Investigating Circumstellar Atomic Radiation-driven Dynamics

Abstract

The interactions between stars and their orbiting planets, driven by forces such as stellar radiation and gravity, play an essential role in shaping exoplanetary atmospheres and gas-rich debris discs. One way to look into the composition of these environments is to observe how they can contaminate the stellar photospheres. For that, we examine how stellar radiation pressure and gravity influence atomic species and analyse their effects across various stellar effective temperatures. Using the radiative-to-gravitational force ratio, we determined the atomic movement direction and assessed the velocity boost imparted to neutral atoms escaping from exoplanet atmospheres or debris discs. Incorporating the solar far ultraviolet/extreme ultraviolet spectrum to address flux discrepancies in the {\sc{atlas9}} model, we find that radiation affects atoms differently according to their ionisation state, with highly ionised species less affected by stellar radiation. Our results conclude that the stars most suitable for observing stellar contamination are those between 6,500 and 8,000 K, with neutral noble gases and ionised iron-peak elements as the most likely contaminants.

Figures (14)

• Figure 1: Illustration showing different scenarios of gaseous circumstellar material spreading around a star and either accreting onto or being accelerated away from it. Left-hand panel: A gas-rich debris disc (see Section \ref{['disc']}). Right-hand panel: An evaporating giant or rocky exoplanet (see Sections \ref{['jupiter']} & \ref{['rocky']}).
• Figure 2: $\beta$ ratios of species representative of selected groups as a function of stellar effective temperature. Each panel shows three ionisation states for each species. The black dashed line ($\beta = 1$) indicates the equilibrium condition, where the radiative and gravitational forces acting on the atoms are balanced and the grey dashed line ($\beta = 0.5$) indicates the $\beta$ value above which unhindered gas, when released from circular orbit, starts to migrate outwards.
• Figure 3: Panels (A) and (B) show the strengths of neutral helium and iron transitions compared to the fluxes emitted by stars with various effective temperatures. The complementary panels (C) and (D) show $\beta$ ratios of neutral helium and iron, respectively. The black dashed line marks the equilibrium between gravitational and radiation forces. Panels highlight the limitations of the atlas9 models in the ultraviolet regime compared to the $T_{\rm eff} = 8,000$ K model, which is corrected with Solar EUV flux, as explained in Section \ref{['sec:betapic']}. Effective temperatures of $4,000$ K, $10,000$ K and $19,000$ K are indicated above the flux curves in the upper panels and from left to right in the lower panels and are distinguished by different colours.
• Figure 4: Velocity boosts of selected neutral atoms with $\beta > 0.5$ as a function of stellar effective temperature. The atoms are arranged across the panels in descending order of their $\beta$ values at $T_{\mathrm{eff}} = 19,000~\mathrm{K}$. Shown in grey are the escape velocities at distances of $0.1$, $1$, $10$, and $100~\mathrm{AU}$.
• Figure 5: Top: Comparison between the observed (black dots) and synthetic (red circles) photometric fluxes of $\beta$ Pictoris in the Johnson ($U$, $B$, $V$, $R$, $I$) and 2MASS ($J$, $H$, and $K$) bands. The red solid line shows the atlas9 model. Bottom: Fractional residuals $(F_{\mathrm{syn}} - F_{\mathrm{obs}})/F_{\mathrm{obs}}$ between observed and synthetic fluxes.