47,458 research outputs found
Retrograde Accretion and Merging Supermassive Black Holes
We investigate whether a circumbinary gas disc can coalesce a supermassive
black hole binary system in the centre of a galaxy. This is known to be
problematic for a prograde disc. We show that in contrast, interaction with a
retrograde circumbinary disc is considerably more effective in shrinking the
binary because there are no orbital resonances. The binary directly absorbs
negative angular momentum from the circumbinary disc by capturing gas into a
disc around the secondary black hole, or discs around both holes if the binary
mass ratio is close to unity. In many cases the binary orbit becomes eccentric,
shortening the pericentre distance as the eccentricity grows. In all cases the
binary coalesces once it has absorbed the angular momentum of a gas mass
comparable to that of the secondary black hole. Importantly, this conclusion is
unaffected even if the gas inflow rate through the disc is formally
super--Eddington for either hole. The coalescence timescale is therefore always
, where is the secondary black hole mass and
the inflow rate through the circumbinary disc.Comment: 8 pages, 4 figures. Accepted for publication in MNRAS. Movies of the
simulations can be found at:
http://www.astro.le.ac.uk/users/cjn12/RetroBinaryMovies.htm
Age structure, dispersion and diet of a population of stoats (Mustela erminea) in southern Fiordland during the decline phase of the beechmast cycle
The dispersion, age structure and diet of stoats (Mustela erminea) in beech forest in the Borland and Grebe Valleys, Fiordland National Park, were examined during December and January 2000/01, 20 months after a heavy seed-fall in 1999. Thirty trap stations were set along a 38-km transect through almost continuous beech forest, at least 1 km apart. Mice were very scarce (nights, C/100TN) along two standard index lines placed at either end of the transect, compared with November 1999 (>60/100TN), but mice were detected (from footprints in stoat tunnels) along an 8 km central section of the transect (stations 14-22). Live trapping with one trap per station (total 317.5 trap nights) in December 2000 caught 2 female and 23 male stoats, of which 10 (including both females) were radio collared. The minimum range lengths of the two females along the transect represented by the trap line were 2.2 and 6.0 km; those of eight radio-tracked males averaged 2.9 ± 1.7 km. Stations 14-22 tended to be visited more often, by more marked individual stoats, than the other 21 stations.
Fenn trapping at the same 30 sites, but with multiple traps per station (1333.5 trap nights), in late January 2001 collected carcasses of 35 males and 28 females (including 12 of the marked live-trapped ones). Another two marked males were recovered dead. The stoat population showed no sign of chronic nutritional stress (average fat reserve index = 2.8 on a scale of 1-4 where 4 = highest fat content); and only one of 63 guts analysed was empty. Nevertheless, all 76 stoats handled were adults with 1-3 cementum annuli in their teeth, showing that reproduction had failed that season. Prey categories recorded in descending frequency of occurrence were birds, carabid beetle (ground beetle), weta, possum, rat, and mouse. The frequencies of occurrence of mice and birds in the diet of these stoats (10% and 48%, respectively) were quite different from those in stoats collected in Pig Creek, a tributary of the Borland River (87%, 5%), 12 months previously when mice were still abundant. Five of the six stoat guts containing mice were collected within 1 km of stations 14-22
Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systems Yb2Ti2O7 and Yb2Sn2O7
We report a positive muon spin relaxation and rotation (\muSR) study of the
quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field
response. In agreement with earlier reports, data recorded in small
longitudinal fields evidence anomalously slow spin dynamics in the microsecond
range below the temperature T_c at which the specific heat displays an intense
peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We
found that slow dynamics extends above T_c up to at least 0.7 K for both
compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the
\muSR spectra recorded above T_c. At lower temperatures a published analytical
extension of the Gaussian Kubo-Toyabe model provides a good description,
consistent with the existence of short-range magnetic correlations. While the
physical response of the two systems is qualitatively the same, Yb2Ti2O7
exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c.
