The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this p...The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this paper describes testing results of key fundamental parameters for FAST, aiming to provide basic support for observation and data reduction of FAST for scientific researchers. The 19-beam receiver covering 1.05–1.45 GHz was utilized for most of these observations. The fluctuation in electronic gain of the system is better than 1% over 3.5 hours, enabling enough stability for observations. Pointing accuracy,aperture efficiency and system temperature are three key parameters for FAST. The measured standard deviation of pointing accuracy is 7.9′′, which satisfies the initial design of FAST. When zenith angle is less than 26.4°, the aperture efficiency and system temperature around 1.4 GHz are ~0.63 and less than 24 K for central beam, respectively. The sensitivity and stability of the 19-beam backend are confirmed to satisfy expectation by spectral HI observations toward NGC 672 and polarization observations toward 3 C 286. The performance allows FAST to take sensitive observations for various scientific goals, from studies of pulsars to galaxy evolution.展开更多
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.Th...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.展开更多
Discovery of pulsars is one of the main goals for large radio telescopes.The Five-hundredmeter Aperture Spherical radio Telescope(FAST),that incorporates an L-band 19-beam receiver with a system temperature of about 2...Discovery of pulsars is one of the main goals for large radio telescopes.The Five-hundredmeter Aperture Spherical radio Telescope(FAST),that incorporates an L-band 19-beam receiver with a system temperature of about 20 K,is the most sensitive radio telescope utilized for discovering pulsars.We designed the snapshot observation mode for a FAST key science project,the Galactic Plane Pulsar Snapshot(GPPS)survey,in which every four nearby pointings can observe a cover of a sky patch of 0.1575 square degrees through beam-switching of the L-band 19-beam receiver.The integration time for each pointing is 300 seconds so that the GPPS observations for a cover can be made in 21 minutes.The goal of the GPPS survey is to discover pulsars within the Galactic latitude of±10∘from the Galactic plane,and the highest priority is given to the inner Galaxy within±5∘.Up to now,the GPPS survey has discovered 201 pulsars,including currently the faintest pulsars which cannot be detected by other telescopes,pulsars with extremely high dispersion measures(DMs)which challenge the currently widely used models for the Galactic electron density distribution,pulsars coincident with supernova remnants,40 millisecond pulsars,16 binary pulsars,some nulling and mode-changing pulsars and rotating radio transients(RRATs).The follow-up observations for confirmation of new pulsars have polarization-signals recorded for polarization profiles of the pulsars.Re-detection of previously known pulsars in the survey data also leads to significant improvements in parameters for 64 pulsars.The GPPS survey discoveries are published and will be updated at http://zmtt.bao.ac.cn/GPPS/.展开更多
The age pattern across spiral arms is one of the key observational features utilised to study the dynamic nature of the Galaxy’s spiral structure.With the most updated samples of high-mass star formation region(HMSFR...The age pattern across spiral arms is one of the key observational features utilised to study the dynamic nature of the Galaxy’s spiral structure.With the most updated samples of high-mass star formation region(HMSFR)masers,O stars and open clusters,we investigated their distributions and kinematic properties in the vicinity of the Sun.We found that the Sagittarius-Carina Arm traced by HMSFRs,O stars((?)10 Myr)and young open clusters(<30 Myr)seem to deviate gradually towards the Galactic Anticenter(GAC)direction.The Local Arm traced by HMSFRs,O stars,young clusters and also mediumyoung clusters(30-100 Myr)are inclined to gradually deviate toward the Galactic Center(GC)direction.The properties for the Local Arm are supported by a simplified simulation of cluster motions in the Galaxy.Indications of systematic motions in the circular and radial velocities are noticed for the old open clusters(>200 Myr).These results are consistent with the idea that star formation can be triggered by spiral shocks of density waves,and indicate that the corotation radius of the Galaxy is located between the SagittariusCarina Arm and the Local Arm,close to the Solar circle.展开更多
H Ⅱ regions made of gas ionized by radiations from young massive stars,are widely distributed in the Milky Way.They are tracers for star formation,and their distributions are correlated with the Galactic spiral struc...H Ⅱ regions made of gas ionized by radiations from young massive stars,are widely distributed in the Milky Way.They are tracers for star formation,and their distributions are correlated with the Galactic spiral structure.Radio recombination lines(RRLs) of hydrogen and other atoms allow for the most precise determination of physical parameters such as temperature and density.However,RRLs at around 1.4 GHz from HⅡ regions are weak and their detections are difficult.As a result,only a limited number of detections have been obtained yet.The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical radio Telescope(FAST) can simultaneously cover 23 RRLs for Hnα,Henα,and Cnα(n=164-186),respectively.This,combined with its unparalleled collecting area,makes FAST the most powerful telescope to detect weak RRLs.In this pilot survey,we use FAST to observe nine HⅡ regions at L band.We allocate20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of2.0 km s^(-1).In total,21 RRLs for Hnα and Cnα at 1.0-1.5 GHz have been simultaneously detected with strong emission signals.Overall,the detection rates for the H167α and C167α RRLs are 100%,while that for the He167α RRL is 33.3%.Using hydrogen and helium RRLs,we measure the electron density,electron temperature,and pressure for three HⅡ regions.This pilot survey demonstrates the capability of FAST in RRL measurements,and a statistically meaningful sample with RRL detection,through which knowledge about Galactic spiral structure and evolution can be obtained,is expected in the future.展开更多
Radio recombination lines（RRLs） are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the ...Radio recombination lines（RRLs） are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the Sagittarius Arm tangent by using the 65-m Shanghai Tian Ma Radio Telescope（TMRT） equipped with broadband feeds and a digital backend. Six hydrogen RRLs（H96α-H101α）at C band（6289 MHz-7319 MHz） were observed simultaneously toward a sky area of 2°× 1.2° by using on-the-fly mapping mode. These transitions were then stacked together for detection of ionized gas. Star forming complexes G48.6＋0.1 and G49.5-0.3 were detected in the integrated intensity map.We found agreements between our measured centroid velocities and previous results for the 21 known HII regions in the mapped area. For more than 80 cataloged HII region candidates without previous RRL measurements, we obtained new RRL spectra at 30 targeted positions. In addition, we detected 25 new discrete RRL sources with spectral S/N 〉 5σ, and they were not listed in the catalogs of previously known HII regions. The distances for 44 out of these 55 new RRL sources were estimated.展开更多
基金supported by the National Key R & D Program of China (No. 2017YFA0402701)the National Natural Science Foundation of China (NSFC, Nos. 11673039, 11803051 and 11833009)+1 种基金supported by the CAS “Light of West China” programsupported by the Youth Innovation Promotion Association CAS
文摘The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this paper describes testing results of key fundamental parameters for FAST, aiming to provide basic support for observation and data reduction of FAST for scientific researchers. The 19-beam receiver covering 1.05–1.45 GHz was utilized for most of these observations. The fluctuation in electronic gain of the system is better than 1% over 3.5 hours, enabling enough stability for observations. Pointing accuracy,aperture efficiency and system temperature are three key parameters for FAST. The measured standard deviation of pointing accuracy is 7.9′′, which satisfies the initial design of FAST. When zenith angle is less than 26.4°, the aperture efficiency and system temperature around 1.4 GHz are ~0.63 and less than 24 K for central beam, respectively. The sensitivity and stability of the 19-beam backend are confirmed to satisfy expectation by spectral HI observations toward NGC 672 and polarization observations toward 3 C 286. The performance allows FAST to take sensitive observations for various scientific goals, from studies of pulsars to galaxy evolution.
基金sponsored by the MOST (Grant No.2017YFA0402701)the NSFC (Grant Nos.11873019, 11673066 and 11503033)+4 种基金the CAS (Grant No.QYZDJ-SSW-SLH047)the Youth Innovation Promotion Association of CASsupported by the Key Laboratory for Radio Astronomy, CASthe Open Project Program of the Key Laboratory of FAST, NAOC, CASFunding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement
文摘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.
基金This project,as one of five key projects,is being carried out by using FAST,a Chinese national mega-science facility built and operated by the National Astronomical Observatories,Chinese Academy of Sciences.J.L.Han is supported by the National Natural Science Foundation of China(NSFC,Nos.11988101 and 11833009)the Key Research Program of the Chinese Academy of Sciences(Grant No.QYZDJ-SSW-SLH021)+4 种基金C.Wang is partially supported by NSFC No.U1731120X.Y.Gao is partially supported by NSFC No.U1831103P.F.Wang is partially supported by the NSFC No.11873058 and the National SKA program of China No.2020SKA0120200.Jun Xu is partially supported by NSFC No.U2031115H.G.Wang is partially supported by the National SKA program of China(No.2020SKA0120100)R.Yuen is partly supported by Xiaofeng Yang's Xinjiang Tianchi Bairen project and CAS Pioneer Hundred Talents Program.L.G.Hou thanks the support from the Youth Innovation Promotion Association CAS.
文摘Discovery of pulsars is one of the main goals for large radio telescopes.The Five-hundredmeter Aperture Spherical radio Telescope(FAST),that incorporates an L-band 19-beam receiver with a system temperature of about 20 K,is the most sensitive radio telescope utilized for discovering pulsars.We designed the snapshot observation mode for a FAST key science project,the Galactic Plane Pulsar Snapshot(GPPS)survey,in which every four nearby pointings can observe a cover of a sky patch of 0.1575 square degrees through beam-switching of the L-band 19-beam receiver.The integration time for each pointing is 300 seconds so that the GPPS observations for a cover can be made in 21 minutes.The goal of the GPPS survey is to discover pulsars within the Galactic latitude of±10∘from the Galactic plane,and the highest priority is given to the inner Galaxy within±5∘.Up to now,the GPPS survey has discovered 201 pulsars,including currently the faintest pulsars which cannot be detected by other telescopes,pulsars with extremely high dispersion measures(DMs)which challenge the currently widely used models for the Galactic electron density distribution,pulsars coincident with supernova remnants,40 millisecond pulsars,16 binary pulsars,some nulling and mode-changing pulsars and rotating radio transients(RRATs).The follow-up observations for confirmation of new pulsars have polarization-signals recorded for polarization profiles of the pulsars.Re-detection of previously known pulsars in the survey data also leads to significant improvements in parameters for 64 pulsars.The GPPS survey discoveries are published and will be updated at http://zmtt.bao.ac.cn/GPPS/.
基金the National Key Research and Development Program of China(No.2017YFA0402701)the National Natural Science Foundation of China(Grant Nos.11988101,11933011,11833009)support from the Youth Innovation Promotion Association,CAS。
文摘The age pattern across spiral arms is one of the key observational features utilised to study the dynamic nature of the Galaxy’s spiral structure.With the most updated samples of high-mass star formation region(HMSFR)masers,O stars and open clusters,we investigated their distributions and kinematic properties in the vicinity of the Sun.We found that the Sagittarius-Carina Arm traced by HMSFRs,O stars((?)10 Myr)and young open clusters(<30 Myr)seem to deviate gradually towards the Galactic Anticenter(GAC)direction.The Local Arm traced by HMSFRs,O stars,young clusters and also mediumyoung clusters(30-100 Myr)are inclined to gradually deviate toward the Galactic Center(GC)direction.The properties for the Local Arm are supported by a simplified simulation of cluster motions in the Galaxy.Indications of systematic motions in the circular and radial velocities are noticed for the old open clusters(>200 Myr).These results are consistent with the idea that star formation can be triggered by spiral shocks of density waves,and indicate that the corotation radius of the Galaxy is located between the SagittariusCarina Arm and the Local Arm,close to the Solar circle.
基金support from the National Key R&D Program of China (2018YFE0202900)support by the NAOC Nebula Talents Program+2 种基金the Cultivation Project for FAST Scientific Payoff and Research Achievement of CAMS-CASthe support from the Youth Innovation Promotion Association of CASsupported by the National Natural Science Foundation of China (Grant Nos.11703040,W820301904,11988101,11933011 and 11833009)。
文摘H Ⅱ regions made of gas ionized by radiations from young massive stars,are widely distributed in the Milky Way.They are tracers for star formation,and their distributions are correlated with the Galactic spiral structure.Radio recombination lines(RRLs) of hydrogen and other atoms allow for the most precise determination of physical parameters such as temperature and density.However,RRLs at around 1.4 GHz from HⅡ regions are weak and their detections are difficult.As a result,only a limited number of detections have been obtained yet.The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical radio Telescope(FAST) can simultaneously cover 23 RRLs for Hnα,Henα,and Cnα(n=164-186),respectively.This,combined with its unparalleled collecting area,makes FAST the most powerful telescope to detect weak RRLs.In this pilot survey,we use FAST to observe nine HⅡ regions at L band.We allocate20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of2.0 km s^(-1).In total,21 RRLs for Hnα and Cnα at 1.0-1.5 GHz have been simultaneously detected with strong emission signals.Overall,the detection rates for the H167α and C167α RRLs are 100%,while that for the He167α RRL is 33.3%.Using hydrogen and helium RRLs,we measure the electron density,electron temperature,and pressure for three HⅡ regions.This pilot survey demonstrates the capability of FAST in RRL measurements,and a statistically meaningful sample with RRL detection,through which knowledge about Galactic spiral structure and evolution can be obtained,is expected in the future.
基金supported by the National Natural Science Foundation of China (11303035, 11473034, 11503033 and 11503070)supported by the Key Research Program of the Chinese Academy of Sciences, Grant No. QYZDJSSW-SLH021+1 种基金additionally supported by the FAST Fellowshipthe Young Researchers Grant of National Astronomical Observatories, Chinese Academy of Sciences
文摘Radio recombination lines（RRLs） are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the Sagittarius Arm tangent by using the 65-m Shanghai Tian Ma Radio Telescope（TMRT） equipped with broadband feeds and a digital backend. Six hydrogen RRLs（H96α-H101α）at C band（6289 MHz-7319 MHz） were observed simultaneously toward a sky area of 2°× 1.2° by using on-the-fly mapping mode. These transitions were then stacked together for detection of ionized gas. Star forming complexes G48.6＋0.1 and G49.5-0.3 were detected in the integrated intensity map.We found agreements between our measured centroid velocities and previous results for the 21 known HII regions in the mapped area. For more than 80 cataloged HII region candidates without previous RRL measurements, we obtained new RRL spectra at 30 targeted positions. In addition, we detected 25 new discrete RRL sources with spectral S/N 〉 5σ, and they were not listed in the catalogs of previously known HII regions. The distances for 44 out of these 55 new RRL sources were estimated.
