共查询到20条相似文献，搜索用时 718 毫秒
1.
2.
E. P. Kurbatov 《Astronomy Reports》2010,54(7):590-598
The ejection of stars from spheroidal and disk dwarf galaxies resulting from the decay of OB associations is studied. This
has substantial observational consequences for disk galaxies with escape velocities up to 20 km/s, or dynamical masses up
to 108
M
⊙. The ejection of stars can (i) reduce the abundances of the products of Type Ia supernovae and, to a lesser degree, Type
II supernovae, in disk stars, (ii) chemically enrich the galactic halo and intergalactic medium, (iii) lead to the loss of
50% of the stellar mass in galaxies with masses ∼107
M
⊙ and the loss of all stars in systems with masses ≲105
M
⊙, (iv) increase the mass-to-luminosity ratio of the galaxy. 相似文献
3.
A. B. Tutukov 《Astronomy Reports》2002,46(8):691-695
The conditions for the formation of planets and brown dwarfs around single main-sequence stars are considered in two scenarios. The formation of planets and brown dwarfs requires that the initial specific angular momentum of a solar-mass protostar be (0.32)×1018 cm2/s. The accreted matter of the protostar envelope forms a compact gas ring (disk) around the young star. If the viscosity of the matter in this ring (disk) is small, increasing its mass above a certain limit results in gravitational instability and the formation of a brown dwarf. If the viscosity of the gas is sufficiently large, the bulk of the protostar envelope material will be accreted by the young star, and the gas disk will grow considerably to the size of a protoplanetary dust disk due to the conservation of angular momentum. The formation of dust in the cool part of the extended disk and its subsequent collisional coalescence ultimately results in the formation of solar-type planetary systems. 相似文献
4.
L. E. Pirogov 《Astronomy Reports》2009,53(12):1127-1135
Radial density profiles for the sample of dense cores associated with high-mass star-forming regions from southern hemisphere
have been derived using the data of observations in continuum at 250 GHz. Radial density profiles for the inner regions of
16 cores (at distances ≲0.2−0.8 pc from the center) are close on average to the ρ ∝ r
−α dependence, where α = 1.6 ± 0.3. In the outer regions density drops steeper. An analysis with various hydrostatic models showed that the modified
Bonnor-Ebertmodel, which describes turbulent sphere confined by external pressure, is preferable compared with the logotrope
and polytrope models practically in all cases. With a help of the Bonnor-Ebert model, estimates of central density in a core,
non-thermal velocity dispersion and core size are obtained. The comparison of central densities with the densities derived
earlier from the CS modeling reveals differences in several cases. The reasons of such differences are probably connected
with the presence of density inhomogenities on the scales smaller than the telescope beam. In most cases non-thermal velocity
dispersions are in agreement with the values obtained from molecular line observations. 相似文献
5.
We present images of the star-forming regionG23.01–0.41 at 6.7GHz in the Class II methanol maser transition 51–60
A
+, produced from archival observations on the European VLBI Network. Our map of the source and its maser spots contains 24
maser components. The data for each spot—absolute coordinates, coordinates relative to the calibration feature, peak flux
and flux integrated over the spot, size, position angle, velocity along the line of sight, and line full width at half-maximum—are
collected in tabular form. The spatial region occupied by the maser spots is approximated by a 200×130 milliarcsec ellipse
in position angle PA = −0.40°, centered on the absolute coordinates α
0 = 18h34m40.282s, δ
0 = −09°00′38.27″ (J2000). If the source is a protoplanetary disk, then, for the distance estimate derived from trigonometric
parallax, its diameter is 1800 AU, and the mass of the central protostar is 23.5M
⊙. 相似文献
6.
M. V. Charykova V. G. Krivovichev O. S. Yakovenko W. Depmeier 《Geology of Ore Deposits》2011,53(7):501-513
The behavior of arsenic at the Earth’s surface and nearby at low temperatures and pressures depends mainly on the redox potential
and the acidity-alkalinity of the crystallization media. These parameters determine the migration of arsenic compounds and
their precipitation as various solid phases. Understanding the mechanism of arsenic’s behavior under surface conditions, which
is important for solving environmental problems, is an urgent task of contemporary mineralogy and geochemistry. The activities
of the components in natural waters beyond the zones of natural (oxidation zones) and man-made contamination with arsenic
(a
ΣAs = 3 × 10−8, a
ΣFe = 10−5, a
ΣCu = 10−7, a
ΣZn = 5 × 10−7, a
ΣCo = 10−8, a
ΣNi = 6 × 10−8, a
ΣPb = 10−8) and in waters formed in the oxidation zone (a
ΣSe = 10−3, a
ΣFe = 10−2, a
ΣCu = 10−2, a
ΣZn = 5 × 10−2, a
ΣCo = 10−3, a
ΣNi = 10−2, a
ΣPb = 10−4) have been estimated. Eh-pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 7.0) software package.
