arXiv subjects

Phil Arras

Publications and source records attributed to Phil Arras.

At least 55 records · Page 3Linked to original sources

arXiv2013

Hα Absorption in Transiting Exoplanet Atmospheres

Absorption of stellar Hα by the upper atmosphere of the planet HD189733b has recently been detected by Jensen et al. Motivated by this observation, we have developed a model for atomic hydrogen in the n=2 state and compared the resulting Hα line profile to the observations. The model atmosphere is in hydrostatic balance, as well as thermal and photoionization equilibrium. Collisional and radiative transitions are included in the determination of the n=2 state level population. We find that Hα absorption is dominated by an optical depth τ ~ 1 shell, composed of hydrogen in the metastable 2s state that is located below the hydrogen ionization layer. The number density of the 2s state within the shell is found to vary slowly with radius, while that of the 1s state falls rapidly. Thus while the Lyαabsorption, for a certain wavelength, occurs inside a relatively well defined impact parameter, the contribution to Hα absorption is roughly uniform over the entire atomic hydrogen layer. The model can approximately reproduce the observed Lyα and Hα integrated transit depths for HD189733b by using an ionization rate enhanced over that expected for the star by an order of magnitude. For HD 209458b, we are unable to explain the asymmetric Hα line profile observed by Jensen et al., as the model produces a symmetric line profile with transit depth comparable to that of HD 189733b. In an appendix, we study the effect of the stellar Lyα absorption on the net cooling rate.

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arXiv2013

Modules for Experiments in Stellar Astrophysics (MESA): Giant Planets, Oscillations, Rotation, and Massive Stars

We substantially update the capabilities of the open source software package Modules for Experiments in Stellar Astrophysics (MESA), and its one-dimensional stellar evolution module, MESA Star. Improvements in MESA Star's ability to model the evolution of giant planets now extends its applicability down to masses as low as one-tenth that of Jupiter. The dramatic improvement in asteroseismology enabled by the space-based Kepler and CoRoT missions motivates our full coupling of the ADIPLS adiabatic pulsation code with MESA Star. This also motivates a numerical recasting of the Ledoux criterion that is more easily implemented when many nuclei are present at non-negligible abundances. This impacts the way in which MESA Star calculates semi-convective and thermohaline mixing. We exhibit the evolution of 3-8 Msun stars through the end of core He burning, the onset of He thermal pulses, and arrival on the white dwarf cooling sequence. We implement diffusion of angular momentum and chemical abundances that enable calculations of rotating-star models, which we compare thoroughly with earlier work. We introduce a new treatment of radiation-dominated envelopes that allows the uninterrupted evolution of massive stars to core collapse. This enables the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. We substantially modify the way in which MESA Star solves the fully coupled stellar structure and composition equations, and we show how this has improved MESA's performance scaling on multi-core processors. Updates to the modules for equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions are also provided. We describe the MESA Software Development Kit (SDK) that packages all the required components needed to form a unified and maintained build environment for MESA. [Abridged]

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arXiv2012

Nonlinear Tides in Close Binary Systems

We study the excitation and damping of tides in close binary systems, accounting for the leading order nonlinear corrections to linear tidal theory. These nonlinear corrections include two distinct effects: three-mode nonlinear interactions and nonlinear excitation of modes by the time-varying gravitational potential of the companion. This paper presents the formalism for studying nonlinear tides and studies the nonlinear stability of the linear tidal flow. Although the formalism is applicable to binaries containing stars, planets, or compact objects, we focus on solar type stars with stellar or planetary companions. Our primary results include: (1) The linear tidal solution often used in studies of binary evolution is unstable over much of the parameter space in which it is employed. More specifically, resonantly excited gravity waves are unstable to parametric resonance for companion masses M' > 10-100 M_Earth at orbital periods P = 1-10 days. The nearly static equilibrium tide is, however, parametrically stable except for solar binaries with P < 2-5 days. (2) For companion masses larger than a few Jupiter masses, the dynamical tide causes waves to grow so rapidly that they must be treated as traveling waves rather than standing waves. (3) We find a novel form of parametric instability in which a single parent wave excites a very large number of daughter waves (N = 10^3[P / 10 days]) and drives them as a single coherent unit with growth rates that are ~N times faster than the standard three wave parametric instability. (4) Independent of the parametric instability, tides excite a wide range of stellar p-modes and g-modes by nonlinear inhomogeneous forcing; this coupling appears particularly efficient at draining energy out of the dynamical tide and may be more important than either wave breaking or parametric resonance at determining the nonlinear dissipation of the dynamical tide.

