The workshop intended to review, interpret and understand the use of the brightest sub-mm spectral line emission - that of C+ - as tracer of gas density and star-formation rate in galaxies. As a tracer for many different conditions in the interstellar medium, C+ allows to constrain the cloud microphysics such as the gas heating efficiency, the dust grain size distribution, the abundance of PAHs the electron density, i.e. the ionization degree, and the total gas density. During the workshop two main questions drove most of the discussion: What is the relative contribution of  photon-dominated regions (PDRs), HII regions, diffuse clouds, shocks, and CO-dark molecular gas to the global C+ emission of a galaxy? And how realiably does the C+ emission measure the star-formation rate in the Milky Way and other galaxies?

With more than 50 participants, the workshop brought together experts on C+ observations and  models with young researchers entering the field. A large fraction of the overall time was covered by 10 review and 23 short talks that were thematically grouped around specific questions. A poster session, time during extended lunch breaks and in three sessions with two parallel splinters was heavily used for discussions to identify questions that can be answered today on the one hand and identify the required input to answer the other questions in the future.

These discussions cleared the state in a rapidly evolving field providing a common level to all participants by summarizing the knowns and the unknowns of today. On the modeling side, it was easy to identify the missing pieces of information, mainly in terms of state-to-state reaction rates, the H2 formation rate including the produced ortho/para ratio, and better constraints on the geometry of the sources. To model specific regions it is moreover required to obtain high-resolution maps of H2 emission and better constraints on the structure of polycyclic aromatic hydrocarbons (PAHs) in the region. A consensus was also reached in the question of the excitation of C+. O and B stars were identified as the main source of energy establishing C+ emission as a star-formation tracer with a memory of about 100 million years. On the scales of normal-type galaxies, the different phases provide a convenient statistical mixture so that C+ can be used as a direct quantitative measure for the star formation activity unaffected by variations in the local gas conditions. A strong impact of metallicity changes is observed and expected, but a full quantitative model is still lacking for this effect as well as for the line emission deficiency in ultra-luminous galaxies. More open points became obvious when dealing with the details of the microphysics for individual regions in the Milky Way. The heating efficiency, and consequently the C+ brightness, depends strongly on the unknown properties of the PAHs in the clouds. There was still a strong debate on the contribution of the different phases of the interstellar medium to the C+ emission, in particular the relative fraction of the CO-dark molecular gas, the contribution of turbulent dissipation regions to the C+ formation, and the pressure equilibrium between the different phases. The discussion identified observations of O++, N+, atomic oxygen, and the total far infrared flux as critical complementary data needed to estimate the role of the different phases. These observations and the need for larger maps of C+ ask for  new missions and observatories. Prospects and requirements for future possible air-borne and space missions were identified.

Therefore the workshop provided input to the planning of several new proposals, both for scientific missions and new model developments. The future will show how successful they were. The participants appreciated the workshop format that followed the normal Lorentz-Center frame with more than one third of the total time available for open and coordinated discussions fed by the input from the reviews, contributed talks, and posters. The comprehensive support from the Lorentz-Center personal provided a perfect framework to enable discussions focusing on the topic of the workshop. A possible follow-up workshop in the future, e.g. on [OI], should probably use the same concept.

Jonathan Braine (Bordeaux, France)
Maryvonne Gerin (Paris, France)
Simon Glover (Heidelberg, Germany)
Paul Goldsmith (Pasadena, USA)
Michael Kaufman (San Jose, USA)
Suzanne Madden (Gif-sur-Yvette, France)
Bhaswati Mookerjea (Mumbai, India)
Volker Ossenkopf (Köln, Germany)
Floris van der Tak (Groningen, The Netherlands)
Thangasamy Velusamy (Pasadena, USA)