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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.
| 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-[24] 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.
| Far-Ultraviolet Observations of the Monogem Ring
We present the results of far-ultraviolet (FUV) observations of theMonogem ring made with the FIMS/SPEAR instrument. The global mapconstructed from the C IV λλ1548, 1550 emission linespresents clear evidence of the interaction of the Monogem ring with theambient medium. The image shows a half-ring feature in the low-latituderegion, with its peak along the distorted boundary of the X-ray ring,where hot gas is in direct contact with the newly found Hα ring(the Gemini Hα ring) centered at (l, b) ~ (191.5°, +5.0°).Only a small portion of the X-ray ring is bright in C IV in thehigh-latitude region, where hot gas is seen to extend to a greatdistance from the ring. We have also detected other ionic emission linesin several regions of the Monogem ring, such as C III λ977, O VIλλ1032, 1038, Si II* λ1533, He II λ1640, andO III] λ1666.
| An X-Ray Imaging Study of the Stellar Population in RCW 49
We present the results of a high-resolution X-ray imaging study of thestellar population in the Galactic massive star-forming region RCW 49and its central OB association Westerlund 2. We obtained a ~40 ks X-rayimage of a ~17'×17' field using the ChandraX-Ray Observatory and deep NIR images using the Infrared Survey Facilityin a concentric ~8.3'×8.3' region. Wedetected 468 X-ray sources and identified optical, NIR, and Spitzer MIRcounterparts for 379 of them. The unprecedented spatial resolution andsensitivity of the X-ray image, enhanced by optical and infrared imagingdata, yielded the following results: (1) The central OB associationWesterlund 2 is resolved for the first time in the X-ray band. X-rayemission is detected from all spectroscopically identified early-typestars in this region. (2) Most (~86%) X-ray sources with optical orinfrared identifications are cluster members in comparison with acontrol field in the Galactic plane. (3) A loose constraint (2-5 kpc)for the distance to RCW 49 is derived from the mean X-ray luminosity ofT Tauri stars. (4) The cluster X-ray population consists of low-masspre-main-sequence and early-type stars as obtained from X-ray and NIRphotometry. About 30 new OB star candidates are identified. (5) Weestimate a cluster radius of 6'-7' based on the X-ray surface numberdensity profiles. (6) A large fraction (~90%) of cluster members areidentified individually using complimentary X-ray and MIR excessemission. (7) The brightest five X-ray sources, two Wolf-Rayet stars andthree O stars, have hard thermal spectra.
| A Spitzer Census of the IC 348 Nebula
Spitzer mid-infrared surveys enable an accurate census of young stellarobjects by sampling large spatial scales, revealing very embeddedprotostars, and detecting low-luminosity objects. Taking advantage ofthese capabilities, we present a Spitzer-based census of the IC 348nebula and embedded star cluster, covering a 2.5 pc region andcomparable in extent to the Orion Nebula. Our Spitzer censussupplemented with ground-based spectra has added 42 Class II T Taurisources to the cluster membership and identified ~20 Class 0/Iprotostars. The population of IC 348 likely exceeds 400 sources afteraccounting statistically for unidentified diskless members. Our Spitzercensus of IC 348 reveals a population of Class I protostars that isanticorrelated spatially with the Class II/III T Tauri members, whichcomprise the centrally condensed cluster around a B star. The protostarsare instead found mostly at the cluster periphery about ~1 pc from the Bstar and spread out along a filamentary ridge. We further find that thestar formation rate in this protostellar ridge is consistent with thatrate which built the older exposed cluster, while the presence of 15cold, starless, millimeter cores intermingled with this protostellarpopulation indicates that the IC 348 nebula has yet to finish formingstars. Moreover, we show that the IC 348 cluster is of order 3-5crossing times old, and, as evidenced by its smooth radial profile andconfirmed mass segregation, is likely relaxed. While it seems apparentthat the current cluster configuration is the result of dynamicalevolution and its primordial structure has been erased, our finding of afilamentary ridge of Class I protostars supports a model in whichembedded clusters are built up from numerous smaller subclusters.Finally, the results of our Spitzer census indicate that the suppositionthat star formation must progress rapidly in a dark cloud should notpreclude these observations that show it can be relatively long lived.
