You might have heard that Senator Warren is introducing a bill to allow for student loan refinancing. This is very important in easing the debt burden of millions. The New York Times has a good write-up of it a long with a couple other things that are in the works to help with student loans –

http://www.nytimes.com/2014/06/08/us/politics/obama-plans-steps-to-ease-student-debt.html

It’s obviously going to have a hard time in the Senate and, if it makes it, in the House. So it’s important to write to your Senators to urge support. Here’s the letter that I wrote for Senators Boxer and Feinstein, 

 

Dear Senator ,

 

My name is Alexa Villaume and I am writing to urge your support of the bill sponsored by Senator Warren to refinance student loans at lower interest rates.

 

I graduated in 2013 and like many people my age I had to take on debt to get complete my undergraduate education. I am lucky in that I am able to make timely payments on my student loans that cover the principle in addition to the interest. Not everyone is so lucky and it takes a significant amount of my monthly take home pay to cover my payments.

 

My experience is the experience of millions. We went to college to make a better life for ourselves and many of us our held back because of debt burdens. We can’t buy cars and certainly not houses. We can’t take risks in our professional lives. I trust you understand that this is not a sustainable way for our country to proceed.

 

I know that many Senators and Representatives would say that we cannot increase taxes on these so-called “Job Creators”. But what about the young people who want to be job creators but can’t take the risk? What about the young people who want to buy houses and start families but can’t? Easing the debt burden would help millions who have graduated in recent years and that in turn will help make a more equal and productive country.

 

Best regards,

Alexa Villaume

 

Explaining UX Orionis Star Variability with Self-Shadowed Disks

C.P. Dullemond, M.E. Van Den Ancker, B. Acke, and R. van Boekel

2003

 

The purpose of this paper is to delineate the problems with the current models of where the dust filaments responsible for the UXOR characteristics originate and propose a better model. The models that the authors are critiquing are that the obscuration events are caused by protocometary clouds or comets. Which they reject noting that the model is not consistent with observed SEDs of UXORs. They then go into discussing the idea of protoplanetary disks around the stars and what geometry could explain the observed phenomena. The main proposal for the disk geometry around these stars was a flared disk. However, if that was the case then the obscuring dust would be in the outer regions of the disk and thus would not be able to account for the variations seen in UXORs. Even adding a puffed up inner rim like Natta et al. (2001) then flared disk would still obscure the star totally and thus would not be seen as HAeBe star. So it’s a logical impossibility for UXORs to be the product of a flared disk.

What the authors propose is a self-shadowed disk with a puffed up inner rim.

[Aside: I really need to get some sort of drawing program on my computer so I can make diagrams.]

The puffed inner rim would be the source of obscuring clouds and the outer, self-shadowed part would not obscure the cloud and inner rim.

What the authors do to provide evidence for this model is try to correlate SED shape with UXOR stars. Our of 86 HAeBe stars they determined that 47 belong to Group I and 39 belong to Group II. Then by defining UXORs as stars spectral type B9 or later and experience variations of ~1 magnitude on time scales of days or weeks they determined that 10 UXORs are in in Group II and 1 in Group I. The authors associate self-shadowed disks with modest FIR excess. So they conclude that “UXOR-type phenomena should only occur in in self-shadowed disk,”

I think that overall the self-shadowed disk is an good way of looking at the system. And I have now sorted out most of my confusion from my  previous post. It’s not disk geometry vs. obscuration by dust clouds, it’s about understanding where in the disk geometry the dust clouds originate. And I think I’m understanding that these “dust clouds” or “obscuring clouds” are the planetesismals that other papers refer to.

There are a couple things that bother me about this paper. First, the sample in this paper did not include any T Tauri stars and T Tauri stars produce UXORs as well. So T Tauri stars would have to have a self-shadowed disk. The authors touch on this point at the very end of the paper by qualitatively saying that there would be a lot less T Tauri UXORs than HAeBe UXORs. Which is true but it is unsatisfying that they did not go into understanding how a T Tauri star would produce the same disk structure of a HAeBe star. Also, I just want to double check there use of Group I and Group II.

And the talk about the different time scales from the different dust cloud origins interested me. I wonder where dust clouds would have to originate to explain the ~100 day variation we observed at MMO.

As part of my independent study I’m going through “Radiative Processes in Astrophysics” by George B. Rybicki and Alan P. Lightman as a way to inform my work. This is in tandem with the other things I’m doing like the literature review and working with HYPERION so this is a pretty light introduction. I plan on working on one problem for each chapter that seems particularly informative or relevant. This first problem is from chapter 1 Fundamentals of Radiative Transfer and it’s problem 4.

1.4a] Show that the condition that an optically thin cloud of material can be ejected by radiation pressure from a nearby luminous object is that the mass to luminosity ration (M/L) for the object be less than,
where G = gravitational constant, c = speed of light, k = mass absorption coefficient of the cloud material (assumed to be independent of frequency)
To begin let’s assume that the luminous object is spherically symmetric. In that case we can use the standard definition of flux,

From equation (1.34) the force per unit mass is given by,

Since cloud is independent of frequency of the frequency dependencies falloff and we’re left with,

In order for material to get objected the radiative pressure needs to be larger than the force of gravity being exerted from the object to the cloud,

Then simple algebra shows that,

1.4b] Calculate the terminal velocity v attained by such a cloud under radiation and gravitational forces alone, if starts from rest and a distance R from the object.

