Michael M. Briley

STELLAR POPULATION MODELS AND INDIVIDUAL ELEMENT ABUNDANCES. II. STELLAR SPECTRA AND INTEGRATED LIGHT MODELS

The first paper in this series explored the effects of altering the chemical mixture of the stellar population on an element-by-element basis on stellar evolutionary tracks and isochrones to the end of the red giant branch. This paper extends the discussion by incorporating the fully consistent synthetic stellar spectra with those isochrone models in predicting integrated colors, Lick indices, and synthetic spectra. Older populations display element ratio effects in their spectra at higher amplitude than younger populations. In addition, spectral effects in the photospheres of stars tend to dominate over effects from isochrone temperatures and lifetimes, but, further, the isochrone-based effects that are present tend to fall along the age-metallicity degeneracy vector, while the direct stellar spectral effects usually show considerable orthogonality.

C and N Abundances in Stars at the Base of the Red Giant Branch in M15

We present an analysis of a large sample of moderate-resolution Keck Low Resolution Imaging Spectrometer spectra of subgiants and stars at the base of the red giant branch (RGB) in the Galactic globular cluster (GC) M15 (NGC?7078), most within the range 16.5 < V < 19.5 (1.2 < MV < 4.2), with the goal of deriving C abundances (from the G band of CH) and N abundances (from the NH band at 3360 ?). Star-to-star stochastic variations with significant range in both [C/Fe] and [N/Fe] are found at all luminosities extending to the subgiants at MV ~ +3. The C and N abundances appear anticorrelated, as would be expected from the CN-cycle processing of stellar material. Yet these M15 stars are considerably fainter than the RGB bump, the point at which deep mixing is believed to set in. On this basis, while the observed abundance pattern is consistent with proton-capture nucleosynthesis, we infer that the site of the reactions is likely not within the present sample. The range of variation of the N abundances is very large, and the sum of C + N increases as C decreases. To reproduce this requires the incorporation not only of CN but also of ON-processed material. Combining our work with that of Trefzger and coworkers for the brighter giants in M15, we find strong evidence for additional depletion of C among the most luminous giants. This presumably represents the first dredge-up (with enhanced deep mixing) expected for such luminous cluster RGB stars in the course of normal stellar evolution as they cross the RGB bump. We compare the behavior of these patterns for C and N in GCs covering a wide range of metallicity and current mass. While all clusters studied show strong anticorrelated variations of C and N at all luminosities probed, the metal-rich clusters (M71, 47 Tuc, and M5) do not show evidence for the first dredge-up among their most luminous giants, while the metal-poor ones (M13, M92, and M15, plus M5) do. Conversely, the metal-poor clusters do not show evidence for the bimodality in CH and CN line strengths seen in the metal-rich clusters. The collected data on C and N abundances in low-luminosity GC stars cannot be explained by the commonly invoked models for the chemical evolution of GC stars; in particular, pollution of existing low-mass stars by ejecta from intermediate-mass asymptotic giant branch (AGB) stars can be ruled out. Pollution of cluster gas by such stars prior to the formation of the lower mass stars we observe today can also be ruled out unless current models of nucleosynthesis and dredge-up into the surface layers of AGB stars are flawed; such models agree qualitatively but disagree quantitatively with our data. We are forced to assume that there was an extended period of star formation in GCs, and that a previous generation of more massive stars evolved, ejected mass, and polluted the GC gas with light elements; the low-mass stars we see today formed afterward. A tentative scenario is developed involving an initial phase of star formation heavily biased toward high-mass stars, with subsequent formation of intermediate-mass, then low-mass stars.

