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Treasures of Orion.
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Methods for analysing structure in molecular clouds
We have previously reported a dimensionless measure, , which can bothquantify, and distinguish between, the extent to which a star cluster iscentrally concentrated, and the extent to which it contains small-scalesubclusters. is the ratio of the normalized correlation length, , (i.e.the mean projected separation between stars, divided by the overallradius of the cluster), to the mean length, , of the segments of aminimal spanning tree (MST) joining all star positions: .In this paper, we attempt to adapt the correlation-length method to thecharacterization of gas clouds, with a view to comparing directly thestructures of gas clouds and star clusters. We also compare the resultsof the correlation-length method with fractal dimensions estimated usingthe more familiar perimeter-area method whereby the lengths of closedcontours are plotted against the areas they enclose, on a log-log plot.We find that the normalized correlation length, when modified to dealwith pixellated grey-scale data, is a robust indicator of either centralconcentration or fractal subclustering of gas clouds, but cannotdistinguish between the two types of structure. It is, however,extremely reliable, easy to implement and works accurately at all scalesand over all dynamic ranges, even with poorly sampled data. Itimplicitly incorporates edge effects, so all the data in the completecloud are used, and it therefore provides a useful method for comparingthe structures of molecular clouds and star clusters.The normalized correlation length produces comparable results to theperimeter-area method when used on molecular cloud data. However, theperimeter-area method is unable to distinguish the degree of clusteringin three-dimensional objects with fractal dimensions greater than 2.0.It also suffers from measurement noise and lack of objectivity,particularly if only a few contours are selected for analysis. It cannotbe used to compare clouds with star clusters.It is not found possible to construct an MST algorithm which worksreliably for grey-scale data and is immune to scaling problems. Thepreviously reported parameter is therefore not useful when consideringgas clouds.

Early Evolution of Stellar Groups and Clusters: Environmental Effects on Forming Planetary Systems
This paper studies the dynamical evolution of young groups/clusters,with N=100-1000 members, from their embedded stage out to ages of ~10Myr. We use N-body simulations to explore how their evolution depends onthe system size N and the initial conditions. Motivated by recentobservations suggesting that stellar groups begin their evolution withsubvirial speeds, this study compares subvirial starting states withvirial starting states. Multiple realizations of equivalent cases (100simulations per initial condition) are used to build up a robuststatistical description of these systems, e.g., the probabilitydistribution of closest approaches, the mass profiles, and theprobability distribution for the radial location of cluster members.These results provide a framework from which to assess the effects ofgroups/clusters on the processes of star and planet formation and tostudy cluster evolution. The distributions of radial positions are usedin conjunction with the probability distributions of the expectedfar-ultraviolet (FUV) luminosities (calculated here as a function ofcluster size N) to determine the radiation exposure of circumstellardisks. The distributions of closest approaches are used in conjunctionwith scattering cross sections (calculated here as a function of stellarmass using ~105 Monte Carlo scattering experiments) todetermine the probability of disruption for newly formed solar systems.We use the nearby cluster NGC 1333 as a test case in this investigation.The main conclusion of this study is that clusters in this size rangehave only a modest effect on forming planetary systems. The interactionrates are low, so that the typical solar system experiences a singleencounter with closest approach distance b~1000 AU. The radiationexposure is also low, with median FUV flux G0~900 (1.4 ergss-1 cm-2), so that photoevaporation ofcircumstellar disks is only important beyond 30 AU. Given the lowinteraction rates and modest radiation levels, we suggest that solarsystem disruption is a rare event in these clusters.

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.

Large-scale molecular shocks in galaxies: the SiO interferometer map of IC 342
We present the first high-resolution (5.6 arcsec×5.1 arcsec )images of the emission of silicon monoxide (SiO) in the nucleus of thenearby spiral IC 342, obtained with the IRAM Plateaude Bure Interferometer (PdBI). Using a two-field mosaic, we havesimultaneously mapped the emission of the SiO(v=0, J=2-1) andH13CO+(J=1-0) lines in a region of 0.9 kpc× 1.3 kpc (RA × Dec) centered around the nucleus ofIC 342. The bulk of the emission in the two linescomes from a ˜290 pc spiral arm located to the North and a centralcomponent that forms the southern ridge of a {r˜80} pc nuclear ringthat was identified in other interferometer maps of the galaxy. Wedetect continuum emission at 86.8 GHz in a ˜80-180 pc centralsource. The continuum emission, dominated by thermal free-freebremsstrahlung, is mostly anticorrelated with the observed distributionof SiO clouds. The SiO-to-H13CO+ intensity ratiois seen to increase by an order of magnitude from the nuclear ring (0.3) to the spiral arm ( 3.3). Furthermore the gas kinematics showsignificant differences between SiO and H13CO+over the spiral arm, where the linewidths of SiO are a factor of 2larger than those of H13CO+. The average abundanceof SiO in the inner {r˜320} pc of IC 342 isX(SiO) ≳2×10-10. This shows that shock chemistryis at work in the inner molecular gas reservoir of IC342. To shed light on the nature of shocks in IC342, we have compared the emission of SiO with another tracerof molecular shocks: the emission of methanol (CH3OH). We find that thesignificant difference of the abundance of SiO measured between thespiral arm (X(SiO) a few 10-9) and the nuclear ring (X(SiO) 10-10) is not echoed by a comparable variation in theSiO-to-CH3OH intensity ratio. This implies that the typical shockvelocities should be similar in the two regions. In contrast, thefraction of shocked molecular gas should be 5-7 times larger in thespiral arm (up to 10% of the available molecular gas mass over the armregion) compared to the nuclear ring. In the light of these results, werevise the validity of the various scenarios that have been proposed toexplain the onset of shock chemistry in galaxies and study theirapplicability to the nucleus of IC 342. We concludethat the large-scale shocks revealed by the SiO map of IC342 are mostly unrelated to star formation and arise insteadin a pre-starburst phase. Shocks are driven by cloud-cloud collisionsalong the potential well of the IC 342 bar. Thegeneral implications for the current understanding of galaxy evolutionare discussed.

