Feb. 10th occultation shows (71) Niobe probably has a large
satellite
An occultation by the same Niobe satellite might have been
videotaped in Japan last November
New: 2005 February 24
The occultation observations described below are among the
strongest yet indicating a satellite of an asteroid. If it is a
satellite, it's relatively large, about 16 km across or 0.2 times
the diameter of (71) Niobe, and about 570 km or 0.27" away from the
asteroid. The satellite should be only about 4 magnitudes fainter
than Niobe, so adaptive optics observations of this asteroid are
strongly encouraged.
The occultation of 8.0-mag. SAO 38784 = HIP 15831 in Perseus,
a few deg. from Algol, by the S-class asteroid Niobe was observed
photoelectrically by Bob Cadmus with a 61-cm telescope at Gale
Observatory, Grinnell College, Grinnell, Iowa, close to Steve
Preston's predicted time. Bob's 3-second occultation, when
considering the (probable) circular geometry and maximum predicted
duration of 5.2 seconds, indicates that he was probably near the
eastern edge of the actual path, which must have shifted less than a
quarter path-width east of Preston's updated path in order for the
event to have occurred there with that duration.
About half a path-width farther east, Doug Kniffen, south of
Warrenton, MO, timed a 1.0-second occultation using a 16-inch
telescope. He observed visually with a low-power eyepiece so that
he could watch several nearby field stars as well as the target
star; he recorded WWV time signals along with his calls of the
disappearance and reappearance. But he was surprised because his
event occurred half a minute before the predicted time, while the
expected error in the predicted time was less than 2 seconds (1-
sigma). And after taking into account the motion of the asteroid
(that is, the difference in predicted closest approach times), his
event occurred 35 seconds earlier than it would have if it had been
by the same object timed at Grinnell. Both observers have very
solid records of their timings; an error as much as even a full
second is very unlikely. And this was no faint star at the limit of
visibility; it was very bright in the large telescopes used and the
occultation events were sharp and deep. Even if a 35-sec.
correction is applied to one of the observations, a circle fitted to
them would have a diameter of 58 km, much less than the 83-km
diameter estimated from the IRAS satellite observations of Niobe,
which is very unlikely; IRAS diameters virtually always agree with
occultation diameters within 20% or less. Unfortunately, there are
no other observations of the occultation; an observer in Arkansas
near the Grinnell chord had equipment problems so was unable to
monitor the event. Five other observers who did monitor the star
were all well west of the predicted (and even farther west of the
actual) path and (with one curious exception described below)
reported no occultation.
The observations are listed below, first the locations and then
the timings. Then a sky-plane view of the observations is
described, followed by Cadmus's and Kniffen's original messages
reporting their observations, their messages when asked for more
details about their timings and reliability of their observations,
Patrick O'Connor's curious observation.
Sta. Observer and Location E. Longitude N. Latitude h
# o ' " o ' " m
1 Bob Cadmus, Gale Obs., Grinnell, Iowa - 92 43 09.2 +41 45 20.1 318
2 Doug Kniffen, Warrenton, Missouri - 91 13 16.1 +38 43 29.8 290
3 Bob Sandy, Blue Springs, Missouri - 94 19 59.5 +39 1 24.0 292
4 Rob Robinson, Bonner Springs, Kansas - 94 53 35.5 +39 3 28.4 250
5 Terry Atwood, Shreveport, Louisiana - 93 43 31.8 +32 28 43.0 60
6 Brad Young, Tulsa, Oklahoma - 95 59 2.4 +36 8 20.4 250
7 Patrick O'Connor, Pinawa, Manitoba - 95 54 19.7 +50 09 23.2 275
8 Prediction, Steve Preston, 2005 Jan - 93 13 32. +39 28 28. 0
Scope
Sta. ap Event Univ.Time Event Univ.Time
# cm h m s h m s
1 31 D 5 56 20.33 R 5 56 23.52
2 29 D 5 56 01.7 R 5 56 02.7
3 20 M 5 56 41.5
4 25 M 5 56 42.3 Event M = miss (no occultation) with U.T.
5 6 M 5 57 19.2 being the closest approach time at the
6 25 M 5 57 1.7 center of station 1's observed chord
7 20 M 5 55 33.4
8 P 5 56 36. Event P = Steve Preston's prediction
The observations projected in the plane of the sky at Niobe show the
star when it was visible for each observer with the lines numbered
with the station numbers given above, in a file that is here.
Another miss line, by Richard Wilds and Craig McManus at Topeka,
Kansas, is not shown because I don't have their coordinates; it must
be near the path for sta. 5. The expected 83-km circle of Niobe is
fit to station 1's timings. Kniffen's (sta. 2) event is shown as the
16-km break in the line in the lower left part of the figure, about
570 km (less than 7 diameters) from Niobe's center. Steve Preston's
prediction (#8) is rather close to the center of the Niobe circle,
showing that the prediction was very good and the possible binary
nature of the star suspected from some Hipparcos data, causing us to
doubt the accuracy of the prediction, was a red herring; the
occultation observations showed that the star was single.
_______________________________________
Bob Cadmus's original message:
David -
I have data for the Niobe event. The data were acquired with a
photoelectric photometer (in V) on the 0.61 m Cassegrain reflector
at Grinnell's Grant O. Gale Observatory. The star and WWVB time
signals were recorded on a digital oscilloscope and the data were
then transferred to a computer for additional analysis. The
conditions were good and the event was conspicuous. There was no
evidence for anything other than instantaneous transitions. The
data record extends from 10s before until 6s after the occultation
and no secondary events are apparent. Visual observation of the
signal for a longer period prior to the event also revealed no such
events, but is less reliable.
Longitude:__W 92 43 9.21__ Latitude:___N 41 45 20.12___
Height above sea level:_318 m_ How determined?:__professional GPS__
The UT times are, with an uncertainty of about 0.02 s:
Start - 5:56:20.33 End - 5:56:23.52
Bob Cadmus
**********************************************
Robert Cadmus ............. Professor of Physics
Department of Physics ..... Grinnell College ..... Grinnell, IA 50112
Office: 641-269-3016 ...... Observatory: 641-269-3014
FAX: 641-269-4285 ......... Home: 641-236-8398
_______________________________________
Doug Kniffen's original message:
---- Original Message -----
To:
Sent: Wednesday, February 09, 2005 9:59 PM
Subject: Still need help to cover good Niobe occ'n SOON
David Dunham wrote:
> Only a few observers have said that they'll be trying tonight's
> good occultation by (71) Niobe, given in the list below, and all of
> them are outside of Steve Preston's updated path.
This location must have been right at the eastern edge of the path.
The target star was under constant observation from 5:38 to 5:59 UT.
With absolute certainty; An occultation was observed here visually
(16" telescope) between 5:56:01.7 UT and 5:56:02.7 UT (timing
correction applied).
Doug Kniffen
North 38.725
West 91.221
278 meters asml
_______________________________________
And his 2nd message:
----- Original Message -----
To: "Cadmus Jr., Robert" ;
Cc: ; ;
;
Sent: Tuesday, February 15, 2005 1:01 AM
Subject: Feb. 10th Niobe occultation disagreement
David Dunham wrote:
> Bob and Doug,
>
> I've analyzed your observations and unfortunately there
> is a large disagreement.
It would be nice to resolve this by adjusting my reported times, but
I've reviewed the tape and have to stick by the report. Having seen
both a few definite occultations, and a number of uncertain
possibilities, I have to say the reported observation was a definite
occultation. A widefield eyepiece was used specifically to eliminate
the chance of being misled by a terrestrial occulting object. Not
even a flicker was observed from any of the numerous field stars, so
I'm certain that isn't a possibility. I do remember being somewhat
surprised that the observed occultation occurred so much earlier
than the prediction time. There's always a fraction of a second
uncertainty with a visual observation, no where near enough to
explain this discrepancy. The location coordinates you used appear
to be for my first observatory (dismantled in 1994). Although it
won't help with this discrepancy, the present location is 38 deg. 43'
29.8" North and 91 deg. 13' 16.1" West.
Doug Kniffen
_______________________________________
and Bob Cadmus's 2nd message:
David and Doug -
Here is a .jpg image of my data as recorded on video from
the storage scope and (via mirrors) the WWVB clock. The version
from which I extract timing information is sharper than this video
image, whose purpose is only to establish the relationship between
the seconds pulses from the WWVB clock and real clock time from the
display of the same WWVB clock. The trace rolls from right to left
so the time on the clock refers to the right edge of the trace. As
you can see, the hand-drawn arrow points at the 25 seconds pulse, so
the event began approximately 5 seconds earlier than that -- near
5:56:20 as I reported originally. This entire display is generated
in real time so it's not easy to see how the signal and the timing
information could get misaligned. It looks as if we have an
interesting situation here. I'll be eager to see how it develops,
if it can.
Bob
_________________________
I don't know what to make of Patrick O'Connor's report, given below.
With Niobe's motion, his event durations are hundreds of km long,
which can't be celestial for an asteroid known to be about 83 km in
diameter (from the IRAS measurements). His path is closely bound by
Bob Sandy, 20 km to the east, and by Rob Robinson, 20 km to the
west, and both of them reported no dimming of the star's light under
good conditions and with telescopes the same size as, or larger
than, O'Connor's.
_____________
I have filled in a report form on the Internet, but I am also
sending you this Email because my observations do not match the
format of the report form. Also my observations did not match what
was expected, which is why I watched the star for an extra 7 minutes
to see if I was seeing things due to the cold.
The expectation was a 5 Sec. occultation. But the star dimmed for
about 1 min. then almost disappeared for another minute and then
returned to its normal brightness. The other stars in the field did
not change over this time. I watched the star for an additional 7
minutes, but it remained at its normal brightness during that time.
OBSERVATION: (UT Times are from WWV.)
