Antistars

Do stars made of antimatter exist in the universe? Possibly.

One of the great mysteries of cosmology and astrophysics is that even though equal quantities of matter and antimatter appear to have been produced during the “Big Bang”, today there is only a negligible quantity of antimatter in the observable universe. We do not appear to live in a matter-antimatter symmetric universe.

If antimatter stars, “antistars”, do exist, how could we distinguish them from stars made of normal matter? The light emitted from an antistar would look identical to the light emitted by a normal-matter star.

But if normal matter were infalling upon an antistar, the contact between matter and antimatter would generate an annihilation spectrum of gamma ray photons that peaks around energy 70 MeV (half the mass of a neutral pion) up to a sharp cutoff around 938 MeV (mass of the proton).

A recent analysis of data collected by the Fermi Gamma-ray Space Telescope found fourteen possible antistars. These fourteen point sources produce a gamma-ray signature indicative of matter-antimatter annihilation.  These point sources do not exhibit the characteristics of other known gamma-ray sources.  For example, they are not, ostensibly, pulsars, active galactic nuclei, or black holes.

The positional error ellipses for these fourteen point sources range from 11×10 arcminutes up to 128×68 arcminutes (95% confidence). Here are optical images of these sources from the Palomar Digital Sky Survey, in order of right ascension (epoch 2000 coordinates).

4FGL J0548.6+1200
5 48 38.8 +12 00 10
29.6’×23.6′ error ellipse
field of view 48.5′, Orion
bright star near crosshairs is HD 38797
4FGL J0948.0-3859
9 48 03.6 -38 59 57
53.7’×45.9′ error ellipse
field of view 48.5′, Antlia
bright star near crosshairs is TYC 7693-3238-1 ;
nebulous streak through the field is unidentified, 11˚ from the galactic plane
4FGL J1112.0+1021
11 12 03.1 +10 21 31
128.3’×67.9′ error ellipse
field of view 1.63˚, Leo
brightest star in field is HD 97502
4FGL J1232.1+5953
12 32 06.1 +59 53 03
15.4’×13.0′ error ellipse
field of view 24.11′, Ursa Major
brightest star in field is TYC 3847-229-1 ;
the galaxy is LEDA 2595040
4FGL J1348.5-8700
13 48 30.7 -87 00 47
10.6’×9.7′ error ellipse
field of view 11.99′, Octans
4FGL J1710.8+1135
17 10 50.5 +11 35 57
30.7’×26.7′ error ellipse
field of view 48.49′, Ophiuchus
brightest star near crosshairs is HD 155411
4FGL J1721.4+2529
17 21 24.7 +25 29 25
36.4’×25.2′ error ellipse
field of view 48.49′, Hercules
brightest star in field is HR 6455
4FGL J1756.3+0236
17 56 21.2 +02 36 52
19.0’×14.1′ error ellipse
field of view 24.11′, Ophiuchus
4FGL J1759.0-0107
17 59 03.7 -01 07 11
25.7’×22.8′ error ellipse
field of view 24.11′, Serpens
brightest star in field is HD 163914
4FGL J1806.2-1347
18 06 14.7 -13 47 36
19.2’×11.5′ error ellipse
field of view 24.11′, Serpens
4FGL J2029.1-3050
20 29 09.6 -30 50 06
31.0’×21.4′ error ellipse
field of view 48.49′, Microscopium
brightest star in field is HD 194640
4FGL J2047.5+4356
20 47 32.0 +43 56 33
58.9’×34.0′ error ellipse
field of view 1.63˚, Cygnus
brightest star in field is 56 Cyg ;
behind it is the Pelican Nebula (IC 5070)
4FGL J2237.6-5126
22 37 39.4 -51 26 05
20.7’×16.8′ error ellipse
field of view 24.11′, Grus
brightest star near crosshairs is TYC 8452-1160-1 ;
the edge-on galaxy is LEDA 92766
4FGL J2330.5-2445
23 30 35.6 -24 45 15
28.5’×20.5′ error ellipse
field of view 48.49′, Aquarius
brightest star near crosshairs is HD 221258

Since there appears to be no known way to distinguish a star made of antimatter from one made of matter—except for the gamma-ray signature of matter infalling onto the antimatter star, a higher-resolution gamma-ray telescope or interferometer (10 – 1000 MeV) needs to be developed to localize these candidate sources to within a few arcseconds. Higher spectral resolution will help as well, allowing a more detailed characterization of the gamma-ray spectrum.

References

S. Dupourqué, L. Tibaldo and P. von Ballmoos. Constraints on the antistar fraction in the solar system neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog. Physical Review D. Published online April 20, 2021. doi: 10.1103/PhysRevD.103.083016.
https://arxiv.org/abs/2103.10073

M. Temming (2021, June 5). Antistars could lurk in Milky Way. Science News, 199(10), 8-9.
https://www.sciencenews.org/article/antimatter-stars-antistars-milky-way-galaxy-space-astronomy

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