RR Lyrae stars in M3 globular cluster - variability type of IO CVn

Science Short Period Pulsators
1

From march 4th to may 3th of 2025 I have been recording M3 globular cluster for its RR Lyrae stars in V and I-band. I have managed to record 77 of those over 21 nights.

One of the stars was IO CVn. It is catalogued in the VSX as a RRab star. But I found the phase plot to be much more of a RRc type with a nearly perfect sinusoïdal phase plot. . On what basis are stars categorized as RRab, RRc or RRd in VSX?

IO CVn_phase_plot_with_errorbars
IO CVn_phase_plot_with_errorbars5579×2719 2.75 MB

Is I wanted to create my own PL relation of these stars I ended up leaving IO CVn out because I am unsure of the variability type.

For those interested all my measurments are uploaded to the AAVSO under observerscode KREA. see a list of those stars beneath, with the average magnitudes retrieved from the phase plots.

Column 1 Column 2 Column 3 Column 4 E F G H
Starname Period (days) Period ( logP) Type I-mag error I V-mag error V
V0421 CVn 0,238855 -0,622 RRc 15,576 0,013 15,665 0,023
QQ CVn 0,287744 -0,541 RRc 15,511 0,011 15,518 0,017
OU CVn 0,289794 -0,538 RRc 15,582 0,005 15,547 0,008
V0478 CVn 0,292659 -0,534 RRc 15,638 0,017 15,649 0,028
MQ CVn 0,309035 -0,51 RRc 15,615 0,016 15,644 0,029
V0361 CVn 0,3179347 -0,498 RRc 15,526 0,017 15,513 0,027
KK CVn 0,3266358 -0,486 RRc 15,643 0,016 15,663 0,026
KT CVn 0,329601 -0,482 RRc 15,555 0,017 15,595 0,027
KV CVn 0,334927 -0,475 RRc 15,689 0,015 15,738 0,024
V0462 CVn 0,349816 -0,456 RRc 15,576 0,014 15,657 0,023
V0468 CVn 0,355823 -0,449 RRc 15,468 0,015 15,49 0,026
V0394 CVn 0,3559732 -0,449 RRd 15,607 0,015 15,691 0,024
V0465 CVn 0,36113 -0,442 RRd 15,473 0,014 15,626 0,019
V0477 Cvn 0,4560852 -0,341 RRab 15,534 0,035 15,633 0,063
KX CVn 0,4800641 -0,319 RRab 15,748 0,047 15,802 0,082
V0464 CVn 0,481406 -0,317 RRab 15,617 0,036 15,7 0,06
V0469 CVn 0,4912573 -0,309 RRab 15,532 0,033 15,63 0,056
OS Cvn 0,5035447 -0,298 RRab 15,547 0,031 15,661 0,055
V0467 CVn 0,505834 -0,296 RRab 15,442 0,026 15,55 0,049
V0460 CVn 0,506526 -0,295 RRd 15,432 0,03 15,565 0,05
KR Cvn 0,5078999 -0,294 RRab 15,53 0,037 15,604 0,064
IR CVn 0,511485 -0,291 RRab 15,599 0,034 15,71 0,06
XX CVn 0,513006 -0,29 RRab 15,542 0,032 15,644 0,057
V0377 CVn 0,51311 -0,29 RRab 15,57 0,033 15,704 0,058
V0476 CVn 0,5133518 -0,29 RRab 15,502 0,025 15,669 0,048
KY CVn 0,514328 -0,289 RRab 15,639 0,035 15,739 0,059
IL CVn 0,5148095 -0,288 RRab 15,531 0,033 15,661 0,059
V0471 CVn 0,515766 -0,288 RRab 15,637 0,032 15,754 0,055
MP CVn 0,516236 -0,287 RRab 15,737 0,043 15,784 0,069
V0444 Cvn 0,516455 -0,287 RRab 15,636 0,032 15,743 0,057
V0443 CVn 0,5170315 -0,286 RRab 15,515 0,03 15,653 0,056
V0466 CVn 0,517635 -0,286 RRab 15,422 0,033 15,564 0,05
KL CVn 0,519606 -0,284 RRab 15,559 0,033 15,687 0,055
V0463 CVn 0,52087 -0,283 RRab 15,504 0,023 15,659 0,04
IM CVn 0,5236852 -0,281 RRab 15,504 0,032 15,653 0,056
LQ CVn 0,5245415 -0,28 RRab 15,509 0,03 15,639 0,055
V0470 CVn 0,529118 -0,276 RRab 15,482 0,028 15,629 0,049
KM Cvn 0,529824 -0,276 RRab 15,531 0,032 15,675 0,053
LW CVn 0,530067 -0,276 RRab 15,512 0,031 15,646 0,056
V0345 CVn 0,53012 -0,276 RRab 15,498 0,036 15,621 0,06
LO CVn 0,53056 -0,275 RRab 15,526 0,023 15,657 0,04
IZ CVn 0,5368978 -0,27 RRab 15,557 0,025 15,691 0,041
V0474 CVn 0,5384629 -0,269 RRab 15,449 0,029 15,607 0,051
IU CVn 0,5415572 -0,266 RRab 15,483 0,029 15,631 0,051
IX CVn 0,545491 -0,263 RRab 15,5 0,025 15,651 0,044
V0461 CVn 0,5455972 -0,263 RRab 15,801 0,046 15,82 0,072
V0454 Cvn 0,5482318 -0,261 RRab 15,504 0,015 15,677 0,026
IV CVn 0,551535 -0,258 RRab 15,525 0,026 15,689 0,047
KN CVn 0,5580514 -0,253 RRab 15,556 0,023 15,72 0,04
V0453 CVn 0,5591099 -0,253 RRab 15,361 0,019 15,575 0,036
KU CVn 0,5683071 -0,245 RRab 15,376 0,017 15,597 0,029
V0457 Cvn 0,5695082 -0,245 RRab 15,446 0,022 15,628 0,039
OX CVn 0,569927 -0,244 RRab 15,32 0,028 15,296 0,042
V0411 CVn 0,570392 -0,244 RRab 15,416 0,025 15,593 0,042
V0455 CVn 0,5761617 -0,239 RRab 15,5 0,02 15,674 0,033
V0404 CVn 0,58398 -0,234 RRab 15,453 0,022 15,648 0,038
IY CVn 0,587067 -0,231 RRab/bl 15,515 0,017 15,699 0,028
LN CVn 0,5888109 -0,23 RRab 15,559 0,021 15,732 0,035
V0376 CVn 0,5977218 -0,224 RRab 15,403 0,023 15,603 0,039
V0440 CVn 0,6005285 -0,221 RRab 15,476 0,023 15,641 0,039
IQ CVn 0,6023073 -0,22 RRab 15,481 0,019 15,681 0,033
V0447 Cvn 0,6054515 -0,218 RRab 15,427 0,021 15,648 0,034
V0451 CVn 0,6278299 -0,202 RRab 15,264 0,013 15,512 0,026
V0472 CVn 0,631963 -0,199 RRab 15,46 0,014 15,689 0,022
NP CVn 0,635903 -0,197 RRab 15,675 0,036 15,753 0,058
IS CVn 0,6401389 -0,194 RRab 15,378 0,012 15,644 0,02
V0439 CVn 0,652413 -0,185 RRab 15,359 0,017 15,595 0,027
V0450 CVn 0,6683344 -0,175 RRab 15,324 0,026 15,511 0,044
IT CVn 0,707743 -0,15 RRab 15,189 0,016 15,451 0,029
IO CVn 0,7735618 -0,112 RRab 15,235 0,005 15,5 0,008
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Hello Remco!
Welcome to the forum.

