arXiv subjects

Beth A. Sargent

Publications and source records attributed to Beth A. Sargent.

3 recordsLinked to original sources

arXiv2025

A Mid-Infrared Spectroscopic Study of Young Stellar Objects in the SMC Region NGC 346: JWST Detects Dust, Accretion, Ices and Outflows

We present mid-infrared spectroscopic observations of intermediate- to high-mass young stellar objects (YSOs) in the low-metallicity star-forming region NGC 346 located within the Small Magellanic Cloud (SMC). We conduct these integral-field-unit observations with the Mid-Infrared Instrument Medium Resolution Spectroscopy instrument on board JWST. The brightest and most active star-forming region in the SMC, NGC 346 has a metallicity of $\sim$1/5 $Z_{\odot}$, analogous to the era when star formation in the early Universe ($z$$\simeq$2) peaked. We discuss the emission and absorption features present in the spectral energy distributions (SEDs) of five YSOs with coverage from 4.9-27.9$μ$m and three other sources with partial spectral coverage. Via SED model-fitting, we estimate their parameters, finding masses ranging from 2.9-18.0 M$_{\odot}$. These targets show dusty silicates, polycyclic aromatic hydrocarbons and ices of CO$_2$, CO, H$_2$O and CH$_3$OH in their protostellar envelopes. We measure emission from H$_2$ and atomic fine-structure lines, suggesting the presence of protostellar jets and outflows. We detect H I lines indicating ongoing accretion and estimate accretion rates for each source which range from 2.50x10$^{-6}$-2.23x10$^{-4}$ M$_{\odot}$yr$^{-1}$ based on H I (7-6) line emission. We present evidence for a $\sim$30,000AU protostellar jet traced by fine-structure, H I and H$_2$ emission about the YSO Y535, the first detection of a resolved protostellar outflow in the SMC, and the most distant yet detected.

astro-ph.GA↗

arXiv2024

Imaging of I Zw 18 by JWST: II. Spatially resolved star formation history

The blue compact dwarf galaxy I Zw 18 is one of the most metal-poor ($Z \sim 3% Z_{\sun}$) star-forming galaxies in the local Universe. Its evolutionary status has sparked debate within the astronomical community. We aim to investigate the stellar populations of I Zw 18 in the near-IR using JWST/NIRCam's high spatial resolution and sensitivity. Additionally, we aim to derive the galaxy's spatially resolved star formation history (SFH) over the last 1 Gyr and provide constraints for older epochs. We used DOLPHOT to measure positions and fluxes of point sources in the F115W and F200W filters' images of I Zw 18. To derive I Zw 18's SFH, we applied the color-magnitude diagram (CMD) fitting technique SFERA 2.0, using two independent sets of stellar models. Our analysis reveals three main stellar populations: one younger than $\sim30$ Myr, mainly in the northwest star-forming (SF) region; an intermediate-age population ($\sim 100 - 800$ Myr) in the southeast SF region; and a red and faint population linked to the underlying halo, older than 1 Gyr and possibly as old as 13.8 Gyr. The main body of the galaxy shows a very low star formation rate (SFR) of $\sim 10^{-4} M_{\odot} \text{yr}^{-1}$ between 1 and 13.8 Gyr ago. In the last billion years, I Zw 18 shows increasing SF, with strong bursts around $\sim10$ and $\sim100$ Myr ago. Component C mirrors the main body's evolution but with lower SFRs. Our findings confirm that I Zw 18 contains stars of all ages, indicating it is not a young galaxy but has an old stellar halo, similar to other BCDs. The low SF activity over the past billion years supports the "slow cooking" dwarf scenario, explaining its low metal content. Currently, the galaxy is undergoing its strongest SF episode ($\sim 0.6 M_{\odot} \text{yr}^{-1}$) mainly in the northwest region, likely due to a recent gravitational interaction with Component C.

astro-ph.GA↗

arXiv2023

JWST MIRI flight performance: The Medium-Resolution Spectrometer

The Medium-Resolution Spectrometer (MRS) provides one of the four operating modes of the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST). The MRS is an integral field spectrometer, measuring the spatial and spectral distributions of light across the 5-28 $μm$ wavelength range with a spectral resolving power between 3700-1300. We present the MRS's optical, spectral, and spectro-photometric performance, as achieved in flight, and we report on the effects that limit the instrument's ultimate sensitivity. The MRS flight performance has been quantified using observations of stars, planetary nebulae, and planets in our Solar System. The precision and accuracy of this calibration was checked against celestial calibrators with well-known flux levels and spectral features. We find that the MRS geometric calibration has a distortion solution accuracy relative to the commanded position of 8 mas at 5 $μm$ and 23 mas at 28 $μm$. The wavelength calibration is accurate to within 9 km/sec at 5 $μm$ and 27 km/sec at 28 $μm$. The uncertainty in the absolute spectro-photometric calibration accuracy was estimated at 5.6 +- 0.7 %. The MIRI calibration pipeline is able to suppress the amplitude of spectral fringes to below 1.5 % for both extended and point sources across the entire wavelength range. The MRS point spread function (PSF) is 60 % broader than the diffraction limit along its long axis at 5 $μm$ and is 15 % broader at 28 $μm$. The MRS flight performance is found to be better than prelaunch expectations. The MRS is one of the most subscribed observing modes of JWST and is yielding many high-profile publications. It is currently humanity's most powerful instrument for measuring the mid-infrared spectra of celestial sources and is expected to continue as such for many years to come.

astro-ph.IM↗