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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

STIS Observations of the Disk and Outflow of the Herbig Ae Star HD163296

Coronagraphic imaging with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope reveals a approx. 450 AU radius circumstellar disk around the Herbig Ae star HDI63296. The disk includes an annulus of reduced scattering at 325 AU and exhibits a flat trend of surface brightness in to 180-222 AU, consistent with a cleared central zone. The disk is accompanied by a chain of nebulosities ("knots") oriented roughly perpendicular to the disk. Followup spectroscopy in the visible and the far ultraviolet confirms the Herbig-Haro nature of the observed knots. The FUV data in particular reveal the presence of a collimated, bipolar outflow, with a blue-shifted Lyman alpha jet that can be traced to within 0.06 arcseconds (7.3 AU) of HD163296 and a red-shifted Lyman alpha component that may be due to infall or to a poorly collimated wind. If the estimated age of this system is correct, these data indicate that protostellar outflows may last up to 10 times longer than previously believed.

Kimble, Randy↗
NASA NTRS1996

Protecting Venus from Asteroids, Comets, and Meteors

It is well accepted that the dense, thick atmosphere of Venus prevents most small cosmic bodies from reaching the surface and forming craters. We have examined this atmospheric intervention in detail, incorporating the lessons learned from the extensive modeling of impactor deceleration and flattening motivated by the SL-9 impacts with Jupiter. We employ a "pancake" model, which best matches detailed code simulations of atmospheric energy deposition, and Schmidt-Holsapple crater scaling modified for complex (flattened) craters. We adopt the distributions of Venus-crossing asteroids and comets determined by E.M. Shoemaker and co-workers, as well as generalizations of these distributions. Our nominal simulation of the venusian crater record is shown below, calibrated to the total number of venusian craters (940). As nearly all craters on Venus are well-preserved and relatively uniformly distributed, such simulations constrain the age of the surface. The fit is reasonable, with a nominal crater retention age of approx. 700 Ma. The fit at the large-crater end is improved if the number of large asteroids is increased, which Shoemaker argues is in fact more representative of the long-term (over several 100 Ma) average, and if Halley-family comets are included. The ages we obtain under a variety of modeling choices that produce good fits (including using Shoemaker's preferred crater scaling) are approx. 700-900 Ma, substantially greater than the most widely cited age estimate in the literature (-300 Ma). The key difference is that we find very large depletions in the production of 20-30-km craters (see figure) compared with previous estimates, the size range at which atmospheric effects are often calibrated or assumed nearly negligible. As venusian global resurfacing recedes deeper into history, the likelihood that Venus is resting between bouts of activity diminishes. Venus, like Mars, may instead be dying or dead.

McKinnon, William B.↗
NASA NTRS2000

Orthostatic intolerance: a disorder of young women

Orthostatic intolerance (OI) is a cause of significant disability in otherwise healthy women seen by gynecologists. Orthostatic tachycardia is often the most obvious hemodynamic abnormality found in OI patients, but symptoms may include dizziness, visual changes, discomfort in the head or neck, poor concentration, fatigue, palpitations, tremulousness, anxiety, and, in some cases, fainting (syncope). It is the most common disorder of blood pressure regulation after essential hypertension, and patients with OI are traditionally women of childbearing age. Estimates suggest that at least 500,000 Americans suffer from some form of OI, and such patients comprise the largest group referred to centers specialized in autonomic disorders. This article reviews recent advances made in the understanding of this condition, potential pathophysiological mechanisms contributing to orthostatic intolerance, and therapeutic alternatives currently available for the management of these patients.

Review, Tutorial↗
NASA NTRS2004

Fragmentation hierarchy of bright sungrazing comets and the birth and orbital evolution of the Kreutz system. I. Two-superfragment model

