RASNZ Occultation Section - Doris Occultation (October 28) Update

ASTROMETRIC UPDATE:
OCCULTATION BY (48) DORIS - 1999 OCTOBER 28

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[Prediction by Edwin Goffin]

[Prediction using OCCULT software]

Both the Goffin and OCCULT predictions suggest a path crossing South Australia in the region of Adelaide and northern NSW/southern Queensland in the region of Brisbane.

UPDATE: 24 October 1999

This prediction update is supplied by Jan Manek of the Stefanik Observatory, Prague, and is based on USNO/Flagstaff astrometry for the minor planet and the GSC 1.2 star position.

Summary:

The updated path is shifted about 2.5 path widths south with a time approx. 0.9 minutes earlier compared to Goffin's nominal prediction.

The path now crosses Melbourne and Canberra, with Sydney not far from the northern edge. Manek comments that the MP orbit itself is good, but the most uncertain factor is the star position. Thus, all observers within 4 path widths of the updated track have a chance to see this event. (i.e. all observers from southern Queensland to Tasmania).

Note that rather large telescopes will be needed because the star is magnitude 12.5 and the brighness drop will be only approximately ONE magnitude - we don't know the exact delta m expected as we do not have any accurate photometry for this star. The magnitude given on Edwin Goffin's chart is in fact the approximately V band magnitude.

Approx time for Melbourne: 11h 25m UT

THE EVENT AT ONE GLIMPSE:

Date and approx. UT time of event: October 28, 1999 @ 11:25 UT
Magnitude of target star: 12.5
Magnitude drop [mag]: 1.1
Estimated maximum duration [s]: 8.5
Path description: Across southeastern Australia.

The Occultation Path:

Approximate width [km]: 250
Uncertainty [path widths]: 4
Uncertainty in time [s]: 35
Map: See below
Remarks: Uncertainities are given on the basis of the nominal star position errors and the expected asteroid positional accuracy.
Circles along the path are OCCULT calculated exact centerline points.

Data for the target star:

Name: GSC 6286.1735
Constellation: Sagittarius
J2000 position [h,m,s; o,',"]: 19h 01m 39.045s; -17^o 10' 05.60"
Position source: GSC 1.2
Estimated accuracy ["]: 0.03"
V mag [mag]: 12.5
B-V [mag]: ??
Remarks:
1. The target star is in northern Sagittarius, about 4.3 degrees NW from the 2.9 mag star pi Sgr. The target is 7' SW of 8.7 mag star SAO 162102 or 1.7' SW of 10.4 mag star PPM 720004. Be careful to identify the right star - try to use a more detailed chart than Goffin's original.

Data for the minor planet:

Number, name: (48) Doris
Approx. diameter [km]: 219
Orbit source: calculation Manek
Source of used astrometry: USNO/Flagstaff (Ron Stone) + TMO (Bill Owen)
Number of used observations: 37 USNO + 43 TMO
Number of rejected observations: 3 USNO
Time covered by the observations: 1998 01 29 - 1999 09 13
Rms residuals ["] (RA,DE): 0.07", 0.05"
Estimated positional accuracy at epoch of event ["]: 0.07"

Data for the event:

UT date and time of least geocentric approach: 11:22:03 UT
Approx. V mag of minor planet at event [mag]: 13.1
Geocentric parallax of minor planet ["]: 2.524
Magnitude drop [mag]: 1.1
Estimated maximum duration [s]: 8.5
Apparent motion of minor planet ["/h]: 36.81
Angular distance to moon, phase of moon [deg,%]: 159, 83%

Doris Update (October 28)

IMPORTANT NOTE!

Astrometric updates such as these should not be taken as definitive, but rather only as an indication of where the true track may lie relative to the original predicted track. Observers must bear in mind that later astrometry, in which the target star is measured in the same field as the asteroid, may still reveal substantial changes to the predicted track and time of the event. For this reason it is most important that observers far from the predicted track still monitor the event.

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