174 research outputs found
Radiative Transfer in a Translucent Cloud Illuminated by an Extended Background Source
No full textWe discuss the radiative transfer theory for translucent clouds illuminated by an extended background source. First, we derive a rigorous solution based on the assumption that multiple scatterings produce an isotropic flux. Then we derive a more manageable analytic approximation showing that it nicely matches the results of the rigorous approach. To validate our model, we compare our predictions with accurate laboratory measurements for various types of well-characterized grains, including purely dielectric and strongly absorbing materials representative of astronomical icy and metallic grains, respectively, finding excellent agreement without the need to add free parameters. We use our model to explore the behavior of an astrophysical cloud illuminated by a diffuse source with dust grains having parameters typical of the classic ISM grains of Draine & Lee and protoplanetary disks, with an application to the dark silhouette disk 114-426 in Orion Nebula. We find that the scattering term modifies the transmitted radiation, both in terms of intensity (extinction) and shape (reddening) of the spectral distribution. In particular, for small optical thickness, our results show that scattering makes reddening almost negligible at visible wavelengths. Once the optical thickness increases enough and the probability of scattering events becomes close to or larger than 1, reddening becomes present but is appreciably modified with respect to the standard expression for line-of-sight absorption. Moreover, variations of the grain refractive index, in particular the amount of absorption, also play an important role in changing the shape of the spectral transmission curve, with dielectric grains showing the minimum amount of reddening
Wide-field imaging at mid-infrared wavelengths: reconstruction of chopped and nodded data
No full textEvidence of photoevaporation and spatial variation of grain sizes in the Orion 114-426 protoplanetary disk
No full textDeep Hubble Space Telescope broadband images taken with Advanced Camera for Surveys (ACS) and WFPC2 of the giant (∼1000 AU diameter) dark silhouette proplyd 114-426 in the Orion Nebula show that this system is tilted, asymmetric, warped, and photoevaporated. The exquisite angular resolution of ACS allows us to map the
distribution of dust grains at the northern translucent edge of the disk, dominated by the photoevaporative flow.
Using the Mie theory for standard circumstellar disk grains, we find evidence for a spatial gradient in grain size. The typical dust radius 0.2–0.7μm (less than what was reported by previous studies) becomes smaller as the distance from the disk center increases, consistent with the expectations for the dynamic of dust entrained in a gaseous photoevaporative wind. Our analysis of the disk morphology and location within the nebula indicates that this system is photoevaporated by the diffuse radiation field of the Orion Nebula, while being shielded from the radiation coming directly from the central Trapezium stars. We estimate the mass-loss rate from the disk surface and the timescale for total disk dissipation, which turns out to be of the order of 104 yr. Such a short time, of the order of 1/100 of the cluster age, indicates that this system is seen on the verge of destruction. This is compatible
with the exceptional nature of the disk, namely its combination of huge size and lowmass. Finally, we briefly discuss the viability of possible mechanisms that may lead to the peculiar morphology of this system: external UV flux,binary star, and past close encounter
Unveiling the disk-jet system in the massive (proto)star IRAS 20126+4104
No full textWe present the results of line and continuum observations towards the source IRAS 20126+4104, performed at 1.3 mm and 3.5 mm with the Plateau de Bure interferometer, from 350 mu m to 2 mm with the James Clerk Maxwell telescope, and at 10 and 20 mu m with the United Kingdom infrared telescope. The results fully confirm the findings of Cesaroni et al. (\cite{cftwo}), namely that IRAS 20126+4104 is a very young stellar object embedded in a dense, hot core and lying at the centre of a rotating disk. The bipolar jet imaged by Cesaroni et al. (\cite{cftwo}) in the 2.122 mu m H_2 line is seen also in the SiO(2-1) transition, which allows to study the velocity field in the jet. A simple model is developed to obtain the inclination angle of the jet (and hence of the disk axis), which turns out to be almost perpendicular to the line of sight. By studying the diameter of the disk in different transitions and the corresponding line widths and peak velocities, one can demonstrate that the disk is Keplerian and collapsing, and thus compute the mass of the central object and the accretion luminosity. We show that if all the mass inducing the Keplerian rotation is concentrated in a single star, then this cannot be a ZAMS star, but more likely a massive protostar which derives its luminosity from accretion
IRAS and near infrared observations of peculiar nebulosities
No full textThe authors used the IRAS survey to search for IR emission from 95
peculiar nebulosities with CO emission. Fifty-two IRAS sources are
associated with them. The authors discuss the nature of the sources on
the basis of their IRAS color-color diagram and of near-IR observations
Applications of DMDs for astrophysical research
No full textA long-standing problem of astrophysical research is how to simultaneously obtain spectra of thousands of sources randomly positioned in the field of view of a telescope. Digital Micromirror Devices, used as optical switches, provide a most powerful solution allowing to design a new generation of instruments with unprecedented capabilities. We illustrate the key factors (opto-mechanical, cryo-thermal, cosmic radiation environment,...) that constrain the design of DMD-based multi-object spectrographs, with particular emphasis on the IR spectroscopic channel onboard the EUCLID mission, currently considered by the European Space Agency for a 2017 launch date
The clustering of merging star-forming haloes: dust emission as high frequency arcminute CMB foreground
No full textNot availabl
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