AAVSO: American Association of Variable Star Observers

Expansion and Reinterpretation of the O–C Diagram of the High Amplitude δ Scuti Star RS Gruis

Volume 48 number 2 (2020)

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Roy A. Axelsen
AAVSO; Astronomical Association of Queensland; Variable Stars South, P. O. Box 706, Kenmore, Queensland 4069, Australia; reaxelsen@gmail.com
Tim Napier-Munn
Astronomical Association of Queensland, 49 Limosa Street, Bellbowrie, Queensland 4070, Australia; tgnm@bigpond.cocm


Photometry obtained with a digital single lens reflex (DSLR) camera and a CMOS astronomical camera from 2016 to 2020 expands the O–C diagram of RS Gru to cover a span of nearly 68 years. Analysis of the data reveals that a previous report of a constant rate of increase of the period between October 2003 and July 2013 may not be correct. Instead, evidence is presented that there was a constant rate of increase of the period from October 2003 to October 2010 followed by a constant but slower rate of increase from July 2011 to July 2020, with the latter described by the expression dP / dt = 2.99 (0.96) × 10–8 d yr–1 or dP / Pdt = 2.04 (0.65) × 10–7 yr–1. A new quadratic ephemeris derived from the 2011 to 2020 observations is TOM (HJD) = 2459045.1247 (0.0002) + 0.14701221 (0.00000004) E + 6.0X10–12 (1.9 × 10–12) E2 for which zero time occurred on 14 July 2020 UTC.