4 Clustering

 

4.1 Searching for clusters

 (X, Y, Z) distributions range from -125 pc to +125 pc and are binned in a Sun centered orthonormal frame, X-axis towards the galactic center, Y-axis in the rotation direction and Z-axis towards the north galactic pole. The discrete wavelet analysis is performed on five scales: 9.7, 13.6, 21.5, 37.1 and 68.3 pc. These values correspond to the size of the dilated filter h at each scale.

Stars belonging to each volume defined by significant coefficients are collected. Due to the over-sampling of the signal by the "à trou'' algorithm, some structures are detected on several scales. A cross-correlation has been done between all scales to keep the structure at the largest scale provided that there is no sub-structure at a lower scale (higher resolution). We control a posteriori the efficiency of the detection by calculating for each over-density the probability to find an identical concentration from the total volume of the sample in a uniform random population (see probability P₂ in Table 1). The method also succeeds in finding over-densities with a high probability to be generated by a poissonian process. An iterative 2.5 sigma clipping procedure is done on tangential velocity distributions for each group to remove field stars and select structures with coherent kinematics (Table 1 and Figs. 6, 7, 8, 9 and 10).

  

Figure 6: Lower Centaurus-Crux after selection on tangential velocities (top) and its age distribution (bottom)

  

Table 1: Main characteristics of detected spatial structures after iterative 2.5 sigma clipping procedure on tangential velocities: galactic coordinates (l,b), distance D, mean tangential velocities ($\overline{T_{\rm l}}$, $\overline{T_{\rm b}}$), tangential velocity dispersions ($\sigma_{T_{\rm l}}$, $\sigma_{T_{\rm b}}$), number of selected stars $N_{\rm sel}$. Other parameters are given: scales of detection S, volume v occupied by the total over-density, number of expected stars $N_{\rm e}$ in this volume according to the mean density, number of observed stars $N_{\rm o}$ before selection on tangential velocities, probability P₁ to find such a concentration from a poissonian distribution in this v volume and probability P₂ to find this volume v with this concentration within the 125 pc radius sphere. A value of .001 indicates a probability $P~\leq~0.001$

4.2 Detailed cluster characteristics

 The volumes selected by the segmentation procedure content 10 per cent of the stars. After the 2.5 sigma clipping procedure on the tangential velocities, only 7 per cent are still in clusters or groups. Most of them are well known: Hyades, Coma Berenices, Ursa Major open clusters (hereafter OCl) and the Scorpio-Centaurus association. The following cases are the most interesting either because of newly discovered features associated to well known structures (Scorpio-Centaurus association) or simply because they were undetected up to now (Bootes and Pegasus 1 and 2).

1.

The Scorpio-Centaurus association shows three precisely limited over-densities with an elongated shape crossing the galactic disc (see scale 4 on Paper II, Fig. 5): lower Cen-taurus-Crux, upper Centaurus-Lupus which both have identical proper motions and a third unknown clump at $(l=325.4^{\rm o}$,$b=-34.4^{\rm o})$ with slightly different proper motions along l axis (see cluster 9 in Table 1). We do not find previous mention of this extension of the Scorpio-Centaurus. Unfortunately very few of these stars have Strömgren photometry. In the case of Centaurus-Crux (see Figs. 6), among 33 stars, 3 stars having a Strömgren age are rather old. One of these stars is flagged as a double system in the Hipparcos catalogue, which could alter the Strömgren photometry. The two others probably do not belong to the association but are not rejected by the selection on tangential velocities because of proper motions close to the association ones. However this association is known to be an O-B association, which means that it is younger than a few tens of million years. The palliative age distributions show that all the three clumps are composed by the youngest stars of the sample. The analysis of the velocity field (see Sect. 5.3.1) reveals that these density inhomogeneities are embedded in a stream-like structure moving through all the 125 pc sphere.

Two new structures are found both probably on their way towards disruption. In the following these loose clusters are designated after the constellation they are found in.

  

Figure 7: Coma Berenices open cluster after selection on tangential velocities ( top) and its age distribution (bottom)

  

Figure 8: Bootes loose cluster after selection on tangential velocities (top) and its age distribution (bottom)

2.

Bootes (Fig. 8) is composed of only 5 stars at a mean distance of 105 pc and centred on $(l=97.6^{\rm o},b=+56.6^{\rm o})$. This small clump is interesting for several reasons. It is located at the same x and z coordinates as Coma Ber. OCl (Fig. 7) but 62 pc ahead along y-axis with exactly the same space velocity components (U, V, W)=(-3.1, -7.8, -0.8) km s^-1. These coincidences suggest a common origin for these two structures. But their respective age distributions (Figs. 7 and 8) show that Bootes is probably younger (peak at 10⁷ yr for the palliative age distribution) than Coma Ber. OCl which is $\sim 6~10^{8}$ yr old. Moreover, the smaller tangential velocity dispersions of Bootes tends to confirm this hypothesis. A more detailed investigation among other spectral types is necessary to better define Bootes in velocity space as well as in age content.

  

Figure 9: Pegasus 1 after selection on tangential velocities (top) and its age distribution (bottom)

  

Figure 10: Pegasus 2 after selection on tangential velocities (top) and its age distribution (bottom)

3.

Pegasus appears to be composed of two different velocity components (Figs. 9, 10). This feature do not permit to extract coherent velocity structures by means of the sigma clipping procedure. A hand selection was realized on the basis of the whole tangential velocity distribution. Pegasus 1 with 10 stars and Pegasus 2 with 8 stars are obtained with mean distances of respectively 97 and 89 pc and centred on $(l=60.7^{\rm o},-32.5^{\rm o})$ and $(54.2^{\rm o},-31.3^{\rm o})$. Their age distributions are not very precise because of the lack of Strömgren ages. However, it seems that Pegasus 1 has an age peak at 8 10⁸ yr. It has no very young ages but 3 Strömgren ages are between 8 10⁸ and 1.2 10⁹ yr which would mean an age older than Pegasus 1. This complex structure gives an instantaneous snapshot of the interactions affecting groups, clusters or associations: merging-like process can take part in dissolving spatial inhomogeneities.