DOCSLIB.ORG

Narrowband Imaging and Spectrophotometry of Galactic Emission Nebulae

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Narrowband Imaging and Spectrophotometry of Galactic Emission Nebulae 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.
Load more

Recommended publications
  • Ira Sprague Bowen Papers, 1940-1973
    http://oac.cdlib.org/findaid/ark:/13030/tf2p300278 No online items Inventory of the Ira Sprague Bowen Papers, 1940-1973 Processed by Ronald S. Brashear; machine-readable finding aid created by Gabriela A. Montoya Manuscripts Department The Huntington Library 1151 Oxford Road San Marino, California 91108 Phone: (626) 405-2203 Fax: (626) 449-5720 Email: [email protected] URL: http://www.huntington.org/huntingtonlibrary.aspx?id=554 © 1998 The Huntington Library. All rights reserved. Observatories of the Carnegie Institution of Washington Collection Inventory of the Ira Sprague 1 Bowen Papers, 1940-1973 Observatories of the Carnegie Institution of Washington Collection Inventory of the Ira Sprague Bowen Paper, 1940-1973 The Huntington Library San Marino, California Contact Information Manuscripts Department The Huntington Library 1151 Oxford Road San Marino, California 91108 Phone: (626) 405-2203 Fax: (626) 449-5720 Email: [email protected] URL: http://www.huntington.org/huntingtonlibrary.aspx?id=554 Processed by: Ronald S. Brashear Encoded by: Gabriela A. Montoya © 1998 The Huntington Library. All rights reserved. Descriptive Summary Title: Ira Sprague Bowen Papers, Date (inclusive): 1940-1973 Creator: Bowen, Ira Sprague Extent: Approximately 29,000 pieces in 88 boxes Repository: The Huntington Library San Marino, California 91108 Language: English. Provenance Placed on permanent deposit in the Huntington Library by the Observatories of the Carnegie Institution of Washington Collection. This was done in 1989 as part of a letter of agreement (dated November 5, 1987) between the Huntington and the Carnegie Observatories. The papers have yet to be officially accessioned. Cataloging of the papers was completed in 1989 prior to their transfer to the Huntington.
    [Show full text]
  • Messier Objects
    Messier Objects From the Stocker Astroscience Center at Florida International University Miami Florida The Messier Project Main contributors: • Daniel Puentes • Steven Revesz • Bobby Martinez Charles Messier • Gabriel Salazar • Riya Gandhi • Dr. James Webb – Director, Stocker Astroscience center • All images reduced and combined using MIRA image processing software. (Mirametrics) What are Messier Objects? • Messier objects are a list of astronomical sources compiled by Charles Messier, an 18th and early 19th century astronomer. He created a list of distracting objects to avoid while comet hunting. This list now contains over 110 objects, many of which are the most famous astronomical bodies known. The list contains planetary nebula, star clusters, and other galaxies. - Bobby Martinez The Telescope The telescope used to take these images is an Astronomical Consultants and Equipment (ACE) 24- inch (0.61-meter) Ritchey-Chretien reflecting telescope. It has a focal ratio of F6.2 and is supported on a structure independent of the building that houses it. It is equipped with a Finger Lakes 1kx1k CCD camera cooled to -30o C at the Cassegrain focus. It is equipped with dual filter wheels, the first containing UBVRI scientific filters and the second RGBL color filters. Messier 1 Found 6,500 light years away in the constellation of Taurus, the Crab Nebula (known as M1) is a supernova remnant. The original supernova that formed the crab nebula was observed by Chinese, Japanese and Arab astronomers in 1054 AD as an incredibly bright “Guest star” which was visible for over twenty-two months. The supernova that produced the Crab Nebula is thought to have been an evolved star roughly ten times more massive than the Sun.
    [Show full text]
  • Arxiv:1805.06071V2 [Astro-Ph.GA] 19 Nov 2018
    DRAFT: NOVEMBER 20, 2018 Preprint typeset using LATEX style emulateapj v. 01/23/15 THE KECK LYMAN CONTINUUM SPECTROSCOPIC SURVEY (KLCS): THE EMERGENT IONIZING SPECTRUM OF GALAXIES AT Z ∼ 31 CHARLES C. STEIDEL2 ,MILAN BOGOSAVLJEVIC´ 2,9 ,ALICE E. SHAPLEY3 ,NAVEEN A. REDDY4,5 ,GWEN C. RUDIE6 , MAX PETTINI8 ,RYAN F. TRAINOR7 ,ALLISON L. STROM2,6 DRAFT: November 20, 2018 ABSTRACT We present results of a deep spectroscopic survey quantifying the statistics of the escape of hydrogen-ionizing photons from star-forming galaxies at z ∼ 3. The Keck Lyman Continuum Spectroscopic Survey (KLCS) includes spectra of 124 galaxies with hzi = 3:05 ± 0:18 and -22:1 ≤ Muv ≤ -19:5, observed in 9 independent < fields, covering a common rest-wavelength range 880 ≤ λ0=Å ∼ 1750. We measure the ratio of ionizing to non-ionizing UV flux density h f900= f1500iobs, where f900 is the mean flux density evaluated over the range λ0 = [880;910] Å. To quantify h f900= f1500iout– the emergent ratio of ionizing to non-ionizing UV flux density – we use detailed Monte Carlo modeling of the opacity of H I in the intergalactic (IGM) and circumgalactic (CGM) medium as a function of source redshift. By analyzing high-S/N composite spectra formed from sub-samples exhibiting common observed properties and numbers sufficient to reduce the uncertainty in the IGM+CGM correction, we obtain precise values of h f900= f1500iout, including a full-sample average h f900= f1500iout = 0:057± 0:006. We further show that h f900= f1500iout increases monotonically with Lyα rest equivalent width Wλ(Lyα), inducing an inverse correlation with UV luminosity as a by-product.
    [Show full text]
  • Guide Du Ciel Profond
    Guide du ciel profond Olivier PETIT 8 mai 2004 2 Introduction hjjdfhgf ghjfghfd fg hdfjgdf gfdhfdk dfkgfd fghfkg fdkg fhdkg fkg kfghfhk Table des mati`eres I Objets par constellation 21 1 Androm`ede (And) Andromeda 23 1.1 Messier 31 (La grande Galaxie d'Androm`ede) . 25 1.2 Messier 32 . 27 1.3 Messier 110 . 29 1.4 NGC 404 . 31 1.5 NGC 752 . 33 1.6 NGC 891 . 35 1.7 NGC 7640 . 37 1.8 NGC 7662 (La boule de neige bleue) . 39 2 La Machine pneumatique (Ant) Antlia 41 2.1 NGC 2997 . 43 3 le Verseau (Aqr) Aquarius 45 3.1 Messier 2 . 47 3.2 Messier 72 . 49 3.3 Messier 73 . 51 3.4 NGC 7009 (La n¶ebuleuse Saturne) . 53 3.5 NGC 7293 (La n¶ebuleuse de l'h¶elice) . 56 3.6 NGC 7492 . 58 3.7 NGC 7606 . 60 3.8 Cederblad 211 (N¶ebuleuse de R Aquarii) . 62 4 l'Aigle (Aql) Aquila 63 4.1 NGC 6709 . 65 4.2 NGC 6741 . 67 4.3 NGC 6751 (La n¶ebuleuse de l’œil flou) . 69 4.4 NGC 6760 . 71 4.5 NGC 6781 (Le nid de l'Aigle ) . 73 TABLE DES MATIERES` 5 4.6 NGC 6790 . 75 4.7 NGC 6804 . 77 4.8 Barnard 142-143 (La tani`ere noire) . 79 5 le B¶elier (Ari) Aries 81 5.1 NGC 772 . 83 6 le Cocher (Aur) Auriga 85 6.1 Messier 36 . 87 6.2 Messier 37 . 89 6.3 Messier 38 .
