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Cataclysmic variable ASASSN–14dx contains a massive pulsating white dwarf, observations find

An international team of astronomers has performed optical photometric, polarimetric and spectroscopic observations of a cataclysmic variable system known as ASASSN–14dx. Results of the new observations, April 9 on the arXiv preprint server, indicate that the system harbors a massive pulsating white dwarf.

Cataclysmic variables (CVs) are binary star systems consisting of a white dwarf primary that is accreting matter from a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state.

ASASSN–14dx is a poorly characterized cataclysmic variable first identified in 2014. The system was found at a distance of about 265 light years away, which makes it one of the closest CVs to Earth. The orbital period of ASASSN–14dx was measured to be approximately 82.8 minutes.

Given that many properties of ASASSN–14dx remain unknown, a group of astronomers led by Pasi Hakala of the University of Turku in Finland, decided to take a closer look at this CV. For this purpose, they employed various ground-based observing facilities, including the Nordic Optical Telescope (NOT).

"Here, we present the results of our time-series photometry, circular polarimetry, and optical spectroscopy of ASASSN–14dx, which we use to derive parameters for both the binary system and the white dwarf itself," the researchers write in the paper.

The observations reveal that ASASSN–14dx, in addition to the orbital period, also exhibits pulsation periods—the strongest of which are around four and 14 minutes. Both pulsations are observed more than once, and one of them is always present.

In general, it was found that ASASSN–14dx displays complex optical variability. According to the astronomers, such variability could be best explained by the presence of a massive white dwarf showcasing non-radial pulsations.

The results suggest that the white dwarf in the system has a mass at a level of 1.1 solar masses and that its effective temperature is about 16,140 K. This temperature is higher than that of any accreting white dwarf pulsator detected so far.

The researchers assume that the white dwarf in ASASSN–14dx was either born massive or has accreted more mass than typical white dwarfs in CVs during its evolutionary history.

They note that there is not enough mass left in the secondary star to ever push the white dwarf over the Chandrasekhar limit—the maximum mass a white dwarf star can have before it collapses under its own gravity.

The authors of the paper add that near-infrared circular polarimetric observations are required in order to inspect the possible magnetic nature of the white dwarf in this CV.