Astrophys.J. 579 (2002) 741-751; Astrophys.J. 579 (2002) 760-763 We examine the AAVSO light curve of U Geminorum from 1908 to 2002, with
particular focus on the October 1985 outburst. This outburst ...was longer than
any other seen in U Gem by about a factor of 2, and appears to be unique among
all dwarf nova outbursts seen in systems with orbital periods longer than 3 hr
in that one can measure the decay time scale during the initial slow decay.
This rate is ~26+-6 d/mag. Using estimates of the rate of accretion during
outburst taken from Froning et al., one can show that ~1e24 g of gas was
accreted onto the white dwarf during the outburst. When coupled with the
viscous time inferred from the (short orbital period) SU UMa stars, the U Gem
viscous time scale lends support to the standard model in which the decays in
dwarf novae can either be viscous or thermal, with the ratio between them being
roughly h/r where h is the vertical pressure scale height in the disk.
We consider vertical heat transport in Keplerian accretion disks, including
the effects of radiation, convection, and turbulent mixing driven by the
Balbus-Hawley instability, in astronomical systems ...ranging from dwarf novae
(DNe), and soft X-ray transients (SXTs), to active galactic nuclei (AGN). We
propose a modified, anisotropic form of mixing-length theory, which includes
radiative and turbulent damping. We also include turbulent heat transport,
which acts everywhere within disks, regardless of whether or not they are
stably stratified, and can move entropy in either direction. We have generated
a series of vertical structure models and thermal equilibrium curves using the
scaling law for the viscosity parameter $\alpha$ suggested by the exponential
decay of the X-ray luminosity in SXTs. We have also included equilibrium curves
for DNe using an $\alpha$ which is constant down to a small magnetic Reynolds
number ($\sim 10^4$). Our models indicate that weak convection is usually
eliminated by turbulent radial mixing. The substitution of turbulent heat
transport for convection is more important on the unstable branches of thermal
equilibrium S-curves when $\alpha$ is larger. The low temperature turnover
points $\Sigma_{max}$ on the equilibrium S-curves are significantly reduced by
turbulent mixing in DNe and SXT disks. However, in AGN disks the standard
mixing-length theory for convection is still a useful approximation when we use
the scaling law for $\alpha$, since these disks are very thin at the relevant
radii. In accordance with previous work, we find that constant $\alpha$ models
give almost vertical S-curves in the $\Sigma-T$ plane and consequently imply
very slow, possibly oscillating, cooling waves.
Astrophys.J. 585 (2003) 1169-1176 Estimates made in the 1970's indicated that a supernova occurring within tens
of parsecs of Earth could have significant effects on the ozone layer. Since
that time, ...improved tools for detailed modeling of atmospheric chemistry have
been developed to calculate ozone depletion, and advances have been made in
theoretical modeling of supernovae and of the resultant gamma-ray spectra. In
addition, one now has better knowledge of the occurrence rate of supernovae in
the galaxy, and of the spatial distribution of progenitors to core-collapse
supernovae. We report here the results of two-dimensional atmospheric model
calculations that take as input the spectral energy distribution of a
supernova, adopting various distances from Earth and various latitude impact
angles. In separate simulations we calculate the ozone depletion due to both
gamma-rays and cosmic rays. We find that for the combined ozone depletion
roughly to double the ``biologically active'' UV flux received at the surface
of the Earth, the supernova must occur at <8 pc. Based on the latest data, the
time-averaged galactic rate of core-collapse supernovae occurring within 8 pc
is ~1.5/Gyr. In comparing our calculated ozone depletions with those of
previous studies, we find them to be significantly less severe than found by
Ruderman (1974), and consistent with Whitten et al. (1976). In summary, given
the amplitude of the effect, the rate of nearby supernovae, and the ~Gyr time
scale for multicellular organisms on Earth, this particular pathway for mass
extinctions may be less important than previously thought.
We examine the AAVSO light curve of U Geminorum from 1908 to 2002, with particular focus on the October 1985 outburst. This outburst was longer than any other seen in U Gem by about a factor of 2, ...and appears to be unique among all dwarf nova outbursts seen in systems with orbital periods longer than 3 hr in that one can measure the decay time scale during the initial slow decay. This rate is ~26+-6 d/mag. Using estimates of the rate of accretion during outburst taken from Froning et al., one can show that ~1e24 g of gas was accreted onto the white dwarf during the outburst. When coupled with the viscous time inferred from the (short orbital period) SU UMa stars, the U Gem viscous time scale lends support to the standard model in which the decays in dwarf novae can either be viscous or thermal, with the ratio between them being roughly h/r where h is the vertical pressure scale height in the disk.
Astrophys.J. 622 (2005) L153-L156 Based on cosmological rates, it is probable that at least once in the last Gy
the Earth has been irradiated by a gamma-ray burst in our Galaxy from within 2
kpc. ...Using a two-dimensional atmospheric model we have performed the first
computation of the effects upon the Earth's atmosphere of one such impulsive
event. A ten second burst delivering 100 kJ/m^2 to the Earth penetrates to the
stratosphere and results in globally averaged ozone depletion of 35%, with
depletion reaching 55% at some latitudes. Significant global depletion persists
for over 5 years after the burst. This depletion would have dramatic
implications for life since a 50% decrease in ozone column density results in
approximately three times the normal UVB flux. Widespread extinctions are
likely, based on extrapolation from UVB sensitivity of modern organisms.
