Hubble Space Telescope UV and H$\alpha$ Measurements of the Accretion Excess Emission from the Young Giant Planet PDS 70 b

TL;DR

This study presents the first direct UV and Hα imaging of the young exoplanet PDS 70 b, revealing accretion signatures and estimating its mass accretion rate, thereby advancing methods to study planet formation.

Contribution

It introduces novel UV observations of an accreting exoplanet, demonstrating the effectiveness of UV imaging for detecting planetary accretion and estimating accretion rates.

Findings

First UV detection of an exoplanet in accretion.

Hα emission accounts for 36% of accretion luminosity.

Estimated mass accretion rate of 1.4×10^{-8} M_Jup/yr.

Abstract

Recent discoveries of young exoplanets within their natal disks offer exciting opportunities to study ongoing planet formation. In particular, a planet's mass accretion rate can be constrained by observing the accretion-induced excess emission. So far, planetary accretion is only probed by the H$\alpha$ line, which is then converted to a total accretion luminosity using correlations derived for stars. However, the majority of the accretion luminosity is expected to emerge from hydrogen continuum emission, and is best measured in the ultraviolet (UV). In this paper, we present HST/WFC3/UVIS F336W (UV) and F656N (H$\alpha$) high-contrast imaging observations of PDS 70. Applying a suite of novel observational techniques, we detect the planet PDS 70 b with signal-to-noise ratios of 5.3 and 7.8 in the F336W and F656N bands, respectively. This is the first time that an exoplanet has been directly imaged in the UV. Our observed H$\alpha$ flux of PDS 70 b is higher by $3.5\sigma$ than the most recent published result. However, the light curve retrieved from our observations does not support greater than 30% variability in the planet's H$\alpha$ emission in six epochs over a five-month timescale. We estimate a mass accretion rate of $1.4\pm0.2\times10^{-8}M_{\mathrm{Jup}}/\mathrm{yr}$. H$\alpha$ accounts for 36% of the total accretion luminosity. Such a high proportion of energy released in line emission suggests efficient production of H$\alpha$ emission in planetary accretion, and motivates using the H$\alpha$ band for searches of accreting planets. These results demonstrate HST/WFC3/UVIS's excellent high-contrast imaging performance and highlight its potential for planet formation studies.