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At least 1,549 records · Page 86
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DOE OSTI2014

THE CATALINA SURVEYS PERIODIC VARIABLE STAR CATALOG

We present ∼47,000 periodic variables found during the analysis of 5.4 million variable star candidates within a 20,000 deg{sup 2} region covered by the Catalina Surveys Data Release-1 (CSDR1). Combining these variables with type ab RR Lyrae from our previous work, we produce an online catalog containing periods, amplitudes, and classifications for ∼61,000 periodic variables. By cross-matching these variables with those from prior surveys, we find that >90% of the ∼8000 known periodic variables in the survey region are recovered. For these sources, we find excellent agreement between our catalog and prior values of luminosity, period, and amplitude as well as classification. We investigate the rate of confusion between objects classified as contact binaries and type c RR Lyrae (RRc's) based on periods, colors, amplitudes, metallicities, radial velocities, and surface gravities. We find that no more than a few percent of the variables in these classes are misidentified. By deriving distances for this clean sample of ∼5500 RRc's, we trace the path of the Sagittarius tidal streams within the Galactic halo. Selecting 146 outer-halo RRc's with SDSS radial velocities, we confirm the presence of a coherent halo structure that is inconsistent with current N-body simulations of the Sagittarius tidal stream. We also find numerous long-period variables that are very likely associated within the Sagittarius tidal stream system. Based on the examination of 31,000 contact binary light curves we find evidence for two subgroups exhibiting irregular light curves. One subgroup presents significant variations in mean brightness that are likely due to chromospheric activity. The other subgroup shows stable modulations over more than a thousand days and thereby provides evidence that the O'Connell effect is not due to stellar spots.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SPIN-ORBIT ALIGNMENT FOR THE CIRCUMBINARY PLANET HOST KEPLER-16 A

Kepler-16 is an eccentric low-mass eclipsing binary with a circumbinary transiting planet. Here, we investigate the angular momentum of the primary star, based on Kepler photometry and Keck spectroscopy. The primary star's rotation period is 35.1 {+-} 1.0 days, and its projected obliquity with respect to the stellar binary orbit is 1.{sup 0}6 {+-} 2.{sup 0}4. Therefore, the three largest sources of angular momentum-the stellar orbit, the planetary orbit, and the primary's rotation-are all closely aligned. This finding supports a formation scenario involving accretion from a single disk. Alternatively, tides may have realigned the stars despite their relatively wide separation (0.2 AU), a hypothesis that is supported by the agreement between the measured rotation period and the 'pseudosynchronous' period of tidal evolution theory. The rotation period, chromospheric activity level, and fractional light variations suggest a main-sequence age of 2-4 Gyr. Evolutionary models of low-mass stars can match the observed masses and radii of the primary and secondary stars to within about 3%.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

ON A NEW THEORETICAL CALIBRATION OF THE STROeMGREN hk METALLICITY INDEX: NGC 6522 AS A FIRST TEST CASE

We present a new theoretical calibration of the Stroemgren metallicity index hk using {alpha}-enhanced evolutionary models transformed into the observational plane by using atmosphere models with the same chemical mixture. We apply the new metallicity-index-color (MIC) relations to a sample of 85 field red giants (RGs) and find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex with a dispersion of {sigma}= 0.19 dex. The outcome is the same if we apply the MIC relations to a sample of eight RGs in the bulge globular cluster NGC 6522, but the standard deviation ranges from 0.26 (hk, v - -y) to 0.49 (hk, u - -y). The difference is mainly caused by a difference in photometric accuracy. The new MIC relations based on the Ca - -y color provide metallicities systematically more metal-rich than the spectroscopic ones. We found that the Ca band is affected by Ca abundance and possibly by chromospheric activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

