Monday, July 21, 2014

Hot-Jupiters around Red Giant Stars

Figure 1: Artist’s impression of a gas giant planet.

Figure 2: Artist’s impression of a gas giant planet.

After ~6 billion years or so, the Sun will start running out of hydrogen in its core and being to enter its post-main-sequence phase of evolution characterised by a large increase in its luminosity. All the planets circling the Sun will receive much greater insolation than they do now. Presently, Jupiter orbits the Sun at a distance of roughly 5 AU, where 1 AU is the average Earth-Sun separation distance. When the Sun enters post-main-sequence evolution, Jupiter might become so intensely irradiated that it becomes a “hot-Jupiter”.

This occurs because the Sun’s luminosity will increase by a factor of several thousand during two stages in its post-main-sequence evolution - the red giant branch (RGB) stage, followed by the asymptotic giant branch (AGB) stage. During the RGB stage, the Sun’s interior is characterised by an inert helium core surrounded by a hydrogen-burning shell (i.e. hydrogen fusing into helium). For the subsequent AGB stage, the Sun’s interior is characterised by an inert carbon core surrounded by a helium-burning shell (i.e. helium fusing into carbon), and a hydrogen-burning shell.

A study by Spiegel & Madhusudhan (2012) show that Jupiter’s atmosphere can be transiently heated to temperatures of up to ~1000 K or more when the Sun goes through its RGB and AGB stages. Many of the currently known Jupiter-mass planets in wide, several-AU orbits around Sun-like stars (i.e. stars between 1 to 3 times the Sun’s mass) will also experience such a temperature increase when their host stars evolve off the main-sequence. The authors term such planets “red giant hot-Jupiters” (RGHJs) to distinguish them from typical hot-Jupiters that circle in short-period, close-in orbits around main-sequence stars.

Figure 3: Artist’s impression of a gas giant planet.

Figure 4: Orbital separations where RGHJs can be found around a Sun-like star. The first rise in temperature corresponds to the RGB phase and the second rise corresponds to the AGB phase. Spiegel & Madhusudhan (2012).

Gas giant planets like Jupiter start out warm and gradually cool over time. However, the intense heating a RGHJ receives from its post-main-sequence host star could “reset” its evolutionary clock. As a result, RGHJs or post-RGHJs could appear younger than they actually are. Nevertheless, there is a major difference between RGHJs and typical hot-Jupiters around main-sequence stars. Typical hot-Jupiters trap some fraction of incident stellar irradiation to produce bulk heating (i.e. internal heating) on timescales spanning tens of millions of years, allowing these objects to settle into a quasi-steady thermal state. By comparison, RGHJs do not have the luxury of time since the RGB and AGB phases last nowhere as long. A RGHJ would not be intensely irradiated for a long enough time to produce any significant bulk heating, even though their outer layers might appear strongly heated.

When a star enters the RGB stage, and subsequently, the AGB stage, it will undergo a huge increase in mass loss. The star loses mass in the form of a continuous stellar wind streaming away from the star. Since stellar wind speeds are typically a few times a star’s surface escape velocity, the stellar wind from a post-main-sequence star would travel much slower compared to the stellar wind from a main-sequence star. This is because a post-main-sequence star has puffed up so greatly in size that its surface escape velocity is small.

In fact, measurements of the stellar wind speeds of AGB stars by Zuckerman & Dyck (1989) show velocities less than 40 km/s, with a clustering around 5 to 25 km/s. For comparison, stellar winds from main-sequence stars, such as the present-day Sun, have speeds of a few 100 km/s. A Jupiter-mass planet around a post-main-sequence star would have sufficient gravity to capture and accrete the slow stellar wind flowing pass it. The total accreted mass is estimated to be of order ~1/10,000th of the planet’s mass for a Jupiter-mass planet.

The stellar wind streaming away from a post-main-sequence star can impose enough drag to change the orbits of small objects circling the star. These objects could then impact the RGHJ and enhance the abundance of heavy elements in the planet’s atmosphere. Furthermore, a post-main-sequence can exhibit a high carbon-to-oxygen ratio due to carbon dredged-up from the star’s interior. A RGHJ accreting the stellar wind from such a star could acquire a carbon-rich atmosphere.

Figure 5: Artist’s impression of a gas giant planet.

 Figure 6: Artist’s impression of a gas giant planet with a few of its moons appearing as points of light.

The intense stellar irradiation experienced by a RGHJ means that wind speeds in its atmosphere are expected to be faster than on present-day Jupiter. However, wind speeds on a RGHJ would not be as strong as on a typical hot-Jupiter around a main-sequence star because a RGHJ is not tidally-locked, and consequently, would not have a large enough day-night temperature contrast to drive strong winds. Roughly estimating, the expected wind speeds for present-day Jupiter, a RGHJ and a typical hot-Jupiter are 40 m/s, ~100 m/s and ~1000 m/s respectively.

Changes in the incident stellar irradiation around an evolving post-main-sequence star can cause interesting changes in the atmospheric chemical properties of a RGHJ. The present-day Jupiter has an atmospheric temperature of 165 K at the 1 bar level. As the Sun’s luminosity increases considerably during its post-main-sequence phase, an important change for a soon-to-be RGHJ would be the enhancement of the H2O abundance in the planet’s atmosphere as it becomes warm enough (i.e. ~300 K) for water-ice to sublimate. The abundance of H2O drops slights for a brief period during the RGB stage when atmospheric temperatures on the RGHJ exceed ~600 K. At such temperatures, some of the oxygen in H2O becomes bounded in silicates. The same drop in H2O abundance might also occur when temperatures rise again during the AGB stage.

Figure 7: Post-main-sequence evolution of Jupiter’s equilibrium temperature and atmospheric chemical composition. Spiegel & Madhusudhan (2012).

Figure 8: Example spectra of Jupiter as a function of equilibrium temperature, as seen from a distance of 5 AU. Spiegel & Madhusudhan (2012).

- Spiegel & Madhusudhan (2012), “Jupiter will become a hot Jupiter: Consequences of Post-Main-Sequence Stellar Evolution on Gas Giant Planets”, arXiv:1207.2770 [astro-ph.EP]
- Zuckerman & Dyck (1989), “Outflow Velocities from Carbon Stars”, Astronomy and Astrophysics, 209, 119-125