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Optical and Near-IR Imaging of the Dark Globule CB 52
The internal structure of the dark globule CB 52 is investigated bymeans of broadband imaging in the optical and near-IR spectral range. Byexploiting the extinction of the stellar light within this object, wederive observational parameters suitable to infer the internal structureof this cloud. Extinction maps were obtained at different wavelengths byusing both stellar counts and two-color diagrams. While in the opticalregion the extinction is better evaluated at the cloud boundaries, theinternal regions are more conveniently probed in the near-IR, so acombined map was derived. The total-to-selective extinction ratioRV was also observed to increase toward the inner regions,and a plot versus the extinction AV suggests that graingrowth processes are active in this cloud. The statistical fluctuationof the AV, estimated in the line of sight of the backgroundstars, is investigated by comparing the observed stellar colors withthose of the unreddened stars. The dispersionσAV, derived from optical observations, isfound to be almost independent of the mean extinction, AV,while by using near-IR data we find a more complex behavior: theσAV versus AV relation increasesuntil AV~6 and then decreases for larger extinctions. This isdiscussed in the framework of a simple model, suggesting that a clumpyand clustered structure can explain the observations in the innerregions, while outside the cloud the mass distribution remains morehomogeneous.

ISO far-infrared observations of the high-latitude cloud L1642 . II. Correlated variations of far-infrared emissivity and temperature of "classical large" dust particles
Aims.Our aim is to compare the infrared properties of big, "classical"dust grains with visual extinction in the cloudL1642. In particular, we study the differences ingrain emissivity between diffuse and dense regions in the cloud. Methods: The far-infrared properties of dust are based on large-scale100 μm and 200 μm maps. Extinction through the cloud was derivedby using the star count method in the B- and I-bands, and colour excessmethod in the J, H, and Ks bands. Radiative transfercalculations were used to study the effects of increasing absorptioncross-section on the far-infrared emission and dust temperature. Results: Dust emissivity, measured by the ratio of far-infrared opticaldepth to visual extinction, τ(far-IR)/A_V, increases with decreasingdust temperature in L1642. There is about a two-foldincrease in emissivity over the dust temperature range of 19 K-14 K.Radiative transfer calculations show that, in order to explain theobserved decrease of dust temperature towards the centre ofL1642, an increase of absorption cross-section ofdust at far-IR is necessary. This temperature decrease cannot beexplained solely by the attenuation of interstellar radiation field.Increased absorption cross-section also manifests itself as an increasedemissivity. We find that, due to temperature effects, the apparent valueof optical depth τ_app(far-IR), derived from 100 μm and 200 μmintensities, is always lower than the true optical depth.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands, and the UK) and with the participation of ISAS and NASA.Appendices A-D are only available in electronic form athttp://www.aanda.org

Young Stellar Groups around Herbig Ae/Be Stars: A Low-Mass YSO Census
We present near-IR and mid-IR observations of eight embedded youngstellar groups around Herbig Ae/Be stars (HAEBEs) using archived SpitzerIRAC data and 2MASS data. These young stellar groups are nearby (<=1kpc) and still embedded within their molecular clouds. In order toidentify the young stellar objects in our sample, we use the color-colordiagram of J-[3.6] versus Ks-[4.5]. The Spitzer images of oursample show that the groups around HAEBEs, spectral types earlier thanB8, are usually associated with bright infrared nebulosity. Within this,there are normally 10-50 young stars distributed close to the HAEBEs(<1 pc). Not only are there young stars around the HAEBEs, there arealso young stellar populations throughout the whole cloud, some of whichare distributed and some of which are clumped. The groups around theHAEBEs are substructures of the large young population within themolecular cloud. The sizes of groups are also comparable with thosesubstructures seen in massive clusters. Young stars in groups aroundHAEBEs have generally larger SED slopes compared to those outside, whichsuggests that the young stars in groups are probably younger than thedistributed systems. This might imply that there is usually a higher andmore continuous star-forming rate in groups, that the formation ofgroups initiates later, or that low-mass stars in groups form slowerthan those outside. Finally, there is no obvious trend between the SEDslopes and the distance to the HAEBEs for those young stars within thegroups. This suggests that the clustering of young stars dominates overthe effect of massive stars on the low-mass young stars at the scale ofour study.

