RESEARCH
My research interests converge on white dwarf stars, the final evolutionary state for more than 97 percent of all stars in our Galaxy, such as the core of the Cat's Eye Nebula in the Hubble telescope image above. I have developed a research program that uses these compact stellar remnants as observational tracers of stars, planets, and binary systems. Their compact nature also allows white dwarfs to probe extreme physics and vary on short timescales.
Research news is also updated at the BU White Dwarf group page.
An artist's impression of a an asteroid breaking apart (credit NASA/JPL-Caltech).
The Final Fate of Planetary Systems
White dwarfs provide unique avenues to investigate the endpoints of exoplanetary systems. We think planets beyond Earth’s orbit will survive the red giant phase and can occasionally scatter in rocky bodies like asteroids that can be tidally disrupted and pollute their host white dwarf star. When this occurs, white dwarf abundances can directly constrain the composition of rocky exoplanet material. We have played a major role in uncovering the active disruption of asteroids involved in an early phase of this process via the discovery of white dwarfs that periodically dim from transiting clouds of debris. This field has exploded since the discovery of the first transiting system in 2015, and our group has played a major role in the discovery of every system discovered since 2015 (Vanderbosch, Hermes, et al. 2020; Guidry, Vanderbosch, Hermes, et al. 2021; Farihi, Hermes, et al. 2022; Hermes et al. 2025). Additionally, we have used Hubble ultraviolet spectroscopy of >250 white dwarfs to show that more massive white dwarfs (descendants of >3.5 solar-mass main sequence stars) show significantly less metal pollution, even accounting for observational biases. Massive white dwarfs provide unique constraints on planet formation and survival around the intermediate-mass stars that cannot be probed with transit or radial-velocity methods (Ould Rouis, Hermes, et al. 2024). Finally, I have also lead the most sensitive search for planetary companions around the final stages of stellar evolution, by monitoring the arrival times of stable pulsating white dwarfs. This method is described in more detail in a book chapter I contributed to the Handbook of Exoplanets (Hermes et al. 2018a).
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).
Failed Type Ia Supernovae
White dwarf stars are the objects that explode as Type Ia supernovae to measure out vast distances in the universe, an approach first used to detect the influence of dark energy. I have discovered and helped investigate the partly burnt runaway stellar remnants of failed Type Ia supernovae (Raddi et al. 2019); we hope abundance analysis can directly constrain nucleosynthetic yields in Type Ia. We have used space telescopes to measure the rotation rate of the prototype object, GD 492, strengthening the evidence that this hypervelocity star is the remnant of an incomplete Type Ia supernova (Hermes et al. 2021). I have also been directly involved with finding signposts of abnormal remnants of binary mergers that did not explode as Type Ia, such as anomalously fast rotation and strong magnetic fields (e.g., Reding, Hermes, et al. 2020; Reding, Hermes et al. 2023; Steen, Hermes, et al. 2024).
An example of one widely separated pair of white dwarf stars (both marked with a yellow box), each analyzed independently to determine their ages. In this case, the two stars move together on the sky and are both at the same distance (59.5 pc); they are separated from each other by roughly 1500 au (credit Aladin / PanSTARRS survey).
Improving Stellar Ages with White Dwarfs
We have engaged in an NSF AAG-funded effort to empirically test how precisely and accurately we can determine white dwarf total ages by comparing the independently determined ages of two widely separated (>100 au), coeval white dwarfs. Like most things in stellar astronomy, Gaia has revolutionized this field by revealing >1500 such wide WD+WD binaries (El-Badry, Rix & Heintz 2021). We have found that state-of-the-art white dwarf age-dating techniques have roughly 25% absolute uncertainties and improve with spectroscopy, though a large fraction (>20%) show evidence of an age reset from stellar merger (Heintz, Hermes, et al. 2022; Heintz, Hermes, et al. 2024). We were most interested in applying these ages to other stars, and we used wide WD+dM binaries to show that gyrochronology likely fails for M dwarfs (dM) at the fully convective boundary (Chiti et al. 2024). We look forward to expanding the use of white dwarfs as reliable age indicators, such as to constrain wide brown dwarf companions (e.g., Zhang, Liu, Hermes, et al. 2020).
The light curve of the pulsating white dwarf GD 1212, from the first publication using K2 data (Hermes et al. 2014a).
White Dwarfs in Kepler/K2/TESS
I am PI of a major program to observe all known and high-probability white dwarfs with the Kepler and TESS space telescopes. We have so far observed more than 2000 white dwarfs with at least 30-min cadence for more than 70 days from space, and hundreds have been sampled every minute. We are cataloging our raw and reduced data at k2wd.org. My primary interest is asteroseismology, but am extremely interested in what these data can say about remnant planetary systems around retired stars. We are also using the data to find and constrain new gravitational wave sources (e.g., Green, Hermes, et al. 2018; Green, Hermes, et al. 2024), short-period binaries (e.g., Parsons, Hermes, et al. 2017; Munday, Tremblay, Hermes, et al. 2023) as well as new spotted white dwarfs with small magnetic field strengths (e.g., Hermes et al. 2017b; Reding, Hermes, et al. 2020). I have served as Deputy Chair of the K2 User's Panel, am co-chair of the working group dedicated to observing compact objects with TESS, and am on the steering committee of the TESS Asteroseismology Science Consortium.
