Observations of structure in the stellar wind of HD 152408

We report on the presence of substantial, low-velocity stellar wind structure in the extreme O supergiant HD 152408, based on optical spectroscopic time-series observations. Systematic variations in the form of migrating optical depth enhancements occur in the absorption trough of the He I 5876 P Cygni profile. These variations start deep in the stellar wind, slowly accelerate bluewards to 0.5v over 1–2 days, and recur at intervals of about 1 day. Sympathetic variations are apparent in the Balmer emission lines. The observations provide constraints on the stability of the low-velocity stellar wind regime, and indicate the presence of large-amplitude perturbations at great depths in the outflow.     

XMM–Newton X-ray observations of the Wolf–Rayet binary system WR 147

We present results of an ≈20-ks X-ray observation of the Wolf–Rayet (WR) binary system WR 147 obtained with XMM–Newton . Previous studies have shown that this system consists of a nitrogen-type WN8 star plus an OB companion whose winds are interacting to produce a colliding wind shock. X-ray spectra from the pn and MOS detectors confirm the high extinction reported from infrared studies and reveal hot plasma including the first detection of the Fe Kα line complex at 6.67 keV. Spectral fits with a constant-temperature plane-parallel shock model give a shock temperature   kT _shock= 2.7  keV (   T _shock≈ 31  MK), close to but slightly hotter than the maximum temperature predicted for a colliding wind shock. Optically thin plasma models suggest even higher temperatures, which are not yet ruled out. The X-ray spectra are harder than can be accounted for using 2D numerical colliding wind shock models based on nominal mass-loss parameters. Possible explanations include: (i) underestimates of the terminal wind speeds or wind abundances, (ii) overly simplistic colliding wind models or (iii) the presence of other X-ray emission mechanisms besides colliding wind shocks. Further improvement of the numerical models to include potentially important physics such as non-equilibrium ionization will be needed to rigorously test the colliding wind interpretation.     

X-ray spectroscopy of stellar coronae

Summary From the early discovery in 1948 of X-rays from the Solar corona, X-ray spectroscopy has proven to be an invaluable tool in studying hot astrophysical and laboratory plasmas. Because the emission line spectra and continua from optically thin plasmas are fairly well known, high-resolution X-ray spectroscopy has its most obvious application in the measurement of optically thin sources such as the coronae of stars. In particular X-ray observations with theEINSTEIN observatory have demonstrated that soft X-ray emitting coronae are a common feature among stars on the cool side of the Hertzsprung-Russell diagram, with the probable exception of single very cool giant and supergiant stars and A-type dwarfs. Observations with the spectrometers aboardEINSTEIN andEXOSAT have shown that data of even modest spectral resolution (/ = 10–100) permit the identification of coronal material at different temperatures whose existence may relate to a range of possible magnetic loop structures in the hot outer atmospheres of these stars. The higher spectral resolution of the next generation of spectrometers aboard NASA'sAXAF and ESA'sXMM will allow to fully resolve the coronal temperature structure and to enable velocity diagnostics and the determination of coronal densities, from which the loop geometry (i.e. surface filling factors and loop lengths) can be derived. In this paper various diagnostic techniques are reviewed and the spectral results fromEINSTEIN andEXOSAT are discussed. A number of spectral simulations forAXAF andXMM, especially high-resolution iron K-shell, L-shell, and2s-2p spectra in the wavelength regions around 1.9 Å, 10 Å, and 100 Å, respectively, are shown to demonstrate the capabilities for temperature, density, and velocity diagnostics. Finally, iron K-shell spectra are simulated for various types of detectors such as microcalorimeter, Nb-junction, and CCD.     

X-ray emission from stellar coronae

Summary Stars of nearly all spectral types and luminosity classes are surrounded by tenuous high-temperature (T10⁶-10⁷K) coronae, which emit most of their radiation in the soft X-ray part of the spectrum. This paper reviews our present observational knowledge and theoretical understanding of stellar coronae, as has emerged from the extensive observations carried out with theEinstein and EXOSAT Observatories. We argue that different physical mechanisms are likely to be responsible for coronal emission in different parts of the HR diagram and we discuss the principal scenarios that have been proposed to account for the data. We show that in spite of the enormous progress made during the past decade, our understanding of stellar coronal emission remains incomplete and largely phenomenological. We outline major unsolved problems to be addressed by future space missions.     

A quantitative analysis of the prototype [WCL] star CPD-56° 8032

We present a detailed, quantitative study of the standard [WC10] Wolf-Rayet central star CPD-56^o 8032 based on new high resolution AAT UCLES observations and the Hillier (1990) WR standard model. Our analysis of CPD-56^o 8032 gives the wind properties (T _*=34500K, lg (L/L )=3.8, lg (M/M a^–1)=–5.4,v =225 km s^–1) and chemistry (C/He=0.5, O/He=0.1, by number), the latter suggesting an intimate relationship with the Ovi PN central stars and the PG 1159-035 objects. A comparison between the wind properties of CPD-56^o 8032 and Sk-66^o 40 (WN 10) indicates that low excitation, low wind velocity WR winds are common to both low mass PN central stars (WC sequence) and high mass post-LBV's (WN sequence).     

ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in γ2 Velorum

We discuss an ASCA observation of the eccentric WC8+O7.5 III binary γ ² Velorum near apastron. The X-ray spectrum is compared with two previous observations obtained when the system was near periastron. All three spectra display a hard-emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of γ ² Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of γ ² Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high-energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in γ ² Vel does not follow the 1/ D distance relation expected from simple models of adiabatic shocks.     

Statistical results from a jcmt survey of MM and sub-MM emission in herbig ae/be stars

We have detected 1.1 mm continuum emission from 24 of 53 Herbig Ae/Be stars surveyed with the JCMT. Survival analysis shows that 1.1 mm luminosity is correlated with bolometric luminosity and with IRAS 25µm luminosity. For those stars that were also detected at 0.45 or 0.8 mm we find a typical flux dependence of the form S _#x03BD; ³, which is steeper than that of most classical T Tauri stars.     

Probing the wind–wind collision in γ2 Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization

We present a new analysis of an archived Chandra HETGS X-ray spectrum of the WR+O colliding wind binary γ² Velorum. The spectrum is dominated by emission lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysis and an analysis of line flux ratios we infer a wide range of temperatures in the X-ray-emitting plasma (∼4–40 MK). As in the previously published analysis, we find the X-ray emission lines are essentially unshifted, with a mean FWHM of  1240 ± 30 km s⁻¹  . Calculations of line profiles based on hydrodynamical simulations of the wind–wind collision predict lines that are blueshifted by a few hundred  km s⁻¹  . The lack of any observed shift in the lines may be evidence of a large shock-cone opening half-angle (>85°), and we suggest this may be evidence of sudden radiative braking. From the R and G ratios measured from He-like forbidden-intercombination-resonance triplets we find evidence that the Mg  xi emission originates from hotter gas closer to the O star than the Si  xiii emission, which suggests that non-equilibrium ionization may be present.     

Low‐mass visual binaries in the solar neighbourhood: The case of HD 141272

We search for stellar and substellar companions of young nearby stars to investigate stellar multiplicity and formation of stellar and substellar companions. We detect common proper‐motion companions of stars via multi‐epoch imaging. Their companionship is finally confirmed with photometry and spectroscopy. Here we report the discovery of a new co‐moving (13 σ) stellar companion ∼17.8 arcsec (350AU in projected separation) north of the nearby star HD141272 (21 pc).With EMMI/NTT optical spectroscopy we determined the spectral type of the companion to be M3±0.5V. The derived spectral type as well as the near infrared photometry of the companion are both fully consistent with a M_⊙ dwarf located at the distance of HD141272 (21 pc). Furthermore the photometry data rules out the pre‐main sequence status, since the system is consistent with the ZAMS of the Pleiades. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sio observations of mass-losing AGB stars

We have observed the first overtone rotation-vibration absorption bands of SiO near = 4µm for a sample of 23 oxygen-rich Mira and Semiregular variables using the ESO NTT and IRSPEC. We discuss the strength of the SiO absorption in terms of the near infrared, IRAS and pulsational properties of the stars. Especially among the Miras there are big differences in the strength of the SiO bands between individual objects, which are probably due to pulsational variability.This work is supported by the Austrian Science Fund Project P9638-AST.     

Modelling forbidden line emission profiles from colliding wind binaries

This paper presents calculations for forbidden emission-line profile shapes arising from colliding wind binaries. The main application is for systems involving a Wolf–Rayet (WR) star and an OB star companion. The WR wind is assumed to dominate the forbidden line emission. The colliding wind interaction is treated as an Archimedean spiral with an inner boundary. Under the assumptions of the model, the major findings are as follows. (i) The redistribution of the WR wind as a result of the wind collision is not flux conservative but typically produces an excess of line emission; however, this excess is modest at around the 10 per cent level. (ii) Deviations from a flat-toped profile shape for a spherical wind are greatest for viewing inclinations that are more nearly face-on to the orbital plane. At intermediate viewing inclinations, profiles display only mild deviations from a flat-toped shape. (iii) The profile shape can be used to constrain the colliding wind bow shock opening angle. (iv) Structure in the line profile tends to be suppressed in binaries of shorter periods. (v) Obtaining data for multiple forbidden lines is important since different lines probe different characteristic radial scales. Our models are discussed in relation to Infrared Space Observatory data for WR 147 and γ Vel (WR 11). The lines for WR 147 are probably not accurate enough to draw firm conclusions. For γ Vel, individual line morphologies are broadly reproducible but not simultaneously so for the claimed wind and orbital parameters. Overall, the effort demonstrates how lines that are sensitive to the large-scale wind can help to deduce binary system properties and provide new tests of numerical simulations.     

Circumstellar shocks: colliding stellar winds

In this paper I will review some recent developments in the field of circumstellar shocks, particularly as they relate to colliding stellar winds. I shall review the basic physics of colliding winds and shocks, and discuss recent developments in hydrodynamic modelling of colliding winds. I shall also report on recent X-ray observations of shock emission in Wolf-Rayet binary systems where high resolution X-ray spectra of colliding wind shock emission is being seen. I will discuss the occurrence of colliding winds to such diverse systems as Wolf-Rayet binaries, pre-main sequence binaries, symbiotic stars as well as the Galactic center object IRS 7, where recent results on interacting winds are yielded insight into the structure of winds in general.     

Modelling the spectra of colliding winds in the Wolf–Rayet WC7+O binaries WR 42 and WR 79

We have obtained complete phase coverage of the WC7+O binaries WR 42 = HD 97152 and WR 79 = HD 152270 with high signal-to-noise ratio (S/N), moderate-resolution spectra. Remarkable orbital phase-locked profile variations of the C  iii λ 5696 line are observed and interpreted as arising from colliding wind effects. Within this scenario, we have modelled the spectra using a purely geometrical model that assumes a cone-shaped wind–wind interaction region which partially wraps around the O star. Such modelling holds the exciting promise of revealing a number of interesting parameters for WR+O binaries, such as the orbital inclination, the streaming velocity of material in the interaction region and the ratio of wind momentum flux. Knowledge of these parameters in turn leads to the possibility of a better understanding of WR star masses, mass-loss rates and wind region characteristics.     

The stellar populations within theρ Oph cloud cores

We have obtained infrared colors and limiting magnitudes from 1.25–4.8µm for a sample of 26 of the cm continuum radio sources located in the core of the Oph molecular cloud. Their colors demonstrate that the majority of the sources appear to be heavily reddened objects surrounded by circumstellar accretion disks. In these cases the radio emission most likely diagnoses accretion driven energetic outflow phenomena: either ionized winds or possibly synchrotron emission from shocked gas associated with stellar jets.     

First Byurakan Spectral Sky Survey. Stars of Late-Spectral Types. XIV. Zone +13° ≤ δ ≤ +33°

We present the fourteenth list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Sky Survey in the zone +13° +33° covering about 4736 sq. degrees. From 260 stars, 118 are newly detected objects: they are 19 carbon stars, 5 carbon star candidates, and 94 M-type stars. Among 118 detected objects 73( 57 PSC + 16 FSC ) are unclassified IRAS sources. Accurate positions, spectral classes, red magnitudes, color indices, and near-infrared J, H and K photometry are given, using several astronomical databases. Finding charts from DSS are given for the most interesting objects.     

Spectral and Photoelectric Studies of the Herbig Ae/Be Star HD 259431

Spectral and photoelectric (ubvy, H, H) observations of the Herbig Ae/Be star HD 259431 are reported. It is found that as its brightness fades, this star becomes bluer in the Paschen continuum and the intensity and equivalent width of the hydrogen emission lines increase. The spectral observations reveal significant variations in the intensity of the Mg II 4481 Å photospheric absorption line. A rise and fall in the luminosity by 0^m.04 within a period of 5-7 minutes was recorded. Radical variations in the H lineshape ("double" "P Cyg") and flare activity are not only observed in this star, but also in a number of HAEBE stars. It is suggested that flare activity may initiate a change in the velocity gradient at the base of the wind and, thereby, induce "double P Cyg" or "P Cyg single" transitions. The nonradial pulsations of this star are also discussed.     

Theoretical X-ray line profiles from colliding wind binaries

We present theoretical X-ray line profiles from a range of model colliding wind systems. In particular, we investigate the effects of varying the stellar mass-loss rates, the wind speeds and the viewing orientation. We find that a wide range of theoretical line profile shapes is possible, varying with orbital inclination and phase. At or near conjunction, the lines have approximately Gaussian profiles, with small widths  (HWHM ∼ 0.1 v _∞)  and definite blueshifts or redshifts (depending on whether the star with the weaker wind is in front or behind). When the system is viewed at quadrature, the lines are generally much broader  (HWHM ∼ v _∞)  , flat-topped and unshifted. Local absorption can have a major effect on the observed profiles – in systems with mass-loss rates of a few times  10⁻⁶ M_⊙ yr⁻¹  the lower energy lines  ( E  ≲ 1 keV)  are particularly affected. This generally results in blueward-skewed profiles, especially when the system is viewed through the dense wind of the primary. The orbital variation of the linewidths and shifts is reduced in a low-inclination binary. The extreme case is a binary with   i = 0°  , for which we would expect no line profile variation.     

Magellanic Cloud WC/WO Wolf–Rayet stars – II. Colliding winds in binaries

A search for evidence of colliding winds is undertaken among the four certain Magellanic Cloud WC/WO spectroscopic binaries found in the companion Paper I, as well as among two Galactic WC/WO binaries of very similar subtype. Two methods of analysis, which allow the determination of orbital inclination and parameters relating to the shock cone from spectroscopic studies of colliding winds, are attempted. In the first method, Lührs' spectroscopic model is fitted to the moderately strong C  iii 5696-Å excess line emission arising in the shock cone for the stars Br22 and WR 9. The four other systems show only very weak C  iii 5696-Å emission. Lührs' model follows well the mean displacement of the line in velocity space, but is unable to reproduce details in the line profile and fails to give a reliable estimate of the orbital inclination. In the second method, an alternative attempt is also made to fit the variation of more global quantities, full width at half-maximum and radial velocity of the excess emission, with phase. This method also gives satisfactory results in a qualitative way, but shows numerical degeneracy with orbital inclination. Colliding wind effects on the very strong C  iv 5808-Å Wolf–Rayet emission line, present in all six binaries, are also found to behave qualitatively as expected. After allowing for line enhancement in colliding wind binaries, it now appears that all Magellanic Cloud WC/WO stars occupy a very narrow range in spectral subclass: WC4/WO3.     

Interferometric observations of HCN and CN towards carbon stars

Using the IRAM interferometer we have observed four carbon stars (U Cam, CIT6, Y CVn, IRC+40540) in the HCN(J=1 0) and CN(N=1 0) lines. Here we present some results for CIT6 and U Cam.     

Millimeter polarimetry of star-forming regions: magnetic field structure around low-mass stars

We have measured polarization of the 1.1 mm and 0.8 mm continuum emission for 3 pre-T Tauri stars and 2 T Tauri stars. Positive detections were made for NGC 1333 IRAS 4 and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. We have found a definite geometrical relation between the polarization and other circumstellar structure.