arXiv subjects

Aristotle Socrates

Publications and source records attributed to Aristotle Socrates.

At least 19 recordsLinked to original sources

arXiv2018

Sub-photospheric fluctuations in magnetized radiative envelopes: contribution from unstable magnetosonic waves

We examine the excitation of unstable magnetosonic waves in the radiative envelopes of intermediate- and high-mass stars with a magnetic field of ~kG strength. Wind clumping close to the star and microturbulence can often be accounted for when including small-scale, sub-photospheric density or velocity perturbations. Compressional waves - with wavelengths comparable to or shorter than the gas pressure scale height - can be destabilized by the radiative flux in optically-thick media when a magnetic field is present, in a process called the Radiation-Driven Magneto-Acoustic Instability (RMI). The instability does not require radiation or magnetic pressure to dominate over gas pressure, and acts independently of sub-surface convection zones. Here we evaluate the conditions for the RMI to operate on a grid of stellar models covering a mass range $3-40M_\odot$ at solar metallicity. For a uniform 1kG magnetic field, fast magnetosonic modes are unstable down to an optical depth of a few tens, while unstable slow modes extend beyond the depth of the iron convection zone. The qualitative behavior is robust to magnetic field strength variations by a factor of a few. When combining our findings with previous results for the saturation amplitude of the RMI, we predict velocity fluctuations in the range ~0.1-10 km/s. These amplitudes are a monotonically increasing function of the ratio of radiation to gas pressure, or alternatively, of the zero-age main sequence mass.

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arXiv2015

The Turbulent Origin of Spin-Orbit Misalignment in Planetary Systems

The turbulent environment from which stars form may lead to misalignment between the stellar spin and the remnant protoplanetary disk. By using hydrodynamic and magnetohydrodynamic simulations, we demonstrate that a wide range of stellar obliquities may be produced as a by-product of forming a star within a turbulent environment. We present a simple semi-analytic model that reveals this connection between the turbulent motions and the orientation of a star and its disk. Our results are consistent with the observed obliquity distribution of hot Jupiters. Migration of misaligned hot Jupiters may, therefore, be due to tidal dissipation in the disk, rather than tidal dissipation of the star-planet interaction.

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arXiv2014

Directly Imaging Tidally Powered Migrating Jupiters

Upcoming direct-imaging experiments may detect a new class of long-period, highly luminous, tidally powered extrasolar gas giants. Even though they are hosted by ~ Gyr-"old" main-sequence stars, they can be as "hot" as young Jupiters at ~100 Myr, the prime targets of direct-imaging surveys. They are on years-long orbits and presently migrating to "feed" the "hot Jupiters." They are expected from "high-e" migration mechanisms, in which Jupiters are excited to highly eccentric orbits and then shrink semi-major axis by a factor of ~10-100 due to tidal dissipation at close periastron passages. The dissipated orbital energy is converted to heat, and if it is deposited deep enough into the atmosphere, the planet likely radiates steadily at luminosity L ~ 100-1000 L_Jup(2 x 10-7-2 x 10-6 L_Sun) during a typical ~ Gyr migration timescale. Their large orbital separations and expected high planet-to-star flux ratios in IR make them potentially accessible to high-contrast imaging instruments on 10 m class telescopes. ~10 such planets are expected to exist around FGK dwarfs within ~50 pc. Long-period radial velocity planets are viable candidates, and the highly eccentric planet HD 20782b at maximum angular separation ~0.''08 is a promising candidate. Directly imaging these tidally powered Jupiters would enable a direct test of high-e migration mechanisms. Once detected, the luminosity would provide a direct measurement of the migration rate, and together with mass (and possibly radius) estimate, they would serve as a laboratory to study planetary spectral formation and tidal physics.

