Allen V. Sweigart

Improved Color-Temperature Relations and Bolometric Corrections for Cool Stars

We present new grids of colors and bolometric corrections for F-K stars having 4000 K ≤ Teff ≤ 6500 K, 0.0 ≤ log g ≤ 4.5, and -3.0 ≤ [Fe/H] ≤ 0.0. A companion paper extends these calculations into the M giant regime (3000 K ≤ Teff ≤ 4000 K). Colors are tabulated for Johnson U-V and B-V, Cousins V-R and V-I, Johnson-Glass V-K, J-K, and H-K, and CIT/CTIO V-K, J-K, H-K, and CO. We have developed these color-temperature relations by convolving synthetic spectra with the best-determined, photometric filter transmission profiles. The synthetic spectra have been computed with the SSG spectral synthesis code using MARCS stellar atmosphere models as input. Both of these codes have been improved substantially, especially at low temperatures, through the incorporation of new opacity data. The resulting synthetic colors have been put onto the observational systems by applying color calibrations derived from models and photometry of field stars that have effective temperatures determined by the infrared flux method. These color calibrations have zero points that change most of the original synthetic colors by less than 0.02 mag, and the corresponding slopes generally alter the colors by less than 5%. The adopted temperature scale, that of Bell & Gustafsson, is confirmed by the extraordinary agreement between the predicted and observed angular diameters of these field stars, indicating that the differences between the synthetic colors and the photometry of the field stars are not due to errors in the effective temperatures adopted for these stars. Thus, we have derived empirical color-temperature relations from the field star photometry, which we use as one test of our calibrated, theoretical, solar-metallicity color-temperature relations. Except for the coolest dwarfs (Teff < 5000 K), our calibrated model colors are found to match these relations, as well as the empirical relations of others, quite well, and our calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to color-magnitude diagrams of M67. We regard this as evidence that our calibrated colors can be applied to many astrophysical problems, including modeling the integrated light of galaxies. Because there are indications that the dwarfs cooler than 5000 K may require different optical color calibrations than the other stars, we present additional colors for our coolest dwarf models that account for this possibility.

Effects of Helium Mixing on the Evolution of Globular Cluster Stars

New noncanonical sequences for the evolution of globular cluster stars have been computed in order to investigate the effects of mixing helium from the hydrogen shell into the envelope during the red giant branch (RGB) phase. The possible occurrence of such mixing, driven by internal rotation, is suggested by the observed abundance variations involving C, N, O, Na, and Al in globular cluster red giants. We find that helium mixing can substantially increase the envelope helium abundance. By increasing the RGB tip luminosity, helium mixing also leads to enhanced mass loss along the RGB. Both of these effects have a potentially large impact on the subsequent horizontal-branch (HB) evolution. In particular, helium mixing produces a bluer HB morphology, thereby making it easier to explain the hot HB population found in various stellar systems, as well as the difference in HB morphology between some second parameter pairs of globular clusters. Helium-mixed sequences show a larger RR Lyrae period shift and predict a smaller age for the metal-poor globular clusters. In addition, such sequences can reproduce the low gravities observed in blue HB stars.

The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy

Using HST ACS, we have obtained deep optical images reaching stars well below the oldest main-sequence turnoff in the spheroid, tidal stream, and outer disk of Andromeda. We have reconstructed the star formation history in these fields by comparing their color-magnitude diagrams to a grid of isochrones calibrated to Galactic globular clusters observed in the same bands. Each field exhibits an extended star formation history, with many stars younger than 10 Gyr but few younger than 4 Gyr. Considered together, the star counts, kinematics, and population characteristics of the spheroid argue against some explanations for its intermediate-age, metal-rich population, such as a significant contribution from stars residing in the disk or a chance intersection with the streams orbit. Instead, it is likely that this population is intrinsic to the inner spheroid, whose highly disturbed structure is clearly distinct from the pressure-supported metal-poor halo that dominates farther from the galaxys center. The stream and spheroid populations are similar, but not identical, with the streams mean age being ~1 Gyr younger; this similarity suggests that the inner spheroid is largely polluted by material stripped from either the streams progenitor or similar objects. The disk population is considerably younger and more metal-rich than the stream and spheroid populations, but not as young as the thin-disk population of the solar neighborhood; instead, the outer disk of Andromeda is dominated by stars of age 4-8 Gyr, resembling the Milky Ways thick disk. The disk data are inconsistent with a population dominated by ages older than 10 Gyr and in fact do not require any stars older than 10 Gyr.

