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1.
A.G.G.M. Tielens L.B.F.M. Waters F.J. Molster K. Justtanont 《Astrophysics and Space Science》1997,255(1-2):415-426
This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal
the presence of many new emission features between 10 and 45 μm. These bands are generally much narrower than the well-known
10 and 20 μm silicates features. The strength of these features relative to the underlying broad continuum varies from source
to source (≅ 5-50%). The 10 μm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are
dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting
grains consist of the Mg-rich end-members of these minerals with an Fe-content of < 10%. The underlying continuum is attributed
to amorphous silicate grains.
These observations of aluminum-rich and magnesium-rich compounds compare well with the thermodynamic condensation sequence
of minerals expected for O-rich outflows. The observations also imply that freeze out (ie., kinetics) of this condensation
sequence at different temperatures is an important characteristic of dust formation in these objects. It is suggested that
the absence of Fe-rich silicates is a natural consequence of the low temperature at which gaseous Fe reacts with Mg-rich silicates
in these outflows, resulting in amorphous grains with little characterizing spectral detail.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
2.
K. Justtanont A. G. G. M. Tielens C. J. Skinner M. R. Haas 《Astrophysics and Space Science》1995,224(1-2):305-308
We present a preliminary interpretation of high CO rotational line data obtained from KAO. The possibility of either a PDR or a shock model is considered in order to explain the observations. 相似文献
3.
M.E. Van Den Ancker P.R. Wesselius A.G.G.M. Tielens L.B.F.M. Waters 《Astrophysics and Space Science》1997,255(1-2):69-75
We present SWS grating scans of pure H2 rotational lines, as well as several infrared fine-structure lines for two embedded
Young Stellar Objects, S106 IR and Cep A East. Excitation temperatures and masses were derived from the low-lying pure rotational
levels of H2 and are 490 and 740 K and 0.04 and 0.007 M⊙ for S106 and Cep A, respectively. The observations were compared
to theoretical models for PDRs and dissociative and non-dissociative shocks. The infrared spectrum of S106 IR is dominated
by PDR emission while that of Cep A East has a large shock component. We suggest that the difference between these two objects
could reflect an evolutionary trend.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
We have developed a model for reflection nebulae around luminous infrared sources embedded in dense dust clouds. The aim of this study is to determine the sizes of the scattering grains. In our analysis, we have adopted an MRN-like power-law size distribution (Mathis, Rumpl, and Nordsieck) of graphite and silicate grains, but other current dust models would give results which were substantially the same. In the optically thin limit, the intensity of the scattered light is proportional to the dust column density, while in the optically thick limit, it reflects the grain albedo. The results show that the shape of the infrared spectrum is the result of a combination of the scattering properties of the dust, the spectrum of the illuminating source, and foreground extinction, while geometry plays a minor role. Comparison of our model results with infrared observations of the reflection nebula surrounding OMC-2/IRS 1 shows that either a grain size distribution like that found in the diffuse interstellar medium, or one consisting of larger grains, can explain the observed shape of the spectrum. However, the absolute intensity level of the scattered light, as well as the observed polarization, requires large grains (approximately 5000 angstroms). By adding water ice mantles to the silicate and graphite cores, we have modeled the 3.08 micrometers ice band feature, which has been observed in the spectra of several infrared reflection nebulae. We show that this ice band arises naturally in optically thick reflection nebulae containing ice-coated grains. We show that the shape of the ice band is diagnostic of the presence of large grains, as previously suggested by Knacke and McCorkle. Comparison with observations of the BN/KL reflection nebula in the OMC-1 cloud shows that large ice grains (approximately 5000 angstroms) contribute substantially to the scattered light. 相似文献
5.
6.
