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Register a new userHigh contrast imaging at 10 microns, a search for exoplanets around: Eps Indi A, Eps Eri, Tau Ceti, Sirius A and Sirius B
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The direct imaging of rocky exoplanets is one of the major science goals for upcoming large telescopes. The contrast requirement for imaging such planets is challenging. However, the mid-IR (InfraRed) regime provides the optimum contrast to directly detect the thermal signatures of exoplanets in our solar neighbourhood. We aim to exploit novel fast chopping techniques newly developed for astronomy with the aid of adaptive optics to look for thermal signatures of exoplanets around bright stars in the solar neighbourhood. We use the upgraded VISIR (Very Large Telescope Imager and Spectrometer for the mid-InfraRed) instrument with high contrast imaging (HCI) capability optimized for observations at 10~$mu$m to look for exoplanets around five nearby ($d$ < 4 pc) stars. The instrument provides an improved signal-to-noise (S/N) by a factor of $sim$4 in the N-band compared to standard VISIR for a given S/N and time. In this work we achieve a detection sensitivity of sub-mJy, which is sufficient to detect few Jupiter mass planets in nearby systems. Although no detections are made we achieve most sensitive limits within $<2$ for all the observed targets compared to previous campaigns. For $epsilon$ Indi A and $epsilon$ Eri we achieve detection limits very close to the giant planets discovered by RV, with the limits on $epsilon$ Indi A being the most sensitive to date. Our non-detection therefore supports an older age for $epsilon$ Indi A. The results presented here show the promise for high contrast imaging and exoplanet detections in the mid-IR regime.
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The active K2V star $epsilon$ Eri hosts the most nearby known extrasolar planet. With an angular separation of about 1 on average, and an age of a few to several hundred Myrs, $epsilon$ Eri b is one of the prime candidates for becoming the first definitive extrasolar planet imaged directly. We present a multi-epoch deep differential imaging survey performed with NACO-SDI at the VLT with the aim of finding the planet. The results are combined with recent astrometry in an attempt to further constrain the detection limits. No convincing candidate is found among the many coherent structures that constitute the residual speckle noise, which is the dominant noise at small angular scales. We present our detection limits, compare them with the estimated brightness of $epsilon$ Eri b, and analyze how the limits can be improved further. It is found that integration time remains a very important parameter for achieving good results, even in the speckle-dominated regimes. The results yield new, improved upper 3$sigma$ limits on the absolute H-band (1.6 $mu$m) brightness of the 1.55 $M_{rm jup}$ companion of 19.1 to 19.5 mag, depending on the specific age of the system.
Sirius is the brightest star in the sky and a strong source of diffuse light for modern telescopes so that the immediate surroundings of the star are still poorly known. We study the close surroundings of the star (2 to 25 arcsec) by means of adaptive optics and coronographic device in the near-infrared, using the ESO/ADONIS system. The resulting high contrast images in the JHKs bands have a resolution of ~ 0.2 arcsec and limiting apparent magnitude ranging from mK = 9.5 at 3 arcsec, from Sirius-A to mK = 13.1 at 10 arcsec. These are the first and deepest images of the Sirius system in this infrared range. From these observations, accurate infrared photometry of the Sirius-B white dwarf companion is obtained. The JH magnitudes of Sirius-B are found to agree with expectations for a DA white dwarf of temperature (T=25000K) and gravity (log(g) = 8.5), consistent with the characteristics determined from optical observations. However, a small, significant excess is measurable for the K band, similar to that detected for dusty isolated white dwarfs harbouring suspected planetary debris. The possible existence of such circumstellar material around Sirius-B has still to be confirmed by further observations. These deep images allow us to search for small but yet undetected companions to Sirius. Apart from Sirius-B, no other source is detected within the total 25 arcsec field. The minimum detectable mass is around 10 MJup inside the planetary limit, indicating that an extrasolar planet at a projected distance of ~ 25 AU from Sirius would have been detected (abridged abstract).
Due to its proximity, youth, and solar-like characteristics with a spectral type of K2V, Eps Eri is one of the most extensively studied systems in an extrasolar planet context. Based on radial velocity, astrometry, and studies of the structure of its circumstellar debris disk, at least two planetary companion candidates to Eps Eri have been inferred in the literature (Eps Eri b, Eps Eri c). Some of these methods also hint at additional companions residing in the system. Here we present a new adaptive optics assisted high-contrast imaging approach that takes advantage of the favourable planet spectral energy distribution at 4 microns, using narrow-band angular differential imaging to provide an improved contrast at small and intermediate separations from the star. We use this method to search for planets at orbits intermediate between Eps Eri b (3.4 AU) and Eps Eri c (40 AU). The method is described in detail, and important issues related to the detectability of planets such as the age of Eps Eri and constraints from indirect measurements are discussed. The non-detection of companion candidates provides stringent upper limits for the masses of additional planets. Using a combination of the existing dynamic and imaging data, we exclude the presence of any planetary companion more massive than 3 Mjup anywhere in the Eps Eri system. Specifically, with regards to the possible residual linear radial velocity trend, we find that it is unlikely to correspond to a real physical companion if the system is as young as 200 Myr, whereas if it is as old as 800 Myr, there is an allowed semi-major axis range between about 8.5 and 25 AU.
The relative roles of metallicity and surface gravity on the near-infrared spectra of late-T brown dwarfs are not yet fully understood, and evolutionary models still need to be calibrated in order to provide accurate estimates of brown dwarf physical parameters from measured spectra. The T-type brown dwarfs Eps Indi Ba and Bb forming the tightly bound binary Eps Indi B, which orbits the K4V star Eps Indi A, are nowadays the only such benchmark T dwarfs for which all important physical parameters such as metallicity, age and mass are (or soon will be) known. We present spatially resolved VLT/NACO images and low resolution spectra of Eps Indi B in the J, H and K near-infrared bands. The spectral types of Eps Indi Ba and Bb are determined by direct comparison of the flux-calibrated JHK spectra with T dwarf standard template spectra and also by NIR spectral indices. Eps Indi Bb is confirmed as a T6 while the spectral type of Eps Indi Ba is T1.5 so somewhat later than the previously reported T1. Constrained values for surface gravity and effective temperature are derived by comparison with model spectra. The evolutionary models predict masses around about 53 M_J for Eps Indi Ba and about 34 M_J for Eps Indi Bb, slightly higher than previously reported values. The suppressed J-band and enhanced K-band flux of Eps Indi Ba indicates that a noticeable cloud layer is still present in a T1.5 dwarf while no clouds are needed to model the spectrum of Eps Indi Bb.
Sirius has always attracted a lot of scientific interest, especially after the discovery of a companion white dwarf at the end of the 19th century. Very early on, the existence of a potential third body was put forward to explain some of the observed properties of the system. We present new coronagraphic observations obtained with VLT/SPHERE that explore, for the very first time, the innermost regions of the system down to 0.2 (0.5 AU) from Sirius A. Our observations cover the near-infrared from 0.95 to 2.3 $mu$m and they offer the best on-sky contrast ever reached at these angular separations. After detailing the steps of our SPHERE/IRDIFS data analysis, we present a robust method to derive detection limits for multi-spectral data from high-contrast imagers and spectrographs. In terms of raw performance, we report contrasts of 14.3 mag at 0.2, ~16.3 mag in the 0.4-1.0 range and down to 19 mag at 3.7. In physical units, our observations are sensitive to giant planets down to 11 $M_{Jup}$ at 0.5 AU, 6-7 $M_{Jup}$ in the 1-2 AU range and ~4 $M_{Jup}$ at 10 AU. Despite the exceptional sensitivity of our observations, we do not report the detection of additional companions around Sirius A. Using a Monte Carlo orbital analysis, we show that we can reject, with about 50% probability, the existence of an 8 $M_{Jup}$ planet orbiting at 1 AU. In addition to the results presented in the paper, we provide our SPHERE/IFS data reduction pipeline at http://people.lam.fr/vigan.arthur/ under the MIT license.
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