Other
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8462p&link_type=abstract
HST Proposal ID #8462
Other
Hst Proposal Id #8462 Stellar Ejecta
Scientific paper
We are following the remarkable event occurring in the central star of the LMC-PN N66. Before 1987, it showed a weak continuum with a T_eff 120000 K and L_bol 30000 L_sun and in few years its continuum increased by large factors and it developed strong WR features {P-Cygni profiles in NV 1240 A and CIV 1550 A lines, wide He II lines, etc.} resembling a WN4.5. The wind has persisted since 1990, showing some dramatic variations. HST FOS data of this object were obtained in Cycles 5 {prop. 6055} and 6 {prop. 6409} from which we have constructed expanding model atmospheres for the star; the best-fit for Cycle 6 data are: T_eff 93300 K, L_*=34000 L_sun, R_*=0.71 R_sun, mass loss rate = 10E-5 M_sun/yr, v_infty=2000 km/s. For Cycle 5 data the stellar parameters are the same, but the mass loss rate is slightly larger. The atmosphere is very H deficient {X/Y < 0.1}, with N abundance of 0.004 by mass and C and O abundances lower than 0.0002 by mass. This chemical composition is most unusual since all the known PNe with WR nuclei show He- and C-rich winds. Model atmospheres have been calculated for the pre-outburst epoch and for maximum stellar brightness} and we found that the stellar luminosity has not changed substantially with time. The atmospheric H-deficiency and the constant L_bol have allowed us to discard the thermal pulse possibility as the triggering mechanism of the outburst. We propose that the star is suffering a violent atmospheric instability with large variations of the mass loss rate. Long-slit STIS observations, covering the spectral range 1100-7000A were performed in January 99 {Cycle 7}. These data are being analized. Preliminar results show that since 1997 the star has been slowly fading. The stellar spectrum still shows WR features at NV 1240, CIV 1550, all the UV and visible HeII lines, and others, but much fainter than before showing that the stellar wind is diminishing. The `false photosphere' generated by the optically thick wind receds, while the surface temperature increases. To re-observe the object in Cycle 8 is paramount to follow its evolution, specially at UV. HST observations are needed to isolate the stellar emission from the nebular spectrum. Thus we can study the star and the nebula separately. Three CVZ orbits have been granted for Cycle 8. WE REQUEST THAT MAMA VISIT AND CCD VISIT ARE PROGRAMMED AS CLOSE AS POSSIBLE Qu estions
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