Showing papers in "Astronomische Nachrichten in 2007"

Astrophysical supplements to the ASCC‐2.5: Ia. Radial velocities of ∼55000 stars and mean radial velocities of 516 Galactic open clusters and associations

Abstract: We present the 2nd version of the Catalogue of Radial Velocities with Astrometric Data (CRVAD-2). This is the result of the cross-identification of stars from the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5) with the General Catalogue of Radial Velocities and with other recently published radial velocity lists and catalogues. The CRVAD-2 includes accurate J2000 equatorial coordinates, proper motions and trigonometric parallaxes in the Hipparcos system, B, V photometry in the Johnson system, spectral types, radial velocities (RVs), multiplicity and variability flags for 54907 ASCC-2.5 stars. We have used the CRVAD-2 for a new determination of mean RVs of 363 open clusters and stellar associations considering their established members from proper motions and photometry in the ASCC-2.5. For 330 clusters and associations we compiled previously published mean RVs from the literature, critically reviewed and partly revised them. The resulting Catalogue of Radial Velocities of Open Clusters and Associations (CRVOCA) contains about 460 open clusters and about 60 stellar associations in the Solar neighbourhood. These numbers still represent less than 30% of the total number of about 1820 objects currently known in the Galaxy. The mean RVs of young clusters are generally better known than those of older ones. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical simulations of fast and slow coronal mass ejections

Abstract: Solar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux-rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force-free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

What to look for in the seismology of solar active regions

Abstract: Using a newly developed extension of ray theory which accounts for mode transmission and conversion between fast and slow magnetoacoustic waves, as well as simple wave mechanical calculations, we find that strong surface magnetic fields, as may be found in solar and stellar active regions, have several related but distinct effects on seismic waves: transmission/conversion, shortened travel times, a directional filtering of acoustic waves entering the overlying atmosphere, and a tendency to more closely align velocities with the field as height increases in the atmosphere. Magnetic field inclination is particularly relevant to these effects. Here, we briefly review these findings, and present some new results on ray travel times and magnetic filtering. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Heating of the solar and stellar coronae: a review

Abstract: Despite great advances in observations and modelling, the problem of solar and stellar heating still remains one of the most challenging problems of space physics. To find a definite answer to what sort of mechanisms act to heat the plasma to a few million degrees requires a collaborative effort of small scales observations, large capacity numerical modelling and complicated theoretical approaches. A unique theory should incorporate aspects such as the generation of energy, its transport and dissipation. Up to now, the first two problems are rather clarified. However, the modality of transfer of magnetic or kinetic energy into heat is a question still awaiting for an answer. In the present paper we review the various popular heating mechanisms put forward in the existing extensive literature. The heating processes are, somewhat arbitrarily, classified as hydrodynamic, magnetohydrodynamic or kinetic based on the characteristics of the model medium. These mechanisms are further divided based on the time scales of the ultimate dissipation involved (i.e. AC and DC heating, turbulent heating). In particular, special attention is paid to discuss shock dissipation, mode coupling, resonant absorption, phase mixing, and, reconnection. Finally, we briefly review the various heating mechanisms proposed to heat other stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Linear sensitivity of helioseismic travel times to local flows

Abstract: Time-distance helioseismology is a technique for measuring the time for waves to travel from one point on the solar surface to another. These wave travel times are affected by advection by subsurface flows. Inferences of plasma flows based on observed travel times depend critically on the ability to accurately model the effects of subsurface flows on time-distance measurements. We present a Born-approximation based computation of the sensitivity of time-distance travel times to weak, steady, inhomogeneous subsurface flows. Three sensitivity functions are obtained, one for each component of the 3D vector flow. We show that the depth sensitivity of travel times to horizontally uniform flows is given approximately by the kinetic energy density of the oscillation modes which contribute to the travel times. For flows with strong depth dependence, the Born approximation can give substantially different results than the ray approximation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Differential rotation on rapidly rotating stars

