453 research outputs found
HUBBLE SPACE TELESCOPE MEASURES OF MASS ACCRETION RATES IN THE ORION NEBULA CLUSTER
No full textThe present observational understanding of the evolution of the mass accretion rates ([dot over M][subscript acc]) in pre-main-sequence stars is limited by the lack of accurate measurements of [dot over M][subscript acc] over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on Hubble Space Telescope/WFPC2 observations, we present a study of [dot over M][subscript acc] for a sample of ~700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M [subscript *] ~ 2 M [subscript ☉]. We derive [dot over M][subscript acc] from both the U-band excess and the Hα luminosity (L [subscript Hα]), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L [subscript acc]) and L [subscript Hα], which is L [subscript acc]/L [subscript ☉] = (1.31 ± 0.03) · L [subscript Hα]/L [subscript ☉] + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between [dot over M][subscript acc] and the parameters of the central star such as mass and age. We clearly find [dot over M][subscript acc] to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of [dot over M][subscript acc] with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log ([dot over M][subscript acc]/M [subscript ☉] yr) = (–5.12 ± 0.86) – (0.46 ± 0.13) · log (t/yr) – (5.75 ± 1.47) · log (M [subscript *]/M [subscript ☉]) + (1.17 ± 0.23) · log (t/yr) · log (M [subscript *]/M [subscript ☉]). These results also suggest that the similarity solution model could be revised for sources with M [subscript *] [> over ~] 0.5 M [subscript ☉]. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources
Geology of the Selk crater region on Titan from Cassini VIMS observations
No full textInternational audienceObservations of Titan obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) have revealed Selk crater, a geologically young, bright-rimmed, impact crater located similar to 800 km north-northwest of the Huygens landing site. The crater rim-crest diameter is 90 km; its floor diameter is similar to 60 km. A central pit/peak, 20-30 km in diameter, is seen; the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on Ganymede and Callisto, all of which are dome craters. The VIMS data, unfortunately, are not of sufficient resolution to detect such a dome. The inner rim of Selk crater is fluted, probably by eolian erosion, while the outer flank and presumed ejecta blanket appear dissected by drainages (particularly to the east), likely the result of fluvial erosion. Terracing is observed on the northern and western walls of Selk crater within a 10-15 km wide terrace zone identified in VIMS data; the terrace zone is bright in SAR data, consistent with it being a rough surface. The terrace zone is slightly wider than those observed on Ganymede and Callisto and may reflect differences in thermal structure and/or composition of the lithosphere. The polygonal appearance of the crater likely results from two preexisting planes of weakness (oriented at azimuths of 21 degrees and 122 degrees east of north). A unit of generally bright terrain that exhibits similar infrared-color variation and contrast to Selk crater extends east-southeast from the crater several hundred kilometers. We informally refer to this terrain as the Selk "bench." Both Selk and the bench are surrounded by the infrared-dark Belet dune field. Hypotheses for the genesis of the optically bright terrain of the bench include: wind shadowing in the lee of Selk crater preventing the encroachment of dunes, impact-induced cryovolcanism, flow of a fluidized-ejecta blanket (similar to the bright crater outflows observed on Venus), and erosion of a streamlined upland formed in the lee of Selk crater by fluid flow. Vestigial circular outlines in this feature just east of Selk's ejecta blanket suggest that this might be a remnant of an ancient, cratered crust. Evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the Belet dune field. We conclude that this feature either represents a relatively high-viscosity, fluidizedejecta flow (a class intermediate to ejecta blankets and long venusian-style ejecta flows) or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west-northwest
A Model for Rotation and Shape of Asteroid 9969 Braille from Ground-Based Observations and Images Obtained during the Deep Space 1 (DS1) Flyby
