Evolutionary Status of the Pre-protostellar Core L1498

Abstract

L1498 is a classic example of a dense cold pre-protostellar core. To study the evolutionary status, the structure, dynamics, and chemical properties of this core we have obtained high spatial and high spectral resolution observations of molecules tracing densities of 10(3)-10(5) cm-3. We observed CCS, NH3, C3H2, and HC7N with NASA's DSN 70 m antennas. We also present large-scale maps of C18O and 13CO observed with the AT&T 7 m antenna. For the high spatial resolution maps of selected regions within the core we used the VLA for CCS at 22 GHz, and the Owens Valley Radio Observatory (OVRO) MMA for CCS at 94 GHz and CS (2-1). The 22 GHz CCS emission marks a high-density [n(H2) > 10(4) cm -3] core, which is elongated with a major axis along the SE-NW direction. NH3 and C3H2 emissions are located inside the boundary of the CCS emission. C18O emission traces a lower density gas extending beyond the CCS boundary. Along the major axis of the dense core, CCS, NH3 and C3H2 emission show evidence of limb brightening. The observations are consistent with a chemically differentiated onion-shell structure for the L1498 core, with NH3 in the inner and CCS in the outer parts of the core. The high angular resolution (9"-12") spectral line maps obtained by combining NASA Goldstone 70 m and VLA data resolve the CCS 22 GHz emission in the southeast and northwest boundaries into arclike enhancements, supporting the picture that CCS emission originates in a shell outside the NH3 emitting region. Interferometric maps of CCS at 94 GHz and CS at 98 GHz show that their emitting regions contain several small-scale dense condensations. We suggest that the differences between the CCS, CS, C3H2, and NH3 emission are caused by a time-dependent effect as the core evolves slowly. We interpret the chemical and physical properties of L1498 in terms of a quasi-static (or slowly contracting) dense core in which the outer envelope is still growing. The growth rate of the core is determined by the density increase in the CCS shell resulting from the accretion of the outer low-density gas traced by C18O. We conclude that L1498 could become unstable to rapid collapse to form a protostar in less than 5 x 10(6) yr.

Dense core formation by fragmentation of velocity-coherent filaments in L1517

“…Work carried out over the past decade has revealed that most inconsistencies between tracers result from chemical changes that occur in the gas at dense-core densities and, in particular, from the selective freeze-out of molecules onto cold dust grains (Kuiper et al 1996;Caselli et al 1999;Bergin et al 2002;Tafalla et al 2002;Aikawa et al 2005). As a result of this work, a reasonably consistent picture of the chemical behavior of the different gas tracers has emerged.…”

Limits on the spatial variations of the electron-to-proton mass ratio in the Galactic plane

“…The source coordinates were taken from Paper I and Levshakov et al (2013), except for L1498A, for which they were estimated from Fig. 3 in Kuiper et al (1996). The source L1498A is a gas condensation within the L1498 dark cloud separated by 2 from the NH 3 reference position.…”

The ratio ofN(C$\mathsf{^{18}}$O) andA$\mathsf{_{V}}$ in Chamaeleon I and III-B

“…Little et al 1979;Burke & Hollenbach 1983). This phenomenon is most obvious in dense and dark globules, in which abundances of molecules such as CO and CS can be decreased by one or two orders of magnitude towards the globule center (Kuiper et al 1996;Willacy et al 1998;Kramer et al 1999;Alves et al 1999;Caselli et al 1999;Bergin et al 2001;Jessop & Ward-Thompson 2001;Hotzel et al 2001;Hotzel et al 2002;Tafalla et al 2002). On the other hand, molecules frozen on the grain surfaces can be evaporated because of heating by energetic particles or radiation.…”