arXiv subjects

L. Duong

Publications and source records attributed to L. Duong.

12 recordsLinked to original sources

arXiv2019

HERBS II: Detailed chemical compositions of Galactic bulge stars

This work explores the detailed chemistry of the Milky Way bulge using the HERMES spectrograph on the Anglo-Australian Telescope. Here we present the abundance ratios of 13 elements for 832 red giant branch and clump stars along the minor bulge axis at latitudes $b=-10^{\circ}, -7.5$ and $-5^{\circ}$. Our results show that none of the abundance ratios vary significantly with latitude. We also observe {\color{red}disk-like} [Na/Fe] abundance ratios, which indicates the bulge does not contain helium-enhanced populations as observed in some globular clusters. Helium enhancement is therefore not the likely explanation for the double red-clump observed in the bulge. We confirm that bulge stars mostly follow abundance trends observed in the disk. However, this similarity is not confirmed across for all elements and metallicity regimes. The more metal-poor bulge population at [Fe/H] $\lesssim -0.8$ is enhanced in the elements associated with core collapse supernovae (SNeII). In addition, the [La/Eu] abundance ratio suggests higher $r$-process contribution, and likely higher star formation in the bulge compared to the disk. This highlights the complex evolution in the bulge, which should be investigated further, both in terms of modelling; and with additional observations of the inner Galaxy.

astro-ph.GA↗

arXiv2019

HERBS I: Metallicity and alpha enhancement along the Galactic bulge minor axis

To better understand the origin and evolution of the Milky Way bulge, we have conducted a survey of bulge red giant branch and clump stars using the HERMES spectrograph on the Anglo-Australian Telescope. We targeted ARGOS survey stars with pre-determined bulge memberships, covering the full metallicity distribution function. The spectra have signal-to-noise ratios comparable to, and were analysed using the same methods as the GALAH survey. In this work, we present the survey design, stellar parameters, distribution of metallicity and alpha-element abundances along the minor bulge axis at latitudes $b$ = $-10^{\circ}, -7.5^{\circ}$ and $-5^{\circ}$. Our analysis of ARGOS stars indicates that the centroids of ARGOS metallicity components should be located $\approx$0.09 dex closer together. The vertical distribution of $α$-element abundances is consistent with the varying contributions of the different metallicity components. Closer to the plane, alpha abundance ratios are lower as the metal-rich population dominates. At higher latitudes, the alpha abundance ratios increase as the number of metal-poor stars increases. However, we find that the trend of alpha-enrichment with respect to metallicity is independent of latitude. Comparison of our results with those of GALAH DR2 revealed that for [Fe/H] $\approx -0.8$, the bulge shares the same abundance trend as the high-$α$ disk population. However, the metal-poor bulge population ([Fe/H] $\lesssim -0.8$) show enhanced alpha abundance ratios compared to the disk/halo. These observations point to fairly rapid chemical evolution in the bulge, and that the metal-poor bulge population does not share the same similarity with the disk as the more metal-rich populations.

astro-ph.GA↗

arXiv2018

The GALAH survey: An abundance, age, and kinematic inventory of the solar neighbourhood made with TGAS

The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey & $Gaia$ provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, & sub-giant stars. [...] We investigate correlations between chemical compositions, ages, & kinematics for this sample. Stellar parameters & elemental abundances are derived from the GALAH spectra with the spectral synthesis code SME. [...] We report Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba & we note that we employ non-LTE calculations for Li, O, Al, & Fe. We show that the use of astrometric & photometric data improves the accuracy of the derived spectroscopic parameters, especially $\log g$. [...] we recover the result that stars of the high-$α$ sequence are typically older than stars in the low-$α$ sequence, the latter spanning $-0.7<$[Fe/H]$<+0.5$. While these two sequences become indistinguishable in [$α$/Fe] vs. [Fe/H] at the metal-rich regime, we find that age can be used to separate stars from the extended high-$α$ & the low-$α$ sequence even in this regime. [...] we find that the old stars ($>8$ Gyr have lower angular momenta $L_z$ than the Sun, which implies that they are on eccentric orbits & originate from the inner disk. Contrary to some previous smaller scale studies we find a continuous evolution in the high-$α$-sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate "high-$α$ metal-rich" population. Stars in our sample that are younger than 10 Gyr, are mainly found on the low $α$-sequence & show a gradient in $L_z$ from low [Fe/H] ($L_z>L_{z,\odot}$) towards higher [Fe/H] ($L_z<L_{z,\odot}$), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.