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arXiv2013

Warm Jupiters Need Close "Friends" for High-Eccentricity Migration -- A Stringent Upper Limit on the Perturber's Separation

We propose a stringent observational test on the formation of warm Jupiters (gas-giant planets with 10 d <~ P <~ 100 d) by high-eccentricity (high-e) migration mechanisms. Unlike hot Jupiters, the majority of observed warm Jupiters have pericenter distances too large to allow efficient tidal dissipation to induce migration. To access the close pericenter required for migration during a Kozai-Lidov cycle, they must be accompanied by a strong enough perturber to overcome the precession caused by General Relativity (GR), placing a strong upper limit on the perturber's separation. For a warm Jupiter at a ~ 0.2 AU, a Jupiter-mass (solar-mass) perturber is required to be <~ 3 AU (<~ 30 AU) and can be identified observationally. Among warm Jupiters detected by Radial Velocities (RV), >~ 50% (5 out of 9) with large eccentricities (e >~ 0.4) have known Jovian companions satisfying this necessary condition for high-e migration. In contrast, <~ 20 % (3 out of 17) of the low-e (e <~ 0.2) warm Jupiters have detected additional Jovian companions, suggesting that high-e migration with planetary perturbers may not be the dominant formation channel. Complete, long-term RV follow-ups of the warm-Jupiter population will allow a firm upper limit to be put on the fraction of these planets formed by high-e migration. Transiting warm Jupiters showing spin-orbit misalignments will be interesting to apply our test. If the misalignments are solely due to high-e migration as commonly suggested, we expect that the majority of warm Jupiters with low-e (e <~0.2) are not misaligned, in contrast with low-e hot Jupiters.

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arXiv2013

Relationship Between Thermal Tides and Radius Excess

Close-in extrasolar gas giants -- the hot Jupiters -- display departures in radius above the zero-temperature solution, the radius excess, that are anomalously high. The radius excess of hot Jupiters follows a relatively close relation with thermal tidal tidal torques and holds for ~ 4-5 orders of magnitude in a characteristic thermal tidal power in such a way that is consistent with basic theoretical expectations. The relation suggests that thermal tidal torques determine the global thermodynamic and spin state of the hot Jupiters. On empirical grounds, it is shown that theories of hot Jupiter inflation that invoke a constant fraction of the stellar flux to be deposited at great depth are, essentially, falsified.

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arXiv2013

Nonlinear Evolution of the Radiation-Driven Magneto-Acoustic Instability (RMI)

We examine the nonlinear development of unstable magnetosonic waves driven by a background radiative flux -- the Radiation-Driven Magneto-Acoustic Instability (RMI, a.k.a. the "photon bubble" instability). The RMI may serve as a persistent source of density, radiative flux, and magnetic field fluctuations in stably-stratified, optically-thick media. The conditions for instability are present in a variety of astrophysical environments, and do not require the radiation pressure to dominate or the magnetic field to be strong. Here we numerically study the saturation properties of the RMI, covering three orders of magnitude in the relative strength of radiation, magnetic field, and gas energies. Two-dimensional, time-dependent radiation-MHD simulations of local, stably-stratified domains are conducted with Zeus-MP in the optically-thick, highly-conducting limit. Our results confirm the theoretical expectations of Blaes and Socrates (2003) in that the RMI operates even in gas pressure-dominated environments that are weakly magnetized. The saturation amplitude is a monotonically increasing function of the ratio of radiation to gas pressure. Keeping this ratio constant, we find that the saturation amplitude peaks when the magnetic pressure is comparable to the radiation pressure. We discuss the implications of our results for the dynamics of magnetized stellar envelopes, where the RMI should act as a source of sub-photospheric perturbations.

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arXiv2013

Photon Feedback: Screening and the Eddington Limit

Bright star-forming galaxies radiate well below their Eddington Limit. The value of the flux-mean opacity that mediates the radiation force onto matter is orders of magnitude smaller than the UV or optical dust opacity. On empirical grounds, it is shown that high-redshift ULIRGs radiate at two orders of magnitude below their Eddington Limit, while the local starbursters M82 and Arp 220 radiate at a few percent of their Eddington Limit. A simple model for the radiative transfer of UV and optical light in dust-rich environments is considered. Radiation pressure on dust does not greatly affect the large-scale gas dynamics of star-forming galaxies.

