| 1. |
- Rauer, H., et al.
(författare)
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The PLATO 2.0 mission
- 2014
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 38:1-2, s. 249-330
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Tidskriftsartikel (refereegranskat)abstract
- PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s cadence) providing a wide field-of-view (2232 deg(2)) and a large photometric magnitude range (4-16 mag). It focuses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e. g.: - complete our knowledge of planet diversity for low-mass objects, - correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,- constrain the influence of planet migration and scattering on the architecture of multiple systems, and - specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such a low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmospheres. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science.
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| 2. |
- Su, Zhan, et al.
(författare)
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Common variants at the MHC locus and at chromosome 16q24.1 predispose to Barrett's esophagus.
- 2012
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Ingår i: Nature Genetics. - : Springer Science and Business Media LLC. - 1061-4036 .- 1546-1718. ; 44:10
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Tidskriftsartikel (refereegranskat)abstract
- Barrett's esophagus is an increasingly common disease that is strongly associated with reflux of stomach acid and usually a hiatus hernia, and it strongly predisposes to esophageal adenocarcinoma (EAC), a tumor with a very poor prognosis. We report the first genome-wide association study on Barrett's esophagus, comprising 1,852 UK cases and 5,172 UK controls in the discovery stage and 5,986 cases and 12,825 controls in the replication stage. Variants at two loci were associated with disease risk: chromosome 6p21, rs9257809 (Pcombined=4.09×10(-9); odds ratio (OR)=1.21, 95% confidence interval (CI)=1.13-1.28), within the major histocompatibility complex locus, and chromosome 16q24, rs9936833 (Pcombined=2.74×10(-10); OR=1.14, 95% CI=1.10-1.19), for which the closest protein-coding gene is FOXF1, which is implicated in esophageal development and structure. We found evidence that many common variants of small effect contribute to genetic susceptibility to Barrett's esophagus and that SNP alleles predisposing to obesity also increase risk for Barrett's esophagus.
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| 3. |
- Birney, Ewan, et al.
(författare)
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Identification and analysis of functional elements in 1% of the human genome by the ENCODE pilot project
- 2007
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 447:7146, s. 799-816
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Tidskriftsartikel (refereegranskat)abstract
- We report the generation and analysis of functional data from multiple, diverse experiments performed on a targeted 1% of the human genome as part of the pilot phase of the ENCODE Project. These data have been further integrated and augmented by a number of evolutionary and computational analyses. Together, our results advance the collective knowledge about human genome function in several major areas. First, our studies provide convincing evidence that the genome is pervasively transcribed, such that the majority of its bases can be found in primary transcripts, including non-protein-coding transcripts, and those that extensively overlap one another. Second, systematic examination of transcriptional regulation has yielded new understanding about transcription start sites, including their relationship to specific regulatory sequences and features of chromatin accessibility and histone modification. Third, a more sophisticated view of chromatin structure has emerged, including its inter-relationship with DNA replication and transcriptional regulation. Finally, integration of these new sources of information, in particular with respect to mammalian evolution based on inter- and intra-species sequence comparisons, has yielded new mechanistic and evolutionary insights concerning the functional landscape of the human genome. Together, these studies are defining a path for pursuit of a more comprehensive characterization of human genome function.
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| 4. |
- 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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| 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. |
- 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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| 7. |
- Askar, Abbas, et al.
(författare)
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Formation of supermassive black holes in galactic nuclei – I. Delivering seed intermediate-mass black holes in massive stellar clusters
- 2021
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 502:2, s. 2682-2700
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Tidskriftsartikel (refereegranskat)abstract
- Supermassive black holes (SMBHs) are found in most galactic nuclei. A significant fraction of these nuclei also contains a nuclear stellar cluster (NSC) surrounding the SMBH. In this paper, we consider the idea that the NSC forms first, from the merger of several stellar clusters that may contain intermediate-mass black holes (IMBHs). These IMBHs can subsequently grow in the NSC and form an SMBH. We carry out N-body simulations of the simultaneous merger of three stellar clusters to form an NSC, and investigate the outcome of simulated runs containing zero, one, two, and three IMBHs. We find that IMBHs can efficiently sink to the centre of the merged cluster. If multiple merging clusters contain an IMBH, we find that an IMBH binary is likely to form and subsequently merge by gravitational wave emission. We show that these mergers are catalyzed by dynamical interactions with surrounding stars, which systematically harden the binary and increase its orbital eccentricity. The seed SMBH will be ejected from the NSC by the recoil kick produced when two IMBHs merge, if their mass ratio q ≳ 0.15. If the seed is ejected then no SMBH will form in the NSC. This is a natural pathway to explain those galactic nuclei that contain an NSC but apparently lack an SMBH, such as M33. However, if an IMBH is retained then it can seed the growth of an SMBH through gas accretion and tidal disruption of stars.
