Unusual variable J2229.3+1853 in Pegasus

The White Dwarf + Main Sequence (WDMS) object CSS J222918.9+185340 listed in catalogs as an ellipsoid variable with the period about 4.54 hr and the range 16.2-16.6m has dramatically changed the shape of the light curve increasing the amplitude almost 5x, from 0.4m to 2m! It is likely to undergo the transition from WDMS to cataclysmic variable stage right now. This evolutionary stage must be short lived, and we might have a unique chance to follow it. Observations during the late summer - early fall 2016 are required!

Introduction

This variable has attracted attention after the detection of an outburst from CSS J162117.4+441254 by Catalina Sky Survey on 2016 June 03 (ATel #9112). J1621.3+4412 was originally discovered as a short-period (P=0.207852 d) eclipsing variable of W UMa (EW) type in LINEAR data or an ellipsoid (ELL) variable by CSS. However, it has shown a dwarf nova outburst from 15.0 to 13.3m in June 2016. It has an UV counterpart GALEX J162117.4+441254 with GALEX magnitudes FUV=20.13 and NUV=19.65.

Following the detection of an outburst from J1621.3+4412, the author has searched for other short-period variables from Catalina surveys with the similar properties in ultraviolet. Among 136 candidate ellipsoidal variables with P below 0.22d (Drake et al., 2014) 6 objects have (FUV-NUV) color index less than 1.0 in GALEX GR6/GR7 data release. It was logical to suggest there may be additional cataclysmic variables hiding among those ELL type variables. The list of candidates was posted in [vsnet-alert 19859] on 2016 June 04 with the call for monitoring these variables and checking the archival data for possible outbursts. Among these six candidates three stars reside in Pegasus and are best suited for observations in late summer - early autumn.

Observations

On the night of 2016 July 18/19 the variable CSS J222918.9+185340 = GALEX J222918.9+185340 was observed using 0.32-m T18 instrument of iTelescope.Net equipped with SBIG STXL-6303E CCD. A total of 150 unfiltered 60-sec exposures were obtained from 2016-07-18 23:32 UT to 2016-07-19 02:49 UT. Despite the 0.36-m amplitude shown before in Catalina data during 2005-2013, the variable has unexpectedly revealed a much larger variability, reaching 18.2+/-0.3 at the minimum light. Thus, the amplitude has increased by ~5x from 0.4m to ~2m.

The images below show the phased light curve of CSS J222918.9+185340 from publicly available Catalina Realtime Transient Survey (CRTS) data folded with an orbital period P=0.189184 d (4.5404 hr) and observations with T18 on 2016 July 18/19.

Range of variability is ~16.2-16.6m. Note the asymmetric shape of light curve which is unusual for an ellipsoid variable.

Range of variability is ~16.2-18.2m. Comparison star 135 was used with unfiltered CSS magnitude 13.4m.

Since the observations on July 18/19 lasted for 3.3 hr, they cover only about 2/3rd of J2229.3+1853 orbital period, and the maximum was likely missed. Magnitude at minimum is uncertain by 0.3m. Further observations are required to improve the range of variability and to monitor the changes of light curve shape.

Further analysis

Data from Catalina Sky Survey were chopped into two parts (one in 2005-2008 and the other in 2009-2013) and the phased curves were plotted for both data sets. The comparison of the light curves is shown at the animation. Apparently some changes in the system were already taking part then.

Note the change of relative heights of maxima over the time

Formal data from catalogs

  • SDSS J222918.95+185340.2 (u=18.85 g=17.90 r=16.98 i=15.91 z=15.25)
  • GALEX J222918.9+185340 (FUV=20.30 NUV=19.63)
  • 2MASS J22291893+1853403 (J=14.03 H=13.39 K=13.20)

    The variable is obviously the binary system consisting of a white dwarf and red dwarf close to filling its Roche lobe. According to Parsons et al., 2015, the spectral type of the red component is M3, and the star is "most likely right on the verge of starting mass transfer".

    Possible interpretation

    It looks like either the red component is growing on its way to finally filling Roche lobe and occulting the larger part of the white dwarf, or the geometry of system is changing (due to the orbit precession or rotation of the red companion). In the first case, we can measure directly the expansion rate of the M3 dwarf at the moment of mass transfer start.

    Call for observations

    The variable is likely to undergo the transition from the WDMS binary to cataclysmic variable right now. This evolution stage of the binary star should be short by the nature, and dynamic changes of the light curve on the scale of weeks are expected. Regular monitoring to measure the minimum depth is required. To reach the maximun signal/noise ratio, please observe unfiltered. If you have a telescope with an aperture of 40-cm or larger, please run time series in B and R filters, changing them after each exposure. The suggested exposure times are from 60 to 180 sec, as the period is about 272 min. Please submit the observations of CSS_J162117.4+441254 to AAVSO database using AUID 000-BLY-357.

    Finder charts

    Update on July 28: magnitudes of comparison stars are now from the latest AAVSO sequence X16339CBB. 

    135  V=13.547    137  V=13.741    143  V=14.260
157  V=15.695    165  V=16.535    171  V=17.068
175  V=17.466    179  V=17.872    182  V=18.160

    High resolution SDSS chart showing the r=19.5m neighbor within 5" to the West of the variable

    Related links

  • VSX details for CSS J222918.9+185340
  • Catalina Sky Survey light curve of CSS J222918.9+185340
  • iTelescope Network
  • Catalina Surveys Data Release 2
  • GALEX Catalog Search
  • AAVSO WebObs - submit your observations!

    References

  • A. Drake et al., ATel #9112
  • D. Denisenko, [vsnet-alert 19859]
  • S.G. Parsons et al., 2015, MNRAS Vol. 449, p. 2194

    Denis Denisenko (d.v.denisenko at gmail dot com)
    Sternberg Astronomical Institute of Moscow State University (SAI MSU)

    Page created: July 22nd, 2016 10:25+0300
    Last updated: July 28th, 2016 10:00+0300