|PHOENIX was used
to sample multiple stellar atmospheres and has revealed tantalizing clues
that neutrino interaction during the supernovae explosion of massive stars
is involved in the genesis of fluorine. PHOENIX uses a Gemini supplied 1024
x 1024 InSb Aladdin II array.
our awareness of fluorine's unique properties, its astrophysical origin is
not well understood. Like most heavy elements, fluorine is a product of nuclear
reactions in the hot cores of stars during various phases of their evolution.
Furthermore, fluorine is not easy to observe in the universe, and very few
spectroscopic measurements of fluorine in stars exist. Until the recent Gemini
work reported here, only a meager set of data for a few stars in our own
Milky Way was available.
program conducted at Gemini South, located on Cerro Pachón, Chile,
has changed this picture. Katia Cunha (Observatório Nacional, Brazil)
led a PHOENIX team of researchers on Gemini South to measure the abundance
of fluorine in our own galaxy's satellite, the Large Magellanic Cloud (LMC),
and in the massive galactic globular cluster
The team studied fluorine in its most accessible form, hydrofluoric acid
(HF), which is detectable through vibration-rotation transitions falling
in the near infrared close to the wavelength of 2.3 microns (main figure and Figure 1). These Gemini observations
provide unique insight on how fluorine behaves as a function of the abundance
of the other elements, specifically metals, helping to identify how fluorine
understand how the abundance of fluorine depends on metallicity and stellar
populations, Cunha and her collaborators compared their Gemini measurements
with known K and M type stars of our own Milky Way. The total sample of stars
with fluorine abundance determinations contains 23 red giant stars across
three stellar populations including the solar neighborhood,
and the LMC. It is to be noted that the abundance of fluorine, like that
of other elements, does not only depend on the yield from nucleosynthesis
processes. Star formation history and element dispersal mechanisms can also
play important roles.
giants stars in the LMC and two in
were observed with PHOENIX. The behavior of the fluorine abundance as a function
of oxygen (Figure 2) and iron (Figure 3) show definite trends. The figures
clearly reveal that the ratio of fluorine to oxygen [F/O] decreases from
the near-solar metallicity galactic stars, to lower metallicity LMC giants
and Arcturus (
Oxygen is produced in the supernova explosions of stars with masses greater
than ~8-10 Msun (SN II). In contrast, iron is mostly from lower
mass stars supernovae (SNI).
mechanisms for producing fluorine in stars are: (1) neutrino-induced spallation
of a proton from Neon-20, referred to as the neutrino process; (2) synthesis
from helium capture by Nitrogen-14 during asymtoptic giant branch (AGB) thermal
pulses; and (3) production of Fluorine-19 in the cores of massive Wolf-Rayet
(W-R) stars. Because the
stars show depletion of fluorine instead of enrichment, the authors easily
discard the AGB stars as an important cradle of fluorine. For Wolf-Rayet
stars to be major providers of fluorine, they need to undergo, more substantial
mass loss than currently accepted.
a W-R source cannot be excluded at this stage, the best bet is the neutrino
process of spallation in supernovae of massive stars (SN II). Models of neutrino
nucleosynthesis predict that the [F/O] ratio declines steadily as oxygen
declines. This is shown very clearly in Figure 4.
red giants stand out from this prediction and trend. We know that
had 3 or 4 major isolated episodes of star formation, which ended several
billions years ago, giving the cluster its wide range of metal abundance.
In contrast, the Milky and the LMC are undergoing continuous star formation.
Taking into account the different star-formation histories, the authors conclude that the dependence of [F/O] on A(O) - as measured in Galactic LMC and
red giant stars is best explained by the chemical evolution models using
the known neutrino spallation present in massive supernovae, and that these
explosions must be the principal source of fluorine-19.
details, see the paper "Fluorine
Abundances in the Large Magellanic Cloud and
Centauri: Evidence for Neutrino Nucleosynthesis," by Katia Cunha (Observatório
Nacional, Brazil), Verne V. Smith (University of Texas, El Paso), David L.
Lambert (University of Texas at Austin), and Kenneth H. Hinkle (NOAO), The
Astronomical Journal, September 2003.
et al. 1998, Proc. SPIE, 3354, 164) was built by NOAO.
The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai`i (Gemini North) and the Gemini South telescope is located on Cerro Pachón in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.