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In our former paper I, we showed on the Sun that different active regions possess unique intensity profiles on the Ca  ii H & K lines. We now extend the analysis by showing how those properties can be used on real stellar observations, delivering more powerful activity proxies for radial velocity correction. More information can be extracted on rotational time-scale from the Ca  ii   H & K lines than the classical indicators: S-index and |$\log (R^{^{\prime }}_{\text{HK}})$|⁠. For high-resolution HARPS observations of |$\alpha$| Cen B, we apply a principal and independent component analysis on the Ca  ii   H & K spectra time-series to disentangle the different sources that contribute to the disc-integrated line profiles. While the first component can be understood as a denoised version of the Mount-Wilson S-index, the second component appears as powerful activity proxies to correct the radial velocities induced by the inhibition of the convective blueshift in stellar active regions. However, we failed to interpret the extracted component into a physical framework. We conclude that a more complex kernel or bandpass than the classical triangular of the Mount Wilson convention should be used to extract activity proxies. To this regard, we provide the first principal component activity profile obtained across the spectral type sequence between M1V to F9V type 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]

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]

Context. The properties of nearby stars bear the imprint of the formation and evolution of the Milky Way (MW). Reconstructing its history requires the determination of precise ages for large samples of stars over long periods. Aims. The present study aims to address the evolution of the MW disc in the region where the Sun and nearby Sun-like stars formed. Methods. The evolution of the disc composition in that region during the last 6 Gyr was inferred from the mean abundances of various chemical elements in nearby Sun-like stars. Their age was estimated from their mean chromospheric activity index using an empirical age–activity relationship derived from stellar rotation period measurements in intermediate-age open clusters. The mean abundances versus age of the sample stars were compared with chemical evolution models of metal-rich gaseous discs experiencing an infall of pristine gas after a quenching period of star formation. Results. The chemical composition of the sample stars reveals two distinct evolutionary trends. Light α elements and iron-peak elements show increasing abundances relative to iron with age. In contrast, the abundance ratios of s-process elements decay with age. Models that best fit the mean abundances of the sample stars as a function of age concur to a gas infall and a concomitant burst of star formation that occurred between 6.2 and 5.5 Gyr ago. Conclusions. This timeline is consistent with a scenario where the first close pericentric passage of the Sagittarius (Sgr) dwarf galaxy ~5.7 Gyr ago induced an infall of metal-poor gas onto the MW disc and a major burst of star formation. The most massive stars that formed in this event rapidly released α elements via type II supernovae explosions, while intermediate-mass stars returned s-process elements on much longer timescales. The first encounter of the Sgr galaxy with the MW played an important role in determining the long-term evolution of the interstellar medium (ISM) composition in the region of the disc where the Sun and Sun-like stars formed, thus explaining the observed correlations between their chemical abundances and their age. [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]

The precise measurement of stellar abundances plays a pivotal role in providing constraints on the chemical evolution of the Galaxy. However, before spectral lines can be employed as reliable abundance indicators, particularly for challenging elements such as helium, they must undergo thorough scrutiny. Galactic open clusters, representing well-defined single stellar populations, offer an ideal setting for unfolding the information stored in the helium spectral line feature. In this study, we characterise the profile and strength of the helium transition at around 10 830 Å (He 10 830) in nine giant stars in the Galactic open cluster Stock 2. To remove the influence of weak blending lines near the helium feature, we calibrated their oscillator strengths (log 푔f) by employing corresponding abundances obtained from simultaneously observed optical spectra. Our observations reveal that the He 10 830 in all the targets is observed in absorption, with line strengths categorised into two groups. Three stars exhibit strong absorption, including a discernible secondary component, while the remaining stars exhibit weaker absorption. The lines are in symmetry and align with or near their rest wavelengths, suggesting a stable upper chromosphere without a significant systematic mass motion. We find a correlation between the He 10 830 strength and the Ca II log R′HK index, with a slope similar to that reported in previous studies on dwarf stars. This correlation underscores the necessity of accounting for the stellar chromosphere structure when employing He 10 830 as a probe for the stellar helium abundance. The procedure of measuring the He 10 830 we developed in this study is applicable not only to other Galactic open clusters but also to field stars, and we plan to use it to map the helium abundance across various types of stars in the future. [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]

Among highly irradiated exoplanets, some have been found to undergo significant hydrodynamic expansion traced by atmospheric escape. To better understand these processes in the context of planetary evolution, we propose NIGHT (the Near-Infrared Gatherer of Helium Transits). NIGHT is a high-resolution spectrograph dedicated to surveying and temporally monitoring He  i triplet absorption at 1083 nm in stellar and planetary atmospheres. In this paper, we outline our scientific objectives, requirements, and cost-efficient design. Our simulations, based on previous detections and modelling using the current exoplanet population, determine our requirements and survey targets. With a spectral resolution of 70 000 on a 2-m telescope, NIGHT can accurately resolve the helium triplet and detect 1 per cent peak absorption in 118 known exoplanets in a single transit. Additionally, it can search for 3σ temporal variations of 0.4 per cent in 66 exoplanets in-between two transits. These are conservative estimates considering the ongoing detections of transiting planets amenable to atmospheric characterization. We find that instrumental stability at 40 m s−1, less stringent than for radial velocity monitoring, is sufficient for transmission spectroscopy in He  i. As such, NIGHT can utilize mostly off-the-shelf components, ensuring cost-efficiency. A fibre-fed system allows for flexibility as a visitor instrument on a variety of telescopes, making it ideal for follow-up observations after JWST or ground-based detections. Over a few years of surveying, NIGHT could offer detailed insights into the mechanisms shaping the hot Neptune desert and close-in planet population by significantly expanding the statistical sample of planets with known evaporating atmospheres. First light is expected in 2024. [ABSTRACT FROM AUTHOR]

Aims. Stellar activity cycles and rotation periods are important parameters for characterising the stellar dynamo, which operates in late-type main-sequence stars. However, the number of stars with well-known cycle and rotation periods is rather low, so new detections are still important. Methods. To find activity cycles and rotation periods, we utilised the TIGRE telescope to monitor stars for periodic variations in chromospheric activity indicators. We employed the widely used CaII H&K lines and the CaII infrared triplet lines as stellar activity indicators. To verify a periodic variation and to determine the corresponding period, we performed a frequency analysis via the generalised Lomb-Scargle method of the taken time series. Results. We studied CaII data of the G5V star HD 111395 and derive an activity cycle period of 949 ± 5 d (≈2.6 yr). This cycle is coincident with coronal measurements from the X-ray telescope eROSITA on board SRG. Furthermore, the TIGRE CaII time series show a long-term trend that indicates an additional long-term cycle. Using the few available literature S-index data points, we estimate a probable cycle length of 12–15 yr for this potential long-term cycle. Finally, we determined rotation periods from each observation season. We computed a mean rotation period of 16.76 ± 0.36 d averaged over all observation seasons and chromospheric indicators. However, we also find a strong variation in the mean seasonal rotation periods, which follows the derived cycle period; therefore, we interpret this behaviour as a sign of surface differential rotation. [ABSTRACT FROM AUTHOR]

