Table of contents

As one of the most violent activities in the solar atmosphere, white-light flares (WLFs) are generally known for their enhanced white-light (or continuum) emission, which primarily originates in the solar lower atmosphere. However, we know little about how white-light emission is produced. In this study, we aim to investigate the response of the continua at 3600 Å and 4250 Å and also the Hα and Lyα lines during WLFs modeled using radiative hydrodynamic simulations. We take non-thermal electron beams as the energy source for the WLFs in two different initial atmospheres and vary their parameters. Our results show that the model with non-thermal electron beam heating clearly shows enhancements in the continua at 3600 Å and 4250 Å as well as in the Hα and Lyα lines. A larger electron beam flux, a smaller spectral index, or an initial penumbral atmosphere leads to a stronger emission increase at 3600 Å, 4250 Å and in the Hα line. The Lyα line, however, is more obviously enhanced in a quiet-Sun initial atmosphere with a larger electron beam spectral index. It is also notable that the continua at 3600 Å and 4250 Å and the Hα line exhibit a dimming at the start of heating and reach their peak emissions after the peak time of the heating function, while the Lyα line does not show such behaviors. These results can serve as a reference for the analysis of future WLF observations.

We investigated the scenario of time-dependent diffusive interaction between dark matter and dark energy and showed that such a model can be accommodated within the observations of luminosity distance-redshift data in Supernova Ia (SN Ia) observations. We obtain constraints on different relevant parameters of this model from the observational data. We consider a homogeneous scalar field ϕ(t) driven by a k-essence Lagrangian of the form L = V(ϕ)F(X) with constant potential V(ϕ) = V, to describe the dynamics of dark energy in this model. Using the temporal behaviour of the FRW scale factor, the equation of state and total energy density of the dark fluid, extracted from the analysis of SN Ia (JLA) data, we have obtained the time-dependence of the k-essence scalar field and also reconstructed the form of the function F(X) in the k-essence Lagrangian.

We investigate a hybrid numerical algorithm aimed at large-scale cosmological N-body simulation for on-going and future high precision sky surveys. It makes use of a truncated Fast Multiple Method (FMM) for short-range gravity, incorporating a Particle Mesh (PM) method for long-range potential, which is applied to deal with extremely large particle number. In this work, we present a specific strategy to modify a conventional FMM by a Gaussian shaped factor and provide quantitative expressions for the interaction kernels between multipole expansions. Moreover, a proper Multipole Acceptance Criterion for the hybrid method is introduced to solve potential precision loss induced by the truncation. Such procedures reduce the amount of computation compared to an original FMM and decouple the global communication. A simplified version of code is introduced to verify the hybrid algorithm, accuracy and parallel implementation.

We present a study of the X-ray emission for a sample of radio-detected quasars constructed from the cross-matches between SDSS, FIRST catalogs and XMM-Newton archives. A sample of radio-quiet SDSS quasars without FIRST radio detection is also assembled for comparison. We construct the optical and X-ray composite spectra normalized at rest frame 4215 Å (or 2200 Å) for both radio-loud quasars (RLQs) and radio-quiet quasars (RQQs) at z ≤ 3.2, with matched X-ray completeness of 19%, redshift and optical luminosity. While the optical composite spectrum of RLQs is similar to that of RQQs, we find that RLQs have a higher X-ray composite spectrum than RQQs, consistent with previous studies in the literature. By dividing the radio-detected quasars into radio loudness bins, we find the X-ray composite spectra are generally higher with increasing radio loudness. Moreover, a significant correlation is found between the optical-to-X-ray spectral index and radio loudness, and there is a unified multi-correlation between the radio and X-ray luminosities and radio loudness in radio-detected quasars. These results could be possibly explained with the corona-jet model, in which the corona and jet are directly related.

The LE is the low energy telescope that is carried on Insight-HXMT. It uses swept charge devices (SCDs) to detect soft X-ray photons. LE's time response is caused by the structure of the SCDs. With theoretical analysis and Monte Carlo simulations we discuss the influence of LE time response (LTR) on the timing analysis from three aspects: the power spectral density, the pulse profile and the time lag. After the LTR, the value of power spectral density monotonously decreases with the increasing frequency. The power spectral density of a sinusoidal signal reduces by a half at frequency 536 Hz. The corresponding frequency for quasi-periodic oscillation (QPO) signals is 458 Hz. The root mean square (RMS) of QPOs holds a similar behaviour. After the LTR, the centroid frequency and full width at half maxima (FWHM) of QPOs signals do not change. The LTR reduces the RMS of pulse profiles and shifts the pulse phase. In the time domain, the LTR only reduces the peak value of the cross-correlation function while it does not change the peak position; thus it will not affect the result of the time lag. When considering the time lag obtained from two instruments and one among them is LE, a 1.18 ms lag is expected caused by the LTR. The time lag calculated in the frequency domain is the same as that in the time domain.

