An optical spectrum of VVV1256-62A was observed with the Gemini South telescope, one half of the International Gemini Observatory, supported in part by the U.S. National Science Foundation and operated by NSF NOIRLab. "The spectrum looks featureless, because the white dwarf is too cold to have detectable Balmer lines," said Dr. Sarah Casewell of the University of Leicester, a co-author of the research paper. VVV1256-62A has a temperature of 4440 K according to white dwarf atmospheric model fitting to its spectral energy distribution observed by the Dark Energy Camera Plane Survey (DECaPS) and the VVV. The white dwarf is cooler than our Sun (5800 K), and should have a light orange colour in visible light, not white or blue like the majority of white dwarfs.
"We usually assume that ultracool subdwarfs have ages between 8-14 billion years like other stars in the Galactic halo. But that assumption was never confirmed by observation," said Prof. Zhang. VVV1256-62AB provides the first opportunity for us to determine the age of an ultracool subdwarf from its white dwarf companion, as white dwarfs are well modelled and we can have precise age constraints of them. The team determined that VVV1256-62A has a total age of 10.5 billion years. "The age of the white dwarf is composed of the main sequence lifetime (2 billion years) of its progenitor star (1.9 solar mass), and the cooling time of the white dwarf (8.5 billion years)", explained Dr. Raddi.
VVV1256-62B has a temperature of 2200-2300 K, and a sub-solar metallicity according to fitting its spectrum to atmosphere models. The team inferred the chemical abundance of VVV1256-62B to be only 15% of an average nearby star. Thus VVV1256-62B has a more transparent atmosphere than ultracool dwarfs. VVV1256-62A is seven times more massive but seven times smaller than VVV1256-62B. Thus the white dwarf is much denser and has a much higher surface gravity (74,000 g, where g is the surface gravity of the Earth) than the ultracool subdwarf (270 g).
"VVV1256-62B is the first L dwarf age benchmark at sub-solar metallicities, and it is a very important source for characterising other ultracool subdwarfs and for calibrating atmosphere models," said Prof. Adam Burgasser of the University of California, San Diego, a co-author of the research paper. "We are looking forward to observing this object with the James Webb Space Telescope (JWST) in an upcoming approved program," Prof. Burgasser added.
The Gaia mission measures stars in optical light, and could only observe perhaps tens of L subdwarfs as these objects are very faint in optical light and emit most of their light in the near infrared. "A new astrometry space observatory that works in the near infrared, like the proposed GaiaNIR mission, would be able to observe many more ultracool subdwarfs," said Dr. Richard Smart of Osservatorio Astronomico di Torino, a co-author of the research paper.
This binary was identified by accident, during a research course for junior students at Nanjing University. Prof. Zhang said, "I was giving an instruction to three undergraduates on how to find wide binaries with the Gaia Catalogue of Nearby Stars. As a show-how, on a whim, I decided to search for white dwarf + ultracool dwarf wide binaries with very tight criteria, and found 5 pairs. VVV1256-62B is a companion on the list, which surprised me, as I had written a paper on this object in 2019 and knew it was an L subdwarf with a very special Galactic orbit." Prof. Zhang recorded the discovery process through screen recording (see the video on ).
ESA Gaia also posted the discovery on the "Image of the Week":
This work was recently published in the Monthly Notices of the Royal Astronomical Society (MNRAS) as the 8th paper of a series titled "Primeval very low-mass stars and brown dwarfs".
Research paper:
The "Primeval" series focus on discoveries and characterization of ultracool subdwarfs, and is collected in the astrophysics data system (ADS) library:
"Primeval" series
Source: Office of Science and Technology
Correspondent: Wang Feihao
