Search

The spectrum of neutral iron is critical to astrophysics, yet furnace laboratory experiments cannot reach many high-lying Fe I levels. Instead, Peterson & Kurucz (2015) and Peterson, Kurucz & Ayres (2017) turned to UV and optical spectra of warm stars to identify and assign energies for 124 Fe I levels with 1900 detectable Fe I lines, and to derive astrophysical gf values for over a thousand of these. An energy value was assumed for each unknown Fe I level, and confirmed if it shifted the predicted positions in updated Kurucz (2011) Fe I calculations to match exactly in wavelength the positions of four or more unidentified lines in the observed spectra. Nearly all these identifications were for LS levels characterized by spin-orbit coupling, whose lines fall primarily at UV and optical wavelengths. This extension of these searches provides nearly a hundred new Fe I level identifications. Thirty-nine LS levels are identified largely by incorporating published positions of unidentified laboratory Fe I lines with wavelengths < 2000A. Adding infrared spectra provided sixty Fe I jK levels, where a single isolated outer electron orbits a compact core. Their weak, blended lines fall mostly in the infrared, but are searchable because their mutual energies obey tight relationships. For each new Fe I level, this work again provides and makes publicly available its identification, its energy, and a list of over 16,000 of its potentially detectable lines with theoretical gf values. For over 2,000 of these, this work also includes astrophysical gf values, ones adjusted semi-empirically to fit the stellar spectra. The potential impact of this work on modeling UV and IR stellar spectra is noted., 23 pages, 1 figure. Near-final ApJS version. Section 9 characterizes and quantifies uncertainties in deduced level energies and astrophysical dgf values. Table 3 includes >16,000 Fe I lines potentially detectable in solar-metallicity dwarfs and giants, and dgf values for >2,000 of these. It is currently available from the website of R. Kurucz at http://kurucz.harvard.edu/atoms/2600/PK21/table3.txt

Cowan et al. (2021) review how roughly half the elements heavier than iron found in the Sun are produced by rapid neutron capture and half by slow neutron capture, the r- and s-processes. In the Sun, their relative contribution to individual elemental abundances is well understood, except for the lightest and heaviest elements beyond iron. Their contributions are especially uncertain for the heaviest non-radioactive element, lead (Pb, Z=82). This is constrained by deriving lead abundances in metal-poor stars. For in the most metal-poor halo stars, strontium and heavier elements are found in the solar r-process proportion; s-process elements appear only at metallicities above one-thirtieth solar. In unevolved metal-poor stars of roughly solar heavy-element content, only two UV Pb lines are detectable. Four such stars have high-resolution spectra of the strongest line, Pb II at 2203.53A. Roederer et al. (2020) analyzed this line in one star, deriving a lead-to-iron abundance ratio ten times solar. This and its blue-shifted profile suggested strong s-process production. This work analyzes the UV spectra of all four stars. Calculations including a predicted Fe I line blueward of the Pb II line, and assuming the lead abundance scales with r-process abundances, match all four profiles extremely well. A scaled s-process contribution might improve the match to the much lower lead abundance found in the unevolved star analyzed previously, but its s-process excess is modest. An Fe II line blends the other lead line, Pb I at 2833.05A, which constrains the lead abundance only in the coolest star., 9 pages, 3 figures. Accepted for publication in the Astrophysical Journal Letters as a gold open access article under a CC BY license

The spectrum of the Fe I atom is critical to many areas of astrophysics and beyond. Measurements of the energies of its high-lying levels remain seriously incomplete. Here we update the work by Peterson & Kurucz (2015) to identify such levels using new and archival high-resolution absorption-line UV and optical spectra of stars whose warm temperatures favor Fe I lines. Our GO-14161 Hubble program will obtain UV spectra for four metal-poor turnoff stars, archive them at ASTRAL, and draw from them the new identifications of 250 – 500 Fe I levels.

Photoelectric B and V photometry of RR Lyrae was obtained over a 77 day interval in 1996. In addition to the 0.5668 day primary period, RR Lyr displayed a prominent 40 day Blazhko cycle during this time span. The observed light curve can be well but not perfectly described by frequency triplets of the form kf0 + jfB, where k = 0, 1, 2, ..., j = -1, 0, 1, f0 is the primary pulsation frequency, and fB is the Blazhko frequency.

