Helium Emission Lines Research Articles

The mass of the white dwarf in the recurrent nova U Scorpii

We present spectroscopy of the eclipsing recurrent nova U Sco. The radial velocity semi-amplitude of the primary star was found to be K_W = 93 \pm 10 kms^{-1} from the motion of the wings of the HeII\lambda4686\AA emission line. By detecting weak absorption features from the secondary star, we find its radial velocity semi-amplitude to be K_R = 170 \pm 10 kms^{-1}. From these parameters, we obtain a mass of M_1 = 1.55 \pm 0.24M_\odot for the white dwarf primary star and a mass of M_2 = 0.88 \pm 0.17M_\odot for the secondary star. The radius of the secondary is calculated to be R_2 = 2.1\pm0.2R_\odot, confirming that it is evolved. The inclination of the system is calculated to be i = 82.7^\circ\pm2.9^\circ, consistent with the deep eclipse seen in the lightcurves. The helium emission lines are double-peaked, with the blue-shifted regions of the disc being eclipsed prior to the red-shifted regions, clearly indicating the presence of an accretion disc. The high mass of the white dwarf is consistent with the thermonuclear runaway model of recurrent nova outbursts, and confirms that U Sco is the best Type Ia supernova progenitor currently known. We predict that U Sco is likely to explode within \sim 700,000 years.

The Spectral Characteristics of Stripped‐Envelope Supernovae

Supernovae (SNe) of Type Ib and Ic are distinguished by what they lack. They do not have the hydrogen lines indicative of Type II SNe, nor do they show the silicon absorption feature near 6150 A that is the hallmark of Type Ia SNe. (Type Ib SNe show helium absorption lines, while Type Ic SNe do not.) Type Ib and Ic SNe are rare and remain fairly mysterious objects. As they are thought to result from progenitors that have lost their massive envelopes, they are referred to as “stripped-envelope” SNe. We present various studies of optical spectra of SNe Ib/c and the metamorphosing SNe that link them to SNe II as core-collapse events. The spectroscopic characteristics of SNe Ib and Ic may help resolve questions about the nature of their progenitors. SN 1993J appeared originally to be a Type II SN, but it underwent a dramatic transformation to one resembling a Type Ib, although the hydrogen lines never completely faded away. This metamorphosis is seen in the 42 low-dispersion spectra shown by T. Matheson et al. (2000, AJ, 120, 1487). Detailed analysis of the SN 1993J spectra is presented by T. Matheson et al. (2000, AJ, 120, 1499). The spectra show that the ejecta are clumped, especially the synthesized oxygen. The calcium emission is smooth, implying that only preexisting calcium contributes to the emission. The late-time spectra exhibit signs of circumstellar interaction with a roughly spherical shell. Some emission lines develop a two-horned profile, indicating the formation of a flattened, disklike emission source. SN 1999cq exhibits the first unambiguous detection of helium emission lines in SNe Ic (T. Matheson et al. 2000, AJ, 119, 2303). They are of intermediate velocity width and appear to show the presence of dense circumstellar material composed of almost pure helium. This material may be stellar matter lost through a dense wind or mass transfer to a companion. We also present more than 80 spectra of various SNe Ib and Ic (T. Matheson et al. 2001, AJ, 121, 1648). SNe Ib are found to exhibit fairly homogeneous spectra and helium line strength. The relative depths of the helium absorption lines appear to provide a measurement of the temporal evolution of SNe Ib. SNe Ib and Ic are shown to have distinguishing characteristics other than the presence or absence of helium lines. SNe Ic have clearly broader nebular-phase emission lines, indicating that SNe Ic have a higher explosion energy and/or lower envelope mass than SNe Ib. SNe Ic also have stronger permitted oxygen lines at early times, implying that they are less diluted by a massive helium envelope. This lends strength to the argument that the difference between SNe Ib and SNe Ic arises from the constituents of the overlying envelope of the star at the time of core collapse, rather than the extent of mixing of radioactive nickel into the helium layers of the progenitor. SNe Ic themselves are heterogeneous, with diverse spectroscopic characteristics. No strong evidence for helium is found in SNe Ic, although some objects may have a low level of helium. These may be SNe that bridge the gap between SNe Ib and Ic.

Non-axisymmetric accretion on the classical TTS RW Aur A

(Abridged) High-resolution spectroscopic monitoring of RW Aur A was carried out in 1996, 1998 and 1999 with simultaneous B, V photometry. A multicomponent spectrum is revealed with a veiled photospheric spectrum, broad emissions, narrow emission lines of helium, and accretion, wind and shell features. Periodic modulations in many spectral features were found. The photospheric absorption lines show sinusoidal variations in radial velocity with an amplitude of +-6 km/s and a period of about 2.77 days. The radial velocities of the narrow emission lines of He vary with the same period but in anti-phase to the photospheric lines. The equivalent widths of the narrow emissions vary with a phase-shift with respect to the velocity curve. The strength of the red-shifted accretion components of Na D and other lines is also modulated with the same period. The broad emission lines of metals vary mostly with the double period of about 5.5 days. One unexpected result is that no correlation was found between the veiling and the brightness, although both varied in wide ranges. This is partly due to a contribution of the shell absorption to the photospheric line profiles, which make them vary in width and depth thus simulating lower veiling. Most of the observed features can be interpreted in the framework of non-axisymmetric magnetospheric accretion. We consider two possible models. In the first the asymmetry is induced by orbital motion of an invisible, low mass secondary, which also influences the gasflows around the star, the second considers rotational modulation of a single star with an inclined or asymmetric magnetosphere.

Emission-line oscillations in the dwarf nova V2051 Ophiuchi

We have detected coherent oscillations, at multiple frequencies, in the line and continuum emission of the eclipsing dwarf nova V2051 Ophiuchi using the 10-m Keck II telescope. Our own novel data acquisition system allowed us to obtain very fast spectroscopy using a continuous readout of the CCD on the LRIS spectrograph. This is the first time that dwarf nova oscillations have been detected and resolved in the emission lines. The accretion disc is highly asymmetric with a stronger contribution from the blueshifted side of the disc during our observations. The disc extends from close to the white dwarf out to the outer regions of the primary Roche lobe. Continuum oscillations at 56.12 s and its first harmonic at 28.06 s are most likely to originate on the surface of a spinning white dwarf with the fundamental period corresponding to the spin period. Balmer and helium emission lines oscillate with a period of 29.77 s at a mean amplitude of 1.9 per cent. The line kinematics and the eclipse constraints indicate an origin in the accretion disc at a radius of 12±2Rwd. The amplitude of the emission-line oscillation modulates (0–4 per cent) at a period of 488 s, corresponding to the Kepler period at R=12Rwd. This modulation is caused by the beating between the white dwarf spin and the orbital motion in the disc. The observed emission-line oscillations cannot be explained by a truncated disc as in the intermediate polars. The observations suggest a non-axisymmetric bulge in the disc, orbiting at 12Rwd, is required. The close correspondence between the location of the oscillations and the circularization radius of the system suggests that stream overflow effects may be of relevance.

Accretion stream mapping of HU Aquarii

We present a new mapping algorithm, the Accretion Stream Mapping (ASM), which uses the full phase-coverage of a light curve to derive spatially resolved intensity distributions along the accretion stream in magnetic cataclysmic variables of AM Herculis type (polars). The surface of the accretion stream is approximated as a 12-sided (duodecadon-shaped) tube. After successfully testing this method on artificial data we applied it to emission-line light curves of Hβ, Hγ and He iiλ4686 of the bright eclipsing polar HU Aqr. We find hydrogen and helium line emission bright in the threading region of the stream where the stream couples on to magnetic field lines. It is particularly interesting that the stream is bright on the irradiated side facing the white dwarf, which highlights the interplay of collisional and radiative excitation/ionization.

The continuing saga of Sakurai's object (V4334 Sgr): dust production and helium line emission

We also report strong He i emission at 1.083mm, in contrast to the strong absorption in this line in 1998. We conclude that the excitation is collisional, and is probably caused by a wind, consistent with the P Cygni profile observed by Eyres et al. in 1998.

Helium Emission Lines in the Type I[CLC]c[/CLC] Supernova 1999[CLC]cq[/CLC

We present the first unambiguous detection of helium emission lines in spectra of Type Ic supernovae (SNe Ic). The presence of He I lines with FWHM ≈ 2000 km s-1 and the distinct absence of any other intermediate-width emission (e.g., Hα) implies that the ejecta of SN Ic 1999cq are interacting with dense circumstellar material composed of almost pure helium. This strengthens the argument that the progenitors of SNe Ic are core-collapse events in stars that have lost both their hydrogen and helium envelopes, either through a dense wind or mass transfer to a companion. In this way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that helped firmly establish a physical connection between Type Ib and Type II supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid rise followed by a quick decline. SN 1999cq is also found to exhibit a high level of emission at blue wavelengths (5500 A), likely resulting from either an unusually large amount of iron and iron-group element emission or uncharacteristically low reddening compared with other SNe Ic.

