tag:blogger.com,1999:blog-71419595793138960322023-11-15T05:18:04.207-08:00astro-ph.SR : Solar and Stellar AstrophysicsSite for <a href="http://communitypeerreview.blogspot.com/">Community Peer Review</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.comBlogger6594125tag:blogger.com,1999:blog-7141959579313896032.post-29225868041939834542013-08-06T00:16:00.019-07:002013-08-06T00:16:08.096-07:001308.0599 (Dimitri Veras et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0599">An Exoplanet's Response to Anisotropic Stellar Mass-Loss During Birth
and Death</a> [<a href="http://arxiv.org/pdf/1308.0599">PDF</a>]</h2>Dimitri Veras, John D. Hadjidemetriou, Christopher A. Tout<a name='more'></a><blockquote class="abstract">The birth and death of planets may be affected by mass outflows from their parent stars during the T-Tauri or post-main-sequence phases of stellar evolution. These outflows are often modelled to be isotropic, but this assumption is not realistic for fast rotators, bipolar jets and supernovae. Here we derive the general equations of motion for the time evolution of a single planet, brown dwarf, comet or asteroid perturbed by anisotropic mass loss in terms of a complete set of planetary orbital elements, the ejecta velocity, and the parent star's co-latitude and longitude. We restrict our application of these equations to 1) rapidly rotating giant stars, and 2) arbitrarily-directed jet outflows. We conclude that the isotropic mass-loss assumption can safely be used to model planetary motion during giant branch phases of stellar evolution within distances of hundreds of au. In fact, latitudinal mass loss variations anisotropically affect planetary motion only if the mass loss is asymmetric about the stellar equator. Also, we demonstrate how constant-velocity, asymmetric bipolar outflows in young systems incite orbital inclination changes. Consequently, this phenomenon readily tilts exoplanetary orbits external to a nascent disc on the order of degrees.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0599">http://arxiv.org/abs/1308.0599</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-5462932323534282652013-08-06T00:16:00.017-07:002013-08-06T00:16:07.058-07:001308.0620 (Bradley W. Hindman et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0620">Equilibrium models of coronal loops that involve curvature and buoyancy</a> [<a href="http://arxiv.org/pdf/1308.0620">PDF</a>]</h2>Bradley W. Hindman, Rekha Jain<a name='more'></a><blockquote class="abstract">We construct magnetostatic models of coronal loops in which the thermodynamics of the loop is fully consistent with the shape and geometry of the loop. This is achieved by treating the loop as a thin, compact, magnetic fibril that is a small departure from a force-free state. The density along the loop is related to the loop's curvature by requiring that the Lorentz force arising from this deviation is balanced by buoyancy. This equilibrium, coupled with hydrostatic balance and the ideal gas law, then connects the temperature of the loop with the curvature of the loop without resorting to a detailed treatment of heating and cooling. We present two example solutions: one with a spatially invariant magnetic Bond number (the dimensionless ratio of buoyancy to Lorentz forces) and the other with a constant radius of curvature of the loop's axis. We find that the density and temperature profiles are quite sensitive to curvature variations along the loop, even for loops with similar aspect ratios.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0620">http://arxiv.org/abs/1308.0620</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-53189439389951483262013-08-06T00:16:00.015-07:002013-08-06T00:16:06.280-07:001308.0644 (Christina Hedges et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0644">Oscillations and Surface Rotation of Red Giant Stars</a> [<a href="http://arxiv.org/pdf/1308.0644">PDF</a>]</h2>Christina Hedges, Savita Mathur, Michael J. Thompson, Keith B. MacGregor<a name='more'></a><blockquote class="abstract">More than 15000 red giants observed by Kepler for a duration of almost one year became public at the beginning of this year. We analysed a subsample of 416 stars to determine the global properties of acoustic modes (mean large separation and frequency of maximum power). Using the effective temperature from the Kepler Input Catalog, we derived a first estimation of the masses and radii of these stars. Finally, we applied wavelets to look for signature of surface rotation, which relies on the presence of spots or other surface features crossing the stellar visible disk.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0644">http://arxiv.org/abs/1308.0644</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-84282738982693724872013-08-06T00:16:00.013-07:002013-08-06T00:16:05.497-07:001308.0645 (S. Mathur)<h2 class="title"><a href="http://arxiv.org/abs/1308.0645">Study of stellar magnetic activity with CoRoT and Kepler data</a> [<a href="http://arxiv.org/pdf/1308.0645">PDF</a>]</h2>S. Mathur<a name='more'></a><blockquote class="abstract">Our knowledge of magnetic activity of stars is mostly based on the study of the Sun and some spectroscopic surveys of a few hundreds of stars. However, the detailed mechanisms of the solar magnetic activity cycle are not fully understood. With the Kepler mission that is providing exquisite photometric data for hundred of thousands of stars monitored continuously for more than three year, we can study changes in the light curves that can be related to magnetic activity. We show here a few examples of stars for which we did a time-frequency analysis to look for such signature.