RESPONSE OF THE PHOTOSPHERIC MAGNETIC FIELD TO THE X2.2 FLARE ON 2011 FEBRUARY 15

It is well known that the long-term evolution of the photospheric magnetic field plays an important role in building up free energy to power solar eruptions. Observations, despite being controversial, have also revealed a rapid and permanent variation of the photospheric magnetic field in response to the coronal magnetic field restructuring during the eruption. The Helioseismic and Magnetic Imager instrument (HMI) on board the newly launched Solar Dynamics Observatory produces seeing-free full-disk vector magnetograms at consistently high resolution and high cadence, which finally makes possible an unambiguous and comprehensive study of this important back-reaction process. In this study, we present a near disk-center, GOES-class X2.2 flare, which occurred in NOAA AR 11158 on 2011 February 15. Using the magnetic field measurements made by HMI, we obtained the first solid evidence of a rapid (in about 30 minutes) and irreversible enhancement in the horizontal magnetic field at the flaring magnetic polarity inversion line (PIL) by a magnitude of {approx}30%. It is also shown that the photospheric field becomes more sheared and more inclined. This field evolution is unequivocally associated with the flare occurrence in this sigmoidal active region, with the enhancement area located in between the two chromospheric flare ribbons and the initial conjugate hard X-ray footpoints. These results strongly corroborate our previous conjecture that the photospheric magnetic field near the PIL must become more horizontal after eruptions, which could be related to the newly formed low-lying fields resulting from the tether-cutting reconnection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MULTIDIMENSIONAL MODELING OF CORONAL RAIN DYNAMICS

We present the first multidimensional, magnetohydrodynamic simulations that capture the initial formation and long-term sustainment of the enigmatic coronal rain phenomenon. We demonstrate how thermal instability can induce a spectacular display of in situ forming blob-like condensations which then start their intimate ballet on top of initially linear force-free arcades. Our magnetic arcades host a chromospheric, transition region, and coronal plasma. Following coronal rain dynamics for over 80 minutes of physical time, we collect enough statistics to quantify blob widths, lengths, velocity distributions, and other characteristics which directly match modern observational knowledge. Our virtual coronal rain displays the deformation of blobs into V-shaped features, interactions of blobs due to mostly pressure-mediated levitations, and gives the first views of blobs that evaporate in situ or are siphoned over the apex of the background arcade. Our simulations pave the way for systematic surveys of coronal rain showers in true multidimensional settings to connect parameterized heating prescriptions with rain statistics, ultimately allowing us to quantify the coronal heating input.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

FINE STRUCTURE OF FLARE RIBBONS AND EVOLUTION OF ELECTRIC CURRENTS

Emission of solar flares across the electromagnetic spectrum is often observed in the form of two expanding ribbons. The standard flare model explains flare ribbons as footpoints of magnetic arcades, emitting due to interaction of energetic particles with the chromospheric plasma. However, the physics of this interaction and properties of the accelerated particles are still unknown. We present results of multiwavelength observations of the C2.1 flare of 2013 August 15, observed with the New Solar Telescope of the Big Bear Solar Observatory, and the Solar Dynamics Observatory, GOES, and Fermi spacecraft. The observations reveal previously unresolved sub-arcsecond structure of flare ribbons in regions of strong magnetic field consisting from numerous small-scale bright knots. We observe a red-blue asymmetry of H{sub α} flare ribbons with a width as small as ∼100 km. We discuss the relationship between the ribbons and vertical electric currents estimated from vector magnetograms, and show that Joule heating can be responsible for energization of H{sub α} knots in the ribbons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

VERTICAL KINK OSCILLATION OF A MAGNETIC FLUX ROPE STRUCTURE IN THE SOLAR CORONA

Vertical transverse oscillations of a coronal magnetic rope, observed simultaneously in the 171 Å and 304 Å bandpasses of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO), are detected. The oscillation period is about 700 s and the displacement amplitude is about 1 Mm. The oscillation amplitude remains constant during the observation. Simultaneous observation of the rope in the bandpasses corresponding to the coronal and chromospheric temperatures suggests that it has a multi-thermal structure. Oscillatory patterns in 171 Å and 304 Å are coherent, which indicates that the observed kink oscillation is collective, in which the rope moves as a single entity. We interpret the oscillation as a fundamental standing vertically polarized kink mode of the rope, while the interpretation in terms of a perpendicular fast wave could not be entirely ruled out. In addition, the arcade situated above the rope and seen in the 171 Å bandpass shows an oscillatory motion with the period of about 1000 s.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

