3,335 research outputs found
The Accreting White Dwarf in SS Cygni Revealed
We have carried out a combined Hubble Space Telescope (HST/GHRS) and Far
Ultraviolet Spectroscopic Explorer FUSE) analysis of the prototype dwarf nova
SS Cygni during quiescence. The FUSE and HST spectra were obtained at
comparable times after outburst and have matching flux levels where the two
spectra overlap. In our synthetic spectral analysis, we have used SS Cygni's
accurate HST FGS parallax giving d = 166pc, a newly determined mass for the
accreting white dwarf (Bitner et al. 2007) of Mwd=0.81Msun (lower than the
previous, widely used 1.2 Msun) and the reddening E_{B-V} values 0.04 (Verbunt
1987; La Dous 1991) and 0.07 (Bruch and Engel 1994) derived from the 2175A
absorption feature in the IUE LWP spectra. From the best-fit model solutions to
the combined HST + FUSE spectral energy distribution, we find that the white
dwarf is reaching a temperature Teff of 45-55,000K in quiescence, assuming
Log(g)= 8.3 with a solar composition accreted atmosphere. The exact temperature
of the WD depends on the reddening assumed and on the inclusion of a quiescent
low mass accretion rate accretion disk. Accretion disk models alone fit badly
in the FUSE range while, and if we take the distance to be a free parameter,
the only accretion disk model which fits well is for a discordant distance of
at least several hundred pc and an accretion rate (1.E-8 Msun/yr which is
unacceptably high for a dwarf nova in quiescence. We discuss the implications
of the white dwarf's temperature on the time-averaged accretion rate and long
term compressional heating models.Comment: ApJ, accepte
Parallax and Distance Estimates for Twelve Cataclysmic Variable Stars
We report parallax and distance estimates for twelve more cataclysmic
binaries and related objects observed with the 2.4m Hiltner telescope at MDM
Observatory. The final parallax accuracy is typically about 1 mas. For only one
of the twelve objects, IR Gem, do we fail to detect a significant parallax.
Notable results include distances for V396 Hya (CE 315), a helium double
degenerate with a relatively long orbital period, and for MQ Dra
(SDSSJ155331+551615), a magnetic system with a very low accretion rate. We find
that the Z Cam star KT Persei is physically paired with a K main-sequence star
lying 15 arcsec away. Several of the targets have distance estimates in the
literature that are based on the white dwarf's effective temperature and flux;
our measurements broadly corroborate these estimates, but tend to put the stars
a bit closer, indicating that the white dwarfs may have rather larger masses
than assumed. As a side note, we briefly describe radial velocity spectroscopy
that refines the orbital period of V396 Hya to 65.07 +- 0.08 min.Comment: Accepted for Astronomical Journal. 19 pages, no figure
An Irradiation Effect in Nova DN Gem 1912 and the Significance of the Period Gap for Classical Novae
Continuous CCD photometry of the classical nova DN Gem during 52 nights in
the years 1992-98 reveals a modulation with a period 0.127844 d. The
semi-amplitude is about 0.03 mag. The stability of the variation suggests that
it is the orbital period of the binary system. This interpretation makes DN Gem
the fourth nova inside the cataclysmic variable (CV) period gap, as defined by
Diaz and Bruch (1997), and it bolsters the idea that there is no period gap for
classical novae. However, the number of known nova periods is still too small
to establish this idea statistically. We eliminate several possible mechanisms
for the variation, and propose that the modulation is driven by an irradiation
effect. We find that model light curves of an irradiated secondary star, fit
the data well. The inclination angle of the system is restricted by this model
to 10 deg < i < 65 deg. We also refine a previous estimate of the distance to
the binary system, and find d=1.6+/-0.6 kpc.Comment: 7 pages, Latex file, 2 .ps files and 3 .eps files. accepted for
publication in MNRAS. also available at:
ftp://ftp.astro.keele.ac.uk/pub/preprints/preprints.htm
The masses of the cataclysmic variables AC Cancri and V363 Aurigae
