red circle) in the SW corner of the southern constellation of Telescopium
Epoch J2000.0 Equinox J2000.0
|Right ascension||18h 17m 07.53179s|
|Declination||−56° 01′ 24.0876″|
|Apparent magnitude (V)||5.36 (5.32 to 5.39)|
|Spectral type||B3IIIpe or B3II/III|
|B−V color index||−0.050±0.018|
|Radial velocity (Rv)||+9.4±0.5 km/s|
|Proper motion (μ)|| RA: -3.667 mas/yr |
Dec.: +11.120 mas/yr
|Parallax (π)||2.9148 ± 0.1828 mas|
|Distance||1,120 ± 70 ly |
(340 ± 20 pc)
|Absolute magnitude (MV)||−1.47|
|Primary||QV Tel Aa|
|Companion||QV Tel Ab|
|Period (P)||40.333±0.004 d|
|Argument of periastron (ω)|
|QV Tel Aa|
|Surface gravity (log g)||3.5-4.0 cgs|
|Rotational velocity (v sin i)||50±1 km/s|
|QV Tel Ab (black hole)|
|QV Tel B|
HR 6819, also known as HD 167128 or QV Telescopii (abbreviated QV Tel), is a double or triple star system in the southern constellation of Telescopium. It is in the south-western corner of the constellation, near Pavo to the south and Ara to the west. The system appears as a variable star that is dimly visible to the naked eye with an apparent magnitude that ranges from 5.32 down to 5.39, which is comparable to the maximum brightness of the planet Uranus. It is about 1,120 light years from the Sun, and is drifting farther away at a rate of 9.4 km/s. Due to its location in the sky, it is visible only to observers south of 33°N latitude.
A May 2020 study hypothesized that the system has the closest known black hole, and the first one in a system visible to the naked eye. But a study in July of the same year concluded that rather than a triple system it was much more likely to be a black hole and a star orbiting at one distance from here and another star at a different distance. Three further 2020 papers argued that HR 6819 is a binary system with two mainstream stars and no black hole at all.
HR 6819 is the Bright Star Catalogue designation for this star. It also has the Henry Draper Catalogue designation HD 167128 and the Hipparcos designation HIP 89605. The General Catalogue of Variable stars includes this system as a variable similar to but not properly of the Gamma Cassiopeiae type, with the variable star designation QV Telescopii, indicating that it is the 330th confirmed variable star (excluding stars with Bayer designations) in the constellation Telescopium.
History of discoveries
HR 6819 was originally considered a single star but it was noted by astronomers Dachs and Slettebak that its absorption spectrum displays characteristics of both Be and B3III stars. In 2003, Monika Maintz concluded the spectrum of HR 6819 contained the signatures of two stars, though there were not enough observations to deduce an orbital period. Further observations by Rivinius et al. in 2009 were able to disentangle the spectra of the two stars, and the same team conducted thorough radial velocity measurements in 2019 suggesting the presence of an unseen stellar-mass black hole within the system. The triple star hypothesis was challenged in 2020 by Mohammadtaher Safarzadeh, Sivia Toonen, and Abraham Loeb.
According to the interpretation of Rivinius et al. in 2019, HR 6819 would be a hierarchical triple containing a classical Be star in a wide orbit of unknown period around an inner 40.3 day binary, a B3 III star and a non-emitting (non-accreting) black hole (≥5±0.4 M☉), designated Ab. Although the HR 6819 system has been described as a member of the Sco OB2 association of co-moving stars, a more recent analysis indicates it is an older system and not part of the association.
The spectrum of HR 6819 contains both narrow and broadened lines. The broad lines originate from the rapidly-rotating Be star, while the narrow lines are from a more slowly-rotating B-class star. The radial velocity variations of the lines indicate that the normal B-class star is in a 40-day orbit, but the Be star does not show detectable oscillation. Assuming that these two stars have similar masses, this suggests that there is a third, invisible, body in the system, as the other component in the 40-day orbit. Analysis of the orbital parameters suggests that the third body is so massive that it can only be a black hole.
However, later work found another explanation, namely that the moving star is actually much lighter, but emitting a similar amount of light. In that case there is no need to invoke the presence of a black hole. This explanation is supported by the detection of motion in the hydrogen disk surrounding the other star, with the 40-day period.
QV Tel A or Aa
Depending on whether there is a black hole or not, the oscillating star is designated QV Tel A or QV Tel Aa, and is a B3 III blue star. If it is a normal star of this type, it has a mass of approximately 6 M☉, but if it is a stripped-down helium star it has a mass of around 0.5 M☉. If there is a black hole, the two orbit one another with a period of 40.33 days.
The spectral type of this component is well-defined at about B3 from the distinct narrow lines in the composite spectrum. Its temperature is 16 to 18 kK.
QV Tel Ab (black hole)
Radial velocity measurements of the above-mentioned component indicate that it has a massive companion. If this is a black hole (rather than component QV Tel B) then its designation is Ab. Being 1,120 light years distant from the Sun, this would make it the closest known black hole to the Sun. As the host stellar system has an apparent magnitude of 5.36, similar to the maximum brightness of planet Uranus, this black hole would be the first discovered in one of the 9,000 stellar systems visible to the naked eye. No black hole is detected in the spectrum and no x-rays have been observed, so any accretion disk around it would have to be a very weak one.
The orbital motion of the visible B3III star shows that the minimum mass of the unseen component Ab is almost as large as that of component A or Aa. If that star has a mass of 5 M☉, this means a minimum mass of 4.2 M☉ for the other orbiting component. If the inclination of the orbit is not edge-on to us, then the mass is higher. Any single star with a mass that high would be easily detected in the spectrum, and objects which might be undetectable, such as neutron stars, cannot be that massive. If it is not QV Tel B, then the object would seem to be a black hole.
But as mentioned, two published papers and one pre-print paper (as of October 2020) argue that HR 6819 is not a triple system, including a black hole, but rather a binary system with two visible stars.
QV Tel B
The second stellar component, designated as QV Tel B, is a type Be star with a stellar classification of B3IIIpe. The 'e' suffix indicates emission lines in its spectrum. It is a rapidly rotating blue-white star with a hot decretion disk surrounding it. It is estimated to be 50 million years old, with a projected rotational velocity of 50 km/s.
The emission lines in the spectrum are strong, but absorption lines from the Be star are weak and so the exact spectral type is difficult to determine. Overall the spectral class is similar to the inner blue giant, but the relative weakness of some luminosity-dependent lines suggest that it is a main sequence star. It appears to be slightly hotter and slightly less luminous than the inner giant star, but the exact properties are difficult to determine due to its rapid rotation, the weak absorption lines, and the presence of strong emission lines from the disk.
- LB-1, a binary system with a B star and possibly a non-accreting black hole or a neutron star.
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- Michelle Starr (2020-10-20). "The Closest Black Hole to Earth May Not Actually Be a Black Hole After All". Retrieved 2020-12-20.
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- Irrgang, A.; Geier, S.; Kreuzer, S.; Pelisoli, I.; Heber, U. (January 2020). "A stripped helium star in the potential black hole binary LB-1". Astronomy and Astrophysics (Letter to the Editor). 633: L5. arXiv:1912.08338. Bibcode:2020A&A...633L...5I. doi:10.1051/0004-6361/201937343.
- Astronomer tells Wikinews about discovery of closest black hole known so far at Wikinews (Friday, 2020 May 22)
| Least distant black hole