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The surprising magnetic topology of τ Sco: fossil remnant or dynamo output?
We report the discovery of a medium-strength (~0.5 kG) magnetic field onthe young, massive star τ Sco (B0.2V), which becomes thethird-hottest magnetic star known. Circularly polarized Zeemansignatures are clearly detected in observations collected mostly withthe ESPaDOnS spectropolarimeter, recently installed on the 3.6-mCanada-France-Hawaii Telescope; temporal variability is also clearlyestablished in the polarimetry, and can be unambiguously attributed torotational modulation with a period close to 41 d. Archival ultraviolet(UV) spectra confirm that this modulation repeats over time-scales ofdecades, and refine the rotation period to 41.033 +/- 0.002 d.Despite the slow rotation rate of τ Sco, we none the less succeed inreconstructing the large-scale structure of its magnetic topology. Wefind that the magnetic structure is unusually complex for a hot star,with significant power in spherical-harmonic modes of degree up to 5.The surface topology is dominated by a potential field, although amoderate toroidal component is probably present. We fail to detectintrinsic temporal variability of the magnetic structure over the 1.5-yrperiod of our spectropolarimetric observations (in agreement with thestable temporal variations of the UV spectra), and infer that anydifferential surface rotation must be very small.The topology of the extended magnetic field that we derive from thephotospheric magnetic maps is also more complex than a global dipole,and features in particular a significantly warped torus of closedmagnetic loops encircling the star (tilted at about 90° to therotation axis), with additional, smaller, networks of closed-fieldlines. This topology appears to be consistent with the exceptional X-rayproperties of τ Sco and also provides a natural explanation of thevariability observed in wind-formed UV lines. Although we cannotcompletely rule out the possibility that the field is produced throughdynamo processes of an exotic kind, we conclude that its magnetic fieldis most probably a fossil remnant from the star formation stage.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Science de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.E-mail: donati@ast.obs-mip.fr (J-FD); idh@star.ucl.ac.uk (IDH);mmj@st-andrews.ac.uk (MMJ); petit@ast.obs-mip.fr (PP);claude.catala@obspm.fr (CC); jlandstr@uwo.ca (JDL);jean-claude.bouret@oamp.fr (J-CB); evelyne.alecian@obspm.fr (EA);jrb3@st-andrews.ac.uk (JRB); forveill@cfht.hawaii.edu (TF);fpaletou@ast.obs-mip.fr (FP); manset@cfht.hawaii.edu (NM)
| Identification of the Infrared Counterpart to a Newly Discovered X-Ray Source in the Galactic Center
We present first results of a campaign to find and identify new compactobjects in the Galactic center. Selecting candidates from a combinationof Chandra and 2MASS survey data, we search for accretion disksignatures via infrared spectroscopy. We have found the infraredcounterpart to the Chandra source CXO J174536.1-285638, the spectrum ofwhich has strong Brγ and He I emission. The presence of C III, NIII, and He II indicate a binary system. We suspect that the system issome form of high-mass binary system, either a high-mass X-ray binary ora colliding-wind binary.
| Rotational and Cyclical Variability in γ Cassiopeia
γ Cas is an unusual classical Be star for which the optical-bandand hard X-ray fluxes vary on a variety of timescales. We report resultsof a 9 yr monitoring effort on this star with a robotic ground-based(APT) telescope in the B, V filter system, as well as simultaneousobservations in 2004 November with this instrument and the RXTE. Ourobservations disclosed no correlated optical response to the rapid X-rayflares in this star, nor did the star show any sustained flux changesany time during two monitored nights in either wavelength regime.Consistent with an earlier study by Robinson et al., optical lightcurves obtained in our new APT program revealed that γ Casundergoes ~3% amplitude cycles with lengths of 50-91 days. Ourobservations in 2004 showed a similar optical cycle. Over the 9 days wemonitored the star with the RXTE, the X-ray flux varied in phase withits optical cycle and with an amplitude predicted from the data inRobinson et al. In general, the amplitude of the V magnitude cycles are30%-40% larger than the corresponding B amplitude, suggesting that theproduction site of the cycles is circumstellar. The cycle lengthsconstantly change and can damp or grow on timescales as short as 13days. We have also discovered a coherent period of 1.21581+/-0.00004days in all our data, which appears consistent only with rotation. Thefull amplitude of this variation is 0.0060 in both filters, and,surprisingly, its waveform is almost sawtooth in shape. This variationis likely to originate on the star's surface. This circumstance hints atthe existence of a strong magnetic field with a complex topology and apossible heterogeneous surface distribution of metals.
