18,645 research outputs found
The growth and hydrodynamic collapse of a protoplanet envelope
We have conducted three-dimensional self-gravitating radiation hydrodynamical
models of gas accretion onto high mass cores (15-33 Earth masses) over hundreds
of orbits. Of these models, one case accretes more than a third of a Jupiter
mass of gas, before eventually undergoing a hydrodynamic collapse. This
collapse causes the density near the core to increase by more than an order of
magnitude, and the outer envelope to evolve into a circumplanetary disc. A
small reduction in the mass within the Hill radius (R_H) accompanies this
collapse as a shock propagates outwards. This collapse leads to a new
hydrostatic equilibrium for the protoplanetary envelope, at which point 97 per
cent of the mass contained within the Hill radius is within the inner 0.03 R_H
which had previously contained less than 40 per cent. Following this collapse
the protoplanet resumes accretion at its prior rate. The net flow of mass
towards this dense protoplanet is predominantly from high latitudes, whilst at
the outer edge of the circumplanetary disc there is net outflow of gas along
the midplane. We also find a turnover of gas deep within the bound envelope
that may be caused by the establishment of convection cells.Comment: 16 pages, 16 figures. Accepted for publication in MNRA
Ejection of high-velocity stars from the Galactic Center by an inspiraling Intermediate-Mass Black Hole
The presence of young stars in the immediate vicinity and strong tidal field
of SgrA* remains unexplained. One currently popular idea for their origin
posits that the stars were bused in by an Intermediate-Mass Black Hole (IMBH)
which has inspiraled into the Galactic Center a few million years ago.
Yu and Tremaine (2003) have argued that in this case some of the old stars in
the SgrA* cusp would be ejected by hard gravitational collisions with the IMBH.
Here we derive a general expression for the phase-space distribution of the
ejected high-velocity stars, given the distribution function of the stars in
the cusp. We compute it explicitly for the Peebles-Young distribution function
of the cusp, and make a detailed model for the time-dependent ejection of stars
during the IMBH inspiral. We find that (1) the stars are ejected in a burst
lasting a few dynamical friction timescales; if the ejected stars are detected
by Gaia they are likely to be produced by a single inspiral event, (2) if the
inspiral is circular than in the beginning of the burst the velocity vectors of
the ejected stars cluster around the inspiral plane, but rapidly isotropise as
the burst proceeds, (3) if the inspiral is eccentric, then the stars are
ejected in a broad jet roughly perpendicular to the Runge-Lenz vector of the
IMBH orbit. In a typical cusp the orbit will precess with a period of \sim 10^5
years, and the rate of ejection into our part of the Galaxy (as defined by e.g.
the Gaia visibility domain) will be modulated periodically. Gaia, together with
the ground-based follow-up observations, will be able to clock many
high-velocity stars back to their ejection from the Galactic Center, thus
measuring some of the above phenomena. This would provide a clear signature of
the IMBH inspiral in the past 10--20 Myr.Comment: 12 pages, including 7 figure
Separating the conjoined red clump in the Galactic Bulge: Kinematics and Abundances
We have used the AAOMEGA spectrograph to obtain R spectra of 714
stars that are members of two red clumps in the Plaut Window Galactic bulge
field . We discern no difference between the clump
populations based on radial velocities or abundances measured from the Mg
index. The velocity dispersion has a strong trend with Mg-index metallicity,
in the sense of a declining velocity dispersion at higher metallicity. We also
find a strong trend in mean radial velocity with abundance. Our red clump
sample shows distinctly different kinematics for stars with [Fe/H] , which
may plausibly be attributable to a minority classical bulge or inner halo
population. The transition between the two groups is smooth. The
chemo-dynamical properties of our sample are reminiscent of those of the Milky
Way globular cluster system. If correct, this argues for no bulge/halo
dichotomy and a relatively rapid star formation history. Large surveys of the
composition and kinematics of the bulge clump and red giant branch are needed
to define further these trends.Comment: 5 pages, 4 figures, Accepted for Publication in the Astrophysical
Journal Letters Fixed typos, updated affiliations and added some references
upon reques
A new multi-dimensional general relativistic neutrino hydrodynamics code for core-collapse supernovae. I. Method and code tests in spherical symmetry
We present a new general relativistic (GR) code for hydrodynamic supernova
simulations with neutrino transport in spherical and azimuthal symmetry
(1D/2D). The code is a combination of the CoCoNuT hydro module, which is a
Riemann-solver based, high-resolution shock-capturing method, and the
three-flavor, energy-dependent neutrino transport scheme VERTEX. VERTEX
integrates the neutrino moment equations with a variable Eddington factor
closure computed from a model Boltzmann equation and uses the ray-by-ray plus
approximation in 2D, assuming the neutrino distribution to be axially symmetric
around the radial direction, and thus the neutrino flux to be radial. Our
spacetime treatment employs the ADM 3+1 formalism with the conformal flatness
condition for the spatial three-metric. This approach is exact in 1D and has
been shown to yield very accurate results also for rotational stellar collapse.