Substantial Disc Loss in the Be/X-ray Binary System A0535+26

AbstractInfrared JHK photometry from October 1998 revealed a remarkable change in the Be/X-ray binary system A0535+26, manifesting itself as a dramatic reduction in emission from the circumstellar disc. Optical and IR spectra presented here have confirmed this disc-loss in greater detail showing radically reduced Hydrogen and Helium line emission, to a degree never before seen in this system, and only rarely in a handful of Be/X-ray binaries. Optical spectroscopy has revealed the re-appearance of a small, stable inner disc, and IR spectra/photometry show the eventual regrowth of a robust circumstellar disc resembling the pre-disc-loss state.

New Models for Massive Star Populations in Young Starbursts: Summary of the Models and Applications

Using the latest stellar evolution models, theoretical stellar spectra, and a compilation of observed emission line strengths from Wolf-Rayet (WR) stars, we have constructed evolutionary synthesis models for young starbursts (Schaerer & Vacca 1997; see also Schaerer 1996). We provide detailed predictions of UV and optical emission line strengths for both the WR stellar lines and the major nebular hydrogen and helium emission lines, as a function of several input parameters related to the starburst episode.

Ultraviolet Observations of the Hot R Coronae Borealis–Type Star V348 Sagittarii during a Deep Minimum

We obtained three high-quality ultraviolet spectra of the hot R Coronae Borealis type star V348 Sgr using the Faint Object Spectrograph of the Hubble Space Telescope (HST). The first of these spectra was obtained when the star was in a shallow minimum (ΔV less than 1 mag), whereas the other two were obtained during a deep minimum (ΔV ≈ 6 mag). While strong carbon and helium emission lines are seen in the shallow minimum, during the deep minimum these lines are apparently eclipsed by a dust cloud. In the first deep minimum spectrum a few weak nebular emission lines are present, but in the second (a month later) these lines have mostly disappeared. This is consistent with a progressive eclipse caused by an expanding dust cloud. Comparison with International Ultraviolet Explorer (IUE) spectra obtained at maximum light allowed three extinction curves to be calculated for the newly formed dust. The extinction curves obtained using the spectra at deep minimum are quite different from those obtained using spectra at shallow minima. Extinction curves from shallow minima (which have been previously calculated using IUE data) show stellar flux attenuated by amorphous carbon dust in front of the star at wavelengths above 2000 A, while at wavelengths below 2000 A they show contributions from light scattered by dust outside the line of sight. During the deep minimum, however, nearly all of the light seen in the ultraviolet below 3200 A is due to scattering into the field of view. The data are consistent with (1) an optically thick cloud of amorphous carbon dust in front of the star and (2) either a few additional optically thick clouds that are outside a direct line from the star to the earth, or an optically thin cloud that surrounds a small fraction of the star and scatters light into the HST field of view. The data taken together are consistent with the dust forming close to the star.

New Models for Wolf‐Rayet and O Star Populations in Young Starbursts

Using the latest stellar evolution models, theoretical stellar spectra, and a compilation of observed emission line strengths from Wolf-Rayet (WR) stars, we construct evolutionary synthesis models for young starbursts. We explicitly distinguish between the various subtypes (WN, WC, WO), and we treat O and Of stars separately. We provide detailed predictions of UV and optical emission line strengths for both the stellar lines and the major nebular hydrogen and helium emission lines, as a function of several input parameters related to the starburst episode. We also derive the theoretical frequency of WR-rich starbursts. We then discuss: nebular HeII 4686 emission, the contribution of stars to broad Balmer line emission, techniques used to derive the and O star content from integrated spectra, and explore the implications of the formation of stars through mass transfer in close binary systems in instantaneous bursts. The observational features predicted by our models allow a detailed quantitative determination of the massive star population in a starburst region (particularly in so-called WR galaxies) from its integrated spectrum and provide a means of deriving the burst properties (e.g., duration, age) and the parameters of the initial mass function of young starbursts. (Abridged abstract)

Is Eta Carinae a long-period binary?

