H. J. van Langevelde

Parallaxes and proper motions of interstellar masers toward the Cygnus X star-forming complex - I. Membership of the Cygnus X region

Context. Whether the Cygnus X complex consists of one physically connected region of star formation or of multiple independent regions projected close together on the sky has been debated for decades. The main reason for this puzzling scenario is the lack of trustworthy distance measurements. Aims. We aim to understand the structure and dynamics of the star-forming regions toward Cygnus X by accurate distance and proper motion measurements. Methods. To measure trigonometric parallaxes, we observed 6.7 GHz methanol and 22 GHz water masers with the European VLBI Network and the Very Long Baseline Array. Results. We measured the trigonometric parallaxes and proper motions of five massive star-forming regions toward the Cygnus X complex and report the following distances within a 10% accuracy: 1.30 +0.07 ―0.07 kpc for W 75N, 1.46 +0.09 ―0.08 kpc for DR 20, 1.50 +0.08 ―0.07 kpc for DR 21, 1.36 +0.12 ―0.11 kpc for IRAS 20290+4052, and 3.33 +0.11 ―0.11 kpc for AFGL 2591. While the distances of W 75N, DR 20, DR 21, and IRAS 20290+4052 are consistent with a single distance of 1.40 ± 0.08 kpc for the Cygnus X complex, AFGL 2591 is located at a much greater distance than previously assumed. The space velocities of the four star-forming regions in the Cygnus X complex do not suggest an expanding Stromgren sphere.

A direct localization of a fast radio burst and its host

Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities that are orders of magnitude larger than those of all known short-duration radio transients. So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts and tidal disruption events. However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.

Trigonometric parallaxes of 6.7 GHz methanol masers

Aims. Emission from the 6.7 GHz methanol maser transition is very strong, is relatively stable, has small internal motions, and is observed toward numerous massive star-forming regions in the Galaxy. Our goal is to perform high-precision astrometry using this maser transition to obtain accurate distances to their host regions.

The magnetic field around late-type stars revealed by the circumstellar H2O masers

Through polarization observations, circumstellar masers are excellent probes of the magnetic field in the envelopes of late-type stars. Whereas observations of the polarization of the SiO masers close to the star and on the OH masers much further out were fairly commonplace, observations of the magnetic field strength in the intermediate density and temperature region where the 22 GHz H2O masers occur have only recently become possible. Here we present the analysis of the circular polarization, due to Zeeman splitting, of the H2O masers around the Mira variable stars U Her and U Ori and the supergiant VX Sgr. We present an upper limit of the field around U Her that is lower but consistent with previous measurements, reflecting possible changes in the circumstellar envelope. The field strengths around U Ori and VX Sgr are shown to be of the order of several Gauss. Moreover, we show for the first time that large scale magnetic fields permeate the circumstellar envelopes of an evolved star; the polarization of the H2O masers around VX Sgr reveals a dipole field structure. We discuss the coupling of the magnetic field with the stellar outflow, as such fields could possibly be the cause of distinctly aspherical mass-loss.

Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths

Received ?? ; Accepted ?? Abstract. Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study.

Circular polarization of water masers in the circumstellar envelopes of late type stars

We present circular polarization measurements of circumstellar H2O masers. The magnetic fields in circumstellar envelopes are generally examd. by polarization observations of SiO and OH masers. SiO masers C probe the high temp. and d. regime close to the central star. OH masers are found at much lower densities and temps., generally much further out in the circumstellar envelope. The circular polarization detected in the (616-523) rotational transition of the H2O maser can be attributed to Zeeman splitting in the intermediate temp. and d. regime. The magnetic fields are derived using a general, LTE Zeeman anal. as well as a full radiative transfer method (non-LTE), which includes a treatment of all hyperfine components simultaneously as well as the effects of satn. and unequal populations of the magnetic substates. The differences and relevances of these interpretations are discussed extensively. We also address a non-Zeeman interpretation as the cause for the circular polarization, but this is found to be unlikely. We favor the non-LTE anal. The H2O masers are shown to be unsatd., on the basis of their line widths and the lack of linear polarization. The field strengths are compared with previous detections of the magnetic field on the SiO and OH masers. Assuming a r-2 dependence of the magnetic field on the distance to the star, similar to a solar-type magnetic field, our results seem to indicate that we are probing the highest d. maser clumps at the inner edge of the H2O maser region. This allows us to est. the d. of the clumps, and the extent of the H2O maser region. We show that the magnetic pressure dominates the thermal pressure by a factor of 20 or more. We also give an order of magnitude est. of the magnetic field on the surface of the stars. In particular we discuss the differences between supergiants and Mira variable stars. [on SciFinder (R)]

