Yoshifusa Ita

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

The Infrared Camera (IRC) for AKARI–Design and Imaging Performance

The Infrared Camera (IRC) is one of two focal-plane instruments on the AKARI satellite. It is designed for wide-field deep imaging and low-resolution spectroscopy in the nearto mid-infrared (1.8–26.5 m) in the pointed observation mode of AKARI. The IRC is also operated in the survey mode to make an All-Sky Survey at 9 and 18 m. It comprises three channels. The NIR channel (1.8–5.5 m) employs a 512 412 InSb array, whereas both the MIR-S (4.6–13.4 m) and MIR-L (12.6–26.5 m) channels use 256 256 Si:As impurity band conduction arrays. Each of the three channels has a field-of-view of about 100 100, and they are operated simultaneously. The NIR and MIR-S share the same field-of-view by virtue of a beam splitter. The MIR-L observes the sky about 250 away from the NIR/MIR-S field-of-view. The IRC gives us deep insights into the formation and evolution of galaxies, the evolution of planetary disks, the process of star-formation, the properties of interstellar matter under various physical conditions, and the nature and evolution of solar system objects. The in-flight performance of the IRC has been confirmed to be in agreement with the pre-flight expectation. This paper summarizes the design and the in-flight operation and imaging performance of the IRC.

Variable stars in the Magellanic Clouds: results from OGLE and SIRIUS

We have performed a cross-identification between Optical Gravitational Lensing Experiment II (OGLE-II) data and single-epoch Simultaneous three-colour Infrared Imager for Unbiased Surveys (SIRIUS) near-infrared (NIR) JHK survey data in the Large and Small Magellanic Clouds (LMC and SMC, respectively). After eliminating obvious spurious variables, variables with too few good data and variables that seem to have periods longer than the available baseline of the OGLE-II data, we determined the pulsation periods for 8852 and 2927 variables in the LMC and SMC, respectively. Based on these homogeneous data, we studied the pulsation properties and metallicity effects on period–K magnitude (PK) relations by comparing the variable stars in the LMC and SMC. The sample analysed here is much larger than the previous studies, and we found the following new features in the PK diagram. (1) Variable red giants in the SMC form parallel sequences on the PK plane, just like those found by Wood in the LMC. (2) Both sequences A and B of Wood have discontinuities, and they occur at the K-band luminosity of the tip of the red giant branch. (3) The sequence B of Wood separates into three independent sequences B± and C′. (4) A comparison between the theoretical pulsation models and observational data suggests that the variable red giants on sequences C and newly discovered C′ are pulsating in the fundamental and first overtone modes, respectively. (5) The theory cannot explain the pulsation mode of sequences A± and B±, and they are unlikely to be the sequences for the first and second overtone pulsators, as was previously suggested. (6) The zero-points of PK relations of Cepheids in the metal deficient SMC are fainter than those of the LMC by ≈0.1 mag but those of SMC Miras are brighter than those of the LMC by ≈0.13 mag (adopting the distance modulus offset between the LMC and SMC to be 0.49 mag and assuming the slopes of the PK relations are the same in the two galaxies), which are probably due to metallicity effects.

Variable stars in the Magellanic Clouds – II. The data and infrared properties

The data of 8852 and 2927 variable stars detected by the OGLE survey in the Large and Small Magellanic Clouds are presented. They are cross-identified with the SIRIUS JHK survey data, and their infrared properties are discussed. Variable red giants are well separated on the period-(J - K) plane, suggesting that it could be a good tool to distinguish their pulsation mode and type.

Asymptotic giant branch stars in the Fornax dwarf spheroidal galaxy

We report on a multi-epoch study of the Fornax dwarf spheroidal galaxy, made with the Infrared Survey Facility, over an area of about 42 × 42 arcmin 2 . The colour–magnitude diagram shows a broad well-populated giant branch with a tip that slopes downwards from red to blue, as might be expected given Fornax’s known range of age and metallicity. The extensive asymptotic giant branch (AGB) includes seven Mira variables and 10 periodic semiregular variables. Five of the seven Miras are known to be carbon rich. Their pulsation periods range from 215 to 470 d, indicating a range of initial masses. Three of the Fornax Miras are redder than typical Large Magellanic Cloud (LMC) Miras of similar period, probably indicating particularly heavy mass-loss rates. Many, but not all, of the characteristics of the AGB are reproduced by isochrones from Marigo et al. for a 2 Gyr population with a metallicity of Z = 0.0025. An application of the Mira period–luminosity relation to these stars yields a distance modulus for Fornax of 20.69 ± 0.04 (internal), ±0.08 (total) (on a scale that puts the LMC at 18.39 mag) in good agreement with other determinations. Various estimates of the distance to Fornax are reviewed.

