共查询到20条相似文献,搜索用时 31 毫秒
1.
We have studied the chemical evolution in the central core of a contracting cloud representing W3 IRS4. We modified the equation of temperature to satisfy the physical conditions of W3 IRS4. The chemical rate equations and the hydrodynamics are integrated simultaneously. The contraction is followed from a very low density of n = 10 cm −3 to high core density of n = 10 6 cm −3. The chemical evolution is studied for various chemical conditions, including both the effects of varying cosmic ray ionization rate and the effect of ion–dipole molecule collisions. The main results by using the more extensive chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with observation at intermediate and enhanced cosmic ray ionization rate at high depletion of elemental atoms. 相似文献
2.
We have calculated intensity ratios for emission lines of Fexviii in the 13–94 Å wavelength range at electron temperatures characteristic of the solar corona, T
e = 2–10 x 10 6 K. Our model ion includes data for transitions among the 2 s
22 p
5 , 2 s2 p
6, 2 s
22 p
43l, and 2 s2 p
53l (l = s, p, and d) states. Test calculations which omit the 2 s2 p
53l levels show that cascades from these are important. We compare our results with observed ratios determined from four solar X-ray instruments, a rocket-borne spectrograph, and spectrometers on the P78–1, OV1–17 and Solar Maximum Mission (SMM) satellites. In addition, we have generated synthetic spectra which we compare directly with flare observations from SMM. Agreement between theory and observation is generally quite good, with differences that are mostly less than 30%, providing limited support for the accuracy of the atomic physics data used in our calculations. However, large discrepancies are found for ratios involving the 2 s
22 p
5
2P 3/2- 2 s2 p
6
2S line at 93.84 Å, which currently remain unexplained. Our analysis indicates that the FeXVIII feature at 15.83 Å is the 2 s
22 p
5
2P 3/2 - 2 s
22 p
4( 3P)3s 4P 3/2 transition, rather than 2 s
22 p
5
2P 3/2 - 2 s
22 p
4( 3P)3s 2P 3/2, as suggested by some authors. 相似文献
3.
The thermal regime of the baryons behind shock waves arising in the process of virialization of dark matter halos is governed at certain conditions by radiation of HD lines. A small fraction of the shocked gas can cool down to the temperature of the cosmic microwave background (CMB). We estimate an upper limit for this fraction: at z = 10 it increases sharply from about qT ∼ 10 –3 for dark halos of M = 5 × 10 7 M ⊙ to ∼ 0.1 for halos with M = 10 8 M ⊙. Further increase of the halo mass does not lead however to a significant growth of qT – the asymptotic value for M ≫ 10 8 M ⊙ is 0.3. We estimate the star formation rate associated with such shock waves, and show that they can provide a small but not negligible fraction of the star formation. We argue that extremely metal‐poor low‐mass stars in the Milky Way may have been formed from primordial gas behind such shocks. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
4.
The well-known shell supernova remnant (SNR) HB3 is part of a feature-rich star-forming region together with the nebulae W3, W4, and W5. We study the HI structure around this SNR using five RATAN-600 drift curves obtained at a wavelength of 21 cm with an angular resolution of 2′ in one coordinate over the radial-velocity range ?183 to +60 km s ?1 in a wider region of the sky and with a higher sensitivity than in previous works by other authors. The spatial-kinematic distribution of HI features around the SNR clearly shows two concentric expanding shells of gas that surround the SNR and coincide with it in all three coordinates ( α, δ, and V). The outer shell has a radius of 133 pc, a thickness of 24 pc, and an expansion velocity of 48 km s ?1. The mass of the gas in it is ≈2.3 × 10 5M⊙. For the inner shell, these parameters are 78 pc, 36 pc, 24 km s ? 1, and 0.9 × 10 5M⊙, respectively. The inner shell is immediately adjacent to the SNR. Assuming that the outer shell was produced by the stellar wind and the inner shell arose from the shock wave of the SNR proper, we estimated the age of the outer shell, ≈1.7 × 10 6 yr, and the mechanical luminosity of the stellar wind, 1.5 × 10 38 erg s ?1. The inner shell has an age of ≈10 6 yr and corresponds to a total supernova explosion energy of ≈10 52 erg. 相似文献
5.
