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A close look at the heart of RCW 108
Context: The IRAS 16362-4845 star-forming site in the RCW 108 complexcontains an embedded compact cluster that includes some massive O-typestars. Star formation in the complex, and in particular in IRAS16362-4845, has been proposed to be externally triggered by the actionof NGC 6193. Aims: We present a photometric study of the IRAS16362-4845 cluster sensitive enough to probe the massive brown dwarfregime. In particular, we try to verify an apparent scarcity ofsolar-type and low-mass stars reported in a previous paper(Comerón et al. 2005, A&A, 433, 955). Methods: Using NACO atthe VLT we have carried out adaptive optics-assisted imaging in the{JHK}_SL' bands, as well as through narrow-band filters centered on theBrγ and the H2 S(1) v=1 → 0 lines. We estimate individualline-of-sight extinctions and, for stars detected in the three{JHK}S filters, we estimate the contribution to theKS flux caused by light reprocessed in the circumstellarenvironment. We also resolve close binary and multiple systems. We usethe K luminosity function as a diagnostic tool for the characteristicsof the underlying mass function. Results: IRAS 16362-4845 does containyoung low-mass stars. Nevertheless, they are far less than thoseexpected from the extrapolation of the bright end of the K luminosityfunction towards fainter magnitudes. We estimate a total stellar mass of370 M_ȯ. Nearly all the cluster members display L' excesses,whereas KS excesses are in general either absent or moderate(< 1 mag). We also detect an extremely red object with (K_S-L')>9, likely to be a Class I source. Conclusions: The fact that solar-typeand low-mass stars are present in numbers much smaller than thoseexpected from the number of more massive members hints at an initialmass function deficient in low mass stars as compared to that of otheryoung clusters such as the Trapezium. The origin of this difference isunclear, and we speculate that it might be due to external triggeringhaving started star formation in the cluster, perhaps producing atop-heavy initial mass function. We also note that there are nodetectable systematic differences between the spatial distributions ofbright and faint cluster members. Such absence of mass segregation inthe spatial distribution of stars may also support external triggeringhaving played an important role in the history of the RCW 108 region.Based on observations obtained at the European Southern Observatoryusing the Very Large Telescope (VLT) (programme 077.C-0660(A)), on CerroParanal, Chile. Table 2 is only available in electronic form athttp://www.aanda.org
| The Young Stellar Population in M17 Revealed by Chandra
We report here results from a Chandra ACIS observation of the stellarpopulations in and around the M17 H II region. The field reveals 886sources with observed X-ray luminosities (uncorrected for absorption)between ˜29.3 ergs s-1< log LX<32.8ergs s-1, 771 of which have stellar counterparts in infraredimages. In addition to comprehensive tables of X-ray source properties,several results are presented:1. The X-ray luminosity function is calibrated to that of the OrionNebula Cluster population to infer a total population of roughly8000-10,000 stars in M17, one-third lying in the central NGC 6618cluster.2. About 40% of the ACIS sources are heavily obscured withAV>10 mag. Some are concentrated around well-studiedstar-forming regions -- IRS 5/UC1, the Kleinmann-Wright Object, andM17-North -- but most are distributed across the field. As previouslyshown, star formation appears to be widely distributed in the molecularclouds. X-ray emission is detected from 64 of the hundreds of Class Iprotostar candidates that can be identified by near- and mid-infraredcolors. These constitute the most likely protostar candidates known inM17.3. The spatial distribution of X-ray stars is complex: in addition tothe central NGC 6618 cluster and well-known embedded groups, we find anew embedded cluster (designated M17-X), a 2 pc long arc of young starsalong the southwest edge of the M17 H II region, and 0.1 pc substructurewithin various populations. These structures may indicate that thepopulations are dynamically young.4. All (14/14) of the known O stars but only about half (19/34) of theknown B0-B3 stars in the M17 field are detected. These stars exhibitthe long-reported correlation between X-ray and bolometric luminositiesof LX˜10-7Lbol. While many O andearly-B stars show the soft X-ray emission expected from microshocks intheir winds or moderately hard emission that could be caused bymagnetically channeled wind shocks, six of these stars exhibit very hardthermal plasma components (kT>4 keV) that may be due to collidingwind binaries. More than 100 candidate new OB stars are found, including28 X-ray detected intermediate- and high-mass protostar candidates withinfrared excesses.5. Only a small fraction (perhaps 10%) of X-ray selected high- andintermediate-mass stars exhibit K-band-emitting protoplanetary disks,providing further evidence that inner disks evolve very rapidly aroundmore massive stars.
