Line profile variability of OB stars: Pulsation,rotation, clumps and magnetic fields

The variability of line profiles in spectra of bright OB stars has been studied.We obtain more than 1000 high quality spectra of δ Ori A, λ Ori A, α Cam, 19 Cep, ι Her, ρ Leo and other target stars. We revealed the line profile microvariability of small amplitude (0.5–3% in the adjacent continuum units) for all observed stars. For most stars only cyclic components of the line profile variability (LPV) at the time scales from hours to days were detected. These components seem to be connected with both the non-radial pulsations (NRP) and rotation line profile modulation. In the spectra of δ Ori A and λ Ori A we found the evidences of the stochastic LPV in spectra, probably connected with the small clumps in the stellar wind. On the basis of recent observations we discuss the origin of the magnetic field of early-type stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Line variability in the spectrum of supergiant α Cam

CCD spectra taken with the PFES and CEGS echelle spectrographs attached to the 6-m Special Astrophysical Observatory (Russian Academy of Sciences) telescope and the 2-m Shamakha Astrophysical Observatory (National Academy of Sciences of Azerbaijan) telescope, respectively, were used to study the line-profile variations in the spectrum of the hot supergiant α Cam. No fast (≤1.5 h) line-profile and radial-velocity variations were found. Some of the systematic effects that cause spurious variability are considered. The Hα-profile variability appears symmetric relative to the radial velocity of the star’s center of mass and is attributable to variable blueshifted and redshifted emission and/or absorption components superimposed on a variable photospheric profile. The Hα line shows evidence of a large-scale mass ejection from the stellar surface, which is also traceable in other spectral lines. The He II 4686 line exhibits an inverse P Cyg profile, while the red wing of the He I 5876 line shows weak and variable emission. The fast (on characteristic time scales of shorter than an hour) variability of the He II 4686 profile that was previously revealed by our observations (Kholtygin et al. 2000) is called into question. A comparison of the observational data on the variability of ultraviolet and optical line profiles for the supergiant αCam suggests that nonradial motions are mainly responsible for the radial-velocity and line-profile variability.     

Spectroscopic observations of the classical symbiotic star V1413 AQL at the phase of its transition to quiescence

We present the results of our spectroscopic and photometric observations of the classical symbiotic star V1413 Aql in 2003–2007, which cover the transition phase of its hot component from activity to quiescence. Various quiescence criteria for the hot component of the system are analyzed. Judging by its photometric characteristics, the system had not yet returned to quiescence by 2007, although a fairly strong He II λ4686 Å line observed previously during quiescence in 1993 appeared in its spectrum. We model the continuum energy distribution for V1413 Aql based on a standard three-component model and a model with an accretion disk. Analysis of the forbidden [O III] and [Ne III] lines shows that the neon abundance in V1413 Aql may be enhanced with respect to the oxygen abundance. The cool component of V1413 Aql has been found to be a variable M5-type red giant, with the variability amplitude at λ = 7500 Å being at least 2 ^m .     

