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An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster
We report new Spitzer Space Telescope observations, using the IRAC andMIPS instruments, of the young (~3 Myr) σ Orionis cluster. Weidentify 336 stars as members of the cluster, using optical andnear-infrared color-magnitude diagrams. Using the spectral energydistribution slopes in the IRAC spectral range, we place objects intoseveral classes: non-excess stars, stars with optically thick disks(such as classical T Tauri stars), class I (protostellar) candidates,and stars with ``evolved disks'' the last exhibit smaller IRAC excessesthan optically thick disk systems. In general, this classificationagrees with the location expected in IRAC-MIPS color-color diagrams forthese objects. We find that the evolved disk systems are mostly acombination of objects with optically thick but nonflared disks,suggesting grain growth and/or settling, and transition disks, systemsin which the inner disk is partially or fully cleared of small dust. Inall, we identify seven transition disk candidates and three possibledebris disk systems. As in other young stellar populations, the fractionof disks depends on the stellar mass, ranging from ~10% for stars in theHerbig Ae/Be mass range (>2 Msolar) to ~35% for those inthe T Tauri mass range (1-0.1 Msolar). The IRAC infraredexcesses found in stellar clusters and associations with and withoutcentral high-mass stars are similar, suggesting that externalphotoevaporation is not very important in many clusters. Finally, wefind no correlation between the X-ray luminosity and the disk infraredexcess, suggesting that the X-rays are not strongly affected by diskaccretion.
| 25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a
We report here on the photometric and kinematic properties of awell-defined group of nearly 200 low-mass pre-main-sequence stars,concentrated within ~1° of the early-B star 25 Ori, in the OrionOB1a subassociation. We refer to this stellar aggregate as the 25Orionis group. The group also harbors the Herbig Ae/Be star V346 Ori anda dozen other early-type stars with photometry, parallaxes, and somewith IR excess emission, indicative of group membership. The number ofhigh- and low-mass stars is in agreement with expectations from astandard initial mass function. The velocity distribution for thelow-mass stars shows a narrow peak at 19.7 km s-1, offset~-10 km s-1 from the velocity characterizing the youngerstars of the Ori OB1b subassociation, and -4 km s-1 from thevelocity of widely spread young stars of the Ori OB1a population; thisresult provides new and compelling evidence that the 25 Ori group is adistinct kinematic entity, and that considerable space and velocitystructure is present in the Ori OB1a subassociation. The low-massmembers follow a well-defined band in the color-magnitude diagram,consistent with an isochronal age of ~7-10 Myr. The ~2 time drop in theoverall Li I equivalent widths and accretion fraction between theyounger Ori OB1b and the 25 Ori group is consistent with the latterbeing significantly older. In a simple-minded kinematic evolutionscenario, the 25 Ori group may represent the evolved counterpart of theyounger σ Ori cluster. The 25 Ori stellar aggregate is the mostpopulous ~10 Myr sample yet known within 500 pc, setting it as anexcellent laboratory to study the evolution of solar-like stars andprotoplanetary disks.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by CIDA for theMinisterio de Ciencia y Tecnología the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona;and the Fred Lawrence Whipple Observatory of the SmithsonianInstitution.
| 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.
| Remarks on Rapid vs. Slow Star Formation
We discuss problems with some observational estimates indicating longprotostellar core lifetimes and large stellar age spreads in molecularclouds. We also point out some additional observational constraintswhich suggest that protostellar cores do not have long lifetimes beforecollapsing. For external galaxies, we argue that the widths of spiralarms do not imply a long star-formation process, since the formation ofmassive stars will disrupt molecular clouds, move material around,compress it in other regions which produce new star-forming clouds.Thus, it seems unavoidable that this cyclical process will result in anextended period of enhanced star formation, which does not represent thesurvival time of any individual molecular cloud. We argue that the rapidstar formation indicated observationally is also easier to understandtheoretically than the traditional scenario of slow quasi-staticcontraction with ambipolar diffusion.
