James K. McCarthy

First stellar abundances in NGC 6822 from VLT-UVES and Keck-Hires spectroscopy

We have obtained the first high-resolution spectra of individual stars in the dwarf irregular galaxy NGC 6822. The spectra of the two A-type supergiants were obtained at the Very Large Telescope and Keck Observatories, using the Ultraviolet-Visual Echelle Spectrograph and the High Resolution Echelle Spectrometer, respectively. A detailed model atmospheres analysis has been used to determine their atmospheric parameters and elemental abundances. The mean iron abundance from these two stars is = -0.49 ± 0.22 (±0.21), with Cr yielding a similar underabundance, = -0.50 ± 0.20 (±0.16). This confirms that NGC 6822 has a metallicity that is slightly higher than that of the SMC and is the first determination of the present-day iron group abundances in NGC 6822. The mean stellar oxygen abundance, 12 + log(O/H) = 8.36 ± 0.19 (±0.21), is in good agreement with the nebular oxygen results. Oxygen has the same underabundance as iron, = +0.02 ± 0.20 (±0.21). This O/Fe ratio is very similar to that seen in the Magellanic Clouds, which supports the picture that chemical evolution occurs more slowly in these lower mass galaxies, although the O/Fe ratio is also consistent with that observed in comparatively metal-poor stars in the Galactic disk. Combining all of the available abundance observations for NGC 6822 shows that there is no trend in abundance with galactocentric distance. However, a subset of the highest quality data is consistent with a radial abundance gradient. More high-quality stellar and nebular observations are needed to confirm this intriguing possibility.

A New Spin on the Problem of Horizontal-Branch Gaps: Stellar Rotation along the Blue Horizontal Branch of Globular Cluster M13* **

We have determined the projected rotational velocities of 13 blue horizontal-branch (BHB) stars in the globular cluster M13 via rotational broadening of metal absorption lines. Our sample spans the photometric gap observed in the horizontal-branch distribution at Teff 11,000 K and reveals a pronounced difference in stellar rotation on either side of this feature—blueward of the gap, all the stars show modest rotations, v sin i < 10 km s-1, while to the red side of the gap, we confirm the more rapidly rotating population (v sin i 40 km s-1) previously observed by R. C. Peterson and coworkers. Taken together with these prior results, our measurements indicate that a stars rotation is indeed related to its location along the HB, although the mechanism behind this correlation remains unknown. We explore possible connections between stellar rotation and mass-loss mechanisms which influence the photometric morphology of globular cluster HBs.

Striking photospheric abundance anomalies in blue horizontal-branch stars in globular cluster m13

High-resolution optical spectra of 13 blue horizontal-branch stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (Teff > 12,000 K), helium is depleted by factors of 10-100 below solar, while iron is enhanced to 3 times the solar abundance, 2 orders of magnitude above the canonical metallicity of [Fe/H]-1.5 dex for this globular cluster. Nitrogen, phosphorus, and chromium exhibit even more pronounced enhancements, and other metals are also mildly overabundant, with the exception of magnesium, which stays very near the expected cluster metallicity. These photospheric anomalies are most likely due to diffusion—the gravitational settling of helium and the radiative levitation of the other elements—in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the clusters horizontal branch and on estimates of its age and distance.

Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15.

High-resolution optical spectra of 18 blue horizontal-branch stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff approximately 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the vsini rotations appear to have a bimodal distribution, with eight stars at vsini<15 km s-1 and two stars at vsini approximately 35 km s-1. Most of the stars at Teff>/=10,000 K, however, are slowly rotating, vsini<7 km s-1, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of Teff, and helium is depleted by factors of 10-30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars.

Counts and colours of faint galaxies in the U and R bands

Ground-based counts and colors of faint galaxies in the U and R bands in one field at high Galactic latitude are presented. Integrated over flux, a total of 1.2x10^5 sources per square degree are found to U=25.5 mag and 6.3x10^5 sources per square degree to R=27 mag, with d log N/dm ~ 0.5 in the U band and d log N/dm ~ 0.3 in the R band. Consistent with these number-magnitude curves, sources become bluer with increasing magnitude to median U-R=0.6 mag at 24<U<25 mag and U-R=1.2 mag at 25 < R < 26 mag. Because the Lyman break redshifts into the U band at z~3, at least 1.2x10^5 sources per square degree must be at redshifts z<3. Measurable U-band fluxes of 73 percent of the 6.3x10^5 sources per square degree suggest that the majority of these also lie at z < 3. These results require an enormous space density of objects in any cosmological model.

FG Sagittae: A Newborn R Coronae Borealis Star?*

We have monitored FG Sges spectroscopic changes since the time just prior to its dramatic fading in 1992 August. The most significant qualitative changes in the spectrum include large variations in the strength of the C2 molecular bands and the gradual appearance of broad blueshifted high-velocity (~200 km s-1 relative to the photosphere) absorption components of the Na I D lines. During the deep minima of 1994 May and 1996 June, an emission-line spectrum temporarily appeared superimposed on a weak continuum; in addition to the previously reported nebular emission lines, the spectra displayed the C2 and rare earth element lines in emission. Much of the behavior exhibited by FG Sge since 1992 resembles that seen in R CrB stars, including the photometric behavior, the evolution of the Na I D line profiles, variations of the C2 band strengths, and the appearance of narrow emission lines. The results of our abundance analysis (using model atmospheres with a solar He/H ratio) indicate that the carbon abundance is currently greater than that determined by Langer et al., who reported on the dramatic increase in the abundances of the rare earth elements in FG Sge. We derive higher relative abundances of the rare earths ([Me/Fe] ~ 3) than either Langer et al. or Kipper & Kipper, which we attribute to some enhancement of these elements since ~1970. We confirm previous claims that the relative scandium abundance is high ([Sc/Fe] ~ 1) in FG Sge and suggest that it is the result of neutron captures by the light elements leading up to 45Sc. The Hα profile of FG Sge is very similar to that of V854 Cen, a R CrB star deficient in H by 2-3 dex. This is the first evidence pointing toward H-deficiency in the atmosphere of FG Sge, which further strengthens its link with the R CrB class. Additional study is required before we can say definitively whether or not its atmosphere is H deficient.

