arXiv subjects

Enrico Ramirez-Ruiz

Publications and source records attributed to Enrico Ramirez-Ruiz.

At least 217 records · Page 12Linked to original sources

arXiv2012

What Sets the Initial Rotation Rates of Massive Stars?

The physical mechanisms that set the initial rotation rates in massive stars are a crucial unknown in current star formation theory. Observations of young, massive stars provide evidence that they form in a similar fashion to their low-mass counterparts. The magnetic coupling between a star and its accretion disk may be sufficient to spin down low-mass pre-main sequence (PMS) stars to well below breakup at the end stage of their formation when the accretion rate is low. However, we show that these magnetic torques are insufficient to spin down massive PMS stars due to their short formation times and high accretion rates. We develop a model for the angular momentum evolution of stars over a wide range in mass, considering both magnetic and gravitational torques. We find that magnetic torques are unable to spin down either low or high mass stars during the main accretion phase, and that massive stars cannot be spun down significantly by magnetic torques during the end stage of their formation either. Spin-down occurs only if massive stars' disk lifetimes are substantially longer or their magnetic fields are much stronger than current observations suggest.

astro-ph.SR↗

arXiv2012

How the merger of two white dwarfs depends on their mass ratio: orbital stability and detonations at contact

Despite their unique astrophysical relevance, the outcome of white dwarf binary mergers has so far only been studied for a very restricted number of systems. Here we present the results of a survey with more than two hundred simulations systematically scanning the white dwarf binary parameter space. We consider white dwarf masses ranging from 0.2 to 1.2 $M_\odot$ and account for their different chemical compositions. We find excellent agreement with the orbital evolution predicted by mass transfer stability analysis. Much of our effort in this paper is dedicated to determining which binary systems are prone to a thermonuclear explosion just prior to merger or at surface contact. We find that a large fraction of He-accreting binary systems explode: all dynamically unstable systems with accretor masses below 1.1 $M_\odot$ and donor masses above $\sim$ 0.4 $M_\odot$ are found to trigger a helium detonation at surface contact. A substantial fraction of these systems could explode at earlier times via detonations induced by instabilities in the accretion stream, as we have demonstrated in our previous work. We do not find definitive evidence for an explosion prior to merger or at surface contact in any of the studied double carbon-oxygen systems. Although we cannot exclude their occurrence if some helium is present, the available parameter space for a successful detonation in a white dwarf binary of pure carbon-oxygen composition is small. We demonstrate that a wide variety of dynamically unstable systems are viable type Ia candidates. The next decade thus holds enormous promise for the study of these events, in particular with the advent of wide-field synoptic surveys allowing a detailed characterization of their explosive properties.

astro-ph.HE↗

arXiv2011

Gamma-Ray Burst Dynamics and Afterglow Radiation from Adaptive Mesh Refinement, Special Relativistic Hydrodynamic Simulations

We report on the development of Mezcal-SRHD, a new adaptive mesh refinement, special relativistic hydrodynamics (SRHD) code, developed with the aim of studying the highly relativistic flows in Gamma-Ray Burst sources. The SRHD equations are solved using finite volume conservative solvers. The correct implementation of the algorithms is verified by one-dimensional (1D) shock tube and multidimensional tests. The code is then applied to study the propagation of 1D spherical impulsive blast waves expanding in a stratified medium with $ρ\propto r^{-k}$, bridging between the relativistic and Newtonian phases, as well as to a two-dimensional (2D) cylindrically symmetric impulsive jet propagating in a constant density medium. It is shown that the deceleration to non-relativistic speeds in one-dimension occurs on scales significantly larger than the Sedov length. This transition is further delayed with respect to the Sedov length as the degree of stratification of the ambient medium is increased. This result, together with the scaling of position, Lorentz factor and the shock velocity as a function of time and shock radius, is explained here using a simple analytical model based on energy conservation. The method used for calculating the afterglow radiation by post-processing the results of the simulations is described in detail. The light curves computed using the results of 1D numerical simulations during the relativistic stage correctly reproduce those calculated assuming the self-similar Blandford-McKee solution for the evolution of the flow. The jet dynamics from our 2D simulations and the resulting afterglow lightcurves, including the jet break, are in good agreement with those presented in previous works. Finally, we show how the details of the dynamics critically depend on properly resolving the structure of the relativistic flow.

