Etude et mise en oeuvre des techniques Temps-Distance en Heliosismologie [SSA]

In the same way that seismologists study the interior of the earth from the waves generated by earthquakes, heliosismology is the science that is interested in the study of oscillatory waves inside the Sun. Temperature, chemical composition, rotational velocities at different depths are all factors that influence the oscillation frequencies of the waves trapped inside the Sun. The observation and measurement of the frequencies (or velocities) of oscillations of the principal modes on the surface of the Sun allow us to study the internal properties of the star. In our study, we use a local helioseismic method, called “Time-Distance”; the latter, after appropriate treatment (remapping, tracking, and filtering) of the data, which carried out from the GONG (Global Oscillation Network Group) observation network, allows us to deduce, by correlation of the observed signals, the relationship between the time of the wave path and its crossed distance (hence the name of the method) between different points on the surface as a function of the different solar sub-surface parameters, via the dispersion relation. Once the travel times have been established by approximation of the correlation function by a Gaussian wave packet and based on the Fermat principle dealing with acoustic waves, we find the relationship between these times and the internal parameters of the crossed medium. Two models are considered in this study. The first does not include the effects of the magnetic field and takes into account the flow velocity and the sound speed. The second, on the other hand, includes the magnetic effects, by means of the Alfven velocity (magnetic field perturbation velocity), in addition to the parameters mentioned above. The last part of this paper was devoted to the inversion of the obtained times, which allows us to go back to the solar internal parameters.

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M. Hadjara
Fri, 22 Sep 17

Comments: Master’s thesis (2006), University of Sciences and Technology Houari Boumediene, Algeria. in French

Spectral Variability of Two Rapidly Rotating Brown Dwarfs: 2MASS J08354256-0819237 and 2MASS J18212815+1414010 [SSA]

L dwarfs exhibit low-level, rotationally-modulated photometric variability generally associated with heterogeneous, cloud-covered atmospheres. The spectral character of these variations yields insight into the particle sizes and vertical structure of the clouds. Here we present the results of a high precision, ground-based, near-infrared, spectral monitoring study of two mid-type L dwarfs that have variability reported in the literature, 2MASS J08354256-0819237 and 2MASS J18212815+1414010, using the SpeX instrument on the Infrared Telescope Facility. By simultaneously observing a nearby reference star, we achieve <0.15% per-band sensitivity in relative brightness changes across the 0.9–2.4um bandwidth. We find that 2MASS J0835-0819 exhibits marginal (< ~0.5% per band) variability with no clear spectral dependence, while 2MASS J1821+1414 varies by up to +/-1.5% at 0.9 um, with the variability amplitude declining toward longer wavelengths. The latter result extends the variability trend observed in prior HST/WFC3 spectral monitoring of 2MASS J1821+1414, and we show that the full 0.9-2.4 um variability amplitude spectrum can be reproduced by Mie extinction from dust particles with a log-normal particle size distribution with a median radius of 0.24 um. We do not detect statistically significant phase variations with wavelength. The different variability behavior of 2MASS J0835-0819 and 2MASS J1821+1414 suggests dependencies on viewing angle and/or overall cloud content, underlying factors that can be examined through a broader survey.

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E. Schlawin, A. Burgasser, T. Karalidi, et. al.
Thu, 21 Sep 17

Comments: ApJ, accepted

Convection and Overshoot in models of gamma Doradus and delta Scuti stars [SSA]

We investigate the pulsation properties of stellar models representative of $\delta$ Scuti and $\gamma$ Doradus variables. We have calculated a grid of stellar models from 1.2 to 2.2 M$_{\odot}$, including the effects of both rotation and convective overshoot using MESA, and we investigate the pulsation properties of these models using GYRE. We discuss observable patterns in the frequency spacing for $p$ modes and the period spacings for g modes. Using the observable patterns in g mode period spacings, it may be possible to observationally constrain the convective overshoot and rotation of a model. We also calculate the pulsation constant (Q) for all models in our grid, and investigate the variation with convective overshoot and rotation. The variation in Q values of radial modes can be used to place constraints on the convective overshoot and rotation of stars in this region. As a test case, we apply this method to a sample of 22 high amplitude $\delta$ Scuti stars (HADS), and provide estimates for the convective overshoot of the sample.

