Pulsations and Dust Formation in R Coronae Borealis Stars

1995 ◽  
Vol 155 ◽  
pp. 433-434
Author(s):  
Geoffrey C. Clayton ◽  
Barbara A. Whitney ◽  
Warrick A. Lawson
Keyword(s):  
Time Scales ◽  
Amorphous Carbon ◽  
Thermodynamic Equilibrium ◽  
Stellar Atmosphere ◽  
Dust Formation ◽  
Carbon Condensate ◽  
Close Proximity ◽  
Stellar Pulsations ◽  
R Coronae Borealis Stars ◽  
Ir Emission

The observed correlation between pulsational phase in RCB stars and the timing of their declines shows empirically that the stellar pulsations and dust formation are intimately connected (e.g., Lawson et al. 1992). However, the nature of this relationship and the process of dust formation itself are not well understood. We have shown that it is likely that dust is forming in close proximity (< 2 stellar radii) to the RCB star photosphere, based on time scales for acceleration of the dust, eclipse of the chromospheric region, and dispersal of the dust (Clayton et al. 1992, 1993; Whitney, Balm & Clayton 1993; Whitney et al. 1993). The temperature at which amorphous carbon forms can be as high as 4000 K, and can occur in conditions far removed from thermodynamic equilibrium, as long as a mechanism exists to contain carbon atoms within a given volume. A likely form of carbon condensate is fullerenes such as C60 (Whitney, Balm & Clayton 1993). Shocks in the stellar atmosphere due to the pulsations may provide such a mechanism for containing the carbon. However, no spectroscopic signature related to the dust formation has been seen. We have searched unsuccessfully for IR emission bands of C60 (Clayton et al. 1995a).

scholarly journals The Dust in the Hydrogen-Poor Ejecta of Abell 30

1993 ◽  
Vol 155 ◽  
pp. 386-386
Author(s):  
J.P. Harrington ◽  
K.J. Borkowski ◽  
W.P. Blair ◽  
J. Bregman
Keyword(s):  
High Resolution ◽  
Amorphous Carbon ◽  
Ir Spectrum ◽  
Stellar Wind ◽  
Thermal Emission ◽  
Central Star ◽  
Near Ir ◽  
High Resolution Images ◽  
Small Grains ◽  
Ir Emission

High-resolution images in [O III] λ5007 of the hydrogen-poor knots of Abell 30 reveal comet-like structures which may be indicative of interaction with the stellar wind. In the near IR, new, higher-resolution, K-band images show an equatorial ring of hot dust that corresponds closely to optical knots 2 and 4 of Jacoby and Ford, while their polar knots 1 and 3 show no comparable IR emission. Both the thermal IR emission and the heavy internal extinction of the central star demands an extremely dusty ejecta. Greenstein showed that the UV extinction curve is fit by amorphous carbon. Our comprehensive dust models consider both the UV extinction and the IR emission from a population of carbon grains. The thermal emission from larger grains produces the far IR emission, while the stochastic heating of very small grains to high temperatures is essential to explain the near IR flux. We are able to reproduce the shape of the near IR spectrum with an a−3.0 distribution of grain radii which extends down to a minimum grain radius of 8 Å.

scholarly journals Central Stars of Young Planetary Nebulae - A New Class of Variables

2003 ◽  
Vol 209 ◽  
pp. 237-238 ◽  
Author(s):  
G. Handler
Keyword(s):  
Mass Loss ◽  
Radial Velocity ◽  
Time Scales ◽  
Variable Star ◽  
Planetary Nebulae ◽  
Stellar Mass ◽  
New Class ◽  
Stellar Pulsation ◽  
Stellar Pulsations ◽  
Central Stars

A new class of variable star is proposed. These are variable central stars of young Planetary Nebulae exhibiting roughly sinusoidal (semi)regular photometric and/or radial velocity variations with time scales of several hours. Fourteen of these objects have been identified. Their temperatures are between 25000 and 50000 K and most show hydrogen-rich spectra. The most likely reason for the variability is stellar pulsation. Another possibility would be variable stellar mass loss, but in that case the mechansism causing it must be different from that operating in massive O stars. We speculate that it actually is the stellar pulsations which cause mass loss mdulations.

scholarly journals VARIABLE WINDS AND DUST FORMATION IN R CORONAE BOREALIS STARS

2013 ◽  
Vol 146 (2) ◽  
pp. 23 ◽  
Author(s):  
Geoffrey C. Clayton ◽  
T. R. Geballe ◽  
Wanshu Zhang
Keyword(s):  
Dust Formation ◽  
R Coronae Borealis Stars

The benefits of performing continuous gravity measurements at active volcanoes using superconducting gravimeters

