MBCAA Observatory

V884 Her: an improved ephemeris

Observed: 2 sessions in 2018, 3 in 2017, 6 in 2016, 6 in 2015

Michel Bonnardeau
10 May 2018

Abstract

Photometric observations of the cataclysmic polar V884 Her are presented. An improved ephemeris for the eclipses is obtained.

Introduction

V0884 Her is a cataclysmic system, with an accreting orbiting white dwarf. The white dwarf is magnetic, with its spin period equal to the orbital period, and no accreting disc: this is a polar or AM Her type system.

V884 Her has an orbital/spin period of 1.884 hr (Greiner et al, 1998, hereafter G98). In 2015 I found that the positions of the eclipses were offset from the ephemeris of G98.

Observations

The observations were carried out with a 203mm f/6.3 Schmidt-Cassegrain telescope, a Clear filter (or, sometimes, through polarizers for the previous seasons), a ST7E camera (KAF-401E CCD). The exposure duration is 60s.

For the differential photometry the comparison star is 14CMC180220.8+180726, with a Rc magnitude of 12.194, computed from the CMC14 r' magnitude and the 2MASS magnitudes, owing to the transformations formula of Bilir et al (2008) and Smith et al (2002). The Rc magnitude is used as the unfiltered magnitude because of the mostly red sensitive CCD. The check star is 14CMC180147.5+180440.

An example of a light curve:

Red: V884 Her, Blue: the check star, shifted by +1.8mag. The error bars are the quadratic sum of the 1-sigma statistical uncertainties on the variable and of the comparison star. There is an eclipse at THJD 1214.626.

V884 Her was much fainter in 2018 than in the previous seasons (e.g., a drop of half a magnitude from 2017).

20 times of minima (ToM) for the eclipses are observed. They are converted from HJD to BJD using the University of Ohio on-line tool. The results are, with the number of cycles in the G98 ephemeris:

Season Filter ToM (BJD) Cycle

2015 CR 2457073.6828 99788

POL 2457088.6723 99979

2457094.6358 100055

2457119.5968 100373

2457170.3718 101020

2457170.4468 101021

2457170.5238 101022

2016 CR 2457466.6298 104795

2457513.4798 105392

2457513.5573 105393

POL 2457549.4238 105850

2457549.5038 105851

2017 CR 2457902.4273 110348

2457917.4268 110539

2457917.4988 110540

2457917.5768 110541

POL 2457922.4398 110603

2457922.5258 110604

2018 CR 2458214.6260 114326

2458226.6336 114479

AAVSO observations

The AAVSO database is searched for eclipses and the ToMs are converted from JD to BJD using the University of Ohio on-line tool. 11 ToMs are found:

Observer
code Filter ToM (BJD) Cycle Season

DGSA CV 2456517.490 92701 2013

2457321.278 102943 2015

V 2456446.545 91797 2013

MIW V 2456005.646 86179 2012

PXR V 2455888.241 84683 2011

2456178.382 88380 2012

2456132.468 87795

2456132.547 87796

2457532.472 105634 2016

2457532.552 105635

2457513.558 105393

O-C analysis and a new ephemeris

With the ephemeris of G98 (converted in BJD) the O-C diagram is:

Red dots: observations from the AAVSO of DGSA, Blue: MIW, Green: PXR. Open red circle: my observations.

A new ephemeris is derived from the 9 ToMs of G98, from 1990 to 1993, (converted into BJD) and the above 31 ToMs. A least squares method gives the new O-C diagram:

The offset disappears with the new ephemeris.

An improved ephemeris ToM(e)=T+P*e is obtained with a Monte Carlo method:
-3*10⁶ trials of random sets of T and P are tested and the set that gives the best fit is retained;
-10 such runs are executed;
-the adopted P and T are the averages from the 10 runs and the uncertainties are given by the standard deviations.
The results are:
T=2449242.314929(70) BJD
P=0.07848003378(59) d

Acknowledgements

The use of the on-line tool of the University of Ohio to convert JD and HJD into BJD, at http://astroutils.astronomy.ohio-state.edu/time/hjd2bjd.html, is acknowledged.

The use of the AAVSO International Database is acknowledged.