Abstract. We present results of the analysis of data collected in 57-ks XMM-Newton and 505-ks Chandra observations of the nearby ( 120 pc) isolated neutron star RX J1856.5-3754. We confirm most of the statements made by Burwitz et al. (2001) who discussed the original 55-ks Chandra data. Detailed spectral analysis of the combined X-ray and optical data rules out the currently available nonmagnetic light and heavy element neutron star atmosphere (LTE) models with hydrogen, helium, iron and solar compositions. We find that strongly magnetized atmosphere models also are unable to represent the data. The X-ray and optical data show no spectral features and are best fitted with a two-component blackbody model with kT

Abstract.We report on a detailed study of the X-ray spectrum of the nearby eclipsing spectroscopic binary YY Gem. Observations were obtained simultaneously with both large X-ray observatories, XMM-Newton and Chandra. We compare the high-resolution spectra acquired with the Reflection Grating Spectrometer onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer onboard Chandra, and evidence in direct comparison the good performance of both instruments in terms of wavelength and flux calibration. The strongest lines in the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the presence of a low-temperature component (1−4 MK) with density n e ≤ 2 × 10 10 cm −3 . The X-ray lightcurve reveals two flares and a dip corresponding to the secondary eclipse. An increase of the density during phases of high activity is suggested from time-resolved spectroscopy. Time-resolved global fitting of the European Photon Imaging Camera CCD spectrum traces the evolution of temperature and emission measure during the flares. These medium-resolution spectra show that temperatures >10 7 K are relevant in the corona of YY Gem although not as dominant as the lower temperatures represented by the strongest lines in the high-resolution spectrum. Magnetic loops with length on the order of 10 9 cm, i.e., about 5% of the radius of each star, are inferred from a comparison with a one-dimensional hydrodynamic model. This suggests that the flares did not erupt in the (presumably more extended) inter-binary magnetosphere but are related to one of the components of the binary.

We report the discovery in near-infrared (NIR) with SofI at the New Technology Telescope (NTT) of a resolved circumstellar dust disk around a 2MASS source at the periphery of the Ophiuchi dark cloud. We present follow-up observations in J, H, and K s bands, obtained with ISAAC at the Very Large Telescope (VLT) under 0>4 seeing conditions, which unveil a dark dust lane oriented east-west between two characteristic northern and southern reflection nebulae. This new circumstellar dust disk has a radius of 2>15 (300 AU at 140 pc) and a width of 1>2 (170 AU at 140 pc). Thanks to its location at the periphery of the dense cores, it suffers small foreground visual extinction (A V ¼ 2:1 AE 2:6 mag). Although this disk is seen close to edge-on, the two reflection nebulae display very different colors. We introduce a new NIR data visualization called '' pixel NIR color mapping '' (PICMap for short), which allows us to visualize directly the NIR colors of the nebula pixels. Thanks to this method, we identify a ridge, 0>3 (40 AU at 140 pc) to the north of the dark lane and parallel to it, that displays an NIR color excess. This ridge corresponds to an unusual increase of brightness from J to K s , which is also visible in the NTT observations obtained 130 days before the VLT ones. We also find that the northern nebula shows $3 mag more extinction than the southern nebula. We compute axisymmetric disk models to reproduce the VLT scattered-light images and the spectral energy distribution from optical to NIR. Our best model, with a disk inclination i ¼ 86 AE 1 , correctly reproduces the extension of the southern reflection nebula, but it is not able to reproduce either the observed NIR color excess in the northern nebula or the extinction difference between the two reflection nebulae. We discuss the possible origin of the peculiar, asymmetrical NIR color properties of this object.

We report on a 30 ksec XMM-Newton observation of the central region of the Cha I star forming cloud. The field includes a substantial fraction of the known pre-main sequence population of Cha I South, including all thirteen known very-low mass Hα emitters. We detect two bona-fide brown dwarfs (spectral types M7.5 and M8) and seven Hα emitting objects near the hydrogen burning mass limit, including six of seven earlier detections by ROSAT. Three objects classified as Cha I candidate members according to their NIR photometry are revealed by XMM-Newton, providing further evidence for them being truely young stars. A total of 11 new X-ray sources without known optical/IR counterpart may comprise further as yet unrecognized faint cloud members. Spectral analysis of the X-ray bright stars shows that previous X-ray studies in Cha I have underestimated the X-ray luminosities, as a result of simplified assumptions on the spectral shape. In particular, the extinction is variable over the field, such that the choice of a uniform value for the column density is inappropriate. We establish that the X-ray saturation level for the late-type stars in Cha I is located near Lx/L bol ∼ 10 −2.5 , with a possible decline to Lx/L bol ∼ 10 −3 for the lowest mass stars. A group of strongly absorbed stars with unusual hard X-ray emission is clustered around HD 97048, a HAeBe star and the only confirmed intermediate-mass star in the field. While the X-ray properties of HD 97048 are indistinguishable from its lower-mass neighbors, another presumably A-type star (identified as such based on NIR photometry) stands out as the softest X-ray emitter in the whole sample. This suggests that various X-ray emission mechanisms may be at work in intermediate-mass pre-main sequence stars. We find that X-ray luminosity follows a tight correlation with age, effective temperature, and mass. No dramatic changes in these correlations are seen at the substellar boundary, suggesting that the same dynamo mechanism operates in both low-mass stars and brown dwarfs, at least at young ages. The variability of the lowest-mass objects is also similar to that of higher-mass T Tauri stars. X-ray flares are seen on about 1/10th of the Cha I members in the field.

