Physical Sciences and Mathematics Commons^™

Variability In X-Ray Line Ratios In Helium-Like Ions Of Massive Stars: The Radiation-Driven Case., K. T. Hole, R. Ignace

Richard Ignace

Line ratios in "fir" triplets of helium-like ions have proven to be a powerful diagnostic of conditions in X-ray emitting gas surrounding massive stars. Recent observations indicate that these ratios can be variable with time. The possible causes of variation in line ratios are limited: changes in the radiation field or changes in density, and changes in mass-loss or geometry. In this paper, we investigate the ability of changes in the radiation field to induce variability in the ratio R=f/i. To isolate the radiative effect, we use a heuristic model of temperature and radius changes in variable stars in the …

Early Magnetic B-Type Stars: X-Ray Emission And Wind Properties., L. M. Oskinova, H. Todt, R. Ignace, J. C. Brown, J. P. Cassinelli, W.-R. Hamann

Richard Ignace

We present a comprehensive study of X-ray emission by, and wind properties of, massive magnetic early B-type stars. Dedicated XMM–Newton observations were obtained for three early-type B-type stars, ξ¹ CMa, V2052 Oph and ζ Cas, with recently discovered magnetic fields. We report the first detection of X-ray emission from V2052 Oph and ζ Cas. The latter is one the softest X-ray sources among the early-type stars, while the former is one of the X-ray faintest. The observations show that the X-ray spectra of our programme stars are quite soft with the bulk of X-ray emitting material having a temperature …

Time-Dependent Behavior Of Linear Polarization In Unresolved Photospheres, With Applications For The Hanle Effect., R. Ignace, K. T. Hole, J. P. Cassinelli, G. D. Henson

Richard Ignace

Aims: This paper extends previous studies in modeling time varying linear polarization due to axisymmetric magnetic fields in rotating stars. We use the Hanle effect to predict variations in net line polarization, and use geometric arguments to generalize these results to linear polarization due to other mechanisms. Methods: Building on the work of Lopez Ariste et al., we use simple analytic models of rotating stars that are symmetric except for an axisymmetric magnetic field to predict the polarization lightcurve due to the Hanle effect. We highlight the effects for the variable line polarization as a function of viewing inclination and …

Discovery Of The First Τ Sco Analogues: Hd 66665 And Hd 63425., V. Petit, D. L. Massa, W. L. F. Marcolinino, G. A. Wade, R. Ignace

Richard Ignace

The B0.2 V magnetic star τ Sco stands out from the larger population of massive OB stars due to its high X-ray activity, peculiar wind diagnostics and highly complex magnetic field. This Letter presents the discovery of the first two τ Sco analogues – HD 66665 and HD 63425, identified by the striking similarity of their ultraviolet (UV) spectra to that of τ Sco. ESPaDOnS spectropolarimetric observations were secured by the Magnetism in Massive Stars CFHT Large Program, in order to characterize the stellar and magnetic properties of these stars. cmfgen modelling of optical ESPaDOnS spectra and archived IUE UV …

Early Magnetic B-Type Stars: X-Ray Emission And Wind Properties, Lidia Oskinova, Helge Todt, Richard Ignace, John Brown, Joseph Cassinelli, Wolf-Rainer Hamann

Richard Ignace

We present a comprehensive study of X-ray emission and wind properties of magnetic early B-type stars. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. For the first time we analyze the UV spectra of B stars with magnetic fields by means of non-LTE iron-blanketed stellar atmosphere model that account for the X-rays at the intensity and temperatures observed. The mass-loss rates inferred from the analysis of UV lines are significantly lower …

The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes. V. Thin Lines From Keplerian Disks., R. Ignace

Richard Ignace

This paper focuses on the polarized profiles of resonance scattering lines that form in magnetized disks. Optically thin lines from Keplerian planar disks are considered. Model line profiles are calculated for simple field topologies of axial fields (i.e., vertical to the disk plane) and toroidal fields (i.e., purely azimuthal). A scheme for discerning field strengths and geometries in disks is developed based on Stokes Q − U diagrams for the run of polarization across line profiles that are Doppler-broadened by the disk rotation. A discussion of the Hanle effect for magnetized disks in which the magnetorotational instability (MRI) is operating …

