Monthly Notices of the Royal Astronomical Society Letters

Katz, J I

doi: 10.1093/mnrasl/slac080pmid: N/A

ABSTRACTRecent CHIME/FRB observations of the periodic repeating fast radio bursts (FRB) 180916B have produced a homogeneous sample of 44 bursts. These permit a redetermination of the modulation period and phase window, in agreement with earlier results. If the periodicity results from the precession of an accretion disc, in analogy with those of Her X-1, SS 433, and many other superorbital periods, the width of the observable phase window indicates that the disc axis jitters by an angle of about 0.14 of the inclination angle, similar to the ratio of 0.14 in the well-observed jittering jet source SS 433.

Hunt, Jason A S; Price-Whelan, Adrian M; Johnston, Kathryn V; Darragh-Ford, Elise

doi: 10.1093/mnrasl/slac082pmid: N/A

ABSTRACTGaia Data Release 2 revealed that the Milky Way contains significant indications of departures from equilibrium in the form of asymmetric features in the phase space density of stars in the Solar neighbourhood. One such feature is the z–vz phase spiral, interpreted as the response of the disc to the influence of a perturbation perpendicular to the disc plane, which could be external (e.g. a satellite) or internal (e.g. the bar or spiral arms). In this work, we use Gaia Data Release 3 to dissect the phase spiral by dividing the local data set into groups with similar azimuthal actions, Jϕ, and conjugate angles, θϕ, which selects stars on similar orbits and at similar orbital phases, thus having experienced similar perturbations in the past. These divisions allow us to explore areas of the Galactic disc larger than the surveyed region. The separation improves the clarity of the z–vz phase spiral and exposes changes to its morphology across the different action-angle groups. In particular, we discover a transition to two armed ‘breathing spirals’ in the inner Milky Way. We conclude that the local data contain signatures of not one, but multiple perturbations with the prospect to use their distinct properties to infer the properties of the interactions that caused them.

Zhang, Chen; Han, Wen-Biao; Yang, Shu-Cheng

doi: 10.1093/mnrasl/slac100pmid: N/A

ABSTRACTGravitational waves from precessing binary black holes exhibit new features that are absent in non-precessionary systems. All current waveform models take into account only the modulation of the signal due to precession. In this letter, we find that this effect has its own signature, by gravitational emission of a short and transient signal, or burst. The frequency of the burst is comparable to that of the late stage of the inspiral. We show that under certain conditions, this signal is strong enough to be detected by Advanced LIGO. For third-generation detectors like the Einstein telescope, the calculated signal-to-noise ratio can reach higher values. Measurements of precession would provide valuable insights into the intrinsic structure of black holes, and therefore into astrophysical binary formation mechanisms.

Camisassa, Maria E; Raddi, Roberto; Althaus, Leandro G; Isern, Jordi; Rebassa-Mansergas, Alberto; Torres, Santiago; Córsico, Alejandro H; Korre, Lydia

doi: 10.1093/mnrasl/slac078pmid: N/A

ABSTRACTUltra-massive white dwarfs ($1.05\, \rm M_\odot \lesssim M_{WD}$) are particularly interesting objects that allow us to study extreme astrophysical phenomena such as type Ia supernovae explosions and merger events. Traditionally, ultra-massive white dwarfs are thought to harbour oxygen–neon (ONe) cores. However, recent theoretical studies and new observations suggest that some ultra-massive white dwarfs could harbour carbon–oxygen (CO) cores. Although several studies have attempted to elucidate the core composition of ultra-massive white dwarfs, to date, it has not been possible to distinguish them through their observed properties. Here, we present a new method for revealing the core-chemical composition in ultra-massive white dwarfs that is based on the study of magnetic fields generated by convective mixing induced by the crystallization process. ONe white dwarfs crystallize at higher luminosities than their CO counterparts. Therefore, the study of magnetic ultra-massive white dwarfs in the particular domain where ONe cores have reached the crystallization conditions but CO cores have not, may provide valuable support to their ONe core-chemical composition, since ONe white dwarfs would display signs of magnetic fields and CO would not. We apply our method to eight white dwarfs with magnetic field measurements and we suggest that these stars are candidate ONe white dwarfs.

Thuan, T X; Guseva, N G; Izotov, Y I

doi: 10.1093/mnrasl/slac095pmid: N/A

ABSTRACTRecently, Kojima and co-authors have reported a record low oxygen abundance, 12 + logO/H = 6.90 ± 0.03, or 1.6 per cent of solar metallicity, in the low-mass star-forming galaxy HSC J1631 + 4426. This exceptionally low oxygen abundance was obtained by the direct method, using the [O iii]λ4363Å emission line. However, using the strong-line method by Izotov et al. (2019b), these authors have derived a significantly higher metallicity 12 + logO/H = 7.175 ± 0.005. To clarify the situation, we have obtained new observations of HSC J1631 + 4426 with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). We have derived a higher oxygen abundance, 12 + logO/H = 7.14 ± 0.03, using the direct method, a value similar to the oxygen abundance obtained by the strong-line method. Thus, HSC J1631 + 4426 has a metallicity close to that of the well known blue compact dwarf galaxy I Zw 18.

