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1.
A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc−1 and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established.
Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal-rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal-rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.  相似文献   

2.
Taking advantage of recent Hubble Space Telescope ( HST ) data for field stars in the region of the Galactic globular cluster NGC 6397, we tested the predictions of several Galactic models with star counts reaching a largely unexplored range of magnitudes, down to V ∼26.5. After updating the input stellar ( V − I ) colours, we found that the two-component Bahcall–Soneira (B&S) model can be put into satisfactory agreement with observations for suitable choices of disc/spheroid luminosity functions (LFs). However, if one assumes the disc LF of Gould, Bahcall and Flynn together with the spheroid LF of Gould, Flynn and Bahcall, there is no way to reconcile the predicted and observed V -magnitude distribution. We also analysed the agreement between observed and predicted magnitude and colour distributions for two selected models with a thick disc component. Even in this case there are suitable combinations of model parameters and faint-magnitude LFs that can give a reasonable agreement with observational star counts in both magnitude and colour. However, the above-quoted combination of Gould et al. LFs again gives predictions in clear disagreement with observations.  相似文献   

3.
The kinematics of Galactic C-Miras are discussed on the basis of the bolometric magnitudes and radial velocities of Papers I and II of this series. Differential Galactic rotation is used to derive a zero-point for the bolometric period–luminosity relation which is in satisfactory agreement with that inferred from the Large Magellanic Cloud (LMC) C-Miras. We find for the Galactic Miras,   M bol=−2.54 log  P + 2.06(±0.24)  , where the slope is taken from the LMC. The mean velocity dispersion, together with the data of Nordström et al. and the Padova models, leads to a mean age for our sample of C-Miras of  1.8 ± 0.4 Gyr  and a mean initial mass of  1.8 ± 0.2 M⊙  . Evidence for a variation of velocity dispersion with period is found, indicating a dependence of period on age and initial mass, the longer period stars being younger. We discuss the relation between the O- and C-Miras and also their relative numbers in different systems.  相似文献   

4.
Purely gravitational perturbations are considered in a thin rotating disc composed of gas and several stellar components. The dispersion relation for the axisymmetric density waves propagating through the disc is found and the criterion for the local axisymmetric stability of the whole system is formulated. In the appropriate limit of two-component gas we confirm the findings of Jog & Solomon and extend consideration to the case when one component is collisionless. Gravitational stability of the Galactic disc in the solar neighbourhood based on the multicomponent instability condition is explored using recent measurements of the stellar composition and kinematics in the local Galactic disc obtained by the Hipparcos satellite.  相似文献   

5.
A new method to measure the epicycle frequency κ in the Galactic disc is presented. We make use of the large data base on open clusters completed by our group to derive the observed velocity vector (amplitude and direction) of the clusters in the Galactic plane. In the epicycle approximation, this velocity is equal to the circular velocity given by the rotation curve, plus a residual or perturbation velocity, of which the direction rotates as a function of time with the frequency κ. Due to the non-random direction of the perturbation velocity at the birth time of the clusters, a plot of the present-day direction angle of this velocity as a function of the age of the clusters reveals systematic trends from which the epicycle frequency can be obtained. Our analysis considers that the Galactic potential is mainly axis-symmetric, or in other words, that the effect of the spiral arms on the Galactic orbits is small; in this sense, our results do not depend on any specific model of the spiral structure. The values of κ that we obtain provide constraints on the rotation velocity of the disc; in particular, V 0 is found to be  230 ± 15 km s−1  even if the short scale  ( R 0= 7.5 kpc)  of the Galaxy is adopted. The measured κ at the solar radius is  43 ± 5 km s−1 kpc −1  . The distribution of initial velocities of open clusters is discussed.  相似文献   

6.
We determine the velocity distribution and space density of a volume-complete sample of A and F stars, using parallaxes and proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density of 0.102±0.010 pc−3, which may be compared with an estimate of 0.095 M⊙ pc−3 in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Crézé et al. and Pham, also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.  相似文献   

