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Spectroscopic Determination of Capella's Photospheric Abundances

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Spectroscopic Determination of Capella's Photospheric Abundances SPECTROSCOPIC DETERMINATION OF CAPELLA’S PHOTOSPHERIC ABUNDANCES: POSSIBLE INFLUENCE OF STELLAR ACTIVITY Yoichi Takeda1,2, Osamu Hashimoto3, and Satoshi Honda4 [email protected], [email protected], [email protected] ABSTRACT Capella is a spectroscopic binary consisting of two G-type giants, where the primary (G8 iii) is a normal red-clump giant while the secondary (G0 iii) is a chromospherically-active fast rotator showing considerable overabundance of Li as Li-enhanced giants. Recently, Takeda & Tajitsu (2017) reported that abun- dance ratios of specific light elements (e.g., [C/Fe] or [O/Fe]) in Li-rich giants of high activity tend to be anomalously high, which they suspected to be noth- ing but superficial caused by unusual atmospheric structure due to high activity. Towards verifying this hypothesis, we determined the elemental abundances of the primary and the secondary of Capella based on the disentangled spectrum of each component, in order to see whether any apparent disagreement exists between the two, which should have been formed with the same chemical com- position. We found that the primary is slightly supersolar (by +0.1 dex) while the secondary is subsolar (by several tenths dex) for heavier ele∼ments such as Fe, resulting in a marked discrepancy between the primary and secondary, though such a trend is not seen for light elements (e.g., C or O). These observational facts suggest that anomalously large [X/Fe] ratios found in Li-rich giants were mainly arXiv:1806.09036v1 [astro-ph.SR] 23 Jun 2018 due to an apparent decrease of Fe abundance, which we speculate is caused by the overionization effect due to chromospheric UV radiation. We thus conclude that conventional model atmosphere analysis would fail to correctly determine the abundances of fast-rotating giants of high activity, for which proper treatment of chromospheric effect is required for deriving true photospheric abundances. Subject headings: stars: abundances — stars: activity — stars: atmospheres — stars: binaries: spectroscopic — stars: individual (Capella) —stars: late-type 1National Astronomical Observatory, 2-21-1 2SOKENDAI, The Graduate University for Osawa, Mitaka, Tokyo 181-8588, Japan Advanced Studies, 2-21-1 Osawa, Mitaka, Tokyo 1 1. INTRODUCTION high chromospheric activity (though less active slow rotators similar to normal gi- Capella (= α Aur = HD 34029 = ants do exist). Curiously, they found that HR 1708 = HIP 24608) is a spectroscopic abundance ratios of specific light elements binary system (orbital period: 104 d) (e.g., [C/Fe] or [O/Fe] derived from high- iii iii consisting of G8 and G0 giants, excitation lines) in such Li-rich giants of where the more evolved former/primary high rotation/activity tend to be unusu- has a slightly larger mass and luminosity ally high (typically by several tenths dex) (2.6 M⊙ and 79 L⊙) than that of the lat- and do not follow the mean trend exhib- ter/secondary (2.5 M⊙ and 73 L⊙). The ited by normal red giants (see Figure 14 primary is a typical late-G giant presum- in Paper I). Since the initial compositions ably in the He-burning stage (red clump), of these stars should not be so anoma- which is Li-deficient and a slow rotator lous (most of the sample stars were of nor- as other normal giants. In contrast, the mal thin-disk population) and such over- secondary is a fast rotator (projected ro- abundances of these elements are unlikely tational velocity is v sin i 35 km s−1) e ∼ to be produced by dredge-up of nuclear- with high stellar activity (characterized by processed material, they suspected as a conspicuous chromospheric emission lines possibility that this anomaly is nothing but in UV; see, e,g., Sanad 2013), and shows a superficial phenomenon (i.e., not the real a remarkably strong Li line, which indi- abundance peculiarity) caused by unusual cates that the initial Li content is almost atmospheric structure due to high chromo- retained without being diluted (the sur- spheric activity. face Li composition for the secondary is It occurred to us that the Capella sys- by 100 times as high as the primary). ∼ tem is an adequate testbench to verify this That is, the secondary star belongs to the hypothesis, because the primary is a quite unusual group of Li-rich giants. normal giant star while the secondary is Recently, Takeda & Tajitsu (2017; here- a Li-rich giant of high rotation and high inafter referred to as Paper I) conducted activity. That is, these two components a spectroscopic study on 20 Li-rich gi- should have been formed with the same ants, in order to clarify their observational initial composition, and the abundances of characteristic in comparison with a large the normal primary are expected to be re- number of normal red giants. Accord- liably determined. Then, if we could de- ing to their study, a significant fraction tect any apparent discrepancy between the of Li-rich giants rotate rapidly and show abundances of the primary and the sec- ondary, we may state that some problem 181-8588, Japan exists in the abundances derived