Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| 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.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| The long-period companions of multiple stars tend to have moderate eccentricities
We examined the statistics of an angle gamma between the radius vectorof a visual companion of a multiple star and the vector of its apparentrelative motion in the system. Its distribution f(gamma ) is related tothe orbital eccentricity distribution in the investigated sample. Wefound that for the wide physical subsystems of the 174 objects from theMultiple Star Catalogue f(gamma ) is bell-shaped. The Monte-Carlosimulations have shown that our f(gamma ) corresponds to the populationof the moderate-eccentricity orbits and is not compatible with thelinear distribution f(e)=2e which follows from stellar dynamics andseems to hold for wide binaries. This points to the absence of highlyelongated orbits among the outer subsystems of multiple stars. Theconstraint of dynamical stability of triple systems is not sufficient toexplain the ``rounded-off'' outer orbits; instead, we speculate that itcan result from the angular momentum exchange in multiple systems duringtheir early evolution.
| On the delta Scuti variability in the pre-main sequence Herbig Ae star HR 5999
A reanalysis of photometric data for the bright, southern Herbig Ae starHR 5999 obtained by Praderie et al. (\cite{r17}) at ESO in 1985 showsdelta Scuti pulsation with the same period, 4.812 d-1, andsemi-amplitude, 6 mmag, found by Kurtz & Marang (\cite{r10}) in dataobtained from SAAO in 1994. HR 5999 is, therefore, a potential candidateto test Breger & Pamyatnykh's (\cite{r4}) predictions ofevolutionary period changes 10 to 100 times faster in pre-main sequencedelta Scuti stars than in post-main sequence stars. We argue that thelow frequency, low amplitude alpha 2 CVn variability of HR6000 with a period near 2 d, and its long-term variability are nohindrance to using it as a comparison star for study of the delta Scutivariability of HR 5999. With a separation of 44 arcsec between HR 5999and HR 6000, and similar brightnesses for the two stars, we urge the useof small telescopes with CCD photometers to obtain the long-term datanecessary for the study of period change. We point out that this can bedone under less-than-ideal photometric conditions and urban lightingwith CCD photometers on small telescopes which are widely available atsmall observatories, at universities and at the home observatories ofserious amateur astronomers.
| 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
| Excitation and visibility of high-degree modes in stars
Observational evidence for excitation of non-radial modes in stars isconfronted with the results of linear stability surveys for stellarmodels. We consider various types of pulsators on the upper mainsequence as well as stars in the Cepheid strip. Our stability surveycovers the whole range of spherical harmonic degrees, l, whereinstability is found. There is fair agreement between the theoreticalinstability strip and the location of ζ Oph stars, but the observedand calculated periods do not agree in some stars. We suggest thateither pulsation is not responsible for the ζ Oph phenomenon orelse there are serious errors in mode identification in these cases. Wedo not find instability at long periods for early B-type stars,supporting the idea that pulsation is not responsible for the periodicvariations in Be stars. The agreement between the observed andcalculated periods of high-degree modes in δ Sct stars is not verysatisfactory. This is attributed to problems in mode identification. Wediscuss unstable modes of high degree in Cepheid models as a possiblemechanism for the low-amplitude radial velocities seen in some starswithin the instability strip. We find, however, that the observedperiods are at least a factor of 2 longer than the calculated periods.Finally, we discuss the possibility of observing modes of high degreephotometrically. We suggest that a large number of high-degree modes maybecome detectable by future space-borne photometric missions. Theconfusion arising from these modes may greatly reduce the value of suchobservations for asteroseismology. However, they will be very importantin studying the mechanism of mode selection.
| UV observations of B to F-type stars.
Not Available
| 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.
| 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.
