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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Dynamical heating across the Milky Way disc using APOGEE and Gaia

Abstract The kinematics of the Milky Way disc as a function of age are well measured at the solar radius, but have not been studied over a wider range of Galactocentric radii. Here, we measure the kinematics of mono-age, mono-[Fe/H] populations in the low and high [α/Fe] discs between 4 ≲ R ≲ 13 kpc and |z| ≲ 2 kpc using 65 719 stars in common between APOGEE DR14 and Gaia DR2 for which we estimate ages using a Bayesian neural network model trained on asteroseismic ages. We determine the vertical and radial velocity dispersions, finding that the low and high [α/Fe] discs display markedly different age–velocity dispersion relations (AVRs) and shapes σz/σR. The high [α/Fe] disc has roughly flat AVRs and constant σz/σR = 0.64 ± 0.04, whereas the low [α/Fe] disc has large variations in this ratio that positively correlate with the mean orbital radius of the population at fixed age. The high [α/Fe] disc component’s flat AVRs and constant σz/σR clearly indicate an entirely different heating history. Outer disc populations also have flatter radial AVRs than those in the inner disc, likely due to the waning effect of spiral arms. Our detailed measurements of AVRs and σz/σR across the disc indicate that low [α/Fe], inner disc ($R \lesssim 10\, \mathrm{kpc}$) stellar populations are likely dynamically heated by both giant molecular clouds and spiral arms, while the observed trends for outer disc populations require a significant contribution from another heating mechanism such as satellite perturbations. We also find that outer disc populations have slightly positive mean vertical and radial velocities likely because they are part of the warped disc.

Mackereth, J. Ted↗
DOE OSTI2013

THE NATURE OF THE SECOND PARAMETER IN THE IRX-{beta} RELATION FOR LOCAL GALAXIES

We present an analysis of 98 galaxies of low-dust content, selected from the Spitzer Local Volume Legacy survey, aimed at examining the relation between the ultraviolet (UV) color and dust attenuation in normal star-forming galaxies. The IRX-{beta} diagram relates the total dust attenuation in a galaxy, traced by the far-IR (FIR) to UV ratio, to the observed UV color, indicated by {beta}. Previous research has indicated that while starburst galaxies exhibit a relatively tight IRX-{beta} relation, normal star-forming galaxies do not, and have a much larger spread in the total-IR to far-UV (FUV) luminosity for a fixed UV color. We examine the role that the age of the stellar population plays as the ''second parameter'' responsible for the observed deviation and spread of star-forming galaxies from the starburst relation. We model the FUV to FIR spectral energy distribution of each galaxy according to two broad bins of star formation history (SFH): constant and instantaneous burst. We find clear trends between stellar population mean age estimators (extinction-corrected FUV/NIR, U - B, and EW(H{alpha})) and the UV color {beta}; the trends are mostly driven by the galaxies best-described by instantaneous burst populations. We also find a significant correlation between {beta} and the mean age directly determined from the best-fit instantaneous models. As already indicated by other authors, the UV attenuation in star-forming galaxies may not be recovered with the UV color alone and is highly influenced by the stellar population's mean age and SFH. Overall, the scatter in the IRX-{beta} diagram is better correlated with {beta} than with the perpendicular distance, d{sub p}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FREQUENCY OF MAUNDER MINIMUM EVENTS IN SOLAR-TYPE STARS INFERRED FROM ACTIVITY AND METALLICITY OBSERVATIONS

We consider the common proposition that the fraction of chromospherically very inactive stars in a solar-type sample is analogous to the fraction of the Sun's main-sequence lifetime spent in a grand minimum state. In a new approach to this proposition, we examine chromospheric activity log R'{sub HK} in a stellar sample having Hipparcos parallax measurements, and having spectroscopically determined metallicity close to solar (-0.1 {<=} [Fe/H] {<=} 0.1). We evaluate height above the Hipparcos main sequence, and estimate age using isochrones, to identify the most Sun-like stars in this sample. As a threshold below which a star is labeled very inactive, we use the peak of the HK activity distribution mapped over the quiet Sun during the 1968 epoch. We estimate the fraction of Maunder Minimum (MM) analog candidates in our sample at 11.1%. Given the 70 yr duration of the historical MM, this suggests that in any given year there is a 1/630 chance of entering a similar grand minimum. There are three important cautions with this type of estimate. First, recent investigation using actual activity and photometric time series has suggested that very low activity may not be a necessary criterion for identifying a non-cycling MM analog candidate. Second, this type of estimate depends very strongly on the choice of very low activity threshold. Third, in instantaneous measurements of log R'{sub HK}, it is not always clear whether a star is a viable MM analog candidate or merely an older star nearing the end of its main-sequence lifetime.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

