arXiv subjects

M. J. Reid

Publications and source records attributed to M. J. Reid.

At least 109 records · Page 6Linked to original sources

arXiv2008

The nature of the methanol maser ring G23.657-00.127

Methanol masers are associated with young high-mass stars and are an important tool for investigating the process of massive star formation. The recently discovered methanol maser ring in G23.657-00.127 provides an excellent ``laboratory'' for a detailed study of the nature and physical origin of methanol maser emission, as well as parallax and proper motion measurements. Multi-epoch observations of the 12.2 GHz methanol maser line from the ring were conducted using the Very Long Baseline Array. Interferometric observations with milliarcsecond resolution enabled us to track single maser spots in great detail over a period of 2 years. We have determined the trigonometric parallax of G23.657-00.127 to be 0.313+/-0.039 mas, giving a distance of 3.19{+0.46}{-0.35} kpc. The proper motion of the source indicates that it is moving with the same circular velocity as the LSR, but it shows a large peculiar motion of about 35 km/s toward the Galactic center.

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arXiv2008

Discovery of an HI Counterpart to the Extended Tail of Mira

We report the detection of an HI counterpart to the extended, far-ultraviolet-emitting tail associated with the asymptotic giant branch star Mira (o Ceti). Using the Nancay Radio Telescope (NRT), we have detected emission as far as 88' north of the star, confirming that the tail contains a significant atomic component (M_HI ~ 4x10e-3 M_sun). The NRT spectra reveal a deceleration of the tail gas caused by interaction with the local interstellar medium. We estimate an age for the tail of ~1.2x10e5 years, suggesting that the mass-loss history of Mira has been more prolonged than previous observational estimates. Using the Very Large Array (VLA) we have also imaged the HI tail out to ~12' (0.4 pc) from the star. The detected emission shows a ``head-tail'' morphology, but with complex substructure. Regions with detected HI emission correlate with far-ultraviolet-luminous regions on large scales, but the two tracers are not closely correlated on smaller scales (<1'). We propose that detectable tails of HI are likely to be a common feature of red giants undergoing mass-loss.

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arXiv2007

A multi-transition submillimeter water maser study of evolved stars - detection of a new line near 475 GHz

Context: Maser emission from the H2O molecule probes the warm, inner circumstellar envelopes of oxygen-rich red giant and supergiant stars. Multi-maser transition studies can be used to put constraints on the density and temperature of the emission regions. Aims: A number of known H2O maser lines were observed toward the long period variables R Leo and W Hya and the red supergiant VY CMa. A search for a new, not yet detected line near 475 GHz was conducted toward these stars. Methods: The Atacama Pathfinder Experiment telescope was used for a multi-transition observational study of submillimeter H2O lines. Results: The 5_33-4_40 transition near 475 GHz was clearly detected toward VY CMa and W Hya. Many other H2O lines were detected toward all three target stars. Relative line intensity ratios and velocity widths were found to vary significantly from star to star. Conclusions: Maser action is observed in all but one line for which it was theoretically predicted. In contrast, one of the strongest maser lines, in R Leo by far the strongest, the 437 GHz 7_53-6_60 transition, is not predicted to be inverted. Some other qualitative predictions of the model calculations are at variance with our observations. Plausible reasons for this are discussed. Based on our findings for W Hya and VY CMa, we find evidence that the H2O masers in the AGB star W Hya arise from the regular circumstellar outflow, while shock excitation in a high velocity flow seems to be required to excite masers far from the red supergiant VY CMa.

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arXiv2007

Toward a New 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 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/yr indicating an origin within 13 degrees of the disk midline (the perpendicular to the line-of-sight to the black hole). Comparison of high-velocity emission projected positions 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/yr, consistent with emission originating from a concavity where the thin, warped disk is tangent to the line-of-sight. 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.

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arXiv2007

The distance to the Orion Nebula

We have used the Very Long Baseline Array to measure the trigonometric parallax of several member stars of the Orion Nebula Cluster showing non-thermal radio emission. We have determined the distance to the cluster to be 414 +/- 7 pc. Our distance determination allows for an improved calibration of luminosities and ages of young stars. We have also measured the proper motions of four cluster stars which, when accurate radial velocities are measured, will put strong constraints on the origin of the cluster.

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arXiv2007

Imaging the Radio Photospheres of Mira Variables

We have used the VLA at 43 GHz to image the radio continuum emission from o Ceti, R Leo, and W Hya and to precisely locate their SiO maser emission with respect to the star. The radio continuum emission region for all three stars has a diameter close to 5.6 AU. These diameters are similar to those measured at infrared wavelengths in bands containing strong molecular opacity and about twice those measured in line-free regions of the infrared spectrum. Thus, the radio photosphere and the infrared molecular layer appear to be coextensive. The 43 GHz continuum emission is consistent with temperatures near 1600 K and opacity from H-minus free-free interactions. While the continuum image of o Ceti appears nearly circular, both R Leo and W Hya display significant elongations. The SiO masers for all three stars show partial rings with diameters close to 8 AU.

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arXiv2007

Proper Motions in the Andromeda Subgroup

This article presents results of VLBI observations of regions of H2O maser activity in the Local Group galaxies M33 and IC10. Since all position measurements were made relative to extragalactic background sources, the proper motions of the two galaxies could be measured. For M33, this provides this galaxy's three dimensional velocity, showing that this galaxy is moving with a velocity of 190 +/- 59 km\s relative to the Milky Way. For IC10, we obtain a motion of 215 +/- 42 km/s relative to the Milky Way. These measurements promise a new handle on dynamical models for the Local Group and the mass and dark matter halo of Andromeda and the Milky Way.

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arXiv2007

Redshifted formaldehyde from the gravitational lens B0218+357

The gravitational lens toward B0218+357 offers the unique possibility to study cool moderately dense gas with high sensitivity and angular resolution in a cloud that existed half a Hubble time ago. Observations of the radio continuum and six formaldehyde (H2CO) lines were carried out with the VLA, the Plateau de Bure interferometer, and the Effelsberg 100-m telescope. Three radio continuum maps indicate a flux density ratio between the two main images, A and B, of ~ 3.4 +/- 0.2. Within the errors the ratio is the same at 8.6, 14.1, and 43 GHz. The 1_{01}-0_{00} line of para-H2CO is shown to absorb the continuum of image A. Large Velocity Gradient radiative transfer calculations are performed to reproduce the optical depths of the observed two cm-wave "K-doublet" and four mm-wave rotational lines. These calculations also account for a likely frequency-dependent continuum cloud coverage. Confirming the diffuse nature of the cloud, an n(H2) density of < 1000 cm^{-3} is derived, with the best fit suggesting n(H2) ~ 200 cm^{-3}. The H2CO column density of the main velocity component is ~5 * 10^{13} cm^{-2}, to which about 7.5 * 10^{12} cm^{-2} has to be added to also account for a weaker feature on the blue side, 13 km/s apart. N(H2CO)/N(NH3) ~ 0.6, which is four times less than the average ratio obtained from a small number of local diffuse (galactic) clouds seen in absorption. The ortho-to-para H2CO abundance ratio is 2.0 - 3.0, which is consistent with the kinetic temperature of the molecular gas associated with the lens of B0218+357. With the gas kinetic temperature and density known, it is found that optically thin transitions of CS, HCN, HNC, HCO+, and N2H+ (but not CO) will provide excellent probes of the cosmic microwave background at redshift z=0.68.

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arXiv2007

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 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 VLA and the 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 is occurring via disk-mediated accretion.

