The MIRI Excesses around Degenerates (MEAD) Survey I: A candidate cold brown dwarf in orbit around the nearby white dwarf 2MASS J09424023-4637176

TL;DR

This paper introduces MEAD, a Cycle 2 JWST/MIRI survey targeting Gaia-confirmed white dwarfs within 25 pc to detect ultracool companions and infrared signatures. In the first target MEAD 62, a red candidate companion MEAD 62 B is detected at 40 AU with mid-IR photometry suggesting a $T_ ext{eff}\approx343$ K brown dwarf of $\sim14\,M_ ext{Jup}$, though its status is not yet confirmed due to a near 50/50 false-positive odds with background galaxies. The authors perform CMDs, PSF analyses, and WD SED modeling to constrain the object’s nature and derive BD parameters via ATMO2020 evolutionary models, yielding a mass of $0.014^{+0.002}_{-0.003}\,M_\odot$ and an age of $7.6^{+1.7}_{-2.2}$ Gyr. A false-positive estimate across the 56-target MEAD fields yields an expected $0.66$ red, unresolved interlopers within $2''$, making common proper-motion follow-up essential. If confirmed, MEAD 62 B would be among the coldest known ultracool companions to white dwarfs, providing a rare benchmark for BD atmospheres and informing formation scenarios, while highlighting JWST’s capability to uncover such faint objects around nearby WDs.

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~\micron. This survey gathered observations for 56 white dwarfs within 25~pc at 10 and 15~\micron, probing each white dwarf for unresolved IR excesses, IR flux deficits indicative of collision induced absorption, or resolved substellar companions. We present in this paper observations of our first target, 2MASS J09424023-4637176, (also UCAC4 217-039132), henceforth called MEAD 62. It is a magnetic DA white dwarf with an estimated age of $7.6^{+1.7}_{-2.2}$\,Gyr. A red candidate companion, MEAD 62B, about 2 magnitudes fainter at 15\,$μ$m than the white dwarf is detected at an apparent separation of 1.95". If confirmed, MEAD 62B, would be a $0.014^{+0.002}_{-0.003}$\,\Msun\, brown dwarf with T$_{\rm eff} = 343^{+7}_{-11}$\,K, according to ATMO2020 evolutionary models. Although its red F1000W$-$F1500W color is similar to background galaxies, MEAD 62B, is consistent with being an unresolved point-source from empirical PSF fitting. A false positive analysis yields an expectation number of 0.66 red (F1000W$-$F1500$ \geq +0.80$\,mag) unresolved sources within the same separation (r$\leq2$ arcsec) for the entire MEAD survey. Thus, this candidate companion as likely to be an actual companion as a false-positive unresolved background galaxy. Additional observations to measure common proper motion or sample the SED are warranted to confirm the nature of MEAD 62B. A deep near-infrared imaging detection is achievable from the ground while JWST is needed at longer infrared wavelengths.

Figures (5)

• Figure 1: Color composite of the MEAD 62 MIRI field with the F1000W band coded in blue and the F1500W coded in red showing the color diversity of sources in the field. A candidate companion is found 1.9 away from the white dwarf.
• Figure 2: Apparent F1500W (left) and F1000W (right) magnitudes versus F1000W-F1500W colors for all sources detected in both filters at $\geq10$-$\sigma$ in the MIRI field of view for MEAD 62. MEAD 62 B is consistent with a $\sim300$ K brown dwarf when compared to an ATMO-2020 evolutionary model in these color-magnitude diagrams. Individual catalogue sources are in black. The resolved sources (galaxies) are tagged as orange squares while unresolved point-sources are assigned green circles. The MEAD 62 white dwarf is shown as a large green circle while the candidate companion, MEAD 62 B, is the large red star symbol. The ATMO2020 evolutionary model track for $\log g=4.5$ with strong chemical disequilibrium is overlaid as a navy blue dashed line for substellar objects ranging between 1000 K and 300 K phillips.2020. In addition, actual MIRI measurements of brown dwarfs in the same temperature range are plotted as pale blue diamonds and confirm that the ATMO2020 model can be trusted, especially at T$_{\rm eff}\leq 600$ K beiler.2024. Plotted as large blue diamonds are WD $0806-661$ B, the sole Y-type brown dwarf associated to a white dwarf voyer.2025; W0855, the coldest known brown dwarf luhman.2014leggett.2021; and the $6.3\pm0.6$ M$_{\mathrm{Jup}}$ exoplanet Eps Ind Ab matthews.2024. They represent the coldest objects having measured F1000W and F1500W magnitudes.
• Figure 3: Empirical point-spread function fitting indicates that MEAD 62 B is an unresolved point-source. State of the art model PSFs libralato.2024 were used to fit the individual exposure for each of the 4 dither positions, assuming a single point source. The left panel is for the F1000W images while the right panel is for F1500W images. For each row of each panel, the observation is on the left, the best-fit PSF model, in the middle, and the observation $-$ model residuals, on the right. No apparent flux excess is seen in the residuals map which is consistent with the obtained range of ePSF fitting reduced chi-squares: 0.99-1.27 (F1000W) and 1.00-1.49 (F1500W).
• Figure 4: White dwarf physical properties. Top) Fit of the white dwarf spectral energy distribution probed using 6 photometric bands (W1 and W2 excluded from the fit) converges to a DA white dwarf with $T_{\rm eff} = 5968\pm124$K and surface gravity of $\log g = 8.02\pm0.04$. The fit has $\chi^2=0.75$. Bottom) Best estimates of the final white dwarf mass, initial main sequence mass, white dwarf cooling age, main sequence lifetime and total system age based on the wdwarfdate Python package kiman.2022 using as inputs $T_{\rm eff} = 5968\pm124$ K and $\log g=8.02\pm0.04$.
• Figure 5: Left: Inferred mass for the candidate companion MEAD 62 B. Corner plot of the inferred age, mass and error scaling factor, log_f, obtained from a MCMC run using the measured F1000W and F1500W magnitudes, the MEAD 62 distance, the prior age distribution obtained by modeling the white dwarf SED, as well as using the ATMO2020 (strong non-equilibrium) evolutionary models. Right: Plots of the F1000-F1500W, absolute F1000W and F1500W magnitudes versus age filled with lines of iso-masses predicted by the same ATMO2020 models overplotted with the MCMC inferred mass for MEAD 62 B.