John E. Norris

Extremely Metal-Poor Stars. II. Elemental Abundances and the Early Chemical Enrichment of The Galaxy*

We have obtained high-resolution spectra of 23 very metal-poor stars and present an abundance analysis for 19 of these for elements between Mg and Eu. The sample comprises roughly equal numbers of dwarfs and giants. All stars have [Fe/H] 0 deserve further study. CS 22897–008 has high Sr, Y, and C abundances for its [Fe/H] but normal Ba. This signature may have arisen from the weak s-process in M > 15 M stars or by r-processing. By combining an analytic description of gaseous supernova remnants with supernova yields, we show that enrichment of the interstellar medium is influenced more by supernova physics (explosive energy) than by environmental conditions (cloud density). If supernova iron-peak yields are correlated with explosion energy, we can accommodate the well-defined abundance trends with a chaotic picture for halo formation involving independently evolving clouds, as was envisaged by Searle & Zinn. We calculate that a typical enrichment in the protohalo will produce [Fe/H] = −2.7. This coincides with larger abundance variations in field stars of lower metallicity and the lower abundance limit for Galactic globular clusters.

The Milky Way Tomography with SDSS. II. Stellar Metallicity

In addition to optical photometry of unprecedented quality, the Sloan Digital Sky Survey (SDSS) is producing a massive spectroscopic database which already contains over 280,000 stellar spectra. Using eectiv e temperature and metallicity derived from SDSS spectra for 60,000 F and G type main sequence stars (0:2 < g r < 0:6), we develop polynomial models, reminiscent of traditional methods based on the UBV photometry, for estimating these parameters from the SDSS u g and g r colors. These estimators reproduce SDSS spectroscopic parameters with a root-mean-square scatter of 100 K for eectiv e temperature, and 0.2 dex for metallicity (limited by photometric errors), which are similar to random and systematic uncertainties in spectroscopic determinations. We apply this method to a photometric catalog of coadded SDSS observations and study the photometric metallicity distribution of 200,000 F and G type stars observed in 300 deg 2 of high Galactic latitude sky. These deeper (g < 20:5) and photometrically precise ( 0.01 mag) coadded data enable an accurate measurement of the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, that correspond to disk and halo. The best-t number ratio of the two components is consistent with that implied by the decomposition of stellar counts proles into exponential disk and power-law halo components by Juri c et al. (2008). The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component can be modeled as a spatially invariant Gaussian distribution with a mean of [F e=H] = 1:46 and a standard deviation of 0.3 dex. The disk metallicity distribution is non-Gaussian, with a remarkably small scatter (rms 0.16 dex) and the median smoothly decreasing with distance from the plane from 0:6 at 500 pc to 0:8 beyond several kpc. Similarly, we nd using proper motion measurements that a nonGaussian rotational velocity distribution of disk stars shifts by 50 km/s as the distance from the plane increases from 500 pc to several kpc. Despite this similarity, the metallicity and rotational velocity distributions of disk stars are not correlated (Kendall’s = 0:017 0:018). This absence of a correlation between metallicity and kinematics for disk stars is in a conict with the traditional decomposition in terms of thin and thick disks, which predicts a strong correlation ( = 0:30 0:04) at 1 kpc from the mid-plane. Instead, the variation of the metallicity and rotational velocity distributions can be modeled using non-Gaussian functions that retain their shapes and only shift as the distance from the mid-plane increases. We also study the metallicity distribution using a shallower (g < 19:5) but much larger sample of close to three million stars in 8500 sq. deg. of sky included in SDSS Data Release 6. The large sky coverage enables the detection of coherent substructures in the kinematics{ metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of [F e=H] = 0:95, with an rms scatter of only 0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper motion measurements accurate to 0.2-0.5 mas/yr, for about 200 million F/G dwarf stars within a distance limit of 100 kpc (g < 23:5). Subject headings: methods: data analysis | stars: statistics | Galaxy: halo, kinematics and dynamics, stellar content, structure

Two stellar components in the halo of the Milky Way

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components—an inner and an outer halo—that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.

