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Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| The young open cluster Markarian 50
We report on a deep CCD UBV(RI)C photometric survey in theregion of the young open cluster Markarian 50. The new photometric dataallow us to extend the cluster membership down to V~ 17.5, about 2 magdeeper than any previous investigation. On the basis of these data wederive a distance d= 3460 +/- 350 pc (VO-MV= 12.7+/- 0.2), which turns out to be only slightly lower than previousestimates. The cluster presents differential reddening, with E(B-V)values ranging from 0.69 to 1.1. The brightest member (HD 219460) is adouble star, which we separate photometrically for the first time,providing individual magnitudes and colours for each component. One ofthem is a Wolf-Rayet (WR) star and, according to evolutionary models,the mass of its progenitor should be greater than ~20 Msolar.The age obtained for the cluster is 7.5 +/- 2 Myr and the mass functionfor the most massive stars (M > 1 Msolar) presents a slopex~ 1.0.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| Interstellar extinction in the open clusters towards galactic longitude around 130deg
In this paper we present a detailed study of the intra-cluster reddeningmaterial in the young open clusters located around l ~ 130degusing colour-excess diagrams and two-colour diagrams. The study supportsthe universality of the extinction curves for lambda >= lambdaJ, whereas for shorter wavelengths the curve depends upon thevalue of the Rcluster (total-to-selective absorption in thecluster region). The value of Rcluster in the case of NGC654, NGC 869 and NGC 884 is found to be normal, whereas the value ofRcluster in the cluster regions NGC 1502 and IC 1805indicates an anomalous reddening law in these regions. In the case ofNGC 1502 the value of Rcluster is found to be lower(2.57+/-0.27) whereas in the case of IC 1805 it is higher (3.56+/-0.29)than the normal value of 3.1. Although the intra-cluster materialindicates a higher value of Rcluster in the NGC 663 region,the error in the estimation of Rcluster is too large toconclude anything. It is also found that the extinction process in the Uband in the case of NGC 663 seems to be less efficient, whereas in thecase of NGC 869 the process is more efficient.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| Oxygen and helium abundances in Galactic Hii regions - II. Abundance gradients
The absolute integrated fluxes that we obtained (Paper I) for 34 Hiiregions, with Galactocentric distances RG in the 6.6-17.7kpcrange, in the emission lines [Oii] λλ3726 and 3729, [Oiii]λλ4363 and 5007, Hei λ5876, Hα and Hβ,are analysed to derive the extinctions, the electron densities andtemperatures, the ionic abundances O+/H+,O++/H+ and He+/H+ and theO/H abundances. The electron temperature has been derived from the[Oiii] λ4363/λ5007 ratio in six Hii regions withRG between 6.6 and 14.8kpc. These new measurements ofTe, which are in good agreement with those from radiorecombination lines, widen the RG range for which reliableTe measurements exist. Combining our newTe([Oiii]) results with the radio values, we obtain thefollowing temperature relationship: TeK]=(372+/-38)RG+4260+/-350. The O+/H+and O++/H+ abundances have been obtained assuminga two-temperature Hii region model. Our O/H relationship for5kpc=180) measured in the Galaxyby Peimbert et al.
| Pre-Main-Sequence Stars in the Young Galactic Cluster IC 4996: A CCD Photometric Study
The results of a UBV CCD photometric study of the cluster IC 4996 arepresented. We obtain new values for the cluster parameters: E(B-V) =0.71 +/- 0.08, V_0 - M_V = 11.9 +/- 0.1, and age = (7.5 +/- 3) x 10^6yr. The combined evidence from the photometric diagrams suggests thepresence of a number of pre-main-sequence (PMS) cluster members withspectral types ~A0 to F5. The interpretation of these objects as PMScandidates is further supported by independent results from CCD uvbyHβ observations. If confirmed, these stars would bridge theexisting gap in the sample of PMS stars, between the coolest Herbig AeBestars (HAeBe), and the hottest T Tauri stars. These PMS candidates arelocated some 0.5 and 1 mag above the main sequence in the V-(B-V)diagram, around the location of spectral types AF. This feature,together with their locations in the (U-B)-(B-V) diagram and theinterpretation that they are PMS members, suggest an additionalreddening law with a slope alpha = E(U-B)/E(B-V) ~= 0.55, probablycaused by circumstellar material.
| Galactic Clusters with Associated Cepheid Variables. VI. Anonymous van den Bergh (C0634+031) and CV Monocerotis
A photometric UBVI_KC study, based upon a compilation of new andpreviously published photometry, is presented for the clustersurrounding the 5.38 day Cepheid CV Mon. Photoelectric, photographic,and CCD observations are presented for 200 stars lying within ~8' of CVMon, and the data are used with spectroscopic observations for starsnear the Cepheid to investigate the reddening and distance of clustermembers. The reddening law for the cluster field is described by therelation E_U - B/E_B - V = 0.77, and the ratio of total to selectiveextinction for the dust clouds obscuring cluster stars is characterizedby R = A_V/E_B - V = 3.20 +/- 0.04 s.e. The foreground dust complex islocated at a distance of ~900-1100 pc, whereas the cluster itself is1.65 +/- 0.03 kpc distant ( = 11.08 +/- 0.03 s.e.). CVMon shares not only a spatial coincidence with cluster stars-a featureconfirmed by star counts-but also a similar radial velocity andestimated age. Its derived luminosity as a cluster member is = -3.37 +/- 0.08 s.e., and its derived space reddening is E_B - V = 0.75+/- 0.02 [E_B - V(B0) = 0.80 +/- 0.02]. There is evidence for thepresence of two background groups in the field, one lying immediatelybeyond the cluster (d = 1.86 +/- 0.03 kpc) and a more distant group (d =3.02 +/- 0.08 kpc). The cluster itself exhibits photometricproperties-upper-main-sequence dispersion and main-sequencegaps-consistent with rapid rotation for hot cluster members. Suchfeatures are common in Cepheid-age clusters.
| IC 1590, A Young Cluster Embedded in the Nebulosity of NGC 281
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113.2116G&db_key=AST
| JHK Photometry of Three BOK Globules in NGC 281
IC1590 is a very sparse, young, galactic cluster, centered about the O6Vtrapezium system HD5005, embedded in the HII region NGC281, and obscuredin the south by a dust cloud. According to star counts, the clusternucleus has a diameter of 6 parsec and contains only 45 +/- 9 memberswith M_v <= 3.6 mag. An IR survey was initiated in order to detectany other probable cluster members which are obscured by the dust. ThreeBok globules (at various distances from the center) are visible againstthe nebulosity and have been investigated in the JHK bandpasses, usingthe USNO HgCdTe IR camera. Among the many field stars observed are threeobjects which are probable early--type members according to UBV data.Only one IR source was found and is located in the globule closest toHD5005. The question of whether this star is a very heavily reddenedcluster member or a background object will be discussed as well as thesignificance and implications of these results.
| Spectral Types and Hγ Observations for Stars in Eight Associations
Not Available
| A general study of diffuse galactic nebulae.
Not Available
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