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Infrared Extinction toward Nearby Star-forming Regions
We present an independent estimate of the interstellar extinction lawfor the Spitzer IRAC bands, as well as a first attempt at extending thelaw to the 24 μm MIPS band. The source data for these measurementsare observations of five nearby star-forming regions: the Orion A cloud,NGC 2068/2071, NGC 2024/2023, Serpens, and Ophiuchus. Color excessratios EH-Ks/EKs-[λ]were measured for stars without infrared excess dust emission fromcircumstellar disks/envelopes. For four of these five regions, theextinction laws are similar at all wavelengths and differ systematicallyfrom a previous determination of the extinction law, which was dominatedby the diffuse ISM, derived for the IRAC bands. This difference could bedue to the difference in the dust properties of the dense molecularclouds observed here and those of the diffuse ISM. The extinction law atlonger wavelengths toward the Ophiuchus region lies between that to theother four regions studied here and that for the ISM. In addition, weextended our extinction law determination to 24 μm for Serpens andNGC 2068/2071 using Spitzer MIPS data. We compare these results againstseveral ISO extinction law determinations, although in each case thereare assumptions which make absolute comparison uncertain. However, ourwork confirms a relatively flatter extinction curve from 4 to 8 μmthan the previously assumed standard, as noted by all of these recentstudies. The extinction law at 24 μm is consistent with previousmeasurements and models, although there are relatively largeuncertainties.

A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster
We report new Spitzer Space Telescope observations, using the IRAC andMIPS instruments, of the young (~3 Myr) σ Orionis cluster. Weidentify 336 stars as members of the cluster, using optical andnear-infrared color-magnitude diagrams. Using the spectral energydistribution slopes in the IRAC spectral range, we place objects intoseveral classes: non-excess stars, stars with optically thick disks(such as classical T Tauri stars), class I (protostellar) candidates,and stars with ``evolved disks'' the last exhibit smaller IRAC excessesthan optically thick disk systems. In general, this classificationagrees with the location expected in IRAC-MIPS color-color diagrams forthese objects. We find that the evolved disk systems are mostly acombination of objects with optically thick but nonflared disks,suggesting grain growth and/or settling, and transition disks, systemsin which the inner disk is partially or fully cleared of small dust. Inall, we identify seven transition disk candidates and three possibledebris disk systems. As in other young stellar populations, the fractionof disks depends on the stellar mass, ranging from ~10% for stars in theHerbig Ae/Be mass range (>2 Msolar) to ~35% for those inthe T Tauri mass range (1-0.1 Msolar). The IRAC infraredexcesses found in stellar clusters and associations with and withoutcentral high-mass stars are similar, suggesting that externalphotoevaporation is not very important in many clusters. Finally, wefind no correlation between the X-ray luminosity and the disk infraredexcess, suggesting that the X-rays are not strongly affected by diskaccretion.

25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a
We report here on the photometric and kinematic properties of awell-defined group of nearly 200 low-mass pre-main-sequence stars,concentrated within ~1° of the early-B star 25 Ori, in the OrionOB1a subassociation. We refer to this stellar aggregate as the 25Orionis group. The group also harbors the Herbig Ae/Be star V346 Ori anda dozen other early-type stars with photometry, parallaxes, and somewith IR excess emission, indicative of group membership. The number ofhigh- and low-mass stars is in agreement with expectations from astandard initial mass function. The velocity distribution for thelow-mass stars shows a narrow peak at 19.7 km s-1, offset~-10 km s-1 from the velocity characterizing the youngerstars of the Ori OB1b subassociation, and -4 km s-1 from thevelocity of widely spread young stars of the Ori OB1a population; thisresult provides new and compelling evidence that the 25 Ori group is adistinct kinematic entity, and that considerable space and velocitystructure is present in the Ori OB1a subassociation. The low-massmembers follow a well-defined band in the color-magnitude diagram,consistent with an isochronal age of ~7-10 Myr. The ~2 time drop in theoverall Li I equivalent widths and accretion fraction between theyounger Ori OB1b and the 25 Ori group is consistent with the latterbeing significantly older. In a simple-minded kinematic evolutionscenario, the 25 Ori group may represent the evolved counterpart of theyounger σ Ori cluster. The 25 Ori stellar aggregate is the mostpopulous ~10 Myr sample yet known within 500 pc, setting it as anexcellent laboratory to study the evolution of solar-like stars andprotoplanetary disks.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by CIDA for theMinisterio de Ciencia y Tecnología the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona;and the Fred Lawrence Whipple Observatory of the SmithsonianInstitution.