7.
Chemical evolution of the Magellanic Clouds: analytical models   总被引：1，自引：0，他引：1  
We have extended our analytical chemical evolution modelling ideas for the Galaxy to the Magellanic Clouds. Unlike previous authors (Russell &38; Dopita, Tsujimoto et al. and Pilyugin), we assume neither a steepened initial mass function nor selective galactic winds, since among the α-particle elements only oxygen shows a large deficit relative to iron and a similar deficit is also found in Galactic supergiants. Thus we assume yields and time delays identical to those that we previously assumed for the solar neighbourhood. We include inflow and non-selective galactic winds and consider both smooth and bursting star formation rates, the latter giving a better fit to the age–metallicity relations. We predict essentially solar abundance ratios for primary elements and these seem to fit most of the data within their substantial scatter. Our model for the Large Magellanic Cloud also gives a remarkably good fit to the anomalous Galactic halo stars discovered by Nissen &38; Schuster.   Our models predict current ratios of Type Ia supernova to core-collapse supernova rates enhanced by 50 and 25 per cent respectively relative to the solar neighbourhood, in fair agreement with ratios found by Cappellaro et al. for Sdm–Im relative to Sbc galaxies, but these ratios are sensitive to detailed assumptions about the bursts and a still higher enhancement in the Large Magellanic Cloud has been deduced from X-ray studies of remnants by Hughes et al. The corresponding ratios integrated over time up to the present are slightly below 1, but they exceed 1 if one compares the Magellanic Clouds with the Galaxy at times when it had the corresponding metallicities.  相似文献   

8.
The central arcminute of the Perseus cooling flow galaxy, NGC 1275, has been mapped with the JCMT in 12CO(2–1) at 21-arcsec resolution, with detections out to at least 36 arcsec (12 kpc). Within the limits of the resolution and coverage, the distribution of gas appears to be roughly east–west, consistent with previous observations of CO, X-ray, Hα and dust emission. The total detected molecular hydrogen mass is ∼ 1.6 × 1010 M⊙, using a Galactic conversion factor. The inner central rotating disc is apparent in the data, but the overall distribution is not one of rotation. Rather, the line profiles are bluewards-asymmetric, consistent with previous observations in H  i and [O  iii ]. We suggest that the blueshift may be due to an acquired mean velocity of ∼ 150 km s−1 imparted by the radio jet in the advancing direction. Within the uncertainties of the analysis, the available radio energy appears to be sufficient, and the interpretation is consistent with that of Bo¨hringer et al. for displaced X-ray emission. We have also made the first observations of 13CO(2–1) and 12CO(3–2) emission from the central 21-arcsec region of NGC 1275 and combined these data with IRAM data supplied by Reuter et al. to form line ratios over equivalent, well-sampled regions. An LVG radiative transfer analysis indicates that the line ratios are not well reproduced by single values of kinetic temperature, molecular hydrogen density and abundance per unit velocity gradient. At least two temperatures are suggested by a simple two-component LVG model, possibly reflecting a temperature gradient in this region.  相似文献   

9.
Large samples of field horizontal branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ∼7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially dominated orbits to tangentially dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic Centre might help to differentiate them.  相似文献   

10.
The diameter-velocity dispersion relation in B, V, and K for three early-type galaxies in the Leo I (M96) group is derived from published photometry and kinematic data. The relations in all three colors have slopes which agree well with those for the Coma cluster. The RMS scatter of the Leo I galaxies in each color is extremely small, consistent with the group's compactness. These relations yield estimates of the Coma-Leo I distance ratio of 9.01 ± 0.51, 8.77 ± 0.43, and 8.82 ± 0.31, respectively, with a weighted mean of 8.84 ± 0.23. The general agreement among the three colors indicates that the early-type galaxies in Leo I and Coma have similar stellar populations.

The Coma-Leo I distance ratio coupled with estimates of the absolute distance to the Leo I group allows the Hubble constant to be determined, free of the uncertainties which arise when working with the Virgo cluster. Several high quality distance estimates are available from a variety of techniques: Cepheids in M96 (Tanvir, N.R., et al., 1995, Natur, 377, 27) and M95 (Graham, J.A., et al., 1997, ApJ, 477, 535), surface brightness fluctuations (Tonry, J.L., et al., 1997, ApJ, 475, 399), planetary nebulae luminosity functions (Ciardullo, R., et al., 1993, ApJ, 419, 479), and the luminosity of the red giant branch tip (Sakai, S., Freedman, W.L., & Madore, B.F., 1996, in: Formation of the Galactic Halo, Inside and Out, eds. H. Morrison & A. Sarajedini, PASP Conf. Series Vol. 92). Adopting a cosmic recession velocity of the Coma cluster in the microwave background frame of 7200 ± 300 km s−1, these distance estimates lead to values of the Hubble constant ranging from 70 to 81 km s−1 Mpc−1, with an unweighted mean of 75 ± 6 km s−1 Mpc−1.  相似文献   