Abstract: Theories of meridional circulation and differential rotation in stellar convective zones predict trends in surface flow patterns on main-sequence stars that are amenable to direct observational testing. Here I summarise progress made in the last few years in determining surface differential rotation patterns on rapidly-rotating young main-sequence stars of spectral types F, G, K and M. Differential rotation increases strongly with increasing effective temperature along the main sequence. The shear rate appears to increase with depth in the sub-photospheric layers. Tidal locking in close binaries appears to suppress differential rotation, but better statistics are needed before this conclusion can be trusted. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Multidimensional indexing tools for the virtual observatory

Abstract: The last decade has seen a dramatic change in the way astronomy is carried out. The dawn of the the new microelectronic devices, like CCDs has dramatically extended the amount of observed data. Large, in some cases all sky surveys emerged in almost all the wavelength ranges of the observable spectrum of electromagnetic waves. This large amount of data has to be organized, published electronically and a new style of data retrieval is essential to exploit all the hidden information in the multiwavelength data. Many statistical algorithms required for these tasks run reasonably fast when using small sets of in-memory data, but take noticeable performance hits when operating on large databases that do not fit into memory. We utilize new software technologies to develop and evaluate fast multidimensional indexing schemes that inherently follow the underlying, highly non-uniform distribution of the data: they are layered uniform indices, hierarchical binary space partitioning, and sampled flat Voronoi tessellation of the data. These techniques can dramatically speed up operations such as finding similar objects by example, classifying objects or comparing extensive simulation sets with observations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An Elementary Introduction to the JWKB Approximation

Abstract: Asymptotic expansion of the second-order linear ordinary differential equation ψ″ + k2f (z)ψ = 0, in which the real constant k is large and f = O(1), can be carried out in the manner of Liouville and Green provided f does not vanish. If f does vanish, however, at x0 say, then Liouville-Green expansions can be carried out either side of the turning point z = z0, but it is then necessary to ascertain how to connect them. This was first accomplished by Jeffreys, by a comparison of the differential equation with Airy's equation. Soon afterwards, the situation was found to arise in quantum mechanics, and was discussed by Brillouin, Wentzel and Kramers, after whom the method was initially named. It arises throughout classical physics too, and is encountered frequently when studying waves propagating in stars. This brief introduction is aimed at clarifying the principles behind the method, and is illustrated by considering the resonant acoustic-gravity oscillations (normal modes) of a spherical star. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coronal “wave”: A signature of the mechanism making CMEs largescale in the low corona?

Abstract: We analyse one of the first coronal waves observed by STEREO/EUVI associated with a source region just behind the limb, NOAA 10940. We apply the coronal “wave” model proposed by Attrill et al. (2007) to explain the evolution of the observed bright fronts, thereby arguing that the bright fronts and dimmings are due to magnetic reconnections between the expanding CME core and surrounding magnetic structures. We offer a discussion showing that this model provides a mechanism via which CMEs, expanding from a small source region can naturally become large-scale in the low corona. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry

Abstract: We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M 3 M⊙ seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M⊙ and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities β is detected in both higher and lower mass stars. It is quite possible that the angle β becomes close to 0. in slower rotating stars of mass M > 3 M⊙ too, analog to the behaviour of angles β in slowly rotating stars of M 3 M⊙, no similar trend is found for stars with M < 3 M⊙. However, the predominance of high values of β at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SLiM: a code for the simulation of wave propagation through an inhomogeneous, magnetised solar atmosphere

Abstract: In this paper we describe the semi-spectral linear MHD (SLiM) code which we have written to follow the interaction of linear waves through an inhomogeneous three-dimensional solar atmosphere. The background model allows almost arbitrary perturbations of density, temperature, sound speed as well as magnetic and velocity fields. We give details of several of the tests we have used to check the code. The code will be useful in understanding the helioseismic signatures of various solar features, including sunspots. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rotating convection in f‐boxes: Faster rotation

