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

Using the Hubble Space Telescope (HST), astronomers have made spectrophotometric observations of an eclipsing white dwarf and brown dwarf, known as WD1032+011. The results of their observing campaign, published on September 10 on the preprint server arXivprovide important information about the nature of this system.

Binary systems consisting of a white dwarf (ND) and a brown dwarf (NB) are generally rare, because the NBs in these systems must survive engulfment by the ND’s progenitor. Although several sky surveys have identified thousands of white dwarfs, it is estimated that only 0.1–0.5% of them are accompanied by a brown dwarf.

WD1032+011 is a tidally synchronized WD–BD eclipsing binary with an orbital period of 0.09 days and an inclination of 87.5 degrees. The two objects are separated by about 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, used the Hubble Space Telescope to take a closer look at WD1032+011. They did this using the Hubble Space Telescope’s Wide Field Camera 3 (WFC3).

“We present time-resolved spectrophotometry of the white dwarf and brown dwarf binary WD1032+011 obtained with the Hubble Space Telescope’s Wide Field Camera 3. We obtain a broadband light curve that shows the primary eclipse, where the brown dwarf completely occults the white dwarf,” the researchers wrote in the paper.

Observations revealed that the brown dwarf WD1032+011B has daytime and nighttime temperatures of 1,748 and 1,555 K, respectively. The spectral type of the brown dwarf was found to be most likely a special L1 type, and the collected data suggest a cloudless atmosphere.

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

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

Based on the results obtained, the authors of the study assume that WD1032+011 could 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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