Evidence of grain growth in the disk of the bipolar proto-planetary nebula M 1--92

We investigate the dust size and dust shell structure of the bipolar proto-planetary nebula M 1--92 by means of radiative transfer modeling. Our models consists of a disk and bipolar lobes that are surrounded by an AGB shell, each component having different dust characteristics. The upper limit of the grain size $a_\mathrm{max}$ in the lobes is estimated to be $0.5 \mu$m from the polarization value in the bipolar lobe. The $a_\mathrm{max}$ value of the disk is constrained with the disk mass (0.2 M_{\sun}), which was estimated from a previous CO emission line observation. We find a good model with $a_\mathrm{max}=1000.0 \mu$m, which provides an approximated disk mass of 0.15 M_{\sun}. Even taking into account uncertainties such as the gas-to-dust mass ratio, a significantly larger dust of $a_\mathrm{max}>100.0 \mu$m, comparing to the dust in the lobe, is expected. We also estimated the disk inner radius, the disk outer radius, and the envelope mass to be 30 $R_\star$(=9 AU), 4500 AU, and 4 M_{\sun}, respectively, where $v_\mathrm{exp}$ is the expansion velocity. If the dust existing in the lobes in large separations from the central star undergoes little dust processing, the dust sizes preserves the ones in the dust formation. Submicron-sized grains are found in many objects besides M 1--92, suggesting that the size does not depend much on the object properties, such as initial mass of the central star and chemical composition of the stellar system. On the other hand, the grain sizes in the disk do. Evidence of large grains has been reported in many bipolar PPNs, including M 1--92. This result suggests that disks play an important role in grain growth.Comment: 8 pages with 3 figures. Accepted for publication in A&

The Mass-Loss Return From Evolved Stars to The Large Magellanic Cloud VI: Luminosities and Mass-Loss Rates on Population Scales

We present results from the first application of the Grid of Red Supergiant and Asymptotic Giant Branch ModelS (GRAMS) model grid to the entire evolved stellar population of the Large Magellanic Cloud (LMC). GRAMS is a pre-computed grid of 80,843 radiative transfer (RT) models of evolved stars and circumstellar dust shells composed of either silicate or carbonaceous dust. We fit GRAMS models to ~30,000 Asymptotic Giant Branch (AGB) and Red Supergiant (RSG) stars in the LMC, using 12 bands of photometry from the optical to the mid-infrared. Our published dataset consists of thousands of evolved stars with individually determined evolutionary parameters such as luminosity and mass-loss rate. The GRAMS grid has a greater than 80% accuracy rate discriminating between Oxygen- and Carbon-rich chemistry. The global dust injection rate to the interstellar medium (ISM) of the LMC from RSGs and AGB stars is on the order of 1.5x10^(-5) solar masses/yr, equivalent to a total mass injection rate (including the gas) into the ISM of ~5x10^(-3) solar masses/yr. Carbon stars inject two and a half times as much dust into the ISM as do O-rich AGB stars, but the same amount of mass. We determine a bolometric correction factor for C-rich AGB stars in the K band as a function of J - K color, BC(K) = -0.40(J-K)^2 + 1.83(J-K) + 1.29. We determine several IR color proxies for the dust mass-loss rate (MLR) from C-rich AGB stars, such as log (MLR) = (-18.90)/((K-[8.0])+3.37)-5.93. We find that a larger fraction of AGB stars exhibiting the `long-secondary period' phenomenon are O-rich than stars dominated by radial pulsations, and AGB stars without detectable mass-loss do not appear on either the first-overtone or fundamental-mode pulsation sequences.Comment: 19 pages, 19 figure

High resolution CO images of Seyfert Galaxies

The CO (J = 1-0) emission of three Seyfert galaxies, NGC 3227, NGC 7469, and NGC 5033 was imaged. The CO emission in NGC 3227 and NGC 7469 appears as compact structures centered on the active nuclei, containing substantial fractions of the single-dish flux. In NGC 3227, 10 percent of the CO flux detected by the interferometer is contained within the ionized narrow-line region. The unresolved molecular gas concentrations in the nucleus of NGC 3227 imply a CO mass of 65 million solar masses concentrated within a diameter less than 50 pc. The CO emission in NGC 5033 is not detected at this resolution, implying a CO structure size of 20 to 60 arcsec. Continuum emission at 2.7 mm is not detected in any of the three galaxies. In the center of NGC 7469, the H2 mass is comparable to the dynamical mass. Kinematic studies of the detected gas reveal a rotational motion of the gas in NGC 3227 and NGC 7469, allowing identification of the gas in NGC 7469 with a nuclear starburst. These data are consistent with the idea that interactions between galaxies cause gas to concentrate in their nuclei thereby feeding starburst and Seyfert activity

Lateral current density fronts in asymmetric double-barrier resonant-tunneling structures

We present a theoretical analysis and numerical simulations of lateral current density fronts in bistable resonant-tunneling diodes with Z-shaped current-voltage characteristics. The bistability is due to the charge accumulation in the quantum well of the double-barrier structure. We focus on asymmetric structures in the regime of sequential incoherent tunneling and study the dependence of the bistability range, the front velocity and the front width on the structure parameters. We propose a sectional design of a structure that is suitable for experimental observation of front propagation and discuss potential problems of such measurements in view of our theoretical findings. We point out the possibility to use sectional resonant-tunneling structures as controllable three-terminal switches.Comment: to appear in J.Appl.Phy

XMM-Newton Detection of Hot Gas in the Eskimo Nebula: Shocked Stellar Wind or Collimated Outflows?

The Eskimo Nebula (NGC 2392) is a double-shell planetary nebula (PN) known for the exceptionally large expansion velocity of its inner shell, ~90 km/s, and the existence of a fast bipolar outflow with a line-of-sight expansion velocity approaching 200 km/s. We have obtained XMM-Newton observations of the Eskimo and detected diffuse X-ray emission within its inner shell. The X-ray spectra suggest thin plasma emission with a temperature of ~2x10^6 K and an X-ray luminosity of L_X = (2.6+/-1.0)x10^31 (d/1150 pc)^2 ergs/s, where d is the distance in parsecs. The diffuse X-ray emission shows noticeably different spatial distributions between the 0.2-0.65 keV and 0.65-2.0 keV bands. High-resolution X-ray images of the Eskimo are needed to determine whether its diffuse X-ray emission originates from shocked fast wind or bipolar outflows.Comment: 4 pages, 2 figures, accepted in Astronomy and Astrophysics Letter

Resonant control of stochastic spatio-temporal dynamics in a tunnel diode by multiple time delayed feedback

We study the control of noise-induced spatio-temporal current density patterns in a semiconductor nanostructure (double barrier resonant tunnelling diode) by multiple time-delayed feedback. We find much more pronounced resonant features of noise-induced oscillations compared to single time feedback, rendering the system more sensitive to variations in the delay time $\tau$. The coherence of noise-induced oscillations measured by the correlation time exhibits sharp resonances as a function of $\tau$, and can be strongly increased by optimal choices of $\tau$. Similarly, the peaks in the power spectral density are sharpened. We provide analytical insight into the control mechanism by relating the correlation times and mean frequencies of noise-induced breathing oscillations to the stability properties of the deterministic stationary current density filaments under the influence of the control loop. Moreover, we demonstrate that the use of multiple time delays enlarges the regime in which the deterministic dynamical properties of the system are not changed by delay-induced bifurcations