1,347 research outputs found
New numerical solver for flows at various Mach numbers
Many problems in stellar astrophysics feature flows at low Mach numbers.
Conventional compressible hydrodynamics schemes frequently used in the field
have been developed for the transonic regime and exhibit excessive numerical
dissipation for these flows. While schemes were proposed that solve
hydrodynamics strictly in the low Mach regime and thus restrict their
applicability, we aim at developing a scheme that correctly operates in a wide
range of Mach numbers. Based on an analysis of the asymptotic behavior of the
Euler equations in the low Mach limit we propose a novel scheme that is able to
maintain a low Mach number flow setup while retaining all effects of
compressibility. This is achieved by a suitable modification of the well-known
Roe solver. Numerical tests demonstrate the capability of this new scheme to
reproduce slow flow structures even in moderate numerical resolution. Our
scheme provides a promising approach to a consistent multidimensional
hydrodynamical treatment of astrophysical low Mach number problems such as
convection, instabilities, and mixing in stellar evolution.Comment: 16 pages, 8 figures, accepted for publication by A&
Hot subdwarf stars in close-up view. I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)The origin of hot subdwarf B stars (sdBs) is still unclear. About half of the known sdBs are in close binary systems for which common envelope ejection is the most likely formation channel. Little is known about this dynamic phase of binary evolution. Since most of the known sdB systems are single-lined spectroscopic binaries, it is difficult to derive masses and unravel the companions' nature, which is the aim of this paper. Due to the tidal influence of the companion in close binary systems, the rotation of the primary becomes synchronised to its orbital motion. In this case it is possible to constrain the mass of the companion, if the primary mass, its projected rotational velocity as well as its surface gravity are known. For the first time we measured the projected rotational velocities of a large sdB binary sample from high resolution spectra. We analysed a sample of 51 sdB stars in close binaries, 40 of which have known orbital parameters comprising half of all such systems known today. Synchronisation in sdB binaries is discussed both from the theoretical and the observational point of view. The masses and the nature of the unseen companions could be constrained in 31 cases. We found orbital synchronisation most likely to be established in binaries with orbital periods shorter than . Only in five cases it was impossible to decide whether the sdB's companion is a white dwarf or an M dwarf. The companions to seven sdBs could be clearly identified as late M stars. One binary may have a brown dwarf companion. The unseen companions of nine sdBs are white dwarfs with typical masses. The mass of one white dwarf companion is very low. In eight cases (including the well known system KPD1930+2752) the companion mass exceeds , four of which even exceed the Chandrasekhar limit indicating that they may be neutron stars. Even stellar mass black holes are possible for the most massive companions. The distribution of the inclinations of the systems with low mass companions appears to be consistent with expectations, whereas a lack of high inclinations becomes obvious for the massive systems. We show that the formation of such systems can be explained with common envelope evolution and present an appropriate formation channel including two phases of unstable mass transfer and one supernova explosion. The sample also contains a candidate post-RGB star, which rotates fast despite its long orbital period. The post-RGB stars are expected to spin-up caused by their ongoing contraction. The age of the sdB is another important factor. If the EHB star is too young, the synchronisation process might not be finished yet. Estimating the ages of the target stars from their positions on the EHB band, we found PG 2345+318, which is known not to be synchronised, to lie near the zero-age extreme horizontal branch as are the massive candidates PG 1232-136, PG 1432+159 and PG 1101+249. These star may possibly be too young to have reached synchronisation. The derived large fraction of putative massive sdB binary systems in low inclination orbits is inconsistent with theoretical predictions. Even if we dismiss three candidates because they may be too young and assume that the other sdB primaries are of low mass, PG 1743+477 and, in particular, HE 0532-4503 remain as candidates whose companions may have masses close to or above the Chandrasekhar limit. X-ray observations and accurate photometry are suggested to clarify their nature. As high inclination systems must also exist, an appropriate survey has already been launched to find such binaries.Peer reviewe
Three-dimensional simulations of the interaction between Type Ia supernova ejecta and their main sequence companions
The identity of the progenitor systems of SNe Ia is still uncertain. In the
single-degenerate (SD) scenario, the interaction between the SN blast wave and
the outer layers of a main sequence (MS) companion star strips off H-rich
material which is then mixed into the ejecta. Strong contamination of the SN
ejecta with stripped material could lead to a conflict with observations of SNe
Ia. This constrains the SD progenitor model. In this work, our previous
simulations based on simplified progenitor donor stars have been updated by
adopting more realistic progenitor-system models that result from fully
detailed, state-of-the-art binary evolution calculations. We use Eggleton's
