Level Of Chromospheric Activity Research Articles

Orbits and dynamical masses for the active Hyades multiple system HD 284163

ABSTRACT We report near-infrared long-baseline interferometric observations of the Hyades multiple system HD 284163, made with the Center for High Angular Resolution Astronomy array, as well as almost 43 yr of high-resolution spectroscopic monitoring at the Center for Astrophysics. Both types of observations resolve the 2.39 d inner binary, and also an outer companion in a 43.1 yr orbit. Our observations, combined with others from the literature, allow us to solve for the 3D inner and outer orbits, which are found to be at nearly right angles to each other. We determine the dynamical masses of the three stars (good to better than 1.4 per cent for the inner pair), as well as the orbital parallax. The secondary component (0.5245 ± 0.0047 M⊙) is now the lowest mass star with a dynamical mass measurement in the cluster. A comparison of these measurements with current stellar evolution models for the age and metallicity of the Hyades shows good agreement. All three stars display significant levels of chromospheric activity, consistent with the classification of HD 284163 as an RS CVn object. We present evidence that a more distant fourth star is physically associated, making this a hierarchical quadruple system.

Understanding Sun-as-a-Star Variability of Solar Balmer Lines

Precise, high-cadence, long-term records of stellar spectral variability at different temporal scales lead to better understanding of a wide variety of phenomena including stellar atmospheres and dynamos, convective motions, and rotational periods. Here, we investigate the variability of solar Balmer lines (Hα, β, γ, δ) observed by space-borne radiometers (OSIRIS, SCIAMACHY, OMI, and GOME-2), combining these precise, long-term observations with high-resolution data from the ground-based NSO/ISS spectrograph. We relate the detected variability to the appearance of magnetic features on the solar disk. We find that on solar-rotational timescales (about 1 month), the Balmer line activity indices (defined as line-core to line-wing ratios) closely follow variations in the total solar irradiance (which is predominantly photospheric), thus frequently (specifically, during passages of sunspot groups) deviating from behavior of activity indices that track chromospheric activity levels. On longer timescales, the correlation with chromospheric indices increases, with periods of low correlation or even anticorrelation found at intermediate timescales. Comparison of these observations with estimates from semiempirical irradiance reconstructions helps quantify the contributions of different magnetic and quiet features. We conclude that both the lower sensitivity to network and in part the higher sensitivity to filaments and prominences, may result in complex, time-dependent relationships between Balmer and other chromospheric indices observed for the Sun and solar-like stars. The fact that core and wings contribute in a similar manner to the variability, and current knowledge of Balmer-lines formation in stellar atmospheres, supports the notion that Balmer line core-to-wing ratio indices behave more like photospheric rather than chromospheric indices.

Impact of M Dwarfs Ultraviolet Radiation on Prebiotic Chemistry: The Case of Adenine.

To date, several exoplanets have been found to orbit within the habitable zone of main sequence M stars (M dwarfs). These stars exhibit different levels of chromospheric activity that produces ultraviolet (UV) radiation. UV may be harmful to life, but it can also trigger reactions of prebiotic importance on the surface of a potentially habitable planet (PHP). We created a code to obtain the adenine yield for a known adenine synthesis route from diaminomaleonitrile (DAMN). We used computational methods to calculate the reaction coefficient rates (photolysis rate J and rate constant K) for the intermediate molecules DAMN, diaminofumaronitrile (DAFN), and 4-aminoimidazole-5-carbonitrile (AICN) of the adenine synthesis route. We used stellar UV sources and a mercury lamp to compare the theoretical results with experiments performed with lamps. The surface UV flux of planets in the habitable zone of two active M dwarfs (Proxima Centauri and AD Leonis) and the prebiotic Earth was calculated using the photochemical model ATMOS, considering a CO2-N2-H2O atmosphere. We obtained UV absorption coefficients for DAMN and DAFN and thermodynamic parameters that are useful for prebiotic chemistry studies. According to our results, experiments using UV lamps may underestimate the photolysis production of molecules of prebiotic importance. Our results indicate that photolysis reactions are fast with a yield of 50% of AICN in 10 s for the young Sun and ∼1 h for Proxima Centauri b. Planets around active M dwarfs may provide the most favorable environment for UV-mediated production of compounds relevant to the origins of life. The kinetic reaction AICN + HCN  adenine is the bottleneck of the pathway with reaction rates <10-22 L/(mol·s).

