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Written by Sean M. Andrews and Jonathan P. Williams, Institute for Astronomy, University of Hawaii
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Jun 12, 2005 at 04:49 PM |
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We present a sensitive, multiwavelength submillimeter continuum survey of 153 young stellar objects in the Taurus-Auriga star formation region. The submillimeter detection rate is 61% to a completeness limit of <10mJy ({tex}3-\large\sigma{/tex}) at 850 µm. The inferred circumstellar disk masses are log-normally distributed with a mean mass of {tex}~5\times10^{-3}M\odot{/tex} and a large dispersion (0.5 dex). Roughly one third of the submillimeter sources have disk masses larger than the minimal nebula from which the solar system formed. The median disk to star mass ratio is 0.5%. The empirical behavior of the submillimeter continuum is best described as {tex}F_v\propto v^{2.0\pm 0.5}{/tex} between 350 µm and 1.3 mm, which we argue is due to the combined effects of the fraction of optically thick emission and a flatter frequency behavior of the opacity compared to the interstellar medium. This latter effect could be due to a substantial population of large dust grains, which presumably would have grown through collisional agglomeration. In this sample, the only stellar property that is correlated with the outer disk is the presence of a companion. We find evidence for significant decreases in submillimeter flux densities, disk masses, and submillimeter continuum slopes along the canonical infrared spectral energy distribution evolution sequence for young stellar objects. The fraction of objects detected in the submillimeter is essentially identical to the fraction with excess nearinfrared emission, suggesting that dust in the inner and outer disk are removed nearly simultaneously. |
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Written by P. Montañés-Rodriguez, E. Pallé and P.R. Goode, J. Hickey, S.E. Koonin
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May 07, 2005 at 08:57 PM |
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We report spectroscopic observations of the earthshine reflected from the Moon. By applying our well-developed photometry methodology to spectroscopy, we were able to precisely determine the Earth’s reflectance, and its variation as a function of wavelength through a single night as the Earth rotates. These data imply that planned regular monitoring of earthshine spectra will yield valuable, new inputs for climate models, which would be complementary to those from the more standard broadband measurements of satellite platforms. For our single night of reported observations, we find that Earth’s albedo decreases sharply with wavelength from 500 to 600 nm, while being almost flat from 600 to 900 nm. The mean spectroscopic albedo over the visible is consistent with simultaneous broadband photometric measurements. Unlike previous reports, we find no evidence for an appreciable "red" or "vegetation edge" in the Earth’s spectral albedo, and no evidence for changes in this spectral region (700 -740 nm) over the 40º of Earth's rotation covered by our observations. Whether or not the absence of a vegetation signature in disk-integrated observations of the earth is a common feature awaits the analysis of more earthshine data and simultaneous satellite cloud maps at several seasons. If our result is confirmed, it would limit efforts to use the red-edge as a probe for Earth-like, extra-solar planets. Water vapor and molecular oxygen signals in the visible earthshine, and carbon dioxide and methane in the near-infrared, are more likely to be a powerful probe. |
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Written by Felix Ryde
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Apr 23, 2005 at 11:25 PM |
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The prompt emission of gamma-ray bursts has yet defied any simple explanation, despite the presence of a rich observational material and great theoretical efforts. Here we show that all the types of spectral evolution and spectral shapes that have been observed can indeed be described with one and the same model, namely a hybrid model of a thermal and a non-thermal component. We further show that the thermal component is the key emission process determining the spectral evolution. Even though bursts appear to have a variety of, sometimes complex, spectral evolutions, the behaviors of the two separate components are remarkably similar for all bursts, with the temperature describing a broken power-law in time. The non-thermal component is consistent with emission from a population of fast cooling electrons emitting optically-thin synchrotron emission or non-thermal Compton radiation. This indicates that these behaviors are the fundamental and characteristic ones for gamma-ray bursts. |
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