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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.
| Discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480
The chemically peculiar (CP) stars HD 98851 and HD 102480 have beendiscovered to be unusual pulsators during the `Naini Tal-Cape Survey'programme to search for pulsational variability in CP stars. Time seriesphotometric and spectroscopic observations of these newly discoveredstars are reported here. Fourier analyses of the time series photometryreveal that HD 98851 is pulsating mainly with frequencies 0.208 and0.103 mHz, and HD 102480 is pulsating with frequencies 0.107, 0.156 and0.198 mHz. The frequency identifications are all subject to 1d-1 cycle count ambiguities. We have matched the observedlow-resolution spectra of HD 98851 and HD 102480 in the range 3500-7400Å with theoretical synthetic spectra using Kurucz models withsolar metallicity and a micro-turbulent velocity of 2 km s-1.These yield Teff= 7000 +/- 250 K, log g= 3.5 +/- 0.5 for HD98851 and Teff= 6750 +/- 250 K, log g= 3.0 +/- 0.5 for HD102480. We determined the equivalent H-line spectral class of thesestars to be F1 IV and F3 III/IV, respectively. A comparison of thelocation of HD 98851 and HD 102480 in the HR diagram with theoreticalstellar evolutionary tracks indicates that both stars are about1-Gyr-old, 2-Msolar stars that lie towards the red edge ofthe δ Sct instability strip. From comparison between the observedand calculated physical parameters, we conclude that HD 98851 and HD102480 are cool, evolved Am pulsators. The light curves of thesepulsating stars have alternating high and low amplitudes, nearlyharmonic (or subharmonic) period ratios, high pulsational overtones andAm spectral types. This is unusual for both Am and δ Sctpulsators, making these stars interesting objects for furtherobservational and theoretical studies.
| An oscillating model suggestion for 63 Her
An attempt has been made towards explaining the observed frequencies in63 Her. The evolution of rotating stars of 1.96, 1.98, 2.00, 2.05 and2.10 Msun have been studied up to a point where stellarparameters match the observed luminosity and effective temperature of 63Her. Radial and nonradial adiabatic oscillation frequencies wereobtained in low harmonic degrees ( l= 0,1,2,3 ). One radial and threenonradial frequency values that match with the observed values werefound for the model of mass 2.00 Msun.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| δ Scuti stars and their related objects
δ Scuti stars are a group of stars located on or a little abovethe main sequence of H-R diagram with spectral type from A3 to F5. Theyare low amplitude single or multi period pulsators with period shorterthan 0.3 d. Within the same area there are several groups of variablesor special stars correlated with them, e.g., Dwarf Cepheids, γ Dorvariables, Blue Stragglers, Am stars, Ap stars, ROAp variables, λBoo variables and δ Del variables. In this paper a general reviewin this field, including the number of new variables discovered after1995, is presented. The most reliable period variation rates for all thehigh amplitude variables and several low amplitude variables are listed.Statistic shows the higher the rotation rate v sin i is, the lower thelight variation amplitude is. Thus within young open clusters highamplitude variables cannot be found. The amplitudes-periods distributionhave 3 peaks with the highest of 1.0 mag in V at 0.17 d in period. Forδ Scuti variables in stellar systems the shorter the averageperiod is, the lower the metallicity and the older the age of thestellar system are.
