| 26. |
- Beer, Martin E, et al.
(författare)
-
Red giant depletion in globular cluster cores
- 2004
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 348:2, s. 679-686
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the observed depletion of red giants in the cores ofpost-core-collapse globular clusters. In particular, the evolutionaryscenario we consider is a binary consisting of two low-mass stars whichundergoes two common-envelope phases. The first common-envelope phaseoccurs when the primary is a red giant resulting in a helium white dwarfand main-sequence star in a detached binary. The second common-envelopephase occurs shortly after the secondary becomes a red giant. During thesecond common-envelope phase, the degenerate helium cores mergeresulting in a core mass greater than the helium burning limit and theformation of a horizontal branch star. We show that stellar encountersenhance this evolutionary route in post-core-collapse clusters. Theseencounters increase the population of binary secondaries which wouldhave evolved on to the red giant branch in the recent past.
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| 27. |
- Bobrick, Alexey, et al.
(författare)
-
Mass transfer in compact binaries
- 2012
-
Ingår i: Mass transfer in compact binaries.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 28. |
- Bobrick, Alexey, et al.
(författare)
-
Mass transfer in white dwarf-neutron star binaries
- 2017
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 467:3, s. 3556-3575
-
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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| 29. |
|
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| 30. |
- Bobrick, Alexey, et al.
(författare)
-
Transients from ONe white dwarf - neutron star/black hole mergers
- 2022
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 510:3, s. 3758-3777
-
Tidskriftsartikel (refereegranskat)abstract
- We conduct the first 3D hydrodynamic simulations of oxygen-neon white dwarf-neutron star/black hole mergers (ONe WD-NS/BH mergers). Such mergers constitute a significant fraction, and may even dominate, the inspiral rates of all WD-NS binaries. We post-process our simulations to obtain the nuclear evolution of these systems and couple the results to a supernova spectral synthesis code to obtain the first light curves and spectra for these transients. We find that the amount of 56Ni synthesized in these mergers grows as a strong function of the WD mass, reaching typically 0.05 and up to 0.1M⊙ per merger. Photodisintegration leads to similar amounts of 4He and about a ten times smaller amount of 1H. The nuclear yields from these mergers, in particular those of 55Mn, may contribute significantly to Galactic chemical evolution. The transients expected from ONe WD-NS mergers are dominantly red/infrared, evolve on month-long time-scales and reach bolometric magnitudes of up to -16.5. The current surveys must have already detected these transients or are, alternatively, putting strong constraints on merger scenarios. The properties of the expected transients from WD-NS mergers best agree with faint type Iax supernovae. The Vera Rubin Observatory (LSST) will be detecting up to thousands of merging ONe WD-NS systems per year. We simulate a subset of our models with 2D axisymmetric FLASH code to investigate why they have been challenging for previous studies. We find that the likely main challenge has been effectively modelling the nuclear statistical equilibrium regime in such mergers.
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| 31. |
- Bortolas, Elisa, et al.
(författare)
-
Global torques and stochasticity as the drivers of massive black hole pairing in the young Universe
- 2020
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 498:3, s. 3601-3615
-
Tidskriftsartikel (refereegranskat)abstract
- The forthcoming Laser Interferometer Space Antenna (LISA) will probe the population of coalescing massive black hole (MBH) binaries up to the onset of structure formation. Here, we simulate the galactic-scale pairing of ∼106 M☉ MBHs in a typical, non-clumpy main-sequence galaxy embedded in a cosmological environment at z = 7-6. In order to increase our statistical sample, we adopt a strategy that allows us to follow the evolution of six secondary MBHs concomitantly. We find that the magnitude of the dynamical-friction-induced torques is significantly smaller than that of the large-scale, stochastic gravitational torques arising from the perturbed and morphologically evolving galactic disc, suggesting that the standard dynamical friction treatment is inadequate for realistic galaxies at high redshift. The dynamical evolution of MBHs is very stochastic, and a variation in the initial orbital phase can lead to a drastically different time-scale for the inspiral. Most remarkably, the development of a galactic bar in the host system either significantly accelerates the inspiral by dragging a secondary MBH into the centre, or ultimately hinders the orbital decay by scattering the MBH in the galaxy outskirts. The latter occurs more rarely, suggesting that galactic bars overall promote MBH inspiral and binary coalescence. The orbital decay time can be an order of magnitude shorter than what would be predicted relying on dynamical friction alone. The stochasticity and the important role of global torques have crucial implications for the rates of MBH coalescences in the early Universe: both have to be accounted for when making predictions for the upcoming LISA observatory.
