CIV Line Research Articles

Discovery of highly ionized line emission in the far ultraviolet

We report the discovery of highly ionized line emission from CIVλ1550Å, OIII] λ1663Å, and OVI/SiIV λ1400Å, observed by the Berkeley UVX experiment on Shuttle mission 61-C. The CIV and OIII] lines were detected in 4 out of 8 directions, and the OVI/SiIV in 2 directions. The CIV line intensity exhibits a definite anticorrelation with HI column density, and shows significant pole brightening (a factor of 5 at least). There is evidence that the gas is not isothermal. When compared to observed CIV absorption column densities, we derive intrinsic densities of .01–.05 cm −3, and pressure p/k=1000–8000 (T/10 sup 5 K) cm −3 K, assuming the gas has a scale height of 2 kpc. Much higher densities and pressures are indicated if the gas is local (100–200 pc), which would give pressures in violation of canonical limits. Finally, we show that photoionized models, which break down at high densities because of charge exchange with HI, are inconsistent with the observed densities.

Shock-heated gas in the I Per OB association

In response to a suggestion by Phillips & Gondhalekar (1981) that hotgas is receding from the I Per OB association with a velocity of ∼30 km s−1, we have examined the IUE high-dispersion spectra of stars in the association for blueshifted lines of interstellar CIV, SiIV and AlIII. As a result, the high-velocity ions are now known to be present in front of at least five stars. Our analysis of the radial velocities of the ions and of the column density ratio N(CIV)/N(SiIV) in the different sight-lines leads to a conclusion similar to that of Phillips & Gondhalekar, namely that the motion of the highly ionized gas is due to a supernova. We believe, however, that the expansion of the HI shell in front of I Per is unrelated and that it is probably due to a recombining HII region within which the supernova event has occurred.

Interpretation of oscillations in UV lines observed above sunspot umbrae

Our theory of a resonator for slow magneto-atmospheric waves in the chromosphere of a sunspot umbra has been used to check different models of the structure of the chromosphere and transition region. Oscillations of velocity and intensity in Civ, Siiv, and Oiv lines observed by Gurman et al. (1982) on the SMM spacecraft have been compared with the calculated oscillations. The observed spectrum of resonant peaks could well be explained by a gradient model of the umbral chromosphere. Different assumptions concerning the structure of the transition region do not influence the calculated resonance periods, but the amplitudes and phases of oscillations are modified. There is strong evidence for a concentration of the observed oscillations in cold fine structure elements of the transition region, even if the filling factor of such elements is very small (some few percent). Isothermal rather than adiabatic oscillations in the cold elements should be assumed in order to explain the observed fluctuations of line intensity; the relative amplitudes of pressure oscillations in the hot main component with a steep gradient of temperature are too small to explain the observed intensity fluctuations.

Dynamic evolution of recurrent mass ejections observed in H? and Civ lines

During a coordinated SMY program, the consecutive formation of two new active centers merging together within AR 2646 was observed from 28 August, to 5 September, 1980. The two preceding spots compressed an inverse polarity spot on 1 September 1980, causing recurrent ejecta of matter with time intervals around 10 min. The observations of the MSDP spectrograph operating in Hα at the Meudon Solar tower and of the UVSP spectrometer on SMM in the Civ 1548 A line show that cold and hot material had the same projection, although the upward Civ velocity structure was more extended than the Hα one. We present evidence that observed contrasts of the Hα absorbing structure can be interpreted in terms of a dynamic ‘cloud model’ overlying the chromosphere. Hα matter follows a magnetic channel with upward velocity around 20–30 km s−1 in the first phase of the event and with downward velocity (∼ - 40 km s−1) in the second phase. The stored energy is not sufficient to trigger a flare, nor even to propulse matter along the full length of an arch, because of the periodic reorganisation of the magnetic field.

SMM/UVSP observations of the distribution of transition region oscillations and other properties in a sunspot

Observations of a sunspot in the Civ line at 1548 A formed in the transition region have been analyzed to obtain the time variations and/or mean values of the velocity, intensity, longitudinal magnetic field, and line width. Oscillations with periods between approximately 110 and 200 s are observed only over the umbra where the transition region magnetic field is highest and the line width is smallest. When periodic intensity variations occur at the same frequency as the velocity oscillations, the peak intensities occur slightly before the maximum upward motions. No periodic variations in the transition region magnetic field have been detected. Scatter diagrams are presented which show possible relationships between the flow velocity, emission line intensity, line width, and transition region magnetic field.

Coronae of the Magellanic Clouds

A hot gaseous corona was postulated by Spitzer (1956) for the Milky Way as a consequence of the detection of cool interstellar clouds (seen in Call absorption) on paths to stars in the Milky Way halo. Halos around extra galactic systems were proposed by Bahcall and Spitzer (1969) as a possible explanation for the wealth of high redshift absorption line systems in the spectra of quasars. The recording with the International Ultraviolet Explorer (Boggess et al 1978) in 1978 of echelle spectra of R136 and HD38282 in the LMC showed strong absorption lines of the high ionization stage ions CIV and SiIV, due to material with velocities clearly pertaining to locations outside the Milky Way disk (Savage and de Boer 1979). Thus the reality of the galactic corona became established (review de Boer 1984). There is confusion with the words corona (gas – massive component) and halo (location – stars) and I (1984) proposed for the large mass from dynamics MASsive DArk Component, MASDAC.

