Living with Goldilocks K-dwarfs: Rotation, Coronal X-ray, Chromospheric UV Emissions and Stellar Winds over Time: Effects on Hosted Planets
Over the last 30-yr we have carrying out X-ray-UV & photometric studies of F8-G8V stars in the Villanova Sun in Time Program (Guinan & Engle 2009). Over the last decade, we have carried out corresponding studies of M-dwarfs (M0-6 V) in the Living with a Red Dwarf Program (e.g. Engel and Guinan 2016). These programs focused on studying solar/stellar dynamo physics & angular momentum loss from magnetic-winds. We developed Age-Rotation-Activity relations that include X-ray-UV (X-UV) irradiances as a function of age and spectral type. These data have been utilized to investigate the effects of X-UV radiation on planet atmospheres and potential for life on the hosted planets. (Lammer et al. 2003, Ribas et al. 2005). To complete spectral coverage, we have been gathering similar data on K-stars. Currently we have data on 180 G8-K8 V stars. As discussed by Cuntz & Guinan (2017) and others, K-dwarfs may be the best suited (i.e. Goldilocks stars) to host biologically-active planets. K-stars appear to be in the "sweet spot" with properties intermediate between more luminous, but shorter-lived & rarer G-stars and the more numerous, low luminosity, but highly magnetically-active M-dwarfs. Unlike G stars, K-stars evolve slowly and have long MS-lifetimes ~25 - 80 Gyr. For comparison, the luminosity of a K5 star increases only by ~10% over the Sun's MS-lifetime of 9-10 Gyr. Unlike M-stars, K-stars are less magnetically active and flare less frequently. Importantly the habitable zones of M-stars are close-in (~0.04-0.3 au) exposing the hosted HZ planets often exposing them to extreme levels of X-UV radiation -in some cases causing the complete loss the planets' atmospheres and water inventories. Even though M-stars host many HZ planets, it is concerning that many M-star HZ planets may not be suitable for life due to the extreme (hazardous) levels of high energy radiation and plasma incident on them especially when young. We present Age-Rotation relations and X-ray and FUV irradiances [(Fx@1au) -Age and Ffuv@1au -Age)] of G8V - K5V stars. We also calibrated the expected stellar wind flux densities (F-wind@1au) of K-stars vs. age using available mass loss rates from B. Wood (2018). As an examples, we apply these relations to some interesting planet hosting K-stars (e.g. Kepler-442, tau Ceti and several others) to determine their ages and the hosted planets' X-ray - UV irradiances and wind densities fluxes. We evaluate the potential for life on these planets. This research is supported by grants from NASA for HST, XMM-Newton, & Chandra. We greatly appreciate this support.
American Astronomical Society Meeting Abstracts
- Pub Date:
- January 2020