arXiv subjects

Alice C. Quillen

Publications and source records attributed to Alice C. Quillen.

At least 91 records · Page 5Linked to original sources

arXiv2007

An infrared survey of brightest cluster galaxies: Paper I

We report on an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission. These galaxies are located in the cores of X-ray luminous clusters selected from the ROSAT All-Sky Survey. We find that about half of these sources have a sign of excess infrared emission; 22 objects out of 62 are detected at 70 microns, 18 have 8 to 5.8 micron flux ratios above 1.0 and 28 have 24 to 8 micron flux ratios above 1.0. Altogether 35 of 62 objects in our survey exhibit at least one of these signs of infrared excess. Four galaxies with infrared excesses have a 4.5/3.6 micron flux ratio indicating the presence of hot dust, and/or an unresolved nucleus at 8 microns. Three of these have high measured [OIII](5007A)/Hbeta flux ratios suggesting that these four, Abell 1068, Abell 2146, and Zwicky 2089, and R0821+07, host dusty active galactic nuclei (AGNs). 9 objects (including the four hosting dusty AGNs) have infrared luminosities greater than 10^11 L_sol and so can be classified as luminous infrared galaxies (LIRGs). Excluding the four systems hosting dusty AGNs, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation.

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arXiv2007

Spitzer IRS mapping of the central kpc of Centaurus A

We report on the results of infrared spectroscopic mapping observations carried out in the nuclear region of Centaurus A (NGC5128). The 500 pc bipolar dust shell discovered by Quillen et al.(2006) is even more clearly seen in the 11.3 micron dust emission feature than previous broad band imaging. The pure rotational lines of molecular hydrogen other than the S(0) line are detected above the dusty disk and associated with the oval dust shell. The molecular hydrogen transitions indicate the presence of warm gas at temperatures 250--720K. The ratio of the dust emission features at 7.7 and 11.3 micron and the ratio of the [NeII](12.8) and 11.3 dust emission feature are lower in the 500 pc dust shell than in the star forming disk. The clearer shell morphology at 11.3 micron, warm molecular hydrogen emission in the shell, and variation in line ratios in the shell compared to those in the disk, confirm spectroscopically that this shell is a separate coherent entity and is unlikely to be a chance superposition of dust filaments. The physical conditions in the shell are most similar to Galactic supernova remnants where blast waves encounter molecular clouds. The lines requiring the highest level of ionization, [NeV] and [OIV], are detected 20--25 arcsec north-east and south-west of the nucleus and at position angles near the radio jet axis. Fine structure line ratios and limits from this region suggest that the medium is low density and illuminated by a hard radiation field at low ionization parameter. These higher S molecular hydrogen pure rotational transitions are also particularly bright in the same region as the [OIV] and [NeV] emission. This suggests that the gas associated with the dust shell has been excited near the jet axis and is part of an ionization cone.

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arXiv2007

The Minimum Gap-opening Planet Mass in an Irradiated Circumstellar Accretion Disk

We consider the minimum mass planet, as a function of radius, that is capable of opening a gap in an $α$-accretion disk. We estimate that a half Jupiter mass planet can open a gap in a disk with accretion rate $\dot M \la 10^{-8} M_\odot$/yr for viscosity parameter $α=0.01$, and Solar mass and luminosity. The minimum mass is approximately proportional to $\dot M^{0.48} α^{0.8} M_*^{0.42} L_*^{-0.08}$. This estimate can be used to rule out the presence of massive planets in gapless accretion disks. We identify two radii at which an inwardly migrating planet may become able to open a gap and so slow its migration; the radius at which the heating from viscous dissipation is similar to that from stellar radiation in a flared disk, and the radius at which the disk has optical depth order 1 in a self-shadowed disk. If a gap opening planet cuts off disk accretion allowing the formation of a central hole or clearing in the disk then we would estimate that the clearing radius would approximately be proportional to the stellar mass.

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arXiv2007

The Total Number of Giant Planets in Debris Disks with Central Clearings

Infrared spectra from the Spitzer Space Telescope (SSC) of many debris disks are well fit with a single black body temperature which suggest clearings within the disk. We assume that inside the clearing orbital instability due to planets removes dust generating planetesimal belts and dust generated by the outer disk that is scattered or drifts into the clearing. From numerical integrations we estimate a minimum planet spacing required for orbital instability (and so planetesimal and dust removal) as a function of system age and planet mass. We estimate that a 10^8 year old debris disk with a dust disk edge at a radius of 50 AU hosted by an A star must contain approximately 5 Neptune mass planets between the clearing radius and the iceline in order to remove all primordial objects within it. We infer that known debris disk systems contain at least a fifth of a Jupiter mass in massive planets. The number of planets and spacing required is insensitive to the assumed planet mass. However an order of magnitude higher total mass in planets could reside in these systems if the planets are more massive.

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arXiv2007

Isophotal Structure and Dust Distribution in Radio-Loud Elliptical Galaxies

We investigate isophotal properties and dust morphology in the nuclear regions of 84 radio galaxies, imaged in the optical and near-infrared as part of Hubble Space Telescope snapshot surveys. We present a sample-wide trend between host galaxy isophotal structure and the inclination of dusty circumnuclear disks at the centers of 13 of these objects. We find that galaxies containing edge-on disks are invariably seen to possess boxy isophotes, while round, face-on disks are seen exclusively in objects with round or elliptical isophotes. Dust-rich sources with disky isophotes are observed only to possess dust in the form of extended filamentary lanes, and not in settled distributions like disks. As we do not expect that edge-on and face-on disks reside in different populations of galaxies, we conclude that perceived isophotal boxiness is dependent upon the angle at which the observer views the host galaxy's axis of symmetry. We discuss our results in the context of dissipative merger scenarios, and infer that dusty disks primarily reside in old, boxy remnants of gas-poor galaxy mergers, whereas filamentary dust lanes reside in younger disky remnants of gas-rich mergers.

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arXiv2007

Planetary embryos and planetesimals residing in thin debris disks

We consider constraints on the planetesimal population residing in the disks of AU Microscopii, Beta Pictoris and Fomalhaut taking into account their observed thicknesses and normal disk opacities. We estimate that bodies of radius 5, 180 and 70 km are responsible for initiating the collisional cascade accounting for the dust production for AU-Mic, Beta-Pic and Fomalhaut's disks, respectively, at break radii from the star where their surface brightness profiles change slope. Larger bodies, of radius 1000km and with surface density of order 0.01 g/cm^2, are required to explain the thickness of these disks assuming that they are heated by gravitational stirring. A comparison between the densities of the two sizes suggests the size distribution in the largest bodies is flatter than that observed in the Kuiper belt. AU Mic's disk requires the shallowest size distribution for bodies with radius greater than 10km suggesting that the disk contains planetary embryos experiencing a stage of runaway growth.

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arXiv2007

Diffusive low optical depth particle disks truncated by planets

Two dimensional particle disks in proximity to a planet are numerically integrated to determine when a planet in a circular orbit can truncate a particle disk. Collisions are treated by giving each particle a series of velocity perturbations during the integration. We estimate the mass of a planet required to truncate a particle disk as a function of collision rate, related to the disk optical depth, and velocity perturbation size, related to the disk velocity dispersion. We find that for particle disks in the regime estimated for debris disks, a Neptune mass planet is sufficiently massive to truncate the disk. If both the velocity dispersion and the disk optical depth are low (dispersion less than approximately 0.02 in units of circular motion, and optical depth less than 10^-4) then an Earth mass planet suffices. We find that the disk is smooth and axisymmetric unless the velocity perturbation is small and the planet mass is of order or greater than a Neptune mass in which case azimuthal structure is seen near prominent mean motion resonances.

