E. M. L. Humphreys

Toward a New Geometric Distance To the Active Galaxy NGC 4258. III. Final Results and The Hubble Constant

We report a new geometric maser distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight (LOS) velocities and sky positions from 18 epochs of very long baseline interferometry observations, and LOS accelerations measured from a 10 yr monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is 7.60 ± 0.17 ± 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is H0 = 72.0 ± 3.0 km s −1 Mpc −1 .

A FEATURE MOVIE OF SiO EMISSION 20-100 AU FROM THE MASSIVE YOUNG STELLAR OBJECT ORION SOURCE I

We present multi-epoch Very Long Baseline Array imaging of the 28SiO v = 1 and v = 2, J = 1-0 maser emission toward the massive young stellar object (YSO) Orion Source I. Both SiO transitions were observed simultaneously with an angular resolution of ~0.5 mas (~0.2 AU for d = 414 pc) and a spectral resolution of ~0.2 km s–1. Here we explore the global properties and kinematics of the emission through two 19-epoch animated movies spanning 21 months (from 2001 March 19 to 2002 December 10). These movies provide the most detailed view to date of the dynamics and temporal evolution of molecular material within ~20-100 AU of a massive (8 M ☉) YSO. As in previous studies, we find that the bulk of the SiO masers surrounding Source I lie in an -shaped locus; the emission in the south and east arms is predominantly blueshifted, and emission in the north and west is predominantly redshifted. In addition, bridges of intermediate-velocity emission are observed connecting the red and blue sides of the emission distribution. We have measured proper motions of over 1000 individual maser features and found that these motions are characterized by a combination of radially outward migrations along the four main maser-emitting arms and motions tangent to the intermediate-velocity bridges. We interpret the SiO masers as arising from a wide-angle bipolar wind emanating from a rotating, edge-on disk. The detection of maser features along extended, curved filaments suggests that magnetic fields may play a role in launching and/or shaping the wind. Our observations appear to support a picture in which stars with masses as high as at least 8 M ☉ form via disk-mediated accretion. However, we cannot yet rule out that the Source I disk may have been formed or altered following a recent close encounter.

A MULTI-EPOCH STUDY OF THE RADIO CONTINUUM EMISSION OF ORION SOURCE I: CONSTRAINTS ON THE DISK EVOLUTION OF A MASSIVE YSO AND THE DYNAMICAL HISTORY OF ORION BN/KL

We present new λ7 mm continuum observations of Orion BN/KL with the Very Large Array. We resolve the emission from the young stellar objects radio Source I and BN at several epochs. Radio Source I is highly elongated northwest-southeast, and remarkably stable in flux density, position angle, and overall morphology over nearly a decade. This favors the extended emission component arising from an ionized edge-on disk rather than an outwardly propagating jet. We have measured the proper motions of Source I and BN for the first time at 43 GHz. We confirm that both sources are moving at high speed (12 and 26 km s–1, respectively) approximately in opposite directions, as previously inferred from measurements at lower frequencies. We discuss dynamical scenarios that can explain the large motions of both BN and Source I and the presence of disks around both. Our new measurements support the hypothesis that a close (~50 AU) dynamical interaction occurred around 500 years ago between Source I and BN as proposed by Gomez et al. From the dynamics of encounter, we argue that Source I today is likely to be a binary with a total mass on the order of 20 M ☉ and that it probably existed as a softer binary before the close encounter. This enables preservation of the original accretion disk, though truncated to its present radius of ~50 AU. N-body numerical simulations show that the dynamical interaction between a binary of 20 M ☉ total mass (Source I) and a single star of 10 M ☉ mass (BN) may lead to the ejection of both and binary hardening. The gravitational energy released in the process would be large enough to power the wide-angle, high-velocity flow traced by H2 and CO emission in the BN/KL nebula. Assuming that the proposed dynamical history is correct, the smaller mass for Source I recently estimated from SiO maser dynamics (7 M ☉) by Matthews et al., suggests that non-gravitational forces (e.g., magnetic) must play an important role in the circumstellar gas dynamics.

Toward a New Geometric Distance to the Active Galaxy NGC 4258. II. Centripetal Accelerations and Investigation of Spiral Structure

We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is the determination of a new geometric maser distance to NGC 4258, accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range –0.7 to +0.7 km s−1 yr−1, indicating an origin within 13° of the disk midline (the perpendicular to the line of sight [LOS] to the black hole). Comparison of the projected positions of high-velocity emission in VLBI images with those derived from acceleration data provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km s−1 yr−1, consistent with emission originating from a concavity where the thin, warped disk is tangent to the LOS. A trend in accelerations of low-velocity emission, as a function of Doppler velocity, may be associated with disk geometry and orientation or with the presence of spiral structure.

Maser Emission from SiO Isotopologues Traces the Innermost 100 AU Around Radio Source I in Orion Becklin–Neugebauer/Kleinmann-Low

We have used the Very Large Array at 7 mm wavelength to image five rotational transitions (J = 1-0) from three SiO isotopologues toward Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL): 28SiO v = 0, 1, 2; and 29SiO and 30SiO v = 0. For the first time, we have mapped the 29SiO and 30SiO J = 1-0 emission, established the maser nature of the emission, and confirmed association with the deeply embedded high-mass young stellar object commonly denoted radio Source I. The 28SiO v = 0 maser emission shows a bipolar structure that extends over ~700 AU along a northeast-southwest axis, and we propose that it traces a bipolar outflow driven by Source I. The high-brightness isotopic SiO maser emission imaged with a 02 resolution has a more compact distribution, generally similar to that of the 28SiO v = 1, 2 emission, and it probably traces bulk gas flows in a region of diameter 100 AU centered on Source I. On small scales of 10 AU, however, compact 29SiO/30SiO v = 0 and 28SiO v = 1, 2 emission features may be offset from one another in position and line-of-sight velocity. From a radiative transfer analysis based on a large velocity gradient pumping model, we derive similar temperatures and densities for the optimum excitation of both 29SiO/30SiO v = 0 and 28SiO v = 1, 2 masers, significantly higher than required for 28SiO v = 0 maser excitation. In order to account for the small-scale differences among the isotopologues (v = 0) and the main species (v = 1, 2), follow-up radiative transfer modeling that incorporates nonlocal line overlap among transitions of all SiO isotopic species may be required.

