Harvard Astronomy 201b

Discussion of Assignment 1: Schematic of the Milky Way (Chris Beaumont’s version)

In Uncategorized on February 15, 2011 at 10:17 am

The list below is a fairly complete subsample of the questions asked during assignment one. I have arranged them into three categories, correlated with my own ignorance on various subjects. I speculate (or, in rare cases, know) that questions in the hardest group are open research issues. Questions in the second category are likely the kinds of questions a subject-area expert could answer. Questions in the last category are the most well-understood, and will likely be covered during this course. I have added commentary where I think I have something to add

Very Hard (i.e. I don’t know, and I’m not sure if anyone does)

  • Is the Parker Instability a viable mechanism for the formation of giant molecular clouds?
  • How do GMCs form? (Cloud formation theories are discussed in McKee & E. Ostriker’s 2007 Annual Review article on star formation, though this is an open research question).
  • At what length scales in a molecular cloud does self gravity become important? (it is suprisingly difficult to assess whether a given region is gravitationally bound. This is due to uncertainty about distances and flux / mass conversions, the difficulty in defining an “edge” to any cloud region, and the problem of line-of-sight projection. Gravity certainly is important at the protostellar stage and beyond, and many people believe clouds as a whole are bound. Different theories make different predictions about which of the intermediate scales are bound)
  • What drives and supports turbulence in GMCs? At what scales is energy injected / dissipated? (some combination of Galactic shear, stellar feedback, global infall)
  • At this scale (100s of parsecs), how does the physical structure of the ISM evolve over time?  What galactic properties are most important in determining the structure of the ISM?
  • How well can we predict how the ISM will behave during galactic mergers?
  • How important are chemical networks in star formation? (very! But it’s extremely hard to observe or simulate chemical processes)
  • How do grains catalyze reactions and change ISM chemsitry?
  • Is one IMF universal? (At the current level of precision, more or less. However, we don’t know whether it is truly universal, or (more importantly) whether the shape of the IMF diagnoses star formation mechanisms)
  • Does every galaxy undergo a merger-induced starburst phase?
  • How will the ISM evolve in the future?
  • What will the ISM look like in 10 Gyr?
  • What sets the galaxy stellar mass / gas metalicity relation?
  • What is the source of the large-scale B field?
  • What determines the relative distribution of phases in the ISM?
  • How do dust grains grow? (Early dust grain growth is better understood, and proceeds via electrostatic attraction. Nobody knows how to grow “grains” beyond the size of pebbles.)
  • What determines the star formation rate? What scale is relevant?
  • Does jet formation affect a disk? (They are closely related, for sure. Jets seem necessary to shed excess angular momentum contained within disks. The detailed picture of how jets form, however, is unknown)
  • How does gas lose magnetic fields and angular momentum  as it collapses?
  • How do planetary disks evolve into planetary systems?
  • Does dark matter have a direct influence on the ISM?
  • Does the dark matter interact with the baryons anywhere in the Galaxy?

Hard question (i.e. I don’t know, but I bet somebody does)

  • What sets the IMF?
  • What sets the Core Mass Function?
  • What sets the mass function of massive cloud fragments?
  • What are the dominant factors that determine the mass of a new star? (for all of the above, some interplay between fragmentation, molecular support, turbulent support and/or fragmentation, gravitational collapse and accretion, and converging fluid flows. The details are unknown, and likely do NOT correlate cleanly with forms of the CMF/IMF)
  • How different is the Milky Way’s ISM from the ISM of other galaxies?
  • What types of chemical reactions take place on the boundary between a nebula and the ISM?
  • How quickly can O stars drive off gas and end star formation in their region?
  • How are cosmic rays formed in an SNR?
  • What are the compositions of grains? (we know the basic atomic constituents, but they are arranged into very complicated molecules)
  • What is the size distribution of grains?
  • What sets the dust/gas ratio?
  • What is the magnetic field in the IGM?
  • Where does the IGM magnetic field come from?
  • Are B fields connected throughout the ISM? (Read the papers by your fellow classmate Sui Mao, who has been trying to measure this)
  • How do starbursts affect the properties of the ISM?
  • What is the typical value of magnetic fields inside dense cores? Can these fields be strong and ordered enough to induce MRI in protoplanetary disks?
  • Does the presence of ice on interstellar dust fundamentally change how they stick together?
  • How does the distribution of ISM phases vary in different Galaxies?
  • How do spiral density waves form / persist without breaking or winding up?
  • What is the infall timescale for supernova gas ejected at high latitudes? How does this vary for different galaxy morphologies?

Easier question (i.e. I think I know)

  • Young stars strongly influence their surroundings — how does this feed back into star formation?
  • As young hot stars push material away, does it trigger more star formation in now denser adjacent regions, leading to a starburst? (stars are certainly capable of triggering star formation by sweeping up and/or compressing material. It is less-well understood how relevant triggered star formation is on a global scale)
  • Do these effects change the initial mass function?
  • What is the distribution of stars in a typical HII region?
  • Do what extent to the CNM, WNM, and WIM coexist in equilibrium?
  • How well-mixed is the ISM? (very [in that the phases are continually exchanging mass] and not at all [in that they never really reach an equilibrium distribution])
  • How does the supernova shock interact with the boundaries of the molecular cloud? (see, e.g., http://adsabs.harvard.edu/abs/1992ApJ…386..158W)
  • How well does the distribution of dark matter match the distribution of luminous matter? (dark matter is much more smoothly distributed)
  • How much variation is observed in the number of dark cores per molecular cloud? (the raw star formation efficiency, as well as the star formation efficiency per unit mass, varies by several orders of magnitudes between clouds. There is a corresponding variance in core count)
  • How symmetric are supernova shocks? (Not at all! Fluid instabilities cause the shocks to fragment into filaments, and inhomogeneities in the surrounding ISM cause them to expand anisotropically)
  • Where do grains come from?
  • How well is the gas correlated with the dust
  • Are molecular clouds turbulent? If so, at what spatial scales? (Yes, from cloud scales down to ~.1 pc scales)
  • How much mass do stars lose to the ISM?
  • Is the Kennicutt-Schmidt law universal?
  • Where do cosmic rays come from?
  • Can the WNM penetrate into molecular clouds? If so, how far? (Molecular clouds are irregular and filamentary, and thus can get “tangled” in the other phases of the ISM)
  • How can supernova heating be such a dominant force if the supernova rate is only ~1 / Galaxy / century?
  • Why does star formation occur in only a small fraction of the gas? (very dense gas “seeds” are probably needed to trigger localized collapse. These seeds are probably generated by random turbulent motions, and the density threshhold is high compared to the mean cloud density. Thus, star formation sites are few and far between within clouds)
  • How does the CMF map to the IMF? Are they related? (They look similar and, while the CMF is often claimed to derive from the IMF, this is likely an overinterpretation. See, e.g., Swift and Williams (2008) or Swift and Beaumont (2009))
  • How do rotation and spiral density waves influence star formation? (gas rotates into spiral arms, at which point it is compressed and undergoes enhanced star formation)
  • What are the heating/cooling relationships between the various components of an HII region? (will be covered in this course, for sure!)
  • What processes disperse and mix the material from ABG stars, Wolf-Rayet stars, and SNRs throughout the ISM and into the IGM?

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