Slowly Rotating Ap Stars, Prospects for Observing Them with the Tess Space Mission
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SLOWLY ROTATING Ap STARS, PROSPECTS FOR OBSERVING THEM WITH THE TESS SPACE MISSION
I. S. Savanov
The feasibility of detecting candidate slow-rotating magnetic Ap stars with periods from a few tens of days to several hundred years is considered. The question of the fraction of objects of this kind among all Ap stars is important from the standpoint of understanding the origin and evolution of magnetic stars. The search for such objects can be based on analysis of their photometric variability using observations from the TESS space mission. For this purpose, it is first planned to examine all the objects from the corresponding catalogs. It is shown how the proposed method may be used for the example of the slowrotating Ap star HD 50169, with a detailed analysis of its variability and for which observations from the TESS mission are already available. Keywords: stars: photometry: magnetic stars: variability: rotation: magnetic fields
1. Introduction
Studies in recent years have led to the conclusion that the rotation period P for a significant fraction of chemically peculiar magnetic Ap stars can exceed a month and in a number of cases it is suspected that they can reach hundreds of years. For example, a separate group of 33 Ap stars has been identified [1] with periods longer than 30 days; for 16 of them P exceeded 1000 days.
Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia; e-mail: [email protected]
Original article submitted October 22, 2019; accepted for publication June 24, 2020. Astrofizika, Vol. 63, No. 3, pp. 391-399 (August 2020)
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0571-7256/19/6303-0349 ©2020 Springer Science+Business Media, LLC
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Another circumstance worthy of attention is that the range of variation in P is at least 5 orders of magnitude [2]. If stars with rotation periods on the order of days (and less) are sufficiently convenient for observation and subsequent analysis, then obviously obtaining a light curve for the entire rotation period of the most slowly rotating stars is still an unsolved problem in some cases. Even given that the measurement of the magnetic fields of some Ap stars of this kind were begun in the middle of the last century, the data corresponding to the full rotation period have still not yet been obtained (we can only have estimates of P). The best known example of objects of this kind may be the chemically peculiar star J Equ, which has become the subject of numerous studies in recent years (for details, see Ref. 3 with references). It is among the brightest Ap stars (V = 4m.66) (spectral class A9p, subclass SrCrEu) and is a rapidly oscillating roAp star. J Equ has a strong magnetic field, the longitudinal component Bz of which has changed slowly from 1000 G to -1600 G over the last 60 years. J Equ is assumed to be a prototype of stars with an extremely long rotation periods (P is at least 60 years). Polarimetric measurements [4] have indicated a rotation period of 77 years, which can now be regarded as a lower bound. In an analysis of our results and all the available publ
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