Showing papers by "Mark J. Reid published in 2008"

Trigonometric Parallaxes of Massive Star Forming Regions: I. S 252 & G232.6+1.0

Abstract: We are conducting a large program with the NRAO Very Long Baseline Array (VLBA) to measure trigonometric parallaxes of massive star-forming regions across the Milky Way. Here we report measurement of the parallax and proper motion of methanol masers in S 252 and G232.6+1.0. The parallax of S 252 is 0.476 +/- 0.006 mas (2.10 [+0.027/-0.026] kpc), placing it in the Perseus spiral arm. The parallax of G232.6+1.0 is 0.596 +/- 0.035 mas (1.68 [+0.11/-0.09] kpc), placing it between the Carina-Sagittarius and Perseus arms, possibly in a Local (Orion) spur of the Carina-Sagittarius arm. For both sources, kinematic distances are significantly greater than their parallax distances. Our parallaxes and proper motions yield full space motions accurate to about 1 km/s. Both sources orbit the Galaxy about 13 km/s slower than circular rotation.

Tropospheric Delay Calibrations for VERA

Abstract: We present techniques for the tropospheric delay calibration, which is the key to increasing the accuracy of the phase-referencing astrometry with Very Long Baseline Interferometry (VLBI). We study three methods, and make a comparison of these methods to discuss the accuracy in calibration. Our results show that all three methods can calibrate the tropospheric zenith delay within accuracy of � 2 cm. We also present simulations of positional errors in VLBI Exploration of Radio Astrometry (VERA) at the presence of an error in the tropospheric zenith delay, showing that parallax measurements with accuracy of 10 � as can be readily achieved for sources at high declination and with

Trigonometric Parallaxes of Massive Star Forming Regions: III. G59.7+0.1 and W 51 IRS2

Abstract: We report trigonometric parallaxes for G59.7+0.1 and W 51 IRS2, corresponding to distances of 2.16^{+0.10}_{-0.09} kpc and 5.1^{+2.9}_{-1.4} kpc, respectively. The distance to G59.7+0.1 is smaller than its near kinematic distance and places it between the Carina-Sagittarius and Perseus spiral arms, probably in the Local (Orion) spur. The distance to W 51 IRS2, while subject to significant uncertainty, is close to its kinematic distance and places it near the tangent point of the Carina-Sagittarius arm. It also agrees well with a recent estimate based on O-type star spectro/photometry. Combining the distances and proper motions with observed radial velocities gives the full space motions of the star forming regions. We find modest deviations of 5 to 10 km/s from circular Galactic orbits for these sources, both counter to Galactic rotation and toward the Galactic center.

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

Abstract: 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.

Discovery of an H I Counterpart to the Extended Tail of Mira

Abstract: We report the detection of an H I 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 (MH I ~ 4 × 10−3 M☉). 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 ~1.2 × 105 yr, suggesting that the mass-loss history of Mira has been more prolonged than previous observational estimates. Using the Very Large Array we have also imaged the H I 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 H I 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 H I are likely to be a common feature of red giants undergoing mass loss.

Is there a Supermassive Black Hole at the Center of the Milky Way

Abstract: This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4 million solar masses within a radius of 100 Astronomical Units. Orbital speeds have been observed to exceed 5,000 km/s. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c^2). This radio source is motionless at the ~1 km/s level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.

Discovery of an HI Counterpart to the Extended Tail of Mira

Abstract: 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.

The nature of the methanol maser ring G23.657-00.127 - I. The distance through trigonometric parallax measurements

Abstract: Context. Methanol masers are associated with young high-mass stars and are an important tool for investigating the process of massive star formation. Aims. 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. Methods. 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. Results. We have determined the trigonometric parallax of G23.657-00.127 to be 0.313±0.039 mas, giving a distance of 3 19 +0.46 035 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 ≈35 km s -1 toward the Galactic center.

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

Abstract: 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.

A 1.3 cm wavelength radio flare from a deeply embedded source in the Orion BN/KL region

Abstract: Aims. Our aim was to measure and characterize the short-wavelength radio emission from young stellar objects (YSOs) in the Orion Nebula Cluster and the BN/KL star-forming region. Methods. We used the NRAO Very Large Array at a wavelength of A = 1.3 cm and we studied archival X-ray, infrared, and radio data. Results. During our observation, a strong outburst (flux increasing > 10 fold) occurred in one of the 16 sources detected at A = 1.3 cm, while the others remained (nearly) constant. This source does not have an infrared counterpart, but has subsequently been observed to flare in X-rays. Curiously, a very weak variable double radio source was found at other epochs near this position, one of whose components is coincident with it. A very high extinction derived from modeling the X-ray emission and the absence of an infrared counterpart both suggest that this source is very deeply embedded.

Submillimeter water and ammonia absorption by the peculiar z ≈ 0.89 interstellar medium in the gravitational lens of the PKS 1830−211 system

Abstract: Using the Atacama Pathfinder Experiment (APEX) telescope, we have detected the rotational ground-state transitions of ortho-ammonia and ortho-water toward the redshift ≈ 0.89 absorbing galaxy in the PKS 1830-211 gravitational lens system. We discuss our observations in the context of recent space-borne data obtained for these lines with the SWAS and Odin satellites toward Galactic sources. We find commonalities, but also significant differences between the interstellar media in a galaxy at intermediate redshift and in the Milky Way. Future high-quality observations of the ground-state ammonia transition in PKS 1830-211, together with inversion line data, will lead to strong constraints on the variation in the proton to electron mass ratio over the past 7.2 Gyr.

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

Abstract: Context. Maser emission from the H 2 O 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 H 2 O 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 H 2 O lines. Results. The 5 33 -4 40 transition near 475 GHz was clearly detected toward VY CMa and W Hya. Many other H 2 O 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 H 2 O 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.

Submillimeter water and ammonia absorption by the peculiar z~0.89 interstellar medium in the gravitational lens of the PKS 1830-211 system

Abstract: Using the Atacama Pathfinder Experiment (APEX) telescope we have detected the rotational ground-state transitions of ortho-ammonia and ortho-water toward the redshift ~0.89 absorbing galaxy in the PKS 1830-211 gravitational lens system. We discuss our observations in the context of recent space-borne data obtained for these lines with the SWAS and Odin satellites toward Galactic sources. We find commonalities, but also significant differences between the interstellar media in a galaxy at intermediate redshift and in the Milky Way. Future high-quality observations of the ground-state ammonia transition in PKS 1830-211, together with inversion line data, will lead to strong constraints on the variation of the proton to electron mass ratio over the past 7.2 Gyr.

Limits on the position wander of sgr a

Abstract: We present measurements with the Very Long Baseline Array of the variability in the centroid position of Sgr A* relative to a background quasar at 7 mm wavelength. We find an average centroid wander of 71 ± 45 μ as for timescales between 50 and 100 minutes and 113 ± 50 μ as for timescales between 100 and 200 minutes, with no secular trend. These are sufficient to begin constraining the viability of the accretion hot spot model for the radio variability of Sgr A*. It is possible to rule out hot spots with orbital radii above 15GMSgr A*/c2 that contribute more than 30% of the total 7 mm flux. However, closer or less luminous hot spots remain unconstrained. Since the fractional variability of Sgr A* during our observations was ~20% on timescales of hours, the hot spot model for Sgr A*'s radio variability remains consistent with these limits. Improved monitoring of Sgr A*'s centroid position has the potential to place significant constraints on the existence and morphology of inhomogeneities in a supermassive black hole accretion flow.

