arXiv subjects

Timothy C. Beers

Publications and source records attributed to Timothy C. Beers.

396 records · Page 22Linked to original sources

arXiv2001

Extremely Metal-Poor Stars. VIII. High-Resolution, High-Signal-To-Noise Analysis of Five Stars with [Fe/H] < -3.5

High-resolution, high-signal-to-noise ( = 85) spectra have been obtained for five stars -- CD-24:17504, CD-38:245, 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 et al. (1995) 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 effects; (2) We confirm that there is {\it no upward trend} in [$α$/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 effects, are in fair accord with theoretical prediction; (4) We confirm 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 find, however, that [Ni/Fe] does not vary with [Fe/H], and at [Fe/H] ~ -3.7, [Ni/Fe] = +0.08 +/- 0.06. This result appears to be inconsistent with the supernova models of Nakamura et al. (1999) 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. (abbreviated)

astro-ph↗

arXiv2001

Neutron Capture Elements in s-Process-Rich, Very Metal-Poor Stars

We report abundance estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP625-44 and LP706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion AGB stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z=38) to Pb (Z=82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron-capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP706-7 by a re-analysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure tau is estimated to be about 0.7mb^-1, significantly larger than that which best fits solar-system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed abundance pattern can be explained by a few recurrent neutron exposures, and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r~0.1).

astro-ph↗

arXiv2001

Galaxy Formation: One Star at a Time. New Information from the Kinematics of Field Stars in the Galaxy

We present a review of recent studies of the formation and evolution of the Milky Way galaxy, in particular based on large samples of non-kinematically selected stars with available proper motions. The Milky Way is argued to be a reasonable template for the formation of large spiral galaxies, the only one in which complete kinematical and abundance information can be readily obtained. Ongoing and future projects to obtain proper motions and spectral information for much larger samples of stars that will sharpen our perspective are discussed.

astro-ph↗

arXiv2000

Structure of the Galactic Stellar Halo Prior to Disk Formation

We develop a method for recovering the global density distribution of the ancient Galactic stellar halo prior to disk formation, based on the present orbits of metal-poor stars observed in the solar neighborhood. The method relies on the adiabatic invariance of the action integrals of motion for the halo population during the slow accumulation of a disk component, subsequent to earlier halo formation. The method is then applied to a sample of local stars with [Fe/H]<=-1.5, likely to be dominated by the halo component, taken from Beers et al.'s recently revised and supplemented catalog of metal-poor stars selected without kinematic bias. We find that even if the Galactic potential is made spherical by removing the disk component in an adiabatic manner, the halo density distribution in the inner halo region (R <= 15 kpc) remains moderately flattened, with axial ratio of about 0.8 for stars in the abundance range [Fe/H]<=-1.8 and about 0.7 for the more metal-rich interval -1.8<[Fe/H]<=-1.5. The outer halo remains spherical for both abundance intervals. We also find that this initial flattening of the inner halo is caused by the anisotropic velocity dispersions of the halo stars. These results suggest that the two-component nature of the present-day stellar halo, characterized by a highly flattened inner halo and nearly spherical outer halo, is a consequence of both an initially two-component density distribution of the halo (perhaps a signature of dissipative halo formation) and of the adiabatic flattening of the inner part by later disk formation. Further implications of our results for the formation of the Galaxy are also discussed, in particular in the context of the hierarchical clustering scenario of galaxy formation.

astro-ph↗

arXiv2000

Ultra-Lithium-Deficient Halo Stars and Blue Stragglers: A Common Origin?

We present data for four ultra-Li-deficient, warm, halo stars. The Li deficiency of two of these is a new discovery. Three of the four stars have effective temperatures Teff ~ 6300 K, in contrast to previously known Li-deficient halo stars which spanned the temperature range of the Spite Plateau. In this paper we propose that these, and previously known ultra-Li-deficient halo stars, may have had their surface lithium abundances reduced by the same mechanism as produces halo field blue stragglers. Even though these stars have yet to reveal themselves as blue stragglers, they might be regarded as "blue-stragglers-to-be." In our proposed scenario, the surface abundance of Li in these stars could be destroyed (a) during the normal pre-main-sequence single star evolution of their low mass precursors, (b) during the post-main-sequence evolution of a evolved mass donor, and/or (c) via mixing during a mass-transfer event or stellar merger. The warmest Li-deficient stars at the turnoff would be regarded as emerging "canonical" blue stragglers, whereas cooler ones represent sub-turnoff-mass "blue-stragglers-to-be." The latter are presently hidden on the main sequence, Li depletion being possibly the clearest signature of their past history and future significance. Eventually, the main sequence turnoff will reach down to their mass, exposing those Li-depleted stars as canonical blue stragglers when normal stars of that mass evolve away. Arguing against this unified view is the observation that the three Li-depleted stars at Teff ~ 6300 K are all binaries, whereas very few of the cooler systems show evidence for binarity; it is thus possible that two separate mechanisms are responsible for the production of Li-deficient main-sequence halo stars.

