Overview of CHIPS Science

Contents:
1. Motivation & Mission Objectives
2. Hot Diffuse Gas in the Universe
3. Cooling Mechanisms
4. The Hot Local Bubble Gas
5. Mission Goals and Scientific Return

The Holy Grail for studies of the Local Bubble is the spectrum of the hot gas, which contains the key to its origin.... The nature of the cooling of this hot gas is important not only for our understanding the interstellar medium of galaxies, but also for hot gas in clusters of galaxies and in active galactic nuclei.
- Prof. Christopher McKee at the IAU Colloquium ``The Local Bubble and Beyond,'' ESO (Garching) 1997.

In a process that profoundly shapes the host galaxy's interstellar medium, supernovae and stellar winds inject about 10⁴² ergs s^-1 into a typical spiral galaxy. Most of this power is ultimately converted into thermal energy, creating large volumes of tenuous gas with an electron temperature around 10⁶ K. The observed X-ray luminosity of spiral galaxies is less than 5% of the input power (Bregman-94, Cui-96). Our goal is to determine the processes that extract the remaining 95% of the energy from the hot interstellar medium (ISM), using the plasma within about 100 pc of the sun as an astrophysical laboratory.

The observational motivation for a new instrument to study diffuse plasma in the extreme ultraviolet (EUV) is made strikingly clear by Figures 1 and 2. In Figure 1 we show the emissivity of a 10⁶ K low-density plasma with cosmic elemental abundances in collisional ionization equilibrium. Because the very local neutral interstellar medium acts as a filter in front of all such plasmas, we have attenuated the spectrum appropriately. In Figure 2 we show the fraction of the observable radiated power in various bands as a function of temperature. Within a broad range around 10⁶ K, the short-wavelength EUV region spanned by the proposed CHIPS instrument holds the unobserved bulk of the luminosity iceberg; X-rays and the far ultraviolet comprise only the tip.

Figure 1: Emissivity (erg cm³ s^-1) in 1 Å bins for 10⁶ K plasma attenuated by local ISM. There is virtually no power longward of the O VI doublet at 1032, 1038 Å.

CHIPS, the Cosmic Hot Interstellar Plasma Spectrometer, is a simple, low-cost instrument with unparalleled sensitivity and spectral resolution in the critical but comparatively unsurveyed spectral band near 170 Å. CHIPS uses an array of grazing-incidence optics to achieve a peak resolving power of /150 for diffuse emission filling its field of view (5° × 26.7°). CHIPS will provide key spectral diagnostic information on the cooling process, or processes, that take place in the Local Bubble. The primary goal of the CHIPS mission will be to address the following key questions:

• At what wavelengths does the majority of the power radiated by local hot gas emerge?
• What are the physical processes by which the hot interstellar gas of the Local Bubble cools?
• What is the thermal pressure of hot gas in the Local Bubble ?
• What is the morphology and distribution of hot gas within 100 pc of the Sun?
• What is the ionization history of the Local Bubble cavity?
• How can this knowledge be applied to other diffuse hot plasmas in the Universe?

Figure 2: Fraction of total observable power (after attenuation by the LISM) in the CHIPS band (wavelengths between 90 and 260 Å, solid), X-rays (wavelengths below 90 Å, dashed) and FUV (wavelengths above 912 Å, dotted). The small difference between the sum of the three and 100% is carried in the 260 - 912 Å band

Because of the far-reaching effect of the hot interstellar medium in shaping the structure of spiral galaxies, the CHIPS mission primarily supports NASA's Structure & Evolution of the Universe theme. Through the star formation connection, CHIPS secondarily supports NASA's Origins theme. The study of how hot gas cools is directly relevant to Science Goals 2 and 4 of the recently published NASA Space Science Enterprise Strategic Plan, in which we will ``test physical theories and reveal new phenomena throughout the Universe.'' CHIPS also supports the NASA Space Science Enterprise Technology goals 2 & 3 in that: (a) ``it spawns new measurement concepts and mission opportunities, and creates new ways of doing space science,'' and (b) ``it develops and nurtures an effective science-technology partnership... to dramatically lower mission cost and risk.''

2. Hot Diffuse Gas in the Universe

Hot (~10⁶ K) diffuse gas - often termed ``coronal'' gas - is ubiquitous in the Universe. On our astrophysical doorstep, the hot solar wind that streams through the solar system cools through adiabatic expansion. When it reaches a distance of 40 A.U. it has cooled to about 10,000 K, comparable to the temperature in the ambient local cloud. Further afield, the solar neighborhood within ~ 100 pc (the Local Bubble) is thought to be filled primarily with hot, tenuous gas that produces substantial diffuse emission at energies below about 1/4 keV (Snowden-98). As will be discussed in Section 4, the detailed temperature, ionization conditions, and elemental abundances in the Bubble gas are largely unknown.

Supernova remnants such as Puppis A and the Vela SNR are important sources of hot (shocked) gas. Their subsequent evolution is critical to theoretical models of the general ISM; such models are entwined with global parameters such as the interstellar porosity, pressure, and filling factors of the various phases. The CHIPS study of cooling mechanisms in the local bubble will be suggestive of what occurs in young SNRs.

SNRs participate in the galactic system of dying superbubbles that may occupy a large fraction of interstellar space. To date, however, the thermal characteristics of superbubbles have been inadequately measured and modeled. Key uncertainties include the quantity of matter that is heated and the mechanisms by which it cools, yielding large uncertainties in the hot gas emissivity, cooling spectrum, and bubble lifetime.

An extension of hot gas into the Galactic halo was first postulated to provide thermal pressure support to cool or warm clouds at large distances from the disk (Spitzer-56). Most models for the production of hot halo gas fall into three broad categories: (1) ``hot'' models in which both support and ionization are provided by hot gas heated in the disk by supernovae and possibly flowing in a galactic fountain (Bregman-90), (2) ``cool'' models in which the support is provided by non-thermal means such as cosmic-ray pressure and the ionization is purely photo-ionization (Hartquist-84), and (3) other models that invoke magnetic fields and/or shocks (Edgar-93). Copernicus, IUE, HST/GHRS, and ORFEUS have observed highly ionized-gas species such as O VI, N V, C IV and Si IV toward halo stars and extragalactic objects (Jenkins-78, Sembach-97, Savage-87, Hurwitz-96). Although the scale height of the hottest ions is not well determined, substantial column densities exist along some halo sight lines. It is important to recognize that even the absorption lines of the hottest ion, O VI, do not arise in the truly diffuse hot ISM. Theoretical calculations of its peak abundance, observations of the line width, and the clumpiness of the absorption cells all support the conclusion that these intermediate-temperature species arise in discrete ``events'' such as the interfaces between the hot ISM and cool clouds and/or the edges of supernova shells (Benjamin-96, York-77, Shelton-94).

FUSE will soon observe O VI absorption toward nearby white dwarfs, cataclysmic variables, and other stars, where the smooth stellar continuum and/or comparatively simple line of sight structure could dramatically improve our understanding of interfaces. Such observations will remain ambiguous, however, without reliable constraints on the physical conditions in the diffuse hot medium that CHIPS will provide. These uncertainties in our understanding of interstellar processes in our own Galaxy extend to other spirals as well.

Looking to superclusters of galaxies, X-ray observations reveal a diffuse hot (10⁷ - 10⁸ K) gas, containing more mass than all of the cluster's visible stars. In the central region, where the gas is densest, a cooling flow may form (Fabian-94). While the X-ray-emitting gas is well studied, material cooler than about 10⁶ K emits only at EUV and FUV wavelengths and is too faint to be observed. Cold gas dropping out of the flow should accumulate as dense clouds, low-mass stars, or dust grains, but these, too, are undetected. Extended (up to 10 Kpc) optical and ultraviolet emission-line nebulae are found at the centers of many cooling flows in rich clusters (Heckman-89). These nebulae appear far more luminous than would be expected based on cooling rates derived for the X-ray cooling flows of their parent clusters. A complete description of cluster cooling flows and the many phenomena that they power or constrain will require an understanding of the mechanisms by which cooling-flow gas cools through the critical regime of 10⁵ to 10⁶ K. In a recent review, Fabian-94 states of the cooling that, ``The chemical and physical conditions in galaxy clusters are sufficiently different from those in our Galaxy that a simple extrapolation from the situation in our Galaxy may be inadequate.'' This optimistic statement ignores the fact that the situation in the Galaxy is not sufficiently well understood to allow extrapolation!

Finally, there is growing evidence that cooling flows and the distribution of hot and cold gas surrounding a massive central object power, provoke, and shape luminous sources such as radio galaxies and quasars (Fabian-94).

3. Cooling Mechanisms

A variety of mechanisms have been proposed to describe the process by which hot interstellar gas sheds its reservoir of thermal energy. These include the following:

1. If electromagnetic radiation is the sole source of energy loss, and the gas remains in collisional ionization equilibrium (CIE), then the instantaneous cooling rate depends only on the current temperature and density. The relationship between these two quantities depends on whether the gas is cooling isochorically (at constant volume), isobarically (at constant thermal pressure), or in some intermediate regime. The textbook ``interstellar cooling curve'' (Spitzer-78) and the popular plasma emission codes exemplify CIE models.

2. If cool clouds are embedded in the hot medium, conduction (Cowie-77, McKee-77), or turbulence (Slavin-93) can bring hot electrons into contact with elements in lower ionization stages. We will refer to these as ``under-ionized'' conditions. Because the lower ionization stages typically have resonance transitions that are accessible to the characteristic electron energy, this can lead to more efficient cooling compared to CIE for a given electron temperature.

3. When the recombination time exceeds the cooling time, high ionization stages can become frozen in among cooler electrons (Shapiro-76). We will call this ``over-ionized'' gas. The models usually start with CIE at some initial high temperature then track the populations in each ionization stage as the gas cools. The instantaneous cooling rate (or equivalently, the emitted spectrum) is no longer a function solely of density and temperature, but of the dynamical history of the gas. As in CIE models, the nature of the cooling (isochoric/ isobaric/ intermediate) plays an important role (Benjamin-97).

   Over-ionization can also result from the presence of dust in the medium. Grain heating followed by infrared emission drains energy from the thermal electrons while not accelerating recombination. For a standard dust-to-gas ratio, dust cooling exceeds CIE radiation at temperatures above about 10^6.3 K (Ostriker-73), but the survival of grains in shocks and against subsequent sputtering is the subject of ongoing study (Dwek-96).

   Breitschwerdt & Schmutzler (1994; hereafter BS94) have presented an extreme class of over-ionized models. They invoke mechanisms that enable the impulsively-heated gas to undergo physical expansion. Adiabatic cooling becomes a new ``sink'' for the thermal energy, and the gas enters conditions of wild mismatch between the ionization stage and the electron temperature.

Observational determination of the electron temperature and ionization state will reveal an echo of the dynamic history of the local bubble gas. If evaporative cooling can be ruled out as an important mechanism, this would eliminate an important tenet of the widely referenced McKee- Ostriker model of the ISM.

Million-degree gas is sometimes referred to as ``X-ray gas.'' This misnomer belies the uncomfortable reality that extrapolation of X-ray observational limits to the total plasma luminosity is critically dependent on details of cooling processes that are not well understood. Under the simplest assumptions of CIE illustrated in Figures 1 and 2, million-degree gas is ``EUV gas.''

If the gas is cooler, or if under-ionized, nonequilibrium prevails, and the X-ray luminosity can become a vanishingly small fraction of the total. Our CHIPS observations may reveal that the extreme ultraviolet carries the vast majority of the supernova input power. Alternatively, we may discover that highly over-ionized nonequilibrium dominates. This revelation would dramatically reduce the X-ray extrapolation factor, and demonstrate that the supernova power input to galaxies must be dissipated through expansion, down-conversion of high-energy photons to longer wavelengths, or other processes.

4. The Hot Local Bubble Gas

Interstellar absorption prohibits us from studying extragalactic sources or young supernova remnants spectroscopically in the important short-wavelength EUV band. The only diffuse interstellar plasma that can be studied here is the tenuous gas in the local interstellar medium within ~100 pc, often termed the Local Bubble (LB).

Absorption-line studies toward stars within 100 pc demonstrate that the LB is deficient in neutral gas (Welsh-94). Although photoionized clouds are common, such gas cannot comprise the sole non-neutral component of the LB. Copernicus O VI absorption data indicate intermediate-temperature gas within the LB (perhaps at cloud interfaces) or at its boundary (Shelton-94). Observations of the diffuse soft X-ray background provide strong evidence for local 10⁶ K gas (Bloch-86, Cox-87, Snowden-98), and recent X-ray shadowing studies demonstrate conclusively that a significant fraction of the observed diffuse emission must be formed at distances less than 100 pc (Kuntz-97).

The actual size and shape of the LB are only slowly becoming defined. Observations of the diffuse soft X-ray B-Band have been modeled to reproduce contours of the observed negative correlation between X-ray intensity and neutral interstellar hydrogen column density out to 300 pc (Snowden-90). Similarly, ROSAT Wide Field Camera observations of the distribution of EUV sources have been modeled to infer neutral hydrogen column densities as a function of distance to 150 pc. These observations are in agreement with the contours of neutral Na I gas, which indicate a possible mid-plane boundary of the LB shown in Figure 3 (Sfeir-99). It is apparent from this figure that the LB (as defined by a Na I absorption wall) is asymmetric in shape and size. A neutral gas-free tunnel extends some 250 pc toward the star Beta CMa. Due to a lack of data at high galactic latitudes, the contours of the LB are ill-defined. Soft X-ray shadowing data toward Ursa Major suggest that the hot LB gas extends at least 300 pc into the halo (Benjamin-96), and the clustering of extragalactic EUV sources at high galactic latitudes is consistent with the view that the Local Bubble may actually be a ``Local Chimney'' (Welsh-99). The CHIPS spectral observations will produce a sky map of the diffuse EUV background emission in specific spectral lines which can be compared to other tracers of the morphology of the LB.

Figure 3: Contours of the Local Bubble, derived from Na I D absorption-line data, projected on the Galactic plane. Note the extension to 250 pc in the direction of l = 230°. From Sfeir-99.

5. Mission Goals and Scientific Return

Over its 12-month mission, CHIPS will generate approximately 316 spectra of the quality shown in Figure 4, mapping the sky at about 5° × 26.7° resolution. If the plasma temperature is near 10⁶ K and the CIE model predictions are accurate, four independent sky maps in the lines of Fe IX, X, XI, and XII will be created. The S/N per sky resel is expected to be 20 to 45. These lines are comparatively close in wavelength (see Figure 4), so their relative fluxes are not much affected by differential absorption. The predicted S/N is sufficient to determine the CIE plasma temperature to ± 0.011 dex (3 ). At other temperatures or under other cooling assumptions, alternative lines should be bright enough for high S/N sky mapping. Spectra of each sky resel will reveal features of other elements generally less subject to depletion effects (S, Si). CHIPS data will therefore measure the emission measure and temperature (and thus the thermal pressure) and abundances of the local diffuse plasma. Depending on the nature of the observational results, CHIPS data alone or in concert with measurements from other instruments will provide key constraints on the cooling mechanisms in the Local Bubble. These findings directly support NASA's Structure and Evolution of the Universe theme and will help to answer one of the key questions posed in the recently-published Space Science Enterprise Strategic Plan, which is to ``understand... the exchange of matter and energy among the stars and the interstellar medium.''

Figure 4: Simulated CHIPS spectra (150 - 260 Å band only) of each sky resel for (top) 10⁶ K CIE plasma, (middle) underionized cooling, and (bottom) the overionized nonequilibrium model of BS94. Fully illuminated plasmasphere viewing is assumed. Vertical scale is kept constant to illustrate spectral differences.

Other scientific results expected from analysis and modeling of the CHIPS data include

1. A determination of the pressure structure in the local hot gas.

2. Morphological studies of the emission from the LB hot gas to determine regions of anomalous EUV intensity. These will be compared with other galactic emission maps (such as ROSAT) and absorption maps (e.g. Welsh-98) to determine possible correlations.

3. Abundance studies of the elements Fe, Si, S, Ne, and Ni to determine if the observed depletion effects can be attributed to supernova shocks. Abundance values will also be compared with those derived from ultraviolet absorption studies of interstellar clouds in the LISM.

In Figure 5 we show the simulated spectrum for the entire sky, integrated over the 12-month mission. We have assumed a canonical CIE plasma at 10⁶ K. The complete instrument bandpass is shown. Features identified in the previous figures are not labeled. This spectacular integrated CHIPS spectrum would provide a bounty of new plasma diagnostics beyond those discussed above.

References