[Prev][Next][Index][Thread]

(meteorobs) Fwd: New Observations Of Comet Hale-Bopp From La Silla


------- Forwarded Message

Date: Thu, 22 Oct 1998 17:57:25 GMT
From: Ron Baalke <BAALKE@kelvin.jpl.nasadot gov>
To: mplist@bitnik.com
Subject: New Observations Of Comet Hale-Bopp From La Silla


ESO Press Release 16/98

For immediate release: 22 October 1998

New Observations of Comet Hale-Bopp from La Silla
http://www.eso.org/outreach/press-rel/pr-1998/pr-16-98.html

Methanol and Hydrogen Cyanide Detected at Record Distance

Observations of famous Comet Hale-Bopp continue with the 15-m Swedish-ESO
Submillimetre Telescope (SEST) at the La Silla Observatory. They show
amazingly strong activity of this unusual object, also at the present, very
large distance from the Sun.

The radio observations document in detail the release of various molecules
from the comet's icy nucleus. Of particular interest is the observed
emission from methanol (CH3OH) and hydrogen cyanide (HCN) molecules, never
before detected in any comet this far away.

Comet Hale-Bopp still going strong

Just over 18 months after its perihelion passage on April 1, 1997, Comet
Hale-Bopp (official designation C/1995 O1) is continuing its outward journey
through the Solar System. It is now about 1,000 million kilometres (6.7 AU)
from the Sun and the Earth, i.e. almost at the same distance as when it was
first discovered in July 1995.

After having traversed the northern sky in 1996 and 1997, the comet passed
the celestial equator in late June 1997 and is now seen in the southern
constellation Volans (The Flying Fish), i.e. just east of the Large
Magellanic Cloud. It can only be observed from southern latitudes.

The comet's brightness has decreased by a factor of more than 10,000 since
it was at its brightest in March 1997, just before perihelion. However, the
magnitude is still around 9-10, or only about 20-40 times fainter than
what can be seen with the unaided eye. Hale-Bopp is therefore visible in
binoculars to southern observers as a fuzzy object with a diameter of a few
arcminutes.

New observations from La Silla

Several telescopes at La Silla are following the evolution of the activity
of Comet Hale-Bopp as it recedes from the Sun. In particular, the comet is
observed monthly with SEST, a 15-m diameter submillimetre telescope operated
jointly by the Onsala Space Observatory (OSO, Chalmers University of
Technology, Gothenburg, Sweden) and ESO; it is the only telescope of its
type in the southern hemisphere.

Alternating each month, a Swedish team (headed by Anders Winnberg, OSO) and
a European team (headed by Dominique Bockelie-Morvan, Observatoire de Paris)
observe emission lines in the radio region of the spectrum from some of the
molecules in the comet's coma (the cloud of gas and dust around the cometary
"dirty-snowball" nucleus). These data are of great importance for
understanding the mechanisms that are responsible for the outgassing
(sublimation) of ices inside the nucleus of Comet Hale-Bopp.

The observations began at SEST in September 1997 and constitute a follow-up
programme of a long-term monitoring project at radio wavelengths that was
started in August 1995 at the telescopes of the Institut de RadioAstronomie
Millimitrique (IRAM), the James Clerk Maxwell Telescope (JCMT), the Caltech
Submillimeter Observatory (CSO) and the Nangay radio telescope by several
teams of astronomers in Europe and US [1]. Radio emission from nine
molecules in the coma were studied: H2O (water; by means of observations of
the radical OH), CO (carbon monoxide), CH3OH (methanol), H2CO
(formaldehyde), HCN (hydrogen cyanide), HNC (isomeric hydrogen cyanide),
CH3CN (methyl cyanide), H2S (hydrogen sulphide) and CS (carbon sulphide).

