| 31. |
- Dale, James E., et al.
(author)
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Red giant stellar collisions in the Galactic Centre
- 2009
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 393:3, s. 1016-1033
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Journal article (peer-reviewed)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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| 32. |
- Davies, Melvyn B, et al.
(author)
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Blue straggler production in globular clusters
- 2004
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 349:1, s. 129-134
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Journal article (peer-reviewed)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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| 33. |
- Davies, Melvyn B, et al.
(author)
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Stellar encounters involving massive stars in young clusters
- 2006
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:4, s. 2038-2046
-
Journal article (peer-reviewed)abstract
- We model collisions between pre-main-sequence stars using a smoothed particle hydrodynamics method. Assuming that all collisions lead to simple mergers, we use derived merger cross-sections to calculate the time-scale to make a 50-M-circle dot star by collisions within the core of a stellar cluster as a function of stellar number density. We show that a 50-M-circle dot star may be produced in this manner within 106 yr beginning with a cluster core of 200 1-M-circle dot stars within a radius of 0.0025 pc. Encounters between one high-mass star and one low-mass star tend to result in the tidal shredding of the latter, producing a massive disc around the former. This disc spreads viscously and provided a much larger target than any star for subsequent collisions. If a star strikes the disc, it is likely to be captured, and so forms a binary with the other star. Subsequent encounters between the binary and single stars lead either to exchanges or to the formation of merged objects. The inclusion of this effect leads to a significant reduction in the time taken to produce a 50-M-circle dot star. We also consider the role played by primordial binaries. We show that the time-scale required to produce a 50-M-circle dot star decreases with increasing binary fraction. We find that the number of primordial binaries is reduced by encounters. The core of a cluster must therefore contain a very high binary fraction initially if a large fraction of the massive stars are to be contained within binaries when the 50-M-circle dot star is produced.
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| 34. |
- Davies, Melvyn B, et al.
(author)
-
The ultimate outcome of black hole - neutron star mergers
- 2005
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 356:1, s. 54-58
-
Journal article (peer-reviewed)abstract
- We present a simple, semi--analytical description for the final stagesof mergers of black hole (BH) -- neutron star (NS) systems. Such systemsare of much interest as gravitational wave sources and gamma--ray burstprogenitors. Numerical studies show that in general the neutron star isnot disrupted at the first phase of mass transfer. Instead, what remainsof the neutron star is left on a wider, eccentric, orbit. We considerthe evolution of such systems as they lose angular momentum viagravitational radiation and come into contact for further phases of masstransfer. During each mass transfer event the neutron star mass isreduced until a critical value where mass loss leads to a rapid increasein the stellar radius. At this point Roche lobe overflow shreds whatremains of the neutron star, most of the mass forming a disc around theblack hole. Such a disc may be massive enough to power a gamma--rayburst. The mass of the neutron star at the time of disruption (andtherefore the disc mass) is largely independent of the initial masses ofthe black hole and neutron star, indicating that BH--NS star mergers maybe standard candles.
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| 35. |
- Dray, L. M., et al.
(author)
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Young stars in the Galactic Centre: a potential intermediate-mass star origin
- 2006
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:1, s. 31-44
-
Journal article (peer-reviewed)abstract
- There has been recent speculation that the cores of intermediate-mass stars stripped of their envelopes by tidal interaction with the supermassive black hole in the Galactic Centre could form a population observationally similar to the so-called Sgr A* cluster or 'S' stars, which have close eccentric orbits around the hole. We model the evolution of such stars, and show that the more luminous end of the population may indeed appear similar to young B stars within the observational limits of the Galactic Centre region. Whether some or all of these cluster stars can be accounted for in this manner depends strongly on the assumed initial mass function (IMF) of the loss cone stars and the scattering rate. If most of the observed stars are in fact scattered from the Galactic Centre inner cusp region itself then the population of similar to 20 to current observational limits may be reproduced. However, this only works if the local relaxation time is small and relies on the cusp stars themselves being young, i.e. it is dependent on some star formation being possible in the central few parsecs. Conversely, we obtain a possible constraint on the tidal stripping rate of 'normal' IMF stars if there are not to be red stars visible in the Sgr A* cluster.
