The mid-infrared emission from a photodissociation region (PDR) viewed edge-on in the Orion Nebula is examined through 8.7-20.6 km images and 8-13 km spectra. The polycyclic aromatic hydrocarbon ...(PAH) emission is located between the edges of H II regions and layers of C I emission, agreeing with PDR theory. Using a simple model, the spatial variations in the emission from PAHs detected at 8.6, 11.2, and 12.7 km are demonstrated to be directly proportional to the material column density and the intensity of the UV field. For a homogeneous, neutral cloud illuminated by a bright OB star, PDR theory predicts that the ultraviolet (UV) radiation is attenuated exponentially (e super(-1.8 Aw)). The predicted UV attenuation is confirmed by observations of broad PAH emission features found at 8.6, 11.2, and 12.7 km. The PAH emission is found in cool regions having greater optical depths relative to regions where mid-infrared emission from ionized gas is observed. Through modeling we determine a gas density of 9.7 x 10 super(4) cm super(-3). On large and small size scales, the relative strengths of the 8.6, 11.2, and 12.7 km PAH features at the bar of the Orion Nebula indicate that there is not a simple transition from ionized to neutral PAHs across the PDR.
IC 4406: A Radio-Infrared View Cerrigone, Luciano; Hora, Joseph L; Umana, Grazia ...
The Astrophysical journal,
08/2008, Letnik:
682, Številka:
2
Journal Article
Recenzirano
IC 4406 is a large (about image) southern bipolar planetary nebula composed of two elongated lobes extending from a bright central region where there is evidence for the presence of a large torus of ...gas and dust. We show new observations of this source performed with IRAC (Spitzer Space Telescope) and the Australia Telescope Compact Array. The radio maps show that the flux from the ionized gas is concentrated in the bright central region and originates in a clumpy structure previously observed in H alpha , while in the infrared images filaments and clumps can be seen in the extended nebular envelope, the central region showing toroidal emission. Modeling of the infrared emission leads to the conclusion that several dust components are present in the nebula.
Though pairs of dynamically associated asteroids in the Main Belt have been identified and studied for over a decade, very few pair systems have been identified in the near-Earth asteroid population. ...We present data and analysis that supports the existence of two genetically related pairs in near-Earth space. The members of the individual systems, 2015 EE7 – 2015 FP124 and 2017 SN16 – 2018 RY7, are found to be of the same spectral taxonomic class, and both pairs are interpreted to have volatile-poor compositions. In conjunction with dynamical arguments, this suggests that these two systems formed via YORP spin-up and/or dissociation of a binary precursor. Backwards orbital integrations suggest a separation age of <10 kyr for the pair 2017 SN16 – 2018 RY7, making these objects amongst the youngest multiple asteroid systems known to date. A unique separation age was not realized for 2015 EE7 – 2015 FP124 due to large uncertainties associated with these objects' orbits. Determining the ages of such young pairs is of great value for testing models of space weathering and asteroid spin-state evolution. As the NEO catalog continues to grow with current and future discovery surveys, it is expected that more NEO pairs will be found, thus providing an ideal laboratory for studying time dependent evolutionary processes that are relevant to asteroids throughout the Solar System.
•Two new asteroid pairs have been identified in the near-Earth object population.•Spectral data show that members of these systems have the same spectral type.•These two pairs likely formed via YORP spin-up and/or binary dissociation.•Orbit integrations of pair 2017 SN16–2018 RY7 suggest a separation age < 10 kyr.•Continued growth of the NEO catalog will lead to identification of more NEO pairs.
Extrasolar planets must be imaged directly if their nature is to be better understood. But this will be difficult, as the bright light from the parent star (or rather its diffracted halo in the ...imaging apparatus) can easily overwhelm nearby faint sources. Bracewell has proposed a way of selectively removing starlight before detection, by superposing the light from two telescopes so that the stellar wavefronts interfere destructively. Such a 'nulling' interferometer could be used in space to search for extrasolar Earth-like planets through their thermal emission and to determine through spectroscopic analysis if they possess the atmospheric signatures of life. Here we report mid-infrared observations using two co-mounted telescopes of the Multiple Mirror Telescope that demonstrate the viability of this technique. Images of unresolved stars are seen to disappear almost completely, while light from a nearby source as close as 0.2 arcsec remains, as shown by images of Betelgeuse. With this star cancelled, there remains the thermal image of its surrounding, small dust nebula. In the future, larger ground-based interferometers that correct for atmospheric distortions (using adaptive optics) should achieve better cancellation, allowing direct detection of warm, Jupiter-size planets and faint zodiacal dust around other nearby stars.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Since its discovery, the feature at 21 μm has been detected in all C-rich proto-PNe of intermediate spectral type (A–G) and – weakly – in a few PNe and AGB stars, but the nature of its carriers ...remains unknown. In this paper, we show the detection of this feature in the spectra of three new stars obtained with the Spitzer Space Telescope. In the attempt to relate the unidentified feature to other dust features, we retrieved mid-IR spectra of all the 21 μm sources currently known from ISO and Spitzer archives and noticed a correlation between the flux emitted in the 21 μm feature and that emitted at 7 and 11 μm (PAH bands and HAC broad emission). Such a correlation may point to a common nature of the carriers.
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