Analysis of the HST ultraviolet spectra for T Tauri stars: RY Tau and HD 115043

The ultraviolet spectra of the stars RY Tau and HD 115043 from the Hubble Space Telescope are analyzed. RY Tau belongs to the classical T Tauri stars, while HD 115043 is a young (t~3×10⁸ years), chromospherically active star. The most intense emission lines were identified, and their fluxes were measured. Low-resolution spectra of RY Tau and HD 115043 in the wavelength range 1160–1760 Å exhibit almost the same set of emission lines. However, first, the luminosity of RY Tau in these lines is approximately a factor of 300 higher than that of HD 115043, and, second, the relative line intensities differ greatly. The intensity ratio of the C IV λ1550, Si IV λ1400, and NV λ1240 doublet components is close to 1: 2 in the spectra of both stars. Judging by the continuum energy distribution, the spectral type of RY Tau is later than that of HD 115043. Synchronous flux variability in the C IV λ1550 and He II λ1640 lines in a time of ~20 min was detected in RY Tau. The flux rise in these lines was accompanied by a redshift of the intensity peak in the profiles by~50 km s^?1. Intermediate-resolution spectra are used to study line profiles in the spectrum of RY Tau. In particular, the profiles of (optically thin) Si III]λ1892 and C III]λ1909 lines were found to be asymmetric and about 300 km s^?1 in width. The (optically thick) C IV λ1550 doublet lines have similar profiles. The Mg II λ2800 doublet lines are also asymmetric, but their shape is different: they consist of a broad (?750 km s^?1 at the base) emission component on which an interstellar absorption line shifted from the line symmetry center by about 20 km s^?1 is superimposed. The intensity ratio of the Mg II λ2800 doublet components is?1.4. Whether there are molecular hydrogen lines in the spectrum of RY Tau is still an open question. It is shown that the emission lines in the ultraviolet spectrum of RY Tau cannot originate in a hydrostatically equilibrium chromosphere. It is argued that quasi-steady accretion of circumstellar matter is responsible for the emission.     

Long-term infrared variability of the UX Ori-type star SV Cep

We investigate the long-term optical–infrared variability of SV Cep and explain it in the context of an existing UX Ori (UXOR) model. A 25-month monitoring programme was completed with the Infrared Space Observatory in the 3.3–100 μm wavelength range. Following a careful data reduction, the infrared light curves were correlated with the variations of SV Cep in the V band. A remarkable correlation was found between the optical and the far-infrared light curves. In the mid-infrared regime, the amplitude of variations is lower, with a hint for a weak anti-correlation with the optical changes. In order to interpret the observations, we modelled the spectral energy distribution of SV Cep assuming a self-shadowed disc with a puffed-up inner rim, using a two-dimensional radiative transfer code. We found that modifying the height of the inner rim, the wavelength dependence of the long-term optical–infrared variations is well reproduced, except the mid-infrared domain. The origin of variation of the rim height might be fluctuation in the accretion rate in the outer disc. In order to model the mid-infrared behaviour, we tested adding an optically thin envelope to the system, but this model failed to explain the far-infrared variability. Infrared variability is a powerful tool to discriminate between models of the circumstellar environment. The proposed mechanism of variable rim height may not be restricted to UXOR stars, but might be a general characteristic of intermediate-mass young stars.     

Spectrophotometry of some lines in Beta Lyrae

A spectrophotometric study of the spectral lines Hγ, He I λ 4388 Å, He I λ 4472 Å, Fe II λ 4352 Å and Mg II λ 4481 Å in the spectrum of β Lyrae has been made by using fifteen spectra taken with a spectrograph (8 Å/mm) attached to the 1-metre reflector of the Hamburger Sternwarte. The profiles of the above lines are given. Also the probable variations of these profiles with time are examined on the basis of a comparison of our observational data with data existing in the literature. More attention was paid to the emission components of Hγ and the Helium lines. The views previously described in a general way by Sahade as to the structure of the emission lines are rather supported.     

Study of nonstationarity of the atmosphere of <Emphasis Type="Italic">κ</Emphasis> Cas. I. Variability of profiles of photospheric and He I wind lines

Temporal variations of radial velocities and line profiles in the spectrum of the supergiant κ Cas were investigated. Variability of radial velocities and profiles of photospheric lines Si III, OII, He I, H₁₀–Hδ and wind lines He I λ 5875, 6678 Å ismainly caused by non-radial pulsations. For photospheric lines quasisinusoidal variabilities of the radial velocity were found. Temporal variability of radial velocity of the wind lines He I λ 5875, 6678 A? differ from each other and from the photospheric lines. Gamma velocities and amplitudes of radial velocity variability were determined. The amplitude of variability and the velocity of expansion increase from lower to upper layers of the atmosphere. Emission components are superimposed on the line profiles at positions about ?135 ± 10.0, ?20 ± 20 and 135 ± 10.0 kms^?1 respectively. They are more obvious in the wind line profiles, although, there are signs of emissions also in the photospheric lines. Such a character of variability of all the lines in the κ Cas spectrum confirms its Be nature.     

