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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Poking the Beehive from Space: K2 Rotation Periods for Praesepe

We analyze K2 light curves for 794 low-mass (1 ≳M{sub ∗} ≳0.1 M{sub ⊙}) members of the ≈650 Myr old open cluster Praesepe and measure rotation periods (P {sub rot}) for 677 of these stars. We find that half of the rapidly rotating ≳0.3 M{sub ⊙} stars are confirmed or candidate binary systems. The remaining ≳0.3 M{sub ⊙} fast rotators have not been searched for companions, and are therefore not confirmed single stars. We previously found that nearly all rapidly rotating ≳0.3 M{sub ⊙} stars in the Hyades are binaries, but we require deeper binary searches in Praesepe to confirm whether binaries in these two co-eval clusters have different P {sub rot} distributions. We also compare the observed P {sub rot} distribution in Praesepe to that predicted by models of angular-momentum evolution. We do not observe the clear bimodal P {sub rot} distribution predicted by Brown for >0.5 M{sub ⊙} stars at the age of Praesepe, but 0.25−0.5 M{sub ⊙} stars do show stronger bimodality. In addition, we find that >60% of early M dwarfs in Praesepe rotate more slowly than predicted at 650 Myr by Matt et al., which suggests an increase in braking efficiency for these stars relative to solar-type stars and fully convective stars. The incompleteness of surveys for binaries in open clusters likely impacts our comparison with these models since the models only attempt to describe the evolution of isolated single stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

K2 Ultracool Dwarfs Survey. II. The White Light Flare Rate of Young Brown Dwarfs

We use Kepler K2 Campaign 4 short-cadence (one-minute) photometry to measure white light flares in the young, moving group brown dwarfs 2MASS J03350208+2342356 (2M0335+23) and 2MASS J03552337+1133437 (2M0355+11), and report on long-cadence (thirty-minute) photometry of a superflare in the Pleiades M8 brown dwarf CFHT-PL-17. The rotation period (5.24 hr) and projected rotational velocity (45 km s{sup −1}) confirm 2M0335+23 is inflated (R⩾0.20 R{sub ⊙}) as predicted for a 0.06 M{sub ⊙}, 24 Myr old brown dwarf βPic moving group member. We detect 22 white light flares on 2M0335+23. The flare frequency distribution follows a power-law distribution with slope −α=−1.8±0.2 over the range 10{sup 31} to 10{sup 33} erg. This slope is similar to that observed in the Sun and warmer flare stars, and is consistent with lower-energy flares in previous work on M6–M8 very-low-mass stars; taking the two data sets together, the flare frequency distribution for ultracool dwarfs is a power law over 4.3 orders of magnitude. The superflare (2.6×10{sup 34} erg) on CFHT-PL-17 shows higher-energy flares are possible. We detect no flares down to a limit of 2×10{sup 30} erg in the nearby L5γ AB Dor moving group brown dwarf 2M0355+11, consistent with the view that fast magnetic reconnection is suppressed in cool atmospheres. We discuss two multi-peaked flares observed in 2M0335+23, and argue that these complex flares can be understood as sympathetic flares, in which fast-mode magnetohydrodynamic waves similar to extreme-ultraviolet waves in the Sun trigger magnetic reconnection in different active regions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51–0.73 M {sub ⊙} white dwarfs, which evolved from 1.7–3.0 M {sub ⊙} ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://k2wd.org.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Evidence from K2 for Rapid Rotation in the Descendant of an Intermediate-mass Star

Using patterns in the oscillation frequencies of a white dwarf observed by K2, we have measured the fastest rotation rate (1.13 ± 0.02 hr) of any isolated pulsating white dwarf known to date. Balmer-line fits to follow-up spectroscopy from the SOAR telescope show that the star (SDSSJ0837+1856, EPIC 211914185) is a 13,590±340 K, 0.87 ± 0.03 M {sub ⊙} white dwarf. This is the highest mass measured for any pulsating white dwarf with known rotation, suggesting a possible link between high mass and fast rotation. If it is the product of single-star evolution, its progenitor was a roughly 4.0 M {sub ⊙} main-sequence B star; we know very little about the angular momentum evolution of such intermediate-mass stars. We explore the possibility that this rapidly rotating white dwarf is the byproduct of a binary merger, which we conclude is unlikely given the pulsation periods observed.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

The far-ultraviolet emission spectrum of the K2 III star, Arcturus.