Using multi-band photometric images of M51 and its companion NGC 5195 from ultraviolet to optical and infrared, we investigate spatially resolved stellar population properties of this interacting system with stellar population synthesis models. The observed infrared excess (IRX) is used to constrain dust extinction. Stellar mass is also inferred from the model fitting. By fitting observed spectral energy distributions (SEDs) with synthetical ones, we derive two-dimensional distributions of stellar age, metallicity, dust extinction and stellar mass. In M51, two grand-designed spiral arms extending from the bulge show young age, rich metallicity and abundant dust. The inter-arm regions are filled with older, metal-poorer and less dusty stellar populations. Except for the spiral arm extending from M51 into NGC 5195, the stellar population properties of NGC 5195 are quite featureless. NGC 5195 is much older than M51, and its core is very dusty with A_V up to 1.67 mag and dense in stellar mass surface density. The close encounters might drive the dust in the spiral arm of M51 into the center of NGC 5195.

We conducted photometric and spectroscopic observations of Ross 15 in order to further study the flare properties of this less observed flare star. A total of 28 B-band flares are detected in 128 hr of photometric observations, leading to a total flare rate of ${0.22}_{-0.04}^{+0.04}$ hr⁻¹, more accurate than that provided by previous work. We give the energy range of the B-band flare (10^29.5 – 10^31.5 erg) and the flare frequency distribution (FFD) for the star. Within the same energy range, the FFD is lower than that of GJ 1243 (M4) and YZ CMi (M4.5), roughly in the middle of those of three M5-type stars and higher than the average FFDs of spectral types ≥ M6. We performed, for the first time for Ross 15, simultaneous high-cadence spectroscopic and photometric observations, resulting in detection of the most energetic flare in our sample. The intensity enhancements of the continuum and Balmer lines with significant correlations between them are detected during the flare, which is the same as those of other deeply studied flare stars with similar spectral type.

It is surprising to find an instance of migration in the peak positions of synchrotron spectral energy distribution components during the activity epochs of Markarian 421 (Mrk 421), accompanying an orphan flare at the X-ray and GeV-TeV γ-ray bands. A geometric interpretation and standard shock or stochastic acceleration models of blazar emission have difficulty reproducing these observed behaviors. The present paper introduces a linear acceleration by integrating the reconnection electric field into the particle transport model for the observed behaviors of Mrk 421. We note that strong evidence for evolution in characteristic of multi-wavelength spectral energy distribution including shifting the peak frequency, accompanying an orphan flare at the X-ray and GeV-TeV γ-ray bands provides an important electrostatic acceleration diagnostic in a blazar jet. Assuming suitable model parameters, we apply the results of the simulation to the 13-day flaring event in March 2010 of Mrk 421, concentrating on the evolution of multi-wavelength spectral energy distribution characteristic by shifting the peak frequency. It is clear that the ratio of the electric field and magnetic field strength plays an important role in temporal evolution of the peak frequency of synchrotron spectral energy distribution component. We suggest it is reasonable that the electrostatic acceleration is responsible for the evolution of multi-wavelength spectral energy distribution characteristic by shifting the peak frequency. Based on the model results, we assert that the peak frequency of the synchrotron spectral energy distribution component may signify a temporary characteristic of blazars, rather than a permanent one.

The age pattern across spiral arms is one of the key observational features utilised to study the dynamic nature of the Galaxy's spiral structure. With the most updated samples of high-mass star formation region (HMSFR) masers, O stars and open clusters, we investigated their distributions and kinematic properties in the vicinity of the Sun. We found that the Sagittarius-Carina Arm traced by HMSFRs, O stars (≲ 10 Myr) and young open clusters (<30 Myr) seem to deviate gradually towards the Galactic Anticenter (GAC) direction. The Local Arm traced by HMSFRs, O stars, young clusters and also medium-young clusters (30–100 Myr) are inclined to gradually deviate toward the Galactic Center (GC) direction. The properties for the Local Arm are supported by a simplified simulation of cluster motions in the Galaxy. Indications of systematic motions in the circular and radial velocities are noticed for the old open clusters (>200 Myr). These results are consistent with the idea that star formation can be triggered by spiral shocks of density waves, and indicate that the corotation radius of the Galaxy is located between the Sagittarius-Carina Arm and the Local Arm, close to the Solar circle.

