Issei Yamamura

We combine data from two all-sky surveys in order to study the connection between the infrared and hard X-ray (>10keV) properties for local active galactic nuclei (AGNs). The Swift Burst Alert Telescope all-sky survey provides an unbiased, flux-limited selection of hard X-ray-detected AGNs. Cross-correlating the 22month hard X-ray survey with the AKARI all-sky survey, we studied 158 AGNs detected by the AKARI instruments. We find a strong correlation for most AGNs between the infrared (9, 18, and 90 μm) and hard X-ray (14-195keV) luminosities, and quantify the correlation for various subsamples of AGNs. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution. The correlation for radio galaxies has a slope and normalization identical to that for Seyfert 1 galaxies, implying similar hard X-ray/infrared emission processes in both. In contrast, Compton-thick (CT) sources show a large deficit in the hard X-ray band, because high gas column densities diminish even their hard X-ray luminosities. We propose two photometric diagnostics for source classification: one is an X-ray luminosity versus infrared color diagram, in which type 1 radio-loud AGNs are well isolated from the others in the sample. The other uses the X-ray versus infrared color as a useful redshift-independent indicator for identifying CT AGNs. Importantly, CT AGNs and starburst galaxies in composite systems can also be differentiated in this plane based upon their hard X-ray fluxes and dust temperatures. This diagram may be useful as a new indicator to classify objects in new and upcoming surveys such as WISE and NuSTAR. © 2012. The American Astronomical Society. All rights reserved..

Recent observations with the infrared astronomical satellite AKARI have shown that the CO2 bands at 4.2 mu m in three brown dwarfs are much stronger than expected from the unified cloudy model (UCM) based on recent solar C & O abundances. This result has been a puzzle, but we now find it is simply due to the effect of C & O abundances. We show that these strong CO2 bands can be explained with the UCMs based on the classical C & O abundances (log A(C) and log A(O)), which are about 0.2 dex larger compared to the recent values. Since three other brown dwarfs could be interpreted fairly well with the recent solar C & O abundances, we require at least two model sequences based on the different chemical compositions to interpret all the AKARI spectra. The reason is that the CO2 band is especially sensitive to C & O abundances, since the CO2 abundance depends approximately on A(C)A(O)(2)-the cube of C & O abundances. For this reason, even low-resolution spectra of very cool dwarfs, especially of CO2, cannot be understood unless a model with proper abundances is applied. For the same reason, CO2 is an excellent indicator of C & O abundances, and we can now estimate C & O abundances of brown dwarfs as follows: Three of the six brown dwarfs observed with AKARI should have high C & O abundances similar to the classical solar values (e. g., log A(C) = 8.60 and log A(O) = 8.92), but the other three may have low C & O abundances similar to the recent solar values (e. g., log A(C) = 8.39 and log A(O) = 8.69). This result implies that three of the six brown dwarfs are highly metal-rich relative to the Sun, if the recent solar C & O abundances are correct.

We report mid-infrared (MIR) imaging observations of the carbon star U Ant made with the Infrared Camera (IRC) on board AKARI. Subtraction of the artifacts and extended point-spread function of the central star reveals the detached dust shell around the carbon star at MIR wavelengths (15 and 24 mu m) for the first time. The observed radial brightness profiles of the MIR emission are well explained by two shells at 43 '' and 50 '' from the central star detected in optical scattered light observations. Combining Herschel/PACS, AKARI/FIS, and AKARI/IRC data, we obtain the infrared spectral energy distribution (SED) of the thermal emission from the detached shell of U Ant in a wide infrared spectral range of 15-160 mu m. Thermal emission of amorphous carbon grains with a single temperature cannot account for the observed SED from 15 to 160 mu m: it underestimates the emission at 15 mu m. Alternatively, the observed SED is fitted by the model in which amorphous carbon grains in the two shells have different temperatures of 60 and 104 K, which allocates most dust mass in the shell at 50 ''. This supports the previous suggestion that the 43 '' shell is gas-rich and the 50 '' shell is dust-rich. We suggest a possibility that the segregation of the gas and dust resulting from the drift motion of submicron-sized dust grains relative to the gas and that the hot dust component associated with the gas-rich shell is composed of very small grains that are strongly coupled with the gas.

