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θβ Ori (Mizan Batil ath Thani)



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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.

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.

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.

A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i ˜100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.

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.

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.

A deep survey of brown dwarfs in Orion with Gemini
We report the results of a deep near-infrared (JHK) survey of the outerparts of the Trapezium Cluster with Gemini South/Flamingos. 396 sourceswere detected in a 26 arcmin2 area, including 138 brown dwarfcandidates, defined as M < 0.075 Msolar for an assumed ageof 1 Myr. Only 33 of the brown dwarf candidates are planetary masscandidates with estimated masses in the range 0.003 < M < 0.012Msolar. In an extinction-limited sample (AV <5) complete to approximately 0.005 Msolar (5MJup)the mass function appears to drop by a factor of 2 at the deuteriumburning threshold, i.e. at planetary masses. After allowing forbackground contamination it is likely that planetary mass objects at3-13MJup number <10 per cent of the cluster population,with an upper limit of 13 per cent. Analysis of the spatial distributionof stars and brown dwarf candidates suggests that brown dwarfs and verylow-mass stars (M < 0.1 Msolar) are less likely than moremassive stars to have wide (>150 au) binary companions. This resulthas modest statistical significance (96 per cent) in our data but issupported at 93 per cent confidence by analysis of a completelyindependent sample taken from the Subaru data of Kaifu et al. There is astatistically very significant excess of both stars and brown dwarfswith small separations from each other (<6 arcsec or 2600 au). Thisappears to be due to the presence of small-N subgroups, which are likelyto be dynamically unstable in the long term. Hence these results areconsistent with the `ejected stellar embryo' hypothesis for brown dwarfformation. We also report the discovery of two new bipolar nebulae,which are interpreted as Class I protostars.

Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association
A 90-ks Chandra High Energy Transmission Grating observation of theyoung stellar cluster NGC 6193 in the southern Ara OB1 associationdetected 43 X-ray sources in a 2 × 2 arcmin2 coreregion centred on the massive O stars HD 150135 (O6.5V) and HD 150136(O3 + O6V). The cluster is dominated by exceptionally bright X-rayemission from the two O stars, which are separated by only 10 arcsec.The X-ray luminosity of HD 150136 is logLX= 33.39 (ergs-1), making it one of the most luminous O-star X-ray sourcesknown. All of the fainter X-ray sources in the core region havenear-infrared (near-IR) counterparts, but existing JHK photometryprovides little evidence for near-IR excesses. These core sources havetypical mean photon energies ~ 2 keV and about one-third arevariable. It is likely that some are young low-mass stars in thecluster, but cluster membership remains to be determined. Gratingspectra show that the X-ray properties of HD 150135 and HD 150136 aresimilar, but not identical. Both have moderately broadened unshiftedemission lines and their emission is dominated by cool plasma at kT~ 0.3keV, pointing to a wind-shock origin. However, the emission of HD 150136is slightly hotter and four times more luminous than its optical twin HD150135. We discuss the possibility that a radiative colliding wind shockcontributes to the prodigious X-ray output of the short-period (2.66 d)spectroscopic binary HD 150136. A surprising result is that the X-rayemission of HD 150136 is slowly variable on a time-scale of <1 d. Theorigin of the variability is not yet known but the observed behavioursuggests that it is an occultation effect.

