On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B

By M. Tafalla, P. Myers, P. Caselli, C. Walmsley. 2004

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We have characterized the internal structure of two close-to-round starless cores in Taurus, L1498 and L1517B, setting constraints on the initial conditions of star formation and on models of core condensation. Our analysis is based on high angular resolution observations in at least two transitions of NH3, N2H+, CS, C34S, C18O, and C17O, together with maps of the 1.2 mm continuum. For both cores, we derive radial profiles of constant temperature and constant turbulence, and density distributions close to those of non-singular isothermal spheres. Using a Monte Carlo radiative transfer model, we derive abundance profiles for all species and find a pattern of strong chemical differentiation. NH3 has a higher abundance toward the core centers while N2H+ has a constant abundance over most of the cores. Both C18O and CS (and isotopomers) are strongly depleted in the core interiors, most likely due to their freeze out onto cold dust grains. Concerning the kinematics of the dense gas, we find (in addition to constant turbulence) a pattern of internal motions at the level of 0.05 km s−1. These motions seem correlated with asymmetries in the pattern of molecular depletion, and we interpret them as residuals of core contraction. Their distribution and size suggest that core formation happens in rather irregular manner. A comparison with supersonic turbulence models of core formation shows that our observed cores are much more quiescent than allowed by these models.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse (2006, background): “…, which suggest that, during an initial, approximately isothermal phase of the collapse process, molecular species containing the heavy elements, notably carbon and oxygen , are removed from the gas phase by 'freeze–out' on to the surfaces of grains (Bacmann et al. 2002, Tafalla et al. 2004).”
The Large- and Small-Scale Structures of Dust in the Star-forming Perseus Molecular Cloud (2006, background): “Tafalla et al. (2004) show that non-thermal support is negligible in the cores in the quiescent star-forming region Taurus.”
Observing the gas temperature drop in the high-density nucleus of L 1544 (2007, methodology): “We note however that the ammonia single dish data for several sources including L 1544 show no evidence for gradients (Tafalla et al. 2002, 2004) and thus the existence of temperature gradients in pre-stellar cores is uncertain.”
Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “…data on molecular distributions in L1544 better than local volume abundances, especially for NH3, for which observations are consistent with N(NH3)/N(H2) increase towards the center of the core (Tafalla et al. 2004), while the N(N2H +)/N(H2) column density ratio appears to be constant.”
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.”
Our astrochemical heritage (2012, neutral): “Others show expanding motions (Tafalla et al. 2004).”
The Relationship between Molecular Gas Tracers and Kennicutt-Schmidt Laws (2007, neutral): “One potential source of variation in molecular abundance is freezeout onto grain surfaces at high densities and low temperatures (e.g., Tafalla et al. 2004a, 2004b).”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, methodology): “…the pressure required by the BE fits usually exceed the thermal (plus subsonic nonthermal) pressure available from the gas by a factor of a few, again indicating the presence of an additional force component or the lack of equilibrium (Tafalla et al. 2004; Kirk, Ward-Thompson & André 2005).”
Structural Analysis of Molecular Clouds: Dendrograms (2008, background): “…MA 02138 2 University of British Columbia Okanagan, 3333 University Way, Kelowna, BC V1V 1V7, Canada 3 Initiative for Innovative Computing, Harvard University,60 Oxford St., Cambridge, MA 02138 than turbulent motions) onℓ ∼ 0.1 pc scales (Goodman et al. 1998; Tafalla et al. 2004; Lada et al. 2007).”
Theory of Star Formation (2007, background): “For low-mass cores, Barranco & Goodman (1998) (see also Goodman et al. 1998; Tafalla et al. 2004) found that the nonthermal linewidth decreases and then reaches a minimum “plateau” level at a finite impact parameter ∼ 0.1 pc from the center of the core.”
Ammonia in infrared dark clouds (2006, methodology): “An analytical expression (Tafalla et al. 2004) has been used to estimate the kinetic temperature from the rotational temperature.”
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey (2012, background): “The kinetic temperature is obtained from the rotational temperature (Tafalla et al. 2004): Tkin = Trot(1, 2) 1 − Trot(1,2)42 K ln ( 1 + 1.1 exp ( −16 K Trot(1,2) )) , (6) where the energy difference between the NH3 (1, 1) and (2, 2) inversion levels is 42 K. Tafalla et al. (2004) ran different…”
Dense core formation by fragmentation of velocity-coherent filaments in L1517 (2011, methodology): “The data for core L1517B have already been presented in Tafalla et al. (2004), while the data for cores L1517A and L1517C are newly reported here.”
Star formation in Perseus - Clusters, filaments and the conditions for star formation (2005, background): “On the other hand at high densities the line becomes optically thick and in addition in low temperature, high density regions the C18O depletes onto dust grains (e.g. Tafalla et al. 2004) and so ceases to trace the gas.”
On the internal structure of starless cores II. A molecular survey of L1498 and L1517B (2006, methodology): “In Tafalla et al. (2004) (paper 1 hereafter), we used millimeter continuum data together with C(18)O, CS, N2H , and NH3 line observations of these cores to derive their radial profiles of density, temperature, turbulent linewidth, and line-of-sight velocity.”
Thermal physics, cloud geometry and the stellar initial mass function (2005, background): “Observations have generally indicated still higher temperatures of at least 8 K and typically about 10 K (Benson & Myers 1989; Ward-Thompson, André & Kirk 2002), even in the densest cloud cores (Tafalla et al. 2004).”
An Ammonia Spectral Atlas of Dense Cores in Perseus (2007, background): “Tafalla et al. (2004) note that the kinetic temperature of the two cores they study in detail is constant, while there are radial variations in ammonia abundance and excitation temperatures in cores.”
The Stellar Mass Spectrum from Non-Isothermal Gravoturbulent Fragmentation (2004, background): “…gas would be very difficult to observe; various efforts to model the observations have suggested central temperatures between 6 K and 10 K for the densest observed prestellar cores, whose peak densities may approach 10−17 g cm−3 (e.g., Zucconi et al. 2001; Evans et al. 2001; Tafalla et al. 2004).”
Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region (2013, methodology): “To compensate for CO depletion, the contribution from positions with bright N2H+ emission (SNR ≥ 3) has been supplemented with a mass estimated using the N2H+ emission and assuming standard excitation and abundance values (Caselli et al. 2002; Tafalla et al. 2004).”