Narrowband Imaging and Spectrophotometry of Galactic Emission Nebulae Julien Chemouni Bach Independent Project Tufts University May 2009 Abstract The goal of this paper is to provide an introduction to the field of narrowband imaging of emission nebulae. We consider the emission properties of HII regions, planetary nebulae, and shock-excited nebulae (e.g. supernova remnants), particularly in the light of H-alpha, [SII], and [OIII]. We have carried out optical observations of these nebulae using the 0.6-m telescope at the Clay Center Observatory along with special narrowband filters. These observa- tions serve as the basis for explaining our observation methods and procedures in the context of narrowband imaging and spectrophotometry of emission nebulae. We begin by discussing the physical theory of emission nebula. We then look more directly at the specific factors involved in narrowband imaging and the observations and measurements required for any sort of analysis. We detail how measurements can be used to deduce specific nebular properties. Next, we discuss the instrumentation and filters used in this research and the observing strategy. We present electronic imaging data obtained of the HII region M43 and proceed to demonstrate the process of data reduction using the Image Reduction and Analysis Facility (IRAF) suite of software. Reduction steps include using dark, bias, and flat-field frames to remove instrumental artifacts from the target frames. We conclude by discussing further analysis that can be conducted in order to obtain spectra, combined images and final scientific results. 1 Contents 1 Introduction 3 2 Spectrophotometric Properties of Ionized Emission Nebulae 4 2.1 Photoionized Nebulae (HII Regions and Planetary Nebulae) . 6 2.1.1 HII Regions . 6 2.1.2 Planetary Nebulae . 12 2.1.3 Extinction by Dust . 12 2.2 Shock Excited Nebulae . 13 3 Observational Investigation 15 3.1 Optical Narrowband Imaging . 15 3.1.1 Introduction to Narrowband Imaging . 15 3.1.2 General Measurement Techniques . 16 3.1.3 Hα - λ6563 . 18 3.1.4 [SII] - λ6716; 6731 . 19 3.1.5 [OIII] - λ5007 . 19 3.2 Instrumental Configuration . 19 3.2.1 The Telescope . 20 3.2.2 The Filters . 20 3.2.3 The Detector . 20 3.2.4 Data Collection Software . 20 3.3 Observing Strategy . 22 3.3.1 Targets, Exposure Times, and Calibrators . 22 3.3.2 Signal Correction . 23 3.4 Observation Log . 27 4 Data Reduction 27 5 Further Analysis 29 5.1 Spectrophotometric Results . 29 5.2 Analytic Results . 35 6 Acknowledgements 36 2 1 Introduction Any type of observational study in astronomy involves a series of incremental steps. A research plan must first be made, in which the general goal of the research is specified, along with the data needed. Sufficient knowledge is needed to properly assess the steps and data required to accomplish the goal, obtained either through previous experience or literature. Data must then be acquired, usually involving a combination of literary investigations and new data collection. The data collection process first requires locating and obtaining a means of observing the target, such as a telescope or an antenna. Then, the targets can be observed and data is recorded. For optical imaging studies1, this typically involves a series of nighttime observations that can last from minutes to days. At this point, a set of raw measurements is obtained. These unprocessed images must be corrected via a sequence of image reduction steps to account for a variety of signal-altering effects. From the final reduced images, quantitative data can be extracted in a variety of useful forms and rigorously examined and manipulated. In this final stage, the research goal can be directly addressed and scientific results are produced. This paper presents an introduction to optical narrowband imaging of emission nebulae in the Milky Way. We do this by describing our research process step by step, with explanations at each point. We thus aim to present the research process in a straightforward manner, such that it can be used as a reference or manual. We do this with strong emphasis on the initial stages of the research process and focus primarily on data acquisition and initial image reduction. In this way, those seeking to conduct narrowband imaging will understand the theory and methods involved in such a study, and be able to replicate the steps shown here or extrapolate them to their own research. Narrowband imaging is most useful in studying emission nebulae and we focus on ob- serving the prominent emission lines of Hα (λ6563), [SII](λ6716; 6731), and [OIII](λ5007).2 The first section of the paper discusses the physics of emission nebulae, namely HII regions and shock excited nebulae. Great care is taken to ensure that any scientific terms are properly defined and we attempt to keep the discussion of the theory within the context of narrowband imaging. Section 3 details our plan of investigation. We explain why narrowband imaging is useful, and how to take advantage of it to produce scientific results. We then proceed to describe the equipment and software that will be used for the data collection, reduction, and analysis process. Our data collection took place at the Clay Center Observatory, and the reduction and analysis were performed using the Image Reduction and Analysis Facility (IRAF) suite of software packages created at the National Optical Astronomy Observatory (NOAO). The final part of this section outlines our observing strategy, including target 1Optical here refers to light in the portion of the electromagnetic spectrum that is visible to the human eye (typically, 380nm-750nm). 