Systematic Molecular Differentiation in Starless Cores

By M. Tafalla, P. Myers, P. Caselli, C. Walmsley, C. C. Oan, Spain, CfA, Usa, Arcetri, Italy and 2 more. 2001

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We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Some molecules, including CO and CS, almost vanish near the core centers, where the abundance decreases by at least 1 or 2 orders of magnitude with respect to the value in the outer core. At the same time, the N2H+ molecule has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a systematic study of five mostly round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mapped in C18O (1-0), CS (2-1), N2H+ (1-0), NH3 (1, 1) and (2, 2), and the 1.2 mm continuum [complemented with C17O (1-0) and C34S (2-1) data for some systems]. For each core we have built a spherically symmetric model in which the density is derived from the 1.2 mm continuum, the kinetic temperature is derived from NH3, and the abundance of each molecule is derived using a Monte Carlo radiative transfer code, which simultaneously fits the shape of the central spectrum and the radial profile of integrated intensity. Regarding the cores for which we have C17O (1-0) and C34S (2-1) data, the model fits these observations automatically when the standard isotopomer ratio is assumed. As a result of this modeling, we also find that the gas kinetic temperature in each core is constant at approximately 10 K. In agreement with previous work, we find that if the dust temperature is also constant, then the density profiles are centrally flattened, and we can model them with a single analytic expression. We also find that for each core the turbulent line width seems constant in the inner 0.1 pc. The very strong abundance drop of CO and CS toward the center of each core is naturally explained by the depletion of these molecules onto dust grains at densities of (2-6) × 104 cm-3. N2H+ seems unaffected by this process up to densities of several times 105 cm-3, or even 106 cm-3, while the NH3 abundance may be enhanced by its lack of depletion and by reactions triggered by the disappearance of CO from the gas phase. With the help of the Monte Carlo modeling, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem that 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 line width of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent line width with radius, our models show that it can arise in conditions of constant gas turbulence.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/81209db28cf4e644f1a2959c7836dc9e7e313363

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Citations in the corpus

Enhanced Deuterium Fractionation in Dense Interstellar Cores Resulting from Multiply Deuterated H3+ (2003, background): “Tafalla et al. (2002) obtain a density of a few times 106 cm 3 for the core of L1544, and so D/H ratios for selected molecules from models 1 and 2, usingn(H2) p 1.5# 10 6 cm 3, are also listed in Table 1.”
On the internal structure of starless cores II. A molecular survey of L1498 and L1517B (2006, methodology): “Most dense cores, in fact, suffer from severe depletion (e.g., Bacmann et al. 2002; Tafalla et al. 2002), while only a minority seem unaffected by it (Hirota et al. 2002; Tafalla & Santiago 2004).”
Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “This behaviour is typical of low-mass dense cores, including L1544, as found by Tafalla et al. (2002).”
Probing the Evolutionary Status of Starless Cores through N2H+ and N2D+ Observations (2004, methodology): “The most evident feature of Figures 8–13 is that the C18O does not trace the dust continuum while the N2H + and N2D + emission morphology is similar to that of the dust, as already shown by several authors (e.g., Caselli et al. 1999; Bergin et al. 2001; Tafalla et al. 2002).”
Molecular inventories and chemical evolution of low-mass protostellar envelopes (2003, methodology): “Such physical models can then be used as the basis for determining the molecular excitation and for deriving abundances relative to H2 by comparing to molecular line observations (e.g., Bergin et al. 2001; Jørgensen et al. 2002; Schöier et al. 2002; Tafalla et al. 2002; Lee et al. 2003).”
The degree of CO depletion in pre-stellar cores ? (2002, background): “These results considerably extend the previous works on depletion in pre-stellar cores by Kramer et al. (1999), Caselli et al. (1999) and more recently by Tafalla et al. (2002).”
The H2O Southern Galactic Plane Survey (HOPS) – I. Techniques and H2O maser data (2011, background): “In cold, dense regions most molecules and ions are known to freeze out on to dust grains, with CO, CS and HCO+ being classic examples (Bergin & Langer 1997; Willacy, Langer & Velusamy 1998; Caselli et al. 1999; Tafalla et al. 2002; Bergin et al. 2002).”
Dynamic star formation in the massive DR21 filament (2010, background): “However, HCO+ is known to be depleted in cold environments and at densities higher than ∼105 cm−3 (Tafalla et al. 2002).”
Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region (2013, background): “This anticorrelation between the C18O and N2H+ emissions is typical of regions of core formation, and indicates that CO has depleted from the dense gas (Caselli et al. 1999; Bergin et al. 2002; Tafalla et al. 2002).”
Molecular Evolution in Collapsing Prestellar Cores. III. Contraction of a Bonnor-Ebert Sphere (2004, background): “Tafalla et al. (2004) observed L1517B and L1498 with higher angular resolution than Tafalla et al. (2002), and found that their data are consistent with a step function distribution, as can be seen in Figure 4.”
A computer program for fast non-LTE analysis of interstellar line spectra With diagnostic plots to interpret observed line intensity ratios (2007, background): “Using such programs one can constrain temperature, density, and velocity gradients within sources (e.g., Van der Tak et al. 1999; Tafalla et al. 2002; Jakob et al. 2007), and, if enough observations are available, even molecular abundance profiles (e.g., Van der Tak et al. 2000a; Schöier et al.…”
MAMBO Mapping Of Spitzer c2d Small Clouds And Cores (2008, background): “Given that thermal dust continuum emission at ≈1 mm wavelength is a better mass tracer than molecular emission lines (e.g., Tafalla et al. 2002), and that modern bolometer cameras enable complete maps of the dust emission from dense cores to be made within a short time, dust emission maps are a…”
Observing the gas temperature drop in the high-density nucleus of L 1544 (2007, methodology): “…the structures of density, temperature and abundance in the following way: n(r) = n0 1 + (r/r0,n)αn XNH3 (r) = X0(n(r)/n0) αX T (r) = Tout − Tout − Tin1 + (r/r0,T )1.5 · The forms for the density and abundance profiles where chosen for consistency with the results of Tafalla et al. (2002).”
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey (2012, result): “In contrast to those, the fractional gas abundance of ammonia remains constant in prestellar cores (Tafalla et al. 2002).”
Probing the formation of intermediate- to high-mass stars in protoclusters - A detailed millimeter study of the NGC 2264 clumps (2005, background): “The CS and HCO+ molecules are known to be depleted onto dust grains above a density nH2 ∼ 105 cm−3 in starless cores (e.g. Tafalla et al. 2002).”
A “Starless” Core that Isn't: Detection of a Source in the L1014 Dense Core with the Spitzer Space Telescope (2004, methodology): “Be ause it o ers a good t to the density stru ture, we use the an- alyti al approximation given by Tafalla et al. (2002) for an isothermal BE sphere, n(r) = n 1+ r r 0 and found a good t for n = 1:5 10 5 m 3 ; for this entral density, a BE sphere is approximated with r 0 = 4090 AU and = 2:32.”
A PARAMETRIC MODELING APPROACH TO MEASURING THE GAS MASSES OF CIRCUMSTELLAR DISKS (2013, positive): “CO depletion via freeze-out onto dust grains was first characterized in molecular cores (Caselli et al. 1999; Tafalla et al. 2002; Jørgensen et al. 2002), then inferred in circumstellar disks (Qi et al. 2008, 2011), and now directly resolved (Rosenfeld et al. 2013a).”