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Runaway Stars, Trapezia, and Subtrapezia
We studied the internal motions of the components of 44 OBtrapezium-type systems using all available measures of position anglesand separations over a time up to 160 years. We found that most of thetrapezia have the internal motions expected for small, bound andvirialized clusters. However, in some systems, we detected escapingcomponents. We studied these components in detail and found that thefastest components among them can be classified as runaway stars. Therelatively high number of runaways results from the dynamicalinteractions that occur in the unresolved sub-trapezia, which, inanalogy with the Orion trapezium, likely constitute the brightestcomponents of our sample of trapezia.
| Strong magnetic field in W75N OH maser flare
A flare of OH maser emission was discovered in W75N in 2000. Itslocation was determined with the Very Long Baseline Array (VLBA) to bewithin 110 au from one of the ultracompact HII regions, Very Large Array2 (VLA2). The flare consisted of several maser spots. Four of the spotswere found to form Zeeman pairs, all of them with a magnetic fieldstrength of about 40mG. This is the highest ever magnetic field strengthfound in OH masers, an order of magnitude higher than in typical OHmasers. Three possible sources for the enhanced magnetic field arediscussed: (i) the magnetic field of the exciting star dragged out bythe stellar wind; (ii) the general interstellar field in the gascompressed by the magnetohydrodynamic shock; and (iii) the magneticfield of planets which orbit the exciting star and produce maseremission in gaseous envelopes.
| Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis
X-ray line-profile analysis has proved to be the most direct diagnosticof the kinematics and spatial distribution of the very hot plasma aroundO stars. The Doppler-broadened line profiles provide information aboutthe velocity distribution of the hot plasma, while thewavelength-dependent attenuation across a line profile providesinformation about the absorption to the hot plasma, thus providing astrong constraint on its physical location. In this paper, we applyseveral analysis techniques to the emission lines in the Chandra HighEnergy Transmission Grating Spectrometer (HETGS) spectrum of the late-Osupergiant ζ Ori (O9.7 Ib), including the fitting of a simpleline-profile model. We show that there is distinct evidence forblueshifts and profile asymmetry, as well as broadening in the X-rayemission lines of ζ Ori. These are the observational hallmarks of awind-shock X-ray source, and the results for ζ Ori are very similarto those for the earlier O star, ζ Pup, which we have previouslyshown to be well fit by the same wind-shock line-profile model. The moresubtle effects on the line-profile morphologies in ζ Ori, ascompared to ζ Pup, are consistent with the somewhat lower densitywind in this later O supergiant. In both stars, the wind optical depthsrequired to explain the mildly asymmetric X-ray line profiles implyreductions in the effective opacity of nearly an order of magnitude,which may be explained by some combination of mass-loss rate reductionand large-scale clumping, with its associated porosity-based effects onradiation transfer. In the context of the recent reanalysis of thehelium-like line intensity ratios in both ζ Ori and ζ Pup, andalso in light of recent work questioning the published mass-loss ratesin OB stars, these new results indicate that the X-ray emission fromζ Ori can be understood within the framework of the standardwind-shock scenario for hot stars.
| Far-ultraviolet scattering by dust in Orion
We have modelled diffuse far-ultraviolet (FUV) spectrum observed by theFar Ultraviolet Spectroscopic Explorer (FUSE) near M42 as the scatteringof the starlight from the Trapezium stars by dust in front of thenebula. The dust grains are known to be anomalous in Orion withRV= 5.5 and these are the first measurements of the FUVoptical properties of the grains outside of `normal' Milky Way dust. Wefind an albedo varying from 0.3 +/- 0.1 at 912 Å to 0.5 +/- 0.2 at1020 Åwhich is consistent with theoretical predictions.
