As hoped, the James Webb Space Telescope is helping to solve outstanding puzzles, including some left by the Hubble Telescope. At the moment, the answers he gives repeatedly concern brown dwarfs and in this case, those whose existence was suspected in the famous Orion Nebula, one of the closest star-forming nebulae of our Solar System, located 1,300 light years away.
From the start of the Hubble saga in the 1990s, the telescope helped to support the theory of the formation of the Solar System from thecollapsecollapse of a cloudcloud of gazgaz and rotating dust giving a central star surrounded by a protoplanetary diskprotoplanetary disk illuminated and partially ionized by radiation ultravioletultraviolet intense of the young star.
Indeed, Hubble images have revealed large numbers of these UV-illuminated protoplanetary disks, called proplydes pour ionized protoplanetary disk in English, in the Orion Nebula. The hunt for ionized protoplanetary disks was so good in this nebula that nearly 180 were found there.
Hubble, the proplydes and Orion. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © HubbleWebbESA
Stars too weakly luminous in infrared for Hubble
However, the astrophysicistsastrophysicists still wondering if some of them really surrounded young stars or even proto-starsproto-stars in training… or brown dwarfsbrown dwarfs. Remember that these starsstars which can have the size of gas giantsgas giants of the Solar System, as JupiterJupiter or SaturnSaturnare significantly more massive and therefore dense, while not being dense enough to sustainably trigger reactions of fusionfusion thermonuclear, apart from for a very short time, that of the combustioncombustion deuterium – this isotopeisotope heavyhydrogenhydrogen formed during the Big BangBig Bang and on which the work of Hubert Reeves focused.
The answer to this question has therefore been given by observations made with the James-Webb space telescope, as can be seen by reading an open access article on arXiv but accepted for publication in theAstrophysical Journal and which we owe to an international team of researchers, including one astronomerastronomer from Penn State University in the USA.
Some of the protoplanetary disks in Orion actually surround brown dwarfs. The JWST established it because its capabilities to observe in theinfraredinfrared are much better than those of Hubble, in particular because it has a mirrormirror larger and therefore capable of collecting more photonsphotons from faint objects.
The JWST instruments can also measure the temperature of the central star at the heart of Orion’s proplydes and it turned out that in some cases it was too low for a normal star, but not at all for a brown dwarf .
JWST, a key to the enigmas of brown dwarfs
As the Penn State press release explains:
« The team of astronomers performed infrared spectroscopy measurements on a small sample of brown dwarf candidates in Orion using the spectrographespectrographe Webb’s near infrared. This data confirmed that 20 objects are cold enough to be brown dwarfs, the smallest of which could have a massemasse of only 0.5% of that of SoleilSoleil of Earth, or five masses of Jupiter. Two other objects are near the minimum mass for merger (7.5% of the Sun’s mass), so it is unclear whether they are small stars or large brown dwarfs. The sample of new brown dwarfs includes two weak proplydes brightnessbrightness detected by Hubble in the 1990s, making them two of the coldest and least massive proplydes discovered to date ».
Kevin Luhman, professor of astronomy andastrophysicsastrophysics at Eberly College of Science from Penn State and one of the leaders of the research team, also adds in this press release:
« Webb’s new observations have only scratched the surface in terms of brown dwarfs in Orion. The nebula contains a few hundred faint objects that could be brown dwarfs, which are ripe for spectroscopy with Webb. Future observations of Orion with Webb could potentially find many more examples of proplyds around brown dwarfs and determine the smallest mass at which brown dwarfs exist. This information will help us fill gaps in our knowledge about the formation of brown dwarfs and their relationship to stars and planets ».
A presentation of the world of brown dwarfs presented on May 7, 2020 by Frédérique Baron, from the Institute for Research on Exoplanets – iREx. © Program channel Discovering the Universe
Did you know?
The existence of brown dwarfs was predicted theoretically by Indian astronomer Shiv S. Kumar during his dissertation during the period 1958-1962. He was interested in the theory of very low mass stars (M leading figures of Seti.
Brown dwarfs are interesting because they are intermediate stars between a star and a planet in terms of mass and which are not found in our Solar System. They are also useful for studying the evolution and atmosphere of giant planets, because Jupiter-like planets and the lighter brown dwarfs are expected to have similar characteristics.
Brown dwarfs are more or less always part of the debate concerning, on the one hand, the limit in mass beyond which a star is one of the stars (and not brown dwarfs) and on the other hand, concerning the limit below which the star is a gas giant. Astrophysicists, however, agree on one point: what differentiates a star from a brown dwarf is the fact that it is sufficiently massive for lasting thermonuclear fusion reactions, such as those described by the proton-proton chain or the cycle of Bethe-Weizsäcker, snap into place. We then find masses between 75 and 80 times the mass of Jupiter (MJ), that is to say approximately 0.07 solar masses.
When it comes to the criterion for distinguishing a gas giant from a brown dwarf, scientists generally use the threshold of 13 MJ. Temporary fusion reactions, in this case that of deuterium, can then occur, like that of lithium from 65 MJ.
Brown dwarfs are subdivided into several spectral types like ordinary stars. The hottest and most luminous are thus part of the so-called M dwarfs, therefore close to red dwarfs of the same type. Then come the two main kinds of brown dwarfs with firstly those of type L, which have temperatures roughly between 1,500 K and 2,500 K, and type T dwarfs, again with temperatures roughly between 1,500 and 500 K. Y dwarfs have temperatures below 500 K.
There are, however, slight variations in the literature on them; it is sometimes found for T dwarfs that they must have surface temperatures lower than 1200 K.
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