I found the phase plot to be much more of a RRc type with a nearly perfect sinusoïdal phase plot. See the phase plot in the attachement. On what basis are stars categorized as RRab, RRc or RRd in VSX?

They have different pulsation modes. RRAB pulsate in the fundamental mode with larger amplitudes, longer periods (~0.4-1.0 d) and assymetric light curves. RRC pulsate in the first overtone mode, with sinusoidal (although still a bit assymetric) light curves, smaller amplitudes and shorter periods (~0.17 - 0.5 d). RRD show both modes at the same time.
You can classify them based on light curve shape and period.

I can’t find any attachment in your post, but I have checked your light curve.
Indeed the rise duration value (which indicates the degree of assymetry) is too large for an RRAB and the amplitude is too small.
Small amplitudes in globular cluster may be a result of blending with a close companion, so that alone, wouldn’t be enough to rule out the RRAB classification.
RRAB stars have larger amplitudes and shorter rising branchs around P= 0.5, and as the periods grow longer, the amplitude decreases (from 1.3 mag. to less than 0.5) and the rise duration increases (from ~0.1 to 0.3 or so). Even then, what we see for this star doesn’t add up.
Amplitude ~0.17V, rise duration ~40%.

This doesn’t look like an RR Lyrae. It looks like the typical first overtone anomalous cepheid, ACEPS. However, these are a couple of magnitudes brighter than RR Lyraes, and since they all belong to M3, we know they are at the same distance, and then their apparent magnitudes should depend on their intrinsic luminosity. And IO CVn has the same magnitude as the other RR Lyrae stars in the cluster.
I will download data from surveys in the hopes of finding something else, but otherwise, I guess I will revise this to RR only because we have problems with classifying as any of the subtypes.

Cheers,
Sebastian

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Thank you for the reaction Sebastion. I guess I will keep IO CVn out of the PL relation then :slightly_smiling_face: . In the mean while I managed to add the phase plot in the attachement of the original post.

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It is often useful to plot the RR Lyrae stars in a globular cluster in a so-called Bailey diagram of amplitude versus the log of the period. The RRab and RRc variables will have different trends in such a diagram. However, as Sebastian noted, blending of star images can mess things up. You might still get the right period for a blended star, but its observed amplitude and color can be altered. Such blended stars can create unusual scatter in the Bailey diagram. There are, however, a very few variables which look like RR Lyraes, and which seem unblended as far as we can tell, but which deviate from the usual Bailey diagram trends. I might add that keeping track of how globular cluster variables change over time is a valuable contribution to their understanding.-- Horace

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