We present time-resolved spectroscopy and photometry of the double-lined eclipsing cataclysmic variables AC Cnc and V363 Aur (= Lanning 10). There is evidence of irradiation on the inner hemisphere of the secondary star in both systems, which we correct for using a model that reproduces the observations remarkably well. We find the radial velocity of the secondary star in AC Cnc to be KR= 176 ± 3 km sâ1 and its rotational velocity to be v sin i= 135 ± 3 km sâ1. From these parameters we obtain masses of M1= 0.76 ± 0.03 Mâ for the white-dwarf primary and M2= 0.77 ± 0.05 Mâ for the K2 ± 1V secondary star, giving a mass ratio of q= 1.02 ± 0.04. We measure the radial and rotational velocities of the G7 ± 2V secondary star in V363 Aur to be KR= 168 ± 5 km sâ1 and v sin i= 143 ± 5 km sâ1, respectively. The component masses of V363 Aur are M1= 0.90 ± 0.06 Mâ and M2= 1.06 ± 0.11 Mâ, giving a mass ratio of q= 1.17 ± 0.07. The mass ratios for AC Cnc and V363 Aur fall within the theoretical limits for dynamically and thermally stable mass transfer. Both systems are similar to the SW Sex stars, exhibiting single-peaked emission lines with transient absorption features, high-velocity S-wave components and phase-offsets in their radial-velocity curves. The Balmer lines in V363 Aur show a rapid increase in flux around phase 0 followed by a rapid decrease, which we attribute to the eclipse of an optically thick region at the centre of the disc. This model could also account for the behaviour of other SW Sex stars where the Balmer lines show only a shallow eclipse compared to the continuum
State Estimation For An AgonisticâAntagonistic Muscle System
Research on assistive technology, rehabilitation, and prosthetics requires the understanding of human machine interaction, in which human muscular properties play a pivotal role. This paper studies a nonlinear agonisticâantagonistic muscle system based on the Hill muscle model. To investigate the characteristics of the muscle model, the problem of estimating the state variables and activation signals of the dual muscle system is considered. In this work, parameter uncertainty and unknown inputs are taken into account for the estimation problem. Three observers are presented: a high gain observer, a sliding mode observer, and an adaptive sliding mode observer. Theoretical analysis shows the convergence of the three observers. Numerical simulations reveal that the three observers are comparable and provide reliable estimates
Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence
We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the
Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a
normal outburst, when the underlying white dwarf accreter is clearly exposed in
the far ultraviolet. However, model fitting show that a uniform temperature
white dwarf does not reproduce the overall spectrum, especially at the shortest
wavelengths. A better approximation to the spectrum is obtained with a model
consisting of a white dwarf and a rapidly rotating ``accretion belt''. The
white dwarf component accounts for 83% of the total flux, has a temperature of
23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit
accretion belt component accounts for 17% of the total flux, has a temperature
of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000
km/s. The requirement of two components in the modeling of the spectrum of VW
Hyi in quiescence helps to resolve some of the differences in interpretation of
ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of
a second component (and its exact nature) in VW Hyi itself is still relatively
uncertain, given the lack of better models for spectra of the inner disk in a
quiescent dwarf nova.Comment: 6 figures, 10 printed page in the journal, to appear in APJ, 1 Sept.
2004 issue, vol. 61
Cigarette smoking and gastric cancer in the stomach cancer pooling (StoP) project
Tobacco smoking is a known cause of gastric cancer, but
several aspects of the association remain imprecisely
quantified. We examined the relation between cigarette
smoking and the risk of gastric cancer using a uniquely large
dataset of 23 epidemiological studies within the âStomach
cancer Pooling (StoP) Projectâ, including 10 290 cases and
26 145 controls. We estimated summary odds ratios (ORs)
and the corresponding 95% confidence intervals (CIs) by
pooling study-specific ORs using random-effects models.
