Radio Observations Toward The Regions At Early Stage Of Star Formation

Stars are the building block of the universe.How stars form is one of the key problems of astrophysics.The formation of stars determines the structure and evolution of galaxies,and dominates the luminosity of galaxies.Moreover,the processes of star formation are closely related to the formation and early evolution of planetary system.There are still many unresolved problems on the equilibrium state of cold dark clouds in the early stage of star formation and the way they collapse to form stars under gravity.There are two opinions related to the state of cloud: one is that the evolution of cloud to star formation is quasi-static,and the other is that the cloud is dynamic and evolves to form stars in a cross time.There are many unclear issues in early stage of star formation,such as the equilibrium state of the early core,the role of turbulence,gravity and magnetic field in star formation,the star formation rate,and the establishment of the initial mass function.In addition,most of studies of organic molecules mainly focused on the hot core,but there are few studies on how to retain/regenerate organic molecules during star formation.In this thesis,we will discuss the physical and chemical properties of the early stage of star formation based on the data of two radio telescopes in China.The 13188 Planck Galactic cold clumps(PGCCs)surveyed and released by Planck satellite are the best samples to study the early cold dark clouds.Using CO as a probe to observe these PGCCs can help us understand the physical properties of the early cores.Sixty five PGCCs from the first quadrant(IQuad)and thirty nine PGCCs from the Anti-Center direction region(ACent)were observed in12 CO,13CO and C18 O J=1-0lines using the 13.7-m telescope of Purple Mountain Observatory(PMO).All the targets were detected with all the three lines,except for 12 IQuad and 8 ACent PGCCs without C18 O detection.Seventy six and 49 velocity components were obtained in IQuad and ACent respectively.One-hundred and forty-six cores were extracted from 76 IQuad clumps and 100 cores from 49 ACent clumps.The average Texof IQuad cores and ACent cores are 12.4 K and 12.1 K,respectively.The average line width of13 CO of IQuad cores and ACent cores are 1.55 km s-1and 1.77 km s-1,respectively.Among the detected cores,24 in IQuad and 13 in ACent have asymmetric line profiles.The small blue excesses,～0.03 in IQuad and 0.01 in ACent,indicate that the star formation is not active in these PGCC cores.Power-law fittings of core mass function to the high mass end give indexes of –0.57 in IQuad and –1.02 in ACent which are flatter than the slope of initial mass function given by Salpeter[29].The large turnover masses with value of 28 M⊙ for IQuad cores and 77 M⊙ for ACent cores suggest low star formation efficiencies in PGCCs.The correlation between virial mass and gas mass indicates that most of PGCC cores in both regions are not likely pressure-confined.We performed a carbon-chain molecule(CCM)survey toward four low-mass outflow sources of IRAS 04181+2655(I04181),HH211,L1524 and L1598 using the PMO13.7-m telescope and the 65-m Tian Ma Radio telescope(TMRT)of Shanghai Observatory.C2 H,C4H,c–C3H2,HC2n+1N(n=1,2),CnS(n=2,3),and SO,HNC,N2H+were observed.Hydrocarbons and HC3 N are detected in all the sources,except for L1598 with marginal detection of C4 H and non-detection of HC3N(J=2–1).HC5 N and CCCS are only detected in I04181 and L1524.SO is only detected in HH211.L1598 has the lowest detection rate of CCMs and is regarded as a lacking of CCMs source.The ratio of N(HC3N)/N(N2H+)increases with evolution in low-mass star-forming cores.I04181 and L1524 are carbon-chain-rich star-forming cores and are possible candidates for cores with the warm carbon-chain chemistry.In I04181 and L1524,the abundant CCCS can be explained by shocked carbon-chain chemistry.In HH211,the abundant SO suggests that SO is formed by sublimated S+.We also mapped HNC,C4 H,c–C3H2and HC3 N with the PMO.HNC and NH3 concentrate on L1524 S and L1524 N,respectively.We discussed the chemical differences between I04181 SE and I04181 W.The co-evolution between linear hydrocarbon and cyanopolyynes are found in I04181 SE.Using the PMO 13.7m telescope,we have observed and studied the J = 1–0 transition of CO and its isotope molecules in 104 PGCCs in the IQuad and ACent regions.We have identified 246 starless cores.The key point is that some of the 246 starless cores are in the state of gravitational bounding,namely the prestellar cores.They are good samples for studying the early stages of star formation.The study of their physical properties makes us have a better understanding of the early environment of the formation of low-mass stars.In recent years,we have also made follow-up research on warm carbon chain Chemistry(WCCC)in low-mass star-forming cores.Using the TMRT and the PMO 13.7m telescope,we observed the carbon-chain molecules toward four low mass stars with high-velocity molecular outflow.We found that after the surrounding gas was heated,not only the abundance of simple hydrocarbon chain molecules increased due to the combined carbon atom/ion reaction(WCCC mechanism)after methane molecules evaporated to the gaseous state,the abundance of cyanoacetylene is also increasing.