Contents
Cover
Half Title page
Title page
Copyright page
Preface
The Road to Earth Twins
1 The Discovery of a Rich Population of Low Mass Planets on Tight Orbits
2 The HARPS Program to Search for Very Low Mass Planets
3 Emerging Characteristics of Low-Mass Planets and Their Host Star
Acknowledgements
References
Stellar Forensics with the Supernova-GRB Connection1
1 Introduction: The Importance of Stellar Forensics
2 Solid Cases of SN-GRB
3 Do all SNe Ic-bl have an Accompanied GRB?
4 Aspherical Explosions: Only in SN-GRBs?
5 Progenitor Mass as the Culprit?
6 Metallicity as the Culprit?
7 SN 2008D/XRT080109: Stellar Forensics by Witnessing the Death Throes of a Stripped Star
8 The Future is Now: The Golden Age of Transient Surveys and Corresponding Host Galaxy Studies
Acknowledgements
References
Accretion, Jets and Winds: High-Energy Emission from Young Stellar Objects1
1 Introduction
2 Classical T Tauri Stars
3 Herbig Ae/Be Stars
4 Summary
Acknowledgements
References
The Physics and Astrophysics of Supernova Explosions
1 Introduction – Some Observational Facts
2 Physical Classification
3 Numerical Simulations
4 Summary and Conclusions
References
The Facility for Antiproton and Ion Research: A New Era for Supernova Dynamics and Nucleosynthesis
1 The Facility for Antiproton and Ion Research FAIR
2 Introductory Remarks to Nuclear Astrophysics
3 Electron Capture in Core-Collapse Supernovae
4 Electron Capture in Core-Collapse Supernovae
5 Supernova Nucleosynthesis
6 Summary
References
The Bar and Spiral Structure Legacy (BeSSeL) Survey: Mapping the Milky Way with VLBI Astrometry1
1 Introduction
2 Galactic Distances
3 VLBI Astrometry
4 A New Model for the Milky Way
5 The Bar and Spiral Structure Legacy Survey
Acknowledgements
References
On the Origin of Gaseous Galaxy Halos – Low-Column Density Gas in the Milky Way Halo
1 Introduction
2 Motivation of Our Project
3 Data
4 Results
5 Conclusions and Outlook
Acknowledgements
References
Radio Studies of Galaxy Formation: Dense Gas History of The Universe
1 Introduction
2 Tools of radio astronomy
3 Molecular Gas at High Redshift
4 Extreme Starbursts: Massive Galaxy Formation in the Early Universe
5 Secular Galaxy Formation During the Epoch of Galaxy Assembly
6 Dense Gas History of the Universe
7 ALMA and EVLA
Acknowledgements
References
Water in Star-Forming Regions with Herschel1, 2
1 Introduction
2 Observational strategy
3 Results
4 Conclusions
References
Light-Element Abundance Variations in Globular Clusters1
1 Introduction
2 Development of the Observational Data Set
3 Current Models for Globular Cluster Formation
4 Recent Observational Progress
5 Evolution of the Galactic Globular Cluster System
6 Future Challenges
References
Massive Black Holes and the Evolution of Galaxies
1 Introduction
2 Massive Black Hole Formation
3 Understanding the Effect of Environment on Black Hole Growth
4 Black Holes and Their High Redshift Hosts
5 Blazars at Early Cosmic Times
6 Conclusions
7 Acknowledgments
References
High-Energy Astrophysics
1 Introduction
2 The Origin of Cosmic Rays in the GeV–TeV Energy Band
4 The Hunt for Dark-Matter Signals
Acknowledgements
References
Star Formation at High Resolution: Zooming into the Carina Nebula, the Nearest Laboratory of Massive Star Feedback
1 Introduction
2 The Chandra Carina Complex Project
3 HAWK-I Near-Infrared Observations of the Carina Nebula Complex
4 LABOCA sub-mm mapping of the Carina Nebula Complex
5 Future Herschel Observations of the Carina Nebula Complex
6 Conclusions and Outlook
Acknowledgements
References
Characteristic Structures in Circumstellar Disks -Potential Indicators of Embedded Planets
1 Introduction
2 (Proto-)Planets in Young, Gas-Rich Disks
3 Planets in Debris Disks
References
Index of Contributors
General Table of Contents
General Index of Contributors
Reviews in Modern Astronomy Vol. 23
The Series Reviews in Modern Astronomy
Vol. 22: Deciphering the Universe through Spectroscopy
2010
ISBN: 978-3-527-41055-2
Vol. 21: Formation and Evolution of Cosmic Structures
2009
ISBN: 978-3-527-40910-5
Vol. 20: Cosmic Matter
2008
ISBN: 978-3-527-40820-7
Vol. 19: The Many Facets of the Universe - Revelations by New Instruments
2006
ISBN: 978-3-527-40662-3
Vol. 18: From Cosmological Structures to the Milky Way
2005
ISBN: 978-3-527-40608-1
Vol. 17: The Sun and Planetary Systems – Paradigms for the Universe
2004
ISBN: 978-3-527-40476-6
Vol. 16: The Cosmic Circuit of Matter
2003
ISBN: 978-3-527-40451-3
Vol. 15: Astronomy with Large Telescopes from Ground and Space
2002
ISBN: 978-3-527-40404-9
Edited on behalf of the Astronomische Gesellschaft by
Regina von Berlepsch
Leibniz Institute for Astrophysics Potsdam
Potsdam, Germany
RBerlepsch@aip.de
Cover
Artist conception of the Milky Way (R. Hurt: NASA/JPL-Caltech/SSC) showing all sources currently measured (green), including unpublished sources, and all sources observed in the first year of BeSSeL (red), based on their kinematic distances (A. Brunthaler et al.; this book).
