Tile Letfers and PAPERS of JAN HENDRIK OORT AS ARCHIVED in the UNIVERSITY LIBRARY, LEIDEN ASTROPHYSICS and SPACE SCIENCE LIBRARY
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Pos(Westerbork)002 Historical Introduction Historical S 4.0 International License (CC BY-NC-ND 4.0)
Historical Introduction PoS(Westerbork)002 Richard Strom ASTRON Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands E-mail: [email protected] 50 Years Westerbork Radio Observatory, A Continuing Journey to Discoveries and Innovations Richard Strom, Arnold van Ardenne, Steve Torchinsky (eds) Published with permission of the Netherlands Institute for Radio Astronomy (ASTRON) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). Chapter 1 Historical introduction Richard Strom* rom the English longbows at the battle of Crécy (1346) to Winston Chur- chill’s world war I mobilized cannon (its true identity hidden behind the Fpseudonym “[water] tank”), warfare has always pushed technological innovation to new fronts. The second world war (WWII) was no exception. It gave us technology ranging from the dynamo-powered flashlight (a Philips invention) to jet engines, and space-capable rockets (Germany’s V2), not to mention (in a completely different realm) the mass production of Penicillin. In fact, it could be argued that WWII inventions marked the inception of the modern technological era1. In the field of electronics, the war led to innovations such as radio navigation, aircraft landing systems, and radar. It was these developments which were to * ASTRON, Univer- sity of Amsterdam, have a revolutionary impact on astronomy, initially in Britain, Australia and the The Netherlands United States. But the story begins in the US, with the electronics of the 1920s and ‘30s. Figure 1. Karl G. Jansky (c. 1933) Around 1930, there was increasing interest in the use of radio frequencies for communication. One of the main players, the Bell Telephone Laboratories in New Jersey, asked their research engineer, Karl Jansky (Figure 1), to investigate the inter- ference environment in the “short-wave” band around 20 MHz. -
Relaxation Time
Today in Astronomy 142: the Milky Way’s disk More on stars as a gas: stellar relaxation time, equilibrium Differential rotation of the stars in the disk The local standard of rest Rotation curves and the distribution of mass The rotation curve of the Galaxy Figure: spiral structure in the first Galactic quadrant, deduced from CO observations. (Clemens, Sanders, Scoville 1988) 21 March 2013 Astronomy 142, Spring 2013 1 Stellar encounters: relaxation time of a stellar cluster In order to behave like a gas, as we assumed last time, stars have to collide elastically enough times for their random kinetic energy to be shared in a thermal fashion. But stellar encounters, even distant ones, are rare on human time scales. How long does it take a cluster of stars to thermalize? One characteristic time: the time between stellar elastic encounters, called the relaxation time. If a gravitationally bound cluster is a lot older than its relaxation time, then the stars will be describable as a gas (the star system has temperature, pressure, etc.). 21 March 2013 Astronomy 142, Spring 2013 2 Stellar encounters: relaxation time of a stellar cluster (continued) Suppose a star has a gravitational “sphere of influence” with radius r (>>R, the radius of the star), and moves at speed v between encounters, with its sphere of influence sweeping out a cylinder as it does: v V= π r2 vt r vt If the number density of stars (stars per unit volume) is n, then there will be exactly one star in the cylinder if 2 1 nV= nπ r vtcc =⇒=1 t Relaxation time nrvπ 2 21 March 2013 Astronomy 142, Spring 2013 3 Stellar encounters: relaxation time of a stellar cluster (continued) What is the appropriate radius, r? Choose that for which the gravitational potential energy is equal in magnitude to the average stellar kinetic energy. -
In-Class Worksheet on the Galaxy
