Non-LTE in Stars

1. Non-LTE in Stars • The Sun • Early-type stars • Other spectral types

2. NLTE in the Sun • Chromosphere and corona obviously in NLTE • Photosphere? - LTE still dominant assumption for “stellar” work, but not for “solar” work • Photospheric departures from LTE occur even independent of chromosphere and corona, because of boundary through which photons escape to space • Weak lines of low-abundance species often show departures from LTE (e.g. they reverse to emission lines on the solar disk just inside the limb) • Cores of strong lines may depart from LTE while the wings may remain in LTE

3. The Sun’s Continuous Opacity • Free-free continuous absorbers generally in LTE • Balmer continuum forms deep in photosphere where bound-free and free-free Lyman transitions are in detailed balance. Thus, solar Balmer and Paschen continua formed in LTE • H- stays close to LTE in the photosphere but departs at small optical depths (t<10-5)

4. NLTE in Solar Line Formation • Damped lines in the solar spectrum (lines with well-developed wings) have line center opacities at least 104 x the continuum opacity • The chromospheric temperature rise begins around t=10-4 • So most damped lines have Doppler cores formed where temperature is increasing outward • These lines should have emission cores • Most don’t. Why? (The low density in the chromosphere for most species) • The Ca II H and K lines DO show central reversals, as do the Mg II H and K lines at 2800A. Why? • In both cases NLTE is needed to model the lines correctly to match observed profiles

5. NLTE Corrections

6. Oxygen Variations of 0.1 dex depending on which lines are included or not Individual lines vary from 8.697 to 8.921 NLTE effects generally strengthen the lines Also affected by granulation Carbon, Nitrogen - ditto Magnesium – Use Mg II as dominant species Neon EUV spectra of emerging active regions Iron – still controversial! Why? Values range from 7.42 to 7.50 If Fe II is used, NLTE effects very small Disagreements about choice of lines, f-values Details of Solar Abundances

7. Is the Sun Metal-Rich? • Is the Sun metal-rich for its age and position in the Galaxy? Possibly… • Comparison to unbiased sample • Detailed calibration of metallicity scale • Volume-limited sample from the solar neighborhood • Strictly differential spectroscopic analyses • The Sun is 0.1 dex more metal-rich • But the solar neighborhood sample has a mean age twice the age of the Sun • Most chemical enrichment models predict less than 0.1 dex increase in Fe in 5 Gyr • Maybe the Sun is metal-poor?

8. Summary for the Sun • Departures from LTE occur in all lines formed above tc=1 • Cores of lines with well-developed wings show strong departures from LTE • Weak, low-excitation lines of trace elements are likely to show departures from LTE • Weak, high-excitation lines of abundance elements are less susceptible to NLTE • Far wings of strong lines are less susceptible to NLTE • Forbidden lines are likely to have LTE source functions, but opacity may depart markedly from LTE

9. NLTE in O & B Stars • Low atmospheric density • High temperature • LTE not valid • Methods for NLTE in O stars date from the late ’60’s • NLTE effects not subtle • observable even at low spectral resolution

10. Continuum Effects • Balmer and Paschen continua formed at low optical depth – spectral energy distributions affected by departures from LTE • U-B vs. spectral type relation is affected • Stromgren colors affected at lower gravities

11. Hydrogen & Helium Lines • Balmer line cores deeper in B stars than predicted by LTE calculations, by a factor of 2 • Limb darkening behavior also differs • In O stars, LTE profiles are too small (by a factor of 5 at O5!) • In O stars, departures from LTE make He I and He II lines much stronger (this allows a fit to He lines with N(He)/N(H)=0.1 • In the blue-violet spectra of B stars, some He I lines are formed in LTE, or departures are only in line cores; wings are fit by LTE • However, departures become large for red and IR lines (these lines are not collision dominated, but rather are dominated by photoionization-recombination processes) • Weaker lines may be affected more (more dominated by line core)

12. Red He I lines are much stronger than predicted by LTE models

13. Light-Ion Spectra • LTE analyses of metal-lines suggested over-abundances of x 10 or more (e.g. Mg 4481) • LTE analyses of Si III and Si IV in early B and O stars gave abundances 5-6 times too high • LTE analyses of the Ca II K line in B stars gave abundances 3-5 x solar • These spurious abundances disappear with NLTE analyses

14. NLTE in Other Spectral Types • Observable effects have been hard to demonstrate in solar-type and late-type stars except the Sun • Continua appear to be formed in LTE • Consistency between colors and spectroscopic temperatures • Ionization equilibria generally in LTE • Nagging concerns about surface gravities in cool giants • Some NLTE effects HAVE been demonstrated

15. What’s Been Found? • O I lines in “warmer” solar-type stars • High excitation triplet (8 eV) • Departure increases at higher luminosities • Small effects (<0.1 dex) on many elements (e.g. sodium) • Not-so-small effects…. • B I in metal-poor dwarfs ~+0.5 dex • Ionization equilibrium in EMP dwarfs • Error from Fe I lines ~0.1 dex per dex

16. Balmer Line Profiles in White Dwarfs • Balmer line profiles in DA white dwarfs are very strong • Broadened by extreme Stark broadening at high densities (log g=8) • But Ha has a sharp core due to NLTE • DA atmospheres are thin • Opacities low because of absence of metals (gravitational settling) • Can use NLTE cores to measure vsini