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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.
| The spatial distribution of substellar objects in IC 348 and the Orion Trapezium cluster
Context: Some theoretical scenarios suggest the formation of browndwarfs as ejected stellar embryos in star-forming clusters. Such aformation mechanism can result in different spatial distributions ofstars and substellar objects. Aims: We aim to investigate the spatialstructure of stellar and substellar objects in two well-sampled andnearby embedded clusters, namely IC 348 and the Orion Trapezium cluster(OTC), to test this hypothesis. Methods: Deep near-infrared K-band datathat are complete enough to sample the substellar population in IC 348and OTC are obtained from the literature. The spatial distribution ofthe K-band point sources is analysed using the minimum spanning tree(MST) method. The Q parameter and the spanning trees are evaluated forstellar and substellar objects as a function of cluster core radius R_c. Results: The stellar population in both IC 348 and OTC display aclustered distribution, whereas the substellar population is distributedhomogeneously in space within twice the cluster core radius. Althoughthe substellar objects do not appear to be bound by the clusterpotential well, they are still within the limits of the cluster and notsignificantly displaced from their birth sites. Conclusions: Thespatially homogeneous distribution of substellar objects is bestexplained by assuming higher initial velocities, distributed in a randommanner and going through multiple interactions. The overall spatialcoincidence of these objects with the cluster locations can beunderstood if these objects are nevertheless travelling slowly enough tofeel the gravitational influence of the cluster. The observationssupport the scenario of substellar objects forming as "ejected stellarembryos". Higher ejection velocities are necessary, but net spatialdisplacements may not be needed to explain the observational data.
| Spitzer: Accretion in Low-Mass Stars and Brown Dwarfs in the λ Orionis Cluster
We present multiwavelength optical and IR photometry of 170 previouslyknown low-mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster(λ Orionis). The new photometry supports cluster membership formost of them, with less than 15% of the previous candidates identifiedas probable nonmembers. The near-IR photometry allows us to identifystars with IR excesses, and we find that the Class II population is verylarge, around 25% for stars (in the spectral range M0-M6.5) and 40% forbrown dwarfs, down to 0.04 Msolar, despite the fact that theHα equivalent width is low for a significant fraction of them. Inaddition, there are a number of substellar objects, classified as ClassIII, that have optically thin disks. The Class II members aredistributed in an inhomogeneous way, lying preferentially in a filamentrunning toward the southeast. The IR excesses for the Collinder 69members range from pure Class II (flat or nearly flat spectra longwardof 1 μm), to transition disks with no near-IR excess but excessesbeginning within the IRAC wavelength range, to two stars with excessonly detected at 24 μm. Collinder 69 thus appears to be at an agewhere it provides a natural laboratory for the study of primordial disksand their dissipation.Based on observations collected by the Spitzer Space Telescope, at theGerman-Spanish Astronomical Center of Calar Alto jointly operated by theMax-Planck-Institut für Astronomie Heidelberg and the Instituto deAstrofísica de Andalucía (CSIC), and at the WHT operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| Terrestrial Zone Debris Disk Candidates in h and χ Persei
We analyze eight sources with strong mid-infrared excesses in the 13 Myrold double cluster h and χ Persei. New optical spectra and broadbandspectral energy distributions (SEDs; 0.36-8 μm) are consistent withcluster membership. We show that the material with T~300-400 K andLd/L*~10-4 to 10-3 producesthe excesses in these sources. Optically thick blackbody disk models,including those with large inner holes, do not match the observed SEDs.The SEDs of optically thin debris disks produced from terrestrial planetformation calculations match the observations well. Thus, some h andχ Persei stars may have debris from terrestrial zone planetformation.
