Investigation of the helium enhancement in a super lithium-rich giant HD 77361
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J. Astrophys. Astr. (2020)41:39 https://doi.org/10.1007/s12036-020-09667-2
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Investigation of the helium enhancement in a super lithium-rich giant HD 77361 B. P. HEMA* and GAJENDRA PANDEY Indian Institute of Astrophysics, Koramangala II Block, Bengaluru 560 034, India. *Corresponding author. E-mail: [email protected] MS received 6 September 2020; accepted 1 October 2020 Abstract. In this work, the helium-enhancement (He-enhancement) in the lithium-rich (Li-rich) K-giant HD 77361 is investigated using the strengths of the MgH band and the Mg I lines. The detailed abundance analysis and also the synthesis of the MgH band and the Mg I lines has been carried out for HD 77361. One would expect, within uncertainities, same Mg abundance from both the MgH and Mg I lines. But, we found that Mg abundance derived from MgH lines is significantly less than the abundance from Mg I lines, and this difference cannot be reconciled by changing the stellar parameters within the uncertainties, implying He enhancement in star’s photosphere. The He enhancement in the atmospheres is estimated by using models of different He/H ratios so that both the lines, MgH as well as Mg I, return the same Mg abundance for the adopted model’s He/H ratio. We found He=H ¼ 0:4 0:1 as the value for HD 77361, the normal value of He=H ¼ 0:1. Knowing the amount of He-enhancement in the Li-rich giants is a strong clue for understanding the scenarios responsible for the Li and He enrichment. The analysis and results are discussed. Keywords. Stellar evolution—chemically-peculiar stars.
1. Introduction Lithium (Li) is a fragile element that gets destroyed at temperatures of about 2.5 106 K. The initial value of Li abundance from the interstellar medium is about log ðLiÞ ¼ 3:3 dex. Due to the mixing in the pre-main sequence phase, main-sequence phase, and during the first dredge-up, the Li gets diluted and the log (Li) reduces to about 1.5–1.8 dex in the low-mass G-K giants (Lambert et al. 1980; Charbonnel and Balachandran 2000). This Li abundance is the upper limit for most of the G-K giants. But there is a group of G-K giants having very strong Li lines in their spectra. They have unexpectedly high values of the Li abundance of about log ðLiÞ [ 2:53:0 dex. These are the enigmatic group of stars, for which the source/ process of the Li-enhancement is not clear yet. One of This article is part of the Topical Collection: Chemical Elements in the Universe: Origin and Evolution.
the scenarios that explains the Li-enhancement is dredge-up from the deeper layers, where Li is produced. In this work we have investigated the amount of helium (He) in the atmosphere of one such super Li-rich giant HD 77361. We suspect that during the mixing process from the deeper layers, the convection zone dredges-up the He along with the other hydrogen burning elements such as Li, C, N, and 13 C. The analysis and the results are discussed in the following sections.
2. Observations The high-resolution optical spectrum of
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