NIRSpec IFU and MIRI MRS Observations of SN1987A
This example science program presents an application of the IFU Roadmap, showing how to create the cross-instrument MIRI/NIRSpec GTO observing program to observe supernova SN 1987A.
The IFU Roadmap guides the reader, step by step, through the process of creating a JWST observing program that uses the MIRI medium resolution spectrometer (MRS) and/or the NIRSpec IFU. Whichever IFU the reader decides to use, there are a sequence of steps to follow for creating an observing program for the chosen IFU(s). Here we demonstrate this process, showing how choices are made at each step, for a cross-instrument science use case to observe the supernova remnant SN 1987A with MIRI and NIRSpec.
We present a science case of stellar evolution: the study of Supernova 1987A. This observing program was developed as a joint program between M. Meixner (STScI/JHU) and the MIRI-EC community led by Patrice Bouchet (CEA) and Mike Barlow (UCL). In this use case, 3 JWST instruments are involved: MIRI, NIRSpec, and NIRCam. The main science goals of this project are to understand how massive stars age and explode, how their ejecta form dust and molecules, and how the blast wave from their violent explosion affects their surroundings. JWST MIRI imaging, medium resolution spectroscopy (MRS) and NIRSpec IFU spectroscopy will provide key emission line diagnostics and dust feature and continuum measurements of SN 1987A. The central stellar ejecta of SN 1987A is surrounded by a ring of progenitor gas and dust that is being shocked by the blast wave of the explosion. A large quantity (0.4–0.7 solar masses; Matsuura et al. 2011) of dust in the stellar ejecta has an unknown composition, and these observations may provide the first constraints through imaging and spatially resolved spectroscopy. Both MRS and NIRSpec IFU spectroscopy will measure key shocked line diagnostics that will further constrain the shock physics as well as the elemental abundances in both the ring and the stellar ejecta. This program also includes observations with MIRI imaging as the Prime observation and NIRCam imaging as the coordinated parallel observation.
This example science program focuses on the IFU observations.
Steps for creating observations
Step 1 - Pick one or both of the JWST IFU observing modes based on needed wavelength coverage
In this case, since we want observations at both near-infrared and mid-infrared wavelengths, we will want to use both IFUs: MIRI MRS and the NIRSpec IFU. There are emission lines of interest at both near- and mid-infrared wavelengths, and there is interesting dust emission present at mid-infrared wavelengths. Below, we will present the steps necessary to create observations for both IFUs.
Step 2 - Pick wavelength setting(s)
We would like to use all 3 grating settings of the MRS in order to observe all emission lines present in the mid-infrared and dust emission from various types of dust. Bouchet et al. (2006) find multiple emission lines in Spitzer-IRS spectra of SN 1987A, including [Ne II] at 12.8 μm, [Ne III] at 15.6 μm, and [O IV] at 25.9 μm, among others; in addition, the spectra suggest emission from solid-state dust of various kinds of compositions, including silicates.
Bouchet et al (2006) note that the Cas A supernova remnant, which they assume as an analog for SN 1987A, has emission at 1.083 μm from He I and 1.03 μm from [S II], among other lines. They also note that H2 has emission lines in the near-infrared and going to longer wavelengths. For the science goals of this project, it is sufficient to observe with medium spectral resolving power gratings. Therefore, we plan for NIRSpec IFU observations using the following disperser/filter combinations: G140M/F100LP, G235M/F170LP, and G395M/F290LP.
Step 3 - (For MIRI MRS) Determine whether you should choose simultaneous imaging with the MIRI imager
Main article: MIRI MRS Simultaneous Imaging
We would like to do simultaneous imaging along with the MRS observations, as it allows for deep imaging of nearby fields. MIRI simultaneous imaging is a default, and it is recommended. Different filters are used for each of the 3 simultaneous imaging observations (corresponding to the 3 MRS grating settings): F560W, F770W, and F1000W.
Step 4 - Decide whether you need to do a mosaic
The supernova remnant includes stellar ejecta near the center, which is surrounded by an approximately 2" diameter ring of gas and dust. Channel 1 of the MRS has the smallest field of view (3.3" × 3.7"), and it is sufficiently large to observe the 2" diameter ring in one tile, so a mosaic is not necessary.
The NIRSpec IFU field of view is 3" × 3". Like MRS channel 1, this is sufficiently large to observe the 2" diameter ring in one tile, so a mosaic is not necessary.
