Carbon and nitrogen abundance variations among Galactic globular cluster stars

Thesis/Dissertation · Mon Jan 01 04:00:00 EST 1990

OSTI ID:5312990

Briley, Jr, M M

The spectra of stars within Galactic globular clusters spanning a wide range in metallicity (namely 47 Tucanae; (Fe/H) equals {minus}0.8, and NGC 6397; (Fe/H) equals {minus}2.0) were analyzed for differences in molecular features directly related to carbon and nitrogen abundances. A clear pattern of decreasing surface carbon abundance with evolutionary state (brightness) is observed in the red giant branch (RGB) stars of NGC 6397. This is typical of the metal poor clusters, as such trends also were seen by others in M92 and M15, and appears to be due to the mixing of carbon poor nitrogen rich material from the interiors of the stars to their surfaces. The weak G-band phenomenon is found not to be the result of anomalously low carbon abundances, but rather, due to a combination of both carbon abundances typical of RGB tip stars and the lower surface gravities of the asymptotic giant branch (AGB) stars. The carbon and nitrogen variations in the metal rich cluster 47 Tucanae are seen to be somewhat more complicated. A bimodal distribution in the strengths of the 3883 A CN-bands is observed down to M(sub v) is approximately equal to 5 (V is approximately equal to 18), which is well before the cluster main-sequence (MS) turnoff. Such variations occurring before the MS turnoff are very difficult for any theory based on internal mixing to account for. Thus a primordial origin for the CN-band strength differences may be necessary. However, on the basis of 3883 and 4215 A CN-band strengths in subgiant branch (SGB) stars, a difference in the C-12/C-13 ratio may exist between the CN-strong and normal stars. As large amounts of C-13 are expected during C yields N cycle processing, the difference in C-12/C-13 may be the result of varying degrees of mixing, which may additionally explain the CN differences. Observations of the 2.345 micron (13)CO band of four bright RGB stars confirm large amounts of C-13 in the atmospheres of CN-strong RGB tip stars.

• Research Organization: Maryland Univ., College Park, MD (United States)
• OSTI ID: 5312990
• Country of Publication: United States
• Language: English

Related Subjects

640102* -- Astrophysics & Cosmology-- Stars & Quasi-Stellar
Radio & X-Ray Sources
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ABUNDANCE
BRIGHTNESS
CARBON
CARBON 13
CARBON ISOTOPES
DATA
ELEMENTS
EVEN-ODD NUCLEI
EXPERIMENTAL DATA
GIANT STARS
INFORMATION
ISOTOPES
LIGHT NUCLEI
MAIN SEQUENCE STARS
NITROGEN
NONMETALS
NUCLEI
NUMERICAL DATA
OPTICAL PROPERTIES
PHYSICAL PROPERTIES
RED GIANT STARS
STABLE ISOTOPES
STAR CLUSTERS
STARS