A zoom in on an outburst from PG 1149+057, a completely new phenomenon we have discovered from Kepler monitoring of pulsating white dwarfs (Hermes et al. 2015b). The effect is likely related to nonlinear mode coupling via parametric resonance.
White Dwarf Asteroseismology
Just as we can explore the interior of the Earth using seismology from earthquakes, we can unravel the inner secrets of stars using stellar pulsations, global stellar oscillations that respond differently depending on the internal composition of these objects. I specialize in observations of pulsating white dwarfs. As a graduate student, I discovered a new class of pulsating extremely low-mass white dwarfs (Hermes et al. 2012b), as well as the most massive, potentially ONe-core white dwarf known to pulsate (Hermes et al. 2013c). I have used Kepler and K2 for the longest monitoring of pulsating white dwarfs ever taken: our raw and reduced data are compiled at k2wd.org. With Kepler we are exploring white dwarf rotation in a whole new regime, including for extremely low-mass, He-core white dwarfs (Lopez, Hermes et al. 2021). We have also discovered a radical new outburst phenomenon in the coolest pulsating white dwarfs, likely from mode coupling via parametric resonance (Bell, Hermes et al. 2015; Hermes et al. 2015b; Bell, Hermes et al. 2016). I am fundamentally interested in what the starquakes of white dwarfs can tell us about the core and envelope structure of stars at the end stages of the life cycles, and how binary interaction can affect stellar structure (e.g., Hermes et al. 2015a).
Using new K2 data paired with spectroscopy from the 4.1-m SOAR telescope, we have put the first constraints on white dwarf rotation as a function of mass (Hermes et al. 2017d). Most white dwarfs rotate at roughly 1.5 days, but the most massive white dwarfs appear to rotate faster (Hermes et al. 2017c).
Endpoints of Stellar Rotation
White dwarfs offer us a glimpse into the future of stars like the Sun. Using the high-quality light curves from Kepler, we are exploring white dwarf rotation as a function of mass (Hermes et al. 2017c; AAS Nova article), as well as differential rotation as a function of radius in stellar remnants (Hermes et al. 2017a). We hope to use these observations to understand how and when stars lose their angular momentum. For example, if you completely conserve the angular momentum of a 2.5 solar-mass star initially rotating at 10 hr, that compact remnant should be rotating at just a few minutes, much faster than what we observe. We have shown with K2 that the vast majority of white dwarfs (which descended from 1.0-3.0 solar-mass stars) rotate at roughly 1.5 days (Hermes et al. 2017d). Our white dwarf rotation observations will hopefully shed new light on the endpoints of the internal transfer of angular momentum in stars.
All white dwarfs <0.3 solar masses are by necessity the product of close binary interaction. This artist's impression shows an extremely low-mass white dwarf (left) and a smaller, more normal-mass white dwarf companion (Illustration: CfA/David A. Aguilar).
Extremely Low-Mass, He-Core White Dwarfs
I am interested in accurately determining the fundamental parameters of extremely low-mass (ELM) white dwarfs (<0.3 solar masses). These underweight, He-core white were first inferred (and later directly observed) as companions to millisecond pulsars, and are by necessity formed in close binaries. I have exploited brightness changes caused by their tidal deformations to constrain the radii of ELM white dwarfs (Hermes et al. 2014b). I have also explored why the lowest-gravity white dwarfs all show metals; it appears related to their relatively rapid rotation (Hermes et al. 2014c). But I have most enjoyed using the pulsations we discovered in several ELM white dwarfs (Hermes et al. 2013d) -- including one with a pulsar companion (Kilic et al. 2015) -- to explore their interiors using asteroseismology.
The folded light curve of the 12.75-min WD+WD binary J0651+2844. In Hermes et al. (2012c) we showed the orbit of these two white dwarfs was shrinking rapidly due to the emission of gravitational waves.
Ultracompact Binaries
The degenerate nature of white dwarfs allows them to reside in extremely close orbits, too tight for stars like the Sun to exist without transferring material. Some white dwarfs have close low-mass or even substellar companions (such as the 71.2-min white dwarf plus brown dwarf binary described by Parsons, Hermes, et al. 2017). However, even more compact are WD+WD binaries -- these are the shortest-period binaries known where both stars are still detached. I have spent several years searching for and monitoring eclipses in short-period binaries. My favorite system is J0651+2844, a 12.75-min doubly eclipsing WD+WD binary. In 2012 we used the changing mid-eclipse times to show that the orbit of both stars is decaying, exactly in line with the prediction from gravitational wave radiation (Hermes et al. 2012c; BBC News article) -- this is the cleanest optical detection of the effects of gravitational waves. We continue monitoring J0651+2844, and searching for new short-period eclipsing WD+WD binaries in large surveys such as Gaia, TESS, ZTF, and eventually the LSST/Rubin and the Roman mission.
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