% abstracts.tex
% paper titles & abstracts
\input /home/worthey/tex/lyxtex/lyxtex
\def\Ibar{$\overline{I}$}
\def\Lbar{$\overline{L}$}
\def\Mbar{$\overline{M}$}
\def\simgt{\mathrel{\hbox to 0pt{\lower 3.5pt\hbox{$\mathchar"218$}\hss}
      \raise 1.5pt\hbox{$\mathchar"13E$}}}


\quad
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\centerline{\bf Guy Worthey }
\centerline{\bf Major Papers with Abstracts}
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\centerline{ {\bf Hubble Space Telescope Observations of M32:} }
\centerline{ {\bf The Color-Magnitude Diagram } }
\centerline{ Carl J. Grillmair et al.}
\centerline{ 1996 AJ, 112, 1975 }
\vskip 0.2in

We present a $VI$ color-magnitude diagram for a region $1'-2'$ south of the
center of M32 based on {\it Hubble Space Telescope} WFPC2 images.  The
broad color-luminosity distribution of red giants shows that the
stellar population comprises stars with a wide range in metallicity.
This distribution cannot be explained by a spread in age.  The blue
side of the giant branch rises to $M_I \approx -4.0$ and can be fitted
with isochrones having [Fe/H] $\approx-1.5.$ The red side consists of
a heavily populated and dominant sequence that tops out at $M_I
\approx -3.2,$ and extends beyond $V-I=4.$ This sequence can be fitted
with isochrones with $-0.2 <$ [Fe/H] $< +0.1,$ for ages running from
15~Gyr to 5~Gyr respectively.  We do not find the {\it optically}
bright asymptotic giant branch stars seen in previous ground-based
work and argue that the majority of them were artifacts of crowding.
Our results are consistent with the presence of the {\it infrared}-luminous
giants found in ground-based studies, though their existence cannot be
directly confirmed by our data. The tip of the metal-poor portion of
the giant branch occurs at the luminosity expected if M32 is at the
same distance as M31 but is too sparsely sampled by this data set to
provide a precise distance estimate.  At fainter magnitudes, the
rising giant branch is significantly wider (FWHM$_{V-I}\sim 0.6$ mag
down to $M_I\sim-1.0$) than can be accounted for by photometric
uncertainties, again due to a metallicity spread.  There is little
evidence for an extended or even red horizontal branch, but we find a
strong clump on the giant branch itself, as expected for the high
metallicities inferred from the giant branch.  If the age spread is
not extreme, the distribution of metallicities in M32 is considerably
narrower than that of the closed-box model of chemical evolution, and
also appears somewhat narrower than that of the solar neighborhood.
Overall, the M32 {\it HST} color-magnitude diagram is consistent with
the average luminosity-weighted age of 8.5~Gyr and
[Fe/H]$\approx-0.25$ inferred from integrated spectral indices,
extrapolated to this radius and analyzed with the same population
models.

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%8
\centerline{ {\bf The G Dwarf Problem Exists in Other Galaxies} }
\centerline{ Guy Worthey, Ben Dorman, and Lewis A. Jones}
\centerline{ 1996 AJ, 112, 948}
\vskip 0.2in

Stellar population models with abundance distributions determined from
the analytic Simple model of chemical evolution fail to match
observations of the nuclei of bulge-dominated galaxies in three
respects. First, the spectral energy distribution in the
mid-ultraviolet range $2000 < \lambda < 2400$ \AA\ exceeds observation by $\sim
0.6$ mag. Most of that excess is due to metal-poor main sequence
stars. Second, the models do not reproduce metal-sensitive optical
absorption features that arise mainly from red giant stars. Third, the
strength of a Ca II index sensitive to hot stars does not jibe with
the predicted number of A-type horizontal branch stars.  The number of
metal poor stars in galaxies is at least a factor of two less than
predicted by the Simple model, exactly similar to the ``G Dwarf
problem'' in the solar cylinder. Observations at larger radii in local
group galaxies indicate that the paucity of metal poor stars applies
globally, rather than only in the nuclei.

Because of the dominance of metal rich stars, primordial galaxies will
have a plentiful dust supply early in their star formation history,
and thus will probably have weak Ly$\alpha$ emission, as is apparently
observed.  We confirm that early-type galaxies cannot have been formed
exclusively from mergers of small all-stellar subsystems, a result
already established by dynamical simulations. The constraint of peaked
abundance distributions
will limit future chemical evolution models. It
will also make age estimates for the stellar populations in early type
galaxies and bulges more secure.


