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At least 973 records · Page 54
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DOE OSTI2014

Overstable librations can account for the paucity of mean motion resonances among exoplanet pairs

We assess the multi-planet systems discovered by the Kepler satellite in terms of current ideas about orbital migration and eccentricity damping due to planet-disk interactions. Our primary focus is on first order mean motion resonances, which we investigate analytically to lowest order in eccentricity. Only a few percent of planet pairs are in close proximity to a resonance. However, predicted migration rates (parameterized by τ{sub n}=n/| n-dot |) imply that during convergent migration most planets would have been captured into first order resonances. Eccentricity damping (parameterized by τ{sub e}=e/| e-dot |) offers a plausible resolution. Estimates suggest τ {sub e}/τ {sub n} ∼ (h/a){sup 2} ∼ 10{sup –2}, where h/a is the ratio of disk thickness to radius. Together, eccentricity damping and orbital migration give rise to an equilibrium eccentricity, e {sub eq} ∼ (τ {sub e}/τ {sub n}){sup 1/2}. Capture is permanent provided e {sub eq} ≲ μ{sup 1/3}, where μ denotes the planet to star mass ratio. But for e {sub eq} ≳ μ{sup 1/3}, capture is only temporary because librations around equilibrium are overstable and lead to passage through resonance on timescale τ {sub e}. Most Kepler planet pairs have e {sub eq} > μ{sup 1/3}. Since τ {sub n} >> τ {sub e} is the timescale for migration between neighboring resonances, only a modest percentage of pairs end up trapped in resonances after the disk disappears. Thus the paucity of resonances among Kepler pairs should not be taken as evidence for in situ planet formation or the disruptive effects of disk turbulence. Planet pairs close to a mean motion resonance typically exhibit period ratios 1%-2% larger than those for exact resonance. The direction of this shift undoubtedly reflects the same asymmetry that requires convergent migration for resonance capture. Permanent resonance capture at these separations from exact resonance would demand μ(τ {sub n}/τ {sub e}){sup 1/2} ≳ 0.01, a value that estimates of μ from transit data and (τ {sub e}/τ {sub n}){sup 1/2} from theory are insufficient to match. Plausible alternatives involve eccentricity damping during or after disk dispersal. The overstability referred to above has applications beyond those considered in this investigation. It was discovered numerically by Meyer and Wisdom in their study of the tidal evolution of Saturn's satellites.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

CSI 2264: simultaneous optical and infrared light curves of young disk-bearing stars in NGC 2264 with CoRoT and Spitzer—evidence for multiple origins of variability

We present the Coordinated Synoptic Investigation of NGC 2264, a continuous 30 day multi-wavelength photometric monitoring campaign on more than 1000 young cluster members using 16 telescopes. The unprecedented combination of multi-wavelength, high-precision, high-cadence, and long-duration data opens a new window into the time domain behavior of young stellar objects. Here we provide an overview of the observations, focusing on results from Spitzer and CoRoT. The highlight of this work is detailed analysis of 162 classical T Tauri stars for which we can probe optical and mid-infrared flux variations to 1% amplitudes and sub-hour timescales. We present a morphological variability census and then use metrics of periodicity, stochasticity, and symmetry to statistically separate the light curves into seven distinct classes, which we suggest represent different physical processes and geometric effects. We provide distributions of the characteristic timescales and amplitudes and assess the fractional representation within each class. The largest category (>20%) are optical 'dippers' with discrete fading events lasting ∼1-5 days. The degree of correlation between the optical and infrared light curves is positive but weak; notably, the independently assigned optical and infrared morphology classes tend to be different for the same object. Assessment of flux variation behavior with respect to (circum)stellar properties reveals correlations of variability parameters with Hα emission and with effective temperature. Overall, our results point to multiple origins of young star variability, including circumstellar obscuration events, hot spots on the star and/or disk, accretion bursts, and rapid structural changes in the inner disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Kepler and the long-period variables

