A team of astronomy researchers from Stony Brook University, the National Astronomical Observatory of Japan, and Tsuru University are the first to reveal clear details about the rapidly changing plasma tail of the comet C/2013 R1 (Lovejoy). The observation and details behind the discovery are published in a paper in the March 2015 edition of the Astronomical Journal.
The team, Led by Jin Koda, PhD, Assistant Professor in the Department of Physics and Astronomy at Stony Brook University, captured the images by using the Subaru Telescope’s wide-field prime-focus camera, called Suprime-Cam, which resulted in gaining new knowledge regarding the extreme activity in that tail as the comet neared the Sun.
“My research is on galaxies and cosmology, but I always want to explore beyond these boundaries. Lovejoy was up in the sky after my targets were gone, and we started taking other images for educational and outreach purposes, and for curiosity,” said Dr. Koda. “The single image from one night revealed such delicate details along the tail that it inspired us further to take a series of images on the following night. When we analyzed these additional images, we realized that the tail was displaying rapid motion in a matter of only a few minutes. This was an incredible discovery.”
In their paper, “Initial Speed of Knots in the Plasma Tail of C/213 R1 (Lovejoy),” the researchers report short-time variations in the plasma tail of Lovejoy.
They suggest that “these rapid motions suggest the need for high time-resolution studies of comet plasma tails with a large telescope,” and that, “A series of short (2-3 minutes) exposure images with the 8.2 m Subaru telescope shows faint details of filaments and their motions over a 24 minutes observing duration. We identified rapid movements of two knots in the plasma tail near the nucleus. Their speeds are 20 and 25 kms along the tail and 2.8 and 2.2 kms across it respectively. These set a constraint on an acceleration model of plasma tail and knots as they set the initial speed just after their formation. We also found a rapid narrowing of the tail.”
Dr. Koda explained that the plasma tail of a comet forms when gas molecules and atoms coming out from the comet encounter the solar wind. Changes and disturbances in the solar wind can affect the behavior and appearance of this plasma tail, causing it to form clumps of ionized material. The material in the plasma tail departed from the comet’s coma and floats away on the solar wind. At these times, the plasma tail can take on a “kinked” or twisted look.
In 2013, the team reported highly resolved fine details of this comet captured in B-band filter in Subaru Telescope’s Image Captures the Intricacy of Comet Lovejoy’s Tail. They used I-band filter which includes H2O+ line emissions and V-band filter which includes CO+ and H2O+ line emissions. During the observations, the comet exhibited very rapid changes in its tail in the course of only 20 minutes (Figure 1). Such extreme short-term changes are the result of the comet’s interactions with the solar wind where charged particles constantly sweeping out from the Sun. They explain that the reason for the rapidity of these changes is not well understood.
By using the Subaru Telescope, they also discovered that clumps located in the plasma tail at about 300 thousand kilometers from the nucleus moved fairly slow speed at about 20-25 kilometers per second (Figure 2). That is much slower than reported in other comets, such as P/Halley, which gave off clumps that moved as fast as 58 kilometers per second or the value 44 +/- 11 kilometers per second (Note 2) as derived from several bright comets in the past.
The speed of the solar wind ranges from 300 to 700 kilometers per second, and the intensity and velocity that the comet encounters depends on where it is located with respect to the Sun. The solar wind helps to accelerate the clumps in the tail out away from the Sun. Dr. Koda explained that eventually the clumps in the comet’s tail reach this high speed.
The observation team believes they witnessed the beginning of the acceleration of the clumps by the solar wind, however it is still under investigation how these ion clumps form and what parameters determine the initial speed of them.
The team concluded that because of the Subaru Telescope capacity for large photon collection coupled with the wide field-of-view camera they were able to and fortunate enough to catch the rare tail condition before it disappeared. Dr. Koda says their discovery is the first such demonstration underscoring the need for use of a large telescope to capture rapid motions of comets’ tails in action. They also conclude that with such a powerful instrument, more observations will help to contribute to the better understanding of comets. Such observations would include a series of images for longer periods of time, which would help the team learn more about how the comet tail moves and evolves.