%R 1997PASP..109..849G
%J-856
%T Probing the Lhs Catalog. I. New Nearby Stars and the Coolest Subdwarf
%A GIZIS J.E., REID I.N.
%F contents_97_08
%K Stars
%B \begin{tex}We present moderate resolution spectroscopy of 112 cool dwarf stars to supplement the observations we have already
presented in the Palomar/MSU Nearby-Star Spectroscopic Survey. The sample consists of 72 suspected nearby stars added to
the {\it The Preliminary Third Catalog of Nearby Stars} since 1991 as well as 40 faint red stars selected from the LHS
catalog. LHS 1826 is more metal-poor and cooler than the coolest previously known extreme subdwarf, LHS 1742a. LHS 2195
is a very late M dwarf of type ${\rm M} 8~{\rm V}$, probably at a distance of ten parsecs. LHS 1937 is an ${\rm M}
7~{\rm V}$ star at $\sim 20$ parsecs. Three other previously unobserved LHS stars have estimated distances that place
them within 25 parsecs. 
\end{tex}

%R 1997PASP..109..857M
%J-867
%T The Absolute Magnitudes of the rr Lyrae Stars
%A MCNAMARA D.H.
%F contents_97_08
%K Stars
%B \begin{tex}We present a new calibration of the relationship between the absolute magnitudes and metal abundances, [Fe/H], of RR
Lyrae stars. Temperatures inferred from optical and near infrared color indices and the new Kurucz models are utilized
in conjunction with Baade-Wesselink solutions to derive absolute magnitudes. Temperature inferred from the optical and
near infrared photometry are $\sim$ 200K - 300K higher for the variables than those given from ($V-K$) color indices and
the older Kurucz models. We find $M_v$ = 0.287 [Fe/H] + 0.964. This equation gives higher luminosities ($\sim$ 0.2 mag)
for the metal-poor variables than previous B-W calibrations. The RR Lyrae stars in the Large Magellanic Cloud in
conjunction with our calibration yield a distance modulus of 18.53. Thus, the Cepheids and RR Lyrae stars give identical
distance moduli at $\leq$ 0.1 mag level. The repercussions of the higher temperatures and higher luminosities of RR
Lyrae stars on the distances and ages of Globular clusters, distance to the galactic center, the Hubble constant and age
of the universe, and the masses of RR Lyrae stars are discussed. 
\end{tex}

%R 1997PASP..109..868V
%J-882
%T High s/n Echelle Spectroscopy in Young Stellar Groups. I. Observations and Data Reduction
%A VERSCHUEREN W., BROWN A.G.A., HENSBERGE H., DAVID M., LE POOLE R.S., DE GEUS E.J., DE ZEEUW P.T.
%F contents_97_08
%K Stellar Clusters and Associations
%B This is the first paper in a series in which we study the kinematical structure and dynamical evolution of OB
associations and young stellar clusters. By means of high-resolution high-S/N spectroscopic observations, we aim at
obtaining accurate stellar radial and rotational velocities and information on binarity in a selected number of such
systems. In the present paper, we discuss the observations and data reduction for a sample of 156 early-type,
established or probable, members of the Sco OB2 association. These stars form a subset of a larger sample that was
observed by HIPPARCOS satellite. The observations presented here were preformed at the ESO 1.5 m telescope using the
ECHELEC echelle CCD spectrograph. The formal S/N obtained is typically 70 at 3800 \AA and 300 at 4050 \AA. A new data
reduction package was developed. For the sake of qualilty control over the final spectra, we critically evaluated the
performace of each procedure in the reduction. Emphasis is on techniques for detecting systematic errors in an empirical
way and for maximally eliminating them, at least on a differential level. The accuracy of our reduced spectra is limited
by intrinsic instrumental imperfections. Local random errors are given and the amplitude of systematic residuals that
(may) occur in certain spectral regions is estimated in considerable detail. In anticipation of next papers in this
series, we finally show that the accuracy of differential radial velocities (for spectra of the same star) is
essentially limited by centering and zero-point uncertainties amounting to greater or equal to 1km/s (rms).

