Precision kinematics and related parameters of the α Persei open cluster

A kinematical study of the nearby open cluster α Persei is presented based on the astrometric proper motions and positions in the Tycho-2 catalog and Second USNO CCD Astrographic Catalog (UCAC2). Using the astrometric data and photometry from the Tycho-2 and ground-based catalogs, 139 probable members of the cluster are selected, 18 of them new. By the classical convergent point method, systematic motions of stars inside the cluster and velocity dispersions are estimated. As directly observed, the upper limit on the internal velocity dispersion per coordinate is 1.1 km s^(-1). The actual velocity dispersion is much smaller than that value, since all of it appears to come from the expected errors of the astrometric proper motions. The relative position of the convergent point with respect to the cluster stars yields the "astrometric" radial velocity, which turns out larger by a few km s^(-1) than the mean observed spectroscopic radial velocity. This implies an overall contraction of the cluster. Kinematic parallaxes are computed for each member, and an improved H-R diagram is constructed. An age of 52 Myr is determined by isochrone fitting. The star α Per itself fits an isochrone of this age computed with overshooting from the boundary of the convective zone. The theoretical mass of the star α Per is 6.65 M_⊙. With respect to the common center of mass, half of the higher mass members (earlier than G) are located within a radius of 10.3 pc. The cluster appears to be roughly twice as large, or as sparse, as the Pleiades, retaining nonetheless a similar dynamical coherence. The low rate of binaries is another feature of this cluster, where we find only about 20% of members to be known or suspected spectroscopic, astrometric, or visual binaries or multiple systems. X-ray emitters in the cluster appear to have the same dispersion of internal velocities as the rest of the membership. The cluster is surrounded by an extended, sparse halo of comoving dwarfs, which are found by combining the proper-motion data from UCAC2 with Two Micron All-Sky Survey infrared photometry. Since many of these external stars are outside the tidal radius, the cluster being in an active stage of disintegration or evaporation could be considered. This hypothesis is not supported by the weak compression and the nonmeasurable velocity dispersion found in the kinematic analysis. A search for stars ejected from the α Persei cluster is carried out by tracking a large number of nearby stars 70 Myr back in time and matching their positions with the past location of the cluster. Only one plausible ejection is found prior to 10 Myr ago. The nearby star GJ 82, an active M dwarf with a strong Hα emission, is likely a former member ejected 47 Myr ago at 5 km s^(-1)

Binarity of the Nearby 30 Myr Old Solar Analog HIP 16853 in the Tucana-Horologium Stream

A robust unconstrained orbital solution is obtained for the G2 V star HIP 16853 = HD 22705 at 42 pc, which is a probable member of the 28-30 Myr old Tucana-Horologium stream of post-T Tauri stars. The solution yields an apparent semimajor axis of 5.1 ± 0.7 mas, a period of 201 ± 2 days, and an inclination of 80° ± 7°. Assuming a mass of 1 M_⊙ for the primary, the close companion is only 0.4 M_⊙, which implies a spectral type M0.5 at this age. The expected maximum separation (a) between the companions is 18 mas, which makes this system amenable for high-resolution observations. The wide companion HIP 16853 B at 14" is investigated as a possible tertiary component but rejected on account of the near-infrared photometric data inconsistent with the well-defined H-R diagram of the Tucana-Horologium group

Unraveling the Origins of Nearby Young Stars

A systematic search for close conjunctions and clusterings in the past of nearby stars younger than the Pleiades is undertaken, which may reveal the time, location, and mechanism of formation of these often isolated, disconnected from clusters and star-forming regions, objects. The sample under investigation includes 101 T Tauri, post-TT, and main-sequence stars and stellar systems with signs of youth, culled from the literature. Their Galactic orbits are traced back in time and near approaches are evaluated in time, distance, and relative velocity. Numerous clustering events are detected, providing clues to the origin of very young, isolated stars. Each star's orbit is also matched with those of nearby young open clusters, OB and TT associations and star-forming molecular clouds, including the Ophiuchus, Lupus, Corona Australis, and Chamaeleon regions. Ejection of young stars from open clusters is ruled out for nearly all investigated objects, but the nearest OB associations in Scorpius-Centaurus, and especially, the dense clouds in Ophiuchus and Corona Australis have likely played a major role in the generation of the local streams (TWA, Beta Pic, and Tucana-Horologium) that happen to be close to the Sun today. The core of the Tucana-Horologium association probably originated from the vicinity of the Upper Scorpius association 28 Myr ago. A few proposed members of the AB Dor moving group were in conjunction with the coeval Cepheus OB6 association 38 Myr ago

Chaotic rotation and evolution of asteroids and small planets in high-eccentricity orbits around white dwarfs

Observed planetary debris in white dwarf atmospheres predominately originate from the destruction of small bodies on highly eccentric (>0.99) orbits. Despite their importance, these minor planets have coupled physical and orbital evolution, which has remained largely unexplored. Here, we present a novel approach for estimating the influence of fast chaotic rotation on the orbital evolution of high-eccentricity triaxial asteroids, and formally characterize the propagation of their angular rotation velocities and orbital elements as random time processes. By employing the impulse approximation, we demonstrate that the violent gravitational interactions during periastron passages transfer energy between the orbit and asteroid's rotation. If the distribution of spin impulses were symmetric around zero, then the net result would be a secular decrease of the semimajor axis and a further increase of the eccentricity. We find evidence, however, that the chaotic rotation may be self regulated in such a manner that these effects are reduced or nullified. We discover that asteroids on highly eccentric orbits can break themselves apart—in a type of YORP-less (Yarkovsky–O'Keefe–Radzievskii–Paddack) rotational fission—without actually entering the Roche radius, with potentially significant consequences for the distribution of debris and energy requirements for gravitational scattering in metal-polluted white dwarf planetary systems. This mechanism provides a steady stream of material impacting a white dwarf without rapidly depleting the number of small bodies in the stellar system