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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.

Bias-free Measurement of Giant Molecular Cloud Properties
We review methods for measuring the sizes, line widths, and luminositiesof giant molecular clouds (GMCs) in molecular-line data cubes with lowresolution and sensitivity. We find that moment methods are robust andsensitive, making full use of both position and intensity information,and we recommend a standard method to measure the position angle, majorand minor axis sizes, line width, and luminosity using moment methods.Without corrections for the effects of beam convolution and sensitivityto GMC properties, the resulting properties may be severely biased. Thisis particularly true for extragalactic observations, where resolutionand sensitivity effects often bias measured values by 40% or more. Wecorrect for finite spatial and spectral resolutions with a simpledeconvolution, and we correct for sensitivity biases by extrapolatingproperties of a GMC to those we would expect to measure with perfectsensitivity (i.e., the 0 K isosurface). The resulting method recoversthe properties of a GMC to within 10% over a large range of resolutionsand sensitivities, provided the clouds are marginally resolved with apeak signal-to-noise ratio greater than 10. We note that interferometerssystematically underestimate cloud properties, particularly the fluxfrom a cloud. The degree of bias depends on the sensitivity of theobservations and the (u,v) coverage of the observations. In an Appendixto the paper we present a conservative, new decomposition algorithm foridentifying GMCs in molecular-line observations. This algorithm treatsthe data in physical rather than observational units (i.e., parsecsrather than beams or arcseconds), does not produce spurious clouds inthe presence of noise, and is sensitive to a range of morphologies. As aresult, the output of this decomposition should be directly comparableamong disparate data sets.

Circumstellar discs in the young σ Orionis cluster
We present new K- and L'-band imaging observations for members of theyoung (3-5Myr) σOrionis cluster, obtained at the United KingdomInfrared Telescope (UKIRT) with the UKIRT 1-5 micron Imager Spectrometer(UIST). We determine (K - L') colour excesses with respect to thephotospheres, finding evidence for warm circumstellar dust around 27 outof 83 cluster members that have masses between 0.04 and1.0Msolar. This indicates a circumstellar disc frequency ofat least (33+/-6) per cent for this cluster, consistent with previousdeterminations from smaller samples and also consistent with the 3-Myrdisc half-life suggested by Haisch et al. There is marginal evidencethat the disc frequency declines towards lower masses, but the data arealso consistent with no mass dependence at all. There is no evidence forspatial segregation of objects with and without circumstellar discs.

Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy
High spatial resolution spectroscopy at 8-13 μm with T-ReCS onGemini-S has revealed striking variations in the mid-infrared emissionand absorption in the nucleus of the Circinus galaxy (hereafterCircinus) on subarcsecond scales. The core of Circinus is compact andobscured by a substantial column of cool silicate dust. Weak extendedemission to the east and west coincides with the coronal line region andarises from featureless dust grains which are probably heated by lineemission in the coronal emission zone. The extended emission on the eastside of the nucleus displays a much deeper silicate absorption than thaton the west, indicating significant columns of cool material along theline of sight and corresponding to an additional extinction ofAV~ 25 mag. Emission bands from aromatic hydrocarbons are notsubject to this additional extinction, are relatively weak in the coreand in the coronal line region, and are much more spatially extendedthan the continuum dust emission; they presumably arise in thecircumnuclear star-forming regions. These data are interpreted in termsof an inclined disc-like structure around the nucleus extending overtens of parsecs and possibly related to the inner disc found fromobservations of water masers by Greenhill et al..

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.

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.