| 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
| Silicate Dust in Evolved Protoplanetary Disks: Growth, Sedimentation, and Accretion
We present the Spitzer IRS spectra for 33 young stars in Tr 37 and NGC7160. The sample includes the high- and intermediate-mass stars withMIPS 24 μm excess, the only known active accretor in the 12 Myr oldcluster NGC 7160, and 19 low-mass stars with disks in the 4 Myr oldcluster Tr 37. We examine the 10 μm silicate feature, present in thewhole sample of low-mass stars and in three of the high- andintermediate-mass targets, and we find that PAH emission is detectableonly in the Herbig Be star. We analyze the composition and size of thewarm photospheric silicate grains by fitting the 10 μm silicatefeature and study the possible correlations between the silicatecharacteristics and the stellar and disk properties (age, SED slope,accretion rate, and spectral type). We find indications of dust settlingwith age and of the effect of turbulent enrichment of the diskatmosphere with large grains. Crystalline grains are only smallcontributors to the total silicate mass in all disks and do not seem tocorrelate with any other property, except maybe binarity. We alsoobserve that spectra with very weak silicate emission are at least 3times more frequent among M stars than among earlier spectral types,which may be evidence of inner disk evolution. Finally, we find thatfive of the high- and intermediate-mass stars have SEDs and IRS spectraconsistent with debris disk models involving planet formation, whichcould indicate debris disk formation at ages as early as 4 Myr.
| 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.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| 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.
| X-Ray Study of Triggered Star Formation and Protostars in IC 1396N
The IC 1396N cometary globule (CG) within the large nearby H II regionIC 1396 has been observed with the ACIS detector on board the ChandraX-Ray Observatory. We detect 117 X-ray sources, of which ~50-60 arelikely members of the young open cluster Trumpler 37 dispersedthroughout the H II region, and 25 are associated with young starsformed within the globule. Infrared photometry (2MASS and Spitzer) showsthat the X-ray population is very young: 3 older Class III stars, 16classical T Tauri stars, and 6 protostars including a Class 0/I system.We infer a total T Tauri population of ~30 stars in the globule,including the undetected population, with a star formation efficiency of1%-4%. An elongated source spatial distribution with an age gradientoriented toward the exciting star is discovered in the X-ray populationof IC 1396N, supporting similar findings in other cometary globules. Thegeometric and age distribution is consistent with the radiation-drivenimplosion (RDI) model for triggered star formation in CGs by H II regionshocks. The inferred velocity of the shock front propagating into theglobule is ~0.6 km s-1. The large number of X-ray-luminousprotostars in the globule suggests either an unusually high ratio ofClass I/0 to Class II/III stars or a nonstandard initial mass functionfavoring higher mass stars by the triggering process. We find that theChandra source associated with the luminous Class 0/I protostar IRAS21391+5802 is one of the youngest stars ever detected in the X-ray band.We also establish for the first time that the X-ray absorption inprotostars arises from the local infalling envelopes rather than fromambient molecular cloud material.
| The first CO(1-0) mapping of the globule IC 1396 W
Near-infrared observations indicate that three H2 outflowsand their driving sources are present in the globule IC 1396 W, wherethe existence of molecular outflows has also been suggested by someauthors. We made the first CO(1-0) map of IC 1396 W, and found that itsCO molecular cloud may consist of three physically distinct componentswith different velocities. We detected neither molecular outflows northe dense cores associated with candidate driving sources. One possiblereason is that CO(1-0) and its isotopes cannot trace high density gas,and another is that the beam of our observation is too large to observethem. The CO cloud may be one part of the natal molecular cloud of IC1396 W, in the process of disrupting and blowing away. The CO cloudseems to be in the foreground of the H2 outflows.
| Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr Old Upper Scorpius OB Association
We present 4.5, 8, and 16 μm photometry from the Spitzer SpaceTelescope for 204 stars in the Upper Scorpius OB association. The dataare used to investigate the frequency and properties of circumstellardisks around stars with masses between ~0.1 and 20 Msolar atan age of ~5 Myr. We identify 35 stars that have emission at 8 or 16μm in excess of the stellar photosphere. The lower mass stars(~0.1-1.2 Msolar) appear surrounded by primordial opticallythick disks based on the excess emission characteristics. Stars moremassive than ~1.8 Msolar have lower fractional excessluminosities suggesting that the inner ~10 AU of the disk has beenlargely depleted of primordial material. None of the G and F stars(~1.2-1.8 Msolar) in our sample have an infrared excess atwavelengths <=16 μm. These results indicate that the mechanismsfor dispersing primordial optically thick disks operate lessefficiently, on average, for low-mass stars, and that longer timescalesare available for the buildup of planetary systems in the terrestrialzone for stars with masses <~1 Msolar.