We have studied a sample containing ~6000 OB stars with proper motions and trigonometric parallaxes from the Gaia DR2 catalogue. The following parameters of the angular velocity of Galactic rotation have been found: Ω0 = 29.70 ± 0.11 km s-1 kpc-1, Ω'0 = -4.035 ± 0.031 km s-1 kpc-2, and Ω 0 = 0.620 ± 0.014 km s-1 kpc-3. The circular rotation velocity of the solar neighborhood around the Galactic center is V0 = 238 ± 5 km s-1 for the adopted Galactocentric distance of the Sun R0 = 8.0 ± 0.15 kpc. The amplitudes of the tangential and radial velocity perturbations produced by the spiral density wave are fθ = 4.4 ± 1.4 kms-1 and fR = 5.1 ± 1.2 kms-1, respectively; the perturbation wavelengths are λθ = 1.9 ± 0.5 kpc and λR = 2.1 ± 0.5 kpc for the adopted four-armed spiral pattern. The Sun's phase in the spiral density wave is χ⊙ = -178° ± 12°. 相似文献
6.
We present new spectral (FPI and long‐slit) data on the Eastern optical filament of the well known radionebula W50 associated with SS433. We find that on sub‐parsec scales different emission lines are emitted by different regions with evidently different physical conditions. Kinematical properties of the ionized gas show evidence for moderately high ( V ∼ 100 km s –1) supersonic motions. [O III] λ 5007 emission is found to be multi‐component and differs from lowerexcitation [S II] λ 6717 line both in spatial and kinematical properties. Indirect evidence for very low characteristic densities of the gas ( n ∼ 0.1 cm –3) is found. We propose radiative (possibly incomplete) shock waves in low‐density, moderately high metallicity gas as the most probable candidate for the power source of the optical filament. Apparent nitrogen overabundance is better understood if the location of W50 in the Galaxy is taken into account (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
Supernova Remnants (SNRs) are the most likely sources of the galactic cosmic rays up to energies of about 10 15 eV/nuc. The large scale shock waves of SNRs are almost ideal sites to accelerate particles up to these highly non-thermal
energies by a first order Fermi mechanism which operates through scattering of the particles at magnetic irregularities. In
order to get an estimate on the total amount of the explosion energy E
SNconverted into high energy particles the evolution of a SNR has to be followed up to the final merging with the interstellar
medium. This can only be done by numerical simulations since the non-linear modifications of the shock wave due to particle
acceleration as well as radiative cooling processes at later SNR stages have to be considered in such investigations. Based
on a large sample of numerical evolution calculations performed for different ambient densities n
ext, SN explosion energies, magnetic fields etc. we discuss the final ‘yields’ of cosmic rays at the final SNR stage where the
Mach number of the shock waves drops below 2. At these times the cosmic rays start to diffuse out of the remnant. In the range
of external densities of10 -2 ≤ n
ext/[cm -3] ≤ 30 we find a the total acceleration efficiency of about 0.15 E
SN with an increase up to 0.24 E
SN at maximum for an external density of n
ext = 10 cm -3. Since for the larger ambient densities radiative cooling can reduce significantly the total thermal energy content of the
remnant dissipation of Alfvén waves can provide an important heating mechanism for the gas at these later stages. From the
collisions of the cosmic rays with the thermal plasma neutral pions are generated which decay subsequently into observable
γ-rays above 100 MeV. Hence, we calculate these γ-ray luminosities of SNRs and compare them with current upper limits of ground
based γ-raytelescopes. The development of dense shells due to cooling of the thermal plasma increases the γ-ray luminosities
and e.g. an external density of n
ext = 10 cm -3 with E
SN = 10 51 erg can lead to a γ-ray flux above 10 -6 ph cm -2 s -1 for a remnant located at a distance of 1 kpc.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
The physical conditions under which suprathermal grains may loose energy and the processes involving the grains ( a3×10 –6 cm) destruction are investigated. It is found that the dust grain once attaining the velocity U (10 5 cm s –1) may attain suprathermal energy ( v
g3×10 8 cm s –1) and subsequently may also attain relativistic energy are almost indestructible in the accelerating phase. 相似文献
9.
