Talk:V391 Pegasi

Luminosity?
What's the source for the luminosity value of 15.4 times solar? If I do the calculation assuming the star and the Sun are spherical blackbodies, the equation for bolometric luminosity is:

$$\frac { L_\ast } { L_\odot } = \left ( \frac { T_\ast } { T_\odot } \right )^4 \left ( \frac { r_\ast } { r_\odot } \right )^2$$

Using the values given in the article for radius and temperature, I obtain a bolometric luminosity of 35 times solar. However I'm not sure how well the blackbody approximation holds for subdwarf B stars.

On the other hand, if I use the absolute magnitude, the visual luminosity (I'm going to use the symbol l here rather than L to avoid confusion) is given by:

$$\frac { l_\ast } { l_\odot } = 100^{((M_\odot - M_\ast)/5)}$$

Using an absolute visual magnitude of +3.84 for V391 Peg and +4.83 gives a visual luminosity 2.5 times solar.

So what's the source of the 15.4? Chaos syndrome 20:24, 16 September 2007 (UTC)
 * I calculated the bolometric luminosity using the temperature and radius as in the equation above. The temperature and radius come from the Extrasolar Planets Encyclopaedia. BlueEarth 21:24, 16 September 2007 (UTC)


 * == Luminosity: correction necessary ==

The temperature and radius cited in the Extrasolar Planets Encyclopaedia are 29300 (± 500) K and 0.23 (± 0.03) Rsun, respectively. These values are taken from Silvotti et al. 2007, Nature 449, 189 (the original research article). In the supplementary online material to that article (see Table 1 on page 2), the calculation you are looking for has been done. It yields the 35 (± 9) Lsun that Chaos syndrome also recovered. Further numbers of interest are the apparent visual magnitude of 14.57 (± 0.02). The derived absolute visual magnitude of 3.84 ± 0.28 is obtained by using the luminosity and a bolometric correction of -2.95 ± 0.02.

That should also settle the question about the "citation needed" tag: it's Silvotti et al. 2007 (which was there in an earlier version, and from where information that still is in the article right now was taken); the link is http://www.nature.com/nature/journal/v449/n7159/full/nature06143.html.

Further suggestions for improvement that should be discussed
>> blue-white subdwarf star: << was subdwarf B star before, which is the most accurate description. As this might not be easily understood, hot subdwarf B star might be better for most readers: It matches the nomenclature used in the subdwarf article (which uses the correct scientific terms - at the moment, anyway).


 * More specifically: "blue-white subdwarf" star links (via "Subdwarf") to http://en.wikipedia.org/wiki/Subdwarf_star where the cool and hot subdwarfs are rather broadly described. http://en.wikipedia.org/wiki/Subdwarf_B would be much more precise.Bruddl (talk) 16:58, 20 May 2010 (UTC)

>> The star is clarified as an "extreme horizontal branch star." << What is the word clarified supposed to mean? One could say that the star is *classified* as - no, not an extreme horizontal branch star, but as a subdwarf B star (spectral classification). It can be *identified* with the evolutionary status of an "extreme horizontal branch star." The difference, if unclear, has e.g. been described by Østensen 2009, here: http://adsabs.harvard.edu/abs/2009CoAst.159...75O.

>> one-over-four-and-one-thirds the diameter of the Sun << What is one-over-four-and-one-thirds supposed to mean? For the radius see above (0.23 ± 0.03 Rsun), this would obviously also translate into a diameter 0.23 times that of the sun.

>> luminosity << see also above; compare to apparent visual magnitude of 14.57

Expanding on the content: the pulsations in this star were discovered by Østensen et al. 2001 (the first link in the References section) and have been further studied by Silvotti et al. 2002, A&A, 389, 180. The latter work was followed up by Silvotti et al. 2007 (see above) and yielded not only the discovery of the planet that survived its host stars' red giant expansion (as explained in the V391 Pegasi b article), but also the measurement of changes in the star's pulsation periods due to evolutionary effects. In other words, this is one of the relatively rare objects were one can directly "see" the very small and slow effects of stellar evolution on human time-scales of only a few years.

Bruddl (talk) 22:18, 30 December 2009 (UTC) Bruddl (talk) 19:12, 1 January 2010 (UTC)