The diversity of methanol maser morphologies from VLBI observations

Context. The 6.7 GHz methanol maser marks an early stage of high-mass star formation, but the origin of this maser is currently a matter of debate. In particular it is unclear whether the maser emission arises in discs, outflows or behind shocks running into rotating molecular clouds. Aims. We investigated which structures the methanol masers trace in the environment of high-mass protostar candidates by observing a homogenous sample of methanol masers selected from Torun surveys. We also probed their origins by looking for associated H II regions and IR emission. Methods. We selected 30 methanol sources with improved position accuracies achieved using MERLIN and another 3 from the literature. We imaged 31 of these using the European VLBI Network’s expanded array of telescopes with 5-cm (6-GHz) receivers. We used the VLA to search for 8.4 GHz radio continuum counterparts and inspected Spitzer GLIMPSE data at 3.6–8 μm from the archive. Results. High angular resolution images allowed us to analyze the morphology and kinematics of the methanol masers in great detail and verify their association with radio continuum and mid-infrared emission. A new class of “ring-like” methanol masers in starforming regions appeared to be suprisingly common, 29% of the sample. Conclusions. The new morphology strongly suggests that methanol masers originate in the disc or torus around a proto- or a young massive star. However, the maser kinematics indicate the strong influence of outflow or infall. This suggests that they form at the interface between the disc/torus and a flow. This is also strongly supported by Spitzer results because the majority of the masers coincide with 4.5 μm emission to within less than 1 �� . Only four masers are associated with the central parts of UC H II regions. This implies that 6.7 GHz methanol maser emission occurs before H II region observable at cm wavelengths is formed.

The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102

The precise localization of the repeating fast radio burst (FRB 121102) has provided the first unambiguous association (chance coincidence probability p ≲ 3 × 10‑4) of an FRB with an optical and persistent radio counterpart. We report on optical imaging and spectroscopy of the counterpart and find that it is an extended (0.″6–0.″8) object displaying prominent Balmer and [Oiii] emission lines. Based on the spectrum and emission line ratios, we classify the counterpart as a low-metallicity, star-forming, mr‧ = 25.1 AB mag dwarf galaxy at a redshift of z =0.19273(8), corresponding to a luminosity distance of 972 Mpc. From the angular size, the redshift, and luminosity, we estimate the host galaxy to have a diameter ≲4 kpc and a stellar mass of M* ∼ (4–7) × 107 M⊙, assuming a mass-to-light ratio between 2 to 3 M⊙L⊙‑1. Based on the Hα flux, we estimate the star formation rate of the host to be 0.4 M⊙yr‑1 and a substantial host dispersion measure (DM)depth ≲324 pc cm‑3. The net DM contribution of the host galaxy to FRB 121102 is likely to be lower than this value depending on geometrical factors. We show that the persistent radio source at FRB 121102’s location reported by Marcote et al. is offset from the galaxy’s center of light by ∼200 mas and the host galaxy does not show optical signatures for AGN activity. If FRB121102 is typical of the wider FRB population and if futureinterferometric localizations preferentially find them in dwarf galaxies with low metallicities and prominent emission lines, they would share such a preference with long gamma-ray bursts and superluminous supernovae.

VLBI astrometry of circumstellar OH masers: Proper motions and parallaxes of four AGB stars

The main-line OH masers around 4 AGB stars have been observed with the NRAO Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5 yrs. Using a phase referencing technique, the position of the most compact maser spot of each star was monitored with respect to two extragalactic reference sources. For U Her and W Hya, we observe the most blue-shifted maser spot, while for R Cas and S CrB we only detect a compact red-shifted maser spot. We managed to determine an accurate proper motion and parallax for U Her, R Cas and S CrB, while additional motion of the compact blue-shifted maser of W Hya is shown to possibly be related to the stellar pulsation. The motion and radio position are compared with the stellar trajectory and absolute optical position determined by the Hipparcos satellite. For U Her and W Hya, the most blue-shifted maser is consistent with the amplified stellar image. The new distances are compared with several published P−L relations, and in this respect the VLBI distances seem an improvement upon the Hipparcos distances.

A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr

The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration γ-ray bursts (GRBs). Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft γ-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c. Independently, a second mildly relativistic supernova has been reported. Contrary to the radio data, optical observations of SN 2007gr indicate a typical type Ic supernova with ejecta velocities ∼6,000 km s-1, much lower than in GRB-associated supernovae. We conclude that in SN 2007gr a small fraction of the ejecta produced a low-energy mildly relativistic bipolar radio jet, while the bulk of the ejecta were slower and, as shown by optical spectropolarimetry, mildly aspherical.