Search

We present the results of an asteroseismic study of HD 118660 (TIC 171729860), being a chemically peculiar (mild Am) star exhibiting |$\delta$| Scuti (⁠|$\delta$|  Sct) pulsations. It is based on the analysis of two sectors of time-series photometry from the space mission TESS (Transiting Exoplanet Survey Satellite) and seismic modelling. It yielded the detection of 15 and 16 frequencies for TESS sectors 23 and 50, respectively. The identified pulsation modes include four radial (⁠|$\ell =0$|⁠) and five dipolar (⁠|$\ell =1$|⁠) ones. The radial modes are overtones with order n ranging from 3 and 6. Such high values of n are theoretically not expected for stars with the effective temperature of HD 118660 (⁠|$T_{\rm eff}\approx 7550\,\rm K$|⁠) located near the red edge of the |$\delta$|  Sct instability strip. To estimate the asteroseismic parameters, we have generated a grid of stellar models assuming a solar metallicity (⁠|$Z=0.014$|⁠) and different values for the convective overshooting parameter (⁠|$0.1\le \alpha _{\rm ov}\le 0.3$|⁠). We conclude that the analysis of the radial modes is insufficient to constrain |$\alpha _{\rm ov}$| and Z for |$\delta$|  Sct stars. The value for the equatorial velocity of HD 118660 derived from the seismic radius and the rotational frequency is consistent with values found in the literature. [ABSTRACT FROM AUTHOR]

Teegarden's star is a late-type M-dwarf planet host, typically showing only rather low levels of activity. In this paper we present an extensive characterisation of this activity at photospheric, chromospheric, and coronal levels. We specifically investigated TESS observations of Teegarden's star, which showed two very large flares with an estimated flare fluence between 1029 and 1032 erg comparable to the largest solar flares. We furthermore analysed nearly 300 CARMENES spectra and 11 ESPRESSO spectra covering all the usually used chromospheric lines in the optical from the Ca II H & K lines at 3930 Å to the He I infrared triplet at 10 830 Å. These lines show different behaviour: The He I infrared triplet is the only one absent in all spectra, some lines show up only during flares, and others are always present and highly variable. Specifically, the Hα line is more or less filled in during quiescence; however, the higher Balmer lines are still observed in emission. Many chromospheric lines show a correlation with Hα variability, which, in addition to stochastic behaviour, shows systematic behaviour on different timescales including the rotation period. Moreover, we found several flares and also report hints of an erupting prominence, which may have led to a coronal mass ejection. Finally, we present X-ray observations of Teegarden's star (i.e. a discovery pointing obtained with the Chandra observatory) and an extensive study with the XMM-Newton observatory, which observed two large flares. One of these showed clear signatures of the Neupert effect, suggesting the production of hard X-rays in the system. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) are often associated with X-ray (SXR) flares powered by magnetic reconnection in the low corona, while the CME shocks in the upper corona and interplanetary (IP) space accelerate electrons often producing the type II radio bursts. The CME and the reconnection event are part of the same energy release process as highlighted by the correlation between reconnection flux (ϕrec) that quantifies the strength of the released magnetic free energy during the SXR flare, and the CME kinetic energy that drives the IP shocks leading to type II bursts. Unlike the Sun, these physical parameters cannot be directly inferred in stellar observations. Hence, scaling laws between unresolved sun-as-a-star observables, namely SXR luminosity (LX) and type II luminosity (LR), and the physical properties of the associated dynamical events are crucial. Such scaling laws also provide insights into the interconnections between the particle acceleration processes across low-corona to IP space during solar-stellar "flare-CME-type II" events. Using long-term solar data in the SXR to radio waveband, we derived a scaling law between two novel power metrics for the flare and CME-associated processes. The metrics of "flare power" (Pflare = √(LXϕrec)) and "CME power" (PCME = √(LRVCME2)), where VCME is the CME speed, scale as Pflare ∝ PCME0.76 ± 0.04. In addition, LX and ϕrec show power-law trends with PCME with indices of 1.12 ± 0.05 and 0.61 ± 0.05, respectively. These power laws help infer the spatially resolved physical parameters, VCME and ϕrec, from disk-averaged observables, LX and LR during solar-stellar flare-CME-type II events. [ABSTRACT FROM AUTHOR]

Context. RACE-OC (Rotation and ACtivity Evolution in Open Clusters) is a project aimed at characterising the rotational and magnetic activity properties of the late-type members of open clusters, stellar associations, and moving groups of different ages. The evolution in time of rotation and activity at different masses sheds light on the evolution of the stellar internal structure, on magneto-hydrodynamic processes operating in the stellar interior, and on the coupling and decoupling mechanisms between the radiative core and the external convective envelope. As part of this project, in the present paper we present the results of an investigation of a likely member of the AB Doradus association, the M-type star 2MASS J15594729+4403595. Aims. In the present study, we aim to reveal the real nature of our target, which turned out to be a hierarchical triple system, to derive the stellar rotation period and surface differential rotation, and to characterise its photospheric magnetic activity. Methods. We have collected radial velocity and photometric time series, complemented with archive data, to determine the orbital parameters and the rotation period and we have used the spot modelling technique to explore what causes its photometric variability. Results. We found 2MASS J15594729+4403595 to be a hierarchical triple system consisting of a dwarf, SB1 M2, and a companion, M8. The M2 star has a rotation period of P = 0.37 d, making it the fastest among M-type members of AB Dor. The most relevant result is the detection of a periodic variation in the spotted area on opposite stellar hemispheres, which resembles a sort of Rossby wave or Rieger-like cycles on an extremely short timescale. Another interesting result is the occurrence of a highly significant photometric periodicity, P = 0.443 d, which may be related to the stellar rotation in terms of either a Rossby wave or surface differential rotation. Conclusions. 2MASS J15594729+4403595 may be the prototype of a new class of extremely fast rotating stars exibiting short Rieger-like cycles. We shall further explore what may drive these short-duration cycles and we shall also search for similar stars to allow for a statistical analysis. [ABSTRACT FROM AUTHOR]

Context. Starspots, regions of strong magnetic fields, serve as indicators of stellar activity and the dynamo mechanism at play in the interior of stars. The magnetic fields of main-sequence stars play a crucial role in driving stellar activity. An effective approach to better understanding stellar magnetic fields and activity lies in the detailed characterisation of starspot properties. Aims. We propose a new method for estimating the magnetic fields of starspots that employs modelling techniques of planetary transit mapping, which provides estimates of the size, intensity, and location of spots on the stellar photosphere. Methods. A starspot's maximum magnetic field was calculated using the linear relationship with the spot flux deficit, ΔFspot (the spot's brightness times its area) and the well-characterised relation for sunspots determined in this work, Bspot = 1170 + 844log Δ​Fspot (G). Results. Applying this relationship to previously mapped spots on the photospheres of 14 FGK and M stars yields spot maximum magnetic fields ranging from 2700 G to 4600 G, with an overall average of 3900 ± 400 G. We looked for correlations between starspot magnetic fields and stellar properties. We did not find any correlation between a spot's mean extreme magnetic field and effective temperature, nor the differential shear. However, a weak anti-correlation is seen between the spots' magnetic field and stellar age as well as between the magnetic field and the rotation period. Conclusions. When compared with previous results of small-scale magnetic field measurements, the B values obtained here are basically constant and near the saturation limit found for rapid rotators. This implies that it is not the intensity of the magnetic field of starspots that decreases with age but rather the filling factor. This result offers a unique window into the magnetic dynamo of stars. [ABSTRACT FROM AUTHOR]

Context. The long-term solar magnetic activity and its cyclical behaviour, which is maintained by a dynamo mechanism, are both still challenging problems for astrophysics. In particular, an atypical event occurred between 1645 and 1715, when the solar activity was remarkably decreased and the number of sunspots was extremely reduced. However, the exact events that unfolded during the solar cycle remain unclear. The discovery of longer activity minima in cool stars may shed light on the nature of the complex mechanisms involved in the long-term behaviour of the solar-stellar dynamo. Aims. Our aim is to explore whether the G5V solar-like star HD 4915, which showed a striking chromospheric activity pattern in a previous study performed with HIRES data, might be considered a bona fide Maunder Minimum (MM) candidate. Methods. We analysed over 380 spectra acquired between 2003 and 2022 using the HARPS and HIRES spectrographs. We carried out a detailed search for activity signatures in HD 4915 by using the Mount Wilson and the Balmer Hα activity indexes. This task was performed by means of the generalised Lomb-Scargle periodogram. Results. The new HARPS data show that the chromospheric activity of HD 4915 is not decreasing. In fact, the increases in the activity after the broad minimum in three years reaches the level of activity before this phase, suggesting that it is not entering an MM phase. We also calculate a rotation period of 23.4 ± 0.2 d, which has not been reported before. Conclusions. HD 4915 shows a distinctive activity behaviour that was initially attributed to a possible and incipient MM phase. Additional HARPS data allowed us to discard an MM in the star. Our analysis shows that the complex activity pattern of HD 4915 might be ruled by a multiple activity cycle, in which a shorter cycle of 4.8 yr is modulated by a potential longer cycle. More activity surveys with extensive records and suitable cadence are crucial for an accurate identification of stars in magnetic grand minima. [ABSTRACT FROM AUTHOR]