| Rotation and Activity of Pre-Main-Sequence Stars
We present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%.
| A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster
We report new Spitzer Space Telescope observations, using the IRAC andMIPS instruments, of the young (~3 Myr) σ Orionis cluster. Weidentify 336 stars as members of the cluster, using optical andnear-infrared color-magnitude diagrams. Using the spectral energydistribution slopes in the IRAC spectral range, we place objects intoseveral classes: non-excess stars, stars with optically thick disks(such as classical T Tauri stars), class I (protostellar) candidates,and stars with ``evolved disks'' the last exhibit smaller IRAC excessesthan optically thick disk systems. In general, this classificationagrees with the location expected in IRAC-MIPS color-color diagrams forthese objects. We find that the evolved disk systems are mostly acombination of objects with optically thick but nonflared disks,suggesting grain growth and/or settling, and transition disks, systemsin which the inner disk is partially or fully cleared of small dust. Inall, we identify seven transition disk candidates and three possibledebris disk systems. As in other young stellar populations, the fractionof disks depends on the stellar mass, ranging from ~10% for stars in theHerbig Ae/Be mass range (>2 Msolar) to ~35% for those inthe T Tauri mass range (1-0.1 Msolar). The IRAC infraredexcesses found in stellar clusters and associations with and withoutcentral high-mass stars are similar, suggesting that externalphotoevaporation is not very important in many clusters. Finally, wefind no correlation between the X-ray luminosity and the disk infraredexcess, suggesting that the X-rays are not strongly affected by diskaccretion.
| 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.
| Towards absolute scales for the radii and masses of open clusters
Aims. In this paper we derive tidal radii and masses of open clusters inthe nearest kiloparsecs around the Sun. Methods: For each cluster, themass is estimated from tidal radii determined from a fitting ofthree-parameter King profiles to the observed integrated densitydistribution. Different samples of members are investigated. Results:For 236 open clusters, all contained in the catalogue ASCC-2.5, weobtain core and tidal radii, as well as tidal masses. The distributionsof the core and tidal radii peak at about 1.5 pc and 7-10 pc,respectively. A typical relative error of the core radius lies between15% and 50%, whereas, for the majority of clusters, the tidal radius wasdetermined with a relative accuracy better than 20%. Most of theclusters have tidal masses between 50 and 1000 m_ȯ, and for abouthalf of the clusters, the masses were obtained with a relative errorbetter than 50%.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/468/151
| Evidence for a Planetary Companion around a Nearby Young Star
We report evidence for a planetary companion around the nearby youngstar HD 70573. The star is a G-type dwarf located at a distance of 46pc. We carried out spectroscopic observations of this star with FEROS atthe 2.2 m MPG/ESO telescope at La Silla. Our spectroscopic analysisyields a spectral type of G1-1.5 V and an age of about 100 Myr.Variations in stellar radial velocity (RV) of HD 70573 were monitoredfrom 2003 December until 2007 January. HD 70573 shows an RV variationwith a period of 852(+/-12) days and a semiamplitude of 149(+/-6) ms-1. The period of this variation is significantly longerthan its rotational period. Based on the analysis of the Ca II Kemission line, Hα, and Teff variation asstellar-activity indicators, as well as the lack of a correlationbetween the bisector velocity span and the RV, we can exclude rotationalmodulation and nonradial pulsations as the source of the long-period RVvariation. Thus, the presence of a low-mass companion provides the bestexplanation for the observed RV variation. Assuming a primary massm1=1.0+/-0.1 Msolar, we calculated a minimum massof the companion m2sini of 6.1 MJup, which lies inthe planetary-mass regime, and an orbital semimajor axis of 1.76 AU. Theorbit's eccentricity is e=0.4. The planet discovery around HD 70573gives an important input for the study of debris disks around youngstars and their relation to the presence of planets.
| 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.
| Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| 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.
