1,691 research outputs found
Non-adiabatic effects in long-pulse mixed-field orientation of a linear polar molecule
We present a theoretical study of the impact of an electrostatic field
combined with non-resonant linearly polarized laser pulses on the rotational
dynamics of linear molecules. Within the rigid rotor approximation, we solve
the time-dependent Schr\"odinger equation for several field configurations.
Using the OCS molecule as prototype, the field-dressed dynamics is analyzed in
detail for experimentally accessible static field strengths and laser pulses.
Results for directional cosines are presented and compared to the predictions
of the adiabatic theory. We demonstrate that for prototypical field
configuration used in current mixed-field orientation experiments, the
molecular field dynamics is, in general, non-adiabatic, being mandatory a
time-dependent description of these systems. We investigate several field
regimes identifying the sources of non-adiabatic effects, and provide the field
parameters under which the adiabatic dynamics would be achieved.Comment: 16 pages, 16 figures. Submitted to Physical Review
Age spread in Galactic star forming region W3 Main
We present near-infrared JHKs imaging as well as K-band multi-object
spectroscopy of the massive stellar content of W3 Main using LUCI at the LBT.
We confirm 13 OB stars by their absorption line spectra in W3 Main and spectral
types between O5V and B4V have been found. Three massive Young Stellar Objects
are identified by their emission line spectra and near-infrared excess. From
our spectrophotometric analysis of the massive stars and the nature of their
surrounding HII regions we derive the evolutionary sequence of W3 Main and we
find an age spread of 2-3 Myr.Comment: 4 pages, 2 figures, To appear in conference proceedings of "370 years
of Astronomy in Utrecht
Multiple episodes of star formation in the CN15/16/17 molecular complex
We have started a campaign to identify massive star clusters inside bright
molecular bubbles towards the Galactic Center. The CN15/16/17 molecular complex
is the first example of our study. The region is characterized by the presence
of two young clusters, DB10 and DB11, visible in the NIR, an ultra-compact HII
region identified in the radio, several young stellar objects visible in the
MIR, a bright diffuse nebulosity at 8\mu m coming from PAHs and sub-mm
continuum emission revealing the presence of cold dust. Given its position on
the sky (l=0.58, b=-0.85) and its kinematic distance of ~7.5 kpc, the region
was thought to be a very massive site of star formation in proximity of the
CMZ. The cluster DB11 was estimated to be as massive as 10^4 M_sun. However the
region's properties were known only through photometry and its kinematic
distance was very uncertain given its location at the tangential point. We
aimed at better characterizing the region and assess whether it could be a site
of massive star formation located close to the Galactic Center. We have
obtained NTT/SofI JHKs photometry and long slit K band spectroscopy of the
brightest members. We have additionally collected data in the radio, sub-mm and
mid infrared, resulting in a quite different picture of the region. We have
confirmed the presence of massive early B type stars and have derived a
spectro-photometric distance of ~1.2 kpc, much smaller than the kinematic
distance. Adopting this distance we obtain clusters masses of M(DB10) ~ 170
M_sun and M(DB11) ~ 275 M_sun. This is consistent with the absence of any O
star, confirmed by the excitation/ionization status of the nebula. No HeI
diffuse emission is detected in our spectroscopic observations at 2.113\mu m,
which would be expected if the region was hosting more massive stars. Radio
continuum measurements are also consistent with the region hosting at most
early B stars.Comment: Accepted for publication in Astronomy and Astrophysics. Fig. 1 and 3
presented in reduced resolutio
Post-collision Interactions in the Auger Decay of the Ar L-shell
The photoionization cross sections for Ar+ through Ar4+, produced by the Auger decay of a 2p hole in argon, have been measured between 242 eV and 253 eV by the use of synchrotron radiation. The high resolution of the monochromator has allowed a detailed study of the postcollision interactions that occur in this spectral region. The concept of photoelectron recapture by Ar2+ to produce the Ar+ continuum is studied. The relative values of the quantum-mechanical calculations of the photoelectron recapture probability are shown to be in excellent agreement with the present data. The magnitude and shape of the Ar2+ continuum has been explained on the basis that about 67% of the recaptured photoelectrons produce excited states of Ar+ which subsequently reemit the electrons by autoionization
Faecalibacterium prausnitzii Strain HTF-F and Its Extracellular Polymeric Matrix Attenuate Clinical Parameters in DSS-Induced Colitis
Date of Acceptance: 26/02/2015 Funding: The authors received no specific funding for this work.Peer reviewedPublisher PD
Observational indicators of the transition from fully convective stars to stars with radiative cores
We present a discussion of the similarities and key differences between the
transition onto (at the turn-on) and away from (at the turn-off) the main
sequence, the latter termed the Hertzsprung gap. Using a set of model
isochrones and adopting an initial mass function leads us to predict a dearth
of G-type stars for any star forming region. This is caused by the (relatively)
