arXiv subjects

Benjamin C. Bromley

Publications and source records attributed to Benjamin C. Bromley.

At least 73 records · Page 4Linked to original sources

arXiv2007

Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center

We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf < -275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 - 4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is also allowed.

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arXiv2007

Terrestrial Zone Debris Disk Candidates in h and chi Persei

We analyze 8 sources with strong mid-infrared excesses in the 13 Myr-old double cluster h and chi Persei. New optical spectra and broadband SEDs (0.36-8 mu_m) are consistent with cluster membership. We show that material with T ~ 300-400 K and Ld/Lstar ~ 10^-4-10^-3 produces the excesses in these sources. Optically-thick blackbody disk models - including those with large inner holes - do not match the observed SEDs. The SEDs of optically-thin debris disks produced from terrestrial planet formation calculations match the observations well. Thus, some h and chi Persei stars may have debris from terrestrial zone planet formation.

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arXiv2007

Formation and Collisional Evolution of Kuiper Belt Objects

This chapter summarizes analytic theory and numerical calculations for the formation and collisional evolution of KBOs at 20--150 AU. We describe the main predictions of a baseline self-stirring model and show how dynamical perturbations from a stellar flyby or stirring by a giant planet modify the evolution. Although robust comparisons between observations and theory require better KBO statistics and more comprehensive calculations, the data are broadly consistent with KBO formation in a massive disk followed by substantial collisional grinding and dynamical ejection. However, there are important problems reconciling the results of coagulation and dynamical calculations. Contrasting our current understanding of the evolution of KBOs and asteroids suggests that additional observational constraints, such as the identification of more dynamical families of KBOs (like the 2003 EL61 family), would provide additional information on the relative roles of collisional grinding and dynamical ejection in the Kuiper Belt. The uncertainties also motivate calculations that combine collisional and dynamical evolution, a `unified' calculation that should give us a better picture of KBO formation and evolution.

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arXiv2007

Hypervelocity Stars II. The Bound Population

Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way by three-body interactions with the massive black hole in the Galactic center. We describe 643 new spectroscopic observations from our targeted survey for HVSs. We find a significant (3.5 sigma) excess of B-type stars with large velocities +275 10 kpc that are most plausibly explained as a new class of HVSs: stars ejected from the Galactic center on bound orbits. If a Galactic center ejection origin is correct, the distribution of HVSs on the sky should be anisotropic for a survey complete to a fixed limiting apparent magnitude. The unbound HVSs in our survey have a marginally anisotropic distribution on the sky, consistent with the Galactic center ejection picture.

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arXiv2006

Planet formation around low mass stars: the moving snow line and super-Earths

We develop a semi-analytic model for planet formation during the pre-main sequence contraction phase of a low mass star. During this evolution, the stellar magnetosphere maintains a fixed ratio between the inner disk radius and the stellar radius. As the star contracts at constant effective temperature, the `snow line', which separates regions of rocky planet formation from regions of icy planet formation, moves inward. This process enables rapid formation of icy protoplanets that collide and merge into super-Earths before the star reaches the main sequence. The masses and orbits of these super-Earths are consistent with super-Earths detected in recent microlensing experiments.

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arXiv2006

Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities

The disruption of binary stars by the tidal field of the black hole in the Galactic Center can produce the hypervelocity stars observed in the halo. We use numerical models to simulate the full spectrum of observable velocities of stars ejected into the halo by this binary disruption process. Our model includes a range of parameters for binaries with 3-4 M_Solar primaries, consideration of radial orbits of the ejected stars through an approximate mass distribution for the Galaxy, and the impact of stellar lifetimes. We calculate the spectrum of ejection velocities and reproduce previous results for the mean ejection velocity at the Galactic center. The model predicts that the full population of ejected stars includes both the hypervelocity stars with velocities large enough to escape from the Galaxy and a comparable number of ejected, but bound, stars of the same stellar type. The predicted median speeds of the population of ejected stars as a function of distance in the halo are consistent with current observations. Combining the model with the data also shows that interesting constraints on the properties of binaries in the Galactic Center and on the mass distribution in the Galaxy can be obtained even with modest samples of ejected stars.

