In this paper we consider a first application of the Learning Vectorial Quantification Neural method (LVQ) to the problem of studying and distinguishing between different populations within an stellar catalogue of the solar neighbourhood (a complete description can be found in Hernández-Pajares and Monte, 1991, Artificial Neural Networks, Ed. A.Prieto, Lecture Notes in Computer Science 540, Springer-Verlag, p.422). It consists, briefly, in the approximation of a set of vectors in a certain characteristic space that contains continuous elements. The representative points for every cluster are the centroids, calculated in such a way to minimize the distortion. Each of those can be labeled with integer numbers using a 2D representation that preserves the neighbouring property in the characteristic space: the Kohonen Map (Kohonen, 1988, IEEE Computer, 21 Nbr.3).

The main objective of this work has been the connection between the Ogorodnikov-Milne model (OM) and the Spherical Harmonic development (SH) for different kinematic observables. This new approach allow us to benefit from the high orthogonality, easy extension to high orders and no appearance of the distance in the SH expansions; and also to benefit from the intrinsic meaning of the OM parameters. This work (a complete description will appear in Hernández-Pajares & Núñez, 1991, Astron. J., submitted) completes the theoretical studies started in Hernández-Pajares & Núñez (1990a, Astrophys. Space Science 170, 187 and 1990b, Error, bias and uncertainties in Astronomy, ed. Jaschek and Murtagh, Cambridge Univ. Press, 339).

UBV CCD photometry has been obtained for 14 OB associations in the Large and Small Magellanic Clouds. The data have been used to construct color-magnitude diagrams for the purpose of investigating the massive-star content of these extragalactic associations.

The kinematics of the long-period variables (LPVs), both in the LMC (Hughes et al. 1991 AJ 101, 1304) and in the Galaxy (eg. Feast 1963 MNRAS 125, 367) shows that there exists an Age-Period relation for these objects. Low-resolution (FORS) spectra have been obtained for a sample of SMC, LMC and Galactic LPVs using the Anglo-Australian Telescope, with the ultimate aim of using the bandstrengths of the TiO bands, in combination with a temperature indicator derived from the continuum, to estimate their metallicities in a method similar to that used by Mould and Bessell (1982 ApJ 262, 142), but using the TiO bandhead at 8480 å. Metallicity estimates of these variables are essential, both in (1) verifying the finding of Wood et al. (1991 The Magellanic Clouds, eds. R.Haynes and D.Milne, (Kluwer: Dordrecht), p259) who theorise that the scatter in the mean Period-Luminosity and Period-Color relations of the LPVs in the LMC (Feast et al. 1989 MNRAS 241, 375) could be produced by a scatter in abundance, and (2) in investigating whether there exists a correlation between abundance, kinematics and period (age) for the LPVs in the LMC, as appears to be the case in the Galaxy.

The radio object Cepheus A is a known region of star formation, in a region of very heavy optical obscuration. It has been observed at the VLA since 1980, and is known to consist of 2 lines of about 13 objects. In particular, the central region has been monitored since 1988, at L, C, and U-band, with resolutions of 1″.0, 0″.3 and 0″.1, corresponding to linear resolutions of 700, 200 and 70 au. The presence of a highly variable Source 8 had been known since 1980, and was known to appear and disappear in a period of less than 1 year. However, a new and much more intense Source 9 was seen 1988 November 2. Over the period 1990 March 13 – 1990 May 8, it showed increases in flux density at C-band by a factor of nearly six times over a period of 50 days. Spectra have been obtained, and show that the flux density peaks at about 5 × 109 Hz, and an increase in flux density is accompanied by an increase in the frequency of the peak. The variable sources are not resolved at 0.″1 at U-Band. This strongly suggests gyrosynchrotron emission from a region of size ~ 1 au, magnetic field ~ 100 G, temperature ~ 108 K and density ~ 106 cm–3. The variations in flux density are then due to increases in magnetic field, as might occur if the source were a protostar, or pre-main sequence star, which is rotating and shedding its magnetic field, by converting magnetic flux energy into particle energy. This would provide also a mechanism for molecular outflows. The observations and model will be described.

We present recent analysis of star star clusters in M31

Recently, Jablonka, Alloin and Bica (1991) (herafter JAB) aimed at studying the properties of M31's star clusters and at enlarging the data base of star cluster integrated properties (Bica and Alloin, 1987) at high metallicities, for population synthesis purpose.

We have obtained spectra for 9 clusters in M31, 7 globular clusters, G1, G78, G158, G170, G177, G219, G222 and 2 open clusters, C107 and C130. Those spectra cover a wide wavelength range (3500å– 9800å) which allows one to confidently disentangle age, metallicity and reddening effects.

We demonstrate that, contrary to the conventionnal view, late type spirals do not seem to contain more dark matter than early type ones.

A photochemical evolution model has been used to investigate the chemical and photometric evolution of galactic discs which form by prolonged infall of halo material on a timescale that increases with radius, termed biased infall, (Josey & Arimoto, 1991). We find that the decline in the mean age and metallicity of the stellar population with radius generates significant colour gradients in the disc and suggest that biased infall may be responsible for the colour variations which have been observed in a number of spirals. Age variations are found to be the primary factor responsible for gradient production in the U, B and V bands while metallicity effects become increasingly important at longer wavelengths. Our model has been applied to M31, in which strong radial colour gradients have been observed, and we find that its chemical and photometric properties can be largely accounted for if its disc formed on a timescale that increased from 0.7 Gyr at the centre to 5 Gyr at a radius of 10 kpc (see Fig.1).