Oudmaijer, René D; Jones, Emma R M; Vioque, Miguel

doi: 10.1093/mnrasl/slac088pmid: N/A

ABSTRACTThis paper presents the first census of Galactic post-asymptotic giant branch (post-AGB) stars in the Hertzsprung–Russell (HR) diagram. We combined Gaia DR3 parallax-based distances with extinction corrected integrated fluxes and derived luminosities for a sample of 185 stars that had been proposed to be post-AGB stars in the literature. The luminosities allow us to create an HR diagram containing the largest number of post-AGB candidate objects to date. A significant fraction of the objects fall outside the typical luminosity range as covered by theoretical evolutionary post-AGB tracks as well as observed for planetary nebula central stars. These include massive evolved supergiants and lower luminosity objects. Here, we highlight the fact that one-third of the post-AGB candidates are underluminous and we identify these with the recently recognized class of post-red giant branch objects thought to be the result of binary evolution.

Chand, Krishan; , Gopal-Krishna

doi: 10.1093/mnrasl/slac066pmid: N/A

ABSTRACTFlat-spectrum radio quasars (FSRQs), whose brightness is dominated by a relativistically beamed core, are frequently found in the ‘blazar state’ commonly inferred from a high optical polarization (> 3 per cent) and/or a large continuum variability. Here we use these two prime optical markers to investigate continuance of an FSRQ in the blazar (or non-blazar) state over an exceptionally long time baseline spanning four decades. Our basic sample is a well-defined, unbiased set of 80 FSRQs whose blazar state stood confirmed during the 1980s from optical polarimetry. Four decades later, the blazar state of each FSRQ is ascertained here from variability of their optical light curves with a typical duration of ∼3.5 yr, a low noise (rms ∼2 per cent) and good cadence (∼3 d), obtained under the Zwicky Transient Facility project, which has been ongoing since 2018. For about 40 per cent of these FSRQs, the blazar state could be ascertained additionally from the opto-polarimetric survey RoboPol (2013–2017). From both these data bases, it is found that only ∼10 per cent of the FSRQs have undergone a blazar ↔ non-blazar state transition over the past three to four decades. This reinforces the case for the long-term stability of the blazar state in individual FSRQs, despite their state fluctuating more commonly on year-like time-scales.

Koley, Atanu; Roy, Nirupam; Momjian, Emmanuel; Sarma, Anuj P; Datta, Abhirup

doi: 10.1093/mnrasl/slac085pmid: N/A

ABSTRACTMeasurement of magnetic fields in dense molecular clouds is essential for understanding the fragmentation process prior to star formation. Radio interferometric observations of CCS 22.3 GHz emission, from the starless core TMC-1C, have been carried out with the Karl G. Jansky Very Large Array to search for Zeeman splitting of the line in order to constrain the magnetic field strength. Toward a region offset from the dust peak, we report a detection of the Zeeman splitting of the CCS 21–10 transition, with an inferred magnetic field of ∼2 mG. If we interpret the dust peak to be the core of TMC-1C, and the region where we have made a detection of the magnetic field to be the envelope, then our observed value for the magnetic field is consistent with a subcritical mass-to-flux ratio envelope around a core with supercritical mass-to-flux ratio. The ambipolar diffusion time-scale for the formation of the core is consistent with the relevant time-scale based on chemical modelling of the TMC-1C core. This work demonstrates the potential of deep CCS observation to carry out future measurements of magnetic field strengths in dense molecular clouds and, in turn, understand the role of the magnetic field in star formation.

Chan, Man Ho; Del Popolo, Antonino

doi: 10.1093/mnrasl/slac091pmid: N/A

ABSTRACTIn the past few decades, some studies pointed out that magnetic field might affect the rotation curves in galaxies. However, the impact is relatively small compared with the effects of dark matter and the baryonic components. In this letter, we revisit the impact of magnetic field on the rotation curve of our Galaxy. We show that the inner Galactic rotation curve could be affected significantly by the magnetic field. The addition of the inner bulge component, which has been proposed previously to account for the inner rotation curve data, is not necessary. The magnetic field contribution can fully account for the excess of the inner rotation velocity between 5 to 50 pc from the Galactic Centre. Our analysis can also constrain the azimuthal component of the central regular magnetic field strength to $B_0 \sim 50-60\, \mu$G, which is consistent with the observed range.

Parker, Richard J; Daffern-Powell, Emma C

doi: 10.1093/mnrasl/slac086pmid: N/A

ABSTRACTExoplanets display incredible diversity, from planetary system architectures around Sun-like stars that are very different from our Solar system, to planets orbiting post-main-sequence stars or stellar remnants. Recently, the B-star Exoplanet Abundance STudy (BEAST) reported the discovery of at least two super-Jovian planets orbiting massive stars in the Sco Cen OB association. Whilst such massive stars do have Keplerian discs, it is hard to envisage gas giant planets being able to form in such hostile environments. We use N-body simulations of star-forming regions to show that these systems can instead form from the capture of a free-floating planet or the direct theft of a planet from one star to another, more massive star. We find that this occurs on average once in the first 10 Myr of an association’s evolution, and that the semimajor axes of the hitherto confirmed BEAST planets (290 and 556 au) are more consistent with capture than theft. Our results lend further credence to the notion that planets on more distant (>100 au) orbits may not be orbiting their parent star.