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DEPENDENCE OF THE TURBULENT VELOCITY FIELD ON GAS DENSITY IN L1551

We have carried out mapping observations of the entire L1551 molecular cloud with about 2 pc × 2 pc size in the 12CO(1–0) line with the Nobeyama 45 m radio telescope at the high effective resolution of 22″ (corresponding to 0.017 pc at the distance of 160 pc), and analyzed the 12CO data together with the 13CO(1–0) and C18O(1–0) data from the Nobeyama Radio Observatory database. We derived the new non-thermal line width–size relations, σNT ∝ L γ, for the three molecular lines, corrected for the effect of optical depth and the line-of-sight integration. To investigate the characteristic of the intrinsic turbulence, the effects of the outflows were removed. The derived relations are (σNT/km s−1) = (0.18 ± 0.010)(L/pc)0.45 ± 0.095, (0.20 ± 0.020)(L/pc)0.48 ± 0.091, and (0.22 ± 0.050) (L/pc)0.54 ± 0.21 for the 12CO, 13CO, and C18O lines, respectively, suggesting that the line width–size relation of the turbulence very weakly depends on our observed molecular lines, i.e., the relation does not change between the density ranges of 102–103 and 103–104 cm−3. In addition, the relations indicate that incompressible turbulence is dominant at the scales smaller than 0.6 pc in L1551. The power spectrum indices converted from the relations, however, seem to be larger than that of the Kolmogorov spectrum for incompressible flow. The disagreement could be explained by the anisotropy in the turbulent velocity field in L1551, as expected in MHD turbulence. Actually, the autocorrelation functions of the centroid velocity fluctuations show larger correlation along the direction of the magnetic field measured for the whole Taurus cloud, which is consistent with the results of numerical simulations for incompressible MHD flow.