    [Show full text]
  • Astro2020 Science White Paper Lyman Continuum Observations Across Cosmic Time: Recent Developments, Future Requirements
    Astro2020 Science White Paper Lyman continuum observations across cosmic time: recent developments, future requirements Thematic Areas: Galaxy Evolution Cosmology and Fundamental Physics Principal Author: Stephan R. McCandliss Johns Hopkins University, Department of Physics and Astronomy, Center for Astrophysical Sciences, 3400 North Charles Street, Baltimore, MD 21218 [email protected], 410-516-5272 Co-authors: Daniela Calzetti (astro.umass.edu), Henry C. Ferguson (stsci.edu), Steven Finkelstein (astro.as.utexas.edu), Brian T. Fleming (colorado.edu), Kevin France (colorado.edu), Matthew Hayes (astro.su.se), Tim- othy Heckman (jhu.edu), Alaina Henry (stsci.edu), Akio K. Inoue (aoni.waseda.jp), Anne Jaskot (astro.umass.edu), Claus Leitherer, (stsci.edu), Sally Oey (umich.edu), John O’Meara (keck.hawaii.edu), Marc Postman (stsci.edu), Laura Prichard (stsci.edu) Swara Ravindranath (stsci.edu), Jane Rigby (nasa.gov), Claudia Scarlata (astro.umn.edu), Daniel Schaerer (unige.ch), Alice Shapley (astro.ucla.edu), Eros Vanzella (inaf.it) Abstract: Quantifying the physical conditions that allow radiation emitted shortward of the hy- drogen ionization edge at 911.7 A˚ to escape the first collapsed objects and ultimately reionize the universe is a compelling problem for astrophysics. The escape of LyC emission from star-forming galaxies and active galactic nuclei is intimately tied to the emergence and sustenance of the meta- galactic ionizing background (MIB) that pervades the universe to the present day and in turn is tied to the emergence of structure at all epochs. The James Webb Space Telescope (JWST) was built in part to search for the source(s) responsible for reionization, but it cannot observe LyC escape directly, because of the progressive increase in the mean transmission of the intergalactic medium towards the epoch of reionization.
    [Show full text]
  • A Basic Requirement for Studying the Heavens Is Determining Where In
    Abasic requirement for studying the heavens is determining where in the sky things are. To specify sky positions, astronomers have developed several coordinate systems. Each uses a coordinate grid projected on to the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle (the fundamental plane of the geographic system is the Earth's equator) . Each coordinate system is named for its choice of fundamental plane. The equatorial coordinate system is probably the most widely used celestial coordinate system. It is also the one most closely related to the geographic coordinate system, because they use the same fun­ damental plane and the same poles. The projection of the Earth's equator onto the celestial sphere is called the celestial equator. Similarly, projecting the geographic poles on to the celest ial sphere defines the north and south celestial poles. However, there is an important difference between the equatorial and geographic coordinate systems: the geographic system is fixed to the Earth; it rotates as the Earth does . The equatorial system is fixed to the stars, so it appears to rotate across the sky with the stars, but of course it's really the Earth rotating under the fixed sky. The latitudinal (latitude-like) angle of the equatorial system is called declination (Dec for short) . It measures the angle of an object above or below the celestial equator. The longitud inal angle is called the right ascension (RA for short).
    [Show full text]
  • OBSERVING BASICS by GUY MACKIE
    OBSERVING BASICS by GUY MACKIE Observing Reports The colorful and detailed photographs we see of celestial objects are not at all like the ubiquitous "fuzzy blobs" we see at the eyepiece. Nevertheless, you are freezing your buns off and loosing much needed sleep for work, the next day so why not make a description of your observations that will make the hunt worthwhile. Here are some suggestions to fill the empty spaces in your logbook and to imprint the observing experience more deeply in your memory. The Basics Your website www.m51.ca has a downloadable log sheet template that is just super, but you can also make up one for yourself or customize the website version to your own needs. The main things to start your report should be the circumstances under which you observed: Observing Location Time (of observing session and of the observation of each object) Optics (type of instrument, eyepiece, filters, power of magnification) Transparency (page 56 of the Observers Handbook) Seeing (for me this is a subjective rating of the atmospheric stability based on Planet features and double star observations) It is good to know the field of view (FOV) of each of your eyepieces in minutes of degree, then you can estimate the approximate size of the object. The sketchpad I use has the FOV for every eyepiece I use taped to the back, a handy reference. To calculate your field of view there are websites that will punch out the both the magnification and the FOV for most eyepieces. You can do it yourself: With any motor drives turned off, place a star near the celestial equator just outside the field of view in the eyepiece so that it will drift across the middle of the field of view.
    [Show full text]
  • The Interstellar Medium
    The Interstellar Medium http://apod.nasa.gov/apod/astropix.html THE INTERSTELLAR MEDIUM • Total mass ~ 5 to 10 x 109 solar masses of about 5 – 10% of the mass of the Milky Way Galaxy interior to the suns orbit • Average density overall about 0.5 atoms/cm3 or ~10-24 g cm-3, but large variations are seen • Composition - essentially the same as the surfaces of Population I stars, but the gas may be ionized, neutral, or in molecules (or dust) H I – neutral atomic hydrogen H2 - molecular hydrogen H II – ionized hydrogen He I – neutral helium Carbon, nitrogen, oxygen, dust, molecules, etc. THE INTERSTELLAR MEDIUM • Energy input – starlight (especially O and B), supernovae, cosmic rays • Cooling – line radiation and infrared radiation from dust • Largely concentrated (in our Galaxy) in the disk Evolution in the ISM of the Galaxy Stellar winds Planetary Nebulae Supernovae + Circulation Stellar burial ground The The Stars Interstellar Big Medium Galaxy Bang Formation White Dwarfs Neutron stars Black holes Star Formation As a result the ISM is continually stirred, heated, and cooled – a dynamic environment And its composition evolves: 0.02 Total fraction of heavy elements Total 10 billion Time years THE LOCAL BUBBLE http://www.daviddarling.info/encyclopedia/L/Local_Bubble.html The “Local Bubble” is a region of low density (~0.05 cm-3 and Loop I bubble high temperature (~106 K) 600 ly has been inflated by numerous supernova explosions. It is Galactic Center about 300 light years long and 300 ly peanut-shaped. Its smallest dimension is in the plane of the Milky Way Galaxy.
    [Show full text]
  • Journal Für Astronomie ISSN 1615-0880
    Journal für Astronomie www.vds-astro.de ISSN 1615-0880 Nr. 75 4/2020 Zeitschrift der Vereinigung der Sternfreunde e.V. Infrarotastronomie ASTRONOMISCHE VEREINIGUNGEN Die Sternwarte St. Andreasberg KOMETEN Die Entwicklung von C/2020 F3 (NEOWISE) PLANETEN Die Sichtbarkeit von Venus im Frühjahr 2020 Canon EOS 250D & 2000D modifiziert für die Astrofotografie ! Ob mit Kameraobjektiv oder am Teleskop: Modizierte Canon EOS DSLR Kameras bieten Ihnen einen einfachen Einstieg in die Astrofotograe! Die Vorteile im Überblick: • etwa fünffach höhere Empfindlichkeit bei H-alpha und SII • Infrarot Blockung der Kamera bleibt vollständig erhalten • kein Einbau eines teuren Ersatzfilters • mit Astronomik OWB-Clip-Filter uneingeschränkt bei Tag nutzbar • auch ohne Computer am Teleskop einsatzfähig • 14 Bit Datentiefe im RAW-Format, 24 Megapixel • bei der 250Da: Erhalt des EOS Integrated Cleaning System • bei der 250Da: Dreh- und schwenkbarer Bildschirm • kompatibel mit vielen gängigen Astronomieprogrammen • voller Erhalt der Herstellergarantie Weitere Modelle auf Anfrage Wir bauen auch Ihre bereits vorhandene Kamera um! Canon EOS 250Da € 72037 * Canon EOS 2000Da € 54491 * * Tagespreis vom 07. August 2020 mit 200mm 1:4 Reflektor ©Bruno Mattern, Aufnahme Astronomik Deep-Sky RGB Farbfiltersatz Neuentwicklung! + optimierte Bildschärfe + höchster Kontrast + maximale Transmission = optimale Ergebnisse © Michael Sidonio Annals of the Deep Sky - A Survey of Galactic and Extragalactic Objects astro-shop Diese englischsprachige Buchreihe ist als mehrbändiges Werk angelegt, das Das seit zwei Jahrzehnten erfolgreiche Konzept die heutige Sicht auf alle Sternbilder des Himmels und die in Ihnen beobacht- hat inzwischen viele Nachahmer gefunden. baren Objekte darstellt. Deshalb achten Sie unbedingt darauf, dass Sie Die Gliederung erfolgt alphabetisch nach Sternbildern. Vergleichbar sind die auf der richtigen astro-shop Seite landen: Bücher mit "Burnham's Celestial Handbook" oder "Taschenatlas der Sternbilder“, eben aktualisiert auf den Wissens- und Technikstand von heute.
