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The internal structure of stellar jets arising from young stellar objects is characterized by a series of `knots' or condensations which have highly supersonic proper motions. These structures, we believe, are the result of a variable ejection from the source, which leads to the formation of internal working surfaces or small bowshock within the jet beam. In this paper we present a long-slit spectrum of Herbig-Haro object HH 111 obtained with STIS and an interpretation of this observation in terms of an ejection velocity variability model. 相似文献
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J. J. Martinell D. del-Castillo-Negrete A. C. Raga D. A. Williams 《Monthly notices of the Royal Astronomical Society》2006,372(1):213-218
Observations of fluctuations in the redshifted 21-cm radiation from neutral hydrogen (H i ) are perceived to be an important future probe of the universe at high redshifts. Under the assumption that at redshifts z ≤ 6 (post-reionization era) the H i traces the underlying dark matter with a possible bias, we investigate the possibility of using observations of redshifted 21-cm radiation to detect the bispectrum arising from non-linear gravitational clustering and from non-linear bias. We find that the expected signal is ∼ 0.1 mJy at 325 MHz ( z = 3.4) for the small baselines at the Giant Metrewave Radio Telescope, the strength being a few times larger at higher frequencies (610 MHz, z = 1.3) . Further, the magnitude of the signal from the bispectrum is predicted to be comparable to that from the power spectrum, allowing a detection of both in roughly the same integration time. The H i signal is found to be uncorrelated beyond frequency separations of ∼1.3 MHz whereas the continuum sources of contamination are expected to be correlated across much larger frequencies. This signature can in principle be used to distinguish the H i signal from the contamination. We also consider the possibility of using observations of the bispectrum to determine the linear and quadratic bias parameters of the H i at high redshifts, this having possible implications for theories of galaxy formation. 相似文献
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Radar data from Cerro Catedral (a peak close to Mexico City) were used to investigate the properties of convective storms over central Mexico, a region with complex orography. The spatial distribution shows a preference for storms to form and move to the west of radar, over a narrow band of high terrain. However, the storms with the higher volumes and echo-top heights tend to be located southwestward over lower terrain. Each radar feature was matched with the number of cloud-to-ground (CG) lightning produced inside it, as retrieved from the World Wide Lightning Location Network dataset. The storms in which lightning was detected, with an average of more than six lightning bolts, clearly outperform in size and intensity the group of storms in which lightning was not detected, and tend to lie over lower terrain. The sample of over 98 000 identified cells was divided into four elevation groups to look for elevation trends in the mean properties, as reported for other Mexican regions. While the number of storms per unit area increases with terrain height, the average values for properties related to both size (area, volume, echo-top height) and intensity (maximum reflectivity, number of CG bolts, height of maximum reflectivity, maximum height of 30 dBZ echo) decrease. These results could be related to the possible shallower warm-cloud depths over the higher elevations. The diurnal cycles of convection and lightning north of the radar show a nearly typical continental regime of precipitation in that zone, with maxima at 18:00 LT in both variables. However, south of the radar, a maximum in lightning activity occurs during late night and early morning, which is linked with the deeper nocturnal convection over the lower terrain in that zone. 相似文献
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A. Raga L. López-Martín L. Binette J. A. López J. Cantó S. J. Arthur G. Mellema W. Steffen P. Ferruit 《Monthly notices of the Royal Astronomical Society》2000,314(4):681-688
Recent observations show the existence of an increasing number of collimated outflows ejected by young, low-mass stars which are embedded in H ii regions. At distances of a few tens of au from the star, at least one lobe of these outflows will be shielded from the ambient ionizing radiation by the compact, high-extinction circumstellar disc. Within these shielded regions, the jets are probably mostly neutral, similar to the jets in 'normal' Herbig–Haro (HH) objects. At larger distances, these jets emerge into the photoionized nebula, and start to be photoionized by the radiation from the ionizing photon source of the nebula.
In this paper, we model the photoionization of an initially neutral HH jet. This process begins as an ionization front at the side of the jet, which is directed towards the ionizing star of the nebula, and progresses into the beam of the jet. There are two possible solutions. In the first solution, the jet beam becomes fully ionized through the passage of an R-type ionization front. In the second solution, the ionization front slows down enough to become a D-type front (or is already a D-type front at the point in which the jet emerges into the photoionized nebula), forming a partially ionized jet beam, with an expanding photoionized region and a compressed neutral region.