The infrared Hourglass cluster in M8*†
A detailed study of the Hourglass nebula in the M8 star-forming regionis presented. The study is mainly based on recent subarcsec-resolutionJHKs images taken at Las Campanas Observatory andcomplemented with archival Hubble Space Telescope (HST) images andlong-slit spectroscopy retrieved from the European Southern ObservatoryArchive Facility. Using the new numerical code CHORIZOS, we estimate thedistance to the earliest stars in the region to be 1.25 kpc. Infraredphotometry of all the sources detected in the field is given. Fromanalysis of the JHKs colour-colour diagrams, we find that animportant fraction of these sources exhibit significant infrared excess.These objects are candidates to be low- and intermediate-masspre-main-sequence stars. Based on HST observations, the spatialdistribution of gas, dust and stars in the region is analysed. Amorphological analysis of these images also reveals a rich variety ofstructures related to star formation (proplyds, jets, bow shocks),similar to those observed in M16 and M42, along with the detection ofthe first four Herbig-Haro objects in the region. Furthermore, along-slit spectrum obtained with the New Technology Telescope confirmsthe identification of one of them (HH 870) in the core of the Hourglassnebula, providing the first direct evidence of active star formation byaccretion in M8.

theta^1 Orionis E
IAUC 8669 available at Central Bureau for Astronomical Telegrams.

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.

VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue in one sight line but is lower by factors of 10-15 in two others.The abundance of CH in the Magellanic Clouds thus appears to depend onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.

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.

Determining the H+ Region / PDR Equation of State in Star-forming Regions
The emission-line regions of starburst galaxies and active nuclei reveala wealth of spectroscopic information. A unified picture of therelationship among ionized, atomic, and molecular gas makes it possibleto better understand these observations. We performed a series ofcalculations designed to determine the equation of state-therelationship among density, temperature, and pressure-throughemission-line diagnostic ratios that form in the H+ regionand the photodissociation region (PDR). We consider a wide range ofphysical conditions in the H+ region. We connect theH+ region to the PDR by considering two constant pressurecases: one with no magnetic field and one in which the magnetic fieldoverwhelms the thermal pressure. We show that diagnostic ratios canyield the equation of state for single H+ regions adjacent tosingle PDRs, with the results being more ambiguous when consideringobservations of entire galaxies. As a test, we apply our calculations tothe Orion H+/PDR region behind the Trapezium. We find theratio of thermal to magnetic pressure in the PDR to be ~1.2. If magneticand turbulent energy are in equipartition, our results mean that themagnetic field is not the cause of the unexplained broadening in M42,but may significantly affect line broadening in the PDR. Since Orion isoften used to understand physical processes in extragalacticenvironments, our calculations suggest that magnetic pressure should beconsidered in modeling such regions.

Low-Mass Stars and Brown Dwarfs in NGC 2024: Constraints on the Substellar Mass Function
We present results from a near-infrared spectroscopic study of candidatebrown dwarfs and low-mass stars in the young cluster NGC 2024. UsingFLAMINGOS on the KPNO 2.1 m and 4 m telescopes, we have obtained spectraof ~70 new members of the cluster and classified them via the prominentJ- and H-band water absorption features. Derived spectral types rangefrom ~M1 to later than M8 with typical classification errors of 0.5-1subclasses. By combining these spectral types with JHK photometry, weplace these objects on the H-R diagram and use pre-main-sequenceevolutionary models to infer masses and ages. The mean age for thislow-mass population of NGC 2024 is 0.5 Myr, and derived masses rangefrom ~0.7 to 0.02 Msolar with 23 objects falling below thehydrogen-burning limit. The logarithmic mass function rises to a peak at~0.2 Msolar before turning over and declining into thesubstellar regime. There is a possible secondary peak at ~0.035Msolar however, the errors are also consistent with a flatIMF in this region. The ratio of brown dwarfs to stars is similar tothat found in the Trapezium but roughly twice the ratio found in IC 348,leading us to conclude that the substellar IMF in young clusters may bedependent on the local star-forming environment.

