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  • ASTROPHYSICS  (8)
  • silicates  (1)
  • young stellar objects: individual (WL 16)  (1)
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  • 1
    Digitale Medien
    Digitale Medien
    Discovery Of Mg-Rich Pyroxenes In Comet C/1995 O1 (Hale-Bopp): Pristine Grains Revealed At Perihelion (1997)
    Springer
    Earth, moon and planets 78 (1997), S. 285-291 
    Details
    ISSN: 1573-0794
    Schlagwort(e): Comet ; infrared ; dust ; silicates ; olivine ; pyroxene
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract The NASA Ames HIFOGS spectrometer observed comet C/1995 O1 (Hale-Bopp) at epochs including 96 Oct 7–14 UT (2.8 AU), 97 Feb 14–15 UT (1.2 AU), 97 Apr 11 UT (0.93 AU), and 97 Jun 22, 25 UT (1.7 AU). The HIFOGS 7.5–13.5 μm spectrophotometry (R = 360 - 180) of the silicate feature at 2.8 AU is identical in shape to the ISO SWS spectra of comet Hale-Bopp (Crovisier et al., 1997); the strong 11.2 μm peak in the structured silicate feature is identified as olivine. Upon close passage to the sun, the HIFOGS spectra at 1.2 AU and 0.93 AU reveals strong peaks at 9.3 μm and 10.0 μm. The post-perihelion 10 μm silicate feature at 1.7 AU is weaker but has nearly the same shape as the pre-perihelion spectra at 1.2 AU, reverting to its pre-perihelion shape: there is no change in the dust chemistry by close passage to the sun. The appearance of the strong peaks at 9.3 μm and 10.0 μm at rh ≲ 1.7 AU is attributed to the rise in the contribution of pryoxenes (clino-pyroxene and orthopyroxene crystals) to the shape of the feature, and leads to the hypothesis that the pyroxenes are significantly cooler than the olivines. The pyroxenes are radiating on the Wien side of the blackbody at 2.8 AU and transition to the Rayleigh-Jeans tail of the blackbody upon closer approach to the Sun. Composite fits to the observed 10 μm silicate features using IDPs and laboratory minerals shows that a good empirical fit to the spectra is obtained when the pryoxenes are about 150 K cooler than the olivines. The pyroxenes, because they are cooler and contribute signficantly at perihelion, are more abundant than the olivines. The perihelion temperature of the pyroxenes implies that the pyroxenes are more Mg-rich than the other minerals including the olivines, amorphous olivines, and amorphous pyroxenes. The PUMA-1 flyby measurements of comet P/Halley also indicated an overabundance of Mg-rich pryoxenes compared to olivines. Comet Hale-Bopp's pyroxenes are similar to pyroxere IDPs from the ’Spray‘ class, known for their D-richness and their unaltered morphologies: Hale-Bopp's Mg-rich pyroxenes may be pristine relic ISM grains.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1006225722620
    Standort Signatur Erwartet Verfügbarkeit
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    Artikel (Nationallizenzen)
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  • 2
    Digitale Medien
    Digitale Medien
    Extended structures around yso in Mid-IR broad emission features (1995)
    Springer
    Astrophysics and space science 224 (1995), S. 89-92 
    Details
    ISSN: 1572-946X
    Schlagwort(e): young stellar objects: individual (WL 16) ; Herbig Ae/Be stars: individual (TY CrA, MWC 1080) ; young stellar objects: General ; young stellar objects: Infrared
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We present high-spatial-resolution, mid-IR images of young stellar objects (YSO). Images were obtained with MIRAC (Mid-InfraRed Array Camera) at wavelengths of grain continuum and feature emission near 10 and 20µm. Three of the objects observed (WL 16, TY CrA and MWC 1080) show structure and extended emission (on the order of 1200 AU or greater) at wavelengths of the UIR (Unidentified InfraRed) emission features. Mid-IR imaging of other sources in these clouds which do not exhibit UIR features (Elias 29 and R CrA) shows these sources to be point-like. We have produced morphological models based on the mid-IR images for the observed extended structures.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00667828
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
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    CS multitransitional study of density distribution in star-forming regions. 2: The S140 region (1994)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: The S140 molecular cloud was observed in five transitions of CS with resolutions of 11 to 45 arcsec. The data were analyzed with both the LVG and microturbulent models of radiative transfer to derive the density structure. It was found that the CS emission comes from three components of gas: a spherical component centered on the infrared cluster, an arc component along the ionization front between the S140 H II region and the dense molecular cloud core, and a high-velocity component from the dense part of a molecular outflow. The spherical component contributes most to the CS emission and was analyzed in more detail than the other components. Using a temperature distribution derived from an analysis of the dust emission from S140, we fit a power-law density distribution of n(r) = n(sub i)(r/r(sub i))(exp -alpha) to the spherical component. The best fit was for n(sub i) = 1.4 x 10(exp 6) (density at r(sub i) = 0.026 pc) and alpha = 0.8. The density (n(sub i)) was found to be greater than or equal to the density required to account for the dust