101 research outputs found
Two-way coherent Doppler error due to solar corona
Two-way coherent Doppler errors resulting from phase scintillations induced on the uplink by the solar corona are considered. It is shown that this error can be estimated by taking statistics on the differential Doppler measurements. Typical estimates for the error are given for four Sun-Earth-probe angles and for integration times ranging from 1 second to 1 minute. These results are based on data collected during the 1985 Voyager 2 conjunction
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Academics' Experiences of a Respite From Work: Effects of Self-Critical Perfectionism and Perseverative Cognition on Postrespite Well-Being
This longitudinal study examined relations between personality and cognitive vulnerabilities and the outcomes of a respite from work. A sample of 77 academic employees responded to week-level measures of affective well-being before, during, and on 2 occasions after an Easter respite. When academics were classified as being either high or low in a self-critical form of perfectionism (doubts about actions), a divergent pattern of respite to postrespite effects was revealed. Specifically, during the respite, the 2 groups of academics experienced similar levels of well-being. However, during postrespite working weeks, the more perfectionistic academics reported significantly higher levels of fatigue, emotional exhaustion, and anxiety. The greater deterioration in well-being experienced by perfectionist academics when first returning to work was mediated by their tendency for perseverative cognition (i.e., worry and rumination) about work during the respite itself. These findings support the view that the self-critical perfectionist vulnerability is activated by direct exposure to achievement-related stressors and manifested through perseverative modes of thinking
The Deep Space Network: A Radio Communications Instrument for Deep Space Exploration
The primary purpose of the Deep Space Network (DSN) is to serve as a communications instrument for deep space exploration, providing communications between the spacecraft and the ground facilities. The uplink communications channel provides instructions or commands to the spacecraft. The downlink communications channel provides command verification and spacecraft engineering and science instrument payload data
Line Broadening in Field Metal-poor Red Giant and Red Horizontal Branch Stars
We report 349 radial velocities for 45 metal-poor field red giant and red
horizontal branch stars. We have have identified one new spectroscopic binary,
HD 4306, and one possible such system, HD 184711. We also report 57 radial
velocities for 11 of the 91 stars reported on previously by Carney et al.
(2003). As was found in the previous study, radial velocity "jitter" is present
in many of the most luminous stars. Excluding stars showing spectroscopic
binary orbital motion, all 7 of the red giants with M(V) <= -2.0 display
jitter, as well as 3 of the 14 stars with -2.0 <= M(V) <= -1.4. We have also
measured line broadening in all of the new spectra, using synthetic spectra as
templates. The most luminous red giants show significant line broadening, as do
many of the red horizontal branch stars, and we discuss briefly possible
causes.Comment: To appear in the Astronomical Journa
Chemo-dynamical evolution of Globular Cluster Systems
We studied the relation between the ratio of rotational velocity to velocity
dispersion and the metallicity (/\sigma_{v}-metallicity relation) of
globular cluster systems (GCS) of disk galaxies by comparing the relation
predicted from simple chemo-dynamical models for the formation and evolution of
disk galaxies with the observed kinematical and chemical properties of their
GCSs. We conclude that proto disk galaxies underwent a slow initial collapse
that was followed by a rapid contraction and derive that the ratio of the
initial collapse time scale to the active star formation time scale is \sim 6
for our Galaxy and \sim 15 for M31. The fundamental formation process of disk
galaxies was simulated based on simple chemo-dynamical models assuming the
conservation of their angular momentum. We suggest that there is a typical
universal pattern in the /\sigma_{v}-metallicity relation of the GCS
of disk galaxies. This picture is supported by the observed properties of GCSs
in the Galaxy and in M31. This relation would deviate from the universal
pattern, however, if large-scale merging events took major role in
chemo-dynamical evolution of galaxies and will reflect the epoch of such
merging events. We discuss the properties of the GCS of M81 and suggest the
presence of past major merging event.Comment: 25 pages, 8 figures, Accepted for publication in the Astrophysical
Journa
The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Abundance analyses from high-resolution optical spectra are presented for 19
Type II Cepheids in the Galactic field. The sample includes both short-period
(BL Her) and long-period (W Vir) stars. This is the first extensive abundance
analysis of these variables. The C, N, and O abundances with similar spreads
for the BL Her and W Vir show evidence for an atmosphere contaminated with
-process and CN-cycling products. A notable anomaly of the BL Her
stars is an overabundance of Na by a factor of about five relative to their
presumed initial abundances. This overabundance is not seen in the W Vir stars.
