The Spin-orbit alignment of two short period eclipsing binary systems

Abstract: We present a joint analysis of TESS photometry and radial velocity measurements obtained from the Minerva-Australis facility for two short-period eclipsing binaries, TIC 48227288 and TIC 339607421. TIC 339607421 hosts an M-dwarf companion ($M_B = 0.294 \pm 0.013 : M_{\odot}$, $R_B = 0.291 \pm 0.006 : R_{\odot}$) orbiting an F6V star ($M_A=1.09 \pm 0.04 : M_{\odot}$, $R_A=1.21^{+0.03}_{-0.02} : R_{\odot}$). While TIC 48227288 contains a late K class companion ($M_B=0.635 \pm 0.037 : M_{\odot}$, $R_B = 0.605 \pm 0.011 : R_{\odot}$) orbiting an F3V star ($M_A = 1.36^{+0.06}_{-0.08} : M_{\odot}$, $R_A = 1.61 \pm 0.03 : R_{\odot}$). Both companions follow short period, near-circular orbits ($P_B = 2.4-3.0$ d, $e \approx 0.001$). Sky-projected obliquities for each system were derived using a classical analysis of the RV perturbation and the Reloaded Rossiter-McLaughlin (RRM) technique. The classical method indicates minor spin-orbit misalignment for both systems ($\lambda_A = -14.7^{+5.4}_{-5.9}$ deg and $-17.8^{+1.9}_{-2.0}$ deg for TIC 339607421 and TIC 48227288, respectively). The RRM analysis yields smaller obliquities ($\lambda_A = -8.2 \pm 0.2$ deg and $-9.5 \pm 0.2$ deg respectively), but confirms the minor misalignment inferred from the classical analysis. The findings of misaligned, circular orbits are notable even though the misalignments are not large, and suggest potential gaps in current models of binary formation and orbital evolution. As such, further investigation of these and similar systems appears warranted.