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1.
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan Jayaram N. Chengalur 《Journal of Astrophysics and Astronomy》2001,22(1):35-50
Nearby interstellar clouds with high (|ν|≥10km s−1) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption)
in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio
sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random
velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds.
The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s−1, consistent with absorption arising from CNM clouds. 相似文献
2.
Conventional ablation theory assumes that a meteoroid undergoes intensive heating during atmospheric flight and surface atoms
are liberated through thermal processes. Our research has indicated that physical sputtering could play a significant role
in meteoroid mass loss. Using a 4th order Runge-Kutta numerical integration technique, we tabulated the mass loss due to the
two ablation mechanisms and computed the fraction of total mass lost due to sputtering. We modeled cometary structure meteoroids
with masses ranging from 10−13 to 10−3 kg and velocities ranging from 11.2 to 71 km s−1. Our results indicate that a significant fraction of the mass loss for small, fast meteors is due to sputtering, particularly
in the early portion of the light curve. In the past 6 years evidence has emerged for meteor luminosity at heights greater
than can be explained by conventional ablation theory. We have applied our sputtering model and find excellent agreement with
these observations, and therefore suggest that sputtered material accounts for the new type of radiation found at great heights. 相似文献
3.
David G. Russell 《Journal of Astrophysics and Astronomy》2009,30(2):93-118
The value of Hubble parameter (H0) is determined using the morphologically type dependent Ks-band Tully-Fisher Relation (K-TFR). The slope and zero point are
determined using 36 calibrator galaxies with ScI morphology. Calibration distances are adopted from direct Cepheid distances,
and group or companion distances derived with the Surface Brightness Fluctuation Method or Type Ia Supernova. It is found
that a small morphological type effect is present in the K-TFR such that ScI galaxies are more luminous at a given rotational
velocity than Sa/Sb galaxies and Sbc/Sc galaxies of later luminosity classes. Distances are determined to 16 galaxy clusters
and 218 ScI galaxies with minimum distances of 40.0 Mpc. From the 16 galaxy clusters a weighted mean Hubble parameter of H0 = 84.2 ± 6 km s−1 Mpc−1 is found. From the 218 ScI galaxies a Hubble parameter of H0 = 83.4 ± 8 km s−1 Mpc−1 is found. When the zero point of K-TFR is corrected to account for recent results that find a Large Magellanic Cloud distance
modulus of 18.39±0.05, a Hubble parameter of 88.0 ± 6 km s−1 Mpc−1 is found. Effects from Malmquist bias are shown to be negligible in this sample as galaxies are restricted to a minimum rotational
velocity of 150 km s−1. It is also shown that the results of this study are negligibly affected by the adopted slope for the K-TFR, inclination
binning, and distance binning. A comparison with the results of the Hubble Key Project (Freedman et al. 2001) is made. Discrepancies between the K-TFR distances and the HKP I-TFR distances are discussed. Implications for Λ-CDM
cosmology are considered with H0 = 84 km s−1 Mpc−1. It is concluded that it is very difficult to reconcile the value of H0 found in this study with ages of the oldest globular
clusters and matter density of the universe derived from galaxy clusters in the context of Λ-CDM cosmology. 相似文献
4.