\vskip 0.4in

%7
\centerline{ {\bf Uncertainties in the Modeling of Old Stellar
Populations}}
\centerline{St\'ephane Charlot, Guy Worthey, and Alessandro Bressan}
\centerline{ 1996 ApJ, 457, 625}
\vskip 0.2in

We compare three recent models for the spectral evolution of stellar 
populations to assess the origin of serious discrepancies in the colors
predicted for $\simgt1\,$Gyr old populations of the same input age and 
metallicity. To isolate the source of these discrepancies, we investigate 
separately the two main characteristics of each model: the underlying stellar 
evolution prescription and the spectral calibrations used to transform
the theoretical Hertzsprung-Russell diagram into observables. A 0.05~mag
discrepancy in $B-V$ color is caused almost entirely by a known limitation
of theoretical spectra. Differences in spectral calibrations are found
to account for only a small fraction of the 0.25~mag discrepancies in $V-K$
color and of the 25\% dispersion in mass-to-visual light ratio among the 
models considered here. The main source of disagreement for these quantities
is the underlying stellar evolution prescription.

For idealized galaxies containing a single generation of stars, properties
derived from broad-band colors using population synthesis models are found
to be accurate by roughly $\pm$35\% in age at fixed metallicity, 25\% in
metallicity at fixed age if the heavy element mixture is assumed to be
scaled-solar, and 35\% in mass at fixed metallicity and fixed initial mass
function. These indicative uncertainties are based on the dispersion in the
predictions of the models investigated here.

There appear to be persistent problems in virtually every ingredient
of population synthesis models. The most serious are the lifetimes and
luminosities of stars in post-main sequence evolutionary stages; the 
temperature of the red giant branch and color-temperature relation
for cool stars; and the lack of accurate libraries of stellar spectra, 
especially for cool stars and for non-solar metallicities. These 
problems have profound causes. The main one is the high sensitivity of
stellar evolution models on the efficiency of several critical factors
which are either not sufficiently understood or cannot yet be determined
uniquely from comparisons with observations (opacities, heavy element 
mixture, helium content, convection, diffusion, mass loss, rotational
mixing). Other major limitations are the difficult spectral modeling of 
cool stars and the unavailability of calibration stars for metal-rich 
populations and populations with altered chemical mixes.


%6
\vskip 0.4in
\centerline{ {\bf New Age Indicators for Old Stellar Populations}}
\centerline{ Lewis A. Jones and Guy Worthey}
\centerline{ 1995 ApJ (Letters), 446, L31}
\vskip 0.2in

We report on new age indicators in the integrated light of old stellar
populations.  Spectral indices defined by Rose in the 
$\lambda \lambda$ 3800--4500 \AA\ 
region and two broader indices, H$\delta _{A}$ and H$\gamma _{A}$, 
have been measured in a
spectral library of about 500 stars.  The behavior of these indices with 
temperature, gravity, and [Fe/H] was modeled with polynomials, and they
were included in the 
population models of Worthey (1994).  All
definitions of H$\delta$ and H$\gamma$ are sensitive to population age.  One
index in particular, H$\gamma _{HR}$, shows a striking ability to separate
the effects of age and metal abundance in integrated light.


% 5
\vskip 0.4in
\centerline{ {\bf Comprehensive Stellar Population Models}}
\centerline{ {\bf and the Disentanglement of Age and Metallicity Effects} }
\centerline{ 1994 ApJS, 95, 107 }
\vskip 0.2in

The construction of detailed models for intermediate and old stellar
populations is described.  Input parameters include metallicity
($-2<{\rm [Fe/H]}<0.5$), single-burst age (between 1.5 and 17 Gyr),
and IMF exponent.  Quantities output include broad-band magnitudes,
spectral energy distributions, surface brightness fluctuation
magnitudes, and a suite of 21 absorption feature indices. The models
are checked against a wide variety of available observations.
Examinations of model output yield the following conclusions.  (1) If
the percentage change $\Delta$age/$\Delta Z \approx 3/2$ for two
populations, they will appear almost identical in most indices. A few
indices break this degeneracy by being either more abundance-sensitive
(Fe4668, Fe5015, Fe5709, Fe5782) or more age-sensitive (G4300,
H$\beta$, and presumably higher-order Balmer lines) than usual. (2)
Present uncertainties in stellar evolution are of the same magnitude
as the effects of IMF and $Y$ in the indices studied. (3) Changes in
abundance ratios (like [Mg/Fe]) are predicted to be readily apparent
in the spectra of old stellar populations. (4) The $I$ band flux of a
stellar population is predicted to be nearly independent of
metallicity, and only modestly sensitive to age. The $I$ band is
therefore recommended for standard candle work or studies of $M/L$ in
galaxies.

