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Register a new userScience with an ngVLA: Understanding Massive Star Formation through Maser Imaging
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Imaging the bright maser emission produced by several molecular species at centimeter wavelengths is an essential tool for understanding the process of massive star formation because it provides a way to probe the kinematics of dense molecular gas at high angular resolution. Unimpeded by the high dust optical depths that affect shorter wavelength observations, the high brightness temperature of these emission lines offers a way to resolve accretion and outflow motions down to scales as fine as $sim$1-10 au in deeply embedded Galactic star-forming regions, and at sub-pc scales in nearby galaxies. The Next Generation Very Large Array will provide the capabilities needed to fully exploit these powerful tracers.
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Various sign-posts of recent star-formation activity, such as water and methanol maser emission or magnetically active low-mass young stars, can be detected with Very Long Baseline Interferometry (VLBI) radio arrays. The extremely accurate astrometry already attainable with VLBI instruments implies that the trigonometric parallax and the proper motion of these objects can be measured to better than 1% for sources within a few hundred parsecs of the Sun, and better than 10% for objects at a few kiloparsecs. An ngVLA with baselines extending to several thousand km will have a sensitivity more than one order of magnitude better than current VLBI instruments, and will enable such highly accurate astrometric measurements to be performed throughout the Milky Way. This will provide a full six-dimensional view (three spatial and three velocity coordinates) of star-formation in the Galactic disk, and have a transformative impact on our understanding of both star-formation processes and Galactic structure.
Planets assemble in the midplanes of protoplanetary disks. The compositions of dust and gas in the disk midplane region determine the compositions of nascent planets, including their chemical hospitality to life. In this context, the distributions of volatile organic material across the planet and comet forming zones is of special interest. These are difficult to access in the disk midplane at IR and even millimeter wavelengths due to dust opacity, which can veil the midplane, low intrinsic molecular abundances due to efficient freeze-out, and, in the case of mid-sized organics, a mismatch between expected excitation temperatures and accessible line upper energy levels. At ngVLA wavelengths, the dust is optically thin, enabling observations into the planet forming disk midplane. ngVLA also has the requisite sensitivity. Using TW Hya as a case study, we show that ngVLA will be able to map out the distributions of diagnostic organics, such as CH3CN, in nearby protoplanetary disks.
Extraterrestrial amino acids, the chemical building blocks of the biopolymers that comprise life as we know it on Earth are present in meteoritic samples. More recently, glycine (NH$_2$CH$_2$COOH), the simplest amino acid, was detected by the Rosetta mission in comet 67P. Despite these exciting discoveries, our understanding of the chemical and physical pathways to the formation of (pre)biotic molecules is woefully incomplete. This is largely because our knowledge of chemical inventories during the different stages of star and planet formation is incomplete. It is therefore imperative to solidify our accounting of the chemical inventories, especially of critical yet low-abundance species, in key regions and to use this knowledge to inform, expand, and constrain chemical models of these reactions. This is followed naturally by a requirement to understand the spatial distribution and temporal evolution of this inventory. Here, we briefly outline a handful of particularly-impactful use cases in which the ngVLA will drive the field forward.
Detailed mapping of the distributions and kinematics of gases in cometary comae at radio wavelengths can provide fundamental advances in our understanding of cometary activity and outgassing mechanisms. Furthermore, the measurement of molecular abundances in comets provides new insights into the chemical composition of some of the Solar Systems oldest and most primitive materials. Here we investigate the opportunities for significant progress in cometary science using a very large radio interferometer. The ngVLA concept will enable detection and mapping of a range of key coma species in the 1.2-116 GHz range, and will allow for the first time, high-resolution mapping of the fundamental cometary molecules OH and NH$_3$. The extremely high angular resolution and continuum sensitivity of the proposed ngVLA will also allow the possibility of imaging thermal emission from the nucleus itself, as well as large dust/ice grains in the comae, of comets passing within $sim1$ au of Earth.
Galactic winds are ubiquitously observed in galaxies both locally and in the high-redshift Universe. While these winds span many orders of magnitude in both temperature and density, observations of nearby galaxies show that the cold molecular phase tends to dominate both the mass and momentum carried. The capabilities of the ngVLA for the study of molecular outflows at low redshift are described elsewhere in this Volume; here we focus on the ability of the ngVLA to detect and image such outflows in the high-redshift Universe via deep observations of low-J transitions of the CO molecule. The ngVLA is capable of detecting molecular outflows from typical galaxies on the star-forming sequence with log(Mstar/Msun) >~ 10.5 to z~3, and galaxies with higher star formation rates to beyond z~4. The ngVLA will enable an understanding of the feedback processes that shape galaxies throughout the epoch of galaxy assembly when the bulk of the stars in the Universe were formed. While the emission associated with outflows is faint in comparison to the emission from the galaxy, deep observations are also required for high-resolution dynamical studies, allowing for the routine simultaneous detection and imaging of the outflows.
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