A back-and-forth orbit integration technique, developed for our previous investigation of the splitting of the parent of the sungrazers C/1882 R1 and C/1965 S1, is now applied in an effort to understand the history and orbital evolution of the Kreutz sungrazer system, starting with the birth of two subgroups, which show prominently among the bright members and whose inception dates back to the progenitor's breakup into two superfragments. The integration technique is used to reproduce the motion of comet C/1843 D1 - the second brightest sungrazer known and presumably the most massive surviving piece of superfragment I - from the motion of C/1882 R1 - the brightest sungrazer on record and arguably the most massive surviving piece of superfragment II. Running the orbit of C/1882 R1 back to A.D. 326, the progenitor comet is found to have split at a heliocentric distance of 50 AU and nearly 30 yr before perihelion. The superfragments acquired separation velocities of similar to8 m s(-1) in opposite directions. Using the same technique, we show next that (1) the motions of two additional sungrazers, C/1880 C1 and C/1887 B1, are matched extremely well if these objects shared a common parent with C/1843 D1, and (2) C/1963 R1 (Pereyra), the second brightest subgroup I member on record, is more closely related to subgroup II objects (such as C/1882 R1 and C/1965 S1) than to C/1843 D1. This finding raises serious doubts about the major role of the subgroups in the system's orbital history and offers an incentive for considering an alternative dynamical scenario. The fragmentation models for C/1963 R1 and two additional bright sungrazers, C/1945 X1 and C/1970 K1, suggest that (1) these comets may have been the most massive pieces of the fragment populations formed from their respective disintegrating parents, and (2) the course of evolution of the Kreutz system at the upper end of the mass spectrum may be better ascertained from the distribution of the sungrazers' arrival times than from the sources of subgroups. If so, the fragment hierarchy should be determined primarily by the cascading nature of the fragmentation process, which was recently shown by Sekanina to control the evolution of minor fragments as well. The sungrazer system's estimated age is in any case very short, less than 1700 yr.

sungrazing comets↗
NASA NTRS2008

Geologic Mapping of the Martian Impact Crater Tooting

Tooting crater is approximately 29 km in diameters, is located at 23.4 deg N, 207.5 deg E and is classified as a multi-layered ejecta crater. Tooting crater is a very young crater, with an estimated age of 700,000 to 2M years. The crater formed on virtually flat lava flows within Amazonis Planitia where there appears to have been no major topographic features prior to the impact, so that we can measure ejecta thickness and cavity volume. In the past 12 months, the authors have: published their first detailed analysis of the geometry of the crater cavity and the distribution of the ejecta layers; refined the geologic map of the interior of Tooting crater through mapping of the cavity at a scale of 1:1100K; and continued the analysis of an increasing number of high resolution images obtained by the CTX and HiRISE instruments. Currently the authors seek to resolve several science issues that have been identified during this mapping, including: what is the origin of the lobate flows on the NW and SW rims of the crater?; how did the ejecta curtain break apart during the formation of the crater, and how uniform was the emplacement process for the ejecta layers; and, can we infer physical characteristics about the ejecta? Future study plans include the completion of a draft geologic map of Tooting crater and submission of it to the U.S. Geological survey for a preliminary review, publishing a second research paper on the detailed geology of the crater cavity and the distribution of the flows on the crater rim, and completing the map text for the 1:100K geologic map description of units at Tooting crater.

Mouginis-Mark, Peter↗
NASA NTRS2012

Hubble Space Telescope Observations of the HD 202628 Debris Disk

A ring-shaped debris disk around the G2V star HD 202628 (d = 24.4 pc) was imaged in scattered light at visible wavelengths using the coronagraphic mode of the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The ring is inclined by approx.64deg from face-on, based on the apparent major/minor axis ratio, with the major axis aligned along PA = 130deg. It has inner and outer radii (> 50% maximum surface brightness) of 139 AU and 193 AU in the northwest ansae and 161 AU and 223 AU in the southeast ((Delta)r/r approx. = 0.4). The maximum visible radial extent is approx. 254 AU. With a mean surface brightnesses of V approx. = 24 mag arcsec.(sup -2), this is the faintest debris disk observed to date in reflected light. The center of the ring appears offset from the star by approx.28 AU (deprojected). An ellipse fit to the inner edge has an eccentricity of 0.18 and a = 158 AU. This offset, along with the relatively sharp inner edge of the ring, suggests the influence of a planetary-mass companion. There is a strong similarity with the debris ring around Fomalhaut, though HD 202628 is a more mature star with an estimated age of about 2 Gyr. We also provide surface brightness limits for nine other stars in our study with strong Spitzer excesses around which no debris disks were detected in scattered light (HD 377, HD 7590, HD 38858, HD 45184, HD 73350, HD 135599, HD 145229, HD 187897, and HD 201219).

