The Main Sequence and a White Dwarf Sequence in the Globular Cluster NGC 6397

Abstract

view Abstract Citations (96) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Main Sequence and a White Dwarf Sequence in the Globular Cluster NGC 6397 Cool, Adrienne M. ; Piotto, Giampaolo ; King, Ivan R. Abstract We present a deep color-magnitude diagram of the nearby globular cluster NGC 6397, from V and I images obtained with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope (HST). A narrow main sequence extending ∼10 mag below the turnoff is delineated, as well as a white dwarf sequence spanning 3.5 mag. The main sequence has a 1 σ full width of 0.04 mag down to V555 ≃ 24, and can be distinguished from field stars to a limit of V555 ∼ 26 (Mv ∼ 14). This corresponds to 0.10 Msun, which is close to the hydrogen-burning limit for this low-metallicity cluster. The main-sequence luminosity function rises to a maximum at I814 ≃ 20.5 (MI ≃ 8.5) and then drops by a factor of ∼3 over the next 3 mag. The corresponding mass function either rises to the low-mass limit of the observations or flattens out at the very low mass end, depending on the mass-luminosity relation adopted. There is no sign of an end to the main sequence in the present data. The white dwarf sequence contains ∼40 white dwarfs with Mv 10.5-14. The brightest ∼15 white dwarfs form a well-defined sequence that is well matched in position and shape to theoretical cooling sequences for white dwarfs with masses of 0.55±0.05 Msun, assuming standard values of the distance and reddening of the cluster. The 0.03 mag spread in colors along the upper ∼2.5 mag of the sequence is consistent with being due to measurement error alone. The intrinsic dispersion in white dwarf masses is no more than 0.05 Msunand is likely to be significantly smaller. The observed white dwarfs have cooling ages of ∼0.1-2 Gyr and total numbers that are in keeping with their expected rate of formation given the current rate of evolution past the tip of the red giant branch. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177723 Bibcode: 1996ApJ...468..655C Keywords: GALAXY: GLOBULAR CLUSTERS: INDIVIDUAL NGC NUMBER: NGC 6397; STARS: HERTZSPRUNG-RUSSELL DIAGRAM; STARS: LOW-MASS; BROWN DWARFS; STARS: LUMINOSITY FUNCTION; MASS FUNCTION; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (6) MAST (1) ESA (1)