| H2O maser emission from bright rimmed clouds in the southern hemisphere
Context: Water maser emission is a powerful tracer of the presence ofembedded sources in dense clouds since it requires elevated temperatures(>100 K) and densities (>107 cm-3) that canbe found in circumstellar disks and/or jets/outflows associated withYoung Stellar Objects. Bright rimmed clouds compressed by ionizationfronts from nearby massive stars are considered good examples ofexternally triggered star formation, possibly resulting in the formationof massive stars. Aims: We aim to determine the water maser emissionfrequency and characteristics of 45 bright rimmed clouds in the southernhemisphere identified by Sugitani & Ogura (1994, ApJS, 92, 163). Methods: We have used the Tidbinbilla 70-m radiotelescope to perform ahigh sensitivity survey at 22.2 GHz of the maser emission from the616-523 rotational transition of H2O molecules. Results: We found 7 water maser sources out of 44 (16% detection rate),5 being new detections. With the exception of the maser associated withBRC 68, all the other maser are characterized by low integrated fluxesand luminosities. Conclusions: Most maser sources fall below thecorrelation between the H2O and far-infrared luminosity found in otherstudies towards a variety of star forming regions. These results aresimilar to those found in the companion survey of BRCs in the northernhemisphere by Valdettaro et al. (2005, A&A, 443, 535). The lowdetection frequency and the properties of water maser emission from BRCsindicate that low-mass star formation is the most natural outcome of theexternal compression induced by the ionization front from nearby massivestars.Based on observations obtained with the 70-m Tidbinbilla radiotelescope.
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity
Aims.We present new CORAVEL radial-velocity observations andphotoelectric photometry in the UBV and DDO systems for a sample ofpotential members of the red-giant branches of NGC 6192, NGC 6208 andNGC 6268, three open clusters projected close to the Galactic centerdirection. We also examine the properties of a sample of 42 inner diskopen clusters projected towards almost the same direction as the threeclusters. Methods: .Cluster members and red field giants werediscriminated by using the CORAVEL radial-velocity data and by applyingtwo photometric criteria. Interstellar reddening and metal content ofthe clusters were derived from combined BV and DDO data. Results:.Cluster membership for five red giants in NGC 6192, three in NGC 6208and three in NGC 6268 has been confirmed by the analysis of thephotometric and kinematic data. Photometric membership probabilitiesshow very good agreement with those obtained from CORAVEL radialvelocities. Three new spectroscopic binaries were discovered among thered giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B-V)colour excesses were derived. Conclusions: .The overallmetallicities were found to be nearly solar for NGC 6208 and above solarfor NGC 6192 and NGC 6268. Most of the clusters located closer than 2kpc from the Sun in the considered direction are slightly more reddenedthan the absorption resulting from the Baade's window absorption law.
| 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.
