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Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| X-Ray Emission from Early-Type Stars in the Orion Nebula Cluster
The X-ray properties of twenty ~1 Myr old O, B, and A stars of the OrionTrapezium are examined with data from the Chandra Orion UltradeepProject (COUP). On the basis of simple theories for X-ray emission, wedefine two classes separated at spectral type B4: hotter stars havestrong winds that may give rise to X-ray emission in small- orlarge-scale wind shocks, and cooler stars that should be X-ray dark dueto their weaker winds and absence of outer convection zones wheredynamos can generate magnetic fields. Emission by late-type magneticallyactive companions may be present in either class. Sixteen of the 20stars are detected with a wide range of X-ray luminosities,logLX (ergs s-1)~29-33, and X-ray efficiencies,log(LX/Lbol)~-4 to -8. Only two stars,θ1 Ori D (B0.5) and NU Ori (B1), show exclusively theconstant soft-spectrum emission at log(LX/Lbol)~-7expected from the standard model involving many small shocks in anunmagnetized radiatively accelerated wind. Most of the other massiveO7-B3 stars exhibit some combination of soft-spectrum wind emission,hard-spectrum flaring, and/or rotational modulation indicatinglarge-scale inhomogeneity. Magnetic confinement of winds withlarge-scale shocks can be invoked to explain these phenomena. This issupported in some cases by nonthermal radio emission and/or chemicalpeculiarities, or direct detection of the magnetic field(θ1 Ori C). Most of the stars in the weak-wind classexhibit X-ray flares and logLX<31 ergs s-1,consistent with magnetic activity from known or unseen low-masscompanions. In most cases, the X-ray spectra can be interpreted in termsof a two-temperature plasma model with a soft component of 3-10 MK and ahard component up to 40 MK. All nondetections belong to the weak-windclass. A group of stars exhibit hybrid properties-flarelike behaviorsuperimposed on a constant component with logLX~32 ergss-1-which suggest both magnetic activity and wind emission.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| New Silhouette Disks with Reflection Nebulae and Outflows in the Orion Nebula and M43
We report the detection of several new circumstellar disks seen insilhouette against background nebular light in the outskirts of theOrion Nebula and the neighboring H II region M43. These were detected aspart of our Hα survey of Orion with the Advanced Camera forSurveys on board the Hubble Space Telescope. Several of the disks showbipolar reflection nebulae, microjets, or pronounced temporalvariability of their central stars. The relatively large fraction ofbipolar reflection nebulae and microjets in our sample may be aselection effect caused by the faint nebular background far from theTrapezium. Two disks in our sample are large and particularlynoteworthy: A nearly edge-on disk, d216-0939, is located severalarcminutes northwest of M43 and resembles the famous HH 30 disk/jetsystem in Taurus. It drives the 0.15 pc long bipolar outflow HH 667 andexhibits a remarkable asymmetric reflection nebula caused by the tilt ofthe flared disk. With a diameter of ~2.6" (1200 AU), d216-0939 is aslarge as the giant edge-on silhouette disk d114-426 in the core of theOrion Nebula. The large disk d253-1536 is located in a binary systemembedded within an externally ionized giant proplyd in M43. The diskexhibits distortions that we attribute to tidal interactions with thecompanion star. The bipolar jet HH 668 emerges from the proplydionization front in a direction orthogonal to the disk and can be tracedto the young star embedded within it. A bow shock lies 54" south of thisbinary system along the outflow axis. Proper motions over a 1.4 yrbaseline confirm that these emission knots are indeed moving away fromd253-1536 with speeds as high as ~330 km s-1 in the HH 668microjet and slower motion farther from the star.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555.