| High-Resolution Spectroscopy in Tr 37: Gas Accretion Evolution in Evolved Dusty Disks
Using the Hectochelle multifiber spectrograph, we have obtainedhigh-resolution (R~34,000) spectra in the Hα region for a largenumber of stars in the 4 Myr old cluster Tr 37, containing 146previously known members and 26 newly identified ones. We present theHα line profiles of all members, compare them to our IRobservations of dusty disks (Two Micron All Sky Survey JHK + IRAC + MIPS24 μm), use the radial velocities as a membership criterion, andcalculate the rotational velocities. We find a good correlation betweenthe accretion-broadened profiles and the presence of protoplanetarydisks, noting that a small fraction of the accreting stars presentsbroad profiles with Hα equivalent widths smaller than thecanonical limit separating classical T Tauri stars (CTTSs) andweak-lined T Tauri stars (WTTSs). The number of strong accretors appearsto be lower than in younger regions, and a large number of CTTSs havevery small accretion rates (M˙<=10-9 Msolaryr-1). Taking into account that the spectral energydistributions are consistent with dust evolution (grain growth/settling)in the innermost disk, this suggests a parallel evolution of the dustyand gaseous components. We also observe that about half of the``transition objects'' (stars with no IR excesses at λ<=6μm) do not show any signs of active accretion, whereas the other halfis accreting with accretion rates <=10-9 Msolaryr-1. These zero or very low accretion rates reveal importantgas evolution and/or gas depletion in the innermost disk, which could berelated to grain growth up to planetesimal or even planet sizes.Finally, we examine the rotational velocities of accreting andnonaccreting stars, finding no significant differences that couldindicate disk locking at these ages.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
| Spitzer MIPS 24 μm Detection of Photoevaporating Protoplanetary Disks
We present 24 μm images of three protoplanetary disks beingphotoevaporated around high-mass O-type stars. These objects have``cometary'' structure where the dust pulled away from the disk by thephotoevaporating flow is forced away from the O star by photon pressureon the dust and heating and ionization of the gas. Models of the 24 and8 μm brightness profiles agree with this hypothesis. These modelsshow that the mass-loss rate needed to sustain such a configuration isin agreement with or somewhat less than the theoretical predictions forthe photoevaporation process.
| Why Do T Tauri Disks Accrete?
Observations of T Tauri stars and young brown dwarfs suggest that theaccretion rates of their disks scale strongly with the central stellarmass, approximately M˙~M2*. No dependence ofaccretion rate on stellar mass is predicted by the simplest version ofthe Gammie layered disk model, in which nonthermal ionization of upperdisk layers allows accretion to occur via the magnetorotationalinstability. We show that a minor modification of Gammie's model toinclude heating by irradiation from the central star yields a modestdependence of M˙ on the mass of the central star. A purely viscousdisk model could provide a strong dependence of accretion rate onstellar mass if the initial disk radius (before much viscous evolutionhas occurred) has a strong dependence on stellar mass. However, it isfar from clear that at least the most massive pre-main-sequence diskscan be totally magnetically activated by X-rays or cosmic rays. Wesuggest that a combination of effects are responsible for the observeddependence, with the lowest mass stars having the lowest mass disks,which can be thoroughly magnetically active, while the higher mass starshave higher mass disks that have layered accretion and relativelyinactive or ``dead'' central zones at some radii. In such dead zones, wesuggest that gravitational instabilities may play a role in allowingaccretion to proceed. In this connection, we emphasize the uncertaintyin disk masses derived from dust emission and argue that T Tauri diskmasses have been systematically underestimated by conventional analyses.Further study of accretion rates, especially in the lowest mass stars,would help to clarify the mechanisms of accretion in T Tauri stars.
| Effects of dust scattering albedo and 2175-Å bump on ultraviolet colours of normal disc galaxies
We discuss dust properties in the interstellar medium (ISM) of nearbynormal galaxies by comparing observations in the ultraviolet (UV) withsimulations by a radiative transfer model. The observed UV colours ofnearby galaxies show a reddening relative to their expected intrinsiccolours. Some authors argued that the Milky Way dust cannot reproducethe reddening because of the prominent 2175-Å absorption bump.Other authors proposed a reduction mechanism of the bump strength in anattenuation law derived from the ratio of the observed intensity to theintrinsic one through an age-selective attenuation (i.e. young stars aremore attenuated selectively). We find that the wavelength dependence ofthe scattering albedo also has a strong effect on the UV colour; analbedo decreasing towards shorter wavelengths (except for the absorptionbump range) produces a significant UV reddening. After comparing theobserved UV colours of nearby normal galaxies with those expected fromradiative transfer simulations assumed several dust models, we find twosorts of dust suitable for these galaxies: (i) dust with a bump and asmaller albedo for a shorter wavelength (except for the bump range) and(ii) dust without any bump but with an almost constant albedo. If verysmall carbonaceous grains responsible for the common unidentifiedinfrared emission band are also the bump carrier, the former dust isfavourable. Finally, we derive mean attenuation laws of various dustmodels as a function of the UV attenuation, and derive some relationsbetween the UV attenuation and observable/theoretical quantities.
