Milky Way’s Galactic Core, Stellar “Paradox of Youth”

William · #1 (**permalink**) 26-April-2008, 05:23 PM

Milky Way’s Galactic Core, Stellar “Paradox of Youth”

The subject of this thread is what is known as the “paradox of youth” problem which is how to explain the recent discover of very young, massive, short lived stars that are located very, very, close to the galactic core. The “paradox of youth” problem is that the stars in question have a lifetime that is at most 100 MM years and it is very difficult to create a gas cloud with sufficient density to enable that number of very young stars to form so close to the galactic core. Half of stars in questions are located in three tight clusters of stars that are estimated to be 0.5 MM years old. There is no evidence of current star formation in the locations where the young stars in question are found which is puzzling, but perhaps not unexpected as simulations indicate that it is very difficult to get gas clouds to form that close to the Milky Way’s core massive compact object.

http://arxiv.org/abs/astro-ph/0508106v1

The following are excerpts from the above review paper “Stellar Processes Near the Massive Black Hole in the Galactic Center” by Tal Alexander which summarizes the observations and different hypotheses that have been developed to try to explain this paradox.

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About half of the young stars in the region are found today in three particularly massive, young (.5 Myr) clusters: The Quintuplet (∼30 pc from the center in projection, M ∼10^4M⊙…, The Arches (∼30 pc from the center in projection, M &10^4M⊙, R∼0.2 pc, …, and the central cluster around the Milky Way's massive black hole (Figer 2003). The three clusters contain in total hundreds of MS O-stars, tens of Wolf-Rayet (WR) stars and a few luminous blue variable stars (§2.2), which are ∼10% of all the massive stars (initial mass>20M⊙) in the entire Milky Way Galaxy.

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It has proved difficult to find a satisfactory explanation of how they could have formed so close to the massive black hole, or alternatively, of how they could have migrated inward from farther away in the course of their short lifespans. This is the so-called “paradox of youth”. The question applies to any of the young stars in the inner parsec, but particularly so to the central cluster of the “S-stars”, which exist a mere few hundredths of a parsec from the massive black hole. The problem of the young stars has become one of the major outstanding issues in galactic core research.

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The young population in the inner ∼1 pc is often loosely described as the “OB-stars”. This general designation can be misleading, as it fails to convey the significant systematic differences that exist in the population. An important open question is the nature of the connection, if any, between the S-stars inside ∼0.04 pc and the luminous emission line stars further out, on the 0.04–0.4 pc scale. While it is plausible to assume that these are different components of the same parent population, it should be noted the two groups have distinct locations, kinematics and stellar properties. The stars detected so far in the two young star disks at p∼1”–10” are luminous OB supergiants, giants and WR stars of various types (Genzel et al. 2003b; Paumard et al. 2001; Paumard et al. 2004).

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The solutions proposed so far for the riddle of the young stars (see reviews by Genzel et al. 2003b; Ghez et al. 2005) fall into three main categories: unusual modes of star formation near the Milky Way's massive black hole; rejuvenation of old stars from the local population; and dynamic migration or capture from farther out, where stars can form. While each has some attractive features, none is quite satisfactory. The paradox of youth remains unsolved at this time.

Comment:
The overall universe is estimated to be 13.7 billion years old. Our galaxy is one of the oldest galaxies in the universe and is estimated to be 13.6 billion years old. An interesting question is how to explain why the mass of the compact massive object at the centre of the Milky Way SgrA* is only 3.6x10^6 solar masses.

(The abbrievation “SgrA*” is short for Sagittarius A* is used to designate the Milky Way’s black hole. The Milky Way’s black hole (BH) is located in the constellation Sagittarius and is designated as “Sagittarius A*”.)

The Milky Way galaxy formed during the crowded early times in the universe and would therefore have opportunities for multiple mergers with other galaxies, in addition to time to accrete its own very massive compact objects. The current theory of galaxy formation hypothesizes that all galaxies that have a central massive object at their core have gone through a quasar phase. It is therefore natural to compare the Milky Way to galaxies that going through their “quasar phase’. Quasars at z=2, for example when the universe has only 3.3 billion years old, have an average black hole mass in the order of 10^8 to 10^9 solar masses, a hundred to a thousand times larger than the Milky Way’s compact object.

