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Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| The infrared Hourglass cluster in M8*†
A detailed study of the Hourglass nebula in the M8 star-forming regionis presented. The study is mainly based on recent subarcsec-resolutionJHKs images taken at Las Campanas Observatory andcomplemented with archival Hubble Space Telescope (HST) images andlong-slit spectroscopy retrieved from the European Southern ObservatoryArchive Facility. Using the new numerical code CHORIZOS, we estimate thedistance to the earliest stars in the region to be 1.25 kpc. Infraredphotometry of all the sources detected in the field is given. Fromanalysis of the JHKs colour-colour diagrams, we find that animportant fraction of these sources exhibit significant infrared excess.These objects are candidates to be low- and intermediate-masspre-main-sequence stars. Based on HST observations, the spatialdistribution of gas, dust and stars in the region is analysed. Amorphological analysis of these images also reveals a rich variety ofstructures related to star formation (proplyds, jets, bow shocks),similar to those observed in M16 and M42, along with the detection ofthe first four Herbig-Haro objects in the region. Furthermore, along-slit spectrum obtained with the New Technology Telescope confirmsthe identification of one of them (HH 870) in the core of the Hourglassnebula, providing the first direct evidence of active star formation byaccretion in M8.
| The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ``ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments.
| 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.
| Ultraviolet spectroscopy of the extended solar corona
The first observations of ultraviolet spectral line profiles andintensities from the extended solar corona (i.e., more than 1.5 solarradii from Sun-center) were obtained on 13 April 1979 when arocket-borne ultraviolet coronagraph spectrometer of theHarvard-Smithsonian Center for Astrophysics made direct measurements ofproton kinetic temperatures, and obtained upper limits on outflowvelocities in a quiet coronal region and a polar coronal hole. Followingthose observations, ultraviolet coronagraphic spectroscopy has expandedto include observations of over 60 spectral lines in coronal holes,streamers, coronal jets, and solar flare/coronal mass ejection (CME)events. Spectroscopic diagnostic techniques have been developed todetermine proton, electron and ion kinetic temperatures and velocitydistributions, proton and ion bulk flow speeds and chemical abundances.The observations have been made during three sounding rocket flights,four Shuttle deployed and retrieved Spartan 201 flights, and the Solarand Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy ofthe extended solar corona has led to fundamentally new views of theacceleration regions of the solar wind and CMEs. Observations with theUltraviolet Coronagraph Spectrometer (UVCS) on SOHO revealedsurprisingly large temperatures, outflow speeds, and velocitydistribution anisotropies in coronal holes, especially for minor ions.Those measurements have guided theorists to discard some candidatephysical processes of solar wind acceleration and to increase and expandinvestigations of ion cyclotron resonance and related processes.Analyses of UVCS observations of CME plasma properties and the evolutionof CMEs have provided the following: temperatures, inflow velocities andderived values of resistivity and reconnection rates in CME currentsheets, compression ratios and extremely high ion temperatures behindCME shocks, and three dimensional flow velocities and magnetic fieldchirality in CMEs. Ultraviolet spectroscopy has been used to determinethe thermal energy content of CMEs allowing the total energy budget tobe known for the first time. Such spectroscopic observations are capableof providing detailed empirical descriptions of solar energetic particle(SEP) source regions that allow theoretical models of SEP accelerationto be tailored to specific events, thereby enabling in situ measurementsof freshly emitted SEPs to be used for testing and guiding the evolutionof SEP acceleration theory. Here we review the history of ultravioletcoronagraph spectroscopy, summarize the physics of spectral lineformation in the extended corona, describe the spectroscopic diagnostictechniques, review the advances in our understanding of solar windsource regions and flare/CME events provided by ultraviolet spectroscopyand discuss the scientific potential of next generation ultravioletcoronagraph spectrometers.
| Can single O stars produce non-thermal radio emission?
