Figure 1: Mid-infrared image of Comet 17P/Holmes on
October 25, 2007. The mid-infrared observations of the comet by COMICS
on the Subaru Telescope were carried out for four consecutive nights
from two days to five days after its great outburst on October 23, 2007
UT. (Credit: NAOJ)
A team of astronomers led by Yoshiharu Shinnaka of Koyama
Astronomical Observatory of Kyoto Sangyo University, Japan, has
discovered that Comet 17P/Holmes formed in a cold region of the solar
nebula far from the Sun. This suggests that Comet 17P/Holmes probably
includes highly volatile species abundantly (with low sublimation
temperatures below ~50 K) and that sublimation of these volatiles could
be responsible for the comet's explosive releases of dust grains. These
new insights come from re-analysing mid-infrared spectra of the comet
taken by the Cooled Mid-Infrared Camera and Spectrograph (COMICS) on the
Subaru Telescope during October 25 to 28, 2007.
Comet 17P/Holmes is a short-period comet with an orbital period of ~7
years. The comet underwent a great outburst starting on October 23,
2007 (when the comet was 2.5 au from the Sun), five months after
perihelion at 2.05 au on May 5, 2007. The total magnitude of this
outburst reached a maximum brightness of ~2-3 mag in V-band within two
days after the outburst, increasing from an initial brightness of ~17
mag. This huge outburst, with a 15 mag brightness increase, was unlike
any other. Other than the outburst in 2007, Comet 17P/Holmes had
exhibited outbursts in November 1892, when the comet was discovered by
E. Holmes, and January 1893.
This research focuses on the dust components released from Comet
17P/Holmes. A cometary nucleus includes minerals called silicates.
Cometary silicates consist of both amorphous and crystalline forms.
Silicates in amorphous form exist in interstellar space and might have
been incorporated into the cometary nucleus in the solar nebula in the
early Solar System. Meanwhile, it has been thought that crystalline
silicates formed by the annealing of amorphous silicate grains or direct
condensation of gaseous materials in hot regions of the solar nebula
near the Sun and were incorporated into cometary nuclei in the cold
comet-forming region (~5—30 au from the Sun) after radial transportation
of the silicate grains in the solar nebula. Because the mass fraction
of crystalline silicates with respect to the total (amorphous +
crystalline) silicates is expected to be smaller for further distances
from the Sun in the solar nebula, it is thought that a smaller mass
fraction of crystalline silicates in a comet indicates that the comet
formed at a further distance from the Sun in the solar nebula.
The researchers found that dust grains of Comet 17P/Holmes contain a
large amount of amorphous silicates (less crystalline silicates)
compared with grains of other comets. This result is evidence that Comet
17P/Holmes formed in a farther, colder region in the solar nebula than
other comets. At such a region, it is expected that much CO ice (which
has a low sublimation temperature of ~30 K) and amorphous water ices
(which through crystallization in the low temperature conditions become
an energy source for explosive sublimation) would have existed.
Figure 2: The mid-infrared spectrum of Comet 17P/Holmes
on October 25, 2007. Four striking thermal emission features of
silicate dust grains are seen around 10 microns. Narrow peaks originate
from crystalline silicate and the peak position depends on the Mg/Fe
ratio of the silicate grains. The sets of six vertical lines (red,
orange, green, blue, purple, and black in order from left to right)
indicate peak wavelengths of the 10.0, 10.4, 11.2 and 11.9 microns lines
with Mg/(Mg + Fe) ratios of 100% to 0% in 20% intervals. The vertical
gray bar is the absorption band of telluric ozone (O3). (Credit: NAOJ)
These results were published on October 31, 2018 in The Astronomical Journal (Shinnaka et al., 156, 242, "Mid-infrared Spectroscopic Observations of Comet 17P/Holmes Immediately After Its Great Outburst in 2007"). This research paper is also available as a preprint (Shinnaka et al., arxiv: 1808.07606)
on arxiv.org. This research is supported by Grants-in-Aid from Japan
Society for the Promotion of Science Fellows, 15J10864 (YS) and for
Scientific Research (C), 17K05381 (TO). This paper is based on data
collected at the Subaru Telescope and obtained from SMOKA, which is
operated by the National Astronomical Observatory of Japan.
Links:
Source: Subaru Telescope