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.:
applied for
British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2011 Wiley-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Composition Uwe Krieg, Berlin
Print ISBN: 978-3-527-41113-9
Preface
The annual series Reviews in Modem Astronomy of the ASTRONOMISCHE GESELLSCHAFT was established in 1988 in order to bring the scientific events of the meetings of the Society to the attention of the worldwide astronomical community. Reviews in Modem Astronomy is devoted to the Karl Schwarzschild Lectures, the Ludwig Biermann Award Lectures, the invited reviews, and to the Highlight Contributions from leading scientists reporting on recent progress and scientific achievements at their respective research institutes.
The Karl Schwarzschild Lectures constitute a special series of invited reviews delivered by outstanding scientists who have been awarded the Karl Schwarzschild Medal of the Astronomische Gesellschaft, whereas excellent young astronomers are honoured by the Ludwig Biermann Prize.
Volume 23 continues the series with fourteen invited reviews and Highlight Contributions which were presented during the International Scientific Conference of the Society on “Zooming in: The Cosmos at High Resolution” held in Bonn, Germany, September 13 to 17, 2010.
The Karl Schwarzschild medal 2010 was awarded to Professor Michel Mayor, Genf. His lecture with the title “Exoplanets: The road to Earth twins” opened the meeting.
The talk presented by the Ludwig Biermann Prize winner 2010, Dr. Maryam Modjaz, Berkeley, dealt with the topic “Stellar Forensics with the Supernova-GRB connection”.
In 2010 the Doctoral Thesis Award was established by the Astronomische Gesellschaft to honor the author of the most outstandig Doctoral Thesis of the past year. The first awardee was Hans Moritz Günther. His lecture with the title “Accretion, jets and winds: High-energy emission from young stellar objects” was one of the highlights of the conference.
Other contributions to the meeting published in this volume discuss, among other subjects, the gas history of the universe, the facility for antiproton and ion research, the Bar and Spiral Structure Legacy (BeSSeL) survey and star formation at high resolution.
A report on the Herschel Key Program “Water in star-forming regions with Herschel” completes this volume.
The editor would like to thank the lecturers for their stimulating presentations. Thanks also to the local organizing committee from the Argelander Institute for Astronomy and the Max Planck Institute for Radio Astronomy.
Potsdam, Mai 2011
Regina v. Berlepsch
The ASTRONOMISCHE GESELLSCHAFT awards the Karl Schwarzschild Medal. Awarding of the medal is accompanied by the Karl Schwarzschild lecture held at the scientific annual meeting and the publication. Recipients of the Karl Schwarzschild Medal are
1959 | Martin Schwarzschild: Die Theorien des inneren Aufbaus der Sterne. Mitteilungen der AG 12, 15 |
1963 | Charles Fehrenbach: Die Bestimmung der Radialgeschwindigkeiten mit dem Objektivprisma. Mitteilungen der AG 17, 59 |
1968 | Maarten Schmidt: Quasi-stellar sources. Mitteilungen der AG 25, 13 |
1969 | Bengt Strömgren: Quantitative Spektralklassifikation und ihre Anwendung auf Probleme der Entwicklung der Sterne und der Milchstraße. Mitteilungen der AG 27, 15 |
1971 | Antony Hewish: Three years with pulsars. Mitteilungen der AG 31, 15 |
1972 | Jan H. Oort: On the problem of the origin of spiral structure. Mitteilungen der AG 32, 15 |
1974 | Cornelis de Jager: Dynamik von Sternatmosphären. Mitteilungen der AG 36, 15 |
1975 | Lyman Spitzer, jr.: Interstellar matter research with the Copernicus satellite. Mitteilungen der AG 38, 27 |
1977 | Wilhelm Becker: Die galaktische Struktur aus optischen Beobachtungen. Mitteilungen der AG 43, 21 |
1978 | George B. Field: Intergalactic matter and the evolution of galaxies. Mitteilungen der AG 47, 7 |
1980 | Ludwig Biermann: Dreißig Jahre Kometenforschung. Mitteilungen der AG 51, 37 |
1981 | Bohdan Paczynski: Thick accretion disks around black holes. Mitteilungen der AG 57, 27 |
1982 | Jean Delhaye: Die Bewegungen der Sterne und ihre Bedeutung in der galaktischen Astronomie. Mitteilungen der AG 57, 123 |
1983 | Donald Lynden-Bell: Mysterious mass in local group galaxies. Mitteilungen der AG 60, 23 |
1984 | Daniel M. Popper: Some problems in the determination of fundamental stellar parameters from binary stars. Mitteilungen der AG 62, 19 |
1985 | Edwin E. Salpeter: Galactic fountains, planetary nebulae, and warm HI. Mitteilungen der AG 63, 11 |
1986 | Subrahmanyan Chandrasekhar: The aesthetic base of the general theory of relativity. Mitteilungen der AG 67, 19 |
1987 | Lodewijk Woltjer: The future of European astronomy. Mitteilungen der AG 70, 21 |
