arXiv subjects

Edmundo Moreno

Publications and source records attributed to Edmundo Moreno.

31 records · Page 2Linked to original sources

arXiv2012

Tidal effects on the radial velocity curve of HD77581 (Vela X-1)

The mass of the neutron star in Vela X-1 has been found to be more massive than the canonical 1.5 Mo. This result relies on the assumption that the amplitude of the optical component's measured radial velocity curve is not seriously affected by the interactions in the system. In this paper we explore the effect on the radial velocity curve caused by surface motions excited by tidal interactions. We use a calculation from first principles that involves solving the equations of motion of a Lagrangian grid of surface elements. The velocities on the visible surface of the star are projected along the line-of-sight to the observer to obtain the absorption-line profile in the observer's reference frame. The centroid of the line-profiles for different orbital phases is then measured and a simulated RV curve constructed. Models are run for the "standard" (vsini=116 km/s) and "slow" (56 km/s) supergiant rotation velocities. We find that the surface velocity field is complex and includes fast, small-spatial scale structures. It leads to strong variability in the photospheric line profiles which, in turn, causes significant deviations from a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves are in all cases larger than the amplitude of the orbital motion. Keplerian fits to RV curves obtained with the "standard" rotation velocity imply a neutron star >1.7 Mo. However, a similar analysis of the "slow" rotational velocity models allows for m_ns ~ 1.5 Mo. Thus, the stellar rotation plays an important role in determining the characteristics of the perturbed RV curve. Given the observational uncertainty in GP Vel's projected rotation velocity and the strong perturbations seen in the published and the model RV curves, we are unable to rule out a small (~1.5 Mo) mass for the neutron star companion.

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arXiv2012

The Sun was not born in M 67

Using the most recent proper-motion determination of the old, Solar-metallicity, Galactic open cluster M 67, in orbital computations in a non-axisymmetric model of the Milky Way, including a bar and 3D spiral arms, we explore the possibility that the Sun once belonged to this cluster. We have performed Monte Carlo numerical simulations to generate the present-day orbital conditions of the Sun and M 67, and all the parameters in the Galactic model. We compute 3.5 \times 10^5 pairs of orbits Sun-M 67 looking for close encounters in the past with a minimum distance approach within the tidal radius of M 67. In these encounters we find that the relative velocity between the Sun and M 67 is larger than 20 km/s. If the Sun had been ejected from M 67 with this high velocity by means of a three-body encounter, this interaction would destroy an initial circumstellar disk around the Sun, or disperse its already formed planets. We also find a very low probability, much less than 10^-7, that the Sun was ejected from M 67 by an encounter of this cluster with a giant molecular cloud. This study also excludes the possibility that the Sun and M 67 were born in the same molecular cloud. Our dynamical results convincingly demonstrate that M67 could not have been the birth cluster of our Solar System.

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arXiv2011

Pitch Angle Restrictions in Late Type Spiral Galaxies Based on Chaotic and Ordered Orbital Behavior

We built models for low bulge mass spiral galaxies (late type as defined by the Hubble classification) using a 3-D self-gravitating model for spiral arms, and analyzed the orbital dynamics as a function of pitch angle, going from 10$°$ to 60$°$. Testing undirectly orbital self-consistency, we search for the main periodic orbits and studied the density response. For pitch angles up to approximately $\sim 20°$, the response supports closely the potential permitting readily the presence of long lasting spiral structures. The density response tends to "avoid" larger pitch angles in the potential, by keeping smaller pitch angles in the corresponding response. Spiral arms with pitch angles larger than $\sim 20°$, would not be long-lasting structures but rather transient. On the other hand, from an extensive orbital study in phase space, we also find that for late type galaxies with pitch angles larger than $\sim 50°$, chaos becomes pervasive destroying the ordered phase space surrounding the main stable periodic and quasi-periodic orbits and even destroying them. This result is in good agreement with observations of late type galaxies, where the maximum observed pitch angle is $\sim 50°$.

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arXiv2011

Two distinct halo populations in the solar neighborhood. III. Evidence from stellar ages and orbital parameters

In Papers I and II of this series, the existence of two distinct halo populations of stars have been found in the solar neighborhood. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range -1.4 < [Fe/H] < -0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy. Ages are derived by comparing the positions of stars in the logT_{eff}-log(g) diagram with isochrones from the Y^2 models interpolated to the exact [Fe/H] and [alpha/Fe] values of each star. Possible systematic errors in T_{eff} and log(g) are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms. The `high-alpha' halo stars have ages 2-3 Gyr larger than the `low-alpha' ones. The orbital parameters show very distinct differences between the `high-alpha' and `low-alpha' halo stars. The `low-alpha' ones have r_{max}'s to 30-40 kpc, z_{max}'s to approx. 18 kpc, and e_{max}'s clumped at values greater than 0.85, while the `high-alpha' ones, r_{max}'s to about 16 kpc, z_{max}'s to 6-8 kpc, and e_{max} more or less uniformly distributed over 0.4-1.0. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two halo populations, but one remaining defect is that this model is not consistent regarding the r_{max}'s obtained for the in situ `high-alpha' component; the predicted values are too small. It appears that omega Cen may have contributed in a significant way to the existence of the `low-alpha' component; recent models, including dynamical friction and tidal stripping, have produced orbital parameters as great as those of the `low-alpha' component.