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arXiv2006

Outflow Driven Cavities: Numerical Simulations of Intermediaries of Protostellar Turbulence

We investigate the evolution of fossil cavities produced by extinct YSO jets and wide angle outflows. Fossil cavities are ellipsoidal or cylindrical shells of swept-up of ambient (molecular cloud) material moving at low velocities. The cavities form when the momentum in a YSO jet or wide angle outflow decays in time allowing the bowshock or swept-up shell to decelerate to velocities near the turbulent speed in the cloud. It has been suggested in previous studies that cavities provide efficient coupling between the jets/outflows and the cloud and, as such, are the agents by which cloud turbulence is can be re-energized. In this paper we carry forward a series of numerical simulations of jets and outflows whose momentum flux decrease in time. We compare simulations with decaying momentum fluxes to those with constant flux. We show that decaying flux models exhibit deceleration of the outflow head and backfilling via expansion off of the cavity walls. They also have lower density contrast, are longer lived and wider than their continuously driven counterparts. The simulations recover the basic properties of observed fossil cavities. In addition, we provide synthetic observations in terms of P-V diagrams which demonstrate that fossil cavities form both jets and wide angle outflows are characterized by linear "Hubble-law" expansions patterns superimposed on "spur" patterns indicative of the head of a bow shock.

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arXiv2006

Chaotic zone boundary for low free eccentricity particles near an eccentric planet

We consider particles with low free or proper eccentricity that are orbiting near planets on eccentric orbits. Via collisionless particle integration we numerically find the location of the boundary of the chaotic zone in the planet's corotation region. We find that the distance in semi-major axis between the planet and boundary depends on the planet mass to the 2/7 power and is independent of the planet eccentricity, at least for planet eccentricities below 0.3. Our integrations reveal a similarity between the dynamics of particles at zero eccentricity near a planet in a circular orbit and with zero free eccentricity particles near an eccentric planet. The 2/7 law has been previously explained by estimating the semi-major at which the first order mean motion resonances are large enough to overlap. Orbital dynamics near an eccentric planet could differ due to first order corotation resonances that have strength proportional to the planet's eccentricity. However, we find the corotation resonance width at low free eccentricity is small. Also the first order resonance width at zero free eccentricity is the same as that for a zero eccentricity particle near a planet in a circular orbit. This accounts for insensitivity of the chaotic zone width to planet eccentricity. Particles at zero free eccentricity near an eccentric planet have similar dynamics to those at zero eccentricity near a planet in a circular orbit.

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arXiv2006

Predictions for a planet just inside Fomalhaut's eccentric ring

We propose that the eccentricity and sharpness of the edge of Fomalhaut's disk are due to a planet just interior to the ring edge. The collision timescale consistent with the disk opacity is long enough that spiral density waves cannot be driven near the planet. The ring edge is likely to be located at the boundary of a chaotic zone in the corotation region of the planet. We find that this zone can open a gap in a particle disk as long as the collision timescale exceeds the removal or ejection timescale in the zone. We use the slope measured from the ring edge surface brightness profile to place an upper limit on the planet mass. The removal timescale in the chaotic zone is used to estimate a lower limit. The ring edge has eccentricity caused by secular perturbations from the planet. These arguments imply that the planet has a mass between that of Neptune and that of Saturn, a semi-major axis of approximately 119 AU and longitude of periastron and eccentricity, 0.1, the same as that of the ring edge.

