From The Condensation Of Clouds To The Birth Of Stars

When we are fortunate enough to see the sky on a moonless and cloudless night,we are able to see the Milky Way because of its distinctive feature—this is a belt that spans the sky and contains a large number of bright stars and a background of many fainter stars.This paper focuses on the material in the Milky Way,which is not easily seen by our eyes,but is still there and has played a role in the evolution of the Milky Way.The material is the interstellar medium(ISM).The interstellar medium literally means "matter between stars",an all-encompassing definition.Although stars play an important role in galaxies such as the Milky Way,gas and dust play a role in their life cycles and galaxy evolution.About 99%of the gas in the interstellar medium is composed of neutral hydrogen atoms.They could be divided into different states based on their different physical conditions—temperature,density,and ionization,etc.Under the condition of pressure equilibrium,the cloud with lower temperature tends to have higher density of matter.As the dense cloud evolves,interstellar molecules may be forming in the dense part of clouds,a physical environment conducive to the gravitational collapse of the cloud is created,and then the formation process of stars begins,finally new stars are born.In this paper,We are focus on three areas:cold neutral hydrogen clouds,the gravitational collapse of molecular clouds,and the effects of the physical process of star formation on surrounding interstellar molecules.The warm neutral medium and cold neutral medium coexist in our galaxy.The neutral hydrogen self-absorption feature will appear in the spectrum,if the cold neutral hydrogen medium is in front of the warm neutral hydrogen medium in the same line-of-sight direction we observed.How to find these absorption features robustly and automatically has become the focus of this research.The THOR project(The HI/OH/Recombination line survey of the milky way)uses the Very Large Array Telescope(VLA)to observe the Milky Way plane in the northern sky at 21 cm,with an observational area of 15°<1<67° and |b|<±1°.The project provides observational data for searching cold neutral hydrogen clouds.We use the method of Empirical Mode Decomposition(EMD)to decompose a nonlinear,non-stationary natural signal into Intrinsic Mode Functions(IMFs),finding the self-absorption features from IMFs,and isolating the cold neutral hydrogen clouds.From the observational data of THOR,we have found～150 self-absorbed clouds of neutral hydrogen.This work provides sample for future research on cold neutral medium.Molecular clouds are the cradle of new stars,gravity is the main driving force of star formation.Studying how molecular clouds collapse is key to studying star formation.One way to understand the gravitational collapse of molecular clouds is to understand the gravity-derived acceleration mapping of molecular clouds.Simulations of star formation are built in three-dimensional space,however we currently study star formation through two-dimensional observation data.The problem is that the accuracy of those acceleration maps,how does the 2D acceleration maps obtained from observation data reflects the real 3D distribution.Therefore,we use the 3D simulation data of the SFM project and 2D data generated by integrating 3D data along the line of sight to calculate the gravitational fields in fourier space respectively,then derive acceleration fields of a3Dproj and a2D.We found that most of the acceleration size ratios |a2D|/|a3Dproj|<5.Their include angle can be obtained by inner product:a2D-a3Dproj=|a2D‖a3Dproj|cosθ.Most of the included angles θ<25°.These errors may be due to projection effects and sampling effects.Through our calculations,the acceleration mapping calculated using the observed two-dimensional data can be applied to the future study of the three-dimensional distribution of the acceleration field.Interstellar molecules are found in star-forming regions.We use Submillimeter array(SMA)data of nine massive star-forming regions at different stages of evolution to study the effects of star-forming process on surrounding molecules.Star evolution stage is indicated by the luminosity-to-mass ratio of molecular clumps observed by ATLASGAL project,these sources span luminosity-to-mass ratio range from 10 to 154 L⊙/M⊙.Transition lines of CH3CN(12k-11k)were detected in all sources,and we derived the rotational temperature and abundance and other physical parameters of the molecule.We find that the rotation temperature of CH3CN molecule appears to be trending increase between 10 to 40 L⊙/M⊙,and decrease after Lclump/Mclump>40 L⊙/M⊙.This may be caused by star wind.Assuming that the CH3CN molecule is heated by the radiation of the central star,we estimate the effective distance of the CH3CN molecule to the central star.The estimated effective distances range from～0.003 pc to～0.083 pc,which account for～1/100 to～1/1000 of the molecular clump size.The effective distance increases with the increase of the Lclump/Mclump(Reff ∝(Lclump/Mclump)0.5±0.2).We also found that CH3CN abundance is inversely correlated with Lclump/Mclump(XCH3CN∝(Lclump/Mclump)-1.0±0.7).The reason may be due to the photodissociation during star evolution.There is also an inverse relationship between molecular abundance and ionization flux.The strong anti-correlation deserves observational studies with high spatial resolution telescopes,and the CH3CN molecular abundance may be used as a tracer of star evolution.