Bibcode
Mészáros, Sz.; Dupree, A. K.; Szalai, T.
Referencia bibliográfica
The Astronomical Journal, Volume 137, Issue 5, pp. 4282-4295 (2009).
Fecha de publicación:
5
2009
Número de citas
27
Número de citas referidas
25
Descripción
High-resolution spectra of 123 red giant stars in the globular cluster
M13 and 64 red giant stars in M92 were obtained with Hectochelle at the
MMT telescope. Emission and line asymmetries in Hα and Ca II K are
identified, characterizing motions in the extended atmospheres and
seeking differences attributable to metallicity in these clusters and
M15. On the red giant branch, emission in Hα generally appears in
stars with T eff lsim 4500 K and log L/L sungsim
2.75. Fainter stars showing emission are asymptotic giant branch (AGB)
stars or perhaps binary stars. The line-bisector for Hα reveals
the onset of chromospheric expansion in stars more luminous than log
(L/L sun) ~ 2.5 in all clusters, and this outflow velocity
increases with stellar luminosity. However, the coolest giants in the
metal-rich M13 show greatly reduced outflow in Hα most probably
due to decreased T eff and changing atmospheric structure.
The Ca II K3 outflow velocities are larger than shown by
Hα at the same luminosity and signal accelerating outflows in the
chromospheres. Stars clearly on the AGB show faster chromospheric
outflows in Hα than RGB objects. While the Hα velocities on
the RGB are similar for all metallicities, the AGB stars in the
metal-poor M15 and M92 have higher outflow velocities than in the
metal-rich M13. Comparison of these chromospheric line profiles in the
paired metal-poor clusters, M15 and M92, shows remarkable similarities
in the presence of emission and dynamical signatures, and does not
reveal a source of the "second-parameter" effect.