60 research outputs found
The Edge of the Galactic Disc
As part of a stellar population sampling program, a series of photometric
probes at various field sizes and depths have been obtained in a low extinction
window in the galactic anticentre direction. Such data set strong constraints
on the radial structure of the disc. At the forefront of this "drilling"
program, very deep CCD frames probe the most external parts of the disc. Over
the whole effective magnitude range (18 to 25), all contributions in the
statistics which should be expected from old disc stars beyond 6 kpc vanish,
although such stars dominate by far at distances less than 5 kpc. This is the
signature of a sharp cut-off in the star density: the edge of the galactic disc
between 5.5 and 6 kpc. As a consequence, the galactic radius does not exceed 14
kpc (assuming =8.5). Colours of elliptical galaxies measured in
the field rule out the risk of being misled by undetected extinction.Comment: 8 pages, Besancon Observatory preprint n.
Extended HI spiral structure and the figure rotation of triaxial dark halos
The HI disk of the blue compact dwarf (BCD) galaxy NGC 2915 extends to
22 optical scalelengths and shows spiral arms reaching far beyond the optical
component. None of the previous theories for spiral structure provide likely
explanations for these very extended spiral arms. Our numerical simulations
first demonstrate that such large spiral arms can form in an extended gas disk
embedded in a massive triaxial dark matter halo with slow figure rotation,
through the strong gravitational torque of the rotating halo. We then show that
the detailed morphological properties of the developed spirals and rings depend
strongly on the pattern speed of the figure rotation, the shape of the triaxial
halo, and the inclination of the disk with respect to the plane including the
triaxial halo's long and middle axes. These results strongly suggest that the
dark matter halo of NGC 2915 is triaxial and has figure rotation. Based on
these results, we also suggest that dynamical effects of triaxial halos with
figure rotation are important in various aspect of galaxy formation and
evolution, such as formation of polar ring galaxies, excitation of
non-axisymmetric structures in low surface-brightness galaxies, and gas fueling
to the central starburst regions of BCDs.Comment: 13 pages 2 figures (fig.2 = jpg format), accepted by ApJ
High-velocity white dwarfs: thick disk, not dark matter
We present an alternative interpretation of the nature of the extremely cool,
high-velocity white dwarfs identified by Oppenheimer et al (2001) in a
high-latitude astrometric survey. We argue that the velocity distribution of
the majority of the sample is more consistent with the high-velocity tail of a
rotating population, probably the thick disk, rather than a pressure-supported
halo system. Indeed, the observed numbers are well matched by predictions based
on the kinematics of a complete sample of nearby M dwarfs. Analysing only stars
showing retrograde motion gives a local density close to that expected for
white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of
the white dwarfs need be assigned to the dark-matter, heavy halo. However,
luminosity functions derived from observations of these stars can set important
constraints on the age of the oldest stars in the Galactic Disk.Comment: 11 pages, 5 figures; accepted for ApJ, 29 May 200
Local Surface Density of the Galactic Disk from a 3-D Stellar Velocity Sample
We have re-estimated the surface density of the Galactic disk in the solar
neighborhood within 0.4 kpc of the Sun using parallaxes and proper
motions of a kinematically and spatially unbiased sample of 1476 old bright red
giant stars from the Hipparcos catalog with measured radial velocities from
Barbier-Brossat & Figon (2000). We determine the vertical distribution of the
red giants as well as the vertical velocity dispersion of the sample, (14.4
0.26 km/sec), and combine these to derive the surface density of
gravitating matter in the Galactic disk as a function of the galactic
coordinate . The surface density of the disk increases from 10.5 0.5
/ pc within 50 pc to 42 6 / pc
within 350 pc. The estimated volume density of the galactic disk within
50 pc is about 0.1 / pc which is close to the volume
density estimates of the observed baryonic matter in the solar neighborhood.Comment: 24 pages, 15 figures, AJ in pres
The thick disk of the galaxy: sequel of a merging event
Accurate characterization of thick disc properties from recent kinematic and photometric surveys provides converging evidences that this intermediate population is a sequel of the violent heating of early disc populations by a merging satellite galaxy. The thick disc population is revisited under the light of new data in a number of galactic sample fields. Various thick disc hypotheses are fitted to observational data through a maximum likelihood technique. The resulting characteristics of the thick disc are the following: a scale height of 760 ± 50pc, with a local density of 5.6 ± 1% of the thin disc. The scale length is constrained to be 2.8 ± 0.8kpc, well in agreement with the disc scale length (2.5 ± 0.3kpc). The mean metallicity of the thick disc is found to be -0.7 ±0.2 dex, with no significant metallicity gradients. These photometric constraints in combination with kinematic data give new constraints on the thick disc formation. We show that thick disc characteristics are hardly compatible with a top-down formation scenario but fully compatible with a violent merging event arising at the early thin disc life time as described by Quinn, Hernquist & Fullagar (1993)
