Observations of comet impact by reflections from the Galilean satellites

Results of observations of the Galilean satellites during the week of the impact of comet Shoemaker-Levy 9 on Jupiter are reported. These observations have been carried out at the Pico dos Dias observatory, at Brasópolis, Brazil. The satellites's light flux was monitored with a photometer during the time of the predicted impact of fragments B, F, J and L. No conclusive flash was observed in any of the data adquired. This non-detection imposes an upper limit to the energy released during the impact of these fragments.Observations carried out at the Observatório do Pico dos Dias, operated by the Laboratório Nacional de Astrofísica/CNPq/MCT, Brazil     

Spectrograph distortion in sunspot line profiles

We show empirically how the effect of the instrumental polarization of the spectrograph can distort the shape of the intensity profiles in sunspots. In order to avoid the problems an analysis of the polarization of the light should be performed at the entrance slit of the spectrograph.Based on observations made with the Vakuum Tower Teleskop (V.T.T.) operated on the island of Tenerife by the Kiepenheuer-Institut für Sonnenphysik (Freiburg, F.R.G.) in the Spanish Observatorio del Teide of the Institute de Astrofísica de CanariasThe National Center for Atmospheric Research is sponsored by the National Science Foundation     

OH and O2 airglow emissions during the 1998 leonid outburst and the 2002 leonid storm

In order to enhance our understanding of the possible influence of meteor ablation on the enrichment in OH and O₂ of the lower thermosphere we studied intense Leonid meteor activity by using the SATI (Spectral Airglow Temperature Imager) interferometer of the Instituto de Astrofísica de Andalucía. We measured the emission rate and rotational temperature of OH and O₂ airglow emission layers during two observation periods of high meteoric activity: the 1998 Leonid outburst and the 2002 Leonid storm. The results show that there is not a clear relation of O₂ and OH airglow emission and rotational temperature with meteoric activity.     

Five minute microwave solar oscillations

Oscillations with a period of 5.6 min were observed on 10 July, 1978 while tracking at 22 GHz the active region McMath 15403. The oscillations were strong, clearly defined, had no damping, and lasted for about two hours. The rarity of the phenomenon is indicated by the fact that it occurred only once in more than 250 hr of solar observations. The possibility that these oscillations are due to a standing Alfvén wave driven by the photospheric velocity field is discussed.On sabbatical leave from Technion, Haifa, Israel.Formerly: Centro de Radio-Astronomia e Astrofísica Mackenzie, now with Brazilian National Research Agency CNPq, National Observatory.     

New Activity of Chiron: Results from 5 Years of Photometric Monitoring

The results of photometric observations of Centaur object Chiron carried out at the Observatório do Pico dos Dias (OPD, Brazil), the Estación Astrof?́sica de Bosque Alegre (EABA, Argentina), and Complejo Astronómico El Leoncito (CASLEO, Argentina) from 1997 to 2001 are presented here. The analysis of the photometric data shows that the brightness of Chiron reached a minimum value in 1999 and began increasing again in 2000. The absolute magnitude, H_V, varied from 7.26 in June 1999 to 5.78 in April 2001. The data tend to indicate that Chiron is starting a new outburst of activity which is compatible with a sporadic cometary behavior not related to heliocentric distance.     

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.     

A comparison between compton-synchrotron and Compton black-body emission in radio sources

A simple relation between observational parameters of radiosources that allows to state the predominance in the X-ray emission of either the Compton-synchrotron or the Compton black-body effect is derived.The obtained results suggest that the Compton-synchrotron contribution is dominant in compact radiosources.Operated by the CNICT with the participation of the University of Buenos Aires and the Comisión Nacional de Estudios Geoheliofísicos.     

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     

White dwarf evolution and crystallization

We present a numerical study for the evolution of non-DA white dwarfs of 0.4, 0.55, 0.8, 1.0, and 1.2M . We pay special attention to the behaviour of the crystallization front. It is shown that crystallization begins at higher luminosities the higher the white dwarf mass is. The shape of the crystal growth function is very similar, almost independent of the value of the total stellar mass. We also study the crystallization process analytically, finding that it is nicely reproduced by a very simple model that accounts for the numerical results.Member of the Carrera del Investigador Científico, Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (Argentina).Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas (Argentina).     

The effect of charge exchange reaction on carbon lines

The effect of charge exchange reactions: carbon ions with neutral hydrogen and carbon ions with neutral helium, on carbon ionization equations is analysed. Both, optically thin and optically thick, photoionization models for the line emission region of QSOs are considered. From the computed line intensities it is suggested that the charge exchange mechanism should be included in future models.Partially supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil.     

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.     

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.     

4.7s nearly periodic oscillations superimposed on the solar microwave great burst of 28 March 1976

An unusual fast oscillation was found superimposed on the solar great burst of 28 March 1976, as measured at 7 GHz. The period of the osculation was 4.7 ± 0.9s, defined over the entire duration of the event. The amplitude of the oscillation was proportional to the flux density, in the range 50 < S < 3000 solar flux units. The degree of circular polarization has not shown any fast periodic time structures.Presently merged with CNP, Observatório Nacional.     

A quantitative comparison of vector magnetic field measurement and analysis techniques

We make a quantitative comparison between spectral vs filter measurement and analysis techniques for extraction of solar vector magnetic fields from polarimetric data using as a basis the accurately calibrated, high angular resolution Stokes profile data from the Advanced Stokes Polarimeter. It is shown that filter-based measurements deliver qualitative images of the field alignment for sunspots that are visually similar to images derived from the more detailed analysis of the Stokes profiles. However, quantitative comparison with least-squares fits to the full Stokes profiles show that both the strength of the field predicted by the filter-based analysis and its orientation contain substantial errors. These errors are largest for plage regions outside of sunspots, where the field strengths are inferred to be only a fraction of their true values, and errors in the orientation of 40–50° are common. Within sunspots, errors of 20° are commonplace. The greatest source of these errors is the inability of the filter-based measurements to account for the small fill fraction of magnetic fields or, equivalently, scattered light in the instrument, which reduce the degree of polarization. The uncertainties of the full profile fitting methods are also discussed, along with the errors introduced by coarser wavelength sampling of the observed Stokes profiles. The least-squares fitting procedure operates best when the profiles are sampled at least as frequently as one Doppler width of the line.On leave from the Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Spatially resolved spectroscopic observations of Jupiter during the 1987 opposition

Spectroscopic observations in the spectral region 6000–6600 » of major belts and zones of Jupiter have been carried out at Calar Alto Observatory (Spain) during the 1987 opposition, in support to the International Jupiter Watch Program. More than a hundred long-slit, medium resolution spectra have been taken at the coudé focus of the 1.52 m telescope. The longitudinal and latitudinal distribution of the equivalent widths of the methane and ammonia absorption bands at 6190 » and 6450 » have been investigated. Unlike the CH₄-6190 » band, the longitudinal distribution in the NH₃-6450 » band shows asymmetries in some of the Jovian regions analyzed, being the absorption greater at West than at East limbs. These results are interpreted in terms of inhomogeneous scattering models by the simultaneous variation of the NH₃ cloud optical thickness and its altitude level. The north-south scans show little latitudinal dependence on absorption in both methane and ammonia bands, although some relationship could be established between the absorption and relative continuum reflectivity variation.

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Resumen Se presentan los resultados obtenidos de las observaciones espectroscópicas de las principales zonas y cinturones de Júpiter realizadas durante la oposición de 1987 como una contribución al International Jupiter Watch Program. Las observaciones fueron llevadas a cabo haciendo uso del foco coudé del telescopio de 1.52 m del observatorio hispanoalemán de Calar Alto (España), habiéndose obtenido algo más de cien espectros, de resolución media, en la región espectral de 6000–6600 ». Se exponen los resultados de las anchuras equivalentes de las bandas de absorción del metano en 6190 » y del amoniaco en 6450 », estudiándose, al mismo tiempo, la distribución tanto longitudinal como latitudinal de las mismas. Mientras que los resultados para la banda del metano presentan un comportamiento simétrico en todas las regiones analizadas, las anchuras equivalentes para la banda NH₃-6450 » presentan un comportamiento asimétrico en algunas regiones del planeta, con una mayor absorción hacia el limbo del oeste que hacia el del este. Los resultados son interpretados en base a un modelo de scattering no homogéneo, variando, simultáneamente, el espesor óptico y la altitud de la nube de amoniaco. Los espectros tomados en la dirección norte-sur, aunque presentan una dependencia latitudinal pequeña en la absorción de las bandas mencionadas, sugieren una relación entre la absorción y las variaciones de reflectividad en el continuo.

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Also at Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Campus Universitario Fuentenueva, Granada, Spain.     

Photoionization models and chemical abundances of three halo planetary nebulae

Abtracts Photoionization models have been constructed to explain the line emission of the three halo planetary nebulae: K 648, 49+88°.1 and 108–76°.1. The charge transfer reactions between neutral hydrogen and different ions were taken into account in ionization equations. Due to lack of observational information, radiation-bounded as well as matter-bounded models have been considered. Both these models indicate that He and C abundances are close to solar values whereas the heavier elements are depleted.Partially supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil.This paper was entirely written when I received a preprint of Peimbert and Torres-Peimbert (1979) whose results confirm a high abundance of carbon in the planetary nebula 49+88°.1.     

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.     

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.     

Analysis of Neckel and Chini standard stars in theUBVRI photometric system

This paper studies the list of 60 faint standard stars of Neckel and Chini (1980) in theUBVRI Johnson photometric system. These standards have been used due to the collaboration in the HIPPARCOS project of our department, but this work was made in parallel of this and the results were not used.We have re-reduced the standard stars using our own method of reduction from a large number of selected observations made at the Observatorio Astronómico Nacional and at the Centro Astronómico Hispano-Alemán, both at Calar Alto, Spain, and at the Observatorio del Roque de los Muchachos, La Palma Island, Spain.The reduction method used is explained and the results are given for the four colours used: (B-V),(U-B), (V-R), and (V-I), together with theV magnitude, that is, we have corrected many standard stars for each colour and magnitude. These results are supported with graphs where residual colour (o magnitude), is plotted against corresponding colour (or magnitude).Finally, our (V-R) and (V-I) results are compared to those of Tayloret al. (1989), with a good correspondence.Observations were made at Observatorio Astronómico Nacional and Centro Astronómico Hispano-Alemán, at Calar Alto, and in the Observatorio del Roque de los Muchachos at La Palma. This work was supported, in part, by the Comisión Interministerial de Ciencia y Tecnologia of Spain, under contract PB85-0017.     

Putative Meteoritic Craters in Río Cuarto (Central Argentina) Interpreted as Eolian Structures

Ten oblong aligned depressions in the Río Cuarto area (provincia de Córdoba, Argentina) were supposed to be the result of very-low-angle Holocene meteoroid impacts. However, we consider that authors that studied the structures did not demonstrated their extraterrestrial origin. We suggest that an eolian origin for the structures of Río Cuarto is more likely. Actually, these landforms integrate large systems of similar deflation/accumulation geoforms aligned according to predominant winds during different periods.