Space Infrared Telescope for Cosmology and Astrophysics (SPICA), the cryogenic infrared space telescope recently pre-selected for a ‘Phase A’ concept study as one of the three remaining candidates for European Space Agency (ESA's) fifth medium class (M5) mission, is foreseen to include a far-infrared polarimetric imager [SPICA-POL, now called B-fields with BOlometers and Polarizers (B-BOP)], which would offer a unique opportunity to resolve major issues in our understanding of the nearby, cold magnetised Universe. This paper presents an overview of the main science drivers for B-BOP, including high dynamic range polarimetric imaging of the cold interstellar medium (ISM) in both our Milky Way and nearby galaxies. Thanks to a cooled telescope, B-BOP will deliver wide-field 100–350 $\mu$m images of linearly polarised dust emission in Stokes Q and U with a resolution, signal-to-noise ratio, and both intensity and spatial dynamic ranges comparable to those achieved by Herschel images of the cold ISM in total intensity (Stokes I). The B-BOP 200 $\mu$m images will also have a factor $\sim $30 higher resolution than Planck polarisation data. This will make B-BOP a unique tool for characterising the statistical properties of the magnetised ISM and probing the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our Galaxy. B-BOP will also be a powerful instrument for studying the magnetism of nearby galaxies and testing Galactic dynamo models, constraining the physics of dust grain alignment, informing the problem of the interaction of cosmic rays with molecular clouds, tracing magnetic fields in the inner layers of protoplanetary disks, and monitoring accretion bursts in embedded protostars.

Starburst galaxies at z ∼ 2 – 4 are among the most intensely star-forming galaxies in the universe. The way they accrete their gas to form stars at such high rates is still a controversial issue. ALMA has detected the CH+ (J = 1-0) line in emission and/or absorption in all the gravitationally lensed starburst galaxies targeted so far at z ∼ 3. Its unique spectroscopic and chemical properties enable CH+ to highlight the sites of most intense dissipation of mechanical energy. The absorption lines reveal highly turbulent, massive reservoirs of low-density molecular gas. The broad emission lines, arising in myriad UV-irradiated molecular shocks, reveal powerful galactic winds. The CH+ lines therefore probe the fate of prodigious energy releases, due to infall and/or outflows, and primarily stored in turbulence before being radiated by cool molecular gas. The turbulent reservoirs act as mass and energy buffers over the duration of the starburst phase.

The cold diffuse interstellar medium (ISM) is turbulent with supersonic, most likely transAlfvénic motions. All its tracers, including velocity, are structured down to the smallest scales accessible to observations. This structure, though, is not random and can be quantified using the statistics of its Fourier phase increments. The quantity of structure of interstellar clouds is larger than that of fractal Brownian motion (fBm) fields and of mildly compressible turbulence. We propose that this structure is a response of the medium to a general property of turbulence, the space-time intermittency of its dissipation. The bursts of turbulent dissipation, because they temporarily (over a few hundred years) and locally heat the diffuse medium up to ~103 K, irreversibly modify the subsequent thermal and chemical evolution of the gas. Preliminary models of such bursts predict chemical abundances and excitation of a subset of molecules that are in agreement with the observations. Timescales of the velocity fluctuations are short enough to locally decouple the neutrals from the ionized species in the diffuse medium and generate helical magnetic fields. Small scale fluctuations of the polarization properties of the diffuse ISM are therefore expected.

The signatures of intermittent dissipation of turbulent energy have been sought in the translucent environment of a low-mass dense core. Molecular line observations reveal a network of narrow filamentary structures, found on statistical grounds to be the locus of the largest velocity shears. Three independent properties of these structures make them the plausible sites of intermittent dissipation of turbulence: (1) gas there is warmer and more diluted than average, (2) it bears the signatures of a non-equilibrium chemistry triggered by impulsive heating due to turbulence dissipation, and (3) the power that these structures radiate in the gas cooling lines (mostly H2) is so large that it balances the total energy injection rate of the turbulent cascade, for a volume filling factor of only a few percents, consistent with other observations in the Solar Neighborhood. These filamentary structures may act as tiny seeds of gas condensation in diffuse molecular gas. They do not exhibit the properties of steady-state low-velocity magneto-hydrodynamic (MHD) shocks, as presently modelled.

