Detection of a white dwarf orbiting a Carbon-Oxygen depleted blue straggler in 47 Tucanae

TL;DR

This work tests the mass-transfer MT formation channel for blue stragglers in 47 Tuc by combining chemical evidence (CO depletion) with a search for a hot white dwarf companion via deep far-UV HST observations. Using multi-band photometry and SED fitting with ATLAS9 WD and BSS models, the authors find unambiguous UV excess only for BSS4, consistent with a hot WD companion with $T_{\rm WD}$ in the $2$–$3\times10^4$ K range and cooling ages up to about $12$ Myr, implying a very recent MT event. The companion fits span WD masses $M_{\rm WD} = 0.2$–$0.55\,M_\odot$, corresponding to He- or CO-core remnants from different donor evolutionary stages, while other CO-depleted BSSs show no hot WD detection, suggesting older or ongoing MT or discovery limitations due to WD cooling. Overall, the results establish a direct link between chemical MT signatures and photometric MT signatures, providing compelling evidence for MT as a formation channel for at least a subset of BSSs and highlighting the transient nature of these signatures.

Abstract

We have employed deep far-UV observations secured with the Solar Blind Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope to search for hot companions to five blue stragglers stars (BSSs) showing significant surface depletion of carbon (C) and oxygen (O), in the Galactic globular cluster 47 Tucanae. Such a chemical pattern has been interpreted as the chemical signature of the mass transfer formation process for the observed blue stragglers. The mass transfer origin is also expected to leave a "photometric signature" in the form of a UV-excess, as the stripped core of the donor star should be observable as a white dwarf (WD) companion orbiting the newborn BSS. We found strong evidence for the presence of a hot (T > 20000 K) WD companion to one of the investigated BSS, indicating that it likely formed through mass transfer less than 12 Myr ago. This is the first simultaneous evidence of the chemical and the photometric signatures of the mass-transfer formation channel. The lack of evidence for a hot companion to the other investigated blue stragglers is consistent with the expectation that the photometric signature (as well as the chemical one) is a transient phenomenon.

Figures (11)

• Figure 1: Position in the [O/Fe]$-$[C/Fe] diagram of the 43 BSSs (squares and circles) and MS-TO stars (gray shaded region) of 47 Tucanae discussed in Ferraro06. The blue squares mark the 5 targets of this study.
• Figure 2: Drizzled ACS/SBC images in the F165LP filter of the five BSSs investigated in this study. The positions of the 5 targets are marked by green coloured circles: from left to right, BSS1, BSS2, BSS3, BSS4, and BSS5. The scale is indicated in the bottom left corner of each frame.
• Figure 3: Near-UV CMD (left panel) and optical CMD (right panel) of 47 Tucanae with the 5 targets of this study highlighted as blue squares. The 12 Gyr old BaSTI Pietrinferni_2021 isochrone well reproducing the MS-TO region is shown in both CMDs as a red line. The corresponding 40 Myr old isochrone, assumed to be representative of the zero-age MS location, is plotted as a solid blue line. The dashed blue lines are the evolutionary tracks that best reproduce the observed positions of the targets in these CMDs.
• Figure 4: Left Panel: Observed SED of BSS1 (blue circles) compared to the expected fluxes (cyan squares) computed from the convolution of the best-fit synthetic SED (black line) with the adopted photometric filters. The uncertainties on the observed fluxes are marked with vertical errorbars (unless they are smaller than the size of the blue circles). The horizontal cyan-shaded rectangles mark the wavelength width of each photometric filter. For the sake of clarity, they have been associated to the expected fluxes only but, of course, they also hold for the observed points. The vertical errorbar of the cyan squares corresponds to the 1$\sigma$ error on the expected fluxes. The best-fit surface temperature and radius are labelled together with their $1\sigma$ uncertainty in the top-right corner legend. The residuals between the observed and the expected fluxes are plotted in the lower panel. Right Panel:$\chi^2$ map for BSS1 showing the distribution of $\chi^2$ values (colour-coded as in the side bar) obtained for all the explored combinations of R and T. The black lines refer to the $1\sigma$, $2\sigma$ and $3\sigma$$\chi^2$ contours from the minimum values ($\chi_0^2$), which is marked with a black cross and labelled in the bottom-right corner.
• Figure 5: Same as in Figure \ref{['bss1_fit']}, but for BSS2.