2,815 research outputs found
A disk of dust and molecular gas around a high-mass protostar
The processes leading to the birth of low-mass stars such as our Sun have
been well studied, but the formation of high-mass (> 8 x Sun's mass) stars has
heretofore remained poorly understood. Recent observational studies suggest
that high-mass stars may form in essentially the same way as low-mass stars,
namely via an accretion process, instead of via merging of several low-mass (<
8 Msun) stars. However, there is as yet no conclusive evidence. Here, we report
the discovery of a flattened disk-like structure observed at submillimeter
wavelengths, centered on a massive 15 Msun protostar in the Cepheus-A region.
The disk, with a radius of about 330 astronomical units (AU) and a mass of 1 to
8 Msun, is detected in dust continuum as well as in molecular line emission.
Its perpendicular orientation to, and spatial coincidence with the central
embedded powerful bipolar radio jet, provides the best evidence yet that
massive stars form via disk accretion in direct analogy to the formation of
low-mass stars
Molecular Gas in NUclei of GAlaxies (NUGA) I.The counter-rotating LINER NGC4826
We present new high-resolution observations of the nucleus of the
counter-rotating LINER NGC4826, made in the J=1-0 and J=2-1 lines of 12CO with
the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve
0.8''(16pc) resolution in the 2-1 line, fully resolve an inner molecular gas
disk which is truncated at an outer radius of 700pc. The total molecular gas
mass is distributed in a lopsided nuclear disk of 40pc radius and two one-arm
spirals, which develop at different radii in the disk. The distribution and
kinematics of molecular gas in the inner 1kpc of NGC4826 show the prevalence of
different types of m=1 perturbations in the gas. Although dominated by
rotation, the gas kinematics are perturbed by streaming motions related to the
m=1 instabilities. The non-circular motions associated with the inner m=1
perturbations agree qualitatively with the pattern expected for a trailing wave
developed outside corotation ('fast' wave). In contrast, the streaming motions
in the outer m=1 spiral are better explained by a 'slow' wave. A paradoxical
consequence is that the inner m=1 perturbations would not favour AGN feeding.
An independent confirmation that the AGN is not being generously fueled at
present is found in the low values of the gravitational torques exerted by the
stellar potential for R<530pc. The distribution of star formation in the disk
of NGC4826 is also strongly asymmetrical. Massive star formation is still
vigorous, fed by the significant molecular gas reservoir at R<700pc. There is
supporting evidence for a recent large mass inflow episode in NGC4826.
These observations have been made in the context of the NUclei of GAlaxies
(NUGA) project, aimed at the study of the different mechanisms for gas fueling
of AGN.Comment: A&A, 2003, Paper accepted (04/06/03). For a full-resolution version
of this paper see http://www.oan.es/preprint
Cultural Constraints on Brain Development: Evidence from a Developmental Study of Visual Word Processing in Mandarin Chinese
Developmental differences in phonological and orthographic processing in Chinese were examined in 9 year olds, 11 year olds, and adults using functional magnetic resonance imaging. Rhyming and spelling judgments were made to 2-character words presented sequentially in the visual modality. The spelling task showed greater activation than the rhyming task in right superior parietal lobule and right inferior temporal gyrus, and there were developmental increases across tasks bilaterally in these regions in addition to bilateral occipital cortex, suggesting increased involvement over age on visuo-orthographic analysis. The rhyming task showed greater activation than the spelling task in left superior temporal gyrus and there were developmental decreases across tasks in this region, suggesting reduced involvement over age on phonological representations. The rhyming and spelling tasks included words with conflicting orthographic and phonological information (i.e., rhyming words spelled differently or nonrhyming words spelled similarly) or nonconflicting information. There was a developmental increase in the difference between conflicting and nonconflicting words in left inferior parietal lobule, suggesting greater engagement of systems for mapping between orthographic and phonological representations. Finally, there were developmental increases across tasks in an anterior (Broadman area [BA] 45, 46) and posterior (BA 9) left inferior frontal gyrus, suggesting greater reliance on controlled retrieval and selection of posterior lexical representations
Cognitive skills and literacy performance of Chinese adolescents with and without dyslexia