X-Ray Emission from Early-Type Stars in the Orion Nebula Cluster
The X-ray properties of twenty ~1 Myr old O, B, and A stars of the OrionTrapezium are examined with data from the Chandra Orion UltradeepProject (COUP). On the basis of simple theories for X-ray emission, wedefine two classes separated at spectral type B4: hotter stars havestrong winds that may give rise to X-ray emission in small- orlarge-scale wind shocks, and cooler stars that should be X-ray dark dueto their weaker winds and absence of outer convection zones wheredynamos can generate magnetic fields. Emission by late-type magneticallyactive companions may be present in either class. Sixteen of the 20stars are detected with a wide range of X-ray luminosities,logLX (ergs s-1)~29-33, and X-ray efficiencies,log(LX/Lbol)~-4 to -8. Only two stars,θ1 Ori D (B0.5) and NU Ori (B1), show exclusively theconstant soft-spectrum emission at log(LX/Lbol)~-7expected from the standard model involving many small shocks in anunmagnetized radiatively accelerated wind. Most of the other massiveO7-B3 stars exhibit some combination of soft-spectrum wind emission,hard-spectrum flaring, and/or rotational modulation indicatinglarge-scale inhomogeneity. Magnetic confinement of winds withlarge-scale shocks can be invoked to explain these phenomena. This issupported in some cases by nonthermal radio emission and/or chemicalpeculiarities, or direct detection of the magnetic field(θ1 Ori C). Most of the stars in the weak-wind classexhibit X-ray flares and logLX<31 ergs s-1,consistent with magnetic activity from known or unseen low-masscompanions. In most cases, the X-ray spectra can be interpreted in termsof a two-temperature plasma model with a soft component of 3-10 MK and ahard component up to 40 MK. All nondetections belong to the weak-windclass. A group of stars exhibit hybrid properties-flarelike behaviorsuperimposed on a constant component with logLX~32 ergss-1-which suggest both magnetic activity and wind emission.

Global X-Ray Properties of the Orion Nebula Region
Based on the Chandra Orion Ultradeep Project (COUP) observation, weestablish the global X-ray properties of the stellar populationassociated with the Orion Nebula. Three components contribute roughlyequally to the integrated COUP luminosity in the hard (2-8 keV) X-rayband: several OB stars, 822 lightly obscured cool stars in the OrionNebula Cluster (ONC), and 559 heavily obscured stars. ONC stars 0.5-2 pcfrom the center show a spatial asymmetry consistent with violentrelaxation in the stellar dynamics. The obscured COUP sourcesconcentrate around both OMC-1 molecular cores; these small-scalestructures indicate ages t<~0.1 Myr. The X-ray luminosity function(XLF) of the lightly obscured sample is roughly lognormal in shape. Theobscured population is deficient in lower luminosity stars, perhaps dueto localized circumstellar material. Mass-stratified XLFs show thatone-third of the Orion Nebula region hard-band emission is produced bythe bright O6 star θ1 Ori C and half is produced bylower mass pre-main-sequence stars with masses 0.3Msolar

Membership of the Orion Nebula Population from the Chandra Orion Ultradeep Project
The Chandra Orion Ultradeep project (COUP) observation described in acompanion paper by Getman et al. provides an exceptionally deep X-raysurvey of the Orion Nebula Cluster and associated embedded young stellarobjects. Membership of the region is important for studies of thestellar IMF, cluster dynamics, and star formation. The COUP studydetected 1616 X-ray sources. In this study we confirm cloud membershipfor 1315 stars, identify 16 probable foreground field stars havingoptical counterparts with discrepant proper motions, and classify theremaining 285 X-ray sources, of which 51 are lightly and 234 heavilyobscured. The 51 lightly obscured sources without known counterpartsfall into three groups: (i) 16 are likely new members of the OrionNebula Cluster; (ii) 2 with unusually soft and nonflaring X-ray emissionappear to be associated with nebular shocks, and may be new examples ofX-rays produced at the bow shocks of Herbig-Haro outflows; (iii) theremaining 33 are very weak uncertain sources, possibly spurious. Out of234 heavily absorbed sources without optical or near-infraredcounterparts 75 COUP sources are likely new embedded cloud members (withmembership for 42 confirmed by powerful X-ray flares), and the remaining159 are likely extragalactic active galactic nuclei seen through themolecular cloud, as argued by a careful simulation of the extragalacticbackground population. Finally, a few new binary companions to Orionstars may have been found, but most cases of proximate COUP sources canbe attributed to chance superpositions in this crowded field.

Chandra Orion Ultradeep Project: Observations and Source Lists
We present a description of the data reduction methods and the derivedcatalog of more than 1600 X-ray point sources from the exceptionallydeep 2003 January Chandra X-Ray Observatory (Chandra) observation of theOrion Nebula Cluster and embedded populations around OMC-1. Theobservation was obtained with Chandra's Advanced CCD ImagingSpectrometer (ACIS) and has been nicknamed the Chandra Orion UltradeepProject (COUP). With an 838 ks exposure made over a continuous period of13.2 days, the COUP observation provides the most uniform andcomprehensive data set on the X-ray emission of normal stars everobtained in the history of X-ray astronomy.

