Journal Article

On the impossible NGC 4372 V1 and V2: an extended AGB to the [Fe/H] = −2.2 cluster

I. McDonald, A. A. Zijlstra, A. F. Rajoelimanana and C. I. Johnson

in Monthly Notices of the Royal Astronomical Society: Letters

Published on behalf of The Royal Astronomical Society

Volume 429, issue 1, pages L65-L69
Published in print February 2013 |
Published online November 2012 | e-ISSN: 1745-3933 | DOI: http://dx.doi.org/10.1093/mnrasl/sls024

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The asymptotic giant branch (AGB) of the globular cluster NGC 4372 appears to extend to unexpectedly high luminosities. We show, on the basis of proper motions and spatial distribution, that the extended AGB is indeed a likely part of the cluster. We also present the first spectra of the very cool (2600 K), very luminous (8000 L), very dusty, oxygen-rich, purported long-period variable stars V1 and V2 that define the AGB tip. In particular, on the basis of their radial velocities, we conclude that V1 and V2 are probably members. We find that V1 and V2 are likely undergoing the superwind phase that terminates their nuclear-burning evolution. We hypothesize that the mass-loss processes that terminate the AGB are inhibited in NGC 4372 due to a lack of atmospheric pulsation and the high gas-to-dust ratio in the ejecta, leading to a delay in the associated enhanced mass loss and dust production. Previously predicted, but never observed, this explains the high mass of the white dwarf in Pease 1 in M15 without the need to invoke a stellar merger. If commonplace, this phenomenon has implications for the mass return from stars, the production of carbon stars and supernovae through the Universe's history, and the AGB contribution to light from unresolved metal-poor populations.

Keywords: stars: AGB and post-AGB; circumstellar matter; stars: mass-loss; stars: winds, outflows; globular clusters: individual: NGC 4372; infrared: stars

Journal Article.  3161 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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