[ { "id": "authors:7nqcj-5ce50", "collection": "authors", "collection_id": "7nqcj-5ce50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190524-100055784", "type": "article", "title": "Abundances in Stars from the Red Giant Branch Tip to Near the Main-Sequence Turnoff in M71. I. Sample Selection, Observing Strategy, and Stellar Parameters", "author": [ { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "Behr", "given_name": "Bradford B.", "clpid": "Behr-B-B" }, { "family_name": "Briley", "given_name": "Michael M.", "clpid": "Briley-M-M" } ], "abstract": "We present the sample for an abundance analysis of 25 members of M71, with luminosities ranging from the red giant branch tip to the upper main sequence. The spectra are of high dispersion and of high precision. We describe the observing strategy and determine the stellar parameters for the sample stars using both broadband colors and fits of H\u03b1 profiles. The derived stellar parameters agree with those from the Yale^2 stellar evolutionary tracks to within 50\u2013100 K for a fixed log g, which is within the level of the uncertainties.", "doi": "10.1086/322100", "issn": "0004-6256", "publisher": "American Astronomical Society", "publication": "Astronomical Journal", "publication_date": "2001-09", "series_number": "3", "volume": "122", "issue": "3", "pages": "1420-1428" }, { "id": "authors:c6evy-16865", "collection": "authors", "collection_id": "c6evy-16865", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190524-103624108", "type": "article", "title": "An Abundance Analysis for Four Red Horizontal-Branch Stars in the Extremely Metal-Rich Globular Cluster NGC 6528", "author": [ { "family_name": "Carretta", "given_name": "Eugenio", "clpid": "Carretta-E" }, { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "Gratton", "given_name": "Raffaele G.", "clpid": "Gratton-R-G" }, { "family_name": "Behr", "given_name": "Bradford B.", "clpid": "Behr-B-B" } ], "abstract": "We present the results of the first analysis of high-dispersion spectra of four red horizontal-branch stars in the metal-rich globular cluster NGC 6528, located in Baade's window. We find that the mean [Fe/H] for NGC 6528 is +0.07 \u00b1 0.01 dex (error of the mean), with a star-to-star scatter of \u03c3 = 0.02 dex (four stars), although the total error is likely to be larger (~0.1 dex) because of systematic errors related to the effective temperature scale and to model atmospheres. This metallicity is somewhat larger than both the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and that found in our previous paper for NGC 6553. However, we find that the spectra of clump stars in NGC 6528 and 6553 are very similar each other, the slightly different metal abundances found being possibly due to the different atmospheric parameters adopted in the two analyses. Since the present analysis is based on higher quality material, we propose to revise our previous published metal abundance for NGC 6553 to [Fe/H] = -0.06 \u00b1 0.15. For NGC 6528, we find excesses for the \u03b1-process elements Si and Ca ([Si/Fe] = +0.4 and [Ca/Fe] = +0.2), whereas Mn is found to be underabundant ([Mn/Fe] = -0.4). We find a solar abundance of O; however, this is somewhat uncertain because of the dependence of the O abundance on the adopted atmospheric parameters and the coupling between C and O abundances in these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe] ~ +0.4) in all stars examined.", "doi": "10.1086/322116", "issn": "0004-6256", "publisher": "American Astronomical Society", "publication": "Astronomical Journal", "publication_date": "2001-09", "series_number": "3", "volume": "122", "issue": "3", "pages": "1469-1485" }, { "id": "authors:gdjw9-4f348", "collection": "authors", "collection_id": "gdjw9-4f348", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190524-135043421", "type": "article", "title": "Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15", "author": [ { "family_name": "Behr", "given_name": "Bradford B.", "clpid": "Behr-B-B" }, { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "McCarthy", "given_name": "James K.", "clpid": "McCarthy-J-K" } ], "abstract": "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 T_(eff) ~ 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km s^(-1) and two stars at v sin i ~ 35 km s^(-1). Most of the stars at T_(eff) \u2265 10,000 K, however, are slowly rotating, v sin i < 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 T_(eff), 