Stars resembling the Sun

Summary. This review is primarily directed to the question whether photometric solar analogues remain such when subjected to detailed spectroscopic analyses and interpreted with the help of internal stucture models. In other words, whether the physical parameters: mass, chemical composition, age (determining effective temperature and luminosity), chromospheric activity, equatorial rotation, lithium abundance, velocity fields etc., we derive from the spectral analysis of a photometric solar analogue, are really close to those of the Sun. We start from 109 photometric solar analogues extracted from different authors. The stars selected had to satisfy three conditions: i) their colour index must be contained in the interval: –0.69, ii) they must possess a trigonometric parallax, iii) they must have undergone a high resolution detailed spectroscopic analysis. First, this review presents photometric and spectrophotometric researches on solar analogues and recalls the pionneering work on these stars by the late Johannes Hardorp. After a brief discussion on low and high resolution spectroscopic researches, a comparison is made between effective temperatures as obtained, directly, from detailed spectral analyses and those obtained, indirectly, from different photometric relations. An interesting point in this review is the discussion on the tantalilizing value of the of the Sun, and the presentation of a new reliable value of this index. A short restatement of the kinematic properties of the sample of solar analogues is also made. And, finally, the observational diagram, obtained with 99 of the initially presented 109 analogues, is compared to a theoretical diagram. This latter has been constructed with a grid of internal structure models for which, (very important for this investigation), the Sun was used as gauge. In analysing the position, with respect to the Sun, of each star we hoped to find a certain number of stars tightly neighbouring the Sun in mass, chemical composition and state of evolution. The surprising result is that the stars occupy in this HR Diagram a rather extended region around the Sun, many of them seem more evolved and older than the Sun, and only 4 of the evolved stars seem younger. The age of some stars in the sample is also discussed in terms of chromospheric activity and Li-content. Our conclusion is much the same as that contained in previous papers we have written on the subject: in spite of a much larger number of stars, we have not been able to nominate a single star of the sample for a “perfect good solar twin”. Another aim in beginning, 25 years ago, this search for solar analogues, was to have ready a bunch of stars resembling the Sun and analysed spectroscopically in detail, in order that, when planets hunters of solar type stars, finally would have found such a specimen, we would have been able to immediately compare the physical parameters of this star to those of the Sun. We have been lucky enough: one of the good solar analogues we present herewith, is 51 Pegasi (HD 217014) which, according to the very recent observations by Mayor and Queloz (1995), has a planet orbiting around it. And what is more: two other stars possessing planets: 47 Ursae Majoris (HD 95128) and 70 Virginis (HD 117176), have just been discovered by Marcy and Butler (187 Meeting of the AAS, January 1996). One of them, 47 Ursae Majoris, is also included in the list of photometric solar analogues. The other star, 70 Virginis, has only been included after the “Planets News”, because the colour index of this star is slightly higher than the prescribted limit of the selection, (, instead, 0.69). It would have been a pity to leave the third ” planet star out of the competition.     

Possibility of Excitation of Low-ℓ P-Modes by Energetic Solar Transients

We examine the temporal variation of power in low-ℓ modes using GONG data for the period of May 1995–October 2005 and compare this with disk-integrated flare and CME indices. A poor correlation between the running means of Flare Index and mode power is found. A similar result is found for CME Index also. Variations in the running mean mode power corresponding to ℓ = 0 modes with different radial orders are generally stochastic in nature. This behaviour is also reflected in the distribution of mode power.     

Non-Axisymmetric Magnetic Fields and Flip-Flops on the Sun and Cool Stars

The modulation of solar activity closely follows the solar rotation period suggesting the existence of long-lived active regions at preferred longitudes. For instance, two preferred active longitudes in both southern and northern hemispheres are found to be persistent at the century time scale. These regions migrate with differential rotation and periodically alternate their activity levels showing a flip-flop cycle. The pattern and behaviour of active longitudes on the Sun is similar to that on cool, rapidly rotating stars with outer convective envelopes. This suggests that the magnetic dynamo, including non-axisymmetric magnetic fields and flip-flop cycles, is also similar in these stars. This allows us to overview the phenomenon of stellar magnetic activity and to study it in detail on the Sun.     

