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Degen1103
Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 11/05/2022 - 16:16
Do I understand right, that BG Cyg reached minima for two months to predicted date?
I understand that predicted minimum can vary by several weeks or more. I'm not aware of when the current minimum for BG Cyg was expected. My observation from August 31st gave an instrumental (non-transformed) V magnitude of 12.681, which from the AAVSO curve is close to minimum. My following observations show a gradual rise through my last observation on November 2nd of 11.439.
If something unusual is happening here please let the rest of us know.
Jamey (JENJ)
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Degen1103
Affiliation
American Association of Variable Star Observers (AAVSO)
But according to light curve BG Cyg went last maximum in the end of April, however ephemeris shows mid at 13 February 2022. That's why I scratch on the back of the head.
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naillond
Affiliation
Association Française des Observateurs d'Étoiles Variables (AFOEV)
If you look at the "remarks" section for BG Cyg on VSX. You will see that the period is variable, ranging from 288d to 307.3d depending on the date range. Ephemerides are forecast predictions of the time of maximum and are not perfect either statistically or more so in the case of a star with a variable period.
This is one of the reasons we observe to elucidate the behavior of these stars... leading to reasoning it out and finally understanding and knowledge.
BG Cyg is a good star to have on your observing program because the period is variable.
Jim (DEY)
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Degen1103
Affiliation
American Association of Variable Star Observers (AAVSO)
I have updated elements for BG Cyg on my page with O-C diagrams at http://var.saaf.se/mirainfooc2.php. With my period elements the time for maximum have shifted between -60 to +100 days. Since year 1900 has the period fluctuated between 284 and 299 days according to my data at http://var.saaf.se/mirainfoper2.php.
The star is more of what is called a meandering mira, where the period goes up and down, than a mira with a steady period change.
Thomas Karlsson
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Degen1103
Affiliation
American Association of Variable Star Observers (AAVSO)
The 2.5 magnitude definition is kind of an arbitrary dividing line unfortunately. I think it is one of the numerous things we classified or defined before we really understood what was going on. Many SRa stars behave like Miras and seem to be of the same type, but have less than delta 2.5V and others behave more like SRbs or something else.
Here is a good paper on Red Variable classification. Janet Mattei is the lead author.
Yes, indeed, I myself would prefer classification with dimensionless similarity criteria based on physical processes within pulsators, but they are too complex for similarity method.
I understand that predicted minimum can vary by several weeks or more. I'm not aware of when the current minimum for BG Cyg was expected. My observation from August 31st gave an instrumental (non-transformed) V magnitude of 12.681, which from the AAVSO curve is close to minimum. My following observations show a gradual rise through my last observation on November 2nd of 11.439.
If something unusual is happening here please let the rest of us know.
Jamey (JENJ)
Maybe I wrongly thought that "mid" in ephemerides means minima? If so - sorry for the misunderstanding.
I see what you mean...perhaps someone can explain what the "mid" found in the VSX ephemerides is telling us.
Jamey (JENJ)
For mira stars, "Mid" is the predicted maximum, computed from Epoch and Period.
Dominique
But…
Theoretically.
But according to light curve BG Cyg went last maximum in the end of April, however ephemeris shows mid at 13 February 2022. That's why I scratch on the back of the head.
A lot of miras are irregular...
Look at BG Cyg observed maxima by the AFOEV: https://cdsarc.cds.unistra.fr/afoev/activ/obs/lpv/repmira/cygbg.htx
(O = observed max, C = computed max, O-C = -91 to +62 days !!!)
Greetings,
If you look at the "remarks" section for BG Cyg on VSX. You will see that the period is variable, ranging from 288d to 307.3d depending on the date range. Ephemerides are forecast predictions of the time of maximum and are not perfect either statistically or more so in the case of a star with a variable period.
This is one of the reasons we observe to elucidate the behavior of these stars... leading to reasoning it out and finally understanding and knowledge.
BG Cyg is a good star to have on your observing program because the period is variable.
Jim (DEY)
Oh, now I understood, thank you!
BG Cyg is not noted in the "short list" of Mira Variables with Period Changes, that's whyI started to worry :)
For full understanding I kindly ask explain the reason of differense of mid definition for EA and pulsators.
I have updated elements for BG Cyg on my page with O-C diagrams at http://var.saaf.se/mirainfooc2.php. With my period elements the time for maximum have shifted between -60 to +100 days. Since year 1900 has the period fluctuated between 284 and 299 days according to my data at http://var.saaf.se/mirainfoper2.php.
The star is more of what is called a meandering mira, where the period goes up and down, than a mira with a steady period change.
Thomas Karlsson
A lot of thanks, dear Thomas! I've been impressed by variability of miras variability!
I was always under the impression that in order to be a Mira, a star had to have an amplitude of at least 2.5m, which makes BG Cyg a non-mira?
Amplitude of BG Cyg is about 4m, yes?
The 2.5 magnitude definition is kind of an arbitrary dividing line unfortunately. I think it is one of the numerous things we classified or defined before we really understood what was going on. Many SRa stars behave like Miras and seem to be of the same type, but have less than delta 2.5V and others behave more like SRbs or something else.
Here is a good paper on Red Variable classification. Janet Mattei is the lead author.
https://ui.adsabs.harvard.edu/abs/1997ESASP.402..269M/abstract
Here is another one on the Secular Evolution of Mira Pulsations by Templeton & Mattei.
The Astronomical Journal, 130:776 –788, 2005 August
Clear Skies,
Rich
Yes, indeed, I myself would prefer classification with dimensionless similarity criteria based on physical processes within pulsators, but they are too complex for similarity method.