Peculiarities of the UV continuum energy distribution for T Tauri stars

In the UV spectra of BP Tau, GW Ori, T Tau, and RY Tau obtained with the Hubble Space Telescope, we detected an inflection near 2000 Å in the F _λ ^c (λ) curve that describes the continuum energy distribution. The inflection probably stems from the fact that the UV continuum in these stars consists of two components: the emission from an optically thick gas with T<8000 K and the emission from a gas with a much higher temperature. The total luminosity of the hot component is much lower than that of the cool component, but the hot-gas radiation dominates at λ<1800 Å. Previously, other authors have drawn a similar conclusion for several young stars from low-resolution IUE spectra. However, we show that the short-wavelength continuum is determined from these spectra with large errors. We also show that, for three of the stars studied (BP Tau, GW Ori, and T Tau), the accretion-shock radiation cannot account for the observed dependence F _λ ^c (λ) in the ultraviolet. We argue that more than 90% of the emission continuum in BP Tau at λ>2000 Å originates not in the accretion shock but in the inner accretion disk. Previously, a similar conclusion was reached for six more classical T Tau stars. Therefore, we believe that the high-temperature continuum can be associated with the radiation from the disk chromosphere. However, it may well be that the stellar chromosphere is its source.     

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.     

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 H_β profiles to confirm the spectral type A2–A3 of WW Vul.     

Detection of lithium in the spectrum of the symbiotic Mira star V407 Cygni

We detected the resonance lithium doublet, ⁷LiI λ6708, in the spectra of the symbiotic Mira V407 Cyg (P≈763^d) obtained with a resolution R = λ/Δλ ≈ 18 500. The line equivalent width is ～0.34 Å. The presence of lithium in the atmosphere of the Mira, which, judging by its period, appreciably ascended the asymptotic giant branch (AGB), can be explained by the penetration of its convective envelope into the hot-bottom-burning (HBB) hydrogen-shell source. At the same time, the spectrum of V407 Cyg does not reveal the ZrO λ5551, 6474 absorption bands, which are used to classify S-type stars from low-resolution spectra. We found only weak ZrO λ5718, 6412 bands, which are, however, invisible in low-resolution spectra. Thus, the Mira star V407 Cyg should be classified as being of the spectral type M. In general, the spectrum of V407 Cyg is similar to the spectrum of the red giant in the symbiotic star CH Cyg, but the latter exhibits no lithium lines. The switch-on of the HBB process without any significant enrichment of the atmosphere of an AGB star with s-process elements implies that the third dredge-up is not efficient for some of these stars.     

Results of Photometric and Spectroscopic Observations of the Young Variable Star V730 Cep

We present the results of our photometric (V RI) and spectroscopic observations of the young variable star V730 Cep (MisV1147) classified by Uemura et al. (2004) as a Herbig Be star. Our photometry confirms the conclusion of the above authors that this star has a complex pattern of variability including periodic or quasi-periodic brightness variations with a period of about 14 days and deep Algol-like minima typical for UX Ori stars. Our spectroscopy shows that the classification of V730 Cep as a Herbig Be star is wrong. Actually, this star has a much lower temperature and belongs to the family of T Tauri stars. This allows us to explain the nature of the unusual photometric activity of V730 Cep based on a combination of two well-known models of variable circumstellar extinction applied to young stars: AA Tau- and UX Oritype variability. It follows from our observations that the color tracks on the V ?(V ?I) color–magnitude diagram for these models slightly differ: the AA Tau-type variability of circumstellar extinction is caused by larger grains than the UX Ori-type variability. Such a difference can be due to an increase in the characteristic sizes of circumstellar dust as the star is approached and has a simple explanation: small dust grains evaporate faster than large ones.     

Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis

X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.     

