首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
We make use of a previous well-tested Galactic model, but describing the observational behaviour of the various stellar components in terms of suitable assumptions on their evolutionary status. In this way we are able to predict the expected distribution of Galactic white dwarfs (WDs), with results which appear in rather good agreement with recent estimates of the local WD luminosity function. The predicted occurrence of WDs in deep observations of selected Galactic fields is presented, and we discuss the role played by WDs in star counts. The effects on the theoretical predictions of different white dwarf evolutionary models, ages, initial mass functions and relations between progenitor mass and WD mass are also discussed.  相似文献   

2.
We have used the field stars from the open cluster survey BOCCE (Bologna Open Clusters Chemical Evolution), to study three low-latitude fields imaged with the Canada–France–Hawaii telescope (CFHT), with the aim of better understanding the Galactic structure in those directions. Because of the deep and accurate photometry in these fields, they provide a powerful discriminant among Galactic structure models. In the present paper we discuss if a canonical star count model, expressed in terms of thin and thick disc radial scales, thick disc normalization and reddening distribution, can explain the observed colour–magnitude diagrams (CMDs). Disc and thick disc are described with double exponentials, the spheroid is represented with a De Vaucouleurs density law. In order to assess the fitting quality of a particular set of parameters, the colour distribution and luminosity function of synthetic photometry is compared to that of target stars selected from the blue sequence of the observed CMDs. Through a Kolmogorov–Smirnov test, we find that the classical decomposition halo-thin/thick disc is sufficient to reproduce the observations – no additional population is strictly necessary. In terms of solutions common to all three fields, we have found a thick disc scalelength that is equal to (or slightly longer than) the thin disc scale.  相似文献   

3.
A comparison of observed stellar distributions with a three-component model of the Galaxy is presented. The analysis is based on photometric and photoelectric data obtained along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The assumed model considers the Galaxy as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. To model the observed colour distribution, we distinguish main sequence stars and disk red giants as the disk subsystem; white dwarfs, subdwarfs and intermediate giants as the thick disk subsystem; extreme subdwarfs, spheroid giants and horizontal branch stars as the spheroid subsystem. A statistical relation between the apparent and absolute magnitudes of stars which make the maximum contribution to the star counts for a given disk subsystem is derived. In order to achieve the best agreement between the model and observations, we fit the values of the ‘dip’ (aw) of the disk luminosity function, the correction to the absolute magnitude of disk red giants (ΔMVRG) and the expression for interstellar extinction. As the main result, we obtained aw = 0.6 (logarithmic scale) and ΔMVRG = 0.5 mag; the interstellar extinction has to be taken into account by the modified Sandage law.  相似文献   

4.
We performed a photometric multicolour survey of the core of the Canis Major overdensity at     , reaching   V ∼ 22  and covering  0.3 × 1.0  arcmin2. The main aim is to unravel the complex mixture of stellar populations toward this Galactic direction, where in the recent past important signatures of an accretion event have been claimed to be detected. While our previous investigations were based on disjointed pointings aimed at revealing the large-scale structure of the third Galactic Quadrant, we now focus on a complete coverage of a smaller field centred on the Canis Major overdensity. A large wavelength baseline, in the UBVRI bands, allows us to build up a suite of colour–colour and colour–magnitude diagrams, providing a much better diagnostic tool to disentangle the stellar populations of the region. In fact, the simple use of one colour–magnitude diagram, widely employed in all the previous studies defending the existence of the Canis Major galaxy, does not allow one to separate the effects of the different parameters (reddening, age, metallicity and distance) involved in the interpretation of data, forcing to rely on heavy modelling. In agreement with our previous studies, in the same general region of the Milky Way, we recognize a young stellar population compatible with the expected structure and extension of the Local (Orion) and Outer (Norma–Cygnus) spiral arms in the Third Galactic Quadrant. Moreover, we interpret the conspicuous intermediate-age metal-poor population as belonging to the Galactic thick disc, distorted by the effect of strong disc warping at this latitude, and to the Galactic halo.  相似文献   

