We present polarization observations of the 6.7-GHz methanol masers around the massive protostar Cepheus A HW2 and its associated disc. The data were taken with the Multi-Element Radio Linked ...Interferometer Network. The maser polarization is used to determine the full three-dimensional magnetic field structure around Cepheus A HW2. The observations suggest that the masers probe the large-scale magnetic field and not isolated pockets of a compressed field. We find that the magnetic field is predominantly aligned along the protostellar outflow and perpendicular to the molecular and dust disc. From the three-dimensional magnetic field orientation and measurements of the magnetic field strength along the line of sight, we are able to determine that the high-density material, in which the masers occur, is threaded by a large-scale magnetic field of ∼23 mG. This indicates that the protostellar environment at ∼1000 au from Cepheus A HW2 is slightly supercritical (λ≈ 1.7) and the relation between density and magnetic field is consistent with the collapse along the magnetic field lines. Thus, the observations indicate that the magnetic field likely regulates accretion on to the disc. The magnetic field dominates the turbulent energies by approximately a factor of 3 and is sufficiently strong to be the crucial component stabilizing the massive accretion disc and sustaining the high accretion rates needed during massive star formation.
Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum ...transferring binary system offers a plausible solution to how Be stars attained their fast rotation. Although the subdwarf remnants of mass donors in such systems should exist in abundance, only a few have been confirmed due to tight observational constraints. An indirect method of detecting otherwise hidden companions is offered by their effect on the outer parts of Be star disks, which are expected to be disrupted or truncated. In the context of the infrared and radio continuum excess radiation originating in the disk, the disk truncation can be revealed by a turndown in the spectral energy distribution due to reduced radio flux levels. In this work, we search for signs of spectral turndown in a sample of 57 classical Be stars with radio data, which include new data for 23 stars and the longest-wavelength detections so far (λ 10 cm) for two stars. We confidently detect the turndown for all 26 stars with sufficient data coverage (20 of which are not known to have close binary companions). For the remaining 31 stars, the data are inconclusive as to whether the turndown is present or not. The analysis suggests that many if not all Be stars have close companions influencing their outer disks. If confirmed to be subdwarf companions, the mass transfer spin-up scenario might explain the existence of the vast majority of classical Be stars.
Context. The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of ...the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationshipto the spiral structure, is still missing. Aims. We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions. Methods. The SEDIGISM survey covers 78 deg2 of the inner Galaxy (−60°≤ℓ≤ 18°, |b|≤ 0.5°) in the J = 2–1 rotational transition of 13CO. This isotopologue of CO is less abundant than 12CO by factors up to 100. Therefore, its emission has low to moderate optical depths, and higher critical density, making it an ideal tracer of the cold, dense interstellar medium. The data have been observed with the SHFI single-pixel instrument at APEX. The observational setup covers the 13CO(2−1) and C18O(2−1) lines, plus several transitions from other molecules. Results. The observations have been completed. Data reduction is in progress, and the final data products will be made available in the near future. Here we give a detailed description of the survey and the dedicated data reduction pipeline. To illustrate the scientific potential of this survey, preliminary results based on a science demonstration field covering −20°≤ℓ ≤ −18.5° are presented. Analysis of the 13CO(2−1) data in this field reveals compact clumps, diffuse clouds, and filamentary structures at a range of heliocentric distances. By combining our data with data in the (1–0) transition of CO isotopologues from the ThrUMMS survey, we are able to compute a 3D realization of the excitation temperature and optical depth in the interstellar medium. Ultimately, this survey will provide a detailed, global view of the inner Galactic interstellar medium at an unprecedented angular resolution of ~30′′.
Aims. In order to test the nature of an (accretion) disk in the vicinity of Cepheus A HW2, we measured the three-dimensional velocity field of the CH3OH maser spots, which are projected within 1000 ...au of the HW2 object, with an accuracy on the order of 0.1 km s-1. Methods. We made use of the European VLBI Network (EVN) to image the 6.7 GHz CH3OH maser emission toward Cepheus A HW2 with 4.5 milliarcsec resolution (3 au). We observed at three epochs spaced by one year between 2013 and 2015. During the last epoch, in mid-March 2015, we benefited from the newly deployed Sardinia Radio Telescope. Results. We show that the CH3OH velocity vectors lie on a preferential plane for the gas motion with only small deviations of 12° ± 9° away from the plane. This plane is oriented at a position angle of 134° east of north, and inclined by 26° with the line of sight, closely matching the orientation of the previously reported disk-like structure. Knowing the orientation of the equatorial plane, we can reconstruct a face-on view of the CH3OH gas kinematics onto the plane. CH3OH maser emission is detected within a radius of 900 au from HW2, and down to a radius of about 300 au, the latter coincident with the extent of the dust emission at 0.9 mm. The velocity field is dominated by an infall component of about 2 km s-1 down to a radius of 300 au, where a rotational component of 4 km s-1 becomes dominant. We discuss the nature of this velocity field and the implications for the enclosed mass. Conclusions. These findings directly support the interpretation that the high-density gas and dust emission that surrounds Cepheus A HW2 traces an accretion disk.
