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The evaporation and the chemistry of the atmospheres of warm and hot planets are strongly determined by the high-energy irradiation they receive from their parent stars. This is more crucial among young extra-solar systems, due to the high activity of stars at early ages. In particular, the EUV part of the stellar spectra drives significant processes of photo-chemical interaction, but it is not directly measurable due to strong interstellar absorption and the lack of sufficiently sensitive instrumentation. An alternative approach is to derive synthetic spectra from the analysis of FUV and X-ray emission lines, that allow to estimate the missed flux in the EUV band. We performed joint and simultaneous spectroscopy of HIP 67522 with XMM-Newton and HST aimed at reconstructing the full high-energy spectrum of this 17 Myr old solar-type (G0) star, which is the youngest known transiting multi-planet system at present time. We performed a time-resolved spectral analysis of the observations, including quiescent emission and flaring variability. Then, we derived the Emission Measure Distribution (EMD) vs. temperature of the chromospheric and coronal plasma from the high-resolution spectra obtained in X-rays with RGS and in FUV with COS. We derived broad-band X-ray and EUV luminosities from the synthetic spectrum based on the EMD, that allowed us to test alternative EUV vs. X-ray scaling laws available in literature. We also employed the total XUV flux received by the inner planet of the system to estimate its instantaneous atmospheric mass loss rate. We confirm that HIP 67522 is a very active star with a hot corona, reaching plasma temperatures above 20 MK even in quiescent state. Its EUV/X-ray flux ratio falls in between the predictions of the two scaling laws we have tested, indicating an important spread in the stellar properties, that requires further investigation., Comment: 12 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics

Atmospheric mass loss plays a major role in the evolution of exoplanets. This process is driven by the stellar high-energy irradiation, especially in the first hundreds of millions of years after dissipation of the proto-planetary disk. A major source of uncertainty in modeling atmospheric photo-evaporation and photo-chemistry is due to the lack of direct measurements of the stellar flux at EUV wavelengths. Several empirical relationships have been proposed in the past to link EUV fluxes to emission levels in X-rays, but stellar samples employed for this aim are heterogeneous, and available scaling laws provide significantly different predictions, especially for very active stars. We present new UV and X-ray observations of V1298 Tau with HST/COS and XMM-Newton, aimed to determine more accurately the XUV emission of this solar-mass pre-Main Sequence star, which hosts four exoplanets. Spectroscopic data were employed to derive the plasma emission measure distribution vs.\ temperature, from the chromosphere to the corona, and the possible variability of this irradiation on short and year-long time scales, due to magnetic activity. As a side result, we have also measured the chemical abundances of several elements in the outer atmosphere of V1298 Tau. We employ our results as a new benchmark point for the calibration of the X-ray to EUV scaling laws, and hence to predict the time evolution of the irradiation in the EUV band, and its effect on the evaporation of exo-atmospheres., Comment: 17 pages, 14 figures. Accepted for pubblication on ApJ

We present a statistical analysis of the large-scale (up to 2 Mpc) environment of an homogeneous and complete sample, both in radio and optical selection, of wide-angle tailed radio galaxies (WATs) in the local Universe (i.e., with redshifts $z\lesssim$ 0.15). The analysis is carried out using the parameters obtained from cosmological neighbors within 2 Mpc of the target source. Results on WATs large-scale environments are then compared with that of Fanaroff-Riley type I (FR Is) and type II (FR IIs) radio galaxies, listed in two others homogeneous and complete catalogs, and selected with the same criterion adopted for the WATs catalog. We obtain indication that at low redshift WATs inhabit environments with a larger number of galaxies than that of FR Is and FR IIs. In the explored redshift range, the physical size of the galaxy group/cluster in which WATs reside appears to be almost constant with respect to FR Is and FR IIs, being around 1 Mpc. From the distribution of the concentration parameter, defined as the ratio between the number of cosmological neighbors lying within 500 kpc and within 1 Mpc, we conclude that WATs tend to inhabit the central region of the group/cluster in which they reside, in agreement with the general paradigm that WATs are the cluster BCG., Comment: 8 pages, 5 figures, 1 table. Accepted for publication on A&A

X-rays emitted by the coronae of solar-type stars are a feature present in up to late-A types during the main sequence phase. F stars, either with or without hot Jupiters, are usually X-ray emitters. The very low level of X-ray emission of the F5 star WASP-18 despite its relatively young age and spectral type is thus quite peculiar. [Abridged] We observed KELT-24 with \xmm\ for a total of 43 ks in order to test if the X-ray activity of this star is depressed by the interaction with its massive hot Jupiter, as is the case of WASP-18. KELT-24 is detected in combined EPIC images with a high significance level. Its average coronal spectrum is well described by a cool component at 0.36 keV and a hotter component at 0.98 keV. We detected a flare with a duration of about 2 ks, during which the coronal temperature reached 3.5 keV. The unabsorbed quiescent flux in 0.3-8.0 keV is $\sim1.33\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$, corresponding to a luminosity of $1.5\times10^{29}$ erg s$^{-1}$ at the distance of the star. The luminosity is well within the range of the typical X-ray luminosity of F stars in Hyades, which are coeval. We conclude that the activity of KELT-24 appears normal, as expected, and is not affected by any star--planet interaction. From the analysis of TESS light curves, we infer a distribution of optical flares for KELT-24 and WASP-18. Small optical flickering similar to flares is recognized in WASP-18 but at lower levels of energy and amplitude than in KELT-24. We discuss the causes of the low activity of WASP-18. Either WASP-18b could hamper the formation of a corona bright in X-rays in its host star through some form of tidal interaction, or the star has entered a minimum of activity similar to the solar Maunder minimum. This latter hypothesis would make WASP-18 among the few candidates showing such a quench of stellar activity., Comment: 6 pages, 7 figures, A&A accepted

We aim to narrow down the origin of the non-detection of the metastable HeI triplet at about 10830 A obtained for the hot Jupiter WASP-80b. We measure the X-ray flux of WASP-80 from archival observations and use it as input to scaling relations accounting for the coronal [Fe/O] abundance ratio to infer the extreme-ultraviolet (EUV) flux in the 200-504 A range, which controls the formation of metastable HeI. We run three dimensional (magneto) hydrodynamic simulations of the expanding planetary upper atmosphere interacting with the stellar wind to study the impact on the HeI absorption of the stellar high-energy emission, the He/H abundance ratio, the stellar wind, and the possible presence of a planetary magnetic field up to 1 G. For a low stellar EUV emission, which is favoured by the measured logR'HK value, the HeI non-detection can be explained by a solar He/H abundance ratio in combination with a strong stellar wind, or by a sub-solar He/H abundance ratio, or by a combination of the two. For a high stellar EUV emission, the non-detection implies a sub-solar He/H abundance ratio. A planetary magnetic field is unlikely to be the cause of the non-detection. The low EUV stellar flux, driven by the low [Fe/O] coronal abundance, is the likely primary cause of the HeI non-detection. High-quality EUV spectra of nearby stars are urgently needed to improve the accuracy of high-energy emission estimates, which would then enable one to employ the observations to constrain the planetary He/H abundance ratio and the stellar wind strength. This would greatly enhance the information that can be extracted from HeI atmospheric characterisation observations., Comment: Accepted for publication on A&A, 20 pages

