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The transient universe • Sullivan: Supernovae 1: The spiral galaxy M101, home to supernova SN 2011fe, the brightest Type Ia supernova in the sky for more than 25 years. The left-hand image shows the galaxy before the explosion, and the right-hand image shows the supernova near peak brightness. (BJ Fulton [IfA, Univ. Hawaii]/LCOGT/PTF/STScI) Gone in a flash: supernovae in the survey era T he study of supernovae – stellar explosions, the dramatic final moments of a star’s life – is one of the richest and longest running fields of astronomy. Supernovae have been observed, recorded and studied by humans for nearly 2000 years, since the first descriptions of “guest stars” appeared in the records of Chinese astronomers. These relatively nearby galactic supernovae, appearing brighter in the sky than Venus – perhaps bright enough to cast shadows – must have astonished and perplexed observers of the time. Those of 1572 (noted by Tycho Brahe) and 1604 (Kepler’s supernova) were used as arguments against the then commonly held Aristotelian world-view that the heavens were immutable. Even today, the idea of an entire star self-destructing in an explosion five billion times brighter than the Sun is both compelling and awe-inspiring. Milky Way supernovae There are around seven recorded supernovae in the Milky Way for which plausible remnants have been identified (depending on how one counts), including SN 1572, SN 1604 and SN 1054, the explosion that gave birth to the A&G • December 2013 • Vol. 54 The transient universe Mark Sullivan outlines the scientific heritage of supernovae and the potential for future discoveries. Together with Walter Baade, he coined the term “super-novae” in the early 1930s, and over the course of the next 50 years found 120 events – a record not bettered until recently by Scottish amateur astronomer Tom Boles (see page 6.9). The development of dedicated robotic searches Crab pulsar. Some 20 further ancient galactic in the 1960s and 70s saw the discovery rate supernova candidates exist in Chinese, Islamic pick up to a few tens of events per year; CCD and other astronomical records. searches in the 1980s on larger telThe first detected extragalactic escopes provided the first events The supernova, in Andromeda at cosmological distances. idea of a star self(S Andromedae), was not Around this time came the destructing in an discovered until the end of realization of the idea, first the 19th century, although postulated in the 1960s, explosion five billion that Type Ia supernovae it was not recognized for times brighter than what it was at the time. can act as particularly highthe Sun is an aweFor the next few decades quality standard candles. inspiring field of This sparked a significant the discovery rate remained study low – a “blind luck” era, when observational effort in the early finding supernovae was the result 1990s to find larger samples of disof chance rather than any concerted tant events, pushing the boundaries of plan (see figure 2). The first dedicated “super- what was then achievable with the available nova patrol” did not commence until the 1930s, telescopes, instruments and computing. The when the Caltech astronomer Fritz Zwicky used culmination was the announcement by two the Mount Wilson telescopes and an arduous teams in the late 1990s that high-redshift Type manual comparison of photographic plates. Ia supernovae appeared about 40% fainter – or, ‘‘ ’’ 6.17 The transient universe • Sullivan: Supernovae equivalently, more distant – than expected in a flat, matter-dominated universe. This indicated that the rate of expansion of the universe was speeding up, and that the substance of the universe was dominated by a “dark energy” responsible for 70–75% of its energy (Riess et al. 1998, Perlmutter et al. 1999). This result later won the 2011 Nobel Prize in Physics, and has forever changed our understanding of the universe. New sky surveys continue to find supernovae at an extraordinary rate, partly motivated by trying to understand dark energy and partly motivated by an innate curiosity as to how stars explode. Cheaper, larger CCDs and the birth of large-scale computing have made the task of surveying large areas of sky significantly simpler and quicker. Even as recently as the turn of the last century, supernovae were only discovered at the rate of a few tens per year. Modern surveys are enabling events to be located with a frequency that is almost an order of magnitude larger – 2012 saw more than 2000 spectroscopically confirmed supernovae announced, and 2013 is on track to better this (figure 2). Much of this survey work has been performed on dedicated smaller telescopes used with large format cameras. Prime examples include: ● T he Palomar Transient Factory (PTF), using the Palomar 48-inch Schmidt telescope refurbished with a large-format CCD camera ● Pan-STARRS, using a purpose-built 1.8 m telescope ● T he Catalina Real-Time Transient Survey (CRTS), using three dedicated telescopes in the northern and southern hemispheres. Other surveys, such as the La Silla Quest Variability Survey on the ESO 1 m Schmidt telescope on La Silla, and the SkyMapper