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Gas-phase complex organic molecules have been detected toward a range of high- and low-mass star-forming regions at abundances which cannot be explained by any known gas-phase chemistry. Recent laboratory experiments show that UV irradiation of CH3OH-rich ices may be an important mechanism for producing complex molecules and releasing them into the gas phase. To test this ice formation scenario, we mapped the B1-b dust core and nearby protostar in CH3OH gas using the IRAM 30 m telescope to identify locations of efficient non-thermal ice desorption. We find three CH3OH abundance peaks tracing two outflows and a quiescent region on the side of the core facing the protostar. The CH3OH gas has a rotational temperature of [?]10 K at all locations. The quiescent CH3OH abundance peak and one outflow position were searched for complex molecules. Narrow, 0.6-0.8 km s-1 wide, HCOOCH3 and CH3CHO lines originating in cold gas are clearly detected, CH3OCH3 is tentatively detected, and C2H5OH and HOCH2CHO are undetected toward the quiescent core, while no complex molecular lines were found toward the outflow. The core abundances with respect to CH3OH are [?]2.3% and 1.1% for HCOOCH3 and CH3CHO, respectively, and the upper limits are 0.7%-1.1%, which is similar to most other low-mass sources. The observed complex molecule characteristics toward B1-b and the pre-dominance of HCO-bearing species suggests a cold ice (below 25 K, the sublimation temperature of CO) formation pathway followed by non-thermal desorption through, e.g., UV photons traveling through outflow cavities. The observed complex gas composition together with the lack of any evidence of warm gas-phase chemistry provides clear evidence of efficient complex molecule formation in cold interstellar ices.

We present Spitzer InfraRed Spectrograph (IRS) mid-infrared (5-35 mm) spectra of a complete flux-limited sample ([?] 3 mJy at 8 mm) of young stellar object (YSO) candidates selected on the basis of their infrared colors in the Serpens Molecular Cloud. Spectra of 147 sources are presented and classified. Background stars (with slope consistent with a reddened stellar spectrum and silicate features in absorption), galaxies (with redshifted polycyclic aromatic hydrocarbon (PAH) features), and a planetary nebula (with high ionization lines) amount to 22% of contamination in this sample, leaving 115 true YSOs. Sources with rising spectra and ice absorption features, classified as embedded Stage I protostars, amount to 18% of the sample. The remaining 82% (94) of the disk sources are analyzed in terms of spectral energy distribution shapes, PAHs, and silicate features. The presence, strength, and shape of these silicate features are used to infer disk properties for these systems. About 8% of the disks have 30/13 mm flux ratios consistent with cold disks with inner holes or gaps, and 3% of the disks show PAH emission. Comparison with models indicates that dust grains in the surface of these disks have sizes of at least a few mm. The 20 mm silicate feature is sometimes seen in the absence of the 10 mm feature, which may be indicative of very small holes in these disks. No significant difference is found in the distribution of silicate feature shapes and strengths between sources in clusters and in the field. Moreover, the results in Serpens are compared with other well-studied samples: the c2d IRS sample distributed over five clouds and a large sample of disks in the Taurus star-forming region. The remarkably similar distributions of silicate feature characteristics in samples with different environment and median ages--if significant--imply that the dust population in the disk surface results from an equilibrium between dust growth and destructive collision processes that are maintained over a few million years for any YSO population irrespective of environment.

Observations of Lynds Dark Nebula 1221 from the Spitzer Space Telescope are presented. These data show three candidate protostars toward L1221, only two of which were previously known. The infrared observations also show signatures of outflowing material, an interpretation which is also supported by radio observations with the Very Large Array. In addition, molecular line maps from the Five College Radio Astronomy Observatory are shown. One-dimensional dust continuum modeling of two of these protostars, IRS1 and IRS3, is described. These models show two distinctly different protostars forming in very similar environments. IRS1 shows a higher luminosity and a larger inner radius of the envelope than IRS3. The disparity could be caused by a difference in age or mass, orientation of outflow cavities, or the impact of a binary in the IRS1 core.