Abstract: Results based on 36 cases of f-box computations for different latitudes and rotation rates are summarized. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Zeeman‐Doppler imaging of late‐type stars: The surface magnetic field of II Peg

Abstract: Late-type stars in general possess complicated magnetic surface fields which makes their detection and in particular their modeling and reconstruction challenging. In this work we present a new Zeeman-Doppler imaging code which is especially designed for the application to late-type stars. This code uses a new multi-line cross-correlation technique by means of a principal component analysis to extract and enhance the quality of individual polarized line profiles. It implements the full polarized radiative transfer equation and uses an inversion strategy that can incorporate prior knowledge based on solar analogies. Moreover, our code utilizes a new regularization scheme which is based on local maximum entropy to allow a more appropriate reproduction of complex surface fields as those expected for late-type stars. In a first application we present Zeeman-Doppler images of II Pegasi which reveal a surprisingly large scale surface structure with one predominant (unipolar) magnetic longitude which is mainly radially oriented. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evidence for interchange reconnection between a coronal hole and an adjacent emerging flux region

Abstract: Coronal holes are regions of dominantly monopolar magnetic field on the Sun where the field is considered to be ‘open’ towards interplanetary space. Magnetic bipoles emerging in proximity to a coronal hole boundary naturally interact with this surrounding open magnetic field. In the case of oppositely aligned polarities between the active region and the coronal hole, we expect interchange reconnection to take place, driven by the coronal expansion of the emerging bipole as well as occasional eruptive events. Using SOHO/EIT and SOHO/MDI data, we present observational evidence of such interchange reconnection by studying AR 10869 which emerged close to a coronal hole. We find closed loops forming between the active region and the coronal hole leading to the retreat of the hole. At the same time, on the far side of the active region, we see dimming of the corona which we interpret as a signature of field line ‘opening’ there, as a consequence of a topological displacement of the ‘open’ field lines of the coronal hole. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

RFI detection by automated feature extraction and statistical analysis

Abstract: In this paper we present an interference detection toolbox consisting of a high dynamic range Digital Fast-FourierTransform spectrometer (DFFT, based on FPGA-technology) and data analysis software for automated radio frequency interference (RFI) detection. The DFFT spectrometer allows high speed data storage of spectra on time scales of less than a second. The high dynamic range of the device assures constant calibration even during extremely powerful RFI events. The software uses an algorithm which performs a two-dimensional baseline fit in the time-frequency domain, searching automatically for RFI signals superposed on the spectral data. We demonstrate, that the software operates successfully on computer-generated RFI data as well as on real DFFT data recorded at the Effelsberg 100-m telescope. At 21-cm wavelength RFI signals can be identified down to the 4�rms level. A statistical analysis of all RFI events detected in o ur observational data revealed that: (1) mean signal strength is comparable to the astronomical line emission of the Milky Way, (2) interferences are polarised, (3) electronic devic es in the neighbourhood of the telescope contribute signific antly to the RFI radiation. We also show that the radiometer equation is no longer fulfilled in presence of RFI signals.

Meridional flow profile measurements with SOHO/MDI

Abstract: We present meridional flow measurements of the Sun using a novel helioseismic approach for analyzing SOHO/MDI data in order to push the current limits in radial depth. Analyzing three consecutive months of data during solar minimum, we find that the meridional flow is as expected poleward in the upper convection zone, turns equatorward at a depth of around 40 Mm (∼ 0.95 R⊙), and possibly changes direction again in the lower convection zone. This may indicate two meridional circulation cells in each hemisphere, one beneath the other. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Solar Acoustic Simulator: Applications and Results

Abstract: It is important to understand the limits and accuracy of helioseismic techniques in their ability to probe the solar interior The availability of a method that is able to compute the solar acoustic wave eld in the presence of thermal or o w perturbations affords us a means to place bounds on detectability and accuracy of inferences of interior perturbations We describe the technique used to simulate wave propagation within a spherical shell that extends from a desired depth (not including the center) into the solar atmosphere and which possesses a solar like stratication

