Brown dwarf

term in astronomy

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synonym Brown dwarf (Brown dwarf) generally refers to brown dwarf

Brown dwarfs are among the heaviest gas giant A secondary star between the lightest star and the lightest star, in particular, with a mass between 13 and 75 or 80 times Jupiter Mass, or about 2.5 × 10 twenty-eight Kg to about 1.5 × 10 twenty-nine kg。 Below this range are sub brown dwarfs (sometimes called Rogue planet ）, the lightest star above this Red dwarf 。 Brown dwarfs may only have convection without stratification or Chemical differentiation Depth.^[4]

differ Main order band The mass of the brown dwarf star is not enough to maintain the hydrogen in the core（ one H) Heliotropic nuclear fusion Reaction. However, they are between 13 M J And 65 M J The mass between is considered that deuterium can be carried out（ two H) And lithium（ seven Li), people are still debating whether brown dwarfs can be better defined by the formation process rather than by nuclear fusion reaction.^[5]

On April 12, 2021, foreign media reported that, Canada Astronomers have found three brown dwarfs with the fastest rotation speed. The rotation speed of three brown dwarfs is faster than that of any brown dwarfs previously discovered. Each brown dwarf completes about once an hour Complete rotation.^[3]

Chinese name: Brown dwarf
Foreign name: brown dwarf
Alias: Brown dwarf
Discovery time: 1995

Quality: 13 to 80 MJ
surface temperature: Not more than 3000K
Features: Intermediate in mass between stars and planets celestial bodies
Example: 2M1207 ， Gliese 229b

catalog

nine Discovery significance

Research History

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Brown dwarf

Astronomers spent 12 years of research to find these two brown dwarfs, observed more than 300 nights and made 1600 measurements in total, and calculated all the necessary parameters of two relatively young brown dwarfs (less than 1 million years), which are located 1500 light-years away from the Earth Orion 。 binary star The larger brown dwarf has a mass 50 times greater than Jupiter, while the smaller brown dwarf has a mass 30 times greater than Jupiter, and their diameters are 70% and 50% of the sun's diameter, respectively. Although they don't look small at first, their masses are Solar mass 5.5% and 3.5%.

Astronomers also unexpectedly found that the surface temperature of lighter brown dwarfs is higher, although the situation of "ordinary" stars is the opposite: Stellar mass The bigger it gets, the hotter it gets. Maybe it caused this Anomaly The reason lies in a certain physical process, which is not considered in modern stellar structure theory (such as the strong magnetic field of stars). In addition, these two brown dwarfs may not form at the same time or at the same place, but are combined due to some kind of disaster, so their surface temperature Different, but all this is still a hypothesis for the time being. American scientists utilize U.S.A Aerospace Spitzer infrared of the bureau Space telescope , discovered a small brown dwarf star (brown dwarf star) orbiting a star, and obtained its image directly. They said that this is the first time that humans have found such a scene, but this phenomenon is not isolated. The first author of the research report University of Pennsylvania Kevin Ruhman, assistant professor of the Department of Astronomy and Astrophysics, said that in the past 10 years and more, astronomers have been extremely successful in finding planets close to stars by using direct detection methods. With the infrared detection capability of Spitzer infrared space telescope, people can directly detect Planetary system A brown dwarf star (also called T dwarf ）, even detected Megaplanets 。

The discovered T-dwarf star, HD3651B, is located at Pisces Its mass is 50 times that of Jupiter. The star surrounded by it has a mass smaller than that of the sun, and there is a nearby star with a mass slightly smaller than that of the sun Saturn The planet of. The planet's orbit is extremely flat and elliptical. Scientists believe that the reason why it has such an orbit is the gravity of HD3651B, which orbits on the periphery. In the past, people found that planets outside the solar system have extremely flat orbits, and put forward the theory that other objects hidden in the planetary system (such as T dwarfs) lead to extreme planetary orbits. The discovery of Spitzer Infrared Space Telescope provides evidence for this theory for the first time.

The researchers said that after discovering HD3651B, they found other T dwarfs one after another. If in Pegasus HNPegB was discovered, and its mass is 20 times that of Jupiter. Compared with other old brown dwarfs in the constellation that are billions of years old, this T dwarf is quite young, only about 300 million years old.

