The Mass of Exoplanet GJ 676Ab

The mass of planet GJ676A b from ground-based astrometry: A planetary system with two mature gas giants suitable for direct imaging

Authors:

Sahlmann et al

Abstract:

GJ676A is an M0 dwarf hosting both gas-giant and super-Earth-type planets discovered with radial-velocity measurements. Using FORS2/VLT, we obtained position measurements of the star in the plane of the sky that tightly constrain its astrometric reflex motion caused by the super-Jupiter planet `b` in a 1052-day orbit. This allows us to determine the mass of this planet to Mb=6.7+1.8−1.5MJ, which is ∼40 \% higher than the minimum mass inferred from the radial-velocity orbit. Using new HARPS radial-velocity measurements, we improve upon the orbital parameters of the inner low-mass planets `d` and `e` and we determine the orbital period of the outer giant planet `c` to Pc=7340 days under the assumption of a circular orbit. The preliminary minimum mass of planet `c` is Mcsini=6.8MJ with an upper limit of ∼39MJ that we set using NACO/VLT high-contrast imaging. We also determine precise parallaxes and relative proper motions for both GJ676A and its wide M3 companion GJ676B. Despite the probably mature age of the system, the masses and projected separations (∼0.1" -- 0.4") of planets `b` and `c` make them promising targets for direct imaging with future instruments in space and on extremely large telescopes. In particular, we estimate that GJ676A b and GJ676A c are promising targets for directly detecting their reflected light with the WFIRST space mission. Our study demonstrates the synergy of radial-velocity and astrometric surveys that is necessary to identify the best targets for such a mission.

DETECTING PLANET PAIRS IN MEAN MOTION RESONANCES VIA THE ASTROMETRY METHOD

DETECTING PLANET PAIRS IN MEAN MOTION RESONANCES VIA THE ASTROMETRY METHOD

Authors:

Wu et al

Abstract:

Gaia is leading us into a new era with a high astrometry precision of ~10 μas. Under such precision, astrometry can play an important role in detecting and characterizing exoplanets. In particular, we can identify planet pairs in mean motion resonances (MMRs), which constrain the formation and evolution of planetary systems. In accordance with observations, we consider two-Jupiter or two-super-Earth systems in 1:2, 2:3, and 3:4 MMRs. Our simulations show that the false alarm probabilities (FAPs) of a third planet are extremely small, while the two real planets can be fitted well with a signal-to-noise ratio (S/N)$\;\gt \;3$. The probability of reconstructing a resonant system is related to the eccentricities and the resonance intensity. Generally, when the S/N $\geqslant \;10$, if the eccentricities of both planets are larger than 0.01 and the resonance is quite strong, the probability of reconstructing the planet pair in MMRs is $\geqslant \;80 \% $. Jupiter pairs in MMRs are reconstructed more easily than super-Earth pairs with similar S/N when we consider dynamical stability. FAPs are also calculated when we detect planet pairs in or near MMRs. The FAPs for 1:2 MMRs are the largest, i.e., FAP $\gt 15 \% $ when S/N $\leqslant \;10$. Extrapolating from the Kepler planet pairs near MMRs and assuming a S/N ~ 3, we discover and reconstruct a few tens of Jupiter pairs and hundreds of super-Earth pairs in 2:3 and 1:2 MMRs within 30 pc. We also compare the differences between even and uneven data cadence and find that planets are better measured with more uniform phase coverage.

Astrometric Monitoring of the HR 8799 Planets

Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements

Authors:

Konopacky et al

Abstract:

We present new astrometric measurements from our ongoing monitoring campaign of the HR 8799 directly imaged planetary system. These new data points were obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and 2014. In addition, we present updated astrometry from previously published observations in 2007 and 2008. All data were reduced using the SOSIE algorithm, which accounts for systematic biases present in previously published observations. This allows us to construct a self-consistent data set derived entirely from NIRC2 data alone. From this dataset, we detect acceleration for two of the planets (HR 8799b and e) at greater than 3σ. We also assess possible orbital parameters for each of the four planets independently. We find no statistically significant difference in the allowed inclinations of the planets. Fitting the astrometry while forcing coplanarity also returns χ2 consistent to within 1σ of the best fit values, suggesting that if inclination offsets of ≲20o are present, they are not detectable with current data. Our orbital fits also favor low eccentricities, consistent with predictions from dynamical modeling. We also find period distributions consistent to within 1σ with a 1:2:4:8 resonance between all planets. This analysis demonstrates the importance of minimizing astrometric systematics when fitting for solutions to highly undersampled orbits.

Finding Co-Orbital Exoplanets Through Radial Velocity & Astrometic Detection Methods

Detectability of quasi-circular co-orbital planets. Application to the radial velocity technique

Authors:

Leleu et al

Abstract:

Several celestial bodies in co-orbital configurations exist in the solar system. However, co-orbital exoplanets have not yet been discovered. This lack may result from a degeneracy between the signal induced by co-orbital planets and other orbital configurations. Here we determine a criterion for the detectability of quasi-circular co-orbital planets and develop a demodulation method to bring out their signature from the observational data. We show that the precision required to identify a pair of co-orbital planets depends only on the libration amplitude and on the planet's mass ratio. We apply our method to synthetic radial velocity data, and show that for tadpole orbits we are able to determine the inclination of the system to the line of sight. Our method is also valid for planets detected through the transit and astrometry techniques.

