================================================================================ PSF-based approach to Kepler/K2 data. I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158. Libralato M., Bedin L. R., Nardiello D., Piotto G. (2016, MNRAS, 456, 1137) ================================================================================ Keywords: Instrumentation: Techniques: Image processing, Photometric / Binaries: General / Stars: Variables: General / Open clusters and associations: Individual: M 35, NGC 2158 Abstract: Kepler and K2 data analysis reported in the literature is mostly based on aperture photometry. Because of Kepler's large, undersampled pixels and the presence of nearby sources, aperture photometry is not always the ideal way to obtain high-precision photometry and, because of this, the data set has not been fully exploited so far. We present a new method that builds on our experience with undersampled HST images. The method involves a point-spread function (PSF) neighbour-subtraction and was specifically developed to exploit the huge potential offered by the K2 ``super-stamps'' covering the core of dense star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed during the K2 Campaign 0. We present our PSF modeling and demonstrate that, by using a high-angular-resolution input star list from the Asiago Schmidt telescope as the basis for PSF neighbour subtraction, we are able to reach magnitudes as faint as Kp~24 with a photometric precision of 10% over 6.5 hours, even in the densest regions. At the bright end, our photometric precision reaches ~30 parts-per-million. Our method leads to a considerable level of improvement at the faint magnitudes (Kp>15.5) with respect to the classical aperture photometry. This improvement is more significant in crowded regions. We also extracted raw light curves of ~60 000 stars and detrended them for systematic effects induced by spacecraft motion and other artifacts that harms K2 photometric precision. We present a list of 2133 variables. Description: We release: (1) an astro-photometric catalogue of all sources analyzed in our work on K2/Campaign 0/Channel 81 (AIC catalog) (2) the corresponding light curves of the objects in the field (3) a catalogue for all variable stars found in M 35 and NGC 2158 field (4) a stacked image of K2/Campaign 0/Channel 81 field File Summary: ----------------------------------------------------------------------------------------------------------- FileName recl Records Explanations ----------------------------------------------------------------------------------------------------------- ReadMe 118 . This file m35_n2158_N.dat 51 75935 AIC catalog LC.?????? 205 2422 Light curves in M35 & NGC2158 field from K2 data (75935 files) K2_var.dat 297 2849 Variable stars in M35 & NGC2158 field K2_C0_CHANNEL_81.fits 0 0 K2 Stacked image (FITS file) ----------------------------------------------------------------------------------------------------------- Byte-by-byte Description of file: m35_n2158_N.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 5- 12 F8.5 deg RA Right ascension (J2000) 18- 26 F9.6 deg DEC Declination (J2000) 32- 39 F7.4 mag Nmag Instrumental "white light" magnitude 42- 47 I6 --- ID ID number of the star 51- 51 I1 --- FLAG [0/1] K2-field flag (1) -------------------------------------------------------------------------------- (1) 0 = outside K2 field 1 = inside K2 field Byte-by-byte Description of file: LC.?????? ---------------------------------------------------------------------------------------------------------------------- Bytes Format Units Label Explanations ---------------------------------------------------------------------------------------------------------------------- 5- 21 F18.9 day TIME Average BKJD for a given K2 long-cadence reconstructed exposure 25- 36 F12.8 mag M1_det Detrended K2 magnitude obtained with 1-pixel aperture photometry 40- 51 F12.8 mag M1_raw Raw K2 magnitude obtained with 1-pixel aperture photometry 55- 66 F12.8 mag M3_det Detrended K2 magnitude obtained with 3-pixel aperture photometry 70- 81 F12.8 mag M3_raw Raw K2 magnitude obtained with 3-pixel aperture photometry 85- 96 F12.8 mag M5_det Detrended K2 magnitude obtained with 5-pixel aperture photometry 100-111 F12.8 mag M5_raw Raw K2 magnitude obtained with 5-pixel aperture photometry 115-126 F12.8 mag