2,101 research outputs found
Analysis of The Hipparcos Measurements of HD10697 - A Mass Determination of a Brown-Dwarf Secondary
HD10697 is a nearby main-sequence star around which a planet candidate has
recently been discovered by means of radial-velocity measurements (Vogt et al.
1999, submitted to ApJ). The stellar orbit has a period of about three years,
the secondary minimum mass is 6.35 Jupiter masses and the minimum semi-major
axis is 0.36 milli-arc-sec (mas). Using the Hipparcos data of HD10697 together
with the spectroscopic elements of Vogt et al. (1999) we found a semi-major
axis of 2.1 +/- 0.7 mas, implying a mass of 38 +/- 13 Jupiter masses for the
unseen companion. We therefore suggest that the secondary of HD10697 is
probably a brown dwarf, orbiting around its parent star at a distance of 2 AU.Comment: 6 pages, 2 figures, LaTex, aastex, accepted for publication by ApJ
Letter
The Optical Gravitational Lensing Experiment. Red Clump Stars as a Distance Indicator
We present relation of the mean I-band brightness of red clump stars on
metallicity. Red clump stars were proposed to be a very attractive standard
candle for distance determination. The calibration is based on 284 nearby red
giant stars whose high quality spectra allowed to determine accurate individual
metal abundances. High quality parallaxes (\sigma_\pi / \pi < 10%) and
photometry of these very bright stars come from Hipparcos measurements.
Metallicity of the sample covers a large range: -0.6<[Fe/H]<+0.2 dex. We find a
weak dependence of the mean I-band brightness on metallicity (about 0.13
mag/dex).
What is more important, the range of metallicity of the Hipparcos sample
partially overlaps with metallicity of field giants in the LMC, thus making it
possible to determine the distance to the LMC by almost direct comparison of
brightness of the local Hipparcos red clump giants with that of LMC stars.
Photometry of field red clump giants in nine low extinction fields of the LMC
halo collected during the OGLE-II microlensing survey compared with the
Hipparcos red clump stars data yields the distance modulus to the LMC:
(m-M)_LMC=18.24+/-0.08 mag.Comment: 11 pages. Latex+psfig. Accepted for publictation in ApJ Letters.
Major revision: 30% larger sample of Hipparcos red giants with spectroscopic
metallicities and extended photometry of the LMC field red clump stars (9
lines-of-sight). Due to journal space limitation, sections on comparison of
the LMC red clump distance modulus with previous determinations and relations
between the color indices and metallicity removed from this versio
A Concept for an STJ-based Spectrograph
We describe a multi-order spectrograph concept suitable for 8m-class
telescopes, using the intrinsic spectral resolution of Superconducting
Tunneling Junction detectors to sort the spectral orders. The spectrograph
works at low orders, 1-5 or 1-6, and provides spectral coverage with a
resolving power of R~8000 from the atmospheric cutoff at 320 nm to the long
wavelength end of the infrared H or K band at 1800 nm or 2400 nm. We calculate
that the spectrograph would provide substantial throughput and wavelength
coverage, together with high time resolution and sufficient dynamic range. The
concept uses currently available technology, or technologies with short
development horizons, restricting the spatial sampling to two linear arrays;
however an upgrade path to provide more spatial sampling is identified. All of
the other challenging aspects of the concept - the cryogenics, thermal baffling
and magnetic field biasing - are identified as being feasible.Comment: Accepted in Monthly Notices of the Royal Astronomical Society, 12
pages with 10 figure
Diseases of winter linseed : occurrence, effects and importance
In 1998, a survey of the incidence and severity of diseases was carried out on 30 crops of winter linseed at early flowering and again at crop maturity. Five crops each were selected in south west, east, east Midlands, west Midlands and north of England and from Scotland. Crops were predominantly cv. Oliver (90% crops), grown from certified seed (83%) and sown in September (97%). Pasmo (Mycosphaerella) was the most important disease, affecting leaves of 73% crops at early flowering and 90% crops at maturity. Powdery mildew (70% crops), Alternaria (30% crops) on leaves and Botrytis on capsules (70% crops) were also common. Regional differences were apparent for powdery mildew, which was present in all regions except the southwest, whilst Alternaria predominated in the Midlands. Half of the crops surveyed had received fungicide sprays, but this appeared to have made limited impact on disease severity. Pasmo is a new threat to UK linseed crops and this raises concerns about the threat it poses to spring linsee
First Astronomical Application of a Cryogenic TES Spectrophotometer
We report on the first astronomical observations with a photon counting pixel
detector that provides arrival time- (delta t = 100ns) and energy- (delta
E_gamma < 0.15eV) resolved measurements from the near IR through the near UV.
Our test observations were performed by coupling this Transition Edge Sensor
(TES) device to a 0.6m telescope; we have obtained the first simultaneous
optical near-IR phase-resolved spectra of the Crab pulsar. A varying infrared
turnover gives evidence of self-absorption in the pulsar plasma. The potential
of such detectors in imaging arrays from a space platform are briefly
described.Comment: 4 pages, 5 figure
GAIA: AGB stars as tracers of star formation histories in the Galaxy and beyond
We discuss the tracing of star formation histories with ESA's space
astrometry mission GAIA, emphasizing the advantages of AGB stars for this
purpose. GAIA's microarcsecond-level astrometry, multi-band photometry and
spectroscopy will provide individual distances, motions, effective
temperatures, gravities and metallicities for vast numbers of AGB stars in the
Galaxy and beyond. Reliable ages of AGB stars can be determined to distances of
\~200 kpc in a wide range of ages and metallicities, allowing star formation
histories to be studied in a diversity of astrophysical environments.Comment: 4 pages, 1 figure, to be appear in 'Mass-Losing Pulsating Stars and
their Circumstellar Matter', eds. Y. Nakada, M. Honma & M. Seki, Kluwer ASSL
series, vol. 28
Temperature determination via STJ optical spectroscopy
ESA's Superconducting Tunnel Junction (STJ) optical photon-counting camera
(S-Cam2) incorporates an array of pixels with intrinsic energy sensitivity.
Using the spectral fitting technique common in X-ray astronomy, we fit black
bodies to nine stellar spectra, ranging from cool flare stars to hot white
dwarfs. The measured temperatures are consistent with literature values at the
expected level of accuracy based on the predicted gain stability of the
instrument. Having also demonstrated that systematic effects due to count rate
are likely to be small, we then proceed to apply the temperature determination
method to four cataclysmic variable (CV) binary systems. In three cases we
measure the temperature of the accretion stream, while in the fourth we measure
the temperature of the white dwarf. The results are discussed in the context of
existing CV results. We conclude by outlining the prospects for future versions
of S-Cam.Comment: 9 pages, 9 figures (11 files); uses aa.cls; accepted for publication
in A&
Light Propagation in the Gravitational Field of Moving Bodies by means of Lorentz Transformation. I. Mass monopoles moving with constant velocities
We show how to derive the equations of light propagation in the gravitational
field of uniformly moving mass monopoles without formulating and integrating
the differential equations of light propagation in that field. The well-known
equations of light propagation in the gravitational field of a motionless mass
monopole are combined with a suitable Lorentz transformation. The possibility
to generalize this technique for the more complicated case of uniformly moving
body of arbitrary multipole structure is discussed.Comment: 10 page
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