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The electromagnetic radiation associated with the energy dissipated by cosmic rays in interstellar hydrogen
Electromagnetic radiation due to cosmic ray energy dissipation in interstellar hydrogen
On the effect of interstellar material on stellar non-circular velocities in disk galaxies.
Secular increase of stellar noncircular velocities in disk portions of galactic systems, noting gravitational disturbances
Photodestruction of hydrogen molecules in H I regions.
Photodissociation of hydrogen molecules in H I regions of interstellar medium, evaluating lifetime and density
IUE spectra of the hot close binary V Puppis
Low- and high-dispersion ultraviolet spectra obtained by the IUE of the early B-type close binary V Puppis are reported. The continuum derived from the low-dispersion images is consistent with a solar abundance model of effective temperature 25,000 K and log surface gravity 4.0. The high-dispersion spectra show shallow double lines, representing Doppler-resolved stellar lines, which are broadened as expected for the fast rotational velocities given by Levato (1974). The equivalent widths of the silicon lines, however, do not agree with the temperature derived for the star, while the silicon and carbon absorptions indicate a disordered photosphere in the presence of circumstellar gas. Several interstellar lines are also observed for the first time.
A High-resolution Study of Local Interstellar Sodium
A high-resolution spectroscopic investigation was made of interstellar lines of sodium. From identifications of D1 and D2 line components concentrations of interstellar sodium gas were studied. Some preliminary data are provided on the spatial distribution of stronger sodium concentrations.
On the anomalous component
The so-called anomalous cosmic ray component, which occurs at energies of about 10 MeV/nucleon and consists only of He, N, O, and Ne, has been a subject of interest for more than a decade. The origin of this component is generally considered to be interstellar neutral gas that is ionized and accelerated in the solar wind. The mechanism and the location for the acceleration, however, remains an unsolved problem. A model is used which includes the effects of gradient and curvature drifts and considers the implications of observed spatial gradients of the anomalous component for the location of the acceleration region. It is concluded that if drifts are important the acceleration region cannot lie at the solar poles. It is also concluded that there is no single region for the acceleration which can account for both the observed intensities and gradients in models which include drift effects.
X-ray secondary heating and ionization in quasar emission-line clouds
Accurate Monte Carlo computations of the X-ray secondary electron heating, ionization, and excitation of H and He gas in interstellar space and in quasar emission-line clouds, are presented. The fraction of energy deposited in each form is sensitive to the background ionization fraction, x = n(H+)/n(Htot), and can affect the temperature, ionization state, and line emissivities at large depths in X-ray photoionized clouds. Analytic fits are provided for these energy fractions over the range 0.0001-1 for primary electron energies up to many keV. In both broad-line and narrow-line clouds, emission lines sensitive to the energy budget and electron density may be strongly affected.
Evidence for methane and ammonia in the coma of comet P/Halley
Methane and ammonia abundances in the coma of Halley are derived from Giotto ion mass spectrometer data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto high-intensity spectrometer ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01 = 0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(Q2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.
Water vapor in the Orion Molecular Cloud
Infrared observations of interstellar gas-phase H2O in the spectrum of the BN object in Orion are reported. There are absorptions (S/N = 2-5) at the positions of four of the strong lines in the 000-001 nu3 vibration-rotation band. With an estimated excitation temperature of 150 K, the column density of gaseous H2O toward BN in the OMC-1 cloud is (2 + or - 1) x 10 to the 17th/sq cm. The intensities of the lines imply an ortho/para ratio of 1 + or - 0.5 indicating recent sublimation of H2O from low-temperature grains. The results give gas-phase abundance ratios of H2O/CO roughly 0.03 + or - 0.02 and HDO/H2O = 0.001-0.0001 toward BN. The velocities of the H2O absorptions agree with those of the ridge source and CO outflow, but the position along the line of sight is not well constrained. The gas/solid ratio is H2O(gas)/H20(ice) = 0.05 or less. Less than 1 percent of the oxygen is in H2O gas (assuming total cosmic abundance). Most of the H2O in the line of sight to BN, and by inference in quiescent regions of molecular clouds generally, is frozen on grains.
Molecular line emission models of Herbig-Haro objects. II - HCO(+) emission
We present time-dependent models of the chemistry and temperature of interstellar molecular gas clumps that are exposed to the radiation from propagating stellar-jet shocks. The X-ray, EUV, and FUV radiation from the shock initiates ion chemistry and also heats the gas in the clumps. Using representative parameters, we show that, on the shock transit time between the clumps, the abundances of the ionized molecular species that are produced in the clumps can exceed the values determined from steady state models by several orders of magnitude. Collisional excitation by the heated gas can lead to measurable line emission from several ionized species; as in previous investigations of X-ray-irradiated molecular gas, we find that electron impacts contribute significantly to this process. We apply these results to the interpretation of the HCO(+) line emission that has already been detected in several Herbig-Haro objects. We demonstrate that this picture provides a natural explanation of the fact that the line intensity typically peaks ahead of the associated shock, as well as of the reported low line-center velocities and narrow line widths. We tabulate several diagnostic line intensities of HCO(+) and other molecular species that may be used to infer the physical conditions in the emitting gas.
Engineering Ulysses extended mission
The Ulysses Mission is a collaboration between the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). The mission is unique, enabling exploration of the heliosphere within a few astronomical units of the Sun over a full range of heliographic latitudes adding a third dimension to our understanding of the Solar System. The advanced scientific instrumentation on Ulysses continually measures the properties of the heliospheric magnetic field, the solar wind, solar radio bursts and plasma waves, galactic cosmic rays, energetic particles, solar X-rays, and interstellar neutral gas. By the end of 1995, the spacecraft will have completed measurements at heliographic latitudes up to 80 degrees over a single orbit of the Sun. The properties of the heliosphere are solar cycle dependent, and Ulysses' first orbit of the Sun will have taken place around a solar minimum. In order to characterize the heliosphere over a full (11 year) solar cycle, it is desirable to continue measurements over a second orbit of the Sun, a new Odyssey that will extend through 2001. Since the spacecraft was only designed for a five-year mission, a number of technical challenges have been surmounted in order to demonstrate the engineering feasibility of this unparalleled scientific opportunity. This paper describes the changes that were necessary to the Ulysses mission engineering and mission operations in order to ensure continual, effective payload operation throughout 1996-2001.
A critical analysis of interstellar Zn and Cr as galactic abundance benchmarks for quasar absorbers
We present interstellar gas-phase abundances of zinc and chromium in the Milky Way for comparison with the abundances of these elements in damped Ly-alpha systems at redshifts near z approximately 2. We make use of both IUE (International Ultraviolet Explorer) and GHRS (Goddard High Resolution Spectrograph) archival information in deriving the Milky Way values and find that the average difference in (Zn/H) and (Cr/Zn) between the Milky Way and damped Ly-alpha systems at large N(H1) is about 0.9 dex. The Zn/H values we find for the Milky Way are approximately -0.2 to -0.7 dex below solar and are due in large part to depletion of Zn onto dust grains. The amount of this gas-phase depletion is variable and depends upon the H1 column density and molecular fraction of the gas. Milky Way sight lines having low molecular fractions have values of (Zn/H) within approximately 0.2 dex of the solar value. Combined with larger values of (Cr/Zn) found for the damped Ly-alpha systems, this result confirms that measures of (Zn/H) should accurately reflect the lower gas-phase abundances of Zn found in these systems.
Studies of Interstellar Pickup Ions in the Solar Wind
The work under this grant involves studies of the interaction of interstellar pickup ions with the solar wind, with the goal of a comprehensive model of the particle distributions and wave intensities to be expected throughout the heliosphere, as well as the interactions of those distributions with the solar wind termination shock. In the past year, we have completed a number of projects, including observations and modeling of the effects of a large scattering mean free path on the pickup He(+) seen at AMPTE, an analytical model of anisotropic pickup tons in a steady radial magnetic field, and a derivation of a reduced solar wind Mach number due to increased estimates on the inflowing hydrogen density allowing for a weak termination shock. In the next year, we plan to investigate in more detail the correspondence between our models of anisotropic pickup ions and the data on spectra, variations, and proton-He(+) correlation provided by AMPTE, Ulysses, and our instrument on SOHO. We will model the time-dependent pickup ion density resulting from finite periods of radial magnetic field. We will also incorporate the effects of a large mean free path into our analysis of the He(+) focusing cone, leading to more accurate parameter values for the interstellar helium gas. This progress report also includes a discussion of our Space Physics Educational Outreach activities in the past year and plans for the next year.
Modeling of the EUV and X-Ray Emission Spectra Induced by the Solar Winds Ions in the Heliosphere
We have carried out investigation of the EUV and X-ray emission spectra induced in interaction between the Solar Wind (SW) and interstellar neutral gas. The spectra of most important SW ions have been computed for the charge-exchange mechanism of X-ray emission using new accurate spectroscopic data from recent laboratory measurements and theoretical calculations. Total spectra have been constructed as a sum of spectra induced in the charge-exchange collisions by individual O(exp q+), C(exp q+), N(exp q+), Ne(exp q+), Mg (exp q+) and Fe(exp q+) ions. Calculations have been performed for X-ray emission from the heliospheric hydrogen and helium gas. X-ray maps of the heliosphere have been computed. The power density of X-ray sources in the heliospheric ecliptic plane is shown for the H gas and for the He gas. Distances from the Sun (0,0) are given in AU. The helium cone is clear seen in the X-ray map of the charge-exchange emission induced by the solar wind. X-ray emission spectra detected by the Chandra X-ray telescope from the "dark" side of Moon has been identified as a X-ray background emission induced by the solar wind from the geocorona. Spectra and intensities of this charge-exchange X-rays have been compared with the heliospheric component of the X-ray background. Observations and modeling of the SW spectra induced from the geocorona indicate a strong presence of emission lines of highly charged oxygen ions. Anisotropy in distribution of heliospheric X-rays has been predicted and calculated for the regions of the fast and slow solar winds.
Generalized Jeans' Escape of Pick-Up Ions in Quasi-Linear Relaxation
Jeans escape is a well-validated formulation of upper atmospheric escape that we have generalized to estimate plasma escape from ionospheres. It involves the computation of the parts of particle velocity space that are unbound by the gravitational potential at the exobase, followed by a calculation of the flux carried by such unbound particles as they escape from the potential well. To generalize this approach for ions, we superposed an electrostatic ambipolar potential and a centrifugal potential, for motions across and along a divergent magnetic field. We then considered how the presence of superthermal electrons, produced by precipitating auroral primary electrons, controls the ambipolar potential. We also showed that the centrifugal potential plays a small role in controlling the mass escape flux from the terrestrial ionosphere. We then applied the transverse ion velocity distribution produced when ions, picked up by supersonic (i.e., auroral) ionospheric convection, relax via quasi-linear diffusion, as estimated for cometary comas [1]. The results provide a theoretical basis for observed ion escape response to electromagnetic and kinetic energy sources. They also suggest that super-sonic but sub-Alfvenic flow, with ion pick-up, is a unique and important regime of ion-neutral coupling, in which plasma wave-particle interactions are driven by ion-neutral collisions at densities for which the collision frequency falls near or below the gyro-frequency. As another possible illustration of this process, the heliopause ribbon discovered by the IBEX mission involves interactions between the solar wind ions and the interstellar neutral gas, in a regime that may be analogous [2].
Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT 2019qiz
We report the results of ultraviolet (UV) and optical photometric and spectroscopic analysis of the tidal disruption event (TDE) AT 2019qiz. Our follow-up observations started <10 days after the source began to brighten in the optical and lasted for a period of six months. Our late-time host-dominated spectrum indicates that the host galaxy likely harbors a weak active galactic nucleus. The initial Hubble Space Telescope (HST) spectrum of AT 2019qiz exhibits an iron and low-ionization broad absorption line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum also bears a striking resemblance to that of Gaia16apd, a superluminous supernova. Our observations provide insights into the outflow properties in TDEs and show evidence for a connection between TDEs and engine-powered supernovae at early phases, as originally suggested by Metzger & Stone (2016). In a time frame of 50 days, the UV spectra of AT 2019qiz started to resemble those of previous TDEs with only high-ionization BALs. The change in UV spectral signatures is accompanied by a decrease in the outflow velocity, which began at 15,000 km/s and decelerated to ~ 10,000 km/s. A similar evolution in the Ha emission-line width further supports the speculation that the broad Balmer emission lines are formed in TDE outflows. In addition, we detect narrow absorption features on top of the FeLoBAL signatures in the early HST UV spectrum of AT 2019qiz. The measured H I column density corresponds to a Lyman-limit system, whereas the metal absorption lines (such as N V, C IV, Fe II, and Mg II) are likely probing the circumnuclear gas and interstellar medium in the host galaxy.
The depletion of interstellar elements and the interaction between gas and dust in space
Observational data obtained by the Copernicus satellite and ground-based facilities are discussed which show that the depletion of interstellar elements apparently varies with cloud density in the way to be expected if the depletions were due to accretion of gas atoms or ions onto grains. It is shown that the depletion-by-accretion mechanism is supported by a good correlation between depletions and first ionization potentials of the elements (except for lithium, sodium, and potassium) and that this mechanism offers an alternative to the grain-condensation hypothesis in which most of the interstellar medium is required to have been processed through stellar nebulae. It is noted that if the grains do not normally carry positive charges, ion-grain encounters are the most important interactions between gas and dust and may be significant in determining ionization equilibrium, especially if there is a large population of very small grains.