 a & Broadband photometry of open clusters Martin Netopil Institute of Astronomy, University of Vienna - Austria AIPW Dubrovnik

AIPW – Dubrovnik /13 Why open clusters? They are physically related groups of stars held together by gravitational attraction and were formed at roughly the same time. Their evolutionary stages range from clouds where star formation is still ongoing to very old aggregates (~10Gyr). Open clusters offer therefore an unique and easy possibility to examine  the galactic structure as well as  distribution and evolution of several star groups (e.g. Be, CP, WR stars, WD)

AIPW – Dubrovnik /13 What is known/missing? ~1700 clusters (or candidates) known in the Milky Way. ~850 clusters with „known“ distance / age / reddening ~300 clusters with „known“ distance / age / PM and RV ~140 clusters with abundances what is missing:  therefore large efforts are necessary for a reliable image next current and future milestones: SDSS (SEGUE), COROT, GAIA, int. research group for open clusters

AIPW – Dubrovnik /13 The current status of open cluster parameters Paunzen & Netopil (2006) examined the accuracy of ~ 6400 individual parameter estimates of 395 open clusters using clusters with ≥3 determinations each, compiling a set of 72 most accurate clusters for testing isochrones etc. ~90% cl.: error of E(B-V) < 0.1mag ~80% cl.: absolute error in distance < 20% ~10% cl.: absolute error in age < 20%; >30% cl. exhibit error in age > 50% with extreme values larger than 200%!  severe problem: age determination

AIPW – Dubrovnik /13 What are CP stars?  discovery in 1897 by Antonia Maury  peculiar and often variable line strengths  overabundances for heavy elements such as Silicon, Chromium, Strontium and Europium According to Preston (1974) the CP stars were divided into the following groups:

AIPW – Dubrovnik /13 What are CP stars?

AIPW – Dubrovnik /13 The tool of  a photometry Photometry offers an easy possibility for investigating large samples of CP stars, especially through the discovery of characteristic broadband absorption features, the most suitable of them located around 5200Å. g 1 : 5027Å / 222Å g 2 : 5205Å / 107Å y: 5509Å / 120Å The index (g 1 -y) shows an excellent correlation with (b – y) or (B – V) and can be used as an index for the effective temperature (Paunzen, Schnell, Maitzen 2005, 2006). An y versus (g 1 -y) diagram (= cmd) for a cluster can then easily sorts out non-members. Assuming that all stars exhibit the same interstellar reddening, peculiar objects deviate from the normality line more than 3 .

AIPW – Dubrovnik /13 The tool of  a photometry Netopil, Paunzen, Maitzen et al. (2006) submitted advantages: Isochrones available (Claret et al. 2003, Claret 2004) ; accurate photometry (~10 frames each => time consuming); reduction „relative“ simple using available packages (apphot, daophot); no standard fields necessary

AIPW – Dubrovnik /13  a open cluster survey Photoelectric survey: 14 papers until now (first: Maitzen, Hensberge 1981 ; for the time being last: Maitzen 1993 ) ~1240 stars in 38 open clusters => 62 CP candidates CCD survey: 8 papers (2000 – 2006) until now ~11340 stars in 45 open clusters => ~ 90 CP candidates data collection: 17 different telescopes (ESO, CTIO, CASLEO, Hvar etc.) A statistical analysis of the sample is in preparation. However, 83 clusters are still a small sample if one intend to examine their galactic distribution.

AIPW – Dubrovnik /13  a in extragalactic environments  First CP2 discovery by Maitzen et al. (2001) in LMC via  a  Further investigations by Paunzen et al. (2005, 2006) confirm an occurrence of CP stars about half than in the Milky Way (2.2 / 5%). already in preparation:  Spectroscopic verification of detected CP stars in the LMC   a photometry of a 30‘ field (~ stars) within the SMC (ESO 2.2m – WFI) image reduction problems:  extreme crowded fields together with instrumental resolution and seeing conditions makes it impossible to resolve the innermost cluster regions.

AIPW – Dubrovnik /13 Broadband photometry Since about ½ of the known open clusters are not or only marginally investigated up to now, we have started a small survey (BVR photometry) to fill some of the white spots on the galactic map. Netopil, Paunzen, Maitzen et al. (2005): NGC 7296 Netopil, Maitzen, Paunzen et al. (2006): Basel 11b, King 14, Czernik 43 Netopil, Maitzen, Paunzen et al. (in prep.): Czernik 15, Czernik 23, NGC 743 mostly observed for the first time. main aim: determining cluster properties using reasonable magnitude limits based on more observations in each filter => accuracy. used observatories:Figl Observatory for Astrophysics (Austria) Hvar Observatory (Croatia)

AIPW – Dubrovnik /13 Broadband photometry reduction „relative“ simple using available packages (apphot, daophot) „problems“: standard transformation, atmospheric extinction observing strategy: all observations at low airmass back-to-back Netopil, Maitzen, Paunzen et al. (2006)

AIPW – Dubrovnik /13 WEBDA – The database WEBDA (maintained by Ernst Paunzen) is a site devoted to observational data on stars in galactic open clusters. It is intended to provide a reliable image of the available data and knowledge on these objects and to offer a wide access to the existing observations including: stars / individual measurements of 1100 clusters photometry (different systems), astrometric & spectroscopic data, membership probabilities, references etc. using a homogenized star numbering system for an easy identification and comparison. Also available are data of SMC clusters.