HAZMAT. VII. The Evolution of Ultraviolet Emission with Age and Rotation for Early M Dwarf Stars

TL;DR

This study investigates how ultraviolet emission from early M dwarf stars evolves with age and rotation, revealing a significant decline in UV flux over billions of years, which impacts planetary habitability and atmospheric chemistry.

Contribution

It provides the first detailed analysis of UV spectral evolution in early M dwarfs across a wide age range, with models to predict UV emission based on stellar properties.

Findings

UV emission remains elevated for ~240 Myr before declining significantly.

Chromospheric emission declines more gradually than transition region emission.

Predictive models can estimate UV flux with 0.2-0.3 dex accuracy.

Abstract

The ultraviolet (UV) emission from the most numerous stars in the universe, M dwarfs, impacts the formation, chemistry, atmospheric stability, and surface habitability of their planets. We have analyzed the spectral evolution of UV emission from M0-M2.5 (0.3-0.6 Msun) stars as a function of age, rotation, and Rossby number, using Hubble Space Telescope observations of Tucana Horologium (40 Myr), Hyades (650 Myr), and field (2-9 Gyr) objects. The quiescent surface flux of their C II, C III, C IV, He II, N V, Si III, and Si IV emission lines, formed in the stellar transition region, remains elevated at a constant level for 240 $\pm$ 30 Myr before declining by 2.1 orders of magnitude to an age of 10 Gyr. Mg II and far-UV pseudocontinuum emission, formed in the stellar chromosphere, exhibit more gradual evolution with age, declining by 1.3 and 1.7 orders of magnitude, respectively. The youngest stars exhibit a scatter of 0.1 dex in far-UV line and pseudocontinuum flux attributable only to rotational modulation, long-term activity cycles, or an unknown source of variability. Saturation-decay fits to these data can predict an M0-M2.5 star's quiescent emission in UV lines and the far-UV pseudocontinuum with an accuracy of roughly 0.2-0.3 dex, the most accurate means presently available. Predictions of UV emission will be useful for studying exoplanetary atmospheric evolution, the destruction and abiotic production of biologically relevant molecules, and interpreting infrared and optical planetary spectra measured with observatories like the James Webb Space Telescope.