Lines Of Rare-earth Elements Research Articles

Stark broadening data for spectral lines of rare-earth elements: Example of Tb II and Tb IV

Stark broadening data for spectral lines of rare-earth elements: Example of Tb II and Tb IV

Stark broadening data for spectral lines of rare-earth elements: Nb III

The electron-impact widths for 15 doubly charged Nb ion lines have been theoretically determined by using the modified semiempirical method. Using the obtained results, we considered the influence of the electron-impact mechanism on line shapes in spectra of chemically peculiar stars and white dwarfs.

Combining emission and absorption spectroscopy at rare earth spectral lines: plasma temperature measurements in ceramic metal halide lamps

Presently, most high intensity discharge (HID) lamps contain mercury to generate a high pressure buffer gas and thereby an appropriate power input into the arc. Due to its toxicity, the replacement of Hg is of particular interest in recent research on HID lamps. Up to now, the emission coefficient of an atomic Hg double line is widely used to determine the plasma temperature Tpl in HID lamps. Tpl is needed to calculate the total density of atoms and ions of elements inside these lamps. A combination of optical emission and broadband absorption spectroscopy allows us to evaluate Tpl independently of Hg emission lines. The method is required for a determination of Tpl if the Hg line intensity within the investigated lamp is too low, is superimposed by other lines or if environmental-friendly Hg-free lamps are developed.Within this work, phase-resolved plasma temperatures are determined in front of the electrode of Hg-containing MH lamps by emission spectroscopy at atomic Hg lines. Above all, temperatures are measured by a combination of emission and absorption spectroscopy at atomic rare earth lines, namely Dy and Tm. A comparison of Tpl determined by both methods agree within an error margin of <10%. Total phase-resolved rare earth atom densities are obtained by means of the measured ground state densities and Tpl. The combination of emission and absorption spectroscopy is also applied to the bulk plasma of lamps where the intensity of the Hg emission lines is too low for plasma temperature measurements or Hg is absent. It provides the partial rare earth pressure and by comparison with thermodynamic data cold spot temperatures within the lamps.

Chemical composition and evolutionary status of the Ap star HD 138633

We present the results of our study of the atmospheric chemical composition and evolutionary status for the chemically peculiar Ap star HD 138633. In contrast to ordinary Ap stars that exhibit strong lines of rare-earth elements in their spectra, these elements are represented very poorly in the spectrum of HD 138633. The magnetic field is estimated to be less than 700 G, which is also atypical of peculiar Ap stars. We have detected a stratification of such elements as Fe, Si, Ca, and Y in the atmosphere of HD 138633 whose pattern agrees qualitatively with the predictions of the theory of diffusive separation of elements under the joint action of gravity and radiation pressure forces. The nonuniform distribution obtained by assuming a stepwise distribution of elements in depth agrees qualitatively with the stratification distribution for β CrB. The search for pulsations points to the possible existence of lowamplitude pulsations with a period of about 17 min.

Chemical composition and stratification of chemical elements in the atmosphere of the Ap star HD 8441

We present the results of our abundance analysis for the atmosphere of the chemically peculiar Ap star HD 8441. HD 8441 is interesting in that the lines of rare-earth elements in its spectrum are weak and the longitudinal magnetic field component is small, being only several hundred gauss. Our estimates of the evolutionary status for HD 8441 have confirmed that it belongs to the group of Ap stars with weak lines of rare-earth elements in their spectra that leave the main sequence. A stratification analysis of the atmosphere of HD 8441 assuming a stepwise distribution of elements with depth reveals a nonuniform distribution of Si, Ca, Cr,Mn, and Fe with a significant increase in elemental abundances in deeper layers. The derived distribution qualitatively agrees with predictions of the theory of diffusive separation of elements under the action of radiation pressure forces and gravity. Comparison of the chemical composition and evolutionary status of HD 8441 with those of the stars HD 66318 and HD 144897 with strong magnetic fields shows that their atmospheres differ mainly by the abundances of rare-earth elements. The iron-peak elements exhibit large overabundances irrespective of the magnetic field strength.

