Seminar 1. How to observe in the infrared

Observing in the infrared and a distant OB association around RAFGL 2475.

Many infrareds!

• Infrared spans a 1,000-fold range in wavelength, from ~1 micron to ~1 mm.
• Several ranges (NIR, MWIR, FIR):
  • Near-infrared (~1 to ~3 microns)
  • Thermal infrared (~3 to ~30 microns) -> range that the atmosphere makes opaque
  • Mid-infrared (~30 to ~3000 microns)
  • Far-infrared (aka submillimeter: ~300 microns to ~1 mm)

(Ranges defined mainly by detection technology; but also astronomical applications vary across those ranges.)

In this seminar we will focus on the near-infrared region.

Why astronomy in the infrared?

• The peak of the black body spectral energy distribution shifts to the infrared for temperatures below ~3000 K ([math]\displaystyle{ \lambda_\text{peak} = \frac{2898}{T(K)} \text{ (micrometers)} }[/math]).
• Interstellar extinction (caused by dust particles) decreases with increasing wavelength: the interstellar medium becomes more transparent.

There are other reasons too, but those are the most important ones.

Observing in the infrared provides access to many astrophysical objects and phenomena. Without aiming to be exhaustive:

• Star formation deeply embedded in dust
  • It happens in giant molecular clouds. It is permeated by dust, so to penetrate the dust we can use the infrared.
• Cool stars and brown dwarfs.
  • They are cold (they can even reach only hundreds of Kelvins).
• Molecular rotational and vibrational transitions.
  • Due to the fact that the typical energy involved in the transitions is in the order of the infrared spectrum.
• Hot dust in circumstellar disks and envelopes.
  • Temperatures of a few hundred to a thousand Kelvin --> it allows us to reveal those structures in the infrared.
• Visible/ultraviolet spectrum of high-redshift galaxies.
  • Ultraviolet light is redshifted and can be seen from the ground if we look far enough, because it is redshifted into the transparent section of the spectrum.
• Compositional information on the surface of rocky and icy bodies.
  • Like asteroids, comet nuclei, ... They reflect the light from the sun.
• Interstellar and circumstellar dust mineralogy.
  • Due to the fact that different mineral species that compose the grains of dust have different properties which tell us which mineral composites (composites of Magnesium, Sillicates, ...) are part of the dust and the physical conditions of the circumstellar environment or insterstellar medium.

Near-infrared instrumentation

• Instruments are cryogenic: enclosed in a vacuum vessel kept at very low temperature (typically ~100 K, or even below if we're looking at the longer wavelengths) to avoid termal glow of surfaces (due to the thermal emission).
• Minimal number of optical surfaces and moving parts: keep interventions to a minimum! Each thermal cycling damages the detector a little.
  • 1 week out of service because the process is pretty complex and delicate (you have to pump air in, warm it up, etc.).
  • The thermal cycles cause dead pixels because it breaks some bondings.

Detectors

• Infrared array detectors first available to astronomy in the late 1980s, few pixels.
  • Reason: due to the range of temperatures to which they are sensitive, they were "applicable" to militar applications.
  • After the cold war, some of this technology became disclassified and became available to be used by the astronomy community.
• Conventional Si (silicun) or Ge (germanium) detectors not useful in the infrared: gap between valence and conduction bands too large.
• The solution: Hybrid arrays: detector layer of proper semiconductors (typically HgCdTe, InSb), welded to silicon amplifier array (base) through In bumps.
  • Good linearity characteristics
  • Large formats currently available (2k x 2k)
  • Low dark current (~20 e-/hour)
    • Nothing spectacular for visible detectors, but for infrared detectors it is a break-through!
  • Low readout noise (12 electrons, < 3 ADU)
    • Again, pretty good for infrared detectors! (visible detectors can have a readout noise below 1 electron)
  • Good cosmetic quality (very few bad pixels)

Dealing with the infrared sky

• The infrared sky glows: OH emission below ~2.2 microns, thermal emission above.
• Atmospheric absorption by H₂O and CO₂ defines transparency windows.
  • This is why mountains are a good infrared detection spot (clouds are below the top of the mountain).
• Both emission and absorption vary on timescales of minutes.
• Individual exposures must be kept short to keep background level within the linearity regime detector and to mitigate variability; depth obtained by stacking.
  • ~ few seconds or 1/2 minutes, depending in the infrared wavelength we're observing in.
• Specific techniques required to remove atmospheric effects, both in imaging and spectroscopy.

RAFGL 5475

• RAFGL 5475 is a heavily embedded massive star in the process of formation.
• Surveys at radio and infrared wavelengths show other signposts of massive star activity in the interstellar medium of its surroundings.
• All the region is heavily obscured by the interstellar extinction, inaccessible to visible wavelengths.
• Nothing known about the stellar component.

Is RAFGL 5475 part of an OB association?

• Observed with PANIC, the near-infrared camera at the 2.2m Calar Alto telescope.
• Observations spread over four nights in August 2016 (shared with another program)
• Images in the J, H, K infrared bands (1.25, 1.65, 2.15 microns)

RAFGL 5475 if projected against a rich background in the Milky Way! How to identify the (presumably few) stars in the association?

The JHK color-color magnitude is a powerful tool for the identification of intrinsically blue stars...

...and yes!! there is evidence for a population of highly reddened, intrinsically blue stars.

No obvious concentration (apart from a blue star very close to the center of RAFGL 5475).

Spectroscopy? O and B stars have distinctive lines of H, He in the 2 microns window (infrared K-band).

Spectroscopy of stars in RAFGL 5475

...

Confirmed: there is an OB association around RAFGL 5475!

Conclusion

• Infrared instrumentation is an essential tool in the observational astronomer's toolset.
• Present in virtually all fields of astrophysics.
• Infrared instruments exist at most major observatories in the world.