Enrico Ramirez-Ruiz

NASA, Washington, West Virginia, United States

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Publications (258)1305.78 Total impact

  • Morgan MacLeod, Enrico Ramirez-Ruiz
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    ABSTRACT: This paper models the orbital inspiral of a neutron star (NS) through the envelope of its giant-branch companion during a common envelope (CE) episode. These CE episodes are necessary to produce close pairs of NSs that can inspiral and merge due to gravitational wave losses in less than a Hubble time. Because cooling by neutrinos can be very efficient, NSs have been predicted to accumulate significant mass during CE events, perhaps enough to lead them to collapse to black holes. We revisit this conclusion with the additional consideration of CE structure, particularly density gradients across the embedded NS's accretion radius. This work is informed by our recent numerical simulations that find that the presence of a density gradient strongly limits accretion by imposing a net angular momentum to the flow around the NS. Our calculations suggest that NSs should survive CE encounters. They accrete only modest amounts of envelope material, $\lesssim 0.1M_\odot$, which is broadly consistent with mass determinations of double NS binaries. With less mass gain, NSs must spiral deeper to eject their CE, leading to a potential increase in mergers. The survival of NSs in CE events has implications for the formation mechanism of observed double NS binaries, as well as for predicted rates of NS binary gravitational wave inspirals and their electromagnetic counterparts.
    10/2014;
  • Morgan MacLeod, Enrico Ramirez-Ruiz
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    ABSTRACT: This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). These embedded objects spiral to tighter separations because of drag that is generated when gas collides and shocks as it is gravitationally focused. This flow convergence is expected to lead to gas accretion onto the inspiralling object. This process has been studied numerically and analytically in the context of Hoyle-Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of 3D HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by 1-2 orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically-derived rates of drag and accretion as a function of the density gradient, which can be applied to any CE system. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.
    10/2014;
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    ABSTRACT: Investigations of element abundances in the ancient and most metal deficient stars are extremely important because they serve as tests of variable nucleosynthesis pathways and can provide critical inferences of the type of stars that lived and died before them. The presence of r-process elements in a handful of carbon-enhanced metal-poor (CEMP) stars, which are assumed to be closely connected to the chemical yield from the first stars, is hard to reconcile with standard neutron star mergers. Here we show that the production rate of dynamically assembled compact binaries in high-z nuclear star clusters can attain a sufficient high value to be a potential viable source of heavy r-material in CEMP stars. The predicted frequency of such events in the early Galaxy, much lower than the frequency of Type II supernovae but with significantly higher mass ejected per event, can naturally lead to a high level of scatter of Eu as observed in CEMP stars.
    10/2014;
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    ABSTRACT: We present a template-fitting algorithm for determining photometric redshifts, z phot, of candidate high-redshift gamma-ray bursts (GRBs). Using afterglow photometry, obtained by the Reionization and Transients InfraRed (RATIR) camera, this algorithm accounts for the intrinsic GRB afterglow spectral energy distribution, host dust extinction, and the effect of neutral hydrogen (local and cosmological) along the line of sight. We present the results obtained by this algorithm and the RATIR photometry of GRB 130606A, finding a range of best-fit solutions, 5.6 < z phot < 6.0, for models of several host dust extinction laws (none, the Milky Way, Large Magellanic Clouds, and Small Magellanic Clouds), consistent with spectroscopic measurements of the redshift of this GRB. Using simulated RATIR photometry, we find that our algorithm provides precise measures of z phot in the ranges of 4 < z phot 8 and 9 < z phot < 10 and can robustly determine when z phot > 4. Further testing highlights the required caution in cases of highly dust-extincted host galaxies. These tests also show that our algorithm does not erroneously find z phot < 4 when z sim > 4, thereby minimizing false negatives and allowing us to rapidly identify all potential high-redshift events.
    The Astronomical Journal 05/2014; 148(1):2. · 4.97 Impact Factor
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    ABSTRACT: Star clusters larger than $\sim 10^{3}$ $M_\odot$ contain multiple hot stars that launch fast stellar winds. The integrated kinetic energy carried by these winds is comparable to that delivered by supernova explosions, suggesting that at early times winds could be an important form of feedback on the surrounding cold material from which the star cluster formed. However, the interaction of these winds with the surrounding clumpy, turbulent, cold gas is complex and poorly understood. Here we investigate this problem via an accounting exercise: we use empirically determined properties of four well-studied massive star clusters to determine where the energy injected by stellar winds ultimately ends up. We consider a range of kinetic energy loss channels, including radiative cooling, mechanical work on the cold interstellar medium, thermal conduction, heating of dust via collisions by the hot gas, and bulk advection of thermal energy by the hot gas. We show that, for at least some of the clusters, none of these channels can account for more than a small fraction of the injected energy. We suggest that turbulent mixing at the hot-cold interface or physical leakage of the hot gas from the HII region can efficiently remove the kinetic energy injected by the massive stars in young star clusters. Even for the clusters where we are able to account for all the injected kinetic energy, we show that our accounting sets strong constraints on the importance of stellar winds as a mechanism for feedback on the cold interstellar medium.
