Effective temperatures,radii and bolometric magnitudes of Ap and Am stars

The effective temperatures radii and bolometric magnitudes of Ap, Am and normal A stars have been estimated from their energy distribution curves between 478 nm and 680 nm. All the Am stars and one Ap star (i.e. CrB) were found to be affected by line blanketing, a rough estimation of which in the respective (B-V) colours has been found out in each case.The range in effective temperature is 0.45–0.60 in terms of (=5040/T _e), while it is 1.8–4.8R in the case of radius, that in bolometric magnitude being from-0^m.67 to+1^m.61. An approximate estimate of the masses shows that they are between 1.5 and 3.0M . All these estimates are in agreement with those of the normal A stars. The Ap and Am stars are found to be slightly evolved and, therefore, are probably in the hydrogen shell-burning phase.     

Spectrophotometric studies of Am stars

The relationships among the various physical parameters-namely, the effective temperatures, radii and bolometric magnitudes, determined on the basis of the energy distribution curves of 25 Am stars — have been studied. Their effective temperatures are in the range of 7200 K to 9700 K; the radii, 1.5R to 2.5R ; the bolometric magnitudes, 0.75 mag. to 2.25 mag.; and the masses, 1.5M to 2.25M . The Am stars in general, appear redder than their normal counterparts, the blanketing in the blue andUV regions being the major cause. For the relatively cooler stars, the (B-V) colours are found to be less affected by blanketing. They are located in the neighbourhood of the upper edge of the zero-age Main Sequence band and show a fairly wide range in the evolutionary status among themselves. The bolometric corrections which are independent of the uncertainties in the parallax measurements, follow the same trend as that of the Ap stars, with reference to the temperature.     

Spectrophotometric studies of Ap stars

Based on the observed energy distribution curves of about a hundred Ap stars, the various relationships among their physical parameters: namely, the temperature, colour index, bolometric correction and bolometric magnitude have been studied. The hotter Ap stars have been found to be apparently bluer than their normal counterparts, which is possibly due to the broad continuum features at 4200 and 5200 that are generally present in Ap stars only. The bolometric corrections are independent of parallax measurements; the Ap stars as well as the normal stars follow the same sequence of bolometric corrections when related to temperature. The Ap stars appear to beslightly evolved and their position in the HR diagram indicates the hydrogen shell burning phase. The mass range of Ap stars is similar to that of normal A stars.     

Temperature behavior of elemental abundances in the atmospheres of magnetic peculiar stars

We analyze the temperature dependence of the abundances of the chemical elements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line (Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp) stars in the range 6000–15000 K. The Cr and Fe abundances in the atmospheres of Ap stars increase rapidly as the temperature rises from 6000 to 9000–10000 K. Subsequently, the Cr abundance decreases to values that exceed the solar abundance by an order of magnitude, while the Fe abundance remains enhanced by approximately +1.0 dex compared to the solar value. The temperature dependence of the abundances of these elements in the atmospheres of normal and Am stars is similar in shape, but its maximum is several orders of magnitude lower than that observed for Ap stars. In the range 6000–9500 K, the observed temperature dependences for Ap stars are satisfactorily described in terms of element diffusion under the combined action of gravitational settling and radiative acceleration. It may well be that diffusion also takes place in the atmospheres of normal stars, but its efficiency is very low due to the presence of microturbulence. We show that the magnetic field has virtually no effect on the Cr and Fe diffusion in Ap stars in the range of effective temperatures 6000–9500 K. The Ca abundance and its variation in the atmospheres of Ap stars can also be explained in terms of the diffusion model if we assume the existence of a stellar wind with a variable moderate rate of ～(2–4) × 10^{? 15}M_⊙ yr^?1.     