| 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
| On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics of lambda Bootis type stars
Generally, chemical peculiarity found for stars on the upper mainsequence excludes delta Scuti type pulsation (e.g. Ap and Am stars), butfor the group of lambda Bootis stars it is just the opposite. This makesthem very interesting for asteroseismological investigations. The groupof lambda Bootis type stars comprises late B- to early F-type,Population I objects which are basically metal weak, in particular theFe group elements, but with the clear exception of C, N, O and S. Thepresent work is a continuation of the studies by Paunzen et al.(\cite{Pau97}, \cite{Pau98}), who presented first results on thepulsational characteristics of the lambda Bootis stars. Since then, wehave observed 22 additional objects; we found eight new pulsators andconfirmed another one. Furthermore, new spectroscopic data (Paunzen\cite{Pau01}) allowed us to sort out misidentified candidates and to addtrue members to the group. From 67 members of this group, only two arenot photometrically investigated yet which makes our analysis highlyrepresentative. We have compared our results on the pulsationalbehaviour of the lambda Bootis stars with those of a sample of deltaScuti type objects. We find that at least 70% of all lambda Bootis typestars inside the classical instability strip pulsate, and they do sowith high overtone modes (Q < 0.020 d). Only a few stars, if any,pulsate in the fundamental mode. Our photometric results are inexcellent agreement with the spectroscopic work on high-degree nonradialpulsations by Bohlender et al. (\cite{Boh99}). Compared to the deltaScuti stars, the cool and hot borders of the instability strip of thelambda Bootis stars are shifted by about 25 mmag, towards smaller(b-y)_0. Using published abundances and the metallicity sensitiveindices of the Geneva 7-colour and Strömgren uvbybeta systems, wehave derived [Z] values which describe the surface abundance of theheavier elements for the group members. We find that thePeriod-Luminosity-Colour relation for the group of lambda Bootis starsis within the errors identical with that of the normal delta Scutistars. No clear evidence for a statistically significant metallicityterm was detected. Based on observations from the Austrian AutomaticPhotoelectric Telescope (Fairborn Observatory), SAAO and Siding SpringObservatory.
| 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
| A revised catalogue of delta Sct stars
An extensive and up-dated list of delta Sct stars is presented here.More than 500 papers, published during the last few years, have beenrevised and 341 new variables have been added to our last list, sixyears ago. This catalogue is intended to be a comprehensive review onthe observational characteristics of all the delta Sct stars known untilnow, including stars contained in earlier catalogues together with othernew discovered variables, covering information published until January2000. In summary, 636 variables, 1149 references and 182 individualnotes are presented in this new list. Tables 1 and 2 will be accessibleonly in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| CCD photometry of a delta Scuti variable in an open cluster. III. V 465 Persei in the alpha Persei cluster
We present the results of real-time CCD differential photometry for thedelta Scuti variable V 465 Per. The observations were performed forthirteen nights between November, 1994 and January, 1995. Total 3345points of differential V magnitudes were collected during theobservation period of 76.4 hours. From the Fourier analysis, we havedetected four frequencies as follows; f_1=14.040 c/d, f_2=17.208 c/d,f_3=33.259 c/d and f_4=13.721 c/d. Two frequencies of 12.50 c/d and33.49 c/d detected previously by Slovak (1978, ApJ 223, 192) maycorrespond to our f_1 and f_3, respectively. For each frequency,pulsation constants of Q_1=0.0520+/-0.008, Q_2=0.0425+/-0.006,Q_3=0.0220+/-0.003 and Q_4=0.0533+/-0.008 were derived from severalobservational properties of V 465 Per. Only the value of Q_3 is found tobe within the range of theoretical p-mode oscillations and the otherones show large differences relative to the theoretical values. The highQ-values might be interpreted as the occurrence of g-mode oscillationsin V 465 Per, even though it can not rule out the possibility ofsystematic errors (Breger 1990; Delta Scuti Star Newsletter, 2, 13)related to rotational velocity.
| Pulsation, binarity and close frequency spacing in $delta Scuti ta$ Scuti stars: BQ and BW Cancri. Results of the STEPHI VI campaign in Praesepe Cluster
New pulsation modes in two delta Scuti stars of Praesepe cluster, BQ andBW Cnc, have been detected during the STEPHI VI campaign in 1995. Inparticular, 3 frequencies for BQ Cnc and 9 frequencies for BW Cnc havebeen found above a 99% confidence level. The possibility of the presenceof a g-mode present in BQ Cnc is discussed, considering its binarity.The effect of mutual interference between very close detectedfrequencies in BW Cnc during the observations, is also considered. Thislast effect reveals the necessity of long-period observing runs, inorder to avoid its influence in the final number of detected modes. Insuch situation, studies of secular amplitude changes can be stronglyaffected.