| Modelling of isolated radio pulsars and magnetars on the fossil field hypothesis
We explore the hypothesis that the magnetic fields of neutron stars areof fossil origin. For parametrized models of the distribution ofmagnetic flux on the main sequence and of the birth spin period of theneutron stars, we calculate the expected properties of isolated radiopulsars in the Galaxy using as our starting point the initial massfunction and star formation rate as a function of Galactocentric radius.We then use the 1374-MHz Parkes Multi-Beam Survey of isolated radiopulsars to constrain the parameters in our model and to deduce therequired distribution of magnetic fields on the main sequence. We findagreement with observations for a model with a star formation rate thatcorresponds to a supernova rate of 2 per century in the Galaxy fromstars with masses in the range 8-45Msolar and predict 447000active pulsars in the Galaxy with luminosities greater than 0.19 mJykpc2. The progenitor OB stars have a field distribution whichpeaks at ~46 G with ~8 per cent of stars having fields in excess of 1000G. The higher-field progenitors yield a population of 24 neutron starswith fields in excess of 1014 G, periods ranging from 5 to 12s, and ages of up to 100000 yr, which we identify as the dominantcomponent of the magnetars. We also predict that high-field neutronstars (logB > 13.5) originate preferentially from higher-massprogenitors and have a mean mass of 1.6Msolar, which issignificantly above the mean mass of 1.4Msolar calculated forthe overall population of radio pulsars.
| Hydroxyl maser disc and outflow in the Orion-BN/KL region
Multi Element Radio Linked Interferometer Network (MERLIN) measurementsof 1.6-GHz hydroxyl (OH) masers associated withOrion-Becklin-Neugebauer/Kleinmann-Low (Orion-BN/KL) are presented, andthe data are compared with data on other masers, molecular lines,compact radio continuum sources and infrared sources in the region. OHmasers are detected over an area of 30 arcsec in diameter, with themajority lying along an approximately E-W structure that extends for ~18arcsec, encompassing the infrared sources IRc2, IRc6 and IRc7. Radialvelocities range from -13 to +42kms-1. The system of OHmasers shows a velocity gradient together with non-circular motions. Thekinematics are modelled in terms of an expanding and rotating disc ortorus. The rotation axis is found to be in the same direction as themolecular outflow. There is an inner cavity of radius ~1300 au with noOH masers. The inner cavity, like the H2O `shell' masers andSiO masers, is centred on radio source I. Some of the OH masers occur invelocity-coherent strings or arcs that are longer than 5 arcsec (2250au). One such feature, Stream A, is a linear structure at position angle~45°, lying between IRc2 and BN. We suggest that these masers traceshock fronts, and have appeared, like a vapour trail, 200 yr after thepassage of the runaway star BN. The radio proper motions of BN, source Iand source n project back to a region near the base of Stream A that islargely devoid of OH masers. The 1612-MHz masers are kinematicallydistinct from the other OH masers. They are also more widely distributedand appear to be associated with the outflow as traced by H2Omasers and by the 2.12-μm emission from shocked H2. Themagnetic field traced by the OH masers ranges from 1.8 to 16.3mG, with apossible reversal. No OH masers were found associated with even the mostprominent proplyds within 10 arcsec of θ1 Ori C.
| Blueshifted diffuse interstellar bands in the spectrum of HD 34078
In this paper, we report the very first observation of diffuseinterstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur),are blueshifted with respect to the normal position that they have inother objects, where the rest-wavelength velocity frame is determinedusing very sharp interstellar atomic lines or molecular features. Onlyreasonably narrow DIBs seemingly show this effect, which is absent inbroader ones. The result is confirmed independently using threedifferent spectrographs attached to two different telescopes.
| Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of θ1 Orionis C*
From observations made with the ESPaDOnS spectropolarimeter, recentlyinstalled on the 3.6-m Canada-France-Hawaii Telescope, we report thediscovery of a strong magnetic field in the Of?p spectrum variable HD191612 - only the second known magnetic O star (followingθ1 Ori C). The stability of the observed Zeemansignature over four nights of observation, together with thenon-rotational shape of line profiles, argues that the rotation periodof HD 191612 is significantly longer than the 9-d value previouslyproposed. We suggest that the recently identified 538-d spectralvariability period is the rotation period, in which case the observedline-of-sight magnetic field of -220 +/- 38 G implies a large-scalefield (assumed dipolar) with a polar strength of about -1.5 kG. Ifconfirmed, this scenario suggests that HD 191612 is, essentially, anevolved version of the near-zero-age main-sequence magnetic O starθ1 Ori C, but with an even stronger field (about 15 kGat an age similar to that of θ1 Ori C). We suggestthat the rotation rate of HD 191612, which is exceptionally slow byaccepted O-star standards, could be due to angular momentum dissipationthrough a magnetically confined wind.
