arXiv subjects

Guang-Xing Li

Publications and source records attributed to Guang-Xing Li.

At least 73 records · Page 4Linked to original sources

arXiv2021

Radio Recombination Line Observations at $1.0-1.5$ GHz with FAST

HII regions made of gas ionized by radiations from young massive stars, are widely distributed in the Milky Way. They are tracers for star formation, and their distributions are correlated with the Galactic spiral structure. Radio recombination lines (RRLs) of hydrogen and other atoms allows for the precisest determination of physical parameters such as temperature and density. However, RRLs at around 1.4 GHz from HII regions are weak and their detections are difficult. As a result, only a limited number of detections have been obtained yet. The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) can simultaneously cover 23 RRLs for H$nα$, He$nα$, and C$nα$ ($n = 164 - 186$), respectively. This, combined with its unparalleled collecting area, makes FAST the most powerful telescope to detect weak RRLs. In this pilot survey, we use FAST to observe nine HII regions at L band. We allocate 20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of 2.0 km/s. In total, 21 RRLs for H$nα$ and C$nα$ at $1.0 - 1.5$ GHz have been simultaneously detected with strong emission signals. Overall, the detection rates for the H167$α$ and C167$α$ RRLs are 100%, while that for the He167$α$ RRL is 33.3%. Using hydrogen and helium RRLs, we measure the electron density, electron temperature, and pressure for three HII regions. This pilot survey demonstrates the capability of FAST in RRL measurements, and a statistically meaningful sample with RRL detection, through which knowledges about Galactic spiral structure and evolution can be obtained, is expected in the future.

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arXiv2021

Quantifying the anisotropic density structure of the Central Molecular Zone -- a 2D correlation function approach

The Central Molecular Zone (CMZ) is a ring-like structure sitting at the center of the Milky Way. Using the 870 $μ$m continuum map from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), we study the anisotropy of the density structure of the gas in the Central Molecular Zone (CMZ) using the 2D correlation function. To quantify the spatial anisotropy, we define the critical angle $θ_{\rm half}$, as well as the anisotropy parameter $A\equiv \fracπ{4θ_{\rm half}}-1$. We find that the density structure is strongly anisotropic at the large scale (~ 100 pc), and the degree of spatial anisotropy decreases with the decreasing scale. At the scale of ~ 10 pc, the structure is still mildly anisotropic. In our analyses, we provide a quantitative description of the anisotropic density structure of gas in the CMZ, and the formalism can be applied to different regions to study their differences.

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arXiv2021

Molecular contrails -- triggered contraction by passages of massive objects through molecular clouds

We study the effects of passages of compact objects such as stars, star clusters, and black holes through molecular clouds, and propose that the gravitational interaction between the compact object and the ambient gas can lead to the formation of thin and collimated features made of dense gas, which we call "molecular contrails". Supercritical contrails can collapse further leading to triggered star formation. The width of a molecular contrail is determined by the mass and velocity of the compact object and the velocity dispersion of the ambient molecular medium. Under typical conditions in the Milky Way, passages of stellar-mass objects lead to the formation of width $d\gtrsim 0.01\;\rm {parsec}$ contrails, and passages of star clusters lead to the formation of $d\gtrsim 1\;\rm {parsec}$ contrails. We present a few molecular contrail candidates from both categories identified from ALMA 1.3mm continuum observations of star-forming regions and the $^{13}$CO(1-0) map from the Galactic Ring Survey respectively. The contrails represent an overlooked channel where stars and gas in the Galactic disk interact to for structures. They also present a potential way of detecting dark compact objects in the Milky Way.

