Stellar Populations Of Elliptical Galaxies Research Articles

On the deceleration of Fanaroff-Riley Class I jets: mass loading of magnetized jets by stellar winds.

Abstract In this paper we present steady-state RMHD simulations that include a mass-load term to study the process of jet deceleration. The mass-load mimics the injection of a proton-electron plasma from stellar winds within the host galaxy into initially pair plasma jets, with mean stellar mass-losses ranging from 10−14 to 10−9 M⊙ yr−1. The spatial jet evolution covers ∼500 pc from jet injection in the grid at 10 pc from the jet nozzle. Our simulations use a relativistic gas equation of state and a pressure profile for the ambient medium. We compare these simulations with previous dynamical simulations of relativistic, non-magnetised jets. Our results show that toroidal magnetic fields can prevent fast jet expansion and the subsequent embedding of further stars via magnetic tension. In this sense, magnetic fields avoid a runaway deceleration process. Furthermore, when the mass-load is large enough to increase the jet density and produce fast, differential jet expansion, the conversion of magnetic energy flux into kinetic energy flux (i.e., magnetic acceleration), helps to delay the deceleration process with respect to non-magnetised jets. We conclude that the typical stellar population in elliptical galaxies cannot explain jet deceleration in classical FRI radio galaxies. However, we observe a significant change in the jet composition, thermodynamical parameters and energy dissipation along its evolution, even for moderate values of the mass-load.

The Planetary Nebula Spectrograph survey of S0 galaxy kinematics

The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems*. We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases.

Abundance ratios in the hot ISM of elliptical galaxies

To constrain the recipes put forth to solve the theoretical Fe discrepancy in the hot interstellar medium of elliptical galaxies and at the same time explain the [alpha/Fe] ratios. In order to do so we use the latest theoretical nucleosynthetic yields, we incorporate the dust, we explore differing SNIa progenitor scenarios by means of a self-consistent chemical evolution model which reproduces the properties of the stellar populations in elliptical galaxies. Models with Fe-only dust and/or a lower effective SNIa rate achieve a better agreement with the observed Fe abundance. However, a suitable modification to the SNIa yield with respect to the standard W7 model is needed to fully match the abundance ratio pattern. The 2D explosion model C-DDT by Maeda et al. (2010) is a promising candidate for reproducing the [Fe/H] and the [alpha/Fe] ratios. (A&A format)

THE STELLAR POPULATION AND METALLICITY DISTRIBUTION OF THE SOMBRERO GALAXY

Hubble Space Telescope studies of the resolved stellar population of elliptical galaxies have shown that the galaxies form by steady accretion of gas which is all the while forming stars and evolving chemically to a metallicity distribution that is as high as solar composition in the most massive objects that have been analyzed, and much lower for low mass ellipticals. In this paper we study for the first time the stellar content of an early type spiral galaxy, the massive disk galaxy, the Sombrero, NGC 4594. We consider whether the metallicity distribution function (MDF) in the observed field matches that of elliptical galaxies of some luminosity, and what these data imply for the accretion and enrichment model that can be fitted to the MDF. The MDF of NGC 4594 is similar to that of the elliptical galaxy of similar luminosity, NGC 5128. The field we are probing is a combination of the galaxy's bulge and halo.

Cold gas and young stars in tidally disturbed ellipticals at z = 0

Abstract We present an analysis of the neutral hydrogen and stellar populations of elliptical galaxies in the Tal et al. sample. Our aim is to test their conclusion that the continuing assembly of these galaxies at z ∼ 0 is essentially gas free and not accompanied by significant star formation. In order to do so, we make use of H i data and line-strength indices available in the literature. We look for direct and indirect evidence of the presence of cold gas during the recent assembly of these objects and analyse its relation to galaxy morphological fine structure. We find that ≥25 per cent of ellipticals contain H i at the level of M(H i) > 108 M⊙, and that M(H i) is of the order of a few per cent of the total stellar mass. Available data are insufficient to establish whether galaxies with a disturbed stellar morphology are more likely to contain H i. However, H i interferometry reveals very disturbed gas morphology/kinematics in all but one of the detected systems, confirming the continuing assembly of many ellipticals but also showing that this is not necessarily gas free. We also find that all very disturbed ellipticals have a single-stellar-population-equivalent age <4 Gyr. We interpret this as evidence that ∼0.5–5 per cent of their stellar mass is contained in a young population formed during the past ∼1 Gyr. Overall, a large fraction of ellipticals seem to have continued their assembly over the past few Gyr in the presence of a mass of cold gas of the order of 10 per cent of the galaxy stellar mass. This material is now observable as neutral hydrogen and young stars.