Nucleosynthetic signatures of the first stars

The chemically most primitive stars provide constraints on the nature of the first stellar objects that formed in the Universe; elements other than hydrogen, helium and traces of lithium present within these objects were generated by nucleosynthesis in the very first stars. The relative abundances of elements in the surviving primitive stars reflect the masses of the first stars, because the pathways of nucleosynthesis are quite sensitive to stellar masses. Several models have been suggested to explain the origin of the abundance pattern of the giant star HE0107–5240, which hitherto exhibited the highest deficiency of heavy elements known. Here we report the discovery of HE1327–2326, a subgiant or main-sequence star with an iron abundance about a factor of two lower than that of HE0107–5240. Both stars show extreme overabundances of carbon and nitrogen with respect to iron, suggesting a similar origin of the abundance patterns. The unexpectedly low Li and high Sr abundances of HE1327–2326, however, challenge existing theoretical understanding: no model predicts the high Sr abundance or provides a Li depletion mechanism consistent with data available for the most metal-poor stars.

THE SEGUE STELLAR PARAMETER PIPELINE. I. DESCRIPTION AND COMPARISON OF INDIVIDUAL METHODS

We describe the development and implementation of the Sloan Extension for Galactic Exploration and Understanding (SEGUE) Stellar Parameter Pipeline (SSPP). The SSPP is derived, using multiple techniques, radial velocities, and the fundamental stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) for AFGK-type stars, based on medium-resolution spectroscopy and ugriz photometry obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its Galactic extension (SDSS-II/SEGUE). The SSPP also provides spectral classification for a much wider range of stars, including stars with temperatures outside the window where atmospheric parameters can be estimated with the current approaches. This is Paper I in a series of papers on the SSPP; it provides an overview of the SSPP, and tests of its performance using several external data sets. Random and systematic errors are critically examined for the current version of the SSPP, which has been used for the sixth public data release of the SDSS (DR-6).

Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars*

The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C2 molecular bands are determined. Twenty-two stars in our sample satisfy our definition for carbon-enhanced metal-poor (CEMP) stars based on the carbon abundance ratio ([C/Fe]) and the evolutionary status. In addition, we measure Na abundances for nine known carbon-enhanced stars. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. The following results are obtained: (1) All but one of the 37 stars with [Fe/H] ≥ -2.6 exhibit large excesses of barium ([Ba/Fe] > +0.5), while the other 27 stars with lower metallicity exhibit a large scatter in their barium abundance ratios (-1.2 < [Ba/Fe] < +3.3). (2) A correlation between the carbon and barium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhanced objects (comprising 54 stars), suggesting that the origin of the observed carbon excess in Ba-enhanced stars is nucleosynthesis in AGB stars, where the main s-process occurs. (3) The majority of the Ba-enhanced stars have -1.0 < [C/H] < 0.0, and a clear cutoff exists at [C/H] ~ 0, which we take as the limit of carbon enrichment by metal-poor AGB stars. The [C/H] values of Ba-normal stars are relatively low, with a wide distribution. (4) The difference in the distributions of evolutionary status between Ba-enhanced and Ba-normal CEMP stars suggested by our previous work is not statistically confirmed by the present, enlarged sample. (5) Excesses of Na are found in stars with extremely large enhancements of C, N, and Ba, suggesting efficient production of this element by AGB nucleosynthesis. The implications of these results on the origins of carbon in CEMP stars are discussed.

