Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...453..761t&link_type=abstract
Astrophysical Journal v.453, p.761
Astronomy and Astrophysics
Astronomy
13
Stars: Circumstellar Matter, Ism: Individual Alphanumeric: M2-9, Ism: Planetary Nebulae: Individual Alphanumeric: M2-
Scientific paper
We present optical spectropolarimetry of the young bipolar planetary nebula M2-9. The goal of these observations is to determined the origin of the knots or brightness enhancements seen in the lobes of M2-9. The line spectra of the lobes of M2-9 are composed of two components, one that is produced locally in the lobes and one that is scattered from deep in the nebula. The presence of this scattered radiation means that the total flux line ratios do not accurately describe the local conditions in the lobes. We have obtained spectropolarimetric data of the N2 and S2 knots and the adjacent nebula, and we use our data to separate the scattered and unscattered emission-line components.
The spectrum of the core of M2-9 exhibits broad Hα emission lines. In our high-resolution spectra we observe a broad wing on the scattered Hα line profile at all of the positions in the north lobe. This confirms that the scattered line emission originates in the core. In addition, we calculate the outflow velocity of the scatterers, ≍15 km s-1, based on the observed wavelength shift between the Hα peak in scattered and unscattered flux.
Using the unscattered spectra, we derive the local line ratios as a function of position in the north and south lobes. The degree of ionization of the spectra decreases in the off-knot regions. We measure the gas temperature as a function of position and find that it is approximately constant across the lobes. This result rules out the simple recombination tail model proposed by Goodrich for the origin of the knots. We suggest instead that the off-knot positions are ionized by a UV spectrum that is attenuated by material between the off-knot positions and the central UV source. We have used the photoionization code CLOUDY to test this idea and find that attenuation effects alone cannot accurately reproduce the observed unscattered line ratios. To accurately model the observed line ratios in the knots, we require the presence of both high (≥ 105 cm-3) and low (≍ 103 cm-3) density components. For the off-knot positions both attenuation effects and multiple density components are necessary to reproduce the observed line ratios.
We modify the recombination tail model proposed by Goodrich to explain the origin of the knots in M2-9. We propose, as did Goodrich, that at discrete north-south levels, the lobes of M2-9 contain "rings" of higher density material. The UV source illuminates a section of the lobe, causing these rings of material to glow, producing the knots and explaining and naturally leading to their "fixed" north-south positions. Obscuring material near the star keeps one side of the lobe from being illuminated by the full force of UV source, thus explaining the lower excitation off-knot spectra. This model can explain why the knots appear to move only in the east-west plane, while remaining fixed in the north-south direction. In addition, it explains why the observed spectra in the off-knot regions are lower excitation than the knot spectra.
Dinerstein Harriet L.
Goodrich Robert W.
Trammell Susan Rebecca
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