R136a1

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R136a1
The young cluster R136.jpg
A near infrared image of the R136 cluster, obtained at high resolution with the MAD adaptive optics instrument at ESO’s Very Large Telescope. R136a1 is resolved at the center with R136a2 close by, R136a3 below right, and R136b to the left.
Credit: ESO/VLT
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Dorado
Right ascension 5h 38m 42.43s
Declination −69° 06′ 02.2″
Apparent magnitude (V) 12.23[1]
Characteristics
Evolutionary stage Wolf-Rayet star
Spectral type WN5h[2]
Apparent magnitude (U) 11.59
Apparent magnitude (B) 7.77
U−B color index -3.82
B−V color index -0.30[1]
Astrometry
Distance 157,000 ly
(48,500 pc)
Absolute magnitude (MV) -7.6[3]
Absolute bolometric
magnitude
 (Mbol)
-12.6[3]
Details
Mass 265[3] M
Radius 35.4[3] R
Luminosity 8,700,000[3] L
Luminosity (visual, LV) 94,000[3] L
Temperature 53,000[3] K
Metallicity 0.4 XH[2]
Age ~1.7[3] Myr
Other designations
BAT99 108, RMC 136a1, HSH95 3, WO84 1b, Cl* NGC 2070 MH 498, CHH92 1, P93 954.
Database references
SIMBAD data

RMC 136a1 (usually abbreviated to R136a1) is a Wolf-Rayet star located near the center of R136, the central condensation of stars of the large NGC 2070 open cluster in the Tarantula Nebula, a giant H II region in the Large Magellanic Cloud. It has the highest confirmed mass and luminosity of any known star, at 265 M and 8.7 million L, as well as one of the highest surface temperature of any main sequence star, at more than 50,000 K. It lies at a distance of about 48 kiloparsecs (157,000 light-years) based on the currently accepted distance to the Tarantula Nebula. Because of its large mass, it is one of the candidates for a potential supernova or hypernova in the astronomically near future.

Observations[edit]

It was only in 2010 that the star was recognized as the most massive and luminous star known. Previous estimates placed the luminosity as low as 1,500,000 L.[4]

Discovery[edit]

In 1960, a group of astronomers working at the Radcliffe Observatory in Pretoria made systematic measurements of the brightness and spectra of bright stars in the Large Magellanic Cloud. Among the objects cataloged was RMC 136, (Radcliffe observatory Magellanic Cloud catalog number 136) the central "star" of the Tarantula Nebula. Subsequent observations showed that R136 was located in the center of a giant H II region that was a center of intense star formation in the immediate vicinity of the observed stars.[5]

In 1979, ESO's 3.6 meter telescope was used to resolve R136 into three components; R136b, R136c, and R136a. The exact nature of R136a was unknown and a subject of intense discussion. Estimates that the brightness of the central region would require as many as 30 hot O class stars within half a parsec at the centre of the cluster led to speculation that a star 2500 times the mass of the sun was the more likely explanation.[6]

The first evidence that R136a was a star cluster was provided by Weigelt and Beier in 1985.[7] Using the speckle interferometry technique, R136a was shown to be made up of 8 stars, with R136a1 being the brightest found within 1 arcsecond at the centre of the cluster.

Final confirmation of the nature of R136a came after the launch of the Hubble Space Telescope. Advanced technology on the HST enabled astronomers to confirm R136a's nature and resolve it into 24 components.[8]

Visibility[edit]

Sky position of R136a1 viewed from Argentina

In the night sky, R136a1 is spotted within the Large Magellanic Cloud as a magnitude 12.23 whitish spot within the R136 cluster. Due to its faintness, it can only be spotted by a medium-sized amateur telescope or larger.[9] However, it is tightly surrounded by several bright stars whose luminescence largely effect the star's visible disk.[3] In fact, R136a was not recognized to be a cluster until speckle interferometry resolved it in 1980.[7]

South of about 20 degrees South latitude, the LMC is circumpolar, meaning that it can be seen (at least in part) all night every night of the year, weather permitting. In the Northern Hemisphere, it can be visible south of about 20 degrees North latitude. This excludes North America (except southern Mexico), Europe, northern Africa and northern Asia.[10]

At the distance of the LMC, a modest ~1.4 magnitudes are absorbed by the interstellar medium.[11] It is estimated that a visually bright star of 94,000 L would appear to be magnitude 10.82 from 157 kly[11] but R136a1 is about magnitude 12.23. Hence, around 1.41 magnitudes are absorbed by the interstellar medium. The rest is significantly reddened by the intervening gas and dust and most of the radiation reaches us in the infrared.