Near-infrared medium-resolution spectra of seven bright brown dwarfs are presented. The spectra were obtained with the Infrared Camera on board the infrared astronomical satellite AKARI, covering 2.5-5.0 mu m with a spectral resolution of approximately 120. The spectral types of the objects range from L5 to T8 and enable us to study the spectral evolution of brown dwarfs. The observed spectra are in general consistent with predictions from previous observations and photospheric models; spectra of L-type dwarfs are characterized by continuum opacity from dust clouds in the photosphere, while very strong molecular absorption bands dominate the spectra in T-type dwarfs. We find that the CO fundamental band around 4.6 mu m is clearly seen even in the T8 dwarf 2MASS J041519-0935, confirming the presence of a non-equilibrium chemical state in the atmosphere. We also identify the CO2 fundamental stretching-mode band at 4.2 mu m for the first time in the spectra of late-L-and T-type brown dwarfs. As a preliminary step towards interpretation of the data obtained by AKARI, we analyze the observed spectra by comparing with those predicted by the unified cloudy model (UCM). Although overall spectral energy distributions can be reasonably fitted with the UCM, observed CO and CO2 bands in late-L and T dwarfs are unexpectedly stronger than the model predictions assuming local thermodynamical equilibrium. We examine the vertical mixing model and find that this model explains the CO band at least partly in the T dwarfs 2MASS J041519-0935 and 2MASS J055919-1404. The CO fundamental band also shows excess absorption against the predicted one in the L9 dwarf SDSS J083008+4828. Since CO is already highly abundant in the upper photospheres of late-L dwarfs, the extra CO due to vertical mixing has little effect on the CO band strengths, and the vertical mixing model cannot be applied to this L dwarf. A more serious problem is that the significant enhancement of the CO2 4.2 mu m band in both the late-L and T dwarfs cannot be explained at all by the vertical mixing model. The cause of this enhancement of the CO2 band remains to be explained.

Near-infrared medium-resolution spectra of seven bright brown dwarfs are presented. The spectra were obtained with the Infrared Camera on board the infrared astronomical satellite AKARI, covering 2.5-5.0 mu m with a spectral resolution of approximately 120. The spectral types of the objects range from L5 to T8 and enable us to study the spectral evolution of brown dwarfs. The observed spectra are in general consistent with predictions from previous observations and photospheric models; spectra of L-type dwarfs are characterized by continuum opacity from dust clouds in the photosphere, while very strong molecular absorption bands dominate the spectra in T-type dwarfs. We find that the CO fundamental band around 4.6 mu m is clearly seen even in the T8 dwarf 2MASS J041519-0935, confirming the presence of a non-equilibrium chemical state in the atmosphere. We also identify the CO2 fundamental stretching-mode band at 4.2 mu m for the first time in the spectra of late-L-and T-type brown dwarfs. As a preliminary step towards interpretation of the data obtained by AKARI, we analyze the observed spectra by comparing with those predicted by the unified cloudy model (UCM). Although overall spectral energy distributions can be reasonably fitted with the UCM, observed CO and CO2 bands in late-L and T dwarfs are unexpectedly stronger than the model predictions assuming local thermodynamical equilibrium. We examine the vertical mixing model and find that this model explains the CO band at least partly in the T dwarfs 2MASS J041519-0935 and 2MASS J055919-1404. The CO fundamental band also shows excess absorption against the predicted one in the L9 dwarf SDSS J083008+4828. Since CO is already highly abundant in the upper photospheres of late-L dwarfs, the extra CO due to vertical mixing has little effect on the CO band strengths, and the vertical mixing model cannot be applied to this L dwarf. A more serious problem is that the significant enhancement of the CO2 4.2 mu m band in both the late-L and T dwarfs cannot be explained at all by the vertical mixing model. The cause of this enhancement of the CO2 band remains to be explained.

Context. The AKARI, a Japanese infrared space mission, has performed an All-Sky Survey in six infrared-bands from 9 to 180 mu m with higher spatial resolutions and better sensitivities than IRAS. Aims. We investigate the mid-infrared (9 and 18 mu m) point source catalog (PSC) obtained with the infrared camera (IRC) onboard AKARI, in order to understand the infrared nature of the known objects and to identify previously unknown objects. Methods. Color-color diagrams and a color-magnitude diagram were plotted with the AKARI-IRC PSC and other available all-sky survey catalogs. We combined the Hipparcos astrometric catalog and the 2MASS all-sky survey catalog with the AKARI-IRC PSC. We furthermore searched literature and SIMBAD astronomical database for object types, spectral types, and luminosity classes. We identified the locations of representative stars and objects on the color-magnitude and color-color diagram schemes. The properties of unclassified sources can be inferred from their locations on these diagrams. Results. We found that the (B - V) vs. (V - S 9W) color-color diagram is useful for identifying the stars with infrared excess emerged from circumstellar envelopes or disks. Be stars with infrared excess are separated well from other types of stars in this diagram. Whereas (J - L18W) vs. (S 9W - L18W) diagram is a powerful tool for classifying several object types. Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars form distinct sequences in this color-color diagram. Young stellar objects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, and planetary nebulae (PNe) have the largest mid-infrared color excess and can be identified in the infrared catalog. Finally, we plot the L18W vs. (S 9W - L18W) color-magnitude diagram, using the AKARI data together with Hipparcos parallaxes. This diagram can be used to identify low-mass YSOs and AGB stars. We found that this diagram is comparable to the [24] vs. ([8.0] - [24]) diagram of Large Magellanic Cloud sources using the Spitzer Space Telescope data. Our understanding of Galactic objects will be used to interpret color-magnitude diagram of stellar populations in the nearby galaxies that Spitzer Space Telescope observed. Conclusions. Our study of the AKARI color-color and color-magnitude diagrams will be used to explore properties of unknown objects in the future. In addition, our analysis highlights a future key project to understand stellar evolution with a circumstellar envelope, once the forthcoming astronometrical data with GAIA are available.

