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Understanding the parallaxes of Long Period Variables

Understanding the parallaxes of Long Period Variables. C. Babusiaux & A. Jorissen. Understanding the revised Hipparcos parallaxes The case of the parallaxe of Betelgeuse Models for Gaia. The Long Period Variables. Asymptotic giant branch stars crossing the instability strip

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Understanding the parallaxes of Long Period Variables

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  1. Understanding the parallaxes of Long Period Variables C. Babusiaux & A. Jorissen • Understanding the revised Hipparcos parallaxes • The case of the parallaxe of Betelgeuse • Models for Gaia

  2. The Long Period Variables • Asymptotic giant branch stars crossing the instability strip • Population tracers. Chemical evolution. Standard candles? • Pulsation mode? Period-luminosity relation? • Major improvement expected from Gaia !

  3. Astrometric observations of LPVs • Very red colours  difficult chromaticity correction • Colour variability  epoch colour index of chromaticity corr. • Large radii extended sources for Gaia • Time-varying brightness asymmetries  degradation of parallax Betelgeuse 40 mas

  4. Hipparcos Vol.3 figure 16.3: the chromaticity effect

  5. Revised Hipparcos parallaxes for LPVs • chromaticity correction with epoch V-I using Hp and VT (Platais et al. 2003, Pourbaix et al. 2003, Knapp et al. 2003) • Much improved parallaxes • Detection of false VIMs (47%)

  6. Large errors of the revised parallaxes

  7. Large errors of the revised parallaxes

  8. VLTI observations Large errors of the revised parallaxes

  9. The case of Betelgeuse

  10. The case of Betelgeuse • Unresolved feature on the surface of Betelgeuse are known and should be of the order of its parallax • V-I = 2.3, no variation. • R = M2/M1= 0.31 = 10% of the point source value, not expected for a 40 mas diameter (Hestroffer & Mignard 97) • Binarity ? Components at 0.1 and 0.5 arcsec (Karovska 86) too faint : about 3 and 4.5 magnitude differences with respect to the primary. Nothing in the Transit Data cleaned imaged (Quist & Lindegren 99) No variation of R with observation angle. • V-I correction ? Hipparcos vol 1: R rather insensitive to the colour…

  11. The case of Betelgeuse All stars in TD with Hp<4

  12. Models for Gaia : convection models Giant star model: Variation timescale = day Freytag model of supergiants 0.1 AU = 1 mas at 100 pc Variation timescale : year ! Svensson & Ludwig 05 Ludwig 05, in preparation

  13. Models for Gaia : hotspots observations Tuthill, Haniff & Baldwin (1997) interferometric observations of  Ori (Betelgeuse),  Sco (Antares) and  Her: • Flux : between 5 and 20% • Colours : similar to the disk • Variation timescales : a few months • Hipparcos : parallaxes of 5-8 mas. • Spots at about 5 and 20 mas  about 0.7 and 2.6 AU VLTI project of P. deLaverny on Betelgeuse

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