arXiv subjects

H. J. van Langevelde

Publications and source records attributed to H. J. van Langevelde.

83 records · Page 5Linked to original sources

arXiv2002

The Shroud Around the Twin Radio Jets in NGC 1052

(Abridged) We discuss multiple VLBI continuum and spectral line observations and WSRT spectroscopy of NGC 1052. Sub-parsec scale features move outward at approximately 0.26c in bi-symmetric jets, most likely oriented near the plane of the sky. Absorption and emission signatures reveal ionised, atomic, and molecular components of the surrounding medium. Seven-frequency (1.4 to 43 GHz) VLBA observations show free-free absorption in the inner parsec, probably together with synchrotron self-absorption. There is apparently a geometrically thick but patchy structure oriented roughly orthogonal to the jets. The western jet is receding: it is covered more deeply and extensively. HI spectral line VLBI reveals atomic gas in front of both jets. There appear to be three velocity systems. The deepest, at "high velocities" (receding by 125 to 200 km/s), seems restricted to a shell 1 to 2 pc away from the core, within which this gas might be largely ionised. WSRT spectroscopy has revealed 1667 and 1665 MHz OH absorption with their line ratio varying roughly from 1:1 to 2:1 between -35 and 200 km/s. In the high velocity system the OH profiles are similar to HI, suggesting co-location of that atomic and molecular gas, and leaving unclear the connection to the H2O masing gas seen elsewhere. We have also detected both 18cm OH satellite lines in the high velocity system. They have conjugate profiles: 1612 MHz is in absorption, and 1720 MHz in emission.

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arXiv2002

The shroud around the "compact, symmetric" radio jets in NGC 1052

This is a paper on young jet material in a frustratingly complex environment. NGC 1052 has a compact, flat- or GHz-peaked-spectrum radio nucleus consisting of bi-symmetric jets, oriented close to the plane of the sky. Many features on both sides move away at v_app~0.26c (H_0=65 km/s/Mpc). VLBI at seven frequencies shows a wide range of spectral shapes and brightness temperatures; there is clearly free-free absorption, probably together with synchrotron self-absorption, on both sides of the core. The absorbing structure is likely to be geometrically thick and oriented roughly orthogonal to the jets, but it is patchy. HI VLBI shows atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems, at least two of which are local to the AGN environment. The "high velocity system", 125 to 200 km/s redward of systemic, seems restricted to a shell 1 to 2 pc away from the core. Closer to the centre, this gas might be largely ionised; it could cause the free-free absorption. WSRT spectroscopy shows 1667 and 1665 MHz OH absorption over a wide velocity range. OH and HI profile similarity suggests co-location of molecular and atomic ``high velocity'' gas; the connection to H2O masing gas is unclear. Further, at ``high velocity'' we detected the OH 1612 MHz satellite line in absorption and the 1720 MHz line in emission, with complementary strengths.

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arXiv2002

Astrometry of circumstellar masers

The circumstellar masers around evolved stars offer an interesting possibility to measure stellar parameters through VLBI astrometry. In this paper the application of this technique is discussed, including the accuracy and the uncertainties of the method. The different maser species (OH, H_2O, SiO) have slightly different characteristics and applications. This paper does not concern astrometry of maser spots to study the kinematics of the envelope, but concentrates on attempting to measure the motion of the underlying star.

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arXiv2002

Circular Polarization of Water Masers in the Circumstellar Envelopes of Late Type Stars

We present circular polarization measurements of circumstellar H_2O masers. The circular polarization detected in the (6_{16}-5_{23}) rotational transition of the H_{2}O maser can be attributed to Zeeman splitting in the intermediate temperature and density regime. The magnetic fields are derived using a general, LTE Zeeman analysis 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 saturation and unequal populations of the magnetic substates. The differences and relevances of these interpretations are discussed extensively. The field strengths are compared with previous detections of the magnetic field on the SiO and OH masers. We show that the magnetic pressure dominates the thermal pressure by a factor of 20 or more.

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arXiv2002

Astrometry of the stellar image of U Her amplified by the circumstellar 22 GHz water masers

The 22 GHz H_2O masers in the circumstellar envelope of the Mira variable star U Her have been observed with MERLIN using a phase referencing technique to determine accurate astrometric positions. The positions were compared with the optical positions obtained with the Hipparcos satellite to an accuracy of 18 mas. The absolute radio position of the brightest H_2O maser spot is found to match the optical position, indicating that this spot is the stellar image amplified by the maser screen in front of it. The occurrence of an amplified image in the 22 GHz maser can be used to accurately determine the positions of the H_2O with respect to the star as well as with respect to the SiO and OH masers. Our observations seem to indicate that the star is not in the centre of the distribution of maser spots, which has been interpreted as a ring.

