An excess of luminous white dwarfs in the peculiar Galactic globular cluster NGC 2808

TL;DR

This study investigates whether the peculiar Galactic globular cluster NGC 2808 hosts an excess of white dwarfs (WDs) by exploiting deep near-UV photometry from the Hubble Space Telescope and BaSTI evolutionary models. By comparing WD counts along the cooling sequence to lifetimes inferred from the main-sequence turnoff (MSTO) and horizontal-branch (HB) populations, the authors quantify deviations indicative of slowly cooling WDs (SCWDs) or helium-core WDs, potentially linked to the cluster's extended blue HB. They find a robust WD excess of about $60$–$70\%$ when referenced to MSTO lifetimes, and a remaining ~$30$–$40\%$ excess relative to HB lifetimes, with HB and MSTO comparisons also suggesting an HB population excess of ~45–50% under helium-enhanced scenarios. The results support the presence of SCWDs (and possibly He-core WDs) in NGC 2808, likely connected to its helium-enhanced sub-populations and HB morphology, and motivate deeper UV observations and RGB luminosity function analyses to further constrain the WD composition and cooling physics in this and similar clusters.

Abstract

We study the white dwarf (WD) cooling sequence of the Galactic Globular Cluster (GGC) NGC 2808 by using deep near-UV data from the Hubble Space Telescope and theoretical models, to investigate if this cluster hosts an excess of WDs. Excess in WDs is a rare phenomenon that has been found to exist only in a few GGCs. We compared star counts from different evolutionary phases on the near-UV color-magnitude diagram to evolutionary times predicted by BaSTI models. The investigation was carried out over a region within a radii of 1.5 $\arcmin$ of the cluster center and a region of similar dimension located 5$\arcmin$ away. We find a WD excess of $\approx$ 60 - 70\% when comparing star counts and evolutionary models of the WD cooling sequence to the main-sequence turn-off, and by using different values and fractions of Helium enhancement. This excess decreases to $\approx$ 30 - 40\% when the WD cooling sequence is compared to the horizontal branch. The WD excess is slightly larger in the internal field that covers the cluster center; however, the difference with the external field is compatible within the uncertainties. We argue that this excess is possibly related to the existence of SCWDs and Helium-core WDs in NGC~2808, and might be directly associated to the extended blue horizontal branch of this GGC.

Figures (10)

• Figure 1: Stars from the HST HUGS catalog for NGC 2808 are plotted in the RA and DEC plane as black dots ('internal field'), while red dots indicate stars detected and measured from HST program GO-15857 ('external field'). The corners of the internal field are cut since only the region covered by photometry in all WFC3-UVIS near-UV filters is represented.
• Figure 2: Left -- $F275W,\ F275W - F336W$ CMD for the internal field with 145,610 objects whose error is $<$ 1 mag in $F275W$ and $F336W$. Right -- Same plot as the left for the external field with 10,897 objects.
• Figure 3: Left -- The black solid and dashed lines indicate the completeness curves for WDs observed in the $F275W$ filter for the internal and the external field, respectively. The dashed-dotted horizontal line indicates a completeness level of 50%. Right -- Same but for the completeness of the other evolutionary phases, such as MSTO, RGB and HB.
• Figure 4: Top-left -- $F275W,\ F275W - F336W$ CMD of the internal field zoomed in around the MSTO region for all sources with an error in magnitude of $F275W$ and magnitude of $F336W$$< 1$. A BaSTI $\alpha$-enhanced isochrone for $t$ = 12 Gyr, $Z$ = 0.002, and $Y$ = 0.248, is overplotted as a solid light blue line. The model was transformed to the observed plane by using the reddening and extinction values listed in Section \ref{['MSTO Analysis']}. The theoretical MSTO at $M$ = 0.8 $\text{M}_\odot$ is marked with a golden star. The thicker section of the isochrone indicates a mass width $\delta\mathcal{M} = \pm 0.004$ around 0.8 M$_\odot$ of 20.88 $\le$F275W$\le$ 21.03. Stars selected in this MSTO region are marked as green dots. Top-right -- Color histogram of the MSTO selected stars. The dotted blue vertical lines mark the $\pm 1.5\sigma$ color cut used to select the final sample of MSTO stars. Bottom-left -- Final sample of MSTO stars in the same CMD. The $F275W$ magnitude bin is split in half, with the stars in the upper (N = 1,970) and bottom bin (N = 1,902) marked with pink and purple dots, respectively. Stars discarded by the color selection are marked with green dots. Bottom-right -- A BaSTI evolutionary track for $M$ = 0.8 $\text{M}_\odot$ (solid orange line) and the same isochrone (solid light blue) are overplotted on the MSTO region of the CMD. The filled-black circles on the track mark the $F275W$ magnitudes used to calculate the MSTO crossing times (see Table \ref{['Table MSTO']} for more details).
• Figure 5: Left -- $F275W,\ F275W - F336W$ CMD of the internal field zoomed around the WD cooling sequence region. Stars marked as teal asterisk-shaped stars are the objects excluded from this study. Pink and purple triangles mark selected WDs divided into magnitude bins (see text for more details). The dashed horizontal lines indicate the lower bounds of the magnitude bins, and the solid black line is a BaSTI CO-core/H-envelope WD cooling model for a mass of 0.54 $\text{M}_\odot$. Right -- Same zoomed-in CMD for the external field. Light and dark blue triangles mark selected WDs in the two magnitude bins.