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1.
G. Cayrel de Strobel 《Astronomy and Astrophysics Review》1996,7(3):243-288
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. 相似文献
2.
The coronal index of solar activity over the period 1992–1994 is given. The data are a good tool to study solar activity, for the Sun as a star, in the solar corona over a solar cycle and its influence in the heliosphere. 相似文献
3.
An increase in solar activity is shown to be accompanied by a decrease in solar rotation rate. This effect has been established from various indices; it manifests itself as cyclic and secular variations in the global magnetic field, in the observations of the magnetic field of the Sun as a star, and in the observations of the solar corona. Some possible explanations of this effect are discussed. 相似文献
4.
The Sun is the only star for which individual surface features can be observed directly. For other stars, the properties of starspots, stellar rotation, stellar flares, etc, are derived indirectly via variation of star‐integrated spectral line profiles or their luminosity measurements. Solar disk‐integrated and disk‐resolved observations allow for investigations of the contribution of individual solar disk features to sun‐as‐a‐star spectra. Here, we provide a brief overview of three sun‐as‐a‐star programs, currently in operation, and describe recent improvements in observations and data reduction for the Integrated Sunlight Spectrometer (ISS), one of three instruments comprising the Synoptic Optical Long‐term Investigations of the Sun (SOLIS) system. Next, we discuss studies employing sun‐as‐a‐star observations (including Ca II K line as proxy for total unsigned magnetic flux and 2800 MHz radio flux) as well as the effects of flares on solar disk‐integrated spectra. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
5.
The Heliospheric Imager (HI) instruments on the Solar TErrestrial RElations Observatory (STEREO) observe solar plasma as it streams out from the Sun and into the heliosphere. The telescopes point off-limb (from
about 4° to 90° elongation) and so the Sun is not in the field of view. Hence, the Sun cannot be used to confirm the instrument
pointing. Until now, the pointing of the instruments have been calculated using the nominal preflight instrument offsets from
the STEREO spacecraft together with the spacecraft attitude data. This paper develops a new method for deriving the instrument
pointing solutions, along with other optical parameters, by comparing the locations of stars identified in each HI image with
the known star positions predicted from a star catalogue. The pointing and optical parameters are varied in an autonomous
manner to minimise the discrepancy between the predicted and observed positions of the stars. This method is applied to all
HI observations from the beginning of the mission to the end of April 2008. For the vast majority of images a good attitude
solution has been obtained with a mean-squared deviation between the observed and predicted star positions of one image pixel
or less. Updated values have been obtained for the instrument offsets relative to the spacecraft, and for the optical parameters
of the HI cameras. With this method the HI images can be considered as “self-calibrating,” with the actual instrument offsets
calculated as a byproduct. The updated pointing results and their by-products have been implemented in SolarSoft. 相似文献
6.
Kenneth R. Lang 《Earth, Moon, and Planets》1995,70(1-3):1-20
The Sun is enveloped by a hot, tenuous million-degree corona that expands to create a continuous solar wind that sweeps past all the planets and fills the heliosphere. The solar wind is modulated by strong gusts that are initiated by powerful explosions on the Sun, including solar flares and coronal mass ejections. This dynamic, invisible outer atmosphere of the Sun is currently under observation with the soft X-ray telescope aboard the Yohkoh spacecraft, whose results are presented. We also show observations from the Ulysses spacecraft that is now passing over the solar pole, sampling the solar wind in this region for the first time. Two other spacecraft, Voyager 1 and 2, have recently detected the outer edge of the invisible heliosphere, roughly halfway to the nearest star. Magnetic solar activity, the total radiative output from the Sun, and the Earth's mean global surface temperature all vary with the 11-year sunspot cycle in which the total number of sunspots varies from a maximum to a minimum and back to a maximum again in about 11 years. The terrestrial magnetic field hollows out a protective magnetic cavity, called the magnetosphere, within the solar wind. This protection is incomplete, however, so the Sun feeds an unseen world of high-speed particles and magnetic fields that encircle the Earth in space. These particles endanger spacecraft and astronauts, and also produce terrestrial aurorae. An international flotilla of spacecraft is now sampling the weak points in this magnetic defense. Similar spacecraft have also discovered a new radiation belt, in addition to the familiar Van Allen belts, except fed by interstellar ions instead of electrons and protons from the Sun. 相似文献
7.
S. R. Verma 《Journal of Astrophysics and Astronomy》2006,27(2-3):113-114
The Sun is a mysterious star. The high temperature of the chromosphere and corona present one of the most puzzling problems
of solar physics. Observations show that the solar coronal heating problem is highly complex with many different facts. It
is likely that different heating mechanisms are at work in solar corona. Recent observations show that Magnetic Carpet is
a potential candidate for solar coronal heating. 相似文献
8.
9.
D. W. Kurtz 《Solar physics》2008,251(1-2):21-30
Stellar astronomers look on in envy at the wealth of data, the incredible spatial resolution, and the maturity of the theoretical understanding of the Sun. Yet the Sun is but one star, so stellar astronomy is of great interest to solar astronomers for its range of different conditions under which to test theoretical understanding gained from the study of the Sun. The rapidly oscillating peculiar A stars are of particular interest to solar astronomers. They have strong, global, dipolar magnetic fields with strengths in the range 1?–?25?kG, and they pulsate in high-overtone p modes similar to those in the Sun; thus they offer a unique opportunity to study the interaction of pulsation, convection, and strong magnetic fields, as is now done in the local helioseismology of sunspots. Some of them even pulsate in modes with frequencies above the acoustic cutoff frequency, in analogy with the highest frequency solar modes, but with mode lifetimes up to decades in the roAp stars, very unlike the short mode lifetimes of the Sun. They offer the most extreme cases of atomic diffusion, a small, but important ingredient of the standard solar model with wide application in stellar astrophysics. They are compositionally stratified and are observed and modelled as a function of atmospheric depth and thus can inform plans to expand helioseismic observations to have atmospheric depth resolution. Study of this unique class of pulsating stars follows the advanced state of studies of the Sun and offers more extreme conditions for the understanding of physics shared with the Sun. 相似文献
10.
S. V. Chernov 《Astronomy Letters》2017,43(3):186-201
We investigate the change in the orbital period of a binary system due to dynamical tides by taking into account the evolution of a main-sequence star. Three stars with masses of one, one and a half, and two solar masses are considered. A star of one solar mass at lifetimes t = 4.57 × 109 yr closely corresponds to our Sun. We show that a planet of one Jupiter mass revolving around a star of one solar mass will fall onto the star in the main-sequence lifetime of the star due to dynamical tides if the initial orbital period of the planet is less than P orb ≈ 2.8 days. Planets of one Jupiter mass with an orbital period P orb ≈ 2 days or shorter will fall onto a star of one and a half and two solar masses in the mainsequence lifetime of the star. 相似文献
11.
John F. Doorish 《Astrophysics and Space Science》1994,214(1-2):19-33
In this paper we wish to develop an analytical technique for modelling the energy generation rate and the luminosity gradient for a one solar mass star like the Sun. Following the MDV method for approximating the solar/stellar structure (Doorish, 1989) we derive dimensionless variables for the energy generation rate and luminosity which are used in an expansion equation (on the order of a simple general confluent hypergeometric function). Modelling the luminosity of the Sun is complicated due to the nature of the energy generation rate. For this reason, we attempt to first derive approximation of the energy generation rate. The resulting solutions bring about a model of the luminosity gradient and the corresponding energy generation rate for the proton-proton cycle. Also, we are able to derive, analytically, the hydrogen content from the center of the star to the surface using concepts derived from the energy generation rate.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9–13 November, 1992. 相似文献
12.
