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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.
| Ultraviolet spectroscopy of the extended solar corona
The first observations of ultraviolet spectral line profiles andintensities from the extended solar corona (i.e., more than 1.5 solarradii from Sun-center) were obtained on 13 April 1979 when arocket-borne ultraviolet coronagraph spectrometer of theHarvard-Smithsonian Center for Astrophysics made direct measurements ofproton kinetic temperatures, and obtained upper limits on outflowvelocities in a quiet coronal region and a polar coronal hole. Followingthose observations, ultraviolet coronagraphic spectroscopy has expandedto include observations of over 60 spectral lines in coronal holes,streamers, coronal jets, and solar flare/coronal mass ejection (CME)events. Spectroscopic diagnostic techniques have been developed todetermine proton, electron and ion kinetic temperatures and velocitydistributions, proton and ion bulk flow speeds and chemical abundances.The observations have been made during three sounding rocket flights,four Shuttle deployed and retrieved Spartan 201 flights, and the Solarand Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy ofthe extended solar corona has led to fundamentally new views of theacceleration regions of the solar wind and CMEs. Observations with theUltraviolet Coronagraph Spectrometer (UVCS) on SOHO revealedsurprisingly large temperatures, outflow speeds, and velocitydistribution anisotropies in coronal holes, especially for minor ions.Those measurements have guided theorists to discard some candidatephysical processes of solar wind acceleration and to increase and expandinvestigations of ion cyclotron resonance and related processes.Analyses of UVCS observations of CME plasma properties and the evolutionof CMEs have provided the following: temperatures, inflow velocities andderived values of resistivity and reconnection rates in CME currentsheets, compression ratios and extremely high ion temperatures behindCME shocks, and three dimensional flow velocities and magnetic fieldchirality in CMEs. Ultraviolet spectroscopy has been used to determinethe thermal energy content of CMEs allowing the total energy budget tobe known for the first time. Such spectroscopic observations are capableof providing detailed empirical descriptions of solar energetic particle(SEP) source regions that allow theoretical models of SEP accelerationto be tailored to specific events, thereby enabling in situ measurementsof freshly emitted SEPs to be used for testing and guiding the evolutionof SEP acceleration theory. Here we review the history of ultravioletcoronagraph spectroscopy, summarize the physics of spectral lineformation in the extended corona, describe the spectroscopic diagnostictechniques, review the advances in our understanding of solar windsource regions and flare/CME events provided by ultraviolet spectroscopyand discuss the scientific potential of next generation ultravioletcoronagraph spectrometers.
| Observations of Hα, iron, and oxygen lines in B, Be, and shell stars
We carried out a spectroscopic survey of several B, Be, and shell starsin optical and near-infrared regions. Line profiles of the Hα lineand of selected Fe II and O I lines are presented.
| 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.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| Near-infrared line spectropolarimetry of hot massive stars
In order to study the inner parts of the circumstellar material aroundoptically faint infrared bright objects, we present the firstmedium-resolution spectropolarimetric data taken in the near-infrared.In this paper, we discuss Paβ line data of GL 490, a well-knownembedded massive young stellar object, and of MWC 349A and MWC 342, twooptically faint stars that are proposed to be in the pre-main-sequencephase of evolution. As a check on the method, the classical Be starζ Tau, known to display line polarization changes at opticalwavelengths, was observed as well. Three of our targets show a `lineeffect' across Paβ. For ζ Tau and MWC 349A, this line effectis due to depolarization by a circumstellar electron-scattering disc. Inboth cases, the position angle of the polarization is consistent withthat of the larger scale discs imaged at other wavelengths, validatinginfrared spectropolarimetry as a means to detect flattening on smallscales. The tentative detection of a rotation in the polarizationposition angle at Paβ in the embedded massive young stellar objectGL 490 suggests the presence of a small-scale rotating accretion discwith an inner hole - similar to those recently discovered at opticalwavelengths in Herbig Ae and T Tauri stars.