The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution,
624 solid phases, and 10 gases. The Eh-pH diagrams of the Me-As-H2O systems (Me = Co, Ni, Fe, Cu, Zn, Pb) were plotted for the average contents of these elements in the underground water and
for their higher contents in the oxidation zones of sulfide deposits. The formation of Co, Ni, Fe, Cu, Zn, and Pb arsenates
at the surface is discussed. 相似文献
7.
A (2+ 1)-dimensional numerical model for the formation and evolution of young stellar objects with sub-solar masses is presented. The numerical hydrodynamicall code describing the formation and evolution of a pølar disk in a two-dimensional approximation is supplemented by one-dimensional code for the evolution of the star and an algorithm for establishing the vertical structure of the disk. This code is used to investigate the influence of luminosity bursts with intensities similar to those observed in FU Orionis objects (FUors) on the properties and thermal balance of pølar disks. A model with gravitational instability and fragmentation of the disk, with subsequent migration of the fragments onto the protostar, is used as a basic model for FUors. Typical FUor bursts (L ～ 100 L ⊙) can appreciably influence the thermal balance of their disks and parent envelopes, leading to an increase in the disk temperature by more than a factor of two. On the other hand, massive fragments in the disk are only weakly perturbed by such bursts, partially due to screening by the disk and partially due to their high temperature brought about by adiabatic heating. Apart from massive fragments, the characteristic thermal time scales are appreciably shorter than the dynamical time scales throughout the radial extent of the disk and envelope; this enables the use of a stationary radiative-transfer equation when determining the vertical structure of the disk. 相似文献
8.
We use UBVRI CCD photometry to study star-forming regions (SFRs) in the galaxies NGC 5585 and IC 1525. The observations were acquired
with the 1.5-m telescope of the Mt. Maidanak Observatory of the Astronomical Institute of the Uzbek Academy of Sciences (Uzbekistan),
with seeing of 0.8″–1.8″. We identified 47 SFRs in NGC 5585 and 16 SFRs in IC 1525. We estimated the ages and internal extinctions
of the SFRs using the PEGASE2 evolution models. The sizes of the SFRs were also determined. We discuss in detail the techniques
applied to evaluate the SFR parameters from photometric analysis. The age range for the studied SFRs is (2–40) × 106 yrs, and the internal extinctions are A(V ) ≤ 1.5m. The age distributions of the SFRs in both galaxies are typical of stellar systems with intense, extended star formation.
The internal extinction in the SFRs decreases with distance from the galactic centers: A(V ) ∝ −r. For both galaxies, the scale length for the decrease of the dust surface density, estimated from the A(V )−r relation for SFRs, is close to the scale length for the disk brightness decrease in the V and R bands. Relatively larger and older SFRs are observed in the galaxies’ rings, while such SFRs are not found in the spiral
arms. We detected different SFR parameters for different spiral arms of NGC 5585. 相似文献
9.
We study the fragmentation properties in the protoplanetary disk and properties of the resultant self-gravitating clumps using our newly constructed disk model. Our disk model includes the mass inflall term from a molecular cloud core and the photoevaporation winds effect. We adopt the conventional fragmentation criterion to judge whether a protoplanetary disk can fragment. In this work, we follow our previous work to investigate the properties of the resultant self-gravitating clumps. In our calculation, the initial masses of the resultant self-gravitating clumps lie in the range of tens of MJ to more than one hundred of MJ, where MJ is the Jupiter mass. These initial masses can seemingly account for the masses of extrasolar planets in magnitude. We also calculate the subsequent gas accretion of clumps in 1.27 × 104 yr after the formation of self-gravitating clumps. We find that the subsequent gas accretion of self-gravitating clumps is very efficient, and the clump masses grow to hundreds of MJ and the physical radii Rc of clumps increase to about 10 AU. Additionally, we also calculate the orbital migration of clumps. We find that most clumps have short migration timescale to be accreted onto the protostar, and only a small fraction of clumps have long migration timescale (>106 yr) to successfully become gas giant planets. These results are consistent with previous studies. 相似文献
10.
Results of numerical simulations of a collision of the gaseous components of two identical disk galaxies during a head-on
collision of the galaxies in the polar direction are presented. When the relative velocity of the galaxy collision is small,
their gaseous components merge. At high relative velocities (100–500 km/s), the massive stellar components of the galaxies
(M
g
= 109
M
⊙) pass through each other nearly freely, leaving behind the gaseous components, which are decelerated and heated by the collision.