Evidence for Warped Disks of Young Stars in the Galactic Center

Abstract: The central parsec around the super-massive black hole in the Galactic Center hosts more than 100 young and massive stars. Outside the central cusp (R~1") the majority of these O and Wolf-Rayet (WR) stars reside in a main clockwise system, plus a second, less prominent disk or streamer system at large angles with respect to the main system. Here we present the results from new observations of the Galactic Center with the AO-assisted near-infrared imager NACO and the integral field spectrograph SINFONI on the ESO/VLT. These include the detection of 27 new reliably measured WR/O stars in the central 12" and improved measurements of 63 previously detected stars, with proper motion uncertainties reduced by a factor of four compared to our earlier work. We develop a detailed statistical analysis of their orbital properties and orientations. Half of the WR/O stars are compatible with being members of a clockwise rotating system. The rotation axis of this system shows a strong transition as a function of the projected distance from SgrA*. The main clockwise system either is either a strongly warped single disk with a thickness of about 10 degrees, or consists of a series of streamers with significant radial variation in their orbital planes. 11 out of 61 clockwise moving stars have an angular separation of more than 30 degrees from the clockwise system. The mean eccentricity of the clockwise system is 0.36+/-0.06. The distribution of the counter-clockwise WR/O star is not isotropic at the 98% confidence level. It is compatible with a coherent structure such as stellar filaments, streams, small clusters or possibly a disk in a dissolving state. The observed disk warp and the steep surface density distribution favor in situ star formation in gaseous accretion disks as the origin of the young stars.

Distances to Galactic methanol masers

Abstract: We present the first EVN parallax measurements of 6.7 GHz methanol masers in star forming regions of the Galaxy. The 6.7 GHz methanol maser transition is a very valuable astrometric tool, for its large stability and confined velocity spread, which makes it ideal to measure proper motions and parallaxes. Eight well-studied massive star forming regions have been observed during five EVN sessions of 24 hours duration each and we present here preliminary results for five of them. We achieve accuracies of up to 51 $\mu$as, which still have the potential to be proved by more ideal observational circumstances.

Limits on the Position Wander of Sgr A

Abstract: We present measurements with the VLBA of the variability in the centroid position of Sgr A* relative to a background quasar at 7-mm wavelength. We find an average centroid wander of 71 +/- 45 micro-arcsec for time scales between 50 and 100 min and 113 +/- 50 micro-arcsec for timescales between 100 and 200 min, with no secular trend. These are sufficient to begin constraining the viability of the hot-spot model for the radio variability of Sgr A*. It is possible to rule out hot spots with orbital radii above 15GM_SgrA*/c^2 that contribute more than 30% of the total 7-mm flux. However, closer or less luminous hot spots remain unconstrained. Since the fractional variability of Sgr A* during our observations was ~20% on time scales of hours, the hot-spot model for Sgr A*'s radio variability remains consistent with these limits. Improved monitoring of Sgr A*'s centroid position has the potential to place significant constraints upon the existence and morphology of inhomogeneities in a supermassive black hole accretion flow.

Distances to Galactic methanol masers

Abstract: We present the first EVN parallax measurements of 6.7 GHz methanol masers in star forming regions of the Galaxy. The 6.7 GHz methanol maser transition is a very valuable astrometric tool, for its large stability and confined velocity spread, which makes it ideal to measure proper motions and parallaxes. Eight well-studied massive star forming regions have been observed during five EVN sessions of 24 hours duration each and we present here preliminary results for five of them. We achieve accuracies of up to 51 μas, which still have the potential to be improved by more ideal observational circumstances.

