1,405 research outputs found
A Fourier transform spectrometer for visible and near ultra-violet measurements of atmospheric absorption
The development of a prototype, ground-based, Sun-pointed Michelson interferometric spectrometer is described. Its intended use is to measure the atmospheric amount of various gases which absorb in the near-infrared, visible, and near-ultraviolet portions of the electromagnetic spectrum. Preliminary spectra which contain the alpha, 0.8 micrometer, and rho sigma tau water vapor absorption bands in the near-infrared are presented to indicate the present capability of the system. Ultimately, the spectrometer can be used to explore the feasible applications of Fourier transform spectroscopy in the ultraviolet where grating spectrometers were used exclusively
Accretion disc dynamics in extreme mass ratio compact binaries
An analysis is presented of a numerical investigation of the dynamics and
geometry of accretion discs in binary systems with mass ratios q < 0.1,
applicable to ultra-compact X-ray binaries, AM CVn stars and very short period
cataclysmic variables. The steady-state geometry of the disc in the binary
reference frame is found to be quite different from that expected at higher
mass ratios. For q ~ 0.1, the disc takes on the usual elliptical shape, with
the major axis aligned perpendicular to the line of centres of the two stars.
However, at smaller mass ratios the elliptical gaseous orbits in the outer
regions of the disc are rotated in the binary plane. The angle of rotation
increases with gas temperature, but is found to vary inversely with q. At q =
0.01, the major axis of these orbits is aligned almost parallel to the line of
centres of the two stars. These effects may be responsible for the similar disc
structure inferred from Doppler tomography of the AM CVn star GP Com
(Morales-Rueda et al. 2003), which has q = 0.02. The steady-state geometry at
low mass ratios is not predicted by an inviscid, restricted three-body model of
gaseous orbits; it is related to the effects of tidal-viscous truncation of the
disc near the Roche lobe boundary. Since the disc geometry can be inferred
observationally for some systems, it is proposed that this may offer a useful
diagnostic for the determination of mass ratios in ultra-compact binaries.Comment: 17 pages, 9 figures, 7 in colour. Accepted for publication in MNRAS.
Plain article formatting to get round arXiv problems with mn2e.st
Smoothed Particle Hydrodynamics Simulations of Apsidal and Nodal Superhumps
In recent years a handful of systems have been observed to show "negative"
(nodal) superhumps, with periods slightly shorter than the orbital period. It
has been suggested that these modes are a consequence of the slow retrograde
precession of the line of nodes in a disk tilted with respect to the orbital
plane. Our simulations confirm and refine this model: they suggest a roughly
axisymmetric, retrogradely-precessing, tilted disk that is driven at a period
slightly less than half the orbital period as the tidal field of the orbiting
secondary encounters in turn the two halves of the disk above and below the
midplane. Each of these passings leads to viscous dissipation on one face of an
optically-thick disk -- observers on opposite sides of the disk would each
observe one brightening per orbit, but 180 degrees out of phase with each
other.Comment: 11 pages. Accepted for publication in The ApJ Letter
The statistical significance of the superhump signal in U Gem
Although its well determined mass ratio of q=\Msec/\Mwd=0.357\pm0.007
should avoid superoutbursts according to the thermal tidal instability model,
the prototypical dwarf nova U Gem experienced in 1985 an extraordinary long
outburst resembling very much superoutbursts observed in SU UMa systems.
Recently, the situation for the model became even worse as superhump detections
have been reported for the 1985 outburst of U Gem. The superhump signal is
noisy and the evidence provided by simple periodograms seems to be weak.
Therefore and because of the importance for our understanding of superoutbursts
and superhumps, we determine the statistical significance of the recently
published detection of superhumps in the AAVSO light curve of the famous long
1985 outburst of U Gem. Using Lomb-Scargle periodograms, analysis of variance
(AoV), and Monte-Carlo methods we analyse the 160 visual magnitudes obtained by
the AAVSO during the outburst and relate our analyse to previous superhump
detections. The 160 data points of the outburst alone do not contain a
statistically significant period. However, using additionally the
characteristics of superhumps detected previously in other SU UMa systems and
searching only for signals that are consistent with these, we derive a
significance for the superhump signal. The alleged appearance of an
additional superhump at the end of the outbursts appears to be statistically
insignificant. Although of weak statistical significance, the superhump signal
of the long 1985 outburst of U Gem can be interpreted as further indication for
the SU UMa nature of this outburst. This further contradicts the tidal
instability model as the explanation for the superhump phenomenon.Comment: 7 pages, 7 figures, accepted for publication in A&
On Echo Outbursts and ER UMa Supercycles in SU UMa-type Cataclysmic Variables
I present a variation on Osaki's tidal-thermal-instability model for SU UMa
behavior. I suggest that in systems with the lowest mass ratios, the
angular-momentum dissipation in an eccentric disk is unable to sustain the disk
on the hot side of the thermal instability. This decoupling of the tidal and
thermal instabilities in systems with q < 0.07 allows a better explanation of
the `echo' outbursts of EG Cnc and the short supercycles of RZ LMi and DI UMa.
