Neutral Helium Lines Research Articles

Influence of the ionic electric field on Stark shifts of neutral helium lines in plasmas

In this work we provide a contribution to Stark shift for some lines of neutral helium in plasmas. The mentioned shift is caused by the collisions of the unbounded electrons with helium atoms in plasmas. We calculate this contribution by taking into account only weak collisions. This contribution takes into consideration the presence of the local electric microfield created by the ions that constitute in part the plasma. The effect of this microfield modifies the electron trajectory and therefore changes the shift formula developed early by using the straight trajectories in the case of neutral emitters. Our new contribution to the Stark shift caused by electron's collision is applied to some HeI lines: 3188 , 3888 , 6678 and 7065 . Our results are confronted with the experimental and theoretical data from the literature which clearly show that the influence of the local electric microfield created by the ions on the Stark shift caused by electron's collision is not negligible and must be taken into account. It turns out that the correction to the theoretical shift for the lines under consideration tends to red.

Some observational properties of the Herbig Be Star AS 310

The results of spectral and photometric studies of Herbig Be-type star AS 310 have been presented. Photometric observations in the BVRcIc bands performed in 2016–2021 showed that the amplitude of the seasonal brightness variations of the star changes smoothly in different years and reaches a maximum of approximately around ΔV ∼ 0.6 mag in 2018. The star is more often in a weak state of radiation, but from time to time an increase in its brightness is observed. With a decrease in brightness in the V-band, the star’s radiation in the blue color increases, while in the red part of the spectrum, it decreases. Searches for periodicity in the light variations did not reveal definite periods of variability. The emission components observed in the hydrogen and forbidden nitrogen lines are rather formed in the nebula where the star itself is immersed. The spectrum contains strong lines of neutral helium, without any signs of emission components. The spectral class corresponds to the combined spectrum of B3–5 V + B1.5–2 V.

Spectrophotometric analysis of magnetic white dwarf – II. Helium-rich compositions

ABSTRACT We present an analysis of all single white dwarf stars known to exhibit spectroscopic signatures of neutral helium line splitting due to the presence of a strong magnetic field. Using state-of-the-art models taking into account the effects of magnetic fields on the synthetic spectra, we determine effective temperatures, surface gravities, and masses for the stars in our sample. Our analysis uses data from the second and third Gaia (early) data releases, photometric data from diverse surveys such as the Sloan Digital Sky Survey and Pan-STARRS, and archived spectroscopic data. We are able to successfully reproduce the spectra of eight objects using an offset dipole geometry while several others seem to require either a more complex geometry or a different chemical composition. We also highlight a group of hot featureless white dwarfs that are most probably highly magnetic objects whose spectra are completely smeared due to the field strength distribution across the surface.

Gaia white dwarfs within 40 pc – III. Spectroscopic observations of new candidates in the Southern hemisphere

Abstract We present a spectroscopic survey of 248 white dwarf candidates within 40 pc of the Sun; of these 244 are in the Southern hemisphere. Observations were performed mostly with the Very Large Telescope (X-Shooter) and Southern Astrophysical Research Telescope. Almost all candidates were selected from Gaia Data Release 3 (DR3). We find a total of 246 confirmed white dwarfs, 209 of which had no previously published spectra, and two main-sequence star contaminants. Of these, 100 white dwarfs display hydrogen Balmer lines, 69 have featureless spectra, and two show only neutral helium lines. Additionally, 14 white dwarfs display traces of carbon, while 37 have traces of other elements that are heavier than helium. We observe 35 magnetic white dwarfs through the detection of Zeeman splitting of their hydrogen Balmer or metal spectral lines. High spectroscopic completeness (> 97 per cent) has now been reached, such that we have 1058 confirmed Gaia DR3 white dwarfs out of 1083 candidates within 40 pc of the Sun at all declinations.

