Bill to Refinance Student Loans

Posted on by Alexa Villaume

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

 

Email from MMO last night

Posted on by Alexa Villaume

Hello Alexa

We had a clear night tonight, first one since AAS meeting. Anisha arrived, but has come down with a bug from all the delayed travel. We did observe RR Tau tonight. Did it in 7 filters at Vladimir’s request. Ha300, Haw, Han, Ha30, R645, B and V. Got 12 images in each filter. Was a very nice night. Expect good PT. I am going off island for the weekend, but will be back on Monday to teach Anisha how to reduce the data.

We plan to repeat the above observing scenario on every clear night for the next month.

 

Literature Review #8: Self-Shadowed Disks

Posted on by 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.

Literature #7: Analyzing [OI] lines

Posted on by Alexa Villaume

The structure of protoplanetary disks surrounding three young intermediate mass stars

I. Resolving the disk rotation in the [OI] 6300 A line

G. van der Plas, M.E. van den Ancker, D. Fedele, B. Acke, C. Dominik, L. B. F. M. Waters, and J. Bouwman 

HD 172555: detection of 6300 A [OI] emission in a debris disc

P. Riviere-Marichalar, D. Barrado, J.-C. Augereau, W.F. Thi, A. Roberge, C. Eiroa, B. Montesinos, G. Meeus, C. Howard,G . Sandell, G. Duchene, W.R.F. Dent, J. Lebreton, I. Medigutia, N. Huelamo, F. Menard, and C. Pinte.

These two papers caught my eye because the both focus on analysis of [OI] line in circumstellar disks. The van der Plas et al. paper focuses on three Herbig AeBe stars while the Riviere-Marichalar et al. paper focuses on a single star that is a member of the Beta Pictoris moving group. Both papers address the importance of studying the gaseous materials in disk, van der Plas et al. noting that 99% of disk mass is gas, but they diverge in their interests in the [OI] line. It’s interesting to read these two papers together because both teams are essentially studying the same thing, the very beginnings of the formation of planets, but the stars they’re choosing to study are at different evolutionary points. This difference is reflected in their different foci. van der Plas et al. use the [OI] to refine the determination of the disk structures of their three stars while Riviere-Marichalar et al. are focused on determining the composition of the gas in the debris disk.

(I’m going to take an aside and wonder about the disk in the the Riviere-Marichalar paper. The star, HD 17255, is a part of the BMPG and as such as an age around 12-20 Myr so I’m curious to why it has a disk at all. A previous paper I read said that these types of disks dissipate long before that. So why does this star have a disk at all?)

Riviere-Marichalar et al. derived the [OI] mass in the disk by first creating an SED by compiling the observations from numerous sources. By fitting the SED with a blackbody spectrum the authors were able to determine the radius of dust distribution. Then from computing the infrared excess and the mass of the dust disk were able to determine the final dust mass (I really don’t understand how these calculations were done). From there the authors compute the [OI] mass (the paper has an appendix where they go over this computation that I haven’t gone over enough to understand).

The authors of the other paper followed a method created by Acke et al. (2005) which allows for determining disk rotation and distribution of gas in the disk. I haven’t read that paper (it is on my reading list) and van der Plas et al. leave the details of that method out of this paper.  They essentially have expected disk shapes from the SEDs for each star and then they refine the structure of disks using the [OI] line by finding the distance from the star that the [OI] line was emitted.

Both papers discuss the possible origins of the [OI] line – the thing that I’m most interested for my project. van der Plas et al. split the origin of [OI] between broad and narrow lines. According to them, broad lines in T Tauri stars are, “due to a combination of a dense stellar jets and a disk wind or magnetic accretion columns.”  Then they go onto to say the narrow lines in HAeBe stars (I’m not sure if they are saying, in general, T Tauri stars have broad lines and, in general, HAeBe stars have narrow lines or not) are from the photodissociation of OH in the upper layers of the disk.

Both of these papers used the [OI] line in interesting ways. And they both made mention of the difficulty in detecting the line so I’m eager to reduce the data I’ve gotten so far and see if I’ve picked up on [OI]. I will definitely have to think about how each of these terms used the [OI] line and if and how I can apply either of these methods.

Literature Review #6: Parameters Herbig Ae/Be and Vega-type stars

Posted on by Alexa Villaume

B. Montesinos, C. Eiroa, A. Mora, B. Merin

Previous studies involving the modellin of circumstellar disks, including the previously discussed Manoj et al. (2006), have used basic parameters for the central star such as assuming solar metallicity. This will likely lead to incorrect results since the energy received by the disk from the star, determined by the star’s mass, radius, and effective temperature, affects disk characteristics such as geometry and contribution to the SED. The goal of the paper by Montesino et al. is to determine the stellar characteristics of 27 Herbig Ae/Be stars. The characteristics they found are effective temperature, surface gravity, and metallicity. This was interesting for me to read about because, as the paper points out in its introduction, that the circumstellar disks for T Tauri stars and Herbig Ae/Be stars dissipate at different rates. The disk of a classical T Tauri star will be gone when the star is 5-7 Myr while the disk of a HAeBe star will be gone when the star is 3 Myr. I don’t know, and I think nobody knows, why the lifetime of the disks is so different from the low mass to intermediate mass stars. An accurate model of CSDs based on accurate source parameters is very likely key to understanding this.

