Observations provide crucial insights into the nature of a white dwarf–brown dwarf binary

Using the Hubble Space Telescope (HST), astronomers have performed spectrophotometric observations of an eclipsing white dwarf–brown dwarf binary known as WD1032+011. Results of their observational campaign, published September 10 on the preprint server arXiv, yield important information regarding the nature of this system.

Binaries composed of a white dwarf (WD) and a brown dwarf (BD) are generally rare, as BDs in such systems must survive being engulfed by the WD’s progenitor. Although several all-sky surveys have identified thousands of white dwarfs, only about 0.1–0.5% of them are predicted to have a brown dwarf companion.

WD1032+011 is an eclipsing, tidally-locked WD–BD binary with an orbital period of 0.09 days and an inclination of 87.5 degrees. The two objects are separated by approximately 0.003 AU, while the masses of the white dwarf (WD1032+011A) and brown dwarf (WD1032+011B) are estimated to be 0.45 and 0.066 solar masses.

A team of astronomers led by Jenni R. French of the University of Leicester, UK, has employed HST to take a closer look at WD1032+011. For this purpose, they used HST’s Wide Field Camera 3 (WFC3).

“We present Hubble Space Telescope Wide Field Camera 3 time-resolved spectrophotometry of the eclipsing white dwarf–brown dwarf binary WD1032+011. We derive a broadband light curve which shows the primary eclipse, where the brown dwarf fully occults the white dwarf,” the researchers wrote in the paper.

The observations found that the brown dwarf WD1032+011B has dayside and nightside temperatures of 1,748 and 1,555 K, respectively. The spectral type of the brown dwarf was found to be most likely L1 peculiar, and the collected data suggest a cloud-free atmosphere.

According to the study, WD1032+011B has a radius of about 0.1 solar radii, which means that the object is inflated, likely due to the constant irradiation from the white dwarf. This makes WD1032+011B the only known inflated brown dwarf in an eclipsing WD–BD binary.

When it comes to the white dwarf WD1032+011A, it has a radius of approximately 0.015 solar radii, and its effective temperature is at a level of 9,950 K. Astronomers estimate that the system WD1032+011 is located some 1,020 light years away and is at least five billion years old.

Based on the obtained results, the authors of the paper suppose that WD1032+011 may be a cataclysmic variable, which is consistent with the properties of the system. However, so far, no signs of magnetism have been detected in the spectrum of the white dwarf, which could confirm this hypothesis.

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