| Size distribution of circumstellar disks in the Trapezium cluster
In this paper we present results on the size distribution ofcircumstellar disks in the Trapezium cluster asmeasured from HST/WFPC2 data. Direct diameter measurements of a sampleof 135 bright proplyds and 14 silhouettes disks suggest that there is asingle population of disks well characterized by a power-lawdistribution with an exponent of -1.9 ± 0.3 between diskdiameters 100-400 AU. For the stellar mass sampled (from late G to lateM stars) we find no obvious correlation between disk diameter andstellar mass. We also find that there is no obvious correlation betweendisk diameter and the projected distance to the ionizing Trapezium OBstars. We estimate that about 40% of the disks in the Trapezium haveradius larger than 50 AU. We suggest that the origin of the Solarsystem's (Kuiper belt) outer edge is likely to be due to the starformation environment and disk destruction processes (photoevaporation,collisions) present in the stellar cluster on which the Sun was probablyformed. Finally, we identified a previously unknown proplyd and named it266-557, following convention.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| Kinematic Masses of Super-Star Clusters in M82 from High-Resolution Near-Infrared Spectroscopy
Using high-resolution (R~22,000) near-infrared (1.51-1.75 μm) spectrafrom Keck Observatory, we measure the kinematic masses of two super-starclusters in M82. Cross-correlation of the spectra with template spectraof cool evolved stars gives stellar velocity dispersions ofσr=15.9+/-0.8 km s-1 for J0955505+694045(MGG-9) and σr=11.4+/-0.8 km s-1 forJ0955502+694045 (MGG-11). The cluster spectra are dominated by the lightof red supergiants and correlate most closely with template supergiantsof spectral types M0 and M4.5. King model fits to the observed profilesof the clusters in archival Hubble Space Telescope/Near-Infrared Cameraand Multi-Object Spectometer images give half-light radii ofrhp=2.6+/-0.4 pc for MGG-9 and rhp=1.2+/-0.17 pcfor MGG-11. Applying the virial theorem, we determine masses of1.5+/-0.3×106 Msolar for MGG-9 and3.5+/-0.7×105 Msolar for MGG-11 (where thequoted errors include σr, rhp, and thedistance). Population synthesis modeling suggests that MGG-9 isconsistent with a standard initial mass function (IMF), whereas MGG-11appears to be deficient in low-mass stars relative to a standard IMF.There is, however, evidence of mass segregation in the clusters, inwhich case the virial mass estimates would represent lower limits.Based on observations made at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.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/411/447
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Quantitative Stellar Spectral Classification. II. Early Type Stars
The method developed by Stock & Stock (1999) for stars of spectraltypes A to K to derive absolute magnitudes and intrinsic colors from theequivalent widths of absorption lines in stellar spectra is extended toB-type stars. Spectra of this type of stars for which the Hipparcoscatalogue gives parallaxes with an error of less than 20% were observedwith the CIDA one-meter reflector equipped with a Richardsonspectrograph with a Thompson 576×384 CCD detector. The dispersionis 1.753 Å/pixel using a 600 lines/mm grating in the first order.In order to cover the spectral range 3850 Å to 5750 Å thegrating had to be used in two different positions, with an overlap inthe region from 4800 Å to 4900 Å . A total of 116 stars wasobserved, but not all with both grating positions. A total of 12measurable absorption lines were identified in the spectra and theirequivalent widths were measured. These were related to the absolutemagnitudes derived from the Hipparcos catalogue and to the intrinsiccolors (deduced from the MK spectral types) using linear and secondorder polynomials and two or three lines as independent variables. Thebest solutions were obtained with polynomials of three lines,reproducing the absolute magnitudes with an average residual of about0.40 magnitudes and the intrinsic colors with an average residual of0.016 magnitudes.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution
Boron abundances have been derived for seven main-sequence B-type starsfrom Hubble Space Telescope STIS spectra around the B III λ2066line. In two stars, boron appears to be undepleted with respect to thepresumed initial abundance. In one star, boron is detectable but isclearly depleted. In the other four stars, boron is undetectable,implying depletions of 1-2 dex. Three of these four stars are nitrogenenriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions areunaccompanied by nitrogen enrichments. The inferred rate of borondepletion from our observations is in good agreement with thesepredictions. Other boron-depleted nitrogen-normal stars are identifiedfrom the literature. In addition, several boron-depleted nitrogen-richstars are identified, and while all fall on the boron-nitrogen trendpredicted by rotationally induced mixing, a majority have nitrogenenrichments that are not uniquely explained by rotation. The spectrahave also been used to determine iron group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near-solar iron groupabundances, as expected for young stars in the solar neighborhood. Wehave also analyzed the halo B-type star PG 0832+676. We find[Fe/H]=-0.88+/-0.10, and the absence of the B III line gives the upperlimit [B/H]<-2.5. These and other published abundances are used toinfer the star's evolutionary status as a post-asymptotic giant branchstar. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO 07400.
