5.2 Future work
5.2.1 Kinematics
Results
The centroid velocities and line widths have been derived for the lines CH3OH (20,2-10,1, A+), CH3OH (21,2-11,1, E2), CH2DOH (20,2-10,1), CH2DOH (30,3-20,2), and C17O (1-0), by performing a Gaussian fit of the spectral profile, pixel by pixel. In the case of C17O (1-0), the whole hyperfine structure was considered. The maps for the centroid velocities can be found in Figs. 5.1, 5.2, and 5.3 for CH3OH, CH2DOH, and C17O, respectively. The maps for the line widths can be found in Figs. 5.4, 5.5, and 5.6. As a summary, in Table 5.1, the values of the VLSR and σv at the dust and methanol peaks are presented.
CH2DOH presents bluer velocities by ∼0.3 km s−1 than methanol or C17O. Given the spectral resolution (∼0.04-0.07 km s−1), the uncertainties in the rest frequencies and the error in the fit (.0.02 km s−1), this discrepancy for CH2DOH is significant, as already discussed in Bizzocchi et al. (2014).
All molecular lines present redder velocities toward the center compared to the methanol peak. This trend is clearer in the centroid velocity map of C17O (see Fig. 5.3), which shows a clear gradient in velocity from the north-east to south-west. This shows however a different general trend than that of methanol and deuterated methanol (see Figs. 5.1
100 5. Conclusions and future prospects Table 5.1: Results of the centroid velocity and line widths from the Gaussian fit to the spectra at the dust and methanol peaks.
Line
Dust peak Methanol peak
VLSR σ VLSR σ
(km s−1) (km s−1) (km s−1) (km s−1) CH3OH (20,2-10,1, A+) 7.204±0.004 0.135±0.004 7.176±0.003 0.130±0.003
CH3OH (21,2-11,1, E2) 7.195±0.005 0.135±0.005 7.171±0.003 0.123±0.003 CH2DOH (20,2-10,1) 6.93±0.02 0.13±0.02 6.85±0.02 0.11±0.02 CH2DOH (30,3-20,2) 6.94±0.02 0.13±0.02 6.92±0.02 0.12±0.02 C17O (1-0) 7.15±0.02 0.13±0.02 7.13±0.02 0.15±0.02 and 5.2), which present a velocity gradient from south-east to north-west. The map of the centroid velocity of the C17O (1-0) (see Fig. 5.3) provides a large scale picture of the kinematics, while CH2DOH and CH3OH are tracing a smaller scale picture, as they become abundant in dense core regions where CO is already significantly frozen. This indicates the presence of complex motions within the dense core, which will be explored with ALMA.
There is not a significant difference between the velocity dispersion at the center and the methanol peak, probably due to the fact that methanol indeed traces a shell around the dust peak (with size of approximately the radius of the CO depletion zone; Bizzocchi et al.
2014). Nevertheless, looking at the velocity dispersion map of CH2DOH in Fig. 5.5 one can clearly see that CH2DOH shows a gradient in velocity dispersion which peaks toward the south and the south-west.
Discussion
Deuterated methanol line widths show a peculiar behavior toward the south-west (see Fig. 5.5). There is a region that shows wider line widths parallel to the major axis of the core. Clemens et al. (2016) found a steep gradient in the polarization angle which coincides with the major axis of the core. This could be caused by collision or helical pitching of the magnetic field. If it is connected to the cloud dynamics, they expected to find a similar gradient in the gas velocity. However, the kinematic information available at that time did not follow this trend. Now, we see this change in velocity dispersion parallel to this ridge.
Moreover, we also see that the centroid velocity map seen in C17O is consistent with the large scale structure rotating in the direction south-west to north-east (see Fig. 5.3); such direction coincides with the major axis of L1544. Unlike the large scale gas traced by CO, the gas toward the central zones of L1544, traced by CH3OH and CH2DOH, rotates mostly south-east to north-west (see Fig. 5.1). The major axis of L1544 is also a region which separates two different chemical regions (Spezzano et al. 2016). Punanova et al. (2018) found even larger line widths in the methanol peak, only observable with high resolution interferometers, which could perhaps indicate slow shocks in the cloud possibly due to
5.2 Future work 101 material accretion onto the core. These observations were done only toward the methanol peak, so they do not cover the region where deuterated methanol shows large line widths.