HD 97394: a magnetic Ap star with high cerium overabundance★

We report a spectroscopic analysis of the chemically peculiar Ap star HD 97394. The stellar spectrum is rich in lines of rare earth elements with large overabundances, especially cerium, gadolinium and europium. Enhancement of the abundances of these rare earths shows this star to be one of the most peculiar stars. Very large overabundances were found for lines of Ce III and Eu III. Abundances obtained from second ionization lines of Nd, Ce and Eu are about 2 dex higher than for those of the first ionization. From partially split Zeeman components of the Fe II 6149.258 A line and from synthetic modelling, a global magnetic field of 3.1 kG was measured. We tested for pulsation of the star with high time resolution spectroscopy obtained with the ESO Very Large Telescope. We place an upper limit to any pulsation amplitude of 30–40 m s −1 for individual lines of rare earth elements, of 10–20 m s −1 for the combination of several lines, and of 6–10 m s −1 for cross-correlation over large spectral bands.

Identification of absorption lines of rare-earth elements in the region of the spectrum λλ 610.25–610.57 nm roAp stars, HD 101065, HD 134214, HD 137949, and HD 24712

In the spectral region λλ = 610.25−610.57 nm of the roAp star HD 101065, we performed the identification of the rare-earth element absorption lines absent from the atomic spectral line databases VALD and DREAM. The identified lines were used for calculating the synthetic spectra of the roAp stars HD 137949, HD 134214, and HD 24712. The upper limit of the identified line estimates was determined. The rotation/pulsation parameter νsini and magnetic field modulus were determined using the lines Nd III 669.083 nm and Ca I 616.217 nm. The element abundance in the roAp stars HD 134214 and HD 24712 was determined for the first time using Nd III lines. Calculations were carried out with the help of the code SynthM developed by S.A. Khan.

Realistic model atmosphere and revised abundances of the coolest Ap star HD 101065

Among the known Ap stars, HD101065 is probably one of the most interesting objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly peculiar photometric parameters of this star that can not be fully reproduced by any modern theoretical calculations, even those accounting for realistic chemistry of its atmosphere. In this study we investigate a role of missing REE line opacity and construct a self-consistent atmospheric model based on accurate abundance and chemical stratification analysis. We employed the LLmodels stellar model atmosphere code together with DDAFit and SynthMag software packages to derive homogeneous and stratified abundances for 52 chemical elements and to construct a self-consistent model of HD101065 atmosphere. The opacity in REE lines is accounted for in details, by using up-to-date extensive theoretical calculations. We show that REE elements play a key role in the radiative energy balance in the atmosphere of HD101065, leading to the strong suppression of the Balmer jump and energy redistribution very different from that of normal stars. Introducing new line lists of REEs allowed us to reproduce, for the first time, spectral energy distribution of HD101065 and achieve a better agreement between the unusually small observed Str\"omgren c1 index and the model predictions. Using combined photometric and spectroscopic approaches and based on the iterative procedure of abundance and stratification analysis we find effective temperature of HD101065 to be Teff=6400K.

A search for rapid pulsations in the magnetic cool chemically peculiar star HD 3980★

The Ap star HD 3980 appears to be a promising roAp candidate based on its fundamental parameters, leading us to search for rapid pulsations with the VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcos parallax and estimated temperature of 8100 K place HD 3980 in the middle of the theoretical instability strip for rapidly oscillating Ap stars, about halfway through its main-sequence evolution stage. The star has a strong, variable magnetic field, as is typical of the cool magnetic Ap stars. Dipole model parameters were determined from VLT observations using Focal Reducer and low Dispersion Spectrograph (FORS)1. From Doppler shift measurements for individual spectral lines of rare-earth elements and the Hα line core, we find no pulsations above 20–30 m s−1. This result is corroborated by the inspection of lines of several other chemical elements, as well as with cross-correlation for long spectral regions with the average spectrum as a template. Abundances of chemical elements were determined and show larger than solar abundances of rare-earth elements. Further, ionization disequilibria for the first two ionized states of Nd and Pr are detected. We also find that the star has a strong overabundance of manganese, which is typical for much hotter HgMn and other Bp stars. Line profile variability with the rotation period was detected for the majority of chemical species.