| A highly sensitive search for magnetic fields in B, A and F stars
Circular spectropolarimetric observations of 74 stars were obtained inan attempt to detect magnetic fields via the longitudinal Zeeman effectin their spectral lines. The sample observed includes 22 normal B, A andF stars, four emission-line B and A stars, 25 Am stars, 10 HgMn stars,two lambda Boo stars and 11 magnetic Ap stars. Using the Least-SquaresDeconvolution multi-line analysis approach (Donati et al.\cite{donati97etal}), high precision Stokes I and V mean signatures wereextracted from each spectrum. We find absolutely no evidence formagnetic fields in the normal, Am and HgMn stars, with upper limits onlongitudinal field measurements usually considerably smaller than anypreviously obtained for these objects. We conclude that if any magneticfields exist in the photospheres of these stars, these fields are notordered as in the magnetic Ap stars, nor do they resemble the fields ofactive late-type stars. We also detect for the first time a field in theA2pSr star HD 108945 and make new precise measurements of longitudinalfields in five previously known magnetic Ap stars, but do not detectfields in five other stars classified as Ap SrCrEu. We also report newresults for several binary systems, including a new vsin i for therapidly rotating secondary of the Am-delta Del SB2 HD 110951. Based onobservations obtained using the MuSiCoS spectropolarimeter on theBernard Lyot telescope, l'Observatoire du Pic du Midi, France.
| Iron abundance in H II regions
Optical CCD spectra are used to determine the \element{Fe} abundances atseveral positions inside seven bright Galactic H Ii regions. Theobserved [Fe Iii] line ratios are compared with the predictions ofdifferent sets of collision strengths and transition probabilities forthis ion to select the atomic data providing the best fit to theobservations. The values found for the \element[++]{Fe} and\element[+]{Fe} abundances, along with ionization correction factors forthe contribution of \element[3+]{Fe}, obtained from available grids ofphotoionized models, imply that the Fe/O ratio in the ionized gas isbetween 2% and 30% of solar. The \element{Fe} abundances derived foreach area are correlated both with the degree of ionization and thecolour excess. A possible explanation is suggested, namely the presenceof a population of small grains, probably originating from thefragmentation of larger grains. These small grains would release\element{Fe} atoms into the gas after the absorption of energeticphotons; the small grains surviving this destruction process would beswept out of the ionized region by the action of radiation pressure orstellar winds. An indication of a further and more efficient destructionagent is given by the high \element{Fe} abundance derived for a positionsampling the optical jet H 399 in M20, where dust destruction due to shock waves has presumablytaken place. Based on observations made with the Isaac Newton Telescope,operated on the island of La Palma by the Isaac Newton Group in theSpanish Observatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| Comparative morphology of ALIVARS and RCB type stars
Some conclusions were drawn from the analysis of the following specificfeatures of the rapid Algol-Like Irregular VARiable Stars (ALIVARS): 1)general mode of variability for ALIVARS and R CrB, 2) long periods ofnearly constant brightness, 3) specific "color - V-magnitude"dependence,4) Anticorrelation between V brightness and degree ofpolarization, 5) identity of the optical properties of the RCB andALIVARS circumstellar dust, 6) cyclic light variations at normalbrighness, 7) connection between cyclic light variations and the onsetof drastic light fadings. We found that ALIVARS have some morphologicalfeatures more in common with evolved RCB-type stars rather than withyoung Herbig Ae/Be stars. Moreover, the results of the analysis of theALIVARS evolution characteristic - a) space distribution of individualALIVARS and star formation regions, b) loci on the H-R diagram, c)equivalent widths of hydrogen absorption lines, d) relative hydrogendeficiency of two ALIVARS investigated (70% of the solar abundance for V351 Ori and 30% for RZ Psc), e) mean group parameter Vsini of ALIVARS,f) luminosity of ALIVARS �alpha emission envelopes- unambiguously point that ALIVARS are not so young as Herbig Ae/Bestars and they have left the main sequence. Thus, we have encounteredthe fact of stellar mimicry: ALIVARS tend to look out as young stars,having at that time features of rather evolved stars. Relying on suchfindings, we conclude the same ALIVARS should not be further regarded asyoung stars of the HAEBES group.
| Boron Abundances in Early B Stars: Results from the B III Resonance Line in IUE Data
We have used archival International Ultraviolet Explorer high-dispersionSWP spectra to study the B III resonance line at 2065.8 Å in 44early B stars. We find a median boron abundance about half that of solarsystem meteoritic material, consistent with the values found fromprevious studies of boron in early B stars. About one-third of the starsstudied appear to have boron abundances that are a factor of 4 or morelower than this median. Many of these are stars with enhanced nitrogenabundances, confirming the belief that deep envelope mixing can occur inmain-sequence B stars. A few stars with low boron abundances have normalN/C ratios. It is unclear whether all of these can be explained as starswhere mixing depleted the boron but did not go deep enough to bringCN-processed material to the surface or if some stars were actuallyformed with an unusually low boron abundance.
