MBCAA Observatory

RV Cet: a Blazhko pulsating star

Observed: 26 sessions in 2009-2011

Michel Bonnardeau
1 Jan, 7 Aug 2012
21 Aug 2012: web published

Abstract

Photometry observations of this RRAB pulsating star are presented, with a GIF animation showing the Blazhko modulation.

Introduction

RV Cet is a radially pulsating giant, slightly bluer than the Sun (RR Lyrae type), with an asymetric light curve (steep ascending branch) (RRAB type). From the ROTSE-I data (spanning 9 months), Wils et al (2006) derived a pulsation period of 0.62340d and they observed a Blazhko modulation with a period of 117d.

Observations

The observations were carried out with a 203mm SC telescope, Johnson B and V filters in a filter wheel, and a SBIG ST7E camera (KAF401E CCD). The exposure durations are 60s with the V filter, and 200s with the B filter.

List of the different sessions along with the filters used and the number of magnitude measurements:

2009-2010
season 18 Nov, BV
20 Nov, BV
26 Nov, BV
2 Jan, BV
11 Jan, BV
13 Jan, BV
17 Jan, BV
18 Jan, BV
21 Jan, BV
Totals 9 sessions
284 V
214 B

2010-2011
season 31 Aug, BV
1 Sep, V
3 Sep, BV
11 Sep, V
28 Oct, V
5 Nov, V
13 Nov, BV(2x6 only)
5 Feb, V
7 Feb, B(a few)V

Totals 9 sessions
738 V
57 B

2011-2012
season 29 Sep, BV
30 Sep, V
25 Nov, V
26 Nov, V
27 Nov, V
25 Dec, V
27 Dec, BV
28 Dec, V

Totals 8 sessions
1342 V
37 B

Totals 26 sessions
2364 V
308 B

For the differential photometry, the comparison star is GSC 5281-00670 with the magnitudes V=11.991 and B=12.515 (these magnitudes are computed from the CMC14 r' magnitude and the 2MASS magnitudes using the transformations of Bilir et al (2008) and of Smith et al (2002)). The check star is GSC 5281-00661 and is observed as having V=12.484+/-0.024+/-0.034 and B=13.053+/-0.024+/-0.029, where the first +/- is the average 1-sigma statistical uncertainty and the second one is the standard deviation.
Session 26 Nov 2011: the comparison star is GSC 5281-00386 with V=13.118 and B=14.156; this does not introduce any significant offset.

An example of light curves:

Green: with the V filter, Blue: with the B filter. The error bars are the 1-sigma statistical uncertainties.

Pulsation analysis

The pulsation is analyzed the following way:

for each session the data are folded with the GCVS ephemeris (actually from Clube et al (1969));

the resulting phase plots are overlayed. They match when they are at the same Blazhko phase. 5 groups each with very much the same Blazhko phase are found;

the lengths of the phase plots of the different groups are computed and added together, giving a total length;

the pulsation period is slightly changed and the total length is computed as a function of the period. The right period is the one that gives the shortest total length:

another iteration is made with the new period. The result is quite stable.

The resulting pulsation period is:

P = 0.623411 +/- 0.000001 day

Maxima are observed at: 834.52, 892.475, 924.275, 443.608, 498.504 HJD -2,455,000. They are not separed by an exact multiple of P because the maxima swing with the Blazhko phase: a maximum is a little early when the amplitude is high, and a little late when the amplitude is low. For the phase plots below I adopt the ephemeris t(n) = T+P*n with:

T = 2,455,924.275

The resulting phase plots for the V magnitudes:

The 5 different colors correspond to the 5 groups of data each with the same Blazhko phase.

The Blazhko phase origin is arbitrary. One group with a small pulsation phase range is not shown.

From these observations, the Blazhko period is:

P_B = 123 +/- 13 days.

Color analysis

A pulsation phase plot of the B magnitudes, along the V magnitudes measured at the same time:

Blue: the B magnitudes, Green: the V ones. The effect of the Blazhko modulation can be seen.