for the 3 Gunma Astronomical Observatory, 6860-86 secondary star, for which the standard pro- Nakayama, Takayama, Agatsuma, Gunma 377- 0702, Japan cedure of conventional model atmosphere 4Nishi-Harima Astronomical Observatory, analysis would not be sufficient any more. Center for Astronomy, Unfortunately, despite that many pa- University of Hyogo, 407-2 Nishigaichi, Sayo-cho, pers have been published so far regard- Sayo, Hyogo 679-5313, Japan 2 ing the orbital elements and fundamen- component, as recently done by Torres et tal stellar quantities of the Capella sys- al. (2015). tem (see, e.g., Torres et al. 2009, 2015, Motivated by this situation, we decided and the references therein), spectroscopic to carry out a detailed model-atmosphere studies on the photospheric abundances analysis for the primary and secondary are apparently insufficient. Admittedly, components of Capella, closely following the surface Li abundances of both compo- the procedure adopted in Paper I, in or- nents have already been clarified by several der to see whether any meaningful dif- useful articles (e.g., Pilachowski & Sowell ference exists in the photospheric abun- 1992, Torres et al. 2015, and the refer- dances between these two stars. For this ences therein), by which the distinct dif- aim, the genuine single-line spectra for ference between them has been firmly es- both stars were recovered by using the tablished. However, few reliable studies spectrum-disentangle method based on a appear to be available regarding Capella’s set of original double-line spectra obtained abundances of various elements other than at various orbital phases. The purpose of Li. Somewhat surprisingly, even the metal- this paper is to report the outcome of this licity (e.g., Fe abundance) of the sharp- investigation. lined primary star has not yet been estab- The remainder of this article is orga- lished, for which considerably diversified nized as follows: We first explain our ob- results (from metal-poor through metal- servational data in Section 2 and then the rich) have been reported (cf. Torres et al. derivation of the disentangled spectra in 2009, 2015, and the references). Above all, Section 3. Section 4 describes the spectro- it is only Torres et al. (2015) that has ever scopic determination of atmospheric pa- conducted separate determinations of el- rameters by using the equivalent widths emental abundances for the primary and of Fe i and Fe ii lines as done in Pa- secondary components, as far as we know. per I. In Section 5 are described the abun- This scarcity may be closely connected dance determinations for the primary and with the practical difficulty involved with secondary components (along with the Capella (double-line spectroscopic binary Sun) by using two different approaches: with components of similar luminosity), in spectrum-fitting method (as used in Pa- which the line strengths become appar- per I) and the traditional method using ently weaker because of dilution. More- the equivalent widths. Section 6 is the dis- over, determining the abundances of the cussion section, where the resulting abun- secondary star is a very tough matter, be- dances as well as their trends are reviewed cause lines are appreciably wide and shal- and the implications from these observa- low due to large ve sin i. In our opinion, tional facts are discussed. We also argue precise abundance determination is hardly especially about the recent similar abun- possible by directly working on the com- dance study of Capella by Torres et al. plex double-line spectrum. Therefore, the (2015), because they reported a conflicting most promising way would be to use the consequence with ours. The conclusions adequately disentangled spectrum of each are summarized in Section 7. In addition, 3 a supplementary section is also prepared hundred) in all cases. The Julian dates as (Appendix A), where we check from var- well as the corresponding orbital phases of ious viewpoints whether our disentangled these spectra are summarized in Table 1. spectra have been correctly reproduced. 3. SPECTRUM DISENTANGLING 2. OBSERVATIONAL DATA For the purpose of obtaining the pure Most of the observational data of Capella spectrum of each star to be used for our used for this study were obtained by spec- analysis, we made use of the public-domain troscopic observations of 11 times over 4 software CRES2 written by S. Iliji´c. This months in the 2015–2016 season by us- program extracts the pure single-line spec- ing GAOES (Gunma Astronomical Ob- trum of each component based on a set of servatory Echelle Spectrograph) installed observed double-line spectra at various or- at the Nasmyth Focus of the 1.5 m re- bital phases by the spectrum disentangling flector of Gunma Astronomical Observa- technique formulated in the wavelength do- tory, by which spectra with the resolving main, which is computationally more time- power of R 40000 (corresponding to the consuming but more flexible than the one ∼ ′′ slit width of 2 ) covering the wavelength formulated in the Fourier domain (see, e.g., range of 5000–6800 A˚ were obtained (33 Simon & Sturm 1994; Hadrava 1995; Iliji´c echelle orders, each covering 70–90 A).˚ 2004; Hensberge et al.
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