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Appendix A: Outlier analysis

Some objects present proper motions that are clearly discrepant from those of the stars in Groups A and B. In this Appendix, we discuss the reliability of the proper motion measurements for these objects, as listed in Table A.1.

Initially, nine sources were identified as outliers in the vector point diagram: four in Lupus 1, four in Lupus 3, and one more in Lupus 4. To check the reliability of the reported proper motions, we visually inspected these objects with the peculiar proper motions using Aladin. We compared and blinked two sets of images separated several decades in time, from the optical DSS-1 and the near-infrared 2MASS surveys, to easily confirm the high proper motion of our candidates. The 2MASS (Skrutskie et al. 2006) sources and UCAC3 counterparts were superimposed on the images to assess the reliability of the cross-match. Besides, we used other available astro-photometric databases, such as the Astrographic Catalogue AC2000.2 (Urban et al. 1998), the SuperCOSMOS Sky Survey, the USNO-B Catalog and the PPMX catalogue, to verify the peculiar proper motions.

From the original group of nine sources, six turned out to have reliable proper motions that are discrepant from those of Groups A and B. Figures 2 to 4 show their location and their expected displacement after 10⁵ yr. Although most of the outliers are located outside the densest cloud areas, two stars (153940.8-333941 in Lupus 1 and 160908.5-390213 in Lupus 3) are seen towards the cloud cores. The objects from Table A.1 with wrong proper motions are not plotted in these figures.

The proper motion values and directions differ widely among these sources. Three objects are seen not far, but clearly detached, from Groups A and B in the vector point diagram, having μ ≲ 50 mas/yr: 154344.5-335834 in Lupus 1 and 161001.1-384315 and Sz 95 in Lupus 3. We note that the latter has a reported visual companion at a separation of  ~ 3″ according to Comerón (2008), but this second object has no UCAC3 counterpart.

The remaining sources in the list of outliers have 50 < μ < 100 mas/yr, except for 154144.0-343530 in Lupus 1, which has an even higher value, μ ~ 270 mas/yr. Such high proper motions suggest they might be foreground sources.

With a single exception (Sz 95), all these outliers are diskless stars according to the SED analysis (Merín et al. 2008; Comerón et al. 2009), a fact that is consistent with them belonging to the field. However, ejected members from star-forming regions are expected to possess only truncated disks, which would thus dissipate much faster than disks around the in-cloud population. Should some of them be members of the Lupus clouds, these objects would have masses around 0.3 − 0.4   M_⊙ according to the SED-fitting results by Comerón et al. (2009), very close to the median mass of the stars in the region.

© ESO, 2011