اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe Dawn of Dust Astronomy
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We review the development of dust science from the first ground-based astronomical observations of dust in space to compositional analysis of individual dust particles and their source objects. A multitude of observational techniques is available for the scientific study of space dust: from meteors and interplanetary dust particles collected in the upper atmosphere to dust analyzed in situ or returned to Earth. In situ dust detectors have been developed from simple dust impact detectors determining the dust hazard in Earth orbit to dust telescopes capable of providing compositional analysis and accurate trajectory determination of individual dust particles in space. The concept of Dust Astronomy has been developed, recognizing that dust particles, like photons, carry information from remote sites in space and time. From knowledge of the dust particles birthplace and their bulk properties, we learn about the remote environment out of which the particles were formed. Dust Observatory missions like Cassini, Stardust, and Rosetta study Saturns satellites and rings and the dust environments of comet Wild 2 and comet Churyumov-Gerasimenko, respectively. Supplemented by simulations of dusty processes in the laboratory we are beginning to understand the dusty environments in space.
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Dawn is the first NASA mission to operate in the vicinity of the two most massive asteroids in the main belt, Ceres and Vesta. This double-rendezvous mission is enabled by the use of low-thrust solar electric propulsion. Dawn will arrive at Vesta in
2011 and will operate in its vicinity for approximately one year. Vestas mass and non-spherical shape, coupled with its rotational period, presents very interesting challenges to a spacecraft that depends principally upon low-thrust propulsion for trajectory-changing maneuvers. The details of Vestas high-order gravitational terms will not be determined until after Dawns arrival at Vesta, but it is clear that their effect on Dawn operations creates the most complex operational environment for a NASA mission to date. Gravitational perturbations give rise to oscillations in Dawns orbital radius, and it is found that trapping of the spacecraft is possible near the 1:1 resonance between Dawns orbital period and Vestas rotational period, located approximately between 520 and 580 km orbital radius.This resonant trapping can be escaped by thrusting at the appropriate orbital phase. Having passed through the 1:1 resonance, gravitational perturbations ultimately limit the minimum radius for low-altitude operations to about 400 km,in order to safely prevent surface impact. The lowest practical orbit is desirable in order to maximize signal-to-noise and spatial resolution of the Gamma-Ray and Neutron Detector and to provide the highest spatial resolution observations by Dawns Framing Camera and Visible InfraRed mapping spectrometer. Dawn dynamical behavior is modeled in the context of a wide range of Vesta gravity models. Many of these models are distinguishable during Dawns High Altitude Mapping Orbit and the remainder are resolved during Dawns Low Altitude Mapping Orbit, providing insight into Vestas interior structure.
The first LIGO-Virgo detections have confirmed the existence of massive black holes (BHs), with mass $30-40$ M$_odot$. Such BHs might originate from massive metal-poor stars ($Z<0.3$ Z$_odot$) or from gravitational instabilities in the early Universe
. The formation channels of merging BHs are still poorly constrained. The measure of mass, spin and redshift distribution of merging BHs will give us fundamental clues to distinguish between different models. Also, a better understanding of several astrophysical processes (e.g. common envelope, core-collapse supernovae, and dynamical evolution of BHs) is decisive, to shed light on the formation channels of merging BHs.
The surface reflectance of planetary regoliths may increase dramatically towards zero phase angle, a phenomenon known as the opposition effect (OE). Two physical processes that are thought to be the dominant contributors to the brightness surge are s
hadow hiding (SH) and coherent backscatter (CB). The occurrence of shadow hiding in planetary regoliths is self-evident, but it has proved difficult to unambiguously demonstrate CB from remote sensing observations. One prediction of CB theory is the wavelength dependence of the OE angular width. The Dawn spacecraft observed the OE on the surface of dwarf planet Ceres. We characterize the OE over the resolved surface, including the bright Cerealia Facula, and to find evidence for SH and/or CB. We analyze images of the Dawn framing camera by means of photometric modeling of the phase curve. We find that the OE of most of the investigated surface has very similar characteristics, with an enhancement factor of 1.4 and a FWHM of 3{deg} (broad OE). A notable exception are the fresh ejecta of the Azacca crater, which display a very narrow brightness enhancement that is restricted to phase angles $< 0.5${deg} (narrow OE); suggestively, this is in the range in which CB is thought to dominate. We do not find a wavelength dependence for the width of the broad OE, and lack the data to investigate the dependence for the narrow OE. The prediction of a wavelength-dependent CB width is rather ambiguous. The zero-phase observations allow us to determine Ceres visible geometric albedo as $p_V = 0.094 pm 0.005$. A comparison with other asteroids suggests that Ceres broad OE is typical for an asteroid of its spectral type, with characteristics that are primarily linked to surface albedo. Our analysis suggests that CB may occur on the dark surface of Ceres in a highly localized fashion.
We present first results of a study aimed to constrain the star formation rate and dust content of galaxies at z~2. We use a sample of BzK-selected star-forming galaxies, drawn from the COSMOS survey, to perform a stacking analysis of their 1.4 GHz r
adio continuum as a function of different stellar population properties, after removing AGN contaminants from the sample. Dust unbiased star formation rates are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: i) specific star formation rates are constant over about 1 dex in stellar mass and up to the highest stellar mass probed; ii) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects; iii) a single value of the UV extinction applied to all galaxies would lead to grossly underestimate the SFR in massive galaxies; iv) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provide results in very good agreement with the radio derived ones; v) the mean specific star formation rate of our sample steadily decreases by a factor of ~4 with decreasing redshift from z=2.3 to 1.4 and a factor of ~40 down the local Universe. These empirical SFRs would cause galaxies to dramatically overgrow in mass if maintained all the way to low redshifts, we suggest that this does not happen because star formation is progressively quenched, likely starting from the most massive galaxies.
This work describes the correction method applied to the dataset acquired at the asteroid (4) Vesta by the visible channel of the visible and infrared mapping spectrometer. The rising detector temperature during data acquisitions in the visible wavel
engths leads to a spectral slope increase over the whole spectral range. This limits the accuracy of the studies of the Vesta surface in this wavelength range. Here, we detail an empirical method to correct for the visible detector temperature dependency while taking into account the specificity of the Vesta dataset.
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