Gaia's Astrometry Revolution

Gaia: The Astrometry Revolution

Authors:

Sozzetti et al

Abstract:

The power of micro-arcsecond (μas) astrometry is about to be unleashed. ESA's Gaia mission, now headed towards the end of the first year of routine science operations, will soon fulfil its promise for revolutionary science in countless aspects of Galactic astronomy and astrophysics. The potential of Gaia position measurements for important contributions to the astrophysics of planetary systems is huge. We focus here on the expectations for detection and improved characterization of 'young' planetary systems in the neighborhood of the Sun using a combination of Gaia μas astrometry and direct imaging techniques.

Gaia's Great Astrometric Potential

Exoplanets with Gaia: Synergies in the Making

Authors:

Sozzetti et al

Abstract:

The era of high-precision astrometry has dawned upon us. The potential of Gaia μas-level precision in positional measurements is about to be unleashed in the field of extrasolar planetary systems. The Gaia data hold the promise for much improved global characterization of planetary systems around stars of all types, ages, and chemical composition, particularly when synergistically combined with other indirect and direct planet detection and characterization programs.

Expected Astrometric Performance of Gaia

Gaia astrometric science performance - post-launch predictions

Authors:

de Bruijne et al

Abstract:

The standard errors of the end-of-mission Gaia astrometry have been re-assessed after conclusion of the in-orbit commissioning phase of the mission. An analytical relation is provided for the parallax standard error as function of Gaia G magnitude (and V-I colour) which supersedes the pre-launch relation provided in de Bruijne (2012).

How Many Exoplanets is Gaia Expected to Detect?

Astrometric exoplanet detection with Gaia

Authors:

Perryman et al

Abstract:

We provide a revised assessment of the number of exoplanets that should be discovered by Gaia astrometry, extending previous studies to a broader range of spectral types, distances, and magnitudes. Our assessment is based on a large representative sample of host stars from the TRILEGAL Galaxy population synthesis model, recent estimates of the exoplanet frequency distributions as a function of stellar type, and detailed simulation of the Gaia observations using the updated instrument performance and scanning law. We use two approaches to estimate detectable planetary systems: one based on the S/N of the astrometric signature per field crossing, easily reproducible and allowing comparisons with previous estimates, and a new and more robust metric based on orbit fitting to the simulated satellite data.

With some plausible assumptions on planet occurrences, we find that some 21,000 (+/-6000) high-mass (1-15M_J) long-period planets should be discovered out to distances of ~500pc for the nominal 5-yr mission (including at least 1000-1500 around M dwarfs out to 100pc), rising to some 70,000 (+/-20,000) for a 10-yr mission. We indicate some of the expected features of this exoplanet population, amongst them ~25-50 intermediate-period (P~2-3yr) transiting systems.

Follow-up Observations of Directly Imaged Companions to Young Stars and Brown Dwarfs

Astrometric follow-up observations of directly imaged sub-stellar companions to young stars and brown dwarfs

Authors:

Ginski et al

Abstract:

The formation of massive planetary or brown dwarf companions at large projected separations from their host star is not yet well understood. In order to put constraints on formation scenarios we search for signatures in the orbit dynamics of the systems. We are specifically interested in the eccentricities and inclinations since those parameters might tell us about the dynamic history of the systems and where to look for additional low-mass sub-stellar companions. For this purpose we utilized VLT/NACO to take several well calibrated high resolution images of 6 target systems and analyze them together with available literature data points of those systems as well as Hubble Space Telescope archival data. We used a statistical Least-Squares Monte-Carlo approach to constrain the orbit elements of all systems that showed significant differential motion of the primary star and companion. We show for the first time that the GQ Lup system shows significant change in both separation and position angle. Our analysis yields best fitting orbits for this system, which are eccentric (e between 0.21 and 0.69), but can not rule out circular orbits at high inclinations. Given our astrometry we discuss formation scenarios of the GQ Lup system. In addition, we detected an even fainter new companion candidate to GQ Lup, which is most likely a background object. We also updated the orbit constraints of the PZ Tel system, confirming that the companion is on a highly eccentric orbit with e greater than 0.62. Finally we show with a high significance, that there is no orbital motion observed in the cases of the DH Tau, HD 203030 and 1RXS J160929.1-210524 systems and give the most precise relative astrometric measurement of the UScoCTIO 108 system to date.