AGN feedback in numerical simulations

AbstractThe passively evolving stellar population in elliptical galaxies (Es) provides a continuous source of fuel for accretion on the central supermassive black hole (SMBH), which is 1) extended over the entire galaxy life (but declining with cosmic time), 2) linearly proportional to the stellar mass of the host spheroid, 3) summing up to a total gas mass that is > 100 times larger than the currently observed SMBH masses, 4) available independently of merging events. The main results of numerical simulations of Es with central SMBH, in which a physically based implementation of radiative and mechanical feedback effects is considered, are presented.

Stellar Populations of Elliptical Galaxies in Virgo Cluster. I. The Data and Stellar Population Analysis

We have determined spectroscopic ages of elliptical galaxies in the Virgo Cluster using spectra of very high signal-to-noise ratio (S/N > 100 Å-1). We observed eight galaxies with the Subaru Telescope and have combined this sample with six galaxies previously observed with the WHT. To determine their ages, we have used a new method based on the Hγσ age indicator, which is virtually independent of the effects of metallicity. Apart from ages we have estimated abundances of various elements. In this paper we present the observations, the data reduction, and the reliability of the Hγσ method. The results of this investigation are presented in a companion paper.

The Chemical Evolution of Helium

We report on measurements of the 4He abundance toward the outer Galaxy H II region S206 with the NRAO Green Bank Telescope. Observations of hydrogen and helium radio recombination lines between 8 and 10 GHz were made toward the peak radio continuum position in S206. We derive 4He/H = 0.08459 ± 0.00088 (random) ± 0.0010 (known systematic), 20% lower than optical recombination line results. It is difficult to reconcile the large discrepancy between the optical and radio values even when accounting for temperature, density, and ionization structure or for optical extinction by dust. Using only M17 and S206 we determine ΔY/ΔZ = 1.41 ± 0.62 in the Galaxy, consistent with standard chemical evolution models. High helium abundances in the old stellar population of elliptical galaxies can help explain the increase in emission with shorter wavelength between 2000 and 1200 A, called the UV upturn or UVX. Our lower values of ΔY/ΔZ are consistent with a normal helium abundance at higher metallicity and suggest that other factors, such as a variable red giant branch mass loss with metallicity, may be important. When combined with 4He abundances in metal-poor galaxy H II regions, Magellanic Cloud H II regions, and M17 that have been determined from optical recombination lines, including the effects of temperature fluctuations, our radio 4He/H abundance ratio for S206 is consistent with a helium evolution of ΔY/ΔZ = 1.6. A linear extrapolation to zero metallicity predicts a 4He/H primordial abundance ratio about 5% lower than that given by the Wilkinson Microwave Anisotropy Probe and standard big bang nucleosynthesis. The measured 4He abundances may be systematically underestimated by a few percent if clumping exists in these H II regions.

Why Are Radio Galaxies Prolific Producers of Type Ia Supernovae?

An analysis of SN Ia events in early-type galaxies from the database of Cappellaro and coworkers provides conclusive evidence that the rate of SNe Ia in radio-loud galaxies is about 4 times higher than the rate measured in radio-quiet galaxies, i.e., SN Ia rate(radio-loud galaxies) = 0.43 h SNu as compared to SN Ia rate(radio-quiet galaxies) = 0.11 h SNu. The actual value of the enhancement is likely to be in the range ~2-7 (P ~ 10-4). This finding puts on robust empirical grounds the results obtained by Della Valle & Panagia on the basis of a smaller sample of SNe. We analyze the possible causes of this result and conclude that the enhancement of the SN Ia explosion rate in radio-loud galaxies has the same origin as their being strong radio sources, but there is no causal link between the two phenomena. We argue that repeated episodes of interaction and/or mergers of early-type galaxies with dwarf companions, on timescales of about 1 Gyr, are responsible for both inducing strong radio activity observed in ~14% of early-type galaxies and supplying an adequate number of SN Ia progenitors to the stellar population of elliptical galaxies.

