arXiv subjects

Timothy C. Beers

Publications and source records attributed to Timothy C. Beers.

At least 163 records · Page 9Linked to original sources

arXiv2020

The R-Process Alliance: J1521-3538, a very metal-poor, extremely r-process-enhanced star with [Eu/Fe]=+2.2, and the class of r-III stars

We report the discovery of J1521-3538, a bright (V=12.2), very metal-poor ([Fe/H]=-2.8) strongly r-process enhanced field horizontal branch star, based on a high-resolution, high signal-to-noise Magellan/MIKE spectrum. J1521-3538 shows the largest r-process element over-abundance in any known r-process-enhanced star, with [Eu/Fe]=+2.2, and its chemical abundances of 22 neutron-capture elements closely match the scaled solar r-process pattern. J1521-3538 is also one of few known carbon-enhanced metal-poor stars with r-process enhancement (CEMP-r stars), as found after correcting the measured C abundance for the star's evolutionary status. We propose to extend the existing classification of moderately enhanced (+0.3<=[Eu/Fe]<=+1.0) r-I and strongly r-process enhanced ([Eu/Fe]>+1.0) r-II stars to include an r-III class, for r-process stars such as J1521-3538, with [Eu/Fe]>+2.0 and [Ba/Eu]<-0.5, or >100 times the solar ratio of europium to iron. Using cosmochronometry, we estimate J1521-3538 to be 12.5+-5 Gyr and 8.9+-5 Gyr old, using two different sets of initial production ratios. These ages are based on measurements of the Th line at 4019 A and other r-process element abundances. This is broadly consistent with the old age of a low-mass metal-poor field red horizontal branch star. J1521-3538 likely originated in a low-mass dwarf galaxy that was later accreted by the Milky Way, as evidenced by its highly eccentric orbit.

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arXiv2020

The R-process Alliance: First Magellan/MIKE Release from the Southern Search for R-Process-enhanced Stars

Extensive progress has been recently made into our understanding of heavy element production via the $r$-process in the Universe, specifically with the first observed neutron star binary merger (NSBM) event associated with the gravitational wave signal detected by LIGO, GW170817. The chemical abundance patterns of metal-poor $r$-process-enhanced stars provides key evidence into the dominant site(s) of the $r$-process, and whether NSBMs are sufficiently frequent or prolific $r$-process sources to be responsible for the majority of $r$-process material in the Universe. We present atmospheric stellar parameters (using a Non-Local Thermodynamic Equilibrium analysis) and abundances from a detailed analysis of 141 metal-poor stars, carried out as part of the $R$-Process Alliance (RPA) effort. We obtained high-resolution "snapshot" spectroscopy of the stars using the MIKE spectrograph on the 6.5m Magellan Clay telescope at Las Campanas Observatory in Chile. We find 10 new highly enhanced $r$-II (with [Eu/Fe] $> +1.0$), 62 new moderately enhanced $r$-I ($+0.3 < $ [Eu/Fe] $\le +1.0$) and 17 new limited-$r$ ([Eu/Fe] $< +0.3$) stars. Among those, we find 17 new carbon-enhanced metal-poor (CEMP) stars, of which five are CEMP-no. We also identify one new $s$-process-enhanced ([Ba/Eu ]$ > +0.5$), and five new $r/s$ ($0.0 < $ [Ba/Eu] $ < +0.5$) stars. In the process, we discover a new ultra metal-poor (UMP) star at [Fe/H]=$-$4.02. One of the $r$-II stars shows a deficit in $α$ and Fe-peak elements, typical of dwarf galaxy stars. Our search for $r$-process-enhanced stars by RPA efforts, has already roughly doubled the known $r$-process sample.

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arXiv2020

The R-Process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5-455510

We report on the spectroscopic analysis of RAVE J183013.5-455510, an extremely metal-poor star, highly enhanced in CNO, and with discernible contributions from the rapid neutron-capture process. There is no evidence of binarity for this object. At [Fe/H]=-3.57, this is one of the lowest metallicity stars currently observed, with 18 measured abundances of neutron-capture elements. The presence of Ba, La, and Ce abundances above the Solar System r-process predictions suggest that there must have been a non-standard source of r-process elements operating at such low metallicities. One plausible explanation is that this enhancement originates from material ejected at unusually fast velocities in a neutron star merger event. We also explore the possibility that the neutron-capture elements were produced during the evolution and explosion of a rotating massive star. In addition, based on comparisons with yields from zero-metallicity faint supernova, we speculate that RAVE J1830-4555 was formed from a gas cloud pre-enriched by both progenitor types. From analysis based on Gaia DR2 measurements, we show that this star has orbital properties similar to the Galactic metal-weak thick-disk stellar population.