Aims. The circumstellar dust shells of intermediate initial-mass (similar to 1 to 8 M-circle dot) evolved stars are generated by copious mass loss during the asymptotic giant branch phase. The density structure of their circumstellar shell is the direct evidence of mass loss processes, from which we can investigate the nature of mass loss. Methods. We used the AKARI infrared astronomy satellite and the Spitzer space telescope to obtain the surface brightness maps of an evolved star R Cas at far-infrared wavelengths, since the temperature of dust decreases as the distance from the star increases and one needs to probe dust at lower temperatures, i.e., at longer wavelengths. The observed shell structure and the star's known proper motion suggest that the structure represents the interface regions between the dusty wind and the interstellar medium. The deconvolved structures are fitted with the analytic bow shock structure to determine the inclination angle of the bow shock cone. Results. Our data show that (1) the bow shock cone of 1-5 x 10(-5) M-circle dot dust mass is inclined at 68 degrees with respect to the plane of the sky; and (2) the dust temperature in the bow shock cone is raised to more than 20 K by collisional shock interaction in addition to the ambient interstellar radiation field. By comparison between the apex vector of the bow shock and space motion vector of the star we infer that there is a flow of interstellar medium local to R Cas whose flow velocity is at least 55.6 km s(-1), consistent with an environment conducive to dust heating by shock interactions.

We present 10' x 50'scan maps around an M supergiant alpha Ori at 65, 90, 140, and 160 mu m obtained with the AKARI Infrared Astronomy Satellite. Higher spatial resolution data with the exact analytic solution permit us to fit the de-projected shape of the stellar-wind bow shock around alpha Ori to have a stand-off distance of 4'.8, position angle of 55 degrees and inclination angle of 56 degrees. The shape of the bow shock suggests that the velocity of alpha Ori with respect to the local medium is upsilon(*) = 40n(H)(-1/2), where n(H) is the hydrogen nucleus density at alpha Ori. We found that the local medium is of n(H) = 1.5 to 1.9 cm(-3) and the velocity of the local flow is at 11 km s(-1) by using the most recent astrometric solutions for alpha Ori under the assumption that the medium local to alpha Ori is moving away from the Orion OB 1 association. AKARI images may also reveal a vortex ring due to instabilities on the surface of the bow shock, as demonstrated by numerical models. This research exemplifies the potential of AKARI All-Sky data as well as follow-up observations with Herschel Space Telescope and Stratospheric Observatory for Infrared Astronomy for this avenue of research in revealing the nature of the interaction between the stellar wind and the interstellar medium.

The Far-Infrared Surveyor (FIS) is one of two focal-plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 μm, and uses two kinds of array detectors. The FTS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and the performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS. © 2007. Astronomical Society of Japan.

The dependence of stellar molecular bands on the metallicity is studied using infrared L- band spectra of AGB stars ( both carbon- rich and oxygen- rich) and M- type supergiants in the Large and Small Magellanic Clouds ( LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for oxygen- rich stars, and acetylene ( C2H2), CH and HCN bands for carbon- rich AGB stars. The equivalent width of acetylene is found to be high even at low metallicity. The high C2H2 abundance can be explained with a high carbon- to- oxygen ( C/ O) ratio for lower metallicity carbon stars. In contrast, the HCN equivalent width is low: fewer than half of the extra- galactic carbon stars show the 3.5 mu m HCN band, and only a few LMC stars show high HCN equivalent width. HCN abundances are limited by both nitrogen and carbon elemental abundances. The amount of synthesized nitrogen depends on the initial mass, and stars with high luminosity ( i. e. high initial mass) could have a high HCN abundance. CH bands are found in both the extra- galactic and Galactic carbon stars. One SMC post- AGB star, SMC- S2, shows the 3.3 mu m PAH band. This first detection of a PAH band from an SMC post- AGB star confirms PAHs can form in these low- metallicity stars. None of the oxygen- rich LMC stars show SiO bands, except one possible detection in a low quality spectrum. The limits on the equivalent widths of the SiO bands are below the expectation of up to 30 angstrom for LMC metallicity. Several possible explanations are discussed, mostly based on the effect of pulsation and circumstellar dust. The observations imply that LMC and SMC carbon stars could reach mass- loss rates as high as their Galactic counterparts, because there are more carbon atoms available and more carbonaceous dust can be formed. On the other hand, the lack of SiO suggests less dust and lower mass- loss rates in low- metallicity oxygen- rich stars. The effect on the ISM dust enrichment is discussed.