1989 | Sir Martin J. Rees: Is there a massive black hole in every galaxy. Reviews in Modern Astronomy 2, 1 |
1990 | Eugene N. Parker: Convection, spontaneous discontinuities, and stellar winds and X-ray emission. Reviews in Modern Astronomy 4, 1 |
1992 | Sir Fred Hoyle: The synthesis of the light elements. Reviews in Modern Astronomy 6, 1 |
1993 | Raymond Wilson: Karl Schwarzschild and telescope optics. Reviews in Modern Astronomy 7, 1 |
1994 | Joachim Trümper: X-rays from Neutron stars. Reviews in Modern Astronomy 8, 1 |
1995 | Henk van de Hulst: Scaling laws in multiple light scattering under very small angles. Reviews in Modern Astronomy 9, 1 |
1996 | Kip Thorne: Gravitational Radiation – A New Window Onto the Universe. Reviews in Modern Astronomy 10, 1 |
1997 | Joseph H. Taylor: Binary Pulsars and Relativistic Gravity. not published |
1998 | Peter A. Strittmatter: Steps to the LBT – and Beyond. Reviews in Modern Astronomy 12, 1 |
1999 | Jeremiah P. Ostriker: Historical Reflections on the Role of Numerical Modeling in Astrophysics. Reviews in Modern Astronomy 13, 1 |
2000 | Sir Roger Penrose: The Schwarzschild Singularity: One Clue to Resolving the Quantum Measurement Paradox. Reviews in Modern Astronomy 14, 1 |
2001 | Keiichi Kodaira: Macro- and Microscopic Views of Nearby Galaxies. Reviews in Modern Astronomy 15, 1 |
2002 | Charles H. Townes: The Behavior of Stars Observed by Infrared Interferometry. Reviews in Modern Astronomy 16, 1 |
2003 | Erika Boehm-Vitense: What Hyades F Stars tell us about Heating Mechanisms in the outer Stellar Atmospheres. Reviews in Modern Astronomy 17, 1 |
2004 | Riccardo Giacconi: The Dawn of X-Ray Astronomy Reviews in Modern Astronomy 18, 1 |
2005 | G. Andreas Tammann: The Ups and Downs of the Hubble Constant Reviews in Modern Astronomy 19, 1 |
2007 | Rudolf Kippenhahn: Als die Computer die Astronomie eroberten Reviews in Modern Astronomy 20, 1 |
2008 | Rashid Sunyaev: The Richness and Beauty of the Physics of Cosmological Recombination Reviews in Modern Astronomy 21, 1 |
2009 | Rolf-Peter Kudritzki: Dissecting galaxies with quantitative spectroscopy of the brightest stars in the Universe Reviews in Modern Astronomy 22, 1 |
2010 | Michel Mayor: Exoplanets: The road to Earth twins Reviews in Modern Astronomy 23, 1 |
The Ludwig Biermann Award was established in 1988 by the ASTRONOMISCHE GESELLSCHAFT to be awarded in recognition of an outstanding young astronomer. The award consists of financing a scientific stay at an institution of the recipient’s choice. Recipients of the Ludwig Biermann Award are
1989 | Dr. Norbert Langer (Göttingen), |
1990 | Dr. Reinhard W. Hanuschik (Bochum), |
1992 | Dr. Joachim Puls (München), |
1993 | Dr. Andreas Burkert (Garching), |
1994 | Dr. Christoph W. Keller (Tucson, Arizona, USA), |
1995 | Dr. Karl Mannheim (Göttingen), |
1996 | Dr. Eva K. Grebel (Würzburg) and Dr. Matthias L. Bartelmann (Garching), |
1997 | Dr. Ralf Napiwotzki (Bamberg), |
1998 | Dr. Ralph Neuhäuser (Garching), |
1999 | Dr. Markus Kissler-Patig (Garching), |
2000 | Dr. Heino Falcke (Bonn), |
2001 | Dr. Stefanie Komossa (Garching), |
2002 | Dr. Ralf S. Klessen (Potsdam), |
2003 | Dr. Luis R. Bellot Rubio (Freiburg im Breisgau), |
2004 | Dr. Falk Herwig (Los Alamos, USA), |
2005 | Dr. Philipp Richter (Bonn), |
2007 | Dr. Henrik Beuther (Heidelberg) and Dr. Ansgar Reiners (Göttingen), |
2008 | Dr. Andreas Koch (Los Angeles), |
2009 | Dr. Anna Frebel (Cambridge, USA) and Dr. Sonja Schuh (Göttingen), |
2010 | Dr. Maryam Modjaz (Berkely), |
The The Doctoral Thesis Award was established in 2010 by the ASTRONOMISCHE GESELLSCHAFT to honor the author of the most outstandig Doctoral Thesis of the past year. Recipient of the first Doctoral Thesis Award is
2010 | Dr. Hans M. Günther (Cambridge/MA), |
Karl Schwarzschild Lecture
Abstract
A rich population of low-mass planets orbiting solar-type stars on tight orbits has been detected by Doppler spectroscopy. These planets have masses in the domain of super-Earths and Neptune-type objects, and periods less than 100 days. In numerous cases these planets are part of very compact multiplanetary systems. Up to seven planets have been discovered orbiting one single star. These low-mass planets have been detected by the HARPS spectrograph around 3% of solar-type stars. This very high occurrence rate has been recently confirmed by the results of the Kepler planetary transit space mission. The large number of planets of this kind allows us to attempt a first characterization of their statistical properties, which in turn represent constraints to understand the formation process of these systems. The achieved progress in the sensitivity and stability of spectrographs have already led to the discovery of planets with masses as small as 1.5M⊕.
