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Since a factor of 4 difference in the ionization fraction might be significant, as it means that the gas in our model would be less strongly coupled to the magnetic field compared to the gas in the SR10 model and may therefore have a higher infall velocity, in Section 7.

The temperature profiles tested. This is because the ambipolar diffusion model is starting from a much earlier, more diffuse state, and a smaller fraction of the enclosed mass in the cloud would be expected to go into the protostar.

In Section 2 we describe dynamical collapse models studied, and in Section 3 we describe irqm chemical model, the results of which are given in Section 4. Investigating the effects of 24266 on molecular line profiles of infalling low-mass cores J. This irak of irwm has been noted in previous studies e. If the optically thick lines exhibit a double-peaked profile, with the blueshifted peak stronger than the redshifted peak i.

Citing articles via Web of Science 3. We find that even for such high infall velocities in the outer parts of the core, the blueshifted peak of the CS line profiles is almost completely unaffected.

Equilibrium between freeze-out and desorption processes rapidly follows, resulting in extremely depleted abundances.

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For the background radiation field, we have used the standard interstellar radiation field of Mathis et al. In that paper they used a Monte Carlo approach and included layering of the ice, and found that the results were significantly different to the results obtained using a simple rate equation approach.

This solution is valid for a singular isothermal sphere extending to infinity. The temperature profiles are shown in Fig. Related articles in Web of Science Google Scholar. For example, Belloche et al. Since they took into account this depletion in their radiative transfer model, this could explain why their inside-out collapse model failed to reproduce the observations.


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The dotted and dashed lines show the two linear temperature profiles. It should also be noted that the collapse time-scale is approximately one order of magnitude faster for the inside-out model than for the ambipolar diffusion model. The collapse expansion wave CEW solution is a particular solution to this problem, which starts with the initial density distribution of a singular ieam sphere and with an initial velocity of zero everywhere.

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This is because for such igam and rapidly changing densities, the model of SR10 cannot converge on a solution. Material immediately above the collapsing inner region begins to fall, once the material beneath it has given way. The peak intensities of all three CS lines decrease with time. Ideally, we would have liked to model H 2 CO as well, which is another common infall tracer. However, there are several theoretical arguments and observational studies which indicate that the inside-out collapse model may be inappropriate.

Due to the large differences in core sizes and therefore molecular column densities, we cannot make a quantitative comparison to their results, but nevertheless, their resulting line profiles show notable qualitative similarities to those we have derived.

Also, to understand more about the behaviour of the line profiles, we have tested two ad hoc linear temperature profiles, one of which is warm in the centre 12 K 24226 cooler on the outside 8 K and the other which is cold 8 K in the centre and warmer 12 K on the outside.

In such cores, the neutral species do not directly feel the effect of the magnetic field, and therefore drift towards the centre of the core, being impeded by the drag force exerted by the ions which are tied to the magnetic field. A relationship between visual extinction and temperature is assumed, so the radial temperature profile also evolves throughout the collapse, which affects the gas pressure.

In order to obtain a more realistic H 2 CO abundance, sophisticated surface chemistry would have to be implemented in the model, such as that employed in Cuppen et al. There is no mass transfer between the shells. We tested the fact that this was sufficient resolution by running the model with various grid sizes and found that the spectra were invariant for 70 radial grid points.

The inside-out collapse model is also widely used in theoretical studies of collapsing cores e. Obviously, for outer shells, this increase in abundance occurs at later times.


How to protect a building against lightning? The line profiles are plotted in Fig. This has been recognized in previous studies. Freeze-out of species on to grains is included, proceeding at the same rate as given in Roberts iran al.

Therefore, the central infalling blueshifted material does not have an excitation temperature higher than the redshifted infalling material at the edge of the CEW, and this can explain why we do not see the blue asymmetry at this time.

Preceding this is the pre-protostellar phase [often referred to as the pre-stellar phase for brevity Ward-Thompson et al.

What is a lightning rod? These differences appear because the infall velocity in the core envelope of the Pavlyuchenkov et al. The cores were assumed to be at a distance of pc, the distance of the Taurus Molecular Cloud.

These reactions are assumed to occur instantly and the products remain on the grain surface until they are desorbed. The class 0 temperature profile gives the strongest blue asymmetries, which can be explained by the very high excitation temperature in the core centre where there is still a high abundance of CS since we have neglected depletion.

Unlike the inside-out collapse, where the envelope material is static, this redshift in the absorption is the main cause of the blue-skewed asymmetry in the line profiles. These values are applicable to Bok globules which are possible candidates for star formation. Here we concentrate on the commonly used solution derived by Shuwhich was solved by finding similarity solutions to the continuity mass-conservation equation and force equation for an ideal isothermal flow.

The red—blue asymmetries in the lines although not strong broadly reflect the expected dependency on the excitation temperature profile. This paper is organized as follows. For the inside-out collapse model to exhibit the blue asymmetry it is necessary to suppress freeze-out and impose a negative temperature gradient on the core, whereas the ambipolar diffusion model exhibits the blue asymmetry even when there is significant freeze-out, and for all the kinetic temperature profiles tested.

It has been demonstrated in the previous section that the abundance profile of the molecules across the core in particular whether or not there is depletion can have a significant impact upon the line profiles.