Sigma Orionis Star
Credit: IAC
A detailed study on the multiple star system led by Spanish
astrophysicists has identified the period, mass and emission of high
energy photons of the main stars of the system
Some three million years ago hundreds of stars formed from a dense
cloud of gas and dust in the constellation of Orion (“the Hunter”). The
star which swallowed the largest part of the mass was sigma Orionis (sigma Ori), which is now the fourth brightest star in the belt of Orion, and which illuminates the famous Horsehead Nebula. At the same time as sigma Orionis,
a large number of stars with a full range of masses formed in its
neighbourhood, as well as brown dwarfs, and “rogue” planets (objects
similar in mass to that of a Jupiter but which do not rotate around a
star, rather they move freely in the star cluster). The smallest objects
in Orion´s belt have 10,000 times less mass than sigma Orionis.
To understand the frequency of the birth of low mass stars, brown
dwarfs, and rogue planets, and how they evolve, we need first to know
what happens to their high mass, blue neighbours. With this in mind, an
international team of astronomers led by the Spanish researchers Sergio
Simón-Díaz, (IAC/ULL), Jose A. Caballero (CAB, CSIC-INTA), and Javier
Lorenzo ( University of Alicante), with the participation of the
Institute of Astrophysics of Andalusia, have studied in great detail the
multiple star sigma Orionis.
Even eons after their deaths one can observe the unique imprint of high
mass stars on almost everything around them: our own chemical
composition, the distribution in space of the stars and the nebulae
which they leave behind after they have exploded, the form of the spiral
arms of the galaxies, and even, curiously enough, the number of low
mass stars. “This latter effect –explains Sergio Simón-Díaz, who is
first author on the paper- is due to the fact that low mass stars, and
brown dwarfs (which are objects with masses between those of the
smallest stars and the largest planets) are just the left-overs from the
banquets of high mass stars”.
The star sigma Orionis is 3 million years old and has a high
temperature: its surface temperature is 30,000K five times hotter than
that of the Sun. This very high temperature makes the star have a
blueish colour, in contrast to less massive stars which have reddish
colours. “In 2011” –remembers Caballero- “we showed that sigma Orionis
is really a multiple star consisting of six stars instead as five, as
previously thought: two of them are very high mass stars, which are very
close together, rotating mutually around each other with an orbital
period of some 143 days. A third star, also massive, is orbiting at 100
astronomical units (100 times the distance from the Earth to the Sun)
from the other two, and rotates with a much longer period, of some 157
years. The cluster is completed with three other stars, a little cooler
and less massive, as well as numerous stellar remnants”.
Now these researchers, together with 11 others from Spain, Germany,
Chile, the USA, Belgium, and Hungary, have observed in more detail the
central trio of these stars, (sigma Ori Aa, sigma Ori Ab, and sigma Ori B),
and have measured their physical parameters with unprecedented
accuracy. “ The period of the closest pair, of around 143 days, has now
been determined with an uncertainty of only 11 minutes” –points out
Simón-Díaz-,” which makes it feasible to programme specific observations
at certain phases, for example with X-ray telescopes in space at
periastron, the point where the two central stars are separated by their
smallest distance”.
“Gobbler” stars
The study has also allowed us to determine very accurately the masses
of the three stars using different methods. “The sum of the masses of
the trio is bigger than 40 times the mass of the Sun”, emphasizes
Simón-Díaz. “These observations, together with interferometric
observations now in progress, are an excellent input to theoretical
modes which aim at explaining the structure and the fate of these
“gobbler stars”.
“We have also measured” –adds Caballero- “the rate of emission of high energy photons from the trio. These photons, emitted by sigma Orionis Aa, Ab, and B and those which ‘comb the mane’ of the Horsehead Nebula, and announce the start of a new banquet of high mass stars in the region. In just a few million years, when sigma Orionis Aa (and perhaps Ab)
explodes as a supernova, and cleans out the region around it, a large
number of smaller, cooler stars will continue to exist, as well as a few
large, massive, very hot stars, which at the present moment are inside
the dense clouds near the Horsehead Nebula”.
Referente:
Simón-Díaz et al. (2015, ApJ 799, 169)
Contact:
Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll@iaa.es - 958230532
http://www.iaa.es
http://www-divulgacion.iaa.es
