Mass Loss in Galactic Globulars: Red Giant Stragglers and Helium White Dwarfs

Details Mass Loss in Galactic Globulars: Red Giant Stragglers and Helium White Dwarfs Mass Loss in Galactic Globulars: Red Giant Stragglers and Helium White Dwarfs

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994rmxaa..29..153l&link_type=abstract

Revista Mexicana de Astronomia y Astrofisica, 29, 153

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RESUMEN En este trabajo se estudia la evoluci6n de las gigantes rojas de poblaci6n II con perdida de masa, esquematizada a trave's de la formula de Reimers. Se presentan ca'lculos evolutivos para modelos caracterizados por M = 0.8M0, Y = 0.23 y Z = 0.0002, y un amplio conjunto de valores del parametro que determina la tasa de pe'rdida de masa. Se encuentra que, para valores de TI mayores que 0.70 aproximadamente, Ia disminuci6n de Ia masa en la envolvente obliga a la estrella a dejar la rama de las gigantes, antes de que pueda ocurrir el flash del helio. Sin embargo, la evoluci6n del nu'cleo no se afecta por las condiciones de la superficie durante tiempo despue's de la fase de gigante roja, de manera que modelos con menor que 0.80 logran encender el helio en la parte azul del diagrama H-R. Se discute tambien Ia variaci6n de este escenario con la metalicidad. Finalmente, se analiza la producci6n y el enfriamiento de enanas blancas de helio producidas por el mecanismo de perdida de masa. En la ultima secci6n se discuten los vinculos te6ricos sobre la posible existencia de enanas de helio, y se muestra c6mo una fracci6n de "RG stragglers", tan pequeffa como el 10%, puede dar una contribuci6n detectable a las secuencias de enfriamiento de enanas blancas observadas en los cumulos. ABSTRACT The evolution of Population II Red Giants has been studied by adopting Reimers' formalism for the efficiency of mass loss. Evolutionary computations for models of 0.8M0, Y = 0.23 and Z = 0.0002 are presented for a wide range of values of the parameter TI governing the mass loss rate. We find that for values of TI greater than about 0.70 the decrease in the envelope mass forces the star to leave the RGB before undergoing the He flash. However the core evolution is not affected by the surface conditions until long after the RG phase, and models with TI less than 0.80 eventually succeed in igniting the helium on the blue side of the H-R diagram. The variation of sud a scenario with metallicity is also discussed. Finally, we analyze the production and cooling of He white dwarfs produced by the mass loss mechanism. Theoretical constraints concerning the possible occurrence of He dwarfs are discussed, showing that even a 10% of RG stragglers can give contribution to the white dwarfs' cooling sequence observed in clusters. Key words: GLOBULAR CLUSTERS: GENERAL STARS: EVOLUTION - STARS: WHITE DWARFS STARS: MASS LOSS STARS: POPULATION II

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