Interstellar deuteroammonia

Close to 30 deuterated molecules have now been detected in the ISM, including doubly-deuterated species D₂H⁺, ND₂H, D₂CO, CHD₂OH, D₂S, and D₂CS, as well as triply-deuterated ammonia and methanol. We review the current understanding of depletion and deuteration processes in cold, dense interstellar medium (ISM) and discuss the utility of deuteroammonia as a tracer of the physical conditions and kinematics of cold, dense gas.     

Interstellar catalysis

Although it is generally accepted that most, if not all, of the molecular hydrogen in interstellar space is formed through recombination reactions on grains, the exact mechanism by which this is accomplished is far from certain. In the past, great emphasis had been placed on the physical adsorption of H atoms on cold dielectric grains and their subsequent recombination and desorption as H₂ molecules. However, a careful re-examination of the problem leads us to believe that a rate coefficient ofk10^–17 cm³ s^–1—the value usually quoted in the literature—is a very strong overestimate. The same thing can be said for the recombination of H atoms on graphite grains. Since two-body gas phase reactions are not sufficient by themselves to account for the observed abundances of H₂, an alternate mechanism must exist. It is suggested that the chemisorption of hydrogen on transition metal grains may be just that formation mechanism. After separating the adsorption rate equations from those of desorption and using experimentally determined parameters, it is shown that transition metal grains can successfully catalyze as much H₂ as the theoretical maximum predicted for cold ice grains, even though metal grains are probably less than 10% as abundant (by mass) than dielectrics.     

Interstellar catalysis

Since gas-phase reactions alone cannot account for the observed abundances of H₂ in the typical interstellar cloud, one or more surface reactions are probably involved. Of the three possible candidates, only the catalytic production of H₂ on transition metal grains is supported by laboratory evidence. Using the rate equations developed in a previous paper for this process, the steady-state equilibrium abundances of H, H₂,e ^–, H⁺, H^–, H₂ ⁺, and H₃ ⁺ are calculated for large (r>10 pcs;M10² M ), tenuous (n=10²–10⁴ cm^–3) hydrogen dust clouds under a wide variety of conditions. In addition to the four rate equations involved in the catalytic reactions, 18 gas-phase and one additional surface reaction—the physical adsorption of H-atoms on cold, dielectric surfaces and their subsequent recombination and desorption as H₂ molecules—are included in the calculations. It is found that metal grains can produce as much interstellar H₂ as the best physical adsorption mechanism under optimum conditions if the extinction in the visible is less than 5^m.0. The three critical parameters for efficient catalysis (activation energy of desorption, grain temperature, and the number density of available sites) are examined, and it is shown that catalytic reactions are efficient producers of H₂ under all but the most unfavorable conditions.     

Interstellar holography

The dynamic spectrum of a radio pulsar is an in-line digital hologram of the ionized interstellar medium. It has previously been demonstrated that such holograms permit image reconstruction, in the sense that one can determine an approximation to the complex electric field values as a function of Doppler shift and delay, but to date the quality of the reconstructions has been poor. Here we report a substantial improvement in the method which we have achieved by simultaneous optimization of the thousands of coefficients that describe the electric field. For our test spectrum of PSR B0834+06 we find that the model provides an accurate representation of the data over the full 63 dB dynamic range of the observations: residual differences between model and data are noise like. The advent of interstellar holography enables detailed quantitative investigation of the interstellar radio-wave propagation paths for a given pulsar at each epoch of observation. We illustrate this using our test data which show the scattering material to be structured and highly anisotropic. The temporal response of the medium exhibits a scattering tail which extends to beyond  100 μs  , and the centroid of the pulse at this frequency and this epoch of observation is delayed by approximately  15 μs  as a result of multipath propagation in the interstellar medium.     

Interstellar electron density

We impose the requirement that the spatial distribution of pulsars deduced from their dispersion measures using a model of the galactic electron density (n _e ) should be consistent with cylindrical symmetry around the galactic centre (assumed to be 10 kpc from the Sun). Using a carefully selected subsample of the pulsars detected by the II Molonglo Survey (II MS), we test a number of simple models and conclude that (i) the effective mean 〈n_e〉) for the whole galaxy is 0.037_-0.012 ^+0.020 cm^-3, (ii) the scale height of electrons is greater than 300 pc and probably about 1 kpc or more, and (iii) there is little evidence for variation of n_e with galactic radius R_GC for R_Gc ≳ 5 kpc. Further, we make a detailed analysis of the contribution to n_e from H II regions. Combining the results of a number of relatively independent calculations, we propose a model for the galactic electron density of the formn _e (z) = 0.030 + 0.020 exp (- |z|/70) cm^-3 where z(pc) is the height above the galactic plane and the second term describes the contribution from H II regions. We believe the statistical uncertainties in the parameters of this model are quite small.     

Interstellar polarization towardMKN421

Interstellar polarization in the direction of MKN421 reaches a value between .1% and .2% at a distance of 600 pc.     