H2 Pure Rotational Lines in the Orion Bar
Photodissociation regions (PDRs), where UV radiation dominates theenergetics and chemistry of the neutral gas, contain most of the mass inthe dense interstellar medium of our Galaxy. Observations ofH2 rotational and rovibrational lines reveal that PDRscontain unexpectedly large amounts of very warm (400-700 K) moleculargas. Theoretical models have difficulty explaining the existence of somuch warm gas. Possible problems include errors in the heating andcooling functions or in the formation rate for H2. To date,observations of H2 rotational lines smear out the structureof the PDR. Only by resolving the hottest layers of H2 canone test the predictions and assumptions of current models. Using theTexas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission inthe H2 v=0-0 S(1) and S(2) lines toward the Orion Bar PDR at2'' resolution. We also observed H2 v=0-0 S(4) atselected points toward the front of the PDR. Our maps cover a12'' by 40'' region of the bar where H2rovibrational lines are bright. The distributions of H2 0-0S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail.The high spatial resolution (0.002 pc) of our observations allows us toprobe the distribution of warm gas in the Orion Bar to a distanceapproaching the scale length for FUV photon absorption. We use these newobservational results to set parameters for the PDR models described ina companion paper in preparation by Draine et al. The best-fit model canaccount for the separation of the H2 emission from theionization front and the intensities of the ground-state rotationallines, as well as the 1-0 S(1) and 2-1 S(1) lines. This model requiressignificant adjustments to the commonly used values for the dust UVattenuation cross section and the photoelectric heating rate.

Outflows in the Orion Nebula: HH 540 from the Beehive Proplyd
We present Hubble Space Telescope Advanced Camera for Surveys images ofthe giant proplyd 181-826 in the southern Orion Nebula. This objectexhibits a variety of known proplyd properties-an optically visiblecentral star, a silhouette disk, a bright proplyd ionization front, anda bipolar microjet emerging along the disk axis that powers a largerHerbig-Haro flow. The proplyd ionization front is remarkable because ofits large size (a radius of about 800 AU) and because of its corrugatedstructure consisting of a set of coaxial rings with the same orientationas the embedded silhouette disk. The rings are centered on the disk/jetaxis, and we discuss possible mechanisms for their formation. Thesilhouette disk has an outer radius of about 160 AU, and its axis isinclined by an angle of about 60° with respect to our line of sight.A prominent reflection nebula protrudes along the disk axis toward thesouth, marking the base of the approaching outflow, and a faint knot onthe north side of the disk indicates that the nebula is bipolar.Fabry-Pérot images reveal a high-velocity blueshifted jet with amass-loss rate of about 10-8 Msolaryr-1 emerging along the disk axis. It has a velocity(corrected for the outflow inclination angle) of about 160 kms-1 and can be traced for about 1' toward the south. A chainof three lower velocity blueshifted bow shocks, collectively known as HH540, extend several arcminutes south of 181-826 along the same axis,tracing a large-scale outflow powered by this jet.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555.

Size distribution of circumstellar disks in the Trapezium cluster
In this paper we present results on the size distribution ofcircumstellar disks in the Trapezium cluster asmeasured from HST/WFPC2 data. Direct diameter measurements of a sampleof 135 bright proplyds and 14 silhouettes disks suggest that there is asingle population of disks well characterized by a power-lawdistribution with an exponent of -1.9 ± 0.3 between diskdiameters 100-400 AU. For the stellar mass sampled (from late G to lateM stars) we find no obvious correlation between disk diameter andstellar mass. We also find that there is no obvious correlation betweendisk diameter and the projected distance to the ionizing Trapezium OBstars. We estimate that about 40% of the disks in the Trapezium haveradius larger than 50 AU. We suggest that the origin of the Solarsystem's (Kuiper belt) outer edge is likely to be due to the starformation environment and disk destruction processes (photoevaporation,collisions) present in the stellar cluster on which the Sun was probablyformed. Finally, we identified a previously unknown proplyd and named it266-557, following convention.

