Frequently Asked Questions
This page is an initiative by students in Jan Cami’s research group.
This page consists of a lot of questions (which were either asked to us when presenting our research, or questions we think are important to discuss). The idea is that anyone who is getting started on DIBs can give a read and obtain a good idea of what DIBs are and what the EDIBLES program is about. It will also help current students to write introductions for papers/reports, prepare for conferences, get a good idea about the interstellar medium and their work on DIBs etc. This is also a way to document our discussions and make it available to others.
@Collaborators Feel free to add answers/ more questions to the list!
Happy Reading!
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The targets which we have are basically molecular clouds (sometimes near the star-forming region), which are known for their rich chemistry. When we analyze the spectra obtained and filter out the stellar lines, then what we get is basically the absorption lines of the molecules in that target molecular cloud. However, those lines are not a single line, but a set of lines, which appear to be fuzzy, jumbled up and coagulated on the spectra. They are very wide bands and hence we call them “Diffused” Interstellar Bands (DIB). The figure below shows how wide DIBs (black) is compared to Narrow atomic lines like the sodium line (red) shown here.
We are trying to detect molecules that are present in the medium between the stars of the galaxy. So, we focus on lines that are present in the spectra which come only from these molecules, masking or removing the contribution of lines and part of spectra coming from the star itself which we have taken the spectra of.
There are 2 (or more) pieces of evidence that DIBs are interstellar and not stellar (eg: not in the atmosphere of Stars): It was established that they are of interstellar origin because there are no periodic oscillations in the position of absorption bands due to the Doppler shift. Instead, DIBs are found to be stationary (Merrill, 1936). It implies that they are caused by the molecules in the Interstellar medium - gas and dust between the stars. It was further verified when observational surveys (Herbig, 1993; Friedman et al., 2011) found DIBs strength increases with increasing distance and column density of Interstellar medium.
There are more than million possibilities of molecules and even combinations of those due to the environmental conditions of the source under investigation. Potential DIB carriers have bands at almost same wavelength range We need high-resolution data over a wide range of wavelength (like EDIBLES)
EDIBLES data is a large collection of about 400 diffuse absorption bands from spectra of interstellar medium (obtained from spectra of stars after removal of just the spectra of the star) along around 120 lines of sight obtained using spectrograph of VLT. It is to be used to identify the constituent molecules and elements of the interstellar medium present in our galaxy, if one is to compare them with data of bands taken in laboratory under induced. conditions similar to the ISM.
If we look along different LOS, then we may possibly encounter density fluctuations in the molecular cloud, which might affect the strength of the features obtained for a particular DIB. Also, chances of secondary and tertiary ionization increase as some foreign molecule can disrupt the specific lines from the molecule under study.
Rotational temperature varies across different LOS.
Stacking refers to superimposing and averaging different spectral features (temporal) on top of each other. We carry out stacking of different spectrums to enhance our signal-to-noise ratios. By stacking, we cancel out the random noises/ spurious signals in our data, allowing us to enhance the quality of the characteristics of the signals we are looking for.
It’s a way to get detections from a bunch of non-detections.
There could be two factors that would contribute to the band becoming diffused - firstly a molecule itself having many levels so close together they appear as a diffuse band (for example in the above figure we can see multiple bands diffused together) and secondly there is the possibility of having multiple molecules having similar transition levels overlapping to produce a diffuse absorption feature in the spectra.
Why are we interested in knowing about the existence of molecular species which don’t have much impact on the human race?
DIBs are a collection of absorption features, observed in the spectra of the reddened stars (mainly O- and B- types). Given their strength and their widespread occurrence in harsh interstellar environments, the DIB carriers are most likely abundant, stable carbonaceous species such as carbon chains, polycyclic aromatic hydrocarbons (PAHs) or fullerenes. DIBs can be used as tools to get a 3D map of the universe and can be made an important diagnostic tool of the interstellar medium.
Interstellar absorption observed toward stellar targets changes slowly over long timescales, mainly due to the proper motion of the background target relative to the intervening clouds, such that over time, different parts of the intervening cloud are probed. On longer timescales, the slowly changing physical and chemical conditions in the cloud can also cause variation. Detecting such time variations thus provides an opportunity to study cloud structure. (Farhang et al. 2022)
Why do we think DIBs are caused by carbon molecules and not dust grains? What is the consensus about what DIB carriers could be?
Initially, dust grains were thought to be responsible for DIBs, which was later found not to be true, because while bands from dust grains are very sensitive to grain size and composition of ISM, DIBs were found to have constant peak positions and widths along all sightlines (Tielens, 1999). Also, dust grains can slightly polarize the starlight, but DIBs are found to be unpolarized. Due to its strength and ubiquity, the current consensus is that DIBs carrier molecules are most suitably stable, carbonaceous molecules like carbon chain, PAHs, fullerenes, or equivalent species.
The shape of the DIBs can be explained by the rotational transitions of molecules. The transition of the molecule between different rotational levels within the same electronic level. Those transitions can explain the shape of the feature.
(Learn more about P, Q, and R branches: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Rotational_Spectroscopy/Rovibrational_Spectroscopy )
EDIBLES is unique in its combination of spectral resolution, wavelength coverage, sensitivity and sample size.
DIBs are absorption features. DIBs are obtained from the molecules present in the molecular clouds near the reddened stars (mainly O- and B- types). So, the light from the star is observed which reaches us through the molecular cloud where we are searching for the DIB carriers, and the specific diffused, fuzzy regions in the spectra are the DIB imprints. Thus DIBs are essentially absorption features. However, the DIB carriers may also have emission lines either in the same line of sight, or in any other random direction. Thus depending on the emission features will not be a dependable option.
The target stars for the EDIBLES survey are in or near the galactic plane, spread across all galactic longitudes except from 60o to 150o (missing the part of galactic plane in Vulpecula, Cygnus and Cassiopeia) as they were not visible during the night during the observation period. This varied sample of stars should help in getting an idea of the ISM across the galaxy.
How do EDIBLES distinguish interstellar spectra from stellar spectra? On what conditions/characteristics?
During stacking, why are we interested in superimposing signals at different wavelengths on each other?
How do we analyze different characteristic features of molecules in the local standard of rest velocity? What if we decide on an inertial frame for such an analysis?