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arXiv2006

The Warped Nuclear Disk of Radio Galaxy 3C 449

Among radio galaxies containing nuclear dust disks, the bipolar jet axis is generally observed to be perpendicular to the disk major axis. The FR I radio source 3C 449 is an outlier to this statistical majority, as it possesses a nearly parallel jet/disk orientation on the sky. We examine the 600 pc dusty disk in this galaxy with images from the Hubble Space Telescope. We find that a colormap of the disk exhibits a twist in its isocolor contours (isochromes). We model the colormap by integrating galactic starlight through an absorptive disk, and find that the anomalous twist in the isochromes can be reproduced in the model with a vertically thin, warped disk. The model predicts that the disk is nearly perpendicular to the jet axis within 100 pc of the nucleus. We discuss physical mechanisms capable of causing such a warp. We show that precessional models or a torque on the disk arising from a possible binary black hole in the AGN causes precession on a timescale that is too long to account for the predicted disk morphology. However, we estimate that the pressure in the X-ray emitting interstellar medium is large enough to perturb the disk, and argue that jet-driven anisotropy in the excited ISM may be the cause of the warp. In this way, the warped disk in 3C 449 may be a new manifestation of feedback from an active galactic nucleus.

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arXiv2006

Discovery of a 500 pc shell in the nucleus of Centaurus A

Spitzer Space Telescope mid-infrared images of the radio galaxy Centaurus A reveal a shell-like, bipolar, structure 500 pc to the north and south of the nucleus. This shell is seen in 5.8, 8.0 and 24 micron broad-band images. Such a remarkable shell has not been previously detected in a radio galaxy and is the first extragalactic nuclear shell detected at mid-infrared wavelengths. We estimate that the shell is a few million years old and has a mass of order million solar masses. A conservative estimate for the mechanical energy in the wind driven bubble is 10^53 erg. The shell could have created by a small few thousand solar mass nuclear burst of star formation. Alternatively, the bolometric luminosity of the active nucleus is sufficiently large that it could power the shell. Constraints on the shell's velocity are lacking. However, if the shell is moving at 1000 km/s then the required mechanical energy would be 100 times larger.

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arXiv2006

Spitzer observations of the dusty warped disk of Centaurus A

Spitzer mid-infrared images of the dusty warped disk in the galaxy Centaurus A show a parallelogram-shaped structure. We successfully model the observed mid-infrared morphology by integrating the light from an emitting, thin, and warped disk, similar to that inferred from previous kinematic studies. The models with the best match to the morphology lack dust emission within the inner 0.1 to 0.8 kpc, suggesting that energetic processes near the nucleus have disturbed the inner molecular disk, creating a gap in the molecular gas distribution.

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arXiv2005

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. I. A Reverberation-Based Measurement of the Black Hole Mass

A reverberation-mapping program on NGC 4395, the least-luminous known Seyfert 1 galaxy, undertaken with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, yields a measurement of the mass of the central black hole of 360,000 solar masses. The observations consist of two visits of 5 orbits each, in 2004 April and July. During each of these visits, the UV continuum varied by at least 10% (rms) and only C IV 1549 showed corresponding variations large enough to reliably determine the emission-line lag, which was measured to be of order one hour for both visits. The size of the C IV-emitting region is about a factor of three smaller than expected if the slope of the broad-line region radius-luminosity relationship is identical to that for the H-beta emission line. NGC 4395 is underluminous even for its small black hole mass; the Eddington ratio of 0.0012 is lower than that of any other active galactic nucleus for which a black hole mass measurement has been made by emission-line reverberation.

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arXiv2005

Reducing the Probability of Capture into Resonance

A migrating planet can capture planetesimals into mean motion resonances. However, resonant trapping can be prevented when the drift or migration rate is sufficiently high. Using a simple Hamiltonian system for first and second order resonances, we explore how the capture probability depends on the order of the resonance, drift rate and initial particle eccentricity. We present scaling factors as a function of the planet mass and resonance strength to estimate the planetary migration rate above which the capture probability drops to less than 1/2. Applying our framework to multiple extra solar planetary systems that have two planets locked in resonance, we estimate lower limits for the outer planet's migration rate allowing resonance capture of the inner planet. Mean motion resonances are comprised of multiple resonant subterms. We find that the corotation subterm can reduce the probability of capture when the planet eccentricity is above a critical value. We present factors that can be used to estimate this critical planet eccentricity. Applying our framework to the migration of Neptune, we find that Neptune's eccentricity is near the critical value that would make its 2:1 resonance fail to capture twotinos. The capture probability is affected by the separation between resonant subterms and so is also a function of the precession rates of the longitudes of periapse of both planet and particle near resonance.

