Revealing δ Cephei's Secret Companion and Intriguing Past

Abstract

Classical Cepheid variable stars are crucial calibrators of the cosmic distance scale thanks to a relation between their pulsation periods and luminosities. Their archetype, δ Cephei, is an important calibrator for this relation. In this paper, we show that δ Cephei is a spectroscopic binary based on newly-obtained high-precision radial velocities. We combine these new data with literature data to determine the orbit, which has period 2201 days, semi-amplitude 1.5 km/s, and high eccentricity (e = 0.647). We re-analyze Hipparcos intermediate astrometric data to measure δ Cephei's parallax ($\varpi =4.09\pm 0.16$ mas) and find tentative evidence for an orbital signature, although we cannot claim detection. We estimate that Gaia will fully determine the astrometric orbit. Using the available information from spectroscopy, velocimetry, astrometry, and Geneva stellar evolution models ($M_{\delta Cep}5.0-5.25M_{\odot }$), we constrain the companion mass to within $0.2<M_2<1.2M_{\odot }$. We discuss the potential of ongoing and previous interactions between the companion and δ Cephei near pericenter passage, informing reported observations of circumstellar material and bow-shock. The orbit may have undergone significant changes due to a Kozai-Lidov mechanism driven by the outer (visual and astrometric) companion HD 213307. Our discovery of δ Cephei's nature as a spectroscopic binary exposes a hidden companion and reveals a rich and dynamical history of the archetype of classical Cepheid variables.

31 pages (referee format), 10 figures, accepted for publication in The Astrophysical Journal

References in corpus (13)

• Astrometric exoplanet detection with Gaia
• Pulkovo Compilation of Radial Velocities for 35495 Hipparcos Stars in a Common System
• Cepheid Parallaxes and the Hubble Constant
• Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
• On the effect of rotation on populations of classical Cepheids I. Predictions at solar metallicity
• Gaia's potential for the discovery of circumbinary planets
• Baade-Wesselink distances and the effect of metallicity in classical cepheids
• Tuning in on Cepheids: Radial velocity amplitude modulations. A source of systematic uncertainty for Baade-Wesselink distances
• Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes
• Theoretical fits of the δCephei light, radius and radial velocity curves
• The Secret Lives of Cepheids: Evolutionary Changes and Pulsation-Induced Shock Heating in the Prototype Classical Cepheid δ Cep
• HD 5388 b is a 69 M_Jup companion instead of a planet
• Enabling Gaia observations of naked-eye stars

Cited by in corpus (8)

• Discovery of Cycle-to-cycle Modulated Spectral Line Variability and Velocity Gradients in Long-period Cepheids
• Binary Properties from Cepheid Radial Velocities (CRaV)
• The Secret Lives of Cepheids: $\delta$ Cep -- the Prototype of a New Class of Pulsating X-ray Variable Stars
• Vetting Galactic Leavitt Law Calibrators using Radial Velocities: On the Variability, Binarity, and Possible Parallax Error of 19 Long-period Cepheids
• HARPS-N high spectral resolution observations of Cepheids I. The Baade-Wesselink projection factor of δ Cep revisited
• VEGA/CHARA interferometric observations of Cepheids. I. A resolved structure around the prototype classical Cepheid delta Cep in the visible spectral range
• Young and Intermediate-age Distance Indicators
• A Search for Mass Loss on the Cepheid Instability Strip using HI 21-cm Line Observations