astro-ph.HE↗

arXiv2011

Simulations of GRB Jets in a Stratified External Medium: Dynamics, Afterglow Lightcurves, Jet Breaks and Radio Calorimetry

The dynamics of GRB jets during the afterglow phase is most reliably and accurately modelled using hydrodynamic simulations. All published simulations, however, have considered only a uniform external medium, while a stratified external medium is expected around long duration GRB progenitors. Here we present simulations of the dynamics of GRB jets and the resulting afterglow emission for both uniform and stratified external media with $ρ\propto r^{-k}$ for k = 0, 1, 2. The simulations are performed in 2D using the special relativistic version of the Mezcal code. The dynamics for stratified external media are broadly similar to those derived for expansion into a uniform external medium. The jet half-opening angle start increasing logarithmically with time once the Lorentz factor drops below 1/theta_0. For larger k values the lateral expansion is faster at early times and slower at late times with the jet expansion becoming Newtonian and slowly approaching spherical symmetry over progressively longer timescales. We find that contrary to analytic expectations, there is a reasonably sharp jet break in the lightcurve for k = 2 although the shape of the break is affected more by the viewing angle than by the slope of the external density profile. Steeper density profiles are found to produce more gradual jet breaks while larger viewing angles cause smoother and later appearing jet breaks. The counter-jet becomes visible as it becomes sub-relativistic, and for k=0 this results in a clear bump-like feature in the light curve. However, for larger k values the jet decelerates more gradually, causing only a mild flattening in the radio light curve that might be hard to discern when k=2. Late time radio calorimetry is likely to consistently over-estimate the true energy by up to a factor of a few for k=2, and either over-predict or under-predict it by a smaller factor for k = 0,1.

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arXiv2011

An Unexpected Discovery in the Rich Open Cluster NGC 6819 Using XMM-Newton

We present the first study of the X-ray population of the intermediate-age rich open cluster NGC 6819 using the XMM-Newton Observatory. In the past decade, Chandra X-ray observations have shown a relationship between the X-ray population of globular clusters and their internal dynamics and encounter frequency. We investigate the role dynamics possibly play in the formation of X-ray sources in NGC 6819, and compare our results with known properties of field and globular cluster X-ray populations. We implement a multi wavelength approach to studying the X-ray sources, utilizing X-ray and UV data from XMM observations along with the wealth of photometry and radial-velocity data from the WIYN Open Cluster Study (WOCS) and the CFHT Open Cluster Survey. Within the cluster half-light radius we detect 12 X-ray sources down to a luminosity of 10^30 erg/s for cluster members. The sources include a candidate quiescent low-mass X-ray binary (qLMXB), a candidate cataclysmic variable, and two active binary systems. The presence of a qLMXB in an open cluster is previously unexpected given the known relationships between luminous X-ray sources and encounter frequency in globular clusters, and most likely has a dynamical origin.

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arXiv2011

Towards improving the prospects for coordinated gravitational-wave and electromagnetic observations

We discuss two approaches to searches for gravitational-wave (GW) and electromagnetic (EM) counterparts of binary neutron star mergers. The first approach relies on triggering archival searches of GW detector data based on detections of EM transients. We introduce a quantitative approach to evaluate the improvement to GW detector reach due to the extra information gained from the EM transient and the increased confidence in the presence of a signal from a binary merger. We also advocate utilizing other transients in addition to short gamma ray bursts. The second approach involves following up GW candidates with targeted EM observations. We argue for the use of slower but optimal parameter-estimation techniques to localize the source on the sky, and for a more sophisticated use of astrophysical prior information, including galaxy catalogs, to find preferred followup locations.

astro-ph.HE↗

arXiv2011

Fermi/GBM Observations of SGR J0501+4516 Bursts

We present our temporal and spectral analyses of 29 bursts from SGR J0501+4516, detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope during the 13 days of the source activation in 2008 (August 22 to September 3). We find that the T90 durations of the bursts can be fit with a log-normal distribution with a mean value of ~ 123 ms. We also estimate for the first time event durations of Soft Gamma Repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T90s estimated in count space (following a log-normal distribution with a mean value of ~ 124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two black body functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that Epeak decreases with energy flux (and fluence) to a minimum of ~30 keV at F=8.7e-6 erg/cm2/s, increasing steadily afterwards. Two more sources exhibit a similar trend: SGRs J1550-5418 and 1806-20. The isotropic luminosity corresponding to these flux values is roughly similar for all sources (0.4-1.5 e40 erg/s).

