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Register a new userThe M dwarf planet search programme at the ESO VLT + UVES. A search for terrestrial planets in the habitable zone of M dwarfs
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Added by
Mathias Zechmeister
Publication date
2009
fields
Physics
and research's language is
English
Authors
M. Zechmeister MPIA
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We present radial velocity (RV) measurements of our sample of 40 M dwarfs from our planet search programme with VLT+UVES begun in 2000. Although with our RV precision down to 2 - 2.5 m/s and timebase line of up to 7 years, we are capable of finding planets of a few Earth masses in the close-in habitable zones of M dwarfs, there is no detection of a planetary companion. To demonstrate this we present mass detection limits allowing us to exclude Jupiter-mass planets up to 1 AU for most of our sample stars. We identified 6 M dwarfs that host a brown dwarf or low-mass stellar companion. With the exception of these, all other sample stars show low RV variability with an rms < 20 m/s. Some high proper motion stars exhibit a linear RV trend consistent with their secular acceleration. Furthermore, we examine our data sets for a possible correlation between RVs and stellar activity as seen in variations of the Halpha line strength. For Barnards star we found a significant anticorrelation, but most of the sample stars do not show such a correlation.
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We report on the first star discovered to host a planet detected by radial velocity (RV) observations obtained within the CARMENES survey for exoplanets around M dwarfs. HD 147379 ($V = 8.9$ mag, $M = 0.58 pm 0.08$ M$_{odot}$), a bright M0.0V star at a distance of 10.7 pc, is found to undergo periodic RV variations with a semi-amplitude of $K = 5.1pm0.4$ m s$^{-1}$ and a period of $P = 86.54pm0.06$ d. The RV signal is found in our CARMENES data, which were taken between 2016 and 2017, and is supported by HIRES/Keck observations that were obtained since 2000. The RV variations are interpreted as resulting from a planet of minimum mass $m_{rm p}sin{i} = 25 pm 2$ M$_{oplus}$, 1.5 times the mass of Neptune, with an orbital semi-major axis $a = 0.32$ au and low eccentricity ($e < 0.13$). HD 147379b is orbiting inside the temperate zone around the star, where water could exist in liquid form. The RV time-series and various spectroscopic indicators show additional hints of variations at an approximate period of 21.1d (and its first harmonic), which we attribute to the rotation period of the star.
Despite their activity, low-mass stars are of particular importance for the search of exoplanets by the means of Doppler spectroscopy, as planets with lower masses become detectable. We report on the discovery of a planetary companion around HD 180617, a bright J = 5.58 mag, low-mass M = 0.45 M_{sun} star of spectral type M2.5 V. The star, located at a distance of 5.9 pc, is the primary of the high proper motion binary system containing vB 10, a star with one of the lowest masses known in most of the twentieth century. Our analysis is based on new radial velocity (RV) measurements made at red-optical wavelengths provided by the high-precision spectrograph CARMENES, which was designed to carry out a survey for Earth-like planets around M dwarfs. The available CARMENES data are augmented by archival Doppler measurements from HIRES and HARPS. Altogether, the RVs span more than 16 years. The modeling of the RV variations, with a semi-amplitude of K = 2.85-0.25/+0.16m/s yields a Neptune-like planet with a minimum mass of 12.2-1.4/+1.0 M_{Earth} on a 105.90-0.10/+0.09d circumprimary orbit, which is partly located in the host stars habitable zone. The analysis of time series of common activity indicators does not show any dependence on the detected RV signal. The discovery of HD 180617 b not only adds information to a currently hardly filled region of the mass-period diagram of exoplanets around M dwarfs, but the investigated system becomes the third known binary consisting of M dwarfs and hosting an exoplanet in an S-type configuration. Its proximity makes it an attractive candidate for future studies.
