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Stellar Diameters

A scratch note in order to try to estimate the angular diameter of a star from asteroidal occultation timings:

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Occultation of TYC 5757-0353-1 by (420) Bertholda on 2003-Aug-26 @ 21h 46.2m UT
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Estimating the angular diameter Theta of the target star from the observations

Dur. disap. Dur. reapp. Dist Method Observer Note
0.08 +- 0.02 0.04 +- 0.02 62.1 VID + DCF77 A.Knoefel, G.Lehmann -
0.49 +- 0.50 +- 45.7 VID + DCF77 K.Bauer 0
0.4 +- 0.25 0.4 +- 0.25 35.4 CCDscan C.Rinner -
0.04 +- 0.02 0.04 +- 0.02 23.3 VID + DCF77 R.Bitzer 1
0.06 +- 0.02 0.04 +- 0.02 -06.1 VID + DCF77 J.Manek 2
sharp sharp -08.8 VID + DCF77 R.Goncalves, D.Nunes 3
0.3 -20.5 VIS P.Chevalley 4

Dist. means distance from centerline (>0 = north from center) in km according to J.Lecacheux (V0.22)

Notes:
0: analysis error ?
1: 2 half frames for D and R assumed from images on website. 1HF accuracy assumed (0.02s)
2: Manek reports about 3 half frames for D and 2-3 half frames for R. 1HF (0.02s) accuracy assumed.
3: no dimming reported
4: visual observation


Asteroid:
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Distance to Earth: 2.6AU
Shadow speed: 12.6 km/s
Total motion: 23.67 "/h = 6.575 mas/s


Star: TYC 5757-0353-1 = HD 199046
SPEC: F5V (source: TYC) -> mean sequence star
Teff: 6440K (source: TYC)

Photometry:
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TYC-2 DENIS 2-MASS
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B 9.200 - -
V 8.673 - -
K - 7.276+-0.06 7.304+-0.02
V-K - 1.397 1.369
B-K - 1.924 1.896

=> Theta_(V=0) = 7.4 and 7.3 mas -> Theta_theo = 0.13 mas (2-MASS value)
=> Theta_(B=0) = 11.4 and 11.2 mas -> Theta_theo = 0.16 mas (2-MASS value)
Error bars for theoretical value (formular fit parameter + K-mag error) is about 0.05-0.06 mas.

The angular size Theta was calculated using the "zero-magnitude angular size vs V-K,B-K colors" method.
Ref.: G.T. van Belle. Predicting Stellar Angular Sizes. PASP 111(1999)1515-1523.

First maximum of the diffraction pattern is in about
0.4-0.5 km distance (500nm - 900nm wavelength) = 0.19-0.26 mas = 0.03-0.04s pass time
=> diffraction should be smoothed because angular diameter in the same order !???

Angular diameter from reappearance (disapp. might be biased by camera "after glow")
by mean of (Knoefel+Bitzer+Manek) = 0.04s : Theta_obs = 0.26 mas (m.e. >= 0.13 mas = 0.02s in timing)

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Summary: angular diameter of TYC 5757-0353-1
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Theta_obs = 0.26 +- 0.13 mas (m.e. lower limit)
Theta_the = 0.15 +- 0.06 mas (mean from V-K,B-K)

---
M.Kretlow, IOTA-ES, 2003-09-03 [mike@kretlow.de]

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