| Centrifugal Breakout of Magnetically Confined Line-driven Stellar Winds
We present two-dimensional MHD simulations of the radiatively drivenoutflow from a rotating hot star with a dipole magnetic field alignedwith the star's rotation axis. We focus primarily on a model withmoderately rapid rotation (half the critical value) and also a largemagnetic confinement parameter,η*≡B2*R2*/M˙V&infy;=600.The magnetic field channels and torques the wind outflow into anequatorial, rigidly rotating disk extending from near the Keplercorotation radius outward. Even with fine-tuning at lower magneticconfinement, none of the MHD models produce a stable Keplerian disk.Instead, material below the Kepler radius falls back onto the stellarsurface, while the strong centrifugal force on material beyond thecorotation escape radius stretches the magnetic loops outward, leadingto the episodic breakout of mass when the field reconnects. Theassociated dissipation of magnetic energy heats material to temperaturesof nearly 108 K, high enough to emit hard (several keV)X-rays. Such centrifugal mass ejection represents a novel mechanism fordriving magnetic reconnection and seems a very promising basis formodeling X-ray flares recently observed in rotating magnetic Bp starslike σ Ori E.
| Winds from OB Stars: A Two-Component Scenario?
X-ray spectroscopy of several OB stars with massive winds has revealedthat many X-ray line profiles exhibit unexpectedly small blueshifts andare almost symmetric. Moreover, the hottest X-ray lines appear tooriginate closest to the star. These properties appear to beinconsistent with the standard model of X-rays originating in shockedmaterial in line-driven spherically symmetric winds. Here we raise thequestion, can the X-ray line data be understood in terms of atwo-component wind? We consider a scenario in which one component of thewind is a standard line-driven wind that emerges from a broad range oflatitudes centered on the equator. The second component of the windemerges from magnetically active regions in extensive polar caps. Theexistence of such polar caps is suggested by a recent model of dynamoaction in massive stars. We describe how the two-component model isconsistent with a variety of observational properties of OB star winds.
| Multicomponent radiatively driven stellar winds. IV. On the helium decoupling in the wind of σ Orionis E
We study the possibility of the helium decoupling in the stellar wind ofσ Ori E. To obtain reliable wind parameters for this star we firstcalculate NLTE wind model and derive wind mass-loss rate and terminalvelocity. Using corresponding force multipliers we study the possibilityof helium decoupling. We find that helium decoupling is not possible forrealistic values of helium charge (calculated from NLTE wind models).Helium decoupling only seems possible for a very low helium charge. Thereason for this behavior is the strong coupling between helium andhydrogen. We also find that frictional heating becomes important in theouter parts of the wind of σ Ori E due to the collisions betweensome heavier elements and the passive components - hydrogen and helium.For a metallicity ten times lower than the solar one, both hydrogen andhelium decouple from the metals and may fall back onto the stellarsurface. However, this does not explain the observed chemicalpeculiarity since both these components decouple together from theabsorbing ions. Although we do not include the effects of the magneticfield into our models, we argue that the presence of a magnetic fieldwill likely not significantly modify the derived results because in suchcase model equations describe the motion parallel to the magnetic field.