The older plant gets the sun: Age-related changes in Miscanthus × giganteus phenology

Age-related changes are usually overlooked in perennial grass research; when they are considered it is usually as a change in plant size (e.g., biomass). Whether other physiological or developmental aspects change as stands age, and how those aspects may impact long-term stand dynamics, remains unclear. Conventional experimental designs study a single stand over multiple growing seasons and thereby confound age-related changes with growing season conditions. Here we used a staggered-start experimental design with three repeated planting years over two growing seasons to isolate growing season effects. We studied changes in Miscanthus × giganteus phenology during its yield-building stage (first 3 years) and estimated age, growing season and nitrogen (N) effects on development using nonlinear regression parameters. Stand age clearly changed plant growth; faster developmental rates were usually seen in 1-year-old stands (young), but because 2- and 3-year-old stands (mature) emerged 3 months earlier than newly planted stands they produced ~30% more stems with 30%–60% more leaves. Nitrogen fertilization modulated some age-related phenological changes. Fertilized 2-year-old stands reached similar stem densities as unfertilized 3-year-old stands and had fewer number of senesced leaves like 1-year-old stands. In addition, N fertilization had no effect on young M. × giganteus , but extended mature stands’ growing season more than 2 weeks by hastening emergence and delaying senescence. It also delayed flowering regardless of stand age. Our results suggest that, along with changes in size, M. × giganteus stands showed shifts in developmental strategies: young stands emerged later and developed faster, while mature stands grew for longer but more slowly. In temperate regions, where hard frost events are likely to interrupt development in late autumn, rapid early development is critical to plant survival. Nonlinear regression parameter differences proved effective in identifying phenological shifts.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Gems in the outer galaxy: Near-infrared imaging of three young clusters at large galactic radii

Images recorded with the Gemini South Adaptive Optics Imager (GSAOI) and corrected for atmospheric seeing by the Gemini Multi-conjugate Adaptive Optics System are used to investigate the stellar contents of the young outer Galactic disk clusters Haffner 17, NGC 2401, and NGC 3105. Ages estimated from the faint end of the main sequence (MS) and the ridgeline of the pre-main sequence on the (K, J – K) color-magnitude diagrams are consistent with published values that are based on the MS turnoff, with the GSAOI data favoring the younger end of the age range for NGC 2401 in the literature. The mass function (MF) of NGC 2401 is similar to that in the solar neighborhood, and stars spanning a wide range of masses in this cluster have similar clustering properties on the sky. It is concluded that NGC 2401 is not evolved dynamically. In contrast, the MF of Haffner 17 differs significantly from that in the solar neighborhood over all masses covered by these data, while the MF of NGC 3105 is deficient in objects with sub-solar masses when compared with the solar neighborhood. Low-mass objects in Haffner 17 and NGC 3105 are also more uniformly distributed on the sky than brighter, more massive, MS stars. This is consistent with both clusters having experienced significant dynamical evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Detection of Interstellar Ortho-D{sub 2}H{sup +} with SOFIA

We report on the detection of the ground-state rotational line of ortho-D{sub 2}H{sup +} at 1.477 THz (203 μm) using the German REceiver for Astronomy at Terahertz frequencies (GREAT) on board the Stratospheric Observatory For Infrared Astronomy (SOFIA). The line is seen in absorption against far-infrared continuum from the protostellar binary IRAS 16293-2422 in Ophiuchus. The para-D{sub 2}H{sup +} line at 691.7 GHz was not detected with the APEX telescope toward this position. These D{sub 2}H{sup +} observations complement our previous detections of para-H{sub 2}D{sup +} and ortho-H{sub 2}D{sup +} using SOFIA and APEX. By modeling chemistry and radiative transfer in the dense core surrounding the protostars, we find that the ortho-D{sub 2}H{sup +} and para-H{sub 2}D{sup +} absorption features mainly originate in the cool (T < 18 K) outer envelope of the core. In contrast, the ortho-H{sub 2}D{sup +} emission from the core is significantly absorbed by the ambient molecular cloud. Analyses of the combined D{sub 2}H{sup +} and H{sub 2}D{sup +} data result in an age estimate of ∼5 × 10{sup 5} yr for the core, with an uncertainty of ∼2 × 10{sup 5} yr. The core material has probably been pre-processed for another 5 × 10{sup 5} years in conditions corresponding to those in the ambient molecular cloud. The inferred timescale is more than 10 times the age of the embedded protobinary. The D{sub 2}H{sup +} and H{sub 2}D{sup +} ions have large and nearly equal total (ortho+para) fractional abundances of ∼10{sup −9} in the outer envelope. This confirms the central role of H{sub 3}{sup +} in the deuterium chemistry in cool, dense gas, and adds support to the prediction of chemistry models that also D{sub 3}{sup +} should be abundant in these conditions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Constraints on core-collapse supernova progenitors from explosion site integral field spectroscopy

