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The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.
The rapid neutron capture process (r-process) is understood to be responsible for the synthesis of approximately half of all of the isotopes present in Solar System matter in the mass region from approximately zinc through the actinides. While the general features of this process were identified in the classic papers by B2FH (1957) and Cameron (1957), our current understanding of the r-process remains woefully incomplete. We have yet to cleanly identify which of the studied astrophysical sites contribute significantly to the observed abundance pattern. We have yet to reconcile the apparent duplicity of r-process sites with extant models for the operation of the r-process in diverse astronomical environments. While we may still remain theoretically challenged in our attempts to understand the r-process mechanism and to identify its site, significant clues have come from the observational side. Triggered by the first detections of the element europium (formed predominantly by the r-process) in low metallicity stars (Spite & Spite 1978), observations of heavy element abundances in halo stars have since served to provide tremendously important clues to the nature of the r-process mechanism. Identified constraints include: the utter dominance of the r-process contributions (over those of the s-process in extremely metal deficient stars; an extraordinary robustness of the r-process pattern in the mass range A[gsim ]130–140; and the demand for a second r-process site for the production of the A[lsim ]130 r-process nuclei. We will review these observational trends and theoretical models in the context of the Galactic (Cosmic) evolution of r-process abundances.
We have observed seven giants in the metal-poor globular cluster M15 using Subaru/HDS. We confirmed that there are significant star-to-star variations in the neutron-capture elemental abundances. This abundance variation means there were primordial chemical inhomogeneities in the proto-globular cluster cloud of M15. This result implies that there was insufficient time for complete mixing after r-process nucleosynthesis. It suggests that the main r-process occurs probably in supernovae which explode in later stages of globular cluster formation.
Damped Lyman-$\alpha$ (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at $z<1.5$, an epoch spanning $\sim 70$% of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at $z<1.5$. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the Solar value at low redshifts. These surprising results appear to contradict the near-Solar mean metallicity observed for nearby ($z \approx 0$) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly-$\alpha$ imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at $0<z<3.4$ appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies.
Elemental abundance patterns in very metal-poor halo field stars and globular cluster stars play a crucial role both in guiding theoretical models of nucleosynthesis and in providing constraints upon the early star formation and concomitant nucleosynthesis history of our Galaxy. The abundance patterns characterizing the oldest and most metal deficient stars ([Fe/H] ≤ −3) are entirely consistent with their being products of metal-poor massive stars of lifetimes τ ≤ 108 years. This includes both the elevated abundances of the alpha-elements (O, Mg, Si, S, Ca, and Ti) relative to iron-peak elements and the dominance of r-process elements over s-process elements. The nucleosynthetic contributions of lower mass AGB stars of longer lifetimes (τ ≈ 109 years) begin to appear at metallicities [Fe/H] ≈ −2.5, while clear evidence for iron-peak nuclei produced in supernovae Ia (τ ≥ 1-2x109 years?) does not appear until metallicities approaching [Fe/H] ~ −1. Similar trends are also suggested by abundances determined for gas clouds at high redshifts. We review the manner in which a knowledge of the abundances of the stellar and gas components of early populations, as a function of [Fe/H], time, and/or redshift, can be used to set constraints on their star formation and nucleosynthesis histories.
Recent spectroscopic studies of the elemental abundance patterns associated with extremely metal deficient field halo stars and globular cluster stars are briefly reviewed. These metal deficient stellar populations have been found to be characterized by abundance patterns which differ quite distinctly from those of solar system abundances, but are consistent with the view that they reflect primarily the nucleosynthesis products of the evolution of massive stars and associated Type II supernovae. Guided by our current knowledge of nucleosynthesis as a function of stellar mass occurring in stars and supernovae, we identify some interesting constraints upon theories of the formation and early history of our Galaxy.
Theoretical modeling of novae in outburst predicts that they should be active emitters of radiation at soft X-ray wavelengths twice during their outburst. The first time occurs very early in the outburst when only a very sensitive all sky survey will be able to detect them. This period lasts only a few hours for the very fastest novae. They again become bright in X-rays late in the outburst when the remnant object becomes very hot and is still luminous. Both simulations and observations show that novae can remain very hot for months to years. It is important to observe them at these late times because a measurement both of the flux and temperature can provide information about the mass of the white dwarf, the turn-off time scale, and the energy budget of the outburst.
Preliminary results of 1– and 2– dimensional hydrodynamical calculations of the common envelope phase in very slow classical novae are presented. We show that frictional deposition of orbital energy and angular momentum into the envelope can potentially induce mass loss. Specifically, we find that despite rapid initial spin–up of the envelope, ejection of mass in the orbital plane continues at a substantial rate.
Abundance determinations for the ejected shells of classical novae are reviewed. Substantial enrichments (relative to hydrogen) of helium, the CNO elements, and elements in the range from neon to aluminum are found to be characteristic of nova ejecta. The source of these enrichments is believed to be the dredge-up of underlying white dwarf core matter, thus reflecting the white dwarf composition and confirming the presence of oxygen-neon-magnesium (ONeMg) as well as carbon-oxygen (CO) degenerate dwarfs in cataclysmic variable systems. The implications of these abundance enrichments, for the behavior of nova systems in outburst and for nucleosynthesis, are examined.
Using the distribution of white dwarf masses in zero-age cataclysmic variables (CVs) calculated by Politano and Webbink (these proceedings), the relative frequency of classical nova outbursts as a function of white dwarf mass is calculated. These results are compared with the results obtained by Truran and Livio (1986), who calculated the same function, but used a distribution of white dwarf masses in CVs calculated from a Salpeter initial mass function and a particular progenitor mass-white dwarf mass relationship for single stars.
We wish to calculate the frequency of classical nova outbursts as a function of white dwarf mass. To do this, we require two quantities: 1) the recurrence frequency of classical nova outbursts (i.e., how many outbursts per year) and 2) the white dwarf mass spectrum in classical novae (i.e., number of novae per white dwarf mass). We discuss each of these in turn.
The nova outburst requires an energy source that is energetic enough to eject material and is able to recur. The Thermonuclear Runaway (TNR) model, coupled with the binary nature of nova systems, satisfies these conditions. The white dwarf/red dwarf binary nature of novae was first recognized as a necessary condition by Kraft (1963,1964, and these conference proceedings). The small separation characteristic of novae systems allows the cool, red secondary to overflow its Roche lobe. In the absence of strong, funneling magnetic fields, the angular momentum of this material prevents it from falling directly onto the primary, and it first forms a disk around the white dwarf. This material is eventually accreted from the disk onto the white dwarf. As the thickness of this hydrogen-rich layer increases, the degenerate matter at the base reaches a temperature that is high enough to initiate thermonuclear fusion of hydrogen. Thermonuclear energy release increases the temperature which in turn increases the energy generation rate. Because the material is degenerate, the pressure does not increase with temperature, which normally allows a star to adjust itself to a steady nuclear burning rate. Thus the temperature and nuclear energy generation increase and a TNR results. When the temperature reaches the Fermi temperature, degeneracy is lifted and the rapid pressure increase causes material expansion. The hydrogen-rich material either is ejected or consumed by nuclear burning, and the white dwarf returns to its pre-outburst state. The external source of hydrogen fuel from the secondary allows the whole process to repeat.
Significant progress in our understanding of the nature of the outbursts of the classical novae has occurred over the past two decades (see, e.g., reviews by Truran 1982; Starrfield 1986). Their outbursts are now understood to be driven by thermonuclear runaways proceeding in the accreted hydrogen-rich shells on the white dwarf components of close binary systems. Critical parameters which serve to dictate the varied characteristics of the observed outbursts include the intrinsic white dwarf luminosity, the rate of mass accretion, the composition of the envelope matter prior to runaway, and the white dwarf mass.