Step 5 - Pick a dither pattern
Dithering improves both spatial and spectral sampling. Increased spectral sampling is useful because it better resolves velocity structure in the emission lines, and increased spatial sampling is useful because it resolves small spatial structure in the supernova remnant. A 4-point extended source dithering pattern for the MRS is chosen, as it maximizes the common field of view between different pointings while still obtaining good sampling. Since this is an extended source dither pattern, APT offers the user only one choice for the orientation on the sky of the MRS dither pattern (for more, see here) - the only option in APT in this case is the negative dither pattern orientation.
Like for the MRS, dithering improves the spatial sampling of the NIRSpec IFU. In general, to remove detector effects and mitigate any remnant cosmic rays that are not well cleaned in the ramp fitting in data processing and because the PSF for the NIRSpec IFU is undersampled at wavelengths shortward of 3 microns, it is always a good idea to dither with the IFU. In addition, dithering can help mitigate some of the effects of MSA flux leakage, particularly for bright point sources. Dithering is important for finding small scale spatial structure in SN 1987A supernova remnant; therefore, dithering is included. The 4-point small cycling dither pattern is chosen as the optimal pattern. A dither pattern with larger dither separations (a nod pattern or the medium or large cycling dither patterns) would have a region on the sky common to all dithers of insufficient size to include the 2" diameter equatorial ring (ER) of SN 1987A.
Step 6 - Determine whether you need a dedicated background observation.
Thermal background is significant for MIRI, so background subtraction is typically important at these wavelengths. In fact, generally dedicated background observations are recommended whenever an extended source dither pattern is used. However, the local environment near SN 1987A has a lot of structure, which means a dedicated background for the MRS would be measuring ISM emission rather than an observation that could be used to subtract background from the SN 1987A observation. Therefore, we do not seek a dedicated background observation for the MRS.
At near-infrared wavelengths, the background emission is expected to be less than at mid-infrared wavelengths. For this reason and also because of the likelihood that a nearby background observation would measure ISM emission instead of being an observation that could be used to subtract background emission, we do not seek a dedicated background observation for the NIRSpec IFU.
Step 7 - For NIRSpec IFU: Decide whether you need to obtain leakcal observations to mitigate the effects of light that leaks through the NIRSpec micro-shutter assembly (MSA) shutters
There are 3 basic flavors of leakage through the MSA: bright spoilers whose radiation is transmitted through closed MSA shutters, leakage through stuck-open MSA shutters, and pile-up of extended dispersed background. For SN 1987A, bright spoilers are of most concern. The Aladin Viewer in the Astronomers Proposal Tool shows at most 3–4 bright (K < 11.5) stars, depending on the orientation. This is a small enough number of stars that it is anticipated that their contamination can be removed during cube building of the dithered exposures. This being the case, we choose not to obtain a leakcal observation for the NIRSpec IFU.
Step 8 - Decide if you should do a target acquisition (TA)
The smallest field of view for the MRS is 3.3" × 3.7" obtained using channel 1. The fields of view for other MRS channels are larger. The absolute fine-pointing accuracy of JWST is 0.10" (1-σ radial error), so a TA should not be needed for the MRS to observe the 2" diameter gas and dust ring of SN 1987A. Therefore, we do not perform MRS TA.
The NIRSpec IFU field of view, 3" × 3", is smaller than the smallest MRS field of view. Considering the JWST absolute fine-pointing accuracy of 0.10" (1-σ radial error) and the 2" diameter ring of gas and dust, a TA is desired. Performing wide aperture target acquisition (WATA; for more, see here) on the target itself is impractical because the target is extended. However, performing WATA using an offset star is possible, so we choose this option. For more on WATA with offset star, please see here.
Step 9 - Calculate the required exposure time and detector readout parameters using the Exposure Time Calculator (ETC)
To determine the exposure parameters for this observation using the Exposure Time Calculator (ETC), please see the article Step-by-Step ETC Guide for MIRI MRS and NIRSpec IFU Observations of SN1987A.
Step 10 - Fill out the Astronomer's Proposal Tool (APT) for your observation
For details on filling out the APT for this Example Science program, please see the article Step-by-Step APT Guide for MIRI MRS and NIRSpec IFU Observations of SN1987A.
Bouchet, P., et al., 2006, ApJ, 650, 212
SN 1987A after 18 Years: Mid-Infrared Gemini and Spitzer Observations of the Remnant
Matsuura, M., et al., 2011, Science, 333, 1258
Herschel Detects a Massive Dust Reservoir in Supernova 1987A