Investigating Dark Energy with Observations of H 2 O Megamasers

Abstract: As a complement to cosmic microwave background data, a determination of the Hubble constant (H0) to better than ∼ 3% would provide a powerful constraint on the equation of state of dark energy. Water-vapor megamasers provide perhaps the best opportunity for measuring direct distances to galaxies out to about 200 Mpc. The Megamaser Cosmology Project is pursuing the ambitious goal of making a precise measurement of H0 by measuring maser distances using the techniques pioneered on NGC4258 by Herrnstein et al. (1999). In recent surveys we have identified several megamasers appropriate for distance measurements. To get the distance, two types of observations are required: spectral-line monitoring to measure the acceleration of gas in the disk, and sensitive VLBI imaging to measure the angular size of the disk, measure the rotation curve, and model the radial displacement of systemic maser features. We have recently mapped two of the masers with VLBI, and here we present preliminary results of the maser in UGC3789. The maser disk in UGC3789 was discovered and monitored with the Green Bank Telescope (GBT) and subsequently imaged with the High Sensitivity Array (VLBA + GBT + Effelsberg). While uncertainties are still large, our preliminary analysis gives results in the expected range (distance ∼ 49 Mpc corresponding to H0 ∼ 68 km s Mpc). Ongoing observations should reduce the uncertainty in the distance to ∼ 10%.

Trigonometric Parallaxes of Massive Star Forming Regions: V. G23.01-0.41 and G23.44-0.18

Abstract: We report trigonometric parallaxes for the massive star-forming regions G23.01-0.41 and G23.44-0.18, corresponding to distances of 4.59 [+0.38/-0.33] kpc and 5.88 [+1.37/-0.93] kpc, respectively. The distance to G23.01-0.41 is smaller than its near kinematic distance assuming a standard model of the Milky Way and less than half of its far kinematic distance, which has usually been assumed. This places it in the Crux-Scutum spiral arm. The distance to G23.44-0.18 is close to its near kinematic distance and most likely places it in the Norma spiral arm near the end of the Galactic bar. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find large deviations from circular Galactic orbits for these sources: both sources show peculiar motions of 20 to 30 km/s counter to Galactic rotation and toward the Galactic center. These large peculiar motions might be the result of gravitational perturbations from the Galactic bar.

Proper Motions in the Andromeda Subgroup

Abstract: 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 kms-1 relative to the Milky Way. For IC10, we obtain a motion of 215 ± 42 kms-1 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.

Trigonometric Parallaxes of Massive Star Forming Regions: IV. G35.20-0.74 and G35.20-1.74

Abstract: We report trigonometric parallaxes for the high-mass star forming regions G35.20-0.74 and G35.20-1.74, corresponding to distances of 2.19 (+0.24 -0.20) kpc and 3.27 (+0.56 -0.42) kpc, respectively. The distances to both sources are close to their near kinematic distances and place them in the Carina-Sagittarius spiral arm. Combining the distances and proper motions with observed radial velocities gives the locations and full space motions of the star forming regions. Assuming a standard model of the Galaxy, G35.20-0.74 and G35.20-1.74 have peculiar motions of ~13 km/s and ~16 km/s counter to Galactic rotation and ~9 km/s toward the North Galactic Pole.

Mapping the Milky Way and the Local Group

Abstract: Over the past decade, the astrometric accuracy of Very Long Baseline Inteferometry has improved dramatically. Currently relative positions between sources separated by about 1◦ are being measured with accuracies of ∼ 10 μas. With this accuracy, trigonometric parallaxes throughout the Milky Way and proper motions of Local Group galaxies are now being measured. These observations will lead to direct mapping of the structure of the Milky Way, testing of the spiral density wave paradigm, measuring the dark matter halos of Local Group galaxies, and better understanding of the past history and future fate of these galaxies.

Submillimeter narrow emission lines from the inner envelope of IRC+10216

Abstract: A spectral-line survey of IRC+10216 in the 345 GHz band has been undertaken with the Submillimeter Array. Although not yet completed, it has already yielded a fairly large sample of narrow molecular emission lines with line-widths indicating expansion velocities of ~4 km/s, less than 3 times the well-known value of the terminal expansion velocity (14.5 km/s) of the outer envelope. Five of these narrow lines have now been identified as rotational transitions in vibrationally excited states of previously detected molecules: the v=1, J=17--16 and J=19--18 lines of Si34S and 29SiS and the v=2, J=7--6 line of CS. Maps of these lines show that the emission is confined to a region within ~60 AU of the star, indicating that the narrow-line emission is probing the region of dust-formation where the stellar wind is still being accelerated.