Photon scattering in the solar ultraviolet lines of He i and He ii

Observations made with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory ( SOHO ) are used to investigate the behaviour of the intensities of the emission lines of He  i , He  ii and O  iii at the quiet Sun-centre and at  θ= 60°  towards the equatorial limb. The aim is to examine the possible effects of photon scattering on the spatial variation of the optically thick helium lines. At the quiet Sun-centre, we find that, in agreement with previous work, the ratios of the intensities of the He  i 584-Å and He  ii 304-Å lines to those of the O  iii 600-Å line decrease systematically as the intensity of the O  iii line increases. However, we find that the dependence of these ratios on the O  iii intensity is not unique, but differs between the individual regions studied. Similar results are found at  θ= 60°  . We have also used line intensities and intensity ratios to investigate limb-to-disc effects and variations across a sample of supergranulation cell boundaries and adjacent cell interiors at both locations. The results do not exclude photon scattering as the cause of the larger observed ratios in cell interiors. The differences between the apparent widths of boundaries in O  iii at Sun-centre and 60° show that the emitting material is extended in height, which will aid the process of scattering into cell interiors. Photon scattering could also account for the lack of oscillations in the He  i intensities in a cell interior studied by Pietarila & Judge. Three-dimensional radiative transfer calculations in chosen geometries are now needed to account for the observations in detail.     

The evolution of the Lyα forest effective optical depth following He ii reionization

Three independent observational studies have now detected a narrow  (Δ z ≃ 0.5)  dip centred at   z = 3.2  in the otherwise smooth redshift evolution of the Lyα forest effective optical depth. This feature has previously been interpreted as an indirect signature of rapid photoheating in the intergalactic medium (IGM) during the epoch of He  ii reionization. We examine this interpretation using a semi-analytic model of inhomogeneous He  ii reionization and high-resolution hydrodynamical simulations of the Lyα forest. We instead find that a rapid  (Δ z ≃ 0.2)  boost to the IGM temperature  (Δ T ≃ 10⁴ K)  beginning at   z = 3.4  produces a well understood and generic evolution in the Lyα effective optical depth, where a sudden reduction in the opacity is followed by a gradual, monotonic recovery driven largely by adiabatic cooling in the low-density IGM. This behaviour is inconsistent with the narrow feature in the observational data. If photoheating during He  ii reionization is instead extended over several redshift units, as recent theoretical studies suggest, then the Lyα opacity will evolve smoothly with redshift. We conclude that the sharp dip observed in the Lyα forest effective optical depth is instead most likely due to a narrow peak in the hydrogen photoionization rate around   z = 3.2  , and suggest that it may arise from the modulation of either reprocessed radiation during He  ii reionization, or the opacity of Lyman limit systems.     

The WC10 central stars CPD−56° 8032 and He 2–113 –III. Wind electron temperatures and abundances

We present a direct spectroscopic measurement of the wind electron temperatures and a determination of the stellar wind abundances of the WC10 central stars of planetary nebulae CPD−56° 8032 and He 2–113, for which high-resolution (0.15-Å) UCLES echelle spectra have been obtained using the 3.9-m Anglo-Australian Telescope.
The intensities of dielectronic recombination lines, originating from autoionizing resonance states situated in the C²⁺+e⁻ continuum, are sensitive to the electron temperature through the populations of these states, which are close to their LTE values. The high-resolution spectra allow the intensities of fine-structure components of the dielectronic multiplets to be measured. New atomic data for the autoionization and radiative transition probabilities of the resonance states are presented, and used to derive wind electron temperatures in the two stars of 21 300 K for CPD−56°8032 and 16 400 K for He 2–113. One of the dielectronic lines is shown to have an autoionization width in agreement with the theoretical predictions. Wind abundances of carbon with respect to helium are determined from bound–bound recombination lines, and are found to be C/He=0.44 for CPD−56° 8032 and C/He=0.29 for He 2–113 (by number). The oxygen abundances are determined to be O/He=0.24 for CPD−56° 8032 and 0.26 for He 2–113.
The effect of optical depth on the temperature and abundance determinations is investigated by means of a Sobolev escape-probability model. We conclude that the optically thicker recombination lines can still be used for abundance determinations, provided that their upper levels are far from LTE.     