Distance[edit]

The R136 cluster is a massive star forming region in the LMC.

The distance of R136a1 is not well known but is assumed to be 157 kly (48.5 kpc). Determining a precise distance is challenging due to a number of factors. Generally, standard candles are used to measure the distance to a distant object, with Type Ia supernovae and Cepheid variables being the most common. A type Ia supernova has never been recorded in the LMC, so Cepheids would be the most reliable method. The distances to Cepheids can be measured by calibrating the relationship between its absolute luminosity and the period over which its brightness varies. Hence, the apparent magnitude and the luminosity can be adjusted to reveal the distance. However, the Cepheids in the LMC appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period–luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period–luminosity relation are more metal rich than those found in the LMC, so the Cepheid proprieties in the LMC are uncertain.[12]

Recently, the Cepheid absolute luminosity has been re-calibrated using Cepheids in the galaxy Messier 106, whose Cepheids cover a range of metallicities.[13] Using this improved calibration, it was found that the absolute distance modulus was (m-M)0 = 18.41, or 48 kpc (~157,000 light years). This distance, which is slightly shorter than the typically assumed distance of 50 kpc has been confirmed by other authors.[14][15]

In the field of >8-meter-class telescopes, eclipsing binaries have been found throughout the Local Group. Parameters of these systems can be measured without mass or compositional assumptions. The light echoes of supernova 1987A are also geometric measurements, without any stellar models or assumptions.

By cross-correlating different measurement methods, one can bound the distance; the residual errors are now less than the estimated size parameters of the LMC. Further work involves measuring the position of a target star or star system within the galaxy (i.e. toward or away from the observer).

The results of a study using late-type eclipsing binaries to determine the distance more accurately was published in Nature in March 2013. A distance of 49.97 kpc (162,983 light-years) with an accuracy of 2.2% was obtained.[16]

Surroundings[edit]

Zooming in from the Tarantula Nebula to the R136 cluster, with R136a1/2/3 visible as the barely resolved knot at bottom right. The brightest star just to the left of the cluster core is R136c, another extremely massive WN5h star.

R136a1 is located 157,000 light years away from Earth in the Large Magellanic Cloud. It is positioned on the south-east corner of the galaxy in a cluster called R136 at the centre of the Tarantula Nebula, also known as 30 Doradus. R136 itself is just the central condensation of the much larger NGC 2070 open cluster.[17]

R136a1 is a component of the R136a system at the core of R136. This dense luminous knot of stars contains 24 resolved components, the most dominant being R136a1, R136a2, R136a3, and R136c, all of which are extremely luminous and massive. R136a1 is separated from R136a2, the second brightest star in the cluster, by 5,000 AU.[3] R136a2 itself is among the most massive and luminous stars known, at 195 M and 6,000,000 L.[3]

Proprieties[edit]

Binary[edit]

X-ray emission was detected from R136a1 using the Chandra X-ray Observatory. This instrument cannot resolve R136a1 due to the crowded environment, so it is treated as a single star, although the X-rays could indicate colliding winds in a binary system.[2]

R136a1, as well as any stars in R136 over 150 M, are not affirmed as single. However, if the 150 M limit were to (approximately) remain valid, an equal-mass binary system would have to be adopted for R136a1. Rapid Doppler radial velocity variations would be expected from a pair of equal mass stars in a close orbit. However, infrared observations did not detect this, ruling out a pair of equal mass stars in a close orbit. [18]

It is predicted that a pair of equal mass stars in an intermediate orbit would each possess very powerful stellar winds which would collide and produce bright X-rays. A pair of equal mass stars in a wide orbit would also posses X-rays since they would interact with other high mass stars within the center of R136a, reducing the separation and causing them to spiral together into an intermediate separation within <<Myr timescales. However, no X-rays were detected in R136a1, ruling out an equal-mass binary system.[18] With the exclusion of an equal mass binary system, models predict that the primary's mass would be around 10% less than the single-star model of R136a1.[18]

Classification[edit]

Artist's conception of a Wolf-Rayet star surrounded by a nebula.

