Notes from the ISM-PP conference: Day 2

Collapse of molecular clouds and the IMF 40': Matthew Bate (University of Exeter)

Analytical models predict dependence on density, mach number, etc
  • Accretion stopping time + dynamics
  • kinetic feedback, b-fields
Generating the IMF
  • Fragmentation to produce "stellar groups" + how fragmentation proceeds is not importan
  • Only knob that changes the IMF is the initial Jeans mass of the cloud
  • process is always: start with jeans mass, accrete up or get ejected
  • gets multiplicity right
  • Thermal feedback
  • Radiative feedback -> less fragmentation, especially less disk frag
  • radiative feedback erases dependence of IMF on initial conditions + fewer objects -> less dynamics -> less frequent ejection
  • Bate 2012 "turning point" - can reproduce IMF + can try for a predictive theory of SF
  • metallicity comparison + same IMF over 300x difference in Z
  • Krumholz: very high temperatures suppress fragmentation, -> high-mass IMF + outflows "alleviate overheating problem"
  • B-fields increase accretion, enhance thermal feedback
  • Q Hennebelle: What is competitive accretion? How do you conclude that it is the mechanism going on in the sim?
  • A: Competitive accretion is stopping time.
  • Q about terms - competitive accretion governs high mass, but term applied to low mass
  • A: All are competing, but likelihood of losing...
  • Q Paul Clark: Gas-Dust coupling happends at 10^5. Are you including metallicity dependence of coupling? I don't believe metallicity results.
  • A: No. We assume dust is always dominant. Large scale temperatures don't matter. Temperatures matter on the small, self-gravitating scales
  • Clark: Initial cores would have been more massive.
  • Bate: No, not determined by temperature, only by turbulence.
  • Q Zinnecker: Competing theory of turbulent cores. Compare them.
  • A: Fabian Heitsch did that well yesterday. Turbulent core model relies on turbulence providing isotropic pressure.
  • Q Kruijssen: Prediction is that dynamical times vs. disk survival times. Boundedness of cluster? group? has big effect. YMCs don't have top heavy. How do you make bottom-heavy IMFs?
  • A: "Can't believe I'm saying this..." possible you can get bottom-heavy imf with extreme turbulence. Mass reservoirs stripped away by high-mass flows. Prevents long-time accretion.

Tracing the fragmentation of OMC1-north filament with the Submillimeter Array 15: Paula Teixera

with Satoko Takahashi, Luis Zapata, Paul Ho
  • Cores detected in OMC1N NH3 filaments
  • characteristic separation scales
  • "core pods" instead of "clumps"
  • OMC1N has smaller nearest-neighbor separation than OMC2/OMC3
  • Q: Direction of B-field?
  • A: Perpendicular to filament
  • Q: Scales can be interpreted as two different fragmentation scales
  • Q Falgarone: Do you include short-spacing in the maps?
  • A: No.
  • Q: Bias.
  • A: not on short scales.

Filament formation and star formation regulation in collapsing molecular clouds 15': Vazquez-Semadeni

Case for globally contracting molecular clouds
  • "turbulent infall" rather than "turbulence" + no "turbulent support"
  • "turbulence is overrated" ?
  • collapse is pressureless
  • "whole thing collapsing into global potential well..."
  • velocity jumps around local potential wells
  • "two-filament appearance" due to presence of wells. + Hacar gaussian fits yield 2 filaments (incorrectly)
  • How address SFR too high for global collapse? + ionization feedback -> cloud destruction + 10% efficient... THIS IS STILL AN ORDER OF MAGNITUDE TOO HIGH

So are all molecular clouds gravitationally collapsing? Why aren't they forming stars on a free-fall time?

  • Q Keto: Your model is different. Stars forming with accretion/flows along filaments.
  • A: No one has implied filaments are static... no contradiction.

The structure and star-forming fate of the Galactic centre cloud G0.253+0.016 15': Katharine Johnston

Stats...
  • hot gas, cool dust
  • no >B0 stars
  • SCUBA+SMA image + column density PDF + plateau at low column density (could this be pixelation?)
  • shocks in the south
  • hint of cloud-cloud collision
  • cloud-cloud collision comparison: Anathpindika et al 2011 + many claims that super star clusters form from cloud-cloud collisions
  • ring view is reversed + center of galaxy doesn't have to be ring, could be spirals or disk
  • Molinari ring fails
  • Q Xu: Single cloud... surprised you find only one cloud.
  • A: data only cover one cloud...
  • Q Kainulainen: Comment on low SFR. If follow analytic formulations, care about relative density contrast. Despite enormous mean density, the relative density contrast is low. Not surprised by low SFR.

Laboratory Studies of Dust Formation and Processing 40': Cornelia Jäger

There are many types of dust
  • "The ISM is the most dangerous place for dust."
  • 49um band of Forsterite
  • 69um band of crystalline forsterite + these can be used as a dust thermometer
  • beta-T anticorrelation + SiO2 absorption changes strongly with temperature + crystalline material has no beta-T, but amorphous DOES have a beta-T
  • generation of fullerenes in the lab at T>3500K
  • dust growth
  • Helium droplet experiment + "helium droplet beam" + superfluid helium shot through a "pickup oven" at 0.4 K + forms SiO clusters + "barrierless" reactions
  • laser evaporation of particles, condense on a substrate
  • ion-induced processing
  • irradiated silicates lead to iron inclusion
  • Q: Beta/temperature. Observations... beta is effective beta, non-constant temperature is problematic. Observed temperatures are much smaller.
  • A: Yes, there is still dependence at low temperature, but weaker.
  • Q: Are there any features in the mm that we can use to distinguish the species in the submm?
  • A: not yet.

