14 November 2011

Tut-tut, it looks like rain.

Title: Measuring NIR Atmospheric Extinction Using a Global Positioning System Receiver
Authors: C. H. Blake & M. M. Shaw

Ground based astronomical observing has one major obstacle that it must over come in order to produce quality, science quality data: Earth's atmosphere. Methods to correct for atmospheric attenuation are familiar to anyone who has taken data at a ground based telescope, or who have at least studied observational astronomy. As an example, observations of "standard" stars are required in order to calibrate not only the detector, but also to differentially correct for atmospheric effects on a given night.

Figure 1: Components of atmospheric absorption presented in
Blake & Shaw (2011)
Atmospheric absorption and attenuation is particularly noticeable in the near infrared (NIR) where absorption bands due to the molecular species in the atmosphere efficiently absorb much of the incoming flux. Typically, narrow band filters that have transmission peaks between these molecular bands are utilized in order to skirt around the difficult procedure of correcting for molecular absorption. 

However, in this paper, Blake and Shaw propose a very unique and interesting method for correcting astronomical images that have been affected by the absorption due to water molecules. They propose using signals from the Global Positioning Satellite (GPS) system to infer the water content of the atmosphere, allowing for more accurate atmospheric transmission modeling. Relying on the fact that GPS signals must be corrected for atmospheric attenuation, the author's propose that this may then be applied to astronomical studies to correct for the light attenuation of astrophysical sources.

Of greatest interest to the authors is the derivation of the precipitable water vapor (PWV) in the atmosphere. What is PWV? It is actually conceptually very simple - PWV is the column integrated depth of water vapor if all of the water were to precipitate out of the atmosphere instantaneously. As such, it is measured in units of length (typically mm). Basically, what your rain gauge would measure if all of the water vapor in the atmosphere directly above the gauge were to condense and precipitate to the ground. In the 1990s, it was shown that use of multi-wavelength GPS signals combined with a highly accurate barometer could lead to a very accurate derivation of PWV. 

Figure 2: An empirically derived fit for the correlation between
PWV + Airmass and the atmospheric  optical depth of water.
With numerous GPS stations set up across the United States to measure PWV, the authors were easily able to obtain PWV measurements near their location (Apache Point Observatory in NM). An empirical relation was then derived to relate PWV (and the airmass) to the optical depth of water in the atmosphere - assuming the optical depth is related directly to the amount of water in the atmosphere. As you can see on the left, the figure betrays the presence of a fairly obvious correlation (even with the absence of error bars). The empirically derived optical depth can then be input into the atmospheric transmission models, allowing for a more accurate estimation of the correction needed to remove effects due to water molecules.

Figure 3: Differential colors of over 6,000 M stars are binned
according to PWV at the time of observation.
An example correction is presented for numerous stars, but is most evident in the correction for M star colors. From a sample of 6,177 mid-M stars, the authors binned the data as a function of PWV - where the data is the deviation of the stellar color from an assumed stellar color locus. Their corrections are then applied and the result is illustrated to the right.

Overall the technique is very unique and holds a lot of promise, but is strongly dependent on the atmospheric transmission models. While they do provide a very good estimation of the atmospheric transmission, atmosphere models suffer from many uncertainties. However, the authors have demonstrated that their corrections appear to do very well - assuming M star not lying near the color locus are unaffected by other systematics (metallicity, etc). The plan is to have such corrections implemented in large survey telescopes, such as SDSS and LSST in order to allow for more accurate characterizations of M stars in the hunt for exoplanets. Not to mention corrections needed to accurately characterize the transmission spectra of exoplanets - supposing the exoplanets are in a position for this method to be possible (aka: transiting). 

10 November 2011

Chemical Evolution of TN J0924-2201 at z = 5.19

Title: Chemical properties in the most distant radio galaxy
Authors: Matsuoka, K. et al.
Measuring the chemical evolution of galaxies can give clues to their star formation histories. This is often done by measuring the metallicity of galaxies at various redshifts. However, as for all things astronomical, this becomes more difficult at high redshift. To alleviate this, studies have used active galactic nuclei (AGN) to measure the metallicities of high-redshift radio galaxies (HzRGs) because of their high luminosities. Specifically, gas clouds photoionized by the active nucleus emit lines in the ultraviolet (among other wavelengths) that can be observed in the optical. Quite convenient.

