[Paper Review] Hunting for stellar coronal mass ejections

Kosuke Namekata, Hiroyuki Maehara+4 more|arXiv (Cornell University)|Nov 10, 2022

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper reviews indirect observational evidence for stellar coronal mass ejections (CMEs) linked to superflares on cool stars, particularly M/K-dwarfs and a young G-dwarf (EK Dra). It presents the first detection of a stellar filament eruption—associated with a superflare—showing blue-shifted Hα absorption, indicating a CME with a mass of 1.1 × 10¹⁸ g, ten times greater than the largest solar CMEs, suggesting severe impacts on exoplanet habitability.

ABSTRACT

Solar flares are often accompanied by filament/prominence eruptions, sometimes leading to coronal mass ejections (CMEs). By analogy, we expect that stellar flares are also associated with stellar CMEs whose properties are essential to know the impact on exoplanet habitability. Probable detections of stellar CMEs are still rare, but in this decade, there are several reports that (super-)flares on M/K-dwarfs and evolved stars sometimes show blue-shifted optical/UV/X-ray emissions lines, XUV/FUV dimming, and radio bursts. Some of them are interpreted as indirect evidence of stellar prominence eruptions/CMEs on cool stars. More recently, evidence of stellar filament eruption, probably leading to a CME, is reported even on a young solar-type star (G-dwarf) as a blue-shifted absorption of H$α$ line associated with a superflare. Notably, the erupted masses for superflares are larger than those of the largest solar CMEs, indicating severe influence on exoplanet environments. The ratio of the kinetic energy of stellar CMEs to flare energy is significantly smaller than expected from the solar scaling relation and this discrepancy is still in debate. We will review the recent updates of stellar CME studies and discuss the future direction in this paper.

Motivation & Objective

• To assess the existence and properties of stellar coronal mass ejections (CMEs) through indirect observational signatures.
• To understand the impact of stellar CMEs on exoplanet habitability, especially around active M/K-dwarfs and young solar-type stars.
• To identify observational biases and limitations in current detection methods for stellar CMEs.
• To explore discrepancies in kinetic energy scaling between solar and stellar CMEs.
• To propose future observational and modeling strategies for more robust detection and characterization of stellar CMEs.

Proposed method

• Analyzing multi-wavelength archival data from X-ray, EUV, FUV, optical, and radio observations to identify signatures of stellar CMEs.
• Using Doppler shift measurements in Hα lines to detect blue-shifted absorption features indicative of erupting prominence/filament plasma.
• Applying coronal dimming analysis in XUV/EUV bands to infer mass ejection and energy loss from stellar atmospheres.
• Comparing observed CME kinetic energies and masses with solar scaling relations to assess deviations.
• Conducting radiative transfer modeling to understand the visibility of stellar prominences across different stellar types and activity levels.
• Evaluating observational biases in flare-CME association rates using statistical analysis of flare samples.

Experimental results

Research questions

• RQ1 What observational signatures in multi-wavelength data can serve as reliable indicators of stellar CMEs?
• RQ2 Why do the kinetic energies of observed stellar CME candidates fall significantly below solar scaling relations?
• RQ3 How frequently do stellar CMEs occur in relation to superflares, particularly on young solar-type stars?
• RQ4 What physical mechanisms suppress CME velocities or energies on cool stars compared to the Sun?
• RQ5 How can multi-method simultaneous detection (e.g., dimming + Doppler shift) improve confidence in identifying stellar CMEs?

Key findings

• The first detection of a stellar filament eruption on a young G-dwarf (EK Dra) is observed via blue-shifted Hα absorption during a superflare, indicating a CME-like ejection.
• The ejected mass of the prominence on EK Dra is estimated at 1.1 × 10¹⁸ g, ten times greater than the largest observed solar CMEs.
• Not all superflares are accompanied by filament eruptions; a higher-energy superflare on EK Dra showed no such signatures, suggesting variable eruption occurrence.
• Stellar CME candidates exhibit significantly lower kinetic energies than predicted by solar scaling laws, indicating a fundamental difference in energy conversion or dynamics.
• Observational biases, such as limited simultaneous multi-wavelength coverage, likely affect the inferred flare-CME association rate and must be accounted for in statistical studies.
• The visibility of prominence plasma depends on stellar type and activity level, with emission/absorption features differing between M-dwarfs and solar-type stars, requiring radiative transfer modeling for accurate interpretation.