arXiv subjects

Scott J. Kenyon

Publications and source records attributed to Scott J. Kenyon.

At least 145 records · Page 8Linked to original sources

arXiv2006

Infrared Properties of Close Pairs of Galaxies

We discuss spectroscopy and infrared photometry for a complete sample of ~ 800 galaxies in close pairs objectively selected from the CfA2 redshift survey. We use 2MASS to compare near infrared color-color diagrams for our sample with the Nearby Field Galaxy Sample and with a set of IRAS flux-limited pairs from Surace et al. We construct a basic statistical model to explore the physical sources of the substantial differences among these samples. The model explains the spread of near infrared colors and is consistent with a picture where central star formation is triggered by the galaxy-galaxy interaction before a merger occurs. For 160 galaxies we report new, deep JHK photometry within our spectroscopic aperture and we use the combined spectroscopic and photometric data to explore the physical conditions in the central bursts. We find a set of objects with H-K >= 0.45 and with a large F(FIR)/F(H). We interpret the very red H-K colors as evidence for 600-1000 K dust within compact star-forming regions, perhaps similar to super-star clusters identified in individual well-studied interacting galaxies. The galaxies in our sample are candidate ``hidden'' bursts or, possibly, ``hidden'' AGN. Over the entire pair sample, both spectroscopic and photometric data show that the specific star formation rate decreases with the projected separation of the pair. The data suggest that the near infrared color-color diagram is also a function of the projected separation; all of the objects with central near infrared colors indicative of bursts of star formation lie at small projected separation.

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arXiv2006

X-Ray Emitting Blast Wave from the Recurrent Nova RS Ophiuchi

Stellar explosions such as novae and supernovae produce most of the heavy elements in the Universe. Although the onset of novae from runaway thermonuclear fusion reactions on the surface of a white dwarf in a binary star system is understood[1], the structure, dynamics, and mass of the ejecta are not well known. In rare cases, the white dwarf is embedded in the wind nebula of a red-giant companion; the explosion products plow through the nebula and produce X-ray emission. Early this year, an eruption of the recurrent nova RS Ophiuchi[2,3] provided the first opportunity to perform comprehensive X-ray observations of such an event and diagnose conditions within the ejecta. Here we show that the hard X-ray emission from RS Ophiuchi early in the eruption emanates from behind a blast wave, or outward-moving shock wave, that expanded freely for less than 2 days and then decelerated due to interaction with the nebula. The X-rays faded rapidly, suggesting that the blast wave deviates from the standard spherical shell structure[4-6]. The early onset of deceleration indicates that the ejected shell had a low mass, the white dwarf has a high mass[7], and that RS Ophiuchi is a progenitor of the type of supernova integral to studies of the expansion of the universe.

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arXiv2006

Hypervelocity Stars. I. The Spectroscopic Survey

We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Our survey, now half complete, has successfully identified a total of four probable HVSs plus a number of other unusual objects. Here we report the most recently discovered two HVSs: SDSS J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least +508+-12 and +418+-10 km/s, respectively. The other late B-type objects in our survey are consistent with a population of post main-sequence stars or blue stragglers in the Galactic halo, with mean metallicity [Fe/H]=-1.3 and velocity dispersion 108+-5 km/s. Interestingly, the velocity distribution shows a tail of objects with large positive velocities that may be a mix of low-velocity HVSs and high-velocity runaway stars. Our survey also includes a number of DA white dwarfs with unusually red colors, possibly extremely low mass objects. Two of our objects are B supergiants in the Leo A dwarf, providing the first spectroscopic evidence for star formation in this dwarf galaxy within the last ~30 Myr.

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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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arXiv2006

A Successful Targeted Search for Hypervelocity Stars

Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following our discovery of the first HVS, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling with Galactic rest-frame velocities at least +558+-12 and +638+-12 km/s, respectively. Assuming the HVSs are B8 main sequence stars, they are at distances ~75 and ~55 kpc, respectively, and have travel times from the Galactic Center consistent with their lifetimes. The existence of two B8 HVSs in our 1900 deg^2 survey, combined with the Yu & Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster. The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major in-fall event at the Galactic Center in the last \~160 Myr.

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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

The Century Survey Galactic Halo Project II: Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars

We discuss a 175 deg^2 spectroscopic survey for blue horizontal branch (BHB) stars in the Galactic halo. We use the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to select BHB candidates, and find that the 2MASS and SDSS color-selection is 38% and 50% efficient, respectively, for BHB stars. Our samples include one likely run-away B7 star 6 kpc below the Galactic plane. The global properties of the BHB samples are consistent with membership in the halo population: the median metallicity is [Fe/H]=-1.7, the velocity dispersion is 108 km/s, and the mean Galactic rotation of the BHB stars 3<|z|<15 kpc is -4 +- 30 km/s. We discuss the theoretical basis of the Preston, Shectman & Beers M_V-color relation for BHB stars, and conclude that intrinsic shape of the BHB M_V-color relation results from the physics of stars on the horizontal branch. We calculate the luminosity function for the field BHB star samples using the Efstathiou, Ellis, & Peterson maximum-likelihood method which is unbiased by density variations. The field BHB luminosity function exhibits a steep rise at bright luminosities, a peak between 0.8 < M_V < 1.0, and a tail at faint luminosities. We compare the field BHB luminosity functions with the luminosity functions derived from sixteen different globular cluster BHBs. Kolmogorov-Smirnov tests suggest that field BHB stars and BHB stars in globular clusters share a common distribution of luminosities, with the exception of globular clusters with extended BHBs.