Today, more than 500 extrasolar planets have been discovered. Most of the detected exoplanets have been found by using precise measurements of stellar radial velocities. The planetary mass estimate from Doppler measurements is directly proportional to the amplitude of the stellar reflex motion. Our progress to detect very-low-mass planets are directly related to the progress done to improve the sensitivity and stability of spectrographs. In 1989, the detection of HD 114762 b, a companion of 11 Jupiter masses to a metal deficient F star was obtained with spectrographs allowing Doppler measurements with a precision of some 300 ms−1 (Latham et al. 1989). Fifteen years ago, the precision achieved by any team searching for exoplanets was of the order of 15 ms−1. Today, the instrumental precision achieved with the HARPS spectrograph at La Silla Observatory is better than 0.5 ms−1 (Mayor et al. 2003). At this level of precision we are mostly limited by the intrinsic variability of stellar velocities induced by diverse phenomena (acoustic modes, granulation, magnetic activity). However, by adopting an improved observing strategy, we have already some indications that planetary signals as small as a tiny fraction of a meter per second are detectable.
This progress in instrumentation and observing strategy have made possible the discovery of a rich population of super-Earths and Neptune-mass planets in tight orbits around solar-type stars (Mayor & Udry 2008).
The name “super-Earth” is used to qualify planets more massive than the Earth but with masses smaller than 10 Earth masses, a category of planets absent in the solar system. We mention here a few landmark discoveries of these low-mass planets orbiting solar-type stars. Limiting ourself to planets in the super-Earth range we can mention: μ Ara c with a mass of 10.5M⊕ and a period of 9.7 days (Santos et al. 2004b, revised in Pepe et al. 2007), HD 69830 b with a mass of 10.2M⊕ and a period of 8.7 days (Lovis et al. 2006), HD 40307 b, c, d, a system with three super-Earths with masses comprised between 4 and 9M⊕ and periods from 4 to 20 days (Mayor et al. 2009b). We also have to mention the exceptional system around HD 10180, with 7 planets of which one with a mass as small as 1.4M⊕ on a tight orbit with a period of 1.17 day (Lovis et al. 2011). In addition to these early detections of super-Earths orbiting solar-type stars, we also have to mention the discoveries of super-Earths hosted by M dwarfs: GJ 876 d, a planet with a mass of 5.9M⊕ and a period of 1.94 day (Rivera et al. 2005, Correia et al. 2010), GJ 581 c, d, e with masses of 5, 7, and 1.9M⊕ (Udry et al. 2007; Mayor et al. 2009a). It is impressive to see that all these super-Earths are part of rich multi-planetary systems with 3 to 7 planets per system. The remarkable progress of instrumentation in the last 15 years is obvious in Fig. 1. The masses of planetary companions are plotted as a function of the epoch of their discovery. The mass of HD 10180 b (Lovis et al. 2011) is a factor 100 smaller than the mass of 51 Peg b (Mayor & Queloz 1995).
HARPS is a vacuum-operated high-resolution spectrograph (R = 115 000), fiber-fed, optimized to provide stellar radial-velocity measurements with extreme precision (Mayor et al. 2003). As a reward for its construction, the HARPS consortium has received guaranteed observing time (GTO) to carry out an extrasolar planet search in the southern hemisphere (500 observing nights over 5 years). More than 60 % of the total HARPS GTO observing time has been devoted to two sub-programs having the aim of detecting very low-mass planets. The first of these sub-programs comprises some 400 stars which are non-active, slow rotators, not in spectroscopic binary systems, and were selected from the large volume-limited sample measured for several years with the CORALIE spectrograph on the 1.2 m-Euler telescope at la Silla Observatory. The second sub-program consists of a volume-limited sample of about 120 M dwarfs at the bottom of the main sequence, also selected to be slow rotators and not members of spectroscopic binary systems.
What are the limits presently achieved in terms of radial velocity precision? Several sources of noise can be identified:
The global budget of all these errors is difficult to determine. The best estimation of the lower limit of the quadratic sum of the different components of the noise is provided by the residuals observed around fitted radial velocity curves. Several stars with a very large number of velocity measurements spanning several years have residuals with a dispersion as low as 0.6 m s−1 (when binning the data over a few days). For stars with larger chromospheric activity, we can obviously have larger residuals.
This is the precision presently achieved for the HARPS program, for which we have derived preliminary results for the population of low mass planets, as discussed in the next section. If we are searching for low-mass planets on rather long periods, it could be useful to bin the measurements done on N consecutive nights. This procedure could help to damp the noise induced by chromospheric activity, with a time scale comparable to the stellar rotation period. First experiments done on stars with a large number of measurements have shown that the residuals decrease to 0.3–0.5 m s−1 after binning over ten consecutive nights.
