Issei Yamamura

We present a general surface brightness correction method for compact extended sources imaged in the slow-scan pointed observation mode of the Far-Infrared Surveyor (FIS) aboard the AKARI infrared astronomical satellite. Our method recovers correct surface brightness distribution maps by rescaling archived raw FIS maps using the surfacebrightness-dependent inverse FIS response function. The flux of a target source is then automatically corrected for as the simple sum of surface brightnesses within the adopted contour encircling the perimeter of the target (i.e., contour photometry). This correction method is contrasted with the previous aperture photometry method for point sources, which directly corrects for the target flux with a flux-dependent scaling law. The new surface brightness correction scheme is applicable to objects of any shape from unresolved point sources to resolved extended objects, as long as the target is not deemed diffuse, i.e., the total extent of the target source does not exceed too much more than a single FIS scan width of 10. The new correction method takes advantage of the well-defined shape (i.e., the scale invariance) of the point spread function, which enables us to adopt a power-law FIS response function. We analyze the point source photometric calibrator data using the FIS AKARI Slow-scan Tool and constrain the parameters of the adopted power-law FIS response function. We conclude that the photometric accuracy of the new correction method is better than 10% error based on comparisons with the expected fluxes of the photometric calibrators, and that resulting fluxes without the present correction method can lead to up to 230% overestimates or down to 50% underestimates.

The detection of the CO2 absorption band at 4.2 mu(m) in brown dwarf spectra by AKARI has made it possible to discuss CO2 molecular abundance in brown dwarf atmospheres. In our previous studies, we found an excess in the 4.2 mu m CO2 absorption band of three brown dwarf spectra, and suggested that these deviations were caused by high C and O elemental abundances in their atmospheres. To validate this hypothesis, we have constructed a set of models of brown dwarf atmospheres with various elemental abundance patterns, and we investigate the variations of the molecular composition and the thermal structure, and how they affect the near-infrared spectra between 1.0 and 5.0 mu m. The 4.2 mu m CO2 absorption band in some late-L and T dwarfs taken by AKARI is stronger or weaker than predicted by corresponding models with solar abundance. By comparing the CO2 band in the model spectra to the observed near-infrared spectra, we confirm possible elemental abundance variations among brown dwarfs. We find that the band strength is especially sensitive to O abundance, but C is also needed to reproduce the entire near-infrared spectra. This result indicates that both the C and O abundances should increase and decrease simultaneously for brown dwarfs. We find that a weaker CO2 absorption band in a spectrum can also be explained by a model with lower "C and O" abundances.

We derive the radii of 16 brown dwarfs observed by AKARI using their parallaxes and the ratios of observed to model fluxes. We find that the brown dwarf radius ranges between 0.64-1.13 RJ with an average radius of 0.83 RJ . We find a trend in the relation between radii and T eff the radius is at a minimum at T eff ∼ 1600 K, which corresponds to the spectral types of mid- to late-L. The result is interpreted by a combination of radius-mass and radius-age relations that are theoretically expected for brown dwarfs older than 108 yr. © 2013. The American Astronomical Society. All rights reserved..

We investigate variations in the strengths of three molecular bands, CH4 at 3.3 mu m, CO at 4.6 mu m, and CO2 at 4.2 mu m, in 16 brown dwarf spectra obtained by AKARI. Spectral features are examined along the sequence of source classes from L1 to T8. We find that the CH4 3.3 mu m band is present in the spectra of brown dwarfs later than L5, and the CO 4.6 mu m band appears in all spectral types. The CO2 absorption band at 4.2 mu m is detected in late-L and T-type dwarfs. To better understand brown dwarf atmospheres, we analyze the observed spectra using the Unified Cloudy Model. The physical parameters of the AKARI sample, i.e., atmospheric effective temperature T-eff, surface gravity log g, and critical temperature T-cr, are derived. We also model IRTF/SpeX and UKIRT/CGS4 spectra in addition to the AKARI data in order to derive the most probable physical parameters. Correlations between the spectral type and the modeled parameters are examined. We confirm that the spectral-type sequence of late-L dwarfs is not related to T-eff, but instead originates as a result of the effect of dust.

