Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
| Measuring Magnetic Fields in Ultracool Stars and Brown Dwarfs
We present a new method for direct measurement of magnetic fields onultracool stars and brown dwarfs. It takes advantage of the Wing-Fordbands of FeH, which are seen throughout the M and L spectral types.These molecular features are not as blended as other optical molecularbands, are reasonably strong through most of the spectral range, andexhibit a response to magnetic fields that is easier to detect thanother magnetic diagnostics, including the usual optical andnear-infrared atomic spectral lines that have heretofore been employed.The FeH bands show a systematic growth as the star gets cooler. We donot find any contamination by CrH in the relevant spectral region. Weare able to model cool and rapidly rotating spectra from warmer, slowlyrotating spectra utilizing an interpolation scheme based oncurve-of-growth analysis. We show that the FeH features can distinguishbetween negligible, moderate, and high magnetic fluxes on low-massdwarfs, with a current accuracy of about 1 kG. Two different approachesto extracting the information from the spectra are developed andcompared. Which one is superior depends on a number of factors. Wedemonstrate the validity of our new procedures by comparing the spectraof three M stars whose magnetic fluxes are already known from atomicline analysis. The low- and high-field stars are used to produceinterpolated moderate-strength spectra that closely resemble themoderate-field star. The assumption of linear behavior for the magneticeffects appears to be reasonable, but until the molecular constants arebetter understood, the method is subject to that assumption and ratherapproximate. Nonetheless, it opens a new regime of very low mass objectsto direct confirmation and testing of their magnetic dynamos.
| Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| Constraining Disk Parameters of Be Stars using Narrowband Hα Interferometry with the Navy Prototype Optical Interferometer
Interferometric observations of two well-known Be stars, γ Cas andφ Per, were collected and analyzed to determine the spatialcharacteristics of their circumstellar regions. The observations wereobtained using the Navy Prototype Optical Interferometer equipped withcustom-made narrowband filters. The filters isolate the Hαemission line from the nearby continuum radiation, which results in anincreased contrast between the interferometric signature due to theHα-emitting circumstellar region and the central star. Because thenarrowband filters do not significantly attenuate the continuumradiation at wavelengths 50 nm or more away from the line, theinterferometric signal in the Hα channel is calibrated withrespect to the continuum channels. The observations used in this studyrepresent the highest spatial resolution measurements of theHα-emitting regions of Be stars obtained to date. Theseobservations allow us to demonstrate for the first time that theintensity distribution in the circumstellar region of a Be star cannotbe represented by uniform disk or ringlike structures, whereas aGaussian intensity distribution appears to be fully consistent with ourobservations.
| The First High-Resolution Spectra of 1.3 L Subdwarfs
We present the first high-resolution (R~31,000) spectra of the cool Lsubdwarf (sdL) 2MASS 0532+8246 and what was originally identified as anearly-type sdL, LSR 1610-0040. Our work, in combination withcontemporaneous work by Cushing and Vacca, makes it clear that thelatter object is more likely a mid-M dwarf with an unusual compositionthat gives it some subdwarf spectral features. We use the data to deriveprecise radial velocities for both objects and to estimate space motion;both are consistent with halo kinematics. We measure the projectedrotational velocities, revealing a very slow rotation for the old sd?M6object LSR 1610-0040. The object 2MASS 0532+8246 exhibits a rapidrotation of vsini=65+/-15 km s-1, consistent with thebehavior of L dwarfs. This means that the braking time for L dwarfs isextremely long, or that perhaps they never slow down. A detailedcomparison of the atomic Rb and Cs lines with spectra of field L dwarfsshows that the spectral type 2MASS 0532+8246 is consistent with beingmid- to late-L. The Rb I and K I lines of LSR 1610-0040 are like thoseof an early-L dwarf, but the Cs I line is like that of a mid-M dwarf.The appearance of the Ca II triplet in absorption in this object is veryhard to understand if it is not at least as warm as M6. We explain theseeffects in a consistent way using a mildly metal-poor mid-M model. Msubdwarfs have weak metal oxides and enhanced metal hydrides relative tonormal M dwarfs. LSR 1610-0040 exhibits metal hydrides like an M dwarfbut metal oxides like a subdwarf. The same explanation that resolves theatomic-line discrepancy explains this as well. Our spectra cover thespectral region around a previously unidentified absorption feature at9600 Å and the region around 9400 Å in which detection ofTiH has been claimed. We identify the absorption around 9600 Å asbeing due to atomic lines of Ti and a small contribution of FeH, but wecannot confirm a detection of TiH in the spectra of cool sdLs. In 2MASS0532+8246, both metal oxides and metal hydrides are extremely strongrelative to normal L dwarfs. It may be possible to explain the strongoxide features in 2MASS 0532+8246 by invoking effects due to inhibiteddust formation. High-resolution spectroscopy has aided in beginning tounderstand the complex molecular chemistry and spectral formation inmetal-deficient and ultracool atmospheres and the properties of earlyultra-low-mass objects.
