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 52° Congresso Sait - Teramo Acknowledgments: A. Bragaglia, R. Drimmel, A. Vallenari

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 Myr Sanner & Geffert (2001) 170 Myr (3 T.O. stars) Kharchenko et al (2005) 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] =  0.20 Claria et al. (1996)

Interstellar extinction: Reddening map (K&S ‘67) Strong E(B-V) ~ 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

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 < 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 )

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

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)

Reddening map [Implicitly assumed: foreground ISM] Mean reddening over 6x6 subregion (11’x11’) =0.31 = 0.30 (estimated)

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

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

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

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= Age not well constrained, but seems consistent with older ages ( 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)