A.G.W. Cameron 《Icarus》1984,60(2):416-427
A supernova trigger for star formation, such as was discussed by Cameron and Truran a few years ago, lacks observational confirmation, and the nucleosynthesis arguments for suggesting its relevance to the formation of the solar system no longer seem compelling. Observations of star formation currently point toward nearly isolated individual events taking place in the interiors of dense molecular clouds, in which cores are formed in the clouds and collapse to form stars. If the formation of the solar system proceeded in this manner, then a viable theory of cloud core formation must provide circumstances in which it is not uncommon for the cloud cores to be formed with a significant supply of fresh short-lived radioactivities. A review is made of the evidence for the existence of now extinct radioactivities in primitive solar system material and an examination is made of the implications for the early stages of formation of the Sun and solar system. The characteristics of possible disturbances in dense molecular clouds which can initiate the formation of cloud cores is discussed,and in particular those disturbances which can produce fresh radioactivities are considered. A red giant star of roughly one solar mass on the asymptotic giant branch appears to have been the best candidate to account for the short-lived extinct radioactivities in the early solar system. Star formation itself can generate disturbances through the T Tauri stellar winds and bipolar outflows, and it appears likely that several such disturbances in a chain of star-forming events were needed before the radioactive material was diluted enough to be consistent with the abundances seen in early solar system material. 相似文献
13.
N. I. Kobanov 《Experimental Astronomy》1994,5(3-4):301-313
A variant to implement a spacecraft (SC) spatial attitude system with respect to the Sun is discussed. The sunward direction and the solar rotation axis are used as reference points. The system is based on measuring spectral line Doppler shift by scanning the solar image along the limb and is self-adjusting for relative spectral line shifts and instrument band shifts. The first harmonic of the signal serves as a basis for accurate adjustment of filter band. The second harmonic phase is used to measure the spacecraft attitude. The application of this method holds the greatest promise when the SO is stabilized by the sunward spinning because this ensures continuous monitoring of the spacecraft attitude.In addition, the method provides information on the precise coordinates of solar surface details during space-borne observations. 相似文献
14.
Kevin P. Reardon 《Solar physics》2006,239(1-2):503-517
We investigate the effects of atmospheric dispersion on observations of the Sun at the ever-higher spatial resolutions afforded
by increased apertures and improved techniques. The problems induced by atmospheric refraction are particularly significant
for solar physics because the Sun is often best observed at low elevations, and the effect of the image displacement is not
merely a loss of efficiency, but the mixing of information originating from different points on the solar surface. We calculate
the magnitude of the atmospheric dispersion for the Sun during the year and examine the problems produced by this dispersion
in both spectrographic and filter observations. We describe an observing technique for scanning spectrograph observations
that minimizes the effects of the atmospheric dispersion while maintaining a regular scanning geometry. Such an approach could
be useful for the new class of high-resolution solar spectrographs, such as SPINOR, POLIS, TRIPPEL, and ViSP.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
15.
In order to obtain the internal structure of a main-sequence star such as the Sun usually one has to solve the detailed structure equations numerically. This paper is an attempt to construct analytic models for the stellar nuclear energy generation. We give closed-form analytic results for the stellar luminosity and stellar neutrino emission rate when the radial matter density of the spherical star under consideration is linear. For the numerical estimation of the neutrino flux of a specified stellar nuclear reaction we take into account parameters of the standard solar model. The present paper gives for the first time the connection between stellar structure parameters and neutrino fluxes in an analytic stellar model. 相似文献
16.
We present the behaviour of the coronal index (CI) of solar activity over the period 1986–(May) 1996. These data are not only a good tool for studying the activity of the Sun as a star, but they also represent an invaluable source of information in our quest to understand the properties of the heliosphere as a whole. Having compared the variations of CI and of the solar 0.05–0.8 nm soft X-ray flux over the period 1986–1995 we did not find any significant correlation between the two quantities. This favours a scenario in which the sources of the soft X-ray flux are small-scale regions of the corona, and processes of both ionization and recombination do not occur in the same volume of the solar corona as for the green corona. 相似文献
17.