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

Massive stars: their birth sites and distribution
The stellar IMF has been found to be an invariant Salpeter power-law(α=2.35) above about 1 Msolar, but at the same time amassive star typically has more than one companion. This constrains thepossible formation scenarios of massive stars, but also implies that thetrue, binary-star corrected stellar IMF could be significantly steeperthan Salpeter, α>2.7. A significant fraction of all OB starsare found relatively far from potential birth sites which is mostprobably a result of dynamical ejections from cores of binary-rich starclusters. Such cores form rapidly due to dynamical mass segregation, orthey are primordial. Probably all OB stars thus form in stellar clusterstogether with low-mass stars, and they have a rather devastating effecton the embedded cluster by rapidly driving out the remaining gas leavingexpanding OB associations and bound star clusters. The distributedpopulation of OB stars has a measured IMF with α~4, which however,does not necessarily constitute a different physical mode for isolatedstar formation. A steep field-star IMF is obtained naturally becausestars form in clusters which are distributed according to a power-lawcluster mass function.Heisenberg Fellow.

Anomalous dust-to-gas ratios in the Galaxy
Lines of sight with E(B-V)/N(HI) considerably smaller than the averagevalue for the solar neighbourhood have been selected from the catalogueof Diplas & Savage. In order to develop quantitative considerations,estimates of the molecular hydrogen column density were obtained usingthe relation of Savage et al. extended at E(B-V) > 0.4 with therecent data of Rachford et al. Contrary to the prevailing opinion in theliterature for sightlines with similar behaviour, we found that only 22per cent of our sample was characterized by both an average gas densitylarger than 1 cm-3 and a value of RV larger thanthat in the diffuse interstellar medium. By computing extinction models,we were able to fit the E(B-V)/N(HI) by changing the value ofRV only for some sightlines. For the remaining ones, aρd/ρH ratio different from the averageGalactic value must be invoked. The application of the Kramers-Kronigrelation to the observed extinction curves confirmed this possibility.Moreover, attempts to fit such curves with models having grain volumescorresponding to the standard ρd/ρH ratiofailed.We find a linear relation between ρd/ρHand E(B-V)/N(H) for our sightlines. The average Galactic value marks theseparation into two groups characterized by lower abundances of C and Sitrapped into the grains when E(B-V)/N(H) is smaller than the Galacticvalue, and by larger abundances when E(B-V)/N(H) is greater.

A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.

Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lyingbetween us and the main ionizing stars of the Orion Nebula. It isvisible in 21 cm H I absorption and in optical and UV absorption linesof H I and other species. Toward θ1 Ori C, the veilhas two remarkable properties, a high magnetic field (~100 μG) and asurprising lack of H2, given its total column density. Herewe compute photoionization models of the veil to establish its gasdensity and its distance from θ1 Ori C. We use agreatly improved model of the H2 molecule that determineslevel populations in 105 rotational/vibrational levels andprovides improved estimates of H2 destruction via theLyman-Werner bands. Our best-fit photoionization models place the veil1-3 pc in front of the star at a density of103-104 cm-3. Magnetic energy dominatesthe energy of nonthermal motions in at least one of the 21 cm H Ivelocity components. Therefore, the veil is the first interstellarenvironment in which magnetic dominance appears to exist. We find thatthe low ratio of H2/H0 (<10-4) is aconsequence of high UV flux incident on the veil due to its proximity tothe Trapezium stars and the absence of small grains in the region.