Veeger · #2 (**permalink**) 26-April-2008, 05:59 PM

Ok - dumb question.
If stars often form in collapsing hydrogen clouds when sufficient mass and thermal energy are accumulated, why is it beyond understanding to think stars may form in hydrogen clouds collapsing into SgrA's accretion disk?

-Veeger

William · #3 (**permalink**) 26-April-2008, 11:33 PM

In reply to Veeger's question.

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If stars often form in collapsing hydrogen clouds when sufficient mass and thermal energy are accumulated, why is it beyond understanding to think stars may form in hydrogen clouds collapsing into SgrA's accretion disk?

Hi Veeger,

The different clusters of young massive, short lived stars are located very close the Milky Way’s black hole. In that region of space, very, very close to a “black hole” the gravitational field from the massive "black hole" creates tidal forces which will pull apart a gas cloud. As noted in the attached quote, for a gas cloud to have sufficient internal gravitational forces to resist the black hole tidal forces, the density of the gas cloud would need to approach that of the density found in the outer atmospheres of stars which does not seem possible. I.e. A massive gas cloud would have needed to have collapse to form a super density gas cloud. The BH tidal force would have rip the gas cloud apart before it could have theoretically collapse. As noted in the comment, there are other known mechanisms that limit the size of a gas cloud that can collapse. (There is another problem/paradox in that the number of stars found concentrated in this region of space is very very high.) The mechanisms that limit how fast a gas cloud can collapse also limit the number of stars that can be formed in one area of space.

Comments:
1) There are other mechanisms (even if there was not a black hole in the immediate vicinity of the stars in question) that limit the size of the gas cloud that can collapses to such high densities. The collapse of the gas cloud is adiabatic, so the temperature of the gas must increase, which limits the maximum density of the collapsing cloud. (The gas can radiate which helps it cool, however the entire massive gas cloud cannot collapse, as the increase in temperature in one part of the gas cloud causes other of the cloud to expand.) Also when a star starts to form, the energy produced by stellar nuclear reactions heats the gas cloud causing it to expand and dissipate.

2)The stars in question do not form in the black hole accretion disc. The accretion discs' temperature is too high for stars to form in it.

http://arxiv.org/abs/0704.1281v1

The Galactic Center by R.Genzel and V. Karas

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If there is indeed a central black hole associated with Sgr A* the presence of so many young stars in its immediate vicinity constitutes a significant puzzle (Allen & Sanders 1986; Morris 1993; Alexander 2005). For gravitational collapse to occur in the presence of the tidal shear from the central mass, gas clouds have to be denser than ~10^9(R/(10′′))^−3 hydrogen atoms per cm^−3. This ‘Roche’ limit exceeds the density of any gas currently observed in the central region. Recent near-diffraction limited AO spectroscopy with both the Keck and VLT shows that almost all of the cusp stars brighter than K~16 mag appear to be normal, main sequence B stars (Ghez et al 2003; Eisenhauer et al 2005a). If these stars formed in situ, the required cloud densities approach the conditions in outer stellar atmospheres.

neilzero · #4 (**permalink**) 27-April-2008, 02:35 AM

I have not seen the MM years abreviation before; does that mean 1,000,000 years? Are very young , massive, short lived stars more than 100 solar mass each? It does seem that a cloud of hydrogen of 10,000 solar mass or more, should be detectable by some means. Perhaps the last of these hydrogen clouds close to the galactic center became stars a few centuries ago? Neil

William · #5 (**permalink**) 27-April-2008, 04:55 AM

In reply to neilzero's comment:

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Are very young , massive, short lived stars more than 100 solar mass each? It does seem that a cloud of hydrogen of 10,000 solar mass or more, should be detectable by some means. Perhaps the last of these hydrogen clouds close to the galactic center became stars a few centuries ago? Neil

Hi Neil,

There appears to be agreement that a gas cloud of sufficient density could not have formed at that location. Also as noted below there appears to be agreement that the massive young stars could not have migrated to their current locations. (MM is a European abbreviation for a million. As noted below a star of 15M⊙ has a life of approx. 40 million years.)