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arXiv2006

Models of helically symmetric binary systems

Results from helically symmetric scalar field models and first results from a convergent helically symmetric binary neutron star code are reported here; these are models stationary in the rotating frame of a source with constant angular velocity omega. In the scalar field models and the neutron star code, helical symmetry leads to a system of mixed elliptic-hyperbolic character. The scalar field models involve nonlinear terms that mimic nonlinear terms of the Einstein equation. Convergence is strikingly different for different signs of each nonlinear term; it is typically insensitive to the iterative method used; and it improves with an outer boundary in the near zone. In the neutron star code, one has no control on the sign of the source, and convergence has been achieved only for an outer boundary less than approximately 1 wavelength from the source or for a code that imposes helical symmetry only inside a near zone of that size. The inaccuracy of helically symmetric solutions with appropriate boundary conditions should be comparable to the inaccuracy of a waveless formalism that neglects gravitational waves; and the (near zone) solutions we obtain for waveless and helically symmetric BNS codes with the same boundary conditions nearly coincide.

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arXiv2006

A Hybrid N-body--Coagulation Code for Planet Formation

We describe a hybrid algorithm to calculate the formation of planets from an initial ensemble of planetesimals. The algorithm uses a coagulation code to treat the growth of planetesimals into oligarchs and explicit N-body calculations to follow the evolution of oligarchs into planets. To validate the N-body portion of the algorithm, we use a battery of tests in planetary dynamics. Several complete calculations of terrestrial planet formation with the hybrid code yield good agreement with previously published calculations. These results demonstrate that the hybrid code provides an accurate treatment of the evolution of planetesimals into planets.

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arXiv2005

Terrestrial Planet Formation I. The Transition from Oligarchic Growth to Chaotic Growth

We use a hybrid, multiannulus, n-body-coagulation code to investigate the growth of km-sized planetesimals at 0.4-2 AU around a solar-type star. After a short runaway growth phase, protoplanets with masses of roughly 10^26 g and larger form throughout the grid. When (i) the mass in these `oligarchs' is roughly comparable to the mass in planetesimals and (ii) the surface density in oligarchs exceeds 2-3 g/sq cm at 1 AU, strong dynamical interactions among oligarchs produce a high merger rate which leads to the formation of several terrestrial planets. In disks with lower surface density, milder interactions produce several lower mass planets. In all disks, the planet formation timescale is roughly 10-100 Myr, similar to estimates derived from the cratering record and radiometric data.

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arXiv2005

Prospects for Detection of Catastrophic Collisions in Debris Disks

We investigate the prospects for detecting dust from two body collisions during the late stages of planet formation at 1-150 AU. We develop an analytic model to describe the formation of a dusty cloud of debris and use numerical coagulation and n-body calculations to predict observable signals from these events. In a minimum mass solar nebula, collisions of 100-1000 km objects at distances of 3-5 AU or less from the parent star are observable at mid-infrared wavelengths as bright clumps or rings of dust. At 24 microns, the clumps are roughly 0.1-1 mag brighter than emission from dust in the background debris disk. In edge-on systems, dusty clumps produce eclipses with depths of 1.0 mag or less that last for roughly 100 orbital periods. Large-scale surveys for transits from exosolar planets, such as Kepler, can plausibly detect these eclipses and provide important constraints on the terrestrial environment for ages of less than or roughly 100-300 Myr.