astro-ph↗

arXiv2000

Abundances and Evolution of Lithium in the Galactic Halo and Disk

We have measured the Li abundance of 18 stars with -2 <~ [Fe/H] ~< -1 and 6000 K <~ Teff <~ 6400 K, a parameter range that was poorly represented in previous studies. We examine the Galactic chemical evolution (GCE) of this element, combining these data with previous samples of turnoff stars over the full range of halo metallicities. We find that A(Li) increases from a level of \~2.10 at [Fe/H] = -3.5, to ~2.40 at [Fe/H] = -1.0, where A(Li) = log_10 (n(Li)/n(H)) + 12.00. We compare the observations with several GCE calculations, including existing one-zone models, and a new model developed in the framework of inhomogeneous evolution of the Galactic halo. We show that Li evolved at a constant rate relative to iron throughout the halo and old-disk epochs, but that during the formation of young-disk stars, the production of Li relative to iron increased significantly. These observations can be understood in the context of models in which post-primordial Li evolution during the halo and old-disk epochs is dominated by Galactic cosmic ray fusion and spallation reactions, with some contribution from the $ν$-process in supernovae. The onset of more efficient Li production (relative to iron) in the young disk coincides with the appearance of Li from novae and AGB stars. The major challenge facing the models is to reconcile the mild evolution of Li during the halo and old-disk phases with the more efficient production (relative to iron) at [Fe/H] > -0.5. We speculate that cool-bottom processing (production) of Li in low-mass stars may provide an important late-appearing source of Li, without attendant Fe production, that might explain the Li production in the young disk.

astro-ph↗

arXiv2000

s- and r-process elements in two very metal-poor stars

New measurements of neutron-capture elements are presented for two very metal-poor stars ([Fe/H] ~ -3). One (LP 625-44) has an s-process signature believed to be due to mass transfer from a now-extinct metal-poor AGB companion, and the second (CS 22897-008) is one of a number of very metal-poor stars having high [Sr/Ba] ratios which is not expected from the r-process. In the s-process star, many elements including lead have been detected, providing strong constraints on the 13C pocket in the now-extinct AGB star. In the Sr-rich star, Zn, Y, and Zr are also seen to be overabundant, and several possible nucleosynthesis mechanisms are discussed.

astro-ph↗

arXiv2000

Extremely Metal-Poor Stars. VII. The Most Metal-Poor Dwarf, CS 22876-032

We report high-resolution, high-signal-to-noise, observations of the extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system has a long period : P = 424.7 $\pm$ 0.6 days. It comprises two main sequence stars having effective temperatures 6300 K and 5600 K, with a ratio of secondary to primary mass of 0.89 $\pm$ 0.04. The metallicity of the system is [Fe/H] = -3.71 $\pm$ 0.11 $\pm$ 0.12 (random and systematic errors) -- somewhat higher than previous estimates. We find [Mg/Fe] = 0.50, typical of values of less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have significantly lower values, ~ 0.0-0.1, suggesting that the heavier elements might have been underproduced relative to Mg in the material from which this object formed. In the context of the hypothesis that the abundance patterns of extremely metal-poor stars are driven by individual enrichment events and the models of Woosley and Weaver (1995), the data for CS 22876-032 are consistent with its having been enriched by a zero-metallicity supernova of mass 30 M$_{\odot}$. As the most metal-poor near-main-sequence-turnoff star currently known, the primary of the system has the potential to strongly constrain the primordial lithium abundance. We find A(Li) (= log(N(Li)/N(H)) + 12.00) = 2.03 $\pm$ 0.07, which is consistent with the finding of Ryan et al. (1999) that for stars of extremely low metallicity A(Li) is a function of [Fe/H].

astro-ph↗

arXiv2000

Primordial Lithium Abundance as a Stringent Constraint on the Baryonic Content of the Universe