A model for the energy balance and chemical equilibrium of the gas in photodissociation regions at the edge of molecular clouds, which are illuminated by strong FUV fields (6 eV ≦ hv ≦ 13.6 eV), has been developed. This model is used to calculate the emergent intensities in the fine structure lines of OI (63 μm, 145 μm), CI (609 μm, 370 μm), and CII (158 μm) and in the low-lying rotational transitions of CO. The numerical results show that column densities in the range 2 × 1017 to 2 × 1018 cm2 can be expected from the C+/C/CO transition region at the edge of molecular clouds. This difference with previous chemical calculations is partly due to a higher assumed carbon abundance, partly due to the charge exchange reactions of C+ with S and SiO, and partly due to carbon self-shielding which is taken into account. A detailed model is constructed for the Orion photodissociation region, which explains the observed OI (63 μm, 145 μm), CII (158 μm), CI (609 μm), and CO emission. In this model the CI (609 μm) emission originates in the warm (50°K) molecular gas behind Θ1C Ori but near the surface of OMCI. 相似文献
7.
Allamandola LJ Bregman JD Sandford SA Tielens AG Witteborn FC Wooden DH Rank D 《The Astrophysical journal》1989,345(1):L59-L62
We have discovered a new IR emission feature at 1905 cm-1 (5.25 microns) in the spectrum of BD +30 degrees 3639. This feature joins the family of well-known IR emission features at 3040, 2940, 1750, 1610, "1310," 1160, and 890 cm-1 (3.3, 3.4, 5.7, 6.2, "7.7," 8.6, and 11.2 microns). The origin of this new feature is discussed and it is assigned to an overtone or combination band involving C-H bending modes of polycyclic aromatic hydrocarbons (PAHs). Laboratory work suggests that spectral studies of the 2000-1650 cm-1 (5.0-6.1 microns) region may be very useful in elucidating the molecular structure of interstellar PAHs. The new feature, in conjunction with other recently discovered spectral structure, suggests that the narrow IR emission features originate in PAH molecules rather than large carbon grains. Larger species are likely to be the source of the broad underlying "plateaus" seen in many of the spectra. 相似文献
8.
A.G.G.M. Tielens 《Astrophysics and Space Science》1997,251(1-2):1-13
The SWS on ISO has revealed the incredible richness of IR emission features in the spectra of post-AGB objects and planetary
nebulae. Besides the well known features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, a wealth of weaker features is present as well.
These studies also reveal detailed variations from source to source, which may be of key importance in the molecular identification
process. The emission from Polycyclic Aromatic Hydrocarbon molecules and carbon dust are contrasted with an emphasis on the
expected temperature and the feature-to-continuum ratio.
Graphite stardust, isolated from carbonaceous meteorites, consist of a core of microcrystalline aromatic moieties mantled
by crystalline graphite. This structure suggests the importance of two distinct growth stages or processes. The cores resemble
terrestrial soot and indicate formation at relatively low temperature where kinetic factors played an important role, presumably
involving PAH formation through reactions of C2H2 and its radical progeny, followed by coagulation of these PAHs as well as
further chemical growth. The mantle seems to reflect slow chemical growth at higher temperatures of relatively few PAHs accreted
onto these cores.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
9.
P. Ehrenfreund E.F. Van Dishoeck M. Burgdorf J. Cami P. Van Hoof A.G.G.M. Tielens W.A. Schutte W.F. Thi 《Astrophysics and Space Science》1997,255(1-2):83-89
We present ISO observations of bright, high-mass southern starforming regions with the Short Wavelength Spectrometer (SWS)
and Long Wavelength Spectrometer (LWS). The selected targets show a range of pre-main sequence evolutionary phases. Whereas
some objects still show interstellar ice features, the most evolved objects are coincident with ultra-compact HII regions
and show strong emission lines from atomic species and polycyclic aromatic hydrocarbons (PAHs). We discuss the ISO spectra
between 2.5 - 200 μm of selected southern star-forming regions in the context of their evolution.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
W.A. Schutte J.M. Greenberg E.F. Van Dishoeck A.G.G.M. Tielens A.C.A. Boogert D.C.B. Whittet 《Astrophysics and Space Science》1997,255(1-2):61-66
ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24
and 7.40 μm. The position and width of these features indicates that they are due to the XH bending mode (X = C, N, or O)
of molecules in the solid state. Comparison to interstellar ice analogs shows that the 7.24 μm band can be well matched by
solid formic acid (HCOOH), while the 7.40 μm band can be fitted both by the formate ion (HCOO-) and acetaldehyde (CH3HCO).
The laboratory spectra reveal additional strong features of these molecules which should make a more definite identification
straightforward.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献