| A Statistical Study of Threshold Rotation Rates for the Formation of Disks around Be Stars
This paper presents a detailed statistical determination of theequatorial rotation rates of classical Be stars. The rapid rotation ofBe stars is likely to be linked to the ejection of gas that forms densecircumstellar disks. The physical origins of these disks are notunderstood, although it is generally believed that the ability to spinup matter into a Keplerian disk depends on how close the stellarrotation speed is to the critical speed at which the centrifugal forcecancels gravity. There has been recent disagreement between thetraditional idea that Be stars rotate between 50% and 80% of theircritical speeds and new ideas (inspired by the tendency for gravitydarkening to mask rapid rotation at the equator) that their rotation maybe very nearly critical. This paper utilizes Monte Carlo forwardmodeling to simulate distributions of the projected rotation speed(vsini), taking into account gravity darkening, limb darkening, andobservational uncertainties. A χ2 minimization procedurewas used to find the distribution parameters that best reproduceobserved vsini distributions from R. Yudin's database. Early-type(O7e-B2e) Be stars were found to exhibit a roughly uniform spread ofintrinsic rotation speed that extends from 40%-60% up to 100% ofcritical. Late-type (B3e-A0e) Be stars exhibit progressively narrowerranges of rotation speed as the effective temperature decreases; thelower limit rises to reach critical rotation for the coolest Be stars.The derived lower limits on equatorial rotation speed representconservative threshold rotation rates for the onset of the Bephenomenon. The significantly subcritical speeds found for early-type Bestars represent strong constraints on physical models of angularmomentum deposition in Be star disks.
| Properties of the Hα-emitting Circumstellar Regions of Be Stars
Long-baseline interferometric observations obtained with the NavyPrototype Optical Interferometer of the Hα-emitting envelopes ofthe Be stars η Tau and β CMi are presented. For compatibilitywith the previously published interferometric results in the literatureof other Be stars, circularly symmetric and elliptical Gaussian modelswere fitted to the calibrated Hα observations. The models areadequate for characterizing the angular distribution of theHα-emitting circumstellar material associated with these Be stars.To study the correlations between the various model parameters and thestellar properties, the model parameters for η Tau and β CMiwere combined with data for other Be stars from the literature. Afteraccounting for the different distances to the sources and stellarcontinuum flux levels, it was possible to study the relationship betweenthe net Hα emission and the physical extent of theHα-emitting circumstellar region. A clear dependence of the netHα emission on the linear size of the emitting region isdemonstrated, and these results are consistent with an optically thickline emission that is directly proportional to the effective area of theemitting disk. Within the small sample of stars considered in thisanalysis, no clear dependence on the spectral type or stellar rotationis found, although the results do suggest that hotter stars might havemore extended Hα-emitting regions.
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| First VLTI/MIDI observations of a Be star: Alpha Arae
We present the first VLTI/MIDI observations of the Be star alpha Ara (HD158 427), showing a nearly unresolved circumstellar disk in the N band.The interferometric measurements made use of the UT1 and UT3 telescopes.The projected baselines were 102 and 74 meters with position angles of 7° and 55°, respectively. These measurements put an upper limiton the envelope size in the N band under the uniform disk approximationof φmax= 4±1.5 mas, corresponding to 14R*, assuming R*=4.8 Rȯ and theHipparcos distance of 74 pc. On the other hand the disk density must belarge enough to produce the observed strong Balmer line emission. Inorder to estimate the possible circumstellar and stellar parameters wehave used the SIMECA code developed by Stee et al. (1995, A&A, 300,219) and Stee & Bittar (2001, A&A, 367, 532). Optical spectrataken with the échelle instrument Heros and the ESO-50 cmtelescope, as well as infrared ones from the 1.6m Brazilian telescopewere used together with the MIDI spectra and visibilities. Theseobservations place complementary constraints on the density and geometryof the alpha Ara circumstellar disk. We discuss the potential truncationof the disk by a companion and we present spectroscopic indications of aperiodic perturbation of some Balmer lines.