The detection of Earth-like planets with the radial-velocity (RV) method is currently limited by the presence of stellar activity signatures. On rotational time-scales, spots and plages (or faculae) are known to introduce different RV signals, but their corrections require better activity proxies. The best-known chromospheric activity proxies in the visible are the Ca ii H&K lines, but the physical quantities measured by their profiles need to be clarified. We first investigate resolved images of the Sun in order to better understand the spectrum of plages, spots, and the network using the Meudon spectroheliogram. We show that distinct line profiles are produced by plages, spots, and by the network component and we also derived the centre-to-limb variations of the three profiles. Some care is required to disentangle their contributions due to their similarities. By combining disc-integrated spectra from the ISS high-resolution spectrograph with SDO direct images of the Sun, we managed to extract a high-resolution emission spectrum of the different components, which tend to confirm the spectra extracted from the Meudon spectroheliogram datacubes. Similar results were obtained with the HARPS-N Sun-as-a-star spectra. We concluded using a three-component model that the temporal variation of the popular S index contains, on average for the 24th solar cycle: |$70\pm 12{{\ \rm per\ cent}}$| of plage, |$26\pm 12{{\ \rm per\ cent}}$| of network, and |$4\pm 4 {{\ \rm per\ cent}}$| of spots. This preliminary investigation suggests that a detailed study of the Ca ii H&K profiles may provide rich information about the filling factor and distribution of different types of active regions. [ABSTRACT FROM AUTHOR]

The magnetic field of the Sun is generated by internal dynamo process with a cyclic period of 11 years or a 22 year magnetic cycle. The signatures of the Sun's magnetic cycle are observed in the different layers of its atmosphere and in its internal layers. In this review, we use the same diagnostics to understand the magnetic cycles of other stars with the same internal structure as the Sun. We review what is currently known about mapping the surface magnetic fields, chromospheric and coronal indicators, cycles in photometry and asteroseismology. We conclude our review with an outlook for the future. [ABSTRACT FROM AUTHOR]

We present a study of high-resolution spectra of RS Canum Venaticorum (RS CVn), a prototype of active binary systems. Our data were obtained from 1998 to 2017 using different telescopes. We analyse the chromospheric activity indicators |$\rm{Ca~{\small II}}$| IRT, Hα, |$\rm{Na~{\small I}}$| D1, D2 doublet, |$\rm{He~{\small I}}$| D3, and Hβ using a spectral subtraction technique. The chromospheric emission stems mainly from the K2 IV primary star, while the F5 V secondary star only shows weak emission features in a few of our spectra. We find excess absorption features in the subtracted Hα lines and other activity indicators from spectra taken near primary eclipse, which we ascribe to prominence-like material associated with the primary star. We estimate size limits of these tentative prominences based on the geometry of the binary system, and investigate the physical properties of the strongest prominence. An optical flare, characterized by |$\rm{He~{\small I}}$| D3 line emission, together with stronger emission in other activity lines, was detected. The flare energy is roughly comparable to strong flares observed on other RS CVn-type stars. The chromospherically active longitudes of RS CVn most frequently appear near the two quadratures of the system and display changes between observing runs, which indicates an ongoing evolution of its active regions. [ABSTRACT FROM AUTHOR]

Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) aims to detect fast-evolving transients in the Universe, which has led to the discovery of thousands of short-period variables and eclipsing binaries since 2020. In this paper, we present the observed properties of 125 flare stars identified by TMTS within the first two years, with an attempt to constrain their eruption physics. As expected, most of these flares were recorded in late-type red stars with G BP − G RP >2.0 mag; however, the flares associated with bluer stars tend to be on average more energetic and have broader profiles. The peak flux (F peak) of the flare is found to depend strongly on the equivalent duration (ED) of the energy release, i.e. F peak∝ED0.72 ± 0.04, which is consistent with results derived from the Kepler and Evryscope samples. This relation is likely to be related to the magnetic loop emission, while, for the more popular non-thermal electron heating model, a specific time evolution may be required to generate this relation. We notice that flares produced by hotter stars have a flatter F peak - ED relation compared to that from cooler stars. This is related to the statistical discrepancy in light-curve shape of flare events with different colours. In spectra from LAMOST, we find that flare stars have apparently stronger H α emission than inactive stars, especially at the low-temperature end, suggesting that chromospheric activity plays an important role in producing flares. On the other hand, the subclass with frequent flares is found to show H α emission of similar strength in its spectra to that recorded with only a single flare but similar effective temperature, implying that chromospheric activity may not be the only trigger for eruptions. [ABSTRACT FROM AUTHOR]

ASCC 123 is a little-studied young and dispersed open cluster. Recently, we conducted the first research devoted to it. In this paper, we complement our previous work with Transiting Exoplanet Survey Satellite (TESS) photometry for the 55 likely members of the cluster. We pay special attention to seven of these high-probability members, all with FGK spectral types, for which we have high-resolution spectra from our preceding work. By studying the TESS light curves of the cluster members, we determine the rotational period and the amplitude of the rotational modulation for 29 objects. The analysis of the distribution of the periods allows us to estimate a gyrochronological age for ASCC 123 similar to that of the Pleiades, confirming the value obtained in our previous investigation. A young cluster age is also suggested by the distribution of variation amplitudes. In addition, for those stars with spectroscopic data, we calculate the inclination of their rotation axis. These values appear to follow a random distribution, as already observed in young clusters, with no indication of spin alignment. However, our sample is too small to confirm this on more solid statistical grounds. Finally, for these seven stars, we study the level of magnetic activity from the Hα and Ca  ii H&K lines. Despite the small number of data points, we find a correlation of the Hα and Ca  ii flux with Rossby number. The position of these stars in flux–flux diagrams follows the general trends observed in other active late-type stars. [ABSTRACT FROM AUTHOR]

We present optimized physical parameters of the active eclipsing binary system USNO-B1.0 1387-0467554 discovered recently in Yunnan–Hong Kong wide-field photometric (YNHK) survey, based on the analysis of its two colour light curves and low resolution spectra obtained at Yunnan Observatories, China. Its spot distributions are derived by means of the Wilson–Devinney code, using photometric data of the YNHK survey from 2016 to 2021. There exist active longitude belts for the spot activities on the primary star, for which the effect of tidal force seems dominant comparing with the one of common magnetic field of the binary system. We have investigated its chromospheric activities through the indicators including Hα, Hβ, CaII H&K, and HeI D3 lines covered by the low resolution spectroscopic observations. There are obvious spatial correlations between the magnetic activities in its photosphere and chromosphere. The E H α/ E H β values suggest that both plages and prominences led to the emissions seen in the Balmer lines at most phases. A prominence structure appeared around the phase of 0.8 on 2018 October 25 and evolved in November and December, 2018. The spectra demonstrate that there were frequent flare events in the system. Moreover, the decay part of a white-light flare event was hunted by the YNHK survey on 2021 October 26. [ABSTRACT FROM AUTHOR]