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arXiv2002

Magnetic Fields in the envelopes of late-type stars: Circular Polarization of H2O Masers

We present the first circular polarization measurements of circumstellar water masers around a sample of late-type stars. These observations are used to obtain the magnetic field strength in the water maser region with both an LTE and non-LTE analysis. We find fields from a few hundred milliGauss up to a few Gauss, indicating a solar-type r^(-2) dependence of the magnetic field on the distance. No linear polarization is detected to less than 1%.

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arXiv2001

Circular Polarization of Circumstellar Water Masers around S Per

We present the first circular polarization measurements of circumstellar H2O masers. Previously the magnetic field in circumstellar envelopes has been estimated using polarization observations of SiO and OH masers. SiO masers are probes of the high temperature and density regime close to the central star. OH masers are found at much lower densities and temperatures, generally much further out in the circumstellar envelope. The detection of the circular polarization of the (6_16 - 5_23) rotational transition of the H2O maser could be attributed to Zeeman splitting due to the magnetic field in the intermediate temperature and density regime. The fields inferred here agree well with predicted values for a combination of the r^{-2} dependence of a solar-type magnetic field, and the coupling of the field to the high density masing regions. We also discuss the unexpected narrowing of the circular polarization spectrum.

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arXiv2000

VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser

The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.

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arXiv1999

A Thin HI Circumnuclear Disk in NGC4261

We report on high sensitivity, spectral line VLBI observations of the HI absorption feature in the radio galaxy NGC4261. Although absorption is only detectable on the most sensitive baseline, it can be unambiguously associated with the counterjet and is interpreted to originate in a thin atomic circumnuclear disk. This structure is probably a continuation of the dusty accretion disk inferred from HST imaging, which could be feeding the massive black hole. HI column densities in front of the counterjet of the order of 10^{21}(T_sp/100 K) cm^{-2} are derived, consistent with X-ray data and VLBI scale free-free absorption. The data presented here are the result of the first scientific project processed on the new EVN MkIV data processor.

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arXiv1998

Faint Gigahertz peaked spectrum sources and the evolution of young radio sources

GPS sources are the objects of choice to study the initial evolution of extragalactic radio sources, since it is most likely that they are the young counterparts of large scale radio sources. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. The difference in redshift distribution between young and old radio sources must be due to a difference in slope of their luminosity functions, and we argue that this slope is strongly affected by the luminosity evolution of the individual sources. A luminosity evolution scenario is proposed in which GPS sources increase in luminosity and large scale radio sources decrease in luminosity with time. It is shown that such a scenario agrees with the local luminosity function of GPS galaxies.

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arXiv1997

Sub-arcsecond imaging at 267 GHz of a young binary system: Detection of a <70 AU radius dust disk around T Tau N

The young binary system T Tau was observed with the Owens Valley Millimeter Array at 267 GHz continuum and HCO+ J=3-2 emission at 0.8'' resolution, with the JCMT-CSO single-baseline interferometer in 357 GHz continuum, and with the W. M. Keck Telescope at 4 micron. The 267 GHz emission is unresolved with a flux of 397 +/- 35 mJy, located close to the position of the optical star T Tau N. An upper limit of 100 mJy is obtained toward the infrared companion T Tau S. The 357 GHz continuum emission is unresolved, with a flux of 1.35 +/- 0.68 Jy. HCO+ 3-2 was detected from a 2'' diameter core surrounding T Tau N and S. Both stars are detected ar 4 micron, but there is no evidence of the radio source T Tau R. We propose a model in which T Tau S is intrinsically similar to T Tau N, but is obscured by the outer parts of T Tau N's disk. A fit to the spectral energy distribution (SED) between 21 cm and 1.22 micron is constructed on this basis. Adopting a r^-1 surface density distribution and an exponentially truncated edge, disk masses of 0.04 +/- 0.01 Msol and 6e-5 - 3e-3 Msol are inferred for T Tau N and S, respectively. A 0.005-0.03 Msol circumbinary envelope is also required to fit the millimeter to mid-infrared SED.

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