Based on data for 102 OB3 stars with known proper motions and radial velocities, we have tested the distances derived by Megier
et al. from interstellar Ca II spectral lines. The internal reconciliation of the distance scales using the first derivative
of the angular velocity of Galactic rotation Ω′0 and the external reconciliation with Humphreys’s distance scale for OB associations refined by Mel’nik and Dambis show that
the initial distances should be reduced by ≈20%. Given this correction, the heliocentric distances of these stars lie within
the range 0.6–2.6 kpc. A kinematic analysis of these stars at a fixed Galactocentric distance of the Sun, R
0 = 8 kpc, has allowed the following parameters to be determined: (1) the solar peculiar velocity components (u
⊙, v
⊙, ω
⊙) = (8.9, 10.3, 6.8) ± (0.6, 1.0, 0.4) km s−1; (2) the Galactic rotation parameters Ω0 = −31.5 ± 0.9 km s−1 kpc−1, Ω′0 = +4.49 ± 0.12 km s−1 kpc−2, Ω″0 = −1.05 ± 0.38 km s−1 kpc−3 (the corresponding Oort constants are A = 17.9 ± 0.5 km s−1 kpc−1, B = −13.6 ± 1.0 km s−1 kpc−1 and the circular rotation velocity of the solar neighborhood is |V
0| = 252 ± 14 km s−1); (3) the spiral density wave parameters, namely: the perturbation amplitudes for the radial and azimuthal velocity components,
respectively, f
R
= −12.5±1.1 km s−1 and f
ϑ
= 2.0 ± 1.6 km s−1; the pitch angle for the two-armed spiral pattern i = −5.3° ± 0.3°, with the wavelength of the spiral density wave at the solar distance being λ = 2.3 ± 0.2 kpc; the Sun’s phase in the spiral wave x
⊙ = −91° ± 4°. 相似文献
5.
Based on currently available observations of 28 maser sources in 25 star-forming regions with measured trigonometric parallaxes,
proper motions, and radial velocities, we have constructed the rotation curve of the Galaxy. Taking different distances to
the Galactic center R
0, we have estimated the peculiar velocity of the Sun, the angular velocity of Galactic rotation, and its three derivatives.
For R
0 = 8 kpc, we have found the circular velocity of the Sun to be V
0 = 243 ± 16 km s−1, which corresponds to a revolution period of 202 ± 10 Myr. We have obtained the Oort constants A = 16.9 ± 1.2 km s−1 kpc−1 and B = −13.5 ± 1.4 km s−1 kpc−1. Our simulation of the influence of a spiral density wave has shown that the peculiar velocity of the Sun with respect to
the local standard of rest and the component (V
⊙)LSR depend significantly on the Sun’s phase in the spiral wave. 相似文献
6.
A. I. Yatsenko 《Astrophysics and Space Science》1988,142(1-2):85-88
Summary In this paper the results of the research of the stars proper motions Trapezium components are reported. They are: the galactic
coordinates of the solar aprx and the Sun velocity (L
⊙=43±18°,B
⊙=+28±13°,V
⊙=13±4 km s−1), the dispersion of peculiar velocities in the direction of the galactic coordinates for the above mentioned stars (σ
l
=±11 km s−1, σ
b
=±7 km s−1).The attained accuracy of the proper motions (±0.005″ yr−1) is shown to be insufficient to the study of internal space motions in these systems. At present the work to increase the
relative proper motions accuracy for multiple system components and to improve reductions from the relative to absolute proper
motions, is being carried out in the Main Astronomical Observatory (Academy of Sciences of the Ukrainian SSR). The new catalogue
of the AGK3 stars is composed now in the vicinity of the galactic equator in order to improve reductions from the relative
to absolute proper motions. The r.m.s. errors of the proper motions, obtained in the AGK3 system, are ±0.005″ yr−1. 相似文献
7.
We present our results of high temporal resolution spectroscopic observation and study in Hα, Ca II, and He I lines for the 2B/M1.9 confined disk flare on September 9, 2001, combining with GOES soft X-ray (SXR) and Yohkoh hard X-ray (HXR) observations. Apparent redshifted and red-asymmetric profiles were observed in the initial phase. The redshift
lasted until the late phase. The derived velocity depends on both the spectral line and the used method. The redshift velocities
computed from the line centers of the observed emission profiles (υ0) are of the order of 10 km s−1 both inside and outside the streak area. However, the velocities determined from the excess profiles by the bisector method
(υ) are larger in the streak (18–50 km s−1). Both υ and the red full widths (RFWs) derived from the excess profiles show temporal variations similar to the HXR light-curve
in the streak area. Moreover, the Hα line wings of nonthermal characteristics, the redshift velocities, and the lifetime of
impulsive broadening suggest that the streak is related to nonthermal electron bombardment. Spectral simulations reveal that
we cannot reproduce the observed profiles in the three lines simultaneously with a set of parameters, indicating that the
flare atmosphere was not homogeneous along the line-of-sight. Most of the observed Hα profiles showed a ‘flat-top’ structure,
implying the flare plasma was optically thick for this line. The electron temperatures (Te) deduced from the line-center intensity of the three lines are similar and estimated to be higher than 7200 K. The obvious
central reversal of the Hα profiles due to absorption of materials in the impulsive phase lasted more than 2 min. However,
the far blue wings of the Ca II profiles in the impulsive phase showed low-intensity emission, which is suggestive of the
existence of large turbulence or macroscopic motion (> 50 km s−1), which is inconsistent with the current flare model. 相似文献
8.
