The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud Academic Article uri icon

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abstract

  • We present a 9 million star color-magnitude diagram (9M CMD) of the 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 non-variable supergiants, the associated Cepheids, and measure the effective temperatures defining the instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly-evolved AGB stars have a bimodal distribution in brightness, which we interpret as discrete old populations (>1 Gyr). The faint AGB may be metal-poor and very old. We identify the clusters NGC 411 and M3 as templates for the admixture of old stellar populations. However, there are indications that the old and metal-poor field population has a red HB morphology: the RR Lyraes lie on the red edge of the instability strip, the AGB-bump is very red, and the ratio of AGB-bump stars to RR Lyraes is quite large. If the HB second parameter is age, the old and metal-poor field population likely formed after the oldest clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the red HB clump giants with the same old and metal-poor population producing the RR Lyraes and the AGB-bump. In this case, compared to the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyraes and AGB-bump. Last, the surface density profile of RR Lyraes is fit by an exponential, favoring a disk-like rather than spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk. (ABRIDGED)

authors

  • Alcock, C
  • Allsman, RA
  • Alves, DR
  • Axelrod, TS
  • Basu, A
  • Becker, AC
  • Bennett, DP
  • Cook, KH
  • Drake, AJ
  • Freeman, KC
  • Geha, M
  • Griest, K
  • King, L
  • Lehner, MJ
  • Marshall, SL
  • Minniti, D
  • Nelson, CA
  • Peterson, BA
  • Popowski, P
  • Pratt, MR
  • Quinn, PJ
  • Stubbs, CW
  • Sutherland, W
  • Tomaney, AB
  • Vandehei, T
  • Welch, Douglas L

publication date

  • May 2000