We present a model for the non-thermal radio emission from presumablysingle O stars, in terms of synchrotron emission from relativisticelectrons accelerated in wind-embedded shocks. These shocks areassociated with an unstable, chaotic wind. The main improvement withrespect to earlier models is the inclusion of the radial dependence ofthe shock velocity jump and compression ratio, based on one-dimensionaltime-dependent hydrodynamical simulations. The decrease of the velocityjump and the compression ratio as a function of radius produces arapidly decreasing synchrotron emissivity. This effectively prohibitsthe models from reproducing the spectral shape of the observednon-thermal radio emission. We investigate a number of “escaperoutes” by which the hydrodynamical predictions might bereconciled with the radio observations. We find that the observedspectral shape can be reproduced by a slower decline of the compressionratio and the velocity jump, by the re-acceleration of electrons in manyshocks or by adopting a lower mass-loss rate. However, none of theseescape routes are physically plausible. In particular, re-accelerationby feeding an electron distribution through a number of shocks, is incontradiction with current hydrodynamical simulations. Thesehydrodynamical simulations have their limitations, most notably the useof one-dimensionality. At present, it is not feasible to performtwo-dimensional simulations of the wind out to the distances requiredfor synchrotron-emission models. Based on the current hydrodynamicmodels, we suspect that the observed non-thermal radio emission from Ostars cannot be explained by wind-embedded shocks associated with theinstability of the line-driving mechanism. The most likely alternativemechanism is synchrotron emission from colliding winds. That would implythat all O stars with non-thermal radio emission should be members ofbinary or multiple systems.
| Deepsky delights.
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| An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604
We discuss the results of two XMM-Newton observations of the opencluster NGC 6604 obtained in April and September 2002. We concentratemainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). Thesoft part of the EPIC spectrum of this system is thermal with typicaltemperatures of about 2 × 106 to 9 ×106 K. The nature (thermal vs. non-thermal) of the hard partof the spectrum is not unambiguously revealed by our data. If theemission is thermal, the high temperature of the plasma (~2.3 ×107 to 4.6 × 107 K) would be typical of whatshould be expected from a wind-wind interaction zone within a longperiod binary system. This emission could arise from an interactionbetween the combined winds of the O5-8V + O5-8V close binary system andthat of the more distant O8I companion. Assuming instead that the hardpart of the spectrum is non-thermal, the photon index would be rathersteep (~3). Moreover, a marginal variability between our two XMM-Newtonpointings could be attributed to an eclipse of the O5-8V + O5-8V system.The overall X-ray luminosity points to a significant X-ray luminosityexcess of about a factor 4 possibly due to colliding winds. ConsideringHD 167971 along with several recent X-ray and radio observations, wepropose that the simultaneous observation of non-thermal radiation inthe X-ray (below 10.0 keV) and radio domains appears rather unlikely.Our investigation of our XMM-Newton data of NGC 6604 reveals a rathersparse distribution of X-ray emitters. Including the two brightnon-thermal radio emitters HD 168112 and HD 167971, we present a list of31 X-ray sources along with the results of the cross-correlation withoptical and infrared catalogues. A more complete spectral analysis ispresented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-mainsequence star candidates.
| Non-thermal radio emission from O-type stars. I. HD168112
We present a radio lightcurve of the O5.5 III(f^+) star HD 168112, basedon archive data from the Very Large Array (VLA) and the AustraliaTelescope Compact Array (ATCA). The fluxes show considerable variabilityand a negative spectral index, thereby confirming that HD 168112 is anon-thermal radio emitter. The non-thermal radio emission is believed tobe due to synchrotron radiation from relativistic electrons that havebeen Fermi accelerated in shocks. For HD 168112, it is not known whetherthese shocks are due to a wind-wind collision in a binary system or tothe intrinsic instability of the stellar wind driving mechanism.Assuming HD 168112 to be a single star, our synchrotron model shows thatthe velocity jump of the shocks should be very high, or there should bea very large number of shocks in the wind. Neither of these iscompatible with time-dependent hydrodynamical calculations of O starwinds. If, on the other hand, we assume that HD 168112 is a binary, thehigh velocity jump is easily explained by ascribing it to the wind-windcollision. By further assuming the star to be an eccentric binary, wecan explain the observed radio variability by the colliding-wind regionmoving in and out of the region where free-free absorption is important.The radio data presented here show that the binary has a period ofbetween one and two years. By combining the radio data with X-ray data,we find that the most likely period is ~1.4 yr.