Ay 20 / Fall Term 2014-2015 Hillenbrand In-Class Worksheet on The Galaxy Today is another collaborative learning day. As earlier in the term when working on blackbody radiation, divide yourselves into groups of 2-3 people. Then find a broad space at the white boards around the room. Work through the logic of the following two topics. Oort Constants. From the vantage point of the Sun within the plane of the Galaxy, we have managed to infer its structure by observing line-of-sight velocities as a function of galactic longitude. Let’s see why this works by considering basic geometry and observables. • Begin by drawing a picture. Consider the Sun at a distance Ro from the center of the Galaxy (a.k.a. the GC) on a circular orbit with velocity θ(Ro) = θo. Draw several concentric circles interior to the Sun’s orbit that represent the circular orbits of other stars or gas. Now consider some line of sight at a longitude l from the direction of the GC, that intersects one of your concentric circles at only one point. The circular velocity of an object on that (also circular) orbit at galactocentric distance R is θ(R); the direction of θ(R) forms an angle α with respect to the line of sight. You should also label the components of θ(R): they are vr along the line-of-sight, and vt tangential to the line-of-sight. If you need some assistance, Figure 18.14 of C/O shows the desired result. What we are aiming to do is derive formulae for vr and vt, which in principle are measureable, as a function of the distance d from us, the galactic longitude l, and some constants. -
Radio Astronomy
Theme 8: Beyond the Visible I: radio astronomy Until the turn of the 17th century, astronomical observations relied on the naked eye. For 250 years after this, although astronomical instrumentation made great strides, the radiation being detected was still essentially confined to visible light (Herschel discovered infrared radiation in 1800, and the advent of photography opened up the near ultraviolet, but these had little practical significance). This changed dramatically in the mid-20th century with the advent of radio astronomy. 8.1 Early work: Jansky and Reber The atmosphere is transparent to visible light, but opaque to many other wavelengths. The only other clear “window” of transparency lies in the radio region, between 1 mm and 30 m wavelength. One might expect that the astronomical community would deliberately plan to explore this region, but in fact radio astronomy was born almost accidentally, with little if any involvement of professional astronomers. Karl Jansky (1905−50) was a radio engineer at Bell Telephone. In 1932, while studying the cause of interference on the transatlantic radio-telephone link, he discovered that part of the interference had a periodicity of one sidereal day (23h 56m), and must therefore be coming from an extraterrestrial source. By considering the time at which the interference occurred, Jansky identified the source as the Milky Way. This interesting finding was completely ignored by professional astronomers, and was followed up only by the radio engineer and amateur astronomer Grote Reber (1911−2002). Reber built a modern-looking paraboloid antenna and constructed maps of the radio sky, which also failed to attract significant professional attention. -
Messenger-No117.Pdf
ESO WELCOMES FINLANDINLAND AS ELEVENTH MEMBER STAATE CATHERINE CESARSKY, ESO DIRECTOR GENERAL n early July, Finland joined ESO as Education and Science, and exchanged which started in June 2002, and were con- the eleventh member state, following preliminary information. I was then invit- ducted satisfactorily through 2003, mak- II the completion of the formal acces- ed to Helsinki and, with Massimo ing possible a visit to Garching on 9 sion procedure. Before this event, howev- Tarenghi, we presented ESO and its scien- February 2004 by the Finnish Minister of er, Finland and ESO had been in contact tific and technological programmes and Education and Science, Ms. Tuula for a long time. Under an agreement with had a meeting with Finnish authorities, Haatainen, to sign the membership agree- Sweden, Finnish astronomers had for setting up the process towards formal ment together with myself. quite a while enjoyed access to the SEST membership. In March 2000, an interna- Before that, in early November 2003, at La Silla. Finland had also been a very tional evaluation panel, established by the ESO participated in the Helsinki Space active participant in ESO’s educational Academy of Finland, recommended Exhibition at the Kaapelitehdas Cultural activities since they began in 1993. It Finland to join ESO “anticipating further Centre with approx. 24,000 visitors. became clear, that science and technology, increase in the world-standing of ESO warmly welcomes the new mem- as well as education, were priority areas Astronomy in Finland”. In February 2002, ber country and its scientific community for the Finnish government. we were invited to hold an information that is renowned for its expertise in many Meanwhile, the optical astronomers in seminar on ESO in Helsinki as a prelude frontline areas. -