Exploring Magnetic Activity from The Sun to the Stars

Sun-like stars are known to display a wide variety of magnetic activity which is likely to be the signature of a hydromagnetic dynamo mechanism working in stellar interiors. This dynamo mechanism has been studied extensively in the context of the Sun. Here we take ideas and experiences gained from solar dynamo modeling and build upon it to study the inferred scaling laws, involving stellar parameters, from observations of stellar magnetic activity. We also discuss how such a synthesis of theoretical dynamo modeling of Sun-like stars and stellar cycle observations may help us reconstruct the long-term variability of the Sun – an important ingredient for understanding the effects of solar forcing on space and global climate.     

Stars and substars nearest to the Sun: A study review

The structure of the solar neighborhood and its position in the galaxy, including the galactocentric distances, are analyzed. Catalogues are examined of near-sun stars and substars, which are compiled from ground and space based observations in the optical and infrared spectral regions. The problem of classifying celestial bodies in the galaxy by astrophysical and cosmogonic criteria is discussed. The problem of determining the main characteristics of the nearest stars and substars is analyzed. The statistical relationships are investigated between the main characteristics of stars and substars on the basis of their physical evolutionary models. The physical sense of these relationships is discussed. The calculated differential distribution functions of the astrophysical properties of stars and substars are examined.     

Radial Velocities of Six Early Type Evolved Stars

Many early-type main sequence stars are known spectroscopic binaries, while early type spectroscopic binaries with evolved components are rather rare and well-determined masses are available only in some cases. For this reason a program to study stars with suspected variable velocity among early-type evolved stars has been initiated some time ago. In the present paper 54 coudé spectra for a sub-group formed of six early-type stars, namely HD 7902, HD 28446, HD 42087, HD 187983, HD 195592 and HD 208501 have been measured and the results of these plates have been presented.     

The Variability and Radial Velocity of Planetary Nebula Central Stars

The extremely accurate estimates of stellar variability and radial velocity in the Gaia Data Release 3(Gaia DR3)have enabled us to examine the close binarity and radial velocity(RV) of central stars(CSs) of planetary nebulae(PNe). This study is twofold:(1) searching for new close binary CS candidates to better understand how binarity affects the formation and evolution of PNe; and(2) extending the sample size of known RVs of PNe in order to understand their kinematics and the dynamics of the Mil...     

Stochastic Excitation of Gravity Waves by Overshooting Convection in Solar-Type Stars

The excitation of gravity waves by penetrative convective plumes isinvestigated using 2D direct simulations of compressible convection.The oscillation field is measured by a new technique based on theprojection of our simulation data onto the theoretical g-modes solutionsof the associated linear eigenvalue problem. This allows us to determineboth the excited modes and their corresponding amplitudes accurately.     

A Theoretical Probe for Excitation Mechanisms of Sun-like and Mira-like Oscillations of Stars

The linear nonadiabatic oscillations for evolutionary models of 0.6- 3M8 stars are calculated by using a nonlocal and time-dependent convection theory. The results show that in the HR diagram the pulsation-unstable low- temperature stars on the right side of instability strip can be divided into two groups. One group indicates the Sun-like oscillation stars composed of the main- sequence dwarfs, sub-giants and red giants (RGs) of low and intermediate lu- minosities, which are unstable in the intermediate- and high-order (n ≥ 12) p- modes, and stable in the low-order (n ≤ 5) p-modes. Another group indicates the Mira-like stars composed of the bright RGs and asymptotic giant branch (AGB) stars, which are just contrary to Sun-like stars, unstable in low-order (n ≤ 5) p-modes and stable in the intermediate- and high-order (n ≥ 12) p-modes. The oscillations for the red edge of Cepheid (δ Scuti) instability strip, Sun-like and Mira-like stars can be explained uniformly by the coupling between convection and oscillation (CCO). For the low-temperature stars on the right side of in- stability strip, CCO is the dominant excitation and damping mechanism of the oscillations of low- and intermediate-order p-modes, and the turbulent stochas- tic excitation becomes important only for the high-order p-modes of Sun-like oscillations.     

