Under the Microlens, A Binary System (Planetary Science)

Moa-2006-Blg-074 is a gravitational microlensing signal that was thought to be attributable to a Neptunian-type planetary candidate. A new study, conducted by a team of researchers led by Paolo Rota of the University of Salerno, argues instead that it is actually a phenomenon of “xallarap” – parallax in reverse – with a binary system as a source. All the details on The Astronomical Journal

95 percent of the exoplanets have been discovered by the method of transits or that of radial velocities . However, these two observational techniques focus on the inner regions of planetary systems, from burning gas giants to the habitable zone of smaller stars. Compared to them, the microlensing technique is still behind for the number of planets discovered, but great expectations are concentrated on it for the next few years . The microlensingit is in fact the only method capable of investigating the intermediate regions of planetary systems, from the habitable zone of stars similar to the Sun, to the ice worlds of the cold peripheries, passing through gas giants similar to Jupiter or Saturn. Thanks to microlensing , we overcome the so-called ” snow line ” beyond which the planets are formed by also aggregating ice and grow much faster than what happens near the star. Being able to compare the planetary formation patterns of the arid inner regions with the icy ones will allow a great leap forward in our knowledge of the variety of worlds that populate the universe. This is why NASA will invest much of the time of the Nancy Grace Roman Telescope, which in 2025 will undertake a survey of the galactic center with the aim of finding thousands of planets with the microlensing method .

The microlensingit is based on the amplification of the light of a distant star-source caused by a star-lens which intervenes on the line of sight and which acts as a gravitational lens. The light of the source-star undergoes an amplification that can last from a few days to a few months, depending on the mass and speed of the lens-star. If the star-lens is accompanied by a planet, the gravitational field of the planet can induce “anomalies” on the light curve, with characteristic secondary peaks or depressions. However, the faint anomalies induced by small planets are difficult to distinguish from other effects. The case of the event (as astrophysicists call the passage of a “lens” in front of a “source”) Moa-2006-Blg-074 is emblematic from this point of view.Moa ), which operates at the Mount John telescope in New Zealand, was immediately selected as a Neptunian-type planetary candidate from three different analysis platforms: one based at NASA’s Goddard Space Flight Center, one at Osaka University in Japan and one of the University of Salerno in Italy.

Valerio Bozza (University of Salerno), co-author of the study © INAF

At this point, the detailed analysis of the event started, conducted by a team of researchers led by the University of Salerno. However, the initial enthusiasm cooled when an orbital motion too rapid emerged to give rise to a physically consistent planetary system. Evidently, those small anomalies found by Moa’s telescope must have had a different explanation from that suggested by the preliminary analyzes. After sifting through several possibilities, an alternative explanation was found: the source was part of a binary system with a much weaker companion.

“This is not the first time that a binary source has been found in a microlensing event . Sometimes the companion of the main source is also amplified, other times, however, the companion leaves its mark on the microlensing curve through the orbital motion of the main source around the common center of mass “, underlines Valerio Bozza , associate professor at the University of Salerno and co-author of the article, published last week in The Astronomical Journal, which reports the results of the study.

Astronomers have come up with a curious name for this phenomenon: xallarap – like ‘parallax’ spelled backwards . In fact, the parallax effect is due to the orbital motion of the observer in solidarity with the Earth, while the xallarap effect is due to the orbital motion of the source and, in some way, is configured as the opposite case to parallax.

Paolo Rota, PhD student at the University of Salerno and first author of the study published in The Astronomical Journal © INAF

«In the end, all the physical parameters for Moa-2006-Blg-074 return and are consistent with the binary source scenario: the masses, the luminosities of the two components, the period and the orbital radius. In short, there is no longer any doubt », says Paolo Rota , PhD student at the University of Salerno in the group of Valerio Bozza and first author of the publication. “The primary component has a mass of 1.3 solar masses, the secondary 0.44 solar masses. The two stars orbit at a distance of 0.14 astronomical units with a period of 14 days ».

Among the thousands of planets that Roman will discover, it will not be rare to find cases similar to that of Moa-2006-Blg-074. Statistics on small planets could be contaminated by binary sources, and then countermeasures will need to be stepped up to identify these systems, the researchers conclude. The software developed by the University of Salerno will play a central role in the analysis of the Roman telescope data and the experience gained in cases like these will serve to prepare a solid and reliable analysis platform – proof of binary sources.

Featured image: Illustration of the microlensing of a single source by a lens housing a planetary system (left) and a binary source from a single lens (right). Below the light curves, which can be very similar in the two cases. Credits: INAF


To know more:

  • Read on The Astronomical Journal   the article ” MOA-2006-BLG-074: Recognizing Xallarap Contaminants in Planetary Microlensing ” by P. Rota ,  Y. Hirao ,  V. Bozza ,  F. Abe ,  R. Barry ,  DP Bennett ,  A. Bhattacharya ,  IA Bond ,  M. Donachie ,  A. Fukui ,  H. Fujii ,  S. Ishitani Silva ,  Y. Itow 4 ,  R. Kirikawa ,  N. Koshimoto ,  MCA Li ,  Y. Matsubara , S. Miyazaki ,  Y. Muraki ,  G. Olmschenk ,  C. Ranc ,  Y. Satoh ,  T. Sumi ,  D. Suzuki ,  PJ Tristram and  A. Yonehara

Provided by INAF

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