We analyze HST/STIS spectra (see Paper I) of the central region of the dense globular cluster M15. We infer the velocities of 64 individual stars, two-thirds of which have their velocity measured for the first time. This triples the number of stars with measured velocities in the central 1 arcsec of M15 and doubles the number in the central 2 arcsec. Combined with existing ground-based data we obtain the radial profiles of the projected kinematical quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few arcsec, somewhat higher than the values of 10-12 km/s inferred previously from ground-based data. To interpret the results we construct dynamical models based on the Jeans equation, which imply that M15 must have a central concentration of non-luminous material. If this is due to a single black hole, then its mass is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the relation between M_BH and sigma_RMS that has been established for galaxies. Also, the existence of intermediate-mass black holes in globular clusters is consistent with several scenarios for globular cluster evolution proposed in the literature. Therefore, these results may have important implications for our understanding of the evolution of globular clusters, the growth of black holes, the connection between globular cluster and galaxy formation, and the nature of the recently discovered `ultra-luminous' X-ray sources in nearby galaxies. Instead of a single black hole, M15 could have a central concentration of dark remnants (e.g., neutron stars) due to mass segregation. However, the best-fitting Fokker-Planck models that have previously been constructed for M15 do not predict a central mass concentration that is sufficient to explain the observed kinematics.[ABRIDGED], 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in press (Dec 2002). Please note that the results reported here are modified by the Addendum available at astro-ph/0210158 (Astronomical Journal, in press, Jan 2003). This second version submitted to astro-ph is identical to first, with the exception of the preceeding remark

This is the first in a series of three papers presenting a new calculation of the mass of the Galaxy based on radial velocities and distances measured for a sample of some 100 faint 16 30 kpc with measured radial velocities by a factor five. Faint A-type stars in the Galactic halo have been identified from UBR photometry in six UK Schmidt fields. We obtain accurate CCD photometry and spectra to classify these stars. This paper describes our methods for separating out clean samples of BHB stars in a way that is efficient in terms of telescope time required. We use the high signal-to-noise ratio (S/N) spectra of A-type stars of Kinman, Suntzeff & Kraft (published in 1994) to assess the reliability of two methods, and to quantify the S/N requirements. First we revisit, refine and extend the hydrogen line width versus colour relation as a classifier. The second method is new and compares the shapes of the Balmer lines. Using the equivalent width of the Ca II K line as an additional filter we find we can reproduce Kinman, Suntzeff & Kraft's classifications with both methods.

Recent results are reviewed for two methods of luminosity calibration based on high-resolution spectroscopy. The first relies on Teff/log g determinations from model-atmosphere analyses based on high-resolution spectra. This method is physically well founded but operationally demanding, and requires advance knowledge of stellar mass. The second, W-B, stems from the empirical relationship between luminosity and the width of chromospheric emission lines first established by Wilson and Bappu. Its physical basis is only partially understood, however, and the calibration depends on stellar metallicity and on the choice of lines.Both Teff/log g and W-B easily distinguish cool dwarfs from cool giants. Generally reasonable agreement is found between distances derived from Hipparcos parallaxes and those inferred from the log g values derived for nearby dwarfs with relatively well-known Hipparcos parallaxes, σ(π)/π < 0.2. Constraining Hipparcos parallaxes star-by-star is not possible at present. Improvements are suggested for both approaches.

The spectrum of the Fe I atom is critical to many areas of astrophysics and beyond. Measurements of the energies of its high-lying levels remain woefully incomplete, however, despite extensive analysis of ultraviolet laboratory iron absorption spectra, optical laboratory iron emission spectra, and the solar infrared spectrum. In this work we use as sources the high-resolution archival absorption-line ultraviolet and optical spectra of stars, whose warm temperatures favor moderate Fe I excitation. We derive the energy for a particular upper level in Kurucz's semiempirical calculations by adopting a trial value that yields the same wavelength for a given line predicted to be about as strong as that of a strong unidentified spectral line observed in the stellar spectra, then checking the new wavelengths of other strong predicted transitions that share the same upper level for coincidence with other strong observed unidentified lines. To date this analysis has provided the upper energies of 66 Fe I levels. Many new level energies are higher than those accessible to laboratory experiments; several exceed the Fe I ionization energy. These levels provide new identifications for over two thousand potentially detectable lines. Almost all of the new levels of odd parity include UV lines that were detected but unclassified in laboratory Fe I absorption spectra, providing an external check on the energy values. We motivate and present the procedure, provide the resulting new level energies and their uncertainties, list all the potentially detectable UV and optical new Fe I line identifications and their gf-values, point out new lines of astrophysical interest, and discuss the prospects for additional Fe I energy-level determinations in the near future., Comment: v3 is the version submitted to the Astrophysical Journal Oct. 24, 2014, 10 pages, 3 tables, 5 figures, in response to the referee's report; minor corrections to that submission appear in italics. The number of newly-identified levels has nearly doubled to 66 (23 even, 43 odd), with a corresponding increase in the number of newly-identified lines. The full listing of these lines is included

The sensitivity of various methods of oxygen-abundance determination to the structure of the model atmosphere in a classical analysis is described, with emphasis on the role of stellar effective temperature and the treatment of convection. Oxygen-abundance results presented during this meeting are outlined. Because the oxygen diagnostics are sensitive in different ways to the choice of model stellar parameters and to the treatment of convection, it is not surprising that different methods applied by different groups give different results. Most are consistent with an increase in [O/Fe] as [Fe/H] declines, but the slope is uncertain still, and the scatter is large at very low abundances. The trend may continue to low [Fe/H], but more stars and more checks are needed.