A moderate-resolution far-ultraviolet spectrum of the K2 IIIp star Arcturus, obtained with a rocket-borne spectrometer, shows chromospheric emission features. Hydrogen L-alpha and O I (1303 A) are clearly identified. The O I (1304 A) stellar surface brightness is as great or greater than that of the sun. Other metal lines, including those of carbon, are weak compared to the O I line.

Moos, H. W.↗
NASA NTRS1982

A semiclassical study of laser-induced atomic fluorescence from Na2, K2 and NaK

A semiclassical treatment of laser-induced atomic fluorescence for the alkali-dimer systems Na2, K2 and NaK is presented. The variation of the fluorescence intensity with the frequency of the exciting laser photon is studied and a comparison of theoretical results with a set of experimental data is presented.

Yuan, J.-M.↗
NASA NTRS1984

The K2-dwarf V471 TAU: a Stellar Version of Solar Variability

Simultaneous observations of the rotational modulation with a 1/2 day period of chromospheric H alpha emission and of broadband irradiance for the K2-dwarf in V471 Tau are presented. The observations cover eight rotation periods but do not cover the full surface of the dwarf because of timing constraints. Preliminary results show a phase relation between enhanced chromospheric emission and continuum darkening similar to that observed on the Sun. A comparison with chromospheric Mg II resonance emission modulation observed about 2 1/4 years earlier by Guinan and Sion shows that the same active longitude is involved. This is either coincidental due to lucky phasing or it signifies a stable longitude that has persisted for hundreds of rotations.

Skumanich, A.↗
NASA NTRS1989

Evidence for a cool wind from the K2 dwarf in the detached binary V471 Tauri

Evidence for mass loss from the K2 dwarf in V471 Tauri is found in the form of discrete absorption features in lines of various elements (Mg, Fe, Cr, Mn) and ionization stages (Mg I, Mg II, Fe I, Fe II). Resonant Mg II absorption indicates a mass loss rate of at least 10 to the -11th solar masses per year. The wind appears to be cool (no more than a few times 10,000 K).

Mullan, D. J.↗
NASA NTRS1993

The fundamental parameters of the chromospherically active K2 dwarf Epsilon Eridani

A silicon array detector was used to record regions exhibiting calcium and iron lines in the spectrum of the chromospherically active K2 dwarf Epsilon Eri at a resolution of 120,000 and with an SNR of not less than 200. The effective temperature, surface gravity, logarithmic iron and calcium abundances, and microturbulence are determined. Three high-excitation lines of Fe I were found to yield anomalously low iron abundances; it is postulated that the origin of the anomaly lies in the nonthermal excitation of the upper photosphere caused by chromospheric emission. It is shown that Epsilon Eri is in an evolutionary stage consistent with an M/solar mass of 0.85 theoretical zero-age main-sequence model. It is suggested that Epsilon Eri is almost certainly a young star of slightly less than one solar mass.

Drake, Jeremy J.↗
NASA NTRS2016

Campaign 9 of the K2 Mission: Observational Parameters, Scientific Drivers, and Community Involvement for a Simultaneous Space- and Ground-based Microlensing Survey

K2ʼs Campaign 9 (K2C9) will conduct a approximately 3.7 sq. deg survey toward the Galactic bulge from 2016 April 22 through July 2 that will leverage the spatial separation between K2 and the Earth to facilitate measurement of the microlens parallax Pi(sub E) for approximately greater than 170 microlensing events. These will include several that are planetary in nature as well as many short-timescale microlensing events, which are potentially indicative of free-floating planets (FFPs). These satellite parallax measurements will in turn allow for the direct measurement of the masses of and distances to the lensing systems. In this article we provide an overview of the K2C9 space- and ground-based microlensing survey. Specifically, we detail the demographic questions that can be addressed by this program, including the frequency of FFPs and the Galactic distribution of exoplanets, the observational parameters of K2C9, and the array of resources dedicated to concurrent observations. Finally, we outline the avenues through which the larger community can become involved, and generally encourage participation in K2C9, which constitutes an important pathfinding mission and community exercise in anticipation of WFIRST.

