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Wind Variability of B Supergiants (will..1,stronomyI)e inserted,(" ..Lstrophysicsby hand late[')manuscript no. Wind variability of B supergiants IV. A survey of IUE time-series data of 11 B0 to B3 stars R.K. Prinja l, I). Massa 2 and A.W. Fullerton _'4 Depaatment of Physics & Astr moray, [:niversity College London, Gower Street, London WC1E 6BT. U.N. e-mail: rkpQstar, ucl. ac. uk 2 Raytheon ITSS, NASA/GSFC (;ode681, C.reenbeh, MD 20771. USA e-mall: massaOt aot ao.,ona, gsl c. nasa. gov "_ Department of Physics and As1 ronomy, University of Victoria. P.O. Box 3055. Victoria, BC VSW .3P6, Canada e-mail: awf@pha, jhu. edu 4 Center for Astrophysical Sciences, Dept. of Physics and Astronomy. ,Johns Hopkins University,3400 N. Charles St. Baltimore, MD 21218-2686 lrSA Received : accepted Abstract. We present the most suitable data sets available in the International Ultraviolet Explorer(IUE) archive for the study of time-dependent stellar winds in early B supergiants. The UV line profile variability in 11 B0 to Ba stars is analysed, compared and discussed, based on 1B separate data sets comprising over 600 homogeneously re- duced high-resolution spectrograms. The targets include 'normal' stars with moderate rotation rates and examples of rapid rotators. A gallery of grey-scale images (dynamic spectra) is presented, which demonstrates the richness and range of wind variability and highlights different structures in the winds of these stars. This work emphasises the suitability of B supergiants _br wind studies, under-pinned by the fact that they exhibit unsaturated wind lines for a wide range of ionizaticn. The wind activity of B supergiams is substantial and has highly varied characteristics. The variability evident in individual stars is classified and described in terms of discrete absorption components, spontaneous absorption, bowed structures, recurrence, and ionization variability and stratification. Similar structures can occur in stars of different fundamental parameter.,., but also different structures may occur in the same star at a given epoch. We discuss the physical phenomena that may be associated with the spectral signatures, and highlight the challenges that these phenomena present to theoretical studies of time-dependent outflows in massive stars. In addition, SEI line-synthesis modelling of the UV wind lines is used to provide further information about the state of the winds in our program stars. Typically the range, implied by the line profile variability, in the product of mass-loss rate and ion fraction (M q,) is a factor of _ 1.5. when integrated between 0.2 and 0.9 v_¢; it can however be several times larger over localised velocity regions. At a given effective temperature the mean relative ion ratios can differ by a factor of ,5. The general excess in predicted (forward-scattered) emission in the tow velocity regime is discussed in tee'ms of structured outflows. Mean ion fractions are estimated over the B0 to B1 spectral classes, and trends in the ionic ratios as a function of wind velocity are described. The low values obtained for the ion fractions of UV reson.mce tines may reflect the role of clumping in the wind. Key words, stars: early-type - stars: mass-loss ultraviolet: stars 1. Introduction 1983), X-ray emission (Feldmeier et al. 1997, Kahn et al. 2001, Cassinelli et al. 2001), and non-thermal radio emis- The radiation-pressure-driven winds (,t'O and early-B type sion (Bieging et al. 1989, Scuderi et. al. 1998). The small- stars are variable on characteristic time-scales of hours and scale wind structures responsible for these signatures are days, as a result of perturbations in the outflow due to believed to result from a strong instability in the line- small- and large-scale structure, reslc_ectively. Observable driven winds, which operates on linear scales smaller than diagnostics of the former include extended black absorp- the Sobolev length. This instability leads to multiply non- tion troughs m saturated UV P Cygni profiles (Lucy ntonotonic velocity fields in the wind and localised density enhancements which are bounded by pairs of forward and Send offprint requests to: R.N. Prinja 2 t{.IZ. Prinia. l). 3_lassa ,:mcl :\.