* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Core-collapse supernovae and their massive progenitors
Canis Minor wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Constellation wikipedia , lookup
Corona Borealis wikipedia , lookup
Nebular hypothesis wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Corona Australis wikipedia , lookup
Theoretical astronomy wikipedia , lookup
History of gamma-ray burst research wikipedia , lookup
Observational astronomy wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Canis Major wikipedia , lookup
History of supernova observation wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Star catalogue wikipedia , lookup
Stellar classification wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Corvus (constellation) wikipedia , lookup
H II region wikipedia , lookup
Gamma-ray burst wikipedia , lookup
Stellar kinematics wikipedia , lookup
Crowther, Smartt: Meeting report Stellar winds and evolution Relative to lower mass stars, the evolution of high-mass stars is complicated by (a) the metallicity dependence of their radiatively line-driven stellar winds, causing weaker winds at low metallicity, and (b) their initial rotational velocities, providing rotationally induced mixing within their interiors. It is only within the past decade that allowances for both effects have been made within evolutionary models. O star winds are driven primarily by CNO and Fe-peak elements, with a predicted dependence of M· ∝ Z0.7, that is supported by empirical mass-loss rates for stars within the Milky Way, LMC and SMC (e.g. figure 1) drawn from the VLT FLAMES survey of massive stars (Evans et al. 2005, 2006). The agreement between predicted and empiriA&G • February 2007 • Vol. 48 Core-collapse supernovae and their massive progenitors In this summary of the 10 November 2006 RAS Specialist Discussion Meeting, organizers Paul Crowther and Steve Smartt consider aspects of massive stellar evolution, their dependence upon stellar winds, progenitor models and explosion mechanisms. In addition, observations of core-collapse supernovae were discussed alongside the question of whether supernovae form significant quantities of dust, of relevance to the detection of dust in star-forming galaxies in the early universe. 1: Empirical wind momenta (Dmom = M·v∞√R) for luminous O stars in the Milky Way, LMC and SMC (dwarfs: black circles, giants: blue triangles, supergiants: red squares) from Mokiem et al. (2006) and theoretical predictions (dotted lines) for SMC metallicity from Vink et al. Good agreement is achieved after correction by a factor of ~2 to allow for the sensitivity of strong winds to wind clumping (open symbols). For the Milky Way and LMC, the empirical metallicity dependence (dM/dt ∝ Z0.8) agrees well with theory (dM/dt ∝ Z0.7) (Mokiem et al. 2007). 30 29 log Dmom M assive stars form in star clusters within star-forming galaxies, pollute the interstellar medium and inject energy and momentum via powerful stellar winds and core-collapse supernovae. Direct detection of massive stars via the UV continua and indirectly via ionized H ii regions provides some of the most stringent constraints upon the physical properties of galaxies at high-redshift. Within the past few years a direct connection has been established between certain core-collapse supernovae (SNe) and gamma-ray bursts (GRBs), supporting the collapsar model in which the GRB results from the death throes of a rapidly rotating carbon–oxygen (Wolf–Rayet) star. The Initial Mass Function favours the formation of low- and intermediate-mass stars over highmass stars. The division is usually set at 8 M⊙ – the boundary between stars ultimately forming a CO white dwarf or producing a Fe-core and under going a core-collapse SNe. Spectroscopically, stars with initial masses of 8–25 M⊙ are B dwarfs on the main sequence, or O dwarfs at higher initial mass, and are distinguished by the presence of strong stellar winds from radiation pressure, arising from their high surface temperatures and luminosities. These strong winds have a major effect upon the evolution of massive stars. High-mass stars possess convective cores and radiative envelopes, a situation reversed in the Sun and low-mass stars. While there is energy transport from the convective and radiative regions, only the convective core participates in nuclear reactions, unless hydrogen-rich material is mixed downwards from the outer zones. Once the core hydrogen is exhausted, the star leaves the main sequence and becomes a blue supergiant, and ultimately a red supergiant (RSG) for stars with initial mass up to perhaps 20–30 M⊙. Observationally, there is an absence of luminous RSGs, known as the Humphreys–Davidson limit, such that initially more massive stars circumvent the RSG phase, pass through a Luminous Blue Variable stage, before ending their life as Wolf–Rayet stars. 