The processes that lead from molecular clouds to protostars, stellar associations, and the evolution of both, are active research topics. The target of this study, NGC2264, is a benchmark star-forming region (SFR) in which these issues can be profitably studied. We assembled a new extensive sample of NGC 2264 members. To this end we used new X-ray data obtained with the XMM-Newton telescope, GAIA eDR3 data, and an extensive collection of public and published catalogs. Following a previous suggestion that the SFR might extend significantly beyond the better studied areas, our search covers a wide 2.5x2.5 degrees region in the sky. Our catalog comprises more than 2200 candidate members, a ~100% increase over previous determinations. We analyze their spatial distribution and define new substructures. Using GAIA parallaxes we estimate a new average distance to NGC2264 of 722+/-2pc and suggest that the embedded Spokes subregion is ~20pc farther away within the molecular cloud. A complex dynamics is unveiled by the available proper motions and radial velocities: we observe signs of global expansion and rotation. At the same time, we observe the collapse and coalescence of two substructures in a region where active star formation is taking place. The fraction of stars with disks and of those undergoing circumstellar accretion varies significantly across the field, suggesting that star formation has been occurring for several million years. A particularly low disk fraction around the O VII star S Mon might be attributed to external disk photoevaporation or to an older age of the stars in the region. NGC2264 is not dynamically relaxed and its present configuration is the result of multiple dynamical processes. The cloud has been forming stars for several million years and we identify the process that is likely responsible for the ongoing formation activity., Comment: 24 pages, 18 figures, accepted for publication on Astronomy & Astrophysics

Transiting planets around young stars are key benchmarks for our understanding of planetary systems. One of such candidates was identified around the K dwarf HD 18599 by TESS, labeled as TOI-179. We present the confirmation of the transiting planet and the characterization of the host star and of the TOI-179 system over a broad range of angular separations. To this aim, we exploited the TESS photometric time series, intensive radial velocity monitoring performed with HARPS, and deep high-contrast imaging observations obtained with SPHERE and NACO at VLT. The inclusion of Gaussian processes regression analysis is effective to properly model the magnetic activity of the star and identify the Keplerian signature of the transiting planet. The star, with an age of 400+-100 Myr, is orbited by a transiting planet with period 4.137436 days, mass 24+-7 Mearth, radius 2.62 (+0.15-0.12) Rearth, and significant eccentricity (0.34 (+0.07-0.09)). Adaptive optics observations identified a low-mass companion at the boundary between brown dwarfs and very low mass stars (mass derived from luminosity 83 (+4-6) Mjup) at a very small projected separation (84.5 mas, 3.3 au at the distance of the star). Coupling the imaging detection with the long-term radial velocity trend and the astrometric signature, we constrained the orbit of the low mass companion, identifying two families of possible orbital solutions. The TOI-179 system represents a high-merit laboratory for our understanding of the physical evolution of planets and other low-mass objects and of how the planet properties are influenced by dynamical effects and interactions with the parent star., Comment: 25 pages, 24 figures, A&A, in press

Abridged. We observed the 40 Myr old star DS Tuc A with XMM-Newton and recorded two X-ray bright flares, with the second event occurring about 12 ks after the first one. Their duration from the rise to the end of the decay was of about 8-10 ks in soft X-rays (0.3-10 keV). The flares were also recorded in the band 200-300 nm with the UVM2 filter of the Optical Monitor. The duration of the flares in UV was about 3 ks. The observed delay between the peak in the UV band and in X-rays is a probe of the heating phase followed by the evaporation and increase of density and emission measure of the flaring loop. The coronal plasma temperature at the two flare peaks reached 54-55 MK. The diagnostics based on temperatures and time scales of the flares applied to these two events allow us to infer a loop length of 5-7 x 10^10 cm, which is about the size of the stellar radius. We also infer values of electron density at the flare peaks of 2.3-6.5 x 10^11 cm^-3 , and a minimum magnetic field strength of order of 300-500 G needed to confine the plasma. The energy released during the flares was of order of 5-8 x 10^34 erg in the band 0.3-10 keV and 0.9-2.7 x 10^33 erg in the UV band (200-300 nm). We speculate that the flares were associated with Coronal Mass Ejections (CMEs) that hit the planet about 3.3 hr after the flares and dramatically increasing the rate of evaporation of the planet. From the RGS spectra we retrieved the emission measure distribution and the abundances of coronal metals during the quiescent and the flaring states. In agreement with what inferred from time resolved spectroscopy and EPIC spectra, also from the analysis of RGS spectra during the flares we infer a high electron density., Comment: 10 pages, 10 figures, 5 tables, accepted to A&A

KQVel is a binary system composed of a slowly rotating magnetic Ap star with a companion of unknown nature. In this paper, we report the detection of its radio emission. We conducted a multi-frequency radio campaign using the ATCA interferometer (band-names: 16cm, 4cm, and 15mm). The target was detected in all bands. The most obvious explanation for the radio emission is that it originates in the magnetosphere of the Ap star, but this is shown unfeasible. The known stellar parameters of the Ap star enable us to exploit the scaling relationship for non-thermal gyro-synchrotron emission from early-type magnetic stars. This is a general relation demonstrating how radio emission from stars with centrifugal magnetospheres is supported by rotation. Using KQVel's parameters the predicted radio luminosity is more than five orders of magnitudes lower than the measured one. The extremely long rotation period rules out the Ap star as the source of the observed radio emission. Other possible explanations for the radio emission from KQVel, involving its unknown companion, have been explored. A scenario that matches the observed features (i.e. radio luminosity and spectrum, correlation to X-rays) is a hierarchical stellar system, where the possible companion of the magnetic star is a close binary (possibly of RSCVn type) with at least one magnetically active late-type star. To be compatible with the total mass of the system, the last scenario places strong constraints on the orbital inclination of the KQVel stellar system., Comment: Accepted to MNRAS; 16 pages, 9 figures

The characterization of exoplanets, their formation, evolution, and chemical changes is tightly linked to our knowledge of their host stars. In particular, stellar X-rays and UV emission have a strong impact on the dynamical and chemical evolution of planetary atmospheres. We analyzed 25 XMM-Newton observations encompassing about eight years and totaling about 958 ks in order to study the X-ray emission of HD 189733 A. We find that the corona of HD 189733 A has an average temperature of 0.4 keV and it is only during flares that the mean temperature increases to 0.9 keV. Apart from the flares, there is no significant change in the flux and hardness of the coronal emission on a timescale of several months to years. Thus, we conclude that there is no detectable activity cycle on such timescales. We identified the flares and built their energy distribution. The number of flares observed around the phases of the planetary eclipses is not statistically different from the number of flares during transit phases. However, we do find a hint of a difference in the flare-energy distributions, as the flares observed around the planetary eclipses tend to be more energetic than the flares observed around the primary transits of the planet. We modeled the distribution of the number of flares per day with a power law, showing that it is steeper than the one observed in the Sun and in other Main Sequence stars. The steepness hints at a significant fraction of undetected micro-flares. Altogether, the plasma temperatures below 1 keV observed during the flares, along with the slightly larger fraction of energetic flares seen at the secondary transits highlight the peculiarity of the corona of HD 189733 A and points to star-planet interaction as the plausible origin of part of its X-ray emission. However, more observational and modeling efforts are required to confirm or disprove this scenario., Comment: 15 pages, 8 figures, accepted for publication in Astronomy & Astrophysics. Language editor version

