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All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

THE LU ISOTOPIC COMPOSITION OF ACHONDRITES: CLOSING THE CASE FOR ACCELERATED DECAY OF {sup 176}LU

Studies of Lu–Hf isotope systematics in meteorites have produced apparent “ages” that are older than Pb–Pb ages and older than the estimated age of our solar system. One proposed explanation for this discrepancy is that irradiation by cosmic rays caused excitation of {sup 176}Lu to its short-lived isomer that then underwent rapid decay to {sup 176}Hf. This explanation can account for apparent excesses in {sup 176}Hf that correlate with Lu/Hf ratio. Mass balance requires that samples with measurable excess in {sup 176}Hf should also have measurable deficiencies in {sup 176}Lu on the order of 1‰–3‰. To unambiguously test the accelerated decay hypothesis, we have measured the {sup 176}Lu/{sup 175}Lu ratio in terrestrial materials and achondrites to search for evidence of depletion in {sup 176}Lu. To a precision of 0.1‰ terrestrial standards, cumulate and basaltic eucrites and angrites all have the same {sup 176}Lu/{sup 175}Lu ratio. Barring a subsequent mass-dependent fractionation event, these results suggest that the apparent excesses in {sup 176}Hf are not caused by accelerated decay of {sup 176}Lu, and so another hypothesis is required to explain apparently old Lu–Hf ages.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Chromospherically active stars in the RAVE survey. II. Young dwarfs in the solar neighborhood

A large sample of over 38,000 chromospherically active candidate solar-like stars and cooler dwarfs from the RAVE survey is addressed in this paper. An improved activity identification with respect to the previous study was introduced to build a catalog of field stars in the solar neighborhood with an excess emission flux in the calcium infrared triplet wavelength region. The central result of this work is the calibration of the age–activity relation for main-sequence dwarfs in a range from a few 10 Myr up to a few Gyr. It enabled an order of magnitude age estimation of the entire active sample. Almost 15,000 stars are shown to be younger than 1 Gyr and ∼2000 younger than 100 Myr. The young age of the most active stars is confirmed by their position off the main sequence in the J − K versus N{sub UV}−V diagram showing strong ultraviolet excess, mid-infrared excess in the J − K versus W{sub 1}−W{sub 2} diagram, and very cool temperatures (J−K>0.7). They overlap with the reference pre-main-sequence RAVE stars often displaying X-ray emission. The activity level increasing with the color reveals their different nature from the solar-like stars and probably represents an underlying dynamo-generating magnetic fields in cool stars. Of the RAVE objects from DR5, 50% are found in the TGAS catalog and supplemented with accurate parallaxes and proper motions by Gaia. This makes the database of a large number of young stars in a combination with RAVE’s radial velocities directly useful as a tracer of the very recent large-scale star formation history in the solar neighborhood. The data are available online in the Vizier database.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE UV/BLUE EFFECTS OF SPACE WEATHERING MANIFESTED IN S-COMPLEX ASTEROIDS. I. QUANTIFYING CHANGE WITH ASTEROID AGE

Evidence for the manifestation of space weathering in S-complex asteroids as a bluing of the UV/blue reflectance spectrum is extended using high resolution CCD reflectance spectra of 21 main-belt, 1 Mars-crossing, and 3 near-Earth asteroids covering a wavelength range of 320–620 nm. Demonstration of the transition of iron-bearing materials from volume scattering to surface (Fresnel) scattering is apparent as an abrupt downturn at wavelengths just short of 400 nm in reflectance spectra of fresh asteroid surfaces. The weathering away of this downturn is demonstrated by its absence in reflectance spectra of mature S-complex asteroids, consistent with an increase in npFe{sup 0} on the material's surface. Modeling of the effects of the addition of small amounts of npFe{sup 0} to particles from both a hypothetical mineral and a terrestrial basalt shows that evidence of the addition of 0.0001% npFe{sup 0} affects the reflectance at UV/blue wavelengths, while the addition of 0.01% is required to see the visible/near-infrared reddening and diminution of absorption features. Thus, the UV/blue reflectance characteristics allow earlier detection of the onset of space weathering effects. Combining UV/blue spectral characteristics of asteroids and ordinary chondrite meteorites with estimated ages of the young Datura family, we establish a method of dating asteroid surface ages during the early stages of space weathering. We demonstrate by dating the surface of NEA 163249 2002 GT to be 109 (±18) to 128 (±10) Kyr.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