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arXiv2009

Tidal Flows from Asynchronous Rotation in Binaries

Asynchronous rotation in binary stars produces non-radial oscillations that are known to cause observable variability on orbital timescales. The horizontal perturbations of the surface velocity fields are referred to as "tidal flows". In this paper we illustrate the manner in which tidal flows perturb the surface velocity field from that of uniform rotation, using a one-layer stellar model for the calculations. We justify the validity of this simplified model by the striking similarity between the photospheric absorption line-profiles it predicts and observational data of the binary system alpha Virginis. The velocity perturbations are used to compute the mechanical energy dissipation rates, dot-E, due to the shearing flows for the case of a massive (50+28 Mo) binary system having a moderately eccentric (e=0.3) orbit. The largest value of dot-E around periastron phases is found on the hemisphere facing the companion. However, at other orbital phases the maximum dot-E may migrate towards the poles. Assuming that dot-E plays a role in the mass-loss characteristics of massive binary systems, this suggests that peculiar binaries such as HD 5980 and eta Carinae may have a highly non-spherically symmetric mass-loss distribution which, in addition, is time-variable.

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arXiv2009

Tidal effects and periastron events in binary stars

Binary stars in eccentric orbits are frequently reported to present increasing levels of activity around periastron passage. In this paper we present results of a calculation from first principles of the velocity field on the surface of a star that is perturbed by a binary companion. This allows us to follow the orbital phase-dependence of the amount of kinetic energy that may be dissipated through the viscous shear, dot-E, driven by tidal interactions. For stars with relatively small stellar radii compared with the orbital separation at periastron (R/r_per< 0.14), a clear maximum occurs before periastron passage for sub-synchronous rotation and after periastron for super-synchronous rotation. For larger stellar radii however, dot-E oscillates over the orbital cycle and periastron passage does not cause a particularly greater enhancement in energy dissipation rates than some of the other orbital phases. Finally, we perform exploratory calculations for a WR/LBV binary system that in 1993-1994 underwent an LBV-like eruption, HD 5980. Our dot-E calculations reproduce the oscillations that appear around periastron passage in HD5980's recent visual light curve. We suggest that the energy dissipation from tidal flows in asynchronously rotating binary stars may provide a mechanism by which evolving stars may be driven into an active state. Given the nature of the tidal perturbations, the resulting mass-loss distribution is expected to be non-uniform over the stellar surface and highly time-dependent.

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arXiv2007

Six New Galactic Orbits of Globular Clusters in a Milky-Way-Like Galaxy

Absolute proper motions for six new globular clusters have recently been determined. This motivated us to obtain the Galactic orbits of these six clusters both in an axisymmetric Galactic potential and in a barred potential, such as the one of our Galaxy. Orbits are also obtained for a Galactic potential that includes spiral arms. The orbital characteristics are compared and discussed for these three cases. Tidal radii and destruction rates are also computed and discussed.

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arXiv2007

The extraordinary LBV/WR system HD 5980

The LBV/WR system HD 5980 contains a short-period, eccentric binary system with interacting stellar winds. In this paper we summarize results from model calculations of the tidal flows on the LBV component showing that energy dissipation rates, E-dot, associated with turbulent viscosity are orbital-phase dependent as well as variable over the stellar surface. We speculate that if E-dot contributes towards driving mass-loss, the strongest wind-wind interaction effects may occur after periastron passage. In addition, the model suggests the presence of stronger outflows localized at polar angles theta~30 --50 degrees during part of the orbital cycle. Thus, the analysis of wind-wind interactions in this system requires that models be revised to incorporate non-stationary and asymmetric wind structures.

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arXiv2006

Tides in asynchronous binary systems

Stellar oscillations are excited in non-synchronously rotating stars in binary systems due to the tidal forces. Tangential components of the tides can drive a shear flow which behaves as a differentially forced rotating structure in a stratified outer medium. In this paper we show that our single-layer approximation for the calculation of the forced oscillations yields results that are consistent with the predictions for the synchronization timescales in circular orbits. In addition, calibrating our model results to fit Zahn's relationship between synchronization timescales and orbital separation, we are able to constrain the value of the kinematical viscosity parameter. For the 4Mo+5Mo binary modeled in this paper, the kinematical viscosity is in the range 0.0015 -- 0.0043 Ro^2/d for orbital periods in the range 2.5 -- 25 d. Furthermore, the energy dissipation rate due to the shear flow is found to decrease by ~2 orders of magnitude as synchronization is approached, implying that binary systems may approach synchronization relatively quickly but that it takes a much longer timescale to actually attain this condition. Our model can be used to calculate the energy dissipation rates throughout the orbital cycle for arbitrary values of eccentricity and stellar rotational velocity. We suggest that the asymmetric distribution of energy dissipation over the stellar surface may lead to the appearance of localized regions of enhanced surface activity.

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arXiv2006

The orbits of 48 globular clusters in a Milky-Way-Like Barred Galaxy

The effect of a barred potential (such as the one of the Milky Way) on the galactic orbits of forty-eight globular clusters for which absolute proper motions are known is studied. The orbital characteristics are compared with those obtained for the case of an axisymmetric galactic potential. Tidal radii are computed and discussed for both the better known axisymmetric case and that including a bar. The destruction rates due to bulge and disk shocking are calculated and compared in both galactic potentials.

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arXiv2004

On the Galactic Spiral Patterns: Stellar and Gaseous

The gas response to a proposed spiral stellar pattern for our Galaxy is presented here as calculated via 2D hydrodynamic calculations utilizing the ZEUS code in the disk plane. The locus is that found by Drimmel (2000) from emission profiles in the K band and at 240 $μm$. The self-consistency of the stellar spiral pattern was studied in previous work (see Martos et al. 2004). It is a sensitive function of the pattern rotation speed, $Ω_p$, among other parameters which include the mass in the spiral and its pitch angle. Here we further discuss the complex gaseous response found there for plausible values of $Ω_p$ in our Galaxy, and argue that its value must be close to $20 km s^{-1} kpc^{-1}$ from the strong self-consistency criterion and other recent, independent studies which depend on such parameter. However, other values of $Ω_p$ that have been used in the literature are explored to study the gas response to the stellar (K band) 2-armed pattern. For our best fit values, the gaseous response to the 2-armed pattern displayed in the K band is a four-armed pattern with complex features in the interarm regions. This response resembles the optical arms observed in the Milky Way and other galaxies with the smooth underlying two-armed pattern of the old stellar disk populations in our interpretation.

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arXiv2004

Models for the Gravitational Field of the Galactic Bar. An Application to Stellar Orbits in the Galactic Plane and Orbits of Some Globular Clusters

We built three models for the gravitational field of the Galactic bar. These models are an inhomogeneous ellipsoid, an inhomogeneous prolate spheroid, and a superposition of four inhomogeneous ellipsoids. Among the three models, the superposition provides our best approximation to the observed boxy mass distribution of the Galactic bar. Adding the bar component to an axisymmetric Galactic model, we have calculated stellar midplane orbits and orbits of some globular clusters with known kinematical data. For all models we find a secular dispersion effect upon the orbital energy and angular momentum, as measured in the Galactic inertial frame. This effect might be relevant to explain the orbital prograde-retrograde distribution of globular clusters. For the stellar kinematics, we study the connection between the sense of orbital motion in the midplane and the onset of chaos in the presence of the bar. In the inner region of the bar, chaos is induced by an axisymmetric central component (bulge) and it arises in orbits that change its orbital sense from prograde to retrograde and vice versa as seen from an inertial reference frame. Outside the bar region, chaos appears only in prograde orbits. Our results concerning such connection are consistent and extend those obtained for midplane orbits in the presence of only a spiral pattern in the axisymmetric Galactic model.

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arXiv2002

Nonlinear Effects in Models of the Galaxy: 1. Midplane Stellar Orbits in the Presence of 3D Spiral Arms

With the aim of studying the nonlinear stellar and gaseous response to the gravitational potential of a galaxy such as the Milky Way, we have modeled 3D galactic spiral arms as a superposition of inhomogeneous oblate spheroids and added their contribution to an axisymmetric model of the Galactic mass distribution. Three spiral loci are proposed here, based in different sets of observations. A comparison of our model with a tight-winding approximation shows that the self-gravitation of the whole spiral pattern is important in the middle and outer galactic regions. As a first step to full 3D calculations the model is suitable for, we have explored the stellar orbital structure in the midplane of the Galaxy. We present the standard analysis in the pattern rotating frame, and complement this analysis with orbital information from the Galactic inertial frame. Prograde and retrograde orbits are defined unambiguously in the inertial frame, then labeled as such in the Poincaré diagrams of the non-inertial frame. In this manner we found a sharp separatrix between the two classes of orbits. Chaos is restricted to the prograde orbits, and its onset occurs for the higher spiral perturbation considered plausible in our Galaxy.

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