We have identified 139 cool carbon stars in the near-infrared spectrophotometric survey of the Infrared Telescope in Space (IRTS) from the conspicuous presence of molecular absorption bands at 1.8, 3.1, and 3.8 mum. Among them, 14 are new bright (K similar to 4.0-7.0) carbon stars. We find a trend relating the 3.1 mum band strength to the K - L' color index, which is known to correlate with mass- loss rate. This could be an effect of a relation between the depth of the 3.1 mm feature and the degree of development of the extended stellar atmosphere where dust starts to form.

We present the results of observations of the unidentified infrared (UIR) bands in the diffuse Galactic emission across the Galaxy by the Mid-Infrared Spectrometer (MIRS) on board the Infrared Telescope in Space (IRTS). While previous studies on the UIR bands in the Milky Way were limited to the inner Galactic plane, we extend the observing area to the outer Galactic plane. In this paper we analyze the data of four areas of 8degrees; 8degrees around the Galactic plane (\b\ less than or equal to 4degrees; -12degrees less than or equal to l less than or equal to -4degrees, 44degrees less than or equal to l less than or equal to 52degrees, -136degrees less than or equal to l less than or equal to - 128degrees, and 168degrees less than or equal to l less than or equal to 176degrees) and investigate the UIR band intensity relative to the far-infrared ( FIR) intensity, as well as the variation in the band profile. Together with the good correlation between the UIR band and the FIR intensities in the four regions, we have found a systematic variation in the UIR-to-FIR ratio such that the ratio becomes larger in the outer Galactic plane than in the inner Galactic plane. In addition, the 8.6 and 11.3 mum UIR bands were found to be stronger relative to the 6.2 and 7.7 mum bands in the outer Galactic plane, which may be related to differences in the structure or physical conditions of the band carriers. We have also found small shifts ( similar to 0.1 mum) in the peak wavelength of each UIR band to shorter wavelengths from the inner Galactic plane to the outer Galactic plane. Possible interpretations of these variations are discussed.

We investigate carbon isotope effects on the infrared bands of a laboratory analogue of carbonaceous dust, the quenched carbonaceous composite (QCC), synthesized from a plasma gas of methane with various C-12/C-13 ratios. Peak shifts to longer wavelengths due to the substitution of C-12 by C-13 are clearly observed in several absorption bands. The shifts are almost linearly proportional to the C-13 fraction. New features associated with C-13 are not seen, indicating that the infrared bands in the QCC are not very localized vibration modes but come from vibrations associated with rather large carbon structures. An appreciable peak shift (Deltalambda similar to 0.23-0.26 mum per C-13 fraction) is detected in the 6.2 mum band, which is attributed to a carbon-carbon vibration. A peak shift (Deltalambda similar to 0.16-0.18 mum per C-13 fraction) in an out-of-plane bending mode of aromatic C-H at 11.4 mum is also observed, while only a small shift (Deltalambda < 0.015 μm per C-13 fraction) is detected in the 3.3 μm band, which arises from a C-H stretching mode. The present experiment suggests that peak shifts in the unidentified infrared (UIR) bands, particularly in the 6.2 μm band, should be detectable in celestial objects with low C-12/C-13 ratios (< 10). The isotopic shifts seen in the QCC are discussed in relation to the variations in the UIR band peaks observed in post-asymptotic giant branch stars and planetary nebulae. The observed peak shift pattern of the UIR bands is qualitatively in agreement with the isotopic shifts in the QCC except for the 7.7 mum band complex although the observed shifts in the UIR bands are larger than those inferred from derived isotope ratios for individual objects. The poor quantitative agreement may be attributed partly to large uncertainties in the derived C-12/C-13, to possible spatial variations of the isotope abundance within the object, and to combinations of other effects, such as hetero-atom substitutions. The present investigation suggests that part of the observed variations in the UIR band peaks may come from the isotopic effects.

