Our current best PRELIMINARY estimate for the effective diameter of Hale-Bopp's
nucleus is:
If the latter value is correct, then the nucleus of Hale-Bopp is approximately four times larger than that of comet Halley.
However, we must emphasize that measuring Hale-Bopp's size is extremely difficult, even with HST's superb spatial resolution. If you want to learn more about how we derived the above number, you can go directly to the figure from which we estimate the nuclear size . The figure caption describes what assumptions were made and why our estimate could be wrong. We have added a new figure that illustrates a little better HST's possible "photometric" detection of the nucleus.
We should also point out that there is not necessarily a direct relationship between the size of the nucleus and the strength of its activity. We learned from studying comet P/Halley and other periodic comets that, generally, only a small portion of the nuclear surface is "active". A small nucleus that is active over most of its surface area can emit more gas and dust into the coma than a much larger nucleus that is almost completely covered over by an insulating mantle of rubble. Thus, even if our preliminary estimate of the nuclear size is correct, that does not necessarily ensure that the comet will put on a spectacular visual display in early 1997.
On the other hand, a large nucleus (as indicated by the HST observations), and the huge gas and dust production rates at large heliocentric distances (as measured during the ground-based observations of CO and CN), certainly increase the odds that Hale-Bopp will not disappoint us.
Above: The above composite compares the Hubble Space Telescope (HST) images
of comet Hale-Bopp taken in September (left) and October (right). In both
cases the nucleus is centered in the frame, celestial North is straight up,
celestial East is to the left, and each frame is 10 arcsec across.
CLICK on the image to see a larger and improved version.
During September we caught Hale-Bopp only about 60 hours after an outburst,
and the image shows a jet and spiral pattern emanating from the nucleus.
The image from October was taken during a "quiescent" phase and barely
shows any structure in the inner coma. The image in this latter case had
to be strongly stretched in order to see anything (which explains why the
image looks grainier than the September image). The inner coma during October
is clearly elongated nearly due North. There is a very faint patch of
enhanced brightness approximately 3.5 arcsec North of the nucleus, which
is undoubtedly the remnant of the outburst that occurred on October 13th
(i.e., 10 days prior to the HST observations).
If you like you can go directly
to the October 1995 report .
Particular emphasis will be placed on results from the Hubble Space Telescope (HST) . Eleven orbits of HST observing time have been allocated to a Cycle 5 Target of Opportunity program, and the first observations were attempted during 26-27 September 1995.
Above: Hubble Space Telescope (HST) images of comet Hale-Bopp taken
at ~16:00 UTC on 26 September 1995 with the WF3 CCD chip of the
Wide-Field Planetary Camera 2 (WFPC2). The image to the left is from
a 300 second exposure and shows how the ``pinwheel'' structure of the
cometary coma clearly distinguishes Hale-Bopp from the numerous
stars in the field (at the time of these images the comet was passing
near the galactic plane in the constellation Sagittarius). The stars
are streaked because the HST was tracking the motion of the comet.
This image is 51 arcsec on each side.
The magnified view to the right is a composite created from two images,
one exposed for 60 sec and the other for 300 sec, and shows more detail
in the inner coma. Each pixel in this image projects to a distance of
~470 km (= 290 miles) at Hale-Bopp, and the full frame is 70 pixels
(=7 arcsec) on each side. There is a ``clump'' of material about 1.4 arcsec
directly above the nucleus. It appears that this material is the
remnant of a piece of the nucleus that was ejected about 60 hours
before this image was taken and is moving through space with a
projected speed of ~30 meters per sec (=110 km per hour, or 67 miles
per hour).
In both frames celestial North is at 46.7 deg CCW from the straight
up direction and East is at 90 deg CCW from North. The above picture
(
Postscript Version is here )
and its caption
are available as Photo-Release PRC95-41 from the
Office of Public Outreach
of the Space Telescope Science Institute
(STScI) .
(Note: you should CLICK on the above image to see
the larger GIF version of the photo-release.)
The picture was produced with the assistance of Zolt Levay of the STScI.
Incredibly, by pure coincidence we took our HST images shortly after
an outburst. The comet has been monitored daily by a team of observers
(led by Mark Kidger, Miquel Serra-Ricart, and Ruth Torres-Chico) using
the 0.82-meter IAC-80 Telescope at the Teide Observatory in the Canary
Islands. Their observations on 9/25 (about 19 hrs BEFORE the HST observations)
showed no unusual activity, while observations the next night (about
5 hours AFTER the HST observations) showed a prominent jet.
The HST images seem to show a clump of dust ~1.4 arcsec from the nucleus.
It appears that a substantial (TBD) chunk of material was shed by
the nucleus and is disintegrating as it leaves the near-nucleus region.
From the HST and Tenerife data we find that the projected velocity
of the material is ~30 meters per second.
Here are some thumbnail pictures of the comet along with short text
descriptions and links to larger versions of these images.
( CLICK on the thumbnail images to see the larger
versions.)
Above: In this stretch none of the pixels are saturated, so
you can see more clearly the location of the nucleus. The peak pixel
intensity is about 3.5 times brighter than the intensities in the
clump of dust to the NW of the nucleus.
Above: The stretch used here makes the coma look a
little like a question mark.
Above: The intensity stretch was chosen to emphasize the ``clump'' of
dust located 1.35 arcsec directly above the nucleus, which is near the
center of the frame. The peak pixel intensity at the nucleus is about
3.5 times brighter than the brightest pixels in the clump. However, the
intensities in the clump are about 8 times brighter than
expected for dust flowing uniformly out from the nucleus.
Above: This plot shows spatial brightness profiles along the
direction of the jet, on both sides of the nucleus. (On one side, data
averaged over azimuthal angles between 87 and 121 degrees are plotted [where 0
degrees is defined as horizontal and to the right, and positive angles
are measured CCW from the 0 deg direction], while on the other side
data averaged over azimuthal angles between 267 and 301 degrees are plotted.)
The profile for a typical comet, in which the brightness scales
inversely with the projected distance to the nucleus, is also shown
along with the profile for the telescope's point spread function (PSF).
Above: In this plot we have divided the spatial profiles by
that expected for a typical comet to show that the brightness in the
clump is approximately eight times larger than expected by uniform
outflow from the nucleus.
Above: Hubble Space Telescope (HST) images of comet Hale-Bopp taken
at approximately 06:30 UTC on 23 October 1995 with the PC1 CCD chip of the
Wide-Field Planetary Camera 2 (WFPC2). The upper left frame is a
60 sec exposure, the upper right is 300 sec long, and the lower frames
are both 600 secs long. Each frame is 36.4 arcsec on a side, corresponding
to 177,000 km at the comet.
In all frames celestial North is at 130.7 deg CW from the straight
up direction and East is at 90 deg CCW from North. (Sorry but these
frames are rotated by approximately 180 deg from the September images
displayed below. I'll rectify this during subsequent updates.)
These images have not been "cleaned", which means that they
are littered with background stars (the comet is still near the galactic
plane), cosmic ray events, and hot pixels. In the longer exposures the
stars appear trailed since HST was tracking the comet. A first cut
at a cleaned image appears in the report given below.
The nucleus is near the center of the frame in the highly saturated region.
This particular intensity stretch was chosen in order to bring out the
faint feature that's about 3.5 arcsec due north the nucleus (i.e., at
131 deg CW from straight up). This feature is almost certainly the
remnant of the outburst that took place on 10/13 and which was first
reported during observations on 10/14 from the Teide Observatory in
the Canary Islands. The average projected speed of this clump of material
is about 20 meters/sec.
Compared to the September observations, in which the comet was observed
only about 57 hours after an outburst, the near-nucleus region was relatively
dull this time around. If one uses the same intensity stretch for both
the September and October data, the September image shows the now familiar
spiral arm plus a clump of brightness about 1.3 arcsec from the nucleus,
while the October image looks essentially like a point source. (I'll
post a comparison image next week.) One has to really look hard to see
the structure in the October images.
Nevertheless, upon careful inspection one can clearly see the remnants
of the October 13th outburst, which was reported by the Hale-Bopp
observing team at the Teide Observatory. Based on its separation from
the nucleus, the clump associated with this latest outburst has a
projected (on the sky) outflow speed of approximately 20 meters/sec.
We do not see any evidence for any strong outburst on October 23rd, although
the inner coma is clearly elongated along the due north direction.
The direction of this elongation has definitely changed significantly
from its orientation in September, when it was at a position angle
of 315 deg (i.e., 315 deg CCW from North).
Due to the "quiescent" nature of Hale-Bopp during these latest observations,
we are hopeful that these HST images will allow us to set sensitive upper
limits on the size of its nucleus. Our preliminary results are described
below.
The spectroscopic data taken in October show no obvious molecular, atomic,
or ionic emissions. In particular, we don't see OH, CS, CO2+, CO Cameron
band emission, C, or S. We are in the process of looking carefully at
these data for the purpose of setting upper limits on the above species.
Here are some thumbnail pictures of the comet along with short text
descriptions and links to larger versions of these images.
( CLICK on the thumbnail images to see the larger
versions.)
Above: A "cleaned" image of Hale-Bopp has been produced by combining
the two 600 sec exposures. Most of the star trails and other image
artifacts have been removed.
