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Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Detection of X-ray emission from β Pictoris with XMM-Newton: a cool corona, a boundary layer or what?
β Pictoris (HR 2020) is the most prominentprototype of stars with circumstellar disks and has generated particularinterest in the framework of young planetary systems. Given its spectraltype A5, stellar activity is not expected. Nevertheless, resonance linesof C iii and O vi typical for a chromosphere and transition region havebeen unambiguously detected with FUSE. We present results from anXMM-Newton observation of β Pic and find evidence for X-rayemission. In particular, we detected an emission of O vii at 21.6Å with the MOS detectors. These findings present a challenge forthe development of both stellar activity and disk models. We discuss andinvestigate various models to explain the observed emission includingthe presence of a cool corona and a boundary layer.
| On the Flux of Extrasolar Dust in Earth's Atmosphere
Micron-size extrasolar dust particles have been convincingly detected bysatellites. Larger extrasolar meteoroids (5-35 μm) have most likelybeen detected by ground-based radar at Arecibo and New Zealand. Wepresent estimates of the minimum detectable particle sizes and thecollecting areas for both radar systems. We show that particles largerthan ~10 μm can propagate for tens of parsecs through theinterstellar medium, opening up the possibility that ground-based radarsystems can detect AGB stars, young stellar objects such as T Tauristars, and debris disks around Vega-like stars. We provide analyticaland numerical estimates of the ejection velocity in the case of a debrisdisk interacting with a Jupiter-mass planet. We give rough estimates ofthe flux of large micrometeoroids from all three classes of sources.Current radar systems are unlikely to detect significant numbers ofmeteors from debris disks such as β Pictoris. However, we suggestimprovements to radar systems that should allow for the detection ofmultiple examples of all three classes.
| A Far Ultraviolet Spectroscopic Explorer Survey of Late-Type Dwarf Stars
We describe the 910-1180 Å spectra of seven late-type dwarf starsobtained with the Far Ultraviolet Spectroscopic Explorer (FUSE)satellite. The stars include Altair (A7 IV), Procyon (F5 IV-V), αCen A (G2 V), AB Dor (K1 V), α Cen B (K2 V), ɛ Eri (K2 V),and AU Mic (M0 V). We present line identifications, fluxes, Dopplershifts, and widths. Doppler shifts are measured with respect toheliocentric wavelength scales determined from interstellar absorptionlines, and are compared with transition region line shifts seen inHubble Space Telescope (HST) ultraviolet spectra. For the warmer starsthe O VI lines extend the trend of increasing redshift with lineformation temperature, but for the cooler stars the O VI line redshiftsare essentially zero. The C III and O VI lines of most stars in thesample are best fit with two Gaussians, and we confirm the correlationof increasing importance of the broad component with increasing stellaractivity. The nonthermal velocities of the narrow component are subsonicand exhibit a trend toward larger velocities with decreasing surfacegravity, while the nonthermal velocities of the broad components show noobvious trend with stellar gravity. The C III and O VI lines of Altairshow unique broad horned profiles. Two flares were observed on AU Mic.One shows increasing continuum flux to shorter wavelengths, which weinterpret as free-free emission from hot plasma, and relatively narrow,redshifted C III and O VI emission. The other shows very broad lineprofiles.
