Direct imaging and wavefront control- Science Flashcards
What are typical star/planet angular separations?
1 AU = 1 arcsec separation at 1 parsec
Earth separation = 0.1 arcseconds for a star at 10 parsecs
Jupiter separation = 0.5 arcseconds for a star at 10 parsecs
(Ian approx normal values)
10au / 30 parsec ~= .3 arcsec
How do the star/planet angular separations compare to the diffraction limit of current and future ground-based telescopes?
1.22 lambda / D
1 micron / 10m = 1e-7 radian
1 arcsec = 5e-6 radian, so theoretically ~24 milliarcsec achievable, but limit of AO is more like 50-100mas. 30m telescope same problem, would be 8mas theoretical diffraction limit, but AO not likely to do much better.
At what wavelengths do planets peak at? What further info do you need to answer this question?
From Wien’s Law (lambda = b/T), we can derive the peak wavelength by knowing the temperature the planet emits its radiation. The temperature can be derived from Stefan-Boltzmann Law: E = sigmaT^4.
What are differences in spectra of Archaean vs modern Earth?
Archaean Earth: blackbody spectrum
Modern Earth: Rayleigh Scattering by H2O molecules in the optical??? The transmission spectrum would reveal the presence of water vapor (H2O), carbon dioxide (CO2), oxygen (O2), ozone (O3), methane (CH4), and many other molecules in the atmosphere.
Can direct imaging say anything about transiting, RV, or astrometric planets?
- Orbital radius
- Visual evidence of the planet
- Atmospheric composition
(Ian) The stronger the RV or transiting signal, the more edge-on we are seeing the star system, so would need to be “lucky” or try to time to see planets when they are furthest from the star. Whereas a strong astrometric signal should indicate star system has a low inclination angle, so easier for direct imaging in terms of timing
What planets are current direct imaging surveys sensitive to? How does this compare to other detection methods?
Hot young Jupiters, of masses and at distances larger than other detection methods
RV: Masses of 10-1000 Earth masses and distances 0.01 to 1 AU
Transit: Masses of <1-Earth Jupiter masses and distance 0.01 to 1 AU
Direct Imaging: Masses of 500 to 10000 Earth Masses and distances 2 to 1000 AU
Astrometry: Most massive planets with periods slightly less than the survey duration (e.g: Gaia sensitive only to planets with mass several times the mass of Jupiter at separations of several AU)
Current strategies include using gaia accelerations to look for planets. Describe how this works and calculate the magnitude of this effect.
The astrometric acceleration of the star indicates a deviation from its proper motion across the sky, which is most likely due to the presence of a companion. This acceleration depends on the ratio of M/R^2 of the plane (M: mass, R: distance from star). So we can now calculate the “true mass” of the planet.
Directly imaged planets orbit far from young stars for which precision Doppler measurements are impossible, and therefore inferring their mass requires both an estimate of the age of the system and (generally poorly calibrated) evolutionary models. Astrometric measurements with Gaia should change this situation and in turn calibrate the models.
How does direct spectroscopy of an exoplanet atmosphere compare with transmission spectroscopy?
Transmission: As a planet passes in front of its star, some light from the star passes through the atmosphere, probing it thoroughly at a range of depths. Only a tiny fraction of the star’s light goes through the planet’s atmosphere. For a typical hot Jupiter: 1e-4 , for earth-sized planet: 1e-7 . As the expected signal is very small one must perform very careful calibration and data reduction.
Direct spectroscopy can be either of the two kinds:
Reflectance spectroscopy: The basic idea here is to measure the light reflected by a planet from its host star. This planetary contribution will be mixed with the direct light of the star itself, because the host and planet are separated by only tiny tiny fractions of an arcsecond. Therefore, we face the problem of contrast: trying to identify the tiny fraction of the combined light which is due to the planet alone.
Emission spectroscopy: If the planet is hot, either from its own internal heat sources or due to the stellar radiation, it will emit light as a blackbody. But the flux ratio for the planet compared to the star is quite small. For a a typical Hot Jupiter around a sun-like star, it is 1e-8, and for a an Earth-like planet in an Earth-like orbit around a sun-like star, it is 1e-10.
What makes a good direct imaging target star?
- Proximity
- Young age as then planets are their most luminous at early ages, and relative contrast between young giant planets and their host stars is lower than at older ages (because stellar luminosities plateau on MS while planets and brown dwarfs continue to cool, creating luminosity bifurcation)
What do disks look like at optical wavelengths–
- What physical aspects are being probed?
- What causes the flux you measure?
- What actual wavelengths are used?
- substructures, inner disk which is hotter
- scattered light from small dust grains in the inner disk
- 600-800 nm?
What do disks look like at IR wavelengths–
- What physical aspects are being probed?
- What causes the flux you measure?
- What actual wavelengths are used?
- dust emissions, and ices at MIR/FIR as well as substructures
- micron-sized dust grains scattering light
- NIR: 0.6 to 3 or 5 microns, MIR: ~5 to 30-40 microns FIR: all the way up to about 250 micron.
What do disks look like at millimeter wavelengths–
- What physical aspects are being probed?
- What causes the flux you measure?
- What actual wavelengths are used?
- mass, dust concentrations in the midplane, cold gas content like CO from the outer disk
- large, centimeter-sized particles
- Sub-mm: 0.3 to 1.7 mm, Disks can be studied using facilities like ALMA up to 2 cm?
What are the different ways light can scatter?
Geometric Scattering
Mie
Rayleigh
What induces polarization in light?
Polarized light can be produced from the common physical processes that deviate light beams, including absorption, refraction, reflection, diffraction (or scattering), and the process known as birefringence (the property of double refraction).
What is optical depth?
Optical depth describes how much absorption occurs when light travels through an absorbing medium (for example, a planetary atmosphere or a interstellar dust cloud). If the optical depth is large, we say the region is optically thick – light is readily absorbed. If the optical depth is small, the region is optically thin, and light passes through easily
