共查询到20条相似文献，搜索用时 78 毫秒
1.
S. N. Kemp B. Bates J. E. Beckman C. J. Killow R. Barrena D. C. Kennedy J. Rodríguez Alamo 《Monthly notices of the Royal Astronomical Society》2002,333(3):561-574
Here we make a new study of the behaviour of the Na i /K i column density ratio in the interstellar medium, using a sample of new observations of 28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and 1997, and previously published observations (obtained by some of the authors) of 21 stars. The sightlines cover a range of distances and directions, including into the Galactic halo. We make use of new observations of the Na i ultraviolet (UV) doublet for some 18 stars. This doublet is much weaker than the Na i D doublet and so is less susceptible to saturation effects, and it is well known that it can be used to obtain more accurate Na i column densities with a smaller error range. We find an average N (Na i )/ N (K i ) ratio from the Na i UV data of about 90, which is rather higher than that found previously by Hobbs and Lequeux. The Na UV–K i ratio shows a small increase in value with increasing column density, while we also find a sample of low N (Na i )/ N (K i ) ratio clouds generally seen towards distant objects on high-latitude sightlines that reach into the halo, so that the ratio decreases more sharply at lower column densities. As the values of the ratio for these halo clouds (10–20) bracket the cosmic Na/K abundance ratio, we suggest that these ratios result from a harder radiation field in the lower halo, such that the ionized fractions of Na i and K i become similar. Clearly caution needs to be applied in using any kind of 'standard value' for the Na i /K i column density ratio. 相似文献
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N. Lo M. R. Cunningham P. A. Jones I. Bains M. G. Burton T. Wong E. Muller † C. Kramer V. Ossenkopf C. Henkel G. Deragopian S. Donnelly E. F. Ladd 《Monthly notices of the Royal Astronomical Society》2009,395(2):1021-1042
We present multimolecular line maps obtained with the Mopra telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the H ii region RCW 106. We have characterized the GMC by decomposing the 3D data cubes with gaussclumps , and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and linewidth, but a strong correlation between emission luminosity and linewidth. PCA classifies molecules into high- and low-density tracers, and reveals that HCO+ and N2 H+ are anticorrelated. 相似文献
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Deborah P. Ruffle Eric Herbst 《Monthly notices of the Royal Astronomical Society》2000,319(3):837-850
In recent years it has become evident that large differences can exist between model results of grain-surface chemistry obtained from a rate equation approach and from a Monte Carlo technique. This dichotomy has led to the development of a modified rate equation method, in which a key element is the artificial slowing down of the diffusion rate of surface hydrogen atoms. Recent laboratory research into the surface diffusion rate of atomic hydrogen suggests that atomic hydrogen moves more slowly on grains than heretofore assumed. This research appears to lessen the need for modifications to the rate equation method. Based on the new laboratory work, we have developed appropriate models of gas-phase and grain-surface chemistry in quiescent dense cloud cores to examine the chemical effects of slowing down the rate at which atomic H can scan over dust surfaces. Furthermore, we have investigated the effect of slowing down the rate at which all species can move over grain surfaces. 相似文献
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We have obtained a spectrum of the Na D lines of the Seyfert galaxy Mrk 595 with a signal-to-noise ratio (S/N) of about 30. The importance of this sightline is that this galaxy lies in a direction close to the highest column densities of the Cohen high-velocity (HV) stream, and hence represents the best opportunity of detecting this stream in interstellar absorption, as previous searches towards stellar targets up to a distance of 600 pc have not been successful. The H I column density in the direction of this galaxy implies that the HV gas should be detectable in absorption in the Na D lines. No such detections are made (at the 3σ level) and we consider possible explanations. Hence previous non-detections towards stars do not necessarily mean that the HV gas is more distant than the stars, and we emphasize that the distance of this high-velocity cloud (HVC), which is a key parameter to the understanding of the nature and origin of this feature, remains unknown and indeed may be difficult to establish. 相似文献
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W. W. Duley 《Monthly notices of the Royal Astronomical Society》1998,301(4):955-959
An analysis of the intensity and spatial distribution of the discrete 5800-Å emission band in the spectrum of the Red Rectangle has been used to constrain the abundance and physical properties of the carrier of this emission. An origin in a large (>30 C atom) molecule is indicated. This molecule is formed in situ in the Red Rectangle, but is also a component of the diffuse interstellar medium. The UV photodissociation probability for this molecule is ≲10−5 per absorbed photon, and the luminescence efficiency is in the range 10−2 –10−3 . This molecule may be a product of the dissociation of carbonaceous dust. 相似文献
