masers johns hopkins university, tuesday, december 12 microwave amplification by the stimulated...
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MASERS
Johns Hopkins University, Tuesday, December 12
Microwave
Amplification by the
Stimulated
Emission of
Radiation
(S) connotes plural
Outline• Maser General Properties
• Maser Physics
• Masing Regions
• Types of Masers
• Locating Masers
• Two Maser Case Studies
• Megamasers
Masers: General Properties• Nature’s lasers, caused by stimulated emission
• Einstein introduced stimulated emission in 1916, but first (OH) maser not identified until 1966.
• Extremely small in angular size
• Due to nature of stimulated emission, maser light is highly beamed and coherent (like lasers) and quite bright
• Masers require very specific environments (e.g. population inversion, length of path), so they can be variable on the scale of days to 105 years.
• Found in and out of galaxies, on comets, in supernova remnants, and possibly in planetary atmospheres. Usually, they are associated with star forming regions.
Masers Physics: Identifying the Cause
How does Stimulated Emission Amplify Radiation?
- Consider that 1 and 2 are two levels of a particular material, with energies E1 and E2 such that E2 > E1.
- Let I = Intensity of a beam of electromagnetic radiation passing through material
- Let ω = Angular frequency of radiation = (E2 – E1)/ħ
- Let dρa/dt = Rate of change of the average energy density because of absorption from the beam
- Let N1 = Number of atoms in lower energy level per unit volume
- Let W21 = Transition rate per atom for absorption
211 WNdt
d a
211 WNdt
d a 122 WN
dt
d s
IW /12
- Let W12 = Transition rate per atom for stimulated emission
Atomic theory tells us that and that both are proportional to to the intensity I of the incident radiation.
- Let σ = Cross-Section, defined as
2112 WW
Note: σ is characteristic of the particular pair of levels, but is independent of the intensity of the incident radiation.
121212211122 NNINNWWNWNdt
d
dt
d as
Masers Physics: Identifying the Cause
The rate of change of the energy density can be converted to a change in intensity with respect to depth, z.
12 NNIdt
d
dz
dI
dt
cId
dt
d
/
12 NNIdz
dI
If N2 > N1 the incident radiation is amplified exponentially as it passes through the medium, hence, a maser!
Masers Physics: Identifying the Cause
Creating a Population Inversion• Under normal conditions of thermal equilibrium, the number of
particles in lower energy levels is always greater than the number in higher energy levels. This leads to radiation being absorbed rather than amplified.
• One requirement in creating an inversion is “pumping”, either collisionally or radiatively.
Creating a Population Inversion• Three-level pumping is difficult. It takes a LOT to get more than
half of the atoms/molecules in level 2.
• The easier solution is four-level pumping.
• Maser action can continue as long as the population inversion is upheld, or as long as the system is unsaturated.
Requirements for Masing Regions
• Gas density must be considerably higher (105 to 1011 cm-3) than that found in giant molecular clouds.
• Highly luminous sources (> 104 solar luminosities) are needed to pump the masers. In star-forming regions, hot O and B stars can accomplish this, though masers are found around evolved stars.
• Specific Molecules which can sustain population inversions are needed.
- Implication: Expanding gas clouds will cease housing masers, e.g. observations reveal that compact HII regions larger than 3e17 cm never show OH maser emission.
Molecules Known to Exhibit Maser Emission
• OH (hydroxyl)
• H2O
• SiO• CH
• H2CO
• CH3OH (methanol)
• NH3
• HC3N
• HCN
Specifics about Individual Maser Species
• What specific lines should you look for and what causes those particular emissions?
• How should radiative and collisional excitations be accounted for?
• How can masers be classified and grouped?
Detailed analysis provides answers to the following:
Order of magnitude: Over 200 methanol lines have been detected between 360 and 834 GHz. Over 20 of those display maser emission.
H2O Masers: An Important Subgroup
• Example line: Arises from the 616 – 523 rotational transition at 1.3 cm due to nearby rotational states.
• Many more water lines were predicted, and later discovered, partly due to radiation’s difficultly in getting through the atmosphere.
• These trace high densities (n(H2) > 107 cm-3) and with kinetic temperatures Tkin~400K. They have huge brightness temperatures (~1012 K, >109 K) and narrow line widths suggesting non-thermal origin.
• They are quite small. We can see them on scales ~ 10AU.
• Often spatially close to ultra-compact HII regions.
• Spectral appearances change rapidly. Features can vary significantly on a daily basis, and have lifetimes ~ 2 years. OH masers, in contrast, have remained easily recognizable for over a decade.
• Visible at the outer edge of the Galaxy, where other masers are not found.
H2O Masers: An Important Subgroup
Based on the work of Hachisuka et al., 2006
How Do You Find a Maser?• You expect them to reside in dense and bright regions.
• Most astronomical masers have much smaller line widths than expected if caused by thermal motions, i.e. peaks grow faster than wings.
• You are looking for objects that are very intense and extremely small in angular extent. Limitations on making more detections than we have currently is mostly due to instrumental limits.
- Line widths are smaller by factors up to 10, but are smaller than about 5 times on average.
• Radiation is bound to be at least somewhat circularly polarized.
Case Study 1: Magnetic Fields
Strong magnetic fields inside of dense molecular clouds support clouds against gravitational collapse. Protostars begin formation only when gravity overwhelms the magnetic pressure.
Based on the work of Vlemmings et al., 2006
Magnetic fields also play a role in bipolar outflows and circumstellar disk formation.
Motivation
Case Study 1: Magnetic Fields
Based on the work of Vlemmings et al., 2006
Looking at Cepheus A HW2
Case Study 1: Magnetic Fields
Based on the work of Vlemmings et al., 2006
• By analyzing velocities, you can determine the type of shock the maser might be associated with.
• Magnetic field strength is determined from circular polarization measurements. Data is applied to models established in the literature.
Case Study 2: Water Maser Distance Indicator
In order to establish the cosmological distance scale we need to build up the “rungs” of our distance ladder. While we have many relative measures of distance, absolute distance determinations are harder to come by.
Locally, we would like to know the distances to objects within our Galaxy. Parallax is the best direct measurement tool we have.
Based on the work of Hachisuka et al., 2006
The problem with parallax is one of resolution. We can’t see motions that are too small.
d
22.1sin
The best resolution in astronomy, courtesy of Very Long Baseline Interferometry, can determine distances up to a few kpc with greater than 10% uncertainty using the VLBA and maser sources.
Motivation
Case Study 2: Water Maser Distance Indicator
Based on the work of Hachisuka et al., 2006
Case Study 2: Water Maser Distance Indicator
Based on the work of Hachisuka et al., 2006
Hachisuka et al. determined both the distance to and dynamics of W3(OH), a region with both intermediate and high mass stars at different stages of development.
To determine parallax, they took measurements once every three months on average for a year and a half.
Case Study 2: Water Maser Distance Indicator
Based on the work of Hachisuka et al., 2006
Land of Happiness• Measurements such as these allow us to probe Galactic structure
and dynamics.- Determining relative motions of different maser features can be described by an outflow
model by taking the proper motions, radial velocities, and positions of maser features into account.
• Water masers are found at the outer edge of the Galaxy, where other masers are not.
• Water maser parallax measurements achieved through VLBI are accurate to about 10 microarcseconds.
Land of Sadness• Water masers vary significantly with time, making many measurements unreliable, or worse, impossible over a period of years.
- Best results would come if we could find many H2O masers clustered nearby, but in the Milky Way, this requirement is often difficult to meet.
Extragalactic Masers = Megamasers
• A new class of OH sources was found outside the Milky Way in IC 4553. Their luminosities were massive compared to Galactic masers, ~103 solar luminosities.
• Since then, only about 60 of these megamasers have been detected.
• Theoretically, the requirements for megamasers are quite pedestrian. In fact, they may even be widespread throughout our Galaxy and others.
• Detection rate in extragalactic sources is disappointingly low…only a couple percent.
• They seem to be associated most strongly with starburst or Seyfert galaxies, though they fit neatly into neither category.
• OH Megamasers (OHMs) are luminous masers produced in bright, massive, merging galaxies undergoing star formation. They are often associated with AGN, but their environments are otherwise not well understood.
Extragalactic Masers = Megamasers
• Water megamasers are powerful tools for studying regions very close to AGN central engines. They can be broken into three classes:
• Differences between megamasers and regular masers include:
1. Those tracing accretion disks
2. Those in which at least a part of the water emission is believed to be the result of an interaction between the nuclear radio jet and an encroaching molecular cloud.
3. Those, typically with lower luminosities (< 10 solar luminosities) often associated with prominent star forming regions in large-scale galactic disks.
1. We’ve only seen megamaser species OH, H2CO, and H2O.
2. Radiation is less amplified.
3. Lines are more broad.
4. Not bright in comparison to galactic masers.
5. Not polarized.
Summary• Masers occur in regions of above average density and luminosity
with specific molecules that can support population inversions.
• Masers are very bright and very small in size.
• Galactic masers are conspicuous for their narrow line widths.
• Megamasers are similar, yet different than extragalactic masers.
• Uses include probing a region’s kinematics, mass, luminosity, and/or geometry. They are also useful in determining the strength of local magnetic fields and the distances to them through VLBI.