Volume 653, September 2021
|Number of page(s)||17|
|Section||Planets and planetary systems|
|Published online||13 September 2021|
A large sub-Neptune transiting the thick-disk M4 V TOI-2406
Center for Space and Habitability, University of Bern,
2 Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
3 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
4 Universidad Nacional de Córdoba – Observatorio Astronómico de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
5 Consejo Nacional de Investigaciones Cientf́icas y Técnicas (CONICET), Buenos Aires, Argentina
6 Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, México
7 Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA
8 Max Planck Institute for Astronomy (MPIA), Koenigstuhl 17, 69117 Heidelberg, Germany
9 Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
10 Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Universitďe Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
11 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
12 Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800, BC, México
13 Department of Physics & Astronomy, Vanderbilt University, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
14 NASA Ames Research Center, Moffett Field, CA 94035, USA
15 School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmimgham B15 2TT, UK
16 Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
17 Lunar and Planetary Laboratory, The University of Arizona, 1639 E. University Boulevard, Tucson, AZ 85721, USA
18 Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
19 Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, Zhongli, Taoyuan 32001, Taiwan
20 Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
21 Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138, USA
22 Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071, Granada, Spain
23 NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E. California Blvd., Pasadena, CA 91125, USA
24 Vatican Observatory Research Group, University of Arizona, 933 N Cherry Ave., Tucson AZ, 85721-0065, USA
25 Department of Astronomy, Tsinghua University, Beijing 100084, PR China
26 Department of Astronomy, 501 Campbell Hall, University of California at Berkeley, Berkeley, CA, 94720, USA
27 Department of Physics & Astronomy, University of California, Los Angeles, Los Angeles, CA 90095, USA
28 Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
29 Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
30 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
31 Patashnick Voorheesville Observatory, Voorheesville, NY 12186, USA
32 Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
33 SETI Institute, Mountain View, CA 94043, USA
34 Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
Accepted: 23 July 2021
Context. Large sub-Neptunes are uncommon around the coolest stars in the Galaxy and are rarer still around those that are metal-poor. However, owing to the large planet-to-star radius ratio, these planets are highly suitable for atmospheric study via transmission spectroscopy in the infrared, such as with JWST.
Aims. Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. The star’s low metallicity and the relatively large size and short period of the planet make TOI-2406 b an unusual outcome of planet formation, and its characterisation provides an important observational constraint for formation models.
Methods. We first infer properties of the host star by analysing the star’s near-infrared spectrum, spectral energy distribution, and Gaia parallax. We use multi-band photometry to confirm that the transit event is on-target and achromatic, and we statistically validate the TESS signal as a transiting exoplanet. We then determine physical properties of the planet through global transit modelling of the TESS and ground-based time-series data.
Results. We determine the host to be a metal-poor M4 V star, located at a distance of 56 pc, with properties Teff = 3100 ± 75 K, M* = 0.162 ± 0.008M⊙, R* = 0.202 ± 0.011R⊙, and [Fe∕H] = −0.38 ± 0.07, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with Rp = 2.94 ± 0.17R⊕ and P= 3.077 d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3σ, prompting questions about the dynamical history of the system.
Conclusions. This system is an interesting outcome of planet formation and presents a benchmark for large-planet formation around metal-poor, low-mass stars. The system warrants further study, in particular radial velocity follow-up to determine the planet mass and constrain possible bound companions. Furthermore, TOI-2406 b is a good target for future atmospheric study through transmission spectroscopy. Although the planet’s mass remains to be constrained, we estimate the S/N using amass-radius relationship, ranking the system fifth in the population of large sub-Neptunes, with TOI-2406 b having a much lower equilibrium temperature than other spectroscopically accessible members of this population.
Key words: planets and satellites: detection / stars: individual: TOI-2406 / techniques: photometric
© ESO 2021
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