A detailed study of the barium central star of the planetary nebula Abell 70

ABSTRACT We present a detailed study of the barium star at the heart of the planetary nebula Abell 70. Time-series photometry obtained over a period of more than 10 yr demonstrates that the barium-contaminated companion is a rapid rotator with temporal variability due to spots. The amplitude and phasing of the photometric variability change abruptly; however, there is no evidence for a change in the rotation period (P = 2.06 d) over the course of the observations. The co-addition of 17 high-resolution spectra obtained with Ultraviolet and Visual Échelle Spectrograph mounted on the Very Large Telescope allows us to measure the physical and chemical properties of the companion, confirming it to be a chromospherically active, late G-type sub-giant with more than +1 dex of barium enhancement. We find no evidence of radial velocity variability in the spectra, obtained over the course of approximately 130 d with a single additional point some 8 yr later, with the radial velocities of all epochs approximately −10 km s −1 from the previously measured systemic velocity of the nebula. This is perhaps indicative that the binary has a relatively long period (P ≳ 2 yr) and high eccentricity (e ≳ 0.3), and that all the observations were taken around radial velocity minimum. However, unless the binary orbital plane is not aligned with the waist of the nebula or the systemic velocity of the binary is not equal to the literature value for the nebula, this would imply an unfeasibly large mass for the nebular progenitor.

Jones, David (ORCID:0000000339475946)↗
DOE OSTI2025

Investigation of oscillations above the acoustic cut-off frequency for a sample of Kepler stars. II. Identifying temporal frequency shifts

ABSTRACT Stellar oscillations with frequencies greater than the acoustic cut-off frequency are not trapped within the star’s interior. Geometric interference between these high-frequency waves produces a peak-like structure in the power spectrum of the star, known as the pseudo-modes. In this study, we aim to analyse 51 stars previously identified (from a sample of 91 Kepler stars) to contain statistically significant evidence for pseudo-modes, to determine if the pseudo-mode frequencies vary in time. We obtained these frequency variations and the uncertainty by the resampled periodogram approach, where periodogram realisations were created from successive, overlapping 90-d time segments, and a cross-correlation function was generated between them. The computed shifts were compared against temporal p-mode frequency shifts and a stellar magnetic activity proxy, $S_{\mathrm{ph}}$. We found that for 16 per cent of the stars pseudo-mode frequency shifts were significantly anticorrelated with p-mode shifts, as is the case for the Sun. However, we also found pseudo-mode and p-mode shifts to be significantly correlated in-phase in 8 per cent of our sample. The remainder showed no significant correlation. We also searched for trends between the maximum variation of pseudo-mode frequency shifts and stellar parameters. Whilst we found no correlation between the maximum variation and stellar photospheric and chromospheric proxies, we found frequency variations to have a greater amplitude for cooler and slower rotating stars, which is in opposition to p-mode frequency shift variation. Temporal changes to pseudo-mode frequencies may be attributable to variations in stellar magnetic activity, and therefore can contribute to our understanding of stellar activity cycles.

Millson, L. J. (ORCID:0009000342542676)↗
DOE OSTI2019

Kinetic Simulations of Magnetic Reconnection in Partially Ionized Plasmas

Fast magnetic reconnection occurs in nearly all natural and laboratory plasmas and rapidly releases stored magnetic energy. Although commonly studied in fully ionized plasmas, if and when fast reconnection can occur in partially ionized plasmas, such as the interstellar medium or solar chromosphere, is not well understood. Here, this Letter presents the first fully kinetic particle-in-cell simulations of partially ionized reconnection and demonstrates that fast reconnection can occur in partially ionized systems. In the simulations, the transition to fast reconnection occurs when the current sheet width thins below the ion-inertial length in contrast to previous analytic predictions. The peak reconnection rate is ≥ 0.08 when normalized to the bulk Alfvén speed (including both ion and neutral mass), consistent with previous experimental results. However, when the bulk Alfvén speed falls below the neutral sound speed, the rate becomes system size dependent. The normalized inflow velocity is ionization fraction dependent, which is shown to be a result of neutral momentum transport. A model for the inflow is developed which agrees well with the simulation results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Variability of Kepler solar-like stars harboring small exoplanets

