Cool Star Atmospheres

Stellar Atmospheres in the Ultraviolet

The dynamic surface of the Sun and other cool stars is most vividly displayed in the ultraviolet. Emission emanating from the transition region between the photosphere and the corona, where massive magnetic field loops move material and collide to produce massive flares. We are even able to detect signatures of the corona, which normally emits at with high-energy X-rays. Forbidden transitions emit in the far ultraviolet, and we were able to detect FeXVIII and FeXIX (that is an iron atom that has been ionized 17 and 18 times, repectively)! The figure below, from Redfield et al. (2003), is a search for these coronal lines for different classes of stars, in which we were able to detect the three strongest lines.

High signal-to-noise, high resolution spectra provide a lot of detail about stellar atmospheres in the precise shapes and locations of stellar emission lines. Rarely are they symmetric profiles, but instead hide interesting information regarding the dynamic motions and interactions of plasmas in the outer layers of stars.

Recent Papers On This Topic:

1. CARMA CO(J=2-1) Observations of the Circumstellar Envelope of Betelgeuse   O'Gorman, E., Harper, G.M., Brown, J.M., Brown, A., Redfield, S., Richter, M.J., & Requena-Torres, M.A. 2012, AJ, 144, 36

The outer atmospheres of red supergiants can be so cool that molecules can form! In this work, we looked at the outflow or wind of perhaps the most famous red supergiant, the 9th brightest star in the sky, Betelgeuse. I took high resolution spectra of emission lines with P-Cygni profiles to compare to the radio and infrared observations.

2. The Remarkable Far-Ultraviolet Spectrum of FK Comae Berenices: King of Spin   Ayres, T.R., Harper, G.M., Brown, A., Korhonen, H., Ilyin, I.V., Redfield, S., & Wood, B.E. 2006, ApJ, 644, 464 This star has the broadest far-UV emission lines I have ever seen, with a width of about 500 km/s! Rotating close to break-up speed, we were able to detect plasma at even higher rotation speeds because of its location high above the surface, likely suspended by magnetic field loops.

3. Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres   Wood, B.E., Redfield, S., Linsky, J.L., Müller, H.-R., & Zank, G.P. 2005, ApJS, 149, 118

Knowledge of the intrinsic ultraviolet flux of a star is critical for understanding how it will impact a planetary atmosphere, particularly for cool stars, where there is little continuum flux in the UV, other than the strong chromospheric and transition region lines. In many cases, Lyman-α emission is the dominant source of UV emission. However, the Lyman-α profile is a complicated mess of interstellar, heliospheric, and astrospheric absorption, and most of the intrinsic flux is not observed. This paper details how we can calculate the intrinsic flux. I also use it a lot for making flux estimates for potential targets in my HST proposals.

4. A FUSE Survey of Coronal Forbidden Lines in Late-Type Stars   Redfield, S., Ayres, T.R., Linsky, J.L., Ake, T.B., Dupree, A.K., Robinson, R.D., & Young, P.R. 2003, ApJ, 585, 993

This work was a collaboration with Tom Ayres, who was like a second Ph.D. advisor to me. Tom was working on an exciting project to look at highly ionized forbidden transitions of coronal lines with HST (see below). This project grew out of that work and is a search for forbidden lines in the far-ultraviolet with FUSE.

5. STIS Survey of Far-Ultraviolet Coronal Forbidden Lines in Late-Type Stars   Ayres, T.R., Brown, A., Harper, G.M., Osten, R.A., Linsky, J.L., Wood, B.E., & Redfield, S. 2003, ApJ, 583, 963

A comprehensive search for coronal forbidden lines in the HST spectral region. Check out the craftmanship with the figures. Tom is a master figuremaker, and I strive to make my figures as informative, clear, and beautiful as his.

6. A Far Ultraviolet Spectroscopic Explorer Survey of Late-Type Dwarf Stars   Redfield, S., Linsky, J.L., Ake, T.B., Ayres, T.R., Dupree, A.K., Robinson, R.D., Wood, B.E., & Young, P.R. 2002, ApJ, 581, 626

A large-scale survey of cool stars, like the Sun. This was a presentation of some of the first observations taken with FUSE, a spectroscopic space telescope that observed in the far-ultraviolet, at shorter wavelengths than HST could see. As a graduate student, it was a great experience to be involved in the mission when it first started observing and to work with such an experienced team of stellar astronomers.