K2s Campaign 9 (K2C9)↗
NASA NTRS2017

Synergies Between the Kepler, K2 and TESS Missions with the PLATO Mission (Revised)

Two transit survey missions will have been flown by NASA prior to the launch of ESA's PLATO Mission in 2026, laying the groundwork for exoplanet discovery via the transit method. The Kepler Mission, which launched in 2009, collected data on its 100+ square degree field of view for four years before failure of a reaction wheel ended its primary mission. The results from Kepler include 2300+ confirmed or validated exoplanets, 2200+ planetary candidates, 2100+ eclipsing binaries. Kepler also revolutionized the field of asteroseismology by measuring the pressure mode oscillations of over 15000 solar-like stars spanning the lifecycle of such stars from hydrogen-burning dwarfs to helium-burning red giants. The re-purposed Kepler Mission, dubbed K2, continues to observe fields of view in and near the ecliptic plane for 80 days each, significantly broadening the scope of the astrophysical investigations as well as discovering an additional 156 exoplanets to date. The TESS mission will launch in 2017 to conduct an all-sky survey for small exoplanets orbiting stars 10X closer and 100X brighter than Kepler exoplanet host stars, allowing for far greater follow-up and characterization of their masses as well as their sizes for at least 50 small planets. Future assets such as James Webb Space Telescope, and ground-based assets such as ESOs Very Large Telescope (VLT) array, the Exremely Large Telescope (ELT), and the Thirty Meter Telescope (TMT) will be able to characterize the atmospheric composition and properties of these small planets. TESS will observe each 24 X 96 field of view for 30 days and thereby cover first the southern and then the northern hemisphere over 13 pointings during each year of the primary mission. The pole-most camera will observe the James Webb continuous viewing zone for one year in each hemisphere, permitting much longer period planets to be detected in this region. The PLATO mission will seek to detect habitable Earth-like planets with an instrument composed of 26 small telescopes in several 2232 square deg FOVs with a range of observation durations over a mission lifetime of up to eight years. This paper summarizes the findings of the KeplerK2 missions, previews the likely results from the TESS mission, and explores the lessons learned and to be learned from these prior missions that can be incorporated into the observation and data reduction strategy for the PLATO Mission so as to maximize the science return.

exoplanet detection↗
DOE OSTI2020

Materials Data on K2(SO2)3 by Materials Project

(KSO3)2S crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional and consists of four hydrogen sulfide molecules and one KSO3 framework. In the KSO3 framework, there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded in a 9-coordinate geometry to nine O2- atoms. There are a spread of K–O bond distances ranging from 2.86–3.26 Å. In the second K1+ site, K1+ is bonded in a 9-coordinate geometry to nine O2- atoms. There are a spread of K–O bond distances ranging from 2.73–3.08 Å. There are two inequivalent S+3.33+ sites. In the first S+3.33+ site, S+3.33+ is bonded in a trigonal non-coplanar geometry to three O2- atoms. There is two shorter (1.47 Å) and one longer (1.48 Å) S–O bond length. In the second S+3.33+ site, S+3.33+ is bonded in a trigonal non-coplanar geometry to three O2- atoms. All S–O bond lengths are 1.47 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one S+3.33+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one S+3.33+ atom. In the third O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one S+3.33+ atom. In the fourth O2- site, O2- is bonded in a distorted single-bond geometry to three K1+ and one S+3.33+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K2(TaSe2)3 by Materials Project

