Details

Autor(en) / Beteiligte

• Wittrock, Justin M

Titel

Characterizing AU Microscopii Planets With Transit Timing Variations

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2023

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Exoplanetary sciences is a relatively new field of study that has grown beyond just detection method to include, among other things, planetary and atmospheric characterizations. Recent missions such as Kepler, K2, and TESS has resulted in more than 5 000 planets being validated or confirmed, allowing us to delve deeply into studying the properties of these planets and their host systems. In this thesis, I present the Transit Timing Variations (TTVs) of AU Mic b and c and the validation of the candidate planet AU Mic d. AU Mic is a young (22 Myr) nearby exoplanetary system that exhibits excess TTVs that cannot be accounted for by the two known transiting planets b and c nor stellar activity. 37 transit observations (including one ASTEP, one Brierfield, 23 LCOGT, one PEST, three Spitzer, and eight TESS), three Rossiter-McLaughlin (R-M) observations (ESPRESSO, iSHELL, and SPIRou), and nine CHEOPS transit midpoint times are included in our TTV analyses. First, we use EXOFASTv2 to jointly model the transit light curves to obtain the transit midpoint 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+2.3−2.2 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, and do not appear to be due to spots or flares; therefore, we hypothesize the existence of a third planet AU Mic d that is driving the observed excess TTVs. I calculate d’s potential orbital periods by modeling the observed super-period in AU Mic b’s TTVs. Next, we explore several possible configurations for planet d, including having d be interior to b and having d be between b and c. We generate TTV log-likelihood periodograms to explore possible solutions for the orbital period of planet d and then follow those up with detailed TTV and RV MCMC modeling and stability tests. We find several candidate periods for AU Mic d, all of which are near resonances with AU Mic b and c of varying order. Based on our model comparisons, AU Mic b’s TTV super-period, stability tests, and Occam’s razor arguments regarding near-mean motion orbital resonance (MMR) chains and coplanarity of AU Mic system, the most-favored orbital period of AU Mic d is 12.73812 ± 0.00128 days (TC,d = 2458333.32110 ± 0.35836 BJD), which puts the three planets near a 4:6:9 MMR. The mass for d from the most plausible case is Md = 1.013 ± 0.146 M⊕, making this planet Earth-like in mass. This would make AU Mic the first known young star to host an Earth-mass planet. Lastly, I characterize AU Mic b’s atmosphere with Transit Depth Variations and found the effective radius of AU Mic b to be significantly smaller at 4.5 μm than at optical range, a phenomenon that is similarly seen in another young planet K2-33 b. The mass-radius analysis of AU Mic b indicates that it is comparable to that of a 10% H + He planet; however, since it’s atmosphere is known to be evaporating, AU Mic b is evolving into a smaller and denser planet over the next several Myrs or Gyrs. The presence of near-MMRs in a very young system implies that compact planetary systems can develop resonant chains very early on, which can quickly establish the stability of the systems. Additional TTV observations of the AU Mic system are needed to further constrain the planetary masses, search for possible transits of AU Mic d, and detect possible additional planets beyond AU Mic c. If AU Mic d does transit, it will serve as an incredibly valuable case study for characterizing the atmosphere of young terrestrial planets and understanding its evolution.