2 Observations and data analysis

The program stars were taken from the spectroscopic follow-up surveys of ROSAT X-ray sources in Orion and Taurus-Auriga SFRs by A96 and Wi96, respectively, and from a study of EINSTEIN X-ray sources in Scorpius OB2-2 SFR by Wa94 and of X-ray sources east of the Scorpius-Centaurus-Lupus OB association (Walter 1986, hereafter Wa86). Our uvby-$\beta$ observations complement these previous studies. They avoid systematic errors due to different observers, equipment, calibrations or reduction procedures and represent a homogeneous large photometric sample of WTTS and WTTS candidates.

For convenience purposes, the spatial distributions of our program stars in Taurus-Auriga and Scorpius OB2-2 association are shown in Figs. 1 and 2, respectively.

   
2.1 The near IR photometry of Taurus-Auriga stars

The JHK photometric data of 20 active stars in Taurus-Auriga as presented in Table 1 were collected with the 2.1 m telescope of the Observatorio Astronómico Nacional in Sierra San Pedro Mártir (SPMO, Baja California (México) in November/December 1992. The telescope was used in the f/27 IR configuration mode with a N2-cooled InSb diode detector. The chopping frequency was set to 10 Hz and a throw of 30'' in the delta direction and a 14'' diaphragm were used throughout the observing run. The set of standard stars for San Pedro Mártir Observatory given by Tapia et al. (1986) was used here to tie the observations to the standard system (see also Carrasco et al. 1991). At least four standard stars were observed nightly. The photometry was reduced following usual procedures (e.g. Mitchell 1960; Chavarría et al. 2000). Details on the equipment, observing procedures and quality of the resulting photometry are given in A96 (see also references therein and remarks in A98), since the equipment and set of standards used are the same in both works. The standard errors of the IR magnitudes of Table 1 are typically $\sigma_{J}=0.03$, $\sigma_{H}=0.04$ and $\sigma_{K}=0.04$.

Figure 1: The spatial distribution of the RASS WTTS in Taurus-Auriga (open circle) and in Orion SFRs (filled circle)

   
2.2 The uvby-$\beta$ photometry of program stars

The uvby-$\beta$ photometry of 116 active PMS stars associated with Orion, Taurus-Auriga and Scorpius OB2-2 SFRs was collected during four observing runs from 1994 to 1996 at SPMO using the Danish six channel grating spectrophotometer attached to the Harold L. Johnson 1.5 m telescope. The photometer consists of an entrance slit, a grating as its dispersing element, six exit slots and filters that match the Strömgren/Crawford original filters fairly well and six uncooled photomultipliers as light detectors in the photon counting mode. Four channels are used for simultaneous uvby observations and two for the $\beta$-index measurements (the "wide'' and "narrow'' H$\beta$ passbands, which are measured simultaneously). One switches swiftly from one observing mode to the other with a computer command from the data acquisition system. When observing any given star, we made the $\beta$-measurements immediately after the uvby observations. For more details about the instrument see Nissen (1984); Terranegra et al. (1994, T94 hereinafter) and A96. The integration time on each star was normally fixed in order to achieve a photon noise of less than 1$\%$ in the weakest flux of a given passband (usually u or v). Sky was normally measured 30'' east of the star and, if necessary, intercalated between two 30 second integrations of the  star + sky  measurements as many times as necessary, until the photon noise error of 1% could be achieved. Some program stars could not be measured with the quoted accuracy. The $\beta$ index was usually determined with a 2% or better precision. Finally, the seasonal standard stars were each observed twice or more in, at least, one of the four independent runs. The data were reduced to the standard system following usual procedures (e.g. A96, T94, Chavarría et al. 2000). The standard system is taken from the lists by Crawford et al. (1971, 1973) and by Olsen (1983, 1984). On average, about 85 standard stars were observed per season and constitute over 100 standard stars which were used to tie the observations of the different runs to the uvby-$\beta$ standard system. We took care to include reference stars covering the spectral types (F5-M5) and luminosity classes (V and III) of the program stars. The resulting uvby-$\beta$ photometry of the program stars observed here is displayed in Tables 2 to 4. The typical standard errors for the photometry of the program stars with $V\leq 13^{\rm m}$ are $\sigma_{V}$ = 0.010, $\sigma_{b-y}$ = 0.013, $\sigma_{m1}$ = 0.015, $\sigma_{c1}$ = 0.020 and $\sigma_{\beta}$ = 0.02 and thrice as large for stars with $V \geq 14^{\rm m}$.

Figure 2: The spatial distribution of the EO WTTS in Scorpius OB2-2