495 research outputs found
Dynamical structure of the inner 100 AU of the deeply embedded protostar IRAS 16293-2422
A fundamental question about the early evolution of low-mass protostars is
when circumstellar disks may form. High angular resolution observations of
molecular transitions in the (sub)millimeter wavelength windows make it
possible to investigate the kinematics of the gas around newly-formed stars,
for example to identify the presence of rotation and infall. IRAS 16293-2422
was observed with the extended Submillimeter Array (eSMA) resulting in
subarcsecond resolution (0.46" x 0.29", i.e. 55 35~AU) images
of compact emission from the CO (3-2) and CS (7-6) transitions at
337~GHz (0.89~mm). To recover the more extended emission we have combined the
eSMA data with SMA observations of the same molecules. The emission of
CO (3-2) and CS (7-6) both show a velocity gradient oriented
along a northeast-southwest direction with respect to the continuum marking the
location of one of the components of the binary, IRAS16293A. Our combined eSMA
and SMA observations show that the velocity field on the 50--400~AU scales is
consistent with a rotating structure. It cannot be explained by simple
Keplerian rotation around a single point mass but rather needs to take into
account the enclosed envelope mass at the radii where the observed lines are
excited. We suggest that IRAS 16293-2422 could be among the best candidates to
observe a pseudo-disk with future high angular resolution observations.Comment: Accepted for publication in ApJ, 18 pages, 10 figure
Structure of evolved cluster-forming regions
Context. An approach towards understanding the formation of massive
stars and star clusters is to study the structure of their hot core phase, an evolutionary
stage where dust has been heated, but molecules have not yet been destroyed by ultraviolet
radiation. These hot molecular cores are very line-rich, but the interpretation of line
surveys is also hampered by poor knowledge of the physical and chemical structure.
Aims. To constrain the radial structure of high-mass star-forming
regions containing hot cores, we attempt to reproduce by radiative transfer modeling both
the intensity and shape of a variety of molecular lines.
Methods. We observed 12 hot cores with the Atacama Pathfinder EXperiment
(APEX) in lines of HCN, HCO+, CO, and their isotopologues, including
high-J lines and vibrationally excited HCN. We investigate how well the
sources can be modeled as centrally heated spheres with a power-law density gradient,
making use of the radiative transfer code RATRAN and the radial profile of the submm
continuum emission, taken from the APEX Telescope Large Area Survey of the GALaxy
(ATLASGAL).
Results. Most of the observed lines have complicated shapes that
incorporate self-absorption, asymmetries, and line wings. Vibrationally excited HCN is
detected in all sources, and vibrationally excited H13CN in half of the
sources. We are able to successfully model most features seen in the APEX data, such as
the ratio of the isotopologue lines (very high optical depths), self-absorption
(temperature gradient), blue asymmetries (moderate infall), vibrationally excited HCN
(high inner temperatures), and H13CN (high HCN abundance under dense and hot
conditions). Other features could not be reproduced, such as an occasional lack of
self-absorption, the emission from high-J lines in the outer pixels of
the CHAMP+ receiver (15′′−20′′ from the center), the outflow wings, and the red
asymmetric profiles.
Conclusions. The amount of molecular gas, in particular of HCN, at very
high temperatures is larger than previously thought. A complex interplay between infall
and outflow motions is present. Our basic model assumptions of pure central heating and a
power-law radial density distribution can serve as approximations for most sources, but
are too simple to explain all observed lines. In particular, taking into account
clumpiness, multiplicity of heating sources and a more complex velocity field seems to be
necessary to more closely match model calculations to observations. This would require
three-dimensional radiative transfer modeling of high-resolution interferometric data
Demographically explicit scans for barriers to gene flow using gIMble
Identifying regions of the genome that act as barriers to gene flow between recently diverged taxa has remained challenging given the many evolutionary forces that generate variation in genetic diversity and divergence along the genome, and the stochastic nature of this variation. Progress has been impeded by a conceptual and methodological divide between analyses that infer the demographic history of speciation and genome scans aimed at identifying locally maladaptive alleles i.e. genomic barriers to gene flow. Here we implement genomewide IM blockwise likelihood estimation (), a composite likelihood approach for the quantification of barriers, that bridges this divide. This analytic framework captures background selection and selection against barriers in a model of isolation with migration (IM) as heterogeneity in effective population size (N) and effective migration rate (m), respectively. Variation in both effective demographic parameters is estimated in sliding windows via pre-computed likelihood grids. includes modules for pre-processing/filtering of genomic data and performing parametric bootstraps using coalescent simulations. To demonstrate the new approach, we analyse data from a well-studied pair of sister species of tropical butterflies with a known history of post-divergence gene flow: Heliconius melpomene and H. cydno. Our analyses uncover both large-effect barrier loci (including well-known wing-pattern genes) and a genome-wide signal of a polygenic barrier architecture