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.

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.

Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.

2MASS wide field extinction maps. I. The Pipe nebula
Context: . Aims: . We present a 8°×6°, highresolution extinction map of the Pipe nebula using 4.5 million starsfrom the Two Micron All Sky Survey (2MASS) point source catalog. Methods: . The use of Nicer (Lombardi & Alves 2001, A&A, 377,1023), a robust and optimal technique to map the dust column density,allows us to detect a AV = 0.5 mag extinction at a 3-σlevel with a 1 arcmin resolution. Results: . (i ) We find forthe Pipe nebula a normal reddening law, E(J - H) = (1.85 ± 0.15)E(H - K). (ii ) We measure the cloud distance using Hipparchos and Tychoparallaxes, and obtain 130+24-58 pc. This,together with the total estimated mass, 10^4 {M}_ȯ, makes the Pipethe closest massive cloud complex to Earth. (iii ) We compare the Nicerextinction map to the NANTEN {}12CO observations and derivewith unprecedented accuracy the relationship between the near-infraredextinction and the 12CO column density and hence (indirectly)the 12CO X-factor, that we estimate to be 2.91 ×1020 cm-2 K-1 km-1 s in therange AV in [0.9, 5.4] mag. (iv ) We identify approximately1500 OH/IR stars located within the Galactic bulge in the direction ofthe Pipe field. This represents a significant increase of the knownnumbers of such stars in the Galaxy. Conclusions: . Our analysisconfirms the power and simplicity of the color excess technique to studymolecular clouds. The comparison with the NANTEN {}12CO datacorroborates the insensitivity of CO observations to low columndensities (up to approximately 2 mag in A_V), and shows also anirreducible uncertainty in the dust-CO correlation of about 1 mag ofvisual extinction.

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 Distribution of Ortho-H2D+(11,0-11,1) in L1544: Tracing the Deuteration Factory in Prestellar Cores
Prestellar cores are unique laboratories for studying the chemical andphysical conditions preceding star formation. We observed the prestellarcore L1544 in the fundamental transition ofortho-H2D+ (11,0-11,1) atdifferent positions over 100" and found a strong correlation between itsabundance and the CO depletion factor. We also present a tentativedetection of the fundamental transition ofpara-D2H+ (11,0-10,1) at thedust emission peak. Maps in N2H+,N2D+, HCO+, and DCO+ areused and interpreted with the aid of a spherically symmetric chemicalmodel that predicts the column densities and abundances of these speciesas a function of radius. The correlation between the observed deuteriumfractionation of H+3, N2H+,and HCO+ and the observed integrated CO depletion factoracross the core can be reproduced by this chemical model. In addition, asimpler model is used to study the H2D+ortho-to-para ratio. We conclude that, in order to reproduce theobserved ortho-H2D+ observations, the grain radiusshould be larger than 0.3 μm.

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.

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.

The Pre-Main-Sequence Population of L988
L988 is a large (~0.5d×0.7d) dark cloud complex at about 600 pcthat contains several bright pre-main-sequence objects (such as V1331Cyg and LkHα 321), but this paper deals in detail only with asmall region on its eastern edge, near the HAeBe star LkHα 324.That star and its distant companion LkHα 324SE lie at the apex ofa V-shaped area apparently excavated from the edge of L988, and are thebrightest members of a small cluster containing about 50Hα-emission stars. A median age of about 0.6 Myr (with largedispersion) is inferred from its color-magnitude diagram, constructedfrom VRI photometry to V=22. Keck HIRES spectra show that LkHα324SE is probably also an HAeBe. Its image is nonstellar, and within 3"to the northwest are three condensations having complex [S II] and [O I]profiles and radial velocities up to -200 km s-1. Theyprobably originate in an outflow from LkHα 324SE. A bright Ap starwith strong Si II lines is embedded in the heavy obscuration 8' to thewest. It illuminates a small reflection nebulosity, has several faintHα emitters nearby, and shares the radial velocity of L988, soclearly it was formed in that cloud. It demonstrates again that suchchemical peculiarities can be established very early in young stars ofmoderate mass.

Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.

The Young Cluster NGC 2362
An Hα emission survey of the young cluster NGC 2362 resulted inthe detection of 130 Hα emission stars in an11'×11' field approximately centered on thefourth magnitude O9 Ib multiple star τ CMa. The survey was carriedout using the wide-field grism spectrograph on the University of Hawaii2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) onGemini North. Deep optical VRCIC (to V~23.0) andnear-infrared (NIR) photometry (JHK) to K~16 were obtained for severalfields within the cluster. Spectra covering the 6000-8000 Å regionat a resolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were also acquired for ~200pre-main-sequence (PMS) candidates with GMOS. Ages and masses for theHα emitters in NGC 2362 were inferred from the isochrones andevolutionary tracks of D'Antona & Mazzitelli, as well as those ofBaraffe et al. An estimated cluster age of ~1.8 Myr follows from themodels of D'Antona & Mazzitelli and 3.5-5.0 Myr from those ofBaraffe et al. The fraction of the T Tauri star (TTS) population that iscomposed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 forthe TTS population of NGC 2264. On the basis of W(Hα) alone, thefraction of TTSs still undergoing accretion is 5%-9%, comparable to theinner disk fraction determined from JHKL-band excesses by Haisch andcoworkers (12%). Approximately 15% of the PMS sample in this studyexhibits possible NIR excess, having EH-K>0.1 mag. Giventhe lack of NIR excess and strong Hα emission from the majority ofcluster members, it is inferred that the inner disk regions of the TTSpopulation have dissipated significantly. The mean level ofchromospheric activity among the WTTS population of NGC 2362 islog(LHα/Lbol)=-3.65, significantly greaterthan that of the low-mass population of the 600 Myr old Hyades cluster,log(LHα/Lbol)=-3.90. The total mass of theHα emitters and the OB stellar population of NGC 2362 defines alower limit for the cluster mass of ~300 Msolar. Allowancefor A- and F-type stars still on the radiative track, multiplicity,outlying members, and the low-mass population lying below thecompleteness limit of the Hα emission survey increases this lowerlimit to well over 500 Msolar. The derived relaxation,disruption, and evaporation timescales for the cluster imply that NGC2362 will likely survive beyond the age of the Pleiades, but statisticsof galactic cluster lifetimes favor its disruption well before the ageof the Hyades.

From Dusty Filaments to Cores to Stars: An Infrared Extinction Study of Lupus 3
We present deep near-infrared observations of a dense region of theLupus 3 cloud obtained with the ESO NTT and VLT. Using the NICE method,we construct a detailed high angular resolution dust extinction map ofthe cloud. The dust extinction map reveals embedded globules, a densefilament, and a dense ring structure. We derive dust column densitiesand masses for the entire cloud and for the individual structurestherein. We construct radial extinction profiles for the embeddedglobules and find a range of profile shapes from relatively shallowprofiles for cores with low peak extinctions, to relatively steepprofiles for cores with high extinction. Overall, the profiles aresimilar to those of pressure-truncated isothermal spheres of varyingcenter-to-edge density contrast. We apply Bonnor-Ebert analysis tocompare the density profiles of the embedded cores in a quantitativemanner and derive physical parameters such as temperatures, centraldensities, and external pressures. We examine the stability of the coresand find that two cores are likely stable and two are likely unstable.One of these latter cores is known to harbor an active protostar.Finally, we discuss the relation between an emerging cluster in theLupus 3 cloud and the ring structure identified in our extinction map.Assuming that the ring is the remnant of the core within which thecluster originally formed, we estimate that a star formation efficiencyof ~30% characterized the formation of the small cluster. Ourobservations of the Lupus 3 cloud suggest an intimate link between thestructure of a dense core and its state of star-forming activity. Thedense cores in this cloud are found to span the entire range ofevolution from a stable, starless core of modest central concentration,to an unstable, star-forming core that is highly centrally concentrated,to a significantly disrupted core from which a cluster of young stars isemerging.Based on observations carried out at ESO, La Silla and Paranal, Chile.