The magnetic field is one of the most important parameters in solar physics, and a polarimeter is the key device to measure the solar magnetic field. Liquid crystals based Stokes polarimeter is a novel technology, and will be applied for magnetic field measurement in the first space-based solar observatory satellite developed by China, Advanced Space-based Solar Observatory. However, the liquid crystals based Stokes polarimeter in space is not a mature technology. Therefore, it is of great scientific significance to study the control method and characteristics of the device. The retardation produced by a liquid crystal variable retarder is sensitive to the temperature, and the retardation changes 0.09° per 0.1° C. The error in polarization measurement caused by this change is 0.016, which affects the accuracy of magnetic field measurement. In order to ensure the stability of its performance, this paper proposes a high-precision temperature control system for liquid crystals based Stokes polarimeter in space. In order to optimize the structure design and temperature control system, the temperature field of liquid crystals based Stokes polarimeter is analyzed by the finite element method, and the influence of light on the temperature field of the liquid crystal variable retarder is analyzed theoretically. By analyzing the principle of high-precision temperature measurement in space, a high-precision temperature measurement circuit based on integrated operational amplifier, programmable amplifier and 12 bit A/D is designed, and a high-precision space temperature control system is developed by applying the integral separation PI temperature control algorithm and PWM driving heating films. The experimental results show that the effect of temperature control is accurate and stable, whenever the liquid crystals based Stokes polarimeter is either in the air or vacuum. The temperature stability is within ±0.015° C, which demonstrates greatly improved stability for the liquid crystals based Stokes polarimeter.

Observations of the sky irradiation intensity in the visible wavelengths during a solar eclipse permit to model the Sun diameter, a key number to constrain the internal structure of our star. In this paper, we present an algorithm that takes advantage of the precise Moon topography from Lunar Reconnaissance Orbiter to compute, with a high resolution in time, the geometrical part (i.e. top-of-atmosphere, and for a given wavelength) of the sky irradiation at any given location on the Earth during these events. The algorithm is also able to model the Baily's beads. We give as an application the theoretical computation of the light curve corresponding to the solar eclipse observed at Lakeland (Queensland, North Australia) on 2012 November 13. The application to real data, with the introduction of atmospheric and instrumental passbands, will be considered in a forthcoming paper.

In this paper, we propose a long short-term memory (LSTM) deep learning model to deal with the smoothed monthly sunspot number (SSN), aiming to address the problem whereby the prediction results of the existing sunspot prediction methods are not uniform and have large deviations. Our method optimizes the number of hidden nodes and batch sizes of the LSTM network structures to 19 and 20, respectively. The best length of time series and the value of the timesteps were then determined for the network training, and one-step and multi-step predictions for Cycle 22 to Cycle 24 were made using the well-established network. The results showed that the maximum root-mean-square error (RMSE) of the one-step prediction model was 6.12 and the minimum was only 2.45. The maximum amplitude prediction error of the multi-step prediction was 17.2% and the minimum was only 3.0%. Finally, the next solar cycle (Cycle 25) peak amplitude was predicted to occur around 2023, with a peak value of about 114.3. The accuracy of this prediction method is better than that of the other commonly used methods, and the method has high applicability.

As the areas of CCD detectors and CCD mosaics have become larger and larger, the number of readout channels in astronomical cameras has increased accordingly to keep the image readout time within an acceptable range. For the large area cameras or the mosaic cameras, the analog Correlated Double Sampling (aCDS) circuit used in traditional astronomical cameras for suppressing readout noise is difficult to integrate into the camera controllers within the constraints of the space and energy consumption. Recently, digital CDS (dCDS) technology has been developed to solve this problem, which also offers novel analysis and noise suppression methods. In this study, a mathematical model is presented to conveniently analyze the frequency characteristic of a dCDS circuit, which is then simulated by a numerical method for investigating the noise suppression capability with different sampling weights. Importantly, using this model, the extreme point with lowest readout noise can be predicted for a certain dCDS model; and for a specific CCD readout frequency, readout noise can be suppressed by selecting the proper dCDS model. A testing system is then constructed for validating the efficiency of the proposed method.