Carbon Abundances of M92 Red Giant Branch Stars

Using CCD imaging, Fusi Pecci et al. have identified a local maximum at MV = -0.4 in the giant branch luminosity function of three co-added globular clusters having metallicities [Fe/H] ~ -2.2, including M92 (NGC 6341). Theories of deep mixing predict that surface carbon abundance depletions should be produced only within stars brighter than this luminosity function peak. Only in such red giants should the circulation-inhibiting molecular weight discontinuity between the base of the convective envelope and the hydrogen-burning shell be absent. However, spectroscopic analyses of M92 giants by Carbon et al. and Langer et al. indicate that surface carbon depletions may set in as faint as MV > 2.0. In order to further test this potential discord between theory and observations, KPNO 4 m telescope Hydra spectra were obtained of a sample of 50 red giants in M92 covering the absolute magnitude range -2.4 < MV < 1.4. Carbon abundances were determined by comparing a G-band index measured for the program stars to index values computed from synthetic stellar spectra. Analysis of these index values confirms a pattern of decreasing carbon abundance as a function of increasing evolutionary state among cluster members, in accord with the results of Carbon et al. and Langer et al. Our analysis indicates that carbon depletion sets in at absolute magnitudes at least as faint as MV =0.5–1.0, well below that of the luminosity function peak at MV = -0.4 for metal-poor clusters.

An analysis of G-band strengths in NGC 6397 and M55 red giants

A spectroscopic index which measures the strength of the G band has been determined for a number of red giants in the very metal-poor globular clusters NGC 6397 and M55. An analysis of these indices indicates a clear pattern of decreasing carbon abundances as a function of increasing evolutionary state in NGC 6397. The asymptotic giant branch (AGB) stars in NGC 6397 appear to have anomalous weak G bands compared to red giant branch stars of similar color. The weak G-band effect appears to be largely the results of a mild carbon depletion typical of evolved metal-poor stars which has been exaggerated by the lower surface gravities of AGB stars. Nitrogen abundances have also determined for the five brightest stars in NGC 6397. 44 refs.

Carbon Abundances of Faint Stars in M13: Evidence of Two Abundance-altering Mechanisms

We present an analysis of CH band strengths in Keck Low-Resolution Imaging Spectrometer spectra of a sample of 81 stars in M13 within 2 mag of the main-sequence turnoff. The subgiants clearly exhibit a substantial (a factor of ~6) spread in [C/Fe]. Moreover, the bulk of the subgiants possess C abundances larger than those found among their more luminous counterparts. The turnoff stars themselves are too warm for appreciable CH formation, but the relatively small range in the observed CH band strength for stars just below the turnoff nevertheless translates into this same spread in [C/Fe]. Still fainter, the sample size is small, but the same range in [C/Fe] appears to be present. On the basis of these observations we suggest that a process external to the present stars has resulted in a substantial star-to-star dispersion in [C/Fe] (and possibly other light elements) among all stars in M13. In addition, the surface C abundances among the more luminous stars have been further modified by the operation of an internal deep-mixing mechanism during red giant branch ascent. The amplitude of the scatter we find in [C/Fe] at all luminosities may prove difficult to explain via accretion from intermediate-mass asymptotic giant branch stars as the external polluting mechanism.

Anticorrelated CN and CH variations on the 47 Tucanae main-sequence turnoff

Observations of CN and CH band strengths among a random sample of main-sequence turn-off stars (+3.9 < MV < +4.6) in the globular cluster 47 Tuc (NGC 104, C0021-723) were made with the CTIO Argus fiber spectrograph for the purpose of determining the ratio of CN-strong to CN-weak stars and investigating the behavior of CH relative to CN. Of the 20 turn-off stars, 12 were found to be CN-strong while 8 appear to be CN-weak. This ratio of CN-strong to CN-weak stars is similar to the ratios found among the more luminous 47 Tuc stars and implies little change in the overall distribution of CN with evolutionary state, although the present sample size is small. A general anticorrelation between CN and CH is also observed in that the CN-weak main-sequence stars all (with one possible exception) exhibit strong CH bands - a trend similar to that found among the brighter stars. That these variations occur among such relatively un-evolved stars and that the overall CN distribution appears to be independent of evolutionary state presents serious challenges to internal or mixing theories of their origin. We therefore suggest that at least some component of the C and N abundance inhomogeneities observed among the brighter (more evolved) stars of this cluster appears to have been established prior to the commencement of evolution up the red giant branch.

