1 Observational astrophysics

What is this lecture about ?

• Why not just Astronomical techniques ?
• Astronomy + physics = astrophysics
• Astronomy: the art of sky observations
• Physics: unveiling the laws of Nature (Postulate: there are laws)
• We need to observe the sky and derive physical quantities

What do we observe ?

• Different components of the universe: gas, dust, magnetic fields, cosmic rays (dark matter?)
• Essentially electromagnetic waves
• Non-photonic astrophysics: neutrino and gravitational-wave astronomy

1.1 Different properties of light

General properties

• Energy, wavelength
• Polarization
• What can we extract ? image and/or spectrum

Multiwavelength astronomy

• Objects often emit at many wavelengths
• Different wavelengths usually trace different physical processes
• Must combine multi-wavelengths view to build a comprehensive understanding
• Choice of Wavelength
  • Nature of the emitting particule (atom, molecule, grain)
  • Nature of the emission process: spontaneous/induced decay, transition type (electronic, vibrational, rotational, hyperfine, etc); nuclear transition (X-ray, γ-ray)
• Choice of a facility
  • Altitude, mirror diameter, receiving device, spectrometer…
  • Depending on the above, the choice may be reduced.

1.2 Chosing the appropriate wavelength

2 Star formation

ESA/Herschel, IRAM, ALMA

2.1 Clouds and filaments

• Dust emission mapped with the Herschel/SPIRE bolometer camera (200, 350, 500mic)
• Clouds: typical size∼30 pc at d=150 pc
  • how much is this in degrees ? what is the actual size on the sky?
• Filaments: large aspect ratio => high dynamic range i.e. large maps with high angular resolution

2.2 Prestellar cores: chemical factories

• B68 in the visible
• Typical diameter: 0.1 pc or 3x10¹⁷ cm
• Temperature: 10K; rotational + hyperfine transitions; the realm of molecules
• Prestellar cores: first building block of star formation

Multi-wavelength view of B68

• From near IR to far IR: from dust extintion to dust emission !
• Interaction of light with matter: at long wavelength, dust is seen in emission (black body); this emission is optically thin: one can see throughout the core
• Visible: dust opacity → high extinction

Visible/Radio complementarity

2.3 Spatial resolution

Note the different scales

Exercise

• What is the spatial resolution (in arcsec) needed to resolve spatially a dense core ? What is telescope diameter needed at λ=3mm ?
• Same question for a protostar observed at λ=1mm ? For a protoplanetary disk observed at λ=0.8mm ?

2.4 Spectral resolution: Disks in Keplerian rotation

• Protoplanetary disks observed with ALMA
• Rings, spirals: routinely observed in these dust emission maps
• Note: disks are observed both in continuum and lines
• Continuum: means broadband, as opposed to high resolution spectroscopy = usually dust emission; low spectral resolution power R = λ/δλ ∼ few; in disks, (sub)mm continuum dominated by dust emission (there are other continuum emission processes)
• Lines: means high resolution spectroscopy, R>few tens, up to 10⁷

Disks in Keplerian rotation

3 General relativity with ESO/VLT

Figure 15: The Galactic Center (article)

3.1 26 years of astrometry

Motion of S2 during 26 years

S2 pericenter passage in May 2018

3.2 A test of general relativity

• 26 years of measurements of star S2 around the MBH Sgr A* (R_S=10μas or 0.08 AU)
• S2 is a single star: can be used to test Einstein's theory
• Instruments: K-band spectrometry; VLT NIR speckle and AO assisted imaging + spectroscopy
• Astrometry: 4mas in the 1900s (HARP), 0.5mas in the 2000s (NACO) and 0.03mas with VLT/Gravity
• pericenter approach in May 2018: daily motion observed, at ≈ 3% lightspeed
• combined gravitational redshift and relativistic transverse Doppler effect

4 Cosmology with ESA/Planck

CMB power spectrum

ESA/Planck HFI bolometers

4.1 Successful ΛCDM

• ESA/Planck/ mission
• The cold (2.7 K) universe: long wavelength best suited
• Sucess of the spatially-flat 6-parameter ΛCDM model
• Number of a_lm measured: 1 430 000, or 900σ detection (Note: NASA/WMAP measured 150 000 a_lm)
• Review article
• No need (so far) for extra physics (apart from dark matter and dark energy…!)

5 Mapping the Galaxy: the ESA/Gaia mission

The H-R diagram from ESA/GAIA DR2

The Gaia CCDs

• Key: differential measurements, astrometry, redundancy, multi-band

6 Spatial resolution

Planet formation: near infrared

• Planet PDS70b in a transition disk; A planetary-mass companion at 22 au and another gap at 54 au; Probably young giant planet (R=1.4-3.7 R_J); Circumplanetary disk ?
• ESO/Sphere instrument; near IR (λ~1μm)
• Sub-arcsec angular resolution: adaptive optics
• More info

6.1 Techniques to detect planets

• Transits: requires very high sensitivity and excellent flux calibration
• Radial velocity: requires very high spectral resolution and stability (βPic c above; see here and the article).
• Direct imaging: requires very high spatial resolution (adaptive optics)

6.2 Imaging a black hole

• Image of the Messier 87 black hole, a massive galaxy in the nearby Virgo galaxy cluster, d=55e6 ly; M_BH = 6.5e9 M_sun
• Event Horizon Telescope (EHT): planet-scale array of eight ground-based radio telescopes
• Observations at λ1.3mm; data (350TB/day) are recorded with extreme timing precision (hydrogen maser) and recombined afterwards: VLBI

7 Astrochemistry: the evolution of the Universe

• Starburst galaxy NGC 253: SFR∼ 5M/yr
• The most massive galaxy in the Sculptor Group (d=11.4 million light year)

Spectral survey with Atacama Compact Array

8 Practicalities

Outline of the lecture

1. Introduction
2. Distance measurements and coordinates
3. Radiation
4. High-energy sky
5. Observing the dust
6. Observing molecules
7. High-angular resolution
8. Radio astronomy
9. Infrared astronomy
10. Visible astronomy
11. X-ray and gamma-ray astronomy
12. Gravitational waves
13. Observing magnetic fields

Logistique

• Évaluation: examen 2h, QCM/exercices, sans documents, calculatrice autorisée; énoncé en anglais
• Pas de notes écrites de cours; diapos; prenez des notes !
• Étude collective d'articles
• e-mail: pierre.hily-blant@univ-grenoble-alpes.fr
• Office: OSUG-A 121 (in front of the elevator, 1st floor)