The Zero Point of the PeriodLuminosity Curve.
Abstract
From studies of radial velocities the group motion of 67 RR Lyrae variables has been found to be ii~ Â± 15 km/sec, and that of i~ Cepheids, 28.1 Â± 1.4 km/sec. Their respective mean peculiar motions are 72 Â± 5 and 14.0 Â± o.6 km/sec; and their rotational coefficients, 37 Â± 20 and 27.4 Â± 1.4, about a center in galactic longitude 3 26Â°. Mean parallactic and peculiar motions in seconds of arc have been derived from the proper motions of 141 of these stars. Comparisons of these with the values in kilo meters per second derived from the radial velocities give mean parallaxes of the proper motion stars. Assuming Joy's value of the rotational constant, A = 20.9 km/sec per kiloparsec, the rotational coefficients give measures of the mean distances of the radial velocity stars. The two sets of parallaxes, in combination with the corresponding sets of mean photographic magnitudes, determine two sets of absolute magnitudes which are in fair accord. The weighted means of the two determinations indicate corrections to the photographic periodluminosity curve amounting to Â±0.00 Â± 0.2 for the short period variables, and 0.14 Â± 0.2 for those of longer period, if we correct for galactic absorption of 0.85 mag. (photographic) per kiloparsec. When the data are further sub divided into period groups, the computed absolute magnitudes are in good agreement with those derived from the curve. The conclusion is that the radialvelocity and propermotion data now available confirm both the zero point and shape of the photographic periodluminosity curve
 Publication:

The Astrophysical Journal
 Pub Date:
 March 1939
 DOI:
 10.1086/144038
 Bibcode:
 1939ApJ....89..218W