TY - Journal T1 - The rate of period change in pulsating DB white dwarf stars A1 - Córsico, A. H. A1 - Althaus, L. G. JO - Astronomy and Astrophysics VL - 428 Y1 - 2004/12/1 SP - 159 EP - 170 KW - dense matter/ stars: evolution/ stars: white dwarfs/ stars: oscillations UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...428..159C&db_key=AST&high=42a068d47504913 N2 - In this work, we present the theoretically expected rates of pulsation period change for V777 Her (DBV) variable stars. To this end we employ new evolutionary models representative of pulsating DB white dwarf stars computed in a self-consistent way with the predictions of time-dependent element diffusion. At the hot edge of the DB instability strip, the envelopes of the models are characterized by a diffusion-induced double-layered chemical structure. We compute the numerical values of rates of period change by solving the equations of linear, adiabatic, nonradial stellar oscillations. We examine the effects of varying the stellar mass, the mass of the helium envelope and the neutrino emission on the expected period changes. We present extensive tabulations of our results which could be useful for comparison with future detections of the rate of period change in pulsating DB white dwarfs. ER - TY - Journal T1 - Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores A1 - Córsico, A. H. A1 - García-Berro, E. A1 - Althaus, L. G. A1 - Isern, J. JO - Astronomy and Astrophysics VL - 427 Y1 - 2004/12/1 SP - 923 EP - 932 KW - stars: evolution/ stars: white dwarfs/ stars: oscillations UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...427..923C&db_key=AST&high=42a068d47504913 N2 - We explore the adiabatic pulsational properties of massive white dwarf stars with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this end, we compute the cooling of massive white dwarf models for both core compositions taking into account the evolutionary history of the progenitor stars and the chemical evolution caused by time-dependent element diffusion. In particular, for the oxygen/neon models we adopt the chemical profile resulting from repeated carbon-burning shell flashes expected in very massive white dwarf progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles resulting from phase separation upon crystallization. For both compositions we also take into account the effects of crystallization on the oscillation eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs are noticeably different from those made of carbon/oxygen, thus making asteroseismological techniques a promising way to distinguish between the two types of stars and, hence, to obtain valuable information about their progenitors. ER - TY - Journal T1 - The effects of element diffusion on the pulsational properties of variable DA white dwarf stars A1 - Córsico, A. H. A1 - Benvenuto, O. G. A1 - Althaus, L. G. A1 - Serenelli, A. M. JO - Monthly Notices of the Royal Astronomical Society VL - 332 Y1 - 2002/5/1 SP - 392 EP - 398 KW - stars: evolution/ stars: interiors/ stars: oscillations/ white dwarfs UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002MNRAS.332..392C&db_key=AST&high=42a068d47504913 N2 - We explore the effects of element diffusion due to gravitational settling and thermal and chemical diffusion on the pulsational properties of DA white dwarfs. To this end, we employ an updated evolutionary code coupled with a pulsational, finite difference code for computing the linear, non-radial g-modes in the adiabatic approximation. We follow the evolution of a 0.55-Msolar white dwarf model in a self-consistent way with the evolution of chemical abundance distribution as given by time-dependent diffusion processes. Results are compared with the standard treatment of diffusive equilibrium in the trace element approximation. Appreciable differences are found between the two employed treatments. We conclude that time-dependent element diffusion plays an important role in determining the whole oscillation pattern and the temporal derivative of the periods in DAV white dwarfs. In addition, we discuss the plausibility of the standard description employed in accounting for diffusion in most white dwarf asteroseismological studies. ER - TY - Journal T1 - The mode trapping properties of full DA white dwarf evolutionary models A1 - Córsico, A. H. A1 - Althaus, L. G. A1 - Benvenuto, O. G. A1 - Serenelli, A. M. JO - Astronomy and Astrophysics VL - 387 Y1 - 2002/5/1 SP - 531 EP - 549 KW - stars: evolution/ stars: interiors/ stars: white dwarfs/ stars: oscillations UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...387..531C&db_key=AST&high=42a068d47504913 N2 - An adiabatic, non-radial pulsation study of a 0.563 Msun DA white dwarf model is presented on the basis of new evolutionary calculations performed in a self-consistent way with the predictions of time dependent element diffusion, nuclear burning