| 1. |
- de Jong, R. S., et al.
(författare)
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4MOST : Project overview and information for the First Call for Proposals
- 2019
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Ingår i: The Messenger. - : European Southern Observatory. - 0722-6691. ; 175, s. 3-11
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We introduce the 4-metre Multi-Object Spectroscopic Telescope (4MOST), a new high-multiplex, wide-field spectroscopic survey facility under development for the four-metre-class Visible and Infrared Survey Telescope for Astronomy (VISTA) at Paranal. Its key specifications are: a large field of view (FoV) of 4.2 square degrees and a high multiplex capability, with 1624 fibres feeding two low-resolution spectrographs (R = λ/Δλ ~ 6500), and 812 fibres transferring light to the high-resolution spectrograph (R ~ 20 000). After a description of the instrument and its expected performance, a short overview is given of its operational scheme and planned 4MOST Consortium science; these aspects are covered in more detail in other articles in this edition of The Messenger. Finally, the processes, schedules, and policies concerning the selection of ESO Community Surveys are presented, commencing with a singular opportunity to submit Letters of Intent for Public Surveys during the first five years of 4MOST operations.
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| 2. |
- Cordoni, G., et al.
(författare)
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Exploring the Galaxy's halo and very metal-weak thick disc with SkyMapper and Gaia DR2
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 503:2, s. 2539-2561
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of -6.5 <= [Fe/H] <= -2.05 dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of [Fe/H] = - 3.31 and - 3.74, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that similar to 21 per cent of the sample have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Z(max)| <= 3 kpc. Of particular interest is a subsample (similar to 11 per cent of the total) of low |Z(max)| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = -4.30 and the subsample is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Z(max)|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Z(max)|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our sample (similar to 4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.
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| 3. |
- Buldgen, G., et al.
(författare)
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Helioseismic determination of the solar metal mass fraction
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 681
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Tidskriftsartikel (refereegranskat)abstract
- Context. The metal mass fraction of the Sun Z is a key constraint in solar modelling, but its value is still under debate. The standard solar chemical composition of the late 2000s has the ratio of metals to hydrogen as Z/X = 0.0181, and there was a small increase to 0.0187 in 2021, as inferred from 3D non-LTE spectroscopy. However, more recent work on a horizontally and temporally averaged ⟨3D⟩ model claim Z/X = 0.0225, which is consistent with the high values based on 1D LTE spectroscopy from 25 years ago.Aims. We aim to determine a precise and robust value of the solar metal mass fraction from helioseismic inversions, thus providing independent constraints from spectroscopic methods.Methods. We devised a detailed seismic reconstruction technique of the solar envelope, combining multiple inversions and equations of state in order to accurately and precisely determine the metal mass fraction value.Results. We show that a low value of the solar metal mass fraction corresponding to Z/X = 0.0187 is favoured by helioseismic constraints and that a higher metal mass fraction corresponding to Z/X = 0.0225 is strongly rejected by helioseismic data.Conclusions. We conclude that direct measurement of the metal mass fraction in the solar envelope favours a low metallicity, in line with the 3D non-LTE spectroscopic determination of 2021. A high metal mass fraction, as measured using a ⟨3D⟩ model in 2022, is disfavoured by helioseismology for all modern equations of state used to model the solar convective envelope.
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| 4. |
- Buldgen, G., et al.
