Solar Photospheric Spectrum Research Articles

Solar photospheric spectrum microvariability

Solar photospheric spectrum microvariability

Solar photospheric spectrum microvariability

Context. Extreme precision radial-velocity spectrometers enable extreme precision in stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars are limited by various types of physical variability in stellar spectra, such as the short-term jittering of apparent radial velocities on levels of ∼2 m s−1. Aims. To understand the physical origins of radial-velocity jittering, the solar spectrum is assembled, as far as possible, from basic principles. Solar surface convection is modeled with time-dependent 3D hydrodynamics, followed by the computation of high-resolution spectra during numerous instances of the simulation sequence. The behavior of different classes of photospheric spectral lines is monitored throughout the simulations to identify commonalities or differences between different classes of lines: weak or strong, neutral or ionized, high or low excitation, atomic or molecular. Methods. Synthetic spectra were examined. With a wavelength sampling λ/Δλ ∼ 1 000 000, the changing shapes and wavelength shifts of unblended and representative Fe I and Fe II lines were followed during the simulation sequences. The radial-velocity jittering over the small simulation area typically amounts to ±150 m s−1, scaling to ∼2 m s−1 for the full solar disk. Flickering within the G-band region and in photometric indices of the Strömgren uvby system were also measured, and synthetic G-band spectra from magnetic regions are discussed. Results. Most photospheric lines vary in phase, but with different amplitudes among different classes of lines. Amplitudes of radial-velocity excursions are greater for stronger and for ionized lines, decreasing at longer wavelengths. Matching precisely measured radial velocities to such characteristic patterns should enable us to remove a significant component of the stellar noise originating in granulation. Conclusions. The granulation-induced amplitudes in full-disk sunlight amount to ∼2 m s−1; the differences between various line groups are an order of magnitude less. To mitigate this jittering, a matched filter must recognize dissimilar lineshifts among classes of diverse spectral lines with a precision of ∼10 cm s−1 for each line group. To verify the modeling toward the filter, predictions of center-to-limb dependences of jittering amplitudes for different classes of lines are presented, testable with spatially resolving solar telescopes connected to existing radial-velocity instruments.

A frequency comb calibrated solar atlas

The solar spectrum is a primary reference for the study of physical processes in stars and their variation during activity cycles. In Nov 2010 an experiment with a prototype of a Laser Frequency Comb (LFC) calibration system was performed with the HARPS spectrograph of the 3.6m ESO telescope at La Silla during which high signal-to-noise spectra of the Moon were obtained. We exploit those Echelle spectra to study the optical integrated solar spectrum . The DAOSPEC program is used to measure solar line positions through gaussian fitting in an automatic way. We first apply the LFC solar spectrum to characterize the CCDs of the HARPS spectrograph. The comparison of the LFC and Th-Ar calibrated spectra reveals S-type distortions on each order along the whole spectral range with an amplitude of +/-40 m/s. This confirms the pattern found by Wilken et al. (2010) on a single order and extends the detection of the distortions to the whole analyzed region revealing that the precise shape varies with wavelength. A new data reduction is implemented to deal with CCD pixel inequalities to obtain a wavelength corrected solar spectrum. By using this spectrum we provide a new LFC calibrated solar atlas with 400 line positions in the range of 476-530, and 175 lines in the 534-585 nm range. The new LFC atlas improves the accuracy of individual lines by a significant factor reaching a mean value of about 10 m/s. The LFC--based solar line wavelengths are essentially free of major instrumental effects and provide a reference for absolute solar line positions. We suggest that future LFC observations could be used to trace small radial velocity changes of the whole solar photospheric spectrum in connection with the solar cycle and for direct comparison with the predicted line positions of 3D radiative hydrodynamical models of the solar photosphere.