survey at Siding Spring, are also now coming online. This rapid discovery rate has dramatically improved our understanding of supernova physics, but, as might be expected, has also raised more questions to be answered. Here we highlight recent progress on two fronts: finding new ways in which stars can explode, and understanding the progenitor stars that make the cosmologically important Type Ia supernovae. An explosion of supernova types In 1941, Minkowski proposed the first observational classification of supernova into the now familiar Type I (no hydrogen lines in the spectra) and Type II (with hydrogen lines) events. This basic classification scheme survives today, but has acquired ever more complex subdivisions (Ia, Ib, Ic, IIb, IIn, Ia-CSM, Iax, etc) to accommodate the large variety of supernova types now known. This means that the classification of supernovae sometimes seems to require the attention of a black-belt expert – and even then, similar objects can be classified in subtly different ways by experienced observers. 6.18 2: The discovery rate of approximately 14 000 extragalactic supernovae located over the last century, complete up to 31 August 2013. This includes all supernovae announced via IAU circulars and Astronomer’s Telegrams, as well as those discovered by the Palomar Transient Factory, the Supernova Legacy Survey, and Pan-STARRS. Several key dates and observations are highlighted in the figures. The top panel shows an overview of the last 130 years, while the bottom panel focuses on the last 20 years and the future. Historically, supernovae were believed to originate in two physically distinct ways. The first group results from the thermonuclear destruction of a carbon–oxygen white dwarf star, as it accretes or gains material from a companion until carbon burning is ignited at or near its core. These are the Type Ia supernovae (SNe Ia), with distinctive features of silicon (a product of carbon burning) in the spectra. The second group forms through the gravitational core-collapse of a massive star, more than eight times the mass of the Sun, triggered by iron photo-disintegration and the consequent loss of support in the star’s core. This mechanism is believed to produce nearly all the other spectral types. Once the progenitor star has exploded, there must also be some way of generating electromagnetic radiation so the supernova can be detected. There are three basic contributors to the radiation, as presently understood. The first, important in nearly all supernova types, is from the radioactive decay of unstable elements synthesized in the explosive nucleosynthesis. The most important is 56 Ni, which then decays into 56 Co and eventually into stable 56Fe. This decay generates gamma-rays, which are trapped in the ejecta and thermalize it so that it glows. This mechanism is the only power source for SNe Ia – indeed, without radioactive 56 Ni, SNe Ia would never be seen. A second source is the release of internal energy deposited by the explosion via photon diffusion. A final contributor is the A&G • December 2013 • Vol. 54 The transient universe • Sullivan: Supernovae expected from radioactive 56 Ni. A third class, SLSN-R, may actually be closely related to SLSN-I, and are consistent with being powered by the radioactive decay of several solar masses of 56 Ni (here, “R” stands for radioactive). Naturally, several theories as to their origin and power source have emerged to explain this confusing observational picture. At the time of writing, two of these theories seem to be gaining most traction. The first is that SLSN-R result from the death of stars of greater than ~100 M⊙ via the pair instability mechanism (Gal-Yam et al. 2009): an extremely massive star will go through a phase of electron–positron “pair” generation in its core, rendering the star unstable and making it explode, long before it evolves to the point that iron core collapse can occur as happens in lower-mass objects. The second is that SLSN-I (and perhaps, in part, the other SLSN types) may be powered by energy deposition from the spin-down of a magnetar (Kasen and Bildsten 2010). However, this magnetar theory can only be consistent with all the observations with significant tuning – and new 3: The light curves of common supernova types compared to superluminous supernovae. In observations are emerging that may challenge all cases, day zero represents the day of peak brightness. Typical supernova explosions reach the pair instability origin for SLSN-R (Nicholl absolute magnitudes of more than –19.5 and decay over a few weeks, whereas superluminous et al. 2013). supernovae can reach up to –22.5, more than a factor of 10 brighter, and can remain bright for several months. (Reprinted from Gal-Yam 2012) As has happened over several decades with SNe Ia, understanding the detailed physics of SLSNe (and even testing them as standard interaction of the supernova ejecta with any and unrecognizable because its high redshift candles) requires populations of events, rather dense circumstellar medium (CSM) around the meant that it probed the rest-frame ultraviolet than a handful of relatively poorly observed progenitor star, with the kinetic energy