Abstract: We present a homogeneous catalog of global asteroseismic parameters and derived stellar parameters for 765 Kepler main-sequence and subgiant stars. The catalog was produced by re-analyzing all available Kepler DR25 short-cadence data using pySYD, an automated pipeline to extract global asteroseismic parameters. We find 50 new detections, seven of which are also planet candidate host stars. We find excellent agreement between our $\nu_{\text{max}}$ and $\Delta \nu$ measurements and literature values, with an average offset of $0.2 \pm 0.4\%$ ($\sigma=5\%$) and $0.2 \pm 0.7\%$ ($\sigma=2\%$), respectively. In addition, we derive stellar radii and masses with an average precision of $2.7\%$ and $10.4\%$, respectively, and find a median offset of $0.4 \pm 0.4\%$ ($\sigma=10\%$) between our radii derived with asteroseismology and those from Gaia parallaxes. Using spectroscopic $\log{R'_{\text{HK}}}$ activity measurements from Keck/HIRES, we derive a new amplitude scaling relation with an activity term for main-sequence and subgiant stars, which reduces the offset between expected and observed oscillation amplitudes from $9.3 \pm 1.6\%$ to $1.7 \pm 0.9\%$. Our work is the largest and most homogeneous asteroseismic catalog of Kepler main-sequence and subgiant stars to date, including a total of 101 stars hosting planet candidates and 451 stars with measured rotation periods.

Abstract: We present a homogeneous catalog of global asteroseismic parameters and derived stellar parameters for 765 Kepler main-sequence and subgiant stars. The catalog was produced by re-analyzing all available Kepler DR25 short-cadence data using pySYD, an automated pipeline to extract global asteroseismic parameters. We find 50 new detections, seven of which are also planet candidate host stars. We find excellent agreement between our $\nu_{\text{max}}$ and $\Delta \nu$ measurements and literature values, with an average offset of $0.2 \pm 0.4\%$ ($\sigma=5\%$) and $0.2 \pm 0.7\%$ ($\sigma=2\%$), respectively. In addition, we derive stellar radii and masses with an average precision of $2.7\%$ and $10.4\%$, respectively, and find a median offset of $0.4 \pm 0.4\%$ ($\sigma=10\%$) between our radii derived with asteroseismology and those from Gaia parallaxes. Using spectroscopic $\log{R'_{\text{HK}}}$ activity measurements from Keck/HIRES, we derive a new amplitude scaling relation with an activity term for main-sequence and subgiant stars, which reduces the offset between expected and observed oscillation amplitudes from $9.3 \pm 1.6\%$ to $1.7 \pm 0.9\%$. Our work is the largest and most homogeneous asteroseismic catalog of Kepler main-sequence and subgiant stars to date, including a total of 101 stars hosting planet candidates and 451 stars with measured rotation periods.

Abstract: Stellar UV spectra are fundamental diagnostics of physical and magnetic properties of stars. For instance, lines like Mg II at 280 nm serve as valuable indicators of stellar activity, providing insights into the activity levels of Sun-like stars and their potential influence on the atmospheres of orbiting planets. On the other hand, the effective temperature (Teff) is a fundamental stellar parameter, critical for determining stellar properties such as mass, age, composition and evolutionary status. In this study, we investigate the temperature sensitivity of three lines in the mid-ultraviolet range (i.e., Mg II 280.00 nm, Mg I 285.20 nm, and Si I 286.15 nm). Using spectra from the International Ultraviolet Explorer (IUE), we analyze the behavior of the ratios of their corresponding indices (core/continuum) for a sample of calibrating solar-like stars, and find that the ration R = Mg II/Mg I best traces Teff through a log-log relation. The Teff estimated using this relation on a test-sample of solar-like stars agree with the Teff from the literature at the 95% confidence level. The observed results are interpreted making use of Response Functions as diagnostics. This study extends the well-established use of line depth ratio-temperature relationships, traditionally applied in the visible and near-infrared ranges, to the mid-UV spectrum. With the growing interest in stellar UV spectroscopy, results presented in this paper are potentially relevant for future missions as HWO, MANTIS and UVEX.