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0645">http://arxiv.org/abs/1308.0645</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-44401700233807719552013-08-06T00:16:00.011-07:002013-08-06T00:16:04.679-07:001308.0647 (S. Mathur et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0647">Asteroseismic analysis of the CoRoT target HD 169392</a> [<a href="http://arxiv.org/pdf/1308.0647">PDF</a>]</h2>S. Mathur, H. Bruntt, C. Catala, O. Benomar, G. R. Davies, R. A. Garcia, D. Salabert, J. Ballot, B. Mosser, C. Regulo, W. J. Chaplin, Y. Elsworth, R. Handberg, S. Hekker, L. Mantegazza, E. Michel, E. Poretti, M. Rainer, I. W. Roxburgh, R. Samadi, M. Steslicki, K. Uytterhoeven, G. A. Verner, M. Auvergne, A. Baglin, S. Barcelo Forteza, F. Baudin, T. Roca Cortes<a name='more'></a><blockquote class="abstract">The satellite CoRoT (Convection, Rotation, and planetary Transits) has provided high-quality data for almost six years. We show here the asteroseismic analysis and modeling of HD169392A, which belongs to a binary system weakly gravitationally bound as the distance between the two components is of 4250 AU. The main component, HD169392A, is a G0IV star with a magnitude of 7.50 while the second component is a G0V-G2IV star with a magnitude of 8.98. This analysis focuses on the main component, as the secondary one is too faint to measure any seismic parameters. A complete modeling has been possible thanks to the complementary spectroscopic observations from HARPS, providing Teff=5985+/-60K, log g=3.96+/-0.07, and [Fe/H]=- 0.04+/-0.10.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0647">http://arxiv.org/abs/1308.0647</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-72252076787975227292013-08-06T00:16:00.009-07:002013-08-06T00:16:03.916-07:001308.0680 (Joaquim E. R. Costa et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0680">Solar Burst Analysis with 3D Loop Models</a> [<a href="http://arxiv.org/pdf/1308.0680">PDF</a>]</h2>Joaquim E. R. Costa, Paulo J. A. Simões, Tereza S. N. Pinto, Victor F. Melnikov<a name='more'></a><blockquote class="abstract">A sample of Nobeyama flares was selected and analyzed using loop model for important flare parameters. The model for the flaring region consists of a three dimensional dipolar magnetic field, and spatial distributions of non-thermal electrons. We constructed a database by calculating the flare microwave emission for a wide range of these parameters. Out of this database with more than 5,000 cases we extracted general flare properties by comparing the observed and calculated microwave spectra. The analysis of NoRP data was mostly based in the center-to-limb variation of the flare properties with looptop and footpoint electron distributions and for NoRH maps on the resultant distribution of emission. One important aspect of this work is the comparison of the analysis of a flare using an inhomogeneous source model and a simplistic homogeneous source model. Our results show clearly that the homogeneous source hypothesis is not appropriate to describe the possible flare geometry and its use can easily produce misleading results in terms of non-thermal electron density and magnetic field strength. A center darkening of flares was also obtained as a geometrical property of the loop-like sources.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0680">http://arxiv.org/abs/1308.0680</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-89505839247849501882013-08-06T00:16:00.007-07:002013-08-06T00:16:02.888-07:001308.0684 (Iryna Kushniruk et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0684">MgH lines in the spectrum of Arcturus</a> [<a href="http://arxiv.org/pdf/1308.0684">PDF</a>]</h2>Iryna Kushniruk, Yakiv Pavlenko, Bogdan Kaminskiy<a name='more'></a><blockquote class="abstract">The synthetic spectra of MgH lines was computed for the grid of the model atmospheres and compared with observed spectrum of Arcturus. The parameters of the atmosphere of Arcturus log g=1.5 for Teff=4300 K was found by using the minimization procedure and compared with results of other studies.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0684">http://arxiv.org/abs/1308.0684</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-58452072905889766392013-08-06T00:16:00.005-07:002013-08-06T00:16:02.034-07:001308.0757 (Christopher Z. Waters et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0757">ROBOSPECT: Automated Equivalent Width Measurement</a> [<a href="http://arxiv.org/pdf/1308.0757">PDF</a>]</h2>Christopher Z. Waters, Julie K. Hollek<a name='more'></a><blockquote class="abstract">We present ROBOSPECT, a new code to automatically measure and deblend line equivalent widths for both absorption and emission spectra. We test the accuracy of these measurements against simulated spectra as well as manual equivalent width measurements of five stellar spectra over a range of signal-to-noise values and a set of long slit emission spectra. We find that ROBOSPECT accurately matches both the synthetic and manual measurements, with scatter consistent with the expectations based on the data quality and the results of Cayrel (1988).</blockquote>View original: <a href="http://arxiv.org/abs/1308.0757">http://arxiv.org/abs/1308.0757</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-48427819415801456792013-08-06T00:16:00.003-07:002013-08-06T00:16:01.189-07:001308.0825 (Farzana Meru et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0825">Growth and fragmentation of centimetre-sized dust aggregates: the