| Variations of the He II λ1640 line in B0e-B2.5e stars
Using the International Ultraviolet Explorer data archive, we haveexamined the SWP echellograms of 74 B0-B2.5e stars for statisticallysignificant fluctuations in the He II ("Hα") λ1640 lineprofile. In this sample we found that the He II line is occasionallyvariable in 10 stars over short to long timescales. The He II-variablestars discovered are λ Eri, ω Ori, μ Cen, 6 Cep, HD67536, ψ1 Ori, η Cen, π Aqr, 2 Vul, and 19 Mon.The most frequent two types of variability are an extended blue wingabsorption and a weakening of the line along the profile. Other types ofvariability are a weak emission in the red wing and occasionally anarrow emission feature. In the overwhelming number of cases, the C IVresonance doublet exhibits a similar response; rarely, it can exhibit avariation in the opposite sense. Similar responses are also often seenin the Si IV doublet, and occasionally even the Si III λ1206line. We interpret the weakenings of He II and of high-velocityabsorptions of C IV to localized decreases in the photospherictemperature, although this may not be a unique interpretation. Wediscuss the variable blue wing absorptions and red wing emissions interms of changes in the velocity law and mass flux carried by the wind.In the latter case, recent experimental models by Venero, Cidale, &Ringuelet require that during such events the wind must be heated by 35000 K at some distance from the star.
| Stellar magnetosphere reconstruction from radio data. Multi-frequency VLA observations and 3D-simulations of CU Virginis
Aims.In order to fully understand the physical processes in themagnetospheres of the Magnetic Chemically Peculiar stars, we performedmulti-frequency radio observations of CU Virginis.The radio emission of this kind of stars arises from the interactionbetween energetic electrons and magnetic field. Our analysis is used totest the physical scenario proposed for the radio emission from the MCPstars and to derive quantitative information about physical parametersnot directly observable. Methods: .The radio data were acquiredwith the VLA and cover the whole rotational period of CUVirginis. For each observed frequency the radio light curvesof the total flux density and fraction of circular polarization werefitted using a three-dimensional MCP magnetospheric model simulating thestellar radio emission as a function of the magnetospheric physicalparameters. Results: .The observations show a clear correlationbetween the radio emission and the orientation of the magnetosphere ofthis oblique rotator. The radio emission is explained as the result ofthe acceleration of the wind particles in the current sheets just beyondthe Alfvén radius, that eventually return toward the starfollowing the magnetic field and emitting radiation by gyrosyncrotronmechanisms. The accelerated electrons we probed with our simulationshave a hard energetic spectrum (N(E)∝ E-2) and theacceleration process has an efficiency of about 10-3. TheAlfvén radius we determined is in the range of 12-17 R_ast and,for a dipolar field of 3000 Gauss at the magnetic pole of the star, wedetermine a mass loss from the star of about 10-12Mȯ yr-1. In the inner magnetosphere, insidethe Alfvén radius, the confined stellar wind accumulates andreaches temperatures in the range of 10^5-106 K, and adetectable X-ray emission is expected.
| The magnetic Bp star 36 Lyncis. II. A spectroscopic analysis of its co-rotating disk
We report on the physical properties of the disk-like structure of B8IIIp star 36 Lyncis from line syntheses of phase-resolved, highresolution spectra obtained from the International Ultraviolet Explorerarchives and from newly obtained ground-based spectra of the Hαabsorption profile. This disk is highly inclined to the rotational axisand betrays its existence every half rotation cycle as one of twoopposing sectors pass in front of the star. Although the disk absorptionspectrum is at least ten times too weak to be visible in optical ironlines during these occultations, its properties can be readily examinedin a large number of UV "iron curtain" lines because of their higheropacities. The UV Fe II and Fe III lines in particular permit adetermination of the disk temperature: 7500±500 K and a columndensity of 3-1.5+3×1020cm-2. The analysis of the variations of the UV resonancelines brings out some interesting details about the radiative propertiesof the disks: (1) they are optically thick in the C IV and Si IVdoublets, (2) the range of excitation of the UV resonance lines islarger at the primary occultation (φ = 0.00) than at the secondaryone, and (3) the relative strengths of the absorption peaks for the twooccultations varies substantially from line to line. We have modeled theabsorptions of the UV C IV resonance and Hα absorptions by meansof a simulated disk with opaque and translucent components. Oursimulations suggest that a gap separates the star and the inner edge ofthe disk. The disk extends radially out to ≥10 R*. Thedisk scale height perpendicular to the plane is ≈1 R*.However, the sector causing the primary occultation is about four timesthicker than the opposite sector. The C IV scattering region extends toa larger height than the Hα region does, probably because itresults from shock heating far from the cooler disk plane.
