7,986 research outputs found
SCUBA observations of the Horsehead Nebula - what did the horse swallow?
We present observations taken with SCUBA on the JCMT of the Horsehead Nebula
in Orion (B33), at wavelengths of 450 and 850 \mum. We see bright emission from
that part of the cloud associated with the photon-dominated region (PDR) at the
`top' of the horse's head, which we label B33-SMM1. We characterise the
physical parameters of the extended dust responsible for this emission, and
find that B33-SMM1 contains a more dense core than was previously suspected. We
compare the SCUBA data with data from the Infrared Space Observatory (ISO) and
find that the emission at 6.75-\mum is offset towards the west, indicating that
the mid-infrared emission is tracing the PDR while the submillimetre emission
comes from the molecular cloud core behind the PDR. We calculate the virial
balance of this core and find that it is not gravitationally bound but is being
confined by the external pressure from the HII region IC434, and that it will
either be destroyed by the ionising radiation, or else may undergo triggered
star formation. Furthermore we find evidence for a lozenge-shaped clump in the
`throat' of the horse, which is not seen in emission at shorter wavelengths. We
label this source B33-SMM2 and find that it is brighter at submillimetre
wavelengths than B33-SMM1. SMM2 is seen in absorption in the 6.75-\mum ISO
data, from which we obtain an independent estimate of the column density in
excellent agreement with that calculated from the submillimetre emission. We
calculate the stability of this core against collapse and find that it is in
approximate gravitational virial equilibrium. This is consistent with it being
a pre-existing core in B33, possibly pre-stellar in nature, but that it may
also eventually undergo collapse under the effects of the HII region.Comment: 11 pages, 6 figures, accepted by MNRA
SCUBA polarisation observations of the magnetic fields in the prestellar cores L1498 and L1517B
We have mapped linearly polarized dust emission from the prestellar cores
L1498 and L1517B with the James Clerk Maxwell Telescope (JCMT) using the
Submillimetre Common User Bolometer Array (SCUBA) and its polarimeter SCUBAPOL
at a wavelength of 850um. We use these measurements to determine the
plane-of-sky magnetic field orientation in the cores. In L1498 we see a
magnetic field across the peak of the core that lies at an offset of 19 degrees
to the short axis of the core. This is similar to the offsets seen in previous
observations of prestellar cores. To the southeast of the peak, in the
filamentary tail of the core, we see that the magnetic field has rotated to lie
almost parallel to the long axis of the filament. We hypothesise that the field
in the core may have decoupled from the field in the filament that connects the
core to the rest of the cloud. We use the Chandrasekhar-Fermi (CF) method to
measure the plane-of-sky field strength in the core of L1498 to be 10 +/- 7 uG.
In L1517B we see a more gradual turn in the field direction from the northern
part of the core to the south. This appears to follow a twist in the filament
in which the core is buried, with the field staying at a roughly constant 25
degree offset to the short axis of the filament, also consistent with previous
observations of prestellar cores. We again use the CF method and calculate the
magnetic field strength in L1517B also to be 30 +/- 10 uG. Both cores appear to
be roughly virialised. Comparison with our previous work on somewhat denser
cores shows that, for the denser cores, thermal and non-thermal (including
magnetic) support are approximately equal, while for the lower density cores
studied here, thermal support dominates.Comment: 6 pages, 2 figures; accepted for publication by MNRA
Modelling submillimetre spectra of the protostellar infall candidates NGC1333-IRAS2 and Serpens SMM4
We present a radiative transfer model, which is applicable to the study of
submillimetre spectral line observations of protostellar envelopes. The model
uses an exact, non-LTE, spherically symmetric radiative transfer `Stenholm'
method, which numerically solves the radiative transfer problem by the process
of `Lambda-iteration'. We also present submillimetre spectral line data of the
Class 0 protostars NGC1333-IRAS2 and Serpens SMM4. We examine the physical
constraints which can be used to limit the number and range of parameters used
in protostellar envelope models, and identify the turbulent velocity and tracer
molecule abundance as the principle sources of uncertainty in the radiative
transfer modelling. We explore the trends in the appearance of the predicted
line profiles as key parameters in the models are varied.
We find that the separation of the two peaks of a typical infall profile is
dependent not on the evolutionary status of the collapsing protostar, but on
the turbulent velocity dispersion in the envelope. We also find that the line
shapes can be significantly altered by rotation.