If the overall gaseous component of the colliding galaxies is able to cool to the virial temperature during the collision,
a new galaxy forms. At velocities V ≥ 500 km/s, the gaseous component does not have time to cool, and the gas is scattered into intergalactic space, supplying
it with heavy elements produced in supernovae in the colliding galaxies. High-velocity (V ≥ 100 km/s) collisions of identical low-mass galaxies (M
g
≤ 109
M
⊙) whose mass is dominated by the mass of gas lead to the disruption of their stellar components. The overall gaseous component
forms a new galaxy when V ≤ 500 km/s, and is scattered into intergalactic space if the velocity becomes higher than this. A galaxy collision increases
the star-formation rates in the disk galaxies by nearly a factor of 100. Rotation of the colliding galaxies in the same direction
increases the changes of the disruption of both the stellar and gaseous components of the galaxies. The merger of galaxies
during their collision can explain the presence of gaseous disks rotating opposite to the rotation of the stellar component
in some ordinary elliptical galaxies. Moreover, galaxy mergers can help explain the origin of a comparatively young stellar
population in some elliptical galaxies. 相似文献
11.
V. G. Krivovichev M. V. Charykova O. S. Yakovenko W. Depmeier 《Geology of Ore Deposits》2011,53(7):514-527
The behavior of selenium at the Earth’s surface and nearby at low temperatures and pressures is controlled by variations of
the redox potential and the acidity of solutions. These parameters determine the migration of selenium and its precipitation
as various solid phases. Understanding the mechanism of selenium’s behavior under surface conditions, which is important for
solving environmental problems, is an urgent task of contemporary mineralogy and geochemistry. The activities of components
in natural waters beyond the zones of natural (oxidation zones) and man-made contamination with selenium (a
ΣSe = 10−9, a
ΣFe = 10−5, a
ΣCu = 10−7, a
ΣZn = 5 × 10−7, a
ΣCo = 10−8, a
ΣNi = 6 × 10−8, a
ΣPb = 10−8) and in waters formed in the oxidation zone (a
ΣSe = 10−5–10−4, a
ΣFe = 10−2, a
ΣCu = 10−2, a
ΣZn = 5 × 10−2, a
ΣCo = 10−3, a
ΣNi = 10−2, a
ΣPb = 10−4) have been estimated. Eh-pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 7.0) software package.
The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution,
624 solid phases, and 10 gases. The Eh-pH diagrams of the Me-Se-H2O systems (Me = Co, Ni, Fe, Cu, Zn, Pb) were plotted for the average contents of these elements in underground water and for
their contents in oxidation zones of sulfide deposits. The formation of Co, Ni, Fe, Cu, Zn, and Pb selenites and selenates
at the surface is discussed. 相似文献
12.
Harri A. T. Vanhala 《Journal of Earth System Science》1998,107(4):391-400
The scenario of the triggered origin of the solar system suggests that the formation of our planetary system was initiated
by the impact of an interstellar shock wave on a molecular cloud core. The strength of this scenario lies in its ability to
explain the presence of short-lived radionuclides in the early solar system. According to the proposal, the radioactivities
were produced in a stellar source, transported into the molecular cloud core by a shock wave and mixed into the collapsing
system during the interaction between the shock wave and the core. We examine the viability of the scenario by presenting
results from recent numerical simulations. The calculations show that molecular cloud cores can be triggered into collapse
by the impact of a shock wave propagating at the velocity of 10–45 km s−1. Some of the shock wave material incident on the core, typically 10–20%, can be injected into the collapsing system. The
time scale of the process is ∼104–105 years, sufficiently short for the survival of the short-lived radioactivities. The simulations therefore confirm the viability
of the scenario of the triggered origin of the solar system. 相似文献
13.
To determine if toxaphene residues in edible fish tissue decreased after removal of contaminated sediments from an estuarine
site in 1999, 51 composite samples representing six finfish species were collected in 2001 and analyzed using gas chromatography
with electron capture and negative ion mass spectrometric detection. The grand mean total toxaphene residue concentration
on a wet weight basis (ΣTOXwet) was 1,400 ± 3,500 ng g−1 (range: < 18 to 18,000 ng g−1) and was positively correlated with extractable lipid. On a lipid basis, the mean ΣTOXlip was 26 ± 33 μg g1, which decreased with increasing distance from the study site. Although benthically-oriented species, such as spot (Leiostomus xanthurus) and striped mullet (Mugil cephalus), exhibited higher mean ΣTOXwet than those of higher trophic level fish, mean ΣTOXlip were not significantly different among species. The grand mean ΣTOX for 2001 was 3.8 (wet) and 2.6 (lipid) times less than
corresponding preremedial action (1997) concentrations, suggesting that bioavailable toxaphene residues in this system have
been reduced. Forage species, such as croaker (Micropogonias undulatus), mullet, and spot, preferentially accumulate toxaphene residues in this system and may serve as vectors of organochlorine
contaminants in the estuarine and coastal ocean food web. 相似文献
14.