of the – a region poorly explored in supernova science. examples. Determining the rates of SLSNe as ejecta essentially converted into electromagnetic This high redshift also implied an extreme a function of redshift is essential to understand energy. This mechanism is particularly impor- ultraviolet luminosity, making them interest- their progenitors, because both star formation tant in Type IIn supernovae, which also show ing candidates for a new generation of and metallicity evolve over cosmic hisnarrow emission lines in their spectra that indi- cosmological candle. Their bright tory. Meanwhile, improved spectral The apparent magnitudes allow cate interaction with the CSM. and photometric data will allow for studies of the interstellar studies of their chemistry and new surveys are Superluminous supernovae medium of their host galaxenergetics. Their intrinsic rarrevealing a much ity – they occur at rates about The new surveys described above are revealing ies via absorption line speclarger diversity of a much larger diversity of rare explosive tran- troscopy, or perhaps even as 1/1000th of the SN Ia rate – rare explosive sients than this simple picture suggests. Super- standard candles in the verymeans that very large volumes transients luminous supernovae (SLSNe) are a new class of high-redshift universe – beyond of space must be searched in order to find significant numextreme supernova that have emerged over the the reach of our current best tool, past five years (for a review, see Gal-Yam 2012), SNe Ia. bers. But with very large imagers on reaching absolute magnitudes of –23 (50 times To date there are three identified classes 4 m-class facilities (e.g. Megacam on the brighter than a Type Ia supernova). The first of SLSN (Gal-Yam 2012), commonly defined as CFHT, or DECam on the Cerro Tololo Interevent, SCP 06F6, was identified in 2009 and had being brighter than –21 in absolute magnitude American Observatory 4 m Blanco telescope), broad, unexplained spectral absorption lines (figure 3). By analogy to the classification of these objects can be found up to a redshift of 4 and a very slowly evolving light curve. The lack normal SNe, the SLSNe-I have hydrogen-free (Cooke et al. 2012). It should also be possible to of an apparent host galaxy meant that even the optical spectra, and may possibly be related study them in large numbers using projects such redshift was unknown. Consequently, an imagi- to SNe Ic, whereas SLSNe-II show narrow as the Dark Energy Survey (DES). native range of possibilities were postulated to hydrogen lines. The luminosity of these latter explain the object, ranging from a tidally dis- events is probably related to interaction with Progenitors of Type Ia supernovae rupted white dwarf at moderate redshift, to an CSM around the progenitor star, which then Type Ia supernovae are routinely used to asteroid colliding with a white dwarf in our own also causes the observed hydrogen lines. But measure cosmological parameters, such as the galaxy. During the operation of the Palomar SLSNe-I are more puzzling. The normal meth- equation of state of dark energy, w (figure 4). Transient Factory, several similar events were ods of powering supernovae discussed above – The pursuit of w is a goal of many wide-field located (Quimby et al. 2011), this time with host radioactive decay of 56 Ni, or the interaction of surveys, for example: the Supernova Legacy galaxy redshifts. This, in turn, allowed a red- the supernova ejecta with dense CSM – cannot Survey, the Sloan Digital Sky Survey Supershift determination for SCP 06F6 to be made, at reproduce both their extreme brightness and nova Search, the Lick Observatory Supernova z = 1.19. The SCP 06F6 spectrum was so unusual their decay-rates, which are faster than that Search, PTF, SkyMapper, Pan-STARRS, DES, ‘‘ ’’ A&G • December 2013 • Vol. 54 6.19 The transient universe • Sullivan: Supernovae the Large Synoptic Survey Telescope, and Euclid. This represents a significant allocation of observing resources. The technique itself is simple. The SN Ia “standard candle” distances (derived from their apparent brightness and a knowledge of the SN Ia absolute luminosities) can be compared with luminosity distances, calculated from their redshifts together with a set of cosmological parameters and the equations of General Relativity. As the cosmological parameters are, in principle, the only unknowns in this analysis, with a sufficient number of SNe Ia, constraints can be placed on their values. Although SNe Ia provided the first direct evidence for dark energy (and still provide the most mature and constraining measurements), there are several potential drawbacks to the technique – the apparently simple standard candle concept has several less apparent difficulties, fundamentally related to the precision required. Detecting departures in dark energy from w = –1, mathematically equivalent to the cosmological constant, requires an extremely sensitive experiment. A 10% difference in w from –1 is equivalent to a change in SN Ia brightness at z = 0.6 of only 0.04 magnitudes, an absolute precision perhaps not historically or routinely achieved in astronomy. The challenge is even more