Binary trans-Neptunian 1999 TC36

Binary trans-Neptunian 1999 TC36

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This is the first binary trans-Neptunian that has been discovered in 3:2 mean-motion resonance with Neptune since Pluto's moon Charon was discovered. 1999 TC36 is also one of the brightest trans-Neptunians (also called Kuiper belt objects or KBOs) with a visible magnitude of V=20. There have been three other binary trans-Neptunians (also called Kuiper belt objects) discovered: 1998 WW31, 2001 QT297, and 2001 QW322.

IAU Circular 7787 announces 1999 TC36's companion.

Raw Data

The raw data were collected on 2 orbits spanning about 45 minutes on UT 2001 Dec 08 21:44 and UT 2001 Dec 09 07:22 with the Hubble Space Telescope (HST) STIS instrument. Below is a movie of the 16 raw images collected. There are many cosmic rays (white "static") and CCD defects (white background pixels) apparent. The trans-Neptunian 1999 TC36 appears in the center, with its companion towards the upper right. In these images, North is 19 degrees clockwise from vertical (due to the HST rotation angle).

Raw data movie

Processed Data

Each of the 2 processed images are the median combination of the eight images taken each day after subtracting background sources. The two processed images appear below with logarithmic scaling to emphasize the companion, which is about 8 times fainter than the primary (about 20 times less massive, assuming they are made of the same material). The HST STIS instrument has pixels that are 50 milli-arc-seconds (mas) in angular size, which corresponds to a physical size of about 1100 km (710 miles) in these images. The objects are both smaller than a single pixel; the brighter object is probably less than 730 km (450 miles) in diameter. The objects look larger than one pixel because of the optics in the telescope.

Visit A
2001 Dec 08 21:44 (Universal Time),
365.1 +/- 1.1 mas separation (8280 km or 5180 miles).
Visit B
2001 Dec 09 07:22 (Universal Time),
369.8 +/- 0.7 mas separation (8390 km or 5240 miles).

There is a one-quarter pixel motion between visits. This is easy to measure if you are careful, but hard to see in the images. The graph below shows the companion separation from the primary, in milli-arc-seconds (mas). 1 mas is about 23 km (14 miles) in these images. This motion corresponds to about 280 km (170 miles) over the course of about 10 hours, or about 1.7 degrees. In the graph, 1999 TC36 (the brighter object) is at 0,0 while the companion object's position is represented by the points. Statistical measurement errors account for the spread in positions for each visit.

Companion Motion

Recovery

The position of the companion must be measured several more times before its orbit is known (this process is called recovery). The Hubble Space Telescope is the best instrument to use for recovery. Recovery will be very difficult from the ground due to the small separation and large brightness difference between 1999 TC36 and its companion. To illustrate this, below are three simulated images of 1999 TC36 under 0.2, 0.3, and 0.4 arc-second seeing conditions. Each has been scaled to emphasize the secondary object, however, it is apparent that seeing of about 0.4 arc-seconds is needed to even see any asymmetry, while you need at least 0.3 arc-second seeing to start measuring the position of the secondary for orbital information. Typical seeing for the best nights at the best astronomical sites in the world is about 0.6 arc-seconds for visible wavelengths, and may approach 0.4 at red/near infrared wavelengths. Adaptive optics correction during appulse events may be a better way to recover this object from the ground, as this allows 0.1 arc-second seeing or better.

0.2 arc-second seeing
0.2 arc-second seeing
0.3 arc-second seeing
0.3 arc-second seeing
0.4 arc-second seeing
0.4 arc-second seeing

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Chad Trujillo
email: trujillo at gemini dot edu

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