## Sunday, June 22, 2008

### Summer time

The semester has ended and it's time for the summer adventures, certainly this summer promises to be loaded of adventure for me. Hopefully the posting frequency will increase, which is quite a good thing as this blog has been rather inactive lately.

Right now I have just arrived to CRyA (Center for radioastronomy and astrophysics) to continue a collaboration on hypercompact HII regions (this regions are where massive star formation takes place, more on that on a some near post). The cab driver as soon as he heard I was an astrophysicist (student, of course) instantly replied: "but that's just an awful lot of f.....n maths", and proceeded to tell me the stories of "all those crazy geniuses" like Dali, Einstein, Newton and that "nut economist" (don't ask me who that nut economist is), at the end of the trip I guess he had the impression that he thought I'm every bit as crazy as them (but way dumber than them), which is fine, at least I'm in good company.

## Monday, April 21, 2008

### Join the PAM!

There is a nice project started by Julien Girard called wikiPAM, the idea behind this project is creating a wiki for the mexican astronomical community (the name comes from Proyectos Astronómicos Mexicanos, or Mexican Astronomical Projects). Anyone doing any sort of research in astronomy with a mexican group can get an account there and post their work.

This portal has a lot of potential, you can post your code (or modifications to it), your data, an announcement for your future projects... It seems to me like a wonderful tool for a grad student looking for a thesis project.

As far as I know it is possible to post there information only accessible to people in the same research group, so you can also use it to share confidential information to the rest of your group without anyone else seeing the data before it is made public.

There is also a portal for astronomical societies (i.e. amateur astronomers) in Mexico, it is called Cosmowiki. If you can read spanish look at this interesting account of the first mexican connection to internet that I found at cosmowiki, clearly showing its potential for outreach.

I conclude this small post inviting you to join this nice idea, you "only" need to be involved in research that somehow has the participation of the mexican astronomical community in it (for example having mexican astronomers in your group, or using mexican instrumentation like the LMT).

## Monday, April 14, 2008

### National astronomy meeting

Tomorrow starts the national astronomy meeting, this is the only event of the year when all the astronomical community meets in a single place and it's also a great occasion for seeing old friends from other institutions. I'll be participating with some of my work in regions of massive star formation.

Hopefully I'll be posting soon about the interesting stuff that emerges there.

## Saturday, March 01, 2008

### A new kind of supernova

We are sure that at least two kinds of astrophysical black holes exist, one kind correspond to black holes of stellar mass (around 8 times the mass of the sun or bigger) and the black holes at the centers of galaxies with millions of solar masses. For some time there was speculation about the existence of some intermediate mass black holes with masses around few thousands the mass of the Sun, evidence from the rotation of stars in globular clusters gave the first evidence for the existence of intermediate mass black holes.

Despite evidence from the rotation of stars around the centers of globular clusters, the case is far from closed, the centers of globular clusters are regions of high density, filled with degenerate stars (mostly white dwarves) and maybe the rotation speeds can be explained by an overabundance of stars. A new paper (available here) by Stephan Rosswog, Enrico Ramirez-Ruiz, and William R. Hix proposes a radically new method for searching this black holes.

In close binary systems composed by a red giant and a white dwarf, the white dwarf can steal some matter from the red giant star (which has a low surface gravity), matter falling into the white dwarf has angular momentum and as a result of that an accretion disk is formed, white dwarves are supported against gravitational collapse by electron degeneracy (essentially if you try to pack electrons too closely they will start to move faster as a consequence of the uncertainty principle), this degeneracy can only support a mass of about 1.4 solar masses (otherwise the electrons would move faster than light), this is known as the Chandrasekhar limit, so when the white dwarf has "stealed" enough mass from its companion it will eventually surpass this limit, in this case the star is reignited by a mechanism known as carbon deflagration, creating a supernova explosion of the IA type.

Now, enter intermediate mass black holes. Computer simulations including gravity, hydrodynamics and nuclear physics show the effects of a close interaction between a white dwarf and an intermediate mass black holes. The star is heavily disrupted by tidal effects and acquires a pancake-like form, as the star is squeezed there is a dramatic increase in pressure and the star is reignited and creates a new kind of supernova, around half of the mass is ejected and the other half falls into the black hole creating an accretion disk that should emit x-rays.

This image shows the interaction of a white dwarf and a black hole, the star is heavily deformed in the first two frame, in the intermediate frames the star reignites and explodes, the "bubble" is the ejected material and at the bottom left corner an accretion disk is formed, the last two frames follow the evolution of this disk.

