A rotating fast bipolar wind and disk system around the B[e]-type star MWC 922.

We present interferometric observations with the Atacama Large Millimeter Array (ALMA) of the free-free continuum and recombination line emission at 1 and 3 mm of the "Red Square Nebula" surrounding the B[e]-type star MWC922. The unknown distance to the source is usually taken to be d=1.7-3 kpc. The unprecedented angular resolution ( up to ∼ 0 . ″ 02 ) and exquisite sensitivity of these data unveil, for the first time, the structure and kinematics of the emerging, compact ionized region at its center. We imaged the line emission of H30α and H39α, previously detected with single-dish observations, as well as of H51ϵ, H55γ, and H63δ, detected for the first time in this work. The line emission is seen over a full velocity range of ~180 km s-1 arising in a region of diameter < 0 . ″ 14 (less than a few hundred au) in the maser line H30α, which is the most intense transition reported here. We resolve the spatio-kinematic structure of a nearly edge-on disk rotating around a central mass of ~10 M ⊙ (d=1.7 kpc) or ~18 M ⊙ (d=3 kpc), assuming Keplerian rotation. Our data also unveil a fast (~100 km s-1) bipolar ejection (a jet?) orthogonal to the disk. In addition, a slow (<15 km s-1) wind may be lifting off the disk. Both, the slow and the fast winds are found to be rotating in a similar manner to the ionized layers of the disk. This represents the first empirical proof of rotation in a bipolar wind expanding at high velocity (~100 km s-1 ). The launching radius of the fast wind is found to be <30-51 au i.e., smaller than the inner rim of the ionized disk probed by our observations. We believe that the fast wind is actively being launched, probably by a disk-mediated mechanism in a (accretion?) disk around a possible compact companion. We have modelled our observations using the radiative transfer code MORELI. This has enabled us to describe with unparalleled detail the physical conditions and kinematics in the inner layers of MWC 922, which has revealed itself as an ideal laboratory for studying the interplay of disk rotation and jet-launching. Although the nature of MWC 922 remains unclear, we believe it could be a ~15 M ⊙ post-main sequence star in a mass-exchanging binary system. If this is the case, a more realistic value of the distance may be d~3 kpc.

Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines.

CONTEXT: A significant fraction of the molecular gas in star-forming regions is irradiated by stellar UV photons. In these environments, the electron density (n e) plays a critical role in the gas dynamics, chemistry, and collisional excitation of certain molecules. AIMS: We determine n e in the prototypical strongly irradiated photodissociation region (PDR), the Orion Bar, from the detection of new millimeter-wave carbon recombination lines (mmCRLs) and existing far-IR [13Cii] hyperfine line observations. METHODS: We detect 12 mmCRLs (including α, ß, and γ transitions) observed with the IRAM 30m telescope, at ~ 25″ angular resolution, toward the H/H2 dissociation front (DF) of the Bar. We also present a mmCRL emission cut across the PDR. RESULTS: These lines trace the C+/C/CO gas transition layer. As the much lower frequency carbon radio recombination lines, mmCRLs arise from neutral PDR gas and not from ionized gas in the adjacent Hii region. This is readily seen from their narrow line profiles (Δv = 2.6 ± 0.4 km s-1) and line peak velocities (ν LSR = +10.7 ± 0.2 km s-1). Optically thin [13Cii] hyperfine lines and molecular lines - emitted close to the DF by trace species such as reactive ions CO+ and HOC+ - show the same line profiles. We use non-LTE excitation models of [13Cii] and mmCRLs and derive n e = 60 - 100 cm-3 and T e = 500 - 600 K toward the DF. CONCLUSIONS: The inferred electron densities are high, up to an order of magnitude higher than previously thought. They provide a lower limit to the gas thermal pressure at the PDR edge without using molecular tracers. We obtain P th ≥ (2 - 4)·108 cm-3 K assuming that the electron abundance is equal to or lower than the gas-phase elemental abundance of carbon. Such elevated thermal pressures leave little room for magnetic pressure support and agree with a scenario in which the PDR photoevaporates.