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1.
The production of deuterium, He3, lithium, beryllium, and boron by galactic cosmic rays in the interstellar medium, over the life of the Galaxy, is calculated. It is found that high-energy - reactions contribute in an essential way to the observed lithium. When allowance is made for the interchange of material between stars and the interstellar gas and for the change of cosmic-ray intensity with time, the Li6, Be9, and boron produced turn out to be ample to explain the observed abundances, and with remarkable internal consistency. Deuterium and He3 arenot produced in significant amounts, nor Li7 in sufficient amount, however. To explain the Li7/Li6 ratio measured terrestrially and in chondritic meteorites, we invoke cosmological production of Li7. This implies the production of deuterium, He3, and He4 as well, in amounts consistent with observation. The theory in its present form cannot explain a solar-system Li7/Li6 ratio of 12and stellar ratios as low as 3, but additional processes can be adduced to reconcile them. The consistency of the numbers when cosmological production is included lends additional support to the big-bang hypothesis. An incidental result is that the mean luminosity of the Galaxy over its lifetime has been about 3 times its present luminosity.  相似文献   

2.
Supporters of the standard Big Bang theory point to the abundances of light elements, predicted by Big Bang Nucleosynthesis (BBN) as one of the main observational supports of the theory. However, current data no longer confirm BBN. Instead, measurements of the abundances of He3, He4, and D clearly contradict BBN at more than a 3 level, eliminating a key support of the Big Bang.  相似文献   

3.
An empirical model of atomic ion densities (H+, He+, N+, O+) is presented up to 4000 km altitude as a function of time (diurnal, annual), space (position, altitude) and solar flux (F10.7) — using observations of satellites (AE-B, AE-C, AE-D, AE-E, ISIS-2, OGO-6) and rockets during quiet geophysical conditions (K p 3). The numerical treatment is based upon harmonic functions for the horizontal pattern and cubic splines for the vertical structure.The ion densities increase with increasing height up to a maximum (depending roughly on the ion mass) and decrease beyond that with increasing altitude. Above 200 km, O+ is the main ionic constituent being replaced at approximately 800 km (depending on latitude, local time, etc.) by H+. Around polar regions the light ions, H+ and He+, are depleted (polar wind) and the heavier ones enhanced. During local summer conditions the ion densities increase around polar latitudes and correspondingly decrease during local winter, except He+ which reflects the opposite pattern. Diurnal variations are intrinsically coupled to the individual plasma layers: N+ and O+ peak, in general, during daytime, while the amplitudes and phases of H+ and He+ change strongly with altitude and latitude. Earth, Moon and Planets Review article.  相似文献   

4.
Solar abundances of light nuclei and mixing of the Sun   总被引：1，自引：0，他引：1  
Radial profiles of the light nuclei (A 15) are calculated in the non-mixing Sun, taking into account the changes of solar structure with time. The results are discussed in relation to models of solar mixing and compared with abundance determinations at the solar surface or in the solar wind. B cannot be depleted in the outer convective zone without producing a large increase in the He3/He4 ratio. A decrease in He3/He4 would be accompanied by changes in C13/C12 and N15/N14 of a magnitude which is not observed.It is shown that boron could be depleted in the pre-main sequence period of the Sun, if mixing was on a time-scale of 106 yr. The simultaneous small increase in He3/He4 does not contradict observation. However, Be would be depleted more strongly than B.A He3/He4 decrease is always accompanied by large changes in N15/N14 and C13/C12. Since such changes are not observed, it is concluded that the He3/He4 ratio in the outer convective zone is a reliable upper limit for (He3 + D)/He4 in the solar nebula. Thus the D/H ratio in the protosolar material was much lower than it is in sea water or in carbonaceous chondrites.  相似文献   

