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We present results from a study of 94 red giant stars in the globular cluster M53. We use low-resolution spectra to measure the strength of CN and CH features at ∼3800 and 4300 Å, respectively. The strengths of these features are used to classify stars into a CN-enhanced and CN-normal population and to measure C and N abundances in all 94 stars. We find the red giant branch stars to be evenly split between the two populations identified, and observe the presence of CN-enhanced stars on the asymptotic giant branch. In addition, we identify five CH star candidates based on the strength of their CN and CH band features, and the presence of a P-branch in their CH band. We compare our identification of multiple populations to those based on the Na–O anticorrelation and pseudo-color indices in Hubble Space Telescope UV photometry, and find general agreement between all three methods. Our large sample size also allows us to study the radial distribution of each population, and we find that the CN-enhanced population is more centrally concentrated. We use our C and N measurements to compare the evolutionary changes in these elements as a function of magnitude between the two populations, and show that both populations experience similar evolutionary changes to the surface abundances of C and N. Finally, we calculate C+N+O abundances for each population and compare them to similar measurements made in M10; we find that in both clusters, CN-enhanced stars have a slightly enhanced C+N+O (Δ(C+N+O) ∼ 0.2 dex).

We identify 233 X-ray sources, of which 95 are new, in a 222 ks exposure of Omega Centauri with the Chandra X-ray Observatory's ACIS-I detector. The limiting unabsorbed flux in the core is $f_x$ (0.5$-$6.0 keV) $\simeq$ 3 $\times$ 10$^{-16}$ erg s$^{-1}$ cm$^{-2}$ ($L_X$ $\simeq$ 1 $\times$ 10$^{30}$ erg s$^{-1}$ at 5.2 kpc). We estimate that ~$60\pm 20$ of these are cluster members, of which ~30 lie within the core ($r_c$ $=$ 155 arcsec), and another ~30 between 1$-$2 core radii. We identify four new optical counterparts, for a total of 45 likely identifications. Probable cluster members include 18 cataclysmic variables (CVs) and CV candidates, one quiescent low-mass X-ray binary, four variable stars, and five stars that are either associated with w Cen's anomalous red giant branch, or are sub-subgiants. We estimate that the cluster contains $40\pm 10$ CVs with $L_X$ $>$ 10$^{31}$ erg s$^{-1}$, confirming that CVs are underabundant in w Cen relative to the field. Intrinsic absorption is required to fit X-ray spectra of six of the nine brightest CVs, suggesting magnetic CVs, or high-inclination systems. Though no radio millisecond pulsars (MSPs) are currently known in w Cen, more than 30 unidentified sources have luminosities and X-ray colours like those of MSPs found in other globular clusters; these could be responsible for the Fermi-detected gamma-ray emission from the cluster. Finally, we identify a CH star as the counterpart to the second-brightest X-ray source in the cluster and argue that it is a symbiotic star. This is the first such giant/white dwarf binary to be identified in a globular cluster.

We report on the serendipitous discovery of a carbon star near the centre of the low-metallicity globular cluster NGC 6426. We determined its membership and chemical properties using medium-resolution spectra. The radial velocity of −159 km s−1 makes it a member of the cluster. We used photometric data from the literature and the COMARCS stellar atmospheric models to derive its luminosity, effective temperature, surface gravity, metallicity, and approximate C, N and O abundance ratios. According to these properties, we suggest that this star is a genuine carbon-rich low-metallicity asymptotic giant branch star.

We present carbon abundances of red giants in Milky Way globular clusters and dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the clusters decreases with increasing luminosity above log(L/L_sun) ~= 1.6, which can be explained by deep mixing in evolved giants. The same decrease is observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform. Stars in dSphs at lower metallicities have larger [C/Fe] ratios. We hypothesize that [C/Fe] (corrected to the initial carbon abundance) declines with increasing [Fe/H] due to the metallicity dependence of the carbon yield of asymptotic giant branch stars and due to the increasing importance of Type Ia supernovae at higher metallicities. We also identified 11 very carbon-rich giants (8 previously known) in three dSphs. However, our selection biases preclude a detailed comparison to the carbon-enhanced fraction of the Milky Way stellar halo. Nonetheless, the stars with [C/Fe] < +1 in dSphs follow a different [C/Fe] track with [Fe/H] than the halo stars. Specifically, [C/Fe] in dSphs begins to decline at lower [Fe/H] than in the halo. The difference in the metallicity of the [C/Fe] "knee" adds to the evidence from [alpha/Fe] distributions that the progenitors of the halo had a shorter timescale for chemical enrichment than the surviving dSphs., accepted to ApJ; 20 pages, 11 figures, 2 machine-readable tables

