Info

+

+

+

+ + lL+ + +

+ + ±44- + * + + ++jh

Fig. 6.6. The upper plot shows a scatter plot for eccentricities against semimajor axis for exoplanets. The lower plot shows mean eccentricities plotted against semimajor axis for exoplanets. Mean eccentricities are shown binned in the solid line (and median in dotted since there are cases where exoplanet eccentricity fits are set to zero when there is relatively little radial velocity data). The error bars are from y/(number) statistics and are only indicative. While a wide range of eccentricity values can be found across the range of measured semi-major axes it is evident that circularisation processes are evident below 1 au.

0.1 1.0 semimajor axis (au)

10.0

Fig. 6.6. The upper plot shows a scatter plot for eccentricities against semimajor axis for exoplanets. The lower plot shows mean eccentricities plotted against semimajor axis for exoplanets. Mean eccentricities are shown binned in the solid line (and median in dotted since there are cases where exoplanet eccentricity fits are set to zero when there is relatively little radial velocity data). The error bars are from y/(number) statistics and are only indicative. While a wide range of eccentricity values can be found across the range of measured semi-major axes it is evident that circularisation processes are evident below 1 au.

the tidal circularisation timescale is already likely to be many Gyr, for inferred Q values (based on planets having semi-major axes with less than 0.1 au, where Q is the specific dissipation function). The circularisation timescale, tcirc, is a sensitive function of the radius of the planet, Rplanet (tcirc « Rplanet, Goldreich & Soter 1966) and supposedly occurs due to dissipation in the planet, not the star (e.g., Gu et al. 2004). Perhaps, the larger planet radii during contraction, in the first 10 Myr or so, helps to shorten the circularisation time. Thus the rising mean eccentricities versus semi-major axis apparent in Fig. 6.6 from 0.1 to around 0.5 au may be explained by the decreasing effectiveness of the tidal circularisation mechanism.

One might expect that the orbital eccentricities of smaller planets would respond more to eccentricity boosting mechanisms, however, the lower plot in Fig. 6.7 suggests the opposite. While an attempt has been made to correct for the bias against finding high eccentricity planets (by only looking at the more 'complete' sample of higher mass planet) this is a crude correction (e.g. Cumming 2004). Ribas & Miralda-Escude (2007) hypothesize that there are two populations of gaseous planets: the low-mass population which form by gas accretion onto a rock-ice core in a circumstellar disk and is more abundant at high metallicities, and a high-mass population which forms directly by fragmentation of a pre-stellar cloud. Planets

Fig. 6.7. The upper plot shows a scatter plot for Mplanetjup/Mstal0 against eccentricities for exoplanets. The lower plot shows median Mplanetjup /Mstal0 values and suggests that for a given mass host star a larger planet will have a higher eccentricity. The dashed line represents all planets above 1 Mjup sin i. The error bars are from \J(number) statistics and are only indicative.

Fig. 6.7. The upper plot shows a scatter plot for Mplanetjup/Mstal0 against eccentricities for exoplanets. The lower plot shows median Mplanetjup /Mstal0 values and suggests that for a given mass host star a larger planet will have a higher eccentricity. The dashed line represents all planets above 1 Mjup sin i. The error bars are from \J(number) statistics and are only indicative.

of the first population form in initially circular orbits and grow their eccentricities later, and may have a mass upper limit from the total mass of the disk that can be accreted by the core. The second population may have a mass lower limit resulting from opacity-limited fragmentation. This would roughly divide the two populations in mass, although they would likely overlap over some mass range. If most objects in the second population form before the pre-stellar cloud becomes highly opaque, they would have to be initially located in orbits larger than ^30 AU, and would need to migrate to the much smaller orbits in which they are observed. The higher mean orbital eccentricity of the second population might be caused by the larger required intervals of radial migration, and the brown dwarf desert might be due to the inability of high-mass brown dwarfs to migrate inwards sufficiently in radius.

Was this article helpful?

0 0

Post a comment