M. Zechmeister

A terrestrial planet candidate in a temperate orbit around Proxima Centauri

At a distance of 1.295 parsecs, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun’s closest stellar neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days (ref. 3) and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface.

Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf

Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this paper, we investigate the radial velocity variations of Kapteyns star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS, HIRES and PFS Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 days using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indicies and archival ASAS-3 photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyns star is most probably orbited by two super-Earth mass planets, one of which is orbiting in its circumstellar habitable zone, becoming the oldest potentially habitable planet known to date. The presence and long-term survival of a planetary system seems a remarkable feat given the peculiar origin and kinematic history of Kapteyns star. The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.

The planet search programme at the ESO CES and HARPS - IV. The search for Jupiter analogues around solar-like stars

Context. In 1992 we began a precision radial velocity survey for planets around solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES LC) at the 1.4m telescope in La Silla (Chile) resulting in the discovery of the planet iota Hor b. We have continued the survey with the upgraded CES Very Long Camera (VLC) and the HARPS spectrographs, both at the 3.6m telescope, until 2007. Aims. In this paper we present additional radial velocities for 31 stars of the original sample with higher precision. The observations cover a time span of up to 15 years and permit a search for Jupiter analogues. Methods. The survey was carried out with three different instruments/instrument configurations using the iodine absorption cell and the ThAr methods for wavelength calibration. We combine the data sets and perform a joint analysis for variability, trends, and periodicities. We compute Keplerian orbits for companions and detection limits in case of non-detections. Moreover, the HARPS radial velocities are analysed for correlations with activity indicators (CaII H&K and cross-correlation function shape). Results. We achieve a long-term RV precision of 15 m/s (CES+LC, 1992-1998), 9 m/s (CES+VLC, 1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), respectively. This enables us to confirm the known planetary signals in iota Hor and HR 506 as well as the three known planets around HR 3259. A steady RV trend for epsilon Ind A can be explained by a planetary companion and calls for direct imaging campaigns. On the other hand, we find previously reported trends to be smaller for beta Hyi and not present for alpha Men. The candidate planet epsilon Eri b was not detected despite our better precision. Also the planet announced for HR 4523 cannot be confirmed. Long-term trends in several of our stars are compatible with known stellar companions. We provide a spectroscopic orbital solution for the binary HR 2400 and refined solutions for the planets around HR 506 and iota Hor. For some other stars the variations could be attributed to stellar activity, as e.g. the magnetic cycle in the case of HR 8323. Conclusions. The occurrence of two Jupiter-mass planets in our sample is in line with the estimate of 10% for the frequency of giant planets with periods smaller than 10 yr around solar-like stars. We have not detected a Jupiter analogue, while the detections limits for circular orbits indicate at 5 AU a sensitivity for minimum mass of at least 1M(Jup) (2M(Jup)) for 13% (61%) of the stars. (Less)

An Independent Planet Search In The Kepler Dataset II. An extremely low-density super-Earth mass planet around Kepler-87.

Context. The primary goal of the Kepler mission is the measurement of the frequency of Earth-like planets around Sun-like stars. However, the confirmation of the smallest of Kepler’s candidates in long periods around FGK dwarfs is extremely difficult or even beyond the limit of current radial velocity technology. Transit timing variations (TTVs) may offer the possibility for these confirmations of near-resonant multiple systems by the mutual gravitational interaction of the planets. Aims. We previously detected the second planet candidate in the KOI1574 system. The two candidates have relatively long periods (about 114d and 191d) and are in 5:3 resonance. We therefore searched for TTVs in this particularly promising system. Methods. The full Kepler data was detrended with the proven SARS pipeline. The entire data allowed one to search for TTVs of the above signals, and to search for additional transit-like signals. Results. We detected strong anti-correlated TTVs of the 114d and 191d signals, dynamically confirming them as members of the same system. Dynamical simulations reproducing the observed TTVsallowed us to also determine the masses of the planets. We found KOI 1574.01 (hereafter Kepler-87b) to have a radius of 13.49 ± 0.55 R⊕ and a mass of 324.2 ±8.8 M⊕, and KOI 1574.02 (Kepler-87c) to have a radius of 6.14 ± 0.29 R⊕ and a mass of 6.4 ± 0.8 M⊕. Both planets have low densities of 0.729 and 0.152 gcm −3 , respectively, which is non-trivial for such cold and old (7−8 Gyr) planets. Specifically, Kepler-87c is the lowest-density planet in the super-Earth mass range. Both planets are thus particularly amenable to modeling and planetary structure studies, and also present an interesting case where ground-based photometric follow-up of Kepler planets is very desirable. Finally, we also detected two more short-period super-Earth sized (<2 R⊕) planetary candidates in the system, making the relatively high multiplicity of this system notable against the general paucity of multiple systems in the presence of giant planets like Kepler-87b.

Radial velocity signatures of Zeeman broadening

Stellar activity signatures such as spots and plage can significantly limit the search for extrasolar planets. Current models of activity-induced radial velocity (RV) signals focused on the impact of temperature contrast in spots predicting the signal to diminish toward longer wavelengths. Contrary to this is the Zeeman effect on radial velocity measurements: the relative importance of the Zeeman effect on RV measurements should grow with wavelength because the Zeeman displacement itself grows with �, and because a magnetic and cool spot contributes more to the total flux at longer wavelengths. In this paper, we model the impact of active regions on stellar RV measurements including both temperature contrast in spots and line broadening by the Zeeman effect. We calculate stellar line profiles using polarized radiative transfer models including atomic and molecular Zeeman splitting over large wavelength regions from 0.5 to 2.3µm. Our results show that the amplitude of the RV signal caused by the Zeeman effect alone can be comparable to that caused by temperature contrast; a spot magnetic field of �1000G can produce a similar RV amplitude as a spot temperature contrast of �1000K. Furthermore, the RV signal caused by cool and magnetic spots increases with wavelength contrary to the expectation from temperature contrast alone. We also calculate the RV signal due to variations in average magnetic field strength from one observation to the next, for example due to a magnetic cycle, but find it unlikely that this can significantly influence the search for extrasolar planets. As an example, we derive the RV amplitude of the active M dwarf AD Leo as a function of wavelength using data from the HARPS spectrograph. Across this limited wavelength range, the RV signal does not diminish at longer wavelengths but shows evidence for the opposite behavior consistent with a strong influence of the Zeeman effect. We conclude that the RV signal of active stars does not vanish at longer wavelength but sensitively depends on the combination of spot temperature and magnetic field; in active low-mass stars, it is even likely to grow with wavelength.

Calibrating echelle spectrographs with Fabry-Pérot etalons

Over the past decades hollow-cathode lamps have been calibration standards for spectroscopic measurements. Advancing to cm/s radial velocity precisions with the next generation of instruments requires more suitable calibration sources with more lines and less dynamic range problems. Fabry-Perot interferometers provide a regular and dense grid of lines and homogeneous amplitudes making them good candidates for next generation calibrators. We investigate the usefulness of Fabry-Perot etalons in wavelength calibration, present an algorithm to incorporate the etalon spectrum in the wavelength solution and examine potential problems. The quasi periodic pattern of Fabry-Perot lines is used along with a hollow-cathode lamp to anchor the numerous spectral features on an absolute scale. We test our method with the HARPS spectrograph and compare our wavelength solution to the one derived from a laser frequency comb. The combined hollow-cathode lamp/etalon calibration overcomes large distortion (50 m/s) in the wavelength solution of the HARPS data reduction software. Direct comparison to the laser frequency comb bears differences of only maximum 10 m/s. Combining hollow-cathode lamps with Fabry-Perot Interferometers can lead to substantial improvements in the wavelength calibration of echelle spectrographs. Etalons can provide economical alternatives to the laser frequency comb, especially for smaller projects.

The Discovery of Stellar Oscillations in the Planet-hosting Giant Star β Geminorum

We present the results of a long time series of precise stellar radial velocity measurements of the planet hosting K giant star β Geminorum. A total of 20 hr of observations spanning three nights were obtained, and the radial velocity variations show the presence of solar-like stellar oscillations. Our period analysis yields six significant pulsation modes that have frequencies in the range of 30-150 μHz. The dominant mode is at a frequency of 86.9 μHz and has an amplitude of 5.3 m s-1. These values are consistent with stellar oscillations for a giant star with a stellar mass of ≈2 M☉. This stellar mass implies a companion minimum mass of 2.6 MJup. β Gem is the first planet-hosting giant star in which multiperiodic stellar oscillations have been detected. The study of stellar oscillations in planet-hosting giant stars may provide an independent and more accurate determination of the stellar mass.

Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity

Aims. We investigate the nature of the long-period radial velocity variations in Tau first reported over 20 yr ago. Methods. We analyzed precise stellar radial velocity measurements for Tau spanning over 30 yr. An examination of the H and Ca II 8662 spectral lines, and Hipparcos photometry was also done to help discern the nature of the long-period radial velocity variations. Results. Our radial velocity data show that the long-period, low amplitude radial velocity variations are long-lived and coherent. Furthermore, H equivalent width measurements and Hipparcos photometry show no significant variations with this period. Another investigation of this star established that there was no variability in the spectral line shapes with the radial velocity period. An orbital solution results in a period of P = 628.96 0.90 d, eccentricity, e = 0:10 0:05, and a radial velocity amplitude, K = 142:1 7:2 m s 1 . Evolutionary tracks yield a stellar mass of 1.13 0.11 M , which corresponds to a minimum companion mass of 6.47 0.53 MJup with an orbital semi-major axis of a = 1:46 0:27 AU. After removing the orbital motion of the companion, an additional period of 520 d is found in the radial velocity data, but only in some time spans. A similar period is found in the variations in the equivalent width of H and Ca II. Variations at one-third of this period are also found in the spectral line bisector measurements. The 520 d period is interpreted as the rotation modulation by stellar surface structure. Its presence, however, may not be long-lived, and it only appears in epochs of the radial velocity data separated by 10 yr. This might be due to an activity cycle. Conclusions. The data presented here provide further evidence of a planetary companion to Tau, as well as activity-related radial velocity variations.

KIC 4247791: a SB4 system with two eclipsing binaries (2EBs) - A quadruple system?

KIC 4247791 is an eclipsing binary observed by the Kepler satellite mission. We wish to determine the nature of its components and in particular the origin of a shallow dip in its Kepler light curve that previous investigations have been unable to explain in a unique way. We analyze newly obtained high-resolution spectra of the star using synthetic spectra based on atmosphere models, derive the radial velocities of the stellar components from cross-correlation with a synthetic template, and calculate the orbital solution. We use the JKTEBOP program to model the Kepler light curve of KIC 4247791. We find KIC 4247791 to be a SB4 star. The radial velocity variations of its four components can be explained by two separate eclipsing binaries. In contradiction to previous photometric findings, we show that the observed composite spectrum as well as the derived masses of all four of its components correspond to spectral type F. The observed small dip in the light curve is not caused by a transit-like phenomenon but by the eclipses of the second binary system. We find evidence that KIC 4247791 might belong to the very rare hierarchical SB4 systems with two eclipsing binaries.

Flat-relative optimal extraction A quick and efficient algorithm for stabilised spectrographs

Context. Optimal extraction is a key step in processing the raw images of spectra as registered by two-dimensional detector arrays to a one-dimensional format. Previously reported algorithms reconstruct models for a mean one-dimensional spatial profile to assist a properly weighted extraction. Aims. We outline a simple optimal extraction algorithm (including error propagation), which is very suitable for stabilised, fibre-fed spectrographs and does not model the spatial profile shape. Methods. A high signal-to-noise ratio, master-flat image serves as reference image and is directly used as an extraction profile mask. Each extracted spectral value is the scaling factor relative to the cross-section of the unnormalised master flat that contains all information about the spatial profile, as well as pixel-to-pixel variations, fringing, and blaze. The extracted spectrum is measured relative to the flat spectrum. Results. Using echelle spectra of the HARPS spectrograph we demonstrate a competitive extraction performance in terms of a signal-to-noise ratio and show that extracted spectra can be used for high precision radial velocity measurement. Conclusions. Pre- or post-flat-fielding of the data is not necessary, since all spectrograph inefficiencies inherent to the extraction mask are automatically accounted for. Also the reconstruction of the mean spatial profile by models is not needed, thereby reducing the number of operations to extract spectra. Flat-relative optimal extraction is a simple, efficient, and robust method that can be applied easily to stabilised, fibre-fed spectrographs.