We report the results of intensive monitoring of the variability in the Hα line for two F-type stars, τ Boo and υ And, over the last four years 2019–2022, in order to investigate their stellar magnetic activity. The 4 yr Hα line intensity data taken with the 1.88 m reflector at the Okayama Branch Office, Subaru Telescope, shows the existence of a possible ∼123 d magnetic activity cycle for τ Boo. The result of the Hα variability as another tracer of the magnetic activity on the chromosphere is consistent with previous studies of the Ca  ii H&K line and suggests that the magnetic activity cycle persists in τ Boo. For υ And, we suggest a quadratic long-term trend in the Hα variability. Meanwhile, the short-term monitoring shows no significant period corresponding to specific variations likely induced by their hot Jupiter in both cases (≈3.31 and 4.62 d, respectively). In this Hα observation, we could not find any signature of the star–planet magnetic interaction. It is speculated that the detected magnetic activity variability of the two F-type stars is related to the stellar intrinsic dynamo. [ABSTRACT FROM AUTHOR]

Eridani is a highly active young K2 star with an activity cycle of about 3 yr established using Ca II H& K line index measurements (SMWO). This relatively short cycle has been demonstrated to be consistent with X-ray and magnetic flux measurements. Recent work suggested a change in the cyclic behaviour. Here, we report new X-ray flux and SMWO measurements and also include SMWO measurements from the historical Mount Wilson program. This results in an observational time baseline of over 50 yr for the SMWO data and of over 7 yr in X-rays. Moreover, we include Ca II infrared triplet (IRT) index measurements (SCa IRT) from 2013 to 2022 in our study. With the extended X-ray data set, we can now detect the short cycle for the first time using a periodogram analysis. Near-simultaneous SMWO data and X-ray fluxes, which are offset by 20 days at most, are moderately strongly correlated when only the lowest activity state (concerning short-term variability) is considered in both diagnostics. In the SMWO data, we find strong evidence for a much longer cycle of about 34 yr and an 11-yr cycle instead of the formerly proposed 12-yr cycle in addition to the known 3-yr cycle. The superposition of the three periods naturally explains the recent drop in SMWO measurements. The two shorter cycles are also detected in the SCa IRT data, although the activity cycles exhibit lower amplitudes in the SCa IRT than in the SMWO data. Finally, the rotation period of Eri can be found more frequently in the SMWO as well as in the SCa IRT data for times near the minimum of the long cycle. This may be explained by a scenario in which the filling factor for magnetically active regions near cycle maximum is too high to allow for notable short-term variations. [ABSTRACT FROM AUTHOR]

Context. Precision radial velocity (RV) measurements continue to be a key tool for detecting and characterising extrasolar planets. While instrumental precision keeps improving, stellar activity remains a barrier to obtaining reliable measurements below 1-2ms-1 accuracy. Aims. Using simulations and real data, we investigate the capabilities of a deep neural network approach to producing activity-free Doppler measurements of stars. Methods. As case studies we used observations of two known stars, Eridani and AUMicroscopii, both of which have clear signals of activity-induced Doppler variability. Synthetic observations using the starsim code were generated for the observables (inputs) and the resulting Doppler signal (labels), and then they were used to train a deep neural network algorithm to predict Doppler corrections. We identified a relatively simple architecture, consisting of convolutional layers followed by fully connected layers, that is adequate for the task. The indices investigated are mean line-profile parameters (width, bisector, and contrast) and multi-band photometry. Results. We demonstrate that the RV-independent approach can drastically reduce spurious Doppler variability from known physical effects, such as spots, rotation, and convective blueshift. We identify the combinations of activity indices with the most predictive power. When applied to real observations, we observe a good match of the correction with the observed variability, but we also find that the noise reduction is not as good as in the simulations, probably due to a lack of detail in the simulated physics. Conclusions. We demonstrate that a model-driven machine learning approach is sufficient to clean Doppler signals from activityinduced variability for well-known physical effects. There are dozens of known activity-related observables whose inversion power remains unexplored, indicating that the use of additional indicators, more complete models, and more observations with optimised sampling strategies can lead to significant improvements in our detrending capabilities for new and existing datasets. [ABSTRACT FROM AUTHOR]

Aims. In a previous publication, we reported the first detection of emission in the infrared (IR) triplet at 10 830 Å in the spectra of the classical Cepheid X Cyg. Emission is detectable at phases from approximately 0.25 to 0.90. We present and discuss further information on the chromosphere activity of this star, namely International Ultraviolet Explorer (IUE) archive data concerning the emission in the h and k Mg II doublet, and near-ultraviolet (NUV) spectra in the vicinity of the H and K Ca II doublet. We also present rough estimates of the chromosphere conditions and semi-empirical modelling of the observed emissions in He I triplet. Methods. A study of the emissions in UV lines of Ca II and Mg II, and IR lines of He I suggests that the emissions observed at certain phases can be explained within the framework of the Gillet's phenomenological model of shock waves in pulsating atmospheres, which was developed from a study of the Hα behaviour in X Cyg spectra. We used the non-local thermodynamic equilibrium (NLTE) approximation and a simple model of this star's chromosphere for the analysis of the chromosphere indicator profiles. Results. We show that under certain assumptions about the properties of the chromosphere, it is possible to describe the presence or absence of emission in the IR lines of He I and Ca II, and UV lines of Ca II and Mg II. [ABSTRACT FROM AUTHOR]

We characterize twelve young stellar objects (YSOs) located in the Lupus I region, spatially overlapping with the Upper Centaurus Lupus (UCL) sub-stellar association. The aim of this study is to understand whether the Lupus I cloud has more members than what has been claimed so far in the literature and gain a deeper insight into the global properties of the region. We selected our targets using the Gaia DR2 catalog based on their consistent kinematic properties with the Lupus I bona fide members. In our sample of twelve YSOs observed by X-shooter, we identified ten Lupus I members. We could not determine the membership status of two of our targets, namely Gaia DR2 6014269268967059840 and 2MASS J15361110-3444473 due to technical issues. We found out that four of our targets are accretors, among them, 2MASS J15551027-3455045, with a mass of ∼0.03 M⊙, is one of the least massive accretors in the Lupus complex identified to date. Several of our targets (including accretors) are formed in situ and off-cloud with respect to the main filaments of Lupus I; hence, our study may hint that there are diffused populations of M dwarfs around Lupus I main filaments. In this context, we would like to emphasize that our kinematic analysis with Gaia catalogs played a key role in identifying the new members of the Lupus I cloud. [ABSTRACT FROM AUTHOR]