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arXiv2007

Imaging the Ionized Disk of the High-Mass Protostar Orion-I

We have imaged the enigmatic radio source-I (Orion-I) in the Orion-KL nebula with the VLA at 43 GHz with 34 mas angular resolution. The continuum emission is highly elongated and is consistent with that expected from a nearly edge-on disk. The high brightness and lack of strong molecular lines from Orion-I can be used to argue against emission from dust. Collisional ionization and H-minus free-free opacity, as in Mira variables, require a central star with >10^5 Lsun, which is greater than infrared observations allow. However, if significant local heating associated with accretion occurs, lower total luminosities are possible. Alternatively, photo-ionization from an early B-type star and p+/e- bremsstrahlung can explain our observations, and Orion-I may be an example of ionized accretion disk surrounding a forming massive star. Such accretion disks may not be able to form planets efficiently.

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arXiv2007

The Position of Sagittarius A*: III. Motion of the Stellar Cusp

In the first two papers of this series, we determined the position of Sgr A* on infrared images, by aligning the positions of red giant stars with positions measured at radio wavelengths for their circumstellar SiO masers. In this paper, we report detections of 5 new stellar SiO masers within 50" (2 pc) of Sgr A* and new and/or improved positions and proper motions of 15 stellar SiO masers. The current accuracies are ~1 mas in position and ~0.3 mas/y in proper motion. We find that the proper motion of the central stellar cluster with respect to Sgr A* is less than 45 km/s. One star, IRS 9, has a three-dimensional speed of ~370 km/s at a projected distance of 0.33 pc from Sgr A*. If IRS 9 is bound to the inner parsec, this requires an enclosed mass that exceeds current estimates of the sum of the mass of Sgr A* and luminous stars in the stellar cusp by ~0.8 x 10^6 Msun. Possible explanations include i) that IRS 9 is not bound to the central parsec and has "fallen" from a radius greater than 9 pc, ii) that a cluster of dark stellar remnants accounts for some of the excess mass, and/or iii) that Ro is considerably greater than 8 kpc.

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arXiv2007

Toward a New Geometric Distance to the Active Galaxy NGC4258: I. VLBI Monitoring of Water Maser Emission

We report a three year, 18 epoch, VLBI monitoring study of H2O masers in the sub-parsec, warped, accretion disk within the NGC4258 AGN. Our immediate goals are to trace the geometry of the underlying disk, track rotation via measurement of proper motion, and ascertain the radii of masers for which centripetal acceleration may be measured separately. The monitoring includes ~ 4 times as many epochs, ~ 3 times denser sampling, and tighter control over sources of systematic error than earlier VLBI investigations. Coverage of a ~ 2400 km/s bandwidth has also enabled mapping of molecular material ~ 30% closer to the black hole than accomplished previously, which will strengthen geometric and dynamical disk models. Through repeated observation we have also measured for the first time a 5 microarsecond (1 sigma) thickness of the maser medium. Assuming this corresponds to the thickness of the accretion disk, hydrostatic equilibrium requires a disk plane temperature of ~ 600 K. Our long-term goal is a geometric distance to NGC4258 that is accurate to ~ 3%, a ~ 2 times improvement over the current best estimate. A geometric estimate of distance can be compared to distances obtained from analysis of Cepheid light curves, with the intent to recalibrate the extragalactic distance scale with reduced systematic uncertainties. This is the first paper in a series. We present here VLBI observations, data reduction, and temporal and spatial characteristics of the maser emission. Later papers will report estimation of orbital acceleration and proper motion, modeling of disk 3-D geometry and dynamics, and estimation of a "maser distance." Estimation of a "Cepheid distance" is presented in a parallel paper series.

