arXiv subjects

J. Sahlmann

Publications and source records attributed to J. Sahlmann.

At least 37 records · Page 2Linked to original sources

arXiv2016

The SOPHIE search for northern extrasolar planets. XI. Three new companions and an orbit update: Giant planets in the habitable zone

We report the discovery of three new substellar companions to solar-type stars, HD191806, HD214823, and HD221585, based on radial velocity measurements obtained at the Haute-Provence Observatory. Data from the SOPHIE spectrograph are combined with observations acquired with its predecessor, ELODIE, to detect and characterise the orbital parameters of three new gaseous giant and brown dwarf candidates. Additionally, we combine SOPHIE data with velocities obtained at the Lick Observatory to improve the parameters of an already known giant planet companion, HD16175 b. Thanks to the use of different instruments, the data sets of all four targets span more than ten years. Zero-point offsets between instruments are dealt with using Bayesian priors to incorporate the information we possess on the SOPHIE/ELODIE offset based on previous studies. The reported companions have orbital periods between three and five years and minimum masses between 1.6 Mjup and 19 Mjup. Additionally, we find that the star HD191806 is experiencing a secular acceleration of over 11 \ms\ per year, potentially due to an additional stellar or substellar companion. A search for the astrometric signature of these companions was carried out using Hipparcos data. No orbit was detected, but a significant upper limit to the companion mass can be set for HD221585, whose companion must be substellar. With the exception of HD191806 b, the companions are located within the habitable zone of their host star. Therefore, satellites orbiting these objects could be a propitious place for life to develop.

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arXiv2016

The SOPHIE search for northern extrasolar planets. X. Detection and characterization of giant planets by the dozen

We present new radial velocity measurements of eight stars secured with the spectrograph SOPHIE at the 193-cm telescope of the Haute-Provence Observatory allowing the detection and characterization of new giant extrasolar planets. The host stars are dwarfs of spectral types between F5 and K0 and magnitudes between 6.7 and 9.6; the planets have minimum masses M_p sin i between 0.4 to 3.8 M_Jup and orbital periods of several days to several months. The data allow only single planets to be discovered around the first six stars (HD143105, HIP109600, HD35759, HIP109384, HD220842, and HD12484), but one of them shows the signature of an additional substellar companion in the system. The seventh star, HIP65407, allows the discovery of two giant planets, just outside the 12:5 resonance in weak mutual interaction. The last star, HD141399, was already known to host a four-planetary system; our additional data and analyses allow new constraints to be put on it. We present Keplerian orbits of all systems, together with dynamical analyses of the two multi-planetary systems. HD143105 is one of the brightest stars known to host a hot Jupiter, which could allow numerous follow-up studies to be conducted despite this is not a transiting system. The giant planets HIP109600b, HIP109384b, and HD141399c are located in the habitable zone of their host star.

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arXiv2016

The SOPHIE search for northern extrasolar planets IX. Populating the brown dwarf desert

Radial velocity planet search surveys of nearby Solar-type stars have shown a strong deficit of brown dwarf companions within $\sim5\,\mathrm{AU}$. There is presently no comprehensive explanation of this lack of brown dwarf companions, therefore, increasing the sample of such objects is crucial to understand their formation and evolution. Based on precise radial velocities obtained using the SOPHIE spectrograph at Observatoire de Haute-Provence we characterise the orbital parameters of $15$ companions to solar-type stars and constrain their true mass using astrometric data from the Hipparcos space mission. The nine companions not shown to be stellar in nature have minimum masses ranging from ~$13$ to $70\,\mathrm{M}_{\mathrm{Jup}}$, and are well distributed across the planet/brown dwarf mass regime, making them an important contribution to the known population of massive companions around solar-type stars. We characterise six companions as stellar in nature with masses ranging from a minimum mass of $76 \pm 4\,\mathrm{M}_{\mathrm{Jup}}$ to a mass of $0.35 \pm 0.03\,\mathrm{M_\odot}$. The orbital parameters of two previously known sub-stellar candidates are improved.