The radial velocity (RV) method of exoplanet detection requires mitigation of nuisance signals arising from stellar activity. Using analytic cool and facular spot models, we explore the use of central line moments (CLMs) for recovering and monitoring rotation induced RV variability. Different spot distribution patterns, photosphere-spot contrast ratios, and the presence or absence of the convective blueshift lead to differences in CLM signals between M and G dwarfs. Harmonics of the rotation period are often recovered with the highest power in standard periodogram analyses. By contrast, we show the true stellar rotation may be more reliably recovered with string length minimization. For solar minimum activity levels, recovery of the stellar rotation signal from CLMs is found to require unfeasibly high signal-to-noise observations. The stellar rotation period can be recovered at solar maximum activity levels from CLMs for reasonable cross-correlation function (CCF) signal-to-noise ratios >1000–5000. The CLMs can be used to recover and monitor stellar activity through their mutual correlations and correlations with RV and bisector inverse span. The skewness of a CCF, a measure of asymmetry, is described by the third CLM, |$M_3$|⁠. Our noise-free simulations indicate the linear RV versus |$M_3$| correlation is up to 10 per cent higher than the RV versus bisector inverse span correlation. We find a corresponding ∼5 per cent increase in linear correlation for CARMENES observations of the M star, AU Mic. We also assess the effectiveness of the time derivative of the second CLM, |$M_2$|⁠ , for monitoring stellar activity. [ABSTRACT FROM AUTHOR]

We have numerically demonstrated that simulated cool star coronae naturally form condensations. If the star rotates slowly, with a co-rotation radius greater than the Alfvén radius (i.e. |$R_{\mathrm{K}} \gt R_{\mathrm{A}}$|⁠), these condensations will form below the co-rotation radius |$R_{\mathrm{K}}$| and simply fall back to the stellar surface as coronal rain. If, however, the star is more rapidly rotating, (⁠|$R_{\mathrm{K}} \lt R_{\mathrm{A}}$|⁠), not only rain will form but also 'slingshot prominences'. In this case, condensations collect into a large mass reservoir around the co-rotation radius, from which periodic centrifugal ejections occur. In this case, some 51 per cent of the coronal mass is cold gas, either in rain or prominences. We find that 21  per cent of the mass lost by our simulated fast rotating star is cold gas. Studies of stellar mass-loss from the hot wind do not consider this component of the wind and therefore systematically underestimate mass-loss rates of these stars. Centrifugal ejections happen periodically, between every 7.5–17.5 h with masses clustering around |$10^{16}$|  g, These results agree well with observational statistics. Contrasting the fast and slow rotating magnetospheres, we find that there are two distinct types of solutions, high-lying and low-lying loops. Low-lying loops only produce coronal rain whereas high-lying loops produce both rain and slingshots. [ABSTRACT FROM AUTHOR]

Most of the stars in the Universe are M spectral class dwarfs, which are known to be the source of bright and frequent stellar flares. In this paper, we propose new approaches to discover M-dwarf flares in ground-based photometric surveys. We employ two approaches: a modification of a traditional method of parametric fit search and a machine learning algorithm based on active anomaly detection. The algorithms are applied to Zwicky Transient Facility (ZTF) data release 8, which includes the data from the ZTF high-cadence survey, allowing us to reveal flares lasting from minutes to hours. We analyse over 35 million ZTF light curves and visually scrutinize 1168 candidates suggested by the algorithms to filter out artefacts, occultations of a star by an asteroid, and other types of known variable objects. The result of this analysis is the largest catalogue of ZTF flaring stars to date, representing 134 flares with amplitudes ranging from −0.2 to −4.6 mag, including repeated flares. Using Pan-STARRS DR2 colours, we assign a spectral subclass to each object in the sample. For 13 flares with well-sampled light curves and available geometric distances from Gaia DR3, we estimate the bolometric energy. This research shows that the proposed methods combined with the ZTF's cadence strategy are suitable for identifying M-dwarf flares and other fast transients, allowing for the extraction of significant astrophysical information from their light curves. [ABSTRACT FROM AUTHOR]

Context. Young planetary systems represent an opportunity to investigate the early stages of (exo)planetary formation because the gravitational interactions have not yet significantly changed the initial configuration of the system. Aims. TOI-4562 b is a highly eccentric temperate Jupiter analogue orbiting a young F7V-type star of < 700 Myr in age with an orbital period of Porb ∼ 225 days and an eccentricity of e = 0.76, and is one of the largest known exoplanets to have formed in situ. Methods. We observed a new transit of TOI-4562 b using the 0.6-m Zeiss telescope at the Pico dos Dias Observatory (OPD/LNA) in Minas Gerais, Brazil, and combine our data with Transiting Exoplanet Survey Satellite (TESS) and archive data, with the aim being to improve the ephemerides of this interesting system. Results. The O − C diagram for the new ephemeris is consistent with the presence of a giant planet in an outer orbit around TOI-4562. TOI-4562 c is a planet with a mass of M = 5.77 MJup, an orbital period of Porb = 3990 days, and a semi-major axis of a = 5.219 AU. Conclusions. We report the discovery of TOI-4562 c, the exoplanet with the longest orbital period discovered to date via the transit timing variation (TTV) method. The TOI-4562 system is in the process of violent evolution with intense dynamical changes – judging by its young age and high eccentricity – and is therefore a prime target for studies of formation and evolution of planetary systems. [ABSTRACT FROM AUTHOR]

Context. ESPRESSO guaranteed time observations (GTOs) at the 8.2m VLT telescope were performed to look for Earth-like exoplanets in the habitable zone of nearby stars. Barnard's star is a primary target within the ESPRESSO GTO as it is the second closest neighbour to our Sun after the α Centauri stellar system. Aims. We present here a large set of 156 ESPRESSO observations of Barnard's star carried out over four years with the goal of exploring periods of shorter than 50 days, thus including the habitable zone (HZ). Methods. Our analysis of ESPRESSO data using Gaussian process (GP) to model stellar activity suggests a long-term activity cycle at 3200 d and confirms stellar activity due to rotation at 140 d as the dominant source of radial velocity (RV) variations. These results are in agreement with findings based on publicly available HARPS, HARPS-N, and CARMENES data. ESPRESSO RVs do not support the existence of the previously reported candidate planet at 233 d. Results. After subtracting the GP model, ESPRESSO RVs reveal several short-period candidate planet signals at periods of 3.15 d, 4.12 d, 2.34 d, and 6.74 d. We confirm the 3.15 d signal as a sub-Earth mass planet, with a semi-amplitude of 55 ± 7 cm s−1, leading to a planet minimum mass mp sin i of 0.37 ± 0.05 M⊕, which is about three times the mass of Mars. ESPRESSO RVs suggest the possible existence of a candidate system with four sub-Earth mass planets in circular orbits with semi-amplitudes from 20 to 47 cm s−1, thus corresponding to minimum masses in the range of 0.17–0.32 M⊕. Conclusions. The sub-Earth mass planet at 3.1533 ± 0.0006 d is in a close-to circular orbit with a semi-major axis of 0.0229 ± 0.0003 AU, thus located inwards from the HZ of Barnard's star, with an equilibrium temperature of 400 K. Additional ESPRESSO observations would be required to confirm that the other three candidate signals originate from a compact short-period planet system orbiting Barnard's star inwards from its HZ. [ABSTRACT FROM AUTHOR]

Analogous to solar flares, stellar flares are dramatic explosions in the atmosphere, which may be accompanied by prominence eruptions, coronal mass ejections (CMEs), and other forms of plasma motion. Based on time-resolved spectroscopic observations of the RS CVn-type star II Peg, we aim to search for the potential plasma motions associated with flares. In these observations, we detected part of the gradual decay phase of an optical flare, for which we find a lower limit on the energy of the Hα line of 6.03 × 1033 erg. Converting this Hα energy, we find a bolometric white-light energy of 3.10 × 1035 erg. Moreover, a secondary peak is also observed. After removing a quiescence reference, the Hα residual shows an asymmetric behavior, including both a blueshifted and a redshifted emission component. The former component has a bulk velocity of about −180 km s−1 and extends its velocity to more than −350 km s−1. This phenomenon is likely caused by a prominence eruption event or a chromospheric evaporation process. The latter emission component has a bulk velocity of 130–70 km s−1 and extends its velocity to nearly 400 km s−1. We attribute the redshifted emission component to one or a combination of several possible scenarios: flare-driven coronal rain, chromospheric condensation, backward-directed prominence eruption close to the stellar limb, or falling material in a prominence eruption. The minimum masses of the moving plasmas resulting in the blueshifted and redshifted emission components are estimated to be 0.56 × 1020 g and 1.74 × 1020 g, respectively. [ABSTRACT FROM AUTHOR]

We report two epochs of simultaneous near-infrared (IR) and X-ray observations of the low-mass X-ray binary black hole candidate Swift J1753.5–0127 with a subsecond time resolution during its long 2005–2016 outburst. Data were collected strictly simultaneously with VLT/ISAAC (KS band, 2.2 μm) and RXTE (2–15 keV) or XMM-Newton (0.7–10 keV). A clear correlation between the X-ray and the IR variable emission is found during both epochs but with very different properties. In the first epoch, the near-IR variability leads the X-ray by ∼130 ms, which is the opposite of what is usually observed in similar systems. The correlation is more complex in the second epoch, with both anti-correlation and correlations at negative and positive lags. Frequency-resolved Fourier analysis allows us to identify two main components in the complex structure of the phase lags: the first component, characterised by a near-IR lag of a few seconds at low frequencies, is consistent with a combination of disc reprocessing and a magnetised hot flow; the second component is identified at high frequencies by a near-IR lag of ≈0.7 s. Given the similarities of this second component with the well-known constant optical/near-IR jet lag observed in other black hole transients, we tentatively interpret this feature as a signature of a longer-than-usual jet lag. We discuss the possible implications of measuring such a long jet lag in a radio-quiet black hole transient. [ABSTRACT FROM AUTHOR]

Context. Stellar activity is comprised of various phenomena, mainly spots and faculae. It is one of the main sources of noise in exoplanetary observations because it affects both spectroscopic and photometric observations. In studying young active planetary systems, we need to model the activity of the host stars to remove astrophysical noise from our observational data. Aims. We model the contribution of stellar spots in photometric observations. Through the use of multi-band photometry, we aim to extract the geometric properties of the spots and constrain their temperatures. Methods. We analysed multi-band photometric observations acquired with the 80 cm Marcon telescope of the Osservatorio Polifunzionale del Chianti of V1298 Tau, assuming the photometric modulation observed in different bands is attributed to cold spots. Results We constrained the effective temperature of the active regions present on the surface of V1298 Tau, resulting from a combination of spots and faculae. We tested our hypothesis on solar data, verifying that we successfully measured the size of the dominant active region and its averaged effective temperature. [ABSTRACT FROM AUTHOR]

Context. Tidal deformation breaks the axisymmetric structure of stars, and this may affect stellar activity. This effect has been demonstrated in theoretical analyses and simulations, but it lacks observational support. In this paper, we use spot-modulated detached binaries to study the effect of tides on stellar activity. We show this effect by analyzing the properties of the spot lifetime, the harmonic decay timescale, and the orbital parameters. Aims. We aim to explore the differences in spot lifetimes between binaries and single stars, the main mechanisms of spot decay in binaries, and the correlation between orbital parameters and spot lifetimes. These differences will provide clues to the effect of tides on stellar activity. Methods. We collected data of 311 spot-modulated detached binaries and 3272 single stars. The relative orbital parameters of the binaries were derived by combining Kepler photometry, stellar atmospheric parameters from LAMOST DR9 and Gaia DR3, and 2MASS photometry. We then used the ACF method to obtain the rotational periods, lifetimes, and harmonic decay timescales. Finally, we analyzed the correlation between the lifetime of spots and orbital parameters, explored the dominant decay mechanism of spots, and examined the differences in spots for binaries and single stars. Results. The relative lifetime of a starspot is correlated with the sum of the fractional radii, the orbital eccentricity, and the synchronization ratio. Longer lifetimes are observed in close, circular, and synchronous binaries than in the other binaries. The main mechanism for the decay of star spots in binaries is large-scale convective motion. However, on close, cool, and fast-rotating binaries, horizontal diffusion or subphotospheric diffusion are dominant. Compared to single stars, the median lifetime of a starspot on binaries was found to be longer. Moreover, this difference decreases with rotation period. Additionally, it should be noted that spots on binaries experience increased horizontal or subphotospheric diffusion at the same rotation period and effective temperature. Conclusions. According to the observation results, we conclude that the lifetime of starspots on detached close binaries is affected by tidal interactions. [ABSTRACT FROM AUTHOR]

Context. The estimation of rotation periods of stars is a key challenge in stellar astrophysics. Given the large amount of data available from ground-based and space-based telescopes, there is a growing interest in finding reliable methods to quickly and automatically estimate stellar rotation periods with a high level of accuracy and precision. Aims. This work aims to develop a computationally inexpensive approach, based on machine learning techniques, to accurately predict thousands of stellar rotation periods. Methods. The innovation in our approach is the use of the XGBoost algorithm to predict the rotation periods of Kepler targets by means of regression analysis. Therefore, we focused on building a robust supervised machine learning model to predict surface stellar rotation periods from structured data sets built from the Kepler catalogue of K and M stars. We analysed the set of independent variables extracted from Kepler light curves and investigated the relationships between them and the ground truth. Results. Using the extreme gradient boosting (GB) method, we obtained a minimal set of variables that can be used to build machine learning models for predicting stellar rotation periods. Our models have been validated by predicting the rotation periods of about 2900 stars. The results are compatible with those obtained by classical techniques and comparable to those obtained by other recent machine learning approaches, with the advantage of using fewer predictors. When restricting the analysis to stars with rotation periods of less than 45 d, our models are on average wrong less than 5% of the time. Conclusions. We have developed an innovative approach based on a machine learning method to accurately fit the rotation periods of stars. Based on the results of this study, we conclude that the best models generated by the proposed methodology can compete with the latest state-of-the-art approaches, while offering the advantage of being computationally cheaper, easy to train, and reliant only on small sets of predictors. [ABSTRACT FROM AUTHOR]

In recent years, there has been a push to understand how chemical composition affects the magnetic activity levels of main sequence low-mass stars. Results indicate that more metal-rich stars are more magnetically active for a given stellar mass and rotation period. This metallicity dependence has implications for how the rotation periods and activity levels of low-mass stars evolve over their lifetimes. Numerical modelling suggests that at late ages more metal-rich stars should be rotating more slowly and be more magnetically active. In this work, we study the rotation and activity evolution of low-mass stars using a sample of Kepler field stars. We use the gyro-kinematic age dating technique to estimate ages for our sample and use the photometric activity index as our proxy for magnetic activity. We find clear evidence that, at late ages, more metal-rich stars have spun down to slower rotation in agreement with the theoretical modelling. However, further investigation is required to definitively determine whether the magnetic activity evolution occurs in a metallicity dependent way. [ABSTRACT FROM AUTHOR]

This work focuses on the analysis of the mean longitudinal magnetic field as a stellar activity tracer in the context of small exoplanet detection and characterization in radial-velocity (RV) surveys. We use Solar Dynamics Observatory/Helioseismic and Magnetic Imager filtergrams to derive Sun-as-a-star magnetic field measurements, and show that the mean longitudinal magnetic field is an excellent rotational period detector and a useful tracer of the solar magnetic cycle. To put these results into context, we compare the mean longitudinal magnetic field to three common activity proxies derived from HARPS-N Sun-as-a-star data: the full width at half-maximum, the bisector span, and the S-index. The mean longitudinal magnetic field does not correlate with the RVs and therefore cannot be used as a one-to-one proxy. However, with high cadence and a long baseline, the mean longitudinal magnetic field outperforms all other considered proxies as a solar rotational period detector, and can be used to inform our understanding of the physical processes happening on the surface of the Sun. We also test the mean longitudinal magnetic field as a 'stellar proxy' on a reduced solar data set to simulate stellar-like observational sampling. With a Gaussian Process regression analysis, we confirm that the solar mean longitudinal magnetic field is the most effective of the considered indicators, and is the most efficient rotational period indicator over different levels of stellar activity. This work highlights the need for polarimetric time series observations of stars. [ABSTRACT FROM AUTHOR]

The Sun's history is still a subject of interest to modern astrophysics. Observationally constrained rates of Coronal Mass Ejections (CMEs) of young solar analogues are still lacking, as those require dedicated monitoring. We present medium-resolution optical spectroscopic monitoring of a small sample of bright and prominent solar analogues over a period of 3 yr using the 0.5-m telescope at observatory Lustbühel Graz (OLG) of the University of Graz, Austria. The aim is the detection of flares and CMEs from those spectra. In more than 1700 h of spectroscopic monitoring, we found signatures of four flares and one filament eruption on EK Dra which has been reported in previous literature, but we complementarily extended the data to cover the latter phase. The other stars did not reveal detectable signatures of activity. For these non-detections, we derive upper limits of occurrence rates of very massive CMEs, which are detectable with our observational setup, ranging from 0.1 to 2.2 d−1, but these may be even smaller than the given rates considering observational biases. Furthermore, we investigate the detectability of flares/CMEs in OLG spectra by utilizing solar 2D Hα spectra from Mees Solar Observatory. We find that solar-sized events are not detectable within our observations. By scaling up the size of the solar event, we show that with a fractional active region area of 18 per cent in residual spectra and 24 per cent in equivalent width time series derived from the same residuals that solar events are detectable if they had hypothetically occurred on HN Peg. [ABSTRACT FROM AUTHOR]

Excess chromospheric emissions within deep photospheric lines are effective proxies of stellar magnetism for FGK stars. This emission decays with stellar age and is a potential determinant of this important stellar quantity. We report absolutely calibrated H  |$\alpha$| chromospheric fluxes for 511 solar-type stars in a wide interval of precisely determined masses, [Fe/H], ages, and evolution states from high S/N, moderately high |$-$| resolution spectra. The comparison of H  |$\alpha$| and H + K chromospheric fluxes reveals a metallicity bias (absent from H  |$\alpha$|⁠) affecting Ca  ii H + K fluxes thereby metal-rich stars with deep line profiles mimic low chromospheric flux levels, and vice versa for metal-poor stars. This bias blurs the age–activity relation, precluding age determinations for old, inactive stars unless mass and [Fe/H] are calibrated into the relation. The H + K lines being the most widely studied tool to quantify magnetic activity in FGK stars, care should be exercised in its use whenever wide ranges of mass and [Fe/H] are involved. The H  |$\alpha$| age–activity–mass–metallicity calibration appears to be in line with the theoretical expectation that (other parameters being equal) more massive stars possess narrower convective zones and are less active than less massive stars, while more metal-rich stars have deeper convective zones and appear more active than metal-poorer stars. If regarded statistically in tandem with other age diagnostics, H  |$\alpha$| chromospheric fluxes may be suitable to constrain ages for FGK stars with acceptable precision. [ABSTRACT FROM AUTHOR]

Context. Many novel methods have been proposed to mitigate stellar activity for exoplanet detection as the presence of stellar activity in radial velocity (RV) measurements is the current major limitation. Unlike traditional methods that model stellar activity in the RV domain, more methods are moving in the direction of disentangling stellar activity at the spectral level. As deep neural networks have already been proven to be one of the most effective tools in data mining, in this work, we explore their potential in the context of Earth-like planet detection in RV measurements. Aims. The goal of this paper is to present a novel convolutional neural network-based algorithm that efficiently models stellar activity signals at the spectral level, enhancing the detection of Earth-like planets. Methods. Based on the idea that the presence of planets can only produce a Doppler shift at the spectral level while the presence of stellar activity can introduce a variation in the profile of spectral lines (asymmetry and depth change), we trained a convolutional neural network to build the correlation between the change in the spectral line profile and the corresponding RV, full width at half maximum (FWHM) and bisector span (BIS) values derived from the classical cross-correlation function. Results. This algorithm has been tested on three intensively observed stars: Alpha Centauri B (HD 128621), Tau ceti (HD 10700), and the Sun. By injecting simulated planetary signals at the spectral level, we demonstrate that our machine learning algorithm can achieve, for HD 128621 and HD 10700, a detection threshold of 0.5 m s−1 in semi-amplitude for planets with periods ranging from 10 to 300 days. This threshold would correspond to the detection of a ~4 M⊕ in the habitable zone of those stars. On the HARPS-N solar dataset, thanks to significantly more data, our algorithm is even more efficient at mitigating stellar activity signals and can reach a threshold of 0.2 m s−1, which would correspond to a 2.2 M⊕ planet on the orbit of the Earth. Conclusions. To the best of our knowledge, it is the first time that such low detection thresholds are reported for the Sun, but also for other stars, and therefore this highlights the efficiency of our convolutional neural network-based algorithm at mitigating stellar activity in RV measurements. [ABSTRACT FROM AUTHOR]

Studies of the rotation and activity of M type stars are essential in order to enhance our understanding of stellar dynamos and angular momentum evolution. Using the outstanding photometric capabilities of space telescopes, even those rotation signals with low amplitudes can now be investigated in previously unrivaled detail. By combining data of K2 and the TESS prime mission, the star spot activity of M dwarfs can be monitored over a timescale of half a decade. In the framework of our study on the rotation–activity relation for bright and nearby M dwarfs, we also aim to investigate their long-term activity. While K2 was observing fields distributed around the ecliptic plane, the TESS prime mission was oriented along a line of ecliptic longitude, with one camera centered on an ecliptic pole. Due to these different observing strategies, the overlap between K2 and the TESS prime mission is marginal. However, 45 stars from our sample were observed with both missions, and of these, two early M-type stars that fulfill our selection criteria, EPIC 202059229 and EPIC 245919787, were analyzed in more detail. We find that, for both stars, the rotation period did not change, while the rotational phase did change for EPIC 245919787 by ∼0.2. The amplitude of the spot-induced variability changed for both stars but more significantly for EPIC 245919787. By comparing the cumulative flare frequency distributions, we find that the flare activity for EPIC 202059229 is unchanged, while it slightly changes for EPIC 245919787 between the K2 and TESS epochs. Using a combination of light curves from K2 and TESS that span a baseline of up to 4.5 yr, we are able to measure significant differential rotation for EPIC 245919787. Furthermore, we show that combining missions like K2 and TESS is a promising method for detecting stellar activity cycles. [ABSTRACT FROM AUTHOR]

We aim to advance our understanding of magnetic activity and the underlying dynamo mechanism at the end of the main sequence. To this end, we have embarked on collecting simultaneous X-ray and radio observations for a sample of M7..L0 dwarfs in the solar neighborhood using XMM-Newton jointly with the Jansky Very Large Array (JVLA) and the Australia Telescope Compact Array (ATCA). We supplemented the data from these dedicated campaigns with X-ray data from the all-sky surveys of the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG). Moreover, we complement this multiwavelength data set with rotation periods that we measured from light curves acquired with the Transiting Exoplanet Survey Satellite (TESS). We limited the sample to objects with rotation period of < 1 day, focusing on the study of a transition in magnetic behavior suggested by a drastic change in the radio detection rate at υ sin i ≈ 38 km s−1, corresponding to Prot ≈ 0.2 day for a typical ultracool dwarf (UCD) radius of R★ = 0.15 R⊙. Finally, to enlarge the target list, we have compiled archival X-ray and radio data for UCDs from the literature, and we have analyzed the abovementioned ancillary eROSITA and TESS observations for these objects' analogous to the targets from our dedicated X-ray/radio campaigns. We compiled the most up to date radio/X-ray luminosity (LR,ν − Lx) relation for 26 UCDs with rotation periods (Prot) lower than 1 day, finding that rapid rotators lie the furthest away from the "Güdel-Benz" relation previously studied for earlier-type stars. Radio bursts are mainly (although not exclusively) experienced by very fast UCDs (Prot ≤ 0.2 day), while X-ray flares are seen by objects spanning the whole range of rotation. Finally, we examined the Lx/Lbol versus Prot relation, where our sample of UCDs spans a large activity level range, that is log(Lx/Lbol) = −5.5 to log(Lx/Lbol) = −3. Although they are all fast rotating, X-ray activity evidently decouples from that of normal dynamos. In fact, we found no evident relation between the X-ray emission and rotation, reinforcing previous speculations on a bimodal dynamo across late-type dwarfs. One radio-detected object, 2MJ0838, has a rotation period consistent with the range of auroral bursting sources; while it displays moderately circularly polarized emission, there is no temporal variation in the polarized flux. A radio flare from this object is interpreted as gyrosynchrotron emission, and it displays X-ray and optical flares. Among the ten UCDs observed with the dedicated X-ray/radio campaigns, we found a slowly rotating apparent auroral emitter (2MJ0752) that is also one of the X-ray brightest radio-detected UCDs. We speculate that this UCD is experiencing a transition in its magnetic behavior since it produces signatures expected from higher-mass M dwarfs along with emerging evidence of auroral emission. [ABSTRACT FROM AUTHOR]

Context. Stellar variability strongly impacts the search for low-mass exoplanets with radial velocity techniques. Two types of planet-free time series can be used to quantify this impact: models and direct solar observations after a subtraction of the Solar System planetary contribution. Making a comparison among these approaches is necessary to improve the models, which can then be used for blind tests across a broad range of conditions. Aims. Our objective is therefore to validate the amplitude of the convective blueshift in plages used in our previous works, particularly in blind tests, with HARPS-N solar data. Methods. We applied our model to the structures observed at the time of HARPS-N observations and established a direct comparison between the radial velocity time series. To complete our diagnosis, we also studied the observed radial velocities separately for each diffraction order derived from the individual cross-correlation functions, as well as our line-by-line radial velocities. Results. We find that our previous model had been underestimating the amplitude of the convective blueshift inhibition by a factor of about 2. A direct estimation of the convective blueshift in the spectra, which is shown to be correlated with the plage filling factor, allows us to explain the difference with previous estimations obtained with MDI/SOHO Dopplergrams, based on the specific properties of the Ni line used in this mission. In addition, we identified several instrumental systematics, in particular, the presence of a 2 m s−1 peak-to-peak signal with a period of about 200 days in radial velocity and bisector. This signal could be due to periodic detector warmups, a systematic dependence of the long-term trend on wavelength that is possibly related to the variability of the continuum over time, and/or an offset in radial velocity after the interruption of several months in October 2017. Conclusions. A large amplitude in the convective blueshift inhibition of (360 ms−1, namely twice more than in our previous works) must be used when building synthetic times series for blind tests. The presence of instrumental systematics should also be taken into account when using sophisticated methods based on line properties to mitigate stellar activity when searching for very weak signals. [ABSTRACT FROM AUTHOR]

Aims. WASP-18 is an F6V star that hosts a planet with a mass of ~10 Jupiter masses and an orbital period of ~0.94 days. In spite of its relatively fast rotation and young age, the star remains undetected in X-rays, thus implying a very low level of magnetic activity. To account for such unexpected properties, we propose a mechanism that modifies the internal stratification and the photospheric magnetic activity of a late-type main sequence star with a close-by massive planet based on the action of the equilibrium tide. Methods. We speculate that the horizontal flow produced by the equilibrium tide may interact with the convective plumes in the overshoot layer below the stellar outer convective envelope. The interaction is characterised by a very high Reynolds number (Re ~ 1010), leading to the development of turbulent boundary layers at the surface of such structures, whereas turbulent wakes extend over most of the overshoot layer that they straddle. Results. We propose that such a tidally induced turbulence can lead to a reduction of the filling factor of the downdrafts in the overshoot layer. As a consequence, the absolute value of the sub-adiabatic gradient increases in that layer hindering the emergence of magnetic flux tubes responsible for the formation of photospheric starspots. We conjecture that this process is occurring in WASP-18, thus providing a possible mechanism to account for the very low level of magnetic activity observed for such a planet host. [ABSTRACT FROM AUTHOR]

The discrepancy between the observed and the predicted radii in low-mass stars is a well-known and yet to be resolved problem. Although various theoretical approaches have been developed since the first appearance of the problem, there is still no satisfactory mechanism or model which could successfully reproduce observed radii. The stellar magnetic field is among proposed resolutions but has not been investigated comprehensively for detached eclipsing binary stars. In this study, we present analysis of seven low-mass Kepler eclipsing binaries, KIC 4484356, KIC 4678171, KIC 5300878, KIC 6147573, KIC 8543278, KIC 9762519, and KIC 9821078. We investigate the average amplitude of the rotational modulation signal observed at out-of-eclipse phases, which could be used as the proxy for the strength of the stellar magnetic field, and possible discrepancies between observed and theoretically calculated radii of the components of the target systems. Our findings indicate radius discrepancy for both components of KIC 4678171 and KIC 9821078. Moreover, secondary components of KIC 9762519 and KIC 8543278 appear to have quite inflated radii. However, results from this limited sample do not indicate any correlation between average amplitude of the rotation modulation signal and observed radius discrepancies. [ABSTRACT FROM AUTHOR]

We present the discovery of three super-Earth candidates orbiting HD 48948, a bright K-dwarf star with an apparent magnitude of |$m_V$|  = 8.58 mag. As part of the HARPS-N Rocky Planet Search programme, we collect 189 high-precision radial velocity measurements using the HARPS-N spectrograph from 2013 October 6, to 2023 April 16. Various methodologies are applied to extract the radial velocities from the spectra, and we conduct a comprehensive comparative analysis of the outcomes obtained through these diverse extraction techniques. To ensure the robustness of our findings, we employ several methods to address stellar variability, with a focus on Gaussian Process regression. To account for the impact of stellar variability and correlated noise in the radial velocity data set, we include activity indicators, such as |$\log R^{^{\prime }}_{\mathrm{HK}}$| and bisector span, in the multidimensional Gaussian Process regression. Our analysis reveals three planetary candidates with orbital periods of 7.3, 38, and 151 d, and minimum masses estimated at |$4.88 \pm 0.21$| M |$_{\oplus }$|⁠ , |$7.27 \pm 0.70$| M |$_{\oplus }$|⁠ , and |$10.59 \pm 1.00$| M |$_{\oplus }$|⁠ , respectively. The outermost planet resides within the (temperate) habitable zone, positioned at a projected distance of |$0.029\,{\rm arcsec}$| from its star. Given the close proximity of this planetary system, situated at a distance of 16.8 parsecs, HD 48498 emerges as a promising target (closest super-Earth around FGK stars) for future high-contrast direct imaging and high-resolution spectroscopic studies. [ABSTRACT FROM AUTHOR]

We present an exhaustive photometric and spectroscopic analysis of TOI-837, a F9/G0 35 Myr young star, hosting a transiting exoplanet, TOI-837 b, with an orbital period of ∼8.32 d. Utilizing data from the Transiting Exoplanet Survey Satellite and ground-based observations, we determine a planetary radius of |$0.818_{-0.024}^{+0.034}$| R J for TOI-837 b. Through detailed High Accuracy Radial Velocity Planet Searcher spectroscopic time series analysis, we derive a Doppler semi-amplitude of |$34.7_{-5.6}^{+5.3}$| |${\rm m\, s^{-1}}$|⁠ , corresponding to a planetary mass of |$0.379_{-0.061}^{+0.058}$| M J. The derived planetary properties suggest a substantial core of approximately 70 M⊕, constituting about 60 per cent of the planet's total mass. This finding poses a significant challenge to existing theoretical models of core formation. We propose that future atmospheric observations with JWST could provide insights into resolving ambiguities of TOI-837 b, offering new perspectives on its composition, formation, and evolution. [ABSTRACT FROM AUTHOR]

L 98-59b is a transiting exoplanet with radius |${0.85^{+0.061}_{-0.047}}$| R ⊕ and orbital period |${2.2531136^{+1.2e-06}_{-1.5e-06}}$| d. In 2021, a Doppler detection of  L 98-59b was announced, with radial velocity (RV) semi-amplitude |$K_{\rm b}=0.46^{+0.20}_{-0.17}$| |$\textrm {m}\, \textrm {s}^{-1}$| inferred from 164 High Accuracy Radial velocity Planet Searcher and 66 Échelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observation RVs. This translated into a mass |$M_{\rm b}=0.40^{+0.16}_{-0.15}$| M ⊕, or half the mass of Venus, making  L 98-59b then (by far) the lowest mass planet with a putative RV detection. To illuminate the difficulties in securely detecting <1  |$\textrm {m}\, \textrm {s}^{-1}$|  signals, we argue here that the published RV detection of  L 98-59b is not supported by sufficient statistical evidence. We show, under the published modelling assumptions, that Bayesian model comparison (BMC) suggests an |$\sim 50~{{\ \rm per\ cent}}$| false-detection probability. We also often infer ∼40  |$\textrm {cm}\, \textrm {s}^{-1}$| semi-amplitudes for Keplerians with periods not corresponding to any known planet; importantly, though, BMC rejects all these 'detections'. By implementing a more sophisticated stellar activity model and more realistic parameter priors, we infer a mass M b = 0.47 ± 0.14 M ⊕ from semi-amplitude K b = 0.56 ± 0.16  |$\textrm {m}\, \textrm {s}^{-1}$|⁠ , with BMC now indicating substantial yet still not decisive evidence for the detection. Definitive detection and characterization may require many additional RVs, alongside careful modelling and stringent statistical analysis. Our case study has implications for current extreme-precision RV campaigns and the longer term quest to discover the first Earth-twins. [ABSTRACT FROM AUTHOR]

The recently discovered super-Earth Gl 514 b, orbiting a nearby M0.5-1.0 star at 7.6 pc, is one of the best benchmark exoplanets for understanding the potential climate states of eccentric planets. The elongated (e = 0.45 |$^{+0.15}_{-0.14}$|⁠) orbit of Gl 514 b, which only partially lies in the Conservative Habitable Zone, suggests a dynamically young system, where the spin-orbit tidal synchronization may not have yet occurred up to the present time. In the present work, we use a seasonal-latitudinal energy balance model, EOS-ESTM, to explore the potential impact of both constrained and unconstrained planetary, orbital, and atmospheric parameters on the Gl 514 b habitability, mapped in terms of surface temperature. We test three distinct CO2-dominated atmospheres by varying the CH4 concentration values (0 per cent, 0.1 per cent, and 1 per cent) and the total surface pressure. As a general trend, we find that habitable conditions are favoured by high-CH4 and high-pressure regimes. Habitability also increases for high-axis obliquities (at least until the appearance of an icebelt), long-rotation periods, and high-ocean fractional coverage. If the ocean fraction is low, then also the argument of periastron becomes relevant. Our results are robust against changes of the continental distribution. Thus, we conclude that Gl 514 b can potentially maintain temperate surface conditions with modest seasonal temperature variations under a wide variety of planetary, orbital, and atmospheric conditions. Despite no transit have been detected yet, the results found in this work should motivate the community to invest time in future observations. [ABSTRACT FROM AUTHOR]

Absorption lines from exoplanet atmospheres observed in transmission allow us to study atmospheric characteristics such as winds. We present a new high-resolution transit time-series of HD 189733b, acquired with the PEPSI instrument at the LBT and analyse the transmission spectrum around the Na  d lines. We model the spectral signature of the RM-CLV-effect using synthetic PHOENIX spectra based on spherical LTE atmospheric models. We find an Na  d absorption signature between the second and third contact but not during the ingress and egress phases, which casts doubt on the planetary origin of the signal. Presupposing a planetary origin of the signal, the results suggest a weak day-to-nightside streaming wind in the order of 0.7 km s−1 and a moderate super-rotational streaming wind in the order of 3–4 km s−1, challenging claims of prevailing strong winds on HD 189733b. [ABSTRACT FROM AUTHOR]

The dynamical evolution of tight star‐planet systems is influenced by tidal interactions between the star and the planet, as was shown recently. The rate at which spins and orbits in such a system evolve depends on the stellar and planetary tidal dissipation efficiency. Here, we present a method to constrain the modified tidal quality factor Q*′$$ {Q}_{\ast}^{\prime } $$ of a planet‐hosting star whose rotational evolution has been altered by its planet through angular momentum transfer from the planetary orbital motion to the rotation of the stellar convective zone. The altered rotation is estimated from an observed discrepancy of magnetic activity of the planet‐hosting star and a coeval companion star, that is, this method is applicable to star‐planet systems with wide stellar companions. We give an example of the planet‐hosting wide binary system HD189733 and find that the planet host's modified tidal quality factor is constrained to be Q*′≤2.33×107$$ {Q}_{\ast}^{\prime}\le 2.33\times {10}^7 $$. [ABSTRACT FROM AUTHOR]

Young α-rich (YAR) stars within the old Galactic thick disc exhibit a dual characteristic of relative youth determined with asteroseismology and abundance enhancement in α elements measured from high-resolution spectroscopy. The youth origin of YAR stars has been proposed to be binary evolution via mass transfer or stellar mergers. If that is the case, YAR stars should spin rapidly and thus be magnetically active, because they are mass and angular momentum gainers. In this study, to seek this binary footprint, we select YAR stars on the main-sequence turn-off or the subgiant branch (MSTO-SGB) from APOGEE DR17, whose ages and projected rotation velocities (v sin  i) can be precisely measured. With APOGEE v sin  i and LAMOST spectra, we find that YAR stars are indeed fast rotators and magnetically active. In addition, we observe low [C/N] ratios and high Gaia RUWE in some YAR stars, suggesting that these MSTO-SGB stars probably have experienced mass transfer from red-giant companions. Our findings underscore that magnetic activity can serve as a valuable tool for probing the binary evolution for other chemically peculiar stars, such as red giants with lithium anomalies and carbon-enhanced metal-poor stars. [ABSTRACT FROM AUTHOR]

The number of super-Earth and mini-Neptune planet discoveries has increased significantly in the last two decades thanks to transit and radial velocity (RV) surveys. When it is possible to apply both techniques, we can characterise the internal composition of exoplanets, which in turn provides unique insights on their architecture, formation and evolution. We performed a combined photometric and RV analysis of TOI-238 (TYC 6398-132-1), which has one short-orbit super-Earth planet candidate announced by NASA's TESS team. We aim to confirm its planetary nature using radial velocities taken with the ESPRESSO and HARPS spectrographs, to measure its mass, and to detect the presence of other possible planetary companions. We carried out a joint analysis by including Gaussian processes and Keplerian orbits to account for the stellar activity and planetary signals simultaneously. We detected the signal induced by TOI-238 b in the RV time series, and the presence of a second transiting planet, TOI-238 c, whose signal appears in RV and TESS data. TOI-238 b is a planet with a radius of 1.402−0.086+0.084R⊕ and a mass of 3.40−0.45+0.46M⊕. It orbits at a separation of 0.02118 ± 0.00038 au of its host star, with an orbital period of 1.2730988 ± 0.0000029 days, and has an equilibrium temperature of 1311 ± 28 K. TOI-238 c has a radius of 2.18 ± 0.18 R⊕ and a mass of 6.7 ± 1.1 M⊕. It orbits at a separation of 0.0749 ± 0.0013 au of its host star, with an orbital period of 8.465652 ± 0.000031 days, and has an equilibrium temperature of 696 ± 15 K. The mass and radius of planet b are fully consistent with an Earth-like composition, making it a likely rocky super-Earth. Planet c could be a water-rich planet or a rocky planet with a small H-He atmosphere. [ABSTRACT FROM AUTHOR]

Context. The Li abundance observed in pre-main sequence and main sequence late-type stars is strongly age-dependent, but also shows a complex pattern depending on several parameters, such as rotation, chromospheric activity, and metallicity. The best way to calibrate these effects, and with the aim of studying Li as an age indicator for FGK stars, is to calibrate coeval groups of stars, such as open clusters (OCs) and associations. Aims. We present a considerable target sample of 42 OCs and associations – with an age range from 1 Myr to 5 Gyr – observed within the Gaia-ESO survey (GES), and using the latest data provided by GES iDR6 and the most recent release of Gaia that was then available, EDR3. As part of this study, we update and improve the membership analysis for all 20 OCs presented in our previous article. Methods. We perform detailed membership analyses for all target clusters to identify likely candidates, using all available parameters provided by GES, complemented with detailed bibliographical searches, and based on numerous criteria: from radial velocity distributions, to the astrometry (proper motions and parallaxes) and photometry provided by Gaia, to gravity indicators (log g and the γ index), [Fe/H] metallicity, and Li content in diagrams of (Li equivalent widths) EW(Li) versus Teff. Results. We obtain updated lists of cluster members for the whole target sample, as well as a selection of Li-rich giant contaminants obtained as an additional result of the membership process. Each selection of cluster candidates was thoroughly contrasted with numerous existing membership studies using data from Gaia to ensure the most robust results. Conclusions. These final cluster selections will be used in the third and last paper of this series, which reports the results of a comparative study characterising the observable Li dispersion in each cluster and analysing its dependence on several parameters, allowing us to calibrate a Li–age relation and obtain a series of empirical Li envelopes for key ages in our sample. [ABSTRACT FROM AUTHOR]

Transmission spectroscopy is a proven technique for studying a transiting exoplanet's atmosphere. However, stellar surface inhomogeneities – spots and faculae – alter the observed transmission spectra: the stellar contamination effect. The variable nature of the stellar activity also makes it difficult to stitch together multi-epoch observations and evaluate any potential variability in the exoplanet's atmosphere. This paper introduces SAGE, a tool that corrects for the time-dependent impact of stellar activity on transmission spectra. It uses a pixelation approach to model the stellar surface with spots and faculae, while fully accounting for limb-darkening and rotational line-broadening. The current version is designed for low- to medium-resolution spectra. We used SAGE to evaluate stellar contamination for F- to M-type hosts, testing various spot sizes and locations, and quantify the impact of limb-darkening. We find that limb-darkening enhances the importance of the spot location on the stellar disc, with spots close to the disc centre impacting the transmission spectra more strongly than spots near the limb. Moreover, due to the chromaticity of limb-darkening, the shape of the contamination spectrum is also altered. Additionally, SAGE can be used to retrieve the properties and distribution of active regions on the stellar surface from photometric monitoring. We demonstrate this for WASP-69 using TESS data, finding that two spots at midlatitudes and a combined coverage fraction of ~1% are favoured. SAGE allows us to connect the photometric variability to the stellar contamination of transmission spectra, enhancing our ability to jointly interpret transmission spectra obtained at different epochs. [ABSTRACT FROM AUTHOR]

Context. Nova 1670 is a historical transient bearing strong similarities to a recently recognized type of stellar eruption known as the red nova, which is thought to be powered by stellar mergers. The remnant of the transient, CK Vul, is observable today mainly through cool circumstellar gas and dust, and recombining plasma, but we have no direct view on the stellar object. Aims. Within the merger hypothesis, we aim to infer the most likely configuration of the progenitor system that resulted in Nova 1670. Methods. We collected the literature data on the physical properties of the outburst and the remnant (including their energetics), and on the chemical composition of the circumstellar material (including elemental and isotopic abundances). These data, which result from optical and submillimeter observations of the circumstellar gas of CK Vul, are summarized here. We performed simple simulations to analyze the form and the level of mixing within the material associated with the merger. We identified products of nuclear burning, among which we find ashes of hydrogen burning in the CNO cycles and in the MgAl chain, as well as signs of partial helium burning. Results. Based primarily on the luminosity and chemical composition of the remnant, we find that the progenitor primary had to be an evolutionarily advanced red giant branch star of a mass of 1–2 M⊙. The secondary was either a very similar giant, or –more likely– a helium white dwarf. While the eruption event was mainly powered by accretion, we estimate that about 12% of total energy is likely to have come from helium burning activated during the merger. The coalescence of a first-ascent giant with a helium white dwarf created a star with a rather unique internal structure and composition, which resemble those of early R-type carbon stars. Conclusions. Nova 1670 is the result of a merger between a helium white dwarf and a first-ascent red giant and is likely now evolving to become an early R-type carbon star. [ABSTRACT FROM AUTHOR]

We analyse a Chandra observation of the rich globular cluster NGC 362, finding 33 X-ray sources within 1 arcmin (1.2 half-mass radii) of the cluster centre. Spectral analysis of the brightest source (X1) shows blackbody-like emission, indicating it is likely a quiescent low-mass X-ray binary; we find a possible counterpart that falls in the sub-subgiant region. We use Hubble Space Telescope ultraviolet (UV) Globular Cluster Survey photometry to identify 15 potential optical/UV counterparts to these X-ray sources, including two background active galactic nuclei. We identify no likely cataclysmic variables (CVs), probably due to crowding in optical filters in the core, though we predict of order 8 CVs among the detected X-ray sources. We identify three other sub-subgiants and two red straggler counterparts, which are likely powered by coronal activity, along with five other potential coronally active binary counterparts to three X-ray sources. Finally, we note two unusual counterpart candidates that lie to the red of the red giant branch in V 606 − I 814, and shift well to the blue of the red giant branch in ultraviolet colour–magnitude diagrams. These systems seem to contain a red giant with a distorted evolutionary history, plus a bright blue light source, either a blue straggler star (an Algol-like system) or an accreting white dwarf (a long-period CV, or a symbiotic star). [ABSTRACT FROM AUTHOR]

The super-Earth LHS 1140 b is an interesting target for atmospheric observations since it is close to the habitable zone of its star and falls in the gap of the radius distribution of small exoplanets, in the region thought to correspond to the transition between planets with and without atmospheres. Observations of the primary transit with WFC3 onboard of the Hubble Space Telescope (HST) revealed a modulation in the planet transmission spectrum compatible with the presence of water; however, this modulation may be also due to stellar activity-related phenomena. Here, we present a detailed analysis of the WFC3/ HST observations to probe the nature of this modulation and to understand if it can be attributable to the presence of unocculted spots on the stellar surface. Our analysis strongly suggests that LHS1140 is a rather quiet star with subsolar [Fe/H] and enriched in α elements. Therefore, we rule out the possibility that the planetary spectrum is affected by the presence of spots and faculae. This analysis shows the importance of a proper modelling of the stellar spectrum when analysing transit observations. Finally, we modelled the planetary atmosphere of LHS1140 b to retrieve its atmospheric composition. However, the low resolution and the narrow spectral range of HST observations prevented us from definitively determining whether the spectral features are attributable to the presence of water or of other molecules in the planetary atmosphere. [ABSTRACT FROM AUTHOR]

Aims. In this study, we analyse the magnetic field properties of a set of 15 global magnetohydrodynamic (MHD) simulations of solar-type star dynamos conducted using the ASH code. Our objective is to enhance our understanding of these properties by comparing theoretical results to current observations, and to finally provide fresh insights into the field. Methods. We analysed the rotational and magnetic properties as a function of various stellar parameters (mass, age, and rotation rate) in a 'Sun in time' approach in our extended set of 3D MHD simulations. To facilitate direct comparisons with stellar magnetism observations using various Zeeman-effect techniques, we decomposed the numerical data into vectorial spherical harmonics. Results.A comparison of the trends we find in our simulations set reveals a promising overall agreement with the observational context of stellar magnetism, enabling us to suggest a plausible scenario for the magneto-rotational evolution of solar-type stars. In particular, we find that the magnetic field may reach a minimum amplitude at a transition value of the Rossby number near unity. This may have important consequences on the long-term evolution of solar-type stars, by impacting the relation between stellar age, rotation, and magnetism. This supports the need for future observational campaigns, especially for stars in the high Rossby number regime. [ABSTRACT FROM AUTHOR]

We conducted four-night multiwavelength observations of an active M-dwarf star EV Lacertae on 2022 October 24–27 with simultaneous coverage of soft X-rays (NICER; 0.2–12 keV, Swift XRT; 0.2–10 keV), near-ultraviolet (Swift UVOT/UVW2; 1600–3500 Å), optical photometry (TESS; 6000–10000 Å), and optical spectroscopy (Nayuta/MALLS; 6350–6800 Å). During the campaign, we detected a flare starting at 12:28 UTC on October 25 with a white-light bolometric energy of 3.4 × 1032 erg. At about 1 h after this flare peak, our Hα spectrum showed a blueshifted excess component at a corresponding velocity of ∼100 km s−1. This may indicate that the prominence erupted with a 1 h delay of the flare peak. Furthermore, the simultaneous 20 s cadence near-ultraviolet (NUV) and white-light curves show gradual and rapid brightening behaviors during the rising phase at this flare. The ratio of flux in NUV to white light at the gradual brightening was ∼0.49, which may suggest that the temperature of the blackbody is low (<9000 K) or the maximum energy flux of a non-thermal electron beam is less than 5 × 1011 erg cm−2 s−1. Our simultaneous observations of the NUV and white-light flare raise the issue of a simple estimation of UV flux from optical continuum data by using a blackbody model. [ABSTRACT FROM AUTHOR]

While the mission's primary goal was focused on exoplanet detection and characterization, Kepler made and continues to make extraordinary advances in stellar physics. Stellar rotation and magnetic activity are no exceptions. Kepler allowed for these properties to be determined for tens of thousands of stars from the main sequence up to the red giant branch. From photometry, this can be achieved by investigating the brightness fluctuations due to active regions, which cause surface inhomogeneities, or through asteroseismology as oscillation modes are sensitive to rotation and magnetic fields. This review summarizes the rotation and magnetic activity properties of the single main-sequence solar-like stars within the Kepler field. We contextualize the Kepler sample by comparing it to known transitions in the stellar rotation and magnetic-activity evolution, such as the convergence to the rotation sequence (from the saturated to the unsaturated regime of magnetic activity) and the Vaughan-Preston gap. While reviewing the publicly available data, we also uncover one interesting finding related to the intermediate-rotation gap seen in Kepler and other surveys. We find evidence for this rotation gap in previous ground-based data for the X-ray luminosity. Understanding the complex evolution and interplay between rotation and magnetic activity in solar-like stars is crucial, as it sheds light on fundamental processes governing stellar evolution, including the evolution of our own Sun. [ABSTRACT FROM AUTHOR]

Context. Several studies suggest that the emission properties of a star can be significantly affected by its interaction with a nearby planet through magnetic fields or interaction between their respective winds. However, the actual observability of these effects remains a subject of debate. An illustrative example is the HD 189733A system: certain characteristics of its emissions have been interpreted as indicative of ongoing interactions between the star and its associated planet. Other studies attribute these characteristics to the coronal activity of the star. Aims. In this study we aimed to investigate whether the observed stellar X-ray flare events, which appear to be in phase with the planetary period in the HD 189733A system, could be attributed to the accretion of the planetary wind onto the stellar surface or if they resulted from an interaction between the planetary and stellar winds. Methods. We developed a 3D magnetohydrodynamic model with the PLUTO code that describes the system HD 189733A , including the central host star and its hot Jupiter along with their respective winds. The effects of gravity and the magnetic fields of both the star and the planet are taken into account. Results. Our analysis reveals that, in the cases examined in this study, the accretion scenario is only viable when the stellar magnetic field strength is at 5 G and the planetary magnetic field strength is at 1 G. In this scenario, the Rayleigh-Taylor instabilities lead to the formation of an accretion column that connects the star to the planet. Once formed the accretion column remains stable for the duration of the simulation. The accretion column produces an accretion rate of the order of 1012 g s−1 and shows an average density of about 107 cm−3. In the other case explored, the accretion column does not form because the Rayleigh-Taylor instability is suppressed by the stronger magnetic field intensities assumed for both the star and the planet. We synthesized the emission resulting from the shocked planetary wind and found that the total X-ray emission ranges from 5 × 1023 to 1024 erg s−1. Conclusions. In the case of accretion, the emission originating from the hotspot cannot be distinguished from the coronal activity. Also, the interaction between the planetary and stellar winds cannot be responsible for the X-ray emission, as the total emission produced is about four orders of magnitude lower than the average X-ray luminosity of the star. [ABSTRACT FROM AUTHOR]

