Context. Direct imaging of circumstellar disks requires high-contrast and high-resolution techniques. The angular differential imaging (ADI) technique is one of them, initially developed for ...point-like sources but now increasingly applied to extended objects such as disks. This new field of application raises many questions because the disk images reduced with ADI depend strongly on the amplitude of field rotation and the ADI data reduction strategy. Both of them directly affect the disk observable properties. Aims. Our aim is to characterize the applicability and biases of some ADI data reduction strategies for different disk morphologies. A particular emphasis is placed on parameters mostly used for disks such as their surface brightness distribution, their width if the disk is a ring, and local features such as gaps or asymmetries. We first present a general method for predicting and quantifying those biases. In a second step we illustrate them for some widely used ADI algorithms applied to typical debris disk morphologies: inclined rings with various inner/outer slopes and width. Last, our aim is also to propose improvements of classical ADI to limit the biases on extended objects. Methods. Simulated fake disks seen under various observing conditions were used to reduce ADI data and quantify the resulting biases. These conclusions are complemented by previous results from NaCo L’ real-disk images of HR 4796A. Results. As expected, ADI induces flux losses on disks. This makes this technique appropriate only for low- to medium-inclination disks. A theoretical criterion is derived to predict the amount of flux loss for a given disk morphology, and quantitative estimates of the biases are given in some specific configurations. These biases alter the disk observable properties, such as the slopes of the disk surface brightness or the radial/azimuthal extent of the disk. Additionally, this work demonstrates that ADI can very easily create artificial features without involving astrophysical processes. For example, a particularly striking feature appears for a ring when the amplitude of field rotation is too small. The two ring ansae are surrounded by two flux-depleted regions, which makes them appear as bright blobs. This observation does not require any astrophysical process such as dust blown by radiation pressure, as previously proposed in H-band images of HR 4796A. Conclusions. The ADI techniques behave as spatial filtering algorithms and can bias disk observables. Therefore, the filtering process needs to be properly calibrated when deriving disk parameters from processed images.
Context.
Young nearby stars are good candidates in the search for planets with both radial velocity (RV) and direct imaging techniques. This, in turn, allows for the computation of the giant planet ...occurrence rates at all separations. The RV search around young stars is a challenge as they are generally faster rotators than older stars of similar spectral types and they exhibit signatures of magnetic activity (spots) or pulsation in their RV time series. Specific analyses are necessary to characterize, and possibly correct for, this activity.
Aims.
Our aim is to search for planets around young nearby stars and to estimate the giant planet (GP) occurrence rates for periods up to 1000 days.
Methods.
We used the HARPS spectrograph on the 3.6 m telescope at La Silla Observatory to observe 89
A
−
M
young (<600 Myr) stars. We used our SAFIR (Spectroscopic data via Analysis of the Fourier Interspectrum Radial velocities) software to compute the RV and other spectroscopic observables. Then, we computed the companion occurrence rates on this sample.
Results.
We confirm the binary nature of HD 177171, HD 181321 and HD 186704. We report the detection of a close low mass stellar companion for HIP 36985. No planetary companion was detected. We obtain upper limits on the GP (<13
M
Jup
) and BD (∈ 13;80
M
Jup
) occurrence rates based on 83 young stars for periods less than 1000 days, which are set, 2
−2
+3
and 1
−1
+3
%.
Direct imaging has only begun to inventory the population of gas giant planets on wide orbits around young stars in the solar neighborhood. Following this approach, we carried out a deep imaging ...survey in the near-infrared using VLT/NaCo to search for substellar companions. Here we report the discovery of a probable companion orbiting the young (10-17 Myr), dusty, early-type (A8) star HD 95086 at 56 AU in L' (3.8 mu m) images. This discovery is based on observations with more than a year time lapse. Our first epoch clearly revealed the source at Asymptotically = to10sigma, while our second epoch lacks good observing conditions, yielding a Asymptotically = to3sigma detection. Various tests were thus made to rule out possible artifacts. This recovery is consistent with the signal at the first epoch but requires cleaner confirmation. Nevertheless, our astrometric precision suggests that the companion is comoving with the star with a 3sigma confidence level. The planetary nature of the source is reinforced by a non-detection in the K s-band (2.18 mu m) images according to its possible extremely red K s-L' color. Conversely, background contamination is rejected with good confidence level. The luminosity yields a predicted mass of about 4-5 M sub(Jup) (at 10-17 Myr) using "hot-start" evolutionary models, making HD 95086 b the exoplanet with the lowest mass ever imaged around a star.
Context.
Solar simulations and observations show that the detection of long-period Earth-like planets is expected to be very difficult with radial velocity techniques in the solar case because of ...activity. The inhibition of the convective blueshift in active regions (which is then dominating the signal) is expected to decrease toward lower mass stars, which would provide more suitable conditions.
Aims.
In this paper we build synthetic time series to be able to precisely estimate the effects of activity on exoplanet detectability for stars with a wide range of spectral type (F6-K4) and activity levels (old main-sequence stars).
Methods.
We simulated a very large number of realistic time series of radial velocity, chromospheric emission, photometry, and astrometry. We built a coherent grid of stellar parameters that covers a wide range in the (
B
–
V
, Log
R
′
HK
) space based on our current knowledge of stellar activity, to be able to produce these time series. We describe the model and assumptions in detail.
Results.
We present first results on chromospheric emission. We find the average Log
R
′
HK
to correspond well to the target values that are expected from the model, and observe a strong effect of inclination on the average Log
R
′
HK
(over time) and its long-term amplitude.
Conclusions.
This very large set of synthetic time series offers many possibilities for future analysis, for example, for the parameter effect, correction method, and detection limits of exoplanets.
We report deep imaging observations of the young, nearby star AB Pic, a member of the large Tucana-Horologium association. We have detected a faint, red source $5.5''$ South of the star with ...JHK colors compatible with that of a young substellar L dwarf. Follow-up observations at two additional epochs confirm, with a confidence level of 4.7σ, that the faint red object is a companion to AB Pic rather than it being a stationary background object. A low resolution K-band spectrum indicates an early-L spectral type for the companion. Finally, evolutionary model predictions based on the JHK photometry of AB Pic b indicate a mass of 13 to 14 $M_{{\rm Jup}}$ if its age is ~30 Myr. Is AB Pic b a massive planet or a minimum mass brown dwarf?
Context.
The effect of stellar activity on radial velocity (RV) measurements appears to be a limiting factor in detecting Earth-mass planets in the habitable zone of a star that is similar to the Sun ...in spectral type and activity level. It is crucial to estimate whether this conclusion remain true for other stars with current correction methods.
Aims.
We built realistic time series in radial velocity and chromospheric emission for old main-sequence F6-K4 stars. We studied the effect of the stellar parameters we investigate on exoplanet detectability. The stellar parameters are spectral type, activity level, rotation period, cycle period and amplitude, latitude coverage, and spot constrast, which we chose to be in ranges that are compatible with our current knowledge of stellar activity.
Methods.
This very large set of synthetic time series allowed us to study the effect of the parameters on the RV jitter and how the different contributions to the RV are affected in this first analysis of the data set. The RV jitter was used to provide a first-order detection limit for each time series and different temporal samplings.
Results.
We find that the coverage in latitude of the activity pattern and the cycle amplitudes have a strong effect on the RV jitter, as has stellar inclination. RV jitter trends with
B
–
V
and Log R′
HK
are similar to observations, but activity cannot be responsible for RV jitter larger than 2–3 m s
−1
for very quiet stars: this observed jitter is therefore likely to be due to other causes (instrumental noise or stellar or planetary companions, e.g.). Finally, we show that based on the RV jitter that is associated with each time series and using a simple criterion, a planet with one Earth mass and a period of one to two years probably cannot be detected with current analysis techniques, except for the lower mass stars in our sample, but very many observations would be required. The effect of inclination is critical.
Conclusions.
The results are very important in the context of future RV follow-ups of transit detections of such planets. We conclude that a significant improvement of analysis techniques and/or observing strategies must be made to reach such low detection limits.
We report the discovery of a low-mass companion to the nearby (d = 47 pc) F7V star HD 984. The companion is detected 0.19 arcsec away from its host star in the L′ band with the Apodized Phase Plate ...on NaCo/Very Large Telescope and was recovered by L′-band non-coronagraphic imaging data taken a few days later. We confirm the companion is comoving with the star with SINFONI integral field spectrograph H + K data. We present the first published data obtained with SINFONI in pupil-tracking mode. HD 984 has been argued to be a kinematic member of the 30 Myr-old Columba group, and its HR diagram position is not altogether inconsistent with being a zero-age main sequence star of this age. By consolidating different age indicators, including isochronal age, coronal X-ray emission, and stellar rotation, we independently estimate a main-sequence age of 115 ± 85 Myr (95 per cent CL) which does not rely on this kinematic association. The mass of directly imaged companions are usually inferred from theoretical evolutionary tracks, which are highly dependent on the age of the star. Based on the age extrema, we demonstrate that with our photometric data alone, the companion's mass is highly uncertain: between 33 and 96 M
Jup (0.03–0.09 M⊙) using the COND evolutionary models. We compare the companion's SINFONI spectrum with field dwarf spectra to break this degeneracy. Based on the slope and shape of the spectrum in the H band, we conclude that the companion is an M6.0 ± 0.5 dwarf. The age of the system is not further constrained by the companion, as M dwarfs are poorly fit on low-mass evolutionary tracks. This discovery emphasizes the importance of obtaining a spectrum to spectral type companions around F-stars.
Context. High-contrast instruments like SPHERE (Spectro- Polarimetric High-contrast Exoplanet REsearch) enable spatial resolution of young planetary systems and allow us to study the connection ...between planets and the dust contained in debris discs by the gravitational influence a planet can have on its environment. Aims. We present new observations of the edge-on debris disc around HD 15115 (F star at 48.2 pc) obtained in the near-IR. We search for observational evidence for a second inner planetesimal ring in the system. Methods. We obtained total intensity and polarimetric data in the broad bands J and H and processed the data with differential imaging techniques achieving an angular resolution of about 40 mas. A grid of models describing the spatial distribution of the grains in the disc is generated to constrain the geometric parameters of the disc and to explore the presence of a second belt. We perform a photometric analysis of the data and compare disc brightness in two bands in scattered and in polarized light. Results. We observe an axisymmetric planetesimal belt with a radius of ~2′′, an inclination of 85.8° ± 0.7° and position angle of 278.9° ± 0.1°. The photometric analysis shows that the west side is ~2.5 times brighter in total intensity than the east side in both bands, while for polarized light in the J band this ratio is only 1.25. We also find that the J–H colour of the disc appears to be red for the radial separations r ≲ 2′′ and is getting bluer for the larger separations. The maximum polarization fraction is 15–20% at r ~ 2.5′′. The polarized intensity image shows some structural features inside the belt which can be interpreted as an additional inner belt. Conclusions. The apparent change of disc colour from red to blue with an increasing radial separation from the star could be explained by the decreasing average grain size with distance. The presence of an inner belt slightly inclined with respect to the main planetesimal belt is suspected from the data but the analysis and modelling presented here cannot establish a firm conclusion due to the faintness of the disc and its high inclination.
First light of the VLT planet finder SPHERE Zurlo, A.; Vigan, A.; Galicher, R. ...
Astronomy and astrophysics (Berlin),
03/2016, Volume:
587, Issue:
A57
Journal Article, Web Resource
Peer reviewed
Open access
Context. The planetary system discovered around the young A-type HR 8799 provides a unique laboratory to: a) test planet formation theories; b) probe the diversity of system architectures at these ...separations, and c) perform comparative (exo)planetology.Aims. We present and exploit new near-infrared images and integral-field spectra of the four gas giants surrounding HR 8799 obtained with SPHERE, the new planet finder instrument at the Very Large Telescope, during the commissioning and science verification phase of the instrument (July–December 2014). With these new data, we contribute to completing the spectral energy distribution (SED) of these bodies in the 1.0–2.5 μm range. We also provide new astrometric data, in particular for planet e, to further constrain the orbits.Methods. We used the infrared dual-band imager and spectrograph (IRDIS) subsystem to obtain pupil-stabilized, dual-band H2H3 (1.593 μm, 1.667 μm), K1K2 (2.110 μm, 2.251 μm), and broadband J (1.245 μm) images of the four planets. IRDIS was operated in parallel with the integral field spectrograph (IFS) of SPHERE to collect low-resolution (R ~ 30), near-infrared (0.94–1.64 μm) spectra of the two innermost planets HR 8799 d and e. The data were reduced with dedicated algorithms, such as the Karhunen-Loève image projection (KLIP), to reveal the planets. We used the so-called negative planets injection technique to extract their photometry, spectra, and measure their positions. We illustrate the astrometric performance of SPHERE through sample orbital fits compatible with SPHERE and literature data.Results. We demonstrated the ability of SPHERE to detect and characterize planets in this kind of systems, providing spectra and photometry of its components. The spectra improve upon the signal-to-noise ratio of previously obtained data and increase the spectral coverage down to the Y band. In addition, we provide the first detection of planet e in the J band. Astrometric positions for planets HR 8799 bcde are reported for the epochs of July, August, and December 2014. We measured the photometric values in J, H2H3, K1K2 bands for the four planets with a mean accuracy of 0.13 mag. We found upper limit constraints on the mass of a possible planet f of 3–7 MJup . Our new measurements are more consistent with the two inner planets d and e being in a 2d:1e or 3d:2e resonance. The spectra of HR 8799 d and e are well matched by those of L6-8 field dwarfs. However, the SEDs of these objects are redder than field L dwarfs longward of 1.6 μm.
Context. Massive, main-sequence (MS) AF-type stars have so far remained unexplored in past radial velocities (RV) surveys due to their small number of spectral lines and high rotational velocities ...that prevent the classic RV computation method. Aims. Our aim is to search for giant planets (GPs) around AF MS stars, to get primary statistical information on their occurrence rate and to compare the results with evolved stars and lower-mass MS stars. Methods. We used the HARPS spectrograph located on the 3.6 m telescope at ESO La Silla Observatory to observe 108 AF MS stars with B-V in the range -0.04 to 0.58 and masses in the range 1.1 to 3.6 M sub(middot in circle). We used our SAFIR software developed to compute the RV and other spectroscopic observables of these early-type stars. We characterized the detected companions as well as the intrinsic stellar variability. We computed the detection limits and used them as well as the detected companions to derive the first estimates of the close-in brown dwarf (BD) and GP frequencies around AF stars. Results. We report the detection of a m sub(p) sini=4.51M sub(Jup) planetary companion with an ~826-day period to the F6V dwarf HD111998. We also present new data on the two-planet system around the F6IV-V dwarf HD60532. We also report the detections of 14 binaries with long-term RV trends and/or high-amplitude RV variations combined to a flat RV-bisector span diagram. We constrain the minimal masses and semi-major axes of these companions and check that these constraints are compatible with the stellar companions previously detected by direct imaging or astrometry for six of these targets. We get detection limits deep into the planetary domain with 70% of our targets showing detection limits between 0.1 and 10 M sub(Jup) at all orbital periods in the 1- to 10 super(3)-day range. We derive BD (13 < or =m sub(p) sini< or =80 M sub(Jup)) occurrence rates in the 1- to 10 super(3)-day period range of 2 sub(-2) super(+5)% and 2.6 sub(-2.6) super(+6.7)% for stars with Mlow * in the ranges 1.1 to 1.5 and 1.5 to 3 M sub(middot in circle), respectively. As for Jupiter-mass companions (1< or =m sub(p) sini< or = 13 M sub(Jup)), we get occurrence rates in the 1- to 10 super(3)-day period range of 4 sub(-0.9) super(+5.9)% and 6.3 sub(-6.3) super(+15.9)% respectively for the same Mlow * ranges. When considering the same Jupiter-mass companions but periods in the 1- to 100-day range only, we get occurrence rates of 2 sub(-2) super(+5.2)% and 3.9 sub(-3.9) super(+9.9)%. Given the present error bars, these results do not show a significant difference from companion frequencies derived in the same domains for solar-like MS stars.
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