Other conclusions stem from this work. (1) Intercomparison of models
and observations of two TiO indices seem to indicate variation of the
[V/Ti] ratio among galaxies, but it is not clear how this observation
ties into the standard picture of chemical enrichment. (2) Current
estimates of [Fe/H] for the most metal-rich globulars that are based
on integrated indices are probably slightly too high. (3) Colors of
population models from different authors exhibit a substantial range.
At solar metallicity and 13 Gyr, this range corresponds to an age
error of roughly $\pm$7 Gyr. Model colors from different authors
applied in a differential sense have smaller uncertainties. (4) In the present
models the dominant error for colors is probably the transformation from
stellar atmospheric parameters to stellar colors.
(5) Stellar $B-V$ is difficult to model, and current spreads
among different authors can reach 0.2 mag. (6) If known defects in the
stellar flux library are corrected, the population model colors of
this work in passbands redder than $U$ would be accurate to roughly
0.03 mag in an absolute sense. These corrections are not made in the
tables of model output.

% 4
\vskip 0.4in
\centerline{\bf Old Stellar Populations V.}
\centerline{{\bf Absorption Feature Indices for the Complete 
Lick/IDS Sample of Stars} }

\centerline{Guy Worthey, S.M. Faber, J. Jes\'us 
Gonz\'alez, D. Burstein}
\centerline{1994 ApJS, 94, 687 }

\vskip 0.2in
Twenty-one optical absorption features, 11 of which have been
previously defined, are automatically measured in a sample of 460
stars. Following Gorgas et al., the indices are summarized in fitting
functions that give index strengths as functions of stellar
temperature, gravity, and [Fe/H].  This project was carried out with
the purpose of predicting index strengths in the integrated light of
stellar populations of different ages and metallicities, but the data
should be valuable for stellar studies in the Galaxy as well. 
Several of the new indices appear to be promising indicators of
metallicity for old stellar populations.
A complete list of index data and atmospheric parameters is available
in computer-readable form.

% 3
\vskip 0.4in
\centerline{\bf A Method for Quantitative Detection of Hot
Horizontal Branches}
\centerline{\bf in Early-type Galaxies }

\centerline{ 1993 ApJ (Letters) 415, L91 }
\vskip 0.2in

Surface brightness fluctuation magnitudes in the
$\lambda$2800--4000\AA\ region are calculated.  They show that a hot
main sequence can be easily distinguished from a hot horizontal branch
due the relative sensitivity of fluctuation magnitudes to the presence
of giants.  Superluminous post-asymptotic giant branch (PAGB) stars
will need to be subtracted before applying this method, and other
hypothesised late-stage, hot stars may also contribute to the surface
brightness fluctuation magnitudes, complicating the analysis.
Application of this method further in the UV will provide an estimate
of the luminosity of the next-most-luminous subpopulation of stars
after the PAGB stars, which will constrain the possible populations
responsible for the UV flux in ellipticals.


% 2
\vskip 0.4in
\centerline{\bf The Dependence of the }
\centerline{\bf Brightness Fluctuation Distance Indicator}
\centerline{\bf on Stellar Population Age and Metallicity}

\centerline{ 1993 ApJ, 409, 530 }
\centerline{ Erratum 1993 ApJ, 418, 947 }
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We describe how the extragalactic distance indicator \Lbar\ 
depends on stellar
population age, metallicity, and IMF, assuming single-burst ages
between 5 and 17 Gyr and metallicities between --0.5 and +0.5 dex. We
consider Johnson/Cousins $BVR_CI_CJHKLL'M$ passbands.
We find that (1) \Lbar\  is insensitive to changes in IMF, 
(2) \Lbar\  can be reliably calibrated against colors or spectral
indices, due to the similarity of the effects of age and
metallicity on the stellar populations,
(3) the calibration presented here may already give distances good to
10\% for an isolated galaxy measured in one fluctuation passband and
one color or spectral index, not including observational error, and
(4) our theoretical results are consistent with the distance of M32
and with the latest empirical \Ibar\  calibration and require no
empirical corrections.
We summarize our results by giving polynomial calibrations
for \Mbar\  as functions of integrated Mg$_2$, V--I, and J--K.


% 1
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\centerline{\bf Mg and Fe Absorption Features in Elliptical
Galaxies}

\centerline{ Guy Worthey, S.M. Faber, \& J. Jes\'us Gonz\'alez}
\centerline{ 1992 ApJ, 398, 69 }
\vskip 0.2in

Fe and Mg indices from two homogenous collections of nuclear
elliptical galaxy spectra are compared with model indices.  In the
average giant elliptical, the [Mg/Fe] ratio exceeds that of the most
metal-rich stars in the solar neighborhood by $\sim  0.2 - 0.3$ 
dex, with a large spread about this mean. This result implies a
variable ``enrichment effectiveness'' of Type II supernovae (SNe)
compared with Type Ia SNe in the evolution of ellipticals, caused
perhaps by differences in star formation timescales, the initial mass
function, or the amount of Fe vs. Mg ejected in galactic winds.

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