| An XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| On the highly reddened members in six young galactic star clusters - a multiwavelength study
The spectral and reddening properties of 211 highly reddenedproper-motion members with V < 15 mag in six young galactic starclusters are investigated using low-resolution spectroscopic, broad-bandUBVRIJHK and mid-infrared (IR) data. We report emission features in CaIIHK and HI lines for a sample of 29 stars including 11 stars reported forthe first time and also provide either a new or more reliable spectralclass for a sample of 24 stars. CaII triplet width measurements are usedto indicate the presence of an accretion disc for a dozen stars and toindicate luminosity for a couple of stars. On the basis of spectralfeatures, near-IR excesses, dereddened colour-colour diagrams and mid-IRspectral indices we identify a group of 28 pre-main-sequence clustermembers including five highly probable Herbig Ae/Be and six classical TTauri stars. A total of 25 non-emission main-sequence (MS) stars,amounting to ~10 per cent early-type MS members, appears to showVega-like characteristics or are precursors to such a phenomenon. Thevarious membership indicators suggest that ~16 per cent of theproper-motion members are non-members. A significant fraction (>70per cent) of programme stars in NGC 1976, NGC 2244, NGC 6530 and NGC6611 show anomalous reddening with RV= 4.78 +/- 0.10, 3.54+/- 0.04, 3.87 +/- 0.05 and 3.56 +/- 0.02, respectively, indicating thepresence of grain size dust larger than that typical of the diffusemedium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 alsoshow normal behaviour while the cluster NGC 6823 appears to have normalreddening. Three highly luminous late-type giants, one in NGC 2244 andtwo in NGC 6530, appear to be members and are inpost-hydrogen-core-burning stages, suggesting a prolonged duration (~25Myr) of star formation.
| Near-infrared spectroscopic monitoring of WR 140 during the 2001 periastron passage
We present new spectra of WR 140 (HD 193793) in the JHK bands, with somecovering the 1.083-μm HeI emission line at higher resolution,observed between 2000 October and 2003 May to cover its 2001 periastronpassage and maximum colliding-wind activity. The WC7 + O4-5spectroscopic binary WR 140 is the prototype of colliding-wind, episodicdust-making Wolf-Rayet systems, which also show strong variations inradio and X-ray emission. The JHK spectra showed changes in continuumand in the equivalent widths of the WC emission lines, consistent withthe formation of dust, starting between 2001 January 3 and March 26(orbital phases 0.989 and 0.017) and its subsequent fading and cooling.The 1.083-μm HeI line has a P Cygni profile, which showed variationsin both absorption and emission components as WR 140 went throughperiastron passage. The variation of the absorption component of theprofile yielded tight constraints on the geometry of the wind-collisionregion, giving θ= 50°+/- 8° for the opening semi-angle ofthe interaction region `cone', indicating a wind-momentum ratio , aboutthree times larger than previously believed. As the system approachedperiastron, the normally flat-topped emission component of the1.083-μm line profile showed the appearance of a significant subpeak.The movement of the subpeak across the profile was seen to be consistentwith its formation in wind material flowing along the contactdiscontinuity between the WC7 and O4-5 stellar winds and the changingorientation of the colliding-wind region as the stars moved in theirorbits. The flux carried in the subpeak was significant, exceeding theX-ray fluxes measured at previous periastron passages. This additionalsource of radiative cooling of the shock-heated gas probably causes itto depart from being adiabatic around periastron passage, therebyaccounting for the departure of the X-ray flux from its previouslyexpected 1/d dependence.
| A phase-resolved XMM-Newton campaign on the colliding-wind binary HD 152248
We report the first results of an XMM-Newton monitoring campaign of theopen cluster NGC 6231 in the Sco OB 1 association. This first paperfocuses on the massive colliding-wind binary HD 152248, which is thebrightest X-ray source of the cluster. The campaign, with a totalduration of 180 ks, was split into six separate observations, followingthe orbital motion of HD 152248. The X-ray flux from this systempresents a clear, asymmetric modulation with the phase and ranges from0.73 to 1.18 × 10-12 erg s-1 cm-2in the 0.5-10.0 keV energy band. The maximum of the emission is reachedslightly after apastron. The EPIC spectra are quite soft, and peakaround 0.8-0.9 keV. We characterize their shape using severalcombinations of MEKAL models and power-law spectra and we detectsignificant spectral variability in the 0.5-2.5 keV energy band.We also perform 2D hydrodynamical simulations using different sets ofparameters that closely reproduce the physical and orbital configurationof the HD 152248 system at the time of the six XMM-Newton pointings.This allows a direct confrontation of the model predictions with theconstraints deduced from the X-ray observations of the system. We showthat the