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(b)
Figure 4.7: Spectra of the 2k →1k vt=1 CH3OH band at 96.4 GHz (a) and of the 5k →4k band at 241 GHz (b) toward mm1. Overlaid in black the synthetic spectra resulting from the fit.
4.5 Conclusion 121
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Figure 4.8:Spectra of the 2k →1k(b) band at 96 GHz and the 5k →4k (b) band at 241 GHz toward CH3OH -2. Overlaid in black the synthetic spectra resulting from the fit.
(a)
(b)
Figure 4.9:Spectra of the 5k →4kband at 241 GHz toward (5.4 ”,-7.8 ”) (b) and (-11.1 ”,5.1 ”) (c).
Overlaid in black the synthetic spectra resulting from the fit.
Chapter 5
Highlights and prospects
5.1 Summary
This thesis has been a pilot study dedicated to the analysis of the excitation condition of methanol in the interstellar medium and to its tracing properties in the centimeter, millimeter and submillimeter spectra. Since methanol is ubiquitously found in different regimes of star formation and it is associated not only to the inner region of the clouds, close to the proto-stars, but also to the outflows, it is particularly well suited to study high mass star forming regions, whose physical conditions during the early evolutionary stages are still poorly un-derstood.
A diagnostic tool of interstellar clouds. In Chapter 1, we have selected four bands of CH3OH transitions in the millimeter and submillimeter regime and shown that when sev-eral CH3OH lines, in different excitation ranges, are observed towards the same region, the analysis on the spectra lead to estimates of both kinetic temperature and spatial density, thus avoiding problems of different spatial distributions which are known to happen when relying on different molecules. We found that many millimeter line ratios are mainly sensitive to spatial density in the range 105–108 cm−3, while in the submillimeter range a strong depen-dence on kinetic temperature up to 150 K is found. When infrared pumping is taken into account and torsionally excited lines included in the calculations, reliable determinations of temperature are possible by observing vt=1 lines. For complex sources and for lines sur-veys or multi-frequency studies of a molecule, the traditional approach of deriving physical parameter from a spectrum by “by-hand” Gaussian fitting of the lines with multiple com-ponents is impractical. Therefore, we have extended the innovative technique to handle the problem proposed by Schilke et al. (1999) to the Large Velocity Gradient approximation.
This is is based on the simultaneous fit of all the lines in a spectrum with a synthetic spec-trum; it handles multiple components along the line of sight and can simultaneously fit up to 100 000 lines, in different frequency windows and from different telescopes. Local line overlap is taken into account.
Chapter 2 is dedicated to the analysis of the pumping mechanism of Class I CH3OH masers
and to the information on the physical parameters of an interstellar cloud that can be de-rived by observing these transitions: the simultaneous observations of Class I masers in the J2 → J1 −E band and in other transitions allow a reliable estimate of the H2 density, as the J2 → J1−E transitions mase at high density (107 cm−3) and the others at lower values (104−106cm−3).
High mass star forming regions in their early evolutionary stages. In chapter 3, a sample of 13 sources in the early stages of star formation is analysed by single dish ob-servations of centimeter, millimeter and some submm CH3OH lines. The analysis and the technique discussed in the first two chapters are applied to high-mass star forming regions, chosen among the sample of High Mass Protostellar Objects studied by Sridharan et al.
(2002) and the Infrared dark clouds by Carey et al. (1998). Methanol has been successfully detected in all the sources of our sample; non-Gaussian profiles have been found in several sources, probably due to outflows. Among the HMPOs, a variety of different methanol spec-tra has been detected, with linewidths ranging from 2.4 km s−1 to 5 km s−1 and with very different excitation conditions. Therefore, our analysis confirms methanol to be a good indi-cator of the physical conditions in star forming regions. Our models usually reproduce the observations well.
For each source, temperature, H2 density and CH3OH column density are derived by a simultaneously fitting of all the observed transitions. Errors on these quantities are given by aχ2 analysis. Luminosities, H2 masses, column densities and methanol abundances are also given. The derived CH3OH abundance ranges between 10−9 and 10−7, values in good agreement with the results reported by other authors (Menten et al. 1986; Friberg et al. 1988;
van der Tak et al. 2000). In some sources, a jump in the CH3OH abundance is required, from low values at low densities and temperature to three orders of magnitude higher for high temperatures and densities. This jump is usually attributed to be due to evaporation of methanol from ice mantles. High values of the reducedχ2are sometimes found, but they are probably mainly due to the complexity of the sources, which is not really represented by our geometrically simple model; moreover, calibration uncertainties can still play a role in our analysis.
IRAS 05358+3543: a case study. The High Mass Protostellar Object IRAS 05358+3543 has been studied at high resolution with the Plateau de Bure Interferometer, in the 5k → 4k CH3OH band at 1 mm. In the continuum, with the resolution of 1.2600×0.84”, the previously known three millimeter dust condensations split up in at least two other cores, thus revealing a cluster of at least five cores. H2masses range between 39 Mand 4 M.
A first analysis of the methanol data, although affected by the degeneracy between collisions and the IR radiation field in the pumping of the energy levels, indicates hot core physical con-ditions in the surroundings of the main mm core. Highly excited CH3OH lines (lower level energy∼80 K) are detected in the region and a simultaneous fit of all the lines in the 5k → 4k CH3OH vt=0 and in the 2k → 1kCH3OH vt=1 series at 96.5 GHz gives typical hot-core