Extracting science from Gaussians

Extracting science from Gaussians

I have released recently the first version for science production of 30mExplorer, with an impressive set of features that allow to reduce spectra about 100 times faster compared with classic techniques. As with other tools before this the result is far better than I expected, so we are actively taking advantage of it right now.

One project I'm involved is CHESS, the chemical Herschel survey of star forming regions. It is a Key Programme in development with the infrared space telescope Herschel, and its HIFI instrument. Our aim is to study the complex chemistry in the envelopes around intermediate mass protostars (and the hot core itself), and Herschel is suitable since it is located outside the Earth and hence it can detect a lot of molecules that are hidden from ground due to the opacity of our atmosphere. We have done an spectral survey of OMC2-FIR4, which is an extremely young source located inside the Orion Molecular Cloud complex. The survey is extended for a quite large number of molecular species, and our group is working in the CO and CS lines, which are abundant everywhere and suitable to obtain a good global picture of the source. So it is good to finish our work before the other groups since our results will provide the basics to better interpret and characterize this interesting source. One particularity is that this survey is extended from CO 5-4 transition, which is excited at temperatures of around 80 K, up to the 16-15 rotational line of CO, close to 2 GHz of frequency, which tracks the hot core region at temperatures beyond 700 K. So we can obtain valuable information about the chemistry from the outside of the protostar to the protostar itself. As we will see, spectroscopy is a powerful tool for this purpose, specially when optical and infrared observations are not possible due to the opacity at shorter wavelengths of the nebula where these objects are embedded.

CO lines

The first figure shows the spectrum of the rotational transitions 9-8, 10-9, and 14-13 of 12CO towards OMC2-FIR4. Photons can be emited from molecules through electronic, vibrational, and rotational transitions (which means that molecules as a whole can vibrate and rotate), and we study the latter which corresponds, in comparison, to lower energies and longer wavelengths, long enough to scape from these dense and opaque regions. 30mExplorer allows an almost perfect spectrum fit obtaining a set of Gaussians that represents different contributions to the emission. The main red Gaussian represents emission coming from the hot core. We know this because the line is wide (the gas is moving, probably colapsing to the inner core, with large velocities) and it appears also in transitions of higher energies. This emission is partially absorbed at the velocity of the cloud by an external, cold component located in the line of sight. It's cold because this component appears up to the 10-9 transition included (quite weak), but not at higher energies. The green line shows another component visible at all energies. It is probably a clump or hot corino embedded in the main one that also contains gas at very different temperatures. The other less intense components are probably related to the gas in the line of sight that emits slightly at different velocities, or maybe to the outflows, that produces those wide wings visible with pink/magenta colors at both sides of the spectrum. The outflows are ejections of gas at high velocities (shifted to the 'blue' or lower velocities if the gas is moving towards us, or to the red if it is moving away from us) and opposite directions, typical of the first stages in the formation and evolution of stars.


13CO lines

13CO is an isotope of CO, about 80 times less abundant. It is used to track the dense regions where CO appears optically thick, allowing to obtain a better estimate of the amount of mass. The second figure shows the observations and fittings to the emission of this molecule for the rotational transitions 5-4, 9-8, and 11-10. We can see two 'Core' components contributing to the emision at the velocities of the cloud, another two 'large velocity' components: LVR (redshifted) and LVB (blueshifted), and also the two 'high velocity' components HVR and HVB. In the 9-8 transition the LV and HV components start to disappear due to the better resolution of the observations at higher frequencies. The beam or resolution of Herschel goes from 40” to less than 20”, so obviosly at high energies we loose the high velocity wings produced by the outflows. At the same time, at higher frequencies the spectrum becomes more noisier and we have less sensitivity when decomposing it in Gaussian components.


In this isotope we don't see any absorption feature, since the gas in absorption is spread over the line of sight, and not located in the dense and compact regions we are sensitive to with 13CO.

The two core components are located at +11.6 km/s, which is the velocity at which this source moves away from the Sun. This confirms that we have a second core inside this intermediate mass object, as we saw with the CO lines. But interestingly, the properties of the 12CO and 13CO are different in both cores.

First results using the rotational diagram

The rotational diagram is a method to estimate the temperature and column density of the gas assuming certain approximations and LTE (Local Thermodinamic Equilibrium). It can be applied in relatively dense and hot regions where molecules collide with each other efficiently and the gas is considered thermalized. When having observations of different rotational transitions for the same molecule, this technique consists on calculating from the areas of the Gaussians and the properties of the lines (Einstein coefficients for instance, as given in catalogs of molecular spectroscopy like the JPL or COLOGNE catalogs) the abundance of the upper levels of the transitions. This parameter is charted as function of the temperature of the upper level, and allows to calculate the so called rotational temperature, which is the average gas kinetic temperature assuming LTE. This temperature it T = -1 / slope, where slope is the slope of the fit of the rotational diagram to a line.


The chart above shows the rotational diagram for Core 1 and 2 components of CO. It can be seen that the points cannot be fitted to an unique line, since the rotational temperature is not constant and increases towards the inner core (transitions of higher energies). Core 1, the main one, has a temperature of 67 K in the first three transitions and 140 K after, while Core 2 has also two main rotational temperatures: 56 K and 160 K, but the upper energy of the transition where we find the transition from one rotational temperature to the other is around 300 K.


In Fig. 4 the first two rotational diagrams show the Core 1 separated in two regions with different rotational temperatures, being the hottest region obviously the inner one (40 vs 100 K). The Core 2 temperature is similar to Core 1, 40 K, and we could not detect this component in higher energies since it is weak.


The rotational diagrams for the outflow components show that the high velocity component is hotter than the large velocity one, as expected, with temperatures between 40 and 80 K.

We are working on additional modellings using DataCube to better understand the temperature gradient of Core 1 and how the gas is colapsing into the core. We have also observations of C17O, C18O, CS, and C34S, but I will not comment them here since the main information comes from the lines described above. Other groups are working with methanol, N2H+, among other species, and hopefully everything will be published at some time during 2012.

blog/science_from_gaussians.txt · created: 2011/12/02 15:24 (Last modified 2016/05/17 12:41) by Tomás Alonso Albi
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