The Close Binary Central Star of the Young Planetary Nebula HB12 C. H. Hsia a, W. H. Ip a, J.Y. Hu b a Institute of Astronomy, National Central University, Taiwan b Beijing Astronomical Observatory, Beijing, China ABSTRACT Young planetary nebulae play an important role when intermediate-mass stars (0.8~8M ⊙ ) evolve from the proto-planetary nebula (PPN) phase to the planetary nebula (PN) phase (Iben & Renzini, 1983, ARA&A, 21, 271). Many young planetary nebulae displayed marked bipolar structures as they evolve away from the proto-planetary nebulae (PPNe) phase. One possible cause of their bipolarity might have to do with the binary nature of the central stars. We report here our analysis of time-series LOT photometric measurements of the well-known young PN, Hubble 12(HB 12), which is famous for its extended hourglass-like envelope. In addition to the potential presence of a close binary system, the characteristic absorption lines (Mg Ⅰ,CN) of the spectrum show the possibility of a low-mass cool star. WHY IMPORTANT? The dynamical structures of young planetary nebulae (PNe) represent the end state of all kinds of low-mass stars. From their spectral structures, we can study the dynamical ages, history of evolution, physical nature of the central star(s), and mass distribution. Some structures of young planetary nebulae still keep the evolutionary signature of Asymptotic Giant Branch (AGB) phase. Bond et al.(1990) found that half of 13 planetary nebulae with binary nuclei show the butterfly or elliptical morphologies, soker(1997) explained planetary nebula structures using orbital interactions in binary system, and Marco et al.(2004) used a radial-velocity (RV) survey to observe the planetary -nebula nuclei (PNNi), they found that 10 out of 11 well -observed PNNi are binary systems. OBSERVATIONS Time-series photometric measurements of HB 12 using the Lulin One- meter Telescope (LOT). Medium spectral resolution (R~5500) and low spectral resolution (R~2400) measurements of HB 12 using the 2.16m spectrograph at Beijing Astronomical Observatory (BAO). DATA REDUCTION Data were calibrated using standard IRAF procedures. Photometry: The data were processed with the standard APPHOT package. Differential photometric measurements were carried out respect to three comparison stars. Spectra: The spectral data were processed with standard package, which includes bias and flat-field correction, cosmic-ray removal, Wavelength calibration and flux density calibration based on two spectral standards. ACKNOWLEDGMENT This work was partially supported by the National Science Council of Taiwan under NSC M PAE and NSC M We would like to thank Prof. Sun Kwok at University of Calgary, Prof. Chou Yi at National Central University, and Dr. Jinzeng Li at Beijing Astronomical Observatory for useful discussions. RESULTS Fig 2.Two phase plots of R band (Up) and I band (Down) of the adjusted HB 12 data to a frequency of 7.06 c/d ~ d. Fig 3. The power spectrum of the photometric data of HB 12. Fig 4. The observed spectrum of HB 12 in the wavelength range from ~ 3500 A to 9050 A.The spectrum can be decomposed into the white dwarf and the cooler star contributions. Fig 5. (left) The spectral region centered at 4350 A. The characteristic absorption lines (CN λ4216, G band) of late type star(G8~k2) can be easily identified. (right). The spectral region centered at 5450 A. The characteristic absorption lines (the Mg Ⅰ triplet λλ ) of late type star(G8~k2) can be identified. DISCUSSION According to our observation (photometry and spectra), the central object of HB 12 might be a close binary (with a possible period of the multiple of d.). This is the first probable detection of the central stars at HB 12. We found no evidence of photometric variations over one day from our observational data. In view of this, the nucleus of HB 12 is a short-period (< 1 day) binary system. According to our BAO spectra, the companion of central star of HB 12 might be a late type star. FUTURE WORK The spectral type of the companion of central star of HB 12 will be clarified by obtaining a comprehensive set of high resolution spectrographic measurements at BAO in Oct A more accurate period of the central stars of HB 12 will be produced by obtaining a comprehensive set of high-time series photometric measurements at LOT in REFERENCES Bond, Howard E.; Livio,Mario.,ApJ,355,568B,1990 Handler,G.; Mendez,R.H.; Medupe,R.; Costero,R.; Birch,P.V.; and 7 coauthors,A&A,320,125H,1997 Handler,G..,A&AS,135,57H,1999 Iben & Renzini, ARA&A,21, 271,1983 De Marco, Orsola; Bond, Howard E.; Harmer, Dianne; Fleming, Andrew J. ApJL, 93, 602, 2004 Soker, N.,ApJS,112,487,1997 Fig 1. The image of the young planetary nebula HB 12 taken through the narrowband filter [NII] with the HST WFPC2