| Hydrodynamics of Cometary Compact H II Regions
We present numerical radiation-hydrodynamic simulations of cometary H IIregions for a number of champagne flow and bow shock models. For thechampagne flow models we study smooth density distributions with bothsteep and shallow gradients. We also consider cases in which theionizing star has a strong stellar wind and cases in which the staradditionally has a proper motion within the ambient density gradient. Wefind that our champagne flow plus stellar wind models havelimb-brightened morphologies and kinematics that can see theline-of-sight velocities change sign twice between the head and tail ofthe cometary H II region, with respect to the rest frame velocity. Ourbow shock models show that pressure gradients across and within theshell are very important for the dynamics and that simple analyticmodels assuming thin shells in ram pressure balance are whollyinadequate for describing the shape and kinematics of these objects atearly times in their evolution. The dynamics of the gas behind the shockin the neutral material ahead of the ionization front in both champagneflow and bow shock type cometary H II regions is also discussed. Wepresent simulated emission-measure maps and long-slit spectra of ourresults. Our numerical models are not tailored to any particular object,but comparison with observations from the literature shows that, inparticular, the models combining density gradients and stellar winds areable to account for both the morphology and general radial velocitybehavior of several observed cometary H II regions, such as thewell-studied object G29.96-0.02.
| Chandra Study of the Cepheus B Star-forming Region: Stellar Populations and the Initial Mass Function
The Cepheus B (Cep B) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formationwithin 1 kpc, have been observed with the ACIS detector on board theChandra X-Ray Observatory. We detect 431 X-ray sources, of which 89% areconfidently identified as clustered pre-main-sequence (PMS) stars. Twomain results are obtained. First, we provide the best census to date forthe stellar population of the region. We identify many members of tworich stellar clusters: the lightly obscured Cep OB3b association and thedeeply embedded cluster in Cep B, whose existence was previously tracedonly by a handful of radio sources and T Tauri stars. Second, we find adiscrepancy between the X-ray luminosity functions of the Cep OB3b andthe Orion Nebula cluster. This may be due to the different initial massfunctions of the two regions (an excess of ~=0.3 Msolarstars) or different age distributions. Several other results areobtained. A diffuse X-ray component seen in the field is attributed tothe integrated emission of unresolved low-mass PMS stars. The X-rayemission from HD 217086 (O7n), the principle ionizing source of theregion, follows the standard model, involving many small shocks in anunmagnetized radiatively accelerated wind. X-ray source 294 joins anumber of similar superflare PMS stars for which long magneticstructures may connect the protoplanetary disk to the stellar surface.
| Silicate Emission Profiles from Low-Mass Protostellar Disks in the Orion Nebula: Evidence for Growth and Thermal Processing of Grains
We present 8-13 μm low-resolution spectra (R~100) of eight low-massprotostellar objects (``proplyds'') in the Orion Nebula using the LongWavelength Spectrometer at the W. M. Keck Observatory. All but one ofthe sources in our sample show strong circumstellar silicate emission,with profiles that are qualitatively similar to those seen in some TTauri and Herbig Ae/Be stars. The silicate profile in all cases issignificantly flattened compared with the profile for typicalinterstellar dust, suggesting that the dominant emitting grains aresignificantly larger than those found in the interstellar medium. The11.3-to-9.8 μm flux ratio-often used as an indicator of graingrowth-is in the 0.8-1.0 range for all of our targets, indicating thatthe typical grain size is around a few microns in the surface layers ofthe attendant circumstellar disk for each object. Furthermore, thesilicate profiles show some evidence of crystalline features, as seen inother young stellar objects. The results of our analysis show that thegrains in the photoevaporating protostellar disks of Orion haveundergone significant growth and perhaps some annealing, suggesting thatgrain evolution for these objects is not qualitatively different fromother young stellar objects.
