UV observations of sdB stars and prospects of WSO–UV mission for such studies

The hot subdwarf B (sdB) stars are considered to be core He-burning stars with surface temperatures T _eff up to 40?000 K and log?g≥5. They are UV bright and ultraviolet observatories have a significant impact on studies of these objects. Using the technical characteristics of the instruments of the World Space Observatory–Ultraviolet (WSO–UV) mission and data from previous UV studies of sdBs we estimate the limiting apparent magnitudes for these stars that can be observed with the signal-to-noise ratio required in fine spectroscopic analysis. WSO–UV is an international space observatory for observation in UV spectral range 100–350 nm, that is beyond the reach of ground-based instruments but where most of astrophysical processes can be efficiently studied with unprecedented capability. The WSO–UV project is currently funded by national space agencies of Russia and Spain with participation of Germany, Ukraine and China. The WSO–UV consists of a 1.7 m aperture telescope (under responsibility of Russia) with instrumentation designed to carry out high resolution spectroscopy, long-slit low resolution spectroscopy and direct sky imaging. The WSO–UV Ground Segment is under development by Spain and Russia. They will coordinate the Mission and Science Operations and provide the satellite tracking stations for the project. The WSO–UV will work as a targeted scientific observatory. The scientific program of the observatory is open to excellent scientific projects from the world-wide community and occupies up to 40% of total observational time.     

Optical spectroscopy of inhomogeneities in the winds of wolf-rayet stars

Recent high quality spectra in the wavelength range 5300–6000Å of 9 WR stars of different subtypes are presented to study the intrinsic variability of the WR winds. These data clearly show variable subpeaks superimposed on the emission lines of all stars. The global level of variability of the lines decreases while more subpeaks are seen for stars with faster winds. The spectroscopic variations are associated to the presence of perturbations in the stellar winds. An inhomogeneity model is developed to study the dynamic and the physical characteristics of the wind perturbations. In the case of HD193793, for which more structures have been detected, a comparison with the general velocity law givesβ > 3. The internal velocity dispersion is larger in the direction of propagation than perpendicular to it. A histogram of the flux of the inhomogeneities indicates a larger number of wind structures with small flux. A power law fitted to the flux histogram of each stars gives an average exponent which is compatible with the one seen for giant molecular clouds where supersonic turbulence prevails. The physical representation of the WR winds and the origin of the perturbations is questioned.     

WSO/UV spectrographs: the expected performance of HIRDES

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7 m diameter feeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102–310 nm spectral band is split to feed two echelle spectrographs covering the UV range 174–310 nm and the vacuum-UV range 102–176 nm with high spectral resolution (R>55000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering the performance of the instruments.     

Spectral observations using a BTA telescope of chemically peculiar stars in the spectral region near the Li I (6708 Å) resonance doublet

New spectral observations of chemically peculiar (CP) magnetic stars were obtained using an NES echelle spectrometer with a BTA telescope in the Special Astrophysical Observatory (Russian Academy of Sciences). Several stars were shown to have anomalous Li abundances. Testing and monitoring the stars with Doppler shifts Vsini > 10 km/s indicated that the lithium 6708 Å line was variable in the spectra of some roaAP-CP stars. To distinguish variable features in the spectra, the dispersogram technique was used. The most peculiar among the stars studied is HD 12098. The strong and variable lithium 6708 Å line was detected in the spectrum of this star. The star has been shown to have greatly different lithium abundances in two rotation phases corresponding to opposite surface areas. As mentioned earlier, a similar behavior of the Li blend was found in the spectra of HD 83368 and HD 60435 which have lithium spots on their surface. Spectral observations of slowly rotating CP stars with the Doppler shifts Vsini < 10 km/s revealed the strong and nonvariable lithium 6708 Å line in the spectra of these stars. Quantitative spectral analysis using the Li I 6708 Å resonance doublet and the Li I 6103 Å line shows the lithium abundance, as determined by the 6103 Å line, to be somewhat greater than that determined by the 6708 Å line. A higher ratio of ⁶Li/⁷Li amounting to ～0.3–0.5 was found in these stars. ⁶Li production is assumed to be due to spallation reactions on the surface of magnetic CP stars; this isotope ratio remained in strong magnetic fields.     