Krist, John E.↗
NASA NTRS2019

KELT-22Ab: A Massive, Short-Period Hot Jupiter Transiting a Near-solar Twin

We present the discovery of KELT-22Ab, a hot Jupiter from the KELT-South survey. KELT-22Ab transits the moderately bright (V11.1) Sun-like G2V star TYC 7518-468-1. The planet has an orbital period of P = 1.3866529 0.0000027 days, a radius of = -R 1.285+ R P 0.071 J 0.12 , and a relatively large mass of = - M 3.47+ M P 0.14 J 0.15 . The star has = - + R 1.099 0.046 R 0.079 , = - + M 1.092 0.041 M 0.045 , = - T 5767+ eff 49 50 K, = - log g 4.393+0.0600.039 (cgs), and [m/H]=+ -0.259+0.0830.085; thus other than its slightly super-solar metallicity, it appears to be a near-solar twin. Surprisingly, KELT-22A exhibits kinematics and a Galactic orbit that are somewhat atypical for thin-disk stars. Nevertheless, the star is rotating rapidly for its estimated age, and shows evidence of chromospheric activity. Imaging reveals a slightly fainter companion to KELT-22A that is likely bound, with a projected separation of 6″ (∼1400 au). In addition to the orbital motion caused by the transiting planet, we detect a possible linear trend in the radial velocity of KELT-22A, suggesting the presence of another relatively nearby body that is perhaps non-stellar. KELT-22Ab is highly irradiated (as a consequence of the small semimajor axis of a R = 4.97), and is mildly inflated. At such small separations, tidal forces become significant. The configuration of this system is optimal for measuring the rate of tidal dissipation within the host star. Our models predict that, due to tidal forces, the semimajor axis is decreasing rapidly, and KELT-22Ab is predicted to spiral into the star within the next Gyr.

Knicole Colon↗
NASA NTRSDate not supplied

Crater Retention Observations of the Crater Floor–Fractured Rough Unit in Jezero Crater

A main goal of the Mars2020 Perseverance Rover mission is to provide a compelling suite of samples for eventual return to Earth [1]. A high priority goal for Mars Sample Return is calibrating Mars’ crater retention isochrons that are used to date geologic surfaces and events across the planet [2]. Several factors are necessary to consider what type of rock would be most useful for this purpose: ability to retain craters across a large exposure, access to drillable outcrop, identifiable stratigraphic context, and understanding why this specific rock type preferentially retains crater morphology. Now that we’ve seen the surface and sampled some of the main constituent outcrops on the Crater Floor – Fractured Rough unit [3, Simon et al, this conference] (Figure 1), we can begin assessing which rocks can help us achieve this important goal to not only understand the age relationships between stratigraphic units in Jezero crater, but also improve age estimates across Mars.

Mars 2020↗
NASA NTRS2023

The Apparent Absence of Forward Scattering in the HD 53143 Debris Disk

HD 53143 is a mature Sun-like star and host to a broad disk of dusty debris, including a cold outer ring of planetesimals near 90 au. Unlike most other inclined debris disks imaged at visible wavelengths, the cold disk around HD 53143 appears as disconnected "arcs" of material, with no forward-scattering side detected to date. We present new, deeper Hubble Space Telescope Imaging Spectrograph coronagraphic observations of the HD 53143 debris disk and show that the forward-scattering side of the disk remains undetected. By fitting our KLIP-reduced observations via forward modeling with an optically thin disk model, we show that fitting the visible wavelength images with an azimuthally symmetric disk with unconstrained orientation results in an unphysical edge-on orientation that is at odds with recent ALMA observations, while constraining the orientation to that observed by ALMA results in nearly isotropically scattering dust. We show that the HD 53143 host star exhibits significant stellar variations due to spot rotation and revisit age estimates for this system.