Photometric variations from stellar activity as an age indicator for solar-twins

ABSTRACT Stellar ages are elusive to measure, albeit being very important for understanding stellar evolution. We investigate the impact of photospheric activity on 2-min cadence light curves from the TESS/NASA mission of a selected sample of 30 solar-twins with well-determined ages. The photometric variability, $\mathcal {A}_{ {{TESS}}}$, of the light curves due to rotational modulations by the presence of active regions was estimated and correlated with chromospheric activity (Ca ii H&amp;K lines from an extensive High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory (ESO) HARPS/ESO activity time series) and ages. Moreover, these results were compared with the total solar irradiance amplitude behaviour during the solar magnetic cycles 23 and 24, validating our findings for solar-twins. Our results show the photometric amplitude to be strongly correlated to the average level of chromospheric activity for the star sample. Also, we found a good correlation of $\mathcal {A}_{ {{TESS}}}$ with stellar age (in Gyr) described by $\log t = +12.239-0.894 \log \mathcal {A}_{ {{TESS}}}$. In conclusion, stellar photometric variability $\mathcal {A}_{ {{TESS}}}$ may be used as a simple age diagnostic for solar-twins.

Correlation between activity indicators: Hα and Ca II lines in M-dwarf stars

Context. Different approaches have been adopted to study both short- and long-term stellar magnetic activity, and although the mechanisms by which low-mass stars generate large-scale magnetic fields are not well understood, it is known that stellar rotation plays a key role. Aims. There are stars that show a cyclical behaviour in their activity studied on the blue side of the visible spectrum, which can be explained by solar dynamo or αΩ dynamo models. However, when studying late-type dwarf stars, they become redder and it is necessary to implement other indicators to analyse their magnetic activity. In the present work, we perform a comparative study between the best-known activity indicators so far defined from the Ca II and Hα lines to analyse M-dwarf stars. Methods. We studied a sample of 29 M stars with different chromospheric activity levels and spectral classes ranging from dM0 to dM6. To do so, we employed 1796 spectra from different instruments with a median time span of observations of 21 yr. The spectra have a wide spectral range that allowed us to compute the chrosmospheric activity indicators based on Ca II and Hα. In addition, we complemented our data with photometric observations from the TESS space mission for better stellar characterisation and short-term analysis. Results. We obtained a good and significant correlation (ρ = 0.91) between the indexes defined from the two lines for the whole set of stars in the sample. However, we found that there is a deviation for faster rotators (with Prot &lt; 4 days) and higher flare activity (at least one flare per day). For the individual analysis, we found that the indexes computed individually for each star correlate independently of the level of chromospheric emission and the rotation period. Conclusions. There is an overall positive correlation between Ca II and Hα emission in dM stars, except during flare events. In particular, we found that low-energy high-frequency flares could be responsible for the deviation in the linear trend in fast-rotator M dwarfs. This implies that the rotation period could be a fundamental parameter to study the stellar activity and that the rotation could drive the magnetic dynamo in low-mass active stars.

Activity of Selected Solar Twins

We analyze various tracers of magnetic activity for 23 solar twins which are characterized by significant scatter of lithium abundance in their atmospheres. A level of coronal and chromospheric activity has been studied from available X-ray and UV-archival data. It gives us a chance to compare coronae of solar twins of various ages with the solar case. We found a scatter in the X-ray to bolometric luminosity ratio $L_X/L_{bol}$ by several orders of magnitude, which exists in these stars along with a significant spread in Li abundance. This may link the surface activity of stars with phenomena at the base of their convective zones. The TESS data allowed us to reveal rotation modulation of stellar brightness associated with starspots. For some twins of our samples, periods of axial rotation are detected around 6 days, i.e. these stars rotate almost 4 times faster than the contemporary Sun. This indicates their relative youth. Flare activity of solar twins is discovered in the TESS data; we showed existence of various kinds of flares, and present temporal profiles for some of them. We obtained the energy about of $8 \times 10^{33}$ erg for the largest flare of our samples, lasting longer than 4 h. In addition, we discuss also magnetic fields and exoplanets, orbiting these stars.

Stellar Chromospheric Activity Database of Solar-like Stars Based on the LAMOST Low-Resolution Spectroscopic Survey

A stellar chromospheric activity database of solar-like stars is constructed based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Low-Resolution Spectroscopic Survey. The database contains spectral bandpass fluxes and indexes of Ca ii H and K lines derived from 1,330,654 high-quality LRS spectra of solar-like stars. We measure the mean fluxes at line cores of the Ca ii H and K lines using a 1 Å rectangular bandpass and a 1.09 Å FWHM triangular bandpass, as well as the mean fluxes of two 20 Å wide pseudocontinuum bands on the two sides of the lines. Three activity indexes, S rec based on the 1 Å rectangular bandpass and S tri and S L based on the 1.09 Å FWHM triangular bandpass, are evaluated from the measured fluxes to quantitatively indicate the chromospheric activity level. The uncertainties of all the obtained parameters are estimated. We also produce spectrum diagrams of Ca ii H and K lines for all the spectra in the database. The entity of the database is composed of a catalog of spectral sample and activity parameters and a library of spectrum diagrams. Statistics reveal that the solar-like stars with a high level of chromospheric activity (S rec > 0.6) tend to appear in the parameter range of T eff (effective temperature) < 5500 K, 4.3 < log g (surface gravity) < 4.6, and −0.2 < [Fe/H] (metallicity) < 0.3. This database, with more than 1 million high-quality LAMOST LRS spectra of Ca ii H and K lines and basal chromospheric activity parameters, can be further used for investigating activity characteristics of solar-like stars and the solar−stellar connection.

HD 166620: Portrait of a Star Entering a Grand Magnetic Minimum

HD 166620 was recently identified as a Maunder minimum candidate based on nearly 50 years of Ca ii H and K activity data from Mount Wilson and Keck HIRES. These data showed clear cyclic behavior on a 17 yr timescale during the Mount Wilson survey that became flat when picked up later with Keck HIRES planet-search observations. Unfortunately, the transition between these two data sets—and therefore the transition into the candidate Maunder minimum phase—contained little to no data. Here, we present additional Mount Wilson data not present in Baum et al., along with photometry over a nearly 30 yr baseline that definitively traces the transition from cyclic activity to a prolonged phase of flat activity. We present this as conclusive evidence of the star entering a grand magnetic minimum and therefore the first true Maunder minimum analog. We further show that neither the overall brightness nor the chromospheric activity level (as measured by S HK) is significantly lower during the grand magnetic minimum than its activity cycle minimum, implying that an anomalously low mean or instantaneous activity levels are not a good diagnostic or criterion for identifying additional Maunder minimum candidates. Intraseasonal variability in S HK, however, is lower in the star’s grand minimum; this may prove a useful symptom of the phenomenon.

Linking chromospheric activity and magnetic field properties for late-type dwarf stars

ABSTRACT Spectropolarimetric data allow for simultaneous monitoring of stellar chromospheric $\log {R^{\prime }_{\rm {HK}}}$ activity and the surface-averaged longitudinal magnetic field, Bl, giving the opportunity to probe the relationship between large-scale stellar magnetic fields and chromospheric manifestations of magnetism. We present $\log {R^{\prime }_{\rm {HK}}}$ and/or Bl measurements for 954 mid-F to mid-M stars derived from spectropolarimetric observations contained within the PolarBase database. Our magnetically active sample complements previous stellar activity surveys that focus on inactive planet-search targets. We find a positive correlation between mean $\log {R^{\prime }_{\rm {HK}}}$ and mean log |Bl|, but for G stars the relationship may undergo a change between $\log {R^{\prime }_{\rm {HK}}}\sim -4.4$ and −4.8. The mean $\log {R^{\prime }_{\rm {HK}}}$ shows a similar change with respect to the $\log {R^{\prime }_{\rm {HK}}}$ variability amplitude for intermediately active G stars. We also combine our results with archival chromospheric activity data and published observations of large-scale magnetic field geometries derived using Zeeman–Doppler Imaging. The chromospheric activity data indicate a slight under-density of late-F to early-K stars with $-4.75\le \log {R^{\prime }_{\rm HK}}\le -4.5$. This is not as prominent as the original Vaughan–Preston gap, and we do not detect similar under-populated regions in the distributions of the mean |Bl|, or the Bl and $\log {R^{\prime }_{\rm HK}}$ variability amplitudes. Chromospheric activity, activity variability, and toroidal field strength decrease on the main sequence as rotation slows. For G stars, the disappearance of dominant toroidal fields occurs at a similar chromospheric activity level as the change in the relationships between chromospheric activity, activity variability, and mean field strength.

