A Brief History of Star Formation and Chemical Enrichment in the Bulge of the Milky way

Observations of the stellar content of the bulge of the Milky Way can provide critical guidelines for the interpretation of observations of distant galaxies, in particular for understanding their stellar content and evolution. In this brief overview I will first highlight some recent work directed towards measuring the history of star formation and the chemical composition of the central few parsecs of the Galaxy. These observations point to an episodic history of star formation in the central region with several bursts having occurred over the past few 100 Myr (e.g. Blum et al., 1996b). High resolution spectroscopic observations by Ramírez et al. (1998) of luminous M stars in this region yield a near solar value for [Fe/H] from direct measurements of iron lines. Then I will present some results from an ongoing program by my colleagues and myself the objective of which is the delineation of the star formation and chemical enrichment histories of the central 100 parsecs of the Galaxy, the `inner bulge'. From new photometric data we have concluded that there is a small increase in mean [Fe/H] from Baade's Window to the Galactic Center and deduce a near solar value for stars in the central region. For radial distances greater than 1^° from the Galactic Center we fail to find a measurable population of stars that are significantly younger than those in Baade's Window. Within 1^° we find a number of luminous M giants that most likely are the result of a star formation episode not more than one or two Gyr ago. This revised version was published online in July 2006 with corrections to the Cover Date.     

Progressive Transformation of a Flux Rope to an ICME

The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by interplanetary coronal mass ejections (ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic flux and helicity, which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled by neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 – 10 November 2004. This MC was embedded in an ICME. After determining an approximate orientation for the flux rope using the minimum variance method, we obtain a precise orientation of the cloud axis by relating its front and rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic flux between the inbound and outbound branches and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). The MC is also studied using dynamic models with isotropic expansion. We have found (6.2±1.5)×10²⁰ Mx for the axial flux and (78±18)×10²⁰ Mx for the azimuthal flux. Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed by an extended coherent magnetic region. These observations are interpreted by considering the existence of a previously larger flux rope, which partially reconnected with its environment in the front. We estimate that the reconnection process started close to the Sun. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed to the observed ICME (with a remnant flux rope in the front part).     

Problem of the length of the current interglacial

The climate during the past hundreds of thousands of years has been characterized by a rather distinct periodicity of about 100000 yr with brief warming periods (interglacials) lasting approximately 10000–12000 yr. Today, mankind is living in an interglacial period that began about 11 ka ago. In light of the discussion about global warming observed in recent decades, which advocates of an anthropogenic impact associate with emission of greenhouse gases due to combustion of fossil fuel, the question arises concerning the duration of the current interglacial. The data available on climate change and solar radiation on a time scale of the last millions of years are critically analyzed in this article and the problem of the length of the current interglacial is discussed.     

Is there a relationship between the density of primordial black holes in a galaxy and the rate of cosmological gamma-ray bursts?

The rate of accretion of matter from a solar-type star onto a primordial black hole (PBH) that passes through it is calculated. The probability that a PBH is captured into an orbit around a star in a galaxy is found. The mean lifetime of the PBH in such an orbit and the rate of orbital captures of PBHs in the galaxy are calculated. It is shown that this rate does not depend on the mass of the PBH. This mechanism cannot make an appreciable contribution to the rate of observed gamma-ray bursts. The density of PBHs in the galaxy can reach a critical value—the density of the mass of dark matter in the galaxy.     

Late Cretaceous-Cenozoic history of the Lake Baikal depression and formation of its unique biodiversity

Independent methods of geological and molecular-biological chronologies have made it possible to define generally corresponding stages in the geological and biological evolution of the environments and communities of Lake Baikal since the Late Cretaceous, i.e., during the last 70 myr. All the abiotic elements drastically changed during geological evolution, with destruction of existing and formation of new natural complexes. Nevertheless, some specific zones retained relicts of former settings. The resulting present-day natural complex includes elements of different ages and geneses. Similar to different natural zones of the present-day Earth, which are populated by different biocoenoses, stages in the development of abiotic elements are also characterized by different faunal and floral assemblages. Some taxa were replaced by others, and the resulting aqueous biota of Lake Baikal includes different-age and ecologically different elements. The oldest groups of Baikal organisms appeared approximately 70 Ma ago, although the largest proportion of the lake biota started forming 4–3 Ma ago in response to the most drastic changes in the abiotic elements of the environment. The youngest taxa appeared 1.8 to 0.15 Ma ago, i.e., during the period when superdeep lake environments and mountainous glaciations were developing. The chronological coincidence of main stages in development of abiotic and biotic elements of the nature indicates their relationships. Particular transformations of abiotic elements and the probable mechanism of their influence on the evolution of living communities are also considered.     

Size and shape of near-spherical Allegan chondrules

Chondrules were extracted from a disaggregated sample of the Allegan meteorite. Individual chondrules were examined with apparatus incorporating two orthogonal binocular microscopes, and their three major axes measured. Maximum chondrule diameters ranged from 0.15 to 2.75 mm with a peak in distribution between 0.35 and 0.75 mm. The chondrule size distribution was found not to conform to Rosin's law. The chondrules were found to depart from sphericity by only small amounts. The authors still believe that the melting of nebula dust-ball agglomerates by some high-energy event was the most probable mechanism for the formation of chondrules.     

Mars data analysis program special issue no. 2

An extended photometric time series in the J and K bands of Neptune has a complex appearance which appears to require the simultaneous presence of three periodicities plus related harmonics in the (J-K) color. The most apparent of the fundamental periods is N1 = 17.73 hr. The two others are at N2 = 18.56 and N3 = 18.29 hr and may be the result of amplitude modulation of a previously reported period of 18.42 hr. We interpret the presence of multiple periodicity as indicating that distinct systems of zonal winds exist on the planet. We argue that these wind systems are probably confined to moderate or high latitudes on the basis of recent omages of the planet taken in a spectral region of strong CH₄ absorption, and, by analogy to the zonal wind systems that exist in Jupiter's atmosphere, deduce a period of rotation for the body of the planet of 18.2 ± 0.4 hr. Zonal wind contrasts of up to 109 m sec^?1 are implied in the atmosphere of Neptune by these observations.     

Effectiveness of fast electron retention in the magnetic cavern during disturbance of the magnetic field azimuthal symmetry

The loss rate of fast electrons (with an energy much higher than the energy of the plasma electron thermal motion), when they leave a magnetic cavern during a disturbance of the magnetic field azimuthal symmetry, is studied. The cases of point and volume sources of fast particles have been considered. The plasma density in the cavern is supposed to be low, so that collisions of fast electrons with plasma particles can be neglected. The effect of the electric field on particle motion is neglected because it is assumed that the electric charge particles outgoing from the cavern is compensated by the counter current of plasma conductivity electrons. The dependence of the loss value on the harmonic number and the amplitude of the cavern boundary radius disturbance has been obtained.