| The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. IX. The Serpens YSO Population as Observed with IRAC and MIPS
We discuss the combined IRAC/MIPS c2d Spitzer Legacy observations of theSerpens star-forming region. We describe criteria for isolating bonafide YSOs from the extensive background of extragalactic objects. Wethen discuss the properties of the resulting high-confidence set of 235YSOs. An additional 51 lower confidence YSOs outside this area areidentified from the MIPS data and 2MASS photometry. We presentcolor-color diagrams to compare our observed source properties withthose of theoretical models for star/disk/envelope systems and our ownmodeling of the objects that are well represented by a stellarphotosphere plus circumstellar disk. These objects exhibit a wide rangeof disk properties, from many with actively accreting disks to some withboth passive disks and even possibly debris disks. The YSO luminosityfunction extends down to at least a few times 10-3Lsolar or lower. The lower limit may be set more by ourinability to distinguish YSOs from extragalactic sources than by thelack of YSOs at very low luminosities. We find no evidence forvariability in the shorter IRAC bands between the two epochs of our dataset, Δt~6 hr. A spatial clustering analysis shows that thenominally less evolved YSOs are more highly clustered than the laterstages. The background extragalactic population can be fitted by thesame two-point correlation function as seen in other extragalacticstudies. We present a table of matches between several previous infraredand X-ray studies of the Serpens YSO population and our Spitzer dataset. The clusters in Serpens have a very high surface density of YSOs,primarily with SEDs suggesting extreme youth. The total number of YSOs,mostly Class II, is greater outside the clusters.
| Infrared Extinction toward Nearby Star-forming Regions
We present an independent estimate of the interstellar extinction lawfor the Spitzer IRAC bands, as well as a first attempt at extending thelaw to the 24 μm MIPS band. The source data for these measurementsare observations of five nearby star-forming regions: the Orion A cloud,NGC 2068/2071, NGC 2024/2023, Serpens, and Ophiuchus. Color excessratios EH-Ks/EKs-[λ]were measured for stars without infrared excess dust emission fromcircumstellar disks/envelopes. For four of these five regions, theextinction laws are similar at all wavelengths and differ systematicallyfrom a previous determination of the extinction law, which was dominatedby the diffuse ISM, derived for the IRAC bands. This difference could bedue to the difference in the dust properties of the dense molecularclouds observed here and those of the diffuse ISM. The extinction law atlonger wavelengths toward the Ophiuchus region lies between that to theother four regions studied here and that for the ISM. In addition, weextended our extinction law determination to 24 μm for Serpens andNGC 2068/2071 using Spitzer MIPS data. We compare these results againstseveral ISO extinction law determinations, although in each case thereare assumptions which make absolute comparison uncertain. However, ourwork confirms a relatively flatter extinction curve from 4 to 8 μmthan the previously assumed standard, as noted by all of these recentstudies. The extinction law at 24 μm is consistent with previousmeasurements and models, although there are relatively largeuncertainties.
| A Spitzer 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.
| Embedded Clusters: Laboratories for Star Formation
Embedded clusters are the fundamental units of star formation in ourGalaxy, therefore studying their properties is critical forunderstanding how star formation proceeds on both the local and Galacticscale. We have surveyed embedded clusters in local molecular clouds withFLAMINGOS and are investigating the star forming histories, IMF,structure and evolution of these young clusters. In this presentation, Iwill discuss our results for the clusters in the Rosette and Orion starforming complexes where we find evidence for the evolution of clusterstructure and variations in the low mass end of the initial massfunction.
| 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%.
| The Detection of Protostellar Condensations in Infrared Dark Cloud Cores
Infrared dark clouds (IRDCs) are a distinct class of interstellarmolecular cloud identified as dark extinction features against thebright mid-infrared Galactic background. Here we present high angularresolution millimeter continuum images obtained with the IRAM Plateau deBure Interferometer toward four high-mass (200-1800 Msolar)IRDC cores that show evidence for active high-mass star formation(M>8 Msolar). We detect twelve bright (>7 mJy), compact(<~2", <~0.024 pc) condensations toward these cores. Two of thecores (G024.60+00.08 MM1 and G024.60+00.08 MM2) are resolved intomultiple protostellar condensations, while one core (G022.35+00.41 MM1)shows two condensations. The remaining core (G024.33+00.11 MM1) containsa single, compact protostellar condensation with a very rich molecularspectrum, indicating that this is a hot molecular core associated withan early stage in the formation of a high-mass protostar. The derivedgas masses for these condensations suggest that each core is forming atleast one high-mass protostar (Mgas>8 Msolar),and three cores are also forming lower mass protostars(Mgas~2-5 Msolar). A comparison of the ratios ofthe gas masses (MG) to the Jeans masses (MJ) forIRDCs, cores, and condensations, provides broad support for the idea ofhierarchical fragmentation. The close proximity of multiple protostarsof disparate mass indicates that these IRDCs are in the earliestevolutionary states in the formation of stellar clusters.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany),and IGN (Spain).