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arXiv2004

Stellar encounters as the origin of distant solar system objects in highly eccentric orbits

The discovery of Sedna places new constraints on the origin and evolution of our solar system. Here we investigate the possibility that a close encounter with another star produced the observed edge of the Kuiper belt, at roughly 50 AU, and the highly elliptical orbit of Sedna. We show that a passing star probably scattered Sedna from the Kuiper Belt into its observed orbit. The likelihood that a planet at 60-80 AU can be scattered into Sedna's orbit is roughly 50%; this estimate depends critically on the geometry of the flyby. Even more interesting, though, is the roughly 10% chance that Sedna was captured from the outer disk of the passing star. Most captures have very high inclination orbits; detection of these objects would confirm the presence of extrasolar planets in our own Solar System.

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arXiv2004

The Size Distribution of Kuiper Belt Objects

We describe analytical and numerical collisional evolution calculations for the size distribution of icy bodies in the Kuiper Belt. For a wide range of bulk properties, initial masses, and orbital parameters, our results yield power-law cumulative size distributions, N_C propto r^{-q}, with q_L = 3.5 for large bodies with radii of 10-100 km, and q_s = 2.5-3 for small bodies with radii lesss than 0.1-1 km. The transition between the two power laws occurs at a break radius of 1-30 km. The break radius is more sensitive to the initial mass in the Kuiper Belt and the amount of stirring by Neptune than the bulk properties of individual Kuiper Belt objects (KBOs). Comparisons with observations indicate that most models can explain the observed sky surface density of KBOs for red magnitudes, R = 22-27. For R < 22 and R > 28, the model surface density is sensitive to the amount of stirring by Neptune, suggesting that the size distribution of icy planets in the outer solar system provides independent constraints on the formation of Neptune.

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arXiv2003

Collisional Cascades in Planetesimal Disks II. Embedded Planets

We use a multiannulus planetesimal accretion code to investigate the growth of icy planets in the outer regions of a planetesimal disk. In a quiescent minimum mass solar nebula, icy planets grow to sizes of 1000--3000 km on a timescale t = 15-20 Myr (a/30 AU)^3 where a is the distance from the central star. Planets form faster in more massive nebulae. Newly-formed planets stir up leftover planetesimals along their orbits and produce a collisional cascade where icy planetesimals are slowly ground to dust. The dusty debris of planet formation has physical characteristics similar to those observed in beta Pic, HR 4796A, and other debris disks. We derive dust masses for small particles, 1 mm and smaller, and large particles, 1 mm and larger, as a function of the initial conditions in the planetesimal disk. The dust luminosities derived from these masses are similar to those observed in Vega, HR 4796A, and other debris disks. The calculations produce bright rings and dark gaps. Bright rings occur where 1000 km and larger planets have recently formed. Dark gaps are regions where planets have cleared out dust or shadows where planets have yet to form.

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arXiv2002

Dusty Rings: Signposts of Recent Planet Formation

Many nearby stars are surrounded by a bright ring or disk of cold dust. Our calculations show that these disks and rings of dust are signposts of recent planet formation. Bright rings appear because dust associated with the formation of a planet absorbs and scatters light from the central star. The calculations explain the rings observed so far and predict that all nascent solar systems have dusty rings.

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arXiv2001

Collisional Cascades in Planetesimal Disks I. Stellar Flybys

We use a new multiannulus planetesimal accretion code to investigate the evolution of a planetesimal disk following a moderately close encounter with a passing star. The calculations include fragmentation, gas and Poynting-Robertson drag, and velocity evolution from dynamical friction and viscous stirring. We assume that the stellar encounter increases planetesimal velocities to the shattering velocity, initiating a collisional cascade in the disk. During the early stages of our calculations, erosive collisions damp particle velocities and produce substantial amounts of dust. For a wide range of initial conditions and input parameters, the time evolution of the dust luminosity follows a simple relation, L_d/L_{\star} = L_0 / [alpha + (t/t_d)^{beta}]. The maximum dust luminosity L_0 and the damping time t_d depend on the disk mass, with L_0 proportional to M_d and t_d proportional to M_d^{-1}. For disks with dust masses of 1% to 100% of the `minimum mass solar nebula' (1--100 earth masses at 30--150 AU), our calculations yield t_d approx 1--10 Myr, alpha approx 1--2, beta = 1, and dust luminosities similar to the range observed in known `debris disk' systems, L_0 approx 10^{-3} to 10^{-5}. Less massive disks produce smaller dust luminosities and damp on longer timescales. Because encounters with field stars are rare, these results imply that moderately close stellar flybys cannot explain collisional cascades in debris disk systems with stellar ages of 100 Myr or longer.

