A hidden white dwarf found

By JJ HermesSeptember 23rd, 2022
Changes in the arrival time of pulsations (top) of the 342-s pulsation period in the stripped red giant BPM 36430, along with residuals after subtracting the best-fitting sinusoid from the data (bottom). The data reveal that this stripped red giant is being orbited by an unseen, cool, white dwarf that is at least 42% as massive as the Sun (credit Smith et al. 2022).

In September 2022, a manuscript led by undergraduate student at High Point University, Bryce Smith, and including members of the BUWD group has been published (Smith, Barlow, Rosenthal, Hermes & Schaffenroth 2022), which announces the discovery of an unseen, cool white dwarf using the stable pulsations of a stripped red giant star. This work implements the same timing method used to find the first exoplanets around a pulsar --- looking for light-travel-time changes in an object with a steady pulsation period. The discovery was confirmed with spectroscopy showing the star really is wobbling from an unseen companion. The pulsation-timing changes were first discovered by a high-school student working with the BUWD group, Ben Rosenthal, who joined us as a RISE intern in July 2021; Ben is now an undergraduate at Yale University. The work has been accepted for publication in the Astrophysical Journal; a short thread on the discovery can be found here.

Empirically testing WD ages

By JJ HermesAugust 6th, 2022

In June 2022, a manuscript led by BUWD graduate student Tyler Heintz was accepted for publication that empirically tested the reliability of white dwarf stars as age indicators. This extensive work was funded by the NSF and used more than 1250 widely separated (>100 au) pairs of white dwarfs to quantify how accurate their ages are. On the whole, white dwarf ages agree to at least 25% using only existing survey data, but we can do better! We also found that roughly 21-36% of the wide WD+WD binaries had a more massive white dwarf that was hotter than the less massive white dwarf -- so up to 1/3 of WD+WD may have arisen from merged triples! The work has been accepted for publication in the Astrophysical Journal; a short thread on the discovery can be found here.

Planetary bodies observed for first time in habitable zone of dead star

By JJ HermesMarch 24th, 2022
An artist's impression of the white dwarf star WD1054-226 orbited by clouds of planetary debris and a possible major planet in the habitable zone. (credit Mark A. Garlick /

In February 2022, a manuscript led by Jay Farihi from University College London and including members of the BUWD group (Farihi, Hermes, Marsh, et al. 2022) announced the discovery of a remarkable white dwarf that hosts the first planetary debris found in the habitable zone of a retired solar system. The white dwarf, WD1054-226, started out its life like the Sun, but has since evolved through its giant phase and is now a retired white dwarf star. The debris orbits the white dwarf every 25 hours, and features persistent structures in the transit light curve that are most simply explained by a more massive unseen object, in the same way that structures in the rings of Saturn are caused by orbital resonances with moons and moonlets. The discovery was covered by BBC News, NewScientist, Newsweek,, and IFLScience. A nice thread on Twitter by Corey S. Powell summarizes some of the most interesting figures from the manuscript, published in MNRAS.

TESS finds 74 bright pulsating white dwarfs

By JJ HermesJanuary 14th, 2022

In January 2022, a manuscript led by Alejandra Romero from the Universidade Federal do Rio Grande do Sul in Brazil announced the outcome of a large search for bright new pulsating white dwarfs: in the first three years of the TESS mission we have discovered 74 new pulsating hydrogen-atmosphere (ZZ Ceti) white dwarfs. The work involved many members of the BUWD group, as well as observations collected from both the 1.8m Perkins Telescope Observatory as well as the 4.3m Lowell Discovery Telescope, and was coordinated through Working Group 8 of the TESS Asteroseismic Consortium. These bright objects will be studied for the remainder of the TESS mission so that more information can be ascertained from their interiors through asteroseismology. The manuscript announcing these objects has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

Preempting an ‘oops’ with JWST

By JJ HermesJanuary 14th, 2022
A finder chart and TESS light curve of the now-omitted JWST standard star HD 38949 (credit Mullally et al. 2022).

