the abundance of HCO+ decreases by a ular the strong emission observed in the Baehiller, R., & G6mez-Gonzalez, J. 1992, factor of - 20 (Iglesias & Silk, 1978). The methanol lines suggest that these lines A&AR3,257. derived [HCO+]/[CO] ratios in the lobes can be used as powerful signposts of the Baehiller, R., Lieehti, S., Walmsley, C.M., & of Sandqvist 136 then give further sup­ chemical impact of bipolar outflows on Colomer, F. 1995, A&A 295, L51. port to the notion that the abundances the surrounding ambient medium. It ap­ Baehiller, R., Martfn-Pintado, J., & Fuente, A. in the lobes are a product of shock pears that the Sandqvist 136 shock pro­ 1991, A&A 243, L21. chemistry. Bally, J., & Lada, C.J. 1983, ApJ 265,824. duces the evaporation of icy grain man­ Bally, J., Lada, E., & Lane, A.D. 1993, ApJ In addition to the wing emission detect­ tl es resulting in the injection into the gas 418,322. ed toward the lobes of Sandqvist 136 in phase of large amount of ice mantle con­ Bourke, TL., Garay, G., Lehtinen, K.K., Köh• the lines of SiO and CH 30H, we note the stituents, such as methanol. Further, the nenkamp, 1., Launhardt, R, Nyman, L.-A., presence of emission at velocities com­ shock seems to be sufficiently powerful May, J., Robinson, G., & Hyland, A.R. parable to the systemic velocity of the that refractory dust grains are partially 1996, ApJ submitted. globule. In particular for silicon monox­ destroyed, liberating into the gas phase Bourke, TL., Hyland, A.R., Robinson, G., ide, emission in the velocity range of the a significant amount of Si atoms that are James, S.o., & Wright, C.M. 1995, MNRAS 276,1067. ambient cloud is observed only at the later converted to SiO by ion-molecule Charnley, S.B., Tielens, A.G.G.M., & Miliar, P?sition of the lobes. This can be appre­ reactions and/or shock chemistry. Final­ TJ. 1992, ApJ 399, L71. clated in Figure 6 which shows velocity­ Iy, we find that the SiO and CH30H Fukui, Y., Iwata, T, Mizuno, A., Bally, J., & position diagrams of the emission in the emission detected toward the lobes not Lane, A. P. 1993, in Protostars and Planets SiO lines along the symmetry axis of only traces shocked outflowing gas but 1/1, eds. E.H. Levy & J. Lunine (Tueson: the outflow. The strength of the emis­ also ambient medium gas that has been Univ. Arizona Press), 603. sion in the red lobe is roughly constant heated by the UV radiation from the hot Lada, C.J. 1985, ARA&A23, 267. with velocity, with peaks at -4.1 and -0.8 post shock regions. Lada, C.J., & Fieh, M. 1996, ApJ in press. km S-l, while in the blue lobe the emis­ It has been suggested that the strength Hartquist, TW., Oppenheimer, M., & Dalgar- sion peaks at a velocity of -5.0 km s-1, of the emission in diverse trace mole­ no, A. 1980, ApJ 236, 182. Iglesias, E.R. & Silk, J.1978, ApJ226, 851. close to the ambient cloud velocity. This cules might be considered an indicator of Martfn-Pintado, J., Baehiller, R, & Fuente, A. result suggests that the enhancement of the evolutionary stage of bipolar outflows 1992, A&A 254,315. SiO and CH 30H molecules might be due (Bachiller & G6mez-Gonzalez, 1992). 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We Baehiller, R., Cernieharo, J., Martin-Pintado, Zhang, Q., Ho, P.TP., Wright, M.C.H., & ?onclude that the shocks created by the J., Ta/alla, M., & Lazareff, B. 1990, A&A Wilner, D.J. 1995, ApJ 451, L71. Interaction between flows and the sur­ 231,174. rOunding medium play a major role in the Baehiller, R, Fuente, A., & Ta/alla, M., 1995, E-mail address: production of these molecules. In partic- ApJ 445, L51. [email protected] (Guido Garay) On the Optical Emission of the Crab Pulsar EP. NASUT/l, 3, R. MIGNANll, PA. CARAVEOI and G.F. BIGNAMP,2 I/Stituto di Fisica Cosmica deI CNR, Mi/an, Ita/y; 2Dipartimento di /ngegneria /ndustria/e, Universita di Cassino, Ita/y; 3Universita deg/i Studi di Mi/ano, Ita/y 1. Introduction tected at optical wavelengths. It is identi­ summer of 1054. The identification has fied with a star ( V - 16.5) near the cen­ been confirmed by the discovery of The Crab Pulsar (PSR0531 +21) was tre of the Crab Nebula, the remnant of pulsed optical emission at the radio peri­ the first Isolated Neutron Star (INS) de- the supernova explosion observed in the od (Cocke et al., 1969). 37 A. cA) sands of times brighter than Geminga (V 25.5) and PSR0656+14 (V 25). 7000 6500 6000 5500 5000 = = -25 .-----'---I----I----I---~'----+----I----I----I---__, Therefore, it is the only INS within reach of optical spectroscopy. Nevertheless, our knowledge of the optical spectrum of the Grab Pulsar rests mainly on the pio­ neering observations of Oke (1969), and on multicolour photometry (Kristian et al., 1970; Middleditch et al., 1987; Per­ cival et al., 1993). Thus, it seemed ap­ 8 propriate to bring the knowledge of the e> Grab optical emission up to modern as­ ~ tronomy standards. LL: Clo ..J 2. The Observations A 40-minute spectrum of the Grab -26 +------t-------j------+-------+--.....l Pulsar was taken on January 1991 with 14,62 14,66 14,7 14,74 14,78 the ND. The telescope was equipped Log v (Hz) with the ESO Multi Mode Instrument (EMMI) (Melnick, Oekker and O'Odo­ Figure 1: Spectrum of the Grab Pulsar in the wavelength range 4900- 7000 Aas measured with rico, 1991) mounting a "Red" THX 10242 EMMI after a 40-min exposure. The spectrum has been sky subtracled and correcled for inter­ GGO detector. The instrument was oper­ stellar absorption. The (Jux distribution is modelled by apower law with a best fitting spectral ated in the Red Medium Dispersion index a = -0.10 ± 0.01. A broad (. A. - 100 A) absorption feature cenlred on A. = 5900 A is Mode (REMD), with a projected pixel visible. size of 0.44 arcsec. A medium disper­ sion grating blazed at 6200 A was used, providing a spectral resolution of For almost 10 years, the Grab was the However, the Grab remains by far the 2.1 Npixel in the wavelength range only INS seen at optical wavelength. brightest of the optically emitting INSs, 4900-7000 A. According to the seeing The Vela pulsar was observed in 1976 hundreds of limes brighter than conditions (- 1.2 arcsec) the Pulsar was (Lasker, 1976) and PSR0540-69 in 1985 PSR054069 (V = 22.4), Vela (V = 23.6) centred in a 1.5 arcsec slit, with the long (Middleditch and Pennypacker, 1985).