| 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.
| The Role of Mass and Environment in Multiple-Star Formation: A 2MASS Survey of Wide Multiplicity in Three Young Associations
We present the results of a search for wide binary systems among 783members of three nearby young associations: Taurus-Auriga, Chamaeleon I,and two subgroups of Upper Scorpius. Near-infrared (JHK) imagery from2MASS was analyzed to search for wide (1"-30" ~150-4500 AU) companionsto known association members, using color-magnitude cuts to rejectlikely background stars. We identify a total of 131 candidate binarycompanions with colors consistent with physical association, of which 39have not been identified previously in the literature. Our resultssuggest that the wide binary frequency is a function of both mass andenvironment, with significantly higher frequencies among high-mass starsthan lower mass stars and in the T associations than in the OBassociation. We discuss the implications for wide binary formation andconclude that the environmental dependence is not a direct result ofstellar density or total association mass, but instead might depend onanother environmental parameter like the gas temperature. The binarypopulations in these associations generally follow the empiricalmass-maximum separation relation observed for field binaries, but wehave found one candidate low-mass system (USco 160611.9-193532Mtot~0.4 Msolar) that has a projected separation(10.8" 1550 AU) much larger than the suggested limit for its mass.Finally, we find that the binary frequency in the USco-B subgroup issignificantly higher than in the USco-A subgroup and is consistent withthe measured values in Taurus and ChamI. This discrepancy, the absenceof high-mass stars in USco-B, and its marginally distinct kinematicssuggest that it might not be directly associated with the OBassociations of Sco-Cen but instead represents an older analog of theyounger ρ Oph or Lupus associations.
| New pre-main sequence candidates in the Taurus-Auriga star forming region
Aims.We have studied the X-ray source population of the Taurus MolecularCloud (TMC) to search for new members of the Taurus-Auriga star formingregion. Methods: Candidate members have been selected among the X-raysources detected in 24 fields of the XMM-Newton Extended Survey of theTaurus Molecular Cloud, having an IR counterpart in the 2MASS catalog,based on color-magnitude and color-color diagrams. Their X-ray spectralproperties have been compared with those of known members and otherX-ray sources in the same fields but without a NIR counterpart. A searchfor flare-like variability in the time series of all new candidates andthe analysis of the X-ray spectra of the brightest candidates have beenused to identify sources with a high probability of membership. Results: We have found that 347 of 1909 detected X-ray sources have aninfrared counterpart in the 2MASS catalogue. Among them, we haveselected 57 sources that are consistent with being new pre-main sequencestar candidates at the distance of the Taurus-Auriga star formingregion; the X-ray spectral properties of this sample are, on the whole,similar to the properties of known TMC members and different from thoseof X-ray sources without an IR counterpart, most of which are likely tobe of extragalactic origin. For 12 such candidates, the likelihood ofmembership is very high, based on the relatively high plasmatemperatures derived from their X-ray spectra and/or the observation ofpowerful flares in their light curves.Table [see full textsee full textsee full textsee full textsee fulltext] and Fig. [see full textsee full textsee full textsee full textseefull text] are only available in electronic form at http://www.aanda.org
| X-ray emission from the young brown dwarfs of the Taurus molecular cloud
