On the origin of exozodiacal dust – EXOZODI

The emission from exozodiacal dust is tenuous, peaks in the near- and mid-infrared, and originates from a region very close to the star. The only technique that can be used to detect such a faint emission close to a bright star is near/mid-infrared interferometry. We conducted observational surveys with two interferometric instruments, one in California and one in Chile. We have identified previously unknown instrumental limitations, and we have improved the data reduction softwares in order to achieve the accuracy needed for the project. We have fitted the measurements with our radiative transfer model that we upgraded to better describe the sublimation of the dust close to the star. We made the assumption that the observed dust is produced by a steady cometary activity. Our N-body simulations to produce this cometary activity rely on the presence of exoplanets, and take into account the importance of the migration of these exoplanets to extend the cometary shower. Finally, we have overcome the biggest challenge of the project, developing in two years a unique code that can simultaneously handle the collisional and dynamical evolution of comets and dust disks.

We have realised the first two observational surveys of hot, exozodiacal dust disks around a significant number of nearby stars (130). These observations demonstrate the universality of the phenomenon, while previous studies were limited to the solar system and a few stars. Our models show that the grains responsible for these emissions are carbon-rich and tend to accumulate next to the sublimation limit. Our analysis of the data suggests that exo-comets may be a major source of hot dust and that exoplanets could be responsable for this activity. On the modelling side, we have been able to develop two next-generation codes for the study of comets and dust disks around stars. The most sophisticated code, LIDT-DD, is unique, in that it can be used to self-consistently study the collisional and dynamical evolution of these disks.

Thanks to the ANR EXOZODI project, our position in the emerging field of exozodiacal dust is excellent and provides to our team a high visibility on this subject. Our future studies, should they be observational or theoretical, will be based on our experience and on the numerical codes developed in the course of the project. It will also rely on a consolidated and enriched network of collaborators. We have shown that exozodiacal dust disks are common, but we also show that the theoretical framework for understanding their origin remains to be consolidated, which is an essential source of inspiration for further research.

This ANR project led to the publication of 48 papers in refereed journals (more than 10 articles per year). We note that a majority of these articles have a member of the ANR project as first author. We also note that these publications are distributed roughly equally between the 5 different tasks of the project, which highlights that fact that all of them were successful.

This ANR EXOZODI proposal aims to achieve a qualitative step forward
in the studies of warm circumstellar dust disks (exozodis), both from
an observational and a theoretical perspective. This field of research
is relatively new and uncharted: the first interferometric detection
of an annuli of exozodiacal dust has been obtained in 2006 by our team
(Absil et al. 2006) around the star Vega, and the first very bright
case has been detected with Spitzer by Beichman et
al.(2005). Numerical models aimed at understanding these observations
are still in their infancy, and the origin of exozodiacal dust is
still an unresolved issue. However, the richness of this subject, its
connection to the general dynamical evolution of planetary systems, in
particular in the innermost potentially habitable regions, makes it a
fast emerging research field in the community.

The detection of exozodis requires a specific instrumentation,
combining very high angular resolution and high
contrast. Near-Infrared interferometry fulfils these two criteria. For
this project, we shall carry out the first interferometric survey of a
large stellar sample using the only two instruments available during
the course of this project: FLUOR (CHARA) and PIONIER (VLTI). Both
have been built by partners of this project and together cover both
hemispheres. These are the only two instruments able to provide a
sufficient precision on visibility measurements to achieve the
proposed observing program.

Our project has also ambitious objectives in terms of numerical
simulations. Our goal is to understand the mechanisms producing the
high level of dust observed in exozodis. The first step of our study
will be to use collisional particle-in-a-box codes developed within
our team to estimate if such high levels of dustiness could be due to
the “natural” collisional evolution of asteroid-like belts. We shall
in parallel explore alternative scenarios for exozodi dust production,
in which dynamical perturbations or other transitory and/or violent
events are the source for intense collisional activity. For this
second leg, we shall use upgraded versions of N-body codes mostly
developed by our team at the LAOG.

Last but not least, we wish to develop the next generation of
numerical tools, not only for the studies of exozodis but more
generally for those of any collisionaly evolving debris disk. We aim
to create a new model, able to study both the dynamical evolution of
complex gravitational systems and the collisional evolution of their
small body population. This objective is both the most innovative and
the most challenging aspect of this project. However, we are confident
that this decisive step forward can be made during the 4 year
timeframe of our project.

Our team brings together two partner institutions (LAOG and LESIA). It
possesses all the necessary experience and expertise for carrying out
this project, spanning the whole range from the conception of
interferometric instruments to the development of cutting edge
numerical codes. No other team in the world brings together such a
wide spectra of competences, all essential to the study of
exozodis. We shall in addition benefit from active interactions with
external collaborations in Liege, at the ESO (Chile), in Stockholm and
Amsterdam.

We are thus convinced that, by the time this project is completed, we
will have made a significant, quantitative and qualitative leap
forward in the understanding of exozodis. This should put our team in
a leading position in this fast emerging and essential new research
field.