HST/WFC3 H$\alpha$ Direct-Imaging Detection of a Point-like Source in the Disk Cavity of AB Aur

Abstract

Accreting protoplanets enable the direct characterization of planet formation. As part of a high-contrast imaging search for accreting planets with the Hubble Space Telescope (HST) Wide Field Camera 3, we present H$\alpha$ images of AB Aurigae (AB Aur), a Herbig Ae/Be star harboring a transition disk. The data were collected in two epochs of direct-imaging observations using the F656N narrow-band filter. After subtracting the point spread function of the primary star, we identify a point-like source located at a P.A. of $182.5^{\circ }\pm 1.4^{\circ }$ and a separation of $600\pm 22$~mas relative to the host star. The position is consistent with the recently identified protoplanet candidate AB Aur b. The source is visible in two individual epochs separated by $\sim 50$ days and the H$\alpha$ intensities in the two epochs agree. The H$\alpha$ flux density is $F_{\nu }=1.5\pm 0.4$~mJy, $3.2\pm 0.9$ times of the optical continuum determined by published HST/STIS photometry. In comparison to PDS 70 b and c, the H$\alpha$ excess emission is weak. The central star is accreting and the stellar H$\alpha$ emission has a similar line-to-continuum ratio as seen in AB Aur b. We conclude that both planetary accretion and scattered stellar light are possible sources of the H$\alpha$ emission, and the H$\alpha$ detection alone does not validate AB Aur b as an accreting protoplanet. Disentangling the origin of the emission will be crucial for probing planet formation in the AB Aur disk.

Accepted for publication in ApJL

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