Bright X-Ray Flares in Orion Young Stars from COUP: Evidence for Star-Disk Magnetic Fields?
We have analyzed a number of intense X-ray flares observed in theChandra Orion Ultradeep Project (COUP), a 13 day observation of theOrion Nebula Cluster (ONC), concentrating on the events with the higheststatistics (in terms of photon flux and event duration). Analysis of theflare decay allows to determine the physical parameters of the flaringstructure, particularly its size and (using the peak temperature andemission measure of the event) the peak density, pressure, and minimumconfining magnetic field. A total of 32 events, representing the mostpowerful ~=1% of COUP flares, have sufficient statistics and aresufficiently well resolved to grant a detailed analysis. A broad rangeof decay times are present in the sample of flares, withτlc (the 1/e decay time) ranging from 10 to 400 ks. Peakflare temperatures are often very high, with half of the flares in thesample showing temperatures in excess of 100 MK. Significant sustainedheating is present in the majority of the flares. The magneticstructures that are found, from the analysis of the flare's decay, toconfine the plasma are in a number of cases very long, with semilengthsup to ~=1012 cm, implying the presence of magnetic fields ofhundreds of G (necessary to confine the hot flaring plasma) extending tocomparable distance from the stellar photosphere. These very large sizesfor the flaring structures (length L>>R*) are not foundin more evolved stars, where, almost invariably, the same type ofanalysis results in structures with L<=R*. As the majorityof young stars in the ONC are surrounded by disks, we speculate that thelarge magnetic structures that confine the flaring plasma are actuallythe same type of structures that channel the plasma in themagnetospheric accretion paradigm, connecting the star's photospherewith the accretion disk.

Which Are the Youngest Protostars? Determining Properties of Confirmed and Candidate Class 0 Sources by Broadband Photometry
We searched the literature to obtain a complete list of known Class 0sources. A list of 95 confirmed or candidate objects was compiled. Tothe best of our knowledge, all published broadband observations from 1μm to 3.5 mm have been collected and are assembled in a catalog.These data were used to determine physical properties (Tbol,Lbol, Lsmm/Lbol, Menv) andfor a uniform classification. Fifty sources possess sufficientobservational data and are classified as Class 0 or Class 0/1 objects.The source properties are compared with different evolutionary models toinfer ages and masses, and their correlations are investigated. About25% of the sources are found to be in a quiet accretion phase or possessa significantly different time evolution of the accretion rate than theaverage. In Taurus, with its isolated star formation mode, this seemsespecially to be the case.

High-Mass Star Formation. I. The Mass Distribution of Submillimeter Clumps in NGC 7538
We present submillimeter continuum maps at 450 and 850 μm of a12'×8' region of the NGC 7538 high-massstar-forming region, made using the Submillimeter Common-User BolometerArray (SCUBA) on the James Clerk Maxwell Telescope. We used an automatedclump-finding algorithm to identify 67 clumps in the 450 μm image and77 in the 850 μm image. Contrary to previous studies, we find apositive correlation between high spectral index, α, and highsubmillimeter flux, with the difference being accounted for by differenttreatments of the error beam. We interpret the higher spectral index atsubmillimeter peaks as a reflection of elevated dust temperature,particularly when there is an embedded infrared source, although it mayalso reflect changing dust properties. The clump mass-radiusrelationship is well fitted by a power law of the form M~R-xwith x=1.5-2.1, consistent with theories of turbulently supportedclumps. According to our most reliable analysis, the high-mass end(~100-2700 Msolar) of the submillimeter clump mass functionin NGC 7538 follows a Salpeter-like power law with index 2.0+/-0.3. Thisresult agrees well with similar studies of lower mass regions ρ Ophand Orion B. We interpret the apparent invariance of the shape of theclump mass function over a broad range of parent cloud masses asevidence for the self-similarity of the physical processes thatdetermine it. This result is consistent with models that suggest thatturbulent fragmentation, acting at early times, is sufficient to set theclump mass function.