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| 32. |
- Carrera, Daniel, et al.
(författare)
-
How to form planetesimals from mm-sized chondrules and chondrule aggregates
- 2015
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 579
-
Tidskriftsartikel (refereegranskat)abstract
- The size distribution of asteroids and Kuiper belt objects in the solar system is difficult to reconcile with a bottom-up formation scenario due to the observed scarcity of objects smaller than similar to 100 km in size. Instead, planetesimals appear to form top-down, with large 100 1000 km bodies forming from the rapid gravitational collapse of dense clumps of small solid particles. In this paper we investigate the conditions under which solid particles can form dense clumps in a protoplanetary disk. We used a hydrodynamic code to model the interaction between solid particles and the gas inside a shearing box inside the disk, considering particle sizes from submillimeter-sized chondrules to meter-sized rocks. We found that particles down to millimeter sizes can form dense particle clouds through the run-away convergence of radial drift known as the streaming instability. We made a map of the range of conditions (strength of turbulence, particle mass-loading, disk mass, and distance to the star) that are prone to producing dense particle clumps. Finally, we estimate the distribution of collision speeds between mm-sized particles. We calculated the rate of sticking collisions and obtain a robust upper limit on the particle growth timescale of similar to 10(5) years. This means that mm-sized chondrule aggregates can grow on a timescale much smaller than the disk accretion timescale (similar to 10(6)-10(7) years). Our results suggest a pathway from the mm-sized grains found in primitive meteorites to fully formed asteroids. We speculate that asteroids may form from a positive feedback loop in which coagualation leads to particle clumping driven by the streaming instability. This clumping, in turn, reduces collision speeds and enhances coagulation. Future simulations should model coagulation and the streaming instability together to explore this feedback loop further.
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| 33. |
- Carrera, Daniel, et al.
(författare)
-
Planet-planet scattering as the source of the highest eccentricity exoplanets
- 2019
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 629
-
Tidskriftsartikel (refereegranskat)abstract
- Most giant exoplanets discovered by radial velocity surveys have much higher eccentricities than those in the solar system. The planet-planet scattering mechanism has been shown to match the broad eccentricity distribution, but the highest-eccentricity planets are often attributed to Kozai-Lidov oscillations induced by a stellar companion. Here we investigate whether the highly eccentric exoplanet population can be produced entirely by scattering. We ran 500 N-body simulations of closely packed giant-planet systems that became unstable under their own mutual perturbations. We find that the surviving bound planets can have eccentricities up to e > 0.99, with a maximum of 0.999017 in our simulations. This suggests that there is no maximum eccentricity that can be produced by planet-planet scattering. Importantly, we find that extreme eccentricities are not extremely rare; the eccentricity distribution for all giant exoplanets with e > 0.3 is consistent with all planets concerned being generated by scattering. Our results show that the discovery of planets with extremely high eccentricities does not necessarily signal the action of the Kozai-Lidov mechanism.
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| 34. |
- Carrera, Daniel, et al.
(författare)
-
Planetesimal Formation by the Streaming Instability in a Photoevaporating Disk
- 2017
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 839:1
-
Tidskriftsartikel (refereegranskat)abstract
- Recent years have seen growing interest in the streaming instability as a candidate mechanism to produce planetesimals. However, these investigations have been limited to small-scale simulations. We now present the results of a global protoplanetary disk evolution model that incorporates planetesimal formation by the streaming instability, along with viscous accretion, photoevaporation by EUV, FUV, and X-ray photons, dust evolution, the water ice line, and stratified turbulence. Our simulations produce massive (60-130 M ⊕) planetesimal belts beyond 100 au and up to ∼20 M ⊕ of planetesimals in the middle regions (3-100 au). Our most comprehensive model forms 8 M ⊕ of planetesimals inside 3 au, where they can give rise to terrestrial planets. The planetesimal mass formed in the inner disk depends critically on the timing of the formation of an inner cavity in the disk by high-energy photons. Our results show that the combination of photoevaporation and the streaming instability are efficient at converting the solid component of protoplanetary disks into planetesimals. Our model, however, does not form enough early planetesimals in the inner and middle regions of the disk to give rise to giant planets and super-Earths with gaseous envelopes. Additional processes such as particle pileups and mass loss driven by MHD winds may be needed to drive the formation of early planetesimal generations in the planet-forming regions of protoplanetary disks.
