Utilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca ii H and K emission measurements, literature-based abundance estimates, and updated photometry (including recent resolved measurements of close doubles), we revisit the Ursa Major moving group membership status of some 220 stars to produce a final clean list of nearly 60 assured members, based on kinematic and photometric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based membership assignments, indicating the usefulness of criteria based on photometric and chromospheric emission to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, failing to do so only for 15% of objects, perhaps considerably less. Our UMa members demonstrate nonzero vertex deviation in the Bottlinger diagram, behavior seen in older and recent studies of nearby young disk stars and perhaps related to Galactic spiral structure. Comparison of isochrones and our final UMa group members indicates an age of 500 AE 100 Myr, some 200 Myr older than the canonically quoted UMa age. Our UMa kinematic=photometric members' mean chromospheric emission levels, rotational velocities, and scatter therein are indistinguishable from values in the Hyades and smaller than those evinced by members of the younger Pleiades and M34 clusters, suggesting these characteristics decline rapidly with age over 200-500 Myr. None of our UMa members demonstrate inordinately low absolute values of chromospheric emission, but several may show residual fluxes a factor of !2 below a Hyades-defined lower envelope. If one defines a Maunder-like minimum in a relative sense, then the UMa results may suggest that solar-type stars spend 10% of their entire main-sequence lives in periods of precipitously low activity, which is consistent with estimates from older field stars. As related asides, we note six evolved stars (among our UMa nonmembers) with distinctive kinematics that lie along a 2 Gyr isochrone and appear to be late-type counterparts to disk F stars defining intermediate-age star streams in previous studies, identify a small number of potentially very young but isolated field stars, note that active stars (whether UMa members or not) in our sample lie very close to the solar composition zero-age main sequence, unlike Hipparcos-based positions in the H-R diagram of Pleiades dwarfs, and argue that some extant transformations of activity indices are not adequate for cool dwarfs, for which Ca ii infrared triplet emission seems to be a better proxy than H-based values for Ca ii H and K indices.

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The variability of the Hg ii 3984 line in the primary of the binary star And was discovered through the examination of high-dispersion spectra with signal-to-noise ratios greater than 500. This first definitively identified spectrum variation in any mercury-manganese star is not due to the orbital motion of the companion. Rather, the variation is produced by the combination of the 2.38236 day period of rotation of the primary that we determined and a nonuniform surface distribution of mercury that is concentrated in its equatorial region. If the surface mercury distribution exhibits long-term stability, then it is likely that a weak magnetic field operates in its atmosphere, but if changes are observed in the line profile over a period of a few years, then these would constitute direct evidence for diffusion.

Abstract. Elemental abundances analyses are performed for the Mercury-Manganese stars υ Her, φ Her, and HR 7018 consistent with previous studies of this series using spectrograms obtained with Reticon and CCD detectors. Comparisons of the first two analyses with those performed using coadded photographic plates show the general consistency of the derived elemental abundances. For υ Her and for φ Her, abundances were newly found for O, and for Al, V, Zn, and Ce, respectively. HR 7018 is discovered to be a single-lined spectroscopic binary. Its Sc abundance is the smallest of any class member with derived abundances and its Sr abundance the largest of any known HgMn star. A correlation analysis of the most complete abundance sets for 20 HgMn stars shows that the abundances of some elements are correlated with one another and some are functions of the stellar effective temperature.

The formation of long-lasting structures at the surfaces of stars is commonly ascribed to the action of strong magnetic fields. This paradigm is supported by observations of evolving cool spots in the Sun 1 and active late-type stars 2 , and stationary chemical spots in the early-type magnetic stars 3 . However, results of our seven-year monitoring of mercury spots in non-magnetic earlytype star α Andromedae show that the picture of magnetically driven structure formation is fundamentally incomplete. Using an indirect stellar-surface mapping technique, we construct a series of two-dimensional images of starspots and discover a secular evolution of the mercury cloud cover in this star. This remarkable structure-formation process, observed for the first time in any star, is plausibly attributed to a non-equilibrium, dynamical evolution of the heavy-element clouds created by atomic diffusion 4 , and may have the same underlying physics as the weather patterns on terrestrial and giant planets.Since the detection of sunspot magnetic fields 5 , it has become clear that magnetic forces are responsible for the activity and the formation of surface structure in the Sun. Strong magnetic fields, which are generated in the solar interior and brought to the surface by convective motions, inhibit the outward flow of energy, thus creating prominent dark spots with filamentary structure 1 . A very similar relation between magnetic fields and cool surface spots is observed in many active solar-type stars 2 . All these objects possess highly structured magnetic fields, which evolve on timescales from weeks to months. Stars significantly more massive than the Sun do not have an energetic surface convection zone. The lack of mixing in their atmospheres enables diffusive segregation of chemical elements to operate efficiently. The action of the competing forces of downward gravitation acceleration and the outward radiation pressure, collectively referred to as atomic diffusion 6 , leads to selective heavy-element enrichment of stellar atmospheres. This phenomenon of chemical peculiarity is present in most slowly rotating A-and B-type stars. The presence of strong, well-organized magnetic fields in some chemically peculiar (CP) stars alters the efficiency of the atomic transport processes, resulting in the formation of non-uniform distributions of chemical abundances over the surface and with height in their atmospheres. The fields and chemical spots in magnetic CP stars are essentially static, showing no evolutionary changes on the timescales accessible to human observers. Despite a qualitative difference between the magnetic characteristics and related variability patterns of the low-mass solar-type stars and massive early-type objects, an undeniable and seemingly universal link between the presence of magnetic fields and the surface structure formation exists in both groups of stars. A series of recent studies has challenged this picture. A small group of late-B CP stars of mercury-manganese (HgMn) peculiarity type has no convinci...

Context. The study of rotational velocity distributions for normal stars requires an accurate spectral characterization of the objects in order to avoid polluting the results with undetected binary or peculiar stars. This piece of information is a key issue in the understanding of the link between rotation and the presence of chemical peculiarities. Aims. A sample of 47 low v sin i A0−A1 stars (v sin i < 65 km s −1 ), initially selected as main-sequence normal stars, are investigated with high-resolution and high signal-to-noise spectroscopic data. The aim is to detect spectroscopic binaries and chemically peculiar stars, and eventually establish a list of confirmed normal stars. Methods. A detailed abundance analysis and spectral synthesis is performed to derive abundances for 14 chemical species. A hierarchical classification, taking measurement errors into account, is applied to the abundance space and splits the sample into two different groups, identified as the chemically peculiar stars and the normal stars. Results. We show that about one third of the sample is actually composed of spectroscopic binaries (12 double-lined and five singlelined spectroscopic binaries). The hierarchical classification breaks down the remaining sample into 13 chemically peculiar stars (or uncertain) and 17 normal stars.

Observations of the Mercury-Manganese star φ Her with the Navy Prototype Optical Interferometer (NPOI) conclusively reveal the previously unseen companion in this single-lined binary system. The NPOI data were used to predict a spectral type of A8V for the secondary star φ Her B. This prediction was subsequently confirmed by spectroscopic observations obtained at the Dominion Astrophysical Observatory. φ Her B is rotating at 50 ± 3 km s −1 , in contrast to the 8 km s −1 lines of φ Her A. Recognizing the lines from the secondary permits one to separate them from those of the primary. The abundance analysis of φ Her A shows an abundance pattern similar to those of other HgMn stars with Al being very underabundant and Sc,

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