I. Crespo-Chacón

THE NEAREST YOUNG MOVING GROUPS

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 catalog of late-type stars that are possible members of young stellar kinematic groups, studying their membership through spectroscopic and photometric criteria.

Effect of magnetic activity saturation in chromospheric flux–flux relationships

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.

The ultracool dwarf DENIS-P J104814.7-395606 - Chromospheres and coronae at the low-mass end of the main-sequence

Context. Several diagnostics ranging from the radio to the X-ray band re suitable for investigating the magnetic activity of lat etype stars. Empirical connections between the emission at d ifferent wavelengths place constraints on the nature and ef ficiency of the emission mechanism and the physical conditions in differen t atmospheric layers. The activity of ultracool dwarfs, at t he low-mass end of the main-sequence, is poorly understood. Aims. We have performed a multi-wavelength study of one of the near est M9 dwarfs, DENIS-P J104814.7-395606 ( 4 pc), with the aim to examine its position within the group of magnetically ctive ultracool dwarfs, and, in general, to advance our und erstanding of these objects comparing them to early-M type dwarf stars and the Sun. Methods. We have obtained an XMM-Newton observation of DENIS-P J1048 4.7-395606 and a broad-band spectrum from the ultraviolet to the near infrared with X-Shooter. From this d ataset we obtain the X-ray properties, stellar parameters, kinematics and the emission line spectrum tracing chromospheric activity . We integrate these data by a compilation of activity parame ters for ultracool dwarfs from the literature. Results. Our deep XMM-Newton observation has led to the first X-ray det ection of DENIS-P J104814.7-395606 ( logLx = 25.1) as well as the first measurement of its V band brightness ( V = 17.35 mag). 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 agreeme nt of DENISP 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 a s judged from their lower fluxes. The observed Balmer decreme nts of DENIS 1048-3956 are compatible with optically thick plasma in LTE at low, nearly photospheric temperature or optically thin LTE plasma at20000K. Describing the decrements with Case B recombination requ i s different emitting regions for H α and the higher Balmer lines. The high observed H α/Hβ flux ratio is also poorly fitted by the optically thin models. W e derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a chara teristic of ultracool dwarfs. We add DENISP J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relatio n between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the prese nce of two types of ultracool dwarfs with distinct radio and X-ray beha vior.We have obtained an XMM-Newton observation and a broad-band spectrum from the ultraviolet to the near infrared with X-Shooter for one of the nearest M9 dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of activity parameters for ultracool dwarfs from the literature with the aim to advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Our deep XMM-Newton observation has led to the first X-ray detection of DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V band brightness (V = 17.35mag). 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 J1048-3956 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-P J1048-3956 are compatible with optically thick plasma in LTE at low, nearly photospheric temperature or optically thin LTE plasma at 20000K. Describing the decrements with CaseB recombination requires different emitting regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J1048-3956 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Guedel 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 behavior.

Analysis and modeling of high temporal resolution spectroscopic observations of flares on AD Leonis

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^-1). This is consistent with the very important role that flares can play in stellar coronal heating. The detected flares are non white-light flares and, although most solar flares are of this kind, very few such events have been observed previously in stars. The behaviour of different chromospheric lines (Balmer series from H_α to H_11, Ca II H & K, Na I D_1 and D_2, He I 4026 angstrom and He i D_3) was studied in detail for a total of 14 flares. 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.

X-ray flares on the UV Ceti-type star CC Eridani: a "peculiar" time-evolution of spectral parameters ⋆

Context. Weak flares are supposed to be an important heating agent of th e outer layers of stellar atmospheres. However, due to instrumental limitations, only large X-ray flares have been studied in det ail 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 lit erature. 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 usi ng the density-temperature diagram. The loop scaling laws were applied for deriving physical parameters of the flaring plasma. We also e stimated 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 oc curred 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 th e binary system, and even smaller than the radius of each of the stars. The obtained results are consistent with the following ideas: (i) th e 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-u p versions of solar flares.