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arXiv2005

The Warped Circumstellar Disk of HD100546

We propose that the two armed spiral features seen in visible Hubble Space Telescope images of scattered light in HD100546's circumstellar disk are caused by the illumination of a warped outer disk. A tilt of 6-15 degrees from the symmetry plane can cause the observed surface brightness variations providing the disk is very twisted (highly warped) at radii greater than 200 AU where the spiral features are seen. Dust lanes are due in part to shadowing in the equatorial plane from the inner disk within a radius of 100 AU. HD100546's outer disk, if viewed edge-on, would appear similar to that of Beta Pictorus. A disk initially misaligned with a planetary system, becomes warped due to precession induced by planetesimal bodies and planets. However, the twistedness of HD100546's disk cannot be explained by precession during the lifetime of the system induced by a single Jovian mass planet within the clearing at ~13 AU. One possible explanation for the corrugated disk is that precession was induced by massive of bodies embedded in the disk at larger radius. This would require approximately a Jupiter mass of bodies well outside the central clearing at 13 AU and within the location of the spiral features or at radii approximately between 50-200 AU.

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arXiv2005

Driving spiral arms in the circumstellar disks of HD 100546 and HD 141569A

With 2D hydrodynamical simulations of disks perturbed externally by stars, brown dwarfs or planets we investigate possible scenarios that can account for the spiral structure in circumstellar disks. We consider two scenarios, spiral structure driven by an external bound planet or low mass star and that excited by a previous stellar close encounter or flyby. We find that both scenarios produce morphology similar to that observed in the outer disks of HD 141569A and HD 100546; moderately open 2-armed outer spiral structure. The outer two-armed spiral structure observed in the disk of HD 141569A is qualitatively reproduced with tidal perturbations from its companion binary HD 141569B,C on a prograde orbit near periapse. Our simulation accounts for the outer spiral arms, but is less successful than the secular model of Augereau and Papaloizou at matching the lopsidedness or asymmetry of the disk edge at 300AU. The disk has been previously truncated by the tidal force from the binary. A bound object (stellar or planetary) is unlikely to explain the spiral structure in HD 100546. A co-eval planet or brown dwarf in the disk of sufficient mass to account for the amplitude of the spiral structure would be detectable in NICMOS and STIS images, however existing images reveal no such object. A previous encounter could explain the observed structure, provided that the encounter occurred less than a few thousand year ago. The object responsible for causing the spiral structure must then be within a few arcminutes of the star. However, the USNO-B proper motion survey reveals no candidate object. Moreover, the probability that a field star encountered HD 100546 in the past few thousand years is very low.

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arXiv2005

The decay of interplanetary coronal mass ejections and forbush decrease recovery times

We investigate the relation between Forbush cosmic ray decrease recovery time and coronal mass ejection transit time between the Sun and Earth. We identify 17 Forbush decreases from ground based neutron count rates between 1978 and 2003 that occur during the same phase in the solar cycle and can be associated with single coronal mass ejections (CMEs) in the SOHO LASCO CME Catalog or previously published reports, and with specific interplanetary coronal mass ejections (ICMEs) crossing the vicinity of Earth. We find an anti-correlation between Forbush recovery times and CME transit time that contradicts the predictions of simple cosmic ray diffusive barrier models. The anti-correlation suggests that the decay rate of ICMEs is anti-correlated with their travel speed. Forbush recovery times range from seven times the transit time for the fastest disturbance to a fifth the Sun-Earth transit time for the slowest. To account for the large range of measured recovery times we infer that the slowest disturbances must decay rapidly with radius whereas the fastest ones must remain strong. The longest recovery times suggest that the fastest disturbances in our sample decayed less rapidly with radius than the ambient solar wind magnetic field strength.

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