D. L. Lambert

University of Michigan, Ann Arbor, Michigan, United States

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Publications (586)2306.1 Total impact

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    ABSTRACT: DY Cen has shown a steady fading of its visual light by about 1 magnitude in the last 40 years suggesting a secular increase in its effective temperature. We have conducted non-LTE and LTE abundance analyses to determine the star's effective temperature, surface gravity, and chemical composition using high-resolution spectra obtained over two decades. The derived stellar parameters for three epochs suggest that DY Cen has evolved at a constant luminosity and has become hotter by about 5000 K in 23 years. We show that the derived abundances remain unchanged for the three epochs. The derived abundances of the key elements, including F and Ne, are as observed for the extreme helium stars resulting from a merger of an He white dwarf with a C-O white dwarf. Thus, DY Cen by chemical composition appears to be also a product of a merger of two white dwarfs. This appearance seems to be at odds with the recent suggestion that DY Cen is a single-lined spectroscopic binary.
    The Astrophysical Journal 08/2014; 793(2). · 6.73 Impact Factor
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    ABSTRACT: High-resolution optical spectra of the R Coronae Borealis (RCB) star V532 Oph at light maximum are discussed. The absolute visual magnitude M_V of the star is found to be -4.9 \pm 0.5. The elemental abundances suggest the star belongs to the majority class of RCB stars but is among the most O-poor of this class with mild enhancements of heavy elements Y, Zr, Ba and La. The C_2 Swan bands are weak in V532 Oph relative to R CrB. Other aspects of the high-resolution spectrum confirm that V532 Oph is representative of majority RCBs, i.e., the radial velocity is variable, circumstellar material is present and the photosphere feeds a high-velocity stellar wind.
    07/2014;
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    Jens Adamczak, David L. Lambert
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    ABSTRACT: We derived atmospheric parameters and spectroscopic abundances for C and O for a large sample of stars located in the Hertzsprung gap in the Hertzsprung-Russell Diagram in order to detect chemical peculiarities and get a comprehensive overview of the population of stars in this evolutionary state. We have observed and analyzed high resolution spectra (R = 60 000) of 188 stars in the mass range 2 - 5 Msun with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory including 28 stars previously identified as Am/Ap stars. We find that the C and O abundances of the majority of stars in the Hertzsprung gap are in accordance with abundances derived for local lower mass dwarfs but detect expected peculiarities for the Am/Ap stars. The C and O abundances of stars with Teff < 6500 K are slightly lower than for the hotter objects but the C/O ratio is constant in the analyzed temperature domain. No indication of an alteration of the C and O abundances of the stars by mixing during the evolution across the Hertzsprung gap could be found before the homogenization of their atmospheres by the first dredge-up.
    The Astrophysical Journal 07/2014; 791(1). · 6.73 Impact Factor
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    ABSTRACT: We examine the mid-infrared fluxes and spectral energy distributions for metal-poor stars with iron abundances [Fe/H] $\lesssim-5$, as well as two CEMP-no stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excess. These non-detections rule out many types of circumstellar disks, e.g. a warm debris disk ($T\!\le\!290$ K), or debris disks with inner radii $\le1$ AU, such as those associated with the chemically peculiar post-AGB spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e.g. a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 microns is detected at the 2-$\sigma$ level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.
    The Astrophysical Journal 07/2014; 791(2). · 6.73 Impact Factor
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    ABSTRACT: Dynamical information along with survey data on metallicity and in some cases age have been used recently by some authors to search for candidates of stars that were born in the cluster where the Sun formed. We have acquired high-resolution, high signal-to-noise ratio spectra for 30 of these objects to determine, using detailed elemental abundance analysis, if they could be true solar siblings. Only two of the candidates are found to have solar chemical composition. Updated modeling of the stars' past orbits in a realistic Galactic potential reveals that one of them, HD 162826, satisfies both chemical and dynamical conditions for being a sibling of the Sun. Measurements of rare-element abundances for this star further confirm its solar composition, with the only possible exception of Sm. Analysis of long-term high-precision radial velocity data rules out the presence of hot Jupiters and confirms that this star is not in a binary system. We find that chemical tagging does not necessarily benefit from studying as many elements as possible but instead from identifying and carefully measuring the abundances of those elements that show large star-to-star scatter at a given metallicity. Future searches employing data products from ongoing massive astrometric and spectroscopic surveys can be optimized by acknowledging this fact.
