118 research outputs found
Line Profiles of Cores within Clusters. III. What is the most reliable tracer of core collapse in dense clusters?
Recent observational and theoretical investigations have emphasised the
importance of filamentary networks within molecular clouds as sites of star
formation. Since such environments are more complex than those of isolated
cores, it is essential to understand how the observed line profiles from
collapsing cores with non-spherical geometry are affected by filaments. In this
study, we investigate line profile asymmetries by performing radiative transfer
calculations on hydrodynamic models of three collapsing cores that are embedded
in filaments. We compare the results to those that are expected for isolated
cores. We model the five lowest rotational transition line (J = 1-0, 2-1, 3-2,
4-3, and 5-4) of both optically thick (HCN, HCO) as well as optically thin
(NH, HCO) molecules using constant abundance laws. We find
that less than 50% of simulated (1-0) transition lines show blue infall
asymmetries due to obscuration by the surrounding filament. However, the
fraction of collapsing cores that have a blue asymmetric emission line profile
rises to 90% when observed in the (4-3) transition. Since the densest gas
towards the collapsing core can excite higher rotational states, upper level
transitions are more likely to produce blue asymmetric emission profiles. We
conclude that even in irregular, embedded cores one can trace infalling gas
motions with blue asymmetric line profiles of optically thick lines by
observing higher transitions. The best tracer of collapse motions of our sample
is the (4-3) transition of HCN, but the (3-2) and (5-4) transitions of both HCN
and HCO are also good tracers.Comment: accepted by MNRAS; 13 pages, 16 figures, 6 table
The Nature of the Variable Galactic Center Source IRS16SW
We report measurements of the light curve of the variable Galactic Center
source IRS16SW. The light curve is not consistent with an eclipsing binary or
any other obvious variable star. The source may be an example of a high mass
variable predicted theoretically but not observed previously.Comment: 11 pages, 2 figures. Accepted by Ap
Dust SEDs in the era of Herschel and Planck: a Hierarchical Bayesian fitting technique
We present a hierarchical Bayesian method for fitting infrared spectral
energy distributions (SEDs) of dust emission to observed fluxes. Under the
standard assumption of optically thin single temperature (T) sources the dust
SED as represented by a power--law modified black body is subject to a strong
degeneracy between T and the spectral index beta. The traditional
non-hierarchical approaches, typically based on chi-square minimization, are
severely limited by this degeneracy, as it produces an artificial
anti-correlation between T and beta even with modest levels of observational
noise. The hierarchical Bayesian method rigorously and self-consistently treats
measurement uncertainties, including calibration and noise, resulting in more
precise SED fits. As a result, the Bayesian fits do not produce any spurious
anti-correlations between the SED parameters due to measurement uncertainty. We
demonstrate that the Bayesian method is substantially more accurate than the
chi-square fit in recovering the SED parameters, as well as the correlations
between them. As an illustration, we apply our method to Herschel and sub
millimeter ground-based observations of the star-forming Bok globule CB244.
This source is a small, nearby molecular cloud containing a single low-mass
protostar and a starless core. We find that T and beta are weakly positively
correlated -- in contradiction with the chi-square fits, which indicate a
T-beta anti-correlation from the same data-set. Additionally, in comparison to
the chi-square fits the Bayesian SED parameter estimates exhibit a reduced
range in values.Comment: 20 pages, 9 figures, ApJ format, revised version matches ApJ-accepted
versio
Fast deuterium fractionation in magnetized and turbulent filaments
Deuterium fractionation is considered as an important process to infer the
chemical ages of prestellar cores in filaments. We present here the first
magneto-hydrodynamical simulations including a chemical network to study
deuterium fractionation in magnetized and turbulent filaments and their
substructures. The filaments typically show widespread deuterium fractionation
with average values . For individual cores of similar age, we
observe the deuteration fraction to increase with time, but also to be
independent of their average properties such as density, virial or
mass-to-magnetic flux ratio. We further find a correlation of the deuteration
fraction with core mass, average H density and virial parameter only at
late evolutionary stages of the filament and attribute this to the lifetime of
the individual cores. Specifically, chemically old cores reveal higher
deuteration fractions. Within the radial profiles of selected cores, we notice
differences in the structure of the deuteration fraction or surface density,
which we can attribute to their different turbulent properties. High
deuteration fractions of the order may be reached within
approximately ~kyrs, corresponding to two free-fall times, as defined for
cylindrical systems, of the filamentsComment: submitted to MNRAS. Comments welcom
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