The results of numerical studies of the evolution of a close binary system containing a black hole with a mass of ～3000M⊙ are presented. Such a black hole could form in the center of a sufficiently rich and massive globular cluster. The secondary could be a main-sequence star, giant, or degenerate dwarf that fills or nearly fills its Roche lobe. The numerical simulations of the evolution of such a system take into account the magnetic wind of the donor together with the wind induced by X-ray irradiation from the primary, the radiation of gravitational waves by the system, and the nuclear evolution of the donor. Mass transfer between the components is possible when the donor fills its Roche lobe, and also via the black hole’s capture of some material from the induced stellar wind. The computations show that the evolution of systems with solar-mass donors depends only weakly on the mass of the accretor. We conclude that the observed ultra-luminous X-ray sources (L X ? 1038 erg/s) in nearby galaxies could include accreting black holes with masses of 102?104M⊙. Three scenarios for the formation of black holes with such masses in the cores of globular clusters are considered: the collapse of superstars with the corresponding masses, the accretion of gas by a black hole with a stellar initial mass (<100M⊙), and the tidal accumulation of stellar black holes. We conclude that the tidal accumulation of stellar-mass black holes is the main scenario for the formation of intermediate-mass black holes (102?104M⊙) in the cores of globular clusters. 相似文献
15.
The statistical relation between the masses of supermassive black holes (SMBHs) in disk galaxies and the kinematic properties
of their host galaxies is analyzed. Velocity estimates for several galaxies obtained earlier at the 6-m telescope of the Special
Astrophysical Observatory of the Russian Academy of Sciences and the data for other galaxies taken from the literature are
used. The SMBH masses correlate well with the rotational velocities at a distance of R ≈ 1 kpc, V
1, which characterize the mean density of the central region of the galaxy. The SMBH masses correlate appreciably weaker with
the asymptotic velocity at large distances from the center and the angular velocity at the optical radius R
25. We have found for the first time a correlation between the SMBH mass and the total mass of the galaxy within the optical
radius R
25, M
25, which includes both baryonic and “dark” mass. The masses of the nuclear star clusters in disk galaxies (based on the catalog
of Seth et al.) are also related to the dynamical mass M
25; the correlations with the luminosity and rotational velocity of the disk are appreciably weaker. For a given value of M
25, the masses of the central cluster are, on average, an order of magnitude higher in S0-Sbc galaxies than in late-type galaxies,
or than the SMBH masses. We suggest that the growth of the SMBH occurs in the forming “classical” bulge of the galaxy over
a time < 109 yr, during a monolithic collapse of gas in the central region of the protogalaxy. The central star clusters form on a different
time scale, and their stellar masses continue to grow for a long time after the growth of the central black hole has ceased,
if this process is not hindered by activity of the nucleus. 相似文献
16.
REE studies of apatites in two series of granites, including more than ten types of rock and several tens of rock masses in
eight provinces of South China permit us to come to some new understanding and hence the following conclusions have been drawn.
Apatites in granites of different series show significant differences in REE distribution pattern, probably due to different
material sources. Therefore, REE partition characteristics can be used to discriminate granites of different sources. Apatite
in granite of the Nanling series (of shallow-seated source) has a lower content ofΣ REE (4571 ppm on average). Y is the highest of the REE group.Σ Y is rich relative toΣ Ce, thus giving rise to relatively lowΣCe/ΣY ratio (0.68 on average). Eu shows strong depletion, andδ Eu is extremely low (0.135 on average). The REE distribution patterns are represented by a group of “V”-shaped symmetric
curves. Apatite in granites of the Yangtze series are relatively high inΣ REE (8571.3 ppm on average). Ce is the highest of the REE. group.Σ Ce is rich relative toΣ Y and theΣCe/ΣY is relatively high (7.15 on average). The chondrite-normalized REE distribution patterns are characterized by a group of
rightward-inclined curves. 相似文献
17.
