In the late morning, there was an absolutely great Milky Way Group meeting. I love this forum. Ted von Hippel (Embry-Riddle) walked us through the use of white dwarfs as clocks, and the method by which one could constrain (very precisely, in principle) the star-formation history of the Solar Neighborhood, especially post-Gaia. He argued strongly (and correctly) that the right method is not to make a histogram of temperatures! Some of the physics is non-trivial: The less massive WDs are larger, so I would expect them to cool much faster (more surface area, less stuff to store heat), but apparently this isn't so, for detailed reasons about heat capacity.
Also in that meeting, Nicholas Martin (Strasbourg) told us about the possibility that some of the new ultra-faint dwarfs being found in the Local Group might be LMC or SMC satellites, and Haijun Tian (MPIA) showed us the latest on proper motions from PanSTARRS. On the latter, even the first data release from Gaia (that only releases positions for the relevant stars) will be hugely informative.
Mark Allen (CDS Strasbourg) told us about the structure and projects at the CDS, which is one of the most important and long-lived archives and data centers in astronomy. The CDS is composed not just of astronomers, but also data librarians, concerned with long-term preservation and correctness. The CDS is going to be one of the Gaia data-release points, so we pressed him for details!
Over lunch, I discussed with Sara Rezaei (MPIA) and Coryn Bailer-Jones (MPIA) and more of the MPIA Gaia group their non-parametric model for Milky Way dust, which my loyal reader will remember from discussions in summers past. Rezaei has a working system, but (as predicted) the linear algebra is extremely expensive. I showed them a method to collapse their problem to a smaller problem that is instantiated only at the stars, and nowhere else (that is, to live the non-parametric dream!). If we are lucky, this might give them speedups of hundreds or more. Here's hoping.
Yes, this fact about WDs totally confused me at first too.
ReplyDeleteThese sentences from this paper (http://iopscience.iop.org/0004-637X/520/2/680/pdf/39339.web.pdf)
explain it:
"The cooling of helium core white dwarfs is conceptually similar but contains important differences. The first is that the heat content stored in the nondegenerate ions (kTM/Amp) is proportional to the total number of ions for fixed temperature. This means that helium cores (mass number A = 4) contain more heat than do carbon or oxygen cores (A = 12 and 16), and thus the helium core dwarfs are brighter at fixed age."