Comparison of 13CO line and far-infrared continuum emission as a diagnostic of dust and molecular gas physical conditions - III. Systematic effects and scientific implications
Far-infrared (far-IR) continuum data from the COBE/DIRBE instrument werecombined with Nagoya 4-m 13COJ = 1 -> 0 spectral line datato infer the multiparsec-scale physical conditions in the OrionA and Bmolecular clouds, using 140 μm/240 μm dust colour temperatures andthe 240 μm/13COJ = 1 -> 0 intensity ratios. In theory,the ratio of far-IR, submillimetre, or millimetre continuum to that of a13CO (or C18O) rotational line can place reliableupper limits on the temperature of the dust and molecular gas onmultiparsec scales; on such scales, both the line and continuum emissionare optically thin, resulting in a continuum-to-line ratio that suffersno loss of temperature sensitivity in the high-temperature limit asoccurs for ratios of CO rotational lines or ratios of continuum emissionin different wavelength bands.Two-component models fit the Orion data best, where one has a fixedtemperature and the other has a spatially varying temperature. Theinferred physical conditions are consistent with those determined frompreviously observed maps of 12COJ = 1 -> 0 and J = 2 ->1 that cover the entire OrionA and B molecular clouds. The modelsrequire that the dust-gas temperature difference is 0 +/- 2K. If thissurprising result is confirmed with independent studies and applies tomuch of the Galactic interstellar medium (ISM), except in unusualregions such as the Galactic Centre, then there are a number ofimplications. These include dust-gas thermal coupling that is commonlyfactors of 5-10 stronger than previously believed, Galactic-scalemolecular gas temperatures closer to 20K than to 10K, an improvedexplanation for the N(H2)/I(CO) conversion factor (a fulldiscussion of this is deferred to a later paper), and ruling out atleast one dust grain alignment mechanism. The simplest interpretation ofthe models suggests that about 40-50 per cent of the Orion clouds are inthe form of cold (i.e. ~3-10K) dust and gas, although alternativeexplanations are not ruled out. These alternatives include thecontribution to the 240-μm continuum by dust associated with atomichydrogen and reduced 13CO abundance towards the clouds'edges. Even considering these alternatives, it is still likely that coldmaterial with temperatures of ~7-10K still exists. If this cold gas anddust are common in the Galaxy, then mass estimates of the Galactic ISMmust be revised upwards by up to 60 per cent.The feasibility of submillimetre or millimetre continuum to13CO line ratios constraining estimates of dust and moleculargas temperatures was tested. The model fits allowed the simulation ofthe necessary millimetre-continuum and 13COJ = 1 -> 0 mapsused in the test. In certain `hot spots' - that have continuum-to-lineratios above some threshold value - the millimetre continuum to13CO ratio can estimate the dust temperature to within afactor of 2 over large ranges of physical conditions. Nevertheless,supplemental observations of the 13COJ = 2 -> 1 line or ofshorter wavelength continuum are advisable in placing lower limits onthe estimated temperature. Even without such supplemental observations,this test shows that the continuum-to-line ratio places reliable upperlimits on the temperature.

The brightest stars of the σ Orionis cluster
Context: The very young σ Orionis cluster (~3 Ma) is a cornerstonein understanding the formation of stars and substellar objects down toplanetary masses. However, its stellar population is far from beingcompletely known. Aims: This study's purpose is to identify andcharacterise the most massive stars of σ Orionis to complementcurrent and future deep searches for brown dwarfs and planetary-massobjects in the cluster. Methods: I have cross-correlated thesources in the Tycho and 2MASS catalogues in a region of 30 arcminradius with its centre in the O-type star σ Ori A. In this area, Istudied the membership in the Ori OB 1b association of the brighteststars in the optical using astrometric, X-ray, and both infrared andoptical photometric data from public catalogues, and spectroscopic datafrom the literature. Results: A list of 26 young stars, fourcandidate young stars, and 16 probable foreground stars has arisen fromthe study. Seven young stars probably harbour discs (four are new).There is no mass dependence of the disc frequency in the cluster. I havederived the first mass spectrum for σ Orionis from 1.1 to 24M_ȯ (α = +2.0+0.2-0.1; roughlySalpeter-like). I also provide additional proof of the existence ofseveral spatially superimposed stellar populations in the direction ofσ Orionis. Finally, the cluster may be closer and older thanpreviously thought.

