Lack of Emissions
European Southern Observatoryhttp://www.eso.org/
A Spectrum of
11 September 1995
, almost 1,000 million km. Observations have shown that this coma consists of
dust particles of different sizes that have been ejected from the comet's nucleus.
- Comet Hale-Bopp is known to possess a bright coma, despite its large distance from
- But what is causing this outflow of the dust? Most comet specialists believe that a particular
gas must act as the "agent". When molecules evaporate from the solids in the nucleus, either due
to the heating action of the infalling sunlight, or because of some unknown inner energy-
producing mechanism, then they take dust particles along and push them outwards. Although we
may not see the gaseous molecules that quickly diffuse into the surrounding space, we do observe
the reflection of the sunlight from the much larger, slower moving dust particles.
This would also prove the
presence of such material in the nucleus.
- Astronomers would like to know which gas is responsible for the great amount of
dust, now observed around the nucleus of Comet Hale-Bopp.
- Comets are believed to consist mostly of water ice (H2O), but the temperature at the present
distance from the Sun (~ 125 K, or -150 C) is too low for any considerable amount of water ice to
evaporate. Thus water molecules are unlikely to be involved in this process. Cyanic
acid (HCN), carbon monoxide (CO) or carbon dioxide (CO2) are more likely
- However, when they lave the nucleus, these molecules are broken down and/or ionized rather
quickly by the sunlight and/or the solar wind particles. Their presence may instead be indicated
by the detection of CN (radical) and CO+ (ion). Both of these molecules emit radiation in the
ultraviolet part of the spectrum; CN at ~ 3883 A (388.3 nm) and CO+ at ~ 4010 and ~ 4260 A,
respectively. One way to decide which gas has caused the observed dust outflow is therefore to
obtain a spectrum of Comet Hale-Bopp covering this spectral region.
- Press Photo 27/95 [87K] shows such a spectrum, after it has been subjected to preliminary
image processing. It is based on a Hale-Bopp spectrum, obtained in the morning of
September 5, 1995, by visiting astronomer Birgitta Nordström (Copenhagen
University Observatory, Denmark) with a CCD on the Boller & Chivens spectrograph at the ESO
1.5-metre telescope at La Silla. The exposure lasted 30 minutes and the slit
was placed in the East-West direction. Hector Vega (ESO) assisted during this
- This file was transmitted together with the calibration files to Heike Rauer (Observatoire de
Meudon, Paris, France), who performed a provisional reduction. In this process, the instrument and sky
artifacts were removed and the comet spectrum was divided with the spectrum of a G0 star, whose
spectrum closely ressembles that of the Sun. This procedure removes as far as possible the contribution
from the sunlight reflected of the cometary dust and enhances any surplus emission, for instance from the
above mentioned gas molecules.
- The spectrum covers the region from about 3848 A (left) to 4841 A (right). The scale is linear
and the pixel size in the direction of dispersion is 3 A. A few vertical, dark lines are seen, for
instance at 3880, 3933, 3969, 4034 and 4695 A; they are caused by incomplete subtraction of the
sky spectrum, recorded simultaneously.
They would have shown up as white lines, extending
above and below the otherwise "flat" spectrum. The sensitivity of this observation was apparently
not sufficient to detect such emissions, if present at all. Indeed, this equipment was not
optimized for this particular kind of observation which was made in the course of another
observing programme, concerned with galactic stars of low metal abundance.
- As can be seen, there is no sign of CN and CO+ emission lines at the indicated
wavelengths in this spectrum.
- This negative result may be interpreted in several ways. Either there has always been very
little gas of this type in Comet Hale-Bopp and another, so far unknown agent is active, or the gaseous
outflow has stopped in the meantime. Other observations are needed to cast more light on this
European Southern Observatoryhttp://www.eso.org/
- The ESO observations are of many different types and have involved many observers. At the
15-metre Swedish-ESO Submillimetre Telescope (SEST), Albert Nummelin, Anne-Marie
Lagrange and Thierry Forveille searched on August 3-4 and 9-10 for emissions from
the CO molecule. According to one theory, CO gas may possibly be the driving agent
that is responsible for `lifting' dust particles off a comet's nucleus when it is more than about 750
million kilometres from the Sun. However, no emission from CO was seen to
the sensitivity limit of these observations, thus placing important constraints on the proposed
For instance, in Comet Halley, emissions from CN were first seen
at a heliocentric distance of about 725 million kilometres. It would therefore be of great interest to learn
whether CN is already now present in the coma of Comet Hale-Bopp. Spectroscopic observations
with the ESO 1.5-metre telescope were performed by Anne-Marie Lagrange, Jean Luc Beuzit,
Stephane Guisard and Pierpaolo Bonfanti on August 3-4 and 9-10. They have now been reduced
and do not show any such emission. At the present distance of the comet from the Sun, the
temperature is too low for water ice (the major component of cometary nuclei) to evaporate efficiently,
and with the non-detection of CO and CN, the driving gas that has produced the well visible
dust cloud around the nucleus of Comet Hale-Bopp is still unknown.
- Normally, CN is one of the first gaseous molecules to be detected in the coma of
comets approaching the Sun.