| Irradiated Herbig-Haro Jets in the Orion Nebula and near NGC 1333
We report the discovery of a dozen Herbig-Haro jets illuminated by theLyman continuum (λ<912 Å) and/or softer far-ultraviolet(912<λ<2000 Å) radiation fields of nearby high-massstars. Five irradiated outflows lie in the outer parts of the OrionNebula (HH 502-506), and seven lie near the reflection nebula NGC 1333in the Perseus molecular cloud (HH 333-336 and HH 497-499). Thesestellar outflows are powered by optically visible low-mass young starsthat suffer relatively low extinction and seem not to be embedded withinopaque cloud cores. We propose that the UV radiation field has erodedresidual material left over from their formation on a timescale shortcompared to the ages of these star-forming regions. Many of theirradiated jets exhibit unusual C-shaped symmetry. In the outskirts ofthe Orion Nebula, most irradiated jets appear to bend away from the coreof the nebula. On the other hand, in NGC 1333, the C-shaped jets tend tobend back toward the cluster center. Jet bending in the Orion Nebula maybe dominated by either the outflow of material from the nebular core orby the rocket effect pushing on the irradiated portion of a mostlyneutral jet beam. But in NGC 1333, jet bending may indicate that thesource stars have been ejected from the cluster core. Many irradiatedjets are asymmetric with one beam much brighter than the other. Whenfully photoionized, irradiated jets may provide unique insights into thephysical conditions within outflows powered by young stars, permittingthe determination of the density and location of stellar ejecta even inthe absence of shocks. We present a model for the photoionization ofthese outflows by external radiation fields and discuss possiblemechanisms for producing the observed asymmetries. In particular, wedemonstrate that the UV radiation field may alter the amount of cloudmaterial entrained by the jet. Radiation-induced variations in massloading and beam heating can produce differences in the beam velocitiesand spreading rates, which in turn determine the surface brightness ofthe radiating plasma. In a bipolar irradiated jet in which both beamshave the same mass-loss rate and opening angle, the slower beam willappear brighter at a given distance from the source. On the other hand,if both beams spread orthogonal to the jet propagation direction withthe same speed (e.g., both beams have the same internal sound speed orshocks with similar physical conditions), the faster beam will appearbrighter at the same distance from the source. Thus, depending on theparameters, either the faster or slower beam of a jet can be brighter.Finally, we report the discovery of some large-scale bow shocks thatface the core of the Orion Nebula and surround visible young stars.These wind-wind collision fronts provide further evidence for alarge-scale mass flow originating near the nebular core.
| New Proplyds, Outflows, Shocks, and a Reflection Nebula in M43 and the Outer Parts of the Orion Nebula
Hubble Space Telescope WFPC2 images made as planned parallelobservations have produced emission-line images of fields in the outerportions of the Orion Nebula and near the center of the companion H IIregion, M43. Examination of these images have uncovered three new brightproplyds and one silhouette proplyd. Two of the bright proplyds liewithin M43 and are photoionized by its central star, NU Ori. The newbright proplyd in the Orion Nebula shows a monopolar microjet, andanalysis of its size and surface brightness indicates that it lies wellin the foreground. Symmetric shocks indicate bipolar flow around the K8estar V1348 Ori (304-539) near θ2 Ori C. Evidence formultiple outflows from a source southeast of 036-927, well south of thebright bar feature of the nebula, is indicated. A bright reflectionnebula was found around the B1.5 Vp star LP Ori (098-753). The form ofthis object indicates that this star is moving within the veil ofneutral material that lies in front of M42. Based in part onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS 5-26555.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| The Be/X-ray binary LS 992/RX J0812.4-3114: Physical parameters and long-term variability
We present the first long-term optical and infrared study of the opticalcounterpart to the source RX J0812.4-3114, an X-ray pulsar with a Betype companion. During the period covered by the observations theprofile of some Balmer lines changed from absorption to emission andback again to absorption. Contemporaneously, the infrared magnitudesvaried by more than 0.8 mag. This long-term variability is interpretedas the formation and subsequent dissipation of the Be star's disc. Thebuilding up of the disc ended up in an active X-ray state characterisedby regular outbursts occurring at 80 day intervals. The overall durationof the formation/dissipation of the disc is found to be ≲ 4.3years. Optical spectroscopic and infrared photometric observations wereused to refine the spectral type of the primary (B0.2IV) and to monitorthe circumstellar envelope around the Be star. UBVRI and uvbybetaphotometric observations allowed the determination of the astrophysicalparameters of the optical companion. Based on observations collected atthe South African Astronomical Observatory, the European SouthernObservatory, Chile (ESO N64.H-0059) and the Teide Observatory, Tenerife(Spain).
