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A High-Resolution Spectral Atlas of α Persei from 3810 to 8100 Å
We present a high-resolution (λ/δλ=90,000) spectralatlas of the F5 Ib star α Per covering the 3810-8100 Åregion. The atlas, based on data obtained with the aid of the echellespectrograph BOES fed by the 1.8 m telescope at Bohyunsan Observatory(Korea), is the result of the co-addition of a few well-exposed spectra.The final signal-to-noise ratio is ~800 at ~6000 Å. The atlas iscompared with a synthetic spectrum computed using a code based on Kuruczsoftware and databases. The adopted model atmosphere parameters areTeff=6240+/-20 K, logg=0.58+/-0.04, andvmicro=3.20+/-0.05 km s-1. We also derived an ironabundance of [Fe/H]=-0.28+/-0.06. The spectral lines of α Per havebeen identified by matching the synthetic spectrum with the observedone. The atlas is presented in figures and available in digital form onthe World Wide Web, along with the synthetic spectrum and spectral lineidentification tables.Based on data collected with the 1.8 m telescope at Bohyunsan OpticalAstronomy Observatory, South Korea.
| Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.
| The First Spatially Resolved Mid-Infrared Spectra of NGC 1068 Obtained at Diffraction-limited Resolution with the Keck I Telescope Long Wavelength Spectrometer
We present spatially resolved mid-infrared (mid-IR) spectra of NGC 1068with a diffraction-limited resolution of 0.25" using the Long WavelengthSpectrometer (LWS) at the Keck I telescope. The mid-IR image of NGC 1068is extended along the north-south direction. Previous imaging studieshave shown that the extended regions are located inside the ionizationcones, indicating that the mid-IR emission arises perhaps from the innerregions of the narrow-line clouds instead of the proposed dusty torusitself. The spatially resolved mid-IR spectra were obtained at twodifferent slit position angles, +8.0d and -13.0d across the elongatedregions in the mid-IR. From these spectra, we found only weak silicateabsorption toward the northern extended regions but strong absorption inthe nucleus and the southern extended regions. This is consistent with amodel of a slightly inclined cold obscuring torus that covers much ofthe southern regions but is behind the northern extension. While adetailed analysis of the spectra requires a radiative transfer model,the lack of silicate emission from the northern extended regions promptsus to consider a dual dust population model as one of the possibleexplanations in which a different dust population exists in theionization cones compared to that in the dusty torus. Dust inside theionization cones may lack small silicate grains, giving rise to only afeatureless continuum in the northern extended regions, while dust inthe dusty torus has plenty of small silicate grains to produce thestrong silicate absorption lines toward the nucleus and the southernextended regions.
| Precision Kinematics and Related Parameters of the α Persei Open Cluster
A kinematical study of the nearby open cluster α Persei ispresented based on the astrometric proper motions and positions in theTycho-2 catalog and Second USNO CCD Astrographic Catalog (UCAC2). Usingthe astrometric data and photometry from the Tycho-2 and ground-basedcatalogs, 139 probable members of the cluster are selected, 18 of themnew. By the classical convergent point method, systematic motions ofstars inside the cluster and velocity dispersions are estimated. Asdirectly observed, the upper limit on the internal velocity dispersionper coordinate is 1.1 km s-1. The actual velocity dispersionis much smaller than that value, since all of it appears to come fromthe expected errors of the astrometric proper motions. The relativeposition of the convergent point with respect to the cluster starsyields the ``astrometric'' radial velocity, which turns out larger by afew km s-1 than the mean observed spectroscopic radialvelocity. This implies an overall contraction of the cluster. Kinematicparallaxes are computed for each member, and an improved H-R diagram isconstructed. An age of 52 Myr is determined by isochrone fitting. Thestar α Per itself fits an isochrone of this age computed withovershooting from the boundary of the convective zone. The theoreticalmass of the star α Per is 6.65 Msolar. With respect tothe common center of mass, half of the higher mass members (earlier thanG) are located within a radius of 10.3 pc. The cluster appears to beroughly twice as large, or as sparse, as the Pleiades, retainingnonetheless a similar dynamical coherence. The low rate of binaries isanother feature of this cluster, where we find only about 20% of membersto be known or suspected spectroscopic, astrometric, or visual binariesor multiple systems. X-ray emitters in the cluster appear to have thesame dispersion of internal velocities as the rest of the membership.The cluster is surrounded by an extended, sparse halo of comovingdwarfs, which are found by combining the proper-motion data from UCAC2with Two Micron All-Sky Survey infrared photometry. Since many of theseexternal stars are outside the tidal radius, the cluster being in anactive stage of disintegration or evaporation could be considered. Thishypothesis is not supported by the weak compression and thenonmeasurable velocity dispersion found in the kinematic analysis. Asearch for stars ejected from the α Persei cluster is carried outby tracking a large number of nearby stars 70 Myr back in time andmatching their positions with the past location of the cluster. Only oneplausible ejection is found prior to 10 Myr ago. The nearby star GJ 82,an active M dwarf with a strong Hα emission, is likely a formermember ejected 47 Myr ago at 5 km s-1.