Kinematic and Emission Analysis of the Relativistic Jet in the Blazar B1551+130 Using VLBI Data
Kinematic and Emission Analysis of the Relativistic Jet in the Blazar B1551+130 using VLBI Data Shaun Julian Nicol MSci Project Imperial College London Host Institute: Supervisor: Universitat de Val`encia Prof. Eduardo Ros Ibarra June 2012 Abstract Very Long Baseline Interferometry (VLBI) allows the most energetic structures in our universe to be observed with sub-milliarcsecond resolution (parsecs at cosmological distances). These methods are particularly effective in observing the relativistic jets extending from the black hole region of active galactic nu- clei (AGN). Six epochs of VLBI data from the MOJAVE 2 cm survey taken between June 2009 and April 2012 are used to carry out the kinematic analysis of the relativistic jet in the flat spectrum radio quasar B1551+130 with red- shift z = 1:308. The apparent velocities of areas of enhanced radio emission, or `knots', in the jet, calculated through the model fitting technique, are pre- sented along with images of the source obtained through the hybrid mapping process. Superluminal velocities for the `knots' are observed with the highest apparent velocity being 10.21.8c. From this maximum velocity, the viewing angle of 5.6◦ 1◦ is derived, agreeing with the classification of the source as a FSRQ. An estimate of the luminosity Doppler boosting factor gives a value of order 102. Two jet features are observed with negative superluminal velocities, the most significant being the region closest to the base or `core' of the jet ob- served with a velocity of {10.62.7c. The flux of this region undergoes a large increase during the most recent observations, between mid 2011 and mid 2012. -
Galactic Rotation in Gaia DR1
Mon. Not. R. Astron. Soc. 000,1{6 (2017) Printed 10 February 2017 (MN LATEX style file v2.2) Galactic rotation in Gaia DR1 Jo Bovy? Department ofy Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada and Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave, New York, NY 10010, USA 30 January 2017 ABSTRACT The spatial variations of the velocity field of local stars provide direct evidence of Galactic differential rotation. The local divergence, shear, and vorticity of the velocity field—the traditional Oort constants|can be measured based purely on astrometric measurements and in particular depend linearly on proper motion and parallax. I use data for 304,267 main-sequence stars from the Gaia DR1 Tycho-Gaia Astrometric Solution to perform a local, precise measurement of the Oort constants at a typical heliocentric distance of 230 pc. The pattern of proper motions for these stars clearly displays the expected effects from differential rotation. I measure the Oort constants to be: A = 15:3 0:4 km s−1 kpc−1, B = 11:9 0:4 km s−1 kpc−1, C = 3:2 0:4 km s−1 kpc−1 ±and K = 3:3 0:6 km s−−1 kpc−1±, with no color trend over− a wide± range of stellar populations.− These± first confident measurements of C and K clearly demonstrate the importance of non-axisymmetry for the velocity field of local stars and they provide strong constraints on non-axisymmetric models of the Milky Way. Key words: Galaxy: disk | Galaxy: fundamental parameters | Galaxy: kinematics and dynamics | stars: kinematics and dynamics | solar neighborhood 1 INTRODUCTION use Cepheids to investigate the velocity field on large scales (> 1 kpc) with a kinematically-cold stellar tracer population Determining the rotation of the Milky Way's disk is diffi- (e.g., Feast & Whitelock 1997; Metzger et al. -
What Is Dark Matter