(författare)
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In-depth analysis of solar models with high-metallicity abundances and updated opacity tables
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 686
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Tidskriftsartikel (refereegranskat)abstract
- Context. As a result of the high-quality constraints available for the Sun, we are able to carry out detailed combined analyses using neutrino, spectroscopic, and helioseismic observations. These studies lay the ground for future improvements of the key physical components of solar and stellar models because ingredients such as the equation of state, the radiative opacities, or the prescriptions for macroscopic transport processes of chemicals are then used to study other stars in the Universe.Aims. We study the existing degeneracies in solar models using the recent high-metallicity spectroscopic abundances by comparing them to helioseismic and neutrino data and discuss the effect on their properties of changes in the micro and macro physical ingredients.Methods. We carried out a detailed study of solar models computed with a high-metallicity composition from the literature based on averaged 3D models that were claimed to resolve the solar modelling problem. We compared these models to helioseismic and neutrino constraints.Results. The properties of the solar models are significantly affected by the use of the recent OPLIB opacity tables and the inclusion of macroscopic transport. The properties of the standard solar models computed using the OPAL opacities are similar to those for which the OP opacities were used. We show that a modification of the temperature gradient just below the base of the convective zone is required to remove the discrepancies in solar models, particularly in the presence of macroscopic mixing. This can be simulated by a localised increase in the opacity of a few percent.Conclusions. We conclude that the existing degeneracies and issues in solar modelling are not removed by using an increase in the solar metallicity, in contradiction to what has been suggested in the recent literature. Therefore, standard solar models cannot be used as an argument for a high-metallicity composition. While further work is required to improve solar models, we note that direct helioseismic inversions indicate a low metallicity in the convective envelope, in agreement with spectroscopic analyses based on full 3D models.
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| 5. |
- Carlos, M., et al.
(författare)
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The chemical compositions of multiple stellar populations in the globular cluster NGC 2808
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 519:2, s. 1695-1712
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Tidskriftsartikel (refereegranskat)abstract
- Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U, B, I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a sample of 70 red giant branch and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr, and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterization of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another; whereas the iron peak elements show negligible variation between the different populations (at a level of less than or similar to 0.10 dex). Our AGB stars are also characterized by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models that suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manque star.
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| 6. |
- Amarsi, Anish, et al.
(författare)
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The GALAH Survey : non-LTE departure coefficients for large spectroscopic surveys
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 642
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Tidskriftsartikel (refereegranskat)abstract
- Massive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for 13 different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 3756 1D MARCS model atmospheres that spans 3000 <= T-eff/K <= 8000, - 0.5 <= log g/cm s(-2) <= 5.5, and - 5 <= [Fe/H] <= 1. We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of 50 126 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between - 0.7 dex and + 0.2 dex for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the [A/Fe] versus [Fe/H] plane by up to 0.1 dex, and it can remove spurious differences between the dwarfs and giants by up to 0.2 dex. The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy. The grids of departure coefficients are publicly available and can be implemented into LTE pipelines to make the most of observational data sets from large spectroscopic surveys.
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| 7. |
- Bétrisey, Jérôme, et al.
(författare)
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Imprint of the magnetic activity cycle on solar asteroseismic characterisation based on 26 years of GOLF and BiSON data
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 688
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. Asteroseismic modelling will play a key role in future space-based missions, such as PLATO, CubeSpec, and Roman. Despite remarkable achievements, asteroseismology has revealed significant discrepancies between observations and theoretical predictions of the physics used in stellar models, which have the potential to bias stellar characterisation at the precision level demanded by PLATO. The current modelling strategies largely overlook magnetic activity, assuming that its effects are masked within the parametrisation of the so-called ‘surface effects’. Given the presence of activity cycles in multiple solar-like oscillators and activity variations in a significant fraction of Kepler observations of main-sequence stars, it is therefore relevant to investigate systematic errors in asteroseismic characterisations caused by our incomplete understanding of magnetic activity.Aims. Based on 26.5 years of GOLF and BiSON observations, we measured the impact of magnetic activity on the asteroseismic characterisation of the Sun as a star, a reference target for assessing the PLATO mission requirements.Methods. The GOLF and BiSON observations, which fully cover solar cycles 23 and 24, were divided into yearly overlapping snapshots, each delayed by a quarter of a year. For each snapshot, an advanced asteroseismic characterisation, similar to that to be adopted by the PLATO pipeline, was performed with standard prescriptions for the parametrisation of the surface effects. This allowed us to monitor the apparent temporal evolution of fundamental solar parameters such as mass, radius, and age. The correlation of these parameters with the 10.7 cm radio emission flux, a proxy of the solar activity cycle, was then measured.Results. The effects of magnetic activity are partially absorbed into the parametrisation of the surface effects when suitable prescriptions are used, and they do not significantly affect the measured solar mass or radius. However, contrary to literature expectations, we find a significant imprint on the age determination, with variations of up to 6.5% between solar minima and maxima. This imprint persists across both BiSON and GOLF datasets.Conclusions. Considering that the Sun exhibits low levels of activity, our study highlights the looming challenge posed by magnetic activity for future photometry missions, and it prompts a potential reevaluation of the asteroseismic characterisation of the most active Kepler targets.
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| 8. |
- Canocchi, Gloria, 1998-, et al.
(författare)
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3D non-LTE modeling of the stellar center-To-limb variation for transmission spectroscopy studies
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 683
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Tidskriftsartikel (refereegranskat)abstract
- Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-To-limb variations (CLVs) are not properly accounted for. Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D1 and K I resonance lines on transmission spectra. Methods. We modeled the CLV of the Na I D1 and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between μ = 0.1 and μ = 1.0. Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 A and the K I 7699 A resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%). Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-Type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST.
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| 9. |
- Nordlander, Thomas, 1985-, et al.
(författare)
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3D NLTE Analysis of the Most Iron-Deficient Star, SMSS0313-6708
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 597
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Tidskriftsartikel (refereegranskat)abstract
- Context. Models of star formation in the early universe require a detailed understanding of accretion, fragmentation and radiative feedback in metal-free molecular clouds. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low-mass (0.1-10 Msol), to massive (10-100 Msol), or even supermassive (100-1000 Msol). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova.Aims. The photospheres of metal-poor stars are relatively transparent in the UV, which may lead to large three-dimensional (3D) effects as well as departures from local thermodynamical equilibrium (LTE), even for weak spectral lines. If 3D effects and departures from LTE (NLTE) are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 by means of the most realistic methods available, and compare the results to predicted supernova yields.Methods. A 3D hydrodynamical Stagger model atmosphere and 3D NLTE radiative transfer were applied to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters.Results. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] < -6.53 (3 sigma), than previous estimates based on <3D> NLTE (i.e., using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D-<3D> NLTE differences, all of which lead to a higher abundance determination.Conclusions. We find that supernova yields for models in a wide range of progenitor masses reproduce the revised chemical composition. In addition to massive progenitors of 20-60 Msol exploding with low energies (1-2 B, where 1 B = 10^51 erg), we also find good fits for progenitors of 10 Msol, with very low explosion energies (<1 B). We cannot reconcile the new abundances with supernovae or hypernovae with explosion energies above 2.5 B, nor with pair-instability supernovae.
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| 10. |
- Aguado, D. S., et al.
(författare)
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PISN-explorer : hunting the descendants of very massive first stars
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 520:1, s. 866-878
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Tidskriftsartikel (refereegranskat)abstract
- The very massiv e first stars ( m > 100 M-?) were fundamental to the early phases of reionization, metal enrichment, and supermassive black hole formation. Among them, those with 140 <= m / M-? <= 260 are predicted to evolve as Pair Instability Supernovae (PISN) leaving a unique chemical signature in their chemical yields. Still, despite long searches, the stellar descendants of PISN remain elusive. Here we propose a new methodology, the PISN-explorer, to identify candidates for stars with a dominant PISN enrichment. The PISN-explorer is based on a combination of physically driven models, and the FERRE code; and applied to data from large spectroscopic surv e ys (APOGEE, GALAH, GES, MINCE, and the JINA data base). We looked into more than 1.4 million objects and built a catalogue with 166 candidates of PISN descendants. One of which, 2M13593064 + 3241036, was observed with UVES at VLT and full chemical signature was derived, including the killing elements, Cu and Zn. We find that our proposed methodology is efficient in selecting PISN candidates from both the Milky Way and dwarf satellite galaxies such as Sextans or Draco. Further high-resolution observations are highly required to confirm our best selected candidates, therefore allowing us to probe the existence and properties of the very massive First Stars.
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