Winds and mass-loss from evolved,low-gravity cool stars

We summarize results from several programs utilizing the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) to study winds and mass-loss from evolved, low-gravity cool stars. We have found that: (i) the photons for thermally and fluorescently excited UV emission lines are created below the region of wind acceleration, (ii) the self-reversals in optically thick emission lines indicate an outflowing wind with mean velocities of 9–25 km/s, (iii) the profiles of optically thin emission lines indicate a mean chromospheric macroturbulence of 24–35 km/s, anisotropically distributed along the radial-tangential directions, (iv) significant emission from hot material (≈10⁵ K) is seen in both non-coronal and hybrid stars to the right of the Linsky-Haisch dividing line, (v) the weakness of Fe II emission lines in the carbon stars, combined with the presence of the Fe I 2807 Å feature only in carbon stars, suggests that the ionization fraction of iron is significantly lower in the outer atmospheres of carbon stars than in O-rich stars, and (vi) Fe II line profile variations indicate changes in mass-loss rate and wind opacity on a timescale of several years in two typical late-type, low-gravity stars.     

Summary of the First NUVA Conference Space Astronomy: the UV Window to the Universe

In this summary of the conference Space Astronomy: the UV Window to the Universe, held in El?Escorial, Spain, May 28 to June 1, 2007, I identify the important scientific questions posed by the speakers and the corresponding discoveries that future ultraviolet space instruments should enable. The science objectives described by the various speakers naturally fall into groups according to the needed instrumental requirements: wavelength coverage, spectral resolution, sensitivity, rapid access to targets, monitoring, and signal/noise. Although most of the science objectives presented during the conference require UV spectra in the 1,170–3,200 Å range, there are important science objectives that require spectra in the 912–1,170 Å range and at shorter wavelengths. I identify the limitations of present instruments for meeting these requirements. To avoid the upcoming UV dark age, important work must be done to properly build the World Space Observatory (WSO) and to plan future space missions.     

Spectral atlas of O9.5-A1-Type supergiants

High-resolution spectra of nine supergiants and three comparison stars taken with CCD echelle spectrographs in the coude’ foci of the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and 2-m telescope of Terskol Observatory (with R = 40000 and R = 45000, respectively) are reported in a tabular and graphic form. Two hundred (α Cam, O9.5 Ia) to 1000 (HD 12953, A1 Ia-0) stellar and interstellar lines and bands are identified in the 3600–7800 ÅÅ wavelength interval and most of them have their central intensities and heliocentric radial velocities measured. A spectral classification based on weak photospheric absorptions is tested. This is actual for the brightest supergiants and hypergiants, where the formation regions of strong lines, which are traditionally used for classification, also include the bases of stellar winds. Radial gradients of velocity are revealed in the atmospheres of supergiants. The cases of the refinement of the effective wavelengths, analysis of blends, and revealing of wind anomalies in line profiles are illustrated. The atlas is used extensively as a teaching tool.     

Major prospects of exoplanet astronomy with the World Space Observatory–UltraViolet mission

The success of the International Ultraviolet Explorer (IUE) first and then of the STIS and COS spectrographs on-board the Hubble Space Telescope (HST) demonstrate the impact that observations at UV wavelengths had and are having on modern astronomy. Several discoveries in the exoplanet field have been done at UV wavelengths. Nevertheless, the amount of data collected in this band is still limited both in terms of observed targets and time spent on each of them. For the next decade, the post-HST era, the only large (2-m class) space telescope capable of UV observations will be the World Space Observatory–UltraViolet (WSO–UV). In its characteristics, the WSO–UV mission is similar to that of HST, but all observing time will be dedicated to UV astronomy. In this work, we briefly outline the major prospects of the WSO–UV mission in terms of exoplanet studies. To the limits of the data and tools currently available, here we also compare the quality of key exoplanet data obtained in the far-UV and near-UV with HST (STIS and COS) to that expected to obtain with WSO–UV.     

UVMag: stellar formation,evolution, structure and environment with space UV and visible spectropolarimetry

Important insights into the formation, structure, evolution and environment of all types of stars can be obtained through the measurement of their winds and possible magnetospheres. However, this has hardly been done up to now mainly because of the lack of UV instrumentation available for long periods of time. To reach this aim, we have designed UVMag, an M-size space mission equipped with a high-resolution spectropolarimeter working in the UV and visible spectral range. The UV domain is crucial in stellar physics as it is very rich in atomic and molecular lines and contains most of the flux of hot stars. Moreover, covering the UV and visible spectral domains at the same time will allow us to study the star and its environment simultaneously. Adding polarimetric power to the spectrograph will multiply tenfold the capabilities of extracting information on stellar magnetospheres, winds, disks, and magnetic fields. Examples of science objectives that can be reached with UVMag are presented for pre-main sequence, main sequence and evolved stars. They will cast new light onto stellar physics by addressing many exciting and important questions. UVMag is currently undergoing a Research & Technology study and will be proposed at the forthcoming ESA call for M-size missions. This spectropolarimeter could also be installed on a large UV and visible observatory (e.g. NASA’s LUVOIR project) within a suite of instruments.     

