Douglas Geisler

University of Concepción, Concepción, Region del Biobio, Chile

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Publications (14)16.95 Total impact

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    ABSTRACT: The VISTA Variables in the V\'ia L\'actea (VVV) survey is one of six public ESO surveys, and is now in its 4th year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variables stars, microlensing events, new star clusters), others concerning more exotic objects, e.g. novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the actual status of the survey, as well some of the results based on the VVV data, are presented.
    06/2014;
  • Douglas Geisler, S. Villanova
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    ABSTRACT: Globular Clusters (GCs), long considered as ideal Single Stellar Populations, are now known to harbor a wide variety of chemical inhomogeneities. In particular, Carretta et al. (2009) showed that all GCs studied to date have at least a spread (or anticorrelation) in the light-elements O and Na. Indeed, they suggest a new definition of a GC as a cluster which exhibits such an anticorrelation, with the implication that all GCs, at least those above a certain mass limit, must posses this characteristic. The explanation is that GCs had at least two epochs of star formation and thus two generations of stars, where the second was formed from gas polluted by processed material produced by massive stars of the first. This multiplicity can only happen if the initial cluster mass exceeds a threshold above which stellar ejecta are retained and eventually a second generation is formed. A determination of this mass-threshold is mandatory in order to understand how GCs form. However, no convincing case of a single population GC has so far been found. We present a detailed abundance analysis of a large sample of stars from the ''GC" Ruprecht 106 observed with UVES@VLT2. We show that Ruprecht 106 is the first convincing example of a true single population ''GC". It is old (12 Gyrs) and, at odds with other GCs, has no alpha-element enhancement and a much lower C+N+O content. The abundance pattern points toward an extragalactic origin. Its present day mass can be assumed as a lower limit for the initial mass threshold below which no second generation is formed.
    05/2012;
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    ABSTRACT: The putative association between the 10.8 d classical Cepheid TW Nor and the open cluster Lynga 6 has generated considerable debate in the literature. New JHKs photometry in tandem with existing radial velocities for Lynga 6 stars imply cluster membership for TW Nor, and establish the variable as a high-weight calibrator for classical Cepheid relations. Fundamental mean parameters determined for Lynga 6 are: d=1.91+-0.10 kpc, E(J-H)=0.38+-0.02, and log(t)=7.9+-0.1. The Benedict et al.(2007)/Turner (2010) Galactic VIc Wesenheit function was revised using TW Nor's new parameters: W_VI,0=(-3.37+-0.08)log(P_0)-2.48+-0.08. TW Nor/Lynga 6 lie beyond the Sagittarius-Carina spiral arm and occupy the Centaurus arm, along with innumerable young Cepheids and clusters (e.g., VW Cen & VVV CL070).
    The Astrophysical Journal Letters 10/2011; 741(2). · 6.35 Impact Factor
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    Sandro Villanova, Douglas Geisler
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    ABSTRACT: All Globular Clusters (GCs) studied in detail so far host two or more populations of stars. Theoretical models suggest that the second population is formed from gas polluted by processed material produced by massive stars of the first generation. However the nature of the polluter is a matter of strong debate. Several candidates have been proposed: massive main-sequence stars (fast rotating or binaries), intermediate-mass AGB stars, or SNeII. We studied red giant branch (RGB) stars in the GC M4 (NGC 6121) to measure their chemical signature. We confirm the presence of a bimodal population, first discovered by Marino et al. (2008). The two groups have different C,$^{12}$C/$^{13}$C,N,O,Na content, but share the same Li,C+N+O,Mg,Al,Si,Ca,Ti,Cr,Fe,Ni,Zr,Ba and Eu abundance. Quite surprisingly the two groups differ also in their Y abundance. The absence of a spread in $\alpha$-elements, Eu and Ba makes SNeII and AGB stars unlikely as polluters. On the other hand, massive main-sequence stars can explain the bimodality of Y through the weak s-process. This stement is confirmed independently also by literature data on Rb and Pb. Observations suggest that the mass of the polluters is between 20 and 30 M$_{\odot}$. This implies a formation time scale for the cluster of 10$\div$30 Myrs. This result is valid for M4. Other clusters like NGC 1851, M22, or $\omega$ Cen have different chemical signatures and may require other kinds of polluter.
    09/2011;
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    Renee Mateluna, Douglas Geisler, Sandro Villanova
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    ABSTRACT: We have obtained high-resolution VLT/FLAMES spectra of a number of giants in the LMC cluster Hodge 11 and the surrounding field. We present results of a detailed model-atmosphere abundance analysis for a variety of elements, including Fe, Ca, Ti, Si, Sc, Ni and Cr.
    Proceedings of the International Astronomical Union 01/2010; 266:474-476.
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    Renee Mateluna, Douglas Geisler, Sandro Villanova
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    ABSTRACT: We present results of a detailed model atmosphere abundance analysis for a variety of elements, including Fe, Ca, Si, Ti, Sc, Ni, Cr and Ba for a number of giants in the surrounding field of the LMC cluster H11, obtained from high resolution FLAMES@VLT spectra.
    Proceedings of the International Astronomical Union 01/2010; 262:385-387.
  • Globular Clusters - Guides to Galaxies. 01/2009;
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    ABSTRACT: The outer parts of the Milky Way disk are believed to be one of the main arenas where the accretion of external material in the form of dwarf galaxies and subsequent formation of streams is taking place. The Monoceros stream and the Canis Major and Argo over-densities are notorious examples. VLT high resolution spectra have been acquired for five distant open clusters. We derive accurate radial velocities to distinguish field interlopers and cluster members. For the latter we perform a detailed abundance analysis and derive the iron abundance [Fe/H] and the abundance ratios of several $\alpha$ elements. Our analysis confirms previous indications that the radial abundance gradient in the outer Galactic disk does not follow the expectations extrapolated from the solar vicinity, but exhibits a shallower slope. By combining the metallicity of the five program clusters with eight more clusters for which high resolution spectroscopy is available, we find that the mean metallicity in the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H] $\approx -0.35$, with only marginal indications for a radial variation. In addition, all the program clusters exhibit solar scaled or slightly enhanced $\alpha$ elements, similar to open clusters in the solar vicinity and thin disk stars. We investigate whether this outer disk cluster sample might belong to an extra-galactic population, like the Monoceros ring. However, close scrutiny of their properties - location, kinematics and chemistry - does not convincingly favor this hypothesis. On the contrary, they appear more likely genuine Galactic disk clusters. We finally stress the importance to obtain proper motion measurements for these clusters to constrain their orbits. Comment: 19 pages, 9 eps figure, in press in A&A, abstract rephrased to fit in
    Astronomy and Astrophysics 09/2007; · 5.08 Impact Factor
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    ABSTRACT: New photometric data in the Washington system are presented for red giant candidates in NGC 1817 and 2251, two open clusters located towards the Galactic anticentre direction. In the case of NGC 2251, the Washington data are supplemented with new UBV and David Dunlap Observatory (DDO) photoelectric photometry. Published radial velocities are used to separate field stars from cluster giants. The photometric data yield an effective temperature and metal abundance for each cluster member. Five independent Washington abundance indices yield mean metallicities of [Fe/H]= 0.25 ± 0.04 for NGC 1817 and 2251, respectively. From combined BV and DDO data, we also derive E(B−V) = 0.21 ± 0.03 and [Fe/H]DDO=−0.14 ± 0.05 for NGC 2251. Both objects are then found to be on the metal-poor side of the distribution of open clusters, their metallicities being compatible with the existence of a radial abundance gradient in the disc. Using the WEBDA Open Cluster data base and the available literature, we re-examined the overall properties of a sample of 30 clusters located towards the Galactic anticentre with the distances, ages and metallicities available. This cluster sample presents no evidence of an abundance gradient perpendicular to the Galactic plane, nor is an age–metallicity relation found. However, a radial abundance gradient of −0.093 dex kpc−1 is derived over a Galactocentric distance of 14 kpc, a gradient which is in keeping with most recent determinations. This value practically does not change when all clusters with basic parameters known up to this date are considered.
