A&A 383, 1062-1066 (2002)
DOI: 10.1051/0004-6361:20011831

Optical positions of 55 radio stars from astrolabe observations from the Yunnan Observatory

H. Hui - W. Rui

Yunnan Observatory, National Astronomical Observatories, Academia Sinica, Kunming 650011, PR China

Received 6 July 2001 / Accepted 18 December 2001

Abstract
The observations by the photoelectric astrolabe at Yunnan Observatory relative to the Hipparcos Catalogue and the optical positions of 55 radio stars were obtained from observations between 1991 and 2000. They all resulted from processing the photon counts obtained by means of the astrolabe after the automation of the instrument. There are 46 stars in common with the Hipparcos Catalogue[*].

Key words: astrometry - radio continuum: stars


1 Introduction

The photoelectric astrolabe of the Yunnan Observatory was installed at the site and put into operation in 1980. The instrument took part in the determination of the Earth Rotation Parameters and joined the Main Campaign of Project MERIT (a program of international collaboration to monitor earth rotation and compare techniques of observation and analysis; Melbourne et al. 1983). Owing to its high observation accuracy the instrument also took part in compiling the General Catalogue of Photoelectric Astrolabes stars (Working group of GCPA 1983) and afterwards in the Chinese Geodetical Stars Catalogue (Working group of CGSC 1991). Upon S. Debarbat's (1986) call for intensive observations of radio stars by optical astrometry, 50 radio stars have been selected and observed since 1986. The instrument was automated and equipped with a photon counting detector in the later eighties. It can be operated automatically to observe stars as faint as magnitude 11.0 (Xu et al. 1993). The objects contained in the astrometric catalogue of radio stars by Walter et al. (1990) have all been included in the program to contribute to the linking of the optical reference frame to the VLBI reference frame based on extragalactic objects (Walter & Sovers 2000). Despite the rainy period lasting nearly half a year in Kunming we succeeded by the end of the year 2000 in deriving the optical positions of 55 radio stars from two transits each between 1991 and 2000. These observations resulted from the data acquired by the photon counting detector after the automation of the instrument.

2 Observation and reduction

The radio stars were inserted in the program of 16 basic groups. Each basic group lasts 1.5 hours and involves 40 stars, among which more than 32 stars are related to the FK5 (All FK5 stars are part of the Hipparcos Catalogue). The parameters, U (clock error), Y (latitude correction) and Z (correction of zenith distance for the instrument) are estimated using the FK5 stars in a basic group in which a radio star is inserted. Then the residual is calculated from the following expression:

 \begin{displaymath}
V=15 \sin A \cos\varphi_{0} (T-T_{0}+U)+B_{z}+Y \cos A-Z
\end{displaymath} (1)

(Debarbat & Guinot 1970; Li et al. 1983), where T is the recorded time of transit and T0 is the calculated time of transit of a star. Bz accounts for the vacuum correction of the instrument tube and the acceleration correction of the instrument during observation. $\varphi_{0}$ is the adopted latitude of the observing site. A is the transit azimuth of a star, reckoning from north to east, in the range of $0^{\rm o}$ to $360^{\rm o}$.
 

 
Table 1: 55 radio star positions (Mean epoch of observations; equator and equinox J2000).
                    Epoch
Name RSC HIC Mg. $\alpha$ $m_{\alpha}$ $\delta$ $m_{\delta}$ $N_{\rm e}$ $N_{\rm w}$ -1900
        h m s s $\circ$ $\prime$ $\prime\prime$ $\prime\prime$      
UU Psc A 1004 1196 6.02 0 14 58.819 0.001 8 49 15.48 0.02 32 30 96.30
13 Cet   2762 5.20 0 35 14.786 0.002 -3 35 34.00 0.01 22 50 96.51
39 Cet 1010 5951 5.42 1 16 36.303 0.002 -2 30 01.14 0.01 43 31 96.03
HD 8634 1014 6669 6.18 1 25 35.664 0.001     38 25 93.83
o Cet   10826 6.47 2 19 20.795 0.004 -2 58 38.65 0.02 38 37 96.24
92 Cet 1025 14135 2.54 3 02 16.773 0.001 4 05 23.26 0.01 36 35 97.78
$\beta$ Per 1026 14576 2.09 3 08 10.129 0.001 40 57 20.23 0.02 25 34 93.90
UX Ari 1028 16042 6.47 3 26 35.377 0.001     42 36 95.58
HR 1099 A 1029 16846 5.82 3 36 47.292 0.001 0 35 16.27 0.01 25 38 98.08
HD 22403 2654   8.30 3 37 10.707 0.003     28 22 94.30
HD 30455 1044 22349 6.95 4 48 42.112 0.001 18 42 34.86 0.03 47 41 97.58
$\alpha$ Aur   24608 0.08 5 16 41.335 0.002 45 59 54.05 0.02 43 56 97.06
R Aur     6.50 5 17 17.856 0.014 53 35 09.87 0.05 39 31 94.88
$\delta$ Ori A 2113 25930 2.25 5 32 00.395 0.001 -0 17 56.60 0.01 68 50 97.37
$\zeta$ OriA   26727 1.74 5 40 45.517 0.002 -1 56 33.24 0.02 38 32 93.91
HD 37806 1061   8.00 5 41 02.296 0.002 -2 43 00.67 0.01 31 25 95.85
54 Ori 2123 27913 4.39 5 54 23.012 0.001     49 42 96.93
$\pi$ Ori   28404 4.30 5 59 56.092 0.001 45 56 12.25 0.01 38 36 95.82
CQ Aur 2129 26715 9.04 6 03 53.637 0.003     29 32 94.06
RY Gem 2166   8.69 7 27 24.152 0.002 15 39 35.21 0.08 32 40 93.87
$\sigma$ Gem 1084 37629 4.23 7 43 18.709 0.001     43 33 96.72
RZ Cne 2183   8.67 8 39 08.546 0.002     32 42 93.66
RW UMa 1118   10.30 11 40 46.407 0.002 51 59 53.48 0.02 32 31 93.28
RS CVn 1137 64293 8.07 13 10 36.926 0.001     34 26 94.82
59 Vir 2256 64792 5.19 13 16 46.620 0.001 9 25 26.24 0.01 37 21 95.69
FK Com 1140 65915 8.15 13 30 46.813 0.004     34 32 94.62
BH CVn 1143 66257 4.91 13 34 47.777 0.001     49 33 95.22
ZZ Boo 1147 68064 6.78 13 56 09.550 0.001     30 29 95.78
CU Vir 2270 69389 4.99 14 12 15.820 0.002 2 24 34.14 0.02 32 33 94.97
26 Boo 2608 71115 5.91 14 32 32.578 0.001     45 35 96.05
44 Cen   73695 4.83 15 03 47.589 0.001 47 39 14.43 0.01 28 33 93.67
RW CrB 2298   10.22 15 39 15.243 0.002     27 30 93.35
$\sigma$ CrB A 1172 79607 5.23 16 14 40.958 0.002     38 30 94.95
U Her   80488 8.31 16 25 47.484 0.001 18 53 33.12 0.03 42 21 93.32
V729 Her 2340 84014 8.08 17 10 25.609 0.004 48 57 56.24 0.04 24 25 93.30
Z Her 1189 87965 7.24 17 58 06.994 0.001 15 08 21.55 0.04 30 28 94.75
59 Ser 2373 90441 5.20 18 27 12.518 0.001 0 11 46.10 0.01 25 38 96.76
$\beta$ Lyr 1201 92420 3.52 18 50 04.792 0.001     35 30 97.10
HD 178208 2613   7.60 19 05 09.849 0.001 49 55 23.26 0.01 42 27 93.69
HD 179094 1207 94013 5.88 19 08 25.832 0.003 52 25 32.72 0.01 23 39 95.66
U Sge A 2614 94910 6.50 19 18 48.405 0.001     30 42 96.60
$\chi$ Cyg   97629 7.91 19 50 33.934 0.004     21 27 93.93
    100013 6.57 20 17 25.181 0.001 39 35 36.61 0.04 33 35 97.55
$\rho$ Cyg   100044 4.77 20 17 47.203 0.001     31 35 96.95
V444 Cyg 2442 100214 7.93 20 19 32.435 0.001 38 43 53.95 0.04 21 30 93.55
V1687 Cyg 1225 100287 6.78 20 20 27.984 0.001 43 51 16.10 0.02 33 34 97.74
ER Vul 2460 103833 7.33 21 02 25.893 0.001     29 39 97.38
HN Peg 2475 107350 5.96 21 44 31.297 0.001 14 46 19.39 0.02 41 40 97.62
RT Lac 1246 108728 8.93 22 01 30.716 0.003 43 53 25.58 0.06 21 23 94.08
IM Peg 1257 112997 5.86 22 53 02.279 0.001 16 50 28.25 0.04 31 45 98.65
RZ And 2615   10.30 23 09 30.043 0.017 53 02 39.98 0.08 29 27 94.83
SZ Psc 1263 114639 7.40 23 13 23.794 0.004 2 40 31.66 0.04 37 31 95.97
$\lambda$ And 1265 116584 3.81 23 37 33.748 0.001 46 27 31.88 0.02 28 34 93.77
HD 223460 1271 117503 5.86 23 49 40.958 0.001     37 41 95.87
II Peg 1272 117915 7.51 23 55 03.862 0.002     21 42 95.88


Assuming that $V_{\rm e}$ and $V_{\rm w}$ are the mean residuals at east and west transits, respectively, the position corrections of the radio stars in right ascension and declination are determined from double transits by the formulas

\begin{displaymath}\Delta\alpha=\frac{V_{\rm e}-V_{\rm w}}{30\cos\varphi_{0}\vert \sin A \vert}
\end{displaymath} (2)


 

 
Table 2: The external accuracy of the 55 radio star positions.
        HIC-YPA BORD-YPA USNO-YPA
Name RSC HIC Mg. Da Db Da Db Da Db
            (0.01'')      
UU Psc A 1004 1196 6.02 -6 7 -1 6 -3 2
13 Cet   2762 5.20 -3 -12        
39 Cet 1010 5951 5.42 17 7 17 -13 17 9
HD 8634 1014 6669 6.18 -1   2   5  
o Cet   10826 6.47 -7 3        
92 Cet 1025 14135 2.54 1 -3 7 -5    
$\beta$ Per 1026 14576 2.09 2 10 -6 9    
UX Ari 1028 16042 6.47 -3   -11   -10  
HR 1099 A 1029 16846 5.82 3 -3 6 -8 4 2
HD 22403 2654   8.30 2   20      
HD 30455 1044 22349 6.95 -8 1 -9 -4 -2 -1
$\alpha$ Aur   24608 0.08 4 -2        
R Aur     6.50 -5 4        
$\delta$ Ori A 2113 25930 2.25 8 -12 3 -17    
$\zeta$ OriA   26727 1.74 13 -3        
HD 37806 1061   8.00 -13 -8 4 -19 -5 1
54 Ori 2123 27913 4.39 7   15   -2  
$\pi$ Ori   28404 4.30 11 2 8 -11 4 1
CQ Aur 2129 26715 9.04 19   17   18  
RY Gem 2166   8.69 -2 -21        
$\sigma$ Gem 1084 37629 4.23 4   -1   -4  
RZ Cne 2183   8.67 10   -1   0  
RW UMa 1118   10.30 5 -15 -17 -14 -13 6
RS CVn 1137 64293 8.07 4   4   12  
59 Vir 2256 64792 5.19 -9 -9 -4 -20 -6 -14
FK Com 1140 65915 8.15 17   23   16  
BH CVn 1143 66257 4.91 -4   -7   -3  
            (0.01'')      
ZZ Boo 1147 68064 6.78 -2   1   9  
CU Vir 2270 69389 4.99 -1 -4 5 -11 5 -7
26 Boo 2608 71115 5.91     1   9  
44 Cen   73695 4.83 -18 6        
RW CrB 2298   10.22 19   -1   -17  
$\sigma$ CrB A 1172 79607 5.23 5   3   -14  
U Her   80488 8.31 -7 -19 7 -22 -13 -18
V729 Her 2340 84014 8.08 -14 19 -17 13 -11 10
Z Her 1189 87965 7.24 -7 -3 -3 -7 -10 3
59 Ser 2373 90441 5.20 14 8 13 -7    
$\beta$ Lyr 1201 92420 3.52 4   4      
HD 178208 2613   7.60 15 4 -12 13 -3 4
HD 179094 1207 94013 5.88 6 15 8 11 0 21
U Sge A 2614 94910 6.50 4   6   7  
$\chi$ Cyg   97629 7.91 -1   11   -10  
    100013 6.57 -6 13 -14 22    
$\rho$ Cyg   100044 4.77     -1      
V444 Cyg 2442 100214 7.93 -17 3 -14 6    
V1687 Cyg 1225 100287 6.78 -10 18 -8 17 -2 17
ER Vul 2460 103833 7.33 -4   0   0  
HN Peg 2475 107350 5.96 -6 -14 -5 -17 1 -27
RT Lac 1246 108728 8.93 -10 -6 -4 -7    
IM Peg 1257 112997 5.86 -16 8 -7 12 -12 8
RZ And 2615   10.30 -3 -6 -9 -13 -8 -18
SZ Psc 1263 114639 7.40 -19 -18 -12 -18 -19 -16
$\lambda$ And 1265 116584 3.81 -1 9 6 -4    
HD 223460 1271 117503 5.86 3   5      
II Peg 1272 117915 7.51 16   23   16  
Error budget, unit 0.001'':            
Mean difference     -4 8 11 -40 -12 -9
Standard deviation of the mean difference 14 19 15 25 17 29
Standard deviation of the difference 95 103 101 128 99 127


and

\begin{displaymath}\Delta\delta=\frac{V_{\rm e}+V_{\rm w}-2K}{2\cos q}\cdot
\end{displaymath} (3)

Here q denotes the parallactic angle. We used the stars with $\vert \cos q \vert <0.3$ to calculate 2K from the following expression

\begin{displaymath}2K=\frac{ \sum P_{i}(V_{\rm e}+V_{\rm w})_{i}}{ \sum P_{i}}
\end{displaymath} (4)


\begin{displaymath}P_{i}=-\frac{0.1}{m_{\rm e}^{2}+m_{\rm w}^{2}}
\end{displaymath} (5)

where $m_{\rm e}$ and $m_{\rm w}$ are mean errors of $V_{\rm e}$ and $V_{\rm w}$, respectively. In this catalogue there are 36 stars, from which one obtains 2K= 0.0045. Since for $\vert \cos q \vert <0.3$ the declinations are obtained by the astrolabe with a rather low precision, they are not given in the catalogue.

Since the Hipparcos Catalogue is the primary realization of the ICRS at optical wavelengths (IAU, 1998), the stellar data of the basic groups have all been adopted from the Hipparcos Catalogue and all the observations have been rereduced since 1986.

3 Results

The optical positions of the 55 radio stars are presented in Table 1.

The mean number of observations of each star is about 33. The mean precisions are $\pm$ $0\hbox{$.\!\!^{\rm s}$ }$0021 and $\pm0\hbox{$.\!\!^{\prime\prime}$ }$026, respectively. The mean epoch of observations is 1995.52.

To test the external accuracy of the positions given in Table 1 and denoted YPA we made the following three comparisons:

a) HIC-YPA. The difference between the positions of the Hipparcos Catalogue and YPA for the stars in Table 1.

b) BORD-YPA. The positions of stars determined by the Bordeaux meridian circle minus YPA. The Bordeaux results are quoted from the report of Requi $\grave{\rm e}$me & Mazurier (1991).

c) USNO-YPA. The positions of stars determined by the Flagstaff Astrometric Scanning Transit Telescope of the US Naval Observatory (Stone 1997) minus YPA.

For the comparisons, the positions of stars were reduced to the same epoch by means of the proper motions taken from the Hipparcos Catalogue or from Walter et al. (1990, 1997) before computing differences. The differences are given in Table 2. We also made a statistical analysis of the differences and the results are given in last four rows of Table 2. It can be seen that these quantities all are very small. Therefore YPA's external accuracy is very good.

4 Explanation of Tables 1 and 2

Table 1:
Column 1: name of star.
Column 2: the RSC denotes the number in the astrometric catalogue of radio stars.
Column 3: the number in the Hipparcos Catalogue.
Column 4: observed mean visual magnitude.
Columns 5 and 7: right ascension and declination for equator and equinox J2000.0 and epoch of observation.
Columns 6 and 8: mean errors of right ascension and declination.
Columns 9 and 10: the number of the observed transits in east and west, respectively.
Column 11: mean epoch of observations minus 1900.00.

Table 2:
Column 1: name of star.
Column 2: the RSC denotes the number in the astrometric catalogue of radio stars.
Column 3: the number in the Hipparcos Catalogue.
Column 4: observed mean visual magnitude.
Columns 5 and 6: the differences HIC-YPA in right ascension and declination, respectively. Definition of Da: Da=$[\alpha($HIC$)-\alpha($YPA $)]\cos\delta$. The unit of Da and Db is 0.01''.
Columns 7 and 8: the differences BORD-YPA in right ascension and declination, respectively. The unit of Da and Db is 0.01''.
Columns 9 and 10: the differences USNO-YPA in right ascension and declination, respectively. The unit of Da and Db is 0.01''.

Acknowledgements
The authors heartily thank Professor H. G. Walter and Professor Li Dongming for their valuable help. This work is supported by the Chinese Astronomic Committee Foundation.

References

 


Copyright ESO 2002