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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}
| Bispectrum speckle interferometry of IRC +10216: The dynamic evolution of the innermost circumstellar environment from 1995 to 2001
We present new near-infrared (JHK) bispectrum speckle-interferometrymonitoring of the carbon star IRC+10216 obtainedbetween 1999 and 2001 with the SAO 6 m telescope. The J-, H-, and K-bandresolutions are 50 mas, 56 mas, and 73 mas, respectively. The totalsequence of K-band observations covers now 8 epochs from 1995 to 2001and shows the dynamic evolution of the inner dust shell. The presentobservations show that the appearance of the dust shell has considerablychanged compared to the epochs of 1995 to 1998. Four main componentswithin a 0\farcs2 radius can be identified in the K-band images. Theapparent separation of the two initially brightest components A and Bincreased from ~ 191 mas in 1995 to ~ 351 mas in 2001. Simultaneously,component B has been fading and almost disappeared in 2000 whereas theinitially faint components C and D became brighter (relative to peakintensity). The changes of the images can be related to changes of theoptical depth caused, for instance, by mass-loss variations or new dustcondensation in the wind. Our recent two-dimensional radiative transfermodel of IRC +10216 suggests that the observedrelative motion of components A and B is not consistent with the outflowof gas and dust at the well-known terminal wind velocity of 15 kms-1. The apparent motion with a deprojected velocity of 19 kms-1 on average and of recently 27 km s-1 appearsto be caused by a displacement of the dust density peak due to dustevaporation in the optically thicker and hotter environment. The presentmonitoring, covering more than 3 pulsation periods, shows that thestructural variations are not related to the stellar pulsation cycle ina simple way. This is consistent with the predictions of hydrodynamicalmodels that enhanced dust formation takes place on a timescale ofseveral pulsation periods. The timescale of the fading of component Bcan well be explained by the formation of new dust in the circumstellarenvelope.
| The dynamical evolution of the fragmented, bipolar dust shell around the carbon star IRC +10 216 . Rapid changes of a PPN-like structure?
We present high-resolution J-, H-, and K-band observations and the firstH-K color image of the carbon star IRC +10 216. Theimages were reconstructed from 6 m telescope speckle interferogramsusing the bispectrum speckle interferometry method. The H and K imageswith resolutions between 70 mas and 92 mas consist of several compactcomponents within a 0\farcs2 radius and a fainter asymmetric nebula. Thebrightest four components are denoted with A to D in the order ofdecreasing brightness in the 1996 image. A comparison of our images from1995, 1996, 1997, and 1998 gives -- almost like a movie of five frames-- insight into the dynamical evolution of the inner nebula. Forinstance, the separation of the two brightest components A and Bincreased from 191 mas in 1995 to 265 mas in 1998. At the same time,component B is fading and the components C and D become brighter. TheX-shaped bipolar structure of the nebula, most prominently present inthe J-band image, implies an asymmetric mass-loss. Such asymmetries areoften present in protoplanetary nebulae but are unexpected for AGBstars. IRC +10 216 is thus likely to be very advancedin its AGB evolution, shortly before turning into a protoplanetarynebula. The cometary shapes of A in the H and J images and in the 0.79mu m and 1.06 mu m HST images suggest that the core of A is not thecentral star, but the southern lobe of a bipolar structure. The positionof the central star is probably at or near the position of component B,where the H-K color has a value of 4.2. If the star is at or near B,then the components A, C, and D are likely to be located at the innerboundary of the dust shell. Based on observations performed with the 6~mtelecope at the Special Astrophysical Observatory, Russia.
| 76mas speckle-masking interferometry of IRC+10216 with the SAO 6m telescope: Evidence for a clumpy shell structure
We present the first K(') -band image of the carbon star IRC+10216 with76mas resolution. The diffraction-limited image was reconstructed from6m telescope speckle data using the speckle masking bispectrum method.The image shows that the dust shell of IRC+10216 is extremely clumpy.Five individual clouds within a 0farcs21 radius of the central star havebeen resolved for the first time. On the basis of consistent theoreticalmodels we argue that these structures are produced by circumstellar dustformation. The fragmentation of the shell structure gives most likelydirect evidence for an inhomogeneous mass-loss process which may beinterpreted in terms of large-scale surface convection-cells(Schwarzschild \cite{Schwschil_75}) being a common phenomenon for redgiants.
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| Takýmyýldýz: | Aslan |
| Sað Açýklýk: | 10h26m45.22s |
| Yükselim: | +13°37'35.1" |
| Görünürdeki Parlaklýk: | 7.317 |
| Uzaklýk: | 364.964 parsek |
| özdevim Sað Açýklýk: | -35.4 |
| özdevim Yükselim: | -16.7 |
| B-T magnitude: | 9.367 |
| V-T magnitude: | 7.487 |
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