Effective temperatures of red supergiants estimated from line-depth ratios of iron lines in the YJ bands, 0.97-1.32μm

Author:

Taniguchi Daisuke1ORCID,Matsunaga Noriyuki12,Jian Mingjie1ORCID,Kobayashi Naoto234,Fukue Kei2,Hamano Satoshi25ORCID,Ikeda Yuji26ORCID,Kawakita Hideyo27ORCID,Kondo Sohei24,Otsubo Shogo2,Sameshima Hiroaki3ORCID,Takenaka Keiichi8,Yasui Chikako5

Affiliation:

1. Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

2. Laboratory of Infrared High-Resolution Spectroscopy (LiH), Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan

3. Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan

4. Kiso Observatory, The University of Tokyo, 10762-30 Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101, Japan

5. National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

6. Photocoding, 460-102 Iwakura-Nakamachi, Sakyo-ku, Kyoto 606-0025, Japan

7. Department of Astrophysics and Atmospheric Sciences, Faculty of Sciences, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan

8. Division of Science, Graduate School, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan

Abstract

ABSTRACT 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 as a result of the model uncertainty, which originates mainly in the extended atmospheres of RSGs. Here we propose a line-depth ratio (LDR) method in which we use only Fe i lines. As opposed to the conventional LDR method, in which lines of multiple species are involved, this LDR method is insensitive to surface gravity effects and is expected to circumvent the uncertainty originating in the upper atmosphere of RSGs. Therefore, the LDR–Teff relationships that we calibrated empirically with red giants can be directly applied to RSGs, although various differences between the two groups of objects, for example caused by 3D non-local thermodynamic equilibrium (non-LTE) effects, need to be kept in mind. Using the near-infrared YJ-band spectra of nine well-known solar-metallicity red giants observed with the WINERED high-resolution spectrograph, we selected 12 pairs of Fe i lines that were the least contaminated with other lines. After applying their LDR–Teff relation to 10 nearby RSGs, the resulting Teff with an internal precision of $30\!-\!70\, \mathrm{K}$ shows good agreement with previous observational results assuming 1D LTE and with the Geneva 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 that we rejected because the non-LTE effects may be as large as $\sim \! 250\, \mathrm{K}$. Nevertheless, it is difficult to evaluate the systematic bias, and further study is required, for example including the 3D non-LTE calculations of all the lines involved.

Funder

Japan Society for the Promotion of Science

Department of Science and Technology, Government of West Bengal

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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1. Astrophysical calibration of the oscillator strengths of YJ-band absorption lines in classical Cepheids;Monthly Notices of the Royal Astronomical Society;2024-07-09

2. Red supergiant candidates for multimessenger monitoring of the next Galactic supernova;Monthly Notices of the Royal Astronomical Society;2024-03-15

3. WARP: The Data Reduction Pipeline for the WINERED Spectrograph;Publications of the Astronomical Society of the Pacific;2024-01-01

4. M giants with IGRINS;Astronomy & Astrophysics;2023-06-30

5. Effective temperatures of classical Cepheids from line-depth ratios in the H-band;Monthly Notices of the Royal Astronomical Society;2023-06-07

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