| Photometric distances to nine dark globules
Distances to nine dark globules are determined by a method using optical(VRI) and near-infrared (near-IR) (JHK) photometry of stars projectedtowards the field containing the globules. In this method, we computeintrinsic colour indices of stars projected towards the direction of theglobule by dereddening the observed colour indices using various trialvalues of extinction AV and a standard extinction law. Thesecomputed intrinsic colour indices for each star are then compared withthe intrinsic colour indices of normal main-sequence stars and aspectral type is assigned to the star for which the computed colourindices best match with the standard intrinsic colour indices. Distances(d) to the stars are determined using the AV and absolutemagnitude (MV) corresponding to the spectral types thusobtained. A distance versus extinction plot is made and the distance atwhich AV undergoes a sharp rise is taken to be the distanceto the globule. All the clouds studied in this work are in the distancerange 160-400pc. The estimated distances to dark globules LDN 544, LDN549, LDN 567, LDN 543, LDN 1113, LDN 1031, LDN 1225, LDN 1252 and LDN1257 are 180 +/- 35, 200 +/- 40, 180 +/- 35, 160 +/- 30, 350 +/- 70, 200+/- 40, 400 +/- 80, 250 +/- 50 and 250 +/- 50pc, respectively. Using thedistances determined, we have estimated the masses of the globules andthe far-IR luminosity of the IRAS sources associated with them. The massof the clouds studied are in the range 10-200Msolar.
| The SPITZER c2d Survey of Weak-Line T Tauri Stars. I. Initial Results
Using the Spitzer Space Telescope, we have observed 90 weak-line andclassical T Tauri stars in the vicinity of the Ophiuchus, Lupus,Chamaeleon, and Taurus star-forming regions as part of the Cores toDisks (c2d) Spitzer Legacy project. In addition to the Spitzer data, wehave obtained contemporaneous optical photometry to assist inconstructing spectral energy distributions. These objects werespecifically chosen as solar-type young stars with low levels ofHα emission, strong X-ray emission, and lithium absorption, i.e.,weak-line T Tauri stars, most of which were undetected in the mid- tofar-IR by the IRAS survey. Weak-line T Tauri stars are potentiallyextremely important objects in determining the timescale over which diskevolution may take place. Our objective is to determine whether theseyoung stars are diskless or have remnant disks that are below thedetection threshold of previous infrared missions. We find that only5/83 weak-line T Tauri stars have detectable excess emission between 3.6and 70 μm, which would indicate the presence of dust from the innerfew tenths of an AU out to the planet-forming regions a few tens of AUfrom the star. Of these sources, two have small excesses at 24 μmconsistent with optically thin disks; the others have optically thickdisks already detected by previous IR surveys. All of the sevenclassical T Tauri stars show excess emission at 24 and 70 μm althoughtheir properties vary at shorter wavelengths. Our initial results showthat disks are rare among young stars selected for their weak Hαemission.
