Contents
Images
Upload your image
DSS Images Other Images
Related articles
On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| Optical polarimetry of infrared excess stars
We present UBRVI polarimetry measurements for a group of 38 IRASinfrared excess stars and complement these observations with V-band datataken from the literature for 87 additional objects. After correctingthe observed values by the interstellar contribution, we find that 48%of the analyzed sample has polarization excess. In addition, thepolarization of these stars may correlate with infrared color excesses,particularly at 60 and 100 μm. We caution, however, that poor IRASdata quality at longer wavelengths affects this correlation. We analyzethe wavelength dependence of the linear polarization of 15 polarizedobjects in relation to Serkowski's empirical interstellar law. We findthat for 6 to 7 objects (depending on the interstellar model) themeasured polarization differs significantly from the empiricalinterstellar law, suggesting an intrinsic origin. We analyze thepolarimetry distribution of IRAS infrared excess objects in relation tothe Exoplanet host stars (i.e., stars associated with at least onelikely planetary mass object). The corresponding polarimetrydistributions are different within a high confidence level. Finally, wecompare the metallicity distributions of F and G IRAS infrared excess,Exoplanet host and field main sequence stars, and find that F-G IRASinfrared excess objects have metallicities quite similar (although notidentical) to field main sequence stars and significantly different fromthe Exoplanet host group.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| First AU-scale observations of V1647 Orionis with VLTI/MIDI
The young eruptive star V1647 Ori was observed with MIDI, themid-infrared interferometric instrument at the Very Large TelescopeInterferometer (VLTI), on March 2, 2005. We present the first spectrallyresolved interferometric visibility points for this object. Our resultsshow that (1) the mid-infrared emitting region is extended, having asize of ≈7 AU at 10 μm; (2) no signatures of a close companion canbe seen; (3) the 8{-}13 μm spectrum exhibits no obvious spectralfeatures. Comparison with similar observations of Herbig Ae starssuggests that V1647 Ori probably possesses a disk of moderate flaring. Asimple disk model with T r-0.53, Σr-1.5, M_d=0.05 {M}ȯ is able to fit both thespectral energy distribution and the observed visibility valuessimultaneously.
| 2MASS observations of Be stars
A catalogue of almost all known Galactic Be stars in the 2MASS database(1185 in total) and in the MSX database (225 in total) is presented inthis paper. From the color color diagrams and spectral indexes, it isfound that the IR excesses for many samples are due to free free andfree bound emission from the ionized material around a Be star assuggested previously and that the mean spectral index derived for thesamples is 1.50 ± 0.34 between J to K bands and 1.14 ±0.42 between K to A bands, respectively. However, it is also found thatthere are eight Be stars that have large near infrared excess emissionwith spectral index values outside the range expected for free free andfree bound emission. This is likely attributed to circumstellar dustemission.
| Recent astrophysical results from the VLTI.
Not Available
| An asteroseismic study of the β Cephei star θ Ophiuchi: photometric results
We have carried out a three-site photometric campaign for the βCephei star θ Oph from 2003 April to August. 245 h ofdifferential photoelectric uvy photometry were obtained during 77 clearnights. The frequency analysis of our measurements has resulted in thedetection of seven pulsation modes within a narrow frequency intervalbetween 7.116 and 7.973 c d-1. No combination or harmonicfrequencies have been found. We have performed a mode identification ofthe individual pulsations from our colour photometry that shows thepresence of one radial mode, one rotationally split l= 1 triplet andpossibly three components of a rotationally split l= 2 quintuplet. Wediscuss the implications of our findings and point out the similarity ofthe pulsation spectrum of θ Oph to that of another β Cepheistar, V836 Cen.
