Andrew D. Vanture

Abundance Patterns of the S Stars in ω Centauri

The abundance of heavy elements has been determined for seven stars in the Galactic globular cluster ω Cen (NGC 5139). This sample includes five stars classified as S stars by Lloyd Evans, including V6, V17, and ROA 320, which are all variable stars and among the coolest and most luminous stars in the cluster. This study increases the number of S stars in ω Cen for which high-quality abundance analyses exist to eight. The abundance of post-iron-peak elements supports the hypothesis that the spread in metallicities and heavy-element abundances observed among stars in the cluster is due to chemical self-enrichment of ω Cen by supernovae and low-mass asymptotic giant branch (AGB) stars. Furthermore, the post-iron-peak elements suggest that the self-enrichment of the cluster was complex and nonhomogeneous with respect to the s-process elements.

The lithium content and other properties of F2-G5 giants in the Hertzsprung Gap

As stars of 2-5 solar mass evolve across the Hertzsprung Gap they should first deplete their surface lithium by convective dilution and then, when convection penetrates deeper, begin to bring CN processed material to their surfaces. To investigate this process we have observed 52 giants, 25 of which have known C/N ratios, for their Li abundances. After eliminating four stars that may actually be dwarfs and including the two components of Capella analyzed by Pilachowski and Sowell we have compared our (Li/Fe) ratios with models of Swenson. For stars showing v sin i greater than 50 km/s we find (Li/Fe) to be uneffected by mixing for B - V less than 0.7 as predicted. For stars cooler than B - V = 0.7 both v sin i and (Li/Fe) drop to smaller values. For the sharp lined stars (v sin i less than 50 km/s) we find a drop in Li between B - V = 0.45 and 0.60 which cannot be understood in terms of dilution by convection. Various possible causes of such an early depletion or dilution of surface Li are discussed including diffusion at the base of the convection zone, mass loss possibly enhanced by pulsation, and magnetic activity as in the magnetic A and B type stars. The models of Richer & Michaud (1993) with diffusion point toward a satisfactory solution. A few giants with low v sin i values stand out with much higher than expected (Li/Fe) values despite their cool effective temperatures. We do not understand why those stars have not depleted their lithium as have most giants of similar color. The correlation of (N/C) with (Li/Fe) follows expectations in so far as almost all stars with enhanced (N/C) have depleted their Li as well.

ABUNDANCES IN THREE HEAVY-ELEMENT STARS IN OMEGA CENTAURI

The abundances of the iron-peak elements Fe, Ni and Ti, the light metals Mg, Al and K and the s-process elements Rb, Y and Zr are determined for the heavy-element stars ROA 371, ROA 5293 and ROA 3812 in the globular cluster omega Centauri. ROA 3812 and ROA 5293 are classified as S stars while ROA 371 is classified as a K5 barium star. The metallicities of ROA 3812, 5293 and 371 are [Fe/H] = -0.7, -0.8 and -1.0 respectively. Thus, ROA 371 has a metallicity slightly higher than the red giants analyzed by Brown et al., 1991 and ROA 3812 and 5293 have metallicities near the upper end of the range for the cluster. All three stars show an excess of Al, which is common in red giants of omega Cen, and a mean excess of the s-process elements Rb, Y, and Zr of 1.4 dex. In addition, all three stars are too faint to by asymptotic giant branch (AGB) stars according to current theory. Hence, they were either formed with their present composition, as suggested by Lloyd Evans (1983a) or are binaries that have had their atmospheres polluted by a now defunct companion. Based upon the dependence of [s/Fe] on [Fe/H] for various s-process elements over a range from [Fe/H] = -2.0 to [Fe/H] = -0.7, we show that the Lloyd Evans hypothesis is probably correct. This shows that mass-loss from AGB stars was contributing s-process elements to the intracluster gas for as much as 109 years before Type Ia SNe swept the cluster clear of gas and terminated star-formation.

Zirconium to Titanium Ratios in a Large Sample of Galactic S Stars

The [Zr/Ti] ratio for a large sample of Galactic S stars has been determined using high-quality, high-resolution spectra. The pattern of Zr enhancements in intrinsic and extrinsic S stars is found to differ, and the [Zr/Ti] ratio in the extrinsic S stars clearly links them to the strong barium stars. In addition, the pattern of [Zr/Ti] ratios seems to indicate that the progression of spectral type M to MS to S to SC is due largely to an increase in the abundance of s-process elements and not solely to a changing C/O ratio as claimed by some investigators (such as Scalo and Ross).

