An X-Ray Imaging Study of the Stellar Population in RCW 49
We present the results of a high-resolution X-ray imaging study of thestellar population in the Galactic massive star-forming region RCW 49and its central OB association Westerlund 2. We obtained a ~40 ks X-rayimage of a ~17'×17' field using the ChandraX-Ray Observatory and deep NIR images using the Infrared Survey Facilityin a concentric ~8.3'×8.3' region. Wedetected 468 X-ray sources and identified optical, NIR, and Spitzer MIRcounterparts for 379 of them. The unprecedented spatial resolution andsensitivity of the X-ray image, enhanced by optical and infrared imagingdata, yielded the following results: (1) The central OB associationWesterlund 2 is resolved for the first time in the X-ray band. X-rayemission is detected from all spectroscopically identified early-typestars in this region. (2) Most (~86%) X-ray sources with optical orinfrared identifications are cluster members in comparison with acontrol field in the Galactic plane. (3) A loose constraint (2-5 kpc)for the distance to RCW 49 is derived from the mean X-ray luminosity ofT Tauri stars. (4) The cluster X-ray population consists of low-masspre-main-sequence and early-type stars as obtained from X-ray and NIRphotometry. About 30 new OB star candidates are identified. (5) Weestimate a cluster radius of 6'-7' based on the X-ray surface numberdensity profiles. (6) A large fraction (~90%) of cluster members areidentified individually using complimentary X-ray and MIR excessemission. (7) The brightest five X-ray sources, two Wolf-Rayet stars andthree O stars, have hard thermal spectra.
|
Detection of extended very-high-energy γ-ray emission towards the young stellar cluster Westerlund 2
Aims. Results from γ-ray observations by the HESS telescope arrayin the direction of the young stellar cluster Westerlund 2 arepresented. Methods: Stereoscopic imaging of Cherenkov light emission ofγ-ray induced showers in the atmosphere is used to study thecelestial region around the massive Wolf-Rayet (WR) binary WR 20a.Spectral and positional analysis is performed using standard eventreconstruction techniques and parameter cuts. Results: The detection ofa new γ-ray source is reported from HESS observations in 2006.HESS J1023-575 is found to be coincident with the young stellar clusterWesterlund 2 in the well-known HII complex RCW 49. The source isdetected with a statistical significance of more than 9σ, andshows extension beyond a point-like object within the HESS point-spreadfunction. The differential γ-ray spectrum of the emission regionis measured over approximately two orders of magnitude in flux. Conclusions: The spatial coincidence between HESS J1023-575 and theyoung open cluster Westerlund 2, hosting e.g. the massive WR binary WR20a, requires one to look into a variety of potential models to accountfor the observed very-high-energy (VHE) γ-ray emission. Consideredemission scenarios include emission from the colliding wind zone of WR20a, collective stellar winds from the extraordinary ensemble of hot andmassive stars in the stellar cluster Westerlund 2, diffusive shockacceleration in the wind-blown bubble itself, and supersonic windsbreaking out into the interstellar medium (ISM). The observed sourceextension argues against a single star origin of the observed VHEemission.
|
RCW 49 at Mid-Infrared Wavelengths: A GLIMPSE from the Spitzer Space Telescope
The luminous, massive star formation region RCW 49, located in thesouthern Galactic plane, was imaged with the Infrared Array Camera(IRAC) on the Spitzer Space Telescope as part of the Galactic LegacyInfrared Mid-Plane Survey Extraordinaire (GLIMPSE) program. The IRACbands contain polycyclic aromatic hydrocarbon (PAH) features at 3.3,6.2, 7.7, and 8.6 μm, as well as the Brα line. These featuresare the major contributors to the diffuse emission from RCW 49 in theIRAC bands. The Spitzer IRAC images show that the dust in RCW 49 isdistributed in a network of fine filaments, pillars, knots, sharplydefined boundaries, bubbles, and bow shocks. The regions immediatelysurrounding the ionizing star cluster and W-R stars are evacuated ofdust by stellar winds and radiation. The IRAC images of RCW 49 suggestthat the dust in RCW 49 has been sculpted by the winds and radiationfrom the embedded luminous stars in the inner 5' (inner ~6 pc) of thenebula. At projected angular radii φ>5' from thecentral ionizing cluster, the azimuthally averaged infrared intensityfalls off as ~φ-3. Both high-resolution radio and mid-IRimages suggest that the nebula is density bounded along its westernboundary. The filamentary structure of the dust in RCW 49 suggests thatthe nebula has a small dust filling factor and, as a consequence, theentire nebula may be slightly density bounded to H-ionizing photons.