Numerical integration of particle trajectories is performed to evaluate the statistical acceleration coefficients D
TT for 1 to 100 MeV protons in a solar wind corotating interaction region (CIR) seen at 2.5 and 5.0 AU. Acceleration is followed in the solar wind reference frame and is due to random wave-particle interactions and to random drift motion in moderate scale field gradients. D
TT due to the first effect reaches a peak value of 4 × 10 –7 MeV 2 s –1 post shock at 10 MeV at 2.5 AU consistent with estimates based both upon cyclotron resonance and transit time damping theory. D
TT from the second effect is less well established but is of the order of 10 –7 MeV 2 s –1 at 10 MeV, 5 AU. A comparison is made between the time constant for statistical acceleration within this CIR and estimates for diffuse shock acceleration and adiabatic deceleration. All three time constants are of the same order, but deceleration is faster than shock acceleration which in turn is faster than statistical acceleration. 相似文献
10.
We investigate the influence of the following parameters on the crust properties of strange stars: the strange quark mass
( m
s), the strong coupling constant (α c) and the vacuum energy density ( B). It is found that the mass density at the crust base of strange stars cannot reach the neutron drip density. For a conventional
parameter set of m
s=200 MeV, B
1/4 = 145 MeV and α c = 0.3, the maximum density at the crust base of a typical strange star is only 5.5 × 10 10 gcm -3, and correspondingly the maximum crust mass is 1.4 ×10 -6 M ⊙. Subsequently, we present the thermal structure and the cooling behavior of strange stars with crusts of different thickness,
and under different diquark pairing gaps. Our work might provide important clues for distinguishing strange stars from neutron
stars. 相似文献
11.
R-matrix calculations of electron impact excitation rates in 0 III are used to derive the electron-density-sensitive emission-line
ratio R = 1 (2 s
2 2 p
23
P
2-2 s
2
2P
23
P
1)/ I (2 s
22 p
23
P
1 - 2 s
22 p
23
P
0) = I (52μm)/ I (88μm) for a range of electron temperatures (Te = 5000-20000 K) and densities (N
e
= 10–10
5
cm −3) applicable to planetary nebulae. Electron densities deduced from the observed values of R in several planetary nebulae are in excellent agreement with those deduced from C1 I and Ar IV, which provides support for
the accuracy of the atomic data adopted in the calculations. 相似文献
12.
We have followed the chemistry of W3 IRS4, as one of the most famous regions with a chemistry similar to that found in quiescent
molecular clouds. Here the time scale, since star formation, has probably been long enough for the gas-phase ion-molecule
chemistry to become dominant again. We have constructed a reaction system containing the chemical families of H, C, O, N,
S and metals (Me:Fe, Mg, Na and Si). A total of 282 species have been included and a network of 1270 reactions has been used.
The chemical kinetic equations were integrated as a function of pseudo-time-dependent approach. The evolution of the different
chemical species are followed in models of density n equal to 10 6 cm -3 and temperature T equal to 55 K. Different initial abundances have been assumed. Our results are in reasonable agreement
with the observation, especially in Model 6. The resulting molecular abundances are compared with those obtained in the intermediate
mass star NGC 2264 IRS1, as well as the low mass IRAS 16293-2422.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
12CO J=2-1 maps of L379 IRS1 show a molecular outflow seen almost end-on while C 18O J=2-1 emission covers a smaller central region, tracing virially bound material deeper within the cloud. Continuum maps at 450, 800 and 1100 µm all trace an identical double peaked arc west of IRS1 and VLA NH 3 (1,1) & (2,2) integrated intensity maps reveal the same double-peaked structure. An identical velocity gradient is seen in 12CO, 13CO, C 18O and NH 3 (1,1) & (2,2) following the arc-like structure of the continuum emission. 相似文献
14.
