ALMA Resolves C i Emission from the β Pictoris Debris Disk

Cataldi, Gianni; Brandeker, Alexis; Wu, Yanqin; Chen, Christine; Dent, W. R. F.; Vries, B. L. de; Kamp, I.; Liseau, R.; Olofsson, G.; Pantin, E.; Roberge, Aki

doi:10.3847/1538-4357/aac5f3

Cited by 49 publications

(92 citation statements)

References 81 publications

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“…By analyzing spectroscopic observations obtained with the FUSE (Far Ultraviolet Spectrocopic Explorer) satellite, Lecavelier des Etangs et al (2001) were able to set an upper limit of <10 18 cm −2 on the column density of H 2 molecules in the gaseous edgeon disk of β Pic. Comparing this result to the column density of CO molecules detected in absorption resulted in a CO to H 2 ratio of >6×10 −4 indicating -in good accordance with conclusions from other observations (Fernández et al 2006;Matrà et al 2017a;Cataldi et al 2018) -that the gas is of second generation in β Pic. Later, Martin-Zaïdi et al (2008) reported an even stricter upper limit of N H2 < 2.6×10 17 cm −2 for β Pic.…”

Section: A Possible Primordial Origin Of the Gas Componentsupporting

confidence: 91%

“…We note that in debris disks α values could be very different from those in protoplanetary disks for several reasons. Kral & Latter (2016) found that the magnetorotational instability could work in a debris disk environment and could even lead to high α values because of the high ionisation fraction that can be reached in unshielded disks (mostly made of ionised carbon, such as in β Pic, Cataldi et al 2018). In HD 32297 and 49 Cet, the ionisation fraction could be much lower because of the strong shielding leading to drastically lower α values.…”

Section: Upgraded Shielded Secondary Gas Disk Modelmentioning

confidence: 99%

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New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297

Moór

Kral

Ábrahám

et al. 2019

ApJ

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Previous observations revealed the existence of CO gas at nearly protoplanetary level in several dust-rich debris disks around young A-type stars. Here we used the ALMA 7m-array to measure 13 CO and C 18 O emission toward two debris disks, 49 Cet and HD 32297, and detected similarly high CO content (>0.01 M ⊕ ). These high CO masses imply a highly efficient shielding of CO molecules against stellar and interstellar ultraviolet photons. Adapting a recent secondary gas disk model that considers both shielding by carbon atoms and self-shielding of CO, we can explain the observed CO level in both systems. Based on the derived gas densities we suggest that, in the HD 32297 disk, dust and gas are coupled and the dynamics of small grains is affected by the gaseous component. For 49 Cet, the question of coupling remains undecided. We found that the main stellar and disk properties of 49 Cet and HD 32297 are very similar to those of previously identified debris disks with high CO content. These objects constitute together the first known representatives of shielded debris disks.

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Section: A Possible Primordial Origin Of the Gas Componentsupporting

confidence: 91%

Section: Upgraded Shielded Secondary Gas Disk Modelmentioning

confidence: 99%

New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297

Moór

Kral

Ábrahám

et al. 2019

ApJ

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“…This is for two main reasons. First, carbon emission in band 8 seems to be a better tracer of this gas than CO in either band 6 or 7 according to models 6 and to the first few observations of neutral carbon in these disks 7,15,44 . Second, carbon is always expected to spread in the inner region, given enough time, while CO photodissociates in about 100 yr in unshielded disks and remains colocated with the parent belt of planetesimals, implying that the CO cavity observed is then due to photodissociation rather than accreting planets.…”

Section: Abovementioning

confidence: 92%

Formation of secondary atmospheres on terrestrial planets by late disk accretion

2020

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Recently, gas disks have been discovered around main sequence stars well beyond the usual protoplanetary disk lifetimes (i.e., 10 Myrs), when planets have already formed 1-4 . These gas disks, mainly composed of CO, carbon, and oxygen 5-7 seem to be ubiquitous 3 in systems with planetesimal belts (similar to our Kuiper belt), and can last for hundreds of millions of years 8 . Planets orbiting in these gas disks will accrete 9, 10 a large quantity of gas that will transform their primordial atmospheres into new secondary atmospheres with compositions similar to that of the parent gas disk. Here, we quantify how large a secondary atmosphere can be created for a variety of observed gas disks and for a wide range of planet types.We find that gas accretion in this late phase is very significant and an Earth's atmospheric mass of gas is readily accreted on terrestrial planets in very tenuous gas disks. In slightly more massive disks, we show that massive CO atmospheres can be accreted, forming planets with up to sub-Neptune-like pressures. Our new results demonstrate that new secondary atmospheres with high metallicities and high C/O ratios will be created 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 atmosphere to photoevaporation or giant 1 arXiv:2004.02496v1 [astro-ph.EP] 6 Apr 2020 impacts. We therefore propose a new paradigm for the formation of atmospheres on lowmass planets, which can be tested with future observations (JWST, ELT, ARIEL). We also show that this late accretion would show a very clear signature in Sub-Neptunes or cold exo-Jupiters. Finally, we find that accretion creates cavities in late gas disks, which could be used as a new planet detection method, for low mass planets a few au to a few tens of au from their host stars.The discovery of large amounts of gas around main-sequence stars is recent with most detections occurring in the last few years 3, 11 . These late gas disks are observed in systems that have planetesimal belts, which are older than 10 Myr and can last for hundreds of millions of years 8 .It is thought that the observed gas is released from volatile-rich planetesimals when they collide with each other in the system's belts 6, 12 . The gas then viscously evolves 4 , spreading inward and outwards 10, 15 . Hence the observed gas is likely secondary (rather than of primordial origin) and this late disk (main-sequence) phase is different from the younger (<10 Myr) protoplanetary disks that are much more massive, hydrogen-rich and in which giant planets form within a few millions of years 16 .These late gas disks are nearly ubiquitous around A-type stars; Gas has been detected around more than 70% of systems with bright planetesimal belts 3 . As for other stellar type stars or lower mass systems the statistics are still based on too small a sample as these gas disks are harder to detect but gas evolution models 6 predict that all stars surrounded by planetesimal belts s...

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“…with the estimated CO and neutral carbon mass in four discs (β Pic, 49 Ceti, HD 131835, HD32297, Higuchi et al 2017Cataldi et al 2018;Kral et al 2019;Cataldi et al 2019). Overall we find that the observed carbon masses are lower than predicted by our model.…”

Section: High αmentioning

confidence: 95%

“…Only if radiation pressure can remove carbon from the system or at least beyond the planetesimal belt this could solve this apparent inconsistency. Alternatively, the gas distribution could still have large cavities if the gas was released recently in a giant collision, as suggested by Dent et al (2014) and Cataldi et al (2018Cataldi et al ( , 2019, or the disc was stirred only recently (in less than a viscous timescale) and thus gas release has only been in place for a small fraction of the ages of these systems. This however raises even more questions regarding the frequency of these events since we would also expect to observe systems where gas has viscously spread filling the cavities with gas.…”

Section: Resolved Observations Of Atomic Carbonmentioning

confidence: 98%

Population synthesis of exocometary gas around A stars

Marino

Flock

Henning

et al. 2020

Monthly Notices of the Royal Astronomical Society

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The presence of CO gas around 10-50 Myr old A stars with debris discs has sparked debate on whether the gas is primordial or secondary. Since secondary gas released from planetesimals is poor in H 2 , it was thought that CO would quickly photodissociate never reaching the high levels observed around the majority of A stars with bright debris discs. Kral et al. (2019) showed that neutral carbon produced by CO photodissociation can effectively shield CO and potentially explain the high CO masses around 9 A stars with bright debris discs. Here we present a new model that simulates the gas viscous evolution, accounting for carbon shielding and how the gas release rate decreases with time as the planetesimal disc loses mass. We find that the present gas mass in a system is highly dependant on its evolutionary path. Since gas is lost on long timescales, it can retain a memory of the initial disc mass. Moreover, we find that gas levels can be out of equilibrium and quickly evolving from a shielded onto an unshielded state. With this model, we build the first population synthesis of gas around A stars, which we use to constrain the disc viscosity. We find a good match with a high viscosity (α ∼ 0.1), indicating that gas is lost on timescales ∼ 1 − 10 Myr. Moreover, our model also shows that high CO masses are not expected around FGK stars since their planetesimal discs are born with lower masses, explaining why shielded discs are only found around A stars. Finally, we hypothesise that the observed carbon cavities could be due to radiation pressure or accreting planets.

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ALMA Resolves C i Emission from the β Pictoris Debris Disk

Cited by 49 publications

References 81 publications

New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297

New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297

Formation of secondary atmospheres on terrestrial planets by late disk accretion

Population synthesis of exocometary gas around A stars

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