Centrally Condensed Collapse of Starless Cores
Models of self-gravitating gas in the early stages of pressure-freecollapse are compared for initial states that are equilibrium layers,cylinders, and ``Bonnor-Ebert'' spheres. For each geometrical case thedensity profile has an inner region of shallow slope surrounded by anouter region of steep slope, and the profile shape during early collapseremains similar to the profile shape of the initial equilibrium. Thetwo-slope density structure divides the spherical collapse history intoa starless infall phase and a protostellar accretion phase. Thesimilarity of density profiles implies that Bonnor-Ebert fits toobserved column density maps may not distinguish spherical cores fromoblate or prolate cores and may not distinguish static cores fromcollapsing cores. The velocity profiles discriminate better than thedensity profiles between initial geometries and between collapse ages.The infall velocity generally has a subsonic maximum value, which isapproximately equal to the initial velocity dispersion times the ratioof collapse age to central free-fall time. Observations of starless-coreline profiles constrain collapse models. Collapse from initial statesthat are strongly condensed and slightly prolate is consistent with``infall asymmetry'' observed around starless cores and is moreconsistent than collapse from initial states that are weakly condensedand/or oblate. Spherical models match observed inward speeds of0.05-0.09 km s-1 over 0.1-0.2 pc if the collapse has atypical age of 0.3-0.5 free-fall times and if it began from a centrallycondensed state that was not in stable equilibrium. In a collapsingcore, optically thin line profiles should broaden and develop a two-peakstructure, as seen in L1544, once the typical infall velocity approachesthe molecular velocity dispersion or when the collapse age exceeds ~0.4free-fall times, for typical parameters, independent of depletion.

The Outburst of V1647 Orionis Revealed by Spitzer
We present Spitzer Space Telescope observations of V1647 Ori, theoutbursting source lighting McNeil's Nebula, taken near the optical peakof the outburst in early March 2004. The source is easily detected inall Spitzer imaging bands from 3.6 to 70 μm. The fluxes at allwavelengths are roughly a factor of 15 brighter than pre-outburstlevels; we measure a bolometric luminosity of 44 Lsolar. Weposit that this event is due to an increase in the accretion luminosityof the source. Simple models of an accretion disk plus a tenuousenvelope can qualitatively explain the observed pre- and post-outburstspectral energy distributions. The accretion activity implied by ourresults indicates that the outburst may be intermediate between FUor-and EXor-type events. We also report the discovery of a previouslyunknown mid-infrared counterpart to the nearby Herbig-Haro object HH 22.

Photometric Accretion Signatures Near the Substellar Boundary
Multiepoch imaging of the Orion equatorial region by the Sloan DigitalSky Survey has revealed that significant variability in the bluecontinuum persists into the late-M spectral types, indicating thatmagnetospheric accretion processes occur below the substellar boundaryin the Orion OB1 association. We investigate the strength of theaccretion-related continuum veiling by comparing the reddening-invariantcolors of the most highly variable stars against those of main-sequenceM dwarfs and evolutionary models. A gradual decrease in the g-bandveiling is seen for the cooler and less massive members, as expected fora declining accretion rate with decreasing mass. We also see evidencethat the temperature of the accretion shock decreases in the very lowmass regime, reflecting a reduction in the energy flux carried by theaccretion columns. We find that the near-IR excess attributed tocircumstellar disk thermal emission drops rapidly for spectral typeslater than M4. This is likely due to the decrease in color contrastbetween the disk and the cooler stellar photosphere. Since accretion,which requires a substantial stellar magnetic field and the presence ofa circumstellar disk, is inferred for masses down to 0.05Msolar, we surmise that brown dwarfs and low-mass stars sharea common mode of formation.

The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1b
We present results of a large-scale, multiepoch optical survey of theOrion OB1 association, carried out with the QUEST camera at theVenezuela National Astronomical Observatory. We identify for the firsttime the widely spread low-mass, young population in the Ori OB1a andOB1b subassociations. Candidate members were picked up by theirvariability in the V band and position in color-magnitude diagrams. Weobtained spectra to confirm membership. In a region spanning ~68deg2, we found 197 new young stars; of these, 56 are locatedin the Ori OB1a subassociation and 141 in Ori OB1b. The spatialdistribution of the low-mass young stars is spatially coincident withthat of the high-mass members but suggests a much sharper edge to theassociation. Comparison with the spatial extent of molecular gas andextinction maps indicates that the subassociation Ori OB1b isconcentrated within a ringlike structure of radius ~2°(~15 pc at 440pc), centered roughly on the star ɛ Ori in the Orion belt. The ringis apparent in 13CO and corresponds to a region with anextinction AV>=1. The stars exhibiting strong Hαemission, an indicator of active accretion, are found along this ring,whereas the center is populated with weak Hα-emitting stars. Incontrast, Ori OB1a is located in a region devoid of gas and dust. Weidentify a grouping of stars within a ~3 deg2 area located inOri OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Bestar V346 Ori is also associated with this grouping, which could be anolder analog of σ Ori. Using several sets of evolutionary tracks,we find an age of 7-10 Myr for Ori OB1a and of ~4-6 Myr for Ori OB1b,consistent with previous estimates from OB stars. Indicators such as theequivalent width of Hα and near-IR excesses show that the numberof accreting low-mass stars decreases sharply between Ori OB1b and OriOB1a. These results indicate that although a substantial fraction ofaccreting disks remain at ages ~5 Myr, inner disks are essentiallydissipated by 10 Myr.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by Centro deInvestigaciones de Astronomía (CIDA) for the Ministerio deCiencia y Tecnología, and at the Fred Lawrence WhippleObservatory (FLWO) of the Smithsonian Institution.Based on observations obtained at the 3.5 m WIYN Telescope. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory (NOAO).

