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L. J. Greenhill

Publications and source records attributed to L. J. Greenhill.

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arXiv2001

Radio Properties of Pinwheel Nebulae

A small number of dusty Wolf-Rayet stars have been resolved into pinwheel nebulae, defined by their ``rotating'' spiral dust shells observed in the infrared. This morphology is naturally explained by dust formation associated with colliding winds in a binary system. In order to confirm and further explore this hypothesis, we have observed the known pinwheel nebulae (WR 104 and WR 98a) as well as the suspected binary WR 112 at multiple radio wavelengths with the Very Large Array to search for non-thermal radio emission from colliding winds. The spectrum of each target is nearly flat between 5 and 22 GHz, consistent with the presence of non-thermal emission that is reduced at low frequencies by free-free absorption. This emission must lie outside the radio ``photosphere,'' leading us to estimate a lower limit to the physical size of the non-thermal emitting region that is larger than expected from current theory. Based on a radio and infrared comparison to WR 104 and 98a, we conclude that WR 112 is a likely candidate pinwheel nebula, but its temporal variability indicates an eccentric binary orbit or a pinwheel viewed nearly edge-on. A sensitive radio survey of IR-bright WRs would stringently test the hypothesis that colliding winds lie at the heart of_all_ dusty WR systems. We also discuss the effects of dust obscuration in the ultra-violet and how radio-determined mass-loss rates of pinwheel nebulae (and dusty WR stars in general) may be underestimated due to shadowing effects.

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arXiv2001

Extragalactic H2O Masers

Study of extragalactic H2O masers has progressed significantly in the 25 years since their discovery. Existing in star forming regions and in the accretion disks supermassive black holes, they are familiar and unfamiliar at the same time. A review of how our understanding has grown, up to the present day, is followed by comments on future prospects.

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arXiv2001

Radio and Infrared Properties of Dust-Enshrouded Wolf-Rayet Stars

This paper discusses our ongoing efforts to characterize dust-enshrouded Wolf-Rayet (WR) stars in the radio and infrared. We have used the Very Large Array to measure the broadband radio spectrum of WR stars in suspected binary systems and discovered non-thermal emission, which is usually attributed to colliding winds. In addition, infrared imaging using aperture masking interferometry on the Keck-I telescope has resolved the dust shells around a number of WR stars with K-magnitudes brighter than ~6. Although this admittedly small study suffers from selection bias, we note that all the dust-enshrouded WR stars with radio detections show evidence for colliding winds, supporting the theory that wind compression in a binary system is necessary for efficient dust production. A consequence of this hypothesis is that virtually all WC8-10 stars must be in binaries, since most are dusty. Single-star and binary stellar evolution models will have to be modified to accommodate this observational result if confirmed.

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arXiv2000

Accretion and Outflow Traced by Water Masers in the Circinus AGN

The first VLBI images of water maser emission in the Circinus Galaxy AGN show both a warped, edge-on accretion disk and an outflow 0.1 to 1 pc from the central engine. The inferred central mass is 1.3 million suns, while the disk mass may be on the order of 0.1 million suns, based on a nearly Keplerian rotation curve. The bipolar, wide-angle outflow appears to contain ``bullets'' ejected from within <0.1 pc of the central mass. The positions of filaments and bullets observed in the AGN ionization cone on kpc-scales suggest that the disk channels the flow to a radius of about 0.4 pc, at which the flow appears to disrupt the disk.

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arXiv2000

Accretion and Outflow in the Circinus AGN

The first VLBI images of water maser emission in the Circinus Galaxy AGN show both an accretion disk and outflow 0.1 to 1 pc from the central engine. The maser traces a warped, edge-on accretion disk between radii of about 0.1 and 0.4 pc that is bound by a 1.3x10^6 Msun central mass. The rotation curve is somewhat flatter than a Keplerian rotation law and is consistent with a disk mass on the order of 10^5 Msun. Clumpy substructure may display a kinematic signature consistent with spiral arms. Away from the disk, a second population of water masers traces broadly a bipolar, wide-angle outflow that contains (bullet-like) clumps ejected from a region centered on the central engine and <0.1 pc in radius. Out to a radius of about 0.4 pc, the warp of the accretion disk appears to channel the outflow, the orientation of which on the sky coincides with the orientation of the known kiloparsec-scale ionization cone. Beyond this radius, the flow crosses the disk and truncates it. This suggests that the current accretion event and associated disk-outflow geometry have a lifetime on the order of 10^7 years.