| 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
| Close binary stars in ob-association regions i. preliminary investigation
We performed a sample of O- and B-eclipsing binary stars inOB-association regions and obtained the preliminary list of 147 binariesin 45 OB-association regions. We tried to elucidate the question whether(or not) the close binaries belong to corresponding OB-associations,from the commonness of their proper motions, radial velocities anddistances. Based on the completeness of the data,the binaries aredevided into three groups and the scheme for calculation of degree ofbelonging of stars to OB-associations is developed. Necessary data arenot available for nine systems and they are given in a specific table.For 12 cases, the binaries project onto the regions of two associations.We show that 33 (22.3%) close binary stars are members, 65 (43.9%) areprobable members and 39 (26.4%) are less probable members of theOB-associations. We find that 11 binaries belong to the Galaxybackground. The comparison of the distributions of orbital periods forthe binaries in OB-associations and for O-, B-binaries of the Galaxybackground shows their considerable differences in the vicinity of thetwo-day period.
| Amateur Spectroscopy of Hot Stars: Long Term Tracking of Circumstellar Emission
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| High-Precision Optical Interferometry and Application to Be Stars
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| Uber die Variabilitat des H-alpha-Emissionslinienprofils im Spektrum von zeta Tau.
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| Determination of the night sky background around the Crab pulsar using its optical pulsation
The poor angular resolution of imaging γ-ray telescopes is offsetby the large reflector areas of next generation telescopes such as MAGIC(17 m diameter), which makes the study of optical emission associatedwith some γ-ray sources feasible. Furthermore, the extremely fasttime response of photomultipliers (PMs) makes them ideal detectors forfast (subsecond) optical transients and periodic sources like pulsars.The optical pulse of the Crab pulsar was detected with the HEGRA CT1central pixel using a modified PM, similar to the future MAGIC cameraPMs.The HEGRA CT1 telescope is a 2.1 m radius Čerenkov telescope,which has seen the Crab optical pulsations. The correspondingobservation time required for a detection is 200 s, but MAGIC will onlyrequire ~30 s if the same strategy with standard PMs is used.The constancy of the pulsed fraction (with a relatively small error)during the 2 h CT1 measurements shows that the pointing did not changeduring the observations. The purpose of these periodic observations wasto determine the light of the night sky (LONS) for the galacticanticenter Crab region, and to compare it with the LONS for a nearbybright star (ζ Tauri). These obtained LONS values vary between 4.6and4.3×1012phm-2s-1sr-1(with a coarse estimated error of 10%), being the first value from thetiming signature of the Crab pulsar, and the second one was derived fromthe technique using the ζ Tauri standard star. Our results arebetween 2.5 and 3 times larger than the previously measured LONS(outside the galactic plane) of (1.7+/-0.4)×1012phm-2s-1sr-1, asexpected since the Crab pulsar is in the galactic plane, which implies aslightly higher energy threshold for Crab observations, if the highervalue of CT1 measured LONS rate for galactic sources is used.
| The Circumstellar Envelope of ζ Tauri through Optical Interferometry
We present optical interferometric observations of the Be star ζTauri obtained using the Navy Prototype Optical Interferometer (NPOI).The multichannel capability of the NPOI allows a high-quality internalcalibration of the squared visibilities corresponding to the Hαemission from the circumstellar environment. The observations suggest astrong departure from circular symmetry and thus are described by anelliptical Gaussian model. We use a nonlinear least-squares fit to thedata to obtain the likeliest parameters, and the correspondinguncertainties are determined using a Monte Carlo simulation. We obtain3.14+/-0.21 mas for the angular size of the major axis, -62.3d+/-4.4dfor the position angle, and 0.310+/-0.072 for the axial ratio. Bycomparing our results with those already in the literature, we concludethat the model parameters describing the general characteristics of thecircumstellar envelope of ζ Tau appear to be stable on timescalesof years. We also compare our results with the known parametersdescribing the binary nature of ζ Tau, and we conclude that theenvelope surrounds only the primary component and is well within itsRoche lobe.