| Limits on Chromospheres and Convection among the Main-Sequence A Stars
In deeply convective stars, the nonthermal energy required to heat thechromosphere ultimately is supplied by turbulent magnetoconvection.Because the early and middle A stars have very shallow convectivelayers, they are not expected to produce enough magnetoconvective powerto sustain luminous chromospheres or hot coronae. Here we describe asearch for chromospheric emission in the far-ultraviolet (905-1185Å) spectra of seven main-sequence A stars, based on observationsfrom the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope. Oursurvey spans the interval in effective temperature along the mainsequence over which powerful subsurface convection zones and hencechromospheric emission are expected to vanish. The presence or absenceof high-temperature emissions in our FUSE spectra therefore can be usedto identify the locus for the transition from convective to radiativeenvelopes-a change in stellar structure that is difficult to assess byother means. We present our observations and analysis of the subcoronalemission lines of C III λλ977, 1175 and O VIλλ1032, 1037, which bracket a range in formationtemperatures from 50,000 to 300,000 K. To supplement our FUSEobservations, we also report Goddard High Resolution Spectrographmeasurements of Si III λ1206 and H I Lyα λ1215,obtained from archival observations of the Hubble Space Telescope, aswell as X-ray measurements from previous ROSAT survey and pointedobservations. We detected C III and O VI emission features in the FUSEspectra of the coolest stars of our sample, at Teff<~8200K. When normalized to the bolometric luminosities, the detectedemission-line fluxes are comparable to solar values. We detected none ofthe hotter stars in our survey at Teff>=8300 K. Upperlimits on the normalized flux in some instances approach 40 times lessthan solar. Within an uncertainty in the effective temperature scale ofup to several hundred kelvins, our FUSE observations indicate that thetransition between convective and radiative stellar envelopes takesplace at, or very near, the point along the main sequence where stellarstructure models predict and, moreover, that the changeover occurs veryabruptly, over a temperature interval no greater than ~100 K in width.Our FUSE sample also includes two binary stars. In both cases, thenarrow UV line profiles we have observed suggest that thehigh-temperature emission is most likely associated with the late-typecompanions rather than the A stars themselves. Based on observationsmade with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer,operated for NASA by Johns Hopkins University under NASA contractNAS5-32985.
| The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within~30 pc of the Sun is determined from optical and ultraviolet absorptionline data, after omitting from the sample stars with circumstellar disksor variable emission lines and the active variable HR 1099. A total of96 velocity components toward the remaining 60 stars yield a streamingvelocity through the local standard of rest of -17.0+/-4.6 kms-1, with an upstream direction of l=2.3d, b=-5.2d (usingHipparcos values for the solar apex motion). The velocity dispersion ofthe interstellar matter (ISM) within 30 pc is consistent with that ofnearby diffuse clouds, but present statistics are inadequate todistinguish between a Gaussian or exponential distribution about thebulk flow velocity. The upstream direction of the bulk flow vectorsuggests an origin associated with the Loop I supernova remnant.Groupings of component velocities by region are seen, indicatingregional departures from the bulk flow velocity or possibly separateclouds. The absorption components from the cloudlet feeding ISM into thesolar system form one of the regional features. The nominal gradientbetween the velocities of upstream and downstream gas may be an artifactof the Sun's location near the edge of the local cloud complex. The Sunmay emerge from the surrounding gas patch within several thousand years.
| 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
| Ages of A-Type Vega-like Stars from uvbyβ Photometry
We have estimated the ages of a sample of A-type Vega-like stars byusing Strömgren uvbyβ photometric data and theoreticalevolutionary tracks. We find that 13% of these A stars have beenreported as Vega-like stars in the literature and that the ages of thissubset run the gamut from very young (50 Myr) to old (1 Gyr), with noobvious age difference compared to those of field A stars. We clearlyshow that the fractional IR luminosity decreases with the ages ofVega-like stars.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is 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/367/297
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Polarization measurements of Vega-like stars
Optical linear polarization measurements are presented for about 30Vega-like stars. These are then compared with the polarization observedfor normal field stars. A significant fraction of the Vega-like starsare found to show polarization much in excess of that expected to be dueto interstellar matter along the line of sight to the star. The excesspolarization must be intrinsic to the star, caused by circumstellarscattering material that is distributed in a flattened disk. Acorrelation between infrared excess and optical polarization is foundfor the Vega-like stars.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| UV observations of B to F-type stars.
