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|X-Ray Observations of the Young Cluster NGC 2264|
We present results from an X-ray imaging survey of the young cluster NGC2264, carried out with the European Photon Imaging Cameras (EPIC) onboard the XMM-Newton spacecraft. The X-ray data are merged with extantoptical and near-infrared photometry, spectral classifications, Hαemission strengths, and rotation periods to examine theinterrelationships between coronal and chromospheric activity, rotation,stellar mass, and internal structure for a statistically significantsample of pre-main-sequence stars. A total of 300 distinct X-ray sourcescan be identified with optical or near-infrared counterparts. Thesources are concentrated within three regions of the cluster: in thevicinity of S Mon, within the large emission/reflection nebulositysouthwest of S Mon, and along the broad ridge of molecular gas thatextends from the Cone Nebula to the NGC 2264 IRS 2 field. From theextinction-corrected color-magnitude diagram of the cluster, ages andmasses for the optically identified X-ray sources are derived. A medianage of ~2.5 Myr and an apparent age dispersion of ~5 Myr are suggestedby pre-main-sequence evolutionary models. The X-ray luminosity of thedetected sources appears well correlated with bolometric luminosity,although there is considerable scatter in the relationship. Stellar masscontributes significantly to this dispersion, while isochronal age androtation do not. X-ray luminosity and mass are well correlated such thatLX~(M/Msolar)1.5, similar to therelationship found within the younger Orion Nebula Cluster. No strongevidence is found for a correlation between EH-K, thenear-infrared color excess, and the fractional X-ray luminosity, whichsuggests that optically thick dust disks have little direct influence onthe observed X-ray activity levels. Among the X-ray-detected weak-line TTauri stars, the fractional X-ray luminosity,LX/Lbol, is moderately well correlated with thefractional Hα luminosity, LHα/Lbol,but only at the 2 σ level of significance. The cumulativedistribution functions for the X-ray luminosities of the X-ray-detectedclassical and weak-line T Tauri stars within the cluster are comparable,assuming the demarcation between the two classes is at an Hαequivalent width of 10 Å. However, if the nondetections in X-raysfor the entire sample of Hα emitters known within the cluster aretaken into account, then the cumulative distribution functions of thesetwo groups are clearly different, such that classical T Tauri stars areunderdetected by at least a factor of 2 relative to the weak-line TTauri stars. Examining a small subsample of X-ray-detected stars thatare probable accretors based on the presence of strong Hα emissionand near-infrared excess, we conclude that definitive nonaccretors are~1.6 times more X-ray luminous than their accreting counterparts. Inagreement with earlier published findings for the Orion Nebula Cluster,we find a slight positive correlation (valid at the 2 σ confidencelevel) between LX/Lbol and rotation period in NGC2264. The lack of a strong anticorrelation between X-ray activity androtation period in the stellar population of NGC 2264 suggests thateither the deeply convective T Tauri stars are rotationally saturated orthe physical mechanism responsible for generating magnetic fields inpre-main-sequence stars is distinct from the one that operates inevolved main-sequence stars.
|Fast transition between classical and weak lined T Tauri stars due to external UV dissipation|
The discovery of optical jets immersed in the strong UV radiation fieldof the Rosette Nebula sheds new light on, but meanwhile poses challengesto, the study of externally irradiated jets. The jet systems in theRosette are found to have a high state of ionization and show uniquefeatures. In this paper, we investigate the evolutionary status of thejet-driving sources for young solar-like stars. To our surprise, thesejet sources indicate unexpected near infrared properties with no excessemission. They are bathed in harsh external UV radiation such thatevaporation leads to a fast dissipation of their circumstellar material.This could represent a transient phase of evolution of young solar-likestars between classical and weak lined T Tauri stars. Naked T Tauristars formed in this way have indistinguishable evolutionary ages fromthose of classical T Tauri stars resulting from the same episode of starformation. However, it would be hard for such sources to be identifiedif they are not driving an irradiated jet in a photoionized medium.
|The development of a protoplanetary disk from its natal envelope|
Class 0 protostars, the youngest type of young stellar objects, showmany signs of rapid development from their initial, spheroidalconfigurations, and therefore are studied intensively for details of theformation of protoplanetary disks within protostellar envelopes. Atmillimetre wavelengths, kinematic signatures of collapse have beenobserved in several such protostars, through observations of molecularlines that probe their outer envelopes. It has been suggested that oneor more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1,2) may display signs of an embedded region that is warmer and denserthan the bulk of the envelope. Here we report observations that revealdetails of the core on Solar System dimensions. We detect in NGC1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths20-37μm, which indicates an origin in extremely dense, warm gas. Wecan model the emission as infall from a protostellar envelope onto thesurface of a deeply embedded, dense disk, and therefore see thedevelopment of a protoplanetary disk. This is the only example ofmid-infrared water emission from a sample of 30 class 0 objects, perhapsarising from a favourable orientation; alternatively, this may be anearly and short-lived stage in the evolution of a protoplanetary disk.
