Austin F. Gulliver

STELLAR KINEMATIC GROUPS. II. A REEXAMINATION OF THE MEMBERSHIP, ACTIVITY, AND AGE OF THE URSA MAJOR GROUP

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 � 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.

VEGA : A RAPIDLY ROTATING POLE-ON STAR

High-dispersion (2.4 A/mm), ultrahigh signal-to-noise ratio (3000:1) Reticon spectra of Vega revealed two distinct types of profiles. The strong lines exhibit classical rotational profiles with enhanced wings, but the weak lines have distinctly different, flat-bottomed profiles. Using ATLAS9 model atmopheres and SYNTHE synthetic spectra, Vega has been modeled as a rapidly rotating, pole-on star with a gradient in temperature and gravity over the photosphere. By fitting to the flat-bottomed line profiles of Fe 1 lambda 4528 and Ti 2 lambda 4529, we find least-squares fit values of V sin i = 21.8 plus or minus 0.2 km/sec polar T(sub eff) = 9695 plus or minus 25 K, polar log(base 10)g = 3.75 plus or minus 0.02 dex, V(sub eq) = 245 plus or minus 15 km/sec, and inclination 5 deg .1 plus or minus 0 deg .3. The variations in T(sub eff) and log(base 10)g over the photosphere total 390 K and 0.08 dex, respectively. Assuming V sin i = 21.8 km/sec, an independent fit to the observed continuous flux from 1200 to 10,500 A produced a similar set of values with polar T(sub eff) = 9595 plus or minus 20 K, polar log(base 10)g = 3.80 plus or minus 0.03 dex, and inclination 6 deg .0 plus or minus 0 deg .7.

The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae

The variability of the Hg ii � 3984 line in the primary of the binary starAnd 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 compan- ion. Rather, the variation is produced by the combination of the 2.38236 day period of rotation of the pri- mary 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. Subject headings: binaries: general — binaries: spectroscopic — stars: chemically peculiar — stars: individual (� Andromedae)

An elemental abundance analysis of the superficially normal A star Vega

An elemental abundance analysis of Vega has been performed using high-signal-to-noise 2.4 A/mm Reticon observations of the region 4313-4809 A. Vega is found to be a metal-poor star with a mean underabundance of 0.60 dex. The He/H ratio of 0.03 as derived from He I 4472 A suggests that the superficial helium convection zone has disappeared and that radiative diffusion is producing the photospheric abundance anomalies. 45 refs.

Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae

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 and active late-type stars, and stationary chemical spots in the early-type magnetic stars. However, results of our seven-year monitoring of mercury spots in non-magnetic early-type star alpha 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 2-D 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 and may have the same underlying physics as the weather patterns on terrestrial and giant planets.

A STUDY OF VEGA: A RAPIDLY ROTATING POLE-ON STAR

Ultra-high signal-to-noise, high dispersion spectroscopy over the wavelength range λλ4519-4535 shows Vega to be a rapidly rotating star with V eq of 211 km s–1 seen almost pole-on. The analysis of five independent series of spectroscopic data is combined with analyses of the hydrogen lines, Hγ, Hβ, and Hα, and the latest absolute continuum flux for Vega to yield the following results: Vsin i = 20.8 ± 0.2 km s–1, polar Teff = 10, 000 ± 30 K, polar log g = 4.04 ± 0.01 dex, V eq = 211 ± 4 km s–1, breakup fraction = 0.81 ± 0.02, microturbulence (ξ T ) = 1.0 ± 0.1 km s–1, macroturbulence (ζ) = 7.4 ± 0.5 km s–1, and an inclination i = 57 ± 01. The variations in Teff and log g over the photosphere total 1410 K and 0.26 dex, respectively, while the mean temperature is 9560 ± 30 K and log g is 3.95 ± 0.01 dex. Low level variations in the Ti II 4529 A profile are also illustrated.

Metallic line profiles of the AO V star Vega

High dispersion ultrahigh signal to noise Reticon spectra of Vega were obtained with the coude spectrograph of the 1.2 m telescope. A mean signal to noise ratio of 2500 over the spectral region lambdas from 3825 to 5435 was achieved. Examination of the line profiles confirmed the presence of two different types of profiles which were previously seen in IIIaJ and lower signal to noise Reticon spectra. The profiles of the strong lines are essentially classical rotational profiles with enhanced wings which are slightly stronger than expected while those of weak lines are clearly flat-bottomed resulting in a trapezoidal appearance. A few possible theoretical explanations are presented.

The Mercury-Manganese Binary Star φ Herculis: Detection and Properties of the Secondary and Revision of the Elemental Abundances of the Primary

Observations of the mercury-manganese star 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 A8 V for the secondary star Her B. This prediction was subsequently confirmed by spectroscopic observations obtained at the Dominion Astrophysical Observatory. Her B is rotating at 50 ± 3 km s-1, in contrast to the 8 km s-1 lines of Her A. Recognizing the lines from the secondary permits one to separate them from those of the primary. The abundance analysis of Her A shows an abundance pattern similar to those of other HgMn stars, with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr, Y, Zr, Ba, Ce, and Hg being very overabundant.

A spectroscopic atlas of Deneb (A2 Iae)

We present a spectroscopic atlas of Deneb (A2 Iae) obtained with the long camera of the 1.22-m telescope of the Dominion Astrophysical Observatory using Reticon and CCD detectors. For3826-5212 the inverse dispersion is 2.4 A mm 1 with a resolution of 0.072 A. At the continuum the mean signal-to-noise ratio is 1030. The wavelengths in the laboratory frame, the equivalent widths, and the identifications of the various spectral features are given. This atlas should provide useful guidance for studies of other stars with similar spectral types. The stellar and synthetic spectra with their corresponding line identifications can be examined at http://www.brandonu.ca/physics/gulliver/atlases.html

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Elemental abundances analyses are performed for the Mercury-Manganese stars Her, Her, and HR 7018 consistent with previous studies of this series using spectrograms obtained with Reticon and CCD detectors. Comparisons of the rst two analyses with those performed using coadded photographic plates show the general consistency of the derived elemental abundances. For Her and for 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 eective temperature.