Damineli et al. (1997)[NewA, 2, 107] have recently proposed a specific binary model for η Carinae. It is based on wavelength shifts in hydrogen and helium emission lines, and may explain a 5.5-year periodicity involving spectroscopic events. Here I amplify but also modify their discussion with the following points: (1) A more eccentric orbit gives a better fit to the velocities quoted by those authors. The higher eccentricity implies appreciably different behavior near periastron. (2) Intra-system eclipses may explain the 5.5-year spectroscopic events in a way that is different from the Damineli et al. scenario. (3) On the other hand, a 5.5-year binary model entails several implausibilities; a single-star model seems equally promising at this time. (4) Finally, I list a few specific, possibly decisive observations that can be made during the next critical “event”, which is expected to occur around the end of 1997. These and other observations of the event urgently merit detailed and intensive efforts.

The Ultraviolet Spectrum of the Magnetic Cataclysmic Variable RX J1802.1+1804

We observed the counterpart to the X-ray source RX J1802.1+1804, an object recently discovered as part of a survey to identify the optical counterparts to ultrasoft X-ray sources in the ROSAT point-source catalog that was subsequently identified as a magnetic cataclysmic variable, with the ultraviolet spectrometers on IUE in low dispersion mode. We report on the results of these observations and interpret our data within the context of other UV spectroscopic studies of magnetic CVs (polars). RX J1802.1+1804 is a relatively bright UV source, with a UV-to-optical flux ratio and emission-line spectrum similar to other well-studied CVs. Our observations insufficiently sample the binary orbit to discern any phase-dependent effects. We found RX J1802.1+1804 to have an unusually large He II λ2733 emission-line strength, and we have also made probable detections of weaker He II lines at 2511, 3203, and possibly 2386 A, in addition to the more commonly detected He II at 1640 A. We suggest that RX J1802.1+1804 is thus anomalous in terms of its overall He II emission-line spectrum among the sample. Several lines of O III λλ3047 and 3133 are apparently detected as well, which is unusual for CVs at the signal-to-noise ratio levels obtainable with IUE long-wavelength spectrograph. Given the seemingly rich spectrum of helium and oxygen emission lines, and the known relationship between He II Lyα and the oxygen lines, i.e., the Bowen fluorescence mechanism, we can, in principle, constrain the size and thermodynamics of the emitting region. This analysis suggests that R ~ 109 cm and ne ~ 1011 cm-3. This physical scale is consistent with the line-emission originating in the accretion column rather than in a hot spot on the white dwarf surface.

Detection of soft X-rays and a sensitive search for noble gases in comet Hale-Bopp.

An image of comet Hale-Bopp (C/1995 O1) in soft x-rays reveals a central emission offset from the nucleus, as well as an extended emission feature that does not correlate with the dust jets seen at optical wavelengths. Neon was found to be depleted in the cometary ice by more than a factor of 25 relative to solar abundance, which suggests that ices in Hale-Bopp formed at (or later experienced) temperatures higher than 25 kelvin. A helium line emission at a wavelength of 584 angstroms was detected and may be attributable to charge transfer of solar wind alpha particles in the cometary coma. Ionized oxygen and another helium line contribute to an emission observed at 538 angstroms.

Plasma conditions for the deposition of wear-resistant carbon-coatings

The properties of sputter and microwave-ECR plasmas have been investigated with a plasma emission monitor (PEM) in the range between 280 and 820 run with various resolutions. Additional Langmuir probe measurements of the sputter plasma were performed to determine the spatial variation of electron density and temperature. Using helium as an additional measuring gas, the electron temperature has been determined by comparing the intensities of an argon and a helium emission line. The concentration of atomic hydrogen and CH radicals can be determined by actinometry. A comparison between the two types of plasmas shows similar electron temperature dependencies on the plasma power, the argon partial pressure and the acetylene partial pressure. The electron temperatures determined from PEM and Langmuir probe measurements correlate very well. When the microwave ECR plasma is used at the same time as the sputter plasma the electron temperature in the sputter plasma is reduced because of the charged particle current flowing out of the ECR plasma. The electron temperature in the ECR plasma varies only slightly when the sputter plasma is switched on but the concentration of atomic hydrogen increases because of the additional conversion of the acetylene on the target surface.

Infrared spectroscopy of the long-period polar RX J0515.6 + 0105

We present K-band (1.81-2.44 mu m) spectroscopy of the 7.98-h orbital period polar RXJ0515.6 + 0105. Spectra obtained around orbital phase 0.5 reveal strong emission lines of neutral hydrogen and helium from the accretion stream. No obvious spectral features from the secondary star are detected: the K-band emission is dominated instead by cyclotron radiation from the accretion region. We find that the slope of the cyclotron spectrum is consistent with a magnetic field strength of 60 MG, in agreement with previous optical studies and with the results from our modelling of previous white-light polarimetry. We estimate that the secondary contributes less than similar to 20 per cent of the infrared flux, implying a distance greater than similar to 1500 pc. With this result, we estimate the mass transfer rate to be greater than similar to 1.0 x 10(17) g s(-1). We show that it is plausible that the system is synchronized with the derived values of the mass transfer rate and with a primary magnetic field strength of 60 MG.

