Radiation environment of low mass stars
Problem description:
Low-mass stars are not just cooler and fainter than Sun-like stars, but their light is also redder. For lower temperatures, the maximum of the spectral energy distribution (SED) is shifted towards longer wavelengths. This means that M stars have their maximum output in the red/near-infrared region of the spectrum, in contrast to the Sun which has its maximum at visual wavelengths. A comparison between SEDs from F,G,K and M stars can be found in Segura et al. (2005).
- Fig.1: Full (a) and UV (b) spectra for the two active M stars AD Leo and GJ 643 in comparison to a theoretical spectrum of an inactive M dwarf and other spectral types (Segura et al. 2005)
Emission at short wavelengths (XUV, 0.1-120 nm):
On M stars, the only sources of emission at wavelengths shorter than 250 nm are due to dynamo-driven chromospheric and coronal activity. In contrast to G stars, M stars have no photospheric UV emission due to their low temperatures (Guinan & Engle, 2008). Since young M stars are very active they emit high XUV fluxes and are known to flare frequently. These stars emit larger fractions (up to 0.1%) of their total bolometric luminosity in X-rays than G stars of comparable ages. They can remain at a saturated level of activity for up to 1 Gyr before it decreases following a power law similar to solar-like stars (Fig. 2; Scalo et al., 2007).
- Fig. 2: Temporal evolution of activity proxy Lx/Lbol for stars of different masses. The duration of saturated activity increases with decreasing mass (Scalo et al., 2007).
The ?Living with a Red Dwarf? program (http://astronomy.villanova.edu/livingwithareddwarf/Opener.htm) is dedicated to the study of the temporal evolution of the XUV emissions of M dwarfs, similar to ?The Sun in Time? for solar-type stars. In this ongoing study, the evolution of magnetic activity of a sample of M dwarfs with ages from 0.1-13 Gyr will be studied.
Flares:
Low-mass stars are also known for their frequent and powerful flares which can have energies up to 1034 erg. Audard et al. (2000) found that the rate of occurrence of energetic flares scales with the total X-ray luminosity. Overviews of stellar flares can be found in Haisch et al. (1991) and Garcia-Alvarez (2000). The catalogue of Gershberg et al. (1999) provides a collection of data on flare stars.
References:
- Audard, M., Güdel, M., Drake, J. J. & Kashyap, V. L.: Extreme-Ultraviolet Flare Activity in Late-Type Stars, ApJ, 2000, 541, 396-409
- Garcia Alvarez, D.: Modelling of flares on late-type stars, Irish Astronomical Journal, 2000, 27, 117-136
- Gershberg, R. E., Katsova, M. M., Lovkaya, M. N., Terebizh, A. V. & Shakhovskaya, N. I.: Catalogue and bibliography of the UV Cet-type flare stars and related objects in the solar vicinity, A&AS, 1999, 139, 555-558
- Guinan, E. F. & Engle, S. G.: Evolution over Time of Magentic Dynamo Driven UV X-ray Emissions of dG-M Stars and Effects on Hosted Planets, Oral Contribution to IAU 26th General Assembly Joint Discussion 4 - The Ultraviolet Universe: Stars from Birth to Death, ArXiv e-prints, 2007
- Haisch, B., Strong, K. T. & Rodono, M.: Flares on the sun and other stars, ARAA, 1991, 29, 275-324
- Scalo, J., Kaltenegger, L., Segura, A., Fridlund, M., Ribas, I., Kulikov, Y. N., Grenfell, J. L., Rauer, H., Odert, P., Leitzinger, M., Selsis, F., Khodachenko, M. L., Eiroa, C., Kasting, J. & Lammer, H.: M Stars as Targets for Terrestrial Exoplanet Searches and Biosignature Detection, Astrobiology, 2007, 7, 85-166
- Segura, A., Kasting, J. F., Meadows, V., Cohen, M., Scalo, J., Crisp, D., Butler, R. A. H. & Tinetti, G.: Biosignatures from Earth-Like Planets Around M Dwarfs, Astrobiology, 2005, 5, 706-725
Contacts of relevant researchers:
- Mag. Petra Odert:
Institut für Physik (Geophysik, Astrophysik, Meteorologie), Universitätsplatz 5/II, 8010 Graz, Austria.
Email: petra.odert(at)uni-graz.at
- Dr. Ignasi Ribas:
Institut d?Estudis Espacials de Catalunya /CSIC, Campus UAB, Facultat de Ciències, Torre C5-parell-2a planta, 08193 Bellaterra, Spain
Email: iribas(at)ieec.fcr.es
Resources:
Stellar XUV data can be retrieved from the following databases:
NEXXUS 2 - The Database for Nearby X-Ray and extreme UV emitting Stars
MAST - The Multimission Archive at STScI is a NASA funded project to support and provide to the astronomical community a variety of astronomical data archives, with the primary focus on scientifically related data sets in the optical, ultraviolet, and near-infrared parts of the spectrum.
HEASARC - The High Energy Astrophysics Science Archive Research Center is the primary archive for NASA missions dealing with extremely energetic phenomena, from black holes to the Big Bang.
