Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters

Details Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999astl...25..595r&link_type=abstract

Astronomy Letters, Volume 25, Issue 9, September 1999, pp.595-607

Astronomy and Astrophysics

Astronomy

23

Scientific paper

The statistical-parallax 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 period-luminosity relation by Berdnikov et al. (1996). The distance scale of short-period Cepheids (with periods less than 9 days) is shown to require an average correction of 15-20%, whereas statistical parallaxes of Cepheids with periods > 9 days are found to agree well with photometric distances. It is shown that the luminosities of short-period Cepheids must have been underestimated partly due to the contamination of this subsample by a substantial (20 to 40%) fraction of first-overtone pulsators. The statistical-parallax 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 0.12-0.15 mag increase of the distance scales of open clusters and Cepheids would be sufficient to reconcile the statistical-parallax 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 period-luminosity relation by Berdnikov and Efremov (1985) seems to be a good compromise. Extragalactic distances, which rely on long-period 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 open-clusters: solar-motion components (U0 ,V0, W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes (σU; σV; σW) = (15.0, 10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of the subsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constant A = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity, ⋰ω0= 1.15 +/- 0.2 km/s/kpc3.

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