5.
Using the stellar photometry catalogue based on the latest data release (DR4) of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using star counts is carried out for selected areas of the sky. The sample areas are selected along a circle at a Galactic latitude of +60°, and 10 strips of high Galactic latitude along different longitudes. Direct statistics of the data show that the surface densities of ℓ from 180° to 360° are systematically higher than those of ℓ from 0° to 180°, defining a region of overdensity (in the direction of Virgo) and another one of underdensity (in the direction of Ursa Major) with respect to an axisymmetric model. It is shown by comparing the results from star counts in the ( g − r ) colour that the density deviations are due to an asymmetry of the stellar density in the halo. Theoretical models for the surface density profile are built and star counts are performed using a triaxial halo of which the parameters are constrained by observational data. Two possible reasons for the asymmetric structure are discussed.  相似文献   

6.
One of the most promising space missions of the European Space Agency is the astrometric satellite Gaia , which will provide very precise astrometry and multicolour photometry, for all 1.3 billion objects to   V ∼ 20  , and radial velocities with accuracies of a few km s−1 for most stars brighter than   V ∼ 17  . Consequently, full homogeneous six-dimensional phase-space information for a huge number of stars will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs potentially observable by Gaia . From this we assess the white dwarf luminosity functions that Gaia will obtain and discuss in depth the scientific returns of Gaia in the specific field of white dwarf populations. Scientifically attainable goals include, among others, a reliable determination of the age of the Galactic disc, a better knowledge of the halo of the Milky Way and the reconstruction of the star formation history of the Galactic disc. Our results also demonstrate the potential impact of a mission such as Gaia within the context of current understanding of white dwarf cooling theory.  相似文献   

7.
We present the analysis of three-colour optical/near-infrared images, in IJK , taken for the DEep Near Infrared Southern Sky Survey (DENIS) project. The region considered covers 17.4 deg2 and lies within <5°, b <1.°5. The adopted methods for deriving photometry and astrometry in these crowded images, together with an analysis of the deficiencies nevertheless remaining, are presented. The numbers of objects extracted in I , J and K are 748 000, 851 000 and 659 000 respectively, to magnitude limits of 17, 15 and 13. Eighty per cent completeness levels typically fall at magnitudes 16, 13 and 10 respectively, fainter by about 2 mag than the usual DENIS limits as a result of the crowded nature of these fields. A simple model to describe the disc contribution to the number counts is constructed, and parameters for the dust layer are derived. We find that a formal fit of parameters for the dust plane, from these data in limited directions, gives a scalelength and scaleheight of 3.4±1.0 kpc and 40±5 pc respectively, and a solar position 14.0±2.5 pc below the plane. This latter value is likely to be affected by localized dust asymmetries. We convolve a detailed model of the systematic and random errors in the photometry with a simple model of the Galactic disc and dust distribution to simulate expected colourmagnitude diagrams. These are in good agreement with the observed diagrams, allowing us to isolate those stars from the inner disc and bulge. After correcting for local dust-induced asymmetries, we find evidence for longitude-dependent asymmetries in the distant J and K sources, consistent with the general predictions of some Galactic bar models. We consider complementary L -band observations in the companion paper.  相似文献   

8.
Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and α-elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva–Copenhagen survey (GCS) of the solar neighbourhood and a good fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age–metallicity plane, although this plane was not used in the fitting process. Although this model's star formation rate is monotonically declining, its disc naturally splits into an α-enhanced thick disc and a normal thin disc. In particular, the model's distribution of stars in the ([O/Fe], [Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration, and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high  σ z   from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of  σ z   with age than predicted by theory. The metallicity gradient in the interstellar medium is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The models are inconsistent with a cut-off in the star formation rate at low gas surface densities. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.  相似文献   