%R 1997PASP..109..883R
%J-906
%T Galactic Globular Cluster Metallicity Scale from the Ca ii Triplet I. Catalog
%A RUTLEDGE G.A., HESSER J.E., STETSON P.B., MATEO M., SIMARD L., BOLTE M., FRIEL E.D., COPIN Y.
%F contents_97_08
%K Stellar Clusters and Associations
%B \begin{tex}We have obtained 2640 CCD spectra with resolution $\sim$4~\AA\ in the region 7250--9000~\AA\ for 976 stars lying near
the red giant branches in color-magnitude diagrams of 52 Galactic globular clusters. Radial velocities of $\sim$16 \kms\
accuracy per star determined from the spectra are combined with other criteria to assess quantitative membership
probabilities. Measurements of the equivalent widths of the infrared calcium triplet lines yield a relative
metal-abundance ranking with a precision that compares favorably to other techniques. Regressions between our system and
those of others are derived. Our reduction procedures are discussed in detail, and the resultant catalog of derived
velocities and equivalent widths is presented. The metal abundances derived from these data will be the subject of a
future paper.
\end{tex}

%R 1997PASP..109..907R
%J-919
%T Galactic Globular Cluster Metallicity Scale from the Ca ii Triplet ii. Rankings, Comparisons, and Puzzles
%A RUTLEDGE G.A., HESSER J.E., STETSON P.B.
%F contents_97_08
%K Stellar Clusters and Associations
%B \begin{tex}We compare our compilation of the \redew\ calcium index for 71 Galactic globular clusters to the widely used
\markcite{zinn84}Zinn and West (1984 ApJS, 55, 45) \fe\ scale and to Carretta and Gratton's (1997 A\&AS, 121, 95) scale
from high-dispersion spectra analyzed with Kurucz (1992, private communication) model atmospheres. We find our calcium
ranking to be tightly correlated with each comparison set, in a non-linear and a linear fashion, respectively. By
combining our calcium index information with the Zinn and West ranking, we are able to rank the globular clusters in our
sample with a typical precision of $\pm 0.05$ dex for $\fe_{ZW84} \lesssim -0.5$; for clusters more metal rich than
this, the ranking is less precise. The significant differences between these metallicity scales raise important
questions about our understanding of Galactic formation and chemical enrichment processes. Furthermore, in spite of the
apparent improvement in metallicity ranking for the Galactic globular clusters that results from our addition of
information from the \ca\ triplet lines to the potpourri of other metallicity indicators, caution -- perhaps
considerable -- may be advisable when using \redew\ as a surrogate for metallicity, especially for systems where ranges
in age and metallicity are likely.
\end{tex}

%R 1997PASP..109..920K
%J-926
%T Palomar 10 and Ngc 6749: A Study in Contrasts
%A KAISLER D., HARRIS W.E., MCLAUGHLIN D.E
%F contents_97_08
%K Stellar Clusters and Associations
%B \begin{tex}We present new photometry in $V$ and $I$ for Palomar 10 and NGC 6749, two heavily obscured and sparse globular
clusters. The color-magnitude diagram (CMD) of Pal 10 exhibits marked differential reddening with a mean reddening
$E(B-V) = 1.66 \pm 0.1$. Its CMD has the purely red horizontal branch characteristic of a high-metallicity cluster; we
estimate [Fe/H] $\sim -0.1$ on the Zinn-West metallicity scale. NGC 6749 (= Berkeley 42, listed in some other sources as
an old open cluster) has a similarly large reddening ($E(B-V) = 1.5 \pm 0.07$) but its CMD is that of a typically
metal-poor ([Fe/H] $\sim -1.6$) cluster with a purely blue HB. We estimate their distance moduli as $(m-M)_0$ = 13.85 (d
= 5.9 kpc) for Pal 10 and $(m-M)_0$ = 14.45 (7.8 kpc) for NGC 6749. King-model fits to their radial distributions yield
very low central concentrations for both: c = 0.58 (Pal 10) and c = 0.83 (NGC 6749).
\end{tex}

