1 Introduction

The population of CSSs (Compact Steep-spectrum Sources) & GPSs (GHz Peaked-spectrum Sources) (see O'Dea, 1998 for a review) has received recently an increasing attention as at least the two sided (or symmetric) objects of this class are considered to be the young stage in the radio source evolution (Fanti et al. 1995; Readhead et al. 1996), from the Compact (CSO, size <1 h^-1kpc) through the Medium (MSO, 1 h^-1 kpc < size < 20 h^-1 kpc) to the Large Symmetric Object (LSO, size > 20 h^-1 kpc). A strong support for this interpretation comes from the large expansion velocities recently measured in a handful of sub-kpc sources (see compilation in Fanti 2000 and references therein) and from the determination of the radiative ages of a larger number of CSSs spanning a range of linear size from sub-kpc to $\sim$20 h^-1 kpc (Murgia et al. 1999).

Starting from the seminal paper by Baldwin 1982, models for radio source evolution have been presented by several authors which link the CSS/GPS population to the larger size sources, (see e.g. Begelman 1996; Kaiser & Alexander 1997).

All models essentially predict a decrease of the source luminosity with time, and therefore with increasing source linear size. A key test for the models is the ability to reproduce the distribution of sources in the "Radio Power - Linear Size" ( $P_{\rm R} - LS$) plane (Baldwin 1982). CSSs/GPSs seem to be in the right proportion as compared with model expectations (Fanti et al. 1995; Readhead et al. 1996). However, from the samples studied so far the statistics have been not very large and suspicions were raised on some "irregularities" in the LS distribution (O'Dea & Baum 1997) and on the, perhaps too high, number of very small size CSSs, the GPSs (Polatidis et al. 1999; Fanti & Fanti 2001).

Recently we have selected a new sample of CSSs from the B3-VLA catalogue (Vigotti et al. 1989) aimed at increasing significantly the statistics on sources with LS in the range $0.4~h^{-1} \leq LS$(kpc) $\leq 20~h^{-1}$ (Fanti et al. 2001, hereafter referred to as Paper I). This sample, consisting of 87 CSSs ( $LS \leq 20~h^{-1}$ kpc), has VLA observations at 1.4 GHz (A and C configurations), 5 and 8.4 GHz (both A configuration). About 50% of these sources were not resolved or were poorly resolved even at 8.4 GHz (beam size $\sim$0.2$\arcsec$). For them two VLBI observing projects were undertaken with the VLBA and EVN & MERLIN.

This paper reports on the results of the VLBA observations of the most compact sources. The EVN & MERLIN observations instead are presented in a companion paper by Dallacasa et al. (2002, Paper III).

 

 

Table 1: The VLBA sample: see text for a description of the columns.

Source	Id	mR	z	LAS	Log $P_{0.4 \rm GHz}$	$S_{1.67}^{\rm VLA}$	$S_{1.67}^{\rm VLBA}$	$LAS_{\rm VLBA}$	LLS	Morph.
				mas	W/Hz h-2	mJy	mJy	mas	kpc h-1
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)
0039+373	G		1.006	100	27.37	784	779	120	0.50	CSO
0147+400	E			100	>26.6	631	563	95	$\sim$0.40	scJ?$^\ddag$
0703+468	E			80	>26.6	1392	1356	75	$\sim$0.32	CSO
0800+472 $\ast$	E			1000	>26.8	769	447	160	$\sim$0.67	scJ?$^\ddag$
0809+404 $\ast$	G		0.551	1200	26.90	929	522	80	0.28	?
0822+394	G		1.180	50	27.33	1003	942	70	0.30	CSO
0840+424A	E			80	>26.8	1243	1187	135	$\sim$0.57	CSO
1007+422 $\ast$	E			130	>26.4	370	313	265	$\sim$1.12	MSO
1008+423	E			50	>26.5	521	498	115	$\sim$0.48	CSO
1016+443	G	19.7	0.33  R	110	26.11	287	266	155	0.44	CSO
1044+454 $\ast$	G	24.8	4.10  K	1000	28.82	352	311	175	0.55	?
1049+384 $\ast$	G	20.9	1.018	100	27.14	574	502	210	0.89	CSO?
1133+432	E			70	>26.3	1247	1265	45	$\sim$0.20	CSO
1136+383 $\ast$	E			50	>26.4	400	377	70	$\sim$0.28	CSO
1136+420	G	21.7	0.829	1000	a 26.9	402	279	120	0.49	?
1159+395	G	23.4	2.370	50	27.57	535	498	70	0.26	CSO
1225+442	G	18.2	0.22  R	200	27.50	316	291	420	0.95	CSO
1242+410	Q	19.7	0.811	40	27.08	1215	1126	70	0.29	CSO?
1314+453A$\ast$	G	21.8	1.544	160	27.77	574	493	185	0.79	CSO
1340+439	E			70	>26.5	447	397	130	$\sim$0.56	CSO
1343+386 $\ast$	Q	17.5	1.844	110	27.70	793	717	130	0.54	CSO
1432+428B$\ast$	E			40	>26.3	809	706	50	$\sim$0.21	CSO
1441+409	E			100	>26.7	834	768	130	$\sim$0.56	CSO
1449+421	E			80	>26.9	639	592	120	$\sim$0.50	CSO
2304+377	G	19.7	0.40  R	100	26.44	1299	1217	150	0.49	CSO
2330+402	E			70	>26.7	730	705	100	$\sim$0.43	CSO
2348+450 $\ast$	G		0.978	200	27.36	630	520	310	1.32	MSO
2358+406	E			80	>26.8	1159	1161	120	$\sim$0.47	CSO

^a The value given in Fanti et al. (2001) is wrong.

$^\ddag$ Steep-core Jet: elongated structure, very asymmetric in brightness, where the brightest component
   is at one source extremity and has a steep spectrum.

Figure 1: Typical uv coverage.