observed variation of the flux can be explained by a variationof the X-ray emission from the colliding-wind zone, diluted by thesofter X-ray contribution of the two O-type stars of the system. Oursimulations also reveal that the interaction region of HD 152248 shouldbe highly unstable, giving rise to shells of dense gas that areseparated by low-density regions.Finally, we perform a search for short-term variability in the lightcurves of the system and we show that trends are present within severalof the 30-ks exposures of our campaign. Further, most of these trendsare in good agreement with the orbital motion and provide a directconstraint on the first-order derivative of the flux. In the samecontext, we also search for long-range correlations in the X-ray data ofthe system, but we only marginally detect them in the high-energy tailof the signal.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD 168112 (O5.5III(f+))
We report the results of a multiwavelength study of the non-thermalradio emitter HD 168112 (O5.5III(f+)). The detailed analysisof two quasi-simultaneous XMM-Newton and VLA observations reveals strongvariability of this star both in the X-ray and radio ranges. The X-rayobservations separated by five months reveal a decrease of the X-rayflux of 30%. The radio emission on the other hand increases by afactor 5-7 between the two observations obtained roughly simultaneouslywith the XMM-Newton pointings. The X-ray data reveal a hard emissionthat is most likely produced by a thermal plasma at kT 2-3 keVwhile the VLA data confirm the non-thermal status of this star in theradio waveband. Comparison with archive X-ray and radio data confirmsthe variability of this source in both wavelength ranges over a yet illdefined time scale. The properties of HD 168112 in the X-ray and radiodomain point towards a binary system with a significant eccentricity andan orbital period of a few years. However, our optical spectra reveal nosignificant changes of the star's radial velocity suggesting that if HD168112 is indeed a binary, it must be seen under a fairly lowinclination.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA). Also based on observations collected with the VLA, aninstrument of the National Radio Astronomy Observatory, which is afacility of the National Science Foundation operated by AssociatedUniversities, Inc. Optical data were collected at the European SouthernObservatory (La Silla, Chile), and at the ObservatorioAstronómico Nacional of San Pedro Mártir (Mexico).
| Non-thermal radio emission from single hot stars
We present a theoretical model for the non-thermal radio emission fromsingle hot stars, in terms of synchrotron radiation from electronsaccelerated in wind-embedded shocks. The model is described by fiveindependent parameters each with a straightforward physicalinterpretation. Applying the model to a high-quality observation ofCyg OB2 No. 9 (O5 If), we obtain meaningfulconstraints on most parameters. The most important result is that theouter boundary of the synchrotron emission region must lie between 500and 2200 stellar radii. This means that shocks must persist up to thatdistance. We also find that relatively weak shocks (with a compressionratio <3) are needed to produce the observed radio spectrum. Theseresults are compatible with current hydrodynamical predictions. Most ofour models also show a relativistic electron fraction that increasesoutwards. This points to an increasing efficiency of the accelerationmechanism, perhaps due to multiple acceleration, or an increase in thestrength of the shocks. Implications of our results for non-thermalX-ray emission are discussed.
| An XMM-Newton observation of the massive binary HD 159176
We report the analysis of an XMM-Newton observation of the close binaryHD 159176 (O7 V + O7 V). The observed LX/Lbolratio reveals an X-ray luminosity exceeding by a factor 7 theexpected value for X-ray emission from single O-stars, thereforesuggesting a wind-wind interaction scenario. EPIC and RGS spectra arefitted consistently with a two temperature mekal optically thin thermalplasma model, with temperatures ranging from 2 to6×106 K. At first sight, these rather low temperaturesare consistent with the expectations for a close binary system where thewinds collide well before reaching their terminal velocities. We alsoinvestigate the variability of the X-ray light curve of HD 159176 onvarious short time scales. No significant variability is found and weconclude that if hydrodynamical instabilities exist in the windinteraction region of HD 159176, they are not sufficient to produce anobservable signature in the X-ray emission. Hydrodynamic simulationsusing wind parameters from the literature reveal some puzzlingdiscrepancies. The most striking one concerns the predicted X-rayluminosity which is one or more orders of magnitude larger than theobserved one. A significant reduction of the mass loss rate of thecomponents compared to the values quoted in the literature alleviatesthe discrepancy but is not sufficient to fully account for the observedluminosity. Because hydrodynamical models are best for the adiabaticcase whereas the colliding winds in HD 159176 are most likely highlyradiative, a totally new approach has been envisaged, using ageometrical steady-state colliding wind model suitable for the case ofradiative winds. This model successfully reproduces the spectral shapeof the EPIC spectrum, but further developments are still needed toalleviate the disagreement between theoretical and observed X-rayluminosities.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| A Compact Array imaging survey of southern bright-rimmed clouds
