| 1. |
- Martell, S., et al.
(författare)
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The GALAH survey : Scientific motivation
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 449:3
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Tidskriftsartikel (refereegranskat)abstract
- The Galactic Archaeology with HERMES (GALAH) survey is a large high-resolution spectroscopic survey using the newly commissioned High Efficiency and Resolution Multi-Element Spectrograph (HERMES) 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 deg 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, α-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.
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| 2. |
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| 3. |
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| 4. |
- Lattanzio, J. C., et al.
(författare)
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On the numerical treatment and dependence of thermohaline mixing in red giants
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 446:3, s. 2673-2688
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Tidskriftsartikel (refereegranskat)abstract
- In recent years much interest has been shown in the process of thermohaline mixing in red giants. In low- and intermediate-mass stars this mechanism first activates at the position of the bump in the luminosity function, and has been identified as a likely candidate for driving the slow mixing inferred to occur in these stars. One particularly important consequence of this process, which is driven by a molecular weight inversion, is the destruction of lithium. We show that the degree of lithium destruction, or in some cases production, is extremely sensitive to the numerical details of the stellar models. Within the standard 1D diffusion approximation to thermohaline mixing, we find that different evolution codes, with their default numerical schemes, can produce lithium abundances that differ from one another by many orders of magnitude. This disagreement is worse for faster mixing. We perform experiments with four independent stellar evolution codes, and derive conditions for the spatial and temporal resolution required for a converged numerical solution. The results are extremely sensitive to the time-steps used. We find that predicted lithium abundances published in the literature until now should be treated with caution.
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| 5. |
- Angelou, George C., et al.
(författare)
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Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 450:3, s. 2423-2440
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Tidskriftsartikel (refereegranskat)abstract
- We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turn-off and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter $\Delta M_V \,<^>{\rm {MSTO}}_{\rm {bump}}$. We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but $\Delta M_V \,<^>{\rm {MSTO}}_{\rm {bump}}$, which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of $\Delta M_V \,<^>{\rm {MSTO}}_{\rm {bump}}$.
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| 6. |
- Angelou, George C., et al.
(författare)
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The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X. ; 749:2
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Tidskriftsartikel (refereegranskat)abstract
- It is now widely accepted that globular cluster red giant branch (RGB) stars owe their strange abundance patterns to a combination of pollution from progenitor stars and in situ extra mixing. In this hybrid theory a first generation of stars imprints abundance patterns into the gas from which a second generation forms. The hybrid theory suggests that extra mixing is operating in both populations and we use the variation of [C/Fe] with luminosity to examine how efficient this mixing is. We investigate the observed RGBs ofM3, M13, M92, M15, and NGC 5466 as a means to test a theory of thermohaline mixing. The second parameter pair M3 and M13 are of intermediate metallicity and our models are able to account for the evolution of carbon along the RGB in both clusters, although in order to fit the most carbon-depleted main-sequence stars in M13 we require a model whose initial [C/Fe] abundance leads to a carbon abundance lower than is observed. Furthermore, our results suggest that stars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3). In the metal-poor regime only NGC 5466 can be tentatively explained by thermohaline mixing operating in multiple populations. We find thermohaline mixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15. It appears as if extra mixing is occurring before the luminosity function bump in these clusters. To reconcile the data with the models would require first dredge-up to be deeper than found in extant models.
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| 7. |
- Angelou, George C., et al.
(författare)
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Thermohaline Mixing And Its Role In The Evolution Of Carbon And Nitrogen Abundances In Globular Cluster Red Giants: The Test Case Of Messier 3
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X. ; 728:2
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Tidskriftsartikel (refereegranskat)abstract
- We review the observational evidence for extra mixing in stars on the red giant branch (RGB) and discuss why thermohaline mixing is a strong candidate mechanism. We recall the simple phenomenological description of thermohaline mixing and aspects of mixing in stars in general. We use observations of M3 to constrain the form of the thermohaline diffusion coefficient and any associated free parameters. This is done by matching [C/Fe] and [N/Fe] along the RGB of M3. After taking into account a presumed initial primordial bimodality of [C/Fe] in the CN-weak and CN-strong stars, our thermohaline mixing models can explain the full spread of [C/Fe]. Thermohaline mixing can produce a significant change in [N/Fe] as a function of absolute magnitude on the RGB for initially CN-weak stars, but not for initially CN-strong stars, which have so much nitrogen to begin with that any extra mixing does not significantly affect the surface nitrogen composition.
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| 8. |
- Angelou, George C., et al.
(författare)
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Thermohaline mixing and the variation of [C/Fe] with magnitude in M3
- 2010
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Ingår i: 11th Symposium on Nuclei in the Cosmos, NIC 2010; Heidelberg; Germany; 19 July 2010 through 23 July 2010.
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Konferensbidrag (refereegranskat)abstract
- We use observations of M3 to constrain thermohaline mixing. This is done by matching [C/Fe] and [N/Fe] along the RGB of M3. We find our models can explain observations if it is assumed there is a spread of ∼ 0.3 - 0.4 dex in [C/Fe] in the stars in M3 from their birth. We reproduce the full spread in [C/Fe] at the tip of the RGB. Thermohaline mixing can produce a significant change in [N/Fe] as a function of absolute magnitude on the RGB for initially CN-weak stars, but not for initially CN-strong stars, which have so much nitrogen to begin with that any deep mixing does not significantly affect the surface nitrogen composition.
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| 9. |
- Church, Ross, et al.
(författare)
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Which physics determines the location of the mean molecular weight minimum in red giants?
- 2014
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 443:2, s. 977-984
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Tidskriftsartikel (refereegranskat)abstract
- Stars ascending the red giant branch develop an inversion in mean molecular weight (mu) owing to the burning of He-3 in the region immediately above their hydrogen-burning shells. This inversion may drive thermohaline mixing and thereby be responsible for the extra mixing which is observationally indicated on the red giant branch. In this paper, we investigate the physical influences that determine the mass and temperature at which the inversion in mu develops. We find that it depends most strongly on the thermal structure of the envelope - the profiles of density and temperature in the region of the star immediately above the shell - and is otherwise relatively insensitive to abundances and nuclear reaction rates. The changes in the effects of thermohaline mixing as stars proceed up the giant branch can mostly be understood in terms of their changing thermal structure, driven by their increasing core mass.
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| 10. |
- Doherty, Carolyn L., et al.
(författare)
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Monash chemical yields project (Monχey) element production in low- and intermediate-mass stars
- 2015
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213. ; 11:A29B, s. 164-165
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Tidskriftsartikel (refereegranskat)abstract
- The Monχey project will provide a large and homogeneous set of stellar yields for the low- and intermediate- mass stars and has applications particularly to galactic chemical evolution modelling. We describe our detailed grid of stellar evolutionary models and corresponding nucleosynthetic yields for stars of initial mass 0.8 M⊙ up to the limit for core collapse supernova (CC-SN) ≈ 10 M⊙. Our study covers a broad range of metallicities, ranging from the first, primordial stars (Z = 0) to those of super-solar metallicity (Z = 0.04). The models are evolved from the zero-age main-sequence until the end of the asymptotic giant branch (AGB) and the nucleosynthesis calculations include all elements from H to Bi. A major innovation of our work is the first complete grid of heavy element nucleosynthetic predictions for primordial AGB stars as well as the inclusion of extra-mixing processes (in this case thermohaline) during the red giant branch. We provide a broad overview of our results with implications for galactic chemical evolution as well as highlight interesting results such as heavy element production in dredge-out events of super-AGB stars. We briefly introduce our forthcoming web-based database which provides the evolutionary tracks, structural properties, internal/surface nucleosynthetic compositions and stellar yields. Our web interface includes user- driven plotting capabilities with output available in a range of formats. Our nucleosynthetic results will be available for further use in post processing calculations for dust production yields.
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