LOW-METALLICITY YOUNG CLUSTERS IN THE OUTER GALAXY. I. Sh 2-207

To study star formation in low-metallicity environments ([M/H] ∼ −1 dex), we obtained deep near-infrared (NIR) images of Sh 2-207 (S207), which is an H ii region in the outer Galaxy with a spectroscopically determined metallicity of [O/H] ≃ −0.8 dex. We identified a young cluster in the western region of S207 with a limiting magnitude of K{sub S} = 19.0 mag (10σ) that corresponds to a mass detection limit of ≲0.1 M{sub ⊙} and enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. From the fitting of the K-band luminosity function (KLF), the age and distance of the S207 cluster are estimated at 2–3 Myr and ∼4 kpc, respectively. The estimated age is consistent with the suggestion of small extinctions of stars in the cluster (A{sub V} ∼ 3 mag) and the non-detection of molecular clouds. The reasonably good fit between the observed KLF and the model KLF suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit is not significantly different from the typical IMFs in the solar metallicity. From the fraction of stars with NIR excesses, a low disk fraction (<10%) in the cluster with a relatively young age is suggested, as we had previously proposed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE

We present new maps of the Milky Way disk showing the distribution of metallicity ([Fe/H]), α-element abundances ([Mg/Fe]), and stellar age, using a sample of 66,496 red giant stars from the final data release (DR17) of the Apache Point Observatory Galactic Evolution Experiment survey. We measure radial and vertical gradients, quantify the distribution functions for age and metallicity, and explore chemical clock relations across the Milky Way for the low-α disk, high-α disk, and total population independently. The low-α disk exhibits a negative radial metallicity gradient of -0.06 ± 0.001 dex kpc -1 , which flattens with distance from the midplane. The high-α disk shows a flat radial gradient in metallicity and age across nearly all locations of the disk. The age and metallicity distribution functions shift from negatively skewed in the inner Galaxy to positively skewed at large radius. Significant bimodality in the [Mg/Fe]–[Fe/H] plane and in the [Mg/Fe]–age relation persist across the entire disk. The age estimates have typical uncertainties of ~0.15 in log(age) and may be subject to additional systematic errors, which impose limitations on conclusions drawn from this sample. Nevertheless, these results act as critical constraints on galactic evolution models, constraining which physical processes played a dominant role in the formation of the Milky Way disk. We discuss how radial migration predicts many of the observed trends near the solar neighborhood and in the outer disk, but an additional more dramatic evolution history, such as the multi-infall model or a merger event, is needed to explain the chemical and age bimodality elsewhere in the Galaxy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The Age of Evapotranspiration: Lower‐Bound Constraints From Distributed Water Fluxes Across the Continental United States

Abstract Unlike streamflow, which can be sampled in aggregate at the catchment outlet, evapotranspiration (ET) is spatially dispersed, challenging large‐scale age estimation. Here, we introduce an approach for constraining the age of ET via mass balance and present the minimum flux‐weighted age of ET across the continental United States using distributed, publicly available water flux data sets. The lower‐bound constraint on ET age can be calculated by assuming that ET is preferentially sourced from the most recent precipitation through a last‐in, first‐out algorithm. From 2012 to 2017, ET was at least several months old across large areas of the western continental United States, including in Mediterranean and (semi‐)arid climate zones and shrub and evergreen needleleaf plant communities. The primary limitation of this approach is that it provides only a minimum flux‐weighted average age to satisfy the mass balance of outgoing fluxes; true ET fluxes are composed of distributions of ages and may be composed of much older water. The primary advantage of the approach is that flux time series of precipitation and ET are sufficient to constrain ET age, and model parameterization is unnecessary. ET ages can be used to validate tracer‐aided and modeling approaches and inform studies of biogeochemistry, water‐rock interactions, and plant water sourcing under drought.

Hahm, W. J.↗
NASA NTRS2008

Martian Chronology and Atmospheric Composition: In Situ Measurements versus Sample Return

I examine two significant issues of martian science from the point of view of in situ measurements by robotic spacecraft versus sample return and analysis in terrestrial labs. (1) To define martian history, ages of geological processes and surface features are required. Estimated ages from surface crater densities have limitations, and the ages measured for martian meteorites cannot be associated with specific martian locales. Whereas returned martian rocks could be accurately dated, some have suggested sending a robotic spacecraft to Mars to measure rock ages using the classical K- Ar-40 technique, considered the easiest to implement. (2) To understand the evolution of the martian atmosphere and its interactions with the surface, requires precise measurements of atmospheric composition. A significant amount of information has derived from measurements by Viking and of martian meteorites. Instrumentation on the Mars Science Lander (MSL) spacecraft to be launched in the near future promises to determine atmospheric composition even more precisely. If MSL is successful, which questions about atmospheric composition will remain and thus will require atmospheric sample return to answer?

