4.7 Article

Long Secondary Periods in variable red giants

Journal

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 399, Issue 4, Pages 2063-2078

Publisher

WILEY-BLACKWELL PUBLISHING, INC
DOI: 10.1111/j.1365-2966.2009.15401.x

Keywords

stars: AGB and post-AGB; binaries: close; stars: oscillations

Funding

  1. Australian Research Council's Discovery Projects [DP0663447]
  2. US Department of Energy through the University of California
  3. Lawrence Livermore National Laboratory [W-7405-Eng-48]
  4. National Science Foundation through the Center for Particle Astrophysics of the University of California [AST-8809616]
  5. Mount Stromlo and Siding Spring Observatory
  6. STFC [ST/G002622/1] Funding Source: UKRI
  7. Science and Technology Facilities Council [ST/G002622/1] Funding Source: researchfish
  8. Australian Research Council [DP0663447] Funding Source: Australian Research Council

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We present a study of a sample of Large Magellanic Cloud red giants exhibiting Long Secondary Periods (LSPs). We use radial velocities obtained from VLT spectral observations and MACHO and OGLE light curves to examine properties of the stars and to evaluate models for the cause of LSPs. This sample is much larger than the combined previous studies of Hinkle et al. and Wood, Olivier & Kawaler. Binary and pulsation models have enjoyed much support in recent years. Assuming stellar pulsation, we calculate from the velocity curves that the typical fractional radius change over an LSP cycle is greater than 30 per cent. This should lead to large changes in T(eff) that are not observed. Also, the small light amplitude of these stars seems inconsistent with the radius amplitude. We conclude that pulsation is not a likely explanation for the LSPs. The main alternative, physical movement of the star - binary motion - also has severe problems. If the velocity variations are due to binary motion, the distribution of the angle of periastron in our large sample of stars has a probability of 1.4 x 10(-3) that it comes from randomly aligned binary orbits. In addition, we calculate a typical companion mass of 0.09 M(circle dot). Less than 1 per cent of low-mass main-sequence stars have companions near this mass (0.06-0.12 M(circle dot)) whereas similar to 25-50 per cent of low-mass red giants end up with LSPs. We are unable to find a suitable model for the LSPs and conclude by listing their known properties.

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