Study of the Carbon Star T Draconis

Abstract

We present the results of the infrared photometric observations in the $JHKLM$ bands and infrared spectroscopic observations in the range 1-2.5 $μ$m for the carbon Mira star T Dra performed from 2019 to 2023. An analysis of the photometric observations shows the presence of both pulsational brightness fluctuations with an amplitude falling from $1.2^m$ in the $J$ band to $0.84^m$ in the $L$ and $M$ bands and a linear trend in the mean brightness with a value of $0.0007^m/d$ in the $J$ band. In the infrared spectrum of T Dra we have identified the absorption bands of C${}₂$H${}₂$, HCN, CN, CO, and C${}₂$ molecules. The depth of the absorption band at 1.53 $μ$m has been found to depend on the star's brightness. We show that the CO $λ 2.29$$μ$m bands have a high contrast, suggesting their formation not in the stellar atmosphere but in the circumstellar dust envelope. We present the spectral energy distribution of T Dra in a wide spectral range from which the bolometric fluxes at maximum and minimum brightness have been estimated: $4.8× 10^{-10}$ and $2.5× 10^{-10}$ W m${}^{-2}$, respectively. For the distance to T Dra of 944 pc they correspond to the star's luminosity at maximum brightness $L_{\textrm{max}}≈ 13 300L_{\odot}$ and at minimum brightness $L_{\textrm{min}}≈ 6900L_{\odot}$. We have modeled the radiative transfer in the circumstellar envelope of T Dra and estimated the parameters of the star and the envelope: $T_{\textrm{eff}}=2400$ K, $R_{*}=670R_{\odot}$, $R_{\textrm{in}}=5{-}6$ AU, $R_{\textrm{out}}∼ 50 000$ AU, $\tau_V=3.5$, $M_{\textrm{dust}}=4{-}8× 10^{-5}M_{\odot}$, and $dM/dt∼ 1.5× 10^{-6}$$M_{\odot}$ yr${}^{-1}$.