We have serendipitously discovered an extremely lithium-rich star on the red giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet of 520 milli-Angstrom equivalent width, and our analysis places the star among the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the elemental abundances of this star, IV-101, and three other cluster members of similar luminosity and color, and conclude that IV-101 has abundance ratios typical of giants in M3 and M13 that have undergone significant mixing. We discuss mechanisms by which a low-mass star may be so enriched in Li, focusing on the mixing of material processed by the hydrogen-burning shell just below the convective envelope. While such enrichment could conceivably only happen rarely, it may in fact regularly occur during giant-branch evolution but be rarely detected because of rapid subsequent Li depletion., 7-page LaTeX file, including 2 encapsulated ps figures + 1 table; accepted for publication in the Astrophysical Journal Letters

We report the discovery of hot horizontal branch stars in the nuclear bulge of the Milky Way. Spectra from the 2dF instrument of the Anglo-Australian Telescope allow us to confirm their membership in the bulge through radial velocities. We also review the current observational information on hot horizontal branch stars in Milky Way clusters and the Local Group, and discuss the relevance of star-by-star studies in the bulge for the ultraviolet-excess phenomenon seen in large ellipticals.

This work sketches how SDSSugrcolors and Kepler 2 in halo fields can identify red giants 50 – 100 kpc distant with minimal metallicity bias. For these mildly-reddened, metal-poor giants, (g-r)oyields the effective temperatureTeffto 100 K. K2 can detect the p-mode oscillations of red giants and measure their frequency of maximum power νmax. This sets the luminosityLboland thus the distance, plus an estimate of metallicity [Fe/H].

The old open cluster NGC 6791 is believed to be more metal-rich than any other, yet several hot blue horizontal-branch (BHB) stars are probable members. We have performed an abundance analysis of the coolest BHB candidate, 2-17, whose proper motion and radial velocity both support cluster membership. Its luminosity and its low rotational velocity, vsin${r sin}$ -->${r sin}$ -->i=16 ± 1 km s-1, suggest that it is a BHB star rather than a very luminous blue straggler. We find an effective temperature Teff${r eff}$ -->$t SUBgt {r eff}t/SUBgt $ -->=7300 ± 50 K, gravity log${r log}$ -->${r log}$ -->g=3.6 ± 0.3 dex, and an iron abundance more than twice solar, [Fe/H]=+0.4 ± 0.1 dex. This result is free from the serious line blending and continuum suppression that hamper spectroscopic analyses of metal-rich giants. The Ca abundance is in the solar proportion with respect to iron, ruling out A-star peculiarities in 2-17. The relative abundances of C, O, and Al are nearly solar; mixing effects are probably small but not ruled out. The light elements Mg and Si are enhanced, N and Na especially so, as seen in metal-rich galaxies. NGC 6791 thus provides an excellent template for the study of the stellar content of metal-rich galactic systems. Both its abundances and its continued existence suggest that metal-rich systems might generally form in locally deep potential wells. Further study of its hot population should clarify the mechanisms responsible for producing hot stars in a metal-rich environment and thus assist the interpretation of the integrated light and the ultraviolet upturn in elliptical galaxies.

The boron abundances of two giants and one turno† star in the Hyades, and the -eld K0 III giant b Gem, have been measured from spectra obtained with the Hubble Space Telescope in the vicinity of the B I lines near 2500 This provides a test of the dilution of giant envelopes predicted by standard stellar ". evolutionary models: since B is destroyed by (p, a) reactions at a temperature near 5 ) 106 K, it is expected to be preserved only in an outer shell on the main sequence and diluted by B-free interior material on the giant branch. We -nd that the B abundance in each giant is reduced below solar by the factor of 10 predicted by standard stellar evolution models. Li, which is more fragile than B, is reduced in the same stars by a factor of 160; the prediction is 60. However, when non-LTE e†ects are accounted for, the observed Li dilution factor becomes 80, equaling the predicted value within measurement error and resolving a long-standing discrepancy. No unusual mass loss or nonstandard internal mixing is required to match the observations. Subject headings: stars: abundances E stars: evolution E stars: interiors E stars: late-type

