Abstract.A spectroscopic survey of a small area at the center of the Lupus 3 star-forming core has revealed four new mid-tolate M-type members, including a M7.5 brown dwarf. One of the new members, classified as M5, displays prominent forbidden lines and strong Hα emission (EW(Hα) = 410 Å), in addition to other permitted lines, and its luminosity is far below that of other members of the region with similar or later spectral types. We estimate a mass accretion rate rate of ∼1.4 × 10 −9 M yr −1 for this object, although with uncertainties that do not exclude values as low as 10 −10 M yr −1 . Based on the Hα/[SII] ratio, the detection of HeI, and the CaII infrared triplet, we argue that most of the Hα emission is produced near the surface of the object, probably in accretion columns or at the base of jets, rather than in a low-density extended region. The strong emission-line spectrum superimposed on an unusually faint photospheric continuum thus seems to be a real, intrinsic feature rather than a result of the viewing geometry caused by an edge-on disk blocking the light from the central object. Other Lupus 3 late-type members also display noticeable underluminosity, all of them having EW(Hα) > 100 Å as a result of the faint underlying continuum. We tentatively interpret these findings as evidence for the pre-main sequence evolution of objects with very low (possibly substellar) initial masses being significantly modified by accretion.

Abstract. We present the results of the first extensive mid-infrared (IR) imaging survey of the ρ Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main ρ Ophiuchi molecular cloud L1688, as well as the two secondary clouds L1689N and L1689S, have been completely surveyed for point sources at 6.7 µm and 14.3 µm. A total of 425 sources are detected in ∼0.7 deg 2 , including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fν ∼ 10-15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the ρ Ophiuchi central region. There are, however, reasons to believe that several tens of Class III YSOs remain to be identified below L ∼ 0.2 L . The mid-IR luminosities of most (∼65%) Class II objects are consistent with emission from purely passive circumstellar disks. The stellar luminosity function of the complete sample of Class II YSOs is derived with good accuracy down to L ∼ 0.03 L . It is basically flat (in logarithmic units) below L ∼ 2 L , exhibits a possible local maximum at L ∼ 1.5 L , and sharply falls off at higher luminosities. A modeling of the luminosity function, using available pre-main sequence tracks and plausible star formation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the initial mass function (IMF) of the embedded cluster. After correction for the presence of unresolved binary systems, we estimate that the IMF in ρ Ophiuchi is well described by a two-component power law with a low-mass index of −0.35 ± 0.25, a high-mass index of −1.7 (to be compared with the Salpeter value of −1.35), and a break occurring at M flat = 0.55 ± 0.25 M . This IMF is flat with no evidence for a low-mass cutoff down to at least ∼0.06 M .

Abstract. We present results from an ISOCAM survey in the two broad band filters LW2 (5−8.5 µm) and LW3 (12−18 µm) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 µm band and 139 in the 14.3 µm band to a limiting sensitivity of ∼2 mJy. We identified 53 Young Stellar Objects (YSOs) with mid-IR excess from the single colour index [14.3/6.7], and 8 additional YSOs from the H − K/K − m 6.7 diagram. Only 32 of these 61 sources were previously known to be YSO candidates. Only about 50% of the mid-IR excess sources show excesses in the near-IR J − H/H − K diagram. In the 48 square arcmin field covering the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as ρ Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW−SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for ρ Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a "microburst" of star formation has taken place very recently, probably within the last 10 5 yrs.Key words. stars: formation -stars: pre-main-sequence -stars: luminosity function, mass functionstars: low-mass, brown dwarfs -ISM: individual objects: Serpens Cloud Core Send offprint requests to: A. A. Kaas, e-mail: akaas@not.iac.es Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with participation of ISAS and NASA. Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via