|The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VI. Perseus Observed with MIPS|
We present observations of 10.6 deg2 of the Perseus molecularcloud at 24, 70, and 160 μm with Spitzer MIPS. The images showprominent, complex extended emission dominated by illuminating B starson the east side of the cloud and by cold filaments of 160 μmemission on the west side. Of 3950 point sources identified at 24 μm,1141 have 2MASS counterparts. A quarter of these populate regions of theKs versus Ks- diagram that are distinct fromstellar photospheres and background galaxies and thus are likely to becloud members with infrared excess. Nearly half (46%) of these 24 μmexcess sources are distributed outside the IC 348 and NGC 1333 clusters.A significant number of IRAS PSC objects are not recovered by SpitzerMIPS, most often because the IRAS objects were confused by brightnebulosity. The intercluster region contains several tightly clumped(r~0.1 pc) young stellar aggregates whose members exhibit a wide varietyof infrared SEDs characteristic of different circumstellar environments.This could be explained by a significant age spread among the aggregatemembers, or if the members formed at the same time, a remarkably rapidcircumstellar evolution would be required to account for the associationof Class I and Class III sources at ages <~1 Myr. We highlightimportant results for the HH 211 flow, where the bow shocks are detectedat both 24 and 70 μm, and for the debris disk candidate BD +31 643,where the MIPS data show the linear nebulosity to be an unrelatedinterstellar feature. Our data, mosaics, and catalogs are available atthe Spitzer Science Archive for use by interested members of thecommunity.
|Observations of Water Masers in the NGC 1333 IRAS 4 Region|
The NGC 1333 IRAS 4 region was observed in the 22 GHz H2Omaser line with an angular resolution of about 0.08". Two groups ofmasers were detected, one near IRAS 4A and the other near BI. Among theeight maser spots detected near IRAS 4A, six spots are located close toA2, within 100 AU, and the maser velocities are also near the systemicvelocity of the cloud core. These masers are probably related with thecircumstellar disk. Since there is no maser spot detected around A1, thestar-forming process is relatively more active in A2 than in A1. Fourmaser spots were detected near IRAS 4BI. Since most of them aredistributed along a straight line in the direction of the outflow, BImasers are most likely related with the jet or outflow. The disk-outflowdichotomy of H2O masers is discussed briefly. No maser wasdetected near H2O(C), another maser source reportedpreviously.
|The Unusual Hydrocarbon Emission from the Early Carbon Star HD 100764: The Connection between Aromatics and Aliphatics|
We have used the Infrared Spectrograph (IRS) on the Spitzer SpaceTelescope to obtain spectra of HD 100764, an apparently single carbonstar with a circumstellar disk. The spectrum shows emission featuresfrom polycyclic aromatic hydrocarbons (PAHs) that are shifted to longerwavelengths than normally seen, a characteristic of ``class C'' systemsin the classification scheme of Peeters et al. All seven of the knownclass C PAH sources are illuminated by radiation fields that are coolerthan those which typically excite PAH emission features. The observedwavelength shifts are consistent with hydrocarbon mixtures containingboth aromatic and aliphatic bonds. We propose that the class C PAHspectra are distinctive because the carbonaceous material has not beensubjected to a strong ultraviolet radiation field, allowing relativelyfragile aliphatic materials to survive.
|The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VIII. Serpens Observed with MIPS|
We present maps of 1.5 deg2 of the Serpens dark cloud at 24,70, and 160 μm observed with the Spitzer Space Telescope MIPS camera.We describe the observations and briefly discuss the data processingcarried out by the c2d team on these data. More than 2400 compactsources have been extracted at 24 μm, nearly 100 at 70 μm, andfour at 160 μm. We estimate completeness limits for our 24 μmsurvey from Monte Carlo tests with artificial sources inserted into theSpitzer maps. We compare source counts, colors, and magnitudes in theSerpens cloud to two reference data sets: a 0.50 deg 2 set ona low-extinction region near the dark cloud, and a 5.3 deg2subset of the SWIRE ELAIS N1 data that was processed through ourpipeline. These results show that there is an easily identifiablepopulation of young stellar object candidates in the Serpens cloud thatis not present in either of the reference data sets. We also show acomparison of visual extinction and cool dust emission illustrating aclose correlation between the two and find that the most embedded YSOcandidates are located in the areas of highest visual extinction.
|Star Clusters with the GTC|
Star clusters are becoming very useful probes in the studies of manytopics in modern Astronomy. Actually, they are providing invaluableinformation about the origin, structure and star formation processes inextragalactic systems. Otherwise, most of our knowledge on howindividual stars are formed is derived from the analysis of stellarclusters resolved into stars. In this contribution we review the mainscientific objectives in this field, in the light of the firstgeneration instruments for the Gran Telescopio Canarias.
