scholarly journals Asymmetric shocks in χ Cygni observed with linear spectropolarimetry

2019 ◽  
Vol 632 ◽  
pp. A30
Author(s):  
A. López Ariste ◽  
B. Tessore ◽  
E. S. Carlín ◽  
Ph. Mathias ◽  
A. Lèbre ◽  
...  
Keyword(s):  
Spherical Geometry ◽  
Careful Analysis ◽  
Spectral Lines ◽  
Unexpected Result ◽  
Stellar Spectra ◽  
Asymmetric Shocks ◽  
Stellar Convection ◽  
Stellar Photosphere ◽  
Molecular Lines ◽  
Linear Polarisation

Aims. We derive information about the dynamics of the stellar photosphere, including pulsation, from a coherent interpretation of the linear polarisation detected in the spectral lines of the Mira star χ Cyg. Methods. From spectropolarimetric observations of χ Cyg, we performed a careful analysis of the polarisation signals observed in atomic and molecular lines, both in absorption and emission, using radiative transfer in the context of polarisation produced through two mechanisms: intrinsic polarisation and continuum depolarisation. We also explain the observed line doubling phenomenon in terms of an expanding shell in spherical geometry, which allows us to pinpoint the coordinates over the stellar disc with enhanced polarisation. Results. We find that the polarised spectrum of χ Cyg is dominated by intrinsic polarisation and has a negligible continuum depolarisation. The observed polarised signals can only be explained by assuming that this polarisation is locally enhanced by velocity fields. During the pulsation, radial velocities are not homogeneous over the disc. We map these regions of enhanced velocities. Conclusions. We set an algorithm to distinguish the origin of this polarisation in any stellar spectra of linear polarisation and to find a way to increase the signal by coherently adding many lines with an appropriated weight. Applied to the Mira star χ Cyg, we reached the unexpected result that during the pulsation, velocities are radial but not homogeneous over the disc. The reason for these local velocity enhancements are probably related to the interplay between the atmospheric pulsation dynamics and the underlying stellar convection.

scholarly journals Laboratory astrophysics for the interpretation of stellar spectra

2019 ◽  
Vol 15 (S350) ◽  
pp. 345-349
Author(s):  
Ulrike Heiter
Keyword(s):  
Milky Way ◽  
Chemical Elements ◽  
Spectral Lines ◽  
Laboratory Astrophysics ◽  
Atomic Data ◽  
Milky Way Galaxy ◽  
Infrared Wavelength ◽  
Stellar Spectra ◽  
Continuous Activities ◽  
Host Stars

AbstractHigh-resolution stellar spectra are important tools for studying the chemical evolution of the Milky Way Galaxy, tracing the origin of chemical elements, and characterizing planetary host stars. Large amounts of data have been accumulating, in particular in the optical and infrared wavelength regions. The observed spectral lines are interpreted using model spectra that are calculated based on transition data for numerous species, in particular neutral and singly ionized atoms. We rely heavily on the continuous activities of laboratory astrophysics groups that produce high-quality experimental and theoretical atomic data for the relevant transitions. We give examples for the precision with which the chemical composition of stars observed by different surveys can be determined, and discuss future needs from laboratory astrophysics.

scholarly journals Ammonia in circumstellar environment of V Cyg

2020 ◽  
Vol 10 (1) ◽  
pp. 7-11
Author(s):  
B. Etmański ◽  
M. Schmidt ◽  
R. Szczerba
Keyword(s):  
Background Radiation ◽  
Spectral Energy Distribution ◽  
Careful Analysis ◽  
Asymptotic Giant Branch ◽  
Spectral Energy ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Theoretical Predictions ◽  
Red Supergiants ◽  
Molecular Lines

The HIFI instrument on board of the Herschel Space Observatory (HSO) has been very successful in detecting molecular lines from the circumstellar envelopes around evolved stars, like massive red supergiants, Asymptotic Giant Branch (AGB) and post-AGB stars, as well as the planetary nebulae. Among others, ammonia have been found in the circumstellar envelopes of C-rich AGB stars in amounts that significantly exceeded the theoretical predictions for C-rich stars. Few scenarios have been proposed to resolve this problem: formation of ammonia behind the shock front and photochemical processes in the inner part of the envelope partly transparent to UV background radiation due to the clumpy structure of the gas and formation of ammonia on dust grains. Careful analysis of observations may help to put the constraints on one or another mechanism of ammonia formation. Here, we present results of the non-LTE radiative transfer modeling of ammonia transitions including the crucial process of radiative pumping via the v2=1 vibrational band (at ∼10 μm) for V Cyg. Only the ground-based ammonia transition NH3 J = 10-00 at 572.5 GHz has been observed by HIFI. Therefore, to determine the abundance of ammonia we estimate the photodissociation radius of NH3 using chemical model of the envelope consistent with the dust grain properties concluded from the spectral energy distribution.