    The Astrophysical Journal 05/2014; 787(2):154. · 6.73 Impact Factor
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    ABSTRACT: Residual Spitzer/IRS spectra for a sample of 31 R Coronae Borealis (RCB) stars are presented and discussed in terms of narrow emission features superimposed on the quasi-blackbody continuous infrared emission. A broad ~6-10 um dust emission complex is seen in the RCBs showing an extreme H-deficiency. A secondary and much weaker ~11.5-15 um broad emission feature is detected in a few RCBs with the strongest ~6-10 um dust complex. The Spitzer infrared spectra reveal for the first time the structure within the ~6-10 um dust complex, showing the presence of strong C-C stretching modes at ~6.3 and 8.1 um as well as of other dust features at ~5.9, 6.9, and 7.3 um, which are attributable to amorphous carbonaceous solids with little or no hydrogen. The few RCBs with only moderate H-deficiencies display the classical 'unidentified infrared bands (UIRs)' and mid-infrared features from fullerene-related molecules. In general, the characteristics of the RCB infrared emission features are not correlated with the stellar and circumstellar properties, suggesting that the RCB dust features may not be dependent on the present physical conditions around RCB stars. The only exception seems to be the central wavelength of the 6.3 um feature, which is blue-shifted in those RCBs showing also the UIRs, i.e., the RCBs with the smallest H deficiency.
    The Astrophysical Journal 07/2013; 773(2). · 6.73 Impact Factor
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    ABSTRACT: We report the first spectroscopic identification of massive Galactic asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP) phase. These stars are the most Li-rich massive AGBs found to date, super Li-rich AGBs with logE(Li)~3-4. The high Li overabundances are accompanied by weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our observations with the most recent hot bottom burning (HBB) and s-process nucleosynthesis models confirms that HBB is strongly activated during the first TPs but the 22Ne neutron source needs many more TP and third dredge-up episodes to produce enough Rb at the stellar surface. We also show that the short-lived element Tc, usually used as an indicator of AGB genuineness, is not detected in massive AGBs which is in agreement with the theoretical predictions when the 22Ne neutron source dominates the s-process nucleosynthesis.
    Astronomy and Astrophysics 06/2013; · 5.08 Impact Factor
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    ABSTRACT: We have analyzed high-resolution echelle spectra of red giant members for seven open clusters in the Galactic anticentre direction to explore their chemical positions. Cluster membership has been confirmed by radial velocity. The spread in temperatures and gravities being very small among the red giants, nearly the same stellar lines were employed for all stars thereby reducing the abundance errors: the errors of the average abundance for a cluster were generally in the 0.02 to 0.05 dex range. Our present sample covers Galactocentric distances of 8.3 to 11.3 kpc and an age range of 0.2 to 4.3 Gyr. A careful comparison of our results for the cluster NGC 2682 (M 67) to other high-resolution abundance studies in the literature shows general good agreement for almost all elements in common.
    Monthly Notices of the Royal Astronomical Society 03/2013; 431(4). · 5.52 Impact Factor
  • Timur Sahin, David L. Lambert, Carlos Allende Prieto
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    ABSTRACT: We aim to reveal the nature of the Sloan Digital Sky Survey (SDSS) stars: SDSSJ100921.40+375233.9, SDSSJ015717.04+135535.9, and SDSSJ171422.43+283657.2, showing apparently high NUV excesses for their g-z colors, as expected for extremely low-metallicity stars. High resolution (R=60 000) spectra of the stars with a wide wavelength coverage were obtained to determine their chemical compositions with the Tull echelle spectrograph on the 2.7 m telescope at the McDonald Observatory. We derived the spectroscopic parameters Teff =5820+-125 K, log g =3.9+-0.2, and vt =1.1+-0.5 km/s for SDSSJ100921.40+375233.9, Teff=6250+-125 K, log g =3.7+-0.2, and vt =4.0+-0.5 km/s for SDSSJ015717.04+135535.9, and Teff=6320+-125 K, log g =4.1+-0.3, and vt =1.5+-0.5 km/s for SDSS J171422.43+283657.2, and elemental abundances were computed for 21 elements for J100921 and J171422 and for 19 elements for J015717 for the first time. We find metallicities of [Fe/H]= -1.30, -0.94, and -0.80 for SDSSJ100921.40+375233.9, J015717.04+135535.9, and J171422.43+283657.2, respectively. On the basis of calculated abundance ratios for J171422.43+283657.2 and J015717.04+135535.9, we also report that these two program stars have the expected composition of main-sequence halo turnoff stars, but with low-alpha abundances, i.e., the [alpha/Fe] ratio is ~0.0 for J171422.43+283657.2 and ~0.1 for J015717.04+135535.9. The latter one shows typical halo or thick-disk alpha-element abundances, but has a substantial rotational line broadening and vsini=40 +- 0.5 km/s.
