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BigDon
2011-Feb-04, 03:14 PM
A small argument came up when some friends and I were discussing the size and proportions of objects in our solar system.

The disagreement occured when comparing the Sun and the Earth and I mentioned that as insignificant in size as it is, and Earth-like planetary body spiralling into a Sol sized star would make changes to the star's spectrum Earth observers could see from clear across the galaxy. (Assuming a commonly looked at star.) If not in neighboring galaxies.

I was thinking, in particular, iron and silicon would show up a lot heavier than "earlier observations". (Maybe oxygen as well.)

The naysayer's argument is an Earth-like body is still too small to have such an effect on the overall light output as iron and silicon lines are already impurities in the Sun's output.

My counter argument was as the planet spiraled in it would shed all kinds of material directly to the star's atmosphere, as opposed to a rogue object center punching directly into the mantle, and act like a "flourescent tube". Though I imagine each type of event would have a different look to it.

And would there be a significant radio or x-ray component to such an event? Or I guess a better question would be what would be the first sign this was not merely a little extra "ash" being circulated into the spectrum?

I guess a better title for this thread would have been: What Happens When you Throw A Big Rock At A Star?

Ken G
2011-Feb-04, 04:21 PM
I think the problem is that the matter would not actually leave the Earth if it fell into the Sun-- my guess is, the Earth would stay more or less intact, but would sink like a rock deep into the solar interior. It would eventually melt, but probably more slowly than it falls in, but even so, it's own self-gravity would hold the molten ball together until it fell deep into the interior. Then I guess the question would be, would it break apart before it fell beneath the convection zone, or while it was still in it? If the latter, then convection would mix the metals throughout the convection zone. That would have a fairly insignificant effect, though perhaps not completely insignificant-- the total iron in the convection zone of the Sun is perhaps about 10 times the mass of the whole Earth, and the Earth's iron is only a fraction of that (10% maybe?), so I'm thinking we'd be looking at something like a 1% increase in the iron in the convection zone if the Earth broke up in it. Note that higher mass stars have much thinner convection zones, so perhaps a star a bit more massive than the Sun would show a more pronounced effect. These numbers are loose enough though that I could not rule out the possibility you might be able to detect it if the Earth fell into the Sun, but probably it would be a little below our ability to detect.

George
2011-Feb-04, 05:41 PM
I think the problem is that the matter would not actually leave the Earth if it fell into the Sun-- my guess is, the Earth would stay more or less intact, but would sink like a rock deep into the solar interior. It would eventually melt, but probably more slowly than it falls in, but even so, it's own self-gravity would hold the molten ball together until it fell deep into the interior. Then I guess the question would be, would it break apart before it fell beneath the convection zone, or while it was still in it? Yes, this makes sense given that the Roche limit is about 550,000 km for the Earth. Since this is inside the Sun, I would assume the tidal stress becomes less and less so that the 550,000 km point is no longer accurate and the Earth might indeed plunge deeper as it heats to plasma.


If the latter, then convection would mix the metals throughout the convection zone. That would have a fairly insignificant effect, though perhaps not completely insignificant-- the total iron in the convection zone of the Sun is perhaps about 10 times the mass of the whole Earth, and the Earth's iron is only a fraction of that (10% maybe?), so I'm thinking we'd be looking at something like a 1% increase in the iron in the convection zone if the Earth broke up in it. Since the convective zone would lift the ablating(?) elements from its gobbled Earth, I'd bet that the resulting burp would have much higher concentrations, and that spectrometry would easily notice the difference, but perhaps only if that specific region was being studied. Spectrometry of the entire disk might still reveal a temporary bump, though the heavier elements like iron might not make it to the surface in any significant concentrations.

[For the chromatically inclined, I would assume such a "burp" would not reveal a pale blue dot. ;)]

ngc3314
2011-Feb-04, 06:03 PM
The answer has been worked out in detail for a white dwarf - even a big asteroid's worth of iron, silicon, and magnesium would be blindingly obvious until it gravitationally settled through the thin (tens of meters) hydrogen atmosphere. Working up a Galaxy Zoo post on DZ stars dealing with this - until it goes "live" on Feb. 8, the draft is here (http://www.galaxyzooforum.org/index.php?topic=278892.0).

George
2011-Feb-04, 09:09 PM
The answer has been worked out in detail for a white dwarf - even a big asteroid's worth of iron, silicon, and magnesium would be blindingly obvious until it gravitationally settled through the thin (tens of meters) hydrogen atmosphere. Working up a Galaxy Zoo post on DZ stars dealing with this - until it goes "live" on Feb. 8, the draft is here (http://www.galaxyzooforum.org/index.php?topic=278892.0).

The link seems problematic. Is it Mitch's Mystery Star (http://www.galaxyzooforum.org/index.php?topic=277052)?

I see Universe Today (http://www.universetoday.com/61789/mitchs-mystery-star-curiouser-and-curiouser/) had recapped some of the excitement up until last April, including many of your comments.

I suspect the problem is that it looks like it might be a yellow white dwarf. (hehe)

Spaceman Spiff
2011-Feb-07, 04:23 AM
For reference, the standard solar model predicts that the Sun's outer convection zone contains about 2.5% of the Sun's mass.