We investigate carbon isotope effects on the infrared bands of a laboratory analogue of carbonaceous dust, the quenched carbonaceous composite (QCC), synthesized from a plasma gas of methane with various C-12/C-13 ratios. Peak shifts to longer wavelengths due to the substitution of C-12 by C-13 are clearly observed in several absorption bands. The shifts are almost linearly proportional to the C-13 fraction. New features associated with C-13 are not seen, indicating that the infrared bands in the QCC are not very localized vibration modes but come from vibrations associated with rather large carbon structures. An appreciable peak shift (Deltalambda similar to 0.23-0.26 mum per C-13 fraction) is detected in the 6.2 mum band, which is attributed to a carbon-carbon vibration. A peak shift (Deltalambda similar to 0.16-0.18 mum per C-13 fraction) in an out-of-plane bending mode of aromatic C-H at 11.4 mum is also observed, while only a small shift (Deltalambda < 0.015 μm per C-13 fraction) is detected in the 3.3 μm band, which arises from a C-H stretching mode. The present experiment suggests that peak shifts in the unidentified infrared (UIR) bands, particularly in the 6.2 μm band, should be detectable in celestial objects with low C-12/C-13 ratios (< 10). The isotopic shifts seen in the QCC are discussed in relation to the variations in the UIR band peaks observed in post-asymptotic giant branch stars and planetary nebulae. The observed peak shift pattern of the UIR bands is qualitatively in agreement with the isotopic shifts in the QCC except for the 7.7 mum band complex although the observed shifts in the UIR bands are larger than those inferred from derived isotope ratios for individual objects. The poor quantitative agreement may be attributed partly to large uncertainties in the derived C-12/C-13, to possible spatial variations of the isotope abundance within the object, and to combinations of other effects, such as hetero-atom substitutions. The present investigation suggests that part of the observed variations in the UIR band peaks may come from the isotopic effects.

We are using the 2002 data-release from the Japanese space experiment IRTS to investigate the spatial distribution of galactic mass-losing (>2 x 10(-8) M-circle dot yr(-1)) AGB stars and the relative contribution of C-rich and O-rich ones to the replenishment of the ISM. Our sample contains 126 C-rich and 563 O-rich sources which are sorted on the basis of the molecular bands observed in the range 1.4-4.0 mum, and for which we estimate distances and mass loss rates from near-infrared photometry (K and L'). There is a clear dependence on galactocentric distance, with O-rich sources outnumbering C-rich ones for r(GC) < 8 kpc, and the reverse for r(GC) > 10 kpc. The contribution to the replenishment of the ISM by O-rich AGB stars relative to C-rich ones follows the same trend. Although they are rare (similar to10% in our sample), sources with 10(-6) M-circle dot yr(-1) <. M < 10(-5) M-circle dot yr(-1) dominate the replenishment of the ISM by contributing to similar to50% of the total of the complete sample. We find 2 carbon stars at more than 1 kpc from the Galactic Plane, that probably belong to the halo of our Galaxy.

We are using the 2002 data-release from the Japanese space experiment IRTS to investigate the spatial distribution of galactic mass-losing (>2 x 10(-8) M-circle dot yr(-1)) AGB stars and the relative contribution of C-rich and O-rich ones to the replenishment of the ISM. Our sample contains 126 C-rich and 563 O-rich sources which are sorted on the basis of the molecular bands observed in the range 1.4-4.0 mum, and for which we estimate distances and mass loss rates from near-infrared photometry (K and L'). There is a clear dependence on galactocentric distance, with O-rich sources outnumbering C-rich ones for r(GC) < 8 kpc, and the reverse for r(GC) > 10 kpc. The contribution to the replenishment of the ISM by O-rich AGB stars relative to C-rich ones follows the same trend. Although they are rare (similar to10% in our sample), sources with 10(-6) M-circle dot yr(-1) <. M < 10(-5) M-circle dot yr(-1) dominate the replenishment of the ISM by contributing to similar to50% of the total of the complete sample. We find 2 carbon stars at more than 1 kpc from the Galactic Plane, that probably belong to the halo of our Galaxy.

We identify H2O, CO, SiO and CO2 bands in near-infrared ISO/SWS spectra of the RV Tau star R Sct. These molecules originate from an atmosphere around the photosphere. RV Tau stars are commonly considered as post-AGB stars. We argue that R Sct may be a thermal-pulsing AGB star, observed in a helium-burning phase.

We identify H2O, CO, SiO and CO2 bands in near-infrared ISO/SWS spectra of the RV Tau star R Sct. These molecules originate from an atmosphere around the photosphere. RV Tau stars are commonly considered as post-AGB stars. We argue that R Sct may be a thermal-pulsing AGB star, observed in a helium-burning phase.