Not Available
| Infrared Space Observatory Photometric Search of Main-Sequence Stars for Vega-Type Systems
We obtained 3.6-20 μm photometry of 38 bright [IRAS F_nu(12μm)>0.7 Jy] main-sequence stars with the Infrared SpaceObservatory (ISO). Observations were conducted with the ISOPHOTinstrument, in the single-pointing photometry mode, through filters at3.6, 11.5, and 20.0 mum. We searched for excess (Vega-type) emissionfrom dust at temperatures >~100 K, located at ~1-60 AU from thestars. We thus sampled dust at warm, terrestrial material temperaturesand at cool (~100 K) temperatures of possible Kuiper Belt-type regionsin these systems. We detected 20 μm excesses from ~14% of oursources, but we did not detect 11.5 μm excesses from any of them. Wepresent single-temperature blackbody models of the location and densityof dust emission around 10 stars, two of them (29 Cyg and Gl 816) withexcesses newly reported here. We make a thorough comparison of ISO andIRAS data on our target stars and propose a new calibration procedurefor ISOPHOT staring measurements at 3.6, 11.5, and 20 mum.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Spectroscopy of Hot Stars in the Galactic Halo. II. The Identification and Classification of Horizontal-Branch and Other A-Type Stars
We discuss a spectroscopic and photometric technique that enables theidentification and classification of field horizontal-branch (FHB) andother A-type stars, even from relatively low signal-to-noise ratiomedium-resolution spectra. This technique makes use of broadband UBVcolors predicted from model atmosphere calculations and Balmer lineprofiles and Ca II K equivalent widths determined from synthetic spectrato estimate the physical parameters T_eff, log g, and [Fe/H] for starsin the effective temperature range 6000-10,000 K. A comparison of ourmethod with high signal-to-noise ratio spectra of standard starsindicates a scatter in the derived parameters of sigma(T_eff)=+/-250 K,sigma(log g)=+/-0.14 dex, and sigma([Fe/H])=+/-0.12 dex. This precisionallows for a separation of low surface gravity FHB and other, generallyhigher surface gravity, A-type (and somewhat later) stars. We alsodevelop a synthetic-template comparison technique, which is veryeffective in the identification of metallic-line and peculiar A-typestars. A detailed investigation of the influence of noise in the spectraon the determination of physical parameters shows that, for spectra withsignal-to-noise ratios in the range 10
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Determination of the temperatures of selected ISO flux calibration stars using the Infrared Flux Method
Effective temperatures for 420 stars with spectral types between A0 andK3, and luminosity classes between II and V, selected for a fluxcalibration of the Infrared Space Observatory, ISO, have been determinedusing the Infrared Flux Method (IRFM). The determinations are based onnarrow and wide band photometric data obtained for this purpose, andtake into account previously published narrow-band measures oftemperature. Regression coefficients are given for relations between thedetermined temperatures and the photometric parameters (B2-V1), (b-y)and (B-V), corrected for interstellar extinction through use ofHipparcos parallaxes. A correction for the effect of metallicity on thedetermination of integrated flux is proposed. The importance of aknowledge of metallicity in the representation of derived temperaturesfor Class V, IV and III stars by empirical functions is discussed andformulae given. An estimate is given for the probable error of eachtemperature determination. Based on data from the ESA HipparcosAstrometry Satellite.
