Proefstuderen Huiswerkopdracht Voorblad Sterrenkunde Voorjaar 2019
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Proefstuderen Sterrenkunde Huiswerkopdracht In dit document vind je het artikel Molecules from clouds to disks and planets van Prof.dr. Ewine van Dishoeck, hoogleraar Moleculaire Astrofysica bij de Universiteit Leiden. 1. Lees het artikel aandachtig. 2. Verzin een goede vraag die met het artikel te maken heeft. Wees nieuwsgierig en creatief! Let op: formuleer je vraag in het Engels! 3. Stuur je vraag uiterlijk maandag 25 maart 2019 per e-mail naar: [email protected]. Vermeld ook je voornaam en achternaam. De 3 beste vragen worden behandeld in het Sterrenkunde proefcollege tijdens Proefstuderen. De inzenders van de winnende vragen ontvangen een gesigneerd exemplaar van het boek Reisbureau Einstein van Prof. dr. Vincent Icke, emeritus hoogleraar Theoretische Sterren- kunde aan de Universiteit Leiden!* Reisbureau Einstein Wie Aarde voor gezien houdt en een kijkje wil nemen voorbij de sterren, kan terecht bij Reis- bureau Einstein. Met ruimteschepen die bijna zo snel gaan als het licht kunnen we misschien in de toekomst het hele Heelal doorkruisen. Naar onze sterrenburen, naar het centrum van de Melkweg, naar sterrenstelsels op een miljard lichtjaar afstand. Is er 'iemand' daarginds? * De prijs wordt alleen toegekend als de inzender van de winnende vraag ook daadwerkelijk aanwezig is bij het Sterrenkunde proefcollege tijdens Proefstuderen. Let op! Het hierboven genoemde e-mailadres is alléén voor het insturen van je vraag voor de Sterrenkunde huiswerkopdracht. Neem voor algemene vragen over Proefstuderen contact op met [email protected]. Volg voor eventuele afmeldingen de instructies die je hierover hebt ontvangen bij je inschrijving. Kavli 2018 Astrophysics Prize Lecture Molecules from clouds to disks and planets Ewine F. van Dishoeck Leiden Observatory, Leiden University, the Netherlands; E-mail: [email protected] and Max-Planck-Institute fur¨ Extraterrestrische Physik, Garching,Germany Abstract. This paper presents a brief overview of the physics and chemistry in regions where new stars and planets are formed. New telescopes at infrared and submillimeter wavelengths reveal a rich chemistry, including simple molecules like water, complex (organic) gases, ices, polycyclic aromatic hydrocarbons, and silicates. The journey of these molecules from dark clouds to the planet-forming zones of disks is described. Ultimately, they may become part of new planetary systems where they form the building blocks for life. The continued importance of the ‘golden triangle’ of observations, models and laboratory experiments to advance the field is emphasized. 1 Introduction When Fred Kavli looked up to the sky from his hometown Eresfjord in the mountains of Norway, he experienced ‘the world at its most magnificent’ and ‘pondered the mysteries of the Universe, the planet, nature, and man.’1 Many people across the world share Fred Kavli’s fascination with the night sky and want to know our place in the Universe, in particular the possibility for life elsewhere in space. But few people ask the question what actually lies in between the stars, and how they form or die. The space between the stars is not empty, but is filled with a very dilute gas, the so-called interstellar medium. The colder and denser concentrations of the gas are called interstellar clouds, and this is where molecules are detected and where new generations of stars like our Sun and planets like Jupiter or Earth are formed. With densities of 104 − 107 particles per cm3, interstellar clouds are still more tenuous than a typical ultra-high vacuum laboratory experiment on Earth. Thus, besides its astrophysical significance, interstellar space provides a unique environment in which chemistry can be studied under extreme conditions. This combination makes astrochemistry such a fascinating research field, for both chemists and astronomers alike. One of the most important developments in astronomy has been the discovery of exoplanets, i.e., planets orbiting stars other than our Sun. Although they were the subject of speculation for many centuries, the first exoplanet was found only in 1995 and study of these objects is now one of the most fast growing fields in astronomy. Today, nearly 4000 exoplanets have been discovered, and surveys have shown that on average every star has at least one planet orbiting around it. This begs the questions: how did these planetary systems form? Are they similar to our Solar System? How unique is the Solar System? And which of these planets could potentially host life, like our Earth? Thanks to a new generation of telescopes, we can now finally address these questions scientifically. Astrochemistry, also known as molecular astrophysics, is ‘the study of the formation, destruction and excitation of molecules in astronomical environments and their influence on the structure, dynamics and evolution of astronomical objects’ as stated by Dalgarno (2008) (the grandfather of this field and my PhD supervisor). This definition stresses not only chemistry but also the fact that molecules are excellent diagnostics of the physical conditions and processes in the regions where they reside. The main questions in astrochemistry therefore include: how, when and where are these molecules produced and excited? How far does this chemical complexity go? How are they cycled through the various phases of stellar evolution, from birth to death? And, most far-reaching, can they indeed become part of new planetary systems and form the building blocks for life elsewhere in the Universe? 