Pregalactic-primordial low-mass stars

The Main-Sequence positions as well as the evolutionary behavior of Population III stars up to an evolution age of 2×10¹⁰ yr, taking this time as the age of the Universe, have been investigated in the mass range 0.2 and 0.8M . While Population III stars with masses greater than 0.3M develop a radiative core during the approach to the Main Sequence, stars with masses smaller than 0.3M reach the Main Sequence as a wholly convective stars. Population III stars with masses greater than 0.5M show a brightening of at most 2.2 in bolometric magnitude when the evolution is terminated as compared to the value which corresponds to zero-age Main Sequence. The positions of stars with masses smaller than 0.5M remain almost the same in the H-R diagram.If Population III stars have formed over a range of redshifts, 6相似文献   

The evolutionary behaviour of population III intermediate mass stars

Assuming the Big-Bang nucleosynthesis was responsible for the formation of helium, the evolution of first-generation intermediate-mass stars of 5, 7, and 9M with no metals have been studied from the threshold of stability through the stage of helium exhaustion in the cores of the stars. Hydrogen Main-Sequence positions are marked at effective temperatures higher than those of normal stars. The evolutionary tracks during the hydrogen burning phase start to be similar to those of normal stars when the CN-cycle reactions, which are controlled by the triple-alpha reactions, become operative for hydrogen depletion. Helium Main Sequence of Population III stars of intermediate mass occurs at the high effective temperature region of the H-R diagram and stars stay as blue stars until the end of the core helium exhaustion phase. The total time elapsed is in the range of 3×10⁷ and 10⁸yr. The stars with the initial masses of 5, 7, and 9M developed a moderately electron degenerate complete hydrogen-exhausted region with masses of 0.77, 1.06, and 1.42M , respectively, in which the most abundant element is carbon.     

Mass loss from coronae and its effect upon stellar rotation

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.     

The evolution of hydrogen-helium stars

According to the work of Truran and Cameron, and of others, on the chemical evolution of the Galaxy, the first generation of stars in the Galaxy contained principally massive objects. If big-bang nucleosynthesis was responsible for the formation of helium, the first generation of stars would contain about 80% hydrogen and 20% helium, to be consistent with the approximately 22% helium found in recent stellar evolutionary studies of the Sun. The present investigation has followed the pre-main sequence evolution and the main sequence evolution of stars of 5, 10, 20, 30, 100, and 200M . Normal stars in this entire mass range normally convert hydrogen into helium by the CN-cycle on the main sequence. the present hydrogen-helium stars of 5 and 10M must reach higher central temperatures in order to convert hydrogen to helium by the proton-proton chains. Consequently, the mean densities in the stars are greater, and the surface temperatures are higher than in normal stars. In the stars of 20M and larger, the proton-proton chains do not succed in supplying the necessary luminosity of the stars by the time the contraction has produced a central temperature near 10⁸K. At that point triple-alpha reactions generate small amounts of C¹², which then acts as a catalyst in the CN-cycle, the rate of which is then limited by the beta-decays occurring within the cycle. During the evolution of these more massive stars, the central temperature remains in the vicinity of 10⁸ K, and the surface temperature on the main sequence approaches 10⁵ K. The star of 200M becomes unstable against surface mass loss through radiation pressure in the later stages of its main sequence evolution, and these mass loss effects were not followed. Young galaxies containing these massive stars will have a very high luminosity, but if they have formed at one-tenth the present age of the universe or later, then the light from them will mainly reside in the visible or ultraviolet, rather than in the infrared as has been suggested by Partridge and Peebles.     

On the radii of Ap and Am stars

The radii of several Ap and Am stars have been compared with those of the normal A stars of the Main Sequence. Though the brighter Ap stars have a little larger radii than the Main-Sequence stars, they may not be much different from those of the slightly evolved normal A stars. The Am stars have radii with which they appear to be merging with those of the cooler A stars of the Main Sequence. The Ap stars have radii predominantly in the range of 1.8 to 3.4R _⊙, while the Am stars are mainly concentrated between 1.8 and 2.2R _⊙.     

Statistical results from a jcmt survey of MM and sub-MM emission in herbig ae/be stars

We have detected 1.1 mm continuum emission from 24 of 53 Herbig Ae/Be stars surveyed with the JCMT. Survival analysis shows that 1.1 mm luminosity is correlated with bolometric luminosity and with IRAS 25µm luminosity. For those stars that were also detected at 0.45 or 0.8 mm we find a typical flux dependence of the form S _#x03BD; ³, which is steeper than that of most classical T Tauri stars.     

