Intrinsic properties of carbon stars. I. Effective temperature scale of N-type carbon stars |
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| Authors: | Takashi Tsuji |
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| Institution: | (1) Tokyo Astronomical Observatory, University of Tokyo, Mitaka, 181 Tokyo, Japan |
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| Abstract: | It is shown that the infrared flux method for determining stellar effective temperatures (Blackwell and Shallis 1977; Blackwell,
Petford and Shallis 1980) can be applied to cool carbon stars. Although the spectra of cool carbon stars are highly line blanketed,
the spectral region between 3 and 4 μm (L-band in the infrared photometry system) is found to be relatively free from strong
line absorption. The ratioR
L of bolometric flux toL flux can then be used as a measure of effective temperature. On the basis of the predicted line-blanketed flux based on model
atmospheres, with an empirical correction for the effect of 3 μm absorption due to polyatomic species (HCN, C2H2), it is shown thatR
L is roughly proportional to T3
eff. The high sensitivity ofR
L to Teff makes it a very good measure of effective temperature, and the usual difficulty due to differential line blanketing effect
in the analyses of photometric indices of cool carbon stars can be minimized.
It is found that the majority of N-type carbon stars with small variability (SRb and Lb variables) are confined to the effective
temperature range between 2400 and 3200 K, in contrast to M-giant stars (M0 III - M6 III, including SRb and Lb variables)
that are confined to the effective temperature range between 3200 and 3900 K. The effective temperatures based on the infrared
flux method show good agreement with those derived directly from angular diameter measurements of 5 carbon stars. On the basis
of the new effective temperature scale for carbon stars, it is shown that the well known C-classification does not represent
a temperature sequence. On the other hand, colour temperatures based on various photometric indices all show good correlations
with our derived effective temperatures.
An erratum to this article is available at . |
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| Keywords: | carbon stars— effective temperatures— line blanketing— spectral classification— stellar atmospheres |
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