arXiv subjects

Nguyen Bich Ngoc

Publications and source records attributed to Nguyen Bich Ngoc.

22 records · Page 2Linked to original sources

arXiv2021

Revealing the diverse magnetic field morphologies in Taurus dense cores with sensitive sub-millimeter polarimetry

We have obtained sensitive dust continuum polarization observations at 850 $μ$m in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope (JCMT), as part of the BISTRO (B-fields in STar-forming Region Observations) survey. These observations allow us to probe magnetic field (B-field) at high spatial resolution ($\sim$2000 au or $\sim$0.01 pc at 140 pc) in two protostellar cores (K04166 and K04169) and one prestellar core (Miz-8b) that lie within the B213 filament. Using the Davis-Chandrasekhar-Fermi method, we estimate the B-field strengths in K04166, K04169, and Miz-8b to be 38$\pm$14 $μ$G, 44$\pm$16 $μ$G, and 12$\pm$5 $μ$G, respectively. These cores show distinct mean B-field orientations. B-field in K04166 is well ordered and aligned parallel to the orientations of the core minor axis, outflows, core rotation axis, and large-scale uniform B-field, in accordance with magnetically regulated star formation via ambipolar diffusion taking place in K04166. B-field in K04169 is found to be ordered but oriented nearly perpendicular to the core minor axis and large-scale B-field, and not well-correlated with other axes. In contrast, Miz-8b exhibits disordered B-field which show no preferred alignment with the core minor axis or large-scale field. We found that only one core, K04166, retains a memory of the large-scale uniform B-field. The other two cores, K04169 and Miz-8b, are decoupled from the large-scale field. Such a complex B-field configuration could be caused by gas inflow onto the filament, even in the presence of a substantial magnetic flux.

astro-ph.GA↗

arXiv2020

Observations of magnetic fields surrounding LkH$α$ 101 taken by the BISTRO survey with JCMT-POL-2

We report the first high spatial resolution measurement of magnetic fields surrounding LkH$α$ 101, a part of the Auriga-California molecular cloud. The observations were taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 $μ$m is found to be mostly associated with the red-shifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar-Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of $B_{\rm POS}\sim 115$ $\mathrmμ$G. The mass-to-magnetic-flux ratio in critical value units, $λ\sim0.3$, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkH$α$ 101 region is sub-critical and the magnetic field is strong enough to prevent gravitational collapse. The inferred $δB/B_0\sim 0.3$ implies that the large scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power-law with an index of $α=0.82\pm0.03$, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkH$α$ 101) in the region. The magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques are potential of explaining such a decreasing trend.

astro-ph.GA↗

arXiv2020

Grain alignment and disruption by radiative torques in dense molecular clouds and implication for polarization holes

Dust polarization induced by aligned grains is widely used to study magnetic fields in various environments, including star-forming regions. However, the question of to what optical depth grain alignment still exists in a dense molecular cloud (MC) is unclear. In this paper, we aim to achieve analytical formulae for the minimum size of aligned grains ($a_{\rm align}$) and rotational disruption ($a_{\rm disr}$) by RAdiative Torques (RATs) as a function of the local physical parameters within dense MCs. We first find the analytical approximations for the radiation strength and the mean wavelength of the attenuated radiation field in a dense MC without and with embedded stars and then derive analytical formulae for $a_{\rm align}$ and $a_{\rm disr}$ as functions of the visual extinction $A_{V}$ and the gas density. We find that within a starless core of density $n_{\rm H}\sim 10^{5}\rm cm^{-3}$, grains of size $a<0.25μm$ can be aligned up to $A_{V}\sim 5$ by RATs, whereas micron-sized grains can still be aligned at $A_{V}\sim 50$. The increase in the alignment size with $A_{V}$ can explain the presence of polarization holes observed toward starless cores. For MCs with an embedded protostar, we find that the efficiency of both alignment and rotational disruption increases toward the protostar due to the increasing radiation strength. Such a disruption effect results in the decrease of the polarization degree with $A_{V}$ or emission intensity, which reproduces the popular polarization holes observed toward the location of protostars. Finally, we derive the formula for the maximum $A_{V}$ where grain alignment still exists in a starless core and discuss its potential for constraining grain growth.

astro-ph.GA↗

arXiv2019

Spectral properties of the surface reflectance of the northern polar region of Mercury

We analyse MESSENGER reflectance measurements covering the northern polar region of Mercury, the least studied region of the northern mercurian hemisphere. We use observations from the Mercury Dual Imaging System Wide-Angle Camera (MDIS/WAC) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS/VIRS) to study the spectral dependence of the surface reflectance. The results obtained from the observations made by both instruments are remarkably consistent. We find that a second degree polynomial description of the measured reflectance spectra gives very good fits to the data and that the information that they carry can best be characterized by two parameters, the mean reflectance and the mean relative spectral slope, averaged over the explored range of wavelengths. The properties of the four main types of terrains known to form Mercury's regolith in the northern region, smooth plains (SP), heavily cratered terrain (HCT), fresh ejecta/materials and red pitted ground (RPG) are examined in terms of these two parameters. The results are compared, and found consistent with those obtained by earlier studies in spite of difficulties met in obtaining accurate reflectance measurements under the large incidence angle condition characteristic of polar regions. These results will help with the preparation of the BepiColombo mission and with supporting its observational strategy.

astro-ph.EP↗