A New Nearby Candidate Star Cluster in Ophiuchus at d = 170 pc
The recent discoveries of nearby star clusters and associations within a few
hundred pc of the Sun, as well as the order of magnitude difference in the
formation rates of the embedded and open cluster populations, suggests that
additional poor stellar groups are likely to be found at surprisingly close
distances to the Sun. Here I describe a new nearby stellar aggregate found by
virtue of the parallel proper motions, similar trigonometric parallaxes, and
consistent color-magnitude distribution of its early-type members. The 120
Myr-old group lies in Ophiuchus at 170 pc, with its most massive
member being the 4th-magnitude post-MS B8II-III star Oph. The group may
have escaped previous notice due to its non-negligible extinction (
0.9 mag). If the group was born with a normal initial mass function,
and the nine B- and A-type systems represent a complete system of
intermediate-mass stars, then the original population was probably of order
200 systems. The age and space motion of the new cluster are very similar
to those of the Pleiades, Per cluster, and AB Dor Moving Group,
suggesting that these aggregates may have formed in the same star-forming
complex some yr ago.Comment: 23 pages, 3 figs., to appear in Nov. 2006 A
Malondialdehyde and anion patterns in exhaled breath condensate among subway workers.
Underground transportation systems can contribute to the daily particulates and metal exposures for both commuter and subway workers. The redox and metabolic changes in workers exposed to such metal-rich particles have yet to be characterized. We hypothesize that the distribution of nitrosative/oxidative stress and related metabolic biomarkers in exhaled breath condensate (EBC) are modified depending on exposures.
Particulate number and size as well as mass concentration and airborne metal content were measured in three groups of nine subway workers (station agents, locomotive operators and security guards). In parallel, pre- and post-shift EBC was collected daily during two consecutive working weeks. In this biological matrix, malondialdehyde, lactate, acetate, propionate, butyrate, formate, pyruvate, the sum of nitrite and nitrate (ΣNO <sub>x</sub> ) and the ratio nitrite/nitrate as well as metals and nanoparticle concentrations was determined. Weekly evolution of the log-transformed selected biomarkers as well as their association with exposure variables was investigated using linear mixed effects models with the participant ID as random effect. The professional activity had a strong influence on the pattern of anions and malondialdehyde in EBC. The daily number concentration and the lung deposited surface area of ultrafine particles was consistently and mainly associated with nitrogen oxides variations during the work-shift, with an inhibitory effect on the ΣNO <sub>x</sub> . We observed that the particulate matter (PM) mass was associated with a decreasing level of acetate, lactate and ΣNO <sub>x</sub> during the work-shift, suggestive of a build-up of these anions during the previous night in response to exposures from the previous day. Lactate was moderately and positively associated with some metals and with the sub-micrometer particle concentration in EBC.
These results are exploratory but suggest that exposure to subway PM could affect concentrations of nitrogen oxides as well as acetate and lactate in EBC of subway workers. The effect is modulated by the particle size and can correspond to the body's cellular responses under oxidative stress to maintain the redox and/or metabolic homeostasis
Ubv CCD Standards Near 5C1 Radio Sources at Intermediate Galactic Latitude-I
We have been doing a photometric and astrometric sample survey programme on
Schmidt plates in a few selected directions in the Galaxy with an aim to study
galactic stellar populations. In order to calibrate Schmidt plates
photometrically, it is necessary to have a number of photometric standards
which cover the entire range of magnitudes to be studied on the plates. As
Schmidt plates cover a wide area of the sky, the standards should also be
spread over the surface of the plate so as to minimise geometrical effects
present on the plate. This paper is first in a series of papers aimed at to
provide photometric standards in the selected galactic directions to be
studied. In this paper we present photometric magnitudes obtained through CCD
observation in the galactic direction (l=170deg, b=45deg) centered at
alpha(2000) = 9h 41m 20s and delta(2000) = +49deg 54' 20'' and covering an area
of 14 sq degs. B and V magnitudes have been obtained for a total of 214 stars
out of which for 73 stars U magnitudes also have been obtained. The magnitude
range covered is 11<V<21 and 0.18<B-V<1.85.Comment: Postscript file, 13 pages, 1 figure (figure available on request),
paper accepted for publication in the Bulletin of Astronomical Society of
Indi
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