Abstract: M dwarfs are the most common type of star in the solar neighborhood, and many exhibit frequent and highly energetic flares. To better understand these events across the electromagnetic spectrum, a campaign observed AU Mic (dM1e) over 7 days from the X-ray to radio regimes. Here, we present high-time-resolution light curves from the Karl G. Jansky Very Large Array (VLA) Ku band (12--18 GHz) and the Australia Telescope Compact Array (ATCA) K band (16--25 GHz), which observe gyrosynchrotron radiation and directly probe the action of accelerated electrons within flaring loops. Observations reveal 16 VLA and 3 ATCA flares of varying shapes and sizes, from a short (30 sec) spiky burst to a long-duration ($\sim$5 hr) decaying exponential. The Ku-band spectral index is found to often evolve during flares. Both rising and falling spectra are observed in the Ku-band, indicating optically thick and thin flares, respectively. Estimations from optically thick radiation indicate higher loop-top magnetic field strengths ($\sim$1 kG) and sustained electron densities ($\sim$10$^{6}$ cm$^{-3}$) than previous observations of large M-dwarf flares. We estimate the total kinetic energies of gyrating electrons in optically thin flares to be between 10$^{32}$ and 10$^{34}$ erg when the local magnetic field strength is between 500 and 700 G. These energies are able to explain the combined radiated energies from multi-wavelength observations. Overall, values are more aligned with modern radiative-hydrodynamic simulations of M-dwarf flares, and future modeling efforts will better constrain findings.

Abstract: The X-rays and Extreme Ultraviolet (XUV) emission from M stars can drive the atmospheric escape on planets orbiting them. M stars are also known for their frequent emission of stellar flares, which will increase the high-energy flux received by their orbiting planets. To understand how stellar flares impact the primordial atmospheres of planets orbiting young M stars, we use UV spectroscopic data of flares from the Habitable Zones and M dwarf Activity across Time (HAZMAT) and Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) programs as a proxy to the XUV flare emission. Using the software package VPLanet, we simulate the young AU Mic planetary system composed of two Neptune-sized and one Earth-sized planet orbiting a 23-Myr-old M1 star. Our findings show that the Earth-sized planet AU Mic d should be in the process of losing completely its atmosphere in the next couple million years, solely due to the quiescent emission, with flares not significantly contributing to its atmospheric escape due to the small size of AU mic d and its close-in distance from the star. However, our results indicate that flares would play a crucial role for such planets further away, in the habitable zone (i.e. 0.2935 AU) of AU Mic-like stars during the post-saturation phase, accelerating the total atmospheric loss process by a few billion years. For planets between 0.365 AU and the HZ outer edge, the additional XUV from flares is necessary to deplete primordial atmospheres fully since the quiescent emission alone is insufficient.

Abstract: Photoevaporation caused by X-rays and ultraviolet radiation from the central star has attracted attention as a key process driving the dispersal of protoplanetary discs. Although numerous models have been used to investigate the photoevaporation process, their conclusions vary, partly due to differences in the adopted radiation spectra of the host star in particular in the extreme ultraviolet (EUV) and soft X-ray bands. This study aims to construct the EUV and (soft) X-ray emission spectrum from pre-main-sequence stars using a physics-based model. While the high-energy radiation sources of pre-main-sequence stars include accretion shocks and magnetically heated coronae, this study focuses on the latter. An MHD model capable of reproducing the coronal emission of main-sequence stars is applied to a pre-main-sequence star TW Hya, and its feasibility is assessed by comparing the predicted and observed emission-line intensities. We find that the emission lines formed at coronal temperatures ($T = 4-13 \times 10^6$ K) are reproduced in intensity within a factor of three. Emission lines from lower-temperature ($T < 4 \times 10^6$ K) plasmas are systematically underestimated, with typical intensities at 10-30% of observed values, consistent with previous findings that these emissions predominantly originate from accretion shocks. Emission lines emitted at extremely high temperatures ($T > 13 \times 10^6$ K) account for only about 1-10% of the observed values, likely due to the neglect of transient heating associated with flares. These results indicate that the quiescent coronal emission of pre-main-sequence stars can be adequately modeled using a physics-based approach.

Abstract: Main sequence stars of spectral types F, G, and K with low to moderate activity levels exhibit a recognizable pattern known as the first ionization potential effect (FIP effect), where elements with lower first ionization potentials are more abundant in the stellar corona than in the photosphere. In contrast, high activity main sequence stars such as AB Dor (K0), active binaries, and M dwarfs exhibit an inverse pattern known as iFIP. We aim to determine whether or not the iFIP pattern persists in moderate-activity M dwarfs. We used XMM-Newton to observe the moderately active M dwarf HD 223889 that has an X-ray surface flux of log FX,surf = 5.26, the lowest for an M dwarf studied so far for coronal abundance patterns. We used low-resolution CCD spectra of the star to calculate the strength of the FIP effect quantified by the FIP bias (Fbias) to assess the persistence of the iFIP effect in M dwarfs. Our findings reveal an iFIP effect similar to that of another moderately active binary star, GJ 338 AB, with a comparable error margin. The results hint at a possible plateau in the Teff-Fbias diagram for moderately active M dwarfs. Targeting stars with low coronal activity that have a coronal temperature between 2 MK and 4 MK is essential for refining our understanding of (i)FIP patterns and their causes.

Abstract: Despite the energetic significance of Lyman-alpha (Ly<inline-formula id="IEq1"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula>; 1216 Å) emission from solar flares, regular observations of flare related Ly<inline-formula id="IEq2"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> have been relatively scarce until recently. Advances in instrumental capabilities and a shift in focus over previous solar cycles mean it is now routinely possible to take regular co-observations of Ly<inline-formula id="IEq3"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission in solar flares. Thus, it is valuable to examine how the instruments selected for flare observations may influence the conclusions drawn from the analysis of their unique measurements. Here, we examine three M-class flares each observed in Ly<inline-formula id="IEq4"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> by GOES-14/EUVS-E, GOES-15/EUVS-E, or GOES-16/EXIS-EUVS-B, and at least one other instrument from PROBA2/LYRA, MAVEN/EUVM, ASO-S/LST-SDI, and SDO/EVE-MEGS-P. For each flare, the relative and excess flux, contrast, total energy, and timings of the Ly<inline-formula id="IEq5"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission were compared between instruments. It was found that while the discrepancies in measurements of the relative flux between instruments may be considered minimal, the calculated contrasts, excess fluxes, and energetics may differ significantly – in some cases up to a factor of five. This may have a notable impact on multi-instrument investigations of the variable Ly<inline-formula id="IEq6"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission in solar flares and estimates of the contribution of Ly<inline-formula id="IEq7"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> to the radiated energy budget of the chromosphere. The findings presented in this study will act as a guide for the interpretation of observations of flare-related Ly<inline-formula id="IEq8"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> from upcoming instruments during future solar cycles and inform conclusions drawn from multi-instrument studies.

Abstract: There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index $S_{\rm ph}$. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than $S_{\rm ph}$ are correlated with the Rossby number, $Ro$, or ratio of rotation period to convective overturn timescale. We compute the latter using the Yale Rotating Evolution Code stellar models. We observe different $S_{\rm ph}$-$Ro$ relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus $Ro$ is recovered, we find a localized dip in $S_{\rm ph}$ around $Ro/Ro_{\odot} \sim$\,0.3 for the G and K dwarfs. F dwarfs show little to no dependence of $S_{\rm ph}$ on $Ro$ due to their shallow convective zones; further accentuated as $T_{\rm eff}$ increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in $S_{\rm ph}$ could be associated with the redistribution of angular momentum between the core and convective envelope inside stars. For G-type stars, we observe enhanced magnetic activity above solar $Ro$. Compared to other Sun-like stars with similar effective temperature and metallicity, we find that the Sun's current level of magnetic activity is comparable to its peers and lies near the transition to increasing magnetic activity at high $Ro$. We confirm that metal-rich stars have a systematically larger $S_{\rm ph}$ level than metal-poor stars, which is likely a consequence of their deeper convective zones.

Abstract: Despite the energetic significance of Lyman-alpha (Ly<inline-formula id="IEq1"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula>; 1216 Å) emission from solar flares, regular observations of flare related Ly<inline-formula id="IEq2"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> have been relatively scarce until recently. Advances in instrumental capabilities and a shift in focus over previous solar cycles mean it is now routinely possible to take regular co-observations of Ly<inline-formula id="IEq3"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission in solar flares. Thus, it is valuable to examine how the instruments selected for flare observations may influence the conclusions drawn from the analysis of their unique measurements. Here, we examine three M-class flares each observed in Ly<inline-formula id="IEq4"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> by GOES-14/EUVS-E, GOES-15/EUVS-E, or GOES-16/EXIS-EUVS-B, and at least one other instrument from PROBA2/LYRA, MAVEN/EUVM, ASO-S/LST-SDI, and SDO/EVE-MEGS-P. For each flare, the relative and excess flux, contrast, total energy, and timings of the Ly<inline-formula id="IEq5"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission were compared between instruments. It was found that while the discrepancies in measurements of the relative flux between instruments may be considered minimal, the calculated contrasts, excess fluxes, and energetics may differ significantly – in some cases up to a factor of five. This may have a notable impact on multi-instrument investigations of the variable Ly<inline-formula id="IEq6"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission in solar flares and estimates of the contribution of Ly<inline-formula id="IEq7"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> to the radiated energy budget of the chromosphere. The findings presented in this study will act as a guide for the interpretation of observations of flare-related Ly<inline-formula id="IEq8"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> from upcoming instruments during future solar cycles and inform conclusions drawn from multi-instrument studies.

Abstract: There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index $S_{\rm ph}$. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than $S_{\rm ph}$ are correlated with the Rossby number, $Ro$, or ratio of rotation period to convective overturn timescale. We compute the latter using the Yale Rotating Evolution Code stellar models. We observe different $S_{\rm ph}$-$Ro$ relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus $Ro$ is recovered, we find a localized dip in $S_{\rm ph}$ around $Ro/Ro_{\odot} \sim$\,0.3 for the G and K dwarfs. F dwarfs show little to no dependence of $S_{\rm ph}$ on $Ro$ due to their shallow convective zones; further accentuated as $T_{\rm eff}$ increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in $S_{\rm ph}$ could be associated with the redistribution of angular momentum between the core and convective envelope inside stars. For G-type stars, we observe enhanced magnetic activity above solar $Ro$. Compared to other Sun-like stars with similar effective temperature and metallicity, we find that the Sun's current level of magnetic activity is comparable to its peers and lies near the transition to increasing magnetic activity at high $Ro$. We confirm that metal-rich stars have a systematically larger $S_{\rm ph}$ level than metal-poor stars, which is likely a consequence of their deeper convective zones.

Abstract: Despite the energetic significance of Lyman-alpha (Ly<inline-formula id="IEq1"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula>; 1216 Å) emission from solar flares, regular observations of flare related Ly<inline-formula id="IEq2"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> have been relatively scarce until recently. Advances in instrumental capabilities and a shift in focus over previous solar cycles mean it is now routinely possible to take regular co-observations of Ly<inline-formula id="IEq3"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission in solar flares. Thus, it is valuable to examine how the instruments selected for flare observations may influence the conclusions drawn from the analysis of their unique measurements. Here, we examine three M-class flares each observed in Ly<inline-formula id="IEq4"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> by GOES-14/EUVS-E, GOES-15/EUVS-E, or GOES-16/EXIS-EUVS-B, and at least one other instrument from PROBA2/LYRA, MAVEN/EUVM, ASO-S/LST-SDI, and SDO/EVE-MEGS-P. For each flare, the relative and excess flux, contrast, total energy, and timings of the Ly<inline-formula id="IEq5"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission were compared between instruments. It was found that while the discrepancies in measurements of the relative flux between instruments may be considered minimal, the calculated contrasts, excess fluxes, and energetics may differ significantly – in some cases up to a factor of five. This may have a notable impact on multi-instrument investigations of the variable Ly<inline-formula id="IEq6"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> emission in solar flares and estimates of the contribution of Ly<inline-formula id="IEq7"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> to the radiated energy budget of the chromosphere. The findings presented in this study will act as a guide for the interpretation of observations of flare-related Ly<inline-formula id="IEq8"><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula> from upcoming instruments during future solar cycles and inform conclusions drawn from multi-instrument studies.

Abstract: There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index $S_{\rm ph}$. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than $S_{\rm ph}$ are correlated with the Rossby number, $Ro$, or ratio of rotation period to convective overturn timescale. We compute the latter using the Yale Rotating Evolution Code stellar models. We observe different $S_{\rm ph}$-$Ro$ relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus $Ro$ is recovered, we find a localized dip in $S_{\rm ph}$ around $Ro/Ro_{\odot} \sim$\,0.3 for the G and K dwarfs. F dwarfs show little to no dependence of $S_{\rm ph}$ on $Ro$ due to their shallow convective zones; further accentuated as $T_{\rm eff}$ increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in $S_{\rm ph}$ could be associated with the redistribution of angular momentum between the core and convective envelope inside stars. For G-type stars, we observe enhanced magnetic activity above solar $Ro$. Compared to other Sun-like stars with similar effective temperature and metallicity, we find that the Sun's current level of magnetic activity is comparable to its peers and lies near the transition to increasing magnetic activity at high $Ro$. We confirm that metal-rich stars have a systematically larger $S_{\rm ph}$ level than metal-poor stars, which is likely a consequence of their deeper convective zones.

Abstract: There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index $S_{\rm ph}$. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than $S_{\rm ph}$ are correlated with the Rossby number, $Ro$, or ratio of rotation period to convective overturn timescale. We compute the latter using the Yale Rotating Evolution Code stellar models. We observe different $S_{\rm ph}$-$Ro$ relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus $Ro$ is recovered, we find a localized dip in $S_{\rm ph}$ around $Ro/Ro_{\odot} \sim$\,0.3 for the G and K dwarfs. F dwarfs show little to no dependence of $S_{\rm ph}$ on $Ro$ due to their shallow convective zones; further accentuated as $T_{\rm eff}$ increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in $S_{\rm ph}$ could be associated with the redistribution of angular momentum between the core and convective envelope inside stars. For G-type stars, we observe enhanced magnetic activity above solar $Ro$. Compared to other Sun-like stars with similar effective temperature and metallicity, we find that the Sun's current level of magnetic activity is comparable to its peers and lies near the transition to increasing magnetic activity at high $Ro$. We confirm that metal-rich stars have a systematically larger $S_{\rm ph}$ level than metal-poor stars, which is likely a consequence of their deeper convective zones.

Abstract: Surface convection is important for the presence of magnetic activity at stars. So far, this convection is thought to be a result of heating from below, where convection cells rise and break up. New models reveal that surface convection is instead strongly driven by cooling from above. We compare two simulations of surface convection, one with a significant heating from below and one without. We obtain surface convection in both cases, and they show similar granulation patterns. The deep convection driven by heating from below is still evolving and asymptotically approaches a steady-state solution. We find that convection from below is not needed at all to form typical photospheric granulation. This indicates the possibility of a surface dynamo acting on stars without a convecting envelope. Even stars without a convecting envelope could therefore exhibit stronger magnetic and coronal activity than expected so far.

Abstract: Context. The magnetic cycle on the Sun consists of two consecutive 11-yr sunspot cycles and exhibits a polarity reversal around sunspot maximum. Although solar dynamo theories have progressively become more sophisticated, the details as to how the dynamo sustains magnetic fields are still the subject of research. Observing the magnetic fields of Sun-like stars can bring useful insights to contextualise the solar dynamo. Aims. With the long-term spectropolarimetric monitoring of stars, the BCool survey studies the evolution of surface magnetic fields to understand how dynamo-generated processes are influenced by key ingredients, such as mass and rotation. Here, we focus on six Sun-like stars with masses between 1.02 and 1.06 M<SUB>⊙</SUB> and with rotation periods of 3.5–21 d (or 0.3–1.8 in Rossby numbers), a practical sample with which to study magnetic cycles across distinct activity levels. Methods. We analysed high-resolution spectropolarimetric data collected with ESPaDOnS, Narval, and Neo-Narval between 2007 and 2024 within the BCool programme. We measured longitudinal magnetic field from least-squares deconvolution line profiles and we inspected its long-term behaviour with both a Lomb-Scargle periodogram and a Gaussian process. We then applied Zeeman-Doppler imaging to reconstruct the large-scale magnetic field geometry at the stellar surface for different epochs. Results. Two of our slow rotators, namely HD 9986 and HD 56124 (P<SUB>rot</SUB> ∼ 20 d), exhibit repeating polarity reversals in the radial or toroidal field component on shorter timescales than the Sun (5–6 yr). HD 73350 (P<SUB>rot</SUB> ∼ 12 d) has one polarity reversal in the toroidal component and HD 76151 (P<SUB>rot</SUB> = 17 d) may have short-term evolution (2.5 yr) modulated by the long-term (16 yr) chromospheric cycle. Our two fast rotators, HD 166435 and HD 175726 (P<SUB>rot</SUB> = 3 ‑ 5 d), manifest complex magnetic fields without an evident cyclic evolution. Conclusions. Our findings indicate the potential dependence of the magnetic cycles' nature on the stellar rotation period. For the two stars with likely cycles, the polarity reversal timescale seems to decrease with a decreasing rotation period or Rossby number. These results represent important observational constraints for dynamo models of solar-like stars.

Abstract: We present an analysis of four Chandra observations of the 45 Myr old DS Tuc binary system. We observed X-ray variability of both stars on timescales from hours to months, including two strong X-ray flares from star A. The implied flaring rates are in agreement with past observations made with XMM-Newton, though these rates remain imprecise due to the relatively short total observation time. We find a clear, monotonic decline in the quiescent level of the star by a factor 1.8 across eight months, suggesting stellar variability that might be due to an activity cycle. If proven through future observations, DS Tuc A would be the youngest star for which a coronal activity cycle has been confirmed. The variation in our flux measurements across the four visits is also consistent with the scatter in empirical stellar X-ray relationships with Rossby number. In simulations of the possible evolution of the currently super-Neptune-sized planet DS Tuc Ab, we find a range of scenarios for the planet once it reaches a typical field age of 5 Gyr, from Neptune-size down to a completely stripped super-Earth. Improved constraints on the planet's mass in the future would significantly narrow these possibilities. We advocate for further Chandra observations to better constrain the variability of this important system.

Abstract: There is an intricate relationship between the organization of large-scale magnetic fields by a stellar dynamo and the rate of angular momentum loss due to magnetized stellar winds. An essential ingredient for the operation of a large-scale dynamo is the Coriolis force, which imprints organizing flows on the global convective patterns and inhibits the complete cancellation of bipolar magnetic regions. Consequently, it is natural to expect a rotational threshold for large-scale dynamo action and for the efficient angular momentum loss that it mediates through magnetic braking. Here we present new observational constraints on magnetic braking for an evolutionary sequence of six early K-type stars. To determine the wind braking torque for each of our targets, we combine spectropolarimetric constraints on the large-scale magnetic field, Ly-alpha or X-ray constraints on the mass-loss rate, as well as uniform estimates of the stellar rotation period, mass, and radius. As identified previously from similar observations of hotter stars, we find that the wind braking torque decreases abruptly by more than an order of magnitude at a critical value of the stellar Rossby number. Given that all of the stars in our sample exhibit clear activity cycles, we suggest that weakened magnetic braking may coincide with the operation of a subcritical stellar dynamo.

Abstract: Surface convection is important for the presence of magnetic activity at stars. So far, this convection is thought to be a result of heating from below, where convection cells rise and break up. New models reveal that surface convection is instead strongly driven by cooling from above. We compare two simulations of surface convection, one with a significant heating from below and one without. We obtain surface convection in both cases, and they show similar granulation patterns. The deep convection driven by heating from below is still evolving and asymptotically approaches a steady-state solution. We find that convection from below is not needed at all to form typical photospheric granulation. This indicates the possibility of a surface dynamo acting on stars without a convecting envelope. Even stars without a convecting envelope could therefore exhibit stronger magnetic and coronal activity than expected so far.

Abstract: We present an analysis of four Chandra observations of the 45 Myr old DS Tuc binary system. We observed X-ray variability of both stars on timescales from hours to months, including two strong X-ray flares from star A. The implied flaring rates are in agreement with past observations made with XMM-Newton, though these rates remain imprecise due to the relatively short total observation time. We find a clear, monotonic decline in the quiescent level of the star by a factor 1.8 across eight months, suggesting stellar variability that might be due to an activity cycle. If proven through future observations, DS Tuc A would be the youngest star for which a coronal activity cycle has been confirmed. The variation in our flux measurements across the four visits is also consistent with the scatter in empirical stellar X-ray relationships with Rossby number. In simulations of the possible evolution of the currently super-Neptune-sized planet DS Tuc Ab, we find a range of scenarios for the planet once it reaches a typical field age of 5 Gyr, from Neptune-size down to a completely stripped super-Earth. Improved constraints on the planet's mass in the future would significantly narrow these possibilities. We advocate for further Chandra observations to better constrain the variability of this important system.

Abstract: We present an analysis of four Chandra observations of the 45 Myr old DS Tuc binary system. We observed X-ray variability of both stars on timescales from hours to months, including two strong X-ray flares from star A. The implied flaring rates are in agreement with past observations made with XMM-Newton, though these rates remain imprecise due to the relatively short total observation time. We find a clear, monotonic decline in the quiescent level of the star by a factor 1.8 across eight months, suggesting stellar variability that might be due to an activity cycle. If proven through future observations, DS Tuc A would be the youngest star for which a coronal activity cycle has been confirmed. The variation in our flux measurements across the four visits is also consistent with the scatter in empirical stellar X-ray relationships with Rossby number. In simulations of the possible evolution of the currently super-Neptune-sized planet DS Tuc Ab, we find a range of scenarios for the planet once it reaches a typical field age of 5 Gyr, from Neptune-size down to a completely stripped super-Earth. Improved constraints on the planet's mass in the future would significantly narrow these possibilities. We advocate for further Chandra observations to better constrain the variability of this important system.

Abstract: Stellar superflares are energetic outbursts of electromagnetic radiation that are similar to solar flares but release more energy, up to 10<SUP>36</SUP> erg on main-sequence stars. It is unknown whether the Sun can generate superflares and, if so, how often they might occur. We used photometry from the Kepler space observatory to investigate superflares on other stars with Sun-like fundamental parameters. We identified 2889 superflares on 2527 Sun-like stars, out of 56,450 observed. This detection rate indicates that superflares with energies &gt;10<SUP>34</SUP> erg occur roughly once per century on stars with Sun-like temperature and variability. The resulting stellar superflare frequency-energy distribution is consistent with an extrapolation of the Sun's flare distribution to higher energies, so we suggest that both are generated by the same physical mechanism.

Abstract: Context. The intense X-ray and UV emission of some active M stars has raised questions about the habitability of planets around M-type stars. Aims. We aim to determine the unbiased distribution of X-ray luminosities in complete, volume-limited samples of nearby M dwarfs, and compare them to those of K and G dwarfs. Methods. We constructed volume-complete samples of 205 M stars with a spectral type $\leq$ M6 within 10 pc of the Sun, 129 K stars within 16 pc, and 107 G stars within 20 pc. We used X-ray data from Chandra, XMM-Newton, eROSITA, and ROSAT to obtain the X-ray luminosities of the stars. Results. Our samples reach an X-ray detection completeness of 85%, 86%, and 80% for M, K, and G stars, respectively. The fractional X-ray luminosities relative to the bolometric luminosities, $\log(L_\mathrm{X}/L_\mathrm{bol})$, of the M stars show a bimodal distribution, with one peak at around -5, mostly contributed by early M stars (M0--M4), and another peak around -3.5, contributed mainly by M4--M6 stars. The comparison of the different spectral classes shows that 63% of all M stars in our sample (80% of the M stars with a spectral type $<$ M4) have $L_\mathrm{X}/L_\mathrm{bol}$ values that are within the central 80% quantile of the distribution function for G stars. In addition, 55% of all M stars in our sample (and 72% of the M stars with a spectral type $<$ M4) have $L_\mathrm{X}/L_\mathrm{bol}$ less than 10 times the solar value. Conclusions. The X-ray activity levels of the majority ($\ge 60\%$) of nearby M dwarfs no later than M6 are actually not higher than the typical (80% quantile) levels for G-type stars. The X-ray irradiation of habitable-zone planets around these stars should therefore not present a specific problem for their habitability.

Abstract: One of the primary sources of stellar spectral variability is magnetic activity. While our current understanding of chromospheric activity is largely derived from specific lines sensitive to chromospheric heating, such as the Ca II HK doublet, previous observational studies have shown that other spectral lines are also affected. To investigate the influence of activity on line formation in greater detail, we constructed a set of stellar models for hypothetical G2 dwarf stars with varying levels of activity and calculated their synthetic spectra. A comparison of these spectra revealed two spectral regions most significantly impacted by activity: approximately 3300-4400 A and 5250-5500 A. By calculating the total contribution function of the lines, we determined that the emergence of a secondary chromospheric contribution to line formation is the primary mechanism driving these changes. Based on our calculations and analysis, we compiled a list of transition lines and their corresponding changes due to chromospheric activity. This list could serve as a valuable tool for selecting spectral lines applicable to a wide range of astrophysical studies.

Abstract: We present results from fitting $p$-mode spectra derived from 7-d segments of Sun-as-a-star helioseismic observations from the Birmingham Solar Oscillations Network covering 32 yr. The results show a clear dependence of the mode frequencies on solar activity, and the frequency dependence of the sensitivity to activity can also be seen. Because we use data segments that cover less than half of a solar rotation, we are able to test for the effect of activity on the solar far side. By fitting with a model that takes into account activity on the far side of the Sun, we show that the frequency shifts are sensitive to activity from the whole Sun, not just the side facing the observer. Our results suggest that there is potential to investigate activity-related asteroseismic frequency shifts in solar-like oscillators using short time series of observations.