Cool stars like the Sun show chromospheric activity, which is largely attributed to nonradiative sources of energy linked to rotationally driven magnetic fields. However, a nonmagnetic process, such as dissipation of acoustic waves, may operate side by side with the first process. We report detections of chromospheric emission reversals in Mg II 2800 A spectra for nine metal-poor solar-type stars, some believed to be among the oldest stars known, whose great age and slow rotation argue against strong magnetic activity. Although interstellar absorption is always present, in seven stars with high radial velocities each reversal profile is seen to be doubly peaked, with the blue peak stronger than the red, much like such profiles from the quiet Sun. Despite having as little as 1/300th as much metal as the Sun and being 3 times as old, these stars have chromospheric activity comparable to that in solar quiet regions. While these data do not rule out magnetic fields, they support an acoustic origin of chromospheric emission and show that relatively inactive solar-type stars of all ages have chromospheres whose characteristics are largely independent of metallicity.

The ratios of elemental abundances observed in metal-poor stars of the Galactic halo provide a unique present-day record of the nucleosynthesis products of its earliest stars. While the heaviest elements were synthesized by the r- and s-processes, dominant production mechanisms of light trans-ironic elements were obscure until recently. This work investigates further our 2011 conclusion that the low-entropy regime of a high-entropy wind (HEW) produced molybdenum and ruthenium in two moderately metal-poor turnoff stars that showed extreme overabundances of those elements with respect to iron. Only a few, rare nucleosynthesis events may have been involved. Here we determine abundances for Mo, Ru, and other trans-Fe elements for 28 similar stars by matching spectral calculations to well-exposed near-UV Keck HIRES spectra obtained for beryllium abundances. In each of the 26 turnoff stars with Mo or Ru line detections and no evidence for s-process production (therefore old), we find Mo and Ru to be three to six times overabundant. In contrast, the maximum overabundance is reduced to factors of three and two for the neighboring elements zirconium and palladium. Since the overproduction peaks sharply at Mo and Ru, a low-entropy HEW is confirmed as its origin. The overabundance level of the heavy r-process elements varies significantly, from none to a factor of four, but is uncorrelated with Mo and Ru overabundances. Despite their moderate metallicity, stars in this group trace the products of different nucleosynthetic events: possibly very few events, possibly events whose output depended on environment, metallicity, or time., Accepted April 2, 2013, for publication in the Astrophysical Journal Letters (7 pages, 3 figures)

We report upper limits to rotation from the measurement of line breadths via cross-correlation analysis for 27 RR Lyrae variables. The eight best-observed stars of type RRab show the same variation of breadth with phase: the breadth peaks sharply during the rise to maximum light, drops gradually, and reaches a broad minimum during the phase of maximum radius. During this phase the breadth is always narrow, consistent with instrumental resolution and turbulence alone. For the three well-observed RRc variables, the breadth is this narrow at all phases except for a slight increase during the rise to maximum light. The remaining stars also conform to these patterns, albeit with sparse phase coverage and lower signal-to-noise ratio. We interpret these results as indicating that lines are broadened in RRab stars by shock-induced plumes or turbulence during the rise to maximum light, and perhaps by other causes as well, but not by rotation in RR Lyrae stars of either type. We estimate an upper limit of v sin i 10 km s-1 in three out of six well-studied field stars, and to the more rapid rotation, often exceeding 100 km s-1, of the Population I δ Scuti variables which occupy adjacent regions of the instability strip.

The sensitivity of various methods of oxygen-abundance determination to the structure of the model atmosphere in a classical analysis is described, with emphasis on the treatment of convection. Oxygen-abundance results presented during this meeting are outlined. Because the oxygen diagnostics are sensitive in different ways to the choice of model stellar parameters and to the treatment of convection, it is not surprising that different methods applied by different groups give different results. Most are consistent with an increase in [O/Fe] as [Fe/H] declines, but the slope is uncertain still, and the scatter is large at very low abundances. The trend may continue to low [Fe/H], but more stars and more checks are needed.

We report determinations of the molybdenum abundances in five mildly to extremely metal-poor turnoff stars using five Mo II lines near 2000A. In two of the stars, the abundance of molybdenum is found to be extremely enhanced, as high or higher than the neighboring even-Z elements ruthenium and zirconium. Of the several nucleosynthesis scenarios envisioned for the production of nuclei in this mass range in the oldest stars, a high-entropy wind acting in a core-collapse supernova seems uniquely capable of the twin aspects of a high molybdenum overproduction confined to a narrow mass range. Whatever the details of the nucleosynthesis mechanism, however, this unusual excess suggests that very few individual nucleosynthesis events were responsible for the synthesis of the light trans-Fe heavy elements in these cases, an unexpected result given that both are only moderately metal-poor., Comment: updated in v2, including text missing from the third-to-last paragraph in the published version

As part of NASA and Hubble Treasury programs, we are generating theoretical spectra for globular clusters and galaxies from first principles. To check their validity, we are assessing how well they match high-resolution echelle spectra of the Sun and nearby stars. Once agreement is reasonable, spectra at fixed metallicity and light-element enhancement are coadded with weights representing the relative contribution of various types of stars in old stellar systems, to generate theoretical composite spectra for single stellar populations. These are compared to globular cluster spectra, notably in M31, to derive properties such as age and metallicity.

We are calculating stellar spectra for types A through K using Kurucz codes, Castelli models, and Kurucz laboratory lines plus guessed identifications for other lines in the spectra. Weighted coadditions of these spectra are being constructed to match spectra observed in integrated light of old stellar systems such as elliptical galaxies and globular clusters. Grids of theoretical spectra, both stellar and composite, that include an enhancement of light elements and span a wide metallicity range will be calculated over 2200A–9000A, and will be archived on MAST at the Hubble Space Telescope website. Here we summarize our results and describe how we automate the fit to our grid of an observed high-resolution stellar or globular-cluster spectrum, to determine the stellar parameters or to break the age-metallicity degeneracy.

We report new chemical abundances of 23 bright red giants of the globular cluster M3, based on high-resolution spectra obtained with the Keck I telescope. Combining these data with a previously-reported small sample of M3 giants obtained with the Lick 3m telescope, we compare [X/Fe] ratios for 28 M3 giants with 35 M13 giants, and with halo field stars. All three groups exhibit C depletion with advancing evolutionary state beginning at the RGB bump region. but the overall depletion in the clusters is larger than that of the field stars. The behaviors of O, Na, Mg and Al are distinctively different among the three stellar samples. Both M3 and M13 show evidence of high-temperature proton capture synthesis from the ON, NeNa, and MgAl cycles, while there is no evidence for such synthesis among halo field stars. But the degree of such extreme proton-capture synthesis in M3 is smaller than it is in M13, and no indication that O depletions are a function of advancing evolutionary state as has been claimed for M13. We have also considered NGC 6752, for which Mg isotopic abundances have been reported by Yong et al. (2003). Giants in NGC 6752 and M13 satisfy the same anticorrelation of O abundances with the ratio (25Mg+26Mg)/24Mg. This suggests that these abundance ratios arose in the ejected material of 3-6 solar mass cluster stars. It also suggests that the low oxygen abundance seen among the most evolved M13 giants arose in hot-bottom O to N processing in these same intermediate-mass cluster stars. Thus mixing is required by the dependence of some abundance ratios on luminosity, but an earlier nucleosynthesis process in a hotter environment than giants or main-sequence stars is required by the variations previously seen in stars near the main sequence., 55 pages, 18 figures, to be published in AJ

We present a progress report on an extensive survey to find and characterize all types of blue horizontal-branch stars in the nuclear bulge of the Galaxy. We have obtained wide, shallow imaging in UBV of ~12 square degrees in the bulge, with follow-up spectroscopy for radial velocities and metal abundance determinations. We have discovered a number of metal-rich blue HB stars, whose presence in the bulge is expected by the interpretation of the extragalactic ultraviolet excess. Very deep images have been obtained in UBV and SDSS u along the bulge minor axis, which reveal a significant number of EHB candidates fainter than B = 19, i.e., with the same absolute magnitudes as EHB stars in several globular clusters., To appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluwer Academic Publishers, proceedings of the meeting held in Keele, UK, June 16-20, 2003

Twenty years ago, Burstein et al. (1984)suggested that strong CN and Hbeta absorption meant younger ages among globular clusters in the Andromeda galaxy (M31), unless blue stars above the main-sequence turnoff or on the horizontal branch were uncommonly prominent. Here we test these suggestions by fitting the detailed mid-ultraviolet (2280-3120A) and optical (3850-4750A) spectra of one moderately metal-rich M31 globular cluster, G1. We explore the effects of a wide range of non-solar temperatures and abundance ratios, by combining a small set of theoretical stellar spectra like those of Peterson et al. (2001) that were calculated using extensively updated atomic-line constants. To match the mid-UV fluxes of G1, we find that hot components with Teff >= 8000K must be included. We obtain a very good fit with cool and hot blue horizontal branch (BHB) stars, but less satisfactory fits for blue straggler stars, those hotter than the main-sequence turnoff. The G1 color-magnitude diagram does show cool BHB stars, and the color of its giant branch supports the metallicity of one-sixth the solar value that we deduce. The turnoff temperature of the best-fit model is consistent with that of turnoff stars in galactic globular clusters and the field halo, indicating G1 is comparably old. Because metal-rich cool BHB and extremely blue HB stars have now been found within our own Galaxy, we suggest that these hot horizontal-branch stars be considered in fitting spectra of metal-rich populations such as the Andromeda globular clusters, to avoid possible underestimates of their ages. We plan to make the relevant spectral calculations available as part of our Hubble Treasury Program., 18 pages plus 3 figures, each 2 panels. Accepted by The Astrophysical Journal