K3(TaSe2)4K(TaSe2)2 crystallizes in the hexagonal P-6m2 space group. The structure is two-dimensional and consists of one K(TaSe2)2 sheet oriented in the (0, 0, 1) direction and one K3(TaSe2)4 sheet oriented in the (0, 0, 1) direction. In the K(TaSe2)2 sheet, K1+ is bonded in a 12-coordinate geometry to six equivalent Se2- atoms. All K–Se bond lengths are 3.21 Å. Ta+3.33+ is bonded to six Se2- atoms to form distorted edge-sharing TaSe6 pentagonal pyramids. There are three shorter (2.62 Å) and three longer (2.64 Å) Ta–Se bond lengths. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to three equivalent K1+ and three equivalent Ta+3.33+ atoms to form a mixture of distorted corner, edge, and face-sharing SeK3Ta3 octahedra. The corner-sharing octahedral tilt angles are 38°. In the second Se2- site, Se2- is bonded in a distorted T-shaped geometry to three equivalent Ta+3.33+ atoms. In the K3(TaSe2)4 sheet, there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded in a 12-coordinate geometry to six Se2- atoms. There are three shorter (3.20 Å) and three longer (3.22 Å) K–Se bond lengths. In the second K1+ site, K1+ is bonded in a 12-coordinate geometry to six equivalent Se2- atoms. All K–Se bond lengths are 3.22 Å. There are two inequivalent Ta+3.33+ sites. In the first Ta+3.33+ site, Ta+3.33+ is bonded to six Se2- atoms to form distorted edge-sharing TaSe6 pentagonal pyramids. There are three shorter (2.62 Å) and three longer (2.64 Å) Ta–Se bond lengths. In the second Ta+3.33+ site, Ta+3.33+ is bonded to six Se2- atoms to form distorted edge-sharing TaSe6 pentagonal pyramids. All Ta–Se bond lengths are 2.63 Å. There are four inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a distorted T-shaped geometry to three equivalent Ta+3.33+ atoms. In the second Se2- site, Se2- is bonded in a 6-coordinate geometry to three equivalent K1+ and three equivalent Ta+3.33+ atoms. In the third Se2- site, Se2- is bonded to three equivalent K1+ and three equivalent Ta+3.33+ atoms to form a mixture of distorted corner, edge, and face-sharing SeK3Ta3 octahedra. The corner-sharing octahedral tilt angles are 37°. In the fourth Se2- site, Se2- is bonded to three equivalent K1+ and three equivalent Ta+3.33+ atoms to form a mixture of distorted corner, edge, and face-sharing SeK3Ta3 octahedra. The corner-sharing octahedral tilt angles are 37°.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Chromospheric activity and rotational modulation on the young, single K2 dwarf LQ Hya

High-resolution echelle spectra of LQ Hya, obtained during several observing runs from 2006 to 2012, have been analyzed to study its chromospheric activity. Using the spectral subtraction technique, we derived information about chromospheric activity of LQ Hya from several optical chromospheric activity indicators (including the H{sub β}, He I D{sub 3}, Na I D{sub 1}, D{sub 2}, H{sub α}, and Ca II infrared triplet (IRT) lines). No optical flares were found during our observations. The equivalent widths (EWs) of the excess emissions in the chromospheric activity lines have been measured. The ratios of EW{sub 8542}/EW{sub 8498} are generally small, which indicates that the Ca II IRT emission arises from plage-like regions, while the E {sub Hα}/E {sub Hβ} values suggest that the emission of the Balmer lines is due to both plage and prominence structures for the observations in 2012. We find that clear rotational modulation of chromospheric emission exists, which suggests the presence and change of chromospheric active regions over the surface of LQ Hya. Moreover, the active regions were associated with the photospheric spots in spatial structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

HII 2407: AN ECLIPSING BINARY REVEALED BY K2 OBSERVATIONS OF THE PLEIADES

The star HII 2407 is a member of the relatively young Pleiades star cluster and was previously discovered to be a single-lined spectroscopic binary. It is newly identified here within Kepler/K2 photometric time series data as an eclipsing binary system. Mutual fitting of the radial velocity and photometric data leads to an orbital solution and constraints on fundamental stellar parameters. While the primary has arrived on the main sequence, the secondary is still pre-main sequence and we compare our results for the M/M{sub ⊙} and R/R{sub ⊙} values with stellar evolutionary models. We also demonstrate that the system is likely to be tidally synchronized. Follow-up infrared spectroscopy is likely to reveal the lines of the secondary, allowing for dynamically measured masses and elevating the system to benchmark eclipsing binary status.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

The evolution of the Milky Way’s thin disc radial metallicity gradient with K2 asteroseismic ages