We explore these two types of solutions both analytically and numerically, and discuss the observational effects introduced by this jet photoionization process, concentrating in a region of parameter space that straddles the parameters deduced for HH 444 (the jet from V 510 Orionis). 相似文献
In this paper, we model the photoionization of an initially neutral HH jet. This process begins as an ionization front at the side of the jet, which is directed towards the ionizing star of the nebula, and progresses into the beam of the jet. There are two possible solutions. In the first solution, the jet beam becomes fully ionized through the passage of an R-type ionization front. In the second solution, the ionization front slows down enough to become a D-type front (or is already a D-type front at the point in which the jet emerges into the photoionized nebula), forming a partially ionized jet beam, with an expanding photoionized region and a compressed neutral region.
We explore these two types of solutions both analytically and numerically, and discuss the observational effects introduced by this jet photoionization process, concentrating in a region of parameter space that straddles the parameters deduced for HH 444 (the jet from V 510 Orionis). 相似文献
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J. Cantó R. F. González A. C. Raga E. M. de Gouveia Dal Pino A. Lara J. A. González-Esparza 《Monthly notices of the Royal Astronomical Society》2005,357(2):572-578
In this paper we present a simple, analytic model for the dynamical evolution of supersonic velocity fluctuations at the base of the ambient solar wind. These fluctuations result in the formation of dense working surfaces that travel down the wind. It is shown how the initial parameters of the fluctuations (velocity, density and duration) are related to the characteristics of the working surfaces far from the Sun (for instance at the Earth). We apply the model to the evolution of the coronal mass ejections in the IP medium, finding that the model is in good agreement with satellite observations of these phenomena, thus providing physical insight into their dynamical evolution. Our model may contribute to future 'space weather forecasting' on the Earth, based on detailed satellite monitoring of the solar corona. 相似文献
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A. Rodríguez-González J. Cantó A. Esquivel A. C. Raga P. F. Velázquez 《Monthly notices of the Royal Astronomical Society》2007,380(3):1198-1206
We present the analytic and numerical models of the 'cluster wind' resulting from the multiple interactions of the winds ejected by the stars of a dense cluster of massive stars. We consider the case in which the distribution of stars (i.e. the number of stars per unit volume) within the cluster is spherically symmetric, has a power-law radial dependence, and drops discontinuously to zero at the outer radius of the cluster. We carry out comparisons between an analytic model (in which the stars are considered in terms of a spatially continuous injection of mass and energy) and 3D gasdynamic simulations (in which we include 100 stars with identical winds, located in 3D space by statistically sampling the stellar distribution function). From the analytic model, we find that for stellar distributions with steep enough radial dependencies, the cluster wind flow develops a very high central density and a non-zero central velocity, and for steeper dependencies, it becomes fully supersonic throughout the volume of the cluster (these properties are partially reproduced by the 3D numerical simulations). Therefore, the wind solutions obtained for stratified clusters can differ dramatically from the case of a homogeneous stellar distribution (which produces a cluster wind with zero central velocity, and a fully subsonic flow within the cluster radius). Finally, from our numerical simulations, we compute predictions of X-ray emission maps and luminosities, which can be directly compared with observations of cluster wind flows. 相似文献
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We study the internal structure of coronal mass ejections (CMEs) using wavelet analysis. We derive wavelet spectra, spatially
integrated over regions of interest within LASCO C2 white-light coronographic images. These spectra show an inflection point,
which we use to characterize each spectrum. In a diagram of flux vs. spatial scale of the inflection point, we find that the analyzed structures fall into two, distinct groups: a low-flux,
small-spatial-scale group (which we call the “homogeneous” type), and a high-flux, larger-spatial-scale group (the “collimated”
type). Interestingly, if we study different regions of a given image, all of the structures fall into one of the two groups
described above. From a qualitative comparison with the images, it is clear that the two groups identified by the wavelet
analysis correspond to two types of morphologies, which are seen as either more-homogeneous or more-collimated structures. 相似文献