# Evidence for abnormal H$\alpha$ variability during near-transit observations of HD 189733 b

## TL;DR

This study investigates the variability of H-alpha line in the star HD 189733 to determine if observed near-transit signals are due to circumplanetary material or stellar activity, finding such variability is uncommon but significant.

## Contribution

The paper provides high-resolution, short-cadence out-of-transit H-alpha spectra to assess stellar activity variations related to planetary transits, clarifying the origin of near-transit absorption signatures.

## Key findings

- H-alpha variability near transit is uncommon in out-of-transit data.
- Observed near-transit signatures may be due to circumplanetary material or star-planet interactions.
- Further high signal-to-noise monitoring is needed for definitive conclusions.

## Abstract

Changes in levels of stellar activity can mimic absorption signatures in transmission spectra from circumplanetary material. The frequency and magnitude of these changes is thus important to understand in order to attribute any particular signal to the circumplanetary environment. We present short-cadence, high-resolution out-of-transit H$\alpha$ spectra for the hot Jupiter host HD 189733 in order to establish the frequency and magnitude of intrinsic stellar variations in the H$\alpha$ line core. We find that changes in the line core strength similar to those observed immediately pre- and post-transit in two independent data sets are uncommon. This suggests that the observed near-transit signatures are either due to absorbing circumplanetary material or occur preferentially in time very near planetary transits. In either case, the evidence for abnormal H$\alpha$ variability is strengthened, although the short-cadence out-of-transit data do not argue for circumplanetary absorption versus stellar activity caused by a star-planet interaction. Further out-of-transit monitoring at higher signal-to-noise would be useful to more strictly constrain the frequency of the near-transit changes in the H$\alpha$ line core.