    Monthly Notices of the Royal Astronomical Society 09/2005; 363(4):1247 - 1256. · 5.52 Impact Factor
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    ABSTRACT: We intend to use SIM to make definitive measurements of fundamental structural and dynamical parameters of the Milky Way. The important niche in dynamical parameter space afforded by SIM can be exploited to resolve, with unprecedented precision, a number of classical problems of Galactic astronomy. In addition, we have developed new tests of the Galactic mass distribution specifically designed for data with the special properties of SIM products. Our proposed suite of experiments will utilize the SIM Astrometric Grid as well as complementary observations of star clusters and other strategically-selected, distant "test particles" for a definitive characterization of the major components (bulge, disk, halo, satellite system) of the Milky Way. Specifically, our goals will be: 1) The determination of two fundamental parameters that play a central role in virtually every problem in Galactic astronomy, namely (a) the solar distance to the center of the Milky Way, R(sub 0); (b) the solar angular velocity around the Galactic: center, omega(sub 0). 2) The measurement of fundamental dynamical properties of the Milky Way, among them (a) the pattern speed of the central bar (b) the rotation field and velocity-dispersion tensor in the disk (c) the kinematics (mean rotational velocity and velocity dispersion tensor) of the halo as a function of position 3. The definition of the mass distribution of the Galaxy, which is dominated by the presence of dark matter. We intend to measure (a) the relative contribution of the disk and halo to the gravitational potential (b) the local volume and surface mass density of the disk (c) the shape, mass and extent of the dark halo of the Milky Way out to 250 kpc.
    01/2005;
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    ABSTRACT: We propose to measure radial velocities, abundances, and gravities for halo and thick disk giant and HB star candidates identified through a separate photometric survey. The NASA-sponsored Grid Giant Star Survey (GGSS) to find V~12, 5-10 kpc distant giant stars for the Astrometric Grid of the Space Interferometry Mission (SIM) has created a large, systematically assembled catalogue of V≲17.5 giant and HB star candidates. M_V~ -2, metal poor giants at V=17 probe to > 50 kpc. The final catalogue of ~70,000 giant and HB stars provides an unparalleled database for studying the structure, substructure, and chemodynamics of the Milky Way in all directions of the sky. We propose to continue a pilot program of HYDRA spectroscopy of giants and HB stars in 4% of GGSS fields to: (1) assess the richness of GGSS+HYDRA data for probing stellar populations and halo substructure and (2) explore the region where the Sloan Survey has found evidence for a northern extension of the Sgr dSph, and which includes fields where we have found evidence of a new tidal stream, possibly a foreground, > 360°-wrapped Sgr arm. The several thousand new giant/HB spectra will allow us to verify both of these tantalizing tidal debris findings.
    NOAO Proposal. 08/2002;
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    ABSTRACT: We have been awarded a NASA grant to obtain followup photometry of the Grid Giant Star Survey (GGSS) candidates we are establishing in our Space Interferometry Mission (SIM) Preparatory Science program. The GGSS will yield ~ 4000 metal-poor G-K giants as SIM Astrometric Grid candidates, based on single-epoch photometry and low resolution spectroscopy. %(being obtained at LCO). Multi-epoch photometric followup observations of these candidates are essential for verifying their photometric stability and thus guaranteeing their utility as SIM Grid stars. We propose here to continue our followup photometric observations which started in late 2000. These data are not only critical for the SIM project but will also allow us to improve our very limited knowledge of binary and planetary frequency in metal-poor giants. Because the kinematic properties of these stars will eventually be determined to unprecedented accuracy, a detailed understanding of their characteristics will not only be important for the SIM project, but also crucial for future studies of the GGSS database. These observations will greatly improve our knowledge of the atmospheric parameters of the Grid stars.
    NOAO Proposal. 02/2001;
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    01/2001; 246:65.
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    ABSTRACT: We will discuss results based on VLT/UVES spectra of giants in the Sculptor dwarf spheroidal galaxy. These high resolution spectra allow us to examine details of the chemical abundances in Sculptor including Fe O alpha elements Ba etc. Implications for chemical evolution in Sculptor and the Galaxy will be presented.