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. I. Large Magellanic Cloud, field of NGC 2004
Aims.To statistically study the effects of the metallicity,star-formation conditions, and evolution on the behaviour of massivestars and, more particularly, of B and Be stars, we observed largesamples of stars in the Magellanic Clouds for the first time. In thisarticle we present the first part of this study. Methods:.Spectroscopic observations of hot stars belonging to the young clusterLMC-NGC 2004 and its surrounding region were carried out with theVLT-GIRAFFE facilities in MEDUSA mode. We determined the fundamentalparameters (T_eff, log~g, V sin i, and radial velocity) for all B and Bestars in the sample thanks to a code developed in our group. The effectof fast rotation (stellar flattening and gravitational darkening) aretaken into account in this study. We also determined the age of observedclusters. We then compared the mean V sin i obtained for field andcluster B and Be stars in the Large Magellanic Cloud (LMC) with the onesin the Milky Way (MW). Results: .We find, in particular, that Bestars rotate faster in the LMC than in the MW, in the field as well asin clusters. We discuss the relations between V sin i, metallicity,star-formation conditions, and stellar evolution by comparing the LMCwith the MW. We conclude that Be stars began their main sequence lifewith an initial rotational velocity higher than the one for B stars. Itis probable that only part of the B stars, those with a sufficientinitial rotational velocity, can become Be stars. This result mayexplain the differences in the proportion of Be stars in clusters withsimilar ages.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Radio Detection of Colliding Wind Binaries
Four massive, early-type stars, three of which are confirmed binaries,have been observed with the Australia Telescope Compact Array at 1.4,2.4, 4.8, and 8.6 GHz. The earliest star cataloged so far, HD 93129A,was also observed at 17.8 and 24.5 GHz. Here we present an analysis ofthe spectra as well as the structure of the stellar systems. All fourspectra show clear evidence of non-thermal emission, indicative of abinary system with a colliding wind region. We discuss the magneticfield of the emitting region of HD 93129A and make predictions on theradiation at high energies. Archive X-ray observations towards thetarget sources are also investigated and interpreted in the light of thenon-thermal emission detected.
| 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.
| New fundamental parameters for the inner disk open cluster Lyngå 11
We have obtained CCD BVIKC photometry down to V ˜ 21.5for the open cluster Lyngå 11 and its surrounding field. Thispreviously unstudied object appears to be rich and highly absorbed. Acluster angular radius of 4.5′ ± 0.5′, equivalent to(3.0 ± 0.3) pc, is estimated from star counts in 100-pixel a sideboxes distributed throughout the whole observed field. Our analysissuggests that Lyngå 11 is moderately young and probably of solarmetallicity. Adopting the theoretical metal content Z = 0.02, whichprovides the best global fit, we derive an age of (630 ± 70) Myr.Simultaneously, colour excesses E(B ‑ V) = 0.70 and E(V ‑ I)= 0.85 and an apparent distance modulus V ‑ MV = 14.0are obtained. These results place Lyngå 11 at a distance of (2.3± 0.5) kpc from the Sun and ˜6.5 kpc from the Galacticcentre. The properties of a sample of clusters aligned along theline-of-sight of Lyngå 11 are examined as well.
| The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters
Be stars are a class of rapidly rotating B stars with circumstellardisks that cause Balmer and other line emission. There are threepossible reasons for the rapid rotation of Be stars: they may have beenborn as rapid rotators, spun up by binary mass transfer, or spun upduring the main-sequence (MS) evolution of B stars. To test the variousformation scenarios, we have conducted a photometric survey of 55 openclusters in the southern sky. Of these, five clusters are probably notphysically associated groups and our results for two other clusters arenot reliable, but we identify 52 definite Be stars and an additional 129Be candidates in the remaining clusters. We use our results to examinethe age and evolutionary dependence of the Be phenomenon. We find anoverall increase in the fraction of Be stars with age until 100 Myr, andBe stars are most common among the brightest, most massive B-type starsabove the zero-age main sequence (ZAMS). We show that a spin-up phase atthe terminal-age main sequence (TAMS) cannot produce the observeddistribution of Be stars, but up to 73% of the Be stars detected mayhave been spun-up by binary mass transfer. Most of the remaining Bestars were likely rapid rotators at birth. Previous studies havesuggested that low metallicity and high cluster density may also favorBe star formation. Our results indicate a possible increase in thefraction of Be stars with increasing cluster distance from the Galacticcenter (in environments of decreasing metallicity). However, the trendis not significant and could be ruled out due to the intrinsic scatterin our data. We also find no relationship between the fraction of Bestars and cluster density.
