Harvard Astronomy 201b

ARTICLE: A Filament Runs Through It: EVLA observations of B5

In Journal Club, Journal Club 2013 on April 1, 2013 at 4:37 am



Post by Zachary Slepian

0. Main points

Like a good poem, this paper is short but thought-provoking. For a cosmologist, perhaps the most interesting take-aways relate to star formation. First, the paper helps with a long-standing problem (trace it back to Larson’s Laws!) in how stars form—lots of molecular clouds have velocity dispersions too high to allow star formation (or to be explained by thermal motions alone).  But Pineda et al. find that in at least one dense core (within a molecular cloud), velocity dispersions are small enough to make it likely that a Jeans-like instability could form stars.  As evidence for this, the authors find that the Jeans length is on the order of the separation between a young stellar object in the core and a starless condensation to its north, the latter perhaps being a precursor for another star.  The fact that these two objects are separated by a Jeans length suggests they both stem from Jeans-like collapse and fragmentation.

The paper’s second major conclusion is that there is a 5000 AU long filament at the center of the core.  This is interesting on its own, because the filament turns out to be well fit by Ostriker’s 1964 model of an isothermal cylinder in hydrostatic equilibrium, in contrast to a different filament observed in 2011 in another star-forming region, which is not fit by this model and is also much smaller than Pineda et al.’s.   More significantly, though, the filament Pineda et al. find is an additional piece of evidence for the efficacy of a Jeans-like instability, as it could easily have resulted from Jeans collapse.

Ultimately, the low velocity dispersion is the common factor between these two pieces of evidence for Jeans collapse: it is only in the absence of turbulent support that Jeans collapse occurs in cores, and the low velocity dispersion implies this absence.

In what follows, I 1) offer some questions, with linked answers on separate pages, to help you think through the paper’s text and figures, and 2) take-aways summarizing the main point of each figure. Details on how the observations were done, which aren’t essential to understanding the physics,  are here, while I go through Ostriker’s 1964 isothermal filament model here.  I close with some recommended further reading and a linked summary of Alyssa and collaborators’ model of how coherence emerges in cores.

1. Introduction

As noted earlier, molecular clouds previously have been found to have “supersonic” velocity dispersions—what does this mean? (1)

These motions must be dissipated to allow collapse and star formation.  Why?  After all, such motions will increase the Jeans length.   But can’t we just treat them as producing an extra effective pressure, and presume that above the Jeans length these regions still collapse? (2)

The technique used to study these dense cores is NH_3 mapping, specifically the (1,1) transition.  Where is another place you can find NH_3? Why use NH_3 rather than our more popular friend CO?  What the heck is the (1,1) transition? (3)

The authors note that previous work with the Green Bank Telescope (GBT) has already mapped the ammonia in B5—why do it again? Hint: they used another array, the Expanded Very Large Array (EVLA), which has better resolution.  Why would they want better resolution if they are interested in studying velocity dispersions? (4)

2. Results

When in a rush, it is apparently standard astronomy practice to scan an article’s figures, introduction, and conclusion—after all, astronomers, unlike particle physicists, are visual people.  So I’ll focus my discussion on guiding you through the figures.

Figure 1

Figure 1 from Pineda et al. 2011.

Figure 1 from Pineda et al. 2011.

. . . explains why this paper was done even though observations had already been made of B5 with the GBT.  Compare left to right panel: what is the difference? (5)

The figures say they are integrated intensity, but the right-hand panel is in “mJy beam inverse km/s”.  Help!  That does not look like units of intensity.  Try resolving this yourself before clicking the answer! (6)

  • Take-aways: The regions with greater integrated intensity are those with higher density, so this figure is showing us where the structures are likely to form or have formed (e.g. the filament), since the densest regions will collapse first (\tau_J\propto 1/\sqrt{\rho}).  The right panel has zoomed in on the regions with subsonic velocity dispersions, which are bounded in the left panel by the orange contour.

Figure 2


Figure 2 from Pineda et al. 2011.

