Astronomical Science An Unbiased Near-infrared Interferometric Survey for Hot Exozodiacal Dust Steve Ertel1,2,3 composition and distribution of the dust vide clues pointing towards the presence Jean-Charles Augereau2,3 produced in debris discs through colli- of a planet, but clumps in the dust distri- Olivier Absil4 sions and outgassing of these larger bod- bution may also be misinterpreted as Denis Defrère5 ies is a powerful tool that can help to actual planets due to the limited resolu- Jean-Baptiste Le Bouquin2,3 constrain the architecture, dynamics and tion and sensitivity of current instruments. Lindsay Marion4 evolution of extrasolar planetary systems. Thus, detecting and characterising exo- Amy Bonsor6 However, debris discs that are relatively zodiacal dust systems provides critical Jérémy Lebreton7,8 easy to detect are located several astro- input for the design of such space mis- nomical units (au) to a few hundreds of au sions. However, detecting the dust itself from their host stars, and thus only trace with present instruments is complicated 1 ESO the outer regions of planetary systems. by the fact that the warm and hot emis- 2 Université Grenoble Alpes, France In order to study the inner regions close sion peaks at near- to mid-infrared wave- 3 CNRS, Institut de Planétologie et to the habitable zones, one has to study lengths where the dust emission is out- d’Astrophysique de Grenoble, France warm and hot dust closer to the star. This shone by the host star. Thus, only a few 4 Départment d’Astrophysique, Géo- dust is called exozodiacal dust, or exo- very bright — and perhaps unrepresenta- physique et Océanographie, Université zodi for short, by analogue with the Zodi- tive — systems can be detected photo- de Liège, Belgium acal dust in the Solar System. metrically. 5 Department of Astronomy, Steward Observatory, University of Arizona, USA The Zodiacal light can be observed on 6 School of Physics, H. H. Wills Physics dark nights directly after dusk and before Infrared interferometric detection of Laboratory, University of Bristol, United dawn as a cone of faint light stretching e x o z o d i s Kingdom from the horizon in the west (after dusk) 7 NASA Exoplanet Science Institute, or in the east (before dawn). It is caused Due to the small extent of exozodiacal California Institute of Technology, by sunlight scattered off small dust parti- systems — one au at a typical distance Pasadena, USA cles close to the orbit of the Earth. More of 10 pc for nearby stars corresponds to 8 Infrared Processing and Analysis Center, generally, Zodiacal dust is distributed in an angular size of 100 milliarcseconds — California Institute of Technology, a disc inside the asteroid belt, extending only interferometry is currently able to Pasadena, USA all the way down to the sublimation dis- spatially resolve them. When used at tance of the dust from the Sun, which baselines of a few tens of metres, near- corresponds to a few Solar radii. The infrared interferometry is able to fully Exozodiacal dust is warm or hot dust dust temperatures range from about resolve the extended emission of the dust found in the inner regions of planetary 100 K to about 2000 K, depending on disc while the star still remains largely systems orbiting main sequence stars, the distance from the Sun. In the inner- unresolved. The result is a small deficit in in or around their habitable zones. The most regions it forms the Fraunhofer the measured squared visibilities (the main dust can be the most luminous compo- corona (F-corona) of the Sun, a region of observable of infrared interferometry) nent of extrasolar planetary systems, the corona where the prominent absorp- compared to the values expected from but predominantly emits in the near- to tion lines in the Solar spectrum are visible the star alone (see Figure 1 for an expla- mid-infrared where it is outshone by because the light seen there is nearly nation). Using this technique the disc can the host star. Interferometry provides a unaltered sunlight scattered by the dust be spatially disentangled from the star, unique method of separating this dusty particles. It is noteworthy that the Zodia- allowing the disc to be detected and the emission from the stellar emission. cal light is the most luminous component ratio between the disc and stellar emis- The visitor instrument PIONIER at the of the Solar System after the Sun itself. sion measured. This method has so far Very Large Telescope Interferometer been the most powerful and efficient tool (VLTI) has been used to search for hot Not unlike the Zodiacal dust, exozodiacal in the search for faint exozodiacal dust. exozodiacal dust around a large sample dust is located in the inner regions of of nearby main sequence stars. The extrasolar planetary systems, within a few However, the dust detected by this results of this survey are summarised: au of main sequence stars. This region method in the