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Astrophysical Masers

I thought I'd take care of my daily blog post business earlier today so, here's the 2 page report I made for a presentation as part of a course on Laser physics & Applications -

Astronomical Masers
by Poruri Sai Rahul

PH5814 - Laser Physics & Applications

Masers are the microwave analogues of Lasers. So far, as part of the course, we’ve been looking at optical and IR lasers. Similar to optical lasers, masers can be produced by atomic/molecular species such as Ammonia, Hydrogen, Rubidium & Free electron masers and by solid state masers such as the Ruby maser & the Fe-sapphire maser. While lasers emit in the Thz regime, masers emit in the Ghz regime.

Coming to the astronomical significance of masers, Astronomers equipped with spectroscopes started observing very strong spectral lines from astronomical sources. These sources were extremely bright in the radio spectrum while their optical counterparts looked like young stars. This emission was also discrete and not a continuum which is what caused astronomers to suggest masing of light from the background star by a medium between us and the star. Further studies revealed that such maser lines were peculiar to young type stars and were constrained mostly to star-forming regions in the galaxy. It was concurred that such maser lines are being produced by the amplification of stellar light by an expanding shell of atoms around the star, in our line of sight.

To understand the differences, let’s go back to the theory of lasers. The basic design of a laser is that of two mirrors and a lasing medium between them to provide the necessary amplification or gain. The mirrors are also highly reflective causing multiple reflections of light before it exits the cavity. We can refer to such a laser as a Multi-pass laser. Drawing this analogy, we can look at an astronomical maser as a Single-pass laser. Unlike multi-pass lasers where the pump beam is bounced back and forth multiple times through the gain medium, in single pass astronomical masers, the pump light from the background star only passes through once through the gain medium. In normal lasers, we needed multi-pass to be able to achieve meaningful gain. In astronomical masers, this is achieved using an astronomically long gain medium!

Another important thing to note is the condition of the star-forming region or the masing region. The molecules along the line of sight, which are causing the masing, should have similar velocities. This is another reason why masers are observed specifically around young stars as young stars have expanding gas shells around them, where species are collectively moving together.

Now, to get an understanding of the kind of numbers we are dealing with, let’s look at the strength of the pump, density of the gain medium, velocities of the masing species and overall size of the gain medium we’re looking at. Astronomical masers usually have a pump with a luminosity of 10^4 solar luminosity. Density of the masing medium is of the order of 10^9 per cm^3. Velocities of species are of the order 100-300km/s. Brightness of the maser lines is of the order of 10^12 K and radius of the gain medium/shell is of the 10^17 cm. Temperatures of the gain medium on the other hand should be ~160K.

NOTE : While the temperature mentioned seems absurd, it is infact the temperature of a blackbody with the intensity of the observed line at the particular frequency. This is merely a convention in Radio astronomy. But one can understand the

As you can understand by now, we overcame the inadequacies of single pass over multi-pass laser by having an extremely strong pump, a dense and cold masing medium over a very long length.

Commonly observed species that emit astronomical masers are - OH, H_2O, CH, H_2CO CH_3OH, NH_3, HC_3N and HCN.

Coming to interesting properties of such masers, as is common to lasers, line narrowing occurs in maser lines and the maser lines show high polarization. If a global magnetic field exits in the gain medium, as is the case in most star-forming regions, the magnetic field will give us an accurate quantization axis along which the species in the gain medium can emit radiation. Line narrowing is common in masers i.e the peak of the maser line goes up and the width of the line decreases, a characteristic of any maser line.

Looking at the applications of astronomical masers, they’ve been used for distance measurement, to study galactic properties such as the galactic rotation curve, the galactic magnetic field, to study interstellar scattering and to study the evolutionary schemes of late-type stars.

This small report relies largely on the review article Astronomical Masers i.e the reference [1]. As any review article on a field is, it has extensive references to other papers on topics ranging from the discovery of astronomical masers to their properties, unique properties of the different species observed and to the applications of astronomical masers. Reference [4] is a good undergraduate level introduction to the theory of lasers and basic concepts in Astronomical Masers. References [5] and [6] are also short and concise introductions to astronomical masers while references [2] and [3] can be read for a preliminary understanding of masers and astronomical masers, in general.

PS - Do get back to me if there are any corrections that need to be made or if I said something absolutely horribly wrong which needs to be corrected!

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