The currently accepted model of a typical AGN is as follows. Material from an accretion disk falls onto the central black hole, which is encircled by a large torus of gas and dust. Thus, if we observe an AGN from a pole, we see all the way down into the center, where the velocity dispersions of the gas create broad emission lines (this is known as the broad line region, or BLR). However, if we are fortuitous enough to view an AGN edge-on, the torus obscures the BLR, and instead we see emission lines resulting from the slower-moving gas clouds farther away from the black hole. This, naturally, is known as the narrow line region (NLR).

Figure 1: Line ratios showing possible metallicity evolution.
Studies using AGN have found no metallicity evolution up to z ~ 6. However, this may be due to the fact that many of these studies focused on the BLR, which could have evolved faster than the rest of the galaxy. Matsuoka et al. (2011), then, concentrate on the NLR of the most distant radio galaxy at z = 5.19, TN J0924-2201 (catchy name). Using the Faint Object Camera and Spectrograph (FOCUS), they detect Lyα and CIV lines, the first time CIV has been detected from a galaxy with z > 5. This indicates that a significant amount of carbon exists even in this high-z galaxy. Additionally, the Lyα/C IV ratio is slightly lower than that from lower-z HzRGs (Figure 1), suggesting possible metallicity evolution because of the higher amounts of carbon. However, this could also be attributed to weaker star-formation activity or Lyα absorption. Upper limits of NV/CIV and CIV/HeII were also measured, but these agree with lower-z HzRG measurements.

The authors also investigate the [C/O] abundance ratio by comparing observational limits of NV/CIV and CIV/HeII to photoionization models, the results of which are shown in Figure 2. Carbon enrichment in these galaxies is delayed compared to α elements, because much of the production of carbon comes from intermediate-mass stars (which have longer lives compared to those stars that create α elements). Thus, [C/O] is a good measure of star formation. The analysis finds a lower limit on [C/O] of -0.5, suggesting that this galaxy has experienced some chemical evolution. Comparison of this limit to previous models suggests an age of TN J0924-2201 of a few hundred million years old.

Figure 2: Lower limit of [C/O] abundance from photoionization models.

01 November 2011

JWST Passes Senate

Title: JWST Bill
Authors CJS Subcommittee Chairwoman Mikulski

The James Webb Space Telescope (JWST), the purported successor to Hubble, has been in the news a lot over the past year. As the budget for the project increased once again, the will of the US Government to actually finish the project was called into question. Well, after a long battle, a bill which explicitly outlines supporting and funding a 2018 launch of JWST was passed by the Senate. If you are interested in reading the Appropriations Committee's press release, see the title link above. It must still be approved by the House of Representatives, which might prove to be the more difficult task.

An artists conception of JWST. Credit: NASA/ESA

However, it is very interesting to note that not everyone is in favor of JWST (politicians, astronomers, scientists, the public, etc). There seems to be a divide since the money to fund JWST must come from somewhere - possibly other NASA projects (e.g., smaller satellites, research grants). Other are adamantly in favor of it because the scientific questions which will be answered (or plausibly answered) are important.

This topic came up at astro lunch this afternoon, before we knew about the bill passing. Now, I propose the question to the readers. Is JWST a worthwhile investment? I hope to hear all of your comments and opinions.

Magically Spotted

Title: No magnetic field in the spotted HgMn star μ Leporis
Authors O. Kochukhov et al.

Chemically peculiar stars are, well, peculiar. What makes them so peculiar? When analyzing abundances of stars (say, via spectroscopy) we generally have a good idea of the relative proportions of all the elements. There is no general rule that can be followed, as there are various scenarios which can occur that lead to stars with different relative abundances of particular elements. For instance, stars which are the progeny of the first generation of supernova that exploded in the Universe, will typically be metal poor but have a higher abundance of what are known as α capture elements (O, Ne, Mg, Si, S, Ar, Ca, and Ti). One can easily convince themselves that this is consistent with our view of Type II supernova (the death of a massive star) when considering the by-products of a run-away nuclear reaction chain. However, this is but one scenario of a few that we can use to generalize the relative abundance patterns of stars. When these generalizations are ignored by nature, we get chemically peculiar stars - stars that show a chemical signature (relative abundance pattern) drastically different from what we have come to expect.

