322 research outputs found
Interferometric Observations of V838 Monocerotis
We have used long-baseline near-IR interferometry to resolve the peculiar
eruptive variable V838 Mon and to provide the first direct measurement of its
angular size. Assuming a uniform disk model for the emission we derive an
apparent angular diameter at the time of observations (November-December 2004)
of milli-arcseconds. For a nominal distance of kpc,
this implies a linear radius of . However, the data are
somewhat better fit by elliptical disk or binary component models, and we
suggest that the emission may be strongly affected by ejecta from the outburst.Comment: 12 pages, 1 two-part encapsulated postscript figure. Accepted by
ApJL. Added a table of observation
Momentum Spectroscopy of Phase Fluctuations of an Elongated Bose-Einstein Condensate
We have measured the momentum distribution of an elongated BEC (aspect ratio
of 152), for temperatures below the critical temperature. The corresponding
coherence length is significantly smaller than the condensate length in a wide
range of temperature, in quantitative agreement with theoretical predictions.
The Lorentzian shape of the momentum spectrum supports the image of a phase
fluctuating quasicondensate.Comment: Proceedings of the International Conference on Laser Spectroscopy
(ICOLS 03), Cairns, Australia, july 200
One-dimensional behavior of elongated Bose-Einstein condensates
We study the properties of elongated Bose-Einstein condensates. First, we
show that the dimensions of the condensate after expansion differs from the 3D
Thomas-Fermi regime. We also study the coherence length of such elongated
condensates.Comment: proceeding of Quantum Gases in Low Dimension, Les Houches 2003, 8
pages, 5 figure
Constraining jet theories using nova outbursts
It is commonly accepted that jets have not been observed in CVs so far. This
absence was recently explained by their low mass transfer rates compared with
objects with jets. A mass accretion limit for jets in CVs was proposed to be
about 1e-7-1e-6 Mo/yr. There was, however, a report of evidence for jets in
V1494 Aql=Nova Aql 1999/2. We estimate the mass transfer rate of this system
around the reported event and show that it is consistent with the theoretical
limit for jets. We further propose that the X-ray flare that was observed in
this object may be connected with a jet as well. The appearance of jets in
novae is actually expected since during the early decline from outburst some
are alike supersoft X-ray sources, in which jets have been found. The detection
of jets in novae also fits the suggestion that in addition to the presence of
an accretion disk, a hot central source is required for the formation of jets.
The observations of jets during the transition phase in V1494 Aql can be
regarded as evidence for the early existence of the accretion disk in the
system. This conclusion supports our previous suggestion for a link between the
transition phase in novae and the re-establishment of the accretion disk. We
further speculate that jets may be restricted to transient novae. If our ideas
are confirmed, jets should be common in transient novae and may be formed
several times during the transition phase and perhaps even long after it ends.
In classical novae jets may be launched and observed in real time. As binary
systems, these objects are easy to study. Novae may, therefore, be key systems
in understanding the formation and evolution of jets and ideal targets to test
and constrain jet theories.Comment: 4 pages, 1 .eps figure, 2 .sty files, ApJL accepted, abstract was
truncated to the 24 lines limi
A model of an expanding giant that swallowed planets for the eruption of V838 Monocerotis
In early 2002 V838 Monocerotis had an extraordinary outburst whose nature is
still unclear. The optical light curve showed at least three peaks and imaging
revealed a light echo around the object - evidence for a dust shell which was
emitted several thousand years ago and now reflecting light from the eruption.
Spectral analysis suggests that the object was relatively cold throughout the
event, which was characterized by an expansion to extremely large radii. We
show that the three peaks in the light curve have a similar shape and thus it
seems likely that a certain phenomenon was three times repeated. Our suggestion
that the outburst was caused by the expansion of a red giant, followed by the
successive swallowing of three relatively massive planets in close orbits,
supplies a simple explanation to all observed peculiarities of this intriguing
object.Comment: 5 pages, 1 LaTex file, 2 .eps figures, accepted for publication in
MNRA
Population and Phase Coherence during the Growth of an Elongated Bose-Einstein Condensate
We study the growth of an elongated phase-fluctuating condensate from a
non-equilibrium thermal cloud obtained by shock-cooling. We compare the growth
of the condensate with numerical simulations, revealing a time delay and a
reduction in the growth rate which we attribute to phase fluctuations. We
measure the phase coherence using momentum Bragg spectroscopy, and thereby
observe the evolution of the phase coherence as a function of time. Combining
the phase coherence results with the numerical simulations, we suggest a simple
model for the reduction of the growth rate based on the reduction of bosonic
stimulation due to phase fluctuations and obtain improved agreement between
theory and experiment
Evidence for large superhumps in TX Col and V4742 Sgr
Since the discovery of the largest positive superhump period in TV Col, we
have started a program to search for superhumps in CVs with large orbital
periods. Here, we summarize preliminary results of TX Col and V4742 Sgr. TX Col
is an intermediate polar with a 5.7-h orbital period. V4742 Sgr is a recent
nova with no known periods. CCD unfiltered continuous photometry of these 2
objects was carried out during 56 nights in 2002-3. In TX Col, in addition to
the orbital period of 5.7 h, we found peaks at 7.1 h and 5.0 h. These are
interpreted as positive and negative superhumps correspondingly, although the
effects of the quasi-periodic oscillations at about 2 h were not taken into
consideration. In the light curve of V4742 Sgr 2 long periods are detected --
6.1 and 5.4 h as well as a short-term period at 1.6 h. This result suggests
that V4742 Sgr is an intermediate polar candidate and a permanent superhump
system with a large orbital period (5.4 h) and a superhump period excess of 13
percent. If these results are confirmed, TX Col, V4742 Sgr and TV Col form a
group of intermediate polars with extremely large superhump periods. There
seems to be now growing evidence that superhumps can occur in intermediate
polars with long orbital periods, which is very likely inconsistent with the
theoretical prediction that superhumps can only occur in systems with mass
ratios below 0.33. Alternatively, if the mass ratio in these systems is
nevertheless below the theoretical limit, they should harbour undermassive
secondaries and massive white dwarfs, near the Chandrasekhar limit, which would
make them excellent candidates for progenitors of supernovae type Ia.Comment: 9 pages, 8 figures, 3 sty files, To appear in the proceedings of IAU
JD5, `White Dwarfs: Galactic and Cosmological Probes', eds. Ed Sion, Stephane
Vennes and Harry Shipman, Full abstract in pape
Detection of orbital and superhump periods in Nova V2574 Ophiuchi (2004)
We present the results of 37 nights of CCD unfiltered photometry of nova
V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 d (~3.40 h)
and 0.14773 d (~3.55 h) in the 2005 data. The 2004 data show variability on a
similar timescale, but no coherent periodicity was found. We suggest that the
longer periodicity is the orbital period of the underlying binary system and
that the shorter period represents a negative superhump. The 3.40 h period is
about 4% shorter than the orbital period and obeys the relation between
superhump period deficit and binary period. The detection of superhumps in the
light curve is evidence of the presence of a precessing accretion disk in this
binary system shortly after the nova outburst. From the maximum magnitude -
rate of decline relation, we estimate the decay rate t_2 = 17+/-4 d and a
maximum absolute visual magnitude of M_Vmax = -7.7+/-1.7 mag.Comment: 6 pages, 6 figures, 2 .sty files, AJ accepted, minor change to one of
reference
Experimental Validation of a Numerical Phase Change Model for Microchannel Slug Flow Boiling
Thermal management of high-power electronic devices continues to be a critical challenge. Flow boiling in microchannel heat sinks has been demonstrated to be an effective method for removing high heat fluxes from these devices owing to utilization of the latent heat of the fluid and the large surface area enhancement for heat exchange. However, microchannel flow boiling technologies have yet to be broadly implemented due to a lack of experimentally validated prediction and design tools. The goal of this study is to use high-fidelity experimental data to validate a previously developed numerical phase change model, to help enable physics-based prediction of flow boiling heat transfer characteristics and reduce the reliance on empirical-based correlations. A novel experimental facility was used to generate archetypal microchannel slug flow boiling and capture high-speed flow visualizations for a range of heat fluxes and flow rates. Image processing of the flow visualizations was performed to extract time-resolved hydrodynamic and heat transfer parameters, such as vapor bubble length and liquid film thickness. The experimental boundary, initial, and operating conditions are input into the numerical model, implemented via a user-defined function in a commercial finite-volume software package, to predict the vapor bubble growth by phase change and the liquid film thickness. A direct comparison of the model prediction and experimental results is performed and good agreement is obtained
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