Width Of He Research Articles

Inferring the presence of very massive stars in local star-forming regions

Context. Very massive stars (VMS) have masses in excess of 100 M⊙ and are rare. However, owing to their powerful winds, very high luminosity, and efficient nucleosynthesis, they are key players of star-forming regions. In particular, their strong ionizing fluxes impact the surrounding interstellar medium. Aims. We aim at detecting VMS in local star-forming region from the imprint they leave on the integrated UV and optical light. Methods. We analyzed a sample of 27 star-forming regions and galaxies in the local Universe. We selected sources with a metallicity close to 12+log(O/H) = 8.3, which is typical of the Large Magellanic Cloud. We defined empirical criteria to distinguish sources dominated by VMS and Wolf-Rayet stars (WR), using template spectra of VMS- and WR-dominated regions. We subsequently built population synthesis models with an updated treatment of VMS. We investigated the successes and failures of these new models in accounting for the UV-optical spectroscopy of our sample sources. Results. We show that the UV range alone is not sufficient to distinguish between VMS- and WR-dominated sources because their spectra are almost identical in this range. The region of the WR bumps in the optical breaks the degeneracy. In particular, the morphology of the blue bump at 4640–4686 Å is a key diagnostic. Beyond the prototypical R136 region, which contains VMS, we identify two galaxies showing clear signatures of VMS. In two other galaxies or regions the presence of VMS can be suspected, as already discussed in the literature. The stellar population is clearly dominated by WR stars in seven other sources. The most recent BPASS population synthesis models can neither account for the strong He II 1640 emission, nor for the shape of the blue bump in VMS- and WR-dominated sources. Our models that include VMS more realistically reproduce the UV-optical spectra of VMS-dominated sources. Conclusions. We conclude that VMS are present in some local star-forming regions, but that separating them from WR-dominated populations requires optical spectroscopy with a high signal-to-noise ratio. A high equivalent width of He II 1640 is not a sufficient condition for identifying VMS. Populations synthesis models need to take VMS into account by incorporating not only evolutionary tracks, but also dedicated spectral libraries. Finally, we stress that the treatment of WR stars needs to be improved as well.

A Systematic Study of Ia-CSM Supernovae from the ZTF Bright Transient Survey

Among the supernovae (SNe) that show strong interaction with a circumstellar medium (CSM), there is a rare subclass of Type Ia supernovae, SNe Ia-CSM, which show strong narrow hydrogen emission lines much like SNe IIn but on top of a diluted Type Ia spectrum. The only previous systematic study of this class identified 16 SNe Ia-CSM, eight historic and eight from the Palomar Transient Factory (PTF). Now using the successor survey to PTF, the Zwicky Transient Facility (ZTF), we have classified 12 additional SNe Ia-CSM through the systematic Bright Transient Survey (BTS). Consistent with previous studies, we find these SNe to have slowly evolving optical light curves with peak absolute magnitudes between −19.1 and −21, spectra having weak Hβ and large Balmer decrements of ∼7. Out of the 10 SNe from our sample observed by NEOWISE, nine have 3σ detections, with some SNe showing a reduction in the red wing of Hα, indicative of newly formed dust. We do not find our SN Ia-CSM sample to have a significantly different distribution of equivalent widths of He i λ5876 than SNe IIn as observed in Silverman et al. The hosts tend to be late-type galaxies with recent star formation. We derive a rate estimate of 29−21+27 Gpc−3 yr−1 for SNe Ia-CSM, which is ∼0.02%–0.2% of the SN Ia rate. We also identify six ambiguous SNe IIn/Ia-CSM in the BTS sample and including them gives an upper limit rate of 0.07%–0.8%. This work nearly doubles the sample of well-studied Ia-CSM objects in Silverman et al., increasing the total number to 28.

Study of H-mode pedestal model for helium plasmas in DIII-D

Helium (He) plasmas will be used to test the operationally relevant systems and plasma diagnostics during the ITER non-nuclear phase. Analyses of pedestal width and height in recent DIII-D H-mode experiments in He plasmas find good agreement with between the pedestal width Δ ψ and the square root of the poloidal pedestal beta from experimental observations. The EPED/REPED pedestal model is found to be applicable to predict the pedestal structure of the He plasma from comparison of predictions with experiments. Furthermore, it indicates that the pedestal height of He and deuterium (D) plasmas are similar from the REPED predictions with the same global parameters, and the pedestal width of He is approximately 6% larger than that of D.