Coherent radio emission from a population of RS Canum Venaticorum systems

Coherent radio emission from stars can be used to constrain fundamental coronal plasma parameters, such as plasma density and magnetic field strength. It is also a probe of chromospheric and magnetospheric acceleration mechanisms. Close stellar binaries, such as RS Canum Venaticorum (RS CVn) systems, are particularly interesting as their heightened level of chromospheric activity and possible direct magnetic interaction make them a unique laboratory to study coronal and magnetospheric acceleration mechanisms. RS CVn binaries are known to be radio-bright but coherent radio emission has only conclusively been detected previously in one system. Here, we present a population of 14 coherent radio emitting RS CVn systems. We identified the population in the ongoing LOFAR Two Metre Sky Survey as circularly polarised sources at 144 MHz that are astrometrically associated with known RS CVn binaries. We show that the observed emission is powered by the electron cyclotron maser instability. We use numerical calculations of the maser’s beaming geometry to argue that the commonly invoked ‘loss-cone’ maser cannot generate the necessary brightness temperature in some of our detections and that a more efficient instability, such as the shell or horseshoe maser, must be invoked. Such maser configurations are known to operate in planetary magnetospheres. We also outline two acceleration mechanisms that could produce coherent radio emission, one where the acceleration occurs in the chromosphere and one where the acceleration is due to an electrodynamic interaction between the stars. We propose radio and optical monitoring observations that can differentiate between these two mechanisms.

Three years of HARPS-N high-resolution spectroscopy and precise radial velocity data for the Sun

Context. The solar telescope connected to HARPS-N has been observing the Sun since the summer of 2015. Such a high-cadence, long-baseline data set is crucial for understanding spurious radial-velocity signals induced by our Sun and by the instrument. On the instrumental side, this data set allowed us to detect sub- m s−1 systematics that needed to be corrected for. Aims. The goals of this manuscript are to (i) present a new data reduction software for HARPS-N, (ii) demonstrate the improvement brought by this new software during the first three years of the HARPS-N solar data set, and (iii) release all the obtained solar products, from extracted spectra to precise radial velocities. Methods. To correct for the instrumental systematics observed in the data reduced with the current version of the HARPS-N data reduction software (DRS version 3.7), we adapted the newly available ESPRESSO DRS (version 2.2.3) to HARPS-N and developed new optimised recipes for the spectrograph. We then compared the first three years of HARPS-N solar data reduced with the current and new DRS. Results. The most significant improvement brought by the new DRS is a strong decrease in the day-to-day radial-velocity scatter, from 1.27 to 1.07 m s−1; this is thanks to a more robust method to derive wavelength solutions, but also to the use of calibrations closer in time. The newly derived solar radial-velocities are also better correlated with the chromospheric activity level of the Sun in the long term, with a Pearson correlation coefficient of 0.93 compared to 0.77 before, which is expected from our understanding of stellar signals. Finally, we also discuss how HARPS-N spectral ghosts contaminate the measurement of the calcium activity index, and we present an efficient technique to derive an index free of instrumental systematics. Conclusions. This paper presents a new data reduction software for HARPS-N and demonstrates its improvements, mainly in terms of radial-velocity precision, when applied to the first three years of the HARPS-N solar data set. Those newly reduced solar data, representing an unprecedented time series of 34 550 high-resolution spectra and precise radial velocities, are released alongside this paper. Those data are crucial to understand stellar activity signals in solar-type stars further and develop the mitigating techniques that will allow us to detect other Earths.

