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So I'm currently working around a specific variable star, α Ori and I want to determine it's luminosity...I do only have the light curve (driven from V-optical photometric band - Johnson V from AAVSO)... Is it allowed to utilize it to compute for Luminosity L? The distance modulus doesn't seem to fit since it is for 'visual' magnitude (Using our eyes) so I guess it requires a different approach?

I tried to check in SDSS tho for the other photometric system...But since it avoids the galactic plane and α Ori is "bright" enough, I'm sure the detectors will be saturated so I opted it.

Since I'm new to this kind of stuff, I'm looking forward for your help/s, suggestions and others. Also, I may have missed out something so I apologize.

Best regards,

CGHA

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  • $\begingroup$ If you need a distance, it doesn't matter how it was derived as the object ultimately can only have one distance (it only matters for error estimates). Btw: Alf is a very strange abbreviation for alpha $\endgroup$
    – planetmaker
    Aug 13, 2020 at 8:04
  • $\begingroup$ ah yah it is a strange abbreviation haha (Even AAVSO and VSX does that). I should have indicated "a" instead lol. But I do not need distance at all since I do have the value for that. I think what matters most is to get the "L"..and getting the L through the optical standard passband is a little bit problematic for me C: $\endgroup$
    – CGHA
    Aug 13, 2020 at 8:17
  • $\begingroup$ @planetmaker - "alf" is pretty standard, e.g. SIMBAD uses it. $\endgroup$
    – user24157
    Aug 13, 2020 at 8:19
  • $\begingroup$ but yep, it is pretty much standard tho. c: Kinda seems strange at first. $\endgroup$
    – CGHA
    Aug 13, 2020 at 8:21
  • $\begingroup$ For my understanding of your question: you have V-band photometry data on the star - but you are concerned that you cannot use it to derive a magnitude and subsequently a luminosity? Where is the difference between the brightness derived from V-band data and the visual magnitude? Thx for the hint with 'Alf' = Alpha. One never stops to learn, though this curls my nails ;) $\endgroup$
    – planetmaker
    Aug 13, 2020 at 8:26

1 Answer 1

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To calculate a "luminosity light curve" from a time series of V-band photoetry, you need two things.

  1. You need to know the distance. The distance to Betelgeuse is uncertain and that means the absolute value of the luminosity you get will also be shifted systematically up or down by whatever distance you adopt.

  2. The V-band only contains a small fraction of the luminous flux from Betelgeuse (most is in the red and near infrared). To convert a V-band magnetude to a "bolometric magnitude" requires you to add a bolometric correction, which is dependent on the spectrum of the star. In principle, if the star is varying, in the V-band, that might be because the bolometric correction is varying (i.e. the luminosity is constant, but the distribution of flux with wavelength is changing). Or it could be that the bolometric correction is constant and the total luminosity is varying. You can't tell from the V-band data alone.

In summary - you can't do it.

The best you can do is assume a constant bolometric correction and turn your V-band light curve into a bolometric magnitude light curve and hence into a luminosity light curve. But then, since the absolute luminosity is uncertain because of the distance uncertainty, you may as well just work with the V-band flux.

To do any better requires nearly-simultaneous photometry in more bands, especially in the red and near-infrared.

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  • $\begingroup$ Even though it is an instrumental magnitude 'minst', it can be 'converted' to Mv then Mbol to L? Does it still require some color indices, optical thickness and color excess, atmospheric extinction, and reddening corrections? Pretty helpful btw so thank you. c: $\endgroup$
    – CGHA
    Aug 14, 2020 at 0:12
  • $\begingroup$ @GenerichCapuli it requires knowledge of the intrinsic spectrum of the object. $\endgroup$
    – ProfRob
    Aug 14, 2020 at 6:35

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