359 research outputs found
Producing type Iax supernovae from a specific class of helium-ignited WD explosions?
It has recently been proposed that one sub-class of type Ia supernovae (SNe
Ia) is sufficiently both distinct and common to be classified separately from
the bulk of SNe Ia, with a suggested class name of "type Iax supernovae" (SNe
Iax), after SN 2002cx. However, their progenitors are still uncertain. We study
whether the population properties of this class might be understood if the
events originate from a subset of sub-Chandrasekhar mass explosions. In this
potential progenitor population, a carbon--oxygen white dwarf (CO WD)
accumulates a helium layer from a non-degenerate helium star; ignition of that
helium layer then leads to ignition of the CO WD. We incorporated detailed
binary evolution calculations for the progenitor systems into a binary
population synthesis model to obtain rates and delay times for such events. The
predicted Galactic event rate of these explosions is ~1.5\times10^{-3}{yr}^{-1}
according to our standard model, in good agreement with the measured rates of
SNe Iax. In addition, predicted delay times are ~70Myr-800Myr, consistent with
the fact that most of SNe Iax have been discovered in late-type galaxies. If
the explosions are assumed to be double-detonations -- following current model
expectations -- then based on the CO WD masses at explosion we also estimate
the distribution of resulting SN brightness (-13 \gtrsim M_{bol} \gtrsim
-19mag), which can reproduce the empirical diversity of SNe Iax. We speculate
on why binaries with non-degenerate donor stars might lead to SNe Iax if
similar systems with degenerate donors do not. We suggest that the high mass of
the helium layer necessary for ignition at the lower accretion rates typically
delivered from non-degenerate donors might be necessary to produce SN
2002cx-like characteristics, perhaps even by changing the nature of the CO
ignition.Comment: 8 pages, 10 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
On the role of recombination in common-envelope ejections
The energy budget in common-envelope events (CEEs) is not well understood,
with substantial uncertainty even over to what extent the recombination energy
stored in ionised hydrogen and helium might be used to help envelope ejection.
We investigate the reaction of a red-giant envelope to heating which mimics
limiting cases of energy input provided by the orbital decay of a binary during
a CEE, specifically during the post-plunge-in phase during which the spiral-in
has been argued to occur on a time-scale longer than dynamical. We show that
the outcome of such a CEE depends less on the total amount of energy by which
the envelope is heated than on how rapidly the energy was transferred to the
envelope and on where the envelope was heated. The envelope always becomes
dynamically unstable before receiving net heat energy equal to the envelope's
initial binding energy. We find two types of outcome, both of which likely lead
to at least partial envelope ejection: "runaway" solutions in which the
expansion of the radius becomes undeniably dynamical, and superficially
"self-regulated" solutions, in which the expansion of the stellar radius stops
but a significant fraction of the envelope becomes formally dynamically
unstable. Almost the entire reservoir of initial helium recombination energy is
used for envelope expansion. Hydrogen recombination is less energetically
useful, but is nonetheless important for the development of the dynamical
instabilities. However, this result requires the companion to have already
plunged deep into the envelope; therefore this release of recombination energy
does not help to explain wide post-common-envelope orbits.Comment: 17 pages, 10 figures, submitted to MNRAS. Comments are welcom
Globular cluster formation efficiencies from black-hole X-ray binary feedback
We investigate a scenario in which feedback from black-hole X-ray binaries
(BHXBs) sometimes begins inside young star clusters before strong supernova
feedback. Those BHXBs could reduce the gas fraction inside embedded young
clusters whilst maintaining virial equilibrium, which may help globular
clusters (GCs) to stay bound when supernova-driven gas ejection subsequently
occurs. Adopting a simple toy model with parameters guided by BHXB population
models, we produce GC formation efficiencies consistent with
empirically-inferred values. The metallicity dependence of BHXB formation could
naturally explain why GC formation efficiency is higher at lower metallicity.
