Zi Zhu

Nanjing University | NJU · Department of Astronomy

Pulsars are special objects whose positions can be determined independently from timing, radio interferometric, and Gaia astrometry at sub-milliarcsecond (mas) precision; thus, they provide a unique way to monitor the link between dynamical and kinematic reference frames. We aimed to assess the orientation consistency between the dynamical referenc...

Aims. At the beginning of 2019 the third realization of the International Celestial Reference Frame (ICRF3) replaced the previous version (ICRF2). In the current International Celestial Reference Frame (ICRF), a major improvement is the inclusion of the Galactic aberration (GA) effect. The Earth orientation parameters (EOP), which link the celestia...

We present an updated study on assessing the axes stability of the third generation of the International Celestial Reference Frame (ICRF3) in terms of linear drift and scatter based on the extragalactic source position time series from analyses of archival very long baseline interferometry observations. Our results show that the axes of the ICRF3 a...

Context. The celestial reference frame is established on the basis of the absolute positions of extragalactic sources that are assumed to be fixed in space. The fixing of the axes is one of the crucial points for the concept behind the International Celestial Reference System (ICRS). However, due to various effects such as its intrinsic activity, t...

The celestial reference frame is realized by absolute positions of extragalactic sources that are assumed to be fixed in the space. The fixing of the axes is one of the crucial points for the International Celestial Reference System (ICRS) concept. However, due to various effects such as its intrinsic activity, the apparent position of the extragal...

For detecting exoplanets with high precision, using the angular distance between the two stars to detect the periodic motion of the star will be a better choice. This approach can avoid importing the position error of the reference catalog in the process that using the traditional photographic plate to derive the star position. At the precision lev...

We examine the vertical structure of Galactic disk kinematics over a Galactocentric radial distance range of R = 5-15 kpc and up to 3 kpc away from the Galactic plane, using the K-type giants surveyed by LAMOST. Based on robust measurements of three-dimensional velocity moments, a wobbly disk is detected in a phenomenological sense. An outflow domi...

Context. Comparisons of optical positions derived from the Gaia mission and radio positions measured by very long baseline interferometry (VLBI) probe the structure of active galactic nuclei (AGN) on the milliarcsecond scale. So far, these comparisons have focused on using the S ∕ X -band (2/8 GHz) radio positions, but did not take advantage of the...

We examine the vertical structure of Galactic disk kinematics over a Galactocentric radial distance range of $R=5-15$ $\rm{kpc}$ and up to $3$ $\rm{kpc}$ away from the Galactic plane, using the K-type giants surveyed by LAMOST. Based on robust measurements of three-dimensional velocity moments, a wobbly disk is detected in a phenomenological sense....

We extend previous works by considering two additional radio frequencies (K band and X/Ka band) with the aim to study the frequency dependence of the source positions and its potential connection with the physical properties of the underlying AGN. We compared the absolute source positions measured at four different wavelengths, that is, the optical...

Aims. We aim to study the relative positions of quasar emission centers at different wavelengths in order to help link the various realizations of the International Celestial Reference System (ICRS), and to unveil systematic uncertainties and individual source behavior at different wavelengths. Methods. We based our study on four catalogs represent...

Aims. In order to check the astrometric solution quality, dependences of parallaxes and proper motions on precision, reliability, and consistency of sample solutions are studied for the quasars in the celestial reference frame of the second release of Gaia data (Gaia-CRF2). Methods. Astrometric statistics (the number of visibility periods, the semi...

Context. The third generation of the ICRF – ICRF3 – was published in 2018. This new fundamental catalog provides radio source positions measured independently at three bands: S / X , K , and X / Ka , representing three independent radio celestial frames which altogether constitute a multi-frequency ICRF. Aims. We aim to investigate the overall prop...

We aim to investigate the overall properties of the ICRF3 with the help of the Gaia Data release 2 (Gaia DR2). This could serve as an external check of the quality of the ICRF3. The radio source positions of the ICRF3 catalog were compared with the Gaia DR2 positions of their optical counterparts at G < 18.7. Their properties were analyzed in terms...

Aims . This work aims to explore the possibilities of determining the long-period part of the precession-nutation of the Earth with techniques other than very long baseline interferometry (VLBI). Lunar laser ranging (LLR) is chosen for its relatively high accuracy and long period. Results of previous studies could be updated using the latest data w...

