1,032 research outputs found
The mass and radius of the M-dwarf in the short period eclipsing binary RR Caeli
We present new photometry and spectroscopy of the eclipsing white dwarf -
M-dwarf binary star RR Cae. We use timings of the primary eclipse from
white-light photo-electric photometry to derive a new ephemeris for the
eclipses. We find no evidence for any period change greater than Pdot/P ~ 5E-12
over a timescale of 10 years. We have measured the effective temperature of the
white dwarf, T_WD, from an analysis of two high resolution spectra of RR Cae
and find T_WD = (7540 +- 175)K. We estimate a spectral type of M4 for the
companion from the same spectra. We have combined new spectroscopic orbits for
the white dwarf and M-dwarf with an analysis of the primary eclipse and cooling
models for helium white dwarfs to measure the mass and radius of the M-dwarf.
The mass of the M-dwarf is (0.182 - 0.183) +- 0.013 Msun and the radius is
(0.203 - 0.215) +- 0.013 Rsun, where the ranges quoted for these values reflect
the range of white dwarf models used. In contrast to previous studies, which
lacked a spectroscopic orbit for the white dwarf, we find that the mass and
radius of the M-dwarf are normal for an M4 dwarf. The mass of the white dwarf
is (0.440 +-0.022) Msun. With these revised masses and radii we find that RR
Cae will become a cataclysmic variable star when the orbital period is reduced
from its current value of 7.3 hours to 121 minutes by magnetic braking in 9-20
Gyr. We note that there is night-to-night variability of a few seconds in the
timing of primary eclipse caused by changes to the shape of the primary
eclipse. We speculate as to the possible causes of this phenomenon. (Abridged)Comment: Accepted for publication in MNRAS. The paper contains 10 figures and
3 table
Simultaneous X-ray/optical observations of GX 9+9 (4U 1728-16)
We report on the results of the first simultaneous X-ray (RXTE) and optical
(SAAO) observations of the luminous low mass X-ray binary (LMXB) GX 9+9 in 1999
August. The high-speed optical photometry revealed an orbital period of 4.1958
hr and confirmed previous observations, but with greater precision. No X-ray
modulation was found at the orbital period. On shorter timescales, a possible
1.4-hr variability was found in the optical light curves which might be related
to the mHz quasi-periodic oscillations seen in other LMXBs. We do not find any
significant X-ray/optical correlation in the light curves. In X-rays, the
colour-colour diagram and hardness-intensity diagram indicate that the source
shows characteristics of an atoll source in the upper banana state, with a
correlation between intensity and spectral hardness. Time-resolved X-ray
spectroscopy suggests that two-component spectral models give a reasonable fit
to the X-ray emission. Such models consist of a blackbody component which can
be interpreted as the emission from an optically thick accretion disc or an
optically thick boundary layer, and a hard Comptonized component for an
extended corona.Comment: 19 pages, 13 figures; accepted for publication in MNRA
Dairy Resource Management: A Comparison of Conventional and Pasture-Based Systems
Facing rapid and significant change in the sector, U.S. dairy production trends from 1993-2005 were tracked and performance measures (scale and technical efficiency and returns on assets) were estimated for conventional and pasture-based dairy farms using data from USDA's Agricultural Resource Management Survey. Comparisons of relative economic performance of dairy farms by size and type are made.dairy operations, pasture-based systems, technical efficiency, Livestock Production/Industries,
Evidence for a 2 hr Optical Modulation in GS1826-24
We report the discovery of a 2.1hr optical modulation in the transient source
GS1826-24, based on two independent high time-resolution photometric observing
runs. There is additional irregular variability on shorter timescales. The
source also exhibited an optical burst during each observation, with peak
fluxes consistent with those of the three X-ray bursts so far detected by
BeppoSAX. We compare the low-amplitude variation (0.06 mag) to that seen on the
orbital periods of the short period X-ray bursters, X1636-536 and X1735-444, as
well as the similarity in their non-periodic fluctuations. Other transient
neutron star LMXBs possess short periods in the range 3.8-7.1 hrs. However, if
confirmed as the orbital, a 2.1 hr modulation would make GS1826-24 unique and
therefore of great interest within the context of their formation and
evolution.Comment: 7 pages, 4 figures, to be published in MNRA
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