HD 5980 AB(C?)
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et al, 2002, U. Liege,
Larger x-ray image.
The x-ray bright, HD 5980 star system
lies next to a hot gas cloud that spans
around 100 light-years (more at CXC
and Astronomy Picture of the Day).
On February 16, 2007, astronomers using the European Space Agency's XMM-Newton X-ray observatory, with help from NASA's Chandra X-ray Observatory, announced their confirmation that the collision of supersonic winds from extremely massive but close-orbiting, stars A and B was creating multi-million-degree hot gas that radiates very brightly in X-rays with a periodic pattern (ESA press release; and Nazé et al, 2007 -- more below).
Intergalactic Region around HD 5980
In 1994, a star in the Small Magellanic Cloud (SMC or NGC 292) became brighter than everything else in that nearby galaxy, which lies about 210,000 light-years away (based on corrected Cepheid distances derived from HIPPARCOS satellite data published in early 1997). The SMC is a small irregular galaxy and orbiting satellite of the Milky Way visible to naked-eye observers from Earth's Southern Hemisphere, slightly north of the globular cluster 47 Tucanae. It is located in the southeastern part (0:52.7-72:49, ICRS 2000.0) of Constellation Tucana, the Toucan -- south of Zeta, Epsilon, and Beta Tucanae, west of Kappa Tucanae, northeast of Beta Hydri, southeast of Delta Tucanae, and slightly north of globular cluster 47 Tucanae (NGC 104). The galaxy is one of the most distant objects that can be seen with the naked eye by Humans and so has been known since pre-historic times.
© Anglo-Australian Observatory
Larger image; new image at APOD.
The Small Magellanic Cloud contains
an abundance of young hot blue stars
(more at Astronomy Picture of the Day
The second brightest and largest of the Milky Way's satellite galaxies (after the Large Magellanic Cloud), the SMC will eventually be torn apart and integrated gravitationally into the Milky Way within a few billion years. Gravitational tides induced by the much more massive Milky Way have already distorted the SMC's original shape by moving around its gas, dust, and stars, which may have helped to foster stellar formation in nebulae like NGC 346 as the galaxy has a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. The SMC may not be be gravitationally bound to its neighbor, the Large Magellanic Cloud (Demers and Irwin, 1993). (More images and information are available on the SMC, galaxies in general, and the absorption of satellites and their globular clusters).
Within the SMC, the HD 5980 system can be found as the visually brighest stellar member of the dusty nebula and young star cluster NGC 346 (Moffat et al, 1998). Although relatively metal-poor, the cluster contains a lot of young, massive stars including about half of all the early type-O stars in the SMC. Indeed, the hot stars of the cluster are responsible for the ionization of N66, which also surrounds NGC 346 as the largest and most luminous H-II region in the SMC (Ye et al, 1991). (More discussion about observing this distant star cluster from Andrew Murrell's NGC 346 and the Astronomical Society of New South Wales.)
The HD 5980 System
The system was once estimated to be located at least 180,000 ly (Mathewson et al, 1986) but, like most of the SMC, it is probably located at an HIPPARCOS updated distance of around 210,000 ly away from Sol. It appears to lie next to or within a hot and x-ray bright, heart-shaped cloud gas about 100 ly across. At about eight million °C and bright in x-rays, the gas cloud is probably the remnant of a supernova explosion that occurred thousands of years ago (i.e., an expanding hot shell of ejecta) or a nebula of ejecta associated with a luminous blue variable -- the system primary (Star A) was resembled in 1994. Moreover, its spatial proximity to the system suggests that the gas cloud is related to the system.
et al, 2002,
Larger x-ray image.
The x-ray bright, hot gas
cloud is either a supernova
remnant or the type of nebula
associated with the ejecta of
luminous blue variables (more
at CXC and HEASARC).
A compact, x-ray bright object at the location of the system has been found that is somewhat less luminous than the one found at the location of Eta Carinae. If colliding stellar winds from its two confirmed stars (A and B) are not responsible for producing the observed radiation (as is found with Eta Carinae, which may also be a binary star system), then the x-ray brightness of the system may be coming from a compact object orbiting as a third companion "C" (or "D") -- e.g., a neutron star or black hole -- which also requires the existence of a fourth companion "D" (or "C") in a wider orbit (Nazé et al, 2002, in pdf). On February 16, 2007, astronomers using the European Space Agency's XMM-Newton X-ray observatory, with help from NASA's Chandra X-ray Observatory, announced their confirmation that the collision of supersonic winds from extremely massive but close-orbiting, stars A and B was creating multi-million-degree hot gas that radiates very brightly in X-rays with a periodic pattern (ESA press release; and Nazé et al, 2007).
