163 research outputs found
Monte Carlo Radiative Transfer in Embedded Prestellar Cores
We implement a Monte Carlo radiative transfer method, that uses a large
number of monochromatic luminosity packets to represent the radiation
transported through a system. These packets are injected into the system and
interact stochastically with it. We test our code against various benchmark
calculations and determine the number of packets required to obtain accurate
results under different circumstances. We then use this method to study cores
that are directly exposed to the interstellar radiation field (non-embedded
cores) and find similar results with previous studies. We also explore a large
number of models of cores that are embedded in the centre of a molecular cloud.
Our study indicates that the temperature profiles in embedded cores are less
steep than those in non-embedded cores. Deeply embedded cores (ambient cloud
with visual extinction larger than 15-25) are almost isothermal at around 7-8
K. The temperature inside cores surrounded by an ambient cloud of even moderate
thickness (Av~5) is less than 12 K, which is lower than previous studies have
assumed. Thus, previous mass calculations of embedded cores (for example in the
rho Ophiuchi protocluster), based on mm continuum observations, may
underestimate core masses by up to a factor of 2. Our study shows that the best
wavelength region to observe embedded cores is between 400 and 500 microns,
where the core is quite distinct from the background. We also predict that very
sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate
how deeply a core is embedded in its parent molecular cloud. The upcoming
HERSCHEL mission (ESA, 2007) will, in principle, be able to detect these
features and test our models.Comment: 15 pages, 18 figures, accepted by A&
Revised analysis of SPIRE observations for 2M1207
We have revised our analysis of the SPIRE observations of 2MASSW
J1207334-393254 (2M1207). Recent PACS observations show a bright source located
~25" east of 2M1207. There are issues in terms of the detection/non-detection
of the bright source when comparing the Spitzer, WISE, and PACS observations.
It is apparently inconsistent, perhaps due to variability or low
signal-to-noise of the data. We have looked into the possible misidentification
of the target, and have revised the measured SPIRE fluxes and the disc
parameters for 2M1207. We have also reviewed which among the various formation
mechanisms of this system would still be valid.Comment: Revised SPIRE fluxe
Controlling Artificial Viscosity in SPH simulations of accretion disks
We test the operation of two methods for selective application of Artificial
Viscosity (AV) in SPH simulations of Keplerian Accretion Disks, using a ring
spreading test to quantify effective viscosity, and a correlation coefficient
technique to measure the formation of unwanted prograde alignments of
particles. Neither the Balsara Switch nor Time Dependent Viscosity work
effectively, as they leave AV active in areas of smooth shearing flow, and do
not eliminate the accumulation of alignments of particles in the prograde
direction. The effect of both switches is periodic, the periodicity dependent
on radius and unaffected by the density of particles. We demonstrate that a
very simple algorithm activates AV only when truly convergent flow is detected
and reduces the unwanted formation of prograde alignments. The new switch works
by testing whether all the neighbours of a particle are in Keplerian orbit
around the same point, rather than calculating the divergence of the velocity
field, which is very strongly affected by Poisson noise in the positions of the
SPH particles.Comment: 8 pages, 5 figure
A natural formation scenario for misaligned and short-period eccentric extrasolar planets
Recent discoveries of strongly misaligned transiting exoplanets pose a
challenge to the established planet formation theory which assumes planetary
systems to form and evolve in isolation. However, the fact that the majority of
stars actually do form in star clusters raises the question how isolated
forming planetary systems really are. Besides radiative and tidal forces the
presence of dense gas aggregates in star-forming regions are potential sources
for perturbations to protoplanetary discs or systems. Here we show that
subsequent capture of gas from large extended accretion envelopes onto a
passing star with a typical circumstellar disc can tilt the disc plane to
retrograde orientation, naturally explaining the formation of strongly inclined
planetary systems. Furthermore, the inner disc regions may become denser, and
thus more prone to speedy coagulation and planet formation. Pre-existing
planetary systems are compressed by gas inflows leading to a natural occurrence
of close-in misaligned hot Jupiters and short-period eccentric planets. The
likelihood of such events mainly depends on the gas content of the cluster and
is thus expected to be highest in the youngest star clusters.Comment: 7 pages, 4 figures. Accepted for publication in MNRAS. Updated to
match published versio
How to identify the youngest protostars
We study the transition from a prestellar core to a Class 0 protostar, using
SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer
code to generate the SED and isophotal maps. For a prestellar core illuminated
by the standard interstellar radiation field, the luminosity is low and the SED
peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to
a growing contribution from accretion onto the protostar) and the peak of the
SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the
Class 0 phase, the accretion rate is falling, the luminosity has decreased, and
the peak of the SED shifts back towards longer wavelengths (90-150 micron). In
our simulations, the density of material around the protostar remains
sufficiently high well into the Class 0 phase that the protostar only becomes
visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm
maps of Class 0 protostars tend to be much more centrally condensed than those
of prestellar cores. However, when convolved with a typical telescope beam, the
difference in central concentration is less marked, although the Class 0
protostars appear more circular. Our results suggest that, if a core is deemed
to be prestellar on the basis of having no associated IRAS source, no cm radio
emission, and no outflow, but it has a circular appearance and an SED which
peaks at wavelengths below ~170 micron, it may well contain a very young Class
0 protostar.Comment: Accepted by A&A (avaliable with high-res images at
http://carina.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics
We perform SPH simulations of the collapse and fragmentation of low-mass
cores having different initial levels of turbulence
(alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation
which captures the transport of cooling radiation against opacity due to both
dust and gas (including the effects of dust sublimation, molecules, and H^-
ions). We also perform comparison simulations using a standard barotropic
equation of state. We find that -- when compared with the barotropic equation
of state -- our more realistic treatment of the energy equation results in more
protostellar objects being formed, and a higher proportion of brown dwarfs; the
multiplicity frequency is essentially unchanged, but the multiple systems tend
to have shorter periods (by a factor ~3), higher eccentricities, and higher
mass ratios. The reason for this is that small fragments are able to cool more
effectively with the new treatment, as compared with the barotropic equation of
state. We find that the process of fragmentation is often bimodal. The first
protostar to form is usually, at the end, the most massive, i.e. the primary.
