Pressure shifts and abundance gradients in the atmosphere of the DAZ white dwarf GALEX J193156.8+011745

We present a detailed model atmosphere analysis of high-dispersion and high signal-to-noise ratio spectra of the heavily polluted DAZ white dwarf GALEX J1931+0117. The spectra obtained with the VLT-Kueyen/UV-Visual Echelle Spectrograph show several well-resolved Si II spectral lines enabling a study of pressure effects on line profiles. We observed large Stark shifts in silicon lines in agreement with theoretical predictions and laboratory measurements. Taking into account Stark shifts in the calculation of synthetic spectra we reduced the scatter in individual line radial velocity measurements from ~ 3 to < 1 km/s. We present revised abundances of O, Mg, Si, Ca, and Fe based on a critical review of line broadening parameters and oscillator strengths. The new measurements are generally in agreement with our previous analysis with the exception of magnesium with a revised abundance a factor of two lower than previously estimated. The magnesium, silicon and iron abundances exceed solar abundances, but the oxygen and calcium abundances are below solar. Also, we compared the observed line profiles to synthetic spectra computed with variable accretion rates and vertical abundance distributions assuming diffusive steady-state. The inferred accretion rates vary from dM/dt = 2x10^6 for calcium to 2x10^9 g/s for oxygen. We find that the accretion flow must be oxygen-rich while being deficient in calcium relative to solar abundances. The lack of radial velocity variations between two measurement epochs suggests that GALEX J1931+0117 is probably not in a close binary and that the source of the accreted material resides in a debris disc.Comment: Accepted for publication in MNRA

Measured, calculated and predicted stark widths along a beryllium isoelectronic sequence in the 3s-3p transition

On the basis of the observed satisfactory agreement between recent experimental and calculated Stark width values with our earlier predicted ones along the beryllium isoelectronic sequence (BeI, BII, CIII, NIV, OV FVI, NeVII,..) new Stark width values have been predicted for high ionized emitters (NaVIII, MgIX, AlX, SiXI and PXII) in the 3s-3p transition interesting for astrophysical investigations

Experimental transition probabilities in N III, N IV and N V spectra

We have obtained transition probabilities (Einstein's A values) of thirteen transitions in doubly (N III), six in triply (N IV) and two in four times (N V) ionized nitrogen spectra belonging to the $3s{-}3p$ and $3p{-}3d$ transitions using a relative line intensity ratio (RLIR) technique. The linear low-pressure pulsed arc was used as an optically thin plasma source operated at 51 400 K electron temperature and $2.2 \times 10^{23}$ m-3 electron density in nitrogen plasma. Our A values are compared to recent theoretical and experimental data

Stark broadening and transition probability ratios in the Mg I spectrum

Stark widths (W) and shifts (d) of the six astrophysically important 285.212, 383.230, 383.829, 516.732, 517.268, and 518.360 nm neutral magnesium (Mg I) spectral lines in the $3{\rm s}^2\,{^{1}}{\rm S}{-}3{\rm p}^{1}{\rm P}{^0}, 3{\rm p}^{3}{\rm P}{^0}{-}3{\rm d} ^{3}{\rm D}$ and $3{\rm p}^{3}{\rm P}{^0}{-}4{\rm s}^{3}{\rm S}$ transitions have been measured in laboratory helium plasma at about 50 000 K electron temperature and 1 $\times$ 1023 m-3 electron density. They represent the first measured values and, also, the first experimental W and d data in the mentioned transitions. Using the relative line intensity ratios of the lines in the mentioned transitions we have obtained the ratios of corresponding transition probability values (Einstein's A values). They represent the first experimental data based on the analysis of the Mg I emission spectral lines. We have found agreement with theoretical transition probability ratios tabulated by NIST

Line broadening in the Si I, Si II, Si III, and Si IV spectra in the helium plasma

Context. The neutral and ionized silicon spectral line shapes have been investigated in the laboratory helium plasma at electron densities ranging between $3.7\times 10^{22}$ m-3 and $1.1\times 10^{23}$ m-3 and electron temperatures between 12 500 K and 19 000 K, both interesting for astrophysics. Aims. The aim of this work is to present experimental Stark FWHM (full-width at half of the maximum line intensity, W) for number of spectral lines from neutral (Si I), singly (Si II), doubly (Si III), and triply (Si IV) ionized silicon spectra emitted by the pulsed helium discharge, which is optically thin at the wavelengths of the investigated ionic silicon lines. A specific method for estimating self-absorbtion is presented in detail. For investigated Si I spectral lines, applying the proposed method, an optical depth of less than 0.38 is found. Appropriate corrections of the Si I Stark widths were made. The Stark widths of different ionic species, presented in this paper, are measured for the first time in the essentially same laboratory plasma. Methods. The silicon atoms were evaporated from the walls of the specially designed pyrex discharge tube in the pulsed helium discharge at a pressure of 665 Pa in a flowing regime. The Si I, Si II, Si III, and Si IV spectral line profiles were recorded using the McPherson model 209 spectrograph and the Andor ICCD camera as the detection system. Results. The Stark FWHMs of 13 Si I, 15 Si II, 28 Si III, and 9 Si IV spectral lines were measured in the wavelength interval between 206 nm and 640 nm. Five Si I, four Si II, eleven Si III, and one Si IV W values from the above set not had measured or calculated. Our W values are compared with the existing theoretical and experimental data. Conclusions. At the mentioned plasma parameters tolerable agreement was found (within the accuracy of the experiment and uncertainties of the theoretical approaches used) between measured and calculated Stark FWHM values. We recommend the Stark FWHMs of the intense 254.182 nm, 308.624 nm, and 309.342 nm Si III, and 314.956 nm and also 316.571 nm Si IV lines for the plasma diagnostic purposes

