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Wikipedia's Astrophysical_maser#comets mentions some anecdotes of notable masers associated with comets, and Maser emissions from comets begins:

The 18-cm lines of the OH radical are the only well-documented masers in comets. They have been observed in more than 65 comets since 1973. The good knowledge of their excitation mechanisms and their linear regime allow us to estimate the water production rate and its variation with heliocentric distance from radio observations of OH in comets…

I'm guessing that OH around comets starts as water and is dissociated by UV light from the Sun. Either it's left in a metastable excited state or it's excited (pumped) up to one by other photons from the Sun.

What is it about the 18-cm lines that allows one to conclude that this is masing; that there is microwave amplification by stimulated emission rather than just strong fluorescence?

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  • $\begingroup$ The same Wikipedia article list the properties of astrophysical master emission, including very high brightness and polarisation. $\endgroup$
    – ProfRob
    Commented Apr 8, 2020 at 20:12
  • $\begingroup$ @RobJeffries okay those are aspects associated with masers, but in terms of a real "How do we know..." is there some specific threshold on brightness temperature that can never be reached by pumping, cascades and spontaneous emission alone and can only be the result of amplification by stimulated emission, and comets exceed this threshold? And are there constraints on the type and degree of polarization that can not be produced by any other mechanism and can only occur via amplification by stimulated emission? $\endgroup$
    – uhoh
    Commented Apr 8, 2020 at 23:17
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    $\begingroup$ @uhoh Just a tip to put in comments: masing has a lot to do with hydroxides forming from water and yes it has something to do with light from a light source. I see potential for expansion in my answer though, so I'll keep updating it with new sources and information! :3 $\endgroup$
    – DialFrost
    Commented Oct 6, 2022 at 10:42

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TLDR;

Yes it does indeed "mase"! Note this answer will cover only the masing of comets.


Long answer;

According to this paper on Maser emissions from comets:

In contrast with interstellar OH masers, the cometary OH maser has a well know excitation mechanism. It is a weak maser operating in the linear regime. Its quantitative modelling permits a reliable estimation of OH column density in a first step, the production rate of water from the cometary nucleus in a second step.

Since 1973, the 18-cm OH lines have been observed in more than 65 comets.

Now for the important part which deduces the OH maser:

Studies with realistic modelling of the OH radio lines in comets started with the discovery by Biraud et al. (1974) and Turner (1974) of the 1667 & 1665 MHz transitions arising from the ground state $\Lambda$-doublet in comet Kohoutek (C/1973 E1). Mies (1974) and Biraud et al. (1974) invoked the solar UV pumping of the $A^2\Sigma^+$ excited state from the $X^2\Pi_{3/2}$, followed by subsequent cascades, as the main mechanism to populate the OH $\Lambda$-doublet energy levels. They succeeded to explain the observed line reversals (absorption to emission or vice versa) by the Swings effect, namely the population distribution dependence on the Doppler shifted solar UV spectrum, due to the heliocentric velocity variations.

Following Schloerb & Gerard (1985), the flux $F$ of a OH 18-cm line is:

$$F=\frac{A_{ul}kT_{BG}}{4\pi\Delta^2} \frac{2F_u+1}{8} \iint B(x,y) \int i(s)n_{OH}(x,y,s) \text{ } ds \text{ } dx \text{ } dy$$

To be clear, fluorescence occurs when there is emission of light as the substance had absorbed light or other electromagnetic radiation, while specifically on comets, masing occurs when ultraviolet light from the Sun breaks down some water molecules to form hydroxides that can mase. In 1997, 1667-MHz maser emission characteristic of hydroxide was observed from comet Hale-Bopp.

So basically for comets, masing has to do with hydroxides formed from water molecules.

enter image description here

Occultation event observed with the VLA on 12 February 1997: 1667 MHz (dashed line) and 1665 MHz (full line) OH line variations are observed when comet Hale-Bopp (C/1995 O1) was passing in front of the background source J2013+220. The OH main lines areas are shown versus the distance measure, the projected distance between the background source and the point of closest approach $6.1\times10^5$km (Butler et al. 1998)

The full pdf file website can be found here

You may be asking now, how does the 18-cm wavelength has anything to do with OH maser of comets? Well according to this paper by EPSC - Observations of OH maser comets in 1.6GHz frequency band:

The 18 cm line is the result of an excitation from resonance fluorescence, whereby molecules absorb solar radiation and then reradiate the energy. The OH molecule absorbs the UV solar photons and cascades back to the ground state Lambda doublet, where the relative populations of the upper and lower levels strongly depend upon the heliocentric radial velocity (the “Swings effect”).

Definitions

NOTE: Resonance flourescence is different than normal fluorescence, but related:

the process in which a two-level atom system interacts with the quantum electromagnetic field if the field is driven at a frequency near to the natural frequency of the atom.

To clarify:

An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum.

The "stimulated spectral line emission" definition from Wikipedia:

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.

Now, the 18-cm wavelength used actually stimulates this emission and this paper on A Masing Hale-Bopp explains the rest:

Sublimated $H_2O$ becomes dissociated by solar radiation to create OH radicals. The OH is then excited by solar UV emission lines to decay by fluorescence to upper and lower levels of a rotational ground state.

So in a sense you are correct to say a sort of "fluorescence" occurs, but it does "mase".

Proof

Evidence that a comet mases:

enter image description here

Here as the velocity nears 0, we see a sudden spike in the radio flux, which is in the equation I provided earlier, due to a rise in radiation and decay from the OH on the comet.

A seriously long explanation of the graph can be found on page 3 here.


Additional information from Wikipedia - Astronomical maser#Comets

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    $\begingroup$ Okay I'm starting to get it now. I'll have to read further to understand, but the argument seems to involve spectral lines in the solar spectrum and details of Doppler shifts of the pumping radiation from the Sun as well as with the line-of-sight velocity of the gas.. And finally, with a line like "Despite these complications one can see that there is significant OH emission and that it is indeed a-masing!" who can argue!? :-) $\endgroup$
    – uhoh
    Commented Oct 6, 2022 at 12:42
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    $\begingroup$ Hehe masing is truly a-masing! @uhoh Appreciate the bounty, I'm still going to add a lot more information, do you mind? Cuz I'm afraid long answers are too length and users don't end up reading them $\endgroup$
    – DialFrost
    Commented Oct 6, 2022 at 12:45
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    $\begingroup$ Oh go for it! Long answers are definitely read by those future readers who come here looking for good answers. $\endgroup$
    – uhoh
    Commented Oct 6, 2022 at 12:47

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