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arXiv2001

Polarimetric Imaging of the Massive Black Hole at the Galactic Center

The radio source Sgr A* in the Galactic center emits a polarized spectrum at millimeter and sub-millimeter wavelengths that is strongly suggestive of relativistic disk accretion onto a massive black hole. We use the well-constrained mass of Sgr A* and a magnetohydrodynamic model of the accretion flow to match both the total flux and polarization from this object. Our results demonstrate explicitly that the shift in the position angle of the polarization vector, seen at wavelengths near the peak of the mm to sub-mm emission from this source, is a signal of relativistic accretion flow in a strong gravitational field. We provide maps of the polarized emission to illustrate how the images of polarized intensity from the vicinity of the black hole would appear in upcoming observations with very long baseline radio interferometers (VLBI). Our results suggest that near-term VLBI observations will be able to directly image the polarized Keplerian portion of the flow near the horizon of the black hole.

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arXiv2001

Measuring the Black Hole Spin in Sgr A*

The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a Keplerian structure whose peak emission occurs within several Schwarzschild radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if confirmed, is presumably the self-Comptonized component from this region. In this paper, we suggest that sub-mm timing observations could yield a signal corresponding to the period P_0 of the marginally stable orbit, and therefore point directly to the black hole's spin a. Sgr A*'s mass is now known to be (2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital frequency with the innermost stable circular orbit is made feasible by the transition from optically thick to thin emission at sub-mm wavelengths. With stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum is thus produced at the smallest radius. We caution, however, that theoretical uncertainties in the structure of the emission region may still produce some ambiguity in the timing signal. Given that Sgr A*'s flux at $ν\sim 1$ mm is several Jy, these periods should lie within the temporal-resolving capability of sub-mm telescopes using bolometric detectors. A determination of P_0 should provide not only a value of a, but it should also define the angular momentum vector of the orbiting gas in relation to the black hole's spin axis. In addition, since the X-ray flux detected by Chandra appears to be the self-Comptonized mm to sub-mm component, these temporal fluctuations may also be evident in the X-ray signal.

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arXiv2000

The Tully-Fisher Relation as a Measure of Luminosity Evolution: A Low Redshift Baseline for Evolving Galaxies

We use optical rotation curves to investigate the R-band Tully-Fisher properties of a sample of 90 spiral galaxies in close pairs. The galaxies follow the Tully-Fisher relation remarkably well, with the exception of eight distinct 3-sigma outliers. Although most of the outliers show signs of recent star formation, gasdynamical effects are probably the dominant cause of their anomalous Tully-Fisher properties. Four outliers with small emission line widths have very centrally concentrated line emission and truncated rotation curves; the central emission indicates recent gas infall after a close galaxy-galaxy pass. These four galaxies may be local counterparts to compact, blue galaxies at intermediate redshift. The remaining galaxies have a negligible offset from the reference Tully-Fisher relation, but a shallower slope (2.6-sigma significance) and a 25% larger scatter. We characterize the non-outlier sample with measures of distortion and star formation to search for third parameter dependence in the residuals of the TF relation. Severe kinematic distortion is the only significant predictor of TF residuals; this distortion is not, however, responsible for the slope difference from the reference distribution. Because the outliers are easily removed by sigma clipping, we conclude that even in the presence of some tidal distortion, detection of moderate luminosity evolution should be possible with high-redshift samples the size of this 90-galaxy study. (Abridged.)

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