Bimodality and Gaps on Globular Cluster Horizontal Branches. II. The Cases of NGC 6229, NGC 1851, and NGC 2808

The outer halo globular cluster NGC 6229 has a peculiar horizontal-branch (HB) morphology, with clear indications of a bimodal HB and a gap on the blue HB. In this paper, we present extensive synthetic HB simulations to determine whether peculiar distributions in the underlying physical parameters are needed to explain the observed HB morphology. We find that a unimodal mass distribution along the HB can satisfactorily account for the observed HB bimodality, provided the mass dispersion is substantially larger than usually inferred for the Galactic globular clusters. In this case, NGC 6229 should have a well-populated, extended blue tail. A truly bimodal distribution in HB masses can also satisfactorily account for the observed HB morphology, although in this case the existence of an extended blue tail is not necessarily implied. The other two well-known bimodal-HB clusters, NGC 1851 and NGC 2808, are briefly analyzed. While the HB morphology of NGC 1851 can also be reproduced with a unimodal mass distribution assuming a large mass dispersion, the same is not true of NGC 2808, for which a bimodal, and possibly multimodal, mass distribution seems definitely required. The problem of gaps on the blue HB is also discussed. Applying the standard Hawarden and Newell χ2 test, we find that the NGC 6229 gap is significant at the 99.7% level. However, in a set of 1000 simulations, blue-HB gaps comparable to the observed one are present in ~6%-9% of all cases. We employ a new and simple formalism, based on the binomial distribution, to explain the origin of this discrepancy, and conclude that Hawardens method, in general, substantially overestimates the statistical significance of gaps.

The development of the red giant branch. II - Astrophysical properties

Evolutionary sequences developed in another paper are used here to investigate the properties of the red giant branch (RGB) phase transition. Results are found for compositions in the range Y(MS) between 0.20 and 0.30 and Z between 0.004 and 0.04. The transition mass M(HeF) increases as either Y(MS) decreases or Z increases. The stellar population transition age t(HeF) is virtually independent of composition and close to 0.6 Gyr. The RGB phase transition occurs almost abruptly over a mass range of only a few tenths of a solar mass or, equivalently, over a time interval of about 0.2 Gyr in the life of a stellar population. During the RGB phase transition the core mass Mc at helium ignition increases very rapidly by about 0.15 solar mass, while the luminosity at the tip of the RGB increases by about one order of magnitude. Absolute minima are found for the values of Mc and the RGB tip luminosity.

The Second-Parameter Effect in Metal-Rich Globular Clusters

Recent Hubble Space Telescope observations have found that the horizontal branches (HBs) in the metal-rich globular clusters NGC 6388 and NGC 6441 slope upward with decreasing B-V. Such a slope is not predicted by canonical HB models and cannot be produced by either a greater cluster age or enhanced mass loss along the red giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to explore three noncanonical scenarios for explaining the sloped HBs in NGC 6388 and NGC 6441: (1) a high cluster helium abundance scenario, in which the HB evolution is characterized by long blue loops; (2) a rotation scenario, in which internal rotation during the RGB phase increases the HB core mass; and (3) a helium-mixing scenario, in which deep mixing on the RGB enhances the envelope helium abundance. All three of these scenarios predict sloped HBs with anomalously bright RR Lyrae variables. We compare this prediction with the properties of the two known RR Lyrae variables in NGC 6388. Additional observational tests of these scenarios are suggested.