Elliptical Galaxies at z = 0 from Self-consistent Hydrodynamic Simulations: Clues on Age Effects in Their Stellar Populations

We present results of a study of the stellar age distributions in the sample of elliptical-like objects (ELOs) identified at z = 0 in four hydrodynamic, self-consistent simulations operating in the context of a concordance cosmological model. The simulations show that the formation of most stars in each ELO of the sample is a consequence of violent dynamical events, either fast multiclump collapse at high z's, or mergers at lower z's. The latter can explain the age spread as well as the dynamical peculiarities observed in some ellipticals, but its relative weight is never dominant and decreases as the ELO mass at the halo scale, Mvir, increases, to such an extent that some recent mergers contributing an important fraction to the total ELO mass can possibly contribute only a small fraction of newborn stars. More massive objects have older means and narrower spreads in their stellar age distributions than less massive objects. The ELO sample shows also a tight correlation between Mvir and the central stellar line-of-sight velocity dispersion, σ. This gives a trend of the means and spreads of ELO stellar populations with σ that is consistent, even quantitatively, with the age effects observationally detected in the stellar populations of elliptical galaxies. Therefore, these effects can be explained as the observational manifestation of the intrinsic correlations found in the ELO sample between Mvir and the properties of the stellar age distribution, on the one hand, and Mvir and σ, on the other hand. These correlations hint, for the first time, at a possible way to reconcile age effects in ellipticals, and, particularly, the increase of α/Fe ratios with σ, with the hierarchical clustering paradigm. We briefly discuss the origin of the intrinsic correlations shown by ELOs in terms of the adhesion model.

Line-of-Sight Reddening Predictions: Zero Points, Accuracies, the Interstellar Medium, and the Stellar Populations of Elliptical Galaxies

Revised (B-V)_0-Mg_2 data for 402 elliptical galaxies are given to test reddening predictions which can also tell us both what the intrinsic errors are in this relationship among gE galaxy stellar populations, as well as details of nearby structure in the interstellar medium (ISM) of our Galaxy and of the intrinsic errors in reddening predictions. Using least-squares fits, the explicit 1-sigma errors in the Burstein-Heiles (BH) and the Schlegel et al. (IR) predicted reddenings are calculated, as well as the 1-sigma observational error in the (B-V)_0-Mg_2 for gE galaxies. It is found that, in directions with E(B-V) = 0.100 mag, significantly better than those of the BH predictions (0.024-0.025). Gas-to-dust variations that vary by a factor of 3, both high and low, exist along many lines-of-sight in our Galaxy. The approx 0.02 higher reddening zero point in E(B-V) previously determined by Schlegel et al. is confirmed, primarily at the Galactic poles. Despite this, both methods also predict many directions with E(B-V)<0.015 mag. Independent evidence of reddening at the North Galactic pole is reviewed, with the conclusion that there still exists directions at the NGP that have E(B-V)<<0.01. Two lines of evidence suggest that IR reddenings are overpredicted in directions with high gas-to-dust ratios. As high gas-to-dust directions in the ISM also include the Galactic poles, this overprediction is the likely cause of the E(B-V) = 0.02 mag larger IR reddening zero point.