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arXiv2012

The Stability of Massive Main Sequence Stars as a Function of Metallicity

We investigate the pulsational stability of massive (M >~ 120 Msun) main sequence stars of a range of metallicities, including primordial, Population III stars. We include a formulation of convective damping motivated by numerical simulations of the interaction between convection and periodic shear flows. We find that convective viscosity is likely strong enough to stabilize radial pulsations whenever nuclear-burning (the epsilon-mechanism) is the dominant source of driving. This suggests that massive main sequence stars with Z <~ 2 x 10^-3 are pulsationally stable and are unlikely to experience pulsation-driven mass loss on the main sequence. These conclusions are, however, sensitive to the form of the convective viscosity and highlight the need for further high-resolution simulations of the convection-oscillation interaction. For more metal-rich stars (Z >~ 2 x 10^-3), the dominant pulsational driving arises due to the kappa-mechanism arising from the iron-bump in opacity and is strong enough to overcome convective damping. Our results highlight that even for oscillations with periods a few orders of magnitude shorter than the outer convective turnover time, the "frozen-in" approximation for the convection-oscillation interaction is inappropriate, and convective damping should be taken into account when assessing mode stability.

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arXiv2011

Tidal asteroseismology: Kepler's KOI-54

We develop a general framework for interpreting and analyzing high-precision lightcurves from eccentric stellar binaries. Although our methods are general, we focus on the recently discovered Kepler system KOI-54, a face-on binary of two A stars with $e=0.83$ and an orbital period of 42 days. KOI-54 exhibits strong ellipsoidal variability during its periastron passage; its lightcurve also contains ~20 pulsations at perfect harmonics of the orbital frequency, and another ~10 nonharmonic pulsations. Analysis of such data is a new form of asteroseismology in which oscillation amplitudes and phases rather than frequencies contain information that can be mined to constrain stellar properties. We qualitatively explain the physics of mode excitation and the range of harmonics expected to be observed. To quantitatively model observed pulsation spectra, we develop and apply a linear, tidally forced, nonadiabatic stellar oscillation formalism including the Coriolis force. We produce temporal power spectra for KOI-54 that are semi-quantitatively consistent with the observations. Both stars in the KOI-54 system are expected to be rotating pseudosynchronously, with resonant nonaxisymmetric modes providing a key contribution to the total torque; such resonances provide a possible explanation for the two largest-amplitude harmonic pulsations observed in KOI-54, although we find quantitative problems with this interpretation. We show in detail that the nonharmonic pulsations observed in KOI-54 can be produced by nonlinear three-mode coupling. The methods developed in this paper can be generalized in the future to determine the best-fit stellar parameters given pulsation data. We also derive an analytic model of KOI-54's ellipsoidal variability, including both tidal distortion and stellar irradiation, which can be used to model other similar systems.

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arXiv2011

The Radial Velocity Signature of Tides Raised in Stars Hosting Exoplanets

Close-in, massive exoplanets raise significant tides in their stellar hosts. We compute the radial velocity (RV) signal due to this fluid motion in the equilibrium tide approximation. The predicted radial velocities in the observed sample of exoplanets exceed 1 m/s for 17 systems, with the largest predicted signal being ~30 m/s for WASP-18 b. Tidally-induced RV's are thus detectable with present methods. Both tidal fluid flow and the epicyclic motion of a slightly eccentric orbit produce an RV signal at twice the orbital frequency. If care is not taken, the tidally induced RV may, in some cases, be confused with a finite orbital eccentricity. Indeed, WASP-18 b is reported to have an eccentric orbit with small e=0.009 and pericenter longitude ω=-π/2. Whereas such a close alignment of the orbit and line of sight to the observer requires fine tuning, this phase in the RV signal is naturally explained by the tidal velocity signature of an e=0 orbit. Additionally, the equilibrium tide estimate for the amplitude is in rough agreement with the data. Thus the reported eccentricity for WASP-18 b is instead likely a signature of the tidally-induced RV in the stellar host. Measurement of both the orbital and tidal velocity for non-transiting planets may allow planet mass and inclination to be separately determined solely from radial velocity data. We suggest that high precision fitting of RV data should include the tidal velocity signal in those cases where it may affect the determination of orbital parameters.