Star formation in a clustered environment around the UCH {II} region in IRAS 20293+3952
Aims.We aim at studying the cluster environment surrounding the UCH IIregion in IRAS 20293+3952, a region in the first stages of formation ofa cluster around a high-mass star. Methods: BIMA and VLA were used toobserve the 3 mm continuum, N2H+ (1-0), NH3 (1, 1), NH3 (2,2), and CH3OH (2-1) emission of the surroundings of the UCH II region.We studied the kinematics of the region and computed the rotationaltemperature and column density maps by fitting the hyperfine structureof N2H+ and NH3. Results: The dense gas traced byN2H+ and NH3 shows two different clouds, a main cloud to theeast of the UCH II region, of ~0.5 pc and ~250 Mȯ, and awestern cloud, of ~0.15 pc and ~30 Mȯ. The dust emissionreveals two strong components in the northern side of the main cloud,BIMA 1 and BIMA 2, associated with Young Stellar Objects (YSOs) drivingmolecular outflows, and two fainter components in the southern side,BIMA 3 and BIMA 4, with no signs of star forming activity. Regarding theCH3OH, we found strong emission in a fork-like structure associated withoutflow B, as well as emission associated with outflow A. The YSOsassociated with the dense gas seem to have a diversity of age andproperties. The rotational temperature is higher in the northern side ofthe main cloud, around 22 K, where there are most of the YSOs, than inthe southern side, around 16 K. There is strong chemical differentiationin the region, since we determined low values of the NH3/N2H+ratio, ~50, associated with YSOs in the north of the main cloud, andhigh values, up to 300, associated with cores with no detected YSOs, inthe south of the main cloud. Such a chemical differentiation is likelydue to abundance/depletion effects. Finally, interaction between thedifferent sources in the region is important. First, the UCH II regionis interacting with the main cloud, heating it and enhancing the CN(1-0) emission. Second, outflow A seems to be excavating a cavity andheating its walls. Third, outflow B is interacting with the BIMA 4 core,likely producing the deflection of the outflow and illuminating a clumplocated ~0.2 pc to the northeast of the shock. Conclusions: .The starformation process in IRAS 20293+3952 is not obviously associated withinteractions, but seems to take place where density is highest.

Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.

Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made byHST STIS and FUSE to determine the column densities and rotationalexcitation temperatures for carbon monoxide and molecular hydrogen,respectively, along 23 sight lines to Galactic O and B stars. Thereddening values range from E(B-V)=0.07 to 0.62, sampling the diffuse totranslucent interstellar medium (ISM). We find that the H2column densities range from 5×1018 to8×1020 cm-2 and the CO from upper limitsaround 2×1012 cm-2 to detections as high as1.4×1016 cm-2. CO increases with increasingH2, roughly following a power law of factor ~2. TheCO/H2 column density ratio is thus not constant, ranging from10-7 to 10-5, with a mean value of3×10-6. The sample segregates into ``diffuse'' and``translucent'' regimes, the former with molecular fraction <~0.25and AV/d<1 mag kpc-1. The mean CO/H2for these two regimes are 3.6×10-7 and9.3×10-6, respectively, significantly lower than thecanonical dark cloud value of 10-4. Six sight lines show theisotopic variant 13CO, and the isotopic ratio we observe(~50-70) is consistent with, if perhaps a little below, the average12C/13C for the ISM at large. The averageH2 rotational excitation temperature is 74+/-24 K, agreeingwell with previous studies, and the average CO temperature is 4.1 K,with some sight lines showing temperatures as high as 6.4 K. The higherexcitation CO is observed with higher column densities, consistent withthe effects of photon trapping in clouds with densities in the 20-100cm-3 range. We discuss the implications for the structure ofthe diffuse/translucent regimes of the ISM and the estimation ofmolecular mass in galaxies.

The Galactic distribution of magnetic fields in molecular clouds and HII regions
Aims.Magnetic fields exist on all scales in our Galaxy. There is acontroversy about whether the magnetic fields in molecular clouds arepreserved from the permeated magnetic fields in the interstellar medium(ISM) during cloud formation. We investigate this controversy usingavailable data in the light of the newly revealed magnetic fieldstructure of the Galactic disk obtained from pulsar rotation measures(RMs). Methods: We collected measurements of the magnetic fields inmolecular clouds, including Zeeman splitting data of OH masers in cloudsand OH or HI absorption or emission lines of clouds themselves. Results: The Zeeman data show structures in the sign distribution of theline-of-sight component of the magnetic field. Compared to thelarge-scale Galactic magnetic fields derived from pulsar RMs, the signdistribution of the Zeeman data shows similar large-scale fieldreversals. Previous such examinations were flawed in the over-simplifiedglobal model used for the large-scale magnetic fields in the Galacticdisk. Conclusions: .We conclude that the magnetic fields in the cloudsmay still "remember" the directions of magnetic fields in the GalacticISM to some extent, and could be used as complementary tracers of thelarge-scale magnetic structure. More Zeeman data of OH masers in widelydistributed clouds are required.Tables 1 and 2 are only available in electronic form athttp://www.aanda.org

