Figure 1：This GIF animation shows changes in the global
structure of Comet Lovejoy's (C/2013 R1) plasma tail. There are three,
2-minute exposures taken in the I-band. The image is aligned so that the
nucleus of the comet is at the same position and the tail lies
vertically. The time stamp at the bottom right shows the start time of
each exposure in Hawai'i time on the morning of December 4, 2013. Bright
parts of the sky are shown as black, and dark parts are shown as white,
allowing astronomers to see details in the object more clearly. The
white tilted grid is a gap between CCD detectors. In the image, the tail
narrows with time, especially downstream of the nucleus (which is at
the bottom of the image). Moreover, two clumps were detected forming
formed at about 0.3 million kilometers from the nucleus. They drifted
toward downstream about 20 - 25 kilometers per second (Figure 2). (Credit: NAOJ)
Figure 2: (left): A 2-second I-band exposure of the
comet. The cyan rectangle shows the region in the right panel. (right):
This shows movement of two clumps in the plasma tail. Images produced
from 2-minute exposures are further processed; background star trails
are masked, and unsharp-masked to enhance detailed structures. The
masked star trails are seen as short tilted white lines. Time stamps in
yellow show the start time of the exposure. White circles indicate the
clumps detected in this study. They move away from the nucleus over
time. The size of the cutout is about 2500 x 5600 kilometers. From the
data, the research team calculated the speed of the clumps at 20 - 25
kilometers per second. (Credit: NAOJ)
Images from a December 2013 observation of the comet C/2013 R1 (Lovejoy) (Note 1)
reveal clear details about rapidly changing activity in that comet's
plasma tail. To get this image, astronomers used Subaru Telescope's
wide-field prime-focus Suprime-Cam to zero in on 0.8 million kilometers
of the comet's plasma tail, which resulted in gaining precious knowledge
regarding the extreme activity in that tail as the comet neared the
Sun. Their results are reported this week in a paper in the March 2015
edition of the Astronomical Journal.
Team of researchers from National Astronomical
Observatory of Japan, Stony Brook University (The State University of
New York) and Tsuru University reported highly resolved find details of
this comet captured in B-band in 2013 (Subaru Telescope's Image Captures the Intricacy of Comet Lovejoy's Tail). They used I-band filter which includes H2O+ line emissions and the 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, which consists of charged particles
constantly sweeping out from the Sun. The reason for the rapidity of
these changes is not well understood.
Dr. Jin Koda, the principal investigator of these
nights, says "My research is on galaxies and cosmology, so I rarely
observe comets. But Lovejoy was up in the sky after my targets were gone
on our observing nights, and we started taking images for educational
and outreach purposes. The single image from the previous night revealed
such delicate details along the tail it inspired us further to take a
series of images on the following night. As we analyzed the images, we
realized that the tail was displaying rapid motion in a matter of only a
few minutes! It was just incredible!"
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 departs 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.
A good candidate for a detailed study of activity in
the plasma tail must be a bright comet with an orbit that takes it close
enough to the Sun to form such a tail. In addition, the best viewing
angles for astronomers to capture views of plasma tail changes occur
when the comet also approaches close to Earth. As a result, comets that
allow good viewing of the plasma tail are relatively rare - about one or
two per year. During its passage, Comet Lovejoy's plasma tail was
almost perpendicular (83.5 degrees) to the line of sight from Earth.
That made it a prime candidate for close-up observations of its plasma
tail structure using Suprime-Cam.
Another discovery is that clumps located in the
plasma tail at about 300 thousand kilometers from the nucleus moved at a
fairly slow speed -- 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 wind 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. Eventually the clumps in the comet's tail reach this high speed.
The observation team thinks they witnessed the beginning of the
acceleration of the clumps by the solar wind.
It is still under study how these ion clumps form and
what parameters determine the initial speed of them. Dr. Masafumi Yagi,
the first author of the paper noted "Comets are often observable only
during the twilight as they come near the Sun. On the other hand, it
becomes difficult to observe faint objects like galaxies during the
twilight hours because of the brighter sky background. Well-designed
telescope scheduling like this case makes an effective use of the Subaru
Telescope's time and will enable us to collect more data of comets when
the opportunity arises in the future."
The team's research paper titled “Initial Speed of
Knots in the Plasma Tail of C/2013 R1 (Lovejoy) will be published in
Astronomical Journal in its March 2015 issue.
Authors:
- Masafumi Yagi (National Astronomical Observatory of Japan)
- Jin Koda (Stony Brook University)
- Reiko Furusho (National Astronomical Observatory of Japan and Tsuru University)
- Tsuyoshi Terai (National Astronomical Observatory of Japan and Subaru Telescope)
- Hideaki Fujiwara (Subaru Telescope)
- Jun-ichi Watanabe (National Astronomical Observatory of Japan)
Notes:
- Comets are generally referred to by designations according to the
order of discovery while IAU adopted a guideline that comets are named
for the family name of their individual discoverer(s) (guideline by IAU).
There are several bright "comet Lovejoy", which were discovered by Mr. Terry Lovejoy. For example, C/2014 Q2 (Lovejoy), which is still bright at the time of this press release (March 2015) (Jan 16, 2015 Comet Lovejoy Photographed at ALMA OSF, and C/2011 W3 (Lovejoy), which approached the Sun in 2012 Jan 05, 2012 Comet Lovejoy and Southern Cross are different from C/2013 R1 (Lovejoy) in this study. - References
Saito et al. 1987, "Structure and dynamics of the plasma tail of comet P/Halley. I - Knot event on December 31, 1985", Astronomy and Astrophysics, 187, 209.
Niedner 1981, "Interplanetary gas. XXVII - A catalog of disconnection events in cometary plasma tails", Astrophysical Journal Supplement Series, 46, 141.
Source: Subaru Telescope
