He star evolutionary channel to intermediate-mass binary pulsar PSR J1802-2124

The intermediate-mass binary pulsars (IMBPs) are characterized by relatively long spin periods (10 - 200 ms) and massive ($\ga 0.4 M_{\odot}$) white dwarf (WD) companions. Recently, precise mass measurements have been performed for the pulsar and the WD in the IMBP PSR J1802-2124. Some observed properties, such as the low mass of the pulsar, the high mass of the WD, the moderately long spin period, and the tight orbit, imply that this system has undergone a peculiar formation mechanism. In this work, we attempt to simulate the detailed evolutionary history of PSR J1802-2124. We propose that a binary system consisting of a neutron star (NS, of mass $1.3 M_{\odot}$) and an He star (of mass $1.0 M_{\odot}$), and with an initial orbital period of 0.5 d, may have been the progenitor of PSR J1802-2124. Once the He star overflows its Roche lobe, He-rich material is transferred onto the NS at a relatively high rate of $\sim 10^{-7}-10^{-6} M_{\odot},\rm yr^{-1}$, which is significantly higher than the Eddington accretion rate. A large amount of the transferred material is ejected from the vicinity of the NS by radiation pressure and results in the birth of a mildly recycled pulsar. Our simulated results are consistent with the observed parameters of PSR J1802-2124. Therefore, we argue that the NS + He star evolutionary channel may be responsible for the formation of most IMBPs with orbital periods $\la 3 \rm d$.

💡 Research Summary

The paper addresses the formation of intermediate‑mass binary pulsars (IMBPs), focusing on the well‑characterized system PSR J1802‑2124, which exhibits a relatively long spin period (12.6 ms), a massive white‑dwarf companion (≈0.78 M⊙), and a short orbital period (≈0.70 d). Traditional recycling scenarios—where a neutron star (NS) in a low‑mass X‑ray binary (LMXB) accretes matter over long timescales—cannot easily account for the combination of a low NS mass, a high‑mass white dwarf, and a tight orbit. The authors therefore propose an alternative evolutionary channel involving a binary composed initially of a 1.3 M⊙ NS and a 1.0 M⊙ helium star (He star) with an orbital period of 0.5 d.

First, a population‑synthesis study using the rapid binary‑star evolution (BSE) code explores the parameter space that can produce NS + He star systems. By simulating one million primordial binaries over 12 Gyr, they find that most viable NS + He star progenitors have He‑star masses between 0.5 and 3 M⊙ and initial orbital periods between 0.01 and 1 d. The efficiency of common‑envelope (CE) ejection (α_CE) and the natal kick velocity dispersion are critical: a high α_CE (≈3) greatly enhances survival through the CE phase, while low kick velocities (σ≈20 km s⁻¹ for electron‑capture supernovae or accretion‑induced collapse) prevent binary disruption.

Next, detailed binary evolution is computed with Eggleton’s stellar‑evolution code for a representative system (M_NS,i = 1.3 M⊙, M_He,i = 1.0 M⊙, P_orb,i = 0.5 d). When the He star fills its Roche lobe, mass transfer proceeds at rates of 10⁻⁷–10⁻⁶ M⊙ yr⁻¹, far exceeding the Eddington limit for a neutron star (≈3 × 10⁻⁸ M⊙ yr⁻¹). The excess material is assumed to be expelled isotropically from the vicinity of the NS under radiation pressure, carrying away the specific orbital angular momentum of the NS. Consequently, only about 4 % of the transferred mass (≈0.01 M⊙) is actually accreted. This modest accretion is sufficient to spin the NS up to a period of ≲16 ms and to reduce its magnetic field, producing a mildly recycled pulsar. The mass transfer episode lasts roughly 0.25 Myr; during this time the orbit expands because mass flows from the less massive donor to the more massive accretor. At the end of the episode the He star has been stripped to a 0.81 M⊙ carbon‑oxygen white dwarf, and the orbital period has increased to 0.71 d.

The final system parameters (NS mass ≈1.31 M⊙, WD mass ≈0.81 M⊙, P_orb ≈0.71 d, spin ≈12 ms) match the observed values of PSR J1802‑2124 within uncertainties. The authors argue that this NS + He star channel naturally explains the formation of short‑period IMBPs, whereas previous models that invoke long‑term mass transfer from a more massive donor (intermediate‑mass X‑ray binaries) often require a common‑envelope phase that would either merge the binary or produce wider orbits inconsistent with observations.

In the discussion, the paper emphasizes that the key ingredients enabling this channel are (i) a low natal kick associated with electron‑capture supernovae or accretion‑induced collapse, and (ii) a high CE ejection efficiency that prevents coalescence during the two CE episodes required to produce the compact NS + He star progenitor. The authors suggest that most IMBPs with orbital periods ≲3 d can be formed via this route, and they encourage future observational surveys and detailed binary‑evolution modeling to test the prevalence of this channel.

In summary, the study provides a coherent and quantitatively supported evolutionary pathway—starting from a neutron star and a helium star in a tight orbit—that reproduces the observed properties of PSR J1802‑2124 and likely accounts for a substantial fraction of short‑period intermediate‑mass binary pulsars.