Bogard, Donald D.↗
DOE OSTI2017

RAVE J203843.2–002333: The First Highly R-process-enhanced Star Identified in the RAVE Survey

We report the discovery of RAVE J203843.2−002333, a bright (V = 12.73), very metal-poor ([Fe/H] = −2.91), r-process-enhanced ([Eu/Fe] = +1.64 and [Ba/Eu] = −0.81) star selected from the RAVE survey. This star was identified as a metal-poor candidate based on its medium-resolution (R ∼ 1600) spectrum obtained with the KPNO/Mayall Telescope, and followed up with high-resolution (R ∼ 66,000) spectroscopy with the Magellan/Clay Telescope, allowing for the determination of elemental abundances for 24 neutron-capture elements, including thorium and uranium. RAVE J2038−0023 is only the fourth metal-poor star with a clearly measured U abundance. The derived chemical abundance pattern exhibits good agreement with those of other known highly r-process-enhanced stars, and evidence suggests that it is not an actinide-boost star. Age estimates were calculated using U/X abundance ratios, yielding a mean age of 13.0 ± 1.1 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

IRAS studies of galactic supershells

Using IRAS Skyflux images and a new catalog of OB stars in the Cygnus region, a complete infrared supershell surrounding the Cyg OB1 Association was identified. This supershell is seen as a conspicuous, well-defined 2 deg x 5 deg region deficient of IR emission, with a limb-brightened edge and dimensions of about 50 x 130 pc at 1.5 kpc. The shell's elongated morphology is consistent with OB-star subclustering over the approximately 10(exp 6) yr age of the bubble. With a parent star cluster still visible (10 O stars between 25 and 45 solar mass, 3 - 4 Wolf-Rayet stars, and the possibility of 3 - 5 more massive stars that died as supernovae) the Cyg OB1 supershell is an excellent object for studying the formation and evolution of Galactic supershells. A discrepancy between the less than or equal to 1 Myr bubble age estimated from its size and the 5 Myr cluster turnoff age (45 solar mass) may require non-coeval massive star formation to explain the number of post-main-sequence stars and limit the number of past supernovae.

Saken, Jon M.↗
NASA NTRS2005

Discovery of a 105-ms X-ray Pulsar in Kesteven-79: On the Nature of Compact Central Objects in Supernova Remnants

We report the discovery of 105-ms X-ray pulsations from the compact central object (CCO) in the supernova remnant \snr\ using data acquired with the {\it Newton X-Ray Multi-Mirror Mission). Using two observations of the pulsar taken 6-days apart we derive an upper limit on its spin-down rate of $\dot P < 9 \times 10"{-14}$-s-~$~{-l)$,a nd find no evidence for binary orbital motion. The implied energy loss rate is $\dot E < 3 \times 10A{36)$-ergs-s$A{-1)$, polar magnetic field strength is $B-{\rm p) < 3 \times 10A{12)$-G, and spin-down age is $\tau > 18.5$-kyr. The latter exceeds the remnant's estimated age, suggesting that the pulsar was born spinning near its current period. The X-ray spectrum of \psr\ is best characterized as a blackbody of temperature $kT {BB) =, 0.43\pm0.02$ keV, radius $R-{BB) \approx 1.3$-km, and $~I{\rm bol) = 5.2 \times 10A{33)$ ergs-sSA{-1)$ at $d = 7.1$-kpc. The sinusoidal light curve is modulated with a pulsed fraction of $>45\%$, suggestive of a small hot spot on the surface of the rotating neutron star. The lack of a discernible pulsar wind nebula is consistent with an interpretation of \psr\ as a rotation-powered pulsar whose spin-down luminosity falls below the empirical threshold for generating bright wind nebulae, $\dot E-{\rm c) = 4 \times 10A{36)$-ergs-sSA{-I)$. The age discrepancy suggests that its $\dot E$ has always been below $\dot E c$, perhaps a distinguishing property of the CCOs. Alternatively, the X-ray spectrum of \psr\ suggests a low-luminosity AXP, but the weak inferred $B-{\rm p)$ field is incompatible with a magnetar theory of its X-ray luminosity. The ordinary spin parameters discovered from \psr\ highlight the inability of existing theories to explain the high luminosities and temperatures of CCO thermal X-ray spectra.

