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Asteroseismology of red giants in the globular cluster 47 Tuc using the HST

Dennis Stello, Timothy R. Bedding, Ronald L. Gilliland

Abstract

Globular clusters are complex stellar populations that provide unique opportunities to study stellar evolution -- as the second brightest cluster, 47 Tuc is a prime target. Asteroseismology can be used to measure precise masses of stars and has recently been applied to red giants in globular clusters, but so far not for 47 Tuc. Here, we present a search for solar-like oscillations in red giants of 47 Tuc using 8.3 days of high-cadence Hubble Space Telescope data. We detect oscillations in two out of the five giants falling in the field of view, 5 arcmin from the cluster center. One is on the horizontal branch (HB) while the other is on the red giant branch (RGB) at a similar brightness. From the seismic signal, we measure the stellar masses to be $0.78\pm0.13\,$M$_\odot$ (HB) and $0.94\pm0.15\,$M$_\odot$ (RGB), and hence an inferred integrated mass loss along the upper RGB of $0.16\pm0.20\,$M$_\odot$. A mass uncertainty of less than 0.05M$_\odot$ would be required to obtain a useful estimate of the mass loss, while an uncertainty below 0.01M$_\odot$ would be required to measure the mass difference between the cluster's multiple chemical populations. The former would be attainable with observations of about 100 times more stars to form ensemble-averaged values (assuming a similar length campaign), or alternatively a longer campaign observing fewer stars. Detecting mass differences between the chemical sub-populations, would require a 20-day campaign observing several hundreds of stars. Our clear detection of oscillations and the prospects presented here warrant dedicated high-cadence campaigns of 47 Tuc, which are possible with NASA's Roman mission and future missions like HAYDN.

Asteroseismology of red giants in the globular cluster 47 Tuc using the HST

Abstract

Globular clusters are complex stellar populations that provide unique opportunities to study stellar evolution -- as the second brightest cluster, 47 Tuc is a prime target. Asteroseismology can be used to measure precise masses of stars and has recently been applied to red giants in globular clusters, but so far not for 47 Tuc. Here, we present a search for solar-like oscillations in red giants of 47 Tuc using 8.3 days of high-cadence Hubble Space Telescope data. We detect oscillations in two out of the five giants falling in the field of view, 5 arcmin from the cluster center. One is on the horizontal branch (HB) while the other is on the red giant branch (RGB) at a similar brightness. From the seismic signal, we measure the stellar masses to be M (HB) and M (RGB), and hence an inferred integrated mass loss along the upper RGB of M. A mass uncertainty of less than 0.05M would be required to obtain a useful estimate of the mass loss, while an uncertainty below 0.01M would be required to measure the mass difference between the cluster's multiple chemical populations. The former would be attainable with observations of about 100 times more stars to form ensemble-averaged values (assuming a similar length campaign), or alternatively a longer campaign observing fewer stars. Detecting mass differences between the chemical sub-populations, would require a 20-day campaign observing several hundreds of stars. Our clear detection of oscillations and the prospects presented here warrant dedicated high-cadence campaigns of 47 Tuc, which are possible with NASA's Roman mission and future missions like HAYDN.
Paper Structure (8 sections, 2 equations, 6 figures, 1 table)

This paper contains 8 sections, 2 equations, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observations and analysis
  3. Observations
  4. Time series and Fourier analysis
  5. Gaia data and isochrone matching
  6. Mass estimations
  7. Mass loss and multiple stellar populations
  8. Requirements for dedicated asteroseismic campaigns

Figures (6)

  • Figure 1: Field of view of 47 Tuc (12x12 arcmin centered on $\mathrm{RA}=00$:24:06.5, $\mathrm{Dec}=-72$:04:38). The HST fields for program GO-8267 are shown in orange. The STIS field studied in this paper, centered at $\mathrm{RA}=00$:23:24, $\mathrm{Dec}=-72$:01:24, is in the upper right.
  • Figure 2: Colour-magnitude diagram of 47 Tuc for the stars with time series data. Stars for which light curves were investigated in detail are shown with red circles.
  • Figure 3: Light curves from the CLEAR filter for the three stars with a brightness around $V=14$.
  • Figure 4: Power spectra of star 1, 2 and 3. The first two spectra smoothed by a Gaussian of FWHM=10$\mu$Hz are shown in orange.
  • Figure 5: Colour-magnitude diagram of 47 Tuc based on Gaia DR3 photometry. The PARSEC isochrone (12.75Gyr, $Z=0.0056$) is shifted by a distance modulus of 13.27mag and reddening of 0.03mag. The three stars discussed in the text are highlighted.
  • ...and 1 more figures