    03/2013;
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    ABSTRACT: Among the distinguishing characteristics of the remarkable hot R Coronae Borealis star DY Cen, which was recently found to be a spectroscopic binary, is the presence of nebular forbidden lines in its optical spectrum. A compilation of photometry from 1970 to the present suggests that the star has evolved to higher effective temperatures. Comparison of spectra from 2010 with earlier spectra show that between 2003 and 2010, the 6717 and 6730 A emission lines of [S II] underwent a dramatic change in their fluxes suggesting an increase in the nebula's electron density of 290 cm-3 to 3140 cm-3 from 1989 to 2010 while the stellar temperature increased from 19500 K to 25000 K. The nebular radius is about 0.02 pc, 60000 times bigger than the semimajor axis of DY Cen binary system. Rapid changes of stellar temperature and its response by the nebula demonstrate stellar evolution in action.
    Monthly Notices of the Royal Astronomical Society 01/2013; 431(1). · 5.52 Impact Factor
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    Jens Adamczak, David L. Lambert
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    ABSTRACT: We determined the chemical composition of a large sample of weak G band stars -- a rare class of G and K giants of intermediate mass with unusual abundances of C, N, and Li. We have observed 24 weak G band stars with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory and derived spectroscopic abundances for C, N, O, and Li, as well as for selected elements from Na - Eu. The results show that the atmospheres of weak G band stars are highly contaminated with CN-cycle products. The C underabundance is about a factor of 20 larger than for normal giants and the $^{12}$C/$^{13}$C ratio approaches the CN-cycle equilibrium value. In addition to the striking CN-cycle signature the strong N overabundance may indicate the presence of partially ON-cycled material in the atmospheres of the weak G band stars. The exact mechanism responsible for the transport of the elements to the surface has yet to be identified but could be induced by rapid rotation of the main sequence progenitors of the stars. The unusually high Li abundances in some of the stars are an indicator for Li production by the Cameron-Fowler mechanism. A quantitative prediction of a weak G band star's Li abundance is complicated by the strong temperature sensitivity of the mechanism and its participants. In addition to the unusual abundances of CN-cycle elements and Li we find an overabundance of Na that is in accordance with the NeNa chain running in parallel with the CN-cycle. Apart from these peculiarities the element abundances in a weak G band star's atmosphere are consistent with those of normal giants.
    The Astrophysical Journal 01/2013; 765(2). · 6.73 Impact Factor
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    I. Ramirez, C. Allende Prieto, D. L. Lambert
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    ABSTRACT: Atmospheric parameters and oxygen abundances of 825 nearby FGK stars are derived using high-quality spectra and a non-LTE analysis of the 777 nm O I triplet lines. We assign a kinematic probability for the stars to be thin-disk (P1), thick-disk (P2), and halo (P3) members. We confirm previous findings of enhanced [O/Fe] in thick-disk (P2>0.5) relative to thin-disk (P1>0.5) stars with [Fe/H]<-0.2, as well as a "knee" that connects the mean [O/Fe]-[Fe/H] trend of thick-disk stars with that of thin-disk members at [Fe/H]>-0.2. Nevertheless, we find that the kinematic membership criterion fails at separating perfectly the stars in the [O/Fe]-[Fe/H] plane, even when a very restrictive kinematic separation is employed. Stars with "intermediate" kinematics (P1<0.7, P2<0.7) do not all populate the region of the [O/Fe]-[Fe/H] plane intermediate between the mean thin-disk and thick-disk trends, but their distribution is not necessarily bimodal. Halo stars (P3>0.5) show a large star-to-star scatter in [O/Fe]-[Fe/H], but most of it is due to stars with Galactocentric rotational velocity V<-200 km/s; halo stars with V>-200 km/s follow an [O/Fe]-[Fe/H] relation with almost no star-to-star scatter. Early mergers with satellite galaxies explain most of our observations, but the significant fraction of disk stars with "ambiguous" kinematics and abundances suggests that scattering by molecular clouds and radial migration have both played an important role in determining the kinematic and chemical properties of solar neighborhood stars.