Chandra Observation of the Trifid Nebula: X-Ray Emission from the O Star Complex and Actively Forming Pre-Main-Sequence Stars
The Trifid Nebula, a young star-forming H II region, was observed for 16hr by the ACIS-I detector on board the Chandra X-Ray Observatory. Wedetected 304 X-ray sources, 30% of which are hard sources and 70% ofwhich have near-infrared counterparts. Chandra resolved the HD 164492multiple system into a number of discrete X-ray sources. X-ray emissionis detected from components HD 164492A (an O7.5 III star that ionizesthe nebula), B and C (a B6 V star), and possibly D (a Be star).Component C is blended with an unidentified source to the northwest. HD164492A has a soft spectrum (kT~0.5 keV), while the component C blendshows much harder emission (kT~6 keV). This blend and other hard sourcesare responsible for the hard emission and Fe K line seen by ASCA, whichwas previously attributed entirely to HD 164492A. The soft spectrum ofthe O star is similar to emission seen from other single O stars and isprobably produced by shocks within its massive stellar wind. Lack ofhard emission suggests that neither a magnetically confined wind shocknor colliding wind emission is important in HD 164492A. A dozen starsare found to have flares in the field, and most of them arepre-main-sequence stars (PMS). Six sources with flares have both opticaland Two Micron All Sky Survey counterparts. These counterparts are notembedded, and thus it is likely that these sources are in a later stageof PMS evolution, possibly Class II or III. Two flare sources did nothave any near-IR, optical, or radio counterparts. We suggest that theseX-ray flare stars are in an early PMS stage (Class I or earlier). Wealso detected X-ray sources apparently associated with two massivestar-forming cores, TC 1 and TC 4. The spectra of these sources showhigh extinction and X-ray luminosities of (2-5)×1031ergs s-1. If these sources are Class 0 objects, it is unclearwhether their X-ray emission is due to solar-type magnetic activities,as in Class I objects, or to some other mechanism.

The Angular Momentum Evolution of 0.1-10 Msolar Stars from the Birth Line to the Main Sequence
Projected rotational velocities (vsini) have been measured for a sampleof 145 stars with masses between 0.4 and greater than 10Msolar (median mass 2.1 Msolar) located in theOrion star-forming complex. These measurements have been supplementedwith data from the literature for Orion stars with masses as low as 0.1Msolar. The primary finding from analysis of these data isthat the upper envelope of the observed values of angular momentum perunit mass (J/M) varies as M0.25 for stars on convectivetracks having masses in the range ~0.1 to ~3 Msolar. Thispower law extends smoothly into the domain of more massive stars (3-10Msolar), which in Orion are already on the zero-age mainsequence. This result stands in sharp contrast to the properties ofmain-sequence stars, which show a break in the power law and a sharpdecline in J/M with decreasing mass for stars with M<2Msolar. A second result of our study is that this break isseen already among the pre-main-sequence stars in our Orion sample thatare on radiative tracks, even though these stars are only a few millionyears old. A comparison of rotation rates seen for stars on either sideof the convective-radiative boundary shows that stars do not rotate assolid bodies during the transition from convective to radiative tracks.As a preliminary demonstration of how observations can be used toconstrain the processes that control early stellar angular momentum, weshow that the broad trends in the data can be accounted for by simplemodels that posit that stars (1) lose angular momentum before they aredeposited on the birth line, plausibly through star-disk interactions;(2) undergo additional braking as they evolve down their convectivetracks; and (3) are subject to core-envelope decoupling during theconvective-radiative transition.

Internal Velocities in the Orion Nebula: Large Radial Velocity Features
A map of high-velocity features in the 3'×5'central region of the Orion Nebula was created from slit spectra with avelocity resolution of 8 km s-1. We identified two newbipolar flows, Herbig-Haro (HH) 725 and HH 726, as well as a highlyredshifted flow, HH 512. We also found multiple new high-velocityco-moving features, all lying in the velocity range of -30 to -100 kms-1. The newly discovered Big Arc appears as a slightlyblueshifted feature. Unlike the other objects found, the Big Arc is nota shock; rather it is a result of a structural alteration in the nebulanearly extended across the sampled region. Spatial velocities of 19features belonging to 11 HH objects were obtained by combining theseradial velocity data with existing proper motion data. Most HH objectsin the H II region exhibit spatial velocities ranging from 50 to 150 kms-1. By analyzing their three-dimensional paths, HH 202 andHH 203 + 204 were found to be formed on the curved main ionizationfront, not on the Veil, as previously proposed. We have been able tolocate the source of the HH 201 outflow (possibly the IRc2 featurewithin the BN/KL region) at 0.21 pc behind the ionization front of thenebula. This is in contrast to the Optical Outflow Source, which givesrise to most of the HH objects in the Orion Nebula and lies only a fewhundredths of a parsec behind the ionization front of the nebula.Based in part on observations obtained at the Kitt Peak NationalObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agreement with theNational Science Foundation and based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.