Abstract— We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon‐rich asymptotic giant branch (AGB) stars employing a newly‐derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core‐graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core‐graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 < n? 0.1 ( M5/ζ) ?1/2L41/4 < 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s ?1, M5 the mass loss rate in 10 ?5 M⊙ year ?1, L4 the stellar luminosity in 10 4 L⊙, and M/ζ is the effective mass loss rate taking account of non‐spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M/ζ = 10 ?5‐10 ?3 M⊙ year ?1 correspond to 0.01 < Pc < 0.9 dyn cm ?2, implying that the observed TiC core‐graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the C/O abundance ratio, 1 < ? ? 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the C/O ratios inferred from the astronomical observations of carbon‐rich AGB stars. Brief discussion is given on other types of graphite spherules. 相似文献
15.
We have compared the frequency distribution of the dynamical observedquantity log ( V
z
2
r
p), for a sample of 46 pairs of elliptical galaxies, to the distribution of this quantity obtained from numerical simulations
of pairs of galaxies. From such an analysis, where we have considered the structure of the galaxies and its influence in the
orbital evolution of the system, we have obtained the characteristic mass and the mass-luminosity ratio for the sample. Our
results show that the hypothesis of point-mass in elliptical orbits is, for this sample, an approximation as good as the model
that takes into account the structure of the galaxies. The statistical method used here gives an estimate of a more reliable
mass, it minimizes the contamination of spurious pairs and it considers adequately the contribution of the physical pairs.
We have obtained a characteristic mass to the 46 elliptical pairs of 1.68 × 10 12 ± 7.01 ×10 11 M ⊙ with M/L = 17.6 ± 7.3 ( H
0 = 60 km s -1Mpc -1).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
In some gradual hard X-ray bursts with high intensity, hard X-ray source (15–40 keV) is steadily located in the corona along with softer X-ray source (5–10 keV).Two stationary models, high density and high temperature models, are proposed to solve the difficult problem of confinement of hot (or nonthermal) plasma in the direction of the magnetic field along the loops in the corona. In both models, an essential point is that the effective X-ray source is composed of fine dense filamentary loops imbeded in a larger rarefied coronal loop, and the electron number density in the filaments is so high as 10 11–10 12 cm -3. If the density is so high heat conduction can be as reasonably small as of the order of 10 27 erg s -1 for the given emission measures of observed X-rays, since the required cross-sectional area is small and also classical conduction is valid. Collisional confinement of thermal tail, and nonthermal electrons if any, up to 50–60 keV in the filaments is also possible, so that the hard X-ray images can be loop like structure instead of double source (foot points).High density model is applicable to the coronal filamentary loops with temperature T
m < 5 × 10 7 K at the loop summit. The heat flow from the summit downwards is lost almost completely by the radiation from the loop during the conduction to the foot points. A continuous energy release is assumed near the summit to maintain the stationary temperature T
m, and pressure balance is maintained along the loop. In this model, the number density at the summit is given by n
m - 10 6
T
m
2
/s m, where s
m is the length of the loop from the summit to the foot point, and the distribution of temperature and density along the loop are given by T = T
m(s/s m) 1/3 and n = n
m(s/s m) -1/3, respectively.High temperature model is applicable to the filamentary loops with higher temperature up to about 10 8.5 K and comparatively lower number density as 10 11 cm -3 for the requirement of magnetic confinement of the hot plasma in radial direction. The radiation from the loop is negligibly small in this model so that the heat flux is nearly conserved down to the foot points. In this case, temperature gradient is smaller than that of the high density model, depending on the tapering of the magnetic bottle.In both models, the differential emission measure is maximum at the highest temperature T
m and the brightness distribution along the loop shows a maximum around the summit of the loop if some magnetic tapering is taken into account. 相似文献
17.
A comparison of Skylab S082A observations for several solar flares with calculations of the electron temperature sensitive emission line ratio R
1 = I(2 s2 p
1
P – 2 s
2
1
S)/ I(2 s2 p
3
P
1 - 2 s
2
1
S) = = I(256.68 Å)/ I(491.45 Å) in Be-like SXIII reveals good agreement between theory and experiment, which provides observational support for the accuracy of the adopted atomic data. However, observed values of the electron density sensitive ratio R
2 = I(2 s2 p
1
P – 2 s
2
1
S)/ I(2 p
2
3
P
2 - 2 s2 p
3
P
2) = = I(256.68 Å)/ I(308.96 Å) all lie below the theoretical high density limit, which is probably due to blending in the 308.96 Å line. 相似文献
18.
