2,026 research outputs found
Variability of eta Carinae III
Spectra (1951-78) of the central object in eta Car, taken by A.D. Thackeray,
reveal three previously unrecorded epochs of low excitation. Since 1948, at
least, these states have occurred regularly in the 2020 day cycle proposed by
Damineli et al. They last about 10 percent of each cycle. Early slit spectra
(1899-1919) suggest that at that time the object was always in a low state.
JHKL photometry is reported for the period 1994-2000. This shows that the
secular increase in brightness found in 1972-94 has continued and its rate has
increased at the shorter wavelengths. Modulation of the infrared brightness in
a period near 2020 days continues. There is a dip in the JHKL light curves near
1998.0, coincident with a dip in the X-ray light curve. Evidence is given that
this dip in the infrared repeats in the 2020 day cycle. As suggested by
Whitelock & Laney, the dip is best interpreted as an eclipse phenomenon in an
interacting binary system; the object eclipsed being a bright region (`hot
spot'), possibly on a circumstellar disc or produced by interacting stellar
winds. The eclipse coincides in phase and duration with the state of low
excitation. It is presumably caused by a plasma column and/or by one of the
stars in the system.Comment: 10 pages, 7 postscript figures, accepted for MNRA
SN2002kg -- the brightening of LBV V37 in NGC 2403
SN2002kg is a type IIn supernova, detected in October 2002 in the nearby
spiral galaxy NGC 2403. We show that the position of SN2002kg agrees within the
errors with the position of the LBV V37. Ground based and HST ACS images
however show that V37 is still present after the SN2002kg event. We compiled a
lightcurve of V37 which underlines the variablity of the object, and shows that
SN2002kg was the brightening of V37 and not a supernova. The recent brightening
is not a giant eruption, but more likely part of an S Dor phase. V37 shows
strong Halpha +[NII] emission in recent images and in the SN2002kg spectrum,
which we interprete as the signature of the presence of an LBV nebula. A
historic spectrum lacks emission, which may hint that we are witnessing the
formation of an LBV nebula.Comment: 4 pages, 5 figures, accepted in A&A Letter, paper with images in full
resolution at http://www.astro.ruhr-uni-bochum.de/kweis/publications.htm
On the Structure and Kinematics of Nebulae around LBVs and LBV Candidates in the LMC
We present a detailed analysis of the morphology and kinematics of nebulae
around LBVs and LBV candidates in the Large Magellanic Cloud. HST images and
high-resolution Echelle Spectra were used to determine the size, shape,
brightness, and expansion velocities of the LBV nebulae around R127, R143, and
S61. For S Dor, R71, R99, and R84 we discuss the possible presence of nebular
emission, and derive upper limits for the size and lower limits on the
expansion velocities of possible nebulae. Including earlier results for the LBV
candidates S119 and SK-69 279 we find that in general the nebulae around LBVs
in the LMC are comparable in size to those found in the Milky Way. The
expansion velocities of the LMC nebulae, however, are significantly lower--by
about a factor of 3 to 4--than those of galactic nebulae of comparable size.
Galactic and LMC nebulae show about the same diversity of morphologies, but
only in the LMC do we find nebulae with outflow. Bipolarity--at least to some
degree--is found in nebulae in the LMC as well as in the Milky Way, and
manifests a much more general feature among LBV nebulae than previously known.Comment: paper accepted by A&A, 25 pages, 24 figures; paper with images in
full resolution available at
http://www.astro.ruhr-uni-bochum.de/kweis/publications.htm
A spectroscopic event of eta Car viewed from different directions: The data and first results
We present spectroscopic observations with high spectral resolution of eta
Car as seen by the SE lobe of the Homunculus nebula over the 2003.5
"spectroscopic event". The observed spectra represent the stellar spectrum
emitted near the pole of the star and are much less contaminated with nebular
emission lines than direct observations of the central object. The "event" is
qualitatively similar near the pole to what is observed in direct spectra of
the star (more equator-on at 45 degree), but shows interesting differences. The
observations show that the equivalent width changes of H alpha emission and
other lines are less pronounced at the pole than in the line of sight. Also the
absorption components appear less variable. A pronounced high-velocity
absorption is present near the event in the He I lines indicating a
mass-ejection event. This feature is also seen, but less pronounced, in the
hydrogen lines. HeII4686 emission is observed for a brief period of time near
the event and appears, if corrected for light travel time, to precede similar
emission in the direct view. Our observations indicate that the event is
probably not only a change in ionization and excitation structure or a simple
eclipse-like event.Comment: 10 pages, 16 figures, accepted by A&
An outflow from the nebula around the LBV candidate S 119
We present an analysis of the kinematic and morphological structure of the
nebula around the LMC LBV candidate S 119. On HST images, we find a
predominantly spherical nebula which, however, seems to be much better confined
in its eastern hemisphere than in the western one. The filamentary western part
of the nebula is indicative of matter flowing out of the nebula's main body.
