AbstractUnusual activity connected with infalling mass motions was recorded for the B0.5 IVe star FY CMa in February 1987 both from archival IUE spectrograms of circumstellar N V resonance lines and from optical spectra of H-alpha- emission and He I 5876 lines showing inverse P Cygni-type profiles. We estimate the mass loss rate, -dM/dt to be about 4.1 x 10-9 Msun yr -1 for a spherically symmetric wind. If we assume instead conical shape for the wind, for a cone half angle of ~30o then -dM/dt ~ 3.4 x 10-10 Msun yr -1 , or -dM/dt ~ 2.2 x 1015 gm s -1 . We attempt to give a qualitative explanation of activity observed for FY CMa in terms of circumstellar matter raining down to the star. IntroductionSince 1949, the spectrum of the Be star FY CMa (HR 2855, HD 58978, MWC 179, B0.5 IVe, v sin i ~ 280 km s-1) has changed quite often. Merrill & Burwell (1949) noted that the hydrogen emission lines in the photographic region had disappeared. Burbidge & Burbidge (1954) reported that this object showed double emission lines at H-beta and H-gamma with R/V > 1, the Balmer lines from H9 onward appeared as absorption only. Occasional optical studies of Balmer emission lines profiles obtained for this star at moderate resolution (1-2 Angstrom at H-alpha ) between 1981-1983 showed FY CMa to be in a rather stable state of optical emission-lines formation in its circumstellar disk (Dachs et al. 1986), while a number of high-resolution IUE spectrograms obtained for the star since 1981 demonstrated strong variations of fine structure of C IV, Si IV and N V resonance absorption doublets, indicating variable mass loss (Grady et al. 1987). In the present work we once more want to draw attention to the fact that FY CMa was observed in a state of unusually high activity of mass-loss variations during early-1987, as already noted by Peters (1988) and by Grady et al. (1988), and that these mass-loss variation were accompanied by strong infalling motions of circumstellar matter in front of the star, simultaneously visible in ultraviolet N V resonance absorption line profiles and in an optical He I 5876 inverse P Cygni-type line profile obtained by Dachs et al. (1992). Data Obtained and ReductionSix high resolution (lambda/ Delta lambda ~ 10000 ) ultraviolet spectra obtained between 1981 and 1987 by the International Ultraviolet Explorer (IUE) spacecraft were provided by the archives of the ESA Vilspa Data Center at the ESA Satellite Tracking Station at Villafrance del Castillo, Spain. Two high quality optical spectra of H-alpha and He I 5876 were measured with the ESO Coude Echelle Spectrometer fed by the 1.4 m Coude Auxiliary Telescope at La Silla, Chile. The instrumentation full width at half maximum (FWHM) measured in the thorium lamp comparison spectrum corresponds to a spectral resolution of 76 milli-Angstrom at either wavelength. These high-resolution optical spectrograms were supplemented by a total of 11 medium-resolution H-alpha emission line-profiles obtained for the star between 1981 and 1987 by various observers of the University of Bochum at the 61-cm telescope at ESO La Silla, using the University of Bochum scanner spectrometer at 1 to 2 Angstrom resolution, as described by Dachs et al. (1986). Both for IUE spectrograms and for optical spectrograms obtained at ESO, reductions were performed bymeans of IHAP and MIDAS image processing software systems provided by ESO. Radial velocities were first calculated in the heliocentric system, and then further corrected by subtracting the radial velocity of the star listed in the Bright Star Catalogue, Vr = +25 km s-1, in order to obtain velocities in the stellar reference frame. Typical mass-loss rates implied for FY CMa by the Si IV profiles were determined by employing the method used by Snow (1981). For the ion number density fraction in the wind, N (Si IV)/ N (H) = 3.5 x 10 -5 was adopted.