This work aims to explore the possibilities of determining the long-period part of the precession-nutation of the Earth with techniques other than very long baseline interferometry (VLBI). Lunar laser ranging (LLR) is chosen for its relatively high accuracy and long period. Results of previous studies could be updated using the latest data with gen...

The local standard of rest (LSR) provides a reference framework for studies of Galactic kinematics. Determination of the LSR corresponds to the measurement of solar peculiar motion, which is under debate due to the fact that different methods and samples have been used. Adopting the astrometric data and line-of-sight velocities of main sequence sta...

The variation of Earth orientation that is characterized by Earth orientation parameters (EOP) is of interest in astronomy and geophysics since the study of EOP opens a window to Earth’s deep interior. In addition, EOP connects the celestial reference frame (CRF) and terrestrial reference frame (TRF) and should be in accordance with realizations of...

The astrometry method has great advantages in searching for exoplanets in the habitable zone around solar-like stars. However, the presence of multiple planets may cause a problem with degeneracy when trying to compute accurate planet parameters from observation data and reduce detectability. The degeneracy problem is extremely critical, especially...

Aims. We propose to estimate the accuracy of current VLBI catalogs. Methods. The difference of source position estimated from two decimation solutions was analyzed to estimate the scale factor and noise floor for the formal error of radio source positions by two different methods. In one method, we investigated the weighted rootsquare-mean (wrms)...

The space telescope Search for Terrestrial Exo-Planets (STEP) employed a method of sub-pixel technology which ensures that the astrometric accuracy of the telescope on the focal plane is at the order of 1 μas. This kind of astrometric precision is promising to detect the earth-like planets beyond the solar system. In this paper, we analyze the infl...

The link between the International Celestial Reference Frame at radio wavelength and the forthcoming Gaia optical reference frame is a mandatory task after the completion of the Gaia mission. Starting from the provisional reference frame in which Gaia astrometric solutions were obtained, we discuss the ways to correct the residual rotation and acce...

The International Celestial Reference Frame (ICRF) is currently realized by the very long baseline interferometry (VLBI) observations of extragalactic sources with the zero proper motion assumption, while Gaia will observe proper motions of these distant and faint objects to an accuracy of tens of microarcseconds per year. This paper investigates t...

Aims. In order to investigate the systematic errors in the very long baseline interferometry (VLBI) positions of extragalactic sources (quasars) and the global differences between Gaia and VLBI catalogs, we use the first data release of Gaia ( Gaia DR1) quasar positions as the reference and study the positional offsets of the second realization of...

The gravitational potential of the Milky Way is non-axisymmetric, caused by a bar or triaxial halo, which dominates elliptical rotation of the Milky Way. Employing a likelihood analysis, we exploit the astrometric data of masers thoroughly and constrain the elliptical rotation of the Galaxy. Masers in high-mass star-forming regions, observed by VLB...

We compare quasar positions of the auxiliary quasar solution with ICRF2 sources using different samples and evaluate the influence on the {\it Gaia} DR1 reference frame owing to the Galactic aberration effect over the J2000.0-J20015.0 period. Then we estimate the global rotation between TGAS with {\it Tycho}-2 proper motion systems to investigate t...

We aim to evaluate the possibility of improving the ICRS realization starting from the ICRF2 catalogue by investigating the coordinate time series of radio sources observed by VLBI between 1979 and 2016. Sources with long observational history are selected as the candidates and the least squares fits with special handling of the weights are perform...

The currently used Galactic coordinate system (GalCS) is based on the FK5 system at J2000.0, which was transformed from the FK4 system at B1950.0. The limitations and misunderstandings related to this transformed GalCS can be avoided by defining a new GalCS that is directly connected to the International Celestial Reference System (ICRS). With more...

We have constructed a catalog containing best available astrometric, photometric, radial velocity and astrophysical data for mainly F-type and G-type stars (called the Astrometric catalog associated with Astrophysical Data, ACAD), which contains 27,553 records, and is used for the purpose of analyzing the stellar kinematics in the Solar neighborhoo...

Accelerations of both the solar system barycenter (SSB) and stars in the Milky Way cause a systematic observational effect on the stellar proper motions, which was first studied in the early 1990s and developed by J. Kovalevsky (aberration in proper motions, 2003, A&A, 404, 743). This paper intends to extend that work and aims to estimate the magni...

Commission 8 has regularly published triennial reports in the past and the current OC therefore voted to adopt a traditional format also for this special Legacy issue of the IAU Transactions. The outgoing President is grateful for the support of many Commission members who contributed to this report. Our contribution consists of 3 parts: 1) this in...

Based on radial velocity data and Hipparcos proper motions, we present a new determination of the Galactocentric distance based on a purely kinematic model. We have selected three subgroups of Galactic thin-disk components (O-B5 stars, classical Cepheids and Galactic open clusters) to trace the local structure and kinematics of the Galactic disk. A...

Context. The curvature of the motion of the solar system barycenter around the Galactic center induces an aberration effect varying linearly with time. It can be called the "Galactic aberration" and is also known as the "secular aberration (drift)" or "aberration in proper motions". This results in a systematic dipole pattern of the apparent proper...

We observed the open clusters NGC 1664 (43 exposures) and M35 (42 exposures) by the Yunnan Faint Object Spectrograph and Camera in the 2.4 m telescope at Yunnan Astronomical Observatory on 2011 January 3, and processed them by a method recently proposed by us. The result shows that there is a geometric distortion effect in the field of view and the...

We describe the target selection algorithm for the low latitude disk portion of the LAMOST Pilot Survey, which aims to test systems in preparation for the LAMOST spectroscopic survey. We use the PPMXL (Roeser et al. 2010) astrometric catalog, which provides positions, proper motions, B/R/I magnitudes (mostly) from USNO-B (Monet et al. 2003) and J/H...

We describe a general target selection algorithm that is applicable to any survey in which the number of available candidates is much larger than the number of objects to be observed. This routine aims to achieve a balance between a smoothly-varying, well-understood selection function and the desire to preferentially select certain types of targets...

We describe the current plans for a spectroscopic survey of millions of stars in the Milky Way galaxy using the Guo Shou Jing Telescope (GSJT, formerly the Large Area Multi-Object Spectroscopic Telescope - LAMOST). The survey will obtain spectra for 2.5 million stars brighter than $r<19$ during dark/grey time, and 5 million stars brighter than $r<1...

Gaia is continuing well with its development and construction and is expected to achieve parallax accuracies of about 10 to 300 μas for 6 to 20 mag. It is scheduled to launch in 2013. The ICRF2 was adopted by the IAU in 2009 as the new fundamental celestial reference frame. The UCAC project concluded with the publication of its 4th data release (al...

The definition of the Galactic coordinate system was announced by the IAU Sub-Commission 33b on behalf of the IAU in 1958. For more than 50 years the definition of the Galactic coordinate system has remained unchanged from this IAU1958 version. On the basis of deep and all-sky catalogs, the position of the Galactic plane can be revised and updated...

Considered to be extensions of the Hipparcos reference system, PPMX and UCAC3 are two of the most important astrometric catalogs released in current years. Extensive analyses of these two large catalogs have been made in order to determine the local and overall systematic biases. The regional and magnitude dependent differences in stellar position...

The business meeting was opened by the President, Irina Kumkova. She presented the agenda, which was approved. This session was attended by 42 participants. The meeting approved Dafydd Evans as secretary of minutes.

Initially defined by the IAU in 1958, the galactic coordinate system was thereafter in 1984 transformed from the B1950.0 FK4-based system to the J2000.0 FK5-based system. In 1994, the IAU recommended that the dynamical reference system FK5 be replaced by the ICRS, which is a kinematical non-rotating system defined by a set of remote radio sources....

The availability of astrometric data and radial velocities of carbon stars near the Galactic plane enables us to investigate the kinematics of the Milky Way, especially the rotation curve. The recently published Third U. S. Naval Observatory CCD Astrograph Catalog (UCAC3) provides the opportunity to test this problem us-ing three-dimensional veloci...

Van Leeuwen has completed and published the new reduction of the Hipparcos data. Parallax accuracies have improved by up to a factor five for the brightest stars and correlations effectively removed.

On the basis of recently published astrophysical parameters of the open clusters, we have selected 301 clusters with measurements of their kinematical parameters to trace the local structure and kinematics of the Galactic disk. The present sample covers a range of over 3.0 kpc from the Sun and gives significant estimates of the disk structure and k...