| The magnetic Bp star 36 Lyncis. I. Magnetic and photospheric properties
Aims.This paper reports the photospheric, magnetic and circumstellar gascharacteristics of the magnetic B8p star 36 Lyncis (HD 79158).Methods: .Using archival data and new polarised and unpolarisedhigh-resolution spectra, we redetermine the basic physical properties,the rotational period and the geometry of the magnetic field, and thephotospheric abundances of various elements. Results: .Based onmagnetic and spectroscopic measurements, we infer an improved rotationalperiod of 3.83475± 0.00002 d. We determine a current epoch of thelongitudinal magnetic field positive extremum (HJD 2 452 246.033), andprovide constraints on the geometry of the dipole magnetic field (i≥56°, 3210~G≤ B_d≤ 3930 G, β unconstrained). Weredetermine the effective temperature and surface gravity using theoptical and UV energy distributions, optical photometry and Balmer lineprofiles (T_eff=13 300± 300 K, log g=3.7-4.2), and based on theHipparcos parallax we redetermine the luminosity, mass, radius and truerotational speed (L=2.54± 0.16~L_ȯ, M=4.0±0.2~M_ȯ, R=3.4± 0.7~R_ȯ, v_eq=45-61.5 kms-1). We measure photospheric abundances for 21 elementsusing optical and UV spectra, and constrain the presence of verticalstratification of these elements. We perform preliminary Doppler Imagingof the surface distribution of Fe, finding that Fe is distributed in apatchy belt near the rotational equator. Most remarkably, we confirmstrong variations of the Hα line core which we interpret as due tooccultations of the star by magnetically-confined circumstellar gas.
| XMM-Newton observations of the σ Orionis cluster. II. Spatial and spectral analysis of the full EPIC field
We present the results of an XMM-Newton observation of the young (2{-}4 Myr) cluster around the hot star σ Orionis. In a previouspaper we presented the analysis of the RGS spectrum of the central hotstar; here we discuss the results of the analysis of the full EPICfield. We have detected 175 X-ray sources, 88 of which have beenidentified with cluster members, including very low-mass stars down tothe substellar limit. We detected eleven new possible candidate membersfrom the 2MASS catalogue. We find that late-type stars have a median logL_X/L_bol -3.3, i.e. very close to the saturation limit. Wedetected significant variability in 40% of late-type members orcandidates, including 10 flaring sources; rotational modulation isdetected in one K-type star and possibly in another 3 or 4 stars.Spectral analysis of the brightest sources shows typical quiescenttemperatures in the range T_1 0.3{-}0.8 keV and T_2 1{-}3 keV,with subsolar abundances Z 0.1{-}0.3 Z_ȯ, similar to what isfound in other star-forming regions and associations. We find nosignificant difference in the spectral properties of classical andweak-lined T Tauri stars, although classical T Tauri stars tend to beless X-ray luminous than weak-lined T Tauri stars.
