The Glory of a Nearby Star – Optical Light from a Hot Stellar Corona Detected with the VLT
Note: Text with all links and the photo is available on the ESO Website at:
http://www.eso.org/outreach/press-rel/pr-2001/pr-17-01.html
Summary
The solar corona is a beautiful sight during total solar eclipses. It is
the uppermost region of the extended solar atmosphere and consists of a
very hot (over 1 million degrees), tenuous plasma of highly ionised
elements that emit strong X-ray radiation. There is also a much weaker
coronal emission in the optical part of the spectrum.
The Sun is a normal star and X-ray observations from rockets and orbiting
X-ray telescopes have shown that many other stars also possess coronae.
But due to observational limits of the telescopes available so far, the
much fainter optical emission from stellar coronae had never been
detected.
Now, however, an optical coronal line from iron ions that have lost 12
electrons (Fe XIII) has for the first time been observed in a star other
than the Sun. The object, a cool star named CN Leonis, is located at a
distance of 8 light-years. This impressive observational feat was
performed with the UV-Visual Echelle Spectrograph (UVES) on the VLT 8.2-m
KUEYEN telescope at the ESO Paranal Observatory, within a programme by
German astronomer Juergen Schmitt and his collaborators at the University
of Hamburg Observatory.
The possibility to observe stellar coronae with ground-based telescopes
opens up new and exciting research opportunities, including the detailed
study of stellar cycles, similar to the 11-year solar period.
PR Photo 24a/01: The solar corona during the August 11, 1999, solar
eclipse.PR Photo 24b/01: The nearby star CN Leonis.
PR Photo 24c/01: Ultraviolet spectrum of CN Leonis, obtained with UVES at
VLT KUEYEN.PR Photo 24d/01: The coronal Fe XIII emission line at 3388 Angstrom in CN
Leonis.The ‘coronium’ mystery
ESO PR Photo 24a/01
Caption: Photo of the solar corona, obtained by Philippe Duhoux (ESO) on
August 11, 1999.
Two years ago, on August 11, 1999, the shadow of the Moon moved rapidly
across Europe and millions of eager observers experienced a total solar
eclipse, many for the first time in their lives. Those who had a clear view
during the 2-min phase of totality were able to see the glorious solar
corona, a shimmering halo of light around the eclipsed solar disk, cf. PR
Photo 24a/01.
Some 130 years earlier, during a total solar eclipse on August, 7, 1869,
American astronomers William Harkness and Charles Young observed a weak
spectral emission line from the solar corona in the green region of the
spectrum; it was visible for a couple of minutes. However, despite an
enormous amount of work, both at the telescope during subsequent eclipses
and in the laboratory, this emission line could not be attributed to any
known chemical element.
As the years passed, the mystery of the origin of this emission line
deepened and some astronomers went as far as introducing an entirely new
element named ‘coronium’ [1]. As better instruments became available, more
coronal lines were seen during later solar eclipses.
A hot corona
It was only after 70 years that the coronium mystery was finally solved by
two astrophysicists, Walter Grotrian from Germany and Bengt EdlÈn from
Sweden. They showed that two observed emission lines arise from iron atoms
which have lost about half their 26 electrons. By 1941, all of the coronal
lines had been found to originate from such highly ‘ionized atoms’.
The successful identification created, however, another puzzle: in order to
strip iron atoms of half of their electrons, temperatures of more than one
million degrees are required, yet the temperature of the surface of the Sun
is only of the order of 5500 C! The astronomers in the 1940’s were well
aware that the Sun’s energy is produced in the interior and that heat flows
outwards from hotter to cooler regions. So how could there be a much hotter
corona above the cooler photosphere?
Since then, much research effort has been aimed at understanding the
transport of energy in the solar atmosphere and it appears that several
mechanisms play a role, including magnetic and other effects. Nevertheless,
a full and detailed explanation of the high temperature of the solar corona
is still outstanding.
X-rays from the solar and stellar coronae
An ionized gas (a ‘plasma’) at temperatures of a million or more degrees
emits most of its energy at short X-ray wavelengths. X-rays do not penetrate
the Earth’s atmosphere and can therefore only be studied from space. Soon
after World War II, the predicted X-ray emission from the solar corona was
detected by American astrophysicist Herbert Friedman and his colleagues,
using an X-ray detector onboard a German V-2 rocket, and hereby inaugurating
the rich field of solar X-ray astronomy [1].
The Sun is a quite normal star and other stars therefore ought to possess
coronae as well. Still, it took nearly 30 years until X-ray emission from
other normal stars was finally detected. While X-rays from several distant
objects (including the Crab Nebula, the Galactic Centre and the quasar
3C273) were discovered during the 1960’s, it was only in 1975 that X-rays
were registered from the bright, normal star Capella (Alpha Aurigae) during
a rocket flight to study other X-ray sources. In fact, this discovery was
accidental, as Capella happened to be used as a ‘guide star’ while the
pointing direction of the rocket was “hopping” from one object to the
next.
Quite surprisingly, Capella was found to be a very strong emitter of X-rays,
corresponding to an intrinsic level of more than 1000 times that of the
solar corona. This discovery laid the foundation for the subsequent
detection of X-ray emission from tens of thousand of stars by means of X-ray
satellites, e.g., by the Einstein Observatory and especially by ROSAT.
All these observations showed that stellar coronae must be a very common
phenomenon.
Observation of stellar coronal lines
Given this widespread occurrence of stellar coronae, Juergen Schmitt and his
collaborators at the University of Hamburg (Germany) asked themselves the
natural question: “What about coronal line emission from other stars in the
optical (visible) region of the spectrum ? Wouldn’t it be a good idea to
observe coronal emission from other stars with ground-based telescopes ? In
any case, observations from the ground are easier to perform and are also
more economical than from space”.
This may be easy to say, but it is much harder to do. The main problem is
the same as when observing the solar corona. The solar coronal emission
lines in the visible region of the spectrum are always observed above the
solar limb. If one were to try to detect these weak lines in front of the
solar disk, they would “drown” in the strong background light from the solar
‘surface’ (the photosphere). The original discovery of coronal emission in
1869 was indeed obtained during a solar eclipse, when this strong light is
completely blocked out by the Moon.
However, current telescopes are unfortunately unable to block out the light
from a stellar disk in a similar way in order to make its corona visible;
the angular size of the disk is too small and the positional accuracy needed
for such an observation is too high for it to be feasible with present
techniques. The only way forward is then a direct attempt to detect the
faint coronal emission against the much higher background of the stellar
disk — and that is exactly why a very large telescope is needed for such
an observational feat.
Selecting the target star: CN Leonis
ESO PR Photo 24b/01
Caption: Images of the nearby, variable star CN Leonis, in which a coronal
emission line has been observed with the UVES spectrograph at the 8.2-m
VLT KUEYEN telescope. This star is relatively nearby (8 light-years) and
moves about 5 arcsec/yr in the sky, approximately towards south-west (the
4 o’clock direction). The motion is clearly visible on these two images
obtained with the UK Schmidt telescope and reproduced from the Second
Digized Sky Survey (DSS-2); the blue image (left) was taken several years
before the red one (right). Moreover, the red colour of the star is
obvious; the red image is clearly brighter than the blue one. The field
measures 5 x 5 arcmin^2; North is up and East is left. These DSS-2 images
are copyright by the UK SERC/PPARC (Particle Physics and Astronomy
Research Council, formerly Science and Engineering Research Council), the
Anglo-Australian Telescope Board and the Association of Universities for
Research in Astronomy (AURA).
In order to increase the chances of success, Juergen Schmitt and his
colleagues decided to focus on optically faint, red dwarf stars. Such stars
may have the same X-ray output (or even larger) than the Sun, and hence
presumably possess pronounced coronae, yet their disks emit over one
thousand times less visible light than does that of the Sun.
They first turned their attention towards an optically faint (visual
magnitude 14) and nearby (distance 8 light-years) red dwarf star (of type
M5.5) known as CN Leonis, cf. PR Photo 24b/01. It is located slightly north
of the celestial equator in the constellation Leo (the Lion) and the name
indicates that it is a variable star. It has been found to undergo sudden
brightenings (it is a ‘flare star’), and exhibits strong magnetic activity.
It is also a source of strong X-rays which the German astronomers had
previously studied with the ROSAT satellite observatory and they therefore
considered this star as an excellent first choice for a coronal study with
the VLT.
UVES detects a coronal line in the visible spectral region
ESO PR Photo 24c/01 ESO PR Photo 24d/01
Caption: Left: A small part of the near-ultraviolet spectrum of CN Leonis,
obtained with UVES at the 8.2-m VLT KUEYEN telescope in January 2001,
showing many emission lines from nickel atoms (Ni I) and titanium ions
(Ti II). Right: “Decomposition” of an emission line at wavelength 3388.1
Angstrom (338.81 nm) into two components. The observed spectral intensity
is indicated by the ‘step’-curve (in blue). As will be seen, the sum
(fully drawn red line) of a strong and narrow line from titanium ions (Ti
II) in the stellar chromosphere (dashed, in red) and an underlying, much
broader, coronal line from 12 times ionised iron (Fe XIII; dashed, in red,
slightly to the right of the titanium line) fits the observed spectral
intensity curve perfectly, cf. the text.
A spectrum of CN Leonis was obtained with the VLT UV-Visual Echelle
Spectrograph on January 6, 2001. The spectrum covers a wide spectral region
and is extremely rich in emission lines, but the team was mainly interested
in one particular emission line, seen in the ultraviolet part of the
spectrum at wavelength 3388.1 Angstrom (338.81 nm). This is the wavelength
at which a coronal emission line arising from 12 times ionised iron (denoted
as Fe12+ or Fe XIII) is seen in the solar spectrum. Would the same line be
visible in the spectrum of CN Leo as well?
When first inspecting the spectrum of CN Leonis (PR Photo 24c/01), Juergen
Schmitt was hopeful: “We saw a strong line, right at the proper location!”
But then, he explains, “we soon learned that life is never as easy as
expected … that line had a rather strange appearance and something
seemed to be wrong”.
Indeed, the early investigation showed that this line feature might be
attributed to emission by singly ionised titanium atoms (Ti+ or Ti II),
located in a lower atmospheric layer (the ‘chromosphere’) and not in the
corona of CN Leo.
However, a subsequent, very careful study definitively proved the presence
of the hoped-for coronal emission line. The titanium line is produced at
lower temperatures than those that reign in the corona, and the individual
velocities of the titanium ions are thus much slower than those of the iron
ions in the corona. The broadening of the titanium line, introduced by the
Doppler effect (the combined lineshifts by all ions), must therefore be
much less. The titanium line must accordingly be much more narrow than any
coronal line.
Many other titanium emission lines are visible in the UVES spectrum, and the
common width of these lines can be determined with high accuracy. It turns
out to be much less than the observed width of the line seen at 3388
Angstrom, and that line can therefore not be due to titanium alone. And
indeed, when ‘subtracting’ the contribution from the narrow titanium line,
an underlying, much broader line emerges and becomes well visible — cf. PR
Photo 24d/01 — it is indeed the coronal emission line from 12 times ionised
iron (Fe XIII).
This is the first time a stellar coronal line has been unambiguously
observed in the optical part of the spectrum.
Prospects
This KUEYEN/UVES detection of a coronal line closes the historical loop to
the discovery of the solar corona as a tenuous, hot envelope around the Sun.
It now opens up a new window for the study of stellar coronae and allows
thermal emission from these hot regions to be studied from the ground and
not only from space, as this was the case until now.
Thus, it is now feasible to use the superb capabilities of ground-based
instrumentation which has much higher spectral resolving power than
currently available X-ray spectrometers. With the new tools at large
telescopes like the VLT, the astronomers may embark on detailed studies of
the dynamics of stellar coronae. They will then also be able to watch the
expected changes in the emission levels of other stars, similar to the
well-known 11-year cycle of the Sun. Eventually, they may also obtain
images of stellar chromospheres and coronae.
More information
The research reported in this Press Release is described in a scientific
article (“Light from Stellar Coronae: Ground-based Discovery of Emission
Lines” by Juergen Schmitt and Reiner Wichmann) that appears in the August 2,
2001, issue of the scientific journal “Nature”. Juergen Schmitt has written
a popular account on stellar X-ray emission in the German language journal
“Sterne und Weltraum” (July 2001, page 544).
Note
[1]: A report on the observations of the 1869 solar eclipse appeared in the
first Nature issue (November 4, 1869) and the interesting story about the
identification of the solar coronal lines is described in a popular article
(John Talbot). A talk by Herbert Friedman about the evolution of X-Ray
Astronomy includes a description of the 1949 detection of solar emission in
this waveband. More details about the solar-stellar connection and X-rays
may be found in the article by Berhard Haisch and Juergen Schmitt in the
October 1999 issue of the journal “Sky & Telescope” (page 46).
Contact
Juergen Schmitt
Hamburger Sternwarte
Universitaet Hamburg
Germany
Tel.: +49-40-42891-4112
E-Mail: jschmitt@hs.uni-hamburg.de
