228 research outputs found
Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha
Stellar evolution models predict the existence of hybrid white dwarfs (WDs)
with a carbon-oxygen core surrounded by an oxygen-neon mantle. Being born with
masses ~1.1 Msun, hybrid WDs in a binary system may easily approach the
Chandrasekhar mass (MCh) by accretion and give rise to a thermonuclear
explosion. Here, we investigate an off-centre deflagration in a near-MCh hybrid
WD under the assumption that nuclear burning only occurs in carbon-rich
material. Performing hydrodynamics simulations of the explosion and detailed
nucleosynthesis post-processing calculations, we find that only 0.014 Msun of
material is ejected while the remainder of the mass stays bound. The ejecta
consist predominantly of iron-group elements, O, C, Si and S. We also calculate
synthetic observables for our model and find reasonable agreement with the
faint Type Iax SN 2008ha. This shows for the first time that deflagrations in
near-MCh WDs can in principle explain the observed diversity of Type Iax
supernovae. Leaving behind a near-MCh bound remnant opens the possibility for
recurrent explosions or a subsequent accretion-induced collapse in faint Type
Iax SNe, if further accretion episodes occur. From binary population synthesis
calculations, we find the rate of hybrid WDs approaching MCh to be on the order
of 1 percent of the Galactic SN Ia rate.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in MNRA
Type Ia supernovae from exploding oxygen-neon white dwarfs
The progenitor problem of Type Ia supernovae (SNe Ia) is still unsolved. Most
of these events are thought to be explosions of carbon-oxygen (CO) white dwarfs
(WDs), but for many of the explosion scenarios, particularly those involving
the externally triggered detonation of a sub-Chandrasekhar mass WD (sub-M Ch
WD), there is also a possibility of having an oxygen-neon (ONe) WD as
progenitor. We simulate detonations of ONe WDs and calculate synthetic
observables from these models. The results are compared with detonations in CO
WDs of similar mass and observational data of SNe Ia. We perform hydrodynamic
explosion simulations of detonations in initially hydrostatic ONe WDs for a
range of masses below the Chandrasekhar mass (M Ch), followed by detailed
nucleosynthetic postprocessing with a 384-isotope nuclear reaction network. The
results are used to calculate synthetic spectra and light curves, which are
then compared with observations of SNe Ia. We also perform binary evolution
calculations to determine the number of SNe Ia involving ONe WDs relative to
the number of other promising progenitor channels. The ejecta structures of our
simulated detonations in sub-M Ch ONe WDs are similar to those from CO WDs.
There are, however, small systematic deviations in the mass fractions and the
ejecta velocities. These lead to spectral features that are systematically less
blueshifted. Nevertheless, the synthetic observables of our ONe WD explosions
are similar to those obtained from CO models. Our binary evolution calculations
show that a significant fraction (3-10%) of potential progenitor systems should
contain an ONe WD. The comparison of our ONe models with our CO models of
comparable mass (1.2 Msun) shows that the less blueshifted spectral features
fit the observations better, although they are too bright for normal SNe Ia.Comment: 6 pages, 5 figure
Common-envelope evolution with an asymptotic giant branch star
Common-envelope phases are decisive for the evolution of many binary systems.
Of particular interest are cases with asymptotic giant branch (AGB) primary
stars, because they are thought to be progenitors of various astrophysical
transients. In three-dimensional hydrodynamic simulations with the moving-mesh
code AREPO, we study the common-envelope evolution of a
early-AGB star with companions of different masses. Although the stellar
envelope of the AGB star is less tightly bound than that of a red giant, we
find that the release of orbital energy of the core binary is insufficient to
eject more than about twenty percent of the envelope mass. Ionization energy
released in the expanding envelope, however, can lead to complete envelope
ejection. Because recombination proceeds largely at high optical depths in our
simulations, it is likely that this effect indeed plays a significant role in
the considered systems. The efficiency of mass loss and the final orbital
separation of the core binary system depend on the mass ratio between the
companion and the primary star. Our results suggest a linear relation between
the ratio of final to initial orbital separation and this parameter.Comment: 12 pages, 9 figures, 5 tables; accepted for publication by A&
Long-term evolution of a magnetic massive merger product
About 10% of stars more massive than have
strong, large-scale surface magnetic fields and are being discussed as
progenitors of highly-magnetic white dwarfs and magnetars. The origin of these
fields remains uncertain. Recent 3D magnetohydrodynamical simulations have
shown that strong magnetic fields can be generated in the merger of two massive
stars. Here, we follow the long-term evolution of such a 3D merger product in a