| A rigidly rotating magnetosphere model for circumstellar emission from magnetic OB stars
We present a semi-analytical approach for modelling circumstellaremission from rotating hot stars with a strong dipole magnetic fieldtilted at an arbitrary angle to the rotation axis. By assuming therigid-field limit in which material driven (e.g. in a wind outflow) fromthe star is forced to remain in strict rigid-body corotation, we areable to solve for the effective centrifugal-plus-gravitational potentialalong each field line, and thereby identify the location of potentialminima where material is prone to accumulate. Applying basic scalingsfor the surface mass flux of a radiatively driven stellar wind, wecalculate the circumstellar density distribution that obtains onceejected plasma settles into hydrostatic stratification along fieldlines. The resulting accumulation surface resembles a rigidly rotating,warped disc, tilted such that its average surface normal lies betweenthe rotation and magnetic axes. Using a simple model of the plasmaemissivity, we calculate time-resolved synthetic line spectra for thedisc. Initial comparisons show an encouraging level of correspondencewith the observed rotational phase variations of Balmer-line emissionprofiles from magnetic Bp stars such as σ Ori E.
| The Rigidly Rotating Magnetosphere of σ Orionis E
We characterize the observed variability of the magnetic helium-strongstar σ Ori E in terms of a recently developed rigidly rotatingmagnetosphere model. This model predicts the accumulation ofcircumstellar plasma in two corotating clouds, situated inmagnetohydrostatic equilibrium at the intersections between the magneticand rotational equators. We find that the model can reproduce well theperiodic modulations observed in the star's light curve, Hαemission-line profile, and longitudinal field strength, confirming thatit furnishes an essentially correct, quantitative description of thestar's magnetically controlled circumstellar environment.
| Chandra HETGS Multiphase Spectroscopy of the Young Magnetic O Star θ1 Orionis C
We report on four Chandra grating observations of the oblique magneticrotator θ1 Ori C (O5.5 V), covering a wide range ofviewing angles with respect to the star's 1060 G dipole magnetic field.We employ line-width and centroid analyses to study the dynamics of theX-ray-emitting plasma in the circumstellar environment, as well asline-ratio diagnostics to constrain the spatial location, and globalspectral modeling to constrain the temperature distribution andabundances of the very hot plasma. We investigate these diagnostics as afunction of viewing angle and analyze them in conjunction with new MHDsimulations of the magnetically channeled wind shock mechanism onθ1 Ori C. This model fits all the data surprisinglywell, predicting the temperature, luminosity, and occultation of theX-ray-emitting plasma with rotation phase.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| On the detection of chemically peculiar stars using Δa photometry
We have summarized all Δ a measurements for galactic field stars(1474 objects) from the literature published over more than two decades.These measurements were, for the first time, compiled and homogeneouslyanalyzed. The Δ a intermediate band photometric system samples thedepth of the 5200 Å flux depression by comparing the flux at thecenter with the adjacent regions with bandwidths of 110 Å to 230Å. Because it was slightly modified over the last three decades,we checked for systematic trends for the different measurements butfound no correlations whatsoever. The Δ a photometric system ismost suitable to detecting magnetic chemically peculiar (CP) stars withhigh efficiency, but is also capable of detecting a small percentage ofnon-magnetic CP objects. Furthermore, the groups of (metal-weak)λ Bootis, as well as classical Be/shell stars, can besuccessfully investigated. In addition, we also analyzed the behaviourof supergiants (luminosity class I and II). On the basis of apparentnormal type objects, the correlation of the 3σ significance limitand the percentage of positive detection for all groups was derived. Wecompared the capability of the Δ a photometric system with theΔ (V1 - G) and Z indices of the Geneva 7-color system to detectpeculiar objects. Both photometric systems show the same efficiency forthe detection of CP and λ Bootis stars, while the indices in theGeneva system are even more efficient at detecting Be/shell objects. Onthe basis of this statistical analysis it is possible to derive theincidence of CP stars in galactic open cluster and extragalactic systemsincluding the former unknown bias of undetected objects. This isespecially important in order to make a sound statistical analysis ofthe correlation between the occurrence of these objects andastrophysical parameters such as the age, metallicity, and strength ofglobal, as well as local, magnetic fields.
