arXiv subjects

H. B. Liu

Publications and source records attributed to H. B. Liu.

25 records · Page 2Linked to original sources

arXiv2016

Outflow detection in a 70 $μ$m dark high-mass core

We present observations towards a high-mass ($\rm >40\,M_{\odot}$), low luminosity ($\rm <10\,L_{\odot}$) $\rm 70\,μ$m dark molecular core G 28.34 S-A at 3.4 mm, using the IRAM 30 m telescope and the NOEMA interferometer. We report the detection of $\rm SiO$ $J=\rm 2\rightarrow1$ line emission, which is spatially resolved in this source at a linear resolution of $\sim$0.1 pc, while the 3.4 mm continuum image does not resolve any internal sub-structures. The SiO emission exhibits two W-E oriented lobes centring on the continuum peak. Corresponding to the red-shifted and blue-shifted gas with velocities up to $\rm 40\,km\,s^{-1}$ relative to the quiescent cloud, these lobes clearly indicate the presence of a strong bipolar outflow from this $\rm 70\,μ$m dark core, a source previously considered as one of the best candidates of "starless" core. Our SiO detection is consistent with ALMA archival data of $\rm SiO$ $J=\rm 5\rightarrow4$, whose high-velocity blue-shifted gas reveals a more compact lobe spatially closer to the dust center. This outflow indicates that the central source may be in an early evolutionary stage of forming a high-mass protostar. We also find that the low-velocity components (in the range of $\rm V_{lsr}$$\rm_{-5}^{+3}\,km\,s^{-1}$) have an extended, NW-SE oriented distribution. Discussing the possible accretion scenarios of the outflow-powering young stellar object, we argue that the molecular line emission and the molecular outflows may provide a better indication of the accretion history when forming young stellar object, than that from a snapshot observations of the present bolometric luminosity. This is particularly significant for the cases of episodic accretion, which may occur during the collapse of the parent molecular core.

astro-ph.GA↗

arXiv2016

Comparing Young Massive Clusters and their Progenitor Clouds in the Milky Way

Young massive clusters (YMCs) have central stellar mass surface densities exceeding $10^{4} M_{\odot} pc^{-2}$. It is currently unknown whether the stars formed at such high (proto)stellar densities. We compile a sample of gas clouds in the Galaxy which have sufficient gas mass within a radius of a few parsecs to form a YMC, and compare their radial gas mass distributions to the stellar mass distribution of Galactic YMCs. We find that the gas in the progenitor clouds is distributed differently than the stars in YMCs. The mass surface density profiles of the gas clouds are generally shallower than the stellar mass surface density profiles of the YMCs, which are characterised by prominent dense core regions with radii ~ 0.1 pc, followed by a power-law tail. On the scale of YMC core radii, we find that there are no known clouds with significantly more mass in their central regions when compared to Galactic YMCs. Additionally, we find that models in which stars form from very dense initial conditions require surface densities that are generally higher than those seen in the known candidate YMC progenitor clouds. Our results show that the quiescent, less evolved clouds contain less mass in their central regions than in the highly star-forming clouds. This suggests an evolutionary trend in which clouds continue to accumulate mass towards their centres after the onset of star formation. We conclude that a conveyor-belt scenario for YMC formation is consistent with the current sample of Galactic YMCs and their progenitor clouds.

astro-ph.GA↗

arXiv2016

What is controlling the fragmentation process in the Infrared Dark Cloud G14.225-0.506? Differet level of fragmentation in twin hubs

We present observations of the 1.3 mm continuum emission toward hub-N and hub-S of the infrared dark cloud G14.225-0.506 carried out with the Submillimeter Array, together with observations of the dust emission at 870 and 350 microns obtained with APEX and CSO telescopes. The large scale dust emission of both hubs consists of a single peaked clump elongated in the direction of the associated filament. At small scales, the SMA images reveal that both hubs fragment into several dust condensations. The fragmentation level was assessed under the same conditions and we found that hub-N presents 4 fragments while hub-S is more fragmented, with 13 fragments identified. We studied the density structure by means of a simultaneous fit of the radial intensity profile at 870 and 350 microns and the spectral energy distribution adopting a Plummer-like function to describe the density structure. The parameters inferred from the model are remarkably similar in both hubs, suggesting that density structure could not be responsible in determining the fragmentation level. We estimated several physical parameters such as the level of turbulence and the magnetic field strength, and we found no significant differences between these hubs. The Jeans analysis indicates that the observed fragmentation is more consistent with thermal Jeans fragmentation compared with a scenario that turbulent support is included. The lower fragmentation level observed in hub-N could be explained in terms of stronger UV radiation effects from a nearby HII region, evolutionary effects, and/or stronger magnetic fields at small scales, a scenario that should be further investigated.

astro-ph.GA↗

arXiv2014

Diversity of chemistry and excitation conditions in the high-mass star forming complex W33

The object W33 is a giant molecular cloud that contains star forming regions at various evolutionary stages from quiescent clumps to developed H II regions. Since its star forming regions are located at the same distance and the primary material of the birth clouds is probably similar, we conducted a comparative chemical study to trace the chemical footprint of the different phases of evolution. We observed six clumps in W33 with the Atacama Pathfinder Experiment (APEX) telescope at 280 GHz and the Submillimeter Array (SMA) at 230 GHz. We detected 27 transitions of 10 different molecules in the APEX data and 52 transitions of 16 different molecules in the SMA data. The chemistry on scales larger than $\sim$0.2 pc, which are traced by the APEX data, becomes more complex and diverse the more evolved the star forming region is. On smaller scales traced by the SMA data, the chemical complexity and diversity increase up to the hot core stage. In the H II region phase, the SMA spectra resemble the spectra of the protostellar phase. Either these more complex molecules are destroyed or their emission is not compact enough to be detected with the SMA. Synthetic spectra modelling of the H$_{2}$CO transitions, as detected with the APEX telescope, shows that both a warm and a cold component are needed to obtain a good fit to the emission for all sources except for W33 Main1. The temperatures and column densities of the two components increase during the evolution of the star forming regions. The integrated intensity ratios N$_{2}$H$^{+}$(3$-$2)/CS(6$-$5) and N$_{2}$H$^{+}$(3$-$2)/H$_{2}$CO(4$_{2,2}$$-$3$_{2,1}$) show clear trends as a function of evolutionary stage, luminosity, luminosity-to-mass ratio, and H$_{2}$ peak column density of the clumps and might be usable as chemical clocks.

astro-ph.GA↗

arXiv2014

Measuring Mass Accretion Rate onto the Supermassive Black Hole in M 87 Using Faraday Rotation Measure with the Submillimeter Array

We present the first constraint on Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M 87. By fitting the polarization position angles ($χ$) observed with the SMA at four independent frequencies around $\sim$230 GHz and interpreting the change in $χ$ as a result of \emph{external} Faraday rotation associated with accretion flow, we determine the rotation measure of the M 87 core to be between $-$7.5$\times$10$^{5}$ and 3.4$\times$10$^{5}$ rad/m$^{2}$. Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the rotation measure constrains the mass accretion rate $\dot{M}$ to be below 9.2$\times$10$^{-4}$ M$_{\odot}$~yr$^{-1}$ at a distance of 21 Schwarzchild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of the accretion flow. Consequently, our result disfavors the classical \emph{advection dominated accretion flow} (ADAF) and prefers the \emph{adiabatic inflow-outflow solution} (ADIOS) or \emph{convection-dominated accretion flow} (CDAF) for the hot accretion flow in M 87.

astro-ph.GA↗

arXiv2014

MUSCLE W49 : A Multi-Scale Continuum and Line Exploration of the Most Luminous Star Formation Region in the Milky Way. I. Data and The Mass Structure of the Giant Molecular Cloud

The Multi-Scale Continuum and Line Exploration of W49 (MUSCLE W49) is a comprehensive gas and dust survey of the giant molecular cloud (GMC) of W49A, the most luminous star-formation region in the Milky Way. It covers the entire GMC at different scales and angular resolutions. In this paper we present: 1) an all-configuration SMA mosaic in the 230-GHz band covering the central 3 arcmin (10 pc, known as W49N), with most of the embedded massive stars; and 2) PMO 14m telescope observations in the 90-GHz band, covering the entire GMC with maps up to 35 arcmin in size, or 113 pc. We also make use of archival data from the VLA, JCMT-SCUBA, IRAM 30m, and the CSO BOLOCAM GPS. Our main findings are: 1) The W49 GMC is one of the most massive in the Galaxy, with a total mass ~1.1x10^6 Msun within a radius of 60 pc. Within a radius of 6 pc, the total gas mass is ~2x10^5 Msun. At these scales only 1% of the material is photoionized. The mass reservoir is sufficient to form several young massive clusters (YMCs) as massive as a globular cluster. 2) The mass of the GMC is distributed in a hierarchical network of filaments. At scales <10 pc, a triple, centrally condensed structure peaks toward the ring of HC HII regions in W49N. This structure extends to scales from ~10 to 100 pc. The W49A starburst most likely formed from global gravitational contraction with localized collapse in a "hub-filament" geometry. 3) Currently, feedback from the central YMCs (with a present mass Mcl > 5x10^4 Msun) is still not enough to entirely disrupt the GMC, but further stellar mass growth could be enough to allow radiation pressure to clear the cloud and halt star formation. 4) The resulting stellar content will probably remain as a gravitationally bound massive star cluster, or a small system of bound clusters. (ABRIDGED)

astro-ph.GA↗

arXiv2012

A study of thinner-THGEM, with some applications

THGEMs (THick Gas Electron Multiplier) of varying thickness, hole diameter and hole pitch have been studied. For a thinner-THGEM of thickness 0.2 mm, with hole diameter 0.2 mm, pitch 0.2 mm and narrow (5-10 μm) rim, the performance of gain versus high voltage with different gas mixtures have been studied using 5.9 keV X-rays. In general, a gain of around 3\times10^3 was obtained with a single board in Ar/iC4H10, and gains higher than 10^4 were obtained in Ne mixture at lower voltage. The dependence of the energy resolution on the drift and induction electric fields was measured and an energy resolution of 15.9% was obtained. A curved thinner-THGEM chamber with one-dimensional readout has been assembled for use in the diffraction studies at the Beijing Synchrotron Radiation Facility (BSRF). A cosmic-ray muon hodoscope based on thinner-THGEM was developed as a teaching experiment for the Graduate University of the Chinese Academy of Sciences (GUCAS).

hep-ex↗