Properties of the solar neighbor WISE J072003.20-084651.2

The severe crowding towards the Galactic plane suggests that the census of nearby stars in that direction may be incomplete. Recently, Scholz reported a new M9 object at an estimated distance d~7 pc (WISE J072003.20-084651.2; hereafter WISE0720) at Galactic latitude b=2.3 degr. Our goals are to determine the physical characteristics of WISE0720, its kinematic properties, and to address the question if it is a binary object, as suggested in the discovery paper. Optical and infrared spectroscopy from the Southern African Large Telescope and Magellan, respectively, and spectral energy distribution fitting were used to determine the spectral type of WISE0720. The measured radial velocity, proper motion and parallax yielded its Galactic velocities. We also investigated if WISE0720 may show X-ray activity based on archival data. Our spectra are consistent with spectral type L0+/-1. We find no evidence for binarity, apart for a minor 2-sigma level difference in the radial velocities taken at two different epochs. The spatial velocity of WISE0720 does not connect it to any known moving group, instead it places the object with high probability in the old thin disk or in the thick disk. The spectral energy distribution fit hints at excess in the 12 and 22 micron WISE bands which may be due to a redder companion, but the same excess is visible in other late type objects, and it more likely implies a shortcoming of the models (e.g., issues with the effective wavelengths of the filters for these extremely cool objects, etc.) rather than a disk or redder companion. The optical spectrum shows some Halpha emission, indicative of stellar activity. Archival X-ray observations yield no detection.Comment: A&A, accepted; 9 pages, 6 figure

Spectra disentangling applied to the Hyades binary Theta^2 Tau AB: new orbit, orbital parallax and component properties

Theta^2 Tauri is a detached and single-lined interferometric-spectroscopic binary as well as the most massive binary system of the Hyades cluster. The system revolves in an eccentric orbit with a periodicity of 140.7 days. The secondary has a similar temperature but is less evolved and fainter than the primary. It is also rotating more rapidly. Since the composite spectra are heavily blended, the direct extraction of radial velocities over the orbit of component B was hitherto unsuccessful. Using high-resolution spectroscopic data recently obtained with the Elodie (OHP, France) and Hermes (ORM, La Palma, Spain) spectrographs, and applying a spectra disentangling algorithm to three independent data sets including spectra from the Oak Ridge Observatory (USA), we derived an improved spectroscopic orbit and refined the solution by performing a combined astrometric-spectroscopic analysis based on the new spectroscopy and the long-baseline data from the Mark III optical interferometer. As a result, the velocity amplitude of the fainter component is obtained in a direct and objective way. Major progress based on this new determination includes an improved computation of the orbital parallax. Our mass ratio is in good agreement with the older estimates of Peterson et al. (1991, 1993), but the mass of the primary is 15-25% higher than the more recent estimates by Torres et al. (1997) and Armstrong et al. (2006). Due to the strategic position of the components in the turnoff region of the cluster, these new determinations imply stricter constraints for the age and the metallicity of the Hyades cluster. The location of component B can be explained by current evolutionary models, but the location of the more evolved component A is not trivially explained and requires a detailed abundance analysis of its disentangled spectrum.Comment: in press, 13 pages, 10 Postscript figures, 5 tables. Table~4 is available as online material. Keywords: astrometry - techniques: high angular resolution - stars: binaries: visual - stars: binaries: spectroscopic - stars: fundamental parameter

Hot stars observed by XMM-Newton I. The catalog and the properties of OB stars

Aims : Following the advent of increasingly sensitive X-ray observatories, deep observations of early-type stars became possible. However, the results for only a few objects or clusters have until now been reported and there has been no large survey comparable to that based upon the ROSAT All-Sky Survey (RASS). Methods : A limited survey of X-ray sources, consisting of all public XMM observations (2XMMi) and slew survey data (XMMSL1), is now available. The X-ray counterparts to hot, massive stars have been searched for in these catalogs. Results : About 300 OB stars were detected with XMM. Half of them were bright enough for a spectral analysis to be possible, and we make available the detailed spectral properties that were derived. The X-ray spectra of O stars are represented well by low (<1keV) temperature components and seem to indicate that an absorption column is present in addition to the interstellar contribution. The X-ray fluxes are well correlated with the bolometric fluxes, with a scatter comparable to that of the RASS studies and thus larger than found previously with XMM for some individual clusters. These results contrast with those of B stars that exhibit a large scatter in the L_X-L_BOL relation, no additional absorption being found, and the fits indicate a plasma at higher temperatures. Variability (either within one exposure or between multiple exposures) was also investigated whenever possible: short-term variations are far more rare than long-term ones (the former affects a few percent of the sample, while the latter concerns between one third and two thirds of the sources). Conclusions : This paper presents the results of the first high-sensitivity investigation of the overall high-energy properties of a sizable sample of hot stars.Comment: 10 pages, 4 figures, accepted for publication by A&

