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Faint emission lines in the Galactic HII regions M16, M20 and NGC 3603*
We present deep echelle spectrophotometry of the Galactic HII regionsM16, M20 and NGC 3603. The data have been taken with the Very LargeTelescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10400Å range. We have detected more than 200 emission lines in eachregion. Physical conditions have been derived using different continuumand line intensity ratios. We have derived He+,C++ and O++ abundances from pure recombinationlines as well as collisionally excited lines (CELs) for a large numberof ions of different elements. We have obtained consistent estimationsof the temperature fluctuation parameter, t2, using differentmethods. We also report the detection of deuterium Balmer lines up toDδ (M16) and to Dγ (M20) in the blue wings of the hydrogenlines, which excitation mechanism seems to be continuum fluorescence.The temperature fluctuation paradigm agrees with the results obtainedfrom optical CELs, and the more uncertain ones from far-infraredfine-structure CELs in NGC 3603, although, more observations coveringthe same volume of the nebula are necessary to obtain solid conclusions.
| Spectacular Spitzer Images of the Trifid Nebula: Protostars in a Young, Massive-Star-forming Region
Spitzer IRAC and MIPS images of the Trifid Nebula (M20) reveal itsspectacular appearance in infrared light, highlighting the nebula'sspecial evolutionary stage. The images feature recently formed massiveprotostars and numerous young stellar objects, and a single O star thatilluminates the surrounding molecular cloud from which it formed, andunveil large-scale, filamentary dark clouds. Multiple protostars aredetected in the infrared, within the cold dust cores of TC3 and TC4,which were previously defined as Class 0. The cold dust continuum coresof TC1 and TC2 contain only one protostar each. The Spitzer color-colordiagram allowed us to identify ~160 young stellar objects (YSOs) andclassify them into different evolutionary stages. The diagram alsorevealed a unique group of YSOs that are bright at 24 μm but have thespectral energy distribution peaking at 5-8 μm. Despite expectationthat Class 0 sources would be ``starless'' cores, the Spitzer images,with unprecedented sensitivity, uncover mid-infrared emission from theseClass 0 protostars. The mid-infrared detections of Class 0 protostarsshow that the emission escapes the dense, cold envelope of youngprotostars. The mid-infrared emission of the protostars can be fit bytwo temperatures of 150 and 400 K; the hot core region is probablyoptically thin in the mid-infrared regime, and the size of hot core ismuch smaller than that of the cold envelope. The presence of multipleprotostars within the cold cores of Class 0 objects implies thatclustering occurs at this early stage of star formation. The mostmassive star in the TC3 cluster is located at the center of the clusterand at the bottom of the gravitational potential well.
| 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.
| Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula
HH 399 is one of the first Herbig-Haro flows recognized to be irradiatedby the UV radiation of the massive O7.5 star in the Trifid Nebula. Wepresent the proper motion of the first irradiated jet, based on twoepochs of Hubble Space Telescope (HST) observations of HH 399 separatedby nearly 5 yr, using Hα and [S II] line filters. High propermotion with continuous velocities between 200+/-55 and 528+/-24 kms-1 is detected in both lines along the 18" extent of the jetaxis. The irradiated fully ionized jet consists of numerous knots alongthe jet but also shows evidence for a number of isolated blob likestructures running immediately outside the jet with lower transversevelocities. The transverse velocities combined with radial velocitymeasurements indicate that the jet axis lies away from the plane of thesky by only a few degrees. We argue that the jet is fully ionized, basedon a [S II]/Hα line ratio, as well as radio continuum emissiondetected from the full extent of the jet at a 3.6 cm wavelength. Thestellar mass-loss rate producing HH 399 is estimated to be~2×10-6 Msolar yr-1.