In January 2022, Deputy Project Scientist for JWST Susan Mullally and members of the BUWD group announced the first results of a project to monitor nearly all possible spectrophotometric standards that are planned to calibrate the high-precision observations undertaken by the recently launched 6.5-meter James Webb Space Telescope. Using another NASA mission, TESS, we found that most planned standards are suitable for calibrations. However, we also discovered four that are intrinsically quite variable, including one especially poor standard that the JWST team has now thrown out (see image above). The work has been accepted for publication in the Astrophysical Journal; a short thread on the discovery can be found here.

TESS watches a low-mass white dwarf pulsate

By JJ HermesOctober 8th, 2021
Eight days of brightness changes as seen from TESS from the pulsating extremely low-mass white dwarf GD 278 (credit Lopez et al. 2021).

In October 2021, former BUWD group member Isaac Lopez led a global collaboration announcing the discovery, characterization with TESS, and asteroseismic modeling of the pulsations in the first extremely low-mass white dwarf observed from space: GD 278. Using a method to select variable white dwarfs from Gaia pioneered by our group, we first saw pulsations from McDonald Observatory in August 2019. We quickly secured TESS data (see above), producing some of the best data on a pulsating white dwarf ever collected. We matched the observed oscillation periods to theoretical models with multiple stellar evolution codes. This is now the lowest-mass white dwarf with a measured rotation rate. The work was accepted for publication and will appear soon in the Astrophysical Journal; a thread can be found here on Twitter:

A 99-min binary from SDSS-V

By JJ HermesSeptember 25th, 2021
Data (left) and the model (right) of the velocity changes of the new 99-min WD+WD binary SDSS J1337+3952 from SDSS-V (credit Chandra et al. 2021).

In one of the first science results from SDSS-V, collaborators led by Harvard graduate student Vedant Chandra have discovered a double-lined, double-white-dwarf binary orbiting one another every 99 minutes in a paper that was recently accepted for publication by the Astrophysical Journal. The two white dwarfs will merge into a roughly 0.85 solar-mass remnant in less than 300 million years, and the system is already likely giving off enough gravitational waves to be detectable by the LISA mission. A nice Twitter summary of the discovery is here. The pair of stars was found in just the first six months of SDSS-V data, so more exciting discoveries await when the robots take over later this year!

Rotation in shrapnel from a supernova

By JJ HermesJuly 10th, 2021
An artist's impression of the explosion and subsequent ejection of a partly burnt runaway star like LP 40-365 (credit University of Warwick/Mark Garlick).

In June 2021 work led by researchers in the BU White Dwarf group discovered that the partly burnt runaway star LP 40-365 (also known as GD 492) rotates every 8.9 hours using archival data from the Transiting Exoplanet Survey Satellite (TESS) and the Hubble Space Telescope (HST). This relatively long rotation period likes adds more evidence that LP 40-365 is actually the bound remnant of an exploded white dwarf itself, the still-simmering embers of a thermonuclear (Type Iax) explosion that slung-shot the star from the Milky Way. An excellent write-up was recently featured in The Brink, and a thread on the discovery of the system is located here: The paper has been published the Astrophysical Journal Letters.

Debris orbiting a white dwarf every 9.9 hours

By JJ HermesJuly 10th, 2021
Individual light curves from McDonald Observatory phased to 9.9370 hr, showing repeated structures in the debris transiting the white dwarf ZTF J0328−1219. Each successive light curve is vertically shifted down for easier comparison between nights and labeled by their corresponding dates (credit Vanderbosch et al. 2021).

In June 2021, collaborators from all over the world led by UT-Austin graduate student Zach Vanderbosch characterized the orbital period of transiting debris around a new white dwarf: ZTF J0328−1219. The debris shows repeating dips that are stable from night-to-night and repeat every 9.937 hr (shown in the figure above over several nights), as well as some other chunks that repeat every 11.2 hr. Spectra collected by BUWD group members were analyzed to show this is a strongly metal polluted white dwarf; new data show the debris shows circumstellar absorption features from the clouds of debris, as well. The work was accepted for publication and will appear soon in the Astrophysical Journal.