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Panel Discussion on the Brown Dwarf { Exoplanet Connection Mem. S.A.It. Vol. 84, 1154 c SAIt 2013 Memorie della Panel discussion on the brown dwarf { exoplanet connection D. J. Pinfield1, J.-P. Beaulieu2, A. J. Burgasser3, P. Delorme4, J. Gizis5, and Q. Konopacky6 1 Centre for Astrophysics Research, University of Hertfordshire, Hatfield, UK, AL10 9AB, UK, e-mail: [email protected] 2 Institut d’Astrophysique de Paris, UMR7095, CNRS, Universite´ Paris VI, 98bis Boulevard Arago, 75014 Paris, France 3 Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA 4 UJF-Grenoble 1/CNRS-INSU, Institut de Plantologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041, Grenoble, France 5 Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA 6 Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 Abstract. We present a summary of the panel discussion session on the brown dwarf ex- oplanet connection, at the Brown Dwarfs Come of Age conference in Fuerteventura. The discussion included an audience vote on the status (planet or brown dwarf) of a selection of interesting objects, as well as a vote on the current components of the IAU definition separating planets and brown dwarfs, and we report the results. In between these two opin- ion tests we discussed a set of key questions that helped us explore a variety of important areas, and we summarise the resulting discussion both from the panel and the conference audience. Key words. Stars: brown dwarfs, planetary systems 1. Introduction also ambiguity from the very start. The rela- tively unambiguous brown dwarf Teide 1 has a Eighteen years on from the discovery of the mass of 40-70 MJup (just below the Hydrogen first brown dwarfs and exoplanet, the Brown burning limit) and is a free-floating M8 dwarf Dwarfs Come of Age conference provides an in the Pleiades cluster (Rebolo et al. 1995). opportunity to re-examine and discuss the con- In contrast the first exoplanet, 51 Peg b, has a nection between these two exotic populations, likely sub-Jupiter mass (∼0.5 MJup) and orbits and consider how we might better understand at a tight (hot) 0.05 AU separation (Mayor & their nature now that the field has matured. The Queloz 1995). Early ambiguity appeared in the very first discovered brown dwarfs and exo- form of Gl229B (Nakajima et al. 1995), with planets showed some clear disconnection, but a mass (20-50 MJup) much closer to the giant Send offprint requests to: D. Pinfield planet regime, a 40 AU orbit, and a spectrum Pinfield et al.: The brown dwarf – exoplanet connection 1155 showing strong similarities with that of Jupiter. 1.2. Structure of the panel discussion So the distinction between brown dwarfs and exoplanets has been blurred since the earliest Our discussion session took the following discoveries, and we would like to explore de- structure, which we describe in the rest of this veloped/growing opportunities for clarity. write-up. – To canvass opinion from the community, 1.1. Where do things stand? - the IAU and more generally encourage clarity over ambiguity we began with a vote on a se- definition ries of interesting objects - brown dwarf or As part of the re-classification exercise that planet? made Pluto a “dwarf planet” following new – We then presented five key questions to Kuiper-Belt discoveries, the IAU Working foster discussion, with panel and audience Group on Extrasolar Planets (WGESP) agreed member participation. on a “Planet” definition in a position statement – And at the end we took a vote on the main first issued on 28 Feb 2001 (and last modified elements of the existing IAU definition, to 28 Feb 2003; see Boss et al. 2003). The state- elicit an overview of how current opinion ment consists of the following points: maps onto existing definition. – Objects with true masses below the lim- iting mass for thermonuclear fusion of 2. Community view - individual deuterium (currently calculated to be 13 objects Jupiter masses for objects of solar metal- To stimulate discussion on what properties licity) that orbit stars or stellar remnants might best distinguish brown dwarfs from ex- are “planets” (no matter how they formed). oplanets, we asked the audience to vote on the The minimum mass/size required for an ex- planethood of a sample of 11 systems which trasolar object to be considered a planet span the brown dwarf/exoplanet “regimes”, should be the same as that used in our Solar covering a variety of masses, primary types System. (including no primary), ages and separations. – Substellar objects with true masses above This was a binary selection - either an object is the limiting mass for thermonuclear fusion or is not a planet. There were 55–60 votes per of deuterium are “brown dwarfs”, no mat- system, implying a sampling error of 4–6%. ter how they formed nor where they are lo- Tables 1 and 2 summarize the results (sys- cated. tem parametres and polling respectively) in – Free-floating objects in young star clusters order of decreasing “planetness”. The bench- with masses below the limiting mass for mark companions to HR 8799 and that of thermonuclear fusion of deuterium are not Gliese 229 were extremities on the planet- “planets”, but are “sub-brown dwarfs” (or brown dwarf spectrum; these systems have whatever name is most appropriate). similar separations and possibly ages, but the The WGESP noted that these statements clear distinguishing factors were companion are a compromise between definitions based mass and mass ratio, the latter differing by an purely on the deuterium-burning mass or on order of magnitude. The presence of more than the formation mechanism, and as such did not one substellar object and a debris disk in HR fully satisfy anyone on the working group. 8799 but not Gliese 229 may also have a signif- Very young free-floating objects are not icant impact on the outcome of the planetness just found in young clusters of course, and have vote. The most split vote came for the com- been identified in the field e.g. as members of panion to WD 0806-661, which is even within kinematic moving groups. As such the final the margin of error; the companion to Corot-3 point is naturally extended to include all free- and the “free-floating” planetary mass objects floating objects, so we assume this modifica- CFBDS 2149−04 and those in σ Orionis were tion hereafter. all within 2.5 σ of the margin of error. The last 1156 Pinfield et al.: The brown dwarf – exoplanet connection of these reflects the even split on whether such desert separates the stellar and planetary pop- unbound objects should be considered planets ulations. Brown dwarf researchers, however, in a later poll (below). are now working in regimes where these def- We examined the correlation between frac- initions are not very helpful. Direct imaging tion of planet votes to the systems’ various searches are detecting objects above and be- physical parameters. The most significant fac- low the deuterium-burning limit at wide sep- tors were object mass and separation from arations from the primary. These may or may primary star, both of which had marginally not have anything to do with formation in a significant (1.6 σ and 1.2 σ) negative correla- disk. tions (larger values decreased “planethood”). The panel discussion emphasizes the prob- Primary mass, system mass ratio and presence lems of formation-based and mass-based defi- of a primary were insignificantly correlated. A nitions. The HR 8799 system had wide support linear regression of these data provides a sim- as a planetary system, as it consists of multi- ple but probably unreliable prediction of this ple low mass objects that seem to have formed audience’s planethood for a given object based in a disk, though not necessarily through core on these parameters: accretion. If disk formation is not important, then 2M1207b seems to meet most planet def- initions. Current work suggests that forma- %Planethood = 54:8 + 9:78MA − 2:22MB tion scenarios are not a simple binary choice. Theorists at this conference predicted a new +57:5q − 11:9 log10 a population of objects that form through disk in- +36:8C (1) stabilities. These may lie in the range 3 to “42” jupiter masses. Many of these are predicted to where M is primary mass in solar masses, M A B be ejected and become isolated, “free-floating” is companion mass in Jupiter masses, q is the objects. Some observers of star forming re- mass ratio M /M , a is the separation in AU B A gions reported evidence that the mass function and C = 1 if a primary is present or 0 if not. increases below the deuterium-burning limit, This is accurate to within 9% for the most sig- which if confirmed, also suggests a new for- nificant factors mation mechanism. There was a discussion of the latest microlensing evidence of a new population of free-floating or wide compan- %Planethood = 105 − 2:30MB ion objects below the deuterium-burning limit. −12:1 log10 a (2) Ultimately, the panel wondered if the disk in- stability should be considered a planetary or a is accurate to within 16%. stellar formation mechanism. There was spirited challenge from some 3. Discussion questions audience members on whether there are truly fundamental differences between planets and 3.1. How might we define the brown dwarfs. Perhaps these limits are arbi- fundamental differences (e.g. trary and astronomers should be more open- properties, formation) between minded. For example, if we lived on an earth- brown dwarfs and giant planets? like planet orbiting Gliese 229A, would we consider Gliese 229B a planet? Perhaps the de- The voting brought out that even at this spe- sire to define planets to be like those in our cialized conference there is a wide range of own system is just an emotional attachment, views on definitions, or rather what differences a psychological bias that astronomers should are fundamental.
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