MBCAA Observatory

V884 Her: a polar

Observed: 11, 19 Feb, 6, 12 Mar, 6 Apr, 27 May 2015

Michel Bonnardeau
19 Oct 2015, revised 1 Nov 2015, 10 May 2018

Abstract

The linear polarization of this cataclysmic polar is detected, and the orbital minima are found to be way off from the 1993 ephemeris.

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 and is one of the brightest polars (in 2015).

Observations

The observations were carried out with a 203 mm f/6.3 Schmidt-Cassegrain telescope, a filter wheel and a SBIG ST7E camera (KAF401E CCD, mostly red sensitive). The observations for the 2 sessions of February were done through a Clear filter. For the other sessions, two polarized filters were used alternatively, one with its axis roughtly parallel to the RA axis (the "X polarization filter"), the other roughtly parallel to the DEC axis (the "Y polarization filter"). The exposure durations were 60s long, whatever the filter.

The comparison star is 14CMC180220.8+180726, with an R_c 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 R_c magnitude is used as the unfiltered magnitude because of the mostly red sensitive CCD. It is also used for the polarization measurements.

The check star is 14CMC180147.5+180440. It is observed with the following magnitudes:
C: 12.967 ± 0.014 ± 0.018
X pol: 12.889 ± 0.025 ± 0.029
Y pol: 12.881 ± 0.023 ± 0.028
where the first ± is the average statistical uncertainty (computed as the quadratic sum of the statistical uncertainties on the check star and the comparison star) and the second ± is the standard deviation.

149 useful images were acquired through the Clear filter (in 2 sessions), 234 through the X polarization filter, 229 through the Y polarization filter, alternatively (in 4 sessions).

An example of a light curve:

Red: through the X polarization filter, Green: through the Y polarization filter. The error bars are the 1-sigma statistical uncertainties.

Phase analysis

An ephemeris for the eclipse minima, t(n)=T+P*n, is given by Greiner et al (1998), with:

T = 2,449,242.3124(21) HJD (11 Sept 1993)
P = 0.07847977(11) day.

However, when used with my data, the orbital minima are no longer at phase 0. So I need to modify T as:
T = 2,457,088.6715(15) HJD (7 Mar 2015)

The phase plot for the data with the Clear filter:

The phase plot for the data with the polarization filters:

Red: X polarization, Green: Y polarization.

V884 appears to be polarized around the orbital minima. To compute the magnitude difference between the 2 polarizations, as the measurements are done alternatively, the data are interpolated so as to be at the same phases. The following are phase plots of the linear polarization:

The object is brighter in the X polarization before the minimum, and dimmer after.

Conclusions

Between the T of the ephemeris of Greiner et al (1998) and mine there are 99979.38 orbital/spin rotations, not an integer number. The 0.38 dephasing is much too large to be explained by the uncertainty on the period. The period may be changing and fairly fast. It would be interesting to have measurements between 1993 and 2015 to see how this is changing.

The linear polarization appears to reach ± 4% around the orbital minima, a fairly common occurence for polars. Howing to the brightness, the polarization may be detetected even with an amateur setup.