Thirty massive clumps associated with bright infrared sources were observed to detect the infall signatures and characterize infall properties in the envelope of the massive clumps by APEX telescope in CO(4–3) and C¹⁷O(3–2) lines. Eighteen objects exhibit a "blue profile" in the CO(4–3) line with virial parameters less than 2, suggesting that global collapse is taking place in these massive clumps. The CO(4–3) lines were fitted via the two-layer model in order to obtain infall velocities and mass infall rates. Derived mass infall rates range from 10⁻³ to 10⁻¹M_⊙ yr⁻¹. A positive relationship between clump mass and infall rate appears to indicate that gravity plays a dominant role in the collapsing process. Higher luminosity clumps have a larger mass infall rate, implying that those clumps with higher mass infall rates possess a higher star formation rate.

In the fourth Fermi Large Area Telescope source catalog (4FGL), 5064 γ-ray sources are reported, including 3207 active galactic nuclei (AGNs), 239 pulsars, 1336 unassociated sources, 92 sources with weak association with blazars at low Galactic latitudes and 190 other sources. We employ two different supervised machine learning classifiers, combined with the direct observation parameters given by the 4FGL fits table, to search for sources potentially classified as AGNs and pulsars in the 1336 unassociated sources. In order to reduce the error caused by the large difference in the sizes of samples, we divide the classification process into two separate steps in order to identify the AGNs and the pulsars. First, we select the identified AGNs from all of the samples, and then select the identified pulsars from the remaining cases. Using the 4FGL sources associated or identified as AGNs, pulsars and other sources with the features selected through the K-S test and the random forest (RF) feature importance measurement, we trained, optimized and tested our classifier models. Then, the models are applied to classify the 1336 unassociated sources. According to the calculation results of the two classifiers, we report the sensitivity, specificity, accuracy in each step and the class of unassociated sources given by each classifier. The accuracy obtained in the first step is approximately 95%; in the second step, the obtained overall accuracy is approximately 80%. Combining the results of the two classifiers, we predict that there are 583 AGN-type candidates, 115 pulsar-type candidates, 154 other types of γ-ray candidates and 484 of uncertain types.

In China's asteroid mission to be launched around 2025, (7968) 133P/Elst-Pizarro (hereafter 133P) will be the second target, after a visit to asteroid (469219) Kamo'oalewa. This paper describes a simulation of precise orbit determination for the spacecraft around comet 133P, as well as estimation of its gravitational parameter (GM) value and the solar radiation pressure coefficient C_r for the spacecraft. Different cometocentric distances of 200, 150 and 100 km orbits are considered, as well as two tracking modes: exclusive two-way range-rate mode (Earth station to spacecraft) and combinations of two-way range-rate and local spacecraft onboard ranging to the comet. Compared to exclusive two-way range-rate, the introduction of local ranging observables improves the final GM uncertainties by up to one order of magnitude. An ephemeris error in the orbit of 133P is also considered, and we show that, to obtain a reliable estimate of the GM for 133P, this error cannot exceed a one km range.

We combine K-nearest neighbors (KNN) with a genetic algorithm (GA) for photometric redshift estimation of quasars, short for GeneticKNN, which is a weighted KNN approach supported by a GA. This approach has two improvements compared to KNN: one is the feature weighted by GA; the other is that the predicted redshift is not the redshift average of K neighbors but the weighted average of median and mean of redshifts for K neighbors, i.e. p × z_median + (1 – p) × z_mean. Based on the SDSS and SDSS-WISE quasar samples, we explore the performance of GeneticKNN for photometric redshift estimation, comparing with the other six traditional machine learning methods, i.e. the least absolute shrinkage and selection operator (LASSO), support vector regression (SVR), multi-layer perceptrons (MLP), XGBoost, KNN and random forest. KNN and random forest show their superiority. Considering the easy implementation of KNN, we make improvement on KNN as GeneticKNN and apply GeneticKNN on photometric redshift estimation of quasars. Finally the performance of GeneticKNN is better than that of LASSO, SVR, MLP, XGBoost, KNN and random forest for all cases. Moreover the accuracy is better with the additional WISE magnitudes for the same method.