emission, depending on the dust opacity laws adopted. The presence of optical emission (Dinerstein, Lester, & Rank 1979) suggests a clumpy structure for the dense gas. Considerations of the virial mass and the lowest amount of column density required to produce dust emission put the volume filling factor (f(sub nu)) of the dense gas at approximately 0.14-0.5. We compared S140 with other regions of star formation where the density structure has been derived from excitation analysis. Source-source variations in density gradients and clumpiness clearly exist, ranging from alpha = 2 and f(sub nu) approximately 1 in B335 to alpha approximately 0, f(sub nu) approximately 0.1 in M17. There is a tendency for more massive star-forming regions to have a flatter density distribution, a more clumpy structure, and a large number of young stars. The implications of this tendency are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 428; 1; p. 219-232
    Format: text
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  • 4
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    Infrared studies of pre-main-sequence intermediate-mass stars - LkH-Alpha 198 (1992)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: The paper presents FIR scans of LkH-Alpha 198 at 50 and 100 microns along several directions; the observations reveal an extended, roughly spherical source diameter of 33 arcsec, or 20,000 AU, at 100 microns and not more than 10 arcsec, or 15,000 AU, at 50 microns. A luminosity of 250 solar luminosities at a distance of 600 pc is estimated on the basis of a comparison of the observed properties at optical, IR, and millimeter wavelengths with the predictions of radiation transfer models. The optical depth at 100 microns is 0.004-0.006, implying a mass of about 2 solar masses with a radius of 10,000 AU. It is argued that the inner radius of the dusty envelope is rather small, less than about 300 AU (0.5 arcsec), and it may be as small as the dust destruction front: in LkH-Alpha 198 there is no evidence of a large region devoid of grains.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 391; 2 Ju; 805-816
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
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    Constraining circumstellar environments: Far-infrared observations of Herbig Ae/Be stars (1994)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-08-28
    Beschreibung: We have observed six Herbig Ae/Be stars at 50 and 100 microns with the Kuiper Airborne Observatory (KAO). All were classified as group I sources by Hillenbrand et al., signifying that their spectral energy distributions could be modeled with stars surrounded only by accretion disks. If the far-infrared emission is assumed to arise in a disk, it should be unresolved at 100 microns, regardless of the size of the disk. In contrast, we find that five out of six sources are clearly resolved at 100 microns. Three sources were also observed at 50 microns and at least two were resolved. Consequently the far-infrared emission must arise in another component, most plausibly a circumstellar envelope. This suggests that the group I sources may be less distinct from the group II sources than previously suggested. Since the presence of an envelope can affect both the actual temperature distribution in a disk and the temperature distribution derived from modeling the emission, more consistent models, including both disks and envelopes, are needed for these sources.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 432; 2; p. 710-719
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 6
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    'Spherical' disks: Moving toward a unified source model for L1551 (1994)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-08-28
    Beschreibung: To predict the effects of a disk on the spectral energy distribution of a deeply embedded protostar, we construct disk models with power-law temperature distributions T is proportional to r(exp -q). We then use the spherically averaged disk emission as the central source for a spherical envelope, hence the term, 'spherical' disk. We then calculate the predicted spectral energy distribution of the disk and envelope, using a spherically symmetric radiative transport code. Applying this procedure to L1551 IRS 5, we find that the predicted far-infrared flux is not very sensitive to the nature of the central source. The best source model is consistent with the far-infrared emission arising from the infalling region in an 'inside-out' collapse model, independent of the nature of the central source. Disk models are superior to the star-only model when we try to match millimeter interferometer data. While disks with various q can reproduce the observed 2.7 mm interferometer flux, only an active disk (q = 0.5) can produce enough emission in a region small enough to match the observed 2.7 mm visibilities. However, if the disk is backwarmed by the envelope, even purely reprocessing disks can meet this constraint. All types of backwarmed disks are virtually in distinguishable in their millimeter properties. We find that all reasonable envelope models are sufficiently opaque in the mid-infrared to attenuate any disk model to a level well below the observations, unless the ratio of the mid-infrared to far-infrared dust opacities is similar to that of the dust opacities advocated by Mathis, Mezger, & Panagia (1983).
