David K. Lai

University of California, Santa Cruz, Santa Cruz, CA, United States

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Publications (20)46.79 Total impact

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    ABSTRACT: In recent years, with massive spectroscopic surveys it is possible to probe the chemical/star-formation history of the early Galaxy. In this work, we derive carbon abundances and kinematics for a subset of 30,000 stars from SDSS-DR7 calibration sample. We confirm the increase in the CEMP fraction at low metallicities. The analysis showed that the CEMP fraction of the outer Halo stars is almost twice the CEMP fraction of the inner Halo stars (Carollo et al. 2011). Similar enhancement of carbon is also seen among the stars of ultra faint dwarf satellites (Lai et al. 2011). This subject recieved an increased attention after the recent discovery of the C-rich metal poor ([Fe/H] < -3.0) DLA, at a redshift of z= 2.3 (Kobayashi et al 2011), with abundance ratios similar to CEMP stars. This indicate contributions from Pop-III faint first supernovae to the early carbon production.
    01/2012;
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    ABSTRACT: We present the results of a low-resolution spectral abundance study of 25 stars in the Boötes I dwarf spheroidal (dSph) galaxy. The data were obtained with the low resolution imaging spectrometer instrument at Keck Observatory and allow us to measure [Fe/H], [C/Fe], and [α/Fe] for each star. We find both a large spread in metallicity (2.1 dex in [Fe/H]) as well as a low average metallicity in this system, [Fe/H] = –2.59, matching previous estimates. This sample includes a newly discovered extremely metal-poor star, with [Fe/H] = –3.8, that is one of the most metal-poor stars yet found in a dSph. We compare the metallicity distribution function of Boötes I to analytic chemical evolution models. While the metallicity distribution function of Boötes I is best fit by an Extra Gas chemical evolution model, leaky-box models also provide reasonable fits. We also find that the [α/Fe] distribution and the carbon-enhanced metal-poor fraction of our sample (12%) are reasonable matches to Galactic halo star samples in the same metallicity range, indicating that at these low metallicities, systems like the Boötes I ultra-faint dSph could have been contributors to the Galactic halo.
    The Astrophysical Journal 08/2011; 738(1):51. · 6.73 Impact Factor
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    ABSTRACT: We present the results of a low-resolution spectral abundance study of 25 stars in the Bootes I dwarf spheroidal (dSph) galaxy. The data were obtained with the LRIS instrument at Keck Observatory, and allow us to measure [Fe/H], [C/Fe], and [alpha/Fe] for each star. We find both a large spread in metallicity (2.1 dex in [Fe/H]) as well as the low average metallicity in this system, <[Fe/H]>=-2.59, matching previous estimates. This sample includes a newly discovered extremely metal-poor star, with [Fe/H]=-3.8, that is one of the most metal-poor stars yet found in a dSph. We compare the metallicity distribution function of Bootes I to analytic chemical evolution models. While the metallicity distribution function of Bootes I is best fit by an Extra Gas chemical evolution model, leaky-box models also provide reasonable fits. We also find that the [alpha/Fe] distribution and the carbon-enhanced metal-poor fraction of our sample (12%) are reasonable matches to Galactic halo star samples in the same metallicity range, indicating that at these low metallicities, systems like the Bootes I ultra-faint dSph could have been contributors to the Galactic halo.
    06/2011;
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    ABSTRACT: We present a method for the determination of [α/Fe] ratios from low-resolution (R = 2000) SDSS/SEGUE stellar spectra. By means of a star-by-star comparison with degraded spectra from the ELODIE spectral library and with a set of moderately high-resolution (R = 15, 000) and medium-resolution (R = 6000) spectra of SDSS/SEGUE stars, we demonstrate that we are able to measure [α/Fe] from SDSS/SEGUE spectra (with S/N>20/1) to a precision of better than 0.1 dex, for stars with atmospheric parameters in the range T eff = [4500, 7000] K, log g = [1.5, 5.0], and [Fe/H] = [–1.4, +0.3], over the range [α/Fe] = [–0.1, +0.6]. For stars with [Fe/H] <–1.4, our method requires spectra with slightly higher signal-to-noise to achieve this precision (S/N>25/1). Over the full temperature range considered, the lowest metallicity star for which a confident estimate of [α/Fe] can be obtained from our approach is [Fe/H] ~–2.5; preliminary tests indicate that a metallicity limit as low as [Fe/H] ~–3.0 may apply to cooler stars. As a further validation of this approach, weighted averages of [α/Fe] obtained for SEGUE spectra of likely member stars of Galactic globular clusters (M15, M13, and M71) and open clusters (NGC 2420, M67, and NGC 6791) exhibit good agreement with the values of [α/Fe] from previous studies. The results of the comparison with NGC 6791 imply that the metallicity range for the method may extend to ~+0.5.