High-precision Kepler photometry is used to explore the details of asymptotic giant branch (AGB) light curves. Since AGB variability has a typical timescale on the order of a year, we discuss at length the removal of long-term trends and quarterly changes in Kepler data. Photometry for a small sample of nine semi-regular (SR) AGB stars is examined using a 30 minute cadence over a period of 45 months. While undergoing long-period variations of many magnitudes, the light curves are shown to be smooth at the millimagnitude level over much shorter time intervals. No flares or other rapid events were detected on a sub-day timescale. The shortest AGB period detected is on the order of 100 days. All the SR variables in our sample are shown to have multiple modes. This is always the first overtone, typically combined with the fundamental. A second common characteristic of SR variables is shown to be the simultaneous excitation of multiple closely separated periods for the same overtone mode. Approximately half the sample had a much longer variation in the light curve, likely a long secondary period (LSP). The light curves were all well represented by a combination of sinusoids. However, the properties of the sinusoids are time variable, with irregular variations present at low levels. No non-radial pulsations were detected. It is argued that the LSP variation seen in many SR variables is intrinsic to the star and linked to multiple mode pulsation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

An unobscured type II quasar candidate: SDSS J012032.19-005501.9

We report the finding of an unobscured type II active galactic nucleus (AGN) candidate, SDSS J012032.19-005501.9, at a relatively high redshift of 0.601, which shows a number of unusual properties. It varies significantly on timescales of years, typical of type I AGNs, and marginally on timescales of weeks. The color–magnitude relation and the structure function are also consistent with that of type I AGNs, which implies that its variability likely originates from the black hole accretion system. However, no broad emission line (BEL) is detected in the Sloan Digital Sky Survey spectrum, and the upper limit of the equivalent width of the Hβ BEL is much less than that of type I AGNs. These properties suggest that SDSS J012032.19-005501.9 may be an unobscured quasar without intrinsically broad emission lines, namely, an unobscured type II AGN or “true” type II AGN. Furthermore, its continuum luminosity is at least one order of magnitude fainter than the average value over the past century from the [O iii] emission line. This indicates that SDSS J012032.19-005501.9 may be switching off. Additional possible scenarios to explain this intriguing source are also discussed. Future deep observations at multiwavelengths are needed to reveal the nature of this peculiar and intriguing AGN.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

HUBBLE SPACE TELESCOPE SNAPSHOT SEARCH FOR PLANETARY NEBULAE IN GLOBULAR CLUSTERS OF THE LOCAL GROUP

Single stars in ancient globular clusters (GCs) are believed incapable of producing planetary nebulae (PNs), because their post-asymptotic-giant-branch evolutionary timescales are slower than the dissipation timescales for PNs. Nevertheless, four PNs are known in Galactic GCs. Their existence likely requires more exotic evolutionary channels, including stellar mergers and common-envelope binary interactions. I carried out a snapshot imaging search with the Hubble Space Telescope (HST) for PNs in bright Local Group GCs outside the Milky Way. I used a filter covering the 5007 Å nebular emission line of [O iii], and another one in the nearby continuum, to image 66 GCs. Inclusion of archival HST frames brought the total number of extragalactic GCs imaged at 5007 Å to 75, whose total luminosity slightly exceeds that of the entire Galactic GC system. I found no convincing PNs in these clusters, aside from one PN in a young M31 cluster misclassified as a GC, and two PNs at such large angular separations from an M31 GC that membership is doubtful. In a ground-based spectroscopic survey of 274 old GCs in M31, Jacoby et al. found three candidate PNs. My HST images of one of them suggest that the [O iii] emission actually arises from ambient interstellar medium rather than a PN; for the other two candidates, there are broadband archival UV HST images that show bright, blue point sources that are probably the PNs. In a literature search, I also identified five further PN candidates lying near old GCs in M31, for which follow-up observations are necessary to confirm their membership. The rates of incidence of PNs are similar, and small but nonzero, throughout the GCs of the Local Group.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RESONANCES, CHAOS, AND SHORT-TERM INTERACTIONS AMONG THE INNER URANIAN SATELLITES