stellar evolution code including the optically thick accretion wind model and
the possibility of the effects of accretion disk instabilities to obtain
realistic models of companions for different progenitor systems. The impact of
the SN blast wave on these companion stars is followed in three-dimensional
hydrodynamic simulations employing the SPH code GADGET3. We find that the
stripped masses range from 0.11 to 0.18 M_sun. The kick velocity is between 51
and 105 km/s. We find that the stripped mass and kick velocity depend on the
ratio of the orbital separation to the radius of a companion. They can be
fitted by a power law for a given companion model. However, the structure of
the companion star is also important for the amount of stripped material. With
more realistic companion star models than in previous studies, our simulations
show that the H masses stripped from companions are inconsistent with the best
observational limits (< 0.01 M_sun) derived from nebular spectra. However, a
rigorous forward modeling based on impact simulations with radiation transfer
is required to reliably predict observable signatures of the stripped H and to
conclusively assess the viability of the considered SN Ia progenitor scenario.Comment: 14 pages, 13 figures, accepted for publication by A&
Three-Dimensional Simulations of Massive Stars: II. Age Dependence
We present 3D full star simulations, reaching up to 90% of the total stellar
radius, for three stars of different ages (ZAMS, midMS and TAMS). A
comparison with several theoretical prescriptions shows the generation spectra
for all three ages are dominated by convective plumes. Two distinct
overshooting layers are observed, with most plumes stopped within the layer
situated directly above the convective boundary (CB); overshooting to the
second, deeper layer becomes increasingly more infrequent with stellar age.
Internal gravity wave (IGW) propagation is significantly impacted in the midMS
and TAMS models as a result of some IGWs getting trapped within their
Brunt-V\"{a}is\"{a}l\"{a} frequency spikes. A fundamental change in the wave
structure across radius is also observed, driven by the effect of density
stratification on IGW propagation causing waves to become evanescent within the
radiative zone, with older stars being affected more strongly. We find that the
steepness of the frequency spectrum at the surface increases from ZAMS to the
older models, with older stars also showing more modes in their spectra.Comment: 24 pages, 14 figures / Accepted at Ap
Tsunami 2004
The Tsunami after the sea quake in Southeast Asia at the 26th of December 2004 represents one of the largest disasters in the modern World. Approximately 228,000 people from the countries surrounding the Indian Ocean have died. A large number of visitors from different European countries staying for their Christmas holidays in Thailand and Sri Lanka became victims of the natural disaster. The large number of foreign victims in these countries required additional forensic investigations which were organized by internationally working DVI (Disaster Victim Identification) teams. Victim identification was a great challenge due to the environmental conditions rapidly leading to heavily decomposed bodies. Thus the forensic medical investigations were very important to identify the victims. The different steps of forensic medical, odonto-stomatological and molecular genetic investigations beginning at the end of 2004 with the identification of a small number of victims and ending with the closing of the TTVI IMC (Thai Tsunami Victim Identification Information Management Center) in Phuket one year later are described and critically discussed. Up to 31 international DVI Teams worked in the TTVI IMC during 2005
Turbulent dynamo action and its effects on the mixing at the convective boundary of an idealized oxygen-burning shell
Convection is one of the most important mixing processes in stellar
interiors. Hydrodynamic mass entrainment can bring fresh fuel from neighboring
stable layers into a convection zone, modifying the structure and evolution of
the star. Under some conditions, strong magnetic fields can be sustained by the
action of a turbulent dynamo, adding another layer of complexity and possibly
altering the dynamics in the convection zone and at its boundaries. In this
study, we used our fully compressible Seven-League Hydro code to run detailed
and highly resolved three-dimensional magnetohydrodynamic simulations of
turbulent convection, dynamo amplification, and convective boundary mixing in a
simplified setup whose stratification is similar to that of an oxygen-burning
shell in a star with an initial mass of . We find that the random
stretching of magnetic field lines by fluid motions in the inertial range of
the turbulent spectrum (i.e., a small-scale dynamo) naturally amplifies the
seed field by several orders of magnitude in a few convective turnover
timescales. During the subsequent saturated regime, the magnetic-to-kinetic
energy ratio inside the convective shell reaches values as high as , and
the average magnetic field strength is . Such strong
fields efficiently suppress shear instabilities, which feed the turbulent
cascade of kinetic energy, on a wide range of spatial scales. The resulting
convective flows are characterized by thread-like structures that extend over a
large fraction of the convective shell. The reduced flow speeds and the
presence of magnetic fields with strengths up to of the equipartition
value at the upper convective boundary diminish the rate of mass entrainment
from the stable layer by as compared to the purely
hydrodynamic case
- …