dependence on aggregate size and porosity</a> [<a href="http://arxiv.org/pdf/1308.0825">PDF</a>]</h2>Farzana Meru, Ralf J. Geretshauser, Christoph Schaefer, Roland Speith, Wilhelm Kley<a name='more'></a><blockquote class="abstract">We carry out three-dimensional Smoothed Particle Hydrodynamics simulations of spherical homogeneous SiO2 dust aggregates to investigate how the mass and the porosity of the aggregates affects their ability to survive an impact at various different collision velocities (between 1 - 27.5m/s). We explore how the threshold velocities for fragmentation vary with these parameters. Crucially, we find that the porosity plays a part of utmost importance in determining the outcome of collisions. In particular, we find that aggregates with filling factors >37% are significantly weakened and that the velocity regime in which the aggregates grow is reduced or even non-existent (instead, the aggregates either rebound off each other or break apart). At filling factors less than ~37% we find that more porous objects are weaker but not as weak as highly compact objects with filling factors >37%. In addition we find that (for a given aggregate density) collisions between very different mass objects have higher threshold velocities than those between very similar mass objects. We find that fragmentation velocities are higher than the typical values of 1m/s and that growth can even occur for velocities as high as 27.5m/s. Therefore, while the growth of aggregates is more likely if collisions between different sized objects occurs or if the aggregates are porous with filling factor <37%, it may also be hindered if the aggregates become too compact.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0825">http://arxiv.org/abs/1308.0825</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-32233414167171058032013-08-06T00:16:00.001-07:002013-08-06T00:16:00.123-07:001308.0830 (Bo Zhao et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0830">Effect of Magnetic Misalignment on Protobinary Evolution</a> [<a href="http://arxiv.org/pdf/1308.0830">PDF</a>]</h2>Bo Zhao, Zhi-Yun Li, Kaitlin M. Kratter<a name='more'></a><blockquote class="abstract">The majority of solar-type stars reside in multiple systems, especially binaries. They form in dense cores of molecular clouds that are observed to be significantly magnetized. Our previous study shows that magnetic braking can tighten the binary separation during the protostellar mass accretion phase by removing the angular momentum of the accreting material. Recent numerical calculations of single star formation have shown that misalignment between the magnetic field and rotation axis may weaken both magnetic braking and the associated magnetically driven outflows. These two effects allow for disk formation even in strongly magnetized cores. Here we investigate the effects of magnetic field misalignment on the properties of protobinaries. Somewhat surprisingly, the misaligned magnetic field is more efficient at tightening the binary orbit compared to the aligned field. The main reason is that the misalignment weakens the magnetically-driven outflow, which allows more material to accrete onto the binary. Even though the specific angular momentum of this inner material is higher than in the aligned case, it is insufficient to compensate for the additional mass. A corollary of this result is that a weaker field is required to achieve the same degree of inward migration when the field is tilted relative to the rotation axis. Large field misalignment also helps to produce rotationally-supported circumbinary disks even for relatively strong magnetic fields, by weakening the magnetically-dominated structure close to the binary. Our result may provide an explanation for the circumbinary disks detected in recent SMA and ALMA observations.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0830">http://arxiv.org/abs/1308.0830</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-4186279970647919512013-08-06T00:15:00.015-07:002013-08-06T00:15:59.087-07:001308.0876 (Yun-Wei Yu et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0876">Bright "merger-nova" from the remnant of a neutron star binary merger: A
signature of a newly born, massive, millisecond magnetar</a> [<a href="http://arxiv.org/pdf/1308.0876">PDF</a>]</h2>Yun-Wei Yu, Bing Zhang, He Gao<a name='more'></a><blockquote class="abstract">A massive millisecond magnetar may survive a merger of a neutron star (NS) binary, which could continuously power the merger ejecta and accelerate it to a mildly-relativistic speed. We develop a generic dynamic model for the merger ejecta, where the transition from sub-relativistic to relativistic is well described. We show that the ejecta emission (the "merger-nova") powered by the magnetar can have an optical peak brightness comparable to a supernova, which is a few tens or hundreds times brighter than the radioactive-powered merger-novae (the so-called macro-nova or kilo-nova). However, the duration of the magnetar-powered merger-nova peaks earlier and has a duration significantly shorter than that of a supernova. An X-ray bump appears even earlier in time. Such merger-novae may be detected with NS-NS merger events with or without a short gamma-ray burst, and could be a bright electromagnetic counterpart for gravitational wave bursts due to NS-NS mergers. If detected, it suggests that the merger leaves behind a massive NS, which has important implications in understanding the equation-of-state of nuclear matter.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0876">http://arxiv.org/abs/1308.0876</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-28344885146911333402013-08-06T00:15:00.013-07:002013-08-06T00:15:58.340-07:001308.0898 (P. D. Kiel et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0898">Accretion, Ablation and Propeller Evolution in Close Millisecond Pulsar
