Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
Abstract
The statisticalparallax method is applied for the first time to space velocities of 270 classical Cepheids with proper motions adopted from HIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances based on the periodluminosity relation by Berdnikov et al. (1996). The distance scale of shortperiod Cepheids (with periods less than 9 days) is shown to require an average correction of 1520%, whereas statistical parallaxes of Cepheids with periods > 9 days are found to agree well with photometric distances. It is shown that the luminosities of shortperiod Cepheids must have been underestimated partly due to the contamination of this subsample by a substantial (20 to 40%) fraction of firstovertone pulsators. The statisticalparallax technique is also applied for the first time to 117 open clusters younger than 100 million years and with proper motions reduced to the HIPPARCOS reference system. It is concluded that a 0.120.15 mag increase of the distance scales of open clusters and Cepheids would be sufficient to reconcile the statisticalparallax results inferred for these two types of objects. Such approach leads to an LMC distance modulus of less than 18.40 mag, which agrees, within the errors, with the short distance scale for RR Lyrae variables and is at variance with the conclusions by Feast and Catchpole (1998) and Feast et al. (1998), who argue that the LMC distance modulus should be increased to 18.70 mag. The distance scale based on the Cepheid periodluminosity relation by Berdnikov and Efremov (1985) seems to be a good compromise. Extragalactic distances, which rely on longperiod Cepheids, seem to require no substantial correction. In addition to statistical parallaxes, kinematical parameters have been inferred for the combined sample consisting of Cepheids and openclusters: solarmotion components (U_{0} ,V_{0}, W_{0}) = (9, 12, 7) km/s (+/ 1 km/s); velocityellipsoid axes (σ_{U}; σ_{V}; σ_{W}) = (15.0, 10.3, 8.5) km/s (+/ 1 km/s); the angular velocity of rotation of the subsystem, ω_{0} = 28.7 +/ 1 km/s/kpc, the Oort constant A = 17.4 +/ 1.5 km/s, and the second derivative of angular velocity, ⋰ω_{0}= 1.15 +/ 0.2 km/s/kpc^{3}.
 Publication:

Astronomy Letters
 Pub Date:
 September 1999
 Bibcode:
 1999AstL...25..595R