scholarly journals Observation of High Doppler Velocity Wings in the Nascent Wind of R Doradus

2020 ◽  
Vol 30 (1) ◽  
pp. 85
Author(s):  
Hoai Thi Do ◽  
Nhung Tuyet Pham ◽  
Tuan-Anh Pham ◽  
Pierre Darriulat ◽  
Diep Ngoc Pham ◽  
...  
Keyword(s):  
Current Knowledge ◽  
Line Emission ◽  
Terminal Velocity ◽  
Spectral Lines ◽  
Line Of Sight ◽  
Doppler Velocity ◽  
Agb Stars ◽  
Gas Streams ◽  
Stellar Pulsations ◽  
Molecular Lines

We study the morpho-kinematics in the nascent wind of AGB star R Doradus in the light of high Doppler velocity wings observed in the spectral lines of several species. We probe distances from the star between ∼10 and ∼100 au using ALMA observations of the emission of five different molecular lines. High Doppler velocity enhancements of the line emission are observed in the vicinity of the line of sight crossing the star, reminiscent of those recently interpreted as gas streams in the nascent wind of a similar AGB star, EP Aqr. They are present in both blue-shifted and red- shifted hemispheres but are not exactly back-to-back. They are accelerated at a typical rate of 0.7 km s\(^{−1}\) au\(^{−1}\) up to some 20 km s\(^{−1}\). Important differences are observed between the emissions of different molecules. We exclude an effect of improper continuum subtraction. However, in contrast to EP Aqr, the line of sight plays no particular role in the R Dor morpho-kinematics, shedding doubt on the validity of a gas stream interpretation. We discuss possible interpretations in terms of stellar pulsations or of rotation of the gas in the environment of the star. We conclude that, in the state of current knowledge, no fully convincing picture of the physics governing the production of such high velocities, typically twice as large as the terminal velocity, can be reliably drawn. New high resolution analyses of observations of the nascent wind of oxygen-rich AGB stars are needed to clarify the issue.

scholarly journals Observations of ultraviolet stellar spectra

1970 ◽  
Vol 36 ◽  
pp. 147-162
Author(s):  
R. Wilson
Keyword(s):  
Spectral Resolution ◽  
Photographic Plate ◽  
Spectroscopic Studies ◽  
Ultraviolet Spectrum ◽  
Spectral Lines ◽  
Resolution Limit ◽  
Stellar Spectra ◽  
Stellar Spectroscopy ◽  
Strong Resonance ◽  
The Many

The dividing line between photometry and spectrometry is not always obvious and for the purpose of this review, I will define ultraviolet stellar spectroscopy as observations with sufficient spectral resolution to allow the detection of individual spectral lines and their measurement in terms of wavelength and strength. From an examination of the existing observations this results in a resolution requirement of δλ < 10 Å. Since the best spectral resolution so far obtained is about 1 Å then this places the results to be discussed within the range 1–10 Å. In terms of λ/δλ this corresponds to a range of about 2000–200 and it is important to bear in mind that these represent low resolution spectra. In fact the limit of 200 that I have imposed would rarely be used for spectroscopic studies in ground based observatories where it corresponds, in the notation of the optical astronomer, to a dispersion of about 1000 Å/mm, the resolution limit being set by the photographic plate, typically taken as 20 μ. Hence, even the faintest objects like quasars are usually studied with a dispersion of a few hundred Å mm−1. The fact that such a resolution can be included here is an indication of the exceptionally strong resonance lines which occur in the ultraviolet and which can be detected with such a resolution. On the other hand, the richness of the ultraviolet spectrum is making and will continue to make, demands on improved resolution in order to separate the many features. The best achieved resolution of about 1 Å goes only part-way to solving this problem.

scholarly journals Stellar Convection Theory

1980 ◽  
Vol 51 ◽  
pp. 1-14
Author(s):  
Jean-Paul Zahn
Keyword(s):  
Theoretical Prediction ◽  
Thermal Convection ◽  
Convective Instability ◽  
Spectral Lines ◽  
Density Stratification ◽  
Late Type ◽  
Stellar Convection

This meeting, which deals with turbulence in stars, opens with a review on thermal convection. There is no better way to state from the start that among all instabilities that are likely to arise in stars, it is thermal convection which is the most firmly established as a cause for the turbulence that we observe on their surface. Our confidence in this comes mainly from the theoretical prediction that convective instability sets in whenever the density stratification becomes superadiabatic, as is expected in late type stars whose outer layers are very opaque, due to the ionization of the two most abundant elements, hydrogen and helium. And, in these stars at least, thermal convection occurs close enough to the photosphere to influence, be it indirectly, the profile of spectral lines.