Time stability characterization of quadrupole mass spectrometers

• Time stability of eight QMSs was investigated over 2-years period. • Stability of sensitivity of Faraday cup detector and SEM gain for He and N 2 . • Stability of peak position and peak width of He and N 2 . • Stability of minimum detectable partial pressure for He. • Measurements were carried out according to ISO TS 20175:2018. This study focuses on the time stability of different metrological characteristics of quadrupole mass spectrometers (QMSs). Investigated parameters were the sensitivity, when using a Faraday cup as a detector, the peak position, the peak width, the gain of a secondary electron multiplier, and the minimum detectable partial pressure. Eight different commercial QMSs were investigated at seven national metrology institutes under three different nominal calibration pressures of 5 × 10 −6 Pa, 5 × 10 −5 Pa, and 5 × 10 −4 Pa for N 2 and He. The measurements were carried out using a periodicity of approximately 3 months for a total duration of 2 years. Sensitivity calibrations were conducted either through a comparison method using calibrated ionization gauges or by applying a primary vacuum calibration system. In the case of ionization gauges, they were periodically recalibrated to ensure their stability and traceability to a primary pressure standard.

Spectroscopic Observations of the Recurrent Symbiotic Nova V407 Cyg During 2001 - 2009

Abstract We analyze the results of our spectroscopic monitoring of V407 Cyg in 2001–2009. the slow decline in optical brightness connected with the evolution of a Z And-type outburst (maximum in 1998) was observed in 2001–2003. the system returned to quiescence in 2004, and its spectrum returned to nearly the same as in 1994. We estimated the hot component’s temperature from the equivalent width of He II 4686 Å to be ~63 000 K . During 2006–2007 there was no clear evidence of the hot component in the spectra. the equivalent width of Hα dropped to 5Å (in 2007). This passive state was similar to previous passive states observed during 2006–2007 and in 1991.

RAMAN SCATTERED He II λ4332 IN THE SYMBIOTIC STAR V1016 CYGNI

Raman scattering of He ii line photons with atomic hydrogen is important in studying the mass loss processes in many symbiotic stars and a number of young planetary nebulae. We calculate the scattering cross sections and branching ratios associated with the Raman scattered He ii λ4332 feature formed through inelastic scattering of He ii λ949 with a hydrogen atom. At the line center of He ii λ949, the total scattering cross section is computed to be σtot = 2.5 × 10−22 cm2, and the branching ratio into the level 2s is 0.12. We also present a high-resolution spectrum of the symbiotic star V1016 Cygni obtained with the 1.8 m telescope at Mt. Bohyun to investigate the Raman scattering origin of the broad feature blueward of He ii λ4338. Based on the atomic calculation, we perform Monte Carlo calculations for the line formation. The scattering region is assumed to be a part of a uniform spherical shell that subtends a solid angle ΔΩ = π steradian with a neutral column density . By adding a far-UV continuum around He ii λ949 normalized by the equivalent width of He ii λ949 to be 2.3 Å, we obtain a good fit for both the Raman scattered He ii λ4332 and the broad wings around Hγ. Our analysis of the Raman feature blueward of Hγ in V1016 Cyg is consistent with the previous study of the Raman features blueward of Hα and Hβ by Jung and Lee.

Measurements of electron density and Stark width of neutral helium lines in a helium arc plasma

Electron densities in an atmospheric helium arc plasma have been measured with the Stark broadening parameters of helium spectral lines. The spatially distributed radiation intensities are converted to plasma emission coefficients at every wavelength by means of Abel inversion. From the inverted profiles of He I lines of 4713 A, 5016 A, and 6678 A electron density has been calculated, which ranges from 0.5 ×1016 to 4 ×1016 cm-3 for a helium arc with current 200 A. Stark widths of He I lines of 3889 A and 7065 A are determined based on the measurements and compared with existing data.

Rapid variability of the line He I λ 587.6 nm in the spectrum of the Herbig Ae/Be star WW Vul

We investigate the behavior of the relatively strong absorption line He I λ 587.6 nm in the spectrum of the rapid irregular variable star WW Vul (Sp A3) with the Algol-like brightness fadings. As many as 76 spectra of this star were acquired with the 2.6-m ZTSh telescope of the Crimean Astrophysical Observatory with the use of the medium-dispersion spectrograph SPEM. We show that the equivalent width of the He I line exhibits not only daily variations but also rapid oscillations with a typical period of several tens of minutes. Quasi-simultaneous observations of several standard stars with known energy distributions allowed us to calculate the monochromatic magnitudes of the variable at λ = 550 nm. There was no correlation between the equivalent widths of the line He I λ 587.6 nm and the monochromatic brightness of the star. At times we could identify the short-wave emission component of the He I line. We used the Hβ profiles to confirm the spectral type A2–A3 of WW Vul.