We have carried out a radio-wavelength imaging survey of 45bright-rimmed clouds (BRCs), using the Australia Telescope Compact Arrayto characterise the physical properties in their ionised boundarylayers. We detected radio emission from a total of 25 clouds and using acombination of Digitised Sky Survey and mid-infrared MSX 8 \mum imagesclassified the emission into that associated with the ionised cloudrims, that associated with embedded possible massive YSOs and thatunlikely to be associated with the clouds at all. A total of 18 cloudsdisplay radio emission clearly associated with the cloud rim and wedetermine the ionising photon flux illuminating these clouds and theelectron density and pressure of their ionised boundary layers. Using aglobal estimate for the interior molecular pressure of these clouds weshow that the majority are likely to be in pressure equilibrium andhence are currently being shocked by photoionisation-induced shocks. Weidentify those clouds where the predicted ionising photon flux isinconsistent with that derived from the observations and show thateither the spectral types of the stars illuminating the BRCs are earlierthan previously thought or that there must be additional ionisingsources within the HII regions. Finally, we identify the radio sourcesembedded within the clouds with infrared stellar clusters and show thatthey contain late O and early B-type stars, demonstrating that a numberof BRCs are intimately involved with high to intermediate-mass starformation.Full Figs. \ref{fig:images} and \ref{fig:sfo86dss} are only available inelectronic form at http://www.edpsciences.org
| High-Resolution X-Ray Spectroscopy with CHANDRA and XMM-NEWTON
The launches of the Chandra X-Ray Observatory in June 1999 and theXMM-Newton Observatory in December 1999 opened a new era in X-rayastronomy. Both of these missions incorporate novel diffraction gratingspectrometers that are providing the first high-resolution X-ray spectraof most classes of astrophysical sources. The spectra obtained to dateexhibit a wealth of discrete detail, yielding sensitive constraints onphysical conditions in the emitting plasmas. We review the essentialcharacteristics of these instruments, the basics of X-ray spectralformation in cosmic sources, and the exciting new results that haveemerged from Chandra and XMM-Newton grating observations of a widevariety of astrophysical systems.
| Molecular Counterparts of Ultracompact H II Regions with Extended Envelopes
We have carried out 13CO J=1-0, CS, and C34S J=2-1and J=3-2 line observations of molecular clouds associated with 16ultracompact (UC) H II regions with extended envelopes. The molecularclouds are the ones that give birth to rich stellar clusters and/or verymassive (O7-O4) stars. Our data show that the clouds are very clumpy andof irregular morphology. They usually have much larger masses, velocitydispersions, and fractions of dense gas than molecular clouds that formearly B or late O stars. This is compatible with earlier findings thatmore massive stars form in more massive cores. The IR luminosity-to-massratio has a mean value of 9 Lsolar/Msolar and islittle correlated with the cloud mass. Most molecular clouds have starformation efficiencies of 1%-2%. We find size-line width andsize-density relations in the forms of Δv~D0.4 andn(H2)~D-1.2. 13CO cores are in generalassociated with compact H II regions regardless of the presence of UC HII regions therein. In contrast, CS cores are preferentially associatedwith compact H II regions that contain UC H II regions. As with the factthat the compact H II regions containing UC H II regions are morecompact than those not associated with UC H II regions, these indicatethat the former may be in an earlier evolutionary phase than the latter.The diffuse extended envelopes of H II regions often develop in thedirection of decreasing molecular gas density. Based on detailedcomparison of molecular line data with radio continuum and recombinationline data, the extended ionized envelopes are likely the results ofchampagne flows in at least 10 sources in our sample. Together theseresults appear to support a published suggestion that the extendedemission around UC H II regions can be naturally understood by combiningthe champagne flow model with the hierarchical structure of molecularclouds, taking into account various inclinations and low resolutions ofour data. In addition, the blister model seems to be still applicable tomost H II regions, even though massive stars usually form in theinteriors rather than on the surfaces of molecular clouds. This ispossible because massive star-forming clouds have hierarchical structureand irregular morphology.
