Late and debris disks
A Deep Polarimetric Analysis of the Asymmetrical Debris Disk HD 106906
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The effect of late gas disks on the late stages of planet formationQuentin 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 DispersaJoshua 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
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Exocomets and gas in debris disks
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Predictions for exozodiacal dust dragged in from an exo-Kuiper beltJessica 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. |