1http://kavliprize.org/about/fred-kavli 1 Fred Kavli established prizes in three areas of interest, ’from the biggest to the smallest, to the most complex’. This lecture will touch on all three aspects: starting with the vast Universe and the gigantic interstellar clouds in galaxies, we will zoom into the smallest molecules and nanometer-sized dust particles, and end with the complex question of the origin of life. In fact, it is fascinating to realize that the macroscopic structures that we see in the sky such as clouds, stars and planets are to a large degree controlled by the microscopic processes within and between atoms and molecules. The contents of this lecture have been described in detail in recent reviews by myself (van Dishoeck, 2006, 2014a,b, 2018; van Dishoeck et al., 2013, 2014) and others Tielens (2013); Caselli & Ceccarelli (2012); Bergin et al. (2013); Herbst & van Dishoeck (2009), which contain references to the hundreds of papers that have made the progress in this field possible over the past decades. Only a few selected references will be given here. Figure 1: Hubble Space Telescope image of the Orion star-forming region.The distance to the cloud is 1500 light years, and the image covers a region on the sky of approximately the angular size of the full moon. The dark regions are dense molecular clouds. The colors represent emission from ionized gas (hydrogen Hα and other atomic lines at optical wavelengths). Credit: NASA/StScI/ESA, M. Robberto and the Orion Treasury Project Team. 2 Birthplaces of stars: interstellar clouds Interstellar clouds are found throughout the Universe. This talk focuses on the solar neighbourhood because that is where the sensitivity and spatial resolution of our instruments is highest. The Sun is one of several hundred billion stars in the Milky Way galaxy and can be found about halfway out from the galactic center to the edge. The clouds discussed here lie at a distance of less than 1500 lightyear from the Sun (1 lightyear=9.5×1017 cm). But the same processes also happen in the rest of the Milky Way and in external galaxies, even out as far as the edge of the observable Universe. Molecules like water and carbon monoxide have been seen at distances corresponding to a time when the Universe was only 500 million years old, less than 5% of its current age. A well-known example of an interstellar cloud is the Orion nebula, which contains a stellar nursery with hundreds of stars just in the process of forming. Beautiful images such as those from the Hubble Space Telescope or European Southern Observatory (ESO) show not only colourful nebulae due to ionized gas that is emitting brightly at visible wavelengths, but also very dark regions (Fig. 1). These dark areas are dense molecular clouds which contain tiny ∼0.1 µm-sized particles of dust composed of silicates and carbonaceous compounds. They absorb and scatter visible and ultraviolet (UV) light, thereby also protecting molecules from dissociating radiation. Molecular clouds can be quite large (tens of light years across) and massive (up to 105 solar masses) but the process of star formation is quite inefficient: only a few % of the gas is eventually turned into stars (Evans et al., 2009). 2 Figure 2: Lifecycle of gas and dust in interstellar space. Some characteristic molecules at each of the star- and planet formation and stellar death stages are indicated. Image by Bill Saxton (NRAO/AUI/NSF) and molecule pictures from the Astrochymist (www.astrochymist.org; this website also contains a list of detected molecules in space). 3 Interstellar molecules 3.1 Composition and chemistry Interstellar clouds consist mostly of gas, about 99% by mass, with 1% of solid materials. The astronomers’ view of the periodic table is fairly restricted. Clouds in the local Universe contain primarily hydrogen (90%) with about 8% helium by number. All other elements are called ‘metals’ by astronomers even if they are obviously not a metal in a chemical sense. The next most abundant elements, important for water and organic chemistry, are oxygen, carbon, and nitrogen at abundances of only about 4.9, 2.7 and 0.7 ×10−4 that of hydrogen. Traditional chemistry would predict that virtually no molecules are formed at typical densities of 104 cm−3 and temperatures of 10 K of dense clouds, with 1000 times more hydrogen than any other chemically interesting element. The detection of nearly 200 different species over the past 45 years (not counting isotopologs) demonstrates the opposite: there is a very rich chemistry in space.