| The AAO/UKST SuperCOSMOS Hα survey
The UK Schmidt Telescope (UKST) of the Anglo-Australian Observatorycompleted a narrow-band Hα plus [NII] 6548, 6584-Åsurvey ofthe Southern Galactic Plane and Magellanic Clouds in late 2003. Thesurvey, which was the last UKST wide-field photographic survey and theonly one undertaken in a narrow-band, is now an online digital dataproduct of the Wide-Field Astronomy Unit of the Royal ObservatoryEdinburgh (ROE). The survey utilized a high specification, monolithicHα interference bandpass filter of exceptional quality. Inconjunction with the fine-grained Tech-Pan film as a detector it hasproduced a survey with a powerful combination of area coverage (4000square degrees), resolution (~1 arcsec) and sensitivity (<=5Rayleighs), reaching a depth for continuum point sources of R~= 20.5.The main survey consists of 233 individual fields on a grid of centresseparated by 4° at declinations below +2° and covers a swatheapproximately 20° wide about the Southern Galactic Plane. Theoriginal survey films were scanned by the SuperCOSMOS measuring machineat the Royal Observatory, Edinburgh, to provide the online digital atlascalled the SuperCOSMOS Hα Survey (SHS). We present the backgroundof the survey, the key survey characteristics, details and examples ofthe data product, calibration process, comparison with other surveys anda brief description of its potential for scientific exploitation.
| 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.
| Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association
A 90-ks Chandra High Energy Transmission Grating observation of theyoung stellar cluster NGC 6193 in the southern Ara OB1 associationdetected 43 X-ray sources in a 2 × 2 arcmin2 coreregion centred on the massive O stars HD 150135 (O6.5V) and HD 150136(O3 + O6V). The cluster is dominated by exceptionally bright X-rayemission from the two O stars, which are separated by only 10 arcsec.The X-ray luminosity of HD 150136 is logLX= 33.39 (ergs-1), making it one of the most luminous O-star X-ray sourcesknown. All of the fainter X-ray sources in the core region havenear-infrared (near-IR) counterparts, but existing JHK photometryprovides little evidence for near-IR excesses. These core sources havetypical mean photon energies ~ 2 keV and about one-third arevariable. It is likely that some are young low-mass stars in thecluster, but cluster membership remains to be determined. Gratingspectra show that the X-ray properties of HD 150135 and HD 150136 aresimilar, but not identical. Both have moderately broadened unshiftedemission lines and their emission is dominated by cool plasma at kT~ 0.3keV, pointing to a wind-shock origin. However, the emission of HD 150136is slightly hotter and four times more luminous than its optical twin HD150135. We discuss the possibility that a radiative colliding wind shockcontributes to the prodigious X-ray output of the short-period (2.66 d)spectroscopic binary HD 150136. A surprising result is that the X-rayemission of HD 150136 is slowly variable on a time-scale of <1 d. Theorigin of the variability is not yet known but the observed behavioursuggests that it is an occultation effect.
| Star formation in RCW 108: Triggered or spontaneous?
We present visible, near infrared and mm-wave observations of RCW 108, amolecular cloud complex in the Ara OB1 association that is being erodedby the energetic radiation of two O-type stars in the nearby cluster NGC6193. The western part of the RCW108 molecular cloud, for which wederive a mass of ~8000 Mȯ, contains an embedded compactHII region, IRAS 16362-4845, ionized by an aggregate of early-type starsfor which we estimate a mass of 210 Mȯ. Thespectral type of the earliest star is O9, as confirmed by the visiblespectrum of the compact HII region. We notice a lack of stars later thanA0 in the aggregate, at least having the moderate reddenings that arecommon among its B-type stars, and we speculate that this might be aconsequence of the extreme youth of the aggregate. We also note theexistence of a dense ionized clump (n > 104cm-3) appearing near the main ionizing star of the compactHII region. We examine the distribution of stars displaying infraredexcesses projected across the molecular cloud. While many of them arelocated in the densest (n 104-5 cm-3) areaof the molecular cloud near the position of IRAS 16362-4845, we alsofind a group concentrating towards the edge of the cloud that faces NGC6193, as well as some other stars beyond the edge of the molecularcloud. The intense ionizing radiation field by the O stars in NGC 6193is a clear candidate trigger of star formation in the molecular cloud,and we suggest that the existence and arrangement of stars in thisregion of the molecular cloud supports a scenario in which theirformation may be a consequence of this. However, infrared excess starsare also present in some areas of the opposite side of the cloud, whereno obvious candidate external trigger is identified. The existence ofsuch tracers of recent star formation scattered across the more massivemolecular cloud associated with IRAS 16362-4845, and the low starformation efficiency that we derive, indicate that it is in a state tostill form stars. This is in contrast to the less massive cloud (660 Mȯ) close to NGC 6193, which seems to be moreevolved and mostly already recycled into stars, and whose internalkinematics show hints of having been perturbed by the presence of themassive stars formed out of it.