| Towards a fundamental calibration of stellar parameters of A, F, G, K dwarfs and giants
I report on the implementation of the empirical surface brightnesstechnique using the near-infrared Johnson broadband { (V-K)} colour assuitable sampling observable aimed at providing accurate effectivetemperatures of 537 dwarfs and giants of A-F-G-K spectral-type selectedfor a flux calibration of the Infrared Space Observatory (ISO). Thesurface brightness-colour correlation is carefully calibrated using aset of high-precision angular diameters measured by moderninterferometry techniques. The stellar sizes predicted by thiscorrelation are then combined with the bolometric flux measurementsavailable for a subset of 327 ISO standard stars in order to determineone-dimensional { (T, V-K)} temperature scales of dwarfs and giants. Theresulting very tight relationships show an intrinsic scatter induced byobservational photometry and bolometric flux measurements well below thetarget accuracy of +/- 1 % required for temperature determinations ofthe ISO standards. Major improvements related to the actual directcalibration are the high-precision broadband { K} magnitudes obtainedfor this purpose and the use of Hipparcos parallaxes for dereddeningphotometric data. The temperature scale of F-G-K dwarfs shows thesmallest random errors closely consistent with those affecting theobservational photometry alone, indicating a negligible contributionfrom the component due to the bolometric flux measurements despite thewide range in metallicity for these stars. A more detailed analysisusing a subset of selected dwarfs with large metallicity gradientsstrongly supports the actual bolometric fluxes as being practicallyunaffected by the metallicity of field stars, in contrast with recentresults claiming somewhat significant effects. The temperature scale ofF-G-K giants is affected by random errors much larger than those ofdwarfs, indicating that most of the relevant component of the scattercomes from the bolometric flux measurements. Since the giants have smallmetallicities, only gravity effects become likely responsible for theincreased level of scatter. The empirical stellar temperatures withsmall model-dependent corrections are compared with the semiempiricaldata by the Infrared Flux Method (IRFM) using the large sample of 327comparison stars. One major achievement is that all empirical andsemiempirical temperature estimates of F-G-K giants and dwarfs are foundto be closely consistent between each other to within +/- 1 %. However,there is also evidence for somewhat significant differential effects.These include an average systematic shift of (2.33 +/- 0.13) % affectingthe A-type stars, the semiempirical estimates being too low by thisamount, and an additional component of scatter as significant as +/- 1 %affecting all the comparison stars. The systematic effect confirms theresults from other investigations and indicates that previousdiscrepancies in applying the IRFM to A-type stars are not yet removedby using new LTE line-blanketed model atmospheres along with the updatedabsolute flux calibration, whereas the additional random component isfound to disappear in a broadband version of the IRFM using an infraredreference flux derived from wide rather than narrow band photometricdata. Table 1 and 2 are only available in the electronic form of thispaper
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| Dust around Main-Sequence Stars: Nature or Nurture by the Interstellar Medium?
Dust from the interstellar medium (ISM) can collide with and destroyparticles in the circumstellar dust disks around main-sequence stars(Vega/ beta Pic stars). Two current theories tying the occurrence of theVega/ beta Pic phenomenon to the erosive influence of the ISM arecritically reconsidered here. Using the local standard of rest frame, wefind little evidence for a correlated motion (streaming) of prominentdisk systems, which one theory suggests would result from a passageabout 107 yr ago of these stars, but not the control A-type stars,through the nearby Lupus-Centaurus interstellar cloud complex. Moreover,the prototype system of beta Pic could not have retained dust producedin such a passage for much longer than 104 yr. We show theoreticallythat the ISM sandblasting of disks has minor importance for thestructure and evolution of circumstellar disks, except perhaps in theiroutskirts (usually >400 AU from the stars), where under favorableconditions it may cause asymmetries in observed brightness and color.The ISM neither produces the disks (as in one theory) nor depletes andeliminates them with time (as in another theory), because typical ISMgrains are subject to strong radiative repulsion from A- and F-typedwarfs (a few to 100 times stronger than gravity). Atypically large ISMgrains are not repelled strongly, but are unimportant on account oftheir small number density. Dust production and destruction in betaPic-type disks results mainly from their collisional nature enhanced bythe radiatively produced eccentricities of particle orbits, rather thanfrom nurture in a hostile ISM. The residence times of the few-microndust grains predominant in the densest part of the beta Pic disk is only104 yr, or a few dozen orbital periods. Submicronic debris is blown outas beta meteoroids, carrying away from this system an equivalent of thesolar system's total mass in solids (~120 Earth masses) in only ~65 Myr.This rate of collisional erosion exceeds almost 108 times that of thezodiacal light disk of our own system. A massive and relatively young(<~102 Myr) planetesimal disk appears to surround beta Pic, destinedto decline in dust density over time comparable to its age. Other dustdisks, like those around Fomalhaut and Vega, contain much less dust andmay be much older than the beta Pic disk, but like the beta Pic diskthey are also derived from and replenished many times during theirlifetimes by unseen parent bodies.
