We study C/O white dwarfs (WDs) with masses of 1.0-1.4 M accreting solar-composition material at very high accretion rates. We address the secular changes in the WDs, and in particular, the question ...whether accretion and the thermonuclear runaways result is net accretion or erosion. The present calculation is unique in that it follows a large number of cycles, thus revealing the secular evolution of the WD system. We find that counter to previous studies, accretion does not give rise to steady-state burning. Instead, it produces cyclic thermonuclear runaways of two types. During most of the evolution, many small cycles of hydrogen ignition and burning build a helium layer over the surface of the WD. This He layer gradually thickens and progressively becomes more degenerate. Once a sufficient amount of He has accumulated, several very large helium burning flashes take place and expel the accreted envelope, leaving no net mass accumulation. Thus, the multicycle evolution has a notable effect on the overall behaviour of the WD system - under the assumptions considered, such a scenario will not lead to an accretion-induced collapse, nor will it produce a Type Ia supernova, unless a major new physical process is found.
There is a growing number of Type IIn supernovae (SNe) which present an outburst prior to their presumably final explosion. These precursors may affect the SN display, and are likely related to ...poorly charted phenomena in the final stages of stellar evolution. By coadding Palomar Transient Factory (PTF) images taken prior to the explosion, here we present a search for precursors in a sample of 16 Type IIn SNe. We find five SNe IIn that likely have at least one possible precursor event (PTF 10bjb, SN 2010mc, PTF 10weh, SN 2011ht, and PTF 12cxj), three of which are reported here for the first time. For each SN we calculate the control time. We find that precursor events among SNe IIn are common: at the one-sided 99% confidence level, >50% of SNe IIn have at least one pre-explosion outburst that is brighter than 3 x 10 super(7) L sub(middot in circle) taking place up to 1/3 yr prior to the SN explosion. The average rate of such precursor events during the year prior to the SN explosion is likely gap1 yr super(-1), and fainter precursors are possibly even more common. Ignoring the two weakest precursors in our sample, the precursors rate we find is still on the order of one per year. We also find possible correlations between the integrated luminosity of the precursor and the SN total radiated energy, peak luminosity, and rise time. These correlations are expected if the precursors are mass-ejection events, and the early-time light curve of these SNe is powered by interaction of the SN shock and ejecta with optically thick circumstellar material.
Observations of marine stratus clouds in clean air off the Californian coast reveal a functional relationship between the number of cloud condensation nuclei (CCN) and supersaturation. ...Satellite‐derived liquid droplet density estimates the number density of CCN. Combining the estimated supersaturation using Köhler theory, global maps of supersaturation and the critical activation size of CCN are estimated. Here, we show that high supersaturation >0.5% persists over the oceans with a critical CCN size of 25–30 nm, which is smaller than the conventional wisdom of 60 nm. Independent support for such high supersaturation in the marine cloud is obtained from CCN measurements provided by the “Atmospheric Tomography Mission.” Higher supersaturation implies smaller activation size for CCN making cloud formation more sensitive to changes in aerosol nucleation.
Plain Language Summary
Clouds in Earth's atmosphere are of fundamental importance for the climate by regulating the reflection of sunlight into space and interacting with thermal radiation from Earth. Clouds form when moist air ascends and gets supersaturated with water vapor that condenses on aerosol particles of sufficient sizes, which then grow into cloud droplets. The aerosol number‐density and size spectrum influence the resulting cloud properties, and the supersaturation determines which aerosols can be activated into cloud drops. Here, we show that the supersaturation in marine liquid clouds is significantly higher than in the conventional view. As a consequence, much smaller aerosols can serve as cloud condensation nuclei. This can make cloud formation more sensitive to changes in aerosol properties than previously thought. Such a result should be of general interest and lead to a better understanding of aerosol‐cloud interactions, which presently constitute the largest uncertainty in our understanding of climate.
Key Points
On average, supersaturation in marine clouds is significantly higher than the conventional view of 0.2%–0.3%
Due to the higher supersaturation, much smaller aerosols get activated into cloud droplets
The results are essential for better understanding aerosols‐cloud interactions
We construct Super-Eddington Slim discs models around both stellar and supermassive black holes by allowing the formation of a porous layer with a reduced effective opacity. Unlike the standard ...scenario in which the discs become thick, super-Eddington discs remain slim. In addition, they accelerate a significant wind with a 'thick disc' geometry. We show that above about 1.5 times the standard critical mass accretion rate (needed to release the Eddington luminosity), the net luminosity released is above Eddington. At above about five times the standard critical rate, the central BH accretes more than the Eddington accretion rate. Above about
, the disc remains slim but the wind becomes spherical, and the present model breaks down.
The Phanerozoic climate Shaviv, Nir J.; Svensmark, Henrik; Veizer, Ján
Annals of the New York Academy of Sciences,
January 2023, Letnik:
1519, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We review the long‐term climate variations during the last 540 million years (Phanerozoic Eon). We begin with a short summary of the relevant geological and geochemical datasets available for the ...reconstruction of long‐term climate variations. We then explore the main drivers of climate that appear to explain a large fraction of these climatic oscillations. The first is the long‐term trend in atmospheric CO2 due to geological processes, while the second is the atmospheric ionization due to the changing galactic environment. Other drivers, such as albedo and geographic effects, are of secondary importance. In this review, we pay particular attention to problems that may affect the measurements of temperature obtained from oxygen isotopes, such as the long‐term changes in the concentration of δ18O seawater.