Nevertheless, Punanova et al. (2018) found these large line widths in the southermost part of the observed region, thus closer to the dust peak and in the same direction as the gradient found in deuterated methanol. This scenario shows that complex dynamics are present in the cloud, and a deeper study is needed to understand what is causing the observed velocity patterns and whether magnetic fields play an important role in shaping the internal motions.
Methanol and CO line widths show a less clear gradient, although they are consistent with CH2DOH; also, CH2DOH present wider lines compared to CH3OH, consistent with the fact that CH2DOH is tracing a more central part of the core, where the contraction velocity is higher (see e.g. Keto & Caselli 2010).
Nevertheless, the kinematics seem to increase in complexity when the spectra is exam-ined carefully at those regions where the line widths of CH3OH and C17O are wider. In these regions the wide line widths are due to the presence of a second velocity component.
Unfortunately, the maps of CH2DOH are not sensitive enough to double check this, but this second velocity component is perfectly seen towards the western parts of the core in CH3OH and C17O. As an example, we show in Fig. 5.7 the spectra at the center and at the region where CH3OH shows the greater line widths (see Fig. 5.4).
Figure 5.1: Central velocity maps derived from fitting the methanol lines A+ (left panel) and E2(right panel) to Gaussian profiles. The contours represent 3σ steps of the integrated intensity maps above the 3σ detection level, with σ=0.02 K km s−1. The HPBW is shown in the bottom right corner of the figure. The black cross marks the dust continuum peak.
Therefore, a deep study of the kinematics, exploring this second velocity component, is needed.
102 5. Conclusions and future prospects
Figure 5.2: VLSR map of the deuterated methanol (20,2-10,1) transition (left panel), and (30,3-20,2) transition (right panel) derived from the Gaussian fit to the data. The black contours represent increasingσ steps of the CH2DOH integrated intensity maps above the 3σ detection level, with σ=0.005 K km s−1 for CH2DOH (20,2-10,1) and σ=0.004 K km s−1 for CH2DOH (30,3-20,2). The HPBW is shown in the bottom right corner of both figures.
The black cross marks the dust continuum peak.
5.2 Future work 103
Figure 5.3: VLSR map of C17O (1-0) line. The black contours represent increasingσsteps of the integrated intensity above the 3σ detection level, withσ=0.014 K km s−1. The HPBW is shown in the bottom right corner of the figure. The black cross marks the dust continuum peak.
104 5. Conclusions and future prospects
Figure 5.4: Line width maps derived from fitting the methanol lines A+ (left panel) and E2 (right panel) to Gaussian profiles. The contours represent 3σ steps of the integrated intensity maps above the 3σ detection level, with σ=0.02 K km s−1. The HPBW is shown in the bottom right corner of the figure. The black cross marks the dust continuum peak, and the white triangle marks the offset position chosen for Fig. 5.7.
Figure 5.5: Line width maps of the deuterated methanol (2-1) transition (left panel), and (3-2) transition (right panel) derived from the Gaussian fit to the data. The black contours represent increasingσ steps of the CH2DOH integrated intensity maps above the 3σ detection level, with σ=0.005 K km s−1 for CH2DOH (20,2-10,1) and σ=0.004 K km s−1 for CH2DOH (30,3-20,2). The HPBW is shown in the bottom right corner of both figures.
The black cross marks the dust continuum peak.
5.2 Future work 105
Figure 5.6: Line width map of C17O (1-0) line. The black contours represent increasing σ steps of the integrated intensity above the 3σ detection level, with σ=0.014 K km s−1. The HPBW is shown in the bottom right corner of the figure. The black cross marks the dust continuum peak.
106 5. Conclusions and future prospects
Figure 5.7: Spectra found at the center of the core (left panels) and at an offset position from the center (right panels) for the C17O (1-0) line (top panels) and the CH3OH (2-1) lines (bottom panels). The offset position has been chosen to be where CH3OH shows wider line widths towards the western part of the core (indicated with a white triangle in Fig. 5.4).