15 research outputs found
On the evolution of intra-cluster gas within Galactic globular clusters
It has been known since the 1950's that the observed gas content of Galactic
globular clusters (GCs) is 2-3 orders of magnitude less than the mass lost by
stars between Galactic disk crossings. In this work we address the question:
What happens to this stellar gas? Using an Eulerian nested grid code, we
present 3D simulations to determine how stellar wind material evolves within
the GC environment. We expand upon work done in the 70's and move a single-mass
King-model GC through the Galactic halo medium, stripping a 10^5 Msun GC of its
intra-cluster medium but predicting a detectable medium for a 10^6 Msun
cluster. We find from new multi-mass King model simulations, the first to
incorporate empirical mass-loss formulae, that the single-mass King model
underestimates the retention of intra-cluster gas in the cluster. Lastly, we
present a simple discretised multi-mass GC model, which yields lower levels of
intra-cluster medium compared to the continuous single- and multi-mass King
models. Our results show that there is still an issue with the predicted
intra-cluster gas content of massive GCs. We conclude that by modelling GC
systems more accurately, in particular the stellar structure and description of
mass loss, we will be able to work towards resolving this issue and begin to
fill in some of the gaps in our understanding of the evolution of globular
clusters.Comment: 19 pages, 19 pdf figures. Accepted for publication in Monthly Notices
of the Royal Astronomical Societ
An analytical study of Bondi-Hoyle-Lyttleton accretion I. Stationary flows
We prove that the sonic surface of axisymmetric meridional stationary flows
is always attached to the accretor, however small, if the adiabatic index of
the gas is gamma=5/3. Using local expansions near a point-like accretor, we
extend Bondi's classification of spherically symmetric flows to axisymmetric
flows, introducing the possibility of angular sectors reached by no flow lines,
and singular directions of infinite mass flux, in addition to the angular
regions of subsonic and supersonic accretion. For gamma<5/3, we show the
impossibility of subsonic accretion onto a point-like accretor when the entropy
of the flow is not uniform. The special case gamma=5/3 is treated separately.
We analyse the influence of the adiabatic index and Mach number of the flow at
infinity on the mass accretion rate of shocked spherical flows. We propose an
interpolation formula for the mass accretion rate of axisymmetric flows as a
function of the Mach number and the adiabatic index, in the range
9/7<gamma<5/3.Comment: 22 pages, A&A LaTeX, submitted to A&
Black Hole - Neutron Star Mergers as Central Engines of Gamma-Ray Bursts
Hydrodynamic simulations of the merger of stellar mass black hole - neutron
star binaries (BH/NS) are compared with mergers of binary neutron stars
(NS/NS). The simulations are Newtonian, but take into account the emission and
backreaction of gravitational waves. The use of a physical nuclear equation of
state allows us to include the effects of neutrino emission. For low neutron
star to black hole mass ratios the neutron star transfers mass to the black
hole during a few cycles of orbital decay and subsequent widening before
finally being disrupted, whereas for ratios near unity the neutron star is
already distroyed during its first approach. A gas mass between about 0.3 and
about 0.7 solar masses is left in an accretion torus around the black hole and
radiates neutrinos at a luminosity of several 10^{53} erg/s during an estimated
accretion time scale of about 0.1 s. The emitted neutrinos and antineutrinos
annihilate into electron-positron pairs with efficiencies of 1-3% percent and
rates of up to 2*10^{52} erg/s, thus depositing an energy of up to 10^{51} erg
above the poles of the black hole in a region which contains less than 10^{-5}
solar masses of baryonic matter. This could allow for relativistic expansion
with Lorentz factors around 100 and is sufficient to explain apparent burst
luminosities of up to several 10^{53} erg/s for burst durations of
approximately 0.1-1 s, if the gamma emission is collimated in two moderately
focussed jets in a fraction of about 1/100-1/10 of the sky.Comment: 8 pages, LaTex, 4 postscript figures, 2 tables. ApJ Letters,
accepted; revised and shortened version, Fig. 2 change
Presentation of binning-based inter-click interval data from passive acoustic monitoring of free-ranging harbour porpoises (Phocoena Phocoena)
Potential audibility of three Acoustic Harassment Devices (AHDs) to marine mammals in Scotland, UK
Proximate underwater soundscape of a North Sea offshore petroleum-exploration jack-up drilling-rig in the Dogger Bank
Dynamics of core accretion
(shortened) We perform 3D hydrodynamic simulations of gas flowing around a
planetary core of mass \mplan=10\me embedded in a near Keplerian background
flow, using a modified shearing box approximation. We employ a nested grid
hydrodynamic code with as many as six nested grids, providing spatial
resolution on the finest grid comparable to the present day diameters of
Neptune and Uranus. We find that a strongly dynamically active flow develops
such that no static envelope can form. The activity is not sensitive to
plausible variations in the rotation curve of the underlying disk. It is
sensitive to the thermodynamic treatment of the gas, as modeled by prescribed
equations of state (either `locally isothermal' or `locally isentropic') and
the temperature of the background disk material. The activity is also sensitive
to the shape and depth of the core's gravitational potential, through its mass
and gravitational softening coefficient. The varying flow pattern gives rise to
large, irregular eruptions of matter from the region around the core which
return matter to the background flow: mass in the envelope at one time may not
be found in the envelope at any later time. The angular momentum of material in
the envelope, relative to the core, varies both in magnitude and in sign on
time scales of days to months near the core and on time scales a few years at
distances comparable to the Hill radius. We show that material entering the
dynamically active environment may suffer intense heating and cooling events
the durations of which are as short as a few hours to a few days. Peak
temperatures in these events range from K to as high as K, with densities g/cm. These time
scales, densities and temperatures span a range consistent with those required
for chondrule formation in the nebular shock model.Comment: Accepted for publication in MNRA