Context. With the recent surge of planetary surveys focusing on detecting Earth-mass planets around M dwarfs, it is becoming more important to understand chromospheric activity in M dwarfs. Stellar chromospheric calcium emission is typically measured using the R′HK calibrations of Noyes et al. (1984), which are only valid for 0.44 ≤ B – V ≤ 0.82. Measurements of calcium emission for cooler dwarfs B – V ≥ 0.82 are difficult because of their intrinsic dimness in the blue end of the visible spectrum. Aims. We measure the absolute Ca II H & K and Hα flux of a sample of 110 HARPS M dwarfs and also extend the calibration of R′HK to the M dwarf regime using PHOENIX stellar atmosphere models. Methods. We normalized a template spectrum with a high signal-to-noise ratio that was obtained by coadding multiple spectra of the same star to a PHOENIX stellar atmosphere model to measure the chromospheric Ca II H & K and Ha flux in physical units. We used three different Teff calibrations and investigated their effect on Ca II H & K and Hα activity measurements. We performed conversions of the Mount Wilson S index to R′HK as a function of effective temperature for the range 2300 K ≤ Teff ≤ 7200 K. Last, we calculated continuum luminosity χ values for Ca II H & K and Hα in the same manner as West & Hawley (2008) for –1.0 ≤ [Fe/H] ≤ + 1.0 in steps of Δ [Fe/H] = 0.5. Results. We compare different Teff calibrations and find ΔΤeff ~ several 100 K for mid- to late-M dwarfs. Using these different Teff calibrations, we establish a catalog of log R′HK and ℱ′Hα/ℱbol measurements for 110 HARPS M dwarfs. The difference between our results and the calibrations of Noyes et al. (1984) is Δ log R′HK = 0.01 dex for a Sun-like star. Our χ values agree well with those of West & Hawley (2008). We confirm that the lower boundary of chromospheric Ca II H and K activity does not increase toward later-M dwarfs: it either stays constant or decreases, depending on the Teff calibration used. We also confirm that for Ha, the lower boundary of chromospheric flux is in absorption for earlier -M dwarfs and fills into the continuum toward later M dwarfs. Conclusions. We confirm that we can effectively measure R′HK in M dwarfs using template spectra with a high signal-to-noise ratio. We also conclude that our calibrations are a reliable extension of previous R′HK calibrations, and effective temperature calibration is the main source of error in our activity measurements. [ABSTRACT FROM AUTHOR]

We present a magnetic activity study of GJ 436 using spectroscopic data from HARPS, spanning over 14 yr, and additional data from NARVAL, falling within the HARPS observations. We study the Ca  ii H&K, He i D3 , Na  i doublet, H α, and Ca  ii IRT triplets lines and explore linear correlations between them. Using the full HARPS data set, we found indices H α versus Ca  ii H&K & H α versus He  i to correlate positively. From the NARVAL data set, covering one observing epoch, we found Ca  ii IRT3 versus Ca  ii IRT2 & Ca  ii IRT3 versus H α index to correlate negatively. We investigate long and short-term periodicity in these index variations, using the Generalized Lomb–Scargle periodogram. For Ca  ii H&K, Na  i and H α indices, we detect long-term periods of 2470.7 d (∼ 6.8 yr), 1861.6 d (∼ 5.1 yr), and 2160.9 d (∼ 5.9 yr) respectively, consistent with GJ 436's photometric cycle of ∼ 7.4 yr. Applying the 'Pooled Variance' technique to H α & Na  i indices, we found ∼ 2500 d to be the period of an activity cycle mechanism, in good agreement with the detected 2470.7 d period. For Ca  ii H&K and H α indices, we detect short-term periods of |$39.47^{+0.11}_{-0.15}$|  d and |$40.46^{+0.44}_{-0.52}$|  d, respectively, identified as the stellar rotation period. The stellar rotation is detected after pre-whitening the long-term periodicity. It is detected as well in the analysis of individual observing epochs. [ABSTRACT FROM AUTHOR]

We examine the acoustic and magnetic energy generation and propagation in β Hydri (G2 IV). The underlying motivation for this work is based on the solar, stellar, and galactic relevance of β Hyi (a star in the Southern hemisphere), which is readily understood as a prime example and proxy of the future Sun – thus allowing assessments and analyses of the secular decay of solar activity. Regarding the magnetic energy generation, we consider longitudinal flux tube waves. We also assess acoustic waves. For the acoustic wave energy flux, the difference between the results obtained for β Hyi and the Sun is significantly smaller than typically attained for main-sequence stars, which is largely due to the gravity-dependence of the acoustic energy generation. Furthermore, we study the height-dependent behavior of the magnetic energy flux for different magnetic filling factors corresponding to different flux tube spreadings. Finally, we comment on possible directions of future research. [ABSTRACT FROM AUTHOR]

Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots and show strong chromospheric emission lines because of their strong surface magnetic field. We discuss the dynamo activities of ZAMS stars with respect to their periodic light variation caused by a starspot and with respect to the strength of the chromospheric emission lines. The light curves of 33 ZAMS stars in IC 2391 and IC 2602 were obtained from TESS (Transiting Exoplanet Survey Satellite) photometric data. The light curves can be grouped into the following four categories: single frequency, possible shape changer, beater, and complex variability. The amplitudes of the light curves are 0.001–0.145 mag, similar to those of ZAMS stars in Pleiades. The starspot coverages are |$0.1\%$| – |$21\%$|⁠. We found that the light variations and Ca  ii emission line strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as large as those of the most active superflare stars and two orders larger than those of the Sun, and are located on the extensions of the superflare stars. These results suggest that superflare stars link the properties of the Sun to those of the ZAMS stars of ages between 30 and 120 Myr. ZAMS stars with a single frequency or possible shape change in the light curve tend to have both large light variation, indicating large spot coverage, and saturated Ca  ii emission line strength. ZAMS stars with beat or complex variability have small spot coverage and a faint Ca  ii emission line. We also detected 21 flares in the TESS light curves of 12 ZAMS stars in IC 2391 and IC 2602, where most of these stars have saturated chromospheric Ca  ii emission lines. The energies of the flares are estimated to be ∼1033–1035 erg, which is comparable with the energy of a superflare. [ABSTRACT FROM AUTHOR]

