Characteristics of the hot components of symbiotic stars
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Sümbiootilised tähed on vastastikmõjustuvad kaksiksüsteemid, mille komponentideks on punane hiid ja kuum kompaktne täht, enamasti valge kääbus. Kääbus püüab hiiu rohket tähetuult ning ergastab seda, mille tulemusena tekivad süsteemi spektrisse kiirgusjooned ning ilmnevad mitmed nähtused nagu ketta moodustumine kuuma komponendi ümber ning sellest väljapaiskuvad kitsad ainejoad.
Selles väitekirjas on spektroskoopiliselt uuritud nelja võimalikult erinevat sümbiootilist tähte. Neis Z Andromedae on nn klassikaline sümbiootiline täht kuuma kaaslasega, millel esinevad iseloomulikud heleduse tõusud (pursked). AG Draconis on samuti hele süsteem nagu Z Andromedae ning sarnaste pursetega, tema doonortäht on aga sümbiootilise kaksiku kohta ebaharilikult kuum kollane hiid. CH Cygni ja EG Andromedae omavad vastupidiselt kahele eelmisele üsna nõrku valgeid kääbuseid. CH Cygni on muutlik täht, kuid tema ebaregulaarsed pursked ei sarnane klassikaliste sümbiootikute omadega. EG Andromedae on üks rahulikest sümbiootikutest, millel ühtki purset pole seni avastatud.
Igaüht neist neljast tähest vaatlesime vähemalt kümne aasta jooksul Tartu Observatooriumi 1,5-meetrise teleskoobiga. Selle aja jooksul olime tunnistajaks mitmetele Z Andromedae ja AG Draconise pursetele ning olulistele muutustele CH Cygni spektris. Vahest üllatavaimaks avastuseks olid kaks kitsaste jugade esinemise episoodi Z Andromedael, esmakordselt sel tähel. CH Cygni puhul leidsime tõendeid akretsiooniketta olemasolust 1998. aastal. Ainsad muutused EG Andromedae spektrites olid põhjustatud orbitaalsest liikumisest.
Leidsime, et mitte kõiki Z Andromedae ning AG Draconise heledusmaksimume ei saada ühesugused muutused spektris: mõne maksimumi ajal läheb täht kuumemaks, mõne ajal jahedamaks. Ketas CH Cygni ümber kinnitas, et sellise struktuuri teke on sümbiootikute puhul võimalik, olgugi teoreetiliselt raskesti seletatav. Lisaks sellele, kuna kitsaid jugasid seostatakse akretsioonikettaga, on jugade avastamine Z Andromedael samuti märk võimalikust kettast. Kuna seni on jugasid leitud peamiselt väikese heledusega sümbiootilistel tähtedel, Z Andromedae aga kuulub väga heledate hulka, võib see viidata sellele, et neid tekitab mingi ühine mehhanism ning nõrgad ja heledad sümbiootikud on üksteisele sarnasemad kui seni arvatud.
Symbiotic stars are interacting binaries whose components are a red giant and a small hot star, usually a white dwarf. The intensive stellar wind from the giant is captured by the companion, giving rise to strong emission lines in the spectra and a range of phenomena, which may include the formation of an accretion disk and the ejection of collimated jets. In this thesis, four symbiotic stars, as different as possible, were chosen for a spectral investigation of the symbiotic phenomenon. Of those, Z Andromedae is a so-called classical symbiotic star with a hot companion that shows a characteristic pattern of brightenings (outbursts). AG Draconis is a bright system like Z Andromedae and shows similar activity, but has an unusually hot yellow donor star. CH Cygni and EG Andromedae have, on the contrary, relatively dim white dwarfs. The former shows irregular outbursts, the origin of which is not easy to explain, the latter is one of the quiet symbiotic stars with no outburst yet recorded. Each of those four stars was observed for at least ten years with the 1.5-m telescope at Tartu Observatory. Several outbursts of Z Andromedae and AG Draconis were witnessed, as well as substantial changes in the CH Cygni spectra. The perhaps most surprising result was the discovery of collimated jets in Z Andromedae spectra on two instances, an event never observed in this star before. In CH Cygni, evidence for the existence of an accretion disk in 1998 was discovered. EG Andromedae stayed quiet and the only changes in its spectra could be ascribed to orbital motion. We found that not all the outbursts of Z Andromedae and AG Draconis are accompanied by similar changes in the spectra: during some brightenings the stars become hotter, during some, cooler. The existence of the disk in CH Cygni in 1998 affirms that the formation of such a structure is possible in symbiotic stars. Moreover, as the ejection of jets is associated to an accretion disk, the jets in Z Andromedae can also be regarded as an indirect proof of the existence of a disk at least for some time. The discovery of jets in Z Andromedae also indicates that the jets do appear not only in faint symbiotics as has been thought, but also in the bright ones, for which there has to be the same mechanism that produces them, similar in both bright and dim stars.
Symbiotic stars are interacting binaries whose components are a red giant and a small hot star, usually a white dwarf. The intensive stellar wind from the giant is captured by the companion, giving rise to strong emission lines in the spectra and a range of phenomena, which may include the formation of an accretion disk and the ejection of collimated jets. In this thesis, four symbiotic stars, as different as possible, were chosen for a spectral investigation of the symbiotic phenomenon. Of those, Z Andromedae is a so-called classical symbiotic star with a hot companion that shows a characteristic pattern of brightenings (outbursts). AG Draconis is a bright system like Z Andromedae and shows similar activity, but has an unusually hot yellow donor star. CH Cygni and EG Andromedae have, on the contrary, relatively dim white dwarfs. The former shows irregular outbursts, the origin of which is not easy to explain, the latter is one of the quiet symbiotic stars with no outburst yet recorded. Each of those four stars was observed for at least ten years with the 1.5-m telescope at Tartu Observatory. Several outbursts of Z Andromedae and AG Draconis were witnessed, as well as substantial changes in the CH Cygni spectra. The perhaps most surprising result was the discovery of collimated jets in Z Andromedae spectra on two instances, an event never observed in this star before. In CH Cygni, evidence for the existence of an accretion disk in 1998 was discovered. EG Andromedae stayed quiet and the only changes in its spectra could be ascribed to orbital motion. We found that not all the outbursts of Z Andromedae and AG Draconis are accompanied by similar changes in the spectra: during some brightenings the stars become hotter, during some, cooler. The existence of the disk in CH Cygni in 1998 affirms that the formation of such a structure is possible in symbiotic stars. Moreover, as the ejection of jets is associated to an accretion disk, the jets in Z Andromedae can also be regarded as an indirect proof of the existence of a disk at least for some time. The discovery of jets in Z Andromedae also indicates that the jets do appear not only in faint symbiotics as has been thought, but also in the bright ones, for which there has to be the same mechanism that produces them, similar in both bright and dim stars.
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