arXiv subjects

Alice C. Quillen

Publications and source records attributed to Alice C. Quillen.

97 records · Page 6Linked to original sources

arXiv2004

On the Planet and the Disk of CoKuTau/4

Spitzer observations of the young star CoKuTau/4 reveal a disk with a 10 AU hole that is most likely caused by a newly formed planet. Assuming that the planet opened a gap in the viscous disk, we estimate that the planet mass is greater than 0.1 Jupiter masses. This estimate depends on a lower limit to the disk viscosity derived from the time scale needed to accrete the inner disk, creating the now detectable hole. The planet migration time scale must at least modestly exceed the time for the spectrally inferred hole to clear. The proximity of the planet to the disk edge implied by our limits suggests that the latter is perturbed by the nearby planet and may exhibit a spiral pattern rotating with the planet. This pattern might be resolved with current ground based mid-infrared cameras and optical cameras on the Hubble Space Telescope. The required sub-Myr planet formation may challenge core accretion formation models. However, we find that only if the planet mass is larger than about 10 Jupiter masses, allowing for a high enough surface density without inducing migration, would formation by direct gravitational instability be possible.

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arXiv2004

The impact of a close stellar encounter on the Edgeworth-Kuiper Belt

We numerically investigate the possibility that a close stellar encounter could account for the high inclinations of the Kuiper belt, as originally proposed by Ida, Larwood and Burkert, however we consider encounters with pericenters within those explored by previous works. A star of mass 0.2 Msol entering the solar system on a nearly parabolic, low inclination, retrograde orbit with perihelion of 50 AU can scatter ~30% of the Kuiper belt into moderate inclination (i>10 degrees) and eccentricity orbits, while leaving the rest of the belt at low eccentricities and inclinations. This scenario provides a possible explanation for the dichotomy of the Kuiper belt, accounting for the scattered and classical populations, including objects with high eccentricities and inclinations. High eccentricity objects that were not placed into resonance with Neptune are more likely to be removed from the system by subsequent long timescale evolution. Consequently this scenario suggests that some Plutinos could have originally been objects in circular orbits which were scattered into resonant high eccentricity and high inclination orbits by the stellar encounter. Objects could have been placed into stable resonant regions with Neptune by the encounter. However long lived regions of dynamical stability should differ from those selected by resonance capture. This scenario predicts the presence of objects in resonances with Neptune that are unlikely to have been captured via Neptune's migration.

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arXiv2003

Spectral Energy Distributions of Seyfert Nuclei

We present nuclear spectral energy distributions (SEDs) in the range 0.4-16micron for an expanded CfA sample of Seyfert galaxies. The spectral indices from 1-16micron range from alpha_IR 0.9 to 3.8. The shapes of the spectra are correlated with Seyfert type in the sense that steeper nuclear SEDs (nu*f_nu increasing with increasing wavelength) tend to be found in Seyfert 2s and flatter SEDs (nu *f_nu constant) in Seyfert 1-1.5s. The galaxies optically classified as Seyferts 1.8s and 1.9s display values of alpha_IR as in type 1 objects, or values intermediate between those of Seyfert 1s and Seyfert 2s. The intermediate SEDs of many Seyfert 1.8-1.9s may be consistent with the presence of a pure Seyfert 1 viewed through a moderate amount (A_V <5mag) of foreground galaxy extinction. We find, however, that between 10 and 20% of galaxies with broad optical line components have steep infrared SEDs. Torus models usually adopt high equatorial opacities to reproduce the infrared properties of Seyfert 1s and 2s, resulting in a dichotomy of infrared SEDs (flat for type 1s, and steep for type 2s). Such a dichotomy, however, is not observed in our sample. The wide range of spectral indices observed in the type 2 objects, the lack of extremely steep SEDs, and the large numbers of objects with intermediate spectral indices cannot be reconciled with predictions from existing optically thick torus models. We discuss possible modifications to improve torus models, including low optical depth tori, clumpy dusty tori, and high-optical-depth tori with an extended optically thin component.