astro-ph.SR↗

arXiv2018

The GALAH Survey: Second Data Release

The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way and designed to deliver chemical information complementary to a large number of stars covered by the $Gaia$ mission. We present the GALAH second public data release (GALAH DR2) containing 342,682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multi-step approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels ($T_\mathrm{eff}$, $\log g$, $\mathrm{[Fe/H]}$, $\mathrm{[X/Fe]}$, $v_\mathrm{mic}$, $v \sin i$, $A_{K_S}$) for a representative training set of stars. This information is then propagated to the whole survey with the data-driven method of $The~Cannon$. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D MARCS stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence in our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from $Gaia$ will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.

astro-ph.SR↗

arXiv2018

The GALAH survey: properties of the Galactic disk(s) in the solar neighbourhood

Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick disks near the solar neighbourhood. The data cover a small range of Galactocentric radius ($7.9 \leq R_\mathrm{GC} \leq 9.5$ kpc), but extend up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude $260 ^\circ \leq \ell \leq 280^\circ$). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined `thick' and `thin' disks of the Galaxy. The thin disk (low-$α$ population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/d$z=-0.18 \pm 0.01$ dex kpc$^{-1}$, which is broadly consistent with previous studies. In contrast, its vertical $α$-abundance profile is almost flat, with a gradient of d[$α$/M]/d$z$ = $0.008 \pm 0.002$ dex kpc$^{-1}$. The steep vertical metallicity gradient of the low-$α$ population is in agreement with models where radial migration has a major role in the evolution of the thin disk. The thick disk (high-$α$ population) has a weaker vertical metallicity gradient d[M/H]/d$z = -0.058 \pm 0.003$ dex kpc$^{-1}$. The $α$-abundance of the thick disk is nearly constant with height, d[$α$/M]/d$z$ = $0.007 \pm 0.002$ dex kpc$^{-1}$. The negative gradient in metallicity and the small gradient in [$α$/M] indicate that the high-$α$ population experienced a settling phase, but also formed prior to the onset of major SNIa enrichment. We explore the implications of the distinct $α$-enrichments and narrow [$α$/M] range of the sub-populations in the context of thick disk formation.

astro-ph.GA↗

arXiv2016

The Galah Survey: Classification and diagnostics with t-SNE reduction of spectral information

Galah is an ongoing high-resolution spectroscopic survey with the goal of disentangling the formation history of the Milky Way, using the fossil remnants of disrupted star formation sites which are now dispersed around the Galaxy. It is targeting a randomly selected, magnitude limited ($V \leq 14$) sample of stars, with the goal of observing one million objects. To date, 300,000 spectra have been obtained. Not all of them are correctly processed by parameter estimation pipelines and we need to know about them. We present a semi-automated classification scheme which identifies different types of peculiar spectral morphologies, in an effort to discover and flag potentially problematic spectra and thus help to preserve the integrity of the survey's results. To this end we employ a recently developed dimensionality reduction technique t-SNE (t-distributed Stochastic Neighbour Embedding), which enables us to represent the complex spectral morphology in a two-dimensional projection map while still preserving the properties of the local neighbourhoods of spectra. We find that the majority (178,483) of the 209,533 Galah spectra considered in this study represents normal single stars, whereas 31,050 peculiar and problematic spectra with very diverse spectral features pertaining to 28,579 stars are distributed into 10 classification categories: Hot stars, Cool metal-poor giants, Molecular absorption bands, Binary stars, H$α$/H$β$ emission, H$α$/H$β$ emission superimposed on absorption, H$α$/H$β$ P-Cygni, H$α$/H$β$ inverted P-Cygni, Lithium absorption, and Problematic. Classified spectra with supplementary information are presented in the catalogue, indicating candidates for follow-up observations and population studies of the short-lived phases of stellar evolution.

