40 research outputs found
The Enigmatic Young Object: Walker 90/V590 Monocerotis
We assess the evolutionary status of the intriguing object Walker 90/V590
Mon, which is located about 20 arcminutes northwest of the Cone Nebula near the
center of the open cluster NGC 2264. This object, according to its most recent
optical spectral type determination (B7), which we confirmed, is at least 3
magnitudes too faint in V for the cluster distance, but it shows the classical
signs of a young pre-main sequence object, such as highly variable Halpha
emission, Mg II emission, IR excess, UV continuum, and optical variability. We
analyzed a collection of archival and original data on Walker 90, covering 45
years including photometry, imaging, and spectroscopic data ranging from
ultraviolet to near-infrared wavelengths. According to star formation
processes, it is expected that, as this object clears its primordial
surroundings, it should become optically brighter, show a weakening of its IR
excess and present decreasing line emissions. This behavior is supported by our
observations and analysis, but timescales are expected to be longer than the
one observed here. Based on photometric data secured in 2007, we find Walker 90
at its brightest recorded optical magnitude. We document an evolution in
spectral type over the past five decades (from A2/A3 to currently B7 and as
early as B4), along with a decrease in the near-infrared K fluxes. From
near-infrared images secured in 2004, Walker 90 appears as a point source
placing an upper limit of 0.1 arcsec for its diameter. We conclude that many
observational features are explained if W90 is a flared disk system, surrounded
by an inclined optically thick accretion disk.Comment: 13 pages, 19 figure
Probing the anomalous extinction of four young star clusters: the use of colour-excess, main sequence fitting and fractal analysis
Four young star clusters were studied in order to characterize their
anomalous extinction or variable reddening that could be due to a possible
contamination by dense clouds or circumstellar effects. The extinction law (Rv)
was evaluated by adopting two methods: (i) the use of theoretical expressions
based on the colour-excess of stars with known spectral type, and (ii) the
analysis of two-colour diagrams, where the slope of observed colours
distribution is compared to the normal distribution. An algorithm to reproduce
the zero age main sequence (ZAMS) reddened colours was developed in order to
derive the average visual extinction (Av) that provides the best fitting of the
observational data. The structure of the clouds was evaluated by means of
statistical fractal analysis, aiming to compare their geometric structure with
the spatial distribution of the cluster members. The cluster NGC 6530 is the
only object of our sample showing anomalous extinction. In average, the other
clusters are suffering normal extinction, but several of their members, mainly
in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The
ZAMS fitting provides Av values that are in good agreement with those found in
the literature. The fractal analysis shows that NGC 6530 has a centrally
concentrated distribution of stars that is different of the sub-structures
found in the density distribution of the cloud projected in the Av map,
suggesting that the original cloud has been changed with the cluster formation.
On the other hand, the fractal dimension and the statistical parameters of
Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation,
when compared to other works based on artificial distribution of points.Comment: 13 pages, 7 figure
8--13 um spectroscopy of YSOs: Evolution of the silicate feature
In order to investigate possible connections between dust processing and disk
properties, 8--13 um spectra of 34 young stars, with a range of circumstellar
environments and spectral types A to M, were obtained using the Long Wavelength
Spectrometer at the W. M. Keck Observatory. The broad 9.7 um amorphous silicate
feature which dominates this wavelength regime evolves from absorption in
young, embedded sources, to emission in optically revealed stars, and to
complete absence in older debris disk systems for both low- and
intermediate-mass stars. The peak wavelength and FWHM are centered about 9.7
and ~2.3 um, corresponding to amorphous olivine, with a larger spread in FWHM
for embedded sources and in peak wavelength for disks. In a few of our objects
that have been previously identified as class I low-mass YSOs, the observed
silicate feature is complex, with absorption near 9.5 um and emission peaking
around 10 um. Although most of the emission spectra show broad classical
features attributed to amorphous silicates, variations in the shape/strength
may be linked to dust processing, including grain growth and/or silicate
crystallization. We study quantitatively the evidence for evolutionary trends
in the 8--13 um spectra through a variety of spectral shape diagnostics. Based
on the lack of correlation between these diagnostics and broad-band infrared
luminosity characteristics for silicate emission sources, we conclude that
although spectral signatures of dust processing are present, they can not be
connected clearly to disk evolutionary stage (for optically thick disks) or
optical depth (for optically thin disks). The diagnostics of silicate
absorption features (other than the central wavelength of the feature),
however, are tightly correlated with optical depth.Comment: 27 pages, 13 figures, accepted for publication by ApJ, formatted with
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Restricted Diffusion in Biophysical Systems: Experiment
The pulsed-gradient spin echo nuclear magnetic resonance (PGSENMR) technique was used to measure restricted diffusion of water in three types of animal tissue: human blood plasma and red cells; rat and rabbit heart; rat and rabbit liver. Characteristic lengths (L) for restriction of diffusion are estimated from dependence on the measuring time. Limitations on the range of observable restrictive lengths (1.5-15 ÎŒm) are discussed. The decrease in diffusivity due to 1 ÎŒm alumina powder (volume fraction = 0.18) in glycerin/water mixtures agrees with the Wang theory assuming spherical particles and no hydration. The characteristic length (L â 4 ÎŒm) is larger than the particle size (1 ÎŒm) or separation (1.8 ÎŒm). Comparison of the diffusivities in tissues at short diffusion times with the Wang theory indicates some bound or trapped water. For packed red blood cells, a restriction (L â 2.3 ÎŒm) was attributed tothe red cell membrane. A permeability p â 0.014 cm/s may be estimated from the decrease in diffusivity. Average values of diffusivity ratio in heart were: 0.36 ± 0.02 for rat; and 0.26 ± 0.03 for rabbit; and in liver: 0.25 ± 0.01 for rat; 0.25 ± .04 for 10-day old rabbit; and 0.195 ± 0.03 for 2-yr old rabbit. A restriction (L â 2.7 ÎŒm) in rat liver probably results from the mitochondria