New insights into the dust properties of the Taurus molecular cloud TMC-2 and its surroundings
We present ISOPHOT observations at 120 and 200 μm of a 31 × 57arcmin2 region, with optical extinction AV rangingbetween ~ 0.5 and 11 mag, that encloses the Taurus molecular cloudTMC-2. The far-infrared emission is separated into a warm and a coldcomponent using the ISOPHOT data and IRAS measurements at 60 and 100μm. This separation is based on the very different morphologies ofthe 60 and 200 μm emission maps. The 60 μm emission is used as aspatial template for the warm component, and the 200 μm emission[Iν(200)] as a template for the cold component. The warmcomponent presents an average colour temperature of nearly 20 K. Thecolour temperature map of the cold component is nearly uniform, with amean temperature of 12.5 K. The optical depths at 200 μm of the warmand cold components (τ200) were determined. The ratiosIν(200)/AV andτ200/AV of the cold component indicate changesin the optical properties of the dust grains, which present afar-infrared emissivity that is a few times larger than that of the biggrains in the diffuse interstellar medium. Comparisons of the emissionsand τ200 of the cold component with carbon monoxidemeasurements, which trace the molecular gas, were performed. The coldcomponent emission, in particular at 100 μm, correlates very wellwith 13CO (J= 1-0) total intensity. Very good correlationsbetween C18O (J= 1-0) total intensity and especiallyτ200 are found for two distinct regions, one thatencloses the core TMC-2 and a northern region that also containsmolecular condensations. These results confirm that τ200is a powerful tracer of dense cores with n(H2) ~104cm-3, and that the change in the properties ofdust grains in the observed region takes place at densities ofn(H2) ~ 103cm-3.

Density and Temperature Structure of TMC-1C from 450 and 850 Micron Maps
We have mapped the central 10'×10' of thedense core TMC-1C at 450 and 850 μm using SCUBA on the James ClerkMaxwell Telescope. The unusually high quality of the 450 μm mapallows us to make a detailed analysis of the temperature and columndensity profiles of the core. We find that the dust temperature at thecenter of TMC-1C is ~7 K, rising to ~11 K at the edges. We discuss thepossibility and effects of a variable emissivity spectral index on thederived mass profile. The low dust temperature of TMC-1C results in ahigh derived mass for the core, significantly larger than the virialmass estimated from the line width of the N2H+(1-0) transition. This result is valid within a wide range of dustproperties and ellipticities of the core. The N2H+(1-0) spectra, taken with the IRAM 30 m telescope, show signs ofself-absorption, which provide evidence of subsonic infall motions. Thederived density profile and infall velocity are compared to thepredictions of several popular star formation models, and theBonnor-Ebert model is the best-fit analytic model.

The T Tauri Star Population of the Young Cluster NGC 2264
An Hα emission survey of the young cluster NGC 2264 in the Mon OB1association resulted in the detection of 490 Hα emission stars ina 25'×40' field approximately centeredbetween the O7 V multiple star S Mon and the Cone Nebula. The survey wascarried out with the wide-field grism spectrograph (WFGS) on theUniversity of Hawaii 2.2 m telescope on Mauna Kea. X-ray observationsmade with the European Photon Imaging Camera on board the European SpaceAgency's XMM-Newton satellite observatory will be discussed in asubsequent paper. Optical (BVRCIC) photometry wasobtained for selected fields to supplement similar data from theliterature. Spectra covering the 6000-8000 Å region at aresolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were obtained for 150 Hα and X-rayemission sources with the Gemini Multi-Object Spectrograph.Near-infrared spectra (1-2.5 μm) of a number of T Tauri stars (TTSs),X-ray sources, and LHα25 (W90) were also obtained using SpeX onthe Infrared Telescope Facility. Ages and masses for the Hαemitters were inferred from the isochrones and evolutionary tracks ofD'Antona & Mazzitelli. The median age for the TTS population isabout 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, existsfor individual objects. Several fields in the cluster were observed withthe WFGS on more than one occasion, permitting an examination ofHα variability over long baselines in time. About 90% of theclassical T Tauri stars (CTTSs) showed changes in W(Hα) of atleast 10%, while 57% varied at levels of 50% or more. No evidence wasfound for a significant pool of dormant Hα emitters. Summing themasses of the TTSs and the OB stellar population of NGC 2264, a lowerlimit for the total stellar mass content of the cluster is about 430Msolar. This is less than 1% of the total mass of the atomicand molecular gas believed to be associated with NGC 2264. Evidence forhierarchical structure within the cluster is suggested by the spatialdistribution of TTSs. Four concentrations of Hα emitters areevident: two near S Mon and two near the Cone Nebula. The median age ofthe TTSs in the immediate vicinity of S Mon was found to be greater thanthat of the TTSs near Allen's infrared source (IRS-1), but a significantdispersion is present. From the rotational data of Lamm et al. andMakidon et al., 241 of the TTSs are periodic variables, 150 weak-line TTauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregularvariables (30 WTTSs and 93 CTTSs). A weak-to-moderate positivecorrelation is found between H-K color and Prot for theCTTSs, in the sense that stars having longer periods tend to have largerH-K colors. A similar positive correlation is found betweenLHα and Prot among the CTTSs. Nostatistically significant correlation is found between Protand theoretical age or between Prot and LX. Othertopics discussed include the fraction of Hα emitters that areWTTSs, f(WTTS)=N(WTTS)/N(TTS), for clusters of different ages; therelative detectability of Hα emission using WFGS and narrowbandfilter imaging techniques; and the correlation of W(Li I) withTe, age, H-K color, and W(Hα).