The linear Jeans stability problem in a turbulent medium is treated using a description of the large-scale motions, with the response of turbulence on the small scales being treated using a renormalization approach. This treatment shows how turbulence at scales smaller than the potentially collapsing scale builds up a turbulent pressure force which effectively resists compression, if the kinetic energy is sufficient to balance the gravitational attraction.

We present the scientific case for a high resolution spectro-imager in the mid-IR that would be dedicated to the mapping of H2 emission in its four lowest rotational transitions, at 28.2, 17.0, 12.3 and 9.7 micron with a spectral resolution ~104 sufficient to provide kinematical distances in galaxies. The proposed instrument on a 2 m-class telescope will be most sensitive to H2 line emission simultaneously extended and structured at small scale, in gas at temperatures higher than 80 K. Colder H2, which may contribute most of the baryonic dark matter in galaxies, will be traced by the emission of glitters of warm H2 heated, throughout the medium, by the dissipation of omnipresent turbulence. The main scientific objectives are to (i) directly measure the mass and temperature distribution of the warm H2, far from star forming regions, in particular in low metallicity environments where the traditional tracers of H2 (CO, dust emission) fail, (ii) trace the dissipation of turbulence in the perspective of building a global view of the star formation process in galaxies and (iii) trace baryonic dark matter in the form of cold H2 in a large sample of galaxies along the Hubble sequence.

Recent CO surveys and measurements of the magnetic field intensity and direction have renewed the description of molecular clouds. They appear as a large self-similar hierarchy of structures with a unique power law mass spectrum over 9 orders of magnitude in masses. No lower threshold has yet been observed for this hierarchy in non star forming clouds. At the opposite, in clouds which have started to form stars, the self-similarity breaks down at the scale of the dense cores. The whole hierarchy is fractal. Molecular clouds appear to be in virial balance. The magnitude of the external pressure terms is large in non star forming complexes and very likely due to magnetic stresses. The need for an external pressure decreases as the star formation activity increases and the equality 2T+Ω=0 is reached only for the most massive substructures of GMCs, on the verge of (or already) forming stars. The gas motions are turbulent and likely trans-Alfvénic, given the observed magnetic field intensities. The whole hierarchy of scales is characterized by an invariant, ρvl3/l, the transfer rate of kinetic energy. Molecular cloud cores appear to be supercritical and a static magnetic field cannot stabilize them against gravitational instability. MHD turbulence captures several of the characteristics of molecular clouds and is able, as long as it is fed, to prevent clouds from collapsing. Its dissipation, alike in hydrodynamical turbulence, is concentrated in bursts, which are sheetlike structures.

We present ISOCAM observations (5-18 μm) of illuminated surfaces of molecular clouds. The emission properties of the transiently heated small particles, which dominate the emission observed by ISOCAM, are analysed in relation with the spatial structure of the material. Striking spatial variations of the infrared colour (5-8.5 μm/12-18 μm) are detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at 7.7 μm) relatively to the continuum emission increasing towards long wavelengths. Compared to the intensity of the continuum emission, the intensity of the aromatic features are significantly fainter at the illuminated surfaces of dense structures than in low density regions surrounding these dense structures. This effect could be the signature of photo-chemical evolution, size segregation due to grain dynamics in uni-directional radiation fields, or abundance variations of very small particles.

Several tracers of the cold interstellar medium in its quiescent phase reveal common unexpected properties: the fractal nature of the emission contour levels, the self-similar brightness distribution, the existence of small unresolved fragments of dense gas in various environments, and a highly turbulent velocity field. In spite of the difficulties met in interpreting these data, they are important bearings in our understanding of the structure, the physics and the chemistry of this medium.