|Protostellar Turbulence Driven by Collimated Outflows|
We investigate the global properties of the outflow-driven protostellarturbulence through 3D MHD simulations. The simulations show that theturbulence in regions of active cluster formation is quickly transformedby the forming stars through protostellar outflows, and that stronglyinfluences and perhaps controls protostellar turbulence clusterformation. We find that collimated outflows are more efficient indriving turbulence than spherical outflows that carry the same amountsof momentum. This is because collimated outflows can propagate fartheraway from their sources, effectively increasing the turbulence drivinglength; turbulence driven on a larger scale decays more slowly. Gravityplays an important role in shaping the turbulence, generating infallmotions that balance the outward motions driven by outflows. Theresulting quasi-equilibrium state is maintained through a slow rate ofstar formation, with a fraction of the total mass converted into starsper free-fall time as low as a few percent. Magnetic fields aredynamically important even in magnetically supercritical clumps,provided that their initial strengths are not far below the criticalvalue for static cloud support. They contain an energy comparable to theturbulent energy and can significantly reduce the rate of starformation. The mass-weighted probability distribution function (PDF) ofthe volume density of the protostellar turbulence is often, although notalways, approximately lognormal. The PDFs of the column density deviatemore strongly from lognormal distributions. There is a prominent breakin the power spectrum, which may provide a way to distinguish it fromother types of turbulence.
|Methyl Formate in the NGC 2264 IRS 1 Region|
Millimeter-wave spectral lines of HCOOCH3 are observed towarda massive star-forming region, NGC 2264 IRS 1, with the Nobeyama 45 mradio telescope and Nobeyama Millimeter Array. The HCOOCH3emission is not detected toward the dense core around IRS 1, which isthe brightest IR source. However, it is definitively detected toward MMS3, which is thought to contain a high-mass equivalent of a Class 0protostar. The column density and the fractional abundance ofHCOOCH3 in MMS 3 are found to be (4-30)×1015cm-2 and (0.7-5.3)×10-8, respectively,assuming that the range of the excitation temperature is from 50 to 250K. The fractional abundance is lower by an order of magnitude than thatin the compact ridge of Orion KL. On the other hand, the upper limit tothe fractional abundance toward IRS 1 is significantly lower than theabundance toward MMS 3. Since MMS 3 is less evolved than IRS 1, thisresult would indicate that HCOOCH3 preferentially exists inthe younger stage of protostellar evolution, as in the case of low-massstar forming regions. The distribution of HCOOCH3 is found tobe slightly offset from the dust continuum peak of MMS 3 by 13". Thissituation is similar to that found in the compact ridge of Orion KL,which would provide us with an important clue in exploring its peculiarchemistry.
|A radiation driven implosion model for the enhanced luminosity of protostars near HII regions|
Context: Molecular clouds near H II regions tend to harbor more luminousprotostars. Aims: We investigate whether a radiation-drivenimplosion mechanism enhances the luminosity of protostars near regionsof high ionizing fluxes. Methods: We performed numericalsimulations to model collapse of cores exposed to UV radiation from Ostars. We investigated the dependence of mass loss rates on the initialdensity profiles of cores and variation of UV fluxes. We derived simpleanalytic estimates of accretion rates and final masses of protostars.Results: The radiation-driven implosion mechanism can increaseaccretion rates of protostars by 1-2 orders of magnitude. On the otherhand, mass loss due to photo-evaporation is not high enough to have asignificant impact on the luminosity. The increase in accretion rateresults in luminosity 1-2 orders of magnitude higher than those ofprotostars that form without external triggering. Conclusions:Radiation-driven implosion can help explain the observed higherluminosity of protostars in molecular clouds near H II regions.
