MIRI MRS Spectroscopy of a Late M Star
This example science program presents an application of the IFU Roadmap, showing how to create an observing program of a late M star with the MIRI Medium Resolution Spectroscopy mode.
The IFU Roadmap guides the reader, step by step, through the process of creating a JWST observing program that makes use of 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 to create an observing program for the chosen IFU(s). Here we demonstrate this process, showing how choices are made at each step for an example science program using MRS spectroscopy to observe a late M-type star.
We want to obtain the mid-infrared spectrum of a late M-type star, to the longest wavelengths and highest spectral resolving power possible with JWST. These are common stars, so it is important to understand their spectra. Their stellar atmospheres are relatively cool, so they allow for the formation of molecules such as H2O, which give rise to interesting spectra over mid-infrared wavelengths (5-30 μm). At low spectral resolving power, H2O absorption bands can be seen in Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of M stars at spectral resolving power, R, of ~90 shortward of 10 μm wavelength (Cushing et al. 2006) and R ~ 600 over 10–19 μms wavelength (Mainzer et al. 2007). For this science example program, IFU spectroscopy with MIRI-MRS is ideal because it is desired to observe late M stars over mid-infrared wavelengths at the highest spectral resolving power possible with JWST to study their molecular bands.
Step 1 - Pick one or both of the JWST IFU observing modes based on needed wavelength coverage
Main article: MIRI Medium Resolution Spectroscopy
The MRS on MIRI offers medium spectral resolution (R ~ 3,000) over roughly the wavelength range 5–28 μms, so the MRS is chosen for this observing program.
Step 2 - Pick Wavelength Setting(s)
The entire 5–28 μm spectrum of a late M-type star is desired, as there are potentially interesting molecular lines (e.g., of H2O; see Introduction) in each of the 12 bands of the MRS. The range 5–19 μms wavelength (the wavelength range over which H2O absorption was found by Cushing et al. 2006 and Mainzer et al. 2007) is covered by all three grating settings of MRS channels 1–3 and the Short grating setting for MRS channel 4. An MRS observation at one grating setting obtains spectra at that grating setting in all four MRS channels. So all three grating settings are desired—short, medium, and long.
Step 3 - Determine whether you should choose simultaneous imaging with the MIRI Imager
Main article: MIRI MRS Simultaneous Imaging
The default MRS operational mode is to obtain simultaneous imaging unless there is a reason to avoid this, such as concerns over saturation or data volume. There are no data volume issues with including simultaneous imaging, and it adds imaging of nearby sky, so it is included. This imaging potentially adds to the astrometric accuracy of the resulting data products and provides archival legacy value. Here we choose the F1130W filter for the imager, when performing simultaneous imaging. Using a longer-wavelength filter can give a Long Wavelength Filter warning in APT, so we want to use a shorter-wavelength filter for simultaneous imaging. The F1130W filter is relatively narrow, so it allows brighter stars to be observed without saturation. Using ETC, we estimate that a M5 V star with a Spitzer-IRAC magnitude of [8.0] = 10.0 could be observed using this filter and not encounter saturation problems; any brighter, and it would begin to experience saturation problems. We choose the Full subarray for simultaneous imaging because we obtain a TA for this observation, and Full subarray is required for simultaneous imaging in that case.
Step 4 - Decide whether you need to do a mosaic.
Main article: MIRI MRS Mosaics
For this observation, we are interested only in the mid-infrared spectrum of a single star. The science target is not extended, so there is no need to perform mosaicking on the target.
Step 5 - Pick a dither pattern.
Main article: MIRI MRS Dithering
Dithering increases the MRS' sampling both spatially and spectrally, so it is desirable to include dithering in MRS observations. Here, we use a 4-point dither pattern optimized for a point source at all wavelengths. In APT, the negative direction is chosen for the MRS point source dithering pattern, though the choice of positive versus negative direction in this case does not matter since there is no local structure on the sky that we anticipate needing to avoid or include in the field-of-view.
Step 6 - Determine whether you need a dedicated background observation.
Main article: MIRI MRS Dedicated Sky Observations
An extended target may need a dedicated background observation, but the science target here is a point source - a late M-type star. A dedicated background observation is not needed here, as the dither pattern will provide observations to use in background subtraction.
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
This step is to be skipped for this program since it does not involve the NIRSpec IFU.
Step 8 - Decide if you should do a Target Acquisition (TA)
Main article: MIRI MRS Target Acquisition
The target is not extended, and the absolute fine pointing accuracy of JWST is 0.3"–0.45" (1-σ radial error). However, since a point-source dither pattern is chosen, the area common to all four dithers is significantly smaller than an MRS field of view; therefore, we choose to perform an MRS TA. For more information on how exposure parameters were determined from the ETC, please see ETC Instructions for the Example Science Program "MIRI IFU Spectroscopy of a Late M Star".
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 Spectroscopy of a Late M Star.
Step 10 - Fill out the Astronomer's Proposal Tool (APT) for your observation
Cushing, M. C., et al., 2006, ApJ, 648, 614
A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs
Mainzer, A. K., et al., 2007, ApJ, 662, 1245
Moderate-Resolution Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs