The old metal-poor open cluster ESO 92-SC05: accreted from a dwarf galaxy ?

The study of old open clusters outside the solar circle can bring constraints on formation scenarios of the outer disc. In particular, accretion of dwarf galaxies has been proposed as a likely mechanism in the area. We use BVI photometry for determining fundamental parameters of the faint open cluster ESO 92-SC05. Colour-magnitude diagrams are compared with Padova isochrones, in order to derive age, reddening and distance. We derive a reddening E(B - V) = 0.17, and an old age of similar to 6.0 Gyr. It is one of the rare open clusters known to be older than 5 Gyr. A metallicity of Z similar to 0.004 or [M/H] similar to -0.7 is found. The rather low metallicity suggests that this cluster might be the result of an accretion episode of a dwarf galaxy.

Measurement of the epicycle frequency in the Galactic disc and initial velocities of open clusters

A new method to measure the epicycle frequency kappa 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 kappa. 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 kappa that we obtain provide constraints on the rotation velocity of the in particular, V(0) is found to be 230 +/- 15 km s(-1) even if the scale (R(0) = 7.5 kpc) of the Galaxy is adopted. The measured kappa at the solar radius is 43 +/- 5 km s(-1) kpc(-1). The distribution of initial velocities of open clusters is discussed.

Open cluster survival within the solar circle: Teutsch 145 and Teutsch 146 star

Teutsch 145 and Teutsch 146 are shown to be open clusters (OCs) orbiting well inside the solar circle, a region where several dynamical processes combine to disrupt most OCs on a time-scale of a few 108 yr. BVI photometry from the GALILEO telescope is used to investigate the nature and derive the fundamental and structural parameters of the optically faint and poorly known OCs Teutsch 145 and 146. These parameters are computed by means of field-star-decontaminated colour-magnitude diagrams and stellar radial density profiles (RDPs). Cluster mass estimates are made based on the intrinsic mass functions (MFs). We derive the ages 200+100(-50) and 400 +/- 100 Myr, and the distances from the Sun d(circle dot) = 2.7 +/- 0.3 and 3.8 +/- 0.2 kpc, respectively, for Teutsch 145 and 146. Their integrated apparent and absolute magnitudes are m(V) approximate to 12.4 and 13.3 and M(V) approximate to -5.6 and -5.3. The MFs (detected for stars with m greater than or similar to 1 M(circle dot)) have slopes similar to Salpeter`s initial mass function. Extrapolated to the H-burning limit, the MFs would produce total stellar masses of similar to 1400 M(circle dot), typical of relatively massive OCs. Both OCs are located deep into the inner Galaxy and close to the Crux-Scutum arm. Since cluster-disruption processes are important, their primordial masses must have been higher than the present-day values. The conspicuous stellar density excess observed in the innermost bin of both RDPs might reflect the dynamical effects induced by a few 108 yr of external tidal stress.

Multi-Conjugate Adaptive Optics VLT imaging of the distant old open cluster FSR 1415

We employ the recently installed near-infrared Multi-Conjugate Adaptive Optics demonstrator (MAD) to determine the basic properties of a newly identified, old and distant, Galactic open cluster (FSR 1415). The MAD facility remarkably approaches the diffraction limit, reaching a resolution of 0.07 arcsec (in K), that is also uniform in a field of similar to 1.8 arcmin in diameter. The MAD facility provides photometry that is 50 per cent complete at K similar to 19. This corresponds to about 2.5 mag below the cluster main-sequence turn-off. This high-quality data set allows us to derive an accurate heliocentric distance of 8.6 kpc, a metallicity close to solar and an age of similar to 2.5 Gyr. On the other hand, the deepness of the data allows us to reconstruct (completeness-corrected) mass functions (MFs) indicating a relatively massive cluster, with a flat core MF. The Very Large Telescope/MAD capabilities will therefore provide fundamental data for identifying/analysing other faint and distant open clusters in the Galaxy III and IV quadrants.

Effect of fires on soil nutrient availability in an open savanna in Central Brazil

Fire is common in savannas but its effects on soil are poorly understood. We analyzed long-term effects of fire on surface soil of an open Brazilian savanna (campo sujo) in plots submitted to different fire regimes during 18 years. The five fire regimes were: unburned, quadrennial fires in middle dry season, and biennial fires in early, middle or late dry season. Soil was collected during the wet and the middle dry season of 2008, and analyzed for pH, organic matter, total N, potential acidity, exchangeable cations and available P, S, Mn, Cu, Zn and Fe. We applied multivariate analysis to search for patterns related to fire regimes, and to local climate, fuel, and fire behavior. Spearman test was used to establish correlations between soil variables and the multivariate analysis gradient structure. Seasonal differences were tested using t-test. We found evidence of long-term fire effects: the unburned plot was segregated mainly by lower soil pH; the quadrennial plot was also segregated by lower soil pH and higher amount of exchangeable cations; the time of burning during the dry season in biennial plots did not significantly affect soil availability of nutrients. Differences in elements amounts due to the season of soil sampling (wet or dry) were higher than due to the effect of fires. Higher availability of nutrients in the soil during the wet season was probably related to higher nutrient inputs via rainfall and higher microbial activity.