| Hubble Space Telescope and United Kingdom Infrared Telescope Observations of the Center of the Trifid Nebula: Evidence for the Photoevaporation of a Proplyd and a Protostellar Condensation
The Trifid Nebula (M20) is a well-known prominent optical H II regiontrisected by bands of obscuring dust lanes and excited by an O7.5 star,HD 164492A. Previous near-IR, mid-IR, and radio continuum observationsof the cluster of stars at the center of the Trifid Nebula indicated thepresence of circumstellar disks associated with hot stars with envelopesthat are photoionized externally by the UV radiation from the hotcentral star, HD 164492A. Using the WFPC2 on the Hubble Space Telescope,we present evidence of a resolved proplyd in Hα and [S II] lineemission from a stellar source emitting cool dust emission. Using theUnited Kingdom Infrared Telescope, an infrared observation of thestellar source with a proplyd indicates a late F to mid-G spectral type.We also note a remarkable complex of filamentary and sheetlikestructures that appear to arise from the edge of a protostellarcondensation. These observations are consistent with a picture in whichthe bright massive star HD 164492A is responsible for thephotoevaporation of protoplanetary disks of other less massive membersof the cluster, as well as the closest protostellar condensation facingthe central cluster. Using the evidence for a proplyd, we argue that themassive and intermediate-mass members of the cluster, HD 164492C (B6star) and HD 164492 (Herbig Be star), have disks associated with them.
| To see or not to see a bow shock. Identifying bow shocks with Hα allsky surveys
OB-stars have the highest luminosities and strongest stellar winds ofall stars, which enables them to interact strongly with theirsurrounding ISM, thus creating bow shocks. These offer us an idealopportunity to learn more about the ISM. They were first detected andanalysed around runaway OB-stars using the IRAS allsky survey by vanBuren et al. (1995, AJ, 110, 2614). Using the geometry of such bowshocks information concerning the ISM density and its fluctuations canbe gained from such infrared observations. As to help to improve the bowshock models, additional observations at other wavelengths, e.g.Hα, are most welcome. However due to their low velocity these bowshocks have a size of 1°, and could only be observed as awhole with great difficulties. In the light of the new Hα allskysurveys (SHASSA/VTSS) this is no problem any more. We developeddifferent methods to detect bow shocks, e.g. the improved determinationof their symmetry axis with radial distance profiles. Using twoHα-allsky surveys (SHASSA/VTSS), we searched for bow shocks andcompared the different methods. From our sample we conclude, that thecorrelation between the direction of both proper motion and the symmetryaxis determined with radial distance profile is the most promisingdetection method. We found eight bow shocks around HD17505, HD 24430, HD48099, HD 57061, HD92206, HD 135240, HD149757, and HD 158186 from 37 candidatestaken from van Buren et al. (1995, AJ, 110, 2614). Additionally to thetraditional determination of ISM parameters using the standoff distanceof the bow shock, another approach was chosen, using the thickness ofthe bow-shock layer. Both methods lead to the same results, yieldingdensities ( 1 cm-3) and the maximal temperatures (104 K), that fit well to the up-to-date picture of the WarmIonised Medium.
| 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.
| A Very Large Array Search for Water Masers in Six H II Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated withyoung low-mass protostars in six H II regions-M16, M20, NGC 2264, NGC6357, S125, and S140. The survey was conducted with the NRAO Very LargeArray from 2000 to 2002. For several of these H II regions, ours are thefirst high-resolution observations of water masers. We detected 16 watermasers: eight in M16, four in M20, three in S140, and one in NGC 2264.All but one of these were previously undetected. No maser emission wasdetected from NGC 6357 or S125. There are two principle results to ourstudy. (1) The distribution of water masers in M16 and M20 does notappear to be random but instead is concentrated in a layer of compressedgas within a few tenths of a parsec of the ionization front. (2)Significantly fewer masers are seen in the observed fields than expectedbased on other indications of ongoing star formation, indicating thatthe maser-exciting lifetime of protostars is much shorter in H IIregions than in regions of isolated star formation. Both of theseresults confirm predictions of a scenario in which star formation isfirst triggered by shocks driven in advance of ionization fronts andthen truncated ~105 yr later when the region is overrun bythe ionization front.