The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the Milky Way

We have compiled a new sample of 240 halo objects with accurate distance and radial velocity measurements, including globular clusters, satellite galaxies, field blue horizontal branch (FHB) stars and red giant stars from the Spaghetti survey. The new data lead to a significant increase in the number of known objects for Galactocentric radii beyond 50 kpc, which allows a reliable determination of the radial velocity dispersion profile out to very large distances. The radial velocity dispersion shows an almost constant value of 120 km s −1 out to 30 kpc and then continuously declines down to 50 km s −1 at about 120 kpc. This fall-off puts important constraints on the density profile and total mass of the dark matter halo of the Milky Way. Fo ra constant velocity anisotropy, the isothermal profile is ruled out, while both a dark halo ∞

The Helium Abundances of ω Centauri

Recent Hubble Space Telescope color magnitude diagrams (CMDs) of ω Centauri, which clearly show three sequences in the region of the main-sequence turnoff together with a double lower main sequence, are explained in terms of three populations having canonical heavy-element abundances ([Fe/H] = -1.7, -1.2, and -0.6), reasonable proportions and ages, but with surprising helium abundances. In particular, the CMD morphology of the second population ([Fe/H] = -1.2) is very well represented with a helium abundance much larger, by ΔY ~ 0.12, than that of the first ([Fe/H] = -1.7), which may hold the key to an understanding of the double lower main sequence.

The Spite Lithium Plateau: Ultrathin but Postprimordial

Original article can be found at: http://www.journals.uchicago.edu/loi/apj--Copyright University of Chicago Press / American Astronomical Society

EXTREMELY METAL-POOR STARS. VIII. HIGH-RESOLUTION, HIGH SIGNAL-TO-NOISE RATIO ANALYSIS OF FIVE STARS WITH (Fe/H) ( (3.5

High-resolution, high signal-to-noise ratio (SS/NT \ 85) spectra have been obtained for —ve stars, CD [24i17504, CD [38i245, CS 22172[002, CS 22885[096, and CS 22949[037, having [Fe/H] [ [3.5 according to previous lower S/N material. LTE model atmosphere techniques are used to determine [Fe/H] and relative abundances, or their limits, for some 18 elements, and to constrain more tightly the early enrichment history of the Galaxy than is possible based on previous analyses. We compare our results with high-quality higher abundance literature data for other metal-poor stars and with the canonical Galactic chemical enrichment results of Timmes and colleagues and obtain the following basic results. (1) Large supersolar values of [C/Fe] and [N/Fe], not predicted by the canonical models, exist at lowest abundance. For C at least, the result is difficult to attribute to internal mixing eUects. (2) We con—rm that there is no upward trend in [a/Fe] as a function of [Fe/H], in contradistinction to some reports of the behavior of [O/Fe]. (3) The abundances of aluminum, after correction for non-LTE eUects, are in fair accord with theoretical prediction. (4) We con—rm earlier results concerning the Fe peak elements that [Cr/Fe] and [Mn/Fe] decrease at lowest abundance while [Co/Fe] increases, behaviors that had not been predicted. We —nd, however, that [Ni/Fe] does not vary with [Fe/H], and at [Fe/H] D [3.7, [Ni/Fe] \ 0.08 ^ 0.06. This result appears to be inconsistent with the supernova models of Nakamura and colleagues that seek to understand the observed behavior of the Fe peak elements by varying the position of the model mass cut relative to the Si-burning regions. (5) The heavy neutron capture elements Sr and Ba exhibit a large scatter, with the eUect being larger for Sr than Ba. The disparate behavior of these two elements has been attributed to the existence of (at least) two diUerent mechanisms for their production. (6) For the remarkable object CS 22949[037, we con—rm the result of McWilliam and colleagues that [C/Fe], [Mg/Fe], and [Si/Fe] are supersolar by D1.0 dex. Further, we —nd [N/Fe] \ 2.7 ^ 0.4. None of these results are understandable within the framework of standard models. We discuss them in terms of partial ejection of supernova mantles and massive (200¨500 M _ ) zero heavy-element hypernovae. The latter model actually predicted overproduction of N and underproduction of Fe peak elements. (7) We use robust techniques to determine abundance trends as a function of [Fe/H]. In most cases one sees an apparent upturn in the dispersion of relative abundance [X/Fe] for [Fe/H] \ [2.5. It remains unclear whether this is a real eUect or one driven by observational error. The question needs to be resolved with a much larger and homogeneous data set, both to improve the quality of the data and to understand the role of unusual stars such as CS 22949[037. . . .