The abundance anomalies running from mild to extreme in W Vir stars but not
seen in the BL Her stars are attributed to dust-gas separation that provides an
atmosphere deficient in elements of high condensation temperature, notably Al,
Ca, Sc, Ti, and -process elements. Such anomalies have previously been seen
among RV Tau stars which represent a long-period extension of the variability
enjoyed by the Type II Cepheids. Comments are offered on how the contrasting
abundance anomalies of BL Her and W Vir stars may be explained in terms of the
stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication
in Ap
Distant field BHB stars and the mass of the Galaxy II: Photometry and spectroscopy of UKST candidates 16<B<19.5, 11<R<52 kpc
This is the second in a series of papers presenting a new calculation of the
mass of the Galaxy based on radial velocities and distances for a sample of
faint 16 < B < 21.3 field blue horizontal-branch (BHB) stars. We present
accurate BV CCD photometry and spectra for 142 candidate A-type stars selected
from ub_jr photometry of UK Schmidt telescope plates in six
high-Galactic-latitude fields. Classification of these candidates produces a
sample of 60 BHB stars at distances of 11-52 kpc from the Sun (mean 28 kpc),
with heliocentric line-of-sight velocities accurate to 15 km/s, and distance
errors < 10%. We provide a summary table listing coordinates and velocities of
these stars. The measured dispersion of the radial component of the
Galactocentric velocity for this sample is 108+-10 km/s, in agreement with a
recent study of the distant halo by Sirko and coworkers. Measurements of the Ca
II K line indicate that nearly all the stars are metal-poor with a mean [Fe/H]
= -1.8 with dispersion 0.5. Subsequent papers will describe a second survey of
BHBs to heliocentric distances 70 < R < 125 kpc and present a new estimate of
the mass of the Galaxy.Comment: 16 pages, 15 figures. Accepted for publication in MNRA
The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo
We present a complete spectroscopic survey of 2414 2MASS-selected blue
horizontal branch (BHB) candidates selected over 4300 deg^2 of the sky. We
identify 655 BHB stars in this non-kinematically selected sample. We calculate
the luminosity function of field BHB stars and find evidence for very few hot
BHB stars in the field. The BHB stars located at a distance from the Galactic
plane |Z|<4 kpc trace what is clearly a metal-weak thick disk population, with
a mean metallicity of [Fe/H]= -1.7, a rotation velocity gradient of
dv_{rot}/d|Z|= -28+-3.4 km/s in the region |Z|<6 kpc, and a density scale
height of h_Z= 1.26+-0.1 kpc. The BHB stars located at 5<|Z|<9 kpc are a
predominantly inner-halo population, with a mean metallicity of [Fe/H]= -2.0
and a mean Galactic rotation of -4+-31 km/s. We infer the density of halo and
thick disk BHB stars is 104+-37 kpc^-3 near the Sun, and the relative
normalization of halo to thick-disk BHB stars is 4+-1% near the Sun.Comment: 12 pages in emulateapj format, accepted for publication in February
A
Distant field BHB stars and the mass of the Galaxy I: Classification of halo A-type stars
This is the first in a series of three papers presenting a new calculation of
the mass of the Galaxy based on radial velocities and distances measured for a
sample of some 100 faint 16 < B < 20 field blue horizontal-branch (BHB) stars.
This study aims to reduce the uncertainty in the measured mass of the Galaxy by
increasing the number of halo objects at Galactocentric distances r>30 kpc with
measured radial velocities by a factor five. Faint A-type stars in the Galactic
halo have been identified from UBR photometry in six UK Schmidt fields. We
obtain accurate CCD photometry and spectra to classify these stars. This paper
describes our methods for separating out clean samples of BHB stars in a way
that is efficient in terms of telescope time required. We use the high
signal-to-noise ratio (S/N) spectra of A-type stars of Kinman, Suntzeff & Kraft
(published in 1994) to assess the reliability of two methods, and to quantify
the S/N requirements. First we revisit, refine and extend the hydrogen line
width versus colour relation as a classifier. The second method is new and
compares the shapes of the Balmer lines. Using the equivalent width of the Ca
II K line as an additional filter we find we can reproduce Kinman, Suntzeff &
Kraft's classifications with both methods.Comment: 12 pages, 14 figures. Accepted for publication in MNRAS Balmer-line
fitting routine available at
http://astro.ic.ac.uk/Research/Extragal/milkyway.htm
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