An analysis of optical and infrared spectra of the peculiar carbon-rich giant TU Gem

1Polinovsky, GO, 1Yakovina, LA, 1Pavlenko, YV
1Main Astronomical Observatory of the National Academy of Sciences of Ukraine, Kyiv, Ukraine
Kinemat. fiz. nebesnyh tel (Online) 2014, 30(4):38-57
Start Page: Physics of Stars and Interstellar Medium
Language: Russian

TU Gem has been known as a peculiar carbon giant of galactic halo, but its belonging to the type of CH stars is still debated. We estimated the TU Gem atmospheric parameters through the simulation of the star’s spectrum and comparison of the simulation result with observations for two broad spectral ranges, namely, λ=400—720 nm and λ= 900 — 2440 nm. For the analysis we used the low-dispersion optical spectrum of TU Gem by Barnbaum et al. (1996) (R ~ 600) and the infrared spectrum by Tanaka et al. (2007) (R ~ 2600). The atmospheric models were calculated by the program SAM12 (Pavlenko, 2003). Because of the estimate ambiguity for metallicity ([Fe/H]) from our spectral data, only effective temperature of TU Gem, which weakly depends on the metallicity, is determined with confidence, namely, Tef = 3000 ± 100 K. The values of C/O, [C/Fe] and [N/Fe] were estimated for the ranges of –2.0 ≤ [Fe/H] ≤ 0.0 with the step Δ[Fe/H] = 0.5. Our evaluation [C/Fe] =0.63—0.67 at the value [Fe/H] = –1 is higher than the estimate [C/Fe] =0.21 at [Fe/H] = –1.1 by Kipper et al. (1996), and the abundance evaluations [N/Fe] =+1.0 at called metallicities coincide. This brings TU Gem closer to CH stars, but for more accurate conclusions a detailed analysis of the chemical composition of the TU Gem atmosphere is required.

Keywords: atmosphere, CH stars, TU Gem

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