Characterization of helium microplasma generated in a flow focusing microfluidic device

Non-thermal microplasmas produced in a microchannel have several potential applications in analytical chemistry, environmental sensing, and surface modification of microfluidic chips for biomedical and lab-on-chip devices. This paper investigates the properties of an atmospheric pressure helium microplasma excited in a polydimethylsiloxane flow focusing microfluidic chip. The influence of input parameters such as applied voltage and gas flow rate on discharge characteristics is investigated in detail. Electron excitation and molecular rotational temperatures are determined with the Boltzmann plot technique. The rotational temperature from the N2+ emission band was calculated in the range of 348–417 K. Electron density and temperature are determined using the well-known plasma diagnostic technique of Stark broadening. The emission lines of hydrogen Balmer (Hα) and neutral helium (501, 667, and 728 nm) are selected to measure the parameters of Stark broadening. The electron density and electron temperature were found to be in the range 0.7×1016–3.39×1016 cm−3 and 10 800–12 493 K, respectively. The evaluated discharge parameters validate the non-thermal equilibrium state of the microplasma. The electrical diagnostics of plasma were performed by monitoring the signals of high voltage and current of the discharge. Moreover, the plasma modified surface (hydrophobic to hydrophilic) was verified by successfully utilizing the microchannel to form an oil-in-water micro-emulsion.

Zodiacal Exoplanets in Time (ZEIT). XIV. He i Transit Spectroscopy of the 650 Myr Hyades Planet K2-136c

Abstract We report the non-detection of excess absorption in the 1083.2 nm line of neutral triplet helium during a transit of K2-136c, a 2.9R ⊕ planet on a 17.3 days orbit around a K-type dwarf in the 650 Myr old Hyades cluster. We limit the equivalent width of any transit-associated absorption in the He i line to <25 mÅ at 99% confidence.

First high-resolution optical spectra of the distant emission-line star VES 723 (IRAS 02110+6212)

ABSTRACT The first high-resolution spectra (resolving powers of R ≥ 60 000) of the emission-line star VES 723, which has an unknown evolutionary status, were taken at the 6-m Big Telescope Alt-Azimuthal and the 3.6-m Canada–France–Hawaii Telescope. The spectrum is dominated with powerful emission lines of neutral hydrogen and helium, and forbidden singly ionized nitrogen. The Hα and He i lines were found to exhibit broad foundations of the emission profiles at least 250 km s−1 wide. The strongest emission lines exhibit no noticeable variations during the observing period (2011–2017) except for the He i lines. The spectra contain multicomponent interstellar absorptions of Na i and K i (mult. 1) and several strongest diffuse interstellar bands, while photospheric absorptions have not been detected. The average radial velocity of the forbidden [N ii] emission line centres was adopted as the systemic velocity (Vsys ≈ −52 km s−1), which indicates a large distance of ≈6 kpc. Our luminosity estimates of log L/L⊙ = 3.7−4.0 for two different Gaia distance measurements, along with an effective temperature of Teff = 25 000 ± 2000 K and a fast decrease of the infrared flux longward of $\lambda \sim 10\, \mu{\rm m}$, allow us to suggest that VES 723 is neither a pre-main-sequence Herbig B[e] star nor a B[e] supergiant but rather a member of the FS CMa group of objects with the B[e] phenomenon.

New Calculations of Stark-broadened Profiles for Neutral Helium Lines Using Computer Simulations

Abstract We present new calculations of Stark-broadened profiles for neutral helium lines using computer simulations that include some important aspects aimed at better representing the dynamical environment of the helium atom. These include the unification of ion and electron treatment, the correction for ion dynamics, the transition of the electron contribution to broadening from the core to the wings of the profile, the numerical integration of the time evolution operator of helium perturbed by a fluctuating electric field, the Debye correction for the correlation of the motion of charged perturbers, local density variations, and particle reinjection. We compare the results of our simulations for the He i λλ4471 and 4922 lines with other results published in the literature. We also test our simulation environment for narrow lines (He i and 6678) and broader lines (He i and 4144). We find that the narrow lines are more difficult to produce adequately than the broader ones.

Development of a multi-color gas puff imaging diagnostic on HL-2A tokamak.

A Multi-Color (MC) gas puff imaging diagnostic has been developed on HL-2A tokamak. This diagnostic can simultaneously measure two-dimensional (2D, radial, and poloidal) electron density and temperature distributions with a good spatial resolution of 2.5 × 2.5 mm2 and a temporal resolution of about 100 µs at best in edge plasmas. The 2D electron density and temperature distributions are inferred from the ratios of intensities of three different neutral helium emission lines; therefore, it is also referred to as helium beam probe or beam emission spectroscopy on thermal helium. A compact light splitter is used to split the inlet visible emission beam into four channels, and the specific neutral helium lines of the wavelengths λ1 = 587.6 nm, λ2 = 667.8 nm, λ3 = 706.5 nm, and λ4 = 728.1 nm are measured, respectively. This MC diagnostic has been experimentally tested and calibrated on a linear magnetic confinement device Peking University Plasma Test device, and the measured 2D electron density and temperature distributions are compared with the Langmuir probe measurements.

Zodiacal exoplanets in time – X. The orbit and atmosphere of the young ‘neptune desert’-dwelling planet K2-100b

ABSTRACT We obtained high-resolution infrared spectroscopy and short-cadence photometry of the 600–800 Myr Praesepe star K2-100 during transits of its 1.67-d planet. This Neptune-size object, discovered by the NASA K2 mission, is an interloper in the ‘desert’ of planets with similar radii on short-period orbits. Our observations can be used to understand its origin and evolution by constraining the orbital eccentricity by transit fitting, measuring the spin-orbit obliquity by the Rossiter–McLaughlin effect, and detecting any extended, escaping the hydrogen–helium envelope with the 10 830 -Å line of neutral helium in the 2s3S triplet state. Transit photometry with 1-min cadence was obtained by the K2 satellite during Campaign 18 and transit spectra were obtained with the IRD spectrograph on the Subaru telescope. While the elevated activity of K2-100 prevented us from detecting the Rossiter–McLaughlin effect, the new photometry combined with revised stellar parameters allowed us to constrain the eccentricity to e < 0.15/0.28 with 90/99 per cent confidence. We modelled atmospheric escape as an isothermal, spherically symmetric Parker wind, with photochemistry driven by ultraviolet radiation, which we estimate by combining the observed spectrum of the active Sun with calibrations from observations of K2-100 and similar young stars in the nearby Hyades cluster. Our non-detection (<5.7 m Å) of a transit-associated He i line limits mass-loss of a solar-composition atmosphere through a T ≤ 10000 K wind to <0.3 M⊕ Gyr−1. Either K2-100b is an exceptional desert-dwelling planet, or its mass-loss is occurring at a lower rate over a longer interval, consistent with a core accretion-powered scenario for escape.

Neutral helium line profiles through the simulation of local interactions

AbstractFor the past 25 years, we have been considering the Stark effect for neutral helium lines in DB white dwarfs using the standard Stark broadening theory in both the impact regime (in the center of the lines) and the quasi-static regime (in the wings) for the electrons, while neglecting the effect of ions in motion. Although this is probably a good approximation based on previous theoretical work, the transition between the two regimes for the electrons and the contribution of the ions very near the core might be poorly represented. To better represent these particularities, we report here the results of a new series of simulations that treat the local dynamics and interactions of both electrons and ions around a neutral helium atom. From these simulations, we produce new improved line profiles, which we compare with our previous analytical results.

Anisotropy of the Shock Radiation of Helium Atoms in the Earth’s Ionosphere

The possibility of observing, in the upper layers of the Earth’s atmosphere, the polarization of the HeI helium line, with a wavelength of 5015 A, is studied. Several helium lines are observed in the visible region. The polarization of HeI neutral helium lines can have a shock character and will occur when plasma is affected by accelerated proton and electron fluxes, e.g., solar wind. The calculations show that the degree of polarization for a singlet line with a wavelength of 5015 A corresponding to the transition 2S1–3P1 is the highest for any line and can reach 20%. The latter means that the polarization in the 5015 A line can be detected by studying the response the Earth’s chromosphere to the solar wind flare effect. The obtained results can be used to analyze the charge composition of plasma tubes in the ionosphere, in which there has been an increased interest in recent years.