We are still far from having a detailed and unbiased view of the population of planets with masses in the range of super-Earths and Neptunes. Nevertheless, we can already notice a few emerging properties. The study of planet hosts themselves also provide additional information to constrain planet formation. In particular the metallicity of the parent stars seems to be of prime importance for models of planetary formation.
The mass distribution of all detected planets is illustrated in Fig. 2. In this plot the contribution of the HARPS program for the detection of very low mass planets is evident. Due to the better detection sensitivity of Doppler spectroscopy for massive and/or short period planets, we still have a strong bias against the detection of low-mass planets, especially if they are on long-period orbits.
The bimodal aspect of the mass distribution is a clear indication that the decrease of the distribution for masses less than about one mass of Jupiter is not the result of a detection bias, but is real. The extrapolation by a power-law distribution, as for example f(m) ~ m−1, to estimate the number of planets with a mass smaller than the mass of Jupiter is certainly not justified. The observed bimodal shape of the mass distribution from gaseous giant planets to the super-Earth regime provides an interesting constraint for planetary formation scenarios. The planetary formation simulations carried out by Mordasini et al. (2009a,b) also predict a bimodal distribution for that range of planetary mass. In addition these simulations also predict a sharp rise in the mass distribution at a few Earth masses and below. This domain of mass is still at the limit of present instrumental sensitivity. Nevertheless, once again the expected shape of this theoretical mass distribution from 10 down to 1 M⊕ is clearly not an exponential and any estimate of the frequency of Earth-twins based on an exponential extrapolation is completely unjustified.
With the HARPS data presently available from the high-precision sample, we have 48 stars with well-characterized planetary systems. More than 50 % of these systems are multiplanetary. Four of them have 4 planets and the amazing system HD 10180 is the host of 7 planets (Lovis et al. 2011a), one of them having a mass as small as 1.5 M⊕.
The correlation between the occurrence of gaseous giant planets and the metallicity of host stars is striking. Based on large planetary surveys this correlation is well established by independent teams (Santos, Israelian & Mayor 2001, 2004a; Fischer & Valenti 2005). We have a completely different result if we examine the metallicity of host stars for systems having all planets less massive than 40M⊕. We do not have any correlation between the presence of these low-mass planets and the host star metallicity (see Fig. 3), a result already mentioned by Udry et al. (2006) and Sousa et al. (2008), based at that time on a very limited number of stars. With the present study, this lack of correlation with the host star metallicity is robust. The mean metallicity of the 28 planetary systems with planets less massive than 40 M⊕ is [Fe/H] = −0.12, a metallicity not so different from the mean metallicity of stars in the solar neighborhood.
The occurrence of low-mass planets on tight orbits has been estimated by Lovis et al. (2009). For planets with masses between ~5 and 50 M⊕ and periods shorter than 100 days, we have detected low-mass planets orbiting about 30% of the stars in the HARPS sample. A more complete estimate is currently in progress, based on the present, more complete survey.
The programme devoted to the study of the population of super-Earths and Neptune-type planets is still continuing at la Silla for four additional years after the end of the GTO time. In addition, a new exploratory program has been initiated with the goal of pushing the HARPS precision a little further and try to detect super-Earths in the habitable zone of very nearby G and K dwarfs. An adequate strategy to damp the acoustic and granulation noise sources has been implemented. The sample is limited to only 10 bright non-active stars. Already, low-mass planets have been detected around three stars members of that small sample, see Pepe et al. (2011). The radial velocity signal for one of these planets is as small as K = 0.56 ms−1. Furthermore, simulations done by Dumusque et al. (2011b) have demonstrated the possibility with the HARPS spectrograph, the present observing strategy and precision, to detect a 2.5 M⊕ planet orbiting a non-active K dwarf in its habitable zone (see Fig. 4).
Some technical improvements are still feasible to increase the sensitivity and stability of cross-correlation spectrographs like HARPS. A better scrambling of the input beam could be achieved by new optical fibers with octagonal cross sections. These new fibers will strongly diminish the already very small effect of input conditions (guiding errors, variable seeing and focus) on the spectrograph illumination, a mandatory condition to achieve 0.1 m s−1 precision. To secure the stability of radial velocity measurements over a span of several years at the level of 0.1 m s−1 or better, we must have a calibration device better than the existing ThAr lamps. Developments of laser frequency combs adapted to the resolution and wavelength coverage of HARPS will provide the requested stability (Wilken et al. 2010).
A photon noise on the Doppler signal at the level of 0.1 m s−1 requires a rather large telescope size to achieve the needed signal-to-noise ratio in a reasonable exposure time. The ESPRESSO project, presently in development, to be implemented on the 8.2-m VLT telescope at Paranal is designed to achieve the 0.1 ms−1 Doppler precision and stability on the long term (Pepe et al. 2010). The ESPRESSO project can also be seen as a precursor for an even more ambitious stable spectrograph, the CODEX project presently at the study phase level for the 42-m E-ELT telescope, to be implemented by ESO at Cerro Armazones (Chile) in the next decade (Pasquini et al. 2010).
We have to keep in mind that for stars with the lowest chromospheric activity, we still do not know the true level of radial velocity jitter. Analysis of the radial velocity scatter of HARPS measurements for non-active stars suggest a minimum jitter of 0.5 ms−1 or less. This stellar variability, depending on the changing number and phase of magnetic spots (or other features) will be difficult to model. Preliminary studies show that non-active K dwarfs will be the most suitable targets to search for Earth twins. A large number of Doppler measurements has the potential to overcome the effects of the stellar intrinsic variability and permit detections of planetary signals of 0.2 ms−1 or less.
