共查询到20条相似文献，搜索用时 31 毫秒
1.
J. R. Dawson N. Mizuno T. Onishi N. M. McClure-Griffiths Y. Fukui 《Monthly notices of the Royal Astronomical Society》2008,387(1):31-44
The 'Carina Flare' supershell, GSH 287+04−17, is a molecular supershell originally discovered in 12 CO( J = 1–0) with the NANTEN 4 m telescope. We present the first study of the shell's atomic ISM, using H i 21-cm line data from the Parkes 64-m telescope Southern Galactic Plane Survey. The data reveal a gently expanding, ∼230 × 360 pc H i supershell that shows strong evidence of Galactic Plane blowout, with a break in its main body at z ∼ 280 pc and a capped high-latitude extension reaching z ∼ 450 pc. The molecular clouds form comoving parts of the atomic shell, and the morphology of the two phases reflects the supershell's influence on the structure of the ISM. We also report the first discovery of an ionized component of the supershell, in the form of delicate, streamer-like filaments aligned with the proposed direction of blowout. The distance estimate to the shell is re-examined, and we find strong evidence to support the original suggestion that it is located in the Carina Arm at a distance of 2.6 ± 0.4 kpc . Associated H i and H2 masses are estimated as M H I ≈ 7 ± 3 × 105 M⊙ and , and the kinetic energy of the expanding shell as E K ∼ 1 × 1051 erg. We examine the results of analytical and numerical models to estimate a required formation energy of several 1051 to ∼1052 erg, and an age of ∼107 yr . This age is compatible with molecular cloud formation time-scales, and we briefly consider the viability of a supershell-triggered origin for the molecular component. 相似文献
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We report the discovery of high-velocity dense gas from a bipolar outflow source near NGC 2068 in the L1630 giant molecular cloud. CO and HCO+ J =3→2 line wings have a bipolar distribution in the vicinity of LBS 17-H with the flow orientated roughly east–west and perpendicular to the elongation of the submillimetre dust continuum emission. The flow is compact (total extent ∼0.2 pc) and contains of the order of 0.1 M⊙ of swept-up gas. The high-velocity HCO+ emission is distributed over a somewhat smaller area <0.1 pc in extent.
A map of C18 O J =2→1 emission traces the LBS 17 core and follows the ambient HCO+ emission reasonably well, with the exception of the direction towards LBS 17-H where there is a significant anticorrelation between the C18 O and HCO+ . A comparison of beam-matched C18 O and dust-derived H2 column densities suggests that CO is depleted by up to a factor of ∼50 at this position if the temperature is as low as 9 K, although the difference is substantially reduced if the temperature is as high as 20 K. Chemical models of collapsing clouds can account for this discrepancy in terms of different rates of depletion on to dust grains for CO and HCO+ .
LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L⊙ and a submillimetre-to-bolometric luminosity ratio of 0.1, comfortably satisfying the criteria for classification as a class 0 protostar candidate. 相似文献
A map of C
LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L
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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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R. Vázquez J. A. López L. F. Miranda J. M. Torrelles J. Meaburn 《Monthly notices of the Royal Astronomical Society》1999,308(4):939-946
High-resolution, spatially-resolved profiles of H α , He ii λ 6560 and [O iii ] λ 5007 and deep narrow-band CCD images in the H α and [O iii ] λ 5007 emission lines have been obtained of the planetary nebula (PN) NGC 4361. In addition, VLA-DnC λ 3.6-cm continuum observations are presented. This material allows one to explore in unprecedented detail the morphology and kinematics of this PN. The morphology of this object is complex given the highly filamentary structure of the envelope, which is confirmed to possess a low mass. The halo has a high expansion velocity that yields incompatible kinematic and evolutionary ages, unless previous acceleration of the nebular expansion is considered. However, the most remarkable result from the present observations is the detection of a bipolar outflow in NGC 4361, which is unexpected in a PN with a Population II low-mass-core progenitor. It is shown that shocks resulting from the interaction of the bipolar outflow with the outer shell are able to provide an additional heating source in this nebula. 相似文献
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恒星形成于分子云之中, 分子外向流是恒星形成正在进行的重要动力学特征, 也是研究和认识恒星形成的重要契入点. 利用紫金山天文台青海观测站德令哈13.7m毫米波望远镜, 采用5种分子谱线探针(包括12CO、13CO、C18O、HCO$^+$ $J=1-0$和CS $J=2-1$, J为角动量量子数), 对一个包含IRAS 19230+1506、IRAS 19232+1504和G050.3179--00.4186这3个源的大质量恒星形成复合体进行了成图观测研究. 通过对以上分子谱线数据并结合红外波段巡天数据的分析, 在这3个源中首次探测到了分子外向流活动, 并确定了分子外向流的中心驱动源. 最后对这3个源进行了分子外向流相关物理量参数的计算, 分析了这些物理量参数之间的关系, 结果表明分子外向流的性质与中心驱动源的性质息息相关. 相似文献
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We present 13 CO J = 1 − 0 line observations of the H ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ∼ + 65 km s−1 of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ∼ 48 × 17 pc2 and ∼ 2.4 × 105 M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v LSR =+ 60 to +73 km s−1 . The velocity gradient of this ring is ∼ 0.5 km s−1 pc−1 , and the mass is ∼ 6.2 × 104 M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ∼ 7 km s−1 , ∼ 13 pc and ∼ 3.0 × 10 49 erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H ii regions, G49.0−0.3 and G48.9−0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure. 相似文献
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We study the linear theory of Kelvin–Helmholtz instability in a layer of ions and neutrals with finite thickness. In the short
wavelength limit the thickness of the layer has a negligible effect on the growing modes. However, perturbations with wavelength
