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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}
| Circumnuclear structure and kinematics in the active galaxy NGC 6951
A study is presented of the central structure and kinematics of thegalaxy NGC 6951, by means of broad band B{\arcminIJK} images and highresolution high dispersion longslit spectroscopy, together with archivalHST WFPC2 V and NICMOS2 J and H images. We find that there is littleongoing star formation inside the bar dominated region of the galaxy,except for the circumnuclear ring at 5 arcsec radius. There is someevidence that this star formation occurs in two modes, in bursts andcontinuously, along the ring and inwards, towards the nucleus. Theequivalent width of the Ca Ii triplet absorption lines show that, in themetal rich central region, the continuum is dominated by a population ofred supergiants, while red giants dominate outside. The gaseouskinematics along three slit position angles, and the comparison with thestellar kinematics, suggest the existence of a hierarchy of disks withindisks, whose dynamics are decoupled at the two inner Linblad resonances(ILR), that we find to be located at 180 pc and at 1100 pc. This issupported by the structure seen in the high resolution HST images. Thenucleus is spatially resolved in the emission line ratio [N Ii]/Hα, and in the FWHM of the emission lines, within a radius of1.5{\arcsec}, just inside the innermost ILR. Outside the iILR, thestellar CaT velocity profile is resolved into two different components,associated with the bar and the disk. Several results indicate that thisis a dynamically old system: the little ongoing star formation insidethe bar dominated part of the galaxy, the very large relative amount ofmolecular to total mass within the inner 6 arcsec radius, ~ 25%, and thegeometry of the circumnuclear ring that leads the stellar bar at aposition angle greater than 90°. It is thus possible that a nuclearbar has existed in NGC 6951 that drove the gas towards the nucleus, asin the bars within bars scenario, but that this bar has alreadydissolved by the gas accumulated within the circumnuclear region. Wediscuss the possibility that the kinematical component inside the iILRcould be due to a nuclear outflow produced by the combined effects of SNand SN remnants, or to a nuclear disk, as in the disk within diskscenario that we propose for the fueling of the AGN in NGC 6951. Basedon observations made with the 4.2m William Herschel Telescope operatedby the Isaac Newton Group, and the 2.6m Nordic Optical Telescopeoperated jointly by Denmark, Finland, Iceland, Norway, and Sweden, onthe island of La Palma in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofísica de Canarias. Also basedon observations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the ESA Space Telescope European CoordinatingFacility.
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Observation and Astrometry data
| Constellation: | Πήγασος |
| Right ascension: | 21h58m33.47s |
| Declination: | +15°18'26.0" |
| Apparent magnitude: | 7.801 |
| Distance: | 192.308 parsecs |
| Proper motion RA: | 1.6 |
| Proper motion Dec: | 9.3 |
| B-T magnitude: | 9.062 |
| V-T magnitude: | 7.906 |
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