TOI 560 : Two Transiting Planets Orbiting a K Dwarf Validated with iSHELL, PFS and HIRES RVs Mohammed El Mufti1, Peter Plavchan1, Angelle Tanner2, Eric Gaidos3, Bryson Cale4, Michael Reefe1, Justin Wittrock1, Kevin Issac Collins1, Patrick Newman1, David James Vermilion1,5, Claire Geneser2, Rena Lee3, Ahmad Sohani2, Farzaneh Zohrabi6 George Mason University1, Mississippi State University2, University of Hawai‘i at Mänoa3, NASA JPL4, NASA Godard Center5, Louisiana State University6 Abstract We validate the presence of a two-planet system orbiting the 0.15–1.4 Gyr K4 dwarf TOI 560 (HD 73583). The system con- sists of an inner moderately eccentric transiting mini-Neptune (TOI 560 b, P = 6.397438 ± 0.000037 days, e = 0.294+0.13 −0.062) initially discovered in the Sector 8 mission observations, and a transiting mini-Neptune (TOI 560 c, P = 18.8779 ± 0.0016 days) discovered in the Sector 34 observations, in a rare 1:3 orbital resonance. We utilize photometric data from Spitzer, and ground-based follow-up observations to confirm the ephemerides and period of the transiting planets and vet false positive scenarios. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with the iSHELL spectrograph at the NASA Infrared Telescope Facility and the HIRES Spectrograph at Keck Observatory to validate the plan- etary nature of these signals, which we combine with published PFS RVs from Magellan Observatory. We place upper limits on the masses of both planets of <2.1 and <4.1 MNep for b and c, respectively. We apply a Gaussian Processes (GP) model to the light curves to place priors on a chromatic radial velocity GP model to constrain the stellar activity of the TOI 560 host star. TOI 560 is a nearby moderately young multi-planet sys- tem with two planets suitable for atmospheric characterization with James Webb Space Telescope (JWST) and other upcom- ing missions. pychell Data Pipeline When star light passes through our gas cell and iSHELL cross-dispersed echelle spectrograph, we obtain high-resolution (R=80k) multi-order spectra. For each of the 29 orders, we do optimal spectral extraction (weighted summation) in the verti- cal direction so that in each column eventually we get a single value, which gives us a 1D spectrum for each order. We then forward model the spectrum to measure the radial velocity for each order, and then co-add the RVs from each order. We an- alyze our extracted spectral data using the following standard forward model where each term is a function of wave- length: Intensity = Blaze Function ×(Star × Gas Cell × Tellurics) ⊗Line Spread Function The blaze function ultimately sets the continuum of our spec- trum. The line spread function effectively blurs the spectrum down to resolution of the iSHELL spectrograph. The star has a Doppler shift, and the gas cell also has a Doppler shift which enables us to calibrate for instrumental drift in the wavelength solution and to constrain the line spread function. The tel- lurics, parameterized by a scaling factor optical depth related to airmass, are the remaining lines that causes by the green- house gases in the Earth’s atmosphere. Our RV pipeline is adapted from the CSHELL RV code de- scribed in [1]. We have rewritten the CSHELL code in a Python script pychell to adapt to iSHELL’s larger spectral grasp with multiple orders [2]. TOI 560 host star Parameter Value Reference Spectral type K4 Scolz et al. 2005 Teff[K] 4579+62 −61 this work R∗(R) 0.679+0.018 −0.017 this work Rotation period 12.2 ± 0.1 this work Age 150Myr - 0.5Gyr this work M∗(M⊕) 0.701+0.026 −0.025 this work Luminosity(L∗) 0.1802 ± 0.0058 Stassun et al. 2019 R.A. 08:38:45.260 Stassun et al. 2019 Dec. -13:15:24.09 Stassun et al. 2019 Data analysis • Standard G.P. Kernel: KQP(ti, tj) = ησ2 Decay z }| { exp  −∆t2 2ητ 2   Periodic z }| { exp  −1 2ηl2 sin2        π∆t ηp          , • Modified Chromatic G.P. Kernel: KJ2(ti, tj, λi, λj) = η2 σ,0  