The lack of short baselines, referred to as the short-spacing problem (SSP), is a well-known limitation of the performance of radio interferometers, causing a reduction of the ux detected from source structure on large angular scales. The very large number of antennas operated in the Atacama Large Millimeter/sub-millimeter Array (ALMA) generates situations for which the impact of the SSP takes a complex form, not simply measurable by a single number, such as the maximal recoverable scale. In particular, extended antenna congurations, complemented by a small group of closeby antennas at the centre of the array, may result in a double-humped baseline distribution with a signicant gap between the two groups. In such cases one should adopt as the effective maximal recoverable scale the one associated with the extended array and use only the central array to recover missing ux, as one would do with single dish or ACA (Atacama Compact Array) observations. The impact of the missing baselines can be very important and may easily be underestimated, or even overlooked. The present study uses ALMA archival data of the 29SiO(8-7) line emission of the AGB star W Hydrae for a demonstration. A critical discussion of the reliability of the observations away from the star is presented together with comments of a broader scope. Properties of the circumstellar envelope of W Hya within 15 au from the star, many of which are not mentioned in the published literature, are brie y described and compared with R Doradus, an AGB star having properties very similar to W Hya.

Low-mass star-formation studies deal with the birth of individual solar-type stars as it occurs in nearby molecular clouds. While this isolated mode of star formation may not represent the most common form of stellar birth, its study often provides rst evidence for the general ingredients of star formation, such as gravitational infall, disk formation, or out ow acceleration. Here I brie y review the current status and the main challenges in our understanding of low-mass star formation, with emphasis in the still mysterious pre-stellar phase. In addition to presenting by-now classical work, I also show how ALMA is starting to play a decisive role driving progress in this eld.

Formation processes of high-mass stars have been long-standing issues in astronomy and astrophysics. This is mainly because of major diculties in observational studies such as a smaller number of high-mass young stellar objects (YSOs), larger distances, and more complex structures in young high-mass clusters compared with nearby low-mass isolated star-forming regions (SFRs), and extremely large opacity of in- terstellar dust except for centimeter to submillimeter wavelengths. High resolution and high sensitivity observations with Atacama Large Millimeter/Submillimeter Array (ALMA) at millimeter/submillimeter wavelengths will overcome these observational diculties even for statistical studies with increasing num- ber of high-mass YSO samples. This review will summarize recent progresses in high-mass star-formation studies with ALMA such as clumps and laments in giant molecular cloud complexes and infrared dark clouds (IRDCs), protostellar disks and out ows in dense cores, chemistry, masers, and accretion bursts in high-mass SFRs.