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We have detected solar-like oscillations in the mid-K-dwarf ϵ Indi A, making it the coolest dwarf to have measured oscillations. The star is noteworthy for harboring a pair of brown dwarf companions and a Jupiter-type planet. We observed ϵ Indi A during two radial velocity campaigns, using the high-resolution spectrographs HARPS (2011) and UVES (2021). Weighting the time series, we computed the power spectra and established the detection of solar-like oscillations with a power excess located at 5265 ± 110 μ Hz—the highest frequency solar-like oscillations so far measured in any star. The measurement of the center of the power excess allows us to compute a stellar mass of 0.782 ± 0.023 M ⊙ based on scaling relations and a known radius from interferometry. We also determine the amplitude of the peak power and note that there is a slight difference between the two observing campaigns, indicating a varying activity level. Overall, this work confirms that low-amplitude solar-like oscillations can be detected in mid-K-type stars in radial velocity measurements obtained with high-precision spectrographs. [ABSTRACT FROM AUTHOR]

In strong stellar and solar flares, flare loops typically appear during the decay phase, providing an additional contribution to the flare emission and, possibly, obscuring the flare emission. Superflares, common in active, cool stars, persist mostly from minutes to several hours and alter the star's luminosity across the electromagnetic spectrum. Recent observations of a young main-sequence star reveal a distinctive cool loop arcade forming above the flaring region during a 27 hr superflare event, obscuring the region multiple times. Analysis of these occultations enables the estimation of the arcade's geometry and physical properties. The arcade’s size expanded from 0.213 to 0.391 R _* at a speed of approximately 3.5 km s ^−1 . The covering structure exhibited a thickness below 12,200 km, with electron densities ranging from 10 ^13 to 10 ^14 cm ^−3 and temperatures below 7600 K, 6400 K, and 5077 K for successive occultations. Additionally, the flare’s maximum emission temperature has to exceed 12,000 K for the occultations to appear. Comparing these parameters with known values from other stars and the Sun suggests the structure’s nature as an arcade of cool flare loops. For the first time, we present the physical parameters and the reconstructed geometry of the cool flare loops that obscure the flaring region during the gradual phase of a long-duration flare on a star other than the Sun.

The study reports velocity ellipsoid parameters including the kinematical properties of K dwarf stars. Herein, two samples of K dwarf stars are presented with the solar neighborhood (d≤25$$ d\le 25 $$ pc) for high (b≥20o$$ b\ge 2{0}^{\mathrm{o}} $$; 106 points) and low galactic latitudes (b<20o$$ b<2{0}^{\mathrm{o}} $$; 199 points). We calculated spatial velocities (U‾$$ \overline{U} $$, V‾$$ \overline{V} $$, and W‾$$ \overline{W} $$; km s‐1$$ {}^{\hbox{-} 1} $$) along the galactic coordinates and the velocity dispersion (σ1$$ {\sigma}_1 $$, σ2$$ {\sigma}_2 $$, σ3$$ {\sigma}_3 $$; km s‐1$$ {}^{\hbox{-} 1} $$), the vertex longitude (l2$$ {l}_2 $$), which have often differed significantly from zero like many of our analyses, i.e., (l2=−0o.006394$$ {l}_2=-{0}^{\mathrm{o}}.006394 $$; 106 points) and (l2=−0o.630221$$ {l}_2=-{0}^{\mathrm{o}}.630221 $$; 199 points). Moreover, we computed the solar motion, i.e., S⊙=23.17±4.82$$ {S}_{\odot }=23.17\pm 4.82 $$ km s‐1$$ {}^{\hbox{-} 1} $$, and other solar elements (e.g., lA$$ {l}_{\mathrm{A}} $$, bA$$ {b}_{\mathrm{A}} $$, αA$$ {\alpha}_{\mathrm{A}} $$, and δA$$ {\delta}_{\mathrm{A}} $$). Finally, we estimated Oort's constants, i.e., A=12.911±0.161$$ A=12.911\pm 0.161 $$ km s‐1$$ {}^{\hbox{-} 1} $$ kpc‐1$$ {}^{\hbox{-} 1} $$ and B=$$ B= $$‐13.163±0.276$$ 13.163\pm 0.276 $$ km s‐1$$ {}^{\hbox{-} 1} $$ kpc‐1$$ {}^{\hbox{-} 1} $$, both indicating that the rotational velocity was Vo=213.807±14.61$$ {V}_{\mathrm{o}}=213.807\pm 14.61 $$ km s‐1$$ {}^{\hbox{-} 1} $$. [ABSTRACT FROM AUTHOR]

Stellar flares from K and M dwarfs release panchromatic radiation characterized by a significantly higher brightness temperature (∼9–20 kK) than the star. The increased frequency of magnetic activity on young low-mass stars results in the energy released during flaring events becoming a notable contributor to the radiation environment. This study focuses on the β Pictoris moving group ( β PMG; 24 ± 3 Myr) for the analysis of young low-mass star flaring rates within the framework of larger flare studies. The calibration of long-term optical flare statistics is crucial to updating flare activity–age relations and the interpretation of exoplanet atmosphere observations. Using the β PMG, we develop a modular flare-extraction pipeline sensitive to low-mass stellar flares in observations from the Transiting Exoplanet Survey Satellite (TESS). This pipeline is built to characterize the flare properties of these stars such as total energy and cumulative flare rate. Consistent with previous studies, this sample ( N = 49) shows higher cumulative flare rates than early-type and old field stars by at least an order of magnitude. Fitted flare frequency distributions for both early- and late-type M dwarfs show an average slope of 1.58 ± 0.23, with earlier stars flaring with lower or similar rates to late types. A typical member in this sample has daily (∼1 day ^−1 ) flares with TESS band energies of 10 ^32 –10 ^33 erg. The optical flare rates and energies for this group provide essential context into the coevolution of host stars and associated planets.