Abstract: Context. The intense X-ray and UV emission of some active M stars has raised questions about the habitability of planets around M-type stars. Aims. We aim to determine the unbiased distribution of X-ray luminosities in complete, volume-limited samples of nearby M dwarfs, and compare them to those of K and G dwarfs. Methods. We constructed volume-complete samples of 205 M stars with a spectral type $\leq$ M6 within 10 pc of the Sun, 129 K stars within 16 pc, and 107 G stars within 20 pc. We used X-ray data from Chandra, XMM-Newton, eROSITA, and ROSAT to obtain the X-ray luminosities of the stars. Results. Our samples reach an X-ray detection completeness of 85%, 86%, and 80% for M, K, and G stars, respectively. The fractional X-ray luminosities relative to the bolometric luminosities, $\log(L_\mathrm{X}/L_\mathrm{bol})$, of the M stars show a bimodal distribution, with one peak at around -5, mostly contributed by early M stars (M0--M4), and another peak around -3.5, contributed mainly by M4--M6 stars. The comparison of the different spectral classes shows that 63% of all M stars in our sample (80% of the M stars with a spectral type $<$ M4) have $L_\mathrm{X}/L_\mathrm{bol}$ values that are within the central 80% quantile of the distribution function for G stars. In addition, 55% of all M stars in our sample (and 72% of the M stars with a spectral type $<$ M4) have $L_\mathrm{X}/L_\mathrm{bol}$ less than 10 times the solar value. Conclusions. The X-ray activity levels of the majority ($\ge 60\%$) of nearby M dwarfs no later than M6 are actually not higher than the typical (80% quantile) levels for G-type stars. The X-ray irradiation of habitable-zone planets around these stars should therefore not present a specific problem for their habitability.

Abstract: We present results from fitting $p$-mode spectra derived from 7-d segments of Sun-as-a-star helioseismic observations from the Birmingham Solar Oscillations Network covering 32 yr. The results show a clear dependence of the mode frequencies on solar activity, and the frequency dependence of the sensitivity to activity can also be seen. Because we use data segments that cover less than half of a solar rotation, we are able to test for the effect of activity on the solar far side. By fitting with a model that takes into account activity on the far side of the Sun, we show that the frequency shifts are sensitive to activity from the whole Sun, not just the side facing the observer. Our results suggest that there is potential to investigate activity-related asteroseismic frequency shifts in solar-like oscillators using short time series of observations.

Abstract: Solar irradiance and its variations in the ultraviolet (UV) control the photochemistry in Earth's atmosphere and influence Earth's climate. The variability of Mg II h and k core-to-wing ratio, also known as the Mg II index, is highly correlated with the solar UV irradiance variability. Because of this, Mg II index is routinely used as a proxy for solar UV irradiance variability, which can help to get insights into the influence of solar UV irradiance variability on Earth's climate. Measurements of the Mg II index, however, have only been carried out since 1978 and do not cover the climate relevant timescales longer than a few decades. Here we present a model to calculate the Mg II index and its variability based on the well-established SATIRE (Spectral And Total Irradiance REconstruction) model. We demonstrate that our model calculations yield an excellent agreement with the observed Mg II index variations, both on the solar activity cycle and on the solar rotation timescales. Using this model, we synthesize Mg II index timeseries on climate relevant timescales of decades and longer. Here we present the timeseries of the Mg II index spanning nearly three centuries.

Abstract: The growth of a large-scale magnetic field in the Sun and stars is usually possible when the dynamo number (D) is above a critical value Dc. As the star ages, its rotation rate and thus D decrease. Hence, the question is how far the solar dynamo is from the critical dynamo transition. To answer this question, we have performed a set of simulations using Babcock-Leighton type dynamo models at different values of dynamo supercriticality and analyzed various features of magnetic cycle. By comparing the recovery rates of the dynamo from the Maunder minimum and statistics (numbers and durations) of the grand minima and maxima with that of observations and we show that the solar dynamo is only about two times critical and thus not highly supercritical. The observed correlation between the polar field proxy and the following cycle amplitudes and Gnevyshev-Ohl rule are also compatible with this conclusion.