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Title: The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud

Abstract

We present a 9 million star color-magnitude diagram (9M CMD) of the Large Magellanic Cloud (LMC) bar. The 9M CMD reveals a complex superposition of different-age and -metallicity stellar populations, with important stellar evolutionary phases occurring over 3 orders of magnitude in number density. First, we count the nonvariable red and blue supergiants and the associated Cepheid variables and measure the stellar effective temperatures defining the Cepheid instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly evolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodal distribution in brightness, which we interpret as discrete old populations ((greater-or-similar sign)1 Gyr). The faint AGB sequence may be metal-poor and very old. Comparing the mean properties of giant branch and horizontal-branch (HB) stars in the 9M CMD with those of clusters, we identify NGC 411 and M3 as templates for the admixture of old stellar populations in the bar. However, there are several indications that the old and metal-poor field population has a red HB morphology: the RR Lyrae variables lie preferentially on the red edge of the instability strip, the AGB bump is very red, and the ratiomore » of AGB bump stars to RR Lyrae variables is quite large. If the HB second parameter is age, the old and metal-poor field population in the bar likely formed after the oldest LMC clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the {approx}1 million red HB clump giants in the 9M CMD with the same old and metal-poor population producing the RR Lyrae stars and the AGB bump. In this case, compared with the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyrae stars and AGB bump. Last, we show that the surface density profile of RR Lyrae variables is fitted by an exponential, favoring a disklike rather than a spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk. (c) 2000 The American Astronomical Society.« less

Authors:
; ; ; ; ; ; ; ; ;  [1]
  1. (and others)
Publication Date:
OSTI Identifier:
20217305
Resource Type:
Journal Article
Journal Name:
Astronomical Journal
Additional Journal Information:
Journal Volume: 119; Journal Issue: 5; Other Information: PBD: May 2000; Journal ID: ISSN 0004-6256
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; MAGELLANIC CLOUDS; STARS; STAR EVOLUTION; ELEMENT ABUNDANCE; LUMINOSITY; COLOR; SPATIAL DISTRIBUTION; EXPERIMENTAL DATA

Citation Formats

Alcock, C., Allsman, R. A., Alves, D. R., Axelrod, T. S., Basu, A., Becker, A. C., Bennett, D. P., Cook, K. H., Drake, A. J., and Freeman, K. C. The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud. United States: N. p., 2000. Web. doi:10.1086/301326.
Alcock, C., Allsman, R. A., Alves, D. R., Axelrod, T. S., Basu, A., Becker, A. C., Bennett, D. P., Cook, K. H., Drake, A. J., & Freeman, K. C. The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud. United States. doi:10.1086/301326.
Alcock, C., Allsman, R. A., Alves, D. R., Axelrod, T. S., Basu, A., Becker, A. C., Bennett, D. P., Cook, K. H., Drake, A. J., and Freeman, K. C. Mon . "The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud". United States. doi:10.1086/301326.
@article{osti_20217305,
title = {The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud},
author = {Alcock, C. and Allsman, R. A. and Alves, D. R. and Axelrod, T. S. and Basu, A. and Becker, A. C. and Bennett, D. P. and Cook, K. H. and Drake, A. J. and Freeman, K. C.},
abstractNote = {We present a 9 million star color-magnitude diagram (9M CMD) of the Large Magellanic Cloud (LMC) bar. The 9M CMD reveals a complex superposition of different-age and -metallicity stellar populations, with important stellar evolutionary phases occurring over 3 orders of magnitude in number density. First, we count the nonvariable red and blue supergiants and the associated Cepheid variables and measure the stellar effective temperatures defining the Cepheid instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly evolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodal distribution in brightness, which we interpret as discrete old populations ((greater-or-similar sign)1 Gyr). The faint AGB sequence may be metal-poor and very old. Comparing the mean properties of giant branch and horizontal-branch (HB) stars in the 9M CMD with those of clusters, we identify NGC 411 and M3 as templates for the admixture of old stellar populations in the bar. However, there are several indications that the old and metal-poor field population has a red HB morphology: the RR Lyrae variables lie preferentially on the red edge of the instability strip, the AGB bump is very red, and the ratio of AGB bump stars to RR Lyrae variables is quite large. If the HB second parameter is age, the old and metal-poor field population in the bar likely formed after the oldest LMC clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the {approx}1 million red HB clump giants in the 9M CMD with the same old and metal-poor population producing the RR Lyrae stars and the AGB bump. In this case, compared with the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyrae stars and AGB bump. Last, we show that the surface density profile of RR Lyrae variables is fitted by an exponential, favoring a disklike rather than a spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk. (c) 2000 The American Astronomical Society.},
doi = {10.1086/301326},
journal = {Astronomical Journal},
issn = {0004-6256},
number = 5,
volume = 119,
place = {United States},
year = {2000},
month = {5}
}