Morphology of the Diffuse Interstellar Medium (DISM)

Small-scale structure of the DISM

 Diffuse environments do show a non-homogeneous density, what we shortly describe as small scale structure. So far we observed such a steep density derivative within a small set of multiple components targets; this has been done with the ESO 3.6m telescope, and a report is being submitted for publication.

The next step, which is the proposed core activity for DISM section, will be the analysis of a set of UVES/FLAMES spectra through the creation of a numerical code which will implement the algorithm of Maximum Entropy Reconstruction (MER). This metod (e.g. see Arabadjis and Bregman 2000), allows the recovering of the "effective tridimensional distribution" of the absorbing material, given a set of many, simultaneous observations and relative projected density measurements of columns with known endpoints. The power of this method resides in the fact that, simply modulating the model cell size, it is possible to recover the underlying structure in the sampled ISM. The lower limit to the size scale of detected structures is set by the particular stellar sampling density. Clearly, the sample of stars must be sufficiently dense such that the structures sizes are at least as large as the mean spacing between stars. In this sense, clusters of stars are ideal candidates for this kind of sampling. Since some members of the Cagliari research unit are responsible of the Data Reduction Software of the fiber spectrograph UVES/FLAMES, a selected number of stars toward omega Cen and NGC6397 clusters has been observed in May 2003, as part of the ITAL-FLAMES consortium GTO program. UVES data are almost completely reduced, while the GIRAFFE set of data still await a reliable Data Reduction Software package.

The link between chemistry and morphology

 The morphology of the diffuse interstellar regions plays a key role in the understanding the processes of formation and dissipation of DISM and is still an open problem in astrophysics. Recently, Heithausen (2002), using the IRAM 30m radiotelescope, has discovered serendipitously the existence of molecular condensations in CO(J=1-0) and CO(J=2-1) with small sizes (~1'), embedded in a low column density medium. The unbiased nature of thediscovery suggests that these condensations could be a rather common features in the interstellar medium. These regions have not been observed so far in any galactic survey due to their small angular extent and weakness of the line emission.

Following a different approach Scappini et al. (2000, 2002) and Casu et al.(2002) detected very similar structures in 13CO(J=1-0) towards the star Cyg OB2 No.12. The search for molecular gas towards Cyg OB2 No.12 was induced by the discovery of the molecular ion H3+ in these apparently diffuse line of sight (McCall et al. 1998) with an abundance comparable to that found for dense clouds (Geballe and Oka 1996). Subsequent observations of several diffuse clouds have confirmed that H3+ is considerably more abundant than expected from chemical models (McCall et al. 2002).

Several models for H3+ chemistry in diffuse clouds have been proposed (McCall et al.1998; Cecchi-Pestellini and Dalgarno 2000; Gredel, Black and Yan 2001). All these models are based on the uncertain values of three key parameters: the rate of H3+ destruction by electrons, the electron fraction, and the cosmic ionization rate. Focusing only on physical parameters, extremely peculiar values of cosmic-ray ionization rate can be derived (McCall et al. 2003). If such a high cosmic-ray flux is present and ubiquitous in diffuse clouds, it would have several implications for the chemistry and physics of interstellar gas increasing the number densities of oxygen compounds, the fraction of deuterium-bearing molecules, and representing an additional heating source for the diffuse interstellar medium.

From a chemical perspective the important question is: what is the factor that supresses (or induces) molecular formation? As showed by Cecchi-Pestellini and Dalgarno (2000), a strong sinergy exists between physical properties and morphology: using a nesting model, these authors reproduced the observed H3+ column density towards Cygnus OB2 No. 12 and predicted a large column density of HCO+, comparable with that observed by Scappini et al (2000). These results showed the possibility of the presence of a clumpy structure towards this line of sight.

With this aim, we started an observational program aimed to investigate the morphology of the region in front of the star Cygnus OB2 No.12. We firstly mapped with the 13CO (J=1-0) emission line the region towards the star (Scappini et al., 2002), and then extended the map in the North-West direction (Casu et al., in preparation), finding unambiguous evidence of a well kinematically defined clumpy structure.

  Fig. 2: Contour map of the 13CO (J=1-0) integrated intensity of the 12 km/s component, observed at the 20m OSO radiotelescope.

 In fig. 2 we show the map of the region: the intensity distribution reveals the presence of 3 cores (core A, B and C ) embedded in a more tenuous envelope. The physical conditions (gas density, kinetic and 13CO excitation temperature) have been obtained using a statistical equilibrium analysis. All these results show that the incorporation of density fluctuations, length scale and filling factors in theoretical models could be crucial for our undestanding of the diffuse cloud chemistry.

Silvia Casu
Last modified: Thu Sep 23 13:45:39 CEST 2004