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arXiv2010

The Fate of Exoplanets and the Red Giant Rapid Rotator Connection

We have computed the fate of exoplanet companions around main sequence stars to explore the frequency of planet ingestion by their host stars during the red giant branch evolution. Using published properties of exoplanetary systems combined with stellar evolution models and Zahn's theory of tidal friction, we modeled the tidal decay of the planets' orbits as their host stars evolve. Most planets currently orbiting within 2 AU of their star are expected to be ingested by the end of their stars' red giant branch ascent. Our models confirm that many transiting planets are sufficiently close to their parent star that they will be accreted during the main sequence lifetime of the star. We also find that planet accretion may play an important role in explaining the mysterious red giant rapid rotators, although appropriate planetary systems do not seem to be plentiful enough to account for all such rapid rotators. We compare our modeled rapid rotators and surviving planetary systems to their real-life counterparts and discuss the implications of this work to the broader field of exoplanets.

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arXiv2010

Hot Jupiter Magnetospheres

(Abridged) The upper atmospheres of close-in gas giant exoplanets are subjected to intense heating/tidal forces from their parent stars. Atomic/ionized hydrogen (H) layers are sufficiently rarefied that magnetic pressure may dominate gas pressure for expected planetary magnetic field strength. We examine the magnetospheric structure using a 3D isothermal magnetohydrodynamic model that includes: a static "dead zone" near the magnetic equator containing magnetically confined gas; a "wind zone" outside the magnetic equator in which thermal pressure gradients and the magneto-centrifugal-tidal effect give rise to transonic outflow; and a region near the poles where sufficiently strong tidal forces may suppress transonic outflow. Using dipole field geometry, we estimate the size of the dead zone to be ~1-10 planetary radii for a range of parameters. To understand appropriate base conditions for the 3D isothermal model, we compute a 1D thermal model in which photoelectric heating from the stellar Lyman continuum is balanced by collisionally-excited Lyman α cooling. This 1D model exhibits a H layer with temperatures T=5000-10000K down to pressures of 10-100 nbar. Using the 3D isothermal model, we compute H column densities and Lyman α transmission spectra for parameters appropriate to HD 209458b. Line-integrated transit depths of 5-10% can be achieved for the above base conditions. Strong magnetic fields increase the transit signal while decreasing the mass loss, due to higher covering fraction and density of the dead zone. In our model, most of the transit signal arises from magnetically confined gas, some of which may be outside the L1 equipotential. Hence the presence of gas outside the L1 equipotential does not directly imply mass loss. Lastly, we discuss the domain of applicability for the magnetic wind model described in this paper and in the Roche-lobe overflow model.

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arXiv2010

Diffusive Nuclear Burning of Helium on Neutron Stars

Diffusive nuclear burning of H by an underlying material capable of capturing protons can readily consume H from the surface of neutron stars (NSs) during their early cooling history. In the absence of subsequent accretion, it will be depleted from the photosphere. We now extend diffusive nuclear burning to He, motivated by the recent observation by Ho \& Heinke of a carbon atmosphere on the NS in the Cassiopeia A supernova remnant. We calculate the equilibrium structure of He on an underlying $α$ capturing material, accounting for thermal, mass defect, and Coulomb corrections on the stratification of material with the same zeroth order $μ_e = A/Z$. We show that Coulomb corrections dominate over thermal and mass defect corrections in the highly degenerate part of the envelope. We also show that the bulk of the He sits deep in the envelope rather than near the surface. Thus, even if the photospheric He abundance is low, the total He column could be substantially larger than the photospheric column, which may have implications for rapid surface evolution ($\approx 1$ yr timescales) of neutron stars. When nuclear reactions are taken into account, we find that for base temperatures $\gtrsim 1.6 \times 10^8$ K, He is readily captured onto C. As these high temperatures are present during the early stages of NS evolution, we expect that the primordial He is completely depleted from the NS surface like the case for primordial H. We also find that magnetic fields $\lesssim 10^{12}$ G do not affect our conclusions. Armed with the results of this work and our prior efforts, we expect that primordial H and He are depleted, and so any observed H or He on the surfaces of these NS must be due to subsequent accretion (with or without spallation). If this subsequent accretion can be prevented, the underlying mid-Z material would be exposed.

