(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap

We present results of an extensive mapping survey of N 2 H + (1-0) in about 60 low mass cloud cores already mapped in the NH 3 (1,1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution of 54 ′′ , about 1.5 times finer than the previous ammonia observations made at the Haystack telescope. The comparison between N 2 H + and NH 3 maps shows strong similarities in the size and morphology of the two molecular species indicating that they are tracing the same material, especially in starless cores. Cores with stars typically have map sizes about a factor of two smaller for N 2 H + than for NH 3 , indicating the presence of denser and more centrally concentrated gas compared to starless cores. The mean aspect ratio is ∼ 2. Significant correlations are found between NH 3 and N 2 H + column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Starless cores are less massive (< M vir > ≃ 3 M ⊙ ) than cores with stars (< M vir > ≃ 9 M ⊙ ). Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been found with NH 3 data, and the ratio β of rotational kinetic energy to gravitational energy have magnitudes between ∼ 10 −4 and 0.07, indicating that rotation is not energetically dominant in the support of the cores. "Local" velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complex motions

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.

Context. Water is a key tracer of dynamics and chemistry in low-mass star-forming regions, but spectrally resolved observations have so far been limited in sensitivity and angular resolution, and only data from the brightest low-mass protostars have been published. Aims. The first systematic survey of spectrally resolved water emission in 29 low-mass (L < 40 L ) protostellar objects is presented. The sources cover a range of luminosities and evolutionary states. The aim is to characterise the line profiles to distinguish physical components in the beam and examine how water emission changes with protostellar evolution. Methods. H 2 O was observed in the ground-state 1 10 -1 01 transition at 557 GHz (E up /k B ∼ 60 K) as single-point observations with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel in 29 deeply embedded Class 0 and I low-mass protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our programme) are used to constrain the spectral energy distribution (SED) of each source. H 2 O intensities are compared to inferred envelope properties, e.g., mass and density, outflow properties and CO 3-2 emission. Results. H 2 O emission is detected in all objects except one (TMC1A). The line profiles are complex and consist of several kinematic components tracing different physical regions in each system. In particular, the profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not in the quiescent envelope. Several sources show multiple shock components appearing in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in seven sources (six Class 0, one Class I) indicative of infalling envelopes, and regular P-Cygni profiles in four sources (three Class I, one Class 0) indicative of expanding envelopes. Molecular "bullets" moving at > ∼ 50 km s −1 with respect to the source are detected in four Class 0 sources; three of these sources were not known to harbour bullets previously. In the outflow, the H 2 O/CO abundance ratio as a function of velocity is nearly the same for all line wings, increasing from 10 −3 at low velocities (<5 km s −1 ) to > ∼ 10 −1 at high velocities (>10 km s −1 ). The water abundance in the outer cold envelope is low, > ∼ 10 −10 . The different H 2 O profile components show a clear evolutionary trend: in the younger Class 0 sources the emission is dominated by outflow components originating inside an infalling envelope. When large-scale infall diminishes during the Class I phase, the outflow weakens and H 2 O emission all but disappears.