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Which Stars Are Ionizing the Orion Nebula? C University of Kentucky UKnowledge Physics and Astronomy Faculty Publications Physics and Astronomy 3-15-2017 Which Stars Are Ionizing the Orion Nebula? C. R. O'Dell Vanderbilt University W. Kollatschny Universität Göttingen, Germany Gary J. Ferland University of Kentucky, [email protected] Right click to open a feedback form in a new tab to let us know how this document benefits oy u. Follow this and additional works at: https://uknowledge.uky.edu/physastron_facpub Part of the Astrophysics and Astronomy Commons, and the Physics Commons Repository Citation O'Dell, C. R.; Kollatschny, W.; and Ferland, Gary J., "Which Stars Are Ionizing the Orion Nebula?" (2017). Physics and Astronomy Faculty Publications. 491. https://uknowledge.uky.edu/physastron_facpub/491 This Article is brought to you for free and open access by the Physics and Astronomy at UKnowledge. It has been accepted for inclusion in Physics and Astronomy Faculty Publications by an authorized administrator of UKnowledge. For more information, please contact [email protected]. Which Stars Are Ionizing the Orion Nebula? Notes/Citation Information Published in The Astrophysical Journal, v. 837, no. 2, 151, p. 1-7. © 2017. The American Astronomical Society. All rights reserved. The opc yright holder has granted the permission for posting the article here. Digital Object Identifier (DOI) https://doi.org/10.3847/1538-4357/aa6198 This article is available at UKnowledge: https://uknowledge.uky.edu/physastron_facpub/491 The Astrophysical Journal, 837:151 (7pp), 2017 March 10 https://doi.org/10.3847/1538-4357/aa6198 © 2017. The American Astronomical Society. All rights reserved. Which Stars Are Ionizing the Orion Nebula? C. R. O’Dell1, W. Kollatschny2, and G. J. Ferland3 1 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235-1807, USA 2 Institut für Astrophysik, Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany 3 Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA Received 2017 January 23; revised 2017 February 10; accepted 2017 February 14; published 2017 March 15 Abstract The common assumption that q1 Ori C is the dominant ionizing source for the Orion Nebula is critically examined. This assumption underlies much of the existing analysis of the nebula. In this paper we establish through comparison of the relative strengths of emission lines with expectations from Cloudy models and through the direction of the bright edges of proplyds that q2 Ori A, which lies beyond the Bright Bar, also plays an important role. q1 Ori C does dominate ionization in the inner part of the Orion Nebula, but outside of the Bright Bar as far as the southeast boundary of the Extended Orion Nebula, q2 Ori A is the dominant source. In addition to identifying the ionizing star in sample regions, we were able to locate those portions of the nebula in 3D. This analysis illustrates the power of MUSE spectral imaging observations to identify sources of ionization in extended regions. Key words: H II regions – ISM: individual objects (Orion Nebula) – stars: individual (θ1OriC, θ2OriA) 1. Introduction photons s-1 (Badnell et al. 2015). For q2 Ori A at O9.5 V (Warren & Hesser 1977), we use values from Table 2.3 of The central region of the Orion Nebula (NGC1976) is Osterbrock & Ferland (2006) of T =34,600 K and arguably the best-studied H II region (O’Dell et al. 2008).As star Q =3.63×1048 photons s-1. We express distances in the the exemplar of its class of gaseous nebulae, it has also been LyC plane of the sky in parsecs, using the adopted distance. subject to many attempts at modeling the physics that occurs and the 3D structure. We understand that it is basically an irregular concave thin layer of ionized gas lying nearer the 2. Using Line Ratios to Identify the Ionizing Star ( ) observer than the main ionization front MIF that marks the The ionized layer on the observer’s side of the MIF is boundary with the host background molecular cloud. There is stratified into layers according to the varying ionization. These ( ) also a nearby foreground thin layer the Veil of partially layers are determined by the energy of the stellar photons ionized gas. reaching them, and this energy distribution is controlled by the The usual assumption is that ionization in this region is absorption of the ions of the most common elements, hydrogen dominated by the brightest star in the compact Trapezium and helium. 