astro-ph.HE↗

arXiv2010

Pulsations in Hydrogen Burning Low Mass Helium White Dwarfs

Helium core white dwarfs (WDs) with mass M <~ 0.20 Msun undergo several Gyrs of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of 2. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions predominantly reside in the helium core. Though we have not carried out a fully nonadiabatic stability analysis, the scaling of the thermal time in the convective zone with surface gravity highlights several low-mass helium WDs that should be observed in search of pulsations: NLTT 11748, SDSS J0822+2753, and the companion to PSR J1012+5307. Seismological studies of these He core WDs may prove especially fruitful, as their luminosity is related (via stable hydrogen burning) to the hydrogen envelope mass, which eliminates one model parameter.

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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

Massive Satellites of Close-In Gas Giant Exoplanets

We study the orbits, tidal heating and mass loss from satellites around close-in gas giant exoplanets. The focus is on large satellites which are potentially observable by their transit signature. We argue that even Earth-size satellites around hot Jupiters may be immune to destruction by orbital decay; detection of such a massive satellite would strongly constrain theories of tidal dissipation in gas giants, in a manner complementary to orbital circularization. The star's gravity induces significant periodic eccentricity in the satellite's orbit. The resulting tidal heating rates, per unit mass, are far in excess of Io's and dominate radioactive heating out to planet orbital periods of months for reasonable satellite tidal $Q$. Inside planet orbital periods of about a week, tidal heating can completely melt the satellite. Lastly, we compute an upper limit to the satellite mass loss rate due to thermal evaporation from the surface, valid if the satellite's atmosphere is thin and vapor pressure is negligible. Using this upper limit, we find that although rocky satellites around hot Jupiters with orbital periods less than a few days can be significantly evaporated in their lifetimes, detectable satellites suffer negligible mass loss at longer orbital periods.

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arXiv2009

The Role of Planet Accretion in Creating the Next Generation of Red Giant Rapid Rotators

Rapid rotation in field red giant stars is a relatively rare but well-studied phenomenon; here we investigate the potential role of planet accretion in spinning up these stars. Using Zahn's theory of tidal friction and stellar evolution models, we compute the decay of a planet's orbit into its evolving host star and the resulting transfer of angular momentum into the stellar convective envelope. This experiment assesses the frequency of planet ingestion and rapid rotation on the red giant branch (RGB) for a sample of 99 known exoplanet host stars. We find that the known exoplanets are indeed capable of creating rapid rotators; however, the expected fraction due to planet ingestion is only ~10% of the total seen in surveys of present-day red giants. Of the planets ingested, we find that those with smaller initial semimajor axes are more likely to create rapid rotators because these planets are accreted when the stellar moment of inertia is smallest. We also find that many planets may be ingested prior to the RGB phase, contrary to the expectation that accretion would generally occur when the stellar radii expand significantly as giants. Finally, our models suggest that the rapid rotation signal from ingested planets is most likely to be seen on the lower RGB, which is also where alternative mechanisms for spin-up, e.g., angular momentum dredged up from the stellar core, do not operate. Thus, rapid rotators on the lower RGB are the best candidates to search for definitive evidence of systems that have experienced planet accretion.