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.", "doi": "10.1086/312524", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2000-03-01", "series_number": "1", "volume": "531", "issue": "1", "pages": "L37-L40" }, { "id": "authors:y8dqv-4ys46", "collection": "authors", "collection_id": "y8dqv-4ys46", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190524-141458208", "type": "article", "title": "A New Spin on the Problem of Horizontal\u2010Branch Gaps: Stellar Rotation along the Blue Horizontal Branch of Globular Cluster M13", "author": [ { "family_name": "Behr", "given_name": "Bradford B.", "clpid": "Behr-B-B" }, { "family_name": "Djorgovski", "given_name": "S. G.", "orcid": "0000-0002-0603-3087", "clpid": "Djorgovski-S-G" }, { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "McCarthy", "given_name": "James K.", "clpid": "McCarthy-J-K" }, { "family_name": "C\u00f4t\u00e9", "given_name": "Patrick", "orcid": "0000-0003-1184-8114", "clpid": "C\u00f4t\u00e9-P" }, { "family_name": "Piotto", "given_name": "Giampaolo", "clpid": "Piotto-G" }, { "family_name": "Zoccali", "given_name": "Manuela", "clpid": "Zoccali-M" } ], "abstract": "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 T_(eff) 11,000 K and reveals a pronounced difference in stellar rotation on either side of this feature\u2014blueward 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 \u2243 40 km s^(-1)) previously observed by R. C. Peterson and coworkers. Taken together with these prior results, our measurements indicate that a star's 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.", "doi": "10.1086/308219", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2000-01-10", "series_number": "2", "volume": "528", "issue": "2", "pages": "849-853" }, { "id": "authors:4a4st-fc128", "collection": "authors", "collection_id": "4a4st-fc128", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190528-134609249", "type": "article", "title": "An Abundance Analysis for Five Red Horizontal-Branch Stars in the Extremely Metal-rich Globular Cluster NGC 6553", "author": [ { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "Gratton", "given_name": "Raffaele G.", "clpid": "Gratton-R-G" }, { "family_name": "Behr", "given_name": "Bradford B.", "clpid": "Behr-B-B" }, { "family_name": "Carretta", "given_name": "Eugenio", "clpid": "Carretta-E" } ], "abstract": "We provide a high-dispersion line-by-line abundance analysis of five red horizontal-branch (HB) stars in the extremely metal-rich Galactic globular cluster NGC 6553. These red HB stars are significantly hotter than the very cool stars near the tip of the giant branch in such a metal-rich globular cluster, and hence their spectra are much more amenable to an abundance analysis than would be the case for red giants. We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the mean abundance in the Galactic bulge found by McWilliam & Rich and considerably higher than that obtained from an analysis of two red giants in this cluster by Barbuy and coworkers. The relative abundance for the best-determined \u03b1-process element (Ca) indicates an excess of \u03b1-process elements of about a factor of 2. The metallicity of NGC 6553 reaches the average of the Galactic bulge and of the solar neighborhood.", "doi": "10.1086/307765", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "1999-10-01", "series_number": "2", "volume": "523", "issue": "2", "pages": "739-751" }, { "id": "authors:3a6s7-ajm31", "collection": "authors", "collection_id": "3a6s7-ajm31", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190528-140622271", "type": "article", "title": "Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13", "author": [ { "family_name": "Behr", "given_name": "Bradford B.", "clpid": "Behr-B-B" }, { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "McCarthy", "given_name": "James K.", "clpid": "McCarthy-J-K" }, { "family_name": "Djorgovski", "given_name": "S. George", "orcid": "0000-0002-0603-3087", "clpid": "Djorgovski-S-G" } ], "abstract": "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 (T_(eff) > 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] \u2243-1.5 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\u2014the gravitational settling of helium and the radiative levitation of the other elements\u2014in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the cluster's horizontal branch and on estimates of its age and distance.", "doi": "10.1086/312052", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "1999-06-01", "series_number": "2", "volume": "517", "issue": "2", "pages": "L135-L138" } ]