1D stellar evolution code. During a thermal relaxation phase after the
coalescence, the merger product reaches critical surface rotation, sheds mass
and then spins down primarily because of internal mass readjustments. The spin
of the merger product after thermal relaxation is mainly set by the
co-evolution of the star-torus structure left after coalescence. This evolution
is still uncertain, so we also consider magnetic braking and other
angular-momentum-gain and -loss mechanisms that may influence the final spin of
the merged star. Because of core compression and mixing of carbon and nitrogen
in the merger, enhanced nuclear burning drives a transient convective core that
greatly contributes to the rejuvenation of the star. Once the merger product
relaxed back to the main sequence, it continues its evolution similar to that
of a genuine single star of comparable mass. It is a slow rotator that matches
the magnetic blue straggler Sco. Our results show that merging is a
promising mechanism to explain some magnetic massive stars and it may also be
key to understand the origin of the strong magnetic fields of highly-magnetic
white dwarfs and magnetars.Comment: 17 pages (incl. appendix), 14 figures, 2 tables; accepted for
publication in MNRA
Formation of sdB-stars via common envelope ejection by substellar companions
Common envelope (CE) phases in binary systems where the primary star reaches
the tip of the red giant branch are discussed as a formation scenario for hot
subluminous B-type (sdB) stars. For some of these objects, observations point
to very low-mass companions. In hydrodynamical CE simulations with the
moving-mesh code AREPO, we test whether low-mass objects can successfully
unbind the envelope. The success of envelope removal in our simulations
critically depends on whether or not the ionization energy released by
recombination processes in the expanding material is taken into account. If
this energy is thermalized locally, envelope ejection eventually leading to the
formation of an sdB star is possible with companion masses down to the brown
dwarf range. For even lower companion masses approaching the regime of giant
planets, however, envelope removal becomes increasingly difficult or impossible
to achieve. Our results are consistent with current observational constraints
on companion masses of sdB stars. Based on a semianalytic model, we suggest a
new criterion for the lowest companion mass that is capable of triggering a
dynamical response of the primary star thus potentially facilitating the
ejection of a common envelope. This gives an estimate consistent with the
findings of our hydrodynamical simulations.Comment: 12 pages, 8 figures, 3 tables; submitted to A&
Do electron-capture supernovae make neutron stars? First multidimensional hydrodynamic simulations of the oxygen deflagration
Context. In the classical picture, electron-capture supernovae and the accretion-induced collapse of oxygen-neon white dwarfs undergo an oxygen deflagration phase before gravitational collapse produces a neutron star. These types of core collapse events are postulated to explain several astronomical phenomena. In this work, the oxygen deflagration phase is simulated for the first time using multidimensional hydrodynamics.
Aims. By simulating the oxygen deflagration with multidimensional hydrodynamics and a level-set-based flame approach, new insights can be gained into the explosive deaths of 8−10 M⊙ stars and oxygen-neon white dwarfs that accrete material from a binary companion star. The main aim is to determine whether these events are thermonuclear or core-collapse supernova explosions, and hence whether neutron stars are formed by such phenomena.
Methods. The oxygen deflagration is simulated in oxygen-neon cores with three different central ignition densities. The intermediate density case is perhaps the most realistic, being based on recent nuclear physics calculations and 1D stellar models. The 3D hydrodynamic simulations presented in this work begin from a centrally confined flame structure using a level-set-based flame approach and are performed in 2563 and 5123 numerical resolutions.
Results. In the simulations with intermediate and low ignition density, the cores do not appear to collapse into neutron stars. Instead, almost a solar mass of material becomes unbound from the cores, leaving bound remnants. These simulations represent the case in which semiconvective mixing during the electron-capture phase preceding the deflagration is inefficient. The masses of the bound remnants double when Coulomb corrections are included in the equation of state, however they still do not exceed the effective Chandrasekhar mass and, hence, would not collapse into neutron stars. The simulations with the highest ignition density (log 10ρc = 10.3), representing the case where semiconvective mixing is very efficient, show clear signs that the core will collapse into a neutron star
Self-consistent MHD simulation of jet launching in a neutron star - white dwarf merger
The merger of a white dwarf (WD) and a neutron star (NS) is a relatively
common event that will produce an observable electromagnetic signal.