|Tracing high energy radiation with molecular lines near deeply embedded protostars|
Aims.The aim is to probe high energy radiation emitted by deeplyembedded protostars. Methods: Submillimeter lines of CN, NO,CO+ and SO^+, and upper limits on SH+ and N2O areobserved with the James Clerk Maxwell Telescope in two high-mass and upto nine low-mass young stellar objects and compared with chemicalmodels. Results: Constant fractional abundances derived from radiativetransfer modeling of the line strengths are x(CN) ≈ a few×10-11-10-8, x(NO) ≈10-9-10-8 and x(CO^+) ≈10-12-10-10. SO+ has abundances of afew × 10-11 in the high-mass objects and upper limitsof ≈10-12-10-11 in the low-mass sources. Allabundances are up to 1-2 orders of magnitude higher if the molecularemission is assumed to originate mainly from the inner region(≲1000 AU) of the envelope. For high-mass sources, the CN,SO+ and CO+ abundances and abundance ratios arebest explained by an enhanced far-ultraviolet (FUV) field impacting gasat temperatures of a few hundred K. The observed column densitiesrequire that this region of enhanced FUV has scales comparable to theobserving beam, such as in a geometry in which the enhanced FUVirradiates outflow walls. For low-mass sources, the requiredtemperatures within the FUV models of T ≳ 300 K are much higherthan found in models, so that an X-ray enhanced region close to theprotostar (r ≲ 500 AU) is more plausible. Gas-phase chemical modelsproduce more NO than observed, suggesting an additional reductionmechanism not included in current models. Conclusions: The observed CN,CO+ and SO+ abundances can be explained witheither enhanced X-rays or FUV fields from the central source. High-masssources likely have low opacity regions that allow the FUV photons toreach large distances from the central source. X-rays are suggested tobe more effective than FUV fields in the low-mass sources. The observedabundances imply X-ray fluxes for the Class 0 objects of LX≈ 1029-1031 erg s-1, comparable tothose observed from low-mass Class I protostars. Spatially resolved dataare needed to clearly distinguish the effects of FUV and X-rays forindividual species.Appendices are only available in electronic form at http://www.aanda.org
|Protostellar Outflow-driven Turbulence|
Protostellar outflows crisscross the regions of star cluster formation,stirring turbulence and altering the evolution of the forming cluster.We model the stirring of turbulent motions by protostellar outflows,building on an observation that the scaling law of supersonic turbulenceimplies a momentum cascade analogous to the energy cascade in Kolmogorovturbulence. We then generalize this model to account for a diversity ofoutflow strengths and for outflow collimation, both of which enhanceturbulence. For a single value of its coupling coefficient, the model isconsistent with turbulence simulations by Li & Nakamura and,plausibly, with observations of the NGC 1333 cluster-forming region.Outflow-driven turbulence is strong enough to stall collapse incluster-forming regions for several crossing times, relieving themismatch between star formation and turbulent decay rates. The predictedline width-size scaling implies radial density indices between -1 and -2for regions supported by outflow-driven turbulence, with a tendency forsteeper profiles in regions that are more massive or have higher columndensities.
|Resolving the Nature of the Rosette HH 1 Jet Facing Strong UV Dissipation|
The Rosette HH 1 jet is a collimated flow immersed in the strong UVradiation field of the Rosette Nebula. We investigate the physicalproperties of the Rosette HH 1 jet using high-quality narrowband imagesand high-dispersion spectroscopy. The new images show that the axis ofthe jet is not precisely aligned with the star near the base of the jet.The high resolution of the spectra allows us to accurately determine thecontributions from the H II region, jet, and star. The approaching andreceding sides of the expanding shell of the Rosette Nebula are atheliocentric velocities of 13 and 40 km s-1, while the jetreaches a maximum velocity offset at a heliocentric velocity of -30 kms-1. The [S II] doublet ratios indicate an electron densityof ~1000 cm-3 in the jet and <=100 cm-3 in theH II region. With a careful subtraction of the nebular and jetcomponents, we find the stellar Hα line is dominated by a broadabsorption profile with little or no emission component, indicating alack of substantial circumstellar material. The circumstellar materialhas most likely been photoevaporated by the strong UV radiation field inthe Rosette Nebula. The evaporation timescale is 103-104 yr. The Rosette HH 1 jet source provides evidence for anaccelerated evolution from a CTTS to a WTTS due to the strong UVradiation field; therefore, both CTTSs and WTTSs can be spatially mixedin regions with massive star formation.
|PROSAC: A Submillimeter Array Survey of Low-Mass Protostars. I. Overview of Program: Envelopes, Disks, Outflows, and Hot Cores|
This paper presents a large spectral line and continuum survey of eightdeeply embedded, low-mass protostellar cores using the SMA.High-excitation line emission from 11 molecular species originating inwarm and dense gas has been imaged at high angular resolution (1"-3",typically 200-600 AU) together with continuum emission at 230 GHz (1.3mm) and 345 GHz (0.8 mm). Compact continuum emission is observed for allsources, which likely originates in marginally optically thickcircumstellar disks, with typical lower limits to their masses of 0.1Msolar (1%-10% of the masses of their envelopes) and a dustopacity law, κν~νβ, withβ~1. Prominent collimated outflows are present in CO 2-1observations in all sources. The most diffuse outflows are found in thesources with the lowest ratios of disk to envelope mass, and it issuggested that these sources are in a phase where accretion of matterfrom the envelope has almost finished and the remainder of the envelopematerial is being dispersed by the outflows. Other characteristicdynamical signatures are inverse P Cygni profiles indicative ofinfalling motions seen in the 13CO 2-1 lines toward NGC 1333IRAS 4A and NGC 1333 IRAS 4B. Outflow-induced shocks are present on allscales in the protostellar environments and are most clearly traced bythe emission of CH3OH in NGC 1333 IRAS 4A and NGC 1333 IRAS4B. These observations suggest that the emission of CH3OH andH2CO from these proposed ``hot corinos'' is related to theshocks caused by the protostellar outflows. Only one source, NGC 1333IRAS 2A, has evidence for hot, compact CH3OH emissioncoincident with the embedded protostar.
