# The OmegaWhite Survey for Short-Period Variable Stars IV: Discovery of the warm DQ white dwarf OW J175358.85-310728.9

## TL;DR

This paper reports the discovery of a bright, variable warm DQ white dwarf with a stable 35.55-minute period, strong magnetic field, and unique spectral features, expanding understanding of magnetic white dwarf variability.

## Contribution

It presents the first detailed observation and analysis of OW J1753-3107, a new warm DQ white dwarf with unique spectral and magnetic properties, including a stable short-period variability.

## Key findings

- Discovered a bright warm DQ white dwarf with a 35.55-minute period.
- Determined physical parameters: T_eff=15430 K, B_z=2.1 MG, log(g)=9.0.
- Spectral analysis reveals strong magnetic field and carbon-rich atmosphere.

## Abstract

We present the discovery and follow-up observations of the second known variable warm DQ white dwarf OW J175358.85-310728.9 (OW J1753-3107). OW J1753-3107 is the brightest of any of the currently known warm or hot DQ and was discovered in the OmegaWhite Survey as exhibiting optical variations on a period of 35.5452 (2) mins, with no evidence for other periods in its light curves. This period has remained constant over the last two years and a single-period sinusoidal model provides a good fit for all follow-up light curves. The spectrum consists of a very blue continuum with strong absorption lines of neutral and ionised carbon, a broad He I 4471 A line, and possibly weaker hydrogen lines. The C I lines are Zeeman split, and indicate the presence of a strong magnetic field. Using spectral Paschen-Back model descriptions, we determine that OW J1753-3107 exhibits the following physical parameters: T_eff = 15430 K, log(g) = 9.0, log(N(C)/N(He)) = -1.2, and the mean magnetic field strength is B_z =2.1 MG. This relatively low temperature and carbon abundance (compared to the expected properties of hot DQs) is similar to that seen in the other warm DQ SDSS J1036+6522. Although OW J1753-3107 appears to be a twin of SDSS J1036+6522, it exhibits a modulation on a period slightly longer than the dominant period in SDSS J1036+6522 and has a higher carbon abundance. The source of variations is uncertain, but they are believed to originate from the rotation of the magnetic white dwarf.