However, frequently a disc-like structure subsequently forms round this
primary, and then, once it has accumulated sufficient mass, quickly fragments
to produce several secondaries. We believe that this delayed fragmentation of a
disc-like structure is likely to be an important source of very low-mass
hydrogen-burning stars and brown dwarfs.Comment: 14 pages, 8 figures. Accepted for publication by A&
Modelling Herschel observations of infrared-dark clouds in the Hi-GAL survey
We demonstrate the use of the 3D Monte Carlo radiative transfer code PHAETHON
to model infrared-dark clouds (IRDCs) that are externally illuminated by the
interstellar radiation field (ISRF). These clouds are believed to be the
earliest observed phase of high-mass star formation, and may be the high-mass
equivalent of lower-mass prestellar cores. We model three different cases as
examples of the use of the code, in which we vary the mass, density, radius,
morphology and internal velocity field of the IRDC. We show the predicted
output of the models at different wavelengths chosen to match the observing
wavebands of Herschel and Spitzer. For the wavebands of the long- wavelength
SPIRE photometer on Herschel, we also pass the model output through the SPIRE
simulator to generate output images that are as close as possible to the ones
that would be seen using SPIRE. We then analyse the images as if they were real
observations, and compare the results of this analysis with the results of the
radiative transfer models. We find that detailed radiative transfer modelling
is necessary to accurately determine the physical parameters of IRDCs (e.g.
dust temperature, density profile). This method is applied to study
G29.55+00.18, an IRDC observed by the Herschel Infrared Galactic Plane survey
(Hi-GAL), and in the future it will be used to model a larger sample of IRDCs
from the same survey.Comment: MNRAS accepted, High resolution paper available at
http://www.astro.cardiff.ac.uk/pub/Dimitrios.Stamatellos/Publications.htm
The initial conditions of isolated star formation -- IX. Akari mapping of an externally heated pre-stellar core
We present observations of L1155 and L1148 in the Cepheus molecular cloud,
taken using the FIS instrument on the Akari satellite. We compare these data to
submillimetre data taken using the SCUBA camera on the JCMT, and far-infrared
data taken with the ISOPHOT camera on board the ISO satellite. All of the data
show a relation between the position of the peak of emission and the wavelength
for the core of L1155. We interpret this as a temperature gradient. We fit
modified blackbody curves to the spectral energy distributions at two positions
in the core and see that the central core in L1155 (L1155C) is approximately 2
degrees warmer at one edge than it is in the centre. We consider a number of
possible heating sources and conclude that the A6V star BD+67 1263 is the most
likely candidate. This star is at a distance of 0.7 pc from the front of L1155C
in the plane of the sky. We carry out radiative transfer modelling of the
L1155C core including the effects from the nearby star. We find that we can
generate a good fit to the observed data at all wavelengths, and demonstrate
that the different morphologies of the core at different wavelengths can be
explained by the observed 2 degree temperature gradient. The L1148 core
exhibits a similar morphology to that of L1155C, and the data are also
consistent with a temperature gradient across the core. In this case, the most
likely heating source is the star BD197053. Our findings illustrate very
clearly that the apparent observed morphology of a pre-stellar core can be
highly dependent on the wavelength of the observation, and that temperature
gradients must be taken into account before converting images into column
density distributions. This is important to note when interpreting Akari and
Spitzer data and will also be significant for Herschel data.Comment: 15 pages, 15 figures, accepted by MNRA
The Long-term Dynamical Evolution of Disk-fragmented Multiple Systems in the Solar Neighborhood
The origin of very low-mass hydrogen-burning stars, brown dwarfs (BDs), and planetary-mass objects (PMOs) at
the low-mass end of the initial mass function is not yet fully understood. Gravitational fragmentation of
circumstellar disks provides a possible mechanism for the formation of such low-mass objects. The kinematic and
binary properties of very low-mass objects formed through disk fragmentation at early times (<10 Myr) were
discussed in our previous paper. In this paper we extend the analysis by following the long-term evolution of diskfragmented
systems up to an age of 10 Gyr, covering the ages of the stellar and substellar populations in the
Galactic field. We find that the systems continue to decay, although the rates at which companions escape or
collide with each other are substantially lower than during the first 10 Myr, and that dynamical evolution is limited
beyond 1 Gyr. By t = 10 Gyr, about one third of the host stars are single, and more than half have only one
companion left. Most of the other systems have two companions left that orbit their host star in widely separated
orbits. A small fraction of companions have formed binaries that orbit the host star in a hierarchical triple
configuration. The majority of such double-companion systems have internal orbits that are retrograde with respect
to their orbits around their host stars. Our simulations allow a comparison between the predicted outcomes of disk
fragmentation with the observed low-mass hydrogen-burning stars, BDs, and PMOs in the solar neighborhood.
Imaging and radial velocity surveys for faint binary companions among nearby stars are necessary for verification
or rejection of the formation mechanism proposed in this paper
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