In this paper, we present a spectroscopic study of six double-lined binaries, five of which were recently discovered in a high-resolution spectroscopic survey of optical counterparts of stellar X-ray sources. Thanks to high-resolution spectra acquired with CAOS spectropolarimeter during 7 yr, we were able to measure the radial velocities of their components and determine their orbital elements. We have applied our code compo2 to determine the spectral types and atmospheric parameters of the components of these spectroscopic binaries and found that two of these systems are composed of main-sequence stars, while the other four contain at least one evolved (giant or subgiant) component, similar to other well-known RS CVn systems. The subtraction of a photospheric template built up with spectra of non-active stars of the same spectral type as those of the components of each system has allowed us to investigate the chromospheric emission that fills in the H α cores. We found that the colder component is normally the one with the largest H α emission. None of the systems show a detectable Li  i λ6708 line, with the exception of TYC 4279-1821-1, which exhibits high photospheric abundances in both components. Photometric time-series from the literature allowed us to assess that the five systems with a nearly circular orbit have also photometric periods close or equal to the orbital ones, indicating spin–orbit synchronization. For the system with a highly eccentric orbit, a possible pseudo-synchronization with the periastron velocity is suggested. [ABSTRACT FROM AUTHOR]

We apply the variational autoencoder (VAE) to the LAMOST-K2 low-resolution spectra to detect the magnetic activity of the stars in the K2 field. After the training on the spectra of the selected inactive stars, the VAE model can efficiently generate the synthetic reference templates needed by the spectral subtraction procedure, without knowing any stellar parameters. Then, we detect the peculiar spectral features, such as chromospheric emissions, strong nebular emissions, and lithium absorptions, in our sample. We measure the emissions of the chromospheric activity indicators, Hα and Ca  ii infrared triplet (IRT) lines, to quantify the stellar magnetic activity. The excess emissions of Hα and Ca  ii IRT lines of the active stars are correlated well to the rotational periods and the amplitudes of light curves derived from the K2 photometry. We degrade the LAMOST spectra to simulate the slitless spectra of the China Space Station Telescope (CSST) and apply the VAE to the simulated data. For cool active stars, we reveal a good agreement between the equivalent widths of Hα line derived from the spectra with two resolutions. The result indicates the ability of identifying the magnetically active stars in the future CSST survey, which will deliver an unprecedented large data base of low-resolution spectra as well as simultaneous multiband photometry of stars. [ABSTRACT FROM AUTHOR]

We present the study on continuous high-resolution spectroscopic observations of two long-period single-lined RS Canum Venaticorum (RS CVn) binary stars IM Pegasi (IM Peg) and σ Geminorum (σ Gem), obtained with the Hertzsprung SONG telescope during the 2015–2016 season. Chromospheric activity indicators Hα, |$\rm{Na\,\,{\small I}}$| D1, D2 doublet, |$\rm{He\,\,{\small I}}$| D3, and Hβ lines have been analysed by using the spectral subtraction technique. The expected chromospheric emission features in the Hα, |$\rm{Na\,\,{\small I}}$| D1, D2 doublet, and Hβ lines confirm that both of two stars are very active systems. In the spectra, the |$\rm{He\,\,{\small I}}$| D3 line had been always detected in absorption feature. Although the behaviour of chromospheric activity indicators is very similar for both stars, the activity level of IM Peg is much stronger than that of σ Gem. Moreover, the equivalent width variations of the Hα, |$\rm{He\,\,{\small I}}$| D3, and Hβ line subtractions correlate well and show different behaviour among different orbital cycles, which indicates the presence and evolution of activity longitudes over the surface of two stars. Furthermore, the subtracted Hα line profile is usually asymmetric. The red-shifted excess absorption features could be interpreted as a strong down-flow of cool absorbing material, while the blue-shifted emission component is probably caused by up-flow of hot materials through microflare events. [ABSTRACT FROM AUTHOR]

We present an analysis of the starspot properties and chromospheric activity on HD 134319 using high-precision photometry by Transiting Exoplanet Survey Satellite (TESS) in sectors 14–16 (T1) and 21–23 (T2) and high-resolution spectroscopy by the ELODIE spectrograph on 1.93-m telescope of Observatoire de Haute-Provence (OHP/ELODIE) and the High Resolution Echelle Spectrometer on Keck 10-m telescope of W. M. Keck Observatory (Keck/HIRES) during the years 1995–2013. We applied a two-spot model with generalized Lomb–Scargle (GLS) periodogram determined period of P  = 4.436391 ± 0.00137 d to model chunks sliding over TESS light curve. We also measured the relative equivalent widths of Ca ii H and K, Hβ and Hα emissions by subtracting the overall spectrum from individual spectra. It was found that a two-spot configuration, namely a primary, slowly evolving and long-lasting spot (P) plus a secondary, rapidly evolving spot (S), was capable of explaining the data, although the actual starspot distribution cannot be derived from the collected data. Despite the spot radius–latitude degeneracy revealed in the best-fitting solutions, a sudden alternation between P and S radii followed by a gradual decrease of S in T1 and a decrease of both P and S from T1 to T2 were significant, corresponding to the evolution of magnetic activity. In addition, S revealed rotation and an oscillatory longitude migration synchronized to P in T1, but had a much larger migration than P in T2. This might indicate the evolution of the internal magnetic configuration. Chromospheric activity indicators were found to be tightly correlated with each other and revealed rotational modulation as well as a long-term decrease of emissions, implying the existence and evolution of magnetic activity on HD 134319. [ABSTRACT FROM AUTHOR]

We have discovered a young, rapidly rotating active star USNO-B1.0 1388−0463685 with a rotational period of 0.508750 ± 0.000153 d in the Yunnan–Hong Kong wide-field photometric survey. Based on the chromospheric activity identified in the dedicated spectroscopic observations, the brightness variation of this star is attributed to cool spots on its photosphere. We derived the spot maps from 14 light curves, which demonstrate that the strong spot tends to appear in the same hemisphere, and the light curve amplitude varies with a period of about 2.4 yr. Low-resolution spectroscopic observations revealed the chromospheric activity of the star through the indicators Hα, Na  i D1 and D2, He  i D3, and Ca  ii H & K lines. Moreover, there was a clear anticorrelation between the equivalent width variation of the Hα, Na  i D1 and D2 lines and the simultaneous light curve, implying the photosphere and chromosphere connection. Using the spectral subtraction technique, the spectral type of the star was determined as K0–2V and its projected rotational velocity v  sin  i is 107.492 ± 7.388 km s−1. Both the lithium equivalent width of 254.0 ± 43.3 mÅ and its location on the Hertzsprung–Russell diagram indicate that this star should be in the stage from late pre-main sequence to zero-age main sequence. [ABSTRACT FROM AUTHOR]