Role of the He I and He II metastables in the resonance 2p

Aims.The aim of this work is to present atomic processes which lead to an extra population of the \rm 2p ~^2P\degr_{1/2, 3/2} B III resonance levels in helium plasma generating intense radiation in the B III 206.578 nm and 206.723 nm lines. Methods.The line profiles were recorded using a step-by-step (7.3 pm) technique which provides monitoring of the line shapes continually during the plasma decay and gives the possibility to compare line shapes at various times in the same plasma. Results. On the basis of the line intensity decays of the doubly ionized boron resonance spectral lines in laboratory nitrogen and helium plasmas, we have found the existence of a permanent energy transfer from He I and He II metastables to the 2p ^2\rm P\degr_{1/2, 3/2} B III resonance levels. The shapes of the mentioned lines are also observed. At electron temperatures of about 18 000 K and electron densities about $1.1\times 10^{23}$ m-3, the Stark broadening was found as a main B III line broadening mechanism. The measured Stark widths $(W)$ are compared with the Doppler width $(W_{\rm D})$ and with the splitting in the hyperfine structure $(\Delta_{\rm hfs}).$ Our measured W data are found to be much higher than results obtained by means of various theoretical approaches. Conclusions. The He I and He II metastables over populate the B III resonance levels leading to populations higher than predicted by LTE model. Consequently, the emitted B III resonance lines are more intense than expected from LTE model. This fact can be of importance if B III resonance line intensities are used for abundance determination purposes in astrophysics. Similar behavior can be expected for some lines emitted by astrophysical interesting emitters: Al III, Si III, Sc III, Cr III, V III, Ti III, Fe III, Co III, Ni III, Ga III, Zr III, Y III, Nb III, In III, Sn III, Sb III, Au III, Pb III and Bi III in hot and dense helium plasmas

Stark broadening parameters of the 381.96 nm He I line

Stark width (W) and shift (d) of the neutral helium (He I) 381.96 nm spectral line in the high lying $2p{-}6d$ transition has been measured in the optically thin linear, low-pressure, pulsed arc discharge operated in the helium at 50 000 K electron temperature and $6.1 \times 10^{22}$ m-3 electron density. These values are the first experimentally obtained W and d related to this line. Unfortunately, comparison with theoretical results is not possible due to the fact that only one existing theoretical calculations has been done at a 1019 m-3 electron density only. We have found negative line shift (toward the blue) that agree well with existing theoretical predictions

On the Bowen fluorescence mechanism in the helium–oxygen plasmas

The dependence of the radiation intensity from the 2p3d 3P$^{\rm o}_{2,1,0}$ doubly ionized oxygen (\ion{O}{iii}) levels on the He/O density ratio has been investigated in optically thin laboratory plasmas. A clear contribution of the astrophysically important Bowen mechanism to the most intensive 313.279 nm \ion{O}{iii} line radiation has been found in the primary \ion{O}{iii} Bowen cascade. We have found that in plasmas with electron temperatures of about 50 000 K and electron densities higher than 1022 m-3, the 312.163 nm \ion{O}{iii} spectral line also shows a fluorescence tendency caused by the Bowen mechanism. On the basis of the established dependence of the fluorescence efficiency on the He/O density ratio we recommend the I(313.279 nm)/I(311.567 nm) and I(312.163 nm)/I(311.567 nm) \ion{O}{iii} line intensity ratios as a measure of the presence of the helium/oxygen density ratio in astrophysical plasmas. The line intensity ratio related to the 344.405 nm and 342.863 nm \ion{O}{iii} lines (which also belong to the primary cascade in the Bowen fluorescence mechanism and originate in the same energy level) has also been monitored in pure oxygen and helium–oxygen plasmas. We have found a good agreement with the results of previous astrophysical observations and recently published theoretical predictions. We have also found that the I(344.405 nm)/I(342.863 nm) line intensity ratio does not depends on the helium presence in plasmas and thus, it represents a convenient value in plasma spectroscopy