תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| The 77th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1152 variable stars recently designated in the system ofthe General Catalogue ofVariable Stars. New designations are alsointroduced for two ``old'' variables whose corrected coordinates bringthem to other constellations.
| Study of Suggested New R Coronae Borealis Stars HAD V98 and HD 172468
Previous observers have suggested that the lightcurve activity of thestars HAD V98 and HD 172468 may indicate that they are R CoronaeBorealis (RCB) stars. The Harvard photographic plate collection wassearched for plates containing these stars, and their activity wasexamined between 1899 and 1989. No fadings were found for either star.However, for HAD V98 in particular, coverage was not complete enough torule out classification as an RCB star. Additional photometry for bothstars was collected from the literature, VSNET and the AAVSO. Newphotometry was obtained using the 31in telescope at Lowell Observatory.Low resolution spectra were also obtained, using Steward Observatory's90in telescope on Kitt Peak. Based on the spectra and long-termlightcurve, we conclude that HAD V98 is indeed an RCB star, while HD172468 is not. The classification of HD 172468 remains to be determined.This project was supported by the NSF/REU grant AST-0097694 and theNantucket Maria Mitchell Association.
| Drei spannende Woche mit HD 172468.
Not Available
| Serendipitous discovery of an irregular and a semi-regular type variable in the field of BY Draconis
We present new evidence of the optical variability of two red giantstars: HD 172468 and HK Dra, based on photometric and spectroscopicobservations. These stars had been included as check stars in ourphotometric monitoring program of BY Dra and turned out to be variable.HD 172468, whereas almost constant for most of the time, suddenlystarted to drop in brightness to such a low level to becomeundetectable. We suspect that such an abrupt event may be an``obscurational'' minimum, that is typical of eruptive RCB stars, or maybe due to the variable extinction by circumstellar dust in a young Oriontype object. HK Dra, already known as an irregular variable, ischaracterised by periodic flux modulation with season-to-season changesof the photometric period, as inferred from a periodogram analysis. Italso shows changes of the light curve peak-to-peak amplitude and shape.Such a behaviour in giant stars is commonly found among semi-regulargiants (SR) at the Asymptotic Giant Branch (AGB). Our radial velocitymeasurements rule out that HK Dra may be a close binary system.
| Photometry of the spotted flare star by DRA during 1985-1991
Spot photometry in the UBV was obtained for the flare star BY Dra during1985-1991 at the Bulgarian National Astronomical Observatory. Lightcurves are presented for 1986, 1987, 1988, and 1991. There aresignificant variations of the amplitudes of the light curves implyingchanges of the spot distribution. The phases of the light minima arevariable, indicating possible latitudinal migration of the spot anddifferential rotation. Rough estimate of the corresponding latitudinalshear is in the range 3 x 10 exp -11 - 5 x 10 exp -10 rad/s/deg. Thephotometric behavior of BY Dra since 1960 shows no compeling evidencefor existence of stellar cycles. This is in contradiction with previousstudies. During 1985-1991 a total of 38.47 hours of monitoring in theU-filter were obtained. Analysis of the flare activity of BY Dra showsno significant changes from previous studies.
| BV Light Curves of BY Dra in 1986
Not Available
| Slow Variations in the Light of By-Draconis
Not Available
| Flare activity of the star BY DRA
Not Available
| BVRI observations of BY Draconis during 1975 and 1976.
Abstract image available at:http://adsabs.harvard.edu/abs/1977ApJ...214..140D
| A Flare Event on the dM0e Variable BY Draconis
Not Available
| On the Variability of HD 234677
Not Available
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | דרקון |
| התרוממות ימנית: | 18h37m41.06s |
| סירוב: | +51°56'45.5" |
| גודל גלוי: | 7.528 |
| מרחק: | 321.543 פארסק |
| תנועה נכונה: | -2 |
| תנועה נכונה: | 4.1 |
| B-T magnitude: | 9.161 |
| V-T magnitude: | 7.663 |
קטלוגים וכינוים:
|