28 27 26 25 4.0 cal mass-loss rates of O stars is impressive, albeit subject to a modest (factor of two) reduction in empirical results from wind clumping in the Hαforming region. The origin of wind structure is thought to be due to the intrinsic instability of radiatively driven winds. However, establishing robust clumping factors for O stars remains challenging. Indeed, far-UV diagnostics suggest massloss rates of up to a factor of 10 less than predicted rates. Clumping aside, empirical mass-loss rates support the theory of radiatively driven winds of O-type stars, although the situation for B supergiants is less satisfactory. Optical Hα line profile and radio free-free continua-derived empirical 4.5 5.0 5.5 log (L/LSun) 6.0 6.5 mass-loss rates fall substantially below theoretical predictions; correction for wind clumping would make the comparison still worse. The FLAMES survey also provides valuable information on the initial rotational velocities of O stars. Evolutionary models have been calculated for various initial rotational velocities. Rotation rates of Milky Way O stars are well known, but their strong winds make the stars spin down rapidly, while few Magellanic Cloud measurements were available before the FLAMES survey. A comparison between LMC and SMC stars, drawn from a homogeneous dataset, indicate higher rotational velocities for 1.35 Crowther, Smartt: Meeting report the SMC stars, typically 175 ± 100 km s–1 (Mokiem et al. 2006). Rotational velocities have not been significantly affected by stellar winds, and are significantly lower than 300 km s–1 presently adopted in evolutionary models of Meynet and Maeder (2000). Although the majority of high-mass stars undergo classical core-collapse during the red supergiant phase, due to iron disintegration in their cores, very massive (Super) Asymptotic Giant Branch (SAGB) stars may explode as electron capture supernovae (Eldridge, this meeting). Such stars, with initial masses of approximately 9 M⊙ undergo carbon burning and a thermal pulse making the core grow until either the entire envelope is ejected (producing a massive white dwarf) or encountering electron captures on 20Ne and 24 Mg, triggering a supernova. Poelarends et al. (2006) indicate up to 10% of core-collapse SNe are due to SAGB stars at solar metallicity, with a higher fraction in metal-poor environments. Type II-P SNe (“P” stands for the plateau phase in their lightcurve) are by far the most common type of core-collapse SNe, with an expected RSG progenitor, due to their extended, massive H-rich envelopes, for which single-star evolutionary models suggest initial masses of up to about 20 M⊙. Observationally, RSG progenitors for a number of Type II-P SNe are confirmed from pre-explosion images (e.g. figure 2). Type IIL and IIb supernovae are less common, with a relatively low-mass hydrogen envelope, and denser circumstellar media. These result either from somewhat more massive single stars (up to approx 25 M⊙), or close binaries, with red, yellow or blue supergiant progenitors. Type IIn SNe are rarer still, with a dense circumstellar environment, arising either from a single H-rich star with a very dense wind (possibly Luminous Blue Variables) or an interacting binary. It is not yet established whether H-rich LBVs may explode as a supernova, without first evolving to the blue to the H-poor Wolf–Rayet (WR) stage. At solar metallicity, stars initially more massive than ~25 M⊙ end their lives as either a nitrogenrich (WN) or carbon-rich (WC) WR star. WN and WC stars are believed to be the immediate progenitors of a subset of Type Ib (H-poor) and Type Ic (H and He-poor) supernovae, respectively, whose circumstellar environment matches that of a WR star. Observationally, SN 2002ap (Type Ic) so far provides the most stringent constraints upon a potential WR progenitor, revealing an upper limit of MB = –4 mag, in common with a subset of Magellanic Cloud WC stars (Eldridge, this meeting). The observed ratio of Type Ib/c SN to Type II lies between expectations for single, rotating stars and close binaries. Wolf–Rayet stars possess higher wind densities than other early-type stars, although they too have been reduced in recent years due to wind clumping, which is more readily measured than for their O star cousins. Evolutionary models 1.36 2: Composite INT image of M74 (NGC 628) showing the pre-explosion star of SN2003gd (top inset, Gemini) and six months after the supernova (bottom inset, INT), for which a RSG progenitor of the type