A trio of transiting Neptunian planets around HD 28109 (aka TOI-282) Florian Rodler (ESO Chile, frodler@eso.org) on behalf of Georgina Dransfield, Amaury H.M.J. Triaud, Davide Gandolfi, Juan Cabrera, Tristan Guillot, Djamel Mekarnia, David Nesvorny, Nicolas Crouzet, Lyu Abe, Karim Agabi, Marco Buttu, Maximilian N. Günther, François-Xavier Schmider, Philippe Stee, Olga Suarez, Karen A. Collins, Martín Dévora-Pajares, Steve B. Howell, Elisabeth C. Matthews, Matthew R. Standing, Keivan G. Stassun, Chris Stockdale, Samuel N. Quinn, Carl Ziegler, Ian J. M. Crossfield, Jack J. Lissauer, Andrew W. Mann, Rachel Matson, Joshua Schlieder and George Zhou Introduction. HD 28109 (TOI-282, where TOI stands for TESS Object of Interest) is a bright (V=9.4) main-sequence star, of spectral type F8/G0V, located in the southern continuous viewing zone of the Transiting Exoplanet Survey Satellite (TESS). TESS observations taken with 2 min cadence initially led to three transiting planet candidate signals: TOI-282.01 with an orbital period of about 56 days, .02 with a period of 31 days, and finally .03 with a period of 84 days. A search for additional candidates in the TESS data as well as in radial velocity data revealed that the candidate feature .02 with a period of 31 days was spurious; instead, a further candidate signal .04 with an orbital period close to 23 days was identified (Fig. 1). We furthermore excluded the presence of background stars of / stellar companions to HD 28109 by using high-resolution imaging data with NACO (VLT), Zorro (Gemini South) and with SOAR. We verified the three candidate signals with photometry and radial velocity (RV) follow-up. Transit-time variations (TTVs). Given the proximity of the orbital periods of the two outer planets to a first order 3:2 mean-motion resonance (84.3 vs 56 days), these planets experience some mutual gravitational influence leading to TTVs. Fig. 2 shows that these planets present very clear and significant anti-correlated TTVs with peak to peak amplitudes of ∼50 min and ∼100 min respectively. This allowed us to put constraints on their masses (see table below). Radial velocity (RV) measurements. We are currently conducting an RV survey with ESO's ultra- stable spectrographs HARPS and ESPRESSO to further characterize the planetary system (Fig. 3). Stellar activity in the star, which exhibits stellar jitter on the order of 2.5 - 3 m/s, plus the low masses of the planets and the resulting small RV semi-amplitudes ranging from only 0.8 to 2.4 m/s, make this RV follow-up a very challenging task. Results. The following table lists the fundamental parameters derived from our analysis, with their error values specified in parentheses. The mass estimates for the planets are derived by RV measurements as well as by TTVs (the latter for planets c and d; marked with asterisk*). Fig. 4 shows that the inner planet b is supposedly a rocky sub-Neptunian with a density of Earth, while the outer two planets are fluffy, gaseous Neptunians. The results are currently published in Dransfield++ (MNRAS, under review). The ongoing RV survey will be published in Rodler++. Figure 1: Global fit of the TESS photometry data for each planet (phase folded). The colored data points present the TESS data taken with 2 minutes cadence, while the white circles are binned data. Figure 2: Transit-time variations (in minutes) for the planets c and d spanning over three years of TESS data plus ground-based photometric monitoring. Candidate → Planet Orbital Period (days) Semi-major axis (AU) Radius (Earth) Mass (Earth) TOI-282.04 → HD 28109 b 22.891 (0.0004) 0.14 (0.003) 2.2 (0.1) 11.4 (2.6) TOI-282.01 → HD 28109 c 56.008 (0.002) 0.31 (0.011) 4.2 (0.11) 10.2 (3.6); 7.9* (3.8)* TOI-282.03 → HD 28109 d 84.26 (0.007) 0.41 (0.016) 3.3 (0.11) 8.1 (4.1); 5.7* (2.5)* Figure 3: Radial velocity fit based on 38 HARPS and 44 ESPRESSO measurements. Figure 4: Mass-radius diagram showing the positions of HD 28109 b, c and d. While the innermos