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these data sets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M). As in Perseus, the mid-infrared sources are located close to the center of the SCUBA cores and the narrowness of the spatial distribution of mid-infrared sources around the peaks of the SCUBA cores suggests that no significant dispersion of the newly formed YSOs has occurred. Embedded YSOs are found in 35% of the SCUBA cores--fewer than in Perseus (58%). On the other hand the mid-infrared sources in Ophiuchus have less red mid-infrared colors, possibly indicating that they are less embedded. We apply a nearest neighbor surface density algorithm to define the substructure in each of the clouds and calculate characteristic numbers for each subregion--including masses, star formation efficiencies, fraction of embedded sources, etc. Generally the main clusters in Ophiuchus and Perseus (L1688, NGC 1333, and IC 348) are found to have higher star formation efficiencies than small groups such as B1, L1455, and L1448, which on the other hand are completely dominated by deeply embedded protostars. We discuss possible explanations for the differences between the regions in Perseus and Ophiuchus, such as different evolutionary timescales for the YSOs or differences, e.g., in the accretion in the two clouds.

We discuss the results of the optical spectroscopic follow-up of pre-main-sequence (PMS) objects and candidates selected in the Chamaeleon II dark cloud based on data from the Spitzer Legacy survey "From Molecular Cores to Planet Forming Disks" (c2d) and from previous surveys. Our sample includes both objects with infrared excess selected according to c2d criteria and referred to as young stellar objects and other cloud members and candidates selected from complementary optical and near-infrared data. We characterize the sample of objects by deriving their physical parameters. The vast majority of objects have masses M [?] 1 M and ages <6 Myr. Several of the PMS objects and candidates lie very close to or below the hydrogen-burning limit. A first estimate of the slope of the initial mass function in Cha II is consistent with that of other T associations. The star formation efficiency in the cloud (1%-4%) is consistent with our own estimates for Taurus and Lupus, but significantly lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds may reflect a different history of star formation. We also find that the Cha II cloud is turning some 8 M into stars every megayear, which is less than the star formation rate in the other c2d clouds. However, the star formation rate is not steady and evidence is found that the star formation in Cha II might have occurred very rapidly. The Ha emission of the Cha II PMS objects, as well as possible correlations between their stellar and disk properties, is also investigated.

We present observations of two areas totalling 0.57 deg2 in the IC 5146 star-forming region at 3.6, 4.5, 5.8, 8.0, 24, and 70 mm observed with the Spitzer Space Telescope. We reexamine the issue of the distance to this cloud and conclude a value of 950 +- 80 pc is most likely. We compare source counts, colors, and magnitudes in our observed region to a subset of the SWIRE data that was processed through our pipeline. We identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, many of which were previously unknown. We compare the colors of these YSOs to the models of Robitaille et al. and perform simple fits to the SED's to estimate properties of the circumstellar disks likely to surround the Class II and III sources. We also compare the mid-IR disk excesses to Ha emission-line data where available. We present a quantitative description of the degree of clustering, estimate the star formation efficiency, and discuss the fraction of YSOs in the region with disks relative to an estimate of the diskless YSO population. Finally, we compare the YSO distribution to the cold dust distribution mapped by SCUBA and briefly describe the diffuse emission likely due to PAHs associated with the H II region.

We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of ~1.75 deg2 and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime ( M [?] 0.03 M). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M pc [?]3 and 5-10 members each. A multiplicity fraction of about 13% +- 3% is observed for objects with separations 0.8'' < th < 6.0'' (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, D t [?] 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered.

We present maps of 14.4 deg2 of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and non-star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 mm and 81 sources at 70 mm at the 10 s level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus K [?] [ 24] color-magnitude diagrams, as compared to 24 mm detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates.

We present maps of 7.78 deg2 of the Lupus molecular cloud complex at 24, 70, and 160 mm. They were made with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) instrument as part of the Spitzer Legacy Program "From Molecular Cores to Planet-Forming Disks" (c2d). The maps cover three separate regions in Lupus, denoted I, III, and IV. We discuss the c2d pipeline and how our data processing differs from it. We compare source counts in the three regions with two other data sets and predicted star counts from the Wainscoat model. This comparison shows the contribution from background galaxies in Lupus I. We also create two color-magnitude diagrams using the 2MASS and MIPS data. From these results, we can identify background galaxies and distinguish them from probable young stellar objects. The sources in our catalogs are classified based on their spectral energy distribution (SED) from 2MASS and Spitzer wavelengths to create a sample of young stellar object candidates. From 2MASS data, we create extinction maps for each region and note a strong correspondence between the extinction and the 160 mm emission. The masses we derived in each Lupus cloud from our extinction maps are compared to masses estimated from 13CO and C18 O and found to be similar to our extinction masses in some regions, but significantly different in others. Finally, based on our color-magnitude diagrams, we selected 12 of our reddest candidate young stellar objects for individual discussion. Five of the 12 appear to be newly discovered YSOs.