What is the most important part of this figure? I’ll give you a hint: it is black and white, and not so exciting looking! (7)

What are the two panels actually showing?  This is not so obvious! (8)

  • Take-aways: the left panel shows that the filament is on order the Jeans length, and also that the dense starless condensation and young stellar object (YSO) are separated by on order a Jeans length.  Hence, Jeans collapse is the likely culprit.  The right panel shows the velocity dispersion: regions with darker blue have smaller \sigma_v and hence more coherent velocities; they become less coherent near the YSO, possibly due to feedback.

Figure 3

Figure 3 from Pineda et al. 2011.

Figure 3 from Pineda et al. 2011.

Is Figure 3 redundant? (9)

Why do the authors do two different histograms in Figure 3?  Why divide into bits of the region near the YSO and not near it? Why might the red histogram (bits of the region near the YSO) have a higher centroid value of \sigma_v and width than that for bits of the region not near the YSO? Extra credit: can you use Figure 3 to estimate how many other stars we should eventually expect to form near the YSO already observed (assume there are no stellar outflows or winds)? (10)

And why do they use the criterion of two beam widths as the cut between objects close to the YSO and far from the YSO? (11)

Finally, in the caption for Figure 3, they note \mu=2.33?  Why?  There’s a very simple answer! (12)

  • Take-aways: This Figure is a different way of seeing the same information as the right panel of Figure 2.  It shows that the velocity dispersion is by and large subsonic, emphasizing that the velocity is fairly coherent, especially in regions away from the YSO.  The red histogram in the Figure emphasizes that near the YSO the velocity dispersion is higher, though still susbsonic, as noted earlier likely due to feedback, e.g. radiation from the YSO or interaction between an outflow or stellar wind and the dense surrounding gas.

Figure 4

Figure 4 from Pineda et al. 2011.

Figure 4 from Pineda et al. 2011.

Is perhaps the most difficult figure at first glance.  Radius of zero on the horizontal axis is the center of the filament, + and – move right and left in the yellow box around the filament in Figure 1. What is the key point of panel a? (13)

Panel b is perhaps more interesting, because it shows the filament is isothermal.  The model with p=4 is a better fit to the filament, which is Ostriker’s predicted value for p in eqn. (1) if the filament is isothermal.  What is the role of the blue curve, the beam response—why should we care about that? (14)

  • Take-aways: Imagine the filament as a cylinder. The top panel shows that as one goes out radial to concentric shells of the cylinder, the velocity dispersions remain sub-sonic  and roughly constant until one is well away from the filament, which is isothermal and resolved.

Pineda et al. note that their filament contrasts with the ones detected by Herschel in other star-forming regions (Arzoumanian et al. 2011), which are not isothermal, and are fit with p=2 instead (see below).

Showing how Herschel filament is better fit with p=2, non-isothermal profile. From Arzoumanian et al. 2011 Fig. 4.

Showing how Herschel filament is better fit with p=2, non-isothermal profile. From Arzoumanian et al. 2011 Fig. 4.

3. Handouts from JC discussion and further reading

Alyssa and collaborators proposed a model of why coherence emerges in “COHERENCE IN DENSE CORES. II. THE TRANSITION TO COHERENCE”, which I go through here.

How does a turbulently-supported core turn into, for instance, a filament? Stella Offner knows!  See “Observing Turbulent Fragmentation in Simulations: Predictions for CARMA and ALMA” by Stella S.R. Offner, John Capodilupo, Scott Schnee, and Alyssa A. Goodman.

For the discovery of the sharp transition to a dense, coherent (velocity) core, alluded to in the paper, see “Direct observation of a sharp transition to coherence in Dense Cores” by Jaime E. Pineda, Alyssa A. Goodman, Héctor G. Arce, Paola Caselli, Jonathan B. Foster, Philip C. Myers, and Erik W. Rosolowsky.

For extremely recent (March 2013) discussion of core formation and filaments in another region, see “Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region” by A. Hacar, M. Tafalla, J. Kauffmann, and A. Kovacs.

For the Herschel core observations alluded to in the paper, see “Characterizing interstellar filaments with Herschel in IC 5146″ by D. Arzoumanian et al., 2011.

For context to  “COHERENCE IN DENSE CORES. II. THE TRANSITION TO COHERENCE”,  you may want also to see the companion paper.

“COHERENT DENSE CORES. I. NH3 OBSERVATIONS” by J.A. Barranco and A.A. Goodman.


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