Aims.We report the X-ray properties of young (~3 Myr) bona fide browndwarfs of the Taurus Molecular Cloud (TMC). Methods: The XMM-NewtonExtended Survey of the TMC (XEST) is a large program designed tosystematically investigate the X-ray properties of youngstellar/substellar objects in the TMC. In particular, the area surveyedby 15 XMM-Newton pointings (of which three are archival observations),supplemented with one archival Chandra observation, allows us to study17 brown dwarfs with M spectral types. Results: Half of this sample (9out of 17 brown dwarfs) is detected; 7 brown dwarfs are detected herefor the first time in X-rays. We observed a flare from one brown dwarf.We confirm several previous findings on brown dwarf X-ray activity: alog-log relation between X-ray and bolometric luminosity for stars (withL* ≤ 10 L_ȯ) and brown dwarfs detected in X-rays,which is consistent with a mean X-ray fractional luminosity =-3.5 ± 0.4; for the XEST brown dwarfs, themedian of log(L_X/L_*) (including upper limits) is -4.0; a shallowlog-log relation between X-ray fractional luminosity and mass; a log-logrelation between X-ray fractional luminosity and effective temperature;a log-log relation between X-ray surface flux and effective temperature.We find no significant log-log correlation between the X-ray fractionalluminosity and EW(Hα). Accreting and nonaccreting brown dwarfshave a similar X-ray fractional luminosity. The median X-ray fractionalluminosity of nonaccreting brown dwarfs is about 4 times lower than themean saturation value for rapidly rotating low-mass field stars. Our TMCbrown dwarfs have higher X-ray fractional luminosity than brown dwarfsin the Chandra Orion Ultradeep Project. Conclusions: The X-rayfractional luminosity declines from low-mass stars to M-type browndwarfs, and as a sample, the brown dwarfs are less efficient X-rayemitters than low-mass stars. We thus conclude that while the browndwarf atmospheres observed here are mostly warm enough to sustaincoronal activity, a trend is seen that may indicate its gradual declinedue to the drop in photospheric ionization degree.Appendices A, B, C, are only available in electronic form athttp://www.aanda.org
| Characterizing the Disk around the TW Hydrae Association Brown Dwarf 2MASSW J1207334-393254
We present detailed modeling of the disk around the TW HydraeAssociation (TWA) brown dwarf 2MASSW J1207334-393254 (2M1207), usingSpitzer observations from 3.6 to 24 μm. The spectral energydistribution (SED) does not show a high amount of flaring. We haveobtained a good fit using a flat disk of mass between 10-4and 10 -6 Msolar, M˙<~10-11Msolar yr-1, and a large inclination angle between60° and 70°. We have used three different grain models to fitthe 10 μm Si emission feature, and have found the results to beconsistent with ISM-like dust. In comparison with other TWA members,this suggests lesser dust processing for 2M1207, which could beexplained by mechanisms such as aggregate fragmentation and/or turbulentmixing. We have found a good fit using an inner disk radius equal to thedust sublimation radius, which indicates the absence of an inner hole inthe disk. This suggests the presence of a small K-L' excess,similar to the observed K-[3.6] excess.
| Evolution of Brown Dwarf Disks: A Spitzer Survey in Upper Scorpius
We have carried out a Spitzer survey for brown dwarf disks in the ~5 Myrold Upper Scorpius (UpSco) star-forming region, using IRS spectroscopyfrom 8 to 12 μm and MIPS photometry at 24 μm. Our sample consistsof 35 confirmed very low mass members of UpSco. Thirteen objects in thissample show clear excess flux at 24 μm, explained by dust emissionfrom a circum-substellar disk. The spectral energy distributions (SEDs)of the remaining objects are consistent with pure photospheric emission.Objects without excess emission have either no disks or disks with inneropacity holes of at least ~5 AU radii. Our disk frequency of 37%+/-9% ishigher than what has been derived previously for K0-M5 stars in the sameregion (1.8 σ confidence level), suggesting a mass-dependent disklifetime in UpSco. The clear distinction between objects with andwithout disks, as well as the lack of transition objects, shows thatdisk dissipation inside 5 AU occurs rapidly, probably on timescales of<~105 yr. For the objects with disks, most SEDs areuniformly flat, with flux levels of a few mJy, well modeled as emissionfrom dusty disks affected by dust settling to the midplane. The silicatefeature around 10 μm is either absent or weak in our SEDs, arguingfor a lack of hot, small dust grains. Compared with younger objects inTaurus, brown dwarf disks in UpSco show less flaring. These resultsclearly demonstrate that we see disks in an advanced evolutionary state.Dust settling and grain growth are ubiquituous in circum-substellardisks at ages of 5 Myr, arguing for planet-forming processes in browndwarf disks.
| Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr
We present results from a mid-infrared imaging survey of the ~5 Myr oldcluster NGC 2362 carried out with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. The archival mid-infrared data weremerged with extant Hα emission data, optical and near-infraredphotometry, and moderate-resolution optical spectroscopy to identify theremnant disk-bearing population of the cluster and to estimate thefraction of stars that still retain primordial circumstellar disks. Theprincipal sample of 232 suspected cluster members with masses rangingfrom ~10 to 0.3 Msolar (B2-M5 spectral types) was drawn fromknown Hα emission stars, X-ray-detected stars from a single 100 ksarchival Chandra observation, and established lithium-rich stars. Asecond sample of 153 stars over a similar mass range whose membershipstatus was based on optical photometry alone was also examined. Measuredfluxes in the optical and infrared passbands were fitted with synthetic,low-resolution spectra created using the NextGen atmospheric models,permitting the detection of infrared excesses relative to predictedstellar photospheric fluxes. Using the measured slope of the stellarspectral energy distribution through the four IRAC channels tocharacterize disk emission for the 195 out of 232activity/lithium-selected stars and the 105 out of 153 photometricmembership candidates having complete IRAC photometry, we derive anupper limit for the primordial, optically thick disk fraction of NGC2362 of ~7%+/-2%, with another ~12%+/-3% of suspected members exhibitinginfrared excesses indicative of weak or optically thin disk emission.The presence of circumstellar disks among candidate members of NGC 2362is strongly mass-dependent, such that no stars more massive than ~1.2Msolar exhibit significant infrared excess shortward of 8μm. An upper limit for the fraction of stars hosting primordial,optically thick disks peaks near 10.7%+/-4% for stars with massesbetween 1.05 and 0.6 Msolar, but the Spitzer IRAC survey issensitivity-limited below ~0.3 Msolar. From Hαemission-line strengths, an upper limit for the accretion fraction ofthe cluster is estimated at ~5%, with most suspected accretorsassociated with primordial, optically thick disks identified withSpitzer. The presence of primordial disk-bearing stars in NGC 2362, someof which are suspected of still experiencing gaseous accretion, mayimply that even within dense cluster environments, sufficient numbers ofinner disks survive to ages consistent with core accretion models ofgiant planet formation to account for the observed frequency ofexoplanets within 5 AU of all FGKM-type stars.
| The Keele-Exeter young cluster survey - I. Low-mass pre-main-sequence stars in NGC 2169
We have used RCIC CCD photometry from the IsaacNewton telescope and intermediate-resolution spectroscopy from theGemini North telescope to identify and characterize low-mass (0.15
| 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.