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arXiv2000

Proper Motion of Water Masers Associated with IRAS 21391+5802: Bipolar Outflow and an AU-Scale Dusty Circumstellar Shell

We present VLBA observations of water maser emission associated with the star-forming region IRAS 21391+5802, which is embedded in a bright rimmed cometary globule in IC1396. The angular resolution of the maps is about 0.8 mas, corresponding to a spatial resolution of about 0.6 AU, at an estimated distance of 750 pc. Proper motions are derived for 10 maser features identified consistently over three epochs, which were separated by intervals of about one month. The masers appear in four groups, which are aligned linearly on the sky, roughly along a northeast-southwest direction, with a total separation of about 520 AU (about 0.7 arcseconds). The 3-D velocities of the masers have a maximum value of about 42 km/s (about 9 AU/yr). The average error on the derived proper motions is about 4 km/s. The overall pattern of proper motions is indicative of a bipolar outflow. Proper motions of the masers in a central cluster, with a projected extent of about 20 AU, show systematic deviations from a radial outflow. However, we find no evidence of Keplerian rotation, as has been claimed elsewhere. A nearly circular loop of masers lies near the middle of the cluster. The radius of this loop is 1 AU and the line-of-sight velocities of the masers in the loop are within 2 km/s of the systemic velocity of the region. These masers presumably exist at the radial distance where significant dust condensation occurs in the outflow emanating from the star.

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arXiv2000

Observational Evidence for Massive Black Holes in the Centers of Active Galaxies

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, < 0.1 km/s, the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3.9 times 10^7 solar masses. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 percent. The status of the search for other maser/black hole candidates is also discussed.

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arXiv2000

Accelerations of Water Masers in NGC4258

The water masers in NGC4258 delineate the structure and dynamics of a sub-parsec-diameter accretion disk around a supermassive black hole. We report acceleration measurements for 12 systemic-velocity spectral features and 19 high-velocity spectral features, using a total of 25 epochs of observations from Effelsberg (5 epochs), the VLA (15 epochs), and the VLBA (5 epochs) spanning the years 1994 to 1997. The measured accelerations of the systemic-velocity features are between 7.5 and 10.4 km/s/yr, and there is no evidence for a dip in the spectrum at the systemic velocity. Such a dip has been attributed in the past to an absorbing layer of non-inverted H2O (Watson & Wallin 1994; Maoz & McKee 1998). The accelerations of the high-velocity features, measured here for the first time, range from -0.77 to 0.38 km/s/yr. From the line-of-sight accelerations and velocities, we infer the positions of these high-velocity masers with a simple edge-on disk model. The resulting positions fall between -13.6 and 9.3 degrees in azimuth (measured from the midline, the diameter perpendicular to the line of sight). A model that suggests a spiral shock origin of the masers (Maoz & McKee 1998), in which changes in maser velocity are due to the outward motion of the shock wave, predicts apparent accelerations of -0.05 times theta_p/2.5 degrees km/s/yr, where theta_p is the pitch angle of the spiral arms. Our data are not consistent with these predictions. We also discuss the physical properties of the high-velocity masers. Most notably, the strongest high-velocity masers lie near the midline, where the velocity gradient is smallest, thereby providing the longest amplification path lengths. (Abridged)

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arXiv1999

A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk

The water maser in the mildly active nucleus in the nearby galaxy NGC4258 traces a thin, nearly edge-on, subparsec-scale Keplerian disk. Using the technique of very long baseline interferometry, we have detected the proper motions of these masers as they sweep in front of the central black hole at an orbital velocity of about 1100 km/s. The average maser proper motion of 31.5 microarcseconds per year is used in conjunction with the observed acceleration of the masers to derive a purely geometric distance to the galaxy of 7.2 +- 0.3 Mpc. This is the most precise extragalactic distance measured to date, and, being independent of all other distance indicators, is likely to play an important role in calibrating the extragalactic distance scale.

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arXiv1998

Coexisting conical bipolar and equatorial outflows from a high-mass protostar

The BN/KL region in the Orion molecular cloud is an archetype in the study of the formation of stars much more massive than the Sun. This region contains luminous young stars and protostars, but it is difficult to study because of overlying dust and gas. Our basic expectations are shaped to some extent by the present theoretical picture of star formation, the cornerstone of which is that protostars acrete gas from rotating equatorial disks, and shed angular momentum by ejecting gas in bipolar outflows. The main source of the outflow in the BN/KL region may be an object known as radio source I, which is commonly believed to be surrounded by a rotating disk of molecular material. Here we report high-resolution observations of silicon monoxide (SiO) and water maser emission from the gas surrounding source I; we show that within 60 AU (about the size of the Solar System), the region is dominated by a conical bipolar outflow, rather than the expected disk. A slower outflow, close to the equatorial plane of the protostellar system, extends to radii of 1,000 AU.

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arXiv1998

Polarimetric Observations of the Masers in NGC 4258: An Upper Limit on the Large-Scale Magnetic Field 0.2 pc from the Central Engine

We report VLA 1 sigma upper limits of 1.5% and 3% on the intrinsic circular and linear fractional polarizations, respectively, of the water vapor maser emission 0.2 pc from the central engine of NGC 4258. A corresponding 0.5% upper limit on any Zeeman-splitting-induced circular polarization translates to a 1 sigma upper limit on the parallel, or toroidal, component of the magnetic field of 300 mG. Assuming magnetic and thermal pressure balance in the disk, this magnetic field upper limit corresponds to a model-dependent estimate of the accretion rate through the molecular disk of 10^-1.9 alpha solar masses per year for the case where the magnetic field lies along the line of sight.