comparable to layer’s thickness are significantly affected by the thickness of the layer. We show that the thickness of the
layer has a stabilizing effect on the two dominant growing modes. Transition between the modes not only depends on the magnetic
strength, but also on the thickness of the layer. 相似文献
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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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Maps are presented of 3 P 1 →3 P 0 [C i ] and J =2→1 C18 O line emission from the interstellar molecular cloud G35.2−0.74N. The maps are interpreted with reference to a previous model for the structure of the cloud in which opposing jets from a central object, embedded in a rotating interstellar disc, precess and drive a bipolar molecular outflow. The C18 O emission traces the rotating interstellar disc, but the [C i ] emission shows several features. An unresolved component is observed which probably results from dissociation of CO in the centre of the disc by UV radiation from the central source. Background [C i ] emission is also observed which shares the rotation of the disc on larger scales. The C i /CO ratio in these components is typically a few per cent. High-velocity [C i ] emission, where C i /CO is high (>0.1–0.4), is observed between the CO molecular outflow and the cavity exacavated by the jet. This material has probably been accelerated by the jet but dissociated by far-UV radiation propagating through the cavity. The C i /CO ratio falls as the shocked outflow later sweeps up CO. 相似文献
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Mohsen Nejad-Asghar 《Monthly notices of the Royal Astronomical Society》2007,379(1):222-228
Frictional heating by the ion-neutral drift is calculated and its effect on the isobaric thermal instability is studied. Ambipolar drift heating of a one-dimensional self-gravitating magnetized molecular slab is used under the assumptions of quasi-magnetohydrostatic and local ionization equilibrium. We see that ambipolar drift heating is inversely proportional to density and its value in some regions of the slab can be significantly larger than the average heating rates of cosmic rays and turbulent motions. The results show that isobaric thermal instability can occur in some regions of the slab, and thus it may produce slab fragmentation and formation of astronomical unit scale condensations. 相似文献
14.
N. Lehner F. P. Keenan K. R. Sembach 《Monthly notices of the Royal Astronomical Society》2001,323(4):904-910
We present intermediate-resolution HST /STIS spectra of a high-velocity interstellar cloud ( v LSR =+80 km s−1 ) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s−1 (FWHM). The high-velocity cloud absorption is observed in the lines of C ii , O i , Si ii , Si iii , Si iv and S iii . Limits can be placed on the amount of S ii and Fe ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C ii and O i , suggests that this high-velocity gas is warm ( T k ∼103 –104 K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si iv . This sightline also intercepts two other high-velocity clouds that produce weak absorption features at v LSR =+113 and +130 km s−1 in the STIS spectra. 相似文献
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CO abundances in a protostellar cloud: freeze-out and desorption in the envelope and outflow of L483
P. B. Carolan M. P. Redman E. Keto J. M. C. Rawlings 《Monthly notices of the Royal Astronomical Society》2008,383(2):705-712
CO isotopes are able to probe the different components in protostellar clouds. These components, core, envelope and outflow have distinct physical conditions, and sometimes more than one component contributes to the observed line profile. In this study, we determine how CO isotope abundances are altered by the physical conditions in the different components. We use a 3D molecular line transport code to simulate the emission of four CO isotopomers, 12 CO J = 2 → 1, 13 CO J = 2 → 1 , C18 O J = 2 → 1 and C17 O J = 2 → 1 from the Class 0/1 object L483, which contains a cold quiescent core, an infalling envelope and a clear outflow. Our models replicate James Clerk Maxwell Telescope (JCMT) line observations with the inclusion of freeze-out, a density profile and infall. Our model profiles of 12 CO and 13 CO have a large linewidth due to a high-velocity jet. These profiles replicate the process of more abundant material being susceptible to a jet. C18 O and C17 O do not display such a large linewidth as they trace denser quiescent material deep in the cloud. 相似文献
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Christopher J. Davis Michael D. Smith & Gerald H. Moriarty-Schieven 《Monthly notices of the Royal Astronomical Society》1998,299(3):825-833
Near-infrared images in H2 line emission and submillimetre maps in CO J = 3–2 emission illustrate the remarkable association between a molecular bow shock and the redshifted molecular outflow lobe in W75N. The flow lobe fits perfectly into the wake of the bow, as one would expect if the lobe represented swept-up gas. Indeed, these observations strongly support the 'bow shock' entrainment scenario for molecular outflows driven by young stars. The characteristics of the bow shock and CO outflow lobe are compared with those of numerical simulations of jet-driven flows. These models successfully reproduce the bulge and limb-brightening in the CO outflow, although the model H2 bow exhibits more structure extending back along the flow axis. We also find that the size of the flow, the high mass fraction in the flow at low outflow velocities (low γ values) and the high CO/H2 luminosity ratio indicate that the system is evolved. We also predict a correlation, in evolved systems, between outflow age and the CO/H2 luminosity ratio. 相似文献