Magnetic activity optimal tracers: from radio to X-ray; the relevance of UV astronomy

et al. 2012

ApJ

We report the discovery of a brown dwarf companion to the young M dwarf 1RXS J235133.3+312720 as part of a high contrast imaging search for planets around nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2. ′′ 4 (∼120 AU) pair is confirmed to be comoving from two epochs of high resolution imaging. Follow-up low-and moderate-resolution near-infrared spectroscopy of 1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of L0 +2 −1 . The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV activity levels comparable to young moving group members with ages of ∼10-100 Myr. U V W kinematics based the measured radial velocity of the primary and the system's photometric distance (50 ± 10 pc) indicate it is likely a member of the ∼50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology shows subtle hints of intermediate surface gravity. The spectrum is also an excellent match to the ∼200 Myr M9 brown dwarf LP 944-20. Assuming an age of 50-150 Myr, evolutionary models imply a mass of 32 ± 6 M Jup for the companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor moving group after the L4 companion CD-35 2722 B and one of the few benchmark brown dwarfs known at young ages.

Section: Uv Activitymentioning

confidence: 99%

Planets Around Low-Mass Stars (Palms). I. A Substellar Companion to the Young M Dwarf 1rxs J235133.3+312720

Bowler

Liu

et al. 2012

ApJ

“…For stars hotter than about 5250 K, the flux in the GALEX bandpasses is made up predominantly from continuum emission (Smith & Redenbaugh 2010) with additional flux provided by strong emission lines (C iv, C ii, Si iv, He ii) originating from the corona, transition region and chromosphere. Cooler stars have FUV and NUV fluxes strongly dominated by stellar activity (e.g., Robinson et al 2005;Welsh et al 2006;Pagano 2009). This makes GALEX an excellent tool with which to study stellar activity, especially since GALEX can detect FGK (and early Ms) at great distances than the existing X-ray missions, out to ∼150 pc for the FUV and between 20 and 500 pc, depending of T eff for the NUV (Findeisen & Hillenbrand 2010;Shkolnik et al 2011).…”

mentioning

confidence: 99%

An Ultraviolet Investigation of Activity on Exoplanet Host Stars

2013

ApJ

Using the far-UV (FUV) and near-UV (NUV) photometry from the NASA Galaxy Evolution Explorer (GALEX), we searched for evidence of increased stellar activity due to tidal and/or magnetic star-planet interactions (SPI) in the 272 known FGK planetary hosts observed by GALEX. With the increased sensitivity of GALEX, we are able probe systems with lower activity levels and at larger distances than what has been done to date with X-ray satellites. We compared samples of stars with close-in planets (a < 0.1 AU) to those with far-out planets (a > 0.5 AU) and looked for correlations of excess activity with other system parameters. This statistical investigation found no clear correlations with a, M p , or M p /a, in contrast to some X-ray and Ca ii studies. However, there is tentative evidence (at a level of 1.8σ ) that stars with radial-velocity-(RV)-detected close-in planets are more FUV-active than stars with far-out planets, in agreement with several published X-ray and Ca ii results. The case is strengthened to a level of significance to 2.3σ when transit-detected close-in planets are included. This is most likely because the RV-selected sample of stars is significantly less active than the field population of comparable stars, while the transit-selected sample is similarly active. Given the factor of 2-3 scatter in fractional FUV luminosity for a given stellar effective temperature, it is necessary to conduct a time-resolved study of the planet hosts in order to better characterize their UV variability and generate a firmer statistical result.

“…However, the EUV flux from stars other than the Sun is predominantly absorbed by the interstellar medium (ISM). Limited data exist in the EUV and must therefore be estimated from empirical relationships, differential emission measure models, or stellar atmospheric models constrained by data at FUV, NUV, or X-ray wavelengths (e.g., Pagano 2009;Linsky & Güdel 2015;Fontenla et al 2016;Peacock et al 2019Peacock et al , 2020Tilipman et al 2021).…”

Section: The Effect Of Uv Radiation On Planetary Atmospheresmentioning

confidence: 99%

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

Richey-Yowell

Loyd

et al. 2022

ApJ

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 Hubble Space Telescope's Cosmic Origins Spectrograph observations of 39 K stars at three distinct ages: 40 Myr, 650 Myr, and ≈5 Gyr. We find that the K star (0.6–0.8 M ⊙) 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 ⊙), which begin to decline after only a few hundred megayears. 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 gigayear when their rotation periods reach ≈10 days, 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.