MODELLING ACTIVITY-INDUCED RADIAL VELOCITIES THROUGH STELLA/WiFSIP SIMULTANEOUS PHOTOMETRY A. Castro-González1, J. Lillo-Box1, and the KOBE team2-6 1Centro de Astrobiología, 2Universidade do Porto, 3Universidad de Cádiz, 4Aix-Marseille Université, 5Instituto de Astrofísica de Andalucía, 6Observatorio de Calar Alto The KOBE experiment (K-dwarfs Orbited by Habitable Exoplanets; Lillo-Box et al. 2022, submitted) is aimed at searching for new worlds within the habitable zone (HZ) of K-dwarf stars through the radial velocity method using the CARMENES spectrograph. The survey is currently monitoring 50 pre-selected late K-dwarf stars, where we expect to find a significant number of planets according to planet occurrence rates in this stellar regime. K-dwarf stars are the ideal hosts to search for planets in the HZ. Contrary to G-dwarfs, the HZ is closer, thus making planet detection easier. And contrary to M-dwarfs, the stellar activity is much smaller, hence having a lower impact in the detectability and in the true habitability of the planet. Despite this, there is a desert of habitable worlds around K-dwarfs due to the lack of surveys devoted to this parameter space. In Fig. 1 we show 2 years of 3-day cadence simulated WiFSIP photometry and 9-day cadence CARMENES data for a typical target of our sample. We used the SOAP2 package (Dumusque et al. 2014) to simulate the photometric and spectroscopic effects of two spots covering 0.38% of the stellar surface, and two plages covering 1.68%. We then injected a simulated Keplerian signal of 4 m/s in an orbital period of 86.5 days. We analysed the simulated dataset by both using and not using the photometric time series. Our results show that in this simulation the planet is detected in both cases but the planet properties are a 30% more precise when using the photometric time series. For lower-mass planet simulations we find that only when using the WiFSIP photometry we can significantly detect the signal. K-dwarfs are among the most quiet stars in terms of activity. However, the low-amplitude RV signals from HZ planets can be disturbed by the presence of active regions. A key technique to achieve very precise RV measurements consists of simultaneously characterizing this stellar activity by measuring the so-called activity indicators. These indicators can be obtained from the spectra from which the RV is acquired or from additional observations (e.g. photometry has been proved to be a good indicator; Aigrain et al. 2012). The vast majority of works that take into account activity indicators, make use of those extracted from the RV spectra alone, due to its observational simplicity. However, the KOBE long-cadence of 9 days is not enough to properly sample the activity of K dwarfs. With typical rotation periods of 40 days (and a range between 10-50 days, McQuillan et al. 2014), the RV cadence would only sample up to 5 data points per stellar rotation. We fit the spectroscopic and photometric data through a Bayesian approach combined with a Markov chain Monte Carlo (MCMC) sampling. The signals caused by stellar activity can be seen as correlated quasi-periodic noise. To deal with it, Gaussian Processes (GP) are an increasingly common tool. Thus, we model the RV through a Keplerian and a GP, and jointly we model the full width of half maximum (FWHM) of the cross correlation function (CCF) and the photometric time series through GPs with shared hyperparameters. In particular, we use a quasi-periodic kernel of the form where 𝜂1 is the amplitude of the correlations, 𝜂2 is the timescale of the correlation decay, 𝜂3 defines the periodic component and 𝜂4 controls the relative importance of the periodic and decaying components. Methodology References. 1. Aigrain S., Pont F., Zucker S., 2012, MNRAS, 419, 3147. 2. McQuillan A., Mazeh T., & Aigrain S., 2014, ApJS, 211, 24. 3. Dumusque X., Boisse I., & Santos N. C. 2014, ApJ, 796, 132. Figure 1. Simulated RV, FWHM and photometry for a typical target