| A catalog of stellar magnetic rotational phase curves
Magnetized stars usually exhibit periodic variations of the effective(longitudinal) magnetic field Be caused by their rotation. Wepresent a catalog of magnetic rotational phase curves, Be vs.the rotational phase φ, and tables of their parameters for 136stars on the main sequence and above it. Phase curves were obtained bythe least squares fitting of sine wave or double wave functions to theavailable Be measurements, which were compiled from theexisting literature. Most of the catalogued objects are chemicallypeculiar A and B type stars (127 stars). For some stars we also improvedor determined periods of their rotation. We discuss the distribution ofparameters describing magnetic rotational phase curves in our sample.All tables and Appendix A are only available in electronic form athttp://www.edpsciences.org
| XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (2-5Myr) and Taurus-Auriga (1-10 Myr) as well as for the OCs IC 2602(30 Myr), alpha Persei (50 Myr), Praesepe (600 Myr) andthe Hyades (600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time
| Photometric Identification of the Low-Mass Population of Orion OB1b. I. The σ Orionis Cluster
We report an optical photometric survey of 0.89 deg2 of theOrion OB1b association centered on σ Ori. This region includesmost of the σ Ori cluster, the highest-density region within OrionOB1b. We have developed a statistical procedure to identify the young,low-mass, pre-main-sequence population of the association. We estimatethat the cluster has ~160 members in the mass range (0.2<=M<=1.0Msolar). The cluster has a radius of ~3-5 pc and an estimatedage of 2.5+/-0.3 Myr. We estimate that the total mass of the cluster is225+/-30 Msolar. This mass is similar to the estimated massof the ~5×105 yr old cluster NGC 2024. NGC 2024 andσ Ori appear to be a well-matched pair of clusters, except for the~2 Myr difference in their ages.
| Extending the radio spectrum of magnetic chemically peculiar stars to the mm range
Magnetic chemically peculiar (MCP) stars can present radio emission atcentimetre wavelengths. The steep decrement of the dominant dipolarcomponent of the photospheric magnetic field results in each radiofrequency being mainly emitted in a well localised shell of thecircumstellar region. To explore the most internal regions of themagnetosphere, observations of a sample of eleven MCP stars known to beradio sources in the 1.4-22.5 GHz range were carried out at 87.7 GHzwith the IRAM interferometer. Millimeter emission, with a flux densityat about 4× the sensitivity limit of our observations, wasdetected towards two of the stars: HD 35298 and HD 124224. Combiningour mm-observations with previous cm-observations, it appears that MCPstars with a relatively weak magnetic field present a radio spectrumthat increases with frequency up to 22.5 GHz and then decreases towardsthe mm range. In presence of strong fields, the radio spectrum is alwaysdecreasing with frequency. A comparison of the observed cm-mm spectrumof HD 124224 with results of numerical simulations of thegyrosynchrotron emission suggests that circumstellar regions emitting inthe mm-range cannot present magnetic fields larger than 1-2 kG.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain).
| XMM-Newton observations of the σ Ori cluster. I. The complex RGS spectrum of the hot star σ Ori AB
We present XMM-Newton observations of the young ({} 2{-}5 Myr)cluster around the hot (O9.5V) star σ Orionis AB, aimed atobtaining a high resolution RGS spectrum of the hot star as well as EPICimaging data for the whole field. We show that the RGS spectrum ofσ Ori AB may be contaminated by weaker nearby sources whichrequired the development of a suitable procedure to extract a clean RGSspectrum and to determine the thermal structure and wind properties ofthe hot star. We also report on the detection of a flare from the B2Vpstar σ Ori E and we discuss whether the flare originated from thehot star itself or rather from an unseen late-type companion. Otherresults of this observation include: the detection of 174 X-ray sourcesin the field of σ Ori of which 76 are identified as clustermembers, including very low-mass stars down to the substellar limit; thediscovery of rotational modulation in a late-type star near σ OriAB; no detectable line broadenings and shifts ( 800 kms-1) in the spectrum of σ Ori AB together with aremarkable low value of the O VII forbidden to intercombination lineratio and unusually high coronal abundances of CNO elements.
