Long-period Ap stars discovered with TESS data: The northern ecliptic hemisphere

Abstract

The rotation periods of the magnetic Ap stars span five to six orders of magnitude. While it is well established that period differentiation must have taken place at the pre-main sequence stage, the physical processes that lead to it remain elusive. The existence of Ap stars that have rotation periods of tens to hundreds of years is particularly intriguing, and their study represents a promising avenue to gain additional insight into the origin and evolution of rotation in Ap stars. Historically, almost all the longest period Ap stars known have been found to be strongly magnetic; very few weakly magnetic Ap stars with very long periods have been identified and studied. To remedy that, we showed how a systematic search based on the analysis of TESS photometric data could be performed to identify super-slowly rotating Ap (ssrAp) stars independently of the strengths of their magnetic fields, with the intention to characterise the distribution of the longest Ap star rotation periods in an unbiased manner. We successfully applied this method to the analysis of the TESS 2-min cadence observations of Ap stars of the southern ecliptic hemisphere. For our present study, we applied the same approach to the analysis of the TESS 2-min cadence observations of Ap stars of the northern ecliptic hemisphere. We confirm that the technique leads to the reliable identification of ssrAp star candidates in an unbiased manner. We find 67 Ap stars with no rotational variability in the northern ecliptic hemisphere TESS data. Among them, 46 are newly identified ssrAp star candidates, which is double the number found in the southern ecliptic hemisphere. We confirm that super-slow rotation tends to occur less frequently in weakly magnetic Ap stars than in strongly magnetic stars. We present new evidence of the existence of a gap between ∼2 kG and ∼3 kG in the distribution of the magnetic field strengths of long period Ap stars. We also confirm that the incidence of roAp stars is higher than average in slowly rotating Ap stars. We report the unexpected discovery of nine definite and five candidate δ Sct stars, and of two eclipsing binaries. This work paves the way for a systematic, unbiased study of the longest period Ap stars, with a view to characterise the correlations between their rotational, magnetic, and pulsational properties.

Photometric variability of the LAMOST sample of magnetic chemically peculiar stars as seen by TESS

“…Not included at the time of this writing are the studies of Bowman et al (2018), Sikora et al (2019a), David-Uraz et al (2019), Bernhard et al (2020), and Mathys et al (2020bMathys et al ( , 2022. Except for two stars from the list of Mathys et al (2022) 4 , there are no matches between the samples of these studies and our sample. The VSX has data for only 64 out of the 782 objects presented here.…”

“…It contains the results of most relevant papers dealing with the periods of ACV variables (including more recent studies, such as Paunzen & Maitzen 1998, Wraight et al 2012, Bernhard et al 2015a,b, 2021and Hümmerich et al 2016). Not included at the time of this writing are the studies of Bowman et al (2018), Sikora et al (2019a), David-Uraz et al (2019), Bernhard et al (2020), and Mathys et al (2020bMathys et al ( , 2022. Except for two stars from the list of Mathys et al (2022) 4 , there are no matches between the samples of these studies and our sample.…”

HD 213258: A new rapidly oscillating, super slowly rotating, strongly magnetic Ap star in a spectroscopic binary

“…While the rate of occurrence of roAp stars among ssrAp stars, which may reach ∼20%, is considerably higher than the fraction of roAp stars among all Ap stars (Mathys et al 2022), roAp stars seldom belong to binary systems (Hubrig et al 2000;Schöller et al 2012;Hey et al 2019). Such binary systems are wide: the shortest orbital period that has been accurately determined for a pair containing an roAp star, HD 42659, is 93 d .2 (Hartmann & Hatzes 2015).…”

“…Originally, this classification referred to stars whose spectra resemble those of Am stars but show an abnormally strong Sr ii λ 4077 Å line. However, as noted by North (1987), a specific definition of the criteria that allow one to distinguish F str λ 4077 stars from Am, Ap Sr, or λ Boo stars was missing at the time 1 . This left some ambiguity in the classification.…”

An Ap Star Catalog Based on LAMOST DR9

“…Among these 72 stars, four stars (TIC 2934856, TIC 239801694, TIC 291258469, and TIC 434372218) do not show detectable rotation features. Mathys et al (2020Mathys et al ( , 2022 found a total of 127 super slowly rotating Ap stars (ssrAp stars) that have long rotation periods so that no obvious rotational variability is seen in one TESS sector, or more. Among our four stars without rotation features detected, TIC 239801694 was identified as an ssrAp star candidate in Mathys et al (2022), and the other three stars may also be candidates, or their rotation inclination angles are quite small.…”