Additional effects include a shot of nitrate fertilizer and NO2 opacity in the
visible providing a cooling perturbation to the climate over a similar
timescale. These results lend support to the hypothesis that a GRB may have
initiated the late Ordovician mass extinction (Melott et al. 2004).
Astrophys.J. 634 (2005) 509-533 Gamma-Ray Bursts (GRBs) are likely to have made a number of significant
impacts on the Earth during the last billion years. We have used a
two-dimensional atmospheric ...model to investigate the effects on the Earth's
atmosphere of GRBs delivering a range of fluences, at various latitudes, at the
equinoxes and solstices, and at different times of day. We have estimated DNA
damage levels caused by increased solar UVB radiation, reduction in solar
visible light due to $\mathrm{NO_2}$ opacity, and deposition of nitrates
through rainout of $\mathrm{HNO_3}$. For the ``typical'' nearest burst in the
last billion years, we find globally averaged ozone depletion up to 38%.
Localized depletion reaches as much as 74%. Significant global depletion (at
least 10%) persists up to about 7 years after the burst. Our results depend
strongly on time of year and latitude over which the burst occurs. We find DNA
damage of up to 16 times the normal annual global average, well above lethal
levels for simple life forms such as phytoplankton. The greatest damage occurs
at low to mid latitudes. We find reductions in visible sunlight of a few
percent, primarily in the polar regions. Nitrate deposition similar to or
slightly greater than that currently caused by lightning is also observed,
lasting several years. We discuss how these results support the hypothesis that
the Late Ordovician mass extinction may have been initiated by a GRB.
We consider vertical heat transport in Keplerian accretion disks, including the effects of radiation, convection, and turbulent mixing driven by the Balbus-Hawley instability, in astronomical systems ...ranging from dwarf novae (DNe), and soft X-ray transients (SXTs), to active galactic nuclei (AGN). We propose a modified, anisotropic form of mixing-length theory, which includes radiative and turbulent damping. We also include turbulent heat transport, which acts everywhere within disks, regardless of whether or not they are stably stratified, and can move entropy in either direction. We have generated a series of vertical structure models and thermal equilibrium curves using the scaling law for the viscosity parameter \(\alpha\) suggested by the exponential decay of the X-ray luminosity in SXTs. We have also included equilibrium curves for DNe using an \(\alpha\) which is constant down to a small magnetic Reynolds number (\(\sim 10^4\)). Our models indicate that weak convection is usually eliminated by turbulent radial mixing. The substitution of turbulent heat transport for convection is more important on the unstable branches of thermal equilibrium S-curves when \(\alpha\) is larger. The low temperature turnover points \(\Sigma_{max}\) on the equilibrium S-curves are significantly reduced by turbulent mixing in DNe and SXT disks. However, in AGN disks the standard mixing-length theory for convection is still a useful approximation when we use the scaling law for \(\alpha\), since these disks are very thin at the relevant radii. In accordance with previous work, we find that constant \(\alpha\) models give almost vertical S-curves in the \(\Sigma-T\) plane and consequently imply very slow, possibly oscillating, cooling waves.
Estimates made in the 1970's indicated that a supernova occurring within tens of parsecs of Earth could have significant effects on the ozone layer. Since that time, improved tools for detailed ...modeling of atmospheric chemistry have been developed to calculate ozone depletion, and advances have been made in theoretical modeling of supernovae and of the resultant gamma-ray spectra. In addition, one now has better knowledge of the occurrence rate of supernovae in the galaxy, and of the spatial distribution of progenitors to core-collapse supernovae. We report here the results of two-dimensional atmospheric model calculations that take as input the spectral energy distribution of a supernova, adopting various distances from Earth and various latitude impact angles. In separate simulations we calculate the ozone depletion due to both gamma-rays and cosmic rays. We find that for the combined ozone depletion roughly to double the ``biologically active'' UV flux received at the surface of the Earth, the supernova must occur at <8 pc. Based on the latest data, the time-averaged galactic rate of core-collapse supernovae occurring within 8 pc is ~1.5/Gyr. In comparing our calculated ozone depletions with those of previous studies, we find them to be significantly less severe than found by Ruderman (1974), and consistent with Whitten et al. (1976). In summary, given the amplitude of the effect, the rate of nearby supernovae, and the ~Gyr time scale for multicellular organisms on Earth, this particular pathway for mass extinctions may be less important than previously thought.
Based on cosmological rates, it is probable that at least once in the last Gy the Earth has been irradiated by a gamma-ray burst in our Galaxy from within 2 kpc. Using a two-dimensional atmospheric ...model we have performed the first computation of the effects upon the Earth's atmosphere of one such impulsive event. A ten second burst delivering 100 kJ/m^2 to the Earth penetrates to the stratosphere and results in globally averaged ozone depletion of 35%, with depletion reaching 55% at some latitudes. Significant global depletion persists for over 5 years after the burst. This depletion would have dramatic implications for life since a 50% decrease in ozone column density results in approximately three times the normal UVB flux. Widespread extinctions are likely, based on extrapolation from UVB sensitivity of modern organisms. Additional effects include a shot of nitrate fertilizer and NO2 opacity in the visible providing a cooling perturbation to the climate over a similar timescale. These results lend support to the hypothesis that a GRB may have initiated the late Ordovician mass extinction (Melott et al. 2004).
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