and the history of the white dwarf progenitor. Emphasis is placed on the role played by the internal chemical stratification of these new models in the behaviour of the eigenmodes, and the expectations for the full g-spectrum of periods. The implications for the mode trapping properties are discussed at length. In this regard, we find that, for high periods, the viability of mode trapping as a mode selection mechanism is markedly weaker for our models, as compared with the situation in which the hydrogen-helium transition region is treated assuming equilibrium diffusion in the trace element approximation. ER - TY - Journal T1 - A New Code for Nonradial Stellar Pulsations and its Application to Low-Mass, Helium White Dwarfs A1 - Córsico, A. H. A1 - Benvenuto, O. G. JO - Astrophysics and Space Science VL - 279 Y1 - 2002/1/1 SP - 281 EP - 300 UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Ap%26SS.279..281C&db_key=AST&high=42a068d47504913 N2 - We present a finite difference code intended for computing linear, adiabatic, nonradial pulsations of spherical stars. This code is based on a slight modification of the general Newton-Raphson technique in order to handle the relaxation of the eigenvalue (square of the eigenfrequency) of the modes and their corresponding eigenfunctions. This code has been tested computing the pulsation spectra of polytropic spheres finding a good agreement with previous work. Then, we have coupled this code to our evolutionary code and applied it to the computation of the pulsation spectrum of a low mass, pure-helium white dwarf of 0.3 M_solar for a wide range of effective temperatures. In making this calculation we have taken an evolutionary time step short enough such that eigenmodes corresponding to a given model are used as initial approximation to those of the next one. Specifically, we have computed periods, period spacing, eigenfunctions, weight functions, kinetic energies and variational periods for a wide range of modes. To our notice this is the first effort in studying the pulsation properties of helium white dwarfs. The solution we have found working with these realistic white dwarf models are in good accord with the predictions of the asymptotic theory of Tassoul (1980) for high order modes. This indicates that the code presented here is able to work adequately also with realistic stellar models. ER - TY - Journal T1 - New DA white dwarf evolutionary models and their pulsational properties A1 - Córsico, A. H. A1 - Althaus, L. G. A1 - Benvenuto, O. G. A1 - Serenelli, A. M. JO - Astronomy and Astrophysics VL - 380 Y1 - 2001/12/1 SP - L17 EP - L20 KW - STARS: EVOLUTION/ STARS: INTERIORS/ STARS: WHITE DWARFS/ STARS: OSCILLATIONS UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001A%26A...380L..17C&db_key=AST&high=42a068d47504913 N2 - In this letter we investigate the pulsational properties of ZZ Ceti stars on the basis of new white dwarf evolutionary models calculated in a self-consistent way with the predictions of time dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to the white dwarf formation. Emphasis is placed on the trapping properties of such models. By means of adiabatic, non-radial pulsation calculations, we find, as a result of time dependent diffusion, a much weaker mode trapping effect, particularly for the high-period regime of the pulsation g-spectrum. This result is valid at least for models with massive hydrogen-rich envelopes. Thus, mode trapping would not be an effective mechanism to explain the fact that all the high periods expected from standard models of stratified white dwarfs are not observed in the ZZ Ceti stars. ER - TY - Journal T1 - The potential of the variable DA white dwarf G117-B15A as a tool for fundamental physics A1 - Córsico, A. H. A1 - Benvenuto, O. G. A1 - Althaus, L. G. A1 - Isern, J. A1 - García-Berro, E. JO - New Astronomy VL - 6 Y1 - 2001/6/1 SP - 197 EP - 213 UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001NewA....6..197C&db_key=AST&high=42a068d47504913 N2 - White dwarfs are well studied objects. The relative simplicity of their physics allows one to obtain very detailed models which can be ultimately compared with their observed properties. Among white dwarfs there are specific classes of stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show periodic variations in their light curves. G117-B15A belongs to this particular set of stars. The luminosity variations have been successfully explained as due to /g-mode pulsations. G117-B15A has recently claimed to be the most stable optical clock ever found, being the rate of change of its 215.2 s period very small: Ṗ=(2.3+/-1.4)×10-15 ss-1, with a stability comparable to that of the most stable millisecond pulsars. The rate of change of the period is closely related to its cooling timescale, which can be accurately computed. In this paper we study the pulsational properties of G117-B15A and we use the observed rate of change of the period to impose constraints on the axion emissivity and thus, to obtain a preliminary upper bound to the mass of the axion. This upper bound turns out to be 4 cos2betameV at the 95% confidence level. Although there are still several observational and theoretical uncertainties, we conclude that G117-B15A is a very promising stellar object to set up constraints on particle physics. ER - TY - Journal T1 - Crustal thinning in the Southwestern Iberia Margin A1 - González, A. A1 - Torné, M. A1 - Córdoba, D. A1 - Vidal, N. A1 - Matias, L. M. A1 - Díaz, J. JO - Geophysical Research Letters VL - 23 Y1 - 1996/1/1 SP - 2477 EP - 2480 KW - Marine Geology and Geophysics: Marine seismics/ Exploration Geophysics: Seismic methods/ Marine Geology and Geophysics: Gravity/ Exploration Geophysics: Data processing UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1996GeoRL..23.2477G&db_key=AST&high=42a068d47524183 N2 - The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide-angle seismic reflection data, whereas offshore we have used coincident steep and wide-angle reflection data along a NE-SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km. ER - TY - Journal T1 - Spectroscopic Binaries and Kinematic Membership in the Open Cluster NGC 3532 A1 - González, Jorge Federico A1 - Lapasset, Emilio JO - Astronomical Journal VL - 123 Y1 - 2002/6/1 SP - 3318 EP - 3324 KW - Stars: Binaries: Spectroscopic/ Galaxy: Open Clusters and Associations: Individual: NGC Number: NGC 3532/ Techniques: Radial Velocities UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002AJ....123.3318G&db_key=AST&high=42a068d47527134 N2 - We report spectroscopic observations for bright stars in the open cluster NGC 3532 up to 1 mag below the turnoff point. We compute radial velocities by cross-correlations and determine spectral types and rotational velocities. Using 21 stars identified as certain members, we derive a mean cluster velocity of +3.4+/-0.3 km s-1. From radial velocities and angular distances to the cluster center, we compute membership probabilities for all but two stars that are radial velocity variables. Only one out of 34 program stars is a clear kinematic nonmember. Three spectroscopic binaries and three additional possible radial velocity variables are detected among the 23 stars measured more than once. We report the star HD 96609 as a double-lined spectroscopic binary. Using the two-dimensional cross-correlation technique TODCOR developed by Zucker & Mazeh, we derive the radial velocity curves for both components and obtain the orbital parameters with errors of 0.3% and 0.7% for the projected orbital semiaxis and masses, respectively. This system is composed of two main-sequence stars in a circular orbit, with a period of 8.19 days. The observations presented here were obtained at the Complejo Astronómico El Leoncito (CASLEO), which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET) and the National Universities of La Plata, Córdoba, and San Juan. ER - TY - Journal T1 - Radial Velocities and Kinematic Membership in the Open Cluster NGC 3114 A1 - González, Jorge Federico A1 - Lapasset, Emilio JO - Astronomical Journal VL - 121 Y1 - 2001/5/1 SP - 2657 EP - 2663 KW - Stars: Blue Stragglers/ Galaxy: Open Clusters and Associations: Individual: NGC Number: NGC 3114/ Techniques: Radial Velocities UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001AJ....121.2657G&db_key=AST&high=42a068d47527134 N2 - Echelle spectroscopic observations for 30 bright stars in the field of the sparse open cluster NGC 3114 are presented. The sample includes main-sequence stars, yellow and red giants, and blue straggler candidates. Radial velocities are derived by cross-correlations using high signal-to-noise ratio standard spectra as templates. The cluster mean velocity is well defined from eight giants and several main-sequence stars whose average is =-3.52+/-0.25 km s-1. The membership probabilities of the observed stars are computed on the basis of the velocity distributions of the cluster and field stars, and the expected percentage of contamination at each position. We classified 19 cluster members and 10 nonmembers; the remaining star is a known spectroscopic binary for which no membership probability was assigned. Among the members, there is a bright yellow giant, seven red giants, and four blue straggler candidates, although they should be considered as turn-off stars. The location of two of them in the color-magnitude diagram (slightly blueward of the turn-off) can be explained by their low rotational velocities. No velocity variations were detected in the 16 stars measured more than once, which indicates that NGC 3114 possess an abnormally low binary frequency. From spectral types of cluster members, a distance modulus (V-Mv)=9.8+/-0.2 mag and a reddening E(B-V)=0.07+/-0.01 mag are derived. The