The "Giraffe": Discovery of a stripped red giant in an interacting binary with a ${\sim}2~M_\odot$ lower giant

Abstract

We report the discovery of a massive ∼2 − 3 𝑀 object as a binary companion to 2M04123153+6738486 (2M0412). 2M0412 is an evolved (𝑇 eff,giant 4200 K), luminous (𝐿 giant 230 𝐿 ) red giant in a circular 𝑃 = 81.2 d binary. 2M0412 is a known variable star previously classified as a semi-regular variable. The ASAS-SN, ATLAS, TESS and ZTF light curves show that the giant is a nearly Roche lobe filling ellipsoidal variable with an inclination of 41.9 • ± 0.1 • , a mass ratio of 𝑞 0.20 ± 0.01, a companion mass of 𝑀 comp = 2.97 ± 0.02 𝑀 , and a giant mass of 𝑀 giant = 0.60 ± 0.01 𝑀 for a distance of 4 kpc. The mass of the giant indicates that its envelope is stripped. The cross-correlation functions of the Keck/HIRES and LBT/PEPSI spectra show a second RV signal implying a consistent mass ratio of 𝑞 0.20 ± 0.01. We also identify an orbital phase dependent, broad H𝛼 emission line. The simplest explanation for the massive companion is a single mass-gap black hole or a high mass neutron star. A sufficiently faint stellar companion can only be made feasible by significantly reducing the distance to 3.5 kpc while simultaneously increasing the mass ratio to 𝑞 0.3, despite the multiple lines of evidence for 𝑞 0.2.

Unicorns and Giraffes in the binary zoo: stripped giants with subgiant companions

“…Phase-dependent variability of the Balmer lines is observed in both systems, suggesting ongoing mass transfer. Previous analyses (Jayasinghe et al 2021(Jayasinghe et al , 2022 concluded that the second luminous source in both systems is too faint to be a stellar companion of the required mass. These works proposed that the second light originates from an accretion disk around a BH companion (in the Giraffe), or from a diffuse circumbinary nebula (in the Unicorn).…”

“…The Giraffe (Jayasinghe et al 2022) is a binary in Camelopardalis containing a luminous red giant (𝐿 giant ≈ 200 𝐿 ) and a companion in an 81-day circular orbit. The spectroscopic mass function, 𝑓 (𝑀 2 ) ≡ 𝑀 2 sin 𝑖 3 /(𝑀 giant + 𝑀 2 ) 2 = 0.58 𝑀 , implies a relatively massive companion.…”

“…The spectroscopic mass function, 𝑓 (𝑀 2 ) ≡ 𝑀 2 sin 𝑖 3 /(𝑀 giant + 𝑀 2 ) 2 = 0.58 𝑀 , implies a relatively massive companion. By modeling the system's SED, light curves, and RVs, Jayasinghe et al (2022) inferred a giant mass of ≈ 0.6 𝑀 , inclination of ≈ 41 degrees, and unseen companion mass of ≈ 3 𝑀 . 1 A main-sequence star of this mass would overwhelm the observed SED at blue wavelengths, so they concluded that the companion is a BH.…”

“…1 A main-sequence star of this mass would overwhelm the observed SED at blue wavelengths, so they concluded that the companion is a BH. When they subtracted the best-fit spectral model for the giant from the observed optical spectra, Jayasinghe et al (2022) found residuals resembling the narrow-lined absorption spectrum of a star. The residual spectrum moves in anti-phase with the giant, but with an RV amplitude 5 times smaller.…”

“…★ E-mail: kareem.el-badry@cfa.harvard.edu Jayasinghe et al 2022). Both systems contain red giant stars in relatively wide orbits (60 and 81 days) and exhibit strong ellipsoidal variability due to tidal deformation of the giants.…”