We report results of a photometric survey with the WIYN telescope for carbon stars in the M 31 dwarf spheroidal (dSph) companions And III, And V, And VI, and And VII, as well as in the relatively isolated Local Group dSph Cetus. We find three carbon-star candidates in And VII and one carbon star in each And VI and Cetus. Comparing the carbon star content with other Local Group dwarf galaxies, we argue against the presence of substantial intermediate-age stellar populations in the all of the galaxies surveyed with the exception of And VII. We discuss these results in the context of the origin of the Andromeda dSphs and conclude that these are ancient galaxies, most of which ceased star formation long before the main merger events in M31. The M31 dSphs therefore show less diversity in star formation histories than the Galactic dSph companions, or the M31 dE companions, as illustrated by NGC147 which was surveyed as a calibration object. All of our dSph targets except And V have candidate carbon stars below the tip of the RGB, which resemble CH stars found in globular clusters. We estimate that 0.3% of stars in the dSphs are CH stars, presumably as a result of C pollution from a binary companion. Comparisons with CH star frequencies in globular clusters could constrain the impact of dense environments on the frequency of this form of binary star evolution., accepted for publication in AJ

We present calibrations of the effective temperatures of giant stars versus [Fe/H] and colours , , , , , , , , , , and . These calibrations are based on a large sample of field and globular cluster stars which roughly cover spectral types from F0 to K5. Their effective temperatures, scaled to direct determinations via reliable angular diameter measurements, were derived by applying the infrared flux method. The empirical relations have been fitted to polynomials of the form by using the least squares method. The precision of the fits ranges from 40 K for to 170 K for . We tabulate intrinsic colours of giant stars in the ranges: 3500 K 8000 K; -3.0 . We also present the calibration of BC(V) as a function of log and metallicity. Finally, we compare the resulting scale of temperatures with previous works.

The strengths of the CN, NH, and CH bands in the spectra of Population II red giants are discussed based on observations of a sample of both globular cluster and field giants. We confirm an anticorrelation between the oxygen abundance and the CN indices and a correlation between the 3883 A CN band and the 3360 A NH band strengths among giants in the globular clusters M15 and M13. We find one M15 star (IV-38) with a strong 3883 A CN band and detectable 4215 A CN and 3360 A NH bands. Abundance analysis of M15 IV-38 reveals a comparable carbon abundance to a comparison star that has much weaker CN bands. One possible reason for this may be that IV-38 is an insipid CH star that has also undergone some deep mixing. Alternatively, the atmosphere of IV-38 may have been extensively processed through the O→N cycle of hydrogen burning such that carbon has been affected by O→C conversion. By combining our data set with material from the literature and high-quality B–V photometry, we find that the CN band strength differences among the M5 red giants do not appear to broaden the giant branch of that cluster within the photometric errors. Using the CN, CH, and NH bands, we are able to confirm that the metal-poor field giant HD 135148 is a CH-strong star and has not undergone deep mixing.

Abundances relative to iron for carbon, nitrogen, strontium and barium are presented for 33 stars on the red giant branch of the globular cluster omega Centauri. They are based on intermediate-resolution spectroscopic data covering the blue spectral region analyzed using spectrum synthesis techniques. The data reveal the existence of a broad range in the abundances of these elements, and a comparison with similar data for main sequence stars enables insight into the evolutionary history of the cluster. The majority of the red giant branch stars were found to be depleted in carbon, i.e. [C/Fe], 20 pages, 13 figures, Accepted to ApJ

Infrared JHK photometry is given for all the proposed LMC CH stars in the list (43 stars) of Hartwick & Cowley (1985, A. J., 334). Their luminosities range up to M(bol) ∼ −6 or brighter, making them the most massive carbon stars in the LMC. However, the kinematic discussion of Hartwick & Cowley suggests an age of a few Gyr for them. They may thus be the products of binary mergers, i.e. evolutionary products of blue stragglers of an intermediate-age population. Some analogous stars in the halo of our Galaxy may have evolved from globular cluster type blue stragglers

We report the discovery of a probable CH star in the core of the Galactic globular cluster M14, identified from an integrated-light spectrum of the cluster obtained with the MOS spectrograph on the CFHT. From a high- resolution echelle spectrum of the same star obtained with the Hydra fiber positioner and bench spectrograph on the WIYN telescope, we measure a radial velocity of $-53.0\pm1.2$ km s$^{-1}$. Although this velocity is inconsistent with published estimates of the systemic radial velocity of M14 (eg, ${\bar {v_r}} \approx -123$ km s$^{-1}$), we use high-precision Hydra velocities for 20 stars in the central 2.6 arcminutes of M14 to calculate improved values for the cluster mean velocity and one-dimensional velocity dispersion: $-59.5\pm1.9$ km s$^{-1}$ and $8.2\pm1.4$ km s$^{-1}$, respectively. Both the star's location near the tip of the red giant branch in the cluster color magnitude diagram and its radial velocity therefore argue for membership in M14. Since the intermediate-resolution MOS spectrum shows not only enhanced CH absorption but also strong Swan bands of C$_2$, M14 joins Omega Cen as the only globular clusters known to contain classical CH stars. Although evidence for its duplicity must await additional radial velocity measurements, the CH star in M14 is probably, like all field CH stars, a spectroscopic binary with a degenerate (white dwarf) secondary. The candidate and confirmed CH stars in M14 and Omega Cen, and in a number of Galactic dSph galaxies, may then owe their existence to the long timescales for the shrinking and coalescence of hard binaries in low-concentration environments., Accepted to the Astrophysical Journal Letters. 13 pages, AAS LaTeX and three postscript figures (numbers 2,3,4). Entire paper (including Figure 1) available at http://www.hia.nrc.ca/DAO/SCIENCE/science.html