| The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars)
| Hot Inner Disks that Appear and Disappear around Rapidly Rotating A-Type Dwarfs
At any one time, approximately one-quarter of the most rapidly rotatingnormal A-type dwarfs (V sin i >= 200 km s-1) show shell lines of TiII in the near-ultraviolet. Our observations during 22 years show thatthe lines appear and disappear on timescales of decades but do notdisplay significant changes within 1 year. This implies that they arenot remnants of the star formation but rather are probably caused bysporadic mass-loss events. A working hypothesis is that all A-type starsthat are rotating near their limits have these shells, but for onlyone-quarter of the time. Because these lines do not appear in stars withsmaller sin i, the shells must be disks. These are hot inner disks thatmay or may not be related to the cool outer disks seen by Smith andTerrile around beta Pic or through infrared excesses around Vega andother A-type dwarfs. The similar, limited line widths indicate that thedisks are ~7 R* above the stellar surfaces.
| Spectroscopic survey of delta Scuti stars. I. Rotation velocities and effective temperatures
Projected rotational velocities and effective temperatures for 68 deltaSct stars as well as 41 non-variable stars of similar spectral type andluminosity are presented here. The rotational velocities have beencalculated following the method developed in \cite[Gray (1992)]{ref38}and effective temperatures have been derived using the Balmer lineprofiles. The temperatures obtained from this method are shown to be inreasonable agreement with those calculated using the Infrared FluxMethod (IRFM) or spectrophotometric methods. This result has allowed usto use our temperatures to compare different uvby beta photometriccalibrations. We find that the calibration given by \cite[Moon \&Dworetsky (1985)]{ref72} is the most consistent. In the second part ofthis paper we have studied the relation between the pulsationalproperties (periods and amplitudes) and the physical parameters (v sin iand Teff). Where pulsation modes have been determined, thelow amplitude $\delta$ Scutis tend to be multimode (radial andnon-radial) pulsators, consistent with the theory that non-linearcoupling between modes acts to limit the amplitude in these stars. Wehave compared the distribution of v sin i for low amplitude $\delta$Scutis and non-variable stars. This shows the $\delta$ Scutis have abroader distribution in v sin i suggesting that a high rotation velocitymay favour pulsation. We find that the large amplitude delta Scuti starstend to have longer periods, cooler temperatures and lower rotationvelocities. Given that the large amplitude stars are also relativelyrare all the above are consistent with the hypothesis that these starsare more evolved (sub-giants) than the low amplitude delta Scutis (mainsequence or early post-main sequence).
| Luminosity and related parameters of δ Scuti stars from HIPPARCOS parallaxes. General properties of luminosity.
The absolute magnitudes of δ Scuti stars derived from parallaxesmeasured by the Hipparcos astrometric satellite are discussed andcompared with the previous estimates based on photometric uvbyβindices. There are significant differences which are related tophotometric effects of metallicity and rotational velocity, but thepossible effect of a close companion on the measured apparent magnitudeshould be also taken into account. The possibility of differentgroupings of δ Scuti stars based on the absolute magnitudes isbriefly discussed. Some high amplitude δ Scuti stars withintermediate or normal metallicity and small and uncertain parallax haveapparently a very low luminosity; this could be a systematic effectrelated to the observational errors.
| Complex behaviour of the δ Scuti star θ Tucanae. I. Frequencies in the light variation.