Comparison of 13CO line and far-infrared continuum emission as a diagnostic of dust and molecular gas physical conditions - I. Motivation and modelling
Determining temperatures in molecular clouds from ratios of COrotational lines or from ratios of continuum emission in differentwavelength bands suffers from reduced temperature sensitivity in thehigh-temperature limit. In theory, the ratio of far-infrared (FIR),submillimetre or millimetre continuum to that of a 13CO (orC18O) rotational line can place reliable upper limits on thetemperature of the dust and molecular gas. Consequently, FIR continuumdata from the COBE/Diffuse Infrared Background Experiment (DIRBE)instrument and Nagoya 4-m 13COJ = 1 -> 0 spectral linedata were used to plot 240 μm/13COJ = 1 -> 0 intensityratios against 140/240 μm dust colour temperatures, allowing us toconstrain the multiparsec-scale physical conditions in the Orion A and Bmolecular clouds.The best-fitting models to the Orion clouds consist of two components: acomponent near the surface of the clouds that is heated primarily by avery large scale (i.e. ~1kpc) interstellar radiation field and acomponent deeper within the clouds. The former has a fixed temperatureand the latter has a range of temperatures that vary from one sightlineto another. The models require a dust-gas temperature difference of 0+/- 2K and suggest that 40-50 per cent of the Orion clouds are in theform of dust and gas with temperatures between 3 and 10K. Theimplications are discussed in detail in later papers and includestronger dust-gas thermal coupling and higher Galactic-scale moleculargas temperatures than are usually accepted, and an improved explanationfor the N(H2)/I(CO) conversion factor. It is emphasized thatthese results are preliminary and require confirmation by independentobservations and methods.

A SCUBA survey of Orion - the low-mass end of the core mass function
We have re-analysed all of the Submillimetre Common User Bolometer Array(SCUBA) archive data of the Orion star-forming regions. We have puttogether all of the data taken at different times by different groups.Consequently, we have constructed the deepest submillimetre maps ofthese regions ever made. There are four regions that have been mapped:Orion A North and South, and Orion B North and South. We find that twoof the regions, Orion A North and Orion B North, have deeper sensitivityand completeness limits, and contain a larger number of sources, so weconcentrate on these two. We compare the data with archive data from theSpitzer Space Telescope to determine whether or not a core detected inthe submillimetre is pre-stellar in nature. We extract all of thepre-stellar cores from the data and make a histogram of the core masses.This can be compared to the stellar initial mass function (IMF). We findthe high-mass core mass function (CMF) follows a roughly Salpeter-likeslope, just like the IMF, as seen in previous work. Our deeper mapsallow us to see that the CMF turns over at, ~1.3Msolar abouta factor of 4 higher than our completeness limit. This turnover hasnever previously been observed, and is only visible here due to our muchdeeper maps. It mimics the turnover seen in the stellar IMF at~0.1Msolar. The low-mass side of the CMF is a power law withan exponent of, 0.35 +/- 0.2 which is consistent with the low-mass slopeof the young cluster IMF of 0.3 +/- 0.1. This shows that the CMFcontinues to mimic the shape of the IMF all the way down to the lowercompleteness limit of these data at ~0.3Msolar.

The Spitzer c2d Survey of Nearby Dense Cores. IV. Revealing the Embedded Cluster in B59
Infrared images of the dark cloud core B59 were obtained with the Spitzer Space Telescope as part of the ``Cores to Disks'' Legacy Scienceproject. Photometry from 3.6-70 μm indicates at least 20 candidatelow-mass young stars near the core, more than doubling the previouslyknown population. Out of this group, 13 are located within ~0.1 pc inprojection of the molecular gas peak, where a new embedded source isdetected. Spectral energy distributions span the range from smallexcesses above photospheric levels to rising in the mid-infrared. Oneother embedded object, probably associated with the millimeter sourceB59-MMS1, with a bolometric luminosity Lbol~2Lsolar, has extended structure at 3.6 and 4.5 μm, possiblytracing the edges of an outflow cavity. The measured extinction throughthe central part of the core is AV>~45 mag. The B59 coreis producing young stars with a high efficiency.

Anisotropy of Magnetohydrodynamic Turbulence and Polarization of the Spectral Lines of Molecules
The anisotropy of velocities in MHD turbulence is demonstratedexplicitly by calculating the velocity gradients as a function ofdirection in representative simulations of decaying turbulence. Itfollows that the optical depths of spectral lines are anisotropic whenthere is MHD turbulence and that this anisotropy influences thepolarization characteristics of the emergent radiation. We calculate thelinear polarization that results for the microwave lines of the COmolecule in star-forming gas and show that it is comparable to thepolarization that is observed. This and our earlier result-that theanisotropy of MHD turbulence may be the cause for the absence of theZeeman π-components in the spectra of OH mainline masers-are thefirst demonstrations of the occurrence of anisotropy in the opticaldepths caused by MHD turbulence. A nonlocal approximation is developedfor the radiative transfer, and the results are compared with those froma local (LVG) approximation.