We have refined the estimate of the primordial level of $^7$Li abundance to an accuracy better than 10%, based on high-precision Li abundances for metal-poor halo stars, and a recent model of post-BBN (Big Bang Nucleosynthesis) chemical evolution that provides a quantitative explanation of the detected gentle ascent of the Spite Plateau for stars with metallicities [Fe/H]$ > -3$. Our maximum likelihood analysis obtains an estimate for the primordial Li abundance of $A({\rm Li})_p=2.07^{+0.16}_{-0.04}$, after taking into account possible systematic errors in the estimation of Li abundances, with the exception of a still-controversial issue regarding stellar depletion. The inferred value of $η$ (the baryon-to-photon number-density ratio in the universe) based on this estimate is more consistent with that derived from the set of reported ``low He'' + ``high D'' from extragalactic sites than that derived from reported ``high He'' + ``low D'' measurements. Since, within current models of stellar depletion processes, it is difficult to account for the observed very small scatter of Li abundance in metal-poor stars, our estimate of $A({\rm Li})_p$ should be taken as an independent constraint on the baryonic mass density parameter in the universe, giving $Ω_b h^2=(0.64-1.4)\times 10^{-2}$ with $h=H_0/100$ km s$^{-1}$Mpc$^{-1}$.

astro-ph↗

arXiv2000

A Search for Stars of Very Low Metal Abundance. IV. uvbyCa Observations of Metal-Weak Candidates from the Northern HK Survey

CCD photometry on the uvbyCa system has been obtained for 521 candidate metal-poor stars from the northern hemisphere HK survey of Beers and colleagues. Reddening corrections, classifications of stars by luminosity class, and the derivation of metal abundances based on Stromgren indices m1 and hk are described and presented, along with preliminary spectroscopic estimates of abundance. A number of extremely metal-poor stars with [Fe/H] less than or equal to -2.50 are identified.

astro-ph↗

arXiv2000

Evidence of Multiple r-Process Sites in the Early Galaxy: New Observations of CS 22892-052

First results are reported of a new abundance study of neutron-capture elements in the ultra-metal-poor (UMP; [Fe/H] = -3.1) halo field giant star CS 22892-052. Using new high resolution, high signal-to-noise spectra, abundances of more than 30 neutron-capture elements (Z>30) have been determined. Six elements in the 40 =56) stable neutron-capture elements in CS 22892-052 match well the scaled solar system r-process abundance distribution. From the observed Th abundance, an average age of ~= 16 +/- 4 Gyr is derived for cs22892-052, consistent with the lower age limit of ~= 11 Gyr derived from the upper limit on the U abundance. The concordance of scaled solar r-process and CS 22892-052 abundances breaks down for the lighter neutron-capture elements, supporting previous suggestions that different r-process production sites are responsible for lighter and heavier neutron-capture elements.

astro-ph↗

arXiv2000

The Light Elements Be and B as Stellar Chronometers in the Early Galaxy

Recent detailed simulations of Galactic Chemical Evolution have shown that the heavy elements, in particular [Fe/H], are expected to exhibit a weak, or absent, correlation with stellar ages in the early Galaxy due to the lack of efficient mixing of interstellar material enriched by individual Type II supernovae. A promising alternative "chronometer" of stellar ages is suggested, based on the expectation that the light elements Be and B are formed primarily as spallation products of Galactic Cosmic Rays.

astro-ph↗

arXiv2000

Ongoing Large Surveys for Metal-Poor Stars in the Galactic Halo

We report on two major surveys for metal-poor stars in the galactic halo, the HK survey, and the Hamburg/ESO survey, which have been undertaken in order to provide targets for high-resolution spectroscopy with the Subaru HDS and other large telescopes. We compare basic properties of these two surveys and their current status, and add some historical remarks. The candidate selection procedures of both surveys are described in detail. We evaluate the candidate selection by comparing effective yields (EYs) of the survey techniques for the identification of metal-poor stars. It is found that EY for stars below [Fe/H]=-2.0 in the HES can be up to 80% for stars selected by automatic classification from machine-scanned unwidened plates, whereas in the HK survey, where stars are selected by visual inspection of widened survey plates, the EY is between 11% and 32%, depending on whether a pre-selection based on BV photometry has been applied. Finally, we describe techniques used for determining stellar parameters of the survey stars by means of moderate resolution follow-up spectroscopy, and additional UBV photometry. While follow-up observations of HES stars have just been started, the HK survey has already produced a list of ~4700 stars with estimates of [Fe/H] typically precise to +/-0.2 dex, some 1000 of which have [Fe/H]<-2.0, and roughly 100 of which have [Fe/H]<-3.0.

astro-ph↗

arXiv1999

(Teff,log g,[Fe/H]) Classification of Low-Resolution Stellar Spectra using Artificial Neural Networks

