N1-0040 Dynamical Stellar and Interstellar Medium through Optical Stellar Spectroscopic Surveys

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Research project is (co) funded by the Slovenian Research Agency.

UL Member: Faculty of Mathematics and Physics
Code: N1-0040
Project: Dynamical Stellar and Interstellar Medium through Optical Stellar Spectroscopic Surveys
Period: 1.9.2015 - 31.8.2018   
Range per year: 1,45 FTE, category D
Head: Tomaž Zwitter
Research activity: Natural sciences and mathematics
Research Organisations: link on SICRIS
Researchers: link on SICRIS
Citations for bibliographic records: link on SICRIS

Project description:

In 2020, the Gaia mission (launched in December 2013) is expected to release 6-dimensional (spatial position + velocity) vectors for a significant fraction of stars on our side of the Galactic centre, thus allowing a computation of stellar orbits and of evolution of the Galaxy as a whole. Traditional studies of the interstellar medium cannot yield information equivalent to stars, as absorption studies get only a 2-dimensional (column density) information by observing one hot star at a time. Here we propose to open up the 3-rd and the 4-th dimension in studies of Galactic interstellar medium (ISM) by studying diffuse interstellar bands (DIBs), weak but numerous absorption lines seen in spectra of background stars which are likely caused by distinct macromolecular carriers. With a new method developed in my team we are able to measure the strength of these weak interstellar lines also for cool stars which by far outnumber hot stars in the Galaxy. By observing a given DIB toward many stars which are nearly in the same direction but at different and known distances one can reconstruct absorption sites along the line of sight. Joining observations in many directions on the sky then gives their spatial distribution. Finally, measurement of radial velocity shift yields a 4-dimensional picture of the ISM for each DIB. Interstellar absorption lines of sodium and potassium atoms yield information equivalent to DIBs, but emission lines or dust absorptions cannot go over 3 dimensions.

Here I propose to address the challenge of obtaining complete information on the best ISM information carriers in the optical and on a massive scale for the first time. A dozen DIBs and the neutral potassium interstellar line at 7699 Å will be measured in a million sight-lines toward stars away from the Galactic plane and with known spectrophotometric distances which are being observed by the ongoing and massive GALAH and Gaia-ESO optical spectroscopic surveys which are the largest optical high resolution stellar surveys in this decade. My team has an unconstrained access to proprietary data of these surveys and a detailed knowledge of data collection and reduction, as we wrote and tested data reduction pipelines for GALAH and we are co-leading research on peculiar stars in Gaia-ESO.

ISM is the place of violent collisions of supernova shells, plus winds from asymptotic giant branch stars and hot-star associations. Head-on collisions in the Galactic plane are difficult to interpret. But our observations are away from the plane where interactions generally result in a net motion perpendicular to the plane. If any shells of absorbing material are identified we can assume that their motion is perpendicular to shell surfaces and reconstruct a complete velocity vector from its radial velocity component. Such information for ISM is then equivalent to the one collected for stars by Gaia.

The main goal is to study past events in the interstellar medium, most notably explosions of supernovae as old as a million years. Our pilot quasi 3-dimensional map of intensity of one of diffuse interstellar bands shows that its distribution is different on either side of the Galactic plane, a witness to asymmetries in placement of recent explosions of supernovae and to incomplete vertical mixing. We aim to identify and characterize Galactic fountains blown away by these ancient supernovae. Such flows are thought to sustain star formation in the disk by entraining fresh gas from the halo, so they provide a mechanism which explains why star formation in our and other similar galaxies did not stop when gas present in the disk has been used up. Comparison of distribution of these events in the interstellar medium to the distribution of peculiar stars of similar age (which are being discovered in the same spectroscopic surveys by my team) could establish links between contemporary events in the interstellar medium and formation and ejection of stars from the Galactic plane.

Work packages:

The workplan is described in detail in part B2b of the original ERC proposal. As explained above we will work on the first 5 Drops (work-packages) introduced there, discrete feature detection will be done on a simplified level, but we will omit detailed modelling of the interstellar medium from the present proposal. Drop 1 will be easier than expected because our detailed tests in the last 6 months (published in internal documents of the Gaia-ESO Hermes/GALAH collaborations, but also quoted under “conferences” in my homepage fiz.fmf.uni-lj.si/zwitter) showed that data are of very high quality, so easier to reduce than expected. Also Drops 3 and 6 will be helped by introduction of a new classification method t-Distributed Stochastic Neighbor Embedding (t-SNE, see van der Maaten & Hilton, 2008, J. of Machine Learning Research 9:2579-2605, and van der Maaten 2014,  J. of Machine Learning Research 15:3221-3245) which is given very promising results on both Gaia-ESO and Hermes/GALAH datasets. Moreover, Drop 4 will be helped by the possible earlier release of results from the Gaia satellite, which is now expected in early 2016.

To summarize, the drops (work-packages) which are explained in detail in the part B2b of the original ERC proposal are as follows:

Drop 1: Data collection and reductions

Drop 2: Diffuse interstellar band measurements

Drop 3: Stellar morphological classification

Drop 4: Stellar distance determination

Drop 5: Three-dimensional maps of values of DIB parameters

Drop 6: Discrete feature detection