Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL

1979 ◽

Vol 46 ◽

pp. 386-408 ◽

Cited By ~ 2

Author(s):

G. V. Coyne ◽

I. S. McLean

Keyword(s):

Wavelength Dependence ◽

Stellar Atmosphere ◽

Molecular Absorption ◽

Absorption Bands ◽

Absorption And Emission ◽

Mira Variables ◽

Stellar Photosphere ◽

Regular Variable ◽

Red Supergiants ◽

Balmer Lines

AbstractIn recent years the wavelength, dependence of the polarization in a number of Mira variables, semi-regular variables and red supergiants has been measured with resolutions between 0.3 and 300 A over the range 3300 to 11000 A. Variations are seen across molecular absorption bands, especially TiO bands, and across atomic absorption and emission lines, especially the Balmer lines. In most cases one can ignore or it is possible to eliminate the effects due to interstellar polarization, so that one can study the polarization mechanisms operating in the stellar atmosphere and environment. The stars Omicron Ceti. (Mira), V CVn (semi-regular variable) and Mu Cephei (M2 la), in addition to other stars similar to them, will be discussed in some detail.Models to explain the observed polarization consider that the continuum flux is polarized either by electron, molecular and/or grain scattering or by temperature variations and/or geometrical asymmetries over the stellar photosphere. This polarized radiation is affected by atomic and molecular absorption and emission processes at various geometric depths in the stellar atmosphere and envelope. High resolution spectropolarimetry promises, therefore, to be a power-rul tool for studying stratification effects in these stars.

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Molecular Envelopes of Planetary Nebulae and Proto-Planetary Nebulae

International Astronomical Union Colloquium ◽

10.1017/s0252921100019345 ◽

1994 ◽

Vol 140 ◽

pp. 152-153

Author(s):

Sun Kwok

Keyword(s):

Planetary Nebulae ◽

Asymptotic Giant Branch ◽

High Rate ◽

Radiative Transfer Model ◽

Spectral Energy ◽

Transfer Model ◽

Energy Distributions ◽

Cool Component ◽

Dust Envelope ◽

Peak Energy

As stars evolve up the asymptotic giant branch (AGB), they begin to lose mass at a high rate, and in the process they create extended circumstellar molecular envelopes. Since the transition from AGB to planetary nebula stages is of the order of 1000 yr, the remnant of such molecular envelopes should still be observable in pro to-planetary nebulae (PPN) and planetary nebulae (PN). Recent ground-based survey of cool IRAS sources have discovered ~30 candidates of PPN (Kwok 1992). These sources show the characteristic “double-peak” energy distribution. The cool component is due to the remnant of the AGB dust envelope, and the hot component represents the reddened photosphere. The fact that the two components are clearly separated suggests that the dust envelope is well detached from the photosphere. Radiative transfer model fits to the spectral energy distributions of PPN suggest a typical separation of ~1 arc sec between the dust envelope and the photosphere, and such “hole-in-the-middle” structure can be mapped by millimeter interferometry in CO.

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What Do We Really Know About the Winds of Massive Stars?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308020371 ◽

2007 ◽

Vol 3 (S250) ◽

pp. 89-96

Author(s):

D. John Hillier

Keyword(s):

Mass Loss ◽

Massive Stars ◽

Standard Theory ◽

Stellar Winds ◽

X Rays ◽

X Ray ◽

Stellar Photosphere ◽

Weak Winds ◽

Ionization Balance ◽

Loss Rates

AbstractThe standard theory of radiation driven winds has provided a useful framework to understand stellar winds arising from massive stars (O stars, Wolf-Rayet stars, and luminous blue variables). However, with new diagnostics, and advances in spectral modeling, deficiencies in our understanding of stellar winds have been thrust to the forefront of our research efforts. Spectroscopic observations and analyses have shown the importance of inhomogeneities in stellar winds, and revealed that there are fundamental discrepancies between predicted and theoretical mass-loss rates. For late O stars, spectroscopic analyses derive mass-loss rates significantly lower than predicted. For all O stars, observed X-ray fluxes are difficult to reproduce using standard shock theory, while observed X-ray profiles indicate lower mass-loss rates, the potential importance of porosity effects, and an origin surprisingly close to the stellar photosphere. In O stars with weak winds, X-rays play a crucial role in determining the ionization balance, and must be taken into account.

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Clumps in stellar winds

ASTRA Proceedings ◽

10.5194/ap-1-39-2014 ◽

2014 ◽

Vol 1 ◽

pp. 39-41 ◽

Cited By ~ 1

Author(s):

J. S. Vink

Keyword(s):

Mass Loss ◽

Massive Stars ◽

Stellar Winds ◽

Present Evidence ◽

Close Proximity ◽

Stellar Photosphere ◽

Loss Rates

Abstract. We discuss the origin and quantification of wind clumping and mass–loss rates (Ṁ), particularly in close proximity to the Eddington (Γ) limit, relevant for very massive stars (VMS). We present evidence that clumping may not be the result of the line-deshadowing instability (LDI), but that clumps are already present in the stellar photosphere.

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Interferometric studies of rapid rotators

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311009963 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 44-55 ◽

Cited By ~ 1

Author(s):

Ming Zhao ◽

John D. Monnier ◽

Xiao Che

Keyword(s):

Fundamental Property ◽

Optical Interferometry ◽

Rotating Stars ◽

Stellar Rotation ◽

Rapid Rotation ◽

Long Baseline ◽

Imaging Results ◽

Basic Physics ◽

Stellar Photosphere

AbstractStellar rotation, like stellar mass and metallicity, is a fundamental property of stars. Rapid rotation distorts the stellar photosphere and affects a star's luminosity, abundances and evolution. It is also linked to stellar wind and mass loss. The distortion of the stellar photosphere due to rapid rotation causes the stellar surface brightness and effective temperature to vary with latitude, leading to a bright pole and a dark equator - a phenomenon known as ‘Gravity Darkening’. Thanks to the development of long baseline optical interferometry in recent years, optical interferometers have resolved the elongation of rapidly rotating stars, and have even imaged a few systems for the first time, directly confirming the gravity darkening effect. In this paper, we review the recent interferometric studies of rapid rotators, particularly the imaging results from CHARA-MIRC. These sub-milliarcsecond resolution observations permit the determination of the inclination, the polar and equatorial radius and temperature, as well as the fractional rotation speed of several rapid rotators with unprecedented precision. The modeling also allows the determination of the true effective temperatures and luminosities of these stars, permitting the investigation of their true locations on the HR diagram. Discrepancies from standard models were also found in some measurements, suggesting the requirement of more sophisticated mechanisms such as non-uniform rotation in the model. These observations have demonstrated that optical interferometry is now sufficiently mature to provide valuable constraints and even model-independent images to shed light on the basic physics of stars.

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A model for the dust envelope of the protoplanetary nebula LSIV-12°111

Astronomy Reports ◽

10.1134/1.1583774 ◽

2003 ◽

Vol 47 (6) ◽

pp. 480-482 ◽

Cited By ~ 2

Author(s):

M. B. Bogdanov

Keyword(s):

Dust Envelope ◽

Protoplanetary Nebula

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Resolving the extended stellar atmospheres of asymptotic giant branch stars at (sub)millimetre wavelengths

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935329 ◽

2019 ◽

Vol 626 ◽

pp. A81

Author(s):

W. H. T. Vlemmings ◽

T. Khouri ◽

H. Olofsson

Keyword(s):

Initial Conditions ◽

Stellar Atmospheres ◽

Asymptotic Giant Branch ◽

High Angular Resolution ◽

Inhomogeneous Layer ◽

Temperature Structure ◽

Asymptotic Giant Branch Stars ◽

Agb Stars ◽

Brightness Temperatures ◽

Stellar Photosphere

Context. The initial conditions for mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. Aims. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We interpreted the observations using a parameterised atmosphere model. Results. We resolve all four AGB stars and determine the brightness temperature structure between 1 and 2 stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures > 3000 to 10 000 K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least 10.6 ± 1.4 km s−1. For all objects but W Hya we find that the temperature-radius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. Conclusions. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere. We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential.

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Spatially Resolved STIS Spectroscopy of Betelgeuse's Outer Atmosphere

Symposium - International Astronomical Union ◽

10.1017/s0074180900182671 ◽

2004 ◽

Vol 219 ◽

pp. 641-645 ◽

Cited By ~ 2

Author(s):

A. Lobel ◽

J. Aufdenberg ◽

A. K. Dupree ◽

R. L. Kurucz ◽

R. P. Stefanik ◽

...

Keyword(s):

Emission Feature ◽

Disk Center ◽

Emission Lines ◽

Small Aperture ◽

Space Telescope ◽

Dust Envelope ◽

Spatially Resolved ◽

Outer Atmosphere ◽

Cool Gas ◽

Gas Component

We present spatially resolved spectra observed with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope of the upper chromosphere and dust envelope of Betelgeuse (α Orionis, M2 Iab). In the fall of 2002 a set of five high-resolution near-UV spectra was obtained by scanning at intensity peak-up position and four off-limb target positions up to one arcsecond, using a small aperture (200 by 63 mas), to investigate the thermal conditions and flow dynamics in the outer atmosphere of this important nearby cool supergiant star.Based on Mg ii h & k, Fe ii λ2716, C ii λ2327, and Al ii] λ2669 emission lines we provide the first evidence for the presence of warm chromospheric plasma at least 1 arcsecond away from the star at ∼40 R* (1 R*≃700 R⊙). The STIS spectra reveal that Betelgeuse's upper chromosphere extends far beyond the circumstellar Hα envelope of ∼5 R*, determined from previous ground-based imaging (Hebden et al. 1987).The flux in the broad and self-absorbed resonance lines of Mg ii decreases by a factor of ∼700 compared to the flux at chromospheric disk center. We observe strong asymmetry changes in the Mg ii h and Si i resonance line profiles when scanning off-limb, signaling the outward acceleration of gas outflow in the upper chromosphere.From the radial intensity distributions of Fe i and Fe ii emission lines we determine the radial non-LTE iron ionization balance. We compute that the local kinetic gas temperatures of the warm chromospheric gas component in the outer atmosphere exceed 2600 K, when assuming local gas densities of the cool gas component we determine from radiative transfer models that fit the 9.7 μm silicate dust emission feature. The spatially resolved STIS spectra directly demonstrate that warm chromospheric plasma co-exisists with cool gas in Betelgeuse's circumstellar dust envelope.

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Physical Interpretation of Light Curves of R Coronae Borealis-Type Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900010196 ◽

1975 ◽

Vol 67 ◽

pp. 149-157 ◽

Cited By ~ 2

Author(s):

Jacek Krełowski

Keyword(s):

Laboratory Experiment ◽

Radiation Field ◽

Physical Interpretation ◽

Light Curves ◽

Circumstellar Dust ◽

Dust Envelope ◽

Preliminary Results ◽

Theoretical Investigations ◽

Grain Formation

The paper contains preliminary results of theoretical investigations of the light curves of RCB variables. A circumstellar dust envelope is assumed to be the source of the great visual opacity as well as the infrared and violet excesses. The processes of grain formation and the interaction with the radiation field are discussed briefly. The results of a laboratory experiment concerning the problem of the violet continuum are also described.

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