As part of an All-Sky Automated Survey for SuperNovae (ASAS-SN) search for sources with large flux decrements, we discovered a transient where the quiescent, stellar source ASASSN-V J192114.84+624950.8 rapidly decreased in flux by |$\sim 55{{\ \rm per\ cent}}$| (∼0.9 mag) in the g band. The Transiting Exoplanet Survey Satellite light curve revealed that the source is a highly eccentric, eclipsing binary. Fits to the light curve using phoebe find the binary orbit to have e  = 0.79, P orb = 18.462 d, and i  = 88.6°, and the ratios of the stellar radii and temperatures to be R 2/ R 1 = 0.71 and T e,2/ T e,1 = 0.82. Both stars are chromospherically active, allowing us to determine their rotational periods of P 1 = 1.52 d and P 2 = 1.79 d, respectively. A Large Binocular Telescope/Multi-Object Double Spectrograph spectrum shows that the primary is a late-G- or early-K-type dwarf. Fits to the spectral energy distribution show that the luminosities and temperatures of the two stars are L 1 = 0.48 L⊙, |$T_1= 5050\, \mathrm{K}$|⁠ , L 2 = 0.12 L⊙, and |$T_{2} = 4190\, \mathrm{K}$|⁠. We conclude that ASASSN-V J192114.84+624950.8 consists of two chromospherically active, rotational variable stars in a highly elliptical eclipsing orbit. [ABSTRACT FROM AUTHOR]

From the Sun, we know that coronal mass ejections (CMEs) are a transient phenomenon, often correlated with flares. They have an impact on solar mass- and angular momentum loss, and therefore solar evolution, and make a significant part of space weather. The same is true for stars, but stellar CMEs are still not well constrained, although new methodologies have been established, and new detections presented in the recent past. So far, probable detections of stellar CMEs have been presented, but their physical parameters which are not directly accessible from observations, such as electron density, optical thickness, temperature, etc. have been so far not determined for the majority of known events. We apply cloud modeling, as commonly used on the Sun, to a known event from the literature, detected on the young dMe star V374 Peg. This event manifests itself in extra emission on the blue side of the Balmer lines. By determining the line source function from 1D NLTE modeling together with the cloud model formulation, we present distributions of physical parameters of this event. We find that except for temperature and area all parameters are at the upper range of typical solar prominence parameters. The temperature and the area of the event were found to be higher than for typical solar prominences observed in Balmer lines. We find more solutions for the filament than for the prominence geometry. Moreover, we show that filaments can appear in emission on dMe stars contrary to the solar case. [ABSTRACT FROM AUTHOR]

We present a sample of 32 stars of spectral types G and K and luminosity classes I–V, with moderate activity levels, covering four orders of magnitude of surface gravity and a representative range of effective temperature. For each star we obtained high signal-to-noise ratio (S/N) spectra from the Telescopio Internacional de Guanajuato Robótico-Espectroscópico (TIGRE–HEROS) with a spectral resolving power of |$R\approx 20\, 000$| and measured the Ca  ii K line widths of interest, W 0 and W 1. The main physical parameters are determined by means of iSpec synthesis and Gaia EDR3 parallaxes. Mass estimates are based on matching to evolution models. Using this stellar sample, which is highly uniform in terms of spectral quality and assessment, we derive the best-fitting relation between emission-line width and gravity g , including a notable dependence on effective temperature T eff, of the form |$W_1 \propto g^{-0.229} T_{\rm eff}^{+2.41}$|⁠. This result confirms the physical interpretation of the Wilson–Bappu effect as a line saturation and photon redistribution effect in the chromospheric Ca  ii column density, under the assumption of hydrostatic equilibrium at the bottom of the chromosphere. While the column density (and hence W 1) increases towards lower gravities, the observed temperature dependence is then understood as a simple ionization effect: in cooler stars, Ca  ii densities decrease in favour of Ca  i. [ABSTRACT FROM AUTHOR]

There are several strong K I lines found in the spectra of M dwarfs, among them the doublet near 7700 Å and another doublet near 12 500 Å. We study these optical and near-infrared doublets in a sample of 324 M dwarfs, observed with CARMENES, the high-resolution optical and near-infrared spectrograph at Calar Alto, and investigate how well the lines can be used as photospheric and chromospheric diagnostics. Both doublets have a dominant photospheric component in inactive stars and can be used as tracers of effective temperature and gravity. For variability studies using the optical doublet, we concentrate on the red line component because this is less prone to artefacts from telluric correction in individual spectra. The optical doublet lines are sensitive to activity, especially for M dwarfs later than M5.0 V where the lines develop an emission core. For earlier type M dwarfs, the red component of the optical doublet lines is also correlated with Hα activity. We usually find positive correlation for stars with Hα in emission, while early-type M stars with Hα in absorption show anti-correlation. During flares, the optical doublet lines can exhibit strong fill-in or emission cores for our latest spectral types. On the other hand, the near-infrared doublet lines very rarely show correlation or anti-correlation to Hα and do not change line shape significantly even during the strongest observed flares. Nevertheless, the near-infrared doublet lines show notable resolved Zeeman splitting for about 20 active stars which allows to estimate the magnetic fields B. [ABSTRACT FROM AUTHOR]

Based on the high signal-to-noise ratio (S/N) and high-dispersion spectroscopic observations, we derived the equivalent width of the Li I (λ = 6707.8 Å) and the lithium abundance for 9 active late-type stars. The correlation between the lithium abundance and the X-ray luminosity (lg L X) indicates that the lithium abundance of these 9 active late-type stars with a higher X-ray luminosity tends to be larger than that of those with lower X-ray luminosity. That is to say that the lithium abundances analogs progressively increase as the activity increases for these late-type stars. Considering both the lithium abundance and the rotational velocity decrease with increasing stellar age for main-sequence stars, and activity increasing with increasing rotation velocity for active late-type stars, like lithium abundance, the rotational velocity and stellar activity are also considered good indicators of age for late-type stars. [ABSTRACT FROM AUTHOR]

For the first time, we describe a detection of the emission in the infrared (IR) He  i triplet at 10 830 Å in the classical Cepheid X Cyg. Emissions are clearly seen at phases approximately from 0.25 to 0.85. The IR redshifted He  i emission is excited by shock wave in the uppermost layers of the star's envelope, in its falling layers. This is a first detection of the helium IR emission in the classical Cepheids. [ABSTRACT FROM AUTHOR]

In this study, we consider chromospheric heating models for 55 Cancri in conjunction with observations. The theoretical models, previously discussed in Paper I , are self-consistent, non-linear, and time-dependent ab-initio computations encompassing the generation, propagation, and dissipation of waves. Our focus is the consideration of both acoustic waves and longitudinal flux tube waves amounting to two-component chromosphere models. 55 Cancri, a K-type orange dwarf, is a star of low activity, as expected by its age, which also implies a relatively small magnetic filling factor. The Ca  ii  K fluxes are computed (multi-ray treatment) assuming partial redistribution and time-dependent ionization. The theoretical Ca  ii H+K fluxes are subsequently compared with observations. It is found that for stages of lowest chromospheric activity the observed Ca  ii fluxes are akin, though not identical, to those obtained by acoustic heating, but agreement can be obtained if low levels of magnetic heating – consistent with the expected photospheric magnetic filling factor – are considered as an additional component; this idea is in alignment with previous proposals conveyed in the literature. [ABSTRACT FROM AUTHOR]

