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L. MatrЮ1, M. Wyatt1, O. Pani1, B. Dent2 "
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Institute of Astronomy, University of Cambridge, UK

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ALMA Santiago Central Offices, Chile"

Motivation!
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Debris is detected in the form of disks and belts around ~20%[1] of main-sequence stars (including our Sun) and represents the final product of a collisional cascade that originates back at the time when planets form. These debris disks are generally considered gas-poor: most of the gas is expected to dissipate at the end of the protoplanetary stage of disk evolution. However, several young (<50 Myr old) debris disks have been recently observed to contain considerable amounts of gas. Its origin is still poorly constrained: is it the product of collisions between cometary bodies (e.g. Pic[2]), or is it a remnant of the primordial disk (e.g. HD21997[3])? ALMA has the potential to detect and resolve CO rotational line emission in debris disks, potentially providing key information to determine its provenance. "
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(e.g. LTE, CO/H2 abundance ratio) do not necessarily apply in low gas density environments such as debris disks, and can lead to orders of magnitude errors in CO mass estimations. The 3 upper limit on the total CO mass for the Fomalhaut system is of 5.8 x 10-5 Mearth.

! Prospects for detection with ALMA! "

In the case of Fomalhaut, we find that low rotational transitions are best suited for future CO detection, since they probe a wider range of the density-temperature parameter space. Indeed, 1 hour on-source with the full ALMA array can potentially detect CO masses down to ~10-7 Mearth.!
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Figure 3: Sensitivity per beam per spectral channel (3) required to probe the same amount of mass in the Fomalhaut system as our upper limit, for different CO rotational transitions. Different colours represent different collisional partner to dust mass ratios, while different line styles represent different gas kinetic temperatures (solid: 100 K, dotted: 70 K, dashed: 40 K, dash-dotted: 10 K). Left: collisions with H2. Right: collisions with H2O. Shaded areas represent emission levels detectable in the future using ALMA (light grey: 1 hour on-source, dark grey: 2 hours).

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Figure 1: The Fomalhaut 0continuum debris -20 g seen in the sub-mm by ALMA (left) and in the optical by rin 20 10 -10 [4] (right). North is up, east is left. In the ALMA image, white contours represent pixels selected in our Hubble cos offset (") analysis, yellow lines are the ring inner and outer radii, and the red dashed circle is the primary beam.

ALMA Cycle-0 observations[5] of the north-west side of the Fomalhaut ring were retrieved to search for CO J=3-2 emission, but no detection was achieved. We set an upper limit of 0.16 Jy km s-1 on its flux, integrated spatially over the whole ring, and spectrally over the expected Keplerian ring velocities." "

Modelling CO excitation!
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In general, ALMA will be able to set significantly tighter 10 constraints on the amount Dent+ 2005 Moor+ 2011 of CO present not only in Dent+ 2014 10 Panic+ 2010 debris disks, but also in Matra+ in prep. 10 Herbig Ae/Be stars, shining new light on the gas 10 dissipation process and during the transition 10 between protoplanetary 10 and debris systems. This becomes clear if we 10 compare our CO flux upper 10 to previous CO 10 10 10 10 10 10 limit Age (Myr) detections and upper limits Figure 4: CO integrated flux vs age for a sample of Herbig Ae/Be and debris disks. Filled dots are detections, from the literature."
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Figure 2: Upper limits on the CO mass present in the Fomalhaut ring, as a function of the collisional partner to dust mass ratio. Solid lines show results in the most general scenario, whereas dotted lines show results in the LTE regime. Different colours represent different gas kinetic temperatures. Left: Collisions with H2; the grey dashed line represents a CO/H2 abundance ratio of 10-4. Right: Collisions with H2O; the grey shaded area represents CO/H2O abundance ratios observed in Solar System comets.

In order to set an upper limit on the total mass of CO in the system, we build a physical model of the Fomalhaut disk that calculates the non-LTE excitation of CO in the optically thin regime. This allows us to explore the extremely low gas collider densities n expected in debris discs, for a range of gas kinetic temperatures Tkin, and for collisions with either H2 or H2O. We show that common assumptions typical of protoplanetary disks
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" Fig. 2 shows how CO mass estimates can be strongly dependent on n and Tkin, making determination of the total CO mass in debris disks problematic, if only a single optically thin line is detected. However, this degeneracy can be broken via observations of multiple optically thin transitions. In fact, measurements of one line ratio can resolve the dependence of the mass on one between the two unknowns, whereas detections of more than one line ratio can completely solve the problem. Remarkably, ALMA should be able to spatially and spectrally resolve CO emission, allowing us to create maps of n and Tkin within the nearest, brightest disks. This will represent a big step forward towards understanding the origin of gas in debris systems.!
References! [1 [2 [3 [4 ] ] ] ] Eiroa et al. 2013, A&A, 555, A11" Dent et al. 2014, Science" Kospal et al. 2013, ApJ, 776, 77" Kalas et al. 2013, ApJ, 775, 56" [5 [6 [7 [8 ] ] ] ] Boley et al. 2012 Moor et al. 2011, Panic et al. 2010 Dent et al. 2005, , ApJ, 750, L21" ApJ, 740, L7" , A&A, 557, A68" MNRAS, 359, 663"

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Acknowledgements! ! This work was made possible thanks to support from STFC through a Ph.D. studentship and from the European Union through ERC grant number 279973. "

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