11.
We use proper motions and parallaxes from the new reduction of Hipparcos data and Geneva–Copenhagen radial velocities for a complete sample of  ∼15 000  main-sequence and subgiant stars, and new Padova isochrones to constrain the kinematics and star formation history of the solar neighbourhood. We rederive the solar motion and the structure of the local velocity ellipsoids. When the principal velocity dispersions are assumed to increase with time as   t β  , the index β is larger for  σ W (β W ≈ 0.45  ) than for  σ U (β U ≈ 0.31)  . For the three-dimensional velocity dispersion, we obtain  β= 0.35  . We exclude saturation of disc heating after  ∼3 Gyr  as proposed by Quillen & Garnett. Saturation after  ≳4 Gyr  combined with an abrupt increase in velocity dispersion for the oldest stars cannot be excluded. For all our models, the star formation rate (SFR) is declining, being a factor of 2–7 lower now than it was at the beginning. Models in which the SFR declines exponentially favour very high disc ages between 11.5 and 13 Gyr and exclude ages below  ∼10.5 Gyr  as they yield worse fits to the number density and velocity dispersion of red stars. Models in which the SFR is the sum of two declining exponentials representing the thin and thick discs favour ages between 10.5 and 12 Gyr with a lower limit of  ∼10.0 Gyr  . Although in our models the SFR peaked surprisingly early, the mean formation time of solar-neighbourhood stars is later than in ab initio models of galaxy formation, probably on account of weaknesses in such models.  相似文献   

12.
The space motions of Mira variables are derived from radial velocities, Hipparcos proper motions and a period–luminosity relation. The previously known dependence of Mira kinematics on the period of pulsation is confirmed and refined. In addition, it is found that Miras with periods in the range 145–200 d in the general Solar neighbourhood have a net radial outward motion from the Galactic Centre of 75±18 km s−1. This, together with a lag behind the circular velocity of Galactic rotation of 98±19 km s−1, is interpreted as evidence for an elongation of their orbits, with their major axes aligned at an angle of ∼17° with the Sun–Galactic Centre line, towards positive Galactic longitudes. This concentration seems to be a continuation to the Solar circle and beyond of the bar-like structure of the Galactic bulge, with the orbits of some local Miras probably penetrating into the bulge. These conclusions are not sensitive to the distance scale adopted. A further analysis is given of the short-period (SP) red group of Miras discussed in companion papers in this series. In Appendix A the mean radial velocities and other data for 842 oxygen-rich Mira-like variables are tabulated. These velocities were derived from published optical and radio observations.  相似文献   

13.
We present and analyse the kinematics and orbits for a sample of 488 open clusters (OCs) in the Galaxy. The velocity ellipsoid for our present sample is derived as  (σ U , σ V , σ W ) = (28.7, 15.8, 11.0) km s−1  which represents a young thin-disc population. We also confirm that the velocity dispersions increase with the age of a cluster subsample. The orbits of OCs are calculated with three Galactic gravitational potential models. The errors of orbital parameters are also calculated considering the intrinsic variation of the orbital parameters and the effects of observational uncertainties. The observational uncertainties dominate the errors of derived orbital parameters. The vertical motions of clusters calculated using different Galactic disc models are rather different. The observed radial metallicity gradient of clusters is derived with a slope of   b =−0.070 ± 0.011   dex kpc−1. The radial metallicity gradient of clusters based on their apogalactic distances is also derived with a slope of   b =−0.082 ± 0.014   dex kpc−1. The distribution of derived orbital eccentricities for OCs is very similar to that derived for the field population of dwarfs and giants in the thin disc.  相似文献   

14.
We present mid-infrared (MIR) photometry for 367 Galactic disc, bulge and Large Magellanic Cloud (LMC) planetary nebulae (PNe), determined using data acquired with the Spitzer Space Telescope , and through the Legacy Programs GLIMPSE II (Galactic Legacy Infrared Mid-plane Survey Extraordinaire II) and SAGE (Surveying the Agents of the Galaxy's Evolution). This has permitted us to make a comparison between the luminosity functions of bulge and LMC PNe, and between the MIR colours of all three categories of source. It is determined that whilst the  3.6 μm  luminosity functions of the LMC and bulge sources are likely to be closely similar, the [3.6]–[5.8] and [5.8]–[8-0] indices of LMC nebulae are different from those of their disc and bulge counterparts. This may arise because of enhanced  6.2 μm  polycyclic aromatic hydrocarbon emission within the LMC sources, and/or as a result of further, and more radical differences between the spectra of LMC and Galactic PNe. We also determine that the more evolved disc sources listed in the Macquarie/AAO/Strasbourg (MASH) catalogues of Parker et al. and Miszalski et al. have similar colours to those of the less evolved (and higher surface brightness) sources in the catalogue of Acker et al., a result which appears at variance with previous studies of these sources.  相似文献   