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arXiv1997

VLBA Imaging of the OH Maser in IIIZw35

We present a parsec-scale image of the OH maser in the nucleus of the active galaxy IIIZw35, made using the Very Long Baseline Array at a wavelength of 18 cm. We detected two distinct components, with a projected separation of 50 pc (for D=110 Mpc) and a separation in Doppler velocity of 70 km/s, which contain 50% of the total maser flux. Velocity gradients within these components could indicate rotation of clouds with binding mass densities of ~7000 solar masses per cubic parsec, or total masses of more than 500,000 solar masses. Emission in the 1665-MHz OH line is roughly coincident in position with that in the 1667-MHz line, although the lines peak at different Doppler velocities. We detected no 18 cm continuum emission; our upper limit implies a peak apparent optical depth greater than 3.4, assuming the maser is an unsaturated amplifier of continuum radiation.

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arXiv1997

The Distribution of H2O Maser Emission in the Nucleus of NGC 4945

We present the first interferometer map of the water maser emission in the active nucleus of NGC 4945. Although the declination of the galaxy is about -49 degrees, we were able to make the observations with the southernmost antennas of the Very Long Baseline Array. Strong maser emission is present in three velocity ranges, one near the systemic velocity and two shifted roughly symmetrically by +/-(100-150) km/s. This is the first detection of highly blue-shifted water emission in NGC 4945. We determined the position of the maser to be RA(B1950)= 13 02 32.28 +/- 0.02 ; Dec(B1950)= -49 12 01.9 +/- 0.1. The uncertainties in earlier estimates are at least several arcseconds. The maser lies within 2'' (36 pc at a distance of 3.7 Mpc) of the peaks in 1.4 GHz continuum and 1.6 micron emission from the nucleus. The mappable maser emission is distributed roughly linearly over about 40 milliarcseconds (0.7 pc) at a position angle of about 45 degrees, which is close to the 43 +/- 2 degree position angle of the galactic disk. The red and blue-shifted emission symmetrically stradle the systemic emission on the sky, which suggests material in edge-on circular motion around a central object. The position-velocity structure indicates a binding mass of about one million Suns, within a volume of radius about 0.3 pc. This implies that the central engine radiates on the order of 10% of its Eddington luminosity.

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arXiv1996

Extremely Rapid Variations of Water Maser Emission from the Circinus Galaxy

The water maser lines in the nucleus of the Circinus galaxy vary on time scales as short as a few minutes, at least two orders of magnitude less than for other Galactic or extragalactic water masers. The amplitude of one line more than doubled in about 10 minutes. The intensity changes cannot be attributed easily to a mechanism of intrinsic fluctuations. The variability may be the result of strong interstellar diffractive scintillation along the line of sight within our Galaxy, which would be the first example of diffractive scintillation for any source at 22 GHz and for any source other than a pulsar. However, only the very shortest timescales for interstellar scintillation, obtained from pulsar observations and scaled to 22 GHz, correspond to the observed maser variability. Alternatively, the intensity changes may be a reaction to fluctuations in compact background or radiative pump sources and thereby may be related to variability of the central engine. The maser spectral features symmetrically bracket the systemic velocity of the galaxy, offset by about +/-(100-200) km/s. The spectrum of the Circinus maser is similar in some respects to that of the maser in NGC 4258, which probably traces a molecular disk rotating around a supermassive object. VLBI observations could reveal whether the maser source in the heart of the Circinus galaxy is part of a similar dynamical system.

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arXiv1996

VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068

We have made the first VLBI synthesis images of the H2O maser emission associated with the central engine of the Seyfert galaxy NGC 1068. Emission extends about +/-300 km/s from the systemic velocity. Images with submilliarcsecond angular resolution show that the red-shifted emission lies along an arc to the northwest of the systemic emission. (The blue-shifted emission has not yet been imaged with VLBI.) Based on the maser velocities and the relative orientation of the known radio jet, we propose that the maser emission arises on the surface of a nearly edge-on torus, where physical conditions are conducive to maser action. The visible part of the torus is axially thick, with comparable height and radius. The velocity field indicates sub-Keplerian differential rotation around a central mass of about 1e7 Msun that lies within a cylindrical radius of about 0.65 pc. The estimated luminosity of the central engine is about 0.5 of the Eddington limit. There is no detectable compact radio continuum emission near the proposed center of the torus (T_B< 5e6 K on size scales of about 0.1 pc), so that the observed flat-spectrum core cannot be direct self-absorbed synchrotron radiation.

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