How to identify the youngest protostars
We study the transition from a prestellar core to a Class 0 protostar,using SPH to simulate the dynamical evolution, and a Monte Carloradiative transfer code to generate the SED and isophotal maps. For aprestellar core illuminated by the standard interstellar radiationfield, the luminosity is low and the SED peaks at ~190 μ m. Once aprotostar has formed, the luminosity rises (due to a growingcontribution from accretion onto the protostar) and the peak of the SEDshifts to shorter wavelengths (80~to 100 μ m). However, by the end ofthe Class 0 phase, the accretion rate is falling, the luminosity hasdecreased, and the peak of the SED shifts back towards longerwavelengths (90 to 150 μ m). In our simulations, the density ofmaterial around the protostar remains sufficiently high well into theClass 0 phase that the protostar only becomes visible in the NIR if itis displaced from the centre dynamically. Raw submm/mm maps of Class 0protostars tend to be much more centrally condensed than those ofprestellar cores. However, when convolved with a typical telescope beam,the difference in central concentration is less marked, although theClass 0 protostars appear more circular. Our results suggest that, if acore is deemed to be prestellar on the basis of having no associatedIRAS source, no cm radio emission, and no outflow, but it has a circularappearance and an SED which peaks at wavelengths below ~170 μ m, itmay well contain a very young Class 0 protostar.

Enhanced X-ray variability from V1647 Ori, the young star in outburst illuminating McNeil's Nebula
We report a 38 ks X-ray observation of McNeil's Nebula obtained withXMM-Newton on 2004 April 4. V1647 Ori, the young star in outburstilluminating McNeil's Nebula, is detected with XMM-Newton and appearsvariable in X-rays. We investigate the hardness ratio variability andtime variations of the event energy distribution with quantile analysis,and show that the large increase of the count rate from V1647 Oriobserved during the second half of the observation is not associatedwith any large plasma temperature variations as for typical X-ray flaresfrom young low-mass stars. X-ray spectral fitting shows that the bulk (75%) of the intrinsic X-ray emission in the 0.5-8 keV energy band comesfrom a soft plasma component, with kT_soft=0.9 keV (0.7-1.1 keV, at the90% confidence limit), reminiscent of the X-ray spectrum of theclassical T Tauri star TW Hya, for which X-ray emission is believed tobe generated by an accretion shock onto the photosphere of a low-massstar. The hard plasma component, with kThard=4.2 keV (3.0-6.5keV), contributes 25% of the total X-ray emission, and can beunderstood only in the framework of plasma heating sustained by magneticreconnection events. We find a hydrogen column density ofN_H=4.1×1022 cm-2(3.5{-}4.7×1022 cm-2), which points out asignificant excess of hydrogen column density compared to the valuederived from optical/IR observations, consistent with the picture of therise of a wind/jet unveiled from ground optical spectroscopy. The X-rayflux observed with XMM-Newton ranges from roughly the flux observed byChandra on 2004 March 22 (i.e. 10 times greater than the pre-outburstX-ray flux) to a value two times greater than that caught by Chandra on2004 March 7 (i.e. 200 times greater than the pre-outburst X-ray flux).The X-ray variability of V1647 Ori in outburst is clearly enhanced. Wehave investigated the possibility that V1647 Ori displays a periodicvariation in X-ray brightness as suggested by the combinedChandra+XMM-Newton data set. Assuming that the X-ray flux density isperiodic, the folding of the two Chandra observed X-ray flux densitieswith the XMM-Newton ones leads to three periodic X-ray light curvesolutions. Our best period candidate is 0.72 day, which corresponds tothe time scale of the Keplerian rotation at a distance of 1 and 1.4stellar radius for a one solar mass star aged of 0.5 and 1 Myrs,respectively. We propose that the emission measure, i.e. the observedX-ray flux, is modulated by the Keplerian rotation of the inner part ofthe V1647 Ori accretion disk.

X-Ray Flares from a Late B-Type Star, HD 38563S, in NGC 2068 Observed with Chandra X-Ray Observatory
A report is made on the Chandra archival data analysis of NGC 2068,which is one of the nearest (˜ 400 pc) and X-ray active star-formingregions, with an estimated age of 105‑6yr. Theobservation was made for 100ks. Using the wavdetect tool, 152 X-raysources were detected, of which 52 had counterparts in the 2MASS survey.One of the brightest X-ray sources is apparently identified with a V =10.42 late B-type main-sequence star, HD 38563S. During the observation,it exhibited two X-ray flares, with the 0.5-8.0keV luminosity reaching˜ 1031.6 erg s-1. The flare X-rays evolved ona time scale of several hours, and showed a thermal spectrum oftemperature of ˜ 5 keV. These results suggests that this star formsan active binary, although contamination by a T Tauri star also remainsan alternative possibility.