The differential rotation of ε Eri from MOST data

Abstract: From high-precision MOST photometry spanning 35 days the existence of two spots rotating with slightly differing periods is confirmed. From the marginal probability distribution of the derived differential rotation parameter k its expectation value as well as confidence limits are computed directly from the data. The result depends on the assumed range in inclination i, not on the shape of the prior distributions. Two cases have been considered: (a) The priors for angles, inclination i of the star and spot latitudes β 1,2, are assumed to be constant over i, β 1, and β 2; (b) the priors are assumed to be constant over cos i, sin β 1, and sin β 2. In both cases the full range of inclination is considered: 0° ≤ i ≤ 90°. Scalefree parameters, i. e. periods and spot areas (in case of small spots) are taken logarithmically. Irrespective of the shape of the prior, k is restricted to 0.03 ≤ k ≤ 0.10 (1 σ limits). The inclination i of the star is photometrically ill-defined. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent photometry of symbiotic stars

Abstract: We present new photometric observations of 15 symbiotic stars covering their last orbital cycle(s) from 2003.9 to 2007.2. We obtained our data by both classical photoelectric and CCD photometry. Main results are: EG And brightened by ∼0.3 mag in U from 2003. A ∼0.5 mag deep primary minimum developed in the U light curve (LC) at the end of 2006. ZAnd continues its recent activity that began during the 2000 autumn. A new small outburst started in summer of 2004 with a peak U magnitude of ∼ 9.2. During the spring of 2006 the star entered a massive outburst. It reached its historical maximum at U ∼ 8.0 in 2006 July. AEAra erupted in 2006 February with Δmvis ∼ 1.2 mag. BF Cyg entered a new active stage in 2006 August. A brightness maximum (U ∼ 9.4) was measured during 2006 September. CH Cyg persists in a quiescent phase. During 2006 June–December a ∼ 2 mag decline in all colours was measured. CI Cyg started a new active phase during 2006 May–June. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On-line database of photometric observations of magnetic chemically peculiar stars

Abstract: We present our extensive project of the On-line database of photometric observations of magnetic chemically peculiar stars to collect published data of photometric observations of magnetic chemically peculiar (mCP) stars in the optical and near IR regions. Now the nascent database contains more than 107 000 photometric measurements of 102 mCP stars and will be continually supplemented with published or new photometric data on these and about 150 additional mCP stars. This report describes the structure and organization of the database. Moreover, for the all included data we estimated the error of measurements and the effective amplitudes of the light curves.

The search for a cometary outbursts mechanism: a comparison of various theories

Abstract: In the paper the potential sources of energy of cometary outbursts have been reviewed. Considerations focus on four probable sources of the outbursts' energy. These are the polymerization of hydrogen cyanide HCN, impacts with the meteoroids, destruction of cometary grains in the field of strong solar wind and the transformation of amorphous water Ice into the crystalline one. The values of released energy and jumps of cometary brightness caused by these mechanisms have been discussed. A modern approach to the problem of the thermodynamical evolution of the comet nucleus which includes amorphous water ice is considered as the starting point in the discussion presented in the paper. The main characteristics of an outburst of a hypothetical comet belonging to the Jupiter family comet are calculated. The obtained results are in a good agreement with the characteristics observed during the real outbursts of comets. The main conclusion of this paper confirms a general presumption that the cometary outbursts can have different causes. However, the hypothesis concerning the amorphous water ice transformation appears to be the most probable one. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The orbit of the visual binary ADS 8630 (γ Vir)