Near‐Infrared Line‐Strength Indices and Their Usefulness for Studying the Stellar Populations of Elliptical Galaxies

Until recently, the potential of the near-infrared (NIR) spectral range for interpreting the stellar populations of elliptical galaxies (Es) was still almost unexploited. Previous calibrations of NIR line-strength indices suffer from limitations that translate into severe uncertainties on the predictions of stellar population synthesis models. In order to overcome the above problems, in this thesis we have calibrated new line-strength indices on the basis of a new, extensive NIR stellar library that consists of 706 spectra (ll8348–9020; FWHM p 1.50 A) with a wide range of atmospheric parameters. The new indices have been designed to measure the Ca ii triplet at ll8498, 8542, 8662, the H Paschen series, the Mg i line at l8807, and the TiO bands. Unlike previous definitions, they exhibit reliable pseudocontinua for all spectral types, and the Ca ii index has been corrected for the contamination with Paschen lines in early spectral types. The large sample of spectra, as well as the wide range of atmospheric parameters, allowed us to compute accurate empirical fitting functions for the indices. These, together with the stellar spectra, have been implemented into a recent single stellar population (SSP) synthesis model, concluding that, for SSPs older than 3 Gyr, (1) the age is a secondary parameter, (2) the Ca ii indices of SSPs with intermediate to high metallicity mainly depend on the initial mass function (IMF) slope (m; assuming a Salpeter-like IMF) in the sense that they decrease with increasing m, and (3) the TiO index is an excellent metallicity indicator. The inclusion of M-type stars in our stellar library reveals unprecedented behaviors for metal-rich SSPs, such as the saturation—or even a decreasing trend—of the integrated Ca ii with increasing metallicity. Finally, new spectroscopic data for a sample of 35 Es are interpreted in the light of our model predictions. The most surprising result is the significant anticorrelation between the

Integrated spectroscopy of bulge globular clusters and fields

An empirical calibration is presented for the synthetic Lick indices (e.g. Mg2, <Fe>, Hβ, etc.) of Simple Stellar Population (SSP) models that for the first time extends up to solar metallicity. This is accomplished by means of a sample of Milky Way globular clusters (GCs) whose metallicities range from ~ to , thanks to the inclusion of several metal rich clusters belonging to the Galactic bulge (e.g., NGC 6553 and NGC 6528). This metallicity range approaches the regime that is relevant for the interpretation of the integrated spectra of elliptical galaxies. It is shown that the spectra of both the globular clusters and the Galactic bulge follow the same correlation between magnesium and iron indices that extends to elliptical galaxies, showing weaker iron indices at given magnesium indices with respect to the predictions of models that assume solar-scaled abundances. This similarity provides robust empirical evidence for enhanced [ α/Fe] ratios in the stellar populations of elliptical galaxies, since the globular clusters are independently known to have enhanced [ α/Fe] ratios from spectroscopy of individual stars. We check the uniqueness of this α-overabundance solution by exploring the whole range of model ingredients and parameters, i.e. fitting functions, stellar tracks, and the initial mass function (IMF). We argue that the standard models (meant for solar abundance ratios) succeed in reproducing the Mg-Fe correlation at low metallicities () because the stellar templates used in the synthesis are Galactic halo stars that actually are α-enhanced. The same models, however, fail to predict the observed Mg-Fe pattern at higher metallicities () (i.e., for bulge clusters and ellipticals alike) because the high-metallicity templates are disk stars that are not α-enhanced. We show that the new set of SSP models which incorporates the dependence on the [ α/Fe] ratio (Thomas et al. 2003) is able to reproduce the Mg and Fe indices of GCs at all metallicities, with an α-enhancement α/Fe=+0.3, in agreement with the available spectroscopic determinations. The Hβ index and the higher-order Balmer indices are well calibrated, provided the appropriate morphology of the Horizontal Branch is taken into account. In particular, the Balmer line indices of the two metal rich clusters NGC 6388 and NGC 6441, which are known to exhibit a tail of warm Horizontal Branch stars, are well reproduced. Finally, we note that the Mg indices of very metal-poor () populations are dominated by the contribution of the lower Main Sequence, hence are strongly affected by the present-day mass function of individual globular clusters, which is known to vary from cluster to cluster due to dynamical effects.