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| Attempts to measure the magnetic field of the pulsating B star ν Eridani
We report on attempts to measure the magnetic field of the pulsating Bstar ν Eridani with the Musicos spectropolarimeter attached to the 2m telescope at the Pic du Midi, France. This object is one of the mostextensively studied stars for pulsation modes, and the existence of amagnetic field was suggested from the inequality of the frequencyseparations of a triplet in the stars' oscillation spectrum. We showthat the inferred 5-10 kG field was not present during our observations,which cover about one year. We discuss the influence of the strongpulsations on the analysis of the magnetic field strength and set anupper limit to the effective longitudinal field strength and to thefield strength for a dipolar configuration. We also find that theobserved wind line variability is caused by the pulsations.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| A rigidly rotating magnetosphere model for circumstellar emission from magnetic OB stars
We present a semi-analytical approach for modelling circumstellaremission from rotating hot stars with a strong dipole magnetic fieldtilted at an arbitrary angle to the rotation axis. By assuming therigid-field limit in which material driven (e.g. in a wind outflow) fromthe star is forced to remain in strict rigid-body corotation, we areable to solve for the effective centrifugal-plus-gravitational potentialalong each field line, and thereby identify the location of potentialminima where material is prone to accumulate. Applying basic scalingsfor the surface mass flux of a radiatively driven stellar wind, wecalculate the circumstellar density distribution that obtains onceejected plasma settles into hydrostatic stratification along fieldlines. The resulting accumulation surface resembles a rigidly rotating,warped disc, tilted such that its average surface normal lies betweenthe rotation and magnetic axes. Using a simple model of the plasmaemissivity, we calculate time-resolved synthetic line spectra for thedisc. Initial comparisons show an encouraging level of correspondencewith the observed rotational phase variations of Balmer-line emissionprofiles from magnetic Bp stars such as σ Ori E.
| Dynamo-generated magnetic fields at the surface of a massive star
Spruit has shown that an astrophysical dynamo can operate in thenon-convective material of a differentially rotating star as a result ofa particular instability in the magnetic field (the Tayler instability).By assuming that the dynamo operates in a state of marginal instability,Spruit has obtained formulae which predict the equilibrium strengths ofazimuthal and radial field components in terms of local physicalquantities. Here, we apply Spruit's formulae to our previously publishedmodels of rotating massive stars in order to estimate Tayler dynamofield strengths. There are no free parameters in Spruit's formulae. Inour models of 10- and 50-Msolar stars on the zero-age mainsequence, we find internal azimuthal fields of up to 1 MG, and internalradial components of a few kG. Evolved models contain weaker fields. Inorder to obtain estimates of the field strength at the stellar surface,we examine the conditions under which the Tayler dynamo fields aresubject to magnetic buoyancy. We find that conditions for Taylerinstability overlap with those for buoyancy at intermediate to highmagnetic latitudes. This suggests that fields emerge at the surface of amassive star between magnetic latitudes of about 45° and the poles.We attempt to estimate the strength of the field which emerges at thesurface of a massive star. Although these estimates are very rough, wefind that the surface field strengths overlap with values which havebeen reported recently for line-of-sight fields in several O and Bstars.
| La supernova de 1572, une diversite d'interpretations.