    The Astronomical Journal 02/2011; 141(3):90. · 4.97 Impact Factor
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    ABSTRACT: We present analysis of high-resolution spectra of a sample of stars in the globular cluster M5 (NGC 5904). The sample includes stars from the red giant branch (RGB; seven stars), the red horizontal branch (two stars), and the asymptotic giant branch (AGB; eight stars), with effective temperatures ranging from 4000 K to 6100 K. Spectra were obtained with the HIRES spectrometer on the Keck I telescope, with a wavelength coverage from 3700 Å to 7950 Å for the HB and AGB sample, and 5300 Å to 7600 Å for the majority of the RGB sample. We find offsets of some abundance ratios between the AGB and the RGB branches. However, these discrepancies appear to be due to analysis effects, and indicate that caution must be exerted when directly comparing abundance ratios between different evolutionary branches. We find the expected signatures of pollution from material enriched in the products of the hot hydrogen burning cycles such as the CNO, Ne-Na, and Mg-Al cycles, but no significant differences within these signatures among the three stellar evolutionary branches especially when considering the analysis offsets. We are also able to measure an assortment of neutron-capture element abundances, from Sr to Th, in the cluster. We find that the neutron-capture signature for all stars is the same, and shows a predominately r-process origin. However, we also see evidence of a small but consistent extra s-process signature that is not tied to the light-element variations, pointing to a pre-enrichment of this material in the protocluster gas.
    The Astronomical Journal 01/2011; 141(2):62. · 4.97 Impact Factor
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    ABSTRACT: The metal-poor population II stars in the Galactic halo formed during the first few generations of star formation following the Big Bang, and can thus provide key insights into conditions in the early Universe. Carbon-enriched metal-poor (CEMP) stars, a subset of this population, are of even further interest because of their peculiar chemical abundance patterns. High-resolution spectra of ten known CEMP stars were obtained on the Keck High Resolution Echelle Spectrometer (HIRES) for this study. Stellar parameters for our model atmospheres were determined using Fe I and Fe II equivalent width measurements. We derived chemical abundances from measured equivalent widths whenever possible, or from synthetic spectra in the cases where no equivalent widths could be measured for an element's spectral lines. Using these methods, we have obtained preliminary abundances for up to 28 elements in our sample. Further work remains in introducing carbon into the model atmospheres and repeating the abundance analysis using these revised models. We can then study the relative abundances of heavy elements in comparison to solar system r-process and s-process abundance patterns in order to infer possible mechanisms by which these stars became carbon-enriched.
    01/2011;
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    ABSTRACT: We present the results of an abundance analysis for a sample of stars with $-4<$[Fe/H]$<-2$. The data were obtained with the HIRES spectrograph at Keck Observatory. The set includes 28 stars, with effective temperature ranging from 4800 to 6600 K. For 13 stars with [Fe/H]$<-2.6$, including nine with [Fe/H]$<-3.0$ and one with [Fe/H]$=-4.0$, these are the first reported detailed abundances. For the most metal-poor star in our sample, CS 30336-049, we measure an abundance pattern that is very similar to stars in the range [Fe/H]$\sim-3.5$, including a normal C+N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutron-capture activity even at this low of a metallicity. We explore this issue further by examining other very neutron-capture-deficient stars, and find that at the lowest levels, [Ba/Sr] exhibits the ratio of the main r-process. We also report on a new r-process-enhanced star, CS 31078-018. This star has [Fe/H]$=-2.85$, [Eu/Fe]$=1.23$, and [Ba/Eu]$=-0.51$. CS 31078-018 exhibits an ``actinide boost'', i.e. much higher [Th/Eu] than expected and at a similar level to CS 31082-001. Our spectra allow us to further constrain the abundance scatter at low metallicities, which we then use to fit to the zero-metallicity Type II supernova yields of Heger & Woosley (2008). We find that supernovae with progenitor masses between 10 and 20 M$_{\odot}$ provide the best matches to our abundances. Comment: 48 pages, 30 figures, 17 tables. Updated to ApJ version. Multiple typos and errors fixed