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| 8. |
- Bobrick, Alexey, et al.
(författare)
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Mass transfer in white dwarf-neutron star binaries
- 2017
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 467:3, s. 3556-3575
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Tidskriftsartikel (refereegranskat)abstract
- We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of M-WD, (crit) = 0.2 M-circle dot undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs.
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| 9. |
- Christleib, Norbert, et al.
(författare)
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4MOST Consortium Survey 2: The Milky Way Halo High-Resolution Survey
- 2019
-
Ingår i: Messenger. - 0722-6691. ; 175, s. 26-29
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We will study the formation history of the Milky Way, and the earliest phases of its chemical enrichment, with a sample of more than 1.5 million stars at high galactic latitude. Elemental abundances of up to 20 elements with a precision of better than 0.2 dex will be derived for these stars. The sample will include members of kinematically coherent substructures, which we will associate with their possible birthplaces by means of their abundance signatures and kinematics, allowing us to test models of galaxy formation. Our target catalogue is also expected to contain 30 000 stars at a metallicity of less than one hundredth that of the Sun. This sample will therefore be almost a factor of 100 larger than currently existing samples of metal-poor stars for which precise elemental abundances are available (determined from high-resolution spectroscopy), enabling us to study the early chemical evolution of the Milky Way in unprecedented detail.
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| 10. |
- Church, Ross P., et al.
(författare)
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Detailed models of the binary pulsars J1141-6545 and B2303+46
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:2, s. 715-727
-
Tidskriftsartikel (refereegranskat)abstract
- We have modelled the formation of the eccentric double-degenerate binaries J1141-6545 and B2303+46 using the Henyey-type full stellar evolution code STARS and the population synthesis code BSE. We find that the outcome depends strongly on the common envelope (CE) evolution efficiency parameter alpha(CE) and show that both systems can be modelled with a single value of alpha(CE). The final orbit of the system depends critically on the order of Roche lobe filling events. The phase space of progenitors and the different evolutionary pathways followed by binary stars that form eccentric double-degenerate binaries in the two codes are compared. We show that the pathways are similar between the codes and that the distribution of progenitors in mass and separation phase space is qualitatively the same, thus validating the use of BSE-like population synthesis for simulations of this type. The phase space of initial parameters is very different to that obtained using ad hoc arguments about the evolution, which shows that such arguments are insufficient to model evolutionary pathways of this complexity. There are some differences associated with the prescription adopted for CE evolution but these are not qualitatively significant. We investigate the dependence of the formation mechanism on wind mass loss and the CE efficiency parameter alpha(CE), showing that it depends strongly on the latter but rather less on the former.
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| 11. |
- Church, Ross P., et al.
(författare)
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Formation Constraints Indicate a Black Hole Accretor in 47 Tuc X9
- 2017
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 851:1
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Tidskriftsartikel (refereegranskat)abstract
- The luminous X-ray binary 47 Tuc X9 shows radio and X-ray emission consistent with a stellar-mass black hole (BH) accreting from a carbon-oxygen white dwarf. Its location, in the core of the massive globular cluster 47 Tuc, hints at a dynamical origin. We assess the stability of mass transfer from a carbon-oxygen white dwarf onto compact objects of various masses, and conclude that for mass transfer to proceed stably, the accretor must, in fact, be a BH. Such systems can form dynamically by the collision of a stellar-mass BH with a giant star. Tidal dissipation of energy in the giant's envelope leads to a bound binary with a pericenter separation less than the radius of the giant. An episode of common-envelope evolution follows, which ejects the giant's envelope. We find that the most likely target is a horizontal-branch star, and that a realistic quantity of subsequent dynamical hardening is required for the resulting binary to merge via gravitational wave emission. Observing one binary like 47 Tuc X9 in the Milky Way globular cluster system is consistent with the expected formation rate. The observed 6.8-day periodicity in the X-ray emission may be driven by eccentricity induced in the ultra-compact X-ray binary's orbit by a perturbing companion.
