Multiplicity of Galactic Cepheids from long-baseline interferometry

Abstract

Aims. We aim at detecting and characterizing the main-sequence companions of a sample of known and suspected Galactic binary Cepheids. The long-term objective is to accurately and independently measure the Cepheid masses and distances. Methods. We used the multi-telescope interferometric combiners CHARA/MIRC and VLTI/PIONIER to detect and measure the astrometric positions of the high-contrast companions orbiting 16 bright Galactic Cepheids. We made use of the CANDID algorithm to search for the companions and set detection limits from interferometric observations. We also present new high-precision radial velocity measurements which were used to fit radial pulsation and orbital velocities. Results. We report the detection of the companions orbiting the Cepheids U Aql, BP Cir, and S Mus, possible detections for FF Aql, Y Car, BG Cru, X Sgr, V350 Sgr, and V636 Sco, while no component is detected around U Car, YZ Car, T Mon, R Mus, S Nor, W Sgr, and AH Vel. For U Aql and S Mus, we performed a preliminary orbital fit combining their astrometric measurements with newly obtained high-precision single-line radial velocities, providing the full set of orbital elements and pulsation parameters. Assuming the distance from a period-luminosity (P-L) relation for both Cepheids, we estimated preliminary masses of M U Aql = 4.97 ± 0.62 M and M S Mus = 4.63 ± 0.99 M . For YZ Car, W Sgr, V350 Sgr, and V636 Sco, we revised the spectroscopic orbits using new high-precision radial velocities, while we updated the pulsation parameters for BP Cir, BG Cru, S Nor, and AH Vel. Our interferometric observations also provide measurements of the angular diameters, which can be used in a Baade-Wesselink type analysis. Conclusions. We now have several astrometric detections of Cepheid companions. When radial velocities of the companions are available, such systems will provide accurate and independent masses and distances. Orbital parallaxes with an accuracy better than 5% will be particularly useful for a better calibration of the P-L relation. The final Gaia parallaxes will also be particularly helpful for single-line spectroscopic systems, where mass and distance are degenerate. Mass measurements are necessary for a better understanding of the age and evolution of Cepheids.Article published by EDP Sciences A164, page 1 of 21 A&A 622, A164 (2019)

Consistent radial velocities of classical Cepheids from the cross-correlation technique

“…Instr. (2) (1) G18a, G18b stand for Gallenne et al (2019) and Gallenne et al (2018). (2) H, S, C, F, HM, and HN stand for HARPS, SOPHIE, CORALIE, FEROS, HERMES, and HARPS-North, respectively.…”

“…Our best spline curve is displayed as a red solid line. The v rad are corrected only from the Keplerian orbit (no offset correction), based on the orbital parameters recently computed by Gallenne et al (2019). In the red box is one of our few CORALIE v rad outliers (see text), that we did not take into account to build the spline curve.…”

Precision angular diameters for 16 southern stars with VLTI/PIONIER

“…Combining data they found a relative difference of 0.35% in the measured separations, consistent with Kervella et al (2017), which was taken to be the systematic uncertainty of the PIONIER wavelength calibration. The authors do not report a systematic offset equivalent to γ from Kervella et al (2017), only a relative uncertainty, with subsequent work involving authors of both investigations using only this relative value (Gallenne et al 2019). Lachaume et al (2019), and the associated Rabus et al (2019), undertook investigation into the statistical uncertainties and systematics when using PIONIER to measure diameters for under-resolved low-mass stars.…”

Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2

“…This favorable configuration provides a stringent test of the reliability of the PMa analysis. In addition, Gallenne et al (2019) recently obtained high-accuracy interferometric astrometric orbits from interferometry for U Aql and S Mus. We briefly discuss them in Sect.…”

“…An analysis of the GDR2 parallaxes of Galactic CCs in the context of the extragalactic distance scale was recently presented by Riess et al (2018). Classical Cepheids are intermediate-mass stars (typically 5-10 M ) and this class therefore comprises Full Tables A.1 and A.3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/623/A116 a high fraction of binary and multiple stars (Gieren 1982;Szabados 2003a;Evans et al 2013Evans et al , 2015Sana 2017). RR Lyrae stars are short-period (P ≈ 0.5 d) low-mass pulsators (m ≈ 0.6 M ) that are abundant in the Galaxy and especially in globular clusters (Bailey & Leland 1899;Pickering et al 1901).…”