Observationally, supernovae (SNe) are divided into subclasses according to their distinct characteristics. This diversity naturally reflects the diversity in the progenitor stars. It is not entirely clear, however, how different evolutionary paths leading massive stars to become an SN are governed by fundamental parameters such as progenitor initial mass and metallicity. Furthermore, this paper places constraints on progenitor initial mass and metallicity in distinct core-collapse SN subclasses through a study of the parent stellar populations at the explosion sites. Integral field spectroscopy (IFS) of 83 nearby SN explosion sites with a median distance of 18 Mpc has been collected and analysed, enabling detection and spectral extraction of the parent stellar population of SN progenitors. From the parent stellar population spectrum, the initial mass and metallicity of the coeval progenitor are derived by means of comparison to simple stellar population models and strong-line methods. Additionally, near-infrared IFS was employed to characterise the star formation history at the explosion sites. No significant metallicity differences are observed among distinct SN types. The typical progenitor mass is found to be highest for SN type Ic, followed by type Ib, then types IIb and II. Type IIn is the least associated with young stellar populations and thus massive progenitors. However, statistically significant differences in progenitor initial mass are observed only when comparing SNe IIn with other subclasses. Stripped-envelope SN progenitors with initial mass estimates lower than 25 M ⊙ are found; they are thought to be the result of binary progenitors. Confirming previous studies, these results support the notion that core-collapse SN progenitors cannot arise from single-star channels only, and both single and binary channels are at play in the production of core-collapse SNe. Near-infrared IFS suggests that multiple stellar populations with different ages may be present in some of the SN sites. As a consequence, there could be a non-negligible amount of contamination from old populations, and therefore the individual age estimates are effectively lower limits.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release

The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of ∼2000 UV-selected objects that are candidate nearby (D≲150 pc), young (age ∼ 10–100 Myr), late-type stars. Here, we evaluate the distances and ages of the subsample of (19) GALNYSS stars with Gaia Data Release 1 (DR1) parallax distances D⩽120 pc. The overall youth of these 19 mid-K to early-M stars is readily apparent from their positions relative to the loci of main-sequence stars and giants in Gaia-based color-magnitude and color-color diagrams constructed for all stars detected by Galex and the Wide-field Infrared Space Explorer for which parallax measurements are included in DR1. The isochronal ages of all 19 stars lie in the range ∼10–100 Myr. Comparison with Li-based age estimates indicates a handful of these stars may be young main-sequence binaries rather than pre-main sequence stars. Nine of the 19 objects have not previously been considered as nearby, young stars, and all but one of these are found at declinations north of +30°. The Gaia DR1 results presented here indicate that the GALNYSS sample includes several hundred nearby, young stars, a substantial fraction of which have not been previously recognized as having ages ≲100 Myr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

HST/WFC3 near-infrared spectroscopy of quenched galaxies at z ∼ 1.5 from the WISP survey: Stellar population properties

We combine Hubble Space Telescope (HST) G102 and G141 near-IR (NIR) grism spectroscopy with HST/WFC3-UVIS, HST/WFC3-IR, and Spitzer/IRAC [3.6 μm] photometry to assemble a sample of massive (log (M {sub star}/M {sub ☉}) ∼ 11.0) and quenched (specific star formation rate <0.01 Gyr{sup –1}) galaxies at z ∼ 1.5. Our sample of 41 galaxies is the largest with G102+G141 NIR spectroscopy for quenched sources at these redshifts. In contrast to the local universe, z ∼ 1.5 quenched galaxies in the high-mass range have a wide range of stellar population properties. We find that their spectral energy distributions (SEDs) are well fitted with exponentially decreasing star formation histories and short star formation timescales (τ ≤ 100 Myr). Quenched galaxies also show a wide distribution in ages, between 1 and 4 Gyr. In the (u – r){sub 0}-versus-mass space quenched galaxies have a large spread in rest-frame color at a given mass. Most quenched galaxies populate the z ∼ 1.5 red sequence (RS), but an important fraction of them (32%) have substantially bluer colors. Although with a large spread, we find that the quenched galaxies on the RS have older median ages (3.1 Gyr) than the quenched galaxies off the RS (1.5 Gyr). We also show that a rejuvenated SED cannot reproduce the observed stacked spectra of (the bluer) quenched galaxies off the RS. We derive the upper limit on the fraction of massive galaxies on the RS at z ∼ 1.5 to be <43%. We speculate that the young quenched galaxies off the RS are in a transition phase between vigorous star formation at z > 2 and the z ∼ 1.5 RS. According to their estimated ages, the time required for quenched galaxies off the RS to join their counterparts on the z ∼ 1.5 RS is of the order of ∼1 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