ABSTRACT The radial metallicity distribution of the Milky Way’s disc is an important observational constraint for models of the formation and evolution of our Galaxy. It informs our understanding of the chemical enrichment of the Galactic disc and the dynamical processes therein, particularly radial migration. We investigate how the metallicity changes with guiding radius in the thin disc using a sample of red giant stars with robust astrometric, spectroscopic, and asteroseismic parameters. Our sample contains 668 stars with guiding radii 4 < Rg < 11 kpc and asteroseismic ages covering the whole history of the thin disc with precision ${\approx} 25 {{\, \rm per\ cent}}$. We use MCMC analysis to measure the gradient and its intrinsic spread in bins of age and construct a hierarchical Bayesian model to investigate the evolution of these parameters independently of the bins. We find a smooth evolution of the gradient from ≈−0.07 dex kpc−1 in the youngest stars to ≈−0.04 dex kpc−1 in stars older than 10 Gyr, with no break at intermediate ages. Our results are consistent with those based on asteroseismic ages from CoRoT, with that found in Cepheid variables for stars younger than 1 Gyr, and with open clusters for stars younger than 6 Gyr. For older stars we find a significantly lower metallicity in our sample than in the clusters, suggesting a survival bias favouring more metal-rich clusters. We also find that the chemical evolution model of Chiappini '09 is too metal poor in the early stages of disc formation. Our results provide strong new constraints for the growth and enrichment of the thin disc and radial migration, which will facilitate new tests of model conditions and physics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

PacWave South SeaRAY k2 Risk Registers

The SeaRAY is a deployable power system for maritime sensors, monitoring equipment, communications, unmanned underwater vehicles, and other similar payloads. This project is to design, deliver, and test a prototype low-power WEC that lowers the total cost of ownership and provides robust, new capabilities for customers in the maritime environment. Failure Modes, Effects, and Criticality Analysis (FMECA) is conducted to systematically identify all potential failure modes and their effects on the system, and to analyze the criticality of each risk based on the likelihood of the event and the severity of the impact. Actions may then be recommended to mitigate the criticality of a risk, either by reducing the likelihood of the risk or the severity of its impact. Risk assessment is executed iteratively as an integral part of the design process. By incorporating risk assessment early in the development cycle, mitigation of risk can be achieved cost effectively. The actions recommended to mitigate risk may be subsequently executed, and as the design progresses the risk assessment is reviewed and revised. Review of the risk assessment is integrated into structured design reviews, ensuring that critical risks are comprehended and that the Project will not progress to e.g. fabrication while intolerable risks remain. The risk assessment process results in the population and maintenance of Risk Registers (RRs). Each major system (and as needed, subsystem) will have a distinct RR. This allows each system or subsystem to be assessed individually, rendering the RRs to a manageable size for review.

16 TIDAL AND WAVE POWER↗
DOE OSTI2016

ROTATION AND GRANULATION OF THE K2 GIANT α SER

The red giant α Ser was observed over 10 seasons, 2001–2010, at the Elginfield Observatory with the high-resolution coudé spectrograph. Season-mean radial velocities appear to show a small secular rise ∼11 ± 3 m s{sup 1} yr{sup 1}. The absolute spectroscopic radial velocity with convective blueshifts taken into account is 2730 m s{sup 1}. Ten line-depth ratios were investigated and show that the star's temperature is constant with any secular variation below 1.3 ± 1.0 K over the 11 years of observation. Fourier analysis of the line broadening yields v sin i = 2.0 ± 0.3 km s{sup 1} and a radial-tangential macroturbulence dispersion ζ {sub RT} = 4.50 ± 0.10 km s{sup 1}. The third-granulation-signature plot shows that the granulation velocities of α Ser are only 0.55 ± 0.10 as large as the Sun's. The line bisector of Fe i λ 6253 has the usual “C” shape and when mapped onto the third-signature plot results in a flux deficit that is slightly broader than seen in other measured K giants. The deficit fractional area of 12.3 ± 1.5% suggests a temperature difference between granules and lanes of 105 K as seen averaged over the stellar disk.

79 ASTRONOMY AND ASTROPHYSICS↗