The present study sought to identify cognitive abilities that might distinguish Hong Kong Chinese adolescents with dyslexia and to assess how these abilities were associated with Chinese word reading, word dictation, and reading comprehension. The cognitive skills of interest were morphological awareness, visual-orthographic knowledge, rapid naming, and verbal working memory. A total of 90 junior secondary school students, 30 dyslexic, 30 chronological age controls, and 30 reading level controls was tested on a range of cognitive and literacy tasks. Dyslexic students were less competent than the control students in all cognitive and literacy measures. The regression analyses also showed that verbal working memory, rapid naming, morphological awareness, and visual-orthographic knowledge were significantly associated with literacy performance. Findings underscore the importance of these cognitive skills for Chinese literacy acquisition. Overall, this study highlights the persistent difficulties of Chinese dyslexic adolescents who seem to have multiple causes for reading and spelling difficulties
A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system
We present five epochs of VLBI water maser observations around the massive
protostar Cepheus A HW2 with 0.4 mas (0.3 AU) resolution. The main goal of
these observations was to follow the evolution of the remarkable water maser
linear/arcuate structures found in earlier VLBI observations. Comparing the
data of our new epochs of observation with those observed five years before, we
find that at "large" scales of > 1" (700 AU) the main regions of maser emission
persist, implying that both the surrounding medium and the exciting sources of
the masers have been relatively stable during that time span. However, at
smaller scales of < 0.1" (70 AU) we see large changes in the maser structures,
particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly
elliptical patchy ring of ~ 70 mas size (50 AU) with expanding motions of ~ 5
mas/yr (15 km/s). This structure is probably driven by the wind of a still
unidentified YSO located at the centre of the ring (~ 0.18" south of HW2). On
the other hand, the R5 expanding bubble structure (driven by the wind of a
previously identified YSO located ~ 0.6" south of HW2) is currently dissipating
in the circumstellar medium and losing its previous degree of symmetry,
indicating a very short-lived event. In addition, our results reveal, at scales
of ~ 1" (700 AU), the simultaneous presence of a relatively slow (~ 10-70 km/s)
wide-angle outflow (opening angle of ~ 102 deg, traced by the masers, and the
fast (~ 500~km/s) highly collimated radio jet associated with HW2 (opening
angle of ~ 18 deg, previously observed with the VLA. This simultaneous presence
of a wide-angle outflow and a highly collimated jet associated with a massive
protostar is similar to what is found in some low-mass YSOs. The implications
of these results in the study of the formation of high-mass stars are
discussed.Comment: 28 pages, 7 figures. Animations will be included as supporting
material online (MNRAS web page
Uncoupling Protein-4 (UCP4) Increases ATP Supply by Interacting with Mitochondrial Complex II in Neuroblastoma Cells
Mitochondrial uncoupling protein-4 (UCP4) protects against Complex I deficiency as induced by 1-methyl-4-phenylpyridinium (MPP+), but how UCP4 affects mitochondrial function is unclear. Here we investigated how UCP4 affects mitochondrial bioenergetics in SH-SY5Y cells. Cells stably overexpressing UCP4 exhibited higher oxygen consumption (10.1%, p<0.01), with 20% greater proton leak than vector controls (p<0.01). Increased ATP supply was observed in UCP4-overexpressing cells compared to controls (p<0.05). Although state 4 and state 3 respiration rates of UCP4-overexpressing and control cells were similar, Complex II activity in UCP4-overexpressing cells was 30% higher (p<0.05), associated with protein binding between UCP4 and Complex II, but not that of either Complex I or IV. Mitochondrial ADP consumption by succinate-induced respiration was 26% higher in UCP4-overexpressing cells, with 20% higher ADP:O ratio (p<0.05). ADP/ATP exchange rate was not altered by UCP4 overexpression, as shown by unchanged mitochondrial ADP uptake activity. UCP4 overexpression retained normal mitochondrial morphology in situ, with similar mitochondrial membrane potential compared to controls. Our findings elucidate how UCP4 overexpression increases ATP synthesis by specifically interacting with Complex II. This highlights a unique role of UCP4 as a potential regulatory target to modulate mitochondrial Complex II and ATP output in preserving existing neurons against energy crisis
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