| MSC - a catalogue of physical multiple stars
The MSC catalogue contains data on 612 physical multiple stars ofmultiplicity 3 to 7 which are hierarchical with few exceptions. Orbitalperiods, angular separations and mass ratios are estimated for eachsub-system. Orbital elements are given when available. The catalogue canbe accessed through CDS (Strasbourg). Half of the systems are within 100pc from the Sun. The comparison of the periods of close and widesub-systems reveals that there is no preferred period ratio and allpossible combinations of periods are found. The distribution of thelogarithms of short periods is bimodal, probably due to observationalselection. In 82\% of triple stars the close sub-system is related tothe primary of a wide pair. However, the analysis of mass ratiodistribution gives some support to the idea that component masses areindependently selected from the Salpeter mass function. Orbits of wideand close sub-systems are not always coplanar, although thecorresponding orbital angular momentum vectors do show a weak tendencyof alignment. Some observational programs based on the MSC aresuggested. Tables 2 and 3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Nonvariability among lambda Bootis stars
With asteroseismic techniques it is possible to investigate the interiorand the evolutionary status of stars via their frequency spectrum. Bothinformation would be very much needed for lambda Bootis stars, a groupof metal-poor Population I, A-type stars, since no conclusive theoryexists explaining the observed abundance anomalies. Geneva and Stromgrenphotometry place these stars inside the classical instability strip orat least very close to it. We therefore have started an extensivephotometric survey for pulsation in lambda Bootis stars and havediscovered so far 13 new variables. In this paper we present results forstars which presumably are constant, because we are able to establishonly an upper level for possible variability. A typical noise level of 3mmag for Stromgren b was achieved in the relevant frequency domain up to100 d^{-1}. Considering the given noise level of our survey, we concludethat at least 50% of all investigated lambda Bootis stars inside theinstability strip are pulsating, making this group remarkable comparedto stars with similar spectral types. This may suggest that a low(surface) metallicity has an influence on the pulsation Based onobservations obtained at ESO-La Silla, CTIO, SAAO, McDonald Observatory,Instituto Astrofisica Andalucia Observatory.
| A search for beta Pictoris-like CA II circumstellar gas around Ursa Major Stream stars
We carried out an optical spectroscopic survey of twenty one stars tosearch for Caii circumstellar gas at the stellar velocity similar tothat found in the beta Pictoris disk. The surveyed stars are A-typestars with large v sin i in the Ursa Major stellar kinematic group. Noevidence of circumstellar gas was found around any of the stars. Thisresult suggests that beta Pictoris either is a very peculiar objectsurrounded by an anomalous amount of gas, or is younger than the starsin the Ursa Major Group ( ~ 3\ 10(8) years). Based on observationscollected at Observatoire de Haute Provence, CNRS, France
| 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.
| The Line-Profile Variations of Theta2 Tauri
With a 5.4-hour time series of high-resolution CFHT spectra, theoscillations of the rapidly-rotating delta Scuti star Theta2 Tauri (A7IV + A5 V) are investigated. Line-profile variations are discovered andidentified with the spectrum of the primary star of this binary system.Radial velocity variations are derived from the data and analyzed toreveal an oscillation frequency near 13.7 cycles day^-1. Variationswithin the absorption profiles are analyzed with a Fourier-DopplerImaging (FDI) technique to reveal that high-degree oscillations with afrequency of 16.1 cycles day ^-1 and an apparent degree of about l = 8are also present in the star. Whereas the low-degree variations ofTheta2 Tau have been shown to be consistent with p-modes of radial ordern=2 or 3, the identification of the high-degree variations in terms ofpressure modes requires confirmation with theoretical models. (SECTION:Stars)
| Nonvariability among lambda Bootis Stars III.: CTIO (1995) and McDonald (1995) Data
Not Available
| UV survey of δ scuti stars: final results and open problems
Not Available
| The photoelectric astrolabe catalogue of Yunnan Observatory (YPAC).
The positions of 53 FK5, 70 FK5 Extension and 486 GC stars are given forthe equator and equinox J2000.0 and for the mean observation epoch ofeach star. They are determined with the photoelectric astrolabe ofYunnan Observatory. The internal mean errors in right ascension anddeclination are +/- 0.046" and +/- 0.059", respectively. The meanobservation epoch is 1989.51.
| 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
| Lithium Abundances in delta Scuti Stars and Their Relationship with the Lithium Dip
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...451..747R&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.
| CCD photometry of the δ-Scuti star κ^2^ Bootis.
CCD time-series differential photometry of the binary star κBoohas been carried out from a small in town situated telescope. Onecomponent of the system is known to be a δ-Scuti pulsator. Due tothe high S/N data, four frequencies have been detected even though thewindow function is quite complicated. A search for a standard model thatfits all available data, including the observed mode frequencies,produces a model, which fits everything well, but with an unlikelycombination of modes. This demonstrates the necessity of redundant dataof very high quality, if the seismic studies shall lead to progress inthe understanding of stellar evolution. An interesting byproduct is thatthe analysis leads to precise prediction for observables that we expectto have in the near future, such as the distance to be published in theHipparcos catalog.
| Asteroseismology
Not Available
| 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.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Ökörhajcsár |
| Rektaszcenzió: | 14h13m29.00s |
| Deklináció: | +51°47'25.0" |
| Vizuális fényesség: | 4.54 |
| Távolság: | 47.551 parszek |
| RA sajátmozgás: | 60.5 |
| Dec sajátmozgás: | -10.6 |
| B-T magnitude: | 4.754 |
| V-T magnitude: | 4.535 |
Katalógusok és elnevezések:
|
INHABITED SKY