\V. lqfllerton: Wind variability of B supra'giants reverse shocks (_.g. Owo('ki er al. 1!),'_8: F_'htmei,r I!)95). _ho wind .d_sr,rption result from material flowing through l,arge-scale structure in hot star wimls is i)rimarily stud- a semi-permanent. ('o-rotatin_ wind s_ructure. icd via systenlatic variability of lr\: r_'s()nall('_' ;111([opti- ,'al emission lines. These changes diagnose organised, spa- 1.1. Winds of early B-type supergiants tially extended perturbations in the wind. which may be induced by' magnetic or pulsational processes arising at the The stellar winds of early B supergiants (BIs) are par- stellar surface. We document in this paper the nature of ticularly suitable for a study of wind dynamics and the large-scale wind structure in L1 early-type B supergiant effects of large-scale wind structure since, (i) their IT\: stars, based on high-resolution lnt_:rnutwnal [ltrm'_olet wind lines are often well-developed but unsaturated, (ii) Explorer ( [U E) time-series spectroscopy. these diagnostic lines cover a wide range of ionization, One of the major discoveries made witl, I['/2 was the close to the dominant stage of the wind (see Table 2), systematic nature of variability' m the P Cvgni-type line (iii) it is generally easier to follow the progression of profiles of OB stars. The key developmental sequence was large-scale structure in the UV lines of Bls compared defined in a number of time-series UV investigations (e.g. to O-type stars, (iv) the winds have lower terminal ve- Prinja et al. 1987, Prinja & Howarth 1988, tIenrichs et. al. locities, so individual structures evolve more slowly and 1988, Kaper 1993, Kaper et al. 1996). A relatively broad, the important Silv &A1400 resonance line doublets are shallow absorption feature appears at < 0.3 of the terminal mostly decoupled, (v) their strong UV photospheric lines velocity (v_), which then evolves blue-wards over several are excellent effective temperature-luminosity diagnostics, hours and perhaps even days, while narrowing in velocity and (vi) the strongest UV photospheric lines sample the width. The velocity at which these 'discrete absorption wind/photosphere 'interface', often displaying wind be- haviour. components' (DACs) are first detected is roughly constant for a given star, but may vary from star-to-star. Motivated by these attractions, we describe in this paper the time-dependent wind characteristics of eleven Subsequent IUE studies established a connection be- B0 to B3 supergiants, based on 16 separate IUE high- tween stellar projected rotation velocity (re sin(i)) and resolution time-series data sets, involving the inspection of the rate of recurrence and acceleration of DACs (Prinja 638 spectrograms in total. This work extends our knowl- 1988; Kaper et al. 1996). More recent IUE monitoring edge of the characteristics of variable UV resonance lines campaigns extending over a few weeks have shown that in line-driven winds to cooler and less luminous stars. The the stellar wind lines vary continuously, implying that OB data are used to demonstrate the presence of a range of star winds are structured over a range of spatial and tem- empirical wind properties and implied varied perturba- poral scales. They have also shown that variations in the tions in the outflows. We report here on the incidence profiles modulate on rather long time-scales (i.e. several and repetition of DACs, evidence for rotational modula- days). Furthermore, the variability evident in _ Per (07.5 tion, ionization-state changes, shock formation, and spon- Ill(f); Henrichs et al. 1998) and ( Pup (04 If: Howarth et taneous wind enhancements. The diverse wind variability al. 199,5) appears to modulate on the stellar rotation rate. signatures in early BIs are compared and contrasted to Rotation is plausibly also the key factor in the repetitive those previously noted in luminous O-type stars. The later wind activity seen in HD 64760 (B0.5 Ib: Fullerton et al. sections of this paper present SEI line-synthesis modelling, 1997), HD 9a843 (05 III; Prinja et al. 1998) and HD 91969 which is used to explore further the ionization mixture and (B0 la; Massa et al. 1998). RotationMly inodulated wind structured nature of the winds. fluctuations are also apparent in late B and early A super- giants (e.g. Kaufer et al. 1996), which have nmch larger 2. The time-series data sets radii and longer rotation periods. IUE The fundamental origin of wind variability on large Our objective was to carry out a systematic survey of the spatial scales in hot stars remains uncertain. The causal temporal nature of wind activity in early B supergiants. link between inhomogeneities at the stellar surface and We therefore restricted our selection to time-series of high- large-scale wind structure has not been directly estab- resolution (,VAA _ 104) IUE SWP (._AllS0 to 1900A) lished observationally or theoretically. A currently pop- data sets. A list of the program stars is given in Table 1. Mar idea is that the large-scale variations discussed above They include luminosity classes Ia and Ib for a spectral (and some of those discussed in this paper) are spawned range B0 to B3, and have a wide range of projected ro- by photospheric irregularities which cause the wind from tation velocities. The corresponding 16 data sets analysed different longitudinal sectors on the surface to emerge with are also outlined in Table 1, and include 4 cases where different densities and/or velocities. The surface structure short-term wind variability may be compared over two or may, for example, be due to the action of non-radial pulsa- more epochs separated by a few years.
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