Furthermore, the compactness of these stellar objects makes them an interesting
candidate for gravitational wave (GW) astronomy, potentially being in the
frequency range of LISA and other missions. To date, three-dimensional
simulations of these mergers have not fully modelled the WD disruption, or have
used lower resolutions and have not included magnetic fields even though they
potentially shape the evolution of the merger remnant. In this work, we
simulate the merger of a 1.4 NS with a 1 carbon oxygen WD in
the magnetohydrodynamic moving mesh code \AREPO. We find that the disruption of
the WD forms an accretion disk around the NS, and the subsequent accretion by
the NS powers the launch of strongly magnetized, mildly relativistic jets
perpendicular to the orbital plane. Although the exact properties of the jets
could be altered by unresolved physics around the NS, the event could result in
a transient with a larger luminosity than kilonovae. We discuss possible
connections to fast blue optical transients (FBOTs) and long-duration gamma-ray
bursts. We find that the frequency of GWs released during the merger is too
high to be detectable by the LISA mission, but suitable for deci-hertz
observatories such as LGWA, BBO or DECIGO.Comment: Accepted for publication in A&A. 13 pages, 11 figure
Bipolar planetary nebulae from common envelope evolution of binary stars
Asymmetric shapes and evidence for binary central stars suggest a
common-envelope origin for many bipolar planetary nebulae. The bipolar
components of the nebulae are observed to expand faster than the rest and the
more slowly expanding material has been associated with the bulk of the
envelope ejected during the common-envelope phase of a stellar binary system.
Common-envelope evolution in general remains one of the biggest uncertainties
in binary star evolution and the origin of the fast outflow has not been
explained satisfactorily. We perform three-dimensional magnetohydrodynamic
simulations of common-envelope interaction with the moving-mesh code AREPO.
Starting from the plunge-in of the companion into the envelope of an asymptotic
giant branch star and covering hundreds of orbits of the binary star system, we
are able to follow the evolution to complete envelope ejection. We find that
magnetic fields are strongly amplified in two consecutive episodes. First, when
the companion spirals in the envelope and, second, when it forms a contact
binary with the core of the former giant star. In the second episode, a
magnetically-driven, high-velocity outflow of gas is launched self-consistently
in our simulations. The outflow is bipolar and the gas is additionally
collimated by the ejected common envelope. The resulting structure reproduces
typical morphologies and velocities observed in young planetary nebulae. We
propose that the magnetic driving mechanism is a universal consequence of
common envelope interaction responsible for a substantial fraction of observed
planetary nebulae. Such a mechanism likely also exists in the common-envelope
phase of other binary stars that lead to the formation of Type Ia supernovae,
X-ray binaries and gravitational-wave merger events.Comment: 9 pages, 11 figures; accepted for publication by A&
In-situ characterization of the Hamamatsu R5912-HQE photomultiplier tubes used in the DEAP-3600 experiment
The Hamamatsu R5912-HQE photomultiplier-tube (PMT) is a novel high-quantum
efficiency PMT. It is currently used in the DEAP-3600 dark matter detector and
is of significant interest for future dark matter and neutrino experiments
where high signal yields are needed.
We report on the methods developed for in-situ characterization and
monitoring of DEAP's 255 R5912-HQE PMTs. This includes a detailed discussion of
typical measured single-photoelectron charge distributions, correlated noise
(afterpulsing), dark noise, double, and late pulsing characteristics. The
characterization is performed during the detector commissioning phase using
laser light injected through a light diffusing sphere and during normal
detector operation using LED light injected through optical fibres
Does the Integration of Haptic and Visual Cues Reduce the Effect of a Biased Visual Reference Frame on the Subjective Head Orientation?
The selection of appropriate frames of reference (FOR) is a key factor in the elaboration of spatial perception and the production of robust interaction with our environment. The extent to which we perceive the head axis orientation (subjective head orientation, SHO) with both accuracy and precision likely contributes to the efficiency of these spatial interactions. A first goal of this study was to investigate the relative contribution of both the visual and egocentric FOR (centre-of-mass) in the SHO processing. A second goal was to investigate humans' ability to process SHO in various sensory response modalities (visual, haptic and visuo-haptic), and the way they modify the reliance to either the visual or egocentric FORs. A third goal was to question whether subjects combined visual and haptic cues optimally to increase SHO certainty and to decrease the FORs disruption effect.Thirteen subjects were asked to indicate their SHO while the visual and/or egocentric FORs were deviated. Four results emerged from our study. First, visual rod settings to SHO were altered by the tilted visual frame but not by the egocentric FOR alteration, whereas no haptic settings alteration was observed whether due to the egocentric FOR alteration or the tilted visual frame. These results are modulated by individual analysis. Second, visual and egocentric FOR dependency appear to be negatively correlated. Third, the response modality enrichment appears to improve SHO. Fourth, several combination rules of the visuo-haptic cues such as the Maximum Likelihood Estimation (MLE), Winner-Take-All (WTA) or Unweighted Mean (UWM) rule seem to account for SHO improvements. However, the UWM rule seems to best account for the improvement of visuo-haptic estimates, especially in situations with high FOR incongruence. Finally, the data also indicated that FOR reliance resulted from the application of UWM rule. This was observed more particularly, in the visual dependent subject. Conclusions: Taken together, these findings emphasize the importance of identifying individual spatial FOR preferences to assess the efficiency of our interaction with the environment whilst performing spatial tasks
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