A Correlation between Pre-Main-Sequence Stellar Rotation Rates and IRAC Excesses in Orion
Early observations of T Tauri stars suggested that stars with evidenceof circumstellar accretion disks rotated slower than stars without suchevidence, but more recent results are not as clear. Near-IRcircumstellar disk indicators, although the most widely available, aresubject to uncertainties that can result from inner disk holes and/orthe system inclination. Mid-infrared observations are less sensitive tosuch effects, but until now, these observations have been difficult toobtain. The Spitzer Space Telescope now easily enables mid-infraredmeasurements of large samples of PMS stars covering a broad mass rangein nearby star-forming regions. Megeath and collaborators surveyed theOrion Molecular Clouds (~1 Myr) with the IRAC instrument (3.6, 4.5, 5.8,8 μm) as part of a joint IRAC and MIPS GTO program. We examine therelationship between rotation and Spitzer mid-IR fluxes for ~900 starsin Orion for stars between 3 and 0.1 Msolar. We find in theseSpitzer data the clearest indication to date that stars with longerperiods are more likely than those with short periods to have IRexcesses suggestive of disks.

Dust Processing in Disks around T Tauri Stars
The 8-14 μm emission spectra of 12 T Tauri stars in the Taurus/Aurigadark clouds and in the TW Hydrae association obtained with the InfraredSpectrograph (IRS) on board Spitzer are analyzed. Assuming that the 10μm features originate from silicate grains in the optically thinsurface layers of T Tauri disks, the 8-14 μm dust emissivity for eachobject is derived from its Spitzer spectrum. The emissivities are fitwith the opacities of laboratory analogs of cosmic dust. The fitsinclude small nonspherical grains of amorphous silicates (pyroxene andolivine), crystalline silicates (forsterite and pyroxene), and quartz,together with large fluffy amorphous silicate grains. A wide range inthe fraction of crystalline silicate grains, as well as large silicategrains among these stars, are found. The dust in the transitional-diskobjects CoKu Tau/4, GM Aur, and DM Tau has the simplest form ofsilicates, with almost no hint of crystalline components and modestamounts of large grains. This indicates that the dust grains in theseobjects have been modified little from their origin in the interstellarmedium. Other stars show various amounts of crystalline silicates,similar to the wide dispersion of the degree of crystallinity reportedfor Herbig Ae/Be stars of mass <2.5 Msolar. Late spectraltype, low-mass stars can have significant fractions of crystallinesilicate grains. Higher quartz mass fractions often accompany lowamorphous olivine to amorphous pyroxene ratios. Lower contrast of the 10μm feature accompanies greater crystallinity.

Water Maser Motions in W3(OH) and a Determination of Its Distance
We report phase-referencing VLBA observations of H2O masersnear the star-forming region W3(OH) to measure their parallax andabsolute proper motions. The measured annual parallax is 0.489+/-0.017mas (2.04+/-0.07 kpc), where the error is dominated by a systematicatmospheric contribution. This distance is consistent with photometricdistances from previous observations and with the distance determinedfrom CH3OH maser astrometry presented in a related paper. Wealso find that the source driving the H2O outflow, the``TW-object,'' moves with a three-dimensional velocity of >7 kms-1 relative to the ultracompact H II region W3(OH).

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.

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.

Physical Conditions in Orion's Veil. II. A Multicomponent Study of the Line of Sight toward the Trapezium
Orion's Veil is an absorbing screen that lies along the line of sight tothe Orion H II region. It consists of two or more layers of gas thatmust lie within a few parsecs of the Trapezium cluster. Our previouswork considered the Veil as a whole and found that the magnetic fielddominates the energetics of the gas in at least one component. Here weuse high-resolution STIS UV spectra that resolve the two velocitycomponents in absorption and determine the conditions in each. We derivea volume hydrogen density, 21 cm spin temperature, turbulent velocity,and kinetic temperature for each. We combine these estimates withmagnetic field measurements to find that magnetic energy significantlydominates turbulent and thermal energies in one component, while theother component is close to equipartition between turbulent and magneticenergies. We observe H2 absorption for highly excited v, Jlevels that are photoexcited by the stellar continuum, and detectblueshifted S+2 and P+2 ions. These ions mustarise from ionized gas between the mostly neutral portions of the Veiland the Trapezium and shields the Veil from ionizing radiation. We findthat this layer of ionized gas is also responsible for He I λ3889absorption toward the Veil, which resolves a 40 year old debate on theorigin of He I absorption toward the Trapezium. Finally, we determinethat the ionized and mostly atomic layers of the Veil will collide inless than 85,000 yr.