The basic method that the authors used to characterize the properties of the stars is to compare observed spectra with synthetic spectra. They observed the sample set using the Calar Alto Faint Object Spectrograph (CAFOS) to get intermediate resolution spectra to use Balmer line profiles in order to estimate stellar gravities. To determine the metallicities of the stars high-resolution echelle spectra was taken with the Utrech Echelle Spectrograph and the William Herschel Telescope.

The first parameter that needed to be found was the effective temperature of each star. To do this the authors compared observed spectral energy distributions with a grid of low-resolution synthetic spectra with different effective temperatures and then choose the best fit.

Stellar gravities, which is the surface gravity of the star not the potential well created by the star, were found by comparing the wings of the Balmer lines with synthetic profiles from Kurucz (1993). Since the wings of the lines are related to how the gas is moving a high surface gravity corresponds to a broader line.

Metallicities were found by comparing synthetic spectra found using SYNTHE (Sbordone et al 2004) with observed high-resolution spectra. Basically the made different spectra with the same effective temperature and surface gravity of the stars computed for different metallicities with the observed spectrum.

In the paper the authors specifically point out RR Tau for the difficulties involved in using the above methods to determine its parameters. Because RR Tau varies in a complex way the authors tried to single out spectral features that had as little variation as possible, features that would originate from the photosphere, to determine the parameters.

The paper presents a single SED for RR Tau and I don’t think it can ever be completely accurate to present a single thing of anything for RR Tau. I understand that they tried to minimize the influence of the variation but I think it would be better to make SEDs of RR Tau at many different places in its brightness. I talked to Hugh about the idea of doing this with HYPERION and my Gemini observations but he said that because my wavelength region is so narrow it’s unlikely that the SED shape will change that much. So he suggested I do full spectral fits instead of SEDs using line indices. So I’m going to start looking into that.

A late Christmas present

Posted on by Alexa Villaume

I wrote this because we wanted to do Secret Santa this year but Joe’s daughter’s live in California and wouldn’t be getting here until Dec 22nd. Joe used Bandcamp email voodoo to use the file generated to send the emails anonymously.

# Pair names for secret santa presents

import random as r
import sys

f = open('TestSanta.txt','w')

names = ["Joe","Alexa","Hannah","Xander","Kate","Soley","Max"]
secretsanta = ["Joe","Alexa","Hannah","Xander","Kate","Soley","Max"]

for name in names:
        x=r.randint(0,len(secretsanta)-1)
        receiver=secretsanta[x]
        while name is receiver:
                if len(secretsanta) == 1:
                        print "Run again"
                        sys.exit(0)
                else:
                        # If you get your own name put it back and pick again
                        # Want to go back to beginning of loop
                        x=r.randint(0,len(secretsanta)-1)
                        receiver=secretsanta[x]
        f.write(name +','+ receiver + '\n')
    del secretsanta[x]

f.close()

Literature Review #5: Emission Lines and Accretion Disks

Posted on by Alexa Villaume

EVOLUTION OF EMISSION-LINE ACTIVITY IN INTERMEDIATE-MASS YOUNG STARS
P Manoj, H.C. Bhatt, G. Maheswar, S. Muneer

This paper is centered around the question: How long does emission line activity persist in young stars? This is a relevant question because emission line activity is associated with the presence of an active accretion disk around the star in both T Tauri and HAeBe stars. Since the matter that feeds accretion disks is taken from the the gas rich disk that surrounds the star we can characterize how long it takes Circumstellar disks to get to the planet forming phase by looking at when their emission line activity drops off. As the circumstellar disk loses material to the beginning stages of planet formation and other processes the rate of accretion goes down. This paper cites various other papers to say that the most stars lose their inner disks by 5 Myr. They don’t discuss what happens to to the outer disk and they also don’t explicitly say that once the inner disk is gone so is the accretion disk.

The authors compare low-mass T Tauri stars to intermediate mass HAeBe stars. Apparently, the origin of emission lines in pre-main sequence T Tauri stars is understood to be from magnetospheric accretion (Uchida & Shibata 1985, and several other papers). There is no such general understanding for emission lines in HAeBe stars but it is thought to be similar. In Muzerolle et al. (2004) the authors were able to apply the magnetospheric accretion model to HAeBe stars.

The Manoj et al. sample includes 91 HAeBe stars including RR Tau and a couple other UXors such as KK Oph. They found the equivalent width of H-Alpha for each of these stars and compared those values to the stellar age of each star. They found that H-Alpha line strength decreases with increasing stellar age and thus accretion gradually declines during the pre main sequence phase. They also find that inner disks dissipate on a similar timescale which they say suggests that inner disks dissipate after the accretion activity has fallen below a certain level. This seems like a backwards interpretation to me if it’s the inner disk is what feeds the accretion disk.

I’m also curious about the validity of only including one equivalent width value for H-Alpha per star. For UXors like RR Tau the equivalent width of the H-Alpha line varies so it’s curious to me at what point in RR Tau’s variation did the authors get the equivalent width.

October Project Update

Posted on by Alexa Villaume

My proposal was accepted and I have completed the Phase II process. So now I’m just waiting for reasonably bad weather on Mauna Kea.

I’m getting started on my Hyperion model. I have Hyperion almost installed except for the FORTRAN dependencies. In the meantime I’m just mocking up a very simple model which I’ll write about once I can run it.