Untangling mysterious emission in some cool white dwarfs

By JJ HermesMarch 18th, 2021

In March 2021 work led by University College London graduate student Nik Walters was accepted to MNRAS analyzing GD 356, the prototype of a new class of just four white dwarfs that exhibit Balmer emission lines despite being apparently isolated stars. Here we provide strong evidence that this emission is not the result of a current generated by a close-in rocky planet; instead, this process may reveal a new phase of white dwarf evolution. This work includes multiple BUWD group researchers, as well as data collected remotely during the COVID-19 pandemic from BU's 1.8-meter Perkins Telescope Observatory, which helped rule out large spin-period changes in this rapidly rotating white dwarf.

More transiting debris found in ZTF

By JJ HermesMarch 18th, 2021
Seven different white dwarf stars showing evidence of dimming from transiting debris, on long (left) and short (right) timescales (credit Guidry et al. 2021).

In December 2020 work led by UT-Austin undergraduate student Joseph Guidry and collaborators was posted to arXiv announcing up to five new white dwarfs showing transiting debris from the Zwicky Transient Facility (ZTF), more than tripling the number of such systems known! That work was accepted for publication in March 2021 and will appear soon in ApJ, and includes spectroscopy from the Lowell Discovery Telescope confirming that at least one of the white dwarfs with transiting debris is also heavily polluted by rocky debris. A thread can be found here on Twitter: as well as a thread on how we used Gaia's empirical uncertainties to select variable stars located here:

Planetary debris transiting a second white dwarf

By JJ HermesNovember 28th, 2020
An artist's impression of a an asteroid breaking apart (credit NASA/JPL-Caltech).

In July 2020, collaborators led by PhD student Zachary Vanderbosch at the University of Texas at Austin have published in The Astrophysical Journal only the second white dwarf known to show transits from an asteroid or planetesimal that got too close to its retired host star. The transits recur roughly every 100 days as the cloud of debris passes in front of the white dwarf. A thread first announcing the submitted paper is here,, and the system continues to provide interesting surprises, including a deep new transit in November 2020:

Seeing the interiors of massive A stars with TESS

By JJ HermesNovember 28th, 2020

Collaborators led by Tim Bedding at the University of Sydney published in Nature in May 2020 exciting new results from NASA's TESS mission that are some of the first convincing identifications of the oscillations of a class of massive A stars that pulsate, named after the prototype star delta Scuti. The 2-minute cadence of TESS has finally allowed astronomers to identify the pulsations in delta Scuti stars, especially those that are young. Besides a NASA press release and a Nature News & Views for more general audiences, a thread contextualizing the discovery is here:

A puzzling, fast-spinning white dwarf with emission

By JJ HermesNovember 28th, 2020
An artist's impression of a strongly magnetic white dwarf (credit ESO/L. Calçada).

In May 2020, collaborators led by PhD student Josh Reding at the University of North Carolina at Chapel Hill published in The Astrophysical Journal the fastest-rotating, isolated white dwarf. The star spins on its axis every 317 seconds, and is only the second white dwarf to show Balmer emission lines -- the origin of this emission is still puzzling, and could be connected to the induction of a current from a close-in rocky planet. A thread detailing this highly magnetic stellar remnant is here:

Pulsations & eclipses in a white dwarf

By JJ HermesNovember 28th, 2020

In March 2020, collaborators led by Steven Parsons at the University of Sheffield published in Nature Astronomy the first pulsating white dwarf ever found in a binary system that also exhibits eclipses. Having both pulsations and eclipses in this low-mass (0.325 solar-mass) white dwarf will put some exceptional constraints on the star. We already know the radius of the star to 2% precision, less than 300 km, and know its mass to better than 4% -- this will be an excellent benchmark to understand how stars are stripped of mass at the end of their lives. Aside from a press release with an artist's impression from the University of Sheffield, a thread with more details can be found here:

An old failed star

By JJ HermesFebruary 26th, 2020

Collaborators, led by PhD student ZJ Zhang at the University of Hawaii, have discovered a bound pair of stars that serve as an exciting benchmark: one is a T4 (<1300 K) brown dwarf, the other is a very cool (5100 K) white dwarf. White dwarfs can serve as useful age indicators, and so we used this very cool white dwarf (which we suspect is roughly 7.3 billion years old) to constrain the age of the pair of stars. This old age puts unique constraints on interpreting the brown dwarf -- modeling the atmosphere of these failed stars is very complicated! A thread with more information from is located on Twitter:

A class of supernova survivors

By JJ HermesFebruary 1st, 2020

Group members and collaborators have discovered a new class of zombie stars that are likely the shattered remnants of thermonuclear supernova that did not fully disrupt the white dwarf! Led by Roberto Raddi at Dr.Karl-Remeis-Sternwarte Bamberg, we have now published at least three more members of this new class of star named after the prototype LP 40-365.

The research is well summarized by Shannon Hall writing in Scientific American: This “newly discovered class of odd stars appears to have persevered through supernova explosions — providing a rare glimpse into these astrophysical catastrophes.” Another summary is detailed in a Twitter thread:

White dwarf core crystallization

By JJ HermesFebruary 1st, 2020

Last year we published in Nature the most comprehensive evidence that white dwarf stars, at the end of their lives, solidify into crystals! The research is best summarized in this quote from Deborah Netburn of The Los Angeles Times: “New research suggests that long after our roiling, boiling life-giving star runs out of fuel, it will slowly transform into a cold, dead, super-dense crystal sphere about the size of Earth that will linger like a translucent tombstone for close to eternity.”

An artist's illustration of a white dwarf in the process of solidifying (credit University of Warwick/Mark Garlick).

The manuscript was led by Pier-Emmanuel Tremblay at the University of Warwick, and was made possible by the excellent data from the second data release of the Gaia mission. Another write-up of the results can be found here: