Varuna.20100219 Occultation February 19, 2010

(last updated 2009 11 06)

Across the globe pictured above, the three solid lines correspond to the northern limit, centerline, and southern limit of Varuna's shadow. The northern and southern limits correspond to a radius of 483 km. The upper and lower dashed lines indicate the effects of unknown systematic errors such as catalog zone errors. Previous KBO appulse analyses have shown the observed error to be 1.5-2 times that of the prediction's random error. The shaded area represents where the sun is more than 12 degrees below the horizon.

The scientific goals of this event are (i) to accurately determine the diameter of Varuna, (ii) to probe for a tenuous atmosphere of Varuna, and (iii) to detect possible nearby satellites of Varuna.

Observations are worthwhile and encouraged from any location at which the event is visible. Due to the possibilities of the occultation star being a multiple star system or the KBO having a satellite; either of these could move the event by more than the systematic effect uncertainties shown.
Table 1: Prediction Details
Geocentric Mid-time (yyyy month dd hh:mm:ss)

2010 February 19 23:05:54± 00:01:111 UT
Minimum Geocentric Separation 0.0830± 0.05411 arcsec
Position Angle (Varuna relative to the star; measured north through east) -171.73 degrees
Geocentric Velocity 18.66 km/sec
Occultation Star UCAC2 magnitude 11.12
KBO Visual Magnitude 20.2
KBO Angular Radius 0.015 arcseconds (approximate)
1One standard deviation of random error.
2The UCAC bandpass (579-642nm) is between V and R.
Table 2: Reference Star Position
Reference star position:
(UCAC2, at epoch of event)		 RA (h:m:s; J2000) Dec (d:m:s; J2000) Notes
Varuna.20100219 Catalog 07 29 22.4701± 0.0177 26 07 23.229± 0.018
Varuna.20100219 Measured3 07 29 22.4702± 0.0117 26 07 23.206± 0.025
From 5 SMARTS 0.9m telescope frames. See Note 4.
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  
Varuna

–0.1334± 0.0175

 –0.147± 0.011 See Notes 5 and 6

3Measured position corresponds to RA offset of +0.0010'' and Dec. offset of -0.023''.

4All "offsets" are defined in the ("corrected" – "reference") or ("observed" – "calculated") sense. The offsets should be added to reference positions to get the measured positions, which we use to calculate the prediction.

4Data analyzed using UCAC2 reference network. A weighted average of the data from the two telescopes was used to calculate the RA and DEC. The errors given are 1 standard deviation.

5The reference position for Varuna is that given by JPL Horizon's ephemeris (Varuna source file: JPL#13; Earth center source file: DE405).

6Data from the SMARTS 0.9m, Lowell 42-inch, SWOPE 1m, and du Pont 2.5m telescopes over the span of 5 years were reduced with respect to stars in the UCAC2 catalog. A model was developed to fit the residuals obtained from our measured positions compared to that of the object's JPL ephemeris. All residuals obtained from the different telescopes were consistent with the model. The model includes the first-order effects of errors in the orbital elements of Varuna: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, and (iii) sinusoidal terms with the Earth's orbital period. The model was propagated to obtain the predicted position and error of the KBO at the time of the occultation. The errors listed for the KBO are 1 standard deviation.

 

 

Table 4: Site Information

Site
East Longitude
Latitude

Site Altitude6

(km)

Distance7

(km)

Velocity

(km/s)
Geocenter
----------
---------
center of Earth
2540 N.
18.67

6Altitude of each observatory is measured in kilometers above sea level.

7"Distance" refers to the closest approach distance of the "Site" to the center of Varuna's shadow in the shadow plane. The errors on all closest approach distances are ±838 km (one standard deviation). "S." means the site is south of the center of Varuna's shadow. "N." means the site is north of the center of Varuna's shadow.

Table 5: Varuna.20100219 Occultation Predictions for Individual Sites

Site
Varuna Immersion (UT)9
UT Mid-Time
KBO Altitude
Solar Altitude9

Varuna Emersion (UT)9
Geocenter
-----------
23:05:54
------
-----------
-----------

9The errors on all times are ±0:35 (35 seconds; one standard deviation). The solar altitude is given for locations where it is relevant (solar altitude greater than -18°). No entry in the immersion and emersion columns indicates that the occultation is not predicted to be visible at that site. But due to unknown effects (stellar or KBO duplicity), some of these sites may see an occultation event that would be centered on the time in the "UT Mid-Time" column.

Last updated by Carlos Zuluaga (czuluaga@mit.edu) 2009-10-20 12:50

Please direct all inquiries to Jim Elliot (jle@mit.edu)