| Nonthermal High-Energy Emission from Colliding Winds of Massive Stars
Colliding winds of massive star binary systems are considered aspotential sites of nonthermal high-energy photon production. Motivatedby the detection of synchrotron radio emission from the colliding windlocation, we here investigate the properties of high-energy photonproduction in colliding winds of long-period WR+OB systems. Analyticalformulae for the steady state proton- and electron-particle spectra arederived assuming diffusive particle acceleration out of a pool ofthermal wind particles, taking into account adiabatic and all relevantradiative losses, and include advection/convection out of the windcollision zone. This includes analytical approximations for the electronenergy losses in the Klein-Nishina transition regime. For the first timein the context of CWB systems, our calculations use the fullKlein-Nishina cross section and account for the anisotropy of theinverse Compton scattering process. This leads to orbital fluxvariations by up to several orders of magnitude, which may, however, beblurred by the system's geometry. Both anisotropy and Klein-Nishinaeffects may yield characteristic spectral and variability signatures inthe γ-ray domain. Since propagation effects lead to a deficit oflow-energy particles in the convection-dominated zone, one expectsimprints in the radiation spectra. If protons are accelerated to atleast several GeV, π0-decay γ-rays might beobservable, depending on the injected electron-to-proton ratio. We showthat photon-photon pair production is generally not negligible,potentially affecting the emitted spectrum above ~50 GeV, depending onorbital phase and system inclination. The calculations are applied tothe archetypal WR+OB systems WR 140 and WR 147 to predict their expectedspectral and temporal characteristics and to assess their detectabilitywith current and upcoming γ-ray experiments.
| A Photoionized Herbig-Haro Object in the Orion Nebula
The spectra of Herbig-Haro objects are usually characteristic ofionization and excitation in shock-heated gas, whether an internal shockin an unsteady outflow or a bow shock interface with the interstellarmedium. We examine the easternmost shock-the leading optically visibleshock-of a Herbig-Haro outflow (HH 529) seen projected on the face ofthe Orion Nebula, using deep optical echelle spectroscopy, showing thatthe spectrum of this gas is consistent with photoionization byθ1 Ori C. By modeling the emission lines, we determinea gas-phase abundance of Fe that is consistent with the depleted(relative to solar) abundance found in the Orion nebula-evidence for thepresence of dust in the nebula and therefore in the Herbig-Haro outflow.The spectrum also allows for the calculation of temperaturefluctuations, t2, in the nebula and the shock. Thesefluctuations have been used to explain discrepancies between abundancesobtained from recombination lines versus those obtained fromcollisionally excited lines, although to date there has not been arobust theory for how such large fluctuations (t2>0.02)can be.
| Hierarchical Star Formation in the Spiral Galaxy NGC 628
The distributions of size and luminosity for star-forming regions in thenearby spiral galaxy NGC 628 are studied over a wide range of scalesusing progressively blurred versions of an image from the Hubble SpaceTelescope Advanced Camera for Surveys. Four optical filters areconsidered for the central region, including Hα. Two filters areused for an outer region. The features in each blurred image are countedand measured using SExtractor. The cumulative size distribution is foundto be a power law in all passbands, with a slope of approximately -1.5over 1.8 orders of magnitude. The luminosity distribution isapproximately a power law as well, with a slope of approximately -1 forlogarithmic intervals of luminosity. The results suggest a scale-freenature for stellar aggregates in a galaxy disk. Fractal models of thindisks reproduce the projected size distribution and suggest a projectedmass distribution slope of approximately -0.5 for these extendedregions. This mass slope converts to the observed luminosity slope if weaccount for luminosity evolution and longer lifetimes in larger regions.