Not Available
| Caroline Herschel as observer
Not Available
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| 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.
| Probing the internal magnetic field of slowly pulsating B-stars through g modes
Context: . Aims: .We suggest that high-order g modes can be usedas a probe of the internal magnetic field of SPB (slowly pulsating B)stars. The idea is based on earlier work by the authors hichanalytically investigated the effect of a vertical magnetic field on pand g modes in a plane-parallel isothermal stratified atmosphere. It wasfound that even a weak field can significantly shift the g-modefrequencies - the effect increases with mode order. Methods: .Inthe present study we adopt the classical perturbative approach toestimate the internal field of a 4 solar mass SPB star by looking at itseffect on a low-degree (l=1) and high-order (n=20) g mode with a periodof about 1.5 d. Results: .We find that a polar field strength ofabout 110 kG on the edge of the convective core is required to produce afrequency shift of 1%. Frequency splittings of that order have beenobserved in several SPB variables, in some cases clearly too small to beascribed to rotation. We suggest that they may be due to a poloidalfield with a strength of order 100 kG, buried in the deep interior ofthe star. Conclusions: .
| Integral field spectroscopy of the ultraluminous X-ray source Holmberg II X-1
We present optical integral field observations of the H II regioncontaining the ultraluminous X-ray source Holmberg II X-1. We confirmthe existence of an X-ray ionized nebula as the counterpart of thesource owing to the detection of an extended He II λ4686 region(21× 47 pc) at the Chandra ACIS-S position. An extended blueobject with a size of 11× 14 pc is coincident with the X-ray/He IIλ4686 region, which could indicate that it is either a youngstellar complex or a cluster. We have derived an X-ray to opticalluminosity ratio of L_X/LB≥170, and presumable it isL_X/LB300{-}400 using the recent HST ACS data. We find acomplex velocity dispersion at the position of the ULX. In addition,there is a radial velocity variation in the X-ray ionized region foundin the He II emission of ±50 km s-1 on spatial scalesof 2 3primeprime. We believe that the putative black hole notonly ionizes the surrounding HII gas, but also perturbs it dynamically(via jets or the accretion disk wind). The spatial analysis of thepublic Chandra ACIS-S data reveals a point-like X-ray source and givesmarginal indication of an extended component (ll15% of the total flux).The XMM-Newton EPIC-PN spectrum of HoII X-1 is best fitted with anabsorbed power law in addition to either a thermal thick plasma or athermal thin plasma or a multi-colour disk black body (MCD). In allcases, the thermal component shows a relatively low temperature(kT0.14{-}0.22 keV). Finally we discuss the optical/X-rayproperties of HoII X-1 with regards to the possible nature of thesource. The existence of an X-ray ionized nebula coincident with the ULXand the soft X-ray component with a cool accretion disk favours theinterpretation as an intermediate-mass black hole (IMBH). However, thecomplex velocity behaviour at the position of the ULX indicates adynamical influence of the black hole on the local HII gas.
| HS 2331+3905: The cataclysmic variable that has it all
We report detailed follow-up observations of the cataclysmic variable HS2331+3905, identified as an emission-line object in the Hamburg QuasarSurvey. An orbital period of 81.08 min is unambiguously determined fromthe detection of eclipses in the light curves of HS 2331+3905. A secondphotometric period is consistently detected at P ≃ 83.38 min, 2.8% longer than Porb, which we tentatively relate tothe presence of permanent superhumps. High time resolution photometryexhibits short-timescale variability on time scales of ≃5-6 minwhich we interpret as non-radial white dwarf pulsations, as well as acoherent signal at 1.12 min, which is likely to be the white dwarf spinperiod. A large-amplitude quasi-sinusoidal radial velocity modulation ofthe Balmer and Helium lines with a period 3.5 h is persistentlydetected throughout three seasons of time-resolved spectroscopy.However, this spectroscopic period, which is in no way related to theorbital period, is not strictly coherent but drifts in period and/orphase on time scales of a few days. Modeling the far-ultraviolet toinfrared spectral energy distribution of HS 2331+3905, we determine awhite dwarf temperature of Teff≃10,500 K (assumingMwd=0.6 Mȯ), close to the ZZ Ceti instabilitystrip of single white dwarfs. The spectral model implies a distance ofd=90±15 pc, and a low value for the distance is supported by thelarge proper motion of the system, μ=0.14 arcsec yr-1. Thenon-detection of molecular bands and the low J, H, and K fluxes of HS2331+3905 make this object a very likely candidate for a brown-dwarfdonor.Based in part 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, on observations made at the 1.2mtelescope, located at Kryoneri Korinthias, and owned by the NationalObservatory of Athens, Greece, and with the Isaac Newton Telescope andWilliam Herschel Telescope, which are operated on the island of La Palmaby the Isaac Newton Group in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Astrometry of the 1572 supernova (B Cassiopeiae)
Contemporary European measurements of the Milky Way supernova of 1572 (BCas) have been analyzed to compute a modern position for the star in1572. This work adds to that done by previous 20th-century authors byusing a new procedure and previously unused astrometry by 16th-centuryEuropean observers. Comparison is made with earlier analyses and tomodern positional data on the supernova remnant (3C 10).