We present an ensemble of recent observational results on molecular clouds which, taken separately, could all be understood by invoking various unrelated physical processes, but taken all together form a coherent ensemble stressing the imprints of turbulence in the physics of the cold interstellar medium. These results are first, the existence of wings in the molecular line profiles, which can be interpreted on statistical grounds as the signature of the intermittency of the velocity field in turbulent flows, second the fractal geometry of the cloud edges, with properties reminiscent of those of various surfaces studied in turbulent laboratory flows, and third, the fact that the dense gas fills only a very small fraction of the space. The last points are supported by CO multitransition observations of a few fields in nearby molecular clouds. They show that the excitation conditions are the same for the gas emitting in the linewings and in the linecores and are also remarkably uniform over a large range (factor 10) of column densities. An attractive interpretation of the molecular line data is that most of the 12CO(J=2—1) and (J=3—2) emissions arise in cold (Tk ≥ 10K) and dense (nH2 ∼ 104 cm —3 or more) structures distributed on a fractal set with no characteristic scale size greater than about 1000 AU.

Above masses of the order of lOO M⊙, molecular clouds have masses and sizes which scale like those of self-gravitating polytropes bounded by an external pervading pressure. It is unlikely that this scaling is due to mere observational bias. But the physics underlying this behaviour is far from being understood. In particular, the possible contribution of turbulence to both the ambient pressure and the internal pressure (whose dependence with the density would mimic a polytropic behavior) is a difficult and much debated issue. The clouds mass, size and internal velocity dispersion are such that they are observed to be in approximate virial balance between their self-gravity, the surface energy term due to the ambient pressure and their internal energy. The latter is dominated by the kinetic energy of disordered internal motions. However, there has been little evidence so far that these motions are actually turbulent rather than simply disordered. The transition to turbulence in a flow occurs when the non linear advection term in the momentum equation, v.Δv, considerably exceeds the viscous dissipation term, vΔv (where v is the kinematic viscosity). Non linearities therefore dominate the physics of a turbulent flow and the velocities are not randomly distributed. Most of the previous attempts to determine a well-defined correlation length in the velocity field (Kleiner and Dickman 1985, a and b; Scalo 1984), which is predicted to be close to the scale at which the energy is injected, or to characterize the expected hierarchical structure (Pérault et al. 1986) have been plagued by the lack of dynamical range in the data set and the range of scales over which the correlation functions have been computed. The most plausible determination, that of Kleiner and Dickman (1987) who claim to have found a correlation length of 0.2 pc in the Taurus cloud, gives a result which is so close to the angular resolution of the observations that it is doubtful.

We propose a hierarchical model of large molecular complexes (mean density ∼ 10 cm−3, masses larger than 105 M⊙ relevant to their “cold phase”) which preceeds the formation of massive stars and H II regions, and investigate the gravitational stability of the different scales.

The kinetic temperatures in molecular clouds are usually considered to range upward from about 10 K (e.g., Dickman 1975). These temperatures are generally measured by observing the CO J = 1 → 0 transition and assuming that this line is optically thick and thermalized. This assumption also underlies estimates of the total mass and distribution of molecular material in our galaxy based on CO surveys. Because a significant amount of molecular material could in principle be missed by galactic CO surveys, a search was undertaken for “ultra-cold” molecular gas, by which is meant an excitation temperature, Tex < 5 K. No evidence was found for a large amount of such material (Evans, Rubin, and Zuckerman 1980), but many clouds with Tex between 5 and 10 K were found. To determine if this low Tex is due to low kinetic temperature, low density, or low CO abundance, we have undertaken observations of a large number of clouds in the J = 2 → 1 CO line and the J = 1 → 0 13CO and CO lines. These observations will be analyzed to determine the properties of these clouds.

Three young stars are still embedded in the small (0.7 pc wide) and densest region of the Rho Ophiuchi dark cloud. One of these stars coincides with the continuum source Oph 4 (Brown and Zuckerman, 1975). Their existence is indicated by the following: i) thermal continuum sources detected at 21 cm and 6 cm with the Westerbork Synthesis Radiotelescope (Netherlands) which display the presence of compact HII regions surrounding each of three stars; ii) peaks of CO emission (J = 1 → 0 transition) detected with the MWO antenna of MacDonald (Texas) which are associated with asymmetrically shaped lines; iii) C 158α (and S 158α for Oph 4) line emission mapped with the Nancay Radiotelescope. All these observations imply that the three stars have B3 - B2 spectral types.