| Chandra Observation of the Trifid Nebula: X-Ray Emission from the O Star Complex and Actively Forming Pre-Main-Sequence Stars
The Trifid Nebula, a young star-forming H II region, was observed for 16hr by the ACIS-I detector on board the Chandra X-Ray Observatory. Wedetected 304 X-ray sources, 30% of which are hard sources and 70% ofwhich have near-infrared counterparts. Chandra resolved the HD 164492multiple system into a number of discrete X-ray sources. X-ray emissionis detected from components HD 164492A (an O7.5 III star that ionizesthe nebula), B and C (a B6 V star), and possibly D (a Be star).Component C is blended with an unidentified source to the northwest. HD164492A has a soft spectrum (kT~0.5 keV), while the component C blendshows much harder emission (kT~6 keV). This blend and other hard sourcesare responsible for the hard emission and Fe K line seen by ASCA, whichwas previously attributed entirely to HD 164492A. The soft spectrum ofthe O star is similar to emission seen from other single O stars and isprobably produced by shocks within its massive stellar wind. Lack ofhard emission suggests that neither a magnetically confined wind shocknor colliding wind emission is important in HD 164492A. A dozen starsare found to have flares in the field, and most of them arepre-main-sequence stars (PMS). Six sources with flares have both opticaland Two Micron All Sky Survey counterparts. These counterparts are notembedded, and thus it is likely that these sources are in a later stageof PMS evolution, possibly Class II or III. Two flare sources did nothave any near-IR, optical, or radio counterparts. We suggest that theseX-ray flare stars are in an early PMS stage (Class I or earlier). Wealso detected X-ray sources apparently associated with two massivestar-forming cores, TC 1 and TC 4. The spectra of these sources showhigh extinction and X-ray luminosities of (2-5)×1031ergs s-1. If these sources are Class 0 objects, it is unclearwhether their X-ray emission is due to solar-type magnetic activities,as in Class I objects, or to some other mechanism.
| Circular Polarization of Starlight
Of the 7500 stars cited in the Catalog of starlight polarization, thosewhich satisfy the condition P obs % and A V 0m.5 are selected. It ispresumed that the selected stars (n=216) have circularly polarizedlight.
| 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.
| X-ray observations of massive colliding wind binaries
Massive stars in binary systems can generate X-ray emission in theregion between the two stars where stellar winds collide. Collidingwind X-ray emission acts as an in-situ probe of important windparameters like mass-loss rates, chemical abundances, wind velocities,and possibly magnetic field strengths. Variations in observedcolliding-wind X-ray emission can be produced by the changingline-of-sight to the colliding wind region as the stars move in orbitand/or the changing emission measure of the shocked gas in the windcollision zone produced if the separations between the two stars change.X-ray variability depends on the wind and orbital parameters and so canin principal allow the direct measurement of mass functions even for lowinclination systems and provide a connection between the stellar andwind parameters. X-ray emission from colliding wind systems can thus beused to refine our understanding of the evolutionary state of massivestars and to help test evolutionary models. We discuss recent advancesin defining the X-ray spectral and temporal signatures of colliding windX-ray emission using ROSAT, ASCA, RXTE, Chandra and XMM data.
| 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.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.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/410/905
| Optical spectroscopy of X-Mega targets - IV. CPD - 59°2636: a new O-type multiple system in the Carina Nebula
High-resolution optical spectroscopy of CPD - 59°2636, one of theO-type stars in the open cluster Trumpler 16 in the Carina Nebula,reveals this object to be a multiple system displaying triple lineswhich we label as components A, B and C of spectral types O7 V, O8 V andO9 V, respectively. From our radial velocity measurements we find thatthe components A and B form a close binary with a period of 3.6284 d,and we obtain the first circular radial velocity orbit for this systemwith semi-amplitudes of 184 and 192 km s-1, leading tominimum masses of 10.1 and 9.7 Msolar. We find that thecomponent C is a single lined binary with a period of 5.034 d andsemi-amplitude of 48 km s-1. We also analyse the X-rayradiation from CPD - 59°2636, finding neither appreciableoverluminosity nor phase-related X-ray flux variations.