Observational manifestations of early mixing in B- and O-type stars

The helium and nitrogen enrichment of the atmospheres of early B-type stars during the main sequence (MS) evolutionary phase is re-analysed. It is confirmed that the effect depends on both the aget and the stellar massM. For example, the helium abundanceHe/H increases by 0.04 (60–70% of initial value) for stars withM=8–13M and by 0.025 (about 30%) for stars withM=6M . The nitrogen abundance rises by three times forM=14M and by, two times forM=10M . According to the latest theoretical computations, the observed appearance of CNO-cycled material in surface layers of the stars can be a result of the rotationally induced mixing, in particular, of the turbulent diffusion. Carbon is in deficiency in B stars, but unexpectedly does not show any correlation with the stellar age. However it is shown that the total C+N abundance derived for early B stars conflicts with the theory.Basing on modern data the helium enrichment is first examined in O-type MS stars, as well as in components of binaries. As compared with early B stars, the He abundance for more massive O stars and for components of binaries show a different relation with the relative aget/t _MS . Namely during short time betweent/t _MS 0.5 and 0.7 a sharp jump is observed up toHe/H=0.2 and more. In particular, such a jump is typical for fast rotating O stars (v sini200 km s^–1),. Therefore the effect of mixing depends on massM, relative aget/t _MS , rotational velocityv and duplicity.The mass problem (the discrepancy betweenM _ev andM _sp ) is also analysed, because some authors consider it as a possible evidence of early mixing, too. It is shown that the accurate data for components of binaries lead to the conclusion that the discrepancy is less than 30%. Such a difference can be removed at the expense of theM _ev lowering, if the displacement of evolutionary tracks, owing to the rotationally induced mixing is taken into consideration.     

Some applications of Jacobi dynamics to the stellar evolution

The stars in the Main Sequence are seen as a hierarchy of objects with different massesM and effective dynamical radiiR _eff=R/ given by the stellar radii and the coefficients for the inner structure of the stars.As seen in a previous work (Paper I), during the lifetime in the Main SequenceR _eff(t) remains a near invariant when compared to the variation in the time ofR(t) and (t).With such an effectiveR _eff one obtains the amounts of actionA _c(M), the effective densities _eff(M)=(M)³(M), the densities of action and of energy (or mean presures in the stellar interior)a _c(M),e _c(M), and the potential energiesE _p(M).The amounts of action areA _cM ^k withk1.87 for the M stars,k5/3 for the KGF stars, andk1.83 for the A and earlier stars, representing very simples conditions for the other dynamical parameters. For instancek5/3 means a near invariant effective density _eff for the KGF stars, while for such stars the mean densities and coefficients present the strongest variations with masses (M)M ^–1.81, (M)M^0.6.The cases for the M stars (e _c(M)M ^–1) and for the A and earlier stars (betweena _c(M)=constant and _eff(M)M ^–1) and also discussed. These conditions for the earlier stars also represent reasonable mean values for the whole stellar hierarchy in the range of masses 0.2M M25M .With all this, one can build dynamical HR diagrams withA _c(M), E_p(M), _eff M ^–p , etc., whose characteristics are analogous to these in the photometrical HR diagram. A comparison is made betweenA _c(M) from the models here and the HR diagram with the best known stars of luminosity classes IV, V, and white dwarfs.The comparison of the potential energiesE _p(M)M ^–p according to the stellar models used here and the observed frequency function (MM _–q (number of stars in a given interval of masses) from different authors suggests the possibility that the productE _p(M)(M) is a constant, but this must be confirmed with further studies of the function (M) and its fine structure.There are analogies between the formulation used here for the stellar hierarchy and other physical processes, for instance, in modified forms of the Kolmogorov law of turbulence and in the formulation used for the hierarchy of molecular clouds in gravitational equilibrium. Besides, the function of actionA _c(M) for the stars has analogous properties to the relations of angular momenta and massesJ(M) for different types of objects. The cosmological implications of all this are discussed.     

