Transit timing variations for AU Microscopii b and c

Justin M. Wittrock*, Stefan Dreizler, Michael A. Reefe, Brett M. Morris, Peter P. Plavchan, Patrick J. Lowrance, Brice-Olivier Demory, James G. Ingalls, Emily A. Gilbert, Thomas Barclay, Bryson L. Cale, Karen A. Collins, Kevin I. Collins, Ian J. M. Crossfield, Diana Dragomir, Jason D. Eastman, Mohammed El Mufti, Dax Feliz, Jonathan Gagné, Eric GaidosPeter Gao, Claire S. Geneser, Leslie Hebb, Christopher E. Henze, Keith D. Horne, Jon M. Jenkins, Eric L. N. Jensen, Stephen R. Kane, Laurel Kaye, Eder Martioli, Teresa A. Monsue, Enric Pallé, Elisa V. Quintana, Don J. Radford, Veronica Roccatagliata, Joshua E. Schlieder, Richard P. Schwarz, Avi Shporer, Keivan G. Stassun, Christopher Stockdale, Thiam-Guan Tan, Angelle M. Tanner, Andrew Vanderburg, Laura D. Vega, Songhu Wang

*Corresponding author for this work

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We explore the transit timing variations (TTVs) of the young (22 Myr) nearby AU Mic planetary system. For AU Mic b, we introduce three Spitzer (4.5 μm) transits, five TESS transits, 11 LCO transits, one PEST transit, one Brierfield transit, and two transit timing measurements from Rossiter–McLaughlin observations; for AU Mic c, we introduce three TESS transits. We present two independent TTV analyses. First, we use EXOFASTv2 to jointly model the Spitzer and ground-based transits and obtain the midpoint transit times. We then construct an O − C diagram and model the TTVs with Exo-Striker. Second, we reproduce our results with an independent photodynamical analysis. We recover a TTV mass for AU Mic c of 10.8−2.2+2.3 M. We compare the TTV-derived constraints to a recent radial velocity (RV) mass determination. We also observe excess TTVs that do not appear to be consistent with the dynamical interactions of b and c alone or due to spots or flares. Thus, we present a hypothetical nontransiting “middle-d” candidate exoplanet that is consistent with the observed TTVs and candidate RV signal and would establish the AU Mic system as a compact resonant multiplanet chain in a 4:6:9 period commensurability. These results demonstrate that the AU Mic planetary system is dynamically interacting, producing detectable TTVs, and the implied orbital dynamics may inform the formation mechanisms for this young system. We recommend future RV and TTV observations of AU Mic b and c to further constrain the masses and confirm the existence of possible additional planet(s).
Original languageEnglish
Article number27
Number of pages30
JournalAstronomical Journal
Issue number1
Early online date30 Jun 2022
Publication statusPublished - 1 Jul 2022


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