Probing the Evolutionary Status of Starless Cores through N2H+ and N2D+ Observations

By A. Crapsi, P. Caselli, C. Walmsley, P. Myers, M. Tafalla, C. W. Lee, T. L. B. Unifi, Cfa, O. Arcetri, Oan and 2 more. 2004

306 citations recorded by the source corpus.

We have undertaken a survey of N2H+ and N2D+ toward 31 low-mass starless cores using the IRAM 30 m telescope. Our main objective has been to determine the abundance ratio of N2D+ and N2H+ toward 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 × 104 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)1.2 mm for the nuclei of 14 starless cores. We thus identified a set of properties that characterize the most evolved, or "prestellar," 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, Oph D, L429, L694, L183, L1544, and TMC 2) 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/5b0c6d6484d47b161bf0e916333c4cb11cbd04d3

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Citations in the corpus

FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE (2012, methodology): “These so-called pre-stellar cores, as defined by Crapsi et al. (2005), are centrally concentrated, with central densities 105 cm−3, they have central visual extinctions 50 mag, large degrees of CO freeze out ( 90%), and deuterium fraction ( 20%), and they show clear signs of contraction motions in…”
CHEMISTRY IN INFRARED DARK CLOUD CLUMPS: A MOLECULAR LINE SURVEY AT 3 mm (2011, background): “Although for N2H + J = 1 → 0 there are 15 allowed hyperfine transitions (Daniel et al. 2006), only seven features can be resolved in low-mass star forming regions (e.g., Caselli et al. 1995; Crapsi et al. 2005) and three in high-mass star forming regions (e.g., Foster et al. 2011).”
The Spitzer c2d Survey of Nearby Dense Cores. II. Discovery of a Low-Luminosity Object in the “Evolved Starless Core” L1521F (2006, background): “A similar result is found for the other evolved starless cores identified by Crapsi et al. (2005).”
THE ORIGIN OF COMPLEX ORGANIC MOLECULES IN PRESTELLAR CORES (2014, background): “It is characterised by a central high density (2 × 106 cm−3), low temperature (∼ 7 K; Crapsi et al. 2007), and high CO depletion, accompanied by a large degree of molecular deuteration (Caselli et al. 2003; Crapsi et al. 2005; Vastel et al. 2006).”
Water in Star-forming Regions with the Herschel Space Observatory (WISH). I. Overview of Key Program and First Results (2010, background): “Several of these so-called prestellar cores are thought to be on the verge of collapse and thus represent the earliest stage in the starformation process (e.g., Tafalla et al. 1998; Caselli et al. 2002; Crapsi et al. 2005).”
Observing the gas temperature drop in the high-density nucleus of L 1544 (2007, background): “These properties, combined with kinematical probes of infall (differences in velocity between thin and thick tracers, and increasing line widths of thin tracers toward the core centre) makes this core a good candidate for being on the point of becoming unstable (Crapsi et al. 2005).”
Identifying the Low-Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores (2007, methodology): “…identifying those cores on the verge of collapse since VeLLOs have been discovered in cores that were classified as starless prior to Spitzer observations but were not always those cores believed to be the most evolved and nearest to the onset of collapse (Crapsi et al. 2005b; Bourke et al. 2006).”
Our astrochemical heritage (2012, methodology): “These are the so-called prestellar cores, first identified by Ward-Thompson et al. (1994) in the sub-millimeter continuum and then chemically and kinematically labelled by Crapsi et al. (2005) using millimeter spectroscopy.”
Dense core formation by fragmentation of velocity-coherent filaments in L1517 (2011, background): “N2H+ is a so-called latetime molecule, and its abundance is further enhanced when CO freezes out (e.g., Bergin & Tafalla 2007), so bright N2H+ emission is commonly associated with evolved cores (Crapsi et al. 2005).”
MAMBO Mapping Of Spitzer c2d Small Clouds And Cores (2008, background): “We also list the effective diameter for the area above 70% of the peak intensity, r70% (Crapsi et al. 2005).”
On the internal structure of starless cores II. A molecular survey of L1498 and L1517B (2006, background): “…striking similarity between the mm continuum and N2H+(1–0) maps in many starless cores (e.g., Caselli et al. 1999; Tafalla et al. 2002; Bacmann et al. 2002; Crapsi et al. 2005, this work), and makes the 1–0 line the ideal choice for studying core interiors as long as its optical depth remains low.”