An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula
Young, low-mass stars are luminous X-ray sources whose powerful X-rayflares may exert a profound influence over the process of planetformation. The origin of the X-ray emission is uncertain. Although many(or perhaps most) recently formed, low-mass stars emit X-rays as aconsequence of solar-like coronal activity, it has also been suggestedthat X-ray emission may be a direct result of mass accretion onto theforming star. Here we report X-ray imaging spectroscopy observationswhich reveal a factor ~50 increase in the X-ray flux from a young starthat is at present undergoing a spectacular optical/infrared outburst(this star illuminates McNeil's nebula). The outburst seems to be due tothe sudden onset of a phase of rapid accretion. The coincidence of asurge in X-ray brightness with the optical/infrared eruptiondemonstrates that strongly enhanced high-energy emission from youngstars can occur as a consequence of high accretion rates. We suggestthat such accretion-enhanced X-ray emission from erupting young starsmay be short-lived, because intense star-disk magnetosphericinteractions are quenched rapidly by the subsequent flood of newmaterial onto the star.

Population and dynamical state of the η Chamaeleontis sparse young open cluster
We report new results in our continuing study of the unique compact (1pc extent), nearby (d= 97 pc), young (t= 9 Myr) stellar clusterdominated by the B9 star η Chamaeleontis. An optical photometricsurvey spanning 1.3 × 1.3 pc adds two M5-M5.5 weak-lined T Tauri(WTT) stars to the cluster inventory which is likely to be significantlycomplete for primaries with masses M > 0.15 Msolar. Thecluster now consists of 17 primaries and ~=9 secondaries lying within100 au of their primaries. The apparent distribution of 9:7:1single:binary:triple systems shows 2-4 times higher multiplicity than inthe field main-sequence stars, and is comparable to that seen in otherpre-main-sequence populations. The initial mass function (IMF) isconsistent with that of rich young clusters and field stars. Byextending the cluster IMF to lower masses, we predict 10-14 additionallow-mass stars with 0.08 < M < 0.15 Msolar and 10-15brown dwarfs with 0.025 < M < 0.08 Msolar remain to bediscovered. The η Cha cluster extends the established stellardensity and richness relationship for young open clusters. The radialdistribution of stars is consistent with an isothermal sphere, but masssegregation is present with >50 per cent of the stellar mass residingin the inner 6 arcmin (0.17 pc). Considering that the η Cha clusteris sparse, diffuse and young, the cluster may be an ideal laboratory fordistinguishing between mass segregation that is primordial in nature, orarising from dynamical interaction processes.

The V1647 Orionis (IRAS 05436-0007) Protostar and Its Environment
We present Sloan Digital Sky Survey (SDSS) and United States NavalObservatory (USNO) observations of the V1647 Ori protostar and itssurrounding field near NGC 2068. V1647 Ori, the likely driving sourcefor HH 23, brightened significantly in 2003 November. Analysis of SDSSimaging acquired in 1998 November and 2002 February during the quiescentstate, recent USNO photometry, and published Two Micron All Sky Survey(2MASS) and Gemini data show that the color changes associated withbrightening suggest an EX Lupi type (EXor) outburst rather than a simpledust-clearing event.

Collapse and Fragmentation in Finite Sheets
We present two-dimensional simulations of finite, self-gravitatinggaseous sheets. Unlike the case of infinite sheets, such configurationsdo not constitute equilibrium states but instead are subject to globalcollapse unless countered by pressure forces or rotation. The initialeffect of finite geometry is to promote concentrations of material atthe edges of the sheet. If the sheet is not perfectly circular,gravitational focusing results in enhanced concentrations of mass. Inthe second-most simple geometry, that of an elliptical outer boundary,the general result is collapse to a filamentary structure with thedensest concentrations of mass at the ends of the filament. We suggestthat these simple calculations have interesting implications for thegravitational evolution of overall molecular cloud structure,envisioning that such clouds might originate as roughly sheetlikesections of gas accumulated as a result of large-scale flows in thelocal interstellar medium. We show some examples of local clouds withoverall filamentary shape and denser concentrations of mass and starclusters near the ends of the overall extended structure, suggestive ofour simple ellipse collapse calculations. We suggest thatcluster-forming gas is often concentrated as a result of gravity actingon irregular boundaries; this mechanism can result in very rapid infallof gas, which may be of importance to the formation of massive stars.This picture suggests that much of the supersonic ``turbulence''observed in molecular clouds might be gravitationally generated. Ourresults may provide impetus for further theoretical explorations ofglobal gravitational effects in molecular clouds and their implicationsfor generating the substructure needed for fragmentation into stars andclusters.