|
CCD Photometry of the Open Clusters NGC 2658, NGC 2849, and NGC 3247
We present CCD photometry of the galactic open clusters NGC 2658, NGC2849, and NGC 3247. By means of the comparison between the observedcluster sequences and the Padova theoretical isochrones, we derive thefollowing parameters: for NGC 2658, E(B-V)=0.35+0.05-0.10 , log(age) =8.50+0.25-0.05, and (m - M)0 =13.32+0.36-0.52; for NGC 2849, the reddening is inthe range 0.46 to 0.57 with an uncertainty of +/-0.12 mag, (m -M)0 = 14.02+0.38-0.40, and log(age) =8.8+0.1-0.05; finally, for NGC 3247 we obtainE(B-V)=0.39±0.07 , log(age) = 7.60+0.4-0.3, and(m - M)0 = 11.89+0.51-0.45. In thisstudy the isochrones have been renormalized to the solar values(B-V)ȯ =0.65 and M[V] = 4.84; the values ({V-R)ȯ =0.37 and (V-I)ȯ =0.70 areproposed as zero points in order to renormalize those colors as well.
|
Not Available
Not Available
|
Millimeter-Wavelength Galactic Observations with the Mobile Anisotropy Telescope
Spatially filtered maps of a portion of the Galactic plane are presentedin Ka band (26-36 GHz), Q band (36-46 GHz), and D band (138-150 GHz)near a declination of -60° with resolutions ranging from 0.2d to0.9d. The maps are optimized to detect unresolved source emission andare calibrated through observations of the planet Jupiter. Sixmillimeter-bright regions are detected, including the Carina Nebula andRCW 57, of which three are visible in all bands. Variability of fluxdensity over a 3 month period has an upper limit of 3.8% for all sourcesseen in Ka and Q bands and 10% in D band. A fractional polarizationupper limit of 0.08 is placed for all sources seen in Ka and Q bands.
|
A ROSAT medium-sensitivity Galactic plane survey at 180°< l < 280°
We have performed a moderately deep soft X-ray (0.1-2keV) survey of theGalactic plane using pointed observations with the ROSAT PositionSensitive Proportional Counter (PSPC). The survey is more than an orderof magnitude more sensitive than previous X-ray surveys near theGalactic plane. The data consist of nine fields each of ~10ks exposure,pointed at positions on or very close to the Galactic plane(|b|<0°.3) in the longitude range 180°<~l<~280°.This region has relatively low X-ray absorbing material out to distancesof several hundred pc and presents fewer source-confusion problems thanat other longitudes. The total sky area surveyed was 2.5deg2this yielded 93 sources, 89 of which were detected in the `hard'(0.4-2.0keV) band. Nine sources were detected in both `soft'(0.1-0.4keV) and hard bands. In the hard band, the survey coverage is>~90 per cent for sources brighter than 0.002counts-1(~2×10-14ergcm-2s-1), but fallssteeply below this value, with the weakest sources being~0.001counts-1. The median limiting flux is~0.0013counts-1(~1.3×10-14ergcm-2s-1). There are64 sources with hard-band count rates >0.002counts-1. Wepresent the catalogue of X-ray sources and the number-flux relations(logN-logS). Eighteen sources have possible identifications from theSIMBAD data base. We have searched the Tycho-2 and USNO-A2.0 cataloguesto find all possible optical counterparts brighter than ~19th magnitude,and attempt to classify these on the basis oflog(fX/fopt) versus optical colour diagrams andnear-infrared photometry from the 2MASS Second Incremental Data Release.Hence, we have found the majority of these sources to be consistent withbeing late-type main-sequence stars, as previous studies have proposedfrom incompletely identified surveys. Comparison of the measurednumber-flux relations with predictions of Galactic (stellar) andextragalactic populations supports the view that the population of youngstars in the plane is denser than previously thought.
|
Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
|
Probable binary open star clusters in the Galaxy.
The existence of double/binary clusters in the Magellanic Clouds isfairly well established, whereas only one such pair, h + χ Persei,is known in the Galaxy. From the catalogues of open clusters of theGalaxy, we have identified 18 probable pairs of clusters (with knowndistances), with spatial separations less than 20pc. The tidaldisruption timescales for these pairs, due to Galactic differentialrotation are calculated, using cluster data where available or byassuming typical values. In some cases, these timescales are larger thanthe average open cluster lifetime, =~10^8^yr. About 8% of open clustersappear to be members of binary systems, and hence binary cluster systemsmay not be very uncommon in the Galaxy.
|
Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
|
The Velocity Field of the Outer Galaxy
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...275...67B
|
Kinematics of molecular clouds. II - New data on nearby giant molecular clouds
The best currently available data on positions, distances, andvelocities of giant molecular clouds within 3 kpc of the sun areanalyzed to yield a one-dimensional rms cloud-to-cloud velocitydispersion of 7.8 +0.6, -0.5. Velocity dispersion is defined here as theroot mean square of cloud peculiar velocities, a quantity which includessmall-scale streaming. It is argued that this value for the velocitydispersion is pausible, based on examples of clouds whose velocitiescannot be explained purely by galactic rotation. The mean motion ofnearby molecular clouds is drifting by about 4 km/s with respect to theLSR.
|
Component Analysis of Open Clusters
Not Available
|
The Sagittarius-Carina Arm Structure and Parameters of Spiral Pattern
Not Available
|
Young stellar-gas complexes in the Galaxy
It is found that about 90 percent of OB-associations and o-b2 clusterssituated within 3 kpc of the sun can be united into complexes withdiameters of 150-700 pc. Almost all of these clusters contain giantmolecular clouds with a mass greater than about 100,000 solar masses. Anumber of complexes are associated with giant H I clouds; a few of thesmall complexes are situated in the HI caverns. The concentration ofOB-associations and young clusters in star complexes attests to theircommon origin in the supergiant gaseous clouds.
|
A cluster analysis of open clusters
The Galactic distribution of 361 open clusters is studied using acluster analysis method. It is shown that more than half of the clustersenter groups with characteristic dimensions of several hundred parsecs.To distinguish physical clusters from random condensations, criteriabased on age similarity, the color of the main-sequence blue end, andthe integrated color and radial velocity of the clusters are used. Theproximity of these values suggests a physical unity and common origin ofclusters in a group.
|
A cluster analysis of young open clusters
Cluster analysis methods are used to consider the galactic distributionof 224 open clusters with an age up to 10 to the 8th yrs. Most of theseclusters enter condensations with characteristic dimensions of a fewhundred parsecs. Some condensations are so similar in terms of the age,integrated color, and radial velocity of their components, that thiscannot be considered a coincidence. This suggests that each condensationis a physical entity consisting of clusters apparently linked by acommon origin.
|
The velocity field of the outer Galaxy in the Southern Hemisphere. II - CO observations of galactic nebulae
CO observations of 308 objects (77 percent) from a catalogue (Brand etal., 1985) of galactic emission and reflection nebulae are presented; COwas detected in the direction of 234 nebulae (76 percent). For 194 ofthese objects (63 percent) the emission could actually be associatedwith the nebula. Fifteen objects (5 percent) have associated CO emissionwith velocity in excess of 50 km/s (VLSR). These objects areof crucial importance as they are potentially very distant, and nearlyall of them have been newly identified. In the course of this surveyseveral sources with interesting line profiles were found. Eighteen ofthem exhibit wings and/or plateaus and are potential CO outflow sources.
|
The velocity field of the outer Galaxy in the Southern Hemisphere. I - Catalogue of nebulous objects
A multifrequency program to measure the velocity field of the outerGalaxy in the Southern Hemisphere (l = 230-305 deg) is outlined. Thispaper, the first in a series, presents a catalog of HII regions andreflection nebulae used as a basis for the study. Most of the entriesare previously uncatalogued; the emphasis is on small objects likely tobe very distant from the sun.
|
Catalogue of Eclipsing and Spectroscopic Binary Stars in the Regions of Open Clusters
Not Available
|
Open clusters and galactic structure
A total of 610 references to 434 clusters are employed in thecompilation of a catalog of open clusters with color-magnitude diagramson the UBV or RGU systems. Estimates of reddening, distance modulus, ageand number of cluster members are included. Although the sample isconsidered representative of the discoverable clusters in the galaxy,the observed distribution is nonuniform because of interstellarobscuration. Cluster distribution in the galactic plane is found to bedominated by the locations of dust clouds rather than by spiralstructure. The distributions of clusters as a function of age andrichness class show that the lifetimes of poor clusters are much shorterthan rich ones, and that clusters in the outer disk survive longer thanthose in the inner disk. An outer disk age which is only about 50% theage of the globular clusters is indicated by cluster statistics. Thethickening of the galactic disk with increasing galactocentric distancemay be due to either a younger dynamical age or a lower gravitationalpotential in the outer regions.
|
A catalogue of galactic clusters observed in three colours
This catalogue contains photometric data for 190 galactic clusters, allobserved in UBV or RGU. The distances of the young clusters (with spless than b3) have been calculated or recalculated according to method Aof Becker (1963). The galactic distribution of these clusters confirmstheir role of being good spiral tracers.
|
RGU photometry of the galactic star cluster NGC 3247
Abstract image available at:http://adsabs.harvard.edu/abs/1977A&AS...29..379G
|
A low-latitude galactic survey from lII=46o to 61o and 190o to 290o at 2700 MHz
Not Available
|
A Survey of H 109cc Recombination Line Emission in Galactic H N Regions of the Southern Sky
Abstract image available at:http://adsabs.harvard.edu/abs/1970A&A.....6..364W
|
