The chemistry of compact planetary nebulae

Abstract

Abstract. We report high-sensitivity millimetre observations of several molecular species ( 13 CO, HCN, HNC, CN, HCO + and N 2 H + ) in a sample of compact planetary nebulae. Some species such as HCO + and CN are particularly abundant compared to envelopes around AGB stars or even interstellar clouds. We have estimated the following average values for the column densities ratios: CN/HCN ∼ 2.6, HCO + /HCN ∼ 0.5, and HNC/HCN ∼ 0.4. Thus, the chemical composition of the molecular envelopes in these compact PNe appears somewhat intermediate between the composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such as the Ring, M4-9, or the Helix). From observations of the CO isotopomers, we have estimated that the 12 C/ 13 C ratio is in the range 10 < ∼ 12 C/ 13 C < ∼ 40. These values are below those expected from standard asymptotic giant branch models and suggest non-standard mixing processes. The observed molecular abundances are compared to very recent modelling work, and we conclude that the observations are well explained, in general terms, by time-dependent gas-phase chemical models in which the ionization rate is enhanced by several orders of magnitude with respect to the average interstellar value. Thus, our observations confirm that the chemistry in the neutral shells of PNe is essentially governed by the high energy radiation from the hot central stars. The complexity of the chemical processes is increased by numerous factors linked to the properties of the central star and the geometry and degree of clumpiness of the envelope. Several aspects of the PN chemistry that remains to be understood are discussed within the frame of the available chemical models.

A Spectral Line Survey of NGC 7027 at Millimeter Wavelengths

“…The HCN/HNC ratio is estimated to be larger than 30. The abnormally low HNC abundance in NGC 7027 was also noted by Bachiller et al (1997) and Josselin & Bachiller (2003), who obtained N (HCN)/N (HNC) > 17. As proposed by Bachiller et al (1997), HNC may have been destroyed in NGC 7027 through…”

“…The emission feature at 227.2 GHz is only marginally above our limit of detection and may be a combination of the CN (N ¼ 2Y1; J ¼ 5/2Y1/2) and the C 3 H 2 (4 32 Y3 21 ) transitions. Other previous detections of CN in NGC 7027 include the J ¼ 3Y2 and J ¼ 1Y0 transitions at 340 and 114 GHz, respectively (Hasegawa & Kwok 2001;Bachiller et al 1997;Thronson & Bally 1986;Cox et al 1993;Josselin & Bachiller 2003).…”

“…This was supported by the observations of Herpin et al (2002) with the Infrared Space Observatory (ISO). However, a different conclusion was obtained by Josselin & Bachiller (2003), who suggested that most of molecules can survive during the transition from AGB to PN. Redman et al (2003) modeled the chemical evolution from AGB to PPN and PN considering the effect of clumpiness.…”

“…Unambiguously detected molecular species in NGC 7027 include H 2 , OH, CO, CO + , CH, CH + , HCN, HNC, HCO + , H 2 O, N 2 H + , CN, CS, C 2 H, C 3 H 2 , SiS, and their isotopomers. A systematic molecular-line observation of PNs by Bachiller et al (1997) and Josselin & Bachiller (2003) has found that the column densities of molecules observed in NGC 7027 are generally larger than those in other PNs. Compared with other PNs, NGC 7027 shows a low HNC abundance and an abnormally high N 2 H + (Bachiller et al 1997).…”

Molecules in bipolar proto-planetary nebulae

“…The 12 CO/ 13 CO is low (<10) in several PPNe (e.g., Kahane et al 1992) compared to 10 < 12 CO/ 13 CO < 25 in evolved PNe (Palla et al 2000;Bachiller et al 1997a), 10 < 12 CO/ 13 CO < 40 in compact PNe (Josselin & Bachiller 2003) and 10 12 CO/ 13 CO 50 in AGB stars (e.g., Kahane et al 1988;Greaves & Holland 1997;Schöier & Olofsson 2000). From the radiative transfer analysis 12 CO/ 13 CO ratios of 30 and 7 are derived for the PPNe IRAS 16594-4656 and b Signifies a non-detection, but the calculation of an upper limit to the fractional abundance has been made (see Table 7).…”