|Infrared Nebulae around Young Stellar Objects|
We present a K-band atlas of 106 reflection nebulae, 41 of which are newdiscoveries. We observed these nebulae with the University of Hawaii 2.2m telescope in the course of an imaging survey of 197 objects that wereselected as nearby young Class I sources. K-band images andflux-calibrated surface brightness contour plots of each nebula arepresented. We found that the near-IR luminosities and physical sizes ofthe nebulae increase with the bolometric luminosity of the illuminatingsources. Only 22 nebulae, about 10% of these candidate Class I sources,have indications of shocked H2 emission. The great variety ofnebulae that we observed prevented us from classifying them based onmorphology. However, we note that as the spectral index decreases, thecentral star is more frequently visible at K band, and the flux from thecentral star tends to be dominant over the flux from the nebula. Forobjects that have a higher spectral index, most of the K-band flux isfrom the reflection nebula, and the central star is less frequentlyvisible. The nebula around IRAS 05450+0019 has a unique morphology, andwe speculate that it may be an example of a disk shadow being projectedinto the surrounding cloud. We present J-, H-, and K-band images of thisobject with surface brightness contours, as well as its spectral energydistribution from 1.2 to 100 μm.
|Hubble Space Telescope NICMOS Observations of NGC 1333: The Ratio of Stars to Substellar Objects|
We present an analysis of NICMOS photometry and low-resolution grismspectroscopy of low-mass stars and substellar objects in the youngstar-forming region NGC 1333. Our goal is to constrain the ratio oflow-mass stars to substellar objects down to 20MJup in thecluster, as well as constrain the cluster initial mass function (IMF)down to 30MJup, in combination with a previous survey of NGC1333 by Wilking et al. Our survey covers four fields of 51.2" ×51.2", centered on brown dwarf candidates previously identified inWilking et al. We extend previous work based on the use of a water vaporindex for spectral typing to wavelengths accessible with NICMOS on theHubble Space Telescope. Spectral types were derived for the 14 brightestobjects in our fields, ranging from <=M0 to M8, which, at the age ofthe cluster (0.3 Myr), correspond to a range in mass of >=0.25-0.02Msolar. In addition to the spectra, we present an analysis ofthe color-magnitude diagram using pre-main-sequence evolutionary modelsof D'Antona & Mazzitelli. Using an extinction-limited sample, wederive the ratio of low-mass stars to brown dwarfs. Comparisons of theobserved ratio to that expected from the field IMF of Chabrier indicatethat the two results are consistent. We combine our data with those ofWilking et al. to compute the ratio of intermediate-mass stars (0.1-1.0Msolar) to low-mass objects (0.03-0.1 Msolar) inthe cluster. We also report the discovery of a faint companion to thepreviously confirmed brown dwarf ASR 28, as well as a possible outflowsurrounding ASR 16. If the faint companion is confirmed as a clustermember, it would have a mass of ~5MJup (mass ratio 0.15) at aprojected distance of 350 AU, similar to that of 2MASS 1207-3923B.
|Molecular CO Outflows in the L1641-N Cluster: Kneading a Cloud Core|
We present results of 1.3 mm interferometric and single-dishobservations of the center of the L1641-N cluster in Orion. Single-dishwide-field continuum and CO(2-1) observations reveal the presence ofseveral molecular outflows driven by deeply embedded protostellarsources. At higher angular resolution, the dominant millimeter source inthe cluster center is resolved into a pair of protostars (L1641-N MM1and L1641-N MM3), each driving a collimated outflow, and a moreextended, clumpy core. Low-velocity CO line-wing emission is spreadwidely over much of the cluster area. We detect and map the distributionof several other molecular transitions (13CO,C18O, 13CS, SO, CH3OH,CH3CN, and OCS). CH3CN and OCS may indicate thepresence of a hot corino around L1641-N MM1. We tentatively identify avelocity gradient over L1641-N MM1 in CH3CN and OCS, orientedroughly perpendicular to the outflow direction, perhaps indicative of acircumstellar disk. An analysis of the energy and momentum load of theCO outflows, along with the notion that apparently a large volumefraction is affected by the multiple outflow activity, suggests thatoutflows from a population of low-mass stars might have a significantimpact on clustered (and potentially high-mass) star formation.