The Metallicity and Lithium Abundances of the Recurring Novae T CrB and RS Oph

We report on high-resolution spectra of the two recurring novae, T CrB and RS Oph, obtained in 2004 when no outbursts were in progress. Selected regions of the spectra between 6500 and 8800 A were measured for equivalent widths and analyzed for metallicity. Lines of Fe I, Ni I, Si I, and Ti I were used to establish the effective temperature. The metallicity as derived using models is near solar with an uncertainty estimated to be near a factor of 2 for both stars. Both stars show a strong lithium line at 6707.8 A. Approximate Li abundances were derived using model atmospheres for a direct comparison with the nearby Fe I line at 6710.31 A and the Ca I ground level line at 6572.78 A. The Li abundances are near log N(Li) = 1.2 for RS Oph and 0.8 for T CrB on the scale of log N(H) = 12.0. Such Li abundances are high for single K and M giants. A survey of symbiotic stars with cool components of types K and M showed no recognizable Li line in 28 stars with high quality spectra. This makes the two repeating novae different from the other symbiotics that consist of a red giant and a white dwarf.

A Reinvestigation of the Possible Metallicity Spread in NGC 3201

We have conducted a reanalysis of the metallicity of six red giants within NGC 3201 on the basis of a new homogeneous set of spectra with resolution ~30,000. Iron line strengths and the MOOG software suite were used to derive metallicities based on spectroscopically and photometrically estimated values of Teff and log g. We cannot confirm the presence of a significant spread in metallicity within the cluster greater than about 0.3 dex nor any significant correlation between metallicity and temperature, as first suggested by Gonzalez & Wallerstein. We call attention to the possibility that differences in boundary temperature for stars of the same Teff may lead to apparent differences in metallicity.

A Search for r-Process Elements in the VELA Supernova Remnant

Author(s): Wallerstein, G; Vanture, AD; Jenkins, EB; Fuller, GM | Abstract: After a description of recent developments in the physics of rapid neutron capture in Type II supernovae and a discussion of the detectability of supernovae ejecta, we present the data from a search for Ge, Kr, Yb, Os, and Hg in five stars behind or within the Vela remnant. Only Ge II was detected, but its column density and the upper limits of the other species show no excess above the estimated contribution of ambient gas. Finally, we discuss the extent that clumping and the improved performance of the GHRS may make r-process ejecta from Vela detectable in the future.

Correlations between Lithium and Technetium Absorption Lines in the Spectra of Galactic S Stars

Correlations between the presence of the 6707 A line of lithium and the resonance lines of technetium (4238 and 4262 A) in a large sample of Galactic S stars are analyzed. Half of the sample stars are intrinsic S stars (those exhibiting technetium in their spectra), and 1/3 of these stars also have strong lithium lines in their spectra. Stars having both lithium and technetium in their spectra are interpreted as intermediate-mass thermally pulsating asymptotic giant branch (TP-AGB) stars in which lithium is produced by the Cameron-Fowler mechanism. The production of lithium is predicted to occur in high-luminosity (Mbol ≤ −6) TP-AGB stars by the hot-bottom burning (HBB) mechanism. Data on the carbon isotope ratios of stars in our sample agree with the predictions of HBB; however, oxygen isotope ratios in these stars do not agree with the predictions of HBB. Furthermore, the available luminosities for our sample stars are below the minimum value necessary for HBB to occur in available models. Cool-bottom processing (CBP) is one possible explanation for the presence of lithium in the spectra of these stars. Intrinsic S stars having technetium but no lithium in their spectra are interpreted as lower mass (1.5-3 M⊙) thermally pulsating AGB stars that have not undergone CBP. Extrinsic S stars constitute the remaining half of the sample. Carbon and oxygen isotope ratios, as well as the lack of technetium and lithium in the spectra of these stars, are consistent with these being low-mass red giant branch stars (1-2 M⊙), with mass transfer from a now extinct thermally pulsating AGB star being responsible for the enhanced abundance of s-process elements.

Abundances of Tc and related elements in stars of type M, MS, and S

Abundances of Co, Zr, Nb, and Tc relative to V have been derived for a sample of ten M and MS stars from 0.10 A pixel −1 and 0.04 A pixel −1 Reticon spectra. On a scale with log N H = 12.0 and log N V = 4.0, we find log N Tc ∼ 1.4 for M stars and 1.0 for MS stars with Tc. It has been suggested that the presence of Tc in M stars which show no enhancements of other s-process elements can be explained by a mini s-process. Detailed calculations and the observed abundances of Tc and related elements presented in this study put this interpretation in doubt.

AN ABUNDANCE ANALYSIS OF A GLOBULAR-CLUSTER CH STAR: M22 III-106

An analysis of the CNO and s-process abundances in the M22 star III-106 has been completed. This star has been identified on the basis of DDO photometry by Hesser, Hartwick, and McClure, and on the basis of a low-resolution spectrum by McClure and Norris as a CH star. We find the star to have C = 7.1, N = 7.65,.