Long-term stellar activity variations can affect the detectability of long-period and Earth-analogue extrasolar planets. We have, for 54 stars, analysed the long-term trend of five activity indicators: log  |$R^{\prime }_\mathrm{{HK}}$|⁠ , the cross-correlation function (CCF) bisector span, CCF full-width-at-half-maximum, CCF contrast, and the area of the Gaussian fit to the CCF; and studied their correlation with the RVs. The sign of the correlations appears to vary as a function of stellar spectral type, and the transition in sign signals a noteworthy change in the stellar activity properties where earlier type stars appear more plage dominated. These transitions become more clearly defined when considered as a function of the convective zone depth. Therefore, it is the convective zone depth (which can be altered by stellar metallicity) that appears to be the underlying fundamental parameter driving the observed activity correlations. In addition, for most of the stars, we find that the RVs become increasingly redshifted as activity levels increase, which can be explained by the increase in the suppression of convective blueshift. However, we also find a minority of stars where the RVs become increasingly blueshifted as activity levels increase. Finally, using the correlation found between activity indicators and RVs, we removed RV signals generated by long-term changes in stellar activity. We find that performing simple cleaning of such long-term signals enables improved planet detection at longer orbital periods. [ABSTRACT FROM AUTHOR]

While most of the exoplanets have been found orbiting around solar-type stars, low-mass stars have recently been recognized as ideal exo-life laboratory. Currently, stellar activity is one of the limiting factors for the characterization of Earth-twins and for assessing their habitability: understanding the activity of M dwarfs is thus crucial. In this contribution I present the spectroscopic analysis of the quiet early-M dwarfs monitored within the HADES (HArps-n red Dwarf Exoplanet Survey) radial velocity survey. The spectra allow us to analyze simultaneously the Ca ii H&K doublet and the Hydrogen Balmer series, while the intensive follow up gives us a large number of spectra (100) for each target. We complement this dataset with ground-based follow-up photometry and archival X-ray data. I present our results on the activity-rotation-stellar parameters and flux-flux relationships, and discuss the correlation of emission fluxes at low activity levels and the evolution timescales of active regions. [ABSTRACT FROM AUTHOR]

We present high-precision light curves of several M- and K-type, active detached eclipsing binaries (DEBs), recorded with 2-minute cadence by the Transiting Exoplanet Survey Satellite (TESS). Analysis of these curves, combined with new and literature radial velocity (RV) data, allows to vastly improve the accuracy and precision of stellar parameters with respect to previous studies of these systems. Results for one previously unpublished DEB are also presented. [ABSTRACT FROM AUTHOR]

We present theoretical models of chromospheric heating for 55 Cancri, an orange dwarf of relatively low activity. Self-consistent, non-linear, and time-dependent ab initio numerical computations are pursued encompassing the generation, propagation, and dissipation of waves. We consider longitudinal waves operating among arrays of flux tubes as well as acoustic waves pertaining to non-magnetic stellar regions. Additionally, flux enhancements for the longitudinal waves are also taken into account as supplied by transverse tube waves. The Ca  ii  K fluxes are computed (multi-ray treatment) assuming partial redistribution as well as time-dependent ionization (TDI). The self-consistent treatment of TDI (especially for hydrogen) greatly impacts the atmospheric temperatures and electron densities (especially behind the shocks); it also affects the emergent Ca  ii fluxes. Particularly, we focus on the influence of magnetic heating on the stellar atmospheric structure and the emergent Ca  ii emission, as well as the impact of non-linearities. Our study shows that a higher photospheric magnetic filling factor entails a larger Ca  ii emission; however, an increased initial wave energy flux (e.g. associated with mode coupling) is of little difference. Comparisons of our theoretical results with observations will be conveyed in forthcoming Paper II. [ABSTRACT FROM AUTHOR]

We present an analysis of the starspot evolution, the surface differential rotation (SDR), the correlation between chromospheric activity indicators and the spatial connection between chromospheric and photospheric activities on the active star Kepler-411, using time-series photometry over four years from Kepler, and spectroscopic data from Keck I 10-m and Lijiang 2.4-m telescopes. We constructed the light curve (LC) by re-performing photometry and reduction from the Target Pixel Files and Cotrending Basis Vectors with a manually redefined aperture using the software pyke 3. An efficient program, gemc_lcm , was developed to apply a two-spots model to chosen LC segments with three spot groups at fixed latitudes (30○, 45○), (30○, 60○) and (45○, 60○). We found a periodic variation of the starspots at the period of about 660 d which independs on spot latitudes, and estimated the lower limit of SDR rate α  = 0.1016(0.0023) and equatoral rotation period P eq = 9.7810(0.0169) d. Simultaneously, the relative variations of chromospheric activity indicators were derived by subtracting the overall mean spectrum from individual spectrum. It is found that Ca  ii H and K emissions are strongly correlated with each other, and there also exists a correlation between H α and Ca  ii H & K emissions, with large dispersion, in accordance with previous results. Furthermore, we find the correlation between Ca  ii H and K emissions is different in 2011 and 2012. The chromospheric emission variation shows a highly spatial anticorrelation with the LC, suggesting a spatial connection between the chromospheric active region and spot region. [ABSTRACT FROM AUTHOR]

Context. Understanding stellar activity in M dwarfs is fundamental to improving our knowledge of the physics of stellar atmospheres and for planet search programmes. High levels of stellar activity (also frequently associated with flare events) can cause additional variations in the stellar emission that contaminate the signal induced by a planet and that need to be corrected. The study of activity indicators in active stars can improve our capability of modelling the signal generated by magnetic activity. Aims. In this work we present measurements of activity indicators at visible wavelength for a star with a high activity level, AD Leonis, observed with HARPS in 2006, and HARPS-N in 2018. Our aim is to understand the behaviour of stellar chromospheres of M stars, studying the more sensitive chromospheric activity indicators. We also focus on characterising their variability and on finding the correlations among these indicators to obtain information on the origin of the magnetic activity in low-mass stars. Methods. We performed a study of the main optical activity indicators (Ca II H&K, Balmer lines, Na I D1,2 doublet, He I D3, and other helium lines) measured for AD Leonis using the data provided by the HARPS-N high-resolution spectrograph at the Telescopio Nazionale Galileo in 2018, and by the HARPS instrument at La Silla observatory in 2006. Spectra were flux-calibrated in units of flux at the stellar surface. We measured excess flux of the selected activity indicators. The correlations between the different activity indicators as well as the temporal evolution of fluxes were analysed. A stellar flare was identified during the 2018 observing run and the Hα, Hβ, He I 4471 Å, and He I 5876 Å lines were analysed in detail by fitting the line profiles with two Gaussian components. Results. We found that the Ca II H&K flux excesses are strongly correlated with each other, but the Ca II H&K doublet is generally less correlated with the other indicators. Moreover, Hα is correlated with Na I doublet and helium lines. Analysing the time variability of flux of the studied lines, we found a higher level of activity of the star during the observations in 2018 than in 2006, while Ca II H&K showed more intense emission on spectra obtained during the observations in 2006. Thanks to a detailed analysis of selected line profiles, we investigated the flare evaluating the mass motion during the event. [ABSTRACT FROM AUTHOR]

