1,927 research outputs found
Conclusions and Recommendations
Rosana G. Moreira, Editor-in-Chief; Texas A&M UniversityThis is a paper from International Commission of Agricultural Engineering (CIGR, Commission Internationale du Genie Rural) E-Journal Volume 7 (2005): Conclusions and Recommendations by E. Gasparett
HST/NICMOS Paschen-alpha Survey of the Galactic Center: Overview
We have recently carried out the first wide-field hydrogen Paschen-alpha line
imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard
the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 around
the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 Micron narrow
bands with a spatial resolution of 0.01 pc at a distance of 8 kpc. Here we
present an overview of the observations, data reduction, preliminary results,
and potential scientific implications, as well as a description of the
rationale and design of the survey. We have produced mosaic maps of the
Paschen-alpha line and continuum emission, giving an unprecedentedly high
resolution and high sensitivity panoramic view of stars and photo-ionized gas
in the nuclear environment of the Galaxy. We detect a significant number of
previously undetected stars with Paschen-alpha in emission. They are most
likely massive stars with strong winds, as confirmed by our initial follow-up
spectroscopic observations. About half of the newly detected massive stars are
found outside the known clusters (Arches, Quintuplet, and Central). Many
previously known diffuse thermal features are now resolved into arrays of
intriguingly fine linear filaments indicating a profound role of magnetic
fields in sculpting the gas. The bright spiral-like Paschen-alpha emission
around Sgr A* is seen to be well confined within the known dusty torus. In the
directions roughly perpendicular to it, we further detect faint, diffuse
Paschen-alpha emission features, which, like earlier radio images, suggest an
outflow from the structure. In addition, we detect various compact
Paschen-alpha nebulae, probably tracing the accretion and/or ejection of stars
at various evolutionary stages.Comment: accepted for publication in MNRAS; a version of higher resolution
images may be found at http://www.astro.umass.edu/~wqd/papers/hst/paper1.pd
Experimental and theoretical study of filtered optical feedback in a semiconductor laser
We report on the systematical investigation of the steady-state regime and the dynamical behavior of a semiconductor laser subject to delayed filtered optical feedback. We study a Fabry-Perot (FP) interferometer type of filter placed in the external feedback loop of a diode laser. The effects of the filter on the locking of the diode laser frequency to the external cavity modes are described. We report and observe hysteresis, bistability, and multistability and show that all these are well described by a set of rate equations for the coupled laser and FP cavity system. We also present an experimental stability diagram that summarizes the dynamical behavior of the syste
K-shell x-ray spectroscopy of atomic nitrogen
Absolute {\it K}-shell photoionization cross sections for atomic nitrogen
have been obtained from both experiment and state-of-the-art theoretical
techniques. Due to the difficulty of creating a target of neutral atomic
nitrogen, no high-resolution {\it K}-edge spectroscopy measurements have been
reported for this important atom. Interplay between theory and experiment
enabled identification and characterization of the strong
resonance features throughout the threshold region. An experimental value
of 409.64 0.02 eV was determined for the {\it K}-shell binding energy.Comment: 4 pages, 2 graphs, 1 tabl
Self-bias modulates saccadic control.
We present novel data on the role of attention in eliciting enhanced processing of stimuli associated with self. Participants were required to make pro- or anti-saccades according to whether learned shape-label pairings matched or mismatched. When stimuli matched participants were required to make an anti-saccade, and when the stimuli mismatched a pro-saccade was required. We found that anti-saccades were difficult to make to stimuli associated with self when compared to stimuli associated with a friend and a stranger. In contrast, anti-saccades to friend-stimuli were easier to make than anti-saccades to stranger-stimuli. In addition, a correct anti-saccade to a self-associated stimulus disrupted subsequent pro-saccade trials, relative to when the preceding anti-saccade was made to other stimuli. The data indicate that self-associated stimuli provide a strong cue for explicit shifts of attention to them, and that correct anti-saccades to such stimuli demand high levels of inhibition (which carries over to subsequent pro-saccade trials). The self exerts an automatic draw on attention
Partial Ion Yield Sspectroscopy around the Cl 2p and C 1s Ionization Thresholds in CF3Cl
We present a partial ion yield experiment on freon 13, CF3Cl, excited in the vicinity of the C 1s and Cl 2p ionization thresholds. We have collected a large amount of cationic fragments and a few anionic fragments at both edges. We have observed a strong intensity dependence of Rydberg transitions with ion fragment size for the CFnCl+ and CFn+/F+ (n=0–3) series at both the Cl 2p and C 1s ionization edges. Selectivity in the fragmentation processes involving the C–Cl and C–F bonds are highlighted by the intensities of the C 1s to lowest unoccupied molecular orbital (LUMO) and LUMO+1 transitions measured on the CFnCl+ and CFn+ yields. Equally, by comparison with their cation counterpart, we discuss possible bond-length dependence for the anion formation at the carbon 1s edge
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
Deep near-infrared imaging of W3 Main: constraints on stellar cluster formation
Embedded clusters like W3 Main are complex and dynamically evolving systems
that represent an important phase of the star formation process. We aim at the
characterization of the entire stellar content of W3 Main in a statistical
sense to identify possible differences in evolutionary phase of the stellar
populations and find clues about the formation mechanism of this massive
embedded cluster. Methods. Deep JHKs imaging is used to derive the disk
fraction, Ks-band luminosity functions and mass functions for several
subregions in W3 Main. A two dimensional completeness analysis using artificial
star experiments is applied as a crucial ingredient to assess realistic
completeness limits for our photometry. We find an overall disk fraction of 7.7
2.3%, radially varying from 9.4 3.0 % in the central 1 pc to 5.6
2.2 % in the outer parts of W3 Main. The mass functions derived for three
subregions are consistent with a Kroupa and Chabrier mass function. The mass
function of IRSN3 is complete down to 0.14 Msun and shows a break at M
0.5 Msun. We interpret the higher disk fraction in the center as evidence for a
younger age of the cluster center. We find that the evolutionary sequence
observed in the low-mass stellar population is consistent with the observed age
spread among the massive stars. An analysis of the mass function variations
does not show evidence for mass segregation. W3 Main is currently still
actively forming stars, showing that the ionizing feedback of OB stars is
confined to small areas ( 0.5 pc). The FUV feedback might be influencing
large regions of the cluster as suggested by the low overall disk fraction.Comment: 15 pages, 13 figures, accepted by A&
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
- …