Diffuse Interstellar Bands (DIBs)

The composition of the interstellar matter and the processes involved are far from being fully understood. In particular, one of the most challenging problems in terms of ISM composition is the ubiquitous presence of the numerous, unidentified Diffuse Interstellar Bands (DIBs) detected in our Galaxy and other galaxies. There is a large consensus recently reached about the nature of their carriers, thought to be large carbonaceous molecules in the gaseous phase of the interstellar medium (ISM). Such macro-molecules are key elements of the life cycle of interstellar matter from stellar ejecta to star formation and understanding their role and evolution would help elucidating unclear aspects of star and planet formation (Tielens 2014).

Up to recently, most of the DIB studies had as a unique goal the identification of the carriers, and therefore, focused on a limited number of hot and reddened stars. My research work marks a turning point in the methods and goals associated with the DIBs and ISM spectral features and turns these molecular features themselves into powerful probes of the conditions in their surroundings.

This evolution is now possible and motivated by the increasing number of stellar surveys with high multiplexed instruments.

A part of my overall research interests presents research regarding mostly the observational side of this topic. It was based on the exploitation of spectroscopic data from select ground-based surveys: 1) The SDSS-III APOGEE, the VLT/KMOS and VLT/CRIRES surveys which are optimized for near-infrared (NIR) spectroscopy, and 2) the EDIBLES (VLT/UVES) survey which is optimized for near UV and optical spectroscopy.

All types of spectral analysis of DIBs have common data cleaning/processing requirements, in particular the removal of telluric lines, the separation of stellar/interstellar signatures and extraction of DIBs using specific codes and automated methods, "stacking" methods in different reference frames, and finally the search for information on dust, gaseous species and physical properties associated with each line of sight.

Here, I highlight some of these data treatments and describe some of the main products of my research work:

Relative strengths of observed diffuse interstellar bands. The origin of most DIBs remains unknown, with common suggestions being polycyclic aromatic hydrocarbons (PAHs) and other large carbon-bearing molecules. @G. Zasowski

Strong constraints on DIB carriers

In Elyajouri et al. (2018), I led the analysis of high-resolution spectroscopic data from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) at VLT/UVES. This included the first characterization of DIB substructures. Using the UVES unique dataset with exceptional signal-to-noise ratios (800-1500), I analyzed a set of weak DIBs linked to the C2 molecule. I showed that these DIBs exhibit profiles with substructures, and clear variability from one DIB to another and along different sightlines. I discussed these substructures in the context of unresolved rotational bands, typically produced by molecular carriers.

Modeling these profiles established a connection between the observed spectral variations and temperature changes, leading to the deduction of molecular constants, including constraints on the size and geometry of the carriers. These results are essential for guiding experimental research, for example in the COSmIC Simulation Chamber (NASA/Ames). This ongoing collaboration with Farid Salama focuses on testing the hypothesis of polycyclic aromatic hydrocarbons (PAHs) as potential DIB carriers.

Extraction methods and massive data processing

I developed robust reduction and analysis techniques to extract as much as possible information from stellar spectra. Such methods have been applied to NIR spectra, in particular, from the Data Release 12 (DR12) of the SDSS/APOGEE survey in the H band to study the distribution of the 15273 Å NIR DIBs (Elyajouri et al. 2016). First trained on the Telluric Standard Star (TSS) sample and then adapted to the specificities of later type stars and series of archival data, my work has led to the production of catalogs, to the characterization of very weak DIBs that were previously only barely suggested in a few sightlines, and to the discovery of new bands. In addition, I generated the first homogenous catalog for a series of NIR DIBs weaker than the DIB15273.

DIB-DIB correlations among the NIR DIBs in this database and a correlation study of the NIR 15273 DIB with 5 extensively-studied optical DIBs (DIB5780, DIB5797, DIB6196, DIB6614 and DIB6283) revealed that the NIR DIBs seemed best correlated with the DIB6283, an optical DIB known to trace the skin of ISM clouds. This study reveals striking similarities between the behaviour of the optical and the NIR DIBs considered here and showcased this NIR DIB as a viable tracer of the ISM physical conditions at Galactic scales (Elyajouri et al. 2017b).

While working as an A.T.E.R, I was able to improve my automated fitting codes dedicated to massive data and apply them to the SDSS/APOGEE DR14 survey. One result of my analysis was the production of the most comprehensive database of 124 064 new measurements (band wavelength, band width, equivalent width) of the 15273 Å infrared DIB. Subsequently, I used this extensive DIB catalog to constrain the 3D structure of the ISM in the Galactic disk by comparing this DIB equivalent width distribution (EW15273) with the distribution of known ISM tracers such as the color excess or the dust opacity as measured by Planck. A series of extinction laws relating EW15273 to Av and the Planck τ353 opacity are derived and discussed in the context of the “skin-effect” in my paper Elyajouri & Lallement (2019).

These new databases, the environmental and tomographic studies can open the way to more extensive work based on future surveys from ground and space. I have explored these studies and reached two further steps.

A first step is the first detailed tomographic study of individual clouds. A second step is the full 3D mapping of ISM using DIBs.

Colour composite of visible (left) and infrared (right) images of the dark cloud Barnard 68 (B68). Credit: ESO

Tomography of dark clouds: Barnard 68

For a pilot project (ESO 096.C-0931(A), PI: N.L.J Cox) we have obtained VLT/KMOS moderate resolution (R =4000) near-infrared (H-band) spectra of 85 stars located behind the Barnard 68 dark globule – from the edge to the centre. My contributions were as follows: the data has been fitted by the convolved product of a telluric transmission and a stellar model. The telluric spectrum is adjusted for airmass and the observing site. The stellar model was built from the SDSS/APOGEE stellar library. Based on a simplified inversion assuming sphericity, I found that the volume density of the DIB carrier at 15 273 Å is 2.7 and 7.9 times lower than this expected average value in the external and central regions of the cloud which have n(H)=0.4 and 3.5 x 10^5 cm−3, respectively (Elyajouri et al. 2017a). The results demonstrate the absence of the 15 273 Å DIB carrier in the denser parts of such dark clouds. This demonstrates that future measurements using multiplex NIR spectrographs and extended to more DIBs, more targets, and various types of clouds should allow one to probe the volume density distribution of the DIB carriers precisely enough to shed light on the processes that govern their carrier formation and destruction. Such a tomography constitutes a prototype of future three-dimensional studies of DIB carriers with JWST’s NIRSpec. Indeed, the advent of space multi-object spectrographs such as NIRSpec will provide massive amounts of data and new detections that will serve the basis for a wide range of analysis about NIR DIBs.

DIBs ?

DIBs

Credit: R. Lallement

Contributions to 3D ISM Tomography

Despite the absence of identifications, optical and NIR DIBs are interstellar cloud tracers and I have tested their use for 3D mapping. I developed the appropriate spectral analysis tools based on state-of-the-art synthetic spectra to make use of stars of all types (early-type stars and the more numerous late-type stars) using the methodology presented in Elyajouri et al. (2016) as a starting point. We merged the resulting catalogue of DIBs derived from Tycho targets with the extinction data, using a conversion factor based on our DIB-extinction correlation study and taking into account the dispersion around the mean relationship. The merged dataset was inverted to produce a 3D map of the local ISM (Capitanio et al. 2017). This illustrated the potential of DIB carriers mapping and its comparison with dust, gas, radiation and star 3D distributions to better understand the conditions of formation and disappearance of DIB carriers and to probe the spatial distribution of the ISM (Lallement et al. 2018).

Project: STILISM - https://stilism.obspm.fr/

ANR Programme blanc: 2012-2017 (ANR-12-BS05-0016)

ANR TEAM: P.I. Lallement Rosine

Partners:

  • GEPI /Paris Observatory: Arenou F., Babusiaux C., Gomez A., Katz D., Haywood M., Lallement R., Leclerc N., Sartoretti P.

  • ACRI/ST: Vergely JL., Ferron S.

  • AIM/ IRFU: Grenier I., Casandjian J.M.

  • PHD students: Remy Q

  • Post-doc: Monreal-Ibero A.

  • New members of the GEPI team: Elyajouri M., Capitanio L.