Study of chemically peculiar stars – I. High-resolution spectroscopy and K2 photometry of Am stars in the region of M44

Abstract

We present a study based on the high-resolution spectroscopy and K2 space photometry of five chemically peculiar stars in the region of the open cluster M44. The analysis of the high-precision photometric K2 data reveals that the light variations in HD 73045 and HD 76310 are rotational in nature and caused by spots or cloud-like co-rotating structures, which are non-stationary and short-lived. The time-resolved radial velocity measurements, in combination with the K2 photometry, confirm that HD 73045 does not show any periodic variability on time-scales shorter than 1.3 d, contrary to previous reports in the literature. In addition to these new rotational variables, we discovered a new heartbeat system, HD 73619, where no pulsational signatures are seen. The spectroscopic and spectropolarimetric analyses indicate that HD 73619 belongs to the peculiar Am class, with either a weak or no magnetic field, considering the 200-G detection limit of our study. The least-squares deconvolution profiles for HD 76310 indicate a complex structure in its spectra, suggesting that this star is either part of a binary system or surrounded by a cloud shell. When placed in the Hertzsprung–Russell diagram, all studied stars are evolved from the main sequence and situated in the δ Scuti instability strip. This work is relevant for further detailed studies of chemically peculiar stars, for example on inhomogeneities (including spots) in the absence of magnetic fields and the origin of the pulsational variability in heartbeat systems.

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“…This was one of the largest ground-based photometric surveys ever taken with the aim to search and study the photometric variability in a large sample of chemically peculiar (CP) stars (Ashoka et al 2000, Martinez et al 2001Joshi et al 2003Joshi et al , 2006Joshi et al , 2009Joshi et al , 2010Joshi et al , 2012Joshi et al , 2016Joshi et al , 2017. By combining time-series of ground-and space-based photometry with high-resolution spectroscopy of a set of samples observed using multiple telescopes around the globe, Joshi et al (2022a) discovered a new heartbeat system (HD 73619), for which no pulsation signatures are seen. Such works not only aim to study the stellar structure and atmospheres of CP stars in the presence of magnetic fields, inhomogeneities (such as spots), and tidal interactions but also demonstrate the importance of international (13 countries) co-operation led by Indians and Belgians.…”