astro-ph.HE↗

arXiv2011

Electromagnetic Transients Powered by Nuclear Decay in the Tidal Tails of Coalescing Compact Binaries

The possibility that long tidal tails formed during compact object mergers may power optical transients through the decay of freshly synthesized r-process material is investigated. Precise modeling of the merger dynamics allows for a realistic determination of the thermodynamic conditions in the ejected debris. The results of hydrodynamic and full nuclear network calculations are combined to calculate the resultant r-process abundances and the heating of the material by their decays. The subsequent homologous structure is mapped into a radiative transfer code to synthesize emergent model light curves and determine how their properties (variability and color evolution) depend on the mass ratio and orientation of the merging binary. The radiation emanating from the ejected debris, though less spectacular than a typical supernova, should be observable in transient surveys and we estimate the associated detection rates. The case for (or against) compact object mergers as the progenitors of short gamma-ray bursts can be tested if such electromagnetic transients are detected (or not) in coincidence with some bursts, although they may be obscured by on-axis afterglows.

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arXiv2011

A relativistic jetted outburst from a massive black hole fed by a tidally disrupted star

While gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, the vast majority of MBHs are considered dormant. Occasionally, a star passing too near a MBH is torn apart by gravitational forces, leading to a bright panchromatic tidal disruption flare (TDF). While the high-energy transient Swift J164449.3+573451 ("Sw 1644+57") initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that the observations (Levan et al. 2011) suggest a sudden accretion event onto a central MBH of mass ~10^6-10^7 solar masses. We find evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 with a smaller-scale blazar. The phenomenologically novel Sw 1644+57 thus connects the study of TDFs and active galaxies, opening a new vista on disk-jet interactions in BHs and magnetic field generation and transport in accretion systems.

astro-ph.HE↗

arXiv2011

External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies and Galaxy Clusters

Accretion studies have been focused on the flow around bodies with point mass gravitational potentials, but few general results are available for non-point mass distributions. Here, we study the accretion flow onto non-divergent, core potentials moving through a background medium. We use Plummer and Hernquist potentials as examples to study gas accretion onto star clusters, dwarf and large galaxy halos and galaxy clusters in a variety of astrophysical environments. The general conditions required for a core potential to collectively accrete large quantities of gas from the external medium are derived using both simulations and analytic results. The consequences of large mass accumulation in galaxy nuclei, dwarf galaxies and star clusters are twofold. First, if the gas cools effectively star formation can be triggered, generating new stellar members in the system. Second, if the collective potential of the system is able to alter the ambient gas properties before the gas is accreted onto the individual core members, the augmented mass supply rates could significantly alter the state of the various accreting stellar populations and result in an enhanced central black hole accretion luminosity.

astro-ph.CO↗

arXiv2011

What Drives the Expansion of Giant HII Regions?: A Study of Stellar Feedback in 30 Doradus

Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular clouds. The mechanism of this stellar feedback is debated theoretically: possible sources of pressure include the classical warm HII gas, the hot gas generated by shock-heating from stellar winds and supernovae, direct radiation of stars, and the dust-processed radiation field trapped inside the HII shell. In this paper, we measure observationally the pressures associated with each component listed above across the giant HII region 30 Doradus in the Large Magellanic Cloud. We exploit high-resolution, multi-wavelengh images (radio, infrared, optical, and X-ray) to map these pressures as a function of position. We find that radiation pressure dominates within 75 pc of the central star cluster, R136, while the HII gas pressure dominates at larger radii. By contrast, the dust-processed radiation pressure and hot gas pressure are generally weak and not dynamically important, although the hot gas pressure may have played a more significant role at early times. Based on the low X-ray gas pressures, we demonstrate that the hot gas is only partially confined and must be leaking out the HII shell. Additionally, we consider the implications of a dominant radiation pressure on the early dynamics of 30 Doradus.

astro-ph.SR↗

arXiv2011

Prelude to a double degenerate merger: the onset of mass transfer and its impact on gravitational waves and surface detonations