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan 《Journal of Astrophysics and Astronomy》2004,25(3-4):185-201
We have carried out a sensitive high-latitude (|b| > 15°) HI 21 cm-line absorption survey towards 102 sources using the GMRT. With a 3σ detection limit in optical depth of
∼ 0.01, this is the most sensitive HI absorption survey. We detected 126 absorption features most of which also have corresponding
HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen. The histogram of random velocities of the
absorption features is well-fit by two Gaussians centered at V1sr ∼ 0 km s−1 with velocity dispersions of 7.6 ± 0.3 km s−1 and 21 ± 4 km s−1 respectively. About 20% of the HI absorption features form the larger velocity dispersion component. The HI absorption features
forming the narrow Gaussian have a mean optical depth of 0.20 ± 0.19, a mean HI column density of (1.46 ± 1.03) × 1020 cm−2, and a mean spin temperature of 121 ± 69 K. These HI concentrations can be identified with the standard HI clouds in the
cold neutral medium of the Galaxy. The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04
± 0.02, a mean HI column density of (4.3 ± 3.4) × 1019 cm−2, and a mean spin temperature of 125 ± 82 K. The HI column densities of these fast clouds decrease with their increasing random
velocities. These fast clouds can be identified with a population of clouds detected so far only in optical absorption and
in HI emission lines with a similar velocity dispersion. This population of fast clouds is likely to be in the lower Galactic
Halo. 相似文献
9.
Any calibration of the present value of the Hubble constant (H
0) requires recession velocities and distances of galaxies. While the conversion of observed velocities into true recession
velocities has only a small effect on the result, the derivation of unbiased distances which rest on a solid zero point and
cover a useful range of about 4–30 Mpc is crucial. A list of 279 such galaxy distances within v < 2,000 km s−1 is given which are derived from the tip of the red-giant branch (TRGB), from Cepheids, and/or from supernovae of type Ia
(SNe Ia). Their random errors are not more than 0.15 mag as shown by intercomparison. They trace a linear expansion field
within narrow margins, supported also by external evidence, from v = 250 to at least 2,000 km s−1. Additional 62 distant SNe Ia confirm the linearity to at least 20,000 km s−1. The dispersion about the Hubble line is dominated by random peculiar velocities, amounting locally to <100 km s−1 but increasing outwards. Due to the linearity of the expansion field the Hubble constant H
0 can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of 78 galaxies above this limit give H
0 = 63.0 ± 1.6 at an effective distance of 6 Mpc. They compensate the effect of peculiar motions by their large number. Support
for this result comes from 28 independently calibrated Cepheids that give H
0 = 63.4 ± 1.7 at 15 Mpc. This agrees also with the large-scale value of H
0 = 61.2 ± 0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of H
0 = 62.3 ± 1.3 is adopted. Because the value depends on two independent zero points of the distance scale its systematic error
is estimated to be 6%. Other determinations of H
0 are discussed. They either conform with the quoted value (e.g. line width data of spirals or the D
n
−σ method of E galaxies) or are judged to be inconclusive. Typical errors of H
0 come from the use of a universal, yet unjustified P–L relation of Cepheids, the neglect of selection bias in magnitude-limited
samples, or they are inherent to the adopted models. 相似文献
10.
We present SOHO/CDS observations taken during the gradual phase of the X17 flare that occurred on October 28, 2003. The CDS
data are supplemented with TRACE and ground-based observations. The spectral observations allow us to determine velocities
from the Doppler shifts measured in the flare loops and in the two ribbon kernels, one hour and a half after the flare peak.