The interstellar C18O/C17O ratio in the solar neighbourhood: The ρ Ophiuchus cloud
Observations of up to ten carbon monoxide (CO and isotopomers)transitions are presented to study the interstellarC18O/C17O ratio towards 21 positions in the nearby(d˜140 pc) low-mass star forming cloud ρ Oph. A map of theC18O J=1-0 distribution of parts of the cloud is also shown.An average 12C18O/12C17Oisotopomeric ratio of 4.11 ± 0.14, reflecting the18O/17O isotope ratio, is derived from LargeVelocity Gradient (LVG) calculations. From LTE column densities wederive a ratio of 4.17±0.26. These calculations also show thatthe kinetic temperature decreases from about 30 K in the cloud envelopeto about 10 K in the cloud cores. This decrease is accompanied by anincrease of the average molecular hydrogen density from 104cm-3 to ⪆105 cm-3. Towards somelines of sight C18O optical depths reach values of orderunity.Based on observations collected with the Swedish/ESO SubmillimeterTelescope (SEST) at the European Southern Observatory, Chile (ESO62.I-0752). All spectra (some of which are shown in Fig. \ref{spectra})are available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/549

The Structure of the Small Dark Cloud CB 107
This paper presents the near-IR imaging observations of CB 107, a smalldark globule projected against a rich stellar background. By means ofaccurate photometry, the near-IR two-color diagram J-H versus H-K wasobtained for the stellar background. This information was used toestimate the color excesses of the detected stars so that, given thereddening curve, it was possible to derive the extinction map of thecloud. The structural properties of the dark globule were investigatedby plotting the extinction dispersion σAV,obtained in a given spatial box, as a function of the mean extinctionAV. This relationship has shown quite a definite linearbehavior, with the slope increasing with the box size. The results ofthe present analysis, compared with those obtained by other authors onlarger dark clouds, suggest that for a given spatial scale the slope ofthe σAV versus AV relation isgreater in CB 107 than in larger clouds. The so-called Δ-variancemethod was also used to investigate the structure of the dark globule byevaluating the drift behavior of its extinction map. In this way, wehave found that the power spectrum of the extinction map ischaracterized by a power law with exponent β~2.7. This value islower than expected, for the same range of spatial scales, on the basisof previous work on large molecular clouds.

