Odd Little Star has Magnetic Personality (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 7 Jan 2008 19:34:04 -0500
Gemini Observatory
Hilo, Hawaii
Media Contact:
Peter Michaud
Gemini Observatory
(808) 974-2510 (desk)
Science Contact:
Edo Berger
Carnegie-Princeton Postdoctoral Fellow
Princeton University
609-258-9027
Wednesday, 28 November 2007
Odd Little Star has Magnetic Personality
A dwarf star with a surprisingly magnetic personality and a huge hot spot
covering half its surface area is showing astronomers that life as a cool
dwarf is not necessarily as simple and quiet as they once assumed.
Simultaneous observations made by four of the most powerful Earth- and
space-based telescopes revealed an unusually active magnetic field on the
ultracool low-mass star TVLM513-46546. A team of astronomers, led by Dr. Edo
Berger, a Carnegie-Princeton postdoctoral fellow at Princeton University, is
using these observations to explain the flamboyant activity of this M-type
dwarf that lies about 35 light-years away in the constellation Boes.
The team's observations of TVLM513-46546 combine radio data from the Very
Large Array, optical spectra from the Gemini North 8-meter telescope,
ultraviolet images from the orbiting Swift observatory and x-ray data from
NASA's Chandra X-ray Observatory. This is the first time that such a
powerful set of telescopes has been trained on one of the smallest known
stars. The study is part of a program that looks at the origins of magnetic
fields in ultracool dwarfs, stars that astronomers always assumed were
simple, quiet, and more tranquil than their hotter and more massive
siblings.
"With such a unique set of observations you always expect to find the
unexpected," said Berger, "but we were shocked at the level of complexity
that this object exhibits."
The star's steady radio emission is interrupted with spectacular fireworks
displays of minute-long flares. These flares come from the catastrophic
collisions and merging of the magnetic fields in the corona of the star;
these actions drive the annihilation of magnetic energy like a giant
short-circuits in the fields. The team also observed soft x-ray emission and
an x-ray flare.
Also for the first time, the group charted optical hydrogen-alpha emission
with a period of two hours that matches the two-hour rotation period of the
star. "We find a hot spot that covers half of the surface of the star like a
giant lighthouse that rotates in and out of our field of view," said Berger.
"We still do not know why only half of the star is lit up in hydrogen and if
this situation remains unchanged over days, weeks, years, or centuries."
Berger describes the dwarf star's magnetic field as probably being a simple
dipole (north-south orientation, like the Earth's much weaker magnetic
field) that extends out at least one stellar radius above the surface. There
is also a smaller-scale field that has loops similar to those seen on the
Sun, but smaller. "Those loops and arcs occur on random places on the
surface of the star, "said Berger. "That's where the flares originate that
last only a few minutes, whereas the overall field doesn't get disturbed."
Objects like TVLM513-46546 were once thought to be models of stellar
quiescence and simplicity, with little to no magnetic field activity.
"Theory has always said that as we look at cooler and cooler stars, the
coolest will be essentially dead," said Berger. "It turns out that stars
like TVLM513-46546 have very complex magnetic activity around them, activity
more like our Sun than that of a star that is barely functional."
This one's complicated magnetic field environment and possible hot spot may
indicate some unusual activity beneath the star's surface (in its dynamo) or
possibly even the existence of a still-hidden companion. The idea of an
unseen companion as an explanation for the star's excitable magnetic
disposition is an intriguing one, says Berger, but no such object has yet
been detected. "The main idea to consider here is an analogy to other
systems where the presence of a companion directly or indirectly excites
magnetic activity," he said.
Like other ultracool dwarf stars, TVLM513-46546 is an M-type star with
surface temperatures below about 2400K (2127 Celsius) and a mass of only 8
to 10% that of our Sun. By contrast, the Sun is a G-type star with an
average surface temperature of 6000K (5727 Celsius).
Imagine the interior of the Sun layered like an onion. Its internal
convection is the process by which heat from the nuclear fusion at the core
is transported by large spinning currents that move through the Sun's outer
layers. Differential rotation is simply the term for the different spin
rates of different layers. Together these motions of electrically charged
gas spin up the magnetic field structures we see at the Sun.
By contrast, an ultracool M-type star like TVLM513-46546 is fully
convective. That is, the zone that transports heat to the surface of the
star extends all the way from the stellar surface into the center, like the
bubble of a huge boiling pot. Such a simple structure has been predicted to
generate a very basic magnetic field structure, perhaps more like the
Earth's than the complex fields we see on the Sun. Why TVLM513-46546 has
such a complex field and activity remains to be studied.
In order to find out if this star is just a stellar oddity, or if it might
turn out be a typical prototype of ultracool dwarfs, the research team plans
to continue with observations of other such stars. The team expects the
larger sample to show how other candidate low-mass stars (and brown dwarfs,
objects too hot to be planets and too cool to be stars) generate magnetic
fields. Berger also notes that he'd like to get more observations to try and
spot any possible companions to such stars. "The issue of a possible
companion is really pure speculation at this point," he said. "However, I am
trying to get observations that will assess this possibility."
These results are being published in the February 10, 2008 issue of the
Astrophysical Journal. A preprint of the paper can be found here.
Partial studies of magnetic activity on these types of stars have been
performed previously, but this is the first time that such a powerful set of
telescopes has been simultaneously pointed at the same object.
The Gemini Observatory provides the astronomical communities in each partner
country with state-of-the-art astronomical facilities that allocate
observing time in proportion to each country's contribution. in addition to
financial support, each country also contributes significant scientific and
technical resources. The national research agencies that form the Gemini
partnership include: the US National Science Foundation (NSF), the UK
Science and Technology Facilities Council (STFC), the Canadian National
Research Council (NRC), the Chilean Comision Nacional de investigacion
Cientifica y Tecnologica (CONiCYT), the Australian Research Council (ARC),
the Secretaria de Ciencia, Tecnologia e Innovacion productiva (SECYT) and
the Brazilian Conselho Nacional de Desenvolvimento Cientifico e Tecnologico
(CNPq). The Observatory is managed by the Association of Universities for
Research in Astronomy, inc. (AURA) under a cooperative agreement with the
NSF. The NSF also serves as the executive agency for the international
partnership.
QuickFacts about TVLM513-46546
* Mass is about 8-10% that of the Sun
* Luminosity is about 0.02% that of the Sun's brightness
* Spectral Type is M9 dwarf
* Age is at least 1 billion years old
* Located about 35 light years away in constellation Bootes
* Surface temperature is about 2400K (2127 Celsius)
* Rotation period about 2 hours
IMAGE CAPTIONS:
[Figure 1:
http://www.gemini.edu/images/stories/press_release/pr2007-8/fig1med.jpg
(683KB)]
Artist's rendition of what the magnetic fields and surface might look like
on TVLM513-46546. Note that the hot-spot that is estimated to cover up to
50% of the surface area of the star is oriented to the left of the star and
is not entirely visible in this orientation. Gemini Observatory artwork by
Dana Berry, SkyWorks Digital Animation.
[Figure 2:
http://www.gemini.edu/images/stories/press_release/pr2007-8/fig2.jpg (48KB)]
Top: Time series of Hydrogen-alpha observations from the Gemini North
telescope showing the periodic signal that results from a hot spot covering
half of the surface of TVLM 513-46546. The high points are when the hot spot
faces Earth, and the low points are when the hot spot is on the far side of
the star. Bottom: Time series of radio emission from observed with the Very
Large Array. The minute-long flares are clearly visible.
.
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