| A theoretical approach for the interpretation of pulsating PMS intermediate-mass stars
Context: The investigation of the pulsation properties ofpre-main-sequence intermediate-mass stars is a promising tool toevaluate the intrinsic properties of these stars and to constraincurrent evolutionary models. Many new candidates of this class have beendiscovered during the last decade and very accurate data are expectedfrom space observations obtained for example with the CoRoT satellite. Aims: In this context we aim at developing a theoretical approach forthe interpretation of observed frequencies, both from the alreadyavailable ground-based observations and from the future more accurateand extensive CoRoT results. Methods: To this purpose we have started aproject devoted to the computations of fine and extensive grids ofasteroseismic models of intermediate mass pre-main-sequence stars. Theobtained frequencies are used to derive an analytical relation betweenthe large frequency separation and the stellar luminosity and effectivetemperature and to develop a tool to compare theory and observations inthe echelle diagram. Results: The predictive capabilities of theproposed method are verified through the application to two test stars.As a second step, we apply the procedure to two true observations frommultisite campaigns and we are able to constrain their stellarparameters, in particular the mass, in spite of the small number offrequencies. Conclusions: We expect that with a significantly highernumber of frequencies both the stellar mass and age could be constrainedand, at the same time, the physics of the models could be tested.
| Empirical isochrones and relative ages for young stars, and the radiative-convective gap
We have selected pre-main-sequence (PMS) stars in 12 groups of notionalages ranging from 1 to 35 Myr, using heterogeneous membership criteria.Using these members we have constructed empirical isochrones in V, V - Icolour-magnitude diagrams. This allows us to identify clearly the gapbetween the radiative main sequence and the convective PMS (the R-Cgap). We follow the evolution of this gap with age and show that it canbe a useful age indicator for groups less than ~=15 Myr old. We alsoobserve a reduction in absolute spreads about the sequences with age.Finally, the empirical isochrones allow us to place the groups in orderof age, independently of theory. The youngest groups can be collatedinto three sets of similar ages. The youngest set is the ONC, NGC6530and IC5146 (nominally 1 Myr); next Cep OB3b, NGC2362, λ Ori andNGC2264 (nominally 3 Myr); and finally σ Ori and IC348 (nominally4-5 Myr). This suggests Cep OB3b is younger than previously thought, andIC348 older. For IC348 the stellar rotation rate distribution andfraction of stars with discs imply a younger age than we derive. Wesuggest this is because of the absence of O-stars in this cluster, whosewinds and/or ionizing radiation may be an important factor in theremoval of discs in other clusters.
| Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made byHST STIS and FUSE to determine the column densities and rotationalexcitation temperatures for carbon monoxide and molecular hydrogen,respectively, along 23 sight lines to Galactic O and B stars. Thereddening values range from E(B-V)=0.07 to 0.62, sampling the diffuse totranslucent interstellar medium (ISM). We find that the H2column densities range from 5×1018 to8×1020 cm-2 and the CO from upper limitsaround 2×1012 cm-2 to detections as high as1.4×1016 cm-2. CO increases with increasingH2, roughly following a power law of factor ~2. TheCO/H2 column density ratio is thus not constant, ranging from10-7 to 10-5, with a mean value of3×10-6. The sample segregates into ``diffuse'' and``translucent'' regimes, the former with molecular fraction <~0.25and AV/d<1 mag kpc-1. The mean CO/H2for these two regimes are 3.6×10-7 and9.3×10-6, respectively, significantly lower than thecanonical dark cloud value of 10-4. Six sight lines show theisotopic variant 13CO, and the isotopic ratio we observe(~50-70) is consistent with, if perhaps a little below, the average12C/13C for the ISM at large. The averageH2 rotational excitation temperature is 74+/-24 K, agreeingwell with previous studies, and the average CO temperature is 4.1 K,with some sight lines showing temperatures as high as 6.4 K. The higherexcitation CO is observed with higher column densities, consistent withthe effects of photon trapping in clouds with densities in the 20-100cm-3 range. We discuss the implications for the structure ofthe diffuse/translucent regimes of the ISM and the estimation ofmolecular mass in galaxies.
| Probing the formation of intermediate- to high-mass stars in protoclusters. II. Comparison between millimeter interferometric observations of NGC 2264-C and SPH simulations of a collapsing clump
Aims.The earliest phases of massive star formation in clusters are stillpoorly understood. Here, we test the hypothesis for high-mass starformation proposed in our earlier paper (Peretto et al. 2006) statingthat a massive, ultra-dense core may be currently forming at the centerof the collapsing NGC 2264-C protocluster via the gravitationalcoalescence of several intermediate-mass Class 0 objects. Methods: Inorder to confirm the physical validity of this hypothesis, we carriedout IRAM Plateau de Bure interferometer observations of NGC 2264-C andperformed SPH numerical simulations of the collapse of a Jeans-unstable,prolate dense clump. A detailed comparison between these hydrodynamicsimulations and both our earlier IRAM 30 m observations and the newinterferometer observations is presented. Results: Our Plateau de Bureobservations provide evidence for disk emission in three of the sixClass 0-like objects identified earlier with the 30 m in the NGC 2264-Cclump. Furthermore, they reveal the presence of a new compact source(C-MM13) located only ~10 000 AU away, but separated by ~1.1 kms-1 in (projected) velocity, from the most massive Class 0object (C-MM3) lying at the very center of NGC 2264-C. Detailedcomparison with our numerical SPH simulations supports the view that NGC2264-C is an elongated cluster-forming clump in the process ofcollapsing and fragmenting along its long axis, leading to a strongdynamical interaction and possible protostar merger in the centralregion of the clump. The marked velocity difference observed between thetwo central objects C-MM3 and C-MM13, which can be reproduced in thesimulations, is interpreted as an observational signature of thisdynamical interaction. The present study also sets several quantitativeconstraints on the initial conditions of large-scale collapse in NGC2264-C. Our hydrodynamic simulations indicate that the observed velocitypattern characterizes an early phase of protocluster collapse whichsurvives for an only short period of time (i.e., ≤1×105 yr). To provide a good match to the observations thesimulations require an initial ratio of turbulent to gravitationalenergy of only ~5%, which strongly suggests that the NGC 2264-C clump isstructured primarily by gravity rather than turbulence. The required"cold" initial conditions may result from rapid compression by anexternal trigger. Conclusions: .We speculate that NGC 2264-C is not anisolated case but may point to key features of the initial phases ofhigh-mass star formation in protoclusters.
| Rotational periods of solar-mass young stars in Orion
Context: The evolution of the angular momentum in young low-mass starsis still a debated issue. The stars presented here were discovered asX-ray sources in the ROSAT All-Sky Survey (RASS) of the Orion complexand subsequently optically identified thanks to both low and highresolution spectroscopy. Aims: The determination of the rotationalperiods in young low-mass stars of different age is fundamental for theunderstanding of the angular momentum evolution. Methods: We performeda photometric monitoring program on a sample of 40 solar-mass youngstars in the Orion star-forming region, almost all previously identifiedas weak T Tauri stars (WTTS) candidates. Photometric B and V data werecollected from 1999 to 2006 at Catania Astrophysical Observatory (OAC).Data were also acquired in December 1998 at Calar Alto Observatory (CA)and in 1999, 2000, and 2003 at San Pedro Martir (SPM). From the observedrotational modulation, induced by starspots, we derived the rotationperiods, using both the Lomb-Scargle periodogram and the CLEANdeconvolution algorithms. Results: In total, we were able to determinethe rotation periods for 39 stars, spanning from about 0.5 to 13 days,showing a rather flat distribution with a peak around 1-2 days. Thoughsome of these stars were found to be spectroscopic binaries, only thesystems with shorter orbital periods and circular orbits turned out tobe synchronized. In the other cases, the rotational period is shorterthan the period of pseudo-synchronization at periastron. Conclusions:.The new data provide further evidence for the spin up of solar-massstars predicted by models of angular momentum evolution of pre-mainsequence (PMS) stars.Based on observations collected at the Catania Astrophysical Observatory(Italy), at the Estación de Observación de Calar Alto(Spain), and San Pedro Martir Observatory (México). Appendix A isonly available in electronic form at http://www.aanda.org
| The Monitor project: searching for occultations in young open clusters
The Monitor project is a photometric monitoring survey of nine young(1-200Myr) clusters in the solar neighbourhood to search for eclipses byvery low mass stars and brown dwarfs and for planetary transits in thelight curves of cluster members. It began in the autumn of 2004 and usesseveral 2- to 4-m telescopes worldwide. We aim to calibrate the relationbetween age, mass, radius and where possible luminosity, from the Kdwarf to the planet regime, in an age range where constraints onevolutionary models are currently very scarce. Any detection of anexoplanet in one of our youngest targets (<~10Myr) would also provideimportant constraints on planet formation and migration time-scales andtheir relation to protoplanetary disc lifetimes. Finally, we will usethe light curves of cluster members to study rotation and flaring inlow-mass pre-main-sequence stars.The present paper details the motivation, science goals and observingstrategy of the survey. We present a method to estimate the sensitivityand number of detections expected in each cluster, using a simplesemi-analytic approach which takes into account the characteristics ofthe cluster and photometric observations, using (tunable) best-guessassumptions for the incidence and parameter distribution of putativecompanions, and we incorporate the limits imposed by radial velocityfollow-up from medium and large telescopes. We use these calculations toshow that the survey as a whole can be expected to detect over 100 younglow and very low mass eclipsing binaries, and ~3 transiting planets withradial velocity signatures detectable with currently availablefacilities.
| A GLIMPSE of the Southern Jellyfish Nebula and Its Massive YSO
In Spitzer/IRAC images obtained under the GLIMPSE Legacy Survey, we haveidentified a unique and provocative nebular object we call the``Southern Jellyfish Nebula.'' The Southern Jellyfish Nebula ischaracterized by a fan of narrow tendrils with extreme length-to-widthratios that emanate from the vicinity of a bright infrared point sourceembedded in a smaller resolved nebula. From CO observations of theNebula's morphologically associated molecular cloud, we have derived akinematic distance of 5.7+/-0.8 kpc and a cloud mass of3.2+/-0.9×103 Msolar. The tendril-like ropesof the Nebula have widths of ~0.1 pc and lengths of up to ~2 pc. We haveintegrated the infrared spectral energy distribution (SED) of the pointsource to establish it as a massive young stellar object (MYSO), mostlikely forming alone, but possibly masking fainter cluster members. Theshape of the SED is consistent with the shape of a late Class 0 SEDmodel. Based on its far-IR luminosity of 3.3+/-0.9×104Lsolar, the Southern Jellyfish's MYSO has a zero-age mainsequence (ZAMS) spectral type of B0. Given the curious nature of thisnebula, we suspect its peculiar IR-bright structure is directly relatedto its current state of star formation.
| The onset of X-ray emission in young stellar objects. A Chandra observation of the Serpens star-forming region
Aims. To study the properties of X-ray emission from young stellarobjects (YSOs) through their evolution from Class I to Class III anddetermine whether Class 0 protostars emit X-rays. Methods: A deepChandra X-ray observation of the Serpens star-forming region wasobtained. The Serpens Cloud Core is ideally suited for this type ofinvestigation, being populated by a dense and extremely young clusterwhose members are found in all evolutionary stages, including sixwell-studied Class 0 sources. Results: None of the six Class 0protostars is detected in our observations, excluding the presence ofsources with typical X-ray luminosities ⪆ 0.4 ×1030 erg s-1 (for column densities of the order of4 × 1023 cm-2, or AV ˜200). A total of 85 X-ray sources are detected and the light curves andspectra of 35 YSOs are derived. There is a clear trend of decreasingabsorbing column densities as one moves from Class I to Class IIIsources, and some evidence of decreasing plasma temperatures, too. Weobserve a strong, long-duration, flare from a Class II low-mass star,for which we derive a flaring loop length of the order of 20 stellarradii. We interpret the flaring event as originating from a magneticflux tube connecting the star to its circumstellar disk. The presence ofsuch a disk is supported by the detection, in the spectrum of this star,of 6.4 keV Fe fluorescent emission.Section 6, Tables 5 and 6, and Fig. 13 are only available in electronicform at http://www.aanda.org
| Discovery of δ Scuti pulsation in the Herbig Ae star VV Serpentis
Context: .The study of pulsation in pre-main-sequence intermediate massstars represents an important tool for deriving information on thestellar parameters and structure, as well as for testing the validity ofcurrent theoretical models. The interest in this class of variable starshas significantly increased during the last decade and about 30 membersare presently known in the literature. Aims: .A new observationalstudy of the Herbig Ae star VV Ser has been performed to detect andaccurately measure pulsation frequencies in the δ Scuti range,thus enlarging the sample of known pulsators and contributing to theempirical definition of the pre-main-sequence instability strip. As itbelongs to the continuous field of view of the asteroseismologicalsatellite COROT, this study also aims at characterizing the propertiesof VV Ser as a potential "COROT additional program" candidate.Methods: .CCD time series photometry in the Johnson V filter has beenobtained for three consecutive years. The resulting light curves havebeen subject to detailed frequency analysis and the derived frequencieshave been compared to model predictions. Results: .Seven pulsationfrequencies have been measured on the basis of the best data setobtained in 2004, ranging from ~31 to ~118 μHz, with an accuracy ofthe order of 0.5 μHz. The comparison with an extensive set ofasteroseismological models shows that all the observed periodicities canbe reproduced if the stellar mass is close to 4~ Mȯ.Conversely, the measured frequencies can be associated with p modes onlyif the effective temperature is significantly lower than that obtainedfrom the spectral type conversion. Conclusions: .The presentresults seem to suggest that more accurate spectral type determinationis necessary to discriminate the best-fit model solution. In any case,the stellar mass of VV Ser is close to the upper mass limit (˜ 4~Mȯ) for this class of pulsators.Tables 3, 5 and 6 are only available in electronic form athttp://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.