| Rotational Velocities of B, A, and Early-F Narrow-lined Stars
Projected rotational velocities for 58 B, A, and early-F stars have beendetermined from high-resolution spectroscopic observations made at KittPeak National Observatory with the coudé feed telescope. All thestars are slowly rotating with vsini<60 km s-1. Because oftheir low rotational velocities, 15 of the stars have been observed asprospective, early-type, radial velocity standards.
| Discovery of weak magnetic fields in early B-type stars
We summarize recent results of magnetic measurements of three brightearly B-type stars, β Cep, ζ Cas, and V2052 Oph, which werefound to be oblique rotators with a weak magnetic dipole field withtypical strength of a few hundred Gauss. From stellar wind studies wecould derive their rotational periods very accurately, and match thestellar wind with the magnetic phase. From model atmosphere fits wederive the angles of rotational and magnetic axis. All three stars showsome chemical abundance anomaly, presumably associated with the magneticproperties. The stars are also pulsating variables. This is of highasteroseismological interest, since these are the only early-type starsknown with observationally determined rotation, pulsation and magneticproperties.
| Kinematical Structure of the Local Interstellar Medium: The Galactic Anticenter Hemisphere
A survey of interstellar Na I D1 and D2 absorption features in thespectra of 104 early-type stars in the second and third Galacticquadrants reveals the large-scale kinematics of the interstellar gaswithin the Galactic anticenter hemisphere at distances from the Sunbetween ~70 and ~250 pc. Employing a technique that uses both the radialvelocities and column densities of the Na I absorptions produced by theintervening gas we have identified the velocity vectors and determinedthe spatial distribution of eight interstellar clouds in the volumeexplored. The average internal H I+H2 densities of the cloudsrange between 0.03 and 1.7 cm-3, and their masses between 80and 104 Msolar, although uncertainties in thesizes of the clouds, their possible extension beyond the regionexplored, and the presence of denser gas embedded in the larger cloudsimply that these will tend to be lower limits. We have clearlyidentified clumps of denser gas immersed in the low-density gas in oneof the clouds; these clumps show internal H I+H2 densities oforder 50 cm-3. Although we are not able to detect anyinterstellar Na I within 70 pc, the sizes of some of the clouds implythat their near edges are within that range of distances from the Sun.With respect to the local standard of rest the clouds move withvelocities between 19 and 54 km s-1. Their velocity vectorsdo not support the view of a local interstellar medium uniquelydominated by expansion from centers in the Scorpio-Centaurus OBassociation; our results suggest that this expansion is present in theGalactic center hemisphere but in the Galactic anticenter hemisphere isrestricted to the immediate neighborhood of the Sun.
| Rotation, pulsations and magnetic field in V 2052 Ophiuchi: A new He-strong star
V 2052 Oph is a beta Cep star with v sin i ~ 60 kms-1. The behavior of its stellar wind was studied in theultraviolet wavelength region with the IUE satellite. It revealedperiodic variations in the equivalent widths (EW) of the resonance linesof wind-sensitive ions with a period of 3.638833 d, which is identifiedas the rotational period. These variations are typical of magneticstars. Therefore time-resolved circular spectropolarimetric observationswere obtained with the Musicos échelle spectropolarimeter at the2-m Télescope Bernard Lyot (TBL) to search for a magnetic fieldin the star. Stokes V patterns were observed, the inclination andmagnetic angles were derived and a value was determined for the polarmagnetic field (250 +/- 190 G) using an oblique rotator dipole model.The spectroscopic information was used to search for periodicity inline-profile variations (lpv), radial velocity and minimum intensitycurves. Multiperiodicity was found, corresponding to radial (f_1=7.15 cd-1) and non-radial (f_2=6.82 c d-1, l=3 or 4)pulsation modes. The rotation period is also detected in rotationallymodulated observables because of the magnetic poles passing through theobserver's line of sight. We determined the stellar parameters of thestar, which was found to be chemically peculiar, in particularHe-enriched. This makes V 2052 Oph the first magneticHe-strong early B star with known pulsational properties.Based on observations obtained with the MuSiCoS spectropolarimeter atthe Observatoire du Pic du Midi, France, and by the InternationalUltraviolet Explorer, collected at NASA Goddard Space Flight Center andVillafranca Satellite Tracking Station of the European Space Agency,retrieved from the INES database.}\fnmsep\thanks{Table \ref{tabmagV} isonly available in electronic form athttp://www.edpsciences.org
| Rotation and magnetic field in the Be star omega Orionis
omega Ori is a B2IIIe star for which rotationalmodulation and non-radial pulsations (NRP) have been recentlyinvestigated from two independent observational campaigns in 1998 and1999. Putting the data of these 2 campaigns together, and adding dataobtained in 2001, we search for multiperiodicity in the line profilevariations and evidence for outbursts. From new spectropolarimetric dataobtained at the Télescope Bernard Lyot (TBL, Pic du Midi, France)in 2001 we also measure the Stokes V parameter in the polarised light.We find evidence for the presence of a weak magnetic field inomega Ori sinusoidally varying with a period of 1.29d. The equivalent widths (EW) of the wind sensitive UV resonance linesalso show a variation with the same period, which we identify as therotational period of the star. We propose an oblique rotator model andderive Bpol =530 +/- 230 G to explain the observations.Moreover, we carry out an abundance analysis and find the star to beN-enriched, a property which is shared with other magnetic stars. Wepropose omega Ori as the first known classical Bestar hosting a magnetic field.Based on observations obtained using the Musicos spectropolarimeter atthe Observatoire du Pic du Midi (France), during the MuSiCoS 98 campaign\citep{neiner}, and by \cite{balona}. Based on INES data from theInternational Ultraviolet Explorer (IUE) satellite.Table 7 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/275
| 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
| Discovery of a magnetic field in the Slowly Pulsating B star zeta Cassiopeiae
zeta Cas is a B2 IV star with vsin i = 17 kms-1. Time-resolved circular spectropolarimetric observationsof zeta Cas obtained in 2001 and 2002 with theMusicos échelle spectropolarimeter at the 2 m TélescopeBernard Lyot (TBL) show a sinusoidally varying longitudinal magneticfield with a strength between 10 G and -46 G for the averagedline-of-sight component, corresponding toB_pol=335+120-65 G. The period corresponds veryaccurately with the 5.37045 day period as derived from stellar windvariations observed in the ultraviolet. The epoch of the positivemaximum field corresponds in phase with the maximum emission in the UVwind lines. This gives compelling evidence for a magnetic rotator modelfor this star, with an unambiguous rotation period of 5.37 days. Wesearched for periodicity in line-profile variations (lpv), radialvelocity and minimum intensity curves in the ~ 400 optical spectra. Wefound a non-radial pulsation mode with l=2 +/- 1 at the frequency f=0.64c d-1. From this periodicity and from stellar parametersderived from model fits, we propose to classify zetaCas as a Slowly Pulsating B (SPB) star. This is the thirddetection of a magnetic field in an early B-type pulsating star and thefirst one in a SPB star.Based on observations obtained using the Musicos spectropolarimeter atthe Observatoire du Pic du Midi, France, and on INES data from theInternational Ultraviolet Explorer (IUE) satellite.
| Radial velocities of early-type stars in the Perseus OB2 association
We present radial velocities for 29 B- and A-type stars in the field ofthe nearby association Perseus OB2. The velocities are derived fromspectra obtained with AURELIE, via cross correlation with radialvelocity standards matched as closely as possible in spectral type. Theresulting accuracy is ~ 2-3 km s-1. We use thesemeasurements, together with published values for a few other early-typestars, to study membership of the association. The mean radial velocity(and measured velocity dispersion) of Per OB2 is 23.5 +/- 3.9 kms-1, and lies ~ 15 km s-1 away from the meanvelocity of the local disk field stars. We identify a number ofinterlopers in the list of possible late-B- and A-type members which wasbased on Hipparcos parallaxes and proper motions, and discuss thecolour-magnitude diagram of the association.Based on observations made at the Observatoire de Haute-Provence (CNRS),France.
| 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.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Kassziopeia |
| Rektaszcenzió: | 00h36m58.30s |
| Deklináció: | +53°53'49.0" |
| Vizuális fényesség: | 3.66 |
| Távolság: | 183.15 parszek |
| RA sajátmozgás: | 0 |
| Dec sajátmozgás: | 0 |
| B-T magnitude: | 3.421 |
| V-T magnitude: | 3.628 |
Katalógusok és elnevezések:
|
INHABITED SKY