Abstract: We present a new orbit for the visual binary ADS 8630 = γ Vir. Although it is one of the first visual double stars discovered, its orbital elements were still poorly known. Indeed the very high eccentricity of the orbit and the difficulty of observing the pair at periastron passage in 1836 has meant that it is only now that sufficient measures of the recent close approach in 2005 have allowed an orbital analysis which predicts the angular motion to an acceptable degree of accuracy. We present a series of 35 speckle measurements of ADS 8630 obtained with PISCO in Merate between 2004 and 2006. Those measures have been crucial for determining the new orbital elements since they cover an arc of 130 degrees in the apparent orbit and include the periastron passage of 2005. The masses of the individual F0V components of the binary are found to be 1.40 M⊙ with an accuracy of about 3%. We also investigate in detail the possibility of the presence of a third body in the system, that was proposed by other authors. The high-angular resolution infra-red image of γ Vir that we obtained in June 2006 with the LuckyCam instrument on the ESO NTT shows the absence of any companion as faint as a M0V star at a distance larger than 0. ′′ 4. Combined with the analysis of the residuals of our orbit, the values found for the masses of the individual components and the radial velocity measurements, this observation rules out the presence in the system of a third companion with a mass larger than 0.3 M⊙.

Effective temperature vs. line‐depth ratio for ELODIE spectra: Gravity and rotational velocity effects

Abstract: The dependence on the temperature of photospheric line-depth ratios (LDRs) in the spectral range 6190–6280 A is investigated by using a sample of 174 ELODIE Archive stellar spectra of luminosity class from V to III. The rotational broadening effect on LDRs is also studied. We provide useful calibrations of effective temperature versus LDRs for giant and main sequence stars with 3800 ≃ Teff ≃6000 K and v sin i in the range 0–30 km s–1. We found that, with the exception of very few line pairs, LDRs, measured at a spectral resolution as high as 42 000, depend on v sin i and that, by neglecting the rotational broadening effect, the Teff determination can be wrong by ∼100 K in the worst cases. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Model atmosphere parameters of the binary systems COU1289 and COU1291

Abstract: The spectral energy distributions between λ 3700 A and λ 8100 A of the binary systems COU1289 and COU1291 have been measured with the Carl-Zeiss-Jena 1 m telescope of the Special Astrophysical Observatory. Their B, V, R magnitudes and B – V colour indices were computed and compared with earlier investigations. Model atmospheres of both systems were constructed using a grid of Kurucz blanketed models, their spectral energy distributions in the continuous spectrum were computed and compared with the observational ones. The model atmosphere parameters for the components of COU1289 were derived as: Taeff = 7100 K, Tbeff = 6300 K, log ga = 4.22, log gb = 4.22, Ra = 1.50 R⊙, Rb = 1.40 R⊙, and for the components of COU1291 as: Taeff = 6400 K, Tbeff = 6100 K, log ga = 4.20, log gb = 4.35, Ra = 1.47 R⊙, Rb = 1.12 R⊙. The spectral types of both components of the system COU1289 were concluded as F1 and F7, and of the system COU1291 as F6 and F9. Finally the formation and evolution of the systems were discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mitteilungen Über neuentdeckte Veränderliche Sterne

Abstract: The discovery of this list's 351 new variables was announced in Mitteilungen uber Veranderliche Sterne. Here some more details are given, including epochs for short period and eclipsing variables and charts of the surroundings.

Towards using modern data assimilation and weather forecasting methods in solar physics

Abstract: We discuss how data assimilation and forecasting methods developed in Earth's weather prediction models could be used to improve our capability to anticipate solar dynamical phenomena and assimilate the huge amount of data that new solar satellites, such as SDO or Hinode, will provide in the coming years. We illustrate with some simple examples such as the solar magnetic activity cycle, the eruption of CMEs, the real potential of such methods for solar physics. We believe that we now need to jointly develop solar forecasting models, whose purpose are to assimilate observational data in order to improve our predictability power, with “first principle” solar models, whose purpose is to understand the underpinning physical processes behind the solar dynamics. These two complementary approaches should lead to the development of a solar equivalent of Earth's general circulation model. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Telescope and instrument robotization at Dome C