Pulsation properties of the rapidly oscillating Ap star 10 Aquilae (HD 176232)

We have studied the pulsation behaviour of radial velocities of individual spectral lines of different chemical elements for the well known, bright rapidly oscillating Ap (roAp) star 10 Aql (HD 176232) based on high-quality Very Large Telescope spectra. We found that only lines of rare earth elements and the narrow Hα core show detectable rapid variations of radial velocity with the known photometric pulsation period, 11.7 min. The highest pulsation amplitudes of more than 0.5 km s−1 are found for rather weak lines of Tb iii and Dy iii. The pulsational phases obtained for lines of Tb iii and Dy iii differ by π rad, perhaps an indication of a radial node between their line-forming layers. Weak lines of Sm ii also display amplitudes near 0.5 km s−1. Surprisingly, the lines of Pr iii show the lowest detected pulsation amplitude in strong contrast with the majority of roAp stars for which this ion has some of the highest radial velocity amplitudes. Lines of Nd iii with different intensities have pulsation amplitudes different to the weaker lines that are formed more deeply in the atmosphere having higher amplitudes. Line bisectors for strong Nd iii line profiles show significant changes of phase, and even phase jumps for some lines, indicating complex variations in the pulsation phase as a function of atmospheric depth. The low-measured rotational velocity of this star indicates that there is little Doppler resolution of the line profiles, hence that the large range of pulsation amplitudes and phases of radial velocities determined for line bisectors are primarily caused by atmospheric depth effects.

On the nature of the upper atmospheric variability in the rapidly oscillating Ap star HD 134214★

Recently, we discovered an entirely new type of variability in the upper atmospheres of rapidly oscillating Ap (roAp) stars. This manifests itself in amplitude modulation of the radial velocities that has not been previously detected in photometric studies of the same stars. To study this new variability further we obtained a full night, 8.85 h, of high time resolution (70s), high spectral resolution (R = 105 000), high signal-to-noise ratio (on average S/N ∼ 130) data with Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) for the roAp star HD 134214. We also obtained 4.2 h of new photometric data in Johnson B with the South African Astronomical Observatory (SAAO) 0.5-m telescope 2d later. HD 134214 has been known for years to be singly periodic with a relatively stable amplitude in photometry; it has the highest pulsation frequency of any roAp star of 2.949 mHz (P = 5.65 min). Our new UVES data show this principal frequency, plus five other frequencies in amplitude spectra of rare earth elements lines and the Ha line. The new frequencies are stable over the 8.85 h of observation, and the highest of them v 2 = 2.782 mHz is the same as found in data taken 2 yr earlier with UVES presented in the discovery paper. The amplitudes of the new frequencies drop faster with atmospheric depth than does the amplitude of the principal frequency, hence explaining why they are generally not seen in broad-band photometric measurements that sample on average more deeply in the atmosphere. Our new photometric measurements also detect v 2 for the first time in photometric data. Our analysis suggests that the new frequencies are associated with pulsation modes, but the nature of those modes and why they increase in amplitude with atmospheric height more strongly than the principal frequencies is not yet known.

