Effective temperatures of red supergiants estimated from line-depth ratios of iron lines in the YJ bands, 0.97–1.32 micron. (arXiv:2012.07856v2 [astro-ph.SR] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Taniguchi_D/0/1/0/all/0/1">Daisuke Taniguchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Matsunaga_N/0/1/0/all/0/1">Noriyuki Matsunaga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jian_M/0/1/0/all/0/1">Mingjie Jian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kobayashi_N/0/1/0/all/0/1">Naoto Kobayashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fukue_K/0/1/0/all/0/1">Kei Fukue</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hamano_S/0/1/0/all/0/1">Satoshi Hamano</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ikeda_Y/0/1/0/all/0/1">Yuji Ikeda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kawakita_H/0/1/0/all/0/1">Hideyo Kawakita</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kondo_S/0/1/0/all/0/1">Sohei Kondo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Otsubo_S/0/1/0/all/0/1">Shogo Otsubo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sameshima_H/0/1/0/all/0/1">Hiroaki Sameshima</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Takenaka_K/0/1/0/all/0/1">Keiichi Takenaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yasui_C/0/1/0/all/0/1">Chikako Yasui</a>
Determining the effective temperatures (Teff) of red supergiants (RSGs)
observationally is important in many fields of stellar physics and galactic
astronomy, yet some significant difficulties remain due to model uncertainty
originating majorly in the extended atmosphere of RSGs. Here we propose the
line-depth ratio (LDR) method in which we use only Fe I lines. As opposed to
the conventional LDR method with lines of multiple species involved, the LDR of
this kind is insensitive to the surface gravity effects and expected to
circumvent the uncertainty originating in the upper atmosphere of RSGs.
Therefore, the LDR–Teff relations that we calibrated empirically with red
giants may be directly applied to RSGs, though various differences, e.g.,
caused by the three-dimensional non-LTE effects, between the two groups of
objects need to be kept in mind. Using the near-infrared YJ-band spectra of
nine well-known solar-metal red giants observed with the WINERED
high-resolution spectrograph, we selected 12 pairs of Fe I lines least
contaminated with other lines. Applying their LDR–Teff relations to ten nearby
RSGs, the resultant Teff with the internal precision of 30–70 K shows good
agreement with previous observational results assuming one-dimensional LTE and
with Geneva’s stellar evolution model. We found no evidence of significant
systematic bias caused by various differences, including those in the size of
the non-LTE effects, between red giants and RSGs except for one line pair which
we rejected because the non-LTE effects may be as large as ~250 K.
Nevertheless, it is difficult to evaluate the systematic bias, and further
study is required, e.g., with including the three-dimensional non-LTE
calculations of all the lines involved.
Determining the effective temperatures (Teff) of red supergiants (RSGs)
observationally is important in many fields of stellar physics and galactic
astronomy, yet some significant difficulties remain due to model uncertainty
originating majorly in the extended atmosphere of RSGs. Here we propose the
line-depth ratio (LDR) method in which we use only Fe I lines. As opposed to
the conventional LDR method with lines of multiple species involved, the LDR of
this kind is insensitive to the surface gravity effects and expected to
circumvent the uncertainty originating in the upper atmosphere of RSGs.
Therefore, the LDR–Teff relations that we calibrated empirically with red
giants may be directly applied to RSGs, though various differences, e.g.,
caused by the three-dimensional non-LTE effects, between the two groups of
objects need to be kept in mind. Using the near-infrared YJ-band spectra of
nine well-known solar-metal red giants observed with the WINERED
high-resolution spectrograph, we selected 12 pairs of Fe I lines least
contaminated with other lines. Applying their LDR–Teff relations to ten nearby
RSGs, the resultant Teff with the internal precision of 30–70 K shows good
agreement with previous observational results assuming one-dimensional LTE and
with Geneva’s stellar evolution model. We found no evidence of significant
systematic bias caused by various differences, including those in the size of
the non-LTE effects, between red giants and RSGs except for one line pair which
we rejected because the non-LTE effects may be as large as ~250 K.
Nevertheless, it is difficult to evaluate the systematic bias, and further
study is required, e.g., with including the three-dimensional non-LTE
calculations of all the lines involved.
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