Fits are found for the observed line profiles of IRAS2 and SMM4 using
plausible infall model parameters. The density and velocity profiles in our
best fit models are inconsistent with a singular isothermal sphere model. We
find better agreement with a form of collapse which assumes non-static initial
conditions. We also find some evidence that the infall velocities are retarded
from free-fall towards the centre of the cloud, probably by rotation, and that
the envelope of SMM4 is rotationally flattened.Comment: Accepted by MNRA
A Far-Infrared Survey of Molecular Cloud Cores
We present a catalogue of molecular cloud cores drawn from high latitude,
medium opacity clouds, using the all-sky IRAS Sky Survey Atlas (ISSA) images at
60 and 100~m. The typical column densities of the cores are cm and the typical volume densities are cm. They are therefore significantly less dense than
many other samples obtained in other ways. Those cloud cores with IRAS point
sources are seen to be already forming stars, but this is found to be only a
small fraction of the total number of cores. The fraction of the cores in the
protostellar stage is used to estimate the prestellar timescale - the time
until the formation of a hydrostatically supported protostellar object. We
argue, on the basis of a comparison with other samples, that a trend exists for
the prestellar lifetime of a cloud core to decrease with the mean column
density and number density of the core. We compare this with model predictions
and show that the data are consistent with star formation regulated by the
ionisation fraction.Comment: 13 pages, 7 figure
An empirical model for protostellar collapse
We propose a new analytic model for the initial conditions of protostellar
collapse in relatively isolated regions of star formation. The model is
non-magnetic, and is based on a Plummer-like radial density profile as its
initial condition. It fits: the observed density profiles of pre-stellar cores
and Class 0 protostars; recent observations in pre-stellar cores of roughly
constant contraction velocities over a wide range of radii; and the lifetimes
and accretion rates derived for Class 0 and Class I protostars. However, the
model is very simple, having in effect only 2 free parameters, and so should
provide a useful framework for interpreting observations of pre-stellar cores
and protostars, and for calculations of radiation transport and time-dependent
chemistry. As an example, we model the pre-stellar core L1544.Comment: To appear in Astrophysical Journal, Jan 20th, 2001. 18 pages incl. 3
fig
A turbulent MHD model for molecular clouds and a new method of accretion on to star-forming cores
We describe the results of a sequence of simulations of gravitational
collapse in a turbulent magnetized region. The parameters are chosen to be
representative of molecular cloud material. We find that several protostellar
cores and filamentary structures of higher than average density form. The
filaments inter-connect the high density cores. Furthermore, the magnetic field
strengths are found to correlate positively with the density, in agreement with
recent observations. We make synthetic channel maps of the simulations and show
that material accreting onto the cores is channelled along the magnetized
filamentary structures. This is compared with recent observations of S106, and
shown to be consistent with these data. We postulate that this mechanism of
accretion along filaments may provide a means for molecular cloud cores to grow
to the point where they become gravitationally unstable and collapse to form
stars.Comment: Accepted by MNRA
The optical polarisation of RCRA and TCRA - NGC6729
A study is presented of the reflection nebula NGC 6729, using the techniques of imaging polarimetry, with the aim of attempting to discover the geometry of the nebula and the role which each of the two stars seen within the nebula - R & T CrA - plays in its illumination. The polarisation results may be interpreted in terms of both stars having circumstellar disks, aligned parallel to one another. From observations of the total intensity of light emitted from the nebula, "jets' are observed to be emanating from the two stars. These 'jets' are interpreted as outflows of matter from the stars, and are compared with similar features noted by other observers elsewhere. These comparisons yield one major difference between the R and T CrA jets and those found elsewhere: These jets are not perpendicular to their respective circumstellar disks. Two alternative hypotheses are presented to account for this, and it is concluded that further observational evidence is required in order to decide between the two
Simulating star formation in molecular cloud cores I. The influence of low levels of turbulence on fragmentation and multiplicity
We present the results of an ensemble of simulations of the collapse and
fragmentation of dense star-forming cores. We show that even with very low
levels of turbulence the outcome is usually a binary, or higher-order multiple,
system. We take as the initial conditions for these simulations a typical
low-mass core, based on the average properties of a large sample of observed
cores. All the simulated cores start with a mass of , a
flattened central density profile, a ratio of thermal to gravitational energy
and a ratio of turbulent to gravitational energy
. Even this low level of turbulence is sufficient to
produce multiple star formation in 80% of the cores; the mean number of stars
and brown dwarfs formed from a single core is 4.55, and the maximum is 10. At
the outset, the cores have no large-scale rotation. The only difference between
each individual simulation is the detailed structure of the turbulent velocity
field. The multiple systems formed in the simulations have properties
consistent with observed multiple systems. Dynamical evolution tends
preferentially to eject lower mass stars and brown dwarves whilst hardening the
remaining binaries so that the median semi-major axis of binaries formed is
au. Ejected objects are usually single low-mass stars and brown
dwarfs, yielding a strong correlation between mass and multiplicity. Our
simulations suggest a natural mechanism for forming binary stars that does not
require large-scale rotation, capture, or large amounts of turbulence.Comment: 20 pages, 12 figures submitted to A&
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