There are about ~750 ppm by mass of Si (~650 ppm) and Fe (~100 ppm) in the Sun, and this amounts to about 6.2 Earth masses (if my maths are right) in those two elements in the Sun's convection zone. Earth would melt, ablate, and vaporize and its contents would become well mixed within the convection zone over several months to a few years time -- assuming all of this occurred within the convection zone. In that case observers would indeed note substantial increases in the Si, Fe, O, Mg abundances. If much of its mass survived the plunge into the deep radiative interior, then correspondingly lower changes would be noted in the elemental abundances via spectroscopic analysis. I've not seen a serious computational model of such a scenario, but one might be out there.

Ken G
2011-Feb-07, 07:46 AM
For reference, the standard solar model predicts that the Sun's outer convection zone contains about 2.5% of the Sun's mass.

There are about ~750 ppm by mass of Si (~650 ppm) and Fe (~100 ppm) in the Sun, and this amounts to about 6.2 Earth masses (if my maths are right) in those two elements in the Sun's convection zone. Earth would melt, ablate, and vaporize and its contents would become well mixed within the convection zone over several months to a few years time -- assuming all of this occurred within the convection zone. In that case observers would indeed note substantial increases in the Si, Fe, O, Mg abundances.Thanks for firming up the numbers, but remember that a 1/6 increase in metal abundances might not be terribly detectable, given the uncertainties. Solar metallicities themselves suffer significant alterations every time someone new conducts a careful study! But perhaps there is an important difference between absolute abundances and relative changes in abundance-- the former might be uncertain at the 16% level of precision, but the latter might be much easier to detect, if we were lucky enough to have "before and after" data. Usually, the expectation is that the event would have been well in the past, so we would need to see an absolute variation moreso than a relative one.

Nereid
2011-Feb-07, 02:01 PM
If instead of being thrown in, the Earth eased in, gradually, its passing may be more noticeable. For example, how much of it would be ripped off, melted, and worse, by being slowly broiled in an orbit very close to the Sun (I don't know if 'contact binary' could ever apply, but something similar)? IOW, there may be a way to get more of the Fe, Si, and Mg into the photosphere quickly, resulting in a temporary spike in abundance, as determined by distant spectroscopists, perhaps even one that is constrained spatially as well?

Ken G
2011-Feb-07, 04:40 PM
That would be true if you could "boil off" the Earth faster than a mixing time in the convection zone. I think the convection zone mixing time is pretty quick, maybe a year or something (that's a guess, better numbers would help).

Spaceman Spiff
2011-Feb-08, 03:08 AM
Thanks for firming up the numbers, but remember that a 1/6 increase in metal abundances might not be terribly detectable, given the uncertainties. Solar metallicities themselves suffer significant alterations every time someone new conducts a careful study! But perhaps there is an important difference between absolute abundances and relative changes in abundance-- the former might be uncertain at the 16% level of precision, but the latter might be much easier to detect, if we were lucky enough to have "before and after" data. Usually, the expectation is that the event would have been well in the past, so we would need to see an absolute variation moreso than a relative one.

[BOLD] Precisely. A change in elemental abundance of this magnitude might well be noticed in before and after spectra, especially in the weaker transitions for which the line strength and elemental abundance are more nearly linear in relation. Depending on the circumstances, the overall increase in the opacity due to the infusion of Si, Fe, Mg, O, etc, could affect significantly the physical structures of the photosphere (and perhaps even that of the convection zone), assuming the infused material is eventually mixed back up into the photosphere.

The question in my mind is what actually happens to an Earth plunging into the Sun (after Dr. Evil removes nearly all of Earth's angular momentum at a distance of 1AU)? Certainly, the Earth will enter the Sun supersonically. Over what depths is this heavy element rich slug mixed within the Sun? Besides the convective motions in the outermost envelope, the Sun rotates and so there are various types of circulations in the deep interior as well.

We could "watch" such an event with helioseismological measurements.

ngc3314
2011-Feb-08, 03:31 AM
The link seems problematic. Is it Mitch's Mystery Star (http://www.galaxyzooforum.org/index.php?topic=277052)?

I see Universe Today (http://www.universetoday.com/61789/mitchs-mystery-star-curiouser-and-curiouser/) had recapped some of the excitement up until last April, including many of your comments.

I suspect the problem is that it looks like it might be a yellow white dwarf. (hehe)

The very one.

My bad - that was a link to the beta-polishing location, which I didn't realize had restricted access. The full post (http://www.galaxyzooforum.org/index.php?topic=278909.0) is now up, on what matter drizzling in slowly from an accretion disk with asteroidal (or, more morbid, terrestrial-planet) abundances does to the atmosphere of a white dwarf.

Until tracking down this star after Mitch's GZ pst, I had no idea DZ stars existed, much less how much one needs to know about the limitation of various van der Waals force approximations in modeling their spectra. Glad it's being done and glad it's being done by someone else...

Ken G
2011-Feb-08, 01:59 PM
The question in my mind is what actually happens to an Earth plunging into the Sun (after Dr. Evil removes nearly all of Earth's angular momentum at a distance of 1AU)? Certainly, the Earth will enter the Sun supersonically. Over what depths is this heavy element rich slug mixed within the Sun? Besides the convective motions in the outermost envelope, the Sun rotates and so there are various types of circulations in the deep interior as well.Perhaps when Shoemaker-Levy hit Jupiter we get some indication of what happens-- that was only a small comet with little gravitational self-binding, and it also didn't contain nearly as much material as the Earth, so it might not be a fair test, but at least we have a metal-rich object hitting a hydrogen-rich photosphere. I don't know if that object had any significant effect on spectra of Jupiter after it was mixed into Jupiter's belts and zones, but it certainly had a local effect if one could spatially resolve the surface of the photosphere.