Stars As Planetary Hosts

Now that it appears that essentially all stars have planets in orbit around them, we need to adjust our perspective from thinking about stars and planets are separate, independent, research areas. Instead, stars, and the planets around them, are integrated units. In particular, the host star has a dramatic influence on the origins and evolution of its planets and their atmospheres. We are keenly aware of this on Earth, as we watch coronal mass ejections (CMEs) hurdle toward the Earth, interact with the Earth's magnetic field, and cause the aurora, damage our satellites, and increase the cosmic ray flux at the surface of our planet. Radio observations of solar CMEs were the subject of my senior thesis at Tufts University (see figure below from Willson et al. 1998).

The figure above shows a simultaneous image of solar emission from opposite sides of the electromagnetic spectrum. The image is from SOHO and is taken in the extreme ultraviolet, almost in the x-ray at 195 Å, the contours are taken in the radio at 74 MHz or a wavelength of 4 meters! I had done an REU at NRAO the summer before, and was able to spend a little more time at the AOC in Socorro to reduce these radio observations in preparation for my thesis. A multiwavelength approach is crucial to understanding stellar emission and how it impacts planets and their atmospheres.

Recent Papers On This Topic:

1. Activity and Magnetic Field Structure of the Sun-Like Planet Hosting Star HD 1237   Alvarado-Gómez, J.D., Hussain, G.A.J., Grunhut, J., Fares, R., Donati, J.-F., Alecian, E., Kochukhov, O., Oksala, M., Morin, J., Redfield, S., Cohen, O., Drake, J.J., Jardine, M., Matt, S., Petit, P., & Walter, F.M. 2015, A&A, 582, 38

This collaboration grew out of discussions with Gaitee Hussain, in which we wanted to couple the work I had been doing on astrospheres of planetary hosting stars in order to get a global measure of their stellar winds, with the Zeeman Doppler Imaging she and her colleagues do which can reconstruct the spatial distribution of magnetic field structure on stars. It is incredible that we can take high resolution and high signal-to-noise spectroscopy and produce a spatial map of a distant, unresolved star.

2. Planet-Induced Emission Enhancements in HD 179949: Results from McDonald Observations   Gurdemir, L., Redfield, S., & Cuntz, M. 2012, PASA, 29, 141

Some planets are so close to their host stars, that their magnetic fields could be intertwined and cause activity back on each other. There are stellar binaries that do this called RS CVns, which have much high activity levels as a pair than the sum of two individual stars. This work looked for such an interaction between a close-in hot Jupiter and its host star.

3. Rocky Planetesimals as the Origin of Metals in DZ Stars   Farihi, J., Barstow, M.A., Redfield, S., Dufour, P., & Hambly, N.C. 2010, MNRAS, 404, 2123

There is a fascinating population of white dwarfs which show evidence of recent accretion. Their surface gravity is so strong that heavily elements, like iron and calcium, settle too low in the atmosphere to see within a period of just months to years. So, when you see some, you know it had to be deposited recently. This work confirmed it could not be accreted from the ISM, but instead from disintegrated rocky planets.

4. The Spin-Orbit Alignment of the HD17156 Transiting Eccentric Planetary System   Cochran, W.D., Redfield, S., Endl, M., & Cochran, A.L. 2008, ApJ, 683, L59

This work used the Rossiter-McLaughlin Effect, which is the slight systematic aysmmetry in stellar absorption lines due to a transiting object, which in this case was a planet. This requires high resolution and high signal-to-noise spectra. The alignment or misalignment of planets with the rotation axis of the star is an interesting constraint on planet and star formation and on long-term changes in orbital properties of planets.

5. First VLA Observations of Nonthermal Metric Bursts Associated with Coronal Mass Ejections Detected by the SoHO   Willson, R.F., Redfield, S., Lang, K.R., Thompson, B.J., & St. Cyr, O.C. 1998, ApJL, 504, L117

A paper which includes work I did for my senior thesis at Tufts University. I got to go to the headquarters for the VLA in Socorro, NM to reduce the 74 MHz (4 meter) radio observations. We observed the Sun and looked for correlations of the radio emission with X-ray observations of coronal mass ejections (CMEs).

6. Doppler Acoustic Diagnostics of Subsurface Solar Magnetic Structure   Lindsey, C., Braun, D.C., Jefferies, S.M., Woodard, M.F., Fan, Y., Gu, Y., & Redfield, S. 1996, ApJ, 470, 636

My first published paper! This work came out of my summer REU experience at NSO in Tucson, AZ, working with Charlie Lindsey. This was the experience that cemented my desire to go to graduate school in astrophysics. I can remember being inspired by working with Charlie, meeting the scientists at NSO and NOAO, observing at Kitt Peak National Observatory, and meeting other undergraduates interested in astronomy. I helped develop an algorithm to use helioseismic data to predict solar activity and sunspots on the far side of the solar surface.