With the Submillimeter Array (SMA) we have made high angular resolution (~1'' = 160 AU) observations of the protobinary system IRAS 16293-2422 in the HCN (4-3), HC15N (4-3), and 354.5 GHz continuum emission. The HCN (4-3) line was also observed using the JCMT to supply missing short-spacing information. The submillimeter continuum emission is detected from the individual binary components of source A in the southeast and source B in the northwest, with a separation of ~5''. The optically thin HC15N (4-3) emission taken with the SMA has revealed a compact (~500 AU) flattened structure (P.A. = -16deg) at source A. This compact structure shows a velocity gradient along the projected minor axis, which can be interpreted as an infalling gas motion. Our HCN image including the short-spacing information shows an extended (~3000 AU) circumbinary envelope, as well as the compact structure at source A. A toy model consisting of a flattened structure with radial infall toward a 1 M central star reproduces the HCN/HC15N position-velocity diagram along the minor axis of the HC15N emission. In the extended envelope there is also a northeast (blue) to southwest (red) velocity gradient across the binary alignment, which is likely to reflect gas motion in the swept-up dense gas associated with the molecular outflow from source A. Only a weak and narrow (~2 km s-1) compact HC15N emission is associated with source B, where no clear molecular outflow is identified, suggesting the different evolutionary starges between sources A and B. Our study demonstrates the importance of adding short-spacing data to interferometer data in order to probe the detailed structure and kinematics of low-mass protostellar envelopes.

This paper presents a large spectral line and continuum survey of eight deeply embedded, low-mass protostellar cores using the SMA. High-excitation line emission from 11 molecular species originating in warm and dense gas has been imaged at high angular resolution (1''-3'', typically 200-600 AU) together with continuum emission at 230 GHz (1.3 mm) and 345 GHz (0.8 mm). Compact continuum emission is observed for all sources, which likely originates in marginally optically thick circumstellar disks, with typical lower limits to their masses of 0.1 M (1%-10% of the masses of their envelopes) and a dust opacity law, kn [?] nb, with b [?] 1. Prominent collimated outflows are present in CO 2-1 observations in all sources. The most diffuse outflows are found in the sources with the lowest ratios of disk to envelope mass, and it is suggested that these sources are in a phase where accretion of matter from the envelope has almost finished and the remainder of the envelope material is being dispersed by the outflows. Other characteristic dynamical signatures are inverse P Cygni profiles indicative of infalling motions seen in the 13CO 2-1 lines toward NGC 1333 IRAS 4A and NGC 1333 IRAS 4B. Outflow-induced shocks are present on all scales in the protostellar environments and are most clearly traced by the emission of CH3OH in NGC 1333 IRAS 4A and NGC 1333 IRAS 4B. These observations suggest that the emission of CH3OH and H2CO from these proposed ``hot corinos'' is related to the shocks caused by the protostellar outflows. Only one source, NGC 1333 IRAS 2A, has evidence for hot, compact CH3OH emission coincident with the embedded protostar.

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Perseus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the Cores to Disks (c2d) legacy team and JCMT SCUBA submillimeter maps from the COMPLETE team. The mid-infrared sources detected at 24 mm and having [3.6] - [4.5] > 1 are located close to the center of the SCUBA cores, typically within 15'' of their peaks. The narrowness of the spatial distribution of mid-infrared sources around the peaks of the SCUBA cores suggests that no significant dispersal of the newly formed YSOs has occurred. This argues against the suggestion that motions of protostars regulate the timescales over which significant (Bondi-Hoyle) accretion can occur. The YSOs are found to have red [3.6] - [4.5] and [8.0] - [24] colors, but not comparable red [5.8] - [8.0] colors. The most deeply embedded YSOs are found in regions with high extinction, AV [?] 5, similar to the extinction threshold observed for the SCUBA cores. All the SCUBA cores with high concentrations have embedded YSOs, but not all cores with low concentrations are starless. From the above considerations, a relatively unbiased sample of 49 deeply embedded YSOs is constructed. Embedded YSOs are found in 40 of the 72 SCUBA cores, with only three cores harboring multiple embedded YSOs within 15''. The equal number of SCUBA cores with and without embedded YSOs suggests that the timescale for the evolution through the dense prestellar stages, where the cores are recognized in the submillimeter maps and have central densities of 5 x 104-1 x 105 cm-3, is similar to the timescale for the embedded protostellar stages. The current star formation efficiency of cores is estimated to be approximately 10%-15%. In contrast, the star formation efficiency averaged over the cloud lifetime and compared to the total cloud mass is only a few percent, reflecting also the efficiency in assembling cloud material into the dense cores actually forming stars.

