C. Bergfors

University College London, Londinium, England, United Kingdom

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Publications (22)64.35 Total impact

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    ABSTRACT: Direct imaging surveys for exoplanets commonly exclude binary stars from their target lists, leaving a large part of the overall planet demography unexplored. To address this gap in our understanding of planet formation and evolution, we have launched the first direct imaging survey dedicated to circumbinary planets: SPOTS, the Search for Planets Orbiting Two Stars. In this paper, we discuss the theoretical context, scientific merit, and technical feasibility of such observations, describe the target sample and observational strategy of our survey, and report on the first results from our pilot survey of 26 targets with the VLT NaCo facility. While we have not found any confirmed substellar companions to date, a number of promising candidate companions remain to be tested for common proper motion in upcoming follow-up observations. We also report on the astrometry of the three resolved binaries in our target sample. This pilot survey constitutes a successful proof of concept for our survey strategy and paves the way for a second stage of exploratory observations with VLT SPHERE.
    09/2014;
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    ABSTRACT: Orbital monitoring of M-type binaries is essential for constraining their fundamental properties. This is particularly useful in young systems, where the extended pre-main sequence evolution can allow for precise isochronal dating. Here, we present the continued astrometric monitoring of the more than 200 binaries of the AstraLux Large Multiplicity Survey, building both on our previous work, archival data, and new astrometric data spanning the range of 2010-2012. The sample is very young overall -- all included stars have known X-ray emission, and a significant fraction (18%) of them have recently also been identified as members of young moving groups in the Solar neighborhood. We identify ~30 targets that both have indications of being young and for which an orbit either has been closed or appears possible to close in a reasonable timeframe (a few years to a few decades). One of these cases, GJ 4326, is however identified as probably being substantially older than has been implied from its apparent moving group membership, based on astrometric and isochronal arguments. With further astrometric monitoring, these targets will provide a set of empirical isochrones, against which theoretical isochrones can be calibrated, and which can be used to evaluate the precise ages of nearby young moving groups.
    The Astrophysical Journal Supplement Series 09/2014; 214(2). · 16.24 Impact Factor
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    ABSTRACT: The distribution of multiplicity among low-mass stars is a key issue to understanding the formation of stars and brown dwarfs, and recent surveys have yielded large enough samples of nearby low-mass stars to study this issue statistically to good accuracy. Previously, we have presented a multiplicity study of ~700 early/mid M-type stars observed with the AstraLux high-resolution Lucky Imaging cameras. Here, we extend the study of multiplicity in M-type stars through studying 286 nearby mid/late M-type stars, bridging the gap between our previous study and multiplicity studies of brown dwarfs. Most of the targets have been observed more than once, allowing us to assess common proper motion to confirm companionship. We detect 68 confirmed or probable companions in 66 systems, of which 41 were previously undiscovered. Detections are made down to the resolution limit of ~100 mas of the instrument. The raw multiplicity in the AstraLux sensitivity range is 17.9%, leading to a total multiplicity fraction of 21%-27% depending on the mass ratio distribution, which is consistent with being flat down to mass ratios of ~0.4, but cannot be stringently constrained below this value. The semi-major axis distribution is well represented by a log-normal function with μa = 0.78 and σa = 0.47, which is narrower and peaked at smaller separations than for a Sun-like sample. This is consistent with a steady decrease in average semi-major axis from the highest-mass binary stars to the brown dwarf binaries.
    The Astrophysical Journal 06/2014; 789(2):102. · 6.73 Impact Factor
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    ABSTRACT: We report the confirmation of the binary nature of the nearby, very low-mass system NLTT 33370 with adaptive optics imaging and present resolved near-infrared photometry and integrated light optical and near-infrared spectroscopy to characterize the system. VLT-NaCo and LBTI-LMIRCam images show significant orbital motion between 2013 February and 2013 April. Optical spectra reveal weak, gravity sensitive alkali lines and strong lithium 6708 Angstrom absorption that indicate the system is younger than field age. VLT-SINFONI near-IR spectra also show weak, gravity sensitive features and spectral morphology that is consistent with other young, very low-mass dwarfs. We combine the constraints from all age diagnostics to estimate a system age of ~30-200 Myr. The 1.2-4.7 micron spectral energy distribution of the components point toward T_eff=3200 +/- 500 K and T_eff=3100 +/- 500 K for NLTT 33370 A and B, respectively. The observed spectra, derived temperatures, and estimated age combine to constrain the component spectral types to the range M6-M8. Evolutionary models predict masses of 113 +/- 8 M_Jup and 106 +/- 7 M_Jup from the estimated luminosities of the components. KPNO-Phoenix spectra allow us to estimate the systemic radial velocity of the binary. The Galactic kinematics of NLTT 33370AB are broadly consistent with other young stars in the Solar neighborhood. However, definitive membership in a young, kinematic group cannot be assigned at this time and further follow-up observations are necessary to fully constrain the system's kinematics. The proximity, age, and late-spectral type of this binary make it very novel and an ideal target for rapid, complete orbit determination. The system is one of only a few model calibration benchmarks at young ages and very low-masses.
    01/2014; 783(1).
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    ABSTRACT: The census of young, M dwarfs in the solar neighborhood is incomplete. Many constituents of the known sample are members of nearby, young moving groups (NYMGs), loose associations of coeval stars with common Galactic kinematics. Thus, new candidates of the under-sampled low-mass star population can be found via their possible association with NYMGs. We have therefore launched the Cool Astrometrically Selected Targets Optimal For Follow-up Spectroscopy (CASTOFFS) survey to identify and characterize previously unrecognized young, low-mass stars. We combine astrometry, photometry, and activity to find candidates and use dedicated spectroscopic follow-up to verify their youth. We are now 1.5 years into CASTOFFS and present early results to highlight new discoveries and provide a taste of what's to come. These results include two low-mass multiple systems: the first, a young hierarchical triple where the primary hosts a candidate debris disk, the other, a 1 AU separation, ˜100 Myr old, late-M binary. We also briefly introduce likely new members of NYMGs that represent key targets for further follow-up.
    01/2014;
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    ABSTRACT: Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope, and AstraLux Sur at the ESO 3.5-m New Technology Telescope at La Silla. We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered candidate companions with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.
    Monthly Notices of the Royal Astronomical Society 09/2012; 428(1). · 5.52 Impact Factor
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    ABSTRACT: We present the results of an extensive high-resolution imaging survey of M-dwarf multiplicity using the Lucky Imaging technique. The survey made use of the AstraLux Norte camera at the Calar Alto 2.2m telescope and the AstraLux Sur camera at the ESO New Technology Telescope in order to cover nearly the full sky. In total, 761 stars were observed (701 M-type and 60 late K-type), among which 182 new and 37 previously known companions were detected in 205 systems. Most of the targets have been observed during two or more epochs, and could be confirmed as physical companions through common proper motion, often with orbital motion being confirmed in addition. After accounting for various bias effects, we find a total M-dwarf multiplicity fraction of 27+/-3% within the AstraLux detection range of 0.08-6" (semi-major axes of ~3-227 AU at a median distance of 30 pc). We examine various statistical multiplicity properties within the sample, such as the trend of multiplicity fraction with stellar mass and the semi-major axis distribution. The results indicate that M-dwarfs are largely consistent with constituting an intermediate step in a continuous distribution from higher-mass stars down to brown dwarfs. Along with other observational results in the literature, this provides further indications that stars and brown dwarfs may share a common formation mechanism, rather than being distinct populations.
    The Astrophysical Journal 05/2012; 754(1). · 6.73 Impact Factor
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    ABSTRACT: Binary/multiple properties provide clues to the formation of stars. In the AstraLux binary survey, we use the Lucky Imaging technique to search for companions to a large sample of young, nearby M dwarfs. We present results from observations of the first sub-sample, consisting of 124 M dwarfs in the southern sky.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):460-461.
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    ABSTRACT: Binary or multiple stars are common in our neighbourhood, and many of the exoplanets we know of belong to a star in such a system. The influence of a second star on planet formation can be probed by comparing properties of planets in binary/multiple-star systems with those of single-star planets. We present some of the results from our Lucky Imaging survey for binary companions to hosts of transiting exoplanets.
    Proceedings of the International Astronomical Union 04/2012; 7(S282):193-194.
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    ABSTRACT: Context. HR 8799 is so far the only directly imaged multiple exoplanet system. The orbital configuration would, if better known, provide valuable insight into the formation and dynamical evolution of wide-orbit planetary systems. Aims: We present data which add to the astrometric monitoring of the planets HR 8799 b, c and d. We investigate how well the two simple cases of (i) a circular orbit and (ii) a face-on orbit fit the astrometric data for HR 8799 d over a total time baseline of ~2 years. Methods: The HR 8799 planetary system was observed in L'-band with NACO at VLT. Results: The results indicate that the orbit of HR 8799 d is inclined with respect to our line of sight, and suggest that the orbit is slightly eccentric or non-coplanar with the outer planets and debris disk. Based on observations collected at the European Southern Observatory, Chile, under programme ID 084.C-0072.
    Astronomy and Astrophysics 01/2011; 528. · 5.08 Impact Factor
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    ABSTRACT: A close stellar companion influences the formation of planets in the system. The occurrence of stellar companions and characteristics of the stars and planets in the system provide constraints on the formation processes. We present results from our high-resolution Lucky Imaging survey for binary exoplanet host stars, including the discovery of stellar companion candidates to the transiting planet hosts WASP-12 and HAT-P-8.
    Proceedings of the International Astronomical Union 01/2011; 276:397-398.
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    ABSTRACT: The Mid-Infrared ELT Imager and Spectrograph (METIS) will be the mid-infrared instrument for the European Extremely Large Telescope (E-ELT). It will provide diffraction limited imaging, high and low-resolution spectroscopy and phase mask coronography. Our science case for this instrument aims at direct detections of exoplanets and the study of their physical and chemical properties. We plan to observe a sample of M-dwarfs in the solar neighbourhood. In case of a direct detection of an exoplanet, we will conduct spectroscopic follow up observations with the low-resolution spectrograph provided by METIS.
    Coudé du Foresto, Vincent ; Gelino, Dawn M.; Ribas, Ignasi: Pathways Towards Habitable Planets, ASP, 534-538 (2010). 10/2010;
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    ABSTRACT: The 2nd generation Very Large Telescope Interferometer instrument GRAVITY aims at achieving micro-arcsecond accuracy astrometry. In a study of the GRAVITY science cases we investigate the astrometric detection of exoplanets around very low mass stars and closure phase variations during transits . This includes sample selection of M-dwarfs to be surveyed and simulations to identify the "best" survey strategy. With GRAVITY astrometric capability of 10 microarcsec, we expect to detect planets with 4 earth masses in 5 years survey.
    10/2010;
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    ABSTRACT: Astrometric planet searches with the GRAVITY VLTI instrument will be able to detect planets in the range from a few Earth-masses to Neptune-masses around nearby low-mass stars. High-resolution Lucky Imaging with AstraLux has provided a sample of nearby M dwarf binaries which defines a potential target list for astrometric planet detection with GRAVITY. We present the GRAVITY planet detection limits for one of the M dwarf binaries detected in the AstraLux survey.
    Coudé du Foresto, Vincent; Gelino, Dawn M. ; Ribas, Ignasi: Pathways Towards Habitable Planets, ASP, 405-406 (2010). 10/2010;
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    ABSTRACT: HR 8799 is the first multi-planet system that has been directly imaged. It contains three known giant planets enclosed by an inner and outer debris belt -- an architecture reminiscent of a scaled-up version of our own Solar system. We will discuss the first spatially resolved spectrum of the middle planet, HR 8799 c, acquired with VLT/NACO at 3.9-4.1 microns, where the continuum shape of the spectrum is suggestive of chemical non-equilibrium conditions in the atmosphere of the planet. We also describe our plans to characterize the other planets in the HR 8799 system, and to extend the wavelength coverage.
    10/2010;
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    ABSTRACT: We present results of direct imaging observations for HAT-P-7 taken with the Subaru HiCIAO and the Calar Alto AstraLux. Since the close-in transiting planet HAT-P-7b was reported to have a highly tilted orbit, massive bodies such as giant planets, brown dwarfs, or a binary star are expected to exist in the outer region of this system. We show that there are indeed two candidates for distant faint stellar companions around HAT-P-7. We discuss possible roles played by such companions on the orbital evolution of HAT-P-7b. We conclude that as there is a third body in the system as reported by Winn et al. (2009, ApJL, 763, L99), the Kozai migration is less likely while planet-planet scattering is possible. Comment: 8 pages, 3 figures, 2 tables, PASJ in press
    Publications- Astronomical Society of Japan 04/2010; · 2.44 Impact Factor
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    ABSTRACT: HR 8799 is a multi-planet system detected in direct imaging, with three companions known so far. Here, we present spatially resolved VLT/NACO 3.88--4.10 micron spectroscopy of the middle planet, HR 8799 c, which has an estimated mass of ~10 Mjup, temperature of ~1100 K and projected separation of 38 AU. The spectrum shows some differences in the continuum from existing theoretical models, particularly longwards of 4 microns, implying that detailed cloud structure or non-equilibrium conditions may play an important role in the physics of young exoplanetary atmospheres. Comment: 4 pages, 4 figures, accepted for publication in ApJ Letters
    The Astrophysical Journal Letters 01/2010; · 6.35 Impact Factor
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    ABSTRACT: Context. While M dwarfs are the most abundant stars in the Milky Way, there is still large uncertainty about their basic physical properties (mass, luminosity, radius, etc.) as well as their formation environment. Precise knowledge of multiplicity characteristics and how they change in this transitional mass region, between Sun-like stars on the one side and very low mass stars and brown dwarfs on the other, provide constraints on low mass star and brown dwarf formation. Aims: In the largest M dwarf binary survey to date, we search for companions to active, and thus preferentially young, M dwarfs in the solar neighbourhood. We study their binary/multiple properties, such as the multiplicity frequency and distributions of mass-ratio and separation, and identify short period visual binaries, for which orbital parameters and hence dynamical mass estimates can be derived in the near future. Methods: The observations are carried out in the SDSS i' and z' band using the Lucky Imaging camera AstraLux Sur at the ESO 3.5 m New Technology Telescope. Lucky Imaging is a very efficient way of observing a large sample of stars at an angular resolution close to the diffraction limit. Results: In the first part of the survey, we observed 124 M dwarfs of integrated spectral types M 0-M 6 and identified 34 new and 17 previously known companions to 44 stars. We derived relative astrometry and component photometry for these binary and multiple systems. More than half of the binaries have separations smaller than 1'' and would have been missed in a simply seeing-limited survey. Correcting our sample for selection effects yields a multiplicity fraction of 32±6% for 108 M dwarfs within 52 pc and with angular separations of 0.1''-6.0'', corresponding to projected separations of 3-180 AU at median distance 30 pc. Compared to early-type M dwarfs (M ⪆ 0.3 M&sun;), later-type (and hence lower mass) M dwarf binaries appear to have closer separations, and more similar masses. Based on observations made with ESO Telescopes at La Silla or Paranal Observatories under programme ID 082.C-0053.Tables 1 and 2 are only available in electronic form at http://www.aanda.org
    Astronomy and Astrophysics 01/2010; · 5.08 Impact Factor
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    ABSTRACT: Context. Sun-like stars are promising candidates to host exoplanets and are often included in exoplanet surveys by radial velocity (RV) and direct imaging. In this paper we report on the detection of a stellar companion to the nearby solar-analogue star HD 104304, which previously was considered to host a planetary mass or brown dwarf companion. Aims: We searched for close stellar and substellar companions around extrasolar planet host stars with high angular resolution imaging to characterize planet formation environments. Methods: The detection of the stellar companion was achieved by high angular resolution measurements, using the ``Lucky Imaging'' technique at the ESO NTT 3.5 m with the AstraLux Sur instrument. We combined the results with VLT/NACO archive data, where the companion could also be detected. The results were compared to precise RV measurements of HD 104304, obtained at the Lick and Keck observatories from 2001-2010. Results: We confirmed common proper motion of the binary system. A spectral type of M4V of the companion and a mass of 0.21 M_&sun; was derived. Due to comparison of the data with RV measurements of the unconfirmed planet candidate listed in the Extrasolar Planets Encyclopaedia, we suggest that the discovered companion is the origin of the RV trend and that the inclination of the orbit of i≈35°explains the relatively small RV signal. Based on observations made with ESO Telescopes at the La Silla and Paranal Observatory under programme IDs 083.C-0145 and 084.C-0812, and on data obtained from the ESO Science Archive Facility.
    Astronomy and Astrophysics 01/2010; · 5.08 Impact Factor
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    ABSTRACT: Lucky Imaging is an observational technique that aims to achieve nearly-diffraction-limited image quality from the ground on 2-4-metre-class telescopes. While diffraction-limited observations from the ground are usually accomplished with the help of adaptive optics in the near-infrared spectral range at almost all 8-10-metre-class telescopes, Lucky Imaging aims for a similar imaging performance at shorter wavelengths, in particular from 0.7-1.1 microns. AstraLux Sur, a visitor instrument for the NTT, built at the Max-Planck-Institute for Astronomy, is described and some early results are presented.
    The Messenger, v.137, 14-17 (2009). 09/2009;