What is Dark Ma,er? History of what the universe is made of, through 2003 Zackary Scholl October 8th, 2009 Early astronomy methods • Measuring distances to galaxies – “Standard candle” Type Ia Supernova used to measure distance to galaxies (1-100 Mps) • Measuring velociQes of galaxies – Speed is proporQonal to amount of Doppler shiR in emission and absorpQon lines Early astronomy methods • EsQmaQng masses of galaxies – From mass to luminosity raQo (M/L) • Based on the sun (Sun’s M/L = 1) • Our galaxy has M/L is 6 – From rotaQonal velocity of gas clouds and stars • Kepler’s laws of moQon say that faster stellar moQons are caused by larger mass concentraQon First menQon of “dark ma,er” • In 1933 Fritz Zwicky analyzed velociQes of galaxies in Coma cluster • Calculated mass was 50 Qmes larger than mass esQmated from luminosity. “If this is confirmed we would arrive at the astonishing conclusion that dark maer is present with much greater density than luminous ma,er.” – Zwicky, 1933 More missing mass… • In 1936 Sinclair Smith concluded Virgo galaxy has 10 Qmes the mass esQmated from luminosity. • In 1940 Jan Oort concluded NGC 3115 has 25 Qmes mass esQmated from luminosity. “[T]he distribuQon of mass in this system appears to bear almost no relaQon to that of light.“ – Oort 1940 Be,er astronomy methods • In 1950’s astronomers started using radio telescopes • In 1960’s astronomers started using X-ray telescopes • Astronomers began invesQgaQng dynamics of galaxies • Evidence accumulated for idea of missing mass Evidence of “missing ma,er” • In -
Great Discoveries Made by Radio Astronomers During the Last Six Decades and Key Questions Today
17_SWARUP (G-L)chiuso_074-092.QXD_Layout 1 01/08/11 10:06 Pagina 74 The Scientific Legacy of the 20th Century Pontifical Academy of Sciences, Acta 21, Vatican City 2011 www.pas.va/content/dam/accademia/pdf/acta21/acta21-swarup.pdf Great Discoveries Made by Radio Astronomers During the Last Six Decades and Key Questions Today Govind Swarup 1. Introduction An important window to the Universe was opened in 1933 when Karl Jansky discovered serendipitously at the Bell Telephone Laboratories that radio waves were being emitted towards the direction of our Galaxy [1]. Jansky could not pursue investigations concerning this discovery, as the Lab- oratory was devoted to work primarily in the field of communications. This discovery was also not followed by any astronomical institute, although a few astronomers did make proposals. However, a young electronics engi- neer, Grote Reber, after reading Jansky’s papers, decided to build an inno- vative parabolic dish of 30 ft. diameter in his backyard in 1935 and made the first radio map of the Galaxy in 1940 [2]. The rapid developments of radars during World War II led to the dis- covery of radio waves from the Sun by Hey in 1942 at metre wavelengths in UK and independently by Southworth in 1942 at cm wavelengths in USA. Due to the secrecy of the radar equipment during the War, those re- sults were published by Southworth only in 1945 [3] and by Hey in 1946 [4]. Reber reported detection of radio waves from the Sun in 1944 [5]. These results were noted by several groups soon after the War and led to intensive developments in the new field of radio astronomy. -
Govind Swarup: Radio Astronomer, Innovator Par Excellence and a Wonderfully Inspiring Leader
LIVING LEGENDS IN INDIAN SCIENCE Govind Swarup: Radio astronomer, innovator par excellence and a wonderfully inspiring leader G. Srinivasan They are ill discoverers that think there the important contributions to cosmology is no land, when they can see nothing but being made using this telescope. He sea. mentioned in very flattering terms the Francis Bacon Ph D thesis of one of Swarup’s students (Vijay Kapahi) which had come to him I must say at the outset that I have no for evaluation. That is how I came to special credentials to write an article know of Swarup. about as famous a person as Govind As it turned out, I moved from Cam- Swarup. I am not one of his numerous bridge to the Raman Research Institute students. Nor have I collaborated with (RRI), Bangalore in the beginning of him in research. Indeed, I am not even an 1976. Within weeks after my coming, I astronomer, let alone a radio astronomer. received an invitation from Govind Swa- But I have been one of his great admir- rup to attend a small meeting he had ers, and he has been a beacon of inspira- convened at RRI to discuss a Summer tion for me during the past four decades. School in Astronomy he was organizing I was therefore delighted – although sur- in Bangalore in June 1976. I was rather prised – when the Editor of Current Sci- surprised because I was still working in ence invited me to write this article. The Condensed Matter Physics. There were K. S. Krishnan, B. N. -
Galactic Rotation II
1 Galactic Rotation • See SG 2.3, BM ch 3 B&T ch 2.6,2.7 B&T fig 1.3 and ch 6 • Coordinate system: define velocity vector by π,θ,z π radial velocity wrt galactic center θ motion tangential to GC with positive values in direct of galactic rotation z motion perpendicular to the plane, positive values toward North If the galaxy is axisymmetric galactic pole and in steady state then each pt • origin is the galactic center (center or in the plane has a velocity mass/rotation) corresponding to a circular • Local standard of rest (BM pg 536) velocity around center of mass • velocity of a test particle moving in of MW the plane of the MW on a closed orbit that passes thru the present position of (π,θ,z)LSR=(0,θ0,0) with 2 the sun θ0 =θ0(R) Coordinate Systems The stellar velocity vectors are z:velocity component perpendicular to plane z θ: motion tangential to GC with positive velocity in the direction of rotation radial velocity wrt to GC b π: GC π With respect to galactic coordinates l +π= (l=180,b=0) +θ= (l=30,b=0) +z= (b=90) θ 3 Local standard of rest: assume MW is axisymmetric and in steady state If this each true each point in the pane has a 'model' velocity corresponding to the circular velocity around of the center of mass. An imaginery point moving with that velocity at the position of the sun is defined to be the LSR (π,θ,z)LSR=(0,θ0,0);whereθ0 = θ0(R0) 4 Description of Galactic Rotation (S&G 2.3) • For circular motion: relative angles and velocities observing a distant point • T is the tangent point V =R sinl(V/R-V /R ) r 0 0 0 Vr Because V/R drops with R (rotation curve is ~flat); for value 0<l<90 or 270<l<360 reaches a maximum at T max So the process is to find Vr for each l and deduce V(R) =Vr+R0sinl For R>R0 : rotation curve from HI or CO is degenerate ; use masers, young stars with known distances 5 Galactic Rotation- S+G sec 2.3, B&T sec 3.2 • Consider a star in the midplane of the Galactic disk with Galactic longitude, l, at a distance d, from the Sun. -
Measurement of the Epicycle Frequency in the Galactic Disc and Initial Velocities of Open Clusters
Mon. Not. R. Astron. Soc. 386, 2081–2090 (2008) doi:10.1111/j.1365-2966.2008.13168.x Measurement of the epicycle frequency in the Galactic disc and initial velocities of open clusters J. R. D. Lepine,´ 1⋆ Wilton S. Dias2 and Yu. Mishurov3 1Instituto de Astronomia, Geof´ısica e Cienciasˆ Atmosfericas,´ Universidade de Sao˜ Paulo, Cidade Universitaria,´ Sao˜ Paulo SP, Brazil 2UNIFEI, Instituto de Cienciasˆ Exatas, Universidade Federal de Itajuba,´ Itajuba´ MG, Brazil 3South Federal University (Rostov State University), Rostov-on-Don, Russia Accepted 2008 February 28. Received 2008 February 27; in original form 2007 October 30 ABSTRACT A new method to measure the epicycle frequency κ in the Galactic disc is presented. We make use of the large data base on open clusters completed by our group to derive the observed velocity vector (amplitude and direction) of the clusters in the Galactic plane. In the epicycle approximation, this velocity is equal to the circular velocity given by the rotation curve, plus a residual or perturbation velocity, of which the direction rotates as a function of time with the frequency κ. Due to the non-random direction of the perturbation velocity at the birth time of the clusters, a plot of the present-day direction angle of this velocity as a function of the age of the clusters reveals systematic trends from which the epicycle frequency can be obtained. Our analysis considers that the Galactic potential is mainly axis-symmetric, or in other words, that the effect of the spiral arms on the Galactic orbits is small; in this sense, our results do not depend on any specific model of the spiral structure.