Comparison of planetary Hα-emission models: A new correlation with accretion luminosity

Abstract

Accreting planets have been detected through their hydrogen-line emission, specifically H$\alpha$. To interpret this, stellar-regime empirical correlations between the H$\alpha$ luminosity $L_{\mathrm{H}\alpha }$ and the accretion luminosity $L_{\mathrm{acc}}$ or accretion rate $\dot{M}$ have been extrapolated to planetary masses, however without validation. We present a theoretical $L_{\mathrm{acc}}$--$L_{\mathrm{H}\alpha }$ relationship applicable to a shock at the surface of a planet. We consider wide ranges of accretion rates and masses and use detailed spectrally-resolved, non-equilibrium models of the postshock cooling. The new relationship gives a markedly higher $L_{\mathrm{acc}}$ for a given $L_{\mathrm{H}\alpha }$ than fits to young stellar objects, because Ly-$\alpha$, which is not observable, carries a large fraction of $L_{\mathrm{acc}}$. Specifically, an $L_{\mathrm{H}\alpha }$ measurement needs ten to 100 times higher $L_{\mathrm{acc}}$ and $\dot{M}$ than previously predicted, which may explain the rarity of planetary H$\alpha$ detections. We also compare the $\dot{M}$--$L_{\mathrm{H}\alpha }$ relationships coming from the planet-surface shock or implied by accretion-funnel emission. Both can contribute simultaneously to an observed H$\alpha$ signal but at low (high) $\dot{M}$ the planetary-surface shock (heated funnel) dominates. Only the shock produces Gaussian line wings. Finally, we discuss accretion contexts in which different emission scenarios may apply, putting recent literature models in perspective, and also present $L_{\mathrm{acc}}$--$L_{\mathrm{line}}$ relationships for several other hydrogen lines.

13 pages, accepted to ApJL

References in corpus (17)

• Two accreting protoplanets around the young star PDS 70
• X-Shooter spectroscopy of young stellar objects in Lupus: Accretion properties of class II and transitional objects
• Meridional flows in the disk around a young star
• Accreting Circumplanetary Disks: Observational Signatures
• Constraining the Nature of the PDS 70 Protoplanets with VLTI/GRAVITY
• Characterization of exoplanets from their formation III: The statistics of planetary luminosities
• Including Dust Coagulation in Hydrodynamic Models of Protoplanetary Disks: Dust Evolution in the Vicinity of a Jupiter-mass Planet
• A Systematic Study of the Final Masses of Gas Giant Planets
• Observability of Forming Planets and their Circumplanetary Disks II. -- SEDs and Near-Infrared Fluxes
• Thermodynamics of Giant Planet Formation: Shocking Hot Surfaces on Circumplanetary Disks
• The widest H$\alpha$ survey of accreting protoplanets around nearby transition disks
• The Planetary Accretion Shock: I. Framework for Radiation-hydrodynamical Simulations and First Results
• Strong H$\alpha$ emission and signs of accretion in a circumbinary planetary mass companion from MUSE
• Detectability of embedded protoplanets from hydrodynamical simulations
• A measure of the size of the magnetospheric accretion region in TW Hydrae
• Searching for proto-planets with MUSE
• Observing the gas component of circumplanetary disks around wide-orbit planet-mass companions in the (sub)mm regime

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