15.
The huge size and uniformity of the Sloan Digital Sky Survey (SDSS) make possible an exacting test of current models of galaxy formation. We compare the predictions of the galform semi-analytical galaxy formation model for the luminosities, morphologies, colours and scalelengths of local galaxies. galform models the luminosity and size of the disc and bulge components of a galaxy, and so we can compute quantities which can be compared directly with SDSS observations, such as the Petrosian magnitude and the Sérsic index. We test the predictions of two published models set in the cold dark matter cosmology: the Baugh et al. model, which assumes a top-heavy initial mass function (IMF) in starbursts and superwind feedback, and the Bower et al. model, which uses active galactic nucleus feedback and a standard IMF. The Bower et al. model better reproduces the overall shape of the luminosity function, the morphology–luminosity relation and the colour bimodality observed in the SDSS data, but gives a poor match to the size–luminosity relation. The Baugh et al. model successfully predicts the size–luminosity relation for late-type galaxies. Both models fail to reproduce the sizes of bright early-type galaxies. These problems highlight the need to understand better both the role of feedback processes in determining galaxy sizes, in particular the treatment of the angular momentum of gas reheated by supernovae, and the sizes of the stellar spheroids formed by galaxy mergers and disc instabilities.  相似文献   

16.
The analysis of the kinematics of solar neighbourhood stars shows that the low- and high-metallicity tails of the thin disc are populated by objects which orbital properties suggest an origin in the outer and inner Galactic disc, respectively. Signatures of radial migration are identified in various recent samples, and are shown to be responsible for the high-metallicity dispersion in the age–metallicity distribution. Most importantly, it is shown that the population of low-metallicity wanderers of the thin disc (−0.7 < [Fe/H] < −0.3 dex) is also responsible for the apparent hiatus in metallicity with the thick disc (which terminal metallicity is about −0.2 dex). It implies that the thin disc at the solar circle has started to form stars at about this same metallicity. This is also consistent with the fact that 'transition' objects, which have α-element abundance intermediate between that of the thick and thin discs, are found in the range [−0.4, −0.2] dex. Once the metal-poor thin disc stars are recognized for what they are – wanderers from the outer thin disc – the parenthood between the two discs can be identified on stars genuinely formed at the solar circle through an evolutionary sequence in [α/Fe] and [Fe/H]. Another consequence is that stars that can be considered as truly resulting of the chemical evolution at the solar circle have a metallicity restricted to about [−0.2, +0.2] dex, confirming an old idea that most chemical evolution in the Milky Way have preceded the thin disc formation.  相似文献   

17.
We present new B ,  V and R linear polarimetric observations for 61 stars towards the region of the young open cluster NGC 654. In this study we found evidence for the presence of at least two layers of dust along the line of sight to the cluster. The distances to the two dust layers are estimated to be ∼200 pc and ∼1 kpc which are located much closer to the Sun than the cluster (∼2.4 kpc). Both the dust layers have their local magnetic field orientation nearly parallel to the direction of the Galactic plane. The foreground dust layer is found to have a ring morphology with the central hole coinciding with the centre of the cluster. The foreground dust grains are suggested to be mainly responsible for both the observed differential reddening and the polarization towards the cluster.  相似文献   

18.
We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of  1.7 ± 0.2 h  , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc.  相似文献   

19.
We show how the continuity equation can be used to determine pattern speeds in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine & Weinberg in the context of external galaxies, requires projected positions, ( l , b ), and line-of-sight velocities for a spatially complete sample of relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the radial direction ( u LSR), then the quantity that is measured is  Δ V ≡Ωp R 0- V LSR  , where Ωp is the pattern speed of the non-axisymmetric feature, R 0 is the distance of the Sun from the Galactic centre and V LSR is the tangential motion of the LSR, including the circular velocity. We use simple models to assess the reliability of the method for measuring a single, constant pattern speed of either a bar or spiral in the inner MWG. We then apply the method to the OH/IR stars in the ATCA/VLA OH 1612-MHz survey of Sevenster et al., finding  Δ V =252±41 km s-1,  if   u LSR=0  . Assuming further that   R 0=8 kpc  and   V LSR=220 km s-1,  this gives  Ωp=59±5 km s-1 kpc-1  with a possible systematic error of perhaps 10 km s−1 kpc−1. The non-axisymmetric feature for which we measure this pattern speed must be in the disc of the MWG.  相似文献   

20.
We present new determinations of bolometric corrections and effective temperature scales as a function of infrared optical colours, using a large data base of photometric observations of about 6500 Population II giants in Galactic globular clusters (GGCs), covering a wide range in metallicity (−2.0 < [Fe/H] < 0.0).   New relations for BC K versus ( V  −  K ) , ( J  −  K ) and BC V versus ( B  −  V ), ( V  −  I ), ( V  −  J ), and new calibrations for T eff, using both an empirical relation and model atmospheres, are provided.   Moreover, an empirical relation to derive the R parameter of the infrared flux method as a function of the stellar temperature is also presented.  相似文献   

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