Lecture04: Infrared Astrophysics

OUTLINE

1. Model Atmospheres for Very Low Mass Stars
2. Opacities in the Atmospheres of Brown Dwarfs
3. Evolution of Brown Dwarfs and Planets
4. Some Other Interesting IR Topics
5. Spitzer Space Telescope
6. Other Infrared Space Observatories and IPAC
7. Problems for the Students

MODEL ATMOSPHERES FOR VERY LOW MASS STARS

Basic references

• Model atmospheres of very low mass stars and brown dwarfs Allard, Hauschildt, Alexander, and Starrfield (1997) Ann Rev Astronomy and Astrophysics 35, 137.
• Observations of brown dwarfs Basri (2000) Ann Rev Astron Astrophysics 38, 485.
• Theory of low-mass stars and substellar objects Chabrier and Baraffe (2000) Ann Rev Astron Astrophysics 38, 337.

Nomenclature

• M dwarfs: Long-term H burning cores. Stars later than about M5 have fully convective interiors.
• Brown dwarfs: H and D burning cores only when young. Degenerate fully-convective interiors. Heat source is gravitational contraction. In late-M dwarfs TiO bands become important features in the near-IR.
• L dwarfs: TiO and H2O bands important. Atmospheres are dusty. Transition to T dwarfs at 1300-1700 K.
• T dwarfs: Methane (CH4) bands important. Dust partly or completely gone by settling. Deuterium burning minimum mass about 0.012 solar masses (12 Jupiter masses) is considered the transition to a "planet", but lower mass free objects are seen in young clusters.
• Planets: Metallic H/He mixture, but for <1 Jupiter mass, planets develop a rock-ice core.

Observations and Models

• The observed spectral sequence
• The effect of molecular opacities
• Predicted absolute fluxes of brown dwarfs

OPACITIES IN THE ATMOSPHERES OF BROWN DWARFS

• Molecular opacity at temperatures above 2000 K:
  • Water vapor opacity becomes important below 3000 K. Important bands centered at 1.4 and 1.9 microns, but water lines almost everywhere.
  • TiO bands in the red portion of the spectrum.
  • VO and other metal oxide bands also in the red spectral region.
  
  
• Molecular opacity below 2000 K:
  • TiO, VO, and CaH bands vanish as a result of the condensation of these refractory elements.
  • Onset of 3.3 micron methane band absorption at about 2200 K, and the 2.2 micron band at about 1800 K in the L6 to L8 stars (methane bands at 1900-1600 K and 2300-2000 K).
  • The collision-induced opacities of H2 and the growth of CH4 bands become increasingly important at 1.6-3.5 microns (H, K, and L bandpasses).
  • As a result cooler stars become bluer (i.e., radiation is forced to appear in the near-IR and optical rather than in the IR. Color-magnitude diagram from Baraffe et al. (2003). M dwarfs are green dots. L dwarfs are black squares. Pink triangles are T dwarfs. J band centered at 1.26 microns. K band centered at 2.21 microns.
  • Synthetic spectra (2500-200 K with no dust)
  • Synthetic spectra (2500-1500 K with dust. Here the strong heating effects of dust opacities prevent the formation of methane bands and dissociates H2O while producing a hotter water vapor opacity profile, which is much weaker and transparent to radiation.
  
  
• Collision induced opacity of molecular hydrogen (Borysow (2002) A+A 390, 779).
  • H2 has no dipole moment so no rotation or vibration-rotation spectrum.
  • Collisions with other species (H2, He, H) can induce transient dipole moments and thus collision-induced opacity. Since collisions have short duration,individual spectral lines are very broad and overlap.
  • Fundamental VR band centered at 4162 wavenumbers (2.4 microns).
  • First overtone VR band centered at 8089 wavenumbers (1.2 microns).
  • Second overtone VR band centered at 11786 wavenumbers (0.2 microns).
  • Pure rotation and translation bands in the far-IR.
  • Total opacity of the three VR bands.
  • H2CIO is important for computing the temperature structure of brown dwarfs because it is a near continuous opacity source that fills in the opacity gaps between the molecular absorption lines.
  
  
• The resonance doublet lines of neutral sodium (5889, 5896 A) and potassium (7665, 7699 A) are perturbed by helium (at 3000, 2000, 1000, and 500 K) and H2 producing very broad profiles that match (example of a T_eff = 1000 K, log g = 5.5 model) observations of the star SDSS 1624, but still a problem of missing opacity in the 8500-11,000 A region of T dwarfs (Allard et al. (2003) A+A 411, L473).
  
