Late and debris disks

A Deep Polarimetric Analysis of the Asymmetrical Debris Disk HD 106906

Katie Crotts (University of Victoria)

HD 106906 is a young, binary stellar system, located at ~103.3 parsecs away in the Lower Centaurus Crux (LCC) group. This system is completely unique among discovered systems in that it contains an asymmetrical debris disk, as well as an ejected 11 M(Jup) planet companion, at a separation of ~735 AU. Only 4 other systems are known to contain both a disk and detected planet, where HD 106906 is the only one in which the planet has apparently been ejected. Furthermore, the debris disk is nearly edge on, and extends roughly from 70 AU to >500 AU, where previous polarimetric studies with HST have shown the outer regions to have high asymmetry. The presence of an ejected planet sparks questions about the origin of this asymmetry. To better understand the structure and composition of the disk, deeper data have been taken with the Gemini Planet Imager (GPI), which we have used to perform a deep polarimetric study of HD 106906’s asymmetrical debris disk. The data were taken in the H-band, and supplemented with both J and K1-band polarimetric data. An empirical analysis of our data, supports a disk that is asymmetrical in surface brightness and structure, where fitting an inclined ring model to the disk spine shows the disk to be highly eccentric. A comparison of the magnitude of the disk with the stellar magnitude in each band, suggests a blue color, where we also find no difference in color between either side. These results will be discussed in terms of possible sources of asymmetry, such as dynamical interaction with the planet companion HD 106906b.

The effect of late gas disks on the late stages of planet formation

Quentin Kral (Paris Observatory)

The external supply of gas to planetary atmospheres may be important to set their final compositions. In this talk, I will summarize recent works that quantified in an exoplanetary context, how much gas can be delivered to planets from late gas disks, which appear to be rather ubiquitous around main-sequence stars with bright planetesimal belts. This new gas component is indeed found to be present for tens and sometimes hundreds of millions of years around main-sequence stars. The gas is thought to be released from planetesimals when they collide together in their parent belt, which creates a gas disk (made of volatiles) that can viscously spread further in the system and encounter the already formed planets that can capture this gas, which will affect the primordial atmospheres of these planets. Kral et al. (2020,NatAst) show that this very late accretion onto planets is very efficient and may allow capturing large quantities of carbon and oxygen (and potentially some nitrogen and hydrogen) leading to new atmospheric masses onto capturing terrestrial planets between that of the Earth's atmosphere to planets with massive atmospheres having sub-Neptune-like pressures. New secondary atmospheres with high metallicities will be created on terrestrial planets bathing in these late gas disks, resetting their primordial compositions inherited from the protoplanetary disk phase, and providing a new birth to planets that lost their atmospheres to photoevaporation, core cooling or giant impacts. This volatile delivery for tens of Myr may also be favourable to the development of the first bricks of life. It will also affect the metallicity and C/O ratio of giant planets accreting late gas, which is an effect that may be observable in the close future with the JWST. This very efficient accretion opens the way to a new planet detection method (for planets down to Earth masses at a few au from their stars) that I will present in this talk.

ALMA Survey of Lupus Class III Stars: Early Planetesimal Belt Formation and Rapid Disk Dispersa

Joshua Lovell (Institute of Astronomy, University of Cambridge) 

Class III stars are those in star forming regions without large non-photospheric infrared emission, suggesting recent dispersal of their protoplanetary disks. We observed 30 class III stars in the 1-3 Myr Lupus region with ALMA at ~856um, resulting in 4 detections that we attribute to circumstellar dust, with inferred dust masses ~1 order of magnitude lower than any previous measurements. One disk is resolved with a radius ~80 au. Two class II sources in the field of view were also detected. We searched for gas emission from the CO~J=3-2 line, and present its detection to NO~Lup. Combining our survey with class II sources shows a gap in the disk mass distribution, evidence of rapid dispersal of mm-sized dust from protoplanetary disks. The class III disk mass distribution is consistent with a population model of planetesimal belts that go on to replenish the debris disks seen around main sequence stars. This suggests that planetesimal belt formation does not require long-lived protoplanetary disks, i.e., planetesimals form within ~2 Myr. While all 4 class III disks are consistent with collisional replenishment, for two the gas and/or mid-IR emission could indicate primordial circumstellar material in the final stages of protoplanetary disk dispersal. Two class III stars without sub-mm detections exhibit hot emission that could arise from ongoing planet formation processes inside ~1 au.

Hot exozodis: trapping dust in gas around main-sequence stars

Tim Pearce (Friedrich Schiller University Jena)

Excess near-infrared emission is detected around one fifth of main-sequence stars, but its nature is a mystery. These excesses are interpreted as populations of small, hot dust very close to their stars, but such grains should rapidly sublimate or be blown out of the system. We investigate a mechanism that could mitigate this problem: dust migrating inwards under radiation forces sublimates near the star, releasing modest quantities of gas which then traps subsequent grains. This model naturally explains many features of the inferred dust populations, and can reproduce observations of near-infrared excesses around Sun-like stars. However, for A-type stars our simulated grains are 5−10 times too large to reproduce observations. To date, no hot dust model has fully explained both the near-infrared phenomenon and its ubiquity around such a diverse range of star types and ages; further progress for A stars requires a means for grains to become very hot without either rapidly sublimating or being blown out of the system.

Exocomets and gas in debris disks

Isabel Rebollido (Space Telescope Science Institute)

Planetary systems are formed not only by planets and their star(s), but also host minor bodies (moons, asteroids and comets), dust and considerable amounts of gas. This gas, observed both at long and optical wavelengths, can be located at different distances from the star, and have very different temperatures. Since most of the primordial gas is depleted in the protoplanetary phase, there must be secondary processes that replenishes it, such as collisions or evaporation of small bodies. For the past 30 years we have been detecting variable spectral features in the optical range of approx. 26 A-type stars, that could be arising from exocomet evaporation, at close distances from the star, releasing very hot gas. We investigate here if the gas detected at longer wavelengths, with a much cooler temperature, might somehow be connected to exocometary processes, and therefore to a delivery mechanism of materials within the system.

Predictions for exozodiacal dust dragged in from an exo-Kuiper belt

Jessica Rigley (Institute of Astronomy, University of Cambridge)

Many nearby stars show mid-infrared excesses which are indicative of the presence of warm habitable zone dust, known as exozodiacal dust, in analogy to the zodiacal cloud. The presence of even low levels of this dust will be problematic for future attempts at detection and characterisation of Earth-like planets. Therefore, the study of exozodiacal dust allows us to better target planet detection missions and learn more about the inner regions of planetary systems. Such dust should disappear rapidly due to its proximity to the star via collisions and radiation pressure, and several mechanisms have been suggested to explain the presence of exozodiacal dust. Given the observed correlation between the presence of warm exozodiacal dust and cold planetesimal belts, we present an analytical model for exozodiacal dust dragged inwards by Poynting-Robertson drag from an exo-Kuiper belt undergoing a collisional cascade. We show that Poynting-Robertson drag should produce detectable levels of exozodiacal dust in systems which have a previously detected outer planetesimal belt, such that non-detections could imply the presence of intervening planets. Our model is applied to the HOSTS survey of exozodiacal dust for systems with known planetesimal belts, and we find that some detections can be explained by our Poynting-Robertson drag model. We also discuss the likelihood of drag producing exozodiacal dust levels which are problematic for direct imaging of exo-Earths.