Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aj....106..142b&link_type=abstract
Astronomical Journal (ISSN 0004-6256), vol. 106, no. 1, p. 142-153.
Other
17
Asymptotic Giant Branch Stars, Cyano Compounds, Globular Clusters, Methylidyne, Red Giant Stars, Stellar Magnitude, Absorption Spectra, Spectral Bands, Stellar Atmospheres, Variations
Scientific paper
Blue spectra of a sample of 24 bright M55 red giants within the magnitude range of V between 11.8 and 14 have been obtained with the CTIO 4 m telescope and Argus multifiber spectrograph. Indices sensitive to absorption by the 3883 A CN band were measured from these spectra and combined with the CN data of Smith and Norris (1982) to yield CN band strength measurements for 47 stars. Aside from the CH star L2406, no stars with unusually strong 3883 A CN bands were detected. Analysis with the aid of synthetic spectra indicates a star-to-star spread in (N/A) of less than 0.3 dex. A similar treatment of the CTIO G-band (CH) strengths suggests a decrease in C abundance of 0.1 to 0.3 dex with increasing luminosity on the red giant branch from M(V) about -0.1 to -2.3. Similar trends have also been noted in other metal-poor clusters and are believed to be the result of the progressive mixing of CN-processed material into the stellar atmosphere. The (C/A) abundances of the asymptotic giant branch stars are consistent with those of the brightest red giants in our sample, implying little mixing during horizontal and asymptotic branch evolution. The overall star-to-star variation in (C/A) on the red giant branch for M(V) less than -0.8 appears to be 0.2 dex. From these data it appears that M55 may be atypical of the Galactic globular clusters in terms of its homogeneity of star-to-star surface abundances of both C and N.
Bell Roger A.
Briley Michael M.
Hesser James Edward
Smith Graeme H.
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