| The nearest star of spectral type O3: a component of the multiple system HD 150136
From radial velocities determined in high signal-to-noise digitalspectra, we report the discovery that the brightest component of thebinary system HD 150136 is of spectral type O3. We also present thefirst double-lined orbital solution for this binary. Our radialvelocities confirm the previously published spectroscopic orbital periodof 2.6 d. HeII absorptions appear double at quadratures, but singlelines of NV and NIV visible in our spectra define a radial velocityorbit of higher semi-amplitude for the primary component than do theHeII lines. From our orbital analysis, we obtain minimum masses for thebinary components of 27 and 18 Msolar. The neutral Heabsorptions apparently do not follow the orbital motion of any of thebinary components, thus they most probably arise in a third star in thesystem.
| High-mass star formation within the bright-rimmed cloud SFO 79
We report Radio Recombination Line (RRL) and continuum observationstoward the IRAS point source 16362-4845, embedded within theBright-Rimmed Cloud (BRC) SFO 79, a small molecular cloud lying at theedge of the HII region RCW 108. High resolution observations of theH92α hydrogen recombination line and of the continuum emission(3.6 and 6 cm) confirm the presence of a resolved Ultra Compact (UC) HIIregion embedded within the molecular cloud. The integrated radio fluxessuggest the source of the ionisation to be an O9 Zero Age Main Sequence(ZAMS) star. Millimetre observations of 12CO, 13COand C18O (J=1-0) molecular lines reveal the presence of amolecular condensation offset 30 arcsec to the north of the IRASposition on the boundary of the UC HII region. Analysis of 2MASS datahas led to the identification of a small IR cluster of Young StellarObjects (YSOs) that are positionally coincident with the UC HII region,lying to the south east of the peak of the radio emission. Moreover, theUC HII region appears to be extended in the direction of the IR cluster,which suggests that the radio emission and the IR cluster are in someway related to each other. MSX 8.3 μm and 21.3 μm images have beenused to trace the large scale structure of the BRC, revealing thepresence of a Photo Dominated Region (PDR) and three embedded thermalsources within the molecular cloud. The PDR has a plane parallelmorphology which correlates extremely well with the morphology of theionised gas traced by the optical emission. The three thermal sources(labelled A, B, C) all lie at a similar projected distance from theinterface between the HII region and the molecular gas of the cloud.Thermal sources A and C are positionally coincident with the IRAS pointsources 16362-4845 and 16362-4841 respectively, both of which have IRAScolours consistent with the presence of UC HII regions. Given that UCHII regions are relatively short lived (105 yrs) it isreasonable to suggest that these two UC HII regions are of a similarage. The alignment of the three thermal sources along a line parallel tothe bright rim suggests that they could have been triggered by thepropagation of a plane parallel shock through the cloud.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| A 12CO (J = 1 -> 0) study towards the Ara OB1 region
Intermediate angular resolution (HPBW=8\farcm7 ) carbon monoxide(12CO J=1->0) line observations towards the OB associationAra OB1 are reported. The molecular line observations cover an area of2\fdg75 x 3\fdg00 , and disclose a rich and complex distribution of themolecular line emission. The molecular features likely to be associatedwith the OB-association Ara OB1 span the velocity range from -28 to -20km s-1. The most negative radial velocities are observedalong NGC 6188 (equiv Rim nebula). This nebula marks the interfacebetween the HII region RCW 108 and the highly absorbing molecularmaterial located westwards of the ionised region. The dominant COstructures, labeled E and F, have a radial velocity of about -23.5 and-21.8 km s-1, respectively. The former harbours a brightoptical knot, the infrared cluster RCW-108 and the strong IRAS source16362-4845. IRAS point sources are also seen in projection onto12CO concentrations F and H. This may indicate that the starforming process is a widespread phenomenon in this molecular complex.Extended continuum emission, having a high degree of spatial correlationwith both the infrared and molecular emission, is observed in theregion. We believe that this emission, thermal in nature, arises fromthe ionised surface layers of the molecular complex. The ionising agentsare the high mass stars of NGC 6193, the nucleus of Ara OB1. The totalamount of molecular gas related to Ara OB1 is about ~ 1.4x104 Msun.
