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Register a new userProbing the evolutionary status of starless cores through N2H+ and N2D+ observations
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A. Crapsi
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We have undertaken a survey of N2H+ and N2D+ towards 31 low-mass starless cores using the IRAM 30m telescope. Our main objective has been to determine the abundance ratio of N2D+ and N2H+ 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(N2D+)/N(N2H+) ratio is larger in more centrally concentrated cores with larger peak H2 and N2H+ 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 C18O. We find a good correlation between deuterium fractionation and N(C18O)/N(H2) 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 N2H+ and N2D+ column densities, higher N(N2D+)/N(N2H+), more pronounced CO depletion, broader N2H+ lines with infall asymmetry, higher central H2 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.
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We have measured the deuterium fractionation (through the column density ratio N(N2D+)/N(N2H+)) and the CO depletion factor (ratio between expected and observed CO abundance) in a sample of 10 high-mass protostellar candidates, in order to understand whether the earliest evolutionary stages of high-mass stars have chemical characteristics similar to those of low-mass ones. The observations were carried out with the IRAM-30m telescope and the JCMT. We have detected N2D+ emission in 7 of the 10 sources of our sample, and found an average value N(N2D+)/N(N2H+)~0.015. This value is 3 orders of magnitude larger than the interstellar D/H ratio, indicating the presence of cold and dense gas, in which the physical-chemical conditions are similar to those observed in low-mass pre-stellar cores. Also, the integrated CO depletion factors show that in the majority of the sources the expected CO abundances are larger than the observed values, with a median ratio of 3.2. In principle, the cold gas that gives origin to the N2D+ emission can be the remnant of the massive molecular core in which the high-mass (proto-)star was born, not yet heated up by the central object. If so, our results indicate that the chemical properties of the clouds in which high-mass stars are born are similar to their low-mass counterparts. Alternatively, this cold gas can be located into one (or more) starless core (cores) near the protostellar object. Due to the poor angular resolution of our data, we cannot decide which is the correct scenario.
Deuterated ions are abundant in cold (T=10 K), dense (n=10^5 cm^-3) regions, in which CO is frozen out onto dust grains. In such environments, the deuterium fractionation of such ions can exceed the elemental abundance ratio of D/H by a factor of 10^4. In this paper we use the deuterium fractionation to investigate the evolutionary state of Class 0 protostars. In a sample of 20 protostellar objects, we found a clear correlation between the N2D+/N2H+ ratio and evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show the largest deuterium fractionation. Furthermore, we find that sources with a high N2D+/N2H+ ratio show clear indication for infall.
Context : Dynamical studies of prestellar cores search for small velocity differences between different tracers. The highest radiation frequency precision is therefore required for each of these species. Aims : We want to adjust the frequency of the first three rotational transitions of N2H+ and N2D+ and extrapolate to the next three transitions. Methods : N2H+ and N2D+ are compared to NH3 the frequency of which is more accurately known and which has the advantage to be spatially coexistent with N2H+ and N2D+ in dark cloud cores. With lines among the narrowests, and N2H+ and NH3 emitting region among the largests, L183 is a good candidate to compare these species. Results : A correction of ~10 kHz for the N2H+ (J:1-0) transition has been found (~0.03 km/s) and similar corrections, from a few m/s up to ~0.05 km/s are reported for the other transitions (N2H+ J:3-2 and N2D+ J:1-0, J:2-1, and J:3-2) compared to previous astronomical determinations. Einstein spontaneous decay coefficients (Aul) are included.
Chemical models predict that the deuterated fraction (the column density ratio between a molecule containing D and its counterpart containing H) of N2H+, Dfrac(N2H+), is high in massive pre-protostellar cores and rapidly drops of an order of magnitude after the protostar birth, while that of HNC, Dfrac(HNC), remains constant for much longer. We tested these predictions by deriving Dfrac(HNC) in 22 high-mass star forming cores divided in three different evolutionary stages, from high-mass starless core candidates (HMSCs, 8) to high-mass protostellar objects (HMPOs, 7) to Ultracompact HII regions (UCHIIs, 7). For all of them, Dfrac (N2H+) was already determined through IRAM-30m Telescope observations, which confirmed the theoretical rapid decrease of Dfrac(N2H+) after protostar birth (Fontani et al. 2011). Therefore our comparative study is not affected by biases introduced by the source selection. We have found average Dfrac(HNC) of 0.012, 0.009 and 0.008 in HMSCs, HMPOs and UCHIIs, respectively, with no statistically significant differences among the three evolutionary groups. These findings confirm the predictions of the chemical models, and indicate that large values of Dfrac(N2H+) are more suitable than large values of Dfrac(HNC) to identify cores on the verge of forming high-mass stars, likewise what found in the low-mass regime.
Seven isolated, nearby low-mass starless molecular cloud cores have been observed as part of the Herschel key program Earliest Phases of Star formation (EPoS). By applying a ray-tracing technique to the obtained continuum emission and complementary (sub)mm emission maps, we derive the physical structure (density, dust temperature) of these cloud cores. We present observations of the 12CO, 13CO, and C18O (2-1) and N2H+ (1-0) transitions towards the same cores. Based on the density and temperature profiles, we apply time-dependent chemical and line-radiative transfer modeling and compare the modeled to the observed molecular emission profiles. CO is frozen onto the grains in the center of all cores in our sample. The level of CO depletion increases with hydrogen density and ranges from 46% up to more than 95% in the core centers in the core centers in the three cores with the highest hydrogen density. The average hydrogen density at which 50% of CO is frozen onto the grains is 1.1+-0.4 10^5 cm^-3. At about this density, the cores typically have the highest relative abundance of N2H+. The cores with higher central densities show depletion of N2H+ at levels of 13% to 55%. The chemical ages for the individual species are on average 2+-1 10^5 yr for 13CO, 6+-3 10^4 yr for C18O, and 9+-2 10^4 yr for N2H+. Chemical modeling indirectly suggests that the gas and dust temperatures decouple in the envelopes and that the dust grains are not yet significantly coagulated. We observationally confirm chemical models of CO-freezeout and nitrogen chemistry. We find clear correlations between the hydrogen density and CO depletion and the emergence of N2H+. The chemical ages indicate a core lifetime of less than 1 Myr.
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