| Tomographic Separation of Composite Spectra. X. The Massive Close Binary HD 101131
We present the first orbital elements for the massive close binary HD101131, one of the brightest objects in the young open cluster IC 2944.This system is a double-lined spectroscopic binary in an ellipticalorbit with a period of 9.64659+/-0.00012 days. It is a young system ofunevolved stars (approximately 2 million yr old) that are well withintheir critical Roche surfaces. We use a Doppler tomography algorithm toreconstruct the individual component optical spectra, and we applywell-known criteria to arrive at classifications of O6.5 V((f)) and O8.5V for the primary and secondary, respectively. We compare thereconstructed spectra of the components to single-star spectrumstandards to determine a flux ratio off2/f1=0.55+/-0.08 in the V band. Both componentsare rotating faster than synchronously. We estimate the temperatures andluminosities of the components from the observed spectralclassifications, composite V magnitude, and cluster distance modulus.The lower limits on the masses derived from the orbital elements and thelack of eclipses are 25 and 14Msolar for the primary andsecondary, respectively. These limits are consistent with the somewhatlarger masses estimated from the positions of the stars in theHertzsprung-Russell diagram and evolutionary tracks for single stars.
| Faint X-Ray Sources Resolved in the ASCA Galactic Plane Survey and Their Contribution to the Galactic Ridge X-Ray Emission
The X-ray emission from the central region of the Galactic plane,|l|<~45° and |b|<~0.4d, was studied in the 0.7-10 keV energyband with a spatial resolution of ~3' with the Advanced Satellite forCosmology and Astrophysics (ASCA) observatory. We developed a newanalysis method for the ASCA data to resolve discrete sources from theextended Galactic ridge X-ray emission (GRXE). We successfully resolved163 discrete sources with an X-ray flux down to 10-12.5 ergscm-2 s-1 and determined the intensity variationsof the GRXE as a function of the Galactic longitude with a spatialresolution of about 1°. The longitudinal intensity variation in theenergy band above 4 keV, for which there is little absorption in theGalactic plane, shows a large enhanced feature within |l|<~30°.This suggests a strong enhancement of X-ray emissivity of the GRXEinside the 4 kpc arm of the Galaxy. Searches for identifications of theresolved X-ray sources with cataloged X-ray sources and optical starsshow that the 66% are unidentified. Spectral analysis of each sourceshows that a large number of the unidentified sources have hard X-rayspectra obscured by the Galactic interstellar medium. We classified thesources into several groups by the flux, the hardness and the softnessof the spectra, and performed further detailed analysis for the spectrasummed within each group. Possible candidates of X-ray origins of theseunidentified sources are discussed based on the grouping spectralanalysis. Also, we derived the logN-logS relations of the resolvedsources in the energy bands below and above 2 keV separately. ThelogN-logS relation of the Galactic X-ray sources above 2 keV wasobtained for the first time with this study. It is represented by apower-law with an index of -0.79+/-0.07 after correction for thecontribution of extragalactic X-ray sources. This flat power-lawrelation suggests that the spatial distribution of the X-ray sourcesshould have an armlike structure in which the solar system is included.The integrated surface brightness of the resolved sources is about 10%of the total GRXE in both energy bands. The approximately 90% of theemission remaining is still unresolved.
| X-Rays and Protostars in the Trifid Nebula
The Trifid Nebula is a young H II region, recently rediscovered as a``pre-Orion'' star-forming region, containing protostars undergoingviolent mass ejections visible in optical jets as seen in images fromthe Infrared Space Observatory and the Hubble Space Telescope. We reportthe first X-ray observations of the Trifid Nebula using ROSAT and ASCA.The ROSAT image shows a dozen X-ray sources, with the brightest X-raysource being the O7 star, HD 164492, which provides most of theionization in the nebula. We also identify 85 T Tauri star and young,massive star candidates from near-infrared colors using theJHKs color-color diagram from the Two-Micron All-Sky Survey(2MASS). Ten X-ray sources have counterpart near-infrared sources. The2MASS stars and X-ray sources suggest there are potentially numerousprotostars in the young H II region of the Trifid. ASCAmoderate-resolution spectroscopy of the brightest source shows hardemission up to 10 keV with a clearly detected Fe K line. The best modelfit is a two-temperature (T=1.2×106 K and39×106 K) thermal model with additional warm absorbingmedia. The hotter component has an unusually high temperature for eitheran O star or an H II region; a typical Galactic H II region could not bethe primary source for such hot temperature plasma and the Fe XXV lineemission. We suggest that the hotter component originates in either theinteraction of the wind with another object (a companion star or a denseregion of the nebula) or from flares from deeply embedded young stars.
