Wouter Vlemmings

An Eccentric Binary Millisecond Pulsar in the Galactic Plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric (e = 0.44) 95-day orbit around a solar mass (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{M}_{{\odot}}\) \end{document}) companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster, then ejecting it into the Galactic disk, or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74 ± 0.04 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{M}_{{\odot}}\) \end{document}, an unusually high value.

A magnetically collimated jet from an evolved star.

Planetary nebulae often have asymmetric shapes, even though their progenitor stars were symmetric; this structure could be the result of collimated jets from the evolved stars before they enter the planetary nebula phase. Theoretical models have shown that magnetic fields could be the dominant source of jet-collimation in evolved stars, just as these fields are thought to collimate outflows in other astrophysical sources, such as active galactic nuclei and proto-stars. But hitherto there have been no direct observations of both the magnetic field direction and strength in any collimated jet. Here we report measurements of the polarization of water vapour masers that trace the precessing jet emanating from the asymptotic giant branch star W43A (at a distance of 2.6 kpc from the Sun), which is undergoing rapid evolution into a planetary nebula. The masers occur in two clusters at opposing tips of the jets, ∼1,000 au from the star. We conclude from the data that the magnetic field is indeed collimating the jet.

Precision Astrometry with the Very Long Baseline Array: Parallaxes and Proper Motions for 14 Pulsars

Astrometry can bring powerful constraints to bear on a variety of scientific questions about neutron stars, including their origins, astrophysics, evolution, and environments. Using phase-referenced observations at the Very Long Baseline Array (VLBA), in conjunction with pulsar gating and in-beam calibration, we have measured the parallaxes and proper motions for 14 pulsars. The smallest measured parallax in our sample is 0.13 ? 0.02 mas for PSR B1541+09, which has a most probable distance of 7.2+1.3 ?1.1 kpc. We detail our methods, including initial VLA surveys to select candidates and find in-beam calibrators, VLBA phase-referencing, pulsar gating, calibration, and data reduction. The use of the bootstrap method to estimate astrometric uncertainties in the presence of unmodeled systematic errors is also described. Based on our new model-independent estimates for distance and transverse velocity, we investigate the kinematics and birth sites of the pulsars and revisit models of the Galactic electron density distribution. We find that young pulsars are moving away from the Galactic plane, as expected, and that age estimates from kinematics and pulsar spindown are generally in agreement, with certain notable exceptions. Given its present trajectory, the pulsar B2045 ? 16 was plausibly born in the open cluster NGC 6604. For several high-latitude pulsars, the NE2001 electron density model underestimates the parallax distances by a factor of 2, while in others the estimates agree with or are larger than the parallax distances, suggesting that the interstellar medium is irregular on relevant length scales. The VLBA astrometric results for the recycled pulsar J1713+0747 are consistent with two independent estimates from pulse timing, enabling a consistency check between the different reference frames.

Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris.

The asymptotic-giant-branch star R Sculptoris is surrounded by a detached shell of dust and gas. The shell originates from a thermal pulse during which the star underwent a brief period of increased mass loss. It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse—parameters that determine the lifetime of the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3″. What was previously thought to be only a thin, spherical shell with a clumpy structure is revealed to also contain a spiral structure. Spiral structures associated with circumstellar envelopes have been previously seen, leading to the conclusion that the systems must be binaries. Combining the observational data with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse about 1,800 years ago, lasting approximately 200 years. About 3 × 10−3 solar masses of material were ejected at a velocity of 14.3 km s−1 and at a rate around 30 times higher than the pre-pulse mass-loss rate. This shows that about three times more mass was returned to the interstellar medium during and immediately after the pulse than previously thought.

VLBA DETERMINATION OF THE DISTANCE TO NEARBY STAR-FORMING REGIONS. V. DYNAMICAL MASS, DISTANCE, AND RADIO STRUCTURE OF V773 Tau A