The Local Interstellar Medium

Summary. Substantial progress in the field of the Local Interstellar Medium has been largely due to recent launches of space missions, mostly in the UV and X–ray domains, but also to ground-based observations, mainly in high resolution spectroscopy. However, a clear gap seems to remain between the wealth of new data and the theoretical understanding. This paper gives an overview of some observational aspects, with no attempt of completeness or doing justice to all the people involved in the field. As progress rarely evolves in straight paths, we can expect that our present picture of the solar system surroundings is not definitive. Received 30 October 1998     

The Interstellar Heliopause Probe: Heliospheric Boundary Explorer Mission to the Interstellar Medium

The Sun, driving a supersonic solar wind, cuts out of the local interstellar medium a giant plasma bubble, the heliosphere. ESA, jointly with NASA, has had an important role in the development of our current understanding of the Suns’ immediate neighborhood. Ulysses is the only spacecraft exploring the third, out-of-ecliptic dimension, while SOHO has allowed us to better understand the influence of the Sun and to image the glow of interstellar matter in the heliosphere. Voyager 1 has recently encountered the innermost boundary of this plasma bubble, the termination shock, and is returning exciting yet puzzling data of this remote region. The next logical step is to leave the heliosphere and to thereby map out in unprecedented detail the structure of the outer heliosphere and its boundaries, the termination shock, the heliosheath, the heliopause, and, after leaving the heliosphere, to discover the true nature of the hydrogen wall, the bow shock, and the local interstellar medium beyond. This will greatly advance our understanding of the heliosphere that is the best-known example for astrospheres as found around other stars. Thus, IHP/HEX will allow us to discover, explore, and understand fundamental astrophysical processes in the largest accessible plasma laboratory, the heliosphere.     

Interstellar water and interstellar ice

There are two ways that water ice can form in the interstellar medium: H₂O molecules can form in the gas phase and then freeze out onto dust grain surfaces, or O and OH can be converted at the surfaces of grains to form H₂O, which is then retained. Bergin et al. (1998) have recently shown that shocks passing through interstellar clouds sufficiently frequently can make the first method effective. However, we present results from a similar chemical model which indicate that this requires significant optical shielding because of the high ionization fraction in regions exposedto a high UV flux. We deduce, therefore, that grain surface reactions probably represent the main source of H₂O ice on lines of sight with visual extinction up to about 6 magnitudes to an embedded source or 12 magnitudes to a background object. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Model for Interstellar Extinction

From an analysis of the interstellar extinction we conclude that interstellar grains are of three main kinds: graphite spheres of radii ∼ 0.02 μmmaking up ∼ ;10% of the total grain mass, small dielectric spheres of radius about 0.04 μm making up ∼ 25% of the mass, and hollow dielectric cylinders containing metallic iron with diameters of ∼ 2/3 μmmaking up ∼ 45% of the mass. The remaining ∼ 20%consists of other metals and metal oxides. The main dielectric component of the grains appears to be comprised of organic material. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formaldehyde Polymers in Interstellar Space

Refractory grains of graphite and silicates ejected from cool giant stars acquire mantles of organic polymers typified by polyoxymethyline in interstellar clouds. Infrared emission from theTrapezium nebula is consistent with the occurrence of such polymers. This revised version was published online in July 2006 with corrections to the Cover Date.     

椭圆星系中的星系际尘埃

该文总结了近年来椭圆星系中尘埃起源研究中的一些关键问题,包括尘埃大小的分布及其组成、椭圆星系恒星质量损失速率、尘埃的两种加热机制(恒星辐射加热和电子-尘埃碰撞加热),以及由此导致的尘埃温度分布;尘埃的总质量估算;由热离子导致的尘埃破坏速率,以及与此相关联的椭圆星系热气体分布,等等.综合考虑这些因素,作者认为热气体分布对于椭圆星系中尘埃存在和破坏的影响不可忽视,椭圆星系中星系内部恒星质量损失产生尘埃的起源可能占据着非常重要的地位.     

Turbulence in the Interstellar Medium

Interstellar turbulence is expected to dissipate quickly in theabsence of continuous energy input. I examine the energy availablefor driving the turbulence from six likely mechanisms:magnetorotational instability, gravitational instability, protostellaroutflows, HII region expansion, stellar winds, and supernovae.I conclude that supernovae contribute far more energy than the othermechanisms, and so form the most likely drving mechanism.     

Interstellar polarization: New approximation

A new, physically valid, approximation of the wavelength dependences of the degree of linear interstellar polarization P _λ observed in a wavelength range from 0.2 to 4 μm is suggested. The data obtained from the ground-based observations and the HPOL and WUPPE space missions on P _λ of 104 stars located in a galactic latitude range B from ?20° to +20° are used. This approximation provides better agreement with the observational P _λ data than the models by Serkowski (1973) and Whittet et al. (1992) proposed before.     

星际红外消光律

该文综述了星际尘埃在红外波段的消光律。现有的观测表明,在0.7μm≤λ≤5μm范围内的星际消光可以近似用幂律函数来拟合,而对5μm≤A≤8μm范围内的星际消光争论比较多,但现在已明确该范围内消光律是随视线方向的变化而变化,这与以往认为在整个0.7μm≤λ≤8μm范围内星际消光为各处一致的结论是相左的。在8μm≤λ≤30μm范围内的消光主要由硅酸盐在9.7μm和18μm处的吸收特征所决定,其中9.7μm处的吸收所引起的消光强度(相对可见光波段消光)在太阳邻近弥漫星云和银心方向上有较大变化,其原因仍有争论;同时,由脂肪族碳氢化合物的C-H键伸缩振动引起的在3.4μm处的谱特征强度也和9.7μm硅酸盐谱特征类似,在太阳邻近弥漫星云和银心方向有较大变化。在λ≥30μm以上的远红外范围,由于观测的限制,人们主要利用尘埃红外辐射理论来间接研究消光率,不同尘埃模型给出的红外辐射明显地不同,对λ≥30μm的红外观测将提高该范围内消光律的准确性。