• Journal of The Korean Astronomical Society
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• v.21 no.1
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• pp.79-95
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• 1988

To examine relations between stellar activity and rotation we estimated parameters of stellar activity such as $R'_{HK}$, $R'_{MgII}$, $R'_{CII}$, $R'_{CIV}$ and $R'_{X-ray}$ from the published data which measure the activity levels of stellar chromospheres, transition regions and coronae. In the present study we considered only the main sequence stars in an attempt to minimize the influence of other stellar parameters such as radius, age and stellar convection on stellar activity since they are also known to affect the magnetic field generation. In the present analysis we selected only those stars that satisfy the following conditions: (1) flux measurements are available together with Ca II fluxes and (2) rotation periods are determined by Ca II observations. We derived relations between the ${\bar{R}}ossby$ number $R_o$ and stellar activity $R'_{HK}$, $R'_{MgII}$, $R'_{CII}$, $R'_{CIV}$ and $R'_{X-ray}$ and assessed the relations by plotting $R'_{HK}$, $R'_{MgII}$ and $R'_{X-ray}$ against rotation period $P_{rot}$ for comparison with observations. From the comparison it is found that as far as the rotation-activity relation is concerned, (1) normalized surface flux $R'_{HK}$ is better than the surface flux $F'_{HK}$, in the sense that $R'_{HK}$ differentiates the color dependence better and (2) $R'_{HK}$ defined by Rutten (1984) describes the observations notably better than $R'_{HK}$ of Noyes et al. (1984).

• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.131-133
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• 2017

We present the latest results from the Mission Program NIRLT (PI: I.Yamamura), the near-infrared spectroscopy of brown dwarfs using the AKARI/IRC grism mode with the spectral resolution of ~ 120. The near-infrared spectra in the wavelength range between 2.5 and $5.0{\mu}m$ are especially important to study the brown dwarf atmospheres because of the presence of major molecular bands, including $CH_4$ at $3.3{\mu}m$, $CO_2$ at $4.2{\mu}m$, CO at $4.6{\mu}m$, and $H_2O$ around $2.7{\mu}m$. We observed 27 sources, and obtained 16 good spectra. Our model fitting reveals deviations between theoretical model and observed spectra in this wavelength range, which may be attributed to the physical condition of the upper atmosphere. The deviations indicate additional heating, which we hypothesize to be due to chromospheric activity. We test this effect by modifying the brown dwarf atmosphere model to artificially increase the temperature of the upper atmosphere, and compare the revised model with observed spectra of early- to mid-L type objects with $H{\alpha}$ emission. We find that the chemical structure of the atmosphere changes dramatically, and the heating model spectra of early-type brown dwarfs can be considerably improved to match the observed spectra. Our result suggests that chromospheric activity is essential to understand early-type brown dwarf atmospheres.