We present multi-epoch Very Long Baseline Array (VLBA) observations of V773 Tau A, the 51 day binary subsystem in the multiple young stellar system V773 Tau. Combined with previous interferometric and radial velocity measurements, these new data enable us to improve the characterization of the physical orbit of the A subsystem. In particular, we infer updated dynamical masses for the primary and the secondary components of 1.55 +/- 0.11 M-circle dot and 1.293 +/- 0.068 M-circle dot, respectively, and an updated orbital parallax distance to the system of 135.7 +/- 3.2 pc, all consistent with previous estimates. Using the improved orbit, we can calculate the absolute coordinates of the barycenter of V773 Tau A at each epoch of our VLBA observations, and fit for its trigonometric parallax and proper motion. This provides a direct measurement of the distance to the system almost entirely independent of the orbit modeling. The best fit yields a distance of 129.9 +/- 3.2 pc, in good agreement (i.e., within 1 sigma) with the distance estimate based on the orbital fit. Taking the mean value of the orbital and trigonometric parallaxes, we conclude that V773 Tau is located at d = 132.8 +/- 2.3 pc. The accuracy of this determination is nearly one order of magnitude better than that of previous estimates. In projection, V773 Tau and two other young stars (Hubble 4 and HDE 283572) recently observed with the VLBA are located toward the dark cloud Lynds 1495, in the central region of Taurus. These three stars appear to have similar trigonometric parallaxes, radial velocities, and proper motions, and we argue that the weighted mean and dispersion of their distances (d = 131.4 pc and sigma(d) = 2.4 pc) provide a good estimate of the distance to and depth of Lynds 1495 and its associated stellar population. The radio emission from the two sources in V773 Tau A is largely of gyrosynchrotron origin. Interestingly, both sources are observed to become typically five times brighter near periastron than near apastron (presumably because of increased flaring activity), and the separation between the radio sources near periastron appears to be systematically smaller than the separation between the stars. While this clearly indicates some interaction between the individual magnetospheres, the exact mechanisms at play are unclear because even at periastron the separation between the stars (similar to 30 R-*) remain much larger than the radius of the magnetospheres around these low-mass young stars (similar to 6 R-*).

UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data

Context. The analysis of astronomical interferometric data is often performed on the images obtained after deconvolving the interferometer’s point spread function. This strategy can be understood (especially for cases of sparse arrays) as fitting models to models, since the deconvolved images are already non-unique model representations of the actual data (i.e., the visibilities). Indeed, the interferometric images may be affected by visibility gridding, weighting schemes (e.g., natural vs. uniform), and the particulars of the (non-linear) deconvolution algorithms. Fitting models to the direct interferometric observables (i.e., the visibilities) is preferable in the cases of simple (analytical) sky intensity distributions. Aims. We present UVMULTIFIT, a versatile library for fitting visibility data, implemented in a Python-based framework. Our software is currently based on the CASA package, but can be easily adapted to other analysis packages, provided they have a Python API. Methods. The user can simultaneously fit an indefinite number of source components to the data, each of which depend on any algebraic combination of fitting parameters. Fits to individual spectral-line channels or simultaneous fits to all frequency channels are allowed. Results. We have tested the software with synthetic data and with real observations. In some cases (e.g., sources with sizes smaller than the diffraction limit of the interferometer), the results from the fit to the visibilities (e.g., spectra of close by sources) are far superior to the output obtained from the mere analysis of the deconvolved images. Conclusions. UVMULTIFIT is a powerful improvement of existing tasks to extract the maximum amount of information from visibility data, especially in cases close to the sensitivity/resolution limits of interferometric observations.

A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system

We present five epochs of VLBI water maser observations around the massive protostar Cepheus A HW2 with 0.4 mas (0.3 au) resolution. The main goal of these observations was to follow the evolution of the remarkable water maser linear/arcuate structures found in earlier VLBI observations. Comparing the data of our new epochs of observation with those observed 5 yr before, we find that at ‘large’ scales of 1 arcsec (700 au) the main regions of maser emission persist, implying that both the surrounding medium and the exciting sources of the masers have been relatively stable during that time-span. However, at smaller scales of 0.1 arcsec (70 au) we see large changes in the maser structures, particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly elliptical patchy ring of ∼70 mas size (50 au) with expanding motions of ∼5 mas yr −1 (15 km s −1 ), consistent with previous results of Gallimore and collaborators. This structure is probably driven by the wind of a still unidentified YSO located at the centre of the ring (∼0.18 arcsec south of HW2). On the other hand, the R5 expanding bubble structure (driven by the wind of a previously identified YSO located ∼0.6 arcsec south of HW2) is currently dissipating in the circumstellar medium and losing its previous degree of symmetry, indicating a very short lived event. In addition, our results reveal, at scales of ∼1 arcsec (700 au), the simultaneous presence of a relatively slow (∼10– 70 km s −1 ) wide-angle outflow (opening angle of ∼102 ◦ ), traced by the masers, and the fast (∼500 km s −1 ) highly collimated radio jet associated with HW2 (opening angle of ∼18 ◦ ), previously observed with the VLA. This simultaneous presence of a wide-angle outflow and a highly collimated jet associated with a massive protostar is similar to what is found in some low-mass YSOs. There are indications that the primary wind(s) from HW2 could be rotating. The implications of these results in the study of the formation of high-mass stars are discussed.

