676 research outputs found
Evidence for Accretion in the High-resolution X-ray Spectrum of the T Tauri Star System Hen 3-600
We present high-resolution X-ray spectra of the multiple T Tauri star system
Hen 3-600, obtained with the High Energy Transmission Grating Spectrograph on
the Chandra X-ray Observatory. Two binary components were detected in the
zeroth-order image. Hen 3-600-A, which has a large mid-infrared excess, is a
2-3 times fainter in X-rays than Hen 3-600-B, due to a large flare on B. The
dispersed X-ray spectra of the two primary components overlap spatially;
spectral analysis was performed on the combined system. Analysis of the
individual spectra was limited to regions where the contributions of A and B
can be disentangled. This analysis results in two lines of evidence indicating
that the X-ray emission from Hen 3-600 is derived from accretion processes:
line ratios of O VII indicate that the characteristic density of its
X-ray-emitting plasma is large; a significant component of low-temperature
plasma is present and is stronger in component A. These results are consistent
with results obtained from X-ray gratings spectroscopy of more rapidly
accreting systems. All of the signatures of Hen 3-600 that are potential
diagnostics of accretion activity -- X-ray emission, UV excess, H-alpha
emission, and weak infrared excess -- suggest that its components represent a
transition phase between rapidly accreting, classical T Tauri stars and
non-accreting, weak-lined T Tauri stars.Comment: latex, 27 pages, 12 figures, 6 tables; accepted by Ap
Attention, Awareness, and the Perception of Auditory Scenes
Auditory perception and cognition entails both low-level and high-level processes, which are likely to interact with each other to create our rich conscious experience of soundscapes. Recent research that we review has revealed numerous influences of high-level factors, such as attention, intention, and prior experience, on conscious auditory perception. And recently, studies have shown that auditory scene analysis tasks can exhibit multistability in a manner very similar to ambiguous visual stimuli, presenting a unique opportunity to study neural correlates of auditory awareness and the extent to which mechanisms of perception are shared across sensory modalities. Research has also led to a growing number of techniques through which auditory perception can be manipulated and even completely suppressed. Such findings have important consequences for our understanding of the mechanisms of perception and also should allow scientists to precisely distinguish the influences of different higher-level influences
Infrared Space Observatory Polarimetric Imaging of the Egg Nebula (RAFGL 2688)
We present polarimetric imaging of the protoplanetary nebula RAFGL 2688
obtained at 4.5 microns with the Infrared Space Observatory (ISO). We have
deconvolved the images to remove the signature of the point spread function of
the ISO telescope, to the extent possible. The deconvolved 4.5 micron image and
polarimetric map reveal a bright point source with faint, surrounding
reflection nebulosity. The reflection nebula is brightest to the
north-northeast, in agreement with previous ground- and space-based infrared
imaging. Comparison with previous near-infrared polarimetric imaging suggests
that the polarization of starlight induced by the dust grains in RAFGL 2688 is
more or less independent of wavelength between 2 microns and 4.5 microns. This,
in turn, indicates that scattering dominates over thermal emission at
wavelengths as long as ~5 microns, and that the dust grains have characteristic
radii < 1 micron.Comment: 27 pages, 9 figures; to appear in the Astronomical Journal, May 2002
issu
Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I
We report the discovery of quiescent emission from molecular hydrogen gas
located in the circumstellar disks of six pre-main sequence stars, including
two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon
I star forming region. For two of these stars, we also place upper limits on
the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence
sources now known to be sources of quiescent near-infrared hydrogen emission,
four possess transitional disks, which suggests that detectable levels of H
emission and the presence of inner disk holes are correlated. These H
detections demonstrate that these inner holes are not completely devoid of gas,
in agreement with the presence of observable accretion signatures for all four
of these stars and the recent detections of [Ne II] emission from three of
them. The overlap in [Ne II] and H detections hints at a possible
correlation between these two features and suggests a shared excitation
mechanism of high energy photons. Our models, combined with the kinematic
information from the H lines, locate the bulk of the emitting gas at a few
tens of AU from the stars. We also find a correlation between H detections
and those targets which possess the largest H equivalent widths,
suggesting a link between accretion activity and quiescent H emission. We
conclude that quiescent H emission from relatively hot gas within the disks
of TTS is most likely related to on-going accretion activity, the production of
UV photons and/or X-rays, and the evolutionary status of the dust grain
populations in the inner disks.Comment: 12 pages, emulateapj, Accepted by Ap
Evidence for Accretion: High-Resolution X-ray Spectroscopy of the Classical T Tauri Star TW Hydrae
We present high resolution X-ray spectra of the X-ray bright classical T
Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 AA. The
differential emission measure derived from fluxes of temperature-sensitive
emission lines shows a plasma with a sharply peaked temperature distribution,
peaking at log T = 6.5. Abundance anomalies are apparent, with iron very
deficient relative to oxygen, while neon is enhanced relative to oxygen.