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Avinash A. Deshpande 《Monthly notices of the Royal Astronomical Society》2000,317(1):199-204
During the past decade or so, measurements of Galactic H i absorption using VLBI against extragalactic sources, as well as multi-epoch observations in pulsar directions, have detected small-scale transverse variations corresponding to tens of au at the distance of the absorbing matter. Hitherto these measurements have been interpreted as small-scale structure in the H i distribution with densities n H i ∼104 –105 cm−3 , orders of magnitude greater than those of the pc-scale structure. Naturally, it is difficult to imagine how such structures could exist in equilibrium with other components of the ISM.
In this paper we show that structure on all scales contributes to the differences on neighbouring lines of sight, and that the observed differences can be accounted for by a natural extension of the distribution of irregularities in the distribution of H i opacities at larger scales, using a single power law. This, in our opinion, should put an end to the decades-long puzzle of the so-called small-scale structure in H i and other species in the Galaxy. 相似文献
In this paper we show that structure on all scales contributes to the differences on neighbouring lines of sight, and that the observed differences can be accounted for by a natural extension of the distribution of irregularities in the distribution of H i opacities at larger scales, using a single power law. This, in our opinion, should put an end to the decades-long puzzle of the so-called small-scale structure in H i and other species in the Galaxy. 相似文献
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Serena Viti David A. Williams 《Monthly notices of the Royal Astronomical Society》1999,310(2):517-526
Compact regions of enhanced HCO+ and NH3 emission have been detected close to a number of Herbig–Haro objects. An interpretation of these detections is the following: a transient clump within the molecular cloud has been irradiated by the shock that generates the Herbig–Haro object. The irradiation releases icy mantles from the grains within the transient clump and initiates a photochemistry. On the basis of this picture, we have developed an extensive chemical model which predicts that a wide range of species, other than NH3 and HCO+ , should also be detectable. These include CH3 OH, H2 S, C3 H4 , H2 CO, SO, SO2 , H2 CS and NS. The chemical effects should last ∼ 104 yr . 相似文献
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M. G. Rawlings M. Juvela K. Mattila K. Lehtinen D. Lemke 《Monthly notices of the Royal Astronomical Society》2005,356(3):810-828
We present isophot spectrophotometry of three positions within the isolated high-latitude cirrus cloud G 300.2−16.8, spanning from the near- to far-infrared (NIR to FIR). The positions exhibit contrasting emission spectrum contributions from the unidentified infrared bands (UIBs), very small grains (VSGs) and large classical grains, and both semi-empirical and numerical models are presented. At all three positions, the UIB spectrum shapes are found to be similar and the large grain emission may be fitted by an equilibrium temperature of ∼17.5 K . The energy requirements of both the observed emission spectrum and optical scattered light are shown to be satisfied by the incident local interstellar radiation field (ISRF). The FIR emissivity of dust in G 300.2−16.8 is found to be lower than in globules or dense clouds and is even lower than model predictions for dust in the diffuse interstellar medium (ISM). The results suggest physical differences in the ISM mixtures between positions within the cloud, possibly arising from grain coagulation processes. 相似文献
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Deborah P. Ruffle Eric Herbst † 《Monthly notices of the Royal Astronomical Society》2001,324(4):1054-1062
Solid CO2 is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO2 at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO2 can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently. 相似文献
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C.Codella F.Scappini R.Bachiller M.Benedettini 《Monthly notices of the Royal Astronomical Society》2002,331(4):893-900
The globule CB 34, which harbours a cluster of class 0 young stellar object (YSO) protostars, has been investigated through a multiline SO and SiO survey at millimetre wavelengths. The SO data reveal that the globule consists of three quiescent high-density (∼105 cm−3 ) clumps, labelled A, B and C, with sizes of ∼ 0.2–0.3 pc . The SiO data provide evidence for high-velocity gas across the globule. Most likely, the high-velocity gas is distributed in three distinct high-velocity outflows associated with the YSOs in each of the three clumps. High-velocity SO features have been detected only towards the two brightest SiO outflows. These broad SO components exhibit spatial and spectral distributions which are consistent with those of the SiO emission, so they can also be used as tracers of the outflows.
