https://zcal.co/i/Xj0XIVbf








http://w.astro.berkeley.edu/~ay216/05/NOTES/Lecture24.pdf






Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation.
Aims. The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100–400 au).
Methods. We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5–3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture.
Results. The protostellar envelope is well traced by C18O, DCO+, and N2D+, which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH3OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H2CO. The cavity walls show tracers of UV-irradiation such as C2H, c-C3H2 and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H2S, SO, OCS, and H13CN, which most likely are related to ice sublimation and high-temperature chemistry.
Conclusions. Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST–MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources.









Class 0 sources are defined by  Class I sources are defined by having an infrared spectral index that indicates strong reddening (Lada 1987), with bolometric temperatures of 70-650 K (Chen et al. 1995). These systems have already converted most of their envelope mass into the disk and protostar (Crapsi et al. 2008; van Kempen et al. 2009b; Maury et al. 2011). For the typical envelope masses of the sources presented here and for the aver- age disk masses found by Tychoniec et al. (2020), the masses are Mdisk/Menv $=$ 1\% for Class 0 and $=$20\% for Class I, with values up to 75–98\% in cases of rotationally supported disks (Jorgensen et al. 2009).
The different components of protostellar systems vary sig- nificantly in their physical conditions, such as density and temperature, molecular enrichment, and dynamics. Our current
knowledge about them is described briefly below to set the scene for the interpretation of our data.
Envelope. Theenvelopesurroundingaprotostaristhemate- rial that fuels the accretion process onto the star and disk. The physical conditions in the outer envelope on scales of a few 1000au are reminiscent of those of starless cores with heavy freeze-out, and their chemical composition is directly inherited from the cloud out of which the star is being born (Caselli \& Ceccarelli 2012). Systematic motions such as infall or expan- sion can occur, but otherwise, they are characterized by low turbulence and narrow (FWHM $<$ 0.5-1 km s1) line profiles indicative of quiescent gas (Jorgensen et al. 2002).
Warminnerenvelope. Intheinnermostpartoftheenvelope on scales of the disk, the temperatures rise above 100 K so that any water and complex organic molecules (COMs) contained in ices are released from the grains back into the gas, where they are readily observed at submillimeter wavelengths. This region with its unique chemical richness is called the hot core, or to dis- tinguish it from its high-mass counterpart, the hot corino (Herbst \& van Dishoeck 2009).
Jets and outflows. As the material is accreting from the envelope onto the disk, excess angular momentum has to be transported in a still unexplained process to allow material to accrete onto the growing protostar. Jets and outflows consti tute compelling candidates for extracting angular momentum through magnetic fields. In the earliest stages when the mass loss is highest, the densities are high enough to form molecules in the internal shocks in the jet (Bachiller \& Gomez-Gonzalez 1992; Tafalla et al. 2010). Much slower ($<$20 km s1) and less colli- mated gas moving away from the protostar is called an outflow. The origin of the outflows remain debated. Large-scale outflows reveal bow-shock shells and cavities that may be driven by the fast intermittent jet (Gueth et al. 1996; Gueth \& Guilloteau 1999; Tychoniec et al. 2019). The temperatures in shocked regions are much higher than in the surrounding envelope, up to a few thou- sand Kelvin, and sputtering of grain cores and ice mantles can further result in unique chemical signatures (Arce et al. 2008; Flower \& Pineau des Forets 2013).
Outflowcavitywalls. Thesearethenarrowzonesinbetween the cold dense quiescent envelope material and the lower-density warm cone where outflows are propagating at high velocities. Cavity walls are exposed to UV radiation from the accreting star-disk boundary layer, which can escape through the outflow cavity without being extincted (Spaans et al. 1995). This creates conditions similar to those found in photon-dominated regions (PDRs), which occur throughout the interstellar medium near sources of intense UV radiation (Hollenbach \& Tielens 1997). In units of the interstellar radiation field (ISRF, Draine 1978), typical values of 102–103 are found on scales of 1000 au (van Kempen et al. 2009a; Yıldız et al. 2012; Benz et al. 2016; Karska et al. 2018).
Young disk. In the inner envelope, a protoplanetary disk starts to form as the natural outcome of a rotating collapsing core (Ulrich 1976; Cassen \& Moosman 1981; Terebey et al. 1984). A young disk should be rotating in Keplerian motion. At early stages, it is difficult to identify whether the so-called embedded disk is rotationally supported because any molecular emission from the disk is entangled with that from the envelope. In recent years, several embedded disks have been identified to have Keplerian rotational structure on scales of 100 au (Tobin et al. 2012; Murillo et al. 2013; Ohashi et al. 2014; Yen et al. 2017). Molecular tracers in young disks can probe their temperature structure in addition to providing kinematic information (van't Hoff et al. 2018b).
This work is organized as follows. In Sect. 2 we describe the observations we used, and Sect. 3 presents the results of this work, including detections and morphology of the targeted molecules. In Sect. 4 the results are discussed, with special focus on which molecular tracers correspond to each of the physical components. The focus is on a qualitative description, rather than quantitative analyses, for which source specific models and more rotational transitions of a given molecule are required. We summarize our work in Sect. 5.