High-Resolution Abundance Analysis of Very Metal-rich Stars in the Solar Neighborhood

We report detailed analysis of high-resolution spectra of nine high velocity metal-rich dwarfs in the solar neighborhood. The stars are super metal-rich and 5 of them have [Fe/H]>=+0.4, making them the most metal-rich stars currently known. We find that alpha-elements decrease with increasing metallicity; s-elements are underabundant by about [s-elements/Fe]=-0.3. While exceeding the [Fe/H] of current bulge samples, the chemistry of these stars has important similarities and differences. The near-solar abundances of the alpha-capture elements places these stars on the metal-rich extension of McWilliam & Rich (1994) [ApJS, 91, 749], but their s-process abundances are much lower than those of the bulge giants. These low s-process values have been interpreted as the hallmark of an ancient stellar population.

Blue channel of the Keck low-resolution imaging spectrometer

This paper summarizes the optical, mechanical, electrical, and software design of LRIS-B, the blue channel of the Keck Low Resolution and Imaging Spectrograph. The LRIS-B project will shortly be completing the existing LRIS instrument through the addition of dichroic beamsplitters, grisms to disperse light on the blue channel, broad-band u, B, and V photometric filters, a blue and near-UV transmitting camera lens, and a large format blue-sensitive CCD detector. LRIS-B will also introduce piezoelectric xy-actuation of the CCD detector inside its Dewar, in order to compensate for flexure in the existing instrument; ultimately the red-side CCD detector will be similarly equipped, its PZT xy-stage being independently programmed. The optical design of the LRIS-B camera uses only fused silica and calcium fluoride elements, and includes a decentered meniscus element to compensate for coma introduced by the LRIS off-axis paraboloid collimator. The design of the blue channel grisms have been optimized for maximum blaze efficiency, the highest dispersion grism having a groove density of 1200 gr/mm. Optical elements not in use at any given time will be stowed in carousels externally mounted to the instrument sidewalls. The entire instrument is designed to permit remote operation.

Dynamical Correlations for Globular Clusters in M31

We present internal velocity dispersion measurements for a set of 21 globular clusters in the Andromeda galaxy (M31). We combine them with structural and photometric cluster parameters measured earlier with the Hubble Space Telescope and from the ground to explore correlations of cluster properties and to compare them with the equivalent correlations for the Galactic globular clusters. We find that the M31 globulars follow the same correlations between velocity dispersion and luminosity, central, and average surface brightness, as do their Galactic counterparts. This suggests a common physical origin for these correlations. They may be produced by the same astrophysical conditions and processes operating at the epoch of globular cluster formation in both galaxies. The very existence of these excellent correlations, and their quantitative form as scaling laws, represent challenges and constraints for theories of globular cluster formation. Preliminary estimates of the cluster M/L ratios show correlations with the cluster metallicity, in the sense of more metal-rich clusters having lower M/L, particularly in the near-infrared. At a given metallicity, there is no detectable systematic difference between the M31 globulars and their Galactic counterparts, which suggests a great similarity of their stellar populations. The observed scatter around these trends is comparable to the expected errors, which implies a small intrinsic scatter and thus which leaves little room for possible variations in the cluster age or stellar IMF at a given metallicity.

Palomar 13: An Unusual Stellar System in the Galactic Halo*

We report the first results of a program to study the internal kinematics of globular clusters in the outer halo of the Milky Way. Using the Keck telescope and High Resolution Echelle Spectrometer, we have measured precise radial velocities for 30 candidate red giants in the direction of Palomar 13, an object traditionally cataloged as a compact, low-luminosity globular cluster. We have combined these radial velocities with published proper motion membership probabilities and new CCD photometry from the Keck and Canada-France-Hawaii telescopes to isolate a sample of 21 probable members. We find a systemic velocity of vrs = 24.1 ? 0.5 km s-1 and a projected, intrinsic velocity dispersion of ?p = 2.2 ? 0.4 km s-1. Although modest, this dispersion is nevertheless several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, it implies a mass-to-light ratio of V = 40 based on the best-fit King-Michie model. The surface density profile of Palomar 13 also appears unusual compared to most Galactic globular clusters; depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of extratidal stars, or is considerably more spatially extended than previously suspected. The full surface density profile is equally well fitted by a King-Michie model having a high concentration and large tidal radius, or by a Navarro-Frenk-White model. We examine?and tentatively reject?a number of possible origins for the observed characteristics of Palomar 13 (e.g., velocity jitter among the red giant branch stars, spectroscopic binary stars, nonstandard mass functions, modified Newtonian dynamics) and conclude that the two leading explanations are either catastrophic heating during a recent perigalacticon passage or the presence of a dark matter halo. The available evidence therefore suggests that Palomar 13 is either a globular cluster that is now in the process of dissolving into the Galactic halo or a faint, dark matter-dominated stellar system.