| Mid-infrared observations of young stellar objects in the vicinity of σ Orionis
We present new mid-infrared observations of objects in the vicinity ofthe O-star σ Orionis, obtained with TIMMI-2 at ESO. Byconstraining their near- and mid-infrared spectral energy distributions,we established the nature of previously known IRAS sources andidentified new mid-infrared sources as young stellar objects withcircumstellar disks, likely massive members of the σ Ori cluster.For two of these objects we have obtained spectroscopy in the 8-13 μmrange in order to investigate the chemistry of the dust grains. TX Oriexhibits a typical silicate emission feature at 10 μm, with a featureat about 11.2 μm that we identify as due to crystalline olivine. TheIRAS 05358-0238 spectrum is very unusual, with a weak silicate featureand structure in the range 10-12 μm that may be explained as due toself-absorbed forsterite. We also provide the first evidence for thepresence of circumstellar disks in the jet sources Haro 5-39/HH 447,V510 Ori/HH 444 and V603 Ori/HH 445.Based on observations collected at the European Southern Observatory,Chile (ESO No 70.C-0623).
| A three-dimensional model for the radio emission of magnetic chemically peculiar stars
In this paper we present a three-dimensional numerical model for theradio emission of Magnetic Chemically Peculiar stars, on the hypothesisthat energetic electrons emit by the gyrosynchrotron mechanism. For thisclass of radio stars, characterized by a mainly dipolar magnetic fieldwhose axis is tilted with respect to the rotational axis, the geometryof the magnetosphere and its deformation due to the stellar rotation aredetermined. The radio emitting region is determined by the physicalconditions of the magnetosphere and of the stellar wind. Free-freeabsorption by the thermal plasma trapped in the inner magnetosphere isalso considered. Several free parameters are involved in the model, suchas the size of the emitting region, the energy spectrum and the numberdensity of the emitting electrons, and the characteristics of the plasmain the inner magnetosphere. By solving the equation of radiativetransfer, along a path parallel to the line of sight, the radiobrightness distribution and the total flux density as a function ofstellar rotation are computed. As the model is applied to simulate theobserved 5 GHz lightcurves of HD 37479 andHD 37017, several possible magnetosphereconfigurations are found. After simulations at other frequencies, inspite of the large number of parameters involved in the modeling, twosolutions in the case of HD 37479 and only onesolution in the case of HD 37017 match the observedspectral indices. The results of our simulations agree with themagnetically confined wind-shock model in a rotating magnetosphere. TheX-ray emission from the inner magnetosphere is also computed, and foundto be consistent with the observations.Appendices A and B are only available in electronic form athttp://www.edpsciences.org
| Discovery of X-ray flaring on the magnetic Bp-star σ Ori E
We report the detection of an X-ray flare on the Bp star σ Ori Ewith the ROSAT high resolution imager (HRI). The flare is shown to havelikely occurred on the early-type star, rather than on an hypothesizedlate-type companion. We derive flare parameters such as total energyrelease, coarse estimates of size and density, and also presentarguments for a magnetic origin of the flare. We place our observationsin the context of a magnetic character of Bp-type stars and speculate ona common physical basis and connection between Bp and Be stars.
| Distribution of magnetically confined circumstellar matter in oblique rotators
We consider the mechanical equilibrium and stability of matter trappedin the magnetosphere of a rapidly rotating star. Assuming a dipolarmagnetic field and arbitrary inclination of the magnetic axis withrespect to the axis of rotation we find stable equilibrium positions a)in a (warped) disk roughly aligned with the magnetic equatorial planeand b) at two locations above and below the disk, whose distance fromthe star increases with decreasing inclination angle between dipole androtation axis. The distribution of matter is not strongly affected byallowing for a spatial offset of the magnetic dipole. These resultsprovide a possible explanation for some observations of corotatinglocalized mass concentrations in hot magnetic stars.
