Computer Science
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7482p&link_type=abstract
HST Proposal ID #7482
Computer Science
Hst Proposal Id #7482 Hot Stars And Stellar Corpses
Scientific paper
We propose to follow 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 emission, 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 central 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 IUE data of 1983 {pre-outburst epoch} and 1995 {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 which produces large variations of the mass loss rate. The spectrum changes with time scales of months, therefore it is paramount to re-observe the object in Cycles 7 and 8 to follow its evolution, specially at UV wavelengths. HST observations are needed to isolate the stellar emission from the nebular spectrum. We require long-slit STIS observations to cover from 1150 to 11000 Angstrom with a resolution of 1000 to 5000. Thus we can study the star and the nebula separately. Three orbits {CVZ} are requested for each cycle.
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