Strong downflows (>70 km s−1) are observed along the loop legs at transition-region temperatures. The velocities are close to those expected for free
fall. Observations and results from a hydrodynamic simulation are consistent with the heating taking place for a short time
near the top of the arcade. Slight upflows are observed in the outer edges of the ribbons (<60 km s−1) in the EUV lines formed at log T < 6.3. These flows could correspond to the so-called “gentle evaporation.” At “flare” temperatures (Fe xix, log T = 6.9), no appreciable flows are observed. The observations are consistent with the general standard reconnection models
for two-ribbons flares. 相似文献
11.
Halton Arp 《Journal of Astrophysics and Astronomy》1987,8(3):241-255
Galaxies of redshiftz ≲ 1000 km s−1 are investigated. In the South Galactic Hemisphere there are two large concentrations of these galaxies. One is in the direction
of the centre of the Local Group, roughly aligned with M 31 and M 33. The other concentration is centred almost 80 degrees
away on the sky and involves the next nearest galaxies to the Local Group, NGC 55, NGC 300 and NGC 253.
The large scale and isolation of these concentrations, and the continuity of their redshifts require that they are all galaxies
at the same, relatively close distance of the brightest group members. The fainter members of the group have higher redshifts,
mimicking to some extent a Hubble relation. But if they are all at the same average distance the higher redshifts must be
due to a cause other than velocity.
The redshifts of the galaxies in the central areas of these groups all obey a quantization interval of δcz0 = 72.4 kms−1. This is the same quantization found by William Tifft, and later by others, in all physical groups and pairs which have been
tested. The quantization discovered here, however, extends over a larger interval in redshift than heretofore encountered.
The majority of redshifts used in the present analysis are accurate to ± 8 km s−1. The deviation of those redshifts from multiples of 72.4 km s-1 averages ±8.2 km s−1. The astonishing result, however, is that for those redshifts which are known more accurately, the deviation from modulo
72.4 drops to a value between 3 and 4 km s−1! The amount of relative velocity allowed these galaxies is therefore implied to be less than this extremely small value. 相似文献
12.
V. V. Bobylev 《Astronomy Letters》2010,36(9):634-644
We analyze the three-dimensional kinematics of about 82 000 Tycho-2 stars belonging to the red giant clump (RGC). First, based
on all of the currently available data, we have determined new, most probable components of the residual rotation vector of
the optical realization of the ICRS/HIPPARCOS system relative to an inertial frame of reference, (ω
x
, ω
y
, ω
z
) = (−0.11, 0.24, −0.52) ± (0.14, 0.10, 0.16) mas yr−1. The stellar proper motions in the form μα cos δ have then be corrected by applying the correction ω
z
= −0.52 mas yr−1. We show that, apart from their involvement in the general Galactic rotation described by the Oort constants A = 15.82 ± 0.21 km s−1 kpc−1 and B = −10.87 ± 0.15 km s−1 kpc−1, the RGC stars have kinematic peculiarities in the Galactic yz plane related to the kinematics of the warped stellar-gaseous Galactic disk. We show that the parameters of the linear Ogorodnikov-Milne
model that describe the kinematics of RGC stars in the zx plane do not differ significantly from zero. The situation in the yz plane is different. For example, the component of the solid-body rotation vector of the local solar neighborhood around the
Galactic x axis is M
32− = −2.6 ± 0.2 km s−1 kpc−1. Two parameters of the deformation tensor in this plane, namely M
23+ = 1.0 ± 0.2 km s−1 kpc−1 and M
33 − M
22 = −1.3 ± 0.4 km s−1 kpc−1, also differ significantly from zero. On the whole, the kinematics of the warped stellar-gaseous Galactic disk in the local
solar neighborhood can be described as a rotation around the Galactic x axis (close to the line of nodes of this structure) with an angular velocity −3.1 ± 0.5 km s−1 kpc−1 ≤ ΩW ≤ −4.4 ± 0.5 km s−1 kpc−1. 相似文献
13.