scholarly journals The Calibration of Stellar Spectra Used in Measurements of the Generalized Compton Shift

1985 ◽  
Vol 111 ◽  
pp. 595-597
Author(s):  
M. Missana
Keyword(s):  
Spectral Range ◽  
Planetary Nebula ◽  
Thomson Scattering ◽  
Compton Effect ◽  
Spectral Lines ◽  
Comparison Spectrum ◽  
Stellar Spectra ◽  
The Continuum

In the study of the changes induced by the Compton effect and Thomson scattering on the shape of spectral lines in light traversing a chromosphere or a planetary nebula it is necessary to have accurate wavelength measurements, central intensities and half widths (FWHM) of the lines. In the comparison of FWHM measures belonging to different spectra it is useful to have the intensities of the continuum at the wavelengths of the lines. The studied spectra should have a dispersion higher than 10 Å/mm and a spectral range larger than 700 Å. A short recommendation is also given about the comparison spectrum and the calibration plate.

scholarly journals Definition of the Types of Problems That Exist in Steady-State Extended Atmospheres

1970 ◽  
Vol 2 ◽  
pp. 1-37
Author(s):  
Anne B. Underhill
Keyword(s):  
Source Function ◽  
Main Sequence ◽  
Coherent Scattering ◽  
Careful Consideration ◽  
Spectral Lines ◽  
Reference Level ◽  
Stellar Spectra ◽  
Radiative Processes ◽  
Level Atom ◽  
Upper Level

AbstractIn section 1 practical details concerning the equivalence of observational and theoretical descriptions of stellar spectra are reviewed, particularly the difficulty of identifying the observed reference level (continuum) with the theoretical continuum in the case when many lines are present. In this connection thought must be given to how integrals over frequency should be normalised and evaluated because the effective continuous absorption coefficient does not remain constant over the range from 0 to ∞. The choice of spectroscopic details by which to determine Teff, log g and abundances requires careful consideration.In section 2 the factors by which an extended atmosphere are recognized are summarized and the question is posed do all stars have extended atmospheres. Another question requiring an answer is whether the concepts microturbulence and macroturbulence are physically real concepts or whether they are merely fitting parameters to make a simple LTE theory account for the observed spectra of supergiants in which rather wide lines occur and many multiplets show rather steep gradients. In section 3 the types of line sensitive to non-LTE conditions are described. These are resonance lines, lines arising from metastable levels, subordinate lines for which the upper level is sufficiently separated from the continuum and other levels that this upper level is chiefly populated by radiative processes from the ground or other low lying levels and lines which go into emission in low density atmospheres as a result of optical-pumping (fluorescent) processes. Such lines should not be used for abundance determinations by means of LTE theory though this is frequently done.Theoretical considerations are discussed in Section 4 where first the problem of the two-level atom is sketched and then the problem is generalised to a many-level atom. The parameter λ which gives the probability that a photon is lost from the line by de-excitation processes other than spontaneous emission is defined and it is pointed out that non-LTE physics has the effect of adding a scattering term to the expression for the source function. One example is given of the effect of changing the line source function from the Planck function to a form suitable for isotropic coherent scattering. The line becomes deeper and wider for the same number of atoms. Interpretational problems in stellar spectra are discussed in section 5. It is noted that many lines in main-sequence early type spectra show the effects of departures from LTE. These effects are shown to a conspicuous degree by the spectra of shell stars. The example of He I 5876 in 10 Lacertae, 09V, is discussed and the implication for interpreting the He I lines in all B type main-sequence stars are touched upon. Helium-weak and helium-strong spectra probably indicate variations in density of the outer U atmosphere rather than true abundance differences. The spectra of supergiants are also considered and it is pointed out that the Ia supergiants of type B may be hydrogen-poor.Finally in section 6 the problem of choosing simplified physical representations of line forming when non-LTE physics must be used is discussed. Some relevant points concerning the observed spectral lines used for spectral classification are illustrated by means of partial energy-level diagrams.

scholarly journals Identification of phase-independent spectral lines in close binary V455 CYG: I. Telluric lines

2007 ◽  
pp. 77-81
Author(s):  
A. Cséki ◽  
I. Vince ◽  
O. Latkovic ◽  
I.I. Antokhin
Keyword(s):  
Binary Star ◽  
Spectral Lines ◽  
Close Binary ◽  
Spectral Features ◽  
Hitran Database ◽  
The Earth ◽  
Atmo Sphere ◽  
Molecular Lines ◽  
Close Binary Star ◽  
Peculiar Behavior