| Rotation, pulsations and magnetic field in V 2052 Ophiuchi: A new He-strong star
V 2052 Oph is a beta Cep star with v sin i ~ 60 kms-1. The behavior of its stellar wind was studied in theultraviolet wavelength region with the IUE satellite. It revealedperiodic variations in the equivalent widths (EW) of the resonance linesof wind-sensitive ions with a period of 3.638833 d, which is identifiedas the rotational period. These variations are typical of magneticstars. Therefore time-resolved circular spectropolarimetric observationswere obtained with the Musicos échelle spectropolarimeter at the2-m Télescope Bernard Lyot (TBL) to search for a magnetic fieldin the star. Stokes V patterns were observed, the inclination andmagnetic angles were derived and a value was determined for the polarmagnetic field (250 +/- 190 G) using an oblique rotator dipole model.The spectroscopic information was used to search for periodicity inline-profile variations (lpv), radial velocity and minimum intensitycurves. Multiperiodicity was found, corresponding to radial (f_1=7.15 cd-1) and non-radial (f_2=6.82 c d-1, l=3 or 4)pulsation modes. The rotation period is also detected in rotationallymodulated observables because of the magnetic poles passing through theobserver's line of sight. We determined the stellar parameters of thestar, which was found to be chemically peculiar, in particularHe-enriched. This makes V 2052 Oph the first magneticHe-strong early B star with known pulsational properties.Based on observations obtained with the MuSiCoS spectropolarimeter atthe Observatoire du Pic du Midi, France, and by the InternationalUltraviolet Explorer, collected at NASA Goddard Space Flight Center andVillafranca Satellite Tracking Station of the European Space Agency,retrieved from the INES database.}\fnmsep\thanks{Table \ref{tabmagV} isonly available in electronic form athttp://www.edpsciences.org
| Rotation and magnetic field in the Be star omega Orionis
omega Ori is a B2IIIe star for which rotationalmodulation and non-radial pulsations (NRP) have been recentlyinvestigated from two independent observational campaigns in 1998 and1999. Putting the data of these 2 campaigns together, and adding dataobtained in 2001, we search for multiperiodicity in the line profilevariations and evidence for outbursts. From new spectropolarimetric dataobtained at the Télescope Bernard Lyot (TBL, Pic du Midi, France)in 2001 we also measure the Stokes V parameter in the polarised light.We find evidence for the presence of a weak magnetic field inomega Ori sinusoidally varying with a period of 1.29d. The equivalent widths (EW) of the wind sensitive UV resonance linesalso show a variation with the same period, which we identify as therotational period of the star. We propose an oblique rotator model andderive Bpol =530 +/- 230 G to explain the observations.Moreover, we carry out an abundance analysis and find the star to beN-enriched, a property which is shared with other magnetic stars. Wepropose omega Ori as the first known classical Bestar hosting a magnetic field.Based on observations obtained using the Musicos spectropolarimeter atthe Observatoire du Pic du Midi (France), during the MuSiCoS 98 campaign\citep{neiner}, and by \cite{balona}. Based on INES data from theInternational Ultraviolet Explorer (IUE) satellite.Table 7 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/275
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Discovery of a dust cloud next to sigma Orionis
We report on the discovery of a mid-infrared source at a projecteddistance of only 1200 AU from the O9.5 V star sigma Orionis. Thespatially resolved, fan-shaped morphology and the presence of anionization front, as well as evidence in the spectrum for processed dustgrains, all suggest that it is a proto-planetary disk being dispersed bythe intense ultraviolet radiation from sigma Orionis. We compute themass budget and the photo-evaporation timescale, and discuss thepossible nature of this remarkable object.
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
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