On the basis of more than 2300 new Stroemgren y and Johnson Vphotometric observations collected during 246 hours spread over 42nights at three sites in 1993, we present a frequency analysis of thelight variation of θ Tuc. 10 frequencies were found in the rangeof 15.8 to 20.28cycles/day displaying an extreme regularity. Frequenciesare situated in groups and these groups are equally spaced. Some kind ofsplitting is definitely involved. The frequencies are constant inamplitude on a short time scale. Two frequencies, 0.282 and0.142cycles/day (3.56 and 7.04days period) were found to be responsiblefor the mean light level variation of θ Tuc. Although θTuc is known as a single star, the length and shape of the mean lightlevel variation and the unusual behaviour of θ Tuc in ultravioletand infrared suggest that the δ Scuti star θ Tuc is aprimary in a binary system with a late F type companion.
| γ Doradus and δ Scuti stars: cousins or twins?
γ Doradus stars are a group of slowly pulsating early F dwarfswith typical low frequencies near 1c/d. The pulsation modes are probablyg modes. These stars are slightly cooler than the typical δ Scutivariables, which pulsate with higher frequencies of typically 10c/d(mostly nonradial p modes). This paper examines the relationship betweenthe two groups of variables. Additional low-frequency variability hasbeen reported for fewer than 10% of the 300 δ Scuti starsexamined. In at least one case (1 Mon), the observational data areconsistent with an alternate interpretation in terms of a modeinteraction, f_1_-f_2_, between two p modes of higher frequency, ratherthan by the excitation of g modes. In several other stars (e.g. 4 CVn),the reported low-frequency variability may originate in the comparisonstar used. Arguments are given that for a number of stars the observedlow-frequency variations are caused by observational errors (such asinstrumental drift and transparency variations). A few δ Scutistars remain for which intrinsic γ Doradus-type variability mayalready have been discovered, although no case is well-studied. One ofthe best candidates is the star BI CMi, which is situated in the smallregion of overlap of the two groups of pulsators in theHertzsprung-Russell diagram. The star deserves further study.
| Can we measure the rotation rate inside stars ?
We examine the possibility of obtaining localized information on therotation rate inside stars through asteroseismic observations fromspace. Attention is focused on δ Scuti stars where bothopacity-driven and solar-like turbulence-driven modes may be detected.Plausible sets of modes with attendant rotational splitting data arededuced from the results of linear stability calculations, effects ofamplitude averaging for modes with higher l's, and information gatheredfrom ground-based photometry. For such sets of modes, optimallylocalized averaging kernels are constructed, and we show that fairlydetailed information about the behavior of the rotation rate can beobtained only if opacity-driven modes like those anticipated are indeeddetected. The turbulence-driven modes are essentially irrelevant forprobing rotation.
| The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..135A&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| A catalogue of variable stars in the lower instability strip.
Identifications, positions, photometry, spectra, some pulsationalfeatures, other astrophysical parameters and literature for 302pulsating variable stars in the lower instability strip, near the ZAMS,are given. About 185 stars have near homogeneous photometric informationin the Stroemgren's uvby-β photometric system. Thiscatalogue/database covers information published until November 1993.
| δ Scuti stars: a new revised list
An extensive and up to date list of δ Sct stars is presented. Thiscatalogue is intended to be a comprehensive review of observationalcharacteristics of all the δ Sct stars known until now, includingstars contained in earlier catalogues together with other new discoveredvariables, covering information published until November 1993. Globalinformation in the form of histograms and diagrams are also shown.
| Multi-period pulsation of the delta Scuti star 63 Herculis
Multisite photoelectric B and V photometry of the delta Scuti star 63Her was obtained on three continents. Three frequencies of pulsation(f1 = 11.32, f2 = 20.07 and f3 = 7.72c/d) were detected with a signal/noise amplitude ratio greater than or =4.0. The three frequencies fit all the available data ranging back to1968. Reports in the literature of discepant behavior of the 1968measurements were probably caused by overinterpretation. The newfrequency values are in excellant agreement with the results found byReed & Welch from their 1983 data, except for a 1 c/d alias in theirvalue for f2. Two additional pulsation frequencies in thelow-frequency g-mode domain with amplitudes near two millimag reportedby the STEPHI group are not seen in the new campaign data. The new datashow average noise levels just below one millimag in this frequencydomain, so that the existence of these two frequencies cannot bedisproved from the new data alone. However, a reanalysis of the STEPHIdata also suggests that these frequencies might not be real. The resultsfrom the two multisite campaigns should be free of 1 c/d aliasing, sothat a resonance hypothesis involving f2 = f1 +f3 can be rejected. The value of the pulsation constantassociated with f3, Q = 0.046 +/- 0.008 d, is too high by 2standard deviations to be explained by radial pulsation. The Q value isincompatible with those of nonradial p1 to p4modes commonly found in delta Scuti stars.