| Mid-Infrared Spectra of Polycyclic Aromatic Hydrocarbon Emission in Herbig Ae/Be stars
We present spectra of four Herbig Ae/Be stars obtained with the InfraredSpectrograph (IRS) on the Spitzer Space Telescope. All four of thesources show strong emission from polycyclic aromatic hydrocarbons(PAHs), with the 6.2 μm emission feature shifted to 6.3 μm and thestrongest CC skeletal-mode feature occurring at 7.9 μm instead of at7.7 μm, as is often seen. Remarkably, none of the four stars hassilicate emission. The strength of the 7.9 μm feature varies withrespect to the 11.3 μm feature among the sources, indicating that wehave observed PAHs with a range of ionization fractions. The ionizationfraction is higher for systems with hotter and brighter central stars.Two sources, HD 34282 and HD 169142, show emission features fromaliphatic hydrocarbons at 6.85 and 7.25 μm. The spectrum of HD 141569shows a previously undetected emission feature at 12.4 μm that may berelated to the 12.7 μm PAH feature. The spectrum of HD 135344, thecoolest star in our sample, shows an unusual profile in the 7-9 μmregion, with the peak emission to the red of 8.0 μm and no 8.6 μmPAH feature.
| An Excess Due to Small Grains around the Nearby K0 V Star HD 69830: Asteroid or Cometary Debris?
Spitzer photometry and spectroscopy of the star HD 69830 reveal anexcess of emission relative to the stellar photosphere between 8 and 35μm dominated by strong features attributable to crystalline silicateswith an emitting surface area more than 1000 times that of our zodiacalcloud. The spectrum closely resembles that of the comet C/1995 O1(Hale-Bopp). Since no excess is detected at 70 μm, the emittingmaterial must be quite warm, be confined within a few AU of the star,and originate in grains with low, long-wavelength emissivity, i.e.,grains much smaller than 70μm/2π~10 μm. The strongmineralogical features are evidence for even smaller, possiblysubmicron-sized grains. This small grain size is in direct contrast tothe 10-100 μm grains that dominate the relatively featureless spectraof our zodiacal dust cloud and most other main-sequence stars withexcesses. The upper limit at 70 μm also implies that any Kuiper Beltanalog must be either very cold or less massive than ~5 times our ownKuiper Belt. With collisional and Poynting-Robertson drag times of lessthan 1000 yr for small grains, the emitting material must either (1) becreated through continual grinding down of material in a dense asteroidbelt, or (2) originate in cometary debris arising from either a single``supercomet'' or a very large number of individual comets arriving froma distant reservoir. In the case of a cometary origin for the emission,the mass requirements for continuous generation by many individualcomets are unreasonable, and we favor the capture of a single supercomet into a 0.5-1 AU orbit, where it can evolve a large number of smallgrains over a 2 Myr period.
| A photometric survey of stars with circumstellar material
We present the result of a follow-up Strömgren photometric surveyof sixteen southern bright stars with circumstellar material, in orderto detect possible weak photometric variations. We found new variationsof the β~ Pictoris brightness from 1999 to 2002 with a weaklong-term variation of ~-0.8× 10-3 mag per year, overabout 3 years. These variations look similar to those seen from 1975 to1981 and from 1995 to 1998 (Nitschelm et al. 2000, A&AS, 145, 275).They can be due to differential occultation by dust inhomogeneitiestransiting the star through the years. We detected new periodicvariations for HD 256 (HR 10) with periods ranging from 0.35 day to 6.69days during several months. These variations may also be interpreted interms of variable obscuration due to structures in the circumstellardisk suspected to be surrounding this star.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Evidence for a hot dust-free inner disk around 51 Oph
We report on the observation of CO bandhead emission around 51Oph (Δ v=2). A high resolving power (R≃10 000)spectrum was obtained with the infrared spectrometer ISAAC mounted onVLT-ANTU. Modeling of the profile suggests that the hot(Tgas= 2000-4000 K) and dense(nH>1010 cm-3) molecular material asprobed by the CO bandhead is located in the inner AU of a Keplerian diskviewed almost edge-on. Combined with the observation of cooler gas(Tgas= 500-900 K) by ISO-SWS and the lack of cold material,our data suggest that the disk around 51 Oph isessentially warm and small. We demonstrate the presence of a dust-freeinner disk that extents from the inner truncation radius until the dustsublimation radius. The disk around 51 Oph may be ina rare transition state toward a small debris disk object.Based on observations collected at the European Southern Observatory atLa Silla and Paranal, Chile (ESO Programme 68-C-0474).
