SETI bioastro: USING GRAVITY TO PROBE A DISTANT STAR

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From: Larry Klaes (larry.klaes@incent.com)
Date: Mon Apr 30 2001 - 06:58:24 PDT


USING GRAVITY TO PROBE A DISTANT STAR

An international team of astronomers have studied the atmosphere of a
star 25,000 light-years away. This feat was accomplished thanks in
large part to two smaller stars that happened to be in the way.

For several years, teams of astronomers have monitored fields of stars
looking for gradual brightenings. The sought magnitude changes aren't
due to any variable nature in the stars themselves, but because of
gravitational lensing. When a massive, but dim, object crosses our
line of sight to a background star, the gravity of the intervening
star distorts the light from the more-distant object. The effect is a
focusing of the starlight. The cycle of brightening and dimming of
this so-called microlensing can last several weeks. Astronomers hope
that such microlensing searches will help estimate the amount of dark
matter in the galaxy by finding evidence for dwarf stars and other
bodies we can't detect through other means.

On May 5, 2000, astronomers of the EROS program found a microlensing
candidate and soon other observing programs were monitoring the event,
designated EROS-BLG-2000-5. After about a month, the star brightened
significantly, indicating that the event was in fact a pair of dwarf
stars passing in front of a red giant in the central bulge of the
Milky Way. Furthermore, researchers predicted that the star would have
another brightening a few weeks later. Astronomers at the European
Southern Observatory's Very Large Telescope geared up for the event,
and when the star did indeed brighten again, they took spectra
throughout several nights in early July 2000. The lensing effect
enhanced emission from different parts of the giant star as the
foreground dwarfs moved across the disk, in effect peering into the
structure of the star. The ESO astronomers traced changes in hydrogen
emission from different atmospheric depths, which were consistent with
stellar models.

GROWING PLANETS IN A BAD NEIGHBORHOOD

It seems that the three most important factors in forming planetary
systems in nebulae just may be: location, location, location.
According to a study conducted by Henry Throop (Southwest Research
Institute) and his colleagues, the environment surrounding a
protoplanetary disk dramatically effects the type of system that will
form.

Using the Hubble Space Telescope, Throop looked at various
million-year-old disks within the Orion Nebula. In the disks, he found
dust grains as large as 5 microns in size, about the one-tenth the
width of a human hair. For comparison, normal interstellar dust grains
are only 0.1 to 0.2 micron. The apparent grain growth implies that
they are in the early stages of planetary growth.

However, the Orion Nebula is also home to more than two dozen O-type
stars. These stellar giants cause an extreme ruckus in any neighboring
disk within 0.3 light-year or so. The star's tremendous energy outflow
blows away the gas in the system, and the giant star's intense
ultraviolet radiation bakes away any ice. According to models
calculated by Throop and others, the resulting system is quite
strange: there is no gas to form Jupiterlike gas giant planets, nor is
there ice to form a Kuiper Belt or any comets. Instead, all that
remains is a group of atmosphereless rocky bodies, similar in many
respects to Mercury.

But, if the disk is fortunate to reside in a "shady spot," says
Throop, then gas giants and normal planetary formation can proceed.
Because O-type stars have such short life spans (up to 100 million
years), disks could easily form after the stars had wreaked their
havoc. Details of the study appear in this week's Science.


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