J0140: An Evolved Cataclysmic Variable Turned Extremely Low Mass White Dwarf (Planetary Science)

Kareem El-Badry and colleagues, using the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), presented the discovery of a close binary binary, LAMOST J0140355+392651 (also called J0140), containing a bloated, low-mass (0.15 M) proto-white dwarf and a massive (about M = 0.95 M) white dwarf companion. Their paper recently appeared on Journal arXiv.

Extremely low-mass white dwarfs (ELM WDs) are rare helium-core WDs with masses below 0.25 solar masses. They are degenerate and semi-degenerate helium stars that never ignited core helium burning. They are assumed to be formed in binary systems via stable or unstable mass transfer, given that the universe is too young to produce such objects by single-star evolution. Therefore, it is thought that ELM WDs are the stripped cores of stars that were initially more massive but lost most of their envelope to their companions.

Now, Kareem El-Badry and colleagues reported on the discovery of J0140 that may be a newly found ELM WD. J0140 first came to their attention when LAMOST observations of this object suggested large epoch-to-epoch radial velocity (RV) variability. Then, after that it was observed/monitored by photometric surveys, Catalina Sky Survey (in between 2005-2013), Palomar Transient Factor (PTF) and is regularly observed by All-Sky Automated Survey for Supernova (ASAS-SN), to obtain more information regarding its parameters.

Kareem et al. found that, J0140 is a close binary with orbital period of approximately 3.81 hours. It contains a bloated, proto-white dwarf with a mass of around 0.15 solar masses and WD companion about 5 percent less massive than the sun. The radius of the proto-WD is estimated to be 0.29 solar radii. The system is located approximately 5,000 light years away and its orbit is inclined 80 degrees.

They also revealed that the proto-WD is cooler (Teff=6800 K) and more puffy (log[g/(cms−²)]=4.74±0.07) than any known extremely low mass (ELM) WD, but hotter than any known cataclysmic variable (CV) donor. It either completely or very nearly fills its Roche lobe (R/RRochelobe=0.99±0.01), suggesting ongoing or recently terminated mass transfer. Hence, the properties of J0140 are transitional between that of known CVs and ELM WDs.

In addition, to investigate the formation and evolution of J0140, astronomers calculated a set of MESA binary evolution models and found that the properties of the system are well-matched by MESA binary evolution models of CVs with donors that underwent significant nuclear evolution before the onset of mass transfer. In these models, the bloated proto-WD is either still losing mass via stable Roche lobe overflow or was doing so until very recently. In either case, it is evolving toward higher temperatures at near-constant luminosity to become an ELM WD.

If the system is detached, mass transfer likely ended when the donor became too hot for magnetic braking to remain efficient.

— wrote authors of the study

Figure 1. Future evolution of J0140. Colored tracks show the MESA models. Normal CVs reach a period minimum at about 80 minutes, when they become degenerate. Systems with evolved donors are smaller when they become degenerate (due to higher mass and lower 𝑌𝑒), allowing them to reach much shorter minimum periods. These models suggest that J0140 will reach a minimum period 𝑃orb ≲ 15 minutes, when (if it avoids a merger) it will be classified as an AM CVn binary. © Kareem et al.

Moreover, evolutionary models predict that the binary will shrink to Porb ≲ 10 minutes within a few Gyr, when it will either merge or become an AM Canum Venaticorum star (AM CVn) binary. J0140 provides an observational link between the formation channels of CVs, ELM WDs, detached ultracompact WD binaries, and AM CVn systems.

“Further observations are necessary to better understand the nature of the system. In particular, higher-resolution and higher-SNR (signal-to-noise ratio) spectra will allow a deeper search for emission features associated with accretion, which will enable a more conclusive determination of whether there is ongoing mass transfer,”

— concluded authors of the study

Featured image: Light curve of J0140, showing data from several time-domain surveys. Credit: El-Badry et al., 2021.

Reference: Kareem El-Badry, Eliot Quataert, Hans-Walter Rix, Daniel R. Weisz, Thomas Kupfer, Ken Shen, Maosheng Xiang, Yong Yang, Xiaowei Liu, “LAMOST J0140355+392651: An evolved cataclysmic variable donor transitioning to become an extremely low mass white dwarf”, arXiv, pp. 1-24, 2021. https://arxiv.org/abs/2104.07033

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