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Register a new userObservations of the Optical Transient in NGC 300 with AKARI/IRC: Possibilities of Asymmetric Dust Formation
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We present the results of near-infrared (NIR) multi-epoch observations of the optical transient in the nearby galaxy NGC300 (NGC300-OT) at 398 and 582 days after the discovery with the Infrared Camera (IRC) onboard AKARI. NIR spectra (2--5 um) of NGC300-OT were obtained for the first time. They show no prominent emission nor absorption features, but are dominated by continuum thermal emission from the dust around NGC300-OT. NIR images were taken in the 2.4, 3.2, and 4.1 um bands. The spectral energy distributions (SED) of NGC300-OT indicate the dust temperature of 810 (+-14) K at 398 days and 670 (+-12) K at 582 days. We attribute the observed NIR emission to the thermal emission from dust grains formed in the ejecta of NGC300-OT. The multi-epoch observations enable us to estimate the dust optical depth as larger than about 12 at 398 days and larger than about 6 at 582 days at 2.4 um, by assuming an isothermal dust cloud. The observed NIR emission must be optically thick, unless the amount of dust grains increases with time. Little extinction at visible wavelengths reported in earlier observations suggests that the dust cloud around NGC300-OT should be distributed inhomogeneously so as to not screen the radiation from the ejecta gas and the central star. The present results suggest the dust grains are not formed in spherically symmetric geometry, but rather in a torus, a bipolar outflow, or clumpy cloudlets.
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A luminous optical transient (OT) that appeared in NGC 300 in early 2008 had a maximum brightness, M_V ~ -12 to -13, intermediate between classical novae and supernovae. We present ground-based photometric and spectroscopic monitoring and adaptive-optics imaging of the OT, as well as pre- and post-outburst space-based imaging with HST and Spitzer. The optical spectrum at maximum showed an F-type supergiant photosphere with superposed emission lines of hydrogen, Ca II, and [Ca II], similar to the spectra of low-luminosity Type IIn supernova impostors like SN 2008S, as well as cool hypergiants like IRC +10420. The emission lines have a complex, double structure, indicating a bipolar outflow with velocities of ~75 km/s. The luminous energy released in the eruption was ~10^47 ergs, most of it emitted in the first 2 months. By registering new HST images with deep archival frames, we have precisely located the OT site, and find no detectable optical progenitor brighter than broad-band V magnitude 28.5. However, archival Spitzer images reveal a bright, non-variable mid-IR pre-outburst source. We conclude that the NGC 300 OT was a heavily dust-enshrouded luminous star, of ~10-15 Msun, which experienced an eruption that cleared the surrounding dust and initiated a bipolar wind. The progenitor was likely an OH/IR source which had begun to evolve on a blue loop toward higher temperatures, but the precise cause of the outburst remains uncertain.
We present low spectral resolution molecular interferometric observations at 1.2 mm obtained with the Combined Array for Research in Millimetre-wave Astronomy (CARMA) towards the C-rich AGB star IRC+10216. We have mapped the emission of several lines of SiS, H13CN, SiO, and SiC2 in the ground and first excited vibrational states with a high angular resolution of 0.25 arcsec. These observations have allowed us to partially resolve the emission of the envelope at distances from the star <50 stellar radii (R*), where the stellar wind is mainly accelerated. The structure of the molecular emission has been modelled with a 3D radiation transfer code. The emission of line SiS(v=0,J=14-13) is best reproduced with a set of maser emitting arcs arranged between 5 and 20 R*. The abundance of H13CN with respect to H2 decreases from 8e-7 at 1-5 R* to 3e-7 at 20 R*. The SiO observations are explained with an abundance <2e-8 in the shell-like region between 1 and 5 R*. At this point, the SiO abundance sharply increases up to (2-3)e-7. The vibrational temperature of SiO increases by a factor of 2 due North-East between 20 and 50 R*. SiC2 is formed at the stellar surface with an abundance of 8e-7 decreasing down to 8e-8 at 20 R* probably due to depletion on to dust grains. Several asymmetries are found in the abundance distributions of H13CN, SiO, and SiC2 which define three remarkable directions (North-East, South-Southwest, and South-East) in the explored region of the envelope. There are some differences between the red- and blue-shifted emissions of these molecules suggesting the existence of additional asymmetries in their abundance distributions along the line-of-sight.
We present the characterization and calibration of the slow-scan observation mode of the Infrared Camera (IRC) on-board AKARI. The IRC slow-scan observations were operated at the S9W (9 $mu$m) and L18W (18 $mu$m) bands. We have developed a toolkit for data reduction of the IRC slow-scan observations. We introduced a self-pointing reconstruction method to improve the positional accuracy to as good as 1. The sizes of the point spread functions were derived to be $sim6$ at the S9W band and $sim7$ at the L18W bands in full width at half maximum. The flux calibrations were achieved with the observations of 3 and 4 infrared standard stars at the S9W and L18W bands, respectively. The flux uncertainties are estimated to be better than 20% from comparisons with the AKARI IRC PSC and the WISE preliminary catalog.
We present an asteroidal catalog from the mid-infrared wavelength region using the slow-scan observation mode obtained by the Infrared Camera (IRC) on-board the Japanese infrared satellite AKARI. An archive of IRC slow-scan observations comprising about 1000 images was used to search for serendipitous encounters of known asteroids. We have determined the geometric albedos and diameters for 88 main-belt asteroids, including two asteroids in the Hilda region, and compared these, where possible, with previously published values. Approximately one-third of the acquired data reflects new asteroidal information. Some bodies classified as C or D-type with high albedo were also identified in the catalog.
The Spitzer Space Telescope was used to study the mid-infrared to far-infrared properties of NGC 300, and to compare dust emission to Halpha to elucidate the heating of the ISM and the star formation cycle at scales < 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 micron dust emission with respect to 24 micron dust and to HII regions traced by the Halpha light. The 8 micron emission highlights the rims of HII regions, and the 24 micron emission is more strongly peaked in star forming regions than at 8 microns. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 microns, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star forming regions. When averaging over regions larger than ~ 1 kpc, the ratio of Halpha to Aromatic Feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid-to-far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.
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