1,671 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
Differential Emission Measure Determination of Collisionally Ionized Plasma: II. Application to Hot Stars
In a previous paper we have described a technique to derive constraints on
the differential emission measure (DEM) distribution, a measure of the
temperature distribution, of collisionally ionized hot plasmas from their X-ray
emission line spectra. We apply this technique to the Chandra/HETG spectra of
all of the nine hot stars available to us at the time this project was
initiated. We find that DEM distributions of six of the seven O stars in our
sample are very similar but that theta Ori has an X-ray spectrum characterized
by higher temperatures. The DEM distributions of both of B stars in our sample
have lower magnitudes than those of the O stars and one, tau Sco, is
characterized by higher temperatures than the other, beta Cru. These results
confirm previous work in which high temperatures have been found for theta Ori
and tau Sco and taken as evidence for channeling of the wind in magnetic
fields, the existence of which are related to the stars' youth. Our results
demonstrate the utility of our method for deriving temperature information for
large samples of X-ray emission line spectra.Comment: The contents of this paper were formerly part of astro-ph/0403603
which was split into two paper
VLBI observations of weak extragalactic radio sources for the alignment of the future GAIA frame with the ICRF
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between the optical and radio
positions, it will be important to align the GAIA frame and the International
Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is
found that only 10% of the ICRF sources are suitable to establish this link,
either because they are not bright enough at optical wavelengths or because
they have significant extended radio emission which precludes reaching the
highest astrometric accuracy. In order to improve the situation, we have
initiated a VLBI survey dedicated to finding additional high-quality radio
sources for aligning the two frames. The sample consists of about 450 sources,
typically 20 times weaker than the current ICRF sources, which have been
selected by cross-correlating optical and radio catalogues. This paper presents
the observing strategy and includes preliminary results of observation of 224
of these sources with the European VLBI Network in June 2007
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
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
Covid-19: dogma over potential for prolonged droplet dispersal in air
Background / Outline: Many of us will have seen a plume of smoke drifting on the breeze from somebody smoking but it is much harder to show where the air breathed out, sneezed or coughed by another person has travelled in a street or aisle of a supermarket. Smoke can indicate where breath and tiny droplets coughed or sneezed may travel to but there are important differences to their dispersal due to the size of droplets compared to smoke particles. This perspectives article briefly explores evidence of dispersal of fine spray droplets in air, including not only microscopic droplets normally considered to form an aerosol and disperse in air, but also relatively large âballistic droplets produced by coughing that might travel up to only a few metres, and then focuses on an under-reported intermediate-sized droplet that previous research investigating rain-splashed plant pathogens has shown can be blown significant distances in wind. The article gives examples of fall-speeds calculated for different sized droplets in still air and reports distances fine spray droplets were blown in a wind tunnel experiment. The article is very timely in that it discusses this issue in the context of the Covid-19 epidemic and the role of guidance on separation distances and use of facemasks to block a potential transmission route. This proposed âperspectivesâ article is not only timely but also of importance to help ensure that good science is conducted to help researchers working on the current Covid-19 epidemic by making them aware of the wealth of existing knowledge produced by experts in other disciplines
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
Sub-Saturn Planet Candidates to HD 16141 and HD 46375
Precision Doppler measurements from the Keck/HIRES spectrometer reveal
periodic Keplerian velocity variations in the stars HD 16141 and HD 46375. HD
16141 (G5 IV) has a period of 75.8 d and a velocity amplitude of 11 m/s,
yielding a companion having Msini = 0.22 Mjup and a semimajor axis, a = 0.35
AU. HD 46375 (K1 IV/V) has a period of 3.024 d and a velocity amplitude of 35
m/s, yielding a companion with Msini=0.25 Mjup, a semimajor axis of a = 0.041
AU, and an eccentricity of 0.04 (consistent with zero). These companions
contribute to the rising planet mass function toward lower masses.Comment: 4 Figure
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
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