1,351 research outputs found
Recommended from our members
Immunocytochemical localization of dromyosuppressin (DMS) in Phormia regina (Meigen) and effect of DMS and benzethonium chloride on crop muscle contractions.
Globular Clusters in NGC 1275
We present the results of a deep photometric study of the outer halo of NGC
1275, the highly active cD galaxy at the center of the Perseus cluster. We find
a modest excess of faint () starlike objects in its halo, indicating
a population of old-halo globular clusters. However, the total estimated
cluster population corresponds to a specific frequency of ,
no larger than that of normal giant ellipticals and three times lower than that
of other central cD galaxies such as M87. We discuss several ideas for the
origin of this galaxy. Our results reinforce the view that high (ie:
highly efficient globular cluster formation) is not associated with cooling
flows, or with recent starburst or merger phenomena.Comment: 25 pages, latex, postscript figures, tarred, Unix compressed,
postscript version of paper and figures available at
http://www.physics.mcmaster.ca/Grads/DKaisler/office.htm
Measurement of Antenna Surfaces from In- and Out-Of-Focus Beam Maps using Astronomical Sources
We present a technique for the accurate estimation of large-scale errors in
an antenna surface using astronomical sources and detectors. The technique
requires several out-of-focus images of a compact source and the
signal-to-noise ratio needs to be good but not unreasonably high. For a given
pattern of surface errors, the expected form of such images can be calculated
directly. We show that it is possible to solve the inverse problem of finding
the surface errors from the images in a stable manner using standard numerical
techniques. To do this we describe the surface error as a linear combination of
a suitable set of basis functions (we use Zernike polynomials). We present
simulations illustrating the technique and in particular we investigate the
effects of receiver noise and pointing errors. Measurements of the 15-m James
Clerk Maxwell telescope made using this technique are presented as an example.
The key result is that good measurements of errors on large spatial scales can
be obtained if the input images have a signal-to-noise ratio of order 100 or
more. The important advantage of this technique over transmitter-based
holography is that it allows measurements at arbitrary elevation angles, so
allowing one to characterise the large scale deformations in an antenna as a
function of elevation.Comment: 6 pages, 5 figures (accepted by Astronomy & Astrophysics
The JCMT dense gas survey of the Perseus Molecular Cloud
We present the results of a large-scale survey of the very dense gas in the
Perseus molecular cloud using HCO+ and HCN (J = 4 - 3) transitions. We have
used this emission to trace the structure and kinematics of gas found in pre-
and protostellar cores, as well as in outflows. We compare the HCO+/HCN data,
highlighting regions where there is a marked discrepancy in the spectra of the
two emission lines. We use the HCO+ to identify positively protostellar
outflows and their driving sources, and present a statistical analysis of the
outflow properties that we derive from this tracer. We find that the relations
we calculate between the HCO+ outflow driving force and the Menv and Lbol of
the driving source are comparable to those obtained from similar outflow
analyses using 12CO, indicating that the two molecules give reliable estimates
of outflow properties. We also compare the HCO+ and the HCN in the outflows,
and find that the HCN traces only the most energetic outflows, the majority of
which are driven by young Class 0 sources. We analyse the abundances of HCN and
HCO+ in the particular case of the IRAS 2A outflows, and find that the HCN is
much more enhanced than the HCO+ in the outflow lobes. We suggest that this is
indicative of shock-enhancement of HCN along the length of the outflow; this
process is not so evident for HCO+, which is largely confined to the outflow
base.Comment: 25 pages, 14 figures, 9 table
Proper motion measurements as indicators of binarity in open clusters
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyr using
UCAC2 proper motion data and 2MASS photometry. For each cluster we consider the
projected velocity distributions in the core and off-core regions separately.
In the projected velocity distribution of all sample clusters we find a
well-defined low-velocity peak, as well as an excess in the number of stars at
larger velocities. The low-velocity peak is accounted for by the random motion
of the single stars, while the high-velocity excess can be attributed to the
large velocity changes produced by a significant fraction of unresolved
binaries in a cluster. We derive kinematic parameters of the single-star
distribution, in particular the projected velocity dispersion. The relatively
large velocity dispersions derived in this work may reflect the non-virialized
state of the clusters. Based on the relative number of high-velocity (binary)
and single stars, we inferred for the sample clusters unresolved binary
fractions in the range , for both core and off-core regions. The
present results suggest that care must be taken when applying proper-motion
filters to sort out members, especially binaries in a star cluster. This paper
shows that proper motions turn out to be a useful tool for identifying
high-velocity stars as unresolved binary cluster members, and as a consequence,
map and quantify the binary component in colour-magnitude diagrams.Comment: 9 pages and 9 figures. Astronomy and Astrophysics, accepted
(25/10/2004
Probing Dark Matter
Recent novel observations have probed the baryonic fraction of the galactic
dark matter that has eluded astronomers for decades. Late in 1993, the MACHO
and EROS collaborations announced in this journal the detection of transient
and achromatic brightenings of a handful of stars in the Large Magellanic Cloud
that are best interpreted as gravitational microlensing by low-mass foreground
objects (MACHOS). This tantalized astronomers, for it implied that the
population of cool, compact objects these lenses represent could be the elusive
dark matter of our galactic halo. A year later in 1994, Sackett et al. reported
the discovery of a red halo in the galaxy NGC 5907 that seems to follow the
inferred radial distribution of its dark matter. This suggested that dwarf
stars could constitute its missing component. Since NGC 5907 is similar to the
Milky Way in type and radius, some surmised that the solution of the galactic
dark matter problem was an abundance of ordinary low-mass stars. Now Bahcall et
al., using the Wide-Field Camera of the recently repaired Hubble Space
Telescope, have dashed this hope.Comment: 3 pages, Plain TeX, no figures, published as a News and Views in
Nature 373, 191 (1995
RĂ©sultats des campagnes MUSORSTOM : volume 15
La campagne MUSORSTOM 8, réalisée à bord du N.O. "Alis", s'est déroulée dans les eaux de Vanuatu du 19 septembre au 14 octobre 1994. Cent quatre-vingt-six opération de dragages et de chalutages ont eu lieu dans la zone bathyale supérieure, sur les pentes des îles et sur le sommet du guyot Bougainville. De grandes superficies chalutables ont été découvertes entre 300 et 1000 m de profondeur. La faune benthique est riche, bien diversifiée, mais semble d'une composition très différente de celle de Nouvelle-Calédonie : les Spongiaires et les Stylastérides, en particulier, ne contribuent pas de façon significative à la physionomie des peuplements. (Résumé d'auteur
NGC 2419, M92, and the Age Gradient in the Galactic Halo
The WFPC2 camera on HST has been used to obtain deep main sequence photometry
of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A
differential fit of the NGC 2419 CMD to that of the similarly metal-poor \
standard cluster M92 shows that they have virtually identical principal
sequences and thus the same age to well within 1 Gyr. Since other
low-metallicity clusters throughout the Milky Way halo have this same age to
within the 1-Gyr precision of the differential age technique, we conclude that
the earliest star (or globular cluster) formation began at essentially the same
time everywhere in the Galactic halo throughout a region now almost 200 kpc in
diameter. Thus for the metal-poorest clusters in the halo there is no
detectable age gradient with Galactocentric distance. To estimate the absolute
age of NGC 2419 and M92, we fit newly computed isochrones transformed through
model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance
scales that represent the range currently debated in the literature.
Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age
of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would
further reduce this estimate by about 1 Gyr. A distance scale as bright as
M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave
several serious problems which have no obvious solution in the context of
current stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in
AJ, September 1997. Also available by e-mail from [email protected]
- …