Context. The best example of a massive star with an ionized outflow launched from its photoevaporating disk is MWC349A. The large amount of reported radio-continuum and radio-recombination line (RRL) observations toward this galactic UC-HII region offers a unique possibility to build a model of the ionized envelope of this source. Aims. To understand the physical conditions and kinematics of the ionized region of the circumstellar disk and also of the outflow of MWC349A. Methods. We compared the bulk of radio-continuum maps, RRL profiles, and the H30α centroid map published to date with the predictions of our non-LTE 3D radiative transfer model, MOdel for REcombination LInes (MORELI), which we describe here in detail. Results. Our non-LTE 3D radiative transfer model provides new evidence that the UC-HII region of MWC349A is composed of an ionized circumstellar disk rotating in Keplerian fashion around a star of 38 M , and an ionized outflow expanding with a terminal velocity of 60 km s −1 and rotating in the same sense as the disk. The model shows that while maser amplification is the dominant process involved for Hnα RRL emission with quantum numbers n < 41, stimulated emission is relevant for the emission of RRLs with n > 41 up at least the H76α line. Conclusions. For the first time, we present a model of MWC349A which satisfactorily explains the vast amount of reported observational data for a very wide range of frequencies and angular resolutions.

Context. Recombination-line maser emission arising from MWC 349 A offers a unique possibility to study the disk kinematics and the origin of ionized outflows driven by massive stars. Aims. We aim to constrain the disk inclination and its kinematics as well as the main parameters of the outflow launching processes. Methods. We used the IRAM interferometer to measure the relative positions of the H30α centroid emission as a function of the radial velocity with an accuracy of ∼2 mas (2.4 AU) for the strongest maser features and ∼5 mas (6 AU) for the weaker line wings.Results. In addition to the east-west velocity gradient expected for a rotating disk, our data reveal for the first time the complex velocity gradients perpendicular to the disk that are related to the ejection of the ionized gas from the disk. Conclusions. From the comparison of the data with non-LTE 3D radiative transfer model predictions of the H30α line we conclude that the kinematics in the outer parts of the disk is represented by pure Keplerian rotation. We constrain the wind launching radius to less than 25 AU, much smaller than the gravitational radius of ∼150 AU. The ionized outflow seems to be launched from the disk surface because it is rotating in the same sense than the disk. Disk wind models seem to explain the inferred kinematics.

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 [ 13 C ii] hyperfine line observations. Methods. We detect 12 mmCRLs (including α, β, and γ transitions) observed with the IRAM 30 m telescope, at ∼ 25 angular resolution, toward the H / H 2 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 H ii region. This is readily seen from their narrow line profiles (∆v = 2.6 ± 0.4 km s −1 ) and line peak velocities (v LSR = +10.7 ± 0.2 km s −1 ). Optically thin [ 13 C ii] 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 [ 13 C ii] 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) · 10 8 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.

We report the detection of a new radio recombination line (RRL) maser object toward the IRS2 source in the MonR2 ultracompact HII region. The continuum emission at 1.3 mm and 0.85 mm and the H30α and H26α lines were observed with the Submillimeter Array (SMA) at angular resolutions of ∼0.5"-3". The SMA observations show that the MonR2-IRS2 source is very compact and remains unresolved at spatial scales ≤400 AU. Its continuum power spectrum at millimeter wavelengths is almost flat (α=-0.16, with S ν ∝ν α ), indicating that this source is dominated by optically thin free-free emission. The H30α and H26α RRL emission is also compact and peaks toward the position of the MonR2-IRS2 source. The measured RRL profiles are double-peaked with the H26α line showing a clear asymmetry in its spectrum. Since the derived line-to-continuum flux ratios (∼80 and 180 km s −1 for H30α and H26α, respectively) exceed the LTE predictions, the RRLs toward MonR2-IRS2 are affected by maser amplification. The amplification factors are however smaller than those found toward the emission line star MWC349A, indicating that MonR2-IRS2 is a weakly amplified maser. Radiative transfer modelling of the RRL emission toward this source shows that the RRL masers arise from a dense and collimated jet embedded in a cylindrical ionized wind, oriented nearly along the direction of the line-of-sight. High-angular resolution observations at sub-millimeter wavelengths are needed to unveil weakly amplified RRL masers in very young massive stars.

We present the first detection of the H40α, H34α and H31α radio recombination lines (RRLs) at millimeter wavelengths toward the high-velocity, ionized jet in the Cepheus A HW2 star forming region. From our single-dish and interferometric observations, we find that the measured RRLs show extremely broad asymmetric line profiles with zero-intensity linewidths of ∼1100 km s −1 . From the linewidths, we estimate a terminal velocity for the ionized gas in the jet of ≥500 km s −1 , consistent with that obtained from the proper motions of the HW2 radio jet. The total integrated line-tocontinuum flux ratios of the H40α, H34α and H31α lines are 43, 229 and 280 km s −1 , clearly deviating from LTE predictions. These ratios are very similar to those observed for the RRL maser toward MWC349A, suggesting that the intensities of the RRLs toward HW2 are affected by maser emission. Our radiative transfer modeling of the RRLs shows that their asymmetric profiles could be explained by maser emission arising from a bi-conical radio jet with a semi-aperture angle of 18 • , electron density distribution varying as r −2.11 and turbulent and expanding wind velocities of 60 and 500 km s −1 .