HgMn stars are a particular class of chemically peculiar stars. It turns out these stars just happen to be late-B stars, likely a key to understanding the nature of the peculiarities. Specifically, as one might guess, they show anomalous amounts of Hg and Mn (singly ionized to be precise) - these two chemical species are over abundant. Even more interesting, is that they show signs of spot modulation (variations in the light output of the star due to temperature variations on the stellar photosphere). Spot activity is nothing new for anyone familiar with a starspot. Unfortunately, the spots on these stars are not necessarily magnetic in origin (more on this later).

Most chemically peculiar stars are thought to be fairly well understood. The physics involved in creating such star is related to diffusion physics. Plasma is constantly on the move within stars - there is no reason for stellar plasma to remain static. Gravity, temperature gradients, and concentration gradients all force particles to reorganize into the most likely configuration (maximization of entropy!). Thus, plasma particles are shifting around attempting to find this state. Well, one other type of process which can occur is called radiative acceleration (or levitation), and it turns out to solve a few problems.

B stars are hot. Super hot. About 12,000 K hot. This means that photons have a lot of energy and are capable of transferring that energy to anything in their way (say, some gas particle). Depending on the opacity of radiation to a particular chemical species at a certain point in a star, photons can actually "push" gas particles towards the surface of the star, aiding in the particle diffusion process. This substantial side-effect, strong radiation pressure, is what is thought to "correct" our standard approach to particle diffusion in order to remedy the discrepancies with chemically peculiar stars.

Unfortunately, this theory doesn't seem to do a great job of reproducing HgMn the properties of HgMn stars. Magnetic fields have also been shown to help produce chemical inhomogeneities in Ap stars (the "classic" chemically peculiar star). We then have to wonder if magnetic fields are at work in the HgMn stars. To do so, one just has to look for the presence of magnetic fields around a star. The clue which betrays the presence of a magnetic field is the amount of polarized radiation.

I am by no means an expert in E&M, so I cannot provide an intuitive description of the physical process of photon polarization. Instead, I point you to a trustworthy source. To probe a stellar environment for polarized radiation and the presence of a magnetic field, observers use a setup called spectropolarization. Essentially, there is a polarization filter which feeds a spectrograph. From this, one can analyze the profiles of particular lines which are sensitive to magnetic fields (typically lines formed due to ionized atoms). The figure on the right displays what reduced data looks like. Note the squiggly lines on the right. In the presence of a magnetic field, the amplitude of the variations if far larger - thus, the authors concluded there is no sign of a strong field around this star.

Interestingly, though, when they look at the line profiles, while there is no clear indication of a magnetic field stronger than 3 G, there is an odd shape to some of the spectral lines. These odd shapes do not appear to be an artifact of their reduction procedures, as they use the same procedures on a set of theoretical spectra in order to derive theoretical line profiles. Here is what they find:

Figure 2: From Kochukhov et al. (2011), TiII and YII line profiles from both synthetic spectra (dashed) and observed, time averaged spectra (solid).

The dashed line is the theoretical line profile while the solid line is that derived from observations. Indents can be seen near the trough of the line. The authors suggest the shape is derived from the abundance distributions being axisymmetric. This, coupled with rotation, they claim provides the explanation. However, no theoretical line profiles were developed with this hypothesis.

No matter how you look at it, though, there is a lack of a large-scale magnetic field with a strength greater than 3 G. As such, magnetic processes are an unlikely solution to the peculiar abundance pattern on the surface of these stars. To date, not good hypothesis exists to explain the anomalies we observe. The next lines of attack are to look at more rigorous radiative diffusion scenarios (time dependence) and at the effects of hydrodynamical instabilities/circulation, such as meridonal circulation.