On the binarity of Herbig Ae/Be stars

We present high-resolution spectro-astrometry of a sample of 28 Herbig Ae/Be and three F-type pre-main-sequence stars. The spectro-astrometry, which is essentially the study of unresolved features in long-slit spectra, is shown from both empirical and simulated data to be capable of detecting binary companions that are fainter by up to 6 mag at separations larger than ∼0.1 arcsec. The nine targets that were previously known to be binary are all detected. In addition, we report the discovery of six new binaries and present five further possible binaries. The resulting binary fraction is 68 ± 11 per cent. This overall binary fraction is the largest reported for any observed sample of Herbig Ae/Be stars, presumably because of the exquisite sensitivity of spectro-astrometry for detecting binary systems. The data hint that the binary frequency of the Herbig Be stars is larger than that of the Herbig Ae stars. The Appendix presents model simulations to assess the capabilities of spectro-astrometry and reinforces the empirical findings. Most spectro-astrometric signatures in this sample of Herbig Ae/Be stars can be explained by the presence of a binary system. Two objects, HD 87643 and Z CMa, display evidence for asymmetric outflows. Finally, the position angles of the binary systems have been compared with available orientations of the circumprimary disc and these appear to be coplanar. The alignment between the circumprimary discs and the binary systems strongly suggests that the formation of binaries with intermediate-mass primaries is due to fragmentation as the alternative, stellar capture, does not naturally predict aligned discs. The alignment extends to the most massive B-type stars in our sample. This leads us to conclude that formation mechanisms that do result in massive stars, but predict random angles between the binaries and the circumprimary discs, such as stellar collisions, are also ruled out for the same reason.     

The Risk Atlas of Mexico City,Mexico: a tool for decision-making and disaster prevention

Natural Hazards - We present a Risk Atlas of Mexico City based on a Geographical Information System (RA-GIS). We identified the prevalent social risk to the more relevant hazards in Mexico City...     

The discovery of a low-mass, pre-main-sequence stellar association around γ Velorum

We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf–Rayet/O-star binary system γ ²  Vel and the Vela OB2 association. We argue that γ ²  Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 σ level with the recent Hipparcos parallax of γ ²  Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ ²  Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.     

Volcanic hazards zonation of the Nevado de Toluca Volcano,Central Mexico

Nevado de Toluca Volcano (NTV), located in central Mexico, is a large stratovolcano, with an explosive history. The area is one of the most important developing centers (>2 millions) in Mexico and in the last 30 yrs large population growth and expansion have increased the potential risk in case of a reactivation of the volcano. As part of a study to assess volcanic risk, this paper presents the results of the volcanic hazard analysis for the NTV. A total of 150 stratigraphic sections were made in the field and three new ages were obtained. Eruptions from NTV produced a complex sequence of pyroclastic deposits that have affected the area at least 18 times during the last 100,000 yrs. Eight vulcanian, four plinian and one-ultraplinian eruptions as well as the destruction of at least three domes occurred in the last 42,000 yr BP as well as two sector collapses in the last 100,000 yrs. Isopach and isopleth maps for the main ulraplinian eruption were also made. The original cone height (5,080 m.a.s.l) was reconstructed through geomorphologic methods. The maximum distance calculated with the energy line for the block and ash flows was 41 km, 35 km for pumice flows and 45 km for debris avalanches. The dominant wind direction at altitudes of 20–30 km is to the east-northeast from November to March, west-northwest in April and west from May to October. Five hazards maps (block and ash flows, pumice flows, ash fall, debris avalanches, and lahars) were made for the NTV. The pyroclastic flows and lahars represent very high to medium hazard for Toluca, Villa Guerrero, Coatepec, Tianguistengo, Metepec, Tenango, Lerma and Zinacantepec. A new debris avalanche would probably affect the south and northeast because of active faulting (E–W and NW–SE) and existing topographic differences in height.     

Influence of volcanic activity on spring water chemistry at Popocatepetl Volcano, Mexico

The results of the 7 years (1994–2000) of monthly monitoring of spring water before and during eruptions show response to volcanic activity. Low salinity and temperature characterize most of the springs, which are located on the flanks of Popocatepetl Volcano. The pH ranges from 5.8 to 7.8 and temperature from 3 to 36 °C. Oxygen and hydrogen isotopic data show that the water is of meteoric origin, but SO₄²⁻, Cl⁻, F⁻, HCO₃⁻, B, and SO₄²⁻/Cl⁻ variations precede main eruptive activity, which is considered linked to influx of magmatic gases and acid fluids that react with sublimates and host rock and mix with the large water system. Na⁺, Ca²⁺, SiO₂ and Mg²⁺ concentrations in the water also increased before eruptive activity. The computed partial pressure of CO₂ in equilibrium with spring waters shows values higher than air-saturated water (ASW), with the highest values up to 0.73 bar of pCO₂. Boron is detected in the water only preceding the larger eruptions. When present, boron concentration is normally under health standard limits, but in two cases the concentration was slightly above. Other components are within health standard limits, except for F⁻ in one spring.     

The significance of phenocryst diversity in tephra from recent eruptions at Popocatepetl volcano (central Mexico)

Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO₂=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo_89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic plagioclase (An_33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#_melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#_melt ≈ 72–76) and an evolved component magma (mg#_melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#_melt=50–72. The mafic end member (mg#_melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H₂O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg# clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing. The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano. The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that the Popo mafic end member is a magnesian andesite with variable, but high SiO₂ contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#_melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#_melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites, such as the low CaO, Al₂O₃, FeO*, high Na₂O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central to quantifying the subcrustal mass fluxes beneath Popo. Received: 13 December 1999 / Accepted: 11 August 2000