Abstract: We are requesting ground-based photometry and spectra of any/all of the 16 stars in our target list. Low-or medium-resolution spectra and time-series photometry in virtually any waveband can be used to complement the TESS observations this work supports, but we are especially hoping for filters bluer than TESS (700-1000 nm). The aim is to measure the amplitude of rotation-induced flux variation for each of these stars in multiple bandpasses using time-series photometry. Many of the targets have rotation periods < 1 day and could be observed for much or all of their phase within a single night.
Justification: Multi-color photometry is useful in characterizing stellar surfaces because starspots, the driving factor in stellar rotational flux modulation, have a spectrum unique from the surrounding star. Differences in the flux contrast between spot and photosphere are wavelength-dependent, and the amplitude of rotational variation is therefore also wavelength-dependent. By observing the multi-color rotation of these 16 stars and jointly analyzing the AAVSO observations with the TESS data, we can measure the temperature of starspots for these stars and better understand the complex surfaces of M dwarfs.
Not speaking for the proposers here, but the information they seem to want involves colors, that is, color-indices. Thus separate magnitudes from many sources will produce a jumble because of calibration offsets. For the M-dwarfs involved, probably B-V and V-I color-indices together will give them some leverage on the temperature changes. But it’ll all have to be done in a highly consistent manner, since the color changes will be modest. Another issue is the degeneracy between spot coverage and the temperature-difference between the spots/active regions and the nominally unblemished photosphere. Bigger spots, smaller temperature difference versus smaller, darker spots can give the same results within the scatter of the photometry.
This sort of thing has been done quite a lot back in the literature. A fair example is for EV Lac from almost 50 years ago:
…which shows UBVRj lightcurve data, including U-B, B-V, and V-Rj color curves for this star, which is very similar to the proposed targets. Notice in Figure 2 of the paper that the color curves are nearly flat, implying that quite high internal precision will be necessary to see anything on similar targets in terms of color/temperature changes.
To some extent you can get some temperature information simply from the change in lightcurve amplitudes going from the violet redward. I’ll mention this paper:
…where Figure 3 shows photometry of mine in B,V,I for the T Tauri-type star DF Tau. Text section 2.4 describes the photometry. The bottom panel of Figure 3 shows the phased lightcurves becoming gradually smaller in amplitude at progressively longer wavelengths. Notice how jittery the B filter data are in this active binary.
Hi,
A few days ago I made a spectrum of YZ Cmi. Below the 1D file.
I will try today again if I have clear Sky.
This was done with a Takahashi TOA 130 @0.7X with a LISA spectrograph.
Do I upload it to AAVSO?
Thanks Rainer
I can’t find any object at the specified coordinates. G 13-22 should be visible inside the yellow square. According to the catalogs, G 13-22 varies between V = 13.5 and 14.5 mag. The whole image shows objects down to V = 16 mag, but there is no object at the expected position. The Moon was bright, but not so bright that it would have prevented me from capturing a 14th-magnitude.
Maybe G 13-22 is the star to the right of the yellow square?
At least based on the chart I downloaded from AAVSO, that should be it. And the magnitude makes sense.
P.S. After checking the data on its proper motion, I saw that it shifts by about 1 arcsecond per year across the sky. So it makes sense that catalogs might place it in the wrong position!
Hi Nikola,
I had the same problem and did a new astrometric solution of my images to determine the correct position of the stars I have chosen out f the list. G 13-22 was among them.
Regards,
Josch
This campaign is mostly on red dwarfs that are high proper motion stars, thus this problem is expected, since the stars have moved since the year 2000, and the epochs in VSX are J2000.0.
According to Gaia DR3, G 13-22 has a proper motion of -951.336 (RA), -284.049 (DEC) mas/yr, this means that it has really moved a lot between 2000 and 2026.
VSP plots stars at their 2015.5 position (using Gaia DR2 proper motion information) so it should be much closer than the J2000.0 position that you obtain from VSX.
The ideal thing would be that software applied the ppm correction, but if that is not the case, checking a real sky image, e.g. from Aladin, will show you what the star is. You might even see more than one star there, as a result of different color images having been taken at different epochs. For red dwarfs you often see a red image and a blue image very well-separated (and indication the direction of motion in space).
Try a DSS image (with the star to the E of the VSX position) and a Pan-STARRS1 image (more recent, with the star to the W of the VSX position) to see what I mean. The Pan-STARRS1 color image will also show the red color clearly making the identification secure.
BTW, the star does not vary between V= 13.5 and 14.5 as you said, the VSX range is in Sloan g. The star is 0.7-0.8 mag. brighter in V.
PS: I saw your final edit just now, you seem to have found the answer to your question yourself.
