The Tycho-Gaia astrometric solution - How to get 2.5 million parallaxes with less than one year of Gaia data

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Table 1

Uncertainties of the astrometric parameters when processing 0.5 yr of simulated Gaia data jointly with Tycho and Hipparcos priors, nominal scenario.

Mag.	Number^a	Position	Parallax	Prop. motion
		[μas]	[μas]	[μas yr^-1]

Subset Tycho without Hipparcos
6–7	411	244	399	198
7–8	8072	198	348	264
8–9	63 630	191	327	403
9–10	257 243	230	407	680
10–11	686 866	329	601	1145
11–12	993 139	379	722	1522
≥12	302 511	349	702	1615
all (≥6)	2 311 872	332	631	1259

Subset Hipparcos
6–7	9381	116	180	17
7–8	23 679	120	192	21
8–9	40 729	125	198	29
9–10	27 912	133	217	39
10–11	8563	154	253	58
11–12	2501	128	211	87
≥12	630	151	248	135
all (≥6)	113 395	127	203	32

Notes. Nominal scenario refers to the results obtained from a simulation without data gaps (see Sect. 4.2) and using the full Hipparcos prior (see Sect. 4.3). Uncertainties are calculated as the Robust Scatter Estimate (RSE; see footnote 18 in Lindegren et al. 2012) of the differences between estimated parameters and “true” values.

^(a)

A small fraction of stars present in the Hipparcos and Tycho Catalogues is not observed in this simulated 0.5 yr interval of Gaia observations.

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