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| 36. |
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| 37. |
- Fletcher, A., et al.
(author)
-
Magnetic fields and spiral arms in the galaxy M51
- 2011
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 412:4, s. 2396-2416
-
Journal article (peer-reviewed)abstract
- We use new multiwavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from 200 pc to several kpc. Interferometric and single-dish data are combined to obtain new maps at lambda lambda 3, 6 cm in total and polarized emission, and earlier lambda 20 cm data are rereduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the lambda 20 cm emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at lambda 6 cm must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy; the disc is dominated by a combination of azimuthal modes, m = 0 + 2, but in the halo only an m = 1 mode is required to fit the observations. We discuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the spiral shocks. In the inner spiral arms, the strong arm-interarm contrasts in total and polarized radio emission are roughly consistent with expectations from shock compression of the regular and turbulent components of the magnetic field. However, the average arm-interam contrast, representative of the radii r > 2 kpc where the spiral arms are broader, is not compatible with straightforward compression: lower arm-interarm contrasts than expected may be due to resolution effects and decompression of the magnetic field as it leaves the arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of 50 pc for the size of the turbulent eddies.
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| 38. |
- Flynn, Chris, et al.
(author)
-
White dwarfs and Galactic dark matter
- 2003
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 339:3, s. 817-824
-
Journal article (peer-reviewed)abstract
- We discuss the recent discovery by Oppenheimer et al. of old, cool white dwarf stars, which may be the first direct detection of Galactic halo dark matter. We argue here that the contribution of more mundane white dwarfs of the stellar halo and thick disc would contribute sufficiently to explain the new high velocity white dwarfs without invoking putative white dwarfs of the dark halo. This by no means rules out that dark matter has been found, but it does constrain the overall contribution by white dwarfs brighter than MR
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| 39. |
- Hannikainen, D C, et al.
(author)
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The X-ray source population of the globular cluster M15: Chandra high-resolution imaging
- 2005
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 357:1, s. 325-332
-
Journal article (peer-reviewed)abstract
- The globular cluster M15 was observed on three occasions with the High Resolution Camera on- board Chandra in 2001 in order to investigate the X- ray source population in the cluster centre. After subtraction of the two bright central sources, four faint sources were identified within 50 arcsec of the core. One of these sources is probably the planetary nebula K648, making this the first positive detection of X- rays from a planetary nebula inside a globular cluster. Another two are identified with UV variables (one previously known), which we suggest are cataclysmic variables (CVs). The nature of the fourth source is more difficult to ascertain, and we discuss whether it is possibly a quiescent soft X- ray transient or also a CV.
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| 40. |
- Harper, GM, et al.
(author)
-
Lines of O IV and S IV in the Goddard High-Resolution Spectrograph spectrum of RR Tel: constraints on atomic data
- 1999
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 303:3, s. 41-46
-
Journal article (peer-reviewed)abstract
- High signal-to-noise ratio spectra of RR Tel obtained at medium resolution with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) are used to test available atomic data for the O IV 2s(2)2p P-2-2s2p(2) P-4 multiplet (UV 0.01). The fine-structure intervals of the 2s2p(2) P-4 term given by Moore (1983) appear to need revision. The flux ratios of lines within multiplet UV (0.01), which have a common upper level, depend only on transition probabilities. The observed flux ratio of lines from the P-4(3/2) level differs from that predicted by theory, but this difference cannot be attributed to a blend with a line of S IV]. At the electron densities in the RR Tel nebula, other flux ratios give information on the relative electron excitation rates between the P-2 and P-4 fine-structure levels. Using the collision strengths calculated by Zhang, Graziani & Pradhan, the rate to the P-4(5/2) level, relative to the rates to the other J states, appears to be underestimated by similar to 10 per cent, which is within the expected uncertainty of 20 per cent. We also discuss the S IV 3s(2)3p P-2-3s3p(2) P-4 multiplet.
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