arXiv subjects

Hao Gong

Publications and source records attributed to Hao Gong.

22 records · Page 2Linked to original sources

arXiv2012

The Wigner solution of quark gap equation at nonzero current quark mass and partial restoration of chiral symmetry at finite chemical potential

According to the generally accepted phase diagram of QCD, at low temperature and high baryon number density the chiral phase transition of QCD is of first order and the co-existence of the Nambu-Goldstone phase and the Wigner phase should appear. This is in conflict with the usual claim that the quark gap equation has no Wigner solution in the case of nonzero current quark mass. In this paper we analyze the reason why the Wigner solution does not exist in the usual treatment and try to propose a new approach to discuss this question. As a first step, we adopt a modified Nambu-Jona-Lasinio (NJL) model to study the Wigner solution at finite current quark mass. We then generalize this approach to the case of finite chemical potential and discuss partial restoration of chiral symmetry at finite chemical potential and compare our results with those in the normal NJL model.

hep-ph↗

arXiv2011

Dense core formation in supersonic turbulent converging flows

We use numerical hydrodynamic simulations to investigate prestellar core formation in the dynamic environment of giant molecular clouds, focusing on planar post-shock layers produced by colliding turbulent flows. A key goal is to test how core evolution and properties depend on the velocity dispersion in the parent cloud; our simulation suite consists of 180 models with inflow Mach numbers Ma=v/c_s=1.1-9. At all Mach numbers, our models show that turbulence and self-gravity collect gas within post-shock regions into filaments at the same time as overdense areas within these filaments condense into cores. This morphology, together with the subsonic velocities we find inside cores, is similar to observations. We extend previous results showing that core collapse develops in an ``outside-in'' manner, with density and velocity approaching the Larson-Penston asymptotic solution. The time for the first core to collapse varies as 1/sqrt(v), consistent with analytic estimates. Core building takes 10 times as long as core collapse, consistent with observed prestellar core lifetimes. Core shapes change from oblate to prolate as they evolve. To define cores, we use isosurfaces of the gravitational potential. We compare to cores defined using the potential computed from projected surface density, finding good agreement for core masses and sizes; this offers a new way to identify cores in observed maps. Cores with masses varying by three orders of magnitude (0.05 - 50 M_sun) are identified in our simulations. Stability analysis of post-shock layers predicts that the first core to collapse will have mass M \propto v^-1/2, and that the minimum mass for cores formed at late times will have M\propto v^-1. From our simulations, the median mass lies between these two relations.

astro-ph.SR↗

arXiv2009

Protostar Formation in Supersonic Flows: Growth and Collapse of Spherical Cores

We present a unified model for molecular core formation and evolution, based on numerical simulations of converging, supersonic flows. Our model applies to star formation in GMCs dominated by large-scale turbulence, and contains four main stages: core building, core collapse, envelope infall, and late accretion. During the building stage, cores form out of dense, post-shock gas, and become increasingly centrally stratified as the mass grows over time. When the shock radius defining the core boundary exceeds $R\approx 4 a (4πG ρ_{mean})^{-1/2}$, where $a$ is the isothermal sound speed, a wave of collapse propagates from the edge to the center. During the building and collapse stages, density profiles can be fit by Bonnor-Ebert profiles with temperature 1.2 - 2.9 times the true value. As found previously for initially static equilibria, outside-in collapse leads to a Larson-Penston density profile $ρ\approx 8.86 a^2/(4 πG r^2)$. The third stage, consisting of an inside-out wave of gravitational rarefaction leading to $ρ\propto r^{-3/2}$, $v\propto r^{-1/2}$, is also similar to that for initially-static spheres, as originally described by Shu. We find that the collapse and infall stages have comparable duration, $\sim t_{ff}$, consistent with estimates for observed prestellar and protostellar (Class 0/I) cores. Core building takes longer, but does not produce high-contrast objects until shortly before collapse. The time to reach core collapse, and the core mass at collapse, decrease with increasing inflow Mach number. For all cases the accretion rate is $\gg a^3/G$ early on but sharply drops off; the final system mass depends on the duration of late-stage accretion, set by large-scale conditions in a cloud.

astro-ph.SR↗

arXiv2008

Magnetically Aligned Velocity Anisotropy in the Taurus Molecular Cloud

Velocity anisotropy induced by MHD turbulence is investigated using computational simulations and molecular line observations of the Taurus molecular cloud. A new analysis method is presented to evaluate the degree and angle of velocity anisotropy using spectroscopic imaging data of interstellar clouds. The efficacy of this method is demonstrated on model observations derived from three dimensional velocity and density fields from the set of numerical MHD simulations that span a range of magnetic field strengths. The analysis is applied to 12CO J=1-0 imaging of a sub-field within the Taurus molecular cloud. Velocity anisotropy is identified that is aligned within 10 degrees of the mean local magnetic field direction derived from optical polarization measurements. Estimated values of the field strength based on velocity anisotropy are consistent with results from other methods. When combined with new column density measurements for Taurus, our magnetic field strength estimate indicates that the envelope of the cloud is magnetically subcritical. These observations favor strong MHD turbulence within the low density, sub-critical, molecular gas substrate of the Taurus cloud.

astro-ph↗