Transiting planets at young ages are key targets for improving our understanding of the evolution of exo-atmospheres. We present results of a new X-ray observation of V1298 Tau with XMM-Newton, aimed to determine more accurately the high-energy irradiation of the four planets orbiting this pre-main-sequence star, and the possible variability due to magnetic activity on short and long time scales. Following the first measurements of planetary masses in the V1298 Tau system, we revise early guesses of the current escape rates from the planetary atmospheres, employing our updated atmospheric evaporation models to predict the future evolution of the system. Contrary to previous expectations, we find that the two outer Jupiter-size planets will not be affected by any evaporation on Gyr time scales, and the same occurs for the two smaller inner planets, unless their true masses are lower than $\sim 40$ M$_\oplus$. These results confirm that relatively massive planets can reach their final position in the mass-radius diagram very early in their evolutionary history., Comment: 9 pages, 4 figures. ApJ, in press

In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broadband radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner-magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broadband radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultra-cool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons., Comment: Accepted to MNRAS; 26 pages, 13 figures

We performed a radial velocity (RV) monitoring of the 40 Myr old star DS Tuc A with HARPS at the ESO-3.6m to determine the planetary mass of its 8.14-days planet, first revealed by TESS. We also observed two planetary transits with HARPS and ESPRESSO at ESO-VLT, to measure the Rossiter-McLaughlin (RM) effect and characterise the planetary atmosphere. We measured the high-energy emission of the host with XMM observations to investigate models for atmospheric evaporation. We employed Gaussian Processes (GP) regression to model the high level of the stellar activity, which is more than 40 times larger than the expected RV planetary signal. We extracted the transmission spectrum of DS Tuc A b from the ESPRESSO data and searched for atmospheric elements/molecules either by single-line retrieval and by performing cross-correlation with a set of theoretical templates. Through a set of simulations, we evaluated different scenarios for the atmospheric photo-evaporation of the planet induced by the strong XUV stellar irradiation. While the stellar activity prevented us from obtaining a clear detection of the planetary signal from the RVs, we set a robust mass upper limit of 14.4 M_e for DS Tuc A b. We also confirm that the planetary system is almost (but not perfectly) aligned. The strong level of stellar activity hampers the detection of any atmospheric compounds, in line with other studies presented in the literature. The expected evolution of DS Tuc A b from our grid of models indicates that the planetary radius after the photo-evaporation phase will fall within the Fulton gap. The comparison of the available parameters of known young transiting planets with the distribution of their mature counterpart confirms that the former are characterised by a low density, with DS Tuc A b being one of the less dense., Comment: 24 pages, 19 figures, Accepted for publication on Astronomy and Astrophysics

The most elusive and extreme sub-class of active galactic nuclei (AGNs), known as BL Lac objects, shows features that can only be explained as the result of relativistic effects occurring in jets pointing at a small angle with respect to the line of sight. A long standing issue is the identification of the BL Lac parent population, having jets oriented at larger angles. According to the "unification scenario" of AGNs, radio galaxies with low luminosity and edge-darkened radio morphology are the most promising candidates to be the parent population of BL Lacs. Here we compare the large-scale environment, an orientation independent property, of well-defined samples of BL Lacs with samples of radio-galaxies all lying in the local Universe. Our study reveals that BL Lacs and radio galaxies live in significantly different environments, challenging predictions of the unification scenario. We propose a solution to this problem proving that large-scale environments of BL Lacs is statistically consistent with that of compact radio-sources, known as FR0s, sharing similar properties. This implies that highly relativistic jets are ubiquitous and are the natural outcome of the accretion of gas into the deep gravitational potential well produced by supermassive black holes., Comment: 6 pages, 1 table, 4 figures (accepted for publication in ApJ Letters - pre-proofs version)

The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multi-wavelength (1.6-16.7 GHz) ATCA measurements of the early-type magnetic star rho OphC, which is a flat-spectrum non-thermal radio source. The rho OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about 60% at the lowest frequency sub-band (1.6 GHz) while is undetected at 16.7 GHz. This is clear evidence of coherent Auroral Radio Emission (ARE) from the rho OphC magnetosphere. Interestingly, the detection of the rho OphC's ARE is not related to a peculiar rotational phase. This is a consequence of the stellar geometry, which makes the strongly anisotropic radiation beam of the amplified radiation always pointed towards Earth. The circular polarization sign evidences mainly amplification of the ordinary mode of the electromagnetic wave, consistent with a maser amplification occurring within dense regions. This is indirect evidence of the plasma evaporation from the polar caps, a phenomenon responsible for the thermal X-ray aurorae. rho OphC is not the first early-type magnetic star showing the O-mode dominated ARE but is the first star with the ARE always on view., Comment: 5 pages, 4 figures; accepted to MNRAS Letter

In our previous analysis we investigated the large-scale environment of two samples of radio galaxies (RGs) in the local Universe (i.e. with redshifts z<0.15), classified as FR I and FR II on the basis of their radio morphology. The analysis was carried out using i) extremely homogeneous catalogs and ii) a new method, known as cosmological overdensity, to investigate their large-scale environments. We concluded that, independently by the shape of their radio extended structure, RGs inhabit galaxy-rich large-scale environments with similar characteristics and richness. In the present work, we first highlight additional advantages of our procedure, that does not suffer cosmological biases and/or artifacts, and then we carry out an additional statistical test to strengthen our previous results. We also investigate properties of RG environments using those of the cosmological neighbors. We find that large-scale environments of both FRIs and FRIIs are remarkably similar and independent on the properties of central RG. Finally, we highlight the importance of comparing radio sources in the same redshift bins to obtain a complete overview of their large-scale environments., Comment: 15 pages, 21 figures, 3 tables, ApJS in press (pre-proof version)

We present new ATCA multi-wavelength radio measurements (range 2.1-21.2 GHz) of the early-type magnetic star rho Oph A, performed in March 2019 during 3 different observing sessions. These new ATCA observations evidence a clear rotational modulation of the stellar radio emission and the detection of coherent auroral radio emission from rho Oph A at 2.1 GHz. We collected high-resolution optical spectra of rho Oph A acquired by several instruments over a time span of about ten years. We also report new magnetic field measurements of rho Oph A that, together with the radio light curves and the temporal variation of the equivalent width of the HeI line (lambda=5015 Angstrom), were used to constrain the rotation period and the stellar magnetic field geometry. The above results have been used to model the stellar radio emission, modelling that allowed us to constrain the physical condition of rho Oph A's magnetosphere. Past XMM measurements showed periodic X-ray pulses from rho Oph A. We correlate the X-ray light curve with the magnetic field geometry of rho Oph A. The already published XMM data have been re-analyzed showing that the X-ray spectra of rho Oph A are compatible with the presence of a non-thermal X-ray component. We discuss a scenario where the emission phenomena occurring at the extremes of the electromagnetic spectrum, radio and X-ray, are directly induced by the same plasma process. We interpret the observed X-ray and radio features of rho Oph A as having an auroral origin., Comment: 20 pages, 13 figures; accepted to MNRAS

[Abridged] We investigated the X-ray characteristics of the Class I YSO Elias 29 with joint XMM-Newton and NuSTAR observations of 300 ks and 450 ks, respectively. These are the first observations of a very young (<1 Myr) stellar object in a band encompassing simultaneously both soft and hard X-rays. In addition to the hot Fe complex at 6.7 keV, we observed fluorescent emission from Fe at $\sim6.4$ keV, confirming the previous findings. The line at 6.4 keV is detected during quiescent and flaring states and its flux is variable. The equivalent width is found varying in the $\approx 0.15--0.5$ keV range. These values make unrealistic a simple model with a centrally illuminated disk and suggest a role of the cavity containing Elias 29 and possibly reverberation processes that could occur in it. We observed two flares, with duration of 20 ks and 50 ks, respectively. We systematically observed an increase of $N_H$ during the flares of a factor five. This behavior has been observed during flares previously detected in Elias 29 with XMM-Newton and ASCA. The phenomenon hints that the flaring regions could be buried under the accretion streams and at high stellar latitudes, as the X-rays from flares pass through gas denser than the gas along the line of sight of the quiescent corona. In a different scenario, a contribution from scattered soft photons to the primary coronal emission could mimic a shallower $N_H$ in the quiescent spectrum. In the spectrum of the full NuSTAR exposure, we detect hard X-ray emission in the band $\approx20-80$ keV in excess with respect to the thermal emission. The hard X-ray emission could be due to a population of energetic electrons accelerated by the magnetic field along the accretion streams. These particles could concur to pumping up the Fe fluorescence of cold Fe of the disk along with X-ray photons with $E>7.11$ keV., Comment: 15 pages, 14 figures, accepted for publication on A&A

Flares are powerful events ignited by a sudden release of magnetic energy. With the aim of studying flares in the 125-Myr-old stars in the Pleiades observed simultaneously in optical and X-ray light, we obtained new XMM-Newton observations of this cluster during the observations of Kepler K2 Campaign 4. Our objective is to characterize the most powerful flares observed in both bands and to constrain the energy released in the optical and X-ray, the geometry of the loops, and their time evolution. We aim to compare our results to existing studies of flares occurring in the Sun and stars at different ages. We selected bright X-ray/optical flares occurred in 12 known members of the Pleiades from their K2 and XMM-Newton light curves. The sample includes ten K-M stars, one F9 star, and one G8 star. Flare average properties were obtained from integrated analysis of the light curves during the flares. The time evolution of the plasma in the magnetic loops is constrained with time-resolved X-ray spectral analysis. Most of the flares studied in this work emitted more energy in optical than in X-rays, as in most solar flares, even if the Pleiades flares output a larger fraction of their total energy in X-rays than typical solar flares do. Additionally, the energy budget in the two bands is weakly correlated. We also found comparable flare duration in optical and X-rays and observed that rapidly rotating stars (e.g., with rotation period shorter than 0.5 days) preferentially host short flares. We estimated the slope of the cooling path of the flares in the log(EM)-versus-log(T) plane. The values we obtained are affected by large uncertainties, but their nominal values suggest that the flares analyzed in this paper are mainly due to single loops with no sustained heating occurring during the cooling phase. We also observed and analyzed oscillations with a period of 500 s during one of the flares., Comment: Accepted for publication by Astronomy & Astrophysics. Some figures are included with low resolution. The abstract is shorter than that of the orginal paper

The role played by the large-scale environment on the nuclear activity of radio galaxies (RGs), is still not completely understood. Accretion mode, jet power and galaxy evolution are connected with their large-scale environment from tens to hundreds of kpc. Here we present a detailed, statistical, analysis of the large-scale environment for two samples of RGs up to redshifts $z_\mathrm{src}$=0.15. The main advantages of our study, with respect to those already present in the literature, are due to the extremely homogeneous selection criteria of catalogs adopted to perform our investigation. This is also coupled with the use of several clustering algorithms. We performed a direct search of galaxy-rich environments around RGs using them as beacon. To perform this study we also developed a new method that does not appear to suffer by a strong $z_\mathrm{src}$ dependence as other algorithms. We conclude that, despite their radio morphological (FR\,I $vs$ FR\,II) and/or their optical (HERG $vs$ LERG) classification, RGs in the local Universe tend to live in galaxy-rich large-scale environments having similar characteristics and richness. We highlight that the fraction of FR\,Is-LERG, inhabiting galaxy rich environments, appears larger than that of FR\,IIs-LERG. We also found that 5 out of 7 FR\,II-HERGs, with $z_\mathrm{src}\leq$0.11, lie in groups/clusters of galaxies. However, we recognize that, despite the high level of completeness of our catalogs, when restricting to the local Universe, the low number of HERGs ($\sim$10\% of the total FR\,IIs investigated) prevent us to make a strong statistical conclusion about this source class., Comment: 21 pages, 25 figures, accepted for publication on the Astrophysical Journal Supplement Series - pre-proof version

Sco OB2 is the nearest OB association, extending over approximately 2000 sq.deg. on the sky. Only its brightest members are already known (from Hipparcos) across its entire size, while studies of its lower-mass population refer only to small portions of its extent. In this work we exploit the capabilities of Gaia DR2 measurements to search for Sco OB2 members across its entire size and down to the lowest stellar masses. We use both Gaia astrometric and photometric data to select association members, using minimal assumptions derived mostly from the Hipparcos studies. Gaia resolves small details in both the kinematics of individual Sco OB2 subgroups and their distances from the Sun. We develop methods to explore the 3D kinematics of stellar populations covering large sky areas. We find ~11000 pre-main sequence (PMS) Sco OB2 members (with <3% contamination), plus ~3600 MS candidate members with a larger (10-30%) field-star contamination. A higher-confidence subsample of ~9200 PMS (and ~1340 MS) members is also selected (<1% contamination for the PMS), affected however by larger (~15%) incompleteness. We classify separately stars in compact and diffuse populations. Most members belong to a few kinematically distinct diffuse populations, whose ensemble outlines the association shape. Upper Sco is the densest part of Sco OB2, with a complex spatial and kinematical structure, and no global pattern of motion. Other dense subclusters are found in Upper Centaurus-Lupus and in Lower Centaurus-Crux. Most clustered stars appear to be younger than the diffuse PMS population, suggesting star formation in small groups which rapidly disperse and dilute, while keeping memory of their original kinematics. We also find that the open cluster IC 2602 has a similar dynamics to Sco OB2, and its PMS members are evaporating and forming a ~10 deg halo around its double-peaked core., Comment: 27 pages, 37 figures. Accepted for publication in Astronomy and Astrophysics

WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age ($<$1 Gyr), the star presents an anomalously low stellar activity level: the measured logR'$_{\rm HK}$ activity parameter lies slightly below the basal level; there is no significant time-variability in the logR'$_{\rm HK}$ value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of Hubble Space Telescope aimed at explaining this anomaly. From the star's spectral energy distribution, we infer the extinction (E(B-V) $\approx$ 0.01 mag) and then the interstellar medium (ISM) column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (CII, CIII, CIV, SiIV). Comparing the CII/CIV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old ($>$5 Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the SiIV line, yields an XUV integrated flux at the planet orbit of 10.2 erg cm$^{-2}$ s$^{-1}$. We employ the rescaled XUV solar fluxes to models of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10$^{-20}$ $M_{\rm J}$ Gyr$^{-1}$. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape., Comment: Accepted for publication on AJ

