DRY ICE, WETTER MARS
NASA
A new-found reservoir of dry ice on Mars suggests that the planet's
surface has been wetter in the relatively recent past, though not
necessarily warmer than it is today. The new study adds to evidence
that Mars once had a carbon dioxide atmosphere thick enough to keep
liquid water on the surface from evaporating. It is not clear whether
the planet would have been hospitable for life, however, because
temperatures on Mars may actually have been slightly colder during
times when the atmosphere had a greater amount of carbon dioxide.
Those findings are based on radar studies by the Mars Reconnaissance
Orbiter of the layered deposits of the south polar cap. Earlier
studies had indicated that a veneer of frozen carbon dioxide covers
part of the cap, with a thin layer of water ice beneath it. But radar
reflected from different layers indicates that beneath the frozen
water lies a volume of carbon dioxide ice 30 times greater than
previously estimated. That reservoir of dry ice is unexpected,
because it is thought that the inclination of Mars' rotational axis
varies substantially with a period of about 100,000 years. At times
of high inclination, enough sunlight could be expected to fall on the
poles to vaporize the frozen carbon dioxide and release it into the
atmosphere, roughly doubling the atmospheric pressure. With a denser
atmosphere, liquid water could persist on the surface rather than
evaporating, and might account for some of the features that appear to
have been carved by water. The associated alterations in climate
would, however, be 'modest'. Warmer conditions would require a much
thicker carbon dioxide atmosphere supplied by an additional source,
possibly carbonates in Martian rocks.
MATERIAL POURS ONTO A YOUNG STAR
McDonald Observatory
Astronomers at the University of Texas have been following a rare
massive flare from a nascent star similar to the early Sun. They have
found that the proto-star, called HBC 722, is situated in a tightly
packed region of gas and proto-stars. HBC 722 lies 2000 light-years
away in the 'Gulf of Mexico' region of the North America Nebula (NGC
7000), in Cygnus. In early 2009, it appeared to be an ordinary young
star in a cloud of similarly young stars. Like most stars less than a
few million years old, it is surrounded by a disc of gas and dust. It
began to brighten, slowly at first, increasing dramatically during the
summer of 2010. By late September it was three magnitudes brighter
than it had been a year before. Since that time, it has begun to
fade. The event provided astronomers with a rare opportunity to
observe the evolution of a flaring young star. Such objects are
called FU Orionis (FUor) objects, after the prototype, which flared up
in 1936. The HBC 722 flare is the first such event discovered in more
than 30 years.
Observations from the Herschel space telescope and other observatories
have documented its behaviour from the early stages, within a few
months of its brightening episode. HBC 722 has been dimming faster
than other FUor objects, which are still bright decades after their
eruptions. It seems likely that the flare was caused when a large
amount of material built up in the surrounding disc and suddenly
reached a critical point where it overflowed and poured onto the star
at a rate 20 times greater than usual, releasing a lot of heat and
ejecting excess material and momentum into the surrounding cloud.
Outbursts are often considered to be an important part of the process
by which a young star acquires its final mass, through a small trickle
of material punctuated by short, repeated floods.
Herschel revealed the busy environment of HBC 722, which is composed
of large amounts of molecular material such as carbon monoxide and
water, thought to be heated by ultraviolet light from the evolving
stars in the vicinity. In Herschel's infrared view, the flare of
HBC 722 may highlight flows of material from nearby cooler and even
younger stars. In the future, the increased radiation from HBC 722
may further heat the gas in the vicinity as it flows past.
Coordinating with observations by ground-based telescopes, astronomers
are looking for signs of the shock waves that should have been
launched from HBC 722. Herschel will observe HBC 722 again in June.
TYCHO SUPERNOVA HAD A COMPANION
Chandra X-Ray Center
A group of Chinese astronomers has proposed that the progenitor of the
Galactic supernova that exploded in 1572 and was observed in
particular by Tycho Brahe (from whom it takes its name) had a
companion star, which was not a white dwarf but probably a solar-type
star. Understanding that supernova is important because it was a type
Ia explosion, the type used as a 'standard candle' in the distance
estimates used to determine the rate of expansion of the Universe.
Although it is thought that such explosions are due either to the
fusion of two white dwarfs or to the transfer of mass from a normal
star onto a white dwarf, it is difficult to distinguish between
those two possibilities observationally.
The team studied features in an X-ray image of the Tycho supernova
remnant, which is about 20 light-years across. X-ray emission was
caused by the shock wave which formed as the ejected material collided
with the gas and dust in the surrounding medium. The evidence for the
presence of a companion star is an arc of X-ray emission that is
interpreted as coming from material that was stripped from a companion
star that was very near the white dwarf that exploded. The explosion
tore away material from the companion and accelerated it in the 'wind'
of the explosion. The impact between the wind and the stripped
material created a shock, which we see because of its X-ray emission.
Previous studies with optical telescopes have found within the remnant
a star that is moving much faster than all the others (greater proper
motion) and is now tentatively identified as the remnant of the
companion to the supernova. From the motion of the star and the
properties of the arc, the team has estimated that the companion had
an orbital period of around 5 days and an orbital radius of less than
1/10 of an astronomical unit. If the ideas of the Chinese team are
correct, then the association of a companion star with the supernova
favours the mass-transfer mechanism for the explosion.
FIRST STELLAR GENERATIONS
Leibniz Institute for Astrophysics
Soon after the Big Bang, the composition of the Universe was simpler
than at present, consisting almost exclusively of hydrogen and
helium. The addition of other elements began after about 300 million
years, when the first generation of massive stars completed their
evolution and enriched the primordial gas with new chemical elements,
which were incorporated into the ensuing generations of stars.
Astronomers have recently analyzed VLT spectra of a group of old stars
in the Galactic Bulge that are so old that only massive, short-lived
stars with masses larger than 10 solar masses should have had time to
evolve and enrich the gas from which the observed stars then formed.
As expected, the stars' spectra show elements typical for enrichment
by massive stars. However, they also show elements usually thought
to be produced only by stars of smaller masses. Fast-rotating massive
stars, on the other hand, would be able to manufacture those elements
themselves. Although it is so far only one tentative possibility, a
case has been made for supposing that massive stars of the first
generations were rapidly rotating. Their rotation would have
influenced the properties and appearance of the first galaxies that
were formed in the Universe.
Topic: Early May Astronomy Bulletin (Read 698 times)