ABSTRACT

The Imager for Mars Pathfinder (IMP) was a mast-mounted instrument on the Mars Pathfinder (MPF) lander which landed on Mars' Ares Vallis floodplain on July 4, 1997. During the 83 sols of MPF landed operations, the IMP collected over 16 600 images. Multispectral images were collected using 12 narrowband filters at wavelengths between 400 and 1000 nm in the visible and near-infrared (VNIR) range. The IMP provided VNIR spectra of the materials surrounding the lander including rocks, bright soils, dark soils, and atmospheric observations. During the primary mission, only a single primary rock spectral class, “Gray Rock,” was recognized; since then, “Black Rock” has been identified. The Black Rock spectra have a stronger absorption at longer wavelengths than do Gray Rock spectra. A number of coated rocks have also been described, the Red and Maroon Rock classes, and perhaps indurated soils in the form of the Pink Rock class. A number of different soil types were also recognized with the primary ones being Bright Red Drift, Dark Soil, Brown Soil, and Disturbed Soil. Examination of spectral parameter plots indicated two trends which were interpreted as representing alteration products formed in at least two different environmental epochs of the Ares Vallis area. Subsequent analysis of the data and comparison with terrestrial analogs have supported the interpretation that the rock coatings provide evidence of earlier Martian environments.

Models of planetary growth are based upon data from our own Solar System, as well as observations of extrasolar planets and the circumstellar environments of young stars. Collapse of molecular cloud cores leads to central condensations (protostars) surrounded by higher specific angular momentum circumstellar disks. Planets form within such disks, and play a major role in disk evolution. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies—until the planetary orbits become separated enough that the configuration is stable for the age of the system. Giant planets begin their growth as do terrestrial planets, but they become massive enough to accumulate substantial amounts of gas before the protoplanetary disk dissipates. A potential hazard to planetary systems is radial decay of planetary orbits, resulting from interactions between the planets and the natal disk. Massive planets can sweep up disk material in their vicinity, eject planetesimals and small planets into interstellar space or into their star, and confine disks in radius and azimuth. Small planetary bodies (asteroids and comets) can sequester solid grains for long periods of time and subsequently release them.

ABSTRACT

The twin Mars Exploration Rovers (MER) Spirit (Gusev crater) and Opportunity (Meridiani Planum) used miniaturized Mössbauer spectrometers (MIMOS II) to analyze Martian surface materials in the first application of extraterrestrial Mössbauer (MB) spectroscopy. The instruments acquired spectra that identified the speciation of Fe according to oxidation state, coordination state, and mineralogical composition and provided quantitative information about the distribution of Fe among oxidation states, coordination states, and Fe-bearing phases. A total of 12 unique Fe-bearing phases were identified: Fe2 + in olivine, pyroxene, and ilmenite; Fe2 + and Fe3 + in magnetite and chromite; Fe3 + in nanophase ferric oxide (npOx), hematite, goethite, jarosite, an unassigned Fe3 + sulfate, and an unassigned Fe3 + phase associated with jarosite; and Fe0 in kamacite. Weakly altered basalts at Gusev crater (SO3 = 2.5 ± 1.4 wt.% and Fe3 +/FeT = 0.24 ± 0.11) are widespread on the Gusev plains and occur in less abundance on West Spur and Husband Hill in the Columbia Hills. Altered low-S rocks (SO3 = 5.2 ± 2.0 wt.% and Fe3 +/FeT = 0.63 ± 0.18) are the most common type of rock in the Columbia Hills. Ilm-bearing, weakly altered basalts were detected only in the Columbia Hills, as was the only occurrence of chromite in an altered low-S rock named Assemblee. Altered high-S rocks (SO3 > 14.2 wt.% and Fe3 +/FeT = 0.83 ± 0.05) are the outcrop rocks of the ubiquitous Burns formation at Meridiani Planum.

Gravitational torques between a planet and gas in the protoplanetary disk result in orbital migration of the planet, and modification of the disk surface density. Migration via this mechanism is likely to play an important role in the formation and early evolution of planetary systems. For masses comparable to those of observed giant extrasolar planets, the interaction with the disk is strong enough to form a gap, leading to coupled evolution of the planet and disk on a viscous time scale (Type II migration). Both the existence of hot Jupiters and the statistical distribution of observed orbital radii are consistent with an important role for Type II migration in the history of currently observed systems. We discuss the possibility of improving constraints on migration by including information on the host stars' metallicity, and note that migration could also form a population of massive planets at large orbital radii that may be indirectly detected via their influence on debris disks. For lower mass planets with Mp ~ M⊕, surface density perturbations created by the planet are small, and migration in a laminar disk is driven by an intrinsic and apparently robust asymmetry between interior and exterior torques. Analytic and numerical calculations of this Type I migration are in reasonable accord, and predict rapid orbital decay during the final stages of the formation of giant planet cores.

ABSTRACT

The Alpha Particle X-Ray Spectrometers (APXSs) on board the Mars Exploration Rovers (MERs) determine the elemental compositions of Martian samples. Improvements to the version of the instrument flown on the Mars Pathfinder (MPF) mission allow, for the first time, in situ detection and quantification of trace elements such as nickel, zinc, and bromine. The APXS measurements are performed by placing the sensor head against or immediately above the sample surface. A wealth of compositional diversity has been discovered at the two MER landing sites. At Gusev crater, fresh rock surfaces in the plains resemble primitive basalts, while rocks in the Columbia Hills are significantly weathered and enriched in mobile elements such as phosphorus, sulfur, chlorine, and bromine. Sandstones cemented by sulfates as well as evidence for clay formation have also been found in the Columbia Hills. At Meridiani Planum, the layered sedimentary rocks were found to consist primarily of sulfates mixed with siliciclastic debris. Iron-rich spherules and their fragments, confirmed to be hematitic by the Mössbauer spectrometer (MB), are found armoring the soil bedforms as well as embedded in the outcrop rocks. A variety of unusual objects, including an iron-nickel meteorite and a likely ejecta fragment similar to a Martian meteorite, have also been discovered. The elemental compositions of soils analyzed at both sites are remarkably similar, indicative of global-scale homogenization or the similarity of the soil precursors.

Introduction

Dust and molecule formation in the winds of classical novae is now a well-observed, if poorly understood, phenomenon. Although the presence of dust in nova ejecta was not confirmed observationally until the 1970 infrared observations of FH Ser (Geisel, Kleinmann & Low, 1970), the subject has a much longer history. Dust formation in a classical nova was first proposed by McLaughlin (1935) to explain the precipitous deep minimum in the visual light curve of DQ Her; coincidentally, DQ Her was also the first nova to display observational evidence of a molecule (CN; Wilson & Merrill, 1935).

At first sight, the hostile environment of a classical nova is not an obvious place to find molecules and dust. This is because the chemistry that leads to the formation of first diatomic, then polyatomic, molecules, and eventually dust, requires an environment that is well shielded from the increasingly hard radiation field of the stellar remnant.

In this chapter the observational evidence for the presence of dust and molecules in the environments of classical novae is reviewed, together with the current state of our theoretical understanding of chemistry in nova winds, and of the growth and processing of the dust.

Molecules in nova ejecta

Preamble

Although CN holds the distinction of being the first molecule to be observed in the spectrum of a nova, in the optical spectrum of DQ Her in 1933 (Wilson & Merrill, 1935), molecules are in general most easily detected in the near- and mid-infrared, where common diatomic molecules (such as CO, H2, SiO) have rotational–vibrational transitions.

Introduction

The most energetic photons from the whole electromagnetic spectrum are the γ-rays, with energies larger than about 10 keV, i.e. wavelengths shorter than 1.2 Å and frequencies larger than 2.4×1018 Hz. These photons trace the most energetic phenomena of the Universe and, in particular, stellar explosions such as classical novae or supernovae.

The potential of novae as γ-ray emitters was first pointed out by Clayton and Hoyle (1974). In that paper the authors stated that observable γ-rays from novae would come from electron–positron annihilation, with positrons from 13N, 14O, 15O, and 22Na decays, as well as a result of the decay of 14O and 22Na to excited states of 14N and 22Ne nuclei, which de-excite by emitting photons at 2.312 and 1.274 MeV respectively. The need for fast mixing within the nova envelope, in order to have a favorable interplay between the transparency of the expanding envelope and the short lifetimes of the radioactive nuclei (22Na excluded), was emphasized. The main ideas presented in that seminal work have remained unchanged; but some aspects have changed in the past 20 years, mainly related to new detailed nucleosynthesis studies of novae, as will be explained in this review.

Seven years later, in a new important paper, Clayton (1981) noted that another γ-ray line could be expected from novae when 7Be transforms (through an electron capture) to an excited state of 7Li, which de-excites by emitting a photon of 478 keV.

Introduction

The classical nova outburst is one consequence of the accretion of hydrogen-rich material onto a white dwarf (WD) in a close binary system. Over long periods of time the accreting material gradually forms a layer of fuel on the WD and the bottom of this layer is gradually compressed and heated by the strong surface gravity of the WD. Ultimately, the bottom of the layer becomes electron-degenerate. The degeneracy of the material then contains the explosion so that, once nuclear burning in the layer bottom reaches thermonuclear runaway (TNR) conditions, the temperatures in the nuclear burning region will exceed 108 K under almost all circumstances. As a direct result, a major fraction of the nuclei in the envelope capable of capturing a proton (C, N, O, Ne, Mg …) are transformed into β+-unstable nuclei, which limits nuclear energy generation on the dynamical time-scale of the runaway and yields distinctively non-solar CNO isotopic abundance ratios in the ejected gases.

Observations of the outburst show that a classical nova explosively ejects metal enriched gas and grains and this material is a source of heavy elements for the interstellar medium (ISM). The observed amount of metal enrichment demands that mixing of the accreted material with core material occur at some time during the evolution of the outburst.

Introduction

X-ray observations of novae in outburst have turned out to be a very powerful tool for the study of novae. The X-ray regime is best suited to study the hot phases in a nova outburst. Unfortunately, the present status of our knowledge is still rudimentary, quite different from that of other spectral ranges such as the ultraviolet, visible, or infrared. Few novae have been observed so far in X-rays and essentially all of them lack observations over a complete outburst cycle. In the visible spectral range novae have been systematically observed over more than a century, and in the infrared and ultraviolet many systematic observations which cover the whole outburst cycle have been obtained. So the picture which has emerged from X-ray observations is far less systematic than that from the abovementioned spectral regions (cf. for example Chapters 9 and 8). However, although by no means plentiful, the X-ray observations carried out so far have provided many fundamental and sometimes totally unexpected new results.

GQ Mus was the first classical nova from which X-ray emission was detected during the outburst (Ögelman, Beuermann & Krautter, 1984). In 1984 April GQ Mus was detected at about 460 days after outburst at a 4.5σ level with the low-energy telescope (0.04–2 keV) and the Channel Multiplier Array (CMA) detector aboard EXOSAT.

Introduction

Infrared observations have contributed substantially to our understanding of how classical novae participate in the chemical evolution of the Galaxy. We describe how infrared observations, combined with optical measurements, can provide quantitative measurements of the primary physical parameters that characterize the outburst, the abundances of elements that are present in the ejecta, and the properties of the grains that condense in the nova wind. We summarize recent evidence that novae are capable of producing large over-abundances of some metals and that they are potential sources of ‘stardust’ similar to the small grains that populate comet comae.

Nova explosions in the context of Galactic chemical evolution

Galactic classical novae take part in a cycle of Galactic chemical evolution in which grains and gas in the ejecta of evolved stars enrich the metal abundance of the Galactic ‘ecosystem’ (see Figure 8.1). Metals produced in stars during post-main-sequence (PMS) nucleosynthesis are ejected into the interstellar medium (ISM) by stellar winds and explosive events. Some of these metals remain in the gas phase, but others can condense to form grains. In the ISM, the gas and grains may become a component of the giant molecular clouds that give birth to new generations of young stars and planetary systems during the star formation process.

Introduction

Nova, abbreviated from stella nova, means new star (the plural form is [stellae] novae). Although the Merriam-Webster dictionary indicates its etymological origin to be in New (Renaissance) Latin, the term is in fact found in C. Plinius Secundus, Naturae Historia, Book 2, chapter XXIV, written around AD 75 (Pliny, 1855)

although the somewhat obscure text would also permit an identification with a meteor or comet.

Because of the Aristotelian doctrine of the immutability of the translunar regions, such an object in the stellar regions would not fit into Aristotle's world view, and other objects now known to be translunar, such as comets, were considered to be atmospheric objects and logically discussed in his book on meteorology (and meteors do indeed belong in that book!).

Introduction

We discuss the evolution of both stellar components of cataclysmic variables (CVs) and symbiotic stars from formation to termination, identifying the modes and estimating the rates of mass tranfer as functions of the period of the system, and suggesting how the composition of the nova ejecta depends on the rate of mass transfer, the processes of mixing between accreted material and material in the underlying white dwarf, and the mass-transfer, mixing, and prior outburst history of the system.

Left completely out of the discussion is the disk component which, in CVs and perhaps in some symbiotic stars, mediates mass transfer between the mass donor and the white dwarf accretor. We begin with an outline of the topics to be discussed.

1. Definitions: CVs are here defined as close binary systems in which one component is a CO or ONe white dwarf and the other is a Roche-lobe-filling (or nearly Roche-lobe-filling) star which can be a main-sequence star, a small hydrogen-rich white dwarf, a red giant (Algol-like CV) or a helium white dwarf or helium main sequence star (helium CV). In symbiotic stars, the companion of the white dwarf is a red giant or Asymptotic Giant Branch (AGB) star which, in general, does not fill its Roche lobe.