Proton-Capture Nucleosynthesis in Globular Cluster Red Giant Stars

Observational evidence suggests that many of the variations of the surface abundances of light- to intermediate-mass elements (A < 28) in globular cluster red giant branch (RGB) stars can be attributed to noncanonical mixing between the surface and the deep stellar interior during the RGB phase. As a first step to studying this mixing in more detail, we have combined a large nuclear reaction network with four detailed stellar evolutionary sequences of different metallicities in order to follow the production and destruction of the C, N, O, Ne, Na, Mg, and Al isotopes around the hydrogen-burning shell (H shell) of globular cluster RGB stars. The abundance distributions determined by this method allow for the variation in the temperature and density around the H shell as well as for the dependence on both the stellar luminosity and cluster metallicity. Because our nuclear reaction network operates separately from the stellar evolution code, we are able to more readily explore the effects of the uncertainties in the reaction rates on the calculated abundances. We discuss implications of our results for mixing in the context of the observational data. Our results are qualitatively consistent with the observed C versus N, O versus N, Na versus O, and Al versus O anticorrelations and their variations with both luminosity and metallicity. We see evidence for variations in Na without requiring changes in O, independent of metallicity, as observed by Norris and Da Costa in 1995 and Briley and coworkers in 1997. Also, we derive 12C/13C ratios near the observed equilibrium value of 4 for all sequences and predict the temperature-dependent 16O/17O equilibrium ratio based on new data for the 17O(p, α)14N reaction rate. Additionally, we discuss the Mg isotopic abundances in light of the recent observations by Shetrone of M13 and NGC 6752.

Synthetic Spectra and Color-Temperature Relations of M Giants

As part of a project to model the integrated spectra and colors of elliptical galaxies through evolutionary synthesis, we have refined our synthetic spectrum calculations of M giants. After critically assessing three effective temperature scales for M giants, we adopted the relation of Dyck et al. for our models. Using empirical spectra of field M giants as a guide, we then calculated MARCS stellar atmosphere models and SSG synthetic spectra of these cool stars, adjusting the band absorption oscillator strengths of the TiO bands to better reproduce the observational data. The resulting synthetic spectra are found to be in very good agreement with the K-band spectra of stars of the appropriate spectral type taken from Kleinmann & Hall as well. Spectral types estimated from the strengths of the TiO bands and the depth of the band head of CO near 2.3 μm quantitatively confirm that the synthetic spectra are good representations of those of field M giants. The broadband colors of the models match the field relations of K and early-M giants very well; for late-M giants, differences between the field star and synthetic colors are probably caused by the omission of spectral lines of VO and H2O in the spectrum synthesis calculations. Here, we present four grids of K-band bolometric corrections and colors—Johnson U-V and B-V, Cousins V-R and V-I, Johnson-Glass V-K, J-K, and H-K, and CIT/CTIO V-K, J-K, H-K, and CO—for models having 3000 K ≤ Teff ≤ 4000 K and -0.5 ≤ log g ≤ 1.5. These grids, which have [Fe/H] = +0.25, 0.0, -0.5, and -1.0, extend and supplement the color-temperature relations of hotter stars presented in a companion paper.

Evolutionary sequences for horizontal branch stars

A new grid of canonical evolutionary horizontal branch (HB) sequences is presented. Sequences are computed for each combination of the following helium and heavy-element abundances, respectively: Y(main sequence) = 0.20, 0.25, 0.30, and Z = 0.0001, 0.001, and 0.01. The results show that the bifurcation point at which the HB morphology changes from redward-evolving tracks to tracks with blueward loops shifts to higher effective temperatures with increasing helium abundance or metallicity. The sequences can be used to study in more detail how a number of HB properties such as the HB lifetime, the effective temperature at the bifurcation point in the track morphology, the luminosity dropoff of the blue HB, and the luminosity width of the red HB depend on the composition. 55 references.

Variable stars in the unusual, metal-rich globular cluster NGC 6441

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6388 using time-series BV photometry. Twenty-eight new variables were found in this survey, increasing the total number of variables found near NGC 6388 to approx. 57. A significant number of the variables are RR Lyrae (approx. 14), most of which are probable cluster members. The periods of the fundamental mode RR Lyrae are shown to be unusually long compared to metal-rich field stars. The existence of these long period RRab stars suggests that the horizontal branch of NGC 6388 is unusually bright. This implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388 are indeed metal-rich. We consider the alternative possibility that the stars in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids were also found. If they are members of the cluster, NGC 6388 would be the most metal-rich globular cluster to contain Population II Cepheids. The mean V magnitude of the RR Lyrae is found to be 16.85 +/- 0.05 resulting in a distance of 9.0 to 10.3 kpc, for a range of assumed values of (M(sub V)) for RR Lyrae. We determine the reddening of the cluster to be E(B - V) = 0.40 +/- 0.03 mag, with differential reddening across the face of the cluster. We discuss the difficulty in determining the Oosterhoff classification of NGC 6388 and NGC 6441 due to the unusual nature of their RR Lyrae, and address evolutionary constraints on a recent suggestion that they are of Oosterhoff type II.