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arXiv2012

Physical Basis for a Constant Lag Time

We show that the constant time lag prescription for tidal dissipation follows directly from the equations of motion of a tidally-forced viscous fluid body, given some basic assumptions. They are (i) dissipation results from a viscous force that is proportional to the velocity of the tidal flow (ii) tidal forcing and dissipation are weak and non-resonant (iii) the equilibrium structure of the forced body is spherically-symmetric. The lag time is an intrinsic property of the tidally-forced body and is independent of the orbital configuration.

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arXiv2012

Q in Other Solar Systems

A significant fraction of the hot Jupiters with final circularized orbital periods of less than 5 days are thought to form through the channel of high-eccentricity migration. Tidal dissipation at successive periastron passages removes orbital energy of the planet, which has the potential for changes in semi-major axis of a factor of ten to a thousand. In the equilibrium tide approximation we show that, in order for high-eccentricity migration to take place, the relative level of tidal dissipation in Jupiter analogues must be at least 10 times higher than the upper-limit attributed to the Jupiter-Io interaction. While this is not a severe problem for high-e migration, it contradicts the results of several previous calculations. We show that these calculations of high-e migration inadvertently over-estimated the strength of tidal dissipation by three to four orders of magnitude. These discrepancies were obscured by the use of various parameters, such as lag time τ, tidal quality factor Q and viscous time t_V. We provide the values of these parameters required for the Jupiter-Io interaction, tidal circularization and high-e migration. Implications for tidal theory as well as models of the inflated radii of hot Jupiters are discussed. Though the tidal Q is not, in general, well-defined, we derive a formula for it during high-eccentricity migration where Q is approximately constant throughout evolution.

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arXiv2012

Swift J1644+57: An Ultra-Luminous X-ray Event

The photon spectral energy distribution of the powerful transient Sw J1644+57 resembles those of the brightest Ultra-Luminous X-ray sources (ULXs). The transient nature of Sw J1644+57 is likely the result of a tidal disruption of a star by a super-massive black hole. The stellar disk generates accretion power at super-Eddington rates and the observational properties of Sw J1644+57 indicate -- in analogy with ULXs -- that the accretion flow maintains a high level of radiative efficiency with a corresponding super-Eddington luminosity. Due to its similarity to ULXs, this powerful transient may be thought of as an Ultra-Luminous X-ray event (ULX-E). Observational tests for this ULX-E model are proposed as well.

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arXiv2012

Exploring a Stream of Highly-Eccentric Binaries with Kepler

With 16-month Kepler data, 14 long-period (40 d - 265 d) eclipsing binaries on highly eccentric orbits (minimum e between 0.5 and 0.85) are recognized from their closely separated primary and secondary eclipses (Δt_I,II = 3 d - 10 d). These systems confirm the existence of a previously hinted binary population situated near a constant angular momentum track at P(1-e^2)^(3/2) ~ 15 d, close to the tidal circularization period P_circ. They may be presently migrating due to tidal dissipation and form a steady-state stream (~1% of stars) feeding the close-binary population (few percent of stars). If so, future Kepler data releases will reveal a growing number (dozens) of systems at longer periods, following dN/dlgP \propto P^(1/3) with increasing eccentricities reaching e -> 0.98 for P -> 1000d. Radial-velocity follow up of long-period eclipsing binaries with no secondary eclipses could offer a significantly larger sample. Orders of magnitude more (hundreds) may reveal their presence from periodic "eccentricity pulses", such as tidal ellipsoidal variations, near pericenter passages. Several new few-day-long eccentricity-pulse candidates with long period (P = 25 d - 80 d) are reported.

