9,305 research outputs found
A Sea Change in Eta Carinae
Major stellar-wind emission features in the spectrum of Eta Car have recently
decreased by factors of order 2 relative to the continuum. This is
unprecedented in the modern observational record. The simplest, but unproven,
explanation is a rapid decrease in the wind density.Comment: 11 pages, 3 figures, accepted for publication in ApJ
Bose Einstein condensation of the classical axion field in cosmology?
The axion is a motivated cold dark matter candidate, which it would be
interesting to distinguish from weakly interacting massive particles. Sikivie
has suggested that axions could behave differently during non-linear galaxy
evolution, if they form a Bose-Einstein condensate, and argues that
``gravitational thermalisation'' drives them to a Bose-Einstein condensate
during the radiation dominated era. Using classical equations of motion during
linear structure formation, we explore whether the gravitational interactions
of axions can generate entropy. At linear order in Newton's constant, we
interpret that the principle activities of gravity are to expand the Universe
and grow density fluctuations. To quantify the rate of entropy creation we use
the anisotropic stress to estimate a short dissipation scale for axions, which
does not confirm previous estimates of their gravitational thermalisation rate.Comment: version accepted for publication, modified intro to Bose Einstein
condensatio
Mid-Cycle Changes in Eta Carinae
In late 2006, ground-based photometry of Car plus the Homunculus
showed an unexpected decrease in its integrated apparent brightness, an
apparent reversal of its long-term brightening. Subsequent HST/WFPC2 photometry
of the central star in the near-UV showed that this was not a simple reversal.
This multi-wavelength photometry did not support increased extinction by dust
as the explanation for the decrease in brightness. A spectrum obtained with
GMOS on the Gemini-South telescope, revealed subtle changes mid-way in
Car's 5.5 yr spectroscopic cycle 0when compared with HST/STIS spectra at the
same phase in the cycle. At mid-cycle the secondary star is 20--30 AU from the
primary. We suggest that the spectroscopic changes are consistent with
fluctuations in the density and velocity of the primary star's wind, unrelated
to the 5.5 yr cycle but possibly related to its latitude-dependent morphology.
We also discuss subtle effects that must be taken into account when comparing
ground-based and HST/STIS spectra.Comment: 34 pages, 9 Figure
Secular Changes in Eta Carinae's Wind 1998-2011
Stellar wind-emission features in the spectrum of eta Carinae have decreased
by factors of 1.5-3 relative to the continuum within the last 10 years. We
investigate a large data set from several instruments (STIS, GMOS, UVES)
obtained between 1998 and 2011 and we analyze the progression of spectral
changes in the direct view of the star, in the reflected polar-on spectra at
FOS4, and at the Weigelt knots. We find that the spectral changes occurred
gradually on a time scale of about 10 years and that they are dependent on the
viewing angle. The line strengths declined most in our direct view of the star.
About a decade ago, broad stellar wind-emission features were much stronger in
our line-of-sight view of the star than at FOS4. After the 2009 event, the
wind-emission line strengths are now very similar at both locations.
High-excitation He I and N II absorption lines in direct view of the star
strengthened gradually. The terminal velocity of Balmer P Cyg absorption lines
now appears to be less latitude-dependent and the absorption strength may have
weakened at FOS4. Latitude-dependent alterations in the mass-loss rate and the
ionization structure of eta Carinae's wind are likely explanations for the
observed spectral changes.Comment: 42 pages, 12 figures, 2 table
Critical Differences and Clues in Eta Car's 2009 Event
We monitored Eta Carinae with HST WFPC2 and Gemini GMOS throughout the 2009
spectroscopic event, which was expected to differ from its predecessor in 2003
(Davidson et al. 2005). Here we report major observed differences between
events, and their implications. Some of these results were quite unexpected.
(1) The UV brightness minimum was much deeper in 2009. This suggests that
physical conditions in the early stages of an event depend on different
parameters than the "normal" inter-event wind. Extra mass ejection from the
primary star is one possible cause. (2) The expected He II 4687 brightness
maximum was followed several weeks later by another. We explain why this fact,
and the timing of the 4687 maxima, strongly support a "shock breakup"
hypothesis for X-ray and 4687 behavior as proposed 5-10 years ago. (3) We
observed a polar view of the star via light reflected by dust in the Homunculus
nebula. Surprisingly, at that location the variations of emission-line
brightness and Doppler velocities closely resembled a direct view of the star;
which should not have been true for any phenomena related to the orbit. This
result casts very serious doubt on all the proposed velocity interpretations
that depend on the secondary star's orbital motion. (4) Latitude-dependent
variations of H I, He I and Fe II features reveal aspects of wind behavior
during the event. In addition, we discuss implications of the observations for
several crucial unsolved problems.Comment: 45 pages, 9 figures, submitted to Ap
A simulation study of the flight dynamics of elastic aircraft. Volume 1: Experiment, results and analysis
The simulation experiment described addresses the effects of structural flexibility on the dynamic characteristics of a generic family of aircraft. The simulation was performed using the NASA Langley VMS simulation facility. The vehicle models were obtained as part of this research. The simulation results include complete response data and subjective pilot ratings and comments and so allow a variety of analyses. The subjective ratings and analysis of the time histories indicate that increased flexibility can lead to increased tracking errors, degraded handling qualities, and changes in the frequency content of the pilot inputs. These results, furthermore, are significantly affected by the visual cues available to the pilot
A simulation study of the flight dynamics of elastic aircraft. Volume 2: Data
The simulation experiment described addresses the effects of structural flexibility on the dynamic characteristics of a generic family of aircraft. The simulation was performed using the NASA Langley VMS simulation facility. The vehicle models were obtained as part of this research project. The simulation results include complete response data and subjective pilot ratings and comments and so allow a variety of analyses. The subjective ratings and analysis of the time histories indicate that increased flexibility can lead to increased tracking errors, degraded handling qualities, and changes in the frequency content of the pilot inputs. These results, furthermore, are significantly affected by the visual cues available to the pilot
N II 5668-5712, a New Class of Spectral Features in Eta Carinae
We report on the N II 5668-5712 emission and absorption lines in the spectrum
of Eta Carinae. Spectral lines of the stellar wind regions can be classified
into four physically distinct categories: 1) low-excitation emission such as H
I and Fe II, 2) higher excitation He I features, 3) the N II lines discussed in
this paper, and 4) He II emission. These categories have different combinations
of radial velocity behavior, excitation processes, and dependences on the
secondary star. The N II lines are the only known features that originate in
"normal" undisturbed zones of the primary wind but depend primarily on the
location of the hot secondary star. N II probably excludes some proposed
models, such as those where He I lines originate in the secondary star's wind
or in an accretion disk.Comment: 4 figures, 1 tabl
Prediction for the He I 10830A Absorption Wing in the Coming Event of Eta Carinae
We propose an explanation to the puzzling appearance of a wide blue
absorption wing in the He I 10830A P-Cygni profile of the massive binary star
Eta Carinae several months before periastron passage. Our basic assumption is
that the colliding winds region is responsible for the blue wing absorption. By
fitting observations, we find that the maximum outflow velocity of this
absorbing material is ~2300 km/s. We also assume that the secondary star is
toward the observer at periastron passage. With a toy-model we achieve two
significant results. (1) We show that the semimajor axis orientation we use can
account for the appearance and evolution of the wide blue wing under our basic
assumption. (2) We predict that the Doppler shift (the edge of the absorption
profile) will reach a maximum 0-3 weeks before periastron passage, and not
necessarily exactly at periastron passage or after periastron passage.Comment: 15 pages, 6 figures. Accepted for publication in MNRA
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