Kinematical studies of the low-mass X-ray binary GR Mus (XB 1254–690)

We present simultaneous high-resolution optical spectroscopy and X-ray data of the X-ray binary system GR Mus (XB 1254–690), obtained over a full range of orbital phases. The X-ray observations are used to re-establish the orbital ephemeris for this source. The optical data include the first spectroscopic detection of the donor star in this system through the use of the Doppler Tomography technique on the Bowen fluorescence blend (∼4630–4650 Å). In combination with an estimate for the orbital parameters of the compact object using the wings of the He  ii λ4686 emission line, dynamical mass constraints of  1.20 ≤ M _X /M_⊙≤ 2.64  for the neutron star and  0.45 ≤ M ₂/M_⊙≤ 0.85  for the companion are derived.     

Infrared helium–hydrogen line ratios as a measure of stellar effective temperature

We have observed a large sample of compact planetary nebulae in the near-infrared to determine how the 2¹P–2¹S He  i line at 2.058 μm varies as a function of stellar effective temperature, T _eff. The ratio of this line with H  i Br γ at 2.166 μm has often been used as a measure of the highest T _eff present in a stellar cluster, and hence of whether there is a cut-off in the stellar initial mass function at high masses. However, recent photoionization modelling has revealed that the behaviour of this line is more complex than previously anticipated. Our work shows that in most aspects the photoionization models are correct. In particular, we confirm the weakening of the 2¹P–2¹S line as T _eff increases beyond 40 000 K. However, in many cases the model underpredicts the observed ratio when we consider the detailed physical conditions in the individual planetary nebulae. Furthermore, there is evidence that there is still significant 2¹P–2¹S He  i line emission even in the planetary nebulae with very hot     central stars. It is clear from our work that this ratio cannot be considered as a reliable measure of effective temperature on its own.     

Spectroscopy of SN 1987A at 0.9–2.4 μm: days 1348–3158

We present near-infrared spectroscopic observations of SN 1987A covering the period 1358 to 3158 d post explosion. This is the first time that IR spectra of a supernova have been obtained to such late epochs. The spectra comprise emission from both the ejecta and the bright, ring-shaped circumstellar medium (CSM). The most prominent CSM emission lines are recombination lines of H  i and He  i , and forbidden lines of [S  iii ] and [Fe  ii ]. The ejecta spectra include allowed lines of H  i , He  i and Na  i and forbidden lines of [Si  i ], [Fe  i ], [Fe  ii ] and possibly [S  i ]. The intensity ratios and widths of the H  i ejecta lines are consistent with a low-temperature Case B recombination spectrum arising from non-thermal ionization/excitation in an extended, adiabatically-cooled H envelope, as predicted by several authors. The slow decline of the ejecta forbidden lines, especially those of [Si  i ], indicates that pure non-thermal excitation was taking place, driven increasingly by the decay of ⁴⁴Ti. The ejecta iron exhibits particularly high velocities  (4000–4500 km s^-1)  , supporting scenarios where fast radioactive nickel is created and ejected just after the core bounce. In addition, the ejecta lines continue to exhibit blueshifts with values ∼−200 to −800 km s⁻¹ to at least day 2000. These blueshifts, which first appeared around day 600, probably indicate that very dense concentrations of dust persist in the ejecta, although an alternative explanation of asymmetry in the excitation conditions is not ruled out.     

