1,293 research outputs found
On the Radial Distribution of White Dwarfs in the Globular Cluster NGC 6397
We have examined the radial distribution of white dwarfs over a single
HST/ACS field in the nearby globular cluster NGC 6397. In relaxed populations,
such as in a globular cluster, stellar velocity dispersion, and hence radial
distribution, is directly dependent on stellar masses. The progenitors of very
young cluster white dwarfs had a mass of ~0.8 solar masses, while the white
dwarfs themselves have a mass of ~0.5 solar masses. We thus expect young white
dwarfs to have a concentrated radial distribution (like that of their
progenitors) that becomes more extended over several relaxation times to mimic
that of ~0.5 solar mass main-sequence stars. However, we observe young white
dwarfs to have a significantly extended radial distribution compared to both
the most massive main sequence stars in the cluster and also to old white
dwarfs.Comment: 13 pages including 1 table and 3 figures. Accepted for publication in
the MNRAS Letter
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
A Cluster of Compact Radio Sources in NGC 2024 (Orion B)
We present deep 3.6 cm radio continuum observations of the H II region NGC
2024 in Orion B obtained using the Very Large Array in its A-configuration,
with angular resolution. We detect a total of 25 compact radio
sources in a region of . We discuss the nature of these sources
and its relation with the infrared and X-ray objects in the region. At least
two of the radio sources are obscured proplyds whose morphology can be used to
restrict the location of the main ionizing source of the region. This cluster
of radio sources is compared with others that have been found in regions of
recent star formation.Comment: 21 pages, 7 figure
Measurement of the magnetic field direction in the NGC2024-FIR5 protostellar outflow
Molecular outflows from young protostars are widely believed to be collimated
by magnetic fields, but there has been little observational evidence to support
this hypothesis. Using the new technique of millimetre-wavelength
spectro-polarimetry, we demonstrate the existence of a magnetic field in the
NGC2024-FIR5 outflow lobe. The 1.3mm J=2-1 transition of carbon monoxide (CO)
is polarized at a level of approximately 1%, in a direction within 10-15
degrees of the outflow axis. This agrees with theoretical models where the
magnetic field channels the outflowing gas, and shows that the process can be
effective as far as 0.1pc from the protostar.Comment: Accepted by ApJL. 10 pages, including 2 figure
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
Completion of a SCUBA survey of Lynds dark clouds and implications for low-mass star formation
We have carried out a survey of optically-selected dark clouds using the
bolometer array SCUBA on the James Clerk Maxwell Telescope, at 850 microns
wavelength. The survey covers a total of 0.5 square degrees and is unbiased
with reference to cloud size, star formation activity, or the presence of
infrared emission. Several new protostars and starless cores have been
discovered; the protostars are confirmed through the detection of their
accompanying outflows in CO(2-1) emission. The survey is believed to be
complete for Class 0 and Class I protostars, and yields two important results
regarding the lifetimes of these phases. First, the ratio of Class 0 to Class
protostars in the sample is roughly unity, very different from the 1:10 ratio
that has previously been observed for the rho Ophiuchi star-forming region.
Assuming star formation to be a homogeneous process in the dark clouds, this
implies that the Class 0 lifetime is similar to the Class I phase, which from
infrared surveys has been established to be approximately 200,000 yr. It also
suggests there is no rapid initial accretion phase in Class 0 objects. A burst
of triggered star formation some 100,000 yr ago can explain the earlier results
for rho Ophiuchus. Second, the number of starless cores is approximately twice
that of the total number of protostars, indicating a starless core lifetime of
approximately 800,000 yr. These starless cores are therefore very short-lived,
surviving only two or three free-fall times. This result suggests that, on size
scales of 10,000 AU at least, the dynamical evolution of starless cores is
probably not controlled by magnetic processes.Comment: 67 pages including 32 figures (highly compressed). Accepted for
publication in the Astronomical Journal. Available with full resolution
(legible) figures at http://www.mrao.cam.ac.uk/%7ejsr/papers/lynds2.ps.g
Deep HST Imaging in NGC 6397: Stellar Dynamics
Multi-epoch observations with ACS on HST provide a unique and comprehensive
probe of stellar dynamics within NGC 6397. We are able to confront analytic
models of the globular cluster with the observed stellar proper motions. The
measured proper motions probe well along the main sequence from 0.8 to below
0.1 M as well as white dwarfs younger than one gigayear. The observed
field lies just beyond the half-light radius where standard models of globular
cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution)
make very robust predictions for the stellar proper motions as a function of
mass. The observed proper motions show no evidence for anisotropy in the
velocity distribution; furthermore, the observations agree in detail with a
straightforward model of the stellar distribution function. We do not find any
evidence that the young white dwarfs have received a natal kick in
contradiction with earlier results. Using the observed proper motions of the
main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397
of kpc and a mass of the cluster of at the photometric distance of 2.53 kpc. One of the
main-sequence stars appears to travel on a trajectory that will escape the
cluster, yielding an estimate of the evaporation timescale, over which the
number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The
proper motions of the youngest white dwarfs appear to resemble those of the
most massive main-sequence stars, providing the first direct constraint on the
relaxation time of the stars in a globular cluster of greater than or about 0.7
Gyr.Comment: 25 pages, 20 figures, accepted for publication in Astrophysical
Journa
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