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1.
M. E. Sachkov 《Astronomy Letters》2002,28(9):589-593
Based on our radial-velocity measurements and on published photometric observations, we calculated the radii of 64 classical Cepheids that were previously assumed to be fundamental-mode pulsators. Our detailed analysis of the period-radius diagram shows that the sample of Cepheids with pulsation periods shorter than 9 days probably contains a significant fraction (up to 30%) of stars pulsating in the first overtone. This fact leads to incorrect luminosity estimates for Cepheids and may be partly responsible for the discrepancy between the short and long distance scales. 相似文献
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银河系厚盘的发现,对于研究银河系以至星系的结构和演化具有重要意义。在简单回顾银河系结构研究史和厚盘发现过程的基础上,综合介绍了人们对银河系厚盘各方面性质认识的现状,并对迄今为止所提出的几种厚盘形成机制作了比较详细的说明和讨论。就目前来看,与伴星系的并合可能是形成厚盘最为可能的机制。 相似文献
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Summary. Due to the foreground extinction of the Milky Way, galaxies appear increasingly fainter the closer they lie to the Galactic
Equator, creating a “zone of avoidance” of about 25% in the distribution of optically visible galaxies. A “whole-sky” map
of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity
of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor
region.
Various dynamically important structures behind the Milky Way have only recently been made “visible” through dedicated deep
surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and
X-ray) for galaxies in the Zone of Avoidance are reviewed, including a discussion on the limitations and selection effects
of these partly complementary approaches. The uncovered and suspected large-scale structures are summarized. Reconstruction
methods of the density field in the Zone of Avoidance are described and the resulting predictions compared with observational
evidence. The comparison between reconstructed density fields and the observed galaxy distribution allow derivations of the
density and biasing parameters and b.
Received 4 April 2000 / Published online 18 July 2000 相似文献
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Michael Feast 《Monthly notices of the Royal Astronomical Society》1998,293(1):L27-L28
It is pointed out that a Cepheid period–luminosity relation with a zero-point from Hipparcos trigonometrical parallaxes and a consistent reddening system zero-point implies that some recent estimates of H 0 based on the Cepheid scale should be increased by ∼8 per cent. This result avoids using the distance to the Large Magellanic Cloud (LMC) as an intermediary point but is not significantly different from the result obtained by Feast & Catchpole via the LMC. A number of other issues are discussed, including metallicity effects on Cepheid distances and reddenings, and the age of metal-poor globular clusters. 相似文献
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《天文和天体物理学研究(英文版)》2018,(12)
The morphology and kinematics of the spiral structure of the Milky Way are long-standing problems in astrophysics.In this review we firstly summarize various methods with different tracers used to solve this puzzle.The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties, as we can currently obtain accurate parallaxes(a few μas) and proper motions(≈1 km s~(-1)) by using Very Long Baseline Interferometry(VLBI).On the other hand, the Gaia mission is providing the largest, uniform sample of parallaxes for O-type stars in the entire Milky Way.Based upon the VLBI maser and Gaia O-star parallax measurements, nearby spiral structures of the Perseus, Local, Sagittarius and Scutum Arms are determined in unprecedented detail.Meanwhile, we estimate fundamental Galactic parameters of the distance to the Galactic center,R_0, to be 8.35 ± 0.18 kpc, and circular rotation speed at the Sun, Θ_0, to be 240±10 km s~(-1).We found kinematic differences between O stars and interstellar masers: the O stars, on average, rotate faster,8 km s~(-1) than maser-traced high-mass star forming regions. 相似文献
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We use an N -body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given present observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years – within the Sun's lifetime. During the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late-forming elliptical galaxies. 相似文献
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造父变星周光(PL)关系对于宇宙距离尺度研究和哈勃常数测定具有重要意义,所涉及的内容颇为广泛,可包括两类不同星族造父变星的PL关系,PL关系的绝对定标,周光色(PLC)关系,PL关系的金属度效应,多波段PL关系,极大光度PL关系和多相PL关系,以及非线性形式的PL关系,等等。 相似文献
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The combination of dispersion measures of pulsars, distances from the model of Cordes & Lazio (2002) and emission measures from the WHAM survey enabled a statistical study of electron densities and filling factors of the diffuse ionized gas (DIG) in the Milky Way. The emission measures were corrected for absorption and contributions from beyond the pulsar distance. For a sample of 157 pulsars at |b | > 5. and 60° < ℓ < 360°, located in mainly interarm regions within about 3 kpc from the Sun, we find that: (1) The average volume filling factor along the line of sight and the mean density in ionized clouds are inversely correlated: ( ) = (0.0184 ± 0.0011) –1.07 ± 0.03 for the ranges 0.03 < < 2 cm–3 and 0.8 > > 0.01. This relationship is very tight. The inverse correlation of and causes the well‐known constancy of the average electron density along the line of sight. As (z ) increases with distance from the Galactic plane |z |, the average size of the ionized clouds increases with |z |. (2) For |z| < 0.9 kpc the local density in clouds n c(z ) and local filling factor f (z ) are inversely correlated because the local electron density n e(z ) = f (z )n c(z ) is constant. We suggest that f (z ) reaches a maximum value of >0.3 near |z | = 0.9 kpc, whereas n c(z ) continues to decrease to higher |z |, thus causing the observed flattening in the distribution of dispersion measures perpendicular to the Galactic plane above this height. (3) For |z | < 0.9 kpc the local distributions n c(z ), f (z ) and (z ) have the same scale height which is in the range 250 < h ≲ 500 pc. (4) The average degree of ionization of the warm atomic gas (z ) increases towards higher |z | similarly to (z ). Towards |z | = 1 kpc, (z ) = 0.24 ± 0.05 and (z ) = 0.24 ± 0.02. Near |z | = 1 kpc most of the warm, atomic hydrogen is ionized. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Yang-Shyang Li Simon D. M. White 《Monthly notices of the Royal Astronomical Society》2008,384(4):1459-1468
We use the very large Millennium Simulation of the concordance Λ cold dark matter cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way. From a large number of isolated spiral–spiral pairs similar to the Milky Way/Andromeda system, we find the interquartile range of the ratio of timing mass to true mass to be a factor of 1.8, while the 5 and 95 per cent points of the distribution of this ratio are separated by a factor of 5.7. Here, we define true mass as the sum of the 'virial' masses, M 200 , of the two dominant galaxies. For present best values of the distance and approach velocity of Andromeda, this leads to a median likelihood estimate of the true mass of the Local Group of 5.27 × 1012 M⊙ or log M LG /M⊙ = 12.72 , with an interquartile range of [12.58, 12.83] and a 5–95 per cent range of [12.26, 13.01]. Thus, a 95 per cent lower confidence limit on the true mass of the Local Group is 1.81 × 1012 M⊙ . A timing estimate of the Milky Way's mass based on the large recession velocity observed for the distant satellite Leo I works equally well, although with larger systematic uncertainties. It gives an estimated virial mass for the Milky Way of 2.43 × 1012 M⊙ with a 95 per cent lower confidence limit of 0.80 × 1012 M⊙ . 相似文献
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Adam Ružčka 《Astrophysics and Space Science》2003,284(2):519-522
A simple sticky-particle numerical model has been developed in order to check whether extended structures of gas created due
to the dynamical evolution of the Galaxy and the Magellanic Clouds system can be explained as remnants of a tidal interaction.
Influence of dissipative nature of gaseous medium has been taken into account. The most remarkable features are: the Magellanic
Stream, the common HI envelope surrounding both the LMC and SMC and the bridge extended between the Clouds. In contrast to
previous works of Murai and Fujimoto (1980), Gardiner et al. (1994) and H and Rohlfs (1994) no presumptions were done on the
actual galactocentric velocities of the Magellanic Clouds. The mean values of the LMC and SMC velocity vectors obtained from
the Hipparcos proper motion measurements (Kroupa and Bastian, 1997) were used in order to verify whether they allow to reproduce
the observed HI distribution. Numerical simulations showed that tidal forces are really significant for the evolution of extended
structures such as the Magellanic Stream but this approach becomes unsufficient for the internal regions of galaxies where
self-gravity and dissipative properties of the gas cannot be neglected. More precise proper motion measurements are urgently
needed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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A parametrized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scalelength R d* of the stellar disc to R 0 . The disc and bulge dominate v c ( R ) at R ≲ R 0 only for R d,* / R 0 ≲0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R ≪ R 0 to affect profoundly the structure of the best-fitting model at R ≫ R 0 . For example, changing the disc slightly from an exponential surface-density profile significantly changes the form of v c ( R ) at R ≫ R 0 , where the disc makes a negligible contribution to v c . Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct haloes rather than outwards-increasing mass-to-light ratios. The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z ≳ R and at R > R 0 are especially important. 相似文献
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Konrad Kuijken 《Astrophysics and Space Science》1999,267(1-4):217-226
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Manuel Metz Pavel Kroupa Helmut Jerjen 《Monthly notices of the Royal Astronomical Society》2007,374(3):1125-1145
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Globulettes are small (radii \({<} 10\) kAU) dark dust clouds, seen against the background of bright nebulae. A majority of the objects have planetary mass. These objects may be a source of brown dwarfs and free floating planetary mass objects in the galaxy. In this paper we investigate how many globulettes could have formed in the Milky Way and how they could contribute to the total population of free floating planets. In order to do that we examine H-alpha images of 27 H II regions. In these images, we find 778 globulettes.We find that a conservative value of the number of globulettes formed is \(5.7\times 10^{10}\). If 10% of the globulettes form free floating planets then they have contributed with \(5.7\times 10^{9}\) free floating planets in the Milky Way. A less conservative number of globulettes would mean that the globulettes could contribute \(2.0\times 10^{10}\) free floating planets. Thus the globulettes could represent a non-negligible source of free floating planets in the Milky Way. 相似文献