II-P SN was deduced, based additionally upon HST WFPC2/ACS imaging. (Smartt et al. 2004) (ING). for the Wolf–Rayet stage have typically assumed metallicity independent mass-loss rates, which both observational (Crowther et al. 2002) and theoretical (Vink and de Koter 2005) evidence now challenges. The metallicity dependence of WN winds appears to be similar to O stars, with a somewhat weaker dependence for WC stars due to their high (primary) carbon and oxygen abundances, of relevance to the observed ratio of WC to WN stars predicted by evolutionary models (e.g. Eldridge and Vink 2006). Gamma-ray bursts The explosion mechanism for core-collapse supernovae is now receiving close attention due to the clear link now established between some long-duration gamma-ray bursts (GRBs) and bright Type Ic supernovae. The two discoveries that revolutionized this field were SN 1998bw/ GRB 980425 (Galama et al. 1998) – the first discovery of an explosion that produced both a GRB and a SN – and SN 2003dh/GRB 030329 (Stanek et al. 2003). In more distant GRBs, although the afterglow is normally observed, the supernovae are intrinsically much fainter and hence are often undetected. This confirmed the collapsar model, involving a compact, rotating hydrogendeficient massive progenitor, i.e. Wolf–Rayet star (MacFadyen and Woosley 1999). Rotation is critically important since the collapsar model involves highly collimated jets produced along the polar axes, arising from a dense, equatorial accretion disc feeding the central black hole. The dense stellar winds from WR stars hinder the direct measurement of rotational velocities, but polarimetry favours negligible deviation from spherical symmetry in most solar metallicity WR stars. An unsolved challenge to evolutionary models involves the requirement of a high angular momentum within the Wolf–Rayet core in the collapsar model. Evolutionary models allowing for magnetic fields involve cores that are efficiently spun down before collapsing, in most models, either due to the shear between the slowly rotating RSG envelope and core, or loss of angular momentum during the WR phase as a result of its high mass-loss rate. These permit the observed rotational rates of young pulsars (e.g. a period of 33 ms for the Crab pulsar, figure 3) to be reproduced. However, collapsars would “The mission lifetime of SWIFT combined with follow-up from ground and space promises more exciting discoveries and solutions” A&G • February 2007 • Vol. 48 Crowther, Smartt: Meeting report nova did occur but that it was extremely faint, of similar magnitude to faint Type II SNe. The location of long-duration GRBs in HST images of their host galaxies suggests that they are more likely to be associated with the brightest regions of star formation in irregular host galaxies than normal Type II SNe (Fruchter et al. 2006). The mission lifetime of SWIFT combined with follow-up from ground and space promises more exciting discoveries and solutions to the outstanding puzzles in this area linking high-energy physics to the last stages of massive stellar evolution. Supernovae explosions 3: This image of the Crab nebula encompasses our knowledge of supernovae. A bright new star was recorded in 1054 by Chinese astronomers, four times brighter than Venus at visible in the daylight. This was the supernova that created the Crab nebula. The nebula is rich in heavy elements such as oxygen, silicon, neon and iron. A spinning neutron star is visible at the core of the remnant, from the collapse of massive star progenitor. Warm dust glowing at mid-infrared wavelengths has been detected and the mass estimated at about 0.01 M⊙. This composite image of the Crab nebula (6 arcmin or 3.7 pc across) uses data from WFPC2 aboard the Hubble Space Telescope, with blue, green and red images sensitive to neutral oxygen, ionized sulphur and doubly ionized oxygen. (STScI-2005-37, NASA, ESA and J Hester) require an order of magnitude shorter periods of <2 ms (Woosley and Heger 2006). At low metallicity, initially high rotational velocities may be capable of avoiding the extended envelope, leading to a near “chemically homogeneous” evolution (Maeder 1987). The mechanical mass-loss induced spin-down during the He star may be avoided since low-metallicity WR stars are believed to possess weak winds, resulting in sufficient angular momentum in the core upon core-collapse (Yoon and Langer 2005). SMC O stars from the VLT FLAMES survey provide some observational evidence in support of such extreme evolution (Mokiem et al. 2006). Of course, only a tiny fraction of SNe produce a GRB, with an apparent bias towards metal-poor environments. This favours the above scenario, with respect to alternative close binary models that would not necessarily show a low-metallicity bias. Langer and Norman (2006) have shown that the observed statistics of GRBs could be explained by considering all sufficiently high-mass stars at low (<1/10 Z⊙) metallicity, i.e. those whose end product is a black hole. In reality, it is perhaps the A&G • February 2007 • Vol. 48 high rotation velocity tail of less metal-deficient stars (<1/3 Z⊙) that provide GRB progenitors. As such GRBs would trace the low-metallicity star-formation history of the universe. As the SNe are significantly fainter than the burst afterglows, it is only after the power-law afterglow fades (after about 10 days) that the SNe are seen as late bumps or flux excesses. Until recently, all long-duration GRBs that were close enough for SN detection did indeed show Type Ic features. However, the recent, very close events GRB 060505 and GRB 060614 show no sign of supernovae down to magnitudes of around MR = –14 mag (Fynbo et al. 2006). Three possible reasons have been given. The most prosaic is that the GRB was much more distant and its spatial location within an apparent host galaxy is a chance coincidence, and hence the SN would be too faint to be detected (Cobb et al. 2006). The second is that this could be a new explosion mechanism and formation channel for long-duration GRBs that does not involve core-collapse of a massive, rotating progenitor (Gehrels et al. 2006). The third is that a super- Observations of the luminosity and the kinetic energy of core-collapse supernovae are vital to constrain the explosion models and determine if there is any link between the explosion mechanism and mass of the star. There are peculiar Type II-P SNe that have distinctly lower luminosities and kinetic energies (measured by the expansion velocity of the ejecta) than normal (Pastorello et al. 2004). Explosion models of 8–9 M⊙ stars have been made by Kitaura et al. (2006) in which electron capture by an oxygen–neon–magnesium core triggers collapse, for which a SAGB star is a possible progenitor (Poelarends et al. 2006). Another plausible model for these supernovae are high-mass progenitors that form a blackhole, for which fall-back onto the compact object reduces the overall observed energy (Zampieri et al. 2003). The direct detection of the progenitor star of SN 2005cs (8–12 M⊙), which is a faint II-P explosion (Pastorello et al. 2006), suggests the low-mass scenario is valid for this particular supernova. But the lightcurve model for SN 2003Z, for example, suggests the high-mass scenario is required to explain the velocity and lightcurve evolution. A larger sample of events and a detailed characterization of their physical parameters is required to determine which of the progenitor scenarios is the most likely. The physical mechanism that produces a supernova explosion from the last stages of stellar evolution has been studied for several decades. The energy source of the explosion is the reservoir of gravitational potential energy locked up in the Chandrasekhar-mass iron core which is somehow converted into the kinetic, neutrino and electromagnetic energy we can observe. The first models employed a hydrodynamic bounce of the collapsing mantle off the newly formed proto-neutron star with the shock accelerated by deposition of neutrino energy. However, both 1‑D and multi-D models incorporating the best known neutrino physics have not resulted in explosions consistently across a range of progenitor masses. In general, models for stars of higher initial mass than 8–9 M⊙ fail to revive the stalled shock with neutrino heating. Burrows et al. (2006) have recently proposed a new alternative in which the core reverberates 1.37 Crowther, Smartt: Meeting report and generates strong sound waves. They suggest that such acoustic power could potentially transport energy to the mantle to drive the explosion. The Burrows group has developed a 2‑D model of the collapse of an 11 M⊙ star and followed accretion onto the proto-neutron star beyond 500 ms after the bounce (figure 4). An acoustic oscillation arises at 200 ms and the power generated by the oscillating core is enough to drive the explosion. This mechanism was not identified previously as models were stopped at 200–300 ms and the oscillations in the core were either removed or not followed fully in 2‑D, which hence suppressed the acoustic vibrations. Burrows (this meeting) suggests that hybrid models of neutrino-driven and acoustic-driven explosions may be able to explain core-collapse across a wide range of progenitor masses. Their models make definite predictions for the neutrino and gravitational wave spectra that are emitted and