Binary Systems</a> [<a href="http://arxiv.org/pdf/1308.0898">PDF</a>]</h2>P. D. Kiel, R. E Taam<a name='more'></a><blockquote class="abstract">A model for the formation and evolution of binary millisecond radio pulsars in systems with low mass companions (< 0.1 Msun) is investigated using a binary population synthesis technique. Taking into account the non conservative evolution of the system due to mass loss from an accretion disk as a result of propeller action and from the companion via ablation by the pulsar, the transition from the accretion powered to rotation powered phase is investigated. It is shown that the operation of the propeller and ablation mechanisms can be responsible for the formation and evolution of black widow millisecond pulsar systems from the low mass X-ray binary phase at an orbital period of ~0.1 day. For a range of population synthesis input parameters, the results reveal that a population of black widow millisecond pulsars characterized by orbital periods as long as ~0.4 days and companion masses as low as ~0.005 Msun can be produced. The orbital periods and minimum companion mass of this radio millisecond pulsar population critically depend on the thermal bloating of the semi-degenerate hydrogen mass losing component, with longer orbital periods for a greater degree of bloating. Provided that the radius of the companion is increased by about a factor of 2 relative to a fully degenerate, zero temperature configuration, an approximate agreement between observed long orbital periods and theoretical modeling of hydrogen rich donors can be achieved. We find no discrepancy between the estimated birth rates for LMXBs and black widow systems, which on average are $\sim~1.3\times10^{-5} {\rm yr}^{-1}$ and $1.3\times10^{-7} {\rm yr}^{-1}$ respectively.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0898">http://arxiv.org/abs/1308.0898</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-10584634372357029122013-08-06T00:15:00.011-07:002013-08-06T00:15:54.625-07:001308.0918 (L. Rosén et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0918">Strong variable linear polarization in the cool active star II Peg</a> [<a href="http://arxiv.org/pdf/1308.0918">PDF</a>]</h2>L. Rosén, O. Kochukhov, G. A. Wade<a name='more'></a><blockquote class="abstract">Magnetic fields of cool active stars are currently studied polarimetrically using only circular polarization observations. Including linear polarization in the reconstruction of stellar magnetic fields allows more information about the magnetic field to be extracted and significantly improves the reliability of stellar magnetic field maps. The goal of this study is to initiate systematic observations of active stars in all four Stokes parameters and to identify cool stars for which linear polarization can be detected at a level sufficient for Zeeman Doppler imaging (ZDI). Four active RS CVn binaries, II Peg, HR 1099, IM Peg, and sigma Gem, were observed with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope during a time period from February 2012 to January 2013. The least-squares deconvolution procedure was applied to derive mean polarization profiles of all four Stokes parameters. Linear polarization was detected in all four stars in at least one observation. At the same time, II Peg showed an exceptionally strong and highly variable linear polarization signature throughout all observations. This establishes II Peg as the first promising target for ZDI in all four Stokes parameters and suggests the feasibility of such an analysis with existing equipment for at least a few of the most active cool stars.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0918">http://arxiv.org/abs/1308.0918</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-27072961913228549762013-08-06T00:15:00.009-07:002013-08-06T00:15:53.856-07:001308.0919 (Moti R. Dugair et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0919">Detection of a variable QPO at ~41 mHz in the Be/X-ray transient pulsar
4U 0115+634</a> [<a href="http://arxiv.org/pdf/1308.0919">PDF</a>]</h2>Moti R. Dugair, Gaurava K. Jaisawal, Sachindra Naik, S. N. A. Jaaffrey<a name='more'></a><blockquote class="abstract">We report the detection of quasi-periodic oscillation (QPO) at ~41 mHz in the transient high-mass Be/X-ray binary pulsar 4U~0115+634 using data from the Rossi X-Ray Timing Explorer (RXTE) observatory. The observations used in the present work were carried out during X-ray outbursts in 1999 March-April, 2004 September-October and 2008 March-April. This frequency of the newly detected QPO was found to vary in 27-46 mHz range. This ~41 mHz QPO was detected in four of the 36 pointed RXTE Proportional Counter Array (PCA) observations during 1999 outburst where as during 2004 and 2008 outbursts, it was detected in four and three times out of 33 and 26 observations, respectively. Though QPOs at ~2 mHz, and ~62 mHz were reported earlier, the ~41 mHz QPO and its first harmonic were detected for the first time in this pulsar. There are three RXTE/PCA observations where multiple QPOs were detected in the power density spectrum of 4U~0115+634. Simultaneous presence of multiple QPOs is rarely seen in accretion powered X-ray pulsars. Spectral analysis of all the pointed RXTE/PCA observations revealed that the 3-30 keV energy spectrum was well described by Negative and Positive Power-law with EXponential cutoff (NPEX) continuum model along with interstellar absorption and cyclotron absorption components. During the three X-ray outbursts, however, no systematic variation in any of the spectral parameters other than the earlier reported anti-correlation between cyclotron absorption energy and luminosity was seen. Presence of any systematic variation of QPO frequency and rms of QPO with source flux were also investigated yielding negative results.