Duvvuri, Girish M.; Pineda, John Sebastian; Garciá Soto, Aylin; Berta-Thompson, Zachory K.; Youngblood, Allison A.; France, Kevin C.; Newton, Elisabeth R.; Stassun, Keivan Guadalupe. (2025). FUMES. IV. Optical and Far-ultraviolet Spectra of a Flare on the M Dwarf GJ 4334. Astronomical Journal, 170(4), 249. https://doi.org/10.3847/1538-3881/adff74
On September 20, 2017, astronomers observed the star GJ 4334, a small, cool M5V dwarf star that rotates once every ~23.5 days. The observations were part of a larger survey of moderately active M dwarfs and were conducted simultaneously using the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (covering far-ultraviolet light from 1160–1710 Å) and the Dual Imaging Spectrograph on the 3.5-meter telescope at Apache Point Observatory (covering optical light from 3750–5050 Å and 5800–6950 Å).
During the observation, GJ 4334 produced a flare—a sudden, intense burst of energy. The flare began with an increase in optical chromospheric emission lines (light emitted from the star’s lower atmosphere), followed by a rapid rise and decay of multiple far-ultraviolet emission lines formed in the transition region (the layer between the star’s chromosphere and corona). Afterward, the optical lines decayed more slowly. The flare caused noticeable broadening and asymmetry in the optical lines, likely due to bulk motions of plasma on the star’s surface, and elevated fluxes (brightness) persisted after the flare in both optical and far-ultraviolet light. Higher-order Balmer lines (specific hydrogen emission lines) were found to rise earlier and decay faster than lower-order lines.
The flare’s equivalent durations in individual emission lines ranged from ~800 to 30,000 seconds, corresponding to energies of ~10²⁸–3×10²⁹ ergs per line. Comparing GJ 4334’s flare behavior with TESS observations of EV Lacertae, a similar-mass but faster-rotating M dwarf, revealed that GJ 4334 produces relatively more large flares than expected from the power-law trend of smaller flares.
This dataset provides a rare opportunity to study flares near a key stage in the evolution of stellar magnetic activity, helping astronomers understand how magnetic energy is released in low-mass stars.
Figure 1. The quiescent APO optical spectrum is plotted in dark blue while the spectrum at flare peak is plotted in orange, with separate panels for the two arms of DIS. The flare only seems to brighten lines relative to the continuum level, leaving the majority of the continuum unchanged. The exception is the continuum below 4500 Å, which appears slightly enhanced. A subset of the optical lines analyzed in this work have been highlighted with ion labels above the line peak.