2014 programme

From light curves to light elements: New ephemerides for the Delta Scuti stars RS Gru and BS Aqr

Roy Axelsen

Astronomical Association of Queensland; Variable Stars South

RS Gru and BS Aqr are bright, high amplitude monoperiodic delta Scuti stars with periods of 3.5 hr and 4.75 hr, and amplitudes in V of 0.6 and 0.4 magnitude respectively. Previous studies of both stars revealed changing periods, a recognized feature of some delta Scuti variables. It was previously suggested that an unseen companion of BS Aqr may account for the pattern of period change.

For BS Aqr, 5 times of maximum were determined by DSLR photometry between 7 September and 26 October 2013, and were combined with 22 other times of maximum obtained by other observers between September 1973 and August 1995. An O-C diagram of these 27 times of maximum revealed a linear ephemeris which when calculated had the following light elements (with standard errors in brackets):

Tmax = HJD 2,456,543.0250(0.0005)+0.1978227646(0.0000000096)E

For RS Gru, 16 times of maximum were determined by photoelectric and DSLR photometry between September 2007 and July 2013, and were combined with 12 times of maximum obtained by three other observers between October 2003 and October 2010. An O-C diagram of these 28 times of maximum revealed a quadratic (second order polynomial) ephemeris which when calculated had the following light elements:

Tmax = HJD 2,452,920.02019(0.00064)+0.14701118(0.00000011)E+2.513 (0.466)E2

The ephemeris indicates that the period of RS Gru is increasing at a rate of 6.244(1.157)x10-8 d/yr, or 42.477(7.870) cycles/yr. To the author's knowledge this is the highest rate of increase in the pulsational period of this type of delta Scuti star ever recorded.