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Constraints on the gas content of the Fomalhaut debris belt Can gas-dust interactions explain the belt's morphology?
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
Vise andre og tillknytning
2015 (engelsk)Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 574, artikkel-id L1Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Context. The 440 Myr old main-sequence A-star Fomalhaut is surrounded by an eccentric debris belt with sharp edges. This sort of a morphology is usually attributed to planetary perturbations, but the orbit of the only planetary candidate detected so far, Fomalhaut b, is too eccentric to efficiently shape the belt. Alternative models that could account for the morphology without invoking a planet are stellar encounters and gas-dust interactions. Aims. We aim to test the possibility of gas-dust interactions as the origin of the observed morphology by putting upper limits on the total gas content of the Fomalhaut belt. Methods. We derive upper limits on the CII 158 mu m and 01 63 pint emission by using non detections from the Photocletector Array Camera and Spectrometer (PACS) onboard the Herschel Space Observatory. Line fluxes are converted into total gas mass using the non-local thermodynamic equilibrium (non-LTE) code RADEX. We consider two different cases for the elemental abundances of the gas: solar abundances and abundances similar to those observed for the gas in the beta Pictoris debris disc. Results. The gas mass is shown to be below the millimetre dust mass by a factor of at least similar to 3 (for solar abundances) respectively similar to 300 (for beta Pic-like abundances). Conclusions. The lack of gas co-spatial with the dust implies that gas-dust interactions cannot efficiently shape the Fomalhaut debris belt. The morphology is therefore more likely due to a yet unseen planet (Fomalhaut c) or stellar encounters.

sted, utgiver, år, opplag, sider
2015. Vol. 574, artikkel-id L1
Emneord [en]
circumstellar matter, planetary systems, stars: individual: Fomalhaut, methods: observational, hydrodynamics, infrared: general
HSV kategori
Forskningsprogram
astronomi
Identifikatorer
URN: urn:nbn:se:su:diva-115933DOI: 10.1051/0004-6361/201425322ISI: 000349467000149OAI: oai:DiVA.org:su-115933DiVA, id: diva2:804962
Merknad

AuthorCount:8;

Tilgjengelig fra: 2015-04-14 Laget: 2015-04-08 Sist oppdatert: 2018-03-08bibliografisk kontrollert
Inngår i avhandling
1. Debris disks and the search for life in the universe
Åpne denne publikasjonen i ny fane eller vindu >>Debris disks and the search for life in the universe
2016 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Circumstellar debris disks are the extrasolar analogues of the asteroid belt and the Kuiper belt. These disks consist of comets and leftover planetesimals that continuously collide to produce copious amounts of circumstellar dust that can be observed as infrared excess or in resolved imaging. As an obvious outcome of the planet formation process, debris disks can help us constrain planet formation theories and learn about the history of our own solar system. Structures in the disks such as gaps or warps can hint at the presence of planets. Thus, the study of debris disks is an important branch of exoplanetary science. In this thesis, some aspects of debris disks are considered in detail.

A handful of debris disks show observable amounts of gas besides the dust. One such case is the edge-on debris disk around the young A-type star β Pictoris, where the gas is thought to be of secondary origin, i.e. derived from the dust itself. By observing this gas, we can thus learn something about the dust, and therefore about the building blocks of planets. In paper I, spectrally resolved observations of C II emission with Herschel/HIFI are presented. The line profile is used to constrain the spatial distribution of carbon gas in the disk, which helps understanding the gas producing mechanism. In paper II, we analyse C II and O I emission detected with Herschel/PACS and find that the oxygen must be located in a relatively dense region, possibly similar to the CO clump seen by ALMA. An upcoming analysis of our ALMA C I observations will give us a clearer picture of the system.

Another famous debris disk is found around the nearby, 440 Myr old A-star Fomalhaut. Its morphology is that of an eccentric debris belt with sharp edges, suggesting shaping by a planet. However, gas-dust interactions may result in a similar morphology without the need to invoke planets. We test this possibility in paper III by analysing non-detections of C II and O I emission by Herschel/PACS. We find that there is not enough gas present to efficiently sustain gas-dust interactions, implying that the morphology of the Fomalhaut belt is due to a yet unseen planet or alternatively stellar encounters.

One of the biggest challenges in exoplanetary research is to answer the question whether there are inhabited worlds other than the Earth. With the number of known rocky exoplanets in the habitable zone increasing rapidly, we might actually be able to answer this question in the coming decades. Different approaches exist to detect the presence of life remotely, for example by studying exoplanetary atmospheres or by analysing light reflected off the surface of an exoplanet. In paper IV, we study whether biosignatures (for example, certain minerals or microorganisms) ejected into a circumstellar debris disk by an impact event could be detected. We consider an impact similar to the Chicxulub event and model the collisional evolution of the ejected debris. Dust from such an event can potentially be detected by current telescopes, but analysis of the debris composition has to wait for future, advanced instruments.

sted, utgiver, år, opplag, sider
Stockholm: Department of Astronomy, Stockholm University, 2016. s. 73
Emneord
debris disks, astrobiology
HSV kategori
Forskningsprogram
astronomi
Identifikatorer
urn:nbn:se:su:diva-127263 (URN)978-91-7649-366-3 (ISBN)
Disputas
2016-04-15, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (engelsk)
Opponent
Veileder
Merknad

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Submitted.

Tilgjengelig fra: 2016-03-21 Laget: 2016-02-29 Sist oppdatert: 2017-02-20bibliografisk kontrollert

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