The discovery of radial velocity variations associated with solar cycle analogues with full amplitude as large as 10 ms−1 seems a priori to be casting doubts on our ability to detect Earth analogues in the habitable zone. However, using parameters of the cross-correlation function it has been possible to correct the magnetic cycle effects to less than 1 ms−1. In addition, for some domain of stellar masses (K dwarfs), we observe that the amplitude of the radial velocity effect is vanishing despite quite noticeable magnetic cycles. Finally, we notice that the periods of magnetic cycles are much longer (about a factor 10) than the expected periods of habitable planets orbiting K dwarfs. We are thus still convinced that Doppler spectroscopy has the potential to detect rocky planets in the habitable zone of K dwarfs.
The medium- or long-term scientific goal to search for chemical signatures of life in the atmospheric spectra of Earth twins via space experiments as the ESA-DARWIN concept will first require identification of targets. It seems that at the moment Doppler spectroscopy is the only method with the potential to detect Earth-type planets in the habitable zone of stars as close as possible to the Sun. The last condition is mandatory, if we want to have a star-planet angular separation large enough for the need of planetary atmosphere spectroscopy, as well as bright enough targets to maximize the signal-to-noise ratio.
From Doppler surveys we know that super-Earths on tight orbits are frequent. We have first hints from microlensing searches that super-Earths could also be frequent at large semi-major axis (Gould et al. 2010). But we do not have any estimate of the frequency of Earth-twins in the habitable zone of solar-type stars and no ideas on their orbital eccentricity distribution. The orbital eccentricity of Earth-twins is also relevant in the frame of life-search experiments. The ESA-PLATO space project is, in that context, the most interesting experiment, complementary to Doppler surveys to explore the domain of Earth-type planets orbiting relatively close stars.
We would like to thank the Swiss National Science Foundation for its continuous support.
Correia, A., Udry, S., Mayor, M., et al.: 2009, A&A 496, 521
Correia, A.C.M., Couetdic, J., Laskar, J., et al.: 2010, A&A 511, A21
Dumusque, X., Udry, S., Lovis, C., Santos, N.C., Monteiro, M.J.P.F.G.: 2011a, A&A 525, A140
Dumusque, X., Santos, N.C., Udry, S., Lovis, C., Bonfils, X.: 2011b, A&A 527, A82
Fischer, D., Valenti, J.: 2005, ApJ 622, 1102
Gould, A., Dong, S., Gaudi, B.S., et al.: 2010, ApJ 720, 1073
Latham, D.W., Stefanik, R.P., Mazeh, T., Mayor, M., Burki, G.: 1989, Nature 339, 38
Lovis, C., Pepe, F.: 2007, A&A 468, 1115
Lovis, C., Mayor, M., Pepe, F., et al.: 2006, Nature 441, 305
Lovis, C., Mayor, M., Bouchy, F., et al.: 2009, in: F. Pont, D.D. Sasselov, M.J. Holman (eds.), Transiting Planets, IAU Symp. 253, p. 502
Lovis, C., Ségransan, D., Mayor, M., et al.: 2011a, A&A 528, A112
Lovis, C., et al.: 2011b, in prep.
Mayor, M., Queloz, D.: 1995, Nature 378, 355
Mayor, M., Udry, S.: 2008, Physica Scripta T 130, 014010
Mayor, M., Pepe, F., Queloz, D., et al.: 2003, The Messenger 114, 20
Mayor, M., Bonfils, X., Forveille, T., et al.: 2009a, A&A 507, 487
Mayor, M., Udry, S., Lovis, C., et al.: 2009b, A&A 493, 639
Mordasini, C., Alibert, Y., Benz, W.: 2009a, A&A 501, 1139
Mordasini, C., Alibert, Y., Benz, W., Naef, D.: 2009b, A&A 501, 1161
Pasquini, L., Cristiani, S., García-López, R., Haehnelt, M., Mayor, M.: 2010, The Messenger 140, 20
Pepe, F., Correia, A.C.M., Mayor, M., et al.: 2007, A&A 462, 769
Pepe, F., Cristiani, S., Rebolo Lopez, R., et al.: 2010, in: I.S. McLean, S.K. Ramsay, H. Takami (eds.), Ground-based and Airborne Instrumentation for Astronomy III, SPIE 7735, p. 77350F
Pepe, F., et al.: 2011, A&A, subm.
Rivera, E.J., Lissauer, J.J., Butler, R.P, et al.: 2005, ApJ 634, 625
Santos, N.C., Israelian, G., Mayor, M.: 2001, A&A 373, 1019
Santos, N.C., Israelian, G., Mayor, M.: 2004a, A&A 415, 1153
Santos, N.C., Bouchy, F., Mayor, M., et al.: 2004b, A&A 426, L19
Sousa, S.G., Santos, N.C., Mayor, M., et al.: 2008, A&A 487, 373
Udry, S., Mayor, M., Benz, W., et al.: 2006, A&A 447, 361
Udry, S., Bonfils, X., Delfosse, X., et al.: 2007, A&A 469, L43
Wilken, T., Lovis, C., Manescau, A., et al.: 2010, MNRAS 405, L16
1This article has already appeared in Astron. Nachr./AN 332, no. 5 (2011).
