skip to main content

Search for: All records

Creators/Authors contains: "Zhang, Qizhou"

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract We observed the high-mass protostellar core G335.579–0.272 ALMA1 at ∼200 au (0.″05) resolution with the Atacama Large Millimeter/submillimeter Array (ALMA) at 226 GHz (with a mass sensitivity of 5 σ = 0.2 M ⊙ at 10 K). We discovered that at least a binary system is forming inside this region, with an additional nearby bow-like structure (≲1000 au) that could add an additional member to the stellar system. These three sources are located at the center of the gravitational potential well of the ALMA1 region and the larger MM1 cluster. The emission from CH 3 OH (and many othermore »tracers) is extended (>1000 au), revealing a common envelope toward the binary system. We use CH 2 CHCN line emission to estimate an inclination angle of the rotation axis of 26° with respect to the line of sight based on geometric assumptions and derive a kinematic mass of the primary source (protostar+disk) of 3.0 M ⊙ within a radius of 230 au. Using SiO emission, we find that the primary source drives the large-scale outflow revealed by previous observations. Precession of the binary system likely produces a change in orientation between the outflow at small scales observed here and large scales observed in previous works. The bow structure may have originated from the entrainment of matter into the envelope due to the widening or precession of the outflow, or, alternatively, an accretion streamer dominated by the gravity of the central sources. An additional third source, forming due to instabilities in the streamer, cannot be ruled out as a temperature gradient is needed to produce the observed absorption spectra.« less
    Free, publicly-accessible full text available April 1, 2023
  2. ABSTRACT Young massive clusters (YMCs) are compact (≲1 pc), high-mass (>104 M⊙) stellar systems of significant scientific interest. Due to their rarity and rapid formation, we have very few examples of YMC progenitor gas clouds before star formation has begun. As a result, the initial conditions required for YMC formation are uncertain. We present high resolution (0.13 arcsec, ∼1000 au) ALMA observations and Mopra single-dish data, showing that Galactic Centre dust ridge ‘Cloud d’ (G0.412 + 0.052, mass = 7.6 × 104 M⊙, radius = 3.2 pc) has the potential to become an Arches-like YMC (104 M⊙, r ∼ 1 pc), but is not yet forming stars. This would mean it is the youngestmore »known pre-star-forming massive cluster and therefore could be an ideal laboratory for studying the initial conditions of YMC formation. We find 96 sources in the dust continuum, with masses ≲3 M⊙ and radii of ∼103 au. The source masses and separations are more consistent with thermal rather than turbulent fragmentation. It is not possible to unambiguously determine the dynamical state of most of the sources, as the uncertainty on virial parameter estimates is large. We find evidence for large-scale (∼1 pc) converging gas flows, which could cause the cloud to grow rapidly, gaining 104 M⊙ within 105 yr. The highest density gas is found at the convergent point of the large-scale flows. We expect this cloud to form many high-mass stars, but find no high-mass starless cores. If the sources represent the initial conditions for star formation, the resulting initial mass function will be bottom heavy.« less
    Free, publicly-accessible full text available June 6, 2023
  3. Abstract We present Very Large Array C- , X- , and Q -band continuum observations, as well as 1.3 mm continuum and CO(2-1) observations with the Submillimeter Array toward the high-mass protostellar candidate ISOSS J23053+5953 SMM2. Compact centimeter continuum emission was detected near the center of the SMM2 core with a spectral index of 0.24(± 0.15) between 6 and 3.6 cm, and a radio luminosity of 1.3(±0.4) mJy kpc 2 . The 1.3 mm thermal dust emission indicates a mass of the SMM2 core of 45.8 (±13.4) M ⊙ , and a density of 7.1 (±1.2)× 10 6 cm −3more ». The CO(2-1) observations reveal a large, massive molecular outflow centered on the SMM2 core. This fast outflow (>50 km s −1 from the cloud systemic velocity) is highly collimated, with a broader, lower-velocity component. The large values for outflow mass (45.2 ± 12.6 M ⊙ ) and momentum rate (6 ± 2 × 10 −3 M ⊙ km s −1 yr −1 ) derived from the CO emission are consistent with those of flows driven by high-mass YSOs. The dynamical timescale of the flow is between 1.5 and 7.2 × 10 4 yr. We also found from the C 18 O to thermal dust emission ratio that CO is depleted by a factor of about 20, possibly due to freeze-out of CO molecules on dust grains. Our data are consistent with previous findings that ISOSS J23053 + 5953 SMM2 is an emerging high-mass protostar in an early phase of evolution, with an ionized jet and a fast, highly collimated, and massive outflow.« less
    Free, publicly-accessible full text available November 1, 2022
  4. null (Ed.)
  5. null (Ed.)
    ABSTRACT G0.253+0.016, aka ‘the Brick’, is one of the most massive (>105 M⊙) and dense (>104 cm−3) molecular clouds in the Milky Way’s Central Molecular Zone. Previous observations have detected tentative signs of active star formation, most notably a water maser that is associated with a dust continuum source. We present ALMA Band 6 observations with an angular resolution of 0.13 arcsec (1000 AU) towards this ‘maser core’ and report unambiguous evidence of active star formation within G0.253+0.016. We detect a population of eighteen continuum sources (median mass ∼2 M⊙), nine of which are driving bi-polar molecular outflows as seen via SiO (5–4) emission.more »At the location of the water maser, we find evidence for a protostellar binary/multiple with multidirectional outflow emission. Despite the high density of G0.253+0.016, we find no evidence for high-mass protostars in our ALMA field. The observed sources are instead consistent with a cluster of low-to-intermediate-mass protostars. However, the measured outflow properties are consistent with those expected for intermediate-to-high-mass star formation. We conclude that the sources are young and rapidly accreting, and may potentially form intermediate- and high-mass stars in the future. The masses and projected spatial distribution of the cores are generally consistent with thermal fragmentation, suggesting that the large-scale turbulence and strong magnetic field in the cloud do not dominate on these scales, and that star formation on the scale of individual protostars is similar to that in Galactic disc environments.« less