The ALMA-QUARKS Survey.Ⅰ.Survey Description and Data Reduction

This paper presents an overview of the QUARKS survey,which stands for Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures."The QUARKS survey is observing139 massive clumps covered by 156 pointings at Atacama Large Millimeter/submillimeter Array(ALMA)Band 6(λ~1.3 mm).In conjunction with data obtained from the ALMA-ATOMS survey at Band 3(λ~3 mm),QUARKS aims to carry out an unbiased statistical investigation of massive star formation process within protoclusters down to a scale of 1000 au.This overview paper describes the observations and data reduction of the QUARKS survey,and gives a first look at an exemplar source,the mini-starburst Sgr B2(M).The wide-b and width(7.5 GHz)and high-angular-resolution(~0."3)observations of the QUARKS survey allow for the resolution of much more compact cores than those could be done by the ATOMS survey,and to detect previously unrevealed fainter filamentary structures.The spectral windows cover transitions of species including CO,SO,N_(2)D^(+),SiO,H_(30)α,H_(2)CO,CH_(3)CN,and many other complex organic molecules,tracing gas components with different temperatures and spatial extents.QUARKS aims to deepen our understanding of several scientific topics of massive star formation,such as the mass transport within protoclusters by(hub-)filamentary structures,the existence of massive starless cores,the physical and chemical properties of dense cores within protoclusters,and the feedback from already formed high-mass young protostars.

The ALMA-QUARKS Survey.Ⅱ.The ACA 1.3 mm Continuum Source Catalog and the Assembly of Dense Gas in Massive Star-Forming Clumps

Leveraging the high resolution,sensitivity,and wide frequency coverage of the Atacama Large Millimeter/submillimeter Array(ALMA),the QUARKS survey,standing for“Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures”,is observing 139 massive starforming clumps at ALMA Band 6(λ~1.3 mm).This paper introduces the Atacama Compact Array(ACA)7 m data of the QUARKS survey,describing the ACA observations and data reduction.Combining multiwavelength data,we provide the first edition of QUARKS atlas,offering insights into the multiscale and multiphase interstellar medium in high-mass star formation.The ACA 1.3 mm catalog includes 207 continuum sources that are called ACA sources.Their gas kinetic temperatures are estimated using three formaldehyde transitions with a non-LTE radiation transfer model,and the mass and density are derived from a dust emission model.The ACA sources are massive(16–84 percentile values of 6–160 M_(⊙)),gravity-dominated(M∝R^(1.1))fragments within massive clumps,with supersonic turbulence(M>1)and embedded star-forming protoclusters.We find a linear correlation between the masses of the fragments and the massive clumps,with a ratio of 6%between the two.When considering fragments as representative of dense gas,the ratio indicates a dense gas fraction(DGF)of 6%,although with a wide scatter ranging from 1%to 10%.If we consider the QUARKS massive clumps to be what is observed at various scales,then the size-independent DGF indicates a self-similar fragmentation or collapsing mode in protocluster formation.With the ACA data over four orders of magnitude of luminosity-to-mass ratio(L/M),we find that the DGF increases significantly with L/M,which indicates clump evolutionary stage.We observed a limited fragmentation at the subclump scale,which can be explained by a dynamic global collapse process.

Resolution-dependent subsonic non-thermal line dispersion revealed by ALMA

We report here Atacama Large Millimeter/submillimeter Array(ALMA)N2H+(1-0)images of the Orion Molecular Cloud 2 and 3(OMC-2/3)with high angular resolution(3"or 1200 au)and high spatial dynamic range.Combining a dataset from the ALMA main array,Atacama Compact Array(ACA),Nobeyama 45-m Telescope and Very Large Array(VLA)(providing temperature measurement on matching scales),we find that most of the dense gas in OMC-2/3 is subsonic(σQNT/cs=0.62)with a mean line width(△v)of 0.39 kms-1 full width at half maximum(FWHM).This is markedly different from the majority of previous observations of massive star-forming regions.In contrast,line widths from the Nobeyama Telescope are transonic at 0.69 km s-1(σNT/cs=1.08).We demonstrated that the larger line widths obtained by the single-dish telescope arose from unresolved sub-structures within their respective beams.The dispersions from larger scalesσls(as traced by the Nobeyama Telescope)can be decomposed into three components such thatσls2=σss2+σbm2+σrd2,where small-scaleσss is the line dispersion of each ALMA beam,bulk motionσbm is dispersion between peak velocity of each ALMA beam andσrd is the residual dispersion.Such decomposition,though purely empirical,appears to be robust throughout our data cubes.Apparent supersonic line widths,commonly found in massive molecular clouds,are thus likely due to the effect of poor spatial resolution.The observed non-thermal line dispersion(sometimes referred to as’turbulence’)transits from supersonic to subsonic at~0.05 pc scales in the OMC-2/3 region.Such transition could be commonly found with sufficient spatial(not just angular)resolution,even in regions with massive young clusters,such as the Orion molecular clouds studied here.