Composite model for the magnetic field of HD 21699

A model is constructed for the magnetic field of the He-weak CP star HD 21699. This star has the field structure of a dipole displaced by 0.4 radii from the center perpendicular to its axis. The magnetic poles appear to be close to one another on the surface; they are separated by 55°, not by 180° as in the case of a central dipole. The phase dependences of the equivalent widths of He and Si lines have extrema at the phases corresponding to passage through the visible meridian of zero magnetic field between the magnetic poles. At the magnetic poles, the intensity of the helium lines is maximal and of the silicon lines, minimal. The silicon abundance is maximal in the regions where the magnetic field is predominantly tangential to the star’s surface. Because of averaging over the visible hemisphere and owing to the closeness of the magnetic poles, only one wave of variation in the intensity of the spectral lines of these chemical elements, one wave of photometric variability, and an average surface magnetic field Bs are observed.

Measurement of the [formula omitted] decay width of [formula omitted

We have precisely measured the Λ → p π − decay width of He Λ 5 and demonstrated significantly larger α– Λ overlap than expected from an α– Λ potential with a central repulsion derived from YNG Λ–nucleon interaction.

The Discovery of a 12th Wolf‐Rayet Star in the Small Magellanic Cloud

ABSTRACTWe report the discovery of a relatively faint (V = 15.5) early‐type WN star in the SMC. The line strength and width of He ii λ4686 emission are similar to those of the other SMC WN stars, and the presence of N v λλ4603, 4619 emission (coupled with the lack of N iii) suggests that this star is of spectral type WN3–4.5, and thus is similar in type to the other SMC WR stars. Also like the other SMC WN stars, an early‐type absorption spectrum is weakly present. The absolute magnitude is comparable to that of other (single) Galactic early‐type WN stars. The star is located in the Hodge 53 OB association, which is also the home of two other SMC WN stars. This star, which we designate SMC‐WR12, was actually detected at a high significance level in an earlier interference‐filter survey, but the wrong star was observed as part of a spectroscopic follow‐up, and this case of mistaken identity resulted in its Wolf‐Rayet nature not being recognized until now.

Optical spectroscopy of GX 339-4 - I. Orbital modulation

Optical spectroscopic observations of GX 339-4 were carried out between 1998 May (X-ray high state) and 1999 May (X-ray low state) over seven epochs. The equivalent width of Ha increased during the high state and then decreased during the low state. The equivalent width of He II λ4686 decreased over both states. The full-width at half-maximum of Hα, Hβ, He II λ4686 and the Bowen blend increased from 1998 to 1999, indicating that the emission-line regions moved closer to the compact object. Hβ shows a redshifted absorption feature at λ4880 at all epochs. This line remains unidentified. Analysis of individual spectra from 1998 May 28-31 shows modulation of the radial velocities, equivalent width and V/R ratios of Ha on the 14.86-h orbital period. The equivalent width of He II λ4686 also varies on the orbital period. The semi-amplitude of the Ha radial velocities is K 1 = 14 km s - 1 . Hence the mass function = 2 × 10 - 4 M O ..

On the Baldwin effect of He ii emission lines in WR (WN) stars

We investigate the relation between the equivalent width of He ii emission lines and the monochromatic continuum luminosity at the line wavelength in the spectra of Wolf-Rayet stars. Model stellar atmospheres and spectra are used to show that the equivalent width inversely correlates with the monochromatic continuum luminosity. We find the effect in Wolf-Rayet star models over a large range of stellar parameters. The effect is shown to be connected to density differences among Wolf-Rayet star winds. The conclusion is drawn that the Baldwin effect may provide a new method for the determination of Wolf-Rayet star distances.

The autoionisation widths of He I below the N=3 He+ threshold

The saddle-point technique is extended to calculate autoionisation energies and widths of helium below the n=2 and the n=3 hydrogenic thresholds. The width of the lowest 1Po state below the n=3 threshold is determined to be 114.2 meV, but the experimental value of Dhez and Ederer (1973) is (132+or-14) meV. The width of the lowest 1Po state below the n=2 threshold is 0.0366 eV, which agrees with the experimental value of 0.0384+or-0.004 eV given by Madden and Codling (1965).