A 42.3-43.6 GHz spectral survey of Orion BN/KL: First detection of the v=0 J=1-0 line from the isotopologues 29SiO and 30SiO

We have surveyed molecular line emission from Orion BN/KL from 42.3 to 43.6 GHz with the Green Bank Telescope. Sixty-seven lines were identified and ascribed to 13 different molecular species. The spectrum at 7 mm is dominated by SiO, SO2 ,C H 3OCH3, and C2H5CN. Five transitions have been detected from the SiO isotopologues 28 SiO, 29 SiO, and 30 SiO. We report here for the first time the spectra of the 29 SiO and 30 SiO v = 0 J = 1–0 emission in Orion BN/KL, and show that they have double-peaked profiles with velocity extents similar to the main isotopologue. The main motivation for the survey was the search of high-velocity (100–1000 km s −1 ) outflows in the BN/KL region as traced by SiO Doppler components. Some of the unidentified lines in principle could be highvelocity SiO features, but without imaging data their location cannot be established. Wings of emission are present in the v = 0 28 SiO, 29 SiO, and 30 SiO profiles, and we suggest that the v = 0 emission from the three isotopologues might trace a moderately high-velocity (∼ 30–50 km s −1 ) component of the flows around the high-mass protostar Source I in the Orion BN/KL region. We also confirm the 7 mm detection of a complex oxygen-bearing species, acetone (CH3COCH3), which has been recently observed toward the hot core at 3 mm, and have found further indications of the presence of long cyanopolyynes (HC5N and HC7N) in the quiescent cold gas of the extended ridge.

First Detection of Millimeter/Submillimeter Extragalactic H2O Maser Emission

We report the first detection of an extragalactic millimeter wavelength H2O maser at 183 GHz toward NGC 3079 using the Submillimeter Array (SMA) and a tentative submillimeter wave detection of the 439 GHz maser toward the same source using the James Clerk Maxwell Telescope (JCMT). These H2O transitions are known to exhibit maser emission in star-forming regions and evolved stars. NGC 3079 is a well-studied nuclear H2O maser source at 22 GHz with a time-variable peak flux density in the range 3-12 Jy. The 183 GHz H2O maser emission, with a peak flux density of ~0.5 Jy (7 σ detection), also originates from the nuclear region of NGC 3079 and is spatially coincident with the dust continuum peak at 193 GHz (53 mJy integrated). Peak emission at both 183 and 439 GHz occurs in the same range of velocity as that covered by the 22 GHz spectrum. We estimate the gas-to-dust ratio of the nucleus of NGC 3079 to be ≈150, comparable to the Galactic value of 160. Discovery of maser emission in an active galactic nucleus beyond the long-known 22 GHz transition opens the possibility of future position-resolved radiative transfer modeling of accretion disks and outflows <1 pc from massive black holes.

Submillimeter and millimeter masers

Despite theoretical predictions of the existence of many submillimeter masers, and some pioneering observational discoveries over the past few decades, these lines have remained relatively unstudied due to (i) challenges associated with observing at shorter wavelength; and, (ii) the lack of possibility of high ( 2 O millimeter masers, hydrogen recombination masers, submillimeter masers in star-forming regions, and in the envelopes of evolved stars - and discuss prospects for the future.

A documentary of high-mass star formation: Probing the dynamical evolution of Orion Source I on 10–100 AU scales using SiO masers

A comprehensive picture of high-mass star formation has remained elusive, in part because examples of high-mass young stellar objects (YSOs) tend to be relatively distant, deeply embedded, and confused with other emission sources. These factors have impeded dynamical investigations within tens of AU of high-mass YSOs—scales that are critical for probing the interfaces where outflows from accretion disks are launched and collimated. Using observations of SiO masers obtained with the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), the KaLYPSO project is overcoming these limitations by mapping the structure and dynamical/temporal evolution of the material 10-1000 AU from the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data include ~40 epochs of VLBA observations over a several-year period, allowing us to track the proper motions of individual SiO maser spots and to monitor changes in the physical conditions of the emitting material with time. Ultimately these data will provide 3-D maps of the outflow structure over approximately 30% of the outflow crossing time. Here we summarize recent results from the KaLYPSO project, including evidence that high-mass star formation occurs via disk-mediated accretion.

The Sub-PC Scale Accretion Disk of NGC 4258

Water megamasers have been found to trace parsec/sub-parsec, circumnuclear accretion disks in several AGN (e.g., Circinus, NGC 1068 & NGC 4258). High-spatial (0.5 mas) and velocity resolution (0.2 km s−1) VLBA imaging of the disks reveals thin, warped ‘pannekoeken (pancake)’-style structures as opposed to thick tori in the inner regions of the central engines (40 000 Rsch). In this contribution, I will describe some current investigations into the dynamical and physical attributes of the water maser disk in NGC 4258, as revealed by VLBA, VLA and Effelsberg monitoring over 8 years.