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C. Lovekin and J. Guzik
Thu, 21 Sep 17

Comments: 10 pages, 8 figures. Accepted for publication in ApJ

Near-infrared and optical studies of the highly obscured nova V1831 Aquilae (Nova Aquilae 2015) [SSA]

Near Infrared (NIR) and optical photometry and spectroscopy are presented for the nova V1831 Aquilae, covering the early decline and dust forming phases during the first $\sim$90 days after its discovery. The nova is highly reddened due to interstellar extinction. Based solely on the nature of NIR spectrum we are able to classify the nova to be of the Fe II class. The distance and extinction to the nova are estimated to be 6.1 $\pm$ 0.5 kpc and $A_{\rm v}$ $\sim$ 9.02 respectively. Lower limits of the electron density, emission measure and ionized ejecta mass are made from a Case B analysis of the NIR Brackett lines while the neutral gas mass is estimated from the optical [OI] lines. We discuss the cause for a rapid strengthening of the He I 1.0830 $\mu$m line during the early stages. V1831 Aql formed a modest amount of dust fairly early ($\sim$ 19.2 days after discovery); the dust shell is not seen to be optically thick. Estimates are made of the dust temperature, dust mass and grain size. Dust formation commences around day 19.2 at a condensation temperature of 1461 $\pm$ 15 K, suggestive of a carbon composition, following which the temperature is seen to gradually decrease to 950K. The dust mass shows a rapid initial increase which we interpret as being due to an increase in the number of grains, followed by a period of constancy suggesting the absence of grain destruction processes during this latter time. A discussion is made of the evolution of these parameters, including certain peculiarities seen in the grain radius evolution.

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D. Banerjee, M. Srivastava, N. Ashok, et. al.
Thu, 21 Sep 17

Comments: 14 pages, to appear in MNRAS

The small-scale structure of photospheric convection retrieved by a deconvolution technique applied to Hinode/SP data [SSA]

Solar granules are bright patterns surrounded by dark channels called intergranular lanes in the solar photosphere and are a manifestation of overshooting convection. Observational studies generally find stronger upflows in granules and weaker downflows in intergranular lanes. This trend is, however, inconsistent with the results of numerical simulations in which downflows are stronger than upflows through the joint action of gravitational acceleration/deceleration and pressure gradients. One cause of this discrepancy is the image degradation caused by optical distortion and light diffraction and scattering that takes place in an imaging instrument. We apply a deconvolution technique to Hinode/SP data in an attempt to recover the original solar scene. Our results show a significant enhancement in both, the convective upflows and downflows, but particularly for the latter. After deconvolution, the up- and downflows reach maximum amplitudes of -3.0 km/s and +3.0 km/s at an average geometrical height of roughly 50 km, respectively. We found that the velocity distributions after deconvolution match those derived from numerical simulations. After deconvolution the net LOS velocity averaged over the whole FOV lies close to zero as expected in a rough sense from mass balance.

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T. Oba, T. Riethmuller, S. Solanki, et. al.
Thu, 21 Sep 17

Comments: 32 pages, 13 figures, accepted for publication in ApJ

Ca II 8542 Å brightenings induced by a solar microflare [SSA]

We study small-scale brightenings in Ca II 8542 \AA\ line-core images to determine their nature and effect on localized heating and mass transfer in active regions. High-resolution 2D spectroscopic observations of an active region in the Ca II 8542 \AA\ line were acquired with the GFPI attached to the 1.5-meter GREGOR telescope. Inversions of the spectra were carried out using NICOLE. We identified three brightenings of sizes up to 2″x2″. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or IRIS bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of about 20″. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to -2.2 km/s. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF. Conclusions: We detected Ca II brightenings that coincided with the footpoint location of an MF. The MF event led to a rise of plasma in the upper photosphere, both before and during the impulsive phase. Excess mass, previously raised to at most chromospheric layers, slowly drained downward along arches toward the footpoints of the MF.

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C. Kuckein, A. Diercke, S. Manrique, et. al.
Thu, 21 Sep 17

Comments: Accepted for publication in Astronomy & Astrophysics, 13 pages, 6 figures, 1 online movie

A Search for a Surviving White Dwarf Companion in SN 1006 [SSA]

Multiple channels have been proposed to produce Type Ia supernovae, with many scenarios suggesting that the exploding white dwarf accretes from a binary companion pre-explosion. In almost all cases, theory suggests that this companion will survive. However, no such companion has been unambiguously identified in ancient supernova remnants — possibly falsifying the accretion scenario. Existing surveys, however, have only looked for stars as faint as $\approx 0.1 L_\odot$ and thus would have missed a surviving white dwarf companion. In this work, we present very deep DECAM imaging $(u, g, r, z)$ of the Type Ia supernova remnant SN 1006 specifically to search for a potential surviving white dwarf companion. We find no object within the inner third of the SN 1006 remnant that is consistent with a relatively young cooling white dwarf. We find that if there is a companion white dwarf, it must have formed long ago and cooled undisturbed for $> 10^8$ yr to be consistent with the redder objects in our sample. We conclude that our findings are consistent with the complete destruction of the secondary (such as in a merger) or an unexpectedly cool and thus very dim surviving companion white dwarf.

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W. Kerzendorf, G. Strampelli, K. Shen, et. al.
Thu, 21 Sep 17

Comments: 9 pages, 5 figure, submitted to MNRAS – comments welcome