|Far-Infrared Detection of H2D+ toward Sgr B2|
We report on the first far-IR detection of H2D+,using the Infrared Space Observatory, in the line of sight toward Sgr B2in the Galactic center. The transition at λ=126.853 μmconnecting the ground level of o-H2D+,11,1, with the 21,2 level at 113 K is observed inabsorption against the continuum emission of the cold dust of thesource. The line is broad, with a total absorption covering 350 kms-1, i.e., similar to that observed in the fundamentaltransitions of H2O, OH, and CH at ~179, 119, and 149 μm,respectively. For the physical conditions of the different absorbingclouds, the H2D+ column density ranges from 2 to5×1013 cm-2, i.e., near an order ofmagnitude below the upper limits obtained from ground-basedsubmillimeter telescopes. The derived H2D+abundance is of a few times 10-10, which agrees with chemicalmodel predictions for a gas at a kinetic temperature of ~=20 K.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, theNetherlands, and the UK) and with participation of ISAS and NASA.
|The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. V. Chamaeleon II Observed with IRAC|
We present IRAC (3.6, 4.5, 5.8, and 8.0 μm) observations of theChamaeleon II molecular cloud. The observed area covers about 1deg2 defined by AV>2. Analysis of the data inthe 2005 c2d catalogs reveals a small number of sources (40) withproperties similar to those of young stellar or substellar objects(YSOs). The surface density of these YSO candidates is low, andcontamination by background galaxies appears to be substantial,especially for sources classified as Class I or flat spectral energydistribution (SED). We discuss this problem in some detail and concludethat very few of the candidate YSOs in early evolutionary stages areactually in the Cha II cloud. Using a refined set of criteria, we definea smaller, but more reliable, set of 24 YSO candidates.
|Current Star Formation in the Perseus Molecular Cloud: Constraints from Unbiased Submillimeter and Mid-Infrared Surveys|
We present a census of the population of deeply embedded young stellarobjects (YSOs) in the Perseus molecular cloud complex based on acombination of Spitzer Space Telescope mid-infrared data from the Coresto Disks (c2d) legacy team and JCMT SCUBA submillimeter maps from theCOMPLETE team. The mid-infrared sources detected at 24 μm and having[3.6]-[4.5]>1 are located close to the center of the SCUBA cores,typically within 15" of their peaks. The narrowness of the spatialdistribution of mid-infrared sources around the peaks of the SCUBA coressuggests that no significant dispersal of the newly formed YSOs hasoccurred. This argues against the suggestion that motions of protostarsregulate the timescales over which significant (Bondi-Hoyle) accretioncan occur. The YSOs are found to have red [3.6]-[4.5] and [8.0]-colors, but not comparable red [5.8]-[8.0] colors. The most deeplyembedded YSOs are found in regions with high extinction,AV>=5, similar to the extinction threshold observed forthe SCUBA cores. All the SCUBA cores with high concentrations haveembedded YSOs, but not all cores with low concentrations are starless.From the above considerations, a relatively unbiased sample of 49 deeplyembedded YSOs is constructed. Embedded YSOs are found in 40 of the 72SCUBA cores, with only three cores harboring multiple embedded YSOswithin 15". The equal number of SCUBA cores with and without embeddedYSOs suggests that the timescale for the evolution through the denseprestellar stages, where the cores are recognized in the submillimetermaps and have central densities of5×104-1×105 cm-3, issimilar to the timescale for the embedded protostellar stages. Thecurrent star formation efficiency of cores is estimated to beapproximately 10%-15%. In contrast, the star formation efficiencyaveraged over the cloud lifetime and compared to the total cloud mass isonly a few percent, reflecting also the efficiency in assembling cloudmaterial into the dense cores actually forming stars.
|A Large-Scale Survey of NGC 1333|
We observed the clustered star forming complex NGC 1333 with the BIMAand FCRAO telescopes in the transitions HCO+(1-0) andN2H+(1-0) over an11'×11' area with resolution~10'' (0.015 pc). The N2H+ emissionfollows very closely the submillimeter dust continuum emission, whileHCO+ emission appears more spatially extended and also tracesoutflows. We have identified 93 N2H+ cores usingthe CLUMPFIND algorithm, and we derive N2H+ coremasses between 0.05 and 2.5 Msolar, with uncertainties of afactor of a few, dominated by the adopted N2H+abundance. From a comparison with virial masses, we argue that most ofthese N2H+ cores are likely to be bound, even atthe lowest masses, suggesting that the cores do not trace transientstructures, and implies the entire mass distribution consists of objectsthat can potentially form stars. We find that the mass distribution ofN2H+ cores resembles the field star IMF, whichsuggests that the IMF is locked in at the prestellar stage of evolution.We find that the N2H+ cores associated with starsidentified from Spitzer infrared images have a flat mass distribution.This might be because lower mass cores lose a larger fraction of theirmass when forming a star. Even in this clustered environment, we find noevidence for ballistic motions of the cores relative to their lowerdensity surroundings traced by isotopic CO emission, although thisconclusion must remain tentative until the surroundings are observed atthe same high resolution as the N2H+.
