Carl Melis

Binary stars are the progenitors of exotic objects, such as supernovae and gravitational-wave sources. However, some systems merge earlier, when the primary expands into a giant star at the end of its lifetime on the main sequence and causes its companion to spiral in. TYC 4144-329-2 is a candidate postmerger first-ascent giant star with a circumstellar disk seen in the infrared (IR). We detect weak X-ray and far-ultraviolet (FUV) emission from TYC 4144-329-2, which we interpret as a signature of weak coronal activity. We also find variability in H α line profiles and in the optical light curves, and we suggest an intermediate geometry where the line of sight passes through the upper layers of a flared disk with a time-variable column density. While the X-rays must be coronal, the H α line profiles point to ongoing and variable accretion. We suggest that TYC 4144-329-2’s merger was more recent than those of other stars in this class and that it did not yet have time to develop a deep convection zone. If this scenario holds, TYC 4144-329-2 would be a unique probe of the earliest stages in the evolution of postmerger systems.

Numerous white dwarf stars are known to be orbited by disks of gas and dust. To date, broad, ∼300 km s ⁻¹ wide, gaseous circumstellar absorption features have only been reported for the already iconic WD 1145+017, where one is witnessing the breakup of an extrasolar asteroid in real time. We report here the discovery of absorption from circumstellar gas around a second white dwarf (WD J0234-0406) with similarly broad features. The observed lines are carried by ions of Ca, Cr, Fe, Ti, Mg, Mn, Na, O, Si, Sc, Sr, Ti, and V. In addition, deep, nonphotospheric lines of Si iv are seen in the ultraviolet; we compare these with Si iv lines previously seen in the ultraviolet spectra of various other white dwarfs. The apparent broadband flux of WD 1145+017 is known to change often and rapidly as chunks of the asteroid pass between the star and Earth. No such variations are seen in the brightness of WD J0234-0406. In addition, while the strength/structure of circumstellar absorption features at WD 1145+017 has changed dramatically with time, nothing similar is seen at WD J0234-0406. Excess infrared emission at WD J0234-0406 indicates the presence of circumstellar dust particles.

The MIRI Excesses Around Degenerates Survey is a Cycle 2 James Webb Space Telescope (JWST) survey program designed to image nearby white dwarfs in the mid-IR with the MIRI imaging mode. Only a handful of white dwarfs have previously been observed beyond 8 μ m. This survey gathered observations for 56 white dwarfs within 25 pc at 10 and 15 μ m, probing each white dwarf for unresolved IR excesses, IR flux deficits indicative of collision-induced absorption, or resolved substellar companions. This paper presents observations of our first target, 2MASS J09424023−4637176 (also UCAC4 217-039132), henceforth called MEAD 62. This is a magnetic DA white dwarf with an estimated age of 6 . 5 − 2.0 + 4.0 Gyr. A red candidate companion, MEAD 62 B, about 2 mag fainter than its host white dwarf, is detected at an apparent separation of 1 . ″ 95. If confirmed, MEAD 62 B would be a 0.01 4 − 0.003 + 0.002 M _⊙ brown dwarf with T eff = 34 3 − 11 + 7 K, according to ATMO2020 evolutionary models. Also, at 20.48 ± 0.01 pc, it would be the third nearest substellar companion to a white dwarf. While the red F1000W − F1500W color of MEAD 62 B is similar to background galaxies, it is also consistent with being an unresolved point source from empirical point-spread function fitting. A false positive analysis yields an expectation number of 0.66 red (F1000W − F1500 ≥ +0.80 mag) unresolved sources within the same separation ( r ≤ 2″) for the entire MEAD survey. Thus, this candidate companion is as likely to be an actual companion as a false positive unresolved background galaxy. Follow-up JWST observations to measure common proper motion and sample the spectral energy distribution are warranted to confirm the nature of MEAD 62 B.

The MIRI Excesses Around Degenerates (MEAD) Survey is a Cycle 2 JWST program designed to image nearby white dwarfs with MIRI at 10 and 15 μ m. This survey targeted 56 white dwarfs within 25 pc to search for mid-infrared excesses, flux deficits from collision-induced absorption, and resolved substellar companions. In this paper we present our analysis of WD 0644+025, an unusually massive white dwarf (0.95 M _⊙ ) and the MEAD target exhibiting the most significant mid-infrared excess. The observed JWST MIRI photometry shows a 7.3 σ excess at 15 μ m and a 3.6 σ excess at 10 μ m, which may be associated with either a planetary companion or a circumstellar dust disk. This excess corresponds to a companion mass of 6.8 M _Jup ( T _eff = 261 ±9 K) with orbital distance M _Jup beyond ∼12 au. Metal pollution is confirmed in both archival Keck HIRES spectra from 1999 and new observations from 2025, with no evidence suggesting that the accretion rate has substantially changed over the decades. We explore possible dust disk morphologies to describe the observed IR excess and find that traditional debris disks struggle to fit our data. WD 0644+025 thus represents a compelling case study in the growing population of white dwarfs with cold infrared excesses and highlights JWST’s ability to probe planetary system remnants inaccessible to prior infrared observatories.

The MIRI Excess Around Degenerates (MEAD) Survey is a cycle 2 JWST program designed to image nearby white dwarfs with MIRI at 10 and 15 microns. This survey targeted 56 white dwarfs within 25 pc to search for mid-infrared excesses, flux deficits from collision-induced absorption, and resolved substellar companions. In this paper we present our analysis of WD 0644+025, an unusually massive white dwarf (0.95 Msun) and the MEAD target exhibiting the most significant mid-infrared excess. The observed JWST MIRI photometry shows a 7.3 sigma excess at 15 microns and a 3.6 sigma excess at 10 microns, which may be associated with either a planetary companion or a circumstellar dust disk. This excess corresponds to a companion mass of 6.8 Mjup (Teff=261 +/- 9 K) with orbital distance <11.8 au, although substantially lower masses are possible if we consider a closely orbiting insolated companion. No spatially resolved sources are detected within 200 au, with contrast curve analysis excluding planets more massive than 2 Mjup beyond ~12 au. Metal pollution is confirmed in both archival Keck HIRES spectra from 1999 and new observations from 2025, with no evidence suggesting the accretion rate has substantially changed over the decades. We explore possible dust disk morphologies to describe the observed IR excess, and find that traditional debris disks struggle to fit our data. WD 0644+025 thus represents a compelling case study in the growing population of white dwarfs with cold infrared excesses, and highlights JWST's ability to probe planetary system remnants inaccessible to prior infrared observatories.

Polluted white dwarfs offer a unique way to directly probe the compositions of exoplanetary bodies. We examine the water content of accreted material using the oxygen abundances of 51 highly polluted white dwarfs. Within this sample, we present new abundances for three H-dominated atmosphere white dwarfs that showed promise for accreting water-rich material. Throughout, we explore the impact of the observed phase and lifetime of accretion disks on the inferred elemental abundances of the parent bodies that pollute each white dwarf. Our results indicate that white dwarfs sample a range of dry to water-rich material, with median uncertainties in water mass fractions of ≈15%. Among the He-dominated white dwarfs, 35/39 water abundances are consistent with corresponding H abundances. While for any individual white dwarf it may be ambiguous as to whether or not water is present in the accreted parent body, when considered as a population, the prevalence of water-rich bodies is statistically robust. The population as a whole has a median water mass fraction of ≈25%, and enforcing chondritic parent body compositions, we find that 31/51 WDs are likely to have nonzero water concentrations. This conclusion is different from a similar previous analysis of white dwarf pollution and we discuss reasons why this might be the case. Pollution in H-dominated white dwarfs continues to be more water-poor than in their He-dominated cousins, although the sample size of H-dominated white dwarfs remains small and the two samples still suffer a disjunction in the range of host star temperatures being probed.