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 420; 1; p. 326-335
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
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    Infrared studies of circumstellar matter around Herbig Ae/Be and related stars (1993)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: The study presents high spatial resolution data at 50 and 100 microns for seven young intermediate-mass stars with flat or rising spectral energy distributions. Five stars, namely, LkH-alpha 198, R Mon, CD -42 deg 11721, R CrA, and V645 Cyg, are resolved at 100 microns. The emission comes from spatially extended envelopes, with sizes varying between 0.025 and 0.4 pc. The wavelengths are optically thin at mid-IR and FIR wavelengths but become optically thick in the visual where the extinction amounts to 5-10 mag. In two cases, R CrA and V 645 Cyg, dust density distributions with steep density gradients are obtained, alpha is approximately equal to 2, while for LkH-alpha, R Mon, and CD -42 deg 11721, alpha is approximately equal to 0.5. Model fitting to the spectral energy distributions, combined with the constraints on the size of the emitting regions, indicates that inside the circumstellar envelopes a large fraction of the stellar radiation is already degraded to longer wavelengths.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 406; 2; p. 674-691.
    Format: text
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  • 8
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    Testing models of low-mass star formation - High-resolution far-infrared observations of L1551 IRS 5 (1991)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: A 50 and 100 micron wavelength study of L151 IRS 5 has yielded data consistent with the Adams et al. (1987) theoretical model prediction. It has proven possible to constrain a range of possible density gradients through source-emission modeling on the basis of the spherically-symmetric radiative transfer program of Egan et al. (1988) and a comparison of the observed scans at 50 and 100 microns. Attention is given to the effects of varying the dust grain properties of the spherical energy distribution of the source; the amount of mid-IR emission is highly sensitive to dust opacity and, because of poorly understood dust properties, is not a sensitive test for the presence of disks.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 376; 636-653
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
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    CO observations of L1582 (1988)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: In this paper, the bright-rimmed cloud L1582 is mapped in (C-12)O and (C-13)O, and the embedded NH3 core is mapped in the CS(3 - 2) line. The optical bright rim coincides with the cloud edge seen in CO. Both the kinetic temperature and column density maps show elongated features along the bright rim, and the NH3 core is located right near the edge of the cloud inside a column density plateau region corresponding to an optical extinction patch. The density enhancement in the CS core is about a factor of three over the ambient cloud and the core mass is estimated at eight solar. The dust luminosity can be supplied by radiation from the Gamma Ori OB association. The most plausible heating source for the cloud gas is the photoelectric ejection of energetic electrons from grains induced by the far-UV flux from the Gamma Ori OB association.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 333; 809-820
    Format: text
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  • 10
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    High-resolution, far-infrared observations of NGC 2071 (1990)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-12
    Beschreibung: The far-IR emission of the visible reflection nebula NGC 2071 has been resolved at both 50 and 100 microns along several directions. The observations reveal an extended, roughly spherical source with an average source diameter of about 12 arcsec or 4700 AU at 50 microns and about 16 arcsec or 6200 AU at 100 microns. The source is modeled using a radiative transport code to match scans of the source and previous photometry. The luminosity of the source is 520 solar at a distance of 390 pc. The optical depth at 100 microns is 0.20, implying a mass of 1.2-10 solar within a radius of 5900 AU. The density gradient is in good agreement with theoretical models for infalling envelopes around protostars and in reasonable agreement with other observational determinations.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 364; 164-172
    Format: text
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