Failed star

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Brown dwarf

Brown dwarf is called "failed star". It cannot become a burning star due to its insufficient mass, but its mass is still Much greater than Jupiter, the largest planet in the solar system. Astronomers have found huge planetary storms on these strange planets, which are similar to those on Jupiter Great Red Spot Storm Comparable. As the brown dwarf will cool down over time, the gaseous iron molecules on the planet will condense into liquid iron clouds and rain. With further cooling, huge storms will sweep through these clouds, allowing bright infrared rays to escape into the universe.

The research and observation of low mass stars is one of the research hotspots in the stellar field in the past decade. Brown dwarfs are the most important group among them. They are neither stars nor planets, but objects in between. The study of brown dwarfs has made us have a deeper understanding of the nature of stars and planets, and found that Main preface The colder L type and T type in the lower part. The brown dwarf star Galaxy Li quantity can be compared with Main sequence star Compared, but not Cosmic dark matter The main components of. The formation of brown dwarfs may be different from both stars and planets. The study of their formation can help us understand the formation of stars and planets more thoroughly.

A decade ago, brown dwarf star It was only a theoretical object in astronomy textbooks, and it was not even known at that time whether such an object with a mass between a giant planet and the coldest dwarf existed. Today, the problem we are facing is how to distinguish these low mass objects. On《 Journal of Astrophysics 》Above, McLean et al. proposed a method based on brown dwarfs near infrared spectroscopy The unified classification method of. This classification method also provides information about the Chemical information 。

Astronomical data

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surface temperature

Because there is no nuclear fusion , brown dwarf surface temperature It will not exceed 3000K. The lower the temperature of the brown dwarf, the visible light The brightness of the band is smaller. The radiation of M-type dwarfs is mainly concentrated in the red light band (about 0.75 μ m) The radiation of the lower temperature L-type brown dwarf (1200-2000K) and T-type brown dwarf (800-1200K) is mainly concentrated in the near-infrared band (1-2 μ m) , which makes brown dwarfs very weak in nature. In addition, molecules in the outer atmosphere of brown dwarfs, such as water carbon monoxide , methane and ammonia will absorb the outward radiation, making the brown dwarf darker. These factors make the search for brown dwarfs Astronomical observation A big challenge of.

atmosphere

On March 27, according to foreign media reports, astronomers used NASA's Spitzer and Hubble Space telescope Detect the violent atmosphere To create a detailed“ The weather map ”。

From University of Arizona Daniel Obuy, the main researcher of, said that, similar to stars, brown dwarfs are composed of high concentrations of gas, but because of their insufficient mass, they cannot complete the nuclear fusion of hydrogen and generate energy like stars. However, such objects, which are often called "failed stars", have more complex atmospheres than gas planets.

This new research will not only help us understand brown dwarfs better, but also help us understand them better solar system The atmosphere of an exoplanet.

In fact, researchers pointed the Hubble and Spitzer telescopes at a brown dwarf named 2MASSJ22282889-431026 at the same time. They found that the light emitted by the brown dwarf star changes with time. With its own rotation, the star completes a light and dark change every 90 seconds. What is more surprising is that researchers believe that the timing of its brightness changes depends on the infrared light The wavelength of.

These changes are obviously caused by different material compositions in different atmospheres around brown dwarfs. To be sure, Spitzer and Hubble observed different atmospheres because of the infrared cover steam And high concentration methane Blocking, while other wavelengths of infrared light are emitted from the deeper atmosphere.

NASA Mark Marley, a scientist at Amus Research Center, explained that ammonia Clouds are different. The clouds of brown dwarfs are mainly composed of hot sand grains hot metal And others Not common The composition of the mixture.

spectrum

Despite the complexity of the spectrum of brown dwarfs, their chemical composition can still be recognized, and can also be used to classify brown dwarfs. Up to now, it has not been directly observed that it is smaller in mass and temperature than T8 brown dwarfs（ Effective temperature About 800K) lower objects, to connect the brown dwarf and Jupiter (about 125K). However, the "Spitzer" that has been launched Space telescope Owned Mid infrared Observation ability may help us find this missing link.