While this supernovas should be much scarcer than usual IA supernovas, new surveys like the LSST should detect enough supernovae to have a good chance of detecting this events, and x-ray emission should be detectable by the Chandra Space Telescope. The light curve should be different, although I haven't seen any detailed model. Considering that globular clusters are composed mostly of old stars and large populations of white dwarves this events should be happening relatively frequently around the universe and might give us unequivocal evidence of intermediate mass black holes.

## Sunday, February 17, 2008

### Back to classes

This weeks have been a bit hectic, the new semester has started and I have been busy handling endless tasks (which is why I haven't posted much in these days).

This semester I will lecture for half of the general astronomy course, mostly about celestial mechanics (Keppler's laws, tides, Roche lobe and Lagrange points) and some practical optics (mostly the types of telescopes and the effect of diffraction on a telescope's resolution). The later half of the course is about stars and galaxies but I won't be lucky enough to lecture about that.

This seems to be a good place to hear your opinions/ideas about this kind of courses, I have always believed that these courses should be accessible to sophomores (or even freshmen), and with an emphasis on stars, galaxies and the universe more than an "applied physics" course (specially with central forces being discussed in most classical mechanics courses and the Rayleigh limit in optic courses), this could serve the double purpose of exposing students to the wonderful ideas of astronomy even if they lack a strong physics background and help to lure students in to the more advanced astronomy courses. What do you think?

## Friday, January 25, 2008

### Star formation 101. Part 1

The time has come to pay one of the old debts of this blog: A mini-course on star formation. The night sky is full of little twinkling stars, but despite their obvious abundance its formation is still far from being a closed case.

Gravitational collapse (a cloud of interstellar gas collapses and produces stars) is the obvious way to produce stars , but it leaves us with a startling question, we can estimate the time of this collapse using the expression:

$t_{ff} \sim \frac{1}{\sqrt{G \rho}}$

which is called the free fall time and as you can see it only depends on the density of the cloud and not of its size, for a typical cold cloud this time is around one million years, this is obviously problematic because we know galaxies are much older than that and we can observe star formation in practically every spiral or irregular galaxy.

This indicates us that other forces are acting on molecular clouds, slowing the process of gravitational collapse. There are four hurdles that star formation has to overcome, as we will see in this star formation series it is surprising that even a single star can overcome this hurdles! This hurdles are:

• The pressure hurdle: A homogeneous, spherical gas cloud is stabilized against collapse by its own pressure gradient. This means that the temperature of the cloud despite being small still implies enough energy for the particles in the cloud to overcome their own gravitational attraction. We can estimate the size at which the cloud is doomed to collapse, this is called the Jeans radius and is given by

$r_J=\sqrt{3kT/4 \pi G m^2 n}$

where k is Boltzmann's constant, T the temperature of the cloud, m the mass of the particles and n the numerical density of particles. The Jeans mass is simply $m_J=(4\pi/3)\rho r_J^3$ , when we plug numbers into it we realize that stars similar to our Sun require cooler and/or denser regions than the average of the interstellar medium. This regions are the molecular clouds.
• The dynamical hurdle: As the cloud collapses it is heated, the energy required to heat the cloud comes from its own gravitational potential and it eventually reaches an equilibrium state when

$2K+V=0$

where K is the kinetic energy and V the potential energy, when this condition is satisfied we say that the cloud is virialized. So we can see that star formation requires cooling, otherwise the cloud will get virialized and reach a stable configuration halting the collapse. Thermal conduction and convection are remarkably ineffective in this regions and most of the cooling is radiative.
• The angular momentum hurdle: As gravitational collapse shrinks the cloud, angular momentum conservation amplifies any initial angular momentum (which is usually of the same magnitude of galactic rotation) by an enormous amount, the spin frequency is amplified by around 10¹⁶, this is such an spectacular increase in the spin that the cloud should be teared apart by the centrifugal effects. Dissipation is required to get rid of this excess of angular momentum, it is thought that this excess leads to proto-stellar disks.
• The magnetic flux hurdle: Lorentz force implies that charged particles are free to move in the direction of the magnetic field but have a hard time moving perpendicularly to it, so the magnetic field behaves as a spring refusing to compression. Collisions between the charged particles and the neutral ones eventually transfer this "springy behavior" to the rest of the cloud. A process known as ambipolar diffusion eventually allows the neutral component to fall through the ionized component.
The current state of star formation is centered around two paradigms: The "standard model" based on the collapse of isothermal clouds under ambipolar diffusion and the "turbulent model" based on the redistribution of energy at diverse scales of the cloud, making some "lumps" where star formation takes place.