Chandra Observations of the Eagle Nebula. I. Embedded Young Stellar Objects near the Pillars of Creation
We present and analyze the first high-resolution X-ray images everobtained of the Eagle Nebula star-forming region. On 2001 July 30 theChandra X-Ray Observatory obtained a 78 ks image of the Eagle Nebula(M16) that includes the core of the young galactic cluster NGC 6611 andthe dark columns of dust and cold molecular gas in M16 known as the``Pillars of Creation.'' We find a total of 1101 X-ray sources in the17'×17' ACIS-I field of view. Most of theX-ray sources are low-mass pre-main-sequence or high-mass main-sequencestars in this young cluster. A handful of hard X-ray sources in thepillars are spatially coincident with deeply embedded young stellarobjects seen in high-resolution near-infrared images recently obtainedwith the VLT (McCaughrean & Andersen). In this paper, we focus onthe 40 X-ray sources in and around pillars 1-4 at the heart of the EagleNebula. None of the X-ray sources are associated with the evaporatinggaseous globules (EGGs) first observed by Hester and coworkers) in HSTWFPC2 images of M16, implying either that the EGGs do not containprotostars or that the protostars have not yet become X-ray active.Eight X-ray counts are coincident with the Herbig-Haro object HH 216,implying logLX~30.0.

The cooling of atomic and molecular gas in DR21
Aims. We present an overview of a high-mass star formation regionthrough the major (sub-)mm, and far-infrared cooling lines to gaininsight into the physical conditions and the energy budget of themolecular cloud. Methods: We used the KOSMA 3 m telescope to map thecore (10'× 14') of the Galactic star-forming region DR21/DR21 (OH)in the Cygnus X region in the two fine structure lines of atomic carbon(C I ^3P_1-^3P0 and ^3P_2-^3P_1), in four mid-J transitionsof CO and 13CO, and in CS J=7-6. These observations werecombined with FCRAO J=1-0 observations of 13CO andC18O. Five positions, including DR21, DR21 (OH), and DR21FIR1, were observed with the ISO/LWS grating spectrometer in the [O I]63 and 145 μm lines, the [C II] 158 μm line, and four high-J COlines. We discuss the intensities and line ratios at these positions andapply the local thermal equilibrium (LTE) and non-LTE analysis methodsin order to derive physical parameters such as mass, density andtemperature. The CO line emission was modeled up to J=20. Results: From non-LTE modeling of the low- to high-J CO lines, we identify twogas components, a cold one at temperatures of T_kin˜ 30-40 K andone with T_kin˜ 80-150 K at a local clump density of about n(H2) ~10^4-106 cm-3. While the cold quiescent componentis massive, typically containing more than 94% of the mass, the warm,dense, and turbulent gas is dominated by mid- and high-J CO lineemission and its large line widths. The medium must be clumpy with avolume-filling of a few percent. The CO lines are found to be importantin cooling the cold molecular gas, e.g. at DR21 (OH). Near the outflowof the UV-heated source DR21, the gas cooling is dominated by lineemission of atomic oxygen and of CO. Atomic and ionised carbon play aminor role.

Deficit of Wide Binaries in the η Chamaeleontis Young Cluster
We have carried out a sensitive high-resolution imaging survey of starsin the young (6-8 Myr), nearby (97 pc) compact cluster around ηChamaeleontis to search for stellar and substellar companions. Our datawere obtained using the NACO adaptive optics system on the ESO VeryLarge Telescope (VLT). Given its youth and proximity, any substellarcompanions are expected to be luminous, especially in the near-infrared,and thus easier to detect next to their parent stars. Here, we presentVLT NACO adaptive optics imaging with companion detection limits for 17η Cha cluster members, and follow-up VLT ISAAC near-infraredspectroscopy for companion candidates. The widest binary detected is~0.2", corresponding to the projected separation 20 AU, despite oursurvey being sensitive down to substellar companions outside 0.3", andplanetary-mass objects outside 0.5". This implies that the stellarcompanion probability outside 0.3" and the brown dwarf companionprobability outside 0.5" are less than 0.16 with 95% confidence. Wecompare the wide binary frequency of η Cha to that of the similarlyaged TW Hydrae association and estimate the statistical likelihood thatthe wide binary probability is equal in both groups to be less than2×10-4. Even though the η Cha cluster is relativelydense, stellar encounters in its present configuration cannot accountfor the relative deficit of wide binaries. We thus conclude that thedifference in wide binary probability in these two groups providesstrong evidence for multiplicity properties being dependent onenvironment. In two appendices we derive the projected separationprobability distribution for binaries, used to constrain physicalseparations from observed projected separations, and summarizestatistical tools useful for multiplicity studies.