As the most sensitive single-dish radio telescope, the Five-hundred Aperture Spherical radio Telescope (FAST) is very susceptive to radio frequency interference (RFI) from active radio services. Moreover, due to the rapid development of space applications and research, satellite interference has become one of the main RFI sources for FAST, particularly at the L band. Therefore, we have developed several measures to mitigate satellite RFI. On the one hand, an antenna with 4.5-meter diameter has been constructed and installed at the FAST site to detect the satellite interference in the frequency band between 1 to 5 GHz. Meanwhile, we have developed a satellite RFI database based on the FAST sky coverage, the observing frequency bands, and known satellite systems. By combining the satellite RFI monitoring antenna and the database, we have established a satellite RFI mitigation system. With this system, we can not only track satellites to collect their characteristics and update the database but also help the observer to program the observing plan by predicting satellite interference. During the practical observation of FAST at the L band, the feasibility of this system to mitigate satellite RFI has been proved. In particular, the system effectively avoids strong satellite interference from entering the main beam of the telescope and causing receiver saturation.

Friction torque severely weakens the tracking accuracy and low-speed stability of an m-level TCS (telescope control system). To solve this problem, a friction compensation method is proposed, based on high-precision LuGre friction model parameters identification. Together with dynamometer calibration, we first design a DOB (disturbance observer) to acquire high-accuracy TCS friction value in real time. Then, the PSO-GA (a hybrid algorithm combined particle swarm optimization algorithm and genetic algorithm) optimization algorithm proposed effectively and efficiently realizes the LuGre model parameters identification. In addition, we design a TCS controller including DOB and LuGre model parameters identification based on double-loop PID controller for practical application. Engineering verification tests indicate that the accuracy of DOB calibrated can reach 96.94%of the real measured friction.When azimuth axis operates in the speed cross-zero work mode, the average positive peak to tracking error reduces from 0.8926'' to 0.2252'' and the absolute average negative peak to tracking error reduces from 0.8881'' to 0.3984''. Moreover, the azimuth axis tracking MSE reduces from 0.1155'' to 0.0737'', which decreases by 36.2%. Experimental results validate the high precision, facile portability and high real-time ability of our approach.

Two Li-rich candidates, TYC 1338–1410–1 and TYC 2825–596–1, were observed by the new high-resolution echelle spectrograph, LAMOST/HRS. Based on their high-resolution and high-signal-to-noise ratio (SNR) spectra, we derived stellar parameters and abundances of 14 important elements for the two candidates. The stellar parameters and lithium abundances indicate that they are Li-rich K-type giants, and having A(Li)_NLTE of 1.77 and 2.91 dex, respectively. Our analysis suggests that TYC 1338–1410–1 is probably a red giant branch (RGB) star at the bump stage, while TYC 2825–596–1 is most likely to be a core helium-burning red clump (RC) star. The line profiles of both spectra indicate that the two Li-rich giants are slow rotators and do not show infrared (IR) excess. We conclude that engulfment is not the lithium enrichment mechanism for either star. The enriched lithium of TYC 1338–1410–1 could be created via the Cameron-Fowler mechanism, while the lithium excess in TYC 2825–596–1 could be associated with either non-canonical mixing processes or He-flash.

The early lunar mantle overturn, associated with the sinking of the dense ilmenite-bearing cumulate (IBC) crystallized at the shallow lunar mantle, provides satisfactory explanations for the origination of high-Ti basalt, the abnormally strong magnetic field between ∼ 3.9 and ∼ 3.6 Ga and the low-viscosity zone in the deep lunar mantle, but still poses a debate regarding the initial state of IBC in the early lunar mantle. If the sinking of IBC initiated before the end of lunar magma ocean crystallization, the solidified IBC can acquire a greater thickness and a higher initial velocity at the IBC-mantle boundary. The variation of initial velocity can affect the strain rate of IBC and, correspondingly, the dislocation creep components at the shallow lunar mantle. In this work, we analyze the effects of initial velocity on the dynamics of early lunar mantle by using the theory of Rayleigh-Taylor instability. To couple the effects of diffusion creep and dislocation creep for all major minerals in the lunar mantle, we exploit an improved Minimized Power Geometric (IMPG) model and isostress mixing model to characterize the upper limit and lower limit for the viscosity of the lunar mantle comprising four major minerals, i.e. olivine, orthopyroxene, clinopyroxene and ilmenite. The modeling results suggest that a high initial velocity, in any case, can shorten the onset time, tending to promote the early lunar mantle overturn even in a rheologically-strong lunar mantle. The effect of initial velocity on the overturn wavelength shows a strong dependence on the rheological mixing model. For the isostress mixing model, the increase of initial velocity tends to elongate the overturn wavelength. For the IMPG mixing model, the overturn wavelength is insensitive to the variation of initial velocity. As the actual lunar mantle rheology sandwiches between the rheologies predicted by isostress mixing model and IMPG model, it can be anticipated that the increase of initial velocity tends to elongate the overturn wavelength. In consideration of the importance of the initial velocity on the dynamics of early lunar mantle, future investigations should focus on the dynamics of the solid IBC in the solidifying lunar magma ocean.

