Using ZTF photometry, and Gaia and Pan-STARRS data, a team of international astronomers discovered an eclipsing binary, “ZTF J0038+2030”, composed of a white dwarf and a substellar companion with an orbital period of 10 hours. Their study recently appeared in Arxiv.
A substellar object, sometimes called a substar, is an astronomical object mainly consist of hydrogen gas and are not massive enough to fuse hydrogen in their core (M ≲ MHBL ≈ 0.07 M; 73 Mjup). Substellar objects have masses in the range of ∼0.3–73 Mjup and are generally divided into two classes: brown dwarfs and giant exoplanets. There is no clear separation based on mass, but the distinction is based on the formation history. The formation of a brown dwarf is the same as that of more massive main-sequence stars: they are formed by gravitational instabilities in gas clouds and have elemental abundances similar to that of the interstellar medium. On the other hand, giant planets are formed by core accretion in a disk around a protostar, and have an enhanced metal abundance compared to the host star.
“To identify the eclipses, we used the Zwicky Transient Facility lightcurves. The system was identified in a search for deep eclipsing white dwarfs. We searched the combined PSF-photometry and alert photometry lightcurves of white dwarfs for deep eclipses and identified the period using the BLS algorithm.”
In the current study, astronomers used follow-up photometry and spectroscopy to measure the binary parameters. They found that, white dwarf has a mass of 0.50 M and a temperature of 10900 K. While, the substellar companion has a mass of 0.059 M and a small radius of 0.0783 R (as shown in table 1 below). It is one of the smallest transiting brown dwarfs known and likely old, ≳ 8 Gyr.
From the mass of white dwarf, astronomers believe that, it has likely a CO core which allows the two formation scenarios. In the first scenario, the white dwarf could have formed during a common envelope phase on the Asymptotic Giant branch (AGB) after helium core exhaustion. The second scenario is that the common envelope happened at the tip of the Red giant branch (RGB), just after the helium flash which would result in a white dwarf with a mass close to 0.5 M. In that scenario the white dwarf would have after the common envelope evolved into a hot sub-dwarf (sdB) and appeared as an HW Vir system before it evolved into a white dwarf with a brown dwarf companion after helium exhaustion in the sdB.
Finally, they concluded that, the system is relatively bright, and a good prototype system where the brown dwarf suffers minimal irradiation. It is also a useful target for eclipse timing to find circumbinary objects as brown dwarf are not expected to show eclipse time variations due to Apple gate’s mechanism.
Featured image: Artist’s impression of one of this study’s superlative discoveries, the oldest known wide-separation white dwarf plus cold brown dwarf pair. The small white orb represents the white dwarf (the remnant of a long-dead Sun-like star), while the brown/orange foreground object is the newly discovered brown dwarf companion. This faint brown dwarf was previously overlooked until it was spotted by citizen scientists, because it lies right within the plane of the Milky Way. © NOIRLab/NSF/AURA/P. Marenfeld Acknowledgement: William Pendrill
Jan van Roestel, Thomas Kupfer, Keaton J. Bell, Kevin Burdge, Przemek Mróz, Thomas A. Prince, Eric C. Bellm, Andrew Drake, Richard Dekany, Ashish A. Mahabal, Michael Porter, Reed Riddle, Kyung Min Shin, David L. Shupe, “ZTFJ0038+2030: a long period eclipsing white dwarf and a substellar companion”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2105.08687
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