We review the long‐term climate variations during the last 540 million years (Phanerozoic Eon). We first summarize the geological and geochemical datasets available for reconstruction of long‐term climate variations. We then explore the main drivers of climate that appear to explain a large fraction of these climatic oscillations. The first is the long‐term trend in atmospheric CO2 due to geological processes, while the second is the atmospheric ionization due to the changing galactic environment.
We show that inhomogeneity of cosmic ray (CR) sources, due to the concentration of supernova remnants (SNRs) towards the galactic spiral arms, can naturally explain the anomalous increase in the ...positron/electron ratio observed by PAMELA. We consistently recover the observed positron fraction between 1 and 100 GeV using SNRs as the sole source of CRs. The contribution of a few known nearby SNRs dominates the CR electron spectrum above approximately 100 GeV, leading to the relatively flat spectrum observed by Fermi and to the sharp cutoff observed by H.E.S.S.
Abstract
Atmospheric ionization produced by cosmic rays has been suspected to influence aerosols and clouds, but its actual importance has been questioned. If changes in atmospheric ionization have a ...substantial impact on clouds, one would expect to observe significant responses in Earth’s energy budget. Here it is shown that the average of the five strongest week-long decreases in atmospheric ionization coincides with changes in the average net radiative balance of 1.7 W/m
$$^2$$
2
(median value: 1.2 W/m
$$^2$$
2
) using CERES satellite observations. Simultaneous satellite observations of clouds show that these variations are mainly caused by changes in the short-wave radiation of low liquid clouds along with small changes in the long-wave radiation, and are almost exclusively located over the pristine areas of the oceans. These observed radiation and cloud changes are consistent with a link in which atmospheric ionization modulates aerosol's formation and growth, which survive to cloud condensation nuclei and ultimately affect cloud formation and thereby temporarily the radiative balance of Earth.
The detection by Rest et al. of light echoes from η Carinae has provided important new observational constraints on the nature of its 1840s era giant eruption. Spectra of the echoes suggest a ...relatively cool spectral temperature of about 5500 K, lower than the lower limit of about 7000 K suggested in the optically thick wind-outflow analysis of Davidson. This has led to a debate about the viability of this steady wind model relative to alternative, explosive scenarios. Here we present an updated analysis of the wind-outflow model using newer low-temperature opacity tabulations and accounting for the stronger mass-loss implied by the >10 M⊙ mass now inferred for the Homunculus. A major conclusion is that, because of the sharp drop in opacity due to recombination loss of free electrons for T < 6500 K, a low temperature of about 5000 K is compatible with, and indeed expected from, a wind with the extreme mass-loss inferred for the eruption. Within a spherical grey model in radiative equilibrium, we derive spectral energy distributions for various assumptions for the opacity variation of the wind, providing a basis for comparisons with observed light echo spectra. The scaling results here are also potentially relevant for other highly optically thick outflows, including those from classical novae, giant eruptions of luminous blue variables and supernovae Type IIn precursors. A broader issue therefore remains whether the complex, variable features observed from such eruptions are better understood in terms of a steady or explosive paradigm, or perhaps a balance of these idealizations.
Abstract
Most of the diffuse Galactic GeV γ-ray emission is produced via collisions of cosmic ray (CR) protons with ISM protons. As such the observed spectra of the γ-rays and the CRs should be ...strongly linked. Recent observations of Fermi-LAT exhibit a hardening of the γ-ray spectrum at around a hundred GeV, between the Sagittarius and Carina tangents, and a further hardening at a few degrees above and below the Galactic plane. However, standard CR propagation models that assume a time-independent source distribution and a location-independent diffusion cannot give rise to a spatially dependent CR (and hence γ-ray) spectral slopes. Here, we consider a dynamic spiral arm model in which the distribution of CR sources is concentrated in the (dynamic) spiral arms, and we study the effects of this model on the π0-decay-produced γ-ray spectra. Within this model, near the Galactic arms the observed γ-ray spectral slope is not trivially related to the CR injection spectrum and energy dependence of the diffusion coefficient. We find unique signatures that agree with the Fermi-LAT observations. This model also provides a physical explanation for the difference between the local CR spectral slope and the CR slope inferred from the average γ-ray spectrum.
We have previously focused on studying the electron-capture isotopes within the dynamic spiral-arms cosmic-rays propagation model and empirically derived the energy dependence of the electron ...attachment rate using the observation of 49Ti/49V and 51V/51Cr ratios in cosmic rays. We have also shown how this relation recovers the energy dependence seen in the lab measurements. In this work, we use this relation to construct the 44Ti/44Ca ratio and place a lower limit on the amount of 44Ti that is required for it to be nucleosynthesized at the source. The results also imply that the acceleration process of the radioisotopes cannot be much longer than a century timescale (or else the required nucleosynthesized amount has to be correspondingly larger). We also provide a similar lower limit on the source 60Fe by comparing to the recently observed 60Fe/56Fe.
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