A high-resolution stellar library for evolutionary population synthesis

We present a library of 1654 high-resolution stellar spectra, with a sampling of 0.3 Å and covering the wavelength range from 3000 to 7000 Å. The library was computed with the latest improvements in stellar atmospheres, incorporating non-local thermodynamic equilibrium (non-LTE) line-blanketed models for hot, massive  ( T _eff≥ 27 500 K)  and line-blanketed models for cool  (3000 ≤ T _eff≤ 4500 K)  stars. The total coverage of the grid is  3000 K ≤ T _eff≤ 55 000 K  and  −0.5 ≤ log g ≤ 5.5  , for four chemical abundance values: twice solar, solar, half solar and 1/10 solar. Evolutionary synthesis models using this library are presented in a companion paper. We tested the general behaviour of the library by calculating and comparing equivalent widths of numerous H and He  i lines, and some of the commonly used metallic indices. We also compared the library with the empirical libraries STELIB and Indo-US. The full set of the synthetic stellar spectra is available from our websites ( http://www.iaa.csic.es/~rosa and http://www.astro.iag.usp.br/~lucimara/library.htm ).     

Infrared spectroscopy of cataclysmic variables – III. Dwarf novae below the period gap and nova-like variables

We present K -band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK Lyn, T Leo, SW UMa and WZ Sge, the nova-like variables DW UMa, V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral types ranging from K2 to M6.
The spectra of the dwarf novae are dominated by emission lines of H  i and He  i . The large velocity and equivalent widths of these lines, in conjunction with the fact that the lines are double-peaked in the highest inclination systems, indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which we obtained time-resolved data covering a complete orbital cycle, the emission lines show modulations in their equivalent widths that are most probably associated with the bright spot (the region where the gas stream collides with the accretion disc). There are no clear detections of the secondary star in any of the dwarf novae below the period gap, yielding upper limits of 10–30 per cent for the contribution of the secondary star to the observed K -band flux. In conjunction with the K -band magnitudes of the dwarf novae, we use the derived secondary star contributions to calculate lower limits to the distances to these systems.
The spectra of the nova-like variables are dominated by broad, single-peaked emission lines of H  i and He  i – even the eclipsing systems we observed do not show the double-peaked profiles predicted by standard accretion disc theory. With the exception of RW Tri, which exhibits Na  i , Ca  i and ¹²CO absorption features consistent with a M0V secondary contributing 65 per cent of the observed K -band flux, we find no evidence for the secondary star in any of the nova-like variables. The implications of this result are discussed.     

Spatial fluctuations in the spectral shape of the ultraviolet background at 2 < z < 3 and the reionization of helium

The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H  i and He  ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H  i and He  ii ionization rates. We further show that the large spatial variations and the anticorrelation with H  i absorber density observed in the ratio of the measured He  ii to H  i column densities can be explained in a model where the H  i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He  ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He  ii ionization rate. A significant contribution to UV emission at the He  ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He  ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He  ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He  ii by QSOs.     

The massive Wolf–Rayet binary SMC WR7

We present a study of optical spectra of the Wolf–Rayet star AzV 336a (=SMC WR7) in the Small Magellanic Cloud. Our study is based on data obtained at several Observatories between 1988 and 2001. We find SMC WR7 to be a double-lined WN+O6 spectroscopic binary with an orbital period of 19.56 d. The radial velocities of the He absorption lines of the O6 component and the strong He  ii emission at λ 4686 Å of the WN component describe anti-phased orbital motions. However, they show a small phase shift of ∼1 d. We discuss possible explanations for this phase shift. The amplitude of the radial velocity variations of He  ii emission is twice that of the absorption lines. The binary components have fairly high minimum masses, ∼18 and 34 M_⊙ for the WN and O6 components, respectively.     

H i and optical spectroscopy towards the M15 intermediate-velocity cloud

We present single-dish Arecibo 21-cm H  i observations, covering a 0675×0625 RA–Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V _LSR=70 km s⁻¹ are investigated; it is found to be clumpy and has a peak surface density N _{H  i} ∼8×10¹⁹ cm⁻². Additionally, we have performed a long H  i integration towards HD 203664, a Galactic halo star some 31 from M15, in which optical IVC absorption has previously been detected. No H  i with a velocity exceeding 60 km s⁻¹ was found to a brightness temperature limit of 0.05 K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo H  i pointings and low-resolution spectra in the Ca  ii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity H  i is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤ d ≤1350 pc. No Ca  ii absorption is observed, either because the IVC is further away than ∼1350 pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations.     