Masses of the components of SB2 binaries observed with Gaia. II. Masses derived from PIONIER interferometric observations for Gaia validation

In anticipation of the Gaia astrometric mission, a sample of spectroscopic binaries is being observed since 2010 with the Sophie spectrograph at the Haute--Provence Observatory. Our aim is to derive the orbital elements of double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to finally obtain the masses of the components with relative errors as small as 1 % when combined with Gaia astrometric measurements. In order to validate the masses derived from Gaia, interferometric observations are obtained for three SB2s in our sample with F-K components: HIP 14157, HIP 20601 and HIP 117186. The masses of the six stellar components are derived. Due to its edge-on orientation, HIP 14157 is probably an eclipsing binary. We note that almost all the derived masses are a few percent larger than the expectations from the standard spectral-type-mass calibration and mass-luminosity relation. Our calculation also leads to accurate parallaxes for the three binaries, and the Hipparcos parallaxes are confirmed.Comment: 10 pages, 3 figures, accepted by MNRA

The interface between the stellar wind and interstellar medium around R Cassiopeiae revealed by far-infrared imaging

The circumstellar dust shells of intermediate initial-mass (about 1 to 8 solar masses) evolved stars are generated by copious mass loss during the asymptotic giant branch phase. The density structure of their circumstellar shell is the direct evidence of mass loss processes, from which we can investigate the nature of mass loss. We used the AKARI Infrared Astronomy Satellite and the Spitzer Space Telescope to obtain the surface brightness maps of an evolved star R Cas at far-infrared wavelengths, since the temperature of dust decreases as the distance from the star increases and one needs to probe dust at lower temperatures, i.e., at longer wavelengths. The observed shell structure and the star's known proper motion suggest that the structure represents the interface regions between the dusty wind and the interstellar medium. The deconvolved structures are fitted with the analytic bow shock structure to determine the inclination angle of the bow shock cone. Our data show that (1) the bow shock cone of 1 - 5 x 10^-5 solar masses (dust mass) is inclined at 68 degrees with respect to the plane of the sky, and (2) the dust temperature in the bow shock cone is raised to more than 20 K by collisional shock interaction in addition to the ambient interstellar radiation field. By comparison between the apex vector of the bow shock and space motion vector of the star we infer that there is a flow of interstellar medium local to R Cas whose flow velocity is at least 55.6 km/s, consistent with an environment conducive to dust heating by shock interactions.Comment: 7 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

Tracing the young massive high-eccentricity binary system Theta 1 Orionis C through periastron passage

The nearby high-mass star binary system Theta 1 Orionis C is the brightest and most massive of the Trapezium OB stars at the core of the Orion Nebula Cluster, and it represents a perfect laboratory to determine the fundamental parameters of young hot stars and to constrain the distance of the Orion Trapezium Cluster. Between January 2007 and March 2008, we observed T1OriC with VLTI/AMBER near-infrared (H- and K-band) long-baseline interferometry, as well as with bispectrum speckle interferometry with the ESO 3.6m and the BTA 6m telescopes (B'- and V'-band). Combining AMBER data taken with three different 3-telescope array configurations, we reconstructed the first VLTI/AMBER closure-phase aperture synthesis image, showing the T1OriC system with a resolution of approx. 2 mas. To extract the astrometric data from our spectrally dispersed AMBER data, we employed a new algorithm, which fits the wavelength-differential visibility and closure phase modulations along the H- and K-band and is insensitive to calibration errors induced, for instance, by changing atmospheric conditions. Our new astrometric measurements show that the companion has nearly completed one orbital revolution since its discovery in 1997. The derived orbital elements imply a short-period (P=11.3 yrs) and high-eccentricity orbit (e=0.6) with periastron passage around 2002.6. The new orbit is consistent with recently published radial velocity measurements, from which we can also derive the first direct constraints on the mass ratio of the binary components. We employ various methods to derive the system mass (M_system=44+/-7 M_sun) and the dynamical distance (d=410+/-20 pc), which is in remarkably good agreement with recently published trigonometric parallax measurements obtained with radio interferometry.Comment: 15 pages, 15 figures, accepted by A&

Deposition of nanoscale multilayer CrN/NbN physical vapor deposition coatings by high power impulse magnetron sputtering

