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Abstract The mass function (MF) of isolated objects measured by microlensing consists of both a stellar and a planetary component. We compare the microlensing MFs of A. Gould et al. and T. Sumi et al. to other measurements of the MF. The abundance of brown dwarfs from the tail of the T. Sumi et al. stellar MF is consistent with measurements from the local solar neighborhood. Microlensing free-floating planets (μFFPs) may be free-floating or orbit host stars with semimajor axesa ≳  10 au and therefore can constrain the populations of both free-floating and wide-orbit planets. Comparisons to radial velocity and direct imaging low-mass companion populations suggest that either most of theμFFP population with masses  > 1M_Jupis bound to hosts more massive than M dwarfs, or some fraction of the observed companion population 1M_Jup < m_p <  0.08M_⊙actually comes from the low-mass tail of the stellar MF. TheμFFP population also places strong constraints on planets inferred from debris disks and gaps in protoplanetary disks observed by the Atacama Large Millimeter/submillimeter Array. 

Abstract 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 μm. This survey gathered observations for 56 white dwarfs within 25 pc at 10 and 15 μm, 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 $\stackrel{″}{.}$ 95. If confirmed, MEAD 62 B would be a $0.014_{-0.003}^{+0.002}$ M_⊙brown dwarf withT ${}_{\mathrm{eff}}=343_{-11}^{+7}$ 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. 

Many white dwarfs are observed in compact double white dwarf binaries, and through the emission of gravitational waves, a large fraction are destined to merge. The merger remnants that do not explode in a Type Ia supernova are expected to initially be rapidly rotating and highly magnetized. In this work, we present our discovery of the variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, as a likely merger remnant showing signs of circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: it is hot (35 500 ± 300 K) and massive (1.12 ± 0.03 M_⊙), rapidly rotating with a period of ≈6.6 minutes, and likely possesses exceptionally strong magnetic fields (∼400−600 MG) at its surface. Remarkably, we detect a significant period derivative of (1.80 ± 0.09)×10⁻¹²s/s, indicating that the white dwarf is spinning down, and a soft X-ray emission that is inconsistent with photospheric emission. As the presence of a mass-transferring stellar or brown dwarf companion is excluded by infrared photometry, the detected spin-down and X-ray emission could be tell-tale signs of a magnetically driven wind or of interaction with circumstellar material, possibly originating from the fallback of gravitationally bound merger ejecta or from the tidal disruption of a planetary object. We also detect Balmer emission, which requires the presence of ionized hydrogen in the vicinity of the white dwarf, showing Doppler shifts as high as ≈2000 km s⁻¹. The unusual variability of the Balmer emission on the spin period of the white dwarf is consistent with the trapping of a half ring of ionized gas in the magnetosphere of the white dwarf. 

ABSTRACT WD 0145+234 is a white dwarf that is accreting metals from a circumstellar disc of planetary material. It has exhibited a substantial and sustained increase in 3–5 $$\mu$$m flux since 2018. Follow-up Spitzer photometry reveals that emission from the disc had begun to decrease by late 2019. Stochastic brightening events superimposed on the decline in brightness suggest the liberation of dust during collisional evolution of the circumstellar solids. A simple model is used to show that the observations are indeed consistent with ongoing collisions. Rare emission lines from circumstellar gas have been detected at this system, supporting the emerging picture of white dwarf debris discs as sites of collisional gas and dust production.