Detection of methanol and hydrogen cyanide at record distance

  ESO PR Photo 40a/98

  PR Photo 40a/98 displays a part of the radio spectrum with emission from
  CH3OH molecules in the coma of Comet Hale-Bopp, as observed with the 15-m
  SEST telescope at La Silla from August 16 to 19, 1998. Three lines of this
  molecule were detected at 145.0938, 145.0974 and 145.1032 GHz,
  respectively. The total integration (exposure) time is 708 min. The
  intensity is indicated in units of antenna temperature.

Observations at SEST were performed in July and August 1998 by Emmanuel
Lellouch (Observatoire de Paris) and Marcus Gunnarsson (Uppsala Astronomiska
Observatorium, Sweden), respectively. Three molecules were still detected:
carbon monoxide (CO) at 230 GHz, hydrogen cyanide (HCN) at 89 GHz and
methanol (CH3OH) at 145 GHz. On August 11, when Hale-Bopp was just over 900
million km (6 AU) from the Sun, no less than 2.4  10^28 CO molecules were
released by the comet per second, corresponding to 1100 kg per second.

The measured production rates of HCN and CH3OH were about 200 and 20 times
smaller, respectively. The observations of these two organic species at SEST
constitute the most distant detections ever made in any comet.

The sublimation of water, the main constituent of cometary ices, is
responsible for cometary activity within 3-4 AU from the Sun. However, at
larger distances, this process ceases, due to the low temperature of the
nucleus. At the present large distance from the Sun, the CO molecule is now
the prime source of activity of Hale-Bopp.

When Comet Hale-Bopp was approaching the Sun before perihelion passage in
1997, the long-term monitoring programmes -- in the radio wavelength region
as well as in other spectral domains -- clearly showed the transition from a
CO- to a water-dominated coma, at about the time the comet came within 3-4
AU from the Sun. The CO-production rate now measured at SEST at 6 AU on the
outward leg is about 100 times less than that at perihelion, and close to
the value measured at the same distance from the Sun before perihelion.

While CO was first detected in Hale-Bopp in September 1995 at 6.8 AU from
the Sun, only a few weeks after the discovery, HCN and CH3OH were not
detected until a few months later, when the comet had approached to within
4.8-4.9 AU. It is likely that the convincing detection of these two
molecules in August 1998 (cf., e.g., PR Photo 40a/98) benefitted from an
outburst (a sudden release of material from the nucleus) on August 15-19.

Some other species were observed at SEST out to a distance of 3-4 AU (H2S,
CS, H2CO), but they are no longer easily detectable due to low production
rates and the SEST sensitivity limit.

New data may provide a "look into the nucleus"

  ESO PR Photo 40b/98

  PR Photo 40b/98 displays Hale-Bopp gas production curves (quantity of
  released gas as a function of heliocentric distance) from radio
  observations at the IRAM, JCMT, CSO, SEST and Nangay telescopes.
  Pre-perihelion data are shown on the left, post-perihelion data on the
  right. Adapted from a figure prepared by Nicolas Biver [2].

Comet Hale-Bopp provided the first opportunity in modern times to follow the
activity of a comet over a very large range of heliocentric distances, cf.
PR Photo 40b/98. The new data trace the gas release in some detail as the
temperature and insolation change when the comet moves along its orbit. They
show similarities and differences between individual molecules that in turn
contain useful information about the physical state of cometary ices in the
nucleus and its internal structure.

Some of the current key questions in this research field are concerned with
the degree of separation of different ices ("chemical differentiation") in
the upper layers of the nucleus, the form under which these ices co-exist
and, not least, the still not understood production mechanisms at large
heliocentric distances. These new observations will provide very valuable
support to the theoretical studies of the cometary nucleus, now being
undertaken by several research groups around the world.

The new observations of molecular lines in the radio spectral region also
provide information about the temperature in the coma, if several lines of
the same species are observed. Moreover, they serve to measure the expansion
velocity of the gas and the outgassing pattern of the nucleus.

For instance, the observations of CH3OH in August 1998 show that the coma is
now very cold at about 16 K (-257 C). At perihelion (0.9 AU from the Sun),
the corresponding temperature was of the order of 110 K (-163 C). The
expansion velocity has also considerably decreased since perihelion, from
1.1 km/sec to 0.5 km/sec. There is also evidence of anisotropic outgassing:
more gas is seen to be flowing out from the sunlit hemisphere of the
nucleus.

Observations continue

The monitoring of Comet Hale-Bopp at the SEST telescope will continue, at
least until March 1999. The comet will then be nearly 1,200 million km (7.9
AU) from the Sun.

  ESO PR Photo 40c/98

  PR Photo 40c/98 shows Comet Hale-Bopp, as imaged on October 19, 1998, in
  visible light and with the DFOSC instrument at the Danish 1.5-m telescope
  on La Silla. At this time, the comet was about 1,000 million kilometer
  (6.7 AU) from the Earth and the Sun. Although well beyond Jupiter's orbit,
  it is very obvious that strong nucleus activity is still present -- the
  large coma extends well beyond the field of view (200 x 200 arcsec or
  about 1 million km at the distance of the comet). The image mostly depicts
  cometary dust that reflects the sunlight. The coma is very asymmetric with
  more material in the northern hemisphere (above). There are also some jets
  embedded in the coma which indicate that some of the dust is emitted from
  active regions on the surface of the nucleus. The background stars are
  slightly elongated since the telescope followed the motion of the comet in
  the sky during the exposure. Technical information: 5-min exposure through
  a broadband V-filtre. North is up, East is left. Observers: Kirsten
  Kraiberg Knudsen (Copenhagen University, Denmark) and Hermann Boehnhardt
  (ESO/Chile)

Observations are also made from time to time with other telescopes at La
Silla. As an example, Photo 40c/98 was obtained a few days ago with the
Danish 1.5-m telescope. It shows that a very complex coma structure is still
present.

Due to the large size of the nucleus, probably 40-60 km in diameter, it
will be possible to observe this comet with large optical telescopes for
many years to come.

Information about Hale-Bopp on the web

Additional information about Comet Hale-Bopp is available on the web at many
sites. Some of the most comprehensive websites may be accessed via the ESO
Hale-Bopp site.

- ------------------------------------------------------------------------

Notes:

[1] Other scientists involved in the long-term radio monitoring of Comet
Hale-Bopp are Nicolas Biver (Institute for Astronomy, University of Hawaii,
USA), Pierre Colom, Jacques Crovisier, Eric Girard, Benoit Germain, Emmanuel
Lellouch (Observatoire de Paris, France), Didier Despois (Observatoire de
Bordeaux, France), Gabriel Paubert (IRAM, Granada, Spain), Raphael Moreno,
Joern E. Wink (IRAM, Grenoble, France), John K. Davies (JAC, Hawaii, USA),
William R.F. Dent (Royal Observatory, Edinburgh, UK), Hans Rickman, Marcus
Gunnarsson (Uppsala Astronomiska Observatorium, Sweden), Per Bergman, Lars
E.B. Johansson (OSO, Sweden), Fredrik Rantakyroe (SEST, La Silla), Darek C.
Lis, David Mehringer, Dominic Benford, Martin Gardner, Tom G. Phillips (CSO,
USA), Heike Rauer (DLR, Berlin, Germany).

[2] The figure appears in N. Biver et al.: "Long-term Monitoring of the
Outgassing of C/1995 O1 (Hale-Bopp) at Radio Wavelengths", a poster paper
presented at the DPS meeting on October 11-16, 1998 (Madison, Wisconsin,
USA) and to be published in Vol. 30 of the Bulletin of the American
Astronomical Society.

------------------------------------------------------------------------

ESO Press Photos may be reproduced, if credit is given to the European
Southern Observatory.

------------------------------------------------------------------------


------- End of Forwarded Message