Low-luminosity intrinsic variables with periods less than 0.2 day.

Abstract

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Low-luminosity intrinsic variables with periods less than 0.2 day. Smith, Harlan J. Abstract New photoelectric and spectrographic observations, coupled with other data, provide evidence which indicates that most of the galactic so- called RR Lyrae (or cluster-type) variables with periods less than 0.2 day are not true RR Lyrae stars. On a period-spectrum diagram these variables lie parallel to, but displaced far from, the line corresponding to RR Lyrae types a and C. In addition, while the period-color relation for true galactic RR Lyrae stars is found to match very closely that long known for M3, the short-period variables form a relatively slender band of much smaller slope and again one which is quite displaced. Furthermore, amplitudes appear to diminish with increasing period, from 1.0 mag. at .05 day to 0.3 mag. at .20 day. In this group there are I I stars for which any information exists other than period and rough photographic light curve. A solar motion solution from the nine having known radial velocities gives a drift of only 66 km/sec, and a weakly determined apex which lies about half-way between those for the local system and the high- velocity stars. The proper motions for these objects, when reduced to the same apparent magnitude, decrease almost monatonically with increasing period. This strong suggestion of a period-luminosity relation is borne out by statistical parallax solutions, using components respectively parallel and perpendicular to the apparent solar motion, from which are found concordant absolute magnitudes of about 4.4 for the four variables with periods less than .10 day, and i.6 for those with periods between .075 and .175 day. Trigonometric parallaxes are available only for SX Phe and b Scu, but these give Mpg = 3.9 at .o56 day and 1.6 at .194 day, respectively. Light, color and velocity curves were used in the Wesselink-Stebbins method to derive radii for CY Aqr and VZ Cnc. The resulting sizes, 1.4 and 3.8 solar radii, indicate absolute magnitudes 2.5 and i.o which agree well with those derived by parallax methods. Such small radii also permit the same P p relation to apply both to the RR Lyrae type a and to the short- period variables, provided the masses of the latter are greater by a factor of about 2. The period-color relation for the galactic RR Lyrae variables alone has the theoretical slope corresponding to a single period-density relation and constant absolute magnitude; for the short- period stars the P p relation coupled with their observed period-luminosity relation can explain fully the much smaller slope of their period- color curve. Comparison with globular clusters where RR Lyrae stars are so numerous suggests that with a single possible exception variables such as those discussed above having extremely short periods, light curves of substantial amplitude and roughly type a form, and low absolute magnitude, do not occur in globular clusters. Thus we may conclude either that RR Lyrae stars are, at least in the galaxy, a much less homogeneous group than has previously been supposed; or that we are here dealing with a distinct class of galactic short-period variables characterized by small size and intermediate population, which apart from period resemble RR Lyrae stars in variation, but which trend toward classical cepheids in terms of galactic distribution and perhaps in mass. The differences from either group are sufficient to suggest that the name "dwarf cepheid" may be appropriate to the class. CY Aqr and Al Vel would then be prototype stars showing respectively extremes of simple and complex variation. Yale University Observatory, New Haven, Conn. Publication: The Astronomical Journal Pub Date: 1955 DOI: 10.1086/107170 Bibcode: 1955AJ.....60S.179S full text sources ADS |