Wade Oliver Christopher Fuller David L. Naftz William P. Johnson Ximena Diaz 《Applied Geochemistry》2009
The mass of Se deposited annually to sediment in the Great Salt Lake (GSL) was estimated to determine the significance of sedimentation as a permanent Se removal mechanism. Lake sediment cores were used to qualitatively delineate sedimentation regions (very high to very low), estimate mass accumulation rates (MARs) and determine sediment Se concentrations. Sedimentation regions were defined by comparison of isopach contours of Holocene sediment thicknesses to linear sedimentation rates determined via analysis of 210Pb, 226Ra, 7Be and 137Cs activity in 20 short cores (<5 cm), yielding quantifiable results in 13 cores. MARs were developed via analysis of the same radioisotopes in eight long cores (>10 cm). These MARs in the upper 1–2 cm of each long core ranged from 0.019 to 0.105 gsed/cm2/a. Surface sediment Se concentrations in the upper 1 or 2 cm of each long core ranged from 0.79 to 2.47 mg/kg. Representative MARs and Se concentrations were used to develop mean annual Se removal by sedimentation in the corresponding sedimentation region. The spatially integrated Se sedimentation rate was estimated to be 624 kg/a within a range of uncertainty between 285 and 960 kg/a. Comparison to annual Se loading and other potential removal processes suggests burial by sedimentation is not the primary removal process for Se from the GSL. 相似文献
18.
Data from the revised Geneva-Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences
of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of
the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative
metallicity gradient in the thin disk, are responsible for the observed negative correlation between the metallicities and
angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do
not depend on either age or metallicity. The velocity components of the Sun relative to the Local Standard of Rest derived
using data for stars born at the solar Galactocentric distance are (U
⊙, V
⊙, W
⊙)
LSR
= (5.1 ± 0.4, 7.9 ± 0.5, 7.7 ± 0.2) km/s. The two coordinates of the apex of the solar motion remain equal to 〈l
⊙〉 = 70° ± 7° and 〈b
⊙〉 = 41° ± 2°, within the errors. The indices for the power-law age dependences of them ajor, middle, and minor semi-axes become
0.26±0.04, 0.32±0.03, and 0.07±0.03, respectively. As a result, with age, the velocity ellipsoid for thin-disk stars born
at the solar Galactocentric distance increases only in the plane of the disk, remaining virtually constant in the perpendicular
direction. Its shape remains far from equilibrium, and the direction of the major axis does not change with age: the ellipsoid
vertex deviation remains constant and equal to zero within the errors (〈L〉 = 0.7° ± 0.6°, 〈B〉 = 1.9° ± 1.1°). Such a small increase in the velocity dispersion perpendicular to the Galactic plane with age can probably
be explained by “heating” of the stellar system purely by spiral density waves, without a contribution from giant molecular
clouds. 相似文献
19.
The dynamical, thermal, and chemical evolution of gas in protogalaxies with non-zero angular momentum is considered. It is
shown that, in protogalaxies with a total mass (dark and baryonic) of M = 107
M⊙ at redshifts z = 12 whose gas has rotational angular momentum (spin parameter λ ≳ 0.005), a disk-like structure forms during the initial collapse of the galaxy, in contrast to non-rotating protogalaxies,
whose collapse is spherically symmetric. The existence of initial angular momentumfor gas in protogalaxies increases the cooling
time of the gas, delaying the formation of the first stars. Increasing the rotational angular momentum of the gas leads to
cooling of the gas to lower temperatures (T < 100 K), at which HD molecules dominate in the cooling, while the total mass of cool gas (T < 1000 K) is decreased. The stability of disk-like structures in the central regions of protogalaxies is analyzed. It is
shown that the disk that is formed is gravitationally unstable, and multiple fragmentation at various distances from the center
is possible when the initial rotational angular momentum of the protogalaxy is increased. The possible birth of several stars
in the first protogalaxies is discussed. 相似文献
20.
Preliminary studies have shown that REE geochemical characteristics are useful criteria to discriminate the two types of granitic
rocks in Jiangxi Province. They are also of significant indication of metallization.
The principal REE geochemical indices are ΣREE, ΣCe/ΣY andδEu, as well as REE distribution pattern. Each of the two types of granitic rocks has a metallogenic evolutionary series of
its own with a successive decrease inδEu.
The application of REE geochemistry in the study of petrogenesis and ore genesis has been receiving increasing attention.
As will be shown in this paper, REE geochemical characteristics, especially ΣREE, ΣCe/ΣY,δEu and REE distribution pattern, are efficient indicators for distinguishing between the two series of granitic rocks in Jiangxi,
and thus enable us to have a better understanding of the ore-forming processes in this region. 相似文献