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Context. The comparison of coronal and photospheric abundances in cool stars is an essential question to resolve. In the Sun an enhancement of the elements with low first ionization potential (FIP) is observed in the corona with respect to the photosphere. Stars with high levels of activity seem to show a depletion of elements with low FIP when compared to solar standard values; however, the few cases of active stars in which photospheric values are available for comparison lead to confusing results, and an enlargement of the sample is mandatory for solving this longstanding problem.

Aims. We have performed a set of high- and low-spectral resolution phase-resolved X-ray observations of the magnetic B star βCep, for which theoretical models predict the presence of a confined wind emitting X-rays from stationary shocks. Given the peculiar geometry of βCep, some of the models predict strong rotational modulation of the X-ray emission, while other models predict a much lower amplitude modulation at 90 deg phase shift from the modulation predicted from the first group of models. Our observations were designed to provide a stringent test of such models.

Context. The mechanism of formation of the He I10 830 Å triplet in cool stars has been subject of debate for the last 30 years. A relation between the X-ray luminosity and the He I10 830 Å flux was found in cool stars, but the dominant mechanism of formation in these stars (photoionization by coronal radiation followed by recombination and cascade, or collisional excitation in the chromosphere), has not yet been established.

Aims.The observation of eclipses during X-ray flares taking place in active cool stars binaries allows us to calculate the position and size of the flares. This information cannot be derived by analyzing the decay of the flares, a frequently used approach in the literature that requires the assumption of a physical model. We make use of the eclipsing light curve to constrain the set of possible solutions, from the geometrical point of view, in two flares of Algol, and one flare in VW Cep.

Context.X-ray surveys are a very efficient mean of detecting young stars and therefore allow us to study the young stellar population in the solar neighborhood and the local star formation history in the last billion of years.

We present the results of an XMM-Newtonobservation of the young (${\sim} 2{-}4$Myr) cluster around the hot star σOrionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster members, including very low-mass stars down to the substellar limit. We detected eleven new possible candidate members from the 2MASS catalogue. We find that late-type stars have a median $\log L_\mathrm{X}/L_\mathrm{bol} \sim -3.3$, i.e. very close to the saturation limit. We detected significant variability in ${\sim} 40$% of late-type members or candidates, including 10 flaring sources; rotational modulation is detected in one K-type star and possibly in another 3 or 4 stars. Spectral analysis of the brightest sources shows typical quiescent temperatures in the range $T_1\sim 0.3{-}0.8$keV and $T_2\sim 1{-}3$keV, with subsolar abundances $Z\sim 0.1{-}0.3 \,Z_\odot$, similar to what is found in other star-forming regions and associations. We find no significant difference in the spectral properties of classical and weak-lined T Tauri stars, although classical T Tauri stars tend to be less X-ray luminous than weak-lined T Tauri stars.

The eclipsing active binary SV Cam (G0V/K6V, $P_{\rm orb}=0.593071$d) was observed with XMM-Newtonduring two campaigns in 2001 and 2003. No eclipses in the quiescent emission are clearly identified, but a flare was eclipsed during the 2001 campaign, allowing us to strongly constrain, from purely geometrical considerations, the position and size of the event: the flare is compact and it is formed at a latitude below 65°. The size, temperature and Emission Measure of the flare imply an electron density of $\log n_{\rm e}$(cm$^{-3})\sim 10.6{-}13.3$and a magnetic field of ~65-1400 G in order to confine the plasma, consistent with the measurements that are obtained from density-sensitive line ratios in other similar active stars. Average emission seems to come from either extended or polar regions because of lack of eclipses. The Emission Measure Distribution, coronal abundances and characteristics of variability are very similar to other active stars such as AB Dor (K1V).

We report a detailed analysis of the coronal abundance of 4 stars with varying levels of activity and with accurately known photospheric abundances. The coronal abundance is determined here using a line flux analysis and a full determination of the differential emission measure. Photospheric abundance values are taken from the literature. Previous coronal abundance determinations have generally been compared to solar photospheric abundances; from this a number of general properties have been inferred, such as the presence of a coronal metal depletion with an inverse First Ionization Potential correlated with activity level. Here we show that, when coronal abundances are compared with real photospheric values for the individual stars, the resulting pattern can be very different. Also, we present evidence that, in some cases, the coronal metal abundance may not be uniform in the corona; in particular it can vary with the temperature of the emitting plasma.