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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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arXiv2005

Discovery of an Unbound Hyper-Velocity Star in the Milky Way Halo

We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a heliocentric radial velocity of +853+-12 km/s, the largest velocity ever observed in the Milky Way halo. The star is either a hot blue horizontal branch star or a B9 main sequence star with a heliocentric distance ~55 kpc. Corrected for the solar reflex motion and to the local standard of rest, the Galactic rest-frame velocity is +709 km/s. Because its radial velocity vector points 173.8 deg from the Galactic center, we suggest that this star is the first example of a hyper-velocity star ejected from the Galactic center as predicted by Hills and later discussed by Yu & Tremaine. The star has [Fe/H]~0, consistent with a Galactic center origin, and a travel time of <80 Myr from the Galactic center, consistent with its stellar lifetime. If the star is indeed traveling from the Galactic center, it should have a proper motion of 0.3 mas/yr observable with GAIA. Identifying additional hyper-velocity stars throughout the halo will constrain the production rate history of hyper-velocity stars at the Galactic center.

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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

Tidally-Triggered Star Formation in Close Pairs of Galaxies 2: Constraints on Burst Strengths and Ages

Galaxy-galaxy interactions rearrange the baryons in galaxies and trigger substantial star formation; the aggregate effects of these interactions on the evolutionary histories of galaxies in the Universe are poorly understood. We combine B and R-band photometry and optical spectroscopy to estimate the strengths and timescales of bursts of triggered star formation in the centers of 190 galaxies in pairs and compact groups. Based on an analysis of the measured colors and EW(H-alpha), we characterize the pre-existing and triggered populations separately. The best-fitting burst scenarios assume stronger reddening corrections for line emission than for the continuum and continuous star formation lasting for \gtrsim a hundred Myr. The most realistic scenarios require an initial mass function that is deficient in the highest-mass stars. The color of the pre-existing stellar population is the most significant source of uncertainty. Triggered star formation contributes substantially (probably >= 50%) to the R-band flux in the central regions of several galaxies; tidal tails do not necessarily accompany this star formation. Many of the galaxies in our sample have bluer centers than outskirts, suggesting that pre- or non-merger interactions may lead to evolution along the Hubble sequence. These objects would appear blue and compact at higher redshifts; the older, redder outskirts of the disks would be difficult to detect. Our data indicate that galaxies with larger separations on the sky contain weaker, and probably older, bursts of star formation on average. However, confirmation of these trends requires further constraints on the colors of the older stellar populations and on the reddening for individual galaxies.

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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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arXiv2002

Spectroscopic survey of the Galactic open cluster NGC 6871 I. New emission line stars

We analyze spectra of 44 emission line stars detected in a low resolution optical spectroscopic survey of the galactic open cluster NGC 6871. The survey of 1217 stars is complete to V = 14.9 and includes stars with V < 16.5 between the zero age main sequence (ZAMS) and the 10^7 yr pre-main sequence (PMS) isochrone. Of the 44 emission line stars in this survey, 28 show obvious emission in H alpha and 16 have weak H alpha absorption (compared to H gamma). We use the reddening to separate foreground and background stars from the cluster members; the position in the HR diagram (HRD) or the presence of forbidden emission lines ([NII], [SII]) then yields the evolutionary status of the emission line stars. A comparison of the H alpha spectral index distribution in NGC 6871 with the distribution of young stars in the Taurus-Auriga molecular cloud indicates that the late type PMS stars in our sample may be weak line T Tauri stars (wTTs). Many of these stars show [SII] (36%) and [NII] (45%) emission.

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arXiv2002

A Near-Infrared Imaging Survey of Coalsack Globule 2

We describe a near-infrared imaging survey of Globule 2 in the Coalsack. This Bok globule is the highest density region of this southern hemisphere molecular cloud and is the most likely location for young stars in this complex. The survey is complete for K < 14.0, H < 14.5, and J < 15.5, several magnitudes more sensitive than previous observations of this globule. From the large number of background stars, we derive an accurate near-infrared extinction law for the cloud. Our result, E_{J-H}/E_{H-K} = 2.08 \pm 0.03, is significantly steeper than results for other southern clouds. We use the J-H/H-K color-color diagram to identify two potential young stars with K < 14.0 in the region. We apply H-band star counts to derive the density profile of the Coalsack Globule 2 and use a polytropic model to describe the internal structure of this small cloud. For a gas temperature T \sim 15 K, this globule is moderately unstable.

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arXiv2002

The Eclipsing Binary BG Geminorum: Improved Constraints on the Orbit and the Structure of the Accretion Disk

We describe new optical photometric and spectroscopic observations of the semi-detached eclipsing binary BG Geminorum. A large change in the amount of Mg I absorption at secondary maximum indicates the presence of cool material in the outer edge of the disk surrounding the unseen primary star. Detection of weak He I emission implies a hot radiation source at the inner edge of the disk. If the velocity variations in the Hbeta emission line track the orbital motion of the primary star, the primary star has an orbital semiamplitude of K1 = K(Hbeta) = 16.0 +- 4.6 km/sec. This result yields a mass ratio, q = 0.22 +- 0.07, consistent with the q = 0.1 derived from the large ellipsoidal variation. Despite this progress, the nature of the primary star - B-type star or black hole - remains uncertain.

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arXiv2002

Extended Near-Infrared Emission from Candidate Protostars in the Taurus-Auriga Molecular Cloud

We describe near-IR imaging data for a sample of 23 class I sources in the Taurus-Auriga dark clouds. Combining our data with previous photometry, we detect brightness variations of 0.1-0.5 mag in many sources. The near-IR morphologies are consistent with mm continuum measurements. Most (60%) of the sample are true protostars; the rest may be objects in transition between class I and class II, T Tauri stars with edge-on disks, or heavily reddened T Tauri stars.

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