The Portia group of Uranian satellites, representing 9 of the planet’s 13 tiny innermost moons, form a densely packed dynamical system. Hubble Space Telescope observations indicate that their orbits have changed significantly over two decades, and long-term numerical integrations show that their orbits are unstable over millions of years. To investigate the dynamical interactions of the Portia group satellites on the decade timescale over which orbital changes have been observed, we have performed a suite of 100–1000 yr N-body numerical integrations for a range of assumed satellite masses, which are at present not tightly constrained by observations. As first noted by Dawson et al. and recently investigated independently by Quillen and (Robert) French, the moons are configured in chains of interlinked first- and second-order eccentric resonances that contribute to chaotic behavior. We explore in detail several of the strongest of these interlinked resonances. The first such chain is a quintet of orbital resonances: Bianca is near a resonance with Cressida, which is itself near a resonance with Desdemona. Desdemona, in turn, is near a resonance with Portia, which is itself near a resonance with Juliet. The five participating resonances are: Cressida and Bianca (16:15), Desdemona and Cressida (47:46), Portia and Desdemona (13:12), and Portia and Juliet (51:49). A second such chain is a set of two interlinked resonances: Cupid and Belinda (58:57) and Belinda and Perdita (44:43). We also report the new identification of a companion set of second-order inclination-type resonances (Cressida and Bianca (32:30), Desdemona and Cressida (94:92), Portia and Desdemona (26:24), Portia and Juliet (51:49), Cupid and Belinda (116:114), and Belinda and Perdita (88:86)), some of which result in quite strongly coupled variations in the inclinations of the interacting satellites. Using a robust formulation of orbital elements that accounts for the oblateness of Uranus, we probe the dynamical interactions among the moons in the time and frequency domains, and also in phase space, using numerical integrations of subsets of the inner moons, for a range of assumed masses. Several of the satellites are near mean-motion resonance with more than one neighbor, and undergo orbital variations at two nearly equal resonant frequencies. Such configurations of two interlinked resonances can result in chaotic behavior, associated with the transition of one resonant argument from circulation to libration. We demonstrate that, even on the short timescales investigated here, the dynamical interactions, onset of chaos, and associated Lyapunov times are highly sensitive to the masses of the interacting satellites.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Effect of stellar encounters on comet cloud formation

We have investigated the effect of stellar encounters on the formation and disruption of the Oort cloud using the classical impulse approximation. We calculate the evolution of a planetesimal disk into a spherical Oort cloud due to the perturbation from passing stars for 10 Gyr. We obtain the empirical fits of the e-folding time for the number of Oort cloud comets using the standard exponential and Kohlrausch formulae as functions of the stellar parameters and the initial semimajor axes of planetesimals. The e-folding time and the evolution timescales of the orbital elements are also analytically derived. In some calculations, the effect of the Galactic tide is additionally considered. We also show the radial variations of the e-folding times to the Oort cloud. From these timescales, we show that if the initial planetesimal disk has the semimajor axes distribution dn/da∝a{sup −2}, which is produced by planetary scattering, the e-folding time for planetesimals in the Oort cloud is ∼10 Gyr at any heliocentric distance r. This uniform e-folding time over the Oort cloud means that the supply of comets from the inner Oort cloud to the outer Oort cloud is sufficiently effective to keep the comet distribution as dn/dr∝r{sup −2}. We also show that the final distribution of the semimajor axes in the Oort cloud is approximately proportional to a{sup −2} for any initial distribution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TESTING MICROVARIABILITY IN QUASAR DIFFERENTIAL LIGHT CURVES USING SEVERAL FIELD STARS

Microvariability consists of small timescale variations of low amplitude in the photometric light curves of quasars and represents an important tool to investigate their inner core. Detection of quasar microvariations is challenging because of their non-periodicity, as well as the need for high monitoring frequency and a high signal-to-noise ratio. Statistical tests developed for the analysis of quasar differential light curves usually show either low power or low reliability, or both. In this paper we compare two statistical procedures to perform tests on several stars with enhanced power and high reliability. We perform light curve simulations of variable quasars and non-variable stars and analyze them with statistical procedures developed from the F-test and the analysis of variance. The results show a large improvement in the power of both statistical probes and a larger reliability when several stars are included in the analysis. The results from the simulations agree with those obtained from observations of real quasars. The high power and high reliability of the tests discussed in this paper improve the results that can be obtained from short and long timescale variability studies. These techniques are not limited to quasar variability; on the contrary, they can be easily implemented for other sources, such as variable stars. Their applications to future research and to the analysis of large-field photometric monitoring archives could reveal new variable sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