We examine Kepler light-curve variability on habitable zone transit timescales for a large uniform sample of spectroscopically studied Kepler exoplanet host stars. The stars, taken from Everett et al., are solar-like in their properties and each harbors at least one exoplanet (or candidate) of radius ≤2.5 R{sub e}. The variability timescale examined is typical for habitable zone planets orbiting solar-like stars and we note that the discovery of the smallest exoplanets (≤1.2 R{sub e}) with corresponding transit depths of less than ∼0.18 mmag occur for the brightest and photometrically quietest stars. Thus, these detections are quite rare in Kepler observations. Some brighter and more evolved stars (subgiants), the latter of which often show large radial velocity jitter, are found to be among the photometrically quietest solar-like stars in our sample and the most likely small planet transit hunting grounds. The Sun is discussed as a solar-like star proxy to provide insight into the nature and cause of photometric variability. It is shown that Kepler’s broad, visible light observations are insensitive to variability caused by chromospheric activity that may be present in the observed stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Long-period variations in the radial velocity of spectroscopic binary M giant μ Ursae Majoris

We report that the spectroscopic binary μ Ursae Majoris (μ UMa) has secondary RV variations of 471.2 days in addition to those of 230.0 days already known. Keplerian orbit analysis yields stellar mass companions of 1.6 M{sub ⊙} for the 230 day period and 0.14 M{sub ⊙} for the 471 day period. However, the HIPPARCOS photometries show a period similar to the stellar rotational period, which is one-quarter of the RV period. Variations in the bisector velocity curvature show a period of 463.6 days. We also find ∼473 day variations in the equivalent width (EW) measurements of the H{sub α} and H{sub β} lines, whose origin is probably stellar activity. We note that the nature of 471 day variations is similar to one observed in “Sequence D” of Asymptotic Giant Branch pulsating stars. We therefore conclude that the RV and the EW variations in the spectroscopic binary M giant μ UMa A originate from the complex pulsations and the chromospheric activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ORBITS OF FOUR YOUNG TRIPLE-LINED MULTIPLE SYSTEMS

Each of the nearby triple systems HIP 7601, 13498, 23824, and 113597 (HD 10800, 18198, 35877, 217379) consist of solar-type dwarfs with comparable masses, where all three components are resolved spectrally, while the outer pairs are resolved both visually and spectrally. These stars are relatively young (between 100 and 600 Myr) and chromospherically active (X-ray sources), although they rotate slowly. I determine the spectroscopic orbits of the inner subsystems (periods 19.4, 14.1, 5.6, 20.3 days) and the orbits of the outer systems (periods 1.75, 51, 27, 500 years, respectively). For HIP 7601 and 13498, the combined spectro-interferometric outer orbits produce direct measurement of the masses of all of the components, allowing for a comparison with stellar models. The 6708 Å lithium line is present and its strength is measured in each component individually by subtracting the contributions of the other components. The inner and outer orbits of HIP 7601 are nearly circular, likely co-planar, and have a modest period ratio of 1:33. This study contributes to the characterization of hierarchical multiplicity in the solar neighborhood and provides data for testing stellar evolutionary models and chronology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE NONLINEAR EVOLUTION OF A TWIST IN A MAGNETIC SHOCKTUBE

The interaction between a small twist and a horizontal chromospheric shocktube is investigated. The magnetic flux tube is modeled using 1.5-D magnetohydrodynamics. The presence of a supersonic yet sub-Alfvénic flow along the flux tube allows the Alfvénic pulse driven at the photospheric boundary to become trapped and amplified between the stationary shock front and photosphere. The amplification of the twist leads to the formation of slow and fast shocks. The pre-existing stationary shock is destabilized and pushed forward as it merges with the slow shock. The propagating fast shock extracts the kinetic energy of the flow and launches rapid twists of 10–15 km s{sup −1} upon each reflection. A cavity is formed between the slow and fast shocks where the flux tube becomes globally twisted within less than an hour. The resultant highly twisted magnetic flux tube is similar to those prone to kink instabilities, which may be responsible for solar eruptions. The generated torsional flux is calculated.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