Vertical Chromium Distribution in the Atmosphere of a Chemically Peculiar Star. I. Diagnostics

On the basis of CCD spectrograms obtained with high resolution at the Coudé focus of the 2.6-m telescope of the Crimean Astrophysical Observatory, Cr II lines lying in the wings of the H hydrogen line are investigated. Vertical chromium stratification in the atmospheres of two normal and eight chemically peculiar stars is diagnosed using the method of spectral synthesis. An increase in Cr abundance with depth is found for the cool Ap stars CrB, HR 7575, Equ, and 10 Aql. Some increase in Cr abundance in the upper layers of the atmosphere is presumed for all Am stars and for both Hg—Mn components of 46 Dra. The vertical chromium distribution in the atmospheres of the hot, spotted Ap stars 17 Com and ² CVn is evidently uniform.     

Steady mass-loss from supermassive stars

Structures of Newtonian super-massive stars are calculated with the opacity for Comptor effectK ₀/(1 + T), whereK ₀=0.21(1 +X and =2.2×10^–9K^–1. The track of the Main-Sequence is turned right in the upper part of the HR diagram. Mass loss will occur in a Main-Sequence stage for a star with mass larger than a critical mass. The cause of mass loss and the expansion of the radius is continuum radiation pressure. The critical mass for mass loss is 1.02×10⁶ M for a Population I star, and 1.23×10⁵ M for Population III star. Mass loss rates expected in these stars are 3.3×10^–3 and 4.0×10^–3 M yr^–1, respectively.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Relativistic evolution of 103 M ⊙ star

A fully relativistic evolution of 10³ M is described for initial composition ofY=Z=0. Our results show that (a) a great part of the star is in radiative equilibrium, (b) the maximal red-shift for main-sequence 10³ M stars is significantly less than for isentropic models, and (c) a very low amount of CNO elements (<10^–8 by mass) is formed at any stage before hydrogen is completely consumed and hence such stars cannot be the progenitors of Population II stars.     

The locations of newly formed stars in molecular clouds

Observations of very massive stars (M10M ) are suggestive of a star formation process which requires an external trigger. However, observations pertaining to the formation of stars of lower mass (M9M ) require no such triggering mechanism and are consistent with the idea that such stars form as a natural consequence of the evolution, gravitational collapse and fragmentation of a proto-stellar molecular cloud.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

A mechanism for the production of planetary nebulae

C¹² stars in the range 1.04–1.55M are evolved to simulate the core evolution of the possible precursors of planetary nebulae. The nuclear shell burning in stars above 1.2M advances to within about 0.2M of the surface, where the intense radiation interacts with the surface matter and causes mass loss. Comparison between our theoretical results and observations suggests that this may be a mechanism by which planetary nebulae are formed.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Exosat observations of X-rays from classical novae during outburst stage

The initial discovery of soft X-rays from Nova Muscae 1983 was followed by eight additional observations of the three brightest novae whose outburst stage coincided with the lifetime ofEXOSAT satellite; namely three more observations of Nova Muscae 1983, three observations of Nova Vulpeculae 1984#1 (PW Vul), and two observations of Nova Vulpeculae 1984#2. Through these observations we sampled the soft X-ray light curve of classical novae from optical maximum to 900 days after. The observations seem best explained by the constant bolometric luminosity model of a hot white dwarf remnant. Although the measurements suffer from limited statistics, very broad energy bandpass, and incomplete sampling of any single nova, their constraints on the theories of nova outburst are significant. One constraint is that the lifetime of the white dwarf remnant in Nova Muscae 1983 is 2 to 3 years, which leads to the conclusion that the burned envelope massM _burn should be of the order of . The second constraint is that the maximum temperature, of the white dwarf remnant should approximately be within 200 000 K to 400 000 K. We estimate that a white dwarf remnant evolving like the central star of a planetary nebula, with core mass of 0.8 to 0.9M , core luminosity of 2×10⁴ L , and envelope mass of 10^–6 M , can explain the general characteristics of the X-ray measurements for Nova Muscae 1983. In order to have 1.1M core mass, estimated from the early observations of bolometric luminosity in the UV to infrared range, a wind withM5×10^–7 M yr^–1 appears to be necessary. The few observations of Nova Vulpeculae 1984 #1 and Nova Vulpeculae 1984#2, during the first year after outburst, give a risetime and intensity that is consistent with a constant bolometric luminosity model.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F. R. G., 16–19 June, 1986.     