| The radial velocity curve and the two-dimensional spectral classification of the Delta SCT variable HD 18878
The radial velocities of HD 18878 were measured from spectra of this newDelta Sct type variable. The radial velocity curve with elementsdetermined from the photoelectric observations of Frolov et al. (1990)is derived. The mean radial velocity is -20 km/s, and the amplitude ofits variation is 34 km/s. On the basis of spectral criteria and thelocation of this star on a two-color diagram, it is argued that HD 18878is an A9III star.
| High-precision time-resolved CCD photometry
High-precision time-resolved photometry and detection of low-amplitudestellar oscillations are limited in precision by many factors. The noiseterms for a typical time-series observation contain observational noiseand errors as well as instrumental noise and reduction noise. This paperdescribes some of the problems related to high-precision photometry,especially focusing on CCD photometry both on focused stars (in semi-and noncrowded fields) and defocused field stars. It is shown thatatmospheric transparency variations and instrumental drift are the mainnoise sources at low frequencies (below 1-2 mHz), and that CCDphotometry is the best way of doing differential measurements in orderto detect oscillations at low frequencies. The theoretical descriptionof noise and problems in high-precision photometry is compared with CCDtime-series observations in a noncrowded test field observed with theDanish 1.5-m telescope at ESO La Silla, Chile in 1990. In addition,observations of defocused double stars observed with the Nordic OpticalTelescope in 1991 are described.
| The Hercules H I shell - Distances of the gas clouds and topography of the shell
The structure and distances of high-latitude (l between 35 and 75 deg)neutral gas clouds constituting the galactic H I shell GS 57 + 41(which, due to its location in the constellation of Hercules was named'Hercules shell') are investigated, using the results of an extensivesurvey of interstellar Na I D absorption lines to determine distances tothe various neutral clouds and filaments in this field of the sky. Basedon the distances, conclusions are drawn with regard to the morphologicalstructure and the physical parameters of the GS 57 + 41. The results ofoptical high-resolution absorption and 21 cm emission line measurementsof the various H I clouds were compared with the cirrus of IRAS 100micron dust emission, as well as with the diffuse X-ray backgroundemission in this area.
| UBV photometry of stars whose positions are accurately known. VI
Results are presented from UBV photometric observations of 1000 stars ofthe Bright Star Catalogue and the faint extension of the FK5.Observations were carried out between July 1987 and December 1990 withthe 40-cm Cassegrain telescope of the Kvistaberg Observatory.
| Rapid photometry of the Delta Scuti variable 63 Herculis and of the F 2V star HD 155543 - Observations and analysis of the time series
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991A&A...246...71B&db_key=AST
| A theoretical interpretation of the Delta Scuti-like pulsation of the star 63 Herculis
Theoretical pulsation frequencies derived from a typical modelcomputation are used to identify the periods during which the DeltaScuti star 63 Herculis oscillates. The frequencies are computed in theadiabatic and linear approximations and consideration is given to p andg modes. The possibility of rotational splitting is considered.
| Empirical P-L-C relation for Delta Scuti stars - A catalogue
An extensive and up-to-date list of 192 Delta Scuti stars is presented.Empirical period - luminosity - color (P-L-C) relations are obtained forthe four lowest modes corresponding to radial pulsations. Agreement withpredicted values indicates that, in general, both Stroemgren photometriccalibration and pulsation theory work well for these stars.
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