About UT 5h 45m start observing star.
5h 53m 57s Star Dimmed to half Brightness.
5h 54m 51s Star became fainter, almost disappeared, maybe only saw asteroid.
5h 55m 38s There was a change, may star brightened.
5h 55m 50s Star returned to normal brightness.
I continued to watch the star until 6h 03 minutes, and it remained
at its normal brightness.
[For his location, the closest approach to the center of the
Grinnell (sta. 1) observed chord was 5h 55m 33.4s - D. Dunham]
OBSERVATION COMMENTS:
The observations were made at -18 C (0 F).
When I was observing the star it faded and possibly disappeared,
I was not sure if I was seeing the occultation or if
my C8 telescope was fogging up, or my eye was tearing up
with the cold. But the other stars in the field were
clearly visible while the target star was fading.
Particularly 2 stars: SAO 38809, SAO 38812.
I had watched the target star from 11:45 PM.
I also watched the target star for 7 minutes after the event,
and it was its normal brightness for all that time.
When it was all over I looked at the front of my telescope
with a flashlight and there was no fog.
LOCATION:
Pinawa Cemetery (no lights) Position, revised:
Longitude: 95D 54m 19.7s +/- 0.2s
Latitude: 50D 9m 23.2s +/- 0.2s
Elevation 275 M +/- 3 M.
EQUIPMENT:
I used my C8 telescope with a 26 mm Plossl eyepiece, with an
electrically heated dew cap. For polar alignment I pointed the
tripod towards the North Star, and levelled it. I did not connect my
clock drive. (It was too cold and too late to fool around with polar
alignment and the clock drive.) I also used a Radio Shack DX-440
Radio tuned to 2.5 MHz, and a Cassette Recorder.
FINDING STAR:
To find the star I pointed the telescope at Algol.
I set the R.A. to 4 Hr. 9 Min. (I was 1 hour off, but it was too cold to
move the circle to 3 Hr. And the Dec. settings were not functioning.)
I put Algol at the bottom of the field of view and
I moved the R.A. to 4 Hr. 24 Min. and I saw the star pattern
I was expecting, and I identified the target star.
This is the star I was watching.
(I spent some time during the day checking the finder charts on the Internet
and my Earth Centered Universe to figure out how to find the star.)
___________________________
Prediction information about the Feb. 10th event is here.
___________________________
A SHORT NIOBE OCCULTATION VIDEORECORDED IN JAPAN LAST NOVEMBER
From: Mitsuru SOMA
Date: Mon, 21 Feb 2005 21:26:37 +0900 (JST)
Subject: Re: Feb. 10th occ'n shows (71) Niobe probably has a large satellite
At 04:52 PM 21 Feb 2005 -0500 David Dunham wrote:
> The occultation observations described below are among the
>strongest yet indicating a satellite of an asteroid. If it is a
>satellite, it's relatively large, about 16 km across or 0.2 times
>the diameter of (71) Niobe, and about 570 km or 0.27" away from the
>asteroid.
I forwarded this information to JOIN (Japanese Occultation
Information Network), then Tsutomu Hayamizu points out the
following facts:
An occultation by this asteroid was observed on 2004
Nov. 2 by Hideto Fukui in Japan with a short duration
of 1.1 sec (this corresponds to the chord length of
13.3 km, which is consistent with yours of 16 km) as
shown here.
The event occurred closer to the predicted time than yours
(tomorrow the above figure will be replaced with the one
that shows how close it is to the predicted time, or you
can look at it now here).
It was observed by video and the event was also confirmed
by Hayamizu. Roger Venable pointed out a possibility
that it was due to a satellite of the asteroid as copied
below.
----- Copied message -----
To: IOTAoccultations@yahoogroups.com
From: "Roger Venable"
Date: Thu, 4 Nov 2004 11:47:41 -0600
Subject: Re: [IOTAoccultations] Re: The observations of
TYC3340-00555-1(mag10.6) by (71)Niobe.
David, and everyone:
The 71 Niobe event, of which a single chord was recorded in Japan,
was predicted to have a maximum duration of 6.9 seconds. The recorded
duration was 1.1 seconds. Very short occultations should occur only a
small fraction of the time -- this one was only 0.16 of the predicted
max, and an occultation this short should happen no more than 1.3% of
the time! Of course, occultations near the edge of the profile will
occur, and this was likely such a recording. However, there is a
possibility that it was a more central occultation by an asteroidal
moon. I believe this was a video record, wasn't it?
Roger
___________________________________________
Jean Lecacheux in France notes that there will be one more
occultation by Niobe this year, a 10.1-mag. star on Nov. 10
with an occultation by the satellite visible possibly from the
British Isles, Norway, Belgium, the Netherlands, and northern
France. He notes that a better occultation by Niobe will occur on
2006 Feb. 16 from Iberia, Mexico, and maybe Florida.
________________________
David Dunham, 2005 Feb. 24, 23h UT
Phones home 301-474-4722; cell 301-526-5590