Circumstantial evidence suggests that magnetism and enhanced X-ray emission are likely correlated in early B-type stars: similar fractions of them ($\sim$ 10 %) are strong and hard X-ray sources and possess strong magnetic fields. It is also known that some B-type stars have spots on their surface. Yet up to now no X-ray activity associated with spots on early-type stars was detected. In this Letter we report the detection of a magnetic field on the B2V star $\rho$ Oph A. Previously, we assessed that the X-ray activity of this star is associated with a surface spot, herewith we establish its magnetic origin. We analyzed FORS2 ESO VLT spectra of $\rho$ Oph A taken at two epochs and detected a longitudinal component of the magnetic field of order of $\sim500$ G in one of the datasets. The detection of the magnetic field only at one epoch can be explained by stellar rotation which is also invoked to explain observed periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we derived the fundamental stellar parameters of $\rho$ Oph A and further constrained its age. We conclude that $\rho$ Oph A provides strong evidence for the presence of active X-ray emitting regions on young magnetized early type stars., Comment: 4 pages, 1 figure, 2 tables, accepted as a "Letter to the Editor" to Astronomy & Astrophysics

We present the results of a 40 ks XMM-Newton observation centered on the variable star V1818 Ori. Using a combination of the XMM-Newton and AllWISE catalog data, we identify a group of about 31 young stellar objects around V1818 Ori. This group is coincident with the eastern edge of the dust ring surrounding Kappa Ori. Previously, we concluded that the young stellar objects on the western side of ring were formed in an episode of star formation that started 3-5 Myr ago, and are at a distance similar to that of kappa Ori (250-280 pc) and in the foreground to the Orion A cloud. Here we use the XMM-Newton observation to calculate X-ray fluxes and luminosities of the young stars around V1818 Ori. We find that their X-ray luminosity function (XLF), calculated for a distance of ~270 pc, matches the XLF of the YSOs west of Kappa Ori. We rule out that this group of young stars is associated to Mon R2 as assumed in the literature, but rather they are part of the same Kappa Ori's ring stellar population., Comment: 4 pages, 4 figures, 1 table. Accepted for publication to A&A

Observations indicate that magnetic fields in rapidly rotating stars are very strong, on both small and large scales. What is the nature of the resulting corona? Here we seek to shed some light on this question. We use the results of an anelastic dynamo simulation of a rapidly rotating fully-convective M-star to drive a physics-based model for the stellar corona. We find that due to the several kilo Gauss large-scale magnetic fields at high latitudes, the corona and its X-ray emission are dominated by star-size large hot loops, while the smaller, underlying colder loops are not visible much in the X-ray. Based on this result we propose that, in rapidly rotating stars, emission from such coronal structures dominates the quiescent, cooler but saturated X-ray emission., Comment: 9 pages, 4 figures, accepted to ApJ

X-rays are a powerful probe of activity in early stages of star formation. They allow us to identify young stars even after they have lost the IR signatures of circumstellar disks and provide constraints on their distance. Here we report on XMM-Newton observations which detect 121 young stellar objects (YSOs) in two fields between L1641S and $\kappa$ Ori. These observations extend the Survey of Orion A with XMM and Spitzer (SOXS). The YSOs are contained in a ring of gas and dust apparent at millimeter wavelengths, and in far-IR and near-IR surveys. The X-ray luminosity function of the young stellar objects detected in the two fields indicates a distance of 250-280 pc, much closer than the Orion A cloud and similar to distance estimates of $\kappa$ Ori. We propose that the ring is a 5-8 pc diameter shell that has been swept up by $\kappa$ Ori. This ring contains several groups of stars detected by Spitzer and WISE including one surrounding the Herbig Ae/Be stars V1818 Ori. In this interpretation, the $\kappa$ Ori ring is one of several shells swept up by massive stars within the Orion Eridanus Superbubble, and is unrelated to the southern portion of Orion A / L1641 S., Comment: 6 pages, 3 figures. Accepted for publication by ApJL

The large number of close-in Jupiter-size exoplanets prompts the question whether star-planet interaction (SPI) effects can be detected. We focused our attention on the system HD 17156, having a Jupiter-mass planet in a very eccentric orbit. Here we present results of the XMM-Newton observations and of a five months coordinated optical campaign with the HARPS-N spectrograph. We observed HD 17156 with XMM-Newton when the planet was approaching the apoastron and then at the following periastron passage, quasi simultaneously with HARPS-N. We obtained a clear ($\approx 5.5\sigma$) X-ray detection only at the periastron visit, accompanied by a significant increase of the $R'_{\rm HK}$ chromospheric index. We discuss two possible scenarios for the activity enhancement: magnetic reconnection and flaring or accretion onto the star of material tidally stripped from the planet. In any case, this is possibly the first evidence of a magnetic SPI effect caught in action.

Binary stars hosting exoplanets are a unique laboratory where chemical tagging can be performed to measure with high accuracy the elemental abundances of both stellar components, with the aim to investigate the formation of planets and their subsequent evolution. Here, we present a high-precision differential abundance analysis of the XO-2 wide stellar binary based on high resolution HARPS-N@TNG spectra. Both components are very similar K-dwarfs and host planets. Since they formed presumably within the same molecular cloud, we expect they should possess the same initial elemental abundances. We investigate if the presence of planets can cause some chemical imprints in the stellar atmospheric abundances. We measure abundances of 25 elements for both stars with a range of condensation temperature $T_{\rm C}=40-1741$ K, achieving typical precisions of $\sim 0.07$ dex. The North component shows abundances in all elements higher by $+0.067 \pm 0.032$ dex on average, with a mean difference of +0.078 dex for elements with $T_{\rm C} > 800$ K. The significance of the XO-2N abundance difference relative to XO-2S is at the $2\sigma$ level for almost all elements. We discuss the possibility that this result could be interpreted as the signature of the ingestion of material by XO-2N or depletion in XO-2S due to locking of heavy elements by the planetary companions. We estimate a mass of several tens of $M_{\oplus}$ in heavy elements. The difference in abundances between XO-2N and XO-2S shows a positive correlation with the condensation temperatures of the elements, with a slope of $(4.7 \pm 0.9) \times 10^{-5}$ dex K$^{-1}$, which could mean that both components have not formed terrestrial planets, but that first experienced the accretion of rocky core interior to the subsequent giant planets., Comment: 10 pages, 5 figures, accepted by Astronomy & Astrophysics. Numbering of the series changed

Aims. We observed the $\tau$ Boo system with the HARPS-N spectrograph to test a new observational strategy aimed at jointly studying asteroseismology, the planetary orbit, and star-planet magnetic interaction. Methods. We collected high-cadence observations on 11 nearly consecutive nights and for each night averaged the raw FITS files using a dedicated software. In this way we obtained spectra with a high signal-to-noise ratio, used to study the variation of the CaII H&K lines and to have radial velocity values free from stellar oscillations, without losing the oscillations information. We developed a dedicated software to build a new custom mask that we used to refine the radial velocity determination with the HARPS-N pipeline and perform the spectroscopic analysis. Results. We updated the planetary ephemeris and showed the acceleration caused by the stellar binary companion. Our results on the stellar activity variation suggest the presence of a high-latitude plage during the time span of our observations. The correlation between the chromospheric activity and the planetary orbital phase remains unclear. Solar-like oscillations are detected in the radial velocity time series: we estimated asteroseismic quantities and found that they agree well with theoretical predictions. Our stellar model yields an age of $0.9\pm0.5$ Gyr for $\tau$ Boo and further constrains the value of the stellar mass to $1.38\pm0.05$ M$_\odot$., Comment: 14 pages, accepted for publication in Astronomy and Astrophysics

