G. E. Langer

SODIUM-OXYGEN ABUNDANCE ANTICORRELATIONS AND DEEP MIXING SCENARIOS FOR GLOBULAR CLUSTER GIANTS

We explore and extend the suggstion that 23Na might be produced in the interior of an evolving low-mass globular cluster giant and mixed to the surface (Kudryashov & Tutukov 1988, Denisenkov & Denisenkova 1990). The reaction rates alone show that sodium will be produced by proton captures on 22Ne in the region of a globulr cluster giant in which oxygen is being depleted. A more detailed calculation shows that significant amounts of the much more abundant 20Ne can be transformed into 23Na on somewhat longer timescales. Atmospheric oxygen depletions that are the result of very deep mixing are bound to be accompanied by sodium enhancements due to 22Ne(p,gamma)23Na reactions; they are quite likely to be accopanied by larger sodium enhancements that begin with proton captures on 20Ne. One cannot argue that a Na-O anticorrelation is strong evidence for a primordial account of the observed variations in O and Na. We find that 27Al can also be produced by very deep mixing and that the aluminum enhancements that would result are likely to be smaller than the sodium enhancements. Finally, we suggest a number of ways in which the consequences of this sort of very deep mixing might be distinguished from primordial variations in oxygen, sodium and aluminum.

Oxygen abundances in halo giants. II : Giants in the globular clusters M13 and M3 and the intermediately metal-poor halo field

Oxygen, sodium, iron, vanadium, and scandium abundances are derived for giants in the intermediately metal-poor globular clusters M 3 and M 13 and for giants of comparable metallicity in the local halo field. The data are obtained from Lick Observatory Hamilton Echelle spectra centered on the [O I] doublet. For M13 we derive = -1.51 ± 0.01 from 13 giants with a range of 500 K in effective temperature. For M3, we find = -1.47 ± 0,01, from seven stars with a range of 300 K in effective temperature. There is no compelling evidence for star-to-star variations in [Fe/H] in either cluster

Oxygen abundances in halo giants. I : giants in the very metal-poor globular clusters M92 and M15 and the metal-poor halo field

We derive oxygen, iron, vanadium, and scandium abundances for very metal-poor giants in the globular clusters M92 and M15, and giants of comparable metallicity in the local halo field. We analyze the [O I] doublet (λλ 6300, 6363) and nearby metallic lines in spectra obtained with a TI CCD detector and the Hamilton echelle spectrograph at the Lick Observatory 3.0 m telescope, using line analysis and spectral synthesis codes developed at the University of Texas.

Oxygen abundances in halo giants. III : Giants in the mildly metal-poor globular cluster M5

We derive abundances of oxygen, sodium, iron-peak, and several traditional «α-elements» for giants in the mildly metal-poor globular cluster M5. The data are obtained from Lick Observatory Hamilton Echelle spectra centered on the [O I] doublet. We derive =−1.17±0.01 from 13 giants with a range of 350 K in effective temperature. There is no evidence for star-to-star variations in [Fe/H] in this cluster. The range of oxygen abundances is similar to that found earlier in M3 and M92. The largest values of [O/Fe] are near +0.35 and the smallest are near −0.25

Oxygen abundances in halo giants. IV: The oxygen-sodium anticorrelation in a sample of 22 bright giants in M13

We have analyzed high resolution echelle spectra of nine bright M13 giants which, when added to those we have analyzed previously by similar techniques, bring our M13 sample size to 22. The sample is 88% complete from the red giant tip (at M bol 0 =−3.6) to a point one bolometric magnitude fainter, and is presumably representative down to M bol 0 =−2.1. We find that the brightest M13 giants are predominantly super oxygen poor ([O/Fe]∼−0.4 to −0. 8) and correspondingly sodium rich ([Na/Fe]∼+0.2 to +0.4). We argue that these super O-poor stars are first ascent giants. The most straightforward way to understand (1) the decline in the average oxygen abundance as M13 stars approach the tip of the giant branch and (2) their anticorrelated oxygen and sodium abundances, is that they are the results of very deep mixing

Variations in cold interstellar clouds on subparsec scales

High-resolution spectra in the region of the Na D lines were used to explore the structure of two cold interstellar clouds: One between the sun and the globular cluster M92 (ten target stars), the other between the sun and the globular cluster M15 (seven target stars). It is found that the neutral sodium column density varies by a factor of about 4 over distances less than about 0.2 pc in the M92 cloud. The column density variations are larger in the higher velocity cloud in front of M15, but the distances over which they occur are more uncertain. There is no obvious pattern to the density variations in either cloud. Evidence was found for velocity variations of about 6 km/s within the M15 cloud. 12 refs.

LAMBDA-3883 CN BAND STRENGTHS FOR 238 METAL-POOR HALO GIANTS: EVIDENCE FOR CHEMICAL DIFFERENCES BETWEEN GLOBULAR CLUSTER AND HALO FIELD GIANTS

Cyanogen band strengths mCN for 238 metal-poor giants in the halo field and in 13 globular clusters are tabulated. The distribution of mCN in field giants of intermediate metallicity is compared with that of giants of similar evolutionry state in M79, M3, and M13. The field star population is dominated by weak-CN stars. M79 has mostly giants with CN-bands of intermediate strength, M3 has relatively few giants with strong CN-bands, and M13 is dominated by strong-CN giants. These results add evidence in support of the notion that halo field stars and stars of most globular clusters differ chemically even at equivalent [Fe/H]. In the spectra of giant stars in the very metal-poor clusters M15 and M92, CN-bands tend to disappear, and become noticable only in stars having exceptionally large nitrogen abundances.

Carbon and nitrogen abundances in old, very metal-poor stars. A reprise based on the low-resolution Lick survey

A summary is given of the carbon and nitrogen abundances found in the Lick survey of nearly 300 old, very metal-poor stars (Fe/H of less than -1.3). The survey consists of population samples drawn from various stellar subsystems of the Galaxy as well as different stages of stellar evolution. The question of whether a systematic pattern of carbon depletion and nitrogen enhancement emerges within the data was examined and three conclusions were made: (1) the average value of (C + N)/Fe lies within narrow limits for all stellar subsystems, suggesting that the large nitrogen enhancement found in many evolved metal-poor stars is the result of interior C and N mixing; (2) the degree of carbon depletion and corresponding nitrogen enhancement is much larger than predicted by classical evolutionary theory; and (3) the extensive CN processing that occurs in evolved metal-poor stars proceeds further in some stellar subsystems than in others. This is particularly true of the most metal-poor halo field giants vis-a-vis giants in M15 and M92.

Is the oxygen-poor giant in M 13 nitrogen rich?

Observations of II-67, an oxygen-poor globular-cluster giant in M 13, are reported. Spectra were obtained at 310-470 nm with resolution 6 A FWHM and dispersion 2 A/pixel using a CCD detector and the Miller UV Schmidt Spectrograph on the 3-m Shane telescope at Lick Observatory on June 9-11, 1986. The data are presented in tables and graphs and analyzed, and it is shown (on the basis of comparisons of the 336-nm NH band strength with those of oxygen-normal stars in M 13) that II-67 has a normal nitrogen abundance.