Surface abundances of light elements for a large sample of early B-type stars – I. Spectral observations of 123 stars; measurements of hydrogen and helium lines; infrared photometry

High-resolution spectral observations of 123 B0–B5 stars in the main sequence evolutionary phase were obtained at two observatories, namely the McDonald Observatory (McDO) and the Crimean Astrophysical Observatory (CrAO). Accurate equivalent widths W of two Balmer lines, H β and H γ , and ten He  i lines were obtained for all the stars, as well as of the He  ii   λ 4686 line for the hottest ones. A careful analysis of the measured equivalent widths was performed. It is shown that there is a very good agreement between the W values derived from the McDO and CrAO spectra for 14 common stars. A comparison with published data leads to the conclusion that the W values measured earlier by some authors for strong He  i lines are very likely to be underestimated. Infrared photometric observations in the J , H , and K bands were performed for 70 programme stars. All these data will be used in other papers: in particular for the T _eff and log  g determination and for the He, C, N and O abundance analyses.     

Optical studies of the X-ray transient XTE J2123–058 – II. Phase-resolved spectroscopy

We present time-resolved spectroscopy of the soft X-ray transient XTE J2123–058 in outburst. A useful spectral coverage of 3700–6700 Å was achieved spanning two orbits of the binary, with single-epoch coverage extending to ∼9000 Å. The optical spectrum approximates a steep blue power law, consistent with emission on the Rayleigh–Jeans tail of a hot blackbody spectrum. The strongest spectral lines are He  ii 4686 Å and C  iii /N  iii 4640 Å (Bowen blend) in emission. Their relative strengths suggest that XTE J2123–058 was formed in the Galactic plane, not in the halo. Other weak emission lines of He  ii and C  iv are present, and Balmer lines show a complex structure, blended with He  ii . He  ii 4686-Å profiles show a complex multiple S-wave structure, with the strongest component appearing at low velocities in the lower-left quadrant of a Doppler tomogram. H α shows transient absorption between phases 0.35 and 0.55. Both of these effects appear to be analogous to similar behaviour in SW Sex type cataclysmic variables. We therefore consider whether the spectral line behaviour of XTE J2123–058 can be explained by the same models invoked for those systems.     

The total and relative contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres in the UV and VUV regions

The main aim of this work is to estimate the total contribution of the processes of     molecular ion photodissociation and     collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of     and other relevant radiative absorption processes included in standard models.
The method for the calculations of the molecular ion     photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in     collisions is based on the quasi-static approximation.
Absorption coefficients are calculated in the region  50 nm ≤λ≤ 850 nm  and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.
We examined the spectral ranges in which the total     and     absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of  λ≲ 70 nm  the process of     atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is  1:10⁵  .     

Prediction for the He iλ10830 Å absorption wing in the coming event of η Carinae

We propose an explanation for the puzzling appearance of a wide blue absorption wing in the He  i  λ10830 Å  P Cygni profile of the massive binary star η Car several months before periastron passage. Our basic assumption is that the colliding winds region is responsible for the blue wing absorption. By fitting observations, we find that the maximum outflow velocity of this absorbing material is  ∼2300 km s⁻¹  . We also assume that the secondary star is towards the observer at periastron passage. With a toy model, we achieve two significant results. (1) We show that the semimajor axis orientation we use can account for the appearance and evolution of the wide blue wing under our basic assumption. (2) We predict that the Doppler shift (the edge of the absorption profile) will reach a maximum 0–3 weeks before periastron passage, and not necessarily exactly at periastron passage or after periastron passage.     

Interstellar Na i D lines towards the Southern Coalsack

The interstellar Na  i D absorption-line profiles observed for 15 stars with lines of sight towards the Southern Coalsack are analysed. The method of profile fitting was used in an attempt to determine column densities, linewidths and velocities for the individual interstellar clouds contributing to the observed absorption lines. In common, the observed spectra show a prominent component which is probably associated with the nearest absorbing material composing the Coalsack. The obtained spatial velocity distribution shows great similarity with earlier results from CO emission. In addition, the Na  i D data reveal evidence for the existence of two or three other structures with radial velocities of about −22, −33 and −40 km s⁻¹. Such components may be the counterparts of interstellar structures observed in diffuse H α and CO emission. The assumption that at least one of these components originated in the Carina arm imposes ∼0.9–1.0 kpc as the maximum distance to the near side of that arm.     

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He  i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He  ii absorptions and metal lines (including many selective emission lines but excluding He  ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with   P _orb= 1542 d, e = 0.45  . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.     

Changes in the red giant and dusty environment of the recurrent nova RS Ophiuchi following the 2006 eruption

We present near-infrared spectroscopy of the recurrent nova RS Ophiuchi (RS Oph) obtained on several occasions after its latest outburst in 2006 February. The  1–5 μ  m spectra are dominated by the red giant, but the H  i , He  i and coronal lines present during the eruption are present in all our observations. From the fits of the computed infrared spectral energy distributions to the observed fluxes, we find   T _eff= 4200 ± 200   K for the red giant. The first overtone CO bands at  2.3 μ  m, formed in the atmosphere of the red giant, are variable. The spectra clearly exhibit an infrared excess due to dust emission longward of  5 μ  m; we estimate an effective temperature for the emitting dust shell of 500 K, and find that the dust emission is also variable, being beyond the limit of detection in 2007. Most likely, the secondary star in RS Oph is intrinsically variable.     

The ongoing 2008–09 outburst of CI Cyg

In this paper, we discuss the early phases of the ongoing outburst that CI Cyg, a prototype symbiotic star, is currently undergoing after 30-year quiescence. We have tightly monitored CI Cyg in   B V R _C I _C  bands, starting a whole year before the onset of the outburst, and in addition we obtained numerous Echelle high- and low-resolution absolutely flux-calibrated spectra. The outburst started while the accreting white dwarf (WD) was being eclipsed by the Roche lobe filling M giant companion, and it was discovered during the egress phase on the second half of 2008 August. The system reached peak V -band brightness in early 2008 October and has been characterized by amplitudes  Δ B = 1.9, Δ V = 1.5, Δ R _C= 0.9, Δ I _C= 0.4  mag. At maximum V -band brightness, the outbursting WD had expanded to closely resemble an F3 II/Ib star, with   M_V =−3.5, T _eff∼ 6900 K  and   R = 28 R_⊙  . The high-ionization emission lines ([Ne  v ], [Fe  vii ], He  ii ) disappeared and only lower ionization lines were visible. Balmer and He  i emission lines declined in equivalent width but increased in absolute flux. The output radiated by the hot component during the outburst corresponds to nuclear burning proceeding at a  2 × 10⁻⁸ M_⊙ yr⁻¹  rate.     

Optical spectroscopy of the quiescent counterpart to EXO 0748−676

We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748−676 after the source returned to quiescence in the autumn of 2008. The X-ray light curve displays eclipses consistent in orbital period, orbital phase and duration with the predictions and measurements before the return to quiescence. Hα and He  i emission lines are present in the optical spectra and show the signature of the orbit of the binary companion, placing a lower limit on the radial velocity semi-amplitude of   K ₂ > 405 km s⁻¹  . Both the flux in the continuum and the emission lines show orbital modulations, indicating that we observe the hemisphere of the binary companion that is being irradiated by the neutron star. Effects due to this irradiation preclude a direct measurement of the radial velocity semi-amplitude of the binary companion; in fact, no stellar absorption lines are seen in the spectrum. Nevertheless, our observations place a stringent lower limit on the neutron star mass of   M ₁ > 1.27 M_⊙  . For the canonical neutron star mass of   M ₁= 1.4 M_⊙  , the mass ratio is constrained to  0.075 < q < 0.105  .