Dual-band Unified Exploration of Three CMZ Clouds (DUET). Cloud-wide census of continuum sources showing low spectral indices

The Milky Way's Central Molecular Zone (CMZ) is measured to form stars 10 times less efficiently than in the Galactic disk, based on emission from high-mass stars. However, the CMZ's low-mass protostellar population, which accounts for most of the initial stellar mass budget and star formation rate (SFR), is poorly constrained observationally due to limited sensitivity and resolution. We present the Dual-band Unified Exploration of Three CMZ Clouds (DUET) survey, targeting the 20 km/s Cloud, Sgr C, and Dust Ridge cloud e using the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 and 3 mm. The mosaicked observations achieve a comparable resolution of 0.2-0.3" (~1600-2500 au) and a sky coverage of 8.3-10.4 square arcmin, respectively. We report 563 continuum sources at 1.3 mm and 330 at 3 mm, respectively, and a dual-band catalog with 450 continuum sources. These sources are marginally resolved at the 2,000 au resolution. We find a cloud-wide deviation (>70%) from commonly-used dust modified blackbody (MBB) models, characterized by either low spectral indices or low brightness temperatures. Three possible explanations for the deviation are discussed. (1) Optically thick Class 0/I Young stellar objects (YSOs) with very small beam filling factors can lead to lower brightness temperatures than what MBB models predict. (2) Large (mm/cm-sized) dust grains have more significant self-scattering, and therefore frequency-dependent albedo could cause lower spectral indices. (3) Free-free emission over 30 uJy can severely contaminate dust emission and cause low spectral indices for mJy sources in our sample, although the needed number of massive protostars (embedded UCHII regions) is infeasibly high for the normal stellar initial mass function. A reliable measurement of the SFR at low protostellar masses will require future work to distinguish between these possible explanations.