ROSAT PSPC observations of the Orion Trapezium area II. Source variability and revised source list

A deep ROSAT PSPC image centred on the Orion Trapezium has been reduceda second time using an improved version of the PSF to fit the data. The outer rim of the field of view was also included. The new catalogue contains 316 X‐ray sources which are easily identified with pre‐main sequence stars of the Ori OB1 Ic and Id association. All 316 sources were tested for variability. No variations were found inside the single exposures of about 45 minutes length each. Between the 4 exposures spaced over 5 days about 1/3 of the sources show signs of activities of various forms. As above 25% of these have somewhat regular lights curves (monotonically rising or falling or hill‐shaped) we infer that at least some outbursts with time scales longer than a day are present and that past searches for X‐ray flares of pre‐main sequence stars were biased towards shorter time scales.     

Stars with discrepant <Emphasis Type="Italic">v</Emphasis> sin <Emphasis Type="Italic">i</Emphasis> as derived from the Ca II λ3933 Å and Mg II λ4481 Å lines. V. HD 182255 and HD 214923—SPB stars in binary systems

The axial rotation of a star plays an important role in its evolution, the physical conditions in its atmosphere and the appearance of its spectrum.We analyzed the CCD spectra of two stars for which their projected rotational velocity differs remarkably when derived from Ca II λ3933 Å and Mg II λ4481 Å lines. We estimated the projected rotational velocity of HD182255 to be 15.5 kms^?1, although in various spectra of this star the line widths correspond to values as high as 28.5 km s^?1. We found the HeI λ4471.498 Å line to be shifted to longer wavelengths by 0.046 Å, thus indicating a presence of the ³He I isotope in the atmosphere of this star with the ³He : ⁴He ratio from 0.2 to 0.6.We also found an absorption feature at the position of the forbidden line He I λ4470.02Å. We found the lines ofMg II and CII originating from higher excited levels to be missing in the spectra of HD 182255. For HD 214923 we determined the projected rotational velocity v sin i = 165km s^?1 from the profiles of the metallic lines and Ca II λ3933Å, whereas for helium lines v sin i ≈ 130km s^?1 is more appropriate. Radial velocity analysis results in three long periods of ≈ 105, 34, and 15 days, and a short period of ≈ 22 hours, close to the pulsational one mentioned earlier in the literature.     

Hotspots and a clumpy disc: variability of brown dwarfs and stars in the young σ Ori cluster

The properties of accretion discs around stars and brown dwarfs in the σ Ori cluster (age 3 Myr) are studied based on near-infrared (IR) time series photometry supported by mid-IR spectral energy distributions (SEDs). We monitor ∼30 young low-mass sources over eight nights in the J and K band using the duPont telescope at Las Campanas. We find three objects showing variability with J -band amplitudes  ≥0.5 mag  ; five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus, we establish the long-term nature of their flux changes. The light curves contain periodic components with time-scales of  ∼0.5–8 d  , but have additional irregular variations superimposed – the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hotspots are the dominant cause of variations in two objects (#19 and #33), including one likely brown dwarf, with spot temperatures in the range of 6000–7000 K. For the third one (#2), a brown dwarf or very low-mass star, inhomogeneities at the inner edge of the disc are the likely origin of variability. Based on mid-IR data from Spitzer , we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar discs. They show typical SEDs for T Tauri-like objects. Using SED models, we infer an enhanced scaleheight in the disc for the object #2, which favours the detection of disc inhomogeneities in light curves and is thus consistent with the information from variability. In the σ Ori cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.     