| A new approach to IRAS observations of Be stars
The IRAS associations for 193 Be stars are identified in this paper.From the infrared colors, the IRAS low-resolution spectra (LRS) and thespectral types, some physical properties and environment of the samplesare discussed. It can be concluded that not only free-free emission orfree-bound emission from the circumstellar ionized gas can beresponsible for the large IR excesses of Be stars as suggestedpreviously, but also, for some Be stars, thermal radiation from thecircumstellar dust and/or nebula around the star can produce large IRexcess as well. It is also found that the far IR excess of Be starsincreases with wavelengths.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Detection of Formaldehyde toward the Extreme Carbon Star IRC +10216
We report the detection of H2CO (formaldehyde) around thecarbon-rich asymptotic giant branch star IRC +10216. We find afractional abundance with respect to molecular hydrogen ofx(H2CO)=1.3+1.5-0.8×10-8.This corresponds to a formaldehyde abundance with respect to water vaporofx(H2CO)/x(H2O)=(1.1+/-0.2)×10-2,in line with the formaldehyde abundances found in solar system comets,and indicates that the putative extrasolar cometary system around IRC+10216 may have a similar chemical composition to solar system comets.However, we also failed to detect CH3OH (methanol) around IRC+10216, and our upper limit ofx(CH3OH)/x(H2O)<7.7×10-4 (3σ) indicates that methanol is substantially underabundant in IRC+10216 compared to solar system comets. We also conclude, on the basisof offset observations, that formaldehyde has an extended source in theenvelope of IRC +10216 and may be produced by the photodissociation of aparent molecule, similar to the production mechanism for formaldehyde insolar system comet comae. Preliminary mapping observations also indicatea possible asymmetry in the spatial distribution of formaldehyde aroundIRC +10216, but higher signal-to-noise ratio observations are requiredto confirm this finding. By serendipity, our observations have led tothe detection of the J=17-16 transition of Al37Cl at 241.855GHz. Our analysis of the measured line flux, along with those ofpreviously observed lower frequency transitions, yields a total AlCl(aluminum monochloride) abundance in the range(2-8)×10-8 relative to H2; this range, whichis a factor of 10 smaller than an abundance estimate that has appearedpreviously in the literature, amounts to ~4%-16% of the solar elementalabundance of chlorine, a fraction that is in accord with the predictionsof thermochemical equilibrium models for cool stellar photospheres.This study is based on observations carried out with the IRAM 30 mtelescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), andIGN (Spain).
| A search for circumstellar dust disks with ADONIS
We present results of a coronographic imaging search for circumstellardust disks with the Adaptive Optics Near Infrared System (ADONIS) at theESO 3.6 m telescope in La Silla (Chile). 22 candidate stars, known to beorbited by a planet or to show infrared excess radiation, were examinedfor circumstellar material. In the PSF-subtracted images no clear diskwas found. We further determine the detection sensitivities and outlinehow remaining atmospheric fluctuations still can hamper adaptive opticsobservations.Based on observations collected at the European Southern Observatory, LaSilla, Chile (65.I-0098, 67.C-0240, 68.C-0070).
| Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI
We present the first long baseline mid-infrared interferometricobservations of the circumstellar disks surrounding Herbig Ae/Be stars.The observations were obtained using the mid-infrared interferometricinstrument MIDI at the European Southern Observatory (ESO) Very LargeTelescope Interferometer VLTI on Cerro Paranal. The 102 m baseline givenby the telescopes UT1 and UT3 was employed, which provides a maximumfull spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10μm. The interferometric signal was spectrally dispersed at aresolution of 30, giving spectrally resolved visibility information from8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars andresolved all objects. The warm dust disk of HD 100546 could even beresolved in single-telescope imaging. Characteristic dimensions of theemitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10μm sizes of our sample stars correlate with the slope of the 10-25μm infrared spectrum in the sense that the reddest objects are thelargest ones. Such a correlation would be consistent with a differentgeometry in terms of flaring or flat (self-shadowed) disks for sourceswith strong or moderate mid-infrared excess, respectively. We comparethe observed spectrally resolved visibilities with predictions based onexisting models of passive centrally irradiated hydrostatic disks madeto fit the SEDs of the observed stars. We find broad qualitativeagreement of the spectral shape of visibilities corresponding to thesemodels with our observations. Quantitatively, there are discrepanciesthat show the need for a next step in modelling of circumstellar disks,satisfying both the spatial constraints such as are now available fromthe MIDI observations and the flux constraints from the SEDs in aconsistent way.Based on observations made with the Very Large Telescope Interferometerat Paranal Observatory.