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arXiv2020

The stellar velocity distribution function in the Milky Way galaxy

The stellar velocity distribution function (DF) in the solar vicinity is re-examined using data from the SDSS APOGEE survey's DR16 and \emph{Gaia} DR2. By exploiting APOGEE's ability to chemically discriminate with great reliability the thin disk, thick disk and (accreted) halo populations, we can, for the first time, derive the three-dimensional velocity DFs for these chemically-separated populations. We employ this smaller, but more data-rich APOGEE+{\it Gaia} sample to build a \emph{data-driven model} of the local stellar population velocity DFs, and use these as basis vectors for assessing the relative density proportions of these populations over 5 $<$ $R$ $<$ 12 kpc, and $-1.5$ $<$ $z$ $<$ 2.5 kpc range as derived from the larger, more complete (i.e., all-sky, magnitude-limited) {\it Gaia} database. We find that 81.9 $\pm$ 3.1$\%$ of the objects in the selected \emph{Gaia} data-set are thin-disk stars, 16.6 $\pm$ 3.2$\%$ are thick-disk stars, and 1.5 $\pm$ 0.1$\%$ belong to the Milky Way stellar halo. We also find the local thick-to-thin-disk density normalization to be $ρ_{T}(R_{\odot})$/$ρ_{t}(R_{\odot})$ = 2.1 $\pm$ 0.2$\%$, a result consistent with, but determined in a completely different way than, typical starcount/density analyses. Using the same methodology, the local halo-to-disk density normalization is found to be $ρ_{H}(R_{\odot})$/($ρ_{T}(R_{\odot})$ + $ρ_{t}(R_{\odot})$) = 1.2 $\pm$ 0.6$\%$, a value that may be inflated due to chemical overlap of halo and metal-weak thick disk stars.

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arXiv2020

Insights into the Formation and Evolution History of the Galactic Disk System

We present a kinematic analysis of a sample of 23,908 G- and K-type dwarfs in the Galactic disk. Based on the $α$-abundance ratio, [$α$/Fe], we separated our sample into low-$α$ thin-disk and high-$α$ thick-disk stars. We find a $V_{\rm ϕ}$ gradient of $-$28.2 km s$^{-1}$ dex$^{-1}$ over [Fe/H] for the thin disk, and an almost flat trend of the velocity dispersions of $V_{\rm R}$, $V_{\rm ϕ}$, and $V_{\rm Z}$ components with [Fe/H]. The metal-poor (MP; [Fe/H] $<$ $-$0.3) thin-disk stars with low-$V_{\rm ϕ}$ velocities have high eccentricities ($e$) and small perigalacticon distances ($r_{\rm p}$), while the high-$V_{\rm ϕ}$ MP thin-disk stars possess low $e$ and large $r_{\rm p}$. Interestingly, half of the super metal-rich ([Fe/H] $>$ $+$0.1) stars in the thin disk exhibit low-$e$, solar-like orbits. Accounting for the inhomogeneous metallicity distribution of the thin-disk stars with various kinematics requires radial migration by churning $-$ it apparently strongly influences the current structure of the thin disk; we cannot rule out the importance of blurring for the high-$e$ stars. We derive a rotation velocity gradient of $+$36.9 km s$^{-1}$ dex$^{-1}$ for the thick disk, and decreasing trends of velocity dispersions with increasing [Fe/H]. The thick-disk population also has a broad distribution of eccentricity, and the number of high-$e$ stars increases with decreasing [Fe/H]. These kinematic behaviors could be the result of a violent mechanism, such as a gas-rich merger or the presence of giant turbulent clumps, early in the history of its formation. Dynamical heating by minor mergers and radial migration may also play roles in forming the current thick-disk structure.

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arXiv2020

A Blueprint for the Milky Way's Stellar Populations: The Power of Large Photometric and Astrometric Surveys

