Figure 1: Left: The brightness variation of solar-type
superflare stars (from Kepler data). In addition to the sudden
brightenings caused by flares, quasi-periodic brightness variations with
periods of about 15 days are seen. Right: An artificial image of a
superflare star seen with visible light. This figure shows a large
superflare (shown in white) occurring in the large starspot area.
(Credit: Kyoto University)
Figure 2: Left: Four neutral iron (Fe I) absorption
lines are shown. As mentioned above, spectral lines show some broadening
because of the Doppler effect on the light from the rotating stellar
surface. Slowly rotating stars like the Sun have a narrow line profile,
while rapidly rotating stars have a wide line profile. Measuring these
broadenings allows an estimate of the stellar rotation velocity. Right:
The wavelength of light from the surface of a rotating star shifts
because of the Doppler effect. For example, the wavelength of light from
point A becomes a bit short (is blue-shifted) since this point is
approaching us (the observer). By contrast, the wavelength of light from
point C is a bit long (is red-shifted) since this point moves away from
us. The wavelengths of light from point B have no shifts since this
point moves perpendicular to the line of sight. Finally, this line-shift
effect results in broadening of spectral lines. (Credit: Kyoto
University)
Figure 3: Left: The bottom two images show the Sun in
visible light (left) and the Ca II line (right) (These two pictures are
from the Big Bear Solar Observatory). The upper two images are imaginary
drawings of a superflare star in visible light (left) and the Ca II
line (right) where the areas around the starspots are bright.
Right：Absorption line of Ca II 854.2[nm] (ionized calcium). Superflare
stars (the upper two spectra, shown in red and blue) have a shallow
(bright) core depth compared to the Sun (the bottom spectrum, in black).
This suggests that these two superflare stars have large
starspots. (Credit: Kyoto University)
A team of astronomers has used the High Dispersion
Spectrograph on the Subaru Telescope to conduct spectroscopic
observations of Sun-like "superflare" stars first observed and cataloged
by the Kepler Space Telescope. The investigations focused on the
detailed properties of these stars and confirmed that Sun-like stars
with large starspots can experience superflares.
The team, made up of astronomers from Kyoto
University, University of Hyogo, the National Astronomical Observatory
of Japan (NAOJ), and Nagoya University, targeted a set of solar-type
stars emitting very large flares that release total energies 10-10000
times greater than the biggest solar flares. Solar flares are energetic
explosions in the solar atmosphere and are thought to occur by intense
releases of magnetic energy around the sunspots. Large flares often
cause massive bursts of high-speed plasma called coronal mass ejections
(CMEs), can lead to geomagnetic storms on Earth. Such storms can have
severe impacts on our daily life by affecting such systems as
communications and power grids.
This work follows up on observations made in 2012
(Maehara et al. Nature on 2012 May 24), where the team reported finding
several hundred superflares on solar-type stars by analyzing stellar
observation data from Kepler Space Telescope. This discovery was very
important since it enabled the astronomers to conduct statistical
analysis of superflares for the first time. However, more detailed
observations were needed to investigate detailed properties of
superflare stars and whether such massive flares can occur on ordinary
single stars similar to our Sun.
Based on the initial discovery, the team carried out
spectroscopic observations on 50 solar-type superflare stars selected
from the Kepler Space Telescope's data. From the investigation of the
detailed properties of spectral lines, the team obtained the following
results:
- More than half the observed 50 stars show no evidence of binarity (that is, they are not binary stars). The team confirmed the characteristics of the target stars as similar to those of the Sun.
- On the basis of the Kepler data, superflare stars show somewhat regular, periodic changes in their brightnesses. The typical periods range from one day to a few tens of days. Such variations are explained by the rotation of the star and its starspots. As shown in Figure 1, the stars seem to become dimmer when their starspots are on their visible sides. Moreover, the timescales of the brightness variations should correspond to the stars' rotation speeds. Spectroscopic observations allow observers to estimate the rotation velocity from the broadening of absorption lines (Figure 2), and confirm that a velocity derived from spectroscopic data matches the brightness variation timescale as the star rotates. In addition, the measured rotation velocity of some target superflare stars is as slow as that of the Sun.
- Based on solar observations, astronomers know that if there are large dark star spots on a stellar surface, the "core depth" (the depth and width of a spectral line) of the Ca II 854.2[nm] (ionized Calcium) absorption line becomes shallow. Using this, they investigated the core depth of Ca II 854.2 [nm] line, and found that superflare stars have large starspots compared to sunspots (Figure 3).
The results of these observations and analysis
confirm that stars similar to the Sun can have superflares if they have
large starspots. In the future, in addition to the continuing
spectroscopic observations with Subaru Telescope, the team will conduct
observations with the Kyoto University's Okayama 3.8m telescope, which
is now under construction. This will allow them to investigate more
detailed properties and changes in long-term activity of superflare
stars.
Research Team Members
- Yuta Notsu (Department of Astronomy, Kyoto University)
- Satoshi Honda (Center for Astronomy, University of Hyogo)
- Hiroyuki Maehara (Okayama Astrophysical Observatory, NAOJ)
- Shota Notsu (Department of Astronomy, Kyoto University)
- Takuya Shibayama (Solar-Terrestrial Environment Laboratory, Nagoya University)
- Daisaku Nogami (Department of Astronomy, Kyoto University)
- Kazunari Shibata (Kwasan and Hida Observatories, Kyoto University)
This research is based on the following two research papers.
- Paper 1: "High Dispersion Spectroscopy of Solar-type Superflare Stars. I. Temperature, Surface Gravity, Metallicity, and v sin i"
- Paper 2: "High Dispersion Spectroscopy of Solar-type Superflare Stars. II. Stellar Rotation, Starspots, and Chromospheric Activities"
Authors of both papers:
Notsu, Y., Honda, S., Maehara, H., Notsu, S., Shibayama, T., Nogami, D., and Shibata, K.
To be published on Publications of the Astronomical Society of Japan June 25, 2015 issue (Online versions are published in February 22, 2015 and March 29, 2015, respectively)
This work was supported by the Grant-in-Aids from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No.25287039, 26400231, and 26800096).
Source: Subaru Telescope