Hot Jupiters are subject to strong irradiation from the host stars and, as a consequence, they do evaporate. They can also interact with the parent stars by means of tides and magnetic fields. Both phenomena have strong implications for the evolution of these systems. Here we present time resolved spectroscopy of HD~189733 observed with the Cosmic Origin Spectrograph (COS) on board to HST. The star has been observed during five consecutive HST orbits, starting at a secondary transit of the planet ($\phi$ ~0.50-0.63). Two main episodes of variability of ion lines of Si, C, N and O are detected, with an increase of line fluxes. Si IV lines show the highest degree of variability. The FUV variability is a signature of enhanced activity in phase with the planet motion, occurring after the planet egress, as already observed three times in X-rays. With the support of MHD simulations, we propose the following interpretation: a stream of gas evaporating from the planet is actively and almost steadily accreting onto the stellar surface, impacting at $70-90\deg$ ahead of the sub-planetary point., Comment: 35 pages, 19 Figures. Accepted for publication to ApJ

(Abridged) Low-mass stars have been recognised as promising targets in the search for rocky, small planets with the potential of supporting life. Doppler search programmes using high-resolution spectrographs like HARPS or HARPS-N are providing huge quantities of optical spectra of M dwarfs. We aim to calibrate empirical relationships to determine stellar parameters for early M dwarfs (spectral types M0-M4.5) using the same spectra that are used for the radial velocity determinations. Our methodology consists in the use of ratios of pseudo equivalent widths of spectral features as a temperature diagnostic. Stars with effective temperatures obtained from interferometric estimates of their radii are used as calibrators. Empirical calibrations for the spectral type are also provided. Combinations of features and ratios of features are used to derive calibrations for the stellar metallicity. Our methods are then applied to a large sample of M dwarfs that are being observed in the framework of the HARPS search for extrasolar planets.The derived temperatures and metallicities are used together with photometric estimates of mass, radius, and surface gravity to calibrate empirical relationships for these parameters. A total of 112 temperature sensitive ratios have been calibrated over the range 3100-3950 K, providing Teff values with typical uncertainties of the order of 70 K. Eighty-two ratios of pseudo equivalent widths of features were calibrated to derive spectral types. Regarding stellar metallicity, 696 combinations of pseudo equivalent widths of individual features and temperature-sensitive ratios have been calibrated, over the metallicity range from -0.54 to +0.24 dex, with estimated uncertainties in the range of 0.07-0.10 dex. We provide our own empirical calibrations for stellar mass, radius, and surface gravity., Comment: Accepted for publication by Astronomy and Astrophysics

Abridged. Here we report on the X-ray activity of the primary star, HD189733 A, using a new XMM-Newton observation and a comparison with the previous X-ray observations. The spectrum in the quiescent intervals is described by two temperatures at 0.2 keV and 0.7 keV, while during the flares a third component at 0.9 keV is detected. We obtain estimates of the electron density in the range $n_e = 1.6 - 13 \times 10^{10}$ cm$^{-3}$ and thus the corona of HD189733 A appears denser than the solar one. {For the third time, we observe a large flare that occurred just after the eclipse of the planet. Together with the flares observed in 2009 and 2011, the events are restricted to a small planetary phase range of $\phi = 0.55-0.65$. Although we do not find conclusive evidence of a significant excess of flares after the secondary transits, we suggest that the planet might trigger such flares when it passes close to locally high magnetic field of the underlying star at particular combinations of stellar rotational phases and orbital planetary phases. For the most recent flares, a wavelet analysis of the light curve suggests a loop of length of four stellar radii at the location of the bright flare, and a local magnetic field of order of 40-100 G, in agreement with the global field measured in other studies. The loop size suggests an interaction of magnetic nature between planet and star, separated by only $\sim8 R_*$. We also detect the stellar companion (HD 189733 B, $\sim12"$ from the primary star) in this XMM observation. Its very low X-ray luminosity ($L_X = 3.4\times 10^{26}$ erg s$^{-1}$) confirms the old age of this star and of the binary system. The high activity of the primary star is best explained by a transfer of angular momentum from the planet to the star., Comment: Preprint format, 28 Pages, 10 Figures, accepted for publication on ApJ

We report the detection, with ${\it Chandra}$ and XMM-${\it Newton}$, of faint, soft X-ray emission from four symbiotics stars that were not known to be X-ray sources. These four object show a $\beta$-type X-ray spectrum, i.e. their spectra can be modeled with an absorbed optically thin thermal emission with temperatures of a few million degrees. Photometric series obtained with the Optical Monitor on board XMM-${\it Newton}$ from V2416 Sgr and NSV 25735 support the proposed scenario where the X-ray emission is produced in a shock-heated region inside the symbiotic nebulae., Comment: 5 pages, 1 figure, 2 tables; A&A published

The high resolution non-dispersive spectroscopy and unprecedented sensitivity of Athena+ will revolutionize solar system observing: the origin of the ions producing Jupiter's X-ray aurorae via charge exchange will be conclusively established, as well as their dynamics, giving clues to their acceleration mechanisms. X-ray aurorae on Saturn will be searched for to a depth unattainable by current Earth-bound observatories. The X-ray Integral Field Unit of Athena+ will map Mars' expanding exosphere, which has a line-rich solar wind charge exchange spectrum, under differing solar wind conditions and through the seasons; relating Mars' X-ray emission to its atmospheric loss will have significant impact also on the study of exoplanet atmospheres. Spectral mapping of cometary comae, which are spectacular X-ray sources with extremely line-rich spectra, will probe solar wind composition and speed at varying distances from the Sun. Athena+ will provide unique contributions also to exoplanetary astrophysics. Athena+ will pioneer the study of ingress/eclipse/egress effects during planetary orbits of hot-Jupiters, and will confirm/improve the evidence of Star-Planet Interactions (SPI) in a wider sample of planetary systems. Finally Athena+ will drastically improve the knowledge of the X-ray incident radiation on exoplanets, a key element for understanding the effects of atmospheric mass loss and of the chemical and physical evolution of planet atmospheres, particularly relevant in the case of young systems., Comment: Supporting paper for the science theme "The Hot and Energetic Universe" to be implemented by the Athena+ X-ray observatory (http://www.the-athena-x-ray-observatory.eu). 8 pages, 6 figures

We present an XMM-Newton survey of the part of Orion A cloud south of the Orion Nebula. This survey includes the Lynds 1641 (L1641) dark cloud, a region of the Orion A cloud with very few massive stars and hence a relatively low ambient UV flux, and the region around the O9 III star Iota Orionis. In addition to proprietary data, we used archival XMM data of the Orion Nebula Cluster (ONC) to extend our analysis to a major fraction of the Orion A cloud. We have detected 1060 X-ray sources in L1641 and Iota Ori region. About 94% of the sources have 2MASS & Spitzer counterparts, 204 and 23 being Class II and Class I or protostars objects, respectively. In addition, we have identified 489 X-ray sources as counterparts to Class III candidates, given they are bright in X-rays and appear as normal photospheres at mid-IR wavelengths. The remaining 205 X-ray sources are likely distant AGNs or other galactic sources not related to Orion A. We find that Class III candidates appear more concentrated in two main clusters in L1641. The first cluster of Class III stars is found toward the northern part of L1641, concentrated around Iota Ori. The stars in this cluster are more evolved than those in the Orion Nebula. We estimate a distance of 300-320 pc for this cluster and thus it is closer than the Orion A cloud. Another cluster rich in Class III stars is located in L1641 South and appears to be a slightly older cluster embedded in the Orion A cloud. Furthermore, other evolved Class III stars are found north of the ONC toward NGC 1977., Comment: 42 pages, 14 figures, accepted for publication in ApJ