The Tully–Fisher relationship (TFR) has been shown to have a relatively small observed scatter of ∼±0.35 mag implying an intrinsic
scatter < ±0.30 mag. However, when the TFR is calibrated from distances derived from the Hubble relation for field galaxies
scatter is consistently found to be ±0.64 to ±0.84 mag. This significantly larger scatter requires that intrinsic TFR scatter
is actually much larger than ±0.30 mag, that field galaxies have an intrinsic TFR scatter much larger than cluster spirals,
or that field galaxies have a velocity dispersion relative to the Hubble flow in excess of 1000 km s−1. Each of these potential explanations faces difficulties and contradicted by available data and the results of previous studies.
An alternative explanation is that the measured redshifts of galaxies are composed of a cosmological redshift component predicted
from the value of the Hubble constant and a superimposed intrinsic redshift component previously identified in other studies.
This intrinsic redshift component may exceed 5000 km s−1 in individual galaxies. In this alternative scenario a possible value for the Hubble constant is 55–60 km s−1 Mpc−1. 相似文献
14.
Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar
galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36.
The full width at half minimum of the absorption line is 100 km s−1. The estimated column density of atomic Hydrogen is 0.7×1022(T
s
/100) cm−2. The HI absorption is redshifted by ∼400km s−1 compared to the [OIII] emission line from this system. We attribute this to an infalling cold gas or to an out-flowing ionised
gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster
potential. 相似文献
15.
H. Hamedivafa 《Solar physics》2008,250(1):17-29
An improved method of image segmentation is introduced. The object-tracking algorithm, originally developed by Sobotka, Brandt,
and Simon (Astron. Astrophys. 328, 682, 1997) is modified with special attentions on splitting and merging of umbral dots (UDs), definition of the umbral boundary, and
the birth-frames and the death-frames of UDs. By applying the new method of image segmentation and the object-tracking algorithm
on a 67-min series of white-light images of a large pore (Sobotka et al., Astrophys. J.
511, 436, 1999), the physical characteristics of 20 “resolved” UDs with umbral origin were recorded. The most probable lifetime of the UDs
is between 7 and 10 min. Umbral dots show a typical size of about 230 km. Their mean speeds are smaller than 2 km s−1 with a distribution around a value less than 1 km s−1. However, their average velocities are less than 0.8 km s−1. Brighter (fainter) UDs are formed in the brighter (dimmer) region of the pore. There is no correlation between time-averaged
area or time-averaged speeds and lifetimes. Also, the time-averaged peak intensities of UDs do not show any well-defined dependence
on the corresponding time-averaged areas. It seems that there is a relation between average velocities of UDs and their time-averaged
peak intensities, with brighter UDs moving more slowly. 相似文献
16.
The GMRTHI 21 cm-line observations of galaxies in the Eridanus group are presented. The Eridanus group, at a distance of ≈ 23 Mpc, is
a loose group of ≈200 galaxies. The group extends to more than 10 Mpc in projection. The velocity dispersion of the galaxies
in the group is ≈240 km s−1. The galaxies are clustered into different sub-groups. The overall population mix of the group is 30% (E + S0) and 70% (Sp
+ Irr). The observations of 57 Eridanus galaxies were carried out with the GMRT for ≈ 200 h. HI emission was detected from
31 galaxies. The channel rms of ≈ 1 mJy beam−1 was achieved for most of the image-cubes made with 4 h of data. The corresponding HI column density sensitivity (3σ) is ≈
1 × 1020 cm−2 for a velocity-width of ≈ 13.4 km s−1. The 3σ detection limit of HI mass is ≈ 1.2 X 107 Mpd for a line-width of 50 km s−1. Total HI images, HI velocity fields, global HI line profiles, HI mass surface densities, HI disk parameters and HI rotation
curves are presented. The velocity fields are analysed separately for the approaching and the receding sides of the galaxies.
These data will be used to study the HI and the radio continuum properties, the Tully-Fisher relations, the dark matter halos,
and the kinematical and HI lopsidedness in galaxies. 相似文献
17.
L. V. Glazunova 《Astronomy Letters》2011,37(6):414-419
Based on two high-dispersion spectra of the close binary BW Boo, we have detected lines of the secondary component whose contribution
to the combined spectrum does not exceed 2%. We have determined the rotation velocities of the components and spectroscopic
orbital elements. Numerous lines of neutral and ionized iron have been used to determine the effective temperature and surface
gravity for the primary component. The photometric light curves for this binary have been solved for the first time. Its primary
component is an A2Vm star with a mass of 2 ± 0.1M
⊙ and a radius of 1.9 ± 0.4R
⊙. Its rotation velocity is 2 km s−1, which is a factor of 18 lower than the pseudo-synchronous velocity for this component. The G6 secondary component, a T Tau
star, has a rotation velocity of 17 km s−1, amass of 1.1M
⊙, and a radius of 1 R
⊙. The age of the binary has been estimated to be 107 yr. 相似文献
18.
This is an account of Allan Sandage’s work on (1) The character of the expansion field. For many years he has been the strongest defender of an expanding Universe. He later explained the CMB dipole by a local velocity of 220±50 km s−1 toward the Virgo cluster and by a bulk motion of the Local supercluster (extending out to ∼3500 km s−1) of 450–500 km s−1 toward an apex at l=275, b=12. Allowing for these streaming velocities he found linear expansion to hold down to local scales (∼300 km s−1). (2) The calibration of the Hubble constant. Probing different methods he finally adopted—from Cepheid-calibrated SNe Ia and from independent RR Lyr-calibrated TRGBs—H 0=62.3±1.3±5.0 km s−1 Mpc−1. 相似文献
19.
The population synthesis method is used to study the possibility of explaining the appreciable fraction of the intergalactic
type-Ia supernovae (SN Ia), 20
−15
+12
%, observed in galaxy clusters (Gal-Yam et al. 2003) when close white dwarf binaries merge in the cores of globular clusters. In a typical globular cluster, the number
of merging double white dwarfs does not exceed ∼10−13 per year per average cluster star in the entire evolution time of the cluster, which is a factor of ∼3 higher than that in
a Milky-Way-type spiral galaxy. From 5 to 30% of the merging white dwarfs are dynamically expelled from the cluster with barycenter
velocities up to 150 km s−1. SN Ia explosions during the mergers of double white dwarfs in dense star clusters may account for ∼1% of the total rate
of thermonuclear supernovae in the central parts of galaxy clusters if the baryon mass fraction in such star clusters is ∼0.3%. 相似文献
20.
The Arecibo UHF radar is able to detect the head-echos of micron-sized meteoroids up to velocities of 75 km/s over a height
range of 80–140 km. Because of their small size there are many uncertainties involved in calculating their above atmosphere
properties as needed for orbit determination. An ab initio model of meteor ablation has been devised that should work over
the mass range 10−16 kg to 10−7 kg, but the faint end of this range cannot be observed by any other method and so direct verification is not possible. On
the other hand, the EISCAT UHF radar system detects micrometeors in the high mass part of this range and its observations
can be fit to a “standard” ablation model and calibrated to optical observations (Szasz et al. 2007). In this paper, we present a preliminary comparison of the two models, one observationally confirmable. Among the features
of the ab initio model that are different from the “standard” model are: (1) uses the experimentally based low pressure vaporization
theory of O’Hanlon (A users’s guide to vacuum technology, 2003) for ablation, (2) uses velocity dependent functions fit from
experimental data on heat transfer, luminosity and ionization efficiencies measured by Friichtenicht and Becker (NASA Special
Publication 319: 53, 1973) for micron sized particles, (3) assumes a density and temperature dependence of the micrometeoroids
and ablation product specific heats, (4) assumes a density and size dependent value for the thermal emissivity and (5) uses
a unified synthesis of experimental data for the most important meteoroid elements and their oxides through least square fits
(as functions of temperature, density, and/or melting point) of the tables of thermodynamic parameters given in Weast (CRC
Handbook of Physics and Chemistry, 1984), Gray (American Institute of Physics Handbook, 1972), and Cox (Allen’s Astrophysical
Quantities 2000). This utilization of mostly experimentally determined data is the main reason for calling this an ab initio
model and is made necessary by the fact that individual average meteoroid mass densities are now derivable from Arecibo observations. 相似文献