PI: Marjorie Decleir, postdoc in the ISM* group at STScI - mdecleir@stsci.edu
Group Website: ISM*@ST
Project Duration: 3 separate one year rotation projects for graduate students
Background and Context
Interstellar dust absorbs and scatters a large fraction of the starlight, hence influencing observations of many astrophysical objects. Understanding the amount of dust, the properties of the grains, and the interplay between dust and radiation, is crucial to derive precise knowledge of any object in the Universe that is obscured by dust. In addition, interstellar dust is fundamental to the star formation process and galaxy evolution.
In the Milky Way, we can investigate dust along individual lines of sight through its wavelength-dependent extinction effect on the light of a background star. Extinction curves are not only required to account for the dust in observations of a range of astrophysical objects, but also provide profound insights into the properties of interstellar dust, imposing fundamental constraints on models of dust grains. For example, the slope of an extinction curve gives an estimate of the average dust grain size along the line of sight, while extinction features provide a direct measurement of the composition of the dust grains in the interstellar medium.
Available student projects
I have three available projects related to the study of interstellar dust. All of these projects are suitable for a student-led publication.
- Reconciling spectral multi-wavelength extinction curves
Fig. 1: Left: NIR extinction curve in black with a powerlaw fit in red (adapted from Decleir et al. 2022). Right: MIR extinction curve in blue with a functional fit in black (Gordon et al. 2021)
Extinction curves are commonly measured by comparing the observed spectrum of a dust-extinguished background star to the intrinsic spectrum of a similar star or stellar model (not affected by dust). These measurements are often limited to a specific wavelength range (e.g. the ultraviolet or the optical). Recently, we measured extinction curves in the near-infrared (NIR) using IRTF/SpeX spectra (Decleir et al. 2022), and mid-infrared (MIR) using Spitzer photometry and spectra (Gordon et al. 2021). The main reasons for this wavelength “split” are often the available data sets, taken with different telescopes and instruments, different astronomers being interested in different wavelength regimes, and different stars being suitable only for observations in part of the electromagnetic spectrum. In the NIR and MIR, we found that the extinction curve can be represented by a power law (see Fig. 1). However, for the same targets, we obtained different power law indices in the NIR (<5 micron) and in the MIR (> 5micron), and both pieces of the extinction curve do not always line up. Other studies have also shown that the extinction curve becomes flatter at longer wavelengths, but the transition wavelength varies between different studies. This heterogeneous set of measured extinction curves makes it very challenging for dust modelers to constrain the dust properties using all these separate pieces of constraints.
The goal of this project is to measure and fit extinction curves over a wavelength range as large as possible, for the same stars, and fit all wavelengths simultaneously. We will start from the overlap targets between the NIR and MIR samples, and measure the full NIR-MIR extinction curve using a stellar atmosphere model. We will investigate if the entire wavelength range can be fitted with one single powerlaw, or if multiple powerlaws or other functions are needed. This result will help to constrain dust grain models. If time permits, this project can be expanded to shorter wavelength regions (UV-optical). This project will be in collaboration with Karl Gordon, who is also an expert on dust extinction.
Planned student work:
- Obtain the NIR and MIR spectra for the overlap sample.
- Measure the full NIR-MIR extinction curves using stellar atmosphere models.
- Fit the full NIR-MIR extinction curves.
- Compare the results with the individual studies and with dust grain models.
- Write a short paper with the results.
Technical skills the student will learn:
- Use existing Python code to measure and fit extinction curves (based on Astropy fitting).
- Expanding the code to simultaneously fit NIR and MIR spectra.
- Academic and technical writing.
