Varda.20180910 Occultation September 10, 2018

Finder Charts

(last updated 2018 09 08)

The google map (above left) shows Varda's path. This path takes into account the rotation of the Earth during this relatively slow event.

Across the globe pictured (above right), the three solid black lines correspond to the northern limit, centerline, and southern limit of Varda's shadow. The eastern and western limits correspond to a radius of 370 km. The dashed lines indicate 3-sigma errors. Ilmare, assumed to be 163 km in radius, is outside Varda's 3-sigma errors and illustrated by the three solid red lines. The shaded area on the globe represents where the sun is more than 0 degrees below the horizon. The shadow paths on the globe are an approximation at the geocentric close approach time; they disregard the Earth rotation during the event.

Table 1: Prediction Details
Varda Geocentric Mid-time (yyyy month dd hh:mm:ss)

2018 September 10 03:39:38± 00:00:281 UT
Varda Minimum Geocentric Separation 0.116± 0.0071 arcsec
Position Angle (Varda relative to the star; measured north through east) +73.75 degrees
Geocentric Velocity 10.86 km/sec
Varda apparent magnitude 20.1
Occultation Star Gaia DR1 magnitude 14.562
Occultation Star Gaia DR2 G magnitude 14.7190
Occultation Star Gaia DR2 bp magnitude 15.5774
Occultation Star Gaia DR2 rp magnitude 13.8122
Pred v9.0_2018ECM_DR2_Varda

1One standard deviation of random error.
2The UCAC bandpass (579-642nm) is between V and R.


Table 2: Reference Star Position
Reference star position:
(Gaia DR2)		 RA (h:m:s; J2000) Dec (d:m:s; J2000) Notes
Varda.20180910 DR2 17 18 25.1249 + 0.17 mas –02 05 14.428 + 0.15 mas
epoch of occultation (DR2 proper motion applied)

Potential Star Duplicity:
We would like to thank Gemini North for allocating 30 minutes of DDT, allowing us to study the target star for potential duplicity through speckle imaging. The data acquired with the 'Alopeke instrument indicates that the star is not a binary.
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  
Varda

–0.0195± 0.0072

 +0.1202± 0.0089 See Notes 5 and 6; Includes Varda Center of Light to Center of Body correction

3Measured position corresponds to RA offset of +0.0 and Dec. offset of –0.0.

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 Gaia DR2 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 positions for Varda are those given by JPL Horizon's ephemeris (Varda source file: JPL7; Earth center source file: DE431).

6Data from the 4.3-m Discovery Channel Telescope (DCT) over the span of 8 months were reduced with respect to stars in the Gaia DR2 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 Varda: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, and (iii) sinusoidal terms with Ilmare's 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)
WIRO
–71 29 06
42 36 36
0.107
56 E.
10.82
La Luz Observatory
–101 19 29
21 03 11
2.4
612 E.
10.77
Mayhill, NM
–107 11 05
33 58 36
-
226 E.
10.77
Hereford Arizona Obs
–110 14 16
31 27 08
1.420
108 W.
10.76
UAZ Space Sciences Building
–110 56 54
32 13 58
-
169 W.
10.76
DCT
–111 25 20
34 44 40
2.36
225 W.
10.77
USNO - Flagstaff Station
–111 44 24
35 11 00
2.305
254 W.
10.77
West Mountain Obs. (BYU)
–111 49 33
40 05 14
-
322 W.
10.77
SRO
–119 24 48
37 04 14
1.41
857 W.
10.76
Geocenter
----------
---------
center of Earth
3927 W.
10.86

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 Varda's shadow in the shadow plane. The errors on all closest approach distances are ±249 km (one standard deviation). "S." means the site is south of the center of Varda's shadow. "N." means the site is north of the center of Varda's shadow.

Table 5: Varda.20180910 Occultation Predictions for Individual Sites

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

KBO Emersion (UT)9
West Mountain Obs. (BYU)
03:34:12
03:34:29
39
–21
03:34:46
WIRO
03:33:57
03:34:31
35
–24
03:35:05
SRO
-----------
03:34:40
45
–16
-----------
USNO - Flagstaff Station
03:34:46
03:35:11
42
–23
03:35:36
DCT
03:34:48
03:35:15
43
–23
03:35:43
UAZ Space Sciences Building
03:35:08
03:35:39
44
–25
03:36:09
Mayhill, NM
03:35:16
03:35:43
40
–29
03:36:11
Hereford Arizona Obs
03:35:14
03:35:47
44
–26
03:36:20
La Luz Observatory
-----------
03:37:44
42
–38
-----------
Geocenter
-----------
03:39:38
-----------
-----------
-----------

9The errors on all times are ±0:28 (28 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.

 

Last updated by Carlos Zuluaga (czuluaga@mit.edu) 2018-09-08 13:06 UT

Please direct all inquiries to PAL (planetary-astronomy@mit.edu)

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