The hot DOA1 degenerate HZ 21 - A search for circumstellar/photospheric metals and peculiar absorption at He II

A high-resolution IUE spectrum of the hot DO1 degenerate HZ 21 was obtained by combining US1 + European 2 low-background observing shifts. The SWP image reveals a rich spectrum of interstellar absorption lines with an average velocity in the line of sight to HZ 21 of -30 km/s. However, there is no clear evidence of any highly or lowly ionized metal features which could be attributed to circumstellar, wind, or photospheric absorption. There is, however, a broad absorption trough at He II (1640) which was not unexpected, given the clear presence of He II (4686) absorption in this star's optical spectrum. The velocity width of He II (1640) appears consistent with photospheric absorption wings which appear to flank the geocoronal Ly-alpha emission feature. The He II (1640) feature reveals what appears to be a broad (310 km/s) emission reversal. Evidence is provided that the emission reversal is probably real.

Description of interelectronic correlation with the Pluvinage method

A correlated wave function method, already used for two-electron molecular systems is extended here to the case of two-electron atomic systems. A careful investigation of some exact properties of the wave function is performed (Kato's conditions, Fock's development, fulfilment of the virial theorem), showing its good behaviour at small interelectronic distances or in strongly correlated states. The application to Helium and H− doubly-excited states is developed as a preliminary case of a more general study concerned with alkali-negative ions featuring in the second paper of this series. Energies and some autoionizing widths of He and H− are calculated. Given the simplicity of our model, close agreement is found in all cases with experimental and other theoretical works. This provides a confirmation of the large application field of this method to general correlated two-electron systems.

Surface refractive indices and profile widths of He implanted LiNbO 3 waveguides

He implanted LiNbO 3 waveguides have been investigated by dark and bright line spectroscopy. The refractive index profiles were reconstructed with an improved inverse WKB procedure. In the region of electronic damage the resulting profiles are very reliable. Likewise the surface-side flank of nuclear damage induced refractive index decrease is reproduced with high accuracy. In contrast with other reconstruction schemes we determine refractive indices close to the surface with an accuracy better than 2 × 10 −4. However, the full range of nuclear damage cannot be explored. We discuss how the profile parameters depend on ion energy and irradiation dose. At 3.17 MeV the helium enriched layer seems to saturate for doses above 5 × 10 15 cm −2. Electronic damage increases the ordinary and decreases the extraordinary refractive index, more for higher doses and less for higher energies.

Analysis of 30 Wolf-Rayet Stars

Temperatures, mass-loss rates and luminosities of 30 galactic Wolf-Rayet stars (24 WN, 6 WC) are derived by fitting the observed equivalent widths of He I λ5876 and He II λ5412 and the absolute visual magnitude. A three-dimensional grid (T*-R*-) of model calculations provides the theoretical values.The results are summarized in Figure 1. The different symbols denote the spectral classification of the individual stars, whereas four groups are distinguished: WNEA, WNE-B, WNL and WC stars. The size of the symbols indicates the mass-loss rates. The uncertainties of the results are estimated to be 0.1 dex in T*, 0.4 dex in and 0.5 dex in L. For the ll stars in common with the sample of Abbott et al. (1986) we find our mass-loss rates to be compatible with their radio flux if the correct ionization equilibrium in the radio emitting region is applied (Schmutz and Hamann, 1986). The model calculations show that for all but the WN2 and WN3 stars helium recombines to He+ before the ions enter the radio-emitting region.

Non-LTE Analysis of Massive Stars in the Magellanic Clouds

The massive stars of the Magellanic Clouds are of considerable current interest with regard to questions of initial mass function, star formation mechanisms, stellar evolution with mass loss and the chemical evolution of galaxies. The effective temperatures, surface gravities and helium abundances of 6 main sequence O-type stars, obtained by fitting non-LTE model atmospheres to high quality spectra, are presented here; these are the first results from a long-term program to determine accurately the parameters and chemical abundances of massive stars in the Magellanic Clouds. The program stars were selected to be main sequence objects, according to the classification of Conti et al. (1985, in prep.), with He II λ 4686 Å in absorption, and to have minimal reddening and nebular emission. Spectra were obtained in 1984 December with the Cassegrain echelle spectrograph (CASPEC) and a CCD detector at the ESO 3.6 m telescope. A preliminary analysis of these spectra has been carried out by fitting the equivalent widths of He I λ 4471 Å and the profiles of Hγ and the Pickering lines (for details of the technique, see Kudritzki, 1980). The resulting values of effective temperature and gravity are given in Table I, along with the identification, spectral type and mv, of each star.

Stark broadening measurements of some N(II) and N(III) lines

The Stark widths of seventeen N(II) and ten N(III) spectral lines, emitted from a wall-stabilized arc plasma with electron densities in the range 0.9-2.9 × 10 17 cm -3, were measured. The arc was run with a 1:1N 2:He mixture, and the electron densities were determined from the Stark widths of He(I) 5876 and 6678 A. The Stark widths of the N lines were found to scale approximately linearly with electron density.