M10_det Detrended K2 magnitude obtained with 10-pixel aperture photometry 130-141 F12.8 mag M10_raw Raw K2 magnitude obtained with 10-pixel aperture photometry 145-156 F12.8 mag MPSF_det Detrended K2 magnitude obtained with PSF-fitting photometry 160-171 F12.8 mag MPSF_raw Raw K2 magnitude obtained with PSF-fitting photometry 178-186 F12.4 pxl X x position in the corresponding K2 long-cadence reconstructed exposure (in K2 pixel) 193-201 F12.4 pxl Y y position in the corresponding K2 long-cadence reconstructed exposure (in K2 pixel) 205-205 I1 --- FLAG [0/1] Clean light-curve flag (1) ---------------------------------------------------------------------------------------------------------------------- (1) To obtain a clean light curve as described in Sect. 6.2, use only the points with FLAG = 1. Byte-by-byte Description of file: K2_var.dat ------------------------------------------------------------------------------------------------ Bytes Format Units Label Explanations ------------------------------------------------------------------------------------------------ 2- 6 I5 --- ID ID number of the star from AIC catalog 19- 27 F9.6 deg RA Right ascension (J2000) 40- 48 F9.6 deg DEC Declination (J2000) 53- 61 F9.4 pxl X x position 66- 74 F9.4 pxl Y y position 80- 87 F8.4 mag Mag_Sch Instrumental "white light" magnitude from AIC 94-100 F7.4 mag Mag_K2 Calibrated median K2 magnitude from the light-curve 107-107 I1 --- Phot_K2 K2-light-curve photometry flag (1) 114-114 I1 --- Clas_K2 Light-curve classification flag (2) 122-133 F12.8 day Period Period (3) 145-147 I3 --- ID_Nar ID number from Nardiello et al. (2015) (4) 160-161 I2 --- ID_Hu ID number from Hu et al. (2005) (5) 174-175 I2 --- ID_Hu ID number from Jeon & Lee (2010) (6) 188-189 I2 --- ID_Kim ID number from Kim et al. (2004) (7) 201-203 I3 --- ID_Mei ID number from Meibom, Mathieu & Stassun (2009) (8) 216-217 I2 --- ID_Moc ID number from Mochejska et al. (2004,2006) (9) 225-232 F8.4 mag B Calibrated B magnitude from Nardiello et al. (2015) (10) 238-245 F8.4 mag V Calibrated V magnitude from Nardiello et al. (2015) (11) 251-258 F8.4 mag R Calibrated R magnitude from Nardiello et al. (2015) (12) 264-271 F8.4 mag J_2MASS Calibrated J_2MASS magnitude from Nardiello et al. (2015) (13) 277-284 F8.4 mag H_2MASS Calibrated H_2MASS magnitude from Nardiello et al. (2015) (14) 290-297 F8.4 mag K_2MASS Calibrated K_2MASS magnitude from Nardiello et al. (2015) (15) ------------------------------------------------------------------------------------------------ (1) 1 = 1-pixel aperture 2 = 3-pixel aperture 3 = 5-pixel aperture 4 = 10-pixel aperture 5 = PSF (2) 0 = high probability that it is a blend 1 = candidate variable 2 = difficult to classify 30 = star marked as ``difficult to classify'' and that, by comparison with the literature, could be a possible blend 31 = star marked as ``difficult to classify'', but for which we found a correspondence in the literature 32 = star we classified as candidate variable but it is close to a variable star from the literature and that seems a possible blend (3) If Period = -99.0, the star could be either irregular or was added in the catalogue after the match with the literature (Sect. 8.2) (4) If ID_Nar = -1, the star was not found in Nardiello et al. (2015) catalogue (5) If ID_Hu = -1, the star was not found in Hu et al. (2005) catalogue. The name of the stars in Hu et al. catalogue starts with 'V' (6) If ID_Hu = -1, the star was not found in Jeon & Lee (2010) catalogue. The name of the stars in Jeon & Lee catalogue starts with 'V' (7) If ID_Kim = -1, the star was not found in Kim et al. (2004) catalogue. The name of the stars in Kim et al. catalogue starts with 'V' (8) If ID_Mei = -1, the star was not found in Meibom, Mathieu & Stassun (2009) catalogue (9) If ID_Moc = -1, the star was not found in Mochejska et al. (2004,2006) catalogue. The name of the stars in Mochejska et al. catalogue starts with 'V' (10) B = -99.0 if not found in Nardiello et al. (2015) catalogue (11) V = -99.0 if not found in Nardiello et al. (2015) catalogue (12) R = -99.0 if not found in Nardiello et al. (2015) catalogue (13) J_2MASS = -99.0 if not found in Nardiello et al. (2015) catalogue (14) H_2MASS = -99.0 if not found in Nardiello et al. (2015) catalogue (15) K_2MASS = -99.0 if not found in Nardiello et al. (2015) catalogue