Category Archives: AJ

A Moderate Sample Size, Multielement Analysis of M12

NGC6218 :: new paper

    Johnson, Christian I.; Pilachowski, Catherine A.

A Moderate Sample Size, Multielement Analysis of the Globular Cluster M12 (NGC 6218)

We present chemical abundances of several proton-capture, α-, Fe-peak, and neutron-capture elements and radial velocities for 21 red giant branch (RGB) and asymptotic giant branch members of the Galactic globular cluster M12. Abundances are based on equivalent width measurements and synthetic spectral analyses of moderate-resolution spectra (R~15,000) obtained with the 3.5 m WIYN telescope and Hydra multifiber spectrograph. The stars observed range from the RGB tip (M0v=-2.47) down to about 0.50 mag above the level of the horizontal branch (M0v=+0.11). Our spectroscopic analysis suggests that M12 is a moderately metal-poor cluster with [Fe/H]=-1.54 (σ=0.09). While the Na abundances exhibit a range of 0.90 dex, Mg and Al abundances are enhanced by 0.37 and 0.54 dex and are nearly constant at all RGB luminosities, in contrast to the blue horizontal-branch cluster M13. The α- and Fe-peak elements indicate that M12 has undergone a similar chemical enrichment history to that of globular clusters and field stars of comparable metallicity, with <[α/Fe]〉=+0.33 (σ=0.11). M12 also appears to be slightly r-process-rich, with <[Eu/Ba,La]〉=+0.22 (σ=0.18).

The Astronomical Journal, Volume 132, Issue 6, pp. 2346-2359
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Image-Subtraction Photometry of Variable Stars in NGC 6388 and NGC 6441

New paper (“The Astronomical Journal”, Volume 132):

    Corwin, T. Michael; Sumerel, Andrew N.; Pritzl, Barton J.; Smith, Horace A.; Catelan, M.; Sweigart, Allen V.; Stetson, Peter B.

Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441

We have applied Alard’s image-subtraction method (ISIS ver. 2.1) to the observations of the globular clusters NGC 6388 and NGC 6441 previously analyzed using standard photometric techniques (DAOPHOT and ALLFRAME). In this reanalysis of observations obtained at the Cerro Tololo Inter-American Observatory, besides recovering the variables previously detected on the basis of our ground-based images, we have also been able to recover most of the RR Lyrae variables previously detected only in the analysis of Hubble Space Telescope WFPC2 observations of the inner region of NGC 6441. In addition, we report five possible new variables not found in the analysis of the Hubble Space Telescope observations of NGC 6441. This dramatically illustrates the capabilities of image-subtraction techniques applied to ground-based data to recover variables in extremely crowded fields. We have also detected 12 new variables and 6 possible variables in NGC 6388 not found in our previous ground-based studies. The revised mean period for RRab stars in NGC 6388 is 0.676 days, while the mean period of RRab stars in NGC 6441 is unchanged at 0.759 days. These values are among the largest known for any Galactic globular cluster. Additional probable type II Cepheids were identified in NGC 6388, confirming its status as a metal-rich globular cluster rich in Cepheids.

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