421 research outputs found
Inhomogeneous Big-Bang Nucleosynthesis in Light of Recent Observations
We consider inhomogeneous big bang nucleosynthesis in light of the present
observational situation. Different observations of He-4 and D disagree with
each other, and depending on which set of observations one uses, the estimated
primordial He-4 corresponds to a lower baryon density in standard big bang
nucleosynthesis than what one gets from deuterium. Recent Kamiokande results
rule out a favorite particle physics solution to this tension between He-4 and
D. Inhomogeneous nucleosynthesis can alleviate this tension, but the more
likely solution is systematics in the observations. The upper limit to Omega_b
from inhomogeneous nucleosynthesis is higher than in standard nucleosynthesis,
given that the distance scale of the inhomogeneity is near the optimal value,
which maximizes effects of neutron diffusion. Possible sources of baryon
inhomogeneity include the QCD and electroweak phase transitions. The distance
scale of the inhomogeneities arising from the electroweak transition is too
small for them to have a large effect on nucleosynthesis, but the effect may
still be larger than some of the other small corrections recently incorporated
to SBBN codes.Comment: 12 pages, 8 figures, REVTe
The Musca cloud: A 6 pc-long velocity-coherent, sonic filament
Filaments play a central role in the molecular clouds' evolution, but their
internal dynamical properties remain poorly characterized. To further explore
the physical state of these structures, we have investigated the kinematic
properties of the Musca cloud. We have sampled the main axis of this
filamentary cloud in CO and CO (2--1) lines using APEX
observations. The different line profiles in Musca shows that this cloud
presents a continuous and quiescent velocity field along its 6.5 pc of
length. With an internal gas kinematics dominated by thermal motions (i.e.,
) and large-scale velocity gradients, these results
reveal Musca as the longest velocity-coherent, sonic-like object identified so
far in the ISM. The transonic properties of Musca present a clear departure
from the predicted supersonic velocity dispersions expected in the Larson's
velocity dispersion-size relationship, and constitute the first observational
evidence of a filament fully decoupled from the turbulent regime over
multi-parsec scales.Comment: 12 pages, 6 figures; Accepted for publication in A&
Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL
We present the first study of the relationship between the column density
distribution of molecular clouds within nearby Galactic spiral arms and their
evolutionary status as measured from their stellar content. We analyze a sample
of 195 molecular clouds located at distances below 5.5 kpc, identified from the
ATLASGAL 870 micron data. We define three evolutionary classes within this
sample: starless clumps, star-forming clouds with associated young stellar
objects, and clouds associated with HII regions. We find that the N(H2)
probability density functions (N-PDFs) of these three classes of objects are
clearly different: the N-PDFs of starless clumps are narrowest and close to
log-normal in shape, while star-forming clouds and HII regions exhibit a
power-law shape over a wide range of column densities and log-normal-like
components only at low column densities. We use the N-PDFs to estimate the
evolutionary time-scales of the three classes of objects based on a simple
analytic model from literature. Finally, we show that the integral of the
N-PDFs, the dense gas mass fraction, depends on the total mass of the regions
as measured by ATLASGAL: more massive clouds contain greater relative amounts
of dense gas across all evolutionary classes.Comment: Accepted for publication in A&A (25th June 15) 23 pages, 12 figures.
Additional appendix figures will appear in the journal version of this pape
Mass reservoirs surrounding massive infrared dark clouds: A view by near-infrared dust extinction
Context: Infrared Dark Clouds (IRDCs) harbor progenitors of high-mass stars.
Little is known of the parental molecular clouds of the IRDCs. Aims: We
demonstrate the feasibility of the near-infrared (NIR) dust extinction mapping
in tracing the parental molecular clouds of IRDCs at the distances of D = 2.5 -
8 kpc. Methods: We derive NIR extinction maps for 10 prominent IRDC complexes
using a color-excess mapping technique and NIR data from the UKIDSS/Galactic
Plane Survey. We compare the resulting maps to the 13CO emission line data, to
the 8 \mu m dust opacity data, and to the millimeter dust emission data. We
derive distances for the clouds by comparing the observed NIR source densities
to the Besancon stellar distribution model and compare them to the kinematic
distance estimates. Results: The NIR extinction maps provide a view to the IRDC
complexes over the dynamical range of Av = 2 - 40 mag, in spatial resolution of
30". The NIR extinction data correlate well with the 13CO data and probe a
similar gas component, but also extend to higher column densities. The NIR data
reveal a wealth of extended structures surrounding the dense gas traced by the
8 \mu m shadowing features and sub-mm dust emission, showing that the clouds
contain typically > 10 times more mass than traced by those tracers. The IRDC
complexes of our sample contain relatively high amount of high-column density
material, and their cumulative column density distributions resemble active
nearby star-forming clouds like Orion rather than less active clouds like
California. Conclusions: NIR dust extinction data provide a new powerful tool
to probe the mass distribution of the parental molecular clouds of IRDCs up to
the distances of D = 8 kpc. This encourages for deeper NIR observations of
IRDCs, because the sensitivity and resolution of the data can be directly
enhanced with dedicated observations.Comment: 22 pages, 24 figures, accepted to A&A. A version with full resolution
figures can be downloaded from
http://www.mpia-hd.mpg.de/homes/jtkainul/NexusI/NexusI_v1.pd
Probing the evolution of molecular cloud structure II: From chaos to confinement
We present an analysis of the large-scale molecular cloud structure and of
the stability of clumpy structures in nearby molecular clouds. In our recent
work, we identified a structural transition in molecular clouds by studying the
probability distributions of gas column densities in them. In this paper, we
further examine the nature of this transition. The transition takes place at
the visual extinction of A_V^tail = 2-4 mag, or equivalently, at \Sigma^tail =
40-80 Ms pc^{-2}. The clumps identified above this limit have wide ranges of
masses and sizes, but a remarkably constant mean volume density of n = 10^3
cm^{-3}. This is 5-10 times larger than the density of the medium surrounding
the clumps. By examining the stability of the clumps, we show that they are
gravitationally unbound entities, and that the external pressure from the
parental molecular cloud is a significant source of confining pressure for
them. Then, the structural transition at A_V^tail may be linked to a transition
between this population and the surrounding medium. The star formation rates in
the clouds correlate strongly with the total mass in the clumps, i.e, with the
mass above A_V^tail, dropping abruptly below that threshold. These results
imply that the formation of pressure confined clumps introduces a prerequisite
for star formation. Furthermore, they give a physically motivated explanation
for the recently reported relation between the star formation rates and the
amount of dense material in molecular clouds. Likewise, they give rise to a
natural threshold for star formation at A_V^tail.Comment: 11 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Probing the evolution of molecular cloud structure: From quiescence to birth
Aims: We derive the probability density functions (PDFs) of column density
for a complete sample of prominent molecular cloud complexes closer than 200
pc. Methods: We derive near-infrared dust extinction maps for 23 molecular
cloud complexes, using the "nicest" colour excess mapping technique and data
from the 2MASS archive. The extinction maps are then used to examine the column
density PDFs in the clouds. Results: The column density PDFs of most molecular
clouds are well-fitted by log-normal functions at low column densities (0.5 mag
< A_v < 3-5 mag). However, at higher column densities prominent, power-law-like
wings are common. In particular, we identify a trend among the PDFs: active
star-forming clouds always have prominent non-log-normal wings. In contrast,
clouds without active star formation resemble log-normals over the whole
observed column density range, or show only low excess of higher column
densities. This trend is also reflected in the cumulative PDFs, showing that
the fraction of high column density material is significantly larger in
star-forming clouds. These observations are in agreement with an evolutionary
trend where turbulent motions are the main cloud-shaping mechanism for
quiescent clouds, but the density enhancements induced by them quickly become
dominated by gravity (and other mechanisms) which is strongly reflected by the
shape of the column density PDFs. The dominant role of the turbulence is
restricted to the very early stages of molecular cloud evolution, comparable to
the onset of active star formation in the clouds.Comment: 7 pages, 11 figures, accepted to A&A Letter
Is Centaurus A special? A neutral hydrogen perspective
Due to the proximity, the neutral hydrogen belonging to Centaurus A can be
observed at high resolution with good sensitivity. This allows to study the
morphology and kinematics in detail in order to understand the evolution of
this radio-loud source (e.g. merger history, AGN activity). At the same time,
it is important to compare the results to other sources of the same class (i.e.
early-type galaxies in general and radio galaxies in particular) to see how
Centaurus A fits into the global picture of early-type/radio galaxy evolution.
The amount of HI, the morphology of a warped disk with HI clouds surrounding
the disk and the regular kinematics of the inner part of the HI disk are not
unusual for early-type galaxies. The growing evidence that mergers are not
necessarily responsible for AGN activity fits with the observational result
that the recent merger event in Centaurus A is not connected to the current
phase of activity. Based on these results, we conclude that Centaurus A has
typical neutral hydrogen properties for an early-type and radio galaxy and it
can therefore - from an HI perspective - be seen as a typical example of its
class.Comment: submitted to PASA; 7 pages, 4 figure
Mouthwash Effects on LGG-Integrated Experimental Oral Biofilms
In order to investigate the effects of mouthwashes on oral biofilms with probiotics, we compared in biofilms the susceptibility to mouthwashes of probiotic Lactobacillus rhamnosus GG (LGG) and oral pathogens Streptococcus mutans, Streptococcus sanguinis, and Candida albicans. We also evaluated these pathogens’ susceptibility to the mouthwashes and their recovery after mouthwash-rinsing in biofilms with/without LGG. First, 1-day-/3-day-old LGG-integrated multi-species biofilms were exposed for 1 min to mouthwashes containing chlorhexidine, essential oils, or amine fluoride/stannous fluoride. Cells were plate-counted and relative survival rates (RSRs) of LGG and pathogens calculated. Second, 1-day-/3-day-old multispecies biofilms with and without LGG were exposed for 1 min to mouthwashes; cells were plate-counted and the pathogens’ RSRs were calculated. Third, 1-day-old biofilms were treated for 1 min with mouthwashes. Cells were plate-counted immediately and after 2-day cultivation. Recovery rates of pathogens were calculated and compared between biofilms with/without LGG. Live/Dead® staining served for structural analyses. Our results showed that RSRs of LGG were insignificantly smaller than those of pathogens in both 1-day and 3-day biofilms. No significant differences appeared in pathogens’ RSRs and recovery rates after treatment between biofilms with/without LGG. To conclude, biofilm LGG was susceptible to the mouthwashes; but biofilm LGG altered neither the mouthwash effects on oral pathogens nor affected their recovery
Do Lognormal Column-Density Distributions in Molecular Clouds Imply Supersonic Turbulence?
Recent observations of column densities in molecular clouds find lognormal
distributions with power-law high-density tails. These results are often
interpreted as indications that supersonic turbulence dominates the dynamics of
the observed clouds. We calculate and present the column-density distributions
of three clouds, modeled with very different techniques, none of which is
dominated by supersonic turbulence. The first star-forming cloud is simulated
using smoothed particle hydrodynamics (SPH); in this case gravity, opposed only
by thermal-pressure forces, drives the evolution. The second cloud is
magnetically subcritical with subsonic turbulence, simulated using nonideal
MHD; in this case the evolution is due to gravitationally-driven ambipolar
diffusion. The third cloud is isothermal, self-gravitating, and has a smooth
density distribution analytically approximated with a uniform inner region and
an r^-2 profile at larger radii. We show that in all three cases the
column-density distributions are lognormal. Power-law tails develop only at
late times (or, in the case of the smooth analytic profile, for strongly
centrally concentrated configurations), when gravity dominates all opposing
forces. It therefore follows that lognormal column-density distributions are
generic features of diverse model clouds, and should not be interpreted as
being a consequence of supersonic turbulence.Comment: 6 pages, 6 figures, accepted for publication in MNRA
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