Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 (‘Oumuamua) and 2I/Borisov

Context. The orientation of the spin axis of a comet is defined by the values of its equatorial obliquity and its cometocentric longitude of the Sun at perihelion. These parameters can be computed from the components of the nongravitational force caused by outgassing if the cometary activity is well characterized. The trajectories of known interstellar bodies passing through the Solar System show nongravitational accelerations. Aims. The spin-axis orientation of 1I/2017 U1 (‘Oumuamua) remains to be determined; for 2I/Borisov, the already released results are mutually exclusive. In both cases, the values of the components of the nongravitational force are relatively well constrained. Here, we investigate – within the framework of the forced precession model of a nonspherical cometary nucleus – the orientation of the spin axes of ‘Oumuamua and 2I/Borisov using public orbit determinations that consider outgassing. Methods. We applied a Monte Carlo simulation using the covariance matrix method together with Monte Carlo random search techniques to compute the distributions of equatorial obliquities and cometocentric longitudes of the Sun at perihelion of ‘Oumuamua and 2I/Borisov from the values of the nongravitational parameters. Results. We find that the equatorial obliquity of ‘Oumuamua could be about 93°, if it has a very prolate (fusiform) shape, or close to 16°, if it is very oblate (disk-like). Different orbit determinations of 2I/Borisov gave obliquity values of 59° and 90°. The distributions of cometocentric longitudes were in general multimodal. Conclusions. Our calculations suggest that the most probable spin-axis direction of ‘Oumuamua in equatorial coordinates is (280°, +46°) if very prolate or (312°, −50°) if very oblate. Our analysis favors a prolate shape. For the orbit determinations of 2I/Borisov used here, we find most probable poles pointing near (275°, +65°) and (231°, +30°), respectively. Although our analysis favors an oblate shape for 2I/Borisov, a prolate one cannot be ruled out.

Shape and spin of minihaloes – II. The effect of streaming velocities

ABSTRACT Models of the decoupling of baryons and photons during the recombination epoch predict the existence of a large-scale velocity offset between baryons and dark matter at later times, the so-called streaming velocity. In this paper, we use high resolution numerical simulations to investigate the impact of this streaming velocity on the spin and shape distributions of high-redshift minihaloes, the formation sites of the earliest generation of stars. We find that the presence of a streaming velocity has a negligible effect on the spin and shape of the dark matter component of the minihaloes. However, it strongly affects the behaviour of the gas component. The most probable spin parameter increases from ∼0.03 in the absence of streaming to ∼0.15 for a run with a streaming velocity of three times σrms, corresponding to 1.4 km s−1 at redshift z = 15. The gas within the minihaloes becomes increasingly less spherical and more oblate as the streaming velocity increases, with dense clumps being found at larger distances from the halo centre. The impact of the streaming velocity is also mass-dependent: less massive objects are influenced more strongly, on account of their shallower potential wells. The number of haloes in which gas cooling and runaway gravitational collapse occurs decreases substantially as the streaming velocity increases. However, the spin and shape distributions of gas that does manage to cool and collapse are insensitive to the value of the streaming velocity and we therefore do not expect the properties of the stars that formed from this collapsed gas to depend on the value of the streaming velocity. The spin and shape of this central gas clump are uncorrelated with the same properties measured on the scale of the halo as a whole.

X-Ray and Optical Observations of a New X-Ray Soft Intermediate Polar: RX J0512.2–3241

The V~17.6 mag optical counterpart of the bright, soft, high-galactic latitude X-ray source RX J0512.2–3241 detected during the ROSAT All-Sky Survey, has been identified as a new, asynchronously rotating, magnetic cataclysmic variable (intermediate polar). The X-ray spectrum of RXJ0512–32 is similar to that of polars, it shows a soft component with no intrinsic absorption and a blackbody temperature kTbb~38 eV. From our optical follow-up B and V CCD photometry (cf. Fig. 1) we derive most probable spin and orbital periods of (863.5 ± 0.7) s and (3.45 ± 0.03) h respectively. A lower limit for the distance to the system is d > 740 pc. From this evidence we suggest that RXJ0512-32 is a further member of the ROSAT discovered class of soft X-ray intermediate polars (for details see Burwitz et al., 1996, A&A 310, L25). This still small class of systems (see Haberl and Motch 1995, A&A 297, L37) has X-ray characteristics similar to those of low magnetic field polars and may be their long sought evolutionary progenitors.

Decay of 83Ym and 83Yg to Levels in 83Sr

The decays of 2.85 min 83Ym and of 7.06 min 83Yg have been investigated. Two photons are observed to be associated with the metastable decay, and one depopulates a 4.95 s metastable level at 259.3 keV in 83Sr. Seventeen γ rays are associated with the ground state decay, γ–γ coincidence and internal conversion measurements are carried out. Decay schemes are proposed with deduced log ft values for the various decay branches. Probable spin–parity assignments are given. The nature of the states involved are discussed within the context of the shell model.

Measurement of the directional correlation of the 1038–605 keV gamma cascade in 134Ba

The directional correlation of the 1038–605 keV gamma-ray cascade in 134Ba has been determined using the well-known 1365–605 keV gamma-ray cascade as a reference. The detection system was a conventional fast–slow coincidence system using a 9 cm3 cylindrical Ge(Li) counter and a 5 cm × 5 cm NaI(Tl) counter. The observed correlation coefficients were A2 = 0.285 ± 0.030, A4 = −0.002 ± 0.050. The most probable spin sequence for the 1038–605 cascade is 3 (1, 2) 2 (2) 0 with a quadrupole content of 35–49% for the 1038-keV transition.

The 4509-keV level in 27Al

Levels in 27Al below 4.6 MeV have been studied by triple angular correlations. The 4509-keV level was found to have a probable spin value of 11/2. The quadrupole/dipole mixing ratio for the 4509 → 3003 (11/2 = 9/2) transition was then found to be −0.58 ± 0.08. The value J = 9/2 for the spin of the 3003-keV level has been confirmed. Both levels are likely to belong to a rotational band with a predominant K value of 5/2.

EXPERIMENTAL EVIDENCE FOR AXIAL VECTOR INTERACTION IN THE DISINTEGRATION OF Pr144

A detailed study of the disintegration scheme of Pr144 has been carried out using lens spectrometers, scintillation spectrometers, and fast coincidence techniques. The 2293 kev β-component (1.3%) studied in coincidence with γ 691 has a predominantly first forbidden unique shape and the β–γ directional correlation is strongly anisotropic. The results indicate a probable spin-sequence 0−(β)2+(γ)0+, but the accuracy is not sufficient to exclude completely a 1− assignment for the Pr144 ground state. The β 803 component (1.0%) observed in coincidence with γ 2181 has an allowed shape and the β–γ directional correlation is isotropic. The shape of the total β-spectrum was studied in a double lens spectrometer having good anti-scattering properties using sources of Ce144 oxide sublimed onto 200 μg/cm2 Al leaf. Subtraction of a unique (Bij) shape 2293 kev 1.3% component and an allowed shape 803 kev 1.0% component yields the ground state β-spectrum. Analyses were made using the electronic functions of Zerianova which assume uniform charge distribution with ρ = 1.2 A1/3 10−13 cm as well as the point charge functions of Rose with ρ = 1.41 A1/3 10−13 cm. The observed spectrum shape can be explained on 0− to 0+ selection rules only by the axial vector interaction. Limits on the permissible tensor or pseudoscalar admixtures with the axial vector are discussed. No acceptable fit is possible with tensor, pseudoscalar, or any TP admixture (interfering or not) using Rose and Osborn's pseudoscalar formalism.