This outflow is even more evidenced by our long-slit echelle spectra. They show
that, while most of the nebula has an expansion velocity of 25.5 km/s, the
outflowing material reaches velocities of almost 140 km/s, relative to the
systemic one. A ROSAT HRI image shows no trace of S 119 and thus no indications
of hot or shocked material.Comment: 8 pages, 8 figures, Accepted in A&A, paper with images in full
resolution at http://www.mpifr-bonn.mpg.de/staff/kweis/publications.htm
The characterisation of hydrogen on nickel and cobalt catalysts
We have investigated a series of supported and unsupported nickel and cobalt catalysts, principally using neutron vibrational spectroscopy (inelastic neutron scattering, INS). For an alumina supported Ni catalyst we are able to detect hydrogen on the metal for the first time, all previous work has used Raney Ni. For an unsupported Ni foam catalyst, which has similar behaviour to Raney Ni but with a much lower density, the spectra show that there are approximately equal numbers of (100) and (111) sites, in contrast to Raney Ni that shows largely (111) sites. The observation of hydrogen on cobalt catalysts proved to be extremely challenging. In order to generate a cobalt metal surface, reduction in hydrogen at 250â300 °C is required. Lower temperatures result in a largely hydroxylated surface. The spectra show that on Raney Co (and probably also on a Co foam catalyst), hydrogen occupies a threefold hollow site, similar to that found on Co(101ÂŻ0). The reduced surface is highly reactive: transfers between cells in a high quality glovebox were sufficient to re-hydroxylate the surface.Publisher PDFPeer reviewe
Low-energy pions in nuclear matter and 2pi photoproduction within a BUU transport model
A description of low-energy scattering of pions and nuclei within a BUU
transport model is presented. Implementing different scenarios of medium
modifications, the mean free path of pions in nuclear matter at low momenta and
pion absorption reactions on nuclei have been studied and compared to data and
to results obtained via quantum mechanical scattering theory. We show that even
in a regime of a long pionic wave length the semi-classical transport model is
still a reliable framework for pion kinetic energies greater than ~20-30 MeV.
Results are presented on pion-absorption cross sections in the regime of 10 MeV
< E(kin) < 130 MeV and on photon-induced double-pion production at incident
beam energies of 400-500 MeV.Comment: 22 pages, 20 figures Replaced with a revised version. Accepted for
publication in EPJ A. Added a short section on pion reaction and charge
exchange cross-section
Proton-proton bremsstrahlung below and above pion-threshold: the influence of the -isobar
The proton-proton bremsstrahlung is investigated within a coupled-channel
model with the degree of freedom. The model is consistent with the
scattering up to 1 GeV and the vertex determined in the
study of pion photoproduction reactions. It is found that the
excitation can significantly improve the agreements with the at MeV. Predictions at and MeV are
presented for future experimental tests.Comment: 26 pages Revtex, 12 figures are available from the authors upon
request ([email protected]
Lamination and folding in electromagnetically driven flows of specified geometries
Accepted versio
The Implementation, Interpretation, and Justification of Likelihoods in Cosmology
I discuss the formal implementation, interpretation, and justification of likelihood attributions in cosmology. I show that likelihood arguments in cosmology suffer from significant conceptual and formal problems that undermine their applicability in this context
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