5.
For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour‐magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g ′, r ′, and i ′ show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems, especially in the (r – i) colours. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
The acoustic energy-generation rate from the convective zone was calculated for various models. Results show that chromosphere and corona can be expected around stars with temperature lower than 8000K at the main sequence, and lower than 6500K at logg=2.When a star is rotating rapidly, mass loss from its corona is large, and can be an effective mechanism of braking the stellar rotation. If this mechanism is effective, we can explain the slow rotation of stars later than F2 to be the result of the loss of the angular momentum through a stellar wind that is effective in their main sequence phase. Stars with massM>1.5M lose mass through a stellar wind during their contraction phase. The mass-loss rate is larger than the solar value because of the larger energy input into the chromosphere-corona system and because of the smaller gravitational potential at the surface. T Tauri stars may be the observational counterparts for such stars. As the duration of contraction phase is very short (less than 107 years), the braking mechanism works only in the presence of a strong magnetic field (Ap) or in the presence of a companion (Am).Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.  相似文献   

7.
We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 M ⊙. The high accretion rate used in the models (M ⊙ = 4.4 x 10-3 M⊙ yr-1) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the in falling envelope. Significant nuclear burning does not take place until a protostellar mass of about 80 M ⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands owing to the radiation pressure. Eventually, a final swelling occurs when the stellar mass reaches about 300 M ⊙. This expansion is likely to signal the end of the main accretion phase, thus setting an upper limit to the protostellar mass formed in these conditions. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

8.
In this paper we discuss the ionization equilibrium of hydrogen and helium in a nebula with an arbitrary gas density distribution. If we consider the spectral characteristics of hot stars, a power law is found to provide a good approximation to the Lyman continuum spectrum for stars withT eff≤100 000 K. With this simplification the ionization equilibrium equation is analytically solved first for a pure hydrogen nebula, then for the general case of a nebula containing H, He, and heavy elements. A simple and quite general formula for the determination of the size and the emission of the He+ zone is obtained. Finally, the ionization equilibrium He++?He+ is considered. This problem can be decoupled from that of the ionization of H0 and He0 if the stellar spectrum is steeper thanv ?0.9 or, equivalently, if the star effective temperature is lower than 200 000 K. Within this limit, which surely includes all classicalHii regions and the low-medium excitation planetary nebulae, an analytical solution of the problem can be used.  相似文献   

9.
The diurnal and seasonal variations of H+, He+, N+, O+ and Ne are analyzed at 1400-km altitude. Using longitudinally averaged observations of ISIS-2 (April 1971 to December 1972), the ion and electron densities are decomposed via spherical harmonics and Fourier series into time-independent, seasonal and diurnal terms. The time-independent terms of H+ and He+ show a plateauor trough-like structure at medium to low latitudes and a strong decrease towards the poles; N+ and O+, on the other hand, yield an almost inverse picture with a density increase at high latitudes. All constituents, except He+, show at polar latitudes an enhancement during local summer conditions and a depletion during local winter conditions; He+, however, exhibits a winter bulge and a density minimum during local summer. The diurnal variations are strongly latitude dependent; while the amplitudes (relative) of H+, He+, and Ne are rather small, the heavier ions N+ and O+ show a deep minimum early in the morning and a high but flat maximum during daytime.  相似文献   

10.
We present archival and recent IUE high dispersion spectra of late B stars which reveal the presence of accreting gas with velocities as high as 350 km s–1, collisional ionization of the accreting gas to temperatures above the stellar Teff, and column densities intermediate between those observed toward classical Herbig Ae/Be stars and the nearby proto-planetary system Pictoris. One of the stars, HD 176386, while lacking obvious optical signatures of youth, is a member of the R CrA star formation region, and with an inferred age of 2.8 Myr has not yet arrived on the zero-age main sequence (ZAMS). The other object, an isolated, field B star with pronounced IR excess due to warm, circumstellar dust, 51 Oph, exhibits only modest h emission. The combination of high velocity, accreting gas in systems with IR excesses due to circumstellar dust suggests that not only are these objects candidate proto-planetary systems, but that they may represent an extension to higher stellar masses of the weak-emission pre-main sequence (PMS) stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.  相似文献   

11.
We have performed TreeSPH simulations of galaxy formation in a standard ΛCDM cosmology, including effects of star formation, energetic stellar feedback processes and a meta-galactic UV field, and obtain a mix of disk, lenticular and elliptical galaxies. The disk galaxies are deficient in angular momentum by only about a factor of two compared to observed disk galaxies. The stellar disks have approximately exponential surface density profiles, and those of the bulges range from exponential to r 1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations and likewise are their integrated B-V colours, which have been calculated using stellar population synthesis techniques. Furthermore, we can match the observed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is (M/L I) ≃ 0.6–0.7. The ellipticals and lenticulars have approximately r 1/4 stellar surface density profiles, are dominated by non-disklike kinematics and flattened due to non-isotropic stellar velocity distributions, again consistent with observations. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

