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Archive for November, 2008
Sonntag, November 30th, 2008
The Probabilities of Positive Detection of Occultation for Stations at the Southern Hemisphere Based on Latest Predictions
Jean Lecacheux (IMCCE, France) wrote:
The updated nominal path is shifted by 0″.2 or 6000 km south -unfortunately for all the European observers-, and now is predicted from the south tip of South-Africa (near 02:10 UT in early dawn) to south Brazil (near 02:15 UT at low elevation).
In the same time the „one sigma“ uncertainty was lowered to 0.12 Re (i.e. ten times better than before the new astrometry), whilst in consequence the probability of positive detection was boosted greatly.
Now it could reach up to 44 % from Porto Alegre (south Brazil), 40 % from Sao Paulo (Brazil), 37 %from Cape Town (South-Africa), 14 % from Windhoek (Namibia), 8 % fromRecife (nordeste, Brazil), 1.5 % from Belem (Brazil) at the Amazon mouth…, and will be fully negligible from any place in the northern hemisphere.
Never any occultation by a Kuiper body, Pluto excepted, reached a likehood level so high.
Update as of 29 November 2008, using a recent star position derived from 62 observations of the star at Pic du Midi (T1M François Colas, T60 Bernard Trégon), St Sulpice (T60, Bernard Christophe) and Sozzago (T40, Frederico Manzini) reduced by Marcelo Assafin et al. (Rio group):
and the following offset for Varuna (from 32 images taken at Pic du Midi T1M):
off_RA = +42masoff_DE = -194mas.
Formal errors on this prediction, taken from rms and number of observations, are about 25 mas, or +/- 800 km, as shown by the dotted lines.
Details about this prediction:
On Sat, 29 Nov 2008, Marcelo Assafin wrote:
In the following, I give updated astrometric results for Varuna and the star, from reductions using the PRAIA package. The observational sources are (many thanks to these heroes):
– François Colas, Pic-du-Midi, France (T1M), run from 20 to 28 November 2008
– Bernard Trégon at Pic-du-Midi, France (T60), run 21 November 2008
– Federico Manzini at Sozzago, Italy (A12), run 25 November 2008
– Bernard Christophe at St-Sulpice, France (947), run 25 November 2008
Note that all the observations from our earlier report were re-reduced, so that the results reported here include all the observations made so far in the recent effort to derive offsets for Varuna and for the star. Unfortunately I have not yet been able to access the 36cm-telescope images from Jose Luis. Therefore these images are not yet included in our analysis.
Below we give the details of the astrometric reductions and results for the star and for Varuna.
As before, I take as reference the star position which has been used in the first plots of the occultation by Dave Herald:
RA =7h 29m 48.171sDEC=+25dg 40′ 7.71″
Individual CCD frame solutions were obtained using the UCAC2 catalog and a 3rd degree polynom for the reductions of the large field CCDs. Typically more than 200 stars were used per field. Mean errorsin (RA,Dec) were about 50mas.
Taking the individual solutions from the (T60), (A12) and (947) sets, the offset in the star positions wrt the reference position above was:
off_ra = +15mas (16mas)(n=10)
off_de = +27mas (14mas)(n=10)
off_ra = +20mas (22mas)(n=15)
off_de = +24mas (22mas)(n=15)
off_ra = +20mas (15mas)(n=37)
off_de = +34mas (17mas)(n=37)
all telescopes together
off_ra = +20mas (17mas)(n=62)
off_de = +30mas (18mas)(n=62)
In the global solution all large CCD frames were mixed and a 3rd degree polynom is used to place positions into the UCAC2 frame in the tangent plane. The star position from the global solution gives:
off_ra = +19mas (17mas)(n=62)
off_de = +30mas (18mas)(n=62)
Individual CCD frame solutions (from T1M observations only) were made using as reference frame the 2MASS catalog transformed into the UCAC2 frame by the tangent plane technique (with a 3rd degree polynom). Using this improved 2MASS, the small CCD fields from the T1M observations could be reduced with a 3rd degree polynom. Typically about 15 reference stars were present, which is not ideal for 3rd degree reductions (N=20 or higher), but is nevertheless acceptable.
Mean errors in (RA,Dec) were about 70mas.
Considering these individual solutions, the offsets wrt the JPL ephemeris were:
T1M: (20-21 November)
off_ra = +077mas (093mas)(n=18)
off_de = -170mas (103mas)(n=18)
T1M: (26-27 November)
off_ra = +052mas (169mas)(n=08)
off_de = -156mas (112mas)(n=08)
T1M: (27-28 November)
off_ra = +089mas (91mas)(n=06)
off_de = -191mas (44mas)(n=06)
T1M: all nights together
off_ra = +073mas (113mas)(n=32)
off_de = -170mas (096mas)(n=32)
Note that the use of the improved 2MASS and the 3rd degree polynom here is an improvement over the former solutions with 1rst degree polynom and about 5 UCAC2 stars, in the individual solutions.
In the global solution for Varuna, all the large CCD frames (referred to the UCAC2) were mixed with the T1M individual frames (referred to the improved 2MASS as described above), using a 3rd degree polynom to place the positions into the UCAC2 frame in the tangent plane. The offsets wrt the JPL ephemeris from the global solution gives:
Felipe B. Ribas is starting to contact some stations in Brazil from amateurs which could try to observe the possible event (Sao Paulo, Brasilia city regions).
The event is too low for the LNA observatory at Itajuba (IAU 874), with h=19 degrees, but we are in touch with the technical personnel to see if, and how, we can manage to observe the event with their 60cm automated telescope.
Prediction using the UCAC2 star position and NO offset on Varuna ephemeris
The following two predictions using the same data obtained from observations made at Pic du Midi:
Update as of 25 November 2008, using a recent star position derived from observations made at Pic du Midi, reduced by Marcelo Assafin et al. (Rio group) and an offset for Varuna (18 positions from the Pic du Midi observations):
Star position: RA= 07h 29mn 48.1720secDE= +25d 40' 07.741"(J2000/UCAC2)
Varuna offset: off_RA = +94mas ,off_DE = -279mas
The dotted lines are typical errors on path, roughly +/-2000 km, or about +/-65 mas.
Update 27 November 2008, using a recent star position and an Varuna offset derived from the same observations, but reduced by Raoul Behrend (Geneva):
Star position: RA= 07h 29mn 48.1720secDE= +25d 40' 07.748" (J2000/UCAC2)
Varuna offset: off_RA = +23mas,off_DE = -185mas
How to read the maps:
Black lines: Varuna shadow, dotted lines: typical uncertainty band. The dots on the centre line are plotted every min, shadows move from right to left. The larger dot is geocentric closest approach, occurring at the UT time indicated at the bottom of the figure.
Comparing the second and the third maps with the first, the typical errors on the path are much smaller now.
Preparing the Varuna Event Status Report, 2008 Nov 27, 22:45 U.T.
Steve Prestons new prediction (Nov 25) is based on a recent star position and the position of (20000) Varuna derived from observations made at Pic du Midi, reduced by Marcelo Assafin et al.
Steve gave strong weighting to the Pic du Midi observations at his calculations.
Bruno Sicardy (Observatoire de Paris, LESIA) states that the Varuna event should be prepared and monitored for several reasons anyway:
– to get a feeling of SNR’s obtained by the various people interested in future events of this kind.
– to have a rehearsal, as many things can go wrong in this kind of time critical observations.
– for the larger telescopes, get a better astrometry when Varuna is very close to the UCAC candidate star. Probably a relative accuracy of 20 mas or better might be obtained. The obtained, accurate offset can then be stored for future monitoring on how Varuna’s offset behaves with time.
– who knows, the astrometry can still change…
At the moment we are searching for potential observers in the southern hemisphere (e.g. Brazil, Namibia, South Africa).
There is still hope to catch the shadow of Varuna…
Preparing the Varuna Event – Status Report, 2008 Nov 21, 21:30 U.T.
Raoul Behrend (Observatoire de Genève, Switzerland) reports at Nov 20:
Bernard Trégon is actually using the 60cm telescope at Pic-du-Midi, France. He had taken a few hours ago some images from the Varuna & target area. Unfortunately, the sky got foggy and moon illuminated just before and during the first shoots.
Nevertheless, I could use one frame for the measurement the star’s position on 2008 11 20.196121 : 07 29 48.161 +25 40 07.68 using UCAC2 stars including proper motions and no other corrections. This position is to be taken with the „only indicative“ flag. Varuna is totally lost in the fog. More observations will be available in the coming nights.
Later this day Dave Herald made a prediction according to the position measured by Bernard Trégon:
The main effect – 3 minutes later, with the path being slightly further north (over France). I think this astrometry confirms that the star position is ‚about right‘. The real uncertainty is the position of Varuna – and hopefully someone will succeed in getting a current position
Marcelo Assafin (Valongo Observatory, UFRJ, Brazil) want to perform an independent reduction with his astrometric software PRAIA.
