Masses of the Hyades white dwarfs

Abstract

Context. It is possible to accurately measure the masses of the white dwarfs (WDs) in the Hyades cluster using gravitational redshift, because the radial velocity of the stars can be obtained independently of spectroscopy from astrometry and the cluster has a low velocity dispersion. Aims. We aim to obtain an accurate measurement of the Hyades WD masses by determining the mass-to-radius ratio (M/R) from the observed gravitational redshift, and to compare them with masses derived from other methods. Methods. We analyse archive high-resolution UVES-VLT spectra of six WDs belonging to the Hyades to measure their Doppler shift, from which M/R is determined after subtracting the astrometric radial velocity. We estimate the radii using Gaia photometry as well as literature data. Results. The M/R error associated to the gravitational redshift measurement is about 5%. The radii estimates, evaluated with different methods, are in very good agreement, though they can differ by up to 4% depending on the quality of the data. The masses based on gravitational redshift are systematically smaller than those derived from other methods, by a minimum of ∼0.02 up to 0.05 solar masses. While this difference is within our measurement uncertainty, the fact that it is systematic indicates a likely real discrepancy between the different methods. Conclusions. We show that the M/R derived from gravitational redshift measurements is a powerful tool to determine the masses of the Hyades WDs and could reveal interesting properties of their atmospheres. The technique can be improved by using dedicated spectrographs, and can be extended to other clusters, making it unique in its ability to accurately and empirically determine the masses of WDs in open clusters. At the same time we prove that gravitational redshift in WDs agrees with the predictions of stellar evolution models to within a few percent.

White dwarf-open cluster associations based on Gaia DR2

“…The IFMR is also an important ingredient in the modeling of stellar feedback in galaxy simulations and predicting SN type Ia rates (e.g., Greggio 2010;Agertz & Kravtsov 2015;Cummings 2017). Aside from the IFMR, other possible avenues of research utilizing cluster WDs include studying the effects of metallicity and binarity on WD evolution and measuring WD masses using gravitational redshift (Pasquini et al 2019). Such studies are impossible or very challenging to conduct for Galactic field WDs.…”

A Gravitational Redshift Measurement of the White Dwarf Mass-Radius Relation

“…The degeneracy between Doppler shift and gravitational redshift can be overcome by using other information like co-moving companions to constrain the radial velocity component of the apparent velocity, as was done in the case of Sirius B (Joyce et al 2018b), as well as for white dwarfs in the Hyades cluster (Pasquini et al 2019). These kinds of studies require high resolution spectroscopy to accurately measure individual apparent radial velocities, and have consequently probed narrow mass ranges and had small sample sizes.…”

“…The bottom panels indicate derived mass-radius relations from the respective observables. We overlay theoretical models fromFontaine et al (2001) for thick-envelope C/O cores at two temperatures and O/Ne cores at 5000 K, along with past observations of Sirius B and the Hyades(Joyce et al 2018b;Pasquini et al 2019). Error bars indicate 1-σ confidence intervals.…”

Search for giant planets around seven white dwarfs in the Hyades cluster with the Hubble Space Telescope

“…Present-day mass estimates for Hyades white dwarfs have been derived from analysis of spectroscopic data using atmospheric models (Bergeron et al 2011;Gianninas et al 2011). We note that gravitational redshift measurements for a subset of the Hyades white dwarfs yield 5% to 10% lower masses estimates (Pasquini et al 2019). For the following discussion, we consider this small systematic discrepancy in mass estimates of minor importance.…”