The He I infrared (IR) triplet at 10 830 Å is known as an activity indicator in solar-type stars and has become a primary diagnostic in exoplanetary transmission spectroscopy. He I IR lines are a tracer of the stellar extreme-ultraviolet irradiation from the transition region and corona. We study the variability of the He I triplet lines in a spectral time series of 319 M dwarf stars that was obtained with the CARMENES high-resolution optical and near-infrared spectrograph at Calar Alto. We detect He I IR line variability in 18% of our sample stars, all of which show Hα in emission. Therefore, we find detectable He I variability in 78% of the sub-sample of stars with Hα emission. Detectable variability is strongly concentrated in the latest spectral sub-types, where the He I lines during quiescence are typically weak. The fraction of stars with detectable He I variation remains lower than 10% for stars earlier than M3.0 V, while it exceeds 30% for the later spectral sub-types. Flares are accompanied by particularly pronounced line variations, including strongly broadened lines with red and blue asymmetries. However, we also find evidence for enhanced He I absorption, which is potentially associated with increased high-energy irradiation levels at flare onset. Generally, He I and Hα line variations tend to be correlated, with Hα being the most sensitive indicator in terms of pseudo-equivalent width variation. This makes the He I triplet a favourable target for planetary transmission spectroscopy. [ABSTRACT FROM AUTHOR]

Context. One of the most enigmatic phenomena related to solar activity is the so-called Maunder minimum phase. It consists in the lowest sunspot count ever registered for the Sun and has not been confirmed for other stars to date. Since the spectroscopic observations of stellar activity at the Mount Wilson Observatory, the solar analog HD 43587 has shown a very low and apparently invariant activity level, which makes it a Maunder minimum candidate. Aims. We aim to analyze the chromospheric activity evolution of HD 43587 and its evolutive status, with the intention of unraveling the reasons for this low and flat activity. Methods. We used an activity measurements dataset available in the literature, and computed the S-index from HARPS and NARVAL spectra to infer a cycle period. Additionally, we analyzed the CoRoT light curve of HD 43587, and applied gyrochronology and activity calibrations to determine its rotation period. Finally, based on an evolutionary model and the inferred rotation period, we used the EULAG-MHD code to perform global MHD simulations of HD 43587 to get some insight into its dynamo process. Results. We confirm the almost flat activity profile, with a cycle period Pcyc = 10.44 ± 3.03 yr deduced from the S-index time series, and a long-term trend that might be a period of more than 50 yr. It was impossible to define a rotation period from the light curve, however gyrochronology and activity calibrations allow us to infer an indirect estimate of ¯Prot = 22.6 ± 1.9 P ¯ rot = 22.6 ± 1.9 $ \overline{P}_{\mathrm{rot}} = 22.6 \pm 1.9 $ d. Furthermore, the MHD simulations confirm an oscillatory dynamo with a cycle period in good agreement with the observations and a low level of surface magnetic activity. Conclusions. We conclude that this object might be experiencing a "natural" decrease in magnetic activity as a consequence of its age. Nevertheless, the possibility that HD 43587 is in a Maunder minimum phase cannot be ruled out. [ABSTRACT FROM AUTHOR]

We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050 Å (TiO2) and equivalent width of H α line (EWH α) are used as the tracers of cool spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested that there exist detectable variabilities in TiO2 and EWH α, and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anticorrelation between temporal variations in TiO2 and EWH α. Also, appreciable anticorrelation in the rotational phase between H α emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool star-spots; however, cool stars do not always show such co-location features. Furthermore, spot coverage and H α emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere. [ABSTRACT FROM AUTHOR]

Stellar magnetic activity is an interesting phenomenon in late-type stars. We use the medium-resolution spectroscopic observations of 406 069 late-type stars from LAMOST to explore their properties. We perform a statistical analysis on the magnetic activity of the stars and their associated physical parameters. Our samples are cross-matched with other data bases (LAMOST DR5, VSX, and SDSS DR12) to obtain additional observational properties. The equivalent widths (EWs) of Hα lines, an indicator of chromospheric activity, are calculated. According to the EWs of the Hα line, we detect 8816 spectra with apparent Hα emission for a total of 2115 stars among the 2 108 565 spectra analysed. Furthermore, 1521 of these stars show variability in their Hα lines. In addition, we detect 2132 flare events associated with 41 542 stars from the catalogue by cross-matching our LAMOST medium-resolution samples and the Kepler and K2 data bases. We also confirm a weak relationship of the flare amplitude with increasing Rossby number. There is a clear decease in the strength of chromospheric activity (L Hα/ L bol) with increasing rotation period. Moreover, the ratio of the flare energy and stellar luminosity is found to decrease with the stellar mass. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the possible future Sun's rotational evolution scenario based on the 8-Gyr-old solar twin HIP 102152. Using HARPS high-cadence observations (and TESS light curves), we analysed the modulation of a variety of activity proxies (Ca  ii  , H  i Balmer, and Na  i lines), finding a strong rotational signal of 35.7 ± 1.4 d (log  B factor ∼ 70, in the case of Ca  ii K line). This value matches with the theoretical expectations regarding the smooth rotational evolution of the Sun towards the end of the main sequence, validating the use of gyrochronology after solar age. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) may have major importance for planetary and stellar evolution. Stellar CME parameters, such as mass and velocity, have yet not been determined statistically. So far only a handful of stellar CMEs has been detected mainly on dMe stars using spectroscopic observations. We therefore aim for a statistical determination of CMEs of solar-like stars by using spectroscopic data from the ESO phase 3 and Polarbase archives. To identify stellar CMEs, we use the Doppler signal in optical spectral lines being a signature of erupting filaments that are closely correlated to CMEs. We investigate more than 3700 h of on-source time of in total 425 dF-dK stars. We find no signatures of CMEs and only few flares. To explain this low level of activity, we derive upper limits for the non-detections of CMEs and compare those with empirically modelled CME rates. To explain the low number of detected flares, we adapt a flare power law derived from EUV data to the H α regime, yielding more realistic results for H α observations. In addition, we examine the detectability of flares from the stars by extracting Sun-as-a-star H α light curves. The extrapolated maximum numbers of observable CMEs are below the observationally determined upper limits, which indicates that the on-source times were mostly too short to detect stellar CMEs in H α. We conclude that these non-detections are related to observational biases in conjunction with a low level of activity of the investigated dF-dK stars. [ABSTRACT FROM AUTHOR]