Binary stars in the Orion Nebula Cluster
We report on a high-spatial-resolution survey for binary stars in theperiphery of the Orion Nebula Cluster, at 5-15 arcmin (0.65-2 pc) fromthe cluster center. We observed 228 stars with adaptive optics systems,in order to find companions at separations of 0.13 arcsec-1.12 arcsec(60-500 AU), and detected 13 new binaries. Combined with the results ofPetr (1998), we have a sample of 275 objects, about half of which havemasses from the literature and high probabilities to be cluster members.We used an improved method to derive the completeness limits of theobservations, which takes into account the elongated point spreadfunction of stars at relatively large distances from the adaptive opticsguide star. The multiplicity of stars with masses >2Mȯ is found to be significantly larger than that oflow-mass stars. The companion star frequency of low-mass stars iscomparable to that of main-sequence M-dwarfs, less than half that ofsolar-type main-sequence stars, and 3.5 to 5 times lower than in theTaurus-Auriga and Scorpius-Centaurus star-forming regions. We find thebinary frequency of low-mass stars in the periphery of the cluster to bethe same or only slightly higher than for stars in the cluster core(<3 arcmin from θ^1C Ori). This is in contrast to theprediction of the theory that the low binary frequency in the cluster iscaused by the disruption of binaries due to dynamical interactions.There are two ways out of this dilemma: Either the initial binaryfrequency in the Orion Nebula Cluster was lower than in Taurus-Auriga,or the Orion Nebula Cluster was originally much denser and dynamicallymore active.

The Discovery of Diffuse X-Ray Emission in NGC 2024, One of the Nearest Massive Star-forming Regions
We analyzed deep 75 ks Chandra ACIS-I data of NGC 2024 with the aim ofsearching for diffuse X-ray emission in this most nearby (415 pc) ofmassive star-forming regions. After removing point sources, extendedemission was detected in the central circular region with a radius of0.5 pc, and it is spatially associated with this young massive stellarcluster. Its X-ray spectrum exhibits a very hard continuum (kT>8 keV)and shows signs of having a He-like Fe Kα line with a 0.5-7 keVabsorption-corrected luminosity of 2 × 1031 ergss-1. Undetected faint point sources, estimated from theluminosity function of the detected sources, contribute less than 10% tothis emission. Hence, the emission is truly diffuse in nature. Becauseof the proximity of NGC 2024 and the long exposure, this discovery isone of the strongest pieces of evidence in support of the existence ofdiffuse X-ray emission in massive star-forming regions.

Deep Near-Infrared Imaging toward the Vela Molecular Ridge C. II. New Protostars and Embedded Clusters in Vela C
We present new candidates for protostars and embedded clusters detectedin our deep near-infrared (J, H, KS) imaging survey of cloudC of the Vela Molecular Ridge (Vela C). We selected protostar candidateson the basis of positional coincidences with Midcourse Space Experiment(MSX) and/or IRAS sources and the slopes of their spectral energydistributions from 2 to 25 μm. For embedded clusters, we selectedthem on the basis of positional coincidence with MSX and/or IRAS sourcesand enhancement of stellar number densities. We identified 31 sources asprotostar candidates and 5 clusters as embedded clusters; 28 protostarcandidates and 3 embedded clusters were newly identified. Most of themare associated with the C18O clumps distributed throughoutVela C. We calculated the bolometric luminosities of the protostarcandidates by integrating the observed fluxes from near- tofar-infrared, and then estimated the mass range to be from 0.9 to 4.0Msolar, assuming their ages at the birth line. There is acorrelation between the number of members and the KSmagnitude of the brightest star in the embedded clusters.

Study of photon dominated regions in Cepheus B
Aims. The aim of the paper is to understand the emission from the photondominated regions in Cepheus B, estimate the column densities of neutralcarbon in bulk of the gas in Cepheus B and to derive constraints on thefactors which determine the abundance of neutral carbon relative toCO. Methods: .This paper presents 15 arcmin ×15 arcmin fullysampled maps of [C I] at 492 GHz and 12CO 4-3 observed withKOSMA at 1´ resolution. The new observations have been combinedwith the FCRAO 12CO 1-0, IRAM-30 m 13CO 2-1 andC18O 1-0 data, and far-infrared continuum data fromHIRES/IRAS. The KOSMA-τ spherical PDR model has been used tounderstand the [C I] and CO emission from the PDRs in Cepheus B and toexplain the observed variation of the relative abundances of bothC0 and CO. Results: .The emission from the PDRassociated with Cepheus B is primarily at V_LSR between -14 and -11 kms-1. We estimate about 23% of the observed [C II] emissionfrom the molecular hotspot is due to the ionized gas in the H II region.Over bulk of the material the C0 column density does notchange significantly, (2.0±1.4)×1017cm-2, although the CO column density changes by an order ofmagnitude. The observed C/CO abundance ratio varies between 0.06 and 4in Cepheus B. We find an anti-correlation of the observed C/CO abundanceratio with the observed hydrogen column density, which holds even whenall previous observations providing C/CO ratios are included. Here weshow that this observed variation of C/CO abundance with total columndensity can be explained only by clumpy PDRs consisting of an ensembleof clumps. At high H2 column densities high mass clumps, which exhibitlow C/CO abundance, dominate, while at low column densities, low massclumps with high C/CO abundance dominate.