Compared with never smokers, the ORs were 1.20 (95% CI:
1.09â1.32) for ever, 1.12 (95% CI: 0.99â1.27) for former, and
1.25 (95% CI: 1.11â1.40) for current cigarette smokers. Among
current smokers, the risk increased with number of cigarettes
per day to reach an OR of 1.32 (95% CI: 1.10â1.58) for
smokers of more than 20 cigarettes per day. The risk
increased with duration of smoking, to reach an OR of 1.33
(95% CI: 1.14â1.54) for more than 40 years of smoking and
decreased with increasing time since stopping cigarette
smoking (P for trend<0.01) and became similar to that of
never smokers 10 years after stopping. Risks were somewhat
higher for cardia than noncardia gastric cancer. Risks were
similar when considering only studies with information on
Helicobacter pylori infection and comparing all cases to
H. pylori+ controls only. This study provides the most precise
estimate of the detrimental effect of cigarette smoking on the
risk of gastric cancer on the basis of individual data, including
the relationship with dose and duration, and the decrease in
risk following stopping smoking
The White Dwarf in EM Cygni: Beyond The Veil
We present a spectral analysis of the FUSE spectra of EM Cygni, a Z Cam DN
system. The FUSE spectrum, obtained in quiescence, consists of 4 individual
exposures (orbits): two exposures, at orbital phases phi ~ 0.65 and phi ~ 0.90,
have a lower flux; and two exposures, at orbital phases phi =0.15 and 0.45,
have a relatively higher flux. The change of flux level as a function of the
orbital phase is consistent with the stream material (flowing over and below
the disk from the hot spot region to smaller radii) partially masking the white
dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase
0.45 (when the flux is maximual, using the codes TLUSTY and SYNSPEC. Using a
single white dwarf spectral component, we obtain a white dwarf temperature of
40,000K, rotating at 100km/s. The white dwarf, or conceivably, the material
overflowing the disk rim, shows suprasolar abundances of silicon, sulphur and
possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the
white dwarf temperature could be even as high as 50,000K, contributing more
than 90% of the FUV flux, and the disk contributing less than 10% must have a
mass accretion rate reaching 1.E-10 Msun/yr.In both cases, however, we obtain
that the white dwarf temperature is much higher than previously estimated.Comment: accepted for publication in ApJ, 3 Tables, 12 Figures (including
color figures), 33 pages in present format (possibly 10 pages in ApJ format
U Geminorum: a test case for orbital parameters determination
High-resolution spectroscopy of U Gem was obtained during quiescence. We did
not find a hot spot or gas stream around the outer boundaries of the accretion
disk. Instead, we detected a strong narrow emission near the location of the
secondary star. We measured the radial velocity curve from the wings of the
double-peaked H emission line, and obtained a semi-amplitude value that
is in excellent agreement with the obtained from observations in the
ultraviolet spectral region by Sion et al. (1998). We present also a new method
to obtain K_2, which enhances the detection of absorption or emission features
arising in the late-type companion. Our results are compared with published
values derived from the near-infrared NaI line doublet. From a comparison of
the TiO band with those of late type M stars, we find that a best fit is
obtained for a M6V star, contributing 5 percent of the total light at that
spectral region. Assuming that the radial velocity semi-amplitudes reflect
accurately the motion of the binary components, then from our results: K_em =
107+/-2 km/s; K_abs = 310+/-5 km/s, and using the inclination angle given by
Zhang & Robinson(1987); i = 69.7+/-0.7, the system parameters become: M_WD =
1.20+/-0.05 M_sun,; M_RD = 0.42+/-0.04 M_sun; and a = 1.55+/- 0.02 R_sun. Based
on the separation of the double emission peaks, we calculate an outer disk
radius of R_out/a ~0.61, close to the distance of the inner Lagrangian point
L_1/a~0.63. Therefore we suggest that, at the time of observations, the
accretion disk was filling the Roche-Lobe of the primary, and that the matter
leaving the L_1 point was colliding with the disc directly, producing the hot
spot at this location.Comment: 36 pages, 14 figures, ccepted for publication in A
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