Discovery process

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Brown dwarf

The first brown dwarf star, Gl229 B, was discovered in 1995（ Gliese 229b ）。 Compared with the red M type dwarf star (the star with the lowest temperature and mass) Giant planet The characteristics of sub stellar objects are very obvious. In the late 1990s, thanks to a wide range of infrared surveys, a large number of brown dwarfs were found.

classification

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Brown dwarfs can be further divided into two types: L-type and T-type. L-type brown dwarfs are more similar to giant planets in spectrum M-type dwarf , which includes the star with the smallest mass and the star with the largest mass Substar Celestial bodies. The T-type brown dwarf has a spectrum more similar to that of a giant planet, but its mass is much larger than that of a giant planet. At the same time, according to some temperature related characteristics in the spectrum, brown dwarfs can be divided into nine subcategories from 0 (the highest temperature) to 8 (the lowest temperature). There are about 250 L-type brown dwarfs and about 50 T-type brown dwarfs.

Brown dwarf

At first, the further classification of L-type brown dwarfs was based on the red light spectrum of visible light, while the subdivision of T-type brown dwarfs depended on near infrared spectroscopy 。 McRae et al Keck Telescope II About 50 brown dwarfs were analyzed by the near-infrared spectrometer on, and the unification of L-type brown dwarfs and T-type brown dwarfs was proposed classification method 。 They used high-quality near-infrared spectroscopy, and compared Na, K Ca , Al and Mg Atomic Relative strength And water carbon monoxide 、 methane And FeH Spectral band The brown dwarfs are classified. This observation is the Spectral classification A framework has been established.

Because of the brown dwarf's own small scale (Size vs Jupiter ) and low mass increase the difficulty of detecting brown dwarfs, so these observations themselves represent great progress. The maximum mass of brown dwarfs can only reach 7% of the sun (as a comparison, Jupiter's mass is about 0.1% of the sun's), which is not enough to start and maintain hydrogen nuclear fusion And become a star. If the mass of a brown dwarf exceeds 13 Jupiter masses, it can burn deuterium, but the energy released by deuterium burning is very small compared with hydrogen, and the deuterium burning time for the largest brown dwarf will not exceed 100 million years. In contrast, in stars Hydrogen combustion It can last for billions of years. Most of the energy released by brown dwarfs during their lifetime is released during their formation and contraction Gravitational energy At the same time, brown dwarfs will eventually become cold and old.

formation mechanism

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Astronomers have different opinions on the formation mechanism of brown dwarfs, including the ejection theory, the light induced erosion theory of the former stellar nucleus Opacity The Split Theory of Constraints and the Disagreement of the Original Astrolabe Stability theory Etc. According to the ejection theory, brown dwarfs are formed because the low-mass protostellar embryos collide with other celestial bodies before reaching the mass required for hydrogen nuclear fusion and are ejected from the former stellar cores. This theory has been partially confirmed by the double brown dwarfs system. The light induced erosion theory of pre stellar nuclei is based on High quality Photogenic effects of stellar radiation on pre stellar nuclei Erosion , can explain Ionized hydrogen region The formation mechanism of brown dwarfs in. Brown dwarfs may also be formed by Original astrolabe It is produced by fragmentation under the gravitational action of other stars. Each of these theories can only explain part of the formation of brown dwarfs. Studying the star disks around brown dwarfs can effectively test these theories.

Brown dwarfs can occur Thermonuclear reaction Only because it is not intense, it will not glow. But its infrared radiation It can account for about 1-2 ‰ of the sun, and there is heat energy too close to it.

Famous dwarf

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Y dwarf CFBDS0059

Scientists found a faint, isolated weak star 40 light-years away from the Earth. The mass of this brown dwarf is 15-30 times that of Jupiter, and its surface temperature is 350 centigrade , and Mercury The equatorial surface temperature is similar Venus Lower surface temperature. This discovery has aroused great interest of scientists because this star will Megaplanets Divided from small stars, it is named as a new star type - "Y dwarf" from the theoretical field. Rosie Albert, star researcher of Canada France Hawaii Telescope, said, "This is the last spectral type between stars and planets." He participated in the research team to identify this newly discovered cold brown dwarf star and named it "CFBDS0059". The research report was published in the recently published《 Astronomy and Astrophysics 》Magazine.^[1]

Albert said that because Y dwarfs are born dim, it is very difficult to find and identify them. In fact, no one has achieved significant success in categorically identifying Y dwarfs. Such stars cannot be seen by human eyes, even if they are used Astronomical telescope It cannot be observed. This is because the surface temperature is not high enough to emit red light, similar to Electric furnace wire Heating effect of. However, at this mild temperature, Y dwarfs emit infrared rays. Therefore, astronomers through the Canada France Hawaii Telescope“ Gemini ”Telescope and European Southern Observatory NTT telescope Near infrared CFBDS0059 was identified and studied by X-ray and infrared instruments.