Star A is of uncertain spectral and luminosity type. Once thought to be a Wolf-Rayet star (WN3-11p) (Moffat et al, 1998; and Virpi S. Niemela, 1997), it sometimes looked like a blue to blue-white supergiant (O7-B1.5 Ip) but erupted like a luminous blue variable (LBV) in 1994 with their associated spectrum (Barbá and Niemela, 1994). From June to October 1994, HD 5980 brightened by more than three magnitudes from its "quiescent" state (Koenigsberger et al, 1998; and Bateson and Jones, 1994) (as well as reddened) to become the visually brightest star in the SMC when its spectrum was typed WN11, like that of the Milky Way LBV AG Carinae in its low state (Heydari-Malayeri et al. 1997; and Barbá et al, 1995). Before its 1994 outburst, HD 5980 was observed to be brightening gradually with small glitches, starting from a WN3 + OB spectrum before 1980 and becoming a WN6 (without detectable photospheric absorption lines) by November 1993 and then a WN8 for six weeks and a rapid drop to WN7 just before the major eruption in June 1994. After 1994 October, HD 5980 declined rapidly at first, then more slowly to WN7 for most of 1995 through February 1997.
IPAC (JPL/Caltech), NASA, NSF
Field, infrared false-color and close-up radio images.
In 1994, HD 5980 A erupted like a luminous blue variable (LBV)
with a spectrum resembling that of the Milky Way LBV AG Carinae,
shown at left (more discussion at 2MASS Atlas image gallery).
While Star A is helium-rich (He/H= 0.43), it does not appeared to have ejected all of its outer hydrogen envelope as yet. Thus, its evolutionary characteristics suggest that core fusion has not progressed to helium burning. Possibly born as a 120-Solar-mass (Stothers and Chin, 1993), low metallicity star (at the theoretical-maximum mass for metal-contaminated gas), Star A may have already ejected off enough mass to have only 40 to 62 Solar-masses remaining. The star sometimes swells from 48 to around 160 times Sol's diameter (exceeding its orbital separation from a close companion star) and has around four million times the luminosity of Sol (Koenigsberger et al, 1998, and 1998). The star has a confirmed stellar companion designated as Star B. Useful catalogue numbers and designations for Star A are: SK 78, AzV 229, CSI-72-00576, Flo 382, GSC 09138-01929, JP11 448, LHA 115-S 28, LIN 353, RMC 14, SMC AB 5, and TYC 9138-1929-1.
An eclipsing binary system, Stars A and B have an eccentric orbit (e= 0.28) with period of 19.3 days, that is inclined by more than 88 degrees from the perspective of an observer on Earth (Koenigsberger et al, 1998). Especially significant for observers of the binary system is the strong collision of the two nearly equal winds before the 1994 outburst. The emission-line spectrum generated by this wind collision tends to mask the underlying line spectra of both stars when the system is not violently erupting.
1.2-m Telescope, Whipple Observatory
Larger and field images.
HD 5980 B is a Wolf-Rayet star, like HD 56925 (WN5) at left,
that has a strong stellar wind blowing gobs of gas and dust
from its outer envelope into space, sometimes as a bubble-like
shell nebula like NGC 2359 also at left (see CfA and APOD).
Companion B is a nitrogen-rich, Wolf-Rayet star of type WN4. It may have around 18 to 30 times the mass of Sol, around 40 times Sol's diameter (Koenigsberger et al, 1998), and many times its luminosity (Koenigsberger et al, 1998).
HD 5980 C?
Recently, astronomers were not able to detect radial velocity variations on the 19.265-day, binary orbital timescale of Stars A and B that would be attributable to erupting Star A. They believe that the non-detection confirms that the visible absorption lines in the spectrum are caused by a third stellar component C (Koenigsberger et al, 2002; and 1998).
HD 5980 D?
If the hot, heart-shaped cloud gas around HD 5980 is the remnant of a supernova explosion from a stellar member of the system, then the compact, x-ray bright object found at the location of the system may be a neutron star or black hole member of the system -- i.e., companion "D" or "C" (Nazé et al, 2002).
To paraphrase an excellent summary by astronomer Stephen White, Luminous Blue Variables (LBVs) are among the most massive stars that astronomers know of. LBVs are more than 40 to more than 100 times as massive as Sol and are presumed to have started out as early O-type, main-sequence dwarf stars. Since the most massive stars tend to also burn extremely hot and to consume their core hydrogen the quickest, they live just a short while by astronomical standards -- only a few million years at most.
Morse, Kris Davidson,
Larger infrared image.
Luminous Blue Variables
like Eta Carinae, at left,
appear to eject huge
amounts of their outer
gas envelopes in quick
but violent outbursts.
Once a massive star has fused most of its core hydrogen to helium ash, it becomes very unstable, and they eventually may blow up as spectacular Type-II supernovae. Some of the most massive ones may first pass through an LBV stage when they appear to eject huge amounts of mass from their outer layers of gas in a very short time (even more mass in a shorter period than Wolf-Rayet stars). The ejected gas (mostly hydrogen) moves outwards from the star at speeds of 50 to 500 kilometers per second and may be observed as nebulae that are bright in radio wavelengths. (Radio images of some LBVs and more discussion are available.)
As summarized by Koenigsberger et al (1998), Wolf-Rayet (W-R or WR) stars are classified as massive objects (with at least 20 Solar-masses) that have evolved beyond the burning of core hydrogen that characterize main-sequence dwarfs. Such stars are characterized by exceptionaly massive stellar winds and high relative abundances of chemical elements such as Helium (He), Nitrogen (N), Carbon (C), and Oxygen (O). Eventually, enough of such elements created at the core of these stars reach their surface so that they absorbs so much of the intense light from the star that an enormously strong wind starts to blow from the star's surface. This wind becomes so thick that it totally obscures the star. The three sequences of W-R stars (WN, WC, and WO) correspond to the relative predominance of N, C, and O lines in their spectra, respectively. Moreover, WN stars are further subdivided by the dominant degree of ionization in their winds into WNE ("early" or WN2-5) and WNL ("late" or WN6-11), where the former have been found to be very H-poor or to have no H in their winds (Hamann et al. 1991. However, the most massive stars may also exhibit Wolf-Rayet-like characteristics (de Koter, Heap, & Hubeny 1997).
Gilles Joncas, Agnes Acker,
Larger false-color image.
At least one of the stars of the
HD 5980 system is a Wolf-Rayet
star, like WR124 at left, which
is vigorously ejecting its outer
envelope of gas and dust (more
from STScI and APOD).
The chemical peculiarities of W-R stars are attributed to the
presence of nuclear-processed material at the stellar surface.
According to the standard evolutionary scenarios
Maeder, 1983; and
and Meynet, 1994, and
a massive O star (of 50-120 Solar-masses) is expected to rapidly
swell when evolving toward lower temperatures after core hydrogen
exhaustion. In this hypothesized evolution, such stars are
believed to reach an instability limit when they enter a phase
with even more rapid and massive loss of mass, that has been
identified with the stars known as luminous blue variables or
and Lamers, 1997; and
Moffat, and Lamers, 1989). LBVs are very luminous and
unstable, hot hyper- or supergiants that undergo irregular
eruptive events that lead to the formation of a very extended
photosphere at maximum visual brightness. Generally at that
stage, their winds are expanding slowly (100-200 km per
second) and relatively low-effective surface temperatures
are observed (7000-9000 Kelvins). Subsequent evolution with
continued mass loss (of its outer hydrogen layer) leads to the
creation of a WN star, followed in succession by a WC and a WO
just prior to the supernova explosion that ends the life of the
star. Hence, the expected evolutionary sequence is:
O --> Of --> LBV --> WNL --> WNE --> WC --> WO --> Supernova
Dave Devine, and
Ralph Sutherland, STScI, NASA
Larger false-color image.
Astronomers believe that Wolf-Rayet stars begin
life as main-sequence O-type dwarf stars, like
Theta1 Orionis C, the brightest of the four
central stars of the Trapezium Cluster in Orion
(at left), located around 800 light-years away.
et al (1998), the WNE stage is believed to be reached
only after the helium core of the star is fully exposed,
given the low abundance of hydrogen in WNE stars relative
to WNLs. More recently,
et al (1994) proposed a modified scenario for the
evolution of massive stars that incorporates theoretical
results for violent pulsational instabilities
et al, 1993; and
et al, 1993). Presumed to be driven by the "iron
opacity bump," these instabilities could be producing
pulsationally driven mass loss near the end of the core
hydrogen-burning phase of massive stars. Langer et al
proposed that at this stage, prior to the LBV phase, a
massive star could have a WN-type spectrum, and so
their proposed evolutionary sequence is:
O --> H-rich WN --> LBV --> H-poor WN --> H-free WN --> WC --> Supernova
Larger true-color composite image.
O-type stars evolve very quickly, and
the most massive ones may soon become
eruptive, luminous blue variables, like
the Pistol Star, at left, which may have
already ejected half of its initial 200
Solar-masses since its birth as much
as three million years ago.
Koenigsberger et al (1998) argue that a close relationship between some of the WNLs (mainly WN9-11) and LBVs is now firmly established (Pasquali et al, 1997; (Crowther et al, 1995; and Stahl et al, 1983), supporting the connection between hydrogen-rich WNs and LBVs. In contrast to these theoretical models, the stars in the R136a region of the Large Magellanic Cloud's that display WNE-type spectra have recently been found to be extremely massive main-sequence objects that mimic the more hydrogen-depleted W-R stars (de Koter et al. 1997). Thus, WNE characteristics are not exclusively associated with postmain-sequence stars.
Uncertain Status of the HD 5980 System
In the case of HD 5980, Koenigsberger et al (1998) described the system as a very massive and luminous binary within which a major LBV-type eruptive event was observed (Barbá and Niemela, 1994; Barbá et al. 1995; Bateson & Jones 1994; Koenigsberger et al, 1994 and 1995). The magnitude of the eruption (with a change of magnitude greater than three) identified it as a major eruption, similar to those that occurred in Eta Carinae and P Cygni (where reviews of previous investigations can be found in Barbá et al, 1996, and 1997; Breysacher, 1997; Koenigsberger et al, 1996, and 1998; Moffat, 1997, and Moffat et al, 1998, and Niemela et al, 1997). Moreover, the binary system's orbit is eccentric, with a period of 19.3 days and an inclination of greater than 86° (Breysacher 1997; Moffat et al, 1997), where eclipses occur at orbital phases 0.0 and 0.36 (the star "in front" at orbital phase 0.00 (O7 I) is star A, while the star "in front" at phase 0.36 (WN4) is star B. According to Koenigsberger et al's "crude estimate" of the stellar masses and luminosities of the two stars within the binary system, both stars are very massive and extremely luminous, and assuming evolution with constant luminosity, the erupting star lies near the theoretical evolutionary track of a star with an initial mass of 120 Solar-masses.
Since the first observations in the early 1970s,
the observed spectral type of the system has evolved in the
following manner (Koenigsberger et al. 1994, 1996):
WNE --> WNL --> LBV-type eruption --> B1.5Ia --> WNL
Changes in the wind velocity leading up to the eruptive event (Koenigsberger et al, 1998) show a systematic decrease prior to the eruption and an increase in velocity after the eruption, where both were correlated with the spectral variations. Although the timescale over which the transition has been observed is very short (18 years) in terms of evolutionary timescales, it is possible that stellar evolution into the instability strip at its earliest stages is being observed.
However, there are several complications which conflict with the hypothesis that the system's eruptive behavior is actually a direct consequence of stellar evolutionary processes. First, the spectral type of HD 5980's primary has evolved from WNE (believed to be hydrogen-poor W-Rs) to WNL (which are usually hydrogen-rich), although the evolutionary sequence is in the opposite direction. Second, the pulsational instabilities referred to above are driven by the iron opacity peak. Given the significantly lower iron abundance in HD 5980 with respect to its Galactic counterparts (Koenigsberger, Moffat, & Auer, 1987), it is not clear whether these instabilities can be very effective in a system such as HD 5980. Finally, HD 5980's eccentric orbit has led some to propose that the eruption could have been induced by the oscillating gravitational forces within the system (Koenigsberger and Moreno, 1997; and Moreno, Georgiev, and Koenigsberger, 1997).
The fact that HD 5980 is a binary system has complicated analysis of the evolutionary state of its component stars. Foremost among these is the uncertainty as to which of the two stars in the system is the source of the outburst (Barbá et al, 1996; Koenigsberger et al, 1995; and Moffat et al, 1998). As mentioned above, in the late 1970s the system was composed of WN4 + O7 I stars (Breysacher et al, 1982). According to this classification, the first question would be avoided if the eruption were to have occurred in the O7 supergiant. However, a subsequent classification of the stars in the binary was given by Niemela (1988) and Niemela et al (1997) to be WN4.5 + WN3 (but see Moffat et al, 1998), so that regardless of which of the two stars underwent the eruption, the precursor would have had WNE characteristics. While the presence of a third stellar object within the system is now suspected (Koenigsberger et al, 2002; and Heydari-Malayeri et al, 1997), there is no question that the erupting star forms part of the 19.3 day eclipsing-orbit binary (Koenigsberger et al, 1995). The situation is confused further by the fact that two such massive and close binaries have very strong interaction effects, including wind-wind collisions (cf. Moffat et al, 1996; and 1998), producing phase-dependent variations in the spectrum that do not have a clear interpretation in terms of orbital motion. (For an extended discussion from which the previous exposition was taken, see: Koenigsberger et al, 1998).
Up-to-date technical summaries on this star are available at: NASA's ADS Abstract Service for the Astrophysics Data System; and the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access.
Tucana is a small constellation located around the South Pole. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Tucana. For another illustration, see David Haworth's Tucana.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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