ABSTRACT Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever exceed the Chandrasekhar mass and explode as a SN Ia? We explore the range of accretion ...rates that allow a WD to secularly grow in mass, and derive limits on the accretion rate and on the initial mass that will allow it to reach 1.4M -the Chandrasekhar mass. We follow the evolution through a long series of hydrogen flashes, during which a thick helium shell accumulates. This determines the effective helium mass accretion rate for long-term, self-consistent evolutionary runs with helium flashes. We find that net mass accumulation always occurs despite helium flashes. Although the amount of mass lost during the first few helium shell flashes is a significant fraction of that accumulated prior to the flash, that fraction decreases with repeated helium shell flashes. Eventually no mass is ejected at all during subsequent flashes. This unexpected result occurs because of continual heating of the WD interior by the helium shell flashes near its surface. The effect of heating is to lower the electron degeneracy throughout the WD, especially in the outer layers. This key result yields helium burning that is quasi-steady state, instead of explosive. We thus find a remarkably large parameter space within which long-term, self-consistent simulations show that a WD can grow in mass and reach the Chandrasekhar limit, despite its helium flashes.
A definitive determination of the progenitors of Type Ia supernovae (SNIa) has been a conundrum for decades. The single degenerate scenario -- a white dwarf (WD) in a semi-detached binary system ...accreting mass from its secondary -- is a plausible path; however, no simulation to date has shown that such an outcome is possible. In this study, we allowed a WD with a near Chandrasekhar mass of 1.4 M... to evolve over tens of thousands of nova cycles, accumulating mass secularly while undergoing periodic nova eruptions. We present the mass accretion limits within which an SNIa can possibly occur. The results showed, for each parameter combination within the permitted limits, tens of thousands of virtually identical nova cycles where the accreted mass exceeded the ejected mass, i.e. the WD grew slowly but steadily in mass. Finally, the WD became unstable, the maximal temperature rose by nearly two orders of magnitude, heavy element production was enhanced by orders of magnitude and the nuclear and neutrino luminosities became enormous. We also found that this mechanism leading to WD collapse is robust, with WDs in the range 1.0-1.38 M..., and an accretion rate of 5 x 10... M... yr..., all growing steadily in mass. These simulations of the onset of an SNIa event make observationally testable predictions about the light curves of pre-SN stars, and about the chemistry of SNIa ejecta. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We explore a possible mechanism that may explain sudden depressions of surface areas on a comet nucleus, as suggested by observations of the Rosetta mission on comet ...67P/Churyumov-Gerasimenko (hereafter, 67P/C-G). Assuming the area is covered by a thin, compact dust layer of low permeability to gas flow compared to deeper, porous layers, gas can accumulate below the surface when a surge of gas release from amorphous ice occurs upon crystallization. The gas pressure is found to exceed the hydrostatic pressure down to a depth of a few metres. The rapid build-up of pressure may weaken the already fragile, highly porous structure. Eventually, the high pressure gradient that arises drives the gas out and the pressure falls well below the hydrostatic pressure. The rapid pressure drop may result in collapse. Since the crystallization front lies at some depth below the surface, the location on the orbit when this phenomenon occurs is determined by the thermal lag, which, in turn, depends on the thermal conductivity. Numerical simulations show that mostly such activity occurs post-perihelion, but it may also occur pre-perihelion. When permeability is uniform, crystallization still causes increased gas production, but the gas pressure inside the nucleus remains below hydrostatic pressure.
This paper is a sequel to an earlier paper devoted to multiple, multicycle nova evolution models (Prialnik & Kovetz, Paper I), which showed that the different characteristics of nova outbursts can be ...reproduced by varying the values of three basic and independent parameters: the white dwarf mass, M sub(WD), the temperature of its isothermal core, T sub(WD), and the mass transfer rate, M. Here we show that the parameter space is constrained by several analytical considerations and find its limiting surfaces. Consequently, we extend the grid of multicycle nova evolution models presented in Paper I to its limits, adding multicycle nova outburst calculations for a considerable number of new parameter combinations. In particular, the extended parameter space that produces nova eruptions includes low mass transfer rates down to 5 x 10 super(-13) M sub( )yr super(-1) and more models for low T sub(WD). Resulting characteristics of these runs are added to the former parameter combination results to provide a full grid spanning the entire parameter space for carbon-oxygen white dwarfs. The full grid covers the entire range of observed nova characteristics, even those of peculiar objects, which have not been numerically reproduced until now. Most remarkably, runs for very low M lead to very high values for some characteristics, such as outburst amplitude A 20, high super-Eddington luminosities at maximum, heavy element abundance of the ejecta Z sub(ej) - 0.63, and high ejected masses m sub(ej) - 7 x 10 super(-4) M sub( ).