Nanoscale multilayer CrN/NbN physical vapor deposition (PVD) coatings are gaining reputation for their high corrosion and wear resistance. However, the CrN/NbN films deposited by ABS(TM) (are bond sputtering) technology have some limitations such as macrodroplets, porosity, and less dense structures. The novel HIPIMS (high power impulse magnetron sputtering) technique produces macroparticle-free, highly ionized metal plasma, which brings advantages in both surface pretreatment and coating deposition stages of the PVD process. In this study, nanoscale multilayer CrN/NbN PVD coatings were pretreated and deposited with HIPIMS technology and compared with those deposited by HIPIMS-UBM (unbalanced magnetron) and by the ABSTM technique. In all cases Cr+ etching was utilized to enhance adhesion by low energy ion implantation. The coatings were deposited at 400 degrees C with substrate biased (Ub) at -75 V. During coating deposition, HIPIMS produced significantly high activation of nitrogen compared to the UBM as observed with mass spectroscopy. HIPIMS-deposited coatings revealed a bilayer period of 4.1 nm (total thickness: 2.9 mu m) and hardness of 3025 HK0.025. TEM results revealed droplet free, denser microstructure with (200) preferred orientation for the HIPIMS coating owing to the increased ionization as compared to the more porous structure with random orientation observed in UBM coating. The dry sliding wear coefficient (K-c) of the coating was 1.8 X 10(-15) m(3) N-1 m(-1), whereas the steady state coefficient of friction was 0.32. Potentiodynamic polarization tests revealed higher E-corr values, higher pitting resistance (around potentials +400 to +600 mV), and lower. corrosion current densities for HIPIMS deposited coatings as compared to the coatings deposited by ABS or HIPIMS-UBM. The corrosion behavior of the coatings qualitatively improved with the progressive use of HIPIMS from pretreatment stage to the coating deposition step. (C) 2008 American Vacuum Society

Visual/infrared interferometry of Orion Trapezium stars: Preliminary dynamical orbit and aperture synthesis imaging of the Theta 1 Orionis C system

Located in the Orion Trapezium cluster, Theta 1 Orionis C is one of the youngest and nearest high-mass stars (O5-O7) and also known to be a close binary system. Using new multi-epoch visual and near-infrared bispectrum speckle interferometric observations obtained at the BTA 6 m telescope, and IOTA near-infrared long-baseline interferometry, we trace the orbital motion of the Theta 1 Ori C components over the interval 1997.8 to 2005.9, covering a significant arc of the orbit. Besides fitting the relative position and the flux ratio, we apply aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the Theta 1 Ori C binary system. The orbital solutions suggest a high eccentricity (e approx. 0.91) and short-period (P approx. 10.9 yrs) orbit. As the current astrometric data only allows rather weak constraints on the total dynamical mass, we present the two best-fit orbits. From these orbital solutions one can be favoured, implying a system mass of 48 M_sun and a distance to the Trapezium cluster of 434 pc. When also taking the measured flux ratio and the derived location in the HR-diagram into account, we find good agreement for all observables, assuming a spectral type of O5.5 for Theta 1 Ori C1 (M=34.0 M_sun) and O9.5 for C2 (M=15.5 M_sun). We find indications that the companion C2 is massive itself, which makes it likely that its contribution to the intense UV radiation field of the Trapezium cluster is non-negligible. Furthermore, the high eccentricity of the preliminary orbit solution predicts a very small physical separation during periastron passage (approx. 1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.Comment: 13 pages, 9 figures, Accepted for publication in Astronomy & Astrophysic

On the Corrosion Resistance of Porous Electroplated Zinc Coatings in Different Corrosive Media

The corrosion resistance of an electroplated (EP) Zn coating whose surface was chemically etched to produce surface defects (pores) is investigated in this work. Impedance and DC polarisation measururements were employed to study the behaviour of such coating in various corrosive media (NaCl, NaOH and rain water). Four different faradaic relaxation processes were clearly revealed in different NaCl concentrations (from 0.1M to 1M). In the most concentrated solutions at least three relaxation processes at low frequencies (LF) appeared and were related to zinc deposition and dissolution. At lower concentrations and depending on the pH, only one process was observed. The charge transfer resistance (Rct) and the corrosion current (Icorr) were practically stable in the pH range 5 to 10. In deaerated NaCl 0.1M, the EIS diagrams showed two time-constants at very close frequencies. From the EIS diagrams the porous nature of the coating was highlighted and showed that the dissolution mechanisms occurred at the base of the pores

Abundances of neutron-capture elements in G 24-25. A halo-population CH subgiant

The differences between the neutron-capture element abundances of halo stars are important to our understanding of the nucleosynthesis of elements heavier than the iron group. We present a detailed abundance analysis of carbon and twelve neutron-capture elements from Sr up to Pb for a peculiar halo star G24-25 with [Fe/H] = -1.4 in order to probe its origin. The equivalent widths of unblended lines are measured from high resolution NOT/FIES spectra and used to derive abundances based on Kurucz model atmospheres. In the case of CH, Pr, Eu, Gd, and Pb lines, the abundances are derived by fitting synthetic profiles to the observed spectra. Abundance analyses are performed both relative to the Sun and to a normal halo star G16-20 that has similar stellar parameters as G24-25. We find that G24-25 is a halo subgiant star with an unseen component. It has large overabundances of carbon and heavy s-process elements and mild overabundances of Eu and light s-process elements. This abundance distribution is consistent with that of a typical CH giant. The abundance pattern can be explained by mass transfer from a former asymptotic giant branch component, which is now a white dwarf.Comment: 11 pages, 9 figures, accepted for publication in A&