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E. M. de Gouveia Dal Pino

Publications and source records attributed to E. M. de Gouveia Dal Pino.

At least 91 records · Page 5Linked to original sources
arXiv2005

Multidimensional Hydrodynamical Simulations of radiative cooling SNRs-clouds interactions: an application to Starburst Environments

In this work we are interested to study the by-products of SNR-clouds in a starburst (SB) system. These interactions can have an important role in the recycling of matter from the clouds to the ISM and vice-versa. In the present work, we have focused our attention on the global effects of the interactions between clouds and SN shock waves in the ISM of SB environments, and performed 3-D radiative cooling hydrodynamical simulations with the adaptive YGUAZU grid code. We have also considered the effects of the photo-evaporation due to the presence of a high number of UV photons from hot stars and supernovae (SNe). The results have shown that, in the presence of radiative cooling, instead of an efficient gas mixing with the diffuse ISM, the interactions cause the fragmentation of the clouds into smaller ones. The results have also revealed that the SNR-clouds interactions are less efficient at producing substantial mass loss from the clouds to the diffuse ISM than mechanisms such as the photo-evaporation caused by the UV flux from the hot stars.

astro-ph↗
arXiv2005

The role of Violent Magnetic Reconnection on the Production of the Large Scale Superluminal Ejections of the Microquasar GRS 1915+105

We here propose that the large scale superluminal ejections observed in the galactic microquasar GRS 1915+105 during radio flare events are produced by violent magnetic reconnection episodes in the corona just above the inner edge of the magnetized accretion disk that surrounds the central $\sim 10 M_{\odot}$ black hole. The process occurs when a large scale magnetic field is established by turbulent dynamo in the inner disk region with a ratio between the gas$+$radiation and the magnetic pressures $β\simeq 1$, implying a magnetic field intensity of $\sim 7 \times 10^8$ G. During this process, substantial angular momentum is removed from the disk by the wind generated by the vertical magnetic flux therefore increasing the disk mass accretion to a value near (but below) the critical one ($\dot M \sim 10^{19} $ g s$^{-1}$). Part of the magnetic energy released by reconnection heats the coronal gas ($T_c \lesssim 5 \times 10^8 $ K) that produces a steep, soft X-ray spectrum with luminosity $L_X \simeq 10^{39} $ erg s$^{- 1}$, in consistency with observations. The remaining magnetic energy released goes to accelerate the particles to relativistic velocities ($v \sim v_A \sim c$, where $v_A$ is the Alfvén speed) in the reconnection site through first-order Fermi processes. In this context, two possible mechanisms have been examined which produce power-law electron distributions $N(E) \propto E^{-α_E}$, with $α_E = 5/2, 2$, and corresponding synchrotron radio power-law spectra with spectral indices which are compatible with that observed during the flares ($ S_ν \propto ν^{-0.75, - 0.5}$).

astro-ph↗
arXiv2005

The Role of Magnetic Fields on Astrophysical Jets

Highly collimated supersonic jets and less collimated outflows are observed to emerge from a wide variety of astrophysical objects. They are seen in young stellar objects (YSOs), proto-planetary nebulae, compact objects (such as galactic black holes or microquasars, and X-ray binary stars), and in the nuclei of active galaxies (AGNs). Despite their different physical scales (in size, velocity, and amount of energy transported), they have strong morphological similarities. What is the universal mechanism that can explain their origin? In this lecture, I briefly review the role that magnetic fields seem to play on the formation, structure, and propagation of these jets.

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arXiv2005

Modeling the spectrum of gravitational waves in the primordial Universe

Recent observations from type Ia Supernovae and from cosmic microwave background (CMB) anisotropies have revealed that most of the matter of the Universe interacts in a repulsive manner, composing the so-called dark energy constituent of the Universe. The analysis of cosmic gravitational waves (GW) represents, besides the CMB temperature and polarization anisotropies, an additional approach in the determination of parameters that may constrain the dark energy models and their consistence. In recent work, a generalized Chaplygin gas model was considered in a flat universe and the corresponding spectrum of gravitational waves was obtained. The present work adds a massless gas component to that model and the new spectrum is compared to the previous one. The Chaplygin gas is also used to simulate a $Λ$-CDM model by means of a particular combination of parameters so that the Chaplygin gas and the $Λ$-CDM models can be easily distinguished in the theoretical scenarios here established. The lack of direct observational data is partialy solved when the signature of the GW on the CMB spectra is determined.