Combining experimental techniques for comprehensive astrophysical case studies 15': Holger Kreckel (MPIK, Heidelberg)

Gas phase chemistry
  • terminate branches with recombination with free electrons
  • Cryogenic Storage Ring + electrostatic field + strong vacuum, low (10K) temperature
  • C- beam driven electrically, dissociate electron with a laser + "Don't normally need 2 kW continuous lasers except for death stars."
  • studying cosmic ray ionization rate + CR ionization is missing some rate coefficients
  • Q Glover: Why only go to 40K?
  • A: Divergence of beam limits temperature

Laboratory studies on electron collisions of atomic and molecular ions 15': Andreas Wolf (MPIK, Heidelberg)

HD formation
  • need cooled vacuum chamber to match ISM conditions

Chemical processes in the ISM: Gas and molecules 30': Simon Glover (ZAH/ITA University of Heidelberg)

Noble gas molecule in ISM: ArH+ in SNR Why are molecular clouds cool?

  • Molecular clouds are cool because of dust shielding, NOT CO cooling + C+, CI are just as efficient as CO + CO allows you to cool from 20 to 10K at n~10^2-10^3
  • production of high-density material drives CO formation
Chemistry as a tracer of physical conditions in the ISM
  • CR rate measurement tool: H3+ -> CR rate + if you can measure C+ + diffuse sightlines have values >> "canonical dark cloud value" = 10^-17
  • HF: most evil substance imaginable - seep through skin, melt bones + exothermic: most fluorine lands in HF + depleted in dense gas
  • CH+ puzzler: formation slow, destruction fast + overabundant by 3-4 orders of magnitude
  • Isotopologue chemistry for CO + Fractionation: O prefers to be in 13CO. Can convert 12CO->13CO + photodissociation: less self shielding in 13CO, 13CO->12CO
  • Uncertainties: + Need good rate coefficients + Chemistry not in steady state: memory effects + PDR modeling done with GCs treated as slabs
  • Q: Are there molecules for which the uncertainties are likely small enough now?
  • A: H2 cooling doesn't play a significant role in the thermal balance of the cloud
  • Q: Matthew Bate: Can observers use H2 (HF?) to deal with dark gas?
  • A: Not clear yet.
  • Q: Deuteration important? Influence... things?
  • A: Timer for core collapse
  • Q Edith Falgarone: Disagree. We know C-shocks are heating gas to few hundred K, contributes to C+ emission. Need additional heating source. Balance not between ionization...?
  • A: Depends on assumptions of photoelectric heating efficiency. We find shock dissipation not important in dense stuff. H2 may be important in diffuse gas.

Chemical processes in the ISM: Dust 30' Speaker: Thomas Henning (MPIA Heidelberg)

Small particles heat the gas Formation/destruction balance

  • AGB star dust formation not efficient enough to balance dust destruction
  • NIR extinction law: Fritz et al 2011 towards GC + dust towards GC is different
  • Particles in mol clds grow to few microns at most
Why is there dust in the ISM?
  • Cold ISM formation?
  • Or core-collapse SNe? + Formation agrees with predictions, destruction too much...
  • Low-temperature dust formation? + works for SiO2 in the lab
Grain models:
  • composite grains with different size distributions
  • mixture of graphitic and silicate grains...
  • Q: SNe still not enough for other galaxies.
  • A: Our galaxy needs 0.5 per SNe. More of a problem for low metallicity.
  • Q Falgarone: ???
  • Q Zinnecker: Dusty wolf-rayet stars. Also dust factories?
  • A: Not in normal environments. O-stars may be a solution for high-Z galaxies.
  • Q: How much oxygen in dust?
  • A: More in silicate.

Galactic dust as seen by Planck 15': Marta Alves

All-sky dust temperature map
  • outer galaxy cold, inner galaxy warmish
  • increase of dust temperature towards poles
  • dust opacity anticorrelated with temperature + not correlated with luminosity
  • Planck extinction maps
  • Flattening of SED at mm wavelengths
  • Q Rahul: Have you accounted for degeneracies in fits?
  • A: Beta map was observed at low-resolution, then did a second run of the fit to extrapolate down to high resolution

Formation signatures and carbon budget of molecular clouds 15': Henrik Beuther

Signatures for cloud formation
  • CI, CO, C+ observations of 4 IRDCs
  • G11.11 IRDC + ionized lines very narrow
  • [CII]/[CI]/CO = 1/5/70 + ionized carbon measurement unknown

Svitlana Zhukovska

  • Q Zinnecker: Gas to dust ratio varies from dense to diffuse gas (Meixner LMC).
  • A: Yes,there is dependence

Dependence of star formation on ISM properties 40': Adam Leroy (NRAO Charlottesville)

"Two Bottleneck" view
  • Atomic Gas -> Molecular Gas -> Dense Gas
  • cloud formation is the first regulating step
  • more molecular = more star forming
  • IR/CO vs HCN/CO... weakly correlated
  • HCN/IR approximately constant over wide range of scales
  • factor of 2 scatter in SFE

Sarah Ragan

  • Q Semadeni: different surface density -> different evolutionary stages? Scaling law depends on how you define objects

Amy Stutzki

  • Q Semadeni: Higher lum implies higher SFR?
  • A: lum higher, but maybe that means shorter accretion time.

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