The photometric and spectroscopic data for three double-lined detached eclipsing binaries were collected from the photometric and spectral surveys. The light and radial velocity curves of each binary system were simultaneously analyzed using Wilson-Devinney (WD) code, and the absolute physical and orbital parameters of these binaries were derived. The masses of both components of ASASSN-V J063123.82+192341.9 were found to be M₁ = 1.088 ± 0.016 and M₂ = 0.883 ± 0.016 M_⊙; those of ASAS J011416+0426.4 were determined to be M₁ = 0.934 ± 0.046 and M₂ = 0.754 ± 0.043 M_⊙; and those of MW Aur were derived to be M₁ = 2.052 ± 0.196 and M₂ = 1.939 ± 0.193 M_⊙. Finally, the evolutionary status of these detached binaries was discussed based on their absolute parameters and the theoretical stellar models.

In this paper, we have investigated accreting millisecond X-ray pulsars, which are rapidly rotating neutron stars in low-mass X-ray binaries. These systems exhibit coherent X-ray pulsations that arise when the accretion flow is magnetically channeled to the stellar surface. Here, we have developed the fundamental equations for an accretion disk around accreting millisecond X-ray pulsars in the presence of a dynamo generated magnetic field in the inner part of the disk. We have also formulated the numerical method for the structure equations in the inner region of the disk and the highest accretion rate is enough to form the inner region of the disk, which is overpowered by radiation pressure and electron scattering. Finally, we have examined our results with the effects of dynamo magnetic fields on accreting millisecond X-ray pulsars.

We report here Atacama Large Millimeter/submillimeter Array (ALMA) N₂H⁺ (1–0) images of the Orion Molecular Cloud 2 and 3 (OMC-2/3) with high angular resolution (3'' or 1200 au) and high spatial dynamic range. Combining a dataset from the ALMA main array, Atacama Compact Array (ACA), Nobeyama 45-m Telescope and Very Large Array (VLA) (providing temperature measurement on matching scales), we find that most of the dense gas in OMC-2/3 is subsonic (σ_NT / C_s = 0.62) with a mean line width (Δυ) of 0.39 km s⁻¹ full width at half maximum (FWHM). This is markedly different from the majority of previous observations of massive star-forming regions. In contrast, line widths from the Nobeyama Telescope are transonic at 0.69 km s⁻¹ (σ_NT / C_s = 1.08). We demonstrated that the larger line widths obtained by the single-dish telescope arose from unresolved sub-structures within their respective beams. The dispersions from larger scales σ_ls (as traced by the Nobeyama Telescope) can be decomposed into three components such that ${\sigma }_{{\rm{ls}}}^{2}={\sigma }_{{\rm{ss}}}^{2}+{\sigma }_{{\rm{bm}}}^{2}+{\sigma }_{{\rm{rd}}}^{2}$, where small-scale σ_ss is the line dispersion of each ALMA beam, bulk motion σ_bm is dispersion between peak velocity of each ALMA beam and σ_rd is the residual dispersion. Such decomposition, though purely empirical, appears to be robust throughout our data cubes. Apparent supersonic line widths, commonly found in massive molecular clouds, are thus likely due to the effect of poor spatial resolution. The observed non-thermal line dispersion (sometimes referred to as 'turbulence') transits from supersonic to subsonic at ∼ 0.05 pc scales in the OMC-2/3 region. Such transition could be commonly found with sufficient spatial (not just angular) resolution, even in regions with massive young clusters, such as the Orion molecular clouds studied here.