2020 ◽  
Author(s):  
Filippo Greco ◽  
Daniele Carbone ◽  
Flavio Cannavò ◽  
Alfio Messina ◽  
Danilo Contrafatto ◽  
...  
Keyword(s):  
Time Scales ◽  
Active Volcano ◽  
Continuous Measurements ◽  
Gravity Changes ◽  
Superconducting Gravimeters ◽  
Close Proximity ◽  
Gravity Measurements ◽  
Long Time ◽  
Changes Over Time ◽  
Active Volcanoes

&lt;p&gt;Continuous gravity measurements at active volcanoes are mostly accomplished using spring gravimeters, that can be operated under harsh field conditions. Unfortunately, these instruments do not provide reliable continuous measurements over long time-scales, due to the instrumental drift and artifacts driven by ambient parameters.&lt;/p&gt;&lt;p&gt;An alternative to spring devices for continuous measurements is given by superconducting gravimeters (SGs), that are free from instrumental effects and thus allow to track even small gravity changes over time-scales from minutes to years. Nevertheless, SGs cannot be deployed in close proximity to the active structures of tall volcanoes, since they need host facilities with main electricity and a large installation surface.&lt;/p&gt;&lt;p&gt;The mini-array of three SGs that were installed on Etna between 2014 and 2016 makes the first network of SGs ever installed on an active volcano. Here we present results from these instruments and show that, even though they are installed at relatively unfavorable positions (in terms of distances from the summit active craters), SGs can detect volcano-related gravity changes that would otherwise remain hidden, thus providing unique insight into the bulk processes driving volcanic activity.&lt;/p&gt;

scholarly journals The IR Emission Features and Hydrogenated Amorphous Carbon (HAC) Particles

1989 ◽  
Vol 135 ◽  
pp. 141-146
Author(s):  
W. W. Duley
Keyword(s):  
Amorphous Carbon ◽  
Interstellar Dust ◽  
Near Infrared ◽  
High Energy ◽  
Radiative Relaxation ◽  
Hydrogenated Amorphous Carbon ◽  
Nir Emission ◽  
Extended Red Emission ◽  
Ir Emission ◽  
Hydrogenated Amorphous

Various sources of non-equilibrium radiation from interstellar dust are discussed. It is shown that the existence of cirrus emission at 12 and 25 μm is consistent with the presence of amorphous carbon dust and arises from thermal spikes within ≃ 10å subvolumes of normal (0.01-0.1 μm radius) dust grains. The 3.28 μm unidentified infrared (UIR) feature also arises in this way, as the radiative relaxation of high energy vibrational modes accompanying a thermal spike in hydrogenated amorphous carbon. Extended red emission (ERE) and near-infrared (NIR) emission are also discussed and are postulated to originate as edge and defect luminescence from HAC solids with bandgaps Eg ≲ 2.5eV.

scholarly journals Formation and destruction of dust grains in circumstellar regions

1987 ◽  
Vol 122 ◽  
pp. 515-527 ◽  
Author(s):  
Yu. A. Fadeyev
Keyword(s):  
Thermal Evaporation ◽  
Silicon Monoxide ◽  
Stellar Atmosphere ◽  
Nucleation Theory ◽  
Dust Particles ◽  
Circumstellar Dust ◽  
Dust Grains ◽  
Stellar Pulsations ◽  
Long Time ◽  
Homogeneous Nucleation Theory

The existence of circumstellar dust in late-type stars is connected with stellar pulsations since periodic shocks accompanying pulsations seem to be the most probable mechanism of the gas density increase needed for condensation of dust particles in outer layers of the stellar atmosphere. Most abundant solid materials formed in O-rich stars are forsterite (or enstatite), silicon monoxide and iron, whereas in C-rich stars these are carbon, silicon carbide, magnesium sulphur and iron. Application of the homogeneous nucleation theory shows that condensation proceeds at extremely high departures from thermal equilibrium due to a very long time between collisions of condensable monomers. The most efficient mechanism of destruction of dust grains is thermal evaporation caused by variations of the stellar effective temperature.

scholarly journals Variable dust formation by the double-line spectroscopic binary WR 70 (HD 137603, WC9vd+B0I)

2003 ◽  
Vol 212 ◽  
pp. 261-262
Author(s):  
Peredur M. Williams ◽  
Karel A. van der Hucht ◽  
Fred Marang
Keyword(s):  
Time Scales ◽  
Dust Emission ◽  
Dust Formation ◽  
Infrared Photometry ◽  
Double Line ◽  
Underlying Process ◽  
Spectroscopic Binary ◽  
Show Evidence

Infrared photometry of the WC9vd+B0I spectroscopic binary WR 70 over two decades shows variable dust emission. The variations are relatively slow and show evidence for an underlying process with a period P = 1045 ± 60 d, together with modulation on shorter and longer time-scales.

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