12.
In this paper, a literal analytical solution is developed for the abundances differential equations of the helium burning phase in hot massive stars. The abundance for each of the basic elements 4He,12C,16O and 20Ne is obtained as a recurrent power series in time, which facilitates its symbolic and numerical evaluations. Numerical comparison between the present solution and the numerical integration of the differential equations for the abundances show good agreement.  相似文献   

13.
A study of the structure and kinematics of the Galaxy from Tautenburg Schmidt plates taken towards the Galactic centre (l = 17.0°, b = +0.8°) is presented. Proper motions and B, V magnitudes were determined for about 36 500 stars up to the limiting magnitude V = 16m.8 in a field of 8.95 square degrees. Proper motion accuracy of about 3 mas/year has been obtained for stars brighter than V = 15m. The rms errors of stellar magnitudes and (B–V) colours is about 0.1 mag. The majority of field stars in the survey are main sequence stars and red giants of the disk. They belong to the Local or Sagittarius-Carina arms, or they are located between these spiral arms. Comparing the modelled and observed distributions of magnitudes and colours, we specified the interstellar extinction determined in the preceding study of open clusters in this field. The luminosity function towards the Galactic centre was determined for stars with absolute magnitudes from -4m.35 to +9m. Kinematical and spatial distribution parameters up to 4 kpc from the Sun were obtained as a function of galactocentric distance.  相似文献   

14.
Spectroscopic observations of the Be star Tau (HR 1910), for the period 17–24 January, 1983, are reported.Radial velocities of the measured lines seem to indicate that the inner layers of the shell were falling to the stellar surface, while the outer ones were going away. There are indications about short-term variability of line profiles: a short discussion is carried out about the meaningful of this behaviour. The periodic variations (P=0.d85) of Hei 6678 line seem quite well established.  相似文献   

15.
Extensive calculations have been made of the behaviour of He+ for situations where ion outflow occurs from the topside ionosphere. For these circumstances, steady state solutions for the He+ continuity, momentum and energy equations have been obtained self-consistently, yielding density, velocity and temperature profiles of He+ from 200 to 2000 km altitude. To model the high latitude topside ionosphere, a range of background H+O+ ionospheres was considered with variations in the H+ outflow velocity, the presence of a perpendicular electric field and different peak O+ densities. In addition, the atmospheric density of neutral helium was chosen to model typical observed winter and summer densities. From our studies we have found that: (a) The outflowing He+ has density profiles of similar shape to those of H+, for basically different reasons; (b) The effect of the perpendicular electric field differs considerably for H+ and He+. This difference stems from the fact that an electric field acts to alter significantly the O+ density at high altitudes and this, in turn, changes the H+ escape flux through the O++H charge exchange reaction. A similar situation does not occur for He+ and therefore the He+ escape flux exhibits a negligibly small change with electric field; (c) The fractional heating of He+ due to the He+O+ relative flow is not as effective in heating He+ as the H+O+ relative flow is in heating H+; (d) During magnetospheric disturbances when the N2 density at the altitude of the He+ peak (600 km) can increase by a factor as large as 50, the He+ peak density decreases only by approximately a factor of 2; and (e) The He+ escape flux over the winter pole is approximately a factor of 20 greater than the He+ escape flux over the summer pole. Consequently, on high latitude closed field lines there could be an interhemispheric He+ flux from winter to summer.  相似文献   

