Research in Astron. Astrophys. Vol.xx (20xx) No.XX, 000–000 Research in http://www.raa-journal.org http://www.iop.org/journals/raa Astronomy and Astrophysics Astrometry of three near Earth asteroids with the Lijiang 2.4 m telescope Xi-Liang Zhang1;2;3, Yong Yu3, Xue-Li Wang1;2, Chuan-Jun Wang1;2, Liang Chang1;2 Yu-Feng Fan1;2 and Zheng-Hong Tang3 1 Yunnan Observatories, Chinese Academy of Sciences (CAS), P. O. Box 110, Kunming 650011, China; [email protected] 2 Key Laboratory of the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, P. O. Box 110, Kunming 650011, China 3 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 20030, China Received 2014 April 10; accepted 2014 June 27 Abstract Under the framework of the observational campaigns organized by the GAIA Follow Up Network for Solar System Objects (GAIA-FUN-SSO), three near Earth asteroids of (367943) Duende, (99942) Apophis and 2013 TV135 were observed with the Lijiang 2.4 m telescope of Yunnan Observatories. The software PRISM was used to calibrate the CCD fields and measure the positions of Apophis and 2013 TV135, and our own software was used to Duende. Comparison results have shown that, the ephemerides of INPOP10a and JPL are consistent for Apophis and 2013 TV135, however quite inconsistent for Duende. Moreover, we have found that the mean values of the system errors between the ephemerides of INPOP10a and JPL are about 7200 and ¡19900 in right ascension and declination respectively for Duende, and the ephemeris JPL is reliable with the means of O ¡ C (observed-minus- calculated) residuals in right ascension and declination about 2:7200 and 1:4900. Key words: astrometry — near Earth object — astrometric observation — ephemerides 1 INTRODUCTION The near Earth objects can cross the orbit of Earth and approach Earth very closely, for an example, the least distance of the near Earth asteroid Duende, also known as 2012 DA14 (Wlodarczyk 2012), was only 27700 km from the surface of Earth (closer than satellites in geosynchronous orbit) at 19:25 on February 15, 2013 achieved from JPL, so it’s essential to study and make precise orbit model of the near Earth objects to predict its accurate position. Now monitoring and predicting the alert of the near Earth objects has been becoming an international campaign. The Gaia probe (Lindegren et al. 2008; Jordan 2008; Prusti 2011), being a scanning machine, had been launched on December 19, 2013. Long term ground-based tracking observations are nec- essary for confirming the new moving objects or improving the obits of some critical ones detected by Gaia, and the goal of GAIA-FUN-SSO is to coordinate such observational campaigns ( Thuillot, 2010Thuillot 2011). 2 X. L. Zhang et al. As a part of pre-launch training programs organized by the GAIA-FUN-SSO, three near Earth asteroids of Dudende, Apophis and 2013 TV135 were observed during 2013, with the Lijiang 2.4 m telescope of Yunnan Observatories. This paper will describe the observational information of these three near Earth asteroids and give the comparison results with several current ephemerides. 2 ASTROMETRIC OBSERVATIONS Our observations were carried out with the Yunnan Faint Object Spectrograph and Camera (YFOSC) instrument and PI VersArray 1300B camera (PICCD) attached to the Lijiang 2.4 m telescope of Yunnan Observatories. The sizes and effective fields of view of YFOSC and PICCD are 2148 £ 2200 pixels, 10:1£10:4 square arcmin and 1340 £ 1300 pixels, 4:8£4:7 square arcmin respectively, more detailed instructions can be found in the papers of Bai (2010)Bai & Liu (2011) and Fan (2012)Fan et al. (2013). Table 1 shows the specifications of both instruments, where “F-length” means focal length of the system and “FOV” indicates the fields of view. Table 1 Specifications of Two Cameras Attached to the Lijiang 2.4 m Telescope of Yunnan Observatories Camera F-length CCD FOV Size of Pixel Size of CCD size/pixel YFOSC 9840 mm 10:10 £ 10:40 13:5 ¹m£13:5 ¹m 2148 £ 2200 0:28300 PICCD 19200 mm 4:80 £ 4:70 20:0 ¹m£20:0 ¹m 1340 £ 1300 0:21500 On February 15, the near Earth asteroid Duende was observed with the YFOSC instrument attached to the Lijiiang 2.4 m telescope of Yunnan Observatories, and no filter was used during observations. The exposure was one second and it took almost twelve seconds for one frame. Before and after its closest approach to Earth, the Duende moved very fast and its highest velocity reached almost one arcmin per second. Because of its quick motion, only 23 frames were catched. The near Earth asteroid Apophis (99942), which was listed as a potential hazard in following decades (Bancelin et al. 2012; Souchay et al. 2014), was also observed on February 18-March 2, 2013, with the YFOSC instrument of Lijiang 2.4 m telescope. In six observational nights, 90 frames were observed in all. The near Earth asteroid 2013 TV135, which was discovered on October 12, 2013 by the Crimean Astrophysical Observatory, was observed with the PICCD and YOFSC instruments attached to the Lijiang 2.4 m telescope in seven nights from October 31 to December 6, 2013, and 204 frames were made. The detailed observational information of these three near Earth asteroids are listed in the Table 2, where “N” and “R” indicate no filter or the Johnson R filter was used during the obser- vations. 3 MEASUREMENTS AND COMPARISON 3.1 Measurements The observations of Apophis and 2013 TV35 were reduced and measured the positions with the software PRISM, a professional astronomical software for conducting the astronomical observations and image processing, recommended by the committee of the GAIA-FUN-SSO for the astrometric results to be comparable among different individuals, a six constant model and the catalogue UCAC4 ( Zacharias, 2012Zacharias et al. 2013) was chosen to calibrate the CCD fields. Astrometry of Three Near Earth Asteroids with the Lijiang 2.4 m Telescope 3 Table 2 Observational information of the near Earth asteroids of Duende, Apophis and 2013 TV135. NEA Date Exposure (s) Filter Camera Duende Feb. 15 1 N YFOSC Apophis Feb. 18 120 N YFOSC 25 120 N YFOSC 26 120 N YFOSC 27 120 N YFOSC Mar. 01 120 N YFOSC 02 120 N YFOSC 2013 TV135 Oct. 31 60 N PICCD Nov. 01 50 N PICCD Nov. 02 50 R PICCD Dec. 04 120 R YFOSC Dec. 06 120 R YFOSC Dec. 09 180 R YFOSC Dec. 10 180 R YFOSC Table 3 Mean and StdDev values of (O ¡ C) residuals of the near Earth asteroid Duende. Ephemerides Mean (O ¡ C) StdDev (O ¡ C) RA Dec RA Dec (arcsec) (arcsec) (arcsec) (arcsec) INPOP10a 74.44310 –197.54389 21.61420 96.36600 JPL 2.71566 1.49123 0.59428 7.03829 As introduced above, the exposure of Duende was only one second which resulted in that the reference stars in the observed frames were not enough to make the measurement with the software of PRISM, so we calculated the positions of Duende with our own program, including calibrating the CCD fields, centering algorithm, and the correction of annual aberration and diurnal aberration. During measurements, a six constant model and the catalogue UCAC4 were chosen. 3.2 Comparison The observed positions of the near Earth asteroids Duende, Apophis and 2013 TV135 were compared to the ephemeris of INPOP10a (Fienga et al. 2011) calculated by Institut de mecanique celeste et de calcul des ephemerides (IMCCE), and the JPL ephemeris respectively. The comparison results shows that, the mean values of O ¡ C residuals in right ascension and declination for Duende are almost 7500 and ¡19800 with the ephemeris of INPOP10a, however 2:7200 and 1:4900 with the ephemeris of JPL, as shown in the second and third columns of Table 3, in which, the forth and fifth columns list the standard deviation values of O ¡ C residuals of Duende in right ascension and declination. Figure 1 displays the O ¡ C residuals of Duende with different ephemerides, along with the UT time in hour. As shown in Tables 4 and 5, the ephemerides of INPOP10a and JPL are consistent for the near Earth asteroids of Apophis and 2013 TV135. The mean values of O ¡ C residuals are about 1.9 mas and 2.8 mas for Apophis, 26 mas and 290 mas for 2013 TV135 with respect to the ephemeris INPOP10a, which are about 3.3 mas and –5.6 mas for Apophis, 33 mas and 350 mas for 2013 TV135 relative to the ephemeris JPL. Figures 2 and 3 display the O ¡ C residuals of Apophis and 2013 TV135 in each observational nights. 4 X. L. Zhang et al. 120 100 INPOP10a 80 60 O-C (RA) / arcsec 3.5 3.0 JPL 2.5 2.0 1.5 21.0 21.5 22.0 22.5 UT / Hour -400 INPOP10a -300 -200 -100 O-C (DEC) / arcsec 20 JPL 10 0 -10 21.0 21.5 22.0 22.5 UT / Hour Fig. 1 The O ¡ C residuals of the near Earth asteroid Duende in right ascension (left) and DEC (right) related to the ephemerides of INPOP10a and JPL. Table 4 Mean and StdDev values of (O ¡ C) residuals of the near Earth asteroid Apophis. Ephemerides Mean (O ¡ C) StdDev (O ¡ C) RA Dec RA Dec (mas) (mas) (arcsec) (arcsec) INPOP10a 1.900 2.830 0.00501 0.13453 JPL 3.330 –5.610 0.00501 0.13469 4 DISCUSSION The orbit calculation of the near Earth object is usually inaccurate when it approaches Earth too close (Duende), or when its astrometric data is not enough (2013 TV135).