Stellar winds from hot low-mass stars

Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion.     

MK morphological study of Am stars at 66 Å/mm

The pseudo-luminosity effect in the metallic line A-type stars found by Abt & Morgan (1976) is confirmed in a random sample of 27 Am stars. From a morphological study of their spectra in the wavelength interval 3850-4400 Å at a reciprocal dispersion of 66 Å/mm, revised spectral types are given on the MK system for their K-line and metallic-line spectra. This shows that: (a) our segregation of weak Am from the Am stars largely agrees with that by Cowleyet al. (1969); (b) all the stars in the sample are dwarfs according to their K-line classification; (c) more than 80 per cent exhibit the pseudo-luminosity effect significantly, with their metallic-line spectra resembling a giant or even a supergiant in the violet (3850-4100 Å), and a giant rather than a dwarf in the blue region (4260-4400 Å); (d) in two-thirds of the stars under (c), the Sr n 4077 Å line is found to have a markedly brighter luminosity class compared to any region, and in more than one-third of the sample it is comparable to that in Ap stars; (e) at least five stars exhibit characteristics which might suggest a spectrum variability: among these, the most striking example is 41 Sex A which was found to show a phase-modulated spectrum variation hitherto unknown in Am stars; (f) the metallic-line spectra of another five stars appear to be similar to A-shell type in differing degrees; (g) less than 20 per cent of the sample comprises stars which do not show any significant differential luminosity effect; these stars might have been misclassified or perhaps they are in a quiescent state. We also confirm the conclusion arrived at by Böhm-Vitense & Johnson (1978) that all Am stars may vary and our observations suggest that groups may exist among them.     

Wolf-Rayet stars

This paper reviews the current status of knowledge regarding the basic physical and chemical properties of Wolf-Rayet stars; their overall mass loss and stellar wind characteristics and current ideas about their evolutionary status. WR stars are believed to be the evolved descendents of massive O-type stars, in which extensive mass loss reveals successive stages of nuclear processed material: WN stars the products of interior CNO-cycle hydrogen burning, and WC and WO stars the products of interior helium burning. Recent stellar evolution models, particularly those incorporating internal mixing, predict results which are in good accord with the different chemical compositions observationally inferred for WN, WC and WO stars. WR stars exhibit the highest levels of mass loss amongst earlytype stars: mass loss rates, typically, lie in the range [1–10]×10⁻⁵ M _⊙yr⁻¹. Radiation pressure-driven winds incorporating multi-scattering in high ionisation-stratified winds may cause these levels, but additional mechanisms may also be needed.     

Spectrophotometry of carbon stars

The results of a spectrophotometric study of 13 carbon stars in the wavelength range from 4000 Å to 6800 Å with 2.75 Å resolution are given. The observed energy distributions for these stars relative to the flux at λ₀ = 5556 Å, represented in graphic form, were determined. Their color temperatures were determined from the [5710] ? [6680] color index. A dust shell is assumed to exist around U Cyg. The indices of the C_2’ MS, and CaCl molecular bands and the D_2,1 line were also determined.     

Spectroscopy of B and Be Stars in the Young Open Star Cluster NGC 581 (M 103)

Astronomy Letters - Based on moderate-resolution spectra in the range 4200–5200 Å, we have studied B and Be stars in the young open star cluster NGC 581. The temperatures of the program...     

Identifying Li-rich giants from low-resolution spectroscopic survey

In this paper we discuss our choice of a large unbiased sample used for the survey of red giant branch stars for finding Li-rich K giants, and the method used for identifying Li-rich candidates using low-resolution spectra. The sample has 2000 giants within a mass range of 0.8 to \(3.0\textit{M}_{\odot }\). Sample stars were selected from the Hipparcos catalogue with colour (B–V) and luminosity \((\textit{L}/\textit{L}_{\odot }\)) in such way that the sample covers RGB evolution from its base towards RGB tip passing through first dredge-up and luminosity bump. Low-resolution (R \(\approx \) 2000, 3500, 5000) spectra were obtained for all sample stars. Using core strength ratios of lines at Li I 6707 Å and its adjacent line Ca I 6717 Å  we successfully identified 15 K giants with A(Li) > 1.5 dex, which are defined as Li-rich K giants. The results demonstrate the usefulness of low-resolution spectra to measure Li abundance and identify Li-rich giants from a large sample of stars in relatively shorter time periods.     