Context. We describe the new Swedish-ESO PI Instrument for APEX (SEPIA) receiver, which was designed and built by the Group for Advanced Receiver Development (GARD), at Onsala Space Observatory (OSO) ...in collaboration with ESO. It was installed and commissioned at the APEX telescope during 2015 with an ALMA Band 5 receiver channel and updated with a new frequency channel (ALMA Band 9) in February 2016. Aim. This manuscript aims to provide, for observers who use the SEPIA receiver, a reference in terms of the hardware description, optics and performance as well as the commissioning results. Methods. Out of three available receiver cartridge positions in SEPIA, the two current frequency channels, corresponding to ALMA Band 5, the RF band 158–211 GHz, and Band 9, the RF band 600–722 GHz, provide state-of-the-art dual polarization receivers. The Band 5 frequency channel uses 2SB SIS mixers with an average SSB noise temperature around 45 K with IF (intermediate frequency) band 4–8 GHz for each sideband providing total 4 × 4 GHz IF band. The Band 9 frequency channel uses DSB SIS mixers with a noise temperature of 75–125 K with IF band 4–12 GHz for each polarization. Results. Both current SEPIA receiver channels are available to all APEX observers.
Context. Several rotational transitions of ortho- and para-water have been identified toward evolved stars in the ground vibrational state as well as in the first excited state of the bending mode ...(v2 = 1 in (0, 1, 0) state). In the latter vibrational state of water, the 658 GHz J = 11,0−10,1 rotational transition is often strong and seems to be widespread in late-type stars. Aims. Our main goals are to better characterize the nature of the 658 GHz emission, compare the velocity extent of the 658 GHz emission with SiO maser emission to help locate the water layers and, more generally, investigate the physical conditions prevailing in the excited water layers of evolved stars. Another goal is to identify new 658 GHz emission sources and contribute in showing that this emission is widespread in evolved stars. Methods. We have used the J = 11,0−10,1 rotational transition of water in the (0, 1, 0) vibrational state nearly 2400 K above the ground-state to trace some of the physical conditions of evolved stars. Eleven evolved stars were extracted from our mini-catalog of existing and potential 658 GHz sources for observations with the Atacama Pathfinder EXperiment (APEX) telescope equipped with the SEPIA Band 9 receiver. The 13CO J = 6−5 line at 661 GHz was placed in the same receiver sideband for simultaneous observation with the 658 GHz line of water. We have compared the ratio of these two lines to the same ratio derived from HIFI earlier observations to check for potential time variability in the 658 GHz line. We have compared the 658 GHz line properties with our H2O radiative transfer models in stars and we have compared the velocity ranges of the 658 GHz and SiO J = 2−1, v = 1 maser lines. Results. Eleven stars have been extracted from our catalog of known or potential 658 GHz evolved stars. All of them show 658 GHz emission with a peak flux density in the range ≈50–70 Jy (RU Hya and RT Eri) to ≈2000–3000 Jy (VY CMa and W Hya). Five Asymptotic Giant Branch (AGB) stars and one supergiant (AH Sco) are new detections. Three AGBs and one supergiant (VY CMa) exhibit relatively weak 13CO J = 6−5 line emission while o Ceti shows stronger 13CO emission. We have shown that the 658 GHz line is masing and we found that the 658 GHz velocity extent tends to be correlated with that of the SiO maser suggesting that both emission lines are excited in circumstellar layers close to the central star. Broad and stable line profiles are observed at 658 GHz. This could indicate maser saturation although we have tentatively provided first information on time variability at 658 GHz.
Context. The 6.7 GHz methanol maser is exclusively associated with high-mass star formation. However, it remains unclear what structures harbour the methanol masers. Cepheus A is one of the closest ...regions of massive star formation, making it an excellent candidate for detailed studies. Aims. We determine the dynamics of maser spots in the high-mass star-forming region Cepheus A in order to infer where and when the maser emission occurs. Methods. Very long baseline interferometry (VLBI) observations of the 6.7 and 12.2 GHz methanol masers allows for mapping their spatial and velocity distribution. Phase-referencing is used to determine the astrometric positions of the maser emission, and multi-epoch observations can reveal 3D motions. Results. The 6.7 GHz methanol masers are found in a filamentary structure over ~1350 AU, straddling the waist of the radio jet HW2. The positions agree well with previous observations of both the 6.7 and 12.2 GHz methanol masers. The velocity field of the maser spots does not show any sign of rotation, but is instead consistent with an infall signature. The 12.2 GHz methanol masers are closely associated with the 6.7 GHz methanol masers, and the parallax that we derive confirms previous measurements. Conclusions. We show that the methanol maser emission very likely arises in a shock interface in the equatorial region of Cepheus A HW2 and presents a model in which the maser emission occurs between the infalling gas and the accretion disk/process.
Context. The star-formation rates and stellar densities found in young massive clusters suggest that these stellar systems originate from gas at densities of n(H2) > 106 cm−3. Until today, however, ...the physical characterization of this ultra high density material remains largely unconstrained in observations. Aims. We investigate the density properties of the star-forming gas in the OMC-1 region located in the vicinity of the Orion Nebula Cluster (ONC). Methods. We mapped the molecular emission at 652 GHz in OMC-1 as part of the APEX-SEPIA660 Early Science. Results. We detect bright and extended N2H+ (J = 7–6) line emission along the entire OMC-1 region. Comparisons with previous ALMA data of the (J = 1–0) transition and radiative transfer models indicate that the line intensities observed in this N2H+ (7–6) line are produced by large mass reservoirs of gas at densities n(H2) > 107 cm−3. Conclusions. The first detection of this N2H+ (7–6) line at parsec-scales demonstrates the extreme density conditions of the star-forming gas in young massive clusters such as the ONC. Our results highlight the unique combination of sensitivity and mapping capabilities of the new SEPIA660 receiver for the study of the ISM properties at high frequencies.
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