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Scattered Light Systematically Distorts Conditions Derived from Emission Lines∗ C

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Scattered Light Systematically Distorts Conditions Derived from Emission Lines∗ C The Astronomical Journal, 140:985–1006, 2010 October doi:10.1088/0004-6256/140/4/985 C 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ! SPECTROPHOTOMETRY OF THE HUYGENS REGION OF THE ORION NEBULA, THE EXTENDED ORION NEBULA, AND M 43: SCATTERED LIGHT SYSTEMATICALLY DISTORTS CONDITIONS DERIVED FROM EMISSION LINES∗ C. R. O’Dell1 and Jessica A. Harris2 1 Department of Physics and Astronomy, Vanderbilt University, Box 1807-B, Nashville, TN 37235, USA; [email protected] 2 Department of Physics, Fisk University, 1000 17th Avenue, Nashville, TN 37208, USA; [email protected] Received 2010 June 8; accepted 2010 August 4; published 2010 September 8 ABSTRACT We report on medium resolution spectrophotometry of the Orion Nebula region, including for the first time the Extended Orion Nebula (EON) and the nearby M 43. The 49 long-slit observations were divided into 99 smaller samples, which have allowed determinations of the amount of extinction, extinction-corrected Hβ surface bright- ness, electron temperatures (from [S ii], [N ii], and [O iii]), and electron densities (from [S ii] and [Cl iii]) throughout much of this complex region. We verify an earlier conclusion from a radio/optical study that beyond about 5# from θ 1Ori C local emission begins to be contaminated by scattering of light from the much brighter central Huygens region of M 42, and this scattered light component becomes dominant at large distances. This contamination means that the derived properties for the outer regions are not accurate. From comparison of the light from the dominant star in M 43 with the continuum of that nebula (which is almost entirely scattered starlight), it is determined that scattered light is enhanced in the blue, which can lead to observed Balmer line ratios that are theoretically impossible and erroneous electron temperatures. This blue scattering of emission lines is important even in the Huygens region because it means that at anything except very high spectroscopic resolution the observed lines are a blend of the original and scattered light, with shorter wavelength lines being artificially enhanced. This can lead to overestimates of the electron temperatures derived from the nebular and auroral line ratios of forbidden lines. This phenomenon is probably applicable to many other H ii regions. We have been able to use extinction-insensitive line ratios, the extinction-corrected surface brightness in Hβ, and the equivalent width of the continuum to create for the first time a three-dimensional model of the entire M 42, EON, and M 43 region. This is an irregular concave blister of ionized gas bounded on the outside by apparent walls where the ionization front has curved almost to the direction of the ob- server. M 43 is seen to be shielded from illumination by θ 1Ori C by the northeast portion of the wall bounding M 42. Key words: ISM: individual objects (Orion Nebula, NGC 1976, NGC 1982, M42, M43) 1. INTRODUCTION Because the EON is dominated by regions about two orders of magnitude lower surface brightness than the Huygens re- The Orion Nebula (NGC 1976 and M 42) is commonly gion, it has not been the subject of detailed spectroscopic study. thought of as a small H ii region, the bright Huygens region Whereas there have been many high spectral resolution spec- (Gingerich 1982) being only about 0.7 pc (5# at a distance trophotometric studies of the Huygens region, e.g., Baldwin of 440 pc (O’Dell & Henney 2008), the distance adopted et al. (2000), Esteban et al. (2004), and one Fabry–Perot study in this study). However, when one includes the much fainter of the 3727 Å [O ii] doublet of the entire nebula (Caplan 1972), Extended Orion Nebula (EON, a term introduced by Gudel¨ only a few lower resolution studies have been made beyond a few et al. 2008), it becomes comparable to other Galactic and arcminutes from θ 1Ori C (Peimbert 1967; Peimbert & Costero extragalactic H ii regions since the angular size of about 33.3 1969; Peimbert &Torres-Peimbert 1977; Simpson 1973; # × 23.