Spectroscopy of high proper motion stars in the ground-based UV

Based on high quality spectral data (spectral resolution R≈60000) within the wavelength range of 3550–5000 Å we determined main parameters (effective temperature, surface gravity, microturbulent velocity, and content of chemical elements including heavy metals from Sr to Dy) for 14 metal-deficient G–K stars with large proper motions. The stars studied have a high range of metallicity: [Fe/H]=?0.3÷?2.9. Abundances of Mg, Al, Sr and Ba were calculated with non-LTE line-formation effects accounted for. The abundance both of radioactive element Th and the r-process element Eu were determined through synthetic spectrum calculations. We selected stars that belong to different galactic populations according to the kinematical criterion and parameters determined by us. We found that the studied stars with large proper motions refer to different components of the Galaxy: thin, thick disks and halo. The chemical composition of the star BD+80°?245 far from the galactic plane agrees with its belonging to the accreted halo. For the giant HD?115444 we obtained [Fe/H]=?2.91, an underabundance of Mn, an overabundance of heavy metals from Ba to Dy, and especially a high excess of the r-process element europium: [Eu/Fe]=+1.26. Contrary to its chemical composition typical for halo stars, HD?115444 belongs to the disc population according to its kinematic parameters.     

The ultraviolet radiation environment in the habitable zones around low-mass exoplanet host stars

The EUV (200–911 Å), FUV (912–1750 Å), and NUV (1750–3200 Å) spectral energy distribution of exoplanet host stars has a profound influence on the atmospheres of Earth-like planets in the habitable zone. The stellar EUV radiation drives atmospheric heating, while the FUV (in particular, Lyα) and NUV radiation fields regulate the atmospheric chemistry: the dissociation of H₂O and CO₂, the production of O₂ and O₃, and may determine the ultimate habitability of these worlds. Despite the importance of this information for atmospheric modeling of exoplanetary systems, the EUV/FUV/NUV radiation fields of cool (K and M dwarf) exoplanet host stars are almost completely unconstrained by observation or theory. We present observational results from a Hubble Space Telescope survey of M dwarf exoplanet host stars, highlighting the importance of realistic UV radiation fields for the formation of potential biomarker molecules, O₂ and O₃. We conclude by describing preliminary results on the characterization of the UV time variability of these sources.     

Rosat and EUVE observations of B stars

Recent observations of the X-ray and EUV emission of non-supergiant B stars are summarized. As compared with O stars, the X-rays of most of the near-main-sequence B stars are soft, and the stars show a departure from theL _x = 10^?7 L _bol relation. Using line driven wind models to provide an estimate of the density distribution, it is concluded that a major fraction of the wind emission measure is hot, whereas in shocked wind theory less than 10 percent of the wind emission measure should be hot. The X-ray observations suggest that all of the B stars are X-ray emitters with a basal X-ray luminosity of about 10^?8.5 L _bol . A hard component dominates the X-ray emission of τ Sco, and possible causes are discussed. For the Be stars, the X-ray emission is that which is expected from a normal B-star wind coming from the poles, as in the Wind Compressed Disk (WCD) model of Be stars. None of the stars, including theβ Cep stars, show noticeable variability in their X-rays.EUVE observations of∈ CMa B2 II, find it to be the brightest object in the EUV sky at 500 to 700 Å. It shows a Lyman continuum flux that is a factor of 30 higher than line blanketed model atmospheres. The continuum is seen on both sides of the He I 504 Å edge, and the discrepancy with model atmospheres is even greater shortward of 504 Å. TheEUVE spectra show emission lines both from high stages of ionization ( Feix to Fexvi) and from low stages (Heii and Oiii). The Heii Lymanα results from recombination following X-ray photoionization in the wind, and the Oiii resonance line is found to be present because of the Bowen fluorescence mechanism. Thus, there is and interesting coupling between the wind production by the EUV photospheric emission, the production of X-ray and line EUV emission by winds, and the production of fluorescence by recombination in the wind; all of these processes are now observable in B stars.     

Stellar wind velocities and the radiation-driven winds theory for O stars

In this paper we have carried out an analysis of the predictions of the radiatively driven stellar winds theories on 63 stars belonging to clusters or associations. The spectral types in our sample range from O3 to B0 and all classes of luminosity are considered. The study has been carried out starting from the relationship between the stellar-wind velocity (v _edge) obtained from the resonance doublet ofCIV for stars observed with the IUE, and the escape velocity. The stellar masses have been obtained from the evolutionary tracks of Maeder and Meynet. Results from recent NLTE analyses with blanketing of lines and winds have been used for the effective temperature.Based on data from the International Ultraviolet Explorer, de-archived from the Villafranca Data Archive of the European Space Agency.