In this work we study a non-cylindrical point-axial symmetric stellar system model that verifies the Chandrasekhar postulates [1]. It explains the values for the centered moments of the peculiar velocities and the galactic rotation parameters, non possible with a cylindrical model [2].

Using population synthesis methods the nuclear and circumnuclear spectra of Seyfert galaxies were modeled. The central nonthermal intensity and the surrounding star formation rate are correlated.

We present a study of optical (BV) and infrared (JK) colours and colour gradients in a sample of Seyfert 1 galaxies. The galaxies belong to the hard X-ray selected complete sample of Piccinotti et al. (Ap.J., 253, 485, 1982). We separate the luminosity profiles of the galaxies into an AGN, bulge and disk components (Kotilainen et al., MNRAS, submitted), and determine the stellar BVJK colours in the nucleus and in an annulus around it. We use three colours, B–V, V–K and J–K, which are sensitive to different kinds of stars. We find no correlation between optical, optical/infrared and infrared colours in the nucleus or in the annulus. The recent star formation rate (SFR), which determines the B–V colour, must therefore vary considerably from galaxy to galaxy and be separated from the properties of the older stellar population responsible for the infrared colours. Most of the galaxies show a steep negative V–K and a flatter negative J–K gradient (i.e. redder colour towards the nucleus) and a slight positive B–V gradient. These colour gradients are steeper than in E/SO's and globular clusters. The optical and optical/infrared gradients are well correlated, whereas there is no correlation between B–V and J–K gradients. We compare a model (Arimoto and Yoshii, AA, 164, 260, 1986) with varying initial mass function, star formation rate and age of the galaxy with the observed colours and gradients. While the models can account for normal galaxy colours, the colours of the Seyfert 1 hosts are generally much redder (in V–K and J–K). Explaining the colours and gradients of the sample galaxies requires a combination of internal differential reddening, thermal reradiation from hot dust grains, and metallicity, recent SFR and IMF that change with radius. Extremely red colours of some of the galaxies may need additional contribution from, for example, a very old red stellar population, extreme IMF or an extremely low lower mass cutoff (LMC).

Evolutionary synthesis models have been computed to construct the spectral energy distributions of BCD galaxies in the optical and NIR ranges (0.3 to 3.5μm). Evolutionary tracks for stars having Z = 1/10 Z⊙ have been employed in order to match the observed low metal abundances of BCDs. Gaseous emission from H II regions has been included in the model. A starburst (of duration 5 · 106yr) is superimposed on an underlying component of red stars characterised by continuous star formation. Burst parameters, star formation rates and ionised hydrogen gas masses have been deduced by fitting the models to observed spectral energy distributions (SEDs) of BCDs.

The field dwarf sample is from the original Carney-Latham survey plus its recent extension to about 500 additional stars, a total of about 1450 stars. We have chosen a conservative criterion to isolate halo stars, namely retrograde orbits in the Galaxy, to minimize contamination by thin and thick disk stars, giving a halo sample of 144 stars.

We present a study of stars in the central (2.2′ x 3.5′) area of NGC 205 using BVRI CCD photometry obtained at the prime focus of the CFHT 3.6m.

We present a study of stars in the central (2.2′ x 3.5′) area of NGC 185 using BVRI CCD photometry obtained at the prime focus of the CFHT 3.6m.

In their recent, high-quality spectroscopic observations of 59 RR Lyrae stars in Baade's window field of the Galactic nuclear bulge, Walker and Terndrup (1991) found that the metallicity distribution for their whole sample is sharply peaked at [Fe/H] = −1.0. Comparison of their data with the sample of halo RR Lyraes (Suntzeff et al. 1991) suggests that the metallicity at which the stellar population is most likely to form RR Lyraes increases with decreasing galactocentric distance.

The horizontal-branch (HB) model calculations indicate that this is what one would expect if the radial variation in HB morphology observed in the halo continues to the very center of the Galaxy. In particular, the observed peak of the bulge RR Lyrae abundance distribution at [Fe/H] = −1.0 can only be reproduced by the unique relationship between the HB type and [Fe/H] that is shifted to the right of the mean relationship for inner halo globular clusters (see Fig. 7a of Lee 1991).

The new IRAS database of OH/IR stars (ages between 5 and 10 Gyr) show clearly different population characteristics from those used by Baud (1981) for his sample of OH/IR stars (ages under 1 Gyr) obtained by means of “blind” surveys. For the IRAS group the velocity dispersions are much higher and the z-scaleheight is larger than for Baud's sample.

Thus OH/IR stars span a range of ages and kinematical properties all the way from the youngest to the oldest stellar galactic population.

Using UBVRI photometry and proper motions we have developed a procedure to separate dwarfs and subdwarfs from stars of other luminosity classes. Three independent methods to take into account the effect of metallicity in photometric distance determinations have been applied, giving special attention to the use of infrared colours for stars with (B – V) > 0.8m.

The application of these methods to a sample of 426 stars included in the HIPPARCOS Input Catalogue –for which we have obtained photometric data– allow us to analyze systematic differences.