AU Microscopii in the Far-UV: Observations in Quiescence, during Flares, and Implications for AU Mic b and c

Abstract

High-energy X-ray and ultraviolet (UV) radiation from young stars impacts planetary atmospheric chemistry and mass loss. The active ∼22 Myr M dwarf AU Mic hosts two exoplanets orbiting interior to its debris disk. Therefore, this system provides a unique opportunity to quantify the effects of stellar X-ray and UV irradiation on planetary atmospheres as a function of both age and orbital separation. In this paper, we present over 5 hr of far-UV (FUV) observations of AU Mic taken with the Cosmic Origins Spectrograph (COS; 1070-1360 Å) on the Hubble Space Telescope (HST). We provide an itemization of 120 emission features in the HST/COS FUV spectrum and quantify the flux contributions from formation temperatures ranging from 104 to 107 K. We detect 13 flares in the FUV white-light curve with energies ranging from 1029 to 1031 erg s. The majority of the energy in each of these flares is released from the transition region between the chromosphere and the corona. There is a 100× increase in flux at continuum wavelengths λ < 1100 Å in each flare, which may be caused by thermal Bremsstrahlung emission. We calculate that the baseline atmospheric mass-loss rate for AU Mic b is ∼108 g s−1, although this rate can be as high as ∼1014 g s−1 during flares with L flare ≃ 10 33 erg s−1. Finally, we model the transmission spectra for AU Mic b and c with a new panchromatic spectrum of AU Mic and motivate future JWST observations of these planets.

Prospects of Detecting Nonthermal Protons in Solar Flares via Lyman Line Spectroscopy: Revisiting the Orrall–Zirker Effect

“…Therefore using OrrallZirkerPy and a large number of RADYN+FP flare models, the influence of those parameters can be investigated. Extension to stellar flares, including modeling synthetic Hubble Space Telescope observables is also an exciting direction, especially given that the only confirmed detection of the OZ effect thus far has been on a dMe star (Woodgate et al 1992) and that follow on searches produced null detections (e.g., Robinson et al 1993;Feinstein et al 2022). There are of course model limitations and idealizations that can be improved upon, such as modeling multispecies particle beams, 13 and moving away from the assumptions of a near disk-center vertical beam.…”

“…Those observations of Ly α from a moderately strong stellar flare that occurred on the dMe star AU Mic revealed a short (t ∼ 3 s) but clear enhancement to redward of the line core relative to the blueward side. Unfortunately, follow-up studies of flares on AU Mic had null detections of nonthermal Ly α emission (Robinson et al 1993(Robinson et al , 2001Feinstein et al 2022). As regards solar flares, we have lacked routine high-quality observations of the Lyman lines in flares, and those that we do possess have not revealed the OZ effect.…”

Correcting Exoplanet Transmission Spectra for Stellar Activity with an Optimised Retrieval Framework

“…In recent years the number of observations of exoplanets orbiting YSOs e.g. AU Mic (Szabó et al 2021;Klein et al 2022), V1298 Tau (Feinstein et al 2022;Maggio et al 2022) and K2-33b (Thao et al 2023), has also increased as, despite being strongly affected by stellar activity, studying these systems provides us with a unique opportunity to begin to fill in gaps in our understanding of the stages of planetary formation and evolution (Raymond & Morbidelli 2022).…”

TOI 560: Two Transiting Planets Orbiting a K Dwarf Validated with iSHELL, PFS, and HIRES RVs

“…The discovery of a young multiplanet system can be especially valuable for improving our understanding of exoplanet formation and evolution, both the demographics of the exoplanet architectures, and the atmospheric evolution as a function of stellar age, orbital period, and host star spectral type (Marchwinski et al 2015;Fulton et al 2017;Newton et al 2019;Hirano et al 2020;Carolan et al 2020;Benatti et al 2021;Alvarado-Gomez et al 2022;Cohen et al 2022;Feinstein et al 2022a;Flagg et al 2022;Ilin & Poppenhaeger 2022;Klein et al 2022). We know that the orbital properties of planets change over time as shown by the existence of hot Jupiters; we also see evidence for orbital distance dependent mass loss via photoevaporation, which may be responsible for producing the planetradius gap and may impact our understanding of the occurrence rate of terrestrial planets at larger orbital separations (Pascucci et al 2019;Mann et al 2020;Plavchan et al 2020;Klein et al 2021).…”