The M-type Mira variable star, Z Cyg, was observed with the Short-Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) 7 times at roughly 60 day intervals over one and a half period. The infrared spectrum (2.38-45.2 mum) of Z Cyg shows prominent silicate emission bands at 10 mum and 18 mum and displays quite large variations over the observed period. The variation in the infrared spectrum of Z Cyg is synchronized with the visual light curve. The circumstellar emission and the 10 mum to 18 mum silicate band ratio increases at maximum and decreases at minimum, indicating a variation in the dust temperature with phase. Apart from minor emission features which may be partly due to oxide dust, the observed spectra can be fitted by optically thin dust shell models with one single silicate dust emissivity profile. Thus silicate is the dominant dust component in the circumstellar shell of Z Cyg. The variation in the integrated infrared flux and in the dust temperature derived from the observed spectra can be interpreted in terms of the variation in the luminosity of the central star if proper dust optical properties are adopted. Conversely, the dust emissivity can be estimated from the variations in the infrared spectrum. The derived optical properties are relatively insensitive to the assumptions made in the model analysis because of the optically thin nature of the dust shell. Possible evidence for dust formation near minimum is discussed. The observed variation of the dust shell spectrum of Z Cyg is fitted most consistently with a model in which the inner dust shell temperature is 700 +/- 100 K at maximum. From the model fits we derive a ratio of the 18 mum and 10 mum emission efficiencies of Q(18 mum)/Q(10 mum) = 0.51 +/- 0.08. The Z Cyg dust also has a broad feature in the 20-25 mum region in addition to the 18 mum silicate band. The optical properties of dust grains around Z Cyg are compared with those in circumstellar shells of other oxygen-rich late-type stars and it is shown that there are variations in the 20 mum emissivity of circumstellar dust, possibly related to the presence of another dust component.

The M-type Mira variable star, Z Cyg, was observed with the Short-Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) 7 times at roughly 60 day intervals over one and a half period. The infrared spectrum (2.38-45.2 mum) of Z Cyg shows prominent silicate emission bands at 10 mum and 18 mum and displays quite large variations over the observed period. The variation in the infrared spectrum of Z Cyg is synchronized with the visual light curve. The circumstellar emission and the 10 mum to 18 mum silicate band ratio increases at maximum and decreases at minimum, indicating a variation in the dust temperature with phase. Apart from minor emission features which may be partly due to oxide dust, the observed spectra can be fitted by optically thin dust shell models with one single silicate dust emissivity profile. Thus silicate is the dominant dust component in the circumstellar shell of Z Cyg. The variation in the integrated infrared flux and in the dust temperature derived from the observed spectra can be interpreted in terms of the variation in the luminosity of the central star if proper dust optical properties are adopted. Conversely, the dust emissivity can be estimated from the variations in the infrared spectrum. The derived optical properties are relatively insensitive to the assumptions made in the model analysis because of the optically thin nature of the dust shell. Possible evidence for dust formation near minimum is discussed. The observed variation of the dust shell spectrum of Z Cyg is fitted most consistently with a model in which the inner dust shell temperature is 700 +/- 100 K at maximum. From the model fits we derive a ratio of the 18 mum and 10 mum emission efficiencies of Q(18 mum)/Q(10 mum) = 0.51 +/- 0.08. The Z Cyg dust also has a broad feature in the 20-25 mum region in addition to the 18 mum silicate band. The optical properties of dust grains around Z Cyg are compared with those in circumstellar shells of other oxygen-rich late-type stars and it is shown that there are variations in the 20 mum emissivity of circumstellar dust, possibly related to the presence of another dust component.