| Accretion in young low intermediate mass stars
We present preliminary results of a study of the evolution of accretionrates in some young members of open clusters: NGC1502(1), NGC884(3),Trumpler37(2), Biurakan2(3), Berkeley87(1). Accretion rates werecomputed using UVI photometry and the Gullbring relation. In addition wefound that accretion rates of three members of the MBM12 youngassociation ( ˜ 2Myr) are in agreement with the expected valuesfor CTTS.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| H2 active jets in the near IR as a probe of protostellar evolution
We present an in-depth near-IR analysis of a sample of H2 outflows fromyoung embedded sources to compare the physical properties and coolingmechanisms of the different flows. The sample comprises 23 outflowsdriven by Class 0 and I sources having low-intermediate luminosity. Wehave obtained narrow band images in H2 2.12 μm and [Fe II] 1.64 μmand spectroscopic observations in the range 1-2.5 μm. From [Fe II]images we detected spots of ionized gas in ~74% of the outflows which insome cases indicate the presence of embedded HH-like objects. H2 lineratios have been used to estimate the visual extinction and averagetemperature of the molecular gas. Av values range from ~2 to~15 mag; average temperatures range between ~2000 and ~4000 K. Inseveral knots, however, a stratification of temperatures is found withmaximum values up to 5000 K. Such a stratification is more commonlyobserved in those knots which also show [Fe II] emission, while athermalized gas at a single temperature is generally found in knotsemitting only in molecular lines. Combining narrow band imaging (H2,2.12 μm and [Fe II], 1.64 μm) with the parameters derived from thespectroscopic analysis, we are able to measure the total luminosity ofthe H2 and [Fe II}] shocked regions (L_H2 and L[FeII]) in each flow. H2 is the major NIR coolant with an averageL_H_2/L[Fe II] ratio of ~102. We find that ~83% ofthe sources have a L_H_2/L_bol ratio ~0.04, irrespective of the Class ofthe driving source, while a smaller group of sources (mostly Class I)have L_H_2/L_bol an order of magnitude smaller. Such a separationreveals the non-homogeneous behaviour of Class I, where sources withvery different outflow activity can be found. This is consistent withother studies showing that among Class I one can find objects withdifferent accretion properties, and it demonstrates that the H2 power inthe jet can be a powerful tool to identify the most active sources amongthe objects of this class.
| Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.
| Spitzer Observations of IC 348: The Disk Population at 2-3 Million Years
We present near- and mid-infrared photometry obtained with the SpitzerSpace Telescope of ~300 known members of the IC 348 cluster. We mergethis photometry with existing ground-based optical and near-infraredphotometry in order to construct optical-infrared spectral energydistributions (SEDs) for all the cluster members and present a completeatlas of these SEDs. We employ these observations to investigate boththe frequency and nature of the circumstellar disk population in thecluster. The Spitzer observations span a wavelength range between 3.6and 24 μm, corresponding to disk radii of ~0.1-5 AU from the centralstar. The observations are sufficiently sensitive to enable the firstdetailed measurement of the disk frequency for very low mass stars atthe peak of the stellar initial mass function. Using measurements ofinfrared excess between 3.6 and 8.0 μm, we find the total frequencyof disk-bearing stars in the cluster to be 50%+/-6%. However, only30%+/-4% of the member stars are surrounded by optically thick,primordial disks, while the remaining disk-bearing stars are surroundedby what appear to be optically thin, anemic disks. Both these values arebelow previous estimates for this cluster. The disk fraction appears tobe a function of spectral type and stellar mass. The fraction of starswith optically thick disks ranges from 11%+/-8% for stars earlier thanK6 to 47%+/-12% for K6-M2 stars to 28%+/-5% for M2-M6 stars. The disklongevity and thus conditions for planet formation appear to be mostfavorable for the K6-M2 stars, which are objects of comparable mass tothe Sun for the age of this cluster. The optically thick disks aroundlater type (>M4) stars appear to be less flared than the disks aroundearlier type stars. This may indicate a greater degree of dust settlingand a more advanced evolutionary state for the late M disk population.Finally, we find that the presence of an optically thick dust disk iscorrelated with gaseous accretion, as measured by the strength ofHα emission. A large fraction of stars classified as classical TTauri stars possess robust, optically thick disks, and very few suchstars are found to be diskless. The majority (64%) of stars classifiedas weak-lined T Tauri stars are found to be diskless. However, asignificant fraction (12%) of these stars are found to be surrounded bythick, primordial disks. These results suggest that it is more likelyfor dust disks to persist in the absence of active gaseous accretionthan for active accretion to persist in the absence of dusty disks.
| Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope
We present the results of an infrared imaging survey of two clusters inthe Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPSinstruments on board the Spitzer Space Telescope. Our observations coverthe wavelength range from 3.6 to 24 μm, allowing us to detect diskemission over a typical range of radii ~0.1 to ~20 AU from the centralstar. In Tr 37, with an age of about 4 Myr, about 48% of the low-massstars exhibit detectable disk emission in the IRAC bands. Roughly 10% ofthe stars with disks may be ``transition'' objects, with essentiallyphotospheric fluxes at wavelengths <=4.5 μm but with excesses atlonger wavelengths, indicating an optically thin inner disk. The medianoptically thick disk emission in Tr 37 is lower than the correspondingmedian for stars in the younger Taurus region; the decrease in infraredexcess is larger at 6-8 μm than at 24 μm, suggesting that graingrowth and/or dust settling has proceeded faster at smaller disk radii,as expected on general theoretical grounds. Only about 4% of thelow-mass stars in the 10 Myr old cluster NGC 7160 show detectableinfrared disk emission. We also find evidence for 24 μm excessesaround a few intermediate-mass stars, which may represent so-called``debris disk'' systems. Our observations provide new constraints ondisk evolution through an important age range.
