| 1. |
- Gao, Xudong, et al.
(författare)
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The GALAH survey : a new constraint on cosmological lithium and Galactic lithium evolution from warm dwarf stars
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 497:1, s. L30-L34
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Tidskriftsartikel (refereegranskat)abstract
- Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten constraints on stellar and Galactic evolution models, we present the largest high-resolution analysis of Li abundances A(Li) to date, with results for over 100000 GALAH (Galactic Archeology with HERMES) field stars spanning effective temperatures 5900K≲Teff≲7000K and metallicities −3 ≲ [Fe/H] ≲ +0.5. We separated these stars into two groups, on the warm and cool sides of the so-called Li dip, a localized region of the Kiel diagram wherein lithium is severely depleted. We discovered that stars in these two groups show similar trends in the A(Li)–[Fe/H] plane, but with a roughly constant offset in A(Li) of 0.4dex, the warm group having higher Li abundances. At [Fe/H]≳−0.5, a significant increase in Li abundance with increasing metallicity is evident in both groups, signalling the onset of significant Galactic production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a reduced lithium of around 0.4dex relative to the primordial value predicted from big bang nucleosynthesis (BBN). However, stars in the warm group between [Fe/H] = −1.0 and −0.5 form an elevated plateau that is largely consistent with the BBN prediction. This may indicate that these stars in fact preserve the primordial Li produced in the early Universe.
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| 2. |
- Lin, Jane, et al.
(författare)
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The GALAH survey : temporal chemical enrichment of the galactic disc
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 491:2, s. 2043-2056
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Tidskriftsartikel (refereegranskat)abstract
- We present isochrone ages and initial bulk metallicities ([Fe/H](bulk), by accounting for diffusion) of 163 722 stars from the GALAH Data Release 2, mainly composed of main-sequence turn-off stars and subgiants (7000 K > T-eff > 4000 K and log g > 3 dex). The local age-metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appears to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick discs in the solar neighbourhood. We also present abundance ratio trends for 16 elements as a function of age, across different [Fe/H](bulk) bins. In general, we find the trends in terms of [X/Fe] versus age from our far larger sample to be compatible with studies based on small (similar to 100 stars) samples of solar twins, but we now extend them to both sub- and supersolar metallicities. The a-elements show differing behaviour: the hydrostatic alpha-elements O and Mg show a steady decline with time for all metallicities, while the explosive alpha-elements Si, Ca, and Ti are nearly constant during the thin-disc epoch (ages less than or similar to 12 Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu has the opposite trend, thus favouring a primary production from sources with a short time delay such as core-collapse supernovae over long-delay events such as neutron star mergers.
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| 3. |
- Simpson, Jeffrey D., et al.
(författare)
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The GALAH Survey : Chemically tagging the Fimbulthul stream to the globular cluster omega Centauri
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 491:3, s. 3374-3384
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Tidskriftsartikel (refereegranskat)abstract
- Using kinematics from Gaia and the large elemental abundance space of the second data release of the GALAH survey, we identify two new members of the Fimbulthul stellar stream, and chemically tag them to massive, multimetallic globular cluster omega Centauri. Recent analysis of the second data release of Gaia had revealed the Fimbulthul stellar stream in the halo of the Milky Way. It had been proposed that the stream is associated with the omega Cen, but this proposition relied exclusively upon the kinematics and metallicities of the stars to make the association. In this work, we find our two new members of the stream to be metal-poor stars that are enhanced in sodium and aluminium, typical of second population globular cluster stars, but not otherwise seen in field stars. Furthermore, the stars share the s-process abundance pattern seen in omega Cen, which is rare in field stars. Apart from one star within 1.5 deg of omega Cen, we find no other stars observed by GALAH spatially near omega Cen or the Fimbulthul stream that could be kinematically and chemically linked to the cluster. Chemically tagging stars in the Fimbulthul stream to omega Cen confirms the earlier work, and further links this tidal feature in the Milky Way halo to omega Cen.
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| 4. |
- Wittenmyer, Robert A., et al.
(författare)
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K2-HERMES II : Planet-candidate properties from K2 Campaigns 1-13
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 496:1, s. 851-863
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Tidskriftsartikel (refereegranskat)abstract
- Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. We present complete results for planet-candidate hosts from the K2-HERMES survey, which uses the HERMES multi-object spectrograph on the Anglo-Australian Telescope to obtain R similar to 28 000 spectra for more than 30 000 K2 stars. We present complete host-star parameters and planet-candidate radii for 224 K2 candidate planets from C1-C13. Our results cast severe doubt on 30 K2 candidates, as we derive unphysically large radii, larger than 2R(Jup). This work highlights the importance of obtaining accurate, precise, and self-consistent stellar parameters for ongoing large planet search programs - something that will only become more important in the coming years, as TESS begins to deliver its own harvest of exoplanets.
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