We have undertaken a survey of N 2 H + and N 2 D + towards 31 low-mass starless cores using the IRAM 30-m telescope. Our main objective has been to determine the abundance ratio of N 2 D + and N 2 H + towards the nuclei of these cores and thus to obtain estimates of the degree of deuterium enrichment, a symptom of advanced chemical evolution according to current models. We find that the N (N 2 D + )/N (N 2 H + ) ratio is larger in more "centrally concentrated cores" with larger peak H 2 and N 2 H + column density than the sample mean. The deuterium enrichment in starless cores is presently ascribed to depletion of CO in the high density (> 3 × 10 4 cm −3 ) core nucleus. To substantiate this picture, we compare our results with observations in dust emission at 1.2 mm and in two transitions of C 18 O. We find a good correlation between deuterium fractionation and N (C 18 O)/N (H 2 ) 1.2 mm for the nuclei of 14 starless cores. We thus identified a set of properties that characterize the most evolved, or "pre-stellar", starless cores. These are: higher N 2 H + and N 2 D + column densities, higher N (N 2 D + )/N (N 2 H + ), more pronounced CO depletion, broader N 2 H + lines with infall asymmetry, higher central H 2 column densities and a more compact density profile than in the average core. We conclude that this combination of properties gives a reliable indication of the evolutionary state of the core. Seven cores in our sample (L1521F, OphD, L429, L694, L183, L1544 and TMC2) show the majority of these features and thus are believed to be closer to forming a protostar than are the other members of our sample. Finally, we note that the subsample of Taurus cores behaves more homogeneously than the total sample, an indication that the external environment could play an important role in the core evolution.

To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L $ 0:1Y10 L ) using 3Y38 m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L $ 10 5 L ). The long-known 6.0 and 6.85 m bands are detected toward all sources, with the Class 0Y type YSOs showing the deepest bands ever observed. The 6.0 m band is often deeper than expected from the bending mode of pure solid H 2 O. The additional 5Y7 m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%Y30% for CH 3 OH, 3%Y8% for NH 3 , 1%Y5% for HCOOH, $6% for H 2 CO, and $0.3% for HCOO À relative to solid H 2 O. The 6.85 m band has one or two carriers, of which one may be less volatile than H 2 O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH þ 4 -containing salt is the carrier, its abundance relative to solid H 2 O is $7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-rayYinduced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

We present the results of a search for all embedded protostars with internal luminosities ≤ 1.0 L ⊙ in the full sample of nearby, low-mass star-forming regions surveyed by the Spitzer Space Telescope Legacy Project "From Molecular Cores to Planet Forming Disks" (c2d). The internal luminosity of a source, L int , is the luminosity of the central source and excludes luminosity arising from external heating. On average, the Spitzer c2d data are sensitive to embedded protostars with L int ≥ 4 × 10 −3 (d/140 pc) 2 L ⊙ , a factor of 25 better than the sensitivity of the Infrared Astronomical Satellite (IRAS) to such objects. We present a set of selection criteria used to identify candidates from the Spitzer data and examine complementary data to decide whether each candidate is truly an embedded protostar. We find a tight correlation between the 70 µm flux and internal luminosity of a protostar, an empirical result based on both observations and detailed twodimensional radiative transfer models of protostars. We identify 50 embedded protostars with L int ≤ 1.0 L ⊙ ; 15 have L int ≤ 0.1 L ⊙ . The intrinsic distribution of source luminosities increases to lower luminosities. While we find sources down to the above sensitivity limit, indicating that the distribution may extend to luminosities lower than probed by these observations, we are able to rule out a continued rise in the distribution below L int = 0.1 L ⊙ . Between 75 −85% of cores classified as starless prior to being observed by Spitzer remain starless to our luminosity sensitivity; the remaining 15 − 25% harbor low-luminosity, embedded protostars. We compile complete Spectral Energy Distributions for all 50 objects and calculate standard evolutionary signatures (L bol , T bol , and L bol /L smm ), and argue that these objects are inconsistent with the simplest picture of star formation wherein mass accretes from the core onto the protostar at a constant rate.

Context. The thermal structure of a starless core is crucial for our understanding of the physics in these objects and hence for our understanding of star formation. Theory predicts a gas temperature drop in the inner ∼5000 AU of the pre-stellar core L 1544, but there has been no observational proof of this. Aims. We performed VLA observations of the NH 3 (1, 1) and (2, 2) transitions towards L 1544 in order to measure the temperature gradient between the high density core nucleus and the surrounding core envelope. Our VLA observation for the first time provide measurements of gas temperature in a core with a resolution smaller than 1000 AU. We have also obtained high resolution Plateau de Bure observations of the 110 GHz 1 11 − 1 01 para-NH 2 D line in order to further constrain the physical parameters of the high density nucleus. Methods. We combine our interferometric NH 3 and NH 2 D observations with available single dish measurements in order to estimate the effects of flux loss from extended components upon our data. We have estimated the temperature gradient using a model of the source to fit our data in the u, v plane. As the NH 3 (1, 1) line is extremely optically thick, this also involved fitting a gradient in the NH 3 abundance. In this way, we also measure the [NH 2 D]/[NH 3 ] abundance ratio in the inner nucleus. Results. We find that indeed the temperature decreases toward the core nucleus from 12 K down to 5.5 K resulting in an increase of a factor of 50% in the estimated density of the core from the dust continuum if compared with the estimates done with constant temperature of 8.75 K. Current models of the thermal equilibrium can describe consistently the observed temperature and density in this object, simultaneously fitting our temperature profile and the continuum emission. We also found a remarkably high abundance of deuterated ammonia with respect to the ammonia abundance (50% ± 20%), which proves the persistence of nitrogen bearing molecules at very high densities (2 × 10 6 cm −3 ) and shows that high-resolution observations yield higher deuteration values than single-dish observations. The NH 2 D observed transition, free of the optical depth problems that affect the NH 3 lines in the core center, is a much better probe of the high-density nucleus and, in fact, its map peak at the dust continuum peak. Our analysis of the NH 3 and NH 2 D kinematic fields shows a decrease of specific angular momentum from the large scales to the small scales.