The AGB phase-transition outside the local group:K-band observations of young star clusters in NGC 7252

We have extended the study of the young star clusters observed in the merger remnant galaxy NGC 7252 by obtaining K band photometry for these clusters. Our K band data signicantly complement the optical pho- tometry and spectroscopy in the literature: K band data are fundamental to study the Asymptotic Giant Branch (AGB) population of these clusters, since the AGB phase transition (occuring between the age of200 Myr and 1 Gyr) causes abrupt changes in the near-infrared luminosity of the clusters while producing only small changes in the optical. Therefore, the e.g. V K colour is ideal to study this evolutionary phase of stellar populations. For the present analysis we present models for Simple Stellar Populations which include the contribution of the AGB stellar phase, calibrated with the young and intermediate age star clusters of the Magellanic Clouds. The comparison with the colour distribution of the NGC 7252 star clusters shows that they are indeed intermediate age clusters undergoing the AGB phase transition. The AGB phase transition is observed for the rst time outside the Local Group. Most of the studied clusters span the very narrow age range 300 500 Myr, and likely have metallicities 0:5{1 Z. A very important exception is the cluster W32, which has already completed its AGB epoch, its colours being consistent with an age of1 2 Gyr. This impacts on the duration of the merger-induced starburst. The strengths of the magnesium and iron lines in the spectrum of the best observed cluster W3, and in the spectrum of the diuse central light of NGC 7252, do not show an overabundance in -elements, in contrast to the bulk stellar population of elliptical galaxies.

Spectroscopic constraints on the stellar population of elliptical galaxies in the Coma cluster

Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). COsp (2.3-μm) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated. It is found that the strength of COsp features in elliptical galaxies increases from the core (r 02) of the Coma cluster. Using the Mg2 strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster. Using the COsp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 to 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal. A relation is found between the COsp and V−K colours of ellipticals, corresponding to a slope of 0.036±0.016, significantly shallower than that from the Mg2–(V−K) relation. This is studied using stellar synthesis models.

Theoretical Predictions for Surface Brightness Fluctuations and Implications for Stellar Populations of Elliptical Galaxies

(Abridged) We present new theoretical predictions for surface brightness fluctuations (SBFs) using models optimized for this purpose. Our predictions agree well with SBF data for globular clusters and elliptical galaxies. We provide refined theoretical calibrations and k-corrections needed to use SBFs as standard candles. We suggest that SBF distance measurements can be improved by using a filter around 1 micron and calibrating I-band SBFs with the integrated V-K galaxy color. We also show that current SBF data provide useful constraints on population synthesis models, and we suggest SBF-based tests for future models. The data favor specific choices of evolutionary tracks and spectra in the models among the several choices allowed by comparisons based on only integrated light. In addition, the tightness of the empirical I-band SBF calibration suggests that model uncertainties in post-main sequence lifetimes are less than +/-50% and that the IMF in ellipticals is not much steeper than that in the solar neighborhood. Finally, we analyze the potential of SBFs for probing unresolved stellar populations. We find that optical/near-IR SBFs are much more sensitive to metallicity than to age. Therefore, SBF magnitudes and colors are a valuable tool to break the age/metallicity degeneracy. Our initial results suggest that the most luminous stellar populations of bright cluster galaxies have roughly solar metallicities and about a factor of three spread in age.

Spectral synthesis of the nuclear regions of Seyfert 2 and radio galaxies

We present the results of an optical spectral synthesis analysis for the nuclei of 20 Seyfert 2 and four radio galaxies, using a base of stellar population templates of different ages and metallicities and a power-law continuum. Compared with the stellar population of elliptical galaxies, we find that Seyfert 2s usually have a smaller contribution from old metal-rich stars (age 10 Gyr, Z ≥ Z⊙), and a larger contribution from stars with ages of 100 Myr. We also find that the contributions from stars with ages ≤ 10 Myr and from a power-law continuum are small, rarely exceeding 5 per cent. These results show that the general assumption of elliptical galaxies as stellar population templates for these objects is incorrect, also implying that the excess blue continuum frequently found in their nuclear spectra is probably due to this template mismatch. We find a considerable contribution from 100-Myr-old stars (≈ 5 per cent), which can be interpreted from the point of view of models where the fuelling of the AGN is carried out by interactions/mergers.