Christopher C Stark↗
DOE OSTI2015

Optimizing cosmogenic burial isochrons using muon-inclusive simple burial dating

Isochron burial dating with cosmogenic nuclides presents new opportunities for determining the deposition age of Plio-Pleistocene gravels. The method uses paired measurements of 26 Al and 10 Be from individual clasts and is able to generate ages even if burial histories are complex and poorly constrained. Critical to the isochron method is the assumption that gravel is deposited with a predictable 26 Al/ 10 Be ratio, based on near-surface production rates, that then decreases due to radioactive decay after burial. If all clasts are collected from the same depth in a deposit, they share the same history of post-burial production and decay. Because decay constants are known, a line regressed through a bivariate plot of 26 Al and 10 Be concentrations represents an isochron, the slope of which is related to burial duration. The method is ideal when samples are numerous (≥ 5 data points) and yield a spread of concentrations such that linear regression can resolve a meaningful slope, but logistics, costs, and site geology can preclude such ideal datasets. Additionally, it can be difficult to identify and reject samples that may have been deposited with nuclide ratios resulting from prior burial that have the potential to contaminate isochron age estimates. Here we present a methodology for checking the integrity of small isochron datasets (≤4 samples) by co-applying a muon-inclusive simple burial dating algorithm that scales muogenic production with shielding mass (cover density * depth). This analysis uses the same 26 Al and 10 Be concentrations as the isochron method, but incorporates assumptions based on site geology to independently validate and inform isochron results. We use two examples to demonstrate how these burial calculations can increase confidence in isochron results, show when the isochron method is inappropriately applied, and provide criteria for rejection of data points in isochrons.

58 GEOSCIENCES↗
DOE OSTI2021

A field guide for aging passerine nestlings using growth data and predictive modeling

Abstract Background Accurate nestling age is valuable for studies on nesting strategies, productivity, and impacts on reproductive success. Most aging guides consist of descriptions and photographs that are time consuming to read and subjective to interpret. The Western Bluebird ( Sialia mexicana ) is a secondary cavity-nesting passerine that nests in coniferous and open deciduous forests. Nest box programs for cavity-nesting species have provided suitable nesting locations and opportunities for data collection on nestling growth and development. Methods We developed models for predicting the age of Western Bluebird nestlings from morphometric measurements using model training and validation. These were developed for mass, tarsus, and two different culmen measurements. Results Our models were accurate to within less than a day, and each model worked best for a specific age range. The mass and tarsus models can be used to estimate the ages of Western Bluebird nestlings 0–10 days old and were accurate to within 0.5 days for mass and 0.7 days for tarsus. The culmen models can be used to estimate ages of nestlings 0–15 days old and were also accurate to within less than a day. The daily mean, minimum, and maximum values of each morphometric measurement are provided and can be used in the field for accurate nestling age estimations in real time. Conclusions The model training and validation procedures used here demonstrate that this method can create aging models that are highly accurate. The methods can be applied to any passerine species provided sufficient nestling morphometric data are available.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1999

Secondary Forest Age and Tropical Forest Biomass Estimation Using TM

The age of secondary forests in the Amazon will become more critical with respect to the estimation of biomass and carbon budgets as tropical forest conversion continues. Multitemporal Thematic Mapper data were used to develop land cover histories for a 33,000 Square kM area near Ariquemes, Rondonia over a 7 year period from 1989-1995. The age of the secondary forest, a surrogate for the amount of biomass (or carbon) stored above-ground, was found to be unimportant in terms of biomass budget error rates in a forested TM scene which had undergone a 20% conversion to nonforest/agricultural cover types. In such a situation, the 80% of the scene still covered by primary forest accounted for over 98% of the scene biomass. The difference between secondary forest biomass estimates developed with and without age information were inconsequential relative to the estimate of biomass for the entire scene. However, in futuristic scenarios where all of the primary forest has been converted to agriculture and secondary forest (55% and 42% respectively), the ability to age secondary forest becomes critical. Depending on biomass accumulation rate assumptions, scene biomass budget errors on the order of -10% to +30% are likely if the age of the secondary forests are not taken into account. Single-date TM imagery cannot be used to accurately age secondary forests into single-year classes. A neural network utilizing TM band 2 and three TM spectral-texture measures (bands 3 and 5) predicted secondary forest age over a range of 0-7 years with an RMSE of 1.59 years and an R(Squared) (sub actual vs predicted) = 0.37. A proposal is made, based on a literature review, to use satellite imagery to identify general secondary forest age groups which, within group, exhibit relatively constant biomass accumulation rates.

Nelson, R. F.↗
NASA NTRS1974

A method for estimating the absolute ages of small Copernican craters and its application to the determination of Copernican meteorite flux

Cosmic ray exposure ages of ejecta from Copernican craters in various stages of erosion can be used to construct a curve from which the time of formation of craters in the Copernican System can be estimated on the basis of their morphology. Use of this curve to estimate the flux for particles that formed 75-1000 m craters during six increments of Copernican time in an area of Mare Serenitatis suggests that the flux has steadily increased throughout Copernican time. These results are considered to be preliminary until more work of this kind has been done for other mare areas of the moon.

Swann, G. A.↗
DOE OSTI2017

The Next Generation Virgo Cluster Survey (NGVS). XXVI. The Issues of Photometric Age and Metallicity Estimates for Globular Clusters

Large samples of globular clusters (GC) with precise multi-wavelength photometry are becoming increasingly available and can be used to constrain the formation history of galaxies. We present the results of an analysis of Milky Way (MW) and Virgo core GCs based on 5 optical-near-infrared colors and 10 synthetic stellar population models. For the MW GCs, the models tend to agree on photometric ages and metallicities, with values similar to those obtained with previous studies. When used with Virgo core GCs, for which photometry is provided by the Next Generation Virgo cluster Survey (NGVS), the same models generically return younger ages. This is a consequence of the systematic differences observed between the locus occupied by Virgo core GCs and models in panchromatic color space. Only extreme fine-tuning of the adjustable parameters available to us can make the majority of the best-fit ages old. Although we cannot exclude that the formation history of the Virgo core may lead to more conspicuous populations of relatively young GCs than in other environments, we emphasize that the intrinsic properties of the Virgo GCs are likely to differ systematically from those assumed in the models. Thus, the large wavelength coverage and photometric quality of modern GC samples, such as those used here, is not by itself sufficient to better constrain the GC formation histories. Models matching the environment-dependent characteristics of GCs in multi-dimensional color space are needed to improve the situation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

DETERMINING AGES OF APOGEE GIANTS WITH KNOWN DISTANCES

We present a sample of 705 local giant stars observed using the New Mexico State University 1 m telescope with the Sloan Digital Sky Survey-III/Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectrograph, for which we estimate stellar ages and the local star formation history (SFH). The high-resolution (R ∼ 22,500), near infrared (1.51–1.7 μm) APOGEE spectra provide measurements of stellar atmospheric parameters (temperature, surface gravity, [M/H], and [α/M]). Due to the smaller uncertainties in surface gravity possible with high-resolution spectra and accurate Hipparcos distance measurements, we are able to calculate the stellar masses to within 30%. For giants, the relatively rapid evolution up the red giant branch allows the age to be constrained by the mass. We examine methods of estimating age using both the mass–age relation directly and a Bayesian isochrone matching of measured parameters, assuming a constant SFH. To improve the SFH prior, we use a hierarchical modeling approach to constrain the parameters of the model SFH using the age probability distribution functions of the data. The results of an α-dependent Gaussian SFH model show a clear age–[α/M] relation at all ages. Using this SFH model as the prior for an empirical Bayesian analysis, we determine ages for individual stars. The resulting age–metallicity relation is flat, with a slight decrease in [M/H] at the oldest ages and a ∼0.5 dex spread in metallicity across most ages. For stars with ages ≲1 Gyr we find a smaller spread, consistent with radial migration having a smaller effect on these young stars than on the older stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AGES OF STAR CLUSTERS IN THE TIDAL TAILS OF MERGING GALAXIES

We study the stellar content in the tidal tails of three nearby merging galaxies, NGC 520, NGC 2623, and NGC 3256, using BVI imaging taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. The tidal tails in all three systems contain compact and fairly massive young star clusters, embedded in a sea of diffuse, unresolved stellar light. We compare the measured colors and luminosities with predictions from population synthesis models to estimate cluster ages and find that clusters began forming in tidal tails during or shortly after the formation of the tails themselves. We find a lack of very young clusters (≤10 Myr old), implying that eventually star formation shuts off in the tails as the gas is used up or dispersed. There are a few clusters in each tail with estimated ages that are older than the modeled tails themselves, suggesting that these may have been stripped out from the original galaxy disks. The luminosity function of the tail clusters can be described by a single power-law, dN/dL ∝ L{sup α}, with −2.6 < α < −2.0. We find a stellar age gradient across some of the tidal tails, which we interpret as a superposition of (1) newly formed stars and clusters along the dense center of the tail and (2) a sea of broadly distributed, older stellar material ejected from the progenitor galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