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arXiv2020

A mean density of $112\, M_{\odot}\,\rm pc^{-3}$ for Central Molecular Zone clumps -- Evidences for shear-enabled pressure equilibrium in the Galactic Center

We carry out a systematic study of the density structure of gas in the Central Molecular Zone (CMZ) in the Galactic center by extracting clumps from the APEX Telescope Large Area Survey of the Galaxy survey at 870 $μ$m. We find that the clumps follow a scaling of $m = ρ_0 r^3$ which corresponds to a characteristic density of $n_{\rm H_2} = 1.6 \times 10^3\,\rm cm^{-3}$ ($ρ_0 =112\;M_{\odot}\;\rm pc^{-3}$) with a variation of $\approx 0.5\,\rm dex$, where we assumed a gas-to-dust mass ratio of 100. This characteristic density can be interpreted as the result of thermal pressure equilibrium between the molecular gas and the warm ambient interstellar medium. Such an equilibrium can plausibly be established since shear has approximately the same strength as self-gravity. Our findings may explain the fact that star formation in the CMZ is highly inefficient compared to the rest of the Milky Way disk. We also identify a population of clumps whose densities are two orders of magnitudes higher in the vicinity of the Sgr B2 region, which we propose are produced by collisions between the clumps of lower densities. For these collisions to occur, processes such as compressive tides probably have created the appropriate condition by assembling the clumps together.

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arXiv2020

Probing the initial conditions of high-mass star formation -- IV. Gas dynamics and NH$_2$D chemistry in high-mass precluster and protocluster clumps

The initial stage of star formation is a complex area study because of its high density and low temperature. Under such conditions, many molecules become depleted from the gas phase by freezing out onto dust grains. However, the deuterated species could remain gaseous and are thus ideal tracers. We investigate the gas dynamics and NH$_2$D chemistry in eight massive pre/protocluster clumps. We present NH$_2$D 1$_{11}$-1$_{01}$ (at 85.926 GHz), NH$_3$ (1, 1) and (2, 2) observations in the eight clumps using the PdBI and the VLA, respectively. We find that the distribution between deuterium fractionation and kinetic temperature shows a number density peak at around $T_{\rm kin}=16.1$ K, and the NH$_2$D cores are mainly located at a temperature range of 13.0 to 22.0 K. We detect seven instances of extremely high deuterium fractionation of $1.0 \leqslant D_{\rm frac} \leqslant 1.41$. We find that the NH$_2$D emission does not appear to coincide exactly with either dust continuum or NH$_3$ peak positions, but often surrounds the star-formation active regions. This suggests that the NH$_{2}$D has been destroyed by the central young stellar object (YSO) due to its heating. The detected NH$_2$D lines are very narrow with a median width of $\rm 0.98\pm0.02 km/s$. The extracted NH$_2$D cores are gravitationally bound ($α_{\rm vir} < 1$), are likely prestellar or starless, and can potentially form intermediate-mass or high-mass stars. Using NH$_3$ (1, 1) as a dynamical tracer, we find very complicated dynamical movement, which can be explained by a combined process with outflow, rotation, convergent flow, collision, large velocity gradient, and rotating toroids. High deuterium fractionation strongly depends on the temperature condition. NH$_2$D is a poor evolutionary indicator of high-mass star formation in evolved stages, but a useful tracer in the starless and prestellar cores.

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arXiv2020

Edge collapse and subsequent longitudinal accretion in Filament S242

Filament S242 is 25 pc long with massive clumps and YSO clusters concentrated in its end regions; it is considered a good example of edge collapse. We mapped this filament in the $^{12}$CO(1-0) and $^{13}$CO(1-0) lines. A large-scale velocity gradient along filament S242 has been detected; the relative velocity between the two end-clumps is $\sim$ 3 km s$^{-1}$, indicating an approaching motion between them. These signatures are consistent with the filament S242 being formed through the collapse of a single elongated entity, where an effect known as "gravitational focusing" drives the ends of the filament to collapse (edge collapse). Based on this picture, we estimate a collapse timescale of $\sim$ 4.2 Myr, which is the time needed for a finite and elongated entity evolving to the observed filament S242. For the whole filament, we find that increases in surface densities lead to increases in velocity dispersion, which can be consistently explained as the result of self-gravity. We also calculated the contribution of longitudinal collapse to the observed velocity dispersion and found it to be the dominant effect in driving the gas motion near the end-clumps. We propose that our filament S242 is formed through a two-stage collapse model, where the edge collapse of a truncated filament is followed by a stage of longitudinal accretion toward the dense end-clumps.

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arXiv2019

Using CO line ratios to trace compressed areas in bubble N131

N131 is a typical infrared dust bubble showing an expanding ringlike shell. We study what kinds of CO line ratios can be used to trace the interaction in the expanding bubble. We carry out new $\rm CO\,(3-2)$ observations towards bubble N131 using the JCMT 15-m telescope, and derive line ratios by combining with our previous $\rm CO\,(2-1)$ and $\rm CO\,(1-0)$ data from the IRAM 30-m telescope observations. To trace the interaction between the molecular gas and the ionized gas in the HII region, we use RADEX to model the dependence of CO line ratios on kinetic temperature and H$_2$ volume density, and examine the abnormal line ratios based on other simulations. We present $\rm CO\,(3-2)$, $\rm CO\,(2-1)$, and $\rm CO\,(1-0)$ integrated intensity maps convolved to the same angular resolution (22.5$"$). The three different CO transition maps show apparently similar morphology. The line ratios of $W_{\rm CO\,(3-2)}$/$W_{\rm CO\,(2-1)}$ mostly range from 0.2 to 1.2 with a median of $0.54\pm0.12$, while the line ratios of $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}$ range from 0.5 to 1.6 with a median of $0.84\pm0.15$. The high CO line ratios $W_{\rm CO\,(3-2)}$/$W_{\rm CO\,(2-1)}\gtrsim 0.8 $ and $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}\gtrsim 1.2$ are beyond the threshold predicted by numerical simulations based on the assumed density-temperature structure for the inner rims of ringlike shell, where are the compressed areas in bubble N131. These high CO integrated intensity ratios, such as $W_{\rm CO\,(3-2)}$/$W_{\rm CO\,(2-1)}\gtrsim0.8$ and $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}\gtrsim1.2$, can be used as a tracer of gas compressed regions with a relatively high temperature and density. This further suggests that the non-Gaussian part of the line-ratio distribution can be used to trace the interaction between the molecular gas and the hot gas in the bubble.

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arXiv2019

Probing the initial conditions of high-mass star formation. III. Fragmentation and triggered star formation

Massive clumps tend to fragment into clusters of cores and condensations, some of which form high-mass stars. In this work, we study the structure of massive clumps at different scales, analyze the fragmentation process, and investigate the possibility that star formation is triggered by nearby HII regions. We present a high angular resolution study of a sample of 8 massive proto-cluster clumps. Combining infrared data, we use few-arcsecond resolution radio- and millimeter interferometric data to study their fragmentation and evolution. Our sample is unique in the sense that all the clumps have neighboring HII regions. Taking advantage of that, we test triggered star formation using a novel method where we study the alignment of the centres of mass traced by dust emission at multiple scales. The eight massive clumps have masses ranging from 228 to 2279 $M_\odot$. The brightest compact structures within infrared bright clumps are typically associated with embedded compact radio continuum sources. The smaller scale structures of $R_{\rm eff}$ $\sim$ 0.02 pc observed within each clump are mostly gravitationally bound and massive enough to form at least a B3-B0 type star. Many condensations have masses larger than 8 $M_\odot$ at small scale of $R_{\rm eff}$ $\sim$ 0.02 pc. Although the clumps are mostly infrared quiet, the dynamical movements are active at clump scale ($\sim$ 1 pc). We studied the spatial distribution of the gas conditions detected at different scales. For some sources we find hints of external triggering, whereas for others we find no significant pattern that indicates triggering is dynamically unimportant. This probably indicates that the different clumps go through different evolutionary paths. In this respect, studies with larger samples are highly desired.

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arXiv2018

Scale-free gravitational collapse as the origin of $ρ\sim r^{-2}$ density profile -- a possible role of turbulence in regulating gravitational collapse

Astrophysical systems, such as clumps that form star clusters share a density profile that is close to $ρ\sim r^{-2}$. We prove analytically this density profile is the result of the scale-free nature of the gravitational collapse. Therefore, it should emerge in many different situations as long as gravity is dominating the evolution for a period that is comparable or longer than the free-fall time, and this does not necessarily imply an isothermal model, as many have previously believed. To describe the collapse process, we construct a model called the turbulence-regulated gravitational collapse model, where turbulence is sustained by accretion and dissipates in roughly a crossing time. We demonstrate that a $ρ\sim r^{-2}$ profile emerges due to the scale-free nature the system. In this particular case, the rate of gravitational collapse is regulated by the rate at which turbulence dissipates the kinetic energy such that the infall speed can be $20 - 50 \%$ of the free-fall speed(which also depends on the interpretation of the crossing time based on simulations of driven turbulence). These predictions are consistent with existing observations, which suggests that these clumps are in the stage of turbulence-regulated gravitational collapse. Our analysis provides a unified description of gravitational collapse in different environments.

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arXiv2018

The TOP-SCOPE survey of PGCCs: PMO and SCUBA-2 observations of 64 PGCCs in the 2nd Galactic Quadrant

In order to understand the initial conditions and early evolution of star formation in a wide range of Galactic environments, we carried out an investigation of 64 \textit{Planck} Galactic Cold Clumps (PGCCs) in the second quadrant of the Milky Way. Using the $^{13}$CO and C$^{18}$O $J = 1 - 0$ lines, and 850\,$μ$m continuum observations, we investigated cloud fragmentation and evolution associated with star formation. We extracted 468 clumps and 117 cores from the $^{13}$CO line and 850\,$μ$m continuum maps, respectively. We make use of the Bayesian Distance Calculator and derived the distances of all 64 PGCCs. We found that in general, the mass-size plane follows a relation of $m\sim r^{1.67}$. At a given scale, the masses of our objects are around 1/10 of that of typical Galactic massive star-forming regions. Analysis of the clump and core masses, virial parameters, densities, and mass-size relation suggests that the PGCCs in our sample have a low core formation efficiency ($\sim$3.0\%), and most PGCCs are likely low-mass star-forming candidates. Statistical study indicates that the 850\,$μ$m cores are more turbulent, more optically thick, and denser than the $^{13}$CO clumps for star formation candidates, suggesting that the 850\,$μ$m cores are likely more appropriate future star-formation candidates than the $^{13}$CO clumps.

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arXiv2017

Quantifying the interplay between gravity and magnetic field in molecular clouds - a possible multi-scale energy equipartition in NGC6334

The interplay between gravity, turbulence and the magnetic field determines the evolution of the molecular ISM and the formation of the stars. In spite of growing interests, there remains a lack of understanding of the importance of magnetic field over multiple scales. We derive the magnetic energy spectrum -- a measure that constraints the multi-scale distribution of the magnetic energy, and compare it with the gravitational energy spectrum derived in Li \& Burkert (2016). In our formalism, the gravitational energy spectrum is purely determined by the surface density PDF, and the magnetic energy spectrum is determined by both the surface density PDF and the magnetic-field-density relation. If regions have density PDFs close to $P(Σ)\sim Σ^{-2}$ and a universal magnetic field-density relation $B\sim ρ^{1/2}$, we expect a multi-scale near equipartition between gravity and the magnetic fields. This equipartition is found to be true in NGC6334 where estimates of magnetic fields over multiple scales (from 0.1 pc to a few parsec) are available. However, the current observations are still limited in sample size. In the future, it is necessary to obtain multi-scale measurements of magnetic fields from different clouds with different surface density PDFs and apply our formalism to further study the gravity-magnetic field interplay.

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arXiv2017

Dynamical cooling of galactic discs by molecular cloud collisions -- Origin of giant clumps in gas-rich galaxy discs

Different from Milky-Way-like galaxies, discs of gas-rich galaxies are clumpy. It is believed that the clumps form because of gravitational instability. However, a necessary condition for gravitational instability to develop is that the disc must dissipate its kinetic energy effectively, this energy dissipation (also called cooling) is not well-understood. We propose that collisions (coagulation) between molecular clouds dissipate the kinetic energy of the discs, which leads to a dynamical cooling. The effectiveness of this dynamical cooling is quantified by the dissipation parameter $D$, which is the ratio between the free-fall time $t_{\rm ff}\approx 1/ \sqrt{G ρ_{\rm disc}}$ and the cooling time determined by the cloud collision process $t_{\rm cool}$. This ratio is related to the ratio between the mean surface density of the disc $Σ_{\rm disc}$ and the mean surface density of molecular clouds in the disc $Σ_{\rm cloud}$. When $D <1/3$ (which roughly corresponds to $Σ_{\rm disc} < 1/3 Σ_{\rm cloud}$), cloud collision cooling is inefficient, and fragmentation is suppressed. When $D > 1/3$ (which roughly corresponds to $Σ_{\rm disc} > 1/3 Σ_{\rm cloud}$), cloud-cloud collisions lead to a rapid cooling through which clumps form. On smaller scales, cloud-cloud collisions can drive molecular cloud turbulence. This dynamical cooling process can be taken into account in numerical simulations as a subgrid model to simulate the global evolution of disc galaxies.

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arXiv2017

Radio QPO in the $γ$-ray-loud X-ray binary LS I +61${^\circ}$303

LS I +61${^\circ}$303 is a $γ$-ray emitting X-ray binary with periodic radio outbursts with time scales of one month. Previous observations have revealed microflares superimposed on these large outbursts with periods ranging from a few minutes to hours. This makes LS I +61${^\circ}$303, along with Cyg X-1, the only TeV emitting X-ray binary exhibiting radio microflares. To further investigate these microflaring activity in LS I +61${^\circ}$303 we observed the source with the 100-m Effelsberg radio telescope at 4.85, 8.35, and 10.45 GHz and performed timing analysis on the obtained data. Radio oscillations of 15 hours time scales are detected at all three frequencies. We also compare the spectral index evolution of radio data to that of the photon index of GeV data observed by Fermi-LAT. We conclude that the observed QPO could result from multiple shocks in a jet.

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arXiv2017

Mass-size scaling M~ r^1.67 of massive star-forming clumps -- evidences of turbulence-regulated gravitational collapse

We study the fragmentation of eight massive clumps using data from ATLASGAL 870 $μ$m, SCUBA 850 and 450 $μ$m, PdBI 1.3 and 3.5 mm, and probe the fragmentation from 1 pc to 0.01 pc scale. We find that the masses and the sizes of our objects follow $M \sim r^{1.68\pm0.05}$. The results are in agreements with the predictions of Li (2017) where $M \sim r^{5/3}$. Inside each object, the densest structures seem to be centrally condensed, with $ρ(r)\sim r^{-2}$. Our observational results support a scenario where molecular gas in the Milky Way is supported by a turbulence characterized by a constant energy dissipation rate, and gas fragments like clumps and cores are structures which are massive enough to be dynamically detached from the ambient medium.

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arXiv2016

A multi-wavelength observation and investigation of six infrared dark clouds

Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet they play a crucial role in breeding newly-formed stars. Aims. With the aim of further understanding the dynamics, chemistry, and evolution of IRDCs, we carried out multi-wavelength observations on a small sample. Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m telescopes, with tracers ${\rm HCO^+}$, HCN, ${\rm N_2H^+}$, ${\rm C^{18}O}$, DCO$^+$, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01, G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05. Results. We investigated 44 cores including 37 cores reported in previous work and seven newly-identified cores. Toward the dense cores, we detected 6 DCO$^+$, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution (SED) fits of the $\textit{Herschel}$ 70 to 500 $μ$m, we obtained dust temperature and column density distributions of the IRDCs. We found that ${\rm N_2H^+}$ emission has a strong correlation with the dust temperature and column density distributions, while ${\rm C^{18}O}$ showed the weakest correlation. It is suggested that ${\rm N_2H^+}$ is indeed a good tracer in very dense conditions, but ${\rm C^{18}O}$ is an unreliable one, as it has a relatively low critical density and is vulnerable to freezing-out onto the surface of cold dust grains. The dynamics within IRDCs are active, with infall, outflow, and collapse; the spectra are abundant especially in deuterium species. Conclusions. We observe many blueshifted and redshifted profiles, respectively, with ${\rm HCO^+}$ and ${\rm C^{18}O}$ toward the same core. This case can be well explained by model "envelope expansion with core collapse (EECC)".

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arXiv2016

Revealing a spiral-shaped molecular cloud in our galaxy - Cloud fragmentation under rotation and gravity

The dynamical processes that control star formation in molecular clouds are not well understood, and in particular, it is unclear if rotation plays a major role in cloud evolution. We investigate the importance of rotation in cloud evolution by studying the kinematic structure of a spiral-shaped Galactic molecular cloud G052.24+00.74. The cloud belongs to a large filament, and is stretching over ~ 100 pc above the Galactic disk midplane. The spiral-shaped morphology of the cloud suggests that the cloud is rotating. We have analysed the kinematic structure of the cloud, and study the fragmentation and star formation. We find that the cloud exhibits a regular velocity pattern along west-east direction - a velocity shift of ~ 10 km/s at a scale of ~ 30 pc. The kinematic structure of the cloud can be reasonably explained by a model that assumes rotational support. Similarly to our Galaxy, the cloud rotates with a prograde motion. We use the formalism of Toomre (1964) to study the cloud's stability, and find that it is unstable and should fragment. The separation of clumps can be consistently reproduced assuming gravitational instability, suggesting that fragmentation is determined by the interplay between rotation and gravity. Star formation occurs in massive, gravitational bound clumps.

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arXiv2016

Criteria for gravitational instability and quasi-isolated gravitational collapse in turbulent medium

We study the evolution of structures in turbulent, self-gravitating media, and present an analytical criterion $M_{\rm crit} \approx ε_{\rm cascade}^{2/3} η^{-2/3} G^{-1} l^{5/3}$ (where $M_{\rm crit}$ is the critical mass, $l$ is the scale, $ε_{\rm cascade}\approx ησ_{\rm v}^3 /l $ is the turbulence energy dissipation rate of the ambient medium, $G$ is the gravitational constant, $σ_{\rm v}$ is the velocity dispersion, $l$ is the scale and $η\approx 0.2$ is an efficiency parameter) for an object to undergo quasi-isolated gravitational collapse. The criterion also defines the critical scale ($l_{\rm crit} \approx ε_{\rm cascade}^{1/2} η^{-1/2} G^{-3/4} ρ^{-3/4}$) for turbulent gravitational instability to develop. The analytical formalism explains the size dependence of the masses of the progenitors of star clusters ($M_{\rm cluster} \sim R_{\rm cluster}^{1.67}$) in our Galaxy.

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arXiv2016

Probing the multi-scale interplay between gravity and turbulence - Power-law like gravitational energy spectra of the Orion Complex

Gravity plays a determining role in the evolution of the molecular ISM. In \citet{2016arXiv160304342L}, we proposed a measure called gravitational energy spectrum to quantify the importance of gravity on multiple physical scales. In this work, using a wavelet-based decomposition technique, we derive the gravitational energy spectra of the Orion A and the Orion B molecular cloud from observational data. The gravitational energy spectra exhibit power-law-like behaviours. From a few pc down to $\sim 0.1 $ pc scale, the Orion A and Orion B molecular cloud have $E_{\rm p}(k)\sim k^{-1.88}$ and $E_{\rm p}(k)\sim k^{-2.09}$, respectively. These scaling exponents are close to the scaling exponents of the kinetic energy power spectrum of compressible turbulence (where $E\sim k^{-2}$), with a near-equipartition of turbulent versus gravitational energy on multiple scales. This provides a clear evidence that gravity is able to counteract effectively against turbulent motion for these length scales. The results confirm our earlier analytical estimates. For the Orion A molecular cloud, gravity inevitably dominates over turbulence inside the cloud. Our results provide a clear observational proof that gravity is playing a determining role in the evolution these molecular clouds from the cloud scale down to $\sim 0.1\;\rm pc$. { However, turbulence is likely to dominate in clouds like California. } The method is general and should be applicable to all the astrophysical problems where gravity plays a role.

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