Abstract: This article reviews the situation for robotization of telescopes and instruments at the Antarctic station Concordia on Dome C. A brain-storming meeting was held in Tenerife in March 2007 from which this review emerged.We describe and summarize the challenges for night-time operations of various astronomical experiments at conditions “between Earth and Space” and conclude that robotization is likely a prerequisite for continuous astronomical data taking during the 2000-hour night at Dome C. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Correcting second-order contamination in low-resolution spectra

Abstract: An empirical method for correcting low-resolution astronomical spectra for second-order contamination is presented. The method was developed for correcting spectra obtained with grism#4 of the ALFO ...

Simulations of the anisotropic kinetic and magnetic alpha effects

Abstract: IUCAA, Post Bag 4, Pune University Campus, Ganeshkhind, Pune 411 007, IndiaReceived 2007 March 19, accepted 2007 April 2Abstract. Using simulations of isotropically forced helical turbulence the contributions to kinetic and magnetic alpha effectsare computed. It is shown that for the parameter regimes considered in an earlier publication (Brandenburg & Subramanian2005, A&A 439, 835), the expressions for isotropic and anisotropic alpha effects give quantitatively similar results. Bothkinetic and magnetic alpha effects are proportional to a relaxation time whose value, in units of the turnover time, is shownto be approximately unity and independent of the magnetic Reynolds number.Key words: MHD – turbulence

Estimation of galactic model parameters with the Sloan Digital Sky Survey and the metallicity distribution in two fields in the anti-centre direction of the Galaxy

Abstract: We estimated the galactic model parameters from the Sloan Digital Sky Survey (SDSS) data reduced for two fields in the anti-centre direction of the Galaxy, l = 180°, and symmetric relative to the galactic plane, b = +45° (north field) and b = –45° (south field). The large size of each field, 60 deg2, and the faint limiting apparent magnitude, g0 = 22, give us the chance to determine reliable parameters for three components, thin and thick discs and halo, in the north and south hemispheres of the Galaxy, except the scale lengths for two discs which are adopted from Juric et al. (2005). Metallicities were evaluated by a recent calibration for SDSS, and absolute magnitudes of stars with 4 < M (g) ≤ 8 were derived as a function of (g – r)0 colour and metallicity. A χ2 method was employed to fit the analytical density laws to the observational-based space densities with the addition constraint of producing local densities consistent with those derived from Hipparcos. Conspicuous differences could not be detected between the corresponding galactic model parameters for the thin disc of north and south fields, and our results are consistent with the ones in the literature. The same case is valid for the halo, especially the axis ratios for two fields are exactly equal, κ = 0.45, and close to the one of Juric et al. (2005). However, we revealed differences between the scale heights and local space densities for the thick disc of the north and south fields. The metallicity distribution for unevolved G type stars with 5 < M (g) ≤ 6 shows three substructures relative to the distance from the galactic plane: for z * < 3 kpc, the metallicity gradient for two fields is d[M/H]/dz ∼ –0.20(±0.02) dex kpc–1, consistent with the formation scenario of the thin disc. For 5 < z * < 10 kpc, d[M/H]/dz ∼ –0.03(±0.001) dex kpc–1 for two fields, confirming that the metallicity gradient for the halo component is close to zero. However, the tendency of the metallicity for stars with 3 < z * < 5 kpc, corresponding to the transition region from thick disc to halo is different. For the north field d[M/H]/dz = –0.36(±0.12) dex kpc–1, whereas it is half of this value for the south field, d[M/H]/dz = –0.18(±0.01) dex kpc–1. The origin of this conflict is probably due to the different structure of the thick disc in opposite latitudes of the Galaxy. When we combine these substructures, however, we find a smooth metallicity gradient for two fields, –0.30(±0.04) dex kpc–1. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)