The COMBS survey I : Chemical Origins of Metal-Poor Stars in the Galactic Bulge

19 pages, 5 tables, accepted to MNRASChemistry and kinematic studies can determine the origins of stellar population across the Milky Way. The metallicity distribution function of the bulge indicates that it comprises multiple populations, the more metal-poor end of which is particularly poorly understood. It is currently unknown if metal-poor bulge stars ([Fe/H] <−1 dex) are part of the stellar halo in the inner most region, or a distinct bulge population or a combination of these. Cosmological simulations also indicate that the metal-poor bulge stars may be the oldest stars in the Galaxy. In this study, we successfully target metal-poor bulge stars selected using SkyMapper photometry. We determine the stellar parameters of 26 stars and their elemental abundances for 22 elements using R∼ 47 000 VLT/UVES spectra and contrast their elemental properties with that of other Galactic stellar populations. We find that the elemental abundances we derive for our metal-poor bulge stars have lower overall scatter than typically found in the halo. This indicates that these stars may be a distinct population confined to the bulge. If these stars are, alternatively, part of the innermost distribution of the halo, this indicates that the halo is more chemically homogeneous at small Galactic radii than at large radii. We also find two stars whose chemistry is consistent with second-generation globular cluster stars. This paper is the first part of the Chemical Origins of Metal-poor Bulge Stars (COMBS) survey that will chemodynamically characterize the metal-poor bulge population.Peer reviewedFinal Published versio

A Near-Solar Metallicity, Nitrogen-Deficient Lyman Limit Absorber Associated with two S0 Galaxies

From UV spectra of the bright quasar PHL 1811 recorded by FUSE and the E140M configuration on STIS, we have determined the abundances of various atomic species in a Lyman limit system at z = 0.0809 with log N(H I) = 17.98. Considerably more hydrogen may be in ionized form, since the abundances of C II, Si II, S II and Fe II are very large compared to that of O I, when compared to their respective solar abundance ratios. Our determination [O/H] = -0.19 in the H I-bearing gas indicates that the chemical enrichment of the gas is unusually high for an extragalactic QSO absorption system. However, this same material has an unusually low abundance of nitrogen, [N/O] < -0.59, indicating that there may not have been enough time during this enrichment for secondary nitrogen to arise from low and intermediate mass stars. In an earlier investigation we found two galaxies at nearly the same redshift as this absorption system and displaced by 34 and 87 kpc from the line of sight. An r-band image recorded by the ACS on HST indicates these are S0 galaxies. One or both of these galaxies may be the source of the gas, which might have been expelled in a fast wind, by tidal stripping, or by ram-pressure stripping. Subtraction of the ACS point-spread function from the image of the QSO reveals the presence of a face-on spiral galaxy under the glare of the quasar; although it is possible that this galaxy may be responsible for the Lyman limit absorption, the exact alignment of the QSO with the center of the galaxy suggests that the spiral is the quasar host.Comment: 74 pages, 14 figures; to be published in the Astrophysical Journal (Part 1) May 1, 2005 issue. A version of the paper with figures of better quality may be found at http://www.astro.princeton.edu/~ebj/PHL1811_paper.ps (postscript) or http://www.astro.princeton.edu/~ebj/PHL1811_paper.pdf (pdf

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○, − 7.5○, and −5○. Our results show that none of the abundance ratios vary significantly with latitude. We also observe disc-like [Na/Fe] abundance ratios, which indicate that 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 disc. However, this similarity is not confirmed across all elements and metallicity regimes. The more metal-poor bulge population at [Fe/H] ≲ − 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 disc. 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.LD, MA, and KCF acknowledge funding from the Australian Research Council (projects FL110100012 and DP160103747). LD gratefully acknowledges a scholarship from Zonta International District 24. DMN was supported by the Allan C. and Dorothy H. Davis Fellowship. MA’s work was conducted as part of the research by Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013

Fluorine Abundances in the Large Magellanic Cloud and Omega Centauri: Evidence for Neutrino Nucleosynthesis?