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| 12. |
- Church, Ross P., et al.
(författare)
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Mass transfer in eccentric binaries: the new oil-on-water smoothed particle hydrodynamics technique
- 2009
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 395:2, s. 1127-1134
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Tidskriftsartikel (refereegranskat)abstract
- To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a 0.6 M-circle dot donor star and a 1 M-circle dot white dwarf; such binaries are thought to form cataclysmic variable ( CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of q = 0.6 and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity.
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| 13. |
- Church, Ross P., et al.
(författare)
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Super-Earth ingestion can explain the anomalously high metal abundances of M67 Y2235
- 2020
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 491:2, s. 2391-2402
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the hypothesis that ingestion of a terrestrial or super-Earth planet could cause the anomalously high metal abundances seen in a turn-off star in the open cluster M67, when compared to other turn-off stars in the same cluster. We show that the mass of the convective envelope of the star is likely only 3.45 × 10-3 M⊙, and hence 5.2 M⊙ of rock is required to obtain the observed 0.128 dex metal enhancement. Rocky planets dissolve entirely in the convective envelope if they enter it with sufficiently tangential orbits: we find that the critical condition for dissolution is that the planet's radial velocity must be less than 40 per cent of its total velocity at the stellar surface; or, equivalently, the impact parameter must be greater than about 0.9. We model the delivery of rocky planets to the stellar surface both by planet-planet scattering in a realistic multiplanet system, and by Lidov-Kozai cycles driven by a more massive planetary or stellar companion. In both cases almost all planets that are ingested arrive at the star on grazing orbits and hence will dissolve in the surface convection zone. We conclude that super-Earth ingestion is a good explanation for the metal enhancement in M67 Y2235, and that a high-resolution spectroscopic survey of stellar abundances around the turn-off and main sequence of M67 has the potential to constrain the frequency of late-time dynamical instability in planetary systems.
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| 14. |
- Dale, James E., et al.
(författare)
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Red giant stellar collisions in the Galactic Centre
- 2009
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 393:3, s. 1016-1033
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Tidskriftsartikel (refereegranskat)abstract
- We show that collisions with stellar-mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al. We show that the missing objects are low-mass giants and asymptotic giant branch stars in the range 1-3 M-circle dot. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of smoothed particle hydrodynamic calculations, restricted three-body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main-sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the 10.5 < K < 12 band. However, depleting the brighter (K < 10.5) objects out to the required radius would require a large population of black hole impactors which would in turn deplete the 10.5 < K < 12 giants in a region much larger than is observed. We conclude that collisions with stellar-mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar-mass black holes in the Galactic Centre.
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| 15. |
- Davies, Melvyn B., et al.
(författare)
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The ecology of the galactic centre : Nuclear stellar clusters and supermassive black holes
- 2019
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213. ; 14:351, s. 80-83
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Tidskriftsartikel (refereegranskat)abstract
- Supermassive black holes are found in most galactic nuclei. A large fraction of these nuclei also contain a nuclear stellar cluster surrounding the black hole. Here we consider the idea that the nuclear stellar cluster formed first and that the supermassive black hole grew later. In particular we consider the merger of three stellar clusters to form a nuclear stellar cluster, where some of these clusters contain a single intermediate-mass black hole (IMBH). In the cases where multiple clusters contain IMBHs, we discuss whether the black holes are likely to merge and whether such mergers are likely to result in the ejection of the merged black hole from the nuclear stellar cluster. In some cases, no supermassive black hole will form as any merger product is not retained. This is a natural pathway to explain those galactic nuclei that contain a nuclear stellar cluster but apparently lack a supermassive black hole; M33 being a nearby example. Alternatively, if an IMBH merger product is retained within the nuclear stellar cluster, it may subsequently grow, e.g. via the tidal disruption of stars, to form a supermassive black hole.
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| 16. |
- Dermine, Tyl, et al.
(författare)
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Barium stars revisited
- 2010
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Ingår i: International Conference on Binaries - In Celebration of Ron Webbink's 65th Birthday. - : AIP. - 9780735408692 ; 1314, s. 61-62
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Konferensbidrag (refereegranskat)abstract
- The barium overabundances observed at the surface of barium stars originate from accretion long ago from the wind of a companion AGB star. However, this scenario cannot explain their high eccentricities and short orbital periods that have remained a mystery for decades. We investigate how kicks imparted to the white-dwarf at its birth or the presence of a circumbinary disk can solve this problem.
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| 17. |
- Frantz, Laurent A. F., et al.
(författare)
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Ancient pigs reveal a near-complete genomic turnover following their introduction to Europe
- 2019
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 116:35, s. 17231-17238
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Tidskriftsartikel (refereegranskat)abstract
- Archaeological evidence indicates that pig domestication had begun by similar to 10,500 y before the present ( BP) in the Near East, and mitochondrial DNA ( mtDNA) suggests that pigs arrived in Europe alongside farmers similar to 8,500 y BP. A few thousand years after the introduction of Near Eastern pigs into Europe, however, their characteristic mtDNA signature disappeared and was replaced by haplotypes associated with European wild boars. This turnover could be accounted for by substantial gene flow from local European wild boars, although it is also possible that European wild boars were domesticated independently without any genetic contribution from the Near East. To test these hypotheses, we obtained mtDNA sequences from 2,099 modern and ancient pig samples and 63 nuclear ancient genomes from Near Eastern and European pigs. Our analyses revealed that European domestic pigs dating from 7,100 to 6,000 y BP possessed both Near Eastern and European nuclear ancestry, while later pigs possessed no more than 4% Near Eastern ancestry, indicating that gene flow from European wild boars resulted in a near-complete disappearance of Near East ancestry. In addition, we demonstrate that a variant at a locus encoding black coat color likely originated in the Near East and persisted in European pigs. Altogether, our results indicate that while pigs were not independently domesticated in Europe, the vast majority of human-mediated selection over the past 5,000 y focused on the genomic fraction derived from the European wild boars, and not on the fraction that was selected by early Neolithic farmers over the first 2,500 y of the domestication process.
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| 18. |
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| 19. |
- Gustafsson, Bengt, et al.
(författare)
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Gravitational scattering of stars and clusters and the heating of the Galactic disk
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 593
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Tidskriftsartikel (refereegranskat)abstract
- Context. Could the velocity spread, increasing with time, in the Galactic disk be explained as a result of gravitational interactions of stars with giant molecular clouds (GMCs) and spiral arms? Do the old open clusters high above the Galactic plane provide clues to this question? Aims. We explore the effects on stellar orbits of scattering by inhomogeneities in the Galactic potential due to GMCs, spiral arms and the Galactic bar, and whether high-altitude clusters could have formed in orbits closer to the Galactic plane and later been scattered. Methods. Simulations of test-particle motions are performed in a realistic Galactic potential. The effects of the internal structure of GMCs are explored. The destruction of clusters in GMC collisions is treated in detail with N-body simulations of the clusters. Results. The observed velocity dispersions of stars as a function of time are well reproduced. The GMC structure is found to be significant, but adequate models produce considerable scattering effects. The fraction of simulated massive old open clusters, scattered into orbits with vertical bar z vertical bar > 400 pc, is typically 0.5%, in agreement with the observed number of high-altitude clusters and consistent with the present formation rate of massive open clusters. Conclusions. The heating of the thin Galactic disk is well explained by gravitational scattering by GMCs and spiral arms, if the local correlation between the GMC mass and the corresponding voids in the gas is not very strong. Our results suggest that the high-altitude metal-rich clusters were formed in orbits close to the Galactic plane and later scattered to higher orbits. It is possible, though not very probable, that the Sun formed in such a cluster before scattering occurred.
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| 20. |
- Henkel, Kate, et al.
(författare)
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Thermohaline Mixing in Extremely Metal-poor Stars
- 2018
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 863:1
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Tidskriftsartikel (refereegranskat)abstract
- Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well studied, they make up less than half of all EMP stars with [Fe/H] ∼ -3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coefficient. Our stellar evolution models utilize a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] = -3 and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.
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| 21. |
- Howes, Louise M., et al.
(författare)
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Estimating stellar ages and metallicities from parallaxes and broadband photometry : Successes and shortcomings
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 622
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Tidskriftsartikel (refereegranskat)abstract
- A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information - a prospect made real for more than a billion stars with the second Gaia data release - deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star's age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star's age and metallicity for certain evolutionary stages. It is possible to find well-constrained ages and metallicities for turn-off and sub-giant stars with colours including a U band and a parallax with uncertainty less than ∼20%. Metallicities alone are possible for the main sequence and giant branch. We find good agreement with the literature when we apply our method to the Gaia benchmark stars, particularly for turn-off and young stars. Further tests on the old open cluster NGC 188, however, reveal significant limitations in the stellar isochrones. The ages derived for the cluster stars vary with evolutionary stage, such that turn-off ages disagree with those on the sub-giant branch, and metallicities vary significantly throughout. Furthermore, the parameters vary appreciably depending on which colour combinations are used in the derivation. We identify the causes of these mismatches and show that improvements are needed in the modelling of giant branch stars and in the creation and calibration of synthetic near-UV photometry. Our results warn against applying isochrone fitting indiscriminately. In particular, the uncertainty on the stellar models should be quantitatively taken into account. Further efforts to improve the models will result in significant advancements in our ability to study the Galaxy.
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| 22. |
- Jørgensen, Timmi G., et al.
(författare)
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Stellar escapers from M67 can reach solar-like Galactic orbits
- 2020
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 492:4, s. 4959-4974
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the possibility that the Sun could have been born in M67 by carrying out N-body simulations of M67-like clusters in a time-varying Galactic environment, and following the Galactic orbits of stars that escape from them. We find that model clusters that occupy similar orbits to M67 today can be divided up into three groups, which we call hot, depleted, and scattered clusters. Hot clusters are born with a high initial z-velocity; depleted clusters are born on dynamically cold orbits but are destroyed by GMC encounters in the Galactic disc; and scattered clusters are born on dynamically cold orbits and survive to have more than 1000 stars at an age of 4.6 Gyr. We find that all cluster models in all three cluster groups have stellar escapers that are kinematically similar to the Sun. Hot clusters have the lowest fraction of escapers with solar-like kinematics, f⊙ = 0.06 per cent, whilst depleted clusters have the highest fraction, f⊙ = 6.61 per cent. We calculate that clusters that are destroyed in the Galactic disc have a specific frequency of escapers that end up on solar-like orbits that is ∼ 2 times that of escapers from clusters that survive their journey.
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| 23. |
- Korol, Valeriya, et al.
(författare)
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Merger rates in primordial black hole clusters without initial binaries
- 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. 994-1000
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Tidskriftsartikel (refereegranskat)abstract
- Primordial black holes formed through the collapse of cosmological density fluctuations have been hypothesized as contributors to the dark matter content of the Universe. At the same time, their mergers could contribute to the recently observed population of gravitational-wave sources. We investigate the scenario in which primordial black holes form binaries at late times in the Universe. Specifically, we re-examine the mergers of primordial black holes in small clusters of ∼30 objects in the absence of initial binaries. Binaries form dynamically through Newtonian gravitational interactions. These binaries act as heat sources for the cluster, increasing the cluster's velocity dispersion, which inhibits direct mergers through gravitational-wave two-body captures. Meanwhile, three-body encounters of tight binaries are too rare to tighten binaries sufficiently to allow them to merge through gravitational-wave emission. We conclude that in the absence of initial binaries, merger rates of primordial black holes in the considered scenario are at least an order of magnitude lower than previously suggested, which makes gravitational-wave detections of such sources improbable.
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| 24. |
- Malmberg, Daniel, et al.
(författare)
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Close encounters in young stellar clusters: implications for planetary systems in the solar neighbourhood
- 2007
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 378:3, s. 1207-1216
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Tidskriftsartikel (refereegranskat)abstract
- The stars that populate the solar neighbourhood were formed in stellar clusters. Through N-body simulations of these clusters, we measure the rate of close encounters between stars. By monitoring the interaction histories of each star, we investigate the singleton fraction in the solar neighbourhood. A singleton is a star which formed as a single star, has never experienced any close encounters with other stars or binaries, or undergone an exchange encounter with a binary. We find that, of the stars which formed as single stars, a significant fraction is not singletons once the clusters have dispersed. If some of these stars had planetary systems, with properties similar to those of the Solar System, the planets' orbits may have been perturbed by the effects of close encounters with other stars or the effects of a companion star within a binary. Such perturbations can lead to strong planet-planet interactions which eject several planets, leaving the remaining planets on eccentric orbits. Some of the single stars exchange into binaries. Most of these binaries are broken up via subsequent interactions within the cluster, but some remain intact beyond the lifetime of the cluster. The properties of these binaries are similar to those of the observed binary systems containing extrasolar planets. Thus, dynamical processes in young stellar clusters will alter significantly any population of Solar System-like planetary systems. In addition, beginning with a population of planetary systems exactly resembling the Solar System around single stars, dynamical encounters in young stellar clusters may produce at least some of the extrasolar planetary systems observed in the solar neighbourhood.
