A Helium-shell Burning Blue Horizontal Branch Star Produced from Common Envelope Evolution

Abstract: Observationally, blue horizontal branch (BHB) stars are defined as hot stars occupying a characteristic region between the extreme blue horizontal branch and RR Lyrae variables in the Hertzsprung-Russell diagram. Most of them are interpreted as stripped core-helium-burning stars, but the role of binary interaction in their formation remains unclear. Here, we report the discovery of a metal-rich BHB star in a 0.82628-day binary system (\Feige 64) comprising a $0.35\pm0.03\,M_{\odot}$ BHB star and a likely $1.26\pm0.17\,M_{\odot}$ white dwarf (WD). The BHB star has an effective temperature of $15{,}524\pm310$\, K and a luminosity of $39.7\pm4.1\,L_{\odot}$. Stellar evolution modelling indicates that it is a helium-shell-burning star produced through the common-envelope channel, retaining a hydrogen-rich envelope that is more massive than previously thought for low-mass stars. This finding provides direct evidence for binary interaction in the formation of BHB stars, offering a fresh perspective on interpreting this emerging population.

• The paper identifies Feige 64 as a 0.35 ± 0.03 solar-mass, helium-shell-burning blue horizontal branch star in a 0.826-day binary, using spectroscopy, photometry, astrometry, and MESA modelling.
• The paper finds that the star retains about 0.018 solar masses of hydrogen-rich envelope—above conventional post-common-envelope estimates—and reproduces its properties with a common-envelope efficiency of αCE ≈ 0.90.
• The paper concludes that the unseen companion is most likely a 1.26 ± 0.17 solar-mass white dwarf and that common-envelope evolution is a viable formation channel for metal-rich field blue horizontal branch stars, although the population frequency remains unknown.

Feige 64, a metal-rich blue horizontal branch (BHB) star in a 0.82628-day binary, provides direct observational evidence that common-envelope ejection (CEE) can produce BHB-like stars. Combining multi-epoch spectroscopy from Palomar 200-inch/DBSP, Keck-I/HIRES, and Xinglong 2.16-m/BFOSC with TESS photometry, Gaia DR3 astrometry, SED fitting, and MESA stellar evolution modelling, Li et al. identify the visible component as a stripped, helium-shell-burning star of $0.35\pm0.03\,M_{\odot}$ orbited by a likely $1.26\pm0.17\,M_{\odot}$ white dwarf (2608.11557). The result challenges the assumption that post-CEE remnants of low-mass stars retain at most $\sim0.01\,M_{\odot}$ of hydrogen envelope.

Observational characterization

The system was identified through a radial-velocity monitoring campaign of LAMOST-selected BHB and sdB candidates. A Lomb–Scargle periodogram of the TESS Sector 22 light curve shows a dominant peak at 0.4131391 d; folding at twice this period yields two photometric maxima and a single RV maximum, consistent with ellipsoidal modulation at an orbital period of $P = 0.8262782(110)$ d. A nearby source 8.4″ away contributes third-light contamination ($f = 0.974$), corrected using Gaia $G_{\rm RP}$ magnitudes; the corrected TESS curve agrees well with ZTF $r$- and $i$-band data.

NLTE spectral fitting against the TLUSTY BSTAR2006 grid gives $T_{\rm eff} = 15524 \pm 307$ K (including a 295 K systematic offset from assuming $Z = 0.7\,Z_\odot$ rather than the light-element-dominated $1.26\pm0.17\,M_{\odot}$0), $1.26\pm0.17\,M_{\odot}$1 (cgs), and $1.26\pm0.17\,M_{\odot}$2. The projected rotational velocity is $1.26\pm0.17\,M_{\odot}$3 km s$1.26\pm0.17\,M_{\odot}$4 — notably rapid for a BHB star hotter than 11,500 K, where radiative levitation typically suppresses rotation. This rapid rotation is naturally explained by tidal synchronization with the 0.83-day orbit: the light-curve solution implies a synchronized $1.26\pm0.17\,M_{\odot}$5 of $1.26\pm0.17\,M_{\odot}$6 km s$1.26\pm0.17\,M_{\odot}$7. However, the light curve cannot constrain internal rotation, so whether the core is also synchronized remains open.

SED fitting with SPEEDYFIT over GALEX through ALLWISE photometry, combined with the corrected Gaia parallax ($1.26\pm0.17\,M_{\odot}$8 pc), yields $1.26\pm0.17\,M_{\odot}$9, $\sim0.01\,M_{\odot}$0, and no infrared excess. Light-curve modelling with LCURVE under Gaussian priors on $\sim0.01\,M_{\odot}$1, radius, and $\sim0.01\,M_{\odot}$2 gives an inclination of $\sim0.01\,M_{\odot}$3, hence $\sim0.01\,M_{\odot}$4 and $\sim0.01\,M_{\odot}$5. These results are robust to inflating the $\sim0.01\,M_{\odot}$6 prior to 0.1 dex, which shifts the companion mass to $\sim0.01\,M_{\odot}$7.

