Super-slowly rotating Ap (ssrAp) stars: New spectroscopic observations

TL;DR

The paper tackles the extreme end of Ap-star rotation by spectroscopically confirming ssrAp candidates identified with TESS, measuring magnetic-field moments and constraining rotation via $v\sin i$ upper limits, and assessing binarity through multi-epoch radial velocities. Using high-resolution spectra from HARPS-N, SALT-HRS, UVES, and other archives, the study derives $\langle B\rangle$ and $\langle B_q\rangle$ (and occasionally $v\sin i$) for 18 sharp-lined Ap stars, discovering five new stars with resolved magnetically split lines and identifying multiple SB systems. The results reveal that 17 of 18 targets are magnetic, with several showing significant long-term magnetic-geometry changes and many presenting evidence for long rotation periods, while about half are in binary systems. This dataset provides a foundation for understanding correlations between rotation, magnetism, and binarity in the ssrAp regime and guides future targeted monitoring and archival data exploitation to map the full rotation-period distribution.

Abstract

The rotation periods of Ap stars range over five to six orders of magnitude. The origin of their differentiation remains unknown. We carry out a systematic study of the longest period Ap stars to gain insight into their properties. We analyse newly obtained spectra of a sample of super-slowly rotating Ap (ssrAp) star candidates identified by a TESS photometric survey to confirm that their projected equatorial velocity v sin i is consistent with (very) long rotation periods, to obtain a first determination of their magnetic fields, and to test their binarity. The value of v sin i in 16 of the 18 studied stars is low enough for them to have moderately to extremely long rotation periods. All stars but one are definitely magnetic; for five of them, the magnetic field was detected for the first time. Five new stars with resolved magnetically split lines were discovered. Five stars that were not previously known to be spectroscopic binaries show radial velocity variations; in one of them, lines from both components are observed.

Figures (11)

• Figure 1: Portion of the spectrum of HD 192686 showing the Fe ii$\uplambda\,4385.4$ Å line. The Zeeman pattern of this line, a doublet, is illustrated below the spectrum. The amplitude of the splitting corresponds to the measured value of the mean magnetic field modulus, $\langle B\rangle=3573\,G$. The length of each vertical bar is proportional to the relative strength of the corresponding line component. The $\uppi$ components appear above the horizontal line (in green), the $\upsigma_+$ and $\upsigma_-$ components below it (in blue and yellow respectively).
• Figure 2: Mean magnetic field modulus of HD 143487 against observation date. Different point types and colours are used to distinguish the observations performed with different instruments.
• Figure 3: Comparison of the profiles of the Fe i$\uplambda\,6335.3$ Å and $\uplambda\,6336.8$ Å lines in BD+35 5094. The wavelengths are in the laboratory reference frame. The Zeeman patterns of the lines seen in the considered wavelength range were shown in Fig. 6 of 2024AA...691A.186M. One can clearly see that Fe i$\uplambda\,6336.8$ Å is significantly broader than Fe i$\uplambda\,6335.3$ Å, which reveals the presence of a magnetic field in BD+35 5094.
• Figure 4: Mean magnetic field modulus ( top) and mean quadratic magnetic field ( bottom) of HD 110274 against rotation phase computed with the value $P_{\mathrm{rot}}=265\fd3$ of the rotation period derived by 2008MNRAS.389..441F. The phase origin has been set at JD 2 454 140.0. The dash-dotted blue curve represents the least-squares fit of the measurements by a sine curve. Different point types and colours are used to distinguish the observations performed with different instruments. The size of the error bars for the $\langle B\rangle$ values does not exceed that of the symbols representing them.
• Figure 5: Spectra of HD 117290 at the epochs of the lowest and highest radial velocity values that were recorded in this study, $v_{\mathrm{r}}=-4.47$ km s$^{-1}$ (2008 UVES spectrum) and $v_{\mathrm{r}}=-36.87$ km s$^{-1}$ (2023 SALT-HRS spectrum). If lines of the two components of the binary are visible, this is when the greatest wavelength separation between them has been observed. The laboratory reference frame was used for the wavelengths of the lines of the sharp-lined Ap component. In this reference frame, the blue wing of the H$\alpha$ line is less depressed in the SALT-HRS spectrum than in the UVES one, and conversely, its red wing is more depressed in the SALT-HRS spectrum than in the UVES one. This is consistent with the presence of a (very) broad H$\alpha$ line from the secondary that is blue shifted in 2008 and redshifted in 2023 with respect to the H$\alpha$ line of the Ap star, which has a much narrower core. (The pairs of very narrow lines that appear shifted with respect to each other between the two spectra are of telluric origin.)