The behavior of fluorine with metallicity has not yet been probed in any stellar population. In this work, we present the first fluorine abundances measured outside of the Milky Way from a sample of red giants in the Large Magellanic Cloud (LMC), as well the Galactic globular cluster Omega Centauri. The fluorine abundances are derived from vibration-rotation transitions of HF using infrared spectra obtained with the Phoenix spectrograph on the Gemini South 8.1m telescope. It is found that the abundance ratio of F/O declines as the oxygen abundance decreases. The values of F/O are especially low in the two Omega Cen giants; this very low value of F/O probably indicates that 19F synthesis in asymptotic giant branch (AGB) stars is not the dominant source of fluorine in stellar populations. The observed decline in F/O with lower O abundances is in qualitative agreement with what is expected if 19F is produced via H- and He-burning sequences in very massive stars, with this fluorine then ejected in high mass-loss rate Wolf-Rayet winds. A quantitative comparison of observations with this process awaits results from more detailed chemical evolution models incorporating the yields from Wolf-Rayet winds. Perhaps of more significance is the quantitative agreement between the Galactic and LMC results with predictions from models in which 19F is produced from neutrino nucleosynthesis during core collpase in supernovae of Type II. The very low values of F/O in Omega Cen are also in agreement with neutrino nucleosynthesis models if the ``peculiar'' star formation history of Omega Cen, with 2-4 distinct episodes of star formation, is considered.Comment: 19 pages, 2 tables, 4 figures. In press to the Astronomical Journa

Argos - III. Stellar populations in the galactic bulge of the milky way

We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Λ cold dark matter theory, or from disc instabilities,

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 High Efficiency and Resolution Multi-Element Spectrograph on the Anglo–Australian Telescope. We targeted ARGOS survey stars with predetermined 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◦, − 7.5◦, and −5◦. Our analysis of ARGOS stars indicates that the centroids of ARGOS metallicity components should be located ≈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] ≈ −0.8, the bulge shares the same abundance trend as the high-α disc population. However, the metal-poor bulge population ([Fe/H] −0.8) show enhanced alpha abundance ratios compared to the disc/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 disc as the more metal-rich populations.LD, MA, and KCF acknowledge funding from the Australian Research Council (projects FL110100012 and DP160103747). LD gratefully acknowledges a scholarship from Zonta International District 24. DMN was supported by the Allan C. and Dorothy H. Davis Fellowship. LMH was supported by the project grant ‘The New Milky Way’ from the Knut and Alice Wallenberg foundation. MA’s work was conducted as part of the research by Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013

ARGOS - IV. The kinematics of the Milky Way bulge

We present the kinematic results from our ARGOS spectroscopic survey of the Galactic bulge of the Milky Way. Our aim is to understand the formation of the Galactic bulge. We examine the kinematics of about 17 400 stars in the bulge located within 3.5 kp

Holistic spectroscopy: complete reconstruction of a wide-field, multiobject spectroscopic image using a photonic comb

The primary goal of Galactic archaeology is to learn about the origin of the Milky Way from the detailed chemistry and kinematics of millions of stars. Wide-field multifibre spectrographs are increasingly used to obtain spectral information for huge samples of stars. Some surveys (e.g. GALAH) are attempting to measure up to 30 separate elements per star. Stellar abundance spectroscopy is a subtle art that requires a very high degree of spectral uniformity across each of the fibres. However, wide-field spectrographs are notoriously non-uniform due to the fast output optics necessary to image many fibre outputs on to the detector. We show that precise spectroscopy is possible with such instruments across all fibres by employing a photonic comb – a device that produces uniformly spaced spots of light on the CCD to precisely map complex aberrations. Aberrations are parametrized by a set of orthogonal moments with ∼100 independent parameters. We then reproduce the observed image by convolving high-resolution spectral templates with measured aberrations as opposed to extracting the spectra from the observed image. Such a forward modelling approach also trivializes some spectroscopic reduction problems like fibre cross-talk, and reliably extracts spectra with a resolution ∼2.3 times above the nominal resolution of the instrument. Our rigorous treatment of optical aberrations also encourages a less conservative spectrograph design in the future

The GALAH survey and Gaia DR2: (non-)existence of five sparse high-latitude open clusters

Sparse open clusters can be found at high galactic latitudes where loosely populated clusters are more easily detected against the lower stellar background. Because most star formation takes place in the thin disc, the observed population of clusters far from the Galactic plane is hard to explain. We combined spectral parameters from the GALAH survey with the Gaia DR2 catalogue to study the dynamics and chemistry of five old sparse high-latitude clusters in more detail. We find that four of them (NGC 1252, NGC 6994, NGC 7772, NGC 7826) – originally classified in 1888 – are not clusters but are instead chance projections on the sky. Member stars quoted in the literature for these four clusters are unrelated in our multidimensional physical parameter space; the quoted cluster properties in the literature are therefore meaningless. We confirm the existence of visually similar NGC 1901 for which we provide a probabilistic membership analysis. An overdensity in three spatial dimensions proves to be enough to reliably detect sparse clusters, but the whole six-dimensional space must be used to identify members with high confidence, as demonstrated in the case of NGC 1901