New Keck Telescope Observations of the G2 Cloud


October 14th a group of U.S. astronomers published the results of Keck Observatory imaging of the G2 cloud which they did last March:

(Thanks to Roman who shared this link)

This paper by Witzel, et al. describes several new things that were learned from these observations.  Note that the present posting has been updated to accommodate changes that Witzel et al. made to their paper since their original posting on October 9th.

1) The G2 cloud definitely contains a star.

2) This star has a luminosity of 30 solar luminosities which implies that it has a mass of about twice the Sun's mass.

3) They rule out the possibility that the star is a binary star system.

4) They find that the photosphere of the star has a diameter of 2.6 AU (astronomical units, slightly larger than the size of the Earth's orbit.  This is 100 times larger than the photosphere normally seen for a main sequence star of this luminosity, which they find puzzling.  But over a year ago, in May 2013, I had predicted that stars at perhaps would have such an inflated size; see below.

5) They more accurately define the pericenter distance for the G2 cloud, determining its distance of closest approach to be 215 ± 30 AU.  This is further out than the 144 AU previously estimated by Gillessen, et al.  Witzel, et al. also find that at this distance of closest approach a 2 solar mass star within the G2 cloud should have a tidal radius of 1.3 AU.  The 1 AU radius estimated for the G2 star's photosphere is smaller than this tidal radius, which is consistent with the lack of evidence of tidal stripping.  Although, the possibility still remains that tidal stripping will be observed at the time the star reaches its periapse distance.  Whether it has already passed its orbital pericenter, though, is not known.

6) They conclude that the star will continue to follow its orbital path around the Galactic core and continue on out.

In a May 2013 posting, G2 Cloud Predicted to Approach Twice as Close to GC, it was predicted that a star nearing its closest approach to the Galactic core would have a substantially inflated photosphere due to a number of reasons: a) energy added to the star due to tidal interactions with the Galactic core, b) energy added to the star due to cosmic ray heating of its atmosphere by the Galactic core's cosmic ray flux, and c) internal genic energy production which itself depends on the value of the ambient gravity potential field, this field becoming increasingly negative as the star approaches its pericenter.  At that time I estimated that a one solar mass star approaching within 130 AU of the Galactic core would have its luminosity boosted 37 fold to 37 solar luminosities.

With the current determination of Witzel, et al. that the star will instead come within 215 AU when at its closest approach to the core, this luminosity estimate must be revised downward.  Accordingly, a one solar mass star at a distance of 215 AU from the Galactic core will have a genic energy output reduced to only 60% of what was previously estimated and a cosmic ray heating input reduced to only 36% of what had been previously estimated.  As a result a one solar mass star is estimated to have a total luminosity of 18.5 solar luminosities, which is close to what Witzel, et al. report for the G2 cloud star.  Here we neglect the contribution to luminosity due to tidal heating effects which would be small by comparison.

Previously I had estimated that due to its over luminosity the photosphere of a one solar mass star at pericenter would expand 4 fold to 4 solar radii.  I was apparently being too conservative.  Because here we see that even with a more modest luminosity increase to 18.5 solar luminosities (half of the earlier estimate) that the star's photospheric radius expands to 1 AU, 245 fold larger, about like that of a star going through its red giant phase.  For example, a one solar mass star going through its red giant phase of core helium burning will have a luminosity of around 40 times the Sun's luminosity and a radius of about0.5 AU.

Consequently, my earlier posting quite accurately estimated the luminosity observed for the G2 cloud star (once the pericenter distance is corrected for), but it underestimated its photospheric diameter.  Indeed, my earlier suggestion that the star would attain a diameter of 4 solar radii, was actually just a very conservative guess based on observations of T Tauri stars that lie near our solar system.

So, in view of the above discussion, the G2 cloud appears to contain a star of about one solar mass which is about 30 fold over luminous due to its passage close to the Galactic core, this excess luminosity causing its photosphere to expand approximately 245 fold to a radius of 1.14 AU.  It is unlikely that a binary star is present, but the star's association with a jovian sized planet cannot be ruled out.

One criticism I have of the report by Witzel, et al. is that they still use the term "black hole" to refer to the galactic core.   The black hole theory is dead now; see recent posting. Let’s put flowers on its grave. When referring to this supermassive celestial object, astronomers should now use the term I have been proposing for the past 30 years: “Mother star”, or supermassive galactic core.  Another criticism I have is that, in order to explain the over luminosity of the G2 cloud star, they theorize that this star is the result of the recent fusion of two binary stars.  Such binary fusion happens to be a highly improbable event.  So, which explanation do you want to believe, their a posteriori highly improbable explanation, or my a priori relatively accurate prediction?

It should be pointed out that the lack of activity from the galactic core indicates that the matter that has been stripped off from the G2 star has been exclusively dust and gas.  As I predicted earlier, such material will not fall into the core, but will be blown away by the core's intense cosmic ray wind.  Galactic center astronomers have been routinely ignoring the fact that the core has an intense wind, which is why they have been surprised at the fact that they have seen no dust infall activity from the core.  Evidence that such an energetic wind exists is indicated by the strong Br gamma line emission from the G2 cloud's outer envelope.  Astronomers, such as Gillessen, et al. (2012) attribute this emission to ionizing photons emitted by stars in the vicinity of G2, totally ignoring the fact that this ionizing radiation more likely comes from the Galactic core.  So it is not surprising that no dust infall activity has been observed from the Galactic core.  Recall the news conference held last January when Leo Meyer said he expected to see "fireworks" (

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