A DETAILED STUDY OF THE RISE PHASE OF A LONG DURATION X-RAY FLARE IN THE YOUNG STAR TWA 11B

We analyzed a long duration flare observed in a serendipitous XMM-Newton detection of the M star CD-39 7717B (TWA 11B), member of the young stellar association TW Hya (~8 Myr). Only the rise phase (with a duration of ~35 ks) and possibly the flare peak were observed. We took advantage of the high count rate of the X-ray source to carry out a detailed analysis of its spectrum during the whole exposure. After a careful analysis, we interpreted the rise phase as resulting from the ignition of a first group of loops (event A) which triggered a subsequent two-ribbon flare (event B). Event A was analyzed using a single-loop model, while a two-ribbon model was applied for event B. Loop semi-lengths of ~4 R * were obtained. Such large structures had been previously observed in very young stellar objects (~1-4 Myr). This is the first time that they have been inferred in a slightly more evolved star. The fluorescent iron emission line at 6.4 keV was detected during event B. Since TWA 11B seems to have no disk, the most plausible explanation found for its presence in the X-ray spectrum of this star is collisional- or photo-ionization. As far as we are concerned, this is only the third clear detection of Fe photospheric fluorescence in stars other than the Sun.

Modeling the long duration rise phase of a flare detected on the M star TWA 11 B

We present preliminary results from the analysis of a long duration flare that was observed in a serendipitous XMM‐Newton detection of the M star CD‐39 7717 B (TWA 11 B), member of the young stellar association TW Hya. Only the rise phase (with a duration of ∼35 ks) and possibly the flare peak are present in the light‐curve. The decay phase was not monitored. The fluorescent iron emission line at 6.4 keV was observed during this event. As far as we are concerned, since TWA 11 B seems to have no disk, this is only the third detection of Fe photospheric fluorescence. During the flare, the X‐ray flux increased a factor of ∼4. Taking the light curve and the evolution of the hardness ratio into account, we interpret the rise phase as resulting from the ignition of a first group of loops (part 1) which triggered a subsequent two‐ribbon flare (part 2). Part 1 is analysed using the Reale’s (2007) model, since it has been assumed to be dominated by a single loop. For part 2, the diagnostic method for two‐ribbon fl...

Weak Flares on M-Dwarfs

We have investigated the physics of flares in M-dwarfs by means of optical/X-ray observations and modeling. The great efficiency of current optical spectrographs and detectors has allowed us to detect and analyze a great number of non white-light flares with intermediate spectral resolution and high temporal resolution. Although this kind of flares is the most typical on the Sun, few such events have been so far recorded on stars. We have obtained the physical parameters of the chromospheric flaring plasma (electron temperature, electron density, optical depth and temperature of the underlying source) by using a model that minimizes the difference between the observed Balmer decrements and the calculated ones, which result from solving the radiative transfer equation. On the other hand, the great sensitivity, wide energy range, high energy resolution and continuous time coverage of the EPIC detectors (on-board the XMM–Newton satellite) have enabled us to study both the effect of weak flares on the corona of active stars and the spatial properties of coronal flaring loops. The results are consistent with interpreting stellar flares as scaled-up versions of solar flares and show multiple evidence for flares being an important heating agent of the outer atmospheric stellar layers.

Criteria for spectral classification of cool stars using high-resolution spectra

We have compiled a large number of optical spectra of cool stars taken with different high-resolution echelle spectrographs ( R ≃ 40 000). Many of those are available as spectral libraries (Montes et al . 1997, 1998, 1999, http://www.ucm.es/info/Astrof/invest/actividad/spectra.html )>.

EUVE J0825-16.3 and EUVE J1501-43.6: Two dMe Double–Lined Spectroscopic Binaries

High-resolution echelle spectroscopic observations taken with the FEROS spectrograph at the ESO 2.2-m telescope confirm the binary nature of the dMe stars EUVE J0825−16.3 and EUVE J1501−43.6, previously reported by Christian & Mathioudakis (2002). In these binary systems, emission of similar intensity from both components is detected in the Na i D 1 & D 2 , He i D 3 , Ca II H&K, Ca II IRT and Balmer lines. We have determined precise radial velocities by cross-correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of these systems. Both binaries consist of two nearly equal M0V components with an orbital period shorter than 3.5 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics.