G. Micela

On the origin of the X-ray emission from Herbig Ae/Be stars

Context. Herbig Ae/Be stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Alternative X-ray production mechanisms, related to stellar winds or star-disk magnetospher es have been proposed, but all in all their X-ray emission has remained a mystery. Aims. A study of Herbig Ae/Be stars’ global X-ray properties (such as detection rate, luminosity, temperature, variability) , helps to constrain the emission mechanism by comparison to other types of stars, e.g. similar-age but lower-mass T Tauri stars, similar-mass but more evolved main-sequence A- and B-type stars, and with respect to model predictions. Methods. We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. The superior spatial resolut ion of this satellite with respect to previous X-ray instrumentation h as allowed us to examine also the possible role of late-type companions in generating the observed X-rays. Results. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10 ′′ . The detection fraction of Herbig Ae/Be stars is 76 %, but lowers to 35 % if all emission is attributed to further known and unresolved companions. The spectral analysis confirms the h igh X-ray temperatures (� 20 MK) and large range of fractional X-ray luminosities (log Lx/L∗) of this class derived from earlier studies of individual ob jects. Conclusions. Radiative winds are ruled out as emission mechanism on basis of the high temperatures. The X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known), nor from other young late-type stars used for comparison. Therefore, either a similar kind of process takes place on both classes of objects, or there must be as yet undiscovered companion stars.

A statistical analysis of X-ray variability in pre-main sequence objects of the Taurus Molecular Cloud

Context. This work is part of a systematic X-ray survey of the Taurus star forming complex with XMM-Newton. Aims. We study the time series of all X-ray sources associated with Taurus members, to statistically characterize their X-ray variability, and compare the results to those for pre-main sequence stars in the Orion Nebula Cluster and to expectations arising from a model where all the X-ray emission is the result of a large number of stochastically occurring flares. Methods. The analysis of the lightcurves is based on a maximum likelihood algorithm that segments the time series in intervals of constant signal without the need of binning. Flares are defined with criteria that take into account the amplitude and the derivative of the segmented lightcurves. Variability statistics are evaluated for di erent classes of pre-main sequence stars (protostars, cTTS, wTTS, brown dwarfs), and for di erent spectral type ranges. Flare frequency and energy distribution are computed. Results. We find that roughly half of the detected X-ray sources show variability above our sensitivity limit, and in 26 % of the cases this variability is recognized as flares. Variability is more frequently detected at hard than at soft energies. The variability statistics of cTTS and wTTS are undistinguishable, suggesting a common (coronal) origin for their X-ray emission. The frequency of large flares (E > 10 35 erg) on Taurus members is 1 event per star in 800 ksec. The typical duration of these flares ‐ probably biased by the finite observing time ‐ is about 10 ksec. We have for the first time applied a rigorous maximum likelihood method in the analysis of the number distribution of flare energies on pre-main sequence stars. In its di erential form this distribution follows a power-law with index = 2:4 0:5, in the range typically observed on late-type stars and the Sun. Conclusions. The signature of the X-ray variability in the pre-main sequence stars in Taurus and Orion provides twofold support for coronal heating by flares: (i) The correlation between the maximum variability amplitude and the minimum emission level indicates that both flare and quiescent emission are closely related to the coronal heating process. (ii) The power-law index derived for the flare energy distribution is large enough to explain the heating of stellar coronae by nano-flares ( > 2), albeit associated with a rather large uncertainty that leaves some doubt on this conclusion.