We report on the follow-up XMM-Newton observation of the planet-hosting star HD 189733 we obtained in April 2011. We observe a flare just after the secondary transit of the hot Jupiter. This event shares the same phase and many of the characteristics of the flare we observed in 2009. We suggest that a systematic interaction between planet and stellar magnetic fields when the planet passes close to active regions on the star can lead to periodic variability phased with planetary motion. By mean of high resolution X-ray spectroscopy with RGS we determine that the corona of this star is unusually dense., Comment: accepted for publication on ApJL

We present early results of our multiwavelength study of the star forming region IRAS 20050+2720. While we use X-rays and IR to classify young stellar objects, the optical data can be used to exclude foreground objects. The dataset set contains 57 class I sources, 183 class II sources and 183 X-ray sources, of which 140 are class III candidates. Within IRAS 20050+2720 four subclusters are found. Subcluster A and B are the densest regions, which contain stars of all evolutionary stages. Subcluster C is much younger than the other two. It has not formed any class III objects yet. We newly identify a fourth subcluster, which consists mostly of class II objects and is located about 10 arcmin from the center of the cloud., Comment: Poster Proceedings Cool Stars 16

We present a systematic search for Fe Kalpha emission from young stellar objects of the rho Ophiuchi star forming region observed in the Deep Rho Ophiuchi XMM-Newton Observation (DROXO)., Comment: Poster Proceedings Cool Stars 16

We report on two XMM-Newton observations of the planetary host star HD189733. The system has a close in planet and it can potentially affect the coronal structure via interactions with the magnetosphere. We have obtained X-ray spectra and light curves from EPIC and RGS on board XMM-Newton which we have analyzed and interpreted. We reduced X-ray data from primary transit and secondary eclipse occurred in April 17th 2007 and May 18th 2009, respectively. In the April 2007 observation only variability due to weak flares is recognized. In 2009 HD189733 exhibited a X-ray flux always larger than in the 2007 observation. The average flux in 2009 was higher than in 2007 observation by a factor of 45%. During the 2009 secondary eclipse we observed a softening of the X-ray spectrum significant at level of ~3 sigma. Further, we observed the most intense flare recorded at either epochs. This flare occurred 3 ks after the end of the eclipse.The flare decay shows several minor ignitions perhaps linked to the main event and hinting for secondary loops that emit triggered by the main loop. Magneto-Hydro-Dynamical (MHD) simulations show that the magnetic interaction between planet and star enhances the density and the magnetic field in a region comprised between the planet and the star because of their relative orbital/rotation motion. X-ray observations and model predictions are globally found in agreement, despite the quite simple MHD model and the lack of precise estimate of parameters including the alignment and the intensity of stellar and planetary magnetic fields. Future observations should confirm or disprove this hypothesis, by determining whether flares are systematically recurring in the light curve at the same planetary phase., Comment: Accepted for publication on The Astrophysical Journal

X-rays from very young stars are powerful probes to investigate the mechanisms at work in the very first stages of the star formation and the origin of X-ray emission in very young stars. We present results from a 500 ks long observation of the Rho Ophiuchi cloud with a XMM-Newton large program named DROXO, aiming at studying the X-ray emission of deeply embedded Young Stellar Objects (YSOs). The data acquired during the DROXO program were reduced with SAS software, and filtered in time and energy to improve the signal to noise of detected sources; light curves and spectra were obtained. We detected 111 sources, 61 of them associated with rho Ophiuchi YSOs as identified from infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class I, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21 classified Class III candidates are also detected. At the same time we suggest that 15 Class III candidates that remain undetected at log Lx < 28.3 are not related to the cloud population. The global detection rate is ~64%. We have achieved a flux sensitivity of ~5 x 10^{-15} erg s^{-1} cm^{-2}. The Lx to L_bol ratio shows saturation at a value of ~ -3.5 for stars with T_eff <= 5000 K or 0.7 M_sun as observed in the Orion Nebula. The plasma temperatures and the spectrum absorption show a decline with YSO class, with Class I YSOs being hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft excess in the X-ray spectrum which is best fitted by a cold thermal component less absorbed than the main thermal component of the plasma. Such a soft component hints to the presence of plasma heated by shocks due to jets outside the dense circumstellar material., Comment: Accepted for publication on Astronomy & Astrophysics journal

The infrared [Ne II] and [Ne III] fine structure lines at 12.81um and 15.55um are predicted to trace the circumstellar disk gas subject to X-ray heating and ionization. We investigate the origin of these lines by comparing observations with models of X-ray irradiated disks and by searching for empirical correlations between the line luminosities and stellar and circumstellar parameters. We measure neon line fluxes and X-ray luminosities for 28 young stellar objects in the Rho Ophiuchi star formation region for which good quality infrared spectra and X-ray data have been obtained, the former with the Spitzer IRS and the latter with the Deep Rho Ophiuchi XMM-Newton Observation. We detect the [Ne II] and the [Ne III] lines in 10 and 1 cases, respectively. Line luminosities show no correlation with X-ray emission. The luminosity of the [Ne II] line for one star, and that of both the [Ne II] and [Ne III] lines for a second star, match the predictions of published models of X-ray irradiated disks; for the remaining 8 objects the [Ne II] emission is 1-3 dex higher than predicted on the basis of their L_X. Class I objects show significantly stronger [Ne II] lines than Class II and Class III ones. A correlation is moreover found between the [Ne II] line emission and the disk mass accretion rates. This might point toward a role of accretion-generated UV emission in the generation of the line or to other mechanisms related to mass inflows from circumstellar disks and envelopes and/or to the associated mass outflows (winds and jets). We conclude that the X-ray luminosity is not the only parameter that determines the [Ne II] emission. Explaining the strong [Ne II] emission of Class I objects likely requires the inclusion in the models of additional physical components such as the envelope, inflows, and outflows., Comment: 21 pages, accepted for publication on Astronomy & Astrophysics (appendix to be published only in the electronic edition)

We have analyzed optical spectra of 25 X-ray sources identified as potential new members of the Taurus molecular cloud (TMC), in order to confirm their membership in this SFR. Fifty-seven candidates were previously selected among the X-ray sources in the XEST survey, having a 2MASS counterpart compatible with a PMS star based on color-magnitude and color-color diagrams. We obtained high-resolution optical spectra for 7 of these candidates with the SARG spectrograph at the TNG telescope, which were used to search for Li absorption and to measure the Ha line and the radial and rotational velocities; 18 low-resolution optical spectra obtained with DOLORES for other candidate members were used for spectral classification, for Ha measurements, and to assess membership together with IR color-color and color-magnitude diagrams and additional information from the X-ray data. We found that 3 sources show Li absorption, with equivalent widths of ~500 mA, broad spectral line profiles, indicating v sin i ~20-40 km/s, radial velocities consistent with those for known members, and Ha emission. Two of them are classified as new WTTSs, while the EW (~ -9 Ang) of the Ha line and its broad asymmetric profile clearly indicate that the third star (XEST-26-062) is a CTTS. Fourteen sources observed with DOLORES are M-type stars. Fifteen sources show Ha emission; 6 of them have spectra that indicate surface gravity lower than in MS stars, and their de-reddened positions in IR color-magnitude diagrams are consistent with their derived spectral type and with PMS models at the distance of the TMC. The K-type star XEST-11-078 is confirmed as a new member from the strength of its Ha emission line. Overall, we confirm membership to the TMC for 10 out of 25 X-ray sources observed in the optical. Three sources remain uncertain., Comment: 15 pages, 7 figures, accepted by Astronomy & Astrophysics

