The Caldwell Objects

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Observatory. Indeed, no matter what telescope or
equipment was employed in the years to follow,
the "star" always appeared to be associated with a
distinct mass of dense nebulosity, at least in
visible light. By 1984 astronomers were
suggesting that a disk containing perhaps five
Earth masses of dust surrounded the actual R
Monocerotis star. NGC 2261 thus became a prime
candidate for a site of planetary formation. The
illuminating star was believed to be in an
evolutionary state similar to that of our Sun some
4 billion years ago.
Astronomers continue to unravel the mysteries
that have veiled this enigmatic object for
two centuries. As explained in the November 1,
1997, Astrophysical Journal, Laird M. Close
(University of Hawaii) and his colleagues have
used the Hubble Space Telescope to confirm that
R Monocerotis cannot be seen directly at visual
wavelengths because dense clouds of dust and
gas shield it from view. However, the researchers
were able to penetrate the dust with an infrared
camera and an adaptive-optics system on the 3.6meter
Canada-France-Hawaii Telescope (CFHT),
and they found R Monocerotis to be a binary star
about 2,500 light-years distant. The primary star,
they concluded, is probably a young, hot B-type
luminary 10 times more massive than our Sun,
with matter accreting onto its surface. The
secondary star lies just 0.7" to the west of the
primary and is 200 times fainter; it is probably an
ordinary Τ Tauri star only 300,000 years old, with
a mass of 1.5 Suns. Each star is buried in a dense
doughnut of dust too small to be resolved by
even the Hubble Space Telescope.
The HST and CFHT observations also
suggest that the familiar fan-shaped nebula is
really a large cone hollowed out by a jet of hot
gas flowing out at right angles to the primary's
disk, which is inclined some 20° from edge on.
Deep within the cone, only 1 or 2 a.u. from the
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star, dark filaments spiral around the cone's walls
like a double helix. Close believes these rotating
filaments may follow twisted magnetic field
lines, casting fleeting shadows on the walls of the
surrounding cone. The patterns we see in NGC
2261 are complicated by the irregular shapes of
the nebula's walls and by additional shadows
cast by other very sluggish filaments within the
shell. Close likens this
interplay of light and shadow to that of a rotating
lighthouse beam and cloud banks at different
distances. There is probably a symmetrical
counterpart to the fan-shaped nebula on the
southern side of the star, but it is heavily
obscured from view by dust.
Of course, the dynamics of this fascinating
object remain subjects for the imagination rather
than the eye. Nevertheless, the nebula is a
pleasure to view in telescopes of all sizes. As the
Herschel and NGC descriptions imply, NGC 2261
looks like a comet with a fuzzy 10th-mag-nitude
head and a fan-shaped dust tail. So it's not hard
to imagine how the sight of NGC 2261 through
backyard telescopes has caused many hearts to
flutter; the nebula has probably been mistaken
for a comet more than any other deep-sky object,
and continues to fool (albeit tem-
Deep-Sky Companions: The Caldwell Objects