The young active star AB Dor (K1 IV-V) has been observed 16 times in the last three years with the XMM-Newton and Chandra observatories, totalling 650 ks of high-resolution X-ray spectra. The XMM/RGS observations with the highest and lowest average emission levels have been selected to study the coronal properties of AB Dor in two different activity levels. We compare the results based on the XMM data with those obtained from a higher resolution Chandra/HETG spectrum, using the same line-based analysis technique. We have reconstructed the plasma Emission Measure Distribution vs. temperature (EMD) in the range $\log T({\rm K})\sim6.1$–7.6, and we have determined the coronal abundances of AB Dor, obtaining consistent results between the two instruments. The overall shape of the EMD is also consistent with the one previously inferred from EUVE data. The EMD shows a steep increase up to the peak at $\log T({\rm K})\sim6.9$and a substantial amount of plasma in the range $\log T({\rm K})\sim6.9$–7.3. The coronal abundances show a clear trend of increasing depletion with respect to solar photospheric values, for elements with increasing First Ionization Potential (FIP), down to the Fe value ([Fe/H] = –0.57), followed by a more gradual recovery of the photospheric values for elements with higher FIP. He-like triplets and $\ion{Fe}{xxi}$and $\ion{Fe}{xxii}$lines ratios indicate electron densities $\log n_{\rm e} \sim 10.8$cm-3at $\log T({\rm K})\sim 6.3$and $\log n_{\rm e}\sim 12.5$cm-3at $\log T({\rm K})\sim7$, implying plasma pressures steeply increasing with temperature. These results are interpreted in the framework of a corona composed of different families of magnetic loop structures, shorter than the stellar radius and in isobaric conditions, having pressures increasing with the maximum plasma temperature, and which occupy a small fraction ($f \sim 10^{-4}$–10-6) of the stellar surface.

All the Extreme Ultraviolet Explorer(EUVE) observations of AD Leo, totalling 1.1 Ms of exposure time, have been employed to analyze the corona of this single M dwarf. The light curves show a well defined quiescent stage, and a distribution of amplitude of variability following a power law with a ~-2.4 index. The flaring behavior exhibits much similarity with other M active stars like FK Aqr or YY Gem, and flares behave differently from late type active giants and subgiants. The Emission Measure Distribution (EMD) of the summed spectrum, as well as that of quiescent and flaring stages, were obtained using a line-based method. The average EMD is dominated by material at log T(K) ~ 6.9, with a second peak around log T(K) ~ 6.3, and a large increase in the amount of material with log T(K) $\ga$7.1 during flares, material almost absent during quiescence. The results are interpreted as the combination of three families of loops with maximum temperatures at log T(K) ~ 6.3, ~6.9 and somewhere beyond log T(K) $\ga$7.1. A value of the abundance of [Ne/Fe] = 1.05 ±0.08 was measured at log T(K) ~ 5.9. No significative increment of neon abundance was detected between quiescence and flaring states.

Context. The mechanism of formation of the He?I?10?830???triplet in cool stars has been subject of debate for the last 30?years. A relation between the X-ray luminosity and the He?I?10?830???flux was found in cool stars, but the dominant mechanism of formation in these stars (photoionization by coronal radiation followed by recombination and cascade, or collisional excitation in the chromosphere), has not yet been established.Aims. We use modern instrumentation (NOT/SOFIN) and a direct measurement of the EUV?flux, which photoionizes He?I, to investigate the formation mechanism of the line for the most active stars which are frequently excluded from analysis.Methods. We have observed with an unprecedented resolution (R ~ 170?000) the He?I?10?830???triplet in a set of 15?stars that were also observed with the Extreme Ultraviolet Explorer (EUVE) in order to compare the line strengths with their EUV and X-ray fluxes.Results. Active dwarf and subgiant stars do not exhibit a relation between the EUV?flux and the equivalent width of the He?I?10?830???line. Giant stars however, show a positive correlation between the strength of the He?I?10?830???absorption and the?EUV and X-ray?fluxes. The strength of the C?IV?1550???emission does not correlate with coronal fluxes in this sample of 15?stars.Conclusions. Active dwarf stars may have high chromospheric densities thus allowing collisional excitation to dominate photoionization/recombination processes in forming the He?I?10?830???line. Active giant stars possess lower gravities, and lower chromospheric densities than dwarfs, allowing for photoexcitation processes to become important. Moreover, their extended chromospheres allow for scattering of infrared continuum radiation, producing strong absorption in He?I and tracing wind dynamics.

