Variability of post-AGB objects R. Szczerba & M. Hajduk N. Copernicus Astronomical Center Toruń, Poland Beijing, March 2010 1
NCAC TORUN Kunming, March 2010 TORUŃ - N. COPERNICUS (1473 -1543) 2
NCAC TORUN OVERVIEW • Introduction: post-AGB objects • ASAS & variability of post-AGBs • Variability of 21 mm emitting post-AGBs • Concluding remarks 3
Introduction: post-AGB objects 4
(F. Herwig, 2005 – Mi = 2 Mo, Z=0. 02) 5
Post-AGBs pulsate Introduction 6
NCAC TORUN Post-AGBs: some facts Prior to the IRAS satelite (1983), post-AGBs remained as an observational „missing link” in the post-MS evolution of low- and intermadiate-mass stars. Ø Ø IRAS allowed to identify about 50 of them Now we know about 470 (including RV Tau stars – counted as post-AGBs: Jura, 1986) post-AGBs see: Ø http: //www. ncac. torun/postabg 2
ASAS & variability of post-AGBs 8
ASAS & OGLE telescopes The All Sky Automated Survey (ASAS) system is fully automated system of telescopes located at Las Campanas, Chile. On site assistance by the OGLE observers (the OGLE telescope is visible in the background of the photograph).
ASAS is a low cost project dedicated to constant photometric monitoring of the whole available sky, which is approximately 10^7 stars brighter than 14 magnitude. The project's ultimate goal is detection and investigation of of any kind of the photometric variability. One of the main objectives of ASAS is to find and catalogue variable stars.
ASAS Presently, ASAS consists of two observing stations, one in LCO, Chile (since 1997) and the other on Haleakala, Maui, Hawaii (since 2006). Both are equipped with two wide-field 200/2. 8 instruments, observing simultaneously in V and I band.
ASAS Sky Coverage (δ<28 Deg North) (Pojmanski et al. , Ac. A, 2005)
ASAS Southern hemisphere – observed for ~10 years now
ASAS photometry accuracy vs star brightness, (Pojmanski, Ac. A, 2002)
NCAC TORUN Post-AGBs with ASAS Kiss et al. (2007) analyzed photometric (light curves) data for 29+2 (ASAS + NSVS) (suspected) binary post. AGBs from De Ruyter et a. (2006) to determine pulsational and orbital periods. Ø Almost half of the sample is composd of RV Tau- type variables. Ø They found a strong correlation between V-band amplitudes and Teff. Ø Ø
Kiss et al. (2007) Teff is a good indicator of the post-AGB evolutionary phase open circles – single periodic stars triangles – multiperiodic / semiregular stars squares –variability due to orbital motion
Kiss et al. (2007) Post-AGB objects on H-R diagram together with “classical” instability strip from Christensn-Dalsgaard (2003)
22 post-AGB stars from our catalogue (other than RV Tau type & no-analyzed by KISS et al (2007)) were detected in ASAS database.
Red Rectangle – known binary
Two stars have dominant period longer than ~150 d – candidates for binaries: IRAS 07253 -2001
IRAS 08005 -2356
Some other show possible binary modulation in addition to the stellar pulsation – e. g. V 4728 Sgr
Most of them show several frequencies with rather well defined dominant frequency, e. g. V 0552 Pup
or BT Lib
Temperature (spectral type) was determined for 10 out of 22 object rather weak (no? ) correlation of the period with the temperature, but …
Hrivnak et al. (2010, Ap. J, 709, 1042) carried out ~14 years long V & R monitoring of 12 post-AGBs with 21 mm emission
e. g. , IRAS 22272+5435 – see: http: //www. ncac. torun. p/postagb 2
IRAS 22272+5435 – see: http: //www. ncac. torun. p/postagb 2
Hrivnak et al. (2010) DV – the amplitude of the variations decrease with Teff
Hrivnak et al. (2010) P – period is becoming shorter when the object is hotter
Concluding remarks 31
NCAC TORUN Concluding remarks All (? ) post-AGBs are variable stars, with period from about 30 up to more than 150 days (!) – this upper limit is surprising since evolutionary calculations predict maximum period shorther than about 100 days. Ø Colder post-AGB objects have longer periods and the larger light variations (in agreement with the results of modelling by Fokin et al. 2001). Ø More model calculations, like those done by Fokin et al. (2001), especially for longer periods are needed. Ø Post-AGB stars are rather dim (m. V>10 -12 mag. ), and due to their long periods, are (probably? ) not good candidates for the SONG observations, but. . . Ø
NCAC TORUN Ø Concluding remarks - there are several post-AGBs with m. V < 7 -8 mag. , & - while searching for plantes in crowded regions with SONG, we can get precise photometry for coincident post -AGBs as a by-product. Ø Ø
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