No full textImage data from the DS1 encounter with Asteroid 9969 Braille and data from a coordinated ground-based photometric observing campaign are combined to study the physical properties of this small Mars crosser. From telescope data the object's brightness was found to vary by up to 0.5 mag from night to night, with the most probable synodic rotational period being 226.4+/-1.3 h (9.4 days) and a mean lightcurve magnitude R(1, ±=24°) =17.04+/-0.10. During the flyby of the spacecraft, two frame images from a range of approximately 13,500 km and phase angle 82.4°, which impose strong constraints on size, shape, and albedo of the object, were obtained. Using telescope and flyby data in combination, the asteroid is estimated to have a size of 2.1×1×1 km3 and shown to have photometric properties similar to the asteroid 4 Vesta, notably a comparably high albedo. The high albedo supports the notion (L. Soderblom et al. 1999, Bull. Am. Astron. Soc. 31,) that Braille is of the V or Q taxonomic type
Soils of eagle crater and Meridiani Planum at the Opportunity Rover landing site
No full textThe soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.Additional co-authors: MP Golombek, R Greeley, JP Grotzinger, KE Herkenhoff, DJ Jerolmack, JR Johnson, B Jolliff, G Klingelhöfer, AH Knoll, ZA Learner, R Li, MC Malin, SM McLennan, HY McSween, DW Ming, RV Morris, JW Rice Jr, L Richter, R Rieder, D Rodionov, FP Seelos IV, JM Soderblom, SW Squyres, R Sullivan, WA Watters, CM Weitz, MB Wyatt, A Yen, J Zipfe
Fluvial channels on Titan: Initial Cassini RADAR observations
No full textCassini radar images show a variety of fluvial channels on Titan’s surface, often several hundreds of kilometers in length. Some
(predominantly at low- and mid-latitude) are radar-bright and braided, resembling desert washes where fines have been removed by
energetic surface liquid flow, presumably from methane rainstorms. Others (predominantly at high latitudes) are radar-dark and
meandering and drain into or connect polar lakes, suggesting slower-moving flow depositing fine-grained sediments. A third type, seen
predominantly at mid- and high latitudes, have radar brightness patterns indicating topographic incision, with valley widths of up to
3 km across and depth of several hundred meters. These observations show that fluvial activity occurs at least occasionally at all latitudes,
not only at the Huygens landing site, and can produce channels much larger in scale than those observed there. The areas in which
channels are prominent so far amount to about 1% of Titan’s surface, of which only a fraction is actually occupied by channels. The
corresponding global sediment volume inferred is not enough to account for the extensive sand seas. Channels observed so far have a
consistent large-scale flow pattern, tending to flow polewards and eastwards
Asteroseismology of the hyades with K2: First detection of main-sequence solar-like oscillations in an open cluster
No full textThe Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2 mission, and short-cadence data were collected on a number of cool main-sequence stars. Here, we report results on two F-type stars that show detectable oscillations of a quality that allows asteroseismic analyses to be performed. These are the first ever detections of solar-like oscillations in main-sequence stars in an open cluster
Distribution and interplay of geologic processes on Titan from Cassini radar data
No full textThe Cassini Titan Radar Mapper is providing an unprecedented view of Titan's surface geology. Here we use Synthetic Aperture Radar (SAR) image swaths (Ta-T30) obtained from October 2004 to December 2007 to infer the geologic processes that have shaped Titan's surface. These SAR swaths cover about 20% of the surface, at a spatial resolution ranging from ∼350 m to ∼2 km. The SAR data are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution of processes. They reveal a geologically complex surface that has been modified by all the major geologic processes seen on Earth - volcanism, tectonism, impact cratering, and erosion and deposition by fluvial and aeolian activity. In this paper, we map geomorphological units from SAR data and analyze their areal distribution and relative ages of modification in order to infer the geologic evolution of Titan's surface. We find that dunes and hummocky and mountainous terrains are more widespread than lakes, putative cryovolcanic features, mottled plains, and craters and crateriform structures that may be due to impact. Undifferentiated plains are the largest areal unit; their origin is uncertain. In terms of latitudinal distribution, dunes and hummocky and mountainous terrains are located mostly at low latitudes (less than 30°), with no dunes being present above 60°. Channels formed by fluvial activity are present at all latitudes, but lakes are at high latitudes only. Crateriform structures that may have been formed by impact appear to be uniformly distributed with latitude, but the well-preserved impact craters are all located at low latitudes, possibly indicating that more resurfacing has occurred at higher latitudes. Cryovolcanic features are not ubiquitous, and are mostly located between 30° and 60° north. We examine temporal relationships between units wherever possible, and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountains and Xanadu are likely the most ancient. © 2009 Elsevier Inc
Europa Deep Dive Two
No full textThe focus of the Europa Deep Dive 2: Composition workshop is to assess existing laboratory data and identify gaps where laboratory work is still needed; assess existing data on the Europa surface and exosphere; plan how to realize the needs for new data; encourage dialogue among laboratory, observational, and data-analysis communities; and foster collaborations among complementary laboratories.Institutional Support Lunar and Planetary Institute, Universities Space Research Association ; Conveners Louise M. Prockter, USRA/Lunar and Planetary Institute, Jonathan Kay, USRA/Lunar and Planetary Institute ; Science Organizing Committee Murthy S. Gudipati, NASA Jet Propulsion Laboratory, Reggie L. Hudson, NASA Goddard Space Flight Center, Lynnae C. Quick, Smithsonian Institution/Air and Space Museum, Abigal M. Rymer, Johns Hopkins University/Applied Physics Laboratory, Jason M. Soderblom, Massachusetts Institute of Technology.PARTIAL CONTENTS: Towards a Better Understanding of Composition: Status Update on Laboratory Studies of Europa Environment Analogs / B. L. Henderson -- Laboratory Spectral Measurements Needed to Better Model Europa's Surface Composition / C. A. Hibbitts -- Impurities in Europa's Ice Shell: Salty Tree-Rings? / S. M. Howell and E. J. Leonard -- Diving Deep into Europa's Reaction Chemistry / R. L. Hudson -- Plasma and Thermal Processing of Europa's Surface / R. E. Johnson, A. Oza, C. Schmidt, and F. Leblanc -- Investigating the Surface Composition of Europa with the Europa Clipper / R. L. Klima, J. Soderblom, M. Gudipati, and Europa Clipper Composition Working Group -- New Constraints on the Abundances of Sulfur and Chlorine at Europa / M. A. McGrath, W. B. Sparks, and J. R. Spencer -- Melt Production, Eruption and Degassing from Europa's Interior: Effects on Composition over Time / M. Melwani Daswani and S. D. Vance -- Investigating Detectability of Organics on Europa's Surface / I. Mishra and J. Lunine -- Rifting, Convection, and the Elevation of Bands on Europa / L. G. J. Montesi, S. M. Howell, and R. T. Pappalardo
Science goals and mission concept for the future exploration of Titan and Enceladus
No full textSaturn's moons, Titan and Enceladus, are two of the Solar System's most enigmatic bodies and are prime targets for future space exploration. Titan provides an analogue for many processes relevant to the Earth, more generally to outer Solar System bodies, and a growing host of newly discovered icy exoplanets. Processes represented include atmospheric dynamics, complex organic chemistry, meteorological cycles (with methane as a working fluid), astrobiology, surface liquids and lakes, geology, fluvial and aeolian erosion, and interactions with an external plasma environment. In addition, exploring Enceladus over multiple targeted flybys will give us a unique opportunity to further study the most active icy moon in our Solar System as revealed by Cassini and to analyse in situ its active plume with highly capable instrumentation addressing its complex chemistry and dynamics. Enceladus' plume likely represents the most accessible samples from an extra-terrestrial liquid water environment in the Solar system, which has far reaching implications for many areas of planetary and biological science. Titan with its massive atmosphere and Enceladus with its active plume are prime planetary objects in the Outer Solar System to perform in situ investigations. In the present paper, we describe the science goals and key measurements to be performed by a future exploration mission involving a Saturn-Titan orbiter and a Titan balloon, which was proposed to ESA in response to the call for definition of the science themes of the next Large-class mission in 2013. The mission scenario is built around three complementary science goals: (A) Titan as an Earth-like system; (B) Enceladus as an active cryovolcanic moon; and (C) Chemistry of Titan and Enceladus - clues for the origin of life. The proposed measurements would provide a step change in our understanding of planetary processes and evolution, with many orders of magnitude improvement in temporal, spatial, and chemical resolution over that which is possible with Cassini-Huygens. This mission concept builds upon the successes of Cassini-Huygens and takes advantage of previous mission heritage in both remote sensing and in situ measurement technologie
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