The comparison between the spatial and spectral properties of the SO and SiO emissions in the three clumps suggests different evolutionary stages for the embedded YSOs. In particular, the YSO associated with clump C exhibits some peculiarities, namely smaller SiO linewidths, lower SiO column densities, a lack of extended SiO structure and of SO wings, and the presence of a SO spatial distribution which is displaced with respect to the location of the YSO. This behaviour is well explained if the SiO and SO molecules which were produced at high velocities in the shocked region have been destroyed or slowed down because of the interaction with the ambient medium, and the chemistry is dominated again by low-temperature reactions. Thus our observations strongly suggest that the YSO in clump C is in a more evolved phase than the other members of the cluster. 相似文献
The comparison between the spatial and spectral properties of the SO and SiO emissions in the three clumps suggests different evolutionary stages for the embedded YSOs. In particular, the YSO associated with clump C exhibits some peculiarities, namely smaller SiO linewidths, lower SiO column densities, a lack of extended SiO structure and of SO wings, and the presence of a SO spatial distribution which is displaced with respect to the location of the YSO. This behaviour is well explained if the SiO and SO molecules which were produced at high velocities in the shocked region have been destroyed or slowed down because of the interaction with the ambient medium, and the chemistry is dominated again by low-temperature reactions. Thus our observations strongly suggest that the YSO in clump C is in a more evolved phase than the other members of the cluster. 相似文献
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T. A. Bell E. Roueff S. Viti D. A. Williams 《Monthly notices of the Royal Astronomical Society》2006,371(4):1865-1872
A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H2 , the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the 12 CO( J = 1 → 0) emission line, using a CO-to-H2 conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor. 相似文献
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I. A. Crawford 《Monthly notices of the Royal Astronomical Society》2000,317(4):996-1004
Intermediate-resolution (60 000 R 120 000) observations of interstellar Na i lines towards 29 stars in the general direction of the Lupus molecular clouds (330°≲ l ≲350°; 0°≲ b ≲25°) are presented. Previously published spectra towards an additional seven stars are also included. Based on the Hipparcos distances to these stars, and the minimum distance at which strong interstellar Na i lines appear in the spectra, I obtain a distance of ~150±10 pc to the Lupus molecular complex. While in agreement with a number of other independent estimates, this result is at odds with the value of 100 pc recently obtained by Knude & Høg from a Hipparcos -based study of interstellar extinction. A possible explanation for this discrepancy is discussed, and it is concluded that the value of 150±10 pc obtained here is to be preferred. In addition, these observations have some other implications for the structure of the interstellar medium in this direction, and these are briefly considered. 相似文献
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T. A. Bell S. Viti D. A. Williams 《Monthly notices of the Royal Astronomical Society》2007,378(3):983-994
We present theoretically established values of the CO-to-H2 and C-to-H2 conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a χ2 analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H2 conversion factors are then computed for CO transitions up to J = 9 → 8. For the conventional CO(1–0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H2 conversion factor for this emission in the four galaxy types considered. 相似文献