In future post I expect to explain you how each of this hurdles can be overcome and give an overview of this two paradigms.

## Thursday, January 10, 2008

### No asteroid impact on Mars

We ended last year commenting the possibility of an asteroid impact on Mars, this asteroid (2007 WD5) had rather small change of crashing on Mars.

The latest data has ruled out this possibility, the use of archival photos and observations from the German-Spanish Astronomical Center, Calar Alto, Spain; the Multi-Mirror Telescope, Mt. Hopkins, Arizona; and the University of Hawaii telescope, Mauna Kea, Hawaii yielded and impact probability of cero.

We will need to wait for this kind of serious fireworks in the future!

## Sunday, January 06, 2008

### Milkomeda

The local group of galaxies consists of two big spiral galaxies: our own Milky Way and Andromeda, and a rather small spiral galaxy known as the Triangulum and about 40 small galaxies of varied morphology.

Unlike most galaxies which are redshifted due to the expansion of the universe, Andromeda is blueshifted meaning that it is moving towards us. Eventually the Milky Way and Andromeda will collide, this is a bit uncertain because the only way to know for sure if the local group is bound we need to measure the radial and transverse velocities of its members, we can measure the radial speed from the redshift but transverse speeds are quite complicated to measure. Despite that, the measurements of the radial velocity of Andromeda are of 120 km s-¹ towards us and a transversal velocity of around 100 km s-¹ and it is certainly smaller than 200 km s-¹ (Loeb A., Reid M. J., Brunthaler A., Falcke H., 2005, ApJ,633, 894). Using this values we can conclude that the system is indeed bounded and that the merger is quite likely to happen.

The galaxies of the local group. Andromeda is clearly the biggest member (this doesn't means it is the most massive, we have reasons to believe that the Milky Way has more dark matter and is more massive). The only other major galaxy is Triangulum which is significantly smaller. The rest of galaxies are small and many of them have irregular morphologies like the Small Magellanic Cloud.

Kahn and Woltjer pioneered the study of the dynamics of the local group (Kahn F. D., Woltjer L., 1959, ApJ, 130, 705), they argued that Andromeda and the Milky Way formed quite closely and then separated with the expansion of the universe, then started to approach each other due to their gravitational attraction. From this suppositions they were able to estimate the mass of the local group and the size of the orbit.

A detailed simulation of this merger has been produced by T.J. Cox and Abraham Loeb (arxiv:0705.1170v1). They used a model of the local group proposed by Kyplin et al (Klypin A., Zhao H., Somerville R. S., 2002, ApJ, 573, 597) which has as much as 20 times more dark matter than baryonic matter. The diffuse intragroup medium was supposed to have a mass comparable to the mass of the galaxies. The simulations were carried with the GADGET 2 code (if you are computer and astro savvy, you can download this code and run your own simulations of astrophysical phenomena in your computer).

In this simulation we have the first detailed scenario for the Sun as the merger happens. This merger will start in less than 2 Gyr, first with tidal interactions that will create a stream of matter between the MW and Andromeda. As we mentioned in a previous post, the Earth will be out of the habitable zone in about 1.1 Gyr, unless some advanced civilization enlarges the radius of Earths orbit (Korycansky D. G., Laughlin G., Adams F. C., 2001,Ap&SS, 275, 349). Despite that, let's continue to discuss the fate of the solar system.

During the first close encounter, there is a 12% chance that the Sun will be pulled out of it's current position in the outer arms of the MW and reside in the extended tidal material between the MW and Andromeda, during this phase we expect a burst of star formation. After the second encounter the chance of residing in the tidal material rises to 30% and a more interesting outcome arises, there is a 2.7 % chance that the Sun will be captured by Andromeda. In this scenario any astronomer in the Earth will be able to see the MW (or rather its remains) from Andromeda in the night sky.

This is the simulation by Loeb et al. You can see that the collision won't be a head-on merger, but rather the two galaxies will spiral into each other, the final result of the merger is an elliptical galaxy.

After the merger is completed, the simulation suggests that the Sun will habit in the outer halo of a massive elliptic galaxy, which Cox and Loeb call Milkomeda. This is only a possible scenario using realistic assumptions about the local group, in their paper Cox and Loeb report a dozen of additional runs with different values of the density of the intragroup medium and the transverse medium and find that the outcome is esentially the same. The resulting galaxy has the R^(1/4) brightness distribution that is typical of elliptical galaxies, so our own local group will act as a prototype of the late forming elliptical galaxies.

Tomorrow the AAS anual reunion starts, so we can expect some nice news in astronomy for the next week!