astro
catdynamics | February 20, 2009Yesterday we talked about triples.
It is an interesting thing that as we seem to not find any binaries in clusters, we do find triples... hmm.
First Rosemary discussed stability of triples - dynamical stability of isolated triples to internal spontaneous ejection - looking at analytic criteria for overlapping resonances to second order in the disturbing function
then Ivanova and meself talked - first Ivanova skated through the theoretical zoo of dynamically formed triples, stable and not, finishing with a teaser about 4U1820-303, then I talked about The Triple.
PSR1620-26 in M4 - The Triple…
catdynamics | February 20, 2009the disbursement of the stimulus funding through the science agencies is going to get real interesting
some disparate word has reached me on the stimulus funding
first, the NSF divisions won't hear for a while, like next month, how much they individually get to push out the door (NIH might be different since the bill specifies the funding goes pro-rate to the divisions).
But, the clock on getting the money disbursed is already counting, they basically have 120 days to dispose of the money from the signing of the bill.
Now, some calls for proposal are in current evaluation (oh to have a…
catdynamics | February 19, 2009word going around that Hubble servicing mission scheduled for May is in danger of being reconsidered because of higher mission risk
haven't seen the memo, but it went around:
due to the Iridium vs Kosmos collision there is concern that higher debris levels in LEO make SM4 too risky, and it is to be re-evaluated and possibly postponed, again, or canceled.
that kinda sux
if someone has the memo, do pass it along, there's a good chap
here's the satellite collision story, in case you missed it - was all over the news media
This is going to be a real problem, because there is still no…
catdynamics | February 19, 2009While I was galavanting about the southland yesterday, the program moved on and
John compared N-body and Monte Carlo (video and podcast)
and Sverre and Rainer continued with the theme (video and podcast)
today we move onto triples
'cause you got to know that if two stars are good, three stars are more better...
theoretically a lot of stars are binaries, (cf Matthew's latest)
Hogeveen Monte Carlo'd observed binaries to try to invert the q distribution and concluded the secondary mass function is a steep function of q, suggesting a lot of unobserved low mass secondaries
someone needs to redo…
catdynamics | February 17, 2009long weekend over, and we're back at it.
today's topic is GRBs
and what, you might ask, do they have to do with clusters...?
Well, Bob, as you know, there are two types of gamma-ray bursters, or maybe three depending on how you count
type I and type II the long and the short
"Some say" that short gamma-ray bursts are, clearly, coalescing binary neutron stars, spiraling into contact through gravitational radiation emission.
This can, clearly, happen in globular clusters, as evidenced by PSR M15C, and taking a single data point, inverting the rate, and throwing in some theoretical…
catdynamics | February 12, 2009Latest on white dwarfs in globulars from Brad, John and Stefan.
White Dwarfs in Globular Clusters - Hansen - video and podcast
White Dwarfs in Globular Clusters - Fregeau and Rosswog - video and podcast
As we learned this morning, white dwarf cooling gives an absolute age indicator, essentially from ab initio physics calculations.
So you can independently measure cluster ages if you have very good photometry of very faint stars in very crowded regions. And we do.
cf NGC6397 - had a bit too much of M4 for now...
NB: white dwarf cooling age for NGC6397 is 11.5 Gyrs +/- 0.5
which is 1.2…
catdynamics | February 12, 2009All day discussion on white dwarfs today: cooling, kinematics, destruction etc
Starts off with Lars tutoring us in the morning...
Basics:
white dwarf origins - they are degenerate burnt out cores of low-to-medium mass main seqeunce stars
key issues still not fully resolved:
what is the relationship between the main sequence initial mass and the white dwarf final mass?
to what extent does it depend on main sequence composition?
binary companion? formation of He core white dwarfs due to envelope stripping - see white dwarfs with He cores and masses less than 0.48 solar mass, so no He core…
catdynamics | February 11, 2009Today we step back and Hans runs The Big Questions past us.
It is always good to think about the Big Picture.
The Big Questions
Most likely scenario for cluster formation:
a) compressed/triggered initial conditions: cloud collisions or intersecting shells, or
b) did globular clusters form as nuclei in dwarf galaxies that got accreted by our MW (dE,N hypothesis), or
c) scaled-up version of open cluster formation in mol. clouds? (Pudritz papers, Krumholz talk); the role of high pressure?
Relation of halo stars and globular cluster destruction ... (are Pop II stars dissolved globular…
catdynamics | February 10, 2009today we start turning our minds back to techniques, before we start contemplating what to do about white dwarfs
Stephan tutors us on SPH.
