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外文期刊>The Astrophysical journal
>IS THERE A METALLICITY CEILING TO FORM CARBON STARS?—A NOVEL TECHNIQUE REVEALS A SCARCITY OF C STARS IN THE INNER M31 DISK*
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IS THERE A METALLICITY CEILING TO FORM CARBON STARS?—A NOVEL TECHNIQUE REVEALS A SCARCITY OF C STARS IN THE INNER M31 DISK*
We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of asymptotic giant branch (AGB) stars to develop a new tool for efficiently distinguishing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we find a surprising lack of C?stars, contrary to the findings of previous C?star searches in other regions of M31. We find only one candidate C?star (plus up to six additional, less certain C?star candidates), resulting in an extremely low ratio of C?to M stars () that is one to two orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O 1 in their atmospheres. These observations provide stringent constraints to evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C?stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies.
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