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A. Spagna, F. Cossu, M.G. Lattanzi, G. Massone INAF-Osservatorio Astronomico di Torino

52° Congresso Sait - Teramo. The nearby strongly reddened open cluster Stock2. A new study based on accurate proper motions and 2MASS photometry. A. Spagna, F. Cossu, M.G. Lattanzi, G. Massone INAF-Osservatorio Astronomico di Torino. Acknowledgments: A. Bragaglia, R. Drimmel, A. Vallenari.

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A. Spagna, F. Cossu, M.G. Lattanzi, G. Massone INAF-Osservatorio Astronomico di Torino

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  1. 52° Congresso Sait - Teramo The nearby strongly reddened open cluster Stock2.A new study based on accurate proper motions and 2MASS photometry A. Spagna, F. Cossu, M.G. Lattanzi, G. Massone INAF-Osservatorio Astronomico di Torino Acknowledgments: A. Bragaglia, R. Drimmel, A. Vallenari

  2. Open cluster discovered by J. Stock in 1955 and located in the Orion spiral arm in front of h& Persei • Age: • 100 Myr (Krzeminski & Serkowski 1967, Martini 1971) • 218 +/- 48 Myr (40 stars with B-V<0.05) Piskunov 1980 • 100 Myr Sanner & Geffert (2001) • 170 Myr (3 T.O. stars) Kharchenko et al (2005) • 600-800 Myr (Rosat X-ray LF/ARAP) Sciortino et al (2000) • Distance: • d = 302 pc (Martini 1971) • d = 345  70 pc Hipparcos  (5 stars) (Robichon 1999) • d = 316 (Sanner & Geffert 2001) • d = 380 pc Kharchenko et al (2005) • Metallicity: • [Fe/H] = -0.14  0.20 Claria et al. (1996)

  3. Interstellar extinction: Strong E(B-V) ~ 0.30-0.38 mag, on average  (Av ~ 4 mag/kpc) … and patchy E(B-V) min/Max = 0.15 – 0.50 R = Av / E(B-V) = 3.4 – 3.5 Reddening map (K&S ‘67)

  4. Astrometric & Photometric data • OATo proper motion survey • Area = 70’ x 70’ (expected full size > 3°x3°) • Wide collection of photographic plates: Astrographic Catalog (Vatican Zone), OATo REOSC e Morais, Palomar POSS-I and Quick-V) • T = 80 years • 1782 stars with V < 15-16 mag • () ~ 1 mas/yr comparable to Hipparcos and better than UCAC (NA) 2MASS photometry J ~ 15.8 mag H ~ 15.1 mag Ks ~ 14.3 mag (nominal limits for SNR = 10 )

  5. Kinematic membership Quite easy, thanks to the large field – cluster separation and accurate . 275 “bona fide” cluster members Field stars  (mas/yr) Cluster (=1.5 mas/yr !!)  cos  (mas/yr)

  6. Reddening correction Method: J-H vs. H-Ks of MS stars (Knude & Fabricius 2003) Problem: significant photometric errors ! (low reddening SNR) Reddening law: Cardelli et al (1989) with R = 3.5 (Martini et al 1971)

  7. Reddening map Mean reddening over 6x6 subregion (11’x11’) [Implicitly assumed: foreground ISM] <AJ>=0.31 <E(B-V)> = 0.30 (estimated)

  8. Distance determination Best-fit of Main Sequence (0.05<J-H<0.4) with isochrones from Padova database (Marigo et al. 2008) 178 Myr

  9. Age determination • Large CMD scatter due to • photometric errors (0.03-0.04 mag) • residual differential extinction • intracluster extinction? • binaries • distance spread ((m-M) ~ 0.1) • Age not well constrained by 2MASS photometry • CMD consistent with 200-500 Myr isochrones 178 Myr 300 Myr 500 Myr Notice the large scatter of the three giants: contamination? prolonged star formation?

  10. CMD of 11’x11’ subregions J (mag) Hp: extinction almost constant within 11’x11’ subregions J-H J-H A(J)=0.34 E(J-H)=0.12

  11. CONCLUSIONS • New catalog containing 1782 stars (V<15-16) with accurate proper motions (~ 1.6 mas/yr) and 2MASS JHKs magnitudes (~0.03 mag). • We identified 275 Stock 2 cluster members with a membership probability greater than 0.9 (265 members have a probability > 0.99). • We derive a reddening map (11.7’ resolution) whose average is consistent with literature values (E(B-V)=0.3÷0.38), but it is not able to reduce significantly the CMD scatter. • distance of 350 pc for Z=0.019, or 330 pc for Z=0.013. • Age not well constrained, but seems consistent with older ages (300-500 Myr)  Reddening map to be improved by means of optical CMD  Cluster giants to be investigated individually  Wider and deeper  survey based on GSC-II material (to V=19)

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