Microwave detection of oh in the interstellar medium
Microwave detection of hydroxyl radical in interstellar medium
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Interaction of the Local Interstellar Medium with the Heliosphere: Role of the Interior and Exterior Magnetic Fields
A complete model of the global interaction between the solar wind and the local interstellar medium must take account of interstellar neutral atoms, interstellar ionized gas, solar and galactic magnetic fields, galactic and anomalous cosmic rays. For now, however, in view of the many uncertainties about conditions in the interstellar medium, etc., all models must be regarded as highly idealized and incomplete. In the present review I concentrate on the role of magnetic fields of solar and interstellar origin. The former, the interior field, has negligible influence on the unshocked solar wind; the immediate post-shock solar wind is probably low-beta, so that the interior magnetic field is still unimportant, but this situation changes as the plasma flows through the heliosheath, and a ridge of strong magnetic field may form to separate materials of polar and equatorial origin. The exterior (interstellar) field is likely to play an important role in determining the global morphology of the system outside the termination shock. If the exterior field is strong enough, it can compress the heliosphere (although exterior neutral and/or ionized hydrogen may play the dominant role). Even if the interstellar magnetic field does not provide the dominant pressure, its orientation can substantially affect the configuration of the heliosphere, especially the location and orientation of the heliospheric discontinuities. The configurations can be quite different for the situations in which the field and flow are (a) aligned or (b) transverse. Obliquity of the field produces asymmetry in the geometry of the system; in particular the noses of heliopause and interstellar bow shock are shifted away from the interstellar flow direction, and in opposite directions, due to the asymmetric draping of the magnetic field.
Barnes, Aaron↗
Stochastic histories of dust grains in the interstellar medium
The purpose is to study an evolving system of refractory dust grains within the Interstellar Medium (ISM). This is done via a combination of Monte Carlo processes and a system of partial differential equations, where refractory dust grains formed within supernova remnants and ejecta from high mass loss stars are subjected to the processes of sputtering and collisional fragmentation in the diffuse media and accretion within the cold molecular clouds. In order to record chemical detail, the authors take each new particle to consist of a superrefractory core plus a more massive refractory mantle. The particles are allowed to transfer to and fro between the different phases of the interstellar medium (ISM) - on a time scale of 10(exp 8) years - until either the particles are destroyed or the program finishes at a Galaxy time of 6x10(exp 9) years. The resulting chemical and size spectrum(s) are then applied to various astrophysical problems with the following results. For an ISM which has no collisional fragmentation of the dust grains, roughly 10 percent by mass of the most refractory material survives the rigors of the ISM intact, which leaves open the possibility that fossilized isotopically anomalous material may have been present within the primordial solar nebula. Stuctured or layered refractory dust grains within the model cannot explain the observed interstellar depletions of refractory material. Fragmentation due to grain-grain collisions in the diffuse phase plus the accretion of material in the molecular cloud phase can under certain circumstances cause a bimodal distribution in grain size.
Liffman, Kurt↗
DIISC-II: Unveiling the Connections between Star Formation and Interstellar Medium in the Extended Ultraviolet Disk of NGC 3344
We present our investigation of the extended ultraviolet (XUV) disk galaxy, NGC 3344, conducted as part of Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium survey. We use surface and aperture photometry of individual young stellar complexes to study star formation and its effect on the physical properties of the interstellar medium. We measure the specific star formation rate (sSFR) and find it to increase from 10 –10 yr –1 in the inner disk to >10 –8 yr –1 in the extended disk. This provides evidence for inside-out disk growth. If these sSFRs are maintained, the XUV disk stellar mass can double in ~0.5 Gyr, suggesting a burst of star formation. The XUV disk will continue forming stars for a long time due to the high gas depletion times (τ dep ). The stellar complexes in the XUV disk have high-Σ H I and low-Σ SFR with τ dep ~ 10 Gyr, marking the onset of a deviation from the traditional Kennicutt–Schmidt law. We find that both far-ultraviolet (FUV) and a combination of FUV and 24 μm effectively trace star formation in the XUV disk. Hα is weaker in general and prone to stochasticities in the formation of massive stars. Investigation of the circumgalactic medium at 29.5 kpc resulted in the detection of two absorbing systems with metal-line species: the stronger absorption component is consistent with gas flows around the disk, most likely tracing inflow, while the weaker component is likely tracing corotating circumgalactic gas.
79 ASTRONOMY AND ASTROPHYSICS↗
The stellar radiation field and the ionization of H and He in the local interstellar medium
Detailed photoionization calculations for the local interstellar medium (LISM) are presented based on constraints imposed by H I column density estimates derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs. It is found that the nearby hot white dwarfs dominate the stellar contributions to the EUV radiation field. Considering stellar contribution to the EUV background alone, the resulting lower limits to the hydrogen and helium fractional ionization in the local diffuse cloud, at the sun, are 0.091 and 0.089, respectively. This result is insensitive to the absorbing geometry of the local cloud. The comparable ionizations of H and He near the sun are due to the lack of absorption from photospheric helium in the hot DA white dwarfs, to the difference in optical depths at wavelengths shortward of the H I and He I ionization edges, and to the fact that the hydrogen ionization controls the electron number density. The derived H and He ionization limits have important implications for the interpretation of the He I and H I backscattering results, for which uncertainties in charge-exchange interactions occurring in or near the heliopause reflect additional significant uncertainties in determining the ionization in the LISM.
Bruhweiler, Frederick C.↗
Trajectories for a Near Term Mission to the Interstellar Medium
Trajectories for rapid access to the interstellar medium (ISM) with a Kuiper Belt Object (KBO) flyby, launching between 2022 and 2030, are described. An impulsive-patched-conic broad search algorithm combined with a local optimizer is used for the trajectory computations. Two classes of trajectories, (1) with a powered Jupiter flyby and (2) with a perihelion maneuver, are studied and compared. Planetary flybys combined with leveraging maneuvers reduce launch C3 requirements (by factor of 2 or more) and help satisfy mission-phasing constraints. Low launch C3 combined with leveraging and a perihelion maneuver is found to be enabling for a near-term potential mission to the ISM.
ISM↗
Deuterium in the Local Interstellar Medium: Its Cosmological Significance
We report on our ongoing program to measure the deuterium/hydrogen (D/H) ratio and interstellar gas properties along many lines of sight through the local interstellar medium using the HST Goddard High-Resolution Spectrograph. For the line of sight towards Capella (12.5 pc) we had previously found D/H = 1.65(+0.07, -0.18) x 10(exp -5), T = 7000 K, and turbulent velocity 1.66 km/s. These quantities were determined by modeling the interstellar hydrogen and deuterium Lyman alpha lines and the resonance lines of Fe II and Mg II against the background stellar emission-line profiles. We now report on our preliminary analysis of these spectral lines for the line of sight toward Procyon (3.5 pc). We find that D/H = 1.40 +/- 0.05 x 10(exp -5) (+/- 3 sigma) photometric random errors only), which is lower than but perhaps consistent with the value of D/H derived for the Capella line of sight when the systematic errors associated with the uncertain intrinsic Procyon emission line are included. Further analysis of this and other lines of sight are planned to determine whether the D/H ratio varies within the local interstellar medium. We infer the primordial value of D/H from Galactic evolution models and comment on the derived baryon density of the Universe.
Linsky, Jeffrey L.↗
Energy and mass balance in the three-phase interstellar medium
Details of the energy and mass balances are considered in the context of a three-phase interstellar medium. The rates of mass exchange between the different phases are derived based on the pressure variations created by supernova remnant expansions. It is shown that the pressure-confined warm and cold gases have stable temperatures under a variety of interstellar conditions. The three-phase quasi-static configuration is found to be a natural outcome, and both warm and cold phases generally contribute about half of the total mass density to the diffuse interstellar gas. The model is also likely to be self-regulatory in the sense that variations of the input parameters do not strongly alter the general result, which is consistent with most current observations. The consequences of extreme conditions on this model are considered, and the possible implications for interstellar medium in other galaxies are briefly discussed.
Wang, Zhong↗
The small scale structure of the interstellar medium in the Orion association: The flotsam of star formation
The small scale dynamics and structure of the interstellar medium of the Orion OB1b, c association was studied using IUE high dispersion SWP and LWR/LWP spectra. The area surveyed centers on the Belt and encompasses the Orion Cloak. The dynamics and abundances of the ionized and warm neutral gas surrounding the central parts of the association are determined using 35 B stars. Results are compared with the picture of the Orion interstellar medium gained using COPERNICUS, especially for the structure and dynamics of the Orion Cloak.
Sonneborn, G.↗
Supernova remnant evolution in an interstellar medium with evaporating clouds
A new similarity solution that describes the evolution of an SNR expanding into a cloudy interstellar medium is expounded. The solution incorporates a reasonable model of the conductive evaporation of cold clouds embedded in the hot gas behind the shock. The model has two new parameters in addition to those describing the usual Sedov solution for a uniform interstellar medium. The X-ray, infrared, and optical luminosities of remnants with evaporating clouds are calculated, and some of the expected spectral characteristics in the various wavebands are discussed. The effects described may explain the class of remnants observed to have centrally peaked X-ray emission and shell-like radio emission. The total H-alpha luminosity from evaporating clouds predicted by this model is less than that observed for SNRs with centrally peaked thermal X-ray emission. A detailed calculation of the X-ray and optical line emission from evaporating clouds would be very useful in testing this model against the observations.
White, Richard L.↗
A Type Stars as Probes of the Local Interstellar Medium
With the aim to sample the Local Interstellar Medium (LISM), it was proposed to use A stars as targets. The Mg II UV lines seem to be the best interstellar absorption candidates. Several hundreths of A stars can be reached within 100 pc. First preliminary results (20 lines of sight) are presented, based on previous Copernicus and actual IUE observations.
Ferrero, R. F.↗
Optical Emission Line Studies and the Warm, Ionized Component of the Local Interstellar Medium
Observations of diffuse, galactic H alpha, N2 lambda 6583, and S2 6716 emission lines provide evidence for a warm (10,000K), primarily ionized component of the interstellar medium distribution throughout the galactic disk. This component of the interstellar gas has an electron density approximately equals 0.1-0.2/cu cm and occupies about 10 to 30% of the interstellar volume. Interstellar H alpha emission near the galactic poles, the dispersion measure of a nearby pulsar, and observations of interstellar gas flowing into the solar system indicate that this ionized component is an important constituent of the interstellar medium in the solar neighborhood. The intensity of the H alpha background at high galactic latitudes implies that this component is maintained by an average hydrogen ionization rate in the vicinity of the Sun of (2-4) x 100,000 s(-1) per square cm of galactic disk. The emission measure is 1.3 to 2.3 cm (-6) pc toward the galactic poles. The sources of this ionization were not identified but may include escaping Lyman continuum radiation from planetary nebulae, hot white dwarfs, and early type stars.
Reynolds, R. J.↗
An MHD study of the interaction between the solar wind and the interstellar medium
The overall objective of this research program is to obtain a better understanding of the interaction between the solar wind and the interstellar medium through the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations. The simulated results will be compared with observations where possible and with the results from previous analytic and numerical studies. The primary progress during the first two years has been to develop codes for 2-D models in both spherical and cylindrical coordinates and to apply them to the solar wind-interstellar medium interaction. Computations have been carried out for both a relatively simple gas-dynamic interaction and a flow-aligned interstellar magnetic field. The results have been shown to compare favorably with models that use more approximations and to modify and extend the previous results as would be expected. Work has also been initiated on the development of a 3-D MHD code in spherical coordinates.
Steinolfson, R. S.↗
The Dense, Warm Interstellar Medium (D-WIM): Unravelling a New Phase of the ISM A PRIMA FIR Probe Mission Project
We propose a study of the Dense Warm Interstellar Medium (D-WIM) via the two fine structure lines of ionized nitrogen (\NII). This new phase of the interstellar medium (ISM) has density 10 $\cmv \le n(e) \le$ 100 $\cmv$, and has effectively been identified throughout much of the Milky Way and in nearby galaxies. :The unexpected conditiosn in the D-WIM immediately raise important questions both about this phase of the ISM and its relationship to the ISM as a while. These include: what is the structure of the D-WIM -- e.g. filling factor, distribution, and morphology?; what is the origin of the D-WIM?; what supports its ionization and what is its evolution given huge overpressure relative to the $\ge$ 100 times more diffuse Warm Ionized Medium (WIM)?; what is its possible relationship to stars - is it the the remnants of very old HII regions that were produced by clusters of masive stars? The PRIMA survey requires a coverage of a large fraction of the plane, plus selected regions at high galactic latitudes. A sensitivity of 10$^{-9}$ $Wm^{-2}sr^{-1}$ would be highly desirable. A spectral resolution $R$ of 10$^4$ will isolate individual spectral features. $R$ = 10$^3$ would be acceptable to measure the total intensity. This PRIMA survey could be part of a broader survey of the ISM, and include a number of other important ISM tracers such as [CII] 158 \um, [OI] 63 \um\ and [OI] 146 \um, depending on wavelength coverage of the instrument. The sensitivity requirement for study of the D-WIM is likely more stringent than for the other lines, and may require special consideration for successful implementation as well as for determining the area that can be observed.
Lis, Dariusz↗
The effect of central starbursts on the interstellar medium of dwarf galaxies
Major starburst events can last tens of millions of years, and in the process they can deposit significant amounts of energy into the surrounding interstellar medium. This energy from supernova and stellar winds imparts enough momentum to the interstellar medium (ISM) that portions of the ISM can become unbound and leave the parent galaxy, taking the metal-enriched stellar debris along. In dwarf galaxies, starbursts can produce enough total energy to unbind most or all of the ambient ISM. Whether this actually occurs is a strong function of the ellipticity of the ISM distribution, with flat disks and spheres being the limiting cases. We calculate whether 'blow out' along the symmetry axis of 'blow away' of the entire ISM occurs during a central starburst in dwarf galaxies as a function of galactic mass, starburst energy, ISM density, and ISM ellipticity. The calculations cover a range of 10(exp 7) to 10(exp 9) solar mass for dwarf galaxies and include 'normal' galaxies of 10(exp 11) solar mass as well. No massive dark matter halos are assumed to be present. We find that for physically reasonable values of total ISM mass and starburst energy a blow out along the symmetry axis occurs in the majority of cases, though a significant fraction of small dwarf galaxies can lose most of their ISM. As no dark matter halos or clumpy ISM distributions are included, it is apparent that the ISM in most dwarf galaxies may be generally resistant to significant disruption by a central starburst event. The effects of this range of behavi or on the metallicities that would be observed in these galaxies is discussed.
De Young, David S.↗
Formation of Benzene in the Interstellar Medium
Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block-the aromatic benzene molecule-has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3- butadiene, C2H + H2CCHCHCH2 --> C6H6, + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadlene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.
Jones, Brant M.↗
Further high-resolution Na I observations of the local interstellar medium
High-resolution absorption measurements of the interstellar Na I D lines at 5890 A observed toward 27 early-type stars in the local interstellar medium (LISM) are presented. These results are combined with other high-resolution sodium measurements to map the space distribution of neutral sodium column density for some 118 stars out to less than 200 pc. These measurements indicate an upper limit to the neutral sodium column density of log N(Na I) less than 10.0/sq cm can be inferred out to a distance of 50 pc in most directions in the LISM. Also, the rarefield region of the Local Bubble may extend beyond 60 pc in at least 35 percent of the directions sampled thus far. Evidence is shown for a ubiquitous, comoving vector for neutral NaI gas clouds in the LISM which is in a different direction to LISM vectors previously reported for more ionized local gas clouds. A comparison of the measured sodium columns with those of interstellar Ca II for a sample of 12 stars within 95 pc results in a ratio of Na I/Ca II less than 0.5 for most stars. This value implies that there could be warm, neutral gas with T of about 12,500 K beyond 50 pc in the LISM.
Welsh, B. Y.↗
The diffuse ionized interstellar medium perpendicular to the plane of NGC 891
In an attempt to study the structure and the properties of the diffuse ionized interstellar medium perpendicular to the plane of disk galaxies researchers obtained H alpha images and spectra of NGC 891. Perhaps the most remarkable property of the H alpha emission line in NGC 891 is its extension out of the plane of the galaxy: researchers are able to measure the H alpha line out to more than 30 seconds (1.4 kpc) from the midplane. This means that the ionized hydrogen extends at least four times higher than the neutral hydrogen layer. An anomalously large scale-height for the ionized gas of approx. equals 1 kpc is also found in the Milky Way. The echelle spectra show a changing ratio of NII to H alpha. This excludes the possibility that the large scaleheight of the emission is due to scattering of disk emission by dust high above the plane. The z-extent of the H alpha emission is confirmed by the imaging result. The large z-extent of the ionized gas is confined to the inner half of the visible disk. In this inner region the H alpha distribution also shows a filamentary structure of the diffuse ionized medium. These filaments, sticking out of the plane, originate in HII regions in the plane. The H alpha image also shows a large scale asymmetry if the NE and SW parts of the disk are compared. The NE part is more prominent and extended in H alpha. The same asymmetry is also seen in the radio continuum distribution. This correlation between the diffuse ionized medium and the distribution of relativistic electrons is one example of a relation between star formation processes in the disk and the various components of the halo. Thermal filaments or spurs which are related to HII regions are also known in the Galaxy. These filamentary structures perpendicular to the galactic planes may represent the chimneys which result in the supernova dominated model of the Interstellar Medium by Norman and Ikeuchi (1989).
Dettmar, Ralf-Juergen↗