constant spectral type at which the transition from a fully convective star to
a star with a radiative core begins. We also present analysis of the details of
this transition in the ONC. In particular we show that a gap in the photometric
and spectral type distributions is centred on, and a change in the fractional
X-ray luminosity and rotation rate distribution occurs approximately at, the
position of a peak in radiative core size as a function of mass. Whilst
photometric signatures of this transition are lost at ages over ~20 Myrs, we
show that changes in fractional X-ray luminosity and magnetic field
configuration persist to older ages. Analysis of literature data show that the
mass at which the change in fractional X-ray luminosity is observed decreases
with age.Comment: 9 pages and 6 figures. Accepted for publication in MNRA
Double Photoionization of Helium
The cross sections for double photoionization of helium and the ratios of double to single ionization have been measured from the double-ionization threshold to 820 eV. The results are in very good agreement with several recent calculations
Adiabatic orientation of rotating dipole molecules in an external field
The induced polarization of a beam of polar clusters or molecules passing
through an electric or magnetic field region differs from the textbook
Langevin-Debye susceptibility. This distinction, which is important for the
interpretation of deflection and focusing experiments, arises because instead
of acquiring thermal equilibrium in the field region, the beam ensemble
typically enters the field adiabatically, i.e., with a previously fixed
distribution of rotational states. We discuss the orientation of rigid
symmetric-top systems with a body-fixed electric or magnetic dipole moment. The
analytical expression for their "adiabatic-entry" orientation is elucidated and
compared with exact numerical results for a range of parameters. The
differences between the polarization of thermodynamic and "adiabatic-entry"
ensembles, of prolate and oblate tops, and of symmetric-top and linear rotators
are illustrated and identified.Comment: 18 pages, 4 figure
Proper motions of the Arches cluster with Keck-LGS Adaptive Optics: the first kinematic mass measurement of the Arches
We report the first detection of the intrinsic velocity dispersion of the
Arches cluster - a young (~2 Myr), massive (~10,000 Solar Mass) starburst
cluster located near the Galactic center. This was accomplished using proper
motion measurements within the central region of the cluster, obtained with the
laser guide star adaptive optics system at Keck Observatory over a 3 year time
baseline (2006-2009). This uniform dataset results in proper motion
measurements that are improved by a factor ~5 over previous measurements from
heterogeneous instruments, yielding internal velocity dispersion estimates 0.15
+/- 0.01 mas/yr, which corresponds to 5.4 +/- 0.4 km/s at a distance of 8.4
kpc.
Projecting a simple model for the cluster onto the sky to compare with our
proper motion dataset, in conjunction with surface density data, we estimate
the total present-day mass of the cluster to be 15,000 (+7400 -6000) Solar
masses. The mass in stars observed within a cylinder of radius R=0.4 pc is
found to be 9000 (+4000 -3500) Solar Masses at formal 3-sigma confidence. This
mass measurement is free from assumptions about the mass function of the
cluster, and thus may be used to check mass estimates from photometry and
simulation. When we conduct this check, we find that the present-day mass
function of the Arches cluster is likely either top-heavy or truncated at
low-mass, or both.
Collateral benefits of our data and analysis include: 1. cluster membership
probabilities, which may be used to extract a clean cluster sample for future
photometric work; 2. a refined estimate of the bulk motion of the Arches
cluster with respect to the field, which we find to be 172 +/- 15 km/s, which
is slightly slower than suggested by previous VLT-Keck measurements; and 3. a
velocity dispersion estimate for the field itself, which is likely dominated by
the inner galactic bulge and the nuclear disk.Comment: 73 pages, 28 figures, 12 tables, ApJ accepte
SARS-CoV-2 structural coverage map reveals viral protein assembly, mimicry, and hijacking mechanisms
We modeled 3D structures of all SARS-CoV-2 proteins, generating 2,060 models that span 69% of the viral proteome and provide details not available elsewhere. We found that ˜6% of the proteome mimicked human proteins, while ˜7% was implicated in hijacking mechanisms that reverse post-translational modifications, block host translation, and disable host defenses; a further ˜29% self-assembled into heteromeric states that provided insight into how the viral replication and translation complex forms. To make these 3D models more accessible, we devised a structural coverage map, a novel visualization method to show what is-and is not-known about the 3D structure of the viral proteome. We integrated the coverage map into an accompanying online resource (https://aquaria.ws/covid) that can be used to find and explore models corresponding to the 79 structural states identified in this work. The resulting Aquaria-COVID resource helps scientists use emerging structural data to understand the mechanisms underlying coronavirus infection and draws attention to the 31% of the viral proteome that remains structurally unknown or dark
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