High S/N spectroscopic studies of the abundance patterns characterizing extremely metal-deficient halo field stars and globular cluster stars have served to provide significant clues to and increasingly stringent boundary conditions upon the chemical evolution of the halo population of our galaxy. Guided by our current knowledge of nucleosynthesis as a function of stellar mass occurring in stars and supernovae, we identify some interesting constraints that these combined observational and theoretical considerations impose upon theories of the early history of our galaxy.
Recently a number of studies have been published on the nuclear abundance of nova ejecta, as summarized by Truran and Livio (1986). H is always underabundant (compared to solar) and He is overabundant except for the cases where the heavier elements are far overabundant. The abundances of C, N, and O range from nearly solar to highly overabundant. A few novae are very rich in Ne and Mg as well as O, which has led to the discovery that these novae occur on O/Ne/Mg white dwarfs (Williams, et al., 1985). We will assume that the abundances are an accurate and consistently determined set of data for our purposes. The nova ejecta is a combination of original white dwarf material, remnant material, remaining on the white dwarf from the previous outburst, and accreted material, all of which has undergone thermonuclear processing during the outburst. The question we address here is “Can we untangle the observational abundances to determine the contributions of each source?” A positive answer would allow us to tell whether the white dwarf’s mass is increasing or decreasing and thus have implications on the accreting white dwarf model for a SNI.
The mass and energy of nova ejecta are 10-5 - 10-4M⊚ and 1044 - 1045 ergs, respectively (Payne-Gaposchkin 1957). Comparison of these quantities with the mass (1 M⊚) and binding energy (1051 erg) of the erupting white dwarf implies that the nova outburst is a surface event. From an average of two or three novae detected each year, it is estimated that the rate of novae is 40-50 per year in our galaxy (Payne-Gaposchkin 1954) . Comparing this rate with a white dwarf birth rate of 2 per year in our galaxy (Weidemann 1968), we conclude that the nova outburst is a recurrent phenomenon (cf. Ford 1978). The recurrent nature also implies that the white dwarf cannot be drastically altered from event to event, thus giving further evidence for a surface event. The argument of recurrency become even stronger when it is realized that observations strongly indicate a close binary structure for the nova candidates—the white dwarf and a red companion (Kraft 1964).
Kraft (1963) proposed the following hypothesis. The red companion overflows its Roche lobe and supplies hydrogen-rich material to an accretion disk around the white dwarf. This material eventually accretes onto the white dwarf, forming an hydrogen-rich envelope whose base is electron-degenerate. As the accretion proceeds, the temperature at the base of this envelope increases. This has little effect on the pressure, but greatly increases the thermonuclear energy generation. The thermonuclear energy generation, in turn, increases the temperature. This positive feedback loop leads to a thermonuclear runaway, which Kraft proposed as the cause of the nova outburst. A large number of theoretical studies based on this model have been carried out. These studies are presented in order of increasing realism and complexity in the following sections.
Questions concerning the origin and evolution of cataclysmic variables continue to be the subject of considerable inquiry and debate. Significant boundary conditions upon theoretical models may be imposed by our increasing knowledge of the characteristics of specific systems. It is the purpose of this contribution to argue that, within the framework of the thermonuclear runaway model, the classical novae are most reasonably interpreted as systems characterized by relatively massive white dwarf components.
Scrutiny of the light curves of the common novae yields important clues concerning both the nature of the nova outburst and the characteristics of the underlying white dwarfs. Ultraviolet and infrared observations have served to make available essentially complete bolometric light curves for several recent novae. These data confirm our earlier prediction that, following maximum, both fast and slow novae experience an epoch (of varying duration) of substantially hydrostatic evolution defined by thermonuclear burning of the residual hydrogen fuel at constant bolometric luminosity. Theoretical studies reveal that the luminosity during this phase of a nova’s evolution is well represented by the Paczynski core mass-luminosity relation for such shell burning configurations involving degenerate stellar cores. This luminosity represents, as well, an increasingly significant fraction of the Eddington luminosity with increasing white dwarf mass.
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