A. Maggio

The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs ?

We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the B V range 0.5-2.0, including 110 field stars and 149 members of the Pleiades, Hyades, Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of dierent masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale,e, which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number Re = Prot=e: we show that our empirical time scalee resembles the theoretical convective turnover time for 0:4 M=M 1:2, but it also has the same functional dependence on B V as L 1=2 bol in the color range 0:5 B V 1:5. Our results imply that - for non-saturated coronae - the Lx - Prot relation is equivalent to the Lx=Lbol vs. Re relation.

A Method Based on Wavelet Transforms for Source Detection in Photon-counting Detector Images. I. Theory and General Properties

We have developed a method based on wavelet transforms (WTs) to detect sources in astronomical images obtained with photon-counting detectors, such as X-ray images. The WT is a multiscale transform that is suitable for detection and analysis of interesting image features (sources) spanning a range of sizes. This property of the WT is particularly well suited to the case in which the point-spread function is strongly varying across the image, and it is also effective in the detection of extended sources. The method allows one to measure source count rates, sizes, and ellipticity, with their errors. Care has been taken in the assessment of thresholds for detection, in the WT space, at any desired confidence level, through a detailed semianalytical study of the statistical properties of noise in wavelet-transformed images. The method includes the use of exposure maps to handle sharp background gradients produced by a nonuniform exposure across the detector, which would otherwise yield many spurious detections. The same method is applied to evaluate upper limits to the count rate of undetected objects in the field of view, allowing a sensitivity map for each observation to be constructed.

The XMM-Newton view of stellar coronae: X-ray spectroscopy of the corona of AB Doradus

M. A. acknowledges support from the Swiss National Science Foundation (grants 2100-049343 and 2000-058827), from the Swiss Academy of Sciences, and from the Swiss Commission for Space Research. H. M., K. B, and J. P. acknowledge financial support from PPARC. A. M. and R. P. acknowledge support from the Italian Space Agency. SRON is supported financially by NWO.

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

On Stellar Coronae and Solar Active Regions

Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near peak activity level obtained on 1992 January 6, we search for coronal structures that have emission measure distributions EM(T ) that match the observed stellar coronal emission measure distributions derived for the intermediate-activity stars v Eri (K2 V) and m Boo A (G8 V) from Extreme Ultraviolet Explorer spectro- scopic observations. We —nd that the temperatures of the peaks of the observed stellar distributions EM(T ), as well as their slopes in the temperature range are very similar to those 6.0 ( log T ( 6.5, obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM P T b with b D 4, which is much steeper than predicted by static, uniformly heated loop models. Plasma densities in the coronae of v Eri and m Boo A are also observed to be essentially the same as the plasma densities typical of solar active regions. These data provide the best observational support yet obtained for the hypothesis that solar-like stars up to the activity levels of v Eri (K2 V) and m Boo A are dominated by active regions similar to, though possibly considerably larger than, those observed on the Sun. The surface —lling factor of bright active regions needed to explain the observed stellar emission measures is approximately unity. We speculate on the scenario in which small-scale ii nano—ares ˇˇ dominate the heating of active regions up to activity levels similar to those of v Eri (K2 V) and m Boo A. At higher activity levels still, the interactions of the active regions themselves may lead to increasing —aring on larger scales that is responsible for heating plasma to the observed coronal temperatures of on very active stars. Observations of X-ray and T Z 107 K EUV light curves using more sensitive instruments than are currently available, together with determi- nations of plasma densities over the full range of coronal temperatures (106¨107 K and higher), will be important to con—rm —are heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures and the temperatures that characterize the most (T ( 5 ) 106 K) active stars (T Z 107 K). Subject headings: stars: coronaestars: individual (v Eridani, m Bootis) ¨ Sun: corona ¨ Sun: X-rays, gamma raysX-rays: stars

Three years in the coronal life of AB Dor I. Plasma emission measure distributions and abundances at different activity levels

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 dierent 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 (K) 6:1-7.6, 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 (K) 6:9 and a substantial amount of plasma in the range log T (K) 6:9-7.3. 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 Fe xxi and Fexxii lines ratios indicate electron densities log ne 10: 8c m 3 at log T (K) 6: 3a nd logne 12: 5c m 3 at log T (K) 7, implying plasma pressures steeply increasing with temperature. These results are interpreted in the framework of a corona composed of dierent 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 10 4 -10 6 ) of the stellar surface.