A Multi-Phase Suzaku Study Of Τ Sco., R. Ignace, L. M. Oskinova, M. Jardine, J. P. Cassinelli, D. H. Cohen, J.-F. Donati, R. H. D. Townsend, A. Ud-Doula

Richard Ignace

We obtained relatively high signal-to-noise X-ray spectral data of the early massive star τ Sco (B0.2V) with the Suzaku X-ray Imaging Spectrometer (XIS) instrument. This source displays several unusual features that motivated our study: (1) redshifted absorption in UV P Cygni lines to approximately +250 km s⁻¹ suggestive of infalling gas, (2) unusually hard X-ray emission requiring hot plasma at temperatures in excess of 10 MK whereas most massive stars show relatively soft X-rays at a few MK, and (3) a complex photospheric magnetic field of open and closed field lines. In an attempt to understand the hard component …

Hard X-Ray Emission From The Massive Star-Forming Region On 2: Discovery With Xmm-Newton., L. M. Oskinova, R. A. Gruendl, R. Ignace, Y.-H. Chu, W.-R. Hamann, A. Feldmeier

Richard Ignace

We obtained X-ray XMM-Newton observations of the open cluster Berkeley 87 and the massive star-forming region (SFR) ON 2. In addition, archival infrared Spitzer Space Telescope observations were used to study the morphology of ON 2, to uncover young stellar objects, and to investigate their relationship with the X-ray sources. It is likely that the SFR ON 2 and Berkeley 87 are at the same distance, 1.23 kpc, and hence are associated. The XMM-Newton observations detected X-rays from massive stars in Berkeley 87 as well as diffuse emission from the SFR ON 2. The two patches of diffuse X-ray emission …

Radio Emission From Substellar Companions Of Evolved Cool Stars., Richard Ignace, Mark L. Giroux, Donald G. Luttermoser

Richard Ignace

A number of substellar companions to evolved cool stars have now been reported. Cool giants are distinct from their progenitor main-sequence low-mass stars in a number of ways. First, the mass loss rates of cool giant stars are orders of magnitude greater than for the late-type main-sequence stars. Secondly, on the cool side of the Linsky–Haisch ‘dividing line’, K and M giant stars are not X-ray sources, although they do show evidence for chromospheres. As a result, cool star winds are largely neutral for those spectral types, suggesting that planetary or brown dwarf magnetospheres will not be effective in standing …

The Zeeman Effect In The Sobolev Approximation Ii. Radial Split Monopole Fields And The ‘Heartbeat’ Stokes V Profile., K. G. Gayley, R. Ignace

Richard Ignace

We calculate the circularly polarized Stokes V(λ) profile for emission lines, formed in hot-star winds threaded with a weak radial magnetic field. For simplicity, the field is treated as a split monopole under the assumptions that it has been radially combed by the wind, and rotation is not playing a central role. Invoking the weak-field approximation, we find that the V(λ) profile has a characteristic “heartbeat” shape exhibiting multiple sign inversions, which might be mistaken for noise in the absence of theoretical guidance. We also conclude that there is a tendency for the V(λ) profile to integrate to zero on …

Polarization Variability Arising From Clumps In The Winds Of Wolf-Rayet Stars., Qing-Kang Li, Joseph P. Cassinelli, John C. Brown, Richard Ignace

Richard Ignace

The polarimetric and photometric variability of Wolf-Rayet (WR) stars as caused by clumps in the winds, is revisited. In the model which is improved from Li et al. 2000, the radial expansion of the thickness is accounted for, but we retain the dependence on the beta velocity law, stellar occultation effects. We again search for parameters that can yield results consistent with observations in regards to the mean polarization, the ratio of polarimetric to photometric variability, and the volume filling factor. Clump generation and spatial distribution are randomized by the Monte Carlo method so as to produce clumps which are, …

Scattering Polarization Due To Light Source Anisotropy Ii. Envelope Of Arbitrary Shape., Richard Ignace, M. B. Al-Malki, J. F. L. Simmons, J. C. Brown, D. Clarke, J. C. Carson

Richard Ignace

Aims. We consider the polarization arising from scattering in an envelope illuminated by a central anisotropic source. This work extends the theory introduced in a previous paper (Al-Malki et al. 1999) in which scattering polarization from a spherically symmetric envelope illuminated by an anisotropic point source was considered. Here we generalize to account for the more realistic expectation of a non-spherical envelope shape.

Methods. Spherical harmonics are used to describe both the light source anisotropy and the envelope density distribution functions of the scattering particles. This framework demonstrates how the net resultant polarization arises from a superposition of three basic …

Discovery Of X-Ray Emission From The Wolf-Rayet Star Wr 142 Of Oxygen Subtype., L. M. Oskinova, W.-R. Hamann, A. Feldmeier, R. Ignace, Y.-H. Chu

Richard Ignace

We report the discovery of weak yet hard X-ray emission from the Wolf-Rayet (WR) star WR 142 with the XMM-Newton X-ray telescope. Being of spectral subtype WO2, WR 142 is a massive star in a very advanced evolutionary stage shortly before its explosion as a supernova or gamma-ray burst. This is the first detection of X-ray emission from a WO-type star. We rule out any serendipitous X-ray sources within approximate to 1 '' of WR 142. WR 142 has an X-ray luminosity of L(X) approximate to 7 x 10(30) erg s(-1), which constitutes only less than or similar to 10(-8) …

Spectropolarimetric Variability And Co-Rotating Structure In Hd 92207., Richard Ignace, S. Hubrig, M. Schöller

Richard Ignace

We report on low resolution (R~3000) spectropolarimetry of the A0 supergiant star HD 92207. This star is well-known for significant spectral variability. The source was observed on seven different nights spanning approximately 3 months in time. With a rotation period of approximately 1 year, our data covers approximately a quarter of the star's rotational phase. Variability in the continuum polarization level is observed over this period of time. The polarization across the Halpha line on any given night is typically different from the degree and position angle of the polarization in the continuum. Interestingly, Hbeta is not in emission and …

Modeling Forbidden Line Emission Profiles From Colliding Wind Binaries., R. Ignace, R. Bessey, C. S. Price

Richard Ignace

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 …

Magnetic Field Measurements Of O Stars With Fors 1 At The Vlt., S. Hubrig, M. Schöller, R. S. Schnerr, J. F. González, R. Ignace, H. F. Henrichs

Richard Ignace

Context.The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of these fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Hα emission variations, chemical peculiarity, narrow X-ray emission lines, and non-thermal radio/X-ray emission.
Aims.To investigate the incidence of magnetic fields in O stars, we acquired 38 new spectropolarimetric observations with FORS 1 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Kueyen telescope of the VLT.
Methods.Spectropolarimetric observations were obtained at different phases for a sample of 13 O …

The Effects Of Clumps In Explaining X-Ray Emission Lines From Hot Stars., J. P. Cassinelli, R. Ignace, W. L. Waldron, J. Cho, N. A. Murphy, A. Lazarian

Richard Ignace

It is now well established that stellar winds of hot stars are fragmentary and that the X-ray emission from stellar winds has a strong contribution from shocks in winds. Chandra high spectral resolution observations of line profiles of O and B stars have shown numerous properties that had not been expected. Here we suggest explanations by considering the X-rays as arising from bow shocks that occur where the stellar wind impacts on spherical clumps in the winds. We use an accurate and stable numerical hydrodynamic code to obtain steady state physical conditions for the temperature and density structure in a …

Phase-Dependent X-Ray Observations Of The Beta Lyrae System: No Eclipse In The Soft Band., R. Ignace, L. M. Oskinova, W. L. Waldron, J. L. Hoffman, W.-R. Hamann

Richard Ignace

Aims.We report on observations of the eclipsing and interacting binary beta Lyrae from the Suzaku X-ray telescope. This system involves an early B star embedded in an optically and geometrically thick disk that is siphoning atmospheric gases from a less massive late B II companion.
Methods.Motivated by an unpublished X-ray spectrum from the Einstein X-ray telescope suggesting unusually hard emission, we obtained time with Suzaku for pointings at three different phases within a single orbit.
Results.From the XIS detectors, the softer X-ray emission appears typical of an early-type star. What is surprising is the remarkably unchanging character …

Neon Abundances From A Spitzer/Irs Survey Of Wolf-Rayet Stars., R. Ignace, J. P. Cassinelli, G. Tracy, E. Churchwell, H. J. G. L. M. Lamers

Richard Ignace

We report on neon abundances derived from Spitzer high resolution spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of [Ne III] 15.56 μm. Our targets include four WN stars of subtypes 4–7, and four WC stars of subtypes 4–7. We derive ion fraction abundances γ of Ne²⁺ for the winds of each star. The ion fraction abundance is a product of the ionization fraction Q_i in stage i and the abundance by number A_E of element E relative to all nuclei. Values generally consistent with solar are obtained for the WN stars, and values …

Models Of Forbidden Line Emission Profiles From Axisymmetric Stellar Winds., R. Ignace, A. Brimeyer

Richard Ignace

A number of strong infrared forbidden lines have been observed in several evolved Wolf–Rayet (WR) star winds, and these are important for deriving metal abundances and testing stellar evolution models. In addition, because these optically thin lines form at large radius in the wind, their resolved profiles carry an imprint of the asymptotic structure of the wind flow. This work presents model forbidden line profile shapes formed in axisymmetric winds. It is well known that an optically thin emission line formed in a spherical wind expanding at constant velocity yields a flat-topped emission profile shape. Simulated forbidden lines are produced …

Microlensing Of Circumstellar Envelopes Iii. Line Profiles From Stellar Winds In Homologous Expansion., M. A. Hendry, R. Ignace, H. M. Bryce

Richard Ignace

This paper examines line profile evolution due to the linear expansion of circumstellar material obsverved during a microlensing event. This work extends our previous papers on emission line profile evolution from radial and azimuthal flow during point mass lens events and fold caustic crossings. Both “flavours” of microlensing were shown to provide effective diagnostics of bulk motion in circumstellar envelopes. In this work a different genre of flow is studied, namely linear homologous expansion, for both point mass lenses and fold caustic crossings. Linear expansion is of particular relevance to the effects of microlensing on supernovae at cosmological distances. We …

The Polarization Signature From Microlensing Of Circumstellar Envelopes In Caustic Cossing Events., R. Ignace, J. E. Bjorkman, H. M. Bryce

Richard Ignace

In recent years, it has been shown that microlensing is a powerful tool for examining the atmospheres of stars in the Galactic bulge and Magellanic Clouds. The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarization signal: arising from differential magnification across the otherwise symmetric source. Earlier work has addressed the signal from a scattering photosphere for both point mass lenses and caustic crossings. In a previous paper, …

First Glimpse Results On The Stellar Structure Of The Galaxy., R. A. Benjamin, E. Churchwell, B. L. Babler, R. Indebetouw, M. R. Meade, B. A. Whitney, C. Watson, M. G. Wolfire, M. J. Wolff, R. Ignace, T. M. Bania, S. Bracker, D. P. Clemens, L. Chomiuk, M. Cohen, J. M. Dickey, J. M. Jackson, H. A. Kobulnicky, E. P. Mercer, J. S. Mathis, S. R. Stolovy, B. Uzpen

Richard Ignace

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~ 30 million mid-infrared sources towards the inner Galaxy, 10 < |l| < 65 degrees and |b| < 1 degree, was used to determine the distribution of stars in Galactic longitude, latitude, and apparent magnitude. The counts versus longitude can be approximated by the modified Bessel function N=N_0*(l/l_0)*K_1(l/l_0), where l_0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l_0= 17-30 degrees with a best fit value in the the 4.5 micron band of l_0=24 +/- 4 degrees. Modeling the source distribution as an exponential disk yields a radial scale length of H= 3.9 +/- 0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| < 30 degrees, with ~ 25% more stars in the north. For l=10-30 degrees, there is a strong enhancement of stars of m= 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length R_bar=4.4 +/- 0.5 kpc, tilted by phi=44 +/- 10 degrees to the Sun-Galactic Center line, provides the simplest interpretation of this data. We examine the possibility that enhanced source counts at l=26-28 degrees, 31.5-34 degrees, and 306-309 degrees are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m_K-m_[8.0] >3) peak. In these areas, the counts are reduced by extinction due to molecular gas and/or high diffuse backgrounds associated with star formation.

Optically Thick Clumps – Not The Solution To The Wolf-Rayet Wind Momentum Problem?, J. C. Brown, J. P. Cassinelli, Q. Li, A. F. Kholtygin, R. Ignace

Richard Ignace

The hot star wind momentum problem eta = M-upsiloninfinity/(L/c) much greater than 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate. M, and hence the momentum supply, required to generate a specified radio emission measure epsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in opposition rather …

Free-Free Spectral Energy Distributions Of Hierarchically Clumped Hii Regions., Richard Ignace, Ed Churchwell

Richard Ignace

In an effort to understand unusual power-law spectral slopes observed in some hypercompact HII regions, we consider the radio continuum energy distribution from an ensemble of spherical clumps. An analytic expression for the free-free emission from a single spherical clump is derived. The radio continuum slope (with F_\nu \nu^\alpha) is governed by the population of clump optical depths N(tau), such that (a) at frequencies where all clumps are thick, a continuum slope of +2 is found, (b) at frequencies where all clumps are optically thin, a flattened slope of -0.11 is found, and (c) at intermediate frequencies, a power-law segment …

The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes Iv. Application To Polarized P Cygni Wind Lines., R. Ignace, K. H. Nordsieck, J. P. Cassinelli

Richard Ignace

The Hanle effect has been proposed as a new diagnostic of circumstellar magnetic fields for early-type stars, for which it is sensitive to field strengths in the 1-300 G range. In this paper we compute the polarized P-Cygni line profiles that result from the Hanle effect. For modeling the polarization, we employ a variant of the ``last scattering approximation''. For cases in which the Sobolev optical depths are greater than unity, the emergent line intensity is assumed to be unpolarized; while for smaller optical depths, the Stokes source functions for the Hanle effect with optically thin line scattering are used. …

Constraints From Infrared Space Observatory Data On The Velocity Law And Clumpiness Of Wr 136., R. Ignace, M. F. Quigley, J. P. Cassinelli

Richard Ignace

Observations with the Infrared Space Observatory (ISO) SWS spectrometer are used to constrain the velocity law and wind clumping of the well-studied Wolf-Rayet (WR) star WR 136 (HD 192163) (WN6). Because the free-free continuum opacity in WR winds increases steadily with wavelength in the IR, each point in the continuous spectrum may be regarded as forming in a pseudo-photosphere of larger radius for longer wavelength. Using this idea in combination with an analysis of the Doppler-broadened widths of several \ion{He}{2} recombination lines, we can derive information about the velocity law and clumpiness of the stellar wind of WR 136. Adopting …

Xmm-Newton Observations Of The Nitrogen-Rich Wolf-Rayet Star Wr 1., R. Ignace, L. M. Oskinova, J. C. Brown

Richard Ignace

We present XMM-Newton results for the X-ray spectrum from the N-richWolf-Rayet (WR) star WR1. The EPIC instrument was used to obtain a medium-resolution spectrum. The following features characterize this spectrum: ( a) significant emission "bumps" appear that are coincident with the wavelengths of typical strong lines, such as MgXI, SiXIII, and SXV; (b) little emission is detected above 4 keV, in contrast to recent reports of a hard component in the stars WR 6 and WR 110 which are of similar subtype; and ( c) evidence for sulfur K-edge absorption at about 2.6 keV, which could only arise from absorption …

The Zeeman Effect In The Sobolev Approximation: Applications To Spherical Stellar Winds., R. Ignace, K. G. Gayley

Richard Ignace

Modern spectropolarimeters are capable of detecting subkilogauss field strengths using the Zeeman effect in line profiles from the static photosphere, but supersonic Doppler broadening makes it more difficult to detect the Zeeman effect in the wind lines of hot stars. Nevertheless, the recent advances in observational capability motivate an assessment of the potential for detecting the magnetic fields threading such winds. We incorporate the weak-field longitudinal Zeeman effect in the Sobolev approximation to yield integral expressions for the flux of circularly polarized emission. To illustrate the results, two specific wind flows are considered: (i) spherical constant expansion with v(r) = …

The Conspicuous Absence Of X-Ray Emission From Carbon-Enriched Wolf-Rayet Stars., L. M. Oskinova, R. Ignace, W.-R. Hamann, A. M. T. Pollock, J. C. Brown

Richard Ignace

The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton observation, implying an upper limit to the X-ray luminosity of L-X less than or similar to 2.5 x 10(30) erg s(-1) and to the X-ray to bolometric luminosity ratio of L-X/L(bo)l less than or similar to 4 x 10(-9). This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton and Chandra observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven …