The dynamical state of the interstellar gas and field.
Dynamical requirements for existence of interstellar magnetic field in solar neighborhood
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A perturbation calculation of the dynamical effects of cosmic rays and interstellar gas on the solar wind
The modulation of galactic cosmic rays and other energetic particles by the solar wind produces a gradient in their pressure, which in turn influences the wind dynamics. The basic equations describing this interaction are presented in the 'hydrodynamic' approximation. A perturbation solution of the equations is presented for the case in which both the galactic cosmic ray pressure, and the pressure of the anomalous cosmic ray component accelerated at the wind termination shock, are small compared with the wind ram pressure. Analytical expressions for the deceleration of the wind and the modification of the shock and its location are derived in this case. These effects are estimated to have a relative magnitude of several percent in the solar wind. The interstellar neutral gas which penetrates the heliosphere is ionized, predominantly by photoionization and charge exchange with the wind, and may also have a significant dynamical effect on the wind. The basic equations describing this interaction are also presented, A perturbation solution is presented under the assumption that the mass loading and momentum-loading of the wind by the interstellar pickup ions is small compared with the wind ram pressure. Analytical expressions for the deceleration of the wind, the contribution of the pickup ions to the wind pressure, and the modification of the termination shock location are derived. Again, with the exception of pickup ion pressure which is large compared with solar wind thermal pressure, the effects are estimated to be several percent in relative magnitude in the solar wind.
Lee, M. A.↗
Hot interstellar gas in the Galactic disk and halo
The diagnostic methods used to study the hot interstellar medium (ISM) are discussed. Observational results for hot gas in the Galactic disk and halo are presented with attention given to absorption line and diffuse X-ray emission studies. It is noted that UV measurements of O I and other highly ionized atoms are a good probe of gas with temperatures near 1-300,000 K. An overview is given of the theory of the hot ISM as it might apply to disk and halo gas. A combination of the galactic fountain and the photoionized halo models may describe hot gas in the Galactic halo.
Savage, Blair D.↗
The local distribution of Na I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambda approximately 75,000) of the interstellar Na I D lines at 5890 A toward 80 southern hemisphere early-type stars located in the local interstellar medium (LISM). Combining these results with other sodium measurements taken from the literature, we produce galactic maps of the distribution of neutral sodium column density for a total of 293 stars generally lying within approximately 250 pc of the Sun. These maps reveal the approximate shape of the mid-plane contours of the rarefied region of interstellar space termed the Local Bubble. Its shape is seen as highly asymmetric, with a radius ranging from 30 to 300 pc, and with an average radius of 60 pc. Similar plots of the Galactic mid-plane distribution of sources emitting extreme ultraviolet radiation show that they also trace out similar contours of the Local Bubble derived from Na I absorption measurements. We conclude that the Local Bubble absorption interface can be represented by a hydrogen column density, Nu(sub ETA) = 2 x 10(exp 19) cm(exp -2), which explains both the local distribution of Na I absorption and the observed galactic distribution of extreme ultraviolet sources. The derived mid-plane contours of the Bubble generally reproduce the large-scale features carved out in the interstellar medium by several nearby galactic shell structures.
Welsh, B. Y.↗
The effect of a weak shock on interstellar gas toward the Rho Ophiuchi cloud
The effect of a low-velocity shock on depletion in interstellar clouds is studied. High-resolution Copernicus observations of interstellar absorption lines toward four stars in the Rho Ophiuchi cloud complex were used to measure differential depletion in interstellar clouds separated by velocities of 10-15 km/s. Optical observations of CH and CH(+) were used as indicators of shock strength and direction of propagation. Observations indicate the presence of a shock with velocity about 10 km/s expanding away from the sun into the Rho Oph cloud. The existence of this shock is supported by the other observations quoted in the literature. Two distinct regions in the lines of sight have been found. A low-density, less depleted, predominantly atomic region is associated with preshock gas, while postshock gas accounts for a predominantly molecular, more highly depleted region. Apparently a weak shock, has the effect of enhancing grain formation or grain growth as a result of increased density in the postshock gas. It is possible for grain growth by accretion to occur in the shocked gas on time scales short compared with the cloud-crossing time of the shock.
Meyers, K. A.↗
PAHs molecules and heating of the interstellar gas
Until now it has remained difficult to account for the rather high temperatures seen in many diffuse interstellar clouds. Various heating mechanisms have been considered: photoionization of minor species, ionization of H by cosmic rays, and photoelectric effect on small grains. Yet all these processes are either too weak or efficient under too restricting conditions to balance the observed cooling rates. A major heat source is thus still missing in the thermal balance of the diffuse gas. Using photoionization cross sections measured in the lab, it was shown that in order to balance the observed cooling rates in cold diffuse clouds (T approx. 80 K) the PAHs would have to contain 15 percent of the cosmic abundance of carbon. This value does not contradict the former estimation of 6 percent deduced from the IR emission bands since this latter is to be taken as a lower limit. Further, it was estimated that the contribution to the heating rate due to PAH's in a warm HI cloud, assuming the same PAH abundance as for a cold HI cloud, would represent a significant fraction of the value required to keep the medium in thermal balance. Thus, photoionization of PAHs might well be a major heat source for the cold and warm HI media.
Verstraete, Laurent↗
Neutral-pion decay gamma rays from the galaxy and the interstellar gas content
The total gamma-ray production rate per H atom from the decay of neutral pions produced in interstellar cosmic-ray interactions is recalculated here using the latest accelerator data on the production of neutral pions in pp interactions up to 300 GeV. A simple but accurate approximation used here resolves the past disagreement over the magnitude of this rate. An upper limit is obtained for this rate by considering a maximum galactic cosmic-ray spectrum taking modulation into account.
Stecker, F. W.↗
Neutral-pion-decay gamma rays from the galaxy and the interstellar gas content
Knowledge of the total gamma ray production rate per H atom from the decay of pion(0) produced in interstellar cosmic ray interactions is essential for determining the possible amount of interstellar H2. This production rate is recalculated using the latest accelerator data on pion(0) production in pp interactions up to approximately 300 GeV. A simple but accurate approximation resolves the past disagreement over the magnitude of this rate. An upper limit is obtained of (1.51 + or - 0.23) x 10 to the minus 25th power/sec. consistent with the observed upper limit of 1.6 x 10 to the minus 25th power/sec.
Stecker, F. W.↗
Neutral-pion-decay gamma rays from the Galaxy and the interstellar gas content
Knowledge of the total gamma-ray production rate per H atom from the decay of neutral pions produced in interstellar cosmic-ray interactions is essential for determining the possible amount of interstellar H2. This production rate is recalculated here using the latest accelerator data on neutral pion production in p-p interactions up to about 1500 GeV. A simple but accurate approximation used here resolves the past disagreement over the magnitude of this rate. An upper limit is obtained of (1.51 plus or minus 0.23) times 10 to the -25th power/sec, consistent with the observed upper limit of 1.6 times 10 to the -25th power/sec.
Stecker, F. W.↗
Abundance variations in high-velocity interstellar gas
Based on line-of-sight models and profile fitting, abundance variations of Ca, Si, Fe, and Na in high-velocity interstellar clouds toward Mu Columbae, HD 28497, and HD 50896 are discussed. The correlation of larger ratios N(Ca II)/N(Na I) with increasing radial velocity in clouds with absolute values of radial velocity from 20 to 100 km/s, is shown to quite likely be due to abundance variations in Ca rather than an ionization effect. Copernicus UV data show that the ratios N(Fe II)/N(S II) and N(Si II)/N(S II) have the same velocity correlation as Ca II, and the ambiguity concerning the relative abundance of Na I to hydrogen is removed. Theories of grain disruption are discussed as they relate to the observed Ca, Si, and Fe enhancements and their correlation with cloud velocity.
Shull, J. M.↗
The Evolution of Interstellar Gas: Massive Stars and the Dispersal of Neutral Material
We studied the effects of newly formed O and B stars on their surrounding interstellar material through a combination of observations and theoretical modeling. The observational data came from measurements of absorption seen in the spectra of background, newly formed stars. Particular attention was given to stellar radiation which converts molecular to atomic material. Laboratory data on absorption cross sections relevant to the analysis and interpretation of carbon monoxide formed part of the effort. The grant supported Postdoctoral Fellows, Drs. Min Yan and Yaron Sheffer, and a laboratory technician. Though the students themselves were not supported. one M.S. Thesis and two Ph.D. dissertations from the University of Toledo were based on the research done under the grant. The research accomplished under this grant led directly to other funded programs. An observing proposal to study the chemistry of diffuse molecular clouds in the Large and Small Magellanic Clouds with ESO s Very Large Telescope was another example of a successful outcome of my LTSA program.
Federman, Steven R.↗
Thermal phases of interstellar and quasar gas
Interstellar gas may be in a variety of thermal phases, depending on how it is heated and ionized; here a unified picture of the equation of state of interstellar and quasar gas is presented for a variety of such mechanisms over a broad range of temperatures, densities, and column densities of absorbing matter. It is found that for select ranges of gas pressure, photoionizing flux, and heating, three thermally stable phases are allowed: coronal gas (T above 100,000 K); warm gas (T about 10,000 K); and cold gas (T less than 100 K). With attenuation of ultraviolet and X-ray radiation, the cold phase may undergo a transition to molecules. In quasar broad-line clouds, this transition occurs at column density N(H) = about 10 to the 23rd/sq cm and could result in warm molecular cores and observable emission from H2 and OH. The underlying atomic physics behind each of these phase transitions and their relevance to interstellar matter and quasars are discussed.
