The Brightest Occultation of a Star by the inner parts of
the Pluto/Charon system will occur Thus. Sept. 27 around 15h UT -
Updated 2007 Septmeber 26, 18h UT
The star, SAO 160793, at mag. 8.7 is about 100 times brighter (5
mag. less) than any other star whose occultation by Pluto or Charon
have been observed
New calculations show that Charon PROBABLY WILL occult SAO 160793 from parts
of Asia
The event provides a rare opportunity to check for rings and dust
close to Pluto; these could be of great interest to the New Horizons
mission
On September 27, 2007, around 14.8h UT, Pluto passes extremely close
to an 8.7-magnitude star, SAO 160793. This is the brightest star
approached so closely by Pluto since predictions of occultations by
this dwarf planet were first made over 30 years ago.
First, I give new information then the information that was posted
earlier today.
Jean Lecacheux points out that the new prediction by Bruno Sicardy
shows that the occultation by Charon will occur in central Asia,
over Pakistan, northern India, and western and southern China.
He writes:
Date: Tue, 25 Sep 2007 12:23:47 -0400 23 of 26
From: "jlxpicdumidi"
Subject: [IOTAoccultations] Re: More about the possible occultation of SAO 160793 by Charon, Pluto, & rings
To: IOTAoccultations@yahoogroups.com
From J.Lecacheux (France)
David,
Thank you for having forwarded to IOTA my memo about the incoming
Pluto event.
Here is a map by Bruno Sicardy (Paris Observatory) showing his
prediction of the Charon appulse, based upon independent
astrometry by a team of Rio-de-Janeiro Observatory (Brazil).
This work predicts the shadow of Charon over central Asia
(to be compared with last D.Herald's result).
_____
Relating to the above, Wolfgang Beisker, IOTA/European Section,
writes:
From: Wolfgang Beisker [beisker@gsf.de]
Sent: Wednesday, September 26, 2007 9:53 AM
To: Bruno Sicardy; planoccult
Subject: [PLANOCCULT] Occultation of a 8m7 star by the Pluto-charon
System... A composite view
Dear all,
On our website I put an overview of the occultation by the Puto-
Charon System on the 27th of September. Its a composite view of
Bruno Sicardy's graphs on his webpage. think of the different
occultation timings for Pluto and Charon! They are about 10 minutes
apart.
With best regards from Munich
Wolfgang
_____
Losyuk Alexey, Russia, is working with astronomers at Majdanak
Observatory, Tajikstan, to record the occultation. Here he gives
information about the brightness and color of the target and nearby
stars, recommendations for observers, and references to work on the
Pluto/Charon system by Russian astronomers. On Sept. 26, Jean
Lecacheux sent some remarks about some of Losyuk's comments that I
have now incorporated into the above link. The link above is just
a text file without links, so for his item 3, I give the link, which
is in Russian, here.
Here, Jay Pasachoff notes publications of recent observations of
occultations by Pluto and by Charon (the latter observed from Chile
with large telescopes two years ago) that show that Charon has no
atmosphere and set limits to any rings that Pluto might have.
_________________________________________________________
The information originally posted earlier today is below.
Earth will actually pass "between the shadows" of Pluto and Charon,
with no occultation by either object visible from Earth's surface
[according to the earlier predictions, but now we think that an
occultation by Charon is likely for locations in Asia; see above].
But occultations by possible Plutonian rings might be observed from
the night areas of most of Asia (from Arabia to Vietnam to western
and southern China to the Ural Mountains), the Horn of Africa,
western Australia, part of Antarctica, and the Indian Ocean.
Observers in those and adjacent areas, such as Japan, eastern
Australia, New Zealand, and South Africa, are encouraged to record
the star with CCD, video, or photometric equipment for a few hours
around the time of closest approach; such records can be analyzed
later to find evidence of any fadings of the star's light that might
be due to Plutonian rings; these would be of great interest to
scientists involved with the New Horizons mission to Pluto. Walker
Vaning, San Rafael, California, claims that observations of past
occultations show that Pluto has an extensive ring system that could
cause fadings of SAO 160793's light even more than 12h from the time
of closest approach, so he encourages observers worldwide to monitor
the star while it is high enough above the local horizon; he thinks
that very distant rings may exist that could pass over the star even
3 days from the Pluto closest approach time. The star, also known
as B.D. -16 deg. 4607, P507, and UCAC2 25587116, is at J2000
position is R.A. 17h 44m 38.4s, Dec. -16 deg. 46' 35"; its spectral
type is K5.
This event was mentioned in Sky and Telescope Astroalerts issued on
May 11, Sept. 24, Sept. 25.
and information about it is also given at the Massachusetts
Institute of Technology's Pluto events Web site.
David Dunham
2007 September 24
____________________________________________
New prediction shows possible occultation of SAO 160793 by Charon on Sept. 27
David Herald, Canberra, Australia, has updated his prediction of the
Pluto/Charon appulse on Sept. 27, using a new Pluto ephemeris
including currections from recently observed occultations, including
the one by Pluto observed from the western U.S.A. on March 18. This
shows the southern limit of the occultation by Charon just grazing
the Earth over Mongolia and south-central Siberia. With the
remaining uncertainties, an occultation by Charon lasting over 100s
is possible between 14:55 and 15:10 UT of Sept. 27, for observers in
western China, Mongolia, and southern Siberia, with some chance also
a little farther south, from central Asia to southern China, and
even possibly northern India. He sent some maps and charts below.
Sky-plane view of Pluto/Charon showing Earth
Path of the Pluto occultation - Antarctica?
Path of the Charon occultation - Asia?
David Dunham
2007 September 25
____________________________________________
More information about the occultation, and the effects of the
relatively large angular diameter of the target star on the
phenomena that might be observed, by Jean Lecacheux
Date: Tue, 25 Sep 2007 14:01:12 +0200
From: jlx@meteores.net
To: David Dunham , 1212Lupus@tut.by
Cc: planoccult@aula.com, bruno.sicardy@obspm.fr
Subject: Re: [PLANOCCULT] Occultation of 8.7-mag.
star by possible rings of Pluto this week
From J.Lecacheux
Hi all,
About the incoming exceptional Pluto / SAO 160793 appulse, Aleksey Losyuk asked:
> When it is necessary to begin photometry of of occultated star
> and when to finish it?
1/ By analogy with major planets, we could hope a main ring existing
well inside the orbit of Charon, the innermost satellite of Pluto.
The orbital radius of Charon is 19 400 km.
According to the current prediction by the MIT, based upon the UCAC2
position of "P507"= SAO 160793 and recent USNO astrometry of Pluto,
the closest approach should occur near 14:48 UT on Thursday 27 at
7,700 to 1,100 km above the Pluto surface. This is well inside the
Charon orbit, as roughly midway between Pluto and Charon.
Nevertheless such an appulse at about 7.5 equatorial radii from
Pluto's center might be not close enough, leading to a miss of the
ring. (Something really difficult to forecast, as we don't know what
this hypothetical ring would be like...)
2/ Faint diffuse rings, narrow ringlets, discrete arcs, etc. also
could exist in the region of the outer satellites Nix and Hydra.
Assuming the MIT appulse prediction, I have computed the approximate
GEOCENTRIC instants of the orbit crossings by the beam from the star
:
- Star entering the HYDRA orbit : 13:43 UT
- Star entering the NIX orbit : 14:01
- Star entering the CHARON orbit : 14:37
- Closest approach of the star at 0".39 : 14:48 UT
- Crossing the Pluto-Charon line : 14:55
- Star leaving the CHARON orbit : 15:13 UT
- Star leaving the NIX orbit : 15:48
- Star leaving the HYDRA orbit : 16:07
These geocentric times are valid also from Uzbekistan.
ADD 1-3 min from India, 2 min from La Reunion and Mauritius, about 6
min from south-east Asia, 8 min from west Australia. The uncertainty
probably lies under 5 minutes.
In my opinion any complete monitoring should begin near 12h30 UT, if
possible from the site (for example from Australia), i.e. at about
75 equatorial radii ESE from Pluto, and should end near 17 h (for
example from South-Africa), at the same distance WNW.
3/ Sure, other faint rings could exist even very farther, maybe at
400 equatoral radii as pointed out by David, but seeking them would
consume two or three complete nights, undoubtly with a low chance of
detection.
4/ Some other useful informations to prepare this observation :
The velocity relative to Earth of the Pluto system will be 11.1 km/s
or 0.49 mas/s. This seems favourable, as occultations by Pluto often
are faster. (Note : 1 mas = 1 milli-arc second will correspond to
22.82 km at the current Pluto distance).
The probable apparent diameter of the giant target star should be
close to 0.5 mas or 11 km. (Note : I assumed a K4 III star at 400
parsecs). The Fresnel diffraction scale will be 1.1 km in visible
light or 2.3 km for 2.2 microns (infrared) sensors.
It results that any quick event (for example a brief occultation by
some "Uranian-style" narrow ringlet, or by some "Neptunian-style"
arc) will be lengthened to about one second by the stellar diameter.
Besides - the occultation drop will suffer reduction, - any
diffraction effect should be almost erased.
To be able to detect by further image addition the slow crossing of
a faint tenuous ring of very small optical depth, please take the
following precautions :
- keep in the field of the camera the brighter SAO 160798, abput five
arc min. east (a V= 8.0, R= 7.8 white-yellow subgiant star);
- to beware of any signal saturation, either by this bright comparison
or by the red target itself.
Note 1: This above suggested bright comparison star is a close
visual binary where one component is a magnetic variable star. Its
luminosity period is unknown, but probably equal to the star
rotation. Moreover Hipparcos found only a 0.011 mag. scatter from 57
widely time-spaced measurements. So one may assume the magnitude of
SAO 160798 will not vary at few minutes scale, thus it can be used
as reliable comparison along the Pluto event.
Note 2: Some fainter comparisons within 0.1 degree also are
convenient for large telescope users.
Good luck to every observer.
____________________________________________________________
Bruno Sicardy has updated his Pluto Web site for this event
Date: Tue, 25 Sep 2007 14:37:06 +0200 (CEST)
From: Bruno Sicardy
To: jlx@meteores.net
cc: David Dunham , 1212Lupus@tut.by, planoccult@aula.com
Subject: Re: [PLANOCCULT] Occultation of 8.7-mag. star by possible rings of
Pluto this week
Hello,
FYI, I put some maps of the 27 Sept. 2007 events (Pluto and Charon)
on my page.
go to the bottom of the page ("Section 2 - star astrometry based
on 1.6m and 0.6m LNA telescopes/Brazil"), and click on "Pluto" or
"Charon" cases in the light-blue table
The event is mainly visible low above horizon from Australia,
Indonesia, Asia, China and India, and higher above horizon, but in
evening twilight, from Pakistan, Afghanistan, Iran, Iraq (...),
Saudi Arabia, La Reunion
Cheers,
Bruno
____________________________________________
David Dunham, 2007 September 26, 18h UT
Phones home 301-474-4722; office 240-228-5609; cell 301-526-5590
e-mail david.dunham@jhuapl.edu
home e-mail: dunham@starpower.net .