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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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arXiv2007

Non-dissipative tidal synchronization in accreting binary white dwarf systems

We study a non-dissipative hydrodynamical mechanism that can stabilize the spin of the accretor in an ultra-compact double white dwarf binary. This novel synchronization mechanism relies on a nonlinear wave interaction spinning down the background star. The essential physics of the synchronization mechanism is summarized as follows. As the compact binary coalesces due to gravitational wave emission, the largest star eventually fills its Roche lobe and accretion starts. The accretor then spins up due to infalling material and eventually reaches a spin frequency where a normal mode of the star is resonantly driven by the gravitational tidal field of the companion. If the resonating mode satisfies a set of specific criteria, which we elucidate in this paper, it exchanges angular momentum with the background star at a rate such that the spin of the accretor locks at this resonant frequency, even though accretion is ongoing. Some of the accreted angular momentum that would otherwise spin up the accretor is fed back into the orbit through this resonant tidal interaction. Two modes capable of stabilizing the accretor's spin are the l=4,m=2 and l=5,m=3 CFS unstable hybrid r-modes, which stabilize the spin of the accretor at frequency 2.6 and 1.5 times the binary's orbital frequency respectively. Since the stabilization mechanism relies on continuously driving a mode at resonance, its lifetime is limited since eventually the mode amplitude saturates due to non-linear mode-mode coupling. Rough estimates of the lifetime of the effect lie from a few orbits to millions of years.

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arXiv2007

Ellipsoidal Oscillations Induced by Substellar Companions: A Prospect for the Kepler Mission

Hundreds of substellar companions to solar-type stars will be discovered with the Kepler satellite. Kepler's extreme photometric precision gives access to low-amplitude stellar variability contributed by a variety of physical processes. We discuss in detail the periodic flux modulations arising from the tidal force on the star due to a substellar companion. An analytic expression for the variability is derived in the equilibrium-tide approximation. We demonstrate analytically and through numerical solutions of the linear, nonadiabatic stellar oscillation equations that the equilibrium-tide formula works extremely well for stars of mass <1.4 Msun with thick surface convection zones. More massive stars with largely radiative envelopes do not conform to the equilibrium-tide approximation and can exhibit flux variations $\ga$10 times larger than naive estimates. Over the full range of stellar masses considered, we treat the oscillatory response of the convection zone by adapting a prescription that A. J. Brickhill developed for pulsating white dwarfs. Compared to other sources of periodic variability, the ellipsoidal lightcurve has a distinct dependence on time and system parameters. We suggest that ellipsoidal oscillations induced by giant planets may be detectable from as many as ~100 of the 10^5 Kepler target stars. (Abridged)

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arXiv2006

Pulsational Instabilities in Accreting White Dwarfs

(Abridged) The Cataclysmic Variable (CV) population harbors a diverse range of donor stars and accreting white dwarfs (WDs). A range of WD masses is expected, from low mass Helium core WDs, to massive WDs which have previously accreted at rates high enough for Hydrogen to burn steadily. Furthermore, a wide range of Helium enrichment is expected in the accreted material depending on the degree to which the donor star is evolved. We investigate the impact of this diversity on the range of effective temperatures ($T_{\rm eff}$) for which g-modes are unstable. The critical $T_{\rm eff}$ below which modes are unstable ("blue edge") depends on both surface gravity, $g$, and He abundance, $Y$. The Hydrogen/first Helium ionization instability strip is more sensitive to $g$ than $Y$. We find that (for solar composition envelopes), relative to a fiducial WD mass $0.6 M_\odot$, the blue edge for a $0.4 M_\odot$ He core WD shifts downward by $\approx 1000 {\rm K}$, while that for a massive $\approx 1.2 M_\odot$ WD shifts upward by $\approx 2000 {\rm K}$. The second Helium ionization instability strip exhibits strong dependences on both $g$ and $Y$. Surprisingly, increasing $Y$ by only 10% relative to solar creates an instability strip near $15,000 {\rm K}$. Hence CV's below the period gap with evolved donor stars of $Y\ga 0.4$ may have an "intermediate" instability strip well outside of the isolated DA and DB variables. This "intermediate" instability strip also occurs for low mass He WD with solar composition envelopes. The lack of pulsations in CV's with $T_{\rm eff}$ in the pure Hydrogen ZZ Ceti instability strip is also easily explained.

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