Interaction of three-dimensional magnetic fields, turbulence, and self-gravity in the molecular cloud is crucial in understanding star formation but has not been addressed so far. In this work, we target the low-mass star-forming region L1688 and use the spectral emissions of 12CO, 13CO, C18O, and H i, as well as polarized dust emissions. To obtain the 3D direction of the magnetic field, we employ the novel polarization fraction analysis. In combining with the plane-of-the-sky (POS) magnetic field strength derived from the Davis–Chandrasekhar–Fermi (DCF) method and the new differential measure analysis (DMA) technique, we present the first measurement of L1688’s three-dimensional magnetic field, including its orientation and strength. We find that L1688’s magnetic field has two statistically different inclination angles. The low-intensity tail has an inclination angle ≈55° on average, while that of the central dense clump is ≈30°. We find the global mean value of total magnetic field strength is Btot ≈ $135 \,\mathrm{\mu }{\rm G}$ from DCF and Btot ≈ $75 \,\mathrm{\mu }{\rm G}$ from DMA. We use the velocity gradient technique (VGT) to separate the magnetic fields’ POS orientation associated with L1688 and its foreground/background. The magnetic fields’ orientations are statistically coherent. The probability density function of H2 column density and VGT reveal that L1688 is potentially undergoing gravitational contraction at large scale ≈1.0 pc and gravitational collapse at small scale ≈0.2 pc. The gravitational contraction mainly along the magnetic field resulting in an approximate power-law relation $B_{\rm tot}\propto n_{\rm H}^{1/2}$ when volume density nH is less than approximately 6.0 × 103 cm−3.
Transition from coherent cores to surrounding cloud in L1688
Context. Stars form in cold dense cores showing subsonic velocity dispersions. The parental molecular clouds display higher temperatures and supersonic velocity dispersions. The transition from core to cloud has been observed in velocity dispersion, but temperature and abundance variations are unknown. Aims. We aim to measure the temperature and velocity dispersion across cores and ambient cloud in a single tracer to study the transition between the two regions. Methods. We use NH 3 (1,1) and (2,2) maps in L1688 from the Green Bank Ammonia Survey, smoothed to 1′, and determine the physical properties by fitting the spectra. We identify the coherent cores and study the changes in temperature and velocity dispersion from the cores to the surrounding cloud. Results. We obtain a kinetic temperature map extending beyond dense cores and tracing the cloud, improving from previous maps tracing mostly the cores. The cloud is 4–6 K warmer than the cores, and shows a larger velocity dispersion (Δ σ v = 0.15–0.25 km s −1 ). Comparing to Herschel -based dust temperatures, we find that cores show kinetic temperatures that are ≈1.8 K lower than the dust temperature, while the gas temperature is higher than the dust temperature in the cloud. We find an average p-NH 3 fractional abundance (with respect to H 2 ) of (4.2 ± 0.2) × 10 −9 towards the coherent cores, and (1.4 ± 0.1) × 10 −9 outside the core boundaries. Using stacked spectra, we detect two components, one narrow and one broad, towards cores and their neighbourhoods. We find the turbulence in the narrow component to be correlated with the size of the structure (Pearson- r = 0.54). With these unresolved regional measurements, we obtain a turbulence–size relation of σ v,NT ∝ r 0.5 , which is similar to previous findings using multiple tracers. Conclusions. We discover that the subsonic component extends up to 0.15 pc beyond the typical coherent boundaries, unveiling larger extents of the coherent cores and showing gradual transition to coherence over ~0.2 pc.
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- Award ID(s):
- 1748571
- NSF-PAR ID:
- 10267918
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 648
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A114
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202039897