We introduce new formulations of the energy equation to improve total energy
conservation in relativistic and Newtonian hydro simulations with Eulerian
finite-volume codes. Moreover, a modified version of the VERTEX scheme is
developed that simultaneously conserves energy and lepton number with better
accuracy and higher numerical stability. To verify our code, we conduct a
series of tests, including a detailed comparison with published 1D results for
stellar core collapse. Long-time simulations of proto-neutron star cooling over
several seconds both demonstrate the robustness of the new CoCoNuT-VERTEX code
and show the approximate treatment of GR effects by means of an effective
gravitational potential as in PROMETHEUS-VERTEX to be remarkably accurate in
1D. (abridged)Comment: 36 pages, 19 eps figures; submitted to ApJS (minor revisions; some
typos corrected
Southern GEMS groups II: HI distribution, mass functions and HI deficient galaxies
We investigate the neutral hydrogen (HI) content of sixteen groups for which
we have multi-wavelength data including X-ray observations. Wide-field imaging
of the groups was obtained with the 20-cm multibeam system on the 64-m Parkes
telescope. We have detected ten previously uncatalogued HI sources, one of
which has no visible optical counterpart. We examine the HI properties of the
groups, compared to their X-ray characteristics, finding that those groups with
a higher X-ray temperature and luminosity contain less HI per galaxy. The HI
content of a group depends on its morphological make-up, with those groups
dominated by early-type galaxies containing the least total HI. We determined
the expected HI for the spiral galaxies in the groups, and found that a number
of the galaxies were HI deficient. The HI deficient spirals were found both in
groups with and without a hot intra-group medium. The HI deficient galaxies
were not necessarily found at the centre of the groups, however, we did find
that two thirds of HI deficient galaxies were found within about 1 Mpc from the
group centre, indicating that the group environment is affecting the gas-loss
from these galaxies. We determined the HI mass function for a composite sample
of 15 groups, and found that it is significantly flatter than the field HI mass
function. We also find a lack of high HI-mass galaxies in groups. One possible
cause of this effect is the tidal stripping of HI gas from spiral galaxies as
they are pre-processed in groups.Comment: accepted for publication in MNRAS, 26 pages, 13 Figures, 2 Appendice
The degree-diameter problem for sparse graph classes
The degree-diameter problem asks for the maximum number of vertices in a
graph with maximum degree and diameter . For fixed , the answer
is . We consider the degree-diameter problem for particular
classes of sparse graphs, and establish the following results. For graphs of
bounded average degree the answer is , and for graphs of
bounded arboricity the answer is \Theta(\Delta^{\floor{k/2}}), in both cases
for fixed . For graphs of given treewidth, we determine the the maximum
number of vertices up to a constant factor. More precise bounds are given for
graphs of given treewidth, graphs embeddable on a given surface, and
apex-minor-free graphs
What determines the density structure of molecular clouds? A case study of Orion B with <i>Herschel</i>
A key parameter to the description of all star formation processes is the density structure of the gas. In this Letter, we make use of probability distribution functions (PDFs) of Herschel column density maps of Orion B, Aquila, and Polaris, obtained with the Herschel Gould Belt survey (HGBS). We aim to understand which physical processes influence the PDF shape, and with which signatures. The PDFs of Orion B (Aquila) show a lognormal distribution for low column densities until AV ~ 3 (6), and a power-law tail for high column densities, consistent with a ρα r-2 profile for the equivalent spherical density distribution. The PDF of Orion B is broadened by external compression due to the nearby OB stellar aggregates. The PDF of a quiescent subregion of the non-star-forming Polaris cloud is nearly lognormal, indicating that supersonic turbulence governs the density distribution. But we also observe a deviation from the lognormal shape at AV > 1 for a subregion in Polaris that includes a prominent filament. We conclude that (1) the point where the PDF deviates from the lognormal form does not trace a universal AV -threshold for star formation, (2) statistical density fluctuations, intermittency, and magnetic fields can cause excess from the lognormal PDF at an early cloud formation stage, (3) core formation and/or global collapse of filaments and a non-isothermal gas distribution lead to a power-law tail, and (4) external compression broadens the column density PDF, consistent with numerical simulations
