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1. Kinematics of the Central Stars Powering Bowshock Nebulae and the Large Multiplicity Fraction of Runaway OB Stars

   https://doi.org/10.3847/1538-3881/ac7f2b  

   Kobulnicky, Henry A. ; Chick, William T. ( August 2022 , The Astronomical Journal)

   OB stars powering stellar bowshock nebulae (SBNe) have been presumed to have large peculiar velocities. We measured peculiar velocities of SBN central stars to assess their kinematics relative to the general O-star population using Gaia EDR3 data for 267 SBN central stars and a sample of 455 Galactic O stars to derive projected velocitiesv_2D. For a subset of each sample, we obtained new optical spectroscopy to measure radial velocities and identify multiple-star systems. We find a minimum multiplicity fraction of 36% ± 6% among SBN central stars, consistent with >28% among runaway Galactic O stars. The large multiplicity fraction among runaways implicates very efficient dynamical ejection rather than binary-supernova origins. The medianv_2Dof SBN central stars isv_2D= 14.6 km s⁻¹, larger than the medianv_2D= 11.4 km s⁻¹for non-bowshock O stars. Central stars of SBNe have a runaway (v_2D> 25 km s⁻¹) fraction of 24${}_{-7}^{+9}$%, consistent with the${22}_{-3}^{+3}$% for control-sample O stars. Most (76%) SBNe central stars are not runaways. Our analysis of alignment (Δ_PA) between the nebular morphological andv_2Dkinematic position angles reveals two populations: a highly aligned (σ_PA= 25°) population that includes stars with the largestv_2D(31% of the sample) and a random (nonaligned) population (69%). SBNe that lie within or near Hiiregions comprise a larger fraction of this latter component than SBNe in isolated environments, implicating localized ISM flows as a factor shaping their orientations and morphologies. We outline a new conceptual approach to computing the solar local standard of rest motion, yielding [U_⊙,V_⊙,W_⊙] = [5.5, 7.5,4.5] km s⁻¹.

    

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2. Modeling Dense Star Clusters in the Milky Way and beyond with the Cluster Monte Carlo Code

   https://doi.org/10.3847/1538-4365/ac2edf  

   Rodriguez, Carl L. ; Weatherford, Newlin C. ; Coughlin, Scott C. ; Amaro-Seoane, Pau ; Breivik, Katelyn ; Chatterjee, Sourav ; Fragione, Giacomo ; Kıroğlu, Fulya ; Kremer, Kyle ; Rui, Nicholas Z. ; et al ( January 2022 , The Astrophysical Journal Supplement Series)

   We describe the public release of the Cluster Monte Carlo (CMC) code, a parallel, star-by-starN-body code for modeling dense star clusters.CMCtreats collisional stellar dynamics using Hénon’s method, where the cumulative effect of many two-body encounters is statistically reproduced as a single effective encounter between nearest-neighbor particles on a relaxation timescale. The star-by-star approach allows for the inclusion of additional physics, including strong gravitational three- and four-body encounters, two-body tidal and gravitational-wave captures, mass loss in arbitrary galactic tidal fields, and stellar evolution for both single and binary stars. The public release ofCMCis pinned directly to theCOSMICpopulation synthesis code, allowing dynamical star cluster simulations and population synthesis studies to be performed using identical assumptions about the stellar physics and initial conditions. As a demonstration, we present two examples of star cluster modeling: first, we perform the largest (N= 10⁸) star-by-starN-body simulation of a Plummer sphere evolving to core collapse, reproducing the expected self-similar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution (such as GW190521, a recently reported merger with component masses in the pulsational pair-instability mass gap).

    

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3. INFERNO: Galactic winds in dwarf galaxies with star-by-star simulations including runaway stars

   https://doi.org/10.1093/mnras/stad692  

   Andersson, Eric P. ; Agertz, Oscar ; Renaud, Florent ; Teyssier, Romain ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   The formation and evolution of galaxies have proved sensitive to the inclusion of stellar feedback, which is therefore crucial to any successful galaxy model. We present INFERNO, a new model for hydrodynamic simulations of galaxies, which incorporates resolved stellar objects with star-by-star calculations of when and where the injection of enriched material, momentum, and energy takes place. INFERNO treats early stellar kinematics to include phenomena such as walkaway and runaway stars. We employ this innovative model on simulations of a dwarf galaxy and demonstrate that our physically motivated stellar feedback model can drive vigorous galactic winds. This is quantified by mass and metal loading factors in the range of 10–100, and an energy loading factor close to unity. Outflows are established close to the disc, are highly multiphase, spanning almost 8 orders of magnitude in temperature, and with a clear dichotomy between mass ejected in cold, slow-moving (T ≲ 5 × 104 K, v < 100 km s−1) gas and energy ejected in hot, fast-moving (T > 106 K, v > 100 km s−1) gas. In contrast to massive disc galaxies, we find a surprisingly weak impact of the early stellar kinematics, with runaway stars having little to no effect on our results, despite exploding in diffuse gas outside the dense star-forming gas, as well as outside the galactic disc entirely. We demonstrate that this weak impact in dwarf galaxies stems from a combination of strong feedback and a porous interstellar medium, which obscure any unique signatures that runaway stars provide.

