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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server Mon. Not. R. Astron. Soc. 000, 000–000 (1998) The evolved B[e] star HD 87643 : observations and a radiation driven disk-wind model for B[e] stars Ren´e D. Oudmaijer1, Daniel Proga1, Janet E. Drew1, Dolf de Winter2,3 1 Imperial College of Science, Technology and Medicine, Blackett Laboratory, Prince Consort Road, London, SW7 2BZ, U.K. 2 Dpto. F´ısica T´eorica C–XI, Facultad de Ciencias, Universidad Aut´onoma de Madrid, Cantoblanco, E–28049 Madrid, Spain 3 Centro de Astrof´ısica de Universidade do Porto, Campo Alegre 823, 4150 Porto, Portugal received, accepted ABSTRACT New high resolution spectroscopic and medium resolution spectropolarimetric data, complemented with optical broad and narrow band imaging, of the B[e] star HD 87643 are presented. The spectrum of HD 87643 exhibits the hybrid characteristics well known to be representative of the group of B[e] stars; a fast wind with an expansion velocity in excess of 1000 km s−1 is measured in the hydrogen and helium lines, while a slower component is traced by lower excitation lines and forbidden lines. Clues to the geometry of the rapidly expanding circumstellar shell are provided by the startling polarization changes across Hα. Comparison with published schematic calculations indicates that the polarizing material is located in a slowly rotating, expanding disk structure. A hydrodynamical model is then presented whose results are consistent with the original two-wind concept for B[e] stars and exhibits kinematic properties that may well explain the observed spectral features in HD 87643. The model calculations use as input a B star undergoing mass loss, surrounded by an optically thick disk. The resulting configuration consists of a fast polar wind from the star and a slowly expanding disk wind. The model also predicts that the stellar wind at intermediate latitudes is slower and denser than in the polar region. Key words: stars: circumstellar matter – stars: individual: HD 87643 – stars: emission line, Be – polarization – hydrodynamics 1 INTRODUCTION Balmer series, and displays low excitation forbidden lines. Except for blueshifted P Cygni absorption components and The B[e] phase is one of the phases a massive star might interstellar lines no optical absorption lines have been de- go through during its evolution. Since the spectra of such tected. Optical and ultraviolet IUE spectra of HD 87643 are objects are dominated by emission lines, and generally do presented by de Freitas Pacheco, Gilra & Pottasch (1982) not show any signs of photospheric absorption lines, it was and de Freitas Pacheco et al. (1985). The UV spectrum ap- only due to the work of Zickgraf et al. (1985, 1986, 1996 and pears to be dominated by low ionization absorption lines, references in these papers) on peculiar B-type emission line although features due to Fe iii,Siiii and Si iv are present. stars in the Magellanic Clouds that the luminosities of some McGregor, Hyland & Hillier (1988) find that HD 87643 of these objects began to be known, and their location in displays a large near-infrared excess (J – K ∼ 2.6), which the HR-diagram could be plotted with some accuracy. they attribute to hot circumstellar dust. Published broad h m s o 0 00 HD 87643 (α2000 =1004 30 , δ2000 = –58 38 52 ; band photometry indicates variations over long timescales: l=283o, b=–2.5o) appears to be a Galactic counter-part the V band magnitude was reported as 8.51 in 1968 (Hilt- of the B[e] stars in the Magellanic Clouds. Over the last ner et al. 1968), and as 9.41 in 1995 (Torres et al. 1995). In decades spectroscopic studies of the object were reported the intervening years, the V band magnitude has fluctuated, by Hiltner, Stephenson & Sanduleak (1968), Stephenson but an overall trend towards fainter magnitudes is present (1974), Swings (1974) and Carlson & Henize (1979). The (Miroshnichenko 1998). No colour information has been pub- picture that has emerged is that the optical spectrum of lished however. The emission line spectrum of the star is HD 87643 is dominated by emission lines from Fe ii and the also variable. Lopes, Daminelli Neto & de Freitas Pacheco 2 Ren´e D. Oudmaijer et al. (1992) show the behaviour of the Hβ line. Its equivalent and included the steps of bias subtraction, flatfielding, back- width (EW) changed by a factor of several in the period ground subtraction and wavelength calibration. 