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Tracking Down an Escaped White Dwarf

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photograph of a white dwarf
A white dwarf shines brightly at the center of this Hubble Space Telescope image. [NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI); CC BY 4.0]

Studying the highest-mass white dwarfs is key to drawing the line between stars that explode as supernovae and stars that live out their days as white dwarfs. It’s easiest to determine the ages and masses of white dwarfs in star clusters, but many clusters seem to be missing their white dwarfs — can researchers track down a massive white dwarf that once belonged to a nearby cluster?

Kicked Out of a Cluster

Many stars are born in star clusters, but they may not remain there; just as children leave home, so too do stars depart their natal clusters — though children aren’t usually ejected via gravitational interactions. Researchers suspect that gravitational kicks may explain why many star clusters are missing their white dwarfs, the exposed cores of low- to intermediate-mass stars.

stellar evolution schematic

The main stellar evolution pathways. It’s not yet clear exactly which stars explode as supernovae and which become white dwarfs. Click to enlarge. [ESA]

Since it’s far easier to determine the age of a star that belongs to a star cluster than it is for a single star, tracking down white dwarfs that live (or once lived) in well-studied clusters can help us determine their ages and, by extension, the masses of the stars they came from. This helps us understand which stars will shed their atmospheres and spend the rest of their days as white dwarfs and which will end their lives in supernova explosions.

A Search for High-Mass White Dwarfs

To probe the line between stars that become supernovae and stars that become white dwarfs, David Miller (University of British Columbia) and collaborators conducted a search for high-mass white dwarfs associated with the nearby Hyades star cluster. Using precise position and velocity data from the Gaia spacecraft, the team identified stars moving in the same direction as the cluster and traveling on a path that places the star within the cluster during the cluster’s lifetime.

Color–magnitude diagram and white dwarf escapee candidates

Color–magnitude diagram showing the full sample of white-dwarf candidates with the three leading candidates labeled. Click to enlarge. [Miller et al. 2023]

Of these stars, 145 are likely white dwarfs, but not all white dwarfs are of interest in this study — only the most massive white dwarfs provide useful data for defining the line between supernova progenitors and white-dwarf progenitors. The three most massive white dwarfs in the sample weighed in at 1.1, 1.1, and >1.3 solar masses. After consulting a catalog of white dwarfs, Miller’s team ruled that the two lower-mass white dwarfs are likely interlopers, unrelated to the star cluster, while the most massive of the trio was almost certainly a cluster member.

Placing Constraints

plot of observed photometry and synthetic spectrum

Best-fit synthetic spectrum (black line) with synthetic photometry (red circles) and data (blue error bars). Click to enlarge. [Adapted from Miller et al. 2023]

The team took spectra of the single remaining runaway white dwarf candidate and used models to determine its mass, radius, and how long it has been cooling. With a mass of 1.317 solar masses, it’s one of the most massive known white dwarfs, especially among those thought to result from the evolution of a single star. (Other massive white dwarfs may be the product of two stars merging.)

Model fitting indicates that the white dwarf has been cooling for 556 million years. With the age of the Hyades cluster still uncertain, though, Miller’s team could only loosely constrain the progenitor star’s mass to >7.5 solar masses and its lifetime to <40 million years. The existence of this massive white dwarf can instead be used to constrain the cluster’s age, limiting it to <606 million years.

Miller and coauthors note that there’s nothing exceptional about the Hyades cluster except its proximity; this suggests that similarly massive white dwarfs may be more common than expected, waiting to be discovered.

Citation

“An Extremely Massive White Dwarf Escaped from the Hyades Star Cluster,” David R. Miller et al 2023 ApJL 956 L41. doi:10.3847/2041-8213/acffc4