Abstract
White dwarfs (WDs) are the final evolutionary product of the vast majority of stars in the Universe. They are electron-degenerate structures characterized by no stable thermonuclear activity, and their evolution is generally described as a pure cooling process. Their cooling rate is adopted as cosmic chronometer to constrain the age of several Galactic populations, including the disk, globular and open clusters. By analysing high-resolution photometric data of two very similar Galactic globular clusters (M3 and M13), we find a clear-cut and unexpected overabundance of bright WDs in M13. Theoretical models suggest that, consistent with the horizontal branch morphology, this overabundance is due to a slowing down of the cooling process in ~70% of the WDs in M13, caused by stable thermonuclear burning in their residual hydrogen-rich envelope. The presented observational evidence of quiescent thermonuclear activity occurring in cooling WDs brings new attention on the use of the WD cooling rate as cosmic chronometer for low-metallicity environments.
This is a preview of subscription content, access via your institution
Access options
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 digital issues and online access to articles
$119.00 per year
only $9.92 per issue
Buy this article
- Purchase on Springer Link
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
Data availability
The photometric data that support the plots and other findings of this study are available from the corresponding author upon reasonable request. The catalogues are also publicly downloadable from the web site of the Cosmic-Lab project (http://www.cosmic-lab.eu/Cosmic-Lab/Home.html). All the HST images are publicly available from the Mikulski Archive for Space Telescopes (https://archive.stsci.edu/).
References
Córsico, A. H., Althaus, L. G., Miller Bertolami, M. M. & Kepler, S. O. Pulsating white dwarfs: new insights. Astron. Astrophys. Rev. 27, 7 (2019).
Woosley, S. E. & Heger, A. The remarkable deaths of 9–11 solar mass stars. Astrophys. J. 810, 34 (2015).
Salaris, M., Cassisi, S., Pietrinferni, A., Kowalski, P. M. & Isern, J. A large stellar evolution database for population synthesis studies. VI. White dwarf cooling sequences. Astrophys. J. 716, 1241–1251 (2010).
Bédard, A., Bergeron, P., Brassard, P. & Fontaine, G. On the spectral evolution of hot white dwarf stars. I. A detailed model atmosphere analysis of hot white dwarfs from SDSS DR12. Astrophys. J. 901, 93 (2020).
Althaus, L. G. et al. The formation of DA white dwarfs with thin hydrogen envelopes. Astron. Astrophys. 440, L1–L4 (2005).
Bradley, P. A. Asteroseismological constraints on the structure of the ZZ Ceti stars G117-B15A and R548. Astrophys. J. Suppl. Ser. 116, 307–319 (1998).
Richer, H. B. et al. Deep Advanced Camera for Surveys imaging in the globular cluster NGC 6397: the cluster color–magnitude diagram and luminosity function. Astron. J. 135, 2141–2154 (2008).
Richer, H. B. et al. Comparing the white dwarf cooling sequences in 47 Tuc and NGC 6397. Astrophys. J. 778, 104 (2013).
Bedin, L. R. et al. The bottom of the white dwarf cooling sequence in the old open cluster NGC 2158. Astrophys. J. Lett. 708, L32–L35 (2010).
Bedin, L. R. et al. The HST large programme on NGC 6752 – III. Detection of the peak of the white dwarf luminosity function. Mon. Not. R. Astron. Soc. 488, 3857–3865 (2019).
Bellini, A. et al. A double white-dwarf cooling sequence in ω Centauri. Astrophys. J. Lett. 769, L32 (2013).
Campos, F. et al. A comparative analysis of the observed white dwarf cooling sequence from globular clusters. Mon. Not. R. Astron. Soc. 456, 3729–3742 (2016).
Moehler, S. & Bono, G. White dwarfs in globular clusters. Preprint at https://arxiv.org/pdf/0806.4456.pdf (2011).
Renedo, I. et al. New cooling sequences for old white dwarfs. Astrophys. J. 717, 183–195 (2010).
Miller Bertolami, M. M. et al. Quiescent nuclear burning in low-metallicity white dwarfs. Astrophys. J. Lett. 775, L22 (2013).
Althaus, L. G., Camisassa, M. E., Miller Bertolami, M. M., Córsico, A. H. & García-Berro, E. White dwarf evolutionary sequences for low-metallicity progenitors: the impact of third dredge-up. Astron. Astrophys. 576, A9 (2015).
Ferraro, F. R. et al. Hubble Space Telescope ultraviolet observations of the cores of M3 and M13. Astrophys. J. Lett. 484, L145–L148 (1997).
Ferraro, F. R., Paltrinieri, B., Fusi Pecci, F., Rood, R. T. & Dorman, B. Multimodal distributions along the horizontal branch. Astrophys. J. 500, 311–319 (1998).
Salaris, M. & Weiss, A. Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms. Astron. Astrophys. 388, 492–503 (2002).
Dotter, A. et al. The ACS survey of Galactic globular clusters. IX. Horizontal branch morphology and the second parameter phenomenon. Astrophys. J. 708, 698–716 (2010).
VandenBerg, D. A., Brogaard, K., Leaman, R. & Casagrande, L. The ages of 55 globular clusters as determined using an improved \(\Delta V^{\mathrm{HB}}_{\mathrm{TO}}\) method along with color–magnitude diagram constraints, and their implications for broader issues. Astrophys. J. 775, 134 (2013).
Denissenkov, P. A. et al. Constraints on the distance moduli, helium, and metal abundances, and ages of globular clusters from their RR Lyrae and non-variable horizontal branch stars. II. Multiple stellar populations in 47 Tuc, M3, and M13. Astrophys. J. 849, 159 (2017).
Catelan, M. Horizontal branch stars: the interplay between observations and theory, and insights into the formation of the Galaxy. Astrophys. Space Sci. 320, 261–309 (2009).
Dalessandro, E., Salaris, M., Ferraro, F. R., Mucciarelli, A. & Cassisi, S. The horizontal branch in the UV colour–magnitude diagrams – II. The case of M3, M13 and M79. Mon. Not. R. Astron. Soc. 430, 459–471 (2013).
Pietrinferni, A., Cassisi, S., Salaris, M. & Castelli, F. A large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an α-enhanced metal distribution. Astrophys. J. 642, 797–812 (2006).
Ferraro, F. R. et al. The giant, horizontal, and asymptotic branches of Galactic globular clusters. I. The catalog, photometric observables, and features. Astron. J. 118, 1738–1758 (1999).
Stetson, P. B. DAOPHOT: a computer program for crowded-field stellar photometry. Publ. Astron. Soc. Pac. 99, 191–222 (1987).
Ferraro, F. R. et al. HST observations of blue straggler stars in the core of the globular cluster M 3. Astron. Astrophys. 324, 915–928 (1997).
Ferraro, F. R., D’Amico, N., Possenti, A., Mignani, R. P. & Paltrinieri, B. Blue stragglers, young white dwarfs, and UV-excess stars in the core of 47 Tucanae. Astrophys. J. 561, 337–345 (2001).
Ferraro, F. R., Sills, A., Rood, R. T., Paltrinieri, B. & Buonanno, R. Blue straggler stars: a direct comparison of star counts and population ratios in six Galactic globular clusters. Astrophys. J. 588, 464–477 (2003).
Raso, S. et al. The “UV-route” to search for blue straggler stars in globular clusters: first results from the HST UV Legacy Survey. Astrophys. J. 839, 64 (2017).
Dalessandro, E. et al. IC 4499 revised: spectro-photometric evidence of small light-element variations. Astron. Astrophys. 618, A131 (2018).
Dalessandro, E. et al. The peculiar radial distribution of multiple populations in the massive globular cluster M80. Astrophys. J. 859, 15 (2018).
Cadelano, M. et al. PSR J1641+3627F: a low-mass He white dwarf orbiting a possible high-mass neutron star in the globular cluster M13. Astrophys. J. 905, 63 (2020).
Stetson, P. B. The center of the core-cusp globular cluster M15: CFHT and HST observations, ALLFRAME reductions. Publ. Astron. Soc. Pac. 106, 250 (1994).
Piotto, G. et al. The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters. I. Overview of the project and detection of multiple stellar populations. Astron. J. 149, 91 (2015).
Ferraro, F. R. et al. Dynamical age differences among coeval star clusters as revealed by blue stragglers. Nature 492, 393–395 (2012).
Ferraro, F. R. et al. The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters. XV. The dynamical clock: reading cluster dynamical evolution from the segregation level of blue straggler stars. Astrophys. J. 860, 36 (2018).
Lanzoni, B. et al. Refining the dynamical clock for star clusters. Astrophys. J. Lett. 833, L29 (2016).
Bellazzini, M., Fusi Pecci, F., Messineo, M., Monaco, L. & Rood, R. T. Deep Hubble Space Telescope WFPC2 photometry of NGC 288. I. Binary systems and blue stragglers. Astron. J. 123, 1509–1527 (2002).
Dalessandro, E. et al. No evidence of mass segregation in the low-mass Galactic globular cluster NGC 6101. Astrophys. J. 810, 40 (2015).
Cadelano, M. et al. Radial variation of the stellar mass functions in the globular clusters M15 and M30: clues of a non-standard IMF? Mon. Not. R. Astron. Soc. 499, 2390–2400 (2020).
Renzini, A. & Buzzoni, A. in Spectral Evolution of Galaxies (eds Chiosi, C. & Renzini, A.) 195–235 (Springer, 1986).
