共查询到20条相似文献，搜索用时 53 毫秒
1.
2.
3.
Raul Jimenez Chris Flynn & Eira Kotoneva 《Monthly notices of the Royal Astronomical Society》1998,299(2):515-519
We use the Hipparcos colour–magnitude diagram of field stars with Tycho colours to make a new minimum age estimate for the Galactic disc. The method is based on fits to the red envelope of subgiants in the Hipparcos colour–magnitude diagram with synthetic isochrones covering the range of disc metal abundance. The colours and luminosities of the isochrones as a function of abundance are checked using new techniques involving 'red-clump' stars in the giant branch region and on the main sequence using G and K dwarfs. We derive a minimum disc age of 8 Gyr, in good agreement with other methods. 相似文献
4.
5.
6.
7.
8.
Agostino Renda Daisuke Kawata Yeshe Fenner Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2005,356(3):1071-1078
The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass. 相似文献
9.
I. G. McCarthy A. Babul R. G. Bower M. L. Balogh 《Monthly notices of the Royal Astronomical Society》2008,386(3):1309-1331
X-ray clusters are conventionally divided into two classes: 'cool core' (CC) clusters and 'non-cool core' (NCC) clusters. Yet relatively little attention has been given to the origins of this apparent dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either CC or NCC clusters. The inferred entropy profiles of NCC clusters require that material is 'pre-heated' prior to cluster collapse in order to explain the absence of low-entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their entropy profiles at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent active galactic nuclei (AGN) feedback loop in which the lowest entropy, most rapidly cooling material is heated and rises buoyantly to mix with material at larger radii. The resulting model does not require fine-tuning and is in excellent agreement with a wide variety of observational data from Chandra and XMM–Newton , including entropy and gas density profiles, the luminosity–temperature relation and high-resolution spectra. The spread in cluster core morphologies is seen to arise because of the steep dependence of the central cooling time on the initial level of pre-heating. Some of the other implications of this model are briefly discussed. 相似文献
10.
Nazirah N. Jetha Irini Sakelliou Martin J. Hardcastle Trevor J. Ponman Ian R. Stevens 《Monthly notices of the Royal Astronomical Society》2005,358(4):1394-1404
We present Chandra and Very Large Array observations of two galaxy clusters, Abell 160 and Abell 2462, whose brightest cluster galaxies (BCGs) host wide angle tailed radio galaxies (WATs). We search for evidence of interactions between the radio emission and the hot, X-ray emitting gas, and we test various jet termination models. We find that both clusters have cool BCGs at the cluster centre, and that the scale of these cores (∼30–40 kpc for both sources) is of approximately the same scale as the length of the radio jets. For both sources, the jet flaring point is coincident with a steepening in the host cluster's temperature gradient, and similar results are found for 3C 465 and Hydra A. However, none of the published models of WAT formation offers a satisfactory explanation as to why this may be the case. Therefore, it is unclear what causes the sudden transition between the jet and the plume. Without accurate modelling, we cannot ascertain whether the steepening of the temperature gradient is the main cause of the transition, or merely a tracer of an underlying process. 相似文献
11.
12.
We performed an extensive non-LTE analysis of the neutral sodium lines of Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo stars of types F-K covering a wide metallicity range (-4 (?) [Fe/H] (?) +0.4), using our own data as well as data collected from the literature. For comparatively metal-rich disk stars (-1(?) [Fe/H] (?) +0.4) where the weaker 6154/6161 lines are the best abundance indicators, we confirmed [Na/Fe]- 0 with an "upturn" (i.e., a shallow/broad dip around -0.5 (?) [Fe/H](?) 0) as already reported in previous studies. For the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines subject to considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly "subsolar" [Na/Fe] values down to--0.4 (with a somewhat large scatter of-±0.2 dex) on the average at the typical halo metallicity of [Fe/H] --2, followed by a rise again to a near-solar ratio of [Na/Fe] - 0 at the very metal-poor regime [Fe/H]--3 to -4 相似文献
13.
14.
S. García-Burillo F. Combes J. Graciá-Carpio A. Usero M. Guélin 《Astrophysics and Space Science》2008,313(1-3):261-265
High-resolution CO maps are an essential tool to search for observational evidence of AGN fueling in galaxy nuclei. While
their capabilities will be surpassed by ALMA, current mm-interferometers can already provide relevant information on scales
which are critical for the process of angular momentum transfer in fueling the AGN. In this context we present the latest
results issued from the NUclei of GAlaxies (NUGA) project, a high-resolution (0.5′′–1′′) CO survey of low luminosity AGNs conducted with the IRAM Plateau de Bure interferometer (PdBI). The use of more specific
molecular tracers of dense gas can probe the feedback influence of activity on the chemistry and energy balance in the interstellar
medium of nearby galaxies, a prerequisite to understanding how feedback operate at higher redshift galaxies. We discuss the
results obtained in an ongoing study devoted to probe the feedback of activity from nearby Seyferts to high-redshift QSO. 相似文献
15.