| 50 New Eccentric Eclipsing Binaries Found in the ASAS, Hipparcos and NSVS Databases
This research presents 50 new eccentric binaries binaries found with thehelp of the NSVS, Hipparcos and ASAS-3 databases.
| Spitzer/IRAC Photometry of the η Chameleontis Association
We present IRAC 3.6, 4.5, 5.8, and 8 μm photometry for the 17 A-, K-,and M-type members of the η Chameleontis association. These datashow infrared excesses toward six of the 15 K and M stars, indicatingthe presence of circumstellar disks around 40% of the stars with massesof 0.1-1 Msolar. The two A stars show no infrared excesses.The excess emission around one of the stars is comparable to the medianexcess for classical T Tauri stars in the Taurus association; theremaining five show comparatively weak excess emission. Taking intoaccount published Hα spectroscopy that shows that five of the sixstars are accreting, we argue that the disks with weak mid-infraredexcesses are disks in which the inner disks have been largely depletedof small grains by grain growth or, in one case, the small grains havesettled to the midplane. This suggests that η Cha has a much higherfraction of disks caught in the act of transitioning into optically thindisks than that measured in younger clusters and associations.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| H{2}O maser emission from bright rimmed clouds in the northern hemisphere
We report the results of a multi-epoch survey of water maserobservations at 22.2 GHz with the Medicina radiotelescope from 44 brightrimmed clouds (BRCs) of the northern hemisphere identified by Sugitaniet al. (1989, ApJ, 342, L87) as potential sites of star formation. Thedata span 16 years of observations and allow to draw conclusions aboutthe maser detection rate in this class of objects. In spite of therelatively high far-infrared luminosities of the embedded sources(L_FIR 102 L_ȯ), H2O maser emission was detectedtowards three globules only. Since the occurrence of water masers ishigher towards bright IRAS sources, the lack of frequent H2O maseremission is somewhat surprising if the suggestion of inducedintermediate- and high-mass star formation within these globules iscorrect. The maser properties of two BRCs are characteristic of excitingsources of low-mass, while the last one (BRC 38) is consistent with anintermediate-mass object. We argue that most BRCs host young stellarobjects of low-luminosity, likely in an evolutionary phase later thanthe protostellar Class 0 sources, and that a significant contribution tothe observd IRAS luminosity comes from warm dust heated by the radiationfrom the bright rim.
| An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
In this paper we present a new method for deriving UV through IRextinction curves, based on the use of stellar atmosphere models toprovide estimates of the intrinsic (i.e., unreddened) stellar spectralenergy distributions (SEDs), rather than unreddened (or lightlyreddened) standard stars. We show that this ``extinction withoutstandards'' technique greatly increases the accuracy of the derivedextinction curves and allows realistic estimations of the uncertainties.An additional benefit of the technique is that it simultaneouslydetermines the fundamental properties of the reddened stars themselves,making the procedure valuable for both stellar and interstellar studies.Given the physical limitations of the models we currently employ, thetechnique is limited to main-sequence and mildly evolved B stars.However, in principle, it can be adapted to any class of star for whichaccurate model SEDs are available and for which the signatures ofinterstellar reddening can be distinguished from those of the stellarparameters. We demonstrate how the extinction without standards curvesmake it possible to (1) study the uniformity of curves in localizedspatial regions with unprecedented precision, (2) determine therelationships between different aspects of curve morphology, (3) producehigh-quality extinction curves from low color excess sight lines, and(4) derive reliable extinction curves for mid to late B stars, therebyincreasing spatial coverage and allowing the study of extinction in openclusters and associations dominated by such stars. The application ofthis technique to the available database of UV through IR SEDs, and tofuture observations, will provide valuable constraints on the nature ofinterstellar grains and on the processes that modify them, and it willenhance our ability to remove the multiwavelength effects of extinctionfrom astronomical energy distributions.
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