Nature of the unseen companion

The companion's identity follows by elimination. A main-sequence companion of the required mass would produce eclipses for $\sim0.01\,M_{\odot}$8 (not observed) and a clear infrared excess (also not observed). A hot subdwarf companion would contribute detectable optical flux inconsistent with the spectrum, and the mass function demands $\sim0.01\,M_{\odot}$9 even at $P = 0.8262782(110)$0 — too massive for an sdB. A neutron star is disfavored but not excluded: wind accretion at the Vink-predicted rate of $P = 0.8262782(110)$1 should yield an X-ray flux of $P = 0.8262782(110)$2 erg cm$P = 0.8262782(110)$3 s$P = 0.8262782(110)$4, above the SRG/eROSITA $P = 0.8262782(110)$5 limit of $P = 0.8262782(110)$6 erg cm$P = 0.8262782(110)$7 s$P = 0.8262782(110)$8, and FAST observations (1.75 h on-source) detected no radio pulsations down to DM range 0–200 pc cm$P = 0.8262782(110)$9. The authors conclude the companion is most likely a white dwarf, while explicitly conceding that a neutron star cannot be entirely ruled out.

Evolutionary state and formation history

Galactic orbit integration places Feige 64 in the thin disk ($f = 0.974$0), with a kinematic age of $f = 0.974$1 Gyr. This age rules out isolated evolution from a progenitor below $f = 0.974$2, since such stars have main-sequence lifetimes exceeding the age of the Universe — establishing that binary interaction was required.

MESA models constructed by artificially stripping red-giant-tip envelopes show that total masses of $f = 0.974$3–$f = 0.974$4 reproduce the observed HR- and Kiel-diagram positions. The best-fitting model has a helium-core mass of $f = 0.974$5 and an envelope mass of $f = 0.974$6, remaining in the BHB-like phase for $f = 0.974$7 Myr. Its interior structure shows helium nearly exhausted in the core (central mass fraction below $f = 0.974$8) with active helium-shell burning and luminosity dominated by hydrogen-shell burning. The predicted surface abundances match the Keck-I measurements. This envelope mass exceeds the $f = 0.974$9 conventionally assumed for low-mass post-CE remnants, because the non-degenerate helium ignition in a $G_{\rm RP}$0 progenitor produces an extended core–envelope transition region of order $G_{\rm RP}$1. The retained envelope thus acts as a fossil record of that transition region and constrains CE stripping physics directly.

A proto-ELM WD interpretation is rejected on two grounds: ELM WDs have $G_{\rm RP}$2, and reproducing the observed mass–period relation via CEE would require $G_{\rm RP}$3, energetically implausible. For the favored CE channel, the energy-budget prescription applied to a $G_{\rm RP}$4 progenitor with a $G_{\rm RP}$5 core and the observed final separation yields $G_{\rm RP}$6, confirming feasibility. The full formation scenario involves two CEE episodes: a $G_{\rm RP}$7–$G_{\rm RP}$8 primary first produced the WD, then the $G_{\rm RP}$9 secondary initiated a second CEE at the RGB tip after an initial orbital period of 2000–3000 days. Future evolution modelling indicates the remnant will undergo brief mass transfer, become a double compact binary, and merge via gravitational-wave radiation only after $r$0 Gyr — not within a Hubble time.

Limitations and open questions

Several assumptions bear on the results. The companion mass rests on the ellipsoidal-amplitude–inclination degeneracy resolved with priors on spectroscopic parameters; the companion's temperature and radius are fixed arbitrarily since its flux contribution is unconstrained. The TESS dilution correction assumes the neighbor's flux ratio in the TESS band matches that in $r$1. The neutron-star exclusion depends on uncertain wind-accretion efficiency assumptions. Most significantly, the paper identifies one system; whether the CEE channel contributes substantially to the metal-rich field BHB population, or how many such systems exist among current BHB catalogs, remains quantitatively unaddressed.

Conclusion

Feige 64 demonstrates empirically that common-envelope ejection can produce a helium-shell-burning star occupying the BHB region of the HR diagram, retaining a thicker hydrogen envelope ($r$2) than previously recognized for low-mass post-CE remnants. The system establishes binary interaction as a viable formation channel for metal-rich BHB stars and provides a concrete energetic constraint ($r$3) on CE physics. The key open question is the population-level frequency of such objects among metal-rich BHB candidates identified in large spectroscopic surveys.