Validating AU Microscopii d with Transit Timing Variations

Creators: Wittrock, Justin M.¹; Plavchan, Peter P.¹; Cale, Bryson L.^{2, 3, 4}; Barclay, Thomas^{5, 6}; Ludwig, Mathis R.⁷; Schwarz, Richard P.⁸; Mékarnia, Djamel⁹; J. Triaud, Amaury H. M.¹⁰; Abe, Lyu⁹; Suarez, Olga⁹; Guillot, Tristan⁹; Conti, Dennis M.¹¹; Collins, Karen A.⁸; Waite, Ian A.¹²; Kielkopf, John F.¹³; Collins, Kevin I.¹; Dreizler, Stefan⁷; El Mufti, Mohammed¹; Feliz, Dax L.¹⁴; Gaidos, Eric¹⁵; Geneser, Claire S.¹⁶; Horne, Keith D.¹⁷; Kane, Stephen R.¹⁸; Lowrance, Patrick J.^{3, 4}; Martioli, Eder^{19, 20, 21}; Radford, Don J.²²; Reefe, Michael A.²³; Roccatagliata, Veronica^{24, 25, 26}; Shporer, Avi²³; Stassun, Keivan G.²⁷; Stockdale, Christopher²⁸; Tan, Thiam-Guan^{29, 30}; Tanner, Angelle M.¹⁶; Vega, Laura D.^{6, 31}

1. George Mason University
2. Jet Propulsion Lab
3. Infrared Processing and Analysis Center
4. California Institute of Technology
5. University of Maryland, Baltimore
6. Goddard Space Flight Center
7. University of Göttingen
8. Harvard-Smithsonian Center for Astrophysics
9. Lagrange Laboratory
10. University of Birmingham
11. American Association of Variable Star Observers
12. University of Southern Queensland
13. University of Louisville
14. American Museum of Natural History
15. University of Hawaii at Manoa
16. Mississippi State University
17. University of St Andrews
18. University of California, Riverside
19. Laboratório Nacional de Astrofísica
20. Institut d'Astrophysique de Paris
21. Sorbonne University
22. Brierfield Observatory, Bowral, NSW, Australia
23. Massachusetts Institute of Technology
24. Arcetri Astrophysical Observatory
25. INFN Sezione di Pisa
26. University of Pisa
27. Vanderbilt University
28. Hazelwood Observatory, Hazelwood South, VIC, Australia
29. Perth Exoplanet Survey Telescope Observatory
30. Curtin University
31. University of Maryland, College Park

Abstract

AU Mic is a young (22 Myr), nearby exoplanetary system that exhibits excess transit timing variations (TTVs) that cannot be accounted for by the two known transiting planets nor stellar activity. We present the statistical "validation" of the tentative planet AU Mic d (even though there are examples of "confirmed" planets with ambiguous orbital periods). We add 18 new transits and nine midpoint times in an updated TTV analysis to prior work. We perform the joint modeling of transit light curves using EXOFASTv2 and extract the transit midpoint times. Next, we construct an O−C diagram and use Exo-Striker to model the TTVs. We generate TTV log-likelihood periodograms to explore possible solutions for d's period, then follow those up with detailed TTV and radial velocity Markov Chain Monte Carlo 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, the most-favored orbital period of AU Mic d is 12.73596 ± 0.00793 days (T_C,d = 2458340.55781 ± 0.11641 BJD), which puts the three planets near 4:6:9 mean-motion resonance. The mass for d is 1.053 ± 0.511 M_⊕, making this planet Earth-like in mass. If confirmed, AU Mic d would be the first known Earth-mass planet orbiting a young star and would provide a valuable opportunity in probing a young terrestrial planet's atmosphere. 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.

Copyright and License

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

This paper includes data collected by the TESS mission, which are publicly available from the Mikulski Archive for Space Telescopes (MAST). Funding for the TESS mission is provided by NASA's Science Mission directorate. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products.

This work is based (in part) on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This research has made use of the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

This work makes use of observations from the Las Cumbres Observatory global telescope network. Part of the LCOGT telescope time was granted by NOIRLab through the Mid-Scale Innovations Program (MSIP). MSIP is funded by NSF.

This research made use of the PEST photometry pipeline³⁹by Thiam-Guan Tan.

This work makes use of observations from the ASTEP telescope. ASTEP benefited from the support of the French and Italian polar agencies IPEV and PNRA in the framework of the Concordia station program, from OCA, INSU, and Idex UCAJEDI (ANR-15-IDEX-01).

This research received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 803193/BEBOP), and from the Science and Technology Facilities Council (STFC; grant No. ST/S00193X/1).

This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.

This research has made use of the Exoplanet Follow-up Observation Program (ExoFOP; DOI: 10.26134/ExoFOP5) website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.

This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.

This research has made use of NASA's Astrophysics Data System Bibliographic Services.

This research has made use of an online calculator that converts a list of Barycentric Julian Dates in Barycentric Dynamical Time (BJD_TDB) to JD in UT (Eastman et al. 2010)⁴⁰.

We also give thanks to Trifon Trifonov for his assistance in the use of the Exo-Striker package and analysis of the AU Mic system.

Funding

P.P.P. acknowledges support from NASA (Exoplanet Research Program Award No. 80NSSC20K0251, TESS Cycle 3 Guest Investigator Program Award No. 80NSSC21K0349, JPL Research and Technology Development, and Keck Observatory Data Analysis) and the NSF (Astronomy and Astrophysics grants Nos. 1716202 and 2006517), and the Mt Cuba Astronomical Foundation.

E.G. acknowledges support from the NASA Exoplanets Research Program Award No. 80NSSC20K0251. The material is based upon work supported by NASA under award No. 80GSFC21M0002.

L.D.V. acknowledges funding support from the Heising-Simons Astrophysics Postdoctoral Launch Program, through a grant to Vanderbilt University.

V.R. acknowledges the support of the Italian National Institute of Astrophysics (INAF) through the INAF GTO Grant "ERIS and SHARK GTO data exploitation."

Facilities

ASTEP 400:0.4m (FLI Proline 16800E) - , Brierfield:0.36m (Moravian G4-16000 KAF-16803) - , CFHT (SPIRou) - Canada-France-Hawaii Telescope, CHEOPS - , ExoFOP - , Exoplanet Archive - , IRSA - , IRTF (iSHELL) - Infrared Telescope Facility, LCOGT (CTIO:1m - , SAAO:1m - , SSO:1m - Siding Springs Observatory's 1 meter Telescope, and TO:1 m; Sinistro) - , MAST - , MKO CDK700:0.7m (U16) - , PEST:0.30m (SBIG ST-8XME) - , Spitzer (IRAC) - Spitzer Space Telescope satellite, TESS - , VLT:Antu (ESPRESSO) - Very Large Telescope (Antu)

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