| Optical Polarization Observations of NGC 6231: Evidence for a Past Supernova Fingerprint
We present the first linear multicolor polarization observations for asample of 35 stars in the direction of the Galactic cluster NGC 6231. Wehave found a complex pattern in the angles of the polarimetric vectors.Near the core of this cluster the structure shows a semicircular patternthat we have interpreted as a reorientation of the dust particlesshowing the morphology of the magnetic field. We propose that asupernova event occurred some time ago and produced a shock on the localISM. We discuss in this paper independent confirmations of this event,both from the studies on the diffuse interstellar absorptions and theresults of the pre-main-sequence stars. We also show that a supernova issupported by the evolutionary status of the cluster.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.
| Optical polarization observations in the region of Stock 16
We present (UBVRI) multicolor linear polarimetric data for 26 of thebrightest stars in the area of the open cluster Stock 16 that wereconsidered to study the properties of the ISM (interstellar medium)towards the cluster. Our data yield a mean polarization percentage of P~ 2.5%, close to the polarization value produced by the ISM with normalefficiency (Plambda_max ~ 5 EB-V) undergoing acolor excess of EB-V =0.51. The mean angle of thepolarization vectors, theta = 74fdg9 , agrees quite well with theexpected angle produced by dust particles aligned in the direction ofthe galactic disk (and the magnetic field) in the region. A study of theextinction suffered by the stars in the zone was also performedcombining our new data with previous photometric data. In this sense,our analysis indicates that the visual absorption affecting Stock 16stars is mainly produced in front of the cluster by a dust cloud atapproximately 500 pc from the sun. The large polarization value of thenonmember star, WR 51, confirms its background star nature.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/933
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| New infrared star clusters in the southern Milky Way with 2MASS
We carried out a 2MASS J, H and Ks survey of infrared starclusters in the Milky Way sector 230deg< l <350deg. This zone was the least studied in the literature,previously including only 12 infrared clusters or stellar groups with|b|< 10deg, according to the recent catalogue by Bica etal. (2003). We concentrated efforts on embedded clusters, which arethose expected in the areas of known radio and optical nebulae. Thepresent study provides 179 new infrared clusters and stellar groups,which are interesting targets for detailed future infrared studies. Thesample of catalogued infrared clusters and stellar groups in the Galaxyis now increased by 63%.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Two highly reddened young open clusters located beyond the Sagittarius arm
We present the results of CCD BVI Johnson-Cousins photometry down to V ~19 mag in the regions of the unstudied stellar groups Pismis 23 and BH222, both projected close to the direction towards the Galactic centre.We measured V magnitude and B-V and V-I colours for a total of 928 starsin fields of about 4arcmin x4arcmin . Pismis 23 is conclusively aphysical system, since a clear main sequence and other meaningfulfeatures can be seen in the colour-magnitude diagrams. The reality ofthis cluster is also supported by star counts carried out within andoutside the cluster field. For Pismis 23 we derive colour excessesE(B-V) = 2.0 +/- 0.1 and E(V-I) = 2.6 +/- 0.1, a distance from the Sunof 2.6 +/- 0.6 kpc (Z = -19 pc) and an age of 300 +/- 100 Myr (assumingsolar metal content). BH 222 appears to be a young open cluster formedby a vertical main sequence and by a conspicuous group of luminous,typically red supergiant stars. We derived for this cluster a colourexcess of E(V-I) = 2.4 +/- 0.2, a distance from the Sun of 6.0 +/- 2.7kpc (Z = -46 pc) and an age of 60 +/- 30 Myr. The resulting reddeningand distance estimates place these two young objects among the mostreddened and distant open clusters known in the direction towards theGalactic centre. They are located beyond the Sagittarius arm, close tothe direction where this arm probably bifurcates into two arms. Based onobservations made at the University of Toronto (David DunlapObservatory) 24-inch telescope, Las Campanas, Chile. Tables \ref{t2} and3 are only available 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/388/179
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are 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/388/168
| The Molecular Gas Associated with ARA OB1
Not Available
| 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.
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