DISCOVERY OF X-RAY PULSATIONS FROM THE INTEGRAL SOURCE IGR J11014–6103

We report the discovery of PSR J1101–6101, a 62.8 ms pulsar in IGR J11014–6103, a hard X-ray source with a jet and a cometary tail that strongly suggests it is moving away from the center of the supernova remnant (SNR) MSH 11–61A at v > 1000 km s{sup –1}. Two XMM-Newton observations were obtained with the EPIC pn in small window mode, resulting in the measurement of its spin-down luminosity E-dot =1.36×10{sup 36} erg s{sup –1}, characteristic age τ {sub c} = 116 kyr, and surface magnetic field strength B{sub s} = 7.4 × 10{sup 11} G. In comparison to τ {sub c}, the 10-30 kyr age estimated for MSH 11–61A suggests that the pulsar was born in the SNR with initial period in the range 54 ≤ P {sub 0} ≤ 60 ms. PSR J1101–6101 is the least energetic of the 15 rotation-powered pulsars detected by INTEGRAL, and has a high efficiency of hard X-ray radiation and jet power. We examine the shape of the cometary nebula in a Chandra image, which is roughly consistent with a bow shock at the velocity inferred from the SNR age and the pulsar's E-dot . However, its structure differs in detail from the classic bow shock, and we explore possible reasons for this.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

CHANDRA AND ROSAT OBSERVATIONS OF A194: DETECTION OF AN X-RAY CAVITY AND MAPPING THE DYNAMICS OF THE CLUSTER

Based on Chandra and ROSAT observations, we investigated the nearby poor cluster A194, which hosts two luminous radio galaxies, NGC 547 (3C 40B) and NGC 541 (3C 40A). We demonstrated the presence of a large X-ray cavity (r {approx} 34 kpc) formed by the giant southern radio lobe arising from 3C 40B in NGC 547. The estimated age of the cavity is t = 7.9 Multiplication-Sign 10{sup 7} years and the total work of the active galactic nucleus is 3.3 Multiplication-Sign 10{sup 59} erg, hence the cavity power is P{sub cav} = 1.3 Multiplication-Sign 10{sup 44} erg s{sup -1}. Furthermore, in the Chandra images of NGC 545 and NGC 541 we detected sharp surface brightness edges, identified as merger cold fronts, and extended tails. Using the pressure ratios between inside and outside the cold fronts we estimated that the velocities of NGC 545 and NGC 541 correspond to Mach numbers of M = 1.0{sup +0.3}{sub -0.5} and M = 0.9{sup +0.2}{sub -0.5}, respectively. The low radial velocities of these galaxies relative to the mean radial velocity of A194 imply that their motion is oriented approximately in the plane of the sky. Based on these and earlier observations, we concluded that NGC 545 and NGC 541 are falling through the cluster, whose center is NGC 547, suggesting that A194 is undergoing a significant cluster merger event. Additionally, we detected 20 bright X-ray sources around NGC 547 and NGC 541, a surprisingly large number, since the predicted number of resolved low-mass X-ray binaries and cosmic X-ray background sources is 2.2 and 4.1, respectively. To explain the nature of additional sources, different possibilities were considered, none of which are satisfactory. We also studied the origin of X-ray emission in Minkowski's object (MO) and concluded that it is most likely dominated by the population of high-mass X-ray binaries rather than by hot diffuse interstellar medium. Moreover, in view of the galaxy dynamics in A194, we explored the possibility that the starburst in MO was triggered by its past interaction with NGC 541, and concluded that it may be a viable path.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A study of massive and evolved galaxies at high redshift