Unveiling the Cygnus OB2 stellar population with Chandra

Aims. The aim of this work is to identify the so far unknown low mass stellar population of the∼ 2Myr old Cygnus OB2 star forming region, and to investigate the X-ray and near-IR stellar properties of its members. Methods. We analyzed a 97.7 ksec Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWDetect code and were then positionally correlated with optical and near-IR catalogs from the literature. Source events were then extracted with the Acis Extract package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed thermal plasma models. Results. We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2 members. From near-IR color-color and color-magnitude diagrams we estimate a typical absorption toward Cygnus OB2 of Av≈ 7.0 mag. Although the region is young, very few stars (∼4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in ∼13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at leas t 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O- that early B-type stars are not significantly variable. Typ ical X-ray spectral parameters are log NH∼ 22.25 (cm −2 ) and kT∼ 1.35 keV with 1σ dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT=3.3 and 3.8 keV, respectively. OB stars are typically softer (kT∼ 0.75 keV). X-ray luminosities range between 10 30 and 10 31 erg s −1 for intermediate- and low-mass stars, and 2.5×10 30 and between 6.3×10 33 erg s −1 for OB stars. Conclusions. The Cygnus OB2 region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce. X-ray variability is essentially related to the magnetic ac tivity of low-mass stars (M/M⊙∼0.5 to 3.0) which display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range.

Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), <1 Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ∼1 Mstar. We consider five SFR of varying age (ρ Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and η Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so as to minimize systematic differences in the results. We study trends of LX and LX/Lbol as a function of stellar mass and association age. For low mass stars (M < 1 M� ) we observe an increase in LX/Lbol in the first 3-4 Myr and a subsequent leveling off at the saturation level (LX/Lbol ∼− 3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ∼10 7 years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of LX/Lbol with time.

Simultaneous optical and X-ray observations of a giant flare on the ultracool dwarf LP 412-31

Cool stars are known to produce flares probably as a result of the magnetic reconnection in their outer atmospheres. We present simultaneous XMM-Newton optical V band and X-ray observations of the M8 dwarf LP 412-31. During the observation a giant flare occurred, with an optical amplitude of 6 mag and total energy of 3 × 10 32 erg in both the V band and soft X-rays. Both flare onset and flare decay were completely covered in both wavebands with a temporal resolution of 20 s, allowing determination of the flare time scales, as well as a study of the temperature evolution of the flaring plasma. The data are consistent with an impulsive energy release followed by radiative cooling without any further energy release during the decay phase. Our analysis suggests that the optical flare originates from a small fraction of the surface of LP 412-31, while the characteristic scale size of the flaring X-ray plasma is of the order of the stellar radius or larger. The absence of any small-scale variability in the light curve suggests a non-standard flare number energy distribution.

The relative role of EUV radiation and X-rays in the heating of hydrogen-rich exoplanet atmospheres

Aims. We study the relative role of EUV and X-ray radiation in the heating of hydrogen-rich planet atmospheres with different composition and electron content. Methods. An accurate photo-ionization model has been used to follow the primary photo-electron energy deposition throughout the atmosphere. Results. Heating rates and efficiencies have been computed, together with column density cut-offs at which photons of given energies stop their heating production inside the atmosphere. Assuming 100 eV as the energy borderline between the extreme ultraviolet spectral range and X-rays we find that when the absorbing hydrogen column density is higher than 10 20 cm −2 only X-rays can heat the gas. Extreme ultraviolet photons heat the upper atmospheric layers. Conclusions. Using emission spectra from a sample of solar-type stars of different ages representative of the Sun’s main sequence lifetime, we have derived the corresponding heating rates. We find that the existence of an energetic cross-over in atmospheric heating is present for all stars in the sample.