    05/2014;
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    ABSTRACT: White dwarfs (WDs) can be tidally disrupted only by massive black holes (MBHs) with masses less than approximately $10^5 M_\odot$. These tidal interactions feed material to the MBH well above its Eddington limit, with the potential to launch a relativistic jet. The corresponding beamed emission is a promising signpost to an otherwise quiescent MBH of relatively low mass. We show that the mass transfer history, and thus the lightcurve, are quite different when the disruptive orbit is parabolic, eccentric, or circular. The mass lost each orbit exponentiates in the eccentric-orbit case leading to the destruction of the WD after several tens of orbits and making it difficult to produce a Swift J1644+57-like lightcurve via this channel. We then examine the stellar dynamics of clusters surrounding these MBHs to show that single-passage WD disruptions are substantially more common than repeating encounters in eccentric orbits. The $10^{49}$ erg s$^{-1}$ peak luminosity of these events makes them visible to cosmological distances and means that they may be detectible at rates of as many as tens per year by instruments like Swift. In fact, WD-disruption transients significantly outshine their main-sequence star counterparts, and are the most likely tidal interaction to be detected arising from MBHs with masses less than $10^5 M_\odot$. The detection or non-detection of such WD-disruption transients by Swift is, therefore, a powerful tool to constrain lower end of the MBH mass function. The emerging class of ultra-long gamma ray bursts, such as GRB 101225A, GRB 111209A, and GRB 121027A, all have peak luminosities and durations reminiscent of those arising of WD disruptions, offering a hint that WD-disruption transients may already be present in existing datasets.
    The Astrophysical Journal 05/2014; 794(1). · 6.73 Impact Factor
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    ABSTRACT: Short gamma-ray bursts (sGRBs) are widely believed to result from the mergers of compact binaries. This model predicts an afterglow that bears the characteristic signatures of a constant, low density medium, including a smooth prompt-afterglow transition, and a simple temporal evolution. However, these expectations are in conflict with observations for a non-negligible fraction of sGRB afterglows. In particular, the onset of the afterglow phase for some of these events appears to be delayed and, in addition, a few of them exhibit late-time rapid fading in their lightcurves. These facts have prompted speculation that the central engine activity continues to operate effectively for tens of seconds following the prompt emission. We show that these peculiar observations can be explained independently of ongoing central engine activity if some sGRB progenitors are compact binaries hosting at least one pulsar. The Poynting flux emanating from the pulsar companion can excavate a bow-shock cavity surrounding the binary. If this cavity is larger than the shock deceleration length scale in the undisturbed interstellar medium, then the onset of the afterglow will be delayed. Should the deceleration occur entirely within the swept-up thin shell, a rapid fade in the lightcurve will ensue. We identify two types of pulsar that can achieve the conditions necessary for altering the afterglow: low field, long lived pulsars, and high field pulsars. We find that a sizable fraction ($\approx 20-50\%$) of low field pulsars are likely to reside in neutron star binaries based on observations, while their high field counterparts are not.
    The Astrophysical Journal Letters 04/2014; 790(1). · 6.35 Impact Factor
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    ABSTRACT: Star clusters larger than $\sim 10^{3}$ $M_\odot$ contain multiple hot stars that launch fast stellar winds. The integrated kinetic energy carried by these winds is comparable to that delivered by supernova explosions, suggesting that at early times winds could be an important form of feedback on the surrounding cold material from which the star cluster formed. However, the interaction of these winds with the surrounding clumpy, turbulent, cold gas is complex and poorly understood. Here we investigate this problem via an accounting exercise: we use empirically determined properties of four well-studied massive star clusters to determine where the energy injected by stellar winds ultimately ends up. We consider a range of kinetic energy loss channels, including radiative cooling, mechanical work on the cold interstellar medium, thermal conduction, heating of dust via collisions by the hot gas, and bulk advection of thermal energy by the hot gas. We show that, for at least some of the clusters, none of these channels can account for more than a small fraction of the injected energy. We suggest that turbulent mixing at the hot-cold interface or physical leakage of the hot gas from the HII region can efficiently remove the kinetic energy injected by the massive stars in young star clusters. Even for the clusters where we are able to account for all the injected kinetic energy, we show that our accounting sets strong constraints on the importance of stellar winds as a mechanism for feedback on the cold interstellar medium.
    04/2014;