near-infrared (NIR) is ex­­ 9 out of 85 stars show excess exo- often encompasses their habitable zone. pected to be very hot, close to its subli- zo diacal emission over the stellar Historically, this circumstance has brought mation temperature, and its relation to photo spheric emission. it a lot of attention, because the presence slightly cooler dust in the habitable zone of exozodis is expected to complicate is unclear. This habitable zone dust is the direct-imaging detection and charac- brighter in the mid-infrared (MIR) where it Planetesimals and comets are a major terisation of Earth-like planets in the hab- can be detected by the VLTI MID-infrared component of the Solar System (in the itable zones around other stars by future interferometric instrument (MIDI) for a Kuiper Belt and the asteroid belt), as well space missions. The faint light of these few bright systems (e.g., Smith et al., as of extrasolar planetary systems, where potential planets can be hidden in the 2009) or more efficiently by nulling inter- they occur in debris discs. Besides the extended emission of the dust disc. The ferometry. With this latter method the planets, they are the main outcome of the structures created in the dust distribution stellar light from two telescopes is brought planet formation process. Studying the due to planet–disc interaction may pro- to destructive interference, while light 24 The Messenger 159 – March 2015 Figure 1. (Left) Illustration of the detection strategy for exozodis. The dashed curve shows a realistic SX 9NNL case assuming a uniform disc for both the star and the flux distribution from the exozodiacal dust and HRHAHKH a disc-to-star flux ratio of f = 1%. For the simplified CU case, the solid curve shows the same assumptions, but with the approximation following the equation. Diameters of the star and (face-on) disc have been SX 2PT@QD chosen to be 2.5 milliarcseconds (about an A-type star at 10 pc) and 500 milliarcseconds (5 au at HRHAHKH 10 pc), but exact numbers are not relevant for this CU #HRBNMKX !@RDKHMDL illustration. 2S@Q CHRBQD@K 2S@Q CHRBRHLOKHjDC 2PT@QD 2S@QNMCH@LDSDQ Figure 2. (Below) A diagram (not to scale) to illustrate #HRBRS@QkTWQ@SHN the scattering of planetesimals by an outer planet, that leads to an exchange of angular momentum and ) /Aeh £ the outward migration of that planet (Bonsor et al., 5£¶lE /Aeh 2014). Some of the scattered particles are ejected, whilst some are scattered into the inner planetary system, where they interact with the inner planets. This scattering leads to a flux of material into the !@RDKHMDL exozodi region. 2S@Q $WNYNCH(MMDQOK@MDSR .TSDQOK@MDSR .TSDQOK@MDSDRHL@KADKS off-centre from the star is transmitted, required to prepare future exoEarth imag- inwards to regions where it is detected. which improves the dynamic range of ing missions. The Large Binocular Tele- Furthermore, a higher dust mass would the observations. Both methods have scope Interferometer (LBTI) is designed require a larger number of planetesimals been used in parallel in the past to search to achieve the required sensitivity and will which are colliding more often to pro- for exozodiacal dust, mostly with the soon start a MIR survey of 50 to 60 care- duce the dust present; this process Fiber Linked Unit for Optical Recombi- fully chosen nearby main sequence stars would in turn destroy the planetesimals nation (FLUOR) instrument at the Center (Weinberger et al., 2015). faster, so that at the ages of the systems for High Angular Resolution Astronomy observed, few planetesimals would be (CHARA) array (in the NIR; e.g., Absil et left (Wyatt et al., 2007). The production of al., 2006) on Mt. Wilson, California and Potential origins of exozodiacal dust the observed amounts of dust through the Keck Interferometer Nuller (in the comet evaporation would require a large MIR; Mennesson et al., 2014) on Mauna The fact that many exozodiacal systems number of comets, approximately one Kea, Hawaii. have already been found, given the lim- thousand events per year similar to Hale– ited sensitivity of present instruments, is Bopp reaching its perihelion. Due to the recent development of the vis- surprising (Absil et al., 2013). Such high itor instrument PIONIER (Precision Inte- levels of dust are difficult to sustain. A potential scenario explaining such a grated Optics Near Infrared ExpeRiment; The Zodiacal dust has two main origins: large number of comets would be a con- LeBouquin et al., 2011) at the VLTI, which collisions of asteroids in the asteroid nection to an outer debris disc where operates in the H-band, the search for belt result in dust that is dragged inwards planetesimals are scattered inwards exozodis in the NIR has become more by the interaction with stellar radiation through gravitational interaction with an efficient and of similar accuracy to the (Poynting–Robertson drag) and the evap- existing planetary system (see Figure 2).