Far-ultraviolet observation of the globular cluster NGC 6397

Abstract

We present an observational far-UV (FUV) and near-UV (NUV) study of the core region of the globular cluster NGC 6397. The observations were obtained with the Space Telescope Imaging Spectrograph (STIS, FUV), and the Wide Field Camera 3 (WFC3, NUV) on board the Hubble Space Telescope. Here, we focus on the UV bright stellar populations such as blue stragglers (BSs), white dwarfs (WDs) and cataclysmic variables (CVs). We present the first FUV − NUV color-magnitude diagram (CMD) for this cluster. To support our classification of the stellar populations, we compare our FUV − NUV CMD with optical data from the ACS Survey of Galactic Globular Clusters. The FUV − NUV CMD indicates 16 sources located in the WD area, and ten BSs within the 25 ′′ × 25 ′′ of the STIS FUV data. Eighteen Chandra X-ray sources are located within the FUV field of view. Thirteen of those have a NUV counterpart, of which nine sources also have a FUV counterpart. Out of those, five sources are previously suggested CVs, and indeed all five are located in the WD/CV region in our FUV − NUV CMD. Another CV only has a FUV but no NUV counterpart. We also detect a NUV (but no FUV) counterpart to the MSP located in the core of this cluster. The NUV lightcurves of the CVs and MSP show flickering behaviour typical of CVs. We found that the BSs and CVs are the most centrally concentrated population. This might be an effect of mass segregation or indicate the preferred birth place of BSs and CVs via dynamical interactions in the dense core region of GCs. HB stars are the least centrally concentrated population and absent in the innermost area of the core.

UVIT–HST–GAIAview of NGC 288: a census of the hot stellar population and its properties from UV

“…Gap objects are the stars which are located between the MS and white dwarf (WD) sequence in the optical and UV CMDs. The CVs are expected to lie in this region as pointed out by Haurberg et al (2010), Dieball et al (2010) and Dieball et al (2017) in their study of M15, M80 and NGC 6397 respectively. We have found one bright gap object which is marked as orange star in Figures 2-6.…”

“…Recent studies of GCs (Ferraro et al 2003;Haurberg et al 2010;Dieball et al 2010;Schiavon et al 2012;Piotto et al 2015;Parada et al 2016;Raso et al 2017;Dieball et al 2017) have shown that Ultra-Violet (UV) CMDs are important tools for identifying and studying the properties of UV bright stellar populations such as BSSs and HBs. Schiavon et al (2012) generated FUV−NUV vs FUV CMD of 44 GCs using GALEX data.…”

Matching Globular Cluster Models to Observations

“…NGC 6397 is a nearby (D = 2.44 ± 0.04 kpc, Baumgardt et al 2019b) metal-poor, core-collapsed GC, whose close proximity has attracted significant study of its white dwarf and low-mass stellar populations (Paresce et al 1995;Cool et al 1996;Taylor et al 2001;Hansen et al 2007). In addition, NGC 6397 has 15 known CV candidates, a quiescent LMXB (qLMXB), and 1-2 MSPs (Cool et al 1995;Grindlay et al 2001;Cohn et al 2010;Dieball et al 2017). We locate 11 wellfitting snapshots from the model, N4-RV1-RG8-Z0.01.…”

Identifications of faintChandrasources in the globular cluster M3

“…Signs of optical/UV variability can be helpful to further confirm the nature of the source, especially for CV identifications, since most CVs appear as blue variable stars (see e.g. Cool et al 1998;Edmonds et al 2003b;Dieball et al 2017;Rivera Sandoval et al 2018).…”