Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 10/17/2012 - 15:43
I am helping out on a project from the Lowell Observatory on young M class dwarf stars, trying to determine the rotational period of a number of selected objects. The assumption is these stars have large spots, which makes detection of a rotation period possible. Being M class objects, they are of course very red, as is several (of the very few) available comp stars in the various fields.
I know there is a general prohibition about using reddish comp stars, but how about for for red stars themselves? I am thinking since I don't need a precise corrected magnitude, only a differential magnitude to measure the light changes, it shouldn't make all that much difference.
Any thoughts?
Jerry
That sounds like a really interesting project. My guess would be that to identify the effects of star spots on a rotating red dwarf, you would need to make measuments with no more than a few milimag uncertainty over time spans of several weeks? If your comp star is also red, Obviously it should not be a dwarf with spots itself, and certainly not a pulsating red giant. What kind of other red stars are there that would be stable enough for your purpose?
As you mentioned you are interested only in the change of magnitude over time and not in the absolute value, so I would think that for that purpose the color of the comp str is even less important: any error that is introduced via uncertainties in a color transform can be avoided, you don't need to transform to a standard filter response, right? All you need is a stable calibration source, no matter what color.
Clear skies
HBE
you don't want to use an intrinsically red star - M or Late K. Almost all vary and often on short time frames, flicker, for example. I agree with HBE that if all you are trying to do is record changes in the flux from your red star color difference is not as important but you will still have differences in extinction for millimag level photometry that will occur if the star colors are very different. You will probably have to include second order extinction correction for this level of precision.
If you are pointing in an advantageous direction and get lucky, you might find a reddened non variable star in your field of view. So the star will be an earlier type reddened by interstellar dust. If you are pointing toward the galactic center you have a greater chance of having such a star than if you are pointing toward the near side of the galaxy. In any case, I would not use very late class stars as comps even if they are on the main sequence. You have to check the spectral classification, not just the B-V value because a reddened earlier class star, a G2V for example, can have a b-V greater than, say, 1.0 or 1.1.
Good graphs can be found in Henden and Kaitchuck pages 42 and 43, page 82 in Stars and Their Spectra by James b Kaler or Section 3.5 of Galactic Astronomy by Binney and Merrifield. Henden and Kaitchuck also has a good discussion of reddening at the end of section 2.4, and Binney and Merrifield covers it in section 3.7.
I don't think I will need milli-mag accuracy, as the folks from Lowell believe 0.02 will be good enough in many cases. Still, your points are well taken. I will endeavor to find the least reddish comp stars available to make the measurements.
Thanks
Remember, that it is red according to MKK class that you have to avoid, not necessarily red according to observed B-V due to reddening.
Good point! When 2MASS photometry exists for a star and the J, H, and K magnitudes all are of "A" quality, it is possible to distinguish between reddened, hotter stars and less-reddened, cooler stars. A J-H index smaller than about 0.4 mags means the star is hotter than a K star would be.
I will dig up a diagram I once made showing how these things correlate with spectral type and reddening.
Cheers,
Doug
In my first response to this thread, I listed a few sources that have very good graphs showing the relationship of U-B vs B-V for reddening, and the B-V color indices that correspond to different spectral classes prior to reddening.
"Good graphs can be found in Henden and Kaitchuck pages 42 and 43, page 82 in Stars and Their Spectra by James b Kaler or Section 3.5 of Galactic Astronomy by Binney and Merrifield. Henden and Kaitchuck also has a good discussion of reddening at the end of section 2.4, and Binney and Merrifield covers it in section 3.7. "
I am sure a one can find a bushel basket of graphs through a web search. The ones I listed are ones in books that I have.