    The Astrophysical Journal 01/2013; 764(1). · 6.73 Impact Factor
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    ABSTRACT: We present a non-local thermodynamic equilibrium abundance analysis of the resonance doublet Li I at 6707.8 A for 55 Galactic F and G supergiants and bright giants. The derived lithium abundances log \epsilon(Li) may be considered in three groups, namely: (i) 10 Li-rich giants with log \epsilon(Li) = 2.0-3.2 (all 10 are F-type or A9 stars); (ii) 13 G- to K0-type stars with Li abundances in the narrow range log \epsilon(Li) = 1.1-1.8; (iii) all other stars provide just upper limits to the Li abundance. The derived Li abundances are compared with theoretical predictions of 2-15 Msun stars. Our results are generally in good agreement with theory. In particular, the absence of detectable lithium for the majority of programme stars is explainable. The comparison suggests that the stars may be separated by mass M into two groups, namely M < 6 Msun and M > 6 Msun. All Li-rich giants and supergiants with log \epsilon(Li) > 2.0 have masses M < 6 Msun; 11 of 13 stars with log \epsilon(Li) = 1.1-1.8, specifically the stars with M < 6 Msun, show good agreement with the post-first dredge-up surface abundance log \epsilon(Li) = 1.4 predicted for the non-rotating 2-6 Msun stellar models. An absence of Li-rich stars in the range M > 6 Msun agrees with the theoretical prediction that F and G supergiants and giants with M > 6 Msun cannot show detectable lithium. We note that present theory appears unable to account for the derived Li abundances for some stars, namely for (i) a few relatively low-mass Li-rich giants (M < 6 Msun), whose high Li abundances accompanied by rather high rotational velocities or substantial nitrogen excess contradict theoretical predictions; (ii) the relatively high-mass supergiants HR 461 and HR 8313 (M > 6 Msun) with the detected abundances log \epsilon = 1.3-1.5. It is possible that the lithium in such stars was synthesized recently.
    Monthly Notices of the Royal Astronomical Society 12/2012; 427(1). · 5.52 Impact Factor
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    ABSTRACT: Fundamental parameters and the carbon, nitrogen and oxygen abundances are determined for 22 B-type stars with distances up to 600 pc and slow rotation (vsini up to 66 km/s). The stars are selected according to their effective temperatures Teff and surface gravities log g, namely: Teff is between 15300 and 24100 K and log g is mostly greater than 3.75; therefore, stars with medium masses of 5-11 M are selected. Theory predicts for the stars with such parameters that the C, N and O abundances in their atmospheres should correspond to their initial values. Non-LTE analysis of C II, N II and O II lines is implemented. The following mean C, N and O abundances are obtained: log \epsilon(C) = 8.31+-0.13, log \epsilon(N) = 7.80+-0.12 and log \epsilon(O) = 8.73+-0.13. These values are in very good agreement with recent data on the C, N and O abundances for nearby B stars from other authors; it is important that different techniques are applied by us and other authors. When excluding for the stars HR 1810 and HR 2938, which can be mixed, we obtain the following mean abundances for the remaining 20 stars: log \epsilon(C) = 8.33+-0.11, log \epsilon(N) = 7.78+-0.09 and log \epsilon(O) = 8.72+-0.12; these values are in excellent agreement with a present-day Cosmic Abundance Standard (CAS) of Nieva & Przybilla.