C. Tadhunter T. G. Robinson R. M. González Delgado K. Wills R. Morganti 《Monthly notices of the Royal Astronomical Society》2005,356(2):480-494
We present high-quality long-slit spectra for three nearby powerful radio galaxies – 3C 293, 3C 305 and PKS 1345+12. These were taken with the aim of characterizing the young stellar populations (YSP), and thereby investigating the evolution of the host galaxies, as well as the events that triggered the activity. Isochrone spectral synthesis modelling of the wide wavelength coverage spectra of nuclear and off-nuclear continuum-emitting regions have been used to estimate the ages, masses and luminosities of the YSP component, taking full account of reddening effects and potential contamination by activity-related components. We find that the YSP make a substantial contribution to the continuum flux in the off-nuclear regions on a radial scale of 1–20 kpc in all three objects. Moreover, in two objects we find evidence for reddened post-starburst stellar populations in the near-nuclear regions of the host galaxies. The YSP are relatively old (0.1–2 Gyr), massive (109 < M YSP < 2 × 1010 M⊙ ) and make up a large proportion (∼1–50 per cent) of the total stellar mass in the regions of the galaxies sampled by the observations. Overall, these results are consistent with the idea that the nuclear activity of active galactic nuclei in some radio galaxies is triggered by major gas-rich mergers. Therefore, these radio galaxies form part of the subset of early-type galaxies that is evolving most rapidly in the local Universe. Intriguingly, the results also suggest that the radio jets are triggered relatively late in the merger sequence, and that there is an evolutionary link between radio galaxies and luminous/ultraluminous infrared galaxies. 相似文献
16.
17.
We present two new sets of complete light curves of EQ Tauri (EQ Tau) observed in 2000 October and 2004 December. These were analysed, together with the light curves obtained by Yang & Liu in 2001 December, with the 2003 version of the Wilson–Devinney code. In the three observing seasons, the light curves show a noticeable variation in the time-scale of years. The more massive component of EQ Tau is a solar-type star (G2) with a very deep convective envelope, which rotates about 80 times as fast as the Sun. Therefore, the change can be explained by dark-spot activity on the common convective envelope. The assumed unperturbed part of the light curve and the radial velocities published by Rucinski et al. were used to determine the basic parameters of the system, which were kept fixed for spot modelling in the three sets of light curves. The results reveal that the total spotted area on the more massive component covers 18, 3 and 20 per cent of the photospheric surface in the three observing seasons, respectively. Polar spots and high-latitude spots are found. The analysis of the orbital period has demonstrated that it undergoes cyclical oscillation, which is due to either a tertiary component or periodic magnetic activity in the more massive component. 相似文献
18.
Paolo A. Mazzali 《Monthly notices of the Royal Astronomical Society》2001,321(2):341-346
The spectrum of the normal Type Ia SN 1990N observed very early on (14 days before B maximum) was analysed by Fisher et al., who showed that the large width and the unusual profile of the strong line near 6000 Å can be reproduced if the line is assumed to be due to C ii 6578, 6583 Å and if carbon is located in a high-velocity shell. This line is one of the characterizing features of SNe Ia, and is usually thought to be due to Si ii . A Monte Carlo spectrum synthesis code is used to investigate this suggestion further. The result is that if a standard explosion model is used, the mass enclosed in the shell at the required high velocity (25 000–35 000 km s−1 ) is too small to give rise to a strong C ii line. At the same time, removing silicon has a negative effect on the synthetic spectrum at other wavelengths, and removing carbon from the lower velocity regions near the photosphere makes it difficult to reproduce two weak lines which are naturally explained as C ii , one of them being the line which Fisher et al. suggested is responsible for the strong 6000-Å feature. However, synthetic spectra confirm that although Si ii can reproduce most of the observed 6000-Å line, the red wing of the line extends too far to be compatible with a Si ii origin, and that the flat bottom of the line is also not easy to reproduce. The best fit is obtained for a normal SN Ia abundance mix at velocities near the photosphere (15 500–19 000 km s−1 ) and an outer carbon–silicon shell beyond 20 000 km s−1 . This suggests that mixing is not complete in the outer ejecta of an SN Ia. Observations at even earlier epochs might reveal to what extent a carbon shell is unmixed. 相似文献
19.
20.
Flux Calibration of the BATC Survey and Its Application for Checking the Consistency of the Oke—Gunn Standards 总被引：1，自引：0，他引：1
1 INTRODUCTIONThe B eij lug- Ahs ona~ TaiP ei- C onnect lout Color S urvey of t he S by (Hereaft er B AT C ) ut driesthe 15 intermediate band filters to make CCD forage photometric ohs~ion. The BATCphotometric system ties its maghtude zero P~Oint to the spectro-photometric AB maghtudesystem. The AB system is a monochro~ic fi system fort introduced by Oke in 1969 with aprovisional calibration designated AB69.The AB system selects F subdwarfs around visual magnitude 9 as standa… 相似文献