We use data taken as part of Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) observations of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) to identify massive and evolved galaxies at 3 < z < 4.5. This is performed using the strength of the Balmer break feature at rest-frame 3648 Å, which is a diagnostic of the age of the stellar population in galaxies. Using the WFC3 H-band-selected catalog for the CANDELS GOODS-S field and deep multi-waveband photometry from optical (HST) to mid-infrared (Spitzer) wavelengths, we identify a population of old and evolved post-starburst galaxies based on the strength of their Balmer breaks (Balmer break galaxies, BBGs). The galaxies are also selected to be bright in rest-frame near-IR wavelengths and hence massive. We identify a total of 16 BBGs. Fitting the spectral energy distribution of the BBGs shows that the candidate galaxies have average estimated ages of ∼800 Myr and average stellar masses of ∼5 × 10{sup 10} M {sub ☉}, consistent with being old and massive systems. Two of our BBG candidates are also identified by the criteria that are sensitive to star-forming galaxies (Lyman break galaxy selection). We find a number density of ∼3.2 × 10{sup –5} Mpc{sup –3} for the BBGs, corresponding to a mass density of ∼2.0 × 10{sup 6} M {sub ☉} Mpc{sup –3} in the redshift range covering the survey. Given the old age and the passive evolution, it is argued that some of these objects formed the bulk of their mass only a few hundred million years after the big bang.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Stable Rate of Slip Along the Karakax Section of the Altyn Tagh Fault from Observation of Interglacial and Postglacial Offset Morphology and Surface Dating

Digital elevation maps obtained using TanDEM-X and Pleiades data combined with newly obtained surface age estimates using cosmogenic radionuclide (CRN) and optically simulated luminescence (OSL) methods are used to quantify the slip rate along the western section of the Altyn Tagh Fault in southern Xinjiang. The reconstruction of the conical shape of massive alluvial fans inferred to be from the Eemian (115±7 ka) from CRN dating shows consistent left-lateral offsets of 300±20 m, yielding a slip rate of 2.6±0.3 mm/year. Successive episodes of incision have left cut terraces inset in wide canyons, 10–25 m below the fans' surface. The incision was followed by the deposition of a broad terrace of early Holocene age, which is reincised by modern stream channels. Near the village of Shanxili, a 200 m wide valley is partially dammed by a shutter ridge displaced by the fault. A fill terrace deposited upstream from the ridge has an OSL age of 8.8±0.6 ka. The 23±2 m offset of the riser incising the terrace indicates a minimum postdepositional movement on the fault, yielding a Holocene rate of 2.6±0.5 mm/year, consistent with the 115-ka average slip rate. Furthermore, scarp degradation analysis using mass diffusion reveals a nonlinear relationship between fault displacement and degradation coefficient along the progressively exposed fault scarp, a pattern suggesting either seismic clustering or variable diffusion rate since the Eemian. Together with the Gozha Co-Longmu Co fault to the south, the Karakax section of the Altyn Tagh Fault contributes to the eastward movement of the western corner of Tibet.

58 GEOSCIENCES↗
DOE OSTI2018

Evaluation of micromilling/conventional isotope ratio mass spectrometry and secondary ion mass spectrometry of δ 18 O values in fish otoliths for sclerochronology

Stable oxygen isotope ratios (δ 18 O values) measured in fish otoliths can provide valuable detailed information on fish life history, fish age determination, and ocean thermography. Traditionally, otoliths are sampled by micromilling followed by isotope ratio mass spectrometry (IRMS), but direct analysis by secondary ion mass spectrometry (SIMS) is becoming more common. However, these two methods have not been compared to determine which, if either, is better for fish age validation studies. Hence, the goals were to: (1) determine if the δ 18 O signatures from the two different methods are similar, (2) determine which method is better for fish age validation studies, and (3) examine biogeographic and migration history. Both analytical techniques, micromilling/IRMS and SIMS, were used to measure δ 18 O values in six Pacific cod (Gadus macrocephalus) otoliths. A series of measurements was made from the center of each otolith to its edge to develop a life-history δ 18 O signature for each fish. The sampling resolution of SIMS analyses was 2–3 times greater than that obtained by micromilling/IRMS. We found an offset between SIMS and micromilling/IRMS δ 18 O values, about 0.5‰ on average, with SIMS yielding lower values. However, the δ 18 O patterns from both methods (i.e., the number of δ 18 O maxima) correspond to the estimated age determined by otolith growth-zone counts, validating fish age determination methods. Both techniques resolved δ 18 O life-history signatures and showed patterns consistent with seasonal variation in temperatures and changes due to fish migration. When otoliths are large, micromilling/IRMS can provide adequate resolution for fish age validation. Finally, SIMS is the better option if greater sampling resolution is required, such as when otoliths are small or specimens are longer lived and have compact growth zones.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