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.

Hubble Space Telescope NICMOS Polarization Measurements of OMC-1
We present 2 μm polarization measurements of positions in the IRc2and BN regions of the Orion Molecular Cloud (OMC-1) made with NICMOSCamera 2 (0.2" resolution) on the Hubble Space Telescope. Our resultsare as follows: BN is ~29% polarized by dichroic absorption and appearsto be the illuminating source for most of the nebulosity to its northand for up to ~5" to its south. Although the stars are probably allpolarized by dichroic absorption, there are a number of compact butnon-point-source objects that could be polarized by a combination ofboth dichroic absorption and local scattering of starlight. We identifyseveral candidate YSOs, including an approximately edge-on bipolar YSO8.7" east of BN, and a deeply embedded variable star. Additionalstrongly polarized sources are IRc2-B, IRc2-D, and IRc7, all of whichare obviously self-luminous at mid-infrared wavelengths and may be YSOs.None of these is a reflection nebula illuminated by a star located nearradio source I, as was previously suggested. Other IRc sources areclearly reflection nebulae: IRc3 appears to be illuminated by IRc2-B ora combination of the IRc2 sources, and IRc4 and IRc5 appear to beilluminated by an unseen star in the vicinity of radio source I, or bystar n or IRc2-A. Trends in the magnetic field direction are inferredfrom the polarization of the 26 stars that are bright enough to be seenas NICMOS point sources. The most polarized star has a polarizationposition angle different from its neighbors by ~40°, but inagreement with the grain alignment inferred from millimeter polarizationmeasurements of the cold dust cloud in the southern part of OMC-1.

Massive Protoplanetary Disks in the Trapezium Region
We determine the disk mass distribution around 336 stars in the young(~1 Myr) Orion Nebula cluster by imaging a 2.5 arcinm × 2.5 arcminregion in 3 mm continuum emission with the Owens Valley MillimeterArray. For this sample of 336 stars, we observe 3 mm emission above the3 σ noise level toward 10 sources, six of which have also beendetected optically in silhouette against the bright nebular background.In addition, we detect 20 objects in 3 mm continuum emission that do notcorrespond to known near-IR cluster members. Comparisons of our measuredfluxes with longer wavelength observations enable rough separation ofdust emission from thermal free-free emission, and we find substantialdust emission toward most objects. For the sample of 10 objects detectedat both 3 mm and near-IR wavelengths, eight exhibit substantial dustemission. Excluding the two high-mass stars (θ1 Ori Aand the BN object) and assuming a gas-to-dust ratio of 100, we estimatecircumstellar masses ranging from 0.13 to 0.39 Msolar. Forthe cluster members not detected at 3 mm, images of individual objectsare stacked to constrain the mean 3 mm flux of the ensemble. The averageflux is detected at the 3 σ confidence level and implies anaverage disk mass of 0.005 Msolar, comparable to theminimum-mass solar nebula. The percentage of stars in Orion surroundedby disks more massive than ~0.1 Msolar is consistent with thedisk mass distribution in Taurus, and we argue that massive disks inOrion do not appear to be truncated through close encounters withhigh-mass stars. Comparison of the average disk mass and number ofmassive dusty structures in Orion with similar surveys of the NGC 2024and IC 348 clusters is used to constrain the evolutionary timescales ofmassive circumstellar disks in clustered environments.