9.
The halo structure at high Galactic latitudes near both the north and south poles is studied using Sloan Digital Sky Survey (SDSS) and SuperCOSMOS data. For the south cap halo, the archive of the SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in B J band from 16.5 to 20.5 mag and SDSS data is about 92 per cent, in a common sky area in the south. While that in the R F band is about 85 per cent from 16.5 to 19.5 mag. Transformed to the SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the North Galactic Cap from SDSS show up to an  16.9 ± 6.3  per cent  asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in the B J band, and up to  13.5 ± 6.7  per cent  asymmetric ratio in the R F band. From SuperCOSMOS B J and R F bands, the structure of the Southern Galactic hemisphere does not show the same obvious asymmetric structures as the northern sky does in both the original and downgraded SDSS star counts. An axisymmetric halo model with n = 2.8 and q = 0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17 per cent. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is shown that no asymmetry can be detected in the South Galactic Cap at the accuracy of SuperCOSMOS, and the Virgo overdensity is likely a foreign component in the Galactic halo.  相似文献   

10.
For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour‐magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g ′, r ′, and i ′ show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems, especially in the (r – i) colours. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
We analyse 20 star cluster candidates projected mostly in the bulge direction  (|ℓ| < 60°)  . The sample contains all candidates in that sector classified by Froebrich, Scholz & Raftery with quality flags denoting high probability of being star clusters. Bulge contamination in the colour–magnitude diagrams (CMDs) is in general important, while at lower Galactic latitudes disc stars contribute as well. Properties of the candidates are investigated with Two Micron All Sky Survey (2MASS) CMDs and stellar radial density profiles (RDPs) built with field star decontaminated photometry. To uncover the nature of the structures we decontaminate the CMDs from field stars using tools that we previously developed to deal with objects in dense fields. We confirm in all cases excesses in the RDPs with respect to the background level, as expected from the method the candidates were originally selected. CMDs and RDPs taken together revealed six open clusters, five uncertain cases that require deeper observations, while nine objects are possibly field density fluctuations.  相似文献   

12.
There is increasing evidence of a local population of short duration gamma-ray bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible luminosity functions (LFs) for both neutron star binary mergers and giant flares from soft gamma repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall Burst and Transient Source Experiment (BATSE) sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best-fitting parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population.  相似文献   

13.
We parametrized the total structure of the Galaxy in cylindrical coordinates up to z ∼ 10 kpc by using radial and vertical exponentials, covering the thin and thick disc and the inner spheroid. However, we allowed the scale height and scale length to vary continuously with distance from the Galactic plane. The standard deviations for the differences between stellar space densities estimated by means of the newly defined scale heights and scale lengths and the observed space densities for three absolute magnitude intervals, 5 < M (g ′) ≤ 6, 6 < M (g ′) ≤ 7, and 7 < M (g ′) ≤ 8, of the fields SA 114, ELAIS, and # 0952+5245 are rather small. The uncertainties of the scale heights are also small, indicating that this parameter depends very sensitively on the distance from the Galactic plane, whereas those of the scale lengths are larger. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
History of Star Formation and Chemical Enrichment in the Milky Way Disk   总被引：2，自引：0，他引：2  
Based on a physical treatment of the star formation law similar to that given by Efstathiou, we have improved our two-component chemical evolution model for the Milky Way disk. Two gas infall rates are compared, one exponential, one Gaussian. It is shown that the star formation law adopted in this paper depends more strongly on the gas surface density than that in Chang et al. It has large effects on the history of star formation and gas evolution of the whole disk. In the solar neighborhood, the history of chemical evolution and star formation is not sensitive to whether the infall rate is Gaussian or exponential. For the same infall time scale, both forms predict the same behavior for the current properties of the Galactic disk. The model predictions do depend on whether or not the infall time scale varies with the radius, but current available observations cannot decide which case is the more realistic. Our results also show that it would be inadequate to describe the gradient evolution along the Gala  相似文献   

15.
We present near-infrared colour–magnitude diagrams and star counts for a number of regions along the Galactic plane. It is shown that along the l =27°, b =0° line of sight there is a feature at 5.7±0.7 kpc with a density of stars at least a factor of 2 and probably more than a factor of 5 times that of the disc at the same position. This feature forms a distinct clump on an H versus J − H diagram and is seen at all longitudes from the bulge to about l =28°, but at no longitude greater than this. The distance to the feature at l =20° is about 0.5 kpc further than at l =27°, and by l =10° it has merged with, or has become, the bulge. Given that at l =27° and l =21° there is also a clustering of very young stars, the only component that can reasonably explain what is seen is a bar with half-length of around 4 kpc and a position angle of about 43°±7°.  相似文献   

16.
We present survey data in the narrow-band L filter (nb L ), taken at UKIRT, for a total area of 277 arcmin2, roughly equally divided between four regions at zero Galactic latitude and longitudes ±4.°3 and ±2.°3. The 80 per cent completeness level for these observations is at roughly magnitude 11.0. This magnitude limit, owing to the low coefficient for interstellar extinction at this wavelength ( A nb L =0.047 AV ), allows us to observe bulge giants. We match the nb L magnitudes with DENIS survey K magnitudes, and find 95 per cent of nb L sources are matched to K sources. Constructing colour–magnitude diagrams, we deredden the magnitudes and find evidence for a longitude-dependent asymmetry in the source counts. We find that there are ∼15 per cent and ∼5 per cent more sources at the negative longitude than at the corresponding positive longitude for the fields at ±4.°3 and ±2.°3, respectively. This is compared with the predictions of some Galactic bar models. We find an asymmetry in the expected sense, which favours gas dynamical models and the recent deconvolution of surface photometry data over earlier treatments of photometric data.  相似文献   

17.
This paper presents an analysis of the first 2MASS (The Two Micron All Sky Survey) sampler data as observed at lower Galactic latitude in our Galaxy. These new near-infrared data provide insight into the structure of the thin disk of our Galaxy, The interpretation of star counts and color distributions of stars in the near-infrared with the synthetic stellar population model, gives strong evidence that the Galactic thin disk density scale length,h R , is rather short (2.7 ± 0.1 kpc).  相似文献   

18.
We perform an evolutionary multivariate analysis of a sample of 54 Galactic globular clusters with high-quality colour–magnitude diagrams and well-determined ages. The four parameters adopted for the analysis are: metallicity, age, maximum temperature on the horizontal branch and absolute V magnitude. Our cladistic analysis breaks the sample into three novel groups. An a posteriori kinematical analysis puts groups 1 and 2 in the halo, and group 3 in the thick disc. The halo and disc clusters separately follow a luminosity–metallicity relation of much weaker slope than galaxies. This property is used to propose a new criterion for distinguishing halo and disc clusters. A comparison of the distinct properties of the two halo groups with those of Galactic halo field stars indicates that the clusters of group 1 originated in the inner halo, while those of group 2 formed in the outer halo of the Galaxy. The inner halo clusters were presumably initially the most massive one, which allowed the formation of more strongly helium-enriched second generation stars, thus explaining the presence of Cepheids and of very hot horizontal-branch stars exclusively in this group. We thus conclude that the 'second parameter' is linked to the environment in which globular clusters form, the inner halo favouring the formation of the most massive clusters which subsequently become more strongly self-enriched than their counterparts of the galactic outer halo and disc.  相似文献   

19.
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   

20.
The velocity dispersion of stars in the solar neighbourhood thin disc increases with time after star formation. Nordström et al. performed the most recent observations to constrain the age–velocity dispersion relation. They fitted the age–velocity dispersion relations of each Galactic cardinal direction space velocity component, U (towards the Galactic Centre), V (in the direction of Galactic rotation) and W (towards the North Galactic Pole), with power laws and interpreted these as evidence for continuous heating of the disc in all directions throughout its lifetime. We revisit these relations with their data and use the results of Famaey et al. to show that structure in the local velocity distribution function distorts the in-plane ( U and V ) velocity distributions away from Gaussian so that a dispersion is not an adequate parametrization of their functions. The age–σ W relation can however be constrained because the sample is well phase-mixed vertically. We do not find any local signature of the stellar warp in the Galactic disc. Vertical disc heating does not saturate at an early stage. Our new result is that a power law is not required by the data: disc heating models that saturate after ∼4.5 Gyr are equally consistent with observations.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号