Brian Rachford's research interests

Interstellar medium

(You can also read a non-technical description of this research.)

My ISM work has focused on conditions within "translucent" clouds, which span the visual extinction range of 1-5 magnitudes. I worked as a post-doc at the University of Colorado with Ted Snow on the analysis of FUSE data for several dozen lines of sight. The main goals were to assess molecular hydrogen abundances and how those abundances relate to other molecules and the dust. We have also studied chemical depletions of a variety of species.

Diffuse Interstellar Bands (funded)

Through my translucent cloud work, I am involved in a major project to try to understand the DIBs, which are ubiquitious absorption features associated with interstellar clouds. There are more than 600 known DIBs, but the identification of the chemical species of any of them has remained a nearly century-long puzzle since they were first identified. This research is led by Don York at the University of Chicago and involves researchers at several institutions. In September 2010 we received a 3-year multi-institution NSF collaborative grant for further studies. Part of the money will go towards summer support for undergraduate students here at Embry-Riddle. (A description of the official Embry-Riddle portion of the grant can be found here. Note that it shows up at the Daytona Beach campus because that's where the main University grants office is located.)

The main H2 study

In 2002, we published paper exploring H2 in about half of the planned FUSE sample. Our main findings were strong correlations between the abundances of H2 and carbon-based molecules such as CH, CN, and CO, the H2 molecular fraction and other density indicators such as kinetic temperature, steepness of the far-UV extinction, and the width of the 2175 A "bump" in the extinction curve.

However, we did not see conclusive evidence for single clouds that by themselves would follow the expectation for a "translucent" cloud, i.e., molecular fraction near unity and very cold kinetic temperatures in a single cloud. Thus, we suggested a change in terminology from "translucent cloud" to "translucent line of sight" unless one can establish that there is a single cloud or cloud core that is highly molecular.

In 2009, I finished a paper containing the full sample of lines of sight. The additional observations do not really change our previous conclusions, but they do cover lines of sight with a wider variety of dust characteristics. I was also involved in a study of the HD molecule.

Chemical depletions

I was involved in three papers on the abundances of elements in our translucent sample, one each for iron, oxygen, and nitrogen. These gas-phase abundances set limits on the amount of material left over for dust grains, whose composition is difficult to directly study. The finding of a particularly small gas-phase abundance implies significant deposition of that element onto grains. However, we did not find significantly lower abundances (higher depletions) as a function of increased extinction or molecular hydrogen abundance. As with our H2 results, we find that we were not quite able to reach into the regime of "translucent clouds" with FUSE.

Stellar activity

My dissertation was a study of chromospheric activity in late A- and early F-type stars in nearby open clusters, which resulted in 4 papers. Recently, I have returned to this topic to publish a paper on chromospheric variability in early F-type stars. My co-author on this paper was an Embry-Riddle student who graduated in May 2009 and worked on the project for 3 years as a Arizona NASA Space Grant student.

These stars are somewhat hotter than the Sun and are near the highest temperatures for which convection occurs in stellar atmospheres. We measure a particular spectral feature (the helium "D3" line) as a measurement of activity, which implies the existence of high-temperature gas above the visible surface, which implies convective energy transport.

I have been working on an additional paper or two using a broader dataset, which will focus on issues other than variability. However, since the DIB work above is now funded, the D3 work is somewhat on the back burner.


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