| Nonradial Oscillations on a Pre-Main-Sequence Star
We show that the modes observed on the pulsating pre-main-sequence (PMS)star NGC 6383 170 can be interpreted as a combination of radial andnonradial p-modes. Using a dense grid of PMS models and oscillationspectra, we have identified models that match the observed oscillationspectrum within 1 σ of the frequency uncertainties and areconsistent with the star's position in the H-R diagram.
| Chandra Observations of the Eagle Nebula. I. Embedded Young Stellar Objects near the Pillars of Creation
We present and analyze the first high-resolution X-ray images everobtained of the Eagle Nebula star-forming region. On 2001 July 30 theChandra X-Ray Observatory obtained a 78 ks image of the Eagle Nebula(M16) that includes the core of the young galactic cluster NGC 6611 andthe dark columns of dust and cold molecular gas in M16 known as the``Pillars of Creation.'' We find a total of 1101 X-ray sources in the17'×17' ACIS-I field of view. Most of theX-ray sources are low-mass pre-main-sequence or high-mass main-sequencestars in this young cluster. A handful of hard X-ray sources in thepillars are spatially coincident with deeply embedded young stellarobjects seen in high-resolution near-infrared images recently obtainedwith the VLT (McCaughrean & Andersen). In this paper, we focus onthe 40 X-ray sources in and around pillars 1-4 at the heart of the EagleNebula. None of the X-ray sources are associated with the evaporatinggaseous globules (EGGs) first observed by Hester and coworkers) in HSTWFPC2 images of M16, implying either that the EGGs do not containprotostars or that the protostars have not yet become X-ray active.Eight X-ray counts are coincident with the Herbig-Haro object HH 216,implying logLX~30.0.
| The 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/L17
| WIYN open cluster study. XXVI. Improved kinematic membership and spectroscopy of IC 2391
Context: Young open clusters provide important clues to the interfacebetween the main sequence and pre-main-sequence phases of stellarevolution. The young and nearby open cluster IC 2391 is well-suited tostudies of these two evolutionary phases. Aims: We establish a bonafide set of cluster members and then analyze this set in terms of binaryfrequency, projected rotational velocities, [Fe/H], and lithiumabundance. In the wake of the Hipparcos distance controversy for thePleiades, we compare the main-sequence fitting distance modulus to theHipparcos mean parallax for IC 2391. Methods: We have obtained newproper motions for 6991 stars down to V ˜ 13-16 over a~9-deg2 area of the sky comprising IC 2391. The precision ofproper motions, σ_μ=1.7 mas yr-1, allowed us tocalculate reliable membership probabilities. We also obtained preciseradial velocity and v sin i measurements with Coravel and FEROS for 76probable cluster members. The cluster's mean radial velocity is+14.8±0.7 km s-1. The FEROS high-resolution spectrawere used to determine both the [Fe/H] abundance in the four mainsequence dwarfs of IC 2391 and the Li abundance in 47 stars. Inaddition, new BV CCD photometry was obtained for the majority ofprobable cluster members. Results: The proper-motion survey covers a 6times larger sky area than the prior targeted searches for clustermembers in IC 2391. A total of 66 stars are considered bona fide clustermembers down to a mass equivalent to 0.5 Mȯ. A quarterof them have been newly identified with many in the F2-K5 spectralrange, which is crucial for a main-sequence fit. We find a mean [Fe/H]value of +0.06±0.06, when a solar abundance of log ɛ(Fe)= 7.45 is adopted. The main sequence fitting yields a distance modulusthat is 0.19 mag larger than that derived from Hipparcos parallaxes;thus this offset nearly has the size of a similar offset found for thePleiades. The Li abundance pattern is similar to the earlier findingsand is typical for a 40 Myr old open cluster. Conclusions: .A varietyof new data on the probable members of IC 2391 improve essentially allobservational parameters of this young open cluster.