Abstract. We report here the discovery of a 30 -chain of embedded Herbig-Haro (HH) objects in the ρ Ophiuchi dark cloud. These HH objects were first detected during a deep KS-band observation (completeness magnitude for point source ∼19) made with NTT/SOFI. We confirm their nature with follow-up observations made with H2 v = 1-0 S(1) narrow-band filter. We argue that they belong to two different jets emanating from two Class I protostars: the main component of the recently resolved subarcsecond radio binary YLW15 (also called IRS43), and IRS54. We propose also to identify the [S II] knot HH224NW1 (Gómez et al. 1998) as emanating from a counterjet of YLW15. The alignment between these HH objects and the thermal jet candidate found in YLW15 by Girart et al. (2000) implies that this jet is not precessing at least on timescale ∼(2-4)×10 4 yr.

Abstract. Recently, Neuhäuser et al. (2005a) presented evidence for a sub-stellar, common proper motion companion to GQ Lup. With two theoretical mass estimates, both below the Deuterium burning minimum mass limit, the companion is probably a planet imaged directly. We present here a more detailed astrometric analysis of the GQ Lup system, using all the (different) proper motions published for the primary. The common proper motion is significant in all cases, also when taking into account the error in parallax or distance (140 ± 50 pc). When using the weighted mean, the significance for common proper motion of GQ Lup and its companion is 7 σ + 4 σ for no change in separation plus 8 σ for no change in position angle. We also discuss the question, whether GQ Lup and its common-proper motion companion are not bound, but share the same or similar proper motion as two independent members of the Lupus T association, which is a moving group, where most members should have the same motion anyway. Given our discussion, this hypothesis can be rejected by several σ: The probability to find by chance an L-dwarf fainter than KS = 14 mag within 0. 7325 with (almost) the same proper motion of GQ Lup is only ≤ 3 · 10 −10 . The orbital motion of the system is not yet detected (1.4 ± 2.2 mas/yr), but is probably smaller than the escape velocity (5.3 ± 2.1 mas/yr), so that the system may well be gravitationally bound and stable. This is different for the 2MASSWJ 1207334-393254 system, as we also show.

Using NACO on the VLT in the imaging mode we have detected an object at a distance of only 0.7 arcsec from GQ Lup. The object turns out to be co-moving. We have taken two K-band spectra with a resolution of lambda /Delta lambda=700. In here, we analyze the spectra in detail. We show that the shape of spectrum is not spoiled by differences in the Strehl ratio in the blue and in the red part, as well as differential refraction. We reanalyze the spectra and derive the spectral type of the companion using classical methods. We find that the object has a spectral type between M9V and L4V, which corresponds to a Teff between 1600 and 2500 K. Using GAIA-dusty models, we find that the spectral type derivation is robust against different log(g)-values. The Teff derived from the models is again in the range between 1800 and 2400 K. While the models reproduce nicely the general shape of the spectrum, the 12CO-lines in the spectrum have about half the depth as those in the model. We speculate that this difference might be caused by veiling, like in other objects of similar age, and spectral class. We also find that the absolute brightness of the companion matches that of other low-mass free-floating objects of similar age and spectral type. A comparison with the objects in USco observed by Mohanty et al. (2004) shows that the companion of GQ Lup has a lower mass than any of these, as it is of later spectral type, and younger. The same is as true, for the companion of AB Pic. To have a first estimate of the mass of the object we compare the derived Teff and luminosity with those calculated from evolutionary tracks. We also point out that future instruments, like NAHUAL, will finally allow us to derive the masses of such objects more precisely.Comment: 6 figures, proceeding of the workshop on Ultralow-mass star formation and evolution held in La Palama June 28 to July 1, 200

No abstract