The Low-Mass Initial Mass Function of the Field Population in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent HST WFPC2 stellar photometry of an areain the Large Magellanic Cloud (LMC), located to the west of the bar ofthe galaxy, which accounts for the general background field of its innerdisk. The WFPC2 observations reach magnitudes as faint as V=25 mag, andthe large sample of more than 80,000 stars allows us to determine indetail the present-day mass function (PDMF) of the detectedmain-sequence stars, which is identical to the initial mass function(IMF) for masses M<~1 Msolar. The low-mass main-sequencemass function of the LMC field is found not to have a uniform slopethroughout the observed mass range; i.e., the slope does not follow asingle power law. This slope changes at about 1 Msolar tobecome more shallow for stars with smaller masses down to the lowestobserved mass of ~0.7 Msolar, giving clear indications offlattening for even smaller masses. We verified statistically that forstars with M<~1 Msolar the IMF has a slope Γ around-2, with an indicative slope Γ~=-1.4 for0.7<~M/Msolar<~0.9, while for more massive stars themain-sequence mass function becomes much steeper with Γ~=-5. Themain-sequence luminosity function (LF) of the observed field is in verygood agreement with the Galactic LF as it was previously found. Takinginto account several assumptions concerning evolutionary effects, whichshould have changed through time the stellar content of the observedfield, we reconstruct qualitatively its IMF for the whole observed massrange (0.7<~M/Msolar<~2.3), and we find that the numberof observed evolved stars is not large enough to have affectedsignificantly the form of the IMF, which thus is found almost identicalto the observed PDMF.

Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope
We present the results of an infrared imaging survey of two clusters inthe Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPSinstruments on board the Spitzer Space Telescope. Our observations coverthe wavelength range from 3.6 to 24 μm, allowing us to detect diskemission over a typical range of radii ~0.1 to ~20 AU from the centralstar. In Tr 37, with an age of about 4 Myr, about 48% of the low-massstars exhibit detectable disk emission in the IRAC bands. Roughly 10% ofthe stars with disks may be ``transition'' objects, with essentiallyphotospheric fluxes at wavelengths <=4.5 μm but with excesses atlonger wavelengths, indicating an optically thin inner disk. The medianoptically thick disk emission in Tr 37 is lower than the correspondingmedian for stars in the younger Taurus region; the decrease in infraredexcess is larger at 6-8 μm than at 24 μm, suggesting that graingrowth and/or dust settling has proceeded faster at smaller disk radii,as expected on general theoretical grounds. Only about 4% of thelow-mass stars in the 10 Myr old cluster NGC 7160 show detectableinfrared disk emission. We also find evidence for 24 μm excessesaround a few intermediate-mass stars, which may represent so-called``debris disk'' systems. Our observations provide new constraints ondisk evolution through an important age range.

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.

Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium
We have analyzed the 9.7 and ``18'' μm interstellar silicateabsorption features along the line of sight toward four heavilyextincted galactic WC-type Wolf-Rayet (WR) stars. We construct twointerstellar extinction curves from 1.25 to 25 μm using near-IRextinction measurements from the literature, along with the silicateprofiles of WR 98a (representing the local ISM) and GCS 3 (representingthe Galactic center). We have investigated the mineralogy of theinterstellar silicates by comparing extinction profiles for amorphoussilicates with olivine and pyroxene stoichiometry to the 9.7 and ``18''μm absorption features in the WR 98a spectrum. In this analysis, wehave considered solid and porous spheres and a continuous distributionof ellipsoids. While it is not possible to simultaneously provide aperfect match to both profiles, we find that the best match requires amixture of these two types of compounds. We also consider iron oxides,aluminosilicates, and silicate carbide (SiC) as grain components. Ironoxides cannot be accommodated in the observed spectrum, while the amountof Si in SiC is limited to <4%. Finally, we discuss the cosmicelemental abundance constraints on the silicate mineralogy, grain shape,and porosity.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.