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arXiv2015

The SOPHIE search for northern extrasolar planets VIII. Follow-up of ELODIE candidates: long-period brown-dwarf companions

Long-period brown dwarf companions detected in radial velocity surveys are important targets for direct imaging and astrometry to calibrate the mass-luminosity relation of substellar objects. Through a 20-year radial velocity monitoring of solar-type stars that began with ELODIE and was extended with SOPHIE spectrographs, giant exoplanets and brown dwarfs with orbital periods longer than ten years are discovered. We report the detection of five new potential brown dwarfs with minimum masses between 32 and 83 Jupiter mass orbiting solar-type stars with periods longer than ten years. An upper mass limit of these companions is provided using astrometric Hipparcos data, high-angular resolution imaging made with PUEO, and a deep analysis of the cross-correlation function of the main stellar spectra to search for blend effects or faint secondary components. These objects double the number of known brown dwarf companions with orbital periods longer than ten years and reinforce the conclusion that the occurrence of such objects increases with orbital separation. With a projected separation larger than 100 mas, all these brown dwarf candidates are appropriate targets for high-contrast and high angular resolution imaging.

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arXiv2015

Parallax of the L4.5 dwarf 2M1821$+$14 from high-precision astrometry with OSIRIS at GTC

We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias (GTC) to monitor the astrometric motion of the L4.5 dwarf 2M1821$+$14 over 17 months. The astrometric residuals of eleven epochs have a r.m.s. dispersion of 0.4 mas, which is larger than the average precision of 0.23 mas per epoch and hints towards an additional signal or excess noise. Comparison of the point-spread-functions in OSIRIS and FORS2/VLT images reveals no differences critical for high-precision astrometry, despite the GTC's segmented primary mirror. We attribute the excess noise to an unknown effect that may be uncovered with additional data. For 2M1821$+$14, we measured a relative parallax of $106.15 \pm 0.18$ mas and determined a correction of $0.50\pm0.05$ mas to absolute parallax, leading to a distance of $9.38 \pm0.03$ pc. We excluded at 3-$σ$ confidence the presence of a companion to 2M1821$+$14 down to a mass ratio of 0.1 ($\approx 5\, M_\mathrm{Jupiter}$) with a period of 50--1000 days and a separation of 0.1--0.7 au. The accurate parallax allowed us to estimate the age and mass of 2M1821$+$14 of 120--700 Myr and 0.049$^{+0.014}_{-0.024}$ M$_\odot$, thus confirming its intermediate age and substellar mass. We complement our study with a parallax and proper motion catalogue of 587 stars ($i'\simeq15.5-22$) close to 2M1821$+$14, used as astrometric references. This study demonstrates sub-mas astrometry with the GTC, a capability applicable for a variety of science cases including the search for extrasolar planets and relevant for future astrometric observations with E-ELT and TMT.

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arXiv2015

DE0823$-$49 is a juvenile binary brown dwarf at 20.7 pc

Astrometric monitoring of the nearby early-L dwarf DE0823$-$49 has revealed a low-mass companion in a 248-day orbit that was announced in an earlier work. Here, we present new astrometric and spectroscopic observations that allow us to characterise the system in detail. The optical spectrum shows LiI-absorption indicative of a young age and/or substellar mass for the primary component. The near-infrared spectrum is best reproduced by a binary system of brown dwarfs with spectral types of L1.5 $+$ L5.5 and effective temperatures of $2150\pm100$ K and $1670\pm140$ K. To conform with the photocentric orbit size measured with astrometry and the current understanding of substellar evolution, the system must have an age in the 80--500 Myr range. Evolutionary models predict component masses in the ranges of $M_1\simeq0.028-0.063\,M_\odot$ and $M_2\simeq0.018-0.045\,M_\odot$ with a mass ratio of $q\simeq0.64-0.74$. Multi-epoch radial velocity measurements unambiguously establish the three-dimensional orbit of the system and allow us to investigate its kinematic properties. DE0823$-$49 emerges as a rare example of a nearby brown dwarf binary with orbit, component properties, and age that are characterised well. It is a juvenile resident of the solar neighbourhood, but does not appear to belong to a known young association or moving group.

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arXiv2015

Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18

Using astrometric measurements obtained with the FORS2/VLT camera, we are searching for low-mass companions around 20 nearby ultracool dwarfs. With a single-measurement precision of 0.1 milli-arcseconds, our survey is sensitive to a wide range of companion masses from planetary companions to binary systems. Here, we report the discovery and orbit characterisation of a new ultracool binary at a distance of 19.5 pc from Earth that is composed of the M8.5-dwarf primary DE0630-18 and a substellar companion. The nearly edge-on orbit is moderately eccentric (e=0.23) with an orbital period of 1120 d, which corresponds to a relative separation in semimajor axis of approximately 1.1 AU. We obtained a high-resolution optical spectrum with UVES/VLT and measured the system's heliocentric radial velocity. The spectrum does not exhibit lithium absorption at 670.8 nm, indicating that the system is not extremely young. A preliminary estimate of the binary's physical parameters tells us that it is composed of a primary at the stellar-substellar limit and a massive brown-dwarf companion. DE0630-18 is a new very low-mass binary system with a well-characterised orbit.

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arXiv2014

Enabling Gaia observations of naked-eye stars

The ESA Gaia space astrometry mission will perform an all-sky survey of stellar objects complete in the nominal magnitude range G = [6.0 - 20.0]. The stars with G lower than 6.0, i.e. those visible to the unaided human eye, would thus not be observed by Gaia. We present an algorithm configuration for the Gaia on-board autonomous object observation system that makes it possible to observe very bright stars with G = [2.0-6.0). Its performance has been tested during the in-orbit commissioning phase achieving an observation completeness of ~94% at G = 3 - 5.7 and ~75% at G = 2 - 3. Furthermore, two targeted observation techniques for data acquisition of stars brighter than G = 2.0 were tested. The capabilities of these two techniques and the results of the in-flight tests are presented. Although the astrometric performance for stars with G lower than 6.0 has yet to be established, it is clear that several science cases will benefit from the results of the work presented here.

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arXiv2014

Gaia on-board metrology: basic angle and best focus

The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of $μ$as (prad, micropixel), which is the highest level ever achieved in space. Two Shack-Hartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented

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arXiv2014

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

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.

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arXiv2014

Mid-infrared interferometry with K band fringe-tracking I. The VLTI MIDI+FSU experiment

Context: A turbulent atmosphere causes atmospheric piston variations leading to rapid changes in the optical path difference of an interferometer, which causes correlated flux losses. This leads to decreased sensitivity and accuracy in the correlated flux measurement. Aims: To stabilize the N band interferometric signal in MIDI (MID-infrared Interferometric instrument), we use an external fringe tracker working in K band, the so-called FSU-A (fringe sensor unit) of the PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at VLTI. We present measurements obtained using the newly commissioned and publicly offered MIDI+FSU-A mode. A first characterization of the fringe-tracking performance and resulting gains in the N band are presented. In addition, we demonstrate the possibility of using the FSU-A to measure visibilities in the K band. Methods: We analyzed FSU-A fringe track data of 43 individual observations covering different baselines and object K band magnitudes with respect to the fringe-tracking performance. The N band group delay and phase delay values could be predicted by computing the relative change in the differential water vapor column density from FSU-A data. Visibility measurements in the K band were carried out using a scanning mode of the FSU-A. Results: Using the FSU-A K band group delay and phase delay measurements, we were able to predict the corresponding N band values with high accuracy with residuals of less than 1 micrometer. This allows the coherent integration of the MIDI fringes of faint or resolved N band targets, respectively. With that method we could decrease the detection limit of correlated fluxes of MIDI down to 0.5 Jy (vs. 5 Jy without FSU-A) and 0.05 Jy (vs. 0.2 Jy without FSU-A) using the ATs and UTs, respectively. The K band visibilities could be measured with a precision down to ~2%.