A Theoretical Probe for Excitation Mechanisms of Solar-like and Mira-like Oscillations of Stars

The linear stability analysis of the radial and non-radial oscillations for the evolutionary model of a star with the mass of 0.6∼3 M8 has been per- formed by using the nonlocal and time-dependent convection theory. The results show that the unstable low-temperature stars on the right side of the instabil- ity strip in the HR diagram can be divided into two groups. One is of the stars of solar-like oscillations, composed of the main-sequence dwarfs, subgiants, and the red giants with low- and intermediate-luminosity, which are unstable in the intermediate- and high-order (nr ≥ 12) p-modes, but stable in the low- order (nr ≤ 5) p-modes. Another is of the Mira-like stars, composed of the luminous red giants and AGB stars, which are just contrary to the solar-like stars, unstable in the low-order (nr ≤ 5) p-modes, but stable in the intermediate- and high-order (nr ≥ 12) p-modes. On the red edge of Cepheid (δ Scuti) insta- bility strip, the oscillations of solar-like and Mira-like stars can be explained uniformly by the coupling between convection and oscillations (CCO). For the low-temperature stars on the right side of the instability strip, the CCO is the dominant excitation and damping mechanism for the low- and intermediate-order p-modes, and the stochastic excitation of turbulence becomes important only for the high-order p-modes of solar-like oscillations.     

Towards an Understanding of Radial Velocity Shift in Uv Spectra of Symbiotic Stars

A systematic redshift of high ionization resonance emission lines relative to the intercombination lines was found by Friedjung et al. (1883, A & A 126, 4071983) in the UV spectra of a number of symbiotic binaries. The interpretation was not then clear. We present a study of archival IUE and GHRS/HST spectra of the symbiotic binary CI Cyg. The shift varies during the orbital cycle which can be understood in terms of the presence of strong circum-binary line absorption formed in an outer expanding region.     

恒星类太阳和类长周期变星脉动激发机制的理论探讨

利用一种非局部和非定常的恒星对流理论,计算了0.6～3.0 M☉恒星演化模型的线性非绝热脉动.结果表明,赫罗图上脉动不稳定带右方的脉动不稳定的低温恒星可以分成2大类(群).一类由主序矮星、亚巨星和中低光度红巨星组成的类太阳振荡恒星,它们的中高阶(径向阶nr≥12)p模是脉动不稳定的,而低阶(nr≤ 5)模是脉动稳定的;另一类是由亮的红巨星和渐近巨星支星(AGB)组成的类长周期变星,它们的低阶模(nr≤5)是脉动不稳定的,而高阶模(nr≥12)却是脉动稳定的.能利用对流与脉动的耦合统一解释造父变星脉动不稳定带红边界、类太阳和类长周期变星脉动.对赫罗图右方的低温恒星的中低阶p模振荡,对流与脉动的耦合是主要的脉动激发和阻尼机制,而湍流的随机激发机制仅对类太阳振荡高阶p模重要.     

Kinematics of T Tauri Stars Close to the Sun from the Gaia DR2 Catalogue

Astronomy Letters - The spatial and kinematic properties of a large sample of young T Tauri stars from the solar neighborhood 500 pc in radius have been studied. The following parameters of the...     

An Update of the Catalog of Radial Velocity Standard Stars from the APOGEE DR17

We present an updated catalog of 46,753 radial velocity(RV) standard stars selected from the APOGEE DR17.These stars cover the Northern and Southern Hemispheres almost evenly,with 62% being red giants and 38%being main sequence stars.These RV standard stars are stable on a baseline longer than 200 days(with 54% longer than one year and 10% longer than five years) with a median stability better than 215 m s^-1.The average number of observations of those stars is 5 and each observation i...     

Zero-Metallicity Stars and the Effects of the First Stars on Reionization

We present a model that relates the width of the broad emission lines of active galactic nuclei (AGNs) to the Keplerian velocity of an accretion disk at a critical distance from the central black hole. This critical distance falls in a region bounded on the inward side by the transition radius between the radiation pressure- and the gas pressure-dominated region of the accretion disk and on the outward side by the maximum radius below which a stabilizing, radially accreting and vertically outflowing corona exists. We show that in the framework of this picture, the observed range of Hbeta FWHMs from broad-line to narrow-line type 1 AGNs is well reproduced as a function of the accretion rate. This interval of velocities is the only permitted range and goes from approximately 20,000 km s-1 for sub-Eddington accretion rates to approximately 1000 km s-1 for Eddington accretion rates.