|High Spectral Resolution Near-IR Observations of ESO-Hα 279A and 279B|
We present high spectral resolution near-IR observations of the starsESO-Hα 279A and 279B located in the Serpens star formationcomplex. ESO-Hα 279A is a known young T Tauri star driving aHerbig-Haro flow, while ESO-Hα 279B is a nearby (8", 2500 AU)extremely red companion. Previous work has suggested that this is not aphysically bound system but a chance alignment of a young star and abackground giant. Below, we further investigate the nature ofESO-Hα 279B and probe the emission characteristics of ESO-Hα279A. We find that ESO-Hα 279B shows many narrow absorptionfeatures typical of late-type giant stars and unlike those seen in FUOrionis objects and Herbig-Haro energy sources, the only young starsthat exhibit such deep CO absorption. This result confirms thatESO-Hα 279B is a background M-type giant viewed through themolecular cloud hosting ESO-Hα 279A, and therefore, theassociation of ESO-Hα 279A and 279B is fortuitous. ForESO-Hα 279A we find that the Na doublet lines are broadened andself-absorbed and that the v=2-0 CO overtone band head is similar inshape to that observed in the young pre-main-sequence object SVS 13 inNGC 1333, and not characteristic of a rotating-disk velocity dispersion.We consider the origin of the CO emission in relation to current modelsand suggest that it perhaps is more indicative of creation in a wind orfunnel flow rather than in the inner hot regions of a circumstellardisk.
|Hot corinos in NGC 1333-IRAS4B and IRAS2A|
Context: Complex organic molecules have been detected in massive hotcores for over two decades, and only recently in three hot corinos (theinner regions surrounding Sun-like protostars, where the dusttemperature exceeds 100 K). Since hot corinos have sizes of ~100 AU(i.e., of the order of the extent of the Solar System), it isparticularly relevant to understand whether they are common and toidentify the formation route(s) of complex organic molecules. Much hasyet to be learned on this topic, since even recent models predicted itwas not possible to form these molecules in low-mass protostars. Aims:We aim to enlarge the number of known hot corinos and carry out a firstcomparative study with hot cores. The ultimate goal is to understandwhether complex organic molecules form in the gas phase or on grainsurfaces, and what the possible key parameters are. Methods: Weobserved millimeter rotational transitions of HCOOH, HCOOCH3CH_3OCH3 CH_3CN and C_2H_5CN in a sample of low-massprotostars with the IRAM-30 m. Using the rotational diagram methodcoupled with the information about the sources' structure, we calculatethe abundances of the observed molecules. To interpret these abundances,we review the proposed formation processes of the above molecules. Results: We report the detection of HCOOCH3 and/or CH_3CNtowards NGC 1333-IRAS4B and NGC 1333-IRAS2A. We find that abundanceratios of O-bearing molecules to methanol or formaldehyde in hot corinosare comparable and about unity, and are relatively (depending on how theratios are determined) higher than those in hot cores and in Galacticcenter clouds. Conclusions: .So far, complex organic molecules weredetected in all the hot corinos where they were searched for, suggestingthat it is a common phase for low-mass protostars. While some evidencepoints to grain-surface synthesis (either in the cold or warm-up phase)of these molecules (in particular for HCOOH and HCOOCH_3), the presentdata do not allow us to disregard gas-phase formation. Moreobservational, laboratory, and theoretical studies are required toimprove our understanding of hot corinos.Appendix is only available in electronic form at http://www.aanda.org
|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.
