Astronomers Discover Two Super-Earths and One Sub-Neptune Orbiting A K-dwarf Star (Planetary Science)

Using TESS observations from 2 sectors, ground-based follow-up from LCOGT and NGTS, and space-based follow-up from Spitzer, a team of international astronomers reported the discovery of three new planets TOI-431 b, c, d orbiting a K-dwarf star TOI-431. Their study recently appeared in Arxiv.

The TOI-431 system was observed in TESS Sectors 5 (Nov 15 to Dec 11 2018) and 6 (Dec 15 2018 to Jan 6 2019) on Camera 2 in the 2-minute cadence mode (𝑡exp = 2 min). TOI-431.01 (now TOI-431 d) was flagged on Feb 8 2019 by the MIT Quick-Look Pipeline with a signal-to-noise ratio (SNR) of 58 while, TOI-431.02 (now TOI-431 b) was flagged later, on June 6, after identification by the TESS Science Processing Operations Center (SPOC) pipeline with an SNR of 24 in a combined transit search of Sectors 5-6.

Now, using ground-based follow-up from LCOGT and NGTS, and space-based follow-up from Spitzer, a team of international astronomers led by Ares Osborn confirmed the planetary nature of TOI-431 d and b.

Table 1: Stellar parameters for TOI-431 © Ares Osborn et al.

They also characterized the bright, multi-planet system TOI-431 using photometry and radial velocities and found evidence that the host star is rotating with a period of 30.5 days.

“The photometric data was modelled jointly with RV data from the HARPS spectrograph, and further RVs from iSHELL, FEROS, and Minerva-Australis are included in our analysis.”

Additionally, it has been shown that the TOI-431 b is a super-Earth, with an ultra-short period of 0.49 days, a radius of 1.28 R, a mass of 3.07 M and a density of 8 g cm³. It likely has a negligible envelope due to substantial atmosphere evolution via photoevaporation, and an Earth-like composition.

Table 2: Planetary parameters TOI-431 b, c, and d © Ares Osborn et al.

While, TOI-431 d is a sub-Neptune with a period of 12.46 days, radius of 3.29 R, a mass of 9.90 M and density of 1.36 g cm. It has likely retained a substantial H-He envelope of about 4 per cent of its total mass.

Moreover, they discovered TOI-431 c in the HARPS RV data, but is not seen to transit in the TESS light curves. It has a period of 4.84 days and a minimum mass similar to the mass of TOI-431 b; extrapolating this minimum mass to a radius via the MR relation places it as a likely second super-Earth.

Finally, it has been suggested that, TOI-431 d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterisation, while the super-Earth TOI-431 b may be a stripped core.

“This system is a candidate for further study of planetary evolution, with TOI-431 b and d either side of the radius valley. The system is bright, making it amenable to follow-up observations. TOI-431 b, in particular, would potentially be an interesting target for phase-curve observations with JWST.”

— they concluded.

Featured image: Artwork of exoplanets around nearby star © Getty images

Reference: Ares Osborn, David J. Armstrong, Bryson Cale, Rafael Brahm, Robert A. Wittenmyer, Fei Dai, Ian J. M. Crossfield, Edward M. Bryant, Vardan Adibekyan, Ryan Cloutier, Karen A. Collins, E. Delgado Mena, Malcolm Fridlund, Coel Hellier, Steve B. Howell, George W. King, Jorge Lillo-Box, Jon Otegi, S. Sousa, Keivan G. Stassun, Elisabeth C. Matthews, Carl Ziegler, George Ricker, Roland Vanderspek, David W. Latham, S. Seager, Joshua N. Winn, Jon M. Jenkins, Jack S. Acton, Brett C. Addison, David R. Anderson, Sarah Ballard, David Barrado, Susana C. C. Barros, Natalie Batalha, Daniel Bayliss, Thomas Barclay, Björn Benneke, John Berberian Jr., Francois Bouchy, Brendan P. Bowler, César Briceño, Christopher J. Burke, Matthew R. Burleigh, Sarah L. Casewell, David Ciardi, Kevin I. Collins, Benjamin F. Cooke, Olivier D. S. Demangeon, Rodrigo F. Díaz, C. Dorn, Diana Dragomir, Courtney Dressing, Xavier Dumusque, Néstor Espinoza, P. Figueira, Benjamin Fulton, E. Furlan, E. Gaidos, C. Geneser, Samuel Gill, Michael R. Goad, Erica J. Gonzales, Varoujan Gorjian, Maximilian N. Günther, Ravit Helled, Beth A. Henderson, Thomas Henning, Aleisha Hogan, Saeed Hojjatpanah, Jonathan Horner, Andrew W. Howard, Sergio Hoyer, Dan Huber, Howard Isaacson, James S. Jenkins, Eric L. N. Jensen, Andrés Jordán, Stephen R. Kane, Richard C. Kidwell Jr., John Kielkopf, Nicholas Law, Monika Lendl, M. Lund, Rachel A. Matson, Andrew W. Mann, James McCormac, Matthew W. Mengel, Farisa Y. Morales, Louise D. Nielsen, Jack Okumura, Hugh P. Osborn, Erik A. Petigura, Peter Plavchan, Don Pollacco, Elisa V. Quintana, Liam Raynard, Paul Robertson, Mark E. Rose, Arpita Roy et al., “TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet”, Arxiv, pp. 1-21, 2021.

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