Solar activity and responses observed in Balmer lines

Context. Many stars show Sun-like magnetic activity cycles, which are frequently observed by tracking changes in the chromospherically sensitive CaII H&amp;K doublet. However, relationships between the line profile changes related to the magnetic activity seen in strong spectral transitions in other portions of a stellar spectrum are yet to be understood. Aims. We follow variability patterns in various solar lines in order to relate them to the emergence, passage, and decay of active solar regions. Methods. The line activity indices (core-to-wing ratio) for the upper Balmer lines – Hβ, Hγ, and Hδ – are constructed from the near-daily solar measurements acquired by the Ozone Monitoring Instrument and the TROPOspheric Monitoring Instrument. Results. On solar rotation timescales, the upper Balmer line activity indices closely follow variations in the total solar irradiance, r ∼ −(0.6 − 0.7), and thus frequently deviate from the behavior of the line activity indices that track chromospheric activity levels (e.g., the CH 430 nm band used in this study), specifically during passages of big sunspot groups.

Solar-type Stars Observed by LAMOST and Kepler

Abstract Obtaining measurements of chromospheric and photometric activity of stars with near-solar fundamental parameters and rotation periods is important for a better understanding of solar–stellar connection. We select a sample of 2603 stars with near-solar fundamental parameters from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST spectra to measure their chromospheric activity and Kepler light curves to measure their photospheric activity (i.e., the amplitude of the photometric variability). While the rotation periods of 1556 of these stars could not be measured due to the low amplitude of the photometric variability and highly irregular temporal profile of light curves, 254 stars were further identified as having near-solar rotation periods. We show that stars with near-solar rotation periods have chromospheric activities that are systematically higher than stars with undetected rotation periods. Furthermore, while the solar level of photospheric and chromospheric activity appears to be typical for stars with undetected rotation periods, the Sun appears to be less active than most stars with near-solar rotation periods (both in terms of photospheric and chromospheric activity).

Chromospheric activity in bright contact binary stars

Context. Studying chromospheric activity of contact binaries is an effective way of revealing the magnetic activity processes of these systems. One efficient but somewhat neglected method for this purpose is to follow the changes of the Hα line profiles via optical spectroscopy. Aims. Our goal is to perform a comprehensive preliminary analysis based on the optical spectral signs of chromospheric activity on the largest sample of contact binaries to date. Methods. We collected optical echelle spectra on 12 bright contact binaries in 17 nights. We derived new radial velocity curves from our observations. For quantifying the apparent chromospheric activity levels of the systems, we subtracted self-constructed synthetic spectra from the observed ones and measured the equivalent widths of the residual Hα-profiles at each observed epoch. Our well-sampled data set allowed us to study the short-term variations of chromospheric activity levels as well as to search for correlations between them and some basic physical parameters of the systems. Results. Fitting the radial velocity curves, we re-determined the mass ratios and systemic velocities of all observed objects. We found that chromospheric activity levels of the studied systems show various changes during the orbital revolution: we see either flat, one-peaked, or two-peaked distributions of equivalent width vs. the orbital phase. The first case means that the activity level is probably constant, while the latter two cases suggest the presence of one or two active longitudes at the stellar surfaces. Our correlation diagrams show that mean chromospheric activity levels may be related to the orbital periods, B−V color indices, inverse Rossby numbers, and temperature differences of the components. At the same time, no clear trend is visible with respect to the mass ratios, inclinations, or fill-out factors of the systems. A- and W-type contact binaries in our sample show similar distributions on each of the studied correlation diagrams.