| The circumstellar environment of the star V923 Aquilae
V923 Aquilae is a Be shell star that displaysemission in Hα, cyclic V/R variations and variations in theintensity of the near infrared continuum. The star presents radialvelocity variations arising from a superposition of a long-term cycleand an orbital motion with a period of 214.756 days (Koubský etal. \cite{koub}). To investigate the physical properties and thegeometry of the circumstellar envelope of this star, we have analysedits spectra in the UV and visual range. We have selected Fe II lines toderive temperatures and location of the line-forming regions. Ourresults indicate that the dimensions of the circumstellar envelope varywith time and this variation correlates with the orbital period of214.75 days. On the other hand, we determined a period of 6.8 years forV/R cyclic variations in Hα. This period is in accordance with thelong-term cycle in the radial velocity and U band photometricobservations. We suggest that the V/R variability in the Hα lineis linked to the behavior of the subsonic wind structure.
| A Millennium of Shattered Stars
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| Optical interferometry in astronomy
Here I review the current state of the field of optical stellarinterferometry, concentrating on ground-based work although a briefreport of space interferometry missions is included. We pause both toreflect on decades of immense progress in the field as well as toprepare for a new generation of large interferometers just now beingcommissioned (most notably, the CHARA, Keck and VLT Interferometers).First, this review summarizes the basic principles behind stellarinterferometry needed by the lay-physicist and general astronomer tounderstand the scientific potential as well as technical challenges ofinterferometry. Next, the basic design principles of practicalinterferometers are discussed, using the experience of past and existingfacilities to illustrate important points. Here there is significantdiscussion of current trends in the field, including the new facilitiesunder construction and advanced technologies being debuted. This decadehas seen the influence of stellar interferometry extend beyond classicalregimes of stellar diameters and binary orbits to new areas such asmapping the accretion discs around young stars, novel calibration of thecepheid period-luminosity relation, and imaging of stellar surfaces. Thethird section is devoted to the major scientific results frominterferometry, grouped into natural categories reflecting these currentdevelopments. Lastly, I consider the future of interferometry,highlighting the kinds of new science promised by the interferometerscoming on-line in the next few years. I also discuss the longer-termfuture of optical interferometry, including the prospects for spaceinterferometry and the possibilities of large-scale ground-basedprojects. Critical technological developments are still needed to makethese projects attractive and affordable.
| Classical Be Stars
Recent results for classical Be stars are reviewed and links to generalastrophysics are presented. Classical Be stars are B-type stars close tothe main sequence that exhibit line emission over the photosphericspectrum. The excess is attributed to a circumstellar gaseous componentthat is commonly accepted to be in the form of an equatorial disk. Since1988, when the last such review was published, major progress has beenmade. The geometry and kinematics of the circumstellar environment canbe best explained by a rotationally supported relatively thin disk withvery little outflow, consistent with interferometric observations. Thepresence of short-term periodic variability is restricted to the earliertype Be stars. This variation for at least some of these objects hasbeen shown to be due to nonradial pulsation. For at least one star,evidence for a magnetic field has been observed. The mechanismsresponsible for the production and dynamics of the circumstellar gas arestill not constrained. Observations of nonradial pulsation beatingphenomena connected to outbursts point toward a relevance of pulsation,but this mechanism cannot be generalized. Either the evidence that Bestars do not form a homogeneous group with respect to disk formation isgrowing or the short-term periodic variability is less important thanpreviously thought. The statistics of Be stars investigated in openclusters of the Milky Way and the Magellanic Clouds has reopened thequestion of the evolutionary status of Be stars. The central B star is afast rotator, although theoretical developments have revived thequestion of how high rotational rates are, so the commonly quoted meanvalue of about 70%-80% of the critical velocity may just be a lowerlimit. Be stars are in a unique position to make contributions toseveral important branches of stellar physics, e.g., asymmetricmass-loss processes, stellar angular momentum distribution evolution,astroseismology, and magnetic field evolution.
| The circumstellar envelope of zeta Tauri through optical interferometry.
