IPHASX J052531.2+281946: a new PN towards the Galactic Anticenter

1. IPHASX J052531.2+281946: a new PN towards the Galactic Anticenter Mampaso, A.1, Viironen, K.1, Corradi, R. L. M.2, Rodríguez, M.3, and the IPHAS Collaboration (*) 1 IAC (Tenerife, Spain), 2 ING (La Palma, Spain), 3 INAOE (Puebla, México) Abstract and Conclusions A new Planetary Nebula (PNG 178.13-04.04) has been discovered in the course of an Hα survey of the Galactic Plane; it has been confirmed via optical spectra. It is near to the Anticenter direction: the third nearest known PN toward the Anticenter. Its small diameter, and low Hβ brigthness point to an extremely large distance; the "Shklovskii" galactocentric distance of Dgc =14-20 kpc renders this nebula as one of the farthest galactic PNe where chemical abundances has been measured. Observations of IPHASX J052531.2+281946 IPHAS detected several hundred strong Hα-emission candidates located in a 30 x 10 square degree region around the Anticenter. Most are spurious detections, stars, and known Hα-emitting objects; however some are "bona-fide" small nebula candidates that we are currently studying. One of the first nebulae to be discovered is IPHASX J052531.2+281946. Fig. 1 and 2 show the INT IPHAS Hα (top) and I (bottom) images of the object (N is up and E to the left. The size of the box is 1 arcmin). Spectra were obtained on October 2004 at San Pedro Mártir Observatory (Observatorio Astronómico Nacional, México) with the 2.1m telescope and the B&Ch low-resolution spectrograph (2 Å/pix from 3720 to 7470 Å; Fig. 2, top) plus the MESCAL medium-resolution echelle spectrograph (0.1 Å /pix at the Hα region; Fig. 3, bottom ). The IPHAS Survey The Isaac Newton telescope Photometric Hα survey (IPHAS; http://astro.ic.ac.uk/Research/Halpha/North/) is currently mapping 1800 square degrees of the Northern Galactic plane (a band between l= -5 to +5 degrees) using the INT Wide Field Camera at the Observatorio del Roque de los Muchachos (La Palma, Spain). The survey is an international collaboration among 13 institutes led by J. Drew at Imperial College (London, UK). IPHAS started in August 2003 and is expected to be completed at the end of 2006, with an estimated total of 30 observing weeks (mostly bright nights) shared by the three communities involved, UK, Spain and the Netherlands. A narrow-band Hα and two Sloan R, I filters are used for matched 120, 30, and 10 s exposures, respectively, spanning the range R=13 to 20 mag for point sources. IPHAS is the first fully-photometric Hα survey of the Galactic plane. It will discover around 40,000 new emission-line stars, including young stars (T Tau, Herbig AeBe stars, etc.), evolved ones (post-AGB, LBVs, etc.) as well as binaries (CVs, symbiotic stars, etc.) in addition to thousands of extended nebulae such as planetary nebulae (PNe), H-H objects, HII regions, SN remnants, etc., (Drew et al., 2005. MNRAS in press).  Physico-chemical analysis IPHASX J052531.2+281946 is a moderately reddened (Av=1.5 mag) low density (Ne[SII]=330 cm-3) planetary nebula. The electron temperatures are Te[NII]= 13,000 K and Te[OIII]= 10,000 K, the latter being more uncertain given the faintness of the [OIII] λ 4363 Å line. Ionic and total abundances are calculated with a two-zone analysis using the Ionization Correction Factor method as described by e.g. Kingsburgh and Barlow (1994; MNRAS 271, 257). Chemical abundances (12+log X/H) are typical of Peimbert's Type II PNe: He/H = 11.12; O/H = 8.55; N/H = 7.90; Ne/H = 8.19; S/H = 6.59; Ar/H = 6.19. Nebular expansion, systemic velocity and distance. The velocity-position diagram shows a 12 arcsec diameter shell expanding at 17 km/s. Its radial velocity, corrected for the Local Standard of Rest motion, is 13.5 km/s, i.e. near zero, as expected from the position of the PN, very close to the Anticenter direction. Obviously, kinematical distances are unreliable at this region. The distance to IPHASX J052531.2+281946 is unknown. A crude estimation can be attempted using the "Shklovskii" statistical method, assuming a constant mass for the ionized gas of the PNe. Total Hβ flux was measured (using a very wide slit at the B&Ch spectrograph) and dereddened with Av=1.5 mag, giving F(Hβ) = 1,7 10-13 erg/cm2 s. This locates the PN at D =12.2 kpc if an "average" nebular mass of 0.4 Mo is assumed (c.f. Pottasch, 1984, in “Planetary Nebulae”) or D =7.0 kpc if an extremely low mass of 0.1 Mo is adopted. The corresponding galactocentric distances are Dgc = 20, and 14 kpc, respectively. In any case, IPHASX J052531.2+281946 is located at the same galactocentric distance or farther than any previously known PN for which chemical abundances have been determined (Dgc =14.1 kpc; Costa et al. 2004; A&A 423, 199). (*) The IPHAS collaborators are: Bristol University (UK): M. Masheder, S. Phillipps, R. A. H. Morris. CASU, Cambridge (UK): M. Irwin, N. Walton. Granada University (Spain): A. Zurita. Harvard-Smithsonian Center forAstrophysics (USA): J. Drake, J. Sokoloski, D. Steeghs. Imperial College London (UK): J. Drew, Y. Unruh, E. C. Hopewell, J. S. Vink. Instituto de Astrofisica de Canarias (Spain) : A. Mampaso, K. Viironen, E. Martin, P. Rodríguez-Gil. Isaac Newton Group (Spain): R. Corradi, R. Greimel, P. Leisy, D. Lennon, I. Skillen. Macquarie University (Australia): Q. Parker. Nijmegen University (Netherlands): P. Groot. L. Morales-Rueda, G. Roelofs. Southampton University (UK): C. Knigge, A. Witham. University College London (UK) M. Barlow, A. Hales, N. Wright. UMIST (UK): A. Zijlstra, M. Matsuura. Warwick University (UK): B. Gaensicke, A. Aungwerojwit Acknowledgements: We thank J.A. López for kindly acquiring the MESCAL spectra for us. Background image: IPHAS mosaic of IC 1396B (credits: Nick Wright, UCL)