Chromospheres of two red giants in NGC 6752

Details Chromospheres of two red giants in NGC 6752 Chromospheres of two red giants in NGC 6752

Sep 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...361l...9d&link_type=abstract

Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 361, Sept. 20, 1990, p. L9-L13. Research supported by SERC.

Astronomy and Astrophysics

Astrophysics

20

Chromosphere, Globular Clusters, Red Giant Stars, Emission Spectra, H Alpha Line, Metallicity, Stellar Evolution, Ultraviolet Spectra

Scientific paper

Two red giant stars, A31 and A59, in the globular cluster NGC 6752 exhibit Mg II (2800 A) emission with surface fluxes comparable to those observed among metal-deficient halo field giants, and among low-activity Population I giants. Optical echelle spectra of these cluster giants reveal emission in the core of the Ca II K (3933.7 A) line, and in the wing of the H-alpha (6562.8 A) profile. Asymmetries exist both in the emission profiles and the line cores. These observations demonstrate unequivocally the existence of chromospheres among old halo population giants, and the presence of mass outflow in their atmospheres. Maintenance of a relatively constant level of chromospheric activity on the red giant branch contrasts with the decay of magnetic dynamo activity exhibited by dwarf stars and younger giants. A purely hydrodynamic phenomenon may be responsible for heating the outer atmospheres of these stars, enhancing chromospheric emission, thus extending the atmospheres and facilitating mass loss.

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