1980 – 1990, reaching a maximum value in 1985. An interesting aspect of the surroundings of HD 87643 is discussed by Surdej et al. (1981) and Surdej & Swings (1983). They obtained spectra of two high density knots in 2.2 Spectropolarimetry the large reflection nebula around HD 87643 at offsets from Optical linear spectropolarimetric data were obtained using the star of 2000 NW and 1700S. Surdej & Swings (1983) show the RGO Spectrograph on the 3.9-metre Anglo-Australian that the central peak Hβ emission at the two positions differs Telescope on 31 December 1996 and 1 January 1997. The by about 150 km s−1, and argue that the observed velocity weather was clear. difference confirms their hypothesis that the line forming The instrumental set-up included a half-wave plate ro- region is not isotropic. tator set to various angles to obtain the Stokes QU parame- The layout of this paper is as follows; Sec. 2 describes ters and a calcite block to separate the light leaving the re- our observations of the star. Sec. 3 describes key features of tarder into perpendicularly polarized light waves. Two holes the optical spectrum, and then Sec. 4 discusses the polariza- in the dekker allow for simultaneous observations of the ob- tion of the object. Constraints on the distance and hence on ject and the sky. Four spectra are recorded, the O and E the evolutionary status of the object are provided in Sec. 5. rays of the target object and the sky respectively. One com- Then in Sec. 6 a new dynamical model for the inner circum- plete polarization observation consists of a series of consecu- stellar environment of HD 87643 and B[e] stars in general is tive exposures at four rotator positions. Spectropolarimetric outlined. A virtue of this model is that it offers great promise standards were observed during both nights. The instrumen- of fitting in with the striking spectropolarimetric behaviour tal polarization was found to be negligible. at Hα seen in HD 87643. The paper’s conclusions appear in A 1024 × 1024 pixel TEK-CCD detector was used Sec. 7. which, combined with the 1200V grating, yielded a spectral coverage from 6340 – 6880 A,˚ in steps of 0.54 A.˚ Wavelength calibration was performed by observing a Cu-Ar lamp be- fore each exposure. On 31 December 5 × 4setsofdatawere 2 OBSERVATIONS taken with exposure times of 90 s, a day later 3 × 4sets with exposure times of 120 s were obtained. A slit width of 2.1 Echelle spectroscopy 1.5 arcsec was used. The initial processing of the data was performed in the Spectroscopic data were obtained during the nights of 20 and iraf package. After bias-subtraction and flatfielding, the in- 21 April 1997 employing the Coud´e echelle spectrograph on dividual spectra were extracted by tracing the pixel rows the 7400 telescope of Mount Stromlo Observatory. The ob- containing the data. The wavelength calibration was per- servational set-up included a 2 arcsec slit, the 300 lines/mm formed by fitting a 3rd order polynomial through more than cross-disperser grating and a 2K × 2K TEK CCD. Since the 20 identified lines in the arc spectra, and resulted in disper- cross-disperser is not located in the pupil plane, vignetting sion fits with a σ of 0.07 A.˚ The resulting spectral resolution occurs away from the central order. Hence, the CCD was as measured from arc lines is 60 km s−1. The E and O ray windowed during read-out. To improve signal-to-noise, the data were then extracted and imported into the Time Se- data were binned over two pixels in both the dispersion and ries/Polarimetry Package (tsp)incorporatedinthefigaro cross-dispersion directions. software package maintained by starlink. The Stokes pa- On 20 April, observations at a central wavelength of rameters were determined and subsequently extracted. 6562 A˚ were obtained. The total on-source integration time was 45 minutes, obtained in 3 individual exposures of 15 minutes each. Observations of a bright B star (HR 4199, B0V) were also obtained to correct the spectrum for tel- 2.3 Imaging luric absorption lines, and Th-Ar arc maps were obtained for the dispersion correction. The dispersion fit through more Images of HD 87643 were obtained through V, R,Hαand than 450 identified lines yielded an rms value of 0.009 Aus-˚ [S ii] filters with the Dutch 92 cm telescope stationed at the ing 4th order polynomials in both the dispersion and cross- ESO La Silla. On 12 July 1992 images were taken through dispersion direction. The final spectrum consisted of 20 dif- the R (ESO#421), Hα (ESO#387) and [S ii] (ESO#391) ferent orders covering the range from 5750 to 7220 A,˚ with pass-bands for 4 × 3, 3 × 30 and 3 × 70 sec, respectively. The a spectral resolution as measured from telluric absorption detector used was a GEC 576 × 385-pixel CCD. The pixels lines of 8.5 km s−1. The data were corrected for telluric ab- have a size of 22 µm corresponding to 0.36 arcsec on the sorption by matching the strength of the absorption lines sky.

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