complex when fitting variable w models. Therefore, the SN Ia experiments are not simply about obtaining more data, but about obtaining higher-quality data in which these experimental systematics can be controlled. While these challenges are considerable, it is at least a well-defined and tractable problem on which substantial progress is being made. Coupled with these experimental challenges is the relative confusion over the nature and astrophysics of the SN Ia progenitor system. Although the nature of the exploding star as a carbon–oxygen white dwarf is not seriously challenged, and has recently been confirmed in a nearby SN Ia (SN 2011fe; Nugent et al. 2011; figure 5), this white dwarf must accrete material from a companion until the density and temperature are high enough to ignite a thermonuclear explosion. The identity of this second star is not known and has puzzled astronomers for decades: is it non-degenerate such as a giant or main sequence star donating mass (as in a nova or similar system), or is it a second white dwarf in some form of merger? These two broad classes, “single degenerate” and “double degenerate”, probably trigger the thermonuclear explosion in different ways: different delays between system formation and explosion, different accretion rates and ignition conditions, and perhaps different white dwarf compositions and ages. There has been a growing realization over the last few years that, in fact, it is very likely that both (and possibly even other) channels operate. The remarkable SN Ia PTF 11kx (Dilday et al. 2012) showed significant evidence for inter 6.20 4: Typical confidence contours in the cosmological parameters w (the equation of state of dark energy) and ΩM (the matter density of the universe). Shown are the SN Ia results (blue), constraints from the Hubble constant (red), and those from galaxy redshift surveys (green). Results from observations of the cosmic microwave background (the WMAP satellite) are also used. The data are consistent with w = –1 (the cosmological constant) with a 7% uncertainty, including all identified systematic uncertainties. (Adapted from Sullivan et al. 2011) 5: Constraints on the mass, effective temperature, radius, and average density of the primary star of the Type Ia supernova SN 2011fe. The shaded red region is excluded from the non-detection of an optical counterpart in Hubble Space Telescope (HST) imaging (see also figure 6 for the constraints this places on the companion star). The shaded green region is excluded from considerations of the non-detection of a shock breakout/heating at early times, and the blue region is excluded by the non-detection of a counterpart in Chandra X-ray data. The location of the H, He and C main sequence is shown, with the symbol size scaled for different primary star masses. Several observed white dwarf and neutron stars are shown. The primary radius in units of R⊙ is shown for a primary star mass of 1.4 M⊙. (Reprinted from Bloom et al. 2012) action between the SN Ia ejecta and a dense CSM, with hydrogen emission lines and calcium features switching from absorption to emission during the supernova evolution. This would naturally be expected in a recurrent nova (i.e. single degenerate) system where the supernova ejecta encounters dense shells of material from earlier nova eruptions, although questions remain over the large CSM mass implied by the data. On the other hand, observations of SN Ia remnants many hundreds of years old have failed to locate any surviving companion stars (Schaefer and Pagnotta 2012). These would be expected in SN Ia remnants which originated from sin- gle degenerate systems, as the star from which material is accreted onto the white dwarf should survive the explosion. Furthermore, historical pre-supernova Hubble Space Telescope imaging of M101, the host galaxy of SN 2011fe (see figure 1), also rules out a luminous giant companion star at the site of the supernova explosion (Li et al. 2011; figure 6). Thus, a diversity in SN Ia progenitors seems very likely. Challenges for future surveys A critical component of any supernova survey is the spectroscopic follow-up of candidate events, confirming their nature and measuring the A&G • December 2013 • Vol. 54 The transient universe • Sullivan: Supernovae 6: Constraints on the companion star to the white dwarf that exploded as the Type Ia supernova SN 2011fe. This Hertzprung–Russell diagram is constructed from pre-explosion Hubble Space Telescope imaging of the supernova location. The parameter space above the yellow line is ruled out by the imaging data, excluding most red giants and many known recurrent nova systems (e.g. RS Oph). Other popular potential SN Ia progenitor systems, such as V445 Pup (a helium nova) and U Sco (a recurrent nova with a main sequence companion), are shown as grey shaded areas. The stellar main sequence is shown as a black line, with giant branches for different mass stars shown as coloured dots. (Reprinted from Li et al. 2011) redshifts essential for either their detailed study, confirmed samples, although with the clear posor placement on a Hubble diagram. Perhaps sibility of contamination in the samples from obviously, a larger telescope is usually required events that are not normal SNe Ia. Such potenfor spectroscopy than for discovery – for exam- tial contamination must be carefully accounted ple, events discovered on 1 m class telescopes are for in cosmological studies. However, photometric typing does not allow usually followed up on 3–4 m class facilities, and those fainter objects discovered on 3–4 m studies of the detailed physics of the supertelescopes require 8 m-class telescopes. This nova explosions, for which spectroscopy is quickly generates a bottleneck, with insufficient required to give chemical composition, nor telescope time available to observe all does it allow the identification of odd, the events discovered. As an exampeculiar or extreme events within A ple, the high-redshift Supernova the mainstream of a populasecond approach tion. Such events can often Legacy Survey (SNLS), part of the Canada–France–Hawaii have an important impact in is also needed: Telescope Legacy Survey, ambitious programmes the field. For example, two used some 1600 hours of extreme SNe Ia (the overdedicated to 8 m telescope time to clasluminous SN 1991T and the transient sify around 500 supernova under-lumious SN 1991bg) follow-up helped to establish the nowevents. This situation will be even worse for DES, which will famous brighter-slower/fainterfind 3500 SNe Ia alone. For future faster relation for SNe Ia used in projects finding even more events, the cosmological applications (the “Phillips situation will become impossible. Similar scal- relation”), while the apparently super-Chaning arguments apply to future local searches. drasekhar SN Ia SN 2003fg showed that not all The solution appears to be two-fold. For cos- SNe Ia need explode near the Chandrasekhar mological applications, photometric typing mass (Howell et al. 2006). Finally, of course, techniques have been developed which, with an superluminous supernovae would not be known after-the-event host galaxy spectroscopic red- were it not for spectroscopy. shift obtained with multi-object spectrographs, Thus a second, complementary approach such as AAOmega at the Anglo-Australian is also needed: ambitious programmes (or Telescope, can provide reliable classifications facilities) dedicated to transient follow-up. An (Campbell et al. 2013). This allows Hubble example is a new programme at the European diagrams to extend beyond spectroscopically Southern Observatory 3.6 m New Technology ‘‘ ’’ A&G • December 2013 • Vol. 54 Telescope: the Public ESO Spectroscopic Survey for Transient Objects (PESSTO). This public survey will attempt to address the lack of spectroscopic follow-up, at least for local supernova surveys, classifying some 2000 SNe over the next five years (>10% of all supernovae ever classified), and conducting detailed time-series studies of 150 events. Supernovae have a profound influence upon many diverse areas of astrophysics, including chemical enrichment, feedback in galaxy formation, cosmology and stellar evolution, and we are fortunate to be living through a golden age for their study. The combination of the discovery of the mysterious dark energy and the uncovering of new types of explosive optical transients, has motivated a diverse set of discovery and follow-up programmes; yet significant questions remain about many aspects of supernova rates, their light curves and spectra, their demographics, and the dependence of all of these properties on their environment and progenitor composition and configuration. Indeed, as we have discussed, in many cases even the nature of the star that explodes, or the configuration of the progenitor system, is still unclear. This picture can only improve over the next decade, with new facilities planned that will dwarf existing programmes. In particular, planned upgrades to the PTF to an approximately 40 square-degree imager, tentatively called the Zwicky Transient Factory, will allow thousands of square degrees to be surveyed every night to 21st magnitude. This will provide measurements of supernova evolution on very short, intra-night timescales, including the first hours after explosion probing the epoch of shock break-out. In the more distant future, the Large Synoptic Survey Telescope, an 8 m-class facility, will survey the entire visible sky to 24th magnitude every few nights. Surveys like these will undoubtedly uncover new explosive transient types, and provide a rich dataset to finally answer the outstanding questions in the field. ● Mark Sullivan, School of Physics and Astronomy, University of Southampton, UK; m.sullivan@ soton.ac.uk References Bloom J et al. 2012 ApJ 744 L17. Campbell H et al. 2013 ApJ 763 88. Cooke J et al. 2012 Nature 491 228. Dilday B et al. 2012 Science 337 942. Gal-Yam A 2012 Science 337 927. Gal-Yam A et al. 2009 Nature 462 624. Howell et al. 2006 Nature 443 308. Kasen D and Bildsten L 2010 ApJ 717 245. Li W et al. 2011 Nature in press arXiv:1310.4446. Nicholl M et al. 2011 Nature 480 348. Nugent P et al. 2011 Nature 480 344. Perlmutter S et al. 1999 ApJ 517 565. Quimby R et al. 2011 Nature 474 487. Riess A et al. 1998 AJ 116 1009. Schaefer B and Pagnotta A 2012 Nature 481 16. Sullivan M et al. 2011 ApJ 737 102. 6.21