We present IRAC (3.6, 4.5, 5.8, and 8.0 mm) observations of the Chamaeleon II molecular cloud. The observed area covers about 1 deg2 defined by AV > 2. Analysis of the data in the 2005 c2d catalogs reveals a small number of sources (40) with properties similar to those of young stellar or substellar objects (YSOs). The surface density of these YSO candidates is low, and contamination by background galaxies appears to be substantial, especially for sources classified as Class I or flat spectral energy distribution (SED). We discuss this problem in some detail and conclude that very few of the candidate YSOs in early evolutionary stages are actually in the Cha II cloud. Using a refined set of criteria, we define a smaller, but more reliable, set of 24 YSO candidates.

We investigate the structure of the core surrounding the recently identified deeply embedded young stellar object Barnard 1c. B1c lies within the Perseus molecular cloud at a distance of 250 pc. It is a deeply embedded core of 2.4 M (Kirk et al.) and a luminosity of 4 +- 2 L. Observations (and resolutions) of 12CO J = 1-0 (9.''2 x 5.''9), 13CO J = 1-0, C18O J = 1-0 (14.''3 x 6.''7), HCO+ J = 1-0 (7.''6 x 5.''8), and N2H+ J = 1-0 (5.''9 x 4.''6) were obtained with the BIMA array, together with the continuum at 3.3 mm (6.''4 x 4.''9) and 2.7 mm (9.''5 x 6.''3). Single-dish measurements of N2H+ J = 1-0 and HCO+ J = 1-0 with FCRAO reveal the larger scale emission in these lines with ~60 resolution. The 12CO and HCO+ emission traces the outflow extending over the full field of view (2.'1), which coincides in detail with the S-shaped jet recently found in Spitzer IRAC imaging. The N2H+ emission, which anticorrelates spatially with the C18O emission, originates from a rotating envelope with effective radius ~2400 AU and mass 2.1-2.9 M, as derived from the 3.3 mm continuum emission. N2H+ emission is absent from a 600 AU diameter region around the young star, offset from the continuum peak. The remaining N2H+ emission may lie in a coherent torus of dense material. With its outflow and rotating envelope, B1c closely resembles the previously studied object L483 mm, and we conclude that it is a protostar in an early stage of evolution, i.e., Class 0 or in transition between Class 0 and Class I. We hypothesize that heating by the outflow and star has desorbed CO from grains, which has destroyed N2H+ in the inner region, and surmise that the presence of grains without ice mantles in this warm inner region can explain the unusual polarization signature observed from B1c.

We present observations of 3.86 deg2 of the Perseus molecular cloud complex with the Spitzer Space Telescope Infrared Array Camera (IRAC). The maps show strong extended emission arising from shocked H2 in outflows and from polycyclic aromatic hydrocarbon features. More than 120,000 sources are extracted toward the cloud. Based on their IRAC colors and comparison to off-cloud and extragalactic fields, we identify 400 candidate young stellar objects. About two-thirds of these are associated with the young clusters IC 348 and NGC 1333, while the last third is distributed over the remaining cloud. The young stellar objects are classified according to the slope of their spectral energy distributions. Significant differences are found between the numbers of embedded Class I objects and more evolved Class II objects in IC 348, NGC 1333 and the remaining cloud, with the embedded Class I and "flat-spectrum" YSOs constituting 14%, 36% and 47% of the total number of YSOs identified in each of these regions. The high number of Class I objects in the extended cloud (61% of the Class I objects in the entire cloud) suggests that a significant fraction of the current star formation occurs outside the two main clusters. Finally, we discuss a number of outflows and identify their driving sources, including the deeply embedded Class 0 sources outside the two main clusters. The Class 0 objects are detected by Spitzer and have very red [3.6] - [4.5] colors, but they do not show similarly red [5.8] - [8.0] colors. The Class 0 objects are easily identifiable in color-color diagrams but are problematic to extract automatically due to the extended emission from shocked gas or scattered light in cavities related to the associated outflows.

