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Gaia discovers interesting duo belonging to the Milky Way halo: an ultracool subdwarf with a white dwarf companion

Figure 1. VVV 1256−62AB’s average orbit from the past 2 Gyr (cyan curves) to the future 2 Gyr (red curves). Its current location is near the Sun and indicated by a white circle. The background image is the spiral structure of the Milky Way based on Gaia DR3 (Credits: ESA/Gaia/DPAC, Stefan Payne-Wardenaar, CC BY-SA 4.0 IGO). The edge-on orbit view in [Y, Z] space is plotted on the same scale and shown at the bottom. Credit: Roberto Raddi, Zenghua Zhang, MNRAS.

While studying stars located nearby and within 100 pc of the Sun, researchers found an interesting duo which belongs in the Milky Way halo: a combination of a white dwarf and an L subdwarf, forming a wide binary system. The first of its kind ever found that belongs to the halo. And it’s been there for a while, the system is thought to be about 10 Gyrs old.

The ultracool subdwarf (VVV1256-62B) of this interesting duo is a low-metallicity subdwarf. Low metallicities are an indication of age. At the early stage of the Universe, there were still very little “metals”, elements heavier than hydrogen and helium. If a star lacks these heavy elements, it is expected to be ancient, and therefore can give us hints on our Milky Way’s past. The subdwarf is right at the boundary between stellar and substellar objects, making it extra interesting to follow-up. It can be used as an age benchmark for studying metal-poor ultracool atmospheres.

The ultracool subdwarf forms a wide binary system with a white dwarf. White dwarfs have typically masses between ~0.17 and 1.33 solar masses, but while their mass is comparably to the Sun, their size is closer to the size of our Earth. A white dwarf is at the end of its evolutionary stage. While it is very hot when it forms and hence looks white, it will gradually cool off and redden because it no longer supports active fusion that keeps other stars going.

This white dwarf of the duo (VVV1256-62A) is low-mass, with a mass about half the mass of our Sun, and is located near the bottom right of the white dwarf cooling sequence, hence must be considerably old.

Figure 2. Artist’s conception of VVV1256-62AB. The background image is a section of Gaia’s view of the Milky Way (Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO). The cool white dwarf VVV1256-62A (left) has a light orange colour. The ultracool subdwarf VVV1256-62B (right) is much cooler but also much larger than its white dwarf companion. The surface pattern of the ultracool subdwarf is based on 3D hydrodynamics simulations of ultracool dwarf atmospheres (Bernd Freytag). Note VVV1256-62B likely has a more transparent atmosphere than field ultracool dwarfs as it has fewer heavy elements. Credit: Jiaxin Zhong, Zenghua Zhang.

So how was this interesting duo found? A little bit by accident to be honest, while demonstrating Gaia data selection during a research training course for undergraduates at the 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." While this specific treasure has now been found, there must be many more out there among the two billion stars Gaia has mapped so far.

Gaia’s Catalogue of Nearby Stars, published in December 2020 with Gaia’s Early Data Release 3, contains stars within 100 pc from our Sun and forms the basis for this quest to find primeval very low-mass stars and brown dwarfs. A sample of star combinations with almost identical proper motions (motions across the sky) and very similar distances was selected, and then down selected to contain only combinations of ultracool dwarfs with white dwarfs. For this additional selection the Gaia catalogue of non-single stars, published in June 2022, has been instrumental. From the initial 3199 common proper motion pairs, only five remained matching all these criteria.

Figure 3. VISTA one micron wavelength images (1x1 arcmin) of VVV1256-62 A (left) and B (right) taken in 2010 and 2015. Credits: VISTA, ESO

While all five of them are interesting pairs, the most interesting one is the set with the highest proper motions. Propagating the orbit shows that the duo belongs to the Milky Way halo. The system can be found close to our Sun at about 75 pc, and currently moves in the direction away from the Galactic centre with a speed of 406 km/s on a prograde, highly eccentric orbit. It can move out as wide as 31,000 pc from the Galactic centre, but comes close to the Galactic centre as well, to within 1,000 pc! Though it passes so close to the centre, the binary system has not been broken up yet, which is thanks to the extremely high velocity it has when it passes through this dense area of our Milky Way.

The orbital motion of VVV 1256−62AB (represented with a filled circle) from the past 2Gyr (cyan curves) to the future 2Gyr (red curves) in [X, Y] (upper panel) and [Y, Z] (lower panel) space. The location of the Sun is indicated with a five-pointed star. Credits: Roberto Raddi, Zenghua Zhang, MNRAS.

The Milky Way consists of many structures, groups of stars and peculiarly moving stars. We are learning more and more about the galactic halo and the systems that inhabit this area. Understanding the link between the halo and the Milky Way is important to understand the formation of our Milky Way and the history of the stars in the halo. By studying systems like these, and especially their orbits, we can learn more about the Milky Way as a whole. The origin of such an eccentric orbit has yet to be clarified but could be related to the existence of an in-situ inner halo or to a past merger of the Milky Way with a satellite galaxy.

On their own, both the white dwarf and the ultracool subdwarf are interesting objects. Together, in a wide binary system on its eccentric orbit, one plus one becomes three. The system can now also be used to test stellar evolution models and serves as a unique benchmark for testing both ultracool dwarf and white dwarf atmospheres.

VVV1256-62A was confirmed as DC white dwarf with an optical spectrum 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. VVV1256-62B was confirmed as a sdL3 subdwarf with an optical to near infrared spectrum observed with the Very Large Telescope (VLT) of the European Southern Observatory (ESO). VVV1256-62AB were imaged by the Dark Energy Camera Plane Survey (DECaPS) of the Blanco telescope at NOIRLab’s Cerro Tololo Inter-American Observatory and the Visible and Infrared Survey Telescope for Astronomy’s (VISTA) Variables in the Via Lactea (VVV) survey at ESO’s Paranel Observatory. These optical to near infrared photometric data were used to derive the age of the white dwarf.

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". The "Primeval" series focus on discoveries and characterization of ultracool subdwarfs, and is collected in the astrophysics data system (ADS) library.

Further reading:

• Press release of Nanjing University: "Waltz" in the Milky Way: The first age benchmark ultracool subdwarf with a white dwarf companion
• Press release of Nanjing University's School of Astronomy and Space Science: "Waltz" in the Milky Way: The first age benchmark ultracool subdwarf with a white dwarf companion
• Press release by University of Hertfordshire: "Ancient duo of stars visiting from far reaches of the Milky Way"

Story written by T. Roegiers, Z. Zhang

Credits: ESA/Gaia/DPAC, Z. Zhang

[Published: 20/08/2024]