Ludwig Biermann Award Lecture
Abstract
Long-duration gamma-ray bursts (GRBs) and type Ib/c supernovae (SNe Ib/c) are amongst nature’s most magnificent explosions. While GRBs launch relativistic jets, SNe Ib/c are core-collapse explosions whose progenitors have been stripped of their hydrogen and helium envelopes. Yet for over a decade, one of the key outstanding questions is what conditions lead to each kind of explosion in massive stars. Determining the fates of massive stars is not only a vibrant topic in itself but also impacts using GRBs as star formation indicators over distances up to 13 billion light-years and for mapping the chemical enrichment history of the universe. This article reviews a number of comprehensive observational studies that probe the progenitor environments, their metallicities and the explosion geometries of SN with and without GRBs, as well as the emerging field of SN environmental studies. Furthermore, it discusses SN 2008D/XRT 080109 which was discovered serendipitously with the Swift satellite via its X-ray emission from shock breakout and which generated great interest amongst both observers and theorists while illustrating a novel technique for stellar forensics. The article concludes with an outlook on how the most promising venues of research – with the many existing and upcoming large-scale surveys such as PTF and LSST – will shed new light on the diverse deaths of massive stars.
Stripped supernovae (SNe) and long-duration gamma-ray bursts (GRBs) are nature’s most powerful explosions from massive stars. They energize and enrich the ISM, and, like beacons, they are visible over large cosmological distances. However, the exact mass and metallicity range of their progenitors is not known, nor the detailed physics of the explosion (see reviews by Woosley & Bloom 2006, and by Smartt 2009). Stripped-envelope SNe (i.e, SN IIb, Ib, Ic, and Ic-bl) are core-collapse events whose massive progenitors have been stripped of progressively larger amounts of their outermost H and He envelopes (Fig. 1, Clocchiatti et al. 1996; Filippenko 1997). In particular, broad-lined SNe Ic (SNe Ic-bl) are SNe Ic whose line widths approach 30 000 km s−1 around before and around maximum light and whose optical spectra show no trace of H and He.
The exciting connection between long GRBs and SNe Ic-bl and the existence of SNe Ic-bl without observed GRBs, as well as that of GRBs that surprisingly lack SN signatures raises the question of what distinguishes a GRB progenitor from that of an ordinary SN Ic-b1 with and without a GRB.
Understanding the progenitors of SN Ib/c and of GRB is important on a number of levels:
Stellar and high-energy astrophysics: These stellar explosions leave behind extreme remnants, such as black holes, neutron stars, magnetars, which in themselves are a rich set of phenomena studied over the full wavelength spectrum from gamma-rays to radio. Ideally we would like to construct a map that connects the mass and make-up of a massive star to the kind of death it undergoes and to the kind of remnant it leaves behind. Furthermore, these stellar explosions are sources of gravitational waves and of neutrino emission, and specifically GRBs are leading candidate sites for high-energy cosmic ray acceleration (e.g., Waxman 2004). Thus, it is of broad astrophysical importance to understand the specific progenitor and production conditions for different kinds of cosmic explosions.
Chemical enrichment history of the universe: The universe’s first- and second-generation stars were massive. Since GRBs and SN probably contribute differently to the enrichment of heavy elements (e.g., Nomoto et al. 2006; Pruet et al. 2006), determining the fate of massive stars is fundamental to tracing the chemical history of the universe.
Cosmology: GRBs are beacons and can illuminate the early universe. Indeed, until recently, the object with the highest spectroscopic redshift was a GRB, GRB 090423 at z ~ 8.2 (Salvaterra et al. 2009; Tanvir et al. 2009), which means that this explosion occurred merely 630 million years after the Big Bang. Thus, a clear understanding of the stellar progenitors of SN and GRBs is an essential foundation for using them as indicators of star formation over cosmological distances.
Various progenitor channels have been proposed for stripped SNe and GRBs: either single massive Wolf-Rayet (WR) stars with main-sequence (MS) masses of that have experienced mass loss during the MS and WR stages (e.g., Woosley et al. 1993), or binaries from lower-mass He stars that have been stripped of their outer envelopes through interaction (Fryer et al. 2007; Podsiadlowski et al. 2004, and references therein), possibly given rise to run-away stars as GRB progenitors (e.g., Cantiello et al. 2007; Eldridge et al. 2011). For long GRBs, the main models for a central engine that is powering the GRB include the collapsar model (MacFadyen & Woosley 1999; Woosley 1993) and the magnetar model (e.g., Usov 1992, for a good summary see Metzger et al. 2011), while rapid rotation of the pre-explosion stellar core appears to be a necessary ingredient for both scenarios.
Attempts to directly identify SN Ib/c progenitors in pre-explosion images obtained with the Hubble Space Telescope or ground-based telescopes have not yet been successful (e.g., Gal-Yam et al. 2005; Maund et al. 2005; Smartt 2009), and cannot conclusively distinguish between the two suggested progenitor scenarios. However, the progenitor non-detections of 10 SN Ib/c strongly indicate that the single massive WR progenitor channel (as we observe in the Local Group) cannot be the only progenitor channel for SN Ibc (Smartt 2009). Similar pre-explosion imaging technique is not possible for GRB progenitors given the large distances at which they are observed.
Thus, in order to fully exploit the potential and power of SNe and GRBs, we have to first figure out their stellar progenitors and the explosions conditions that lead to the various forms of stellar death in a massive star, in form of a “stellar forensics” investigation. In the following review, we will be looking at a number of physical properties in order to find those that set apart SN-GRB, which I will discuss in detail in Sect. 2, from SNe without GRBs: geometry of the explosion (Sect. 4), progenitor mass (Sect. 5) and metallicity (Sect. 6), while the role of binaries are discussed through-out, but not that of magnetic fields. In addition, I will discuss the exciting and emerging field of SN metallicity studies as a promising new tool to probe the progenitors of different kinds of SNe and transients and the story of SN 2008D/XRT 080109 (Sect. 7), which generated great interest amongst both observers and theorists while illustrating a novel technique for stellar forensics
Necessarily, this review will not be complete given the page limit, and is driven by the interest and work of the author, so omissions and simplifications will necessarily arise. Furthermore, given the excellent reviews by Woosley & Bloom (2006), and most recently, Hjorth & Bloom (2011), I will concentrate on developments in the field since 2006 and in complimentary areas.
While the explanation for GRBs after their initial discovery included a vast array of different theories, intensive follow-up observations of GRBs over the last two decades have established that long-duration soft-spectra GRBs (Kouveliotou et al. 1993), or at least a significant fraction of them, are directly connected with supernovae and result from the cataclysmic death of massive, stripped stars (see review by Woosley & Bloom 2006). The most direct proof of the SN-GRB association comes from spectra taken of the GRB afterglows, where the spectral fingerprint of SN, specifically that of a broad-lined SN Ic, emerges over time in the spectrum of the GRB afterglow. Near maximum light, GRB-SNe appear to show broad absorption lines of O I, Ca II, and Fe II (see Fig. 1), while there is no photospheric spectrum of a confirmed GRB-SN that indicated the presence of H or showed optical lines of He I (see also below).
Below we briefly list the SN-GRB cases, in order of descending quality of data (see also Table 1 in Woosley & Bloom 2006 and detailed discussions in Hjorth & Bloom 2011). The five most solid cases of the SN-GRB connection, with high signal-to-noise and multiple spectra, are usually at low z: SN1998bw/GRB980425 at z = 0.0085 (Galama et al. 1998), SN2003dh/GRB030329 at z = 0.1685 (Hjorth et al. 2003; Matheson et al. 2003; Stanek et al. 2003), SN2003lw/GRB031203 at z = 0.10058 (Malesani et al. 2004), SN2006aj/GRB060218 at z = 0.0335 (Campana et al. 2006; Cobb et al. 2006; Kocevski et al. 2007; Mirabal et al. 2006; Modjaz et al. 2006; Pian et al. 2006; Sollerman et al. 2006), and most recently, SN2010bh/GRB100316D at z = 0.0593 (Chornock et al. 2011; Starling et al. 2011), where the SN spectra lines were visible as early as 2 days after the GRB, (Chornock et al. 2011). Two special SNe, SNe 2008D and 2009bb, and the potential presence of a jet in them will be discussed below.
Again, it is important to note that the spectra of the observed GRB-SNe are not any kind of core-collapse SNe, but specifically those of SN Ic-bl The fact that there is no longer the large H envelope present when the star explodes as a SN-GRB is a crucial aspect of why and how the jet can punch its way trough the star (Woosley et al. 1993; Zhang et al. 2004). In addition, SN-GRB do not show the optical Helium lines in their spectra. While there is some discussion of He in the spectrum of SN 1998bw/GRB 980425 (Patat et al. 2001), its claim is based on a broad spectral feature at 1 micron, which could be due to lines other than He I λ10830 Å (Gerardy et al. 2004; Millard et al. 1999; Sauer et al. 2006). The NIR spectrum of the most recent SN2010bh/GRB100316D, did not show the 1 micron He line (Chornock et al. 2011).
What’s more, it remains note-worthy and peculiar that almost all of the solid SN-GRB connections are with GRBs that are usually regarded as non-classical: i.e., GRBs that are less beamed (30°–80°), of low gamma-ray luminosity (i.e., Lisoγ ≤ 1049 erg s−1), have very soft spectra, and thus, are also called X-ray Flashes (XRFs) or X-ray rich GRBs, being, discussed below, perhaps more common than cosmological GRBs (Cobb et al. 2006; Guetta & Delia Valle 2007; Soderberg et al. 2006a). Only GRB 030329 connected with SN 2003dh is the one classical GRB whose kin we see at high z. Either those cosmological high-luminosity GRs are rare at low z, where we can see the SN signatures spectroscopically, or the SN-GRB connection is confined to only GRBs that are more isotropic and of low luminosity. For reference, a SN with the same large luminosity as SN 1998bw/GRB 980425 will appear at R ~ 22 mag at z = 0.5, so approaching the limit of obtaining a spectrum with a large-aperture telescope and reasonable exposure times.
The second broad class encompasses cases with only one epoch of low S/N spectra, which are at higher z: XRF020903 at z = 0.25 (Bersier et al. 2006; Soderberg et al. 2005), SN2002lt/GRB 021211 at z = 1.006 (Delia Valle et al. 2003), SN 2005nc/GRB 050525A at zz