astro-ph.SR↗

arXiv2015

The GALAH Survey: Scientific Motivation

The GALAH survey is a large high-resolution spectroscopic survey using the newly commissioned HERMES spectrograph on the Anglo-Australian Telescope. The HERMES spectrograph provides high-resolution (R ~28,000) spectra in four passbands for 392 stars simultaneously over a 2 degree field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V~14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, alpha-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.

astro-ph.GA↗

arXiv2011

Search for inelastic dark matter with the CDMS II experiment

Results are presented from a reanalysis of the entire five-tower data set acquired with the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory, with an exposure of 969 kg-days. The analysis window was extended to a recoil energy of 150 keV, and an improved surface-event background-rejection cut was defined to increase the sensitivity of the experiment to the inelastic dark matter (iDM) model. Three dark matter candidates were found between 25 keV and 150 keV. The probability to observe three or more background events in this energy range is 11%. Because of the occurrence of these events the constraints on the iDM parameter space are slightly less stringent than those from our previous analysis, which used an energy window of 10-100 keV.

astro-ph.CO↗

arXiv2011

Results from a Low-Energy Analysis of the CDMS II Germanium Data

We report results from a reanalysis of data from the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory. Data taken between October 2006 and September 2008 using eight germanium detectors are reanalyzed with a lowered, 2 keV recoil-energy threshold, to give increased sensitivity to interactions from Weakly Interacting Massive Particles (WIMPs) with masses below ~10 GeV/c^2. This analysis provides stronger constraints than previous CDMS II results for WIMP masses below 9 GeV/c^2 and excludes parameter space associated with possible low-mass WIMP signals from the DAMA/LIBRA and CoGeNT experiments.

astro-ph.CO↗

arXiv2009

An Improved Limit on the Muon Electric Dipole Moment

Three independent searches for an electric dipole moment (EDM) of the positive and negative muons have been performed, using spin precession data from the muon g-2 storage ring at Brookhaven National Laboratory. Details on the experimental apparatus and the three analyses are presented. Since the individual results on the positive and negative muon, as well as the combined result, d=-0.1(0.9)E-19 e-cm, are all consistent with zero, we set a new muon EDM limit, |d| < 1.9E-19 e-cm (95% C.L.). This represents a factor of 5 improvement over the previous best limit on the muon EDM.

hep-ex↗

arXiv2009

Analysis of the low-energy electron-recoil spectrum of the CDMS experiment

We report on the analysis of the low-energy electron-recoil spectrum from the CDMS II experiment using data with an exposure of 443.2 kg-days. The analysis provides details on the observed counting rate and possible background sources in the energy range of 2 - 8.5 keV. We find no significant excess in the counting rate above background, and compare this observation to the recent DAMA results. In the framework of a conversion of a dark matter particle into electromagnetic energy, our 90% confidence level upper limit of 0.246 events/kg/day at 3.15 keV is lower than the total rate above background observed by DAMA by 8.9$σ$. In absence of any specific particle physics model to provide the scaling in cross section between NaI and Ge, we assume a Z^2 scaling. With this assumption the observed rate in DAMA differs from the upper limit in CDMS by 6.8$σ$. Under the conservative assumption that the modulation amplitude is 6% of the total rate we obtain upper limits on the modulation amplitude a factor of ~2 less than observed by DAMA, constraining some possible interpretations of this modulation.

astro-ph.GA↗

arXiv2005

Beyond the CDMS-II Dark Matter Search: SuperCDMS

Presently the CDMS-II collaboration's Weakly Interacting Massive Particle (WIMP) search at the Soudan Underground Laboratory sets the most stringent exclusion limits of any WIMP cold dark matter direct-detection experiment. To extend our reach further, to WIMP-nucleon cross sections in the range $10^{-46} - 10^{-44}$cm$^2$, we propose SuperCDMS, which would take advantage of a very deep site. One promising site is the recently approved SNOLab facility in Canada. In this paper we will present our overall plan, identify primary issues, and set the goals that need to be met prior to embarking upon each phase of SuperCDMS.

astro-ph↗