| Star Formation in Space and Time: The Orion Nebula Cluster
We examine the pattern of star birth in the Orion Nebula cluster (ONC),with the goal of discerning the cluster's formation mechanism. Outsidethe Trapezium, the distribution of stellar masses is remarkably uniformand is not accurately described by the field-star initial mass function.The deconvolved, three-dimensional density of cluster members peaks atthe Trapezium stars, which are truly anomalous in mass. Usingtheoretical pre-main-sequence tracks, we confirm the earlier findingthat star formation has accelerated over the past 107 yr. Wefurther show that the rate of acceleration has been the same for allmasses. Thus, there is no correlation between stellar age and mass,contrary to previous claims. Finally, the acceleration has beenspatially uniform throughout the cluster.Our reconstruction of the parent molecular cloud spawning the clustershows that it had a mass of 6700 Msolar prior to itsdestruction by the Trapezium. If the cloud was supported againstself-gravity by mildly dissipative turbulence, then it contracted in aquasi-static but accelerating manner. We demonstrate this contractiontheoretically through a simple energy argument. The mean turbulent speedincreased to its recent value, which is reflected in the present-daystellar velocity dispersion.The current ONC will be gravitationally unbound once cloud destructionis complete, and is destined to become a dispersing OB association. Wehypothesize that similarly crowded groups seen at the centers of distantOB associations are also unbound and do not give rise to the Galacticpopulation of open clusters. Finally, accelerating star formationimplies that most clumps within giant molecular complexes should haverelatively low formation activity. Sensitive infrared surveys couldconfirm this hypothesis.
| Encounter-triggered Disk Mass Loss in the Orion Nebula Cluster
The relevance of encounters on the destruction of protoplanetary disksin the Orion Nebula cluster (ONC) is investigated by combining twodifferent types of numerical simulation. First, star-cluster simulationsare performed to model the stellar dynamics of the ONC, the results ofwhich are used to investigate the frequency of encounters, the massratio and separation of the stars involved, and the eccentricity of theencounter orbits. The results show that interactions that couldinfluence the star-surrounding disk are more frequent than previouslyassumed in the core of the ONC, the so-called Trapezium cluster. Second,a parameter study of star-disk encounters is performed to determine theupper limits of the mass loss of the disks in encounters. For simulationtimes of ~1-2 Myr (the likely age of the ONC) the results show thatgravitational interaction might account for a significant disk mass lossin dense clusters. Disk destruction is dominated by encounters withhigh-mass stars, especially in the Trapezium cluster, where the fractionof disks destroyed due to stellar encounters can reach 10%-15%. Theseestimates are in accord with recent observations of Lada et al., whodetermined a stellar disk fraction of 80%-85%. Thus, it is shown that inthe ONC-a typical star-forming region-stellar encounters do have asignificant effect on the mass of protoplanetary disks and thus affectthe formation of planetary systems.
| 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.
| Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula
The mid-infrared emission from a photodissociation region (PDR) viewededge-on in the Orion Nebula is examined through 8.7-20.6 μm imagesand 8-13 μm spectra. The polycyclic aromatic hydrocarbon (PAH)emission is located between the edges of H II regions and layers of [CI] emission, agreeing with PDR theory. Using a simple model, the spatialvariations in the emission from PAHs detected at 8.6, 11.2, and 12.7μm are demonstrated to be directly proportional to the materialcolumn density and the intensity of the UV field. For a homogeneous,neutral cloud illuminated by a bright OB star, PDR theory predicts thatthe ultraviolet (UV) radiation is attenuated exponentially(e-1.8Av). The predicted UV attenuation isconfirmed by observations of broad PAH emission features found at 8.6,11.2, and 12.7 μm. The PAH emission is found in cool regions havinggreater optical depths relative to regions where mid-infrared emissionfrom ionized gas is observed. Through modeling we determine a gasdensity of 9.7×104 cm-3. On large and smallsize scales, the relative strengths of the 8.6, 11.2, and 12.7 μm PAHfeatures at the bar of the Orion Nebula indicate that there is not asimple transition from ionized to neutral PAHs across the PDR.
| 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.
| X-Ray and Infrared Point Source Identification and Characteristics in the Embedded, Massive Star-Forming Region RCW 38
We report on results of a 96.7 ks Chandra observation of one of theyoungest, most embedded, and most massive young stellar clusters studiedin X-rays: RCW 38. We detect 460 sources in the field, of which 360 areconfirmed to be associated with the RCW 38 cluster. The cluster membersrange in luminosity from 1030 to 1033.5 ergss-1. Over 10% of the cluster members with over 100 countsexhibit flares, while about 15% of the cluster members with over 30counts are variable. Of the sources identified as cluster members, 160have near-infrared (NIR) counterparts either in the Two Micron All SkySurvey database or detected via Very Large Telescope observations. Ofthese, about 20% appear to have optically thick disks. An additional 353members are identified through NIR observations, of which at least 50%possess optically thick disks. We fit over 100 X-ray sources as absorbedRaymond-Smith-type plasmas and find that the column to the clustermembers varies from 1021.5 to 1023cm-2. We compare the gas to dust absorption signatures inthese stars and findNH=AV(2×1021) cm-2. Wefind that the cluster contains 31 candidate OB stars and is centeredabout 10" (0.1 pc) west of the primary source of the ionization, the O5star IRS 2. The cluster has a peak central density of about 400 X-raysources pc-2. We estimate that the total cluster membershipexceeds 2000 stars.
| Irradiated and Bent Jets in the Orion Nebula
We present new images and proper-motion measurements of irradiatedoutflows from young stars in the outskirts of the Orion Nebula obtainedwith the Advanced Camera for Surveys onboard the Hubble Space Telescope.The images reveal many new outflows and new details in previouslydetected flows. The large-scale bipolar flows such as HH 502, 505, 874,and 876 contain multiple shocks whose velocities systematically decreasewith increasing distance from their sources as the flows bend away fromthe core of the Orion Nebula. We consider several mechanisms for bendingjets, including radiation pressure, the rocket effect on a neutral jet,and the impact of a side wind. While mild bends may be explained byeither radiation pressure or the rocket effect, the extreme bends ofsome Orion jets and LL Ori-type bow shocks are best explained by theinteraction of these outflows with a large-scale flow from the nebularcore. The jet Hα emission measures and geometry are used toestimate their mass-loss rates, which range from aboutM˙~10-9 to 10-6 Msolaryr-1. Many of the quasi-parabolic bow shocks that wrap aroundyoung stars in Orion (the LL Ori-type objects) exhibit large gapsbetween the observed jets and the parabolic fronts facing the core ofthe nebula. These may indicate the additional action of a wide-angleoutflow component such as a T Tauri stellar wind, a proplydphotoablation flow, or possibly a photoablation flow from the jetitself. The Hα surface brightness and radii of the LL Ori frontsare used to estimate that the M˙Vw products of thewide-angle flow components have values around 10-6Msolar yr-1 km s-1. The side-winddensity and velocity are also constrained. Outside the nebular core, thegreatest concentration of bent jets and LL Ori-type parabolic fronts islocated south and west of the Trapezium. The nonuniform, clumpy spatialdistribution of jets, outflow sources, and proplyds in the outskirts ofthe Orion Nebula indicate that star formation occurred in smallhierarchical clusters.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| The fate of discs around massive stars in young clusters
Aims.To understand whether there is a difference in the dispersion ofdiscs around stars in high-density young stellar clusters like the OrionNebula Cluster (ONC) according to the mass of the star. Methods:.Two types of simulations were combined - N-body simulations of thedynamics of the stars in the ONC and mass loss results from simulationsof star-disc encounters, where the disc mass loss of all stars isdetermined as a function of time. Results: .We find that in theTrapezium, the discs around high-mass stars are dispersed much morequickly and to a larger degree by their gravitational interaction thanfor intermediate-mass stars. This is consistent with recent observationsof IC 348, where a higher disc frequency was found around solar massstars than for more massive stars, suggesting that this might be ageneral trend in large young stellar clusters.
| The magnetic field and confined wind of the O star θ1 Orionis C
Aims.In this paper we confirm the presence of a globally-ordered,kG-strength magnetic field in the photosphere of the young O starθ1 Orionis C, and examine the properties of itsoptical line profile variations. Methods: .A new series ofhigh-resolution MuSiCoS Stokes V and I spectra has been acquired whichsamples approximately uniformly the rotational cycle ofθ1 Orionis C. Using the Least-Squares Deconvolution(LSD) multiline technique, we have succeeded in detecting variableStokes V Zeeman signatures associated with the LSD mean line profile.These signatures have been modeled to determine the magnetic fieldgeometry. We have furthermore examined the profile variations of linesformed in both the wind and photosphere using dynamic spectra.Results: .Based on spectrum synthesis fitting of the LSD profiles, wedetermine that the polar strength of the magnetic dipole component is1150 B_d 1800 G and that the magnetic obliquity is 27° β 68°, assuming i=45± 20°. The best-fitvalues for i=45° are Bd = 1300 ± 150~G~ (1σ)and β = 50° ± 6° (1σ). Our data confirm theprevious detection of a magnetic field in this star, and furthermoredemonstrate the sinusoidal variability of the longitudinal field andaccurately determine the phases and intensities of the magnetic extrema.The analysis of "photospheric" and "wind" line profile variationssupports previous reports of the optical spectroscopic characteristics,and provides evidence for infall of material within the magneticequatorial plane.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit
Massive X-ray binaries are usually classified by the properties of thedonor star in classical, supergiant and Be X-ray binaries, the maindifference being the mass transfer mechanism between the two components.The massive X-ray binary 4U 2206+54 does not fit inany of these groups, and deserves a detailed study to understand how thetransfer of matter and the accretion on to the compact object takeplace. To this end we study an IUE spectrum of the donor and obtain awind terminal velocity (v_&infy;) of ~350 km s-1, which isabnormally slow for its spectral type. We also analyse here more than 9years of available RXTE/ASM data. We study the long-term X-rayvariability of the source and find it to be similar to that observed inthe wind-fed supergiant system Vela X-1, reinforcingthe idea that 4U 2206+54 is also a wind-fed system.We find a quasi-period decreasing from ~270 to ~130 d, noticed inprevious works but never studied in detail. We discuss possiblescenarios for its origin and conclude that long-term quasi-periodicvariations in the mass-loss rate of the primary are probably drivingsuch variability in the measured X-ray flux. We obtain an improvedorbital period of P_orb=9.5591±0.0007 d with maximum X-ray fluxat MJD 51856.6±0.1. Our study of the orbital X-ray variability inthe context of wind accretion suggests a moderate eccentricity around0.15 for this binary system. Moreover, the low value of v_&infy; solvesthe long-standing problem of the relatively high X-ray luminosity forthe unevolved nature of the donor, BD +53°2790,which is probably an O9.5 V star. We note that changes in v_&infy;and/or the mass-loss rate of the primary alone cannot explain thedifferent patterns displayed by the orbital X-ray variability. Wefinally emphasize that 4U 2206+54, together withLS 5039, could be part of a new population ofwind-fed HMXBs with main sequence donors, the natural progenitors ofsupergiant X-ray binaries.
| The effect of a strong external radiation field on protostellar envelopes in Orion
We discuss the effects of an enhanced interstellar radiation field(ISRF) on the observables of protostellar cores in the Orion cloudregion. Dust radiative transfer is used to constrain the envelopephysical structure by reproducing SCUBA 850 μm emission. Previouslyreported 13CO, C17O and H2CO line observations arereproduced through detailed Monte Carlo line radiative transfer models.It is found that the 13CO line emission is marginallyoptically thick and sensitive to the physical conditions in the outerenvelope. An increased temperature in this region is needed in order toreproduce the 13CO line strengths and it is suggested to becaused by a strong heating from the exterior, corresponding to an ISRFin Orion 103 times stronger than the "standard" ISRF. Thetypical temperatures in the outer envelope are higher than thedesorption temperature for CO. The C17O emission is lesssensitive to this increased temperature but rather traces the bulkenvelope material. The data are only fit by a model where CO isdepleted, except in the inner and outermost regions where thetemperature increases above 30-40 K. The fact that the temperatures donot drop below ≈25 K in any of the envelopes whereas a significantfraction of CO is frozen-out suggest that the interstellar radiationfield has changed through the evolution of the cores. The H2CO lines aresuccessfully reproduced in the model of an increased ISRF with constantabundances of 3-5 × 10-10.
