The Impact of Irradiation on the Radius and Thermal Evolution of Transiting Brown Dwarfs

TL;DR

This work develops a coupled atmospheric and interior evolution framework to quantify how stellar irradiation alters the radii and thermal evolution of transiting brown dwarfs. By integrating PICASO atmospheric models with a SONORA-like interior evolution solver and using a detailed nuclear-reaction treatment, the authors demonstrate that irradiation of $\log_{10}(F/\mathrm{erg\,s^{-1}\,cm^{-2}}) \ge 9$ (equivalently $T_{\rm eq} \ge 1450$ K) inflates radii across the substellar mass range, and shifts Deuterium- and Hydrogen-burning minimum masses downward. The irradiated models better match radii for a sample of 46 transiting brown dwarfs, though $\sim$10 objects remain inconsistent at >$3\sigma$, implying gaps in current understanding. The results imply irradiation plays a dominant role in substellar radius evolution—more influential than clouds or metallicity—and offer a framework to test hot-Jupiter radius anomaly physics using old, low-mass irradiated brown dwarfs and to interpret brown dwarf–white dwarf systems with different host spectra.

Abstract

Masses and radii of transiting brown dwarfs can be measured directly in contrast to isolated field brown dwarfs, whose mass and radius inferences are model dependent. Therefore, transiting brown dwarfs are a testbed for the interior and evolutionary models of brown dwarfs and giant exoplanets. We have developed atmospheric and evolutionary models for this emerging population. We show that intense stellar irradiation can cause a large enhancement in the radius of transiting brown dwarfs at all masses, especially if the incident flux exceeds $log_{10}(F/cgs)\ge$9 ($T_{\rm eq}\ge 1450$ K). Stellar irradiation can significantly alter rates of nuclear burning in irradiated brown dwarfs, making the Deuterium-burning and Hydrogen-burning minimum masses strong functions of incident stellar flux. We show that the D-burning and H-burning minimum masses can decrease by 16% and 13%, respectively, between isolated and strongly irradiated brown dwarfs ( $log_{10}(F/cgs)\ge$10 ($T_{\rm eq}\ge 2570$ K)). This shows that stellar irradiation has a larger impact on the planet-brown dwarf-star mass boundaries than metallicity or clouds. We show that metal cores or migration affect their evolution to a much lesser extent, whereas low mass highly irradiated old sources can help us test the physics of hot Jupiter radius anomaly. We fit the observed radii of 46 transiting brown dwarfs and show that our irradiated evolutionary models fit their radii better than models that ignore the host star, especially for highly irradiated objects. However, the measured radii of 10 objects are still inconsistent at $>3σ$ level, indicating residual gaps in our irradiated evolutionary model.

Figures (14)

• Figure 1: Top panel shows the measured radii of the sample of transiting brown dwarfs as a function of the incident stellar flux on them. The colors of each point corresponds to their mass in Jupiter mass. Bottom panel shows the same quantities in the two axes but the color now corresponds to the median age of the host star of these objects. Objects with masses between 12.9 and 89 $M_{\rm J}$ have been plotted here with colored points whereas the gray points and contours show the transiting exoplanet population with M$<$12.9$M_{\rm J}$.
• Figure 2: Comparison of $T(P)$ profiles between irradiated brown dwarfs and isolated brown dwarfs. The $T(P)$ profiles of the irradiated brown dwarfs and isolated brown dwarfs are shown with solid and dashed lines, respectively. The deepest convective regions for the irradiated brown dwarfs are shown with thick solid lines and the deepest radiative-convective boundaries for the isolated brown dwarfs are shown with circular points. Each panel corresponds to a different incident stellar flux level. Thermal profiles for $T_{\rm int}$= 300, 500, 800, 1100, 1400, 1700, 2000, and 2300 K are shown in each panel. All $T(P)$ profiles shown here have $log(g)$=5.0. Only the deepest radiative--convective boundaries are denoted here.
• Figure 3: Comparison of emission spectra of irradiated brown dwarfs, to spectra of isolated brown dwarfs from the Sonora bobcat model grid marley21. The four panels show model spectra for brown dwarfs located at 0.5, 0.13, 0.04, and 0.02 AU from the host star. The solid lines show the irradiated brown dwarf spectra at $T_{\rm int}$ values between 2400 K and 400 K with a 400 K $T_{\rm int}$ interval in each panel. Solid lines show the spectra of irradiated brown dwarfs whereas the dashed lines show the spectra for an isolated brown dwarf which has the same $T_{\rm eff}$ as the $T_{\rm int}$ of the irradiated brown dwarf model. All models shown here are for $log(g)=5$ and assume thermochemical equilibrium.
• Figure 4: Each panel shows the effect of incident stellar radiation on the radius evolution of irradiated brown dwarfs with varying mass. Radius evolution for 10.48, 13.62, 17.81, 27.24, 36.66, and 52.38 $M_{\rm J}$ are shown in the six panels. The different colored lines in each panel show the evolution due to different incident stellar flux on the object with $log(F)$ varying from 4 to 10, where $F$ is in $erg.s^{-1}.cm^{-2}$. Note that the temporary halting of contraction for M$\ge13.6 M_{\rm J}$ objects is due to the onset of deuterium burning. The radius contraction continues once most of the available deuterium is exhausted.
• Figure 5: Effect of incident stellar flux on the mass-radius relation of irradiated brown dwarfs at various ages is shown. Each colored shaded region shows the variation in the mass-radius relation due to varying incident stellar flux from $log(F)=6$ to $log(F)=10$. The mass-radius relation for ages 21.5 Myrs, 59 Myrs, 166 Myrs, 480 Myrs, and 10 Gyrs is shown. These ages were chosen to encompass a wide range of ages but still be distinctly visible on the figure.