VLT/MUSE Detection of the AB Aurigae b Protoplanet with $H _{\rm \alpha}$ Spectroscopy

TL;DR

This study reports the detection of the AB Aurigae b protoplanet in $H_{\alpha}$ spectroscopy using VLT/MUSE, revealing an inverse P Cygni profile suggestive of accretion, marking a rare observation of a protoplanet in $H_{\alpha}$.

Contribution

First detection of a protoplanet in $H_{\alpha}$ with a spectrum resembling an inverse P Cygni profile, indicating potential accretion activity.

Findings

AB Aur b detected in $H_{\alpha}$ emission and absorption at specific wavelengths.

Spectrum resembles an inverse P Cygni profile, indicative of infalling gas.

This is only the second protoplanetary system observed in $H_{\alpha}$.

Abstract

We analyze high-contrast, medium-spectral-resolution $H_{\rm \alpha}$ observations of the star AB Aurigae using the Very Large Telescope's Multi Unit Spectroscopic Explorer (MUSE). In multiple epochs, MUSE detects the AB Aur b protoplanet discovered from Subaru/SCExAO data in emission at wavelengths slightly blue-shifted from the $H_{\rm \alpha}$ line center (i.e. at 6558.88--6560.13 \AA; $\sim$ -100 km s$^{-1}$) and in absorption at redshifted wavelengths (6562.8--6565.1 \AA; $\sim$ 75 km s$^{-1}$). AB Aur b's $H_{\rm \alpha}$ spectrum is inconsistent with that of the host star or the average residual disk spectrum and is dissimilar to that of PDS 70 b and c. Instead, the spectrum's shape resembles that of an inverse P Cygni profile seen in some accreting T Tauri stars and interpreted as evidence of infalling cold gas from accretion, although we cannot formally rule out all other nonaccretion origins for AB Aur b's MUSE detection. AB Aurigae hosts only the second protoplanetary system detected in $H_{\rm \alpha}$ thus far and the first with a source showing a spectrum resembling an inverse P Cygni profile. Future modeling and new optical data will be needed to assess how much of AB Aur b's emission source(s) originates from protoplanet accretion reprocessed by the disk, a localized scattered-light feature with a unique $H_{\rm \alpha}$ profile, or another mechanism.