The appearance of R136a1 is a high-luminosity WN5h star with a temperature of 53,000 K, placing it on the extreme-left hand corner of the H-R diagram. At birth it would be an O supergiant with a mass of 320 M and a luminosity over half of its current one. The spectrum would have developed into a WN5h after 1-1.9 Myr.[3] Although a hypergiant appearance would be expected at birth, a large luminosity and mass would not be sufficient for it to be classified as a hypergiant. Ordinary supergiants lack the strong H emission and broadened spectral lines that indicate rapid mass loss in the hypergiants. Models predict that these proprieties would not be found in R136a1, and so it would only be classified as a bright supergiant.[3]

A Wolf-Rayet star is distinguished by the strong, broad emission lines in its spectra. This includes ionized nitrogen, helium, carbon, oxygen and occasionally silicon, but with hydrogen lines usually weak or absent.

R136a1 is not a typical Wolf-Rayet star. It is classified as "WNh", which shows a spectrum similar to the "nitrogen flavored" WN stars plus the enhancement of hydrogen. Despite the similar spectra, the WNh stars differ much from the "normal" WN stars. They are much younger, enriched in hydrogen, and are among the most massive and luminous stars known. They still show strong lines of hydrogen in their spectra and are much younger than any other Wolf-Rayet star, sometimes even being referred to as the "top of the main sequence".[19][20]

WNh stars show lines of hydrogen and nitrogen their spectra, the nitrogen being the product of CNO cycle fusion in the core. During the CNO cycle, carbon acts as a sort of catalyst to produce a 4He ion and a 14Ni ion.

Differential rotation, where the core is spun up to a faster rate than the surface, is theorized to occur in R136a1. Energy is transferred to the exterior by the physical movement of plasma rather than through radiative processes. This convection means that the 14Ni left over from the CNO cycle does not accumulate at the core, but is instead circulated throughout the star. The result of this is a strong surface nitrogen enchantment at a very early age.[19]

The hydrogen in R136a1's spectra is due to the fact that it is actually much less evolved than "normal" Wolf-Rayet stars and so it still retains much of its hydrogen. Measurements of its spectra reveal that its surface hydrogen abundance accounts for 30-40% by mass.[3]

Mass[edit]

At 265 M, R136a1 is the most massive star known. It is ~2.2 times the mass of η Car and is ~30 times the limit in which a star becomes a supernova. At birth, its mass was 320 M.[3]

Atmospheric and stellar evolutionary models used to calculate the mass of R136a1 may have systematic uncertainties. However, the atmospheric and stellar evolutionary models used to calculate R136a1’s mass was tested using a binary system known as NGC 3603-A1. The masses measured for NGC 3603-A1 using the atmospheric and stellar evolutionary models closely agree with the binary star measurements.[18]

The acquiring of reliable stellar masses requires knowledge of certain physical properties; the temperature, bolometric luminosity, and evolutionary state of the star. To determine if the masses are reliable, models are needed that describe how stars change their physical properties as they age. The stellar properties derived for R136a1, plus measurements of its surface hydrogen abundance closely matched evolutionary models calculated for rotating, main-sequence LMC-metallicity stars.[18]

Mass loss[edit]

R136a1 is undergoing extreme mass loss through a fast stellar wind. The process through which this occurs is that its large and dense wind region surrounding its photosphere creates a large amount of UV radiation which causes the star’s outer layers to be stripped away in succession. Currently, the star loses 5.1 × 10-5 M/year (3.21 × 1018 kg/s) and has a stellar wind of 2,600 ± 150 km/s. It is expected to have shed over 50 M since its formation.[3]

Luminosity[edit]

Left to right: a red dwarf, the Sun, a blue dwarf, and R136a1. R136a1 is not the largest known star in terms of radius or volume, only in mass and luminosity.

