Sigma Geminorum (σ Gem) is a spectroscopic binary star of the RS CVn type located about 38 parsecs (125 light-years) away, well-known for its photometric and spectroscopic variations caused by intense chromospheric activity and stellar spots on its primary component, a K1III-type giant. The system has an orbital period of 19.6 days and a low eccentricity of 0.014, indicating a nearly circular orbit. The projected rotational velocity (v sin i) of the primary star is approximately 27 km/s, which corresponds to the component of the rotational velocity visible from Earth, depending on the inclination of the rotation axis. With an apparent magnitude of about 4.2, σ Gem is easily accessible to amateur observers equipped for high-resolution (HR) spectroscopy.
We selected this star for a STAROS projects observation campaign in order to monitor its spectral variations, study its chromospheric activity, and take advantage of this opportunity to redefine the orbital parameters of the binary system from new radial velocity measurements.
https://sigmagem.staros-projects.org/
In this STAROS campaign, we analyze continuous spectroscopic observations of the RS CVn-type binary star, σ Gem (HD 62044). This system was classified as a spectroscopic binary with a single line by Herbig & Spalding (1955). The detection of the secondary companion was made through long-baseline optical interferometry in 2015 (Roettenbacher et al.).
The primary star, a magnetic K giant, is particularly well-studied. It exhibits all the typical characteristics of RS CVn binaries: frequent outbursts (e.g., Brown & Brown 2006; Huenemoerder et al. 2013), chromospheric activity (e.g., Bopp et al. 1988; Montes et al. 2000), as well as luminosity variations due to the presence of large stellar spots (e.g., Oláh et al. 1989; Henry et al. 1995; Kajatkari et al. 2014). These cold spots on the surface have also been mapped using Doppler imaging techniques (e.g., Hatzes 1993; Kovári et al. 2001). Furthermore, Roettenbacher et al. (2017) studied in detail the surface structure of σ Gem by combining light curves, Doppler imaging, and interferometry.
Only a few continuous observation campaigns of the evolution of the spots on its surface have been published. The first, conducted by Kovári et al. (2001), focused on the evolution of the spots over 3.6 stellar rotations between 1996 and 1997. However, the authors were unable to clearly characterize the evolution of the spots or the differential rotation, attributing this to a possible masking effect related to the rapid evolution of the spots. A reanalysis of these data by Kovári et al. (2007) revealed a weak anti-solar differential rotation, where the polar regions rotate faster than the equator. Another campaign, published by Kovári et al. (2015), reanalyzed the 1996–1997 observations as well as new data from 2006–2007, once again confirming the anti-solar differential rotation. More recently, Korhonen et al. (2020) studied the evolution of spots and the differential rotation of σ Gem using broadband photometry and continuous spectroscopic observations over 150 nights, highlighting a marked solar-type differential rotation.
To determine the orbital parameters of σ Gem, we used a set of 58 validated spectra from the STAROS database. The analysis was carried out using the SpectroBinaryStarSystem library, which we slightly modified to allow radial velocity (RV) measurements across multiple lines for each spectrum.
Specifically, we used the Hα line located at 6582.82 Å, along with two iron lines at 6581.2 Å and 6663.446 Å. The error bars associated with the RV measurements correspond to the standard deviation calculated from the three measurements obtained for each spectrum.
It is worth noting that the SpectroBinaryStarSystem library relies on the open-source BinaryStarSolver library: https://github.com/NickMilsonPhysics/BinaryStarSolver
https://arxiv.org/abs/2011.13914
Each spectrum was normalized, and the telluric lines were carefully removed using the ISIS software.
It is important to note that at this stage, our results still present significant uncertainty. Indeed, we have not yet covered the phase where the radial velocity reaches its maximum, which limits the precision of the K amplitude measurement, and there is a gap in the phase coverage between 0.4 and 0.6.