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 colored circles: from left to right, BSS1, BSS2, BSS3, BSS4, and BSS5. The scale is indicated in the bottom left corner of each frame. Credit: arXiv (2025). DOI: 10.48550/arxiv.2508.21118
Using the Hubble Space Telescope (HST), Italian astronomers have observed five blue straggler stars in the globular cluster 47 Tucanae. Their results show that one of these stars has a white dwarf companion. The discovery is presented in a paper Aug. 28 on the arXiv preprint server.
Located approximately 15,300 light years away from Earth, 47 Tucanae is a massive, non core-collapsed globular cluster in the constellation Tucana. This cluster shows low extinction, has a high interaction rate and has the most X-ray sources identified so far among any other globular clusters. These properties make 47 Tucanae an excellent target to study close binary systems.
Now, a team of astronomers led by Elisabetta Reggiani of the University of Florence in Italy, has discovered a new binary in 47 Tucanae, after that they identified the presence of a companion star to one of the cluster's blue straggler stars (BSSs), designated BSS4. In general, BSSs are core hydrogen-burning objects more massive than main sequence (MS) and post-MS stars. 47 Tucanae is known to host at least five blue straggler stars.
"The analysis of far-ultraviolet photometric data acquired with the ACS/SBC at HST unambiguously reveals the presence of a hot companion to BSS4," the researchers wrote in the paper.
Based on the collected data, the astronomers found that the companion object to BSS4 is most likely a hot white dwarf (WD). They assume that the WD likely formed through mass transfer less than 12 million years ago.
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蟽 error on the expected fluxes. The best-fit surface temperature and radius are labeled together with their 1蟽 uncertainty in the top-right corner legend. The residuals between the observed and the expected fluxes are plotted in the lower panel. Right Panel: 蠂2 map for BSS1 showing the distribution of 蠂2 values (color-coded as in the side bar) obtained for all the explored combinations of R and T. The black lines refer to the 1蟽, 2蟽 and 3蟽 蠂2 contours from the minimum values (蠂20), which is marked with a black cross and labeled in the bottom-right corner. Credit: arXiv (2025). DOI: 10.48550/arxiv.2508.21118
When it comes to the properties of the newfound white dwarf, scientists estimate that it has a mass between 0.2 and 0.55 solar masses, while its radius is most likely 0.016-0.032 solar radii. The effective temperature of this star is estimated to be greater than 20,000 K.
The authors of the paper noted that both the very young cooling age of the WD and significant rotational velocity of BSS4 suggest that this is a recently formed system. They added that this binary is possibly on the verge of completing the mass transfer process. Therefore, it is expected that in the next few million years the stripped companion will increase its surface temperature because of the complete loss of the envelope around its nucleus.
Summing up the results, the researchers underlined that BSS4 is one of the youngest BSS-WD systems detected yet. According to them, further investigation of BSS4 could provide important insights into the evolution of these rare binaries.
"In particular, the determination of the radial velocity curve of the system (and also of the BSSs with no evidence of ultraviolet excess) could add further precious details for the physical characterization of these puzzling objects and their evolutionary processes, providing crucial constraints to the theoretical modeling of the mass transfer formation channel," the astronomers concluded.
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More information: Elisabetta Reggiani et al, Detection of a white dwarf orbiting a Carbon-Oxygen depleted blue straggler in 47 Tucanae, arXiv (2025).
Journal information: arXiv
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