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Issue
A&A
Volume 573, January 2015
Article Number A127
Number of page(s) 19
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/201423564
Published online 09 January 2015

© ESO, 2015

1. Introduction

Our understanding of the origin and evolution of extrasolar planets (EPs) has drastically transformed in the last decade. Current theories favor the formation of planets within a protoplanetary disk by the accretion of solids, which build up a 10 to 15 M⊕ core followed by rapid agglomeration of gas (Pollack et al. 1996; Alibert et al. 2004), or by gravitational instability of the gas (Boss 1997; Stamatellos & Withworth 2008; Vorobyov 2013). Whereas physical conditions and timescales favor core accretion in the inner disk (≤10 AU), gravitational instability could be the main mechanism to form massive gaseous giants at wider separations (≥10 AU) in the earliest phase of the disk’s lifetime (Boley 2009). The planets could migrate either inward, toward, or outward from the star by disk-planet interactions (Kley & Nelson 2012 and reference therein) or during planet-planet interactions (Naoz et al. 2011; Dawson & Murray-Clay 2013), which alter the original semi-major axis distribution. A wide range of potential planet masses, sizes, locations, and compositions results from this flurry of formation and evolution possibilities. A major goal for exoplanetary science in the next decade is a better understanding of these mechanisms. In this context, the role of observations is crucial in providing constraints that will help to model the diversity of exoplanetary properties. The main observables are the occurrence of EPs, the physical properties and orbital characteristics (composition, mass, radius, luminosity, distribution of mass, period, and eccentricity) but also the properties of the planetary hosts (mass, age, metallicity, lithium abundance, or multiplicity).

Brown dwarfs (BDs) were originally proposed as a distinguishable class of astrophysical objects with intermediate masses between stars and planets. Recent large infrared surveys and high contrast observations have unambiguously revealed the existence of planetary mass objects, which are isolated in the field (Zapatero-Osorio et al. 2000; Liu et al. 2013; Joergens et al. 2013) or wide companions to stars (Chauvin et al. 2005a). Their existence confirms that the formation mechanisms proposed to form stars (gravo-turbulent fragmentation, disk fragmentation, accretion-ejection or photo-erosion; see Whitworth et al. 2007; Luhman 2012 for reviews) can actually form objects down to the planetary mass regime. The details of contraction and subsequent evolution of the cores remain critical and are still under considerable debate. Episodic accretion processes can affect their physical properties (Baraffe et al. 2009). It is now undeniable that the stellar and planetary formation mechanisms overlap in the substellar regime. They can both lead to the formation of planetary mass objects, including companions to stars and BDs. Fossil traces of the formation processes should be revealed by different physical features (presence of core, composition of the atmosphere, system architecture...). Distinct statistical properties such as the occurrence, the mass, separation and eccentricity distributions, should help to identify the dominant mechanism to form substellar companions.

The main statistical constraints on exoplanets originally came from the radial velocity (RV) technique. More than 800 EPs have been now confirmed, which feature a broad range of physical (mass) and orbital (P, e) characteristics around different stellar hosts (Howard et al. 2010; Mayor et al. 2011; Wright et al. 2012; Bonfils et al. 2013). The strong bimodal aspect of the secondary-mass distribution to solar-type primaries has generally been considered the most obvious evidence of different formation mechanisms for stellar and planetary systems. The period distribution of giant exoplanets is basically made of two main features: a peak around 3 days plus an increasing frequency as a function of period (Udry & Santos 2007). The observed pile up of planets with periods around 3 days is believed to be the result of migration and final stopping mechanism. The rise of the number of planets with increasing distance from the parent star reaches up to a separation corresponding to the duration limit of most of the longest surveys (5–6 AU). This extrapolation hints that a large population of yet undetected Jupiter-mass planets may exist beyond 5 AU, suggesting an ideal niche for the direct-imaging surveys. More recently, a plethora of transiting planetary candidates have been revealed by Kepler (more than 2300 candidates known today, Batalha et al. 2013), which probably corroborate how abundant telluric planets are and agrees with Doppler surveys in terms of occurrence at less than 0.25 AU (Howard et al. 2012).

Table 1

Deep imaging surveys of young (<100 Myr) and intermediate-old to old (0.1−5 Gyr), close (<100 pc) stars that are dedicated to the search for planetary mass companions.

Despite the success of the RV and transit techniques, the time spans explored limit the studies to the close (≤5–6 AU) EPs. Within the coming years, direct imaging represents the only viable technique for probing the existence of EPs and BD companions at large (≥5–6 AU) separations. This technique is also unique for the characterization of planetary atmospheres that are not strongly irradiated by the planetary host (Janson et al. 2010; Bowler et al. 2010; Barman et al. 2011a,b; Bonnefoy et al. 2010, 2013; 2014a,b; Konopacky et al. 2013). Young (≤500 Myr), nearby stars are very favorable targets for the direct detection of the lowest mass companions. Since the discovery of the TW Hydrae association (TWA) by Kastner et al. (1997) and Hoff et al. (1998), more than 300 young, nearby stars were identified. They are gathered in several groups (TWA, β Pictoris, Tucana-Horologium, η Cha, AB Dor, Columba, Carinae), sharing common kinematics and photometric and spectroscopic properties (see Zuckerman & Song 2004; Torres et al. 2008). With typical contrast of 10−15 magnitudes for separations beyond 1.0−2.0′′ (50−100 AU for a star at 50 pc), planetary mass companions down to 1−2 Jupiter masses are potentially detectable by current imaging surveys that are very deep. The first planetary mass companions were detected at large distances (≥100 AU) and/or with small mass ratio with their primaries, indicating a probable star-like or gravitational disk instability formation mechanism (Chauvin et al. 2005b; Lafrenière et al. 2008).