| A deep wide-field optical survey in the young open cluster Collinder 359
We present the first deep, optical, wide-field imaging survey of theyoung open cluster Collinder 359, complemented by near-infraredfollow-up observations. This study is part of a large programme aimed atexamining the dependence of the mass function on environment and time.We have surveyed 1.6 square degrees in the cluster, in the I and zfilters, with the CFH12K camera on the Canada-France-Hawaii 3.6-mtelescope down to completeness and detection limits in both filters of22.0 and 24.0 mag, respectively. Based on their location in the optical(I-z, I) colour-magnitude diagram, we have extracted new cluster membercandidates in Collinder 359 spanning 1.3-0.03 Mȯ,assuming an age of 60 Myr and a distance of 450 pc for the cluster. Wehave used the 2MASS database as well as our own near-infrared photometryto examine the membership status of the optically-selected clustercandidates. Comparison of the location of the most massive members inCollinder 359 in a (B-V, V) diagram with theoretical isochrones suggeststhat Collinder 359 is older than α Per but younger than thePleiades. We discuss the possible relationship between Collinder 359 andIC 4665 as both clusters harbour similar parameters, including propermotion, distance, and age.
| Exploring the lower mass function in the young open cluster IC 4665
We present a study of the young (30-100 Myr) open cluster IC 4665 withthe aim to determine the shape of the mass function well into the browndwarf regime. We photometrically select 691 low-mass stellar and 94brown dwarf candidate members over an area of 3.82 square degreescentred on the cluster. K-band follow-up photometry and Two-MicronAll-Sky Survey data allow a first filtering of contaminant objects fromour catalogues. A second filtering is performed for the brightest starsusing proper motion data provided by the Tycho-2 and UCAC2 publiccatalogues. Contamination by the field population for the lowest massobjects is estimated using same latitude control fields. We fit observedsurface densities of various cluster populations with King profiles andfind a consistent tidal radius of 1.0°. The presence of possiblemass segregation is discussed. In most respects investigated, IC 4665 issimilar to other young open clusters at this age: (1) a power law fit tothe mass function between 1 and 0.04 Mȯ results in bestfit for a slope of -0.6; (2) a cusp in the mass function is noticed atabout the substellar boundary with respect to the power law description,the interpretation of which is discussed; (3) a fraction between 10-19%for BDs with M 0.03 Mȯ to total members; (4) abest-fit lognormal function to the full mass distribution shows anaverage member mass of 0.32 Mȯ, if IC 4665 has an age of50 Myr.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Non-LTE Line-Formation and Abundances of Sulfur and Zinc in F, G, and K Stars
Extensive statistical-equilibrium calculations on S I and Zn I were carried out, in orderto investigate how the non-LTE effect plays a role in the determinationof their abundances in F-K stars. Having checked on the spectra ofrepresentative F-type stars (Polaris, Procyon, α Per) and the Sunthat our non-LTE corrections yield a reasonable consistency between theabundances derived from different lines, we tried an extensive non-LTEreanalysis of published equivalent-width data of S I and Zn I lines for metal-poorhalo/disk stars. According to our calculations, S I 9212/9228/9237 lines suffer significant negative non-LTEcorrections (lesssim 0.2‑0.3 dex), while LTE is practically validfor S I 8683/8694 lines. As far as the verymetal-poor regime is concerned, a marked discordance is observed betweenthe [S/Fe] values from these two abundance indicators, in the sense thatthe former attains a nearly flat plateau (or even a slight downwardbending) while the latter shows an ever-increasing trend with a furtherlowering of metallicity. The characteristics of [Zn/Fe] reported fromrecent LTE studies (i.e., an evident/slight increase of [Zn/Fe] with adecrease of [Fe/H] for very metal-poor/disk stars) were almostconfirmed, since the non-LTE corrections for the Zn I 4722/4810 and 6362 lines (tending to be positive andgradually increasing toward lower [Fe/H]) are quantitatively of lesssignificance (lesssim 0.1 dex).