    The Astrophysical Journal 10/2010; 722(2):1984. · 6.73 Impact Factor
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    ABSTRACT: We present the high-resolution spectral analysis of a sample of stars in the globular cluster M5. The sample covers the red giant branch (seven stars, Mv from 0.6 to -1.9), the red horizontal branch (three stars), and the asymptotic giant branch (seven stars, Mv from -0.2 to -2.0). We obtained the spectra with the HIRES spectrograph on Keck I, with a wavelength range from 3700 to 7950 angstroms for the HB and AGB sample, and 5300 to 7600 angstroms for the RGB sample. Given the expanded wavelength coverage over earlier studies, we are able to look for any variations in the neutron-capture ratios in the cluster. M5 is also particularly well-suited for studying evolutionary effects on abundances beyond the RGB because the RGB and AGB are cleanly separated in c-m diagrams. By performing detailed abundance analysis of our HB/AGB stars and comparing these results to our sample of our RGB stars as well results from the literature, we explore if and how surface abundance ratios have been modified during the stellar evolution process. In particular we are examining if the surface abundances can reveal any mixing processes that a star may encounter as it evolves onto and up the AGB. These processes along the AGB are not observationally well studied, and M5 can provide crucial information for the models. One preliminary result is that the 12C/13C ratio remains at a approximately constant value of ˜4 over our entire sample of AGB stars. This is very close to the CN equilibrium value and also close to the 12C/13C found in other cluster red giants and field metal-poor stars.
    01/2010;
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    ABSTRACT: As part of a program to measure abundance ratios in stars beyond 15 kpc from the Galactic center, we have discovered a metal-poor star in the outer halo with a unique chemical signature. We originally identified it in the Sloan Extension for Galactic Understanding and Exploration survey as a distant metal-poor star. We obtained a follow-up spectrum using the Echelle Spectrometer and Imager at the Keck 2 telescope, and measure [Fe/H] =–3.17, [Mg/Fe] = –0.10, and [Ca/Fe] = +1.11. This is one of the largest over-abundances of Ca measured in any star to date; the extremely low value of [Mg/Ca] = –1.21 is entirely unique. To have found such an unusual star in our small sample of 27 targets suggests that there may be previously unobserved classes of stars yet to be found in situ in the Galactic halo.
    The Astrophysical Journal 04/2009; 697(1):L63. · 6.73 Impact Factor
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    ABSTRACT: Carbon-Enhanced Metal-Poor (CEMP) stars have been seen to make up a large fraction of metal-poor stars ( 30% below [Fe/H] = -3.0). These stars often show enhancements of s-process material, which is believed to have formed near the core of an AGB star, is mixed to the surface during third dredge-up, and then transferred via Roche Lobe overflow to a binary companion. Thus, observed CEMP stars are thought to be mainly the surviving secondary stars of binary systems. These stars are thus useful probes of a number of quantities, including the efficiencies of (1) carbon and s-process production as a function of metallicity, (2) mixing during third dredge-up, (3) mass transfer during Roche Lobe overflow, and (4) the subsequent mixing during the evolution of the secondary star. We report here several projects in progress studying stars at a variety of stages along this path, over a range of metallicities, to probe these mechanisms in greater detail.
    01/2009;
  • GRB Coordinates Network. 01/2008;
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    ABSTRACT: We report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which we compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R ~ 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/Ns), we empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km s 1, 130 K (2.2 %), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. We estimate random errors for lower S/N based on numerical simulations. Based on observations obtained with the Hobby-Eberly Telescope (a joint project of the University of Texas at Austin, Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen), the W. M. Keck Observatory (operated as a scientific partnership among the California Institute of Technology, the University of California and NASA), and the Subaru Telescope (operated by the National Astronomical Observatory of Japan).