THE GEMINI NICI PLANET-FINDING CAMPAIGN: DISCOVERY OF A MULTIPLE SYSTEM ORBITING THE YOUNG A STAR HD 1160

We report the discovery of two low-mass companions to the young A0V star HD 1160 at projected separations of 81 {+-} 5 AU (HD 1160 B) and 533 {+-} 25 AU (HD 1160 C) by the Gemini NICI Planet-Finding Campaign. Very Large Telescope images of the system taken over a decade for the purpose of using HD 1160 A as a photometric calibrator confirm that both companions are physically associated. By comparing the system to members of young moving groups and open clusters with well-established ages, we estimate an age of 50{sup +50}{sub -40} Myr for HD 1160 ABC. While the UVW motion of the system does not match any known moving group, the small magnitude of the space velocity is consistent with youth. Near-IR spectroscopy shows HD 1160 C to be an M3.5 {+-} 0.5 star with an estimated mass of 0.22{sup +0.03}{sub -0.04} M{sub Sun }, while NIR photometry of HD 1160 B suggests a brown dwarf with a mass of 33{sup +12}{sub -9} M{sub Jup}. The very small mass ratio (0.014) between the A and B components of the system is rare for A star binaries, and would represent a planetary-mass companion were HD 1160 A to be slightly less massive than the Sun.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Cosmogenic surface exposure ( 10 Be) dating of raised beaches in Marguerite bay, Antarctic Peninsula: Implications for relative sea-level history

Understanding the dynamics of ice mass loss in polar regions is crucial for deciphering climate change and Glacio Isostatic Adjustment patterns. This study focuses on Marguerite Bay, located in the south-central Antarctic Peninsula. We dated raised beaches to investigate relative sea-level changes using the cosmogenic surface exposure ( 10 Be) method. Previous studies have provided valuable insights into the region's glacial history, but limitations in dating techniques and age estimates necessitate further investigation. By analysing raised shingle beaches in Gaul Cove of Horseshoe Island and the southern coast of Calmette Bay, this research aims to contribute relative sea-level change history for these areas. In Horseshoe Island's Gaul Cove, raised beaches clustered on prominent steps reveal a 15 m relative sea-level change over the last 3.31 ka. Differently, Calmette Bay exhibits a 36 m relative sea-level fall over the last 7.29 ka. Finally, these findings indicate significant and differential glacial-isostatic adjustments in both regions during the middle and late Holocene. Additionally, our data reveal accelerated sea-level fall periods corresponding to Holocene deglaciation and glacial advance events, indicating the shorelines' relative sea-level change sensitivity to climate change.

58 GEOSCIENCES↗
DOE OSTI2015

Using naturally occurring radionuclides to determine drinking water age in a community water system

Drinking water quality in a community water system is closely linked to the age of water from initial treatment to time of delivery. However, water age is difficult to measure with conventional chemical tracers; particularly in stagnant water, where the relationship between disinfectant decay, microbial growth, and water age is poorly understood. Using radionuclides that were naturally present in source water, we found that measured activity ratios of 90 Y/ 90 Sr and 234 Th/ 238 U in discrete drinking water samples of known age accurately estimated water age up to 9 days old (σ est : ± 3.8 h, P < 0.0001, r 2 = 0.998, n = 11) and 25 days old (σ est : ± 13.3 h, P < 0.0001, r 2 = 0.996, n = 12), respectively. Moreover, 90 Y-derived water ages in a community water system (6.8 × 10 4 m 3 d –1 capacity) were generally consistent with water ages derived from an extended period simulation model. Radionuclides differ from conventional chemical tracers in that they are ubiquitous in distribution mains and connected premise plumbing. The ability to measure both water age and an analyte (e.g., chemical or microbe) in any water sample at any time allows for new insight into factors that control drinking water quality.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

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}64 Degree-Sign from face-on, based on the apparent major/minor axis ratio, with the major axis aligned along P.A. = 130 Degree-Sign . 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 Almost-Equal-To 0.4). The maximum visible radial extent is {approx}254 AU. With mean surface brightness of V Almost-Equal-To 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).

79 ASTRONOMY AND ASTROPHYSICS↗