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arXiv2014

Chemical abundances of stars with brown-dwarf companions

It is well-known that stars with giant planets are on average more metal-rich than stars without giant planets, whereas stars with detected low-mass planets do not need to be metal-rich. With the aim of studying the weak boundary that separates giant planets and brown dwarfs (BDs) and their formation mechanism, we analyze the spectra of a sample of stars with already confirmed BD companions both by radial velocity and astrometry. We employ standard and automatic tools to perform an EW-based analysis and to derive chemical abundances from CORALIE spectra of stars with BD companions. We compare these abundances with those of stars without detected planets and with low-mass and giant-mass planets. We find that stars with BDs do not have metallicities and chemical abundances similar to those of giant-planet hosts but they resemble the composition of stars with low-mass planets. The distribution of mean abundances of $α$-elements and iron peak elements of stars with BDs exhibit a peak at about solar abundance whereas for stars with low-mass and high-mass planets the [X$_α$/H] and [X$_{\rm Fe}$/H] peak abundances remain at $\sim -0.1$~dex and $\sim +0.15$~dex, respectively. We display these element abundances for stars with low-mass and high-mass planets, and BDs versus the minimum mass, $m_C \sin i$, of the most-massive substellar companion in each system, and we find a maximum in $α$-element as well as Fe-peak abundances at $m_C \sin i \sim 1.35\pm 0.20$ jupiter masses. We discuss the implication of these results in the context of the formation scenario of BDs in comparison with that of giant planets.

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arXiv2014

Astrometric planet search around southern ultracool dwarfs I: First results, including parallaxes of 20 M8-L2 dwarfs

Extrasolar planet searches targeting very low-mass stars and brown dwarfs are hampered by intrinsic or instrumental limitations. Time series of astrometric measurements with precisions better than one milli-arcsecond can yield new evidence on the planet occurrence around these objects. We present first results of an astrometric search for planets around 20 nearby dwarf stars with spectral types M8-L2. Over a timespan of two years, we obtained I-band images of the target fields with the FORS2 camera at the Very Large Telescope. Using background stars as references, we monitored the targets' astrometric trajectories, which allowed us to measure parallax and proper motions, set limits on the presence of planets, and to discover the orbital motions of two binary systems. We determined trigonometric parallaxes with an average accuracy of 0.09 mas (~0.2 %) resulting in a reference sample for the study of ultracool dwarfs at the M/L transition, whose members are located at distances of 9.5-40 pc. This sample contains two newly discovered tight binaries (DE0630-18 and DE0823-49) and one previously known wide binary (DE1520-44). Only one target shows I-band variability >5 mmag r.m.s. We derived planet exclusion limits that set an upper limit of 9 % to the occurrence of giant planets with masses >5 MJup in intermediate-separation (0.01-0.8 AU) orbits around M8-L2 dwarfs. We demonstrated that astrometric observations with an accuracy of 120 micro-arcsec over two years are feasible from the ground and can be used for a planet search survey. The detection of two tight very low-mass binaries showed that our search strategy is efficient and may lead to the detection of planetary-mass companions through follow-up observations.

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arXiv2014

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

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.