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arXiv2003

Sgr A* Companion S0-2: A Probe of Very High-Mass Star Formation

The star S0-2, which is orbiting Sgr A* with a 15-yr period, almost certainly did not form in situ. We propose that it was injected into this close orbit by the tidal disruption of a massive-star binary, whose primary was more massive than S0-2 and at least 60 Msun. From numerical integrations we find that 1-2% of incoming binaries with closest approach equal to 130 AU leave the secondary in an orbit with eccentricity within 0.01 of that of SO-2. If additional stars are found orbiting Sgr A* with relatively short periods, they could be used to probe the formation of massive stars in the Galactocentric region, even though the massive stars themselves have long since perished.

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arXiv2002

Near-IR and Optical Morphology of Spiral Galaxies

We announce the initial release of data from the Ohio State University Bright Spiral Galaxy Survey, a BVRJHK imaging survey of a well-defined sample of 205 bright, nearby spiral galaxies. We present H-band morphological classification on the Hubble sequence for the OSU Survey sample. We compare the H-band classification to B-band classification from our own images and from standard galaxy catalogs. Our B-band classifications match well with those of the standard catalogs. On average, galaxies with optical classifications from Sa through Scd appear about one T-type earlier in the H-band than in the B-band, but with large scatter. This result does not support recent claims made in the literature that the optical and near-IR morphologies of spiral galaxies are uncorrelated. We present detailed descriptions of the H-band morphologies of our entire sample, as well as B- and H-band images for a set of 17 galaxies chosen as type examples, and BRH color-composite images of six galaxies chosen to demonstrate the range in morphological variation as a function of wavelength. Data from the survey are accessible at http://www.astronomy.ohio-state.edu/~survey/

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arXiv2000

The saturation of disk heating in the solar neighborhood and evidence for a merger 9 Gyrs ago

We re-examine the age-velocity dispersion relation in the solar neighborhood using improved stellar age estimates that are based on Hipparcos parallaxes and recent stellar evolution calculations. The resulting relation shows that the Milky Way stellar disk was relatively quiescent, suffering little heating or dispersion increase between 3 and 9 Gyrs. However, at an age of 9 Gyrs there is an abrupt increase in the stellar velocity dispersions. To explain the abrupt increase (by almost a factor of 2) in dispersions we propose that the Milky Way suffered a minor merger 9 Gyrs ago that created the thick disk. The quiescent phase is consistent with inefficient heating caused by scattering from tightly wound transient spiral structure. It is possible that many galaxy disks have radial velocity dispersions at the point where heating from spiral structure becomes inefficient. Surface brightness profiles and dispersion measurements can be used to predict spiral arm winding numbers at this saturation point. A comparison between the observed and predicted winding numbers could test this hypothesis and also search for evidence of merger induced disk heating in nearby galaxies.

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arXiv1999

The Frequency of Barred Spiral Galaxies in the Near-IR

We have determined the fraction of barred galaxies in the H-band for a statistically well-defined sample of 186 spirals drawn from the Ohio State University Bright Spiral Galaxy survey. We find 56% of our sample to be strongly barred at H, while another 16% is weakly barred. Only 27% of our sample is unbarred in the near-infrared. The RC3 and the Carnegie Atlas of Galaxies both classify only about 30% of our sample as strongly barred. Thus strong bars are nearly twice as prevalent in the near-infrared as in the optical. The frequency of genuine optically hidden bars is significant, but lower than many claims in the literature: 40% of the galaxies in our sample that are classified as unbarred in the RC3 show evidence for a bar in the H-band, while for the Carnegie Atlas this fraction is 66%. Our data reveal no significant trend in bar fraction as a function of morphology in either the optical or H-band. Optical surveys of high redshift galaxies may be strongly biased against finding bars, as bars are increasingly difficult to detect at bluer rest wavelengths.

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