Blue spectra of a sample of 24 bright M55 (NGC 6809 = GC 1936-310) red giants within the magnitude range of 11.8

A detailed abundance analysis of seven red-giant CH stars has been conducted. Using high resolution echelle and coude spectrographic data and modern analysis techniques involving model stellar atmospheres and computer codes which model LTE radiative transfer, the abundances of carbon, nitrogen and oxygen, the carbon isotope ratios and the abundances of the s-process elements have been determined. The red-giant CH stars are a class of metal-poor giants with equivalent spectral types of G and K which show enhanced abundances of s-process elements. Studies of their kinematics indicate that the red-giant CH stars are members of the halo of our galaxy (Hartwick & Cowley 1985). Therefore, the CH stars provide valuable information about the early chemical evolution of our galaxy. In addition, it is known that the red-giant CH stars are members of binary systems like the barium stars (McClure 1984), so they are also valuable in the study of mass transfer. The carbon isotope ratios are found to break the red-giant CH stars into two groups--those with 12C 13C >/= 20 and those with 12C 13C ~4 (a value associated with equilibrium burning of the CN cycle) and carbon and nitrogen abundances are found to be greatly enhanced, [C+N/Fe] ~+1.0. In addition, it is found that the red-giant CH stars show large enhancements of the s-process elements with the enhancements of the heavy s-process peak elements (Ba through Sm) being larger than the light s-process peak elements (Sr, Y and Zr) by a factor of approximately six. Taken with the orbital data available for the red-giant CH stars, the carbon, nitrogen and oxygen abundances and the carbon isotope ratios are consistent with a picture in which carbon and s-process elements were dumped onto the incipient red-giant CH star from an AGB star. The accreted material is mixed deep enough into the star's interior to be subjected to CN burning. In addition, the greatly enhanced s-process abundances stars strongly suggest that the 13C (alpha ,n)16O reaction is the source of neutrons for the s-process. In addition, the abundance patterns in the globular-cluster CH star, M22 III-106, have been analyzed. It is found to have an abnormally large carbon abundance, but the C/O ratio is much less than unity and the star shows no evidence for enhanced abundances of the s-process elements. Therefore, M22 III-106 is most likely not a CH star in the sense that term is applied to galactic CH stars. The most probable explanation for the carbon overabundance in M22 III-106 is that CN processing has not been as complete in this star as in other giants of similar composition in M22.

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,.

Using the 1-0 C 2 Swan Band at 4737 A and the CN red system 2-0 band near 8000 A, the ratio of stable carbon isotopes, 12 C/ 13 C, has been determined for eight CH giants. For a majority of the sample stars, we find that 12 C/ 13 C∼3 near the equilibrium value of the CN cycle, though there seems to be a second population of CH stars with high isotope ratios ( 12 C/ 13 C≥25). This range of ratios is the same as found for the Population II giants and globular cluster giant stars

This paper examines the properties of the LMC halo through the kinematics of its CH stars, which are known to be excellent halo tracers in the Galaxy. Velocities have been measured for 40 CH stars lying in the direction of LMC. These supplement 41 previously determined values. The main result of this investigation is that the distribution of CH-star velocities is asymmetrical, possessing a relatively sharp cutoff at the high-velocity end with an extended low-velocity tail. It is speculated that the asymmetry arises from two discrete groups of CH stars, one associated with the old globular cluster population and a second with the LMC disk. 11 refs.

A photograph of $omega$ Centauri made with a transmission-grating slitless spectrograph has revealed a new CH star near the center of this globular cluster. The star's radial velocity is found to be consistent with cluster membership, and a visual absolute magnitude M/subv/=-1.8 is derived. The total number of carbon stars now known in globular clusters is three, all of which are in $omega$ Cen. (AIP)

Carbon isotope ratios for stars in four globular clusters are derived by means of spectrum synthesis analysis of CN features in high-resolution spectra around 8000 A. In the strong-CN stars in the clusters M4, M22, Omega Cen, and 47 Tuc, the isotope ratios are generally quite low, approaching the equilibrium value in the CN cycle, namely 3-8; this includes a barium star and an S star in Omega Cen and a CH star in M22. (C-13)N is not detected in weak-CN stars, but the limits on the (C-12)/(C-13) ratios in these stars are not significant. Ratios as low as 3-8 are not predicted by standard evolution models of low-mass metal-poor stars. 37 refs.

Spectra from 2.1-2.5 microns and broad-band observations from 1.2-3.5 microns are presented for the CH star III-106 in the globular cluster M22. The visual to infrared color implies an effective temperature of about 4600 K. The CO index is 0.11, significantly greater than for other cluster giants of this temperature and metallicity. The strength of the CO can be understood if the carbon abundance in III-106 has been enriched by nucleosynthesis. A carbon enhancement of at least a factor of 4 appears to be required.