A and F stars with circumstellar dust. Ph. D. Thesis T. Sivarani [Ph.D Thesis]

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 2000

Cirumstellar dust plays an important role in stellar evolution. From the analysis
of IRAS (Infra Red Astronomial Satellite) data of several A and F stars whi
h are young (pre-mainsequene) and evolved (AsymptotiGiant Branh (AGB) and post-
AGB), showed the presene ofirumstellar dust shells.theirumstellar dust around starshanges the spetral energy distribution aswell as the spetral line features of the photosphere. Some stars show shell lines. Insomeases theirumstellar dust plays a major role in understanding the peuliarhemialomposition of the photosphere. Studying the
irumstellar environment ofthese stars and the inuene ofirumstellar matter on the photospheri
spetrumhelps us to understand the pre-mainsequene and post-mainsequen
e evolution.Pre-mainsequene stars haveirumstellar envelopes as a natural proess of starformation. But as it reahes the mainsequene stage only some stars retain theirir
umstellar disk. These starsould probably be having a planetary system. Inonne
tion to the planetary formation, it is interesting to study these stars whihhaveirumstellar dust. Reently even the evolved stars whih is a proto-planetary nebula (Red Retangle Nebula (Jura and Turner, 1998)) seem to show eviden
e ofplanetary formation. After the disovery of planets around pulsars, this is the rsteviden
e of planets around a post-mainsequene star. A-F stars have the most simple
atmosphereompared to any other spetral type. Still there are only a few normal A
stars Bidelman says that "He has never seen any normal A-star". boo stars are one
of thehemially puliar stars seen in the mainsequene in the spe
tral range A-F.These stars seem to have got the peuliaromposition as a result of the inueneofirumstellar dust. They show depletion in metals and show normal C,N,O,S andZn abundanes. Many of the boo stars show IR (Infra Red) ex
ess due to dust.Some of the metal poor post-AGB stars also show similarharateristi
s like the
boo stars. And these post-AGB stars are asso
iated with
ir
umstellar dust shells.
So it is very essential to know that the metal poor
hara
ter is due to the population
II nature of the obje
t or due to fra
tionation or both.The pre-mainsequene stars and post-AGB stars have detahed dust shells. Someof the pre-mainsequene stars have similar IRASolors like that of Proto Plane-tary Nebulae (PPN). In order to distinguish the evolutionary stage of the various
type of stars withirumstellar dust shells and to understand the physial proesses(mass-loss, shoks, mixing, and frationation), we need toarry out high resolutionspetrosopistudy.