The GAPS programme with HARPS-N at TNG - I. Observations of the Rossiter-McLaughlin effect and characterisation of the transiting system Qatar-1

Context. Our understanding of the formation and evolution of planetary systems is still fragmentary because most of the current data provide limited information about the orbital structure and dynamics of these systems. The knowledge of the orbital properties for a variety of systems and at di erent ages yields information on planet migration and on star-planet tidal interaction mechanisms. Aims. In this context, a long-term, multi-purpose, observational programme has started with HARPS-N at TNG and aims to characterise the global architectural properties of exoplanetary systems. The goal of this first paper is to fully characterise the orbital properties of the transiting system Qatar-1 as well as the physical properties of the star and the planet. Methods. We exploit HARPS-N high-precision radial velocity measurements obtained during a transit to measure the Rossiter-McLaughlin e ect in the Qatar-1 system, and out-of-transit measurements to redetermine the spectroscopic orbit. New photometric-transit light-curves were analysed and a spectroscopic characterisation of the host star atmospheric parameters was performed based on various methods (line equivalent width ratios, spectral synthesis, spectral energy distribution). Results. We achieved a significant improvement in the accuracy of the orbital parameters and derived the spin-orbit alignment of the system; this information, combined with the spectroscopic determination of the host star properties (rotation, Te , logg, metallicity), allows us to derive the fundamental physical parameters for star and planet (masses and radii). The orbital solution for the Qatar-1 system is consistent with a circular orbit and the system presents a sky-projected obliquity of = 8:4 7:1 deg. The planet, with a mass of 1:33 0:05 MJ, is found to be significantly more massive than previously reported. The host star is confirmed to be metal-rich ([Fe/H] = 0:20 0:10) and slowly rotating (v sinI = 1:7 0:3 km s 1 ), though moderately active, as indicated by the strong chromospheric emission in the Caii H&K line cores (logR 0 4:60). Conclusions. We find that the system is well aligned and fits well within the general versus Te trend. We can definitely rule out any significant orbital eccentricity. The evolutionary status of the system is inferred based on gyrochronology, and the present orbital configuration and timescale for orbital decay are discussed in terms of star-planet tidal interactions.

Magnetic field, differential rotation and activity of the hot-Jupiter-hosting star HD 179949

HD 179949 is an F8V star, orbited by a giant planet at similar to 8 R? every 3.092 514 d. The system was reported to undergo episodes of stellar activity enhancement modulated by the orbital period, interpreted as caused by starplanet interactions (SPIs). One possible cause of SPIs is the large-scale magnetic field of the host star in which the close-in giant planet orbits. In this paper we present spectropolarimetric observations of HD 179949 during two observing campaigns (2009 September and 2007 June). We detect a weak large-scale magnetic field of a few gauss at the surface of the star. The field configuration is mainly poloidal at both observing epochs. The star is found to rotate differentially, with a surface rotation shear of dO= 0.216 +/- 0.061 rad d-1, corresponding to equatorial and polar rotation periods of 7.62 +/- 0.07 and 10.3 +/- 0.8 d, respectively. The coronal field estimated by extrapolating the surface maps resembles a dipole tilted at similar to 70 degrees. We also find that the chromospheric activity of HD 179949 is mainly modulated by the rotation of the star, with two clear maxima per rotation period as expected from a highly tilted magnetosphere. In 2009 September, we find that the activity of HD 179949 shows hints of low-amplitude fluctuations with a period close to the beat period of the system.

X-ray emitting MHD accretion shocks in classical T Tauri stars Case for moderate to high plasma-β values

Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β � 1). Aims. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β > 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. Methods. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). Results. The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β> 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β :f or β> 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β ≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models.

X-ray emission from dense plasma in classical T Tauri stars: hydrodynamic modeling of the accretion shock

Context. High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T ∼ 2−3 × 10 6 K and density ne ∼ 10 11 −10 13 cm −3 , which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere. Aims. We investigate the dynamics and stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region. Methods. We perform one-dimensional hydrodynamic simulations of the impact of an accretion flow on the chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. We present the results of a simulation based on the parameters of the CTTS MP Mus. Results. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8 × 10 9 cm, density ne ∼ 10 11 −10 12 cm −3 , temperature T ∼ 3 × 10 6 K, and uniform pressure equal to the ram pressure of the accretion flow (∼450 dyn cm −2 ). The base of the shocked region penetrates the chromosphere and remains at a position at which the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab