Changes between Version 48 and Version 49 of u/EricasLibrary

Timestamp:

03/11/13 12:25:07 (12 years ago)

Author:

Erica Kaminski

Comment:

—

u/EricasLibrary

@@ -185 +185 @@
   The paper then begins to set up its motivation - namely that research on flat topped isothermal spheres have begun to be pursued, such as F&C. They say F&C gets more accurate results, but that the collapse is triggered artificially. They plan to take the studies further on non-singular spheres, but by initiating collapse in a more physically relevant way (increasing Pext as suggested by Myers et al). They claim that most of the observational constraints (such as decreasing accretion rate, velocity fields, initial condition) are recovered by this model (although in my opinion - they also seem to have been recovered in F&C model too... so maybe they find MORE accurate lifetimes for their collapsing BE spheres). I find it interesting that they do not call the sphere a Bonnor Ebert sphere, but rather an isothermal sphere... is there any significance to this?
 
-  In their results section they present very weak quantitative results/explanations/insight of their plots (interestingly they spend very little time developing equations of BE sphere and also do not mention the critical values of the BE sphere such as Pcrit, Mcrit). Their different runs are broken down into the rate of compression, in how much time (measured in units of sound crossing time -- ostensibly in terms of the BE sphere) does the Pext double. They initiate the collapse of the critical BE sphere by increasing Pext (by any amount would increase Pext beyond Pcrit as the sphere is already critical), but then they continue to increase Pext throughout the course of the simulation (this potentially is similar to what we did in practice by just allowing the simulation to evolve "naturally" with no forced increase in Pext, simply Pext at the sphere's outer edge increased naturally by the infall of material onto that outer edge). In the first couple of cases they show, compression is slow. In the first 3 panels of Fig. 1, a sink has not yet formed, and so they identify this phase as the pre-stellar (aka the pre-protostellar) phase. It appears to me that they get a nice outside in flow beginning to develop in these plots, although the radius of the BE sphere is unclear. No compression wave seems evident in these plots, but instead, it seems to be a re-equilibration of matter in terms of the language I use in my paper. This could be due to early subsonic adjustment of the material into a modified BE profile that exceeds the critical mass, and hence collapses in the canonical fashion. They use no such language, and make no such identification which is curious. They describe this, as well as all other cases, as being a compression wave solution. This might make sense if you follow Whitworth language that all collapse problems are due to compression waves, but some of the waves have 0 amplitude..? This case looks most like some of our lighter ambient runs, where the Pext increases very slowly due to a slow accumulation of matter.. In the next 2 panes of the plot, a central sink has formed that accretes matter in a free-fall manner (vrad~r^1/2) that they say moves outward in time. I can't tell from this plot that this type of flow is moving outward in time. They talk about the mean cruising velocity (Whitworth language) for the different runs, but it is unclear if this is an average over the entire sphere at a given time, or if it is at a given radius over time, or what.. They say that the mean cruising velocity is set up by the compression wave, but how do they know when it penetrated the sphere if they can't even see it in the density plot? They do say that in this case the compression wave converges on the center, at which time the sink forms, so maybe in this limiting case of the Whitworth solution, the compression wave looks like the classic result? 
+  In their results section they present very weak quantitative results/explanations/insight of their plots (interestingly they spend very little time developing equations of BE sphere and also do not mention the critical values of the BE sphere such as Pcrit, Mcrit). Their different runs are broken down into the rate of compression, in how much time (measured in units of sound crossing time -- ostensibly in terms of the BE sphere) does the Pext double. They initiate the collapse of the critical BE sphere by increasing Pext (by any amount would increase Pext beyond Pcrit as the sphere is already critical), but then they continue to increase Pext throughout the course of the simulation (this potentially is similar to what we did in practice by just allowing the simulation to evolve "naturally" with no forced increase in Pext, simply Pext at the sphere's outer edge increased naturally by the infall of material onto that outer edge). In the first couple of cases they show, compression is slow. In the first 3 panels of Fig. 1, a sink has not yet formed, and so they identify this phase as the pre-stellar (aka the pre-protostellar) phase. It appears to me that they get a nice outside in flow beginning to develop in these plots, although the radius of the BE sphere is unclear. No compression wave seems evident in these plots, but instead, it seems to be a re-equilibration of matter in terms of the language I use in my paper. This could be due to early subsonic adjustment of the material into a modified BE profile that exceeds the critical mass, and hence collapses in the canonical fashion. They use no such language, and make no such identification which is curious. They describe this, as well as all other cases, as being a compression wave solution. This might make sense if you follow Whitworth language that all collapse problems are due to compression waves, but some of the waves have 0 amplitude..? This case looks most like some of our lighter ambient runs, where the Pext increases very slowly due to a slow accumulation of matter.. In the next 2 panes of the plot, a central sink has formed that accretes matter in a free-fall manner (vrad~r^1/2^) that they say moves outward in time. I can't tell from this plot that this type of flow is moving outward in time. They talk about the mean cruising velocity (Whitworth language) for the different runs, but it is unclear if this is an average over the entire sphere at a given time, or if it is at a given radius over time, or what.. They say that the mean cruising velocity is set up by the compression wave, but how do they know when it penetrated the sphere if they can't even see it in the density plot? They do say that in this case the compression wave converges on the center, at which time the sink forms, so maybe in this limiting case of the Whitworth solution, the compression wave looks like the classic result? 
 
   Skipping to figure 3, for which the Pext doubles in a crossing time (aka strong compression). I have a hard time understanding their description in the text, p 874, right column, 2nd paragraph. They say a small compression wave moves in (I see this in plot), but they go on to say that before the wave converges on the center to form the sink (and class 0 phase - last 2 panes in figure), the central density has hardly changed "for the inner regions were unaware of Pext increasing". First of all, in the plot the central density is clearing increasing in panes 1-3. Second of all, they would be unaware only for a supersonic compression wave -- for which they did not establish exists. They say again that around the sink, a free fall v field is set up, but in the outer regions a uniform sonic field is established (v~0.12 km/s). Again, I do not see a uniform v field in the outer regions.. I assume what they mean is uniform in r... not for a fixed r over time..  This plot does however, seem to show a change in the velocity field from the early "compression wave phase" to the later "classic phase", in the language of my paper. Although I think these plots overall do a poor job showing the evolution of the flow since they only sample 3 time states before sink formation, they don't capture the dynamics very well.