2 Sample selection

Our list of 28 targets ( $12.3\le I\le19.6$, $T_{\rm eff}$$\sim$4200-2400K) comprises $\sigma$Orionis solar-mass and low mass stars, and brown dwarfs selected from different surveys (see Table 1). All have been identified as genuine cluster members using various techniques. Stars labelled with "4771'' and "r'' were first identified by Wolk (1996). VRIJHKphotometry, spectroscopy, and strong X-ray detections (in many cases) are available. Wolk (1996) provided equivalent widths of the Li I resonance doublet for a few of these stars. However, his spectroscopic data of relatively faint sources have poor signal-to-noise (S/N) ratios, which severely affects the measurements. We decided to re-observe them to improve the quality of the spectra. SOri targets (IAU nomenclature) have been selected from the RIJsurvey of Béjar (2001). They nicely fit in the cluster optical-infrared sequence. Albeit we lack previous spectroscopic data for them, the spectra presented here confirm them as very active, young sources, and therefore, they have to be cluster members. The two brown dwarfs in our sample, SOri27 and 45, have been taken from Béjar et al. (1999), where they are discussed at length.

Names, coordinates and I magnitudes of our sample are provided in Table 1. Their location in the optical color-magnitude diagram is illustrated in Fig. 1, where RI photometry has been taken from Wolk (1996), Béjar et al. (1999) and Béjar (2001). Overplotted are the solar metallicity, and "no-dust'' 3Myr and 5Myr isochrones of D'Antona & Mazzitelli (1997) and Baraffe et al. (1998), respectively. Masses as predicted by the models are indicated in the figure. Comparisons with other tracks are provided in Béjar et al. (1999). Our targets have masses ranging from 1.2$M_{\odot}$ down to roughly 0.02$M_{\odot}$.

Figure 1: Optical color-magnitude diagram of our $\sigma$Orionis targets (large symbols). Open circles indicate sources with H$\alpha$ emissions larger than pEW=10Å. Open triangles stand for cluster members with forbidden emission lines. SOri objects (small crosses) from Béjar et al. (1999) are included in the figure for a better delineation of the $\sigma$Orionis cluster photometric sequence. Overplotted onto the data are the no-dust 5Myr isochrone (dashed line) of Baraffe et al. (1998) and the 3Myr isochrone (solid line) of D'Antona & Mazzitelli (1997), for which masses in solar units have been labelled. Spectral types as a function of the (R-I) color are also provided.

Figure 2: CAHA spectra. Data have been normalized to the counts at around 7050Å and shifted by 1 in the upper panel and by 1.5 in the lower panel for clarity.

Figure 3: ORM spectra. Data have been normalized to the counts at around 6535Å and shifted by 1 for clarity.

Figure 4: Keck spectra of two brown dwarfs in the $\sigma$Orionis cluster. Data have been normalized to the counts at around 7050Å and shifted by 1.5 for clarity. The spectrum of SOri45 has been smoothed with a boxcar of 5 pixels to increase the S/N ratio of the data.

Figure 5: McDonald spectra. Data have been normalized to the counts at around 6540Å and shifted by 1 for clarity.