A measurement of the high energy secondary charged cosmic radiation

An experiment performed with a balloon-borne large plastic scintillator is described. It was launched from Reconquista, province of Santa Fe, on 24 February, 1978. The energy loss spectra of both atmospheric gamma radiation (forE > 4.15 MeV) and the charged component of the secondary cosmic radiation, were alternately measured at different altitudes, during the ascent of the balloon and at ceiling altitude. The atmospheric gamma radiation spectrum is analyzed in an earlier paper (Azcárateet al., 1992). The shape of the energy loss spectrum due to charged radiation is justified, in its more characteristic features, when the path length distribution in the detector of minimum ionization relativistic particles is taken into account. It is concluded that, at the ceiling altitude, the observed peak in the spectrum is due mainly to relativisticµ-mesons incident from the horizontal direction. The growth curve for the counting rate below the peak and the horizontal intensity of relativisticµ-mesons are also obtained.Member of the Carrera del Investigador Científico y Tecnológico, from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) from ArgentinaMember of the Carrera del Investigador Científico y Tecnológico, from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) from Argentina     

Observations of southern Wolf-Rayet stars

The Wolf-Rayet star HD 90657 is found to be a spectroscopic binary with a WN5+O6 spectrum and a period of 6.456 days. Preliminary orbital elements show an elliptic orbit and a mass ratioM _WR/M _o0.5. Evidence is presented for the Wolf-Rayet atmosphere being accelerated outwards and not being spherically symmetric.Operated under an agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), the Universidad de Buenos Aires and the Comisión Nacional de Estudios Geoheliofísicos.     

Southern peculiar galaxies — V

We give astrometric, photometric and spectroscopic information on an apparent triplet of galaxies already discussed by the author. The compact member of the trio has a redshift of one order of magnitude larger than the brighter member and could represent an important case for an investigation of an anomalous redshift.Under contract with the Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina.     

The atmospheric extinction at the complejo astronómico el leoncito and the Bosque Alegre station

The first-and second-order extinction coefficients in theUBVRI and DDO systems are derived for the first time at CASLEO. They define an extinction curve which is discussed in the framework of atmospheric absorption theory. From this curve the first-order extinction coefficients for the Washington and Strömgren systems are inferred. From a photometric point of view, CASLEO occupies a preferred place among the observatories of the world. A comparison of theUBV extinction coefficients obtained at Bosque Alegre with previous determinations supports the conclusion that the sky transparency at this station has deteriorated in the ultraviolet in the last years.Visting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba y San Juan, Argentina.     

Self-consistent solutions of the semi-classical Einstein equations with cosmological constant and perfect fluid

Exact solutions of the semi-classical Einstein equations with cosmological constant for conformally invariant free quantum fields in a spatially flat Robertson-Walker metric are found when a classical perfect fluid is present. The stability of the asymptotically Friedmann-de Sitter solutions is studied. The formers are found to be stable if < 0, while the stability of the latter depends of the signs of , and A.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas.Supported by scholarship of the Universidad de Buenos Aires.     

Stability of the inflation in R2 cosmology with massless conformal quantum fields

A semiclassical higher-derivative theory of gravity is investigated for conformally invariant free quantum fields in homogeneous and isotropic spacetime with no cosmological constant nor classical source. In the spatially flat case the stability of the de Sitter solution is studied. Moreover, the Minkowski solution is found to be unstable.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas.     

PhotoelectricUBV and DDO photometry of NGC 5138

Results ofUBV photoelectric photometry in NGC 5138 are presented for 50 stars brighter than 14.0 mag. In addition, four probable red giants were also observed in the DDO system. Sixteen stars previously considered members by Lindoff (1972), were found not to be physically connected with the cluster. NGC 5138 is located 1.80 kpc from the Sun and the visual interstellar absorption determined from the reddenedB stars amounts toA _v =0.75 mag. Three of the four red stars observed in the DDO system were found to be cluster members. The mean cyanogen anomaly is CN=0.043±0.018 (m.e.), which implies that NGC 5138 is richer in CN than the field K giants in the solar neighbourhood, but poorer than the Hyades giants. The cluster age is estimated to be 1.5×10⁸ yr.Member of the Carrera del Investigador Cientifico del Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina, and visiting astronomer, Cerro Tololo Inter-American Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Radial velocities in the open cluster IC 2391

We have determined radial velocities for 31 stars in the field of the open cluster IC 2391, and derived the percentage of probable radial-velocity variables. We improved the orbit of one previously known spectroscopic binary, and confirmed the existence of two double-line binaries. We discussed with such data the membership to the cluster, the average cluster radial velocity and compared the incidence of short-period binaries among the Main-Sequence members of IC 2391 with the incidence for other clusters with similar average axial rotation of its members.Member of the Carrera del Investigador Científico del Consejo Nacional de Investigaciones Científicas y Técnicas de la República ArgentinaVisiting Astronomer, Cerro Tololo Inter-American Observatory, operated by AURA, Inc. under contract with the National Science Foundation.     