The Giant Molecular Cloud associated with RCW 106. A 1.2 mm continuum mapping study
We have mapped the dust continuum emission from the molecular cloudcovering a region of 28 pc × 94 pc associated with the well-knownHII region RCW 106 at 1.2 mm using SIMBA on SEST. The observations,having an HPBW of 24\arcsec (0.4 pc), reveal 95 clumps, of which about50% have MSX associations and only 20% have IRAS associations. Owing totheir higher sensitivity to colder dust and higher angular resolutionthe present observations identify new emission features and also showthat most of the IRAS sources in this region consist of multiple dustemission peaks. The detected millimeter sources (MMS) include on one endthe exotic MMS5 (associated with IRAS 16183-4958, one of the brightestinfrared sources in our Galaxy) and the bright (and presumably cold)source MMS54, with no IRAS or MSX associations on the other end. Around10% of the sources are associated with signposts of high mass starformation activity. Assuming a uniform dust temperature of 20 K weestimate the total mass of the GMC associated with RCW 106 to be ˜105 Mȯ. The constituent millimeter clumpscover a range of masses and radii between 40 to 104Mȯ and 0.3 to 1.9 pc. Densities of the clumps rangebetween (0.5-6) ×104 cm-3. We havedecomposed the continuum emission into Gaussian and arbitrary shapedclumps using the two independent structure analysis tools gaussclumpsand clumpfind respectively. The clump mass spectrum was found to have anindex α of 1.6±0.3, independent of the decompositionalgorithm used. The index of the mass spectrum for the mass and lengthscales covered here are consistent with results derived from large scaleCO observations.

LkHα 101 and the Young Cluster in NGC 1579
The central region of the dark cloud L1482 is illuminated by LkHα101, a heavily reddened (AV~10 mag) high-luminosity(>=8×103 Lsolar) star having an unusualemission-line spectrum plus a featureless continuum. About 35 muchfainter (mostly between R=16 and >21) Hα emitters have been foundin the cloud. Their color-magnitude distribution suggests a median ageof about 0.5 Myr, with considerable dispersion. There are also at leastfive bright B-type stars in the cloud, presumably of about the same age;none show the peculiarities expected of HAeBe stars. Dereddened, theirapparent V magnitudes lead to a distance of about 700 pc. Radioobservations suggest that the optical object LkHα 101 is in fact ahot star surrounded by a small H II region, both inside an opticallythick dust shell. The level of ionization inferred from the shape of theradio continuum corresponds to a Lyman continuum luminosity appropriatefor an early B-type zero-age main-sequence star. The V-I color isconsistent with a heavily reddened star of that type. However, theoptical spectrum does not conform to this expectation: the absorptionlines of an OB star are not detected. Also, the [O III] lines of an H IIregion are absent, possibly because those upper levels are collisionallydeexcited at high densities. There are several distinct contributors tothe optical spectrum of LkHα 101. The Hα emission line isvery strong, with wings extending to about +/-1700 km s-1,which could be produced by a thin overlying layer of hot electronscatterers. There is no sign of P Cygni type mass ejection. Lines of SiII are narrower, while the many Fe II lines are still narrower and aredouble with a splitting of about 20 km s-1. Lines of [Fe II],[O I], and [S II] are similarly sharp but are single, at the samevelocity as the Fe II average. Work by Tuthill et al. allowed theinference, from K-band interferometry, that the central source isactually a small horseshoe-shaped arc about 0.05" (35 AU) across. Atipped annulus of that size in rotation about a 15 Msolarstar would produce double spectrum lines having about the splittingobserved for Fe II. The totality of observational evidence encouragesthe belief that LkHα 101 is a massive star caught in an earlyevolutionary state.

The relationship of CO abundance to extinction and N(H2):. Observations of globules and the dependence on star formation activity
We have studied the ratio of 13CO and C18O columndensities to the extinction AJ of background field stars inthe direction of three globules: B 133, B335, L 466. Selected positions in theglobules were observed in the 12CO(J=1-0) line and in J=1-0and J=2-1 transitions of 13CO and C18O using the15-m Swedish-ESO Submillimetre Telescope (SEST) in Chile. One of theclouds, L 466, was also mapped in theC18O(J=1-0) line. In addition to theN(13CO)/AJ and N(C18O)/AJratios also the ratios of N(13CO) and N(C18O) toAV and N(H2) are given. These ratios were found tovary from cloud to cloud so that they are larger in B335 than in B 133 and L466. These variations are thought to primarily arise fromvariations of the column density ratio of CO and its isotopes toH2. The alternative explanation in terms of theN(H2)/AJ ratio being larger in starforming regions(B 335) than in more quiescent regions lacking starformation (B 133 and L 466)appears less likely. We also discuss the evidence forN(CO)/AJ variations on the basis of previous observations instar forming and non-star forming dark clouds.Appendix A is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr ( or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/421/1087Appendices B and C are only available in electronic form athttp://www.edpsciences.org