For reasonable assumptions about that metallicity dependence, our toy model can
produce a GC metallicity bimodality in some galaxies without a bimodality in
the field-star metallicity distribution.Comment: Accepted to ApJ Letters on 19th July. 6 pages. The definitive version
is available from: http://iopscience.iop.org/2041-8205/809/1/L16
Episodic mass ejections from common-envelope objects
After the initial fast spiral-in phase experienced by a common-envelope
binary, the system may enter a slow, self-regulated phase, possibly lasting
100s of years, in which all the energy released by orbital decay can be
efficiently transported to the surface, where it is radiated away. If the
remaining envelope is to be removed during this phase, this removal must occur
through some as-yet-undetermined mechanism. We carried out 1-d hydrodynamic
simulations of a low-mass red giant undergoing a synthetic common-envelope
event in such a slow spiral-in phase, using the stellar evolutionary code MESA.
We simulated the heating of the envelope due to frictional dissipation from a
binary companion's orbit in multiple configurations and investigated the
response of the giant's envelope. We find that our model envelopes become
dynamically unstable and develop large-amplitude pulsations, with periods in
the range 3-20 years and very short growth time-scales of similar order. The
shocks and associated rebounds that emerge as these pulsations grow are in some
cases strong enough to dynamically eject shells of matter of up to 0.1
, % of the mass of the envelope, from the stellar
surface at above escape velocity. These ejections are seen to repeat within a
few decades, leading to a time-averaged mass-loss rate of order
which is sufficiently high to
represent a candidate mechanism for removing the entire envelope over the
duration of the slow spiral-in phase.Comment: 24 pages, 15 figures. This article has been accepted for publication
in Monthly Notices of the Royal Astronomical Society, published by Oxford
University Pres
Sub-Chandrasekhar White Dwarf Mergers as the Progenitors of Type Ia Supernovae
Type Ia supernovae (SNe Ia) are generally thought to be due to the thermonuclear explosions of carbonâoxygen
white dwarfs (COWDs) with masses near the Chandrasekhar mass. This scenario, however, has two long-standing
problems. First, the explosions do not naturally produce the correct mix of elements, but have to be finely tuned
to proceed from subsonic deflagration to supersonic detonation. Second, population models and observations
give formation rates of near-Chandrasekhar WDs that are far too small. Here, we suggest that SNe Ia instead
result from mergers of roughly equal-mass CO WDs, including those that produce sub-Chandrasekhar mass
remnants. Numerical studies of such mergers have shown that the remnants consist of rapidly rotating cores that
contain most of the mass and are hottest in the center, surrounded by dense, small disks. We argue that the disks
accrete quickly, and that the resulting compressional heating likely leads to central carbon ignition. This ignition
occurs at densities for which pure detonations lead to events similar to SNe Ia. With this merger scenario, we
can understand the type Ia rates and have plausible reasons for the observed range in luminosity and for the
bias of more luminous supernovae toward younger populations. We speculate that explosions of WDs slowly
brought to the Chandrasekhar limitâwhich should also occurâare responsible for some of the âatypicalâ SNe Ia
Geochemical Analysis of Ironstone Preserved Molluscan Fossils of the Hell Creek Formation (Cretaceous) and Ludlow Member of the Fort Union Formation (Paleogene) of Southwestern North Dakota
The uppermost Cretaceous Hell Creek Formation and Paleocene Ludlow Member of the Fort Union Formation in easternmost Montana and western North Dakota produce ironstone preserved freshwater molluscan fossils. Ironstone preserved mollusks are associated with ironstone nodules, which have been described as composed of iron carbonate, iron oxide, or manganese oxide by various researchers. To date, the exact composition of the ironstone preservation has not been satisfactorily determined to allow agreement between researchers. Freshwater mollusk fossils, preserved as ironstone external casts, molds, and steinkerns, tend to be highly weathered. The poor preservation of the fossils has resulted in little professional interest, with a limited understanding of the geochemical conditions that produced this preservational phenomenon. The intent of this project was to determine the composition of the ironstone, and attempt to constrain the geochemical conditions necessary to produce the ironstone preservation.