The time variation in the water-vapour bands in oxygen-rich Mira variables has been investigated using multi-epoch ISO/SWS spectra of four Mira variables in the 2.5-4.0 mum region. All four stars show H2O bands in absorption around minimum in the visual light curve. At maximum, H2O emission features appear in the similar to3.5-4.0 mum region, while the features at shorter wavelengths remain in absorption. These H2O bands in the 2.5-4.0 mum region originate from the extended atmosphere. The analysis has been carried out with a disk shape, slab geometry model. The observed H2O bands are reproduced by two layers; a "hot" layer with an excitation temperature of 2000 K and a "cool" layer with an excitation temperature of 1000-1400 K. The column densities of the "hot" layer are 6 x 10(20)-3 x 10(22) cm(-2), and exceed 3 x 10(21) cm(-2) when the features are observed in emission. The radii of the "hot" layer (R-hot) are similar to1 R-* at visual minimum and 2 R-* at maximum, where R-* is a radius of background source of the model, in practical, the radius of a 3000 K black body. The "cool" layer has the column density (N-cool) of 7 x 10(20)-5 x 10(22) cm(-2), and is located at 2.5-4.0 R-*. N-cool depends on the object rather than the variability phase. The time variation of R-hot/R-* from 1 to 2 is attributed to the actual variation in the radius of the H2O layer, since the variation in R-hot far exceeds the variation in the "continuum" stellar radius. A high H2O density shell occurs near the surface of the star around minimum, and moves out with the stellar pulsation. This shell gradually fades away after maximum, and a new high H2O density shell is formed in the inner region again at the next minimum. Due to large optical depth of H2O, the near-infrared variability is dominated by the H2O layer, and the L'-band flux correlates with the area of the H2O shell. The infrared molecular bands trace the structure of the extended atmosphere and impose appreciable effects on near-infrared light curve of Mira variables.

The time variation in the water-vapour bands in oxygen-rich Mira variables has been investigated using multi-epoch ISO/SWS spectra of four Mira variables in the 2.5-4.0 mum region. All four stars show H2O bands in absorption around minimum in the visual light curve. At maximum, H2O emission features appear in the similar to3.5-4.0 mum region, while the features at shorter wavelengths remain in absorption. These H2O bands in the 2.5-4.0 mum region originate from the extended atmosphere. The analysis has been carried out with a disk shape, slab geometry model. The observed H2O bands are reproduced by two layers; a "hot" layer with an excitation temperature of 2000 K and a "cool" layer with an excitation temperature of 1000-1400 K. The column densities of the "hot" layer are 6 x 10(20)-3 x 10(22) cm(-2), and exceed 3 x 10(21) cm(-2) when the features are observed in emission. The radii of the "hot" layer (R-hot) are similar to1 R-* at visual minimum and 2 R-* at maximum, where R-* is a radius of background source of the model, in practical, the radius of a 3000 K black body. The "cool" layer has the column density (N-cool) of 7 x 10(20)-5 x 10(22) cm(-2), and is located at 2.5-4.0 R-*. N-cool depends on the object rather than the variability phase. The time variation of R-hot/R-* from 1 to 2 is attributed to the actual variation in the radius of the H2O layer, since the variation in R-hot far exceeds the variation in the "continuum" stellar radius. A high H2O density shell occurs near the surface of the star around minimum, and moves out with the stellar pulsation. This shell gradually fades away after maximum, and a new high H2O density shell is formed in the inner region again at the next minimum. Due to large optical depth of H2O, the near-infrared variability is dominated by the H2O layer, and the L'-band flux correlates with the area of the H2O shell. The infrared molecular bands trace the structure of the extended atmosphere and impose appreciable effects on near-infrared light curve of Mira variables.

ASTRO-F is the second Japanese infrared space mission to be launched in early 2004. One of its focal-plane instruments, the Far-Infrared Surveyor (FIS), will carry out the whole sky survey in four-wavelength bands ranging from 50 to 200 μm. To evaluate the instrument performance and to prepare the data reduction system prior to the launch, we have developed a program to simulate the ASTRO-F/FIS sky survey, considering the basic instrumental proerties such as the diffraction pattern of the telescope, the detector pixel shape, and the readout electronics. In this paper, we focus on the validation of the hardware design of two long-wavelength (LW) bands covering 110-200 μm and 150-200μm. The stressed Ge:Ga detector arrays of 3x5 and 2x15 pixels are used for these two LW bands, respectively, In order to improve he spatial sampling frequency in the cross-scan direction, we tilted the FIS detector arrays by 26.5 degrees with respect to the scan direction. We demonstrate that this configuration actually provides better spatial information in the cross-scan direction. We evaluate the effective resolution of the FIS for the object with the different spectral energy distributions (SEDs). Since the full width at half maximum (FWHM) and the peak power of the images of point sources change significantly depending on the wavelength, color correction should be considered especially for the wide band in the data reduction processes. We scan a point ource locating at different relative positions from the scanning path, and examine how well we can determine the position and the power of the source. We assume an ideal situation; no noise, constant background, and monochromatic light. We find that fitting by the point spread function (PSF) is indispensable for the accurate measurement of the source flux and position. Since PSF changes with the color of the object, the shape of the PSF itself would be a fitting parameter. We find that second scan improves the reliability of the PSF fit.