Magnetic Fields in the Formation of Sun-Like Stars
We report high-angular-resolution measurements of polarized dustemission toward the low-mass protostellar system NGC 1333 IRAS 4A. Weshow that in this system the observed magnetic field morphology is inagreement with the standard theoretical models of the formation ofSun-like stars in magnetized molecular clouds at scales of a few hundredastronomical units; gravity has overcome magnetic support, and themagnetic field traces a clear hourglass shape. The magnetic field issubstantially more important than turbulence in the evolution of thesystem, and the initial misalignment of the magnetic and spin axes mayhave been important in the formation of the binary system.

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.

VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions
High-quality K-band spectra of strongly reddened point sources, deeplyembedded in (ultra-)compact H II, have revealed a population of 20 youngmassive stars showing no photospheric absorption lines, but sometimesstrong Brγ emission. The Brγ equivalent widths occupy a widerange (from about 1 to over 100 Å); the line widths of 100-200 kms-1 indicate a circumstellar rather than a nebular origin.The K-band spectra exhibit one or more features commonly associated withmassive young stellar objects (YSOs) surrounded by circumstellarmaterial: a very red colour (J-K)  2, CO bandhead emission,hydrogen emission lines (sometimes doubly peaked), and Fe II and/or MgII emission lines. The large number of objects in our sample allows amore detailed definition and thorough investigation of the properties ofthe massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) themassive YSO candidates are located in a region delimited by the OBzero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e]supergiants. The massive YSO distribution in the CMD suggests that themajority of the objects are of similar spectral type as the Herbig Bestars, but some of them are young O stars. The spectral properties ofthe observed objects do not correlate with the location in the CMD. TheCO emission must come from a relatively dense (˜ 1010cm-3) and hot (T˜ 2000-5000 K) region, sufficientlyshielded from the intense UV radiation field of the young massive star.The hydrogen emission is produced in an ionised medium exposed to UVradiation. The best geometrical solution is a dense and neutralcircumstellar disk causing the CO bandhead emission, and an ionisedupper layer where the hydrogen lines are produced. We present argumentsthat the circumstellar disk is more likely a remnant of the accretionprocess than the result of rapid rotation and mass loss such as inBe/B[e] stars.

An XMM-Newton view of the young open cluster NGC 6231. I. The catalogue
This paper is the first of a series dedicated to the X-ray properties ofthe young open cluster NGC 6231. Our data set relies on an XMM-Newtoncampaign of a nominal duration of 180 ks and reveals that NGC 6231 isvery rich in the X-ray domain too. Indeed, 610 X-ray sources aredetected in the present field of view, centered on the cluster core. Thelimiting sensitivity of our survey is approximately 6 ×10-15 erg cm-2 s-1 but clearly dependson the location in the field of view and on the source spectrum. Usingdifferent existing catalogues, over 85% of the X-ray sources could beassociated with at least one optical and/or infrared counterpart withina limited cross-correlation radius of 3´´ at maximum. Thesurface density distribution of the X-ray sources presents a slight N-Selongation. Once corrected for the spatial sensitivity variation of theEPIC instruments, the radial profile of the source surface density iswell described by a King profile with a central density of about 8sources per arcmin2 and a core radius close to 3.1 arcmin.The distribution of the X-ray sources seems closely related to theoptical source distribution. The expected number of foreground andbackground sources should represent about 9% of the detected sources,thus strongly suggesting that most of the observed X-ray emitters arephysically belonging to NGC 6231. Finally, beside a few bright but softobjects - corresponding to the early-type stars of the cluster - most ofthe sources are relatively faint (~5 × 10-15 ergcm-2 s-1) with an energy distribution peakedaround 1.0-2.0 keV.

