The latest results in the research of forming planetary systems have led several authors to compile a sample of candidates for searching for planets in the vicinity of the sun. Young stellar associations are indeed excellent laboratories for this study, but some of them are not close enough to allow the detection of planets through adaptive optics techniques. However, the existence of very close young moving groups can solve this problem. Here we have compiled the members of the nearest young moving groups, as well as a list of new candidates from our catalogue of late-type stars possible members of young stellar kinematic groups, studying their membership through spectroscopic and photometric criteria.

Aims. We present a compilation of spectroscopic data from a survey of 144 chromospherically active young stars in the solar neighborhood, which may be used to investigate different aspects of its formation and evolution in terms of kinematics and stellar formation history. The data have already been used by us in several studies. With this paper, we make all these data accessible to the scientific community for future studies on different topics. Methods. We performed spectroscopic observations with echelle spectrographs to cover the entirety of the optical spectral range simultaneously. Standard data reduction was performed with the IRAF echelle package. We applied the spectral subtraction technique to reveal chromospheric emission in the stars of the sample. The equivalent width of chromospheric emission lines was measured in the subtracted spectra and then converted to fluxes using equivalent width-flux relationships. Radial and rotational velocities were determined by the cross-correlation technique. Kinematics, equivalent widths of the lithium line λ6707.8 Å and spectral types were also determined. Results. A catalog of spectroscopic data is compiled: radial and rotational velocities, space motion, equivalent widths of optical chromospheric activity indicators from Ca ii H & K to the calcium infrared triplet and the lithium line in λ6708 Å. Fluxes in the chromospheric emission lines and R HK are also determined for each observation of a star in the sample. We used these data to investigate the emission levels of our stars. The study of the Hα emission line revealed two different populations of chromospheric emitters in the sample, clearly separated in the log F Hα /F bol − (V − J) diagram. The dichotomy may be associated with the age of the stars.

We report the results of a high temporal resolution spectroscopic monitoring of the flare star AD Leo. During 4 nights, more than 600 spectra were taken in the optical range using the Isaac Newton Telescope (INT) and the Intermediate Dispersion Spectrograph (IDS). We observed a large number of short and weak flares occurring very frequently (flare activity > 0.71 h We estimated the physical parameters of the flaring plasma by using a procedure that assumes a simplified slab model of flares. All the obtained physical parameters are consistent with previously derived values for stellar flares, and the areas -less than 2.3% of the stellar surface -are comparable with the size inferred for other solar and stellar flares. We studied the relationships between the physical parameters and the area, duration, maximum flux and energy released during the detected flares.

We present a homogeneous study of chromospheric and coronal flux–flux relationships using a sample of 298 late‐type dwarf active stars with spectral types F to M. The chromospheric lines were observed simultaneously in each star to avoid spread as a result of long‐term variability. Unlike other works, we subtract the basal chromospheric contribution in all the spectral lines studied. For the first time, we quantify the departure of dMe stars from the general relations. We show that dK and dKe stars also deviate from the general trend. Studying the flux–colour diagrams, we demonstrate that the stars deviating from the general relations are those with saturated X‐ray emission and we show that these stars also present saturation in the Hα line. Using several age spectral indicators, we show that these are younger stars than those following the general relationships. The non‐universality of flux–flux relationships found in this work should be taken into account when converting between fluxes in different chromospheric activity indicators.

Context. Several diagnostics ranging from the radio to the X-ray band are suitable for investigating the magnetic activity of late-type stars. Empirical connections between the emission at different wavelengths place constraints on the nature and efficiency of the emission mechanism and the physical conditions in different atmospheric layers. The activity of ultracool dwarfs, at the low-mass end of the main-sequence, is poorly understood. Aims. We perform a multi-wavelength study of one of the nearest M9 dwarfs, DENIS-P J104814.7-395606 (4 pc), to examine its position within the group of magnetically active ultracool dwarfs, and, in general, advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Methods. We obtained an XMM-Newton observation of DENIS-P J104814.7-395606 and a broad-band spectrum from the ultraviolet to the near-infrared with X-Shooter. From this dataset, we derive the X-ray properties, stellar parameters, kinematics, and the emission-line spectrum tracing chromospheric activity. We integrate these data by compiling the activity parameters of ultracool dwarfs from the literature. Results. Our deep XMM-Newton observation provides the first X-ray detection of DENIS-P J104814.7-395606 (log L x = 25.1), as well as the first measurement of its V band brightness (V = 17.35 mag). The flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J104814.7-395606 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS 1048-3956 are compatible with optically thick plasma in local thermal equilibrium (LTE) at low, nearly photospheric temperature or optically thin LTE plasma at 20 000 K. Describing the decrements with case B recombination requires different emitting regions for Hα and the higher Balmer lines. The high observed Hα/Hβ flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behaviors.