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0919">http://arxiv.org/abs/1308.0919</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-52672294864436195992013-08-06T00:15:00.007-07:002013-08-06T00:15:52.769-07:001308.0944 (Simone Dall'Osso et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0944">Binary Pulsar J0737-3039: Evidence for a new core collapse and neutron
star formation mechanism</a> [<a href="http://arxiv.org/pdf/1308.0944">PDF</a>]</h2>Simone Dall'Osso, Tsvi Piran, Nir Shaviv<a name='more'></a><blockquote class="abstract">The binary pulsar J0737-3039 is the only known system having two observable pulsars, thus offering a unique laboratory to test general relativity and explore pulsar physics. Based on the low eccentricity and the position within the galactic plane, Piran & Shaviv (2004, 2005) argued that pulsar B had a non-standard formation scenario with little or no mass ejection. They have also predicted that the system would have a very slow proper motion. Pulsar timing measurements (Kramer et al. 2006; Deller et al. 2009) confirmed this prediction. The recent observations of the alignment between the spin of pulsar A and the binary orbit is also in agreement with this scenario. Detailed simulations of the formation process of pulsar B enable us to show that its progenitor, just before the collapse, was a massive O-Ne-Mg white dwarf surrounded by a tenuous, 0.1-0.16 M_sun, envelope. This envelope was ejected when the white dwarf collapsed to form a neutron star. Pulsar B was born as a slow rotator (spin period ~ 1 s) and a kick received when the pulsar formed changed its spin direction to the current one. This realization sheds light on the angular momentum evolution of the progenitor star, a process which is strongly affected by interaction with the binary companion. The slow proper motion of the system also implies that the system must have undergone a phase of mass transfer in which Star A shed a significant fraction of its mass onto B.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0944">http://arxiv.org/abs/1308.0944</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-989538173891953952013-08-06T00:15:00.005-07:002013-08-06T00:15:51.788-07:001308.0945 (J. J. Walmswell et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0945">A transdimensional Bayesian method to infer the star formation history
of resolved stellar populations</a> [<a href="http://arxiv.org/pdf/1308.0945">PDF</a>]</h2>J. J. Walmswell, J. J. Eldridge, B. J. Brewer, C. A. Tout<a name='more'></a><blockquote class="abstract">We propose a new method to infer the star formation histories of resolved stellar populations. With photometry one may plot observed stars on a colour-magnitude diagram (CMD) and then compare with synthetic CMDs representing different star formation histories. This has been accomplished hitherto by parametrising the model star formation history as a histogram, usually with the bin widths set by fixed increases in the logarithm of time. A best fit is then found with maximum likelihood methods and we consider the different means by which a likelihood can be calculated. We then apply Bayesian methods by parametrising the star formation history as an unknown number of Gaussian bursts with unknown parameters. This parametrisation automatically provides a smooth function of time. A Reversal Jump Markov Chain Monte Carlo method is then used to find both the most appropriate number of Gaussians, thus avoiding avoid overfitting, and the posterior probability distribution of the star formation rate. We apply our method to artificial populations and to observed data. We discuss the other advantages of the method: direct comparison of different parametrisations and the ability to calculate the probability that a given star is from a given Gaussian. This allows the investigation of possible sub-populations.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0945">http://arxiv.org/abs/1308.0945</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-14688412458564348252013-08-06T00:15:00.003-07:002013-08-06T00:15:50.756-07:001308.1033 (Marcio G B de Avellar et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.1033">Entropy, Disequilibrium and Complexity in Compact Stars: An information
theory approach to understand their Composition</a> [<a href="http://arxiv.org/pdf/1308.1033">PDF</a>]</h2>Marcio G B de Avellar, Jorge E Horvath<a name='more'></a><blockquote class="abstract">The composition of neutron stars is an issue that has been studied for decades. Yet we do not know exactly what these very compact objects are made of. At this stage of the technological development the best we can do is to constrain the range of equations of state via the mass-radius diagram. From the theoretical point of view the things are not easier. The theory of matter at high density and temperature is not well established. Keeping this in mind we applied the Information Theory as a novel way to restrict the range of possible equations of state for neutron stars. From our results, we conclude that if order costs energy, then nature should favour exotic strange quark stars over the hadronic neutron stars and that there is a trend for the types of compact stars of classified under the nomenclature "neutrons stars" to be at a state of minimum complexity.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1033">http://arxiv.org/abs/1308.1033</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-59720784480040836632013-08-06T00:15:00.001-07:002013-08-06T00:15:48.464-07:001308.1058 (G. R. Mamatsashvili et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.1058">Revisiting linear dynamics of non-axisymmetric perturbations in weakly
magnetized accretion discs</a> [<a href="http://arxiv.org/pdf/1308.1058">PDF</a>]</h2>G. R. Mamatsashvili, G. D. Chagelishvili, G. Bodo, P. Rossi<a name='more'></a><blockquote class="abstract">We investigate linear dynamics of non-axisymmetric perturbations in incompressible, vertically stratified Keplerian discs with a weak vertical magnetic field in the shearing box approximation. Perturbations are decomposed into shearing waves whose evolution is followed via numerical integration of the linearized ideal MHD equations. There are two basic modes in the system -- inertia-gravity waves and magnetic mode, which displays the magnetorotational instability (MRI). As distinct from previous studies, we introduce `eigenvariables' characterizing each (counter-propagating) component of the inertia-gravity and magnetic modes, which are governed by a set of four first order coupled ordinary differential equations. This allowed us to identify a new process of linear coupling of the two above non-axisymmetric modes due to the disc's differential rotation. We did a comparative analysis of the dynamics of non-axisymmetric and axisymmetric magnetic mode perturbations. It is shown that the growth of optimal and close-to-optimal non-axisymmetric harmonics of this mode, having transient nature, can prevail over the exponential growth of axisymmetric ones (i.e., over the axisymmetric MRI) during dynamical time. A possible implication of this result for axisymmetric channel solutions is discussed. Specifically, the formation of the channel may be affected/impeded by non-axisymmetric modes already at the early linear stage leading to its untimely disruption -- the outcome strongly depends on the amplitude and spectrum of initial perturbation. So, this competition may result in an uncertainty in the magnetic mode's non-linear dynamics. It is also shown that a maximum non-axisymmetric growth is at vertical wavelengths close to the scale-height for which compressibility effects are important. This indirectly suggests that compressibility plays a role in the dynamics of the non-axisymmetric MRI.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1058">http://arxiv.org/abs/1308.1058</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-37514642482158494052013-08-06T00:04:00.001-07:002013-08-06T00:04:46.127-07:001308.1080 (Jörn Warnecke et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.1080">Bipolar magnetic structures driven by stratified turbulence with a
coronal envelope</a> [<a href="http://arxiv.org/pdf/1308.1080">PDF</a>]</h2>Jörn Warnecke, Illa R. Losada, Axel Brandenburg, Nathan Kleeorin, Igor Rogachevskii<a name='more'></a><blockquote class="abstract">We report the spontaneous formation of a bipolar magnetic structure in a direct numerical simulation of stratified forced turbulence with an outer coronal envelope. We impose a weak horizontal magnetic field in the entire domain. The turbulence is forced with transverse random waves in the lower (turbulent) part of the domain. After about 1.5 turbulent diffusion times, a bipolar magnetic region of vertical field develops with two coherent circular structures that live during one turbulent diffusion time, and then decay during 0.5 turbulent diffusion times. The resulting magnetic field strengths are comparable to the equipartition value with respect to the turbulent kinetic energy. The bipolar magnetic region forms a loop-like structure in the corona. We associate the magnetic structure formation with the negative effective magnetic pressure instability in the two-layer model.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1080">http://arxiv.org/abs/1308.1080</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-61548475803533282032013-08-05T00:03:00.021-07:002013-08-05T00:03:43.380-07:001308.0334 (Anthony L. Piro et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0334">Reconciling 56Ni Production in Type Ia Supernovae with Double Degenerate
Scenarios</a> [<a href="http://arxiv.org/pdf/1308.0334">PDF</a>]</h2>Anthony L. Piro, Todd A. Thompson, Christopher S. Kochanek<a name='more'></a><blockquote class="abstract">Binary white dwarf (WD) coalescence driven by gravitational waves or collisions in triple systems are potential progenitors of Type Ia supernovae (SNe Ia). We combine the distribution of 56Ni inferred from observations of SNe Ia with the results of both sub-Chandrasekhar detonation models and direct collision calculations to estimate what mass WDs should be exploding in each scenario to reproduce the observations. These WD mass distributions are then compared with the observed Galactic WD mass distribution and Monte Carlo simulations of WD-WD binary populations. For collisions, we find that the average mass of the individual components of the WD-WD binary must be peaked at ~0.75Msun, significantly higher than the average WD mass in binaries or in the field of ~0.55-0.60Msun. Thus, if collisions indeed produce a large fraction of SNe Ia, then a mechanism must exist that favors large mass WDs. In particular, collisions between WDs of average mass must be highly suppressed. For sub-Chandrasekhar detonations, we find that the average mass of the exploding WDs must be peaked at ~1.1Msun, consistent with the average sum of the masses in WD-WD binaries. This interesting similarity should be tested by future calculations of the 56Ni yield from double degenerate mergers. These models may also explain why SNe Ia are on average dimmer in early-type hosts: in old environments binaries evolve too quickly to have mergers between two high mass WDs at current times. As future simulations explore the 56Ni yield over a wider range of parameters, the general framework discussed here will be an important tool for continuing to assess double degenerate scenarios.