HAZMAT. VIII. A Spectroscopic Analysis of the Ultraviolet Evolution of K Stars: Additional Evidence for K Dwarf Rotational Stalling in the First Gigayear

Abstract

Efforts to discover and characterize habitable zone planets have primarily focused on Sun-like stars and M dwarfs. K stars, however, provide an appealing compromise between these two alternatives that has been relatively unexplored. Understanding the ultraviolet (UV) environment around such stars is critical to our understanding of their planets, as the UV can drastically alter the photochemistry of a planet's atmosphere. Here we present near-UV and far-UV \textit{Hubble Space Telescope}'s Cosmic Origins Spectrograph observations of 39 K stars at three distinct ages: 40 Myr, 650 Myr, and $\approx$5 Gyr. We find that the K star (0.6 -- 0.8 M${}_{\odot }$) UV flux remains constant beyond 650 Myr before falling off by an order of magnitude by field age. This is distinct from early M stars (0.3 -- 0.6 M${}_{\odot }$), which begin to decline after only a few hundred Myr. However, the rotation-UV activity relation for K stars is nearly identical to that of early M stars. These results may be a consequence of the spin-down stalling effect recently reported for K dwarfs, in which the spin-down of K stars halts for over a Gyr when their rotation periods reach $\approx$10 d, rather than the continuous spin down that G stars experience. These results imply that exoplanets orbiting K dwarfs may experience a stronger UV environment than thought, weakening the case for K stars as hosts of potential "super-habitable" planets.

18 pages, 7 figures

References in corpus (22)

• The NumPy array: a structure for efficient numerical computation
• New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star
• Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth
• A Stellar Census of the Tucana-Horologium Moving Group
• Magnetic activity in the HARPS M-dwarf sample. The rotation-activity relationship for very low-mass stars through R'HK
• When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
• HAZMAT I: The Evolution of Far-UV and Near-UV Emission from Early M Stars
• K2 rotation periods for low-mass Hyads and a quantitative comparison of the distribution of slow rotators in the Hyades and Praesepe
• A Temporary Epoch of Stalled Spin-Down for Low-Mass Stars: Insights from NGC 6811 with Gaia and Kepler
• Pale Orange Dots: The Impact of Organic Haze on the Habitability and Detectability of Earthlike Exoplanets
• Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System
• Relation of X-ray activity and rotation in M dwarfs and predicted time-evolution of the X-ray luminosity
• HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from Early M Star
• Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets Around Low-Mass Stars: GJ 832, GJ 176, GJ 436
• The relative emission from chromospheres and coronae: dependence on spectral type and age
• The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets
• The Far Ultraviolet M-dwarf Evolution Survey. I. The Rotational Evolution of High-Energy Emissions
• HAZMAT. V. The Ultraviolet and X-ray Evolution of K Stars
• HAZMAT. VII. The Evolution of Ultraviolet Emission with Age and Rotation for Early M Dwarf Stars
• Semi-Empirical Modeling of the Atmospheres of the M Dwarf Exoplanet Hosts GJ 832 and GJ 581

Cited by in corpus (9)

• Correcting Stellar Flare Frequency Distributions Detected by TESS and Kepler
• Core-envelope decoupling drives radial shear dynamos in cool stars
• HAZMAT. IX. An Analysis of the UV and X-Ray Evolution of Low-Mass Stars in the Era of Gaia
• Evolution of helium triplet transits of close-in gas giants orbiting K-dwarfs
• TOI-712: a system of adolescent mini-Neptunes extending to the habitable zone
• A One-Dimensional Energy Balance Model Parameterization for the Formation of CO2 Ice on the Surfaces of Eccentric Extrasolar Planets
• Long-term stellar activity of M dwarfs: A combined K2 and TESS study of two early M-type stars
• From CO${}_2$- to H${}_2$O-dominated atmospheres and back -- How mixed outgassing changes the volatile distribution in magma oceans around M dwarf stars
• Discovery of a transiting hot water-world candidate orbiting Ross 176 with TESS and CARMENES