| Accretion in brown dwarfs:. a low-resolution criterion
We present evidences of ongoing accretion and outflows in severalobjects at or below the substellar limit, including the width of theHalpha line, the detection of forbidden lines and near-infrared excess.These objects are likely to belong to several SFRs, and have ages in therange 1-8 Myr. In addition, we present an accretion criterion based onlow-resolution optical spectroscopy, which can be applied, from thestatistical point of view, to large samples.
| Caroline Herschel as observer
Not Available
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| Spectroscopic Abundance Analysis of Dwarfs in the Young Open Cluster IC 4665
We report a detailed spectroscopic abundance analysis for a sample of 18F-K dwarfs of the young open cluster IC 4665. Stellar parameters andelement abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe, and Ni have beenderived using the spectroscopic synthesis tool SME (Spectroscopy MadeEasy). Within the measurement uncertainties the iron abundance isuniform, with a standard deviation of 0.04 dex. No correlation is foundbetween the iron abundance and the mass of the stellar convective zoneor between the Li abundance and the Fe abundance. In other words, ourresults do not reveal any signature of accretion and therefore do notsupport the scenario that stars with planets (SWPs) acquire theiron-average higher metallicity compared to field stars via accretion ofmetal-rich planetary material. Instead, the higher metallicity of SWPsmay simply reflect the fact that planetary formation is more efficientin high-metallicity environs. However, since so many details of theplanetary system formation processes remain poorly understood, furtherstudies are needed for a final settlement of the problem of the highmetallicity of SWPs. The standard deviation of [Fe/H] deduced from ourobservations, taken as an upper limit on the metallicity dispersionamong the IC 4665 member stars, has been used to constrainprotoplanetary disk evolution, terrestrial and giant planets formation,and evolution processes. The total reservoir of heavy elements retainedby the nascent disks is limited, and high retention efficiency ofplanet-building material is supported. Under modest surface density, gasgiant planets are expected to form in locally enhanced regions or startefficient gas accretion when they only have a small core of a few Earthmasses. Our results do not support the possibility that the migration ofgas giants and the circularization of terrestrial planets' orbits areregulated by their interaction with a residual population ofplanetesimals and dust particles.
| The Substellar Mass Function: A Bayesian Approach
We report our efforts to constrain the form of the low-mass star andbrown dwarf mass function via Bayesian inference. Recent surveys of M,L, and T dwarfs in the local solar neighborhood are an essentialcomponent of our study. Uncertainties in the age distribution of localfield stars make reliable inference complicated. We adopt a wide rangeof plausible assumptions about the rate of Galactic star formation andshow that their deviations from a uniform rate produce little effect onthe resulting luminosity function for a given mass function. As anancillary result, we calculate the age distribution for M, L, and Tspectral types. We demonstrate that late L dwarfs, in particular, aresystematically younger than objects with earlier or later spectraltypes, with a mean age of 3 Gyr. Finally, we use a Bayesian statisticalformalism to evaluate the probability of commonly used mass functions inthe light of recent discoveries. We consider three functional forms ofthe mass function, including a two-segment power law, a single power lawwith a low-mass cutoff, and a lognormal distribution. Our results showthat at a 60% confidence level the power-law index α for thelow-mass arm of a two-segment power law has a value between -0.6 and 0.6for objects with masses between 0.04 and 0.10 Msolar. Thebest-fit index is α=0.3+/-0.6 at the 60% confidence level for asingle-segment mass function. Current data require this function toextend to at least 0.05 Msolar with no restrictions placed ona lower mass cutoff. Inferences of the parameter values for a lognormalmass function are virtually unaffected by recent estimates of the localspace density of L and T dwarfs. We find that we have no preferenceamong these three forms using this method. We discuss current and futurecapabilities that may eventually discriminate between mass functionmodels and refine estimates of their associated parameter values.
| Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li dataavailable in the literature for main sequence stars (spectral-types fromlate F to K) in open clusters. In the present paper we focus on adetailed investigation of MS Li depletion and its time scales for starsin the 6350-5500 K effective temperature range. For the first time, wewere able to constrain the age at which non-standard mixing processes,driving MS Li depletion, appear. We have also shown that MS Li depletionis not a continuous process and cannot be simply described by at-α law. We confirm that depletion becomes ineffectivebeyond an age of 1-2 Gyr for the majority of the stars, leading to a Liplateau at old ages. We compared the empirical scenario of Li as afunction of age with the predictions of three non-standard models. Wefound that models including only gravity waves as main mixing processare not able to fit the Li vs. age pattern and thus this kind of mixingcan be excluded as the predominant mechanism responsible for Lidepletion. On the other hand, models including slow mixing induced byrotation and angular momentum loss, and in particular those includingalso diffusive processes not related to rotation, can explain to someextent the empirical evidence. However, none of the currently proposedmodels can fit the plateau at old ages.
| A near-infrared survey for new low-mass members in α Per
We present a near-infrared (K'-band) survey of 0.7 square degree area inthe α Persei open cluster (age = 90 Myr, distance = 182 pc)carried out with the Omega-Prime camera on the Calar Alto 3.5-mtelescope. Combining optical data (Rc and I_c) obtained withthe KPNO/MOSA detector and presented in Stauffer et al. (1999) with theK' observations, a sample of new candidate members has been extractedfrom the optical-infrared colour-magnitude diagram. The location ofthese candidates in the colour-colour diagram suggests that two-thirdsof them are actually reddened background giants. About 20 new candidatemembers with masses between 0.3 and 0.04 Mȯ are added tothe ~400 known α Per cluster members. If they are indeed αPer members, four of the new candidates would be brown dwarfs. Wediscuss the advantages and drawbacks of the near-infrared survey ascompared to the optical selection method. We also describe the outcomeof optical spectroscopy obtained with the Twin spectrograph on the CalarAlto 3.5-m telescope for about 30 candidates, including selected membersfrom the optical sample presented in Barrado y Navascués et al.(2002) and from our joint optical/infrared catalogue. These resultsargue in favour of the optical selection method for this particularcluster.
