Search for stellar spots in field blue horizontal-branch stars

Abstract

Context. Blue horizontal-branch stars are Population II objects which are burning helium in their core and possess a hydrogen-burning shell and radiative envelope. Because of their low rotational velocities, diffusion has been predicted to work in their atmospheres. In many respects, blue horizontal-branch stars closely resemble the magnetic chemically peculiar stars of the upper main sequence, which show photometric variability caused by abundance spots on their surfaces. These spots are thought to be caused by diffusion and the presence of a stable magnetic field. However, the latter does not seem to be axiomatic. Aims. We searched for rotationally induced variability in 30 well-established bright field blue horizontal-branch stars in the solar neighbourhood and searched the literature for magnetic fields measurements of our targets. Methods. We employed archival photometric time series data from the All Sky Automated Survey (ASAS), All-Sky Automated Survey for Supernovae (ASAS-SN), and Wide Angle Search for Planets (SuperWASP) surveys. The data were carefully reduced and processed, and a time series analysis was applied using several different techniques. We also synthesized existing photometric and spectroscopic data of magnetic chemically peculiar stars in order to study possible different surface characteristics producing lower amplitudes. Results. In the accuracy limit of the employed data, no significant variability signals were found in our sample stars. The resulting upper limits for variability are given. Conclusions. We conclude that either no stellar surface spots are present in field blue horizontal-branch stars, or their characteristics (contrast, total area, and involved elements) are not sufficient to produce amplitudes larger than a few millimagnitudes in the optical wavelength region. New detailed models taking into account the elemental abundance pattern of blue horizontal-branch stars are needed to synthesize light curves for a comparison with our results.

Hot subdwarf wind models with accurate abundances

“…Despite similar abundance anomalies, in contrast to classical chemically peculiar stars, the subdwarfs and horizontal branch stars do not show rotationally modulated light variability (Pancino et al 2012;Marinoni et al 2016;Paunzen et al 2019). Light variability in classical chemically peculiar stars typically originates due to flux redistribution in surface abundance spots (e.g.…”

Haydn

“…Finally, the high-precision, high-cadence photometric monitoring of stars along the HB in GCs, will also shed light on the nature of the variability of blue horizontal branch (BHB) stars, and its relation to chromospheric activity / chemical peculiarities (see e.g. [157,177]).…”

Clean catalogues of blue horizontal-branch stars using Gaia EDR3

“…This increases the temperature further and the core ceases to be degenerate. At this point, stars move to the blue horizontal-branch with masses in the range of ∼0.5-1.0 M (Montenegro 2019) and display helium burning in their core as well as hydrogen shell burning (Moehler 2001;Ruh-The catalogues are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ land et al 2011;Paunzen et al 2019). The mass of the hydrogen shell may vary from 0.02 M to over 0.2 M .…”

“…The mass of the helium-burning core is ∼0.5 M . Blue horizontal-branch stars with a higher mass of hydrogen shell will appear cooler and, hence, redder (Moehler 2001;Paunzen et al 2019).…”