Stellar X-ray and UV radiation can significantly affect the survival, composition, and long-term evolution of the atmospheres of planets in or near their host star’s habitable zone (HZ). Especially interesting are planetary systems in the solar neighborhood that may host temperate and potentially habitable surface conditions, which may be analyzed by future ground- and space-based direct-imaging surveys for signatures of habitability and life. To advance our understanding of the radiation environment in these systems, we leverage ∼3 Ms of XMM-Newton and Chandra observations in order to measure three fundamental stellar properties at X-ray energies for 57 nearby FGKM stellar systems: the shape of the stellar X-ray spectrum, the luminosity, and the timescales over which the stars vary (e.g., due to flares). These systems possess HZs that will be directly imageable to next-generation telescopes such as the Habitable Worlds Observatory and ground-based Extremely Large Telescopes. We identify 29 stellar systems with L _X / L _bol ratios similar to (or less than) that of the Sun; any potential planets in the HZs of these stars therefore reside in present-day X-ray radiation environments similar to (or less hostile than) modern Earth, though a broader set of these targets could host habitable planets. An additional 19 stellar systems have been observed with the Swift X-ray Telescope; in total, only ∼30% of potential direct imaging target stars has been observed with XMM-Newton, Chandra, or Swift. The data products from this work (X-ray light curves and spectra) are available via a public Zenodo repository (doi: https://doi.org/10.5281/zenodo.11490574 ).

Asteroseismology of dwarf stars cooler than the Sun is very challenging owing to the low amplitudes and rapid timescales of oscillations. Here we present the asteroseismic detection of solar-like oscillations at 4-minute timescales ( ${\nu }_{\max }\sim 4300$ μ Hz) in the nearby K dwarf σ Draconis using extreme-precision Doppler velocity observations from the Keck Planet Finder and 20 s cadence photometry from NASA’s Transiting Exoplanet Survey Satellite. The star is the coolest dwarf star to date with both velocity and luminosity observations of solar-like oscillations, having amplitudes of 5.9 ± 0.8 cm s ^−1 and 0.8 ± 0.2 ppm, respectively. These measured values are in excellent agreement with established luminosity−velocity amplitude relations for oscillations and provide further evidence that mode amplitudes for stars with T _eff < 5500 K diminish in scale following an ( L / M ) ^1.5 relation. By modeling the star’s oscillation frequencies from photometric data, we measure an asteroseismic age of 4.5 ± 0.9 (ran) ± 1.2 (sys) Gyr. The observations demonstrate the capability of next-generation spectrographs and precise space-based photometry to extend observational asteroseismology to nearby cool dwarfs, which are benchmarks for stellar astrophysics and prime targets for directly imaging planets using future space-based telescopes.

The high-energy X-ray and ultraviolet (UV) radiation fields of exoplanet host stars play a crucial role in controlling the atmospheric conditions and the potential habitability of exoplanets. Major surveys of the X-ray/UV emissions from late-type (K and M spectral types) exoplanet hosts have been conducted by the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary systems (MUSCLES) and Mega-MUSCLES Hubble Space Telescope Treasury programs. These samples primarily consist of relatively old, “inactive,” low-mass stars. In this paper we present results from X-ray observations of the coronal emission from these stars obtained using the Chandra X-ray Observatory, the XMM-Newton Observatory, and the Neil Gehrels Swift Observatory. The stars effectively sample the coronal activity of low-mass stars over a wide range of masses and ages. The vast majority (21 of 23) of the stars are detected and their X-ray luminosities measured. Short-term flaring variability is detected for most of the fully convective ( M ≤ 0.35 M _⊙ ) stars but not for the more massive M dwarfs during these observations. Despite this difference, the mean X-ray luminosities for these two sets of M dwarfs are similar, with more massive (0.35 M _⊙ ≤ M ≤ 0.6 M _⊙ ) M dwarfs at ∼5 × 10 ^26 erg s ^−1 compared to ∼2 × 10 ^26 erg s ^−1 for fully convective stars older than 1 Gyr. Younger, fully convective M dwarfs have X-ray luminosities between 3 and 6 × 10 ^27 erg s ^−1 . The coronal X-ray spectra have been characterized and provide important information that is vital for the modeling of the stellar EUV spectra.

We present the first elemental abundance measurements of the K dwarf (K7V) exoplanet-host star WASP-107 using high-resolution ( R ≃45,000), near-infrared ( H - and K -band) spectra taken from Gemini-S/IGRINS. We use the previously determined physical parameters of the star from the literature and infer the abundances of 15 elements—C, N, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Ni, all with precision < 0.1 dex—based on model fitting using MARCS model atmospheres and the Turbospectrum spectral synthesis code. Our results show near-solar abundances and a carbon-to-oxygen ratio (C/O) of 0.50 ± 0.10, which is consistent with the solar value of 0.54 ± 0.09. The orbiting planet, WASP-107b, is a super-Neptune with a mass in the Neptune regime (=1.8 M _Nep ) and a radius close to Jupiter's (=0.94 R _Jup ). This planet is also being targeted by four JWST Cycle 1 programs in transit and eclipse, which should provide highly precise measurements of atmospheric abundances. This will enable us to properly compare the planetary and stellar chemical abundances, which is essential in understanding the formation mechanisms, internal structure, and chemical composition of exoplanets. Our study is a proof-of-concept that will pave the way for such measurements to be made for all of JWST’s cooler exoplanet-host stars.