and Ni.  No features from elements beyond the iron peak are observed.

Dixon_fig1Figure 1.  Comparison of the abundances derived for vZ 1128 (filled symbols) with those of the solar photosphere (short horizontal lines) and of RGB stars in M3 (rectangles).  Stellar abundances derived without an additional source of line broadening are plotted as circles, those allowing for stellar rotation are plotted as triangles, and those allowing for microturbulence are plotted as squares.

In Figure 1, the measured abundances of vZ 1128 (solid symbols) are compared with those of the sun (short vertical lines; Asplund et al. 2009) and the RGB stars in M3 (rectangles).  Cluster C and N abundances are from Smith et al. (1996), and the O, Al, Si, Fe, and Ni values are from Sneden et al. (2004).  The vertical extent of each rectangle represents the star-to-star scatter in the measured abundance (±1σ about the mean).  Beginning with the most massive elements, we see that the abundances of Si, Fe, and Ni are nearly constant along the RGB.  The scatter is much larger for CNO and Al, reflecting well-known abundance variations in globular-cluster giants (Kraft 1994).  For all of these elements, the measured abundances of vZ 1128 are consistent with those of the RGB stars.


Figure 2.  Abundance ratios of red giants in the globular clusters M3 (triangles) and M13 (squares).  CN-rich stars are plotted as solid symbols, CN-poor stars as open symbols.  The abundances of C and O are correlated, N and O are anticorrelated, and the total abundance of (C+N+O) is essentially constant, consistent with the products of CNO-cycle processing.  The abundance ratios of vZ 1128 in M3 (circles) are consistent with the patterns seen in the RGB stars.

In Figure 2, we reproduce a plot from Smith et al. (1996), who studied variations in the CNO abundances of RGB stars in M3 and M13, which have similar metallicities.  They found that the abundances of C and O are correlated, the N abundance is anticorrelated with both C and O, and the total abundance C+N+O is nearly constant.  These patterns can be explained as the result of CNO-cycle hydrogen burning on the RGB, which converts carbon (rapidly) and oxygen (slowly) into nitrogen, but leaves the total C+N+O abundance unchanged.  Some form of mixing brings this CNO-processed material to the surface (Gratton et al. 2000); as the star ascends the RGB, it moves from right to left in Figure 2.  Comparing abundances derived from non-LTE models of an O-type star observed in the FUV with those derived from LTE models of K-type giants observed in the optical is dangerous; nevertheless, we have added vZ 1128 to Figure 2.  Though its carbon abundance is a bit high, the star’s CNO abundances follow the trends seen in the cluster’s RGB stars remarkably well.

The effective temperature and luminosity of vZ 1128 place it on the 0.546 M post-AGB evolutionary track of Schönberner (1983).  This track traces the evolution of a star that leaves the AGB before the onset of thermal pulsing and the attendant churning of its envelope known as third dredge-up.  Such objects are called post-early AGB (post-EAGB) stars.  This scenario is consistent with the abundance pattern seen in the star’s photosphere: its carbon abundance is not enhanced, nor are any s-process elements detected.  We conclude that vZ 1128 has not undergone third dredge-up.  Indeed, it appears that no significant changes in its photospheric composition have occurred since the star left the RGB.

That vZ 1128 is not a bona fide post-AGB star is not really a surprise. Post-AGB stars evolve quickly, remaining luminous for only 103–104 years, while post-EAGB stars remain luminous for 104–105 years (Schönberner 1981, 1983). Of the roughly one dozen UV-bright stars in globular clusters whose spectra have been analyzed to date (see Moehler 2010 for a review), only two show the enhanced carbon abundance expected of a star that underwent third dredge-up. The first, K648 in M15 (Rauch et al. 2002), hosts a planetary nebula, so most certainly evolved to the tip of the AGB. The second, ZNG 1 in M5, lacks a nebula, and its high helium abundance and high rotational velocity suggest an unusual evolutionary history (Dixon et al. 2004). The dearth of carbon-rich post-AGB stars in galactic globular clusters is consistent with the short lifetimes of these rare objects.




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