Environmental Effects in the Stellar Populations of Elliptical Galaxies

While early work successfully modelled the stellar population of elliptical galaxies as old, single-age, metal-rich systems, there is evidence that a small amount of recent (i.e. a few Gyrs ago) star formation has occurred in some nearby ellipticals (O’Connell, 1980; Rose, 1985; Pickles, 1985; Bica, 1988). Elliptical galaxies reside in environments ranging from the dense cores of rich clusters to isolated field regions. It has long been suggested that the global properties of ellipticals depend on environment. For example, McClure and Van den Bergh (1968) noted apparent differences in the 380–450nm colours between a set of cluster ellipticals and three from the field. Recent work has tended to support the view that the properties of ellipticals depend on environment (e.g. de Carvalho and Djorgovski, 1992). The most likely cause of these differences is the presence of a small amount of recent star formation in the field ellipticals. This environmental dependency may have important consequences for the use of ellipticals as distance indicators.

Chemical Evolution of Elliptical Galaxies: Mg2 Gradients and G-Dwarf Problem

Stellar populations of elliptical galaxies were studied extensively in the recent past (eg., Arimoto &amp; Yoshii 1987; Buzzoni et al. 1992; Worthey 1994; Kodama &amp; Arimoto 1997). However, colours and line-strengths analysed by these approaches came mainly from central regions of galaxies and possible variations of stellar population structure within a galaxy were entirely ignored. Kodama &amp; Arimoto (1997) recently show that a colour-magnitude (CM) relation of ellipticals in Coma cluster (Bower, Lucey, &amp; Ellis 1992), and the CM relation of cluster ellipticals in general, should originate from a difference in mean stellar metallicities, as suggested by Arimoto &amp; Yoshii (1987), and cannot be due to an increase of mean stellar age towards luminous galaxies as claimed by Worthey (1994). It is quite often stated that any theory of galaxy formation should explain the origin of CM relation, or in other words, the mass (luminosity) - metallicity relation of elliptical galaxies. However, one should not forget the fact that the CM relation is defined only for stars in the central regions of galaxies; it is possible that the CM relation itself is an observational artifact, since ellipticals show in general radial gradients of metallic-line strengthes, decreasing from a centre towards outer regions (eg., Faber 1977; Davies, Sadler, &amp; Peletier 1993; Gonzalez 1993; Carollo &amp; Danziger 1994). Indeed, Gorgas &amp; Gonzalez (1996) found that ellipticals with larger central Mg2 indices tend to have steeper Mg2 gradient, which suggests that global metallicities of ellipticals are perphaps universal and are independent of galaxy luminosities.

A Grid of Model Atmospheres and Synthetic Spectra for the Far‐Ultraviolet Analysis of Old Stellar Populations

We present a grid of stellar synthetic spectra suitable for detailed comparison to far-ultraviolet (FUV) observations obtained with the Hopkins Ultraviolet Telescope (HUT), IUE, and Hubble Space Telescope (HST). Our specific application is to study the hot stellar populations in elliptical galaxies, but we anticipate that the models will be useful for other purposes. The 1497 spectra span a range of 10,000 K ≤ Teff ≤ 250,000 K and 2 ≤ log g ≤ 8.5, with three metallicities: Z = Z☉, Z = 0.1 Z☉ and Z = 0.01 Z☉. A variety of simplifying assumptions have been made to reduce computer time and improve convergence, at the inevitable expense of some accuracy. Nevertheless, models in the grid reproduce the overall continuum shape and most of the absorption features seen in HUT spectra of four evolved stars at temperatures of 17,000, 29,900, 36,100, and 55,000 K. The most serious discrepancy is in the cores of the Lyman series lines, where the observed lines are not as deep as those obtained from the models. Until this problem is resolved, the Lyman series lines will not provide a very accurate measure of Teff or log g.While the synthetic spectra in this grid may not be appropriate for detailed analysis of high signal-to-noise ratio stellar spectra, they are sufficiently similar to the observed stars to provide a considerable advantage over existing models for the analysis of the FUV spectra of composite systems, such as elliptical galaxies and globular clusters, where the advantages of having a large, well-sampled grid of models tend to outweigh the known inadequacies of the individual grid points.