• The effects of dust in the atmospheres of brown dwarfs for the limiting cases of:
  (1) chemical equilibrium dust with no settling, and
  (2) complete gravitational settling (Allard et al. (2001) ApJ, 556, 357).
  • Formation of 600 gas-phase species and 30 grain species for stars later than M8:
    Gas (solid lines) and dust species at 2600 K
    Gas (solid lines) and dust species at 1800 K.
  • The greenhouse effects of dust opacities provide a natural explanation for the peculiarly red spectroscopic distribution of the latest M dwarfs and young (warm) brown dwarfs.
  • Brown dwarfs hotter than about 1800 K (L dwarfs) have spectra that are well fit with dusty models.
  • Brown dwarfs cooler than about 1800 K (T dwarfs) have spectra that look like little or no dust.
  • Dust opacity vs. wavelength. The opacity also depends on the dust grain size distribution.
  • Comparison of 1500 K models with dust (solid line), no dust (dotted line), and blackbody (dashed line).
  
  
• Molecular hydrogen quadrupole vibration-rotation emission lines
  in the 1.9-2.5 micron region of 047-436 (a young M8-L1 star in the Trapezium with age about 1 Myr.).

EVOLUTION OF BROWN DWARFS AND PLANETS

• M dwarfs can take ~1 Gyr to reach the main sequence and then nearly constant luminosity and T_eff due to H burning in the core.
• Luminosity of brown dwarfs never stops decreasing because core temperatures are too cool for nuclear reactions after a short initial burn of D and maybe H. Models of Baraffe et al. (2003) A+A 402, 701.
• The least massive brown dwarfs (25 to 1 Jupiter masses or 0.025 to 0.001 solar masses)(Burrows, Sudarsky, and Lunine (2003) ApJ, 596, 587).

SOME OTHER INTERESTING IR TOPICS

• Dusty circumstellar envelopes (Zuckerman (2001) ARAA 39, 549).
• Circumstellar photochemistry (Glassgold (1996) ARAA 34, 241).
• Organic molecules in space (Ehrenfreund and Charnley (2000) ARAA 38, 427).

SPITZER SPACE TELESCOPE

• Website for Spitzer main webpage
• Website for Spitzer data archive
• First results from Spitzer (ApJS Sept 2004). This set of articles includes descriptions of the Spitzer instruments and first scientific articles. Papers on PMS stars located on pages 367, 374, 379, 385, and 391.

OTHER INFRARED SPACE OBSERVATORIES AND IPAC

• Infrared Processing and Analysis Center (IPAC) contains archives, program descriptions, and data analysis software for essentially all space IR missions (like HEASARC for high energy and STScI for the UV/optical).
• Infrared Space Observatory (IRAS) obtained an all-sky survey at 12, 25, 60, and 100 microns (350,000 sources detected) and low-resolution 7.5-23 micron spectra of selected targets.
• Two Micron All Sky Survey (2MASS) is a recent very sensitive all-sky survey in the J (1.25 microns), H (1.65 microns), and Ks (2.17 microns) bands with 4 arcsecond resolution. The Point Source catalog contains ~300 million stars. All of the data are public.
• Infrared Space Observatory (ISO) obtained 28,000 pointed observations (images and spectra). All data are public in the ESA ISO data archive.
• Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory (Boeing 747-SP) with a 2.7 m telescope operating in the IR (3-1600 microns) which is diffration limited beyond 15 microns. PI and Observatory class instruments will change over the 20 year lifetime. First light in 2006? First call for proposals in August 2005.
• Herschel Space Observatory is a 3.5 m IR space observatory to be launched by ESA in 2007. Instruments include high and low resolution spectrometers and cameras for the 57-670 micron region.
• Planck is an ESA mission to be launched in 2007 with Herschel. Planck will map the anisotropies in the cosmic microwave background (CMB) radiation.

PROBLEMS FOR THE STUDENTS

1. What are the continuous opacity sources in the L and T dwarfs? Over what temperature ranges are they important. How does the presence or absence of continuous opacity sources change the thermal structure and emergent fluxes of brown dwarfs? The models of Allard (see her papers for the URL address) are available work on this problem.
   
2. Describe in detail how the spectra of the lowest mass brown dwarfs differ from the more massive L and T dwarfs in the infrared. How would the spectra of these loss mass objects change if they were illuminated by a nearby star? (Hint: check papers by Lunine and Allard.) What spectral features would you look for in a gas planet located near a star and for an isolated brown dwarf with the same mass? Samuel
   
3. Determine the observing time needed to obtain good quality (S/N = 30) IR photometry and spectroscopy of brown dwarfs with effective temperatures of 2000, 1500, 1000, 500, and 300 K located in the Pleiades and Orion clusters. In 1 hour observing time per target, what are the most distant brown dwarfs of each of the above types that can be observed by each instrument on Spitzer? Use realistic model atmospheres. Vlad
   
4. Summarize what has been learned from Spitzer observations about brown dwarfs and pre-main sequence stars as described in the September 2004 ApJS issue of first results from Spitzer. Rurik
   
5. Make a list of 10 bright L dwarfs and 10 bright T dwarfs. Which ones are detected sources in the catalogs of IRAS, ISO, 2MASS, and Spitzer? Why are there nondetections? Are young brown dwarfs in the Scorpio-Centaurus Association more easy to detect by 2MASS and Spitzer than nearby free-floating brown dwarfs? What are the observational requirements for a new instrument to fly on SOFIA to study nearby free-floating brown dwarfs?