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1. High mass function ellipsoidal variables in the Gaia Focused Product Release: searching for black hole candidates in the binary zoo

   https://doi.org/10.33232/001c.116170  

   Rowan, Dominick M; Thompson, Todd A; Jayasinghe, Tharindu; Kochanek, Christopher S; Stanek, Krzysztof Z (January 2024, The Open Journal of Astrophysics)

   The recent Gaia Focused Product Release contains radial velocity time-series for more than 9,000 Gaia long-period photometric variables. Here we search for binary systems with large radial velocity amplitudes to identify candidates with massive, unseen companions. Eight targets have binary mass function $f\left(M\right)>1M_{\odot }$, three of which are eclipsing binaries. The remaining five show evidence of ellipsoidal modulations. We fit spectroscopic orbit models to the Gaia radial velocities, and fit the spectral energy distributions of three targets. For the two systems most likely to host dark companions, J0946 and J1640, we use PHOEBE to fit the ASAS-SN light curves and Gaia radial velocities. The derived companion masses are $>3M_{\odot }$, but the high Galactic dust extinctions towards these objects limit our ability to rule out main sequence companions or subgiants hotter than the photometric primaries. These systems are similar to other stellar-mass black hole impostors, notably the Unicorn (V723 Mon) and the Giraffe (2M04123153+6738486). While it is possible that J1640 and J0946 are similar examples of stripped giant star binaries, high-resolution spectra can be used to determine the nature of their companions. 

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2. The Symbiotic X-Ray Binary IGR J16194-2810: A Window on the Future Evolution of Wide Neutron Star Binaries From Gaia

   https://doi.org/10.1088/1538-3873/ad5dfd  

   Nagarajan, Pranav; El-Badry, Kareem; Lam, Casey; Reggiani, Henrique (July 2024, Publications of the Astronomical Society of the Pacific)

   Abstract We present optical follow-up of IGR J16194-2810, a hard X-ray source discovered by the INTEGRAL mission. The optical counterpart is a ∼500L_⊙red giant at a distance of 2.1 kpc. We measured 17 radial velocities (RVs) of the giant over a period of 271 days. Fitting these RVs with a Keplerian model, we find an orbital period ofP_orb= 192.73 ± 0.01 days and a companion mass functionf(M₂) = 0.365 ± 0.003M_⊙. We detect ellipsoidal variability with the same period in optical light curves from the ASAS-SN survey. Joint fitting of the RVs, light curves, and the broadband spectral energy distribution allows us to robustly constrain the masses of both components. We find a giant mass of $M_{\star }={0.99}_{-0.03}^{+0.02}{M}_{\odot }$and a companion mass of $M_2={1.23}_{-0.03}^{+0.05}{M}_{\odot }$, implying that the companion is a neutron star (NS). We recover a 4.06 hr period in the system’s TESS light curve, which we tentatively associate with the NS spin period. The giant does not yet fill its Roche lobe, suggesting that current mass transfer is primarily via winds. Modules for Experiments in Stellar Astrophysics evolutionary models predict that the giant will overflow its Roche lobe in 5–10 Myr, eventually forming a recycled pulsar + white dwarf binary with a ∼900 days period. IGR J16194-2810 provides a window on the future evolution of wide NS + main sequence binaries recently discovered via Gaia astrometry. As with those systems, the binary’s formation history is uncertain. Before the formation of the NS, it likely survived a common envelope episode with a donor-to-accretor mass ratio ≳10 and emerged in a wide orbit. The NS likely formed with a weak kick (v_kick≲ 50 km s⁻¹), as stronger kicks would have disrupted the orbit. 

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3. ESPRESSO Observations of Gaia BH1: High-precision Orbital Constraints and no Evidence for an Inner Binary

   https://doi.org/10.1088/1538-3873/ad1ba7  

   Nagarajan, Pranav; El-Badry, Kareem; Triaud, Amaury H. M. J.; Baycroft, Thomas A.; Latham, David; Bieryla, Allyson; Buchhave, Lars A.; Rix, Hans-Walter; Quataert, Eliot; Howard, Andrew; et al (January 2024, Publications of the Astronomical Society of the Pacific)