Spectroscopic observations of ices around embedded young stellar objects in the Large Magellanic Cloud with AKARI

Aims. The aim of this study is to understand the chemical conditions of ices around embedded young stellar objects (YSOs) in the metal-poor Large Magellanic Cloud (LMC). Methods. We performed near-infrared (2.5-5 μm) spectroscopic observations toward 12 massive embedded YSOs and their candidates in the LMC using the infrared camera (IRC) onboard AKARI. We estimated the column densities of the H 2 O, CO 2 , and CO ices based on their 3.05, 4.27, and 4.67 μm absorption features, and we investigated the correlation between ice abundances and physical properties of YSOs. Results. The ice absorption features of H 2 O, CO 2 , 13 CO 2 , CO, CH 3 0H, and possibly XCN are detected in the spectra. In addition, hydrogen recombination lines and PAH emission bands are detected toward the majority of the targets. The derived typical CO 2 /H 2 O ice ratio of our samples (~0.36 ± 0.09) is greater than that of Galactic massive YSOs (~0.17 ± 0.03), while the CO/H 2 O ice ratio is comparable. It is shown that the CO 2 ice abundance does not correlate with the observed characteristics of YSOs: the strength of hydrogen recombination line and the total luminosity. Likewise, clear correlation is not seen between the CO ice abundance and YSO characteristics, but it is suggested that the CO ice abundance of luminous samples is significantly lower than in other samples. Conclusions. The systematic difference in the C02 ice abundance around the LMCs massive YSOs, which was suggested by previous studies, is confirmed with the new near-IR data. We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC are responsible for the observed high abundance of the CO 2 ice. It is suggested that the internal stellar radiation does not play an important role in the evolution of the CO 2 ice around a massive YSO, while more volatile molecules like CO are susceptible to the effect of the stellar radiation.

The period–luminosity relation for type II Cepheids in globular clusters

We report the result of our near-infrared observations (JHK s ) for type II Cepheids (including possible RV Tau stars) in galactic globular clusters. We detected variations of 46 variables in 26 clusters (10 new discoveries in seven clusters) and present their light curves. Their periods range from 1.2 d to over 80 d. They show a well-defined period-luminosity relation at each wavelength. Two type II Cepheids in NGC 6441 also obey the relation if we assume the horizontal branch stars in NGC 6441 are as bright as those in metal-poor globular clusters in spite of the high metallicity of the cluster. This result supports the high luminosity which has been suggested for the RR Lyr variables in this cluster. The period-luminosity relation can be reproduced using the pulsation equation (P√ρ = Q) assuming that all the stars have the same mass. Cluster RR Lyr variables were found to lie on an extrapolation of the period-luminosity relation. These results provide important constraints on the parameters of the variable stars. Using Two Micron All-Sky Survey (2MASS) data, we show that the type II Cepheids in the Large Magellanic Cloud (LMC) fit our period-luminosity relation within the expected scatter at the shorter periods. However, at long periods (P > 40 d, i.e. in the RV Tau star range) the LMC field variables are brighter by about one magnitude than those of similar periods in galactic globular clusters. The long-period cluster stars also differ from both these LMC stars and galactic field RV Tau stars in a colour-colour diagram. The reasons for these differences are discussed.

NEAR-INFRARED IMAGING OBSERVATIONS OF THE N159/N160 COMPLEX IN THE LARGE MAGELLANIC CLOUD: LARGE CLUSTERS OF HERBIG Ae/Be STARS AND SEQUENTIAL CLUSTER FORMATION

We have carried out deep near-infrared imaging observations of the N159/N160 star-forming region in the Large Magellanic Cloud. We observed an area of ~380 arcmin2 (~80,000 pc2 at the distance of the LMC) in the J, H, and Ks bands. The observations are deep enough to detect Herbig Ae/Be stars down to ~3 M⊙ in the LMC. We discovered a total of 338 and 464 candidate Herbig Ae/Be and OB stars, respectively, based on the near-infrared colors and magnitudes. The Herbig Ae/Be candidates comprise 10 clusters, the OB star candidates 13. We discovered an embedded Herbig Ae/Be cluster in the N159 East giant molecular cloud (GMC) and a Herbig Ae/Be cluster at the northeast tip of the N159 South GMC. Together with two neighboring H II regions, the Herbig Ae/Be cluster at the tip of the N159S GMC provides a hint of the beginning of sequential cluster formation in N159S. The spatial distributions of the Herbig Ae/Be and OB clusters, in conjunction with previously known optical clusters and embedded massive stars, indicate (1) sequential cluster formation within each of the N159 and N160 star-forming regions and (2) large-scale sequential cluster formation over the entire observed region from N160 to N159S. Possible triggers for the large-scale cluster formation are the supergiant shell SGS 19 and an expanding superbubble. Some of the Herbig Ae/Be clusters in the N159/N160 complex are significantly larger in spatial extent than pre–main-sequence clusters of similar age in the Milky Way. Highly turbulent gas motion in the LMC is probably responsible for forming the large young clusters.

AKARI Near-Infrared Spectroscopy: Detection of H2O and CO2 Ices toward Young Stellar Objects in the Large Magellanic Cloud

We present the first results of AKARI Infrared Camera near-infrared spectroscopic survey of the Large Magellanic Cloud (LMC). We detected absorption features of the H2O ice 3.05 μm and the CO2 ice 4.27 μm stretching mode toward seven massive young stellar objects (YSOs). These samples are for the first time spectroscopically confirmed to be YSOs. We used a curve-of-growth method to evaluate the column densities of the ices and derived the CO2/H2O ratio to be 0.45 ± 0.17. This is clearly higher than that seen in Galactic massive YSOs (0.17 ± 0.03). We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC may be responsible for the observed high CO2 ice abundance.

Pulsation at the tip of the first giant branch

The first results of our ongoing near-infrared (NIR) survey of the variable red giants in the Large Magellanic Cloud, using the Infrared Survey Facility (IRSF) and the SIRIUS infrared camera, are presented. Many very red stars were detected and we found that most of them are variables. In the observed colour‐magnitude diagram (J −K, K ) and the stellar K magnitude distribution, the tip of the first giant branch (TRGB), where helium burning in the core starts, is clearly seen. Apart from the genuine AGB variables, we found many variable stars at luminosities around the TRGB. From this result, we infer that a substantial fraction of them are RGB variables.