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arXiv2006

Towards a kinematic model of the Local Group

Measuring the proper motions and geometric distances of galaxies within the Local Group is very important for our understanding of the history, present state and future of the Local Group. Currently, proper motion measurements using optical methods are limited only to the closest companions of the Milky Way. However, Very Long Baseline Interferometry (VLBI) provides the best angular resolution in astronomy and phase-referencing techniques yield astrometric accuracies of ~ 10 micro-arcseconds. This makes a measurement of proper motions and angular rotation rates of galaxies out to a distance of ~ 1 Mpc feasible. This article presents results of VLBI observations of regions of H2O maser activity in the Local Group galaxies M33 and IC10. These measurements promise a new handle on dynamical models for the Local Group and the mass and dark matter halo of Andromeda and the Milky Way. (Abridged)

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arXiv2006

The Proper Motion of the Local Group Galaxy IC 10

We have measured the proper motion of the Local Group galaxy IC 10 with the Very Long Baseline Array by measuring the position of an H2O maser relative to two background quasars over 4.3 years. After correction for the rotation of the Milky Way and IC 10, we obtain a motion of -39 +/- 9 microas/yr toward the East and 31 +/- 8 microas/yr toward the North. This corresponds to a total space velocity of 215 +/- 42 km/s relative to the Milky Way for an assumed distance of 660 +/- 66 kpc. Assuming that IC 10 and M33, for which also a proper motion measurement exists, are bound to M31, we calculate a lower limit for the mass of M31 of 7.5 x 10^11 Msol.

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arXiv2006

Water masers in the Local Group of galaxies

We compare the number of detected 22 GHz H2O masers in the Local Group galaxies M31, M33, NGC6822, IC10, IC1613, DDO187, GR8, NGC185, and the Magellanic Clouds with the water maser population of the Milky Way. To accomplish this we searched for water maser emission in the two Local Group galaxies M33 and NGC6822 using the Very Large Array (VLA) and incorporated results from previous studies. We observed 62 HII regions in M33 and 36 regions with H-alpha emission in NGC6822. Detection limits are 0.0015 and 0.0008 Lsun for M33 and NGC6822, respectively (corresponding to 47 and 50 mJy in three channels with 0.7 km/s width). M33 hosts three water masers above our detection limit, while in NGC6822 no maser source was detected. We find that the water maser detection rates in the Local Group galaxies M31, M33, NGC6822, IC1613, DDO187, GR8, NGC185, and the Magellanic Clouds are consistent with expectations from the Galactic water masers if one considers the different star formation rates of the galaxies. However, the galaxy IC10 exhibits an overabundance of masers, which may result from a compact central starburst.

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arXiv2006

Search for Compact Extragalactic Radio Sources Near Massive Star Forming Regions

We have used the Very Large Array to search for compact milliarcsecond-size radio sources near methanol masers in high-mass star-forming regions. Such sources are required for Very Long Baseline Interferometry phase-referencing observations. We conducted pointed observations of 234 compact sources found in the NVSS survey and find 92 sources with unresolved components and synchrotron spectral indexes. These sources are likely the cores of AGNs and, thus, good candidates for astrometric calibrators.

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arXiv2006

Radio continuum monitoring of the extreme carbon star IRC+10216

We describe Very Large Array observations of the extreme carbon star IRC+10216 at 8.4, 14.9, and 22.5 GHz made over a two year period. We find possible variability correlated with the infrared phase and a cm- to sub-millimeter wavelength spectral index very close to 2. The variability, observed flux densities, and upper limit on the size are consistent with the emission arising from the stellar photosphere or a slightly larger radio photosphere.

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arXiv2006

Water maser motions in W3(OH) and a determination of its distance

We report phase-referencing VLBA observations of H2O masers near the star-forming region W3(OH) to measure their parallax and absolute proper motions. The measured annual parallax is 0.489 +/- 0.017 milli-arcseconds (2.04 +/- 0.07 kpc), where the error is dominated by a systematic atmospheric contribution. This distance is consistent with photometric distances from previous observations and with the distance determined from CH3OH maser astrometry presented in a related paper. We also find that the source driving the H2O outflow, the ``TW-object'', moves with a 3-dimensional velocity of > 7 km/s relative to the ultracompact HII region W3(OH).

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