Sandquist, E. L., Gordon, M., Levine, D. & Bolte, M. A re-evaluation of the evolved stars in the globular cluster M13. Astron. J. 139, 2374–2409 (2010).
Harris, W. E. A catalog of parameters for globular clusters in the Milky Way. Astron. J. 112, 1487–1488 (1996).
Milone, A. et al. The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters. XVI. The helium abundance of multiple populations. Mon. Not. R. Astron. Soc. 481, 5098–5122 (2018).
Greggio, L. & Renzini, A. Clues on the hot star content and the ultraviolet output of elliptical galaxies. Astrophys. J. 364, 35–64 (1990).
Acknowledgements
This research is part of the Cosmic-Lab project at the Physics and Astronomy Department of the University of Bologna (see http://www.cosmic-lab.eu/Cosmic-Lab/Home.html). The research has been funded by project Light-on-Dark, granted by the Italian MIUR through contract no. PRIN-2017K7REXT (F.R.F., M.C., B.L., C.P. and E.D.). J.C. acknowledges the financial support from the China Scholarship Council. The research is based on data acquired with the NASA/ESA HST under projects GO12605 and GO10775 at the Space Telescope Science Institute, which is operated by Aura Inc. under NASA contract no. NAS5-26555.
Author information
Authors and Affiliations
Contributions
J.C. analysed the photometric data sets, F.R.F. designed the study and coordinated the activity. M.S. performed the Monte Carlo simulations, and M.S. and L.G.A. were in charge of the theoretical model development. J.C., M.C., C.P. and E.D. contributed to the data analysis and the computation of the artificial star analysis. J.C., F.R.F. and B.L. wrote the first draft of the paper. M.S., L.G.A. and M.C. critically contributed to the paper presentation. All the authors contributed to the discussion of the results and commented on the manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Peer review information Nature Astronomy thanks Pier-Emmanuel Tremblay and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data
Extended Data Fig. 1 The effect of stable H-burning on a low mass WD.
a, Contribution of stable H-burning15,16 (via PP and CNO chain) to the global luminosity of a low metallicity (Z = 0.001), low mass (0.54 M⊙) WD as a function of its decreasing luminosity. H-burning provides a relevant contribution (larger than 40%) to the WD luminosity in the brightest portion of the cooling sequence, becoming negligible at log(L/L⊙) ≈ −4 and log(Te) ≈ 3.7 (see the temperature scale in the top axis). b, Delay15,16 in the cooling time induced by stable H-burning, with respect to a model without burning. The time delay keeps increasing during the phase of active H-burning and reaches a value as large as ~760 Myr, which then remains constant during the entire subsequent evolution.
Extended Data Fig. 2 Physical parameters of M3 and M13.
From top to bottom, the listed parameters are: metallicity, age, V-band absolute integrated magnitude, logarithm of the central luminosity density (in units of L⊙ pc-3), logarithm of the central relaxation time (in years).
Extended Data Fig. 3 The completeness distribution of the WD populations of M13 and M3.
a, Completeness parameter as a function of the F275W magnitude and colour-coded in terms of the distance from the cluster centre (see colour bars) for each WD detected in M13. b, The same for M3. The mean error (1 s.e.m.) is also reported.
Extended Data Fig. 4 The RGB reference population.
Selection box (red shaded area) adopted to define the RGB ‘reference population’ in the observed and realigned CMDs of M13 and M3. The number of red giants counted in each cluster is also marked. The mean errors (1 s.e.m.) are also marked.
Extended Data Fig. 5 WD cooling time for models with and without hydrogen burning.
Comparison between the cooling times of a low metallicity, 0.54 M⊙ WD with and without hydrogen-burning16 (solid and dashed lines, respectively). The red segment marks the difference in the cooling time at the luminosity of the faintest WD considered in this study, log(L/L⊙) = – 1.7 and reports the absolute difference between the two cooling time values (60 Myr), corresponding to a 75% increase if hydrogen-burning is active, with respect to the ‘standard’ (no-burning) case.
Extended Data Fig. 6 HB and AGB populations in M3 and M13.
a, UV-CMD of M13 zoomed in the HB region. The extreme-HB (E-HB) and the 7 candidate AGB-manqué stars are highlighted as blue circles. b, AGB and HB selection boxes in the optical- and UV-CMD (top and bottom panels, respectively) for the two clusters. The population star counts are also marked in each panel. The mean photometric errors (1 s.e.m.) are also marked in all panels.
Rights and permissions
About this article
Cite this article
Chen, J., Ferraro, F.R., Cadelano, M. et al. Slowly cooling white dwarfs in M13 from stable hydrogen burning. Nat Astron 5, 1170–1177 (2021). https://doi.org/10.1038/s41550-021-01445-6
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41550-021-01445-6