We report the detection of 13 heavy elements (Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Sr) in the photosphere of LSPM J0207+3331, a ∼3 Gyr old hydrogen-rich white dwarf with an effective temperature comparable to that of the Sun. Upper limits on carbon, obtained through the absence of molecular CH, suggest accretion from a carbon-volatile-depleted source. The accreted parent body exhibits slight deficits of Mg and Si relative to Fe but otherwise bulk Earth-like abundance patterns; a reasonable interpretation is that LSPM J0207+3331 is accreting a massive differentiated rocky body that had a core mass fraction higher than the Earth’s. The high level of pollution indicates that substantial accretion events can still occur even after 3 Gyr of cooling. We also detect weak Ca ii H & K line core emission, making this only the second known isolated polluted white dwarf to exhibit this phenomenon and suggesting the presence of additional physical processes in or above the upper atmosphere. Our analysis also highlights the critical importance of including heavy elements in the model atmosphere structure calculations for highly polluted hydrogen-rich white dwarfs. Neglecting their contribution significantly impacts the inferred thermodynamic structure, leading to inaccuracies in derived stellar parameters. Finally, we show that the observed 11.6 μ m infrared excess can be explained by a single silicate dust disk rather than a two-ring disk model.

We report new spectroscopic and interferometric observations of the Pleiades binary star Atlas, which played an important role nearly 3 decades ago in settling the debate over the distance to the cluster from ground-based and space-based determinations. We use the new measurements, together with other published and archival astrometric observations, to improve the determination of the 291 day orbit and the distance to Atlas (136.2 ± 1.4 pc). We also derive the main properties of the components, including their absolute masses (5.04 ± 0.17 M _⊙ and 3.64 ± 0.12 M _⊙), sizes, effective temperatures, projected rotational velocities, and chemical compositions. We find that the more evolved primary star is rotationally distorted, and we are able to estimate its oblateness and the approximate orientation of its spin axis from the interferometric observations. The spin axis may well be aligned with the orbital axis. Models of stellar evolution from the Modules for Experiments in Stellar Astrophysics (or MESA) that account for rotation provide a good match to all of the primary’s global properties, and point to an initial angular rotation rate on the zero-age main sequence of about 55% of the breakup velocity. The current location of the star in the Hertzsprung–Russell diagram is near the very end of the hydrogen-burning main sequence, at an age of about 105 Myr, according to these models. Our spectroscopic analysis of the more slowly rotating secondary indicates that it is a helium-weak star, with other chemical anomalies.

Polluted white dwarfs offer a unique way to directly probe the compositions of exoplanetary bodies. We examine the water content of accreted material using the oxygen abundances of 51 highly polluted white dwarfs. Within this sample, we present new abundances for three H-dominated atmosphere white dwarfs that showed promise for accreting water-rich material. Throughout, we explore the impact of the observed phase and lifetime of accretion disks on the inferred elemental abundances of the parent bodies that pollute each white dwarf. Our results indicate that white dwarfs sample a range of dry to water-rich material, with median uncertainties in water mass fractions of $\approx$15\%. Amongst the He-dominated white dwarfs, 35/39 water abundances are consistent with corresponding H abundances. While for any individual white dwarf it may be ambiguous as to whether or not water is present in the accreted parent body, when considered as a population the prevalence of water-rich bodies is statistically robust. The population as a whole has a median water mass fraction of $\approx$25\%, and enforcing chondritic parent body compositions, we find that 31/51 WDs are likely to have non-zero water concentrations. This conclusion is different from a similar previous analysis of white dwarf pollution and we discuss reasons why this might be the case. Pollution in H-dominated white dwarfs continues to be more water-poor than in their He-dominated cousins, although the sample size of H-dominated white dwarfs remains small and the two samples still suffer a disjunction in the range of host star temperatures being probed.