911 research outputs found
Dielectronic recombination and stability of warm gas in AGN
Understanding the thermal equilibrium (stability) curve may offer insights
into the nature of the warm absorbers often found in active galactic nuclei.
Its shape is determined by factors like the spectrum of the ionizing continuum
and the chemical composition of the gas. We find that the stability curves
obtained under the same set of the above mentioned physical factors, but using
recently derived dielectronic recombination rates, give significantly different
results, especially in the regions corresponding to warm absorbers, leading to
different physical predictions. Using the current rates we find a larger
probability of having thermally stable warm absorber at 10^5 \kel than
previous predictions and also a greater possibility for its multiphase nature.
the results obtained with the current dielectronic recombination rate
coefficients are more reliable because the warm absorber models along the
stability curve have computed coefficient values, whereas previous calculations
relied on guessed averages for the same due to lack of available data.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS Letters. The
definitive version is available at
http://www3.interscience.wiley.com/cgi-bin/hom
Modeling RR Tel through the Evolution of the Spectra
We investigate the evolution of RR Tel after the outburst by fitting the
emission spectra in two epochs. The first one (1978) is characterized by large
fluctuations in the light curve and the second one (1993) by the slow fading
trend. In the frame of a colliding wind model two shocks are present: the
reverse shock propagates in the direction of the white dwarf and the other one
expands towards or beyond the giant. The results of our modeling show that in
1993 the expanding shock has overcome the system and is propagating in the
nearby ISM. The large fluctuations observed in the 1978 light curve result from
line intensity rather than from continuum variation. These variations are
explained by fragmentation of matter at the time of head-on collision of the
winds from the two stars. A high velocity (500 km/s) wind component is revealed
from the fit of the SED of the continuum in the X-ray range in 1978, but is
quite unobservable in the line profiles. The geometrical thickness of the
emitting clumps is the critical parameter which can explain the short time
scale variabilities of the spectrum and the trend of slow line intensity
decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To
appear in "The Astrophysical Journal
The Effects of Dark Matter Decay and Annihilation on the High-Redshift 21 cm Background
The radiation background produced by the 21 cm spin-flip transition of
neutral hydrogen at high redshifts can be a pristine probe of fundamental
physics and cosmology. At z~30-300, the intergalactic medium (IGM) is visible
in 21 cm absorption against the cosmic microwave background (CMB), with a
strength that depends on the thermal (and ionization) history of the IGM. Here
we examine the constraints this background can place on dark matter decay and
annihilation, which could heat and ionize the IGM through the production of
high-energy particles. Using a simple model for dark matter decay, we show
that, if the decay energy is immediately injected into the IGM, the 21 cm
background can detect energy injection rates >10^{-24} eV cm^{-3} sec^{-1}. If
all the dark matter is subject to decay, this allows us to constrain dark
matter lifetimes <10^{27} sec. Such energy injection rates are much smaller
than those typically probed by the CMB power spectra. The expected brightness
temperature fluctuations at z~50 are a fraction of a mK and can vary from the
standard calculation by up to an order of magnitude, although the difference
can be significantly smaller if some of the decay products free stream to lower
redshifts. For self-annihilating dark matter, the fluctuation amplitude can
differ by a factor <2 from the standard calculation at z~50. Note also that, in
contrast to the CMB, the 21 cm probe is sensitive to both the ionization
fraction and the IGM temperature, in principle allowing better constraints on
the decay process and heating history. We also show that strong IGM heating and
ionization can lead to an enhanced H_2 abundance, which may affect the earliest
generations of stars and galaxies.Comment: submitted to Phys Rev D, 14 pages, 8 figure
Illumination in symbiotic binary stars: Non-LTE photoionization models. II. Wind case
We describe a non-LTE photoionization code to calculate the wind structure
and emergent spectrum of a red giant wind illuminated by the hot component of a
symbiotic binary system. We consider spherically symmetric winds with several
different velocity and temperature laws and derive predicted line fluxes as a
function of the red giant mass loss rate, \mdot. Our models generally match
observations of the symbiotic stars EG And and AG Peg for \mdot about 10^{-8}
\msunyr to 10^{-7} \msunyr. The optically thick cross- section of the red giant
wind as viewed from the hot component is a crucial parameter in these models.