FIGURE 1. The spectral variations of FY CMa in resonance doublet lines N V and C IV of the high resolution IUE spectra. Morphology of Stellar Wind VariationsAsymmetric ultraviolet resonance doublet line profiles showing stellar wind absorption were inspected on six selected IUE spectrograms obtained in 1981 (SWP 15053, 15478), 1982 (SWP 15933, 15979) and 1987 (SWP 30183, 30392). Wavelength regions corresponding to N V (1235-47 Angstrom) and to C IV absorption (1539-58 Angstrom) are plotted in Fig. 1, while Si IV doublet regions (1384-1412 Angstrom) are drawn in Fig. 2. As judged from equivalent widths and asymmetries of C IV, N V and Si IV absorption profiles, the stellar wind in FY CMa is very strong, by comparison to other Be stars of similar spectral types, as noted by Grady et al. (1987). While no large variations are seen in the Si IV profiles recorded between 1981 and 1987 (Fig. 2), dramatic variations occurred in the C IV and in particular, in the N V profiles (Fig. 1). Especially the N V profiles are characterized by broad flat bottom absorption troughs in 1981 September and November, while multiple discrete absorption components located at about -30 and -450 km s-1, dominate N V profiles recorded both in 1982 January and in 1987 January (day 28). By contrast, one month after the 1987 January spectrum had been taken, in 1987 February 26 (day 57), N V and also C IV absorption profiles are distinctly different, showing evidence for simultaneous presence of outflowing and inflowing motions, most probably occurring in adjacent regions of the visible circumstellar hemisphere in front of FY CMa. While outflow is observed at N V extending from about -250 to at least -540 km s-1 and at C IV from about -175 km s-1 to around -1500 km s-1, inflow is visible both in C IV and N V at velocities extending from about 0 to +200 km s-1.
FIGURE 2. Circumstellar doublet lines Si IV 1393.755,1402.770 Angstrom had no obvious variation during Jan. 28 - Feb. 26, 1987. It is interesting to note that infalling motion in about the same velocity range was also clearly visible in an optical He I 5876 profile of the star showing inverse P Cygni-type structure (Fig. 3), as obtained already on 1987 February 12 (day 43) and described earlier by Dachs et al. (1992). The sequence of the optical He I 5876 profile taken 1987-43 and the ultraviolet N V 1239 Angstrom profile measured 1987-57 point to an extended phase of infalling motions of highly excited plasma (at electron temperature T > 105 K) from the circumstellar envelope of the Be star raining down to the stellar surface and lasting at least 14 days (Grady et al. 1988). In many other IUE spectrograms of this star, absorption by infalling matter can be detected at similar velocities ranging between +30 and +100 km s-1 from transitions of ions in lower-ionization states, e.g. from Si III 1299.0, 1303.3 1892.0 Angstrom or Fe III 1895.5 Angstrom, pointing to infall of relatively cool plasma. This can be seen, e.g., 1982 January or, as noted by Grady et al. (1988), in 1987 May. Terminal velocities of the expanding wind flows read from the profiles average around -950 km s-1 for the Si IV ion and approximately -1450 km s-1 for C IV. Available medium-resolution H-alpha emission-line profiles collected between 1981 and 1987 usually indicate double-peak structure of the emission, with violet-to-red peak intensity ratios varying between V/R ~ 1.02 and 1.14. Equivalent widths of H-alpha line emission recorded range between We = -7.3 Angstrom measured on 1985 April 07 (R. Hanuschik, unpublished) and -14.5 Angstrom found in 1981 November-December (Dachs et al. 1986).
FIGURE 3. Infalling motion in the circumstellar gas surrounding FY CMa is indicated by the inverse P Cygni profile shown by the He I 5876 Angstrom line measured 1987 Feb. 11/12. DiscussionThe ultraviolet and optical observations clearly show that an abrupt change observed in the circumstellar region around FY CMa involved the simultaneous presence of both infall and outflow of material (Peters 1988). The data which we collected showed that this activity sustained at least 30 days. Combining our observation with Peters' data it can be estimated that this activity persisted more than 3 months. Similar sudden spectral variations have been seen in pre-main sequence objects in which accretion of matter is occurring, but such bursts of activity have rarely been observed in classical Be stars. A more plausible explanation of the abrupt activity is probably a sudden accretion event in the circumstellar envelope, as evidence for infall of matter is clearly seen in several species of moderate ionization. Such spectral change could be due to magnetic activity, i.e., magnetic flares or loops. FY CMa perhaps has large magnetic loops or jets with a flow of material that originates near one of the poles and falling down upon the star in the line of sight. In this case the line profile presents strong blue shift with an inverse P Cygni-type feature. References
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