Prospects for the characterisation of exo-zodiacal dust with the VLTI
Exo-zodiacal dust, exozodi for short, is warm (300K) or hot (up to 2000K) dust found in the inner regions of planetary systems around main sequence stars. In analogy to our own zodiacal dust, it may be located in or near the habitable zone or closer in, down to the dust sublimation distance. The study of the properties, distribution, and evolution of exozodis can inform about the architecture and dynamics of the innermost regions of planetary systems, close to their habitable zones. On the other hand, the presence of large amounts of exo-zodiacal dust may be an obstacle for future space missions aiming to image Earth-like exoplanets. The dust can be the most luminous component of extrasolar planetary systems, but predominantly emits in the near- to mid-infrared where it is outshone by the host star. Interferometry provides a unique method of separating the dusty from the stellar emission. We discuss the prospects of exozodi observations with the next generation VLTI instruments and summarize critical instrument specifications.
Keywords:Interferometry, Circumstellar matter, Exo-zodiacal dust, Planetary systems
The zodiacal dust in our Solar system is distributed between a few AU from the Sun down to 4 Solar radii (where it forms the F-corona) with a shallow surface density profile (Kimura and Mann, 1998; Hahn et al, 2002). In analogy to this dust, we call any dust near the habitable zone of other stars, or closer in, exo-zodiacal dust (or exozodi for short). In other systems we usually observe two distinct types of dust. Warm dust near the habitable zone is detected in the mid-infrared (Millan-Gabet et al, 2011; Mennesson et al, 2014; Ertel et al, 2018a), while hot dust very close to the star is detected in the near-infrared (Absil et al, 2006, 2013; Ertel et al, 2014). The connection between the two is not clear and the distinction might be largely observational. However, no correlation between the detections at the two wavelengths has been found so far, which would be expected from simply scaling the Solar system’s dust distribution to the higher dust levels detectable in other systems. This suggests that other systems have diverse architectures and that the two species might or might not be directly connected.
The spatial dust distribution relates to its location of origin, production mechanism (asteroid collision, comet evaporation), and the dynamical interaction with the stellar radiation, wind, and magnetic field, as well as with potential planets. It thus traces the architecture and dynamics of the inner regions of planetary systems, in particular that of asteroids and comets (Bonsor et al, 2012, 2013, 2014; Lebreton et al, 2013; Faramaz et al, 2017) that are otherwise hard or impossible to study. On the other hand, the presence of large amounts of dust in or near the habitable zone of planetary systems may be an obstacle for the imaging detection and characterization of Earth-like, potentially habitable exoplanets (exo-Earths) by future space missions. The dust emission causes additional photon noise in the observations and clumps in the dust distribution, albeit potentially caused by interaction with planets, can be misinterpreted as actual planets given the limited angular resolution and signal-to-noise ratio expected from such observations (Defrère et al, 2010; Roberge et al, 2012; Stark et al, 2015). Even hot dust close to the star may significantly degrade coronagraphic performance at the level needed for exo-Earth imaging, as it produces emission that is more extended than the star and consequently cannot be perfectly suppressed. The study of the incidence rate of exo-zodiacal dust, its correlation with other, more accessible properties of the systems such as stellar luminosity, age, and the presence of an outer, cold reservoir of dust (debris disk, e.g. Eiroa et al 2013), and of the dust properties is critical for the success of such missions.
While rather small amounts of this dust can be relatively luminous due to its large surface and high temperature of few 100 to 2000 K, the emission of such warm or hot dust peaks in the near- to mid-infrared (nIR to mIR), where the emission of the host star dominates. The emission from the exozodis found so far produces an excess at the 1% level and below. Thus, detecting the dust requires spatially resolving the dust in order to disentangle its emission from that of the star. Because of the small angular scales (1 AU at a distance of 10 pc corresponds to an angular separation from the host star of 0.1 arcsec), only interferometry is currently able to reach the required angular resolution. Near-IR interferometry has produced the largest number of detected exozodis to date. The VLTI instrument PIONIER has been particularly efficient in surveying a large number of stars. The broad wavelength coverage of the second-generation suite of VLTI instruments (and PIONIER) is particularly well suited for the characterization of exozodis. In this article, we discuss the prospects of observing exo-zodiacal dust with these instruments. We briefly summarize the past and current observational efforts to study exozodis with focus on interferometric observations (Sect. 2). For a detailed review of the field of exo-zodiacal dust, we refer to Kral et al (2017). We explain the method used to detect exozodis with optical long baseline interferometry (Sect. 3) which is the relevant method for all current and near future VLTI instruments. In Sect. 4, we discuss the prospects of using the second generation VLTI instruments to further study exo-zodiacal dust. The critical technical requirements of instruments for exozodi observations are reviewed in Sect. 5. We present our conclusions in Sect. 6.
2 Recent observational success
The first detection of faint nIR excess at the 1% level around a main sequence star was reported for Vega by Absil et al (2006). It was achieved using the FLUOR beam combiner on the CHARA array. Motivated by this detection, survey observations were carried out using CHARA/FLUOR and the PIONIER beam combiner on the VLTI that produced the first statistical samples of detected exozodis (Absil et al 2013; Ertel et al 2014; Fig. 1). As of today, more than 200 stars have systematically been surveyed in the nIR with a detection rate of 20% (Nuñez et al 2017; Marion et al, subm.). Follow-up observations of a sample of detections have demonstrated the repeatability of the observations and revealed first signs of significant variability of the excesses (Nuñez et al 2017; Ertel et al 2016). These observations provide detailed information about the presence of exozodis and their correlation with various properties of the host systems, and motivated and informed several theoretical studies (Bonsor et al 2012, 2013, 2014; Lebreton et al 2013; Rieke et al 2016; Faramaz et al 2017; McKinley et al, in prep.). In parallel, first excesses from warm dust near the habitable zone of other stars were detected using mIR nulling interferometry on the MMT (Stock et al, 2010) and the Keck Interferometric Nuller (Millan-Gabet et al, 2011; Mennesson et al, 2014). Combining data from both nIR and mIR interferometric observations provide particularly strong constraints of the dust properties and location (Defrère et al, 2011; Lebreton et al, 2013; Kirchschlager et al, 2017). Recently, the Large Binocular Telescope Interferometer (LBTI) has surveyed a sample of 40 nearby main sequence stars for habitable zone dust at unprecedented sensitivity (Ertel et al 2018a, b; Ertel et al, in prep.) and achieved a detection rate similar to those in the nIR. Strong constraints on the scattered light content of hot exozodi systems have been provided by parts per million accuracy polarimetric observations (Marshall et al, 2016). Additional constraints have been provided by ground and space based spectroscopic observations (e.g., Chen et al 2006; Lisse et al 2007; Ballering et al 2014; Lisse et al, in prep.) and by studying WISE data of a large sample of stars to constrain the incidence rate of exozodis at the bright end of their luminosity function (Kennedy and Wyatt, 2013).
3 Detection using optical long baseline interferometry
By far the largest number of exozodis has been detected using optical long baseline interferometry in the near-infrared as it is used on the VLTI. Using this technique at short baselines (few 10m), the star remains mostly unresolved, resulting in fully coherent emission. In contrast, the extended emission from a dust disk is ideally fully resolved, resulting in incoherent emission and thus a visibility deficit compared to the values expected from the star alone. This visibility deficit allows for detecting the dust and the disk-to-star flux ratio can be measured as half the visibility deficit (Fig. 2). The fact that the star is almost unresolved is important, because in this case the uncertain stellar diameter has very little impact on the prediction of the visibilities from the star alone and thus the visibility deficit can be measured at high accuracy.