X-Ray and Infrared Observations of Embedded Young Stars in L1630
The HH 24-26 star-forming region in the Lynds 1630 (L1630) dark cloud inOrion contains a remarkable collection of rare Class 0 and Class Iprotostars, collimated molecular and ionized jets, and a luminous butspatially unresolved ASCA X-ray source. To study the X-ray properties ofthe embedded protostar population of that region, we have obtained adeep X-ray image with the ACIS-S camera on board the Chandra X-RayObservatory. A number of Hα emission-line objects were detected inthe areas surrounding HH 24-26, of which the weak-line T Tauri star SSV61 was the brightest source, at a steady luminosity ofLX(0.3-10keV)=1031.9 ergs s-1. TwoClass I protostars aligned with optical jets in HH 24, SSV 63E, and SSV63W, were also detected, as was the continuum radio source SSV 63NE,which is very likely an extreme Class I or Class 0 object. We observedno X rays from the Class 0 protostars HH 24-MMS and HH 25-MMS, nor anyfrom regions of the cloud bounded by HH 25 and HH 26, at a 2 σupper limit of LX~1030.0 ergs s-1. HH26-IR, the Class I object thought to be the origin of the HH 26 flow,was not detected. Near-infrared spectroscopy obtained at the NASA IRTFreveals 3 μm ice bands in the spectra of SSV 59, 63E, 63W, and HH26-IR, and 2.3 μm CO overtone absorption bands for SSV 61. SSV 60,which lies astride one end of the great arc of nebulosity forming HH 25,exhibits a deep infrared ice band and CO absorption but is not an X-raysource and is most likely a distant background giant of late spectraltype.

MCNeil's Nebula in Orion: The Outburst History
We present a sequence of I-band images obtained at the Venezuela 1 mSchmidt telescope during the outburst of the nebula recently discoveredby J. W. McNeil in the Orion L1630 molecular cloud. We derive photometryspanning the preoutburst state and the brightening itself, which is aunique record including 14 epochs and spanning a timescale of ~5 years.We constrain the beginning of the outburst at some time between 2003October 28 and November 15. The light curve of the object at the vertexof the nebula, the likely exciting source of the outburst, reveals thatit has brightened ~5 mag in about 4 months. The timescale for the nebulato develop is consistent with the light-travel time, indicating that weare observing light from the central source scattered by the ambientcloud into the line of sight. We also show recent FLWO opticalspectroscopy of the exciting source and of the nearby HH 22. Thespectrum of the source is highly reddened; in contrast, the spectrum ofHH 22 shows a shock spectrum superposed on a continuum, most likely theresult of reflected light from the exciting source reaching the HHobject through a much less reddened path. The blue portion of thisspectrum is consistent with an early B spectral type, similar to theearly outburst spectrum of the FU Orionis variable star V1057 Cygni; weestimate a luminosity of L~219 Lsolar. The eruptive behaviorof McNeil's Nebula, its spectroscopic characteristics and luminosity,suggest that we may be witnessing an FU Ori event on its way to maximum.By further monitoring this object, we will be able decide whether or notit qualifies as a member of this rare class of objects.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by CIDA for theMinisterio de Ciencia y Tecnología, and at the Fred LawrenceWhipple Observatory (FLWO) of the Smithsonian Institution.

IRAS 05436-0007 and the Emergence of MCNeil's Nebula
We present a study of McNeil's Nebula, a newly appeared reflectionnebula in the L1630 cloud, together with photometry and spectroscopy ofits source. New IR photometry compared with earlier Two Micron All SkySurvey data shows that the star has brightened by about 3 mag in thenear-infrared, changing its location in a J-H/H-K' diagramprecisely along a reddening vector. A Gemini Near InfraRed Imager K-bandspectrum shows strong CO band head emission, and Brγ is inemission, indicative of strong accretion. A Gemini Multi-ObjectSpectrograph optical spectrum shows only a red, heavily veiledcontinuum, with Hα strongly in emission and displaying apronounced P Cygni profile, with an absorption trough reachingvelocities up to 600 km s-1. This implies significant massloss in a powerful wind. However, no evidence is found for any shocks,as commonly seen in collimated outflows from young stars. Apparently theeruption has dispersed a layer of extinction, and this, together withthe intrinsic brightening of the IRAS source, has allowed an earlieroutflow cavity to be flooded with light, thus creating McNeil's Nebula.

A Near-Infrared (JHK) Survey of the Vicinity of the H II Region NGC 7538: Evidence for a Young Embedded Cluster
We describe the results of two near-infrared (K-band) imaging surveysand a three-color (JHK) survey of the vicinity of NGC 7538. The limitingmagnitudes are K~=16.5 and 17.5 mag for the K-band surveys and K~=15 magfor the JHK survey. We identify more than 2000 and 9000 near-infrared(NIR) sources on the images of the two K-band surveys and 786 NIRsources in the JHK survey. From color-color diagrams, we derive areddening law for background stars and identify 238 stars with NIRexcesses. Contour maps indicate a high-density peak coincident with aconcentration of stars with NIR excesses. We identify this peak as ayoung embedded cluster and confirm this result with the K-bandluminosity function (KLF), color histograms, and color-magnitudediagrams. The center of the cluster is atR.A.=23h13m39s34,decl.=61deg29'18.9". The cluster radius is ~3',~2.5 pc for an adopted distance d~=2.8 kpc. For d=2.8 kpc and reddeningEJ-K=0.55 mag, the slope of the logarithmic KLF of thecluster, s~0.32+/-0.03, agrees well with previous results for L1630(s=0.34) and M17 (s=0.26) found by C. Lada, E. Lada, and coworkers.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.

Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data Release
The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope haveobtained five-band optical-wavelength imaging near the Galactic planeoutside of the nominal survey boundaries. These additional data wereobtained during commissioning and subsequent testing of the SDSSobserving system, and they provide unique wide-area imaging data inregions of high obscuration and star formation, including numerous youngstellar objects, Herbig-Haro objects, and young star clusters. Becausethese data are outside the survey regions in the Galactic caps, they arenot part of the standard SDSS data releases. This paper presents imagingdata for 832 square degrees of sky (including repeats), in thestar-forming regions of Orion, Taurus, and Cygnus. About 470deg2 are now released to the public, with the remainder tofollow at the time of SDSS Data Release 4. The public data in Orioninclude the star-forming region NGC 2068/NGC 2071/HH 24 and a large partof Barnard's loop.

A study of high velocity molecular outflows with an up-to-date sample
A statistical study of the properties of molecular outflows is performedbased on an up-to-date sample. 391 outflows were identified in publishedarticles or preprints before February 28, 2003. The parameters ofposition, morphology, mass, energy, outflow dynamics and central sourceluminosity are presented for each outflow source. Outflow lobe polarityis known for all the sources, and 84% are found to be bipolar. Thesources are divided into low mass and high mass groups according toeither the available bolometric luminosity of the central source or theoutflow mass. The pace of discovery of outflows over the past sevenyears has increased much more rapidly than in previous periods. Surveysfor outflows are still continuing. The number of high-mass outflowsdetected (139) has considerably increased, showing that they arecommonly associated with massive as well as low mass stars. Energeticmass ejection may be a common aspect of the formation of high mass aswell as low mass stars. Outflow masses are correlated strongly withbolometric luminosity of the center sources, which was obtained for thefirst time. There are also correlations between the central sourceluminosity and the parameters of mechanical luminosity and the thrust orforce necessary to drive the outflow. The results show that flow mass,momentum and energy depend on the nature of the central source. Despitetheir similarity, there are differences between the high mass and lowmass outflows. Low mass outflows are more collimated than high massoutflows. On average, the mass of high mass sources can be more than twoorders of magnitude larger than those of low mass outflows. The relationbetween flow mass and dynamical time appears to differ for the two typesof outflows. Low mass sources make up 90% of outflows associated with HHobjects while high mass outflows make up 61% of the sources associatedwith H_2O masers. Sources with characteristics of collapse or infallcomprise 12% of the entire outflow sample. The spatial distribution ofthe outflow sources in the Galaxy is presented and the local occurrencerate is compared with the stellar birth rate.Tables 1a and 1b are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/503

Radiative transfer models of non-spherical prestellar cores
We present 2D Monte Carlo radiative transfer simulations of prestellarcores. We consider two types of asymmetry: disk-like asymmetry, in whichthe core is denser towards the equatorial plane than towards the poles;and axial asymmetry, in which the core is denser towards the south polethan the north pole. In both cases the degree of asymmetry ischaracterized by the ratio e between the maximum optical depth from thecentre of the core to its surface and the minimum optical depth from thecentre of the core to its surface. We limit our treatment here to mildasymmetries with e = 1.5 and 2.5 . We consider both cores which areexposed directly to the interstellar radiation field and cores which areembedded inside molecular clouds.The SED of a core is essentially independent of the viewing angle, aslong as the core is optically thin. However, the isophotal maps dependstrongly on the viewing angle. Maps at wavelengths longer than the peakof the SED (e.g. 850 μm) essentially trace the column-density. Thisis because at long wavelengths the emissivity is only weakly dependenton temperature, and the range of temperature in a core is small(typically T_max/T_min  2). Thus, for instance, cores withdisk-like asymmetry appear elongated when mapped at 850 μm from closeto the equatorial plane. However, at wavelengths near the peak of theSED (e.g. 200 μm), the emissivity is more strongly dependent on thetemperature, and therefore, at particular viewing angles, there arecharacteristic features which reflect a more complicated convolution ofthe density and temperature fields within the core.These characteristic features are on scales 1/5 to 1/3 of the overallcore size, and so high resolution observations are needed to observethem. They are also weaker if the core is embedded in a molecular cloud(because the range of temperature within the core is then smaller), andso high sensitivity is needed to detect them. Herschel, to be launchedin 2007, will in principle provide the necessary resolution andsensitivity at 170 to 250 μm.

The infrared properties of the new outburst star IRAS 05436-0007 in quiescent phase
We compiled and investigated the infrared/sub-mm/mm SED of the newoutburst star IRAS 05436-0007 in quiescent phase. The star is aflat-spectrum source, with an estimated total luminosity ofLbol ≈ 5.6 Lsun, typical of low-mass T Tauristars. The derived circumstellar mass of 0.5 Msun is ratherhigh among low-mass YSOs. The observed SED differs from the SEDs oftypical T Tauri stars and of 4 well-known EXors, and resembles more theSEDs of FU Orionis objects indicating the presence of a circumstellarenvelope. IRAS 05436-0007 seems to be a class II source with an age ofapproximately 4 × 105 yr. In this evolutionary stage anaccretion disk is already fully developed, though a circumstellarenvelope may also be present. Observations of the present outburst willprovide additional knowledge on the source.

