MAPS: Constraining Serendipitous Time Variability in Protoplanetary Disk Molecular Ion Emission

TL;DR

This study investigates the potential for detecting time variability in HCO+ molecular ion emission in protoplanetary disks, aiming to understand the influence of stellar activity on disk chemistry.

Contribution

We conducted a daily temporal analysis of HCO+ emission in five protoplanetary disks using ALMA data, revealing minimal variability and highlighting the need for dedicated monitoring campaigns.

Findings

No significant flux variability detected (>3σ)

Spectral profile variations observed across all disks

Variability likely driven by stellar X-ray flares or observational factors

Abstract

Theoretical models and observations suggest that the abundances of molecular ions in protoplanetary disks should be highly sensitive to the variable ionization conditions set by the young central star. We present a search for temporal flux variability of HCO+ J=1-0, which was observed as a part of the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. We split out and imaged the line and continuum data for each individual day the five sources were observed (HD 163296, AS 209, GM Aur, MWC 480, and IM Lup, with between 3 to 6 unique visits per source). Significant enhancement (>3\sigma) was not observed, but we find variations in the spectral profiles in all five disks. Variations in AS 209, GM Aur, and HD 163296 are tentatively attributed to variations in HCO+ flux, while variations in IM Lup and MWC 480 are most likely introduced by differences in the \textit{uv} coverage, which impact the amount of recovered flux during imaging. The tentative detections and low degree of variability are consistent with expectations of X-ray flare driven HCO+ variability, which requires relatively large flares to enhance the HCO+ rotational emission at significant (>20%) levels. These findings also demonstrate the need for dedicated monitoring campaigns with high signal to noise ratios to fully characterize X-ray flare driven chemistry.