Wednesday, January 30, 2013

Stratospheres on Hot-Jupiters

Figure 1: Artist’s impression of HD 209458b - a hot-Jupiter. Credit: NASA, ESA, and G. Bacon (STScI)

Hot-Jupiters are a class of extrasolar planets that have similar characteristics to Jupiter but unlike Jupiter, they orbit very close to their parent stars. As a result, hot-Jupiters exhibit high atmospheric temperatures ranging from hundreds to thousands of Kelvin. Additionally, hot-Jupiters are expected to be tidally-locked with respect to their parent stars and this means that a hot-Jupiter always presents the same hemisphere towards its parent star, resulting in a permanent dayside and nightside hemisphere. This establishes a large temperature contrast between the dayside and nightside. Condensable species such as titanium oxide (TiO) and silicates that are stable in the gas phase on the dayside may condense into particles on the much cooler nightside and gravitationally settle there. In the absence of vertical mixing in the atmosphere, condensation on the nightside can deplete the atmosphere of these condensable species. In this regard, the nightside can serve as a cold trap by depleting the atmosphere of these condensable species.

Observations of some transiting hot-Jupiters have revealed the presence of stratospheres on the daysides of these planets. A stratosphere is a layer in a planet’s atmosphere where the temperature increases as altitude increases (thermal inversion). In the atmosphere of a hot-Jupiter, a stratosphere is thought to result from the absorption of starlight by strong visible/ultraviolet absorbers and gaseous TiO is a good candidate for such an absorber. The presence of the nightside cold trap on a hot-Jupiter brings up the question of whether such a cold trap is effective enough to deplete the atmosphere of TiO and prevent the formation of a stratosphere. This is investigated by using 3D global circulation models of HD 209458b - one of the best studied hot-Jupiter which is also though to have a stratosphere and a large day-night temperature contrast.

Figure 2: Temperature and horizontal winds (arrows) in the global circulation model of the hot-Jupiter HD 209458b. The top three panels show the temperature and flow at three different pressures (0.1 millibar, 1 millibar and 10 millibar). The substellar point is located at longitude 0 degrees and latitude 0 degrees while the dayside is between the longitudes of -90 degrees and +90 degrees.

Based on the global circulation models of HD 209458b, a stratosphere is clearly visible on the dayside of the planet at altitudes above the 10 millibar level due to strong absorption by TiO in the atmosphere. Furthermore, at altitudes above the 10 millibar level, temperatures reach 2200 Kelvin around the substellar point on the planet and the day-night temperature contrast is as large as 1600 Kelvin. The large temperature contrast is due to the short radiative timescales at low atmospheric pressures. Centred along the equator of HD 209458b is a strong eastward superrotating jet. A pair of convergence/divergence shock-like features can be seen along the superrotating jet. The convergence/divergence feature located about 40 degrees west of the substellar point creates a region of strong upward flow while the convergence/divergence feature located about 150 degrees east of the substellar point creates a region of strong downward flow.

As shown from the global circulation models of HD 209458b, large scale circulation in the atmosphere naturally creates vertical flows that are strong enough to keep a condensable species such as TiO aloft if it condenses into particles no larger than a few microns on the planet’s nightside. If TiO is able to condense into particles larger than a few microns, then the nightside cold trap will be efficient enough to deplete TiO from the atmosphere. However, if TiO is unable to condense into particles larger than a few microns, there will be sufficient TiO on the planet’s dayside for a TiO-induced stratosphere to be present. Due to the relatively small abundance of TiO in the atmosphere, condensing TiO into particles larger than a few microns is difficult. However, if TiO can be efficiently incorporated into other more abundant condensable species such as silicates, then it can condense into larger particles. In such a scenario, the nightside cold trap can be effective enough to prevent the formation of a TiO-induced stratosphere on the planet’s dayside.

Reference: Vivien Parmentier, Adam P. Showman and Yuan Lian (2013), “3D mixing in hot Jupiter atmospheres I: application to the day/night cold trap in HD 209458b”, arXiv:1301.4522 [astro-ph.EP]