HD 7449Ab: Choreography of a Planetary Dance

Given this site’s predilections, it’s natural to think of Centauri A and B whenever the topic of planets around close binary stars comes up. But systems with somewhat similar configurations can produce equally interesting results. Take what we’re finding around the G-class star HD 7449, some 127 light years from our Sun. In 2011, a planet of roughly eight times Jupiter’s mass was found orbiting the star in an orbit so eccentric that it demanded explanation. A highly eccentric orbit can indicate another object in the system that is affecting the planet.

Exactly what has now been determined. “The question was: is it a planet or a dwarf star?” says Timothy Rodigas (Carnegie Institution for Science), who led the work on the discovery. Rodigas’ team went to work using the Magellan adaptive optics system (MagAO) on the Magellan II (Clay) instrument at Las Campanas in Chile. MagAO allows sharp visible-light images to be acquired, with the instrument capable of resolving objects down to the 0.02 arcsecond level.

The object near HD 7449 was quickly spotted and we learn that the system has a second star, an M-class dwarf. So now we have HD 7449A and the dwarf secondary, HD 7449B, along with the gas giant HD 7449Ab. HD 7449B is a tiny object with about one-twentieth the Sun’s mass, and it’s close enough to the primary (18 AU) to evoke that Alpha Centauri comparison I made above — Centauri A and B move between 11.4 and 36.0 AU as they orbit. I’ll grant that the comparison is strained by the fact that Centauri B is a K-class dwarf, while HD 7449B is a far smaller M-dwarf. Moreover, we have nothing like planet HD 7449Ab in the Alpha Centauri system.

What we’re seeing in HD 7449Ab is a planet that in the words of Rodigas is “‘dancing’ between the two stars.” The gravitational influences at work here evidently go back millions of years. The system, in fact, may be showing us the so-called Kozai mechanism at work. First described by the Russian astronomer Michael Lidov and later by Japanese researcher Yoshihide Kozai, the Kozai mechanism is one factor that can shapes the orbits of multiple-star systems. We see an oscillation between the planet’s orbital eccentricity and its orbital inclination, the ‘dance’ that Rodigas refers to. Have a look at the team’s visualization of the effect.

Image: This animation shows the Kozai mechanism at work in the HD 7449 system. Credit: Carnegie Institution for Science.

From the paper:

If the planet and outer companion were initially on mutually-inclined orbits of at least 39.2°, then the planet’s eccentricity and inclination would oscillate with oppositely-occurring minima and maxima (Holman et al. 1997). Based on the nominal parameters for the planet and M dwarf companion, the length of a Kozai cycle would be ∼ a few hundred years, which is certainly short enough to be plausible given the age of the system (∼ 2 Gyr).

The Kozai mechanism is not the only explanation for the planet’s eccentric orbit, but the paper argues that it is the most convincing:

Another explanation for the planet’s large eccentricity is planet-planet scattering in the inner parts of the system (e.g., Rasio & Ford 1996). In this case, one or more planets may have been ejected from the system, leaving behind the eccentric HD 7449Ab. This scattering scenario would require both the surviving planet and the scattered planet to be relatively massive (7–10 M J ) and the eccentricity damping of the original circumstellar disk to be small (Moorhead & Adams 2005). Given the “smoking gun” (the nearby M dwarf companion), it seems more likely that Kozai cycles are responsible.

The paper calls for continuing monitoring of this system by both radial velocity and direct imaging methods, the latter important because it can provide further constraints on the planet’s orbital eccentricity and inclination, allowing for a more accurate estimate of its mass. The dwarf star is itself interesting, the paper noting that it can become a benchmark object for studies of stellar structure. What this system gives us is a rare case of an M-dwarf with a measurable age (via the primary) and a mass that will be measured by astrometric means. Spectroscopic follow-up should constrain metallicity, improving structure models for similar cool stars.

The paper is Rodigas et al., “MagAO Imaging of Long-period Objects (MILO). I. A Benchmark M Dwarf Companion Exciting a Massive Planet around the Sun-like Star HD 7449,” accepted at The Astrophysical Journal (preprint). Thanks to Dave Moore for alerting me to this story.