The idea might also apply to the soft X-ray transients.Comment: To appear in PASP, April 2001 (6 pages, 4 figs
Hydrodynamics of the stream-disk impact in interacting binaries
We use hydrodynamic simulations to provide quantitative estimates of the
effects of the impact of the accretion stream on disks in interacting binaries.
For low accretion rates, efficient radiative cooling of the hotspot region can
occur, and the primary consequence of the stream impact is stream overflow
toward smaller disk radii. The stream is well described by a ballistic
trajectory, but larger masses of gas are swept up and overflow at smaller, but
still highly supersonic, velocities. If cooling is inefficient, overflow still
occurs, but there is no coherent stream inward of the disk rim. Qualitatively,
the resulting structure appears as a bulge extending downstream along the disk
rim. We calculate the mass fraction and velocity of the overflowing component
as a function of the important system parameters, and discuss the implications
of the results for X-ray observations and doppler tomography of cataclysmic
variables, low-mass X-ray binaries and supersoft X-ray sources.Comment: 16 pages, including 8 figures. 1 color figure as a jpeg. ApJ, in
pres
SDSS J210014.12+004446.0: A New Dwarf Nova with Quiescent Superhumps?
We report follow-up observations of the Sloan Digital Sky Survey Cataclysmic
Variable SDSS J210014.12+004446.0 (hereafter SDSS J2100). We obtained
photometry and spectroscopy in both outburst and quiescent states, providing
the first quiescent spectrum of this source. In both states, non-sinusoidal
photometric modulations are apparent, suggestive of superhumps, placing SDSS
J2100 in the SU UMa subclass of dwarf novae. However, the periods during
outburst and quiescence differ significantly, being 2.099 plus or minus 0.002
hr and 1.96 plus or minus 0.02 hr respectively. Our phase-resolved spectroscopy
during outburst yielded an estimate of about 2 hr for the orbital period,
consistent with the photometry. The presence of the shorter period modulation
at quiescence is unusual, but not unique. Another atypical feature is the
relative weakness of the Balmer emission lines in quiescence. Overall, we find
a close similarity between SDSS J2100 and the well-studied superhump
cataclysmic Variable V503 Cygni. By analogy, we suggest that the quiescent
modulation is due to a tilted accretion disk -- producing negative superhumps
-- and the modulation in outburst is due to positive superhumps from the
precession of an elliptical disk.Comment: 6 pages, 5 eps figures, accepted by PASP Dec. 16th, 200
Reach in and reach out : the story of the MSc in pipeline engineering at Newcastle University
This paper presents an unusual case of university-industry interaction whereby a group of small businesses came together to persuade a university to establish an MSc in Pipeline Engineering. We identify that the course contributed to regional development in four ways. Firstly, it provided graduates for local industry. Secondly, it linked local firms with pipeline engineers world wide and raised the region's profile within that network. Thirdly, it strengthened the research base of the university through the recruitment of pipeline engineers from industry and fourthly, it facilitated the possibility of joint research between the university and local firms. We question whether this model is transferable to other industry sectors/universities. We conclude that this outreach activity has been shaped by the 'reach-in' to the university of the local business community and propose a revised model of university interaction with regional industry. Traditionally universities have been seen as 'reaching out' to regional industry and the collaborations have been viewed as being instigated by the university and often research-based. Our revised model proposes an alternative mechanism whereby collaborations can be instigated by industry and through a teaching-route
An Accurate Determination of the Optical Periodic Modulation in the X-Ray Binary SAX J1808.4-3658
We report on optical imaging of the X-ray binary SAX J1808.4-3658 with the
8-m Gemini South Telescope. The binary, containing an accretion-powered
millisecond pulsar, appears to have a large periodic modulation in its
quiescent optical emission. In order to clarify the origin of this modulation,
we obtained three time-resolved -band light curves (LCs) of the source in
five days. The LCs can be described by a sinusoid, and the long time-span
between them allows us to determine optical period P=7251.9 s and phase 0.671
at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar
orbit), with uncertainties of 2.8 s and 0.008 (90 % confidence), respectively.
This periodicity is highly consistent with the X-ray orbital ephemeris. By
considering this consistency and the sinusoidal shape of the LCs, we rule out
the possibility of the modulation arising from the accretion disk. Our study
supports the previous suggestion that the X-ray pulsar becomes rotationally
powered in quiescence, with its energy output irradiating the companion star,
causing the optical modulation. While it has also been suggested that the
accretion disk would be evaporated by the pulsar, we argue that the disk exists
and gives rise to the persistent optical emission. The existence of the disk
can be verified by long-term, multi-wavelength optical monitoring of the source
in quiescence, as an increasing flux and spectral changes from the source would
be expected based on the standard disk instability model.Comment: 7 pages, 3 figures. Accepted for publication in Ap
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
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