| New magnetic chemically peculiar stars
Spectropolarimetric observations of 96 chemically peculiar (CP)main-sequence stars have been carried out at the 6-m telescope at theSpecial Astrophysical Observatory of the Russian Academy of Sciences(SAO RAS) with the aim of searching for the presence of stellar magneticfields. The stars selected for investigation were CP stars known to havestrong anomalies in the wavelength region of the continuum fluxdepression around λ 5200Å. This selection was conductedwith the aid of low-resolution spectral observations, made with the SAORAS 1-m telescope, and of published differential photometric data.Magnetic fields have been successfully detected in 72 stars of whichonly three stars were previously known to have magnetic fields. For twostars, the longitudinal component of the magnetic field Beexceeds 5 kG: HD178892 - 7.4 kG, and HD258686 - 6.7 kG. We failed toreliably detect the magnetic field in the other 24 CP stars. These starsare mostly fast rotators, a feature which hampers accurate measurementsof Be. It is demonstrated in this paper that selectingcandidate magnetic stars by considering their photometric indices Z orΔa, or alternatively, by inspecting low-resolution spectra aroundthe λ5200Å flux depression, considerably increases thedetection rate.This paper is based on data obtained at the 6-m telescope of the RussianAcademy of Sciences.E-mail: dkudr@sao.ru
| Deep Imaging Surveys of Star-forming Clouds. IV. The Meek and the Mighty: Outflows from Young Stars in Chamaeleon I
We present a survey of shocks and outflows in the Chamaeleon Istar-forming complex using Hα, [S II], and SDSS i' images obtainedfrom the ground, an i' image obtained with the Hubble Space Telescope,and 4.5 μm images obtained with the Infrared Array Camera (IRAC) onthe Spitzer Space Telescope. We find new Herbig-Haro (HH) objects andextensions to the previously cataloged shocks that trace parts of atleast 20 distinct outflows from young stars. Some HH objects mark thepresence of giant outflows, the largest of which is powered by Cha-MMS1and associated with HH 49/50 near the Ced 110 region. Other large flowsare powered by Cha-MMS2 in the Ced 112 region and the IRN in the Ced 111region. Although some shocks exhibit infrared emission in the IRACbands, most notably HH 49/50 (the ``tornado''), most outflows in the ChaI clouds are not detected in the Spitzer IRAC bands. This result isconsistent with the general lack of extensive 2.12 μm H2emission from Cha I.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars
T Tauri stars are variable stars that are in an early phase ofevolution, in which accretion and contraction to the main sequence arestill taking place. Their photometric variability is complex; it takesplace on a variety of timescales, due to a variety of physicalprocesses. Periodic variability occurs due to rotation and the presenceof cool or hot spots on the star. It may also occur due to periodicobscuration of the star by inhomogeneities in the still presentaccretion disk. But the periodicity may be masked by other forms ofvariability, or by time variation in the cool or hot spots, or theobscuring inhomogeneities. For other types of variable stars,self-correlation has proven to be a useful adjunct to Fourier analysisfor studying semiregular variability; it determines the cycle-to-cyclebehavior of the star, averaged over all the data. We have therefore usedit to investigate the photometric variability of about 30 T Tauri starsusing existing data. It has provided useful information about periodsand their coherence, about the amplitude of the periodic variation, orits upper limit, and about the ``profile'' of the amplitude-timescalebehavior. In most cases, it has confirmed periods previously determinedby Fourier analysis, but in some cases it has suggested that thepreviously determined period is spurious.
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