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Dynamical Evolution and Spectral Characteristics of the Stellar Group Mamajek 2
The dynamical evolution of the recently detected stellar group Mamajek 2is studied by means of its past three-dimensional orbit. The past orbitsof the open clusters NGC 2516 and ? Persei, belonging to theso-called "Local Association," were also computed in order to check fora possible common past dynamical evolution of these systems. To completethe data of the Mamajek 2 small group, we have obtained high-resolutionFEROS spectra to measure the radial and also the projected rotationalvelocities of its members; an estimate of its metallicity was obtainedas well. Two exceptionally low-rotating A-type stars turned out to be astrong magnetic Ap star in one case, and a normal A0 star withnear-solar metallicity in the other. The dynamical results showed thatNGC 2516 and Mamajek 2 may have had a common origin at the age of 135± 5 Myr. This dynamical age confirms the individual ages of 140Myr for NGC 2516 and 120 ± 25 Myr for Mamajek 2 obtainedindependently by photometric methods. Both these groups appear to havethe same solar metallicity giving support to a common birth scenario.The dynamical approach is showing that some bound open clusters can formin a coeval fashion with unbound stellar groups or with associations.
| A New Nearby Candidate Star Cluster in Ophiuchus at d~=170 pc
The recent discoveries of nearby star clusters and associations within afew hundred pc of the Sun, as well as the order of magnitude differencein the formation rates of the embedded and open cluster populations,suggests that additional poor stellar groups are likely to be found atsurprisingly close distances to the Sun. Here I describe a new nearbystellar aggregate found by virtue of the parallel proper motions,similar trigonometric parallaxes, and consistent color-magnitudedistribution of its early-type members. The 120 Myr old group lies inOphiuchus at d~=170 pc, with its most massive member being the 4thmagnitude post-main-sequence B8 II-III star μ Oph. The group may haveescaped previous notice due to its nonnegligible extinction(AV~=0.9 mag). If the group was born with a normal initialmass function, and the nine B- and A-type systems represent a completesystem of intermediate-mass stars, then the original population wasprobably of order ~200 systems. The age and space motion of the newcluster are very similar to those of the Pleiades, the α Percluster, and the AB Dor moving group, suggesting that these aggregatesmay have formed in the same star-forming complex some ~108 yrago.
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