|The mass function of dense molecular cores and the origin of the IMF|
Context: Stars form in the cold dense cores of interstellar molecularclouds and the detailed knowledge of the spectrum of masses of suchcores is clearly a key for the understanding of the origin of the IMF.To date, observations have presented somewhat contradictory evidencerelating to this issue. Aims: In this paper we propose to derive themass function of a complete sample of dense molecular cores in a singlecloud employing a robust method that uses uses extinction of backgroundstarlight to measure core masses and enables the reliable extension ofsuch measurements to lower masses than previously possible. Methods: Weuse a map of near-infrared extinction in the nearby Pipe dark cloud toidentify the population of dense cores in the cloud and measure theirmasses. Results: We identify 159 dense cores and construct the massfunction for this population. We present the first robust evidence for adeparture from a single power-law form in the mass function of apopulation of cores and find that this mass function is surprisinglysimilar in shape to the stellar IMF but scaled to a higher mass by afactor of about 3. This suggests that the distribution of stellar birthmasses (IMF) is the direct product of the dense core mass function and auniform star formation efficiency of 30%±10%, and that thestellar IMF may already be fixed during or before the earliest stages ofcore evolution. These results are consistent with previous dustcontinuum studies which suggested that the IMF directly originates fromthe core mass function. The typical density of ~104cm-3 measured for the dense cores in this cloud suggests thatthe mass scale that characterizes the dense core mass function may bethe result of a simple process of thermal (Jeans) fragmentation.Table of core positions is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/L17
|Two Bipolar Outflows and Magnetic Fields in the Multiple Protostar System L1448 IRS 3|
We present spectral line observations of CO J=2-->1, 13COJ=1-->0, and C18O J=1-->0 and polarimetric observationsin the λ=1.3 mm continuum and in CO J=2-->1 toward themultiple protostar system L1448 IRS 3, using the BIMA array. In theλ=1.3 mm continuum, two sources (IRS 3A and 3B) were clearlydetected with estimated envelope masses of 0.21 and 1.15Msolar, and one source (IRS 3C) was marginally detected withan upper mass limit of 0.03 Msolar. In CO J=2-->1, werevealed two outflows originating from IRS 3A and 3B. The masses, meannumber densities, momentums, and kinetic energies of outflow lobes wereestimated. Based on those estimates and outflow features, we concludethat the two outflows are interacting and that the IRS 3A outflow isnearly perpendicular to the line of sight. In addition, we estimate thevelocity, inclination, and opening of the IRS 3B outflow using Bayesianstatistics. Linear polarization was detected in both the λ=1.3 mmcontinuum and CO J=2-->1. The linear polarization in the continuumshows a magnetic field at the central source (IRS 3B) perpendicular tothe outflow direction, and the linear polarization in the CO J=2-->1was detected in the outflow regions either parallel or perpendicular tothe outflow direction. Moreover, we comprehensively discuss whether thebinary system of IRS 3A and 3B is gravitationally bound, based on thevelocity differences detected in 13CO J=1-->0 andC18O J=1-->0 observations and on the outflow features. Thespecific angular momentum of the system is estimated as~3×1020 cm2 s-1, comparable tothe values obtained from previous studies of binary stars and molecularcloud cores.
|Outflow-driven Cavities: Numerical Simulations of Intermediaries of Protostellar Turbulence|
We investigate the evolution of fossil cavities produced by extinctyoung stellar object (YSO) jets and wide-angle outflows. Fossil cavitiesare ellipsoidal or cylindrical shells of swept-up ambient (molecularcloud) material moving at low velocities. The cavities form when themomentum in a YSO jet or wide-angle outflow decays in time, allowing thebow shock or swept-up shell to decelerate to velocities near theturbulent speed in the cloud. It has been suggested in previous studiesthat cavities provide efficient coupling between the jets/outflows andthe cloud and, as such, are the agents by which cloud turbulence can bere-energized. In this paper, we carry forward a series of numericalsimulations of jets and outflows whose momentum flux decreases in time.We compare simulations with decaying momentum fluxes to those withconstant flux. We show that decaying flux models exhibit deceleration ofthe outflow head and back-filling via expansion off of the cavity walls.They also have lower density contrast and are longer lived and widerthan their continuously driven counterparts. The simulations recover thebasic properties of observed fossil cavities. In addition, we providesynthetic observations in terms of position-velocity (PV) diagrams,which demonstrate that fossil cavities form both jets and wide-angleoutflows and are characterized by linear ``Hubble law'' expansionpatterns superimposed on ``spur'' patterns, indicative of the head of abow shock.
|Silicon Monoxide Observations Reveal a Cluster of Hidden Compact Outflows in the OMC 1 South Region|
We present high angular resolution (2.8"×1.7") SiO J=5-->4, v=0line observations of the OMC 1S region in the Orion Nebula made usingthe Submillimeter Array (SMA). We detect for the first time a cluster offour compact bipolar and monopolar outflows that show high-, moderate-,and low-velocity gas and appear to be energized by millimeter andinfrared sources associated with this region. The SiO molecular outflowsare compact (<3500 AU), and in most of the cases, they are locatedvery close to their exciting sources. We thus propose that the SiOthermal emission is tracing the youngest and most highly excited partsof the outflows that cannot be detected by other molecules. Moreover,since the ambient cloud is weak in the SiO line emission, theseobservations can reveal flows that in other molecular transitions willbe confused with the ambient velocity cloud emission. Analysis of theirposition-velocity diagrams show that some components of these outflowsmay be driven by wide-angle winds very close to the exciting object.Finally, we find that some of these SiO outflows seem to be the base ofpowerful Herbig-Haro jets and large-scale molecular flows that emanatefrom a few arcseconds around this zone. In particular, we find astrongly excited SiO bipolar outflow with a P.A. of ~100° that islikely energized by the luminous (~3×103Lsolar) infrared protostar ``B'' and could be the base of theremarkable object HH 269.
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