LBT/PEPSI Spectropolarimetry of a Magnetic Morphology Shift in Old Solar-type Stars*

Abstract Solar-type stars are born with relatively rapid rotation and strong magnetic fields. Through a process known as magnetic braking, the rotation slows over time as stellar winds gradually remove angular momentum from the system. The rate of angular momentum loss depends sensitively on the magnetic morphology, with the dipole field exerting the largest torque on the star. Recent observations suggest that the efficiency of magnetic braking may decrease dramatically in stars near the middle of their main-sequence lifetimes. One hypothesis to explain this reduction in efficiency is a shift in magnetic morphology from predominantly larger to smaller spatial scales. We aim to test this hypothesis with spectropolarimetric measurements of two stars that sample chromospheric activity levels on opposite sides of the proposed magnetic transition. As predicted, the more active star (HD 100180) exhibits a significant circular polarization signature due to a nonaxisymmetric large-scale magnetic field, while the less active star (HD 143761) shows no significant signal. We identify analogs of the two stars among a sample of well-characterized Kepler targets, and we predict that the asteroseismic age of HD 143761 from future Transiting Exoplanet Survey Satellite observations will substantially exceed the age expected from gyrochronology. We conclude that a shift in magnetic morphology likely contributes to the loss of magnetic braking in middle-aged stars, which appears to coincide with the shutdown of their global dynamos.

GALEX far-ultraviolet observations of metal-poor subdwarfs

Far-ultraviolet photometry derived from the GALEX satellite observatory has been compiled for a sample of metal-poor subdwarfs with $\mathrm{[Fe/H]} < -1.0$ . The FUV properties of these subdwarfs are compared with those of a set of Population I dwarfs that are known to have low levels of chromospheric activity. Comparisons are made via a number of photometric plots, including an absolute FUV magnitude versus $(V-K_{s})$ diagram, two-colour diagrams involving both $(m_{ \mathrm{FUV}}-B)$ and $(m_{\mathrm{FUV}}-V)$ versus $B-V$ , and a two-colour diagram composed of $(m_{\mathrm{FUV}}-V)$ versus $(V-K_{s})$ . The warmest subdwarfs with $(V-K_{s}) \sim1.2\mbox{--}1.4$ show FUV excesses ranging from $\sim2\mbox{--}3~\mbox{mag}$ relative to the Population I dwarfs, with the amount of FUV enhancement decreasing among subdwarfs of decreasing effective temperature. The coolest dwarfs that are compared have $(V-K_{s}) \sim1.8$ , and among these stars the subdwarfs with $-2.0 \leq{\mathrm{[Fe/H]}} \leq-1.0$ approach the locus of low activity Population I dwarfs in the $(m_{\mathrm{FUV}}-V, V-K_{s})$ diagram. In the $(m_{\mathrm{FUV}}-B, B-V)$ diagram the subdwarfs in this metallicity range overlap the Population I dwarf sequence for $(B-V) > 0.6$ . The behaviour of the subdwarfs is consistent with their FUV fluxes being determined by a combination of a photospheric FUV spectrum, the strength of which diminishes towards cooler effective temperatures, and a spectrum of emission lines arising from a chromosphere and/or transition region which are of comparable strength between the coolest dwarfs and subdwarfs.

A Radial velocity survey of spatially resolved young, low-mass binaries

The identification and characterisation of low-mass binaries is of importance for a range of astrophysical investigations. Low-mass binaries in young (∼10–100 Myr) moving groups (YMGs) in the solar neighborhood are of particular significance as they provide unique opportunities to calibrate stellar models and evaluate the ages and coevality of the groups themselves. Low-mass M-dwarfs have pre-main sequence life times on the order of ∼100 Myr and therefore are continually evolving along a mass-luminosity track throughout the YMG phase, providing ideal laboratories for precise isochronal dating, if a model-independent dynamical mass can be measured. AstraLux lucky imaging multiplicity surveys have recently identified hundreds of new YMG low-mass binaries, where a subsample of M-dwarf multiples have estimated orbital periods less than 50 yr. We have conducted a radial velocity survey of a sample of 29 such targets to complement the astrometric data. This will allow enhanced orbital determinations and precise dynamical masses to be derived in a shorter timeframe than possible with astrometric monitoring alone, and allow for a more reliable isochronal analysis. Here we present radial velocity measurements derived for our sample over several epochs. We report the detection of the three-component spectroscopic multiple 2MASS J05301858-5358483, for which the C component is a new discovery, and forms a tight pair with the B component. Originally identified as a YMG member, we find that this system is a likely old field interloper, whose high chromospheric activity level is caused by tidal spin-up of the tight BC pair. Two other triple systems with a tight pair exist in the sample, 2MASS J04244260-0647313 (previously known) and 2MASS J20163382-0711456, but for the rest of the targets we find that additional tidally synchronized companions are highly unlikely, providing further evidence that their high chromospheric activity levels are generally signatures of youth.