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0334">http://arxiv.org/abs/1308.0334</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-89838643856442062262013-08-05T00:03:00.019-07:002013-08-05T00:03:42.649-07:001308.0357 (Lovro Palaversa et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0357">Exploring the Variable Sky with LINEAR. III. Classification of Periodic
Light Curves</a> [<a href="http://arxiv.org/pdf/1308.0357">PDF</a>]</h2>Lovro Palaversa, Željko Ivezić, Laurent Eyer, Domagoj Ruždjak, Davor Sudar, Mario Galin, Andrea Kroflin, Martina Mesarić, Petra Munk, Dijana Vrbanec, Hrvoje Božić, Sarah Loebman, Branimir Sesar, Lorenzo Rimoldini, Nicholas Hunt-Walker, Jacob VanderPlas, David Westman, J. Scott Stuart, Andrew C. Becker, Gregor Srdoč, Przemyslaw Wozniak, Hakeem Oluseyi<a name='more'></a><blockquote class="abstract">We describe the construction of a highly reliable sample of approximately 7,000 optically faint periodic variable stars with light curves obtained by the asteroid survey LINEAR across 10,000 sq.deg of northern sky. Majority of these variables have not been cataloged yet. The sample flux limit is several magnitudes fainter than for most other wide-angle surveys; the photometric errors range from ~0.03 mag at $r=15$ to ~0.20 mag at r=18. Light curves include on average 250 data points, collected over about a decade. Using SDSS-based photometric recalibration of the LINEAR data for about 25 million objects, we selected ~200,000 most probable candidate variables and visually confirmed and classified approximately 7,000 periodic variables using phased light curves. The reliability and uniformity of visual classification across eight human classifiers was calibrated and tested using a SDSS Stripe 82 region variable star catalog, and verified using an unsupervised machine learning approach. The resulting sample of periodic LINEAR variables is dominated by 3,900 RR Lyrae stars and 2,700 eclipsing binary stars of all subtypes, and includes small fractions of relatively rare populations such as asymptotic giant branch stars and SX Phoenicis stars. We discuss the distribution of these mostly uncataloged variables in various diagrams constructed with optical-to-infrared SDSS, 2MASS and WISE photometry, and with LINEAR light curve features. An interesting side result is a robust and precise quantitative description of a strong correlation between the light-curve period and color/spectral type for close and contact eclipsing binary stars. These large samples of robustly classified variable stars will enable detailed statistical studies of the Galactic structure and physics of binary and other stars, and we make them publicly available.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0357">http://arxiv.org/abs/1308.0357</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-46865390940754520802013-08-05T00:03:00.017-07:002013-08-05T00:03:41.656-07:001308.0369 (Douglas R. Gies et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0369">KIC 9406652: An Unusual Cataclysmic Variable in the Kepler Field of View</a> [<a href="http://arxiv.org/pdf/1308.0369">PDF</a>]</h2>Douglas R. Gies, Zhao Guo, Steve B. Howell, Martin D. Still, Tabetha S. Boyajian, Abe J. Hoekstra, Kian J. Jek, Daryll LaCourse, Troy Winarski<a name='more'></a><blockquote class="abstract">KIC 9406652 is a remarkable variable star in the Kepler field of view that shows both very rapid oscillations and long term outbursts in its light curve. We present an analysis of the light curve over quarters 1 to 15 and new spectroscopy that indicates that the object is a cataclysmic variable with an orbital period of 6.108 hours. However, an even stronger signal appears in the light curve periodogram for a shorter period of 5.753 hours, and we argue that this corresponds to the modulation of flux from the hot spot region in a tilted, precessing disk surrounding the white dwarf star. We present a preliminary orbital solution from radial velocity measurements of features from the accretion disk and the photosphere of the companion. We use a Doppler tomography algorithm to reconstruct the disk and companion spectra, and we also consider how these components contribute to the object's spectral energy distribution from ultraviolet to infrared wavelengths. This target offers us a remarkable opportunity to investigate disk processes during the high mass transfer stage of evolution in cataclysmic variables.