Surface image and activity-corrected orbit of the RS CVn binary HR 7275: Disentangling activity tracers

Abstract

Quantifying stellar parameters and magnetic activity for cool stars in double-lined spectroscopic binaries (SB2) is not straightforward, as both stars contribute to the observed composite spectra and are likely variable. Disentangled component spectra allow a detailed analysis of a component's magnetic activity. We aim at separating the spectra of the two stellar components of the HR\,7275 SB2 system. Our further aim is a more accurate orbital solution by cleaning the observed radial velocities (RV) from activity perturbations of the spotted primary ("RV jitter") and obtain a surface image of this component. The Doppler image of the primary shows two large cool spots of size $\approx$20\% of the visible hemisphere plus three smaller spots, each still $\approx$13\% in size. In total, HR\,7275a exhibited an impressive spottedness of $\approx$40\%\ of its entire surface in May-June 2022. The RV is modulated by the rotation of the primary with maximum amplitudes of 320\,\ms\ and 650\,\ms\ for two different modulation behaviors during the 250\,d of our observations. This jitter is primarily caused by the varying asymmetries of the apparent disk brightness due to the cool spots. Its removal resulted in roughly ten times higher precision of the orbital elements. Our snapshot magnetic-field measurements reveal phase-dependent (large-scale) surface fields between +0.6$\pm$2.0\,G at phase 0.1 and $-$15.2$\pm$2.7\,G at phase 0.6, indicating a complex magnetic morphology related to the location of the photospheric spots. We also obtain a logarithmic lithium abundance of 0.58$\pm$0.1 for HR\,7275a, indicating considerable mixing, and 0.16$^{+0.23}_{-0.63}$ for HR\,7275b, which is an extremely low value. }

Figures (12)

• Figure 1: Phase folded radial velocities for both stars in the system. Circle data points indicate the STELLA observations; triangle data points show PEPSI observations. The green lines (solid and dashed) represent the orbital fit while the black dashed line is the center of mass velocity.
• Figure 2: Spectral disentangling by median subtraction. Panel a: The mean spectrum of the secondary star is plotted as a black line (top). The composite spectrum (blue) and a spectrum of the primary star (red) are plotted with a vertical shift of 0.15 at orbital phase 0.82. Panel b: Time series composite spectra phase folded with the orbital period.
• Figure 3: Radial velocities of HR 7275a from VATT+PEPSI and STELLA+SES. In the upper panel, panel $a$, the SB1 orbital fit is plotted as a black line along with the data. STELLA+SES observations are indicated with dark blue circles; VATT+PEPSI observations are plotted in red triangles. In panel $b$ the RV residuals are shown after removing the predicted orbital velocities. The RV jitter appears multi-peaked per rotation. The thick grey and orange lines emphasize two different rotational modulation models.
• Figure 4: Lomb-Scargle periodogram (red) of RV residuals from Fig. \ref{['RV_fit']}. The strongest peak appears at the half rotation of the star. Also one third of the rotation appears prominently as the second strongest peak in the periodogram. Phase dispersion minimization (PDM) is shown in blue which shows the strongest peak at 27.2 days.
• Figure 5: Panel a: Doppler image of HR 7275a. The rotational phases are indicated via phase $\varphi$ with a sampling of 0.25. Panel b: Contemporaneous RV modulation due to the spot modulation. Panel c: Relative flux modulation of Ca ii IRT 8542 $\AA$ simultaneous to the DI. Dark blue dots show the STELLA data set and red triangles indicate the PEPSI observations.