YSOVAR: MID-INFRARED VARIABILITY OF YOUNG STELLAR OBJECTS AND THEIR DISKS IN THE CLUSTER IRAS 20050+2720

We present a time-variability study of young stellar objects (YSOs) in the cluster IRAS 20050+2720, performed at 3.6 and 4.5 μm with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability (YSOVAR) project. We have collected light curves for 181 cluster members over 60 days. We find a high variability fraction among embedded cluster members of ca. 70%, whereas young stars without a detectable disk display variability less often (in ca. 50% of the cases) and with lower amplitudes. We detect periodic variability for 33 sources with periods primarily in the range of 2–6 days. Practically all embedded periodic sources display additional variability on top of their periodicity. Furthermore, we analyze the slopes of the tracks that our sources span in the color–magnitude diagram (CMD). We find that sources with long variability time scales tend to display CMD slopes that are at least partially influenced by accretion processes, while sources with short variability timescales tend to display extinction-dominated slopes. We find a tentative trend of X-ray detected cluster members to vary on longer timescales than the X-ray undetected members.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Tilting Saturn without tilting Jupiter: Constraints on giant planet migration

The migration and encounter histories of the giant planets in our solar system can be constrained by the obliquities of Jupiter and Saturn. We have performed secular simulations with imposed migration and N-body simulations with planetesimals to study the expected obliquity distribution of migrating planets with initial conditions resembling those of the smooth migration model, the resonant Nice model and two models with five giant planets initially in resonance (one compact and one loose configuration). For smooth migration, the secular spin–orbit resonance mechanism can tilt Saturn’s spin axis to the current obliquity if the product of the migration timescale and the orbital inclinations is sufficiently large (exceeding 30 Myr deg). For the resonant Nice model with imposed migration, it is difficult to reproduce today’s obliquity values, because the compactness of the initial system raises the frequency that tilts Saturn above the spin precession frequency of Jupiter, causing a Jupiter spin–orbit resonance crossing. Migration timescales sufficiently long to tilt Saturn generally suffice to tilt Jupiter more than is observed. The full N-body simulations tell a somewhat different story, with Jupiter generally being tilted as often as Saturn, but on average having a higher obliquity. The main obstacle is the final orbital spacing of the giant planets, coupled with the tail of Neptune’s migration. The resonant Nice case is barely able to simultaneously reproduce the orbital and spin properties of the giant planets, with a probability ∼0.15%. The loose five planet model is unable to match all our constraints (probability <0.08%). The compact five planet model has the highest chance of matching the orbital and obliquity constraints simultaneously (probability ∼0.3%).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE YOUNG STELLAR POPULATION OF THE CYGNUS-X DR15 REGION

We present a multi-wavelength study of the young stellar population in the Cygnus-X DR15 region. We studied young stars that were forming or recently formed at and around the tip of a prominent molecular pillar and an infrared dark cloud. Using a combination of ground-based near-infrared, space-based infrared, and X-ray data, we constructed a point source catalog from which we identified 226 young stellar sources, which we classified into evolutionary classes. We studied their spatial distributions across the molecular gas structures and identified several groups that possibly belong to distinct young star clusters. We obtained samples of these groups and constructed K-band luminosity functions that we compared with those of artificial clusters, allowing us to make first order estimates of the mean ages and age spreads of the groups. We used a {sup 13}CO(1-0) map to investigate the gas kinematics at the prominent gaseous envelope of the central cluster in DR15, and we inferred that the removal of this envelope is relatively slow compared to other cluster regions, in which the gas dispersal timescale could be similar or shorter than the circumstellar disk dissipation timescale. The presence of other groups with slightly older ages, associated with much less prominent gaseous structures, may imply that the evolution of young clusters in this part of the complex proceeds in periods that last 3–5 Myr, perhaps after a slow dissipation of their dense molecular cloud birthplaces.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

INCLINATION MIXING IN THE CLASSICAL KUIPER BELT

We investigate the long-term evolution of the inclinations of the known classical and resonant Kuiper Belt objects (KBOs). This is partially motivated by the observed bimodal inclination distribution and by the putative physical differences between the low- and high-inclination populations. We find that some classical KBOs undergo large changes in inclination over gigayear timescales, which means that a current member of the low-inclination population may have been in the high-inclination population in the past, and vice versa. The dynamical mechanisms responsible for the time variability of inclinations are predominantly distant encounters with Neptune and chaotic diffusion near the boundaries of mean motion resonances. We reassess the correlations between inclination and physical properties including inclination time variability. We find that the size-inclination and color-inclination correlations are less statistically significant than previously reported (mostly due to the increased size of the data set since previous works with some contribution from inclination variability). The time variability of inclinations does not change the previous finding that binary classical KBOs have lower inclinations than non-binary objects. Our study of resonant objects in the classical Kuiper Belt region includes objects in the 3:2, 7:4, 2:1, and eight higher-order mean motion resonances. We find that these objects (some of which were previously classified as non-resonant) undergo larger changes in inclination compared to the non-resonant population, indicating that their current inclinations are not generally representative of their original inclinations. They are also less stable on gigayear timescales.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

