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Oct 29, 2013 - The J = 1 → 0 transition of HCO+ at 89 GHz has been mapped across the Helix Nebula (NGC 7293) with 70 spatial resolution (1.68 km s. −1.
The Astrophysical Journal, 778:16 (9pp), 2013 November 20  C 2013.

doi:10.1088/0004-637X/778/1/16

The American Astronomical Society. All rights reserved. Printed in the U.S.A.

THE HELIX NEBULA VIEWED IN HCO+ : LARGE-SCALE MAPPING OF THE J = 1 → 0 TRANSITION 1

N. R. Zeigler1 , L. N. Zack1,3 , N. J. Woolf2 , and L. M. Ziurys1,2

Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, AZ 85721, USA; [email protected] 2 Department of Astronomy and Steward Observatory, Arizona Radio Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA Received 2013 August 2; accepted 2013 September 13; published 2013 October 29

ABSTRACT The J = 1 → 0 transition of HCO at 89 GHz has been mapped across the Helix Nebula (NGC 7293) with 70 spatial resolution (1.68 km s−1 velocity resolution) using the Arizona Radio Observatory 12 m telescope. This work is the first large-scale mapping project of a dense gas tracer (n(H2 ) ∼ 105 cm−3 ) in old planetary nebulae. Observations of over 200 positions encompassing the classical optical image were conducted with a 3σ noise level of ∼20 mK. HCO+ was detected at most positions, often exhibiting multiple velocity components indicative of complex kinematic structures in dense gas. The HCO+ spectra suggest that the Helix is composed of a bipolar, barrel-like structure with red- and blue-shifted halves, symmetric with respect to the central star and oriented ∼10◦ east from the line of sight. A second bipolar, higher velocity outflow exists as well, situated along the direction of the Helix “plumes.” The column density of HCO+ across the Helix is Ntot ∼ 1.5 × 1010 –5.0 × 1011 cm−2 , with an average value Nave ∼ 1 × 1011 cm−2 , corresponding to an abundance, relative to H2 , of f ∼ 1.4 × 10−8 . This value is similar to that observed in young PN, and contradicts chemical models, which predict that the abundance of HCO+ decreases with nebular age. This study indicates that polyatomic molecules readily survive the ultraviolet field of the central white dwarf, and can be useful in tracing nebular morphology in the very late stages of stellar evolution. +

Key words: astrochemistry – ISM: molecules – planetary nebulae: individual (NGC 7293) – radio lines: ISM Online-only material: color figures, machine-readable table

cometary-like tails (e.g., Meaburn et al. 1998; Speck et al. 2002; Meixner et al 2005; Hora et al. 2006; Matsuura et al 2009). It is estimated that about 30,000 of these cometary globules exist in this nebula. Millimeter-wave observations of CO have also been conducted. A map encompassing the region of the optical image in the J = 2 → 1 transition of this molecule has been made (Young et al.1999), as well as studies of individual knots (e.g., Huggins & Healy 1986). However, the CO emission was not detected beyond a radius of 500 from the central star, despite the presence of H2 emission extending faintly out to 1200 . It is not clear why CO has not been observed in the outermost regions. More complex molecules have also been detected in the Helix, including HCN, CN, HCO+ , CCH, C3 H2 , and H2 CO (Bachiller et al. 1997; Tenenbaum et al. 2009). Multiple line observations of H2 CO and HCO+ at nine positions in the nebula by Zack & Ziurys (2013) indicate that the polyatomic molecules exist in dense gas with n(H2 ) ∼ 0.1–5 × 105 cm−3 . Understanding the complex kinematic structure of the Helix Nebula has been the subject of numerous studies for several decades. Carranza et al. (1968), for example, proposed two possible geometries based on observations of Hα and [N ii]: a superposition of two concentric rings and a left-handed helix of constant pitch. A few years later, the helical shape was questioned by Warner & Rubin (1975), who favored a structure consisting of a highly ionized zone surrounded by two or three rings of low-ionization material. A study of [O i] emission by Taylor (1977) suggested an “elongated” helix, while Fabian & Hansen (1979) proposed a model composed of pieces of a helical coil. More recent mapping in the J = 2 → 1 transition of CO showed complex velocity structure in the Helix (Young et al. 1999), suggesting a shape consisting of an equatorial ring with adjoining shell-like arcs and filaments. Based on HST images, O’Dell et al. (2004) devised a model of the Helix composed of an inner disk with a diameter ∼499 , tilted 23◦ relative to the plane of the sky, and an outer ring or torus with a diameter

