2,321 research outputs found
Interpreting the sub-linear Kennicutt-Schmidt relationship: The case for diffuse molecular gas
Recent statistical analysis of two extragalactic observational surveys
strongly indicate a sublinear Kennicutt-Schmidt (KS) relationship between the
star formation rate (Sigsfr) and molecular gas surface density (Sigmol). Here,
we consider the consequences of these results in the context of common
assumptions, as well as observational support for a linear relationship between
Sigsfr and the surface density of dense gas. If the CO traced gas depletion
time (tau_mol) is constant, and if CO only traces star forming giant molecular
clouds (GMCs), then the physical properties of each GMC must vary, such as the
volume densities or star formation rates. Another possibility is that the
conversion between CO luminosity and Sigmol, the XCO factor, differs from
cloud-to-cloud. A more straightforward explanation is that CO permeates the
hierarchical ISM, including the filaments and lower density regions within
which GMCs are embedded. A number of independent observational results support
this description, with the diffuse gas comprising at least 30% of the total
molecular content. The CO bright diffuse gas can explain the sublinear KS
relationship, and consequently leads to an increasing tau_mol with Sigmol. If
Sigsfr linearly correlates with the dense gas surface density, a sublinear KS
relationship indicates that the fraction of diffuse gas fdiff grows with
Sigmol. In galaxies where Sigmol falls towards the outer disk, this description
suggests that fdiff also decreases radially.Comment: 8 pages, 4 figures, to appear in MNRAS, comments welcom
On the temperature structure of the Galactic Centre cloud G0.253+0.016
We present a series of smoothed particle hydrodynamical models of
G0.253+0.016 (also known as 'The Brick'), a very dense molecular cloud that
lies close to the Galactic Centre. We explore how its gas and dust temperatures
react as we vary the strength of both the interstellar radiation field (ISRF)
and the cosmic ray ionisation rate (CRIR). As the physical extent of
G0.253+0.016 along our line-of-sight is unknown, we consider two possibilities:
one in which the longest axis is that measured in the plane of the sky (9.4 pc
in length), and one in which it is along the line of sight, in which case we
take it to be 17 pc. To recover the observed gas and dust temperatures, we find
find that the ISRF must be around 1000 times the solar neighbourhood value, and
the CRIR must be roughly 1E-14 /s, regardless of the geometries studied. For
such high values of the CRIR, we find that cooling in the cloud's interior is
dominated by neutral oxygen, in contrast to standard molecular clouds, which at
the same densities are mainly cooled via CO. Our results suggest that the
conditions near G0.253+0.016 are more extreme than those generally accepted for
the inner 500 pc of the galaxy.Comment: 6 pages, 4 figures, 1 table, accepted for publication in ApJ Letter
Using CO line ratios to trace the physical properties of molecular clouds
The carbon monoxide (CO) rotational transition lines are the most common
tracers of molecular gas within giant molecular clouds (MCs). We study the
ratio () between CO's first two emission lines and examine what
information it provides about the physical properties of the cloud. To study
we perform smooth particle hydrodynamic simulations with time
dependent chemistry (using GADGET-2), along with post-process radiative
transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic
emission maps of a MC. has a bimodal distribution that is a
consequence of the excitation properties of each line, given that reaches
local thermal equilibrium (LTE) while is still sub-thermally excited in
the considered clouds. The bimodality of serves as a tracer of
the physical properties of different regions of the cloud and it helps
constrain local temperatures, densities and opacities. Additionally this
bimodal structure shows an important portion of the CO emission comes from
diffuse regions of the cloud, suggesting that the commonly used conversion
factor of between both lines may need to be studied
further.Comment: 10 pages, 8 figures, accepted to MNRA
Prediction of the asymmetry in decay
Of all decays, the decay
has the smallest observed branching ratio as it takes place primarily via the
suppressed -exchange diagram. The asymmetry for this mode is yet to be
measured experimentally. By exploiting the relationship among the decay
amplitudes of decays (using isospin and topological
amplitudes) we are able to relate the asymmetries and branching ratios by
a simple expression. This enables us to predict the asymmetry in
. While the predicted central values of
are outside the physically allowed region, they are currently associated with
large uncertainties owing to the large errors in the measurements of the branching ratio (), the other asymmetries
(of ) and (of ). With a precise determination of , and
, one can use our analytical result to predict with a
reduced error and compare it with the experimental measurement when it becomes
available. The correlation between and is an interesting
aspect that can be probed in ongoing and future particle physics experiments
such as LHCb and Belle II.Comment: 21 pages, 6 figures, accepted for publication in JHE
Principal Component Analysis of Molecular Clouds: Can CO reveal the dynamics?
We use Principal Component Analysis (PCA) to study the gas dynamics in
numerical simulations of typical MCs. Our simulations account for the
non-isothermal nature of the gas and include a simplified treatment of the
time-dependent gas chemistry. We model the CO line emission in a
post-processing step using a 3D radiative transfer code. We consider mean
number densities n_0 = 30, 100, 300 cm^{-3} that span the range of values
typical for MCs in the solar neighbourhood and investigate the slope
\alpha_{PCA} of the pseudo structure function computed by PCA for several
components: the total density, H2 density, 12CO density, 12CO J = 1 -> 0
intensity and 13CO J = 1 -> 0 intensity. We estimate power-law indices
\alpha_{PCA} for different chemical species that range from 0.5 to 0.9, in good
agreement with observations, and demonstrate that optical depth effects can
influence the PCA. We show that when the PCA succeeds, the combination of
chemical inhomogeneity and radiative transfer effects can influence the
observed PCA slopes by as much as ~ +/- 0.1. The method can fail if the CO
distribution is very intermittent, e.g. in low-density clouds where CO is
confined to small fragments.Comment: 12 pages, 8 figures, accepted for publication in MNRA
Interaction Between Motor Domains Can Explain the Complex Dynamics of Heterodimeric Kinesins
Motor proteins are active enzyme molecules that play a crucial role in many
biological processes. They transform the chemical energy into the mechanical
work and move unidirectionally along rigid cytoskeleton filaments.
Single-molecule experiments suggest that motor proteins, consisting of two
motor domains, move in a hand-over-hand mechanism when each subunit changes
between trailing and leading positions in alternating steps, and these subunits
do not interact with each other. However, recent experiments on heterodimeric
kinesins suggest that the motion of motor domains is not independent, but
rather strongly coupled and coordinated, although the mechanism of these
interactions are not known. We propose a simple discrete stochastic model to
describe the dynamics of homodimeric and heterodimeric two-headed motor
proteins. It is argued that interactions between motor domains modify free
energy landscapes of each motor subunit, and motor proteins still move via the
hand-over-hand mechanism but with different transitions rates. Our calculations
of biophysical properties agree with experimental observations. Several ways to
test the theoretical model are proposed.Comment: To appear in New J. Phy
Indications of a sub-linear and non-universal Kennicutt-Schmidt relationship
We estimate the parameters of the Kennicutt-Schmidt (KS) relationship,
linking the star formation rate (Sigma_SFR) to the molecular gas surface
density (Sigma_mol), in the STING sample of nearby disk galaxies using a
hierarchical Bayesian method. This method rigorously treats measurement
uncertainties, and provides accurate parameter estimates for both individual
galaxies and the entire population. Assuming standard conversion factors to
estimate Sigma_SFR and Sigma_mol from the observations, we find that the KS
parameters vary between galaxies, indicating that no universal relationship
holds for all galaxies. The KS slope of the whole population is 0.76, with the
2sigma range extending from 0.58 to 0.94. These results imply that the
molecular gas depletion time is not constant, but varies from galaxy to galaxy,
and increases with the molecular gas surface density. Therefore, other galactic
properties besides just Sigma_mol affect Sigma_SFR, such as the gas fraction or
stellar mass. The non-universality of the KS relationship indicates that a
comprehensive theory of star formation must take into account additional
physical processes that may vary from galaxy to galaxy.Comment: 7 pages, 2 figures, 1 table. Updated to match MNRAS accepted versio
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