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1.
We have made an observational study of the newly identified cyanomethane radical CH2CN and the possibly related species CH3CN with the goals of (1) elucidating the possible role of reactions of the type CnHm(+) + N in astrochemistry, and (2) providing a possible test of Bates's models of dissociative electron recombination. We find a remarkably different abundance ratio CH2CN/CH3CN in TMC-1 and Sgr B2, which we deduce is a result of the large difference in temperature of these objects. Studies of CH2CN and CH3CN in other sources, including two new detections of CH2CN, support this conclusion and are consistent with a monotonic increase in the CH2CN/CH3CN ratio with decreasing temperature over the range 10-120 K. This behavior may be explained by the destruction of CH2CN by reaction with O. If this reaction does not proceed, then CH2CN and CH3CN are concluded to form via different chemical pathways. Thus, they do not provide a test of Bates's conjectures (they do not both form from CH3CNH+). CH2CN is then likely to form via C2H4(+) + N --> CH2CNH+, thus demonstrating the viability of this important reaction in astrochemistry. The T dependence of the CH2CN/CH3CN ratio would then reflect the increasing rate of the C2H4(+) + N reaction with decreasing temperature. 相似文献
2.
D. C. Lis M. Gerin E. Roueff T. G. Phillips D. R. Poelman 《Astrophysics and Space Science》2008,313(1-3):77-80
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. 相似文献
3.
Ronald G. Tabak 《Astrophysics and Space Science》1978,53(2):279-294
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. 相似文献
4.
M. A. Walker L. V. E. Koopmans D. R. Stinebring W. van Straten 《Monthly notices of the Royal Astronomical Society》2008,388(3):1214-1222
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. 相似文献
5.
Ronald G. Tabak 《Astrophysics and Space Science》1978,54(1):211-232
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 5m.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. 相似文献
6.
7.
8.
Mordecai-Mark MacLow 《Astrophysics and Space Science》2004,289(3-4):323-331
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. 相似文献
9.
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. 相似文献
10.
Dieter Breitschwerdt 《Astrophysics and Space Science》2001,276(1):163-176
The state of our observational and theoretical knowledge of the localinterstellar medium (LISM) is reviewed. The LISM stretches out from thelocal Cloud, surrounding the solar system to the boundary of the LocalBubble, which is a region of distinct anticorrelation between HI (N
H< 1020 cm-2) and 1/4 keV diffuse soft X-rays. After discussingsome key observations in soft X-rays, obtained by ROSAT and DXS, and inthe EUV by the EUVE satellite and the EURD instrument onboard Minisat, Iwill critically review models of the LISM. Since we cannot determine theplasma state (temperature, density, etc.) directly, the best plasmadiagnostic tool is high resolution spectroscopy. The interpretationof the data then has to rely heavily on plasma emission models. I willpoint out several caveats in the standard procedure. Preference is givento dynamical models of the Local Bubble in general and to non-equilibriumplasma emission models in particular, which have to be calculatedself-consistently. Such a model can explain (i) the deficiency of EUVlines as observed by EUVE, (ii) the dispersion measure and scintillationproperties of a nearby pulsar, (iii) the existence of local neutral HIclouds, and (iv) OVI absorption line widths. New model spectra of theLISM will be presented and briefly compared with DXS, EUVE and preliminaryEURD results. 相似文献
11.
N.D. Ramesh Bhat Yashwant Gupta A. Pramesh Rao P.B. Preethi 《Astrophysics and Space Science》2001,278(1-2):171-174
Pulsar scintillation measurements from the Ooty Radio Telescope (ORT) are used to investigate the distribution of scattering
in the Local Interstellar Medium (LISM; region of ≲ 1 kpc of the Sun), specifically the region in and around the Local Bubble.
A 3-componentmodel, where the Solar neighborhood is surrounded by a shell of enhanced plasma turbulence, is proposed for the
LISM. Further, the Ooty data, along with those from Parkes and other telescopes are used for investigating thedistribution
of scattering towards the nearby Loop I Superbubble.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
星际红外消光律 总被引：1，自引：0，他引：1
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。 相似文献
13.
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 〈ne〉) 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 ne with galactic radius RGC for RGc ≳ 5 kpc. Further, we make a detailed analysis of the contribution to ne 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. 相似文献
14.
Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The pseudo-plasma formalism, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important.In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well pinch the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together.Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of dust (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A stellesimal accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites.The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems. 相似文献
15.
16.
P. Predehl 《Astrophysics and Space Science》1997,258(1-2):89-95
The interaction of X-rays with interstellar dust leads to small angle scattering. X-ray sources behind sufficiently dense
dust columns are therefore surrounded by haloes of faint and diffuse X-ray emission. Since these X-ray sources are also highly
absorbed, X-ray observations offer the unique opportunity to measure both components of interstellar extinction simultaneously.
This method provides an excellent means of determining interstellar gas to dust ratios, a differentiation between interstellar
and circumstellar matter, and, last but not least, a clue to the physical nature of dust itself. In particular we have a plausible
explanation why the supernova- explosion in Cassiopaea 300 years ago, of which we know today the remnant Cas A, has not been
seen. Furthermore, we find that dust grains must be ‘fluffy’ or porous with voids up to 70%.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
We deduce the mass distribution and total mass density of interstellar dust streaming into the solar system and compare the
results to the conditions of the very local interstellar medium (VLISM). The mass distribution derived from in situ measurements
shows a gentler slope and includes larger grains, compared to a model distribution proposed for the wavelength dependence
of the interstellar extinction. The mass density of grains in the solar system is consistent with that expected from measurements
of the visible interstellar extinction and the abundance constraints of elements in the diffuse interstellar medium (ISM),
instead of those in the VLISM. This may imply that interstellar dust grains are not associated with the VLISM and that the
conditions of the grains are better represented by the ones expected in the diffuse ISM. If this is the case, then the flatter
slope in the mass distribution and the detection of larger interstellar grains in the solar system may even indicate that
coagulation growth of dust in the diffuse ISM is more effective than previously inferred.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
W. J. Baggaley 《Earth, Moon, and Planets》2004,95(1-4):197-209
Dust is an important component of galactic stucture and the cyclic processing of particulate matter leads to stellar and planetary
formation. Though astronomical methods using analysis of dust-penetrating starlight can provide some limited information about
the dust, the prospect of its in-situ sampling within the Solar System by spacecraft and its remote sensing by ground-based techniques open up a new field in galactic
exploration. 相似文献
19.
Simple calculations indicate that the spatial density in the galaxy of extinct civilizations, and of planets inhabited by intelligent creatures who do not have technical civilizations, may be quite large. 相似文献
20.
Interstellar transfer of planetary microbiota 总被引：2，自引：0，他引：2
Max K. Wallis N. C. Wickramasinghe 《Monthly notices of the Royal Astronomical Society》2004,348(1):52-61