The spiral structure of the Milky Way

The morphology and kinematics of the spiral structure of the Milky Way are long-standing problems in astrophysics.In this review we firstly summarize various methods with different tracers used to solve this puzzle.The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties, as we can currently obtain accurate parallaxes(a few μas) and proper motions(≈1 km s-1) by using Very Long Baseline Interferometry(VLBI).On the other hand, the Gaia mission is providing the largest, uniform sample of parallaxes for O-type stars in the entire Milky Way.Based upon the VLBI maser and Gaia O-star parallax measurements, nearby spiral structures of the Perseus, Local, Sagittarius and Scutum Arms are determined in unprecedented detail.Meanwhile, we estimate fundamental Galactic parameters of the distance to the Galactic center,R0, to be 8.35 ± 0.18 kpc, and circular rotation speed at the Sun, Θ0, to be 240±10 km s-1.We found kinematic differences between O stars and interstellar masers: the O stars, on average, rotate faster,>8 km s-1than maser-traced high-mass star forming regions.

Assessing the performance of molecular gas clump identification algorithms

The detection of clumps(cores)in molecular clouds is an important issue in sub-millimetre astronomy.However,the completeness of the identification and the accuracy of the returned parameters of the automated clump identification algorithms are still not clear.In this work,we test the performance and bias of the GaussClumps,ClumpFind,FellWalker,Reinhold,and Dendrograms algorithms in identifying simulated clumps.By designing the simulated clumps with various sizes,peak brightness,and crowdedness,we investigate the characteristics of the algorithms and their performance.In the aspect of detection completeness,FellWalker,Dendrograms,and GaussClumps are the first,second,and third best algorithms,respectively.The numbers of correct identifications of the six algorithms gradually increase as the size and signal-to-noise ratio(SNRs)of the simulated clumps increase and they decrease as the crowdedness increases.In the aspect of the accuracy of retrieved parameters,FellWalker and Dendrograms exhibit better performance than the other algorithms.The average deviations in clump parameters for all algorithms gradually increase as the size and SNR of clumps increase.Most of the algorithms except FellWalker exhibit significant deviation in extracting the total flux of clumps.Taken together,FellWalker,GaussClumps,and Dendrograms exhibit the best performance in detection completeness and extracting parameters.The deviation in virial parameter for the six algorithms is relatively low.When applying the six algorithms to the clump identification for the Rosette molecular cloud,ClumpFind1994,ClumpFind2006,GaussClumps,FellWalker,and Reinhold exhibit performance that is consistent with the results from the simulated test.

Direct Digital Frequency Control Based on the Phase Step Change Characteristic between Signals

We present a new digital phase lock technology to achieve the frequency control and transformation through high precision multi-cycle group synchronization between signals without the frequency transformation circuit. In the case of digital sampling, the passing zero point of the phase of the controlled signal has the phase step characteristic, the phase step of the passing zero point is monotonic continuous with high resolution in the phase lock process, and using the border effect of digital fuzzy area, the gate can synchronize with the two signals, the quantization error is reduced. This technique is quite different from the existing methods of frequency transformation and frequency synthesis, the phase change characteristic between the periodic signals with different nominal is used. The phase change has the periodic phenomenon, and it has the high resolution step value. With the application of the physical law, the noise is reduced because of simplifying frequency transformation circuits, and the phase is locked with high precision. The regular phase change between frequency signals is only used for frequency measurement, and the change has evident randomness, but this randomness is greatly reduced in frequency control, and the certainty of the process result is clear. The experiment shows that the short term frequency stability can reach 10-12/s orders of magnitude.