HST colour–magnitude diagrams of six old globular clusters in the LMC

We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and 2019. Deep exposures with the F555W and F814W filters provide us with colour–magnitude diagrams that reach to an apparent magnitude in V of ∼25, well below the main-sequence turn-off. These particular clusters are associated with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour–magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour–magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main-sequence turn-off. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque &38; Zinn. Our results imply that the LMC formed at the same time as the Milky Way Galaxy.     

Wirbelströmungen in den Hypolimnien der Seen

Ohne Zusammenfassung     

On the practical stability of the triangular points in the restricted three-body problem

This paper contains a proposal of a new way of treating astrodynamical stability problems. A definition of a practical stability and a direct method of its examination are presented. The method has been applied to the triangular points problem for variety of ande values in the case of the linearized equation system as well as in the general one. The results are shown in a form which facilitates the comparison with results published by other authors.     

Thermobarometric and40Ar/39Ar geochronologic constraints on Eohimalayan metamorphism in the Dinggyê area,southern Tibet

Mineral assemblages in the Dinggyê area of southern Tibet (28°N; 88°E) provide new insights regarding the poorly understood Eohimalayan metamorphic event in the eastern Himalayan orogen. Major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750–830 K at depths of 14±3 km, consistent with the presence of kyanite, sillimanite, and andalusite schists in the area. Laser and resistance furnace⁴⁰Ar/³⁹Ar analyses of hornblendes from intercalated amphibolites yield closure ages of 25 Ma. Overlap between the probable range of Ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that Eohimalayan metamorphism in the Dinggyê area occurred in Late Oligocene time, no more than about 10 million years before the main or Neohimalayan phase of metamorphism in Early to Middle Miocene time. Muscovite, biotite, and K-feldspar⁴⁰Ar/³⁹Ar ages indicate an important episode of rapid cooling between 16 and 13 Ma, which is interpreted as a signature of tectonic denudation related to movement on N-dipping extensional structures of the South Tibetan detachment system.     

Desert Pavement Disturbance and Artifact Taphonomy: A Case Study from the Eastern Libyan Plateau,Egypt

Accretionary desert pavements on the eastern Libyan Plateau of central Egypt support a rich Middle and Upper Paleolithic artifact assemblage exhibiting intensive blank production and minimal tool production. These assemblages appear to be in primary context with numerous examples of lithic refits showing on‐site lithic production. However, the smallest (length ≤2.5 cm) archaeological fragments are recovered at a much lower rate on this desert pavement surface than expected given comparable data from lithic assemblages in cave and shelter contexts in France. Excavation of archaeological contexts on the Libyan Plateau reveals the loss of small artifact fragments into the subsurface due to aeolian accumulation of silts, whereas geomorphic examination of desert pavement surfaces suggests a potential for relatively isolated bioturbation as a source of lateral and vertical disturbance of desert pavement surfaces over small areas. Archaeologists should be aware of the potential for long‐term assemblage stability as well as small artifact burial in surficial desert pavement contexts.     

The discovery of Segue 2: a prototype of the population of satellites of satellites

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of   M _v =−2.5  , a half-light radius of 34 pc and a systemic velocity of  ∼−40 km s⁻¹  . Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s⁻¹ and the possible stream as  ∼7 km s⁻¹  . This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites – survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts   z > 10  and are good candidates for fossils of the reionization epoch.