Models ofHii regions from recombination line observations

In order to study the ionized gas with low electron density the H159 and H200 radio recombination lines (v = 1.62 GHz) atl = 30°5 andl = 31°0 in the galactic plane, observed with the NRAO 43 m radiotelescope, are analyzed. The profiles show the LTE/ intensity ratio for the more distant component of the profiles ( _LSR = 100 km s^–1) (Cersosimo and Onello, 1991).To derive the electron density and temperature of the emitting gas a new interpretation of the radio recombination lines is made. We suppose that the emission originates in a superposition of ionized gas layers with different densities along the line of sight. The number of layers in the model is equal to the numbers of different order transitions observed. By solving an equations set, the contribution of different components can be calculated. The method is used to estimate the electron density and temperature of the gas. Eleven models of two non-LTE components are considered. The photon flux required to ionize the regions are calculated and the results are compared with previous observations obtained in the region at 3 cm (H85, H87, H88 lines) (Lockman, 1989).Our results suggest that the necessary photon flux to ionize the extended envelopes of the regions is at least one order of magnitude larger than that needed for ionizing the core of the regions.Member of the Carrera del Investigador Científico y Tecnológico del Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) from Argentina.Also Instituto Argentino de Radioastronomia.     

Neutral hydrogen observations onHii regions

Nine of the cataloguedHii regions in the southern hemisphere have been selected for correlation with neutral hydrogen observed at the 21 cm line.The radiotelescope used for theHi line observations was the 30 m Carnegie telescope of the Instituto Argentino de Radioastronomía and the 56 channel, 10 KHz bandwidth receiver.The observational results are analyzed for eachHii region. They are compared with previous optical and radio results.For three of the nine observed regions it has been possible to find neutral hydrogen in absorption with similar velocities. In three cases absorption has been found but no component at theHii region velocity is seen. Finally in three cases, it has not been possible to find any absorption at all.Member of the Carrera del Investigador Científico del Cosejo Nacional de Investigaciones Científicas y Técnicas.     

Atmospheric gamma-ray spectrum between 10 and 100 MeV

An experiment made with a baloon-borne large volume (16×16×24 cm³) omnidirectional plastic scintillator sensitive to high-energy -radiation is described. Two flights were carried out on 25 November, 1973 and 24 February, 1978 at high geomagnetic cut-off rigidities. For an omnidirectional detector and for energies greater than several MeV, most of the detected photons are secondary -rays produced through interactions of the charged cosmic rays with the atmosphere. To unfold the observed pulse height spectrum and obtain the incident -ray spectrum, the detector response function is calculated numerically. By using this response function a differential flux dJ/dE=(0.33±0.1)E ^–1.2±0.2 photons cm^–2 s^–1 MeV^–1 averaged over all zenith angles, is estimated for the atmospheric -radiation, at 5 g cm^–2, in the 10–100 MeV energy range. This result is compared with other observations made with different types of detectors and at different geomagnetic latitudes.Members of the Carrera del Investigador Cientifico y Tecnológico del Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) from Argentina.     

Study of the region 348°⩽l⩽12°, +3°⩽b⩽+17° in the 21 cm line

The study of the region 348°l12°, +3°b+17° reveals strong kinematic asymmetries of the interstellar gas, with a notorious predominance of positive radial velocities, referred to the LSR. The principal cause of this is a very intense source, a ridge-like object (Ridge) which presents very special characteristics and seems to be identical with Lindblad's feature A. It should, therefore, be related to Gould's Belt. The object's velocity dependence onl is in good agreement with Lindblad's model. It can be easily followed into high northern latitudes and also into low southern latitudes, as well as into values ofl remarkably lower and higher than those considered in this work. It seems also highly probable that feature A presents both the cold as well as the hot component of the interstellar medium. In that case, the mean optical depth and the mean spin temperature of the cold component would be and K in the region considered here.Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina (CONICET).The Instituto operates under an agreement between the CONICET, the CIC and the Universities of Buenos Aires and La Plata, and with the support of the Carnegie Institution of Washington.     

Magnetoacoustic surface gravity waves

The effect of gravity on the surface magnetoacoustic waves may be important when considering applications to solar and laboratory plasmas. The linear magnetoacoustic waves, which may appear in a configuration with an interface between two plasmas or a plasma and an ordinary gas, are studied. Compressibility and gravity are taken into account. The different types of couplings between internal and surface modes are analyzed. Magnetoacoustic surface waves are studied in detail in a configuration consisting of an interface between an isothermal plasma and an ordinary gas. The possible regions where these modes may exist are discussed. A general way of grouping and classifying the complicated spectra of modes is presented. New groups of modes appear as a consequence of gravity and stratification, in addition to those already present in the absence of gravity. The results may be of help in studying more complicated cases.Researcher of the Consejo Nacional de Investigaciones Científicas y Técnicas.     

On the use and abuse of Newton's second law for variable mass problems

We clarify some misunderstandings currently found in the literature that arise from improper application of Newton's second law to variable mass problems. In the particular case of isotropic mass loss, for example, several authors introduce a force that actually does not exist.Facultad de Ciencias Astronómicas y Geofisicas de la Universidad Nacional de La PlataOn a Fellowship from the Comisión de Investigaciones Cientificas de la Provincia de Buenos Aires.Member of the Carrera del Investigador Cientifico del Consejo Nacional de Investigaciones Cientificas y Técnicas de la República Argentina.     

Large-scale brightenings associated with flares

Flare-associated large-scale (>10¹⁰ cm) X-ray brightenings, the so-called giant arches in the nomenclature of vestka and co-workers, were discovered in images obtained by the SMM Hard X-ray Imaging Spectrometer hours after the onset of two-ribbon flares. The apparent correlation between both phenomena suggested that they could be interpreted in the framework of the same model.In this paper we show that large-scale loop brightenings, of sizes similar to the giant arches, occur also in association with confined flares in complex active regions. In these cases, the relation between the large-scale structure and the underlying flare is clearly given by the magnetic field topology. We also show that energization of these structures can be partially due to the injection of suprathermal particles that are accelerated at the separator region.We discuss the implications of these results within the framework of the interacting loops picture of flares and of the giant arch phenomenology.Member of the Carrera del Investigador Científico, CONICET, Argentina.     

On the origin of the flare optical continuum

We show that the bluish-white emission knots, observed in white-light flares, can be due to the existence of a quite large amount of material at a temperature T _e 2×10⁴ K in the Lyman- forming region.Av. Mitre 3100, San Miguel, Pcia de Buenos Aires, Argentina.     

The Web of Periodic Orbits at L 4

We describe and comment the results of a numerical exploration of the numerous natural families of periodic orbits associated with the L ₄ equilibrium point of the restricted problem of three bodies (and of course by symmetry those associated with the L ₅ equilibrium point). These families are organized in a very structured network or coweb and this structure evolves, when the mass ratio varies, in a very organized way.     

Bifurcation points in close binary evolution: Medium-mass binaries

The remaining core hydrogen burning lifetime after case B of mass transfer of the secondary (mass gaining) component in a medium mass close binary star is estimated, for mediummass binaries with primaries in the mass range 5M to 9M . From the comparison of this quantity with the helium burning time-scale of the remnant primary a critical mass ratioq _c is derived such that for larger values ofq, mass transfer from the secondary towards the primary starts before the latter has evolved into a white dwarf. Consequences for the advanced stages of medium mass binaries are discussed.     

Astrophysical properties of red giants in three open clusters older than the hyades

Results fromCMT ₁T₂ 1T2 broad-band and DDO intermediate-band photometry are presented for G and K giants in the old open clusters NGC. 2482, NGC 3680, and IC 4651. Two independent photometric criteria have been used to separate red field stars from the physical members of the clusters. Recent calibrations of the DDO andCMT ₁T₂ systems have been used to derive reddening, distance moduli, metallicities, effective temperatures, and surface gravities. Rough estimates of masses have also been made. The giants of NGC 2482 and IC 4651 have CN strengths nearly identical to the Hyades giants, while those of NGC 3680 are slightly richer in CN than the nearby K giants.CMT1T2 abundance analysis in NGC 2482 and NGC 3680 yield [Fe/H]_MT = - 0.1 ± 0.1 as derived from the iron lines, while abundances derived from the CNO - contaminated (C - M) index are 0.4 dex higher. BothCMT ₁T₂ and DDO data support the conclusion that 1C 4651, with [Fe/H] = + 0.2 ± 0.1, is on the metalrich side of the distribution of intermediate and old open clusters. Finally, the mass results suggest that the clump stars in NGC 3680 and. IC 4651 could have undergone mass loss before reaching their helium core burning phase of evolution. Supported in part by the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) of Argentina. Visiting astronomer of Cerro Tololo Inter-American Observatory supported by the National Science Foundation under contract No. AST 74-04128.     

Dust and atomic hydrogen nearρ Ophiuchi

We discuss the distribution of dust and atomic hydrogen in and around a small dark cloud near Ophiuchi. Although the obscuration reaches values as high as 8 magnitudes, the values ofN _H derived inside the cloud do not differ markedly from those found at nearby, less obscured points. At low values of the obscuration, dust and hydrogen isophotes show satisfactory agreement, i.e.,N _H increases withN _G. However, we find thatN _H starts decreasing for sufficiently large values ofN _G. In the entire range of optical extinctions investigated our observations are satisfied by an expression of the typeN _H=AN _G exp (-BN _G). Interpretation of this relationship suggests that an appreciable part of the hydrogen has been converted to molecular form inside the cloud. A tentative method for calculating the time elapsed since the conversion process began is proposed.Contribution No. 2 from the Argentine-Carnegie Radio Astronomy Station of the Instituto argentino de Radioastronomía and the Department of Terrestrial Magnetism of the Carnegie Institution of Washington.