| The Low-Velocity Wind from the Circumstellar Matter around the B9 V Star σ Herculis
We have obtained Far Ultraviolet Spectroscopic Explorer spectra ofσ Her, a nearby binary system with a main-sequence primary thathas a Vega-like infrared excess. We observe absorption in the excitedfine structure lines C II* at 1037 Å, N II* at 1085 Å, and NII** at 1086 Å that are blueshifted by as much as ~30 kms-1 with respect to the star. Since these features areconsiderably narrower than the stellar lines and broader thaninterstellar features, the C II and N II are circumstellar. We suggestthat there is a radiatively driven wind arising from the circumstellarmatter rather than accretion as occurs around β Pic because ofσ Her's high luminosity. Assuming that the gas is liberated bycollisions between parent bodies at 20 AU, the approximate distance atwhich blackbody grains are in radiative equilibrium with the star and atwhich three-body orbits become unstable, we inferdM/dt~6×10-12Msolar yr-1. Thiswind depletes the minimum mass of parent bodies in less than theestimated age of the system.
| Far-Ultraviolet Observations of the Circumstellar Gas in the 2 Andromedae System
The A5 star β Pictoris is a possible young planetary system and hasthe best-studied circumstellar disk. Our visible and ultravioletobservations of 2 Andromedae indicated that this A3 star has βPictoris-like gas infall. We present the far-ultraviolet spectrum(905-1195 Å) of 2 And we obtained with the NASA-CNES-CSA FarUltraviolet Spectroscopic Explorer (FUSE). Unlike β Pic, 2 And'sFUSE spectrum does not show strong chromospheric emission lines from CIII and O VI. However, 2 And's FUSE spectrum contains manynonphotospheric lines that allow us to probe the circumstellar gas. Forexample, between 1120 and 1140 Å, we detected several Fe IIIabsorption lines arising from hyperfine levels of ground state, whichcannot be formed in the interstellar medium. These lines are gooddiagnostics of the circumstellar gas. We also detected circumstellar FeII, Cr III, Mn III, and O I (1D) lines. The simultaneouspresence of these species suggests that the circumstellar environment of2 And could include regions with different temperatures and densities.
| High resolution spectroscopy of circumstellar material around A stars
We have analysed a time series of high resolution spectra (R=217 000) ofthe CaII K line of 9 stars which are candidates for the presence ofcircumstellar material from our previous studies. We have searched forvariable narrow absorption components similar to those extensivelystudied in the case of beta Pictoris. Our data show long-termvariations in the spectra of HR 2550 and HR 3685 which can be attributedto the dynamics of circumstellar gas. About one third of the samplestars show variable line absorption but only beta Pictoris seems toexhibit uniquely strong variations on short (nightly) timescales. Inorder to examine possible interstellar contributions we compared ourresults with interstellar data from the literature. The column densitiesof our absorption features are up to three orders of magnitude higherthan those found for the Local Interstellar Cloud.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| Photometry of Mercury from SOHO/LASCO and Earth. The Phase Function from 2 to 170 deg.
CCD observations of Mercury were obtained with the large anglespectrometric coronograph (LASCO) on the solar and heliosphericobservatory spacecraft, near superior and inferior solar conjunctions.Whole disk photometry was extracted from the orange and blue filterimages and transformed to V magnitudes on the UBV system. The LASCO datawere combined with ground-based, V-filter photometry acquired at largerelongation angles. The resulting photometric phase function covers thegreatest span of angles to date and is the first wide-range function tobe obtained since the era of visual observation. We analyzed the datausing a polynomial fit and a Hapke function fit, and derived thefollowing photometric results. Mercury's fully lit brightness, adjustedto a distance of 1.0 AU from the Sun and observer, was found to beV=-0.694(+/-0.030), which is more luminous than previously measured. Thecorresponding geometric albedo is 0.142(+/-0.005). The phase integral is0.478(+/-0.005) and resulting spherical albedo is 0.068(+/-0.003). Theupper limit of a possible rotational brightness variation is about 0.05magnitude. Mercury's brightness surges by more than 40% between phaseangles 10 and 2°, while the illuminated fraction of the diskincreases by less than 1%. A set of coefficients for Hapke's functionthat fit most of the phase curve includes h=0.065+/-0.002 indicatingthat Mercury and the Moon have similar regolith compaction states andparticle size distributions, and θ-bar=16°+/-1° implyinga macroscopically smoother surface than the Moon. However, we foundother solutions that fit the observations nearly as well withsignificantly smaller and larger values of h, and with values ofθ-bar around 25°. The wide range for θ-bar is due tothe inability of the model to fit the photometry obtained at large phaseangles. .
