Lecture02: Ultraviolet Astrophysics

OUTLINE

1. Ultraviolet Instruments in Space: Past, Present, and Future
2. Hubble Space Telescope (HST)
3. Far Ultraviolet Spectrograph Explorer (FUSE)
4. Some Examples of Cool Star Science with HST and FUSE
5. Problems for the students

ULTRAVIOLET INSTRUMENTS IN SPACE: PAST, PRESENT, AND FUTURE

• Why observe stars in the ultraviolet when optical observations can be made from the ground?
  1. For stars later than early-A the photospheric emission in the UV becomes weak, so that one can detect emission lines formed in the outer atmosphere layers (chromosphere, corona, wind, and disk) against the weak photospheric background.
  2. Most of the spectral lines of ions that are formed at temperatures hotter than about 8,000 K are located in the UV (e.g., C IV, Si IV, N V).
  3. Most resonance lines are at UV wavelengths (or in EUV or X-rays). Important examples are H I, C I-IV, N I-V, O I, Mg I-II, Si I-II, S I-II, Fe II, etc. ISM research requires resonance lines because no population in excited levels.
  4. Important density sensitive line ratios are in the UV (e.g., O IV 1400A multiplet).
  5. Hot companions (e.g., white dwarfs, subdwarfs, OB main sequence stars) dominate the UV emission of cool stars (e.g., Mira). Many hot companions were unknown before UV spectroscopy.
  6. Wind signatures for hot stars are best studied using UV lines
  7. etc.
• Why do we need high spectral resolution?
  1. Confusion due to overlapping lines (especially for metal rich stars with molecules)(Fe II lines are everywhere).
  2. Accurate line widths, shapes, identify and separate multiple components.
  3. Determine whether lines are optically thick. this is essential for abundance analyses (need to resolve the line).
  4. Existence and column density of interstellar and circumstellar lines (often need 3 km/s resolution or better).
  5. Measure a weak continuum against a noisy background.
  6. etc.
• List of UV Instruments (page 7)
• Summary of instrumental characteristics (page 8)

HUBBLE SPACE TELESCOPE (HST) (pages 1-6)

FAR ULTRAVIOLET SPECTROGRAPH EXPLORER (HST) (pages 1-6)

SOME EXAMPLES OF COOL STAR SCIENCE WITH HST AND FUSE

1. Some useful catalogs of data
   • Catalog of STIS spectra of cool stars (CoolCAT)
   • IUE Atlas of PMS short wavelength UV spectra
   • IUE Atlas of PMS UV spectra: accretion diagnostics
   • IUE Atlas of PMS long wavelength UV spectra
2. Spectrum of a solar twin (Alpha Cen A)
   • STIS Spectrum of Alpha Cen A
   • There is no complete high-resolution UV spectrum of the Sun including a proper sum over active regions, sunspots, coronal holes, off-limb, and center-to-limb behavior. So use a nearby bright G2 V star instead. (The best solar twin is 18 Sco which is 6.15 magnitudes fainter than Alpha Cen A).
   • Alpha Cen A provides an excellent example of a solar-like UV spectrum. A large accurate line list (671 emission lines from 32 atoms/ions and 2 molecules (H2 and CO)). Comparison to solar spectra.
   • One coronal emission line (Fe XII 1241A). Can measure its width (thermal) and Doppler shift (none).
   • Bright transition region lines generally fit by 2 Gaussians.
   • Estimates of electron densities from ratios of lines in O IV], S IV], and O V multiplets.
   • Emission measure distribution analysis gives value of the square of electron density in each T interval, which is needed to determine the total radiated power in each temperature range (a lower limit to the local heating rate).
3. Evolution of a solar mass star
   • X-ray and UV observtions of normal F-K stars
4. At what effective temperature do chromospheres/transition regions end?
   • Limits on chromospheres and convection among the main sequence A stars
   • Chromospheres/transition regions/coronae are presumed to be heated by the conversion of magnetic or acoustic energy into heat. This requires convective motions in the photosphere to generate the waves and to move the footpoints of magnetic loops. But convective energy transport becomes very weak in the A stars, so where does the chromosphere/corona phenomena end?
   • Look for emission lines at the shortest wavelengths where the photosphere background is weakest.
   • Best emission line to use is C III 977A formed at 60,000 K
   • Best estimate for the high temperature limit is about Teff = 8250K (spectral type A4 V).
5. Redshifts: flows in chromospheres and transition regions
   • Redshifts and Implications for heating mechanisms
6. RS CVn systems: tidally synchronous detached binaries
   • "Chromospheric two-component NLTE modelling of HR 1099" Lanzafame et al (2000) A+A 362, 683.
   • Transition regions of Capella
   • FUSE observations of Capella
   • Quiescent and flaring structure in RS CVn systems
7. Flare stars
   • Flares on the Sun and other stars (Review)
   • Flares (rapid increases in flux at many wavelengths) are observed in many types of stars (Sun, G-M dwarfs, brown dwarfs, PMS stars, RS CVn binaries, etc.).
   • UV and X-ray regions are good for observing flares because bright line and continuum emission observed against a very weak photospheric background.
   • Two-ribbon flares model (Figure 3).
   • Nonthermal phenomena due to particle beams impacting the lower atmosphere, gyrosynchrotron radio emission, hard X-ray emission, gamma-rays, etc.
   • Why flaring (nanoflares, microflares, giant flares)? Most likely magnetic reconnection events (cf. "Parker Spaghetti" model (Figure 7).
8. Coronal emission lines
   • Coronal emission lines observed by STIS
   • Coronal emission lines observed by FUSE
9. Fluorescence: TW Hya
   • Fluorescent molecular hydrogen in TW Hya I
   • Fluorescent molecular hydrogen in TW Hya II

PROBLEMS FOR THE STUDENTS

1. What are the criteria for the best solar twin and why is 18 Sco the best example of a solar twin? Plot the 1150-1700A spectra of a representative F dwarf, G dwarf, K dwarf, and M dwarf. Which spectral features have the largest changes between the F dwarf and the M dwarf. You can use CoolCAT. Anna
2. What are the assumptions that underlie emission measure analyses? What is the difference between an emission measure distribution and a differential emission measure? Compute the emission measure distribution for a T Tauri star. Michelle
3. Plot the flux in the C IV 1550A multiplet vs the He II 1640A line for 3 or more supergiants, giants, and dwarfs of similar spectral type (say G). Does stellar luminosity make a difference in the relative strengths of these two emission features? You can use CoolCAT. Remo
4. Plot the relative fluxes of C IV 1550A emission vs. fluorescent molecular hydrogen emission in the UV for a representative sample of PMS stars. Does the ratio of molecular hydrogen to C IV emission depend on rotation period or spectral type? Why? Rurik
5. The Mg II multiplet at 2800A is formed in the chromosphere, while the C IV 1550A multiplet is formed in the transition region near 100,000K. Is there a correlation between the fluxes of these two features and, if so, what is the functional relationship? Choose a sample of dwarfs and RS CVn stars to see whther there is a functional relationship. You can use the data in the paper in the section on the evolution of solar mass stars.
6. Plot the dependence of the luminosity of representative chromospheric and transition region emission lines vs. rotational period, which is a good proxy for stellar age. Samuel
7. What role(s) does the presence or absence of a disk play in determining the strength of transition region lines in PMS stars. You might want to read carefully the three papers on "IUE atlas of PMS...". Martin