In this series of two papers we describe a project with STIS on HST to measure the line-of-sight velocities of stars in the central few arcsec of the dense globular cluster M15. This first paper focuses on the observations and reduction of the data. We `scanned' the central region of M15 spectroscopically by consecutively placing an 0.1 arcsec slit at 18 adjacent positions. Careful and complicated data reduction and analysis were required. The spectral pixel size exceeds the velocity dispersion of M15, which puts the project at the limit of what is feasible with STIS. We applied corrections for the following effects: drifts in the STIS wavelength scale during an HST orbit; the orbital velocity component of HST along the line-of-sight to the cluster, and its variations during the HST orbit; and the apparent wavelength shift that is perceived for a star that is not centered in the slit. To make these corrections we present a detailed study of the HST/STIS point-spread function and line-spread function. We also present a catalog with positions and U, B and V magnitudes of 31,983 stars in M15, created from existing HST/WFPC2 data. We extract 19,200 one-dimensional spectra from the STIS data, each for a different position in M15, and each with a velocity scale accurate to better than 2.5 km/s. We develop an algorithm that co-adds the spectra for individual apertures and use it to extract spectra of individual stars with minimum blending and maximum S/N. In Paper II we use these spectra to extract the line-of-sight velocities of individual stars. The velocities constrain the central structure, dynamics and mass distribution of the cluster, and provide evidence for the existence of an intermediate-mass black hole in the center of the cluster.[ABRIDGED], Comment: 38 pages, LaTeX, with 12 PostScript figures. Astronomical Journal, in press (Dec 2002)

Determining the properties of old stellar systems using evolutionary population synthesis requires a library of reliable model stellar fluxes. Empirical libraries are limited to spectra of stars in the solar neighborhood, with nearly solar abundances and abundance ratios. We report here a first step towards providing a flux library that includes nonsolar abundances, based on calculations from first principles that are calibrated empirically. We have started with main-sequence stars, whose light dominates the mid-ultraviolet spectrum of an old stellar system. We have calculated mid-ultraviolet spectra for the Sun and nine nearby, near-main-sequence stars spanning metallicities from less than 1/100 solar to greater than solar, encompassing a range of light-element abundance enhancements. We first determined temperatures of eight of the stars by analyzing optical echelle spectra together with the mid-ultraviolet. Both could be matched at the same time only when models with no convective overshoot were adopted, and only when an approximate chromosphere was incorporated near the surface of relatively metal-rich models. Extensive modifications to mid-UV line parameters were also required, notably the manual assignment of approximate identifications for mid-UV lines missing from laboratory linelists. Without recourse to additional missing opacity, these measures suffice to reproduce in detail almost the entire mid-UV spectrum of solar-temperature stars up to one-tenth solar metallicity, and the region from 2900A to 3100A throughout the entire metallicity range. Ramifications for abundance determinations in individual metal-poor stars and for age-metallicity determinations of old stellar systems are briefly discussed, with emphasis on the predictive power of the calculations., Proof revision -Minor changes to revised version submitted to Astrophysical Journal May 1, 2001. 29 pages, 4 figures (Fig. 3 with 5 panels and Fig. 4 with 6 panels). Figures 1 and 2 are .gif; postscript versions of Figures 1 and 2 are available from http://www.astro.virginia.edu/~rtr/uv/index.html

We present the first results of a survey of blue horizontal branch (BHB) stars in the Galactic bulge. 164 candidates with 15 < V < 17.5 in a field 7.5deg from the Galactic Center were observed in the blue at 2.4A FWHM resolution with the AAT 2dF spectrograph. Radial velocities were measured for all stars. For stars with strong Balmer lines, their profiles were matched to theoretical spectrum calculations to determine stellar temperature Teff and gravity log g; matches to metal lines yielded abundances. CTIO UBV photometry then gave the reddening and distance to each hot star. Reddening was found to be highly variable, with E(B-V) from 0.0 to 0.55 around a mean of 0.28. Forty-seven BHB candidates were identified with Teff >= 7250K, of which seven have the gravities of young stars, three are ambiguous, and 37 are HB stars. They span a wide metallicity range, from solar to 1/300 solar. The warmer BHB's are more metal-poor and loosely concentrated towards the Galactic center, while the cooler ones are of somewhat higher metallicity and closer to the center. Their red B-V colors overlap main-sequence stars, but the U-B vs. B-V diagram separates them until E(B-V) > 0.5. We detect two cool solar-metallicity HB stars in the bulge of our own Galaxy, the first such stars known. Still elusive are their hot counterparts, the metal-rich sdB/O stars causing excess UV light in metal-rich galaxies; they have V ~ 20.5 in the Bulge., 29 pages, 4 figures (the third with 4 panels, the fourth with 2 panels). To appear in the Astrophysical Journal v571n1, Jan. 20, 2000. Abstract is shortened here, and figures compressed

In metal-rich stars as cool as the Sun, beryllium abundance determinations are difficult due to heavy line blanketing in the near-UV 3130 Å region where the accessible Be II lines reside. We can now attempt such determinations based on improved lists of atomic line identifications and gf-values in the near-UV. Here we report Be determinations for three metal-rich A, F, and G stars plus three solar-metallicity standards. All six stars have beryllium-to-hydrogen ratios at or below solar. More such determinations would provide stronger constraints on trends in Be abundance with temperature, metallicity, and age.

Oxygen and sodium abundances or upper limits have been determined for 17 red giant members of the metal-poor Galactic globular clusters M68 (two stars), M30 (two stars), M15 (two stars), M55 (two stars), and NGC 6397 (five stars), as well as for the moderately metal-poor clusters NGC 4833 (one star), NGC 6144 (one star), and NGC 6752 (three stars). The behavior of [O/Fe] with respect to luminosity and metallicity is investigated. As found in previous works, the oxygen-to-iron ratios show an upper envelope, in agreement with the mean level of field halo stars of similar metallicities ([O/Fe]=+0.4{plus_minus}0.15). Moreover, as is true of the more metal-rich giants in globular clusters, about one-third of the stars do not have enhanced oxygen with respect to iron, in contrast to field giants of similar metallicity, which are almost invariably oxygen-rich. Several explanations for this deficiency are examined, including emission possibly associated with mass-loss filling in the oxygen lines and evolutionary mixing effects. In particular, the abundances of oxygen and sodium are seen to be anticorrelated, in agreement with previous results (Kraft {ital et} {ital al}. 1995). The enhancement of oxygen in these extremely metal-poor clusters appears similar to that of less metal-deficient clusters, anmore » important datum for determinations of their relative ages. {copyright} {ital 1996 The American Astronomical Society.}« less

The internal stellar velocity distribution of the globular cluster M4 is evaluated from nearly 200 new radial velocity measurements good to 1 km/s and a rederivation of existing proper motions. The mean radial velocity of the cluster is 70.9 +/- 0.6 km/s. The velocity dispersion is 3.5 +/- 0.3 km/s at the core, dropping marginally towards the outskirts. Such a low internal dispersion is somewhat at odds with the cluster's orbit, for which the perigalacticon is sufficiently close to the galactic center that the probability of cluster disruption is high; a tidal radius two-thirds the currently accepted value would eliminate the discrepancy. The cluster mass-to-light ratio is also small, M/L(sub V) = 1.0 +/- 0.4 in solar units. M4 thus joins M22 as a cluster of moderate and concentration with a mass-to-light ratio among the lowest known. The astrometric distance to the cluster is also smaller than expected, 1.72 +/- 0.14 kpc. This is only consistent with conventional estimates of the luminosity of horizontal branch stars provided an extinction law R = A(sub V)/E(B-V) approximately 4 is adopted, as has been suggested recently by several authors.

High-resolution echelle spectra have been obtained for four of the extremely metal-deficient stars from the HK objective-prism/interference-filter survey of Beers, Preston, and Shectman. Model atmosphere analysis shows that the abundances of these objects lie in the range -4.2

High-resolution, high-SNR CCD spectra have been obtained for 16 giants in eight metal-poor Galactic globular clusters. Fe abundances accurate to 0.15 dex have been determined by a fully consistent set of model atmospheres and spectrum synthesis techniques. A metallicity scale is presented for metal-poor clusters that should prove useful for calibrating a wide variety of photometric and low-resolution spectroscopic metallicity indicators.

We report the finding of an unusually large lithium abundance in the tidally locked binary halo star HD 89499. The effective temperature of this star has been determined to be 4900±150 K from photometric and spectroscopic analyses. Its metallicity has been determined to be [Fe/H]=−2.08±0.07 from 26 Fe I lines and its rotational velocity is estimated to be 22±4 km s −1 . If the star is a dwarf, its lithium abundance log e(Li)=1.35 which is a factor of 10-30 above that for other halo dwarfs of the same temperature. In this case, the lithium depletion may have been arrested when the rotational period of the star tidally synchronized with the binary orbital period. We discuss applicable models

We present new measurements of the central stellar velocity dispersions (σ) of dwarf elliptical galaxies (dE's), most of them of the nucleated variety. The data consist of photon counting spectra taken at the MMT with spectral resolution of 12 and 32 km s −1 and CCD data from CTIO and KPNO at 8 and 18 km s −1 (FWHM). The measured dispersions range from 14 to 39 km −1 , and are lower than previous measurements done at more coarse resolution by about 50%. M/L ratios are derived for the galaxies. These are not unusually high (1-7, in solar units), but the change in M/L with luminosity for all dE galaxies confirms a prediction made by Dekel and Silk [ApJ, 303, 39 (1986)] (M/L∼L −0.40±0.06 ) as does the observed relation between L and velocity dispersion (L∼σ 5.6±0.9 )

Using opacity distribution functions based on a newly expanded atomic and molecular line list, we have calculated a model atmosphere for Arcturus that reproduces the observed flux distribution. Individual line parameters in the list were adjusted to match the solar spectrum in a preliminary way, in the regions 5000-5500 A, 6000-6500 A, and 7500-9000 A. The Arcturus model spectrum calculated with these adjustments reproduces well the profiles of all lines in the observed spectrum of the Griffin atlas for which the solar gf-values are well determined. The Arcturus model has an iron abundance [Fe/H]=−0.5±0.1, a temperature T eff =4300±30 K, gravity log g=1.5±0.15, and an overabundance of the light metals

The abundances of Fe and other elements are determined for a star of intermediate metallicity and for nine extremely metal poor stars, including two members of the globular cluster M92 and CD -38 deg 245. The accuracy of the transition probabilities for Fe I and other elements is evaluated. The distribution of the abundances of other elements with respect to Fe is the same for most of the cases studied. Manganese is the only element that shows a different relative abundance in an extremely metal poor star. 120 refs.

This paper reports an abundance analysis of spectra of 10 extremely metal-poor giants, including two in the globular cluster M92, in collaboration with Robert Kurucz and Eugene Avrett of the Center for Astrophysics and Bruce Carney of the University of North Carolina. An intercomparison of equivalent widths indicates that Luck and Bond have seriously overestimated the strengths of very weak lines and so deduced a spuriously high nickel abundance in the most metal-poor stars. A review of line-formation effects in the Sun strongly suggests that the rather high metallicities found for some extremely metal-poor stars by Pilachowski and coworkers is due to errors in their solar gf values, which unfortunately are unpublished. In our own analyses, stars with asymmetric H-alpha line profiles tend to have an excitation temperature at odds with the effective temperature found from infrared photometry. The discrepancy is removed by lowering the surface temperature by 100 K or by increasing the microturbulent velocity, either by a constant 0.5 km/s or by an amount which increases toward the surface. These changes have the most impact on those abundances deduced from strong lines or low-excitation lines of the neutral species; this includes sodium and barium as well as all the Fe I lines with very accurate gf values. Despite these difficulties, our findings reveal a low abundance for M92, and intrinsic scatter in the relationship between the relative abundances of sodium and oxygen and the overall iron abundance. In one field star, oxygen is apparently overabundant by an order of magnitude or more, while in the two M92 giants, the sodium abundance appears to differ by nearly this much.

A new study of carbon and nitrogen abundances has been carried out for a sample of halo dwarf stars. This sample was limited to relatively bright high velocity stars with substantial metal deficiencies (-0.65≥[Fe/H]≥ −2.00). Synthesis of CH molecular bands for nine program stars indicates that [c/Fe] = −0.1±0.2 in halo stars. Preliminary results of the NH synthesis in two stars shows discrepant results: a moderate velocity star has [N/Fe]≲ −1.0 while a very high velocity star has [N/Fe]⋍ +0.0. Implications of these abundances for galactic chemical evolution are discussed.

There is spectroscopic evidence for initial abundance inhomogeneities from star to star within M13 and M22. Among three stars in M13, there is an order-of-magnitude range in the sodium-to-iron ratio, which lies well beyond all known random and systematic errors; details will be given in a paper in preparation. In M22, the spread discussed below in logarithmic iron-to-hydrogen ratios, [Fe/H], is 0.6 dex, which is far larger than that found here for any other globular cluster.

With high-precision radial velocities and proper motions, one can equate the proper motion and radial velocity dispersions to obtain astrometric distances independent of any standard candles. We discuss the method and the small distance it yields to M 22.