Hunting for Exoplanets Around Brown Dwarfs & Very Cool M Dwarfs From the Ground & Gaia

Astrometric planet search around M8--L2 dwarfs from the ground and with Gaia

Authors:

Sahlmann et al

Abstract:

Ultra-cool dwarfs are very low-mass stars or brown dwarfs and because of their faintness they are difficult targets for radial velocity and transit planet searches. High-precision astrometry is one way to efficiently discover planets around these objects. We are conducting a planet search survey of 20 M8-L2 using ground-based imaging astrometry with FORS2 at VLT. The realised accuracy of 100 micro-arcseconds allows us to set stringent constraints on the presence of planets, to discover astrometric binaries, and to measure parallaxes with an unprecedented precision of 0.1 %. The obtained detection limits firmly establish that giant planets are rare around UCDs at all separations. The astrometric performance of our programme is comparable to what is expected from Gaia observations of single faint objects and we discuss potential synergies for planet searches around ultracool dwarfs. We estimate that Gaia will be able to characterise ~100 astrometric binaries with an ultracool primary.

How Well Can Gaia Detect Brown Dwarf Binaries?

Gaia Detection Capabilities of Spectroscopic Brown Dwarf Binaries

Authors:

Joergens et al

Abstract:

The astrometric space mission Gaia is expected to detect a large number of brown dwarf binary systems with close orbits and determine astrometric orbit solutions. This will provide key information for the formation and evolution of brown dwarfs, such as the binary frequency and dynamical masses. Known brown dwarf binaries with orbit constraints from other techniques will play an important role. We are carrying out one of the most precise and long-lasting radial velocity surveys for brown dwarf binaries in the Cha I star-forming region at the VLT. We were able to add two orbit determinations to the small group of a handful of brown dwarf and very low-mass binaries with characterized RV orbits. We show here that the astrometric motion of both systems might very well be detectable with Gaia. We predict an astrometric signal of about 1.2 - 1.6 milliarcseconds (mas) for the brown dwarf binary ChaHa8 and of 0.4 - 0.8 mas for the very low-mass binary CHXR74. We take the luminosity of the companion into account for these estimates and present a relation for the astrometric orbit in the case of non-negligible companion luminosity.

Parallaxes of L Class Brown Dwarfs

Trigonometric parallaxes of young field L dwarfs

Authors:

Zapatero Osorio et al

Abstract:

We aim to determine the trigonometric parallaxes and proper motions of a sample of ten field L0-L5 dwarfs with spectroscopic evidence for low-gravity atmospheres. We obtained J and Ks imaging data using 2-4-m class telescopes with a typical cadence of one image per month between 2010 January and 2012 December. We also obtained low resolution optical spectra (R~300, 500-1100 nm) using the 10-m GTCs to assess the presence of lithium absorption in four targets and confirm their young age. Trigonometric parallaxes and proper motions were derived to typical accuracies of 1 mas and +/-10 mas/yr. All ten L dwarfs have large motions, and are located at distances between 9 and 47 pc. They lie above and on the sequence of field dwarfs in the absolute J and K_s magnitude versus spectral type and luminosity versus Teff diagrams, implying ages similar to or smaller than those typical of the field. The detection of atomic lithium in the atmosphere of 2MASS J00452143+1634446 is reported for the first time. Three dwarfs have locations in the HR diagram indicative of old ages and high masses consistent with the observed lithium depletion previously published. We did not find evidence for the presence of astrometric companions with minimum detectable masses typically >=25 Mjup and face-on, circular orbits with periods between 60-90 d and 3 yr around eight targets. The astrometric and spectroscopic data indicate that about 60-70% of the field L-type dwarfs in our sample with evidence for low-gravity atmospheres are indeed young-to-intermediate-age brown dwarfs of the solar neighborhood with expected ages and masses in the intervals 10-500 Myr and 11-45 Mjup. The peaked-shape of the H-band spectra of L dwarfs, a signpost of youth, appears to be present up to ages of 120-500 Myr and intermediate-to-high gravities.

More Results From Exoplanet Hunting Around Ultracool Dwarfs

Astrometric planet search around southern ultracool dwarfs II: Astrometric reduction methods and a deep astrometric catalogue

Authors:

Lazorenko et al

Abstract:

We describe the astrometric reduction of images obtained with the FORS2/VLT camera in the framework of an astrometric planet search around 20 M/L-transition dwarfs. We present the correction of systematic errors, the achieved astrometric performance, and a new astrometric catalogue containing the faint reference stars in 20 fields located close to the Galactic plane. We detected three types of systematic errors in the FORS2 astrometry: the relative motion of the camera's two CCD chips, errors that are correlated in space, and an error contribution of yet unexplained origin. The relative CCD motion has probably a thermal origin and usually is 0.001-0.010 px (~0.1-1 mas), but sometimes amounts to 0.02-0.05 px (3-6 mas). This instability and space-correlated errors are detected and mitigated using reference stars. The third component of unknown origin has an amplitude of 0.03-0.14 mas and is independent of the observing conditions. We find that a consecutive sequence of 32 images of a well-exposed star over 40 min at 0.6" seeing results in a median r.m.s. of the epoch residuals of 0.126 mas. Overall, the epoch residuals are distributed according to a normal law with a chi2~1. We compiled a catalogue of 12000 stars with I-band magnitudes of 16-22 located in 20 fields, each covering ~2x2'. It contains I-band magnitudes, ICRF positions with 40-70 mas precision, and relative proper motions and absolute trigonometric parallaxes with a precision of 0.1 mas/yr and 0.1 mas at the bright end, respectively.