High-contrast L-band Integral Field Spectroscopy of HD 33632 Ab

TL;DR

This work delivers a flux-calibrated L-band–inclusive 1–4 μm spectrum for the directly imaged brown dwarf HD 33632 Ab and conducts a comprehensive atmospheric analysis using both brown-dwarf templates and four cloudy/evolutionary model grids. By leveraging a precise dynamical mass, the authors test how cloud physics and metallicity shape the observed SED, finding that cloudy atmospheres reproduce the data better than cloud-free models, though tensions with evolutionary radii persist. Mass priors drawn from dynamical constraints partially alleviate these tensions, with Exo-REM models most consistent with evolution-based expectations, while other grids require adjustments in radius or cloud structure. The study highlights the critical role of clouds in the L/T transition regime for directly imaged companions with well-constrained masses and sets the stage for building a spectral library via forthcoming JWST observations. Overall, the paper advances our understanding of cloud processes, gravity, and composition in substellar atmospheres by exploiting precise dynamical masses and broad-wavelength spectroscopy.

Abstract

We present LBTI/ALES 3.07-4.08 micron spectroscopic observations of HD~33632~Ab, a ~53 M_Jup directly imaged companion to an F8 star. Spectroscopic measurements of HD 33632 Ab now span 1-4 micron, and we perform the first spectroscopic analysis covering this full range. The data are compared to isolated brown dwarf template spectra, indicating that HD 33632 Ab is similar to L8/9 field brown dwarfs. Synthetic atmosphere model spectra from multiple model families are fit, with cloudy models providing the best fits, consistent with expectations for an L-dwarf. Evolutionary model predictions for the bulk properties of HD 33632 Ab are highly constrained by the precise dynamical mass found for the object. In particular, predictions for surface gravity are narrowly peaked, log(g)=5.21+/-0.05, and not dependent on the effects of clouds or cloud dispersion. We find significant tension between the surface gravities and object radii inferred from atmosphere model fits and those predicted by evolutionary models. We conclude with a comparison to the spectra of the HR 8799 c, d, and e, and emphasize the case that HD 33632 Ab, and other L/T transition directly imaged companions with constrained masses, will serve an essential role in understanding the complex physical processes governing the appearance of clouds in planetary atmospheres.

Figures (9)

• Figure 1: The wavelength stacked image of HD 33632 Ab taken with LMIRCam+ALES. The annular region shows the pixels processed with our PCA-based ADI routine. White regions have been masked for display. Black contours near the origin indicate the position of the unsaturated host star.
• Figure 2: Scale factors quantifying the flux ratio between the companion and host star are determined for each wavelength by minimizing a parabola fit to the results of a grid-fitting algorithm.
• Figure 3: The flux calibrated ALES spectrum and uncertainties of HD 33632 Ab (black markers) compared to the Currie2020 Keck/NIRC2 L-band photometric measurement, shown with a pink marker with horizontal errorbars indicating the filter bandwidth and vertical errorbars indicating photmetric uncertainty. We indicate with a gray swath a region in which we did not recover the companion with signal-to-noise ratio greater than 2. The blue curve is a normalized, uncalibrated spectrum of the central star, highlighting strong telluric absorption near $3.2~\micron$.
• Figure 4: Template brown dwarf spectra compared to HD 33632 Ab.
• Figure 5: Best-fit synthetic atmosphere models from four model grids. In each case, green markers show fits with a gaussian prior on parallax, a log-flat prior on radius and surface gravity and a flat prior on effective temperature. Where applicable, a log-flat prior is used on metallicity and $K_{zz}$ and flat priors on C/O, and cloud parameters. Pink markers use a joint prior on mass and surface gravity informed by dynamical mass constraints. See text for details.