2. […]

Introduction

The modeling and analysis of early nova spectra have made significant progress since the first edition of this book. The main culprit is the author, via the construction of detailed model atmospheres and synthetic spectra for novae (Hauschildt et al., 1992, 1994a,b, 1995, 1996, 1997; Pistinner et al., 1995; Schwarz et al., 1997; Short et al., 1999; Schwarz et al., 2001; Short et al., 2001; Shore et al., 2003).

In the early stages of the nova outburst, the spectrum is formed in an optically very thick (in both lines and continua) shell with a flat density profile, leading to very extended continuum and line-forming regions (hereafter, CFR and LFR respectively). The large variation of the physical conditions inside the spectrum-forming region makes the classical term ‘photosphere’ not very useful for novae. The large geometrical extension leads to a very large electron temperature gradient within the CFR and LFR, allowing for the observed simultaneous presence of several ionization stages of many elements. Typically, the relative geometrical extension Rout/Rin of a nova atmosphere is ∼ 100–1000, which is much larger than the geometrical extension of hydrostatic stellar atmospheres (even in giants Rout/Rin is typically less than 2) or supernovae (SNe).

The electron temperatures and gas pressures typically found in nova photospheres lead to the presence of a large number of spectral lines, predominantly Fe-group elements, in the LFR and a corresponding influence of line blanketing on the emergent spectrum.

Introduction

Unlike core collapse supernovae, we have no observation of the initial moments of a classical nova outburst. But, as described elsewhere in this volume, we can reconstruct – at least in a broad sense – the explosive and mixing phenomena because of the comparative simplicity of the ejection process. The thermonuclear runaway is the foundation of our understanding. Yet many details of the dynamics remain to be explored and some of the signposts for change are observational since at the moment of ejection, the nucleosynthetic products are contained in – and expelled with – the ejecta. They provide an interpretable record of the stages of the explosion that are otherwise inaccessible to direct observation. The ambiguities of this record, understood only through modeling, will be the subject of our discussion in this chapter.

Decoding the abundance patterns in various subclasses of novae and individual objects has proven a great challenge over the decades since the first edition of this book appeared. Considerable progress has been made in obtaining consistent pictures from a variety of analyses, as we will discuss. Here we will highlight a few of these methods, examine their various limitations and systematic uncertainties, and indicate some directions that new methods of spectroscopic analysis and theoretical modeling can take to improve the picture in the near and long term.

Introduction

Astrophysical spectroscopy, and with it our understanding of the cause and progression of the nova event, has progressed apace in the past two decades. When the ink was drying on the first edition, the International Ultraviolet Explorer (IUE) satellite was still in its heyday and many new phenomena related to the outburst were still to be discovered. Some glimpses had been provided by the Copernicus satellite (for V1500 Cyg), but the Hubble Space Telescope, and with it the Goddard High Resolution Spectrograph (GHRS) and the Space Telescope Imaging Spectrograph (STIS), were waiting to be launched and CCD technology was just being developed. Historically, much of the early work on ultraviolet spectra was undertaken with the aim of determining abundances through analyses of the nebular spectra. This is understandable since, before the early 1980s, the optically thick stage was impossible to model. Theoretical models have guided a shift in methodology, as did the development of spectrophotometric capabilities. One point should, however, be emphasized: no nova – classical or recurrent – was observed panchromatically before the Copernicus and IUE satellites were launched in the 1970s and the classical analyses were based entirely on data longward of the atmospheric cut-off. As we will discuss, we now know the role played by the ultraviolet in the details of spectrum formation at all wavelengths, a view that has changed dramatically since the first edition of Classical Novae, so it is this connection that will be stressed throughout this chapter.

Introduction

In this chapter, we review the observation of radio emission from classical novae. In the majority of cases such emission is undoubtedly dominated by that from the thermal bremsstrahlung process and we begin by giving an overview of such emission arising from relatively simple geometries. We then move on to describe various models of the kinematics in novae and show how these have been combined with the thermal emission process described earlier to fit the radio light curves of several well-observed novae. However, it has become apparent that relatively simple spherically symmetric models fail to describe some of the details in the observations of more recent objects and more complex modelling is introduced. This is reinforced when we consider the evolution of the resolved radio remnants as illustrated in Chapter 12.

In the first edition of this book in 1989, radio emission was reported to have been detected in nine classical novae and, of these, two were spatially resolved (Seaquist, 1989). Since then, we have seen the advent of improvements to the sensitivity of radio arrays. Here we consider observations of radio emission from a total of eighteen classical novae, and as reported here and in Chapter 12, at least six of these have published radio imagery.