Revised Hipparcos data were released by van Leeuwen in 2007. In this article revised Hipparcos trigonometrical parallaxes are used to derive a period-luminosity zero-point in the optical V band. Adopting a fixed slope of −2.81 from the LMC, a zero-point value of ρ= −1.37±0.07 is derived based on the method by Feast and Catchpole (1997). This value...

With the new Hipparcos data recently released, we reexamine the kinematics in the solar neighborhood. Two different populations of objects, namely the thin-disk O-B5 stars and the thick-disk K-M giants, are selected for tracing the kinematical parameters of the Galaxy. Using a 3-D kinematical model, the components of the solar motion and the Oort c...

Local stellar kinematic properties are reinvestigated. A main-sequence sample of 12718 objects is picked out from a total of 117955 Hipparcos stars and some local kinematical parameters are re-determined by dividing 19 subsamples mainly according to color index. The results are reasonably consistent with those derived from the initial Hipparcos dat...

Selecting 301 open clusters with complete spatial velocity measurements and ages, we are able to estimate the disk structure and kinematics of the Milky Way. Our analysis incorporates the disk scale height, the circular velocity of the Galactic rotation, the Galactocentric distance of the Sun and the ellipticity of the weak elliptical potential of...

The optical ground-based astrometry of solar system objects may have its accuracy strongly improved by using new methods for making observations and reductions of them. New photometric methods of observating the mutual phenomena occurring in the solar system, may provide astrometric data with a higher precision than the classical direct imaging. In...

Comparing proper motions of the FK5 and Hipparcos, several authors declared that the two proper-motion systems are inconsistent with the value of the precession correction obtained from VLBI and LLR observations. Based on the proper-motion data from the PPM and ACRS catalogues which are constructed on the FK5 system, the inconsistent values of the...

With measuring precision on the order of milli-arcseconds for ground-based survey facilities or even micro-arcseconds for space astrometric satellites, the importance of astrometric education continues to be important. The content of astrometric courses in China during the past fifteen years is reviewed and the current astrometric courses for under...

An attempt is made to determine the scale height of the thin Galactic disk in the solar neighborhood using various stellar samples from the accurate astrometric data of the Hipparcos satellite. Using the Tycho color indices and considering the completeness of the samples, several samples were selected from the main sequence and the horizontal branc...

From photometric observations and star counts, the existence of a bar in the central few kpc of the Galaxy is suggested. It is generally thought that our Galaxy is surrounded by a massive invisible halo. The gravitational potential of the Galaxy is therefore made non-axisymmetric generated by the central triaxial bar, by the outer triaxial halo, an...

Analyzing Hipparcos proper motions of 1352 O–B5 stars other than the Gould belt stars, which are representative of the young disk population, we have found a clear stellar warping motion that is a systematic rotation +3.8 ± 1.1 km s-1 kpc-1 of stars about the axis pointing to the Galactic center in the sense of increasing the inclination of the H I...

We have performed an overall pattern comparison between the proper motions of the FK5 (Fifth Fundamental Catalogue) systems (FK5, Astrographic Catalog Reference Stars [ACRS], and PPM Catalogue) and those of Hipparcos. The global rotation (time dependent) between the two reference frames is found to be different for different stellar samples, depend...

On the basis of the astrophysical parameters of the Galactic open clusters, which were recently published by Kharchenko et al.(2005a) for 520 clusters, by Kharchenko et al.(2005b) for 130 clusters, and by Dias et al.(2002) for 1599 clusters, we have selected about 300 clusters completely with all measurements of their kinematical parameters(proper...

Results of many researches have shown that the relation between the proper motion systems of FK5 and Hipparcos is not consistent with the precession constant corrections determined by VLBI and LLR. We analysed proper motion data of PPM and ACRS based on the FK5 system for many different sub-samples and found that consistent values of the precession...

We present a new determination of the Galactocentric distance by a pure kinematical model. Two subgroups of components from the Galactic thin disk, the O-B5 stars and the Galactic open clusters, were selected for our analysis. On the basis of kinematical data of around 1200 O-B5 stars, we obtained an estimated value of R0=8.25+/-0.79 kpc, while a s...

Based on the Hipparcos proper motions and available radial velocity data of O-B stars, we have re-examined the local kinematical structure of the young disk population of ~1500 O-B stars not including the Gould-belt stars. A systematic warping motion of the stars about the direction to the Galactic center has been reconfirmed. A negative K-term imp...

Post-Hipparcos advances in both of space and ground-based astrometry is briefly described. First, the current progress of Gaia project is given; the programs of JAMSE and OBSS are introduced as well as the status of SIM PlanetQuest project containing the astrometric content is reviewed. The establishment and maintenance of multi-wavelength referenc...

Based on the Hipparcos proper motions and the available radial velocity data of O-B stars, we have reexamined the local kinematical structure of young disk population of ~1500 O-B stars other than the Gould-belt stars. A systematic warping motion of stars about the axis pointing to the Galactic center has been reconfirmed. A negative K-term has bee...

The astrometric research contents and the relationship with other subjects are described. The development of international astrometric researches in the most recent decade and the advanced subjects are given. The four advanced research subjects are: (1) realization of sub-milliarcsecond and even microarcsecond astrometry by astrometric satellites t...

On the basis of Hipparcos data, whose system is believed to be quasi-inertial to within +/-0.25 mas yr-1 and to represent an internally consistent reference frame, we carried out an analysis of the SPM 2.0 proper-motion system. Comparing the Hipparcos proper motions with those of the SPM 2.0 Catalog, which provides absolute proper motions of object...

From Hipparcos proper motions, we have analysed the local kinematics of the Milky Way from young O-B5 stars, classical Cepheids, and the late-type K-M giants. The components of solar motion along the direction pointing to the galactic center from the Sun u0 and along the direction pointing to the north galactic pole w0 are approximately 10 km s−1 a...

We present an intensive analysis of the FK5 proper-motion system via the two large astrographic catalogs, the PPM and ACRS catalogs, compared with the Hipparcos proper motions. Region-, magnitude-, and color-dependent systematic errors in the PPM and ACRS proper motions are found and exhibit similar tendencies for both catalogs. The term of the glo...

Combining Hipparcos proper motions and the radial velocity data, we have studied the Cepheid kinematics on the basis of the three-dimensional Ogorodnikov-Milne model. The results seem to show a slight contracting motion of the Galaxy in the solar neighbourhood, ∂ V θ / ∂θ / R = −2.60 ± 1.07 km s-1 kpc-1, which is along the solar circle. Under the h...

The ground-based modern meridian circles are discussed from the perspectives of the tremendous achievements in astrometry. A method of CCD star position determination by short strips is proposed. An observing programme of 12 - 14 stars around the extragalactic radiosources for the connection of the optical/radio reference frame using the Mykolayiv...

Astronomical refraction is a basic subject in astrometry. It would affect the determination of stellar position. The automatic sampling system for collection of meteorological data is an important device for the research on astronomical refraction and the analysis of the relationship between the meteorological parameters and the errors of determina...

A special design of collimation system has been employed on the DCMT. The collimation system and the principle for determing collimation error are introduced. The repeatability of collimator is about 0"03 according to the measurements in real operation of the DCMT.

A technique of cross-correlation for the determination of stellar positions is proposed. It is a technique suitable for the recording of stellar positions on transit instruments, photoelectric astrolabes, and also on the meridian circles with reversed observing method. An advantage of this process is that there is no hypothesis required for the mod...

A set of exact formulae for systematic corrections of the stellar position is presented.

Concerned with the estimating method of the image position of the division line and with the systematic errors resulted from the optics, mechanics and the electronics, the repeatabilities of the circle reading is studied. By means of analysing the residuals of the measurements, the authors find out some main problems which influence the reading rep...

The sway of the horizontal axis for the DCMT has been analyzed and studied. The direction of the horizontal axis of the DCMT's is determined by the basic plane of the end of the big gear. The deviation from a ideal plane for the basic plane causes the sway of the horizontal axis. There are two kinds of effects of the sway on instrument precision: (...

The variation of the azimuth of the DCMT base frame, related to the piers, with the temperature of the frame is discussed and analyzed. The construction of the base frame and the principle of four points support are described. The test method for the variation of the azimuth of the base frame are introcuced, and the test results are presented.