| Disks around hot stars in the Trifid nebula
We report on mid-IR observations of the central region in the Trifidnebula, carried out with ISOCAM in several broad-band infrared filtersand in the low resolution spectroscopic mode provided by the circularvariable filter. Analysis of the emission indicates the presence of ahot dust component (500 to 1000 K) and a warm dust component at lowertemperatures (~ 150-200 K) around several members of the clusterexciting the H II region, and other stars undetected at opticalwavelengths. Complementary VLA observations suggest that the mid-IRemission could arise from a dust cocoon or a circumstellar disk,evaporated under the ionization of the central source and the excitingstar of the nebula. In several sources the 9.7 kern 0.20em mu m silicateband is seen in emission. One young stellar source shows indications ofcrystalline silicates in the circumstellar dust.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| Radio Continuum Emission from the Central Stars of M20, and the Detection of a New Supernova Remnant near M20
The Trifid Nebula (M20) is a well-known prominent optical H II regiontrisected by obscuring dust lanes. Radio continuum VLA observations ofthis nebula show free-free emission at λ=3.6 and 6 cm from threestellar sources lying close to the O7 V star at the center of thenebula. We argue that neutral material associated with these stars isphotoionized externally by the UV radiation from the hot central star.We also report the discovery of a barrel-shaped supernova remnant, SNRG7.06-0.12, at the northwest rim of the nebula, and two shell-likefeatures, G6.67-0.42 and G6.83-0.21, adjacent to W28 and M20. We discussthe nature of these features and their possible relationship to thepulsar PSR 1801-2306 and W28 OH (1720 MHz) masers.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Interstellar extinction.
Not Available
| Dynamics of astrophysical nebulae
Not Available
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Photometry and spectroscopy of the central star of the Trifid nebula
UBV photometry of the central star of the Trifid nebula - HD 164492,which has so far been suspected of photometric as well as of radialvelocity variability - is presented. The results of our photometry donot confirm any variability. Moreover, based on new high resolution CCDspectra any radial velocity variability can be discarded. Photometry ofseveral other members of the multiple stellar system is included;according to this photometry, the system is a physical one. Equivalentwidths for the main component of HD 164492 are given. Positions measuredon CCD exposures are presented too. Based on observations collected atthe European Southern Observatory, La Silla, Chile.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| A Survey for H alpha Emission in Massive Binaries: The Search for Colliding Wind Candidates
I report the results of the first all-sky survey of H alpha emission inthe spectra of O-type binaries. The survey includes 26 systems, of which10 have emission that extends clearly above the continuum. This is thefirst report of emission for four of these. An additional three systemsshow small distortions in the H alpha profile that may result from weakemission. I compare the distribution of emission systems in H-R diagramsfor both binary and single stars, using a survey of single O-type starsdone by Conti (1974). Emission in main-sequence systems is extremelyrare and is completely absent in my sample of binary stars. Among binarystars, 78% of the systems containing giants show some emission, while nosingle giants in Conti's sample do. In the case of supergiants, 78% ofsingle stars show emission, while all supergiant binaries show strongemission. H alpha emission may come from a variety sources, but the factthat binaries have a higher incidence and strength of emission inpost--main-sequence stages may indicate that wind interactions are acommon source of emission in massive binaries. To ascertain whether ornot colliding winds have been observed, it will be necessary to studythe H alpha line profile throughout several orbits of each candidatecolliding wind system and look for recurring orbital-phase--relatedvariations. Such a study is underway.
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Observation and Astrometry data
| Constellation: | Sagittarius |
| Right ascension: | 18h02m23.55s |
| Declination: | -23°01'51.1" |
| Apparent magnitude: | 7.593 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | -0.1 |
| Proper motion Dec: | -9.3 |
| B-T magnitude: | 7.527 |
| V-T magnitude: | 7.588 |
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NEWS@SKY (Science&Space News)