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arXiv2011

Super-Eccentric Migrating Jupiters

An important class of formation theories for hot Jupiters involves the excitation of extreme orbital eccentricity (e=0.99 or even larger) followed by tidal dissipation at periastron passage that eventually circularizes the planetary orbit at a period less than 10 days. In a steady state, this mechanism requires the existence of a significant population of super-eccentric (e>0.9) migrating Jupiters with long orbital periods and periastron distances of only a few stellar radii. For these super-eccentric planets, the periastron is fixed due to conservation of orbital angular momentum and the energy dissipated per orbit is constant, implying that the rate of change in semi-major axis a is \dot a \propto a^0.5 and consequently the number distribution satisfies dN/dlog a\propto a^0.5. If this formation process produces most hot Jupiters, Kepler should detect several super-eccentric migrating progenitors of hot Jupiters, allowing for a test of high-eccentricity migration scenarios.

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arXiv2009

Response to "Concerning Thermal Tides on Hot Jupiters" (Goodman 2009; arXiv: 0901.3279)

Motivated by the comments of Goodman (2009) on our paper concerning thermal tides (Arras and Socrates 2009a), we have studied an idealized problem to understand the global response of a completely fluid gas giant planet to thermal forcing at the surface (Arras and Socrates 2009b). Our findings disagree with the main claims in Goodman (2009). We find that significant quadrupole moments can indeed be induced as a result of thermal forcing. Furthermore, we find that it is possible for the orientation of the quadrupoles to be such that the planet is torqued away from synchronous rotation. Given these results, we believe our proposed thermal tide mechanism (Arras and Socrates 2009a) provides a viable scenario for generating steady-state asychronous rotation, inflated radii and possibly eccentric orbits of the hot Jupiters.

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arXiv2009

Thermal Tides in Fluid Extrasolar Planets

Asynchronous rotation and orbital eccentricity lead to time-dependent irradiation of the close-in gas giant exoplanets -- the hot Jupiters. This time-dependent surface heating gives rise to fluid motions which propagate throughout the planet. We investigate the ability of this "thermal tide" to produce a quadrupole moment which can couple to the stellar gravitational tidal force. While previous investigations discussed planets with solid surfaces, here we focus on entirely fluid planets in order to understand gas giants with small cores. The Coriolis force, thermal diffusion and self-gravity of the perturbations are ignored for simplicity. First, we examine the response to thermal forcing through analytic solutions of the fluid equations which treat the forcing frequency as a small parameter. In the "equilibrium tide" limit of zero frequency, fluid motion is present but does not induce a quadrupole moment. In the next approximation, finite frequency corrections to the equilibrium tide do lead to a nonzero quadrupole moment, the sign of which torques the planet {\it away} from synchronous spin. We then numerically solve the boundary value problem for the thermally forced, linear response of a planet with neutrally stratified interior and stably stratified envelope. The numerical results find quadrupole moments in agreement with the analytic non-resonant result at sufficiently long forcing period. Surprisingly, in the range of forcing periods of 1-30 days, the induced quadrupole moments can be far larger than the analytic result due to response of internal gravity waves which propagate in the radiative envelope. We discuss the relevance of our results for the spin, eccentricity and thermal evolution of hot Jupiters.

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arXiv2009

The Eddington Limit in Cosmic Rays: An Explanation for the Observed Lack of Low-Mass Radio-Loud Quasars and the M_{BH}-M_{Bulge} Relation