Pulsation in the presence of a strong magnetic field: the roAp star HD 166473★

Phase-resolved high-resolution, high signal-to-noise ratio (S/N) observations of the strongly magnetic roAp star HD 166473 are analysed. HD 166473 was selected as the target of this study because it has one of the strongest magnetic fields of all the roAp stars known with resolved magnetically split lines. Indeed, we show that enhanced pulsation diagnosis can be achieved from consideration of the different pulsation behaviour of the π and a Zeeman components of the resolved spectral lines. This study is based on a time-series of high spectral resolution observations obtained with the Ultraviolet and Visual Echelle Spectrograph of the Very Large Telescope of the European Southern Observatory. Radial velocity variations due to pulsation are observed in rare earth lines, with amplitudes up to 110 m -1 . The variations occur with three frequencies, already detected in photometry, but which can in this work be determined with better precision: 1.833, 1.886 and 1.928 mHz. The pulsation amplitudes and phases observed in the rare earth element lines vary with atmospheric height, as is the case in other roAp stars studied in detail. Lines of Fe and of other (mostly non-rare earth) elements do not show any variation to very high precision (1.5 m s -1 in the case of Fe). The low amplitudes of the observed variations do not allow the original goal of studying differences between the behaviour of the resolved Zeeman line components to be reached; the S/N achieved in the radial velocity determinations is insufficient to establish definitely the possible existence of such differences. Yet the analysis provides a tantalizing hint at the occurrence of variations of the mean magnetic field modulus with the pulsation frequency, with an amplitude of 21 ± 5 G.

Analysis of the lithium lines in the atmosphere of the roAp star HD 24712

We use the synthetic spectrum technique to analyze two lithium lines in the spectrum of the roAp star HD 24712. The synthetic spectra are calculated in the ranges λλ 610.2–610.4 nm and 670.5–670.8 nm. We take into account the magnetic splitting of the lines and their blending by the lines of rare-earth elements. The effect of the magnetic field on absorption lines is estimated by the synthetic spectrum software SynthM developed by S.A. Khan. The upper limit for the lithium abundance turns out to be log N/N tot = −10.0.

A high-precision search for magnetic field oscillations in the roAp star HD 24712

We have obtained a time series of 81 high-cadence circular polarization observations of the rapidly oscillating Ap star HD 24712 with the new ESPaDOnS spectropolarimeter at CFHT. We used the high-S/N, high-resolution Stokes I and V spectra to investigate possible variation of the mean longitudinal field over the pulsation cycle in this roAp star. Our multiline magnetic field and radial velocity measurements utilized 143 spectral lines of rare-earth elements, attaining precision better than 13 G and 19 m s -1 , respectively. A multiperiodic radial velocity variation with an amplitude of 40–136 m s -1 is clearly detected at the known pulsation frequencies of HD 24712. At the same time, no evidence for pulsational changes of the magnetic field can be found. We derive a $3\sigma$ upper limit of 10 G, or about 1% of the mean longitudinal field strength, for magnetic field oscillations in the upper atmosphere of HD 24712. The absence of detectable pulsational variability of the magnetic field provides a valuable constraint for the interaction between pulsations and magnetic field in roAp stars and is compatible with the recent predictions of detailed theoretical models of stellar magnetoacoustic oscillations.

Lithium and the 6Li-7Li isotope ratio in the atmospheres of some sharp-lined roAp stars

The λ 6708 A and 6103 A lithium lines in the high-resolution spectra of some sharp-lined roAp stars are analyzed using three spectral-synthesis codes STARSP, ZEEMAN2, and SYNTHM. The lines from the VALD database were supplemented with lines of rare-earth elements from the DREAM database and new lines calculated using the NIST energy levels. Our synthetic-spectrum calculations take into account magnetic splitting and other line-broadening effects. Lithium overabundances were found in the atmospheres of the stars based on our analysis of both lithium lines, along with high values of the 6Li-7Li isotope ratio (0.2–0.5). This can be explained if lithium is produced in spallation reactions and the surface 6Li and 7Li is preserved by strong magnetic fields in the upper layers of the stellar atmospheres, around the poles of the dipole field. The asymmetry of lithium lines to the red may be due to the action of shocks in the optically thin upper atmosphere, with the shocked material subsequently falling onto the star.

Search for radial velocity and magnetic field pulsational variations in the roAp star γ Equulei

Aims. A new continuous 3.5 h series of high-resolution spectroscopic observations of the roAp star yEqu, with about 12 points per pulsation cycle, is presented to study the mean magnetic field modulus variability with the pulsation period. Methods. Radial velocity variations with amplitudes of up to approximately 400 m s -1 are clearly detected in the rare earth lines present in the covered spectral range. Two frequencies are resolved in their analysis, which are consistent with frequencies previously observed in the star. No significant radial velocity variation is detected for the line Fe II A 6149.2. The mean magnetic field modulus derived from measurement of the wavelength separation of the magnetically resolved components of this line does not show any variation, with a 3σ upper limit of ∼10G. Results. While our results are consistent with the conclusion of the previous studies, difficulties to reconcile this result with those of other works may be attributed, in part at least, to the use of different diagnostic lines in different works and to the failure of all studies so far to fully resolve all the pulsation frequencies of γ Equ.

High time resolution spectroscopy and magnetic variability of the cool Ap star HD 965

We present the results of an investigation of the magnetic Ap star HD 965 with high spectral and time resolution. We determine precise radial velocities using spectra obtained with the Ultraviolet‐Visual Echelle Spectrograph (UVES) on the European Southern Observatory Very Large Telescope. Special attention is given to spectral lines of rare-earth elements which in rapidly oscillating Ap (roAp) stars exhibit the strongest radial velocity variations with pulsation period. Careful time series analysis did not detect any convincing evidence of pulsation in HD 965 with an upper limit in amplitude of 15‐20 m s −1 . All properties of HD 965 are similar to the majority of roAp stars. A likely reason for the apparent lack of pulsation could be connected with the geometrical structure of the magnetic field and the aspect of the star at the time of observation. Longitudinal magnetic field measurements for HD 965 showed that our UVES spectral observations were carried out when the longitudinal field was near zero and therefore, according to the oblique rotator model, near a time when the star was viewed from the magnetic equator. For a dipole oscillation aligned with the magnetic field, as is typical of roAp stars, no variation can be detected at this aspect. We may, therefore, expect to detect rapid oscillations in HD 965 in the future, when the star will present one of the magnetic poles. Ke yw ords: stars: individual: HD 965 ‐ stars: magnetic fields ‐ stars: oscillations ‐ stars: variables: other.

No magnetic field variation with pulsation phase in the roAp starγ Equulei

We present an analysis of 210 high-resolution time-resolved spectropolarimetric observations of the roAp star γ Equ obtained over three nights in August and September 2003. Radial velocity variations due to p-mode non-radial pulsations are clearly detected in the lines of rare-earth elements, in particular Pr  ,N d and Nd . In contrast, we find absolutely no evidence for the variation of the mean longitudinal magnetic field over the pulsation period in γ Equ at the level of 110-240 G which was recently reported by Leone & Kurtz (2003). Our investigation of the variability of circularly polarized profiles of 13 Nd  lines demonstrates that, at the 3σ confidence level, no magnetic field variation with an amplitude above ≈40−60 G was present in γ Equ during our monitoring of this star.

Pulsational velocity fields in the atmospheres of two roAp stars HR1217 and γ Equ

AbstractWe discuss spectroscopic observations of pulsations in the roAp stars γ Equ and HR 1217. In both stars we observe strong variations of rare-earth lines and detect running magneto-acoustic waves, but do not find evidence for a standing pulsational wave. In γ Equ we find strong pulsational variations across line profiles, characteristic of l ≥ 2 modes.

Characteristic features of the spectrum of the unique roAp star HD 101065 near the 6708 Å lithium resonance doublet

Synthetic spectrum of the peculiar roAp star HD 101065 (Przybylski’s Star) have been computed in the wavelength interval 6705.8–6708.7 A in order to describe the observed spectrum in the vicinity of the 6708 A lithium line. Our detailed computations of the synthetic spectrum allowing for the hyperfine structure of the lithium line, lithium isotopic composition, and blending due to lines of rare-earth elements (CeII, NdII, SmII, and others) have yielded estimates of the atmospheric lithium abundance of the star and its 6Li/7Li isotopic ratio.