Physical properties of the outflow sources in taurus

We have searched for CO outflows in eight embedded IRAS sources located in the Taurus molecular cloud using the 45m telescope of Nobeyama Radio Observatory. We have detected CO wing emission in four of these sources. CO outflow associated with TMC1A (04365+2535) is strong and spatially compact (radius 0.04 pc). The dynamical timescale of 2.5 × 10³ yr suggests this outflow is the youngest one in Taurus.We have combined our data with previously published survey data and have analyzed the physical properties of the outflow sources. We found that 12 out of 16 embedded sources ( 75 %) have CO outflows associated with them; this indicates that almost all stars experience a phase of molecular outflow in their embedded stage. The IRAS color of the outflow sources suggests that the outflows appear in considerably early phase of the evolution of YSOs, that is, as early as YSOs became observable with IRAS and that visible outflow sources are in a transient phase of evolution between embedded sources and visible T Tauri stars without outflow. Visible outflow sources are systematically more luminous than visible no-outflow sources, while embedded outflow sources have comparable luminosities with visible no-outflow sources. Such luminosity function suggests that the YSOs with outflow undergo mass accretion and increase their stellar mass as they progress from embedded sources to visible outflow sources. Typical mass accretion rate derived from the bolometric luminosity is 2 ×10^–6 M _yr ^–1. The timescale for mass accretion to acquire typical stellar mass, 0.5 – 0.8M , is 2.5 – 4 × 10⁵ yr.     

Period distributions in pulsating and binary stars

We analyze the hypothesis of quantization in bands for the angular momenta of binary systems and for the maount of actionA _c in stable and pulsating stars. This parameter isA _c=Mv _eff R _eff, where the effective velocity corresponds to the kinetic energy in the stellar interior and the effective radius corresponds to the potential energyGM ²/R _eff. Analogous parameters can be defined for a pulsating star withm=M where is the rate of the massm participating in the oscillation to the total massM andv _osc,R _osc the effective velocity and oscillation radius.From an elementary dimensional analysis one has thetA _c (energy x time) (period)^1/3 independently ifA _c corresponds to the angular momentum in a binary system, or to the oscillation in a pulsating star or the inner energy and its time-scaleP _eff in a stable star.From evolving stellar models one has that P _effP _eff(solar)1.22 hr a near-invariant for the Main Sequence and for the range of masses 0.6M <M<1.6M .With this one can give scalesn _k=kn ₁ withk integers andn ₁=(P/P ₁)^1/3 withP ₁=P _eff1.22 hr. In these scales proportional toA _c, one sees that the periods in binary and pulsating stars are clustered in discrete unitsn ₁,n ₂,n ₃, etc.This can be seen in pulsating Scuti, Cephei, RR Lyrae, W Virginis, Cephei, semi-regular variables, and Miras and in binary stars as cataclysmic binaries, W Ursa Majoris, Algols, and Lyrae with the corresponding subgroups in all these materials. Phase functions (n _k) in RR Lyrae and Cephei are also associated with discrete levelsn _k.the suggested scenario is that the potential energies and the amounts of actionE _p(t), A_c(t) are indeed time-dependent, but the stars remain more time in determinated most proble states. The Main Sequence itself is an example of this. These most probable states in binary systems, or pulsating or stable stars, must be associated with velocities sub-multiplesc/ _F , given by the velocity of light and the fine structure constant.Additional tests for such a hypothesis are suggested when the sufficient amount of observational data are available. They can made with oscillation velocities in pulsating stars and velocity differences of pairs of galaxies.