Demes:A standard format for demographic models
Understanding the demographic history of populations is a key goal in population genetics, and with improving methods and data, ever more complex models are being proposed and tested. Demographic models of current interest typically consist of a set of discrete populations, their sizes and growth rates, and continuous and pulse migrations between those populations over a number of epochs, which can require dozens of parameters to fully describe. There is currently no standard format to define such models, significantly hampering progress in the field. In particular, the important task of translating the model descriptions in published work into input suitable for population genetic simulators is labor intensive and error prone. We propose the Demes data model and file format, built on widely used technologies, to alleviate these issues. Demes provide a well-defined and unambiguous model of populations and their properties that is straightforward to implement in software, and a text file format that is designed for simplicity and clarity. We provide thoroughly tested implementations of Demes parsers in multiple languages including Python and C, and showcase initial support in several simulators and inference methods. An introduction to the file format and a detailed specification are available at https://popsim-consortium.github.io/demes-spec-docs/
The abundance of C18O and HDO in the envelope and hot core of the intermediate mass protostar NGC 7129 FIRS 2
NGC 7129 FIRS 2 is a young intermediate-mass (IM) protostar, which is
associated with two energetic bipolar outflows and displays clear signs of the
presence of a hot core. It has been extensively observed with ground based
telescopes and within the WISH Guaranteed Time Herschel Key Program. We present
new observations of the C18O 3-2 and the HDO 3_{12}-2_{21} lines towards NGC
7129 FIRS 2. Combining these observations with Herschel data and modeling their
emissions, we constrain the C18O and HDO abundance profiles across the
protostellar envelope. In particular, we derive the abundance of C18O and HDO
in the hot core. The intensities of the C18O lines are well reproduced assuming
that the C18O abundance decreases through the protostellar envelope from the
outer edge towards the centre until the point where the gas and dust reach the
CO evaporation temperature (~20-25 K) where the C18O is released back to the
gas phase. Once the C18O is released to the gas phase, the modelled C18O
abundance is found to be ~1.6x10^{-8}, which is a factor of 10 lower than the
reference abundance. This result is supported by the non-detection of C18O 9-8,
which proves that even in the hot core (T_k>100 K) the CO abundance must be 10
times lower than the reference value. Several scenarios are discussed to
explain this C18O deficiency. One possible explanation is that during the
pre-stellar and protostellar phase, the CO is removed from the grain mantles by
reactions to form more complex molecules. Our HDO modeling shows that the
emission of HDO 3_{12}-2_{21} line is maser and comes from the hot core
(T_k>100 K). Assuming the physical structure derived by Crimier et al. (2010),
we determine a HDO abundance of ~0.4 - 1x10^{-7} in the hot core of this IM
protostar, similar to that found in the hot corinos NGC 1333 IRAS 2A and IRAS
16293-2422.Comment: 10 pages, 7 figure
Het betere werk. Economische effecten van een nieuw gevechtsvliegtuig
Deelname aan de Joint Strike Fighter (JSF) brengt voor Nederland tientallen miljarden euro’s omzet met zich mee. De JSF maakt de Nederlandse vliegtuigindustrie innovatiever waardoor additionele spin-offs optreden. Het programma leidt tot verschuiving van werknemers vanuit andere banen; daarbij worden de werknemers productiever. Het werkgelegenheidseffect is op korte termijn positief maar op lange termijn nul. Omzet en werkgelegenheid Met de JSF is voor Nederland een omzet gemoeid van 24 à 38 miljard euro. Het leidt opgeteld over de periode 2013-2064 tot een verschuiving van circa 75.000 arbeidsjaren, vanuit andere banen. Per jaar zijn dat gemiddeld 1.400 banen die verschuiven. Bij deze verschuiving worden de werknemers 2,5 tot 10 procent productiever. Het netto werkgelegenheidseffect van deelnemen aan het F-35 programma is maximaal 1.350 arbeidsjaren (270 banen) tot en met 2017; op lange termijn is het werkgelegenheidseffect nul. Het F-35 programma levert dus vooral hoogwaardiger en productiever werk op, mede door het creëren van kennis en innovaties. Als Nederland stopt met deelnemen en een ander toestel koopt, worden de omzet en toegevoegde waarde ongeveer gehalveerd. Als Nederland uit de testfase stapt, gaat 15 tot 20 procent verloren. Circa twee derde van de productie- en werkgelegenheidseffecten komt voort uit onderhoud van de F-35 of een alternatief toestel. De productiefase betreft circa een derde van de effecten. Kennis en innovatie Participatie in een internationaal programma biedt voor Nederlandse bedrijven de mogelijkheid om op internationale schaal nieuwe marktinzichten, contacten en vaardigheden op te doen. Dit leidt tot marktgerelateerde spin-offs en spillovers, hetgeen zich uit in nieuwe economische activiteiten en extra rendement. De economische meerwaarde van kennisontwikkeling, innovatie, spin-offs en spillovers is echter niet kwantitatief te duiden. Stoppen met de JSF heeft naar verwachting een negatief effect op spin-offs en spillovers vergeleken met doorgaan. De gevolgen van uitstappen uit de testfase zijn lastiger in te schatten. Het effect is negatief maar mogelijk beperkt als Nederland zich stevig blijft committeren aan de andere delen van het JSF programma
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