We present the discovery of a source with broadband infrared photometric characteristics similar to Sakurai's object. WISE J180956.27-330500.2 (hereafter J1810-3305) shows very red WISE colors, but a very blue 2MASS [K] versus WISE [W1 (3.4 mu m)] color. It was not visible during the IRAS era, but now has a 12 mu m flux well above the IRAS point-source catalog detection limit. There are also indications of variability in historical optical photographic plates as well as in multi-epoch AKARI mid-infrared measurements. The broadband infrared spectral energy distribution (SED) shape, post-IRAS brightening, and multiwavelength variability are all characteristics also shared by Sakurai's object-a post-asymptotic giant branch (post-AGB) star which underwent a late thermal pulse and recently ejected massive envelopes of dust that are currently expanding and cooling. Optical progenitor colors suggest that J1810-3305 may have been of late spectral class. Its dramatic infrared brightening and the detection of a late-type optical counterpart are consistent with a scenario in which we have caught an extremely massive dust ejection event (in 1998 or shortly before) during the thermal pulse of an AGB star, thus providing a unique opportunity to observe stellar evolution in this phase. J1810-3305 is the only source in the entire WISE preliminary data release with similar infrared SED and variability, emphasizing the rarity of such sources. Confirmation of its nature is of great importance.

Context. Low-to intermediate-mass stars lose a significant fraction of their mass while they are on the asymptotic giant branch (AGB). This mass loss is considered to determine the final stages of their evolution. The material ejected from the stellar photosphere forms a circumstellar envelope in its surroundings. Layers of circumstellar envelope constitute the footprint of mass-loss history. Aims. Our aim is to probe the mass-loss history in the carbon star U Hya in the last similar to 10(4) years by investigating the distribution of dust in the circumstellar envelope with high spatial resolution. Methods. We observed U Hya in the far-infrared (FIR) at 65, 90, 140, and 160 mu m simultaneously, using the slow scan observing mode of the far-infrared surveyor (FIS) aboard the infrared astronomical satellite AKARI. It produced a map of similar to 10' x 40' in size in each band. Results. The FIS maps reveal remarkably circular, ring-like emission structure almost centered on the star, showing the presence of a detached, spherical dust shell. A hollow dust shell model gives the inner radius R-in of 101-107 '' [(2.5-2.6) x 1017 cm], thickness that covers a half of the total dust mass Delta R-hm of 16-23 '' [(3.8-5.6) x 10(16) cm], which gives Delta R-hm/R-in similar to 0.2, and the power-law index of the dust opacity distribution of 1.10-1.15. The dust mass in the shell is well-constrained to be (0.9-1.4) x 10(-4) (kappa(100)/25)(-1) M-circle dot, where kappa(100) is the dust absorptivity at 100 mu m in units of cm(2) g(-1). The dust mass-loss rate at R-in is found to be (1.89.6) x 10(-8)(kappa(100)/25)(-1)(v(e)/15) M-circle dot yr(-1), which shows that the total mass-loss rate in the shell is at least an order of magnitude higher than the current rate, where v(e) is the outflow velocity at R-in in units of km s(-1). An extension of FIR emission along PA similar to -70 degrees is found out to similar to 5' from the star, which is probably a ram-stripping wake by the ISM wind. We also find excess FIR emission that might indicate the earliest departure from spherical symmetry in the AGB mass loss inside the shell. Conclusions. The dust shell of U Hya is a hollow sphere and has an effective width that is narrower than the spatial resolution of the FIS. It could be formed as either a direct consequence of a thermal pulse, a result of two-wind interaction induced by a thermal pulse, a termination shock, or any combination of these processes.

We report the results of the near-infrared spectroscopic observations of brown dwarfs with the AKARI/IRC. The spectra covering 2.5-5.0 mu m are significantly better quality than any previous observations ever made from the ground. So far spectra of seven objects are available. Their SEDs are consistent in principle with the prediction by the numerical model atmospheres including dust formation (Unified Cloudy Models; UCMs). We confirm the detection of CO absorption band in late-T dwarfs, supporting the idea of non-equilibrium processes contributing on the chemical abundance in these objects. In addition, we detect the CO2 band for the first time in brown dwarfs. The band commonly presents in the T-dwarfs. This could be a hint to understand the chemistry in the brown dwarf atmospheres.

The IRTS data archive has been in public since 2002. IRTS surveyed about 7 per cent of the whole sky with four instruments, which covered from 1.4 to 700 μm. Presently the archive includes the near- and mid-infrared low resolution spectral catalogues of point sources, and image maps in five wavelength bands in the far-infrared. The point source catalogues contains over 14 000 (near-infrared) and 500 (mid-infrared) sources. The majority of detected sources are late-type stars. These large samples of uniform spectra are especially useful for statistical studies of infrared properties of stars. The far infrared image maps are obtained for the 158 μm [C II] line, and continuum bands at 155, 250, 400, and 700 μm. Radiation from the diffuse interstellar components has been studied with these data. More data products will be available in a few year time-scale. The IRTS data can be accessed via ISAS's data archive service DARTS, URL: http://www.darts.isas.ac.jp/.

