J/A+A/419/149 NGC 6996 (Villanova+, 2004) ================================================================================ Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula Villanova S., Baume G., Carraro, G., Geminale, A. =2004A&A...419..149V ================================================================================ ADC_Keywords: Clusters, open ; Photometry, UBVI Keywords: Galaxy: open clusters and associations: individual: NGC 6996 - Galaxy: open clusters and associations: general - Stars: imaging - Stars: luminosity function, mass function Description: We present and discuss broad band CCD UBV(I)c photometry and low resolution spectroscopy for stars in the region of the open cluster NGC 6996, located in the North America Nebula. The new data allow us to tightly constrain the basic properties of this object. We revise the cluster size, which in the past has been significantly underestimated. The width of the Main Sequence is mainly interpreted in terms of differential reddening, and indeed the stars' color excess E(B-V) ranges from 0.43 to 0.65, implying the presence of a significant and evenly distributed dust component. We cross-correlate our optical photometry with near infrared photometry from 2MASS, and by means of spectral classification we are able to build extinction curves for an handful of bright members. We find that the reddening slope and the total to selective absorption ratio Rv toward NGC 6996 are anomalous. Moreover the reddening-corrected colors and magnitudes allow us to derive estimates for the cluster distance and age, which turn out to be 760 ± 70 pc (V0-MV = 9.4 ± 0.2) and ~350 Myr, respectively. Based on our results, we suggest that NGC 6996 is located in front of the North America Nebula, and does not seem to have any apparent relationship with it. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- × ReadMe 80 . This file × table2.dat 208 2699 Photometric catalogue of the NGC 6996 region -------------------------------------------------------------------------------- Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 5- 8 I4 --- Seq Identification number (this work) 15- 17 A3 --- Id_ZS Numbering from Z&S1990 (1) 21- 26 F6.1 pix Xpos X position in arcseconds (2) 30- 35 F6.1 pix Ypos Y position in arcseconds (2) 39- 40 I2 hours RA[h] Right Ascension_2000 [hours] 42- 43 I2 min RA[m] Right Ascension_2000 [minutes] 45- 48 F4.1 sec RA[s] Right Ascension_2000 [seconds] 51- 53 I3 deg Dec[d] Declination_2000 [degrees] 55- 56 I2 min Dec['] Declination_2000 [arcminutes] 58- 61 F4.1 sec Dec[''] Declination_2000 [arcseconds] 66- 70 F5.2 mag Vmag V magnitude 72- 73 A2 --- n_Vmag Comments for V magnitude (3) 78- 82 F5.2 mag B-V ? B-V colour index 84- 85 A2 --- n_B-V Comments for B-V index (3) 90- 94 F5.2 mag U-B ? U-B colour index 96- 97 A2 --- n_U-B Comments for U-B index (3) 102-106 F5.2 mag V-Ic ? V-Ic colour index 108-109 A2 --- n_V-Ic Comments for V-Ic index (3) 111-112 A2 --- Memb Membership assignment (4) 123-127 F5.2 mag E(B-V) ? B-V colour excess 132-136 F5.2 mag E(U-B) ? U-B colour excess 141-145 F5.2 mag E(V-I) ? V-Ic colour excess 147-152 A6 --- SC Spectral Classification 156-171 A16 --- 2MASS 2MASS identification 175-190 A16 --- TYC TYCHO-2 identification 193-208 A16 --- OtherId BD, BSC and/or GSC identification -------------------------------------------------------------------------------- Note (1): Z&S1990 = Zdanavicius & Straizys 1990, Ap&SS 173, 309 Stars separated in two components are identified as 'a' and 'b' Note (2): Adopted center: RA_2000 = 20:56:30; Dec_2000 = +44:38:00 Note (3): Comments on photometry: (: ) error larger than 0.05 (::) error larger than 0.10 Note (4): Membership assignment: lm = likely member pm = probable member nm = non-member ================================================================================ (End) Gustavo L. Baume