Gravitational redshifts in main-sequence and giant stars

Abstract

Context. Precise analyses of stellar radial velocities is able to reveal intrinsic causes of the wavelength shifts of spectral lines (other than Doppler shifts due to radial motion), such as gravitational redshifts and convective blueshifts. Aims. Gravitational redshifts in solar-type main-sequence stars are expected to be some 500 m s −1 greater than those in giants. We search for this difference in redshifts among groups of open-cluster stars that share the same average space motion and thus have the same average Doppler shift. Methods. We observed 144 main-sequence stars and cool giants in the M 67 open cluster using the ESO FEROS spectrograph and obtained radial velocities by means of cross-correlation with a spectral template. Binaries and doubtful members were not analyzed, and average spectra were created for different classes of stars. Results. The M 67 dwarf and giant radial-velocity distributions are each well represented by Gaussian functions, which share the same apparent average radial velocity to within 100 m s −1 . In addition, dwarfs in M 67 appear to be dynamically hotter (σ = 0.90 km s −1 ) than giants (σ = 0.68 km s −1 ). Conclusions. We fail to detect any difference in the gravitational redshifts of giants and MS stars. This is probably because of the differential wavelength shifts produced by the different hydrodynamics of dwarf and giant atmospheres. Radial-velocity differences measured between unblended lines in averaged spectra vary with line-strength: stronger lines are more blueshifted in dwarfs than in giants, apparently removing any effect of the gravitational redshift. Synthetic high-resolution spectra are computed from three dimensional (3D) hydrodynamic model atmospheres for both giants and dwarfs, and synthetic wavelength shifts obtained. In agreement with observations, 3D models predict substantially smaller wavelength-shift differences than expected from gravitational redshifts only. The procedures developed could be used to test 3D models for different classes of stars, but will ultimately require high-fidelity spectra for measurements of wavelength shifts in individual spectral lines.

The open cluster King 1 in the second quadrant

“…This distribution may be affected by differential gravitational red-shifts and convective blue-shifts, since we are combining stars on the upper main sequence and on the red giant branch. The expected impact of these corrections on the derived radial velocities is under 1 km s −1 (see, e. g. Allende Prieto et al 2002;Pasquini et al 2011). This value is a third of our typical uncertainties, and therefore we have made no attempt to correct it.…”

Proxima’s orbit around α Centauri

“…The GRS effect on the RV is a function of the mass m of the star and its radius R through v GRS = G m/(R c). The GRS has been observed by Takeda & Ueno (2012) in the Sun with an amplitude comparable to the expected value of +633 m s −1 , but its detection in main sequence and giant stars of the M 67 stellar cluster remains elusive (Pasquini et al 2011). As it is a function of m/R, compact objects (white dwarfs, neutron stars, and black holes) create the strongest GRS, typically +40 km s −1 for white dwarfs (Falcon et al 2012), while giants and supergiants exhibit very small GRS (Dravins 1999).…”

The completeness-corrected rate of stellar encounters with the Sun from the first Gaia data release

“…We will then be able to estimate systematically actual impulses (Eq. (1)), and also to correct the measured radial velocities for the gravitational redshift, which is of order 0.5-1.0 km s −1 (Pasquini et al 2011). This data release should also include solutions for astrometric and spectroscopic binaries (Pourbaix 2011), thereby permitting a better computation of flight paths for such systems.…”