DYNAMICS OF CORONAL RAIN AND DESCENDING PLASMA BLOBS IN SOLAR PROMINENCES. II. PARTIALLY IONIZED CASE

Coronal rain clumps and prominence knots are dense condensations with chromospheric to transition region temperatures that fall down in the much hotter corona. Their typical speeds are in the range 30–150 km s{sup −1} and of the order of 10–30 km s{sup −1}, respectively, i.e., they are considerably smaller than free-fall velocities. These cold blobs contain a mixture of ionized and neutral material that must be dynamically coupled in order to fall together, as observed. We investigate this coupling by means of hydrodynamic simulations in which the coupling arises from the friction between ions and neutrals. The numerical simulations presented here are an extension of those of Oliver et al. to the partially ionized case. We find that, although the relative drift speed between the two species is smaller than 1 m s{sup −1} at the blob center, it is sufficient to produce the forces required to strongly couple charged particles and neutrals. The ionization degree has no discernible effect on the main results of our previous work for a fully ionized plasma: the condensation has an initial acceleration phase followed by a period with roughly constant velocity, and, in addition, the maximum descending speed is clearly correlated with the ratio of initial blob to environment density.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

LONG-ORBITAL-PERIOD PREPOLARS CONTAINING EARLY K-TYPE DONOR STARS. BOTTLENECK ACCRETION MECHANISM IN ACTION

We studied two objects identified as cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached, and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

MEASUREMENTS OF NON-THERMAL LINE WIDTHS IN SOLAR ACTIVE REGIONS

Spectral line widths are often observed to be larger than can be accounted for by thermal and instrumental broadening alone. This excess broadening is a key observational constraint for both nanoflare and wave dissipation models of coronal heating. Here we present a survey of non-thermal velocities measured in the high temperature loops (1–4 MK) often found in the cores of solar active regions. This survey of Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) observations covers 15 non-flaring active regions that span a wide range of solar conditions. We find relatively small non-thermal velocities, with a mean value of 17.6 ± 5.3 km s{sup −1}, and no significant trend with temperature or active region magnetic flux. These measurements appear to be inconsistent with those expected from reconnection jets in the corona, chromospheric evaporation induced by coronal nanoflares, and Alfvén wave turbulence models. Furthermore, because the observed non-thermal widths are generally small, such measurements are difficult and susceptible to systematic effects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A COLD FLARE WITH DELAYED HEATING

Recently, a number of peculiar flares have been reported that demonstrate significant nonthermal particle signatures with low, if any, thermal emission, which implies a close association of the observed emission with the primary energy release/electron acceleration region. This paper presents a flare that appears “cold” at the impulsive phase, while displaying delayed heating later on. Using hard X-ray data from Konus- Wind , microwave observations by SSRT, RSTN, NoRH, and NoRP, context observations, and three-dimensional modeling, we study the energy release, particle acceleration, and transport, and the relationships between the nonthermal and thermal signatures. The flaring process is found to involve the interaction between a small loop and a big loop with the accelerated particles divided roughly equally between them. Precipitation of the electrons from the small loop produced only a weak thermal response because the loop volume was small, while the electrons trapped in the big loop lost most of their energy in the coronal part of the loop, which resulted in coronal plasma heating but no or only weak chromospheric evaporation, and thus unusually weak soft X-ray emission. The energy losses of the fast electrons in the big tenuous loop were slow, which resulted in the observed delay of the plasma heating. We determined that the impulsively accelerated electron population had a beamed angular distribution in the direction of the electric force along the magnetic field of the small loop. The accelerated particle transport in the big loop was primarily mediated by turbulent waves, which is similar to other reported cold flares.

79 ASTRONOMY AND ASTROPHYSICS↗