which are highly dependent on the physical parameters of the explosion. The acoustic mechanism is by nature highly aspheric and is the most promising model to explain the explosion of massive progenitors and the high space velocities seen in the galactic pulsar population. A critical test for the models will come when next Local Group or Milky Way core-collapse supernova explodes. Cosmic dust production When a massive star explodes as a supernova and the ejecta expand and cool, they may be important sites for the formation of cosmic dust. The detection and characterization of dust in the early universe is important for studies of abundances in damped Lyman-α systems, cooling of molecular clouds and star formation, and the determination of the star formation rate. It seems likely that a rapid injection of dust in the universe occurred during the first 1 Gyr; prime suspects are supernovae from massive stars (Todini and Ferrara 2001). There has been little work on characterizing dust formation in nearby SNe, but Spitzer Space Telescope results are begining to change the field. Two groups with significant UK leadership are studying nearby core-collapse SNe with Spitzer. If core-collapse SNe are indeed important contributors to dust production at high-redshift, then we should be measuring dust masses of around 0.1–1 M⊙ in their ejecta. Sugerman et al. (2006) claim to have detected mid-infrared excesses consistent with cooling dust in the ejecta of the Type II-P SN 2003gd (recall figure 2) during the period 499–678 days after outburst. Their radiation transfer model predicts up to 0.02 M⊙ of dust has formed. In an earlier study, an enormous mass of dust of 0.1–0.15 M⊙ (Barlow et al. 2005) was estimated to have formed in an optically thick dust shell around SN 2002hh (another II-P). However, Meikle et al. (2006) showed that most of the strong mid-IR emission originated in this cool, obscured star-formation region or 1.38 4: Simulation from Burrows et al. (2006) representing an isodensity plot (with a wedge excised) of the exploding core for a 11 M⊙ model in which the sonic power is driven mostly by a core oscillation that is excited by the violent accretion streams, powered by the energy of gravitational infall. molecular cloud along the line-of-sight, such that the mass of the pre-existing dust in the SN circumstellar medium required to produce the observed infrared echo is about 0.04 M⊙. Hence there is no clear evidence of large amounts of dust condensing in the ejecta of SN 2002hh. Further analysis of three other nearby II-P SNe by Meikle (this meeting) also suggest low amounts of dust in the ejecta (of order between 10–3 and 10–4 M⊙), which is not enough to make them cosmologically significant producers. Both teams remark on the possibility that the dust is significantly clumped and are working on how to deal with this issue. In a further development, the molecule SiO has been observed in the mid-infrared spectra of the Type II-P SN 2005af in the 3.6–24 µm window with Spitzer (Kotak et al. 2006). It is thought that this molecule is a key stage in condensation to dust in the ejecta. It is hoped that during the lifetime of Spitzer we may learn if supernovae really are key to high-redshift dust production. Cooler dust may be probed in the sub-mm in the future with ALMA. ● Dr Paul Crowther, University of Sheffield (Paul. [email protected]); Prof. Steve Smartt, Queen’s University Belfast ([email protected]). References Barlow M et al. 2005 ApJ 627 L113. Burrows A et al. 2006 ApJ 640 878. Crowther P et al. 2002 A&A 392 653. Cobb et al. 2006 ApJ 651 L85. Eldridge J and Vink J 2006 A&A 452 295. Evans C et al. 2005 A&A 437 467. Evans C et al. 2006 A&A 456 623. Fruchter A et al. 2006 Nature 441 463. Fynbo J P U et al. 2006 Nature 444 1047. Galama T et al. 1998 Nature 395 670. Gehrels N et al. 2006 Nature 444 1044. Kitaura F S et al. 2006 A&A 450 34. Kotak R et al. 2006 ApJ 651 L117. Langer N and Norman C 2006 ApJ 638 L63. MacFadyen A and Woosley S 1999 ApJ 524 262. Maeder A 1987 A&A 178 159. Meikle et al. 2006 ApJ 649 332. Meynet G and Maeder A 2000 A&A 361 101. Mokiem R et al. 2006 A&A 456 1131. Mokiem R et al. 2007 A&A to be submitted. Pastorello A et al. 2004 MNRAS 347 74. Pastorello A et al. 2006 MNRAS 370 1752. Poelarends A et al. 2006 Mem. Soc. Astr. Ital. 77 846. Smartt S et al. 2004 Science 303 499. Stanek K Z et al. 2003 ApJ 591 L17. Sugerman B et al. 2006 Science 313 196. Todini P and Ferrara A 2001 MNRAS 325 726. Vink J et al. 2001 A&A 369 574. Vink J and de Koter A 2005 A&A 442 587. Woosley S and Heger A 2006 ApJ 637 914. Yoon S and Langer N 2005 A&A 443 643. Zampieri L et al. 2003 MNRAS 338 711. A&G • February 2007 • Vol. 48