Previous exam questions Flashcards
Why observe from space? List at least 4 reasons and explain them briefly. What motivates the researcher in selecting a space platform?
The lack of atmosphere leads to
Possibility of achieving diffraction limited images
Possibility of observing across the electromagnetic spectrum with 100% transmission
Possibility of accessing information with no or almost no thermal background (in IR/μWave)
Long uninterrupted durations
Earth’s atmosphere blocks most radiation at wavelengths shorter than visible light, so we can only make direct ultraviolet, X-ray, and gamma ray observations from space
Telescopes are able to be cooled to very low temperatures in the vacuum of space.
Space programs, even the simplest are a major undertaking, costing enormous amounts of money and decades of the life of people involved. Alternatives should always be selected if available. The alternatives, however, depend on the nature of the observation one is trying to make. Provide alternatives in the case of:
a) Atmosphere is opaque at relevant wavelengths (3 alternatives)
b) You need very high spatial resolution (2 alternatives)
c) You need long duration (»days) observations with no interruptions (1 alternative)
(a) Mountaintops, airplanes, balloons
(b) Adaptive optics, changing wavelength region
(c) Telescope networks (e.g micro-lensing)
The course has described 4 methods of finding exo-planets- from the ground and from space or both. Which are they? Describe each briefly! State the advantages and disadvantages of each method. State which has so far been used from:
a) Only from the ground
b) Only from space
c) Both from the ground and from space
Method 1: Radial velocity method. Using the doppler effect, we can derive the mass of the planet if the orbital inclination is known
Method 2: Transit Method. Using the difference in flux from a star when the planet passes in front, allowing us to derive the radius.
Method 3: Astrometry. This method consists of precisely measuring a star’s position in the sky, and observing how that position changes over time.
Method 4: Polarimetry. Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and become polarized.
Method 5: Direct imaging. Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead. Images have then been made in the infrared, where the planet is brighter than it is at visible wavelengths. Coronagraphs are used to block light from the star, while leaving the planet visible.
Method 6: Gravitational microlensing. Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. If the foreground lensing star has a planet, then that planet’s own gravitational field can make a detectable contribution to the lensing effect.
(I can only find 2 well described methods in the lecture slides, added some more for fun)
(a) -
(b) Direct imaging I guess
(c) Radial velocity, Transit, Microlensing, astrometry
What are the advantages when one can use more than one method to detect and exoplanet? What physics can one do in this case? Describe how ground- and space-based instrumentation is required in this case and why? What are the main problems with this method and how can they be alleviated?
The advantages would be that parameters of the host star and the exoplanet in turn could be determined more precisely (And more of them) and false-positives could be erased.
Not really sure what is meant with the rest of the question here. I think he’s looking for something specific.
State Kepler’s third law? What is the relevant physics? What does it describe? How and when is it used in mordern exo-planetology?
Squares of orbital periods are proportional to cubes of semimajor axes: 4 π^2 a_pl^3 = G M_starP^2
Used to derive how far the planet is from the host star if orbital period is known, can also be used to derive semi-major axis of the system.
Give the equation for the radial velocity component of a star, due to an orbiting object and including the radial velocity component due to galactic motion.
V(t) = V_0,z + (2piaM_psin(i))/((M_p+M_s)Psqrt(1-e^2))(cos(theta(t)+omega_OP)+ecos(omega_OP))
(Lecture 3 slide 32)
What is the transit probability, i.e. what is the chance of observing(from the earth) a transit of an exo-planet across its host stars surface? Does it depend on the distance to the star? Does it depend on the distance between host star and exo-planet?
Transit probability = (R_s + R_p)/a \simeq R_s/a
It does not depend on the distance to the star from us. But it depends on the distance between host star and exo-planet.
What assumptions are made when one observes and models/interprets a transit light curve?
The planet orbit is circular
M_p «_space;M_s and the companion is dark compared to central star
The stellar mass-radius relation is known
The light comes from a single star (no blends)
In order to determine more precisely the stellar masses and radii one can use astroseismology. The PLATO mission is based on the simultaneous detection of the power spectrum of the oscillations in exo-planetary host stars and the transit light curve. There are scaling relation for solar type stars, calibrated in terms of the solar values that will allow bulk determination of stellar masses and radii.
(a) Explain the process
(b) What are the terms nu_max and \delta nu and what is their physical significance?
(c) Give the scaling relations for stellar mass and radii, and calibrated in terms of the sun.
Few answers to this question in the slides, will get back to it later :)))))))
(a)
(b)
(c)