Using 180 interplanetary (IP) shock events associated with coronal mass ejections (CMEs) during 1997 – 2005, we investigate
the influence of the heliospheric current sheet (HCS) upon the propagation and geoeffectiveness of IP shocks. Our preliminary
results are: (1) The majority of CME-driving IP shocks occurred near the HCS. (2) The numbers of shock events and related
geomagnetic storms observed when the Earth and the solar source are located on the same side of the HCS, represented by f
SS and f
SG, respectively, are obviously higher than those when the Earth and the solar source are located on the opposite sides of the
HCS, denoted by f
OS and f
OG, with f
SS/ f
OS=126/54, f
SG/ f
OG = 91/36. (3) Parameter jumps across the shock fronts for the same-side events are also higher than those for the opposite-side
events, and the stronger shocks (Δ V ≥ 200 km s −1) are mainly attributed to be same-side events, with f
SSh/ f
OSh = 28/15, where f
SSh and f
OSh are numbers of stronger shocks which belong to same-side events and opposite-side events, respectively. (4) The level of
the geomagnetic disturbances is higher for the same-side events than for the opposite-side events. The ratio of the number
of intense magnetic storms (Dst < −100) triggered by same-side events to those triggered by opposite-side events is 25/10.
(5) We propose an empirical model to predict the arrival time of the shock at the Earth, whose accuracy is comparable to that
of other prevailing models. These results show that the HCS is an important physical structure, which probably plays an important
role in the propagation of interplanetary shocks and their geoeffectiveness. 相似文献
19.
We have studied the kinematic properties of the candidates for hot subdwarfs (HSDs) selected by Geier et al. from theGaiaDR2 catalogue. We have used a total of 12 515 stars with relative trigonometric parallax errors less than 30%. The HSDs are shown to have different kinematics, depending on their positions on the celestial sphere. For example, the sample of low-latitude (|b| < 20°) HSDs rotates around the Galactic center with a linear velocity V0 = 221 ± 5 km s?1. This suggests that they belong to the Galactic thin disk. At the same time, they lag behind the local standard of rest by ΔV☉ ~ 16 km s?1 due to the asymmetric drift. The high-latitude (|b| ≥ 20°) HSDs rotate with a considerably lower velocity, V☉ = 168 ± 6 km s?1. Their lagging behind the local standard of rest is already ΔV☉ ~ 40 km s?1. Based on the entire sample of 12 515 HSDs, we have found a positive rotation around the x axis significantly differing from zero with an angular velocity ω1 = 1.36±0.24 km s?1 kpc?1. We have studied the samples of HSDs that are complete within r < 1.5 kpc. Based on them, we have traced the evolution of the parameters of the residual velocity ellipsoid as a function of both latitude |b| and coordinate |z|. The following vertical disk scale heights have been found: h = 180 ± 6 and 290 ± 10 pc from the low- and high-latitude HSDs, respectively. A new estimate of the local stellar density Σout = 53 ± 4 M☉ kpc?2 has been obtained for zout = 0.56 kpc from the high-latitude HSDs. 相似文献
20.
We present the results of 10 years of photometric CCD observations of the intermediate polar V709 Cas obtained by using different instruments during 2003–2013. We detected a new variability with a period of Pnew = 0. d016449979(5) which seems to be real. The spin variability is not clearly seen in all our data, so we are unable to study any evolution of the white dwarf rotation. From the best night (in 2010) we obtained a spin period of Pspin = 311. s8(5). We analyzed the orbital variability using ( O – C) analysis. We found no variations of the orbital period on a timescale of 10 years, but the linear fit to the ( O – C) diagram shows that the value of the orbital period is Porb = 0. d2222123(6), which is close to the earlier published values. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
|