%R 1997PASP..109..927W
%J-931
%T The Luminosity Function and Size Distrbution of the H ii Regions in M33
%A WYDER T.K., HODGE P.W., SKELTON B.P.
%F contents_97_08
%K Galaxies
%B \begin{tex}We present H$\alpha$ fluxes and sizes for most of the \ion{H}{2} regions in M33 listed in the catalog of Court\`es et
al. (1987). The observed bright end of the differential luminosity function is approximately a power law with exponent
$a = -2.40 \pm 0.15$, while the number counts turn over at an H$\alpha$ luminosity of approximately 10$^{36.4}$ erg
s$^{-1}$. After accounting for the effects of incompleteness in our sample, we find that the faint end of the corrected
luminosity function remains almost constant instead of turning over. The cumulative size distribution approximately
follows an exponential law with scale size D$_0 = 32 \pm 1$ pc. We briefly discuss the implications of our results for
our understanding of recent high-mass star formation in M33.
\end{tex}

%R 1997PASP..109..932B
%J-932
%T Neural Network Classification of Stellar Spectra
%A BAILER-JONES C.A.L.
%F contents_97_08
%K Dissertation Summaries
%B \begin{tex}The MK classification system as first proposed in 1943 by Morgan, Keenan \& Kellman (1943) has been an important tool
in stellar and galactic astrophysics. Currently, MK classification is a useful approach to obtaining general information
on stellar spectra and selecting stars for further study. However, the existing manual methods of obtaining
classifications are slow, making it infeasible for them to be applied to the vast numbers of spectra which will be
obtained in future spectral surveys. In this project I investigate the application of artificial neural networks to the
automation of MK spectral classification. For the purposes of this project, I digitized and reduced 100 objective prism
plates taken from the Michigan Spectral Survey (Houk 1994). Using software I developed, I extracted a set of over 5000
spectra at a resolution of $\sim$3\,\AA\ covering the wavelength range 3800--5200\,\AA\ (Bailer-Jones, Irwin \& von
Hippel 1997a). These spectra were used in conjunction with their two-dimensional classifications listed in Michigan
Henry Draper Catalogue (Houk \& Smith-Moore 1988 and references therein) to develop supervised neural network
classifiers. I show that neural networks can give accurate spectral type classifications ($1\sigma$ = 0.82 subtypes,
$\sigma_{RMS} = 1.09$ subtypes) across the full range of spectral types present in the database (B2--M7); I show also
that the networks yield correct luminosity classes for over 95\% of both dwarfs and giants with a high degree of
confidence (Bailer-Jones, Irwin \& von Hippel 1997b). The high level of reproducibility of neural network
classifications is demonstrated and an analysis of the effect of the complexity of the neural network on its
classifications is given. For the purposes of many analyses, stellar spectra contain a large amount of redundant
(correlated) information. I investigate the application of Principal Components Analysis (PCA) to the optimal
compression of spectra. I show that PCA can compress the spectra by a factor of over 30 whilst retaining more than 95\%
of the variance in the data set. Furthermore, this compression leads to no decrease in classifier performance,
indicating that the PCA compression from 820 to 25 components results in no significant loss of relevant information. I
also demonstrate how PCA acts as a filter of noise and bogus features in a spectrum and can be used to identify unusual
spectra. The ultimate goal of stellar classification should be a physical parameterization of the stars. I examine the
application of neural networks to the problem of obtaining physical parameters ($T_{\rm{eff}}$, $\log g$ etc.)\ directly
from an observed spectrum, by training a neural network on synthetic spectra and then applying it to observed spectra
(Bailer-Jones et~al.\ 1997). By determining physical parameters in this way, any assumptions and limitations of an
intermediate classification scheme are avoided. However, I have used the effective temperature ($T_{\rm{eff}}$) inferred
by the networks to produce an accurate calibration of the MK system for dwarf, giant and sub-giant stars, based on the
set of 5000 observed spectra. This calibration agrees well with a number of calibrations published in the literature. It
is demonstrated through the metallicity dependence of the derived temperature calibration that the neural networks are
sensitive to the metallicity features in the observed spectra. With further work it is likely that neural networks will
be able to determine metallicity reliably using only the optical stellar spectrum.
\end{tex}