| 10 MK Gas in M17 and the Rosette Nebula: X-Ray Flows in Galactic H II Regions
We present the first high spatial resolution X-ray images of twohigh-mass star forming regions, the Omega Nebula (M17) and the RosetteNebula (NGC 2237-2246), obtained with the Chandra X-Ray ObservatoryAdvanced CCD Imaging Spectrometer instrument. The massive clusterspowering these H II regions are resolved at the arcsecond level intomore than 900 (M17) and 300 (Rosette) stellar sources similar to thoseseen in closer young stellar clusters. However, we also detect softdiffuse X-ray emission on parsec scales that is spatially and spectrallydistinct from the point-source population. The diffuse emission hasluminosity LX~=3.4×1033 ergs s-1in M17 with plasma energy components at kT~=0.13 and ~=0.6 keV (1.5 and7 MK), while in Rosette it has LX~=6×1032ergs s-1 with plasma energy components at kT~=0.06 and ~=0.8keV (0.7 and 9 MK). This extended emission most likely arises from thefast O star winds thermalized either by wind-wind collisions or by atermination shock against the surrounding media. We establish that onlya small portion of the wind energy and mass appears in the observeddiffuse X-ray plasma; in these blister H II regions, we suspect thatmost of it flows without cooling into the low-density interstellarmedium. These data provide compelling observational evidence that strongwind shocks are present in H II regions.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| XMM-Newton high-resolution X-ray spectroscopy of the Wolf-Rayet object WR 25 in the Carina OB1 association
We report the analysis of the first high-resolution X-ray spectra of theWolf-Rayet (WR) object WR 25 (HD 93162, WN6ha+O4f) obtained with theReflection Grating Spectrometers (RGS) and the European Photon ImagingCameras (EPIC-MOS and PN) CCD spectrometers on board the XMM-Newtonsatellite. The spectrum exhibits bright emission lines of the H- andHe-like ions of Ne, Mg, Si and S, as well as Fe XVII i to Fe XX and FeXXV lines. Line fluxes have been measured. The RGS and EPIC spectrahave been simultaneously fitted to obtain self-consistent temperatures,emission measures, and elemental abundances. Strong absorption by thedense WR stellar wind and the interstellar medium (ISM) is observedequivalent to NH = 7 x 1021 cm-2.Multi-temperature (DEM) fitting yields two dominant components aroundtemperatures of 7.0 and 32 MK, respectively. The XMM intrinsic (i.e.unabsorbed, corrected for the stellar wind absorption and the absorptionof ISM) X-ray luminosity of WR 25 is Lx(0.5-10 keV) = 1.3 x1034 erg s-1, and Lx(0.5-10 keV) = 0.85x 1034 erg s-1, (when correcting for the ISM only)assuming d=3.24 kpc. The obtained chemical abundances are subsolar,except for S. This may be real, but could equally well be due to a weakcoupling to the continuum, which is strongly influenced by theabsorption column density and the subtracted background. The expectedhigh N-abundance, as observed in the optical wavelength region, couldnot be confirmed due to the strong wind absorption, blocking out itsspectral signature. The presence of the Fe XXV emission-line complex at~ 6.7 keV is argued as being indicative for colliding winds inside aWR+O binary system.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
| The mineralogy, geometry and mass-loss history of IRAS 16342-3814
We present the 2-200 mu m Infrared Space Observatory (ISO) spectrum and3.8-20 mu m ISAAC and TIMMI2 images of the extreme OH/IR star IRAS16342-3814. Amorphous silicate absorption features are seen at 10 and 20mu m, together with crystalline silicate absorption features up toalmost 45 mu m. No other OH/IR star is known to have crystallinesilicate features in absorption up to these wavelengths. This suggeststhat IRAS 16342-3814 must have, or recently had, an extremely highmass-loss rate. Indeed, preliminary radiative transfer calculationssuggest that the mass-loss rate may be as large as 10-3Msun yr-1. The 3.8 mu m ISAAC image shows abipolar reflection nebula with a dark equatorial waist or torus, similarto that seen in optical images taken with the Hubble Space Telescope(HST). The position angle of the nebula decreases significantly withincreasing wavelength, suggesting that the dominant source of emissionchanges from scattering to thermal emission. Still, even up to 20 mu mthe nebula is oriented approximately along the major axis of the nebulaseen in the HST and ISAAC images, suggesting that the torus must be verycold, in agreement with the very red ISO spectrum. The 20 mu m imageshows a roughly spherically symmetric extended halo, approximately 6''in diameter, which is probably due to a previous phase of mass-loss onthe AGB, suggesting a transition from a (more) spherically symmetric toa (more) axial symmetric form of mass-loss at the end of the AGB. Usinga simple model, we estimate the maximum dust particle sizes in the torusand in the reflection nebula to be 1.3 and 0.09 mu m respectively. Thesize of the particles in the torus is large compared to typical ISMvalues, but in agreement with high mass-loss rate objects like AFGL 4106and HD161796. We discuss the possible reason for the difference inparticle size between the torus and the reflection nebula.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) with the participation of ISAS and NASA. The SWSis a joint project of SRON and MPE. Also based on observations obtainedat the European Southern Observatory (ESO).