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| 35. |
- Carrera, Daniel, et al.
(författare)
-
Survival of habitable planets in unstable planetary systems
- 2016
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; , s. 3226-3238
-
Tidskriftsartikel (refereegranskat)abstract
- Many observed giant planets lie on eccentric orbits. Such orbits could be the result of strong scatterings with other giant planets. The same dynamical instability that produces giant planet scatterings can also alter the orbits of terrestrial planets. For example, a habitable rocky planet in the system can be ejected or transported to an orbit outside the habitable zone. Therefore, there is a link between observed giant planets and the habitability of smaller planets in the system. We say that a habitable planet has resilient habitability if it is able to avoid ejections and collisions and its orbit remains inside the habitable zone. Here we model the orbital evolution of rocky planets in planetary systems where giant planets become dynamically unstable. We measure the resilience of habitable planets as a function of the observed, present-day masses and orbits of the giant planets. We find that the survival rate of habitable planets depends strongly on the giant planet architecture. Systems with three Jupiters (3J) are far more destructive than systems with four giant planets of unequal masses (4G), akin to the architecture of the solar system. In terms of observables, we find that giant planets with eccentricity above 0.4 originate only in 3J systems and very rarely have habitable interior planets. For a giant planet with an present-day eccentricity of 0.2 and semimajor axis of 5 AU orbiting a Sun-like star, 50% of the orbits in the habitable zone are resilient to the instability. As semimajor axis increases and eccentricity decreases, a higher fraction of habitable planets survive and remain habitable. However, if the habitable planet has rocky siblings, there is a significant risk of rocky planet collisions that would sterilize the planet.
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| 36. |
- Carrera, Daniel, et al.
(författare)
-
Towards an initial mass function for giant planets
- 2018
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711. ; 478:1, s. 961-970
-
Tidskriftsartikel (refereegranskat)abstract
- The distribution of exoplanet masses is not primordial. After the initial stage of planet formation, gravitational interactions between planets can lead to the physical collision of two planets, or the ejection of one or more planets from the system. When this occurs, the remaining planets are typically left in more eccentric orbits. In this report we demonstrate how the present-day eccentricities of the observed exoplanet population can be used to reconstruct the initial mass function of exoplanets before the onset of dynamical instability. We developed a Bayesian framework that combines data from N-body simulations with present-day observations to compute a probability distribution for the mass of the planets that were ejected or collided in the past. Integrating across the exoplanet population, one can estimate the initial mass function of exoplanets. We find that the ejected planets are primarily sub-Saturn-type planets. While the present-day distribution appears to be bimodal, with peaks around ~1MJ and ~20M?, this bimodality does not seem to be primordial. Instead, planets around ~60M⊕ appear to be preferentially removed by dynamical instabilities. Attempts to reproduce exoplanet populations using population synthesis codes should be mindful of the fact that the present population may have been depleted of sub-Saturn-mass planets. Future observations may reveal that young giant planets have a more continuous size distribution with lower eccentricities and more sub-Saturn-type planets. Lastly, there is a need for additional data and for more research on how the system architecture and multiplicity might alter our results.
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| 37. |
- Chapman, R., et al.