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| Non-radially pulsating Be stars
Based on more than 3000 high-resolution echelle spectra of 27 early-typeBe stars, taken over six years, it is shown that the short-term periodicline profile variability of these objects is due to non-radialpulsation. The appearance of the line profile variability depends mostlyon the projected rotational velocity v sin i and thus, since all Bestars rotate rapidly, on the inclination i. The observed variability ofthe investigated stars is described, and for some of them line profilevariability periods are given for the first time. For two of theinvestigated stars the line profile variability was successfully modeledas non-radial pulsation with l=m=+2 already in previous works. Since Bestars with similarly low v sin i share the same variability properties,these are in general explainable under the same model assumptions. Theline profile variability of stars with higher v sin i is different fromthe one observed in low v sin i stars, but can be reproduced by thesame model, if only the model inclination is modified to more equatorialvalues. Only for a few stars with periodic line profile variability thel=m=2 non-radial pulsation mode is not able to provide a satisfyingexplanation. These objects might pulsate in different modes (e.g.tesseral ones, l != |m|). Almost all stars in the sample show traces ofoutburst-like variability, pointing to an ephemeral nature of themass-loss phenomenon responsible for the formation of the circumstellardisk of early-type Be stars, rather than a steady star-to-disk masstransfer. In addition to the variability due to non-radial pulsationpresent in most stars, several objects were found to show other periodsresiding in the immediate circumstellar environment. The presence ofthese secondary periods is enhanced in the outburst phases. Short-livedaperiodic phenomena were clearly seen in two stars. But, given theunfavourable sampling of our database to follow rapid variability oftransient nature, they might be more common. Only in two out of 27 starsshort-term spectroscopic variability was not detected at all.Based on observations collected at the European Southern Observatory atLa Silla, Chile, 55.D-0502, 56.D-0381, 58.D-0697, 62.H-0319, 63.H-0080,64.H-0548, and 267.D-5702, the German-Spanish Astronomical Centre, CalarAlto, operated by the Max-Planck-Institute for Astronomy, Heidelberg,jointly with the Spanish National Commission for Astronomy, and onobservations with the Wendelstein 80-cm and the Ondřejov 2-mtelescopes, both equipped with the HEROS spectrograph provided by theLandessternwarte Heidelberg.
| Stellar and circumstellar activity of the Be star omega CMa. II. Periodic line-profile variability
The rapid line-profile variability of the early-type and pole-on Be staromega CMa between 1996 and 2002 is characterizedacross the complete optical spectrum, for quiescent phases as well asfor outbursts. Owing to different and changing line-profile variabilitypatterns, amplitudes and gamma -velocities are different from line toline and are variable on a time scale of months. A comprehensive timeseries analysis was performed on the modes of a set of selected lines(after individual seasonal normalization to avoid biases). At a highlevel of confidence, only the well-known 1.37-d period could be found inphotospheric lines not contaminated by the disk. Outside majoroutbursts, when the star is at its photometric ground state, the phasecoherence of the variability is very robust. During strong outbursts,when the star is visually bright, the period may either be very slightlydifferent or phase jumps may occur. The present observations do not havethe sampling necessary to distinguish between these possibilities.Harmanec's (\citeyear{1998A&A...334..558H}) report of continuous,cyclic period variations cannot be confirmed. Arguments are presentedthat temporary period changes may be related to interactions between thephotospheric non-radial pulsation and the disk when (during outbursts)these two domains are in contact with one another. This result does notseem to be an artifact of the also previously reported transientperiodicities near 1.49-d, which are prominent during outbursts and seemto be anchored in the exo-photosphere. However, if not properly takeninto account, they may easily lead to false conclusions about multipleor variable periods. In the Be star mu Cen, which has a similarspectral type, outbursts are triggered by the beating of two or morenon-radial pulsation modes \citep{1998hcsp.proc..343R}. Since omega CMa,too, undergoes outbursts although its photospheric variability issingle-periodic, the case of mu Cen cannot be generalized to theactivity of all early-type Be stars or to the Be phenomenon at large.Based on observations collected at the European Southern Observatory atLa Silla, Chile (ESO proposals No. 55.D-0502, 56.D-0381, 58.D-0697,64.H-0548, 62.H-0319 and 267.D-5702).
