Helium Line Formation Research Articles

Conditions for Formation of the Helium HeI 10830 Å Line in The Disk and Magnetospheric Winds of Young Herbig Ae Stars

The conditions for formation of helium lines in the disk and magnetospheric winds of Herbig Ae stars are studied. The program package CLOUDY is used to solve this problem. It is shown that the optical thickness of the gas in the HeI 10830 A line needed to explain the observed P Cyg profiles is reached in the disk or magnetospheric winds of Herbig Ae stars for Te ≥ 10000 K. Here the x-ray emission produced at the surface and in the vicinity of a young star is the main ionization source for helium atoms.

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.

Optical and X-ray study of V404 Cyg during its activity in the Summer 2015

Abstract. The black hole X-ray binary V404Cyg was studied during of the 2015 outburst. Optical photometry and spectroscopy were performed by using 1.5-meter Russian-Turkish telescope (RTT-150) facilities at the TUBITAK National Observatory (Antalya, Turkey). From June 22 to June 28, 2015, shell expansion velocity decreased from 650 to 400 km s–1 as measured by Hα and Hβ lines and from 450 to 330 km s–1 as measured by HeI and HeII lines. Thus, the shell expansion occurred with deceleration, where the hydrogen and helium line formation regions are at different radial distances from the center of the star. The correlation of flow variability in the optical and X-ray ranges is caused by fluctuations in the rate of accretion near a compact source where X-ray photons are generated.

Helium Line Formation and Abundance during a C‐Class Flare

During a coordinated campaign which took place in May 2001, a C-class flare was observed both with SOHO instruments and with the Dunn Solar Telescope of the National Solar Observatory at Sacramento Peak. In two previous papers we have described the observations and discussed some dynamical aspects of the earlier phases of the flare, as well as the helium line formation in the active region prior to the event. Here we extend the analysis of the helium line formation to the later phases of the flare in two different locations of the flaring area. We have devised a new technique, exploiting all available information from various SOHO instruments, to determine the spectral distribution of the photoionizing EUV radiation produced by the corona overlying the two target regions. In order to find semiempirical models matching all of our observables, we analyzed the effect on the calculated helium spectrum both of A(He) (the He abundance) and of the uncertainties in the incident EUV radiation (level and spectral distribution). We found that the abundance has in most cases (but not in all) a larger effect than the coronal back-radiation. The result of our analysis is that, considering the error of the measured lines, and adopting our best estimate for the coronal EUV illumination, the value A(He)=0.075 +/- 0.010 in the chromosphere (for T>6300 K) and transition region yields reasonably good matches for all the observed lines. This value is marginally consistent with the most commonly accepted photospheric value: A(He)=0.085.

Hydrogen and helium line formation in OB dwarfs and giants

Aims. Hydrogen and helium line spectra are crucial diagnostic features for the quantitative analysis of OB stars. Hybrid non-LTE line-formation calculations for these elements have not been discussed thoroughly so far, despite their wide use for analyses of metal line spectra. We compute synthetic spectra based on a hybrid non-LTE approach in order to test the ability of these models to reproduce high-resolution and high-S/N spectra of dwarf and giant stars and also to compare them with published grids of non-LTE (OSTAR2002) and LTE (Padova) models. Methods. Our approach solves the restricted non-LTE problem based on classical line-blanketed LTE model atmospheres. State-of-the-art model atoms and line-broadening theories are employed to model the H and He iii spectra over the entire optical range and in the near-IR. Results. A comparison with published line-blanketed non-LTE models validates the suitability of the LTE approximation for modelling the atmospheric structure of late O to early B-type dwarf and giant stars at metallicities down to (at least) 1/5solar. Our hybrid non-LTE synthetic spectra simultaneously match almost all measurable hydrogen and helium lines observed in six test stars over a wide spectral range from the Balmer limit to the near-IR, except for only a few well-understood cases. A robust starting point for studies of the metal spectra is thus established. Our approach reproduces other published non-LTE calculations, but avoids inconsistencies in the modelling of the He i singlets. These have recently been discussed in the literature in the context of O-type stars and we show that they persist in the early B-types. Our approach improves on published pure LTE models – widely applied for OB star analyses – in many aspects: non-LTE strengthening and the use of improved line-broadening data result in significant differences in the line profiles and equivalent widths of the Balmer and helium lines. Where possible, systematic effects on the stellar parameter determination are quantified, e.g. gravities derived from the Hγ wings may be overestimated by up to ~0.2 dex for dwarfs at our upper temperature boundary of 35 000 K in LTE.

Helium Line Formation and Abundance in a Solar Active Region

An observing campaign (SOHO JOP 139), coordinated between ground based and SOHO instruments, has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines. The chromospheric lines CaII K, Halpha and Na D as well as HeI 10830, 5876, 584 and HeII 304 AA lines have been observed.These simultaneous observations allow us to build semi-empirical models of the chromosphere and low transition region of an active region, taking into account the estimated total number of photoionizing photons impinging on the target active region and their spectral distribution. We obtained a model that matches very well all the observed line profiles, using a standard value for the He abundance ([He]=0.1) and a modified distribution of microturbulence. For this model we study the influence of the coronal radiation on the computed helium lines. We find that, even in an active region, the incident coronal radiation has a limited effect on the UV He lines, while it results of fundamental importance for the 5876 and 10830 lines. Finally we build two more models assuming values of He abundance [He]= 0.07 and 1.5, only in the region where temperatures are larger than 1.* 10^4 K. This region, between the chromosphere and transition region, has been indicated as a good candidate for processes that might be responsible for strong variations of [He]. The set of our observables can still be well reproduced in both cases changing the atmospheric structure mainly in the low transition region. This implies that,to choose between different values of [He], it is necessary to constrain the transition region with different observables, independent on the He lines.

The formation of helium lines in the spectrum of COM J1740-5340

The Hei λ5876 Å absorption line recently discovered in the spectrum of the companion to the millisecond pulsar PSR J1740–5340 is tentatively attributed to electron impact excitations due to the irradiation of its atmosphere by γ-rays emitted by the pulsar's magnetosphere. Numerical calculations, similar to those carried out previously for type Ib SNe, indicate that a pulsar beam with photon energies ~1 MeV gives rise to a λ5876 Å line of the observed strength if the beam's spin-down conversion efficiency approaches 1%. However, a significant difficulty for the proposed mechanism is the strength of the singlet line at λ6678 Å. Compared to the corresponding triplets, singlet lines are weak because of the loss of excitation when photons emitted in the series ionize hydrogen atoms, an effect absent in the hydrogen-free atmospheres of type Ib SNe.

The helium line formation in late-type stars - I. The quiet sun

The formation of helium lines and continua in an atmospheric model representing the medium quiet sun has been investigated considering principally the influence of: (i) changes in the temperature gradient at Transition Region (TR); (ii) the coronal radiation and (iii) the overlapping of He II Ly α at 304 Å and the continuum radiation of He I at |$\lambda\leqslant504$| Å. By diminishing the thermal gradient in the Transition Region a large part of the helium observations are reproduced. This is the case for the He II resonance λ 304 line which is collisionally controlled and is formed at |$1.0 \times {10}^{5}\,\text{K},$| and the He I resonance line at 584 Å which is also collisionally controlled but is formed at deeper layers with a mean temperature of |$2.5 \times {10}^{4}\,\text{K}.$| The He II continuum at 228 Å as well as the Lβ 1ine at 256 Å can be adjusted to observations if a characteristic solar coronal flux is incident on the optimized Transition Region. No reasonable coronal radiation can account for the observed He II line or continuum intensities alone.

Formation of Helium Lines and Continua in a Late-Type Giant Star

Due to the atomic structure and properties of neutral helium, its spectrum can be an excellent diagnostic of the hot components of the atmosphere of a late-type giant star. Having this in mind, we have studied the formation of the helium lines and continua in the K giant star β Cet. For that purpose different NLTE atmospheric models were constructed. We have also solved the NLTE problem for the formation of lines and continua for a model helium atom consisting of twelve bound levels and three ionized stages.