High Dispersion Spectroscopy of solar-type superflare stars

From RHESSI Wiki

Jump to: navigation, search

Number: 254
1st Author: Yuta Notsu
2nd Author:
Published: 8 June 2015
Next Nugget: Electrons and Currents
Previous Nugget: Kepler Superflares
List all



As desceribed in last week's Nugget, some solar-type stars observed by the Kepler observatory can support much more powerful flares than the Sun has shown us yet. In this Nugget we follow up on these discoveries with astronomical observations of these faint stars. These unexpected observations - a byproduct of Kepler's main task of planet search - provide a major extension of the parameter space for flaring. Unfortunately RHESSI-style hard X-rays, the key to flare energetics, cannot be observed (or can they?) from these objects because of sensitivity limits.

See last week's Nugget for background information. Here we report detailed astronomical observations of some of the many objects now known to support "superflares," defined in this work as flares of magnitudes 10-10,000 times that of the most powerful solar flare known.

Subaru observations

Based on the initial discovery, we carried out spectroscopic observations on 50 solar-type superflare stars with the Subaru Telescope, an 8.2-m telescope on Mauna Kea, Hawaii. These observations make use of the High Dispersion Spectrograph (Subaru/HDS). The 50 target stars were selected from the Kepler database. From the investigation of the detailed properties of spectral lines, we obtained several results. The present Nugget is based on Refs. [1,2] and a Subaru press release.

1. More than half the observed 50 stars show no evidence of binarity (that is, they are not binary stars). This is important in terms of understanding flare mechanisms. We confirmed the characteristics of the target stars (e.g., temperature, surface gravity) as similar to those of the Sun.

2. 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 can be explained by the rotation of a star with large starspots. As shown in Figure 1, the stars would become dimmer when their starspots are on their visible sides. Moreover, the timescales of the brightness variations should reflect the stars' rotation speeds. Spectroscopic observations allow observers to estimate the rotation velocity from the broadening of absorption lines (Figure 2), and we confirm that such spectroscopic velocities match the brightness-variation timescales. Note that the measured rotation velocity of some of the target superflare stars is as slow as that of the Sun, about 2 km/s at the equator..

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 imagined image of a superflare star as seen from much closer, with a huge flare (white) and mammoth starspots (black). (Credit: Kyoto University)
Figure 2: Left: Four neutral iron (Fe I) absorption lines, broadened because of rotation. Slowly rotating stars like the Sun have a narrow line profile, while rapidly rotating stars have a wide line profile because of the Doppler shifted contributions from the limb regions of the star. The cartoon on the right illustrates this graphically. (Credit: Kyoto University)

3. Based on solar observations, it is known that if there are large dark starspots on a stellar surface, the "core depth" (the depth and width of a spectral line) of the Ca II 854.2 nm (once-ionized calcium) absorption line becomes shallow (Figure 3). We investigated the core depth of this line, and found there to be a correlation between the amplitude of the brightness variation of the star and the intensity of its line (Figure 4). All the targets expected to have large starspots because of their large amplitude of the brightness variation show high chromospheric activities compared with the Sun.

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 schematic drawings of a superflare star in visible light (left) and the Ca II line (right) where the areas around the starspots are bright. Right: the 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 each have large starspots and plage regions associated with them. (Credit: Kyoto University)
Figure 4: The quantity r0(8542) (the residual core flux normalized by the continuum level at the line cores of the Ca II 854.2 nm) as a function of the amplitude of stellar brightness variation estimated from Kepler data. The plot shows the observational results of the target superflare stars classified into three groups on the basis of stellar projected rotational velocity (v sin i). The solar value is plotted as a circled dot.


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, we will conduct observations with the Kyoto University Okayama 3.8 m telescope, which is now under construction. This will allow them to investigate more detailed properties and changes in long-term activity of superflare stars. Can we learn why these particular stars have outsized spots and extreme flare activity?


[1] "High Dispersion Spectroscopy of Solar-type Superflare Stars. I. Temperature, Surface Gravity, Metallicity, and v sin i"

[2] "High Dispersion Spectroscopy of Solar-type Superflare Stars. II. Stellar Rotation, Starspots, and Chromospheric Activities"

Personal tools