gr-qc↗
arXiv2004

Numerical Modeling of Eta Carinae Bipolar Outflows

In this paper, we present two-dimensional gas dynamic simulations of the formation and evolution of the eta-Car bipolar outflows. Adopting the interacting nonspherical winds model, we have carried out high-resolution numerical simulations, which include explicitly computed time-dependent radiative cooling, for different possible scenarios of the colliding winds. In our simulations, we consider different degrees of non-spherical symmetry for the pre-outburst wind and the great eruption of the 1840s presented by the eta-Car wind. From these models, we obtain important differences in the shape and kinematical properties of the Homunculus structure. In particular, we find an appropriate combination of the wind parameters (that control the degree of non-spherical symmetry) and obtain numerical experiments that best match both the observed morphology and the expansion velocity of the eta-Car bipolar shell. In addition, our numerical simulations show the formation of a bipolar nebula embedded within the Homunculus (the little Homunculus) developed from a secondary eruptive event suffered by the star in the 1890s, and also the development of tenuous, high velocity ejections in the equatorial region that result from the impact of the eruptive wind of the 1840s with the pre-outburst wind and that could explain some of the high speed features observed in the equatorial ejecta. The models were, however, unable to produce equatorial ejections associated to the second eruptive event.

astro-ph↗
arXiv2004

Evolution of the ISM of Starburst galaxies: the SN heating efficiency

The interstellar medium heated by SN explosions may acquire an expansion velocity larger than the escape velocity and leave the galaxy through a supersonic wind. SN ejecta are transported out of the galaxies by such winds which thus affect the chemical evolution of the galaxies. The effectiveness of the processes mentioned above depends on the heating efficiency (HE) of the SNe, that is a matter of debate. We have constructed a simple semi-analytic model, considering the essential ingredients of a SB environment which is able to qualitatively trace the thermalisation history of the ISM in a SB region and determine the HE evolution. We find that, as long as the mass-loss rate of the clouds remains larger than the rate at which the SNRs interact one with each other, the SN heating efficiency remains very small, as radiative cooling of the gas dominates. We conclude that the HE value has a time-dependent trend that is sensitive to the initial conditions of the system.

astro-ph↗
arXiv2003

Gas Dynamical Simulations of the Large and Little Homunculus Nebulae of Eta Carinae

We here present two-dimensional, time-dependent radiatively cooling hydrodynamical simulations of the large and little Homunculus nebulae around $η$ Carinae. We employ an alternative scenario to previous interacting stellar wind models which is supported by both theoretical and observational evidence, where a non-spherical outburst wind (with a latitudinal velocity dependence that matches the observations of the large Homunculus), which is expelled for 20 years, interacts with a pre-eruptive slow wind also with a toroidal density distribution, but with a much smaller equator-to-polar density contrast than that assumed in previous models. A second eruptive wind with spherical shape is ejected about 50 years after the first outburst, and causes the development of the little internal nebula. We find that, as a result of an appropriate combination of the parameters that control the degree of asymmetry of the interacting winds, we are able to produce not only the structure and kinematics of both Homunculus, but also the high-velocity equatorial ejecta. These arise from the impact between the non-spherical outburst and the pre-outburst winds in the equatorial plane.

astro-ph↗
arXiv2003

Asymmetric Explosions of Thermonuclear Supernovae

A type Ia supernova explosion starts in a white dwarf as a laminar deflagration at the center of the star and soon several hydrodynamic instabilities (in particular, the Rayleigh-Taylor (R-T) instability) begin to act. In previous work (Ghezzi, de Gouveia Dal Pino, & Horvath 2001), we addressed the propagation of an initially laminar thermonuclear flame in presence of a magnetic field assumed to be dipolar. We were able to show that, within the framework of a fractal model for the flame velocity, the front is affected by the field through the quenching of the R-T instability growth in the direction perpendicular to the field lines. As a consequence, an asymmetry develops between the magnetic polar and the equatorial axis that gives a prolate shape to the burning front. We have here computed numerically the total integrated asymmetry as the flame front propagates outward through the expanding shells of decreasing density of the magnetized white dwarf progenitor, for several chemical compositions, and found that a total asymmetry of about 50 % is produced between the polar and equatorial directions for progenitors with a surface magnetic field B ~ 5 x 10^{7} G, and a composition C12 = 0.2 and O16 = 0.8 (in this case, the R-T instability saturates at scales \~ 20 times the width of the flame front). This asymmetry is in good agreement with the inferred asymmetries from spectropolarimetric observations of very young supernova remnants, which have recently revealed intrinsic linear polarization interpreted as evidence of an asymmetric explosion in several objects,such as SN1999by, SN1996X, and SN1997dt. Larger magnetic field strengths will produce even larger asymmetries. We have also found that for lighter progenitors the total asymmetry is larger.

astro-ph↗
arXiv2002

Asymmetric core combustion in neutron stars and a potential mechanism for gamma ray bursts