1. INTRODUCTION Planetary nebulae (PNe) are remnants of intermediate mass stars (∼1–8 M ), consisting of a detached envelope surrounding a 0.4–1.2 M white dwarf (e.g., Kwok 2000). Therefore, as such stars reach the end of their lifetimes, between 0.6 and 6.8 M of material are expelled into the interstellar medium (ISM). The ionized gas of a PN cannot contain the majority of this matter because it is only typically ∼0.1 M (e.g., Natta & Hollenbach 1998), suggesting that most of the stellar ejecta are in molecular or another neutral form. As a consequence, observations of molecular lines in these objects could be crucial in understanding nebular history. The Helix Nebula is the nearest large, bright PN, at a welldetermined distance of 216 ± 14 pc (Benedict et al. 2009). The central star is currently about twice the size of its final white dwarf stage, and has an effective temperature of about 110,000 K (G´orny et al. 1997; Napiwotzki 1999). The Helix has been extensively studied over a wide range of wavelengths with ground- and space-based telescopes. For example, at optical wavelengths, neutral and ionized atomic gas has been wellcharacterized by the Hubble Space Telescope (HST; e.g., O’Dell et al. 2004; Meixner et al. 2005). The optical image shows an apparent inner void out to a radius of r ∼ 200 from the central star, containing emission from He ii, which is surrounded by a bright, complex region luminous in spectral lines of cooler ionized atoms out to r ∼ 450 , the classic “Helix,” as well as an extensive faint envelope of about r ∼ 1200 or 1.2 pc (Speck et al 2002; O’Dell et al 2004). Observations of H2 and dust at infrared wavelengths from the Infrared Space Observatory, Spitzer Space Telescope, and other facilities have identified the presence of numerous globules in the Helix, often observed with 3 Current address: Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland.

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The Astrophysical Journal, 778:16 (9pp), 2013 November 20 

Zeigler et al.



Table 1 Observations of HCO+ in the Helix Nebula

of 742 , inclined at 53 . The inner disk was thought to be associated with the northwest and southwest plume structures. An “outermost” ring of diameter 1500 encompassing these inner regions was also proposed. Meaburn et al. (2005), on the other hand, dismissed the two tilted-ring model of O’Dell et al. On the basis of [N ii] and Hα spectra, they explained the structure of the Helix as a bipolar nebula, tilted 37◦ with respect to the line of sight, with a toroidal waist. One major obstacle in understanding the complex kinematic structure of the Helix is the lack of high resolution velocity data. With the exception of CO, observations of other lines (atomic and H2 ) were conducted with resolutions >10 km s−1 (e.g., Meaburn et al. 2005). Moreover, the lower rotational lines of CO do not trace the denser material. In order to provide additional insight into nebular structure, as well as define the extent and distribution of dense molecular gas, the J = 1 → 0 transition of HCO+ at 89 GHz has been observed across the Helix. Over 200 positions were measured in this transition with a velocity resolution of 1.68 km s−1 and an angular resolution of either of 70 (full beamwidth) or 35 (half beamwidth). HCO+ emission was detected at about 77% of the observed positions, resulting in a remarkable image resembling that at optical wavelengths. In this paper, these data are presented and their implications for the geometry of the Helix are discussed. The chemical significance of these observations is also evaluated.

Offseta

TR∗

ΔV1/2

VLSR

Ntot

α ( )

δ ( )

(mK)

(km s−1 )

(km s−1 )

(cm−2 )

525 525 525 525 455 455 455 455 455 455

−105 −35 35 105 −210 −140 −140 −70 −70 0

13.7 16.2

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