Fluorine detection in hot extreme helium stars
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J. Astrophys. Astr. (2020)41:40 https://doi.org/10.1007/s12036-020-09659-2
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Fluorine detection in hot extreme helium stars ANIRBAN BHOWMICK1,* , GAJENDRA PANDEY1 and DAVID L. LAMBERT2 1
Indian Institute of Astrophysics, Koramangala, Bengaluru 560 034, India. W.J. McDonald Observatory and Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA. E-mail: [email protected] 2
MS received 5 September 2020; accepted 5 October 2020 Abstract. The origin and evolution of hydrogen-deficient stars are not yet adequately understood. Their chemical peculiarities, along with hydrogen-deficiency, makes them stand out from the rest and sheds light on their possible origin. Severe fluorine enrichment (of the order of 800–8000) is one such characteristic feature of a class of hydrogen deficient stars, mainly the RCBs (R Coronae Borealis stars) and cool EHes (Extreme Helium stars) which enforces their close connection. For hot EHes, this relationship with the cooler EHes, based on their fluorine abundance is unexplored. Here, first estimates of fluorine abundances in hot EHes are presented and discussed in the light of their cooler counterparts to try to establish an evolutionary connection. The relation between these fluorine estimates with the other elemental abundances observed in these stars plays a pivotal role to predict the formation and evolution of these exotic stars. Keyword. Hydrogen-deficient stars—chemical peculiarity—cxtreme helium stars.
1. Introduction Extreme helium stars (EHes) are helium rich, A- and B-type, hydrogen deficient supergiants having effective temperatures in the range of 8000-35000 K. The observed surface composition of these stars are similar to the cooler H-deficient stars, namely the R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars. Apart from sharing extreme hydrogen deficiency, EHe, RCB and HdC stars also exhibit typical chemical peculiarities. The two most extraordinary peculiarities observed in the surface compositions of these H-deficient stars are (i) the extreme overabundance of 18 O in HdC and cool RCBs such that 18 O/16 O [ 1 (Clayton et al. 2007) and (ii) a startling overabundance of F in RCBs and cool EHes such that F relative to Fe is enhanced by 800 to 8000 times (Pandey 2006; Pandey et al. 2008; Hema et al. 2017). The status of
This article is part of the Topical Collection: Chemical Elements in the Universe: Origin and Evolution.
these anomalies in hot EHes are unknown. This work addresses the F abundance of the hot EHes, and hence determine if these peculiarities extend to the hot EHes. EHes are extremely rare in the Galaxy, with 22 discovered till date (Jeffery et al. 1996; Jeffery 2017). There are about 17 known EHes, having effective temperatures hotter than about 14000 K and are classified as hot EHes. In this work, ten hot EHes are examined. For the hot EHes, sufficient knowledge about the two notable chemical peculiarities of the cool H-deficient stars, i.e., 18 O and F wer
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