| On the circum(sub)stellar environment of brown dwarfs in Taurus
Aims.We want to investigate whether brown dwarfs (BDs) form like starsor are ejected embryos. We study the presence of disks around BDs in theTaurus cloud, and discuss implications for substellar formation models. Methods: We use photometric measurements from the visible to the farinfrared to determine the spectral energy distributions (SEDs) of TaurusBDs. Results: We use Spitzer color indices, Hα as an accretionindicator, and models fit to the SEDs in order to estimate physicalparameters of the disks around these BDs. We study the spatialdistribution of BDs with and without disks across the Taurus aggregates,and we find that BDs with and without disks are not distributedregularly across the Taurus cloud. Conclusions: .We find that48%±14% of Taurus BDs have a circumstellar disk signature, aratio similar to recent results from previous authors in other regions.We fit the SEDs and find that none of the disks around BDs in Taurus canbe fitted convincingly with a flaring index β = 0, indicating thatheating by the central object is efficient and that the disks we observeretain a significant amount of gas. We find that BDs with disks areproportionally more numerous in the northern Taurus filament, possiblythe youngest filament. We do not find such a clear segregation forclassical T Tauri stars (CTTS) and weak-lined T Tauri stars (WTTS),suggesting that, in addition to the effects of evolution, anysegregation effects could be related to the mass of the object. Aby-product of our study is to propose a recalibration of the Barrado yNavascués & Martín (2003) accretion limit in thesubstellar domain. The global shape of the limit fits our data points ifit is raised by a factor 1.25-1.30.Based on observations made at ESO, CFHT, 2MASS, & Spitzer.
| 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.
| 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.
| 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]-[24]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 mid-infrared study of very low mass stars and brown dwarfs in Upper Scorpius
Context: To investigate the formation of sub-stellar objects, weobserved a sample of ultracool dwarf members of the Upper Scorpius OBassociation. Aims: The properties of disks, such as their composition,life-time, and frequency compared to the accretor frequency, provideimportant clues to the mechanisms responsible for the formation ofstellar and sub-stellar objects. Methods: We report the results ofmid-IR observations with VISIR at the VLT of 10 ultracool dwarf membersof the nearby Upper Scorpius OB association in four filters ranging from8.59 (PAH1) to 12.8 μm (Ne II) and one brown dwarf obserbed withSpitzer between 3.6 and 24 μm. Results: Seven of our targets weredetected in at least one of the bands, and we derived upper limits onthe fluxes of the remaining 4. These results combined with previousstudies from the literature lead to an improved disk frequency of50±12%. This frequency is significantly higher than for accretors(16.3%±6.2%). Only one object showing mid-IR excess also hasHα emission at a level that indicates it must be accreting. Fourof the detected targets are multiple system candidates. Conclusions:.The observed disk frequency for sub-stellar objects in the UpperScorpius association is similar to that of stars, consistent with acommon formation scenario. It is also similar to the disk fractionsobserved in younger clusters, suggesting that the disk lifetimes mightbe longer for ultracool dwarfs than for higher-mass stars.Based on observations made with ESO Telescopes at the ParanalObservatories under program ID 075.C-0148.
| On the Luminosity of Young Jupiters
Traditional thermal evolution models of giant planets employ arbitraryinitial conditions selected more for computational expediency thanphysical accuracy. Since the initial conditions are eventually forgottenby the evolving planet, this approach is valid for mature planets, ifnot young ones. To explore the evolution at young ages of jovian massplanets, we have employed model planets created by one implementation ofthe core-accretion mechanism as initial conditions for evolutionarycalculations. The luminosities and early cooling rates of young planetsare highly sensitive to their internal entropies, which depend on theformation mechanism and are highly model dependent. As a result of theaccretion shock through which most of the planetary mass is processed,we find lower initial internal entropies than commonly assumed inpublished evolution tracks. Consequently, young Jovian planets aresmaller, cooler, and several to 100 times less luminous than predictedby earlier models. Furthermore, the time interval during which the youngJupiters are fainter than expected depends on the mass of planet.Jupiter mass planets (1MJ) align with the conventional modelluminosity in as little at 20 million years, but 10MJ planetscan take up to 1 billion years to match commonly cited luminosities,given our implementation of the core-accretion mechanism. If ourassumptions, especially including our treatment of the accretion shock,are correct and if extrasolar Jovian planets indeed form with lowentropy, then young Jovian planets are substantially fainter at youngages than currently believed. Furthermore, early evolution tracks shouldbe regarded as uncertain for much longer than the commonly quoted106 yr. These results have important consequences both fordetection strategies and for assigning masses to young Jovian planetsbased on observed luminosities.
| 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.
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