AIMS. While observational evidence shows that most of the decline in a star's X-ray activity occurs between the age of the Hyades (~8 x 10^8 yrs) and that of the Sun, very little is known about the evolution of stellar activity between these ages. To gain information on the typical level of coronal activity at a star's intermediate age, we studied the X-ray emission from stars in the 1.9 Gyr old open cluster NGC 752. METHODS. We analysed a ~140 ks Chandra observation of NGC 752 and a ~50 ks XMM-Newton observation of the same cluster. We detected 262 X-ray sources in the Chandra data and 145 sources in the XMM-Newton observation. Around 90% of the catalogued cluster members within Chandra's field-of-view are detected in the X-ray. The X-ray luminosity of all observed cluster members (28 stars) and of 11 cluster member candidates was derived. RESULTS. Our data indicate that, at an age of 1.9 Gyr, the typical X-ray luminosity of the cluster members with M=0.8-1.2 Msun is Lx = 1.3 x 10^28 erg s^-1, so approximately a factor of 6 less intense than that observed in the younger Hyades. Given that Lx is proportional to the square of a star's rotational rate, the median Lx of NGC 752 is consistent, for t > 1 Gyr, to a decaying rate in rotational velocities v_rot ~ t^-alpha with alpha ~ 0.75, steeper than the Skumanich relation (alpha ~ 0.5) and significantly steeper than observed between the Pleiades and the Hyades (where alpha < 0.3), suggesting that a change in the rotational regimes of the stellar interiors is taking place at t ~ 1 Gyr., Comment: Accepted for publication in A&A (13 pages, 8 figures)

We present the characteristics of the X-ray variability of stars in the cluster NGC2516 as derived from XMM-Newton/EPIC/pn data. The X-ray variations on short (hours), medium (months), and long (years) time scales have been explored. We detected 303 distinct X-ray sources by analysing six EPIC/pn observations; 194 of them are members of the cluster. Stars of all spectral types, from the early-types to the late-M dwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that, on short time scales, only a relatively small fraction (ranging from 6% to 31% for dG and dF, respectively) of the members of NGC2516 are variable with a confidence level $\geq$99%; however, it is possible that the fraction is small only because of the poor statistics. The time X-ray amplitude distribution functions (XAD) of a set of dF7-dK2 stars, derived on short (hours) and medium (months) time scales, seem to suggest that medium-term variations, if present, have a much smaller amplitude than those on short time scales; a similar result is also obtained for dK3-dM stars. The amplitude variations of late-type stars in NGC2516 are consistent with those of the coeval Pleiades stars. Comparing these data with those of ROSAT/PSPC, collected 7-8 years earlier, and of ROSAT/HRI, just 4-5 years earlier, we find no evidence of significant variability on the related time scales, suggesting that long-term variations due to activity cycles similar to the solar cycle are not common among young stars. Indications of spectral variability was found in one star whose spectra at three epochs were available., Comment: 15 pages, 8 figures, published in A&A

We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the "XMM-Newton Extended Survey of the Taurus Molecular Cloud." X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a "soft excess" in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams., Comment: 8 pages, 9 figures, in press, Proc. IAUS 243, Star-Disk Interactions in Young Stars

AIMS. We study the variability of the Fe 6.4 KeV emission line from the Class I young stellar object Elias 29 in the Rho-Oph cloud. METHODS. We analysed the data from Elias 29 collected by XMM during a nine-day, nearly continuous observation of the Rho-Oph star-forming region (the Deep Rho-Oph X-ray Observation, named Droxo). The data were subdivided into six homogeneous time intervals, and the six resulting spectra were individually analysed. RESULTS. We detect significant variability in the equivalent width of the Fe 6.4 keV emission line from Elias 29. The 6.4 keV line is absent during the first time interval of observation and appears at its maximum strength during the second time interval (90 ks after Elias 29 undergoes a strong flare). The X-ray thermal emission is unchanged between the two observation segments, while line variability is present at a 99.9% confidence level. Given the significant line variability in the absence of variations in the X-ray ionising continuum and the weakness of the photoionising continuum from the star's thermal X-ray emission, we suggest that the fluorescence may be induced by collisional ionisation from an (unseen) population of non-thermal electrons. We speculate on the possibility that the electrons are accelerated in a reconnection event of a magnetically confined accretion loop, connecting the young star to its circumstellar disk., Comment: Accepted for publication in A&A (11 pages, 8 figures)

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., Comment: 14 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics

We analyze 19 bright variable X-ray sources detected in the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), in order to characterize the variations with time of their coronal properties and to derive informations on the X-ray emitting structures. We performed time-resolved spectroscopy of the EPIC PN and MOS spectra of the XEST sources, using a model with one or two thermal components, and we used the time evolution of the temperatures and emission measures during the decay phase of flares to derive the size of the flaring loops. The light curves of the selected sources show different types of variability: flares, long-lasting decay or rise through the whole observation, slow modulation or complex flare-like variability. Spectral analysis shows typical quiescent plasma temperatures of 5-10 MK and 15-35 MK; the cool component generally remains constant, while the observed flux changes are due to variations of the hot component. During flares the plasma reaches temperatures up to 100 MK and luminosities up to $\sim 10^{31}$ erg s$^{-1}$. Loop sizes inferred from flare analysis are generally smaller than or comparable to the stellar radius., Comment: 18 pages, 11 figures, 3 tables. Accepted by A&A, to appear in a special section/issue dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)

Mechanisms regulating the evolution of pre-main sequence stars can be understood by studying stellar properties such as rotation, disk accretion, internal mixing and binarity. To investigate such properties, we studied a sample of 332 candidate members of the massive and populous star forming region NGC 6530. We want to select cluster members by using different membership criteria,to study the properties of pre-main sequence stars with or without circumstellar disks. We use intermediate resolution spectra including the Li I 6707.8 Angstroms line to derive radial and rotational velocities, binarity and to measure the Equivalent Width of the lithium line; these results are combined with X-ray data to study the cluster membership. Optical-IR data and Halpha spectra, these latter available for a subsample of our targets, are used to classify CTTS and WTTS and to compare the properties of stars with and without disks. We find a total of 237 certain members including 53 binaries. The rotational velocity distributions of stars with IR excesses are statistically different from that of stars without IR excesses, while the fraction of binaries with disks is significantly smaller than that of single stars. Stars with evidence for accretion show circumstellar disks; youth of cluster members is confirmed by the lithium abundance consistent with the initial content. As indicated by the disk-locking picture, stars with disks have in general rotational velocities lower than stars without disks. Binaries in NGC 6530 seem have undergone a significant disk evolution., Comment: 18 pages, 17 figures, in press in Astronomy and Astrophysics