Tracking magnetic bright point motions through the solar atmosphere
High-cadence, multiwavelength observations and simulations are employed for the analysis of solar photospheric magnetic bright points (MBPs) in the quiet Sun. The observations were obtained with the Rapid Oscillations in the Solar Atmosphere (ROSA) imager and the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope. Our analysis reveals that photospheric MBPs have an average transverse velocity of approximately 1 km s−1, whereas their chromospheric counterparts have a slightly higher average velocity of 1.4 km s−1. Additionally, chromospheric MBPs were found to be around 63 per cent larger than the equivalent photospheric MBPs. These velocity values were compared with the output of numerical simulations generated using the MURAM code. The simulated results were similar, but slightly elevated, when compared to the observed data. An average velocity of 1.3 km s−1 was found in the simulated G-band images and an average of 1.8 km s−1 seen in the velocity domain at a height of 500 km above the continuum formation layer. Delays in the change of velocities were also analysed. Average delays of ∼4 s between layers of the simulated data set were established and values of ∼29 s observed between G-band and Ca II K ROSA observations. The delays in the simulations are likely to be the result of oblique granular shock waves, whereas those found in the observations are possibly the result of a semi-rigid flux tube
Flame Propagation on the Surfaces of Rapidly Rotating Neutron Stars during Type I X-ray Bursts
We present the first vertically resolved hydrodynamic simulations of a
laterally propagating, deflagrating flame in the thin helium ocean of a
rotating accreting neutron star. We use a new hydrodynamics solver tailored to
deal with the large discrepancy in horizontal and vertical length scales
typical of neutron star oceans, and which filters out sound waves that would
otherwise limit our timesteps. We find that the flame moves horizontally with
velocities of order cm s, crossing the ocean in few seconds,
broadly consistent with the rise times of Type I X-ray bursts. We address the
open question of what drives flame propagation, and find that heat is
transported from burning to unburnt fuel by a combination of top-to-bottom
conduction and mixing driven by a baroclinic instability. The speed of the
flame propagation is therefore a sensitive function of the ocean conductivity
and spin: we explore this dependence for an astrophysically relevant range of
parameters and find that in general flame propagation is faster for slower
rotation and higher conductivity.Comment: Accepted for publication by MNRA
Search for Associations Containing Young stars (SACY): Chemical tagging IC 2391 & the Argus association
We explore the possible connection between the open cluster IC 2391 and the
unbound Argus association identified by the SACY survey. In addition to common
kinematics and ages between these two systems, here we explore their chemical
abundance patterns to confirm if the two substructures shared a common origin.
We carry out a homogenous high-resolution elemental abundance study of eight
confirmed members of IC 2391 as well as six members of the Argus association
using UVES spectra. We derive spectroscopic stellar parameters and abundances
for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba.
All stars in the open cluster and Argus association were found to share
similar abundances with the scatter well within the uncertainties, where [Fe/H]
= -0.04 +/-0.03 for cluster stars and [Fe/H] = -0.06 +/-0.05 for Argus stars.
Effects of over-ionisation/excitation were seen for stars cooler than roughly
5200K as previously noted in the literature. Also, enhanced Ba abundances of
around 0.6 dex were observed in both systems. The common ages, kinematics and
chemical abundances strongly support that the Argus association stars
originated from the open cluster IC 2391. Simple modeling of this system find
this dissolution to be consistent with two-body interactions.Comment: 17 pages, 7 figs, accepted for publication in MNRA
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