Evidence for a deficit of young and old stars in the Milky Way inner in-plane disc
We give independent proof of the deficit of stars in the in-planecentral disc (2

Cosmic ray induced explosive chemical desorption in dense clouds
The desorption due to the energy release of free radicals in the icemantles of a dust grain is investigated theoretically by calculating theultraviolet radiation field inside the cloud, the free radicalaccumulation, the cosmic-ray heating of the grain and then thedesorption in this situation starting from the cosmic-ray energyspectra. This model can reproduce the observations of the CO gasabundances and level of depletion in dark clouds such as L977 and IC5146 with a combination of input parameters which are either constrainedby independent observations or have been derived independently fromlaboratory experiments. We investigate other desorption mechanisms andconclude that they cannot explain the observations. The model also showsthat the energy input by the cosmic-ray induced ultraviolet field isalmost one order of magnitude larger than the direct energy input bycosmic-ray particles. This strengthens the conclusion that desorptiondue to the energy release by ultraviolet photon produced radicalsdominates over direct cosmic-ray desorption.

The Compact H II Region S88B: Excitation and Extinction
We have undertaken an investigation into the excitation of, and dustextinction to, the massive star formation region S88B. Studying stellarproperties of the wide-field Two Micron All Sky Survey (2MASS) J (1.23μm), H (1.65 μm), and Ks (2.16 μm) images, we havedetermined the extent of, and extinction to, the molecular cloudassociated with the ultracompact H II regions S88B-1 and S88B-2 and witha newly described infrared H II region. Infrared observations ofBrγ and Brα hydrogen recombination lines provide extinctionmaps to the infrared H II region, and 3.3 and 3.4 μm images show thepolycyclic aromatic hydrocarbon (PAH) emission distribution. We alsoreport the detection of an infrared counterpart to S88B-2.

Continuum and CO/HCO+ Emission from the Disk Around the T Tauri Star LkCa 15
We present Owens Valley Radio Observatory Millimeter Arrayλ=3.4-1.2 mm dust continuum and spectral line observations of theaccretion disk encircling the T Tauri star LkCa 15. The 1.2 mm dustcontinuum emission is resolved and gives a minimum diameter of 190 AUand an inclination angle of 57deg+/-5deg. There isa noticeable but, at present, poorly constrained decrease in thecontinuum spectral slope with frequency that may result from the coupledprocesses of grain growth and dust settling. Imaging of the fairlyintense emission from the lowest rotational transitions of CO,13CO, and HCO+ reveals a rotating disksubstantially larger than that observed in the dust continuum. Emissionextends to ~750 AU and the characteristic radius of the disk isdetermined to be ~425 AU (HWHM), based on model fits to the CO velocityfield. The measured line ratios demonstrate that the emission from thesespecies is optically thick, while that from C18O andH13CO+ is optically thin, or nearly so. The diskmass derived from the CO isotopologues with typical dense cloudabundances is still nearly 2 orders of magnitude less than that inferredfrom the dust emission, the most probable explanation being extensivemolecular depletion in the cold, dense disk midplane. Thus, while CO,HCO+, and their isotopologues are excellent tracers of thedisk velocity field, they are not reliable tracers of the disk mass.N2H+ 1-->0 emission has also been detectedwhich, along with HCO+, sets a lower limit to the fractionalionization of 10-8 in the near-surface regions ofprotoplanetary disks. This first detection of N2H+in circumstellar disks has also made possible a determination of theN2/CO ratio (~2) that is at least an order of magnitudelarger than those in the envelopes of young stellar objects and denseclouds. The large N2/CO ratio indicates that our observationsprobe disk layers in which CO is depleted but some N2 remainsin the gas phase. Such differential depletion can lead to largevariations in the fractional ionization with height in the outer reachesof circumstellar disks and may help to explain the relative nitrogendeficiency observed in comets.

A Catalog of Young Stellar Groups and Clusters within 1 Kiloparsec of the Sun
We present a catalog of near-infrared surveys of young (<~ a few106 yr) stellar groups and clusters within 1 kpc from theSun, based on an extensive search of the literature from the past tenyears. We find 143 surveys from 69 published articles, covering 73different regions. The number distribution of stars in a region has amedian of 28 and a mean of 100. About 80% of the stars are in clusterswith at least 100 members. By a rough classification of the groups andclusters based on the number of their associated stars, we show thatmost of the stars form in large clusters. The spatial distribution ofcataloged regions in the Galactic plane shows a relative lack ofobserved stellar groups and clusters in the range270°

High-Resolution Spectroscopy of FU Orionis Stars
High-resolution spectroscopy was obtained of the FU orionis stars FU Oriand V1057 Cyg between 1995 and 2002 with the SOFIN spectrograph at theNordic Optical Telescope and with HIRES at Keck I. During these years FUOri remained about 1 mag (in B) below its 1938-39 maximum brightness,but V1057 Cyg (B~10.5 at peak in 1970-1971) faded from about 13.5 to14.9 and then recovered slightly. Their photospheric spectra resemblethat of a rotationally broadened, slightly veiled supergiant of abouttype G0 Ib, with veqsini=70 km s-1 for FU Ori, and55 km s-1 for V1057 Cyg. As V1057 Cyg faded, P Cyg structurein Hα and the IR Ca II lines strengthened and a complexshortward-displaced shell spectrum of low-excitation lines of theneutral metals (including Li I and Rb I) increased in strength,disappeared in 1999, and reappeared in 2001. Several SOFIN runs extendedover a number of successive nights so that a search for rapid and cyclicchanges in the spectra was possible. These spectra show rapidnight-to-night changes in the wind structure of FU Ori at Hα,including clear evidence of sporadic infall. The equivalent width of theP Cyg absorption varied cyclically with a period of 14.8 days, withphase stability maintained over three seasons. This is believed to bethe rotation period of FU Ori. The internal structure of itsphotospheric lines also varies cyclically, but with a period of 3.54days. A similar variation may be present in V1057 Cyg, but the data aremuch noisier and that result uncertain. As V1057 Cyg has faded and thecontinuum level fallen, the emission lines of a preexistinglow-excitation chromosphere have emerged. Therefore we believe that the``line doubling'' in V1057 Cyg is produced by these central emissioncores in the absorption lines, not by orbital motion in an inclinedKeplerian disk. No convincing dependence of veqsini onwavelength or excitation potential was detected in either FU Ori orV1057 Cyg, again contrary to expectation for a self-luminous accretiondisk. It was found also that certain critical lines in the near infraredare not accounted for by synthetic disk spectra. It is concluded that arapidly rotating star near the edge of stability, as proposed by Larson,can better account for these observations. The possibility is alsoconsidered that FUor eruptions are not a property of ordinary T Tauristars but may be confined to a special subspecies of rapidly rotatingpre-main-sequence stars having powerful quasi-permanent winds.

A Photometric and [S II] Survey of the Young Cluster Roslund 4
Optical B- and V-band CCD data have been obtained for the young clusterRoslund 4 in conjunction with a large-scale photometric survey of openclusters. In addition, [S II] and off-line continuum CCD emission-lineimaging data have also been obtained, revealing three regions in thecluster field where shock-excited gas is present. Two Herbig-Haro-likeemission features are found, while a more extended emission feature,along an apparent cloud boundary, may be a result of stellar windsencountering the edge of a molecular cloud. The emission-line featuresare located near IRAS sources with properties consistent withstar-forming regions. A previously published age for Roslund 4 (~10 Myr)would seemingly suggest that shocked gas features should not be presentin the cluster region, but an upper limit on the age (less than 4 Myr)derived from the new photometry is more consistent with their presence.Finally, the distance to Roslund 4 is found to be 1700-2000 pc,considerably closer than the previous published estimate.

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Observation and Astrometry data

Right ascension:21h53m32.00s
Apparent magnitude:7.2

Catalogs and designations:
Proper NamesCocoon nebula
Cocoon   (Edit)
ICIC 5146

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