Albert explained that the key basis for confirming whether CFBDS0059 or other cold brown dwarfs are Y dwarfs is atmosphere Whether ammonia is present in. In order to detect whether there is ammonia in the atmosphere, it is necessary to collect a large amount of infrared light and separate its spectrum to observe the dark lines formed by the absorption of light by ammonia. He's accepting《 Discovery Channel 》The reporter said in the interview, "We found signs of ammonia absorption in the atmosphere of CFBDS0059." The analysis of other brown dwarfs by researchers shows that they rarely belong to Y dwarfs. U.S.A Massachusetts Institute of Technology Adam Bogasse said, "This method can really identify the existence of Y dwarfs." Order brown dwarfs are divided into two categories: L dwarfs and T dwarfs, and the difference between the two is very subtle. The surface temperature of L dwarf is higher, reaching 1200-2000 ℃; The surface temperature of T dwarf is 1200 ℃, and its atmosphere is rich in methane gas.^[1]

At the mild temperature of Y dwarf, water condenses into water vapor droplets; In the atmosphere of brown dwarfs, water usually exists in the form of gas; However, giant planets condense water into ice water, so this temperature is lower than Low water The Y dwarfs in the form of water vapor droplets are expected to connect the transition star sequence between the hottest star and the giant planet, which is important for the research Extrasolar planets It is also of far-reaching significance.

Brown dwarf WISE J085510.83-071442.5

In April 2014, NASA used the wide area Infrared telescope (WISE) and Spitzer Space Telescope It is confirmed that a brown dwarf star with the lowest known temperature, a faint star like star, has been found, but its temperature is unexpectedly low, almost as cold as the North Pole on Earth.

Brown dwarf WISE J085510.83-071442.5 position^[2]

Space telescope observed data At the same time, the distance of the detector is about 7.2 light years, making it the fourth closest celestial system to the sun. So far, the celestial system closest to the sun is a trinity system, Centaurus α， The distance is about 4 light years.

Extradepartmental planets and Livable zone Kevin Luhman, an astronomer at the research center, said: "It's exciting to find a neighbor so close to us. Considering its extreme temperature, it should tell us a lot about the planet's atmosphere, which usually has a very low temperature atmosphere."

The newly discovered brown dwarf star is numbered WISE J085510.83-071442.5 。 Its temperature is about - 48~- 13 ℃. The brown dwarf with the lowest temperature previously observed was also found by WISE and Spitzer telescopes, and its temperature is about room temperature.

The mass of WISE J085510.83-071442.5 is about 3~10 times that of Jupiter^[2] 。 With such a small mass, this planet may be a Jupiter like giant planet kicked out by its original planetary system. But scientists think it should be a brown dwarf, because brown dwarfs are quite common. If this is true, it will also be the smallest brown dwarf known so far.

In March 2013, Luhmann's analysis of WISE telescope data also found two other brown dwarfs with much higher temperature, about 6.5 light-years away, making them the third closest celestial system to the sun. The discovery of these fast moving brown dwarfs also suggests that a "hidden planet" has long been thought to exist in the outer solar system, or“ Planet X ”It is very likely that it does not exist.^[2]

Discovery

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according to NASA According to the website, the Wide Area Infrared Survey Explorer (WISE) affiliated to the bureau has found a pair of stars that are very close to each other. They will become the third closest star/star system to the solar system so far. This star is also the closest star system to the solar system that has been discovered by humans since 1916. Both stars in this binary system are brown dwarfs.

Kevin Luhman is an American Pennsylvania State University Astronomy and Astrophysics Professor, and served as a researcher in the Center for Extraplanetary and Livable World Studies of the University. He said, "This pair of brown dwarfs Konductra The distance to the earth is only 6.5 light years. This distance is so close that the TV program signal broadcast on the earth in 2006 has now spread there This is an excellent place to search for planets, because they are too close. So we have a good chance to see if there are any planets orbiting any of them. "

This dual star system has been officially numbered WISE J104915.57-531906 WISE 1049-5319 The reason why WISE is present in the front is that this binary star system was found on the infrared sky survey map made from the data obtained by WISE. The distance between the two stars is only the second Barnard star A little farther, the latter was discovered in 1916 about 6 light-years away from the solar system.