16.
We consider the evolution of a rotating star with a mass of 16M ⊙ and an angular momentum of 3.25 × 1052 g cm2 s?1, along with the hydrodynamic transport of angular momentum and chemical elements in its interiors. When the partial mixing of matter of the turbulent radiative envelope and the convective core is taken into account, the efficiency of the angular momentum transport by meridional circulation in the stellar interiors and the duration of the hydrogen burning phase increase. Depending on the Schmidt number in the turbulent radiative stellar envelope, the ratio of the equatorial rotational velocity to the circular one increases with time in the process of stellar evolution and can become typical of early-type Be stars during an additional evolution time of the star on the main sequence. Partial mixing of matter is a necessary condition under which the hydrodynamic transport processes can increase the angular momentum of the outer stellar layer to an extent that the equatorial rotational velocity begins to increase during the second half of the evolutionary phase of the star on the main sequence, as shown by observations of the brightest stars in open star clusters with ages of 10–25 Myr. When the turbulent Schmidt number is 0.4, the equatorial rotational velocity of the star increases during the second half of the hydrogen burning phase in the convective core from 330 to 450 km s?1.  相似文献   

17.
During the inbound segment of the Ulysses flyby of Jupiter, there were multiple incursions into the dawnside low-latitude boundary layer, as identified by Bame et al. (Science257, 1539–1542, 1992) using plasma electron data. In the present study, ion composition and spectral measurements provide independent collaborative evidence for the existence of distinct boundary layer regions. Measurements are taken in the energy-per-charge range of 0.6–60 keV/e and involve mass as well as mass-per-charge identification by the Ulysses/SWICS experiment. Ion species of Jovian magnetospheric origin (including O+, O2+, S2+, S3+) and sheath origin (including He2+ and high charge state CNO) have been directly identified for the first time in the Jovian magnetospheric boundary layer. Protons of probably mixed origin and He+ of possibly sheath (ultimately interstellar pickup) origin were also observed in the boundary layer. Sheath-like ions are observed throughout the boundary layer; however, the Jovian ions are depleted or absent for portions of two boundary layer cases studied. Ions of solar wind origin are observed within the outer magnetosphere. and ions of magnetospheric origin are found within the sheath, indicating that transport across the magnetopause boundary can work both ways, at least under some conditions. Although their source cannot be uniquely identified, the proton energy spectrum in the boundary layer suggests a sheath origin for the lower energy protons.  相似文献   

18.
We present a method and results of evaluation of the photometric systems (PSs) proposed for the GAIA mission. The method, however, can be applied for the analysis of virtually any multicolour PS designed for the global investigation of the Galactic stellar populations. Performance of the 1F, 2A and 3G PSs is evaluated taking into account their ability to simultaneously determine the main stellar parameters,T eff, log g, [M/H] and E B-V, for a large variety of stars down to G ∼ 20. The representative Galactic stellar populations were constructed and employed for evaluation of the PSs. Despite the fact that the 2A and 3G PSs perform significantly better than the 1F (presently adopted as a baseline PS for GAIA), we conclude that still there is no photometric system proposed to date, which would allow to achieve the scientific objectives of the GAIA mission. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
The Praesepe cluster contains a number of δ Sct and γ Dor pulsators. Asteroseismology of cluster stars is simplified by the common distance, age and stellar abundances. Since asteroseismology requires a large number of known frequencies, the small pulsation amplitudes of these stars require space satellite campaigns. The present study utilizes photometric MOST satellite measurements in order to determine the pulsation frequencies of two evolved (EP Cnc, BT Cnc) and two main‐sequence (BS Cnc, HD 73872) δ Sct stars in the Praesepe cluster. The frequency analysis of the 2008 and 2009 data detected up to 34 frequencies per star with most amplitudes in the submillimag range. In BS Cnc, two modes showed strong amplitude variability between 2008 and 2009. The frequencies ranged from 0.76 to 41.7 cd–1. After considering the different evolutionary states and mean stellar densities of these four stars, the differences and large ranges in frequency remain (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
Recent satellite observations of thermal ion composition in the near-equatorial plasmasphere have shown that He+ comprises 5–10% typically and occasionally 25% or more of the total thermal ion density. A steady state diffusive equilibrium model for the distributions of H+, He+ and O+ along a plasmaspheric flux tube is used to elicit effects that may help explain these observed high He+ fractional concentrations. The model indicates that both the ionospheric composition and the temperature distribution along the flux tubes are important factors controlling the equatorial He+ composition, through the plasma scale height and thermal diffusion effects. Direct comparison of the model results with thermal ion observations by ISEE-1 indicates that the effects incorporated into the model may explain some of the elevated He+ concentrations. In some instances, however, effects not included in the model may also be of importance.  相似文献   

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