Mamajek 18: An Over-Density in Sco-Cen at D = 175 pc

Between the famous Upper Sco OB subgroup of Sco-Cen, and the Lupus and CrA star-forming complexes, lies a region of young (~10 Myr-old) stars at d ~ 100-200 parsecs dubbed "Lower Sco". Within the Lower Sco subregion of Sco-Cen is a very strong over-density of pre-main sequence stars which may be a candidate open cluster (indexed as "Mamajek 18"). High resolution optical spectroscopy of dozens of X-ray-emitting stars in this group were taken with the MIKE spectrograph on the Magellan Clay 6.5-m telescope in July 2007, and low-resolution spectra taken with the SMARTS 1.5-m in 2010, revealing most of the stars to be Li-rich FGK pre-MS stars. The median radial velocity of the 18 Li-rich stars in this clump is +1.7+-1.0 km/s. Cross-referencing the candidate pre-MS stars in this cluster with the Gaia DR1 data (http://www.cosmos.esa.int/web/gaia/dr1; Gaia collaboration and Brown et al. 2016, A&A, in press, http://www.aanda.org/articles/aa/pdf/forth/aa29512-16.pdf) shows that the cluster stars have mean proper motion pmRA, pmDec = -12.2, -21.7 (+-0.3, 0.4) mas/yr. The clipped Gaia DR1 mean parallax of these stars is 5.73+-0.3 mas (approximate distance 175+-9 pc; dominated by systematic parallax error 0.3 mas). The resultant space motion for the group is U,V,W = -3.9, -19.9, -4.2 (+-1.0, 0.9, 0.6) km/s, with total heliocentric space motion 20.7+-0.6 km/s - similar to the other subgroups of Sco-Cen.