New generation large-aperture telescopes, multi-object spectrographs, and large format detectors are making it possible to acquire very large samples of stellar spectra rapidly. In this context, traditional star-by-star spectroscopic analysis are no longer practical. New tools are required that are capable of extracting quickly and with reasonable accuracy important basic stellar parameters coded in the spectra. Recent analyses of Artificial Neural Networks (ANNs) applied to the classification of astronomical spectra have demonstrated the ability of this concept to derive estimates of temperature and luminosity. We have adapted the back-propagation ANN technique developed by von Hippel et al. (1994) to predict effective temperatures, gravities and overall metallicities from spectra with resolving power ~ 2000 and low signal-to-noise ratio. We show that ANN techniques are very effective in executing a three-parameter (Teff,log g,[Fe/H]) stellar classification. The preliminary results show that the technique is even capable of identifying outliers from the training sample.

astro-ph↗

arXiv1999

Population III by Popular Demand - Progress and Previews

I discuss the ongoing search for stars of the Milky Way which have been referred to as members of Population III. Following a discussion of possible definitions for these stars, I consider the reasons why astronomers have undertaken this search, and list some of the numerous astrophysical uses of the extremely metal-poor stars found along the way. I then review survey techniques which have been used in the past, and provide an update on plans for future investigations. Finally, the question of when one might consider the search for Population III Finished is addressed.

astro-ph↗

arXiv1999

R-Process Abundances and Chronometers in Metal-Poor Stars

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (QRPA beta-decay rates and the ETFSI-Q nuclear mass model), have been made. These calculations are compared with ground-based and HST observations of neutron-capture elements in the metal poor halo stars CS 22892--052, HD 115444, HD 122563 and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z >/= 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This further suggests that there is one r-process site in the Galaxy, at least for elements Z >/= 56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, predictions for the zero decay-age abundances of the radioactive elements Th and U are made. We compare these (least-squares-fit ETFSI-Q) predictions with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892--052 and HD 122563. Within the observational errors the observed ratio of [Th/Eu] is the same in both CS 22892--052 and HD 115444. Comparing with the theoretical ratio suggests an average age of these two very metal poor stars to be approximately 15.6 +/- 4.6 Gyr, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates.

astro-ph↗

arXiv1999

The Spite Lithium Plateau: Ultra-Thin but Post-Primordial

We have studied 23 very metal-poor field turnoff stars, specifically chosen to enable a precise measurement of the dispersion in the lithium abundance of the Spite Li plateau. We concentrated on stars having a narrow range of effective temperature and very low metallicities ([Fe/H] < -2.5) to reduce the effects of systematic errors, and have made particular efforts to minimize random errors in equivalent width and effective temperature. A typical formal error for our abundances is 0.033 dex (1-sigma), which represents a factor of two improvement on most previous studies. Our observed spread about the Li Spite plateau is a mere 0.031 dex (1-sigma), hence we conclude the intrinsic spread is effectively zero at the very metal-poor halo turnoff. We recover a robust dependence of lithium abundance with metallicity, dA(Li)/d[Fe/H] = 0.118 +/- 0.023 dex per dex. Previous claims for a lack of dependence of A(Li) on [Fe/H] are shown to have arisen from the use of noisier estimates of effective temperatures and metallicities. Although we cannot rule out a uniform depletion mechanism, the lack of spread in the A(Li) abundances limits permissible depletion by current rotationally- induced mixing models to < 0.1 dex. Correcting for the GCE contribution to both 6Li and 7Li, we infer a primordial lithium abundance of A(Li) = 2.0 dex. We predict that observations of lithium in extremely low-metallicity stars, having [Fe/H] < -3.0, will yield smaller A(Li) values than the bulk of the stars in this sample, consistent with a low primordial abundance.

astro-ph↗

arXiv1998

An Automated Search for Metal-Poor Halo Stars in the Hamburg/ESO Objective-Prism Survey

An automated search for metal-poor stars is carried out in the course of the Hamburg/ESO objective-prism survey (HES), which covers the full southern sky at Galactic latitudes |b| \gtrsim 30 degrees. As the HES reaches approx. 1 magnitude deeper and covers areas of the sky which have not been touched by the HK Survey of Beers and collaborators, the total survey volume of the HES represents an increase by a factor of 4.5 compared to the HK Survey. Because of the limited availability of telescope time for spectroscopic follow-up observations, we will focus on a search for the most metal-poor, unevolved stars. We present a simulation study of the HES selection function P(B_J, T_eff, [Fe/H], log g) for metal-poor stars, and results from a first follow-up campaign at the ESO-NTT.

astro-ph↗