1 group, the complex hot star q Ori C, whose spectral type is Close to the ionizing star hydrogen is ionized and oxygen is usually assigned as about O6. The next hottest and most doubly ionized (easily traced by the [O III] 500.7 nm line). 2 ″ luminous star is q Ori A, which lies 135 southeast of Helium is singly ionized (easily traced by the He I 667.8 nm 1 q Ori C. As one moves farther from the Trapezium, the degree line). This is the HeH+++ zone. For the relatively cool Orion of ionization decreases, consistent with the ionizing star or stars Nebula stars there is no higher ionization zone (where helium being in the central nebula (O’Dell & Harris 2010).Itis would be doubly ionized). important in interpreting the nebula’s spectra to understand Farther from the ionizing star and closer to the MIF is the whether a secondary star or stars play a role in ionization of the narrow HeHo + + zone, where helium is neutral and hydrogen nebula. is ionized. Nitrogen is singly ionized and is most visible in the The inner ionized nebula is actually very complex in [N II] 658.3 nm line. The easily visible Balmer Hα and Hβ structure, there being irregular features in the concave surface, lines arise from both ionization zones. the most famous of which is the Bright Bar, and there is a neutral cloud of material containing very young stars known as 2.1. The Basic Approach the Orion-South cloud (hereafter Orion-S) lying 55″ to the southwest of q1 Ori C. Interpreting the spectra and emission- We selected seven regions that were expected to illuminate line images of the nebula demands knowing the source of the question of the ionizing star in various parts of the Orion ionization for each region. That is the goal of this study. Nebula. These all fall within the region of calibrated Two approaches are adopted. In Section 2 we utilize the monochromatic images obtained with the MUSE (Weilbacher predictions of key tracers of different degrees of ionization. In et al. 2015) multi-aperture spectrograph. Reddening-corrected Section 3 we utilize resolved ionization fronts formed around line ratios from the MUSE database are used throughout this gas surrounding many of Orion’s proplyds. study. Figure 1 shows the location and sizes of these samples. Numerous assumptions have been made in this study. There They include regions near q1 Ori C and others on both sides of are many similar values for the distance to the Orion Nebula, the Bright Bar. but in this study we have adopted 388±8 pc from the recent The line ratios F([O III])/F(Hβ) and F(He I)/F(Hα) within radio results of Kounkel et al. (2017). For q1 Ori C, we have the HeH+++ zone and F([N II])/F(Hα) within the HeHo + + adopted a temperature (Tstar) of 38,950 K and total luminosity in zone are dependent on the temperature of the ionizing star and 48 photons capable of ionizing hydrogen (QLyC) of 7.35×10 a quantity defined as the ionization parameter U(Osterbrock & 1 The Astrophysical Journal, 837:151 (7pp), 2017 March 10 O’Dell, Kollatschny, & Ferland Table 1 Ionization Parametersa and Flux Ratios of the Seven Samples b 1 2 Sample ne logU(q Ori C) logU(q Ori A) F([O III] 500.7 nm)/F(Hβ) F([N II] 658.3 nm)/F(Hα) F(He I 667.8 nm)/F(Hα) A 6160 −0.95 −2.92 3.73 0.120 0.0129 B 4310 −0.24 −2.54 3.58 0.136 0.0126 C 2090 −1.56 −1.28 2.81 0.125 0.0132 D 1230 −1.76 −0.81 1.06 0.307 0.00812 E 1610 −1.71 −1.26 1.96 0.180 0.0118 F 1860 −1.58 −1.22 2.93 0.116 0.013 G 1970 −1.27 −1.70 3.03 0.129 0.0127 Notes. a The ionization parameters are upper limits, being calculated from the distance to the ionizing star as projected on the sky. b − Electron densities are in cm 3 and are derived from the red [S II] doublet ratio using MUSE fluxes. Ferland 2006), which is the ratio of the density of photons that or near edges of proplyds. In this case a more complex model can ionize hydrogen to the local hydrogen density (nH). We can can be developed (Shaw et al. 2009). For simplicity, in this study approximate the local hydrogen density as the electron density we use the Baldwin et al. (1991) approach and remain mindful (ne) since hydrogen is essentially completely ionized and the that it will break down in edge-on cases. »10% contribution from He I is small. We calculated the line ratios F([O III],500.7 nm)/F(Hβ), ( ) ( )/ ( α) ([ ] )/ ( α) We can calculate the photon density rLyC from the total F He I,667.8 nm F H , and F N II ,658.3 nm F H for a ionizing photon luminosity of the ionizing star (QLyC) and the variety of conditions. These assumptions are summarized in true distance to the emitting layer (r), and this leads to the Table 1.
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