Journal Club


"Studies of RR Lyrae Variables in Binary Systems. I. Evidence of a Trimodal Companion Mass Distribution"

Gergely Hajdu (NCAC, Warsaw)

We present 87 candidates for RR Lyrae variable stars in binary systems, based on our new search using the light-travel time effect (LTTE) and observed - calculated (O - C) diagrams in the Galactic bulge time-series photometry of the Optical Gravitational Lensing Experiment. Out of these, 61 are new candidates, while 26 have been announced previously. Furthermore, 12 stars considered as binary candidates in earlier works are discarded from the list, either because they were found to have O - C diagrams incompatible with the LTTE or because their long-term periodicity is definitely caused by the Blazhko effect. This sample of RR Lyrae binary candidates allows us to draw the first firm conclusions about the population of such objects: no candidate has an orbital period below 1000 days, while their occurrence rate steadily increases with increasing period, and peaks between 3000 and 4000 days; however, the decrease in the number of stars toward even longer periods is probably the result of observational biases. The eccentricities show a very significant concentration between 0.25 and 0.3, with a quarter of candidates found in this single bin, overlaid on an otherwise flat distribution between 0.05 and 0.6. Only six stars have inferred eccentricities above 0.6. Lastly, the distribution of the mass functions is highly peculiar, exhibiting strong trimodality. We interpret these modes as the presence of three distinct groups of companions, with typical inferred masses of ~0.6, ~0.2, and ~0.067 M⊙, which can be associated with populations of white dwarf and main sequence, red dwarf, and brown dwarf companions, respectively.

Hajdu, Gergely, et al., The Astrophysical Journal (2021)

"Tidal star-planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems"

Gergely Hajdu (NCAC, Warsaw)

Tidal interaction between an exoplanet and its host star is a possible pathway to transfer angular momentum between the planetary orbit and the stellar spin. In cases where the planetary orbital period is shorter than the stellar rotation period, this may lead to angular momentum being transferred into the star's rotation, possibly counteracting the intrinsic stellar spin-down induced by magnetic braking. Observationally, detecting altered rotational states of single, cool field stars is challenging, as precise ages for such stars are rarely available. Here we present an empirical investigation of the rotation and magnetic activity of a sample of planet-hosting stars that are accompanied by wide stellar companions. Without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval companions, using X-ray observations to measure the stellar activity levels. Employing three different tidal interaction models, we find that host stars with planets that are expected to tidally interact display elevated activity levels compared to their companion stars. We also find that those activity levels agree with the observed rotational periods for the host stars along the usual rotation-activity relationships, implying that the effect is indeed caused by a tidal interaction and not a purely magnetic interaction which would be expected to affect the stellar activity, but not necessarily the rotation. We conclude that massive, close-in planets have an impact on the stellar rotational evolution, while the smaller, more distant planets do not have a significant influence.

Ilic, N., et al., MNRAS (2022)


"The Accretion Flow in M87 is Really MAD"

Angelos Karakonstantakis (NCAC, Warsaw)

The supermassive black holes in most galaxies in the universe are powered by hot accretion flows. Both theoretical analysis and numerical simulations have indicated that, depending on the degree of magnetization, black hole hot accretion flow is divided into two modes, namely SANE (standard and normal evolution) and MAD (magnetically arrested disk). It has been an important question which mode the hot accretion flows in individual sources should belong to in reality, SANE or MAD. This issue has been investigated in some previous works but they all suffer from various uncertainties. By using the measured rotation measure (RM) values in the prototype low-luminosity active galactic nuclei in M87 at 2, 5, and 8 GHz along the jet at various distances from the black hole, combined with three-dimensional general relativity magnetohydrodynamical numerical simulations of SANE and MAD, we show in this paper that the RM values predicted by MAD are well consistent with observations, while the SANE model overestimates the RM by over two orders of magnitude and thus is ruled out.

Yuan, Feng et al., The Astrophysical Journal (2022)

Yuan, FengYuan, Feng


"Acceleration of UHECRs in AGN jets and backflows"

Araudo Anabella (Astronomical Institute, Czech Academy of Sciences)

Ultra high energy cosmic rays (UHECRs) are charged particles with energies larger than 1 EeV. They are extragalactic but their origin remains unclear. Although observational data indicate that the arrival detection of UHECRs is coincident with starburst galaxies and Active Galactic Nuclei (AGN), the former do not have enough power to accelerate particles up to ultra high energies. On the other hand, recent theoretical advances indicate that the backflows in AGN jets can accelerate particles up to the Hillas limit. In addition, the interaction of stars with the AGN jet can serve as an effective mechanism for the jet mass loading. I will review these results and present some studies still in progress.