(författare)
-
Soft Gamma Repeaters and Short Gamma Ray Bursts: Making Magnetars from WD -- WD Mergers
- 2007
-
Ingår i: ASP Conference Proceedings. ; 372, s. 415-415
-
Konferensbidrag (refereegranskat)abstract
- Recent progress on the nature of short duration Gamma Ray Bursts (GRBs) has shown that a fraction of them originate in the local universe. These systems may well be the result of giant flares from Soft Gamma Repeaters (SGRs) believed to be magnetars (neutron stars with extremely large magnetic fields ≥1014G). If these magnetars are formed via the core collapse of massive stars, then it would be expected that the bursts should originate from predominantly young stellar populations. However, correlating the positions of BATSE short bursts with structure in the local universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars forming via the core collapse of massive stars, they also form via accretion induced collapse following the merger of two white dwarfs (WDs), one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via WD--WD mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of magnetar production by core collapse is proportional to the star formation rate, the rate of production via WD--WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD--WD mergers may produce SGR giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short duration GRB population at low redshift.
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| 38. |
|
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| 39. |
- Church, Ross, et al.
(författare)
-
Implications for the origin of short gamma-ray bursts from their observed positions around their host galaxies
- 2011
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 413:3, s. 2004-2014
-
Tidskriftsartikel (refereegranskat)abstract
- We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them with the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence, the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts, the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies. One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is misidentified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters.
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| 40. |
- Church, Ross, et al.
(författare)
-
Interacting Compact Binaries: Modeling Mass Transfer in Eccentric Systems
- 2012
-
Ingår i: Advances in Computational Astrophysics: methods, tools, and outcomes. - 9781583817889 ; 453, s. 175-178
-
Konferensbidrag (refereegranskat)abstract
- We discuss mass transfer in eccentric binaries containing a white dwarf and a neutron star (WD-NS binaries). We show that such binaries are produced from field binaries following a series of mass transfer episodes that allow the white dwarf to form before the neutron star. We predict the orbital properties of binaries similar to the observed WD-NS binary J1141+6545, and show that they will undergo episodic mass transfer from the white dwarf to the neutron star. Furthermore, we describe oil-on-water, a two-phase SPH formalism that we have developed in order to model mass transfer in such binaries.
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| 41. |
- Church, Ross P., et al.
(författare)
-
Detailed models of the binary pulsars J1141-6545 and B2303+46
- 2006
-
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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| 42. |
- Church, Ross P., et al.
(författare)
-
Formation Constraints Indicate a Black Hole Accretor in 47 Tuc X9
- 2017
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 851:1
-
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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| 43. |
- Church, Ross P., et al.
(författare)
-
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
-
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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| 44. |
- Church, Ross, et al.
(författare)
-
Properties of long gamma-ray bursts from massive compact binaries.
- 2013
-
Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Science. - : The Royal Society. - 1364-503X .- 1471-2962. ; 371:1992
-
Tidskriftsartikel (refereegranskat)abstract
- We consider the implications of a model for long-duration gamma-ray bursts in which the progenitor is spun up in a close binary by tidal interactions with a massive black-hole companion. We investigate a sample of such binaries produced by a binary population synthesis, and show that the model predicts several common features in the accretion on to the newly formed black hole. In all cases, the accretion rate declines as approximately t(-5/3) until a break at a time of order 10(4) s. The accretion rate declines steeply thereafter. Subsequently, there is flaring activity, with the flare peaking between 10(4) and 10(5) s, the peak time being correlated with the flare energy. We show that these times are set by the semi-major axis of the binary, and hence the process of tidal spin-up; furthermore, they are consistent with flares seen in the X-ray light curves of some long gamma-ray bursts.
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| 45. |
- Church, Ross, et al.
(författare)
-
The properties of long gamma-ray bursts in massive compact binaries
- 2012
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 425:1, s. 470-476
-
Tidskriftsartikel (refereegranskat)abstract
- We consider a popular model for long-duration gamma-ray bursts, in which the progenitor star, a stripped helium core, is spun up by tidal interactions with a black hole companion in a compact binary. We perform population synthesis calculations to produce a representative sample of such binaries, and model the effect that the companion has on material that falls back on to the newly formed black hole. Taking the results of hydrodynamic models of black hole formation by fallback as our starting point, we show that the companion has two principal effects on the fallback process. First, a break forms in the accretion curve at around 104?s. Secondly, subsequent to the break, we expect to see a flare of total energy around 1050?erg. We show that the break and flare times are set largely by the semimajor axis of the binary at the time of explosion, and that this correlates negatively with the flare energy. Although comparison with observations is non-trivial, we show that our predicted break times are comparable to those found in the X-ray light curves of canonical long-duration gamma-ray bursts. Similarly, the flare properties that we predict are consistent with the late-time flares observed in a subsample of bursts.