Recent advances from astronomical surveys have revealed spatial, chemical, and kinematical inhomogeneities in the inner region of the stellar halo of the Milky Way Galaxy. In particular, large spectroscopic surveys, combined with Gaia astrometric data, have provided powerful tools for analyzing the detailed abundances and accurate kinematics for individual stars. Despite these noteworthy efforts, however, spectroscopic samples are typically limited by the numbers of stars considered; their analysis and interpretation are also hampered by the complex selection functions that are often employed. Here we present a powerful alternative approach $-$ a synoptic view of the spatial, chemical, and kinematical distributions of stars in the Milky Way based on large photometric survey databases, enabled by a well-calibrated technique for obtaining individual stellar metal abundances from broad-band photometry. We combine metallicities with accurate proper motions from the Gaia mission along the Prime Meridian of the Galaxy, and find that various stellar components are clearly separated from each other in the metallicity versus rotation-velocity space. The observed metallicity distribution of the inner-halo stars deviates from the traditional single-peaked distribution, and exhibits complex substructures comprising varying contributions from individual stellar populations, sometimes with striking double peaks at low metallicities. The substructures revealed from our less-biased, comprehensive maps demonstrate the clear advantages of this approach, which can be built upon by future mixed-band and broad-band photometric surveys, and used as a blueprint for identifying the stars of greatest interest for upcoming spectroscopic studies.

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arXiv2020

The Sixteenth Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

This paper documents the sixteenth data release (DR16) from the Sloan Digital Sky Surveys; the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the southern hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey (TDSS) and new data from the SPectroscopic IDentification of ERosita Survey (SPIDERS) programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).

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arXiv2020

Homogeneous Analysis of Globular Clusters from the APOGEE Survey with the BACCHUS Code. II. The Southern Clusters and Overview

We investigate the Fe, C, N, O, Mg, Al, Si, K, Ca, Ce and Nd abundances of 2283 red giant stars in 31 globular clusters from high-resolution spectra observed in both the northern and southern hemisphere by the SDSS-IV APOGEE-2 survey. This unprecedented homogeneous dataset, largest to date, allows us to discuss the intrinsic Fe spread, the shape and statistics of Al-Mg and N-C anticorrelations as a function of cluster mass, luminosity, age and metallicity for all 31 clusters. We find that the Fe spread does not depend on these parameters within our uncertainties including cluster metallicity, contradicting earlier observations. We do not confirm the metallicity variations previously observed in M22 and NGC 1851. Some clusters show a bimodal Al distribution, while others exhibit a continuous distribution as has been previously reported in the literature. We confirm more than 2 populations in $ω$ Cen and NGC 6752, and find new ones in M79. We discuss the scatter of Al by implementing a correction to the standard chemical evolution of Al in the Milky Way. After correction, its dependence on cluster mass is increased suggesting that the extent of Al enrichment as a function of mass was suppressed before the correction. We observe a turnover in the Mg-Al anticorrelation at very low Mg in $ω$ Cen, similar to the pattern previously reported in M15 and M92. $ω$ Cen may also have a weak K-Mg anticorrelation, and if confirmed, it would be only the third cluster known to show such a pattern.

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arXiv2020

A high-resolution spectroscopic study of two new Na- and Al-rich field giants -- likely globular cluster escapees in the Galactic halo

The stars SDSS J064655.6+411620.5 and SDSS J193712.01+502455.5 are relatively bright stars that were initially observed as part of the SDSS/MARVELS pre-survey. They were selected, on the basis of their weak CH $G$-bands, along with a total of 60 others, in the range of halo globular cluster metallicities for high-resolution spectroscopic follow-up as a part of the HESP-GOMPA survey (Hanle Echelle SPectrograph -- Galactic survey Of Metal Poor stArs). The stars exhibit typical nucleosynthesis signatures expected from the so-called second-generation stars of globular clusters. The light-element anti-correlation of Mg-Al is detected, along with elevated abundances of Na. Carbon is found to be depleted, which is compatible with expectation. Lithium is also detected in SDSS J064655.6+411620.5 and SDSS J193712.01+502455.5; the measured abundances are similar to those of normal halo giant stars. These bright escapees provide a unique opportunity to study the nucleosynthesis events of globular clusters in great detail, and shed light on their chemical-enrichment histories.

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arXiv2020

Identification of a Group III CEMP-no Star in the Dwarf Spheroidal Galaxy Canes Venatici I