DEEP MIXING AND METALLICITY: CARBON DEPLETION IN GLOBULAR CLUSTER GIANTS

We present the results of an observational study of the efficiency of deep mixing in globular cluster red giants as a function of stellar metallicity. We determine [C/Fe] abundances based on low-resolution spectra taken with the Kast spectrograph on the 3 m Shane telescope at Lick Observatory. Spectra centered on the 4300 A CH absorption band were taken for 42 bright red giants in 11 Galactic globular clusters ranging in metallicity from M92 ([Fe/H] = –2.29) to NGC 6712 ([Fe/H] = –1.01). Carbon abundances were derived by comparing values of the CH-band strength index S 2(CH) measured from the data with values measured from a large grid of SSG synthetic spectra. Present-day abundances are combined with theoretical calculations of the time since the onset of mixing, which is also a function of stellar metallicity, to calculate the carbon depletion rate across our metallicity range. We find that the carbon depletion rate is twice as high at a metallicity of [Fe/H] = –2.3 than at [Fe/H] = –1.3, which is a result qualitatively predicted by some theoretical explanations of the deep mixing process.

THE CHEMICAL INHOMOGENEITY OF FAINT M13 STARS: CARBON AND NITROGEN ABUNDANCES

Building upon earlier observations that demonstrate substantial star-to-star differences in the carbon abundances of M13 subgiants, we present new Keck LRIS spectra reaching more that 1.5 mag below the M13 main-sequence turnoff (to V ≈ 20). Our analysis reveals a distribution of C abundances similar to that found among the subgiants, implying little change in the compositions of the M13 stars at least through the main-sequence turnoff. We presume these differences to be the result of some process operating early in the cluster history. Additional spectra of previously studied bright M13 giants have been obtained with the 5 m Hale Telescope. A comparison of C abundances derived using the present methods and those from the literature yield a mean difference of 0.03 ± 0.14 dex for four stars in common with the 1996 study by Smith et al. and 0.14 ± 0.07 dex for stars also observed in Suntzeffs 1981 survey (if one extreme case is removed). We conclude that the lower surface C abundances of these luminous giants as compared with the subgiants and main-sequence stars are likely the result of mixing rather than a difference in our abundance scales. NH band strengths have also been measured for a handful of the most luminous M13 turnoff stars. While molecular band formation in such stars is weak, significant star-to-star NH band strength differences are present. Moreover, for the stars with both C and N measurements, differences between stars in these two elements appear to be anticorrelated. Finally, the most recent C and N abundances for main-sequence, main-sequence turnoff, and subgiant stars in 47 Tuc, M71, M5, and the present M13 data are compared.

Calibration of the CH and CN Variations Among Main-Sequence Stars in M71 and in M13

An analysis of the CN and CH band strengths measured in a large sample of M71 and M13 main-sequence stars by Cohen is undertaken using synthetic spectra to quantify the underlying C and N abundances. In the case of M71 it is found that the observed CN and CH band strengths are best matched by the identical C/N/O abundances which fit the bright giants, implying: (1) little if any mixing is taking place during red giant branch ascent in M71, and (2) a substantial component of the C and N abundance inhomogeneities is in place before the main-sequence turn-off. The unlikelihood of mixing while on the main sequence requires an explanation for the abundance variations which lies outside the present stars (primordial inhomogeneities or intracluster self enrichment). For M13 it is shown that the 3883 A CN bands are too weak to be measured in the spectra for any reasonable set of expected compositions. A similar situation exists for CH as well. However, two of the more luminous program stars do appear to have C abundances considerably greater than those found among the bright giants, thereby suggesting deep mixing has taken place on the M13 red giant branch.

CN Bimodality at Low Metallicity: The Globular Cluster M53

We present low resolution UV-blue spectroscopic observations of red giant stars in the globular cluster M53 ([Fe/H] = -1.84), obtained to study primordial abundance variations and deep mixing via the CN and CH absorption bands. The metallicity of M53 makes it an attractive target: a bimodal distribution of 3883 A CN band strength is common in moderate- and high-metallicity globular clusters ([Fe/H]≥-1.6) but unusual in those of lower metallicity ([Fe/H] ≤ -2.0). We find that M53 is an intermediate case, and has a broad but not strongly bimodal distribution of CN band strength, with CN and CH band strengths anticorrelated in the less-evolved stars. Like many other globular clusters, M53 also exhibits a general decline in CH band strength and [C/Fe] abundance with rising luminosity on the red giant branch.