The nearest star system is Centaur α It was identified in 1839 only 4.4 light years away. Centaur α Is a Triad System, including bright Centaur α A and Centaur α A binary star system composed of B, and a slightly distant, very dim Adjacent star 。 However, it is this faint neighbor star that is only 4.2 light-years away from the solar system, making it the nearest star in strict sense, although it is often regarded as Centaurus α Is treated as part of.

come from UCLA Edward Wright of WISE detector Project Chief scientist He said, "One of the scientific goals when the WISE detector scheme was proposed was to search for the star closest to the sun. WISE J1049-5319 is the closest stellar body found using WISE data. In the future, large telescope Equipment, such as foundation Gemini telescope As well as the James Weber Space Telescope, which will be launched in the future, to observe this binary system, we will gain more knowledge about brown dwarfs. "

WISE detector In 2011, WISE successfully completed its planned infrared band sky survey covering the whole sky, during which WISE scanned the whole sky twice. The sky survey star map based on WISE exploration data has been released to the public, and a plan called "AllWISE" that is still being implemented will combine the data obtained from the two sky surveys. The AllWISE project will use WISE data to search the whole day for objects that move faster at close range, such as WISE J104915.57-531906, but also for dim objects that have not been found in the distant universe before. All these data will be released in 2013.

NASA Jet Propulsion Laboratory Responsible for the management and operation of WISE detector project. Edward White of the University of California, Los Angeles is the chief scientist of this project. WISE is an exploration project implemented under NASA's "Explorer" program, which is managed by NASA's Goddard Space Flight Center. The science payload carried by the WISE detector is from the United States Utah Developed by the Space Dynamics Laboratory. The detector body is composed of Colorado Made by Bauer Aerospace Technology Co., Bordeaux. WISE project Specific science Operations and Data distribution Project by the United States California Institute of Technology Infrared processing and analysis are responsible for.

The results of this research will be published in the Astrophysical Bulletin.

Brown dwarf CFBDS0059, scientists concluded that there may be ammonia above.^[1]

Planetary Imagination of Brown Dwarfs

The coldest brown dwarf was found WISE J085510.83-071442.5 With a mass of 3 to 10 Jupiter Quality.^[2]

On April 12, 2021, foreign media reported that, Canada Astronomers found three brown dwarfs with the fastest rotation speed, and the rotation of three brown dwarfs Speed ratio Any brown dwarf discovered before is faster. Each brown dwarf completes about once an hour Complete rotation.^[3]

In December 2023, the European Space Agency issued a bulletin saying that an international research team had found the smallest free floating brown dwarf by far using the James Webb Space Telescope, whose mass was three to four times that of Jupiter.^[6]

Discovery significance

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According to foreign media reports, brown dwarfs in the universe are considered to be failed stars. According to a new study, European Space Agency The scientists of radio telescope Some brown dwarfs may be surprisingly strong“ aurora ”。 The aurora phenomenon we know comes from the high energy of the sun Charged particle flow Earthly Upper atmosphere It is caused by collision, and the aurora phenomenon generally occurs in High latitude And can present a colorful scene. Usually Coronal mass ejection Phenomena can produce powerful High-energy particle , they arrived Earth orbit It interacts with the geomagnetic field and forms in the polar region Beautiful aurora 。

When the solar particle storm is strong, we can even Mid latitude Aurora phenomenon is seen in the area. However, constant Stellar wind Particles are being Planetary atmosphere Before absorption, spiral Plasma , radiation emission at the same time electrical signal Scientists believe that we can detect the existence of these radio signals. It is reasonable to think that any planet with a global magnetic field has the potential to form auroras. Astronomers are very familiar with the aurora phenomenon on Jupiter and Saturn. Since Jupiter's magnetic field is at least 100 times stronger than the Earth's magnetic field Gaseous planet The study of aurora is helpful to explore the aurora phenomenon of extrasolar objects.

britain University of Leicester The team believes that Jupiter's auroral signal can be transmitted to Interstellar space , which can provide a detection way for scientists to explore the aurora in the distant alien world, including the capture of the radio signal of the smallest star brown dwarf. The research results believe that the celestial bodies outside the solar system will also have auroras, and the radio signal released is hundreds of thousands of times stronger than Jupiter, so it can spread across the interstellar distance, which provides a way for scientists to detect the auroras of celestial bodies outside the solar system. In addition, Research Department The aurora of outer celestial bodies also helps to discover other Stellar system The cosmic life and Habitable planet 。

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