Spectroscopic observations of close binary star V455 Cygni reveal many lines that originate in interstellar and circumstellar medium and the atmo?sphere of the Earth; we found over two hundred such spectral features, and in this paper we present the list of telluric lines we identified through comparison with HITRAN database of molecular lines. The lines that remain unidentified or show peculiar behavior will be discussed in the second part of the paper. .

scholarly journals The Belgian repository of fundamental atomic data and stellar spectra (BRASS)

2019 ◽  
Vol 624 ◽  
pp. A60 ◽  
Author(s):  
M. Laverick ◽  
A. Lobel ◽  
P. Royer ◽  
T. Merle ◽  
C. Martayan ◽  
...  
Keyword(s):  
Quality Assessment ◽  
Large Scale ◽  
Spectral Lines ◽  
Oscillator Strengths ◽  
Atomic Data ◽  
Chemical Abundance ◽  
Stellar Spectra ◽  
Atomic Lines ◽  
The Universe ◽  
Selection Of

Context. Fundamental atomic transition parameters, such as oscillator strengths and rest wavelengths, play a key role in modelling and understanding the chemical composition of stars in the universe. Despite the significant work under way to produce these parameters for many astrophysically important ions, uncertainties in these parameters remain large and can limit the accuracy of chemical abundance determinations.Aims. The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide a large systematic and homogeneous quality assessment of the atomic data available for quantitative spectroscopy. BRASS shall compare synthetic spectra against extremely high-quality observed spectra, at a resolution of ∼85 000 and signal-noise ratios of ∼1000, for approximately 20 bright BAFGK spectral-type stars, in order to critically evaluate the atomic data available for over a thousand potentially useful spectral lines.Methods. A large-scale homogeneous selection of atomic lines is performed by synthesising theoretical spectra of literature atomic lines for FGK-type stars including the Sun, resulting in a selection of 1091 theoretically deep and unblended lines in the wavelength range 4200–6800 Å, which may be suitable for quality assessment. Astrophysical log(g f) values are determined for the 1091 transitions using two commonly employed methods. The agreement of these log(g f) values are used to select well-behaved lines for quality assessment.Results. We found 845 atomic lines to be suitable for quality assessment, of which 408 were found to be robust against systematic differences between analysis methods. Around 53% of the quality-assessed lines were found to have at least one literature log(g f) value in agreement with our derived values, though the remaining values can disagree by as much as 0.5 dex. Only ∼38% of Fe Ilines were found to have sufficiently accurate log(g f) values, increasing to ∼70–75% for the remaining Fe-group lines.

Sun-as-a-star observations of the 2017 August 21 solar eclipse

2019 ◽  
Vol 15 (S354) ◽  
pp. 473-480
Author(s):  
Ekaterina Dineva ◽  
Carsten Denker ◽  
Meetu Verma ◽  
Klaus G. Strassmeier ◽  
Ilya Ilyin ◽  
...  
Keyword(s):  
Solar Eclipse ◽  
Spectral Resolution ◽  
Solar Disk ◽  
State Of The Art ◽  
Active Regions ◽  
Spectral Lines ◽  
High Spectral Resolution ◽  
Line Profiles ◽  
Stellar Spectra ◽  
Limb Darkening

AbstractThe Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) is a state-of-the-art, thermally stabilized, fiber-fed, high-resolution spectrograph for the Large Binocular Telescope (LBT) at Mt. Graham, Arizona. During daytime the instrument is fed with sunlight from the 10-millimeter aperture, fully automated, binocular Solar Disk-Integrated (SDI) telescope. The observed Sun-as-a-star spectra contain a multitude of photospheric and chromospheric spectral lines in the wavelength ranges 4200–4800 Å and 5300–6300 Å. One of the advantages of PEPSI is that solar spectra are recorded in the exactly same manner as nighttime targets. Thus, solar and stellar spectra can be directly compared. PEPSI/SDI recorded 116 Sun-as-a-star spectra during the 2017 August 21 solar eclipse. The observed maximum obscuration was 61.6%. The spectra were taken with a spectral resolution of ≈ 250000 and an exposure time of 0.3 s. The high-spectral resolution facilitates detecting subtle changes in the spectra while the Moon passes the solar disk. Sun-as-a-star spectra are affected by changing contributions due to limb darkening and solar differential rotation, and to a lesser extend by supergranular velocity pattern and the presence of active regions on the solar surface. The goal of this study is to investigate the temporal evolution of the chromospheric Na D doublet during the eclipse and to compare observations with synthetic line profiles computed with the state-of-the-art Bifrost code.

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