Terminal velocity is when the gravitational force is equal to the counter force (for skydivers it’s drag, for this example it’s the radiation pressure). So let’s make a force called Geff that’s the radiation pressure subtracted from the gravitational force. This is the force that the cloud actually feels. We can set the Geff force equal to the general velocity equation and solve for v.

1.4c] A minimum value for kappa may be estimated for pure hydrogen as that due to Thomson scatter off free electrons, when the hydrogen is completely ionized. Show that the maximum luminosity that a central mass M can have and still not spontaneously eject hydrogen by radiation pressure is,

This limit is called the Eddington limit! And it’s really cool! It is usually spoken about the context of the accretion disks of black holes. But do you know what else has accretion disks? Pre-main sequence stars such as RR Tau. Basically, when you have a black hole (or star or what have you) with an accretion disk there are photons coming off the disk. This is why Quasars are so bright. Let’s think about why accretion disk have energy to create photons in the first place. An accretion disk forms when an object with a deep gravity well starts sucking objects toward it. The object it has gravitational potential energy which then gets converted to kinetic energy as it gets closer to the object. This is why accretion disks spin and why they’re luminous. The Eddington Limit basically sets a limit for the rate of accretion. The reason is clear. In our everyday life we’re not knocked over by the light coming off of our lamps even though the the photons coming from our lamps have radiation pressure it just simply isn’t enough to affect us in any significant way. But the the combined pressure of photons from an accretion disk can add up to be enough to start counteracting the force of gravity. So when the radiation pressure is larger than the gravitational force the Eddington Limit has been exceeded. When radiation pressure is equal to gravity that is the Eddington Limit. The answer to this problem is simple. You just make the solution to part a an equality as sub in the mass scattering coefficient for kappa.

After putting a lot of thought into my fall tutorial the plan is all set (mostly) and the paper work is filled out and going down the line of approval. I decided to use HYPERION which is a radiative transfer program written by T. Robitaille. It’s really new. The paper introducing the program came out in 2011 and the actual code library was released just within the past few months. I think it will be good for a few reasons 1.) It appears to cull the best parts of former radiative transfer libraries and combine into one package 2.) It has a python wrapper which I, the user, use to make my models 3.) The documentation seems really thorough especially given how new it is.

Instead of just “playing in parameter space” I’m going to actually try to make a model of the circumstellar environment of RR Tau. HYPERION produces synthetic SEDs and images as its results so it might be something that could be useful in conjunction with the observational data. Still, the primary goal is to learn about radiative transfer and understand these environments is greater detail which I think HYPERION will be perfect for.

I spoke with Bernadette today about this and she was very helpful in getting me started with how to approach this project. One of things we talked about is how current SED models of Herbig Ae/Be stars have dips in the SEDs (I’ll find a plot of this to put here to make this clearer) and that has been used as an indication for gaps and holes in the disk. The problem with suppositions like this from SED models is that they are not unique solutions so there is an uncomfortable level of uncertainty. The reason for this is because SEDs are the result of blackbody modeling so the inputs are distance from the material and the density of the material. From the way Bernadette described it the models pick the temperature distribution, I’m unclear of the specifics but basically the lack of constraints prevents the models from being rigorous.

One of the interesting things about HYPERION in light of this is that many of the inputs for the models get into the composition of the material in the system. It will be interesting to see how the results I get compare to previous SEDs produced from models.

Another thing that Bernadette brought up is that for SEDs of disks my optical monitoring won’t be useful in constraining the model because disks fall in the IR region. So I’ll have to see what data is already available from places like ISO and Spitzer.

CLOUDY came up in our conversation as a possible code library to look into. I actually have worked with CLOUDY before when I was doing work on quasar absorption lines at UMass. I was using it to get metallicity values from column densities but Bernadette said that it’s good for understanding winds. That’s definitely something I’m interested in but I think for just the sake of time (and sanity) I’ll focus on HYPERION for now.

Also, I got many many many papers and authors to read. I’ll be starting up Literature Reviews again with a vengeance.

Joe and I moved back from Hawaii to Bennington and I finally met with Hugh. We’ve been meeting semi-regularly the past month or so. He’s been giving me feedback on my proposal for Gemini time as well as helping me sort out the specifics of my tutorial in the Fall. The earliest I’ll be able to start the actually work on my project is probably Field Work Term because I probably won’t have data, or at least a good amount of data, until then. I still want to be doing something with the project in the Fall though. As I’ve said before I want to understand more about the physics of these environments. The current idea for how to go about that is I work with already built radiative transfer code and essentially I’ll play within the parameter space of the program.

I’ll have to some sort of contained project relating to this, it’s not enough to just “learn”. So I think I’ll write a report detailing how changes in the parameters influenced the synthetic observations and why. It will be like a project I did for my Modern Astrophysics class at UMass where we had to change the physics of a stellar equilibrium program and write a report about how that changed the nature of the star being modelled.

My proposal is just about finished. I’m still waiting for approval from Bernadette. I also sent it to Gary and Vladimir. I haven’t heard from Vladimir yet but Gary emailed me back asking what kind of O I filter I want. What kind of bandpass and central wavelength. Until he asked me these questions I didn’t even consider that O I is a doublet. I have no idea if it makes a difference what line we choose to center the filter on. I emailed Bernadette about it but she’s busy with the Gemini Science meeting in SF right now.