On the incidence of magnetic fields in slowly pulsating B, β Cephei and B-type emission-line stars

We have obtained 40 high-resolution circular spectropolarimetric measurements of 12 slowly pulsating B (SPB) stars, eight β Cephei stars and two Be stars with the Echelle Spectropolarimetric Device for the Observation of Stars at CFHT (ESPaDOnS) and Narval spectropolarimeters. The aim of these observations is to evaluate recent claims of a high incidence of magnetic field detections in stars of these types obtained using low-resolution spectropolarimetry by Hubrig et al. The precision achieved is generally comparable to or superior to that obtained by Hubrig et al., although our new observations are distinguished by their resolution of metallic and He line profiles, and their consequent sensitivity to magnetic fields of zero net longitudinal component. In the SPB stars, we confirm the detection of magnetic field in one star (16 Peg), but find no evidence of the presence of fields in the remaining 11. In the β Cep stars, we detect a field in  ξ¹  CMa, but not in any of the remaining seven stars. Finally, neither of the two B-type emission-line stars shows any evidence of magnetic field. Based on our results, we conclude that fields are not common in SPB, β Cep and B-type emission-line stars, consistent with the general rarity of fields in the broader population of main sequence B-type stars. A relatively small, systematic underestimation of the error bars associated with the UV Focal Reducer and Low Dispersion Spectrograph for the Very Large Telescope (FORS1) longitudinal field measurements of Hubrig et al. could in large part explain the discrepancy between their results and those presented here.     

Analysis of HST ultraviolet spectra for T Tauri stars: Estimating the interstellar extinction and the contribution from an accretion shock to the emission-continuum formation

We analyzed the spectra of eight T Tauri stars (T Tau, RY Tau, CO Ori, EZ Ori, GW Ori, GX Ori, V1044 Ori, and SU Aur) in the wavelength range from 1200 to 3100 Å taken with the STIS spectrograph from the Hubble Space Telescope. For each star, we found an upper limit on the interstellar extinction A _v , which proved to be lower than the values obtained by different authors from optical observations. For T Tau and RY Tau, we found the upper limits on their luminosities, masses, and radii as well as the bolometric luminosity of the excess emission continuum. The latter is most likely associated with mass accretion from a protoplanetary disk. We show that the bulk of the emission continuum is radiated in the infrared. For these stars, we determined the ratio of the flux in the C IV 1550 doublet lines to the excess-continuum flux. This ratio proved to be two orders of magnitude lower than its values predicted by the accretion-shock (AS ) models developed by Lamzin (1998) and Calvet and Gullbring (1998). This result leads us to believe that for T Tau and RY Tau, the emission continuum originates in the accretion disk and/or in the boundary layer rather than in the AS, as has been assumed previously. This implies that in these stars, only a small fraction of the accreted matter passes through the AS, while the bulk of this matter settles in the equatorial plane of the star, passing through the boundary layer.     

On the nature of the photometric activity of the T Tauri star V1184 Tau

We present our 2-year-long photometric (V, R _c, I _c) observations of the T Tauri star V1184 Tau. They show that the relatively quiet “photometric life” of this object, which ended in 2004 with a sharp brightness decline by four magnitudes, was succeeded by a qualitatively new (in nature) period of activity characterized by high-amplitude irregular photometric variability. Judging by its color variations, the object belongs to the class of UX Ori stars and, hence, variable circumstellar extinction is responsible for its brightness variations. Moreover, the (V?I _c)/V color-magnitude diagram for the object is identical to that for UX Ori itself, suggesting that the optical properties of dust grains in the circumstellar space of these stars are similar. At the same time, V1184 Tau is quite dissimilar to UX Ori stars in its light curve, variability amplitude (reaching 4.5 magnitudes in the V band), and some other parameters.     

Atlas of spectra of selected stars in ground-based ultraviolet

We present an atlas of spectra of high signal-to-noise ratio and high spectral resolution (R ≥ 60000) in a poorly studied short-wavelength region up to 3055 Å. The spectra of well-studied stars of close temperatures (β Ori, α Lyr and α Cyg) are compared with the spectrum of a low-metallicity A-type supergiant KSPer, the atmosphere of which is poor in hydrogen, H/He = 3 × 10^?5.We study the velocity field in the expanding atmospheres and envelopes of these stars. A complete atlas and detailed identification of spectral features are available in the Internet.