The chemical composition of the sun1This review is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

We have very recently re-determined the abundances of nearly all the available chemical elements in the solar photosphere, from lithium to thorium (Asplund et al. Annu. Rev. Astron. Astrophys. 47, 481 (2009)). This new complete and homogeneous analysis results from a very careful selection of spectral lines of all the indicators of the abundances present in the solar photospheric spectrum, from a discussion of the atomic and molecular data, and from an analysis of these lines based on a new 3D model of the solar outer layers, taking non-LTE effects into account when possible. We present these new results, compare them with other recent solar data as well as with recent results for the solar neighborhood, and discuss some of their most important implications as well as some of the atomic data we still urgently need.

A method for the treatment of supergranulation advection by giant cells

Aims. We present a new method for the treatment of the advection of solar supergranulation by giant cells, a large-scale analogue to the observed property of granule advection by supergranules. Methods. The proposed method is derived from a description of solar convection via spherical harmonics and spectral coefficients, allowing the investigation of the influence of a giant cell component on a realistic supergranule signal. Results. We show that a supergranule pattern derived from real data, as well as a simplified test signal, can be advected by a giant cell component of various sizes. Conclusions. The identified behaviour is in analogy to observed supergranulation patterns, including those based on MDI Dopplergrams, which show wavelike supergranulation patterns, even after the removal of the geometric projection effect. Our method is an important step towards the construction of future models involving supergranule flow patterns advected by a giant cell flow. Nevertheless, additional efforts are required to obtain a final verification of giant cells as a separate component of the solar photospheric convection spectrum.

Solar Rossby Wave "Hills" Identified as Supergranules

We explore the nature of 'hills' observed on the solar surface which had previously been attributed to Rossby waves. We investigate the sol ar hills phenomenon by analyzing the output from a synthetic model ba sed solely on the observed solar photospheric convection spectrum. We show that the characteristics of these hills can be explained by the corrugation of the surface produced by the radial flows of the conve ction. The hills in our simulations are dominated by supergranules, a well-known component of solar convection. Rossby waves have been predicted to exist within the Sun and may play an important role in the d ynamics of the solar interior, including the Sun's differential rotat ion and magnetic dynamo. Our study suggests, however, that the hills observed at the solar limb do not confirm the existence of solar Ross by waves.

Wavelength Shifts in the Solar Photospheric Spectrum

Wavelength shifts converted to velocities between solar lines observed at disc center and laboratory wavelengths of Fe i, Fe ii, Ti i, Ni i, and Fe i lines in the near infrared are plotted as a function of the logarithm of their solar equivalent width in milliangstroms. The need for wavelengths based on the wavelength standards is stressed. A comparison of photographic Fe i solar wavelength is shown to agree, on the average, with Fourier Transform Spectrometer solar wavelengths within less than 0.5 milliangstroms. Using Balthasar's limb effect tables we convert the disc center velocities to limb velocities and find, though the scatter is large, that there is little evidence for a super-gravitational red shift.

Standard Solar Composition

We review the current status of our knowledge of the chemical composition of the Sun, essentially derived from the analysis of the solar photospheric spectrum. The comparison of solar and meteoritic abundances confirms that there is a very good agreement between the two sets of abundances. They are used to construct a Standard Abundance Distribution.

Lithium Abundances in the Solar Twins 16 CYG A and B and the Solar Analog alpha CEN A, Calibration of the 6707 Angstrom Li Region Linelist, and Implications

view Abstract Citations (105) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lithium Abundances in the Solar Twins 16 CYG A and B and the Solar Analog alpha CEN A, Calibration of the 6707 Angstrom Li Region Linelist, and Implications King, Jeremy R. ; Deliyannis, Constantine P. ; Hiltgen, Daniel D. ; Stephens, Alex ; Cunha, Katia ; Boesgaard, Ann Merchant Abstract We present high resolution (R ∼45,000-70,000) and very high S/N (∼1,000) spectroscopy of the Li I 6707 Å region in each component of the binary solar twins 16 Cyg A and B, the solar analog α Cen A, and α Cen B. Spectra of 16 Cyg were obtained with the University of Hawai'i 2.2-m, McDonald Observatory 2.1-m, and Keck 10-m telescopes and have been independently reduced. Comparison of spectral synthesis and the 16 Cyg data, and corresponding similarly obtained solar data, yields 7Li abundances which show excellent concordance between the various data sets. Despite differing in Teff by only 35-40 K, the Li abundances of 16 Cyg A and B differ by a factor of ≥4.5. The solar photospheric abundance is intermediate to the two values. This intermediacy indicates that the Sun, whose highly depleted photospheric Li abundance is in gross conflict with standard stellar models, is not an isolated anomaly in its Li abundance evolution. A similar conclusion is reached via comparison of α Cen A and metal-rich Hyades dwarfs. The difference in the 16 Cyg components' abundances suggests, though does not directly establish, a slow (possibly rotationally-induced) mixing mechanism operating below the surface convection zone in these stars. Indeed, the Li abundance difference can be viewed as an analog to the Li abundance dispersion seen in cool stars of similar Teff in open and globular clusters, and in Galactic field halo stars. It is possible, in principle anyway, that the low Li abundances of the Sun and 16 Cyg B with respect to 16 Cyg A may be related to the presence of a planetary companion; Li abundances of 47 UMa, and HD 114762 might further support such a connection between planets/disks, angular momentum evolution, and photospheric Li abundances. Due to a variety of uncertainties, however, any conclusions remain tenuous and speculative at this time. Finally, as an interesting aside, we show that current line list uncertainties in the δ6707 region suggest that claims of very small 6Li/7Li ratios ( 0.01) inferred from analysis of the solar photospheric spectrum are overly optimistic-though not necessarily incorrect. Publication: The Astronomical Journal Pub Date: May 1997 DOI: 10.1086/118399 Bibcode: 1997AJ....113.1871K Keywords: STARS: ABUNDANCES full text sources ADS | data products SIMBAD (13)

ATOMIC DATA AND SOLAR PHOTOSPHERIC SPECTROSCOPY

This review is an updated version of a review we made a few years ago /I/. We insist on the role atomic data and their accuracy play in the interpretation of the solar photospheric spectrum, leading to a better knowledge of the physical conditions, physical processes and abundances of the elements in the solar outer layers. Astronomers need many more very accurate atomic wavelengths, ionization energies, partition functions, hyperfine structures, transition probabilities, cross sections, line broadening parameters, Land6 g-values, ....

The Spectrum of Al I and Al II in the Infrared (1800 < σ < 9000 cm-1)

The spectrum of Al I and Al II produced by a hollow-cathode discharge has been observed by the Fourier transform spectroscopy technique between 1800 and 9000 cm-1. These observations have allowed an extension of the term systems of Al I and Al II. The results are also used for revising the identification of Al I lines in the solar photospheric spectrum.

The Infrared Spectrum of Magnesium (1800 < σ < 9000 cm-1) and an Extension of the Term Systems of Mg I and Mg II

The spectrum of neutral and singly ionized magnesium produced by a hollow-cathode discharge has been observed by the Fourier transform spectroscopy technique between 1800 and 9000 cm-1. The observations reported here have allowed an extension of the term systems of Mg I and Mg II; more particularly, first observational evidence is reported for 3s ng 1,3G(n ⩽ 8) terms of Mg I and ni 2I(n ⩽ 10) terms of Mg II. The results are also used for refining the identification of magnesium lines in the solar photospheric spectrum.

The 3410 Å band of the PH molecule in the solar photospheric spectrum

Some weak unidentified solar photospheric lines in the wavelength range: (3400–3465) A may be due to PH lines of the (0, 0) band of the PH(A 3 Π i - X 3 ∑-)system. These faint PH molecular lines have resulted an excitation temperature of the order of 4500 K. Using experimental lifetime data for PH in the A 3 Π i state, an absorption oscillator strength f 00 = 0.0075 is derived for the 3410 A band of the PH (A 3 Π i - X 3 ∑-)system. Accurate line positions, oscillator strength and transition probability for the 4.4μ fundamental rotation-vibration band of the PH molecule are obtained. A comparison of positions of some lines of the 4.4μ band with those obtained on new tracings of high resolution solar spectra shows many coincidences with weak solar lines.

24MgH+ in the solar photospheric spectrum

The24MgH+ (A1Σ+ −X1Σ+) molecular lines have been identified in the photospheric spectrum. The rotational excitation temperature determined from the analysis of molecular line intensities of24MgH+ is found to be of the order of 4850 K which corresponds to the photospheric temperature of the Sun. The CNDO/2 dipole moments of24MgH+ for internuclear distance range: (1.3–2.1) A in theX1Σ+ state can be approximated byM(R)=4.92+1.33R. Estimations for the spontaneous emission Einstein coefficients (Av′ v″) and the absorption oscillator strengths (fv′ v″) for the (1, 0), (2, 0), and (2, 1) transitions in theX1Σ+ state of the24MgH+ ion are also made.

Oscillator strengths for Y I and Y II and the solar abundance of yttrium

view Abstract Citations (310) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Oscillator strengths for Y I and Y II and the solar abundance of yttrium. Hannaford, P. ; Lowe, R. M. ; Grevesse, N. ; Biemont, E. ; Whaling, W. Abstract Oscillator strengths for 154 Y I lines and 66 Y II lines with wavelengths between 2948 and 7055 A are determined by combining laboratory measurements of radiative lifetimes and branching ratios. Lifetimes are presented for 34 levels of Y I and 14 levels of Y II; branching ratios are given for all levels below 9000 A with parity change and delta J equals plus or minus 1, 0. Together with equivalent widths measured on a photometric atlas of the solar photospheric spectrum for eight Y I lines and 41 Y II lines with laboratory wavelenths between 2467 and 7882 A, the data are used to determine the solar abundance of yttrium. Mean logarithmic solar abundances of 2.18 plus or minus 0.12 for Y I and 2.25 plus or minus 0.03 for Y II are deduced, and an yttrium abundance of 2.24 plus or minus 0.03 is derived. It is noted that the data presented have important implications for the yttrium abundances in both cool and hot stars, particularly in chemically peculiar stars. Publication: The Astrophysical Journal Pub Date: November 1982 DOI: 10.1086/160384 Bibcode: 1982ApJ...261..736H Keywords: Abundance; Solar Spectra; Transition Probabilities; Yttrium; Absorptivity; Line Spectra; Tables (Data); Solar Physics full text sources ADS |

Solar abundance of praseodymium

16 lines of Pr ii possibly present in the solar photospheric spectrum have been studied. When including hyperfine structure in synthetic calculations, investigations of 9 lines result in an abundance APr = 0.71 ± 0.08 in the log AH = 12.00 scale.

A new method for the analysis of the solar photospheric spectrum

A new numerical method for the analysis of the high dispersion photospheric spectrum is described. In particular the method is applied to study the C2(0, 0) d3Πg-aaΠu molecular band. From measurements of the equivalent widths of C2 lines, a rotational temperature of 4450 ± 305 K is obtained, and the band intensity log W0/S0 = −0.051 ± 0.101 is found.

The solar photospheric spectrum near wavelength 6138.5 A

Abstract. — The blend mentioned in the S.R.R.T. at 6138.43 Å has been resolved with a new decomposition technique. The presence of Ti I and Y I is confirmed. Besides these two lines the results also reveal a line at 6138.661 Å, corresponding with a line of Ar II.

On the presence of Ar and In in the solar photometric spectrum

Abstract. — The results obtained with our newly proposed method for analyzing the solar photospheric spectrum are presented for the regions λ 6108.8- λ 6109.42 and λ 6114.76 - λ 6115.38. Six new weak lines are found, and the analysis suggests the identification of two lines with In II and one with Ar II.

Oscillator strengths for 2pnd?2pn?f transitions of Ci and identifications in the infrared solar photospheric spectrum (1 ? ? ? 3?)

Absolute transition probabilities are calculated for 2p(2 P) nd − 2p(2 P) n′f transitions of Ci (1 ⩽ λ ⩽ 3μ). Pair coupling expressions for line strengths are used and the radial wavefunctions are obtained through the use of the Coulomb approximation. A description of the occurrence of these lines in the solar photospheric spectrum is presented and 47 new identifications are proposed.