Bibcode
Parthasarathy, M.; García-Lario, P.; Sivarani, T.; Manchado, A.; Sanz Fernández de Córdoba, L.
Referencia bibliográfica
Astronomy and Astrophysics, v.357, p.241-254 (2000)
Advertised on:
5
2000
Revista
Número de citas
34
Número de citas referidas
24
Descripción
SAO 85766 (b = +20o) is an IRAS source with far-infrared
colours similar to planetary nebulae. According to the HDE catalogue,
its spectrum in 1940 was that of an A5 star. The UV fluxes and colours
derived from data obtained by the TD1 satellite in 1972 also indicate
that SAO 85766 was an A-type supergiant at that epoch. However, high
resolution spectra of SAO 85766 obtained in 1993 in the wavelength
interval 4350Ä to 8820Ä shows that now it is similar to that
of an early B type post-AGB supergiant. In addition to the absorptions
lines typical of a B1I type star, the spectrum of SAO 85766 is found to
show numerous permitted and forbidden emission lines of several
elements, typically observed in the spectra of young high density low
excitation planetary nebulae. From an analysis of the absorption lines
we have estimated Teff=22000+/-500 K, log g=3.0+/-0.5, xi
t=15+/-2km s-1 and [M/H]=-0.6. Carbon is found to
be strongly underabundant ([C/Fe] = -1.0), similarly to what has been
observed in other high galactic latitude hot post-AGB stars. The
underabundance of carbon and metals, high galactic latitude, high radial
velocity (46 km s-1), the presence of planetary nebula type
detached cold circumstellar dust shell and also the presence of low
excitation nebular emission lines in the spectrum indicate that SAO
85766 is a low mass star in the post-AGB stage of evolution. The above
mentioned characteristics and the variations observed in the spectrum of
SAO 85766 suggest that it has rapidly evolved during the past 50 years
and it is now in the early stages of the planetary nebula phase. The
central star may just have become hot enough to photoionize the
circumstellar envelope ejected during the previous AGB phase. >From
an analysis of the nebular emission lines we find Te=10000+/-
500K and Ne=2.5 104 cm-3. The nebula
also shows an abundance pattern similar to that of the central star. The
rapid post-AGB evolution of SAO 85766 appears to be similar to that
observed in the case of hot post-AGB star Hen 1357 (= SAO 244567). Based
on observations made with the Issac Newton Telescope (INT) operated on
the island of La Palma (Spain) by the Instituto de Astrofísica de
Canarias in the Spanish Observatorio del Roque de Los Muchachos