Beijing, March 2010 1

Variability of post-AGB objectsVariability of post-AGB objects

R. Szczerba & M. HajdukR. Szczerba & M. Hajduk

N. Copernicus Astronomical Center

Toruń, Poland

Kunming, March 2010

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TORUŃ - N. COPERNICUS (1473-1543)

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OVERVIEW

• Introduction: post-AGB objects

• ASAS & variability of post-AGBs

• Variability of 21 m emitting post-AGBs

• Concluding remarks

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Introduction: post-AGB objects

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(F.Herwig, 2005 – Mi = 2 Mo, Z=0.02)

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Introduction

Post-AGBs pulsate

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.

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:

http://www.ncac.torun/postabg2

Post-AGBs: some facts NCACTORUN

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ASAS & variability of post-AGBs

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

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

ASAS Sky Coverage (δ<28Deg North) (Pojmanski et al., AcA, 2005)

ASAS

Southern hemisphere – observed for ~10 years now

ASAS

ASAS photometry accuracy vs star brightness, (Pojmanski, AcA, 2002)

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.

Almost half of the sample is composd of RV Tau- type variables.

They found a strong correlation between V-band amplitudes and Teff.

Post-AGBs with ASAS NCACTORUN

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 ~150d – candidates for binaries: IRAS

07253-2001

IRAS 08005-2356

Some other show possible binary modulation in addition to the stellar

pulsation – e.g. V4728 Sgr

Most of them show several frequencies with rather well defined dominant frequency,

e.g. V0552 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, ApJ, 709, 1042)carried out ~14 years long V & R

monitoring of 12 post-AGBs with 21 m emission

e.g., IRAS 22272+5435 – see:

http://www.ncac.torun.p/postagb2

IRAS 22272+5435 – see: http://www.ncac.torun.p/postagb2

Hrivnak et al. (2010)

V – the amplitude of the variations decrease with Teff

Hrivnak et al. (2010)

P – period is becoming shorter when the object is hotter

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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.

Colder post-AGB objects have longer periods and the larger light variations (in agreement with the results of modelling by Fokin et al. 2001).

More model calculations, like those done by Fokin et al. (2001), especially for longer periods are needed.

Post-AGB stars are rather dim (mV>10-12 mag.), and due to their long periods, are (probably?) not good candidates for the SONG observations, but ...

Concluding remarks NCACTORUN

- there are several post-AGBs with mV < 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.

Concluding remarks NCACTORUN