    Monthly Notices of the Royal Astronomical Society 12/2012; 428(4). · 5.52 Impact Factor
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    B. P. Hema, Gajendra Pandey, David L. Lambert
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    ABSTRACT: The high resolution optical spectra of H-deficient stars, R Coronae Borealis stars and H-deficient carbon stars are analyzed by synthesizing the C2 Swan bands (0,1), (0,0), and (1,0) using our detailed line-list and Uppsala model atmosphere, to determine the C-abundances and the 12C/13C ratios which are potential clues to the formation process of these stars. The C-abundances derived from C2 bands are about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). The carbon abundances derived from C I lines are a factor of four lower than that adopted for the model atmosphere over the same C/He interval, as reported by Asplund et al.: 'the carbon problem'. In principle, the carbon abundances obtained from C2 Swan bands and that adopted for the model atmosphere can be equated for a particular choice of C/He that varies from star to star (unlike C I lines). Then, the carbon problem for C2 bands is eliminated. However, such C/He ratios are in general less than those of the extreme helium stars, the seemingly natural relatives to the RCB and HdC stars. The derived carbon abundances and the 12C/13C ratios are discussed in light of the double degenerate (DD) and the final flash (FF) scenarios. The carbon abundance and the 12C/13C ratios for the FF product, Sakurai's Object is derived. The carbon abundance in the Sakurai's object is 10 times higher than in the RCB star VZ Sgr. On an average, the carbon abundance in the Sakurai's Object is about 10 to 100 times higher than in RCB stars. The 12C/13C ratio in Sakurai's Object is 3.4, the equilibrium value, as expected for FF products.
    11/2012;
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    ABSTRACT: The remarkable hot R Coronae Borealis star DY Cen is revealed to be the first and only binary system to be found among the R Coronae Borealis (RCB) stars and their likely relatives, including the Extreme Helium stars and the hydrogen-deficient carbon stars. Radial velocity determinations from 1982-2010 have shown DY Cen is a single-lined spectroscopic binary in an eccentric orbit with a period of 39.67 days. It is also one of the hottest and most H-rich member of the class of RCB stars. The system may have evolved from a common-envelope to its current form.
    The Astrophysical Journal Letters 10/2012; 760(1). · 6.35 Impact Factor
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    ABSTRACT: We search high-resolution and high-quality VLT/UVES optical spectra of the hot R Coronae Borealis (RCB) star DY Cen for electronic transitions of the C60 molecule and diffuse interstellar bands (DIBs). We report the non-detection of the strongest C60 electronic transitions (e.g., those at ~3760, 3980, and 4024 A). Absence of C60 absorption bands may support recent laboratory results, which show that the ~7.0, 8.5, 17.4, and 18.8 um emission features seen in DY Cen - and other similar objects with PAH-like dominated IR spectra - are attributable to proto-fullerenes or fullerene precursors rather than to C60. DIBs towards DY Cen are normal for its reddening; the only exception is the DIB at 6284 A (possibly also the 7223A DIB) that is found to be unusually strong. We also report the detection of a new broad (FWHM~2 A) and unidentified feature centered at ~4000 A. We suggest that this new band may be related to the circumstellar proto-fullerenes seen at infrared wavelengths.
    The Astrophysical Journal Letters 10/2012; 759(1). · 6.35 Impact Factor
  • Laura M. Stanford, David L. Lambert
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    ABSTRACT: Compositions of F and G dwarf stars in two groups of thick disc stars are presented. The groups identified by Schuster et al. have mean characteristics ([Fe/H], V(rot), Age, σW') of (-0.7 dex, 120 km s-1, 12.5 Gyr, 62 km s-1) and (-0.4, 160, 10.0, 45.8). Abundances for 23 elements obtained from high-resolution spectra are presented for 59 stars in the metal-rich group and 27 stars in the metal-poor group. The run of abundance ratios [X/Fe] versus [Fe/H] for the two groups define a single relation for each element (designated as X) with no intrinsic scatter and without a measurable discontinuity at -0.6 < [Fe/H] < -0.5, the metallicity at which the two groups overlap. The relations [X/Fe] versus [Fe/H] are those determined previously for thick disc stars. It is suggested that these two groups and the thick disc as a whole have a common origin in terms of prior chemical evolution.