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| 25. |
- Malmberg, Daniel, et al.
(författare)
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The Singleton Fraction of Stars Formed in Stellar Clusters
- 2008
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Ingår i: Extreme Solar Systems (Astronomical Society of the Pacific Conference Series). - 9781583816660 - 9781583816677 ; 398, s. 75-76
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Konferensbidrag (refereegranskat)abstract
- Most stars form in some sort of stellar cluster or association. In such clusters the number density of stars can be very high. Thus, single stars in such clusters may undergo frequent close encounters with other stars and exchange encounters with binary systems. The perturbation caused by the other star in a close encounter or by the companion star in a binary can have significant effects on the evolution of any planetary system around the initially single star. If the planetary system which formed around the single star was originally solar-system-like, planet-planet interactions, induced by the perturbation from other stars, may change it significantly and leave it more like some of the planetary systems which are observed around other stars. Only if the host star of an initially solar-system-like planetary system never undergoes any encounters with other stars and is never exchanged into a binary may the planetary system remain solar-system like. We define such a star to be a SINGLETON.
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| 26. |
- Mastrobuono-Battisti, Alessandra, et al.
(författare)
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Close stellar encounters at the Galactic Centre - I. The effect on the observed stellar populations
- 2021
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 505:3, s. 3314-3328
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Tidskriftsartikel (refereegranskat)abstract
- We model the effects of collisions and close encounters on the stellar populations observed in the Milky Way nuclearstellar cluster (NSC). Our analysis is based on N -body simulations in which the NSC forms by accretion of massivestellar clusters around a supermassive black hole. We attach stellar populations to our N -body particles and follow theevolution of their stars, and the rate of collisions and close encounters. The most common encounters are collisionsbetween pairs of main-sequence stars, which lead to mergers: destructive collisions between main-sequence starsand compact objects are rare. We find that the effects of collisions on the stellar populations are small for threereasons. First, our models possess a core which limits the maximum stellar density. Secondly, the velocity dispersionin the NSC is similar to the surface escape velocities of the stars, which minimises the collision rate. Finally, whilstcollisions between main-sequence stars destroy bright giants by accelerating their evolution, they also create themby accelerating the evolution of lower-mass stars. These two effects approximately cancel out. We also investigatewhether the G2 cloud could be a fuzzball: a compact stellar core which has accreted a tenuous envelope in a closeencounter with a red giant. We conclude that fuzzballs with cores below 2 M have thermal times-scales too shortto reproduce G2. A fuzzball with a black-hole core could reproduce the surface properties of G2 but the productionrate of such objects in our model is low.
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| 27. |
- Nicholson, Rhana B, et al.
(författare)
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Rapid destruction of protoplanetary discs due to external photoevaporation in star-forming regions
- 2019
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 485, s. 4893-4905
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Tidskriftsartikel (refereegranskat)abstract
- We analyse N-body simulations of star-forming regions to investigate the effects of externalfar- and extreme-ultraviolet photoevaporation from massive stars on protoplanetary discs. Byvarying the initial conditions of simulated star-forming regions, such as the spatial distribution,net bulk motion (virial ratio), and density, we investigate which parameters most affect the rateat which discs are dispersed due to external photoevaporation. We find that disc dispersal due toexternal photoevaporation is faster in highly substructured star-forming regions than in smoothand centrally concentrated regions. Subvirial star-forming regions undergoing collapse alsoshow higher rates of disc dispersal than regions that are in virial equilibrium or are expanding.In moderately dense (∼100 M pc −3 ) regions, half of all protoplanetary discs with radii≥100 au are photoevaporated within 1 Myr, three times faster than is currently suggested byobservational studies. Discs in lower density star-forming regions (∼10 M pc −3 ) survive forlonger, but half are still dispersed on short time-scales (∼2 Myr). This demonstrates that theinitial conditions of the star-forming regions will greatly impact the evolution and lifetime ofprotoplanetary discs. These results also imply that either gas giant planet formation is extremelyrapid and occurs before the gas component of discs is evaporated, or gas giants only formin low-density star-forming regions where no massive stars are present to photoevaporate gasfrom protoplanetary discs.