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arXiv2013

The SOPHIE search for northern extrasolar planets: VI. Three new hot Jupiters in multi-planet extrasolar systems

We present high-precision radial-velocity measurements of three solar-type stars: HD 13908, HD 159243, and HIP 91258. The observations were made with the SOPHIE spectrograph at the 1.93-m telescope of Observatoire de Haute-Provence (France). They show that these three bright stars host exoplanetary systems composed of at least two companions. HD 13908 b is a planet with a minimum mass of 0.865+-0.035 Mjup, on a circular orbit with a period of 19.382+-0.006 days. There is an outer massive companion in the system with a period of 931+-17 days, e = 0.12+-0.02, and a minimum mass of 5.13+-0.25 Mjup. The star HD 159243, also has two detected companions with respective masses, periods, and eccentricities of Mp = 1.13+-0.05 and 1.9+-0.13 Mjup, $P$ = 12.620+-0.004 and 248.4+-4.9 days, and e = 0.02+-0.02 and 0.075+-0.05. Finally, the star HIP 91258 has a planetary companion with a minimum mass of 1.068+-0.038 Mjup, an orbital period of 5.0505+-0.0015 days, and a quadratic trend indicating an outer planetary or stellar companion that is as yet uncharacterized. The planet-hosting stars HD 13908, HD 159243, and HIP 91258 are main-sequence stars of spectral types F8V, G0V, and G5V, respectively, with moderate activity levels. HIP 91258 is slightly over-metallic, while the two other stars have solar-like metallicity. The three systems are discussed in the frame of formation and dynamical evolution models of systems composed of several giant planets.

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arXiv2013

Astrometric detection of exoplanets from the ground

Astrometry is a powerful technique to study the populations of extrasolar planets around nearby stars. It gives access to a unique parameter space and is therefore required for obtaining a comprehensive picture of the properties, abundances, and architectures of exoplanetary systems. In this review, we discuss the scientific potential, present the available techniques and instruments, and highlight a few results of astrometric planet searches, with an emphasis on observations from the ground. In particular, we discuss astrometric observations with the Very Large Telescope (VLT) Interferometer and a programme employing optical imaging with a VLT camera, both aimed at the astrometric detection of exoplanets. Finally, we set these efforts into the context of Gaia, ESA's astrometry mission scheduled for launch in 2013, and present an outlook on the future of astrometric exoplanet detection from the ground.

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arXiv2013

Astrometric mass ratios for three spectroscopic binaries

The orbits of five single-lined spectroscopic binaries have recently been determined. We now use astrometric measurements that were collected with the Hipparcos satellite to constrain the systems' mass ratios and secondary masses. The barycentric astrometric orbits of three binary systems, HD 140667, HD 158222, and HD 217924, are fully determined and precise estimates of their mass ratios are obtained. Follow-up of these systems with infrared spectroscopy could yield model-independent dynamical masses for all components.

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arXiv2013

Astrometric orbit of a low-mass companion to an ultracool dwarf

Little is known about the existence of extrasolar planets around ultracool dwarfs. Furthermore, binary stars with Sun-like primaries and very low-mass binaries composed of ultracool dwarfs show differences in the distributions of mass ratio and orbital separation that can be indicative of distinct formation mechanisms. Using FORS2/VLT optical imaging for high precision astrometry we are searching for planets and substellar objects around ultracool dwarfs to investigate their multiplicity properties for very low companion masses. Here we report astrometric measurements with an accuracy of two tenths of a milli-arcsecond over two years that reveal orbital motion of the nearby L1.5 dwarf DENIS-P J082303.1-491201 located at 20.77 +/- 0.08 pc caused by an unseen companion that revolves about its host on an eccentric orbit in 246.4 +/- 1.4 days. We estimate the L1.5 dwarf to have 7.5 +/- 0.7 % of the Sun's mass that implies a companion mass of 28 +/- 2 Jupiter masses. This new system has the smallest mass ratio (0.36 +/- 0.02) of known very low-mass binaries with characterised orbits. With this discovery we demonstrate 200 micro-arcsecond astrometry over an arc-minute field and over several years that is sufficient to discover sub-Jupiter mass planets around ultracool dwarfs. We also show that the achieved parallax accuracy of < 0.4 % makes it possible to remove distance as a dominant source of uncertainty in the modelling of ultracool dwarfs.

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