We report the discovery in the ISO/SWS spectrum of the post-AGE star HR 4049 of emission bands due to O-17 and O-18 isotopes locked up in CO2 molecules. It is the first time these isotopomers are detected outside the solar system. Isotopic ratios derived in the optically thin limit are as low as O-16/O-17 = 8.3 +/- 2.3 and O-16/O-18 = 6.9 +/- 0.9. These values are at least one order of magnitude lower than ally previously determined isotopic ratio in any type of evolved star.

We present infrared spectroscopy and millimeter photometry and spectroscopy of the peculiar carbon star IRAS 09425-6040. The 2-15 mum spectrum, as well as the CO millimeter line observations are typical for a (J-type) carbon star with moderate mass-loss rate. The 15-45 mum spectrum is dominated by strong emission bands from Mg-rich and Fe-poor crystalline silicates. IRAS 09425-6040 has the highest abundance of crystalline silicates (75 per cent) observed in any source so far. The ISO data, combined with IRAS and millimeter wavelength photometry indicate the presence of large cold grains. The observations indicate that the carbon star IRAS 09425-6040 is surrounded by a stationary, massive; highly crystalline oxygen-rich dust disk which is depleted of gas. These properties are very similar to those of tl-le disk seen in the Red Rectangle. We propose that IRAS 09425-6040 is the evolutionary progenitor of the central binary of the Red Rectangle nebula.

We report the discovery in the ISO/SWS spectrum of the post-AGE star HR 4049 of emission bands due to O-17 and O-18 isotopes locked up in CO2 molecules. It is the first time these isotopomers are detected outside the solar system. Isotopic ratios derived in the optically thin limit are as low as O-16/O-17 = 8.3 +/- 2.3 and O-16/O-18 = 6.9 +/- 0.9. These values are at least one order of magnitude lower than ally previously determined isotopic ratio in any type of evolved star.

We present infrared spectroscopy and millimeter photometry and spectroscopy of the peculiar carbon star IRAS 09425-6040. The 2-15 mum spectrum, as well as the CO millimeter line observations are typical for a (J-type) carbon star with moderate mass-loss rate. The 15-45 mum spectrum is dominated by strong emission bands from Mg-rich and Fe-poor crystalline silicates. IRAS 09425-6040 has the highest abundance of crystalline silicates (75 per cent) observed in any source so far. The ISO data, combined with IRAS and millimeter wavelength photometry indicate the presence of large cold grains. The observations indicate that the carbon star IRAS 09425-6040 is surrounded by a stationary, massive; highly crystalline oxygen-rich dust disk which is depleted of gas. These properties are very similar to those of tl-le disk seen in the Red Rectangle. We propose that IRAS 09425-6040 is the evolutionary progenitor of the central binary of the Red Rectangle nebula.

Structure of the extended atmosphere in AGB stars has been investigated by analyzingH2O bands in the near-infrared region. Based on simple model analysis, we concluded that the time variation in the H2O bands with the variability phase can be attributed to the variation in the physical structure of the extended atmosphere, possibly caused by the shocks in the inner region. High wavelength/spatial resolution observations are crucial for further investigation of thestructure.

The origin of silicate carbon stars has been a mystery ever since their discovery. We discuss here a full grating spectrum between 2.4 and 45 mum of the silicate carbon star V778 Cyg obtained by the ISO/SWS. The spectrum, taken about 14 years after the IRAS LRS observation, confirms the complex nature of the object. The spectrum is clearly divided into a short wavelength (lambda < 6.5 <mu>m), carbon-rich part and long-wavelength, oxygen-rich part. No obvious change of the 10 and 18 mum silicate features is observed between IRAS and ISO spectra, indicating that the silicate dust is in a steady structure. The 2.7 mum H2O band and the 15 mum CO2 bands are tentatively detected. The near-infrared part of the spectrum indicates that the present-day mass-loss rate is very low. The silicate features can only be fitted by optically thin dust emission from sub-micron size grains. The total oxygen-rich dust mass seen at infrared wavelengths is 2-10x10(-6) M., of which 3-50x10(-8) M. is warm (300-600 K). If the dust is heated by radiation from the central star, the dust should be located as close as about 12 stellar radii from the star. We suggest that the dust responsible for the emission features is in a steady outflow from the system. We show that the dust cannot be located in a circum-binary disk, but is stored in a disk around the companion star during the previous O-rich mass-loss phase. The duration of silicate emission is estimated as similar to 10(4) yr. It is compatible with the fact that not all J-type carbon stars show silicate emission. The evolution of the central star and formation of the disk in AGE binary systems largely depends on the orbital separation. V778 Cyg and other "IRAS discovered" silicate carbon stars probably have wide orbits. In such a case, a disk is formed around the companion. Close-binary systems such as the Red Rectangle form massive equatorial O-rich disks, and the evolution of the central star is largely influenced by the binarity.