| 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
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| Mid-infrared interferometry of the Mira variable RR Sco with the VLTI MIDI instrument
We present the results of the first mid-infrared interferometricobservations of the Mira variable RR Sco with the MID-infraredInterferometer (MIDI) coupled to the European Southern Observatory's(ESO) Very Large Telescope Interferometer (VLTI), together with K-bandobservations using VLTI VINCI. The observations were carried out in June2003, when the variability phase of the object was 0.6, using two unittelescopes (UT1 and UT3), as part of the Science Demonstration Time(SDT) program of the instrument. Projected baseline lengths ranged from73 to 102 m, and a spectral resolution of 30 was employed in theobservations, which enabled us to obtain the wavelength dependence ofthe visibility in the region between 8 and 13 μm. The uniform-diskdiameter was found to be 18 mas between 8 and 10 μm, while itgradually increases at wavelengths longer than 10 μm to reach 24 masat 13 μm. The uniform-disk diameter between 8 and 13 μm issignificantly larger than the K-band uniform-disk diameter of 10.2± 0.5 mas measured using VLTI VINCI with projected baselinelengths of 15-16 m, three weeks after the MIDI observations. Our modelcalculations show that optically thick emission from a warm molecularenvelope consisting of H2O and SiO can cause the apparent mid-infrared diameter to be much larger than the continuum diameter. Wefind that the warm molecular envelope model extending to 2.3R\star with a temperature of 1400 K and column densitiesof H2O and SiO of 3 × 1021 cm-2and 1 × 1020 cm-2, respectively, canreproduce the observed uniform-disk diameters between 8 and 10 μm.The observed increase of the uniform-disk diameter longward of 10 μmcan be explained by an optically thin dust shell consisting of silicateand corundum grains. The inner radius of the optically thin dust shellis derived to be 7-8 R\star with a temperature of 700 K,and the optical depth at 10 μm is found to be 0.025.Based on observations made with the Very Large Telescope Interferometerof the European Southern Observatory.
| On the potassium-rotation connection in late-type Alpha Persei stars
We present measurements of the K I λ 7699 line from spectra of 19late-type members of the αPersei cluster obtained with theIntermediate Dispersion Spectrograph at the Isaac Newton Telescope.These stars span a narrow range of Teff, from 5091 K to 4771K, and a wide range of v sin i values (9 km s-1-170 kms-1). For a given star, we find empirically that largerrotational broadening apparently increases the equivalent width of K Iλ 7699 linearly for v sin i values from 15 km s-1 to75 km s-1. This correlation breaks down for v sin i > 75km s-1. After correction for this effect, we show that thepotassium line equivalent widths do not really correlate with v sin i.Based on observations made with the Isaac Newton Telescope, operated onthe island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Determination of fundamental characteristics for stars of the F, G, and K spectral types. The surface gravities and metallicity parameters.
Not Available
| 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.
| Ten Micron Observations of Nearby Young Stars
We present new 10 μm photometry of 21 nearby young stars obtained atthe Palomar 5 m and at the Keck I 10 m telescopes as part of a programto search for dust in the habitable zone of young stars. Thirteen of thestars are in the F-K spectral type range (``solar analogs''), four haveB or A spectral types, and four have spectral type M. We confirmexisting IRAS 12 μm and ground-based 10 μm photometry for 10 ofthe stars and present new insight into this spectral regime for therest. Excess emission at 10 μm is not found in any of the young solaranalogs, except for a possible 2.4 σ detection in the G5 V star HD88638. The G2 V star HD 107146, which does not display a 10 μmexcess, is identified as a new Vega-like candidate, based on our 10μm photospheric detection, combined with previously unidentified 60and 100 μm IRAS excesses. Among the early-type stars, a 10 μmexcess is detected only in HD 109573A (HR 4796A), confirming priorobservations; among the M dwarfs, excesses are confirmed in AA Tau, CD-40°8434, and Hen 3-600A. A previously suggested N-band excess inthe M3 dwarf CD -33°7795 is shown to be consistent with photosphericemission. We calculate infrared to stellar bolometric luminosity ratiosfor all stars exhibiting mid-infrared excesses and infer the total massof orbiting dust in the cases of optically thin disks. For a derivedmedian photometric precision of +/-0.11 mag, we place an upper limit ofMdust~2×10-5 M⊕ on the dustmass (assuming a dust temperature of 300 K) around solar analogs notseen in excess at 10 μm. Our calculations for the nearby K1 V star HD17925 show that it may have the least massive debris disk known outsideour solar system (Mdust>~7×10-6M⊕). Our limited data confirm the expected tendency ofdecreasing fractional dust excessfd=LIR/L* with increasing stellar age.However, we argue that estimates of fd suffer from adegeneracy between the temperature and the amount of circumstellar dustMdust, and we propose a relation of Mdust as afunction of age instead.
| 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).
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved Stars
A new calibration of the MV-W(O I 7774) relationship hasbeen calculated using better reddening and distance estimates for asample of 27 calibrator stars of spectral types A to G, based onaccurate parallaxes and proper motions from the Hipparcos and Tychocatalogues. The present calibration predicts absolute magnitude withaccuracies of +/-0.38mag for a sample covering a large range ofMV, from -9.5 to +0.35 mag. The color term included in aprevious paper has been dropped since its inclusion does not lead to anysignificant improvement in the calibration. The variation of the O I7774 feature in the classical cepheid SS Sct has been studied. Wecalculated a phase-dependent correction to random phase OI featurestrengths in Cepheids, such that it predicts mean absolute magnitudesusing the above calibration. After applying such a correction, we couldincrease the list of calibrators to 58 by adding MV and O Itriplet strength data for 31 classical Cepheids. The standard error ofthe calibration using the composite sample was comparable to thatobtained from the primary 27 calibrators, showing that it is possible tocalculate mean Cepheid luminosities from random phase observations ofthe O I 7774 feature. We use our derived calibrations to estimateMV for a set of evolved objects to be able to locate theirpositions in the HR diagram.
| Characteristics and classification of A-type supergiants in the Small Magellanic Cloud
We address the relationship between spectral type and physicalproperties for A-type supergiants in the Small Magellanic Cloud (SMC).First, we construct a self-consistent classification scheme for Asupergiants, employing the calcium K to Hɛ line ratio as atemperature-sequence discriminant. Following the precepts of the `MKprocess', the same morphological criteria are applied to Galactic andSMC spectra, with the understanding that there may not be acorrespondence in physical properties between spectral counterparts indifferent environments. Then we discuss the temperature scale,concluding that A supergiants in the SMC are systematically cooler thantheir Galactic counterparts at the same spectral type, by up to ~10 percent. Considering the relative line strengths of Hγ and the CH Gband, we extend our study to F- and early G-type supergiants, for whichsimilar effects are found. We note the implications for analyses ofluminous extragalactic supergiants, for the flux-weightedgravity-luminosity relationship and for population synthesis studies inunresolved stellar systems.
| Are there brown dwarfs in globular clusters?
We present an analytical method for constraining the substellar initialmass function in globular clusters, based on the observed frequency oftransit events. Globular clusters typically have very high stellardensities where close encounters are relatively common, and thus tidalcapture can occur to form close binary systems. Encounters betweenmain-sequence stars and lower-mass objects can result in tidal captureif the mass ratio is >10-2. If brown dwarfs exist insignificant numbers, they too will be found in close binaries, and somefraction of their number should be revealed as they transit theirstellar companions. We calculate the rate of tidal capture of browndwarfs in both segregated and unsegregated clusters, and find that thetidal capture is more likely to occur over an initial relaxation timebefore equipartition occurs. The lack of any such transits in recentHubble Space Telescope monitoring of 47 Tuc implies an upper limit onthe frequency of brown dwarfs (<15 percent relative to stars)which is significantly below that measured in the galactic field andyoung clusters.
| On the Relationship Between Stellar Rotation and Radius in Young Clusters
We have compiled data on rotational velocities for more than 1000 K andM stars in 12 young clusters ranging in age from Orion to the Hyades.These data enable a search for systematic changes in stellar rotationalvelocity vs. age. Taken together, these data show that most pre-mainsequence (PMS) stars spanning ages from about 0.1 to about 1 Myr do notappear to spin up in response to contraction down their convectivetracks, that spin up between 1 and 3 million years is modest at best,and that some stars do not appear to spin up at all.These results extend and reinforce our earlier study (Rebull et al.2002), based on observations of several hundred stars in the OrionFlanking Fields, NGC 2264, and the Orion Nebula Cluster (ONC), whichshowed that the majority of PMS stars in these three groups apparentlydo not conserve stellar angular momentum as they contract, but insteadevolve at nearly constant angular velocity. This result applies both tostars with and without near-IR I-K excesses indicative of disks.
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