  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
$182.5^{\circ}\pm1.4^{\circ}$ and a separation of $600\pm22$~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 $F_{\nu}=1.5\pm0.4$~mJy, $3.2\pm0.9$ 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.