    [Show full text]
  • Environmental Effects on the Ionisation of Brown Dwarf Atmospheres M
    A&A 618, A107 (2018) Astronomy https://doi.org/10.1051/0004-6361/201832685 & c ESO 2018 Astrophysics Environmental effects on the ionisation of brown dwarf atmospheres M. I. Rodríguez-Barrera1,2, Ch. Helling1,2,3 , and K. Wood2 1 Centre for Exoplanet Science, University of St Andrews, UK e-mail: [email protected] 2 School of Physics & Astronomy, University of St. Andrews, St. Andrews KY16 9SS, UK 3 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands Received 22 January 2018 / Accepted 24 April 2018 ABSTRACT Context. Brown dwarfs emit bursts of Hα, white-light flares, and show radio flares and quiescent radio emission. They are suggested to form aurorae, similar to planets in the solar system, but much more energetic. All these processes require a source gas with an appropriate degree of ionisation, which, so far, is mostly postulated to be sufficient. Aims. We aim to demonstrate that the Galactic environment influences atmospheric ionisation, and that it hence amplifies or enables the magnetic coupling of the atmospheres of ultra-cool objects, like brown dwarfs and free-floating planets. Methods. We build on our previous work on thermal ionisation of ultra-cool atmospheres and explore the effect of environmental high-energy radiation on the degree of ionisation in the atmosphere. We consider the effect of photoionisation by Lyman-continuum radiation in three different environments: in the interstellar radiation field (ISRF), O and B stars in star-forming regions, and in white dwarf companions in binary systems. We apply our Monte Carlo radiation transfer code to investigate the effect of Lyman-continuum photoionisation for prescribed atmosphere structures for very low-mass objects.
    [Show full text]
  • Lyman-$Alpha $ Radiative Transfer in Star-Forming Galaxies
    Lyman-a radiative transfer in Star-forming galaxies Florent Duval Lyman-a radiative transfer in Star- forming galaxies Florent Duval Cover image: Composite image of Mrk1486, a nearby star-forming galaxy located at z ∼ 0.0338 and observed with the Hubble Space Telescope in 2012. Mrk1486 has a disk shape which is seen edge-on on this image. The green channel shows the UV con- tinuum emission from the young and massive stars of the galaxy. The red color shows the continuum-subtracted Ha emission, which traces the ionized hydrogen nebulae (as the result of star formation) in which both Ha and Lya photons are produced. Finally, the blue color shows the continuum-subtracted Lya emission of the galaxy. As most of the Lya-emitting galaxies identified in the Universe, the Lya emission of Mrk1486 is very extended and emerges into a large Lya-halo that surrounds the galaxy. This is the result of the resonant scattering process experienced by Lya photons on neutral hydrogen. Abstract This thesis focuses on the intrinsically strongest spectral signature of star- forming galaxies: the Lyman alpha recombination line of the hydrogen atom (hereafter Lya). Located at the wavelength of lLya = 1215.67 Å in the rest- frame far-ultraviolet spectra of star-forming galaxies, the Lya line proves to be a vital tracer and a powerful emission-line window to discover and to study the remote young star-forming galaxies of the early the Universe. Although intrinsically very strong, the Lya line is also a resonant line. As a consequence, the transport of Lya photons inside the interstellar medium (ISM) of star-forming galaxies is very complex and depends on many ISM quantities (HI mass, dust content, HI gas kinematics and ISM clumpiness).
    [Show full text]
  • The Physics of the Hii Regions: the Photoionization Equilibrium in the Hii Regions of M51
    THE PHYSICS OF THE HII REGIONS: THE PHOTOIONIZATION EQUILIBRIUM IN THE HII REGIONS OF M51 LEONEL GUTIERREZ´ 1,2, JOHN E. BECKMAN1,3 1 Instituto de Astrof´ısica de Canarias. C/V´ıa L´actea s/n, La Laguna, 38200, Tenerife (SPAIN) 2 Universidad Nacional Aut´onoma de M´exico, Apartado Postal 877, Ensenada 22800, Baja California (M´exico) 3 Consejo Superior de Investigaciones Cient´ıficas (Spain) Abstract: A general overview of the physical properties of the HII regions is pre- sented here, emphasizing the various general models made to explain their structure and behaviour. The Lyman continuum effective extinction, representing a measure of the extinguished or lost fraction of the ionizing flux, is explained briefly, and the results of a study performed on this matter are presented. This study, based on a sample of about 200 HII regions of the galaxy M51 using high resolution images acquired with the HST, suggests a highly inhomogeneous structure. Keywords: HII regions – Star formation rate – Extinction. 1 Introduction A simple measure of the star formation rate (SFR) in a galaxy is the Hα luminosity emitted by the ionizad gas in the galaxy. However, according to the evidence, the Hα luminosity is not completely satisfactory as a star formation indicator. Several factors, notably the extinction produced by the dust both of the Hα luminosity and of the Lyman continuum before it can ionize the gas, do not permit an exact measurement of the SFR using only the Hα luminosity. For highly obscured star-forming regions, a better tracer of SFR could be the total infrared emission (IR) [1], mainly the mid-IR.
    [Show full text]