At over 8,000,000 L, R136a1 is the most luminous star known. It supplies ~7% of the ionizing flux of the entire 30 Doradus region[3] and, along with R136a2, a3, and c, provides 34-46% of the the radiation power of R136, which contains 100,000 stars in total.[3] In fact, the Lyman continuum of R136a1 is equal to around 70 O7 dwarf stars.[3] It radiates about as much energy in 3.6 seconds as the Sun does in a year.[21]

As non-fusing helium ash accumulates in the core of R136a1, the reduction in the abundance of hydrogen per unit mass results in a gradual lowering of the fusion rate. Since it is the outflow of fusion-supplied energy that supports the higher layers of the star, the core is compressed, producing higher temperatures and pressures. Both factors increase the rate of fusion thus moving the equilibrium towards a smaller, denser, hotter core whose increased outflow pushes the higher layers further out. Thus, there is a steady increase in the luminosity and radius of the star over time.[22]

As the luminosity of stars increases greatly with mass, the luminosity of massive stars often lies very close to the Eddington limit, which is the luminosity at which the radiation pressure expanding the star outward equals the force of the star's gravity collapsing the star inward. This means that the radiative flux passing through the photosphere of a massive star may be nearly strong enough to lift off the photosphere. Above the Eddington limit, the star would generate so much radiation that parts of its outer layers would be thrown off in massive outbursts. This would effectively restrict the star from shining at higher luminosities for longer periods.[23]

Models estimate that a 300 M star would be be 55% of its Eddington limit at birth so the ZAMS luminosity of R136a1 would be over half of its current one.[3] Strong mass loss has caused it to shed >50 M[3] over the past ~1.7 Myr,[3] so its luminosity has slightly increased over time relative to its Eddington limit factor. R136a1 is currently around 80% of its Eddington luminosity[18] and is approximately 8,700,000 L.[3] Its Eddington luminosity at birth was probably 13,000,000 L but since its mass has decreased by 20%, the Eddington luminosity has decreased to around 10,875,000 L.[24]

The apparent magnitude of R136a1, as viewed from Earth, is around 12.23.[1] However, it is 157,000 ly away, and around 1.5 magnitudes are absorbed by dust. If it replaced the Sun in our Solar System, it would outshine the Sun by 94,000 times,[3] and would appear from Earth to be magnitude -39.16.[25] The apparent bolometric magnitude would be around -44.08,[26] due to the fact that 92% of the radiation occurs in the higher energy band.[27] The star's absolute visual magnitude would be about -7.6, or 3 magnitudes brighter than Venus.[3] Its absolute bolometric magnitude would be around -12.6, or almost as bright as the full moon.[3]

The luminosity of R136a1 is rather uncertain due to conflicting measurements. The most recent survey derived a bolometric luminosity of 7,400,000 L using the Potsdam Wolf-Rayet (PoWR) model atmosphere code.[2] However, this measurement is expected to have an underestimation since observations rely on the Radial velocity measurement, and this approach may lead to substantial errors if RV deviates from the adopted value.[20] Observations largely relied on UV wavelengths where the uncertainties in the extinction are very high.[20] The UV spectrum of R136a1 is also largely contaminated by R136a2, so measurements in this range are unreliable.[3] However, in infrared wavelengths, the extinction is almost negligible, so values derived from infrared observations yield a more accurate value than UV observations.[20] Another study used the infrared apparent magnitude of R136a1, the 157 kly distance to R136, and the modest interstellar extinction to derive a bolometric value of 8,700,000 L, with a plus-minus error margin of 1,200,000.[3] The uncertainties that apply to the bolometric luminosity also apply to the visual luminosity. An infrared observation derived a value of 94,000 L[3] while an UV observation derived one of 59,000 L.[2]

Temperature[edit]

The color of an 53,000 K blackbody.[28]

The star was described to have a temperature of 53,000 K (52,700 °C; 94,900 °F), nearly ten times hotter than the sun.[3]

Because of the star's extreme surface temperature, its energy temperature (boltzmann's constant multiplied by its kelvin temperature) is about 4.6 eV,[29] so most of the star's energy is in the near ultraviolet range[30] and not in the blue part of the visible light. However, if viewed in the visual band, the star would appear to be blue[28] (-0.30 on the B-V color index).[1]

Interstellar reddening, caused by the scattering of light by interstellar dust and gas, significantly reddens the color of R136a1. Without correction, the star is +0.17 on the B-V color index, akin to a F-type star of 8,000 K. With the correction, the star is -0.30, within the color boundaries of an O star.[1]

Its surface is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Non-degenerate stars have no solid surface. Therefore, the photosphere is typically used to describe the star's visual surface.

Size[edit]

A size comparison between R136a1 and the Sun.

The size of R136a1 was derived to be 35.4 times the size of the sun (25,000,000 km; 15,000,000 mi) which corresponds to a diameter that is 3860 times larger than Earth[31] and a volume of 44,000 suns,[32][33] or approximately 57,000,000,000 Earths (1.5 × 1022 mi3).[32][34] It is approximately 1/6 of an AU in diameter.[35]

Blue and white supergiants are high luminosity stars somewhat cooler than the most luminous main sequence stars. A star like Deneb, for example, has a luminosity around 200,000 L, a spectral type of A2, and an effective temperature around 8,500 K, meaning it has a radius around 203 R. For comparison, the red supergiant Betelgeuse has a luminosity around 100,000 L, a spectral type of M2, and a temperature around 3,500 K, meaning its radius is about 1,000 R. Red supergiants are the largest type of star, but the most luminous are much smaller and hotter, with temperatures up to 50,000 K and more and luminosities of several million L, meaning their radii are just a few tens of R. An example of this is R136a1, at over 50,000 K and shining at 8,700,000 L, it is only 35.4 R.

Despite the star's high mass, it has an average density of 14.89 kg/m3, which is approximately 12 times more dense than Earth's atmosphere at sea level. However, this is the average density of the star, and some regions are expected to dramatically vary between the average density. For example, the core is expected to be very dense, while the surface would be puffy and diffuse. Compared with the sun, which has an average density of 1,400 kg/m3, it is 94 times less dense.

Rotation[edit]

The rotation rate of R136a1 is subject to controversy due to spectral appearances that complicate the measurement. Although photospheric absorption features are absent, the NV 2.100 µm feature provides another option of measuring the star's rotation. R136a1 was derived to have a feature of FWHM∼15A, corresponding to a slow or non rotating star, or a fast rotating star viewed pole-on. However, models do derive an equatorial speed of ve ∼200 (300) km/s−1 for a star with proprieties similar to R136a1 after ∼1.75 Myr.[3]

Future[edit]

R136a1 is a rare object, with no other stars being close to its mass and luminosity. A star like this is bound to explode as a supernovae or hypernovae, with no other possible outcome, but will exhibit extreme mass loss before undergoing a large explosion. However, current models do not fully reproduce stars as massive as this, so its future is highly uncertain.

Evolution[edit]

The sizes of stars.

It is thought that no star can be formed over 150 M so stars like R136a1 are thought to be "impossible". However, recent computer simulations suggest that instead of forming through the accretion of gas in a nebula, stars like this one were formed through the collision and merger of multiple stars.[36]

A ZAMS appearance would be a 320 M star with a spectral type of O2[37] and an extremely hot surface. Rapid fusion of hydrogen through the CNO cycle would result in a strongly convective star, with most of the elements fused in its core travelling up to the surface.[19] This process would be enhanced by rotation, or possibly by differential rotation where the core is spun up to a faster rotation than the surface. The result of this would be a strong surface nitrogen enhancement at a very early age. Because of this process, the star would eventually transition from O to Of. If the star was sufficiently hot, it would have developed an Of* spectrum, which would quickly transition to WNh. The total amount of time spent as a O star before showing a nitrogen appearance would be around <2 Myr.[19] R136a1 is currently on the top of the main sequence and shows a nitrogen emission profile.[20]

The future spectrum of R136a1 is uncertain. It is thought that WNh stars develop into LBVs after hydrogen at the core becomes depleted, so the future state of R136a1 would probably be an LBV.[19] However, this future spectrum is difficult to clarify since the spectrums of LBVs show an unusual pattern of variability. In outburst, most LBVs share a similar F-type spectrum. In the quiescent state, LBVs have spectrums of B to O.[38] Owing to the large mass of R136a1, it would be an "extreme" LBV, much like η Car, except more massive and luminous.

Most LBVs have large luminosities that place them near the Eddington limit.[39] This characteristic, combined with the low surface gravity results in a substantial mass loss rate[38] which quickly blows off its hydrogen envelope and causes it to loop back into a H-depleted Wolf-Rayet star. Because of this, their lifetimes are very short (<105 years)..[39]

After the LBV stage, LBVs usually evolve back to hotter temperatures as H-depleted Wolf-Rayet stars, with more than half of their initial mass gone. Stars like this would show a similar spectrum to WNh but with the exclusion of hydrogen. After this, the star would show WC (carbon) and maybe even WO (oxygen) lines, before exploding as a supernova.[40]

Models of the evolution and death of single very massive stars predict an increase in temperature during helium core burning, with the outer layers of the star being lost. It becomes a Wolf–Rayet star on the nitrogen sequence, moving from WNL to WNE as more of the outer layers are lost, possibly reaching the WC or WO spectral class as carbon and oxygen from the triple alpha process reach the surface. This process would continue with heavier elements being fused until an iron core develops, at which point the core collapses and the star is destroyed. Subtle differences in initial conditions, in the models themselves, and most especially in the rates of mass loss, produce different predictions for the final state of the most massive stars. They may survive to become a helium-stripped star or they may collapse at an earlier stage while they retain more of their outer layers.[41][42][43] The lack of sufficiently luminous WN stars and the discovery of apparent LBV supernova progenitors has also prompted the suggestion that certain types of LBVs explode as a supernova without evolving further.[44]

The final state of R136a1 is uncertain. It may survive to become to a bare carbon-oxygen core with a WO spectrum or it may collapse at an earlier stage while the star retains more of its outer layers.[41] In total, the mass lost by R136a1 will be large, and the star will be a fraction of its initial mass at core collapse.

Supernova[edit]

Fundamental properties of supernovae depend on the initial mass of their progenitors. For a sufficiently massive star, the radiation-driven mass loss will have a large effect on its evolution. Mass loss would certainly have a large effect on a star with an initial mass of ~300 M, as with R136a1, which has already shed ~50 M. Mass would also have a large effect, as it governs the lifetime and properties of a star. A 260 M star with an age of ~1.7 Myr would be undergoing hydrogen fusion through the CNO cycle with several stages of nuclear fusion to undergo before it explodes as a supernova or hypernova. The total lifetime would be approximately 2.5-3 Myr,[18] with the star shedding a large amount of mass through its stellar wind. However, the fate of massive stars like this is highly uncertain, but evolutionary tracks can be estimated due to the current properties of the star.

Supernovae properties are expected to depend on the mass of the star at core collapse, which, in the case of R136a1, is expected to be below 100 M. The final stage of R136a1 would probably be a WO/WC type star with a temperature over 100,000 K.

Gamma-ray bursts are unlikely when this type of star reaches core collapse because rotation rates decrease significantly due to mass loss and envelope inflation. At low metallicity, many massive stars will collapse directly to a black hole with no visible explosion or a sub-luminous supernova, and a small fraction will produce a pair instability supernova, but at solar metallicity and above there is expected to be sufficient mass loss before collapse to allow a visible supernova of type Ib or Ic.[45] If there is still a large amount of expelled material close to the star, the shock formed by the supernova explosion impacting the circumstellar material can efficiently convert kinetic energy to radiation, resulting in a superluminous supernova (commonly called a hypernova), several times more luminous than a typical core collapse supernova and much longer-lasting. Highly massive progenitors may also eject sufficient nickel to cause a hypernova simply from the radioactive decay.[46] The resulting remnant would be a black hole since it is highly unlikely such a massive star could ever lose sufficient mass for the core not to exceed the limit for a neutron star.[47]

According to independent calculations R136a1 may be composed of material that is too rich in `metals' (elements other than hydrogen or helium) to undergo a pair-instability supernova.[18]

Most Wolf-Rayet stars explode as type Ib or Ic supernovae, which are divided into to two categories; the luminous type, at M = -22; and the normal, at M = -16 to -17.[48][49] R136a1 is most likely to explode as the luminous type and leave behind a black hole.

References[edit]

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Coordinates: Sky map 05h 38m 42.43s, −69° 06′ 02.2″