Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 06/26/2020 - 21:42
When the magnitude range in VSX is specified as an amplitude is this using the convention that amplitude is half the range of variability or the full range. In this case is the magnitude value specified the max magnitude or the mean?
thanks
Cliff
Hi Cliff,
It is usually a mean magnitude. This means that depending on the star type, that value might be closer to the minimum or maximum magnitude (so it is actually not related to the range).
E.g.: an outbursting or flaring object (UG or UV) will be at minimum magnitude in quiescence so the mean magnitude will be very close or even equal to the minimum magnitude. I know that showing a minimum magnitude in the maximum magnitude field may be confusing but we can only list amplitudes in the minimum magnitude field.
In the case of detached eclipsing binaries (EA), it will be the opposite. They spend most of the time at maximum so the mean magnitude will be close to the maximum magnitude value while the amplitude will indicate the depth of the eclipses. Other eclipsing binaries like the EW subtype and pulsating variables will usually have a magnitude closer to the midrange because their variations are continuous. But it depends on the star's behaviour.
It is always better to give a range if available.
Quoted amplitudes are usually only approximate and might not give a real idea of the actual total amplitude of the star so do not take them at face value.
Cheers,
Sebastian
Thanks,. I see why specifying min and max makes the most sense. Just a final point on amplitude. When it is in VSX does it depend on the type of star? Say for an EB, the amplitude specifies the full range while for a sinusoidal delta scuti the amplitude is half the range? Or is amplitude always the full range in VSX.
thanks,
Cliff
Hi Cliff,
The amplitude is always (in theory) the full amplitude, peak to peak, no matter what the variability type is.
You have to keep in mind that -however- amplitudes are usually determined automatedly and represent a lower limit of the actual variations because those determinations may not take into account cycles brighter or fainter than average or may be based on sparse data. When a star is well-studied it will usually have a range published instead of an amplitude.
Cheers,
Sebastian
Do I understand this correctly?
Mag.range 10.00 (0.50) V means the star (approx.) is variable from 9.75 to 10.25 V. Or is it 9.5 to 10.5 V?
Hans-Georg
This topic was raised again in the general discussion forum a couple of days ago so I copy my reply below.
And the given amplitude is indeed a full amplitude, you don't have to multiply it by two.
It is a peak to peak amplitude.
However, the mean magnitude won't always be mid-range (see below).
Also, when amplitudes are given, they usually come from surveys and thus, they may not be accurate. In the case of eclipsing binaries, they are usually underestimated. In the case of stars that are too faint for the survey, they are usually overestimated (scatter not properly taken into account).
Cheers,
Sebastian
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Mean magnitude + amplitude
Yes, in cases like this, when an amplitude is given (instead of a minimum magnitude), the maximum magnitude becomes a mean magnitude.
The mean magnitude of a pulsating variable, or any other variable with a sinusoidal-like light curve, will be more or les midway from maximum and minimum.
In the case of a detached eclipsing binary, the mean magnitude will be nearly the same as the maximum magnitude.
In the case of a dwarf nova, a flare stars or any other object undergoing outbursts, the mean magnitude will be closer to the quiescence state (minimum magnitude).
So when you see an amplitude in the minimum magnitude field, be prepared to open your mind and find things like these, when the maximum magnitude given is actually the minimum (!)
The key is understanding that, as soon as parentheses are seen, the maximum magnitude value is not longer working as such.
Having said that, nowadays we can adopt ranges for most stars, only that it is not a priority to do so, given the huge amount of data waiting to be imported or corrected. But if you find similar cases, you can submit revisions to VSX, e.g. deriving a range from ASAS-3 and ASAS-SN data.
I will do so for this star now, so in 10 minutes you will find a proper range given.
Best wishes,
Sebastian