Context. The extension of the corona of classical T Tauri stars (CTTS) is is being widely discussed. The standard model of magnetic configuration of CTTS predicts that coronal magnetic flux tubes connect the stellar atmosphere to the inner region of the disk. However, differential rotation may disrupt these long loops. The results from hydrodynamic modeling of X-ray flares observed in CTTS that confirm the star-disk connection hypothesis are still controversial. Some authors suggest the presence of the accretion disk prevents the stellar corona extending beyond the co-rotation radius, while others are simply not confident with the methods used to derive loop lengths. Aims. We use independent procedures to determine the length of flaring loops in stars of the Orion Nebula Cluster, which has previously been analyzed using hydrodynamic models. Our aim is to disentangle the two scenarios that have been proposed. Methods. We present a different approach for determining the length of flaring loops that is based on the oscillatory nature of the loops after strong flares. We use wavelet tools to reveal oscillations during several flares. The subsequent analysis of these oscillations is based on the physics of coronal seismology. Results. Our results likely confirm the large extension of the corona of CTTS and the hypothesis of star-disk magnetic interaction in at least three CTTS of the Orion Nebula Cluster. Conclusions. Analyzing oscillations in flaring events is a powerful tool to determine the physical characteristics of magnetic loops in coronae in stars other than the Sun. The results presented in this work confirm the star-disk magnetic connection in CTTS.Key words. magnetohydrodynamics (MHD) -X-rays: stars -stars: magnetic field -stars: variables: T Tauri, Herbig Ae/Beprotoplanetary disks -stars: flare IntroductionClassical T Tauri stars (CTTS) and other young stellar objects are characterized by accreting gas from a surrounding gaseous disk. Accretion takes place through a funnel that connects the star to the inner region of the disk (e.g., Hartman 1989). Koenigl (1991) was the first to propose the scenario of magnetic driven accretion in CTTS. In this scenario, the star accretes material from the disk following stellar magnetic field lines connected to the disk (see Fig. 12 in Camenzind 1990). A similar scenario was previously proposed for accreting neutron stars by Ghosh & Lamb (1978).Some mechanisms of this star-disk interaction are not yet fully understood. Owing to differential rotation between the star and the disk, magnetic connection may be disrupted. Magnetohydrodynamic (MHD) simulations predict that the poloidal field connecting the disk to the star is wrapped up when their angular velocities differ substantially (Lovelace et al. 1995). As a consequence, magnetic loops inflate rapidly (Goodson et al. 1997). This expansion of poloidal field yields a magnetic configuration with three components: a closed stellar magnetosphere, a region of open field connected to the pole of the star, and a region...

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Context. Weak flares are supposed to be an important heating agent of the outer layers of stellar atmospheres. However, due to instrumental limitations, only large X-ray flares have been studied in detail until now. Aims. We used an XMM-Newton observation of the very active BY-Dra type binary star CC Eri in order to investigate the properties of two flares that are weaker than those typically studied in the literature. Methods. We performed time-resolved spectroscopy of the data taken with the EPIC-PN CCD camera. A multi-temperature model was used to fit the spectra. We inferred the size of the flaring loops using the density-temperature diagram. The loop scaling laws were applied for deriving physical parameters of the flaring plasma. We also estimated the number of loops involved in the observed flares. Results. A large X-ray variability was found. Spectral analysis showed that all the regions in the light curve, including the flare segments, are well-described by a 3-T model with variable emission measures but, surprisingly, with constant temperatures (values of 3, 10 and 22 MK). The analysed flares lasted ∼3.4 and 7.1 ks, with flux increases of factors 1.5−1.9. They occurred in arcades made of a few tens of similar coronal loops. The size of the flaring loops is much smaller than the distance between the stellar surfaces in the binary system, and even smaller than the radius of each of the stars. The obtained results are consistent with the following ideas: (i) the whole X-ray light curve of CC Eri could be the result of a superposition of multiple low-energy flares; and (ii) stellar flares can be scaled-up versions of solar flares.