A new probe of magnetic fields during high-mass star formation Zeeman splitting of 6.7 GHz methanol masers

Context. The role of magnetic fields during high-mass star formation is a matter of fierce debate, yet only a few direct probes of magnetic field strengths are available. Aims. The magnetic field is detected in a number of massive star-forming regions through polarization observations of 6.7 GHz methanol masers. Although these masers are the most abundant of the maser species occurring during high-mass star formation, most magnetic field measurements in the high-density gas currently come from OH and H2O maser observations. Methods. The 100-m Effelsberg telescope was used to measure the Zeeman splitting of 6.7 GHz methanol masers for the first time. The observations were performed on a sample of 24 bright northern maser sources. Results. Significant Zeeman splitting is detected in 17 of the sources with an average magnitude of 0.56 m s −1 . Using the current best estimate of the 6.7 GHz methanol maser Zeeman splitting coefficient and a geometrical correction, this corresponds to an absolute magnetic field strength of 23 mG in the methanol maser region. Conclusions. The magnetic field is dynamically important in the dense maser regions. No clear relation is found with the available OH maser magnetic field measurements. The general sense of direction of the magnetic field is consistent with other Galactic magnetic field measurements, although a few of the masers display a change of direction between different maser features. Due to the abundance of methanol masers, measuring their Zeeman splitting provides the opportunity to construct a comprehensive sample of magnetic fields in high-mass star-forming regions.

The structure of the magnetic field in the massive star-forming region W75N

Context. A debated topic in star formation theory is the role of magnetic fields during the protostellar phase of high-mass stars. It is still unclear how magnetic fields influence the formation and dynamics of massive disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from polarized maser emissions, which allows us to investigate the magnetic field on small scales by using very long-baseline interferometry. Aims. The massive star-forming region W75N contains three radio continuum sources (VLA 1, VLA 2, and VLA 3), at three different evolutionary stages, and associated masers, while a large-scale molecular bipolar outflow is also present. Very recently, polarization observations of the 6.7 GHz methanol masers at milliarsecond resolution have been able to probe the strength and structure of the magnetic field over more than 2000 AU around VLA 1. The magnetic field is parallel to the outflow, suggesting that VLA 1 is its powering source. The observations of H 2 O masers at 22 GHz can give more information about the gas dynamics and the magnetic fields around VLA 1 and VLA 2. Methods. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. Results. We detected 124 water masers, 36 around VLA 1 and 88 around VLA 2 of W75N, which indicate two different physical environments around the two sources, where VLA 1 is in a more evolved state. The linear polarization of the masers confirms the tightly ordered magnetic field around VLA 1, which is aligned with the large-scale molecular outflow, and also reveals an ordered magnetic field around VLA 2, which is not parallel to the outflow. The Zeeman-splitting measured on 20 of the masers indicates strong magnetic fields around both sources (the averaged values are |B VLA | ~ 700 mG and |B VLA2 | ~ 1700 mG). The high values of the magnetic field strengths, which come from the shock compression of the gas, are consistent with the methanol and OH magnetic field strengths. Moreover, by studying the maser properties we were also able to determine that the water masers are pumped in C-shocks in both sources.

A strong magnetic field in the jet base of a supermassive black hole

The polarized mark of magnetic fields Powerful twin jets of plasma often reach more than tens of thousands of light-years from their base in an active galactic nucleus (AGN). Astronomers are still at work investigating what can corral the jets so tightly and propel them so far. Martí-Vidal et al. may have found the answer hiding in polarized light signals that show evidence of a phenomenon called Faraday rotation. This measure can indicate the strength of the magnetic field present, which for the AGN PKS 1830-211 is as strong as a few Gauss. The knowledge that magnetic fields have a driving role brings us closer to understanding this phenomenon. Science, this issue p. 311 A polarized signal offers evidence for the agent that boosts and guides powerful jets in a distant active galaxy. Active galactic nuclei (AGN) host some of the most energetic phenomena in the universe. AGN are thought to be powered by accretion of matter onto a rotating disk that surrounds a supermassive black hole. Jet streams can be boosted in energy near the event horizon of the black hole and then flow outward along the rotation axis of the disk. The mechanism that forms such a jet and guides it over scales from a few light-days up to millions of light-years remains uncertain, but magnetic fields are thought to play a critical role. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we have detected a polarization signal (Faraday rotation) related to the strong magnetic field at the jet base of a distant AGN, PKS 1830−211. The amount of Faraday rotation (rotation measure) is proportional to the integral of the magnetic field strength along the line of sight times the density of electrons. The high rotation measures derived suggest magnetic fields of at least tens of Gauss (and possibly considerably higher) on scales of the order of light-days (0.01 parsec) from the black hole.