Due to the small disk-to-star flux ratio of typically 1%, only few instruments reach the accuracy on the visibility measurements necessary to detect the dust. CHARA/FLUOR and VLTI/PIONIER have been used to survey a large sample of stars for hot exozodis. PIONIER in combination with the VLTI architecture was proven to be particularly well suited due to the high efficiency (simultaneous use of four telescopes) and the availability of closure phase measurements which allow one to directly distinguish between a faint companion and an extended dust disk as the cause for the detected excess (Marion et al, 2014).
4 New opportunities with VLT and LBT interferometry
So far, most detections were achieved only in one band. The large spectral separation of the / and the bands and different inner working angles and sensitivities of the instruments used make it difficult to connect detections in the two bands or trace the excess from one band to the other. PIONIER observations have provided a small spectral dispersion across the band, but the spectral separation of the channels is too small to put strong constraints on the spectral shape of the excesses. Only the characterization of exozodis can answer the three most urgent key questions beyond the frequency and abundance of massive dust systems:
What is the connection between the warm and hot dust? This is important because most systems so far have been detected in the nIR, but the implications of this for the presence of habitable zone dust are unclear. A tentative anti-correlation between the presence of hot and warm dust has been suggested (Mennesson et al, 2014), but contradictory results were found by Marion et al (subm.) based on a different strategy and a larger sample.
Where is the dust located? Detailed temperature information can only be derived from multi-wavelength observations that sample well the relevant parts of the dust’s spectral energy distribution (nIR to mIR). In addition, spatial information may be derived from observations at suitable wavelengths and baselines (Kirchschlager et al, 2018). The dust location gives critical insight into the architecture and dynamics of the underlying planetary system. Furthermore, the dust location relative to the habitable zone is critical information for future exo-Earth imaging missions.
What are the dust properties? The dust is detected as thermal emission in the mIR and as a potential combination of thermal emission and scattered light measured in the nIR. Only with a detailed knowledge of the dust properties is it possible to estimate from these observations the amount of scattered light expected in the visible where future exo-Earth imaging missions will operate.
The second-generation VLTI instruments GRAVITY and MATISSE, together with PIONIER, can provide the ideal tools to address these questions through multi-wavelength measurements of the spectral energy distribution (SED) of the excess emission in the nIR to mIR wavelength range (Fig. 3). In this range the emission of warm and hot dust peaks and carries most information about the dust temperature and properties. The well-established detection strategy used for FLUOR and PIONIER can be employed with all instruments. The broad spectral capabilities of GRAVITY and MATISSE will allow for strong constraints on the dust properties through a better characterization of the SED shape and the potential detection of dust emission features (e.g., 3 m and 10 m silicate features). First steps toward a spectral characterization of the excesses have been taken with PIONIER (Defrère et al, 2012; Ertel et al, 2014). Furthermore, a new survey in a wavelength range where the dust emits more strongly than in the band reached with PIONIER will result in a larger sample to characterize and stronger statistical constraints on the incidence, properties, and evolution of the dust. It has been shown already, that the detection rate of hot exozodis is about twice as high in band compared to band at similar accuracy of the visibility measurements (Ertel et al, 2014). The band beam combiner GRAVITY and the short wavelength channels of MATISSE (, bands) will be ideally suited for such surveys assuming a similar accuracy on the measurements as with PIONIER can be reached on a statistically significant number of stars.
First observations with GRAVITY during science verification and open time have demonstrated the required accuracy to detect exozodis and demonstrated the wealth of information contained in the low resolution spectra produced in the K band (Defrère et al, in prep.).
5 Critical technical requirements
There are three main challenges to be overcome for being able to fully characterize hot and warm exozodis with the VLTI:
It is critical to reach an accuracy of at least 1% on the individual measurements of the calibrated squared visibility. This is necessary to reach a sufficient cumulative accuracy on the source visibilities to detect the excess detected in available observations and accurately measure the disk-to-star flux ratio. Previous detections at the 1% level have been achieved in both the near-infrared and the mid-infrared (see Sect. 2). While the emission from blackbody grains emitting in the near-infrared at temperatures 2000 K is expected to increase toward longer wavelengths, this is not seen in spectrally dispersed PIONIER observations (although the constraints on the slope are weak due to the small wavelength coverage). The presence of very small grains would explain this (Fig. 3). As a consequence, the emission is not expected to be above a few percent at any wavelength. While this sensitivity is readily reached with PIONIER and has been demonstrated for GRAVITY, it is not expected to be reached with MATISSE without the use of a fringe tracker. MATISSE is thus currently limited to observing very extreme systems with massive warm dust detections (e.g., Lisse et al 2007, 2008; Olofsson et al 2012).
Pointing (field rotation) dependent polarization effects in the VLTI optical path limit the absolute calibration of single PIONIER observations to 3%. A specific observing strategy and additional calibration of this effect has to be employed to circumvent this limit (Ertel et al, 2014). This, however, requires observations of a large number of targets in a consistent way throughout a whole night which can only be carried out in visitor mode and puts significant limits on the flexibility of such observations. Correcting this effect on the instrument side (as done with GRAVITY) or solving it on the VLTI side is critical for efficient and flexible high accuracy observations.
To reach a sufficient cumulative accuracy on the measured source visibilities requires several measurements of one target. PIONIER has proven to be very efficient due to the simultaneous use of 4 telescopes (6 baselines). Still, several consecutive, calibrated measurements per target are necessary. Moreover, a potential variability of the hot emission has been detected on a timescale at least as short as one year (Ertel et al, 2016). The shortest timescale of these variations is not known. This variability calls at least for quasi-simultaneous observations of a target with all three instruments in order to avoid corruption of the obtained SED data by excess variability between the different observations. To significantly increase the efficiency of the observations for both a survey for new systems and the characterization of known systems, a fully simultaneous use of all three instruments (such as the “i-SHOOTER” concept, to use GRAVITY, MATISSE, and PIONIER simultaneously to obtain to band data of a target at the same time) will be highly beneficial.
In addition to these critical requirements, further increasing the sensitivity to circumstellar excess would be highly beneficial for exozodi science with the VLTI. In particular, it would allow for surveys for exo-zodiacal dust in the Southern hemisphere with a sensitivity similar to that reached in the North with the LBTI and to detect systems only a few times brighter than our own zodiacal dust. In the near-infrared, the sensitivity of VLTI observations to faint circumstellar emission is currently limited by the ability to accurately measure and calibrate visibilities and to predict the stellar visibilities. This is less critical when aiming to detect dust closer to the habitable zone of a system in the mid-infrared, because lower angular resolution is sufficient. The star is then even less resolved and the impact of its uncertain diameter thus further reduced. In addition, nulling interferometry can be used to remove the stellar light in addition to spatially separating it from the disk emission. An instrument employing this technique has been introduced as the hi-5 concept by Defrère et al (2018) and can improve the high contrast capabilities of the VLTI by one order of magnitude. Moreover, such an instrument would be the first to search for faint habitable zone dust in the Southern hemisphere, critically extending the sample of stars that can be searched.
The multi-wavelength capabilities of the second generation VLTI instruments will be vital to further study exo-zodiacal dust. However, in particular for MATISSE the visibility accuracy needed to detect exozodis is not expected to be readily met at the time of commissioning. Additional effort will be needed to enable the exploitation of the VLTI capabilities and to retain its leading role in the characterization of exozodis in the near future. The gain of one order of magnitude in high-contrast capabilities, as it could be achieved with a nulling interferometer (hi-5 concept), would allow for a survey for habitable zone dust at the extreme efficiency demonstrated by PIONIER and at the accuracy currently only reached with lower efficiency and on a limited sample of stars in the Northern hemisphere.
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