We present the results of a systematic numerical study of the onset of mass transfer in double degenerate binary systems and its impact on the subsequent evolution. All investigated systems belong to the regime of direct impact, unstable mass transfer. In all of the investigated cases, even those considered unstable by conventional stability analysis, we find a long-lived mass transfer phase continuing for as many as several dozen orbital periods. This settles a recent debate sparked by a discrepancy between earlier SPH calculations that showed disruptions after a few orbital periods and newer grid-based studies in which mass transfer continued for tens of orbits. As we show that these binaries can survive at small separation for hundreds of orbital periods, their associated gravitational wave signal should be included when calculating the gravitational wave foreground (although expected to below LISA's sensitivity at these high frequencies). We also show that the inclusion of the entropy increase associated with shock-heating of the accreted material reduces the number of orbits a binary survives given the same initial conditions, although the effect is not as pronounced when using the appropriate initial conditions. The use of accurate initial conditions and a correct treatment of shock heating allows for a reliable time evolution of the temperature, density, and angular momentum, which are important when considering thermonuclear events that may occur during the mass transfer phase and/or after merger. Our treatment allows us to accurately identify when surface detonations may occur in the lead-up to the merger, as well as the properties of final merger products.

astro-ph.HE↗

arXiv2010

Jets and Gamma-Ray Burst Unification Schemes

There are several lines of evidence indicating that the ultra-relativistic outflows powering gamma-ray bursts (GRBs) are collimated into narrow jets. However, these are indirect, and the jet structure is rather poorly constrained. What is more, the jet dynamics have still not been investigated in detail. It has been suggested that the observed variety between different long duration events, ranging from bright spectrally hard GRBs, to dimmer and spectrally softer X-ray flashes (XRFs) may be largely due to different viewing angles (or lines of sight) relative to rather similar relativistic jets. Here we describe the current state of knowledge on these topics, explain some of the most relevant physics behind some of the basic principles, and discuss prospects for the future.

astro-ph.HE↗

arXiv2010

Consequences of the Ejection and Disruption of Giant Planets

The discovery of Jupiter-mass planets in close orbits about their parent stars has challenged models of planet formation. Recent observations have shown that a number of these planets have highly inclined, sometimes retrograde orbits about their parent stars, prompting much speculation as to their origin. It is known that migration alone cannot account for the observed population of these misaligned hot Jupiters, which suggests that dynamical processes after the gas disc dissipates play a substantial role in yielding the observed inclination and eccentricity distributions. One particularly promising candidate is planet-planet scattering, which is not very well understood in the non-linear regime of tides. Through three-dimensional hydrodynamical simulations of multi-orbit encounters, we show that planets that are scattered into an orbit about their parent stars with closest approach distance being less than approximately three times the tidal radius are either destroyed or completely ejected from the system. We find that as few as 5 and as many as 18 of the currently known hot Jupiters have a maximum initial apastron for scattering that lies well within the ice line, implying that these planets must have migrated either before or after the scattering event that brought them to their current positions. If stellar tides are unimportant $(Q_\ast \gtrsim 10^7)$, disk migration is required to explain the existence of the hot Jupiters present in these systems. Additionally, we find that the disruption and/or ejection of Jupiter-mass planets deposits a Sun's worth of angular momentum onto the host star. For systems in which planet-planet scattering is common, we predict that planetary hosts have up to a 35% chance of possessing an obliquity relative to the invariable plane of greater than 90 degrees.

astro-ph.EP↗

arXiv2010

The Distribution of Coalescing Compact Binaries in the Local Universe: Prospects for Gravitational-Wave Observations

Merging compact binaries are the most viable and best studied candidates for gravitational wave (GW) detection by the fully operational network of ground-based observatories. In anticipation of the first detections, the expected distribution of GW sources in the local universe is of considerable interest. Here we investigate the full phase space distribution of coalescing compact binaries at $z = 0$ using dark matter simulations of structure formation. The fact that these binary systems acquire large barycentric velocities at birth ("kicks") results in merger site distributions that are more diffusely distributed with respect to their putative hosts, with mergers occurring out to distances of a few Mpc from the host halo. Redshift estimates based solely on the nearest galaxy in projection can, as a result, be inaccurate. On the other hand, large offsets from the host galaxy could aid the detection of faint optical counterparts and should be considered when designing strategies for follow-up observations. The degree of isotropy in the projected sky distributions of GW sources is found to be augmented with increasing kick velocity and to be severely enhanced if progenitor systems possess large kicks as inferred from the known population of pulsars and double compact binaries. Even in the absence of observed electromagnetic counterparts, the differences in sky distributions of binaries produced by disparate kick-velocity models could be discerned by GW observatories, within the expected accuracies and detection rates of advanced LIGO--in particular with the addition of more interferometers.

