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Search Results: 1 - 10 of 200742 matches for " P. Persi "
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M. Tapia,P. Persi
Revista mexicana de astronomía y astrofísica , 2009,
Abstract: The bright nebular complex NGC 6334 contains some of the most active sites of massive star formation known in our Galaxy. It is located at a distance from the Sun of 1.62 kpc and has a total mass of a few 105M . The physical characteristics of the active spots range widely, from well developed expanding HII regions to deeply embedded, still contracting, young objects detected only as millimeter sources, thus at their earliest observable stage of their evolution. The oldest optically visible round HII regions with central O-type stars are found in the southern parts, and the youngest along a molecular ridge. On the latter, no clear spatial evolutionary correlation is apparent.
Young and very young stars in NGC 3372, the Carina Nebula
M. Tapia,M. Roth,P. Persi
Revista mexicana de astronomía y astrofísica , 2004,
M. Tapia,M. Roth,P. Persi
Revista mexicana de astronomía y astrofísica , 2011,
Abstract: Se presentan nuevas imágenes en el cercano infrarrojo obtenidas con PANIC montado en el telecopio Magallanes "Baade" del Observatorio Las Campanas. Estas fueron tomadas a través de filtros anchos JHKs y de filtros angostos centrados en las líneas Br en 2.17 m y H2 en 2.12 m. El estudio se complementó con imágenes del archivo del Satélite Spitzer con IRAC. Se presenta evidencia de la existencia de un cúmulo joven de radio r = 23'' embebido en Tr14-N4. Este contiene varias estrellas de baja y mediana masa y al menos una protoestrella de alta masa. Se presentan sus propiedades derivadas de la fotometría desde 1.2 hasta 11 m. Se encuentran varios nudos de emisión compacta en la línea de H2, lo que evidencia la existencia de al menos un flujo de masa, presumiblemente proveniente del núcleo del cúmulo. Se presentan también resultados recientes del monitoreo de dos jóvenes estrellas variables Car I-136 y 125, embebidas en la nube densa asociada con Car I.
Near and Mid-Infrared Images of the Massive Star-Forming Complex G 9.62 + 0.19
M. Tapia,P. Persi,M. Roth,A. R. Marenzi
Revista mexicana de astronomía y astrofísica , 2003,
Young Stellar Objects in GY 2-18 (IRAS 05439+3035)
P. Persi,V. F. Polcaro,M. Tapia,J. Bohigas
Revista mexicana de astronomía y astrofísica , 2003,
Two Potential Mechanisms of Spatial Attention in Early Visual Areas
Erez Persi
Quantitative Biology , 2008,
Abstract: We investigate theoretically the effect of spatial attention on the contrast-response function (CRF) and orientation-tuning curves in early visual areas.We look at a model of a hypercolumn developed recently (Persi et al., 2008), that accounts for both the contrast response and tuning properties in the primary visual cortex, and extend it to two visual areas. The effect of spatial attention is studied in a model of two inter-connected visual areas, under two hypothesis that do not necessarily contradict. The first hypothesis is that attention alters inter-areal feedback synaptic strength, as has been proposed by many previous studies. A second new hypothesis is that attention effectively alters single neuron input-output properties. We show that with both mechanisms it is possible to achieve attentional effects similarly to those observed in experiments, namely contrast-gain and response-gain effects, while keeping the orientation-tuning curves width approximately contrast-invariant and attention-invariant. Nevertheless, some differences occur and are discussed. We propose a simple test on existing data based on the second hypothesis.
Some things we've learned (about Markov chain Monte Carlo)
Persi Diaconis
Statistics , 2013, DOI: 10.3150/12-BEJSP09
Abstract: This paper offers a personal review of some things we've learned about rates of convergence of Markov chains to their stationary distributions. The main topic is ways of speeding up diffusive behavior. It also points to open problems and how much more there is to do.
ISOCAM-CVF spectroscopy of the circumstellar environment of young stellar objects
R. D. Alexander,M. M. Casali,P. M. Andre,P. Persi,C. Eiroa
Physics , 2003, DOI: 10.1051/0004-6361:20030158
Abstract: We present the results of a mid-infrared imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, Rho Ophiuchi, Serpens and Chamaeleon I. This survey was performed using ISOCAM and its Circular Variable Filters (CVF) and observed 42 YSO candidates. A number of spectral features were measured, most notably the 9.7um silicate feature, the bending modes of both water and CO2 ices at 6.0 and 15.2um respectively and the well-known unidentified feature at 6.8um. The strength of the unidentified feature was observed to correlate very well with that of the water ice bending mode and far less strongly with the CO2 ice bending mode. This suggests, in a manner consistent with previous observations, that the carrier of the unidentified feature is a strongly polar ice. Absorption profiles of the bending mode of CO2 ice are observed to show a significant long wavelength wing, which suggests that a significant fraction of the CO2 ice observed exists in a polar (H2O-rich) phase. The sources observed in RCrA, Rho Oph and Serpens show similar spectral characteristics, whilst the sources observed in Cha I are somewhat anomalous. However this is consistent with previous studies of this region of the Cha I cloud, which contains an unusual cluster of YSOs. We detect an apparent under-abundance of water ice towards the sources in Rho Oph. This may be indicative of differences in chemical composition between the different clouds, or may be due to evaporation. Finally the CO2:H2O ice ratios observed towards the sources in Rho Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in Rho Oph. (Abridged)
Optical spectra of selected Chamaeleon I young stellar objects
C. Saffe,M. Gomez,S. Randich,D. Mardones,P. Caselli,P. Persi,G. Racca
Physics , 2003, DOI: 10.1051/0004-6361:20031150
Abstract: We present optical spectra of eight candidate brown dwarfs and a previously known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral types based on the strength of the TiO or VO absorption bands present in the spectra of these objects as well as on the PC3 index of Martin et al. (1999). Photometric data from the literature are used to estimate the bolometric luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main sequence evolutionary tracks and isochrones to derive masses and ages. Based on the presence of Halpha in emission, we confirm that most of the candidates are young objects. Our sample however includes two sources for which we can only provide upper limits for the emission in Halpha; whereas these two objects are most likely foreground/background stars, higher resolution spectra are required to confirm their true nature. Among the likely cloud members, we detect one new sub-stellar object and three transition stellar/sub-stellar sources.
Mid-infrared imaging of AGB star envelopes. II. Modelling of observed sources
M. Marengo,G. Canil,G. Silvestro,L. Origlia,M. Busso,P. Persi
Physics , 1996,
Abstract: Radiative transfer modelling of AGB circumstellar envelopes is applied to a sample of AGB stars previously observed with the mid-IR imaging camera TIRCAM (Busso et al. 1996: Paper I). We present the results of our simulations, aimed at deriving the physical parameters of the envelope, such as the optical depth and the radial thermal structure, the mass loss and the dust-to-gas mass ratio. The chemical composition of the dust in the observed envelopes is discussed. The ability of different sets of dust opacities to fit the mid-infrared spectra is evaluated. The hypothesis of dust grain aging and annealing in O-rich envelopes is considered in order to explain an apparent inadequacy of the availabe opacities to describe the variety of observed spectra, as previously noted by other authors. Various possible origins of the discrepancies are discussed, together with their consequences on the dust grain formation processes.
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