We present maps of 0.89 deg2 of the Serpens dark cloud at 3.6, 4.5, 5.8, and 8.0 mm observed with the Spitzer Space Telescope Infrared Array Camera (IRAC). We discuss in detail the data processing carried out by the c2d team on IRAC data. More than 100,000 compact sources have been extracted, but we confine most of our discussion to the most reliable subset of these sources. This includes those that are detected above 7 s in all four IRAC bands or those detected in the two shorter IRAC bands together with 2MASS. We estimate completeness limits for our survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets, a 0.10 deg2 set of low-extinction regions near the dark cloud and a 1 deg2 subset of the SWIRE Elais N1 data that was processed through our pipeline. We find that it is possible to identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, most of which were previously unknown. In addition to the dense area of new star formation known before in the "core" region (cluster A), we also find a moderately rich area to the south (cluster B). Our mapped area also includes the Herbig Ae star VV Ser, whose Spitzer images have been carefully modeled in a separate study. The extreme sensitivity of Spitzer IRAC allows us to search to very low luminosity limits for young substellar objects. The comparison of the Serpens region with the reference areas suggests that a population of infrared excess sources exists in Serpens at least down to luminosities of L ~ 10-3 L and possibly lower.

Infrared ~5-35 mm spectra for 40 solar mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar mass stars limited primarily to the 10 mm region. The observed 10 and 20 mm silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with micron-sized grains indicating fast grain growth (from 0.1 to 1.0 mm in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 mm, indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 mm ratios and require even larger grains emitting at 20 mm than at 10 mm. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 mm feature strength versus shape trend is not correlated with age or Ha equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength versus shape trend is related to spectral type, however, with M stars showing significantly flatter 10 mm features (larger grain sizes) than A/B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type-dependent factors (e.g., X-rays, UV radiation, and stellar/disk winds).

The Submillimeter Array has opened a new window to study the innermost warm and dense regions of the envelopes and disks around deeply embedded protostars. This paper presents high angular resolution (<2'') submillimeter observations of the Class 0 young stellar object NGC 1333 IRAS 2A. Dust continuum emission and lines of complex organic molecules such as CH3OCH3 and CH3OCHO, high-excitation CH3OH transitions, and deuterated methanol, CH3OD, as well as lines of CO, HCN, H13CN, SO, and SO2, are detected on [?]200 AU scales. The observations are interpreted using detailed radiative transfer models of the physical and chemical structure, consistent with both single-dish and interferometer data. The continuum emission is explained by an extended envelope and a compact but resolved component, presumably a circumstellar disk with a diameter of 200-300 AU and a mass of ~a few times 0.01-0.1 M. If it is related to the rotation of the envelope, then the size of this disk suggests a centrifugal barrier of 200-300 AU, which implies that the temperature in the envelope does not increase above 100 K. Its large size also suggests that the buildup of disks proceeds rapidly throughout the early protostellar stages. The smaller (<100 AU) disks found around other deeply embedded protostars may be a result of tidal truncation. The high-resolution observations of SO can be explained with a simple constant abundance, ~10-9, constrained through single-dish observations, whereas those of H13CN and the organic species require high abundances, increased by 1-2 orders of magnitude, or an additional compact source of emission at small scales. The compact molecular emission could originate in a hot-core region of the inner envelope, but a more likely reservoir is the circumstellar disk.

We present Gould's Belt (GB) Spitzer IRAC and MIPS observations of the Lupus V and VI clouds and discuss them in combination with near-infrared (2MASS) data. Our observations complement those obtained for other Lupus clouds within the frame of the Spitzer "Core to Disk" (c2d) Legacy Survey. We found 43 young stellar object (YSO) candidates in Lupus V and 45 in Lupus VI, including two transition disks, using the standard c2d/GB selection method. None of these sources was classified as a pre-main-sequence star from previous optical, near-IR, and X-ray surveys. A large majority of these YSO candidates appear to be surrounded by thin disks (Class III; [?]79% in Lupus V and [?]87% in Lupus VI). These Class III abundances differ significantly from those observed for the other Lupus clouds and c2d/GB surveyed star-forming regions, where objects with optically thick disks (Class II) dominate the young population. We investigate various scenarios that can explain this discrepancy. In particular, we show that disk photoevaporation due to nearby OB stars is not responsible for the high fraction of Class III objects. The gas surface densities measured for Lupus V and VI lie below the star formation threshold (AV [?] 8.6 mag), while this is not the case for other Lupus clouds. Thus, few Myr older age for the YSOs in Lupus V and VI with respect to other Lupus clouds is the most likely explanation of the high fraction of Class III objects in these clouds, while a higher characteristic stellar mass might be a contributing factor. Better constraints on the age and binary fraction of the Lupus clouds might solve the puzzle but require further observations.