#8 corresponds to about 4.3 pc 3.1 pc. The Huygens Baldwin et al. 1991; O’Dell & Goss 2009) and none of these region occupies the northeast corner× of the EON and has cover the full EON. In the study described in this paper, we cor- been frequently imaged in narrow-band emission-line filters rect that lack of data of this important region. The smaller and at Hubble Space Telescope (HST) resolution (O’Dell & Wong fainter M 43 has been the subject of many fewer investigations 1996), whereas the EON has largely been the provence of skilled (Peimbert & Costero 1969; Thum et al. 1978; Khallesse et al. amateur astronomers. Much of the EON has been imaged by 1980;Hanel¨ 1987; Smith et al. 1987; Rodr´ıguez 1999) but its the HST with filters typically passing several emission-lines dominance by one star, NU Ori, makes it potentially an easier (Henney et al. 2007), while an excellent example of a full- object to analyze. field ground telescope image is that of Robert Gendler, M.D. We adopt the blister model for the Huygens region. This (http://www.robgendlerastropics.com). The wide field of view model posits that the emission lines arise from a relative layer images often include the nearby low-ionization H ii region M 43 of ionized gas lying on the side of the Orion molecular cloud (NGC 1982) which lies to the northeast of M 42 and this object that faces the observer (Zuckerman 1973; Balick et al. 1974). has not been well studied but is included in this investigation. A key distinguishing feature of this model is the difference of velocities of different species (summarized in O’Dell et al. 2009a), with the low-ionization ions that give rise to [S ii] emission originating from near the main ionization front on ∗ Based in part on observations obtained at the Cerro Tololo Inter-American Observatory, which is operated by the Association of Universities for Research the observer’s side of the dense photon dominated region (PDR) in Astronomy, Inc., under a cooperative agreement with the National Science having little blueshift with respect to the parent molecular cloud. Foundation. 985 986 O’DELL & HARRIS Vol. 140 Emission in the H++Heo zone, such as [N ii] (O’Dell 1998), is (Section 3), and present useful diagnostic observed properties slightly blueshifted, while emission in the H++He+ zone, such (Section 4). We then use these new observations to determine as [O iii] is the most blueshifted as the hot gas expands away the 3D structure of the EON and come to important conclusions from the main ionization front. The spectral type O7Vp θ 1Ori about the role of wavelength-dependent internal scattered light C is the dominant source of ionizing photons and lies about in determining the properties and abundances in this and other 0.2 pc (O’Dell et al. 2009a) on the observer’s side of the main H ii regions. ionization front (this would correspond to a distance in the plane 2. OBSERVATIONS of the sky of 1.#56). In the foreground is a multi-component Veil of nearly neutral material (van der Werf & Goss 1989) that lies The intent of this program was to obtain moderate resolution about 1–3 pc in the foreground (Abel et al. 2006; O’Dell et al. spectroscopic data over a wider range of samples within the 2009a). This Veil produces most of the extinction in this nearby, Huygens region and the EON with the goal of providing accurate high Galactic latitude ( 20◦), H ii region (O’Dell et al. 1992). line ratios for the primary diagnostic lines. This intent was − It is highly variable in optical depth, this being the greatest in expanded to also include a smaller sampling of data in Messier the highly obscured Dark Bay feature lying immediately to the 43, which is associated with the B 0.5 V star NU Ori (Schild east of the Trapezium stars. & Chaffee 1971). The Boller and Chivens spectrograph on the Ferland (Baldwin et al. 1991) pointed out that a flat blister 1.5 m telescope at the Cerro Tololo Inter-American Observatory model would vary in surface brightness in the Hβ line approx- (CTIO) was nearly ideal for this purpose and was made available 2 1 imately as D− , where D is the distance from θ Ori C in the to the authors through the SMARTS consortium. plane of the sky, in approximate agreement with the limited data then available. This approximation holds only very near 2.1. The Spectroscopic Observations the sub-stellar point and a better approximation is D 3 since the − Observations were made in three sets during the 2008 and photons strike the plane more obliquely with distances and the 2009 observing seasons for the Orion Nebula region. This time flux in Lyman continuum (LyC) photons decreases correspond- was nearly ideal for observing from CTIO since this was during ingly. The fact that over greater distances the surface brightness the summer and conditions were photometrically clear during decreases more rapidly than D 2 indicates that a more complex − almost all the scheduled time. In order to determine the best model applies. Determination of the location of the ionization combination of wavelength coverage and spectral resolution, front from the apparent surface brightness was expanded upon two first-order gratings were employed, G58 giving a scale of by Wen & O’Dell (1995), who did a detailed general solution 2.2 Å pixel 1 was used on 2008 November 19–23 and G09 and derived a model of the Huygens region that is an irregular − giving 2.7 Å pixel 1 was used on 2008 November 25 and 2009 concave surface, that drops away from the observer behind the − January 16, December 9–15.
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