MACHO observations of LMC red giants: Mira and semi-regular pulsators, and contact and semi-detached binaries Conference Paper uri icon

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abstract

  • The MACHO data base has been used to examine light curves of all red giant stars brighter than Mbol ∼ −2 in a 0.5° × 0.5° area of the LMC bar. Periods, often multiple, have been searched for in all stars found to be variable. Five distinct period-luminosity sequences have been found on the low mass (M ≲ 2.25M) giant branch. Comparison of observed periods, luminosities and period ratios with theoretical models identifies Miras unambiguously as radial fundamental mode pulsators, while semi-regular variables can be pulsating in the 1st, 2nd or 3rd overtone, or even the fundamental. All these variables lie on just 3 of the 5 distinct sequences, and they all appear to be on the AGB.The fourth sequence contains red giants on the first giant branch (FGB) or at the red end of the core-helium burning loops of intermediate mass stars (M ≳ 2.25M). The light curves of these stars strongly suggest that they are contact binaries, and they make up ∼0.5% of stars within 1 mag. of the FGB tip. Stars on the fifth sequence show semi-regular, eclipse-like light curves. The light curves and periods of these stars suggest that they are in semi-detached binaries, transfering mass to an invisible companion via a stellar wind or Roche lobe overflow. They make up ∼25% of AGB stars. If the existence of these red giant contact and semi-detached binaries is confirmed, then extant theories of binary star evolution will require substantial modification.

authors

  • Wood, PR
  • Alcock, C
  • Allsman, RA
  • Alves, D
  • Axelrod, TS
  • Becker, AC
  • Bennett, DP
  • Cook, KH
  • Drake, AJ
  • Freeman, KC
  • Griest, K
  • King, LJ
  • Lehner, MJ
  • Marshall, SL
  • Minniti, D
  • Peterson, BA
  • Pratt, MR
  • Quinn, PJ
  • Stubbs, CW
  • Sutherland, W
  • Tomaney, A
  • Vandehei, T
  • Welch, Douglas L

publication date

  • 1999