CEMP-no stars, a subclass of carbon-enhanced metal-poor (CEMP) stars, are one of the most significant stellar populations in Galactic Archaeology, because they dominate the low end of the metallicity distribution function, providing information on the early star-formation and chemical-evolution history of the Milky Way and its satellite galaxies. Here we present an analysis of medium-resolution ($R \sim 1,800$) optical spectroscopy for a CEMP giant, SDSS J132755.56+333521.7, observed with the Large Binocular Telescope (LBT), one of the brightest ($g \sim 20.5$) members of the classical dwarf spheroidal galaxy, Canes Venatici I (CVn I). Many CEMP stars discovered to date have very cool effective temperatures ($T_{\mathrm{eff}}< 4500$ K), resulting in strong veiling by molecular carbon bands over their optical spectra at low/medium spectral resolution. We introduce a technique to mitigate the carbon-veiling problem to obtain reliable stellar parameters and validate this method with the LBT medium-resolution optical spectra of the ultra metal-poor ([Fe/H] = $-4.0$) CEMP-no dwarf, G 77-61, and seven additional very cool CEMP stars, which have published high-resolution spectroscopic parameters. We apply this technique to the LBT spectrum of SDSS J132755.56+333521.7. We find that this star is well-described with parameters $T_{\mathrm{eff}}=4530$ K, log $g=$ 0.7, [Fe/H] $ = -3.38$, and absolute carbon abundance $A$(C) = 7.23, indicating that it is likely the first Group III CEMP-no star identified in CVn I. The Group III identification of this star suggests that it is a member of the extremely metal-poor population in CVn I, which may have been accreted into its halo.

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arXiv2020

The Metallicity Gradient and Complex Formation History of the Outermost Halo of the Milky Way

We present an examination of the metallicity distribution function of the outermost stellar halo of the Galaxy based on an analysis of both local (within 4 kpc of the Sun, ~16,500 stars) and non-local (~21,700 stars) samples. These samples were compiled using spectroscopic metallicities from the Sloan Digital Sky Survey and photometric metallicities from the SkyMapper Southern Survey. We detect a negative metallicity gradient in the outermost halo (r > 35 kpc from the Galactic center), and find that the frequency of very metal-poor ([Fe/H] < -2.0) stars in the outer-halo region reaches up to ~60% in our most distant sample, commensurate with previous theoretical predictions. This result provides clear evidence that the outer-halo formed hierarchically. The retrograde stars in the outermost halo exhibit a roughly constant metallicity, which may be linked to the accretion of the Sequoia progenitor. In contrast, prograde stars in the outermost halo exhibit a strong metallicity-distance dependence, indicating that they likely originated from the accretion of galaxies less massive than the Sequoia progenitor galaxy.

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arXiv2020

The Chemical Compositions of Accreted and {\it in situ} Galactic Globular Clusters According to SDSS/APOGEE

Studies of the kinematics and chemical compositions of Galactic globular clusters (GCs) enable the reconstruction of the history of star formation, chemical evolution, and mass assembly of the Galaxy. Using the latest data release (DR16) of the SDSS/APOGEE survey, we identify 3,090 stars associated with 46 GCs. Using a previously defined kinematic association, we break the sample down into eight separate groups and examine how the kinematics-based classification maps into chemical composition space, considering only $α$ (mostly Si and Mg) elements and Fe. Our results show that: (i) The loci of both in situ and accreted subgroups in chemical space match those of their field counterparts; (ii) GCs from different individual accreted subgroups occupy the same locus in chemical space. This could either mean that they share a similar origin or that they are associated with distinct satellites which underwent similar chemical enrichment histories; (iii) The chemical compositions of the GCs associated with the low orbital energy subgroup defined by Massari and collaborators is broadly consistent with an in situ origin. However, at the low metallicity end, the distinction between accreted and in situ populations is blurred; (iv) Regarding the status of GCs whose origin is ambiguous, we conclude the following: the position in Si-Fe plane suggests an in situ}origin for Liller 1 and a likely accreted origin for NGC 5904 and NGC 6388. The case of NGC 288 is unclear, as its orbital properties suggest an accretion origin, its chemical composition suggests it may have formed in situ.

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arXiv2020

Close Binary Companions to APOGEE DR16 Stars: 20,000 Binary-star Systems Across the Color-Magnitude Diagram

Many problems in contemporary astrophysics---from understanding the formation of black holes to untangling the chemical evolution of galaxies---rely on knowledge about binary stars. This, in turn, depends on discovery and characterization of binary companions for large numbers of different kinds of stars in different chemical and dynamical environments. Current stellar spectroscopic surveys observe hundreds of thousands to millions of stars with (typically) few observational epochs, which allows binary discovery but makes orbital characterization challenging. We use a custom Monte Carlo sampler (The Joker) to perform discovery and characterization of binary systems through radial-velocities, in the regime of sparse, noisy, and poorly sampled multi-epoch data. We use it to generate posterior samplings in Keplerian parameters for 232,531 sources released in APOGEE Data Release 16. Our final catalog contains 19,635 high-confidence close-binary (P < few years, a < few AU) systems that show interesting relationships between binary occurrence rate and location in the color-magnitude diagram. We find notable faint companions at high masses (black-hole candidates), at low masses (substellar candidates), and at very close separations (mass-transfer candidates). We also use the posterior samplings in a (toy) hierarchical inference to measure the long-period binary-star eccentricity distribution. We release the full set of posterior samplings for the entire parent sample of 232,531 stars. This set of samplings involves no heuristic "discovery" threshold and therefore can be used for myriad statistical purposes, including hierarchical inferences about binary-star populations and sub-threshold searches.