    The Astronomical Journal 11/2007; · 4.97 Impact Factor
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    ABSTRACT: We present initial results measuring the companion fraction of metal-poor stars ([Fe/H]$<-$2.0). We are employing the Lick Observatory planet-finding system to make high-precision Doppler observations of these objects. The binary fraction of metal-poor stars provides important constraints on star formation in the early Galaxy (Carney et al. 2003). Although it has been shown that a majority of solar metallicity stars are in binaries, it is not clear if this is the case for metal-poor stars. Is there a metallicity floor below which binary systems do not form or become rare? To test this we are determining binary fractions at metallicities below [Fe/H]$=-2.0$. Our measurments are not as precise as the planet finders', but we are still finding errors of only 50 to 300 m/s, depending on the signal-to-noise of a spectrum and stellar atmosphere of the star. At this precision we can be much more complete than previous studies in our search for stellar companions. Comment: To appear in conference proceedings,"First Stars III", eds. B. O'Shea, A. Heger & T. Abel. 3 pages, 5 figures
    08/2007;
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    ABSTRACT: We present carbon and strontium abundances for 100 metal-poor stars measured from R$\sim $7000 spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory. Using spectral synthesis of the G-band region, we have derived carbon abundances for stars ranging from [Fe/H]$=-1.3$ to [Fe/H]$=-3.8$. The formal errors are $\sim 0.2$ dex in [C/Fe]. The strontium abundance in these stars was measured using spectral synthesis of the resonance line at 4215 {\AA}. Using these two abundance measurments along with the barium abundances from our previous study of these stars, we show it is possible to identify neutron-capture-rich stars with our spectra. We find, as in other studies, a large scatter in [C/Fe] below [Fe/H]$ = -2$. Of the stars with [Fe/H]$<-2$, 9$\pm$4% can be classified as carbon-rich metal-poor stars. The Sr and Ba abundances show that three of the carbon-rich stars are neutron-capture-rich, while two have normal Ba and Sr. This fraction of carbon enhanced stars is consistent with other studies that include this metallicity range. Comment: ApJ, Accepted
    The Astrophysical Journal 06/2007; · 6.73 Impact Factor
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    ABSTRACT: We present the results of an abundance analysis for a sample of stars with -2>[Fe/H]> -4. The set includes 29 stars, with effective temperature ranging from 4800 K to 6300 K. The data were obtained with the HIRES spectrograph at Keck Observatory. For most objects our wavelength range reaches from about 3100 angstroms to 5800 angstroms. Our spectra allow us to further constrain the abundance scatter at low metallicities for the light elements including carbon and nitrogen, up through the iron group, and for many neutron-capture elements. Most of our objects have come from the Beers et al. HK survey (1992, AJ, 103, 1987) for metal-poor stars, and for many of them this is the first high-resolution study. This research is based on work supported by the National Science Foundation under the grant AST-0607770.
    12/2006;
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    ABSTRACT: We present a discussion of efforts to obtain external validation of the estimated atmospheric parameters (Teff, log g, [Fe/H]) obtained from medium-resolution (R = 2000) SDSS spectroscopy and ugriz photometry, which are being employed for both the completed SDSS-I and the ongoing SEGUE survey. The SDSS/SEGUE spectroscopic pipeline makes use of a number of methods for the estimation of each parameter, with estimated internal errors on the order of σ(Teff) = 150 K, σ(log g) = 0.4 dex, and σ([Fe/H]) = 0.3 dex. Over the course of the past two years, we have obtained over 100 high-resolution optical spectra of SDSS/SEGUE stars using the HET, KECK and SUBARU telescopes. For the KECK/HIRES spectra, which have R = 40000, we have performed standard high-resolution analyses to estimate the stellar parameters. For the HET and KECK-ESI data, which have R = 15000 and R = 5000, respectively, we have performed synthetic spectra matching in order to to estimate the stellar parameters. We find that the derived stellar parameters agree well with the SDSS/SEGUE pipeline estimates for the temperature range 5000 K < Teff < 6500K; the errors are of the order of the internal errors expected from the SDSS/SEGUE pipeline. For effective temperatures in the range 4000 K to 5000 K the estimated parameters from the high-resolution spectroscopy exhibit offsets relative to the SDSS/SEGUE pipeline values on the order of ΔTeff = 200 K, Δlogg = 0.8 dex, and Δ[Fe/H] = 0.4 dex. Similar offsets exist for stars with T > 6500 K. The main reason for these offsets appears to arise due to varying microturbulence, for which the medium-resolution SDSS spectra are not sensitive. We also have performed external checks on pipline radial velocities. We find that the errors in radial velocities are on the order of 7 km/s for stars, which is at the expected level.
    11/2006; 38:1139.