2The brackets denote emission lines from forbidden electronic transitions, as described in more detail in Section 2.1.1. The wavelength notation denotes angstroms. 3 selection and exposure times. We also explain what is involved in correcting our electronic images and what data is needed to do so. Section 4 is one of the most important sections. Here we go through the steps of our data reduction using IRAF. Command lines are written out for the reader with an explanation of their purpose at each step. Selected results are shown. In the final section we briefly discuss what the next steps in data analysis and research would be. We examine what types of questions we would want to answer with our data and refer to the appropriate method by which to do so. 2 Spectrophotometric Properties of Ionized Emission Neb- ulae Throughout the Milky Way, massive shining clouds of ionized gas can be observed at opti- cal wavelengths. These regions of interstellar gas, called emission nebulae, have unique properties that set them apart from other astronomical objects. Two main types of emis- sion nebulae are photoionized nebulae (which include HII regions and planetary nebulae) and shock excited nebulae (which include stellar-wind bubble and supernova remnants), differentiated from each other by the origin of their emission spectra. These spectra consist of line emissions (increased radiation at one particular wavelength) superimposed over an underlying continuum. The processes behind line emission will be described in detail for each type of nebula, so we briefly explain continuum emission before moving on. Continuum emission refers to the release of photons over a continuous range of en- ergies of the electromagnetic spectrum. A star, idealized as a black body, has a continuum spectrum given by its blackbody curve. This continuum is generated primarily from the contribution of recombination processes, Bremsstrahlung emission, and two-photon emis- sion, as shown in Figures 2 and 3. We also note that scattered starlight from dust will further raise the continuum level. Continuum emission can include contributions from both thermal processes (arising from heated gas) and non-thermal processes (e.g., synchotron radiation from supernova remnants). The recombination process, also called free-bound emission, occurs when a free electron is captured by an ion and consequently decays, releasing photons. This process is described more fully in Section 2.1. Bremsstrahlung emission, or free-free emission occurs when a free electron travels through the electric field of a positively charged ion. The electric forces associated with this field cause the electron to decelerate and thus emit a photon. Two-photon emission occurs during forbidden transitions to the ground state. When the electron decays, it may release two photons, rather than one. The energy of both photons must sum to that of the single photon that would arise from the forbidden bound-bound transition.3 Thus a wide range of energies is available to each of the two photons and a continuum of photons is released. 3Bound-bound transitions are the change in energy levels of a bound electron that is temporarily excited and then decays. 4 Figure 1: The Orion Nebula (M42) imaged in the optical by the Hubble Space Telescope. This naked-eye nebula includes the smaller HII region M43-visible to the upper left in this image. (NASA/ESA) 5 Figure 2: Spectrum of NGC 7027, a planetary nebula. The dashed line is a synthesized theoretical spectrum of recombination line and continuum emission from ionized hydrogen and helium.The step near 4000A_ is due to the Balmer discontinuity, where free-bound contributions from captures of free electrons to the n=2 level are adding to the continuum. Near the Hα line, the continuum is a combination of two-photon emissions and lingering free-bound contributions to the n=3 level from the Paschen discontinuity near 8000A_. Scattered light from dust contributes throughout the spectrum. (Zhang et al. 2005) 2.1 Photoionized Nebulae (HII Regions and Planetary Nebulae) 2.1.1 HII Regions Emission nebulae in which O or early B-type stars power the interstellar gas are called HII regions.