11:00 〜 11:15
[PCG19-02] Investigation of mid-infrared long-term variability of dusty AGB stars by using multi-epoch scan data of AKARI and WISE
キーワード：ダスト、AGB星、中間赤外線
“How has the Universe evolved to today’s material-rich form?” This is one of the fundamental questions in modern astronomy. Dust is a key component of the life cycle of heavy elements in the Universe, but its origin is not well understood yet. Recent studies indicate that asymptotic giant branch (AGB) stars are one of the main suppliers of dust. Those stars show very strong pulsations of the photosphere, and the stellar pulsation is thought to control formation of dust. However the details have not been revealed yet. Especially observational studies of dusty AGB stars are insufficient so far, because they are relatively faint in optical wavelengths due to the heavy circumstellar dust extinction. Infrared observations of dusty AGB stars, especially mid-infrared monitoring observations, are strongly desired.
In this study, we analyzed multi-epoch scan data of AKARI and WISE and produced long-term mid-infrared light curves of AGB stars. Known oxygen-rich (O-rich) AGB stars and OH/IR stars in our Galaxy were selected for this analysis. One technical issue was the difference of photometric systems of AKARI and WISE. We carefully estimated fluxes at 18 microns by using WISE W3 (12 microns) and W4 (22 microns) data and compared with AKARI L18W (18 microns) data. Finally, we obtained 18-micron light curves with at least 4 epochs of 169 O-rich AGB stars and 35 OH/IR stars. Its time coverage is more than 1300 days. Those are the longest and largest mid-infrared light curves of AGB stars.
Based on these light curves, as well as variability data at other wavelengths, we determined the variability amplitude at 18 microns for 169 O-rich AGB stars and 28 OH/IR stars.
We found that 1) there is a strong correlation between the 18-micron amplitude and W3-W4 color, and 2) this relationship is independent of the variability period and their pulsation mode. Therefore, this correlation indicates that stars with stronger variability show stronger dust mass loss, and the variability amplitude is the most important parameter for the dust supply mechanism.
Furthermore, we constructed models in which only the stellar temperature and luminosity were varied, to identify the stellar parameter changes that significantly affect the variability amplitude. The radiative transfer code DUSTY was used for this calculation. As a result the variability amplitude at 18 microns is not dependent on variation of stellar temperature, but strongly affected by the rate of luminosity change. Therefore, the correlation between the 18-micron amplitude and the infrared color suggests that the stellar luminosity change is the most important factor for the dust formation around AGB stars.
There are no detailed theoretical models to explain this correlation. A possible explanation is that AGB stars with larger variation of luminosity have larger region where the temperature decreases during the pulsation, and the larger size of the temperature-decreasing region causes more amount of dust formation.
For the next step, we have a plan of long-term mid-infrared monitoring of dusty AGB stars using the TAO (The University of Tokyo Atacama Observatory) 6.5-m telescope. Since MIMIZUKU, the mid-infrared instrument for the TAO 6.5-m telescope, has high-precision monitor observation capability, we can obtain mid-infrared light curves with sufficient number of epochs. In addition, we will be able to obtain time variation data of mid-infrared spectra including silicate and alumina features. They may be helpful information to understand the detailed process of the dust formation. TAO will start science operations in 2022.
In this study, we analyzed multi-epoch scan data of AKARI and WISE and produced long-term mid-infrared light curves of AGB stars. Known oxygen-rich (O-rich) AGB stars and OH/IR stars in our Galaxy were selected for this analysis. One technical issue was the difference of photometric systems of AKARI and WISE. We carefully estimated fluxes at 18 microns by using WISE W3 (12 microns) and W4 (22 microns) data and compared with AKARI L18W (18 microns) data. Finally, we obtained 18-micron light curves with at least 4 epochs of 169 O-rich AGB stars and 35 OH/IR stars. Its time coverage is more than 1300 days. Those are the longest and largest mid-infrared light curves of AGB stars.
Based on these light curves, as well as variability data at other wavelengths, we determined the variability amplitude at 18 microns for 169 O-rich AGB stars and 28 OH/IR stars.
We found that 1) there is a strong correlation between the 18-micron amplitude and W3-W4 color, and 2) this relationship is independent of the variability period and their pulsation mode. Therefore, this correlation indicates that stars with stronger variability show stronger dust mass loss, and the variability amplitude is the most important parameter for the dust supply mechanism.
Furthermore, we constructed models in which only the stellar temperature and luminosity were varied, to identify the stellar parameter changes that significantly affect the variability amplitude. The radiative transfer code DUSTY was used for this calculation. As a result the variability amplitude at 18 microns is not dependent on variation of stellar temperature, but strongly affected by the rate of luminosity change. Therefore, the correlation between the 18-micron amplitude and the infrared color suggests that the stellar luminosity change is the most important factor for the dust formation around AGB stars.
There are no detailed theoretical models to explain this correlation. A possible explanation is that AGB stars with larger variation of luminosity have larger region where the temperature decreases during the pulsation, and the larger size of the temperature-decreasing region causes more amount of dust formation.
For the next step, we have a plan of long-term mid-infrared monitoring of dusty AGB stars using the TAO (The University of Tokyo Atacama Observatory) 6.5-m telescope. Since MIMIZUKU, the mid-infrared instrument for the TAO 6.5-m telescope, has high-precision monitor observation capability, we can obtain mid-infrared light curves with sufficient number of epochs. In addition, we will be able to obtain time variation data of mid-infrared spectra including silicate and alumina features. They may be helpful information to understand the detailed process of the dust formation. TAO will start science operations in 2022.