| A new template background estimate for source searching in TeV gamma -ray astronomy
A new method is described that permits quickly and easily, a2-dimensional search for TeV gamma -ray sources over large fields ofview ( ~ 6o) with instruments utilising the imagingatmospheric Čerenkov technique. It employs as a backgroundestimate, events normally rejected according to a cosmic-ray backgroundrejection criterion based on image shape, but with reconstructeddirections overlapping the source of interest. This so-called templatebackground model is demonstrated using example data taken with thestereoscopic HEGRA System of Čerenkov Telescopes. Discussionincludes comparisons with a conventional background estimate andlimitations of the model. The template model is well suited to thesearch for point-like, moderately extended sources and combinationsthereof, and compensates well for localised systematic changes incosmic-ray background response.
| Hot stars mass-loss studied with Spectro-Polarimetric INterferometry (SPIN)
We present a prospective work undertaken on Spectro-PolarimetricINterferometry (SPIN). Our theoretical studies suggest that SPIN is apowerful tool for studying the mass loss from early type stars wherestrong Thomson scattering is present. Based on Monte Carlo simulations,we computed the expected SPIN signal for numerous hot star spectraltypes covering a broad range of geometries and optical depths. The SPINtechnique is based on the detection and comparison of the fringecharacteristics (complex visibility) between two perpendiculardirections of polarization. The most obvious advantage is its ability todetermine the polarization distribution in spherical winds for which nodetection of polarization is achievable by classical techniques. Inparticular, we demonstrate that the SPIN technique is very sensitive tothe beta parameter from the so-called ``beta velocity law'' foroptically thin winds. Moreover, the location where the bulk ofpolarization is generated can be defined accurately. The requiredsensitivity for studying main sequence OB star winds is still verydemanding (inferior to 0.5%), but the signal expected from denser windsor extended atmospheres is well within the capabilities of existinginterferometers. The visibility curves obtained in two perpendicularpolarizations for LBVs or WR stars can differ by more than 15%, andtheir corresponding limb-darkened radii obtained by the fit of thesecurves by more than 35%. The signal expected from the extendedcircumstellar environment of Be stars and B[e] appears also to be easyto detect, relaxing the required instrumental accuracy to 1%. For thesespectral types, the SPIN technique provide a good tool to extract thehighly polarized and spatially confined envelope contribution from thebright star emission.It must be pointed out that the astrophysical environments investigatedhere offer a large panel of SPIN observing conditions in terms ofgeometry and polarization degree. The behavior of the SPIN observablescan be transposed, at least qualitatively, to other astronomical objectsfor which important local polarization is foreseen.
| Estimation of the mass loss, opening angle and mass of Be circumstellar disks from Brmathsf γ continuum emission and interferometric measurements
Using the SIMECA code developed by Stee & Araùjo(\cite{stee1}); Stee et al. (\cite{stee2}) for Be stars we obtain acorrelation between the mass loss rates {dot M} and the Brgammacontinuum luminosity as a function of the opening angle of the disk. Weshow that this correlation is similar to those obtained by Scuderi etal. (\cite{scuderi}) for O-B supergiants. We found that the wind densityat the base of the photosphere, from a sample of 8 Be stars, liesbetween 10-13 and 10-12 g cm-3. We alsopresent a relationship between the mass of the circumstellar disk andthe 2.16 mu m flux. Finally we emphasize how interferometricmeasurements can help to estimate the wind density and we present asample of 16 Be stars with predicted visibilities that can be observedwith the VLTI.
| Autocorrelation Analysis of Hipparcos Photometry of Short-Period Be Stars
We have used Hipparcos epoch photometry and a form of autocorrelationanalysis to investigate the amplitude and timescale of the short-periodvariability of 82 Be stars, including 46 Be stars that were analyzed byHubert & Floquet using Fourier and CLEAN analysis and 36 other Bestars that were suspected of short-period variability. Our method hasgiven useful information for about 84% of these stars; for the rest, thetime distribution of the Hipparcos epoch photometry limits thecapability of our technique.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bika |
| Rektaszcenzió: | 05h37m38.70s |
| Deklináció: | +21°08'33.0" |
| Vizuális fényesség: | 3 |
| Távolság: | 127.877 parszek |
| RA sajátmozgás: | 0 |
| Dec sajátmozgás: | 0 |
| B-T magnitude: | 2.759 |
| V-T magnitude: | 2.931 |
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