Considering previously reported ac susceptibility, neutron scattering and \muSR
results, we suggest the existence of anomalously slow spin dynamics to be a
common physical property of pyrochlore magnetic materials. The possibility of
molecular spin substructures to be associated to the slow dynamics and
therefore the short-range correlations is mentioned. The slow spin dynamics
observed under field does not exclude the presence of much faster dynamics
detected in extremely low or zero field.Comment: 11 pages, 10 figure
Self-awareness of driving impairment in patients with cataract or glaucoma
This study compared the driving performance of individuals with the eye diseases cataracts or glaucoma with age-matched controls, as well as the individual’s own perceptions of driving. Participants included drivers over the age of 50 years who had been diagnosed with glaucoma (n=29) or cataracts (n=33) and a control group with no ocular pathology (n=13). Driving performance was measured on a closed road circuit using a range of standardised measures of vehicle control and hazard recognition and avoidance, while visual performance was measured with a battery of tests including visual acuity, contrast sensitivity and visual fields. Perceptions of vision and driving were assessed using the Activities of Daily Vision Scale, Driver Behaviour Questionnaire and a driving exposure questionnaire. Driving performance was significantly poorer (p<0.05) for each of the ocular disease groups compared to the control group. Impaired contrast sensitivity and the higher disease severity scores (for the glaucoma group only) correlated most strongly with poorer driving performance. While participants with cataracts rated their vision significantly more poorly than those in the glaucoma and control groups, there were no significant differences between the participant groups rating of their own driving performance. These findings suggest that there is no direct relationship between self-rated driving ability and actual vision and driving performance. This has serious road safety implications
Atomic swelling upon compression
The hydrogen atom under the pressure of a spherical penetrable confinement
potential of a decreasing radius is explored, as a case study. A novel
counter-intuitive effect of atomic swelling rather than shrinking with
decreasing is unraveled, when reaches, and remains smaller
than, a certain critical value. Upon swelling, the size of the atom is shown to
increase by an order of magnitude, or more, compared to the size of the free
atom. Examples of changes of photoabsorption properties of confined hydrogen
atom upon its swelling are uncovered and demonstrated.Comment: 5 pages, 4 figure
The Li Overabundance of J37: Diffusion or Accretion?
In September 2002 the discovery of a super Li-rich F-dwarf (J37) in NGC 6633,
an iron poor analogue of the better studied Hyades and Praecepe open clusters,
was announced. This unique star was thought to be the smoking gun for the
action of diffusion, models of which predict a narrow "Li-peak" at
approximately the correct temperature. However, with more detailed studies into
J37s abundance pattern this star provides firm evidence for the accretion of
planetesimals or other material from the circumstellar environment of new born
stars.
Thanks to the specific predictions made about the behaviour of Be abundances,
(the most striking of which being no Be in super-Li-rich dwarfs subject to
diffusion) the opposing diffusion/accretion predictions can be tested.
Initial modelling of the Be line indicates that J37 is as Be rich as it is Li
rich; log N(Be) = 2.25 +/- 0.25, and so is broadly consistent with an
accretion-fuelled enhancement. However, that both Li and Be are enhanced by
much more than the iron-peak elements (as determined in previous studies)
suggests that diffusion also plays a role in increasing the abundances of Li
and Be specifically.
Furthermore, a new data set from the UVES/UT2 combination has allowed the
elemental abundance of Iron to be measured, and the set of preliminary stellar
parameters determined; Teff ~ 7340 K, log g ~ 4.1, microturbulence ~ 4.3 km/s,
[Fe/H] ~ 0.50. This again provides distinct evidence for the effects of
accretion in J37 and requires a new synthesis of the Be doublet.Comment: 5 pages, 2 figures. Poster presented at IAU Symposium 224 "The A Star
Puzzle", 7-13 July 2004, Poprad, Slovaki
Beryllium Enhancement as Evidence for Accretion in a Lithium-Rich F Dwarf
The early F dwarf star ``J37'' in the open cluster NGC6633 shows an unusual
pattern of photospheric abundances, including an order of magnitude enhancement
of lithium and iron-peak elements, but an under-abundance of carbon. As a
consequence of its thin convection zone these anomalies have been attributed to
either radiative diffusion or the accretion of hydrogen-depleted material. By
comparing high resolution VLT/UVES spectra of J37 (and other F stars in NGC
6633) with syntheses of the Be ii doublet region at 3131 Ang, we establish that
J37 also has a Be abundance (A(Be)=3.0+/-0.5) that is at least ten times the
cosmic value. This contradicts radiative diffusion models that produce a Li
over-abundance, as they also predict photospheric Be depletion. Instead, since
Be is a highly refractory element, it supports the notion that J37 is the first
clear example of a star that has accreted volatile-depleted material with a
composition similar to chondritic meteorites, although some diffusion may be
necessary to explain the low C and O abundances.Comment: Accepted for publication in MNRAS letters, 5 page
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