Determination of the Electron Density in the Orion Nebula with High Spectral Resolution
We present the results of long-slit echelle spectroscopic observationsof the density-sensitive [S II] doublet in the Orion Nebula. These formpart of a large-scale program to map the entire inner Orion Nebula inthree dimensions at a resolution of 2'' ×1'' ×6 km /s and inmultiple optical emission lines, covering a wide range of ionizationstages.

Temperature variations from Hubble Space Telescope spectroscopy of the Orion Nebula
We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy ofNGC 1976. Our goal is to measure the intrinsic line ratio [OIII]4364/5008 and thereby evaluate the electron temperature (Te)and the fractional mean-square Te variation(t2A)across the nebula. We also measure theintrinsic line ratio [NII] 5756/6585 in order to estimate Teand t2A in the N+ region. Theinterpretation of the [NII] data is not as clear cut as the [OIII] databecause of a higher sensitivity to knowledge of the electron density aswell as a possible contribution to the [NII] 5756 emission byrecombination (and cascading). We present results from binning the dataalong the various slits into tiles that are 0.5 arcsec square (matchingthe slit width). The average [OIII] temperature for our four HST/STISslits varies from 7678 K to 8358 K; t2A variesfrom 0.00682 to at most 0.0176. For our preferred solution, the average[NII] temperature for each of the four slits varies from 9133 to 10232K; t2A varies from 0.00584 to 0.0175. Themeasurements of Te reported here are an average along eachline of sight. Therefore, despite finding remarkably lowt2A, we cannot rule out significantly largertemperature fluctuations along the line of sight. The result that theaverage [NII]Te exceeds the average [OIII]Teconfirms what has been previously found for Orion and what is expectedon theoretical grounds. Observations of the proplyd P159-350 indicate:large local extinction associated; ionization stratification consistentwith external ionization by θ1 Ori C; and indirectly,evidence of high electron density.

X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications
The Orion Trapezium is one of the youngest and closest star-formingregions within our Galaxy. With a dynamic age of ~3×105yr, it harbors a number of very young hot stars, which likely are on thezero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectrain the wavelength range of 1.5-25 Å of three of itsX-ray-brightest members (Θ1 Ori A, C, and E) obtainedwith the High Energy Transmission Grating Spectrometer (HETGS) on boardthe Chandra X-Ray Observatory. We measured X-ray emission lines,calculated differential emission measure distributions (DEMs), andfitted broadband models to the spectra. The spectra from all three starsare very rich in emission lines, specifically from highly ionized Fe,which includes emission from Fe XVII to Fe XXV ions. A complete linelist is included. This is a mere effect of high temperatures rather thanan overabundance of Fe, which in fact turns out to be underabundant inall three Trapezium members. Similarly there is a significantunderabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appearclose to solar. The DEM derived from over 80 emission lines in thespectrum of Θ1 Ori C indicates three peaks located at7.9, 25, and 66 MK. The emission measure varies over the 15.4 day windperiod of the star. For the two phases observed, the low-temperatureemission remains stable, while the high-temperature emission showssignificant differences. The line widths seem to show a similarbifurcation, where we resolve some of the soft X-ray lines withvelocities up to 850 km s-1 (all widths are stated ashalf-width at half-maximum), whereas the bulk of the lines remainunresolved with a confidence limit of 110 km s-1. Thebroadband spectra of the other two stars can be fitted with severalcollisionally ionized plasma model components within a temperature rangeof 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK forΘ1 Ori A. The high-temperature emissivity contributesover 70% to the total X-ray flux. None of the lines are resolved forΘ1 Ori A and E with a confidence limit of 160 kms-1. The influence of the strong UV radiation field on theforbidden line in the He-like triplets allows us to set an upper limiton distance of the line-emitting region from the photosphere. The bulkof the X-ray emission cannot be produced by shock instabilities in aradiation-driven wind and are likely the result of magnetic confinementin all three stars. Although confinement models cannot explain all theresults, the resemblance of the unresolved lines and of the DEM withrecent observations of active coronae in II Peg and AR Lac during flaresis quite obvious. Thus we speculate that the X-ray production mechanismin these stars is similar, with the difference that the Orion stars maybe in a state of almost continuous flaring driven by the wind. Weclearly rule out major effects due to X-rays from a possible companion.The fact that all three stars appear to be magnetic and are near zeroage on the main sequence also raises the issue of whether the Orionstars are simply different or whether young massive stars enter the mainsequence carrying significant magnetic fields. The ratiologLX/Lbol using the ``wind'' component of thespectrum is -7 for the Trapezium stars, consistent with the expectationfrom O stars. This suggests that massive ZAMS stars generate their X-rayluminosities like normal O stars and magnetic confinement provides anadditional source of X-rays.

