High Dispersion Spectroscopy of solar-type superflare stars
From RHESSI Wiki
|1st Author:||Yuta Notsu|
|Published:||8 June 2015|
|Next Nugget:||Electrons and Currents|
|Previous Nugget:||Kepler Superflares|
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.
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..
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.
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?