Smooth Particle Hydrodynamics models fluid flow using a Lagrangian method - rather than constructing a grid, as in Eulerian techniques, SPH is not constrained by prior geometry.
Rather SPH samples the fluid flow with particles, which have a compact smoothing kernel to sample overlapping adjacent fluid elements.
The modern implementation of SPH as used in astrophysics goes back to the classic paper of Hernquist and Katz (1988), who combined the relatively…
catdynamics | February 9, 2009Mark Krumholz's talk is here - video and podcast
Genevieve Permentier's talk is here - cluster formation, video and podcast.
catdynamics | February 6, 2009forming stars is easy, except for the details
Mark Krumholz is schooling us on the intricacies.
Awesome movies...
Binary star formation animation
Making a BIG star
Turbulence, dood - that one is just very pretty, nothing much to do with the talk.
catdynamics | February 6, 2009Yesterday, there was much talking about blue stragglers.
All preserved for posterity.
Francesco on Hubble observations of blue stragglers in galactic globulars - and VLT and GALEX too.
Barbara on more observations and modeling too.
Nathan on modeling and the mystery correlations in the amazing Nature paper too.
All with live high res video also.
This morning we move on to global properties of globulars and star formation processes.
Hm, globual is actually a potentially useful word, methinks.
catdynamics | February 5, 2009Blue straggler stars are, formally, main sequence stars that are too blue and bright on the colour-magnitude diagram - they are more massive than they ought to be given their age, is the other way of looking at it.
They are found in the field, and you might rightly ponder how we can tell, but in globular clusters there are lots, they really stick out, and they are rather fascinating.
And they make great Hubble Space Telescope targets, being rather blue, in crowded regions and teach you about physics, stellar processes etc.
Blue Straggler Stars (wiki):
Nice illustration of Blue Stragglers…
catdynamics | February 3, 2009What is string theory anyway?
Well, now you can find out.
Joe Polchinski at UCSB just gave the Director's Blackboard Seminar at KITP, and as usual it is webcast and podcast.
Go be inspired.
catdynamics | February 2, 2009Globular Clusters - what drives the evolution of the mass function?
Mark Gieles take is online here
Mike Fall's preceding talk on the subject is not available online. Should have been there...
catdynamics | January 30, 2009in which we contemplate a compact accretor and a low mass donor, in theory
if you have a compact object, like a neutron star or black holes, then the potential energy at the surface is very negative, so any mass that falls onto the compact object may release a large amount of energy per unit mass
since this large amount of energy may be radiated (must in the case of a neutron star) and the object is compact, the characteristic temperature of the object is high - so they are typically x-ray bright
however, it turns out to be surprisingly difficult to get a low mass star to be "in contact"…
catdynamics | January 29, 2009in which we ponder the globular cluster mass function and whether the current mass function really is lognormal and how this came about given that everybody but everybody believes the initial mass function must have been a power law...
wednesday afternoon was a good introduction and lively discussion of theoretical processes affecting globular cluster lifetimes and mass evolution - including some pdf of slides.
Just so we all remember what we are talking about:
log-normal
power law
people in other fields might know these as Somebody's Law, but physical scientists do not deign to name the…
catdynamics | January 28, 2009Cosmic Collisions: Galaxies, an astronomy special, one of three,
is airing tonight (wednesday 18th January) at 10 pm eastern/pacific, 9 pm central
oh it is on the discovery channel.
PS: here is the full schedule for Cosmic Collisions
the associated discover space web page
I hear two more "Cosmic Collisions" shows may be shown, don't know when.
I haven't seen the final version, but they had lots of cool graphics and animations, and inteviews with terribly distinguished and erudite scientists explaining clearly and cogently the wonders of it all.
Er, and me.
So, if you can't get to sleep, now…
catdynamics | January 28, 2009I missed the session on tuesday of "Short Talks: Structure of GCs and Their Dynamics",
fortunately the audio and video feed is here for posterity
Fujii, Varri and Waters present.
catdynamics | January 28, 2009how big is a globular cluster?
There are many ways to measure the size of a globular cluster, both observationally and theoretically.
This causes some confusion, especially when theorists try to talk to observers, or vica versa.
rc - the core radius
there are at least three definitions of this:
1) the observer definition is that this is the radius at which the surface brightness has dropped to half its central value; this assumes the surface brightness is flat at small radii and the core is resolved, otherwise the central surface brightness is underestimated by the convolution of the point…