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| 46. |
- Dale, JE, et al.
(författare)
-
Collisions and close encounters involving massive main-sequence stars
- 2006
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 366:4, s. 1424-1436
-
Tidskriftsartikel (refereegranskat)abstract
- We study close encounters involving massive main-sequence stars and the evolution of the exotic products of these encounters as common-envelope systems or possible hypernova progenitors. We show that parabolic encounters between low- and high-mass stars and between two high-mass stars with small periastrons result in mergers on time-scales of a few tens of stellar free-fall times (a few tens of hours). We show that such mergers of unevolved low-mass stars with evolved high-mass stars result in little mass-loss (similar to 0.01 M-circle dot) and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common-envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of similar to 100. If these remnants exist in very densely populated environments (n greater than or similar to 10(7) pc(-3)), they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma-ray burst progenitors and that similar to 0.1 per cent of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma-ray bursts.
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| 47. |
- Dale, James E., et al.
(författare)
-
Red giant stellar collisions in the Galactic Centre
- 2009
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 393:3, s. 1016-1033
-
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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| 48. |
- Davies, Melvyn B
(författare)
-
ASTROPHYSICS Stellar revival in old clusters
- 2009
-
Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 462:7276, s. 991-992
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
|
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| 49. |
- Davies, Melvyn B, et al.
(författare)
-
Blue straggler production in globular clusters
- 2004
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 349:1, s. 129-134
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Tidskriftsartikel (refereegranskat)abstract
- Recent Hubble Space Telescope observations of a large sample of globular clusters reveal that every cluster contains between 40 and 400 blue stragglers.The population does not correlate with either stellar collision rate (as would be expected if all blue stragglers were formed via collisions) or total mass (as would be expected if all blue stragglers were formed via the unhindered evolution of a subset of the stellar population). In this paper, we support the idea that blue stragglers are made through both channels. The number produced via collisions tends to increase with cluster mass. In this paper we show how the current population produced from primordial binaries decreases with increasing cluster mass;exchange encounters with third, single stars in the most massivec lusters tend to reduce the fraction of binaries containing a primary close to the current turn-off mass. Rather, their primaries tend to be somewhat more massive (~1-3 Msolar) and have evolved off the main sequence, filling their Roche lobes in the past, often converting their secondaries into blue stragglers (but more than 1 Gyr or so ago and thus they are no longer visible as blue stragglers). We show that this decline in the primordial blue straggler population is likely to be offset by the increase in the number of blue stragglers produced via collisions. The predicted total blue straggler population is therefore relatively independent of cluster mass, thus matching the observed population. This result does not depend on any particular assumed blue straggler lifetime.
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- Davies, Melvyn B, et al.
(författare)
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Compact binaries, hypernovae, and GRBs
- 2010
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Ingår i: New Astronomy Reviews. - : Elsevier BV. - 1872-9630 .- 1387-6473. ; 54:3-6, s. 181-182
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Tidskriftsartikel (refereegranskat)abstract
- The collapse of a massive stellar core may lead to the production of a black hole surrounded by a torus of material. Such a system is a potential source for the so-called long gamma-ray bursts (GRBs). A torus will form around the black hole if the infalling material contains sufficient angular momentum. This however requires that the core of the massive star rotates extremely rapidly prior to collapse. Here we explore whether tidal locking within binaries can spin stars up sufficiently. We show that the binaries are required to have separations <= 3-4 R-circle dot, hence the massive star would have lost its outer envelope (for example in a common envelope phase). In addition, the companions to the massive stars must themselves be compact. Comparison with observed tight binaries, which contain either two neutron stars or a neutron star and a white dwarf, shows that angular momentum is likely to have played an important role during the core collapse of the secondary in about half the systems, including the recently-discovered neutron star binary J0737-3039. Even if these systems failed to produce a GRB, as they do not contain a black hole, they are relevant to the problem of GRB production as a very similar evolutionary pathway (but with a slightly more massive helium star core) may well produce a GRB. (C) 2010 Elsevier B.V. All rights reserved.
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