Identifying Primordial Substructure in NGC 2264
We present new Spitzer Space Telescope observations of the young clusterNGC 2264. Observations at 24 μm with the Multiband Imaging Photometerhave enabled us to identify the most highly embedded and youngestobjects in NGC 2264. This Letter reports on one particular region of NGC2264 where bright 24 μm sources are spatially configured in curiouslinear structures with quasi-uniform separations. The majority of thesesources (~60%) are found to be protostellar in nature, with Class Ispectral energy distributions. Comparison of their spatial distributionwith submillimeter data from Wolf-Chase et al. and millimeter data fromPeretto et al. shows a close correlation between the dust filaments andthe linear spatial configurations of the protostars, indicating thatstar formation is occurring primarily within dense, dusty filaments.Finally, the quasi-uniform separations of the protostars are found to becomparable in magnitude to the expected Jeans length, suggesting thermalfragmentation of the dense filamentary material.

THE BM Ori system. IV. A new component of the system
Cross correlations between observed and synthetic spectra are used todiscover yet another satellite of BM Ori with the followingcharacteristics: effective temperature Teff = 4000 K, radiusR = 16Rȯ, mass M = 1.8Mȯ, spectral typeK7 III, absolute bolometric stellar magnitude Mb = +4m·0, axial rotation velocity V sini = 85 km/s, andrelative luminosity 0.005 near the V band.

The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. II. Results from Monte Carlo Population Analyses
In this second of a two-paper sequence, we present Monte Carlopopulation simulation results of brown dwarf companion data collectedduring the Cornell High-Order Adaptive Optics Survey (CHAOS) for browndwarf companions. Making reasonable assumptions of orbital parameters(random inclination, random eccentricity, and random longitude ofpericenter) and age distributions and using published mass functions, wefind that the brown dwarf companion fraction around main-sequence starsis 0.0%-9.3% for the 25-100 AU semimajor axis region. We find acorresponding L dwarf companion fraction of 0.0%-3.3%. We compare ourpopulation analysis methods and results with techniques and resultspresented by several other groups. In this comparison we discover thatsystematic errors (most notably resulting from orbital projectioneffects) occur in the majority of previously published brown dwarfcompanion population estimates, leading authors to claim results notsupported by the observational data.

The Secrets of the Nearest Starburst Cluster. II. The Present-Day Mass Function in NGC 3603
Based on deep Very Large Telescope Infrared Spectrometer and ArrayCamera JHK photometry, we have derived the present-day mass function(MF) of the central starburst cluster NGC 3603 YC (Young Cluster) in thegiant H II region NGC 3603. The effects of field contamination,individual reddening, and a possible binary contribution areinvestigated. The MF slopes resulting from the different methods arecompared and lead to a surprisingly consistent cluster MF with a slopeof Γ=-0.9+/-0.15. Analyzing different radial annuli around thecluster core, no significant change in the slope of the MF is observed.However, mass segregation in the cluster is evidenced by the increasingdepletion of the high-mass tail of the stellar mass distribution withincreasing radius. We discuss the indications of mass segregation withrespect to the changes observed in the binned and cumulative stellar MFsand argue that the cumulative function, as well as the fraction of high-to low-mass stars, provides better indicators for mass segregation thanthe MF slope alone. Finally, the observed MF and starburst morphology ofNGC 3603 YC are discussed in the context of massive local star-formingregions such as the Galactic center Arches cluster, R136/30 Dor in theLMC, and the Orion Trapezium cluster, all providing resolved templatesfor extragalactic star formation. Despite the similarity in the observedMF slopes, dynamical considerations suggest that the starburst clustersdo not form gravitationally bound systems over a Hubble time. Both theenvironment (gravitational potential of the Milky Way) and theconcentration of stars in the cluster core determine the dynamicalstability of a dense star cluster, such that the long-term evolution ofa starburst is not exclusively determined by the stellar evolution ofits members, as frequently assumed for globular cluster systems.Based on observations obtained at the ESO Very Large Telescope onParanal, Chile, under programs 63.I-0015 and 65.I-0135.

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Observation and Astrometry data

Right ascension:05h35m16.50s
Apparent magnitude:5.13
Distance:10000000 parsecs

Catalogs and designations:
Proper Names
Bayerθα Ori
Flamsteed41 Ori
HD 1989HD 37022
BSC 1991HR 1895

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