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    ABSTRACT: We describe an ongoing effort to validate the estimated atmospheric parameters (Teff, log g, [Fe/H]) obtained from SDSS spectroscopy (R = 2000) and ugriz photometry, which are being employed for both the previous SDSS-I and the ongoing SEGUE surveys. The spectroscopic pipeline makes use of a number of methods for the estimation of each parameter, with estimated internal errors in the order of σ (Teff) = 150 K, σ (log g) = 0.4 dex, and σ ([Fe/H]) = 0.3 dex. However, several of these methods rely on an uncertain transformation of g-r colors to B-V, and there does not presently exist an external validation of the derived parameters based on high-resolution spectroscopy. In order to address these deficiencies, we have generated two new grids of synthetic spectra and ugriz colors (based on both Kurucz NEWODF models with no covective overshoot and MARCS models) for stellar atmospheric parameters covering the ranges 3500 K ≤ Teff ≤ 10000 K, 0.0 ≤ log g ≤ 5.0, and -5.0 ≤ [Fe/H] ≤ 0.0. The two grids will provide a useful internal check on the dependence of the derived parameters on the adopted stellar models. In addition to these grids, we have also generated a carbon-enriched subgrid, covering the entire parameter space for various values of carbon enhancement ([C/Fe] = 0.5,1.0,1.5,2.0). This subgrid makes use of carbon-enhanced MARCS models instead of scaled solar models, since the atmospheric structures themselves can be altered by enhanced carbon, especially at cooler temperatures. The synthetic colors will be calibrated using a selection of standard stars and open and globular cluster stars covering a wide range of stellar atmospheric parameters. We have already obtained a small number of the high-resolution spectra for SDSS stars needed to calibrate the spectroscopic pipeline; much larger samples of high-resolution data are presently being acquired. Preliminarycomparisons of the estimated atmospheric parameters based on the SDSS/SEGUE spectroscopic pipeline with those derived from the high-resolution specta will be reported. T.S., Y.L., and T.C.B. acknowledge partial support from grant AST 04-06784, as well as from grant PHY 02-16783, Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA), awarded by the US National Science Foundation.
    11/2005; 37:1379.
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    ABSTRACT: We present chemical abundances for 110 stars identified in objective-prism surveys as candidates to be very metal-poor. The abundances are derived from high S/N, intermediate-resolution spectra obtained with the Keck Observatory Echelle Spectrometer and Imager. An additional 25 stars with well-determined abundances ranging from [Fe/H]$=-1.5$ and -3.2 were observed and the results used to help calibrate our analysis and determine the accuracy of our abundance determinations. Abundances for the program stars were measured for Fe, Mg, Ca, Ti, Cr and Ba with an accuracy of approximately 0.3 dex. 53 of the stars in our sample have [Fe/H]<= -2, 22 have [Fe/H]<= -2.5 and 13 stars have [Fe/H]<= -2.9. Surprisingly, approximately one third of the sample is relatively metal rich with [Fe/H]>-1.5. In addition to identifying a number of extremely metal-poor stars, this study also shows that moderate-resolution spectra obtained with the Keck Echelle Spectrometer and Imager yield relatively accurate abundances for stars as faint as V=14 in modest exposure time (~20 minutes). This capability will prove useful if the so-far elusive stars at [Fe/H]<-4 turn out to be mostly fainter than V=15. Comment: 60 pages, 10 figures, Accepted for publication in the AJ
    The Astronomical Journal 08/2004; · 4.97 Impact Factor
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    ABSTRACT: We present a program measuring the abundance ratios of stars in the outer halo of the Milky Way. Using the metal-poor candidates from SDSS-SEGUE and follow-up spectra with ESI on Keck we efficiently measure metallicity, alpha-ratio abundances, and certain neutron-capture abundance ratios for stars out to distances of about 30 kpc, thereby placing them in situ in the outer halo (Carollo et al. 2007). By studying metal-poor stars in this relatively unexplored region we can look for evidence of different star formation environments which can provide a important constraint on current Galaxy formation scenarios (e.g., Bullock & Johnston 2005), and potentially discover interesting individual stars. In an initial sample of 25 stars, we have already discovered one new highly r-process-enhanced metal-poor star and a new type of very metal-poor star with a unique [Ca/Mg] of 1.2. In terms of the larger sample, we are also seeing hints of a different alpha-element population, possible evidence of varied star formation environments in the outer halo. DKL acknowledges the support from the NSF grant AST-0802292 through the Astronomy and Astrophysics Postdoctoral Fellowship program.