Density-sensitive line ratios of Ne IX and O VII indicate densities near log
n_e = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ksec
observation; however, based on analysis of the emission-line spectrum during
quiescent and flaring states, the derived plasma parameters do not appear
strongly time-dependent. The inferred plasma temperature distribution and
densities are consistent with a model in which the bulk of the X-ray emission
from TW Hya is generated via mass accretion from its circumstellar disk.
Assuming accretion powers the X-ray emission, our results for log n_e suggest
an accretion rate of ~10^{-8} M_sun yr^{-1}.Comment: 14 pages, 6 figures; to appear in the Astrophysical Journal, March 1,
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ROSAT X-ray Spectral Properties of Nearby Young Associations: TW Hydrae, Tucana-Horologium, and the beta Pic Moving Group
We present archival ROSAT data for three recently identified, nearby (D<70
pc), young (~10-40 Myr) stellar associations: the TW Hydrae Association, the
Tucana-Horologium Association, and the beta Pic Moving Group. The distributions
of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose
membership is dominated by low-mass, weak-lined T Tauri stars, are tightly
clustered and very similar to one another. The value of HR1 for TW Hya itself
-- the only bona fide classical T Tauri star in any of the nearby groups -- is
clearly anomalous among these nearby young stars. We compare the hardness ratio
distributions of stars in the three nearby groups with those of T Tauri stars,
the Hyades, and main sequence dwarfs in the field. This comparison demonstrates
that the X-ray spectra of F through M stars soften with age, and that F and G
stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It
is as yet unclear whether this trend can be attributed to age-dependent changes
in the intrinsic X-ray spectra of stars of type F and later, to a decrease in
the column density of circumstellar gas (e.g., in residual protoplanetary
disks), or to the diminishing contributions of star-disk interactions to X-ray
emission. Regardless, these results demonstrate that analysis of archival ROSAT
X-ray spectral data can help both to identify nearby, young associations and to
ascertain the X-ray emission properties of members of known associations.Comment: 17 pages, 4 figures; accepted by the Astrophysical Journa
No sex scandals please, we're French: French attitudes towards politicians' public and private conduct
The notion of distinct ‘public’ and ‘private’ spheres underpins much normative and practical engagement with political misconduct. What is less clear is whether citizens draw distinctions between misdemeanours in the ‘public’ and ‘private’ spheres, and whether they judge these in systematically different ways. This paper explores attitudes to political misconduct in France. French citizens are often said to be particularly relaxed about politicians’ private affairs, but there has been little empirical evidence for this proposition. Drawing on original survey data, this paper demonstrates clearly that French citizens draw a sharp distinction between politicians’ public and private transgressions, and are more tolerant of the latter
Structural and Functional MRI Differences in Master Sommeliers: A Pilot Study on Expertise in the Brain
Our experiences, even as adults, shape our brains. Regional differences have been found in experts, with the regions associated with their particular skill-set. Functional differences have also been noted in brain activation patterns in some experts. This study uses multimodal techniques to assess structural and functional patterns that differ between experts and nonexperts. Sommeliers are experts in wine and thus in olfaction. We assessed differences in Master Sommeliers’ brains, compared with controls, in structure and also in functional response to olfactory and visual judgment tasks. MRI data were analyzed using voxel-based morphometry as well as automated parcellation to assess structural properties, and group differences between tasks were calculated. Results indicate enhanced volume in the right insula and entorhinal cortex, with the cortical thickness of the entorhinal correlating with experience. There were regional activation differences in a large area involving the right olfactory and memory regions, with heightened activation specifically for sommeliers during an olfactory task. Our results indicate that sommeliers’ brains show specialization in the expected regions of the olfactory and memory networks, and also in regions important in integration of internal sensory stimuli and external cues. Overall, these differences suggest that specialized expertise and training might result in enhancements in the brain well into adulthood. This is particularly important given the regions involved, which are the first to be impacted by many neurodegenerative diseases
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