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New Observations and Period Change Study for the Anomalous Cepheid in M 92

Osborn W., Kopacki G., Haberstroh J.

Acta Astronomica

2012

Vol. 62, No. 4

377--388

New observations of the variable star V7 in the globular cluster M 92 have been used to determine the parameters of its B, V and IC light curves. The star's classification as an anomalous Cepheid type is confirmed. The asymmetric shape of the light curve and position in the observed period-luminosity relation for anomalous Cepheids indicate the star is pulsating in the fundamental mode. Data from photographic plates, extending back to 1900, have been combined with published material and recent CCD observations to carry out a period change study. No evidence of a period change is found. Expected period change rates have been calculated using theoretical relations for the pulsational periods of anomalous Cepheids and evolutionary tracks for metal-poor horizontal branch stars with masses 1.0≤M/Msun≤1.6. Only models for stars close to central helium exhaustion show measurable rates, which indicates V7 is in the core helium burning phase of evolution.

SX Phoenicis Stars in the Globular Cluster M 92

Kopacki G.

Acta Astronomica

2007

Vol. 57, No. 1

49--60

We present results of a search for SX Phe stars in the globular cluster M 92. In photometric reductions, we used the image subtraction method supplemented with a procedure of searching for short-period variables, based on the examination of each pixel in the CCD frame. In addition to two SX Phe stars already known in the cluster, we have found five new variables of this type. One SX Phe variable, v34, shows double-mode pulsation with a period ratio equal to 0.741 or 0.790. The second value would indicate oscillations in radial fundamental and first overtone modes. Combining our ground based data and the WFPC HST observations, we determine the average V magnitudes and (V-IC) colors for three SX Phe stars. Moreover, from an estimate of the scatter in the HST data of all observed blue stragglers we suspect three stars to be variable.