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Observationally, both the 3.4micron aliphatic hydrocarbon C--H stretching absorption feature and the 9.7micron amorphous silicate Si--O stretching absorption feature show considerable variations from the local diffuse interstellar medium (ISM) to Galactic center (GC): both the ratio of the visual extinction (A_V) to the 9.7micron Si--O optical depth (\tausil) and the ratio of A_V to the 3.4micron C--H optical depth (\tauahc) of the solar neighborhood local diffuse ISM are about twice as much as that of the GC. In this work, we try to explain these variations in terms of a porous dust model consisting of a mixture of amorphous silicate, carbonaceous organic refractory dust (as well as water ice for the GC dust)., Comment: 6 pages, 6 figures, uses eps.cls. Accepted for publication in "Earth, Planets and Space" (special issue on Cosmic Dust)

The 21micron and 30micron bands are the strongest dust emission features detected in evolved low- and intermediate-mass C-rich stars (i.e. asymptotic giant branch [AGB] stars, proto-planetary nebulae [PPN], and planetary nebulae [PN]). While the 21micron feature is rare and exists only in the transient PPN phase, the 30micron feature is more common and seen in the entire late stage of stellar evolution, from AGB to PPN and PN phases, as well as in the low-metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The carriers of these features remain unidentified. Eleven of the twelve well-identified 21micron sources also emit in the 30micron band, suggesting that their carriers may be somewhat related., Comment: 7 pages, 1 figure, uses eps.cls. Accepted for publication in "Earth, Planets and Space" (special issue on Cosmic Dust)

The extinction laws at 7um and 15um are derived for more than 120 sightlines in the inner Galactic plane based on the ISOGAL survey data and the near-infrared data from DENIS and 2MASS. The tracers are the ISOGAL point sources with [7]-[15]<0.4 which are RGB tip stars or early AGB stars with moderate mass loss. They have well-defined intrinsic color indices (J-Ks)_0, (Ks-[7])_0 and (Ks-[15])_0. By a linear fitting of the observed color indices Ks-[7] and Ks-[15] to the observed J-Ks, we obtain the ratio between the E(Ks-[7]) and E(Ks-[15]) color excesses and E(J-Ks). We infer the selective extinctions at 7 and 15um in terms of the near-infrared extinction in the Ks band. The distribution of the derived extinctions around 7 micron (A_7) is well represented by a Gaussian function, with the peak at about 0.47A_Ks and ranging from 0.33 to 0.55A_Ks (using the near-infrared extinctions of Rieke & Lebovsky 1985). There is some evidence that A_7/A_Ks may vary significantly depending on the line of sight. The derived selective extinction at 15um suffers uncertainty mainly from the dispersion in the intrinsic color index (Ks-[15])_0 which is affected by dust emission from mass-losing AGB stars. The peak value of A_15 is around 0.40A_Ks., Comment: 21 pages, 6 figures, accepted for publication in Astronomy and Astrophysics

The mysterious 21mu emission feature seen in 12 proto-planetary nebulae (PPNe) remains unidentified since its first detection in 1989. Over a dozen of candidate materials have been proposed within the past decade, but none of them has received general acceptance. Very recently, silicon carbide (SiC) grains with impurities were suggested to be the carrier of this enigmatic feature, based on recent laboratory data that doped SiC grains exhibit a resonance at \~21mu. This proposal gains strength from the fact that SiC is a common dust species in carbon-rich circumstellar envelopes. However, SiC dust has a strong vibrational band at ~11.3mu. We show in this Letter that in order to be consistent with the observed flux ratios of the 11.3mu feature to the 21mu feature, the band strength of the 21mu resonance has to be very strong, too strong to be consistent with current laboratory measurements. But this does not yet readily rule out the SiC hypothesis since recent experimental results have demonstrated that the 21mu resonance of doped SiC becomes stronger as the C impurity increases. Further laboratory measurements of SiC dust with high fractions of C impurity are urgently needed to test the hypothesis of SiC as the carrier of the 21mu feature., Comment: 14 pages, 3 figures, accepted for publication in ApJL

A 0.35\degr $\times$ 0.29\degr field centered at $l$=--18.63\degr, $b$=0.35\degr was observed during the ISOGAL survey by ISOCAM imaging at 7$\mu$m and 15{\rm $\mu$m}. 648 objects were detected and their brightness are measured. By combining with the DENIS data in the near-infrared J and K$_{\rm S}$ bands, one derives the extinction at 7{\rm $\mu$m} through ${\rm A_{K_{\rm S}}-A_7= 0.35 (A_J-A_{K_{\rm S}})}$ which yields A$_{7}$/A$_{\it V}$ $\sim$0.03 from the near-IR extinction values of van de Hulst--Glass (Glass 1999). The extinction structure along the line of sight is then determined from the values of J--K$_{\rm S}$ or K$_{\rm S}$--[7] of the ISOGAL sources identified as RGB or early AGB stars with mild mass-loss. The distribution of A$_{\it V}$ ranges from 0 to $\sim$45 and it reflects the concentration of the extinction in the spiral arms. Based on their locations in color-magnitude diagrams and a few cross-identifications with IRAS and MSX sources, the nature of objects is discussed in comparison with the case of a low extinction field in Baade's Window. Most of the objects are either AGB stars with moderate mass loss rate or luminous RGB stars. Some of them may be AGB stars with high mass loss rate. In addition, a few young stellar objects (YSOs) are present., Comment: 14 pages, accepted for publication in Astronomy and Astrophysics

Based on the space MSX observation in bands A(8$\mu$m), C(12$\mu$m), D(15$\mu$m) and E(21$\mu$m), and the ground SiO maser observation of evolved stars by the Nobeyama 45-m telescope in the v=1 and v=2 J=1-0 transitions, the relation between SiO maser emission and mid-IR continuum radiation is analyzed. The relation between SiO maser emission and the IR radiation in the MSX bands A, C, D and E is all clearly correlated. The SiO maser emission can be explained by a radiative pumping mechanism according to its correlation with infrared radiation in the MSX band A., Comment: 11 pages, 4 figures, to appear in ApJL

Light variation of the 47 AGB star candidates in the outer Galactic disk has been monitored at I-band for 5 years. Periods were determined well for 18 of them and less reliably for the other 25. The average period of the objects is then 500 days. According to the period-luminosity relation, the mean luminosity of the sample stars is 10000 $\LO$. Based on the absolute luminosity derived from the period-luminosity relation and the apparent luminosity from the observation, the distances to the objects are determined. The distances calculated are slightly greater than those obtained previously on the assumption of constant luminosity of 8000 $\LO$. In addition, SiO maser emission was detected for most of the observed objects so that their radial velocities were known accurately. With the assumption of circular rotation in the Galactic disk, the rotation curve and Oort's constants were derived., Comment: 13 pages including 4 tables, and 13 postscript figures. Accepted for PASJ (Publication of the Astronomical Society of Japan)

This paper reports a detection of the 30 micron emission feature from the C-rich Asymptotic Giant Branch (AGB) star IRAS 03313+6058 based on the ISO-SWS observation. Modeling of the spectral energy distribution shows that this emission starts at about 20 micron and possibly extends to the limit (45 micron) of the observation. By assuming MgS as the carrier, the number ratio of Sulfur atom in MgS to Hydrogen atom in total, n(S)/n(H), is 3. 10{-6} from model fitting. A comparison of this emission feature is made with other AGB and post-AGB objects., Comment: aa-latex, 5 pages with 3 figures