Winds with cross-sections of 2--3 red giant radii reproduce the observed
fluxes, because the wind density is then high, about 10^9 cm^{-3}. Our models
favor winds with acceleration regions that either lie far from the red giant
photosphere or extend for 2--3 red giant radii.Comment: 51 pages, LaTeX including three tables, requires 15 Encapsulated
Postscript figures, to appear in Ap
Silicates in D-type symbiotic stars: an ISO overview
We investigate the IR spectral features of a sample of D-type symbiotic
stars. Analyzing unexploited ISO-SWS data, deriving the basic observational
parameters of dust bands and comparing them with respect to those observed in
other astronomical sources, we try to highlight the effect of environment on
grain chemistry and physic. We find strong amorphous silicate emission bands at
10 micron and 18 micron in a large fraction of the sample. The analysis of the
10 micron band, along with a direct comparison with several astronomical
sources, reveals that silicate dust in symbiotic stars shows features between
the characteristic circumstellar environments and the interstellar medium. This
indicates an increasing reprocessing of grains in relation to specific
symbiotic behavior of the objects. A correlation between the central wavelength
of the 10 and 18 micron dust bands is found. By the modeling of IR spectral
lines we investigate also dust grains conditions within the shocked nebulae.
Both the unusual depletion values and the high sputtering efficiency might be
explained by the formation of SiO moleculae, which are known to be a very
reliable shock tracer. We conclude that the signature of dust chemical
disturbance due to symbiotic activity should be looked for in the outer,
circumbinary, expanding shells where the environmental conditions for grain
processing might be achieved. Symbiotic stars are thus attractive targets for
new mid-infrared and mm observations.Comment: 24 pages, 6 figures, 5 tables - to be published in A
3D simulations of RS Oph: from accretion to nova blast
RS Ophiuchi is a recurrent nova with a period of about 22 years, consisting
of a wind accreting binary system with a white dwarf (WD) very close to the
Chandrasekhar limit and a red giant star (RG). The system is considered a prime
candidate to evolve into an SNIa. We present a 3D hydrodynamic simulation of
the quiescent accretion and the subsequent explosive phase. The computed
circumstellar mass distribution in the quiescent phase is highly structured
with a mass enhancement in the orbital plane of about a factor of 2 as compared
to the poleward directions. The simulated nova remnant evolves aspherically,
propagating faster toward the poles. The shock velocities derived from the
simulations are in agreement with those derived from observations. For v_RG =
20 km/s and for nearly isothermal flows, we derive a mass transfer rate to the
WD of 10% of the mass loss of the RG. For an RG mass loss of 10^{-7} solar
masses per year, we found the orbit of the system to decay by 3% per million
years. With the derived mass transfer rate, multi-cycle nova models provide a
qualitatively correct recurrence time, amplitude, and fastness of the nova. Our
simulations provide, along with the observations and nova models, the third
ingredient for a deeper understanding of the recurrent novae of the RS Oph
type. In combination with recent multi-cycle nova models, our results suggests
that the WD in RS Oph will increase in mass. Several speculative outcomes then
seem plausible. The WD may reach the Chandrasekhar limit and explode as an SN
Ia. Alternatively, the mass loss of the RG could result in a smaller Roche
volume, a common envelope phase, and a narrow WD+WD system. Angular momentum
loss due to graviational wave emission could trigger the merger of the two WDs
and - perhaps - an SN Ia via the double degenerate scenario.Comment: Accepted by Astronomy & Astrophysics Letters, 4 pages, 5 figures;
Version with high resolution figures and movie can be found at
http://www.astro.phys.ethz.ch/staff/folini/private/research/rsoph/rsoph.htm
Accurate Ritz wavelengths of parity-forbidden [Fe II], [Ti II] and [Cr II] infrared lines of astrophysical interest
With new astronomical infrared spectrographs the demands of accurate atomic
data in the infrared have increased. In this region there is a large amount of
parity-forbidden lines, which are of importance in diagnostics of low-density
astrophysical plasmas. We present improved, experimentally determined, energy
levels for the lowest even LS terms of Fe II, Ti II and Cr II, along with
accurate Ritz wavelengths for parity-forbidden transitions between and within
these terms. Spectra of Fe II, Ti II and Cr II have been produced in a hollow
cathode discharge lamp and acquired using high-resolution Fourier Transform
(FT) spectrometry. The energy levels have been determined by using observed
allowed ultraviolet transitions connecting the even terms with upper odd terms.
