Molecular Ions in L1544. II. The Ionization Degree

By P. Caselli, C. Walmsley, A. Zucconi, M. Tafalla, L. Dore, P. Myers. 2001

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The maps presented in Paper I are here used to infer the variation of the column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from the dust peak. These results are interpreted with the aid of a crude chemical model that predicts the abundances of these species as a function of radius in a spherically symmetric model with radial density distribution inferred from the observations of dust emission at millimeter wavelengths and dust absorption in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+) column density ratio is of order 0.2 toward the L1544 dust peak as compared to N(DCO+)/N(HCO+) = 0.04. We conclude that this result, as well as the general finding that N2H+ and N2D+ correlate well with the dust, is caused by CO being depleted to a much higher degree than molecular nitrogen in the high-density core of L1544. Depletion also favors deuterium enhancement, and thus N2D+, which traces the dense and highly CO-depleted core nucleus, is much more enhanced than DCO+. Our models do not uniquely define the chemistry in the high-density depleted nucleus of L1544, but they do suggest that the ionization degree is a few times 10-9 and that the ambipolar diffusion timescale is locally similar to the free-fall time. It seems likely that the lower limit, which one obtains to ionization degree by summing all observable molecular ions, is not a great underestimate of the true ionization degree. We predict that atomic oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in the central highly depleted region of the core.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/427bc30cc5b932e523e6bc6e40f575dd2846ded6

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Citations in the corpus

Probing the Evolutionary Status of Starless Cores through N2H+ and N2D+ Observations (2004, methodology): “Another possibility is that depletion of N2 (and hence of N2H + andN2D +) may occur in regions with density higher than 5 ;105 cm 3 (see also Caselli et al. 1999, 2002c; Bergin et al. 2001; Crapsi et al. 2004; Belloche & André 2004).”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “The effects of freeze-out can alter both the pre-factor and the exponent of the density dependence (Bergin & Langer 1997; Caselli et al. 2002c; Walmsley et al. 2004), but see also Ciolek & Mouschovias (1998).”
Enhanced Deuterium Fractionation in Dense Interstellar Cores Resulting from Multiply Deuterated H3+ (2003, background): “For the well-studied source L1544, it seems that CO and CS freeze out at lower densities, while N2 stays in the gas longer, forming N2H and NH3, which trace the densest regions (Caselli et al. 2002b).”
Molecular Evolution in Collapsing Prestellar Cores. III. Contraction of a Bonnor-Ebert Sphere (2004, methodology): “Caselli et al. (2002a) obtained column density ratios for N(DCO+)/N(HCO+) and N(N2D +)/N(N2H +) towards the dust peak of L1544 of 0.04 and 0.2, respectively.”
The degree of CO depletion in pre-stellar cores ? (2002, background): “CO depletion in dense cores has already been discussed by Kramer et al. (1999), Willacy et al. (1998), and Caselli et al. (1999, 2002a, 2002b) who focussed on L1544, a core similar to the ones we discuss in this work.”
Molecular inventories and chemical evolution of low-mass protostellar envelopes (2003, background): “HCO+ and N2H+ HCO+ is of great importance in chemical models of protostellar environments as it is the primary molecular ion and thus regulator of the electron density/ionization structure (e.g., Caselli et al. 2002) and the most important destroyer of other molecules (e.g., Bergin & Langer 1997).”
H I Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation (2002, background): “The inner 50 core region around the center of L1544 has been well studied and found to exhibit molecular depletion and infall motions (Caselli et al. 2002; Tafalla et al. 1998).”
Our astrochemical heritage (2012, background): “Starless and prestellar cores present evidence of overall (slow) rotation (Arquilla and Goldsmith 1986; Goodman et al. 1993; Caselli et al. 2002), thus they possess an initial angular momentum.”
On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B (2004, unknown): “We have presented detailed models of the internal structure of two close-to-round starless cores in the nearby Taurus-Auriga star formation complex, L1498 and L1517B.”
Observing the gas temperature drop in the high-density nucleus of L 1544 (2007, methodology): “The central velocity information was used to compute the velocity gradient across the core, in analogous fashion to Caselli et al. (2002a) using N2H+ and N2D+.”
The chemistry of multiply deuterated species in cold, dense interstellar cores (2004, methodology): “Our prediction for N(N2H+) is ∼3 times lower than the observation, while our predicted N2D+/ N2H+ and DCO+/HCO+ ratios are 1.2 and 0.2, respectively, ∼5 times higher than the observation towards the dust peak of L1544 (Caselli et al. 2002b).”
Complete depletion in prestellar cores (2004, background): “Estimates of the ionization degree in L 1544 (Caselli et al. 2002a) are of order 2 10−9.”
Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, methodology): “In turn, it appears similar to the measurements of Caselli et al. (2002).”
CO Depletion and Deuterium Fractionation in Prestellar Cores (2003, background): “In the absence of O depletion, the atomic D/H ratio is predicted to be around 0.01 (RMa), which is not sufficient to account for the high levels of deuteration observed here (to account for a [D2CO]/[H2CO] ratio of 0.05, the atomic D/H ratio would have to be as high as 0.3 - Caselli et al. 2002b).”
Star Formation from Galaxies to Globules (2001, background): “At this density, big grains stop gyrating around the magnetic field (Kamaya & Nishi 2000), molecules begin to freeze onto grains (Bergin et al. 2001), and the ionization fraction begins to drop (Caselli et al. 2002).”
Systematic Molecular Differentiation in Starless Cores (2001, methodology): “The cores were selected from previous work (Caselli et al. 2002a) for having nearly circular contour maps.”
Dense Cores in Dark Clouds. XIV. N2H+ (1-0) Maps of Dense Cloud Cores (2002, methodology): “The agreement between dust and N2H + profiles suggests that the molecular gas traced by the N2H + line is not significantly depleted in relation to the dust, unlike CO and CS (Caselli et al. 1999, 2002a, 2002b; Bergin et al. 2001; Tafalla et al. 2002).”