    

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4. Volumetric Rates of Luminous Red Novae and Intermediate-luminosity Red Transients with the Zwicky Transient Facility

   https://doi.org/10.3847/1538-4357/acc2b9  

   Karambelkar, Viraj R. ; Kasliwal, Mansi M. ; Blagorodnova, Nadejda ; Sollerman, Jesper ; Aloisi, Robert ; Anand, Shreya G. ; Andreoni, Igor ; Brink, Thomas G. ; Bruch, Rachel ; Cook, David ; et al ( May 2023 , The Astrophysical Journal)

   Luminous red novae (LRNe) are transients characterized by low luminosities and expansion velocities, and they are associated with mergers or common-envelope ejections in stellar binaries. Intermediate-luminosity red transients (ILRTs) are an observationally similar class with unknown origins, but they are generally believed to be either electron-capture supernovae in super-asymptotic giant branch stars or outbursts in dusty luminous blue variables (LBVs). In this paper, we present a systematic sample of eight LRNe and eight ILRTs detected as part of the Census of the Local Universe (CLU) experiment on the Zwicky Transient Facility (ZTF). The CLU experiment spectroscopically classifies ZTF transients associated with nearby (<150 Mpc) galaxies, achieving 80% completeness form_r< 20 mag. Using the ZTF-CLU sample, we derive the first systematic LRNe volumetric rate of${7.8}_{-3.7}^{+6.5}\times {10}^{-5}$Mpc⁻³yr⁻¹in the luminosity range −16 ≤M_r≤ −11 mag. We find that, in this luminosity range, the LRN rate scales as$\mathit{dN}/\mathit{dL}\propto L^{-2.5\pm 0.3}$—significantly steeper than the previously derived scaling ofL^−1.4±0.3for lower-luminosity LRNe (M_V≥ −10 mag). The steeper power law for LRNe at high luminosities is consistent with the massive merger rates predicted by binary population synthesis models. We find that the rates of the brightest LRNe (M_r≤ −13 mag) are consistent with a significant fraction of them being progenitors of double compact objects that merge within a Hubble time. For ILRTs, we derive a volumetric rate of${2.6}_{-1.4}^{+1.8}\times {10}^{-6}$Mpc⁻³yr⁻¹forM_r≤ −13.5 mag, which scales as$\mathit{dN}/\mathit{dL}\propto L^{-2.5\pm 0.5}$. This rate is ∼1%–5% of the local core-collapse supernova rate and is consistent with theoretical ECSN rate estimates.

    

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5. 4–8 GHz Fourier-domain Searches for Galactic Center Pulsars

   https://doi.org/10.3847/1538-4357/ac74c0  

   Suresh, Akshay ; Cordes, James M. ; Chatterjee, Shami ; Gajjar, Vishal ; Perez, Karen I. ; Siemion, Andrew P. V. ; Lebofsky, Matt ; MacMahon, David H. E. ; Ng, Cherry ( July 2022 , The Astrophysical Journal)

   The Galactic Center (GC), with its high density of massive stars, is a promising target for radio transient searches. In particular, the discovery and timing of a pulsar orbiting the central supermassive black hole (SMBH) of our galaxy will enable stringent strong-field tests of gravity and accurate measurements of SMBH properties. We performed multiepoch 4–8 GHz observations of the inner ≈15 pc of our galaxy using the Robert C. Byrd Green Bank Telescope in 2019 August–September. Our investigations constitute the most sensitive 4–8 GHz GC pulsar survey conducted to date, reaching down to a 6.1 GHz pseudo-luminosity threshold of ≈1 mJy kpc²for a pulse duty cycle of 2.5%. We searched our data in the Fourier domain for periodic signals incorporating a constant or linearly changing line-of-sight pulsar acceleration. We report the successful detection of the GC magnetar PSR J1745−2900 in our data. Our pulsar searches yielded a nondetection of novel periodic astrophysical emissions above a 6σdetection threshold in harmonic-summed power spectra. We reconcile our nondetection of GC pulsars with inadequate sensitivity to a likely GC pulsar population dominated by millisecond pulsars. Alternatively, close encounters with compact objects in the dense GC environment may scatter pulsars away from the GC. The dense central interstellar medium may also favorably produce magnetars over pulsars.

    

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