CoRoT photometry and STELLA spectroscopy of an eccentric, eclipsing, and spotted HgMn binary with sub-synchronized rotation

Abstract

Context. We report the discovery and analysis of very narrow transits in the eccentric spectroscopic binary HSS 348 (IC 4756). Aims. The aim is to characterize the full HSS 348 system. Methods. We obtained high-precision CoRoT photometry over two long runs and multi-epoch high-resolution échelle spectroscopy and imaging with STELLA. Standard radial-velocity extraction, spectrum synthesis, Fourier analysis, and light-curve inversions are applied to the data. Results. HSS 348 is found to be an eccentric (e = 0.18) double-lined spectroscopic binary with a period of 12.47 d in which at least the primary component is a peculiar B star of the HgMn class. The orbital elements are such that the system undergoes a grazing eclipse with the primary in front but no secondary eclipse. The out-of-eclipse light variations show four nearly equidistant but unequal minima stable in shape and amplitude throughout our observations. Their individual photometric periods are all harmonics of the same fundamental period which happens to agree with the transit period to within the errors. We interpret the fundamental period to be the rotation period of at least one if not both stars due to surface inhomogeneities. Due to the non-zero eccentricity of the orbit the two components are rotating sub-synchronously.Conclusions. It appears that HSS 348 is not a member of the IC 4756 cluster but a background B8+B8.5 binary system. Its sharp eclipses every 12.47 days just mimic a small-body transit but are in reality the grazing eclipses of a B-star binary and thus a classical false positive. The system seems to be pre-main sequence with the primary possibly just arrived on the ZAMS. The light curve with four unequal minima can be explained with four cool spots of different size equidistantly positioned in longitude. Our data do not allow to uniquely assign the spots to either of the two stars.

Space-Based Photometry of Binary Stars: From Voyager to TESS

“…A range of other EBs containing pulsating stars was also detected, including CoRoT 102931335 and CoRoT 102918586 which show γ Dor pulsations [104,105] (Figure 5), the oscillating Algol system (oEA [106,107]) CoRoT 102980178 [108], CoRoT 105906206 which shows total eclipses and 50 δ Scuti oscillation frequencies [109], and CoRoT 100866999 for which 89 γ Dor and 35 δ Sct oscillation frequencies were measured [110]. HSS 348 is a spectroscopic binary of spectral type B8 + B8.5 that was found to show shallow eclipses in its CoRoT light curve [111]. It also exhibits rotational modulation due to dark spots on one or both components, which may be related to the fact that at least one is a HgMn (chemically-peculiar) star.…”

Complex Modulation of Rapidly Rotating Young M Dwarfs: Adding Pieces to the Puzzle

“…1 third row). Thus, even the most extreme rotational modulations discovered so far can be described by just a handful of spots (e.g., Rappaport et al 2014;Strassmeier et al 2017).…”

ELISa: A new tool for fast modelling of eclipsing binaries

“…The temperature inhomogeneities can be caused by magnetic activity, such as in the case of variables of an RS CVn type (Pribulla et al 2000(Pribulla et al , 2001Vaňko et al 2007). Additionally, variations in the local chemical composition of the stellar surface, such as in the case of chemically peculiar stars (Strassmeier et al 2017;Paunzen et al 2018), can lead to spot variability. In the case of magnetic field-driven spots, the disturbances in a local magnetic field of the star with a convective envelope is capable of generating local suppression of convection that transfer energy from deeper parts of the star towards the surface.…”