| High Chromospheres of Late A Stars
We report the detection of N V 1239 A transition region emission inHST/Goddard High Resolution Spectrograph spectra of the A7 V stars,alpha Aql and alpha Cep. Our observations provide the first directevidence of 1--3 x 105 K material in the atmospheres of normal A-typestars. For both stars, and for the mid A-type star tau 3 Eri, we alsoreport the detection of chromospheric emission in the Si III 1206 Aline. At a B-V color of 0.16 and an effective temperature of ~8200 K,tau 3 Eri becomes the hottest main-sequence star known to have achromosphere and, thus, an outer convection zone. We see no firmevidence that the Si III line surface fluxes of the A stars are anylower than those of moderately active, solar-type G and K stars. Thiscontrasts sharply with their coronal X-ray emission, which is more than100 times weaker than that of the later type stars. Given the strengthof the N V emission observed here, it now appears unlikely that theX-ray faintness of A stars is due to their forming very cool, <=1 MKcoronae. An alternative explanation in terms of mass loss in coronalwinds remains a possibility, though we conclude from moderate resolutionspectra of the Si III lines that such winds, if they exist, do notpenetrate into the chromospheric Si III--forming layers of the star,since the profiles of these lines are not blueshifted and may well beredshifted with respect to the star.
| Investigation of a group of rapidly rotating A stars having an emission excess at IRAS wavelengths.
Not Available
| Local interstellar cloud electron density from magnesium and sodium ionization: a comparison.
The ambient interstellar plasma density (i.e. the plasma density of theinterstellar medium surrounding the Sun) directly governs the structureand the size of our heliosphere. Information on this density can bederived from the ionization states of the interstellar species which canbe detected in absorption along the paths to the nearby stars, and whichcan be shown to belong to the Local Interstellar Cloud (LIC). Echellespectra around the resonance lines of neutral and singly ionizedmagnesium have been obtained for the nearby star δ Cas with theGoddard High-Resolution Spectrograph (GHRS) on board the Hubble SpaceTelescope. While apparently a unique velocity component (a uniquecloudlet) is detected in both lines of the MgII λλ2800resonance doublet, at the expected Doppler shift for the LIC, anextremely small λλ2853 Mg line is also detected at aDoppler shift compatible with the LIC motion, allowing a measurement ofthe LIC MgII/MgI ratio, here found to be 400 (-130,+190). This ratioimplies a mean electron density of about 0.28 (-0.14, +0.34)cm^-3^ alongthis line-of-sight, if equilibrium conditions prevail, and if T=7000K,when using the most recent recombination and charge-exchange rates. ThisMgII/MgI ratio is larger than for Sirius (R=~220), which lies at 110^o^from δ Cas, providing some evidence for an ionization gradient inthe local cloud. Such an electron density implies a surprisingly largeionization degree, and the upper range of the interval is incompatiblewith the minimum size of our heliosphere. A second and independent wayto derive the electron density along the path to δ Cas uses thesimplicity of the LIC geometry in the sky region surrounding the star,which allows an estimate of the H column-density to the star, as well asprevious ground-based CaII data, and the measured NaI/CaII ratio andcalcium depletion in the LIC. The resulting most probable electrondensity at 7000K, 0.05cm^-3^, provides a new evidence for a significantionization degree of the LIC, but is a factor of four to five smallerthan the value based on magnesium. The upper limit of 0.19cm^-3^ remainsconsistent with the minimum size of our heliosphere. The existence of acommon interval to the two determinations: n_e_=0.14-0.19cm^-3^ impliesthat ionization equilibrium within the LIC is not totally precluded.However, the lack of a real convergence deserves further observations,involving other interstellar species. The common interval is compatiblewith the result of Frisch (1994Sci...265.1443F), from anomalous C and Ocosmic rays abundances, if carbon is not too much filtered at theheliospheric interface. On the other hand, the sodium-based mostprobable value is in agreement with neutral hydrogen deceleration at theheliospheric interface for the Baranov two-shocks model, as well as withthe ionization degree of hydrogen implied by local EUV sources.
