| 1. |
- Guiglion, G., et al.
(författare)
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4MOST Survey Strategy Plan
- 2019
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Ingår i: Messenger. - 0722-6691. ; 175, s. 17-21
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The current status of and motivation for the 4MOST survey strategy, as developed by the Consortium science team, are presented here. Key elements of the strategy are described, such as sky coverage, number of visits and total exposure times in different parts of the sky, and how to deal with different observing conditions. The task of organising the strategy is not simple, with many different surveys that have vastly different target brightnesses and densities, sample completeness levels, and signal-to-noise requirements. We introduce here a number of concepts that we will use to ensure all surveys are optimised. Astronomers who are planning to submit a Participating Survey proposal are strongly encouraged to read this article and any relevant 4MOST Survey articles in this issue of The Messenger such that they can optimally complement and benefit from the planned surveys of the 4MOST Consortium.
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| 2. |
- Kushniruk, Iryna, et al.
(författare)
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Disentangling the Arcturus stream
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 631
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Arcturus stream is an over-density of stars in velocity space and its origin has been much debated recently without any clear conclusion. The (classical) dissolved open cluster origin is essentially refuted; instead the discussions try to distinguish between an accretion, a resonant, or an external-perturbation origin for the stream. As kinematic structures are observational footprints of ongoing and past dynamical processes in disc galaxies, resolving the nature of the Arcturus stream may provide clues to the formation history of the Milky Way and its stellar populations. Aims. We aim to characterise the kinematical and chemical properties of the Arcturus stream in order to resolve its origin. Methods. The space velocities, angular momenta, and actions for a sample of more than 5.8 million stars, composed from Gaia DR2 were analysed with a wavelet transform method to characterise kinematic over-densities in the Galactic disc. The kinematic characteristics of each identified group is used to select possible members of the groups from the GALAH and APOGEE spectroscopic surveys to further study and constrain their chemical properties. Results. In the velocity and angular momentum spaces the already known Sirius, Pleiades, Hyades, Hercules, AF06, Arcturus and KFR08 streams are clearly identified. The Hercules stream appears to be a mixture of thin and thick disc stars. The Arcturus stream, as well as the AF06 and KFR08 streams, are high-velocity and low-angular momentum structures with chemical compositions similar to the thick disc. These three groups extend further from the Galactic plane compared to the Hercules stream. The detections of all the groups were spaced by approximately 20 - 30 km s-1 in azimuthal velocity. Conclusions. A wide spread of chemical abundances within the Arcturus stream indicates that the group is not a dissolved open cluster. Instead the Arcturus stream, together with the AF06 and KFR08 streams, are more likely to be part of a phase-space wave, that could have been caused by a merger event. This conclusion is based on that the different structures are detected in steps of 20 - 30 km s-1 in azimuthal velocity, that the kinematic and chemical features are different from what is expected for bar-originated structures, and that the higher-velocity streams extend further from the disc than bar-originated structures.
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| 3. |
- Kushniruk, Iryna, et al.
(författare)
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The HR 1614 moving group is not a dissolving cluster
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 638
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Tidskriftsartikel (refereegranskat)abstract
- Context. The HR 1614 is an overdensity in velocity space and has for a long time been known as an old (2 Gyr) and metal-rich ([Fe=H] +0:2) nearby moving group that has a dissolving open cluster origin. The existence of such old and metal-rich groups in the solar vicinity is quite unexpected since the vast majority of nearby moving groups are known to be young. Aims. In the light of new and significantly larger data sets than ever before (astrometric, photometric, and spectroscopic), we aim to re-investigate the properties and origin of the HR 1614 moving group. If the HR 1614 overdensity is a dissolving cluster, its stars should represent a single-age and single-elemental abundance population. Methods. To identify and characterise the HR 1614 moving group we use astrometric data from Gaia DR2; distances, extinction, and reddening corrections from the StarHorse code; elemental abundances from the GALAH and APOGEE spectroscopic surveys; and photometric metallicities from the SkyMapper survey. Bayesian ages were estimated for the SkyMapper stars. Since the Hercules stream is the closest kinematical structure to the HR 1614 moving group in velocity space and as its origin is believed to be wellunderstood, we use the Hercules stream for comparison purposes. Stars that are likely to be members of the two groups were selected based on their space velocities. Results. The HR 1614 moving group is located mainly at negative U velocities, does not form an arch of constant energy in the U V space, and is tilted in V. We find that the HR 1614 overdensity is not chemically homogeneous, but that its stars exist at a wide range of metallicities, ages, and elemental abundance ratios. They are essentially similar to what is observed in the Galactic thin and thick discs, a younger population (around 3 Gyr) that is metal-rich (0:2 [Fe=H] 0:4) and alpha-poor. These findings are very similar to what is seen for the Hercules stream, which is believed to have a dynamical origin and consists of regular stars from the Galactic discs. Conclusions. The HR 1614 overdensity has a wide spread in metallicity, [Mg/Fe], and age distributions resembling the general properties of the Galactic disc. It should therefore not be considered a dissolving open cluster, or an accreted population. Based on the kinematic and chemical properties of the HR 1614 overdensity we suggest that it has a complex origin that could be explained by combining several different mechanisms such as resonances with the Galactic bar and spiral structure, phase mixing of dissolving spiral structure, and phase mixing due to an external perturbation.
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| 4. |
- Kushniruk, Iryna
(författare)
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The nature of kinematic structures in the Galactic disc
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The velocity distribution of stars in the Galactic disc is complex and consists of a vast number of kinematic structures, that is, stars that share similar velocity components. The reasons why some stars move together may be different and are related to dynamical processes connected to the Galaxy, both internal as well as external ones, such as resonances with the Galactic bar or the Galactic spiral arm structure, dissolution of open clusters, or merger events with other galaxies. Therefore, studies of kinematic structures of the Galactic disc may provide information about the formation and evolution history of the Milky Way. In this thesis we focus on detecting and chemo-dynamical characterisation of kinematic structures of the Galactic disc using the most recent and up-to-date astrometric, spectroscopic, and photometric surveys. In Paper I we studied kinematic structures in the solar neighbourhood with the wavelet transform method. In Paper II and Paper III we studied origin of the Arcturus stream and HR 1614 moving group respectively by studying chemo-dynamical properties of the structures.The result of Paper I is the detection of old and well-known kinematic groups together with three new velocity structures. The results of Paper II and Paper III show that both the Arcturus and the HR 1614 structures are composed of thin and thick disc stars and are not dissolved open clusters or accreted stellar populations. We conclude that the origin of kinematic structures is complex and might be a combination of several dynamical processes such as resonances and phase mixing.
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| 5. |
- Walcher, C.~J., et al.
(författare)
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4MOST Scientific Operations
- 2019
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Ingår i: Messenger. - 0722-6691. ; 175, s. 12-16
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The 4MOST instrument is a multi-object spectrograph that will address Galactic and extragalactic science cases simultaneously by observing targets from a large number of different surveys within each science exposure. This parallel mode of operation and the survey nature of 4MOST require some distinct 4MOST- specific operational features within the overall operations model of ESO. The main feature is that the 4MOST Consortium will deliver, not only the instrument, but also contractual services to the user community, which is why 4MOST is also described as a facility. This white paper concentrates on information particularly useful to answering the forthcoming Call for Letters of Intent.
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| 6. |
- Youakim, Kris, et al.
(författare)
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Tidal debris from Omega Centauri discovered with unsupervised machine learning
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 524:2, s. 2630-2650
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Tidskriftsartikel (refereegranskat)abstract
- The gravitational interactions between the Milky Way and in-falling satellites offer a wealth of information about the formation and evolution of our Galaxy. In this paper, we explore the high-dimensionality of the GALAH DR3 plus Gaia eDR3 data set to identify new tidally stripped candidate stars of the nearby star cluster Omega Centauri (? Cen ). We investigate both the chemical and dynamical parameter space simultaneously, and identify cluster candidates that are spatially separated from the main cluster body, in regions where contamination by halo field stars is high. Most notably, we find candidates for ? Cen scattered in the halo extending to more than 50(?) away from the main body of the cluster. Using a grid of simulated streams generated with ? Cen-like orbital properties, we then compare the on sky distribution of these candidates to the models. The results suggest that if ? Cen had a similar initial mass as its present day mass, then we can place a lower limit on its time of accretion at t(acc) > 7 Gyr ago. Alternatively, if the initial stellar mass was significantly larger, as would be expected if ? Cen is the remnant core of a dwarf Galaxy, then we can constrain the accretion time to t(acc) > 4 Gyr ago. Taken together, these results are consistent with the scenario that ? Cen is the remnant core of a disrupted dwarf galaxy.
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