| Molecular Hydrogen in the Lagoon: H2 Line Emission from Messier 8
The 2.12μmv=1-0 S(1) line of molecular hydrogen has been imaged inthe Hourglass region of M8. The line is emitted from a roughly bipolarregion, centred around the O7 star Herschel 36. The peak H21-0 S(1) line intensity is 8.2 ×10-15ergs-1cm-2arcsec-2. Theline centre emission velocity varies from -25kms-1 in the SElobe to +45kms-1 in the NW lobe. The distribution is similarto that of the CO J=3-2 line. The H2 line appears to beshock-excited when a bipolar outflow from Herschel 36 interacts with theambient molecular cloud. The total luminosity of all H2 linesis estimated to be ~16Lsolar and the mass of the hotmolecular gas ~9 × 10-4Msolar (without anycorrection for extinction).
| The Structure and Evolution of the Lagoon Nebula. I. Submillimeter Continuum and CO Line Mapping
We present submillimeter- and millimeter-wave maps tracing the moleculargas and dust around the edge of the H II region M8. The molecularmaterial is clumped into cores on the scale of the beam (about 0.1 pc)whose temperatures can be estimated from CO observations. The masses ofthe clumps, estimated from their continuum fluxes, are consistent with apower-law mass function with index -1.7+/-0.6, which agrees withdeterminations for other molecular clouds at similar resolutions, usingmolecular lines as tracers. The submillimeter clumps are sited at theinterface between the H II region and the background molecular cloud,where they are exposed to the ultraviolet flux of OB stars. The physicalparameters of the clumps are compared to published models of molecularclouds undergoing photoevaporation, suggesting that the pressure of theionized gas exceeds the internal pressure of the clumps and, therefore,that a shock front will be driven into the clumps. The clumps themselvescurrently appear to be gravitationally unbound, but the compression maybe sufficient to induce collapse.
| An XMM-Newton observation of the Lagoon Nebula and the very young open cluster NGC 6530
We report the results of an XMM-Newton observation of the Lagoon Nebula(M 8). Our EPIC images of this region reveal a cluster of point sources,most of which have optical counterparts inside the very young opencluster NGC 6530. The bulk of these X-ray sources are probablyassociated with low and intermediate mass pre-main sequence stars. Oneof the sources experienced a flare-like increase of its X-ray fluxmaking it the second brightest source in M 8 after the O4 star 9 Sgr.The X-ray spectra of most of the brightest sources can be fitted withthermal plasma models with temperatures of kT ~ a few keV. Only a few ofthe X-ray selected PMS candidates are known to display Hα emissionand were previously classified as classical T Tauri stars. This suggeststhat most of the X-ray emitting PMS stars in NGC 6530 are weak-line TTauri stars. In addition to 9 Sgr, our EPIC field of view contains alsoa few early-type stars. The X-ray emission from HD 164816 is found to betypical for an O9.5 III-IV star. At least one of the known Herbig Bestars in NGC 6530 (LkHα 115) exhibits a relatively strong X-rayemission, while most of the main sequence stars of spectral type B1 andlater are not detected. We also detect (probably) diffuse X-ray emissionfrom the Hourglass Region that might reveal a hot bubble blown by thestellar wind of Herschel 36, the ionizing star of the Hourglass Region.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA).
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Osservazione e dati astrometrici
| Costellazione: | Sagittario |
| Ascensione retta: | 18h03m52.40s |
| Declinazione: | -24°21'38.0" |
| Magnitudine apparente: | 5.97 |
| Distanza: | 1515.152 parsec |
| Moto proprio RA: | 0.8 |
| Moto proprio Dec: | -2.1 |
| B-T magnitude: | 5.942 |
| V-T magnitude: | 5.96 |
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INHABITED SKY