| Transformations from Theoretical Hertzsprung-Russell Diagrams to Color-Magnitude Diagrams: Effective Temperatures, B-V Colors, and Bolometric Corrections
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...469..355F&db_key=AST
| Characteristics of Nearby Interstellar Matter
There is a warm tenuous partially ionized cloud (T~104 K,n(HI)~0.1 cm-3, n(HII)~0.22-0.44 cm-3) surroundingthe solar system which regulates the environment of the solar system,determines the structure of the heliopause region, and feeds neutralinterstellar gas into the inner solar system. The velocity (V ~ -20 kms-1 from l~335 deg, b~0 deg in the local standard of rest)and enhanced CaII and FeII abundances of this cloud suggest an origin asevaporated gas from cloud surfaces in the Scorpius-CentaurusAssociation. Although the soft X-ray emission attributed to the 'LocalBubble' is enigmatic, optical and ultraviolet data are consistent withbubble formation caused by star formation epochs in theScorpius-Centaurus Association as regulated by the nearby spiral armconfiguration. The cloud surrounding the solar system (the 'localfluff') appears to be the leading region of an expanding interstellarstructure (the 'squall line') which contains a magnetic field causingpolarization of the light of nearby stars, and also absorption featuresin nearby upwind stars. The velocity vectors of the solar system andlocal fluff are perpendicular in the local standard of rest. Combiningthis information with the low column densities seen toward Sirius in theanti-apex direction, and the assumption that the cloud velocity vectoris parallel to the surface normal, suggests that the Sun entered thelocal fluff within the historical past (less than 10,000 years ago) andis skimming the surface of the cloud. Comparison of magnesiumabsorption lines toward Sirius and anomalous cosmic-ray data suggest thelocal fluff is in ionization equilibrium. [Note that a typographicalerror on page 532 incorrectly gives the age of the squall line shell as~400,000 years; the correct age is ~4 Myrs.]
| A microwave survey of southern early-type stars
A multi-epoch survey with the Parkes telescope of a completedistance-limited sample of 57 stars earlier than F6 has detectedpossible 8.4-GHz emission from 16 stars. Single-epoch partial synthesisobservations with the Australia Telescope Compact Array (ATCA) at 4.8GHz on 27 stars from the same sample (including the possible Parkesdetections) found no emission at the stellar positions above a fluxdensity limit of 1.2-1.9 mJy, but the maps show that the Parkesdetections are not merely the results of confusion of sources within theParkes beam. Three early F stars with UV and/or X-ray emission wereobserved simultaneously at 4.8 and 8.4 GHz in 12-h syntheses with the6-element ATCA. Two of these stars were from the above sample and thethird was the supergiant Alpha Carinae. We detected only alphaCar withflux densities of 300+/-65 and 140+/-65 muJy at 4.8 and 8.6 GHz(S~nu^-1.3+/-1.3). We discuss the legitimacy of the Parkes 3-6sigmadetections and show that, although none has been detected by synthesisobservations, there is no compelling reason for rejecting them on theinternal evidence. The power emitted by the supergiant alphaCar issimilar to that of the 16 possible Parkes detections, although itsactivity index is orders of magnitude lower. We show that this emissioncannot be thermal bremsstrahlung from the 10^7.2-K corona of the starbut is probably synchrotron emission from a magnetically maintainedcorona.
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