Thursday, May 11, 2017

Nearby Brown Dwarf Turns Out to Be a Free-Floating Planetary-Like Object

An artist’s conception of SIMP J013656.5+093347, or SIMP0136 for short, which the research team determined is a planetary like member of a 200-million-year-old group of stars called Carina-Near. Image Credit: NASA/JPL, slightly modified by Jonathan Gagné.

One of the nearest brown dwarfs to our solar system, designated SIMP J013656.5+093347 (SIMP0136 for short), turns out to be a planetary-mass object, new study finds. A team of astronomers, led by Jonathan Gagné of the Carnegie Institution for Science in Washington, D.C., has presented evidence proving planetary nature of this object. The findings were recently published in The Astrophysical Journal Letters.

Located some 20 light years away, SIMP0136 was discovered in 2006 by a near-infrared proper motion survey known as SIMP (Sondage Infrarouge de Mouvement Propre) and initially classified as a bright T-type brown dwarf. Three years later, it was detected that SIMP0136 exhibits photometric variability what is interpreted as a signature of weather patterns coming in and out of view over the object's rotation period.

Now, Gagné and his team comprised of scientists from the Institute for Research on Exoplanets (iREx) at Université de Montréal, the American Museum of Natural History, and University of California San Diego, reveal new breakthrough insights into the real nature of this intriguing variable object.

The researches employed the new Bayesian analysis tool known as BANYAN (Bayesian Analysis for Nearby Young AssociatioNs) and conducted radial velocity measurements of SIMP0136 with the Near InfraRed Spectrometer (NIRSPEC) installed on the Keck II Telescope at the Keck Observatory on Maunakea, Hawaii. The study allowed the team to determine that SIMP0136 is a planetary-like member of a 200-million-year-old group of stars called Carina-Near.

“Our tool flagged SIMP0136 as a potential member of Carina-Near, and we realized that its distance was already measured in the literature and its radial velocity was also measured. We were thus able to put all these measurements together and get the velocity of SIMP0136 in 3D, which confirmed that our tool had correctly identified that the velocity of SIMP0136 is consistent with the Carina-Near moving group,” Gagné told Astrowatch.net.

Groups of similarly aged stars moving together through space like Carina-Near are considered by astronomers as prime regions to search for free-floating planetary-like objects. The key to properly classify SIMP0136 was to estimate its mass, but in order to do it, it was necessary to determine its age. One of the methods to precisely constrain the ages of brown dwarfs is to identify those that are members of young stellar associations.

“Because we now think SIMP0136 is a member of Carina-Near, we also have a good estimate for its age, which we can use as the final needed piece to estimate its mass, when we compare all its properties with physical models of brown dwarfs,” Gagné said.

The researchers estimate that at the age of about 200 million years, SIMP0136 has a model-dependent mass of approximately 12.7 Jupiter masses. Therefore, the object is right at the boundary that separates brown dwarf-like properties, primarily the short-lived burning of deuterium in the object’s core, from planet-like properties.

However, Gagné underlined that the estimation of mass is dependent on our current understanding of the physics that govern the core and the atmosphere of brown dwarfs, and how they evolve as the brown dwarf ages. Thus, when the brown dwarf models become more sophisticated, it is possible that the mass estimate of SIMP0136 will change.

Étienne Artigau (University of Montreal, Canada), co-author of the study, highlights the importance of the search for deuterium in SIMP0136. He noted that the object’s low-mass and young age imply that, unlike more massive brown dwarfs, it should not have burned most of its initial deuterium.

“There are lots of known brown dwarfs that have temperatures close to that of SIMP0136 and that could be used for comparison. Confirming the presence of deuterium on SIMP0136 and its absence in field brown dwarfs would provide an excellent test for evolutionary models for these objects and bulletproof confirmation of its low mass. There is a similar test that exists with lithium for low mass stars and young brown dwarfs,” Artigau told Astrowatch.net.

Although SIMP0136’s other fundamental properties, like its radius (1.22 Jupiter radii), rotational period (2.4 hours) and effective temperature (1,098 K) are known, there is still much to uncover when it comes to its composition. The researchers believe that future observations would allow them to precisely measure the chemical composition of the stellar members of Carina-Near and therefore would also give them more information on the composition of SIMP0136 itself. They also hope to get a much better understanding of the object’s atmosphere.

“We already know a few things about the composition of SIMP0136’s atmosphere from separating its light in all its colors, but that doesn’t tell us the whole picture. For instance, it does not tell us about its composition deep below the atmosphere,” Gagné added.

More detailed information about SIMP0136 could be delivered by the James Webb Space Telescope (JWST). This space observatory, which is being developed jointly by NASA, ESA and the Canadian Space Agency, is scheduled to be launched into orbit in October 2018. One of the main goals of JWST is to study exoplanets and their atmospheres.

“Actually, a few months ago we have a meeting in Montreal to organize our team's ‘guaranteed time’ on JWST. The 450 hours come for the Canadian contribution to the project. Four hours were allocated to observe SIMP0136,” Artigau said.

He revealed that the object will be monitored over a little bit more than a rotation period with a spectrograph that covers a large wavelength domain in the near-infrared. Although this mode is mostly used for exoplanet transit spectroscopy, it will also be perfect to get very high accuracy monitoring of SIMP0136.

“This will allow us to get a much better understanding of the clouds patterns that lead to its variability. We should get this dataset sometime between July and September 2019,” Artigau concluded.

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