Lepp, S.↗
Interstellar gas phase abundance of carbon, oxygen, nitrogen, copper, gallium, germanium, and krypton toward Zeta Ophiuchi
An analysis of weak (less than 10 mA) UV interstellar absorption line data obtained for the line of sight to the O9.5 IV star Zeta Oph is presented. Measurements of weak semiforbidden lines of N I, O I, Cu II, and a new UV detection of Na I are reported along with a small upper limit for C II. Interstellar detections of Ga II, Ge II, and Kr I are also presented. Ga, Ge, and Kr represent the heaviest elements detected in the ISM. A comparison of the derived column densities to cosmic abundances shows Ga to be depleted by about -1.2 dex while Ge is overabundant by +0.2 dex. Assuming Kr to be undepleted, a logarithmic cosmic abundance of Kr/H = 2.95 is obtained on the scale where H = 12.00.
Cardelli, Jason A.↗
On the distribution of interstellar gas in the galactic halo
New and existing observations of 21-cm emission lines toward 10 distant, high-latitude OB stars are combined with existing observations of interstellar Lyman-alpha absorption lines, in order to determine the ratio, N21/N-alpha, of the two different column densities of H I. This ratio, which is related to the fraction of the cool, neutral gas in the halo that lies beyond each star, decreases smoothly to about unity with increasing distance from the galactic plane. The column density of neutral gas beyond about 1 kpc can be as much as one-third of the total above the plane, but only relatively small amounts of such gas lie more than 2 kpc from the plane. The distances to, and the possible birthplaces of, these Population I stars in the halo are discussed.
Hobbs, L. M.↗
Highly ionized interstellar gas located in the Galactic disk and halo
High-resolution IUE absorption line spectra have been obtained for 40 distant stars in order to study the distribution of interstellar H I, Si IV, C IV, and N V in the Galactic disk and lower halo. Respective midplane densities of 2 x 10 to the -9th, 7 x 10 to the -9th, and 3 x 10 to the -9th are found for Si IV, C IV, and Ni V. Both column density and velocity data indicate that the highly ionized gas (HIG) is considerably more extended in directions away from the Galactic plane than is H I or Si II. The absorption-line velocities for the halo HIG are consistent with the notion that halo gas in the inner Galaxy rotates more slowly than gas in the underlying disk. The derived column densities suggest an exponential scale height for the HIG of about 3 kpc; however, a simple exponential distribution is a poor representation of the distribution of the gas. It is concluded that a full explanation of the origin of the halo HIG will probably require a blending of ideas from the Galactic fountain and the photoionized halo models.
Savage, Blair D.↗
Ultraviolet absorption by interstellar gas near 30 Doradus
The IUE was used to obtain high-resolution far-UV spectra (1150-2070 A) of two stars in the 30 Dor H II region in the LMC. Interstellar absorption components are distinguished at +20, +220, +250, and +290 km/s. The +20 km/s component is produced by matter in the Galaxy; the high-velocity components are produced by absorbing gas near or in the LMC. A model of the line-of-sight distribution of the absorbing clouds is developed from the velocity pattern of the observed LMC features. The presence of Si IV, Al III, and C IV ions is discussed.
De Boer, K. S.↗
Interstellar gas and dust in the young cluster IC 348
We have completed a multiband absorption- and emission-line study of a star embedded in the young cluster IC 348, to determine the environmental effect of star formation on the interstellar medium (ISM) local to the region. The hottest and youngest star in IC 348 is BD +31 deg 643, a B5 V star which samples the inner bright nebular region. The nearby star omicron Per, which lies only 8 min to the north and is thought to lie beyond IC 348, samples the gas and dust which has not been processed by very recent star formation. We speculate that the ISM throughout the region was originally the same as that currently seen toward omicron Per, but now the constrasting environmental conditions due to the recent star formation have led to marked differences in the atomic, molecular, and dust characteristics of IC 348. These contrasts include what we have termed a 'composite' UV extinction curve for BD +31 deg 643, evidence for enhanced density and enhanced depletions within IC 348 and very different molecular abundances in the interstellar sightline to BD +31 deg 643. Toward BD +31 deg 643, we find a higher column density of CH, but lower CN and very much higher CH(+) than measured toward omicron Per. We conclude that the physical and chemical state of the gas and dust has been altered by local processes and conditions within IC 348. The characteristics of the ISM in IC 348, via our study of the star BD +31 deg 643, closely resembles those seen toward rho Oph, another sight line passing through a bright nebular region. However, the stars are not as hot in IC 348 as in Ophiuchus, so their effect on the local ISM is not as severe.
Snow, Theodore P.↗
On high-velocity interstellar gas toward Delta Sco
Independent observations of the Na I D2 line toward Delta Sco are described which show that the two high-velocity absorption components reported by Hicks et al. (1975) are not of normal interstellar origin. A portion of the spectrum of Delta Sco near the D2 line is presented, and it is proposed that the two absorption components may have been of atmospheric origin, although their separation and relative intensities do not suggest any obvious counterparts within the observed telluric spectrum.
Hobbs, L. M.↗