Chandra observation of an unusually long and intense X-ray flare from a young solar-like star in M 78
LkHα 312 has been observed serendipitously withthe ACIS-I detector on board the Chandra X-ray Observatory with a 26 hcontinuous exposure. This Hα emission line star belongs toM 78 (NGC 2068), one of thestar-forming regions of the Orion B giant molecular cloud at a distanceof 400 pc. From the optical and the near-infrared (NIR) data, we showthat LkHα 312 is a pre-main sequence (PMS) low-mass star with aweak NIR excess. This genuine T Tauri star displayed an X-ray flare withan unusually long rise phase (˜8 h). The X-ray emission was nearlyconstant during the first 18 h of the observation, and then increased bya factor of 13 during a fast rise phase (˜2 h), and reached a factorof 16 above the quiescent X-ray level at the end of a gradual phase(˜6 h) showing a slower rise. To our knowledge this flare, with˜0.4-0.5 cts s-1, has the highest count rate observed sofar with Chandra from a PMS low-mass star. By chance, the sourceposition, 8.2 arcmin off-axis, protected this observation from pile-up.We make a spectral analysis of the X-ray emission versus time, showingthat the plasma temperature of the quiescent phase and the flare peakreaches 29 MK and 88 MK, respectively. The quiescent and flareluminosities in the energy range 0.5-8 keV corrected from absorption(NH ≈ 1.7× 1021 cm-2) are6× 1030 erg s-1 and ˜1032 ergs-1, respectively. The ratio of the quiescent X-rayluminosity on the LkHα 312 bolometric luminosity is very high withlog (LX/Lbol)= -2.9, implying that the corona ofLkHα 312 reached the ``saturation'' level. The X-ray luminosity ofthe flare peak reaches ˜2% of the stellar bolometric luminosity. Thedifferent phases of this flare are finally discussed in the framework ofsolar flares, which leads to the magnetic loop height from 3.1×1010 to 1011 cm (0.2-0.5 Rstar, i.e.,0.5-1.3 Rȯ).

Astral magnetic fields-as observed in starforming nurseries, in stars, and in the Solar system
``Cherchez le champ magnétique'' is trendy in astronomy. Themagnetic field takes the hourglass shape in gravitationally contractingyoung stellar objects, helically wraps up the jets of protostars, guidesparticles in stellar coronae, explodes in stellar flares, causes unrestin pulsar quakes and creates planetary aurorae. In stars and pulsars,dipolar dynamo magnetic fields play a dominant rôle in the gasdynamics. In planetary disks, the field is toroidal or archimedeanspiral. Remanent magnetism is found in meteorites and asteroids. Theastral magnetic fields can reach 1018 G in magnetars,dwarfing anything we can do on Earth in stable conditions(107 G) or in pulsed conditions (1010 G).Magnetism plays a physical rôle in starforming clouds and itsconcomittent structures, from stellar nurseries to protostars, down toplanets and asteroids. Starting with star-forming clouds (~10 pc=32light-years=31×1016 m), this reviews coversprotostellar systems (~1 cpc), circumstellar space (~1 mpc), masers,interplanetary space (~1 μpc), pulsars, stars, planets (~1 npc),asteroids (~30 km; ~1 ppc), and meteorites (~0.3 m; ~10 apc).

Embedded Clusters in Molecular Clouds
Stellar clusters are born embedded within giant molecular clouds (GMCs)and during their formation and early evolution are often only visible atinfrared wavelengths, being heavily obscured by dust. Over the past 15years advances in infrared detection capabilities have enabled the firstsystematic studies of embedded clusters in galactic molecular clouds. Inthis article we review the current state of empirical knowledgeconcerning these extremely young protocluster systems. From a survey ofthe literature we compile the first extensive catalog of galacticembedded clusters. We use the catalog to construct the mass function andestimate the birthrate for embedded clusters within 2 kpc of the sun. Wefind that the embedded cluster birthrate exceeds that of visible openclusters by an order of magnitude or more indicating a high infantmortality rate for protocluster systems. Less than 4-7% of embeddedclusters survive emergence from molecular clouds to become boundclusters of Pleiades age. The vast majority (90%) of stars that form inembedded clusters form in rich clusters of 100 or more members withmasses in excess of 50 Mȯ. Moreover, observations ofnearby cloud complexes indicate that embedded clusters account for asignificant (70-90%) fraction of all stars formed in GMCs. We review therole of embedded clusters in investigating the nature of the initialmass function (IMF) that, in one nearby example, has been measured overthe entire range of stellar and substellar mass, from OB stars tosubstellar objects near the deuterium burning limit. We also review therole embedded clusters play in the investigation of circumstellar diskevolution and the important constraints they provide for understandingthe origin of planetary systems. Finally, we discuss current ideasconcerning the origin and dynamical evolution of embedded clusters andthe implications for the formation of bound open clusters.

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Observation and Astrometry data

Right ascension:05h46m42.00s
Apparent magnitude:10.5

Catalogs and designations:
Proper Names   (Edit)
MessierM 78
NGC 2000.0NGC 2068

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