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    ABSTRACT: The central engine of short gamma-ray bursts (sGRBs) is hidden from direct view, operating at a scale much smaller than that probed by the emitted radiation. Thus we must infer its origin not only with respect to the formation of the trigger - the actual astrophysical configuration that is capable of powering a sGRB - but also from the consequences that follow from the various evolutionary pathways that may be involved in producing it. Considering binary neutron star mergers we critically evaluate, analytically and through numerical simulations, whether the neutrino-driven wind produced by the newly formed hyper-massive neutron star can allow the collimated relativistic outflow that follows its collapse to actually produce a sGRB or not. Upon comparison with the observed sGRB duration distribution, we find that collapse cannot be significantly delayed (<= 100 ms) before the outflow is choked, thus limiting the possibility that long-lived hyper-massive remnants can account for these events. In the case of successful breakthrough of the jet through the neutrino-driven wind, the energy stored in the cocoon could contribute to the precursor and extended emission observed in sGRBs.
    The Astrophysical Journal Letters 04/2014; 788(1). · 6.35 Impact Factor
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    ABSTRACT: Recent observations have revealed several Jupiter-mass planets with highly eccentric and / or misaligned orbits, which clearly suggests that dynamical processes operated in these systems. These dynamical processes may result in close encounters between Jupiter-like planets and their host stars. Using three-dimensional hydrodynamical simulations, we find that planets with cores are more likely to be retained by their host stars in contrast with previous studies which suggested that coreless planets are often ejected. We propose that after a long term evolution some gas giant planets could be transformed into super-Earths or Neptune-like planets, which is supported by our adiabatic evolution models. Finally, we analyze the orbits and structure of known planets and Kepler candidates and find that our model is capable of producing some of the shortest-period objects.
    03/2014;
  • 01/2014; 782(1).
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    ABSTRACT: The discovery of the gas cloud G2 on a near-radial orbit about Sgr A* has prompted much speculation on its origin. In this Letter, we propose that G2 formed out the debris stream produced by the removal of mass from the outer envelope of a nearby giant star. We perform hydrodynamical simulations of the returning tidal debris stream with cooling, and find that the stream condenses into clumps that fall to Sgr A* approximately once per decade. We propose that one of these clumps is the observed G2 cloud, with the rest of the stream being detectable at lower Br-$\gamma$ emissivity along a trajectory that would trace from G2 to the star that was partially disrupted. By simultaneously fitting the orbits of S2, G2, and ~2,000 candidate stars, and by fixing the orbital plane of each candidate star to G2 (as is expected for a tidal disruption), we find that the late-type star S1-34 has an orbit that is compatible with the notion that it was tidally disrupted to produce G2. If S1-34 is indeed the star that was disrupted, it last encountered Sgr A* in the late 18th century, and will likely be disrupted again in several hundred years. However, while S1-34's orbit is compatible with the giant disruption scenario given its measured position and proper motion, its radial velocity is currently unknown. If S1-34's radial velocity is measured to be compatible with a disruptive orbit, it would strongly suggest that a tidal disruption of S1-34 produced G2.
    The Astrophysical Journal Letters 01/2014; 786(2). · 6.35 Impact Factor
  • Melinda Soares-Furtado, J. Naiman, E. Ramirez-Ruiz
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    ABSTRACT: Modeling globular clusters 47 Tucanae, M15, NGC 6440, and NGC 6752, we examine the effects of mass supply from a population of evolved stars with the presence of energy injection from an abundant main sequence stellar population and then compare our results to observational constraints. We find that the energy injection from the main sequence stellar members is capable of sufficiently clearing the evolved stellar ejecta to produce intracluster gas densities consistent with observational constraints. Since main sequence stars are found universally within globular clusters, this may be the driving force responsible for the tenuous medium within clusters. In addition, our analysis is extended to examine the efficiency of pulsar wind feedback in globular clusters. We find that the pulsar wind thermalization efficiency must be extremely low in the cluster's core in order to be in accordance with density constraints.
    01/2014;
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Publication Stats

4k Citations
1,305.78 Total Impact Points

Institutions

  • 2014
    • NASA
      Washington, West Virginia, United States
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2006–2014
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 2013
    • Universidad Nacional Autónoma de México
      • Institute of Nuclear Science
      Ciudad de México, The Federal District, Mexico
  • 2011–2012
    • TASC Inc.
      Chantilly, Virginia, United States
    • University of Hertfordshire
      • Centre for Astrophysics Research (CAR)
      Hatfield, England, United Kingdom
  • 2009
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2008
    • University of California Observatories
      Santa Cruz, California, United States
    • University of Washington Seattle
      • Institute for Nuclear Theory
      Seattle, WA, United States
  • 2001–2008
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, ENG, United Kingdom
  • 1999–2008
    • Los Alamos National Laboratory
      • Space Science and Applications Group
      Los Alamos, California, United States
  • 2007
    • University of California, San Diego
      San Diego, California, United States
    • Stanford University
      • Kavli Institute for Particle Physics and Cosmology (KIPAC)
      Stanford, CA, United States
  • 2005
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
    • University of Southampton
      • Department of Physics and Astronomy
      Southampton, England, United Kingdom