astro-ph.HE↗

arXiv2010

Using the X-ray Morphologies of Young Supernova Remnants to Constrain Explosion Type, Ejecta Distribution, and Chemical Mixing

Supernova remnants (SNRs) are a complex class of sources, and their heterogeneous nature has hindered the characterization of their general observational properties. To overcome this challenge, we use statistical tools to analyze the Chandra X-ray images of Galactic and Large Magellanic Cloud SNRs. We apply two techniques, a power-ratio method (a multipole expansion) and wavelet-transform analysis, to measure the global and local morphological properties of the X-ray line and thermal emission in twenty-four SNRs. We find that Type Ia SNRs have statistically more spherical and mirror symmetric thermal X-ray emission than core-collapse (CC) SNRs. The ability to type SNRs based on thermal emission morphology alone enables, for the first time, the typing of SNRs with weak X-ray lines or with low-resolution spectra. We identify one source, SNR G344.7-0.1, as originating from a CC explosion that was previously unknown, and we confirm the tentative Type Ia classifications of G337.2-0.7 and G272.2-3.2. Although the global morphology is indicative of the explosion type, the relative morphology of the X-ray line emission within SNRs is not: all sources in our sample have well-mixed ejecta, irrespective of stellar origin. In particular, we find that 90% of the bright metal-line emitting substructures are spatially coincident and have similar scales, even if the metals arise from different burning processes. Moreover, the overall X-ray line morphologies within each SNR are the same, with <6% differences. These findings reinforce that hydrodynamical instabilities can efficiently mix ejecta in Type Ia and CC SNRs. The only exception is W49B, which is likely from its jet-driven/bipolar SN explosion. Finally, we describe observational constraints that can be used to test hydrodynamical models of SNR evolution; notably, the filling factor of X-ray emission decreases with SNR age.

astro-ph.HE↗

arXiv2010

Local absorption of high-energy emission from gamma-ray bursts

High-energy photons emitted from gamma-ray bursts (GRBs) are subject to pair-production interactions with lower energy photons, leading to an effective optical depth. In this Letter, we estimate the opacity resulting from photon fields located at various distances from long GRB sites: that of the binary companion to the massive stellar progenitor, that of the star-forming molecular cloud containing the GRB, and the total photon field of the host galaxy. The first two photon fields are found to be transparent for most reasonable sets of assumptions about these systems. In the case of galactic radiation fields, we have performed several numerical simulations to calculate the expected opacities for different line-of-sight geometries through the host galaxy, and include a full accounting of the infrared radiation produced by the absorption and re-radiation of starlight by dust. The optical depth for GeV gamma-rays, due to direct starlight is less than unity for all host galaxies. At higher energies, $>$10 TeV, a spectral cutoff can occur due to the rapidly increasing number of mid- to far-IR intra-galactic photons reradiated by dust. Photons in the extragalactic background light therefore remain the only relevant source of photon-photon opacity for ongoing GRB observations with Fermi LAT, and potential future detections with ground-based gamma-ray telescopes.

astro-ph.HE↗

arXiv2010

Short gamma-ray bursts from dynamically-assembled compact binaries in globular clusters: pathways, rates, hydrodynamics and cosmological setting

We present a detailed assessment of the dynamical pathways leading to the coalescence of compact objects in Globular Clusters (GCs) and Short Gamma-Ray Burst (SGRB) production. We consider primordial binaries, dynamically formed binaries (through tidal two-body and three-body exchange interactions) and direct impacts of compact objects (WD/NS/BH). We show that if the primordial binary fraction is small, close encounters dominate the production rate of coalescing compact systems. We find that the two dominant channels are the interaction of field NSs with dynamically formed binaries, and two-body encounters. We then estimate the redshift distribution and host galaxy demographics of SGRB progenitors, and find that GCs can provide a significant contribution to the overall observed rate. We have carried out hydrodynamical modeling of evolution of close stellar encounters with WD/NS/BH, and show that there is no problem in accounting for the energy budget of a typical SGRB. The particulars of each encounter are variable and lead to interesting diversity: the encounter characteristics are dependent on the impact parameter, in contrast to the merger scenario; the nature of the compact star itself can produce very different outcomes; the presence of tidal tails in which material falls back onto the central object at later times is a robust feature of these calculations, with the mass involved being larger than for binary mergers. It is thus possible to account generically in this scenario for a prompt episode of energy release, as well as for activity many dynamical time scales later (abridged).

astro-ph.HE↗