We study the transition of nuclear matter to strange quark matter (SQM) inside neutron stars (NSs). It is shown that the influence of the magnetic field expected to be present in NS interiors has a dramatic effect on the propagation of a laminar deflagration (widely studied so far), generating a strong acceleration of the flame in the polar direction. This results in a strong asymmetry in the geometry of the just formed core of hot SQM which resembles a cylinder orientated in the direction of the magnetic poles of the NS. This geometrical asymmetry gives rise to a bipolar emission of the thermal neutrino-antineutrino pairs produced in the process of SQM formation. The neutrino-antineutrino pairs annihilate into electron-positron pairs just above the polar caps of the NS giving rise to a relativistic fireball, thus providing a suitable form of energy transport and conversion to gamma emission that may be associated to short gamma ray bursts (GRBs).

astro-ph↗
arXiv2002

Asymmetric Explosions of Type Ia Supernovae

The burning speed of a thermonuclear supernova front could be described by the fractal model of combustion. We have examined the effects of magnetic fields on the fractalization of the front considering a white dwarf with a nearly dipolar magnetic field and found an intrinsic asymmetry on the velocity field of the expanding plasma. For white dwarf's magnetic fields of 10^8-10^9 G at the surface, and assuming a field roughly 10 times greater near the center, we have found asymmetries in the velocity field > 10-20 % at rho ~ 10^8 g cm^(-3), produced between the magnetic polar and the equatorial axis of the remnant. This effect may be related to the asphericities inferred from spectro-polarimetric observations of very young SN Ia remnants (for example: SN 1999by). In the present work, we analyse the dependence of the asymmetry with the composition of the white dwarf progenitor.

astro-ph↗
arXiv2002

The precession of the giant HH34 outflow: a possible jet deceleration mechanism

The giant jets represent a fundamental trace of the historical evolution of the outflow activity over timescales which are comparable to the accretion time of the outflow sources in their main protostellar phase. The study of such huge jets provides the possibility of retrieving important elements related to the life of the outflow sources. In this paper, we study the role of precession (combined with jet velocity-variability and the resulting enhanced interaction with the surrounding environment) as a deceleration mechanism for giant jets using a numerical approach. We obtain predictions of H alpha intensity maps and position-velocity diagrams from 3D simulations of the giant HH 34 jet (including an appropriate ejection velocity time-variability and a precession of the outflow axis), and we compare them with previously published observations of this object. Our simulations represent a step forward from previous numerical studies of HH objects, in that the use of a 7-level, binary adaptive grid has allowed us to compute models which appropiately cover all relevant scales of a giant jet, from the ~ 100 AU jet radius close to the source to the ~ 1 pc length of the outflow. A good qualitative and quantitative agreement is found between the model predictions and the observations. Moreover, we show that a critical parameter for obtaining a better or worse agreement with the observations is the ratio rho_j/rho_a between the jet and the environmental densities. The implications of this result in the context of the current star formation models are discussed (ABRIDGED).

astro-ph↗
arXiv2002

Jet/cloud collision, 3D gasdynamic simulations of HH 110

We present 3D, gasdynamic simulations of jet/cloud collisions, with the purpose of modelling the HH 270/110 system. From the models, we obtain predictions of H$α$ and H$_2$ 1-0 s(1) emission line maps, which qualitatively reproduce some of the main features of the corresponding observations of HH 110. We find that the model that better reproduces the observed structures corresponds to a jet that was deflected at the surface of the cloud $\sim 1000$ yr ago, but is now boring a tunnel directly into the cloud. This model removes the apparent contradiction between the jet/cloud collision model and the lack of detection of molecular emission in the crossing region of the HH 270 and HH 110 axes.

astro-ph↗
arXiv2000

Ultra-High-Energy Cosmic Ray Acceleration by Magnetic Reconnection in Newborn Accretion Induced Collapse Pulsars

We here investigate the possibility that the ultra-high energy cosmic ray (UHECR) events observed above the GZK limit are mostly protons accelerated in reconnection sites just above the magnetosphere of newborn millisecond pulsars which are originated by accretion induced collapse (AIC). We show that AIC-pulsars with surface magnetic fields $10^{12} G < B_{\star} \lesssim 10^{15}$ G and spin periods $1 ms \lesssim P_{\star} < 60 ms$, are able to accelerate particles to energies $\geq 10^{20}$ eV. Because the expected rate of AIC sources in our Galaxy is very small (\sim 10^{-5} yr^{-1}), the corresponding contribution to the flux of UHECRs is neglegible, and the total flux is given by the integrated contribution from AIC sources produced by the distribution of galaxies located within the distance which is unaffected by the GZK cutoff ($\sim 50$ Mpc). We find that the reconnection efficiency factor needs to be $ξ\gtrsim 0.1$ in order to reproduce the observed flux of UHECRs.