The Southern Flanking Fields of the 25 Orionis Group
The stellar group surrounding the Be (B1 Vpe) star 25 Ori was discoveredto be a pre-main-sequence (PMS) population by the CIDA VariabilitySurvey of Orion and subsequent spectroscopy. We analyze Sloan DigitalSky Survey multiepoch photometry to map the southern extent of the 25Ori group and characterize its PMS population. We compare this group tothe neighboring Orion OB1a and OB1b subassociations and to active starformation sites (NGC 2068/NGC 2071) within the LDN 1630 dark cloud. Wefind that the 25 Ori group has a radius of 1.4d, corresponding to 8-11pc at the distances of Orion OB1a and OB1b. Given that thecharacteristic sizes of young open clusters are a few parsecs or less,this suggests that 25 Ori is an unbound association rather than an opencluster. Due to its PMS population having a low classical T Tauri starfraction (~10%), we conclude that the 25 Ori group is of comparable ageto the 11 Myr Orion OB1a subassociation.

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.

L-band (3.5 μm) IR-excess in massive star formation. II. RCW 57/NGC 3576
Context: .We present a JHKsL survey of the massive star forming regionRCW 57 (NGC 3576) based on L-band data at 3.5 μm taken with SPIREX(South Pole Infrared Explorer), and 2MASS JHKs data at 1.25-2.2 μm.This is the second of two papers, the first one concerning a similarJHKsL survey of 30 Doradus. Aims: .Colour-colour andcolour-magnitude diagrams are used to detect sources with infraredexcess. This excess emission is interpreted as coming from circumstellardisks, and hence gives the cluster disk fraction (CDF). Based on the CDFand the age of RCW 57, it is possible to draw conclusions on theformation and early evolution of massive stars. Methods: .Theinfrared excess is detected by comparing the locations of sources inJHKsL colour-colour and L vs. (K_s-L) colour-magnitude diagrams to thereddening band due to interstellar extinction. Results: .A totalof 251 sources were detected. More than 50% of the 209 sources includedin the diagrams have an infrared excess. Conclusions: .Comparisonwith other JHKsL surveys, including the results on 30 Doradus from thefirst paper, support a very high initial disk fraction (>80%) evenfor massive stars, although there is an indication of a possible fasterevolution of circumstellar disks around high mass stars. 33 sources onlyfound in the L-band indicate the presence of heavily embedded, massiveClass I protostars. We also report the detection of diffuse PAHsemission throughout the RCW 57 region.

Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.

Anomalous absorption in H2CO molecule
Snyder et al. (1969) detected H2 CO through its transition110 - 111 at 4.829 GHz in absorption in theinterstellar medium in a number of galactic and extragalactic sources(M17, W3, W3(OH position), W49, NGC 2024, DR 21, W43, W44, W51, Sgr A,Sgr B2, W33, NGC 6334, Cas A, and 3C 123). This transition ofH2 CO was found in anomalous absorption by Palmer et al.(1969) in the direction of four dark nebulae. In some objects, thistransition has however been detected in emission and even as a maserline (Forster et al. 1980; Whiteoak et al. 1983). Evans et al. (1970)reported detection of H2 CO molecule through its transition211 - 212 at 14.488 GHz in absorption in somecosmic objects. This transition was also found in anomalous absorptionby Evans et al. (1975). Since the transition 110 - 111 is considered asa unique probe of high density gas at low temperature, the study ofH2 CO in cosmic objects is of great importance. Garrison etal. (1975) investigated the problem of anomalous absorption of110 - 111 and 211 -212transitions of H2 CO where they accounted for 8 energy levelsconnected by 10 radiative transitions and considered a kinetictemperature of 5 - 20 K. They found weak anomalous absorption of110 - 111 and 211 - 212transitions of H2 CO.

Large Area Mapping at 850 μm. IV. Analysis of the Clump Distribution in the Orion B South Molecular Cloud
We present results from a survey of a 1300 arcmin2 region ofthe Orion B South molecular cloud, including NGC 2024, NGC 2023, and theHorsehead Nebula (B33), obtained using the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).Submillimeter continuum observations at 450 and 850 μm are discussed.Using an automated algorithm, 57 discrete emission features (``clumps'')are identified in the 850 μm map. The physical conditions withinthese clumps are investigated under the assumption that the objects arein quasi-hydrostatic equilibrium. The best-fit dust temperature for theclumps is found to be Td=18+/-4 K, with the exception ofthose associated with the few known far-infrared sources residing in NGC2024. The latter internally heated sources are found to be much warmer.In the region surrounding NGC 2023, the clump dust temperatures agreewith clump gas temperatures determined from molecular line excitationmeasurements of the CO molecule. The bounding pressure on the clumpslies in the range log(k-1P cm3K-1)=6.1+/-0.3. The cumulative mass distribution is steep atthe high-mass end, as is the stellar initial mass function. Thedistribution flattens significantly at lower masses, with a turnoveraround 3-10 Msolar.

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Observation and Astrometry data

Right ascension:05h41m43.00s
Apparent magnitude:99.9

Catalogs and designations:
Proper NamesFlame Nebula
Flame   (Edit)
NGC 2000.0NGC 2024

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