    Monthly Notices of the Royal Astronomical Society 08/2012; 424(3):2118-2129. · 5.52 Impact Factor
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    P. Bonifacio, E. Caffau, K. A. Venn, D. L. Lambert
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    ABSTRACT: Context: Star HE 1327-2326 is a unique object, with the lowest measured iron abundance ([Fe/H] ~ -6) and a peculiar chemical composition that includes large overabundances of C, N, and O with respect to iron. One important question is whether the chemical abundances in this star reflect the chemical composition of the gas cloud from which it was formed or if they have been severely affected by other processes, such as dust-gas winnowing. Aims: We measure or provide an upper limit to the abundance of the volatile element sulphur, which can help to discriminate between the two scenarios. Methods: We observed HE 1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to observe the S I lines of Multiplet 3 at 1045 nm. Results: We do not detect the S I line. A 3sigma$upper limit on the equivalent width (EW) of any line in our spectrum is EW<0.66 pm. Using either one-dimensional static or three-dimensional hydrodynamical model-atmospheres, this translates into a robust upper limit of [S/H]<-2.6. Conclusions: This upper limit does not provide conclusive evidence for or against dust-gas winnowing, and the evidence coming from other elements (e.g., Na and Ti) is also inconclusive or contradictory. The formation of dust in the atmosphere versus an origin of the metals in a metal-poor supernova with extensive "fall-back" are not mutually exclusive. It is possible that dust formation distorts the peculiar abundance pattern created by a supernova with fall-back, thus the abundance ratios in HE 1327-2326 may be used to constrain the properties of the supernova(e) that produced its metals, but with some caution.
    Astronomy and Astrophysics 07/2012; · 5.08 Impact Factor
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    P. Ramya, Bacham E. Reddy, David L. Lambert
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    ABSTRACT: We present abundances for 20 elements for stars in two stellar streams identified by Arifyanto & Fuchs (2006, A&A, 449, 533): 18 stars from the Arcturus stream and 26 from a new stream, which we call AF06 stream, both from the Galactic thick disk. Results show both streams are metal-poor and very old (10$-$14 Gyrs) with kinematics and abundances overlapping with the properties of local field thick disk stars. Both streams exhibit a range in metallicity but with relative elemental abundances that are identical to those of thick disk stars of the same metallicity. These results show that neither stream can result from dissolution of an open cluster. It is highly unlikely that either stream represents tidal debris from an accreted satellite galaxy. Both streams most probably owe their origin to dynamical perturbations within the Galaxy.
    Monthly Notices of the Royal Astronomical Society 07/2012; 425(4). · 5.52 Impact Factor

Publication Stats

10k Citations
2,306.10 Total Impact Points

Institutions

  • 2014
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
  • 4–2014
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 2010–2013
    • Universidad de La Laguna
      • Department of Astrophysics
      San Cristóbal de La Laguna, Canary Islands, Spain
    • Complutense University of Madrid
      Madrid, Madrid, Spain
  • 1987–2013
    • Indian Institute of Astrophysics
      Bengalūru, Karnātaka, India
  • 2012
    • Harvey Mudd College
      Claremont, California, United States
    • Crimean Astrophysical Observatory
      Semeiz, Avtonomna Respublika Krym, Ukraine
  • 2000–2012
    • University of Toledo
      • Department of Physics and Astronomy
      Toledo, Ohio, United States
    • Queen's University Belfast
      • Department of Physics and Astronomy
      Belfast, NIR, United Kingdom
  • 2008
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany
  • 2007
    • Everett Community College
      Austin, Texas, United States
  • 1992–2006
    • New Mexico State University
      Las Cruces, New Mexico, United States
    • Georgia State University
      Atlanta, Georgia, United States
  • 2004
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 2003
    • University of Texas at El Paso
      • Department of Physics
      El Paso, Texas, United States
  • 2001–2002
    • Macalester College
      • Department of Physics and Astronomy
      Saint Paul, Minnesota, United States
  • 1999
    • Instituto de Astrofísica de Canarias
      San Cristóbal de La Laguna, Canary Islands, Spain
  • 1997–1998
    • Austin College
      Austin, Texas, United States
  • 1994
    • Aarhus University
      • Department of Physics and Astronomy
      Aarhus, Central Jutland, Denmark
  • 1988–1992
    • Case Western Reserve University
      Cleveland, Ohio, United States
  • 1991
    • NASA
      Washington, West Virginia, United States
  • 1969–1988
    • California Institute of Technology
      Pasadena, California, United States
    • Carnegie Institution for Science
      Washington, West Virginia, United States
  • 1986
    • Michigan State University
      East Lansing, Michigan, United States
  • 1981–1985
    • Louisiana State University
      Baton Rouge, Louisiana, United States
  • 1984
    • Indiana University Bloomington
      Bloomington, Indiana, United States
  • 1982
    • Leiden University
      Leyden, South Holland, Netherlands
  • 1968
    • University of Oxford
      Oxford, England, United Kingdom