Press Release

Subaru Measures the Spin-Orbit Alignment in a Faint Transiting Extrasolar Planetary System

August 23, 2007

A Japanese/US collaboration led by a researcher from the University of Tokyo observed the transiting extrasolar planetary system TrES-1 and measured the angle between the stellar spin axis and the planetary orbital axis using the Subaru Telescope High Dispersion Spectrograph (HDS). By measuring the degree of alignment in transiting systems, one can constraint planetary formation models proposed to explain the diverse properties of extrasolar planets. It was the third case and the faintest target so far for which the spin-orbit alignment has been measured.

There are over 200 extrasolar planets that have been discovered so far. The discovery and characterization of extrasolar planets have revealed a diversity of planetary systems, and variety of theoretical models have been proposed to explain the complex process of planet formation. The alignment of the stellar spin axis and the planetary orbital axis (Figure 1) is known to be a promising diagnostic for discriminating planet formation mechanisms by observations. For example, models considering giant planet scattering naturally predict tilts from the original orbital axis and thereby such planets would usually have significant misalignments. However planets which form and migrate inward within proto-planetary disks would generally have negligible misalignments.

In transiting extrasolar planetary system (Note 1), one can measure the spin-orbit alignments by exploiting the Rossiter-McLaughlin effect (Figure 2). Such measurements have been conducted for two bright transiting systems with the Keck Telescope by an international research team led by Prof. Joshua Winn, one of the co-investigators of the Subaru team, at MIT. On the other hand, a Japanese/US collaboration led by a graduate student Norio Narita at the University of Tokyo (Japan Society for Promotion of Science Fellow, DC2) observed a faint transiting system TrES-1 (Note 2) with the Subaru Telescope High Dispersion Spectrograph (HDS) at Mauna Kea and the MAGNUM 2 m Telescope at Haleakala, both in Hawaii (Figure 3). The team succeeded in detecting the Rossiter-McLaughlin effect (Figure 4) for the first time for this target and constrained the alignment angle as 30 degrees with an error of plus or minus 21 degrees, clearly indicating the prograde orbital motion of TrES-1b.

It was the third case for which the spin-orbit alignment has been measured, and importantly it is the faintest target (with a visual magnitude of about 12, which is only about 2 % as bright as the previous targets) so far. The team has demonstrated for the first time that such measurements are possible for such a faint target. This is important because most of the newly discovered transiting planets from ongoing transit surveys have relatively faint host stars. Norio Narita says, "by combining future observations of the Rossiter-McLaughlin effect in other transiting systems, we will be able to determine the distribution of the spin-orbit alignment angles for exoplanetary systems. Moreover, further observations would have the potential to discover large spin-orbit misalignments, if any, which would inspire numerous theoretical investigations."

This result will be published in the August 25, 2007 issue of Publications of Astronomical Society of Japan.

Measurement of the Rossiter--McLaughlin Effect in the Transiting Exoplanetary System TrES-1
Narita, N., Enya, K., Sato, B., Ohta, Y., Winn, J. N., Suto, Y., Taruya, A., Turner, E. L., Aoki, W., Tamura, M., Yamada, T., Yoshii, Y. 2007, Publ. Astron. Soc. Japan, vol 59, No. 4, 763-770

Note 1: Extrasolar planetary systems in which a planet's orbit passes in front of its host star (namely, causing eclipse) are called transiting extrasolar planetary systems.

Note 2: TrES-1 is a main sequence K0 star and its planet TrES-1b was discovered by a transit survey in 2004. TrES-1b is a gaseous giant orbiting the host star with a period of about 3 days (one of the so called "hot Jupiter" class of extrasolar planets).

Figure1: An illustration of the concept of the spin-orbit alignment (described by lambda) in an exoplanetary system.

Figure 2: The Rossiter-McLaughlin effect is defined as the radial velocity anomaly during a transit from the known Keplerian orbit caused by the partial occultation of the rotating stellar disk. For example, if a planet occults part of the blue-shifted (approaching) half of the stellar disk, then the radial velocity of the star will appear to be slightly red-shifted, and vice-versa. The radial velocity anomaly depends on the trajectory of the planet across the disk of the host star, and in particular on the spin-orbit alignment of the system. Thus by monitoring the Rossiter-McLaughlin effect one can measure the spin-orbit alignment.

Figure 3: A photometric light curve of TrES-1 from the MAGNUM observation (top), and radial velocities obtained with the Subaru/HDS (bottom). The light curve shows that the observations were conducted around a transit of TrES-1b.

Figure 4: Orbital plots of TrES-1 radial velocities and the best-fitting models including the Kepler motion and the Rossiter-McLaughlin effect. Left panel: A radial velocity plot for the whole orbital phase. Right panel: A close-up of the radial velocity plot around the transit phase. The waveform around the central transit time (phase = 0) is caused by the Rossiter-McLaughlin effect.

Bottom panels: Residuals from the best-fit curve.



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