Context. The intrinsic variability due to the magnetic activity of young active stars is one of the main challenges in detecting and characterising exoplanets. The stellar activity is responsible for jitter effects observed both in photometric and spectroscopic observations that can impact our planetary detection sensitivity. Aims. We present a method able to model the stellar photosphere and its surface inhomogeneities (starspots) in young, active, and fast-rotating stars based on the cross-correlation function (CCF) technique, and we extract information about the spot configuration of the star. Methods. We developed Spot CCF, a tool able to model the deformation of the CCF profile due to the presence of multiple spots on the stellar surface. Within the Global Architecture of Planetary Systems (GAPS) Project at the Telescopio Nazionale Galileo, we analysed more than 300 spectra of the young planet-hosting star V1298 Tau provided by the HARPS-N high-resolution spectrograph. By applying the SpotCCF model to the CCFs, we extracted the spot configuration (latitude, longitude, and projected filling factor) of this star, and provide a new radial velocity (RV) time series for this target. Results. We find that the features identified in the CCF profiles of V1298 Tau are modulated by the stellar rotation, supporting our assumption that they are caused by starspots. The analysis suggests a differential rotation velocity of the star with lower rotation at higher latitudes. Also, we find that SpotCCF provides an improvement in RV extraction, with a significantly lower dispersion with respect to the commonly used pipelines. This allows mitigation of the stellar activity contribution modulated with stellar rotation. A detection sensitivity test, involving the direct injection of a planetary signal into the data, confirms that the SpotCCF model improves the sensitivity and ability to recover planetary signals. Conclusions. Our method enables us to model the stellar photosphere and extract the spot configuration of young, active, and rapidly rotating stars. It also allows the extraction of optimised RV time series, thereby enhancing our detection capabilities for new exoplanets and advancing our understanding of stellar activity. [ABSTRACT FROM AUTHOR]

We present the results of analyzing the activity manifestations of the K3 V spectral type dwarf HIP 113103 (HD 216530) and estimate the matter loss from the atmospheres of two of its planets. HIP 113103 can be classified as a relatively young object with an age of about 470 million years—1 billion years. Using the standard method, based on the observed data from the TESS mission, we estimated the areas of spots on the surface of the star, which exceed the maximum area of spots on the Sun and are 6100 and 4950 MSH for sectors 1 and 28, respectively. According to the data from the observation archive of the All Sky Automated Survey, the activity cycles of HIP 113103 have been established to last about 610, 825, and 1030 days (1.7, 2.3, and 2.8 years, respectively). Both planets in the stellar system HIP 113103 are characterized as objects with radii of 1.829 and 2.400 , with periods of 7.6 and 14.2 days, respectively, lying on opposite sides of the Fulton Gap in the "Period—Radius" diagram. The loss of matter from the planetary atmospheres was calculated using an approximation formula corresponding to the energy-limited atmospheric loss model. To estimate the flux of XUV photons, analytical dependences were used connecting flux values and the parameter. It was found that the values values for planets b and c are equal to g s and g s , respectively. The planet HIP 113103 b attracts special attention because, with its radius , it belongs to a small population of planets with the radii , which characterizes the evolution of super-Earths and mini-Neptunes. HIP 113103 c has a radius of and an equilibrium temperature of K which characterizes the object as an already established warm mini-Neptune. A comparison of the properties of planets b and c is of interest, since they belong to the same system, are irradiated by the same star, probably have similar ages and conditions of formation, etc. [ABSTRACT FROM AUTHOR]

We report intensive monitoring of the activity variability in the Hα line for 10 Sun-like stars using the 1.88 m reflector at Okayama Branch Office, Subaru Telescope, during the last four years (2019–2022). Our aim was to investigate features of the stellar magnetic activity behavior. We correlated the Hα line variability of each star with the stellar activity levels derived from the Ca  ii H&K line, suggesting its efficiency as a magnetic activity indicator. In analyzing the Hα line variation, we observed that some stars exhibited linear or quadratic trends during the observation period. Among several G- and K-type stars expected to have co-existing activity cycles, we confirmed the 2.9 yr short cycle of ϵ Eri (K2V) from the Hα observations. Additionally, we established upper limits on the Hα variability of β Com (G0V) and κ1 Cet (G5V) concerning their expected shorter cycles. We also detected the possibility of short-term activity cycles in two F-type stars, β Vir (F9V; ∼530 d) and α CMi (F5IV-V; ∼130 d). The cycle in α CMi was observed in only one season of our 4 yr observations, suggesting the temporal absence of the cycle period. However, for stars with planets, we did not observe significant magnetic activity variability likely associated with the planetary orbital period. It is speculated that the impact of Hα variability on radial velocity (RV) measurements may vary with spectral type. [ABSTRACT FROM AUTHOR]

We present the characterization of an inner mini-Neptune in a 9.2292005 ± 0.0000063 d orbit and an outer mono-transiting sub-Saturn planet in a 95.50 |$^{+0.36}_{-0.25}$| d orbit around the moderately active, bright (mv  = 8.9 mag) K5V star TOI-2134. Based on our analysis of five sectors of TESS data, we determine the radii of TOI-2134b and c to be 2.69 ± 0.16 R⊕ for the inner planet and 7.27 ± 0.42 R⊕ for the outer one. We acquired 111 radial-velocity (RV) spectra with HARPS-N and 108 RV spectra with SOPHIE. After careful periodogram analysis, we derive masses for both planets via Gaussian Process regression: 9.13 |$^{+0.78}_{-0.76}$| M⊕ for TOI-2134b and 41.89 |$^{+7.69}_{-7.83}$| M⊕ for TOI-2134c. We analysed the photometric and RV data first separately, then jointly. The inner planet is a mini-Neptune with density consistent with either a water-world or a rocky core planet with a low-mass H/He envelope. The outer planet has a bulk density similar to Saturn's. The outer planet is derived to have a significant eccentricity of 0.67 |$^{+0.05}_{-0.06}$| from a combination of photometry and RVs. We compute the irradiation of TOI-2134c as 1.45 ± 0.10 times the bolometric flux received by Earth, positioning it for part of its orbit in the habitable zone of its system. We recommend further RV observations to fully constrain the orbit of TOI-2134c. With an expected Rossiter–McLaughlin (RM) effect amplitude of 7.2 ± 1.3 |$\rm m\, s^{-1}$|⁠ , we recommend TOI-2134c for follow-up RM analysis to study the spin–orbit architecture of the system. We calculate the Transmission Spectroscopy Metric, and both planets are suitable for bright-mode Near Infrared Camera (NIRCam) atmospheric characterization. [ABSTRACT FROM AUTHOR]

We studied the light curve of the star CD-36 3202, which was observed by TESS for the presence of stellar spots and to analyze the rotationally modulated flare that took place on TESS Barycentric Julian Date 1486.93. Our main aims are to model the light curve of this flare and to estimate its location regarding stellar spots. The flare lasted approximately 27 h. Using our new tool, findinc_mc, we managed to estimate the inclination angle of the star to 70° ± 8°. With BASSMAN, we modeled the light curve of the CD-36 3202 and estimated that three spots are present on its surface. The mean temperature of the spots was about 4000 ± 765 K, and their total area amounted to 11.61% ± 0.13% on average. We created a new tool, named MFUEA, to model rotationally modulated flares, and used it to estimate the latitude of the long-duration flare event, finding 69−1+2 deg. Our estimation of the flare location is the first recreation of the exact position of a flare in relation to starspots. The flare is placed 12° from the center of the coolest spot. This means that the flare is related to the magnetic processes above the active region represented by the spot. Removing the effects of rotational modulation from the flare light curve allowed us to correct the estimation of bolometric energy released during the event from (1.15 ± 0.35) × 1035 erg to (3.99 ± 1.22) × 1035 erg. [ABSTRACT FROM AUTHOR]