Coronal structure geometries on pre-main sequence stars

Using a hydrodynamic model we have re-analyzed large flaring events on three dierent categories of pre-main sequence (PMS) stars: the young stellar object (YSO) YLW 15, the classical T Tauri star (CTTS) LkH 92, the weak-line T Tauri star (WTTS) V773 Tau, and the WTTS HD 283572 (the rst three objects were observed by ASCA, the last by ROSAT; all observations have been previously reported in the literature). The rst three flares were previously analyzed on the basis of a quasi-static model mostly used up to now, consistently yielding large loops (L > R) and no evidence of sustained heating. Our hydrodynamic modeling approach, however, shows that the size of the flaring regions must be much smaller (L < R) and moreover this method shows in all cases evidence of vigorous sustained heating during the flare decay, so that the decay of the observed light curve actually reflects the temporal prole of the heating rather than that of the free decay of the heated loop(s). The events on the protostar YLW 15 have durations comparable to the stellar rotation period, so that their limited size and their lack of self-eclipses give evidence of a polar location on the star. This is in contrast with the recently advanced hypothesis that these flares are due to long loops spanning the region between the star and the accretion disk. In general, the present analysis shows that flaring coronae on PMS stars have a structure similar to the coronae on older active stars.

The stellar content of the XMM-Newton bright serendipitous survey

Context. The comparison of observed counts in a given sky direction with predictions by Galactic models yields constraints on the spatial distribution and the stellar birthrate of young stellar populations. The XMM-Newton Bright Serendipitous Survey (XBSS) is an unbiased survey that includes a total of 58 stellar sources selected in the 0.5–4.5 keV energy band, having a limiting sensitivity of 10 −2 cnt s −1 and covering an area of 28.10 sq deg. Aims. We present the results of analysing the stellar content of the XBSS so as to understand the recent star formation history of the Galaxy in the vicinity of the Sun. Methods. We compared the observations with the predictions obtained with XCOUNT, a model of the stellar X-ray content of the Galaxy. The model predicts the number and properties of the stars to be observed in terms of magnitude, colour, population and fx/fv ratio distributions of the coronal sources detected with a given instrument and sensitivity in a specific sky direction. Results. As in other shallow surveys, we observe an excess of stars not predicted by our Galaxy model. Comparing the colours of the identified infrared counterparts with the model predictions, we observe that this excess is produced by yellow (G+K) stars. The study of the X-ray spectrum of each source reveals a main population of stars with the coronal temperature stratification typical of intermediate-age stars. As no assumptions have been made for the selection of the sample, our results must be representative of the entire solar neighbourhood. Some stars show infrared excess due to circumstellar absorption, which is indicative of youth.

Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars

Context. The X-ray emission from B-type main-sequence stars is a longstanding mystery in stellar coronal research. Since there is no theory at hand that explains intrinsic X-ray emission from intermediate-mass main-sequence stars, the observations have often been interpreted in terms of (unknown) late-type magnetically active companion stars. Aims. Resolving the hypothesized companions requires high spatial resolution observations in the infrared and in X-rays. We use Chandra imaging observations to spatially resolve a sample of main-sequence B-type stars with recently discovered companions at arcsecond separation. Methods. Our strategy is to search for X-ray emission at the position of both the B-type primary and the faint companion. Results. We find that all spatially resolved companions are X-ray emitters, but seven out of eleven intermediate-mass stars are also X-ray sources. If this emission is interpreted in terms of additional sub-arcsecond or spectroscopic companions, this implies a high multiplicity of B-type stars. Firm results on B star multiplicity pending, the alternative, that B stars produce intrinsic X-rays, cannot be discarded. An appropriate scenario would be a magnetically confined wind, as suggested for the X-ray emission of the magnetic Ap star IQ Aur. However, the only Ap star in the Chandra sample is not detected in X-rays, and therefore does not support this picture.

Spectral properties of X-ray bright variable sources in the Taurus molecular cloud

Aims. 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. Methods. 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 ares to derive the size of the aring loops. Results. The light curves of the selected sources show di erent types of variability: ares, long-lasting decay or rise through the whole observation, slow modulation or complex are-lik e variability. Spectral analysis shows typical quiescent plasma temperatures of 5 n 10 MK and 15 n 35 MK; the cool component generally remains constant, while the observed ux changes are due to variations of the hot component. During ares the plasma reaches temperatures up to 100 MK and luminosities up to 10 31 erg s 1 . Loop sizes inferred from are analysis are generally smaller than or comparable to the stellar radius.