Non-LTE Effects of Helium Lines in Late-B and A Stars

A helium model atom that includes 55 He I levels and the He II ground level in a detailed consideration has been constructed to investigate the departures from local thermodynamic equilibrium (LTE) in the formation of helium lines in stars with effective temperatures from 9300 to 20 000 K. For eight stars with effective temperatures from 9380 to 17 500 K the helium abundance has been determined from He I lines. The neutral helium lines in B stars cannot be described under LTE conditions using the common helium abundance. Furthermore, the profiles of several lines cannot be described in terms of the LTE approach at all. In contrast, a satisfactory coincidence of the theoretical and observed profiles for the entire set of helium lines observed in a wide spectral range can be achieved using virtually the same helium abundance by taking into account the departures from LTE. The LTE and non-LTE helium abundances can differ by up to a factor of 2–3, depending on the stellar parameters. The higher the stellar temperature, the stronger the departures from LTE. As a rule, the lines in the blue spectral region are less affected by non- LTE effects. In the atmospheres of six stars the helium abundance corresponds, within the error limits, to the present-day solar value. A helium underabundance is observed in the atmospheres of Sirius and HD 72660 classified as hot Am stars.

A collimated wind interpretation for the spectral variability of Z And during its major 2006 eruption

High-resolution observations in the region, centered at 4400 \AA\ and those of the lines HeII 4686, Hbeta and HeI 6678 of the spectrum of the symbiotic binary Z And were performed during its outburst in 2006. The line Hbeta had additional satellite high-velocity emission components situated on either side of its central peak. The lines of neutral helium presented two components, consisting of a nebular emission situated close to the reference wavelength and a highly variable P Cyg absorption. Close to the optical maximum the line HeII 4686 was weak emission feature, but with the fading of the light it changed into an intensive emission consisting of a central narrow component and a broad component with a low intensity. The lines of NIII and CIII were very broadened. We demonstrate that all of these groups of lines with very different profiles can be interpreted in the light of the same model, where a disc-shaped material surrounding the compact object collimates its stellar wind and gives rise to bipolar outflow.

DETECTION OF FORBIDDEN LINE COMPONENTS OF LITHIUM-LIKE CARBON IN STELLAR SPECTRA

ABSTRACT We report the first identification of forbidden line components from an element heavier than helium in the spectrum of astrophysical plasmas. So far, these components were identified only in laboratory plasmas and not in astrophysical objects. Forbidden components are well known for neutral helium lines in hot stars, particularly in helium-rich post-AGB stars and white dwarfs. We discovered that two hitherto unidentified lines in the ultraviolet spectra of hot hydrogen-deficient (pre-) white dwarfs can be identified as forbidden line components of triply ionized carbon (C iv). The forbidden components (3p–4f and 3d–4d) appear in the blue and red wings of the strong, Stark broadened 3p–4d and 3d–4f lines at 1108 Å and 1169 Å, respectively. They are visible over a wide effective temperature range (60,000–200,000 K) in helium-rich (DO) white dwarfs and PG 1159 stars that have strongly oversolar carbon abundances.

Developing an edge-plasma diagnostic tool for the Globus-M tokamak based on measuring ratios of HeI lines

A diagnostic technique that is based on measuring the ratios of neutral-helium line strengths has been developed for peripheral distributions of electron temperature and density of tokamak plasma. The main components of the technique are a four-channel filter-lens imaging polychromator (FLIP-4) and a Phantom Miro M110 high-speed camera for recording the images. The polychromator has been assembled, adjusted, and tested on an optical test bench. The optical system was installed on the spherical Globus-M tokamak. Some preliminary experiments were carried out. Images of the plasma-gun jet were obtained at neutralhelium lines.

Evaluation of thermal helium beam and line-ratio fast diagnostic on the National Spherical Torus Experiment-Upgrade

A 1-D kinetic collisional radiative model with state-of-the-art atomic data is developed and employed to simulate line emission to evaluate the Thermal Helium Beam (THB) diagnostic on NSTX-U. This diagnostic is currently in operation on RFX-mod, and it is proposed to be installed on NSTX-U. The THB system uses the intensity ratios of neutral helium lines 667.8, 706.5, and 728.1 nm to derive electron temperature (eV) and density (cm−3) profiles. The purpose of the present analysis is to evaluate the applications of this diagnostic for determining fast (∽4 μs) electron temperature and density radial profiles on the scrape-off layer and edge regions of NSTX-U that are needed in turbulence studies. The diagnostic is limited by the level of detection of the 728.1 nm line, which is the weakest of the three. This study will also aid in future design of a similar 2-D diagnostic system on the divertor.

Stark shift of neutral helium lines in low temperature dense plasma and the influence of Debye shielding

In this work, we present the results of an experimental study of the Stark shifts of atomic helium lines at 706.519, 728.135, 471.315, 501.568, 667.815 and 447.148 nm in the electron density range (0.6–7) × 1023 m−3. Stark shift is always presented together with the corresponding electron density and electron temperature. Best-fitting formulas of experimental data enable plasma electron density diagnostics with an accuracy of (15–20 per cent) in the temperature range (10 000–30 000) K. The influence of Debye shielding on the Stark shifts is carefully examined and the experimental results are compared with other experimental and theoretical data.

Phase-resolved spectroscopy andKeplerphotometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4

{\it Kepler} satellite photometry and phase-resolved spectroscopy of the ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The average spectra reveal a variety of weak metal lines of different species, including silicon, sulphur and magnesium as well as many lines of nitrogen, beside the strong absorption lines of neutral helium. The phase-folded spectra and the Doppler tomograms reveal an S-wave in emission in the core of the He I 4471 \AA\,absorption line at a period of $P_{\rm orb}=1085.7\pm2.8$\,sec identifying this as the orbital period of the system. The Si II, Mg II and the core of some He I lines show an S-wave in absorption with a phase offset of $170\pm15^\circ$ compared to the S-wave in emission. The N II, Si III and some helium lines do not show any phase variability at all. The spectroscopic orbital period is in excellent agreement with a period at $P_{\rm orb}=1085.108(9)$\,sec detected in the three year {\it Kepler} lightcurve. A Fourier analysis of the Q6 to Q17 short cadence data obtained by {\it Kepler} revealed a large number of frequencies above the noise level where the majority shows a large variability in frequency and amplitude. In an O-C analysis we measured a $\vert\dot{P}\vert\sim1.0\,$x$\,10^{-8}\,$s\,s$^{-1}$ for some of the strongest variations and set a limit for the orbital period to be $\vert\dot{P}\vert<10^{-10}$s\,s$^{-1}$. The shape of the phase folded lightcurve on the orbital period indicates the motion of the bright spot. Models of the system were constructed to see whether the phases of the radial velocity curves and the lightcurve variation can be combined to a coherent picture. However, from the measured phases neither the absorption nor the emission can be explained to originate in the bright spot.

Spectral and photometric studies of the polar USNO-A2.0 0825-18396733

Results of photometric and spectral studies of the new magnetic cataclysmic variable (polar) USNO-A2.0 0825-18396733 are presented. Photometric data in the B, V, and Rc filters show that this object exhibits a red excess of Rc − V = 1m. A red continuum with superposed strong single-peaked Balmer emission lines and HeII λ4686 A emission, weak lines of neutral helium, and lines of heavy elements are observed in the object’s spectra. Doppler maps constructed using the hydrogen and ionized-helium lines indicate that these lines form near the inner Lagrangian point, and that their formation is associated with an accretion stream. The spectra and radial-velocity curves indicate the eclipse of the white dwarf in the system to be partial. Radial-velocity curves derived for emission lines are used to estimate the component masses. The mass of the white dwarf is estimated to be 0.71–0.78M⊙, and the mass of the red dwarf to be 0.18–0.20M⊙.