cluster age is estimated to be 1.6×108 yr. The observations presented here were obtained at the Complejo Astronómico El Leoncito (CASLEO), which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET) and the National Universities of La Plata, Córdoba and San Juan. ER - TY - Journal T1 - Radial Velocities, Binarity, and Kinematic Membership in the Open Cluster NGC 2516 A1 - González, Jorge Federico A1 - Lapasset, Emilio JO - Astronomical Journal VL - 119 Y1 - 2000/5/1 SP - 2296 EP - 2302 KW - STARS: BINARIES: SPECTROSCOPIC/ STARS: BLUE STRAGGLERS/ GALAXY: OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: NGC NUMBER: NGC 2516/ TECHNIQUES: RADIAL VELOCITIES UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000AJ....119.2296G&db_key=AST&high=42a068d47527134 N2 - We present echelle spectroscopic observations for 36 bright (V<9.6) stars in the open cluster NGC 2516, including several blue straggler candidates and four red giants. Radial velocities are derived by cross-correlations using high signal-to-noise ratio standard spectra as templates. From 22 cluster members a mean cluster velocity of +22.0+/-0.2 km s-1 was derived. Membership probabilities of the observed stars are computed on the basis of their distance to the cluster center and kinematic criteria. We report the discovery of three double-lined spectroscopic binaries and several probable binaries among main-sequence stars. A binary frequency of more than 26% is found among the high-mass main-sequence stars. The blue straggler HD 66341 is a slowly rotating cluster member with constant velocity, while HD 66194 is a fast-rotating Be star with probable variations in radial velocity. Other blue straggler candidates, such as HD 65663, 65950, 66066, and 65987, must be considered turnoff stars. The observations presented here were obtained at the Complejo Astronómico El Leoncito (CASLEO), which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET) and the national universities of La Plata, Córdoba, and San Juan. ER - TY - Journal T1 - Magnetic twist and writhe of active regions. On the origin of deformed flux tubes A1 - López Fuentes, M. C. A1 - Démoulin, P. A1 - Mandrini, C. H. A1 - Pevtsov, A. A. A1 - van Driel-Gesztelyi, L. JO - Astronomy and Astrophysics VL - 397 Y1 - 2003/1/1 SP - 305 EP - 318 KW - magnetic fields/ methods: data analysis/ Sun: interior/ Sun: magnetic fields/ Sun: photosphere UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003A%26A...397..305L&db_key=AST&high=42a068d47519327 N2 - We study the long term evolution of a set of 22 bipolar active regions (ARs) in which the main photospheric polarities are seen to rotate one around the other during several solar rotations. We first show that differential rotation is not at the origin of this large change in the tilt angle. A possible origin of this distortion is the nonlinear development of a kink-instability at the base of the convective zone; this would imply the formation of a non-planar flux tube which, while emerging across the photosphere, would show a rotation of its photospheric polarities as observed. A characteristic of the flux tubes deformed by this mechanism is that their magnetic twist and writhe should have the same sign. From the observed evolution of the tilt of the bipoles, we derive the sign of the writhe of the flux tube forming each AR; while we compute the sign of the twist from transverse field measurements. Comparing the handedness of the magnetic twist and writhe, we find that the presence of kink-unstable flux tubes is coherent with no more than 35% of the 20 cases for which the sign of the twist can be unambiguously determined. Since at most only a fraction of the tilt evolution can be explained by this process, we discuss the role that other mechanisms may play in the inferred deformation. We find that 36% of the 22 cases may result from the action of the Coriolis force as the flux tube travels through the convection zone. Furthermore, because several bipoles overpass in their rotation the mean toroidal (East-West) direction or rotate away from it, we propose that a possible explanation for the deformation of all these flux tubes may lie in the interaction with large-scale vortical motions of the plasma in the convection zone, including also photospheric or shallow sub-photospheric large scale flows. ER - TY - Journal T1 - The Counterkink Rotation of a Non-Hale Active Region A1 - López Fuentes, M. C. A1 - Demoulin, P. A1 - Mandrini, C. H. A1 - van Driel-Gesztelyi, L. JO - Astrophysical Journal VL - 544 Y1 - 2000/11/1 SP - 540 EP - 549 KW - Sun: Activity/ Sun: Corona/ Sun: Interior/ Sun: Magnetic Fields UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000ApJ...544..540L&db_key=AST&high=42a068d47519327 N2 - We describe the long-term evolution of a bipolar non-Hale active region that was observed from 1995 October to 1996 January. During these four solar rotations the sunspots and subsequent flux concentrations, during the decay phase of the region, were observed to move in such a way that by December their orientation conformed to the Hale-Nicholson polarity law. The sigmoidal shape of the observed soft X-ray coronal loops allows us to determine the sense of the twist in the magnetic configuration. This sense is confirmed by extrapolating the observed photospheric magnetic field, using a linear force-free approach, and comparing the shape of computed field lines with the observed coronal loops. This sense of twist agrees with that of the dominant helicity in the solar hemisphere where the region lies, as well as with the evolution observed in the longitudinal magnetogram during the first rotation. At first sight the relative motions of the spots may be misinterpreted as the rising of an Omega loop deformed by a kink instability, but we deduce from the sense of their relative displacements a handedness for the flux-tube axis (writhe) that is opposite to that of the twist in the coronal loops and, therefore, to what is expected for a kink-unstable flux tube. After excluding the kink instability, we interpret our observations in terms of a magnetic flux tube deformed by external motions while rising through the convective zone. We compare our results with those of other related studies, and we discuss, in particular, whether the kink instability is relevant to explain the peculiar evolution of some active regions. ER - TY - Journal T1 - Astrometry with Virtual Observatories A1 - López, Carlos E. JO - Astrophysics and Space Science VL - 290 Y1 - 2004/4/1 SP - 439 EP - 447 KW - astrometry/ databases/ new techniques UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004Ap%26SS.290..439L&db_key=AST&high=42a068d47521753 N2 - With the advent of virtual observatories, most astrometry-oriented projects being currently developed---and those to come in the near future---will be greatly enriched. Among these projects it is worth mentioning double stars and multiple systems, proper-motion detections, identification of lost high-proper-motion stars, a detailed census of open cluster members, and precovery of natural satellites, minor planets and comets. A brief discussion of some of these studies is presented. To accomplish the goals proposed by the virtual observatories concept, a massive amount of high-quality astrometric data is needed in advance. It is shown that today's astrometry is about to ready to face these new challenges. ER - TY - Journal T1 - Determination of the linear mass power spectrum from the mass function of galaxy clusters A1 - Sánchez, Ariel G. A1 - Padilla, Nelson D. A1 - Lambas, Diego G. JO - Monthly Notices of the Royal Astronomical Society VL - 337 Y1 - 2002/11/1 SP - 161 EP - 171 KW - cosmology: theory/ large-scale structure of Universe UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002MNRAS.337..161S&db_key=AST&high=42a068d47513999 N2 - We develop a new method to determine the linear mass power spectrum using the mass function of galaxy clusters. We obtain the rms mass fluctuation sigma(M) using the expression for the mass function in the Press & Schechter, Sheth, Mo & Tormen and Jenkins et al. formalisms. We apply different techniques to recover the adimensional power spectrum Delta2(k) from sigma(M) namely the keff approximation, the singular value decomposition and the linear regularization method. The application of these techniques to the tauCDM and LambdaCDM GIF simulations shows a high efficiency in recovering the theoretical power spectrum over a wide range of scales. We compare our results with those derived from the power spectrum of the spatial distribution of the same sample of clusters in the simulations obtained by application of the classical Feldman, Kaiser & Peacock (FKP) method. We find that the mass function based method presented here can provide a very accurate estimate of the linear power spectrum, particularly for low values of k. This estimate is comparable to, or even better behaved than, the FKP solution. The principal advantage of our method is that it allows the determination of the linear mass power spectrum using the joint information of objects of a wide range of masses without dealing with specific assumptions on the bias relative to the underlying mass distribution. ER - TY - Journal T1 - Galaxy groups in the 2dF Galaxy Redshift Survey: luminosity and mass statistics A1 - Martínez, H. J. A1 - Zandivarez, A. A1 - Merchán, M. E. A1 - Domínguez, M. J. L. JO - Monthly Notices of the Royal Astronomical Society VL - 337 Y1 - 2002/12/1 SP - 1441 EP - 1449 KW - galaxies: clusters: general/ galaxies: luminosity function/ mass function/ galaxies: statistics UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002MNRAS.337.1441M&db_key=AST&high=42a068d47529002 N2 - Several statistics are applied to groups and galaxies in groups in the 2° Field Galaxy Redshift Survey. First, we estimate the luminosity functions for different subsets of galaxies in groups. The results are well fitted by a Schechter function with parameters M*- 5 log (h) =-19.90 +/- 0.03 and alpha=-1.13 +/- 0.02 for all galaxies in groups, which is quite consistent with the results of Norberg et al. for field galaxies. When considering the four different spectral types defined by Madgwick et al. we find that the characteristic magnitude is typically brighter than in the field. We also observe a steeper value, alpha=-0.76 +/- 0.03, of the faint end slope for low star-forming galaxies when compared with the corresponding field value. This steepening is more conspicuous, alpha=-1.10 +/- 0.06, for those galaxies in more massive groups than that obtained in the lower-mass subset, . Secondly, we compute group total luminosities using the prescriptions of Moore, Frenk & White. We define a flux-limited group sample using a new statistical tool developed by Rauzy. The resulting group sample is used to determine the group luminosity function and we find a good agreement with previous determinations and semi-analytical models. Finally, the group mass function for the flux-limited sample is derived. An excellent agreement is obtained when comparing our determination with analytical predictions over two orders of magnitude in mass. ER - TY - Journal T1 - Galaxy groups in the 2dF Galaxy Redshift Survey: effects of environment on star formation A1 - Martínez, H. J. A1 - Zandivarez, A. A1 - Domínguez, M. A1 - Merchán, M. E. A1 - Lambas, D. G. JO - Monthly Notices of the Royal Astronomical Society VL - 333 Y1 - 2002/6/1 SP - L31 EP - L34 KW - galaxies: clusters: general/ galaxies: fundamental parameters/ galaxies: general/ galaxies: statistics/ galaxies: stellar content UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002MNRAS.333L..31M&db_key=AST&high=42a068d47529002 N2 - We estimate the fraction of star-forming galaxies in a catalogue of groups, constructed from the 2dF Galaxy Redshift Survey by Merchán & Zandivarez. We use the eta spectral type parameter of galaxies and subdivide the sample of galaxies in groups into four types depending on the values of the eta parameter following Madgwick et al. We obtain a strong correlation between the relative fraction of galaxies with high star formation and the parent group virial mass. We find that even in the environment of groups with low virial mass M~1013Msolar the star formation of their member galaxies is significantly suppressed. The relation between the fraction of early-type galaxies and the group virial mass obeys a simple power law spanning over three orders of magnitude in virial mass. Our results show quantitatively the way that the presence of galaxies with high star formation rates is inhibited in massive galaxy systems. ER - TY - Journal T1 - Quasar-Galaxy and AGN-Galaxy Cross-Correlations A1 - Martínez, Héctor J. A1 - Merchán, Manuel E. A1 - Valotto, Carlos A. A1 - Lambas, Diego G. JO - Astrophysical Journal VL - 514 Y1 - 1999/4/1 SP - 558 EP - 562 KW - GALAXIES: ACTIVE/ GALAXIES: CLUSTERS: GENERAL/ COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE/ GALAXIES: QUASARS: GENERAL UR - http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999ApJ...514..558M&db_key=AST&high=42a068d47529002 N2 - We compute quasar-galaxy and AGN-galaxy cross-correlation functions for samples taken from the Véron-Cetty & Véron catalog of quasars and active galaxies, using tracer galaxies taken from the Edinburgh/Durham Southern Catalog. The sample of active galaxy targets shows positive correlation at projected separations r_p<6 h^-1 Mpc consistent with the usual power law. On the other hand, we do not find a statistically significant positive quasar-galaxy correlation signal except in the range 3 h^-1 Mpclambda_max~1.8%, close to the polarization value produced by the ISM with normal efficiency (Plambda_max ~ 5 EB-V) with a color excess of EB-V =0.51. Meanwhile for Hogg 22, located behind NGC 6204, the mean polarization is Plambda_max~ 2.15%, lower than the expected value for the observed color excess of EB-V =0.68 (Forbes et al. 1996) and the average efficiency of polarization for the interstellar dust. The mean angle of the polarization vectors of Hogg 22 is theta=44.9 °, which agrees with the expected angle produce by dust particles aligned in the direction of the Galactic Plane (theta=48°), while for NGC 6204 a lower value, theta=33.7 °, was found. Therefore, we believe that Hogg 22 is depolarized by the same dust that is polarizing NGC 6204, due to different orientations of the dust particles (and magnetic fields) that polarize the starlight. Based on observations obtanined at Complejo Astronómico El Leoncito (CASLEO), operated under agreement between the CONICET and the National Universities of La Plata, Córdoba, and San Juan, Argentina. ER -