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| Determination of the effective gravity and equatorial rotational velocity by means of the continuum and hydrogen lines
Two simple approaches for determining the effective gravity of singlerotating stars are suggested. In the first case, this gravity can befound by means of the characteristics from a study of a continuum only.In the second case, both characteristics of continuum and those of linespectrum must be used at the same time. Investigations shows that withincalculation errors the two approaches give identical values of theeffective gravity. The knowledge of this characteristic allows toestimate the equatorial rotational velocity and the inclination angle ofrotation axis of a rotator to the line-of-sight, respectively.
| Rotation in the Orion Nebula Cluster
Eighteen small (4' square) fields within the Orion Nebula cluster (ONC)have been photometrically monitored for one or more observing seasonsbetween 1990 and 1999 with a CCD attached to the 0.6 m telescope at VanVleck Observatory on the campus of Wesleyan University. Data wereobtained exclusively in the Cousins I band on between 25 and 40 nightsper season. Results from the first 3 yr of operation of this programwere summarized and analyzed by Choi & Herbst. Here we provide an updatebased on an additional 6 yr of observation and the extensive optical andinfrared study of the cluster by Hillenbrand et al. Rotation periodswith false-alarm probabilities FAP<1% are now available for 134 membersof the ONC. Of these, 67 were detected at multiple epochs with identicalperiods by us, and an additional 15 were confirmed by Stassun et al. intheir study of Ori OB1c and OB1d. Therefore, we have a sample of 82stars with virtually certain rotation periods and another 52 with highlyprobable periods, all of which are cluster members. The bimodal perioddistribution for the ONC reported by Choi & Herbst is confirmed, but wealso find a clear dependence of rotation period on mass. This phenomenoncan be understood as an effect of deuterium burning, which temporarilyslows the contraction and, therefore, spin-up of stars with M<=0.25Msolar and ages ~1 Myr. Stars with M<0.25 Msolarhave not had time to bridge the gap in the period distribution at around4 days. Excess H-K and I-K emissions, as well as Ca II infrared tripletequivalent widths (Hillenbrand et al.), show weak but significantcorrelations with rotation period among stars with M>0.25Msolar. Our results provide new observational support for theimportance of disks in the early rotational evolution of low-mass stars.
| Multiplicity of the massive stars in the Orion Nebula cluster
We present bispectrum speckle interferometry observations of 13 brightOrion Nebula cluster member stars of spectral type O or B.Diffraction-limited images with a resolution lambda/D of 75 mas in theK'-band were obtained with the SAO 6 m telescope. In our speckle imageswe find 8 visual companions in total. Using the flux ratios of theresolved systems to estimate the masses of the companions, we find thatthe systems generally have mass ratios below 1/2. The distribution ofmass ratios seems to be consistent with a companion mass functionsimilar to the field IMF. Considering both, the visual and thespectroscopic companions of the 13 target stars, the total number ofcompanions is at least 14. Extrapolation with correction for theunresolved systems suggests that there are at least 1.5 companions perprimary star on average. This number is clearly higher than the meannumber of ~ 0.5 companions per primary star found for the low-mass starsin the Orion Nebula cluster as well as in the field population. Thissuggests that a different mechanism is at work in the formation ofhigh-mass multiple systems in the dense Orion Nebula cluster than forlow-mass stars.
| Physical Conditions in Regions of Star Formation
The physical conditions in molecular clouds control the nature and rateof star formation, with consequences for planet formation and galaxyevolution. The focus of this review is on the conditions thatcharacterize regions of star formation in our Galaxy. A review of thetools and tracers for probing physical conditions includes summaries ofgenerally applicable results. Further discussion distinguishes betweenthe formation of low-mass stars in relative isolation and formation in aclustered environment. Evolutionary scenarios and theoreticalpredictions are more developed for isolated star formation, andobservational tests are beginning to interact strongly with the theory.Observers have identified dense cores collapsing to form individualstars or binaries, and analysis of some of these cores supporttheoretical models of collapse. Stars of both low and high mass form inclustered environments, but massive stars form almost exclusively inclusters. The theoretical understanding of such regions is considerablyless developed, but observations are providing the ground rules withinwhich theory must operate. The richest and most massive star clustersform in massive, dense, turbulent cores, which provide models for starformation in other galaxies.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
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Membro dei seguenti gruppi:
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Osservazione e dati astrometrici
| Costellazione: | Orione |
| Ascensione retta: | 06h07m34.30s |
| Declinazione: | +14°46'06.0" |
| Magnitudine apparente: | 4.42 |
| Distanza: | 163.934 parsec |
| Moto proprio RA: | 5.9 |
| Moto proprio Dec: | -21 |
| B-T magnitude: | 4.199 |
| V-T magnitude: | 4.371 |
Cataloghi e designazioni:
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