Constraining formation and evolution scenarios

Low-mass giant planets (i.e., planets with masses in the range 10-100M®), are of particular interest as they provide potentially strong constraints on current giant planet formation and evolution models. Indeed, and perhaps contrary to intuition, the formation

Figure 7. Measured radial velocity of HD 102117 in phase with the orbital period of the planet. The fitted orbital solution is shown as well. The residuals of the data points to this solution are only 0.9 ms-1 rms. This value includes photon noise and remaining 'stellar noise.'

of these objects within the current theoretical models appears more difficult than the formation of their more massive counterparts. For this reason, objects with masses within or at the edge of this range—like j Arac and HD 4308 b (Udry et al. 2005)—are of particular interest.

In the direct-collapse scenario, planets form on very short timescales through gravitational collapse of patches of the proto-planetary disk (Boss 2002). High-resolution simulations of this process show that planets tend to form on elliptical orbits with semimajor axis of several astronomical units and masses between 1 and 7 MJup (Mayer et al. 2002, 2004). In this scenario, j Ara-type planets would have to result from subsequent evolution involving migration and very significant mass loss.

In the framework of the core accretion model (e.g., Pollack et al. 1996), the final mass of a planet is actually determined by the amount of gas the core accretes after it has reached a critical mass, which is of order 10-15 M®. In Ida & Lin (2004), this amount is determined by the rate at which gas can be accreted (essentially the Kelvin-Helmholtz timescale) and by the total amount of gas available within the planet's gravitational reach. Since for super-critical cores (even in low-mass disks) the Kelvin-Helmoltz timescale is short and the amount of gas available large (compared to an Earth's mass), planets tend to form that are either less massive or significantly more massive than the critical mass. From a large number of formation model calculations, Ida & Lin (2004) found that only a very few planets form in the mass range 10-100 M®, a range they actually called a planetary desert.

Figure 8. Histogram of the radial-velocity scatter for all targets belonging to the very high-precision HARPS planet-search program. The distribution peaks at 1.5ms"1 and is mainly dominated by stellar oscillations and jitter. We must point out here that i Ara (corrected for the drift due to i Arab) and HD 102117, are part of this distribution, and that the orbital motions of their planets alone induce a radial-velocity scatter of about 2.5 ms"1 and 6 ms"1 rms, respectively!

Figure 8. Histogram of the radial-velocity scatter for all targets belonging to the very high-precision HARPS planet-search program. The distribution peaks at 1.5ms"1 and is mainly dominated by stellar oscillations and jitter. We must point out here that i Ara (corrected for the drift due to i Arab) and HD 102117, are part of this distribution, and that the orbital motions of their planets alone induce a radial-velocity scatter of about 2.5 ms"1 and 6 ms"1 rms, respectively!

In the extended core accretion models of Alibert et al. (2005), due to the planet's migration, it can in principle accrete gas over the entire lifetime of the disk. However, since the latter thins out with time and the planet eventually opens a gap as it grows more massive, the gas supply decreases with time. The growth rate of the planet is actually set by the rate at which the disk can supply the gas, rather than the rate at which the planet can accrete it. Monte-Carlo simulations are ongoing to verify whether these models lead to a different planetary initial mass function as in Ida & Lin (2004).

At first glance, the relatively numerous small-mass objects discovered so far seem to pose a problem to current planet-formation theories (Lovis et al. 2005)—however, the situation is actually more complex. Since all the known very low-mass planets are located close to their star, one cannot exclude the fact that these objects could have formed much more massively and lost a significant amount of their mass through evaporation during their lifetime (see Baraffe et al. 2004, 2005, for a more detailed discussion).

While mass loss from initially more massive objects could possibly account for the light planets very close to their star, it is not clear whether i Arac—located at a distance of 0.09 AU—could actually result from the evaporation of a more massive object. The situation is even more critical for HD 4308 b (Udry et al. 2005), located further away (0.115 AU) from its parent star which is, in addition, less luminous than i Ara by a factor of ~1.8. The effects could possibly be compensated, at least partially, by the very old estimated age of the star. However, as more i Ara-like objects are being discovered, and if they all are the results of the evaporation of larger-mass planets, the question of the

Figure 9. Histogram of the residuals of the best-orbit fit to the measured data for all extrasolar planets discovered since January 2004. The dashed planets have been discovered with HARPS, whereas the planet belonging to the HARPS Program Nr. 1 (high-precision) are distinguished by cross-dashed area.

Figure 9. Histogram of the residuals of the best-orbit fit to the measured data for all extrasolar planets discovered since January 2004. The dashed planets have been discovered with HARPS, whereas the planet belonging to the HARPS Program Nr. 1 (high-precision) are distinguished by cross-dashed area.

probability of catching these systems shortly before complete evaporation will become a central one.

As already stated by Baraffe et al. (2005), the current evaporation models are still affected by large uncertainties—the lack of detailed chemistry treatment, non-standard chemical composition in the envelope, the effect or rocky/icy cores, etc.—that will need to be clarified in order to solve the question of the possible formation of ^ Ara-type planets through evaporation.

Finally, given their close location to their star, the detected small-mass planets are likely to have migrated to their current position from further out in the nebula. The chemical composition of these planets will depend upon the extent of their migration, the thermal history of the nebula, and hence, the composition of the planetesimals along the accretion path of the planet. The situation is made more complicated by the fact that the ice-line itself is moving as the nebula evolves (see e.g., Sasselov & Lecar 2000). Detailed models of planetary formation including these effects have yet to be developed.

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