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0369">http://arxiv.org/abs/1308.0369</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-42061948330316860702013-08-05T00:03:00.015-07:002013-08-05T00:03:40.945-07:001308.0405 (Adam Schneider et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0405">The nearby, young, isolated, dusty star HD 166191</a> [<a href="http://arxiv.org/pdf/1308.0405">PDF</a>]</h2>Adam Schneider, Inseok Song, Carl Melis, Ben Zuckerman, Mike Bessell, Tara Hufford, Sasha Hinkley<a name='more'></a><blockquote class="abstract">We report an in-depth study of the F8-type star HD 166191, identified in an ongoing survey for stars exhibiting infrared emission above their expected photospheres in the Wide-field Infrared Survey Explorer all-sky catalog. The fractional IR luminosity measured from 3.5 to 70 $\mu$m is exceptionally high (L$_{IR}$/L$_{bol}$ $\sim$10%). Near-diffraction limited imaging observations with the T-ReCS Si filter set on the Gemini South telescope and adaptive optics imaging with the NIRC2 Lp filter on the Keck II telescope confirmed that the excess emission coincides with the star. Si-band images show a strong solid-state emission feature at $\sim$10 $\mu$m. Theoretical evolutionary isochrones and optical spectroscopic observations indicate a stellar age in the range 10-100 Myr. The large dust mass seen in HD 166191's terrestrial planet zone is indicative of a recent collision between planetary embryos or massive ongoing collisional grinding associated with planet building.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0405">http://arxiv.org/abs/1308.0405</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-88897270347911717722013-08-05T00:03:00.013-07:002013-08-05T00:03:39.883-07:001308.0406 (K. Maeda et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0406">Properties of Newly Formed Dust Grains in The Luminous Type IIn
Supernova 2010jl</a> [<a href="http://arxiv.org/pdf/1308.0406">PDF</a>]</h2>K. Maeda, T. Nozawa, D. K. Sahu, Y. Minowa, K. Motohara, I. Ueno, G. Folatelli, T. -S. Pyo, Y. Kitagawa, K. S. Kawabata, G. C. Anupama, T. Kozasa, T. J. Moriya, M. Yamanaka, K. Nomoto, M. Bersten, R. Quimby, M. Iye<a name='more'></a><blockquote class="abstract">Supernovae (SNe) have been proposed to be the main production sites of dust grains in the Universe. Our knowledge on their importance to dust production is, however, limited by observationally poor constraints on the nature and amount of dust particles produced by individual SNe. In this paper, we present a spectrum covering optical through near-Infrared (NIR) light of the luminous Type IIn supernova (SN IIn) 2010jl around one and half years after the explosion. This unique data set reveals multiple signatures of newly formed dust particles. The NIR portion of the spectrum provides a rare example where thermal emission from newly formed hot dust grains is clearly detected. We determine the main population of the dust species to be carbon grains at a temperature of ~1,350 - 1,450K at this epoch. The mass of the dust grains is derived to be ~(7.5 - 8.5) x 10^{-4} Msun. Hydrogen emission lines show wavelength-dependent absorption, which provides a good estimate on the typical size of the newly formed dust grains (~0.1 micron, and most likely <~0.01 micron). We attribute the dust grains to have been formed in a dense cooling shell as a result of a strong SN-circumstellar media (CSM) interaction. The dust grains occupy ~10% of the emitting volume, suggesting an inhomogeneous, clumpy structure. The average CSM density is required to be >~3 x 10^{7} cm^{-3}, corresponding to a mass loss rate of >~0.02 Msun yr^{-1} (for a mass loss wind velocity of ~100 km s^{-1}). This strongly supports a scenario that SN 2010jl and probably other luminous SNe IIn are powered by strong interactions within very dense CSM, perhaps created by Luminous Blue Variable (LBV)-like eruptions within the last century before the explosion.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0406">http://arxiv.org/abs/1308.0406</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-7141959579313896032.post-5072096539669600122013-08-05T00:03:00.011-07:002013-08-05T00:03:37.971-07:001308.0411 (Sachindra Naik)<h2 class="title"><a href="http://arxiv.org/abs/1308.0411">X-ray and Infrared Properties of Be/X-ray Binary Pulsars</a> [<a href="http://arxiv.org/pdf/1308.0411">PDF</a>]</h2>Sachindra Naik<a name='more'></a><blockquote class="abstract">During normal Type I outbursts, the pulse profiles of Be/X-ray binary pulsars are found to be complex in soft X-ray energy ranges. The profiles in soft X-ray energy ranges are characterized by the presence of narrow absorption dips or dip-like features at several pulse phases. However, in hard X-ray energy ranges, the pulse profiles are rather smooth and single-peaked. Pulse phase-averaged spectroscopy of the these pulsars had been carried out during Type I outbursts. The broad-band spectrum of these pulsars were well described by partial covering high energy cutoff power-law model with interstellar absorption and Iron K_alpha emission line at 6.4 keV. Phase-resolved spectroscopy revealed that the presence of additional matter at certain pulse phases that partially obscured the emitted radiation giving rise to dips in the pulse profiles. The additional absorption is understood to be taking place by matter in the accretion streams that are phase locked with the neutron star. Optical/infrared observations of the companion Be star during these Type I outbursts showed that the increase in the X-ray intensity of the pulsar is coupled with the decrease in the optical/infrared flux of the companion star. There are also several changes in the IR/optical emission line profiles during these X-ray outbursts. The X-ray properties of these pulsars during Type I outbursts and corresponding changes in optical/infrared wavebands are discussed in this paper.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0411">http://arxiv.org/abs/1308.0411</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0