Because the planets of a system form in a flattened disk, they are expected to share similar orbital inclinations at the end of their formation. The high-precision photometric monitoring of stars known to host a transiting planet could thus reveal the transits of one or more other planets. We investigate here the potential of this approach for the M dwarf GJ 1214 that hosts a transiting super-Earth. For this system, we infer the transit probabilities as a function of orbital periods. Using Monte-Carlo simulations we address both the cases for fully coplanar and for non-coplanar orbits, with three different choices of inclinations distribution for the non-coplanar case. GJ 1214 reveals to be a very promising target for the considered approach. Because of its small size, a ground-based photometric monitoring of this star could detect the transit of a habitable planet as small as the Earth, while a space-based monitoring could detect any transiting habitable planet down to the size of Mars. The mass measurement of such a small planet would be out of reach for current facilities, but we emphasize that a planet mass would not be needed to confirm the planetary nature of the transiting object. Furthermore, the radius measurement combined with theoretical arguments would help us to constrain the structure of the planet.
We present the discovery and characterisation of two transiting planets observed by the Transiting Exoplanet Survey Satellite (TESS) orbiting the nearby (d ~ 22 pc), bright (J ~ 9 mag) M3.5 dwarf LTT 3780 (TOI-732). We confirm both planets and their association with LTT 3780 via ground-based photometry and determine their masses using precise radial velocities measured with the CARMENES spectrograph. Precise stellar parameters determined from CARMENES high resolution spectra confirm that LTT 3780 is a mid-M dwarf with an effective temperature of T_eff = 3360 +- 51 K, a surface gravity of log(g) = 4.81 +/- 0.04 (cgs), and an iron abundance of [Fe/H] = 0.09 +/- 0.16 dex, with an inferred mass of M_star = 0.379 +/- 0.016 M_sun and a radius of R_star = 0.382 +/- 0.012 R_sun. The ultra-short-period planet LTT 3780 b (P_b = 0.77 d) with a radius of 1.35^{+0.06}_{-0.06} R_earth, a mass of 2.34^{+0.24}_{-0.23} M_earth, and a bulk density of 5.24^{+0.94}_{-0.81} g cm^{-3} joins the population of Earth-size planets with rocky, terrestrial composition. The outer planet, LTT 3780 c, with an orbital period of 12.25 d, radius of 2.42^{+0.10}_{-0.10} R_earth, mass of 6.29^{+0.63}_{-0.61} M_earth, and mean density of 2.45^{+0.44}_{-0.37} g cm^{-3} belongs to the population of dense sub-Neptunes. With the two planets located on opposite sides of the radius gap, this planetary system is an excellent target for testing planetary formation, evolution and atmospheric models. In particular, LTT 3780 c is an ideal object for atmospheric studies with the James Webb Space Telescope.
We confirm the planetary nature of TOI-1728b using a combination of ground-based photometry, near-infrared Doppler velocimetry and spectroscopy with the Habitable-zone Planet Finder.TOI-1728 is an old, inactive M0 star with teff{} $= 3980^{+31}_{-32}$ K, which hosts a transiting super Neptune at an orbital period of $sim$ 3.49 days. Joint fitting of the radial velocities and TESS and ground-based transits yields a planetary radius of $5.05_{-0.17}^{+0.16}$ R$_{oplus}$, mass $26.78_{-5.13}^{+5.43}$ M$_{oplus}$ and eccentricity $0.057_{-0.039}^{+0.054}$. We estimate the stellar properties, and perform a search for He 10830 AA absorption during the transit of this planet and claim a null detection with an upper limit of 1.1$%$ with 90% confidence. A deeper level of He 10830 AA ~ absorption has been detected in the planet atmosphere of GJ 3470b, a comparable gaseous planet. TOI-1728b is the largest super Neptune -- the intermediate subclass of planets between Neptune and the more massive gas-giant planets -- discovered around an M dwarf. With its relatively large mass and radius, TOI-1728 represents a valuable datapoint in the M-dwarf exoplanet mass-radius diagram, bridging the gap between the lighter Neptune-sized planets and the heavier Jovian planets known to orbit M-dwarfs. With a low bulk density of $1.14_{-0.24}^{+0.26}$ g/cm$^3$, and orbiting a bright host star (J $sim 9.6$, V $sim 12.4$), TOI-1728b is also a promising candidate for transmission spectroscopy both from the ground and from space, which can be used to constrain planet formation and evolutionary models.
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