| X-rays from HH 210 in the Orion nebula
We report the detection during the Chandra Orion Ultradeep Project(COUP) of two soft, constant, and faint X-ray sources associated withthe Herbig-Haro object HH 210. HH 210 is located at the tip of the NNEfinger of the emission line system bursting out of the BN-KL complex,northwest of the Trapezium cluster in the OMC-1 molecular cloud. Using arecent Hα image obtained with the ACS imager on board HST, andtaking into account the known proper motions of HH 210 emission knots,we show that the position of the brightest X-ray source, COUP703, coincides with the emission knot 154-040a of HH 210,which is the emission knot of HH 210 having the highest tangentialvelocity (425 km s-1). The second X-ray source, COUP704, is located on the complicated emission tail of HH 210close to an emission line filament and has no obvious optical/infraredcounterpart. Spectral fitting indicates for both sources a plasmatemperature of ~0.8 MK and absorption-corrected X-ray luminosities ofabout 1030 erg s-1 (0.5-2.0 keV). These X-raysources are well explained by a model invoking a fast-moving, radiativebow shock in a neutral medium with a density of ~12 000 cm-3.The X-ray detection of COUP 704 therefore reveals, in the complicated HH210 region, an energetic shock not yet identified at other wavelengths.
| Detailed spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. Rotational velocities, stellar parameters, and oxygen abundances
We present the results of a spectroscopic analysis of the Trapeziumcluster stars inside the Orion nebula. The rotational velocities wereobtained using the Fourier analysis method, and we found agreement withvalues derived by the usual method based on linewidth measurements. Therotational velocity derived for θ1 Ori C by thismethod is consistent with the variability of some of its spectralfeatures that have a period of 15.42 days. By means of the fit of H, HeI, and He II observed profiles with Fastwind synthetic profiles, stellarparameters and wind characteristics were derived. This methodology letus estimate the errors associated with these parameters, and we foundthat macroturbulence effects have to be included for a good fit to theHe I-II lines in the spectrum of θ1 Ori C. By means ofa very accurate study, oxygen abundances were derived for the threeB0.5V stars θ1 Ori A, D, and θ2 OriB. Final abundances are consistent with the nebular gas-phase resultspresented in Esteban et al. (2004) and are lower than those given byCunha & Lambert (1994). Our results suggest a lower dust depletionfactor of oxygen than previous estimations for the Orion nebula.
| Chandra X-ray observations of the stellar group near the Herbig Be star MWC 297. A revision of the X-ray properties of MWC 297
We present a Chandra ACIS-I X-ray observation of the region near theHerbig early-Be star MWC 297, where we detect a tight group of pointX-ray sources. These are probably physically associated to MWC 297,because of their obvious clustering with respect to the more scatteredfield-source population. These data are compared to earlier ASCA datawith much poorer spatial resolution, from which the detection of strongquiescent and flaring emission from MWC 297 itself was claimed. We arguethat this star, contributing only 5% to the total X-ray emission of thegroup, was probably not the dominant contributor to the observed ASCAemission, while the X-ray brightest star in the group is a much bettercandidate. This is also supported by the spectral analysis of theChandra data, with reference to the ASCA spectra. We conclude that noneof the X-ray data available for MWC 297 justify the earlier claim ofstrong magnetic activity in this star. The X-ray emission of MWC 297during the Chandra observation is even weaker than that found in otherHerbig stars with the same spectral type, even accounting for its largeline-of-sight absorption.
| Benchmarking atomic data for astrophysics: Fe XXIV
Fe XXIV produces strong L-shell (n=3, 4, 5 to 2) spectral lines in theX-rays. These lines have been observed in solar flares, in laboratoryspectra, and in a variety of astrophysical sources with e.g. the Chandraand XMM-Newton satellites. In this paper atomic data for Fe XXIV L-shellemission are benchmarked against experimental data. The work focuses onreviewing and assessing previous line identifications on a quantitativebasis, paying particular attention to the most important lines forlaboratory and astrophysical applications. Previous identifications areconfirmed, but many lines turn out to be significantly blended. Theagreement between theoretical and observed data in terms of wavelengths,line intensities and level lifetimes is good, within the experimentaluncertainties. Fe XXIV L-shell lines can be used to measure electrondensities in laboratory plasmas and temperatures for a wide range ofastrophysical sources.
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Osservazione e dati astrometrici
| Costellazione: | Orione |
| Ascensione retta: | 05h35m16.46s |
| Declinazione: | -05°23'23.2" |
| Magnitudine apparente: | 5.064 |
| Moto proprio RA: | -3.6 |
| Moto proprio Dec: | -33.1 |
| B-T magnitude: | 5.017 |
| V-T magnitude: | 5.061 |
Cataloghi e designazioni:
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