Gotthelf, E. V.↗
DOE OSTI2024

Stellar Mergers or Truly Young? Intermediate-age Stars on Highly Radial Orbits in the Milky Way’s Stellar Halo

Reconstructing the mass assembly history of the Milky Way relies on obtaining detailed measurements of the properties of many stars in the galaxy, especially in the stellar halo. One of the most constraining quantities is stellar age, as it can shed light on the accretion time and quenching of star formation in merging satellites. However, obtaining reliable age estimates for large samples of halo stars is difficult. We report published ages of 120 subgiant halo stars with highly radial orbits that likely belong to the debris of the Gaia-Enceladus/Sausage (GES) galaxy. The majority of these halo stars are old, with an age distribution characterized by a median of 11.6 Gyr and a 16th (84th) percentile of 10.5 (12.7) Gyr. However, the distribution is skewed, with a tail of younger stars that span ages down to ~6–9 Gyr. All highly radial halo stars have chemical and kinematic/orbital quantities that associate them with the GES debris. Initial results suggest that these intermediate-age stars are not a product of mass transfer and/or stellar mergers, which can bias their age determination low. If this conclusion is upheld by upcoming spectrophotometric studies, then the presence of these stars will pose an important challenge for constraining the properties of the GES merger and the accretion history of the galaxy.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1973

Helium-uranium dating of corals.

Fanale and Schaeffer (1965) determined He/U ages of corals and other aragonitic fossils, and found that the results generally agreed with independent age estimates. As a more extensive and rigorous test of the reliability of He/U ages of fossil corals, I have determined He/U ages of forty-five independently dated Cenozoic corals. Uranium and thorium isotope compositions and Rn-222 retentivities were also determined as consistency checks. The results indicate that reliable ages are obtained when certain consistency tests are met and specified corrections are made.

Bender, M. L.↗
NASA NTRS1978

Martian cratering V - Toward an empirical Martian chronology, and its implications

This paper estimates ages of Martian features by dividing least-square-fitted crater densities by an estimated Martian crater production rate. This method is more rigorous than methods based purely on assumptions about correspondence of lunar and Martian cratering rates. Results are interpreted as supporting the conclusion of Burns et al (in press) that Mars shifted climatic states due to a change in obliguity caused by buildup of massive volcanics. Prior to a few gy. ago, conditions favored erosion, deposition, and fluvial channel formation. Recent volcanics postdate this era, and the surfaces of the major shields are fairly well constrained in age to a few hundred my, in agreement with Masursky et al.

Hartmann, W. K.↗
NASA NTRS2002

SNC Meteorites and Martian Reservoirs

Jones first suggested that the inverse covariation of initial epsilon (Nd-143) and Sr-87/Sr-86 of the shergottites could be explained by interaction between mantle-derived magmas with another isotopic reservoir(s) (i.e., assimilation or contamination). In that model, magmas were generated in a source region that was isotopically similar to the Nakhla source and the second reservoir(s) was presumed to be crust. The text also permitted the second reservoir to be another type of mantle, but I can confirm that a second mantle reservoir was never seriously considered by that author. Other features of this model were that (i) it occurred at a particular time, 180 m.y. ago, and (ii) the interacting reservoirs had been separated at approximately 4.5 b.y. In a later paper Jones explored this mixing model more quantitatively and concluded that magmas from a Nakhla-like source region at 180 m.y. would fall on or near an isotopic Nd-Sr-Pb hyperplane defined by the shergottites. This criterion was a necessary prerequisite for the parent magma(s) of the shergottites to have initially been Nakhla-like isotopically. At this juncture, it is perhaps worthwhile to note that this mixing model was not presented to explain geochemical variations but as a justification for a 180 m.y. crystallization age for the shergottites and a 1.3 b.y. crystallization age for the nakhlites. In the mid-1980's crystallization ages estimated for Nakhla ranged from approximately 1.3 b.y to 4.5 b.y. Similarly, preferred crystallization ages for the shergottites ranged from 360 m.y., to 1.3 b.y., to 4.5 b.y. In all these models, the 180 m.y. event seen in the shergottites was deemed to be metamorphic. The fit between the Nakhla-like source region and the shergottite hyperplane was a validation both of the 1.3 b.y. igneous age of Nakhla and the 180 m.y. igneous age of the shergottites.

Jones, J. H.↗
DOE OSTI2020

A genome assembly and the somatic genetic and epigenetic mutation rate in a wild long-lived perennial Populus trichocarpa

Abstract Background Plants can transmit somatic mutations and epimutations to offspring, which in turn can affect fitness. Knowledge of the rate at which these variations arise is necessary to understand how plant development contributes to local adaption in an ecoevolutionary context, particularly in long-lived perennials. Results Here, we generate a new high-quality reference genome from the oldest branch of a wild Populus trichocarpa tree with two dominant stems which have been evolving independently for 330 years. By sampling multiple, age-estimated branches of this tree, we use a multi-omics approach to quantify age-related somatic changes at the genetic, epigenetic, and transcriptional level. We show that the per-year somatic mutation and epimutation rates are lower than in annuals and that transcriptional variation is mainly independent of age divergence and cytosine methylation. Furthermore, a detailed analysis of the somatic epimutation spectrum indicates that transgenerationally heritable epimutations originate mainly from DNA methylation maintenance errors during mitotic rather than during meiotic cell divisions. Conclusion Taken together, our study provides unprecedented insights into the origin of nucleotide and functional variation in a long-lived perennial plant.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

CO-Dark Star Formation and Black Hole Activity in 3C 368 at z = 1.131: Coeval Growth of Stellar and Supermassive Black Hole Masses

We present the detection of four far-infrared fine-structure oxygen lines, as well as strong upper limits for the CO(2–1) and [N ii] 205 μ m lines, in 3C 368, a well-studied radio-loud galaxy at z = 1.131. These new oxygen lines, taken in conjunction with previously observed neon and carbon fine-structure lines, suggest a powerful active galactic nucleus (AGN), accompanied by vigorous and extended star formation. A starburst dominated by O8 stars, with an age of ∼6.5 Myr, provides a good fit to the fine-structure line data. This estimated age of the starburst makes it nearly concurrent with the latest episode of AGN activity, suggesting a link between the growth of the supermassive black hole and stellar population in this source. We do not detect the CO(2–1) line, down to a level twelve times lower than the expected value for star-forming galaxies. This lack of CO line emission is consistent with recent star formation activity if the star-forming molecular gas has low metallicity, is highly fractionated (such that CO is photodissociated throughout much of the clouds), or is chemically very young (such that CO has not yet had time to form). It is also possible, although we argue it is unlikely, that the ensemble of fine-structure lines is emitted from the region heated by the AGN.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2019

KELT-24b: A 5M(J) Planet on a 5.6 day Well-aligned Orbit around the Young V=8.3 F-star HD 93148

We present the discovery of KELT-24 b, a massive hot Jupiter orbiting a bright (V=8.3 mag, K=7.2 mag) young F-star with a period of 5.6 days. The host star, KELT-24 (HD 93148), has a Teff=-+65094950K, a mass of M*=+1.4600.0590.055Me, a radius of R*=1.506±0.022Re, and an age of +0.780.420.61Gyr. Its planetary companion (KELT-24 b) has a radius of RP=1.272±0.021RJ and a mass of MP=-+5.180.220.21MJ, and from Doppler tomographic observations, we find that the planet’s orbit is well aligned to its host star’s projected spin axis (l=-+2.63.65.1). The young age estimated for KELT-24 suggests that it only recently started to evolve from the zero-age main sequence. KELT-24 is the brightest star known to host a transiting giant planet with a period between 5 and 10 days. Although the circularization timescale is much longer than the age of the system, we do not detect a large eccentricity or significant misalignment that is expected from dynamical migration. The brightness of its host star and its moderate surface gravity make KELT-24b an intriguing target for detailed atmospheric characterization through spectroscopic emission measurements since it would bridge the current literature results that have primarily focused on lower mass hot Jupiters and a few brown dwarfs.

Transit photometry↗
NASA NTRS1977

Sm-Nd-Pu timepieces in the Angra dos Reis meteorite

Isotope ratios and age estimations are presented for the ultramafic pyroxenite Angra dos Reis meteorite. A pyroxene-phosphate internal isochron age of 4.55 plus or minus 0.04 eons was obtained, in agreement with reported Pb-Pb ages. It is suggested that Sm-146 decay led to an increase in the ratio of Nd-142 to Nd-144 in pyroxene, which is greater than the ratio of the Nd isotopes in phosphates. The effect of Pu-244 on the fission xenon components in pyroxene and phosphate is examined. The ratios of Pu-244 to Nd, U-238, and Th-232 are compared for pyroxene and phosphate separates. The exposure age of the meteorite, as obtained by the Kr-81 to Kr-83 method, was found to be 55.5 plus or minus 1.2 million years.

Lugmair, G. W.↗