High resolution, high -$S/N$- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed photospheric parameters and abundances and to compare the characteristics of analysis techniques. The results have been compared with the aim of determining the most robust method to perform complete spectroscopic analyses of K-type stars, and in this perspective the present work must be considered as a pilot study. In this context we have determined the abundance ratios with respect to iron of several elements. In the first method the photospheric parameters ($T_{\rm eff}$, $\log g$, and $\xi$) and metal abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software code. The analysis proceeds in an iterative way, and relies on the excitation equilibrium of the $\ion{Fe}{i}$ lines for determining the effective temperature and microturbulence, and on the ionization equilibrium of the $\ion{Fe}{i}$ and $\ion{Fe}{ii}$ lines for determining the surface gravity and the metallicity. The second method follows a similar approach, but discards the $\ion{Fe}{i}$ low excitation potential transitions (which are potentially affected by non-LTE effects) from the initial line list, and relies on the $B-V$ colour index to determine the temperature. The third method relies on the detailed fitting of the 6162 Å $\ion{Ca}{i}$ line to derive the surface gravity, using the same restricted line list as the second method. Methods 1 and 3 give consistent results for the program stars; in particular the comparison between the results obtained shows that the $\ion{Fe}{i}$ low-excitation potential transitions do not appear significantly affected by non-LTE effects (at least for the subgiant stars), as suggested by the good agreement of the atmospheric parameters and chemical abundances derived. The second method leads to systematically lower $T_{\rm eff}$ and $\log g$ values with respect to the first one, and a similar trend is shown by the chemical abundances (with the exception of the oxygen abundance). These differences, apart from residual non-LTE effects, may be a consequence of the colour-$T_{\rm eff}$ scale used. The $\alpha$-elements have abundance ratios consistent with the solar values for all the program stars, as expected for ”normal” disk stars. The first method appears to be the most reliable one, as it is self-consistent, it always leads to convergent solutions and the results obtained are in good agreement with previous determinations in the literature.

We present XMM-Newton observations of the young (${\sim} 2{-}5$ Myr) cluster around the hot (O9.5V) star $\sigma$ Orionis AB, aimed at obtaining a high resolution RGS spectrum of the hot star as well as EPIC imaging data for the whole field. We show that the RGS spectrum of $\sigma$ Ori AB may be contaminated by weaker nearby sources which required the development of a suitable procedure to extract a clean RGS spectrum and to determine the thermal structure and wind properties of the hot star. We also report on the detection of a flare from the B2Vp star $\sigma$ Ori E and we discuss whether the flare originated from the hot star itself or rather from an unseen late-type companion. Other results of this observation include: the detection of 174 X-ray sources in the field of $\sigma$ Ori of which 76 are identified as cluster members, including very low-mass stars down to the substellar limit; the discovery of rotational modulation in a late-type star near $\sigma$ Ori AB; no detectable line broadenings and shifts (${\la} 800$ km s$^{-1}$) in the spectrum of $\sigma$ Ori AB together with a remarkable low value of the $\ion{O}{vii}$ forbidden to intercombination line ratio and unusually high coronal abundances of CNO elements.

We present Herschel PACS 100 and 160 μm observations of the solar-type stars αMen, HD 88230 and HD 210277, which form part of the FGK stars sample of the Herschel open time key programme (OTKP) DUNES (DUst around NEarby Stars). Our observations show small infrared excesses at 160 μm for all three stars. HD 210277 also shows a small excess at 100 μm, while the 100 μm fluxes of αMen and HD 88230 agree with the stellar photospheric predictions. We attribute these infrared excesses to a new class of cold, faint debris discs. Both αMen and HD 88230 are spatially resolved in the PACS 160 μm images, while HD 210277 is point-like at that wavelength. The projected linear sizes of the extended emission lie in the range from  ~115 to  ≤ 250 AU. The estimated black body temperatures from the 100 and 160 μm fluxes are  ≲22 K, and the fractional luminosity of the cold dust is Ldust/L⋆~ 10-6, close to the luminosity of the solar-system’s Kuiper belt. These debris discs are the coldest and faintest discs discovered so far around mature stars, so they cannot be explained easily invoking “classical” debris disc models.

Context.The current distribution of planet mass vs. incident stellar X-ray flux supports the idea that photoevaporation of the atmosphere may take place in close-in planets. Integrated effects have to be accounted for. A proper calculation of the mass loss rate through photoevaporation requires the estimation of the total irradiation from the whole XUV (X-rays and extreme ultraviolet, EUV) range.

Context.Dusty debris discs around main sequence stars are thought to be the result of continuous collisional grinding of planetesimals in the system. The majority of these systems are unresolved and analysis of the dust properties is limited by the lack of information regarding the dust location.

Aims.Chemical abundances in solar-type stars are still a much debated topic in many respects. In particular, planet-hosting stars are known to be metal-rich, but whether or not this peculiarity applies also to the chemical composition of the outer stellar atmospheres is still to be clarified. More in general, coronal and photospheric abundances in late-type stars appear to be different in many cases, but understanding how chemical stratification effects work in stellar atmospheres requires an observational base larger than currently available.

Context.Different environmental conditions can play a crucial role in determining final products of the star formation process, and in this context, less favorable activities of star formation are expected in the external regions of our Galaxy.