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arXiv2020

The Lazy Giants: APOGEE Abundances Reveal Low Star Formation Efficiencies in the Magellanic Clouds

We report the first APOGEE metallicities and alpha-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds (SMC), the largest Milky Way dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies, and extends to much larger radial distances. These are the first results presented that make use of the newly installed Southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H]=-0.2 to very metal-poor stars with [Fe/H]=-2.5, the most metal-poor Magellanic Clouds (MCs) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range, but rises by ~0.1 dex at -1.0<[Fe/H]<-0.5. We interpret this as a sign of the known recent increase in MC star-formation activity, and are able to reproduce the pattern with a chemical evolution model that includes a recent "starburst". At the metal-poor end, we capture the increase of [alpha/Fe] with decreasing [Fe/H], and constrain the "alpha-knee" to [Fe/H]<-2.2 in both MCs, implying a low star-formation efficiency of ~0.01 Gyr^-1. The MC knees are more metal poor than those of less massive Milky Way (MW) dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW's gravitational potential only recently.

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arXiv2020

Characterizing $r$-Process Sites through Actinide Production

Of the variations in the elemental abundance patterns of stars enhanced with $r$-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in $r$-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an $r$-process event may be. Furthermore, if an $r$-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of $r$-process site is needed to explain all levels of observed relative actinide enhancements.

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arXiv2019

SDSS-IV MaStar -- A Large and Comprehensive Empirical Stellar Spectral Library: First Release

We present the first release of the MaNGA Stellar Library (MaStar), which is a large, well-calibrated, high-quality empirical library covering the wavelength range of 3,622-10,354A at a resolving power of R~1800. The spectra were obtained using the same instrument as used by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) project, by piggybacking on the SDSS-IV/APOGEE-2N observations. Compared to previous empirical libraries, the MaStar library will have a higher number of stars and a more comprehensive stellar-parameter coverage, especially of cool dwarfs, low-metallicity stars, and stars with different [alpha/Fe], achieved by a sophisticated target selection strategy that takes advantage of stellar-parameter catalogs from the literature. This empirical library will provide a new basis for stellar population synthesis, and is particularly well-suited for stellar-population analysis of MaNGA galaxies. The first version of the library contains 8646 high-quality per-visit spectra for 3321 unique stars. Compared to photometry, the relative flux calibration of the library is accurate to 3.9% in g-r, 2.7% in r-i, and 2.2% in i-z. The data are released as part of Sloan Digital Sky Survey Data Release 15. We expect the final release of the library to contain more than 10,000 stars.

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arXiv2019

Evidence for the Third Stellar Population in the Milky Way's Disk

The Milky Way is a unique laboratory, where stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the Solar Neighborhood, where all of the main Galactic components are well-represented. We find that the thick disk comprises two distinct and overlapping stellar populations, with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ~ 150 km s^(-1), corresponding to a rotational lag of 30 km s^(-1) relative to the canonical thick disk (~ 180 km s^(-1)), with a velocity dispersion of 60 km s^(-1). This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the Solar Neighborhood by bar instability or spiral-arm formation mechanisms.

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arXiv2019

Discovery of a new stellar sub-population residing in the (inner) stellar halo of the Milky Way

We report the discovery of a unique collection of metal-poor giant-stars, that exhibit anomalously high levels of $^{28}$Si, clearly above typical Galactic levels. Our sample spans a narrow range of metallicities, peaking at $-1.07\pm 0.06$, and exhibit abundance ratios of [Si,Al/Fe] that are as extreme as those observed in Galactic globular clusters (GCs), and Mg is slightly less overabundant. In almost all the sources we used, the elemental abundances were re-determined from high-resolution spectra, which were re-analyzed assuming LTE. Thus, we compiled the main element families, namely the light elements (C, N), $α-$elements (O, Mg, Si), iron-peak element (Fe), $\textit{s}-$process elements (Ce, Nd), and the light odd-Z element (Al). We also provide dynamical evidence that most of these stars lie on tight (inner)halo-like and retrograde orbits passing through the bulge. Such kinds of objects have been found in present-day halo GCs, providing the clearest chemical signature of past accretion events in the (inner) stellar halo of the Galaxy, formed possibly as the result of dissolved halo GCs. Their chemical composition is, in general, similar to that of typical GCs population, although several differences exist.

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