We examine the possibility that icy super-Earth mass planets, formed over long timescales (0.1-1 Gyr) at large distances (∼200-1000 au) from their host stars, will develop massive H-rich atmospheres. ...Within the interior of these planets, high pressure converts CH4 into ethane, butane, or diamond and releases H2. Using simplified models that capture the basic physics of the internal structure, we show that the physical properties of the atmosphere depend on the outflux of H2 from the mantle. When this outflux is molec cm−2 s−1, the outgassed atmosphere has a base pressure of 1 bar. Larger outflows result in a substantial atmosphere where the base pressure may approach 103-104 bar. For any pressure, the mean density of these planets, 2.4-3 g cm−3, is much larger than the mean density of Uranus and Neptune, 1.3-1.6 g cm−3. Thus, observations can distinguish between a Planet Nine with a primordial H/He-rich atmosphere accreted from the protosolar nebula and one with an atmosphere outgassed from the core.
The dramatic brightenings of classical novae have yielded rich data sets of detailed light curves. Modelling these light curves is a challenge for any theory of classical novae. We have used our ...extended grid of nova outburst calculations to predict the luminosities of erupting novae expected in three electromagnetic bands - the visual, the near UV and the X-ray. Our models predict and explain many features of novae before eruption, as well as detailed characterizations of nova outbursts and post-nova declines. The evolutionary time-scales of eruption features vary by orders of magnitude, and depend on the basic nova parameters: white dwarf mass, luminosity and accretion rate. However, all light curves are found to share common features. Some of these features are unique to only one electromagnetic passband, while others show up in two, or in all three of the analysed bands. One extraordinary feature, common to all of our low-mass white dwarfs (0.65 M) novae, is that all exhibit a sharp rise followed by a more gradual decline in the near-UV luminosity, prior to the eruption in the visual luminosity. This is because the expansion of the outer layers lags behind the rise in bolometric luminosity. These predicted precursor-UV-flashes last between a few hours and a few days, and the predicted luminosity increase is between ∼0.5 and ∼3 mag. These flashes should be easily observable if a nova event is detected early and its time coverage is dense. Many observed novae exhibit a pre-maximum halt, and this feature is found in all three electromagnetic bands of many, but not all, of our nova models. We explain the presence or absence of pre-maximum halts as due to changes in the convective energy transfer regime. Finally we note cases where the maximum visual magnitude reaches as high as −8.5 mag for low-mass white dwarfs. This re-emphasizes the fact that white dwarf mass is not always the determining factor in setting a nova's peak luminosity.
The Palomar Transient Factory (PTF) is a synoptic survey designed to explore the transient and variable sky in a wide variety of cadences. We use PTF observations of fields that were observed ...multiple times (≳10) per night, for several nights, to find asteroids, construct their light curves and measure their rotation periods. Here we describe the pipeline we use to achieve these goals and present the results from the first four (overlapping) PTF fields analysed as part of this programme. These fields, which cover an area of 21 deg2, were observed on four nights with a cadence of ∼20 min. Our pipeline was able to detect 624 asteroids, of which 145 (≈20 per cent) were previously unknown. We present high-quality rotation periods for 88 main-belt asteroids and possible period or lower limit on the period for an additional 85 asteroids. For the remaining 451 asteroids, we present lower limits on their photometric amplitudes. Three of the asteroids have light curves that are characteristic of binary asteroids. We estimate that implementing our search for all existing high-cadence PTF data will provide rotation periods for about 10 000 asteroids mainly in the magnitude range ≈14 to ≈20.
We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid ...belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.
We explore a possible mechanism that may explain the outbursts observed by the Deep Impact mission on comet 9P/Tempel 1 that appear to occur near sunrise on a particular area. Assuming that the area ...is covered by a porous, poorly conducting dust layer, the heat wave generated by solar radiation at local noon propagates through the dust layer towards the ice-rich layer underneath it. The heated ice sublimates and the vapour flows towards the surface. However, by the time the heat wave reaches the ice, the spot has moved out of sunlight and its temperature has started dropping. As the vapour flows outwards, the surface has become so cold that it refreezes. Thus, at night some ice accumulates in the dust layer, very close to the surface. At sunrise, it immediately evaporates, producing a short-lived surge of activity. Numerical simulations of this mechanism provide the duration and water production of such outbursts, which are compatible with the 9P/Tempel 1 observations of small outbursts.
PDF