%R 1997PASP..109..933R
%J-933
%T The Search for Main Sequence Binary Stars in Galactic Globular Clusters
%A RUBENSTEIN E.P.
%F contents_97_08
%K Dissertation Summaries
%B \begin{tex}Binary stars should be present in relatively large numbers in globular clusters (GCs). While individual binary
systems have been previously observed, until now, no large population had been detected in a dense GC. As part of this
thesis, we developed a new technique to determine the fraction of main-sequence stars which are binaries. We analyzed
the redward spread of the main-sequence ridge-line of the post-core-collapse GC NGC~6752. These data, obtained with
post-refurbishment HST data, indicate that in the inner core-radius, the 3-$\sigma$ lower and upper limits on the binary
fraction are 15\% and 38\%, respectively. The region outside the core has a 3-$\sigma$ upper limit on the binary
fraction of 16\%, and is consistent with no binaries. This rapid change in binary fraction is clear evidence of mass
segregation operating within the inner 3 core-radii. We also find that the binary fraction increases at fainter
magnitudes; mass segregation would also account for such behavior. We discuss the likely effects on the cluster's
luminosity function (LF). In particular, we find that if the contribution of binaries to the LF was not removed, the
apparent number of low mass stars would be over-estimated. I also examined the time-series data and discovered that one
of the blue stragglers in NGC~6752 is an SX Phe pulsator. Its pulsation mass is 1-1.2 M$_\odot$, consistent with the
mass of the best fit stellar models from the Yale Isochrones, and also consistent with any of the blue straggler
formation scenarios. No contact or eclipsing binaries were unambiguously detected. However, many low-amplitude variable
candidates were observed at the limit of our ability to detect variability. The putative period of these candidate
objects is consistent with these objects being produced via tidal capture events. In a time-series study of NGC~6397, we
found a contact binary. This W~UMa system probably has a mass greater than that of main sequence turn-off stars implying
that when this system coalesces due to angular momentum loss, it will become a blue straggler star. The frequency of
W~UMas in this cluster is not statistically different from that of other GCs where $\sim0.1\%$ of the main-sequence
stars are contact binaries.
\end{tex}

%R 1997PASP..109..934T
%T A Numerical Study of Counterrotating Disks in Spiral Galaxies
%A THAKAR A.R.
%F contents_97_08
%K Dissertation Summaries
%B The serendipitous discovery in 1992 of a second spiral disk rotating in a sense opposite to the rest of the galaxy,
followed by subsequent discoveries of other such counterrotating disks, poses a formidable challenge to theorists.
Current galaxy formation theories do not permit such a disk to be formed with the rest of the galaxy, and it appears to
have been built up from external matter after the galaxy was formed. The aim of this thesis was to determine how a
counterrotating disk can form in a spiral galaxy without destroying the preexisting disk. The most promising theory
invokes the adiabatic infall, on a retrograde orbit, of intergalactic gas that dissipates energy and precesses to the
plane of the primary disk. Using numerical simulations that incorporate both stars and gas, I have tested two competing
theories for the formation of counterrotating disks: adiabatic gas infall and a regtrograde gas-rich dwarf merger. A
tree code was used for the gravity solver. Sticky particle gas dynamics were used to study the general characteristics
of the formation process, and the structure of the counterrotating disk formed was investigated with smoothed particle
hydrodynamics. I show that under restricted conditions, dissipational infall of gas can indeed produce a counterrotating
disk without excessive damage to the preexisting disk. A cold, thin primary can survive secular gas infall and the
production of a massive counterrotating disk if the infalling gas is well-dispersed in phase-space. A minor merger with
a gas-rich dwarf galaxy can also yield a counterrotating disk if the primary galaxy's disk is massive and compact to
begin with. Although the origin of counterrotating disks is an intriguing question in its own right, it has profound
implications for the evolution and interaction histories of spiral galaxies.