Ritz wavelengths of parity-forbidden lines have then been determined. Energy
levels of the four lowest Fe II terms (aD, aF, aD and
aP) have been determined, resulting in 97 different parity-forbidden
transitions with wavelengths between 0.74 and 87 micron. For Ti II the energy
levels of the two lowest terms (aF and bF) have been determined,
resulting in 24 different parity-forbidden transitions with wavelengths between
8.9 and 130 micron. Also for Cr II the energy levels of the two lowest terms
(aS and aD) have been determined, in this case resulting in 12
different parity-forbidden transitions with wavelengths between 0.80 and 140
micron.Comment: Accepted for publication in A&A, 13 pages, 6 figures, 9 table
Star Formation in M51 Triggered by Galaxy Interaction
We have mapped the inner 360'' regions of M51 in the 158micron [CII] line at
55'' spatial resolution using the Far-infrared Imaging Fabry-Perot
Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is
peaked at the nucleus, but is detectable over the entire region mapped, which
covers much of the optical disk of the galaxy. There are also two strong
secondary peaks at ~43% to 70% of the nuclear value located roughly 120'' to
the north-east, and south-west of the nucleus. These secondary peaks are at the
same distance from the nucleus as the corotation radius of the density wave
pattern. The density wave also terminates at this location, and the outlying
spiral structure is attributed to material clumping due to the interaction
between M51 and NGC5195. This orbit crowding results in cloud-cloud collisions,
stimulating star formation, that we see as enhanced [CII] line emission. The
[CII] emission at the peaks originates mainly from photodissociation regions
(PDRs) formed on the surfaces of molecular clouds that are exposed to OB
starlight, so that these [CII] peaks trace star formation peaks in M51. The
total mass of [CII] emitting photodissociated gas is ~2.6x10^{8} M_{sun}, or
about 2% of the molecular gas as estimated from its CO(1-0) line emission. At
the peak [CII] positions, the PDR gas mass to total gas mass fraction is
somewhat higher, 3-17%, and at the secondary peaks the mass fraction of the
[CII] emitting photodissociated gas can be as high as 72% of the molecular
mass.... (continued)Comment: 14 pages, 6 figures, Accepted in ApJ (for higher resolution figures
contact the author
A novel approach and software component for supporting competence-based learning with serious games
Digital educational games constitute a major opportunity for acquiring knowledge and competences in a different way than traditional classroom- and technology-based methods. This paper presents a novel approach for a game component that structures the game play in an adaptive way. This approach consists of a combination of three learning theories and techniques. First, Competence-based Knowledge Space Theory is used structure a knowledge domain into competences and game situations. Second, the Leitner system of flashcards is used to establish structured and timed repetition of competences to be acquired. Third, the Ebbinghaus forgetting curve is taken into account to model forgetting learned competences. This approach has been implemented as a game component in line with the games component architecture of the RAGE project. The design and development of this component followed the requirements of the French games company Kiupe that includes it in its environment of games and mini-games
A 3D Monte Carlo Photoionization Code for Modeling Diffuse Ionized Gas
We have developed a three dimensional Monte Carlo photoionization code
tailored for the study of Galactic H II regions and the percolation of ionizing
photons in diffuse ionized gas. We describe the code, our calculation of
photoionization, heating & cooling, and the approximations we have employed for
the low density H II regions we wish to study. Our code gives results in
agreement with the Lexington H II region benchmarks. We show an example of a 2D
shadowed region and point out the very significant effect that diffuse
radiation produced by recombinations of helium has on the temperature within
the shadow.Comment: MNRAS accepte
- …