V. V. Bobylev 《Astronomy Letters》2010,36(3):220-226
Based on a new version of the Hipparcos catalog and currently available radial velocity data, we have searched for stars that
either have encountered or will encounter the solar neighborhood within less than 3 pc in the time interval from −2 Myr to
+2 Myr. Nine new candidates within 30 pc of the Sun have been found. To construct the stellar orbits relative to the solar
orbit, we have used the epicyclic approximation. We show that, given the errors in the observational data, the probability
that the well-known star HIP 89 825 (GL 710) encountering with the Sun most closely falls into the Oort cloud is 0.86 in the
time interval 1.45 ± 0.06 Myr. This star also has a nonzero probability, 1 × 10−4, of falling into the region d < 1000 AU, where its influence on Kuiper Belt objects becomes possible. 相似文献
18.
From the XHIP catalogue, we have selected 1872 F-G-K stars with relative parallax measurement errors <20% and absolute values of their space velocities relative to the Sun <15 km s?1. For all these stars, we have constructed their Galactic orbits for 4.5 Gyr into the past using an axisymmetric Galactic potential model with allowance made for the perturbations from the spiral density wave. Parameters of the encounter with the solar orbit have been calculated for each orbit. We have detected three new stars whose Galactic orbits were close to the solar one during a long time interval in the past. These stars are HIP 43852, HIP 104047, and HIP 112158. The spectroscopic binary HIP 112158 is poorly suited for the role of a kinematic sibling of the Sun by its age and spectroscopic characteristics. For the single star HIP 43852 and the multiple system HIP 104047, this role is quite possible. We have also confirmed the status of our previously found candidates for close encounters, HIP 47399 and HIP 87382. The star HIP 87382 with a chemical composition very close to the solar one is currently the most likely candidate, because it persistently shows close encounters with the Sun on time scales of more than 3 Gyr when using various Galactic potential models both without and with allowance made for the influence of the spiral density wave. 相似文献
19.
Global changes of the solar activity can be expressed by the coronal index that is based upon the total irradiance of the coronal 530.3 nm green line from observations at five stations. Daily mean values of the coronal index of solar activity and other well-correlated solar indices are analyzed for the period 1966–1998 covering over three solar cycles. The significant correlation of this index with the sunspot number and the solar flare index have led to an analytical expression which can reproduce the coronal index of solar activity as a function of these parameters. This expression explains well the existence of the two maxima during the solar cycles taking into account the evolution of the magnetic field that can be expressed by some sinusoidal terms during solar maxima and minima. The accuracy between observed and calculated values of the coronal index on a daily basis reaches the value of 71%. It is concluded that the representative character of the coronal index is preserved even when using daily data and can therefore allow us to study long-term, intermediate and short-term variations for the Sun as a star, in association with different periodical solar–terrestrial phenomena useful for space weather studies. 相似文献
20.
Arvind Bhatnagar 《Journal of Astrophysics and Astronomy》2006,27(2-3):59-78
In this presentation we briefly describe the Sun through large number of illustrations and pictures of the Sun taken from
early times to the present day space missions. The importance of the study of the Sun is emphasized as it is the nearest star
which presents unparallelled views of surface details and numerous phenomena. Our Sun offers a unique celestial laboratory
where a large variety of phenomena take place, ranging in temporal domain from a few milliseconds to several decades, in spatial
domain from a few hundred kilometers to thousands of kilometers, and in the temperature domain from a few thousand degrees
to several million degrees. Its mass motion ranges from thousandths to thousands of kilometers per second. Such an object
provides us with a unique laboratory to study the state of matter in the Universe.
The existing solar ground-based and space missions have already revealed several mysteries of the outer environment of our
Sun and much more is going to come in the near future from planned new sophisticated ground-based solar telescopes and Space
missions.
The new technique of helioseismology has unravelled many secrets of the solar interior and has put the Standard Solar Model
(SSM) on firm footing. The long-standing problem of solar neutrinos has been recently sorted out, and even the ‘back side’
view of the Sun can be seen using the technique of holographic helioseismology. 相似文献