astro-ph↗
arXiv1999

Magnetically Driven Outflows in a Starburst Environment

We here investigate the possibility that the observed collimated outflows in luminous infrared galaxies (LIGs) and some Seyfert galaxies can be produced in a starburst (SB) environment. A nuclear disk can be quickly produced by gas infall during star formation in a rotating, stellar cluster. We find that massive nuclear SBs with core disk masses M_d \sim 10^8 - 10^9 M_{\odot}, and supernova rates ν_{SN} \simeq 5 \times 10^{-3} - 2 yr^{-1} (which are consistent with the ν_{SN} values inferred from the observed non-thermal radio power in source candidates) may inject kinetic energies which are high enough to blow out directed flows from the accreting disk surface, within the SB lifetimes. In our models, the acceleration and collimation of the nuclear outflow are provided by magnetic fields anchored into the rotating SB-disk. The emerging outflow carries a kinetic power that is only a small fraction (a few percent) of the supernovae energy rate produced in the SB. Based on conditions determined from observed outflows and disks, we find that moderate disk magnetic fields (\gtrsim 8 \times 10^{-4} G) are able to accelerate the outflows up to the observed terminal velocities (\lesssim few 100 km s^{-1} in the case of the Seyfert galaxies, and \sim 400 - 950 km s^{-1} in the case of the LIGs). The outflow is produced within a wind zone in the disk of radius \lesssim 100 pc in the LIGs, and \lesssim 10 pc in the Seyferts, with wind mass loss to disk accretion rate ratios \dot M_w /\dot M_d \gtrsim 0.1 (where \dot M_d \sim 100 M_{\odot} yr^{-1}). The observation of rotating nuclear disks of gas within few 100 pc scales in source candidates like the LIG Arp 220, and magnetized outflows provide observational support for the picture drawn here.

astro-ph↗
arXiv1997

Galactic Diffuse $γ$-ray Emission at TeV Energies and the Ultra-High Energy Cosmic Rays

Using the cosmic ray (CR) data available in the energy interval $(10 - 2 \times 10^{7})$ GeV/particle, we have calculated the profile of the primary $γ$-ray spectrum produced by the interaction of these CR with thermal nuclei of the ISM. Normalized to the EGRET measurements, this allows an estimate of the galactic diffuse $γ$-ray background due to intermediate and high energy CR at TeV energies. On the other hand, over the last few years, several particles with energies above $10^{20}$ eV (beyond the Greisen-Zatsepin-Kuzmin cut-off) have been detected. These particles are very likely extragalactic protons originated at distances not greater than $30 - 50$ Mpc [e.g., 1]. The propagation of these ultra-high energy protons (UHEP) through the intergalactic medium leads to the development of $γ$-ray cascades and an ultimate signature at TeV energies. To assess the statistical significance of this $γ$-ray signature by the UHEP, we have also simulated the development of electromagnetic cascades triggered by the decay of a $10^{19}$ eV $π^{o}$ in the intergalactic medium after an UHEP collision with a cosmic microwave background photon.

astro-ph↗
arXiv1996

A Possible Mechanism for Wiggling Protostellar Jets from 3D Simulations in a Stratified Ambient Medium

Most collimated supersonic protostellar jets show a collimated wiggling, and knotty structure (e.g., the Haro 6-5B jet) and frequently reveal a long gap between this structure and the terminal bow shock. In a few cases, there is no evidence of such a terminal feature. We present three-dimensional smoothed particle hydrodynamical simulations which suggest that this morphology may be due to the interaction of the propagating cooling jet with a non-homogeneous ambient medium. In regions where the ambient gas has an increasing density (and pressure) gradient, we find that it tends to compress the cold, low-pressure cocoon of shocked material that surrounds the beam, destroy the bow shock-like structure at the head, and enhance beam focusing, wiggling, and internal traveling shocks. In ambient regions of decreasing density (and pressure), the flow widens and relaxes, becoming very faint. This could explain ``invisible'' segments in systems like the Haro 6-5B jet. The bow shock in these cases could be a relic of an earlier outflow episode, as previously suggested, or the place where the jet reappears after striking a denser portion of the ambient medium.

astro-ph↗
arXiv1995

The role of thermal instabilities on the theoretical mass function of MACHOs

We analyze the primordial protogalactic gas to investigate the role of thermal instabilities in the formation of condensations that could contribute to the MACHO mass function. We find that the previrialized protogalaxy could have undergone a thermally unstable regime which defines a peak in mass condensations starting around $0.1 \, M_{\odot}$ after being reprocessed by the environment which may be identified as giving origin to the MACHOs. Effects of magnetic fields on this instability-driven MACHOs scenario are also addressed.

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