In the past years, extensive radio-continuum surveys conducted with the Very Large Array (VLA) have shown that only approximately 10 of the approximately 100 young stellar objects (YSOs) embedded in the central region of the rho Oph molecular cloud have "strong" radio emission above S6 cm = 1 mJy. Two of these rho Oph radio stars, S1 and DoAr 21, have recently been detected and resolved by very long baseline interferometry (VLBI), implying that their radio emission is "peristellar," i.e., originates in magnetic structures up to approximately 25 R* in diameter. We report here VLBI observations of nine other rho Oph YSOs, yielding positive detections in five cases (VSSG 14, WL 5, ROC 16, ROX 39, and VSSG 11), with flux densities ranging from 1.2 to 9 mJy at 3.6 cm. In contrast, while simultaneously detected by the VLA at levels of a few mJy, the protostellar outflow source IRAS 16293 and the "protostar" candidate YLW 15 were apparently overresolved and undetected on the VLA-Goldstone baseline. The radio emission from the young A star VSSG 14 was measured to be circularly polarized, at a level (approximately 6%) comparable to what had previously been observed for the embedded magnetic B star SI. The brightness temperatures (T(b)) measured for the VLBI detected YSOs range from approximately 10(7) to greater than or similar to 8 x 10(7) K, unambiguously establishing that these radio sources are nonthermal, and not the result of circumstellar ionized winds as is probably the case for IRAS 16293 and YLW 15. The present VLBI observations thus confirm our earlier suggestion that most "strong" (S6 cm greater than or similar to 1 mJy) stellar radio sources of rho Oph are magnetic in character (i.e., nonthermal). When optically visible, these radio emitters are associated with " weak-line " T Tauri stars (DoAr 2 1, ROX 39) or young B-A stars without emission lines (S1 and VSSG 14). Others are optically invisible, low-luminosity YSOs with no or little infrared excesses (e.g., WL 5, VSSG 11). It appears that these nonthermal radio emitters make up a specific class of YSOs characterized by the absence of dense circumstellar disks and the presence of extended, organized magnetic structures reminiscent of those found around magnetic Bp-Ap stars. Remarkably, the magnetized radio YSOs of rho Oph are spatially more concentrated toward the cloud core than the majority of T Tauri stars, suggesting they are younger. Some of the youngest YSOs may thus be already "naked." It is tempting to speculate that, compared to classical T Tauri stars, the diskless, nonthermal radio stars have conserved particularly strong fossil magnetic fields, which caused fast clearing of their circumstellar material.