TIDAL DISRUPTION FLARES: THE ACCRETION DISK PHASE

The evolution of an accretion disk, formed as a consequence of the disruption of a star by a black hole, is followed by solving numerically hydrodynamic equations. The present investigation aims to study the dependence of resulting light curves on dynamical and physical properties of such a transient disk during its existence. One of the main results derived from our simulations is that blackbody fits of X-ray data tend to overestimate the true mean disk temperature. In fact, the temperature derived from blackbody fits should be identified with the color X-ray temperature rather than the average value derived from the true temperature distribution along the disk. The time interval between the beginning of the circularization of the bound debris and the beginning of the accretion process by the black hole is determined by the viscous (or accretion) timescale, which also fixes the rising part of the resulting light curve. The luminosity peak coincides with the beginning of matter accretion by the black hole and the late evolution of the light curve depends on the evolution of the debris fallback rate. Peak bolometric luminosities are in the range 10{sup 45}-10{sup 46} erg s{sup -1}, whereas peak luminosities in soft X-rays (0.2-2.0 keV) are typically one order of magnitude lower. The typical timescale derived from our preferred models for the flare luminosity to decay by two orders of magnitude is about 3-4 yr. Predicted soft X-ray light curves reproduce quite well data on galaxies in which a variable X-ray emission possibly related to a tidal event was detected. In the cases of NGC 3599 and IC 3599, data are reproduced well by models defined by a black hole with mass {approx}10{sup 7} M{sub sun} and a disrupted star of about 1 solar mass. The X-ray variation observed in XMMSL1 is consistent with a model defined by a black hole with mass {approx}3 x 10{sup 6} M{sub sun} and a disrupted star of 1 solar mass, while that observed in the galaxy situated in the cluster A1689 is consistent with a model including a black hole of {approx}10{sup 7} M{sub sun} and a disrupted star of {approx}0.5 M{sub sun}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

ON THE INTERACTION BETWEEN A PROTOPLANETARY DISK AND A PLANET IN AN ECCENTRIC ORBIT: APPLICATION OF DYNAMICAL FRICTION

We present a new analytic approach to the disk-planet interaction that is especially useful for planets with eccentricity larger than the disk aspect ratio. We make use of the dynamical friction formula to calculate the force exerted on the planet by the disk, and the force is averaged over the period of the planet. The resulting migration and eccentricity damping timescale agree very well with previous works in which the planet eccentricity is moderately larger than the disk aspect ratio. The advantage of this approach is that it is possible to apply this formulation to arbitrary large eccentricity. We have found that the timescale of the orbital evolution depends largely on the adopted disk model in the case of highly eccentric planets. We discuss the possible implication of our results for the theory of planet formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE STRUCTURE FUNCTION OF VARIABLE 1.4 GHz RADIO SOURCES BASED ON NVSS AND FIRST OBSERVATIONS

We augment the two widest/deepest 1.4 GHz radio surveys, the NRAO VLA Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimeters (FIRST), with the mean epoch in which each source was observed. We use these catalogs to search for unresolved sources that vary between the FIRST and NVSS epochs. We find 43 variable sources (0.1% of the sources) that vary by more than 4{sigma}, and we construct the mean structure function of these objects. This enables us to explore radio variability on timescales between several months and about five years. We find that, on these timescales, the mean structure function of the variable sources is consistent with a flat structure function. A plausible explanation to these observations is that a large fraction of the variability at 1.4 GHz is induced by scintillations in the interstellar medium, rather than by intrinsic variability. Finally, for a subsample of the variables for which the redshift is available, we do not find strong evidence for a correlation between the variability amplitude and the source redshift.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

