Observational Signatures of Circumstellar Gas Tori Formed by Planetary Mass-Loss from Close-In Exoplanets

TL;DR

This paper develops an analytic model for circumstellar gas tori formed from planetary atmospheric mass loss around very close-in exoplanets and uses ray-tracing to predict how such tori attenuate stellar light in optical/near-IR diagnostics. The authors identify the He I $10830$ Å line as a particularly sensitive tracer of circumstellar material and show that stars hosting tori occupy distinct regions in the He I EW versus log$\,R'_{HK}$ space, enabling a practical survey strategy when combined with Ca II H & K measurements. They further demonstrate that tori can damp or distort planetary transits in the He I line, offering a direct observable effect of the torus on planetary atmospheres. Taken together, these results provide a concrete observational pathway to detect and characterize circumstellar gas fed by planetary mass loss and to constrain the wind-mass-loss balance, magnetic effects, and long-term evolution of close-in planets.

Abstract

Close-in exoplanets with H/He atmospheres often undergo hydrodynamic escape. In extreme cases, it is hypothesized that the mass loss can be high enough for the escaping planetary material to wrap around the star, forming a long-lasting circumstellar torus. In this work, we develop a physical model of such circumstellar tori and use a ray tracing scheme to calculate the attenuation of stellar light passing through them. We show that the presence of a circumstellar torus significantly increases the equivalent width of the observed stellar He I 10830~Å~line. When combined with observations of the star's Ca II H & K lines, these systems can typically be distinguished from field stars. Based on these results, we propose a survey of stars hosting close-in planets, combining observations of the He I 10830~Å~and Ca II H & K lines to search for circumstellar tori generated from planetary mass-loss in these systems.

Figures (6)

• Figure 1: Top Panel: Equivalent width of the He i 10830 Å feature versus the chromospheric Ca ii emission index log $R_{\text{HK}}'$, for a sample of dwarf FGK stars with (B-V) $> 0.47$, compiled by Smith2016. A strong correlation is observed, indicated by the grey arrow, which shows the direction of increasing stellar activity. The red arrow illustrates how circumstellar gas absorption would shift a star's observed properties away from the stellar activity trend. Bottom Left Panel: Example spectrum of the core of the solar Ca ii K line in an active region (black solid line), from White1981. The chromospheric emission appears within a broad photospheric absorption feature. Absorption by circumstellar gas would reduce the observed core emission, as illustrated by the red dashed line, and hence the measured log $R_{\text{HK}}'$. Bottom Right Panel: Same as the left panel, but for the solar He i 10830 Å line in an active region from Sanz-Forcada2008. Absorption from circumstellar gas would deepen the line, and hence increase the measured equivalent width.
• Figure 2: A schematic diagram of the circumstellar gas torus, that has formed due to the accumulation of escaping gas from the planet over many orbits. The left and right panel shows the system geometry when observed top-down (left) and side on (right).
• Figure 3: Model He 10830 Å and Ca ii H stellar profiles for a star of $M_* = 0.92\text{M}_{\odot}$ with log $R_{\text{HK}}'$ = -4.89 and He i 10830 Å EW = 170 mÅ. The solid black lines shows the intrinsic stellar line profiles, and the dashed red line shows the lines modified by an obscuring circumstellar torus with a radius of 0.03 AU.
• Figure 4: The predicted He 10830 Å EW and log $R_{\text{HK}}'$ for the synthetic sample of FGK stars hosting circumstellar tori (blue stars, yellow pluses, red crosses respectively). For comparison, observed stars (without circumstellar tori) from the Smith2016 sample are plotted as black dots. The top panel shows models in which the gas is assumed to have no turbulent velocity ($u_{\text{turb}} = 0$), while the bottom panel shows models with $u_{\text{turb}} = 0.5c_s$.
• Figure 5: The synthetic He i 10830 Å transit for a planet with an escaping H/He atmosphere orbiting a star surrounded by a circumstellar torus. The left panel displays the photometric transit for varying optical depths of the circumstellar gas, while the right panel presents the corresponding mid-transit spectra. In both cases, increasing the optical depth of the circumstellar torus reduces the observed depth of the planetary transit.