The following is another excerpt from the galactic center review paper (see above for a link.)

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The young stars are also too short-lived and too light to have formed far from the Milky Way’s black hole, where the tidal field is weak, and then to have migrated in by dynamical friction. The “collection basin” for the migration of young MS stars of mass of M⋆ =15M⊙ and lifespan t⋆ =2×10^7 yr is only maxrdf ∼ 0.2pc, and for stars of mass M⋆ =3M⊙ and lifespan t⋆ =4×10^8 it is only maxrdf ∼0.5pc.

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The unavoidable conclusion is that if the young stars well inside the central parsec were formed locally, then they must have done so by a different mechanism than the collapse of self gravitating cold molecular gas clouds that occurs in normal star forming regions.

RussT · #6 (**permalink**) 28-April-2008, 01:45 AM

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The unavoidable conclusion is that if the young stars well inside the central parsec were formed locally, then they must have done so by a different mechanism than the collapse of self gravitating cold molecular gas clouds that occurs in normal star forming regions.

Yes, and this happens in every Galaxy, and it happens right from the begging!

http://www.narrabri.atnf.csiro.au/pu...mages/ngc2915/

This IS first light...Stars being formed near the MBH of a Galaxy (this is NOT a Dwarf Galaxy!).

That "Core" accretion disc will grow more and more, looking like a larger and larger "Spheroidal Dwarf" (BCD)...Then the question becomes...Where are the "Quiet BCD's???...

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Indeed, so far, even in sheets, no secure candidate for quiet BCDs have been identified (Kunth & Östlin 2000, Legrand et al 2000).

http://www.usm.uni-muenchen.de/people/hopp/dw.html

AND, here is what a "Quite BCD" becomes/looks like...
http://www.nasa.gov/mission_pages/ga...ex-072505.html

When the "Arms" finally have enough star formation to be seen and are not 'hidden' by the brightness of the ever growing core.

This Star formation in the hot region of the core has NEVER even been accounted for in all of cosmology, and this is not only happening in the Milky Way, as William has linked to, BUT as I said, has been happening in the growth of ALL galaxies since they started as New galaxies, when their SMBH's were first Created, which is when and where the Quantum Gravity for String Theory can be "Tested"...Long GRB's from 2 sec's to 500 sec's!

Here is Andromeda doing the same thing.
http://hubblesite.org/newscenter/arc...05/26/image/a/

SO, all galaxies form their stars with a combination of 'Core Stellar Formation' AND, Dwarf Galaxy Cannabalism!!!
http://heasarc.gsfc.nasa.gov/docs/co...laxy_info.html

William · #7 (**permalink**) 29-April-2008, 03:17 AM

Quote:

The unavoidable conclusion is that if the young stars well inside the central parsec were formed locally, then they must have done so by a different mechanism than the collapse of self gravitating cold molecular gas clouds that occurs in normal star forming regions.

In reply to RussT's comment:

Yes, and this happens in every Galaxy, and it happens right from the beginning!

Hello RussT,

Thanks Russ. That is a interesting set of astronomical pictures. There should be some sort of relationship between a galaxy's massive compact object and the blue compact dwarf galaxies. I am going to look for more information.

http://www.narrabri.atnf.csiro.au/pu...mages/ngc2915/

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Much about blue compact dwarf (BCD) galaxies remains mysterious, such as how the neutral hydrogen obtained its shape, what drives current star formation, and why there is so much dark matter. NGC 2915 is located at the relatively nearby distance of 15 million light-years - just outside our Local Group of Galaxies.

Cougar · #8 (**permalink**) 02-May-2008, 03:05 PM

Quote:

Originally Posted by William

There appears to be agreement that a gas cloud of sufficient density could not have formed at that location. Also as noted below there appears to be agreement that the massive young stars could not have migrated to their current locations.

And yet they appear to be there. What's your point?

matt.o · #9 (**permalink**) 02-May-2008, 11:24 PM

First of all, I think you need to be careful about mixing up your scales here RussT. The work quoted in the original post is on interesting young stars in the central parsec regions of our galaxy. The blue compact dwarf image you linked to shows star formation on the ~1000 parsec scale.

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