| Comet Grains: Their IR Emission and Their Relation to ISm Grains
Comets and the chondritic porous interplanetary dust particles (CP IDPs)that they shed in their comae are reservoirs of primitive solar nebulamaterials. The high porosity and fragility of cometary grains and CPIDPs, and anomalously high deuterium contents of highly fragile,pyroxene-rich Cluster IDPs imply these aggregate particles containsignificant abundances of grains from the interstellar medium (ISM). IRspectra of comets (3 40 μm) reveal the presence of a warm (near-IR)featureless emission modeled by amorphous carbon grains. Broad andnarrowresonances near 10 and 20 microns are modeled by warm chondritic (50%Feand 50% Mg) amorphous silicates and cooler Mg-rich crystallinesilicate minerals, respectively. Cometary amorphous silicates resonancesare well matched by IRspectra of CP IDPs dominated by GEMS (0.1 μmsilicate spherules) that are thought to be the interstellar Fe-bearingamorphous silicates produced in AGB stars. Acid-etched ultramicrotomedCP IDP samples, however, show that both the carbon phase (amorphous andaliphatic) and the Mg-rich amorphous silicate phase in GEMS are notoptically absorbing. Rather, it is Fe and FeS nanoparticles embedded inthe GEMS that makes the CP IDPs dark. Therefore, CP IDPs suggestsignificant processing has occurred in the ISM. ISM processing probablyincludes in He+ ion bombardment in supernovae shocks.Laboratory experiments show He+ ion bombardment amorphizescrystalline silicates, increases porosity, and reduces Fe intonanoparticles. Cometary crystalline silicate resonances are well matchedby IR spectra of laboratory submicron Mg-rich olivine crystals andpyroxene crystals. Discovery of a Mg-pure olivine crystal in a ClusterIDP with isotopically anomalous oxygen indicates that a small fractionof crystalline silicates may have survived their journey from AGB starsthrough the ISM to the early solar nebula. The ISM does not have enoughcrystalline silicates (<5%), however, to account for the deducedabundance of crystalline silicates in comet dust. An insufficient sourceof ISMMg-rich crystals leads to the inference that most Mg-rich crystalsin comets are primitive grains processed in the early solar nebula priorto their incorporation into comets. Mg-rich crystals may condense in thehot (1450 K), inner zones of the early solar nebula and then travellarge radial distances out to the comet-forming zone. On the other hand,Mg-rich silicate crystals may be ISM amorphous silicates annealed at1000 K and radially distributed out to the comet-forming zone orannealed in nebular shocks at 5-10 AU. Determining the relativeabundance of amorphous and crystalline silicatesin comets probes therelative contributions of ISM grains and primitive grains to small, icybodies in the solar system. The life cycle of dust from its stardustorigins through the ISM to its incorporation into comets is 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.
| Far Ultraviolet Spectroscopic Explorer Observations of Possible Infalling Planetesimals in the 51 Ophiuchi Circumstellar Disk
We present the first observations of the circumstellar (CS) disk system51 Ophiuchi with the Far Ultraviolet Spectroscopic Explorer (FUSE). Wedetect several absorption lines arising from the unusual metastableatomic species N I (2D), N I (2P), and S II(2D). These levels lie 1.8-3.6 eV above the ground level andhave radiative decay lifetimes of 2 days or less, indicating that thelines arise from warm CS gas. The high S/N FUSE spectra, obtained 6 daysapart, also show time-variable absorption features arising from N I, NII, O I (1D), and Fe III, which are redshifted with respectto the stellar velocity. The resolved redshifted absorption extends overmany tens of km s-1 (40 for N I, 100 for N II, 65 for O I(1D), and 84 for Fe III). We calculate column densities forall the variable infalling CS gasses, using the apparent optical depthmethod. The Fe III and N II infalling gasses must be produced throughcollisional ionization, and the ionization fraction of nitrogen suggestsa gas temperature between 20,000 and 34,000 K. The infalling gas shows apeculiar, nonsolar composition, with nitrogen and iron more abundantthan carbon. We also set upper limits on the line-of-sight columndensities of H2 and CO. These observations strengthen theconnection between 51 Oph and the older debris-disk system βPictoris and indicate that there may be infalling planetesimals in the51 Oph system.
| Polarimetric Studies of Stars with an Infrared Emission Excess
The results of polarimetric and IR (IRAS) observations of 24 B-A-F starsare given. Intrinsic polarization of the light from 11 of the 24 starsis observed. The degree of polarization for the other 13 stars is withinthe measurement errors. Two-color diagrams are also constructed. From acomparison of the degree of polarization with the color index on thetwo-color diagrams it is seen that 8 of these 13 stars probably are ofthe Vega type, while 5 are stars with gasdust shells and/ordiskshells. It is shown that 6 of the aforementioned 11 starswith intrinsic polarization evidently are stars with gasdustshells and/or diskshells, while 5 of them (also including No. 24)are of the Vega type. It is also shown that the IR emission from 10 ofthe stars corresponds to a power-law distribution F . This fact may beexplained both by freefree transitions of electrons and bythermal emission from dust grains in circumstellar gasdust shells(disks).
| Ultraviolet Spectroscopy of Circumstellar Disks
We present a study of gas in several young circumstellar (CS) disksystems, using UV absorption spectroscopy with HST-STIS and FUSE. Thesystems observed are Beta Pictoris, AB Aurigae, 51 Ophiuchi, HD 100546,and HD 163296. This study focusses on molecular gas abundances andstar-grazing planetesimals, and shows that the characteristics of the CSgas can aid in the evolutionary classification of disk systems. CSmolecular gas abundances, which have been the subject of considerablerecent controversy, may also provide information on the composition ofextrasolar planetesimals. Important results on the individual systemsare listed below. The primordial disk system AB Aurigae is surrounded bya diffuse envelope of gas and dust, a remnant of the molecular cloudthat collapsed to form the star. Carbon monoxide and CI are observed inthe Beta Pic debris disk; when compared to the lack of molecularhydrogen gas, this indicates that the CO gas is produced by evaporationof icy, comet-like planetesimals. Volatile-depleted or metal-rich CS gasis seen infalling toward the star in spectra of 51 Oph; this gas islikely produced by vaporization of star-grazing planetesimals. Analysisof the transitional disk systems HD 100546 and HD 163296 showsextraordinary hot, dense molecular hydrogen near these stars.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| The Dispersal of Young Stars and the Greater Sco-Cen Association
We review topics related to the dispersal of young stars from theirbirth-sites, and focus in particular on the entourage of young starsrelated to the ongoing star-formation event in the Sco-Cen OBassociation. We conduct a follow-up kinematic study to that presented inMamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby,isolated, young stars. In addition to the eta Cha and TW Hya groups, wefind several more intriguing Sco-Cen outlier candidates: most notablyβ Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connectionbetween Sco-Cen and the southern ``150 pc Conspiracy'' molecular clouds,and in particular, Corona Australis. The kinematic evidence suggeststhat many of the nearby, isolated ~10 Myr-old stars were born nearSco-Cen during the UCL and LCC starbursts 10-15 Myr ago. We hypothesizethat these stars inherited 5-10 km/s velocities moving away fromSco-Cen, either through molecular cloud turbulence, or through formationin molecular clouds associated with the expanding Sco-Cen superbubbles(e.g. Loop I).
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | へびつかい座 |
| Right ascension: | 17h31m25.00s |
| Declination: | -23°57'46.0" |
| Apparent magnitude: | 4.81 |
| Distance: | 130.719 parsecs |
| Proper motion RA: | 2.6 |
| Proper motion Dec: | -25.8 |
| B-T magnitude: | 4.813 |
| V-T magnitude: | 4.795 |
Catalogs and designations:
|