The problem we face is the usual one in cometary research: how do we
measure the light from the nucleus when it is embedded in a bright
coma? Fortunately, cometary comae are usually optically thin which
means that, in principle, the nucleus can be picked out provided that
sufficient spatial resolution is used. Note that it is not necessary to
resolve the nucleus physically (i.e., the telescope's resolution
element needn't be as small as the nucleus itself); one simply needs
enough resolution so that there is some contrast between light from the
(unresolved) nucleus and light from the coma. As the spatial resolution
is degraded, the contrast between nucleus and coma becomes more and
more diluted and it becomes impossible to discern any signature of the
nucleus.
The case of Hale-Bopp is particularly difficult because it is beyond the
orbit of Jupiter, so that even the Hubble Space Telescope's resolution
is only about 440 km (=270 miles) at the comet. Thus, in order to
determine the brightness of the nucleus, we must extrapolate the coma
brightness from this exterior region (i.e., outside of 440 km) down
to the nucleus. If the coma light distribution is "well-behaved", then
one would have some confidence that the coma brightness was well-represented
in the unresolved region, allowing an accurate nuclear brightness to be
determined. But if the coma shows irregularities, then any determination
of the nuclear brightness is suspect. Temporal variability can cause
irregularities in the coma spatial distribution, and Hale-Bopp has obviously
showed strong temporal variability in the inner coma. The "knee" in the
spatial profile plotted above may be due to temporal variability.
In determining the nuclear diameter, we ASSUMED that the
the core of the image (i.e., the peak pixel and a few adjacent ones)
was not affected by any strong outburst in activity near the time of
our observations. Since the observers at the Teide Observatory reported
seeing a new jet two days after our observations, we must carefully
examine the assumption that our October observations were indeed
characterized by "quiescent" conditions.
We also had to make an assumption about the reflectivity, or albedo, of the
nucleus. We chose a geometric albedo of 4 percent, which is what was
measured for the surface of comet Halley and is close to values derived
for several other comets. (Albedos have been determined for only a
handful of cometary nuclei, but all seem to be very dark.) The darkest
known albedo for comets and asteroids is 2 percent; if Hale-Bopp's nucleus
is this dark then the estimated diameter must be increased to 55 km.
The largest albedo for any known planet or satellite is 90 percent;
if Hale-Bopp's nucleus is this bright then the estimated diameter
must be decreased to 8 km.
A major uncertainty in our analysis is our extrapolation of the coma
brightness into the unresolved region. Clearly some of the observed
intensity at the peak pixel is due to coma. By using the very
conservative assumption that the coma is completely "flat" for the
inner three points, we find that Hale-Bopp's nucleus must be smaller
than 70 km (for an albedo of 4 percent).
Click here (or on the image) for a magnified view.
Above: Here we compare the observed spatial brightness profile for Hale-Bopp
(the "+" symbols) with a model distribution consisting of a coma (the
diamond symbols) and a nucleus (the box symbols) whose diameter
is 40 km. The fit seems acceptable, which is why we said above that
Hale-Bopp's nucleus is roughly 40 km in diameter. However, there is
more to the story.
Click here (or on the image) for a magnified view.
The most important point about this figure is that the peak pixel (the point which has an x-value near zero) intensity is much brighter than would be expected for an extrapolation of the coma profile. This large deviation of the peak pixel value above the dashed line may constitute a "photometric" detection of the nucleus. Although a strong outburst in dust near the time of our observations could produce an anomalous brightening of the peak pixel, there is no evidence that any such outburst occurred. (Remember that the comet was being monitored nearly daily from the Teide Observatory in the Canary Islands.) It is also possible that the coma brightness profile steepens within approximately 400 km of the nucleus (i.e., within our resolution element; at Hale-Bopp's geocentric distance 1 PC pixel subtends 220 km at the comet), and we cannot rule that out. However, temporal variation would be the most likely cause of a steepening in the coma profile and, again, there is no evidence of any strong temporal variability near the time of our observations.
The JPL Hale-Bopp
page seems to be particularly comprehensive and current.
The
ESO Hale-Bopp page was extremely active during the first month following
the discovery of Hale-Bopp and continues to be the best source of information
on observations conducted from the European Southern Observatory.
The UMD Hale-Bopp Bulletin
Board is an officially-designated clearinghouse for Hale-Bopp
information and is brought to you by the same folks who ran a
similar BB for the SL9 campaign.
You should check out the neat
IAC site ,
which is operated by the folks running the Tenerife Observatories
in the Canary Islands. They have been observing Hale-Bopp
every night using the 0.82 meter IAC-80 telescope.
You can find finder charts and other good background information
on Hale-Bopp at the
Sky and Telescope Hale-Bopp page .
Comments on this WWW page can be sent to Hal Weaver via e-mail at weaver@pha.jhu.edu .
Visitors, since 3 November 1995: