PDA

View Full Version : Dwarf stars, with a certain flare.



Noclevername
2018-Oct-07, 03:07 PM
What's the minimal mass a Red Dwarf needs (assuming no companion) to avoid being a flare star?

Roger E. Moore
2018-Oct-07, 04:12 PM
Van Biesbroeck 8 is a confirmed flare star, and it is right around 0.08 solar mass, give or take, so there appears to be no real minimum. If a star can burn hydrogen, it can flare.

https://arxiv.org/pdf/1111.7072.pdf
link to 2012 paper on VB 8, does not list mass.

Noclevername
2018-Oct-07, 04:36 PM
Van Biesbroeck 8 is a confirmed flare star, and it is right around 0.08 solar mass, give or take, so there appears to be no real minimum. If a star can burn hydrogen, it can flare.

https://arxiv.org/pdf/1111.7072.pdf
link to 2012 paper on VB 8, does not list mass.

Is there a stable mass? (Yes, our Sun flares but we don't normally count it as a flare star.)

Roger E. Moore
2018-Oct-07, 06:22 PM
Is there a stable mass? (Yes, our Sun flares but we don't normally count it as a flare star.)

How do you define "stable"?

Noclevername
2018-Oct-07, 06:26 PM
How do you define "stable"?

Not a flare star.

Roger E. Moore
2018-Oct-07, 06:37 PM
I don't think there is a stable mass, as in not flaring. Solar-twin stars have been known to have superflares, for instance. Giant stars flare. It's hard to find a type of star that doesn't flare or have superflares.

http://adsabs.harvard.edu/abs/1998AAS...193.2201S
Superflares on Normal F8-G8 Main Sequence Stars (title is self-explanatory, check kappa-2 Ceti)
Schaefer, B. E.; King, J. R.; Deliyannis, C. P. (1998)

https://arxiv.org/abs/1711.02587
Stellar flares observed in long cadence data from the Kepler mission
Tom Van Doorsselaere, Hoda Shariati, Jonas Debosscher(Submitted on 7 Nov 2017)

We aim to perform a statistical study of stellar flares observed by Kepler. We want to study the flare amplitude, duration, energy and occurrence rates, and how they are related to the spectral type and rotation period. To that end, we have developed an automated flare detection and characterisation algorithm. We have harvested the stellar parameters from the Kepler input catalogue and the rotation periods from McQuillan et al. (2014). We find several new candidate A stars showing flaring activity. Moreover, we find 653 giants with flares. From the statistical distribution of flare properties, we find that the flare amplitude distribution has a similar behaviour between F+G-types and K+M-types. The flare duration and flare energy seem to be grouped between G+K+M-types vs. F-types and giants. We also detect a tail of stars with high flare occurrence rates across all spectral types (but most prominent in the late spectral types), and this is compatible with the existence of "flare stars". Finally, we have found a strong correlation of the flare occurrence rate and the flare amplitude with the stellar rotation period: a quickly rotating star is more likely to flare often, and has a higher chance to generate large flares.

Roger E. Moore
2018-Oct-07, 06:38 PM
All that said, there do seem to be very old M-dwarfs that do not flare. I think Kapteyn's Star is not a flare star.

antoniseb
2018-Oct-07, 09:48 PM
As I understand it, the boundary between M2 & M3 has to do with there being a convection layer in the brighter star that the dimmer ones don't have, and this also makes the difference between whether they are free from flares or not. I'm having trouble finding a precise mass of the boundary, but I have the impression it is about 0.4 solar masses (400 Jupiter masses).

Roger E. Moore
2018-Oct-07, 10:15 PM
https://arxiv.org/abs/1809.04510
The largest M dwarfs flares from ASAS-SN
Sarah J. Schmidt, et al. (Submitted on 12 Sep 2018)

Look at the table on page 6. M-dwarf flare stars at M7 and M8, and an L0 brown dwarf, to my astonishment.