The breakthrough discoveries of closer and/or lighter planetary mass companions like Fomalhaut b (<1 MJup at 177 AU; Kalas et al. 2008, 2013), HR 8799 bcde (10, 10, 10 and 7 MJup at resp. 14, 24, 38, and 68 AU; Marois et al. 2008, 2010), β Pictoris b (8 MJup at 8 AU; Lagrange et al. 2009), or more recently κ and b (14-2+25\hbox{$14^{+25}_{-2}$}MJup at 55 AU; Carson et al. 2013; Bonnefoy et al. 2014b), HD 95086 b (4−5MJup at 56 AU; Rameau et al. 2013a,b), and GJ 504 b (4-1+4.5MJup\hbox{$4^{+4.5}_{-1}~M_{\rm{Jup}}$} at 43.5 AU; Kuzuhara et al. 2013) indicate that we are just initiating the characterization of the outer part of planetary systems between typicaly 5−100 AU. Vast efforts are now devoted to systematic searches of EPs in direct imaging with an increasing number of large scale surveys (see Table 1; nine new surveys published between 2012 and 2013). The number of targets surveyed and the detection performances will increase with the new generation of planet finders LMIRCam at LBT (Skrutskie et al. 2010), MagAO (Close et al. 2012), ScExAO at Subaru (Guyon et al. 2010), SPHERE at VLT (Beuzit et al. 2008), and GPI at Gemini (Macintosh et al. 2008) with the goal to provide better statistics on larger samples and a greater number of giant planets to be characterized. It should enable the testing of alternative mechanisms to the standard planetary formation theories of core accretion and gravitation instability such as pebble accretion (Lambrechts & Johansen 2012; Morbidelli & Nesvorny 2012) or tidal downsizing (Boley et al. 2010; Nayakshin 2010; Forgan & Rice 2013) that are currently proposed to explain the existence of a population of giant planets at wide orbits. In the context of the VLT/SPHERE scientific preparation, we have conducted a large observing program (ESO: 184.C-0157) of 86 stars with NaCo (hereafter the NaCo-LP). Combined with stars already observed in direct imaging, it represents a total of more than ~210 stars for studying the occurrence rate of giant planets and brown dwarf companions at wide (10−2000 AU) orbits. This complete analysis is detailed in a series of four papers: a description of the complete sample (Desidera et al. 2015), the NaCo-LP survey (this paper), the statistical analysis of the giant planet population (Vigan et al., in prep.), and that of the brown dwarf companion population (Reggiani et al., in prep.). We therefore report here the results of the NaCo-LP carried out between 2009 and 2013. In Sect. 2, we describe the target sample selection. In Sect. 3, we describe the details of the observing setup. In Sect. 4, the data reduction strategy and analysis are reported with the results in Sect. 5. Finally, a preliminary statistical analysis of the observed sample is presented in Sect. 6 and our main conclusions in Sect. 7.

thumbnail Fig. 1

Histrograms summarizing the main properties of NaCo-LP observed sample (dark blue) and of the final NaCo-LP statistical sample of ~210 stars (light blue) used by Vigan et al. (in prep.) and Reggiani et al. (in prep.): spectral type, age, distance, H-band magnitude, proper motion amplitude, and galactic latitude.

2. Target properties

Based on a complete compilation of young, nearby stars that have been recently identified in young co-moving groups and from systematic spectroscopic surveys, we have selected a sample of stars according to their declination (δ ≤ 25°), their age (≲200 Myr), their distance (d ≲ 100 pc), and their R-band brightness (R ≤ 9.5). In addition, none of these stars had been observed in a high-contrast imaging survey before. Great care has been taken in the age selection criteria based on different youth diagnostics (isochrones, lithium abundance, Hα emission, X-ray activity, stellar rotation, chromospheric activity, and kinematics). Close visual (0.1−6.0′′) and spectroscopic binaries were rejected as they degrade the VLT/NaCo detection performances and bias the astrophysical interpretation. Among this sample, 86 stars were finally observed during the large program. The main target properties (spectral type, distance, age, H-band magnitude, galactic latitude, and proper motion) are reported in Table 2. They are also shown in Fig. 1 with the properties of the complete statistical sample used by Vigan et al. (in prep.) and Reggiani et al. (in prep.). A complete characterization of the NaCo-LP observed sample and the archive sample, particularly, with regard to the age and distance determination, is determined by Desidera et al. (2015). As can be seen from Fig. 1, the core of the NaCo-LP observed sample is mainly composed of close young (10–200 Myr) solar-type FGK stars.

3. Observations: telescope and instrument

Table 3

Observing campaigns.

We used the NaCo high contrast Adaptive Optics (AO) imager of the VLT-UT4. The NaCo instrument is equipped with the NAOS AO system (Rousset et al. 2002), and the near-infrared imaging camera CONICA (Lenzen et al. 2002). The observations were obtained during various observing runs spread between the end of 2009 and 2013 in visitor and service (queue-observing) modes. The summary of the observing runs is reported in Table 3. The NaCo-LP represents a total of 16.5 observing nights, 10.5 nights obtained in visitor mode and 6 nights in service.

To achieve high contrasts, we used angular differential imaging (ADI) on pupil-stabilized mode of NaCo. A classical Lyot-coronagraph with a diameter of 0.7′′ was used during the first visitor run but then replaced by saturated imaging as the NaCo point spread function (PSF) was unexpectedly drifting with time owing to a technical problem with the instrument. For accurate astrometry, a single observing setup was used, corresponding to the combined use of the H-band filter with the S13 camera (13.25 mas/pix). The time of the observations were chosen to maximize the field rotation. Typical exposure times of 1–10 s were used to saturate the PSF core by a factor 100 (a few pixels in radius) to improve the dynamic range of our images. The NaCo detector cube mode was additionally used to register each individual frame to optimize the final image selection in post-processing. The typical observing sequence was composed of a total of 10–15 cubes of 10–120 frames, which has a total integration time of 35–40 min for an observing sequence of 1–1.5 hrs on target. The parallactic angle variations are reported in Fig. 2 with the airmass, coherent energy, coherent time, as measured by NaCo, and the seeing, as measured by the DIMM seeing monitor at VLT. Non-saturated PSFs were acquired in ADI using a neutral density filter at the beginning of each observing sequence to monitor the image quality. They also served for the calibration of the relative photometric and astrometric measurements.

4. Data reduction and analysis

4.1. Cosmetics and data processing

thumbnail Fig. 2

Histograms summarizing the observing conditions of the NaCo-LP campaigns: airmass, DIMM seeing (ω), parallactic angle variation (Δπ), coherent energy (Ec), and coherent time (τ0).

Three independent pipelines were used to reduce and analyze the ADI data to optimize the PSF subtraction and the detection performances and to check the consistency of the results in terms of astrometry and photometry. These pipelines are described for the LAM-ADI pipeline by Vigan et al. (2012), the IPAG-ADI pipeline by Chauvin et al. (2012), and the Padova-ADI pipeline by Esposito et al. (2013). Each pipeline processed the data in a similar way for the first cosmetic steps of flat-fielding, bad- and hot-pixel removal, and sky subtraction. To determine the central star position for the frame recentring, a Moffat fitting of the non-saturated part of the stellar PSF wing (with a similar threshold) was used. Finally, an encircled energy criteria was considered for the rejection of open-loop and poorly-corrected frames for computing a final mastercube with the correspoding parallactic angle variation. The main differences between the pipelines mostly reside in the various ADI algorithms applied (cADI and sADI, see Marois et al. 2006; LOCI, see Lafrenière et al. 2007) and in the parameters setup. Consistent results within 0.1–0.2 mag in photometry (candidate photometry and detection limits) and 0.2–0.3 pixels in astrometry were found between the different pipelines for a series of targets used as test cases. Non-saturated PSFs were similarly reduced without PSF subtraction.

The results presented in this final analysis have been obtained with the LAM-ADI pipeline using LOCI with optimization regions of NA = 300×FWHM at less than 3′′, NA = 3000×FWHM at more than 3′′, the radial to azimuthal width ratio g = 1, the radial width Δr = 2×FWHM, and a separation criteria of 0.75 ×FWHM. The binning of the data was tuned to apply LOCI on a final mastercube taht is reduced to ~350 frames. An illustration of the final LOCI processed image of the young star TYC 7617-0549-1 (K0V, 76.4 pc and 30 Myr) is shown in Fig. 3.

thumbnail Fig. 3

VLT/NACO ADI observation in H-band of the young star TYC 7617-0549-1 (K0V, 76.4 pc and 30 Myr). A faint (ΔH = 12.7 mag) candidate, resolved at 1.8′′, has been finally identified as a background contaminant (see Fig. 6).

4.2. Relative astrometry and photometry

The relative position and flux of all candidates was determined using Moffat fitting and aperture photometry corrected from the ADI flux loss. This first order analysis was sufficient for assessing the proper motion and nature of the candidate as described in Sect. 5.2. For the most interesting cases (like HD 8049), the injection of fake planets at the location of the candidate signal was done to properly take any local astrometric and photometric biases into account which induced by the ADI-processing described in Chauvin et al. (2012).

To finally calibrate the relative astrometric position of the detected candidates to the primary star, we used the θ1 Ori C field observed with HST by McCaughrean & Stauffer (1994; with the same set of stars TCC058, 057, 054, 034 and 026) as a primary calibrator. The astrometric binary IDS 13022N0107 (van Dessel & Sinachopoulos 1993) was then used as a secondary calibrator when the θ1 Ori C field was not observable and then recalibrated on the θ1 Ori C field when both were observable. Both fields were observed in standard field-stabilized mode and reduced (cosmetics, flat-fielding, bad and hot-pixel removal, sky subtraction, and recentring) using the Eclipse1 reduction software developed by Devillar (1997). Finally, for ADI data, the NaCo rotator offset at the start of each ADI sequence was also calibrated and taken into account as described by Chauvin et al. (2012). The results of the platescale and true north orientation determinations are given in Table 4.

Table 4

Mean plate scale and true north orientation for each observing run of the NaCo-LP.

The throughput of the NaCo neutral density filter was recalibrated on sky using two different datasets taken for the star TYC 9162-0698 during our February 2010 visitor run. Using aperture photometry on the data taken with and without the neutral density, we derived a transmission factor of 1.19 ± 0.05% with the H-band filter. This result is consistent with the one derived by Bonnefoy et al. (2013) and was used to calibrate the candidate photometry and the detection limits using the non-saturated sequence of the primary star with the neutral density filter as a photometric reference.

thumbnail Fig. 4

VLT/NACO deep ADI 5σ detection limits in H-band combined with the S13 camera. The worst, median, and best detection limits are shown with all the candidates detected. Separations of less than 0.1–0.2′′ are generally saturated.

4.3. Detection limit determination

A pixel-to-pixel noise map of each observation was estimated within a box of 5 × 5 pixels sliding from the star to the limit of the NACO field of view. To correct for the flux loss related to the ADI processing, fake planets were regularly injected for every 20 pixels in radius at 10 different position angles for separations smaller than 3′′. At more than 3′′, fake planets were injected for every 50 pixels at four different position angles. The final flux loss was computed with the azimuthal average of the flux losses of fake planets at the same radii. The final detection limits at 5σ were then obtained using the pixel-to-pixel noise map divided by the flux loss and normalized by the relative calibration with the primary star (considering the different exposure times and the neutral density). The LOCI processing leads to residuals whose distribution closely resembles a Gaussian (Lafrenière et al. 2007); therefore, a 5σ threshold is thus adequate for estimating detection performances. The best, worst and median detection limits of the survey are reported in Fig. 4.

5. Results

A total of 86 sources were observed. Sixteen stars were resolved as binaries, including HD 8049 with a newly discovered white dwarf companion. Ten binaries were simply observed in non-saturated ADI imaging to directly derive their relative astrometry and photometry. Seventy-six stars were observed in saturated high-contrast ADI to search for faint substellar companions. In the following sub-sections, we describe the properties of the new stellar multiple systems, the status of the detected candidates in saturated ADI, the characteristics of the white dwarf companion around HD 8049, and finally the fine analysis of the thin debris-disk around HD 61005.

5.1. New stellar close multiple systems

Despite our sample selection to reject close (0.1−6.0′′) binaries, 16 stars were resolved as multiple. Three systems were already known, HIP 108422 AB (Chauvin et al. 2003), TYC 7835-2569-1 AB (Brandner et al. 1996), and TYC 6786-0811-1 (Köhler et al. 2000), and went through our sample selection process by mistake. The system TYC 8484-1507-1 is actually also a known ~8.6′′ binary that was resolved by 2MASS, which is not rejected during our sample selection but resolved in the NaCo FoV, despite its large separation. Then, in the case of HD 8049, the faint comoving companion turned out to be a white dwarf. Its characteristics are briefly described in Sect. 5.3. At the end, a total of eleven new close multiple systems were resolved. All of them were observed in non-saturated ADI to derive their position and H-band photometry relative to the primary star (see Table 5). The visual binaries HIP 108422 AB, TYC 7835-2569-1 AB and TYC 6786-0811-1 are confirmed as physically bound. Deep ADI observations were obtained in addition to six binaries (TYC 0603-0461-1, TYC 7835-2569-1, HD 8049, TYC 8927-3620-1, HIP 80290, and TYC 8989-0583-1).

5.2. Companion candidates

Table 5

Relative positions and H-band contrast of the new binaries resolved during the NaCo-LP.

Among the 76 stars observed in ADI, one companion candidate or more were detected for 43 targets (see Table 2). More than 700 candidates were detected with 90% of them in six very crowded field (see Fig. 4). The galactic contamination rate, predicted by the Besançon galactic population model (Robin et al. 2003) for the NaCo-LP fields and at least one background source, is equal to 51%, which reasonably agrees with 56% (43 systems with at least one candidate for the 76 observed). The model uses the NaCo field of view as input with the typical magnitude limit of the NaCo-LP (Hlim = 21 mag) survey, and the galactic coordinates of all targets. The repartition of these galactic contaminants is given in Fig. 5. Solar-system and extra-galactic contaminants are expected to be significantly less frequent. Moreover, solar system contaminants smear during a 1 h observing sequence, and extra-galactic contaminants are mainly extended background galaxies resolved by NaCo. The most important population of contaminants that can mimic the apparent flux of the giant planet or brown dwarf companions bound to the star are M dwarfs with typical H = 20−22 mag apparent magnitudes.

To identify their nature, we relied on the follow-up observations at additional epochs to distinguish comoving companions from stationary background stars. The candidates were ranked by priority as a function of their predicted masses (higher priority to lower masses), projected physical separations (assuming they would be bound; higher priority to closer candidates) and predicted false alarm probabilities using the Besançon galactic population model (Robin et al. 2003) to guide the follow-up strategy. Follow-up observations with a second epoch were obtained for 29 targets, including the Moth system (HD 61005) that characterized during dedicated follow-up observations. The amplitude of stellar proper motion (larger than 30 mas/yr for 80% of the NaCo-LP target) enabled a rapid identification over a 1 yr interval (see Fig. 1, bottom–middle).

thumbnail Fig. 5

Expected spectral type distribution of field stars from the Besançon galactic population model, as observed during the NaCo-LP. The FoV, the typical magnitude limit of the NaCo-LP (Hlim = 21 mag), and the galactic coordinates of all targets were considered. The predicted repartion is given as a function of the spectral type and the apparent magnitude in H-band.

For the 29 systems with at least 2-epoch observations (including the Moth system), we used a χ2 probability test with 2 ×Nepochs degrees of freedom (corresponding to the measurements: separations in the Δα and Δδ directions for the number Nepochs of epochs). This test considers the uncertainties in the relative positions measured at each epoch and the uncertainties in the primary proper motion and parallax (or distance). Figure 6 gives an illustration of a (Δα, Δδ) diagram that was used to identify a stationary background contaminant around TYC 7617-0549-1. A status has been assigned to each candidate as a background contaminant (B; Pcomoving,χ2< 1% and with a relative motion compatible with a background source), comoving (C; PBKG,χ2< 1%) and with the relative motion compatible with a comoving companion), and undefined (U) when observed at only one epoch or when not satisfying the first two classifications.

Only one comoving companion, the white dwarf companion around HD 8049 described hereafter, was identified. Among the 28 other follow-up fields, ten fields have been completely characterized, and 18 are partially due to detection limits variation from one epoch to another. Fourteen fields still require second epoch observations. The status of all the candidates is given in Table 6.

thumbnail Fig. 6

VLT/NaCo measurements (filled circles with uncertainties) of the offset positions of the companion candidate to TYC 7617-0549-1 (see Fig. 3). The expected variation of offset positions, if the candidate is a background object, is shown (curved line). The variation is estimated based on the parallactic and proper motions of the primary star, as well as the initial offset position of the companion candidate from TYC 7617-0549-1. The companion candidate is clearly identified here as a stationary background contaminant.

5.3. A white dwarf companion around HD 8049

The only comoving companion identified in this survey with a preliminary predicted mass of 35 MJup was discovered around the star HD 8049 (K2, 33.6 pc). The star had a predicted age of 90–400 Myr from its rotational period, H&K emission and X-ray emission. Thanks to the high proper motion of the central star (μα = 65.99 ± 1.18 mas/yr and μδ = 240.99 ± 0.98 mas/yr), a χ2 probability test on Δα and Δδ with respect to the star at two epochs rejected the possibility (at 99% certainty) that the object was a background source. Further analysis using archived data, radial velocity observations spanning a time range of ~30 yr, U-band imaging with EFOSC, and near-infrared spectroscopy of the comoving companion with VLT/SINFONI finally revealed that the companion was actually a white dwarf (WD) with temperature Teff = 18 800± 2100 K and mass MWD = 0.56 ± 0.08 M⊙.

thumbnail Fig. 7

Left: NaCo-LP mean detection probability map (⟨ pj ⟩) as a function of the mass and semi-major axis. Right: mean probability curves for different masses (1, 3, 5, 7, 10, 15, and 20 MJup) as a function of the semi-major axis.

This astrophysical false positive revealed that the system age was much older than initially thought. The age diagnostics have likely been affected, as the central star has been probably rejuvenated by the accretion of some amount of mass and angular momentum at the time of mass loss from the WD progenitor. A complete analysis of the system (evolution and kinematics) by Zurlo et al. (2013) actually reveals that the resulting age of the system to be about 3–6 Gyr.

5.4. The Moth resolved as a thin debris-disk

In the course of the survey, the emblematic star HD 61005 (G8V, 90 Myr, 34.5 pc), known to host The Moth debris disk (Hines et al. 2007), was observed. The NaCo H-band image remarkably resolves the disk component as a distinct narrow ring at inclination of i = 84.3 ± 1.0°, with a semimajor-axis of a = 61.25 ± 0.85 AU and an eccentricity of e = 0.045 ± 0.015. The observations also revealed that the the ring centre is offset from the star by at least 2.75 ± 0.85 AU, which indicates a possibly dynamical perturbation by a planetary companion that perturbs the remnant planetesimal belt. The observations and the detailed disk modeling were published by Buenzli et al. (2010). Subsequent observations did not reveal any giant planet companions. Three other stars of our sample are known to host debris-disks: HIP 11360 (HD 15115; Kalas et al. 2007, Rodigas et al. 2012), HIP 99273 (HD 191089; Churcher et al. 2011), and HIP 76829 (HD 139664; Kalas et al. 2006). No clear detection was obtained with our ADI analysis.

6. Statistical analysis

6.1. Sample definition

To define a meaningful sample for the statistical analysis of the survey, we first removed all visual and spectroscopic binaries from the sample of 76 stars observed in ADI. It includes the six visual multiple systems observed in that mode (TYC 0603-0461-1, TYC 7835-2569-1, HD 8049, HIP 8290, TYC 8927-3620-1 and TYC 8989-0583-1), and seven new spectroscopic binaries unknown at the time of our sample selection. We have then selected two sub-samples:

  • the full-stat sample of 63 stars that includes all single starsobserved in ADI with detection sensitivities down to planetarymasses for physical separations ranging from 10to 2000 AU. The status of all thecandidates detected in these fields have, however, not been fullycompleted, although a large majority are expected to bestationary background contaminants. This sample gives anestimation of the ultimate performances of the survey in terms ofmasses and physical separations, when the candidate statusidentification will be complete, which is probably with SPHEREin the forthcoming years;

  • the complete-stat sample of 51 stars has been restrained to all systems for which the candidate status identification up to 300 AU was complete. This includes cases with no companion candidates detected or with companion candidates properly identified thanks to our follow-up observations as stationary background sources or comoving companions. In the case of follow-up observations with variable detection performances from one epoch to another (therefore with possible undefined faint sources due to the lack of redetection), only the worst detection limit was considered. These selection criteria offered us a meaningful sample at the end for which the detection and the status identification of the candidates was complete.

6.2. Survey detection probability

To correct for the projection effect from the observations, we then ran a set of Monte-Carlo simulations using an optimized version of the MESS code (Bonavita et al. 2012). For the full-stat sample, the code generates a uniform grid of mass (with a sampling of 0.5 MJup in the [1, 75] MJup interval), and semi-major axis (with a sampling of 1 AU between 1 and 1000 AU, and 2 AU between 1000 and 2000 AU for the [1, 2000] AU interval). For the complete-stat sample, the uniform grid is generated in the semi-major axis ranges between [1, 300] AU with a sampling of 1 AU. For each point in the grid, 100 orbits were generated and randomly oriented in space from uniform distributions in sin(i), ω, Ω, e ≤ 0.8, and Tp. The on-sky projected position (separation and position angle) at the time of the observation is then computed for each orbit and compared to our 5σ 2D-detection maps to determine the individual detection probability (pj) of planets around each star. The average of all individual detection limits gives us the typical mean detection probability (⟨ pj ⟩) of the NaCo-LP to the planet and BD companion population. The results for the full-stat and complete-stat samples are shown in Figs. 7 and 8 top) respectively. The detection probabilities in both cases do not significantly differ at less than 300 AU. Most companions more massive than 20 MJup with a semi-major axis between 70 and 200 AU should have been detected during our survey. We are 50% sensitive to massive (≥10 MJup) planets and brown dwarfs with a semi-major axis between 60 and 400 AU. Finally, the detection of giant planets as light as 5 MJup between 50–800 AU is only possible for 10% of the stars observed. The relatively small number of very young stars (see Fig. 1) is responsible for this limited sensitivity to light giant planets.

6.3. Giant planet occurrence at wide orbits

thumbnail Fig. 8

Results for the complete-stat sample. Top left: NaCo-LP mean detection probability map (⟨ pj ⟩) as a function of the mass and semi-major axis. Top right: mean probability curves for different masses (1, 3, 5, 7, 10, 15, and 20 MJup) as a function of the semi-major axis. Bottom left: giant planet and brown dwarf occurrence upper limit (fmax), considering a 95% confidence level, for different masses (3, 5, 7, 10, 15, and 20 MJup) as a function of the semi-major axis considering the null-detection result and an uniform distribution of planets and brown dwarfs in terms of masses and semi-major axis. Bottom right: same occurrence upper limit (fmax) expressed this time in a mass versus semi-major axis diagramme for a 68% and 95% confidence level (following Biller et al. 2007; Nielsen et al. 2008 representation).

To derive the occurrence of giant planets and brown dwarfs in our survey, we only considered the complete-stat sample with a complete census of the candidates status within 300 AU. As no planetary mass or brown dwarf companions were detected, we considered here a null-detection result. We then used the mean detection probability (⟨ pj ⟩) to derive the giant planet and brown dwarf occurrence upper limit (fmax) that is compatible with the survey detection limits. The probability of planet detection for a survey of N stars is described by a binomial distribution, given a success probability fpj with f as the fraction of stars with planets. The parameter pj is the individual detection probability of detecting a planet if it is present around the star j and computed previously. Assuming that the number of expected detected planets is small compared to the number of stars observed, the binomial distribution can be approximated by a Poisson distribution to derive a simple analytical solution for the exoplanet fraction upper limit (fmax). The formalism is described by Carson et al. (2006) and Lafrenière et al. (2007). The result is shown in Fig. 8 (bottom-left and bottom-bight). For this complete-stat sample, we constrain the occurrence of exoplanets that are more massive than 5 MJup to typically less than 15% between 100 and 300 AU. The occurence is less than 10% between 50 and 300 AU for exoplanets that are more massive than 10 MJup. We consider here a uniform input distribution with a confidence level of 95%. These values are consistent with current estimations from various studies with comparable sensitivities around young, solar-type stars (fmax ≤ 9.7% for [ 0.5,13 ]MJup planet between [ 50−250 ] AU by Lafrenière et al. 2007; fmax ≤ 10% for [ 1,13 ]MJup planet between [ 40−150 ] AU by Chauvin et al. 2010; fmax ≤ 6% for [ 1,20 ]MJup planet between [ 10−150 ] AU by Biller et al. 2013).

A more complete analysis, which combines the results of the NaCo-LP with archive data for a total of ~210 observed stars in direct imaging, will be presented in related papers by Vigan et al. (in prep.) and Reggiani et al. (in prep.). This analysis will provide significant and relevant statistical constraints on the population of planets and brown dwarfs around young, nearby solar-type (FGK) stars (single or members of wide binaries) and enable tests of planet and brown dwarf formation models.

7. Conclusion

In the context of the scientific preparation of the VLT/SPHERE guaranteed time, we have conducted a survey of 86 young, close and mostly solar-type stars by using NaCo at the VLT between 2009 and 2013. Our main goals were to detect new giant planets and brown dwarf companions and to initiate a relevant statistical study of their occurrence at wide (10−2000 AU) orbits. The NaCo instrument was used in pupil-stabilized mode to perform angular differential imaging at H-band. It enables us to reach contrast performances as small as 10-6 at 1.5′′. Of the 86 stars observed, the survey led to

  • the discovery of 11 new close binaries that we characterized interms of relative photometry and astrometry;

  • the detection of more than 700 companion candidates with 90% of them being located in six crowded fields. Among the 76 stars observed in deep ADI, 33 systems have no point-source detected in their vicinity, and 43 systems have at least one companion candidate detected. Repeated observations at several epochs enabled us to analyze the candidate status, either completely or partially, around 29 stars. Planetary mass candidates with proper follow-up were all identified as background sources. Additional follow-up observations are still necessary to fully complete the status identification of all candidates detected in the survey owing to the variability of the detection performances from one run to another. It shows that more than two epochs are generally necessary during a survey for a full exploration of the companions content.

  • The discovery of a unique comoving companion to the star HD 8049. This result has been published by Zurlo et al. (2013) and has revealed that the companion was actually a white dwarf with temperature Teff = 18 800 ± 2100 K and mass MWD = 0.56 ± 0.08 M⊙.

  • New high-contrast images of the Moth debris-disk at HD 61005. The NaCo H-band image remarkably resolves the disk component as a distinct narrow ring offset from the star by at least 2.75 ± 0.85 AU, which indicates a possibly dynamical perturbation by a planetary companion. This study was published by Buenzli et al. (2010).

  • Finally, a preliminary statistical analysis of the survey detection probabitlities around the sample of 63 young, single and mostly solar-type (FGK) stars observed in angular differential imaging with detection performances enabling the search for planets and brown dwarfs in the stellar environment. Most companions that are more massive than 20 MJup with a semi-major axis between 70 and 200 AU should have been detected during our survey. We are 50% sensitive to massive (≥10 MJup) planets and brown dwarfs with a semi-major axis between 60 and 400 AU. Finally, the detection of giant planets as light as 5 MJup between 50–800 AU is only possible for 10% of the stars observed. We have then defined a more complete sample of 51 stars restrained to all systems for which the candidate status identification was complete up to 300 AU. This includes cases with no companion candidates detected or with companion candidates properly and completely identified. Based on this complete sample average detection probability, a non-detection result, and the consideration of a uniform distribution of giant planets and brown dwarf companions in terms of semi-major axis and mass, we derive a typical upper limit for the occurrence of exoplanets that are more massive than 5 MJup of 15% between 100 and 300 AU, and a limit of 10% between 50 and 300 AU for EPs that are more massive than 10 MJup with a confidence level of 95%.

Combined with compiled archived data, the results of this survey offer a unique sample of ~210 young, solar-type stars that are observed in deep imaging as a mean to constrain the presence of giant planets and brown dwarfs in their close environment. A more complete statistical analysis will be published in two linked articles by Vigan et al. (in prep.) and Reggiani et al. (in prep.), which will test the relevance of various analytical distributions for describing the giant planet and brown dwarf companion population at wide orbits but will also bring further constraints on current theories of planetary formation. All final products of this survey (images, detection limits, and candidate status) will be released in the Deep Imaging Virtual Archive (DIVA) database with the archive data used for full statistical analysis. We encourage the community to support this effort by sharing the final products (reduced images, detection limits, and candidate relative astrometry, photometry, and status) of their published surveys to optimally prepare the future of planet imaging searches that come with the new generation of planet imagers like LMIRCam, MagAO, SPHERE, GPI, and SCExAO. In the long term, these include JWST (Clampin 2010), TMT-PFI (Simard et al. 2010), and the EELT instruments (METIS or E-MIDIR, Brandl et al. 2010; EPICS or E-PCS, Kasper et al. 2010).

