1 Thüringer Landessternwarte Tautenburg, Sternwarte 5 07778 Tautenburg Germany
2 Dipartimento di Fisica, Universitá di Torino, via P. Giuria 1, 10125 Torino, Italy
3 Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
4 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan
5 Leiden Observatory, Leiden University, 2333CA Leiden, The Netherlands
6 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
7 Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
8 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
9 Institute for Computational Science, Center for Theoretical Astrophysics & Cosmology, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland
10 Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Strasse 209, 50931 Köln, Germany
11 Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
12 Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
13 Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
14 Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
15 Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
16 Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
17 National AstronomicalObservatory of Japan, NINS, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
18 Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20015-1305, USA
19 Department of Astronomy and McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
20 The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
21 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
22 Princeton University, Department of Astrophysical Sciences, 4 Ivy Lane, Princeton, NJ 08540, USA
23 University of Southern Queensland, Computational Science and Engineering Research Centre, Toowoomba QLD 4350, Australia
Received: 29 March 2017
Accepted: 7 September 2017
Aims. Planets in the mass range from 2 to 15 M⊕ are very diverse. Some of them have low densities, while others are very dense. By measuring the masses and radii, the mean densities, structure, and composition of the planets are constrained. These parameters also give us important information about their formation and evolution, and about possible processes for atmospheric loss.
Methods. We determined the masses, radii, and mean densities for the two transiting planets orbiting K2-106. The inner planet has an ultra-short period of 0.57 days. The period of the outer planet is 13.3 days.
Results. Although the two planets have similar masses, their densities are very different. For K2-106b we derive Mb=8.36-0.94+0.96 M⊕, Rb = 1.52 ± 0.16 R⊕, and a high density of 13.1-3.6+5.4 g cm-3. For K2-106c, we find Mc=5.8-3.0+3.3 M⊕, Rc=2.50-0.26+0.27 R⊕ and a relatively low density of 2.0-1.1+1.6 g cm-3.
Conclusions. Since the system contains two planets of almost the same mass, but different distances from the host star, it is an excellent laboratory to study atmospheric escape. In agreement with the theory of atmospheric-loss processes, it is likely that the outer planet has a hydrogen-dominated atmosphere. The mass and radius of the inner planet is in agreement with theoretical models predicting an iron core containing 80-30+20% of its mass. Such a high metal content is surprising, particularly given that the star has an ordinary (solar) metal abundance. We discuss various possible formation scenarios for this unusual planet.
Key words: techniques: photometric / techniques: radial velocities / stars: abundances / stars: individual: TYC 608-458-1 / planetary systems
The results are partly based on observations obtained at the European Southern Observatory at Paranal, Chile in program 098.C-0860(A). This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. The article is also partly based on observations with the TNG, NOT. This work has also made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium).
The RV measurements are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/608/A93
© ESO, 2017