Ironstone preserved fossils and nodules were collected from five localities in Bowman and Slope Counties in southwestern North Dakota. In order to determine the mineralogical composition, samples of mollusks and nodules were analyzed using x-ray diffraction (XRD). Four fossil and four nodule samples were sent to the University of Arizona for 18O and 13C isotope analyses. The elemental composition of three mollusks and two nodules were analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS). Composition information collected from the XRD analyses were entered into the computer program Geochemistâs WorkbenchÂŽ, in which Eh-pH stability diagrams were created to constrain the geochemical conditions necessary to produce ironstone preservation.
XRD analyses have identified the current mineralogical composition of the ironstone nodules and fossils as siderite (FeCO3), quartz (SiO2), and goethite (FeOOH). The original nodule-forming iron mineral was identified as siderite. Analysis of thermodynamic relationships and stability diagrams indicates that siderite formation occurs within a fairly restricted range of ion activities and Eh-pH conditions. Because sulfate will preferentially combine with ferrous iron to form pyrite, the system must have little to no sulfate activity. Similarly, the fugacity of carbon dioxide must be relatively high in order to encourage the precipitation of siderite. The activity of iron must be above 10-6 mol/kg for siderite precipitation; however, increased iron activity beyond 10-6 mol/kg does not appear to increase the overall stability of siderite. From Eh-pH diagrams, it can be determined that siderite is only stable in a neutral to basic and a moderately to severely reducing environment.
Ferrous iron ions may have directly replaced the calcium ions in aragonitic shells without the dissolution of aragonite and precipitation of siderite. A replacement scenario allows for the preservation of shell ornamentation observed on many of the ironstone preserved mollusks. SEM and isotope analyses indicate that the siderite was formed in a completely continental environment. The restrictions for siderite precipitation and stability provide a guide for the geochemical pore water conditions that may have existed during early diagenesis of the Hell Creek Formation and Ludlow Member
Cyclic variation in the flow field behaviour within a direct injection spark ignition engine: a high speed digital particle image velocimetry study
Currently environmental concerns are driving internal combustion engine manufacturers to seek greater fuel efficiency, more refinement and lower emissions. Cyclic variation is a known obstacle to achieving the greatest potential against these goals and therefore an understanding of how to reduce these is sought. It is widely accepted that cyclic variation in in-cylinder flow motions is a key contributor to overall cyclic variation and therefore the characterisation of factors affecting these is an important step in the process of achieving a better understanding and ultimately control of cyclic variation.
This thesis reports the development of a novel optical engine research facility in which high speed digital particle image velocimetry (HSDPIV) has been applied to the study of flow field behaviour within a direct injection spark ignition (DISI) engine. This study investigates the spatial and temporal development of flow structures over and within many engine cycles. Flow field PIV measurements have been captured with a high spatial resolution and temporal frequencies up to 5 kHz from a number of measurement locations at a large range of crank angles.
The major contributions from this work have included the use of the novel measurement technique to investigate spatial and temporal flow field development in the intake runner, valve jet, in-cylinder tumble and swirl planes and the pent roof. The gathered data have been used to investigate cycle by cycle variations in both high and low frequency flow structures. Major findings of this work have included the observation of highly varying flow fields throughout the engine cycle. Frequency analysis of these flows has allowed the low frequency bulk motions and higher frequency turbulent components to be studied. The low frequency flow field components are shown to create varying flow field interactions within the cylinder that also affect the manner in which the flow develops over the course of the cycle. The intensity of the turbulence fluctuations, u , has been calculated based upon the high frequency components within the flow and variations within this are shown to correlate with pressure related combustion parameters
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