Spectral analyses of late–type [WC] central stars: element abundances of N, Ne and Si

The optical spectra of [WC] central stars of the latest subtypes ([WC11], [WC12]) exhibit not only stellar emission lines of hydrogen (in some cases), helium, carbon and oxygen, but also features of nitrogen (N ii, N iii), neon (Ne i), magnesium (Mg ii), aluminium (Al iii), silicon (Si iii, Si iv) and iron (Fe iii). Some of these features are also visible in the peculiar [WC9]–type object SwSt 1 which has a lower final wind velocity and a smaller wind density than normal [WC9] stars.

Helium Enrichment and Carbon Ionization in the W3 Core

We present high-sensitivity images of the H92α, He92α, and C92α recombination line emission from H II regions in the Galactic radio source W3. Our 8.3 GHz VLA observations have 4'' resolution and sufficient sensitivity to image the carbon recombination line in four of the eight H II regions in the complex. Portions of W3A, the largest and probably most evolved of the H II regions, have a helium abundance as high as 20% by number (nearly twice the cosmic abundance) which confirms previous observations made with lower signal-to-noise ratios. W3A also shows large velocity gradients in the H92α line; we conclude that the gradient is probably caused by the evaporation of the dense molecular core associated with IRS 5. Near the edges of W3B and W3C, strong C+ emission is found where no hydrogen or helium line emission is detected. This C+ emission comes from the interface zone between the H II region and the surrouding molecular cloud. We show that the C+ velocity is more typical of the surrounding molecular cloud while the H+ and He+ velocities trace the ionized gas. A H92α velocity gradient is used as a probe of the gasdynamics in W3B; a model is proposed which explains the gradient. Despite the fact that W3C and W3D are separate H II regions, they apparently share a common C+ region; the C92α line intensity is up to 70% that of the H92α line. Three independent methods provide an electron temperature in the range of Te = 8000-9000 K for W3A; a wider range of values is derived for W3B and W3D. W3B shows signs (10 σ) of a decreasing radial electron temperature gradient.

Infrared Observations of Dust Formation and Coronal Emission in Nova Aquilae 1995

We report 1.25-18.5 μm infrared spectrophotometric measurements of Nova Aquilae 1995. Photometric measurements were obtained over a four month period following the formation of an optically thin dust shell. Hydrogen and helium emission lines were evident throughout this period, along with strong near-infrared coronal line emission that appeared approximately 120 days after outburst. Both the photometric and the spectroscopic data suggest that the ejecta were clumpy, and that they contained both a dust component and a hot gas component. The outflow velocity of the ionized ejecta was observed to be ∼1365-1600 km s<SUP>-1</SUP> (FWHM), and no appreciable deceleration of the ejecta was observed over the duration of these observations. Based on a M<SUB>v</SUB>-t<SUB>2</SUB> light decline relationship, we calculate a distance of ∼3.6-4.8 kpc.

Helium partial pressure measurement in a deuterium environment

To study the removal of helium ash in present day, medium sized tokamaks, experiments with deuterium plasmas and helium puffs are conducted. Because D2 and He have almost the same mass, a spectroscopic method had been developed at TEXTOR where the He partial pressure in the exhaust line was measured by exciting the neutral gas in a Penning gauge and observing the characteristic emission lines of helium and deuterium. This method was limited to low pressures and concentrations because broad spectral lines from the deuterium molecules superimpose the He lines. By using a spectrometer and an intensified charge coupled device camera instead of optical filters and photomultipliers, it is now possible to record the interesting part of the spectrum with a time resolution of 20 ms. By subtracting the molecular spectrum of deuterium, helium concentrations from 2% on can be detected over about three orders of magnitude in pressure down to 2×10−6 mbar. He and D2 pressures are measured simultaneously. By another method the 58 nm resonance line of helium is observed and the crosstalk of the deuterium gas could be eliminated. Pressure measurements as low as 5×10−8 mbar and 1% He concentration are possible. A detection system utilizing a vacuum ultraviolet monochromator in combination with a channeltron has been tested.