DISCOVERY OF AN ULTRASOFT X-RAY TRANSIENT SOURCE IN THE 2XMM CATALOG: A TIDAL DISRUPTION EVENT CANDIDATE

We have discovered an ultrasoft X-ray transient source, 2XMMi J184725.1-631724, which was detected serendipitously in two XMM-Newton observations in the direction of the center of the galaxy IC 4765-f01-1504 at a redshift of 0.0353. These two observations were separated by 211 days, with the 0.2-10 keV absorbed flux increasing by a factor of about nine. Their spectra are best described by a model dominated by a thermal disk or a single-temperature blackbody component (contributing {approx}>80% of the flux) plus a weak power-law component. The thermal emission has a temperature of a few tens of eV, and the weak power-law component has a photon index of {approx}3.5. Similar to the black hole X-ray binaries in the thermal state, our source exhibits an accretion disk whose luminosity appears to follow the L{proportional_to}T {sup 4} relation. This would indicate that the black hole mass is about 10{sup 5}-10{sup 6} M{sub sun} using the best-fitting inner disk radius. Both XMM-Newton observations show variability of about 21% on timescales of hours, which can be explained as due to fast variations in the mass accretion rate. The source was not detected by ROSAT in an observation in 1992, indicating a variability factor of {approx}>64 over longer timescales. The source was not detected again in X-rays in a Swift observation in 2011 February, implying a flux decrease by a factor of {approx}>12 since the last XMM-Newton observation. The transient nature, in addition to the extreme softness of the X-ray spectra and the inactivity of the galaxy implied by the lack of strong optical emission lines, makes it a candidate tidal disruption event. If this is the case, the first XMM-Newton observation would have been in the rising phase and the second one in the decay phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

QUENCHING OF CARBON MONOXIDE AND METHANE IN THE ATMOSPHERES OF COOL BROWN DWARFS AND HOT JUPITERS

We explore CO{r_reversible}CH{sub 4} quench kinetics in the atmospheres of substellar objects using updated timescale arguments, as suggested by a thermochemical kinetics and diffusion model that transitions from the thermochemical-equilibrium regime in the deep atmosphere to a quench-chemical regime at higher altitudes. More specifically, we examine CO quench chemistry on the T dwarf Gliese 229B and CH{sub 4} quench chemistry on the hot-Jupiter HD 189733b. We describe a method for correctly calculating reverse rate coefficients for chemical reactions, discuss the predominant pathways for CO{r_reversible}CH{sub 4} interconversion as indicated by the model, and demonstrate that a simple timescale approach can be used to accurately describe the behavior of quenched species when updated reaction kinetics and mixing-length-scale assumptions are used. Proper treatment of quench kinetics has important implications for estimates of molecular abundances and/or vertical mixing rates in the atmospheres of substellar objects. Our model results indicate significantly higher K{sub zz} values than previously estimated near the CO quench level on Gliese 229B, whereas current-model-data comparisons using CH{sub 4} permit a wide range of K{sub zz} values on HD 189733b. We also use updated reaction kinetics to revise previous estimates of the Jovian water abundance, based upon the observed abundance and chemical behavior of carbon monoxide. The CO chemical/observational constraint, along with Galileo entry probe data, suggests a water abundance of approximately 0.51-2.6 x solar (for a solar value of H{sub 2}O/H{sub 2} = 9.61 x 10{sup -4}) in Jupiter's troposphere, assuming vertical mixing from the deep atmosphere is the only source of tropospheric CO.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE ALLEN TELESCOPE ARRAY Pi GHz SKY SURVEY II. DAILY AND MONTHLY MONITORING FOR TRANSIENTS AND VARIABILITY IN THE BOOeTES FIELD

We present results from daily radio continuum observations of the Booetes field as part of the Pi GHz Sky Survey (PiGSS). These results are part of a systematic and unbiased campaign to characterize variable and transient sources in the radio sky. The observations include 78 individual epochs distributed over five months at a radio frequency of 3.1 GHz with a median rms image noise in each epoch of 2.8 mJy. We produce five monthly images with a median rms of 0.6 mJy. No transient radio sources are detected in the daily or monthly images. At 15 mJy, we set an upper limit (2{sigma}) to the surface density of one-day radio transients at 0.025 deg{sup -2}. At 5 mJy, we set an upper limit (2{sigma}) to the surface density of one-month radio transients at 0.18 deg{sup -2}. We also produce light curves for 425 sources and explore the variability properties of these sources. Approximately 20% of the sources exhibit some variability on daily and monthly timescales. The maximum rms fractional modulations on the one-day and one-month timescales for sources brighter than 10 mJy are 2 and 0.5, respectively. The probability of a daily fluctuation for all sources and all epochs by a factor of 10 is less than 10{sup -4}. We compare the radio to mid-infrared variability for sources in the field and find no correlation. Finally, we apply the statistics of transient and variable populations to constrain models for a variety of source classes.

79 ASTRONOMY AND ASTROPHYSICS↗