The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.

Fine-Scale Temperature Fluctuations in the Orion Nebula and the t2 Problem
We present a high spatial resolution map of the columnar electrontemperature (Tc) of a region to the southwest of theTrapezium in the Orion Nebula. This map was derived from Hubble SpaceTelescope images that isolated the primary lines of H I fordetermination of the local extinction and of the [O III] lines fordetermination of Tc. Although there is no statisticallysignificant variation of Tc with distance from the dominantionizing star, θ1 Ori C, we find small-scalevariations in the plane of the sky down to a few arcseconds, which arecompatible with the variations inferred from comparing the value ofTe derived from forbidden and recombination lines, commonlyknown as the t2 problem. We present other evidence forfine-scale variations in conditions in the nebula, these beingvariations in the surface brightness of the nebula, fluctuations inradial velocities, and ionization changes. From our Tc mapand other considerations we estimate that t2 =0.028+/-0.006for the Orion Nebula. Shadowed regions behind clumps close to theionization front can make a significant contribution to the observedtemperature fluctuations, but they cannot account for the t2values inferred from several methods of temperature determination. It isshown that an anomalous broadening of nebular emission lines appears tohave the same sense of correlation as the temperature anomalies,although a causal link is not obvious.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 NAS 5-26555.

High Proper Motion Features in the Central Orion Nebula
The brightest portion of the Orion Nebula has been reimaged with theHubble Space Telescope in Hα, [N II], and [O III]. Comparison withearlier Wide Field Planetary Camera 2 images going back as much as 8 yrhas allowed determination of tangential velocities (proper motions) downto about 10 km s-1 for a variety of sources. Multiple outflowsystems are found associated with individual proplyds in the ionizedportion of the nebula (HH 518, HH 624, possibly HH 507). The Orion-Scomplex of radio and infrared sources is the source of multipleoutflows. A new outflow system (HH 625) has been identified as comingfrom the blueshifted portion of the imbedded high velocity CO flowcoming from the Orion-S region, this object having CO, H2,and low ionization optical components. The low velocity CO outfloworiginating from or near FIR 4 is the likely source of HH 530. A newimbedded source is inferred from this optical data to lie in Orion-S.This optical outflow source (OOS) clearly feeds the systems HH 269 andHH 529, which lie along a straight line. There is evidence that this isalso the source for HH 528, HH 202, and HH 203/204, all of which areblueshifted (except possibly HH 528 whose radial velocity is unknown).There is no strong radio, infrared, or X-ray source within thepositional ellipse of the OOS. Based in part on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.

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Observation and Astrometry data

Right ascension:05h35m22.90s
Apparent magnitude:5.08
Distance:581.395 parsecs
Proper motion RA:3.7
Proper motion Dec:4
B-T magnitude:4.913
V-T magnitude:5.027

Catalogs and designations:
Proper NamesMizan Batil ath Thani
Mizan Batil II   (Edit)
Bayerθβ Ori
Flamsteed43 Ori
HD 1989HD 37041
TYCHO-2 2000TYC 4774-933-1
USNO-A2.0USNO-A2 0825-01591541
BSC 1991HR 1897
HIPHIP 26235

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