We analyze ISO/SWS spectra of the RV Tau star R Scuti. The infrared spectra are dominated by H2O emission bands. The near- and mid-infrared excess is attributed to H2O; the dust contribution is less important. We also identify CO, SiO and CO2 bands. The various molecular emission bands originate from an extended atmosphere, an atmosphere above the photosphere. The extended atmosphere of R Sct is formed from matter which gradually have lifted up from the photosphere through the pulsations of the star. In contrast to the abundant molecules around the star, the silicate dust feature is weak and the dust mass-loss rate is only (M) over dot(d) = 10(-11) M-. yr(-1). This implies that there might be a process to inhibit dust formation from molecules. RV Tau stars are commonly considered as post-AGB stars. While a detached dust envelope around R Sct is consistent with such an interpretation, we show that its period evolution is slower than expected. We argue that R Sct may be a thermal-pulsing AGB star, observed in a helium-burning phase.

The impact of the ISO/SWS on the study of AGE stars is reviewed. The ISO/SWS has provided high quality, intermediate resolution spectra covering a wide infrared wavelength range. These data enable us, for the first time, to perform an extensive study of the outer atmosphere and inner circumstellar regions of AGE stars. Many new and unexpected features hidden by the terrestrial atmosphere are detected, which raise new questions. We here particularly focus on the detection of dioxides in the O-rich stars, of C(3), C(2)H(2), and HCN in carbon stars, and of atomic / ionic fine-structure lines. We discuss the interpretation of these results and list problems to be solved.

The mid-infrared spectrometer on board Infrared Telescope in Space has detected thousands of pointlike sources in the spectral range of 4.5-11.7 mu m. Extraction of point sources from the survey data has been carried out for the observing period after the spin-flip, where accurate pointing information is presently available, and 129 point sources have been extracted with signal-to-noise ratios greater than 3 throughout the MIRS spectral range. A two-color diagram of these objects, derived from bands of 5 mu m, 8 mu m, and 10 mu m of the sources, is presented. The sources can be classified into five groups on the diagram: visible late-type stars, stars with dust envelopes, stars with silicate emission, objects with the unidentified infrared (UIR) emission bands, and very peculiar red objects. With the help of the 5 mu m and 8 Irm colors, objects with the UIR bands can clearly be distinguished from objects without those features. The characteristics of the sources in each group are discussed based on the spectra.

We report the results of mapping observations of a proto-planetary nebula CRL 2688 in the 13CO J = 1-0 line with a beam size of 4''.4 x 3''.9 with Nobeyama Millimeter Array. Emission of 13CO is extended by about 15'', which is about one-fourth of the CO envelope size obseved by single-dish telescopes. The channel velocity maps show that 13CO emission consists of three components: a spherically symmetric high-brightness core at the center, a less bright extended envelope with clumpy structure, and a high-velocity component near the center. The high-brightness central core is found to extend by about 10'' and to appear spherically symmetric. The disk- or torus-like structure which was previously observed by others in several molecular lines is not seen in the 13CO maps. This indicates that the gas density in the envelope does not vary much with the polar angle. The extended component is observed to spread along the symmetry axis. The peak positions of the red- and blueshifted features of the high-velocity component are found to be separated by about 3''.

The circumstellar envelope of the carbon-rich star CRL 618 has been mapped by the (CO)-C-13 J = 1-0 transition with a resolution of about 5''.0 x 4''.3 with the Nobeyama Millimeter Array. Emission is extended in size by about 10'', and the distribution is approximately spherical. No strong elongation of (CO)-C-12 features along or perpendicular to the bipolar axis has been observed. Combining with the (CO)-C-12 J = 1-0 data that were taken before, we have created three-dimensional density and temperature profiles of the CRL 618 circumstellar envelope. For creating the three-dimensional model, it is assumed that the expansion velocity is constant and the large-velocity-gradient model is applicable. The former (CO)-C-12 observations show that the envelope of CRL 618 consists of complex features. On the basis of the present (CO)-C-13 observations, these features are interpreted as the temperature and density irregularities in the outer circumstellar shell. The comparison of the present results with the model density profile given by optical light-scattering maps is made.