   Abstract We present high-precision radial velocity observations of Gaia BH1, the nearest known black hole (BH). The system contains a solar-type G star orbiting a massive dark companion, which could be either a single BH or an inner BH + BH binary. A BH + BH binary is expected in some models where Gaia BH1 formed as a hierarchical triple, which is attractive because they avoid many of the difficulties associated with forming the system through isolated binary evolution. Our observations test the inner binary scenario. We have measured 115 precise RVs of the G star, including 40 from ESPRESSO with a precision of 3–5 m s⁻¹, and 75 from other instruments with a typical precision of 30–100 m s⁻¹. Our observations span 2.33 orbits of the G star and are concentrated near a periastron passage, when perturbations due to an inner binary would be largest. The RVs are well-fit by a Keplerian two-body orbit and show no convincing evidence of an inner binary. UsingREBOUNDsimulations of hierarchical triples with a range of inner periods, mass ratios, eccentricities, and orientations, we show that plausible inner binaries with periodsP_inner≳ 1.5 days would have produced larger deviations from a Keplerian orbit than observed. Binaries withP_inner≲ 1.5 days are consistent with the data, but these would merge within a Hubble time and would thus imply fine-tuning. We present updated parameters of Gaia BH1's orbit. The RVs yield a spectroscopic mass function $f\left(M_{\mathrm{BH}}\right)=3.9358\pm 0.0002M_{\odot }$—about 7000σabove the ∼2.5M_⊙maximum neutron star mass. Including the inclination constraint from Gaia astrometry, this implies a BH mass ofM_BH= 9.27 ± 0.10M_⊙. 

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4. TOI-2015 b: A Warm Neptune with Transit Timing Variations Orbiting an Active Mid-type M Dwarf

   https://doi.org/10.3847/1538-3881/ad55ea  

   Jones, Sinclaire_E; Stefánsson, Guđmundur; Masuda, Kento; Libby-Roberts, Jessica_E; Gardner, Cristilyn_N; Holcomb, Rae; Beard, Corey; Robertson, Paul; Cañas, Caleb_I; Mahadevan, Suvrath; et al (July 2024, The Astronomical Journal)

   Abstract We report the discovery of a close-in (P_orb= 3.349 days) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby (d= 47.3 pc) active M4 star, TOI-2015. We characterize the planet's properties using Transiting Exoplanet Survey Satellite (TESS) photometry, precise near-infrared radial velocities (RVs) with the Habitable-zone Planet Finder Spectrograph, ground-based photometry, and high-contrast imaging. A joint photometry and RV fit yields a radius $R_p={3.37}_{-0.20}^{+0.15}{R}_{\oplus }$, mass $m_p={16.4}_{-4.1}^{+4.1}{M}_{\oplus }$, and density ${\rho }_p={2.32}_{-0.37}^{+0.38}\mathrm{g}{\mathrm{cm}}^{-3}$for TOI-2015 b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period ofP_rot= 8.7 ± 0.9 days and associated rotation-based age estimate of 1.1 ± 0.1 Gyr. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super-period $P_{\sup }\approx 430\mathrm{days}$and amplitude ∼100 minutes. After considering multiple likely period-ratio models, we show an outer planet candidate near a 2:1 resonance can explain the observed TTVs while offering a dynamically stable solution. However, other possible two-planet solutions—including 3:2 and 4:3 resonances—cannot be conclusively excluded without further observations. Assuming a 2:1 resonance in the joint TTV-RV modeling suggests a mass of $m_b={13.3}_{-4.5}^{+4.7}{M}_{\oplus }$for TOI-2015 b and $m_c={6.8}_{-2.3}^{+3.5}{M}_{\oplus }$for the outer candidate. Additional transit and RV observations will be beneficial to explicitly identify the resonance and further characterize the properties of the system. 

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5. Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

   https://doi.org/10.3847/2041-8213/ac9b3d  

   Wu, Samantha_C; Fuller, Jim (November 2022, The Astrophysical Journal Letters)

   Abstract Many core-collapse supernovae (SNe) with hydrogen-poor and low-mass ejecta, such as ultra-stripped SNe and type Ibn SNe, are observed to interact with dense circumstellar material (CSM). These events likely arise from the core collapse of helium stars that have been heavily stripped by a binary companion and have ejected significant mass during the last weeks to years of their lives. In helium star models run to days before core collapse we identify a range of helium core masses ≈2.5–3M_⊙whose envelopes expand substantially due to the helium shell burning while the core undergoes neon and oxygen burning. When modeled in binary systems, the rapid expansion of these helium stars induces extremely high rates of late-stage mass transfer ( $\dot{M}\gtrsim {10}^{-2}{M}_{\odot }{\mathrm{yr}}^{-1}$) beginning weeks to decades before core collapse. We consider two scenarios for producing CSM in these systems: either mass transfer remains stable and mass loss is driven from the system in the vicinity of the accreting companion, or mass transfer becomes unstable and causes a common envelope event (CEE) through which the helium envelope is unbound. The ensuing CSM properties are consistent with the CSM masses (∼10⁻²–1M_⊙) and radii (∼10¹³–10¹⁶cm) inferred for ultra-stripped SNe and several type Ibn SNe. Furthermore, systems that undergo a CEE could produce short-period neutron star binaries that merge in less than 100 Myr. 

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