Abstract
A recent study of hot (20,000 to 30,000 K) extreme horizontal branch (EHB) stars in globular clusters (GCs) has led to the discovery of their variability. It is suggested that this variability is driven by the projected rotation of magnetic spots on the stellar surfaces and is expected to have higher amplitudes at shorter wavelengths. Here, we present the analysis of such hot stars in the massive GC NGC 2808 using the UltraViolet Imaging Telescope (UVIT), aboard AstroSat. We use the UVIT data in combination with the Hubble Space Telescope UV globular cluster survey (HUGS) data for the central region (within \(\sim \)\(2.7'\) \(\times \) \(2.7'\)) and ground-based optical photometry for the outer parts of the cluster. We generate the Far-UV (FUV)–optical colour-magnitude diagrams (CMDs) and in these we find a population of hot EHB stars fainter than the zero-age horizontal branch (ZAHB) model. A comparison of our FUV magnitudes of the already reported variable EHB stars (vEHBs) shows that the longest period vEHBs are the faintest, along with a tentative correlation between rotation period and UV magnitude of spotted stars. In order to firmly establish any correlation, further study is essential.
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03 September 2021
A Correction to this paper has been published: https://doi.org/10.1007/s12036-021-09775-7
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Acknowledgements
The authors gratefully acknowledge Y. Momany for the useful discussions. This publication utilizes the data from AstroSat mission the Indian Space Science Data Centre (ISSDC).This publication uses of UVIT data processed by the payload operations center by the IIA IIA. The unit is built in collaboration between IIA, IUCAA, TIFR, ISRO, and CSA. This research made use of VOSA, developed under the Spanish Virtual Observatory project supported by the Spanish MINECO through grant AyA2017-84089. This research also made use of Topcat (Taylor 2005), “Aladin sky atlas” developed at CDS, Strasbourg Observatory, France (Bonnarel et al. 2000; Boch & Fernique 2014), Matplotlib, NumPy, SciPy and pandas.
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This article is part of the Special Issue on “AstroSat: Five Years in Orbit”.
The original online version of this article was revised: to correct the reference detail for Prabhu et al. 2021.
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Prabhu, D.S., Subramaniam, A. & Sahu, S. UV photometry of spotted stars in the horizontal branch of the globular cluster NGC 2808 using AstroSat. J Astrophys Astron 42, 66 (2021). https://doi.org/10.1007/s12036-020-09679-y
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DOI: https://doi.org/10.1007/s12036-020-09679-y