Online material

Table 2

NaCo-LP target sample and properties.

Table 6

Companion candidate characterization and identification (for multi-epoch observations).

Acknowledgments

We greatly thank the staff of ESO-VLT for their support at the telescope. This publication has made use of the SIMBAD and VizieR database operated at CDS, Strasbourg, France. Finally, we acknowledge supports from: 1) the French National Research Agency (ANR) through project grant ANR10-BLANC0504-01, the CNRS-D2P PICS grant, and the Programmes Nationaux de Planétologie et de Physique Stellaire (PNP & PNPS), in France for G.C., A.V, P.D., J.-L.B., A.-M.L. and D.M.; 2) INAF through the PRIN-INAF 2010 Planetary Systems at Young Ages project grant for S.D., D.M., M.B. and R.G. and 3) the US National Science Foundation under Award No. 1009203 for J.C.

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All Tables

Table 1

Deep imaging surveys of young (<100 Myr) and intermediate-old to old (0.1−5 Gyr), close (<100 pc) stars that are dedicated to the search for planetary mass companions.

In the text
Table 3

Observing campaigns.

In the text
Table 4

Mean plate scale and true north orientation for each observing run of the NaCo-LP.

In the text
Table 5

Relative positions and H-band contrast of the new binaries resolved during the NaCo-LP.

In the text
Table 2

NaCo-LP target sample and properties.

In the text
Table 6

Companion candidate characterization and identification (for multi-epoch observations).

In the text

All Figures

thumbnail Fig. 1

Histrograms summarizing the main properties of NaCo-LP observed sample (dark blue) and of the final NaCo-LP statistical sample of ~210 stars (light blue) used by Vigan et al. (in prep.) and Reggiani et al. (in prep.): spectral type, age, distance, H-band magnitude, proper motion amplitude, and galactic latitude.
In the text
thumbnail Fig. 2

Histograms summarizing the observing conditions of the NaCo-LP campaigns: airmass, DIMM seeing (ω), parallactic angle variation (Δπ), coherent energy (Ec), and coherent time (τ0).
In the text
thumbnail Fig. 3

VLT/NACO ADI observation in H-band of the young star TYC 7617-0549-1 (K0V, 76.4 pc and 30 Myr). A faint (ΔH = 12.7 mag) candidate, resolved at 1.8′′, has been finally identified as a background contaminant (see Fig. 6).

In the text
thumbnail Fig. 4

VLT/NACO deep ADI 5σ detection limits in H-band combined with the S13 camera. The worst, median, and best detection limits are shown with all the candidates detected. Separations of less than 0.1–0.2′′ are generally saturated.
In the text
thumbnail Fig. 5

Expected spectral type distribution of field stars from the Besançon galactic population model, as observed during the NaCo-LP. The FoV, the typical magnitude limit of the NaCo-LP (Hlim = 21 mag), and the galactic coordinates of all targets were considered. The predicted repartion is given as a function of the spectral type and the apparent magnitude in H-band.
In the text
thumbnail Fig. 6

VLT/NaCo measurements (filled circles with uncertainties) of the offset positions of the companion candidate to TYC 7617-0549-1 (see Fig. 3). The expected variation of offset positions, if the candidate is a background object, is shown (curved line). The variation is estimated based on the parallactic and proper motions of the primary star, as well as the initial offset position of the companion candidate from TYC 7617-0549-1. The companion candidate is clearly identified here as a stationary background contaminant.
In the text
thumbnail Fig. 7

Left: NaCo-LP mean detection probability map (⟨ pj ⟩) as a function of the mass and semi-major axis. Right: mean probability curves for different masses (1, 3, 5, 7, 10, 15, and 20 MJup) as a function of the semi-major axis.
In the text
thumbnail Fig. 8

Results for the complete-stat sample. Top left: NaCo-LP mean detection probability map (⟨ pj ⟩) as a function of the mass and semi-major axis. Top right: mean probability curves for different masses (1, 3, 5, 7, 10, 15, and 20 MJup) as a function of the semi-major axis. Bottom left: giant planet and brown dwarf occurrence upper limit (fmax), considering a 95% confidence level, for different masses (3, 5, 7, 10, 15, and 20 MJup) as a function of the semi-major axis considering the null-detection result and an uniform distribution of planets and brown dwarfs in terms of masses and semi-major axis. Bottom right: same occurrence upper limit (fmax) expressed this time in a mass versus semi-major axis diagramme for a 68% and 95% confidence level (following Biller et al. 2007; Nielsen et al. 2008 representation).
In the text

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