We present a feedback mechanism for supermassive black holes and their host bulges that operates during epochs of radio-loud quasar activity. In the radio cores of relativistic quasar jets, internal shocks convert a fraction of ordered bulk kinetic energy into randomized relativistic ions, or in other words cosmic rays. By employing a phenomenologically-motivated jet model, we show that enough 1-100 GeV cosmic rays escape the radio core into the host galaxy to break the Eddington limit in cosmic rays. As a result, hydrostatic balance is lost and a cosmic ray momentum-driven wind develops, expelling gas from the host galaxy and thus self-limiting the black hole and bulge growth. Although the interstellar cosmic ray power is much smaller than the quasar photon luminosity, cosmic rays provide a stronger feedback than UV photons, since they exchange momentum with the galactic gas much more efficiently. The amount of energy released into the host galaxy as cosmic rays, per unit of black hole rest mass energy, is independent of black hole mass. It follows that radio-loud jets should be more prevalent in relatively massive systems since they sit in galaxies with relatively deep gravitational potentials. Therefore, jet-powered cosmic ray feedback not only self-regulates the black hole and bulge growth, but also provides an explanation for the lack of radio-loud activity in relatively small galaxies. By employing basic known facts regarding the physical conditions in radio cores, we approximately reproduce both the slope and the normalization of the M_{BH}-M_{Bulge} relation.

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arXiv2009

Brilliant Pebbles: A Method for Detection of Very Large Interstellar Grains

A photon of wavelength lambda ~1 micron interacting with a dust grain of radius a_p ~ 1 mm (a "pebble") undergoes scattering in the forward direction, largely within a small characteristic diffraction angle theta_s ~ lambda/a_p ~ 100". Though mm-size dust grains contribute negligibly to the interstellar medium's visual extinction, the signal they produce in scattered light may be detectable, especially for variable sources. Observations of light scattered at small angles allows for the direct measurement of the large grain population; variable sources can also yield tomographic information of the interstellar medium's mass distribution. The ability to detect brilliant pebble halos require a careful understanding of the instrument PSF.

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arXiv2009

Thermal Tides in Short Period Exoplanets

Time-dependent insolation in a planetary atmosphere induces a mass quadrupole upon which the stellar tidal acceleration can exert a force. This "thermal tide" force can give rise to secular torques on the planet and orbit as well as radial forces causing eccentricity evolution. We apply this idea to the close-in gas giant exoplanets ("hot Jupiters"). The response of radiative atmospheres is computed in a hydrostatic model which treats the insolation as a time-dependent heat source, and solves for thermal radiation using flux-limited diffusion. Fully nonlinear numerical simulations are compared to solutions of the linearized equations, as well as analytic approximations, all of which are in good agreement. We find generically that thermal tide density perturbations {\it lead} the semi-diurnal forcing. As a result thermal tides can generate asynchronous spin and eccentricity. Our results are as follows: (1) Departure from synchronous spin is significant for hot Jupiters, and increases with orbital period. (2) Ongoing gravitational tidal dissipation in spin equilibrium leads to steady-state internal heating rates up to $\sim 10^{28} {\rm erg\ s^{-1}}$. If deposited sufficiently deep, these heating rates may explain the anomalously large radii of many hot Jupiters in terms of a "tidal main sequence" where cooling balances tidal heating. At fixed stellar type, planet mass and tidal $Q$, planetary radius increases strongly toward the star inside orbital periods $\la 2$ weeks. (3) There exists a narrow window in orbital period where small eccentricities, $e$, grow exponentially with a large rate. This window may explain the $\sim 1/4$ of hot Jupiters which should have been circularized by the gravitational tide long ago, but are observed to have significant nonzero $e$.(Abridged)

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arXiv2008

Relativistic Accretion Mediated by Turbulent Comptonization

Black hole and neutron star accretion flows display unusually high levels of hard coronal emission in comparison to all other optically thick, gravitationally bound, turbulent astrophysical systems. Since these flows sit in deep relativistic gravitational potentials, their random bulk motions approach the speed of light, therefore allowing turbulent Comptonization to be an important effect. We show that the inevitable production of hard X-ray photons results from turbulent Comptonization in the limit where the turbulence is trans-sonic and the accretion power approaches the Eddington Limit. In this regime, the turbulent Compton y-parameter approaches unity and the turbulent Compton temperature is a significant fraction of the electron rest mass energy, in agreement with the observed phenomena.

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