Francois Colas (IMCCE, France) has taken some images at Pic du Midi:
I just have done some images of Varuna this night at Pic du Midi. Unfortunately we still have the old CCD, so it noise, big pixels and small field… I hope it will be useful. I observe 6 hours to get a light curve.
The observational conditions are: 105 cm telescope F (with reducer) = 5.65 m Pixel size 23 x 23 microns so 0.84 “ on the sky Field : 5 x 4 arcmin.“
There is a discussion now how to incorporate any new astrometry. Whether that be by way of a simple offset, or by way of a new orbit computation.
Preparing the Varuna Event – Status Report, 2008 Nov 16, 20:00 U.T.
At the moment position data and data of proper motion of the target star 2UCAC 40846256 are compared from various catalogues.
Dave Herald from Canberrra, Australia, has made a table of Vizier catalogue information. See the text file at the end of this report.
His comments about this table:
„In particular, the nominal catalogue precision is not indicated, and there are data items present in some of these catalogues that are not displayed in this listing. Main thing to conclude, however, is that UCAC2, CMC14 and SDSS give consistent positions with the UCAC2 proper motions. The one area of concern: the position from USNO-A2 would suggest a proper motion in Dec that is about half the UCAC2 value, and the APM-North position (epoch ~1950, internal precision ~0.1″, fit to Tycho2 ~0.5″) would suggest a proper motion of the other sign – I would discount the APM position, as it is inconsistent with all the others.“ (Dave Herald)
Robert McNaught (Australian National University) was asked by Dave Herald whether he might be able to get some astrometry of Varuna in the next couple of weeks – particularly if the asteroid and star could be on the same image. Because of the faintness, Mr. McNaught will have trouble with a precision for a single measurement of better than 0.2″. He will try over several nights, to improve. He will also see if there is an opportunity with the 2.3 meter scope.
Eric Frappa (euraster.net) asked Martine Castets & Bernard Tregon, the amateur astronomers using the T60cm from Pic du Midi Observatory between November 17 and 23, to make an attempt also with Varuna and target star in the same field. They agree to try in case of clear sky…
Possible Occultation by Trans-Neptunian Object VARUNA
at Dec. 7th 2008, 2h 10min UTC
Bruno Sicardy gave the information about an EMAIL from Jean Lecacheux,
concerning a possible occultation of a 14th magnitude star by VARUNA
on 7th of December, 2008 aroun 2h 10min UTC.
a mail from Jean Lecacheux pointing out a potential stellar
occultation by the Trans-Neptunian Object Varuna, nominally visible from
SW Europe (Spain, Portugal, Canaries) and N. Africa on Dec. 7 around 02:10
Uncertainties are large though (1″= 31000 km at Varuna, whose diameter
is ~900 km, corresponding to a mere 29 milliarcsec on the sky…).
So, widespread observations are advisable, as the event might be visible
anywhere from all Europe, middle east (Israel, Turkey, etc…), and lower
above horizon, from East USA coast, Canada, Caribes, Venezuela, Guyanas,
see map attached.
Note that the star is faintish (V=14.7) but the contrast is very high (V
Varuna = 20.2).
Before embarking in more specific plans, astrometric updates are most
En l’absence de prévision par les collègues de Rio des occultations par
VARUNA, j’ai cherché ce qui pourrait advenir cet hiver, et je „redécouvre“
le phénomène de la nuit du samedi 6 au dimanche 7 décembre, dèjà annoncé
par D.Denissenko il y a 4 ans.
L’étoile est malheureusement faible, mais vu la durée de 43 secondes, on
pourra se rattraper en intégrant une fraction de seconde, surtout si le
télescope n’est pas trop petit. L’étoile sera d’ailleurs haute dans le
ciel au méridien de l’Europe de l’ouest.
Les magnitudes sont : V ~ 14.7, R ~ 14.5 , I ~ 14.1, … K= 13.4 .
Le B-I de +1.2 et le V-K de +1.3 caractérisent une étoile plus blanche
que le Soleil (V-K= +1.5) ou que Varuna (B-I= +2.1).
J’essayerai de surveiller cela du Pic.
Il faudrait sans doute prévenir en divers sites d’Europe (incluant les
Canaries, la Turquie,…), ainsi que sur la côte est de l’Amérique du
Amitiés à tous.
As soon, as we have more detailed information, we will forward it to everybody in the community.