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| 28. |
- Sahlholdt, Christian L., et al.
(författare)
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Benchmark ages for the Gaia benchmark stars
- 2019
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 482:1, s. 895-920
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Tidskriftsartikel (refereegranskat)abstract
- In the era of large-scale surveys of stars in the MilkyWay, stellar ages are crucial for studying the evolution of the Galaxy.But determining ages of field stars is notoriously difficult; therefore, we attempt to determine benchmark ages for the extensively studied Gaia benchmark stars which can be used for validation purposes. By searching the literature for age estimates from different methods and deriving new ages based on Bayesian isochrone fitting, we are able to put reliable limits on the ages of 16 out of the 33 benchmark stars. The giants with well-defined ages are all young, and an expansion of the sample to include older giants with asteroseismic ages would be beneficial. Some of the stars have surface parameters inconsistent with isochrones younger than 16 Gyr. Including a-enhancement in the models when relevant resolves some of these cases, but others clearly highlight discrepancies between the models and observations. We test the impact of atomic diffusion on the age estimates by fitting to the actual surface metallicity of the models instead of the initial value and find that the effect is negligible except for a single turn-off star. Finally, we show that our ability to determine isochrone-based ages for large spectroscopic surveys largely mirrors our ability to determine ages for these benchmark stars, except for stars with log g ≳ 4.4 dex since their location in the Hertzsprung-Russell diagram is almost age insensitive. Hence, isochrone fitting does not constrain their ages given the typical uncertainties of spectroscopic stellar parameters.
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| 29. |
- Storck, Anatole, et al.
(författare)
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The effects of Galactic model uncertainties on LISA observations of double neutron stars
- 2023
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 521:2, s. 2368-2377
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Tidskriftsartikel (refereegranskat)abstract
- Observations of binaries containing pairs of neutron stars using the upcoming space-based gravitational wave observatory, LISA, have the potential to improve our understanding of neutron star physics and binary evolution. In this work, we assess the effect of changing the model of the Milky Way’s kinematics and star formation history on predictions of the population of double neutron stars that will be detected and resolved by LISA. We conclude that the spatial distribution of these binaries is insensitive to the choice of Galactic models, compared to the stochastic variation induced by the small sample size. In particular, the time-consuming computation of the binaries’ Galactic orbits is not necessary. The distributions of eccentricity and gravitational-wave frequency are, however, affected by the choice of star formation history. Binaries with eccentricities e > 0.1, which can be measured by LISA observations, are mostly younger than 100 Myr. We caution that comparisons between different predictions for LISA observations need to use consistent star formation histories, and that the Galactic star formation history should be taken into account in the analysis of the observations themselves. The lack of strong dependence on Galactic models means that LISA detection of double neutron star binaries may provide a relatively clean probe of massive binary star evolution.
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| 30. |
- Svensson, Karl M., et al.
(författare)
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The nature of hypervelocity stars as inferred from their Galactic trajectories
- 2008
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 1745-3925 .- 1745-3933. ; 383:1, s. 15-19
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Tidskriftsartikel (refereegranskat)abstract
- We have computed the Galactic trajectories of 12 hypervelocity stars (HVSs) under the assumption that they originated in the Galactic Centre. We show that eight of these 12 stars are bound to the Galaxy. We consider the subsequent trajectories of the bound stars to compute their characteristic orbital period, which is 2 Gyr. All eight bound stars are moving away from the centre of the Galaxy, which implies that the stars' lifetimes are less than 2 Gyr. We thus infer that the observed HVSs are massive main-sequence stars, rather than blue horizontal branch stars. The observations suggest that blue HVSs are ejected from the Galactic Centre roughly every 15 Myr. This is consistent with the observed population of blue stars in extremely tight orbits round the central supermassive black hole (SMBH), the so-called S-stars, if we assume that the HVSs are produced by the breakup of binaries. One of the stars in such a binary is ejected at high velocities to form a HVS; the other remains bound to the SMBH as an S-star. We further show that the one high-velocity system observed to be moving towards the Galactic Centre, SDSS J172226.55+594155.9, could not have originated in the Galactic Centre; rather, we identify it as a halo object.
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