Far Ultraviolet Photometry of F and G Dwarfs in the Hyades, Coma Berenices, and NGC 2281 Open Clusters as Obtained by GALEX

A two-color diagram formed from GALEX FUV and Johnson BV photometry is presented for F and early G dwarfs in three open clusters: the Hyades, Coma Berenices, and NGC 2281. Denoted as (mFUV − B) versus B − V, the position of dwarf stars in such a diagram has been shown by previous work to vary with the level of chromospheric and coronal activity. The upper main sequences of the Hyades and Coma Ber clusters match quite well in this two-color diagram, consistent with their similar ages and metallicities, and effectively zero reddening. While the upper main sequence of NGC 2281 is also fairly well defined, there is some uncertainty in the reddening of this cluster, which hinders an attempt to compare it with the other two. The results of this work suggest that it may become feasible to define isochrones for Population I dwarf stars in a (mFUV − B) versus B − V diagram if a sufficient sample of stars of well-known ages can be used as calibrating sources. However, the utility of such a diagram for age-dating purposes will depend upon the additional availability of accurate reddening measurements.

A spectroscopic survey of the youngest field stars in the solar neighborhood

Context. Star formation in the solar neighborhood is mainly traced by young stars in open clusters, associations, and in the field, which can be identified, for example, by their X-ray emission. The determination of stellar parameters for the optical counterparts of X-ray sources is crucial for a full characterization of these stars. Aims. This work extends the spectroscopic study of the RasTyc sample, obtained by the cross-correlation of the Tycho and ROSAT All-Sky Survey catalogs, to stars fainter than V = 9.5 mag and aims to identify sparse populations of young stars in the solar neighborhood. Methods. We acquired 625 high-resolution spectra for 443 presumably young stars with four different instruments in the northern hemisphere. The radial and rotational velocity (vsini) of our targets were measured by means of the cross-correlation technique, which is also helpful to discover single-lined (SB1), double-lined spectroscopic binaries (SB2), and multiple systems. We used the code ROTFIT to perform an MK spectral classification and to determine the atmospheric parameters (Teff, logg, [Fe/H]) and vsini of the single stars and SB1 systems. For these objects, we used the spectral subtraction of slowly rotating templates to measure the equivalent widths of the Hα and Li I 6708 Å lines, which enabled us to derive their chromospheric activity level and lithium abundance. We made use of Gaia DR1 parallaxes and proper motions to locate the targets in the Hertzsprung-Russell (HR) diagram and to compute the space velocity components of the youngest objects. Results. We find a remarkable percentage (at least 35%) of binaries and multiple systems. On the basis of the lithium abundance, the sample of single stars and SB1 systems appears to be mostly (~60%) composed of stars younger than the members of the UMa cluster. The remaining sources are in the age range between the UMa and Hyades clusters (~20%) or older (~20%). In total, we identify 42 very young (PMS-like) stars, which lie above or very close to the Pleiades upper envelope of the lithium abundance. A significant percentage (~12%) of evolved stars (giants and subgiants) is also present in our sample. Some of these stars (~36%) are also lithium rich (A(Li) &gt; 1.4).

The RoPES project with HARPS and HARPS-N

We report the discovery of a system of two super-Earths orbiting the moderately active K-dwarf HD 176986. This work is part of the RoPES RV program of G- and K-type stars, which combines radial velocities (RVs) from the HARPS and HARPS-N spectrographs to search for short-period terrestrial planets. HD 176986 b and c are super-Earth planets with masses of 5.74 and 9.18 M⊕, orbital periods of 6.49 and 16.82 days, and distances of 0.063 and 0.119 AU in orbits that are consistent with circular. The host star is a K2.5 dwarf, and despite its modest level of chromospheric activity (log10 (RHK' = –4.90 ± 0.04), it shows a complex activity pattern. Along with the discovery of the planets, we study the magnetic cycle and rotation of the star. HD 176986 proves to be suitable for testing the available RV analysis technique and further our understanding of stellar activity.