EPoS 2012 Focus Groups

Ringberg castle features several smaller seminar rooms which are ideal to meet and to discuss a particular topic in detail.

The oral introduction by the moderator(s) should take only a small fraction of the time allocated for the group.


Connecting the Domains: From Galactic to Extragalactic Star Formation Laws
Convenor: Jens Kauffmann
Wednesday 4 July, 11:20-12:40, 80 mins
We must understand molecular clouds in galaxies near and far, if we ever wish to understand the origin of the stars and planets we see today. Unfortunately, even ALMA will often not be able to resolve the relevant spatial scales in extragalactic clouds. The galactic star formation community must thus develop methods that permit to understand poorly resolved extragalactic clouds (e.g., ~2pc to >>10pc) based on our detailed knowledge of galactic clouds.

For example, HCN has been proposed as a selective tracer of the dense gas in clouds that can form stars. However, based on Milky Way examples, is this really a good tracer of dense gas? And what about all the more typical dense gas tracers? Also, are we certain that (column) density thresholds are really the deciding factor? Or are there other relevant parameters controlling the star formation potential of a cloud, such as cloud kinematics?

In this group, we will thus (1) discuss Milky Way "star formation laws", such as (column) density thresholds; (2) examine how such laws translate into trends for (molecular line) emission of clouds on large spatial scales; and (3) debate the future data sets that are needed to make progress (e.g., wide–field line maps in tracers other than CO). At least half of the time will be devoted to open discussion, with interjections of a few minutes/slides length interspersed. Please sign up VERY EARLY at , since slots will go to invited speakers otherwise.

Variable versus constant protostellar accretion. Theory and observations
Convenors: Eduard Vorobyov, Mike Dunham
Wednesday 4 July, 11:20-12:40, 80 mins
Our understanding of the star formation process is incomplete without the knowledge of how individual protostars accumulate their final mass. In the standard model of Shu (1977), protostars accrete at a constant rate proportional to the cube of the sound speed. However, ample evidence, both theoretical and observational, has recently been collected suggesting that protostellar accretion may be highly variable, with short bursts interspersed with longer periods of quiescent low-rate accretion. If this picture is true, it may have far-stretching consequences affecting our estimates of stellar ages, masses and the IMF. We propose to discuss this controversial topic highlighting the following questions:

1) What is the observational evidence for variability and bursts?
2) What are the physical phenomena leading to variability and bursts?
3) Do we really need the bursts or we can get along with the constant accretion paradigm?
4) How significant/important are bursts in the overall star formation process?
5) Do physical properties of pre-main sequence stars depend on the accretion history?


The session will start with short introductions covering the theoretical and observational background on protostellar accretion. Following these introductions participants are invited to present short (approximately 5 minute) contributions on these topics. We will then use the remainder of the time for a general discussion on protostellar accretion, aiming at reaching a consensus on what we currently know and what future observations and theory are still needed. If you like to present a short contribution, please contact .

The star formation rate (SFR) and efficiency (SFE) in molecular clouds (MCs)
Convenors: Paolo Padoan, Doug Johnstone
Wednesday 4 July, 14:00-15:20, 80 mins
Summary: The rate at which MC gas is converted into stars, and the final efficiency of this process, has major implications for our understanding of star formation and its effect on the host galaxies. Despite a wealth of data and nearly complete census of young stellar objects in nearby regions from space telescopes such as Spitzer, Herschel, and Planck, our knowledge of the SFR and SFE in MCs is still poor, with estimated SFR per free-fall time ranging from 0.01 to nearly 1. Theoretical and numerical predictions of the SFR in MCs also span a wide range of values, and are highly dependent on assumptions about initial and boundary conditions.
The goals of this focus group will be:
1) Review the current knowledge on observational estimates of the SFR and SFE in MCs, and possibly reach a consensus on the range of values of both quantities consistent with the latest observational data. This implies we reach a consensus also on the methods of measuring SFR and SFE and their uncertainties. Two or three 5-minute talks by observers, followed by a 15-minute discussion (total duration: 30 minutes).
2) Review of SFR predictions from theoretical and numerical models of star formation, to unveil the dependence of the SFR on physical parameters, but also on assumptions about initial and boundary conditions. Two or three 5-minute talks by theoreticians followed by a 15-minute discussion (total duration: 30 minutes).
3) Compare theoretical values of the SFR/SFE with the observations. Can we draw any constraints for numerical simulations or theoretical models? Are there strong intrinsic variations of SFR and SFE from cloud to cloud or are there mainly cloud age differences? Is the SFR strongly time dependent? What are the minimal ingredients to address the problem of the final SFE in MCs with numerical simulations (radiation-MHD and HII regions feedbacks, SN feedback, etc.)? (final 20-minute discussion).
If you would like to volunteer for one of the 5-minute presentations (either to review SFR and SFE observations or theory, or to present new results), please send an email (and possibly a draft of the contribution) to .

Star formation in turbulent media: role of magnetic fields
Convenors: Alex Lazarian, Dick Crutcher
Thursday 5 July, 17:10-18:30, 80 mins
Magnetic turbulence plays an important role at all stages of star formation. Compressible fluctuations change the mass to flux ratio, turbulence changes the virial mass etc. Magnetic fields change the nature of turbulent cascade, constrain fluid motions and induce new types of motions compared to the hydrodynamic cascade. In addition, turbulent motions violate frozen-in condition of magnetic field and allow fast diffusion of magnetic field and matter.

The goal of the focus group is to reevaluate the role of magnetic turbulence in view of recent numerical simulations and theoretical advances, discuss the role of ambipolar diffusion in turbulent environments and the prospects of numerical simulations in getting insight into physics of different stages of star formation. If you like to present a short contribution and show slides, send them before July 5 to to be put on one laptop.

Linking high-mass star formation to the dominating filamentary structures of molecular clouds
Convenors: Martin Hennemann, Frederique Motte
Thursday 5 July, 17:10-18:30, 80 mins
Herschel has revealed the prolificity of filaments in molecular clouds and has shown that the majority of the massive stars are forming in their densest and most dominating filamentary structures called ridges (several parsec length, crest NH2 > 10^23 cm^-2). These ridges seem to have formed by the connection and the merging of several filaments already forming stars. The coldest of these ridges should correspond to the densest IR dark clouds identified by MSX or Spitzer. Theoretical arguments and numerical simulations also suggest that high-mass stars and their surrounding clusters preferentially form at the junction of filaments also called hubs.

We propose to discuss:
1) the observational definition/characteristics of ridges combining constraints from NH2 maps and kinematics. A special attention will be given to questions regarding the timescale for the formation of such ridges and subsequent lifetime of high-mass prestellar cores/protostars.
2) the existence of such ridges in simulations and their characteristics. An important question remains to know if converging flows are necessary to form them.
3) the possible link of ridges with enhanced star formation activity (SFR and SFE) one can measure.

We invite short contributions (only a few slides each) on any of this subtopics. Please send an email to with the description/title of your proposed contribution.