On the Strength of the CIV 155 nm Resonance Lines in Planetary Nebulae

Ionization models for NGC 6210, 7009, 3242 and II 2003 have been constructed from optical and IUE spectroscopic data. The CIV 155 nm resonance line is predicted about ten times stronger than observed. Radiative transfer calculations of the CIV lines in a spherical nebula, assuming partial frequency redistribution, were made to investigate the effects of dust absorption and an additional depopulation of the upper level.

An emission measure analysis of two sunspots observed by the UVSP instrument on the SMM spacecraft

The EUV observations from the SMM satellite of two sunspots are presented here. These observations show the sunspots (a) to be regions of lower intensity than the surrounding plage, contrary to that found by previous authors, and (b) to have line intensities which vary little over a period of several hours. An upper limit to mass flows of 2km s-1 is derived, indicating a relatively simple energy balance for the chromosphere-corona transition zone with thermal conduction being balanced by radiative losses. Electron densities derived from Niv to Civ line ratios imply electron pressures (log NeTe) of 15.0 to 15.3.

The velocity characteristics of WR stellar winds

The stellar winds of hot early-type stars are characterised spectroscopically by the presence of P-Cygni profiles, which are most marked for resonance transitions of common ions and generally occur in the UV, and typical resonance lines of SiIV, CIV, NV and OVI are well known P-Cygni signatures of stellar winds in luminous OB stars. Analyses of these profiles can lead to important information concerning the velocity, density and temperature structure of such winds. The WR stars have more developed stellar winds than OB stars, supporting higher particle densities at larger distances from the stellar “photosphere”. Recently Barlow, Smith & Willis, hereinafter BSW, (1980), have derived reliable mass loss rates for 21 WR stars. The mean value is 4x10-5 M0 /y and the rates are considerably larger than can be accomodated by soley radiative models. The, as yet unidentified, mechanism initiating the WR mass loss may also be important for Of stars ( Lamers 1980 ).

Ultraviolet Studies of Novae

High dispersion spectra of WZ Sagittae and Nova Cygni 1978, taken by IUE, have been studied by us.Of the two spectra of WZ Sagittae in our possession, the long wave region one shows little besides the interstellar Mg II doublet. The more interesting short wavelength region has relatively wide (130-250 km/s) resonance absorption lines sometimes at the center of wider emission. No clear sign of ejection is seen. The study of the profiles of the CIV, NV, OI, and SiIV resonance lines and of Hell 1640 Å indicates:a) The absorption lines are probably not interstellar (widths, depths).b) A standard accretion disk model can be used. The high ionization lines are probably formed in a region ionized by the boundary layer. The line width indicates that they are formed near the edge of the disk (top of raised edge?), illuminated by radiation from a radius a factor of 10 smaller.

IUE observations of absorption by hot gas in the nebula NGC6888

New UV observations of Wolf–Rayet stars obtained with the IUE satellite show narrow absorption components in the highly ionised species of SiIV and CIV. In the case of the WN6 star HD192163 each resonance transition in the above ions exhibits two components, one undisplaced in wavelength and one blueshifted. The latter components are shown to arise in the nebula NGC6888 surrounding HD192163, and these are the first observations of absorption lines arising in a nebula associated with an early-type star. The strengths of the ‘interstellar’ SiIV and CIV lines in four WR stars and two other early-type stars are compared. There seems to be no correlation in the strengths of these lines with either stellar distance or colour excess, suggesting that they arise not in the general interstellar medium but in material more intimately linked with the stars themselves.

Stellar-wind theory for O and B stars

The rocket-ultraviolet observations of strong Doppler-shifted absorption lines of Siiv, Civ, Nv and other ions in the spectrum of O and B supergiants clearly indicate a high velocity outflow of matter from these stars. The presence of moderate ionisation stages in the stellar wind is conclusive evidence that the flow cannot be due to a high temperature corona as is the case for the solar wind. It is shown that the driving mechanism for the hot-star mass loss is radiation pressure exerted on the gas through absorption in resonance lines occurring at wavelengths near the maximum of the star's continuum flux. In the upper layers of these stars the outward force per gram of matter due to the radiation pressure can greatly exceed the gravitational acceleration making a static atmosphere impossible.The problem of a steady-state moving reversing layer is formulated and the solution leads to predictions of mass-loss rates as a function of effective temperature and gravity for all hot stars. These results are in substantial agreement with the observations.

Observations of Strong Stellar Lines with the OAO

This paper reports on preliminary analysis of spectral scans of early-type stars obtained with the Orbiting Astronomical Observatory. The discussion is confined to the spectra of 50 stars observed with a resolution of approximately 10 Å over the spectral interval from 1050 Å to 2000 Å. Following a qualitative description of the spectra, observed equivalent widths of the Siiv (1400 Å) and Civ (1550 Å) lines are compared with model atmosphere calculations. The results suggest an upward revision of the stellar temperature scale for stars earlier than B3 V.